Surface Characterization: A User's Sourcebook

Surface Characterization Edited by D. Brune, R.Hellborg H. J. Whitlow, 0.Hunderi 639 WILEYVCH Surface Characterizati...

Author: Dag Brune | Ragnar Hellborg | Harry J. Whitlow | Ola Hunderi

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Surface Characterization Edited by D. Brune, R.Hellborg H. J. Whitlow, 0.Hunderi

639 WILEYVCH

Surface Characterization A User’s Sourcebook Edited by D. Brune, R. Hellborg H. J. Whitlow, 0. Hunderi

U

Scandinavian Science Publisher

@ W I LEY-VCH

Weinheim . Berlin . New York * Chichester Brisbane . Singapore Toronto

The support from Arna and Johannes Brune’s Memorial Foundation is gratefully acknowledged Dr. I h g Brune Scandinavian Scicncc Publisher Bakkehaugveien 16 N-0873 Oslo Norway

Dr. Harry J . Whitlow Department of Nuclcar Physics Lunds University and Institute of Technology P.O.Box 118 S-22100 Lund Swcden

Dr. Ragnar Hellborg Department of Physics University of Lund Siilvegatan 14 S-22362 Lund Swcdcn

Prof. Ola Hundcri Department of Physics Norwegian University of Scicnce and Technology N-7034 lrondheim Norway

This book was carcfully produced. Nevertheless, authors, editors and publisher do not warrant the information contained therein to bc frcc of error\. Keaders are advised to keep in mind that statcments, data, illustrations, procedural details o r othcr items may inadvertently be inaccurate. -

Editorial Director: Dr. Christina Dyllick Production Manager: Claudia Griissl Cover illustration: Growth of polycrystalline diamond surface. Three dimensional imagc by Atomic Force Microscopy. Courtesy of Richard G5hlin, Dcpartment of Technology, Uppsala University. Library of Congress Card No. applicd for A cataloguc record for this book is available from the British Library Dic Dcutsche Bibliothek CIP-Einheitsaufnahmc Surface characterization : a uscr’s sourcebook / ed. by D. Brune ... [Oslo] : Scandinavian Scicnce Publ. ; Wcinhcim ; New York ; Chichester ; Brisbanc ; Singapore ; Toronto : Wiley-VCH, 1907 ISBN 3-527-28843-0 ~

~

WILEY-VCH Vcrlag GmbH. D-69469 Weinheim (Federal Republic of Germany), 1997 Printed on acid-free and low chlorinc paper All rights reserved (including those of translation into other languages). No part of this book may he reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translatcd into a machine language without writtcn permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically markcd as such, are not t o be considcred unprotected by law. Printing: bctz-druck GmhH, 11-64291 Darmstadt Bookhinding: Wilh. Osswald & Co., D-67433 Ncustadt Printed in the Fcdcral Kcpublic of Germany

Preface Surface science is interdisciplinary in nature because it enters in some shape or form into every branch of science and technology. This work, which is directed to both academic and industrial audiences working on the development and production of new materials or in process control, seeks to provide a user-oriented guide for identifying and, selecting those techniques best suited to the requirements of a specific analytical problem. This makes the book an invaluable reference guide for scientists and engineers from a broad range of disciplines within the life sciences, chemistry, physics, metallurgy as well as materials, mechanical, electronic, environmental, electrical and other technologies. A special feature of the book is that the individual chapters, written by acknowledged experts from industry and research on the different techniques, and providing practical advice, are organized in parts according to the material property or characteristic and type of information sought. These parts are preceded by an overview comparing the capabilities of the methods and the types of information they provide. This unconventional ordering according to analysed properties or characteristics, rather than technique, provides a simple path for the user to find the information to solve his particular analytical problem. Introductory tutorials help the reader formulate his analytical requirement and give guidance in the selection of the method. Extensive comparative tables allow the reader to compare directly the capabilities and requirements as well as the pit-falls inherent in the different methods. The book constitutes an advanced tool for development and progress in industry, education and research. The natural progression of techniques being developed in the academic laboratory to become standard shop-floor methods has meant that we have included not just wellestablished techniques, such as scanning electron microscopy, but also promising new techniques such as those based on optical reflectance of laser beams, which, although at the academic laboratory stage at present are, because of their dynamic and noncontact in-situ nature, likely to find wide spread industrial application in the near future.

The editors

Dag Brune, Ph.D. Ragnar Hellborg, Ph.D. Harry J. Whitlow, Ph.D. Ola Hunderi, Prof.

Editorial advisory board Sture Hogmark, Prof. K k e Kristiansen, Ph.D. Ragnar Larsson, Prof. Lars Mattson, Ph.D. Ingemar Oleijord, Prof. Emil J. Samuelsen, Prof. Eero J. Suoninen, Prof.

Acknowledgements The editors want to express their sincere gratitude to all authors in the project for their willingness to share their high level of specialist knowledge and competence. The scientists on the editorial advisory board in addition to having given valuable scientific and technological input, have also reviewed manuscripts, which is greatly appreciated. At an early stage of the project one of the contributors, Lars Mattsson, suggested that the various analytical techniques should be collected in groups according to the material property to be elucidated. The development of this fruitful concept was a difficult and tedious task that allowed all the pieces to be put into their right places. The invaluable technical editing of the book by Kristina Hellborg is further gratefully acknowledged.

Table of contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI

.........................

VII

Table of contents. . . . . . . . . . . . . . . . . .

.........................

Contributors . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Guide to surface analysis ................................... Selection of method E. J. Suoninen Reference data tables . . . H.J. Whitlow

1 1

...................................

14

................................

32

Part 1: Microstructure and topography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Optical microscopy . . . . . . . . . . . . . . . . . . . E.J. Suoninen

2. Confocal scanning optical microscopy . . . . . . . N.J. McCormick 3. Scanning probe microscopy . . . . . . . . . . . . . . . L. Mattsson 4. Surface roughness and microtopography . . . . . L. Mattsson 5. Etching for microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . P. T. Zagierski

Part 2: Elemental composition

x

...........

53

...........

........ ........... . . . . . . . 101

.........................

- and wavelength-dispersive spectrom 6. Scanning electron microscope wit K. Kristiansen 7. X-ray fluorescence analysis. . . . .............................. Myint ri, J. Tolgvessy and K. 8. Energy-dispersive X-ray fluorescence analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R.P. Pettersson and E. Selin-Lindgren 9. Particle-induced X-ray emission and particle-induced gamma ray emission. . . . . . . . . K. G. Malmqvist 10. Charged-particle activation analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K. Strijckmans 1 1. Atom-probe field-ion microscopy H-0. Andrkn 12. Ion scattering spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Taglauer 13. Dynamic-mode secondary-ion mass spectrometry . . . . . . . . . ....... A.R. Lodding and U.S. Sodervall ......... 14. Glow-discharge optical-emission spectroscopy . . . A. Bengtson 15. Nuclear reaction anaiysis ...........

129 136 154 169

205

Vlll

Table of contents

H. Whitlow and R. Hellborg 16. Rutherford back-scattering spectrometry and recoil spectrometry . . . . . . . . . . . . . . . 254 H.J. Whitlow and M. &ling ..... . . . . 272 17. Auger electron spectroscopy . . C.-0. A . Olsson, S. E. Horns

Part 3: Chemical bonding and molecular composition. . . . . . . . . . . . . . . . 289 18. X-ray photoelectron spectro 1. Olegord 19. Synchrotron light . . . . . . . I. Lindau 20. Static mode secondary-ion mass spectrometry . . . . . P. Bertrand and L. T. Weng 2 I . Laser-microprobe mass spectrometry L. van Vaeck, W van Roy, H. Str 22. Fourier-transform infrared s .J. 0. Leppinen 23. Raman spectroscopy. . , , . E.J. Samuelsen 24. Mbssbauer spectroscopy. . . . ........... R. Wappling 25. Laminate analysis by chemometrics . . . . . . . . . . . . . . . . . . . . . . . . A.A. Chrisv, F.O. Libnau and O.M. Kvalheim

. . . . . . . . 291

. . . . . . . 354

Part 4: Crystallography and structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

425

Part 5: Surface films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

493

Part 6: Surface reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

563

26. Studies of surface structures by X-ray diffraction and reflectometry E.J. Samuelsen 27. Transmission electron microscopy and diffraction. . . . . . . . . . E. Johnson 28. Electrons for surface diffraction, imaging and vibrational spectroscopy. . . . . . . . . . . 465 C. Nyberg ................................. . . . . . . . . . . 478

30. Optical characterization of surfaces by linear spectroscopy . , . . J. Bremer, 0. Hunderi and E. Wold 3 1. Optical second-harmonic and sum-frequency generation . . . . . . J. F. McGilp 32. Electrical and magnetic properties of thin films S. Hellstrom 33. Measurement and properties of thin films, . . . . . . . . . . . . . . . . S. Hellstrom

....... ................ 34. Adhesion and surface energy. R. Larsson ........ 35. Surface reactivity . . . . . . . . . . ................ R. Larsson ....... 36. Emanation thermal analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . J. Tolgyessy and R. Larsson 37. Electrochemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J Berendson 38. Corrosion measurements by use of thin-layer activatio ................ J. Asher 39. Nuclear-based corrosion monitoring. . . . . . . . ................ D. Brune

565

570 585 590 607

614

Table of contents IX

Part7:Tribology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

623

40. Tribosurface properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624 S. Hogmark, S. Jacobson, P. Hedenqvist and M. Olson 41. Wear measurements using thin-layer activation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 645 J. Asher

Part 8: Life sciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659 42. Biomaterials.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 660 B. Kasemo and J. Lausmaa 43. Biological nuclear microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681 U. Lindh

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

698

Contributors Hans-Olof Andren, Prof. Department of Physics Chalmers University of Technology S-412 96 Gothenburg, Sweden

Jim Asher, Ph.D. AEA Technology plc 477 Harwell, Oxfordshire OX1 1 ORA, UK

Arne Bengtson, Ph.D. Swedish Institute for Metals Research Drottning Kristinas vag 48 S-114 28 Stockholm, Sweden

Jaak Berendson, Ph.D. Laboratory of Applied Electrochemistry Royal Institute of Technology (KTH) S-100 44 Stockholm, Sweden

Patrick Bertrand, Prof. Universite Catholique de Louvain Unit6 de Physico-Chimie et de Physique des Materiaux 1, Place Croix du Sud B-1348 Louvain-la-Neuve, Belgium

Johannes Bremer, Ph.D. Departement of Physics Norwegian University of Science and Technology N-7034 Trondheim, Norway

Dag Brune, Ph.D. Scandinavian Science Publisher Bakkehaugveien I6 N-0873 Oslo, Norway

Alfred A. Christy, Ph.D. Department of Chemistry University of Bergen Alleg. 4 1 N-5007 Bergen, Norway

Nicholas J. McCormick, Ph.D. National Physical Laboratory Queens Road, Teddington Middx TWI 1 OLW. UK

Renaat Gijbels, Prof. Department of Chemistry University of Antwerp (UIA) Universiteitsplein 1 8-2610 Wilrijk, Belgium

John F. McGilp, Prof. Department of Physics Trinity College Dublin, 2, Ireland

Per Hedenqvist, Ph.D. Materials Science Division Uppsala University P.O. Box 534 S-75 1 2 1 Uppsala, Sweden

Ragnar Hellborg, Ph.D. Department of Physics University of Lund Solvegatan 14 5-223 62 Lund, Sweden

Sten Hellstrom, Ph.D. Relectronic-ESD HB Laxvllgen 46 S-18 t 30 LidingB, Sweden

Sture Hogmark, Prof. Materials Science Division Uppsala University P.O. Box 534 5-751 21 Uppsala, Sweden

Sven Erik Htirnstrlim, Ph.D. CITU Dalarna University S-781 88 Borlilnge, Sweden

Contributors XI Ola Hunderi, Prof. Departement of Physics Norwegian University of Science and Technology N-7034 Trondheim, Norway

Staffan Jacobson, Ph.D. Materials Science Division Uppsala University P.O. Box 534 S-751 21 Uppsala, Sweden

Erik Johnson, Ph.D. Niels Bohr Institute for Astronomy, Physics and Geophysics Orsted Laboratory, Universitetsparken 5 DK-2 100 Copenhagen 0, Denmark

Bengt Kasemo, Prof. Department of Applied Physics Chalmers University of Technology and University o f Gothenburg S412 96 Gothenburg, Sweden

Khre Kristiansen, Ph.D. Hydro Research Centre P.O. Box 2560 Hersya N-3901 Porsgrunn, Norway

Olav M. Kvalheim, Prof. Department of Chemistry University of Bergen Alleg. 41 N-5007 Bergen, Norway

Ragnar Larsson, Prof. Group of Catalysis Research Chemical Center University of Lund S-22 1 00 Lund, Sweden

Jukka Lausmaa, Ph.D. Department of Applied Physics Chalmers University of Technology and University of Gothenburg S-4 12 96 Gothenburg, Sweden

Jaakko 0. Leppinen, Ph.D. Technical Research Centre of Finland VTT Chemical Technology, Mineral Processing FIN-83500 Outokumpu, Finland

Fred 0. Libnau, Ph.D. Department of Chemistry University of Bergen Alleg. 4 1 N-5007 Bergen, Norway

Ingolf Lindau, Prof. University of Lund Department of Physics University of Lund Solvegatan 14 S-223 62 Lund, Sweden

Ulf Lindh, Ph.D. Centre for Metal Biology in Uppsala and Division of Biomedical Radiation Sciences Uppsala University P.O. Box 535 S-75 1 2 1 Uppsala, Sweden

Alexander R. Lodding, Prof. SIMS-Laboratory Physics Department Chalmers University of Technology S-412 96 Gothenburg, Sweden

Klas Malmqvist, Prof. Department of Nuclear Physics Lund University and Institute of Technology P.O. Box 118 S-22 1 00 Lund. Sweden

Lars Mattsson, Ph.D. Surface Evaluation Laboratory Institute of Optical Research S-100 44 Stockholm, Sweden

Myint U, Prof. Nuclear Chemistry Research Laboratory Department of Chemistry University of Yangon Yangon, Myanmar

XI1

Contributors

Curt Nyberg, Ph.D. Department of Applied Physics Chalmers University of Technology and University of Gothenburg S-4 12 96 Gothenburg, Sweden

Ingemar Olefjord, Prof. Department of Engineering Metals Chalmers University of Technology S-412 96 Gothenburg, Sweden

Claes-Olof A. Olsson, Ph.D. Materials Centre University College of Falun-Borlange P.O. Box 764 S-781 27 Borllnge, Sweden

Patrik Pettersson, Ph.D. Department of Physics Chalmers University of Technology and University of Gothenburg S-412 96 Gothenburg, Sweden

Katrien Poels, Ph.D. Department of Chemistry University of Antwerp (UIA) Universiteitsplein 1 B-2610 Wilrijk, Belgium

Emil J. Samuelsen, Prof. Department of Physics Norwegian University of Science and Technology N-7034 Trondheim, Norway

Eva Selin Lindgren, Prof. Department of Physics Chalmers University of Technology and University of Gothenburg

Karel Strijckmans, Prof. Lab. Analytical Chemistry Institute Nuclear Sciences University of Gent B-9000 Gent, Belgium

S-412 96 Gothenburg, Sweden Herbert Struyf, Ph.D. Department of Chemistry University of Antwerp (UIA) Universiteitsplein 1 8-2610 Wilrijk, Belgium

Eero J. Suoninen, Prof Departement of Applied Physics University of Turku FIN-20520 Turku, Finland

Ulf S. Sodewall, Ph.D. SIMS-Laboratory Physics Department Chalmers University of Technology S-4 12 96 Gothenburg, Sweden

Edmund Taglauer, Ph.D. Max-Planck-Institut fir Plasmaphysik EURATOM Association D-85748 Garching bei Miinchen, Germany

Juray Tolgyessy, Prof. Department of Environmental Science University of Bratislava Bratislava, Slovakia

Wim Van Roy, Ph.D. Department of Chemistry University of Antwerp (UIA) Universiteitsplein 1 B-2610 Wilrijk, Belgium

Luc Van Vaeck, Ph.D. Department of Chemistry University of Antwerp (UIA) Universiteitsplein 1 8-2610 Wilrijk, Belgium

Lu Tao Weng, Ph.D.

Harry J. Whitlow, Ph.D. Department of Nuclear Physics Lunds University and Institute of Technology P.O. Box 118 S-221 00 Lund, Sweden

Erik Wold, Ph.D. Departement of Physics Norwegian University of Science and Technology N-7034 Trondheim, Norway

UniversitC Catholique de Louvain Unit6 de Physico-Chimie et de Physique des MatCriaux 1, Place Croix du Sud 8-1348 Louvain-la-Neuve, Belgium

Contributors XI11 Roger Wiippling, Prof. Uppsala University Department of physics Box 530 S-75 1 2 1 Uppsala, Sweden Teodor Zagierski, Ph.D. Material Research Center Forskningsparken University of Oslo N-0371 Oslo, Norway

Mikael Ostling, Ph.D. Department of Solid State Electronics The Royal Institute of Technology KTH-Electrum P.O. Box E229 S-164 40 Kista, Sweden

Surface Characterizatioti: A User3 Sourcebook

Edited by D Brune, R Hellborg, H J Whitlow & 0 Hunderi copyright&)WILEY-VCH Vcrlag Grnh11.19Y7

Introduction Guide to surface analysis H.J. Whitlow, K. Kristiansen and 0. Hunderi Surface science and engineering are playing an ever more important part in all aspects of our modern technological society. Today, engineers and scientists in disciplines that have been traditionally far removed from surface science, (such as dentistry, forestry and food science) are becoming involved with surfaces and surface processes and thereby setting new challenges for the surface characterization community. It is often a difficult task for these new users to select appropriate methods of characterization to study their surfaces and start fruitful cross-disciplinary collaborations with practitioners of surface characterization. The object of this work is to highlight for the new user, some pertinent questions, by discussion of fundamental aspects of surface analysis, limitations, the potential of different methods and to aid in establishing collaborations with surface-science groups.

1 The cross-disciplinary nature of surface characterization The study of interfaces and surfaces goes back more than one hundred years and the recognition of the importance of understanding surfaces goes back to the early part of the century, particularly in the fields of catalysis and corrosion. It was, however, not until the 1960s with the development of ultra-high vacuum techniques that we saw the explosion in methods for characterization of surfaces which may leave many researchers bewildered as to what technique to use given a particular problem. Today, it is probably true to say that the most cross-disciplinary area of natural science and technology is concerned with surfaces and interfaces. There is a number of reasons why this is so. From a fundamental viewpoint, all interactions with a finite body of matter must take place through its surface. It follows that if the surface modifies the interaction, then the perceived properties of the body of matter as a whole are also modified. The relation between the properties of a surface and the process history of the material must be understood since control of the properties of the surface is critical in many technological processes. Examples of such processes include all chemical reactions, catalysis, corrosion, adhesion, crystal growth. Hence surface analysis is a necessary tool to determine whether a surface has the desired properties (optical, electrical, chemical, mechanical or visual). The ability to analyse the surface composition and surface geometry is playing an ever increasingly important role in a wide range of industries particularly since the composition of the surface may differ considerably from that of the bulk as minority elements often segregate at free surfaces or at grain boundaries. This dramatic impact of materials science in a wide range of disciplines

2

Introduction

with which it has not been traditionally associated (such as: dentistry, forestry, food proccssing science, etc.) has led to scientists and engineers from these disciplines becoming new users of surface characterization methods. A good example of the cross-disciplinary nature of surface science can be found in research on the consequences of the introduction of methanol-based fuels to reduce environmentally hazardous emissions from motor vehicles. It has led to close collaboration between automobile engineers, lubrication chemists, tribologists and surface characterization specialists: in cold environments, during the starting and initial warmup phases of methanol-fuelled engines, considerable quantities of water, methanol and formic acid, (a product of the partial combustion of methanol) are absorbed by the oil. This mixture degrades the boundary lubrication films on metal tribosurfaces that are formed from special oil additives. This will influence bearing lifetime and may lead to catastrophic failure by seizure. Considerable effort is currently devoted to characterization of the failure mechanisms and the steps taken to overcome them. New users of surface-characterization techniques are confronted by a wealth of different techniques, and it is often a difficult task to select appropriate methods to characterize the surfaces in question. Almost inevitably this involves setting up a collaboration with experts in surface characterization. This work attempts to help foster these interactions by summarizing the fundamental aspects of surface characterization, limitations imposed by choice of method, factors affecting the potential of the various analytical methods and some guidelines on management of cross-disciplinary collaborations. This book is first of all intended to serve as a practical guide for scientists who are not themselves doing basic research in surface science but who are faced with the problem of determining surface properties. Practical examples of how the various techniques are used are given together with how data are analysed and what the limitations of the various techniques are, because the ideal technique does not exist. Therefore results from several techniques and knowledge of sample history often have to be compared and correlated in order to get a satisfactory answer to the question asked. In this book a broad definition of what constitutes an interface or surface is adopted because it also concerns industrially produced 'surfaces' and not only the often well defined single crystal surfaces studied in basic research. In this context, there are two important questions to be asked: 0 0

What are the desired properties of this surface? Does the surface in question have these desired properties?

2 Fundamentals of surface layer characterization 2.1 The definition of surface and surface layer A problem-oriented definition of the surface layer as used in this book, has the natural implication that the definition of layer thickness is governed by the particular appli-

Guide to surface analysis

3

cation. For example, in the catalytic reaction of gas molecules on a metal surface, the extent of the surface layer encompasses the adsorbed gas molecules and a surface layer of metal atoms. An artificial hip-joint treated by nitrogen-ion implantation to enhance resistance to corrosion by body fluids, illustrates that it is natural to consider the surface layer to extend to the depth where the composition is indistinguishable from that of bulk material. A more common definition of the word ‘surfaces’ is a layer of zero thickness forming the boundary of an object. From a materials science viewpoint, this is not a very enlightening description and it is more realistic to consider a surface layer as the interface between two media. The extent of the interface then spans the region where the properties deviate from those of the bulk media. Specific definition of the surface-layer as being the outer atomic layer is not very satisfying either. This is because processes that reduce the free energy of the outer atomic layer may modify the crystallographic structure over a considerable number of deeper atomic layers (4-5 layers for Si( 100)). A further common, and often misleading, assumption is to consider the surface layer to span the depth interval (information depth) from which the analytical signal is obtained. This has the disadvantage that the depth from which the analytical signal is derived may vary from one measured characteristic to another and from one analytical technique to another. This problem-oriented approach implies that the geometric extent of the surface layer of importance in a particular problem will vary from case to case, from atomic dimensions (nm) to hundreds of micrometers as in coatings and surface segregations.

2.2 Problem definition Problem definition means asking the right questions. This is not a trivial task and should be considered carefully. Examples of decisions to be made are: 0

0

0

Is an identification of the chemical compound needed or is the composition sufficient? In some cases it is important to know, for example, whether the surface layer is a sulphide or a sulphate. This question might be answered by quantitative elemental analysis, but some techniques might also be able to indicate whether the sulphur is surrounded by oxygen atoms or not. Still other techniques might identify the compound by its crystallographic signature. Are quantitative results needed, or is qualitative identification sufficient? Quantitative results with high accuracy are difficult and therefore expensive and time-consuming to obtain. The accuracy required should therefore be carefully considered before starting the analytical work. What is the detection limit needed for qualitative identification? Again low detection limit demands special techniques and careful analysis. The requirements should therefore again be considered carefully.

lnlroduction

4

0

If quantitative results are wanted, what are the accuracy and resolution needed? Accurate quantitative results require careful analysis, well documented standards and careful calibration procedures. Again it is therefore important to be realistic in the requirements.

In addition to the selection of technique, these points also have a great impact on sample selection and sample preparation. There is no point in carrying out an accurate and careful analysis if one is not absolutely sure that the saniple is representative and that it has not in any way changed its nature before reaching the analytical laboratory. The definition of precision, repeatability, uncertainty and accuracy according to IS0 353 1- 1 : 1993 are given in Appendix I. ’l’hcncxt important step is a careful definition of the surface layer property that one wants to characterize. On the basis of this and knowledge about the material in question suitable methods can be selected.

2.3 Characterization of surface layer properties: concentrations, maps, profiles and sections The general underlying scheme for surface layer characterization is presented in Fig. 1. A well defined probe is used to introduce a localized disturbance in the material. In the resulting relaxation process(es) various characteristic signals are emitted from the disturbed region. These signals are registered using a suitablc analyser. Some measurements of surface layer characteristics require the use of more than one probe or signal.

I

Material

1 Dcpth profile

Fig. I . Basic principle of surface characterization and schematic illustration of depth profile and elemental map.


5

Table 1. Common acronyms for surface analysis techniques. Main acronym AES AFM APFIM CPAA EDXRF EIS ETA FTIR GD-OES GEXRF GI-XRF HREELS ISS LEED LEEM LMMS ME NCM NRA NUMI PAS PICE PIXE PTS R-XRF RAS RBS WEED RS SEM SFG SHG Dynamic-SIMS SOM SR Static-SIMS STM SVET TEM TLA TPD TXRF XPS XRD XRF XRR

Other acronym SAM FIM-AP, FlM TFA, TFAA XRF, EDX

GDOS, GDS WD-GEXRF AD-TXRF, IAD-TXRF LElS LAMMA MS, NGR NRB PIGME XRF BS

ERD, ERDA, ERA EDS, WDS, EMP SSHG IMMA

SSIMS SAD, CBED SLA TR-XRF ESCA SXRD AD-TXRF, IAD-TXRF

Name of technique Auger Electron Spectroscopy Atomic Force Microscopy Atom Probe Field Ion Microscopy Charged Particle Activation Analysis Energy Dispersive X-ray Fluorescence Analysis Electrochemical lmpendance Spectroscopy Emanation Thermal Analysis Fourier Transform Infrared Spectroscopy Glow Discharge Optical Emission Spectroscopy Grazing-Emission X-ray Fluorescence Grazing-Incidence X-ray Fluorescence High Resolution Electron Energy Loss Spectroscopy Ion Scattering Spectroscopy Low Energy Electron Diffraction Low Energy Electron Microscopy Laser Microprobe Mass Spectroscopy Mossbauer Spectroscopy Nuclear Corrosion Monitoring Nuclear Reaction Analysis Nuclear Microscopy Photoacoustic Spectroscopy Particle Induced Gamma-ray Emission Particle Induced X-ray Emission Photothermal Spectroscopy Radionuclide Induced X-ray Fluorescence Surface Raman Spectroscopy Rutherford Backscattering Spectrometry Reflection High Energy Electron Diffraction Recoil Spectrometry Scanning Electron Microscopy with X-ray Spectrometer Optical Sum Frequency Generation Outical Second Harmonic Generation Dynamic Mode Secondary Ion Mass Spectrometry Confocal Scanning Optical Microscopy Synchrotron Radiation Static Mode Secondary Ion Mass Spectrometry Scanning Tunnelling Microscopy Scanning Vibrating Electrode Technique Transmission Electron Microscopy/Diffraction Thin Layer Activation Temperature Programmed Desorption Total-Reflection X-ray Fluorescence Spectrometry for Chemical Analysis X-ray Photoelectron Spectroscopy X-ray Diffraction and Reflectrometry X-ray Fluorescence X-ray Reflectrometry

6

Introduction

Surface science is fundamentally concerned with the properties of an inhomogeneous system. Thus properties will vary in at least one dimension and generally the property C will be distributed in all three dimensions. The amount of information in a 3-dimensional distribution is enormous and difficult to visualize. We therefore try to reduce the complexity by reducing the number of dimensions through the processes of integration and sampling (sectioning). For example the distribution of concentration perpendicular to the surface plane is termed the depth profile and describcs how C is distributed in depth (Fig. 1). The area over which the integration is carried out is usually determined by the extent of the analysing beam. If one is interested in how the property C is distributed laterally it can be represented by a twodimensional map of the surface. The mapping can be done in two ways: scanning of the probe over the surface or illumination of a wide area with the probe and imaging the characteristic signal directly, or as a diffraction pattern using a suitable detector system. The depth interval over which the integration is carried out is usually determined by the problem, e.g. for gas atoms physisorbed on a surface plane the extent of the integration is only over the extent of the outer atomic layer. These presentation methods have the disadvantage that they smear out the information about the distribution along one or more directions in the integration process. An altcrnative approach is sectioning where one measures the lateral distribution of a property in a thin slice of the material. It should be borne in mind that sectioning is in essence a sampling method where one measures the distribution of a property in the direction perpendicular to the plane of sectioning. The fact that the section has a well defined thickness means that even methods that are only laterally dispersive can be used for depth profiling. When using sectioning, a series of bulk analytical techniques must also be considered, like X-ray fluorescence, atomic absorption spectroscopy and others. A discussion on the selection of the specific technique is given in a separate chapter of this book.

3 Quality assurance and safeguards 3.1 Quality assurance The word quality is used in many contexts. It concerns the fulfilment of expectations, pronounced and perceived. It also means the absence of faults or something of superior class, expensive. In a commercial and industrial context a more stringent definition is needed. IS043402 give the following definitions: Quality is the totality of characteristics of an entity that bear on its ability to satisfy stated and implied needs. Quality control concerns the operational means to fulfil the quality requirements, while quality assurance aims at providing confidence in this fulfilment, both within the


7

organization and externally to customers and authorities. The quality loop involves all interactive activities that influence quality at the various stages ranging from the identification of needs to the assessment of whether these needs have been satisfied. Some other terms relevant to industrial and commercial contexts is given in the appendix.

3.2 Modification induced by the analytical procedure An important question that must always be considered by the user is whether analysing or primary probe influence the sample in such a way as to modify the effect of the probe or induce other undesirable effects. Table 2 indicates some of the effects that the different probes can induce in materials. Table 2. Probe-induced sample modification. Probe keV-electron beam Ion Beam

X-rays Neutrons Stylus Indenter Electrolyte

Effect Electron stimulated adsorption, electron assisted desorption, radiation damage, radiolysis Radiation damage, sputtering, ion assisted adsorption, induced radioactivity (MeV ions only) collision cascade mixing and radiation enhanced diffusion, radiolysis Photon induced chemical modification, radiolysis Induced radioactivity, radiation damage, radiolysis Plastic flow, wear debris deposition, wear scars. Plastic deformation Electrochemical hydrogen charging

The most common deterioration of the sample is associated with the energy input from the analytical probe. This can have many consequences: chemical reactions that redistribute elements within the sample, recrystallization, evaporation of some constituents, deformation due to differential thermal expansion, etc. Radiation damage is associated with displacement of atoms out of their normal sites in a material. This is generally associated with particle irradiation, but may also be induced in some materials by UV and X-ray photons. In crystalline materials radiation damage can be primarily associated with the formation of Frenkel-pairs, that is a vacancy and interstitial in the crystal lattice. These point defects migrate and can coalesce to form extended defects such as dislocation loops, voids and precipitates. The consequences are significant changes in properties that are associated with the electrical structure of the material. The enhanced vacancy concentration associated with radiation damage can also enhance solid state diffusion. Another process, that is often undesirable from an analysis viewpoint, is collision cascade mixing. This results from ion irradiation when the primary ions from the probing beam transfer large amounts of energy to target atoms that subsequently recoil. These in turn undergo further elastic collisions with target atoms generating further recoils. Thus a single primary ion collision can generate a whole cascade of moving

8

Introduction

secondary recoils (Fig. 2). The displacement of atoms in collision cascades leads both to mixing as well as enhancement of diffusion processes. Moreover, energetic backwards directcd recoils may have sufficient energy to leave the material (sputtering).

Pro'?Ie

\

Sputtered atoms 3

Surface

Recoil --

Frenkel pair

"1

Interstitial

>Primary -./

Fig. 2. Schematic illustration of the processes taking place in a collision cascade.

Radiolysis occurs when the energy transfer in electronic processes induces changes in chemical bonding. The energy quanta induce local excitation of the electronic structure. A good example is the breakdown of PVC by the process of side atom abstraction. It is important to bear in mind that such processes do not always require high energies, indeed this particular process is responsible for the breakdown of PVC in sunlight. Other detrimental effects are not restricted to the probes. Wear resistance testing will, for example, lead to work hardening due to the mechanical introduction of dislocations and other defects into the surface layer. The local stress present at the point where the tip of a stylus probe impinges on the material may in soft materials be sufficiently large to distoi-t the material leading to localized plastic flow.

3.3 Limitations imposed by the analysis environment The environment required for the analysis may also present serious limitations. These may arise because either the analysis environment might introduce undesirable changes in the sample, or the sample can introduce undesirable effects in the analysis instrument. The latter often have trivial reasons but are difficult or expensive to overcome. Examples of this are high vapour pressure materials (e.g. arsenic, zinc, mineral oils, etc.) that might contaminate a vacuum system, as well as corrosive or other objectionable materials. The problem can often be overcome through the use of special analysis cells or techniques which prevent or minimize exposure of the analysis system to objectionable influences, e.g. by sealing or fixing the surface of the sample. As surface science is moving into the fields of bioscience and medicine a further consideration is the safe handling of biohazardous materials, e.8. toxin and virus preparations, etc. The analysis procedure may also limit the analysis by introducing undesirable changes in the sample. The effects listed in Table 2 are an obvious source of


9

undesirable changes. However, not all detrimental changes are associated with the probe. Many analysis techniques require the sample to be placed in a vacuum. In some extreme cases the sample may even need to be baked out to achieve UHV conditions. Some sample materials, such as many archaeological artefacts and biological material may be detrimentally altered by such treatment. The sample may also be unwieldy and not fit readily into the vacuum chamber. An obvious example is works of art. Although here, small samples or objects &art for microanalysis may be made available in conjunction with conservation measures. Moreover, in the case of insulating materials it may be necessary to coat the material with a conducting layer such as vacuumdeposited gold, aluminium or carbon to prevent charge build up with charged particle beam probes. Induced radioactivity resulting from analysis with high-energy light-ion beams (>3 MeV p, d) is particularly troublesome because after analysis the samples must be treated as radioactive material. (Often the activity is short-lived (-hours).) The induced activity may lead to difficulties in subjecting the sample to further processing on aesthetic or regulatory grounds (e.g. samples used in the food and drugs industries.) It has also been observed that even if isotopes of the major elements are not associated with induced activity, considerable activity may be associated with isotopes of minor elements. In the case of samples of high strategic or monetary value measures may have to be taken to prevent theft of the sample or the unauthorized access to results of the analysis. In extreme cases it has even proven advantageous to set up analysis facilities within a secure area. The accelerator analysis facility at The Louvre in Paris is a good example of this.

3.4 Surface-layer-induced limitations The nature of the surface layer itself may also cause problems. The ‘ideal’ surface layer for most analytical techniques is homogeneous and has a flat surface topography. Unfortunately few real surfaces are flat, even on a macroscopic scale. Even crystal facets may be vicinal stepped surfaces when viewed on a microscopic scale. Most surface-characterization techniques measure layer thicknesses perpendicular to the surface plane. This can lead to an increase in the apparent surface-layer thicknesses. A good example is a fibre or tubular structure, analysis of these structures may demand the use of microtomographic methods (Fig. 3). The sample surface layer may also modify the characteristic signal(s) from the region below and thereby also give ‘false’ fluorescence signals. Erosion-induced surface-roughening may be a major problem for techniques where a probe is used to successively erode the surface. This can take place even if the initial surface topography is flat, because of differing erosion rates for different crystal faces, or chemical phases of differing composition. It is usually sufficient to check the topography at the bottom of the erosion pit after analysis using a scanning electron microscope.

10 lntroduction

1

Surface layer of uniform thickness

Surface layer Of miforrn perpendicular thickness

Fiber with constant radial thickness

Fig. 3 , Schematic representation of different surfaces and surface layers.

3.5 Other limitations The sample presented for analysis is of necessity a very small part of a large component or one of many small parts. This means that steps should be taken to ensure that the sample is representative. From the moment the sample is taken until it is analysed it usually takes some time, a few days is typical. One should then consider the possibility that the properties of the sample have changed in an unacceptable way during this period. Even dry air will in nlaiiy cases influence the surface of a sample. If exposure to air is not acceptable, transfer vessels and related procedures have in some cases been developed. A related problem is the possibility of contamination of the sample, for instance by improper handling. A fingerprint is in many cases disastrous. In addition to the factors discussed above a wide range of other factors influence the potential of a particular method to help the user (Table 3 ) . These include purely technical factors such as the depth and lateral resolution, accessible depth, minimum detection limits as well as other factors that are more directly connected to the user’s analytical problem. It should be borne in mind that these factors are often very dependent on the local circumstances and may therefore vary widely from one instrument to another using the same technique. Moreover, the factors are often closely interrelated, for example, the reliability, cost and degree of staff training will be influenced by the need for complex specimen preparation, etc. The potential for automation should also be considered. Whilst it may lead to a reduction in overall flexibility, automation is very necessary if large numbers of analyses are to be performed. Even for a relatively small number of analyses it may still be beneficial because it enhances uniformity and reduces the risk for human error. It is self-evident that consideration has to be given ensuring the sample is correctly loaded into the automatic system.


11

Table 3. Factors to be considered to evaluate the potential of a method. Property Result needed

Consideration Type of measurement

Analysis parameters

Extent of analysed region

Sensitivity

Reliability of results

Sample preparation Precision Accuracy

Volume of work

Error risk Number of samples Batch size

Legal issues

Personnel cost

Accessibility Traceability Confidentially Impartiality Ethical considerations TYPe Investment Unit cost

Associated factors Quantitative Qualitative Relative Absolute Analysis parameters Analysable area Accessible depth Probe size Lateral resolution Depth resolution Detectability Sensitive or information depth. Selectivity Sampling uncertainties Reference standards Sample preparation Sample identification Batch vs continuous processing Automation vs manual Cost per analysis Reporting vs archiving On-demand, weekly, monthly Accounting procedures IS0 900019001 Patents Number, level of training Type of instrumentation Sample preparation Archiving vs reporting Number and training of personnel

4 Management of cross-disciplinary collaborations The worker in surface science has essentially two choices: do it alone and develop the procedures and facilities from scratch, or collaborate with a person or group that is working in the surface characterization field. The nature of the problem is the major factor in this choice. Collaboration may be dictated by other factors, such as the need for large-scale facilities, e.g. a synchrotron light source.

12 Introduction

In the authors’ experience successful collaborations are based on four important factors: 0

0

Motivation Clear objectives Realistic plans to achieve the objective(s) Good communications

Probably the most important of these factors is motivation because it represcnts the prime mover for the interaction. It is self-evident that if a collaboration is to bc successful, all partncrs must get some benefits by participating in it. These benefits can take the form of financial reward, joint publications, increased internationalization, transfer of knowledge to and from industry, prestige and attainment of competence. The relative value of these benefits will be to a large extent governed by the type of organizations involved. In some environments it is quite possible to set up and run collaborations to do research applying surface characterization without involving any financial transactions or formal agreements. This is of course provided that the other benefits are judged to be of sufficient value. From a collaboration management viewpoint it is essential the objectives are well defined so that all the parties understand what they are working towards. Moreover, the objectives must be realizable otherwise motivation will sooner or later be lost which will lead to break up of the collaboration. The final essential ingredient is good communications. This means that the collaborators are regularly in touch with each other so that views can be aired and problems discussed at an carly stage and interest maintained. Meetings nced not be face to face, but should be fairly regular. With modern communications it is quite possible to collaborate on a daily basis with a partner on the other side of the world as effectively as if they were in the ncxt city or in another unit in an industrial complex.


Appendix I

13

Some general terms relating to observations and test results (According to I S 0 3534-1 : 1993, section 3) Accuracy. The closeness of agreement between a test result and the accepted reference value. The term involves a combination of random components and a common systematic error or bias component (3.1 1, Numbers in parentheses refers to the ISO-standard). Test result. The value of a characteristic obtained by carrying out a specified test method (3.7). Accepted reference value. A value that serves as an agreed-upon reference for comparison: a theoretical or certified value, an assigned or certified value, a consensus or certified value, the expectation of the quantity, i.e. the mean of a specified population of measurements (3.4). Random error. A component of the error which, in the course of a number of test results for the same characteristic, varies in an unpredictable way (3.9). Systematic error. A component of the error which, in the course of a number of test results for the same characteristic, remains constant or varies in a predictable way (3.10). Bias. The difference between the expectation of the test results and an accepted reference value (3.13). Error of result. The test result minus the accepted reference value. Error is the sum of random errors and systematic errors (3.8). Precision. The closeness of agreement between independent test results obtained under stipulated conditions. Precision depends only on the distribution of random errors and does not relate to the true value or the specified value (3.14). True value. The value which characterizes a quantity perfectly defined in the conditions which exist when that quantity is considered (3.2). Reputability. Precision under repeatability conditions (3.15). Repeatability conditions. Conditions where independent test results are obtained with the same method on identical test items in the same laboratory by the same operator using the same equipment within short intervals of time (3.16). Uncertainty. An estimate attached to a test result which characterizes the range of values within which the true value is asserted to lie (3.25). Examples of accredited surface-characterization methods Charged Particle Activation Analysis, Accepted by EU Bureau of reference CPAA Trace analysis of C,N,O Dynamic Mode Secondary Ion Mass Spectrometry, NIST; EU Bureau of Reference SlMS Trace element analysis e.g. BCR 5573/1/4/382(92/9-S(10)) Static-SIMS Static Secondary-Ion Mass Spectrometry EU-BR In progress TXRF Total-Reflection X-ray Fluorescence Spectrometry, IS0 (work in progress) Contamination mapping on Si wafers I S 0 TC201 WG2 Glow-Discharge Optical Emission Spectrometry, IS0 (work in progress) G D OES Composition and thickness of Zn-based metallic coatings IS0 TC201 WG 1

Selection of method B. J. Suoninen The suitability of the most common existing methods of basic characterization of solids for surface characterization is assessed on the basis of their surface sensitivity, information content and effect on the sample during the analysis. In addition to a general assessment of each method, their suitability for horizontal and lateral microanalysis is discussed in relation to the particular requirements associated with these techniques. The methods available for characterization of the surface crystallography are also discussed. The continuing rapid progress of surface characterization due to development of new methods based on use of synchrotron sources and tunnelling microscopes is described briefly, together with the resulting changes in characterization techniques to be expected within the foreseeable future.

1 General 1.1 Comparison with bulk analysis The problem of choosing a suitable method for each particular task of surface characterization is in principle quite difficult. One of the basic reasons for this is the multitude of methods available. However, the practical choice is essentially simplified by the fact that most of the methods are still inaccessible to most researchers. Furthermore, the rapid development of surface characterization techniques is changing the situation continuously. Another typical feature of the situation in surface characterization is the sudden expansion of surface technology to almost all subfields of materials engineering, with greatly vatying characterization needs. It is nevertheless possible to present, from an assessment of the situation today, some viewpoints for guiding the choice, and perhaps make some predictions of the changes to be expected in the foreseeable future. Here this will focus mainly on the most widely used methods of high surface sensitivity described elsewhere in the book. The emphasis will be on chemical characterization. It is, however, important always to keep in mind that surface characterization is almost always an inherently more difficult task than the corresponding bulk characterization. Hence, the use of one method only is seldom enough for a satisfactory solution. As an example of the difference between surface and bulk samples, consider the task of determining the chemical composition of minerals or metallic materials. By determining (e.g., by X-ray diffraction) the crystal structures and relative amounts of the phases present in a bulk sample and combining this information with the phase diagrams already existing in the literature, it is often

Selection of method

15

possible to deduce fairly accurately the composition of the sample. On the other hand, it is usually impossible to draw reliable conclusions on the chemical composition of the surface of a crystalline sample solely from determination of the surface microstructure of a crystalline sample (e.g.,by LEED). The primary reason for this is lack of necessary thermodynamic data relevant to the situation at the surface to be characterized. Assessment of the thermodynamic equilibrium that exists at the surface of a bulk phase, in contact with another phase (solid, liquid or gas), is therefore an essentially more complex task than that of determining the equilibrium phase structure of a crystalline bulk sample.

1.2 Sample preparation The above comments indicate the desirability of employing several different methods for the comprehensive characterization of a solid surface in contact with its environment. The next question to be considered is the effect of the conditions during the measurement upon the sample to be studied. Again, the situation is more complicated in the case of surface characterization than in bulk solids. In the latter case, kinetics usually prevent changes within a bulk sample due to a change in the environment. Hence, we can prepare a sample for study using, e.g., X-ray diffraction and optical microscopy, by etching it chemically and/or physically (sputtering). This procedure has practically no effect on the bulk structure, but usually results in an altered surface layer. A sufficiently thin layer is effectively transparent to the relatively penetrating X-ray signal and hence of no significance to the diffraction pattern formed. However, the situation changes with the type of the signal. Hence, inspection of the surface by optical microscopy after etching reveals the crystal structure of the underlying bulk. The practice of staining biological samples for microscopic studies is based on the same principle.

1.3 Special requirements of surface characterization Let us proceed to consider the special case in which the subject of the study is the surface layer itself and not the underlying bulk. The first obvious requirement is that the analytical signal has to represent the surface region only (Fig. 1). This means that the relative contribution of this region to the total signal seen by the detection system used for the measurement has to be predominant. In contrast to the case of bulk analysis, it is also important to prevent as far as possible any changes in the surface layer before or during the study. This combination of highly surface specific analytical signals and preservation of the sample surface intact during the measurement usually presents the main problem in surface characterization.

I6 lntroduction

Bulk analysls

+

+

*

+

+

+

Thin lilm analysis

X 30 000000 Surface analysis

Fig. 1. The regimes of surface analysis, thin film analysis and bulk analysis (Seah and Briggs, 1990).

2 Chemical characterization 2.1 Surface sensitivity The necessary surface sensitivity can be achieved either by using a surface-sensitive method of exciting the analytical signal, or by employing a signal of high surfacesensitivity, The most popular methods today are based on the use of surface sensitive signals consisting of charged particles having suitable kinetic energies. Hence, either electron spectroscopic methods (XPS, AES) or analysis of secondary ions (SIMS) are presently the most common routine methods for chemical surface characterization. From the viewpoint of surface sensitivity only, SIMS is clearly the best candidate. Methods using electromagnetic signals (X-rays, UV-, visible or infrared light) are, because of their considerably larger range in the sample material, in their conventional form less suitable for routine surface characterization, although numerous variations of such methods, e.g., surface sensitive X-ray fluorescence, multiple total reflectance IR spectroscopy, with excellent surface sensitivity have been developed for special applications.

Selection of method

17

2.2 Effect of characterization on the sample The requirement of preserving the sample unchanged during the characterization is in most cases a strictly speaking impossible criterion to satisfy in any of the above main methods (XPS, AES, SIMS). One reason for this is that they all require the sample to be maintained in an ultrahigh vacuum environment during the characterization. In practically all characterization problems of applied nature, the sample originates from an environment consisting of a gaseous atmosphere (most usually air), a liquid phase or another solid. Hence, the characterization cannot be made in situ, and maintaining artificially the original, now metastable situation, is essential. The usual method used for achieving this is to lower the sample temperature before and during the measurement in order to prevent vaporization of volatile species, phase changes, surface diffusion or enrichment, etc., due to evacuation. This is particularly important in cases where the sample originates from a liquid environment, e.g., an aqueous solution (Kartio et al., 1992). The need for cooling is very important for methods based on excitation of the signal by a particle beam (electrons in AES, ions in SIMS) because of the large amount of energy per unit volume deposited by these methods in the vicinity of the sample surface. Poor thermal conductivity of the sample and/or high vapour pressure of some surface species further accentuate the need for cooling.

XPS and AES The much greater penetration of X-rays into the target, as compared to charged particles, makes the XPS method preferable from the viewpoint of sample heating. In addition to the effects of evacuation or sample heating, an important possibility to be considered is structural changes caused by ionization of the target atoms by the exciting beam (Pignataro, 1992; King, 1992; Seibt et al., 1994). The effect is most pronounced in heavy sputtering, i.e. when intense beams of charged particles (ions) are used to clean or remove the surface layer of the sample before the actual characterization of the resulting sample by electron spectroscopy. Many metal oxides are known to be reduced to lower oxides or even elemental phases during such pre-treatment. The effect of X-ray quanta, used for excitation of the electron signal, is much smaller, but in certain cases still possible. Bombardment by electrons presents an intermediate case with often considerable risk of sample deterioration. Hence, XPS is from this viewpoint a safer method than conventional AES using an electron beam for excitation. On the other hand, analysis of Auger signals excited by X-rays (Kibel, 1992) avoids the risks associated with electron bombardment and is therefore recently gaining popularity (XPS excited AES). It is used particularly in cases in which the Auger signal is considered for some specific reason superior to the XPS signals available. In other cases, XPS signals are to be preferred because of their simpler interpretation (see below).

18 Introduction

Table 1, Comparison of the analytical features of selected surface characterization methods (modified from Garten and Werner, 1994, Table 4.). The number of + denotes the ranking order of the performance indicator describing a particular property. P indicates that the method yields information of the physical structure. Performance indicator Information depth

+

1 Pm

++

< 100 nm 23nm

+++

Depth resolution +

++

+++ ++++ Lateral resolution

Elemental range I.imit of detection

(+)

53nm monolayers 20 p i

+ ++

1 Pm 100 nm

+++

2nm

+

> Li

++ +++

+

++ I t+

Chemical species information

+

++ P

Specimen damage

t

++

Difficulties in quantitation Flexibility Existing data available

1 vm < 100 nm

+ ++

+++ b

++ +++ + ++

XPS

AES

SIMS dynamic

SIMS static

i

+++

t++

+++

+++

(+t')

+++

++

(+++I

I t++

H...U

2 0.1 Ya

++ ++

++

+

i

PPm PPb small available physical structure considerable moderate small high moderate low moderatc good excellent moderate many

+

++

(++I

+

(++I

++

++ (+++I

++ ,

I

(+++I

++ t +++

+++

I

(k++>

t

+

(++I

(++I

++ P

+

+ti

++

+ +t

+

++

+++

i li

+ ++

+t

(PI

(+++I

++

(+I

(+)

>B

1 He

RBS

++

++ , .

++

++

++

++

+~t

+t

+

+

(++'I

SIMS The most important advantage of SIMS in chemical surface characterization is undoubtedly its superior sensitivity when compared with XPS or AES (Table 1). Although the sensitivity varies greatly with the species and the chemical state of the sample, it is typically at least 2-3 orders of magnitude better than in both XPS or AES.

Selection of method

19

In spite of technical problems (see below) and considerable costs, this fact seems to guarantee SIMS a safe position among the main methods of chemical surface characterization for the foreseeable future. /

TOLUENE ETHYLBENZENE XYLENES PHENYLETHYLENE CYCLOHEPTATRIENE

+i PHENYL ACETYL ENE NAPHTHALENE

ALKYLNAPHTHALENES

BENZENE

fast

ion

5-PHENYL -1.3- CYCLOHEXADIENE 3- PHENYL- 1.4 CYCLOHEXADIENE

-

BIPHENYL DIPHENYLMETHANE

METHYLBIPHENYLS

PHENYLCYCLOHEPTATRIENE PHENYL PYRIDINES

TERPHENY L DIBENZYL POLYMERS

Fig. 2. A partial list of chemical compounds formed at bombardment of a solid benzene target at 77 K by a beam of fast ions. Altogether 36 compounds were detected by gas chromatography (Pignataro, 1992, Scheme 1).

In SIMS, the situation with respect to sample damage during signal emission is basically different from XPS and AES considered above. The signal itself is created by a process representing a permanent change of the original sample, i.e., emission of atomic species from the sample, caused by the incident beam. The method is therefore always destructive. However, the great majority of the emission events takes place at the onset of the bombardment in the static mode, i.e., the emitted species originate from regions without previous damage. With continued bombardment, the emission becomes dominated by contributions from regions already subjected to previous damage (dynamic mode). Hence, the static mode yields much more direct information on the original sample surface (Licciardello et al., 1994), as compared to the dynamic mode. Regardless of the mode, interpretation of the SIMS spectra is nevertheless practically always relatively complicated. The choice of the particular components of the obtained spectrum to be used as the signals is often ambiguous. The main reason is that the SIMS spectrum normally contains, in addition to the species forming the original species of the sample surface, an assembly of their fragments formed by the compli-

20

Introduction

cated chain of events producing the signal. Furthermore, the possibility of new reaction products being neither indigenous nor fragments cannot be excluded (Fig. 2). The relative abundance of the secondary ions in the total emission is the basis of the quantitative analysis. Unfortunately, it depends in a complicated way on the conditions of the measurement. The complications caused by the influence of previous sample damage add another essential problem to the task of interpreting dynamic SIMS spectra. In characterization aiming at monolayer surface sensitivity, a natural choice would therefore be the static mode. Until rather recently, the chances for a truly static measurement have unfortunately often been greatly limited. The main reason for this is the sequential mode of operation and, consequently, insufficient overall sensitivity, of the present instruments. Presently, the situation seems to be quickly changing due to new spectrometer designs (time of flight SIMS = TOF-SIMS; Giles et al., 1994) with essentially improved detection sensitivities and possibly decreased price tags in the future. The complicated problems of interpreting SIMS spectra may also be alleviated by the intensive search for more realistic models for the separate physical events of the chain producing the signal (MacDonald and King, 1994).

2.3 Quantitative analysis of surface species The role of elemental analysis in surface characterization is important for many applications, e.g., in passivation phenomena, toxicity, catalysis, doping of electronic materials and surface enrichment responsible for embrittlement of metal alloys, etc. On the other hand, the properties and behaviour of the surface are known to depend in most cases primarily on the (not necessarily elemental) particular chemical species present in the top molecular or atomic layer of the material. Usually this information cannot be obtained from quantitative elemental analysis only. The question will be treated further in the next section. It is sufficient here to emphasise that the role of elemental analysis is insufficient, particularly in studies of surface properties, for explaining the behaviour of the material.

2.4 Chemical effects in surface spectroscopies Because of the influence of the chemical environment of any atom upon the binding of its orbitals, all emissions resulting from transitions between the orbitals (emission spectroscopy) are to some extent sensitive to changes in the chemical environment (Briggs and Riviere, 1990). In elemental analysis, all s,ignal contributions originating from atoms of the same element in different chemical environments have to be added in order to obtain a quantity representing the atomic abundance of the element in question. The most usual change due to the changing environment is a ‘chemical shift’ in the net energy of the transition, i.e. in the spectral position of the emission. Superposition of signals from different environments therefore typically causes a broadcning of the overall superimposed signal. As long as the smearing is small compared to the sig-

Selection of method

21

nal separation from any other emission occurring in the spectrum, the elemental signal can be handled relatively easily from the integral intensity of the composite signal. This is a valid approach if the only influence of the chemical environment is a possible change in the energy of the emitted signal. Unfortunately, this is strictly speaking never the case, as the chemical environment considerably changes the shape and intensity of the observed overall signal by changing the probability of a particular emission mechanism of the signal, through creating or destroying alternative mechanisms. In extreme cases, such effects may change the sensitivity of the measurement by an order of magnitude, rendering it impossible to obtain a meaningful result. In most cases, they contribute to the relatively high uncertainty of the elemental surface analysis as compared to the bulk analysis, in which the chemical effects are usually negligible (e.g.,X-ray fluorescence).

2.5 Analysis of chemical bonding XPS and AES Although a complication in elemental analysis, the chemical effects in the surface spectra present an important asset in surface characterization by providing information about the chemical bonding of the emitting species. The most usual way of using this information is determination of the valence charge of an atom or ion bonded to its environment, from the chemical shift observed in its XPS or AES spectrum. For XPS spectra, the method is widely used for determination of the formal valency of an ion in a particular bond. In AES spectra, the interpretation is essentially more difficult because of the much more complicated mechanism of Auger processes, which normally yield a final state with at least two holes, as compared with a single hole in the final state of normal XPS processes. While AES spectra may exhibit even larger chemical effects than XPS spectra, the latter are from the discussion above clearly preferable for analysis of chemical bonding. In particularly, XPS excited AES spectra (Kibel, 1992) are nevertheless valuable in special cases. It is also possible to use AES as a ‘fingerprint’ method for establishing the existence of a certain bonding or phase by comparison of the overall appearance of a spectrum with reference spectra. If the AES spectrum is excited by electrons, it is also important to consider the possibility of radiation damage on the chemical bonding discussed above. From the viewpoint of ease and reliability of interpretation, XPS is, because of all the reasons presented above, clearly the preferable electronic spectroscopic method for characterization of the chemical bonding. SZMS Because of the inherently different nature of the signal, chemical shifts do not occur in SIMS spectra as in electron spectra. It is usually difficult to conclude whether the ionization of the detected species originates from the sample, the excitation process, or post-ionization of neutral species usually forming the main part of the emission (the vast majority of sputtered species are electrically neutral). The post-ionization may

22

Introduction

occur in the emission process or be induced by laser radiation. Hence, assessment of the bonding state in the same sense as is possible using electron spectroscopic methods is impossible. Furthermore, establishing the distribution of the net charge of an observed fragment consisting of several atoms is not possible. On the other hand, the distribution of the fragments often allows conclusions on the relative strengths of bonds existing between the atoms of the original surface species and, particularly, on bonds between the species and the substrate. This possibility is particularly valuable in studies of processes occurring, e.g., in adsorption or catalytic reactions of large non-ionic molecules in contact with a substrate. The method is also widely used to study reaction products on polymer surfaces (Pignataro, 1992) due to different treatments (plasma, corona discharge, etc.).

3 Microanalysis 3.1 Lateral microanalysis AES and SIMS microprobes In most spectroscopic techniques, a natural method of meeting the often necessary requirement of selecting a limited surface area to be characterized is to focus the exciting beam at the sample surface. For methods using a beam of charged particles for excitation (AES, SIMS), the task is relatively easy, at least when focusing to spot sizes down to 1 pm. The scanning Auger microprobe (SAM) operating on this principle is the first instrument that has been extensively used for lateral surface microanalysis. The possibility of sample damage and changes caused by the electron beam presents, however, in the characterization of sensitive samples (e.g., organic materials or weakly adsorbed layers), a serious problem due to the high intensity of the focused beam. The method is well suited for characterization of metals or semiconductors. Focusing of the beam of primary ions in SIMS results in an ion microprobe which is widely used despite its high cost and the considerable problems in interpretation of the SIMS spectra. Due to the inherent high sensitivity of the SIMS emission and recent developments in instrument design, a SIMS microprobe presently offers a particularly good possibility of locating small amounts of elemental impurities on the sample surface (Giles el al., 1994).

XPS microprobes Construction of an XPS microprobe is complicated by the much greater difficulties associated with focusing of X-rays, as compared with the focusing of charged particles or visibie light. One possibility is to form a picture of the emitting spot of a microfocus X-ray tube on the sample surface by Bragg reflection from a spherical crystal monochromator. The resulting microprobe does not presently offer an equally good combination of lateral intensity and sensitivity as the SAM or the ion microprobes. On the

Selection of method

23

other hand, the risk of sample damage is smaller, and the XPS microprobes operating on this principle offer a good possibility for characterization of certain types of sample at resolution down to the order of 1 pm. The risk of sample damage is increased, however, by focusing the X-ray beam. On the other hand, increasing the area of analysis by defocusing the X-ray beam impairs the energy resolution of the obtained spectrum. For these reasons, the method is best suited for applications in which only a spot is of interest and the sample is not very sensitive to damage, e.g., characterization of metal samples or electronic microcircuits. Another possible solution for achieving lateral resolution is to limit the part of the sample surface seen by the analyser. The simplest way to do this is to use an aperture system. In this method, the energy resolution does not depend on the size of the analysed area. Since the excitation is not focused, the risk of sample damage is not increased by limiting the area of analysis. However, the signal decreases roughly linearly with the size of the analysed area. Furthermore, the choice of the particular spot to be analysed is more difficult than in the former method due to the more complicated sample geometry. The aperture method is therefore suitable for a wider choice of sample types, but more laborious and essentially slower. Table 2. Comparison of the possible different methods of lateral chemical microanalysis reproduced with permission from Klauber and Smart, 1992, Table 1.7.. Method

Information available

Selected area XPS (SAXPS)

Surface elemental composition and chemical state analysis

X-ray photoelectron microscope Scanning Auger microscopy (SAM) Secondary-ion imaging mass soectrometry

Spatial resolution 20-150 pm depending upon method selected 10 pm

Advantages and disadvantages

Chemical shift information obtainable with a minimum of radiation damage. Physical movement of specimen may be required to obtain an image. Generally suffers from inferior signal to noise ratio. Chemical shift information obtainable with a Surface elemental composition and minimum of radiation damage. Image obtained chemical surface analysis by control of electron optics. Signal to noise ratio superior to that of SAXPS. Surface elemental com- 30-500 nm Chemical state information is difficult to obtain although phase identification is possible from a position and some chemical state analysis multielement correlation diagram analysis. via ‘fingerprinting’ Surface elemental corn- 50 nm Excellent elemental sensitivity. Based on fragments, some chemical state information is position and isotope distributions possible although the interpretation can be

Other methods In addition to the above methods, which can be presently regarded as the main tools of lateral microanalysis of surfaces, several other methods are being developed. For example, methods using laser or infrared signals (Dumas, 1994) seem promising and may in the future compete with the methods based on electron or ion spectroscopes.

24

Introduction

Their routine application in practical surface characterization is, however, not foreseen in the immediate future. Comparison Table 2 presents a comparison of the main methods of lateral surface microanalysis available today (Smart, 1992).

3.2 Microanalysis of depth distributions Depth profiring Most routine spectroscopic methods for determination of the chemical composition as a function of the distance below the surface are based on the principles of either gradually removing material from the surface or varying the surface sensitivity of the signal. The conventional method of eroding the surface is ion bombardment (sputtering; King, 1992). The oldest and still most common method of depth profiling is based on repeated measurements of Auger spectra after successively increasing the time of sputtering the target. Recently, the Auger signals are often replaced by XPS, usually in order to simplitji the interpretation. By replacing the electron signal by SIMS, the elemental sensitivity of the analysis is increascd. This is useful for many applications, e.g.,studies of surface enrichment of small amounts of bulk impurities. A serious problem in most depth-profiling studies based on sputtering is the effect of the bombarding beam on the sample. In addition to chemical changes, the diffusion caused by the bombardment and the associated heating of the target usually distorts and smoothes the vertical concentration profile. Another important reason for distortion of the results is the effect of the selectivity of the sputtering. Furthermore, the topography of the surface may be drastically changed. An originally flat sample surface is, for cxample, often changed by the continued sputtering of a surface having on the microscale a very irregular profile. Some of these effects can be reduced by continuously averaging the directions of the incident and signal beams by rotation of the sample in its surface plane (Zalar, 1986). Nevertheless, the uncertainties introduced by the sputtering are, in most cases, probably more important than the choice of the particular signal to be measured. The range of depth profiling it is possible to reach by methods based on sputtering is in practice limited, because of the slowness of the erosion process, to roughly 1 pm below the original surface. If a larger range is necessary, the sample can be cut obliquely with respect to the original surface (Bennett, 1994) before the measurement. The other possibility is to use methods with larger erosion rates. For example, GD-OES (glow discharge optical spectroscopy) is capable of providing rapid depth profiling of layers up to 100 pm (Suzuki et al., 1994). The surface sensitivity of typically a few atomic layers, obtainable at the onset of the profiling in methods using

Selection of method

25

sputtering, cannot, however, be attained with GD-OES or other methods having high erosion rates. A ngle-dependen t spectroscopy The other main method available for obtaining information on the depth distribution of sample composition is angle-dependent spectroscopy (Hofmann, 1990). It is based on varying the region below the surface from which the analytical signal is derived, by varying the signal takeoff angle, which determines the weight distribution of the relative contributions to the total signal measured. A simple application of this technique often used in XPS spectroscopy is to turn the sample about an axis perpendicular to the direction of the incident exciting beam. It is often convenient for semiquantitative estimates of, e . g . , surface enrichment, but quantitative estimates are difficult because they require exact information on several geometric and physical parameters changing with the takeoff angle. A spectrometer design based on varying the takeoff angle, by changing the position of the analyser instead of the sample, makes the interpretation of the measurement somewhat easier, but seriously limits the efficiency of conventional XPS measurements with the instrument.

Scattering methods In addition to spectroscopic measurements, methods based on scattering of an ion beam by the sample are being used to some extent in surface microanalysis (van IJsendoorn, 1994). The most common of these methods is the Rutherford Backscattering Spectrometry (RBS) which makes possible semi-non-destructive location of buried layers, interfaces or impurity clusters under the sample surface. Nowadays the interpretation of the measurements is usually done with the aid of computer software that yield highly quantitative information even for complex multilayer structures. This limits the possibilities of applying the method to routine characterization. Furthermore, the technique and equipment needed differ completely from the main spectroscopic methods presently forming the backbone of routine surface characterization. Raman scattering of a laser beam and low energy ion scattering (LEIS) are methods which can be adopted for surface characterization purposes and are presently developing fast. Especially the laser Raman technique may in the future become one of the most important methods of chemical microanalysis of surfaces.

3.3 Three-dimensional microanalysis Combining the techniques for lateral microanalysis and depth profiling described above, it is presently possible to prepare three-dimensional concentration maps of the region underneath a flat sample surface (Rudenauer, 1994). This technique is now available in several commercial spectrometers. It is, however, important to be aware of the limitations on the information content of such maps, always determined by the finite accuracy of the measurements.

26

Introduction

3.4 Characterization of grain boundaries or internal interfaces A particularly important and difficult problem of materials characterization is establishing the presence of impurity enrichment or films at the grain boundaries of a crystalline bulk sample. It is obvious that none of the main spectroscopic methods of microanalysis described above is as such suitable for the task. The most usual method is to prepare a sample inside the vacuum chamber of an AES or XPS microprobe by using a special fracture stage (Seah, 1990). An immediate study of the fresh fracture surface at a position of intergranular fracture makes the analysis possible. Because of the considerable problems in producing an intergranular fracture in ductile materials and the difficulties of measuring the irregular fracture surface, attempts are being made to develop methods avoiding the need for fracture, by using a flat polished surface of a bulk sample. In principle, it is possible to assess the concentration in the vicinity of the grain boundaries, appearing as lines on the sample surface, by any method of microanalysis with a sufficiently high lateral resolution. However, since the resolution needed is of the order of interatomic distances, it is obvious that none of the methods described above possesses the necessary resolution. However, the less surface-sensitive scanning transmission electron microscopy (STEM) has shown some promise for developing into a non-destructive method for chemical characterization of grain boundaries in bulk samples (Garratt-Reed, 1986). The final results of this development work remain to be concluded in the future, and the analysis of fractured surfaces by a scanning Auger microprobe is today still the routine method for grain boundary characterization.

3.5 Synchrotron-based methods The rapid and continuing development of synchrotron storage-rings as powerful sources of radiation throughout the electromagnetic spectrum up to energies of a few keV is today providing surface characterization with a multitude of new methods, and modifications of the existing ones, using an incident beam of photons (Norman and King, 1990; Margaritondo, 1994; Kartio el al., 1994). The main advantages of the synchrotron-based methods are most obvious in the techniques that are based on an incident beam of photons in the region of X-ray or vacuum UV energies. The recent large scale introduction of insertion devices at storage ring facilities allows for the first time a continuous choice of monochromatic beam energy of these radiations. This possibility improves considerably both the surface and the elemental sensitivity of all XPS Characterization. Due to the parallel nature and small divergence of the beam, the possibilities for lateral microanalysis are significantly improved. Hence, the point resolution possible in surface microprobes has already been shown to extend to about 100 nm. On the other hand, the possibility of sample damage obviously increases when using the high brilliance synchrotron excitation as compared with the conventional methods with excitation energy densities, usually smaller by orders of magnitude. This situation

Selection of method

27

makes it necessary to consider carefully, for each individual measurement, the need for sample cooling. In some cases, the use of synchrotron excitation may even turn out to be impossible. The greatest obstacle to the increased use of synchrotron excitation is presently, and will probably always remain, the limited accessibility of sources with dedicated facilities for surface analysis. Despite the encouraging development during recent years, this limitation will, at least in the foreseeable future, unfortunately seriously restrict the routine use of synchrotron-based methods in surface characterization.

4 Characterization of surface microstructures 4.1 LEED Use of the coherent scattering of electrons with kinetic energies of the order I00 eV is the predominant method of studying the periodicity and orientation of surface layers (Weiss et al., 1994). This method named LEED (low energy electron diffraction) is the particle beam equivalent of X-ray diffraction, but differs from the latter because of its small range in the target, which gives it a good surface sensitivity. Another consequence of the small range is the necessity always to treat the interaction of the beam with the target using the dynamic theory of diffraction, instead of the kinematic model usually applied in interpretation of X-ray diffraction patterns. The latter theory neglects the interaction between the scattered and incident beam. This difference makes the quantitative interpretation of LEED patterns much more difficult than that of corresponding back-reflection Laue X-ray patterns. Consequently, a complete characterization of two-dimensional surface structures from an LEED pattern alone is, albeit possible, a very laborious task. Application of LEED to studies of the crystallinity and symmetry of the surface structures is, on the other hand, relatively easy and widely used in preparation and characterization of surface samples.

4.2 SEXAFS The interaction of the wave function of the photoelectron, created in the photoionization of a core orbital of an atom in solid, with the neighbouring atoms gives rise to a variation of ionization probability with the energy of the incident photon. In the X-ray absorption spectra, this interference called EXAFS (extended absorption fine structure) is seen at energies well above the main absorption limit. An exact analysis of these variations allows conclusions on the positions in the neighbourhood of the emitting atom. Because of the considerable ranges of X-rays in solids, the method as such is not surface-sensitive, but can be modified to yield a method of surface characterization. The most practical way of doing this is to use, instead of the attenuation of the X-ray beam, the Auger emission caused by the ionization. Choosing an Auger signal with a short mean free path for inelastic scattering (i.e., kinetic energy in the range

28

Introduction

50-150 eV) yields a new method of surface characterization (SEXAFS = surface EXAFS; Norman and King, 1990). The surface sensitivity of this method is equal to that of conventional XPS or AES. The main problem of the method has in the past been the high intensity and continuous wavelength selection of the X-ray beam needed for the measurement. With the advent of the synchrotron sources equipped with insertion devices, the problem has been alleviated. Hence, use of SEXAFS is increasing rapidly and can be expected to become an important method for studies of surface reactions and their products.

4.3 Tunnelling microscopes An ultimate goal of all surface characterization is direct imaging of the individual species on the sample surface. Although modern high resolution transmission electron microscopic methods have demonstrated that this is possible under particular conditions, these methods are difficult to apply to most problems of applied surface characterization. On the other hand, the recent introduction and rapid progress of methods based on the tunnelling interactions between a localized probe and a sample surface (Wiesendanger, 1994) seem to hold a promise for at least considerable progress in reaching the goal. A particular advantage of the tunnelling microscopic methods is that they can be applied in vacuum, liquid, and atmospheric environments, which in principle allows in situ characterization of any interface. In spite of problems complicating the situation, this fact is particularly valuable in characterization of surfaces in, cg., studies of catalysis, electrolysis, adsorption from liquids, etc.

STM Scanning tunnelling microscopy (STM) implies measurement of the quantum mechanical tunnelling current between a surface and the tip of a conducting probe brought near the surface (Sexton, 1992). The current depends both on the distance between the probe tip and the substrate and the conductivity of the substrate. The requirement that a measurable current must be observed prevents the use of STM for insulating substrates. By establishing the variation of either the current or the distance as a variable, while the other parameter is kept constant, images of the sample surface can be prepared by moving the tip horizontally over the substrate. Because of the complicated dependence of the signal on both the sample geometry and the changes in the conductivity, interpretation of the images is a difficult and sometimes ambiguous task. Under favourable circumstances, STM allows, however, mapping of the surface geometry and/or chemical surface species on an atomic scale. The applicability of STM both in vacuum and in situations in which the sample is, during the characterization, in contact with a gaseous or liquid environment allows in principle in situ observations of important technical surface processes operating in gaseous or liquid environments (catalysis, electrolysis, leaching etc.). This unique property of tunnelling microscopes may find extensive use in application to studies of industrial chemical processes in the future.

Selection of method

29

AFM It is also possible to monitor, instead of the electric current, the interaction force between the tip of a probe and an adjacent substrate surface. This atomic force microscopy (AFM; Binnig et al., 1986) is in many respects similar to STM, but does not require any electrical conductivity of the substrate. In fact, it works best with insulating substrates and is therefore being used increasingly in the surface characterization of glasses, ceramics and polymers. It is, however, important to emphasise again the problems of interpreting the results correctly. Especially in studies involving a liquidholid interface, it is essential to include the influence of the liquid on the interaction force. On the other hand, this very fact makes it also possible to study this interaction on a level which has not been possible previously.

5 Conclusions 5.1 Methodology Several excellent comparisons of the advantages of the main methods of surface characterization, discussed above, have been presented in the literature (Seah and Briggs, 1990; Somorjai, 1992; Klauber and Smart, 1992; Garten and Werner, 1994). In rough agreement with these analyses and the considerations presented above, the following thumb rules can be presented as a basis of the choice of method for particular tasks. In chemical surface characterization, the spectroscopic methods XPS, AES and SIMS stand out presently as the primary choice for the method to be used. The information yielded by these methods is different and complementary. Electron spectroscopic methods (particularly XPS) provide, in addition to elemental composition, information on the bonding of the emitting species. This information is difficult to obtain by the SIMS technique, the main advantage of which is its superior analytical and surface sensitivity. Because of it’s great versatility, small risks of sample damage and relatively easy spectral interpretation, XPS has been rapidly gaining popularity as compared to AES, which nevertheless today maintains an important position as a relatively simple method of chemical microanalysis. This position may, however, be seriously challenged in the foreseeable future, mainly because of the promising development of new varieties of SIMS (notably TOF-SIMS) as methods of chemical surface characterization, with excellent sensitivity and lateral resolution. In addition to the above methods, different varieties of IR spectroscopy maintain an important role in chemical surface characterization, because of their unique capability for in situ observations of bonding in surface layers. For the characterization of surface microstructures, the present situation is much less clear. Neither one of the presently existing established techniques based on scattering (LEED, SEXAFS), meets the requirements of reasonably straightforward interpretation and easy availability. Furthermore, so far they have hardly been applied for

30 Introduction

interfaces other than the solid/vacuum. Hence, the situation in routine characterization of surface microstructures remains problematic, particularly in tasks of an applied nature. The main hope for future improvement lies in the present fast development of tunnelling microscopic methods (STM/AFM), which are opening up new vistas for surface microcharacterization. A typical present trend is to combine tunneling microscopy with the already established spectroscopic methods, e.g XPS (Wittstock ef al., 1996) which provide a possibility to check the often problematic interpretation of the tunneling microscopic observations.

5.2 Fields of applications Considering the types of samples and fields of application to which the methods of surface characterization are applied, the development is rather similar to the history of bulk characterization. The types of sample most easily amenable to successful analysis are reasonably conducting substrates with strong binding forces in the surface layer. For these samples, use of the powerful methods of primary choice (XPS, AES, SIMS) is usually not seriously obstructed by problems of sample damage, charging or desorption phenomena during the characterization. Consequently, the chances of successful characterization of samples with metal or semiconductor substrates are usually relatively good. The situation becomes more difficult for inorganic insulating substrates, in which sample charging normally obstructs, at least to some extent, application of methods utilizing charged particles. The most difficult objects of characterization are insulating, inhomogeneous substrates with weakly bonded, complex surface species easily damaged or desorbed. Typical examples of applications corresponding to thesc increasingly difficult levels of conditions are, e.g.: a) characterization of initial chemical reactions of a metal substrate in a (rarefied) gas atmosphere; monitoring chemical vapour deposition on a semiconducting substrate b) in situ characterization of the surface of an insulating inorganic substrate (glass, ceramic) at different temperatures; monitoring the effect of plasma treatment on the surface of a polymer c) most surface problems in medicine, biology or biotechnology, e.g., adsorption of blood cells on synthetic transplants; diffusion through biological membranes. Extensive surveys of application of the different techniques of surface characterization to particular types of material or to processes can be found in the literature (e.g., Giesekke, 1983; Hochella, 1995). These surveys are generally very helpful in describing the particular needs and problems of the field in question, but are, because of the rapid development of the more recent characterization techniques, usually not able to describe the state-of-art situation of surface characterization in the field. This development can be best followed at the topical meetings and conferences organised today in several fields of materials research and engineering.

Selection of method

31

Arkno wledgement Useful comments and interesting discussions with Prof. R.St.C. Smart and Dr. K. Laajalehto are gratefully acknowledged.

References Bennett M.J. (1994), Surf. Interface Anal., 22,421. Binnig G., Quate C.F., Gerber Ch. (1986), Phys. Rev. Lett., 56,930. Briggs D., Riviere J.C. (1990), in: Briggs and Seah, 1990, pp.119-134. Briggs D., Seah M.P. (1990), Practical Surface Analysis, Vol. 1. Chichester:Wiley. Dumas P. (1994), Surf. Interface Anal., 22, 561. Garratt-Reed A.J. (1 986), Mater. Res. SOC.Symp. Proc., 62, 1 15. Garten R.P.H., Werner H.W. (1994), Anal. Chim. Acta, 297, 3. Giesekke E. W. (1 982), Int. J. Mineral Processing, 1 1, 19. Giles R., Sullivan J.L., Pears C.G. (1994), Surf. Interface Anal., 22, 576. Hochetta M.F.J. (1995), in Mineral Surfaces; Vaughan D.W., Pattrick R.A.D. (Eds), London: Chapman & Hall: pp. 17-60. Hofmann S. (1990), in: Briggs and Seah, 1990, pp. 183-186. Kartio l.,Laajalehto K., Suoninen E., Karthe S., Szargan R. (1992), Surf. Interf. Anal., 18, 807. Kartio I., Laajalehto K., Suoninen E. (1994), Colloids and Surfaces A. Kibel M.H. (1 9921, in: OConnor et a/., 1992, pp. 174-1 75. King B.V. (1992), in OConnor et aL, 1992, pp. 97-1 16. Klauber C., Smart R.St.C. (1992), in O'Connor ef a[., 1992, pp. 12-65. Licciardello A., Wenclawiak B., Boes C., Benninghoven A. (1994), Surf. Interface Anal., 22, 528. MacDonald R.J., King B.V. (1992), in: OConnor et al., 1992, pp. 117-147. Margaritondo G. (1 994), Surf. Interface Anal., 22, 1 , Norman D., King D.A. (1990), in: Applications of Synchrotron Radiation: Catlow, C.R.A., Greaves, G.N. (Eds.). Glasgow: Blackie, 1990; pp. 221-240. O'Connor D.J., Sexton B.A., Smart R.St.C. (Eds.) (1992), Surface Analysis in Materials Science, Berlin: Springer Verlag. Pignataro S. (l992), Surf. Interface Anal., 19,275. Riidenauer F.G. (1994), Anal. Chim. Acta, 297, 197. Seah M.D. (1 990), in: Briggs and Seah, 1990, pp. 3 19-326. Seah M.D., Briggs D. (1990), in: Briggs and Seah, 1990, pp. 1-18. Seibt E.W., Zalar A,, Roose N. (1994), Anal. Chim. Acta, 297, 153. Sexton B.A. (1992), in: O'Connor et al., pp. 221-244. Smart R 3 . C . (1992), in: OConnor et al., pp. 3 19-336. Somorjai G.A. (1992), Surf. Interface Anal., 19,493. Suzuki S., Suzuki K., Mitzuno K. (1994), Surf. Interface Anal., 22, 134. van IJsendoorn L.J. (1994), Anal. Chim. Acta, 297, 55. Weiss W.S., Starke U., Somorjai G.A. (1994), Anal. Chim. Acta, 297, 109. Wiesendanger R. ( I 994), Scanning Probe Microscopy and Spectroscopy. Cambridge: Cambridge University Press. Wittstock G., Kartio I., Hirsch D., Kunze S., Szargon R. (1996), Langmuir, 12,5709. Zalar A. (1986), Surface Interface Anal., 9,41.

Reference data tables H.J.Whitlow The following tabulated information is intended as a guide for the new user in identifying potential surface characterization methods for their particular application. The tables have been compiled from a questionnaire submitted by the authors of the chapters in this book. To make the book self-contained, we have chosen to limit the tables to cover only techniques presented in this volume. Table 1 lists the techniques alphabetically. It should be borne in mind that the tabulated data represent a judgement about what is normally achieved with an ‘average’ sample using a standard laboratory system. The actual performance can vary widely depending on the nature of the sample and actual measurement set-up used. Details of the individual techniques can be found in the individual chapters. Each entry in the table gives the full name of the technique followed by a principal acronym, followed by the main secondary acronyms. Although most of the headings are self-explanatory, a few terms required clarification. The characterized parameter refers to the physical property/entity that forms the basis of the measurement. The type of information, on the other hand, indicates which information and properties can be measured using the technique. The measurement environment refers to the environment the specimen is exposed to during analysis. In order to allow the user to evaluate how commonly the technique is used, the resources required and the type of organization using it, estimates of these factors have been included. The cost is taken to be that in ECU for preparation, analysis and evaluation of the results from one single sample. The number of facilities is intended to indicate approximately how commonly the technique is used and available on a world-wide scale. The type of laboratory is denoted in a simplified manner as: small (company quality control, field laboratory), medium (university, company research and development) and large (national and international facilities), Likewise the skill needed is categorized as: unskilled, technician (skilled), specialist (university degree).

Reference data tables

33

Adhesion Testing Characterized parameter: Surfaceenergy

Surface SpecifIcuv Information depth: Detectibility:

Typeof information: Adhesion force, wetability, peeling resistance

Resolution Depth: Lateral:

Measurement environment: Diflculties

Timeneeded for analysis Preo. Measurement Evaluation loinin 10min 10 min Cost [ECU]: No. offacilities: 10-100 5000 Sample Form Type: Size: solidsolid solidliquid 1-3 cm2

Equipment: Tensiometer rig User skill needed: Typeof laboratory: Small Technician Techniquesyielding similar information: “ Scotch tape” test, peel tests T

Atom Probe Field Ion Microscopy

A P m

Other:

F I M A p m ,

Characterized parameter: Mass and direction of atoms ejected !?om tip

Surface Specificuv Information depth: Detectibility: 0.1 pm 10 at. ppm Typeof information: Resolution Depth: Lateral: Other: Small-volume elemental composition, depth profile 0.2 nm I nm AM: 1 u Measurement environment: Diffulties Timeneeded for analysis Prep, Measurement Evaluation UHV,cryogenic temperature Specimen preparation 8h 8h Equipment: and mechanical strength - 8h Cost [ECU]: No. offacilities: APFIM 1000 50 Typeof laboratory: User skill needed: Medium Specialist Sample Techniques yielding similar information: Form Type: Size: Analytical electron microscopy Solid electrical conductor 50 nm need

+ fAtomic Force Microscopy

*

AFM

Characterized parameter: Local Van der Waals force

Suvace Speciflcuv Information depth: Detectibility:

Type of information: Surface topography map, elasticity, friction, magnetic and electrostaticforces Measurement environment: Diffulties Air/ UHV preferred Softness, tip size Equipment: Atomic Force Microscope Typeof ldoratory: User skill needed: Medium Specialist Technlquesyielding similar information: STM, SEM

Resolution Depth: Lateral: Other: 0.1 nm Timeneeded for analysis Prep. Measurement Evaluation lh 2h Ih Cost [ECU]: No. of facilities: 70-500

Sample Form ripe: Solid

3000

Size: 1 cm

34 Introduction

Auger Electron Spectroscopy ~

Characterized parameter: Electron energy spectrum Qpe of information: Elemental composition, spot, line and map analysis depth profile Measurement environment: Diflcultks UHV,electron irradiation Equipment: Dedicated A E S instrument User skill needed: Qpe of laboratory: Medium Specialist Techniquesyielding similar information: XPS, SIMS, GD-OES, RBS; EDS, WDS

-

AES

SAM

~~-

~

Surface Specifliq Information depth: Detectibility. 3m 0.1 at. % Resolution Depth: Lateral: Ofher: 3nm _ _ _ _ 100 _ _ _nm __ -Timeneeded for analysh Preo. Measurement Evaluation 3h 10 min Cost [ECU]: No. offacil&ies: -~

~

~~

SOP

500

~

Sample Form Type:

Size: 1 cm2

Solid

Channelling Characterized parameter: Angle and energy spectrum of scattered ions Qpe of information: Depth profile, impurity atom atomic site, crystal Structure

Measurement environment: Di@kulties Vacuum, ion irradiation Radiation damage Equipment: MeV ion accelerator, gonimeter Ope of laboratory: User skill needed: Medium Technician Techniquesyielding similar information: X-ray d-tion, electron dii3hction

Surface SpeciflUy Information depth: Detectibility: 20 nm 100 ppm Resolution Depth: Lateral: Other: 1 u; M < 30 5 jim 1ttUll Timeneeded for analysis Preo. Measurement Evaluation 45min l h 30min -Cost [ECU]: No. of faciluies: 200

Charged-Particle Activation ..Analysis -

Qpe of information: Elemental content, surface layer composition Measurement environment: Diflculties Ion irradiation, radioactivity Local heating, Equipment: radioactivity Accelerator, radiochem lab Qpe of laboratory: User skill needed: m e Specialist Techniquesyielding similar information: XRF, GD-OES, RBS, AES, Inductively coupled plasma, SIMS, EDX, W D S

~

~-

Size:

Single crystal

r

Characterized parameter: Characteristic fl and y radiation

200

Sample Form Type:

0.25

mm2

W A WAA

CPAA

Surface Specificity Information depth: Detectibility: 0.1 mm 100 ng gl Resolution Depth: Lateral: Other. 1

mm

Timeneededfor anahsis Prep. Measurement Evaluation 0 1 day Cost [ECU]: No. offacilities: 1000

~.

100

Sample Form Qpe: Solid (ideally foil)

Sue: 11111112

Reference data tables r

~~~

~

35 7

~

Chemometric FTIR microscopy

Characterized parameter: Molecular vibration

Surface SpeciflW Information depth: Detectibility:

Typeof information: Elemental and molecular distributions, fingerprints

Resolution Depth: Lateral: Other: 2 pm 5 pm Timeneeded for ana&sis Preu. Measurement Evaluation 1 day 30 min 2 Cost [ECU]: No. offacilities: 500 20 Sample Form Type: Size: Solid film 1 mm-1 cm

Measurement environment: Solid Equipment: FTIR microscope, computer Typeof laboratory: Medium Techniques yielding similar Static SIMS

Di&f3culties

User skill needed: Specialist information:

h

c

Confocal Scanning Optical Microscopy

Characterized parameter: Sequential optical images Typeof information: 3 dimensional topographic map, buried interfaces Measurement environment: Di&f3culties Steep sides with low reflectivity Equipment: SOM User skill needed: Typeof laboratory: Small medium Unskilled Techniquesyielding similar information: 2D scanning profilometers, interferometry, SEM

Characterized parameter: Mass o f sputter-ejected ions Tjpe of information: Elemental composition, Mass spectra, depth profile, point, line and map analysis Measurement envlronment: Difficulties Ion bombardment vacuum Ma& effects, Equipment: insulating samples Multi-purpose SIMS Typeof laboratory: User skill needed: Medium Specialist Techniquesyielding similar information: LAMMA, AES, PIXE, RBS, GD-OES, EDS

SOM Surface Specijicuv Information depth: Detectibility: 1 Clm Resolution Depth: Lateral: Other: 70nm 250nm Timeneeded for ana&sis Prep. Measurement Evaluation 0 3s 10 min Cost [ECU]: No. offacilities: 200 100 Sample Form Qpe: Size: Solid 0.1 mm10 cm

Surface Specif& Information depth: Detectibility: 10 m - 1 0 0 pm ppb-ppm Resolution Depth: Lateral: Other: AMh4 -104 10 m-5 pm0.1- 5pm Timeneededfor ana&sis Prep. Measurement Evaluation 20 min 5 min-20 h 0-2 h Cost [ECU]: No,of facilities: 100 - 2000 500 Sample Form Qpe: Size: Solid 0.1 m 2.5 cm

36

Introduction

lectrical characterization

1

*

~-

-~

Characterized parameter: Charge carrier mobility, conduction mechanisms

Surface Speci#ikiry Information depth: Detectibility:

u p e of information: Defects, band level mismatch, space charge effects, breakdown Measurement environment: Diflculties Kelvin or other probe Interpretation Equipment:


User skill needed: o p e of laboratory: Small medium Specialist Techniquesyielding similar information:

100-2000 ____

Timeneededfor analysis Frep. Measurement ___

~

Sample Form Type: solid film

EIS

~~~

~

Evaluation

No. offacU&ies: 5000

Cost [ECU]:

Electrochemical Impedance Spectroscopy ~

Other:

Size: pm-cm

~-

.__

Characterized parameter: Electrochemical impedance at metaUsolution interface

Surface Speciflciry Information depth: Detectibility:

Typeof information: Polarization resistance, solution resistance, capacitances Measurement environment: Diflculties Electrolyte Equipment: Potentiostat, frequency response analyser User skill needed: Qpe of laboratory: Small Specialist Techniquesyielding similar information:

Resolution Depth: Lateral: Other: 10- I5 wrn nA Cm-2Timeneeded for analysk Prep, Measurement Evaluation 1 hour min-hours -Cost [ECU]: No. offacil&ies: 50-500 2000 Sample Form Qpe: Sue: Coated or uncoated metal 1 mm-1 CN surfaces

\

~~

f

Ellipsometry

Characterized parameter: Refractive indices o p e of information: F i l m thickness, dielectric constants, phase changes Measurement environment: Polished and liquid surfaces Equhment: Ellipsometer Qpe of laboratory: Small I Techniquesyielding similar IRAS,IR Spectroscopy

Diflcuhies Needs specular refl-surfaces User skill needed: Technician information:

Surface Speci#iki@ Information depth: Deiectibilify: 10 nm 12 mm ~

~


Grazing-Emission X-ray Fluorescence

Characterized parameter: Spectrum o f characteristic X-rays Typeof information: Elemental composition, depth profile, layer thickness Measurement environment: Di@kulties X-ray irradiation Surface roughness Equipment: TXRF instrument Typeof laboratory: User skill needed: Medium Specialist Techniquesyielding similar information: AAS, ICP-MS, SIMS, AES, XPS, RBS c

Grazing-Incidence X-ray Fluorescence Characterized parameter: Spectrum of characteristic X-ray Type of information: Elemental composition, density, layer thickness Measurement environment: D i ~ u l t i e s X-ray irradiation Surface roughness Equipment: TXRF instrument Typeof laboratory: User skill needed: Medium Specialist Techniquesyielding simuar information: SIMS, RBS, AES

39 7

GEXRF Surface SpeciFuy Information depth: Detectibility: 1-100 nm 1011 at. cm-2 Resolution Depth: Lateral: Other: 0.1 nm 5 cm Timeneededfor analysis Prep. Measurement Evaluation 30min 1Omin-3 h 1h Cost IECU]: No,of facil&ies: 600-6000 2 Sample Size: Form Type: Thin film >I0 cm*

&

GI-XRF OTXRF,IADTXd Surface Speci&i@ Information depth: Detectibility: 3 nm-1 pm 1012 at. cm-2 Resolution Depth: Lateral: Other: 0.3-100m 1 ~m Timeneeded for ana&slr Prep. Measurement Evaluation 0 2h 1 h - 1 day Cost fECU]: No. offaci&ies: 10 Sample Form Qpe: Size: Solid surface > 0.5 cmz A

Characterized parameter: Electron scattering Typeof infomtathn: Vibrational modes, Local geometry of adsorbates, phonon dispersion Measurement environment: Di@kulties UHV, electron bombardment Equipment: HREELS spectrometer Typeof laboratory: User skill needed: Medium Specialist Techniquesyielding similar information: Analytical TEM

SurfaceSpeci#W@ Information depth: Detectibility: 0.2 nm < 1 monolayer Resolution Depth: Lateral: Other: Energy: 1 - 10 meV Timeneeded for anabsis Prep. Measurement Evaluation days days Weeks Cost [ECU]: No. of facilities: 1000 500 Sample Form i’jpe: Sue: Single crystal 1mm

Introduction

40 r

Infrared Micro-Spectroscopy __

.~

Characterized parameter: Vibrational states of chemical bonds

Surface Specific& Information depth: DetectibiIity:

Type of information: Chemical composition, functional groups Measuremenf environment: Di@lculties Infr-ared radiation, ambient Equipment: FTIR Spectrometer-Microscope User sRUl needed: Typeof laboratory: Small medium Specialist Technkpesyielding sirnilor Information:

_ _ _ ~ -

Other:

T h e needed for ana&sis Prep. Measurement Evaluatior 0_ _ ~ 1 dav Cost [ECUJ: No. of facilities:

-50_____

500

__

Sample Form Type: Solid, thin layers, deposits ~

Size:

lEls ____

-~

~~

characterized parameter: Scattered ion energy and angle

~-

_ _ ~ ~-

ISS

r~. ~ o Scattering n Spectroscopy

~-


~ . -

-~

Sur$ace SpeciflcQ Informationdepth: Detectibility: ~1-2 monolayer 0 . 0 0 1 monolayer _Typeof information: Resolution Lateral: Other: Surface layer elemental composition, surface structure, Depth: 1 monolayer 150 prn1 u M < 30 atomic distances Measurement environment: Di@i’culties Timeneededfor anabslr Prep, Measurement Evaluatior UHV quantification, mass 10 rnin 15 min Equlpment: identification (M > 30) 30 min Cost [ECUJ: No. of facilities: UHV system, ion accelerator I0 o p e of laboratory: User skilI needed: .- _ _ 500 __-_ ~ Medium Technician Sample Techniquesyielding similar informatlon: Form Type: Size: Direct Recoil Spectroscopy,Sputter-Auger,LEED, Solid, poly-single crystals 1 cm* STM, AFM, AES r

Laser Microprobe Mass Spectroscopy

LMMS

~~

Characterized parameter: Mass of ionized molecular fragments Typeof information: Point analysis, chemical structure, elemental composition Measurement environment: Diffrculties Vacuum, laser ablation Molecular Equipment: hgmentation Laser microprobe User skill needed: Typeof luboratory: Small medium Specialist Techniquesyielding similar information: SIMS, FTIR Microscopy, AES, XPS

LAMMA

Surface Specifi& Information depth: Detectibility: 10-50 NSI 107 atoms Resolution Depth: Lateral: Other: 3 fim MIM > 50C Timeneededfor anabsls Prep. Measurement Evaluatioi 10min 1Omin 4h Cost [ECUJ: No. of facilities:

@-

300 ___

Sample Form Vpe: Solid

___

Size:

mm*


LEED

Low-Energy Electron Diffraction

Characterized parameter: Diffracted electron intensity and angle

Suvace Speciflcity

Typeof information: Surface crystal structure, lattice parameters, space groups Measurement environment: D&?7culties UHV, electron bombardment Theoretical Equipment: interpretation LEED system User skill needed: Typeof laboratory: Medium Specialist Techniquesyielding similar information: RHEED, ISS, RBS, STM, AFM

Resolution

1nformation.depth: Detectibility: 1nm Depth:

Lateral:

Other: - Lat.par. 10 pn

Time neededfor analysis

Measurement Evaluatior Prep. Davs Weeks Davs Cost [ECU]: No. of facilities: 1nnn 5000

Sample Form Type: Single crystal

Size: 1INll

vLow-EnergyElectron Microscopy Characterized parameter: Electron a c t i o n and absorption

Surface Specificity

Typeof information: Surface crystal structure, morphology, defect distribution, surface layer formation Measurement environment: Dlfflculties UHV,electron bombardment Equement: LEEM microscope User skill needed: Typeof laboratory: Medium Specialist Techniquesyielding similar information: E M , STM

Resolution

Information depth: Detectibility: 0.1

nm

Depth:

Lateral: 10 nm

Other:

T h e needed for analysis

Prep. Measurement Evaluatior davs davs weeks Cost [ECU]: No. of facilities: 10

15nn

Sample

Form Type: Solid

Size: 10 pm

r

Magnetic Measurements

Characterized parameter: Magnebtion curve Typeof Information: Anisotropy, hysteresis loop, magnetic domain structure Measurement environment: Difpculties Magnetic field Equipment: Magnetometer User skill needed: Typeof laboratory: Medium Specialist Techniquesyielding similar information:

Surfoce Speciflcity

Information depth: Detectibility:

Resolution Depth:

Lateral:

Other:

Timeneededfor analysls

Prep. lh

Measurement Evaluatior; lh 4h Cost [ECU]: No. of facilities: ~

~~

Sample

~

~~

~~

1000

Form Qpe: Ferromagnetic film

Size: 1 cm

41

lntroduction

42

M6ssbauer Spectroscopy ~~

-

~

p

Characterized parameter: Resonant absorption of y -rays

Surface Speciflcig Information depth: Detectibility: 0.1 - 10 pm 1 monolayer Typeof infomatlon: Resolution Depth: Lateral: Other: Chemical and magnetic nature, local order, texture, 50-nm crystallinity, magnetic properties T h e neededfor anabslr Measurement environment: Difficulties Prep. Measurement Evaluation Gamma ray irradiation Restricted to limited lh I-7day~ 2h Equipment: number of elements Cost [ECU]: No. of facilules: Mhsbauer spectrometer wn 1000 User s k u needed: Typeof laboratory: Medium Technician Sample Form Type: Size: Techniquesyielding shllar information: 1 cm* EXAFS (chemical order) Neutron diffraction (magnetic Solid

order)

Nuclear Corrosion -~ Monitoring

NCM

~-

Sueace Specificity Information depth: Detectibilify: ~. mgppResolution Typeof information: Depth: Lateral: Other: Corrosion state ___ Measurement environment: DGrflculties Timeneededfor ana&sis Measurement Evaluation Neutron irradiation, gamma Radiochemical method Prep. lh l h - 1 day 10 min Equipment: Cost [ECU]: No. offacilitltles: Nuclear reactor, gamma ray spectrometer 100 5 User skill needed: Typeof laboratory: Medium large Technician Sumple Form Type: Size: Techniquesyielding similar Information: solid (flats or foils) 10 mmz TLA

Characterized parameter: Elemental release

~

-Nuclear Microscopy -~ .~

~

NUMI

Characterized parameter: Spatially resolved X-ray spectrum

SurfuceSpeclfrcur Information depth: Detectibility: 50_pm 1-10 pg g' _ __Typeof information: Resolution Lateral: Other: Multi-(trace) element distribution, composition, point, Depth: 0.1 pm 1 pm line-scan, map and microtomographic analysis Measurement environment: Difficulties T h e neededfor ana&slr Prep. Measurement Evaluation Vacuum, MeV ion irradiation, Low accessibility 2-4 hours 4-6 hours 1-2 h o r n Equipment: Cost fECU]: No. of facilities: MeV accelerator, nuclear microscope 100-600 -30 Typeof laboratory: User skill needed: -~ ___ Large Specialist Sample Form Vpe: Size: Techniquesyielding shilar Information: SIMS, LAMMA, Autoradiography Thin sections, cells and 1 pm-1 mm particles on support foils ~


NRA

Reaction Analysis Characterized parameter: Characteristicnuclear reactions Typeof information: Elemental composition, depth profile

Characterized parameter: Reflectance,R m V e , phase, ferr0eledC Qpe of information: Point measurement, pixel map, film thickness, topography Measurement environment: Dijjlcuities Model dependent Reflecting surface analysis Equipment: Typeof laboraloty: User skill needed: Small medium Technician Techniquesyielding similar Information: SEM

Characterized parameter: Non-linear photon excitation

\

Type of information: Interface crystal and electronic structure, point group symmetry of interface, resonance structure Measurement environment: Difflcties Transparent medium Sample damage Equipment: High power laser Typeof Iaboratoty: User s k u needed: Medium Specialist Techniquesyielding similar information: HEEELS

NRB

Surface Speciflcuy Information depth: Detectibility: 0.1-1 pm 0.1-100 ppm Resolution Depth: Lateral: Other: 5 - i o o m 1 mm T h e needed for ana&siS Prep. Measurement Evaluatior 0 10 min 15 min Cost [ECU]: No. offacilities: 200 300 Sample Form Type: Sire: Solid 3mm

Surface SpeclJlc#y Information depth: Detectibility: 0.1 pm 0.01 monolayer Resolution Depth: Lateral: Other: 1y.m _Timeneeded for anabsis Prep. Measurement Evahiatiot 0 2 min. Cost (ECU]: No. of facilities: 20

10000

Sample Form Dpe: Solids, liquids, powders

Sire: 1 mm2

Surface SpeciflcQ Information depth: Detectibility: 10 nm-1m 0.1 monolayer Resolution Depth: Lateral: Other: 0.3 nm 10 pm Timeneededfor anabsis Prep. Measurement Evaluatior 30min 2h 30 min CosfIECUJ: No. of facilities: 600 50 Sample Form Dpe: Size: Transparent crystal, solid, 1 m m 2 liquid

43

44 Introduction

ODtical Sum Freauencv Generation

SFG

Characterized parameter: Interface vibrational structure

Surface Speciflcity Information depth: Detectibility: 0.1~. monolayer 10 nm-lm Resolution Depth: Lateral: Other: 0.3 nm 10 pm _ Timeneededfor ana&sS Prep. Measurement Evaluatior 30min 2hour ~30 min Cost [ECU]: No. of faciliies: ~

Typeof informati~n: Energy and symmetry of vibrational modes

~

Measurement environment: D i ~ u l t i e s Transparent medium Sample damage Equipment: High power lasers User skill needed: Typeof laboratory: Medium Specialist Techniquesylelding slmUar information: FTIR

~

~

6%-

~

PIGE

Particle-Induced Gamma-ray Emission ~

Characterized parameter: Characteristicy -radiation

RGbuIE

Surface Spec#k@ Information depth: Detectibility: 50 pm > 10ppm Resolution Depth: Lateral: Other: 10 Clrn__~_.. Timeneeded for andysh Prep. Measurement Evaluatior 0-2h >10min _ - 5 m i n Cost [ECU]: No. offacl#&s:

Typeof information: Elemental (isotopic) composition, layer, bulk, line scan (microprobe), pixel map (microprobe) Measurement environment: Di&’kult&s Vacudatmosphere Low 2 elements only, Equipment: radiation damage MeV-ion accelerator User skill needed: Typeof Iaboratory: Medium Specialist Techniquesyielding slmUar information: CPAA, XRF,PD(E

50

Sample Form Typ: Solid, crystal, pressed powder pellet ~~

PME

~~

Characterized parameter: CharacteristicX-rays

Type of information: (Trace) Elemental composition, layer, bulk, line scan (microprobe). pixel map (microprobe) Measurement environment: Dimcultks Vacuum atmosphere Z 5 11, radiation damage Equipment: MeV ion accelerator Typeof laboratory: User s k u needed: Medium Specialist Techniquesyleldtng similar information: EPMA, SXRF, XRF, RBS, EDS

50

Sample Form Qpe: Size: Transparent crystal, solid, 0.1- 1 nun liquid

r

Particle-Induced X-ray Emission ~-

~~

I

Size: > mg

Surface Specifi@ Information depth: Detectibility: 50 pm 0.1 ppm Resolution Depth: Lateral: Other: 10pm 1 pm Time neededfor ona&sb Prep. Measurement Evaluatioi 2h6ur 1Omin 3 min Cost [ECU]: No. of facilMes: cn 300 Sample Form Qpe: Size: Solid, crystal, powder 1 mm


{Photoacoustic

Spectroscopy

45

PAS

Sutface SpeciflcUy Information depth: Detectibility: 10 nm 1 monolayer Typeof information: Resolution Depth: Lateral: Other: Thermal properties 10 pm Timeneeded for anabsis Measurement environment: Diflcultks Measurement Evaluation Atmosphere, light Calibration for absolute Prep. 1Omin 10min 10 min Equipment: values Cost [ECU]: No. of facilities: Optical spectrometers 60 200 User skill needed: Typeof laboratory: Medium Technician Sample Form Type: Size: Techniquesy&ldmg similar information: 2 mm-1 cm PTS Solids and liquids Characterlted parameter: Optical adsorption

' 1

'

1

1

v

Photothermal Spectroscopy

Characierized parameter: Optical adsorption Typeof information: Thermal properties ofsurfaces Measurement environment: Di&Pculties All surfaces Small signal levels Equipment: Optical spectrometer, IR detectors Typeof laboratory: User skill needed: Medium Technician Techniquesyielding similar information: PAS r

Radionuclide-induced X-ray Fluorescence

Characterized parameter: Characteristic X-rays

PTS Surface Specijcity Information depth: Detectibility: 10 nm 1 monolayer Resolution Depth: Lateral: Other: 10 pm Timeneeded for anabsis Prep. Measurement Evaluation mins mins mins Cost [ECU]: No. of facilities: 60 Sample Form Type: Size: Solids and liquids 0.2-1 cm

R-m

XRF

Typeof information: Elemental composition, area analysis

Surface SpecifikcUy Information depth: Detectibility: 10 )rm 100 ppm Resolution Depth: Lateral: Other:

Measurement environment: Diflculties X- or y -ray bombardment Radioactive source Equipmeat: X-rayspectrometer User skill needed: Typeof laboratory: Small Technician Techniques yielding slmilar information: AES, PIXE, EDS,XPS, GD-OES

Timeneeded for anabsis Prep. Measurement Evaluation 10min 30min 10 min Cost [ECU]: No. of faciliiies: 50 5000 Sample Form Type: Size: Solid, powder 2 cm

46

Introduction

Characterized parameter: M a s and energy of recoil atoms resulting &om MeV ion bombardment I Typeof information: Elemental composition, quantitative depth profile, mass spectra Measurement environment: Diflcuffies 1 Vacuum, MeV ions Radiation damage Equipment: 1 MeVaccelerator User sklll needed: Typeof laboratory: Large Technician Techniquesyielding similar information: Nuclear reaction analysis, SIMS, Sputter Auger, GDOES, XPS,EDS, AES Characterized parameter: DiEwted electron angle and intensity Typeof information: Surface crystal structure, morphology, lattice parameters, surface roughness, reconstruction Measurement environment: Di@lcult&s UHV,electron bombardment Theoretical Equipmwf: interpretation UHV system, keV e-gun User sku1 needed: Typeof laboratory: Medium Specialist Techniquesyielding similar information: LEED, ISS

‘Rutherford Backscattering ~- Spectrometry ~. ~Characterked parameter: Energy spectrum of scattered MeV-ions

v p e of information: Quantitative elemental composition and depth profile Measurement environment: D@culties Vacuum, ion irradiation Radiation damage, Equipment: overlapping signals MeV ion accelerator Typeof laboratory: User skill needed: Medium Technician Techniquesyieldhg shUar information: SIMS, Recoil Spectrometry, AES, GD-OES, X P S

Surface Specilflcity Informatjon depth: Detectibility: 1P 0.1 at. YO _ _ _Resolution Depth: Lateral: Other: 20nm 1mm 1 u ; M z Timeneeded for analysis Prep. Measurement Evaluation 1 min. 10 min 30 min Cost [ECUj: No. offatuities: 100

Sample Form Type: Solid

300

SEe: 0.5 cm2 A

Surface Specijkity Information depth: Detectibility:

Inm

.

~

_

_

_

Resolution Depth: Lateral: Other: Latticepar. : 0.01 ~1 Timeneeded for an&sls Prep. Measurement Evaluation days &YS WWkS Cost [ECUj: No. of facUUes: Sample Form Qpe: Single crystal

RBS

5000

Sue: 1mm BS

Surface Specffliq Information depth: Detectibility: 2 urn 0.1 at. YO Resolution Depth: Lateral: Other. 25-nm 0.5 mm 1 U; M 1 3 0 _ _ _ Timeneeded for ana&sis Prep. Measurement Evaluation 1 min 10min 5min ___ Cost [ECUI: No. of facilities:

70-

-500

Sample Form Dpe. Solid

___._

Size: 0.25 3113132


Characterized parameter: Distribution and energy of scattered electrons and X-rays, respectively Upe of information: Topography, elemental composition,map, line scan, point analysis Measurement environmen1: Dimulties Vacuum, electron irradiation Vacuum, radiation Equipment: damage User skill needed: Typeof laboratory: Medium Technician Techniquesyielding similar information: Light microscope, XRF, AES

47

surface Speci@& Information depth: Detectibility: 1 Pm 0.1 at. YO Resolution Depth: Lateral: Other: 1 pm 0.01-1 pm Timeneeded for analysh Prep. Measurement Evaluation Smin Smin 5 min Cost [ECU]: No. offacilities: 50-500

Sample Form Qpe: Solid

Scanning Tunnelling Microscopy

10000

Size: pm-cm

STM

Characterized parameter: Spatial variation o f electron tunnelling current

Surface Speciflcily Information depth: Detectibility:

Typeof information: Map o f surface electronic structure, local I-V relation, surface topography, surface reconstruction Measurement environment: D@kulties UHV Finite tip size, Equipment: contamination Scanning tunnelling microscope User skill needed: o p e of Zaboratory: Medium Specialist Techniquesyklding similar information: AFM

Resolution Depth: Lateral: Other: 0.1 nm T h e needed for analysis Prep. Measurement Evaluation 1Omin lh 1 day Cost [ECU]: No. of fecilities: 150 500 Sample Form i'j.pe: Sue: Conductor, semiconductor 1 mm

c

4

/Scanning Vibrating Electrode Technique

+

SVET

Characterized parameter: Mapping o f current densities

Surface Speci@& Information depth: Detectibility:

Typeof information: Location of anodic and cathodic sites in a microscale on a metal surface Measurement environment: Difficulties Electrolyte Equipment: Vibrating electrode potentiostat, lock-in amp User skill needed: Typeof laborafory: Small Specialist Techniquesykiding similar information:

Resolution Depth: Lateral: Other: 10-15 pm nA cm-2 Timeneeded for analysis Prep. Measurement Evaluation lh min-h Cost [ECU]: No. of facilities: 100

500

Sample Form Type: Metal in electrolyte

Size:

&mm

Introduction

48 r

Static Mode Secondary-Ion Mass ___ Spectrometry --__ Static-SIMS .SSh& . ~.

Characterized parameter: Mass of sputtered atomic and molecular ions

Typeof information: Molecular and elemental surface composition, mass spectrum, chemically resolved image Measurement environment: Di&Tlculties UHV, ion bombardment Insulator charging Equipnrent: Ion source, mass spectrometer Typeof laboratory: User skUl needed: Medium Specialist Techniques yielding similar information: XPS, Dynamic-SIMS, X P S

Surface Speci&ki@ Information depth: Detectibility: Im 109 at. cm-2 Resolution Depth: Lateral: Other: 0.5 pm WAM >lo4 T h e neededfor ana&srS Prep. Measurement Evaluatiot 30 min 30mh 10min ~- ______ Cost [ECU]: No. of facilities: 150-400 __

100

Sample Form Qpe: All solids, powder ~

__

~

Size: 2 0.5 mm

r

Stvlus Profiling- ..

Characterized parameter: Vertical deflection o f stylus Typeof information: Surface topography and roughness Measurement environment: Difflcrrlties Air Fhite stylus size, Equipment: softness Stylus profilometer Type of laboratoty: User sku1 needed: Small Technician Techniques yieldin# similar information: SOM, SEM, Optical interferometry, Optical scattering

Sutface Speciftcuv Information depth: Detectibility: ~ t nm l h>20pm Resolution Depth: Lateral: Other: 1-2 pm T h e neededfor ana&sls Prep. Measurement Evaluatior 0 10 min 10 min Cost [ECUJ: No. of facUMa: 10-100 5000 Sample Sue: Form Type: Solid mm-cm ~~

RAS

Surface Raman Spectroscopy Characterized parameter: Stokes-shifted optical emission

Sutface Speciflcirv Information depth: Detectibility:

Typeof information: Molecular adsoption on solid reflecting surfaces

Resolution Depth: Lateral: Other: 10 pm T h e neededfor analysis Prep. Measurement Evaluatior 10min 10min Ih Cost fECU]: No. offacUMa:

Measurement environment: Laser bombardment Equipment: Laser, FTIR spectrometer Typeof laboratory: Medium Techniques yielding simUar Static S I M S , LMMA

Di@iculiies Topographic enhancement effects User s k u needed: Specialist information:

1 no

50

Sample Form Vpe: Solidreflectingsurface

Sue: 1 cm


Characterized parameter: Spatial and distribution of secondary-electrons, -photon energy spectra o p e of information: Element, chemical state, surface structure, surface composition Measurement environment: Difficulties UHV, X-ray W photons Surface contamination Equipment: Synchrotron light source Typeof laboratory: User skill needed: m e Specialist Techniquesyielding similar information: UPS,XPS,AES, LEED, PED,EXAFS r

Temperature-Programmed __ Desorption

Characterized parameter: Temperature at which a species is desorbed Typeof information: Adsorption strength of different species Measurement environment: Difficulties Vacuum, Temp. 5 1100 "C Equipment: Mass spectrometer, vacuum system, gas system User skill needed: Typeof laboratory: Medium Technician Techniquesyielding similar information:

Surface Speciflcuv Information depth: Detectibility; 0.2-10 nm Resolution Depth: Lateral: Other: 0.2 nm 0.1-1000 < 1 monolalyc Time needed for analysis Prep. Measurement Evaluation 0 . 5 4 h. 10 min 4h Cost [ECUj: No. offacilities: 50-500

40

Sample Form Type: Gases, solids, liquids

Size: 1 pm-5 mn

TPD Surface Specif7ci@ Information depth: Detectibility: 1 monolayer Px)

(8-1)

In eq. 8-1, I,(E) is the original intensity of the beam while I(E) is the intensity after the beam has travelled a distance x in the sample. The density of the sample is p, and p(E) is the mass-attenuation coefficient of the sample material. The mass attenuation can be written as a sum of coherent, incoherent and photoelectric mass-attenuation coefficients, thus emphasizing that three independent attenuation processes contribute to the total attenuation of the beam. The functional dependence of the photon energy, E, is written in eq. 8-1 to emphasize that the beam attenuation'is dependent on the photon energy. In practical cases, the attenuation of both the impinging and the characteristic X-rays has to be borne in mind. Since the energy of the exciting photon always has to be greater than the energy of the characteristic X-ray, an approximate value of the surface thickness studied with XRF can be found by a simple calculation of the path-length needed for the characteristic photons to be reduced by a certain factor when travelling through a certain material. Table 8-1 shows some typical values of penetration depths for some elements in an iron and a carbon matrix. In this example, the penetration depth varies by three to

138 Part 2: Elemental composition

four orders of magnitude. From Table 8-1, one can also see that small inhomogeneities in the surface structure can cause large uncertainties in the quantitative determination of concentrations for many elements, especially in a matrix of heavier elements. Table 8-1, Depth of penetration in micrometers of characteristic radiation from sulphur, chromium and yttrium in a matrix of iron or graphite (C). The penetration depth for reduction of X-ray intensities to 2/3 of the original has been calculated according to the formula I=Ioexp-(ppxln). The elements which generate characteristic radiation are assumed to be present at trace levels, why the density of the matrix is not affected.

-".-

Characteristic radiation S Ka Cr Ku Y Ka S Ka Cr Ka Y Ka

Matrix Fe Fe Fe C C

C

p 7 -

* From Handbook of Spectroscopy, Vol

Density 7.87 7.87 2.25 2.25

2.25

Mass attenuation*

119

58.5 208 15.3 0.813

4.3 8.8 8.7 120

2200

I,J W Robinson, E d , CRC Press, Inc (1974)

8.3 General aspects of quantitative evaluation In the quantitative evaluation of element concentrations, one has to take several factors into consideration: the energies of the impinging (primary) photons, the energies of the thorescent photons, the mass-attenuation coefficients in the sample for photons of all relevant energies, the photoelectric cross-sections and fluorescence yields for the elements being studied. In some cases, it is also necessary to take into account the fact that the characteristic radiation produced in one element can cause fluorescence in another element (inter-element effect), which may lead to enhancement of one spectral line and further attenuation of another. Furthermore, in every XRFspectrometer one must have information on the geometry, beam attenuation in the spectrometer, presence of impurities in the spectrometer material and properties of the detector. The details of quantitative analysis are beyond the scope of this section but can be found in the literature (Bertin, 1975; Jenkins et al., 1981; van Grieken and Markowicz, 1993). However, some practical aspects of quantification will be described here. Many kinds of sources of primary photons are used in XFS spectrometers. In the general case, the sources may emit more than one excitation energy. In the case of monoenergetic excitation of a homogeneous sample and in the absence of interelement effects, the relationship between the mass per unit area of the analyte, m,, of a specific element denoted by index 'i' and the measured intensity I, of the characteristic peak of the element at energy €3,is expressed by the simple equation:

I, = I&m,D,

(8-2)

8 Energy-dispersiveX-ray fluorescence analysis 139

in which I, is the intensity of the primary beam of energy E, and D, is a correction factor for attenuation of both the impinging X-rays (of energy E,) and the characteristic Xrays of element ‘i’ (of energy El) in the sample. E, is, of course, assumed to be larger than the threshold energy required to produce the characteristic radiation of energy El. K, is a factor which depends on the geometry of the EDXRF-spectrometer, the probability of producing a vacancy in the relevant shell of element ‘i’, the fluorescent yield of element ‘i’, the detector efficiency for the characteristic radiation from element ‘i’ and the attenuation of the impinging and characteristic radiation along the beam paths and in the detector window. The amount of analyte is often written as: m, = W,m

03-31

In eq. 8-3, W, is the weight fraction of element ‘i’ in the sample and m is the mass per unit area of the sample (m = px in eq. 8-1). The essential feature of eq. 8-2 is that the factor K, is independent of the amount, m,, of the analysed element and thus can be determined by calibration of the instrument. If the sample is very thin, no correction for attenuation is needed and the factor D,=l . However, since the characteristic X-rays from different elements have very different energies, the attenuation correction will be different for soft and hard X-rays, and a specific sample may be regarded as infinitely thin for one element but not for another. Compare Table 8-1. Monochromatic or nearly monochromatic X-ray sources used in practical work are either radioisotopes or X-ray tubes combined with a secondary target or monochromators, for example multilayers. (Most laboratories do not have access to synchrotron radiation.) In these cases, eq. 8-2 can be used for quantitative evaluation, after calibration of the spectrometer with known standards. Calibration gives information on the factors K, for different characteristic X-ray energies. These factors can be stored for a special configuration of a given spectrometer and used for element quantification of unknown samples. If an unknown sample is considered to be thin for some elements (DI=l), the amounts of these elements can be readily evaluated. If the sample is of intermediate thickness, the attenuation factors D, will have to be evaluated from either measurements of area thickness (m) or from information contained in the coherently and incoherently scattered radiation as discussed in Section 8.4.2. For polychromatic X-ray sources, for example broad-band excitation from X-ray tubes, eq. 8-2 is not valid. In this case, the intensity I, can be described by an integral over the distribution of excitation energies. In practice, an approach is often used in which the instrument is calibrated with standards as similar as possible to the investigated samples. If inter-element effects are present in a homogeneous sample, these will have to be evaluated from physical data. For heterogeneous samples, quantitative evaluation is more difficult, and this is particularly the case for X-rays from light elements, for which the penetration depths of the characteristic X-rays are small.


8.4 Surface analysis of thick specimens - practical applications For XW-spectrometers, the goal of the designer is to obtain as high an intensity as possible of characteristic X-rays from the elements whose concentrations are to be determined, whilst keeping the background radiation as low as possible. There are many different designs of XRF spectrometer around the world, and in order to illustrate some practical aspects, the principles and applications of two kinds of spectrometer will be described. One is a scanning XRF spectrometer, which can be used for studies of elemental distributions in the upper layer of thick samples. Subsequently, another technique suitable for thin samples and highly reflecting materials, namely totalreflection X-ray fluorescence (TXRF) will be described.

8.4.1 A scanning XRF spectrometer The important parts to consider in any energy dispersive X-ray spectrometer are the X-ray source, spectrometer geometry and the detector. In this presentation, detector performance will not be discussed, although many things have to be considered when choosing an appropriate detection system (Jenkins et al., 1981; Campbell et al., 1984; Selin et al., 1991). The basic features of one scanning XRF spectrometer (Selin et al., 1993) are described below. A conventional X-ray tube with a silver or tungsten anode excites a secondary target of pure metal, which gives rise to characteristic radiation from the metal. For detection of medium-heavy elements, molybdenum is suitable as a secondary target, but a lighter or heavier secondary target material is used to improve and extend the detection capabilities for other elements of the periodic table. The radiation from the secondary target impinges on the sample, and the radiation from the sample is detected by a Si (Li) detector. The beam paths of 1) the radiation from the tube, 2) the radiation from the secondary target and 3) the radiation from the sample form three mutually orthogonal angles. This geometry has been shown to have very favourable peak-to-background conditions (Standzenieks and Selin, 1979). If the geometry of the instrument is made compact with short beam paths, small intensity losses are obtained in the instrument. Another practical point to consider is the position of the sample in the horizontal plane. This has a great advantage since semi-liquid surfaces and powders that cannot be placed in a vertical position, can be investigated. In a scanning XRF system (for example the one described by Selin et al., 1993), the samples are mounted on a sample holder that can be moved in three mutually orthogonal directions in steps of a few microns at a time. The possibility of vertical adjustment of the sample is essential in order that optimum X-ray intensity is achieved at the target spot for samples of different thickness. The movements in the horizontal directions are controlled by computer-driven step motors (Boman et al., 1991). Fig. 8-1 shows a schematic drawing of the scanning spectrometer at the Department of Physics in Goteborg, Sweden. The exciting beam from the secondary target is collimated accord-

8 Energy-dispersive X-ray fluorescence analysis 141

ing to the needs of the particular application. In a study of elemental variations in tree rings, a conical, compact, slit collimator of size 0 . 1 ~ 1mm2 was used (Selin et al., 1993). This collimator gave a beam profile of strips of about 0.2 mm in width at the target spot. In the actual measurements, the X-ray tube in the scanning spectrometer is operated at a voltage of about 50 kV and a current of 25 mA. To obtain a satisfactory number of counts for detection limits at the pprn level, the spectrum in each position was recorded for 300 s when the slit collimator was used. When larger collimators are used, the sampling time can be reduced. The sensitivity of an XRF instrument can be measured as the number of counts per time interval in the characteristic peak of an X-ray spectrum that a certain concentration (for example 1 ppm) of the element gives rise to. The sensitivity varies to a high degree with the difference in energy between the exciting radiation and the characteristic radiation, and is highest if the energy of the exciting radiation is only slightly greater than the energy of the K-edge for the element in question. For the scanning XRF instrument in Fig. 8-1, the sensitivity is comparable to that achieved by use of synchrotron radiation on targets of a similar type (Gilfrich et al., 1991), and the detection limits will be less than 1 ppm for elements with K-radiation energies well matched with the energy of the exciting radiation.

Fig. 8-1. Sketch of the scanning XRF spectrometer. The sample can be moved in three mutually orthogonal directions as indicated. The plane of the sample is horizontal, which allows scanning of thick, soft materials.

8.4.2 Quantitative evaluation of thick specimens In order to determine concentrations of elements in a sample, the net areas under the characteristic peaks in the X-ray spectrum have to be evaluated. Providing the samples can be considered to be thin and homogeneous, and in the absence of inter-element effects, the intensity of the characteristic X-rays from a certain element will grow line-


arly with the amount of analyte. For most real samples, however, these conditions are not fulfilled. For samples such as alloys, mineral ores and ceramics, several elements are usually present in different concentrations which may differ by several orders of magnitude. In a matrix composed of many elements, each element will attenuate the radiation from all other elements. If one of the matrix elements has a characteristic radiation which is on the short wave length side of the absorption edge of another element, conditions are favourable for enhancement of the lines of the latter element. The combined effects of the matrix elements on each other are often called absorption-enhancement effects. Determination of iron (Z=26) in a matrix of chromium (Z=24), nickel (Z=28) and cobalt (Z=27) can be considered as an example. In this case, neither the chromium nor the cobalt Ka-radiation will enhance the iron; their energies are lower than that of the absorption edge for the K-shell in iron. The nickel Ka, on the other hand, will enhance the iron line, because the energy of the absorption edge of iron in the K-shell is just below that of the nickel Ka-radiation. If instead chromium were the focus of interest, enhancement effects would have to be taken into consideration: Fe-Cr, Ni+Cr, Co+Cr and also to Ni+Fe+Cr. Enhancement effects are especially important for cases in which the analyte is present at trace level and the enhancing elements at percentage levels. Although the general problem of inter-element effects seems difficult, one must keep in mind that, for homogenous samples, these effects can be calculated from basic knowledge of physical parameters. As seen from eq. 8-2, quantification of an element is relatively easy for thin samples (DI=l). In reality, few samples are thin with respect to light elements, which have soft X-rays (Compare Table 8-1). Thus, one is usually faced with the problem of evaluating the attenuation properties of the sample. Two methods of performing such an evaluation will be briefly described. A precondition for both methods, however, is that the sample is of ‘intermediate thickness’, by which is meant that an increase in sample thickness will result in an increase in the measured intensity of the characteristic radiation. Samples which are so thick that a further increase in thickness does not give rise to a measurable increase in the characteristic radiation are referred to as ‘infinitely thick’. For these samples, a simplified relationship between characteristic intensity, I,, and weight fraction, W,, can be derived:

In this expression, p(Eo) and ~ ( E oare ) attenuation coefficients for the exciting and characteristic radiation of energies Eo and E,, respectively. (D, is the glancing angle between the exciting radiation and the sample plane and (Dz the glancing angle between the characteristic radiation and the sample plane. (See Fig. 8-2.) The first method, which was one of the earliest, is the emission-transmission method (Markowicz and van Grieken, 1993; Jenkins et al. 1981). l h e attenuation correction, D,, in eq. 8-2 has the form :


Evaluation of sample attenuation by the emission-transmission method implies three measurements, as sketched in Fig. 8-2. First, the characteristic radiation from an element ‘i’ in a sample is measured. Under identical conditions, the same characteristic radiation is measured from the combination of the sample and a target of element ‘i’ behind the sample. In the third measurement, the characteristic radiation is measured from the target alone. As shown in Fig. 8-2, these measurements will provide knowledge of the factor exp-((p(E,) csc + p(Ei) csc Oz)m), which is needed in eq. 8-5. If this factor is known, calculation of the mass per unit area, mi, of element ‘i’ from measurements of I, according to eq. 8-2 is straightforward.

Target Sample

5 m2 n (D2 a1

I0

Iis

m1

Iit

Iio

Fig. 8-2. The attenuation factor D, in eqs. 8-2 and 8-5 of a sample, S , can be determined by measuring the characteristic radiation, I,,, &om the sample alone, the characteristic radiation from the sample plus a target of element ‘i’, I,,, and the characteristic radiation of the target alone, Ilo. The exponential factor, + P(EJ csc Wm),= (IIt-IIs)~lo. exp-((!-@,) csc

Another well known method is to make use of the information contained in the coherent and incoherent scattering of the incident (exciting) radiation. Under certain conditions, the scattering peaks or combinations of these can be used as internal standards. (Markowicz and van Grieken, 1993; Jenkins et d,1981; Giauque et al, 1993; Giauque, 1994). Fig. 8-3 shows an X-ray spectrum from a biological sample, in which molybdenum has been used as a secondary target. As seen in the spectrum, the incoherently scattered radiation is very pronounced. For light elements with the exception of hydrogen, e.g. carbon, nitrogen and oxygen, which are the main constituents in biological samples, the mass of an atom is closely proportional to its atomic number (Z). Furthermore, the binding energies of the electrons in the light elements are small compared to the energy of the exciting radiation (MoKccof 17.5 keV in Fig. 8-3). The implication is that the exciting radiation essentially ‘sees’ a cloud of free electrons, and the probability of incoherent (Compton) scattering will thus be proportional to the number of electrons, which in turn is closely proportional to the mass of the sample. Thus, the intensity of the incoherently scattered radiation in a biological or other lightelement sample is essentially a measure of the mass seen by the impinging beam, provided that the sample is of ‘intermediate thickness’, as discussed earlier.


In the general case of a matrix containing light and heavy elements, the simple picture referred to above is not strictly true. Several methods, however, have been developed in which use is made of the information contained in both coherently and incoherently scattered radiation. For instance, Giauque (1994) and Giauque et al, (1 993) have developed a method in which different combinations of ratios between coherent and incoherent scattering for K a and KP radiation from the source is used in the calculation of mass thickness and mass attenuation effects in complex samples, for example, sediments and obsidian artefacts. Peak max 39 300 CtS

-

2 9000 c

Ca

5

8

Sr

I

Mc KCI

6750 Fe

4500

Ca KB

2250 Fe 1

P CI

0

0

Mn

5.5

KB

Br

z" 11

16.5

Energy, keV Fig. 8-3. X-ray spectrum from a thick sample of plankton material (50 mg ern-*) recorded with a threeaxial XRF spectrometer using a secondary target of Mo. The measuring time was 500 s.

A simplified way of considering the quantification problem in XRF spectra is as follows: The medium-heavy and heavy elements in the sample will reveal their presence by their characteristic radiation. Thus, a first estimate of the relative abundance of these elements can be made directly from the spectrum. Each element present in the sample will also make a contribution to the scattered peaks of the exciting radiation. Thus, the contribution to the scattered radiation from the elements whose characteristic radiation is seen in the spectrum can be evaluated. The remainder of the scattered radiation in the spectrum is ascribed to light elements which do not have any visible


characteristic lines in the spectrum. For these elements, their contribution to the scattered peaks can be used to estimate their contribution to the sample mass per unit area. By combining the information from the ‘seen’ and the light elements in the sample, an evaluation of the total mass per unit area can be obtained. In an interactive process the contributions to the sample mass from the different kinds of elements can be reevaluated until selfconsistency is obtained in the sense that the calculated composition of the sample should give rise to the observed spectrum of characteristic as well as scattered radiation. As noted in Fig. 8-3, the scattered radiation (both coherent and incoherent) is very pronounced in traditional XRF spectra. While the scattered radiation can be used in quantitative work, it is a drawback because it contributes to the general background under the characteristic peaks of the elements of interest. The background radiation degrades the detection limit, often defined as the amount or concentration needed for the number of counts in the characteristic peak Pi to be equal to 3dB, where B is the number of counts in the background under the peak (Jenkins et al., 1981; Bertin, 1975). Since the number of counts in the peak and background are both timedependent, but scale in different manners (P grows linearly with time whereas dB grows with the square root of time), the detection limit will be lowered when the time of analysis is increased.

8.5 Total-reflection X-ray fluorescence (TXRF) spectrometry TXRF is an inherently surface-sensitive method, in which the properties of totally reflected of X-rays are exploited. The applications of TXRF naturally divide into two areas: chemical analysis and surface analysis. In chemical analysis, the sample is present in the form of a thin film on top of a reflecting surface. In surface analysis, the properties of the reflector itself are studied.

8.5.1 Applications of TXRF Instruments for chemical analysis have been available commercially since about 1980 (EXTRA 11, Rich. Seifert & Co. and Atomika Analysetechnik GmbH). In the use of TXRF for chemical analysis, four typical fields of application can be discerned: environmental, forensic, medical and industrial.

Chemical analysis In environmental analysis, TXRF is used to study trace elements in air, water, soil and living organisms. From the atmosphere, airborne particulate matter can be collected directly on TXRF sample carriers by impaction or deposition, or deposited on filters which are analysed after digestion.


Water readily lends itself to TXRF analysis. Rain and limnic waters can be analysed directly or after separation of the suspended particulate matter. For sea water, the salt matrix must be removed in order to achieve low detection limits. Solid samples, such as soil, coastal sediment or plant material, are usually digested in acid or oxygen plasma before analysis. TXKF is used in forensic science because of its non-destructive and ultramicroanalytical capabilities. Examples of applications in this field are the trace element determination of single textile fibres and artists paint pigments. In medical applications, TXRF is used for the analysis of tissue and body fluids. Tissue is conveniently cut, using an ultramicrotome, into 10-pm sections that can be analysed directly. Body fluids (e.g. blood or amniotic fluid) can be analysed after certain preparatory procedures. Some applications of TXRF that could find their way into industrial use, are the analysis of oil, ultrapure reagents, acids and materials.

Surface analysis In the use of TXRF for surface analysis, three fields of application can be distinguished: analysis of metal contamination on silicon wafers, determination of layer structure and investigations of biological model membranes. The information depth of TXRF depends on the surface material and the angle of incidence, and is of the order of 10 to 1000 A. In 1995, about 100 TXRF spectrometers (one third of all TXRF instruments) were used to analyse metal contamination on silicon wafers. Commercial instruments for this purpose have been available since about 1989. The instruments available in 1995 included the TXRF 8010 by Atomika, TREX 610 by Technos and System 3726 by Rigaku. Two methods are employed: either the contaminants in or on the wafer are excited directly by an impinging X-ray beam, in which case a map of the distribution of the contaminants on the wafer surface can be obtained, or the contaminants adsorbed to the surface are collected by vapour phase decomposition (VPD) followed by TXRF analysis. The latter technique has detection limits of the order of lo-*atoms cm'2. Several methods have been developed for the determination of layer structure. When X-rays are incident on a material that contains distinct boundaries between different layers, standing waves of X-rays will form. The standing waves are used to obtain a depth profile of the fluorescence signal as the angle of incidence, the angle of exit or both angles are varied. From the depth profile, information about the layer structure of the surface can be extracted. Common acronyms used for these methods are: Grazing-Incidence X-ray Fluorescence (GIXRF), synonymous with Incident Angle-Dependent XRF (IAD-XRF); Grazing-Exit XRF (GEXRF), synonymous with Takeoff Angle-Dependent XRF (TAD-XRF) and Glancing-Incidence and -Takeoff XRF (GIT-XRF). In studies of layer structures, synchrotrons with crystal or multilayer monochromators are often used as radiation sources. These non-destructive methods can determine an impressive range of parameters of composite layered structures: layer thickness,


density, roughness and even the compositional depth profile. Structural information about biological model membranes, such as LangmuirBlodgett films, can be obtained by the X-ray Standing Wave (XSW) method. By varying the angle of incidence, a fluorescence profile is obtained from the XSW formed above the reflector surface. This profile gives information about the position of heavy, marker atoms in biological films at h g s t r o m resolution. The development of TXRF can be followed in the proceedings of the TXRF workshops (Geesthacht, 1986; Dortmund, 1988; Vienna, 1990; Geesthacht, 1992; Tsukuba, 1994; Eindhoven and Dortmund, 1996).

8.5.2 General aspects As stated above, TXRF is a method which is inherently sensitive for analysis of surface composition since it analyses the properties of totally reflected X-rays. Two areas of application are clearly appropriate for TXRF namely, chemical analysis and surface analysis. For chemical analysis the requirements is that the sample is a thin film placed on a reflecting surface, while for surface analysis the sample itself is the reflector. Already, in the early part of this century, physicists realised that total reflection of X-rays was possible (e.g. E. Niihring, 1930). In the development of the TXRF technique, important contributions were first made by Yoneda and Horiuchi (1971), Aiginger and Wobrauschek (1974) and b o t h and Schwenke (1978). The last achieved detection limits in the order of some ten pg. In recent years, detection limits have improved by a factor of one hundred or greater (Prange and Schwenke, 1992). The TXRF technique is based on the fact that total reflection can occur when radiation is incident on the boundary of an optically less dense medium. For X-rays, the index of refraction, n, consists of two parts: one real (1-6), and one imaginary (p), according to: n=l-G+iP

(8-6)

In the above equation i = G ,and p describes the attenuation of the X-rays in the material. The real part of the index of refraction is slightly smaller than unity (6 is of the order of and this causes total reflection when the X-rays travel from air or vacuum into a denser medium at a small, glancing angle. Both 6 and p depend on the energy of the X-rays and on the properties of the reflecting material. In this context, the critical angle, (bc, is the glancing angle below which total reflection occurs. (bc depends and is of the order of a few mrad for X-rays in the 5-10 keV on 6 through (bc=JZ, range. From Fresnel’s equations, expressions can be derived for the reflectivity, R=I,enected/Ilncldent, and for the penetration depth in the material, in terms of 6, p and (bc (Prange and Schwenke, 1992; Schwenke and Knoth, 1993). For glancing angles smaller than the critical angle, the reflectivity is close to loo%, but it drops sharply at the critical angle. The penetration depth, on the other hand, is extremely small for


glancing angles below the critical angle. For M o K ~X-rays , of 17.5 keV incident on silicon or quartz at a glancing angle of 0.5 +c, the penetration depth is only a few nm. At very small angles, the penetration depth does not vary appreciably with angle, and remains almost constant.

8.5.3 TXRF for chemical analysis A principal sketch of a TXRF spectrometer for chemical analysis is shown in Fig. 8-4 (Pettersson and Wobrauschek, 1999, where a well-collimated X-ray beam lrom a source is incident on a multilayer or glass mirror to obtain radiation of appropriate energy. A glass mirror, serving as high energy cut-off, or a multilayer, serving as a Bragg reflector, reduces the background. The incident beam impinges on the sample at a glancing angle of a few mrad. The sample is positioned on a polished sample carrier. The incident beam is totally reflected by the sample carrier, and the sample is excited a second time by the reflected beam. Fluorescent X-rays are emitted from the sample in all directions. This characteristic fluorescent radiation from the sample is detected by a Si(Li) detector, as shown in Fig. 8-4. The detector is shielded from unwanted scattered radiation from the sample carrier. Thus, in the resulting X-ray spectrum, the contribution from the sample is strong, whilst the background from the sample carrier is weak. However, this explanation is somewhat simplified, since in reality, the incident and the reflected beams form a pattern of standing waves above the surface of the sample carrier.

Fig. 8-4. Schematic diagram of the TXRF set-up: (A) rotating anode; (Cl-3) collimator slits; (ML) multilayer monochromator; (S) sample carrier; (D) Si(Li) detector. (Reprinted from Nucl. Instr. and Meth., A355 R.P. Pettersson and P. Wobrauschek, Total-reflection X-ray fluorescence analysis with 18kW rotating anode source - first results, 665-667 (1996), with kind permission of Elsevier Science - NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)

The sources of X-rays used in laboratory TXRF spectrometers are either highpower, rotating anodes or fine-focus X-ray tubes, Synchrotron radiation, if available, provides a monoenergetic, parallel and polarized beam without the need for any glass or multilayer mirror. Fig. 8-5 shows the set-up of the TXRF spectrometer at the Physics Department of Goteborg University, Sweden, which uses a high-power (18 kW)


0

bI

Fig. 8-5.Drawing of the TXRF spectrometer at the Department of Physics, Gothenburg. The total-reflection unit is supported by the X-ray tube of the rotating anode system. The detector is supported by a frame that rests on three pillars. All components of the set-up are adjustable in angle and position. (Reprinted fiom Nucl. Instr. and Meth., A371, R.P. Pettersson and J. Boman, A total-reflection X-ray fluorescence spectrometer using a rotating anode, 553-559 (1996), with kind permission of Elsevier Science - NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)

rotating anode source. The detection limits (below 1 pg) for elements matching the energy of the exciting beam Are considerably better than for a conventional EDXRFspectrometer. In general, TXRF is about a hundred times more sensitive than conventional EDXRF. This is mainly because of the dramatic reduction in scattered radiation from the sample support. A comparison of the XRF spectrum in Fig. 8-3 with the TXRF spectrum in Fig. 8-6 shows the difference between the two methods: In the thick sample of Fig. 8-3, the penetration depth is larger the heavier the element, therefore the Sr peak is larger than the Fe peak and the P, S and C1 peaks are very small. The sample of

150 Part 2: Elemental composition Peak max

17 300 cts

6750

1 blla

Fe

Ka

h

4500

2250

I Ail

0

Sr

I

4

8

12

16

Energy, keV Fig. 8-6. TXRF spectrum of a thin sample of the same plankton material as in Fig. 8-3. The features characteristic for TXRF, as compared to XRF in Fig. 8-3, are: i) absence of MoKP in the scattered radiation due to monochromatic excitation, ii) better resolution of heavy element peaks due to smaller MoKapeaks, iii) lower background due to total reflection on the sample support, iv) improved sensitivity for light elements due to lower background.

Fig. 8-6 is about one hundredth as thick as the sample of Fig. 8-3, and the better sensitivity of TXRF shows especially in the larger peaks of the light elements. An important limitation in the use of TXRF spectrometry for chemical analysis lies in the fact that the sample being studied has to be in the form of a thin film in order to avoid scattering effects in the sample material itself. Thus TXRF can be used to study surface concentrations in thick specimens only if these can be polished to give rise to total reflection. On the other hand, there are many samples which can be made into thin layers. Among the most obvious are aqueous samples, which can be pipetted onto the sample carrier, spiked with an internal standard and then dried. This procedure leaves very little scattering material for samples such as rain and river water. If, on the other hand, the liquid contains high levels of organic substances, for example blood and humus, digestion and other sample treatment may be needed. With the use of an appropriate preparation method, many kinds of sample can be analysed by TXRF, for example, sediments, soil, sludge, minerals, human tissues, plant leaves, roots and foodstuffs (Prange and Schwenke, 1992).

8 Energy-dispersive X-ray fluorescence analysis 15 1

For thin, liquid and homogeneous samples, in which absorption effects can be neglected, quantification is very simple and can be achieved by spiking with an internal standard containing an element that is not present in the sample itself. If absorption effects are expected, for instance, for light elements giving rise to soft X-rays, a twoelement, internal standard may be needed. If the two elements differ in mass and thus in the energies of their characteristic X-rays, these will contain the information needed concerning the attenuation properties of the sample. From this information, concentrations of all other elements in the sample can easily be obtained.

8.5.4 TXRF for surface analysis One of the most interesting applications of TXRF for surface analysis is that of surface contamination in or on a reflecting material, for example silicon wafers. In this case, the wafer itself is the reflector. As mentioned above, the penetration depth of Xrays into silicon under conditions of total reflection is very small, but TXRF is actually able to distinguish between those cases in which the contamination is deposited on the surface (as, for example, aerosol particles) and those cases in which the contamination is embedded inside the material. When such studies are performed, the angle of the reflector (silicon wafer) versus that of the incoming radiation is varied in small steps, of the order of 0.2 mrad, and the intensity of fluorescence radiation is measured at each point. Fig. 8-7 shows a sketch of the principal differences between the two cases.

4 1

0

2

4

6

0

10

12

14

16

Angle o f Incidence Iminl

Fig. 8-7. Calculated fluorescence signals as a function of glancing angle from: 1) a sample situated on top of the surface, 2) impurities embedded in the surface of a totally reflecting substrate. From Schwenke and Knoth (1993), by permission.


For material deposited on the surface of the wafer, the fluorescence intensity will be a step function of the glancing angle. For angles below the critical angle, &, the fluorescence intensity will be roughly twice as large as for angles larger than $c (See Fig. 8-7). The simple reason is that below the critical angle the sample will be excited by both the incoming and reflected beams, which are almost equal in intensity. Above the critical angle, the beam penetrates the silicon wafer, and the reflected beam is more or less 'lost' for excitation of the sample. In the case of impurities that are embedded in the wafer, the situation is totally different. Below the critical angle only a minor portion of the incoming beam penetrates the wafer (the reflectivity R>0.99 for 17.5 keV X-radiation and 4 of the order of 1 mrad), and the fluorescence intensity is therefore low. However, R increases with increasing glancing angle, and passes a maximum in the way shown in Fig. 8-7. 'The angular dependence enters in a complex manner into the calculations of fluorescence intensity, as shown by Schwenke and Knoth (1993). The measured values of the variations of the fluorescence intensity agree well with theoretical calculations when the divergence of the incoming beam is taken into consideration. TXRF has therefore potential for determining concentration profiles of embedded impurities in silicon wafers. By using the knowledge of the differences in the variation of fluorescence intensity with the glancing angle for impurities on the wafer and inside the wafer, respectively, experimental fluorescence curves can be analysed and the contribution from each kind of impurity assessed. Detailed investigations of this kind have been performed by de Boer, Leenaers and Hoogenhof (1995) using a combination of reflectometry and angle-dependent X-ray fluorescence (AD-XRF) which can provide information about layer thickness and density, interface roughness, crystalline structure and compositional depth profile in layered structures of a few hundred nm thickness. The detection limits for metal impurities in silicon wafers analysed by TXRF are of the order of 10" atoms crn-'. For analysis of metal contamination on the surface of silicon wafers, the method of Vapour Phase Decomposition followed by TXRF is widely used (Hockett, 1995), and has detection limits of the order of 1O8 atoms cm-2.

8.5.5 Membrane-structure studies using X-ray standing waves Langmuir-Blodgett (LB) films have been used extensively as models for biomembranes. Structural information about LB films on an atomic scale has in recent years been obtained by the X-ray standing wave (XSW) method (Bedzyk et al., 1988). When the XSW method was developed in the mid-sixties, it was used to determine the position of heavy atoms implanted in semiconductors. The XSW was then generated by Bragg diffraction from single crystals. However, the spread of synchrotron X-ray sources and the advent of multilayers with plane spacing of 10-200 A meant that XSWs with a period scale suitable for the study of biological membranes could be generated. An XSW generated by total reflection on a mirror (as in TXRF), will have a period that is dependent on the angle of incidence, $. As Cp is increased from 0 to Cpc, the nodes and antinodes of the XSW move towards the mirror. A film with a layer of


heavy atoms situated above the mirror surface will therefore have a fluorescence yield that is periodic in I$. The profile of this fluorescence yield can be used to calculate the distance of the atom layer to the mirror. An example of the success of this method was the location of the metal atoms in a zinc arachidate bilayer at Angstrom resolution at a distance of almost 1000 8, from the surface of a gold mirror (Wang et al., 1991).

References Aiginger H., Wobrauschek P. (1974), Nucl. Instrum. Methods, 114, 157. Bertin E. P. (1975), Principles and Practice of X-ray Spectrometric Analysis. Plenum Press, NY Boman J., Standzenieks P., Selin E. (1991),X-ray Spectrom., 20,337. Bedzyk M.J., Bilderback D.H., Bornmarito G.M., Caffrey M., Schildkraut J.S. (1988), Science, 241, 1788. Campbell J. L., Leigh R. G., Teesdale W. J. (1984), Nucl. Instrum. Methods, B5, 39. Giauque R. D. (1 994), X-ray Spectrom., 23, 160. Giauque R. D., Asaro F., Stross F. H., Hester T. R. (1 993), X-ray Spectrom., 22,44. Gilfrich J. V., Gilfrich N.L., Skelton E. F., Kirkland J.P., QuadriS. B., Nagel D. J. (1991), X-ray Spectrom., 20,203. de Boer D. K. G., Leenaers A. J. G., Hoogenhof W. W. (1995), X-ray Spectrom 24,92 (review). Hockett R. S. (1999, Advances in X-ray Chemical Analysis Japan, 26s, 79. Jenkins R., Gould R.W., Gedcke D. (1981), Quantitative X-ray Spectrometry, Marcel Dekker, Inc. Knoth J., Schwenke H. (1978), Fresenius 2. Analy. Chem., 291,200. Markowicz A.A., van Grieken R. E. (1993), ‘Quantification in XRF Analysis.. .’, in Handbook of X-ray Spectrometry, p 339, Marcel Dekker, Inc. (edited by van Grieken, R.E. and Markowics A.A.) NBhring E. (1930), Phys. Z., 31,401. Pettersson R.P., Wobrauschek P. (1995), Nucl. Instrum. Methods, A355,665. Prange A,, Schwenke H. (1992), Adv. X-ray Anal., 35B, 899. Schwenke H., Knoth J. (1993), Handbook of X-ray Spectrometry, p 453, Marcel Dekker, Inc. (edited by van Grieken R. E. and Markowics A.A.) Selin E., Standzenieks P., Boman J., Teeyasooontranont V. (1993), X-ray Spectrom., 22,281. Selin E., Oblad M., Standzenieks P., Boman J. (1991), X-ray Spectrom., 20,325. Standzenieks P., Selin E. (1979), Nucl. Instnun. Methods, 165,63. van Grieken, R.E., Markowics A.A. (1993), Handbook of X-ray Spectrometry, Marcel Dekker, Inc. Wang J., Bedzyk M.J., Penner T.L., Caffrey M. (1991) Nature, 354,377. Yoneda Y., Horiuchi T. (1971), Rev. Sci. Instr., 42, 1069.

TXRF Workshop

1. Michaelis W., Prange A. (Eds.), Totalreflexions-Rtintgenfluoreszenzanalyse-Proc., GKSS Forschungszentrum Geesthacht GmbH. GKSS 86W6 I (1 987) (in German). 2. Boumans P.W.J.M., KlockenkBmper R. (Eds.), Proc. of the 2nd Workshop on TXRF, Dortmund, May 1988, Spectrochimica Acta 44B, No. 5 (1989). 3. Boumans P., Wobrauschek P., Aiginger H. (Eds.), Proc. of the Third Workshop on TXRF, Vienna, May 1990, Spectrochimica Acta 46B, No. 10 (199 1). 4. Boumans P., Prange A. (Eds.), Proc. of the Fourth Workshop on TXRF, Geesthacht, May 1992, Spectrochimica Acta 48B, No. 2 (1993). 5. Taniguchi K. (Ed.), Proc. of the 5th Workshop on TXRF and Related Spectroscopical Methods, Tsukuba, Oct. 1994, Advances in X-ray Chemical Analysis Japan, 26s (1995). 6. Proc. of the 6th Conf. on Total Reflection X-ray Fluorescence Analysis and Related Methods, Eindhoven and Dortmund, June 1996, to appear in Spectrochimica Acta.



9 Particle-induced X-ray emission and particleinduced gamma ray emission K.G. Malmqvist

9.1 Introduction The development of high resolution, semiconductor detectors in the late 1960s meant a giant step forward in both X-ray and gamma-ray spectroscopy. The significantly improved spectral resolution in both X-ray and gamma-ray spectra facilitated the development of both Particle-Induced X-ray Emission (PIXE) and Particle-Induced Gamma Emission (PIGE). Today, PIXE and PIGE analysis are often carried out simultaneously and the set-ups are designed to accommodate the relevant detectors and other equipment required for this combination. The accelerators used to produce the suitable ion species and energies are normally of the electrostatic type. For more details about suitable accelerators for ion-beam analysis techniques refer to the chapter by Whitlow and Ostling. To understand both PIXE and PIGE the details of the interaction between the impinging ions and matter are crucial (refer to the chapter by Whitlow and Ostling). When a beam of ions penetrates matter, the ions gradualiy lose their energy with depth, until they finally stop within a well-defined range. The cross-sections for atomic interactions (PIXE) vary slowly with ion energy while nuclear reactions/excitation (PIGE) may vary rapidly (resonant) and in a less predictable manner. The genuinely complementary characteristics of the two techniques are based on the overlapping elemental ranges of analysis. The use of nuclear reactions is facilitated by a low Coulomb barrier, i.e., a light ion in combination with a light (2120) target nucleus. The soft X-rays from low-Z elements makes PIXE suitable for analysis mainly of elements with 2212. If employed in a common experimental set-up, it is thus possible to measure both light and heavy elements and to compare independent results from both techniques for quality assurance. Each of the two techniques or a combination of the two have been used extensively in many fields of application, for instance, geology, medicine, environmental studies, materials science, archaeology, etc.

9.2 Particle-induced X-ray emission 9.2.1 Basic principles In 1970 it was demonstrated (Johansson et al., 1970) that the bombardment of a specimen with protons with an energy of a few MeV producing characteristic X-rays

9 PIXE and PIGE 155

can form the basis for a highly sensitive elemental analysis. This pioneering paper formed the starting point of the X-ray emission analysis technique, which became known as particle-induced X-ray emission analysis. As in X-ray fluorescence (XRF) and electron probe microanalysis (EPMA), the characteristic X-rays produced in the de-excitation of the atoms can be measured by either a wavelength-dispersive or an energy-dispersive detection system. In PIXE, an energy-dispersive spectrometer is used almost exclusively.

a)

-1 W Z

z 4

I

1

u

-

Ep

I

F i

oc w

10 keV

100

1

I

200

1

I

I

I

300

400

P

b)

lo3

PROTON- MICROPROBE

1 o2 10’

1

100 __+

200

300

CHANNEL N U M B E R

40G

Fig. 9-1. X-ray spectra from analysis of the same organic specimen using electrons (a) and protons (b) (Bosch et al.,1980).

In contrast to X-rays, ions and electrons can be focused by electrostatic or magnetic lenses and may hence be transported over large distances without loss of beam intensity. As a result, incident beam densities are generally much higher in PIXE than in ordinary tube-excited XRF.Moreover, focusing of particle beams down to micrometer dimensions is possible, so that PIXE analysis can be carried out with high lateral resolution. This variant of PIXE, micro-PIXE, is used in a nuclear microprobe (see Section 9.2.3). With electrons, focusing to nanometer dimensions is possible and has given rise to the widespread EPMA technique (see chapter by Pettersson and Selin-Lindgren), but heavy charged particles have the clear advantage that they give rise to much lower


continuous background intensity in the X-ray spectrum. As a result, the relative detection limits (micrograms per gram) are typically two orders of magnitude better in micro-PIXE than in EPMA (see Fig. 9-1). After the initial experiments the favourable characteristics of PIXE were rapidly realized by many researchers. Besides the traditional bombardment in vacuum, external beam approaches were also attempted and were found to be useful, particularly in examining delicate and/or large objects. The progress of PIXE over the years is demonstrated by the proceedings of seven international conferences exclusively dedicated to the PIXE technique and its applications. Today, PIXE has evolved into a mature technique, as demonstrated by the increasing numbers of research papers in which PIXE provided the analytical results and by two textbooks on the technique (Johansson and Campbell, 1988; Johansson et al., 1995).

Ionization and X-ray production cross-sections Protons or heavier ions of a few MeV/u energy have a high probability of causing the ejection of inner shell electrons. Various theoretical approaches have been used to calculate the cross-sections for inner-shell vacancy creation. The plane wave Born approximation (PWBA) model has been most elaborated. Since then, a series of modifications in the model to correct for its inherent approximations have been incorporated, resulting in the ECPSSR treatment (Brandt and Lapicki, 1981) of K- and L-shell ionization cross-sections. For K-shell ionisation with protons, the cross-sections as predicted by the ECPSSR and other theories have been thoroughly compared with experimental data. Although no theory has emerged that will predict the experimental data within a few percent for all target elements and energies, it is generally agreed that existing data tables are adequate for most K-shell proton PIXE work. In the case of the L ionization, the situation is less favourable, however, as discussed in detail by Campbell, 1988. The probability of X-ray production is also dependent on the fluorescence yield w which expresses the ratio between X-rays and Auger electrons resulting from a shell vacancy. The fluorescence yield varies strongly with atomic number of the target atom. For the K-shell, w varies from below 0.1 at Z50. Continuous background production The characteristic X-ray lines in a PIXE spectrum are superimposed on a continuous background (see Fig. 9-1). It has been shown that the electron bremsstrahlung in PIXE originates essentially from three processes: quasi-free-electron bremsstrahlung (QFEB), secondary-electron bremsstrahlung (SEB), and atomic bremsstrahlung (AB). SEB is formed by a two-step process: the incident particle first ejects an electron from a target atom, and the secondary electron is subsequently scattered in the Coulomb field of a target nucleus, thus producing the bremsstrahlung. The photon spectrum of SEB is characterized by an ‘end-point’ energy: T,

= 4m,E,N,

(9-1)

9 PIXE and PICE 157

where me and M, are the electron and projectile masses and E, the projectile energy. Above T, the intensity of SEB decreases rapidly. QFEB is emitted when an electron of a target atom is scattered by the Coulomb field of the projectile. The QFEB end-point energy T, is equal to T,/4. The process AB occurs when a bound target electron is excited to a continuum state by the projectile and, returning to its original state, emits a photon. AB predominates in the high-energy part of the spectrum whereas QFEB becomes the prevailing component at low photon energies (below Tr). Besides E , the matrix composition of the target also plays a critical role in both the shape and intensity of the electron bremsstrahlung background. The background becomes more intense with increasing average target Z. Furthermore, the electron bremsstrahlung is emitted anisotropically with a maximum intensity at an angle of 90". For this reason, in a typical experimental PIXE chamber, the X-ray detector is positioned at an angle of 135" relative to the beam direction. For a general overview of background production in PIXE spectra refer to Ishii and Morita (1 990). Detection limits and basic formalism In many cases, intense characteristic X-ray peaks are present in the PIXE spectrum. This is particularly true when the matrix consists of element(s) with atomic number above Z 2 1 1. These intense peaks and their escape peaks and pile-up peaks seriously hamper or preclude the detection of other elements with characteristic X-rays of similar energies. Furthermore, incomplete charge collection in the detector and other processes have the effect that each X-ray peak exhibits a low-energy tail. Consequently, tails associated with very intense peaks form a substantial component of the total background. The limit of detection (LOD) is determined by the relationship N, 2 3 &

(9-2)

where N, is the number of counts in a characteristic X-ray peak and NR is the number of background counts in the corresponding interval ( x one standard deviation). For a given proton-induced, characteristic X-ray emitted by an element, its yield can be calculated from the formula:

&

where the photon attenuation:

and Y,=X-ray yield of element p, C = relative concentration by weight of element p, No= Avogadro's number, R = solid angle subtended by the detector, Q = total beam charge hitting the target, E = detector efficiency, including filter effects, W = atomic


weight of the element, e = ion charge, Eo = incident proton energy, Ef final proton energy, 0;( E ) = X-ray production cross-section for a peak p, p = mass attenuation coefficient, S(E) = matrix stopping power calculated using Bragg's rule, and €4 and €lo= angle of the incoming beam and the outgoing X-ray with respect to the normal to the surface of the target.

Target thickness Eq. 9-3 is a general expression, allowing for calculation of the yield of a characteristic X-ray emitted by an element of interest in all situations, i.e. for thin, intermediate or thick targets. Targets of intermediate thickness are those for which beam particles pass through the sample, but their energy loss cannot be neglected. For two extreme cases, simplifications are possible. For very thin targets, projectile energy losses in the sample are negligible and the energy-dependent terms in the integral over dE in eq. 9-3 are constant. The X-ray absorption for the emerging X-ray may also be neglected. Hence, the dependence of yield of a characteristic X-ray is a linear function of the element concentration and no matrix information is necessary. However, preparation of thin samples is often difficult and in many cases impossible and many apparently thin samples violate the thin specimen criterion for protons and even more so for heavier ions. In the second extreme case of the X-ray yield equation, the particle beam comes to a complete stop in the sample, i.e. the final projectile energy in eq. 9-3 becomes zero. Thus it is easier to calculate correctly the integral in eq. 9-3. The possibility of analysing infinitely thick samples in an essentially virgin state is one of the major advantages of PIXE. The only two requirements for thick target preparation are a smooth surface and sample uniformity. The required smoothness depends on the X-rays to be measured, the degree of their absorption in the matrix and the accuracy to be achieved (Campbell and Cookson, 1984).

9.2.2 Quantification In PIXE, elements with Z up to about 50 are generally determined by their K X-rays and the heavier elements are measured by their L X-rays. The basis for a quantitative analysis is that there is a relationship between the net area of an element's characteristic K or L X-ray line in the PIXE spectrum and the amount of element present in the sample as given by eq. 9-3 above. To derive the concentration from its X-ray yield, several approaches are possible. One can solve eq. 9-3 and thus employ the absolute or fundamental parameter method. This requires accurate knowledge of all the parameters involved. Because of the difficulties with the absolute quantification method, many PIXE workers prefer to rely on a relative approach, and they calibrate their experimental PIXE set up using thin-film standards. In the analysis of infinitely thick specimens, one can also utilize experimental thicktarget calibration factors instead of relying on the fundamental parameter approach or on experimental thin-target sensitivities (Johansson et al., 1981). The thick-target cali-

9 PIXE and PIGE 159

bration factors incorporate the integral of eq. 9-3 and are usually expressed in X-ray counts per pC and per pg/g. They are commonly derived from PIXE measurements on standard samples with known matrix and trace element composition. In a strict sense, the thick-target factors are valid only for the analysis of unknown samples with a composition identical to the standards, but in practice, some variability in composition can be tolerated or corrected for.

9.2.3 Precision and accuracy As in any other analytical technique, high precision and accuracy should be aimed for in PIXE. It is therefore essential that careful attention is given to all stages of the analysis. These include sample and specimen preparation, specimen bombardment, spectral data processing, quantification and correction for matrix effects. During the specimen bombardment stage, great care has to be taken to minimize radiation- or heat-induced losses. Such losses are particularly critical for volatile trace elements and, in the case of organic or biological specimens, also for certain matrix elements (mainly H and 0).The current density applied during analysis plays an important role but the dose deposited is even more crucial. The danger or losses is more severe in microPIXE than in macro-PIXE. More information on this subject can be found in a tutorial paper by Maenhaut (1990). In routine PIXE analysis it is possible that the accuracy and precision can drop to the order of 90-95%.

9.2.4 Instrumentation PlXE in vacuum Fig. 9-2 shows the design and principal components of a hypothetical, ‘typical’ setup for PIXE analysis in vacuum combined with other IBA methods. In the following, the details of such a system are outlined. A typical experimental facility for PIXE analysis employs a system for producing an ion beam which irradiates the specimen homogeneously. This is required for quantitative analysis of heterogeneous samples and can be achieved, for instance, by focusing the beam on to a scattering foil. The homogenized beam is defined by a pair of collimators, normally circular, with diameters between 1 and 10 mm. The spectral shape in PIXE can be advantageously modulated by placing an X-ray absorber between sample and detector. When measuring light elements (Z111) the X-rays are soft and only a very thin absorber can be used between the target and the detector. Charged particles back-scattered from the target can then strike the Si(Li) detector and cause problems. The X-ray detectors used for PIXE typically have an effective area from 10 to 80 mm2.The amplifiers and pulse processors in energy-dispersive X-ray spectroscopy require long time constants for optimum energy resolution. This implies that pulse pileup can become a serious problem at relatively low count rates.


When analysing thin specimens, the ions pass through and are dumped in a Faraday cup for charge integration. When the samples are thick enough to stop the ions, the beam current must be measured either on the whole irradiation chamber or through some indirect approach. As will be discussed below PIXE target chambers often also include gamma-ray detectors for measuring prompt y-rays necessary for complementary P E E (see Fig. 9-2).

1 Faradaycup

c ion beam

collimators gamma ray detector

Fig. 9-2. Schematic experimental arrangement in a ‘typical’ system for PIXE and complementary IBA methods, including data acquisition system.

External beam PIXE It is sometimes advantageous to use atmospheric pressure or moderate vacuum instead of high vacuum in ion-beam analysis. The heat conductivity is increased, the target temperature is decreased, the charge is conduced from the target, and the vacuum requirements are less, thus making it easier to design a low-cost chamber or to irradiate very large specimens. Because the accelerator requires a high vacuum, the ion beam must be extracted into the region of moderate vacuum or atmospheric pressure either through a thin exit foil or through a narrow orifice combined with differential pumping. The beam eventually causes deteriorates of the foil, and therefore its material must be carefully selected. A good choice is the commercially available foil Kapton@,which withstands high intensities and a high radiation dose before breakdown. Supporting the foil by a carbon grid and direct flow of liquid nitrogen-cooled helium gas allows the

9 PIXE and PIGE 16 1

use of high beam intensities over extended time intervals (Hyvonen-Dabek et al., 1982). The chamber gas is normally helium or nitrogen. In addition to its better cooling properties, helium produces less bremsstrahlung and causes less X-ray attenuation than nitrogen. An external beam in air is needed when large objects are to be analysed without sectioning or sub-sampling. This is very important for such objects as those of interest in archaeology and history of art, for example. The strong argon K X-ray line produced in air can be used to monitor the beam charge; this is otherwise difficult to do in nonvacuum PIXE. Micro PIXE - the nuclear microprobe In the instrument called a nuclear microprobe (Watt and Grime, 1995) PIXE analysis is the most important analytical technique. It is often called micro-PIXE. With the equipment used in regular, accelerator-based ion beam analysis, beam sizes down to typically a few tenths of a mm in diameter are easily attained. In the nuclear microprobe the ion beam is collimated and/or focused down to dimensions in the range 1-50 pm. The simplest way of producing such a microprobe is to employ a pinhole collimator. For very small collimator sizes, however, the beam intensity obtainable is much too low for practical use. In addition, a substantial fraction of the ions is scattered at the edge of the collimator, and this gives rise to a halo around the central beam. In most nuclear microprobe systems, fine collimation is therefore combined with an electrostatic or magnetic demagnification system, such as doublets, triplets, or quadruplets of quadruples. The best systems currently available reach a spatial resolution of about 0.5 pm at the specimen while maintaining an ion current that is useful for PIXE analysis (>lo0 PA). For more details refer to the chapter by Lindh.

9.2.5 Data Acquisition The data acquisition systems used in PIXE have much in common with the data acquisition systems used in energy-dispersive X-ray analysis in EPMA. The signals from the X-ray detector are converted in analogue-to-digital converters and then stored in computer (PC)-based multi-channel analysers (see Fig. 9-2). The X-ray spectra are then evaluated by sophisticated, spectrum-fitting-computer codes, normally yielding quantitative results (Maxwell et al., 1989; Ryan et al., 1990). Once a PIXE spectrum has been acquired, the first step in the quantification is the extraction of the net peak intensities for the elements of interest. By far the most common approach to spectrum analysis in PIXE is to model the spectrum by an analytical function. This function includes modified Gaussians to describe the characteristic X-ray peaks and a polynomial or exponential polynomial to represent the underlying continuum background. Because of the low continuum background in PIXE, the range of peak heights in a PIXE spectrum can be up to five to six orders of magnitude. This leads to PIXE spectra that often exhibit some details, such as escape and sum peaks,


and low-energy tailing for intense peaks. Despite the many fine details, accurate modelling of PIXE spectra is quite feasible, as was demonstrated in an inter-comparison exercise of five different PIXE spectrum analysis programs (Campbell et al., 1986).

9.3 Particle-induced gamma ray emission spectrometry 9.3.1 Basic principle In the definition of particle-induced gamma-ray emission, it is rather difficult to completely separate it from nuclear reaction analysis in general. For details, refer to the chapter by Whitlow and Hellborg. In this context, PIGE is limited to the detection of prompt gamma rays produced by ion bombardment with particles at MeV energies of a specimen homogeneous in depth, i.e., the resonant reactions are not used for depth profiling. In principle, any ion with appropriate bombarding energy can be used to induce gamma-ray emission. For the reasons discussed above, protons and alpha particles are mostly used, but a few experiments have been done using other light ions such as deuterons, tritons, 3He, N and F. Because of the interaction with the atomic nucleus, the cross sections are lower than for PIXE, and PIGE typically represents a less sensitive analytical technique than PIXE. However, the gamma-ray peaks are generally well isolated and the energy of the emitted gamma-ray photons is high enough that correction for absorption is not necessary. Hence PIGE may in some situations be more favourable than PIXE for elements such as calcium and potassium (Uzunov et al., 1995). The high penetrability of gamma rays also simplifies the experimental arrangements and the nuclear interaction facilitates also the gaining of isotopic information. The main experimental components for PIGE analysis are illustrated in Fig. 9-2, showing the combined experimental facility for various IBA techniques. External beams are now also a standard in conjunction with PIGE (Raisinen, 1989). The external beams offer the same advantages as discussed above for the case of PIXE. It is rather unusual to include PIGE as an option for nuclear microprobes. However, occasionally PIGE has been applied with nuclear reactions of large cross sections, for instance. those of fluorine.

9.3.2 Thick target gamma-ray yields More or less systematic measurements of gamma-ray yield vs bombarding energy in proton or alpha bombardment have been made at several laboratories. The main source of error in determining these absolute yields is the stopping power. The uncertainty can be as much as 20% in the case of compounds. On the other hand, these absolute yields are normally not used for high accuracy analysis since comparison with proper standards with a similar composition to that of the sample is generally possible. The gamma-ray peaks from certain reactions are significantly broader than the resolution of a semiconductor detector. This broadening is due mainly to the short lifetimes

9 PIXE and PIGE 163

of de-excitation states and the high recoil of the light nucleus (Doppler effect) or due to the large resonance width. The broadening of peaks is useful in identifying the origin of the gamma rays, especially for elements which have only one peak.

9.3.3 Quantification Homogeneous concentrations of elements in thick samples, detected by prompt gamma-ray emission, can be obtained by comparison with standards. Many multicomponent, elemental standards exist for the analysis of geological, biological and medical samples. The concentration of the element (isotope) C, can be calculated from the gamma-ray yield for a certain gamma-ray peak in sample Y, and the yield from a standard YSt using this simple formula

where the stopping powers Si (Ziegler, 1992) have to be calculated at the energy E1,2, where the thick target yield has fallen to half of its value at the bombarding energy Eb. If Eb is used instead of E1/2, the error is less than a few percent. If the excitation curve vs bombarding energy is known, a more accurate calculation can be made using a simple computer program, although this is not generally necessary. Even if the relative method described above is used for quantification of PIGE measurements the same precautions as described above for PIXE are crucial. The losses of light elements occurring during intense and large dose deposition, in particular in organic matrices, has to be accounted for.

9.3.4 Sensitivity Examples of sensitivities obtainable in PIGE using protons and alpha particles are given in Table 9-1 a and b. These values should be taken as order of magnitude values, because the actual sensitivities are better than those given in the tables. This is true especially when experimental arrangements and the bombarding energy are optimized for the detection of a specific element. On the other hand, sensitivities can be much worse if the sample contains major amounts of other elements with high gamma-ray yield or if the sample can withstand only low beam currents. Therefore, possible interference from both radioactive and prompt background, as well as from other sample components, should be investigated. The sensitivity of protons and helium ions for elements of medium mass number is somewhat better with higher bombarding energies. This fact, and problems with spectral interference in PIXE measurements, make the PIGE method preferable for the analysis of samples with several components typical of steel. At higher bombarding


energies, 4He+ions can be used also for the detection of heavier elements, with detection limits down to 0.01-0.1% (Giles and Peisach, 1979). Table 9-la. Detection limits (atomic fraction) for PICE using protons (E,I% Pd, Sm. Gd, Hf, W, Au, Pb

0.1 - I % < 0.1 Yo S, K, Sc, Ti, Co, Cu, Ge, Y, Zr, Mo, Li, Be, B, C, N,Na, Mg, Al, Si, C1, Ru, Ag, Sn, I, Ta, Pt Ca, V, Mn, Fe, Ni, Zn, Nb, Cd, In,

Table 9-lb. Detection limits (atomic fraction) for PIGE using alpha particles ( 5 MeV) (Gilles and Peisach, 1979). > 1 Yo 0.1 - 1 % -

[Li]

= 41 f. 2 ppm

K

0

50

CONCENTRATION (pq/g)

100

-

Fig. 9-4. PIGE spectrum from Li analysis of a NIST reference material (top) and normalised yield vs concentration using the standard addition technique (Wong and Robertson, 1993).

A high quality data base of major, minor and trace elements was obtained for more than 1000 mineral pellets, demonstrating the feasibility of producing large batches of samples using these IBA methods. In this study, PIXE spectra were acquired with a specially designed multi-layer X-ray absorber, able to suppress the very strong Fe K-Xray signal without eliminating the X-rays from light elements, for instance, silicon. Recently, the combination of PIXE and PIGE was used in a study of coal and coalderived products using 2.5 MeV protons in an external helium atmosphere (Wong and Robertson, 1993). It was shown that PIXE and PIGE provided rapid, multi-elemental analyses and the authors concluded that the main advantage was the truly instrumental characteristics allowing them to be applied to complex matrices with accurate results. In Fig. 9-4, a PIGE spectrum (a) and the results from using the standard addition technique in combination with PIGE to measure lithium in samples prepared from reference materials of coal fly ash (b) are shown. Tuurnala et al. (1986) applied an external ‘milliprobe’ using a combination of PIGE and PIXE to check the authenticity of oil paintings. A relatively high beam energy of 4 MeV was used to raise the penetration depth and the excitation probability, and Na,


Mg and A1 were measured by PIGE. The advantages of this non-destructive analytical combination were obvious.

References Anttila A,, Hanninen R., RBisanen J . (1981), J. Radioanal. Chem., 62,441. Bosch F., El Goresey A., IHerth W., Martin B., Nobiling R., Povh B., Reiss H.D., Traxel K. (1980), Nucl. Sci. Appl. 1,33. Brandt W., LapickiG. (1981), Phys. Rev. A,23, 1717. Campbell J.L, Cookson J.A. (1984), Nucl. Instrum. Methods B3, 185. Campbell J.L, Maenhaut W., Bombelka E., Clayton E., Malmqvist K., Maxwell J.A., Pallon J., Vandenhaute J. (1986), Nucl. Instrum. Methods 814,204. Campbell J.L. (1988), Nucl. Instrum. Methods 831, 518. Carlsson L.-E., Akselssson K.R. (1981), Nucl. Insmun. Methods 181,204. Giles I., Peisach M. (1979), J. Radioanal. Chem., 50, 307. Hyvonen-Dabek M., Rrisiinen J., Dabek T. (1982), J. Radioanal. Chem., 63, 163. lshii K., Morita S . (1990), Int. J . PIXE, 1, 1. Johansson, G.I., Pallon, J., Malmqvist, K.G., Akselsson, K.R., (1981), Nucl. Instrum. Methods 181,81. Johansson, S.A.E., Campbell, J.L., (1988) PIXE: A novel Technique for Elemental Analysis, J. Wiley, New York,. Johansson, S.A.E., Campbell, J.L., Malmqvist, K.G., (1995) Particle-Induced X-ray Emission Spectrometry (PIXE), J. Wiley, New York. Lappalainen, R., Anttila, A., RSiisBnen,J., (1983). Nucl. Instrum. Methods, 212, 441. Maenhaut, W. (1990), Scanning Microscopy, 4, 43. Maenhaut, W., Vandenhaute, J., Duflou, H., (1987), Fresenius Z. Anal. Chem., 326, 736. Mangelson, N., Hill, M., (1990), Scanning Microscopy, 4,63. Maxwell, J.A., Campbell, J.L., Teesdale, W.J., (1989) Nucl. Instrum. Methods B43, 218. Ryan, C.G. , Cousens, D.R., Sie, S.H., Griffin, W.L., Suter, G.F., Clayton, E., (1990) Nucl. Instrum. Methods B47, S 5 . Raisanen, J . , (1989), Nucl. Instrum. Methods B40/41,638. KlisBnen, J., Hainninen, R. (1983), Nucl. Instrum. Methods 205,259. Tuurnala, T., Hautojarvi, A, Harva, K, (1986) Nucl. Instrum. Methods B14, 70. Uzunov, N.M., Penev, I.P., Artinian, A.M., (1995), Nucl. Instrum. Methods B95,276. Watt, F., Grime, G., (1995), chapter in: eds. Johansson, S.A.E., Campbell, J.L., Malmqvist, K.G., J. Wiley, New York, 101. Volfinger, M., Robert, J.-L.,(1994), J. Radioanal. Nucl. Chem. 185,273. Wong, AS., Robertson, J.D. (1993), Proc. Annu. Int. Pittsburgh Coal Conf. 10, 1122 Ziegler, J.F. (1 992), TRIM - the transport of ions in matter, IBM-Research, New York.



10 Charged-particle activation analysis K. Strijckmans

10.1 Principle A simple picture of CP activation will be considered, in order to explain the differ-

ent possibilities of Charged Particle Activation Analysis (CPAA) for surface characterization and to obtain semiquantitative formulae. It makes use of the following assumptions. Monoenergetic CPs penetrating matter lose their kinetic energy and are all stopped at a depth called the range R (items in bold are explained in Table 10-1). A particular nuclear reaction only occurs if the CP energy exceeds both the threshold energy Et and the Coulomb barrier Ec of the nuclear reaction. Consequently the activated depth in the material, equals the range for the incident energy E, minus the range for the threshold energy Et. The nuclear reaction cross-section, is a constant for a CP energy above the maximum of threshold energy Et and Coulomb barrier Ec and zero below it, this assumption is somewhat approximate. Table 10-1. CPAA terminology Stopping power S: energy loss per unit of thickness for a CP in a target, commonly expressed in MeV/(g/cm2). It can be calculated for protons, deuterons, helium-3 and helium-4 particles in any pure element target (Ziegler, 1977; Ziegler et al., 1985); for mixtures or compounds, the mass fraction is used as a weighing factor. It is a function of the CP energy. Range R: depth at which a monoenergetic CP beam will be stopped in a target, commonly expressed in g cm'2, R = S'dE Threshold energy Et: minimum CP energy required for an exo-ergic (Q 60 are found in this list. For those elements a higher proton energy (1 5 to 20 MeV) can be used, but then possible matrix activation and/or interferences should be considered for that particular matrix. The depth analysed then goes up to 1 mm. For two industrial products the possible application of CPAA as a surface characterization method will be discussed. Plastic foils are made light and air-tight by cover-

10 Charged-particle activation analysis

175

ing it with a very thin (nm) layer of A1 or (in the food industry) A1203. Using the nuclear reactions on A1 and/or 0 (Table 10-3) one could determine the thickness of this thin layer in an independent and absolute way. Other techniques (ESCA, AES) make use of relative measurements or require prior dissolution of the surface layer, followed by ICP-AES. Another high-tech product is a superconducting YBCO (YBa2Cu306+J layer on a flexible support (e.g. PET). Using the nuclear reactions on Y, Ba, Cu and/or 0 (Table 10-3) one could not only determine the absolute thickness, but also determine the 0 content. The latter parameter strongly influences the superconductivity properties of the material. Table 10-3. Nuclear reactions for A120J and YBCO (YBazCu306+,)layers. Reaction

Threshold energy (MeV) 27AKd,p)28A1 Q 0. I ~ o ( ~ , ~ ) ~ ~ 5.5 N P O ‘60(3He,p)’8F 8 9( ~~ J I ) * ~ ” z ~ P O ”Y (p,n)”~r P O O ’38Ba(p,a)L35mC~ P 63 ~ u ( p , n ) ~ ~ ~ n Q>O

Coulomb barrier (MeV)

Isotopic abundance (%)

Half-life

Gamma energy (keV)

3.3 2.5 4.9 7.4 7.4 9.4 6.0

100 99.8 99.8 100 100 72 69

2.24 min 9.97 min 109.8 rnin 4.18 rnin 78.4 h 53 min 38.5 rnin

1779 511 51 1 588 909 787 670

References Barrandon J.N., Benaben P., Debrun J.L. (1976), Anal. Chim. Acta, 83, 157. Blondiaux G., Debrun J.L., Maggiore C.J. (1995), Charged Particle Activation Analysis in: Handbook of Modern Ion Beam Materials Analysis, Tesmer J.R., Nastasi M., Barbour J.C., Maggiore C.J., Mayer J. W. (Ed.). Pittsburgh: Materials Research Society. Debrun J.L., Barrandon J.N., Benaben P. (1976), Anal. Chem., 48, 167. Erdtmann G., Soyka W. (l979), The Gamma Rays of the Radionuclides. Weinheim: Verlag Chemie. Keller K.A., Lange J., Miinzel H., Pfennig G. (1973-74), Q-Values and Excitation Functions of Nuclear Reactions, Parts a, b and c, Schopper, H. (Ed.). Berlin: Springer-Verlag. Seelmann-Eggebert W., Pfennig G., Miinzel H., Klewe-Nebenius H. (1981), Chart of Nuclides, 5th Edition 1981. Karlsruhe: Kernforschungszentrum Karlsruhe. Strijckmans K. (l994), Chemical Analysis by Nuclear Methods, Alfassi Z.B. (Ed.). Chichester: J. Wiley, pp. 215-252. Strijckmans K. (1995), Encyclopedia of Analytical Science, Townshend A., Worsfold P.J., Haswell S.J., Macrae R., Werner H.W., Wilson I.D. (Eds.). London: Academic Press. Vandecasteele C. (1 988), Activation Analysis with Charged Particles, Chalmers, R.A. (Ed.).Chichester: Ellis Honvood. Walker F.W., Parrington J.R., Feiner F. (1989), Nuclides and Isotopes, 14th Edition. San Jose CA: General Electric Company. Ziegler J.F. (1977), The Stopping and Ranges of Ions in Matter. Vol. 3 and 4. New York: Pergamon Press. Ziegler J.F., Biersack J.P., Littmark U. (1985), The Stopping and Range of Ions in Solids, Vol. 1,New York: Pergamon Press.



11 Atom-probe field-ion microscopy H-0. Andren

11.1 Introduction Atom-probe field-ion microscopy (APFIM) has the highest spatial resolution of all methods of microanalysis. A lateral resolution of 1 nm and a depth resolution of 0.2 nm can be achieved simultaneously. All elements can be analysed quantitatively, even the lightest. The sensitivity can be as good as 0.005 at.%. In addition, the specimen surface can be imaged by atomic or near-atomic resolution. These impressive characteristics of the method are of course coupled to a number of limitations: a special specimen shape is required, the specimen must possess a certain degree of electrical conductivity and, perhaps the most severe limitation, the specimen must have a certain mechanical strength. The Atomprobe instrument was invented by Prof. Erwin Muller at Pennsylvania State University in 1968. Now some 30 instruments are in operation world-wide, mostly at universities but also in research institutes and industry. Research institutes equipped with APFIM instruments are, amongst others: Fritz-Haber-Institut der MaxPlanck-Gesellschaft and Hahn-Meitner-Institut, both in Berlin, Germany; Max-PlanckInstitut fur Eisenforschung in Dusseldorf, Germany; Oak Ridge National Laboratory, Tennessee, USA; Sandia National Laboratories in Albuquerque, New Mexico, USA; I.P. Bardin Central Research Institute for Iron and Steel Industry, and Institute of Theoretical and Experimental Physics, both in Moscow, Russia; and Institute of Metal Research in Shenyang, People’s Republic of China. APFIM instruments are also installed in the R&D laboratories of two Japanese companies: Hitachi, Ltd. and Nippon Steel Corporation. Commercial instruments have now been available for many years (from Vacuum Generators, Ltd. and Applied Microscopy, both in the UK). However, the instrument is still very specialized, expensive, and its use is time-consuming. Therefore it sliould be used only when other, more conventional methods fail. It may then yield unique structural and chemical information on a scale approaching atomic dimensions.

11.2 The APFIM method 11.2.1 Principle of operation of APFIM Specimens for APFIM must have the shape of a sharp needle, with a tip radius of less than about 50 nm (Fig. 11-1).

1 1 Atom-probe field-ion microscopy

177

Fig. 11-1. A needle-shaped APFIM specimen of a stainless steel (a) as polished and ( b H f ) after applying short electropolishing pulses in order to bring a grain boundary near to the tip. The pulse length used in each polishing step is given on the micrographs. 100 kV dark field TEM micrographs with the outer grain in contrast. (Reprinted from Karlsson and Nordkn (1988), with kind permission of Elsevier Science Ltd., UK).

+ " ' L_Frr_ li

ION

REFRIGERATOR

DETECTOR /

HT

TRAJECTORIES

I

I0 N

[T I 1

PULSE GENERATOR

trig

Fig. 11-2. Schematic diagram of an APFIM instrument.

start

stop

c

I

I78 Part 2: Elemental composition

The specimen is mounted in a vacuum system and a very high electric field is created at the surface of the specimen tip by applying a high positive voltage to the specimen (Fig. 1 1-2). A field strength of some 2 0 4 0 GV m-' ( 2 4 V A-') is created by a voltage of between approximately 5 and 20 kV, depending on specimen geometry. At such high fields the process offield evaporation may occur: surface atoms are removed from the specimen surface by the action of the field and at the same time become positively ionized. The ions are immediately repelled in a radial direction away from the tip surface and hit an earthed screen some 5 cm from the specimen, see Fig. 11-2. Field evaporation is a thermally activated process, therefore a higher voltage is needed to give field evaporation at a lower temperature. The mass of the surface atoms removed by field evaporation is identified by timeof-flight spectrometry. A hole in the screen then serves as analysing aperture. Field evaporation of some surface atoms is induced by applying a short (1 0-ns) high-voltage (few kV) pulse to the specimen. The flight times of those ions that pass through the aperture and arrive at an ion detector some 1 to 2 m away are measured electronically. The mass-to-charge ratio of an ion is given by: mln = 2eVt2/dz

(11-1)

In this equation m is the mass of the nS ion, e is the electron charge, V is the specimen voltage during evaporation, t is the flight time, and d is the flight distance. The time t is in the range 1 to 5 ps, and the ion charge n is in the range 1 to 5, depending on atomic species and field strength. Pulsing is then repeated and more surface atoms are field evaporated and identified. In this way atomic layer after atomic layer is removed from the surface of the specimen tip surface, and all atoms that pass through the aperture are analysed, down to a depth of perhaps 10 or 50 nm. All ions that pass the analysing aperture will arrive at the detector. However, it is important that field evaporation occurs only during high-voltage pulses, so that all atoms can be identified by the time-of-flight measurement. To ensure that no evaporation occurs between pulses, the specimen is cooled to a cryogenic temperature, and a sufficiently large pulse amplitude is used, between 15 and 25% of the standing voltage between the pulses. The electric field in the region of the tip is so high during the high voltage pulse that all ions released by field evaporation are ionized. Therefore, atoms of all elements can be detected, from hydrogen to uranium. Quantification is in principle very simple, since no yield factors need to be applied provided that the detection efficiency of the ion detector does not vary with ion mass. Field-evaporated ions are projected in an approximately radial direction from the surface of the hemispherical tip on to the screen. The magnification on the screen is then to a first approximation the tip-to-screen distance divided by the tip radius, e.g., 5 cm divided by 50 nm or 1 million times! The analysing aperture in the screen (the 'probe hole') selects atoms for analysis. A hole diameter of 2 mm then corresponds to a surface area of only 2 nm in diameter. The image magnification can be adjusted simply by varying the tip-to-screen distance; in our example a distance of 10 cm would give an analysed area of 1 nm diameter. Note that there are no lenses and therefore the method

11 Atom-probe field-ion microscopy

(0

Z 0 H

LL

0

lY W

m

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3

t

n

Ti0

1

z

ia

zm

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179

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aa

ea

70.

ea.

m/q

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lam

Fig. 11-3. APFIM spectrum of the outermost part of a ‘Tic’ coating deposited on a cemented carbide specimen by chemical vapour deposition (CVD). Note the occurrence of Ti-0 molecular ions. This part of the coating was found to be an oxide-containing carbon; the composition derived from this analysis was Ti(CozOo&. (Reprinted from Henjered et al. (1981), with kind permission of Elsevier Science Ltd, UK).

z

0c

80.

Ions

1

Oxide layer formed in air 28 days

750

loo0

us0

1500

1750

Fig. 11-4. APFIM composition profile through a thin oxide layer formed by exposure to air, on a clean (field evaporated) specimen of zirconium alloy. The thickness of the oxide film was a few nm. (Courtesy of Boel Wadman).

is not sensitive to vibration; if the specimen vibrates with an amplitude of 20 pm the image also vibrates *20 pm, thus the position of the analysed area will vary by only hO.01 nm! During an analysis usually several thousand atoms are analysed, stemming from a cylinder-shaped volume with a diameter of a few nm and several nm long. Data


can be presented in the form of a mass spectrum, i.e., number of atoms as a function of m/n, see Fig. 1 1-3. Fig. 11-4 shows an example of such a depth profile. For very fine scale variations in chemical composition, grouping of data may not be suitable. A cumulative plot is then used, in which the cumulative number of ions of a certain kind is plotted versus the total number of ions detected, see Fig. 11-5. 100

0.05 nm

I

I

15

+::

50

r”

25

0

0

25

50

12s Total number of atoms

15

150

iis

260

Fig. 11-5. Cumulative plot of an APFIM analysis of a silver layer on a GaAs specimen, prepared by physical vapour deposition (PVD). The first =60 atoms analysed were all Ag (a line 45” to the axes on the diagram) and then Ga, As, and 0, but no Ag was registered (the curve for Ag is horizontal). This indicates an abrupt Ag/GaAs interface. (From Hu et al., 1991).

11.2.2 Principle of operation of FIM If a gas at low pressure is introduced into the APFIM vacuum system, gas atoms close to the surface of the specimen tip will ionize, i.e., lose an electron to the specimen. Just as in the case of field-evaporated ions,.the positive gas ions will be repelled radially out from the hemispherical tip and strike the fluorescent screen. The ‘image gases’ mostly used are the inert gases Ne and He. The ionization process is a quantummechanical tunnelling effect, field ionization, and the ionization rate is very sensitive to the local geometry of the tip surface. Thus, the ionization rate for atoms that protrude from the surface is much higher than between protruding atoms. Each surface atom therefore gives rise to an ion beam, which, when it strikes the fluorescent screen, produces an image spot. In this way an image with a high magnification is created - field ion microscopy (FIM) - with a resolution sufficient to resolve edges of atomic planes and even single atoms in not too closely packed planes (Fig. 11-6). Resolution is best for pure metals, in particular refractory metals such as W, Pt and Ir. In the FIM image of these metals, crystal defects such as grain boundaries, dislocations and even single vacancies can be seen with good resolution. Image quality decreases with increasing alloying and highly


18 1

Fig. 11-6. FIM micrograph of pure tungsten, containing a grain boundary (arrows). The (3 11) planes (A) are imaged at atomic resolution, but not the more closely packed (1 10) planes. Helium is the image gas, specimen voltage is 7.6 kV, and specimen temperature, 77 K. (Reprinted from Andr6n and Norden (1979), with kind permission of Munksgaard Int. Publishers Ltd., Denmark).

Fig. 11-7. FIM micrograph of a steel specimen containing small precipitates of a second phase of the type (Ti, Cr)C,.,. The ‘probe hole’ (analysing aperture) of the APFIM instrument is seen at the centre; the diameter of the analysed area is in this case 5 nm. To analyse a precipitate the specimen is tilted so that its image falls on the probe hole. The image gas is neon, specimen voltage 7.7 kV, and specimen temperature 92 K. (Reprinted from Andr6n et al. (1980),with kind permission of Chapman & Hall Ltd, UK).


alloyed materials often appear ‘amorphous’ in the FIM image. It is usually still possible to obtain contrast from different phases and interfaces (Fig. 11-7), so that FIM imaging is useful for selecting an area for analysis.

11.2.3 Working range The very high electrical field strength at the specimen tip induces a high mechanical stress on the surface, often of the same magnitude as, or even higher than, the tensile strength of the specimen material. Fortunately, only a small volume of the specimen is exposed to very high stresses, and if this volume does not contain features that initiate fractures such as surface irregularities or weak interfaces, the specimen may still survive the stress. Obviously, the field required for field evaporation F, must be lower than the field that causes specimen fracture Ff. Furthermore, the field required for field ionization F, must be lower than F, if FIM imaging, and thus selection of the area to be analysed, is to be possible. A condition for APFIM analysis can therefore be formulated:

F, < F, < Ff

(1 1-2)

The ionization field, F,, is in principle independent of temperature but may be changed by selecting a different image gas. The field required for field evaporation, F,, is strongly temperature-dependent and increases with decreasing temperature. The field Ft that causes fracture is usually higher at low temperatures, and the increase is dependent on the material and fracture mechanism. Obviously, by selecting a suitable image gas and choosing a low specimen temperature the criterion F, < F, can often be fulfilled. However, the higher Fe at a low temperature may cause fracture in the specimen, if the condition F, < Ft is not fulfilled. The temperature range for which eq. 2 is fulfilled is called the working range of APFIM. This means that it is difficult to analyse some materials of low mechanical strength, such as lead or tin, with this instrument. However, most materials are readily analysed, but it is important to prepare specimens with a good surface finish to avoid early specimen fracture and thus premature termination of an APFIM analysis. The risk of specimen fracture is probably the most severe limitation of APFIM analysis.

11.2.4 Instrumentation In order to minimize interaction between the specimen surface and the surrounding atmosphere, APFIM instruments are built to ultra high vacuum (UHV) standards, and specimens are introduced into the UHV chamber through an airlock system. Specimen cooling to cryogenic temperatures is necessary to make atom probe analysis possible (Section 11.2.3) and results quantitative (Section 112.1). Cooling is usually brought about by connecting the specimen mount to a refrigerator by a flexible copper braid. The refrigerator is often of the closed-cycle, helium-gas type and is equipped with a control system that maintains a constant temperature in the range 25 to 100 K. The

1 I Atom-probe field-ion microscopy

183

specimen mount is held by a manipulator or goniometer that permits tilting of the specimen around its tip in two perpendicular directions, as well as alignment on to the ion optical axis of the instrument. The short high voltage pulses that are applied to the specimen to induce field evaporation and start the time-of-flight measurement, are usually produced by discharging a capacitor. A mercury-wetted reed relay is used as a switch, and a rise time of 0.5 ns is easily achieved. The pulse length is about 10 ns. Pulses of a few kV amplitude can be produced at a repetition rate of 200 Hz. The ‘image screen’ actually consists of a channel-plate assembly: a microchannel-plate with typically a 25-pm channel diameter converts each incoming ion to a shower of electrons that are proximity-focused on to a phosphorescent screen. The image on the screen is readily photographed with a standard camera and 35 mm film using an exposure time of typically half a second. The analysing aperture is situated at the centre of the channel-plate (and phosphorescent screen) and the whole channel-plate assembly is moveable. In this way the tip-toscreen distance, and thereby the image magnification on the screen, can be selected. The ion detector usually consists of a double channel-plate assembly, which yields a pulse of sufficient amplitude (>lo mV) for each incoming ion. The detection efficiency is approximately equal to the open area of the channel-plate, 60%, and the height distribution of output pulses is only weakly dependent on ion energy. Flight times are measured electronically, often by commercial time-to-digital converters with several channels to allow multiple stops to be registered. This is important when several ions are produced in a single, high-voltage pulse. The time resolution of these instruments is of the order of 1 ns or less. A computer then reads flight times and specimen voltages and calculates m/n values, which are usually presented on-line in the form of a spectrum and can be stored for later evaluation.

11.2.5 Specimen preparation The unusual shape of specimens for APFIM analysis-a needle with a tip radius of 0.05 pm or less-means that special preparation techniques have been developed and that the success of these techniques is crucial to obtaining any information at all from the atom probe instrument. The first step in the specimen preparation is usually to mechanically cut a small rod from a sample of the material, using conventional techniques such as low-speed diamond-sawing or spark-cutting. The rod should have a quadratic cross section of 0.5 x 0.5 mm2 or less and is typically 10 mm long. A sharp tip must then be formed, most commonly by electropolishing, usually employing the ‘floating layer’ technique. The rod is immersed in an electrolyte floating on an inert liquid, and electropolishing then produces a neck on the rod. The neck is then polished off, often in a separate electrolyte at a lower polishing rate, and polishing is stopped when the bottom part of the specimen falls off. Alternative methods that have been used for difficult materials are chemical-etching and ion etching. The depth accessible to APFIM analysis is a few 10 nm, which means that if the interesting features of the microstructure are at a much larger distance than this, con-


trolled preparation must be employed. This is necessary in the study of, for instance, grain boundaries and large precipitates. The polished specimen is then first inspected in the transmission electron microscope and the distance to the nearest feature of interest is determined. The specimen is then exposed to a short electropolishing pulse that removes the outermost part of the specimen. The process is repeated until the feature is positioned close to the surface of the tip, see Fig. 1 1-1.

11.3 Surface studies with APFIM Many studies related to the surface of materials have been made with APFIM. Some of them deal with the properties of the surface itself: surface reconstruction, surface diffusion and surface segregation. In this chapter some examples of such investigations are presented (Sections 11.3.1-11.3.3). In other studies the APFIM specimen has been exposed to a surface change of some kind: oxidation, ion implantation, or radiation damage; surface coating by anodic passivation, electrolytic deposition, RF (Radio Frequency) sputtering, PVD (Physical Vapour Deposition), or CVD (Chemical Vapour Deposition). A few examples of studies of this kind are given in Sections 11.3.411.3.6. The unusual specimen shape, with a small tip radius, should always be taken into account when interpreting results of APFIM studies; for instance, CVD on to a APFIM specimen may give a very different result from deposition onto a flat substrate. On the other hand, the presence of many crystallographic planes in the same specimen may in some cases be an advantage. If the very high electric field used for imaging and APFIM analysis is allowed to remain during the surface treatment, it would in many cases greatly affect the outcome. Fortunately, it is in most cases possible to do the surface treatment under field-free conditions and thus eliminate this complication. Surface analysis with APFIM usually involves a specimen taken from the bulk, which is used as a substrate for some surface treatment. In some cases it has been possible to prepare APFIM specimens directly from the region close to the surface of a sample, such as in a study of epitaxial layers in 111-V semiconductors (Liddle et al., 1988). Development of methods to study coatings and the near-surface region of cemented carbide samples is at present being undertaken at the author’s laboratory.

11.3.1 Surface reconstruction The resolution of FIM is sufficiently good for direct resolution of the atomic positions of loosely packed atomic planes, whereas only edge atoms of the more closely packed planes image well. However, by controlled-field evaporation, it is possible to remove a single close-packed atomic layer slowly and note the positions of edge atoms of the shrinking plane. In this way the positions of all atoms in the plane can be determined. This means that any surface reconstruction that occurs, for instance because of heat treatment or gas adsorption, can be directly imaged with FIM. The treatment is


185

done under field-free conditions, and it must of course be checked that the application of a high electric field does not itself change the structure of the plane. As an example, Voss et al. (1993) studied the oxygen-induced reconstruction of Rh{ 1 10) and { 113) planes. A needle-shaped specimen of Rh was first cleaned using field evaporation, Ne sputtering and thermal annealing. FIM imaging then showed that the planes to be studied had the expected, unreconstructed structure. The specimen was then heated to a temperature of between 400 and 500 K and exposed for 30 s to oxygen gas at a pressure of 1 O4 Pa, corresponding to a dose of about 20 Langmuir (1 Langmuir = 1.3~10-~ Pas). FIM imaging then showed only layer-edge atoms of the (1 10) and (1 13) surfaces, and by controlled field evaporation the position of the surface atoms on these two planes could be determined. It was found that the planes had reconstructed to a ‘missing row’ or (nx2) and (nx3) facet structure, with n = 1,2.

11.3.2 Surface diffusion The resolution of FIM is sufficient to resolve the atomic positions of several atomic planes in the image of many pure elements. Surface diffusion can therefore be studied by direct observation of single atom movement on the surface; self diffusion as well as the diffusion of other atomic species can be studied provided contrast can be obtained in the FIM image. Both diffusion constants and the mechanisms of surface diffusion can be determined in the following way: an atom is first evaporated on to the plane to be investigated and its position is determined by FIM imaging at a low temperature (cryogenic cooling). The electric field is then removed, and the specimen temperature is raised to the temperature to be studied for a pre-determined time. The specimen is then cooled again and imaged by applying the electric field, and another FIM micrograph is taken. This process is repeated many times, and the displacement of the atom during each heating period is measured. The mean of the square of the measured displacements is a direct measure of the diffusion constant, and its variation with temperature gives the activation energy for surface diffusion and the diffusivity. Crystallographic anisotropy can also be measured, as well as the atomic mechanism involved (ordinary hopping or exchange displacement). By evaporating more than one atom on to a single atomic plane, the surface diffusion behaviour of dimers, trimers, etc., can also be studied. As an example, Kellogg (1 993) determined the activation energy for Pt surface diffusion on,Rh (100)to be 0.92eV (Fig. 11-S), and the mechanism was found to be ordinary single atom hopping. The same activation energy was found for Pt dimers, whereas tri-, tetra-, and pentamers had a somewhat higher activation energy, 1.03 eV.

11.3.3 Surface segregation The ability of APFIM to analyse a specimen atomic layer by atomic layer makes it suitable for the study of surface segregation in alloys. In such studies the specimen is first field-evaporated at a low temperature to create a regular and atomically clean sur


0 Q,

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2A 4-

v c -

-39.0 -40.0

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-41 .O

-42.0

2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20 1rr ( K ' )

Fig, 11-8. An Arrhenius plot of the mean-square displacements of Pt adatoms on Rh(100), measured with FIM. The slope and intercept yield the diffusion parameters stated on the figure. (From Kellogg, 1993).

80 -

bB c.

70-

\

60-

P t - 2 0 w t . % [ 32.1 a t . % ] Rh

(100)

Q

.-o 2 c C

c C

Q)

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c

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5040-

30-

o

0

n 0

O

.

O

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Fig. 11-9. APFIM of composition depth profiles of the (100) plane of Pt20wt% Rh alloys. The outermost atomic layer is Rh-enriched after heat treatment at 700 'C for 1 min. (From Sano and Sakurai, 1989).


187

face. The specimen is then heated to the temperature of interest for a certain time, and cooled to the low temperature again. APFIM analysis of the selected atomic plane is then performed. As large an aperture as possible is used to improve counting statistics; 50 to 100 atoms per atomic plane are usually analysed. As an example, Sano and Sakurai (1989) analysed surface segregation of Pt-Rh alloys, important as catalyst materials. Fig. 11-9 shows their results from Pt-20 wt% Rh. The outermost atomic layer is enriched in Rh after heat treatment at 700 "C but not after heating at 600 "C. This result was interpreted by considering the temperature variation of the entropy and enthalpy terms in the expression for the free energy of surface segregation in the Pt-Rh system.

11.3.4 Oxide layers When APFIM is used to study oxidation processes, atomically clean surfaces are first prepared by normal field evaporation in the APFIM instrument. The specimen is then exposed to the oxidizing environment, either outside the vacuum vessel or in a separate preparation chamber attached to the APFIM instrument. In a study of the early stages of oxidation of zirconium alloys, Wadman and Andren (1991) exposed field-evaporated APFIM specimens to dry air or boiling water for different periods of time. After exposure the specimen was reinserted in the APFIM and analysed. Fig. 11-4 shows an APFIM concentration profile through a thin (a few nm) oxide layer, formed during exposure in dry air for 28 days. The equilibrium oxide on zirconium is zirconia, Zr02, but the oxygen content found in Fig. 1 1-4 and in all analyses of oxide layers on zirconium alloys was much lower than 67 at.%. It is believed that this sub-stoichiometric composition is a characteristic feature of the first formed, thin, black, and adherent oxide on zirconium alloys.

11.3.5 PVD coatings Surface layers prepared by various forms of physical-vapour deposition (PVD) may also be studied by APFIM, provided that the microstructure of a thin layer deposited on to a needle-shaped specimen resembles the microstructure of a layer on a real component. Cleaning of the specimen surface is usually done by field evaporation, and the PVD layer may either be deposited in situ in the APFIM chamber or in an attached preparation chamber. Hu ef a/. (1991) studied the interface between layers of silver and gold on GaAs specimen tips. The surface of the tip was first cleaned by field evaporation in a FIM instrument, and metal was deposited in the same vacuum chamber by thermal evaporation. The thickness of the deposited layer was monitored with a quartz crystal microbalance. In the case of Ag/GaAs the interface was found to be atomically abrupt (Fig. 11 - 9 , whereas the AdGaAs interface exhibited considerable intermixing; the outermost 0.1-0.6 nm thick GaAs layer had a gold concentration of 10-40 at.%. However, if the specimen was exposed to air before gold deposition, intermixing was


effectively blocked and almost pure gold layers were formed, covered with small amounts of As and Ga.

11.3.6 CVD coatings Surface layers produced by chemical-vapour deposition (CVD) can be studied in much the same way as PVD coatings. However, preparation of layers in situ in the microscope chamber or in an attached preparation chamber is usually not feasible, because of the pressures and temperatures involved and the reactive nature of the gases used. Specimens are therefore first given a smooth hemispherical shape by field evaporation, than taken out of the APFIM instrument and coated in a separate apparatus, then preferably inspected by TEM and finally analysed in the APFIM instrument. In a study of CVD coatings on cemented carbides, Henjered et al. (1981) coated specimens of WC-Co with a thin layer of T i c in a laboratory-scale CVD apparatus. The specimens had a single WC grain at the tip, and a TIC layer approximately 50 nm thick was deposited on to the specimen by the reaction of methane with titanium tetrachloride at 1000 "C for 30 s. APFIM analysis showed that the outermost layer was mainly an oxide with a composition corresponding to the formula Ti(C0.200.&, see Fig. 11-3, whereas most of the coating was mainly a carbide and had a composition corresponding to Ti(C0.7400.26)0,71.Close to the substrate no oxygen was found, so there the layer had the character of a pure carbide. In the coating-WC interface some Co was found this had diffused from the substrate interior and along the WC surface during the period of specimen heating before deposition.

11.4 Conclusions The use of APFIM in the study of surfaces and surface layers can give unique information on structure and chemical properties with atomic or near-atomic spatial resolution. The major limitations of the technique are those given by the unusual shape of the specimen, a sharp needle, and the high mechanical stress in the specimen, induced by the high electric field. In many cases these limitations can be coped with, and several examples of surface-related studies have been given in Section 11.3. It should be noted that APFIM is a time-consuming method that is not likely to become a widespread standard method of surface analysis. However, when other, more conventional methods of surface analysis do not give results of sufficient detail, APFIM can in many cases yield microstructural and microchemical data of very high spatial resolution.


I89

References AndrCn H.-O., NordCn H. (1979), Scand. J. Metall., 8, 147-152. And& H.-O., Henjered A,, Norden H. (1980), J. Muter. S c i , 15,2365-2368. Henjered A., Kjellsson L., AndrCn H.-O., NordCn H. (1981), Scr. Metall., 15, 1023-1027. Hu Q.-H., Kvist A., AndrCn H.-0. (1991), in: Microscopy of Semiconductor Materials 1991: Cullis, A.G., Long, N., (Eds.). Bristol: The Institute of Physics, 1991; Inst. Phys. Conf. Ser. Nr 117, Section 2, pp.91-96. Karlsson L., Norden H. (l988), Acta Metall., 36, 13-24. Kellogg G.L. (1993), AppL. Surf: Sci., 67, 134-141. Liddle J.A., Norman A., Cerezo A., Grovenor C.R.M. (1988), J. Phys. (Paris), 49-C6, 509-514. Sano N., Sakurai T. (1989), Colloque de Physique, 50-C8,321-325. Voss C., Gaussmann A., Kruse N. (l993), Appl. Surf: Sci., 67, 142-146. Wadman B., Andren H.-0. (1991), in: Zirconium in the Nuclear Industry: Ninth International Symposium. Eucken, C.M., Garde, A.M., (Eds.). Philadelphia: American Society for Testing and Materials, 1991; ASTM STP 1132, pp. 461475.

Key references A modem, general review of the APFIM technique is: Miller M.K., Smith G.D.W. (l989), Atom Probe Microanalysis: Principles and Applications to Materials Problems. Pittsburgh: Materials Research Society.



12 Ion scattering spectroscopy E. Taglauer

12.1 Introduction The scattering of low-energy ions is a useful tool for surface analysis. ‘Low energies’ in this context refer to primary kinetic energies in the range of about 500 eV to 5 keV. The ion species used most frequently are the rare gas ions HeCand Ne’ and the alkali metal ions Li’ and Na+. In this energy range, the fairly large scattering crosssections in combination with the high neutralization probability, ensure that backscattered ions predominantly arise from interactions between the top one or two atomic layers of the surface under analysis. Therefore, the energy and angular distributions of back-scattered ions convey information about the composition and ‘structure of the outermost atomic layers of the studied specimen. It turns out that the ion-surface interaction process can be described with sufficient accuracy by one binary collision or a sequence thereof. Consequently, the concepts of data interpretation are in principle rather simple. Quantitative compositional analysis is not always that easy (mainly due to the neutralization process) and requires proper calibration. Furthermore, detailed structural analysis must frequently be accompanied by numerical model calculations. The effect of the ion beam on the bombarded surface is not generally negligible, but ‘static’ analysis (i.e. without significant perturbation of the analysed surface) is possible and on the other hand the sputtering action of the ion beam can also be used for obtaining near-surface composition depth-profiles. The properties of the method mentioned above are discussed in more detail and demonstrated by related examples in the following with emphasis on practical applications. Surface analysis by low-energy ions was first reported by D.P. Smith (Smith, 1967) and named ‘Ion-Scattering Spectroscopy’, ISS. Noble gas ions and an electrostatic energy analyser were used. Later applications include alkali metal ions and time-of-flight methods for neutral particle detection. The acronym LEIS for Low-Energy Ion Scattering has also become commonly used. For a recent review of the field see, e.g. Niehus et al. (1 993). In the following sections, firstly, some principles of the method and experimental aspects are considered, subsequently surface chemical composition analysis by ion scattering is discussed in view of successful applications such as the analysis of oxide and catalyst surfaces and the analysis of alloy surfaces with respect to segregation. Using ISS it is also possible to obtain mass-selective structural information and that is demonstrated for adsorbate structures, surface reconstruction and thin-film growth.

12 Ion scattering spectroscopy

191

12.2 Basics The principle of ISS is the binary scattering process between an incoming ion and a target surface atom. According to collision kinematics, the kinetic energy E of the projectile after the collision process is a function of the mass ratio A= M ~ / Mof I the projectile (MI) and target (Mz) atomic masses: (12-1) where Eo is the primary ion energy and 91 the experimentally given laboratory scattering angle (the positive sign holds for A 2 1, both signs for A 4 ) . An energy spectrum of the scattered ions therefore yields a peak for each atomic mass present on the surface, the peak height being a measure of its abundance.

HeNe

- Analyzer Section

-v+ !

Target

Charge Neutralization Filament

Fig. 12-1. Schematic diagram of an ISS apparatus using a cylindrical mirror analyser with an integrated ion gun (modified 3M model 5 15). ~

In a typical scattering arrangement, a beam of noble gas ions is directed on to the sample surface, the currents being of the order of lo-* to lo-’ A. Back-scattered ions can be energy-analysed by using electrostatic fields, e.g. in a spherical sector analyser or a cylindrical mirror analyser (CMA), see Fig. 12-1. The ions are usually detected by channeltron-type electron multipliers in the counting mode, count rates being generally between 1O2 and lo5s-l and nA primary current. The background pressure in the system has to meet UHV conditions, which is an inherent requirement for surface investigations with sub-monolayer sensitivity. The pressure from the ion source supply, for


example, is for many systems in the to lo-’ mbar range during operation; this apparently does not cause deterioration of the surface sensitivity. I

I

0

I

I

I

I

ISS

1

Fig. 12-2. ISS spectra (500 eV He+, scattering angle 1 3 7 O ) of an alumina film formed by oxidation in air, with an overlayer of rhodium. On the right side, the ion fluence at the beginning of each spectrum is noted. The arrows indicate the peak positions calculated by use of eq. 12-1 .(Linsmeier et al., 1992).

Fig. 12-2 shows an example of a series of energy spectra of an A1203 sample which is covered by about a monolayer of Rh (due to the roughness of the surface, the nominal amount for a monolayer does not imply complete coverage)(Linsmeier et al., 1992). Several essential features of ISS can be recognised in this example: the binary collision model is obviously applicable for calculation of the peak positions for each atomic mass on the surface. Detailed analysis shows (see, e.g., Niehus et al. (1993)) that these distinct peaks are due to the fact that the vast majority of the detected backscattered ions have undergone collisions with atoms in the topmost layer, whereas ions which penetrate further into the solid are back-scattered as neutrals and not detected in an arrangement of the kind shown in Fig. 12-1. Fig. 12-2 further demonstrates that the method can be applied to insulating material if electrons are offered for charge neu-


193

tralization, e.g. from a filament as shown in Fig. 12-1. The presence of contaminants with low atomic mass (hydrogen, hydrocarbons) has the effect that there is almost no back-scattered ion yield at the beginning of the bombardment. The sputtering action of the ion beam removes this contamination layer and can also be used for etching through the deposited Rh layer. The ion beam is therefore suitable for measuring the kind and amount of atomic species present on the surface and also due to the sputtering effect, obtaining near-surface concentration profiles.

12.3 Quantification In the scattering model mentioned so far, a linear dependence of the scattering signal on the surface density, NI, of species i could be expected and would be desirable for quantitative analysis. This has in fact been observed for various surfaces, but it cannot be generally assumed. For the scattered ion intensity ,:I the following expression can be written: do, 1: =I;KNi-xARxPj dR

(12-2)

Here 1; is the primary current, K a factor for the apparatus transmission, do/dS2 is the differential scattering cross-section and AS2 the acceptance solid angle of the detector. Pi describes the probability that an ion is back-scattered as an ion and not neutralized, its value is generally of the order of a few percent for noble gas ions scattered from surface atoms and at least one order of magnitude less for scattering from deeper layers. If a substrate is covered by an adsorbate with surface density NA,then the substrate signal :1 decreases accordingly: 1; =I;fK(Ns -aNA)(doS/dQ)xAS2xPs

(12-3)

Here, the shadowing factor a indicates the average number of substrate atoms shadowed by one adsorbate atom. A well defined example is the adsorption of CO on Ni(l00) as shown in Fig. 12-3 (Beckschulte et al., 1990). It demonstrates the linear increase of the adsorbate signal (0 from CO) with coverage, according to eq. 12-2 and the corresponding decrease of the Ni substrate signal, according to eq. 12-3. After an exposure of 5 Langmuirs, saturation is reached and a ~ ( 2 x 2 overlayer ) can be identified. It is worth noticing that in these measurements no scattering signal from C was obtained, an observation that nicely demonstrates that CO is adsorbed in an upright orientation, the oxygen pointing away from the surface. Another important observation in this context concerns the change in work function by an adsorbate. Although in these experiments the work function increased by as much as 0.9eV, no deviation from linearity between coverage and signal was observed. Therefore, it must be concluded that such a change in work-function has no strong influence on the efficiency of the main neutralization process which is Auger neutralization (Hagstrum, 1954; MacDonald and O’Connor, 1983). If, however,

194 Part 2 : Elemental composition

the work-function is lowered to an extent that another neutralization mechanism becomes effective, namely resonance neutralisation, a strong decrease of the ion signal is observed (Beckschulte and Taglauer, 1993). The reliability of quantification and the absence of matrix effects can further be demonstrated in favourable cases, e.g. for the surfaces of binary alloys. Here, it is justified to assume that the sum of the surface concentrations c,=N,/N (where N is the density of lattice sites on the surface) is unity: (12-4)

c*+cs=l

CO O N Ni (100) WORK FUNCTION CHANGE (eV)

25t

ISS: He++ Ni (100) + CO E O =l k e V 19 = 100" = 50" < 01 1> direction

I

>

h

.1.0 2 '

0.9

'

0.8

g W

3a Z

. 0.7 0 . 0.6 . 0.5

i=

$? 3

a: 3

. 0.4 0

5 t f

k-

He'frorn

Ni

A

I

I '

A

1

. 0.3 '

0.2

. 0.1 0

5

*

CO

-

10 15 EXPOSURE (L)

30 I

Fig. 12-3. He' ion-scattering intensities and work-function change for the adsorption of CO on Ni(100) at 300 K (Beckschulte et a/., 1990).


195

From the combination of eqs. 12-2 and 12-4, it follows that a linear relationship between the signals of the constituents A and B should be observed. This is in fact the case; an example has been given for CuPd alloys with varying surface concentrations (Brongersma and Leerdam, 1991). Besides the factors K and ASZdefined by the experimental set-up, eq. 12-2 contains two other factors that have to be discussed briefly. The differential scattering cross section do/dSZ can be calculated by using an appropriate scattering potential (Taglauer, 1991). The cross-sections increase with the target atomic number and decrease with increasing energy and scattering angle. Using a Thomas-Fermi-Molihre potential the cross section (in A2 steradian") for helium scattering can be approximately given by (do/dSZ) = 2 x lo7 x EOp x Z:." with Eo in eV and 91 in degrees; the exponent 1.6 for 0 to 1.2 for W.

x

9r3

(12-5)

p varies slightly with target atom, from

-v) 1

c

8

m 1

-

-

-

I

I

I

I

Target Nuclear Charge Z, Fig. 12-4. Yield (counts 5.') of He' ions scattered from various elemental surfaces, given in relation to the target current. Data taken with a cylindrical mirror analyser (scattering angle 137") at three different primary energies (Taglauer, 1982).


The corresponding experimental results given in Fig. 12-4 (Taglauer, 1982) show the same general trend but deviations from a monotonous increase with the target atomic number are obvious. These are due to the differences in the ion escape probabilities, PI, for the different elements. Pi cannot be calculated generally and therefore sensitivity factors have to be determined experimentally. From theoretical considerations (Hagstrum, 1954) of the dominant Auger neutralisation process, a velocity dependence like P cc exp (-VO/VL) has been deduced. Here, vo is a characteristic value for an ion-surface combination and v l is the ion velocity perpendicular to the surface (more exactly the velocities before and after the collision have to be considered). This relation has been confirmed by experiment in many cases (see, e.g., MacDonald and O'Connor (1 983); Beckschulte and Taglauer (1993); Mikhailov et al. (1994)). The neutralization effect provides the exclusive surface sensitivity of ISS to the outermost atomic layer, but it can also cause problems for quantification because changes of the surface or scattering conditions can produce changes of the neutralization probability. In order to deal with this problem, alkali metal ions have been used (Taglauer et al., 1980; Los and Geerlings, 1990; Overbury et al., 1991). Their ionization energy is close to the work-function of most materials, i.e. about 4-5 eV, and therefore their neutralization probability is much lower than for noble gas ions. Another approach is the detection of neutral particles in time-of-flight systems (Buck et al., 1975; Niehus, 1991). Some arrangements offer the possibility of measuring with a scattering angle of 180" (Niehus, 1991; Katayama et al., 1988), which is advantageous for structure analysis, as discussed below. Alkali metal ion-scattering and neutral particle detection circumvent the neutralization problem to some extent, but generally the specificity to the outermost layer is lost. Their energy spectra can contain large contributions from multiple scattering and numerical simulations have to be performed for data interpretation.

12.4 Oxides, catalysts A field in which ISS has been very successfully applied is the analysis of supported catalyst systems (Brongersma and Leerdam, .1991; Taglauer, 1991; Bertrand ef al., 1983). Regarding their catalytic performance, the composition of the outermost atomic layer is of decisive importance. Supported catalysts generally consist of oxide material (e.g. A1203, TiOz, SiO2) with a high surface area on which other metals or oxides are dispersed as the active components. These materials are highly insulating and therefore electron spectroscopy cannot generally be used. Ion scattering, however, is applicable, because charging effects can be overcome by flooding the sample with electrons (see Fig. 12-1). The microscopic roughness of the samples may reduce the ion intensity detected (Margraf et al., 1987), but this is not a problem for the analysis of the surface composition and the layering of the surface species as detected by gradual sputteretching (see Fig. 12-2). An illustrative example is the spreading of the oxides of group Vb, VIb and VIIb metals on the support oxides mentioned above. It has been shown by ISS experiments that during heat treatment (calcination), spreading in the sense of monolayer disper-


197

sion, can occur as a process of solid-solid wetting (Leyrer et al., 1988). A result of that kind is plotted in Fig. 12-5. The near-surface depth profile as a function of He+ ion fluence shows a flat distribution for a physical mixture of molybdena and titania (slightly increasing due to preferential sputtering). The drastic changes in the Mo/Ti intensity ratios after calcination indicate the high dispersion of the molybdenum on the titanium oxide after that treatment: a high Mo signal at the surface is observed; this decays with ion fluence to a value characteristic of sputtering of about one monolayer. These kinds of ISS study are particularly valuable if they are combined with other measurements, such as catalytic activity, laser Raman spectroscopy (Leyrer et al., 1988) or X-ray photoemission spectroscopy, XPS. He++ MoO,/TiO, a: physical mixture b: 720 K, 24 h H,O sat. 0.3

a

ni4

01 0

I

“b

I

I

I

4

Fluence

I

8

I

I

12

I

I

16

ions/cm2)

I

Fig. 12-5. Scattered He’ ion Mo/Ti intensity ratios as a function of ion fluence for a MoO31TiO2 mixture: (a) physical mixture; (b) after calcination for 24 h in H,O-saturated 0 2 (Margraf et al., 1987).

12.5 Alloys The investigation of alloy surfaces is also a field where ISS methods have been used very successfully. This is due mainly to two reasons: firstly, the compositions of the first and second atomic layers are often very different (and also different from the bulk), particularly if strong segregation occurs, and it is to these first layers that ISS is particularly sensitive. Secondly, quantification has been shown to be reliably applicable for metal surfaces, either by using elemental standards or by scattering from single crystal surfaces under various scattering conditions. Consequently, ISS has been applied to a number of studies of surface segregation and preferential sputtering of alloys (Buck, 1982; du Plessis et al., 1989; Schomann and Taglauer, 1996; Weigand et al., 1993).


The possibility of quantification with respect to alloys has already been referred to in Section 12.3. If elemental standards are used, the concentration of species A of a binary system with components A and B is given by ( 12-6)

here N,are the respective surface densities and 1;the signals from the related elemental standards. The validity of this expression has been proven in several cases. For a series of PdPt alloys it could be verified by analysing samples which were prepared by milling in UHV, a treatment that is expected to produce the bulk composition on the surface (Plessis et d., 1989). Hef 1,97 keV, 6 = 90" I

'

I

'

i

'

1

'

I

--2ooQ

-

2000

3

I500

---*--= 45" @

90 1500 2

1000

9 Q

T looo

500

C

d

500

0

0

-500

1600

1700

I800

Energy [eV]

1900

2M)O

Energy [ev]

Fig. 12-6. ISS spectra from the (001) surfaces of Au3Cu (left) and Cu3Au (right) alloy single crystals. The angle of incidence is 45", the scattering angle 90"; Q=O" corresponds to a scattering plane parallel to the [IOO] direction, W 4 5 " to scattering parallel to the [110] direction (SchBmann and Taglauer, 1996).

A good example for alloy investigations is the copper-gold system. It is a classical case of an ordering alloy with a negative enthalpy of mixing and undergoes a bulk first order phase transition, the critical temperature depending on the composition. With respect to surface segregation and its relation to the order-disorder transition, the C q A u alloy was studied using several methods, including ISS (Buck, 1982). For the (001) surface, Au segregation (59%) was found in the top layer and only Cu in the second layer. For the complementary system Au3Cu, complete Au segregation to the first layer and close to 100% Cu in the second layer were observed on the (001) surface (Schomann and Taglauer, 1996). ISS was also used to study details of the structure and


199

segregation kinetics of that system. Fig. 12-6 shows energy spectra from the (001) surfaces of these two alloys. The scattering contributions from the first layer alone (O=O) and from the first plus second layer (@=45”) can be well distinguished. This is an illustrative example of the mass- and layer-specific information obtainable by ISS.

12.6 Structure The basis for obtaining structural information by ion scattering consists in the concept of the shadow cone. It describes the flux distribution downstream of a target atom on which a flux of primary ions impinges, see Fig. 12-7. The radial flux distribution f(R) behind the scattering atom exhibits strong flux peaking at the edge of the cone. For a Coulomb potential the radius Rs of the shadow cone at distance d from the scatterer is: Rs=2(Z1Z2e2d/E)”

(12-7)

The square root singularity at R=Rs is smeared out by beam divergence and by the thermal vibrations of surface atoms, see Fig. 12-7. If the peaked flux of the shadow cone edge is directed on to a neighbouring atom (i.e. at the appropriate impact parameter), a peak in the scattered ion intensity is observed. This situation can be created by gradually increasing the angle of incidence of the ion beam on the surface until the shadow cone edge hits a neighbouring atom, see Figs. 12-7 and 12-8. The technique has been called Impact Collision Ion Scattering Spectroscopy, ICISS (Aono et al., 1981) and allows the measurement of interatomic distances on a surface. If the shadow cone radius Rs(d) is known, either by calculation or by calibration using a known surface structure, the interatomic distance d is found from the critical angle wc for which the maximum scattering intensity is observed: d = R, I sinyr,

(12-8)

Using this technique the local arrangement of surface atoms can be measured with an accuracy of about 0.1 A. In order to determine crystallographic surface structure, e.g. in the frequent case of surface reconstruction, structural models have to be considered (usually deduced from low-energy electron-diffraction (LEED) patterns). Ion scattering then provides the possibility of unambiguous determination of which of various models of the same symmetry exists on the surface. For multicomponent material, it is particularly useful that ISS provides a mass-sensitive signal, i.e. the positions of the various constituents can be identified. An example in the literature is the confirmation of the missing row model for the reconstructed Ni(llO)-0(2xl) surface (Niehus and Comsa, 1985). A simple and useful description of the shadow cone was given by Oen (1983), who calculated the radius R of the shadow cone using a TFM potential. The cone radius at a distance d from the scattering centre is expressed as R = R s (1+0.12k+0.01k2)

(12-9)


with k= Rs/a being between 0 and 4.5 and a similar expression for larger values of k (a is the screening length in the TFM potential.). The critical angle wc can be calculated from a corresponding expression (Fauster, 1988). It varies as vc cc d", the exponent being between 0.7 and 0.8, in agreement with experimental results (Taglauer, 1991). In several cases, two-dimensional numerical codes were able to reproduce experimental results quite well (Taglauer, 1991; Daley et al., 1989; Tromp and van der Veen, 1983; Taglauer et al., 1995).For more complete calculations three-dimensional codes such as the MARLOWE program (Robinson and Torrens, 1974) are useful but their applicability is restricted due to the large amount of computer time needed.

-

He Ni E = 1 keV

i

I

51 i

31

1

I

T = 300 K

2

44

I

3

1

i i d

i""""

3 j

3

0

1

I

3

2

f / /

0

1

Impact Parameter

T

2

[A1

0

I

0

= . I '

=

1

I

I

2

Radius

[A]

I

I

3

Fig. 12-7. Schematic representation of the shadow cone formed by ion trajectories (left) and numerically calculated intensity distributions for 1 keV He at different distances from the Ni scattering centre. Target temperatures 0 and 300 K, Debye temperature 220 K.


a) clean f m 0" 30" 60" 90" angle of incidence Y

201

Fig. 12-8. ICISS spectra for the clean surface and various Fe films on Cu(OOI), azimuth [loo]. All spectra are normalized to the same maximum height. Insert: Fe signal at an angle of incidence of 40° where only the top layer is probed (Memmel and Detzel, 1994).


As an illustrative example that demonstrates the potential of ion scattering for structural investigations, we consider an ISS study of the growth of ultrathin iron films on Cu(OO1) (Memmel and Detzel, 1994). Fig. 12-8 shows ICISS spectra obtained in the way described above using 5 keV Ne' ions at a scattering angle of 160". As a function of the angle of incidence (measured relative to the surface) the first peak relates to scattering from the top most atomic layer, the second peak (at 63') to scattering from the second layer. Both peaks are observed for the clean Cu surface, for the lowest Fe coverage of O.1ML ( 1 M L = 1 . 5 3 ~ 1 0atoms ~ ~ cm-2) and for the continuous film at about 6ML. This demonstrates that the films do not grow layer-by-layer, but Fe is incorporated into the original Cu surface from the beginning, the Cu substrate being at room temperature during evaporation. The insert in Fig. 12-8 shows that at a coverage of IML there is still 50% Cu on the surface and the iron signal only saturates after deposition of 2 monolayers The subsequent growth resembles more the layer-by-layer growth of an ordered iron film. This can also be seen from the scattering intensity at very low angles of incidence (hatched area). There should be no scattering at all into this angular region from a completely ordered surface due to the shadow cone position (see Fig. 12-7). The intensity observed is therefore an indication of defects and adatoms on the surface and it becomes a minimum at a coverage of about 2ML. The results shown in Fig. 12-8 also demonstrate that the interatomic distance (peak positions) is the same for the Cu substrate and the Fe films, (to an accuracy of 0.1 A), i.e. these films grow in the fcc structure. Only after deposition of more than IOML a phase transition from fcc (001) to bcc (110) can be observed (see Fig. 12-8g). This structural development is also correlated with the magnetic properties of these films: no ferromagnetism up to 2ML, ferromagnetism between 5 and lOML, only surface layer ferromagnetism above lOML (Detzel et al., 1993). Studies of this kind demonstrate the assets of low-energy ion scattering for probing surfaces, i.e. sensitivity to the topmost atomic layer, to the structure of the surface and to the masses of surface atoms. It should be noted that such experiments can generally be conducted with current density low enough to avoid considerable damage or sputtering of the surface, although this point has always to be kept in mind. An attractive variation of ISS is direct-recoil spectroscopy (DRS). It refers to the detection of those atoms or ions which are released from the surface in one single collision (as opposed to the collision cascade which is effective in sputtering). For these particles a similar expression to eq. 12-1 holds: (12- 10) 82 being the recoil angle relative to the incident beam. This technique is particularly useful for the detection of light particles such as hydrogen isotopes. For these scattering is restricted to small scattering angles (9 < arcsin) and the scattering cross-section is relatively small, whereas the recoil cross sections can be much higher (Taglauer, 1991; Eckstein, 1987).


203

DRS can also be used in connection with the shadow cone technique described above. It has, for example, been applied to determine the position of H adsorbed on Ru(OOl), where adsorption in a threefold co-ordinated. hollow site at a distance of 1.01 A k 0.07 a above the topmost Ru layer was found (Schulz et al., 1991).

.3 3

0

2

6

4 -

2

2

.3 Y

0 I

. .

0 ,

A A A Inmm

3

6 -

8

A

IP4131P4

0 0

m 3 w

.

5.

1

r;

IN

3

1

P4

3

0

-

3

3

-

2 -

(d

d

2 0

I

I

I

I

I

I

Another example is given in Fig. 12-9 (Roux et al., 1991) for the oxygen induced reconstruction of the Ni( 110)-O(2x 1) surface. The azimuthal intensity distribution of oxygen recoils due to 4 keV Ar+ bombardment shows strong variations which can be related to the structure model plotted on top. Recoils are shadowed in certain crystallographic directions either by surface Ni atoms or by other 0 adsorbates. The correlation with the structure model reveals a perfect correspondence with the missing-row reconstruction, with the oxygen sitting in the long bridge position along the rows.


This result again demonstrates the structural sensitivity of ion scattering and recoiling techniques.

References Aono M., Oshima C., Zaima S . , Otani S., Ishizawa Y. (1981), Jpn.J.Appl.Phys.20, L829. Beckschulte M., Mehl D., Taglauer E. (1990), Vacuum 41,67. Beckschulte M., Taglauer E. (1993), Nucl. Instr. Phys. Res. 878, 29. Bertrand P.,Beuken J.-M., Delvaux M. (1983), Nucl. lnstr. Meth. 218, 249. Brongersma H.H., van Leerdam G.E. (199 I), 'Fundamental Aspects of Hetrogeneous Catalysis Studied by Particle Beams', Eds.H.H.Brongersma, R.A. van Santen, Plenum press, New York, p. 283. Buck T.M. (1982). Chemistry and Physics of Solid Surfaces lV, Eds. R Vanselow and R. Howe (Springer Berlin) p.435. Buck T.M., Chen Y.S., Wheatley G.H., van der Weg W.F. (1975), Surf.Sci.47,244. Daley R.S., Huang J.H., Williams R.S. (1989), Surf.Sci.215, 281. Detzel Th., Memniel N., Fauster Th. (1993), Surf. Sci. 293,227. du Plessis J., van Wyk G.N., Taglauer E. (1989), Surf.Sci.220,381. Eckstein W. (1987), Nucl. Instr. Meth.B27, 78. Fauster Th. (1988), Vacuum 38, 129. Hagstrum H.D. (1954), Phys. Rev. 96,336. Jacobs J.-P., Lindfors L.P., Reintjes J.G.H., Jylha O., Brongersma H.H. (1994), Catal.Lett. 25 315. Katayama K., Momura E., Kanekama N., Soejima H., Aono M. (1988), Nucl. Instr. Meth. B33,857. Leyrer J., Margraf J., Taglauer E., Knozinger H. (1988), Surf.Sci.201,603. Linsmeier Ch., Knozinger H., Taglauer E. (1992), Surf. Sci. 275, 101. Los J., Geerlings J.J.C. (1990), Phys. Rep. 190 133. MacDonald R.J., OConnor D.J. (1983), Surf. Sci.124, 423. Margraf R., Leyrer J., KnBzinger H., Taglauer E.( l987), SurfSci. 108/109, 842. Memmel N., Detzel Th. (1994), Surf. Sci. 307-309,490, Mikhailov S.N., Elfrink R.J.M., Jacobs J.-P., van den Oetelaar L.C.A., Scanlon P.J., H.H. Brongersma (1994), Nucl. Inst. Meth. B39,145. Niehus H. (1991), Appl. Phys. A 53 388. Niehus H., Comsa G. (1985), Surf.Sci.151, L171. Niehus H., Heiland W., Taglauer E. (1993), Surf. Sci. Rep. 17,213. 0enO.S. (1983), Surf.Sci. 131, L407. Overbury S.H., Mullins D.R., Paffet M.T., Koel B.E. (1991) Surf.Sci.254,45. Robinson M.T., Torrens 1.M. (1974), Phys. Rev. B9,5008. Roux C.D., Bu H., Rabalais J.W. (1991), Surf. Sci. 259,253. SchUmann S., Taglauer E. (1 996), Surf. Rev. Lett. Schulz J., Taglauer E., Feulner P., Menzel D. (1991), Nucl. Instr. Meth. B64,588. Smith, D.P. (1967), J.Appl.Phys.18, 340, and Surf.Sci.25, 171 (1971). Taglauer E. (1982), Appl. Surf. Sci. 13, 80. Taglauer E. (l99l), 'Fundamental Aspects of Hetrogenious Catalysis Studied by Particle Beams' Eds.H.H.Brongersma, R.A. van Santen, Plenum press, New York, p. 301. Taglauer E. (1 991), 'Methods of Surface Characterization', V01.2: Ion spectroscopies for Surface Analysis, Eds. A.W. Czanderna and D.M. Hercules, Plenum press, Ney York, p. 363. Taglauer E., Englert W., Heiland W., Jackson D.P. (1980), Phys. Rev. Lett. 45 740. Taglauer E., Reiter St., Liegl A., SchUmann St. (1995), Nucl. Instr. Meth. Phys. Res. B. Tromp R.M., van der Veen J.F. (1983), Surf.Sci.133, 159. Weigand P., Jelinek B., Hofer W., Varga P. (1993), Surf. Sci.295,57.



13 Dynamic-mode secondary-ion mass spectrometry A.R. Lodding and U.S. Sodervall

13.1 Introduction The SIMS technique is particularly esteemed for its outstanding sensitivity of chemical and isotopic detection. Quantitative and semi-quantitative analysis can be performed for small concentrations (sub-ppm) of most elements in the periodic table, including the lightest. However, the great usefulness of modern SIMS is mainly due to the unique combination of high sensitivity and good topographic resolution, both laterally (sub-micron) and in depth (down to a few atomic layers). Because of these assets, the fields and ambitions of SIMS applications in industry and interdisciplinary science have been steadily expanding. The number of major (‘multi-purpose’) instruments in operation to-day (1995) is about 400, distributed over nearly all industrialized countries. Further, various SIMS-type instrumentation is employed in specialized projects and/or as accessories to other research equipment. The uses of Dynamic mode Secondary Ion Mass Spectrometry (SIMS) range over widely differing interests. The distribution of the principal fields of employment is today roughly as follows: semiconductors, electronics: 45%; geo- and cosmosciences: 15%; biology, medicine, environment: 5%; multi-purpose, service 10%. The topic of SIMS has been presented in several books and review articles (Benninghoven et al., 1987; Lodding, 1988; Wilson et al., 1989; Williams, 1992). Since 1975 the ‘state of the art’ has been regularly discussed in the proceedings of dedicated conferences (Heinrich and Newbury, 1975; Benninghoven et ul., 1989, 1982, 1984, 1986, 1988, 1990, 1992, 1994). From the point of view of main modes of employment, one may distinguish between two classes of SIMS. ‘Dynamic SIMS’ is primarily utilized for the determination of concentrations and/or topographic distributions of elements (or isotopes) in layers of up to several micron on solids. During such analysis the specimen is being continuously eroded by sputtering, which offers sensitive in-depth profiling or even (in combination with surface imaging) threedimensional mapping of concentrations. ‘Static SIMS’ (including the time-of-flight technique), on the other hand, is mainly directed at chemical-molecular characterization of the outermost layers of the solid or liquid, and aspires to minimal possible erosion of the analysed surface. This mode, particularly useful in the study of organic and polymer materials, is discussed in a separate chapter of this book (Bertrand, 1996). The purpose of the present chapter is to give an up-to-date orientation, on a non-specialist level, of the main principles, possibilities and problems of SIMS from the viewpoint of practical applications to materials in different disciplines.


13.2 Functioning principles Secondary ion mass spectrometry is based on the following functions: 0

0

Bombardment of sample surface by a focused beam of primary ions; sputtering of the outermost atomic layers. Mass-spectrometric separation of ejected secondary ion species according to their mass-to-charge ratios. Collection of the separated secondary ions as quantifiable mass spectra, as indepth or along-surface concentration profiles, or as distribution images of the sputtered surface.

The primary ions are usually produced by a) plasma discharge (duoplasmatron; Ol', Nz'), by thermionic surface emission (Cs', Li'), or by liquid-metal field emission (Ga', In'). The ions are accelerated, normally through 2-50 kV, and focused to a beam-impact area on the specimen. The beam diameter differs according to the type of instrument and mode of analysis. In most instruments a beam raster is provided to define the size of the bombarded area. The incidence of a primary ion on the specimen results in a collision cascade, involving the reshuffling of some 50-500 near-surface atoms, some of which are emitted from the surface as secondary particles (monatomic, molecular, or clusters). A fraction of the particles emerge positively or negatively ionized. These secondary ions are extracted from the specimen into the mass spectrometer through a system of ion optics. This system controls the size of the area from which the ions are admitted for mass analysis, the focusing of the separation trajectories, and often also the kinetic energy range of the admitted ions. Mass separation is effected according to one of three main classes of spectrometer, viz. those based on electric/magnetic deflection fields, on quadruple orbits, or on the time-of-flight pulse principle (Benninghoven et al.,1987; Wilson et al., 1989; Bertrand 1996). Secondary ions with a given mass-to-charge ratio and within particular intervals of kinetic energy and angle of emission are collected for ion-current measurement (usually via an electron multiplier or a Faraday cage type arrangement) and/or for ionoptic imaging (by means of channel-plate image amplification or by TV-type scanning, see, e.g., Benninghoven et al.,1987). One can distinguish a few main classes of instrumentation for dynamic SIMS: 0-,Art,

0

0

Non-imaging ion probes are used for point analysis or for in-depth profiling, usually on laterally rather homogeneous specimens, and often as accessories to other surface-analytical systems such as AES, ESCA or electron microscopy. Direct-imaging ion microanalysers (Castaing and Slodzian, 1962; Rouberol et al., 1980; Rasser et al., 1994) are based on relatively wide (ca 5-300 pm) homogeneous primary beams. A point-to-point ion microscope knction is achieved by a set of ion-optic lenses. A stigmatic image of the bombarded surface is obtained on the collector plate of the mass spectrometer by the arriving mass-resolved secondary ions. The collector is followed by an image amplifier

13 Dynamic-mode secondary-ion mass spectrometry 207

(a channel-plate arrangement). The lateral resolution in this mode is mainly limited by gating of the imaged area and by the geometry of the channel plate; it is, in modern instrumentation, at best of the order of 0.5 pm. For higher resolution, commercial direct-imaging equipment is often provided with a scanning microbeam complement. Scanning ion microprobes-microscopes (Liebl, 1967; Andersen and Hinthome, 1972; Waugh et al., 1984; Levi-Setti et al., 1985, 1994) utilize a narrow beam (< ca 10 pm) of primary ions. Lateral resolution is given essentially by the beam size. Imaging of the specimen surface can be effected by a TV-type raster of the beam. Figs. 13-1 and 13-2, together with Table 13-1, are intended to illustrate the principal features of modern SIMS instrumentation, as exemplified by commercial equipment of recent design (Cameca IMS-6F, Rasser et al., 1994) and by a vanguard laboratory design (Univ. of Chicago UC-SIM, Levi-Setti et al., 1994). The former is the latest link in a chain of successful commercial designs based on the original idea of a directimaging secondary-ion microscope (Castaing and Slodzian, 1962) and successively modified for great versatility. About 250 instruments in this family (Cameca IMS300,-3F,-4F,-5F,-6F) have been sold. In addition, a modification intended particularly for use in isotope geology, IMS-1270, has recently been developed (de Chambost et al., 1992); it differs from IMS-6F mainly in incorporating an exceptionally large magnet, rendering particularly enhanced mass resolution (WAM in the order of 5x104). About 15 ‘geological’ machines with extra high mass resolution, both direct-imaging and of the scanning microbeam type, have so far been built by different manufacturers.

18’

Fig. 13- 1. Schematic drawing of a direct-imaging secondary-ion analyser.

208 Part 2: Elemental composition Exlt

Electron efining Aperture MAGNETIC

SECTOR

PRIMARY

ION

COLUMN ctupole D e f l e c t o r erentiol Pumping Switchyord Dynamic Ernittance Ouadrupole

R F PUADRUPOLE SPECTROMETER

ACCEL-ZOOM TRANSPORT O P T I C S

Cnonnel Electron Multiplier O e t e c t o r

sample

Fig. 13-2. Schematic drawing of the UC-SIM scanning microprobe-microscope. Table 13-1 , Characteristic renderings of two designs of ‘three-dimensional’ SIMS instrumentation. Instrument 1: Cameca IMS-6F, direct-imaging microanalyser with microbeam-scanning accessories. Instrument 2: University of Chicago UC SIWmag, scanning ion microprobe-microscope. Order-ofmagnitude figures. Primary ions Primary-ion impact energy (keV) Primary ion current (nA) Probe diameter d, (nm) Current density (pNmm2) Optimum size of imaged area (pm’) Best lateral resolution (nm) Sputtering rate in practical profiling (mm h-I) Optimum depth resolution in profiling (nm) Maximum mass resolution MlAM Max. sec. Ion energy window (eV) Maximum energy offset (eV) Maximum instrument transmission (%) Typical element detection limits (in absence of spectral contamination, counted at y rs: 0.01)

Instrument 1 O;, 0-, Cs’, Art, N; 2-20 10-103 10’ - 3x 10’ 10- 10’ (large beam) 103-104(micro beam) 10x10 - 200x200 100

lo2-lo2 0.5

1o4

150 150 10

1ol- 1o - ~

Instrument 2 Gat, In’ 10-60 1.5 (at 20 nm beam) - 50 2 20 3x103 -3x104 500xd,; min. 10x10 20 1 O-I-5 1 3x10’ 30

0 20 1-10.’

In the multi-purpose instrument shown in Fig. 13-1, the double-focusing mass spectrometer consists of an electrostatic deflection sector (ES) for energy focusing, followed by a magnetic sector (EM) for mass focusing. The magnet is of a laminated type for fast response. Even faster alternation between different mass peaks (mainly for precise isotope work) can be achieved by means of an electrostatic peak-switching acces-


sory. The two primary ion sources (a duoplasmatron for ionization of gases, and a thermionic source for Cs’) are interchangeable. A mass filter safeguards the purity of the ion beam. Accessories include an electron gun for neutralization or control of the positive charge build-up on insulating specimens. When negative secondary ions are studied, the electron gun operates in a self-adjusting mode, maintaining constant specimen potential. A transfer optics system in the spectrometer provides an ionoptical ‘zoom’, allowing inter alia the choice of very small specimen areas for point analysis without essential loss of transmission. An energy discrimination facility, between the ES and EM sectors, permits the selection of a suitable ‘energy window’ for ions permitted to proceed to the collector. In particular, low energy ions can be excluded, which suppresses the presence of polyatomic ion species in the mass spectrum. For the direct-imaging mode, the ion lens system of the spectrometer is designed to yield on the collector (a multichannel plate) a point-to-point representation of the sputtered surface, with simultaneous information from all imaged parts. The channel plate, consisting of ‘bundled’ electron-multiplying capillaries, converts each arriving ion event into a laterally localized avalanche of electrons, visible as a flash on a screen beyond the plate. The mass-resolved image can be viewed on a video screen and collected photographically or digitally. In the quantitative mode of analysis, the trajectory of the separated ions is deflected by another electrostatic sector towards an electron multiplier or a Faraday-cage collector. A computer stores and processes the registered ion currents. This may entail, for example, mass spectra with height indication of selected mass peak intensities as function of time or position (in-depth or along-surface concentration profiling) or kinetic energy of the ejected secondary ions (energy distributions). The second example (Fig. 13-2) illustrates the principles of an advanced instrument of the ion microprobe type (Levi-Setti et al., 1994), which is particularly useful in the scanning microscope mode due to its excellent lateral resolution. A very narrow and concentrated primary beam (usually Ga’) is extracted from a liquid-metal field-emission source. The diameter of the impinging beam may be as narrow as ca 20 nm, which is of the same order as the size of the ensuing collision cascade region. The primary ion currents are here considerably lower than in the previous example, but comparable density of current may be achieved. Imaging is effected by digital rastering, which of course means that different parts of the image are not recorded simultaneously. The design shown in Fig. 13-2 is a recent modification of a pioneering scanning ion microprobe-microscope instrument, developed at the University of Chicago, which has asserted itself internationally since 1985 by a multitude of successful applications in numerous fields of science and engineering, supplying elemental mapping by SIMS images with unsurpassed topographic resolution (Levi-Setti et al., 1991). The limitations before very recent modifications lay chiefly in the mass separation, which was effected by a RF quadruple spectrometer. Although considerably less costly than a magnetic/electrostatic sector mass spectrometer, and easier in ion optical adaptation, a quadrupole instrument has relatively poor transmission, a narrow energy pass-band, and low mass-resolution. In the new design, a high-resolution magnetic sector has been

2 10 Part 2: Elemental composition

incorporated; the adaptation to the specimen chamber has required advanced calculations in three-dimensional optics. The modification resulted in an improvement in spectrometer transmission by a factor of four, and the element limits were lowered by about two powers of 10 on average. For ion detection, a channeltron with fast counting is used, furthering quantitative analysis as well as imaging. As complement to elemental mapping the instrumentation also provides sharp-relief topographic imaging of the analysed area by means of integrated (not mass-resolved) secondary ions (ISI) at low exit angle.

13.3 Sensitivity and quantification in SIMS 13.3.1 Mass Spectra The mass peaks in a SIMS spectrum are characteristic of the sputtered solid, but are also influenced by numerous experimental factors in analysis, such as: the type, intensity, energy and incidence angle of the primary ions; exit energy and angle of the emitted secondary ions; acceptance and selectivity of analyser and collector; and ambient vacuum. One may distinguish between the intrinsic spectrum, given by the matrix composition, and the superimposed impurity spectrum. Fig. 13-3 shows a positive-ion spectrum from a silicon wafer coated with germanium (Lodding 1988). The peaks due to Ge are in part affected by overlap of the intrinsic Si spectrum (such as SizO', SiO3'); in addition to monatomic Si', Ge' and various other elemental peaks the spectrum is seen to contain a multitude of dimers, trimers and other molecular peaks, mainly of the form SikGelO,H, (the oxygen originating chiefly from the primary ion beam, the hydrogen from the residual atmosphere). One may also see 'half-mass' peaks due to doubly ionized species (Si2+,Ge2+,Si202+).For some elements even triply or higher ionized species may be seen in positive mass spectra (note: negative spectra show only singly charged ions). When impurity concentrations are measured, the superimposed contributions from the intrinsic spectra, or from other impurities, on the respective peaks of interest, have to be accounted for and minimized. Obviously, instrumentation capable of very high mass resolution (such as in spectrometers with very large magnets or of the time-offlight type) are especially suitable for separating a composite mass peak. If sufficiently high mass-resolution is not available, or entails too great a loss in instrument transmission, one may instead revert to low-energy ion discrimination (by means of an 'offset' in specimen potential). This utilizes the differences in shape in the energy distributions of different kinds of ion (see Fig. 13-6, below): those of di-, tri-, or polyatomic molecular or cluster ions show, in general, steeper decline towards the high energy tail, than do those of monatomic ions. The device of preventing low energy ions from reaching the collector may thus largely eliminate molecular contributions to the spectrum, leaving practically only monatomic (elemental) ions.

13 Dynamic-mode secondary-ion mass spectrometry 2 1 1 lE7

lE6

1E5

lE4

1E3

1E2

1El

1EO

I

10

20

30

40

50

60

i

70

80

90

100

110

120

Fig. 13-3. Positive ion mass spectrum (cps vs integral mass number) fiom a Ge-coated Si wafer. Primary ' ~ Acceptance window of secondary ion kinetic energy: 5-55 eV. ions 0;. Oxygen leak 3 ~ 1 0torr.

13.3.2 Element detection limits The task of analytical SIMS is to convert the intensity of one of several peaks, characteristic of the element L, to the corresponding molar concentration CL. When primary ions hit the sample, collision cascades are initiated, resulting in, inter alia, the emission of secondary ions. These are in part detected as peaks in the mass spectrum. The positive or negative ion current with which the element contributes at the isotope mass number M is: (13-1)

where bM is the isotopic abundance of L at mass M; I, is the total primary current collected by the analysed area A (current density i,=I,/A); SLis the sputter yield (number of secondary particles per collected primary ion); I ~ Mis the effective instrument transmission; and y t is the positive or negative ionizability (ions per ejected particle). The entities S, q and y may be considerably dependent on the kinetic energy and angle (Ee, @,)of exit from the specimen surface. They are known to be only moderately dependent on isotope mass. Elemental differences manifest themselves usually quite strongly in regard of y*, which may vary in a range of several powers of 10 for different elements sputtered from a given matrix.


In principle, detection and quantification of elements can be effected from dimers or larger ions as well as from monatomic peaks. For example, the (12C 14N)-dimq can be used to detect nitrogen in a carbon-containing matrix, as the ionizability of N as well as N- is low, and CN- is often the dominant N-containing ion. Molecular ions are of special interest in studies of surface structures and of adsorption-desorption energetics (static SIMS, time-of-flight SIMS, see Bertrand (1996)). In dynamic SIMS, however, the monatomic peaks are normally dominant in the spectra and preferred for analysis. The arguments to follow will apply, in the main, to monatomic ions. The lower limit of detection, (Cmin), for an element L may be arbitrarily defined as the concentration corresponding to a multiple f, say, of the background level JM (electronic noise or spectral contamination) at mass number M. Then, via eq. 13-1, (Cm,,)L

= f x J M x K d x (i,A)-l

(I 3-2)

where KM=bM&qM.yL. The erosion rate of sputtering, vsp, is given by the primary current density, i,, via vsp/ip = (sx m)/(z x qe x No x P)

(1 3-3)

where zq, is the charge of the primary ion, No is Avogadro's number, m is the mean atomic mass of the matrix, and p is the matrix density. Hence eq. 13-2 can be rewritten as

( 1 3-4) where K,,zl 0 5 m - ' x b ~ x z x p x q ~ xIfy ~spectral . background, intrinsic or other, at mass number M is very low, then JM is given by the electronic noise. The sensitivity, which may be formulated as proportional to the inverse of the detection limit, is then, via the total 'useful' primary current I,, proportional to the rate of material consumption in sputter-erosion of the analysed area, i.e., to vspA. The consequence should be emphasized: high sensitivity of element detection over a small area requires a high rate of sputtering; on the other hand when slow erosion is desired (such as in static SIMS or for shallow profiles), retained sensitivity requires a correspondingly extended area of analysis. In other words, the typical combined assets of SIMS, viz. sensitivity, in-depth resolution and lateral resolution, are mutual trade-off qualities. It may be mentioned here that the trade-off principle also applies to mass resolution, as high M/AM requires adjustments in the ion optics of the secondary ions (narrowing of trajectory slits), which reduces the effective transmission term ?M in eqs. 13-2 and 13-4. As a numerical example, one may calculate an order-of-magnitude sensitivity limit via eq. 13-2 with the following parameters, normal in high-intensity SIMS: I ~ M= I,= lo6; SL= 3; JM= lo-''; and f= 3. One finds here, for an element with moderate ionisability, cmin=ca 1Om*, i.e. about 1Oa2 mole ppm. If the background the intrinsic and contamination spectrum is much above the noise level, JM is likely to be proportional to I,. Under such conditions it may be practical to

13 Dynamic-mode secondary-ion mass spectrometry 2 13

relate the contamination-ion current to a major matrix peak at mass number R, say, where the current is measured as IR. Applying eq. 13-4 and assuming the transmission equal for R and M, one gets: (13-5)

In a realistic analysis, most background peaks can be suppressed so as to give JM/IM values between ca 10-3and 1O-7. Under such premises, in measurements with moderate mass resolution, cmlnmay be expected to lie between ca 10” and lo4 mole ppm, depending mainly on the ionizabilities of the components in the M peak.

13.3.3 Aspects of quantification 13.3.3.1 Sensitivity factors Quantitative analysis by SIMS usually requires the use of well defined calibration standards. This involves the concept of sensitivity factors. For an element L such a factor may be defined, with the nomenclature of the preceding section, as (1 3-6)

Under reproducible conditions of analysis, using standards with compositions and microstructures not too different from those of the analysed specimens, the entities S, q and y may be considered as independent of CL,and useful calibration factors may be obtained, using only the first term in the parentheses. However, instabilities in analytical conditions make the use of absolute sensitivity factors hazardous for other than very well defined systems. It is generally found more practical to utilize the simultaneously measured ion current of a matrix reference element, R, say, of known concentration CR. Hence, one defines the relative sensitivity factor as 0LR = ~ L / Q

(sL/sM)(~L/~MXYL/YM)

(13-7)

It has been found that in many systems the 01.factors are only weakly dependent on CL. Satisfactory quantifications with relative sensitivity factors have been reported, e.g.,for steels, binary alloys, semiconductors, glasses, ceramic superconductors, and some biomineralized tissues. In general, however, concentration-dependence is to be expected especially through the ratio of ionizabilities, y d y ~ This . may necessitate tedious operations of standard preparation and/or theoretically based iterative routines. Among factors likely to affect (TL via qdqi,,~,one may mention counting dead-time and collector sensitivity. 13.3.3.2 Semi-empirical formalism for relative ionizabilities of elements (LTE) It is well known that the yields of positive ions, y,’ are relatively abundant for the alkali and alkalines earth metals and rare earth elements, but very low for the halogens, chalcogens, and Group IB and IIB elements. In the negative spectrum, on the other


hand, y- is generally found to be high for the VIIA, VIA and IB elements, but low for most others. In numerous matrix systems these tendencies are reasonably well represented by the so-called LTE (local thermal equilibrium) formalism (Andersen and Hinthorne 1973), based on the Saha-Eggert equation, drawing an arbitrary analogy between the sputter cascade and the plasma in a flame. The LTE model predicts the yield of positive ions to be largely given by the first ionization potential, E,, of each element, and that of negative ions by the electron affnity, Ea. Although the theoretical basis of the LTE model as applied to the sputtering cascade is generally considered to be very questionable, the formal premises of the model lead to empirical expressions for relative sensitivity factors. These have been found useful in numerous cases where semi-quantitative elemental analysis was performed by SIMS without recourse to external calibration standards. The formalism (Lodding, 1988) leads to o;,

- [(B/Bo)L/(B/Bo)R]exp[(EiR-EdkhTI'

(13 -8a)

-

(13-8b)

and oLR [(g/g~)L/(g/g~)R] eXP[(EaL-EdkTif

where R is a reference element; Ei and Ea are the respective 1st ionization potentials and electron affinities; BEI, and g/go denote, respectively, the thermodynamic partition functions arid the statistical weights in the 1st ionized and ground states (see, e.g., Drawin and Felenbok, 1965; deGalan et al., 1968; Vedenyev et al., 1966); and Ti', Tiare entities of the order 103-104K,usually called 'ionization temperatures'. As the thermodynamic notion of temperature has no evident meaning in the discussion of a sputter cascade, khT, should be understood simply as the gradient in a plot of logo' vs E, or logo- vs E,. 13.3.3.3 Energy-distributions of secondary ions The kinetic energies of ions emerging from a sputtered surface exhibit a more or less sharp maximum, corresponding to the mean information depth in the specimen. An example of typical energy distributions may be seen in Fig. 13-4. The effective range of energies of a secondary ion species is normally in the order of 30-300 eV; narrow distributions characterize mainly polyatomic clusters. This is of particular significance in practical quantification routines: by applying an 'offset' in specimen potential, so that only ions above a certain limit are allowed to reach the collector, the molecular contributions can be effectively discriminated away, and also Iower compound ions, such as triplets or doublets (see Fig. 13-4) may be conveniently suppressed in relation to the monatomic species. Often this technique of spectral background limitation proves more efficient than the use of very high mass resolution.


Fig. 13-4. Example of secondary ion energy distributions. Positive secondary ions from fluorapatite. Primary ions 0'.

0

50

100

150 eV

Fig. 13-5. Energy distributions (schematic) of positive secondary ions sputtered from an alkali borosilicate glass. Primary ions 0'.OFS: energy offset. EWW: energy pass-band window.


Fig. 13-5 illustrates that even among monatomic ions from a given matrix the energy distributions of elements may differ significantly. It is often (but not always) observed that elements of higher valency (such as P, Zr, U, Si) exhibit broader distributions than mono- or divalent elements. Relative sensitivity factors are therefore dependent on exit energy, and it is essential in quantification that the position and width of the pass-band (OFS and EWW in the figure) be well defined. Fo: example, in alkali borosilicate glasses (Lodding et al., 1985), the fitting parameter Ti (‘ionisation temperature’) was found to be about 7500 K at very low offset (i.e. near the distribution maximum), but of the order of I5000 K at OFS = 100 eV. A major practical problem may arise when insulating materials are studied. Due to arriving primary ions and emitted secondary electrons, a surface charge gradually develops, changing the specimen potential and shifting the energy distributions of the secondary ions past the pass-band EWW. This influences the relative sensitivity factors. Quantitative analysis therefore requires charge neutralization or some kind of automatic compensation for shifts in effective offset voltage (see, e.g., Lodding, 1992). 13.3.3.4 Inverse velocity distributions; quantification by the ‘infinite velocity method’ While a knowledge of elemental energy distributions is of importance in LTE-type quantification, it is difficult to predict these distribution shapes from theoretical premises. As regards uncharged sputtered species, their distributions as functions of exit energy can in most cases be satisfactorily predicted by theory (Thompson, 1968; Sigmund, 1969). However, the process of ionization in sputtering, i.e. the entity y* as a function of Ee, is much less clear. Most recent theoretical approaches, tend to suggest that the probability of ionization can be meaningplly formulated in terms of the exit velocity (v,), rather than the kinetic energy (Ee=MV, /2) of the emitted ion. Usually one predicts an exponential dependence on inverse velocity, in the form (1 3-9) where 8 is the emission angle with respect to the sample normal, and v, is a matrixcharacteristic constant, given by the electronic environment of the emerging ion (see, e.g., Norskov and Lundqvist, 1979). Such behaviour would yield a straight-line relationship when the logarithm of ionizability is plotted versus vL1 (under the normally prevailing conditions of constant 8). Recent experimental studies have confirmed that this does indeed apply for monatomic atoms of numerous elements, as sputtered from several types of matrix; the linear relationship conforms at least at moderate-to-high exit velocities (i.e., in normal SIMS practice, at energies E, above some 10-20 eV). The slopes of the linear parts of the log(1M) vs vi’distributions, i.e. the elemental char1994; Sodervall et d,. 1996) to be acteristic velocities v t , are found (Lodding et d., systematically connected with the 1st ionization potential Ei or the electron affinity E, of the ejected species. In fact, the behaviour appears to be well represented by a theoretical approach (Norskov and Lundqvist, 1979), which predicts


Y+ =Yo+ e x P [ ~ (Ei + -~F)/ve]

(13- 1Oa)

y- =Yo-exPIK-(EF -E&]

(1 3- 1Ob)

and

where EF is the work function of the sputtered surface, and :y and K* are constants. When recording multielement mass spectra, the observed (background-free) ion yield on mass M will be given as IMin eq. 13-1. 'Corrected elemental ion yield' may be formulated for predominantly monatomic ion emission (at given ip), as:

4

1t -

IM/~M)@LxvM)-

I

= CL

(13-11)

XYL

Thus, after correction of the measured IM by bM, SL and TM, the dependence of ele' as inverse-vemental ionizability on the exit velocity may be plotted as I: vs v; ', 1.e. locity distribution curves of y+ or of y-. The sputtering yield SL as a function of energy can be calculated from the well established relation (Thompson, 1968; Sigmund, 1969) SL= EJ(E,+ E ~ L ) ~

(13-12)

where EbL is the binding energy at the exit surface. At kinetic energies where the linear dependence of 1; on v; applies, this approximatesto SL E; * for all elements. The transmission correction is composed of two main terms, TM Txr, where T is given by the extraction geometry of the spectrometer and r by the mass discrimination of the secondary ion collector. T(E,) has been calculated for several geometries of some commercial SIMS instruments; at the kinetic energies of interest in this section the approximation T EL was found adequate. The collector response to the mass of the arriving ion is found to vary according to the make of the collector, and is different for positive and negative ions. The mass discrimination, which may range over one or two orders of magnitude, may be obtained empirically by comparing the ion current as recorded by the multiplier with that obtained via a Faraday-cage collector. Thus, a realistic correction factor SLxTxrL can readily be applied to I t in eq. 13-11. Now, if the predictions of eqs. 13-9 and 13-10 are obeyed, the extrapolation of the linear distributions to v-'+ 0 leads to the constant values of y+ or y- as intercepts on the vertical axis for all elements. This has consequences for the possibilities of elemental quantification in standard-free SIMS microanalysis. Again using a matrix element whose concentration CR is known as reference, one obtains the mole concentration of the investigated element L as

-

'

-

-

CL

= cR(I;)o/(I~)o

(13-13)

where 1: are the respective intercepts on the v-'=O axis obtained by extrapolation of the linear portions of the inverse velocity distributions of L and R. This has been found to


give good quantification both in elemental point analysis (Sodervall et al., 1996) and in in-depth profiling (van der Heide et al., 1995). Deviations from agreement with eqs. 13-10 have been observed not only at low E,, but also for a few elements (e.g. F, H) in the whole energy range. Nevertheless, the ‘infinite velocity method’ offers the promise of a new technique for quantification in sensitive elemental analysis by SIMS, with the special virtue of not requiring any external calibration standards or any knowledge of elemental sensitivity factors. 13.3.3.5 Chemical enhancement of ionic yields; matrix effects In the practice of dynamic SIMS, high and constant yields of secondary ions are normally obtained only from ionic matrices or, paradoxically, when the specimen surface is purposely contaminated by the introduction of a reactive species. Thus, on metals, the presence of oxygen, either as implant from the primary beam or from an 0 2 gas leak at the surface, usually stimulates the positive ion yield. Reproducibility of quantification can be maintained only when the surface layers are saturated with oxygen. This ‘chemical enhancement’ is of considerable importance in practical analysis; the positive ion yield from a metal bombarded by oxygen is often higher by 2-4 orders of magnitude than if it were instead sputtered by an Ar’ beam of the same intensity. An oxygen leak improves the yield as well as its stability. The introduction of oxygen may also bring some amorphization of the surface, preventing undesirable crystallographic effects such as channelling. The physical background of yield enhancement by oxygen is not perfectly understood, but the effect appears qualitatively compatible with a bond-breaking model of ionization (see, e.g.,Benninghoven et al., 1987; Yu, 1994). For negative secondary ions, a similar yield enhancement may be achieved if the specimen surface is implanted or flushed with an alkali metal, a fact which has led to the development of Cs primary ion guns, particularly for the detection and analysis by SIMS of electronegative dopants in semiconductors. The physical basis of the effect is believed to be the lowering of the electron exit work function, i.e. an increase in electron concentration in the region of the sputter cascade. The significant role of the chemical enhancement effect, underlines the potential influence of various kinds of surface contaminant on the accuracy of quantitative SIMS analysis. Saturating the surface with purposely introduced reactive species may reduce local effects of impurities. The introduction of a gas or vapour leak, if technically feasible, is vital in safeguarding stable secondary ion yields. The use of an 0 2 leak, up to around lo4 mbar at the specimen surface, is a normal procedure in quantitative SIMS of metals. When introducing the enhancement effect only by means of the primary beam, a limitation is that the efficiency of implantation may be low, especially in fast-sputtered matrices, This may be seen from the relationship (Lodding, 1982) for the steady-state concentration of the implanted species: (13- 14)


where p, S and M are defined as in eqs. 13-1 and 13-3, and p, the implantation factor, is of the order of 0.5. Ion-yield studies have suggested (Lodding, 1988) that effective positive and negative ionizabilities are proportional to (cipljYy, where y is of the order of 3 to 4. It is further evident from eq. 13-14 that, when the reactive species is introduced only by the primary beam, the effective detection sensitivity is promoted by the inverse of the sputtering yield. On the other hand, if the reactive species is deposited by gas or vapour backfilling, this may be inefficient at surfaces of low ‘sticking coefficient’, and is also inhibited if the primary-ion intensity is high. The latter fact is connected with the ratio (Lodding, 1982) linking the arrival frequency f, of atoms (from the residual atmosphere or from a gas leak) with that of the impinging primary ions, fi: 2

fr/fix 10 pr/ip

(13-1 5)

where pr is the effective pressure in mbar at the bombarded surface, and i, is expressed in A cm-2.

13.4 Aspects of depth profiling Several techniques of in-depth profiling analysis utilize sputtering to erode successive layers of the specimen. Most such techniques (e.g. AES or XPS) remove material stepwise, and study the residual surface after each step. SIMS, on the other hand, continuously analyses the sputtered-off material itself, simultaneously as it is being removed from the specimen. This material economy, combined with information depths of the order of one atomic layer and with very high sensitivities of elemental or isotopic detection, makes SIMS a particularly powerful technique for in-depth profiling of element concentrations, indispensable, e.g., in the electronic component industry. An application example (Larsson et al., 1994), showing the high in-depth resolution in the SIMS study of periodically A-doped semiconductor material, is given in Fig. 13-6. The subject of depth profiling by SIMS has been thoroughly reviewed in recent publications (Benninghoven et al., 1987; Wilson et al., 1989). The presentation which follows herein is a brief, application oriented summary.

13.4.1 Dynamic range and sensitivity limits The low concentration limit of the dynamic range is given by the signal-to-background ratio. For elemental detection limits the arguments of the previous section (eq. 13-4) apply, and it is normally desired in profiling to work with relatively fast specimen consumption (high vsp),in order to reach high sensitivity. When, at the same time, high depth resolution is required, a compromise between fast sputtering and a wide analysed area has to be made. Also in line with eq. 13-4, the dynamic range is furthered if the yield parameters (especially ionizability yL) are high.

220 Part 2: Elemental composition I

I

I

I

I

I

O,+, 3.5 keV Be

O i , 3.5 keV Si

U

0

--

200

I

400

600

800

I

I

1000 1200 1400

Depth (nm) Fig. 13-6. SlMS profiles of beryllium and silicon in periodically A-doped gallium arsenide. Distance between consecutive planes 30 nm. Sheet density 3 ~ 1 0 ’ ~ c m0; ’ ~ . primary ions at low acceleration energy. Positive ion spectrum.


A limit on sensitivity is often due to restrictions in the receptivity of the collector and amplifier. The effective capacity of an electron multiplier is normally of the order of lo6 counts per second (cps) and account must be taken of the counting deadline. The effective deadline in profiling differs from the intrinsic instrumental deadline: zep= gz,, where the factor g depends on the relation between the analysed area on one hand, and the size and raster of the primary beam on the other (Odelius and Sodervall, 1984). This factor (essentially the ratio of the counts with a stationary beam, to those with a rastered beam) may in practical cases amount to some 20 or more. The true intensity IM at a mass peak M is thus related to the actually recorded counts Ieff by

IMneft=(1-gdeff)-'

( 13- 16 )

13.4.2 Cyclic profiling Profiling of element concentrations is usually performed cyclically for several selected mass numbers. Within each cycle the intensities at the different mass peaks of the spectrum are counted successively, each during a chosen time interval, tM, for example. The lowest detectable concentration, Cmin, in profiling may be formulated by defining the lowest distinguishable count in each cycle, fmin. Then fmin t l q , replaces f. JM in eq. 13-4, so that: (13- 17)

The order of magnitude of K,, may vary within about two powers of ten from unity. Choosing, as an example, fmjn= 5 s", tM= IS, vs = 10-7 cm s-1 , A = lO-'crn 2, one may assess the sensitivity limit to lie typically within the range of magnitude between and lo2 ppm. The 'intrinsic' in-depth resolution in cyclic profiling is given by the thickness of the layer Az = VsptM, and so the product of the three instrumental assets (i.e., sensitivity, indepth resolution and lateral resolution) may be expressed in the form: (cminx AZX A)-'

- K,,tc

(13-18)

where tc is the total time of each cycle, which in addition to the sum of the respective M t intervals for all cycled masses, also contains the switching and peak-finding times between successive cycles and mass numbers. For profiling in insulator specimens, time is also needed for a voltage compensation for surface charging (Odelius and Sodervall, 1984; Lodding, 1992). Thus tc is considerably longer than tM, which is a significant drawback especially when high accuracy of counting is required, such as in isotope studies (see, e.g., Sodervall, 1991). To reduce the waste ('non-counting') time in each cycle, it is desirable to minimize the peak switching time, either by the use of modem laminated deflection magnets, or, still faster, employing electrostatic rather than magnetic switching.


13.4.3 Quantification aspects in profiling In the very outermost layers of a profiled specimen, i.e., in a transition zone before equilibrium ion emission is attained, special difficulties in quantification may be encountered due to differential sputter yields and/or surface contamination. Elements with high SLmay initially be impoverished relative to the matrix, until the changed concentrations stabilize, which usually happens at depths of the order of 10-20 nm. Theoretical models have been proposed to describe this transition zone (see Benninghoven et al., 1987; Wilson et al., 1989). Beyond this depth, meaningful proportionality between the ion yields and the respective concentrations applies and constant elemental sensitivity factors may be assumed. Similarly, ion yields are influenced by implantation effects from the primary beam (see section on ‘chemical enhancement’ above). The matrix is changed by the presence of the implant, and a constant quantification relationship is attained only at depths beyond the projected range of implantation. Non-equilibrium conditions of quantification are reached whenever the composition of the matrix exhibits significant and abrupt changes with depth, and particularly at an interface between two matrices. The thickness of the transition zones on both sides of the interfaces is usually given by the roughness and misalignment of the outer surface, by cascade mixing and by knock-on effects (Wilson et al., 1989).

13.4.4 Factors affecting depth resolution 13.4.4.1 Atomic mixing; recoil mixing Depth resolution Az is usually defined in terms of a step-function interface, as two standard deviations (2a)of the error curve profile approximation. The ultimate practical limitation to depth resolution is provided by cascade mixing. The large number of atoms involved in a cascade will ‘homogenize’ the region to a depth typically of the order 0.5-10 nm. The mixing range is given by the energy and direction of the primary beam, and is not depth dependent. Optimum depth resolution, if limited by cascade mixing, is obtained with low energy ions incident at a low angle to the surface. In the application example shown in Fig. 13-7, the incident 0; primary ions had an energy of 3.5 keV, arriving at ca 30”to the surface; a resolution of ca 5 nm was achieved. If the first collision between a primary ion and a target atom is a near-head-on hit, a large amount of energy is transferred and the recoiling atom may penetrate relatively deeply into the solid (20 nm or more). Such recoil mixing or knock-on effects, again chiefly conditioned by primary ion energy, are as a rule less dominant than cascade mixing (Wilson et al., 1989).


Fig. 13-7. SIMS distribution of La'on an aluminium ceramic sintered with 1000 ppm lanthanum. Showing the segregation of La to grain boundaries and second phases. Primary ions: 40 keV Ga'. The spacial resolution of the image is about 30 nm. 20 pm full scale.

13.4.4.2 Diffusion, strain and charging effects In a collision cascade, high concentrations of vacancies and interstitials are created, affecting atomic mobilities. The observed depth profiles may consequently be blurred by radiation-enhanced diffusion. The implantation of the primary beam species may, depending inter alia on the direction of incidence, create layers of lattice strain and so cause impurity elements to segregate either at the surface or at the end of the changed layer. Since such an effect may completely prevent the detection of the segregating impurity, care should be taken in regard of the stoichiometric products of implantation. Another type of segregation may apply to mobile ions in films on insulators, such as, e.g.,Na' in films of SiOz on silicon substrate. The deposition of surface charge creates a sharp potential drop through the layer, and ion displacement may take place through electromigration; the sodium ion may tend to be segregated at the surface. Electric fields across thin films have also been reported to induce dissociation of molecular complexes. Since most of the charging is, as a rule, caused by the emission of secondary electrons, the use of negative ions (0-)is usually preferable to positive primaries in the profiling of insulators. Thin conducting coating of the specimen (gold, carbon) is extensively used to reduce charging effects. Positive charging may also be counteracted by the action of an electron spray gun. The particularly difficult problem of negative-ion extraction from insulators, has been tackled by the construction of a self-regulating electron gun, maintaining an equipotential sample surface (Slodzian et al., 1986). Considerable progress, in regard of cyclic in-depth profiling of elements in insulators, has recently been reached by compensating the surface charge-up by means of automatic adjustment of specimen potential, after every cycle; the computer program seeks out a pre-selected


sharp peak in the spectrum, and returns the potential to the value corresponding to the constant position of that peak (see Lodding, 1992). 13.4.4.3 Surface roughening Even when the intensity of the primary beam over the analysed area is uniform, local differences in sputtering rate will arise, due to heterogeneous sample composition and irregularities in the original surface. Consequently the depth resolution Az will progressively deteriorate with increasing depth z (Wilson et al., 1989; Odom and Hitzman, 1994). Ion etching around local defects and impurities may induce spectacular cones or ripples on the crater bottom, even in single crystals (Dorner et al., 1980; Nomachi et al., 1994). When specimens containing several phases, variable in hardness, density, etc., are sputtered, all meaning of depth resolution may be lost. In single-phase polycrystalline specimens, the local erosion rate depends on the angle between the primary beam and the crystal axes. If the analysed area is of the same order as the crystalline dimensions, Adz may be as high as 0.5. If the area of analysis is much wider, the roughening is in the order of crystal size. Even amorphous and flat specimens may to some extent exhibit uneven surface topography on sputtering, due to the inherent instability of local primary beam interactions (Wilson et al., 1989). On very well-prepared specimens, defects on the scale of a single atomic layer (kinks, ledges, dislocations) may generate uneven sputtering. With optimum flatness, however, Adz values as low as 0.01-0.05 have been achieved. In most systems the relative depth resolution Adz is found to be nearly constant. However, at depths much in excess of the projected range of primary ion penetration, the dependence of Az on z has been observed to decrease, even approaching constant Az at continued sputtering. At the penetration of an interface in profiling, previously developed roughening may strongly affect the depth resolution. Regular cone formation has been observed. Even on good single crystals, progressive faceting may develop at particular orientations of the primary beam relative to the crystallographic axes. Such effects, as well as cone formation and preferential yield effects, can often be alleviated by the use of reactive gas backfill or reactive primary ions (see, e.g.,Wilson et al., 1989). 13.4.4.4 Methodological points in profiling (i)Even when the current density in the primary beam is very homogeneous, the sputtered crater is always more or less rounded at the edges. To ensure that simultaneously recorded ions all originate from one given depth, a flat crater bottom should be maintained, either by the use of a wide homogeneous beam, or by rastering the beam over sample surface. Even so, both the depth resolution and the dynamic range are impaired if one cannot avoid contamination by species originating from localities other than the flat crater bottom - from crater walls, from the specimen surface, from residual atmosphere in the sample chamber, or from material sputtered or evaporated from adjacent parts of the apparatus.

13 Dynamicmode secondary-ion mass spectrometry 225

@)In all quantitative SIMS profiling, gating by mechanical or electronic aperture is used to extract into the analyser only ions from a central part of the crater (see, e.g., Wilson et al., 1989). (iii)Impurities in the primary beam get implanted in the specimen and subsequently show up in the spectrum: typically a 1 ppm impurity in the beam will be spuriously recorded as about 0.1 ppm in the specimen. High purification of the gas or vapour in the beam is therefore essential. Ionised impurities in the primary beam are usually suppressed by use of, e.g., a Wien filter. To avoid also the neutral component of the beam, a bend in the beam column line is required; the impact of unfocused neutral species is reported to be a major limitation to the dynamic range in profiling (Wilson et al., 1989). The beam may also entrain evaporated atoms from the specimen, an effect which may to some extent be reduced by inert coating of the surface. (iv)Previously sputtered or evaporated atoms from the specimen, present in the residual atmosphere near the surface, may re-deposit in the crater and then be detected at the wrong depth. Optimum vacuum, combined with relatively high beam intensity, may (see eq. 13-15 ) minimise this effect. (v)The deposition of sputtered specimen material on adjacent mechanical and ionoptical details may cause serious memory effects. Efficient routines for cleaning and ‘baking’ are required. The erosion of the details themselves may bring selective contamination of the mass spectra. For sensitive analysis of, e.g., dopants in semiconductors it has been necessary to introduce an easily interchangeable extraction lens design and to manufacture all ion-optic details near to the specimen from a very pure refractive metal.

13.4.4.5 Depth scale determination As the sputtering rates vspdepend on several instrumental and matrix factors, they may differ even between two consecutive profiles in a given specimen. The problem is relatively trivial if a marker, such as, e.g., an interface, is available at a known depth in the specimen. In most cases, however, the crater depth must be measured, which can be done only after the profile has been completed. The measurement can be made either by optical fringe interferometry (for light-reflecting specimens) or by a mechanical stylus-type surface level monitor. If vspcan be expected to vary due to compositional changes, it may be necessary to sputter and measure several craters, to different depths, on a given specimen. Thus, for instance, for the profiles through corroded layers on nuclear waste glass, sputter rates had to be determined successively for the different precipitation and depletion zones, as well as for the uncorroded bulk glass (Lodding, 1992). In this particular case (Lodding et al., 1990; Wicks et al., 1994) the porous interaction zone, Po, was found to be sputtered about twice as fast as bulk glass. For comparison of different mass peaks in cyclic profiling, the successively recorded intensities have to be carefully interpolated to the ‘common depth’ within


each cycle, according to a switching-time algorithm (Odelius and Sodervall 1984; Sodervall 1991).

13.5 Imaging by SIMS When discussing surface mapping by SIMS, lateral resolution, Ay, may enter an analogy to eq. 13-18, replacing the duration of a profiling cycle by the exposure time, texp,of surface element AA w (Ay)', so that (cminx Az x Ay 2)-'

- Ksptexp-'

( 1 3- 19)

which again expresses the competition between sensitivity on one hand and topographic resolution on the other. In imaging, optimum reduction of (AY)~c,,,~, is required, which necessarily entails a sacrifice in depth resolution, via Adt,,, = v,,. As mentioned in an earlier section, two instrumental approaches to imaging may be distinguished: direct-imaging and beam-scanning.

13.5.1 Direct image SIMS Nearly all instruments capable of direct imaging have been developed (Rouberol el al., 1980; Rasser et al., 1994; de Chambost et al., 1992) from a concept proposed in France (Castaing and Slodzian, 1962). The lateral resolution is here limited by the ionoptical aberrations in the objective (emission lens). An increase in resolution leads to a steep decrease in transmission, which limits the practically obtainable lateral resolution to about 0.5 pm. A similar limit is imposed by the use of channel-plate detectors. Relatively wide (> ca 10 pm) primary ion beams, capable of high integrated current per exposure, are employed. Direct-imaging instruments have therefore asserted themselves, particularly in very sensitive element distribution studies with only moderate lateral resolution, as compared with narrow-beam ion microscopes. Very frequent applications of the direct mode of imaging have been reported during recent decades (see, e.g., Benninghoven et al., 1979-1994). As a rule, distributions of elements even at concentrations at the ppm level can be sharply imaged at exposure times of 10 s or less.

13.5.2 Narrow-beam imaging In scanning microprobes the resolution is given by the size of the primary beam. The usefulness of this mode is conditional on the availability of microbeams with sufficient current densities. The best obtainable lateral resolution (obtained with liquidmetal field-emission sources, (Levi-Setti et al., 1991, 1994)) is of the order of the dimension of a sputtering cascade, i.e. some 10 nm. However, the current density of a microbeam is seldom much higher than that available in the direct-imaging technique

13 Dynamic-mode secondary-ion mass spectromehy 227

(see Table 13-1). Thus the time-averaged flux of impinging ions in the raster mode is much lower than in the direct mode, and exposures are much more time-consuming. Microbeam-scanning SIMS imaging has produced excellent high-resolution micrographs despite of relatively limited element sensitivity. Examples of applications (Steiger et at., 1991; Levi-Setti et al., 1994) are shown in Figs. 13-7 and 13-8.

Fig. 13-8. High resolution SIMS mapping of one spread of photographic AgBr crystals. On the surface of the crystals is a mono-layer of a fluorine-containing molecule. 10 pm full-scale. Ga' primary ions. Upper left: Ag' map of the cubic crystals. Upper right: F- map, before erosion. Fluorine is preferentially deposited near the edges of the crystals. Bottom left: Topographic map (integrated secondary ions, ISI) of the AgBr cubes. The surfaces are not smooth, due to slight ion beam erosion.

13.5.3 Quantification of concentration distributions The imaging SIMS is mainly a microscope; quantification poses particular problems. On every point, or pixel, of the mass-resolved image the recorded brightness is dependent on the analytical parameters discussed in the previous section (y, S, q), each likely to exhibit lateral variations. Chemical enhancement, memory effects, etc., mentioned in connection with depth profiling, are equally important factors in surface mapping. Particular imaging artifacts include microscope relief contrast and depth


sharpness. In direct imaging, additional problems are given by local variations in sensitivity across the multichannel plate and in the viewing equipment. In principle quantification should take place at each point individually, with account taken of all such local factors. This has led to the development of digital ‘encoding’ methods for evaluating ion micrographs in terms of element concentrations. The latest progress in this area has been inspired by the fact that, basically, SIMS is a microanalytical as well as imaging technique in three dimensions. In early realizations of 3-dimensional SIMS, series of mass-resolved ion micrographs, taken at successive depths on a given analysed area, gave information element distributions in depth as well as laterally. However, such studies were in the main qualitative, and quite timeconsuming. More recently, the efficiency of 3-dimensional mapping has been greatly furthered by the introduction of on-line image acquisition and image processing (see, e.g., Odom et al., 1983; Grasserbauer and Hutter, 1994). Prompted by the requirements of industrial applications and precise quantitative research in many disciplines, modem SIMS is steadily developing in the direction of more sophisticated digital control and speed in operation.

References Andersen C.A., Hinthorne J.R. (1973), AnaLChem. 45, 1421. Benninghoven A., Evans Jr C.A., Powell R.A., Shimizu R., Storms H.A. (Eds)(1979), “SIMS 11”, Springer Ser.Chem.Phys. 8, Berlin-Heidelberg-New York. Benninghoven A,, Giber J., LBsz16 J., Riedel M., Werner H.W. (Eds) (1982), “SIMS Ill”, Springer Ser.Chem.Phys. 19, Berlin-Heidelberg-New York. Benninghoven A., Okano J., Shimizu R., Werner H.W. (Eds) (1984), “SIMS IV”, Springer Ser.Chem.Phys. 36, Berlin-Heidelberg-New York. Benninghoven A., Colton R.J., Simons D.S., Werner H.W. (Eds) (1986), “SIMS V”, Springer Ser.Chem.Phys. 44, Berlin-Heidelberg-NewYork-Tokyo. Benninghoven A,, Rudenauer F.G., Werner H.W. (1987), “Secondary Ion Mass Spectrometry”, Chemical Analysis Series 86, J.Wiley & Sons, New York. Benninghoven A,, Huber A.M., Werner H.W. (Eds) (1988), “SIMS VI”, J.Wiley & Sons, ChichesterNewYork-Toronto-Singapore. Benninghoven A,, Evans Jr C.A., Mckeegan K.D., Storms H.A., Werner H.W. (Eds) (1990), “Sims VII”, J.Wiley & Sons, Chichester-NewYork-Brisbane-Toronto-Singapore. Benninghoven A,, Janssen K.T.F., TUmpner J., Werner H.W.(Eds) (1992), J.Wiley & Sons, ChichesterNewY ork-Brisbane-Toronto-Singapore. Benninghoven A., Nihei Y., Shimizu R., Werner H.W. (Eds) (1994), “SIMS IX’, J.Wiley & Sons, Chichester-NewY ork-Brisbane-Toronto-Singapore. Bertrand P. (l996), “Static mode secondary ion mass spectrometry”, chapter in this book. Castaing R., Slodzian G. (1962). J Microscopie 1,395. Chabala J.M. (1992), Phys.Rev.B 46, 11346.. De Chambost E., Hillion F., Rasser B., Migeon H.N. (1992), in “SIMS V11I” (Benninghoven et al., 1992), 207. Dorner P., Gust W., Hintz M.B., Lodding A., Odelius H., Predel B. (1 980), Acta Met. 28,291. Drawin H.W., Felenbok P. (1965), “Data for Plasmas in Local Thermodynamic Equilibrium”, GauthierVillars, Paris. De Galan L., Smith R., Winefordner J.D. (1968), Spectrochim.Acta 23B, 521. Grasserbauer M., Hutter H. (1994), in “SIMS 1 X (Benninghoven et QI., 1994), 545.

13 Dynamic-mode secondary-ion mass spectrometry 229 Heinrich K.F.J., Newbury D.E. (Eds) (l975), “Secondary Ion Mass Spectrometry”, NBS Spec.Publ. 427, US Nat. Bureau of Standards. Larsson A., Jonsson B., Cody J.G., Anderson T.G., Sbdervall U. (1994), Semicond.Sci.Tech. 9,2190. Levi.Setti R., Crow G., Wang Y.L. (1985), Scanning Elec.Microsc. 85-11,535. Levi-Setti R., Hallegot P., Girod C., Chabala J.M., Li J., Sodonis A., Wolbach W. (1991), SurfScience 246,94. Levi-Setti R., Chabala J.M., Gavrilov K., Li J., Soni K.K., Mogilevski R. (1994), in “SIMS 1X’ (Benninghoven el al., 1994), 233. Liebl H.J. (1967), J.Appl.Phys. 38,5277. Lodding A. (l982), Ch.6 in “Reviews on Analytical Chemistry” (L.Niinisto, Ed.), Akadkmiai Kiad6, Budapest. Lodding A,, Odelius H., Clark D.E., Werme L.O. (1985), Mikrochim.Acta Suppl. 1 I , 145. Lodding A. (1988), Ch.4 in “Inorganic Mass Sopectrometry” (F.Adams, R.Gijbels, R.vanGrieken, Eds), Chem.Analysis Ser. 95, J.Wiley & Sons, NewYork-Chichester-Brisbane-Toronto-Singapore. Lodding A., Engstriim E.U., Clark D.E., Wicks G.G. (1990), in “Nuclear Waste Management” (G B Mellinger, Ed.), Cer.Trans. 9, ACerS, Westerville Ohio, 3 17. Lodding A. (1992), Ch.4 in “Corrosion of Glass, Ceramics and Ceramic Semiconductors” (D.E.Clark, B.K.Zoitos, Eds), Noyes Publ., Park Ridge NJ. Lodding A., van der Heide P.A.W., Stidervall U., Brown J.D. (1994), in “SIMS I X (Benninghoven et al., 1994), 32. Nomachi I., Satori K., Miwa S. (1994), in “SIMS I X ’ (Benninghoven et al., 1994), 670. Niirskov J.K., Lundqvist B.I. (1979), Phys.Rev. B 19, 5661. Odelius H., Sbdervall U. (1984), in “SIMS V” (Benninghoven et al., 1984), 31 I . Odom R.W., Furman B.K., Evans Jr C.A, Bryson C.E., Petersen W.A., Kelly M.A., Wayne D.H. (1983), AnalChem. 55,574. Odom R.W., Hitzman C.J. (1994), in “SIMS IX’ (Benninghoven et al., 1994), 593. Rasser B., Renard D., Schuhmacher M. (1994), in “SIMS IX’ (Benninghoven et al., 1994), 278. Rouberol J.M., Lepareur M., Autier B., Gourgout J.M. (1980), in “X-ray Optics and Microanalysis” (D.R.Beaman, R.E.Ogilvie, D.B.Wittry, Eds.), Pendell, Midland MI. Sigmund P. (1969), Phys.Rev. 184,383. Slodzian G., Chaintreau M., Dennebouy R. (1986), in “SIMS V” (Benninghoven et al., 1986), 158. Sodervall U. (l991), “Quantitative Applications of Secondary Ion Mass Spectrometry”, Thesis, Chalmers Univ. of Technology, Gothenburg. Siidervall U., van der Heide P.A.W., Brown J.D., Lodding A. (1996), Mikrochim.Acta, in press. Steiger R., Aebischer J.N., Haselbach E. (1991), J.Imag.Sci. 35, 1. Thompson M.W. (1968), Philos.Mag. 18, 377. Vedenyev V.I. (1 966), “Bond Energies, Ionization Potentials and Electron Affinities”, E.Arnold, London. van der Heide P.A.W. , Ramamurthy S. ,McIntire N.S. (1999, Surf. Interf. And 23, 163. Waugh A.R., Bayly A.R., Anderson K. (1984), Vacuum 34, 103. Wicks G.G., Lodding A,, Molecke M.A. (1994), MRS Bulletin XVIII-9, 32. Williams P. (l992), in “Practical Surface Analysis” (D.Briggs, M.P.Hsieh, Eds.), J.Wiley & Sons, New York. Wilson R.G., Stevie F.A., Magee C.W. (1989), “Secondary Ion Mass Spectrometry; a Practical Handbook for Depth Profiling and Bulk Impurity Analysis”, J.Wiley & Sons, New York. Yu M.L. (1994). in “SIMS IX” (Benninghoven et al., 1994), 10.

A cknowlegements We are indebted to Dr Jan Chabala for providng valuable application examples at our disposal.



14 Glow-discharge optical-emission spectroscopy A. Bengtson

14.1 Introduction Glow Dischurge is a general term for low-power electrical discharges in reduced pressure systems. A large variety of light sources, some of which are familiar from everyday life, can be classified as glow discharge devices. Among these, the ordinary fluorescent tube and neon lights are perhaps the most well known. A glow discharge device consists of a vacuum vessel, within which two physically separated surfaces form a cathode and an anode, see Fig. 14-1. The geometrical shape of the vessel and electrodes can be varied almost infinitely; the figure only illustrates the basic concepts in a simplified way. In a direct current (DC) glow discharge, which is the most common type, the electrodes must be electrically conducting. The vacuum vessel is evacuated and filled with a discharge gas to a pressure ranging from about 10 to 1000 Pa. Practically any type of gas can be used, but noble gases are commonly used in order to avoid chemical reactions inside the device during operation. In order to initiate the discharge, the voltage across the electrodes must exceed a minimum value. The electric field will accelerate electrons leaving the cathode in the direction of the anode, leading to electrical breakdown of the gas (plasma formation) and a flow of current between the electrodes. Once the plasma is established, the operating voltage can often be reduced substantially. The current is low to moderate, varying between approximately 0.1 mA and a few ampere. The glow-discharge plasma is physically divided into more or less distinct regions (see Fig. 14-1). In the cathode dark space, which typically extends a few mm into the gas, the electrons lose most of their kinetic energy through collisions with gas atoms, and practically all of the potential drop takes place here. This region is comparatively dark because the cross-section for electron excitation of the atoms is small at high electron energies. The cross-section for ionization is also small at high electron energies, and relatively little ionization occurs in the dark space. Beyond this region, one finds the intensely luminous negative glow, which varies from several mm to a few cm in length depending on the design of the device. Here, the average electron energy is typically 20-30 eV and the cross sections for ionisation and excitation are at a maximum. The actual degree of ionisation varies, but very seldom exceeds a few tenths of a percent. If the distance between the electrodes is sufficiently long (as it is in a neon tube), the negative glow is followed by a slightly less luminous region known as the positive column, which extends all the way up to the anode, In all of this region, the electrons have energies that are essentially thermal and the plasma is nearly at anode potential. Depending on the voltage-current characteristics, a glow discharge is classified as normal or abnormal. A normal glow discharge is characterized by a constant voltage

14 Glow-discharge optical-emission spectroscopy

23 1

which is independent of the current, which is the case when the current is kept very low, typically < 1 mA. As the current is increased, the discharge gradually takes on an Ohmic voltage-current characteristic, the voltage being proportional to the current. The discharge is then classified as abnormal. As the current is further increased, the voltage suddenly drops to a very low value. At this point, the discharge changes characteristics from a glow discharge to what is known as an arc discharge. In recent years, glow discharge devices which are energized by a radio frequency (RF) alternating electric field, rather than a direct current, have become more common as sources for analytical spectrometry. The major advantage of these devices is their ability to sputter non-conducting materials, which extends the range of applications for Glow Discharge Optical Emission Spectroscopy (GD-OES) enormously. The major fields of applications of RF glow discharges to date have been in bulk analysis, but there is an increasing interest in depth profile analysis of non-conducting surface layers such as paint. For more detailed information about the fundamentals of glow discharges, including RF varieties, the interested reader is referred to Markus (1 993) and Bogaerts et al. (1994).

Negative glow

\

Cathode dark space

Positive column

Fig. 14-1. Principal layout of a glow discharge device.

14.1.1 Sputtering in glow discharges By the beginning of this century, physicists had already discovered that the cathode in an electrical discharge is eroded or etched. Before the advent of modem atomic theory, this effect could not be properly understood. We now know that this erosion is caused by the bombardment of ions and fast neutral species from the plasma; the term for this effect is cathodic sputtering. The sputtering of the cathode is the very basis of the usefulness of glow discharges as tools for surface and depth-profile analysis. Devices can be designed where the sample to be analysed forms the cathode, and the continuous erosion of the sample surface by the discharge is used for elemental depthprofile analysis.


In a glow discharge, the bombarding particles do not have a well defined energy, as is the case when an ion beam in a high vacuum is used for sputtering. This is because the mean free path for atoms and ions is only of the order of 0.01-0.1 mm, which means that the bombarding particles will have suffered energy losses due to collisions before reaching the cathode. As a rule of thumb, the average energy of the ions striking the cathode is around 10% of the voltage drop across the electrodes, but the energy spread of individual ions covers the entire potential drop. The short mean-free path also has the consequence that a substantial fraction of the sputtered atoms return to the cathode surface, where they can be redeposited or cause ‘self-sputtering’. As the discharge is switched on, a ‘dynamic equilibrium’ between sputtering and redeposition processes is rapidly established, and the sample is eroded at an ‘effective’ sputtering rate, sometimes referred to as the mass-loss rate. Despite of the complexity of the sputtering processes involved, the mass loss rate in several types of glow discharge device can be accurately described by the following simple empirical expression: qb = Cabi(U-UOb)

(14-1)

where qb = the mass loss rate of sample b; i = the current; U = the voltage; and Cab, U O ~ are sputtering rate constants characteristic of the sample b. C a b is related to the sputtering yield (probability of a sputtering event per ion impact) of the material; UOb is a threshold voltage, below which the sputtering is reduced to zero. Eq. 14-1 shows that the rate of sample loss increases linearly with current and voltage, as is illustrated graphically by plotting the reduced sputtering rate qb/i against voltage (see Fig. 14-2).

f

400

300

0 200

p!

0

200

400

600

Volt

800

1000

I

1200

Fig. 14-2. Reduced sputtering rate vs voltage, obtained with a glow discharge source of the G r i m type.

Also evident from Fig. 14-2 is the fact that the sputtering rate varies considerably with the material. The relative differences in sputtering rates observed between different materials closely follow results obtained by sputtering with high-energy ion beams.


233

14.1.2 Optical emission in glow discharges As was stated at the beginning of this chapter, glow discharges are frequently used as light sources. The light, or optical emission, originates from atoms and molecules in the plasma which are energetically excited by means of inelastic collision processes. More strictly, optical emission is here defined as emitted electromagnetic radiation in the wavelength range 100-1000 nm, comprising the ultraviolet, the visible and the near-infrared wavelength ranges. Most of the excitation processes giving rise to the optical emission result from collisions with electrons. However, many other types of processes are also of considerable importance, such as charge and energy transfer in ion-atom, ion-ion and atom-atom collisions. The excitation of free atoms and ions to higher electronic energy levels gives rise to optical emission with a wavelength spectrum which is characteristic of the species present in the plasma. Since sputtered atoms from the cathode diffuse into the plasma, and there take part in the emission process, the elemental composition of the cathode can be determined by means of spectral analysis of the optical emission.

14.2 Spectrometer systems for depth-profile analysis based on the Grimm-type source In the majority of publications on depth profile analysis by GD-OES, the GD sources used are based on the design of Grimm (1968). Although originally intended as a source for routine bulk analysis of solid metal samples (for which purpose it works splendidly), the Grimm GD has also proven to be an excellent practical tool for depth profile analysis. The principal layout of this source is shown in Fig. 14-3. The lamp body is normally at ground potential, and the water-cooled, isolated front plate is at negative potential during operation. The tubular-shaped anode fits tightly into the central opening of the front plate, and extends to within approximately 0.2 mm of the front surface. The inner diameter of the ano'de is typically 8 mm, but anodes of 7, 5, 4, 2.5 and 1 mm diameter have also been used. The sample, which forms the cathode, is placed on the front plate, and sealed from the atmosphere by an O-ring of relatively soft material. When the source is operated, the lamp house is first evacuated for a few seconds by a rotary pump (pump-down), after which an argon flush valve is opened. After a few seconds to stabilize the pressure (preflush) at approximately 100 Pa, the discharge is initiated by applying a voltage of typically 500-1000 V. With an 8 mm anode, the operating current is normally in the range 30-100 mA. The geometry of the source restricts the discharge physically to the inside of the anode tube (obstructed discharge). This is because the distance between cathode and anode is smaller than the mean-free path-length of the electrons at the operating pressure. The sample surface, which forms the cathode of the device, is continuously eroded by the cathodic sputtering process during operation. Before being adsorbed on a cold surface inside the source, a substantial fraction of the sputtered atoms diffuse into the negative


Grimm type Glow Discharge Source

Fig. 14-3. Schematic diagram of the Grimm-type glow-discharge source.

glow region of the plasma, where they are excited and produce element-characteristic optical emission. With a relatively high current density of 50-500 mA cm-2, the penetration rate in a Grimm type GD is typically in the range 1-10 pm mid'. The homogeneous electric-field distribution in the active region ensures that the sample surface is sputtered fairly evenly, resulting in a crater with a nearly flat bottom (Fig. 14-4). .--

I

?-

-I-

v

_J

Fig. 14-4. Profilometer trace of sputtering crater in copper.

Thus, by recording the optical emission signals as a function of sputtering time, an elemental depth profile is obtained. To enable full use to be made of the vast amount of analytical information provided from the glow discharge, the spectrometer must be able to sample the emission spectrum at a rate of at least 100 times per second. As a


23 5

consequence of this, multichannel spectrometers are most commonly used. A spectrometer of this type has a fixed grating, and is equipped with individual photomultiplier detectors for up to 60 analytical spectral lines (wavelengths), determined by exit slits in fixed positions. Depending on the analytical applications, one (occasionally two) spectral line for each element of interest is selected for the fixed analytical program. To accommodate sensitive spectral lines for some of the light elements (C, s, P, N), the spectrometer must be of a vacuum type, since the most sensitive lines for these elements are found in the vacuum UV spectral region below 200 nm. For increased flexibility, a complete spectrometer system often incorporates some scanning device, e.g. a monochromator with fixed detector and movable grating. In commercially available GD-OES systems, the source is normally fixed to the spectrometer to form one integrated unit. A schematic diagram of a GD-OES spectrometer system, based on a Grimm-type source, is shown in Fig. 14-5.

CONCAVE HOLOGRAPHIC GRATING

PHOTOMULTIPLIER

GLOW DISCHARGE SOURCE

Fig. 14-5.Schematic diagram of a multichannel GD-OES spectrometer system (courtesy of LECO Corporation).


14.3 Qualitative and quantitative depth-profile analysis by GD-OES When a sample is analysed by a GD-OES technique, the primary data obtained are in the form of intensities as a function of sputtering time from each of the elemental detectors. This information is normally presented in diagrams, which represent elemental depth profiles through the corresponding surface layers in a qualitative way (see Fig. 14-6). The word qualitative is used in order to emphasize that the data presented in this way are not quantified into concentration according to depth, which normally is the desired information. However, for many applications a qualitative depth profile is quite sufficient. This is often the case when the analytical problem is simply to compare ‘good’ samples with ‘bad’ as, for instance, when trouble shooting in a production process. 4000

7

3500 3000

.-b 2500 2

-3

2000

C

1500

1000

500 0

___1

0

10

20

30

40

50

Time (s)

60

70

80

90

100

Fig. 14-6. Qualitative depth profile through a ZnNi layer on steel.

14.3.1 Quantification of GD-OES depth profiles - an introduction As was pointed out above, the basic information obtained in a GD-OES depth profile is the relative intensity of the elemental detectors as a function of sputtering time. The quantification problem can be separated into two parts: 1) elemental concentrations; and 2) sputtered depth. In optical-emission analysis, it is generally not possible to quantify the recorded signals on the basis of fundamental physical principles; the uncertainties involved in such an approach are simply too large. Instead, quantification must rely on the use of standards, or calibration samples of well known chemical composition. This situation is not unique to GD-OES, but common to most types of spectrochemical analysis. Stan-


231

d a d s in the form of a homogeneous bulk material, or as substrates with a well characterized surface coating, can be used. For elemental concentrations, standard OES methods for accurate quantification of bulk materials have existed for decades. However, these methods are of limited use in depth-profile analysis for a number of reasons. In bulk analysis, homogeneous samples of a quite well defined matrix composition are measured. Calibration of the analytical system is carried out with a set of calibration samples having compositions similar to the unknown samples. Normally, a reference channel of the major element is used as an ‘internal’ standard. Consequently, a separate calibration is necessary for each material (alloy) type to be analysed. In depth profile analysis this approach is generally not applicable, since the different layers encountered in a depth profile often represent widely different material types (matrices). This fact can also be expressed by saying that depth profile analysis is a multi-matrix analytical problem. A further complication concerning depth profile analysis, as opposed to bulk analysis, is that the electrical parameters of the source (voltage, current) often vary considerably as layers of different composition are penetrated. Such variations may have a significant influence on the signal intensity, and must be accounted for in a complete quantification method. For quantification of the sputtered depth, much effort has been spent on measurements of the sputtering rates in different materials. Most of this work is based on eq. 14-1, which describes the sputtering in a Grimm type source very well. According to eq. 14-1, the effective sputtering rate in a glow discharge at any excitation condition can be described by the two material-dependent constants Cab’and UOb (see Fig. 14-2). The extensive data base for such constants existing in the literature forms a solid basis for depth calibration, but this is not sufficient for a generally applicable quantification model. The main reason is that it is not practical to measure the sputtering rates of all the vast number of material compositions that occur in real depth profile applications. So, it is clear that a general quantification method for GD-OES elemental depth profiles cannot be based on standard bulk analytical techniques, combined with sputtering rate measurements of reference materials.

14.3.2 The emission yield concept as a basis for quantification Experimentally, it is easy to show that the emission intensity of an element is proportional not only to the concentration of that element but also to its sputtering rate, Intuitively, this observation is easy to accept; the emission intensity should be proportional to the sample atom density in the plasma, which in turn should be proportional to the sputtering rate. If we allow the voltage, current and pressure to vary, the situation becomes more complex. However, if the excitation conditions are fixed, the sputtering rate-intensity proportionality provides for an elegant solution to the quantification problem based on the concept of emission yield, which can be defined as the emitted light per unit sputtered mass of an element. Mathematically, this leads to the relationship: ( 14-2)


where 6w, is the sputtered mass of element n during time increment 6t; I,, is the emission intensity of spectral line m of element n; and R,,, is the emission yield of spectral line m of element n, which is an atomic- and instrument-dependent quantity. Eq. 14-2 is equivalent to: (14-3) where c, is the concentration of element n in sample segment b; and qb(=6wn/6t)is the sputtering rate in sample segment b. Eqs. 14-2 and 14-3 express the assumption that the integrated intensity from one element (and spectral line) is proportional to the sputtered mass of that element, which implies that the emission yield Rnm is independent of the sample matrix (the major elements that make up the sample). This has been investigated by several authors, and is by now widely accepted to be valid, at least to a first approximation. The emission-yield concept as a basis for quantification of depth profiles was first proposed in 1984 by Pons-Corbeau and others from IRSlD in France in Takadoum et al. (1984). However, this concept was first put to effective use for complete quantification (concentration vs depth) by Japanese workers (Takimoto et al., 1987). The method is essentially based on eq. 14-2, which may be augmented by a background correction term. Calibration is effected by determining the emission yields %, by means of calibration samples. The samples may be of bulk type with known concentrations, in which case it is necessary to determine the sputtering rate of each calibration sample. Alternatively, samples with coatings of known composition and thickness may be used. When measuring an unknown sample, the primary measurement obtained is the sputtered mass of each element per time increment. The total sputtered mass of the sample is obtained as the sum of all elements, and the concentrations are easily calculated as fractions of the sum. Conversion to sputtered depth requires a calculated estimate of the density of the material, which introduces some uncertainty. One method frequently used to accomplish this is by summing over the fractional volumes of each pure element, using tabulated data. Alternative algorithms are possible, and will be discussed in a later section of this chapter. The emission-yield model, as described in the literature, does not incorporate any compensation for variations in the excitation parameters, voltage and/or current. Consequently, calibration and measurements must be carried out under nearly constant excitation conditions for quantitative results to be accurate.

14.3.3 The SIMR quantification method and the empirical intensity expression At the Swedish Institute for Metals Research (SIMR), a quantification method has been developed on the basis of the sputtering rate-intensity proportionality expressed in eq. 14-3, rather than the concept of emission yield. Obviously, since eqs. 14-2 and 14-3 are equivalent, the choice between them is mainly a matter of practical considerations. There are two major reasons why eq. 14-3 was selected as the basis for the SIMR


239

model: 1) calibration can be effected by regression analysis applied to a plot of concentration vs intensity, which is the standard procedure in bulk analysis; and 2) compensation for intensity variations due to voltage and current becomes more straightforward in a direct intensity expression. 14.3.3.1 The empirical intensity expression From practical experience with GD-OES systems, it is well known that different applications require different settings of the excitation parameters, current and voltage. Furthermore, one of these parameters will normally vary during the course of an analysis, at least for sources lacking an active pressure-regulation system. In several applications, Zn-coated steels, for instance, these variations can be substantial. For these reasons, it was considered desirable to devise a quantification method capable of compensation for variations in the excitation parameters. Intensities from a large number of reference samples were measured while the voltage and current were systematically varied. The results were fitted to different mathematical models, leading eventually to the empirical intensity expression: ( 14-4)

where k, is an atom- and instrument-dependent constant characteristic of spectral line m; A, is a matrix-independent constant, characteristic of spectral line m only; U is the voltage and f,(U) is a polynomial of degree 1-3, also characteristic of spectral line m. Eq. 14-4 embodies both the sputtering-rate intensity dependence from eq. 14-3 and the direct influence of voltage and current on the excitation processes. The currentdependence is exponential; experimentally determined values of A, are in the range 1.O to 2.5, with a remarkably large proportion relatively close to 2.0. This means that to a first approximation, the intensity increases as the square of the current. The voltagedependence is most conveniently modelled by a polynomial. Experimentally, it has been found that for a large number of analytical lines the intensity increases approximately as the square root of the over-voltage (U-UO) from eq. 14-1, although the threshold voltage UOdoes not appear explicitly in eq. 14-4. Constants A, and f,(U) for a large number of analytical lines have been measured in several laboratories. Tables of these constants are made available by manufacturers of GD-OES spectrometers. 14.3.3.2 The SIMR quantification method Based on eq. 14-4, a quantification method has been developed which compensates for variations in sputtering rate, voltage and current. The method requires that a reference excitation condition for calibration measurements is defined. For the 8 and 7 mm lamps, 60 mA and 700V has been selected; for the 4 mm lamp 20 mA and 700V. These conditions can be changed if necessary. For certain applications other conditions may be more appropriate. Furthermore, a reference matrix (material type) with a wellknown sputtering rate must also be defined. In the standard software, low-alloy steel is defined as the reference, but essentially any material type can be used. The calibration


procedure and quantification calculation are executed according to the scheme described below.

14.3.4 From qualitative to quantitative profil'es - some basic considerations A qualitative depth profile is more or less distorted with respect to the true profile, in basically two respects. First of all, variations in the sputtering rate mean that the depth is not linear with the time scale. An extreme example of this type of nonlinearity occurs in a steel sheet coated by several Zn-based layers, where the sputtering rate may be 5-8 times higher in the coating than in the steel substrate. Secondly, since the emission intensity is proportional to the sputtering rate, the variations in sputtering rate also cause distortion of the apparent concentration variations. In several applications this effect is not quite obvious, since the elements that make up surface coatings are often almost nearly non-existent in the substrate material. However, in the case of ZnFe (Galvanneal) coatings on steel, the distortion in apparent Fe concentration through the profile is very obvious, as is the previously mentioned non-linearity in the depth vs time scale (Fig. 14-7). 100

a0

4000

3000 2000 1000

0

20

0

50

0

100

Time (s)

0

2

4

6

8

ia

Pm

Fig. 14-7. Depth profile through a Zn Fe coating on steel. Qualitative picture to the left, quantitative picture to the right.

14.3.4.1 Analytical figures of merit For any instrumental technique, the analytical figures of merit vary considerably depending on the application and the type of instrument used. Furthermore, technical development tends to improve the performance of instruments, rendering published data obsolete very quickly. Despite these reservations some analytical figures of merit for state-of-the-art GD-OES are given in Table 14-1 .


24 1

Table 14-1. Analytical figures of merit for GD-OES. Figures of merit Lower limit of detection (ppm) Minimum detectable number of atoms (atoms cm-2) Minimum information depth Relative instrumental depth resolution Penetration rate Short term precision (major and minor elements)

1-100 1013-1015

lnm < 10% 1-100 nm s-l < 1% RSD

A further drawback of the data presented here is that the lower limit of detection cannot normally be attained at the minimum information depth.

14.4 Applications of quantitative depth-profile analysis The number of documented applications of quantitative depth-profile analysis by GD-OES is large, and rapidly increasing. Metal coatings of various types make up the majority of the published applications, but the technique has also been successfully applied to oxides, nitrides, and several other non-metal coatings. A few of the more common applications are discussed below.

14.4.1 Electroplated galvanized steels Because of their excellent corrosion resistance, various types of galvanized sheet steel are used extensively in the automotive, home appliances and construction industries. The plating may consist of pure Zn or some Zn-based alloy with coating thicknesses in the range 5-20 pm. These types of material are technically and economically the most important applications of GD-OES depth profile analysis to date. One reason for this is their very high effective sputtering rate (up to 10 pm min-’), enabling complete depth profiling of these rather ‘thick’ coatings within minutes. Compared with almost any other type of equipment for depth-profile analysis, GD-OES is exceptionally fast, giving it the capability for production control in a busy factory laboratory. Another factor contributing to the suitability of the technique for this type of application is that the requirements for sample preparation are kept to a minimum.

14.4.1.1 ZnNi electroplated steels This type of material, which has found widespread use in the automobile industry, is illustrated in the form of a qualitative profile in Fig. 14-6. The same profile as quantified is shown in Fig. 14-8. The major analytical problem is to determine the thickness, or coating weight, and the Ni content of the coating. Surface contamination (mainly hydrocarbons) may also be of interest; in particular contamination on the original steel surface, appearing as enrichments in the Zn/Fe interface, might be studied.


14.4.1.2 Galvanneal This type of material is produced by electroplating steel with pure zinc, followed by annealing at a temperature which enables all of the zinc to form ZnFe intermetallic phases. This is an example of a conversion coating, which is claimed to have wear and corrosion resistance superior to that of the original zinc coating. The major analytical problems are to determine the coating thickness and the variation of Fe content with depth. Qualitative and quantitative depth profiles are shown in Fig. 14-7.

w

20

0

0.00

zoo

4.m

8.00

8.00

1o.w

Inn

Fig. 14-8. Quantitative depth profile of ZnNi electroplated steel.

14.4.3 TiN and other non-metallic coatings on steels and hard materials Investigations of Physical Vapour Deposition (PVD) and Chemical Vapour Deposition (CVD) type coatings are becoming increasingly important analytical applications of quantitative GD-OES. TiN, TiCN, CrN and other nitridic compounds are examples of such ‘high-tech’ surface coating materials used in order to improve the wearresistance and other properties of cutting tools. In order to control the production of such coatings, there is a need for analytical techniques capable of rapid and quantitative depth-profile analyses. Fig. 14-9 shows an example of an essentially stochiometric TiN layer on tungsten carbide. A carbon peak at a depth of 3-4 pm reveals the existence of a thin intermediate layer of TiCN. The technical reason for introducing the intermediate layer in this case is probably to increase the mechanical strength of the film. In other cases, multilayer structures are produced mainly in order to improve the corrosion resistance of the material. An example of such a structure with 15 sublayers of alternating TiN and TiAlN on steel is shown in Fig. 14-10. This example also illustrates the fact that the practical depth resolution which can be attained by GD-OES is quite impressive, considering that the analysed area has a diameter of several mm.


243

"T

10

0

i

0

5

10

20

15

30

25

urn

Fig. 14-9. Quantitative depth profile of a TiN-coated hard material.

_ _ _ -n -- --

Fs2

N3

0

Ar5

0

2

4

6

8

10

12

t

-I

14

16

urn

Fig. 14-10. Quantitative depth profile of a multilayer TIN and TiAlN coating on tool steel.

References Bengtson A. (l994), Spectrochim, Acta, Vol. 49B, 4 11-429. Bengtson A,, Eklund A,, Saric A. (1991), J. Anal. At. Spectrom. 5 , 563. Bogaerts A., van Straaten M., Gijbels R. (1 994), J. Appl. Phys. 77,5. Boumans P.W.J.M. ( 1972), Anal. Chem. 44, 12 19. Grimm W. (1968), Spectrochim. Acta 23B, 443. Markus R.K. (ed) (1993), Glow Discharge Spectroscopies. Plenum Press, New York. Takadoum J., Pirrin J.C., Pons-Corbeau J., Berneron R., Charbonnier J.C. (1'984), Surf. Interf. Anal, 6, 174. Takimoto K.,Nishizaka K., Suzuki K.,Ohtsubo T. (1987), Nippon Steel Technical Report 33,28.



15 Nuclear reaction analysis H. Whitlow and R. Hellborg

15.1 Introduction Nuclear Reaction Analysis (NU)is a generic term for a group of analytical methods that utilize characteristic nuclear reactions to measure the concentration of different isotopes in the target. Although any form of prompt, characteristic, nuclear reaction may be used, reactions in which the incident particle is a MeV ion (e.g. p, d, 4He) are the most commonly used. The ions are produced by a MeV ion accelerator which are discussed briefly in another chapter by Whitlow and Ostling. The term 'prompt' means that the reaction takes place within a microsecond or so of the incident particle impinging on the target nucleus. This distinguishes the technique from charged particle activation analysis (CPAA) discussed by Strijckmans in another chapter where the irradiation forms radionuclides which decay over periods of seconds to years. The fact that characteristic nuclear reactions are used as the basis of the analysis implies that NRA methods are almost completely unaffected by chemical effects and furthermore they measure the concentration of a specific isotope. This makes NRA methods particularly useful in measurements of surface phenomena using tracers of stable lownatural-abundance isotopes. NR4 is very closely related to Rutherford Back-scattering Spectrometry (RBS) and Recoil Spectrometry (Whitlow and Ostling, 1996). The RBS and NRA techniques can be considered to be complementary because NRA is best suited for depth profiling and analysis of light elements such as B, C , N whilst RBS is better suited for depth profiling and analysis of medium-heavy and heavy elements. The main focus of practical application of NRA methods has been to study carbide, hydride nitride and oxide films on metals and semiconductors that result from oxidation, processing with ion- and plasma-beams and hydrogen changing.

15.2 Fundamentals Nuclear processes such as the reactions used in NRA are by convention written in a shorthand form: A(a,bc)D. Here, A, denotes the target isotope, a, the projectile, b and c denotes a reaction in which the products and D the residue. For example, 19F(p,y)2%Je protons incident on 19Fform ' h e in an excited state which subsequently emits a y-ray photon in a characteristic decay to the ground state. p +19F +%e*

+'%e+y

A more complex two-step reaction is 13N(p,ay)'ZC in which,

15 Nuclear reaction analysis

245

p +15N-+160*+a+12C*;'2C*+'2C+ 4.43 MeV y Here the excited state 12C* subsequently decays to the ground state by y emission The reaction may be brought about by a light particle incident on the heavier target nucleus (e.g. "N(p,ay)12C ) this is termed the forward reaction. Alternatively, in the equivalent inverse reaction (e.g. 'H('5N,ay)'2C ) the heavy 15N ion is incident on the lighter IH target nucleus. Inverse reactions are particularly useful for characterising H, D and He isotopes. For the nuclear reaction A(a,bc)B the yield Y of reaction products b,c is given by: YE = TN A

10

(do/dQ)

(15-1)

where NAis the number of target atoms of type A, 10 the number of incident particles and d d d Q the differential cross-section of the reaction. The differential cross section is integrated over the solid angle Qdet subtended by the detector with efficiency q. Suitable nuclear reactions for NRA and their differential cross-sections are tabulated in a number of works (for example Tesmer et al., 1990). The differential cross section can be a function of energy and scattering angle. Three different forms of energy dependence are commonly used for NRA of materials. These are illustrated schematically in Fig. 15-1.

Threshold reaction

%

Energy

p

c

t

i Continious o n

1

Energy

Fig. 15-1. Schematic illustration of different forms of differential cross-section used in NRA.

15.3 Particle-induced gamma emission (PIGE) Particle-Induced Gamma Emission (PIGE) is very similar to Particle-Induced X-ray (PIXE) analysis, the principle difference being that a Ge detector is used to measure the characteristic y-ray photons from light elements. The technique which is discussed in detail in another chapter by Malmqvist, often employs nuclear reactions with a crosssection of the threshold and continuous type (Fig. 15-1).


15.4 Depth profiling with NRA 15.4.1 Depth profiling with resonant nuclear reactions The use of resonant nuclear reactions for depth profiling is often termed resonant nuclear reaction analysis, or Nuclear-Resonance Broadening (NRJ3). This technique exploits a nuclear reaction with a narrow resonance to sample the concentration in a succession of thin layers below the sample surface. The sample is measured using an experimental set-up similar to that shown below in Fig. 15-2. The incident particles are produced by a ion accelerator and directed at the sample. The number of characteristic particles from the nuclear reaction is registered using a suitable detector. (In Fig. 15-2 a scintillation detector is depicted and is arranged so as to detect characteristic y-rays over a large solid angle, however a large-area Si detector could just as well be used to measure charged particle reaction products.)

Sample Evacuated

Ion from accelerator

Fig. 15-

Discriminator

Experimental con..guration for depth profiling using resonant nuclear-reaction analysis.

The basic principle of nuclear reaction analysis with charged particles is illustrated in Fig. 15-3. Consider the case of a resonant reaction where the differential crosssection (do/dQ) has a resonant form such as that shown in the top right of Fig. 15-3. Essentially the reaction can only take place if the energy of the incident ions lies within the energy window at an energy Ere, spanned by the resonance. Outside this energy window it is very improbable that the reaction will take place. Consider an ion beam with energy El which is greater than the resonance energy E,, for the reaction A(a,bc)D. As the ions, of type a, penetrate the material they will lose energy because of nuclear and electron scattering. After penetrating to a depth x1 given by:


247

(15-2)

the ions will have lost energy and the amount remainingwill be equal to the resonance energy E,,,. It follows from eq. 15-1 that the yield of reaction products, bc, is then proportional to the number of target nuclei of the isotope, A, within the depth interval spanned by the resonance window through eq. 15-1. If the sample is bombarded with an energy EZgreater than El, the ions have to penetrate to a greater depth to reach Ere, where the reaction can take place (Fig. 15-2). This enables the concentration of isotope A to be sampled at different depths within the surface layer by measuring the reaction product yield as a function of energy. (Fig. 15-4.) NRA with resonance reaction has found wide application for depth profiling.

Ion Beam

Fig. 15-3. Principle of nuclear reaction analysis using resonant reactions.


FRESH WILLOW -- 0 - -Wear

\

\

Neutral surface trace

\

t 3

7.0

E (MeV) Fig. 15-4. Resonant nuclear reaction analysis of the hydrogen (water) depth profile from the surface of the cutting edge and a neutral surface of a modern flint tool that had been used to debark willow branches . The sample was covered with Au prior to analysis to prevent surface charging. The peaks at 6.5 and 6.6 MeV correspond to hydrogen at the Au surface and at the Adflint interfaces respectively. The alteration in the surface layer resulting from cutting extends to about 0.7 prn corresponding to an energy of7.8 MeV (From Andersen and Whitlow, 1983).

NRA with resonant nuclear reactions is particularly suitable for depth profiling light elements in heavier substrates. Low Pressure Chemical Vapour Deposition (LPCVD) is today a common industrial technique for deposition of refractory metals on semiconductors and other materials. Fig. 15-4 illustrates the fluorine depth profiles determined by resonant NRA using the 340 keV resonance in the " F ( ~ , a y ) ' ~reaction. o The samples were 100 nm thick W films deposited on Si using the LPCVD reaction and subsequently heat-treated at different temperatures:

WF6+ 3Hz--> 2W + 6HF

249


0 1

Depth in W h m ) "

'

A

I

100 I

12

)8 I

,\"

+ m

3

c

)4

U

I

0 Proton Energy (keV)

Fig. 15-5.Fluorine depth-profiles in a 100-nm thick LPCVD tungsten film on a silicon substrate subject to heat treatment at different temperatures. The depth profiles were measured using the 340 keV proton energy resonance in the 19F(p,ay)'60nuclear reaction. The solid circles, triangles, squares and diamonds denote the as-deposited, and 1 h anneals at 500, 600, 700 and 800 "C, resp. (From Whitlow et uf.,1987).

Evidently considerable amounts of fluorine are trapped at the WlSi interface even after heat-treatment at temperatures up to 900 "C where silicide formation has taken place. Fig. 15-5 Illustrates the use of the "N('H,cx~)'~Cinverse nuclear reaction to study the change in the water content of the cutting edges of flint knives associated with the use of the knife to work different materials. The study was undertaken to try to investigate the origin of the different wear traces used in archaeometric studies of the use of a flint tool. The sample was coated with a thin layer of Au to render the surface conductive. I5N2+ions from a tandem accelerator were directed at the knife edge and the characteristic 4.43 MeV y-rays were detected using a NaI (Tl) scintillation counter. The surface corresponds to a I5N energy of -6.5 MeV. Working fresh willow evidently leads to incorporation of a considerable amount of hydrogen in the surface. This might be associated with deposition of silica from silica-gel formed from the deposits in plant cells under the pressure and temperature of cutting with the knife edge. Working fresh


hide resulted in very different H profiles that were only enhanced in hydrogen (water) content at the very surface.

15.4.2 Depth-profiling with threshold nuclear reactions The principle of depth-profiling with threshold nuclear reactions is very similar to NRA with resonant reactions. The yield of the reaction is measured vs the beam energy as the beam energy is increased in a stepwise manner from the threshold energy. The change in the yield can then be determined from the increase in the yield with energy. The technique is difficult to apply because the changes in the signal may be swamped by the statistical uncertainty in the signal. It has only been used for particularly troublesome systems where no suitable resonant or continuous reactions exist. (e.g. tritium profiling).

1

I

I I

I

sl(

R

aRorber f0ll detector

Fig. 15-6. Geometry €or charged particle producing nuclear reaction analysis (the insert is a schematic diagam of the collision).


251

15.4.3 Depth profiling with charged-particle-producing reactions The equipment required for .NRA by charged-particle detection is basically the same as for RBS (see chapter by Whitlow and Ostling), i.e. an MeV accelerator, a scattering chamber, a Si charged-particle detector, standard nuclear electronics, and a multichannel analyser are needed. Resonant, continuous and threshold nuclei reactions may be used, see Fig. 15-1. The geometry for a nuclear-reaction experiment producing charged particles and a schematic representation of the collision process is shown in Fig. 15-6. The energy of the reaction products is usually higher than the energy of the incident beam, therefore the reaction products are normally well separated from the backscattered particles. Usually the cross-section of the nuclear reactions is much smaller than the back-scattering cross-section and therefore a longer measuring time, or higher beam current is necessary to obtain comparable statistics. More than one reaction can take place for a certain nucleus which results in different particles. The spectra can, therefore, sometimes be difficult to interpret. In order to be useful for analytical purposes, the reaction should satisfy the following: 1. The spectra should be easy to interpret, i.e. the peaks should be well-separated in a relatively background-free region. 2. The cross-section should be large. The most important nuclear reactions for charged particle detection are listed in Table 15-1. As can be seen from the list, there are large differences for various nuclear reactions and the best conditions have to be selected carefully. High-resolution depth-profiling requires a primary ion beam with high energystability and small energy-spread. Sometimes filtering is necessary to remove scattered particles. A drawback of using filtering is that the depth resolution is generally very poor. However, the resolution can be improved if resonances exist. The method is with the exception that a charged particle is detected - similar to the resonance profiling by (p,y) as described above. The energy of the primary particle immediately before the reaction at depth x is given by:

where Ea is the primary beam energy before entering the target, dE/dxin(x) is the stopping power of the primary beam and a is the angle of the primary beam relative to the surface normal of the sample surface. After the nuclear reaction the outgoing particle has an energy which is a function of the following parameters: E=f(Min, Mout, Mtarget, Mfinal Ein, Q, 0 )


where Mi", MOut,Mtargetand Mfinalare the mass of the primary particle, the outgoing particle, the target nucleus and the final nucleus, respectively Q is the Q-value of the reaction and Q is the scattering angle. The energy of the particle leaving the target is given by: x/cosP

where p is the angle of the outgoing particle relative to the surface normal of the sample surface and dE/dx,,,(x) is the stopping power of the outgoing beam. Taking into consideration additional energy loss of the particle after it has left the target, e.g. in a stopping foil, AEf,il, the detected energy Edetwill be: Edet=Eout-AEfoil With these four formulae, the scale of the energy of the detected particle can be transformed into a depth scale and the evaluation of depth profiles is possible. Generally, those reactions which have high Q-values, i.e. when the outgoing particles have a higher energy than the elastically scattered ions, are advantageous. The depth resolution is determined by the stopping cross-section dE/dx, the incident energy Eo, the detector angle 0 and the solid angle subtended by the detector. The depth resolution therefore varies with the depth in the target. Thus, the depth resolution has to be calculated for the given conditions of every special case. However, the values are in the same order as those discussed for RBS, i.e. a few nm for optimized geometries at the target surface. For the resonance method, the depth resolution near the surface is determined by the resonance width of the nuclear reaction and the energy-spread of the incident beam. In some cases relatively narrow resonances, in the order of 1 keV or narrower, exist [e.g. 180(p,a)1'N]these allow high-resolution depth-profiling of a few tens of nm. High-resolution depth-profiling requires an accelerator with high energy-stability and small energy-spread. The accessible depth is usually limited by resonances or an increased non-resonant yield at higher energies. Otherwise, the accessible depth will be given by the maximum broadening allowable in the depth resolution due to the energy straggling of the beam. Detailed discussions about charged-particle-production nuclear reactions can be found in three publications: Vizkelethy (1995), Gotz and Gartner (1988) and Tesmer (1 990).


253

Table 15-1. Nuclear reactions used for the detection of charged particles produced in nuclear reactions (from Gotz and GSirtner, 1988). Nucleus

Reaction

Q-value Q [MeV1

Incidentenergy I) El [MeV1

Emitted energy E'I [MeV] 2.3 13.0 13.6 9.7 7.7 4.1 5.57(ao) 3.70(0rl) 3. I 5.8 9.%ao) 6.7(a1) 3.9 2.4(Pd4) 1.6(Pl) 3.4 6.9 2.238 2.734

Cross-section (do/dR)NR [mb/sr]

4.032 1.o 5.2 18.352 0.7 61 0.45 18.352 64 22.374 0.7 6 1.5 1.5 17.347 7.153 0.62' 1 8.586 0.65 0.12(ao) 5.65 0.65 90(al> 2.722 35 1.20 I2C I2C(d,p)l3C 5.951 0.64 0.4 "c 13C(d,p)14C 13.574 14N 14N(d,a)12C 0.6(a0) 1.5 9.146 1.2 1.3(~1) 0.8" I 5N 15N(p,a)12C 4.964 z15 1.917 I6O 160(d,p)170 0.74(PJ 0.90 4.5(Pi) 1.05 0.90 3.980 0.733' 15 I8O 1sO(p,cr)'5N 1.25 "F 19~(p,a)160 8.1 14 0.5 23Na 23Na(p,a)2%e 2.379 0.592 4 1.514 16 3lP 3 1 ~ ( p , a ) 2 8 ~ i 1.917 For laboratory emission angle 150' with recoil nucleus in ground state (excited state) 0.6 MeV is optimum for Be in light Z matrix and 1.6 MeV for high Z matrix 31 Maximum energy for Mylar to stop backscattered proton 4, Measured at 8=164" 5, range of a < range of proton

2H 2H 3He 6Li ' ~ i 9Be "B

2H(d,p)3H 2H(3He,p)4He 3He(d,p)4He 6Li(d,or)4He 7Li(p,cr)4He 'Be(d,~x)~Li 'IB(p,a)'Be

Mylar thickness Zsf

[PI 14 6 8 8 35 6 10 10 I6 0.6 23 16 12 12 12 11 25 6 5)

References Andersen H.H., Whitlow H.J. ( 1983), Nucl. Instr. and Meth., 2 18,468. Chu W.-K., Mayer J.W., Nicolet M.-A. (1978), Backscattering Spectrometry (Academic Press, New York, p.213. Feldman L.C., Picraux S.T. (1977), in Mayer J.W. and E. Rimhi (eds.) Ion Beam Handboookfor Materials Analysis, Academic Press, New York, Ch. 4. Gotz G., Ggrtner K. (1988), High Energy Zon Beam Analysis of Solids. (Akademi Verlag Berlin). Tesmer J. et at. (1990), High energy and heavy ion beams in material analysis, (Material Research Society). Whitlow H.J., Eriksson Th., Ostling M., Petersson C.S., Keinonen J., Anttila A. (1987), J. Appl. Phys. 50,1497. Vizkelethy G. (1995), in Handbook of modem ion beam material analysis, Ed. J.r. Tesmer et ul., (Material Research Society).



16 Rutherford back-scattering spectrometry and recoil spectrometry H.J. Whitlow and M. Ostling

16.1 Introduction 16.1.1 Basic principles Rutherford Back-scattering Spectrometry (RBS) and Recoil Spectrometry (RS) are related techniques which are used for measuring the depth distribution of elements within a micrometer or so of the surface of a body. Both techniques are based on elastic scattering of ions by the sample and are today standard techniques in the characterization of thin-film materials. The techniques are discussed in detail in a number of standard texts (Chu et al., 1978; Gotz et al., 1988; Feldman and Mayer, 1986; Tesmer and Nastasi, 1995). Usually the ions used are protons or Hef, produced by an accelerator with an energy of about one MeV. As these ions penetrate the material, they lose energy (so-called stopping) mostly by being scattered by the electrons in the material. A small fraction of them will also be strongly scattered directly by the nuclei of the atoms that make up the sample. These ion-nuclei collisions are often elastic collisions in which the ion nucleus simply bounces off the nucleus of the atom in the sample (Fig. 16-1) because of the electrostatic (Coulomb) repulsion between the electrical charges carried by the two nuclei. This is termed Rutherford scattering. E"

Fig. 16-1. Schematic illustration of an elastic collision between two atomic nuclei.

The central concept of both RBS and RS is the measurement of the statistical probability of scattering and energy loss. It follows that the formalism for the two tech-

16 Rutherford back-scattering spectrometry and recoil spectrometry 25 5

niques is very closely similar and for this reason RBS will be considered as the prototype technique.

16.1.2 Scattering kinematics In an elastic collision, for which the quantities are defined as in Fig. 16-1, the energy, E', of the scattered ion immediately after scattering is related to the energy E just prior to scattering according to:

E

= K(Mi,Mz,e)E

(16- 1a)

where for MI < M2 (as in RBS) the kinematic factor K is a function of M1, M2, and 0 only:

K(M,,

=

i

cosB+[(M2/M,)Z-sin2Q] 1+ M2 /M,

(16-lb)

16.1.3 Scattering probabilities The number of ions Y scattered into a detector viewing a (small) solid angle interval 6 0 is simply related by a scattering probability to the number of target atoms per unit area NA and the number of incident ions No: (1 6-2) Here do/d0 is the differential cross-section for scattering which is the probability of scattering of a single ion from a single target atom into unit solid angle. The solid angle for a typical RBS set-up is about 1-5x 10" sr (steradians). The differential cross-section for scattering is governed by the form of the interaction between the ion and the target atom. The formalism of this interaction is treated in a number of standard texts and handbooks (Krane, 1988; Chu et al., 1978; Gotz et al., 1988; Tesmer and Nastasi, 1995). For the present purposes we will consider only the case where the interaction is a pure Coulomb repulsion between the nuclear charges. This corresponds to the case where by far the major part of the deflection of the ion trajectory occws when the ion nuclei are within the inner (K) electron shells, so that no shielding by negative electronic charges takes place. The closest approach distance should be sufficiently large, however, that the effects' of short-range nuclear forces are negligible. Elastic scattering under these conditions is termed Rutherford Scattering after Rutherford and Marsden's famous experiment on aIpha-particle scattering (Rutherford, 1911) and is given the name Rutherford Back-scattering Spectrometry (RBS). The conditions are generally very well satisfied by scattering of 0.3-2.5 MeV He' ions in backward directions (0 2 90') from target atoms with M2 larger than about 20u. In SI units the differential


cross-section for Rutherford Scattering of an ion with atomic number ZI from a target atom with atomic number 2 2 is, in the laboratory frame of reference (Chu et al., 1978): (1 6-3a)

where in essence; do/dR a (Z,Z,

)z,

do/dR a E-*,

+...I

(1 6-3b)

do/dS2 a [Sir1-~(0/2)-2(M,/M~)2

It should be noted that do/dS2 depends only on ZI, MI, Z2, Mz, 0 and E. These relationships show that (1) the scattering cross-section improves as the square of the atomic number of the target, making heavy elements very easy to detect. (2) the scattering cross-section decreases in inverse proportion to the square of the energy, which imposes a tilt in all energy spectra. (3) the back-scattering angle should be close to 180" for high sensitivity.

16.2 Rutherford Back-scattering Spectrometry (RBS) 16.2.1 Mass spectrometry of thin layers using RBS The experimental configuration for RBS measurements is illustrated schematically in Fig. 16-2. Vacuum chamber

Sample

/

Detector

Pre-amplifier

I Multi-channel

Fig. 16-2. Schematic illustration of configuration for Rl3S measurement.

16 Rutherford back-scattering spectrometry and recoil spectrometry 257

Ions from the accelerator are directed through an evacuated tube (often termed the beam line) to the scattering chamber. The accelerator is set up to deliver ions with a single atomic number Z1, mass Mi and energy Eo, e.g. 2 MeV 4He+.The collimating aperture defines the size of the ion beam that impinges on the sample at a fixed angle. Usually an ion-beam diameter of 0.5-1 mm is used. The detector is shielded by another collimator aperture (usually 2-5 mm dia.) so that it sees only the ions that have been back-scattered at the back-scattering angle 9. Fixing the values of MI, Z1 and 9 in this way implies that the value of the kinematic factor K (M2) depends only on the atomic mass of the target, while the value of d a d 0 depends only on the atomic species and E of the target. If the target is so thin that energy loss can be neglected then the energy prior to scattering will be the beam energy Eo, and the energy El of the back-scattered particles will be:

Ei = EoK(M2)

(1 6-4)

Note that conservation of momentum requires that M1 S M2 for back-scattering to take place. Fig. 16-3 shows the number of back-scattered ions vs energy for a sub-monolayer of Cu-Pt alloy, sputter-deposited on a carbon foil (Andersson et al., 1983). The peaks correspond to the different K(M2) values for back-scattering from C (M2 = 12), Cu(M2= 63, 65) and Pt (Mz = 195). Thus, from the position of the peaks in the backscattering spectrum the elements present in the surface layer can be identified. (The C foil was actually purposely selected to be so thick that the peak is broadened into a plateau.). I

I

2OkeV Ar'-

I

1

CuPt

8.15"

Jt

cu 50

100

150

2

I

CHANNELS

Fig. 16-3. RBS spectrum of a sub-monolayer Cu-Pt alloy sputter-ejected at 15" to the target surface normal and deposited on a thin carbon film. (From Andersson et ul., 1985).


16.2.1.1 Example: composition of a sub-monolayer, sputter-deposited alloy For the spectrum in Fig. 16-3, the number of back-scattered ions that make up the copper peak, YcU , was 8346 counts, while for platinum, Ypt, it was 64,864 counts. From eq. 16-2, these yields are: (1 6-5a)

(1 6-5b) Where Ncu and Npt are the number of Cu and Pt atoms per unit area of film. Dividing and rearranging eqs. 16-Sa and b:

(1 6-6) For M2 200, da/dQ is nearly proportional to eq. 16-6 as:

Z2

(eq. 16-3b) and we can approximate

(1 6-7) Inserting numerical values of Zcu, ZPt:

Ncu/Npt= (8346/64864) x (78/29)2= 0.9308, i.e. the composition of the film corresponds to CuPtl 074.

16.2.2 Depth scales in RBS The energy loss (stopping) by electron excitation and nuclear collisions as the ions penetrate the target material form the basis of the depth scale in RBS. Consider the energy loss AE as an ion penetrates a thin layer of material with NAatoms per unit volume and thickness x. (Fig. 16-4a) This energy loss can be written:

AE = (NAx)E

(16-8)

In this case the layer thickness is written in terms of the number of atoms per unit area W A X ) . Then the energy loss (stopping) is specified by E, which defines the energy loss in terms of atoms per unit area, and is termed the stopping cross-section. This manner of expressing the relationship between path length in the material, and corresponding energy loss, has two important advantages:

16 Rutherford back-scattering spectrometry and recoil spectrometry 259

1. The relationship between thickness in units of atoms per unit area to energy loss is independent of the density of the material, which in thin-film structures may ' not be accurately known. 2. The stopping cross-sections for compounds is simply the weighted sum of the stopping cross section for the constituent elements (Bragg's law rule).

90,oo

&

I

40,OO

0,oo

ooo,oo

1

I 2000,00

I

3000,OO

Energy [keV] Fig. 16-4. Energy loss of ions passing through a thin layer of material. a) Schematic illustration. b) Stopping cross section e [eV I ( lOI5 at. cm-*)] vs energy for 4Het ions in Si and Co.

The stopping cross-sections depend on the material, the ion species and the energy of the projectile. It is not practical to calculate them ab initio but empirical tables based on experimental measurements are available (Chu et al., 1978; Gotz, 1988; Tesmer and Nastasi, 1995) as well as semi-empirical computer codes (e.g. Ziegler, 1985.) Fig. 16-4 shows the stopping cross sections for 4He+ions in Si and Co. In the low-energy region E is proportional to velocity and increases according to dE. In the high-energy region E


exhibits an E-' dependence and is often termed Bethe-Bloch stopping. The statistical nature of the scattering of the ion from electrons and nuclei leads to a spread in energy as the ions traverse the foil (Fig. 16-4a). This is termed energy straggling and is characterized by the variance Q2 of the ion energy distribution, which, for our purposes, where AE

loll

0

25

120,

100

200

I5

20

300 25

30 I

a0 ao-

-

400

Ga

D

w

-

>

40 40-

Surfore

I

0

400

300 15

wy

500 20

600

2s

700 700

30

160

120 9

80

10

0

300

LOO 500 Energy (channel number1

600

700

Fig. 16-12. The same data as in Fig. 16-1 1 but presented as histograms of, respectively, 27Al,69-7'Gaand 75Asrecoil energy distributions (From Hult et a1 1992).


In an RBS measurement these signals from the light element would be buried in the counting statistical noise from the major elements (Al, Ga and As ). This illustrates one of the most significant advantages of RS over RBS, namely that small concentrations of light elements (C, 0, etc.) can be measured simultaneously with the major elements from a heavy matrix such as GaAs or a ferrous metal.

16.3.4 Coincidence recoil spectrometry A further form of recoil spectrometry is worthy of mention. This is called coincidence RS (Elastic Recoil Coincidence Spectrometry (ECRS) (Tesmer and Nastasi, 1995), where the probing beam impinges at normal incidence on the surface of a sample in the form of a thin foil. The scattered projectile and the recoiling target atom are ejected from the foil in the forward direction in coincidence and can both be detected (Fig. 16-13). The method has found some application for depth profiling. However, the combination of the coincidence requirement of large solid-angle detectors and kinematic restrictions can be used as the basis for very specific, high-sensitivity measurements.

Incident ion beamv

- --

Fig. 16-13. Principle of coincidence recoil spectrometry.

16.4 Numerical simulation methods for the analysis of RBS data For complex, multilayer structures the RBS spectrum can be difficult to interpret because the measured energy spectrum is the superimposition of the back-scattering energy spectrum from each element in the sample that has a mass greater than the projectile ion, Unfortunately the implication of this is that no unique solution to the depth distribution function can be determined from the measured energy spectrum. Since the late 1970s, procedures to analyse these complex RI3S spectra have been developed that rely on the comparison of a simulated RI3S spectrum with the measured spectrum. As a result of the rapid advance in computer technology, this technique is well established

16 Rutherford back-scattering spectrometry and recoil spectrometry 27 1

today and accessible within the realm of lap top computing. The technique can also be readily employed for RS where the energy distribution is measured. Although methods are under development for handling multi-dimensional data from ToF-RS and AE-RS no simulation code is available at the time of writing. Analysis by computer simulation is based on the calculation of the RJ3S spectrum corresponding to a model target structure. The simulated spectrum is then compared with the experimental data and the model subsequently refined. The process is repeated until satisfactory agreement is obtained between calculated and measured data. (This is similar to the procedure used in X-ray crystallographic structure determination.) Fitting of the parameters of the model target structure to the measured experimental spectrum can be carried out using standard numerical analysis procedures.

References Anderssen H.H., Stenum B., Ssrensen T., Whitlow H.J. (1983), Nucl. Instrum. and Methods 209/2 10,487 . Chu W.-K., Mayer J.W., Nicolet M.-A. (1978), Backscatering Spectrometry, Academic Press, New York Feldman L.C., Mayer J.W. (1986), Fundamentals of Surface and Thin Film Analysis, Elsevier Science Publisching Co. Gijtz G., Gilrtner K. (1988), High energy Ion Beam Analysis of Solids. Akademie-Verlag Berlin. Hult M., Whitlow H.J., dstling M., Appl. Phys. Lett. 60,219. Krane K. (1988), introduction to Nuclear Physics, John Wiley & Sons. Rutherford E. (191 I), Philos. Mag. 21,p669. Tesmer J.R., Nastasi M. (l995), Handbook of Modem Ion Beam Analysis. (eds.), Materials Research SOC.Pittsburgh USA. Whitlow H.J. (1989a), in: Proc. High Energy and Heavy Ion Beams in Materials Analysis, Tesmer J.R., Maggiore C.J. Nastasi M., Barbour J.C. and Mayer J.W. Pittsburg PA p.73 Whitlow H.J. (1989b), in: Proc. High Energy and Heavy Ion Beams in Materials Analysis, Tesrner J.R., Maggiore C.J. Nastasi M., Barbour J.C. and Mayer J.W. Pittsburg PA p.243. Ziegler J.F., Biersack J.P., Littmark U. (1985), The stopping and Ranges of ions in solids , The stopping and ranges of ions in matter, Ziegler J.F. (Ed.) New York, Pergamon Press, vol. 1.



17 Auger electron spectroscopy C.-0. A. Olsson, S . E. Hornstrom and S. Hogmark

17.1 Physics of Auger electron spectroscopy 17.1.1 Introduction The detection of the first ‘Auger’ electrons was reported in 1923 by the French scientist Pierre Auger (Auger, 1923), who studied X-ray-induced emission of characteristic electrons in a cloud chamber. He found electrons with an energy independent of the exciting source and explained this behaviour as radiationless de-excitation of ionized atoms. The use of Auger electrons as a tool for surface analysis was first introduced by Lander in 1953. He studied the emission of secondary electrons from different surfaces. The number of Auger electrons is, however, rather small when compared with the background of elastically scattered electrons. This made quantitative analysis impossible until the mid sixties, when lock-in amplifiers were applied to extract the weak signals from the Auger electrons. In the beginning of the 70s, Auger electron spectroscopy, AES, had reached the level where it could be used for applied surface analysis. Further refining of the instrumentation has taken the detectivity for most elements to levels of the order of tenths of a percent. It is possible to detect all elements in the periodic table with the exception of hydrogen and helium. In certain cases, it is even possible to obtain information on the chemical bonding of the surface atoms. In dedicated systems, the detector for Auger electrons is normally combined with a detector for secondary electrons. By rastering the focused primary electron beam, it is possible to get a high resolution secondary electron image of the surface. Elemental maps can be acquired from exactly the same area as seen with the secondary electron detector; Auger electron spectra and sputter-depth profiles are readily acquired from selected points or areas of the surface. Such ‘Scanning Auger Microprobes’, SAM, are available commercially from a number of manufacturers. Over the years, AES has found a wide use for a number of scientific and engineering applications. The main reasons for its popularity are: 0

0 0

0

0

the possibility of detecting all elements from Li in the periodic table high surface sensitivity, approximately 1 nm good lateral resolution, =l Onm (commercially available spring ’95) perfect correlation between the secondary electron image and the point of analysis good sensitivity throughout the periodic table the possibility of combining AES with noble gas ion-sputtering for depth profiling.

17 Auger electron spectroscopy

273

An extensive volume on Auger and X-ray photo-electron spectroscopies has been edited by Briggs and Seah (1990).

17.1.2 The Auger process When a beam of electrons interacts with the atoms in a material, core level electrons can be ejected if the energy of the incident electrons is larger than the ionization threshold. Relaxation of the ionized atom can occur by filling the core vacancy with an electron from an outer shell. The relaxation energy is then dissipated in either of two ways. It can be given to a second electron, an Auger electron, which is emitted from the atom as demonstrated in Fig. 17-1, or it can appear as a characteristic X-ray photon.

0 eV -1 eV -73 eV -118 eV

K

-

f

-1559 eV

Fig. 17-1. Illustration of the Auger KLL process for aluminium. An initial vacancy in the K-shell is filled byan electron from the L-shell.To relaxthe excess angular momentum and energy, an Auger electron is emitted from the outer L-shell.

The conventional way to assign Auger transitions is to use the X-ray spectroscopic nomenclature. Three electron levels are involved in an Auger transition, each of which is designed by its principal quantum number n. The capital letters K, L, M, N ... are used for states with n = 1, 2, 3, 4 ..., respectively. Different subshells are distinguished using the suffices 1, 2, 3, 4 ... which correspond to the spectroscopic levels sl/2,pln,p3/2,d3,2,d512,... Should, for example, a vacancy in the lsln level be filled by a 2sln electron and a 2p1,2 electron be ejected, the corresponding Auger transition would be designated as KlLlL2, or KLlL2. This notation is generally used to assign Auger transitions to all elements in the periodic table, although it is strictly valid only for elements with high atomic numbers, Z. For D m 7 5 and Zw104 at 28 Da ) on a flat and conductive surface such as a silicon wafer. This permits separation of different contributions appearing as a peak from isobar ions (molecular or atomic ions with different chemical composition but with mass differences of only a few Da). Fig. 20-3 shows an example of such discrimination obtained at the 56 Da on a Si wafer. The mass resolution is degraded on rough and insulating samples. Nevertheless, in practice, it is sufficiently high to permit discrimination between pure hydrocarbon and oxygen-containing molecular ions. The separation between hydrocarbon and nitrogen-containing ions is more arduous. Table 20-1. TOF-SIMS typical working conditions. Spectrum acquisition

Image recording

a) 69Ga' (1 5 keV) b) %+( 12 keV) a) 0.5 b) 30 500 3.1~10~ 5

69Ga+(25keV)

ION BOMBARDMENT

Ion Beam diameter (pm) DC current (PA) (ions s-I> Pulse duration (ns) Repetition rate (kHz) AC current (ions S-I) (ions pulse") Rastered area (pm') AC current density (ions cm-' 5.') TOF SPECTROMETER Flight time (ps) Flight path (m) DATAACQUISITION Mass range (Da) Acquisition time (s) Data memory

5

0.06 60 3.8~10~ 25 10

7 . 8 lo4 ~ 15.6 I O O X 100

9.4~10~ 9.4 looxloo

7 . 8 10' ~

9.4x 1o8

2form=lDa 63 form = 1 000 Da 2 1-5000

300 /spectrum 1 . 6 lo6 ~ channelslspectrum dwell time : 156 ps khan Ion fluence (ions cm-2) 2.4~10~'/spectrum 2 . 4 10' ~ ions/spectrum

1-1000

1800 /image 256x256 pixeldimage

1.7x1Ol2/image 2 . 5 ~ 1 0iondpixel ~

The extraction and focusing of the secondary ions uses immersion and transfer lenses that control the direct image magnification and limit with the angular acceptance of the spectrometer by means of the apertures. This leads to an overall secondary ion transmission greater than 40% (Schueler, 1992). To increase the detector efficiency for

20 Static mode secondary-ion mass spectrometry

343

heavy ions (m> 1000 Da) a post-acceleration stage (10-20 keV) can be placed in front of the detector. Typical working conditions are summarized in Table 20-1.

20.3.2 SSIMS imaging With TOF spectrometers, images can be produced either in microprobe or microscope mode. The latter is only possible if the spectrometer has imaging optics for imaging secondary ions so that direct images are formed at the detector position (Schueler, 1992). These images can be directly recorded by the use of a position-sensitive detector such as dual micro-channel plates followed by a resistive anode encoder (RAE). A lateral resolution of a few pm can be obtained in this microscope, direct imaging mode. A better lateral resolution (< 1 pm) can be obtained in the microprobe mode in which a finely focused metal-ion beam (e.g. 25 keV Ga’ beam with a spot diameter of I 0.1 pm) is rastered across the surface and combined with synchronous detection of the secondary ions. Generally the improved lateral resolution in microprobe mode is achieved at the expense of poorer mass resolution (Table 20-1).

20.3.3 Static conditions and sample degradation Considering the sample degradation during the analysis and the requirement of static conditions, studies of polymer modifications under ion beam bombardment have shown that the surfaces undergo a continuous transformation that starts even at very low fluences (Delcorte et al., 1995). This transformation affects the whole spectrum and not only the highly characteristic SIMS peaks, which are, however, the most sensitive to degradation. No plateau regime is seen in the peak intensity, their decrease, preceded or not by a maximum, is commonly observed as a function of the ion fluence. This means that no real ‘static’ regime for polymers exists and that ion formation is intrinsically related to the degradation process. Nevertheless, reproducible molecular information can be obtained from these polymers using ion fluences of less than ions cm-2.

20.3.4 Sample charging For electrically insulating samples, like most polymers, charge compensation is needed. This is achieved by flooding low-energy electrons onto the irradiated area. These can be produced by an electron gun or a heated tungsten filament. With TOF instruments, the electrons are pulsed during the off-cycles of the ion pulses (Hagenhoff et al., 1989), with an adjustable timing and repetition rate. The microscope imaging mode allows control of the neutralization settings by seeking an uniform ion emission from the irradiated zone. The surface charge removal can be improved by placing an earthed metal grid on the sample surface. Fast Atom Bombardment (FAB) has also been applied to reduce charging of the sample and degradation of organic samples (Reed and Vickerman, 1992).

344 Part 3: Chemical bonding and molecular composition

20.4 Specificity of SSIMS for surface analysis 20.4.1 Polymer and organic layers Polymers are finding increasing application, and interfaces involving polymers and other materials abound in many different systems: composite materials (polymer matrix with fibres), elastomer matrix and carbon black filler in tyres, layered structures such as metallized polymers or protective polymer coatings on metals. For all these applications it is important to be able to link the physic0 chemical properties of polymer surfaces to their molecular structure (composition, functional groups, side-group orientation, crystallinity). For this purpose, SSIMS brings new possibilities as a complement to the electronic spectrometries (XPS) more commonly used. 107:

106:

Ag+ lo' 911

105- II

,I

Fingerprint

3

cd -

Polystyrene (Mw=3250) on Ag

Oligomer distribution

Y

n

150-

6 28

27

100-

50-

0 2000

I

24

1'

0 32

19

2500

3000

0

4000

m/Z

Fig. 20-4. 15 keV-Cia' positive-ion TOF-SIMS spectrum of polystyrene deposited on silver.

45


345

Fig. 20-4 shows a typical TOF SIMS spectrum of a thin layer of a low-molecular weight (Mw=3250) polystyrene (PS) deposited on a silver substrate. Three different regions can be distinguished in the spectrum. The low-mass range ( < 500 Da) is called the fingerprint region in which fragments of the macrochains are observed up to the fragmentation of the monomer repeat unit itself. In the fragmentation region (500-2000 Da), the large fragments of macrochains cationized with Ag are observed. zsood----

c3 (hf-H)t

3

Fig. 20-5. 15 keV-Gd positive-ion TOF-SIMS spectra of polyethylene PE, polypropylene PP and polyisobutylene PIB (Delcorte et al., 1995). Reprinted with kind permission of Elsevier Science.

The higher mass range (>2000 Da) is called the oligomer distribution region. In this part, the intact macrochains cationized with Ag' are detected and the spectrum shows the distribution of the molecular mass (Bletsos et al., 1991). Peaks are separated by Am=104 Da corresponding to the mass of the PS monomer repeat unit. The exact mass


determination of these ions also permits determination of the mass and hence the composition of the end groups of the chain (Bletsos et al., 1991). The fingerprint region contains specific information about the structure and chemical composition of the polymer. With bulk polymer samples, only the fingerprint region is observed in the spectra, and higher mass peaks, if detected, usually correspond to molecular additives and/or contamination. The SSIMS fingerprint spectra are very sensitive to the molecular structures of the polymers and subtle modifications have a direct influence on the spectra. For example, polyolefins with molecular structure differing only by the presence of one (polypropylene, PP) or two (polyisobutylene, PIS) methyl pendant groups along the same hydrocarbon backbone, are easily discriminated by their SSIMS spectra; this is not possible by XPS without a careful study of their valence band (Delcorte et al., 1995). Indeed, the SSIMS spectra of these polymers, shown in Fig. 20-5, are different with regard to the relative intensities of the different carbon clusters (CXHi). The maximum intensity is observed for the deprotonated monomer ion (M-H)' at 27 Da for polyethylene (PE), 41 Da for PP and 55 Da for PIB. The pendant methyl group contributes to the high relative intensities at masses 55 and 69 Da for PP and PIB, and the double pendant methyl group of PIB accounts for the peaks at 83 and 97 Da. Their molecular structures have been discussed previously (Van Ooij and Brinkhuis, 1988; Briggs, 1990). Another significant difference between these spectra is the contribution of the CScluster which increases with branching level. The explanation of this effect is related to the higher stability of branched ions as compared with linear ones (Van Ooij and Brinkhuis, 1988). If a polymer STET unsaturated, owing to aromaticity and/or functional groups, its SIMS spectrum gives a direct signature of the unsaturation. Fig. 20-6 shows the TOFSIMS spectrum of poly(ethy1ene terephthalate) (PET). In the mass range (0-200 Da), the positive secondary ion spectrum is interpreted on the basis of the fragmentation scheme of the monomer repeat unit (Briggs et al, 1989). The ions at 191 and 193 Da correspond to (M f H)' where M is the monomer repeat unit. The major peaks correspond to CsHl((D) (76 Da), (D-CO' (104 Da) and CO-0-COOH' (149 Da). These examples prove the fingerprint nature of the characteristic fragmentation pattern observed in the static SIMS spectra of polymers. Greater intensity is observed for the ions able to stabilize their charge by inductive and/or resonant effects. Some fragments can rearrange in a complex way in order to fulfil this requirement. The relative intensity of the different peaks in the spectra may also be influenced by the experimental conditions (type and energy of the primary ions, total ion fluence per spectrum, spectrometer type) (Briggs and Hearn, 1986). Higher primary ion mass (Xe or Cs) favours the emission of heavier fragments. High mass resolution is possible only with TOF-systems.


347

35

C2H3' 27

30

25

to

n

m

0

x

20

c,

C

n

CH2+

1

3 15 a

o+

c1

10

76

5 149

0

50

-4 .

100

d Z

.

.

.

/I

(rnH)-

150

191

2 0

Fig. 20-6. 15 keV-Gat positive-ion TOF-SIMS spectrum of poly(ethy1ene terephthalate) PET.

20.4.2 Molecular materials and catalysts Molecular solids formed by weak van der Waals interactions of large molecules such as organometallics and hllerenes also produce characteristic SSIMS spectra with direct emission of the constituent molecule associated with its fragmentation pattern. Fig. 20-7 shows a positive-ion spectrum of cobalt phthalocyanine (the assignment of some characteristic ions is indicated in the figure). It is seen that the intact molecule is well detected (Weng et al., 1995). These transition metal-N4 chelates, when loaded on carbon black and heat-treated, can be used to replace platinum as catalysts for the electroreduction of 0 2 in solid-polymer-electrolyte fuel cells. SSIMS made it possible to follow the pyrolysis of the molecule at different heat-treatment temperatures thereby helping to elucidate the nature of the catalytically active sites (Weng et al., 1995).


10

Fig. 20-7. 15 keV-Ga' positive-ion TOF-SIMS spectrum of cobalt phthalocyanine (Weng ei a/., 1995). Reprinted with kind permission of Elsevier Science.

20.5 Applications 20.5.1 Polymer-surface contamination and additive migration The sensitivity to molecular additives and surface contamination is illustrated in the spectrum shown in Fig. 20-8, which was recorded for a commercial styrene-butadiene rubber copolymer. Parent ions of palmitic and stearic acids [MH' and (M-OH)'] are directly observed, as is the fingerprint fragmentation pattern of polydimethylsiloxane (PDMS) (28, 73, 131, 147, 191, 207, 221 Da). PDMS is a very common surface contaminant owing to its very low surface free energy. As a consequence, a semi-quantitative interpretation of the SIMS intensity, in terms of the copolymer composition,


349

requires that the sample is washed (extraction in toluene) before analysis. After such a cleaning procedure, linear relationships have been obtained between the SIMS intensity of the deprotonated ions, (M-H)', of each monomer repeat unit and the copolymer bulk composition (Weng et aE., 1995). "U"""

Styrene-Butadiene Rubber

11

40000;

43

Si

41

20000~

0,

. .

" 15 I[!

,

'

39

,I

lOOOj

:'

,

.

!

55 57

1':

;' 1

1

L 4 140 l 4

.1

% I :

C3HgSi

69 73

i

I.I:I

1.;

. '.' ' . :

:.

I

..

CgHlgOSi2

b 160 180 2

"\

(M-OH)+ 267

1000.

Stearic Acid 500: CgH1503Si3 207

Palmitic Acid

C7H2102Si3

A\

(M-OH)+

(MW+

Fig. 20-8. 15 keV-Ga' positive-ion TOF-SIMS spectrum of styrene-butadiene rubber (SBR) showing the presence of additives and PDMS contamination (Weng el ul., 1995).

20.5.2 Surface segregation in polymer blends Polymer blending is an important way of creating new materials with synergetic properties. Since most polymers are immiscible on a molecular scale, a mixture of two polymers results in a two-phase system. The properties of the blend depend on the morphology which is influenced by the elaboration parameters. Although bulk mor-


phology can be controlled, this is not the case for the surface morphology. However the control of surface heterogeneities is crucial if we want tailor-made properties such as adhesion, biocompatibility, printability, etc. For this purpose, it is important to characterize the surface of the blend in terms of chemical composition and morphology, and for this, TOF-SIMS can be used because of its molecular sensitivity and its imaging capabilities (see Section 20.5.5) (Lhoest et al., 1995).

20.5.3 Surface chemistry: functionalization of carbon fibres SSIMS is a very valuable technique for following surface chemistry. Carbon fibres (CF) after extraction in CH2C12 were hnctionalized with the chloride of trimellitic anhydride and, by applying TOF-SIMS, it was possible to show that the anhydride reacted with the m i n e groups present on the CF surface (Weng et al., 1995). Fig. 20-9 shows the mass range of the negative spectra with molecular ions (146 and 190 Da) characteristic of these reactions, as explained in the figure inset.

20.5.4 Polymer surface modification by plasma treatment Plasma treatment of polymer surfaces is extensively used for modifying their surface properties without modifying their bulk properties. The main goal is to enhance the adhesion properties, in particular to improve biocompatibility and to enhance metal adhesion (Strobe1 et al., 1994). Polymers with a broad variety of properties can be produced by varying the operating conditions, the design of the plasma chambers and, in particular, the composition of the gas used in the plasma processing. XPS has been extensively used to characterize the surface modifications produced by plasma treatments. TOF-SIMS can provide complementary information to XPS. For example, TOF-SIMS has been used to monitor semi-quantitatively, the chemical modifications produced at the surface of low-density polyethylene (LDPE) films by SF6-CF4 plasma treatment (Leonard et al., 1995). The influence of the composition of the plasma gas (SF6-CF4plasma) on the surface fluorination has been investigated and compared with results obtained using plasma diagnosis (mass spectrometry and optical emission spectroscopy) as well as from XPS characterization.

20.5.5 SSIMS imaging TOF-SIMS can be used to produce chemically resolved images with a lateral resolution I 1 pm of organic material by mapping the intensity of specific molecular or atomic ions while maintaining static analysis conditions (see Table 20-1). Fig. 20-10 shows the TOF-SIMS images obtained from a thin paraffin layer (hexatriacontane, C36H74) deposited on a silicon wafer. As seen in the different images (Fig. 20-lo), the deposition was not uniform and there is complementarity between the Si' image coming from the substrate and the C4H9' (57 Da) ions originating in the layer. The total-ion image is sensitive to the difference in ion yield between the layer and the substrate and,


35 1

XIlU

0 d z = 190(-)

I

Reaction between (-NHt I

0

-NH)group

II

I 4

and -C-Clgroup

OH

Reaction between (-NH2 I-NH) group

(1)

0

0 miz = 146 (-

I

I I

0

c"

a

CF Fig. 20-9. The (1 00-200 Da) mass range of a 15 keV-Ga' negative-ion TOF-SIMS spectrum of carbon fibres (AS4 6k) after CHzClz extraction and functionalization with trimellitic anhydride; below a schematic representation of the chemical reactions (Weng et al., 1995).

in this case, it reproduces the same features as the Sic image. It can also be influenced by topographic effects.

20.6 Conclusions As shown by the various examples, SSIMS is becoming a mature technique with which to study surface chemistry. It is particularly well adapted to the situation where molecular information is required. Moreover it can also be used for chemical-surface imaging. However, from a fundamental point of view, much work is still needed to understand the exact mechanisms of secondary molecular ion emission and to improve the quantitative interpretation of the spectra. Post-ionization by pulsed UV laser opens new perspectives in quantification (SNMS mode) and in improving the detection limit mainly for elemental species.


L-i---l

25

pm

Fig. 20-10. 25 keV-Gat TOF-SIMS images of hexatriacontane deposited on a Si wafer: a) total positive ion image, b) Sit (28 Da) image, c) C4H9f(57 Da) image.

References Benninghoven A. (1969), Phys. Status Solidi, K169, 34. Benninghoven A. (1982), Springer Series in Chemical Physics 25, Springer Verlag, Berlin p.77. Benninghoven A. (1994) Angew. Chem. Int. Ed. Engl., 33, 1023. Benninghoven A. (1994) Surf. Sci. 299/300,246. Renninghoven A,, Riidenauer F.G., Werner H.W.(1987), "Secondary Ion Mass Spectrometry, Basic Concepts, Instrumental Aspects, Applications and Trends", Wiley, Chichester.


353

BhatiaQS., Burrell M.C. (1990), Surf. Interface Anal., 15, 388. Bletsos I.V., Hercules D.M., van Leyen D., Hagenhoff B., Niehuis E., Benninghoven A. (1991), Anal. Chem., 63, 1959. Briggs D. (1990), Surf. Interface Anal. 15, 734. Briggs D. (1993), in Practical Surface Analysis vol 11, ed. by D. Briggs and M.P. Seah, p. 367, John Wiley & Sons, Chichester. Briggs D., Hearn M.J. (1986), Vacuum 36, 1005. Briggs D., Ratner B.D. (l986), Polym. Commun., 29,6. Briggs D., Wootton A.B. (1982), Surf. Interface Anal. 4, 109. Briggs D., Brown A., Vickerman J.C. (1989), Handbook of Static Secondary Ion Mass Spectrometry (SIMS), Wiley, Chichester, p.44. Brown A,, van den Berg J.A., Vickerman J.C. (1985), Spectrochim. Acta 40B, 871. Chilkoti A., Castner P.G., Ratner B.D., Briggs D. (1990), J. Vac. Sci. Technol. A, 8(3), 2273. Delcorte A., Bertrand P. (1 996), Nucl. Instr. and Phys. Res., B 1 15,246. Delcorte A., Weng L.T., Bertrand P. (1995), Nucl. Instr. and Meth. 100,213. Gardella J.A., Hercules D.M. (1980), Anal. Chem. 52,226. Hagenhoff B., van Leyen D., Niehuis E., Benninghoven A. (1989), J. Vac. Sci. Technol. A, 7,3056. Hearn M.J., Briggs D., Yoon S.C., Ratner B.D. (1987), Surf. Interface Anal. 10,384. LeggettGJ., Vickerman J.C. (1991),Anal.Chem. 63,561. Leggett G.J., Vickerman J.C. (1992), Appl. Surf. Sci. 55, 105. Leggett G.J., Vickerman J.C. (1992), Int. J. Mass Spectrom. Ion Proc. 122,281. LBonard D., Bertrand P., Khairallah Y., Khonsary-Arefi F., Amouroux J. (1995), in press. Surf. Interf. Anal. 23,467. Lhoest J.B., Bertrand P., Weng L.T., Dewez J.L. (1995), Macromolecules, 28,4631. Licciardello A., Pignataro S., Leute A., Benninghoven A. (1993), Surf. Interface Anal. 20,549. Lub J., van Vroonhoven F.C.B.M., van Leyen D., Benninghoven A. (1989), J. Polym. Sci., part B, Polym. Phys., 27,2071. Marletta G . (1990), Nucl. Instr. and Meth. B46,295. Odom R.W. (1993), in "Microscopic and Spectroscopic Imaging of the Chemical State", Ed., M. D. Morris, Marcel Dekker, New York, p. 345. Pachuta S.J., Cooks R.J. (1987), Chem. Rev. 87,647. Pignataro S. (1992), Surf. Interface Anal. 19,275. Reed N.M., Vickerman J.C. (1992), in Practical Surface Analysis, Vol2- Ion and Neutral Spectroscopy, Eds. D. Briggs and M.P. Seah, J. Wiley &Sons publ., chap. 6. Schueler B.W. (1992), Microsc. Microanal. Microstruct. 3, 119. Schwieters J., Cramer H.G., Heller T., JUrgens U., Niehuis E., Benninghoven A. (1991), J. Vac. Sci. Technol. A, 9,2864. Strobel M., Lyons C.S., Mittal K.L. (1994), VSP, Utrecht, Plasma Surface Modification of polymers : Relevance to Adhesion, Eds. Taylor R.S., Brummel C.L., Winograd N.W., Garrison B.J., Vickerman J.C. (1 995), Chem. Phys. Lett. 233, 575. Thompson P.M. (1 993), Anal. Chem., 63,2447. van den Berg J.A. (1986), Vacuum 36,981. van Ooij W.J., Brinkhuis R.H.G. (1988), Surf. Interface Anal. 11,430. Weng L.T., Bertrand P., Lalande G., Guay D., Dodelet J.P. (1995), Appl. Surf. Sci. 84,9. Weng L.T., Bertrand P., Lauer W., Zimmer R., Bussetti S . (1995), Surf. Interface Anal., 23,879. Weng L.T., Poleunis C., Bertrand P., Carlier V., Sclavons M., Franquinet P., Legras R. (l995), J. Adh. Sci. Technol., 9, 859. Vickerman J.C. (1994), Analyst, 119, 513. Yu M.L. (1982), Appl. Surf. Sci., 11/12, 196.



21 Laser-microprobe mass spectrometry L. van Vaeck, W. van Roy, H. Struyf, K. Poels and R. Gijbels

21.1 Introduction A focused UV laser is x e d for the one-step desorption-ionization (DI) of a solid. The subsequent analysis by mass spectrometry (MS) involves the separation of the ions according to their mass-to-charge ( d z )ratio and detection. Depending on the applied puwer density, Laser Microprobe Mass Spectrometry (LMMS) generates elemental ions, cluster ions of higher m/z from inorganic substances, or ionized, structural elements from organic molecules. In general, information on the molecular weight (MW) of the analyte, i.e. the compound of interest in the sample, is available from the detection of intact molecules attached to a stable, charged ion, while structural fragments permit specification of the functional groups, i.e. specific groups such as esters, ethers, amides, etc. Such direct identification of the substances in the microvolume analysed is referred to as structural analysis in the case of organic molecules and speciation when inorganic compounds are involved. Quantitation in LMMS is not obvious. Sample preparation is minimal, and the compatibility of the specimen with the vacuum is the major requirement. Hence, LMMS becomes the method of choice whenever qualitative information on organic and/or inorganic constituents in micro-objects or local heterogeneities is required (Van Vaeck et al., 1994a and b). The first instruments were developed in the late sevt dies and employed a time-of-flight (TOF) MS. TOF LMMS demonstrated the potential of the approach (Hillenkamp and Kaufmann, 1981; Seydel and Lindner, 1983; Adams and Van Vaeck, 1986; Russell, 1989) but also that high-mass resolution capabilities are needed in problem solving applications. Hence, Fourier transform (FT) LMMS emerged recently as the second generation LMMS.

2 1.2 Instrumentation A frequency quadrupled Nd:YAG laser is generally used to generate high-power 5-15 ns UV pulses at 266 nm. Power densities of between lo6 and 10" W cm-2 are applied to the sample. In the transmission geometry, the laser beam impinges on the bottom of the sample, while the ions are extracted from the top surface. Thin specimens such as 1-pm sections or micro-particles on a self-supporting thin film are suitable. Surfaces of bulk samples can be analysed in the reflection mode, where the laser irradiates the sample on the same side as that from which the ions are ejected.

2 1 Laser-microprobe mass spectrometry

355

21.2.1 Time-of-flight laser-microprobe mass spectrometry (TOF LMMS) Leybold-Heraeus (Cologne, now SPECS, Berlin, Germany) market separate instruments with either transmission or reflection geometry, i.e. LAMMA 500@ and LAMMA 1000@, while the LIMA 2a@ from Cambridge Mass Spectrometry (Cambridge, UK, now Kratos, Manchester, UK) can work in either configuration. The UV-beam can be focused down to the diffraction limit of 0.5 pm but a spot of 1-3 pm is more common. The use of micropositioners manipulated under observation with a microscope, make possible the positioning of the sample in the waist of the UV beam, indicated by the collinearly aligned He-Ne laser. The sample is mounted directly in the DC extracting field. Upon acceleration all ions acquire the same kinetic energy so that their velocity is inversely proportional to ( d ~ ) ~ , Hence, ’. light ions travel faster through the field-free drift tube than the heavier ions and arrive first at the detector. The d z is derived from the flight time. The drift tube is fitted with an ion reflector to compensate the initial energy spread of the ions. The TOF LMMS is simple to operate and provides high transmission and panoramic registration. A full mass spectrum is recorded within 0.5 ms of each laser interaction. Nominal mass resolution, i.e. separation of adjacent nominal d z , is achieved up to mfz 500, if the condition that ions are formed during the laser pulse is satisfied. Whenever post-laser DI occurs, the mass resolution decreases and the calibration of the d z scale is no longer valid. These effects limit the application of TOF LMMS to compounds with a MW below roughly 500. A LAMMA 2000 has recently been constructed to overcome some of the handicaps of the former instruments (Kaufmann and Spengler, 1992a and b). Mapping is possible, while the mass resolution has been extended to 4500 for m/z 208.

21.2.2 Fourier-transform laser-microprobe mass spectrometry (FT LMMS) Three arrangements, all using reflection geometry, have been developed (Brenna et al., 1988; Pelletier et al., 1988; Van Vaeck et al., 1993); two are commercialized (LaserProbeTMby Finnigan FTMS, Madison, WI, USA, and MicroFocusTM,Bruker Instruments, Inc., Billerica, MA, USA). Fig. 21-1 illustrates the latter instrument developed at our laboratory. The sample is mounted in an external source, facilitating the manipulation of the optics and the sample positioning devices. The ions are transferred by static electrical fields into a cylindrical FTMS cell inside a 4.7 Tesla magnet. Ions are trapped between the two vertical plates and follow circular orbits as a result of the Lorentz force. The orbiting frequency is inversely proportional to m/z. A radiofrequency excitation field is applied to increase the radius of the orbit of the ion and to form coherent packets, orbiting close to the receiver plates. The frequency of the induced image current reflects the d z and its amplitude the number of ions. Ions with different m/z can be trapped, excited and detected simultaneously. The superimposition

356 Part 3: Chemical bonding and molecular composition Trap

Receiver

Trap OPTICAL INTERFACE Attenuator

-

Filam

I

Grid

'I'

Transmitter

4 7 TESLA SUPERCONDUCTING MAGNET

----+

Transmitter

Receiver

Nd: YAG LASER Diaphragm

i

\/ d l 3 P

h=266nm. 'c=lOns E = 10 mJ per pulse

He-NB d o t l a s e r

I1

Beam steering prisms

ION TRANSFER LINE

ION SOURCE

/I

SAMPLE MICROPOSITIONER

Fig. 2 1-1. Schematic diagram of the MicroFocusTM Fourier-transform laser-microprobe mass spectrometer with external ion source. Reprinted from (Struyf et al., 1993) with permission of Elsevier Science Publishers. The inset shows the cylindrical FT MS cell. (Reprinted from Laukien et al. (1987), with the permission of the American Chemical Society).

of individual signals is detected and resolved afterwards by Fourier transformation into individual frequency components. General information on the principle of FTMS is available (Marshall and Verdun, 1990; Asamoto, 1991). In practice, FT LMMS achieves routinely a mass resolution of over 100000 and a mass accuracy to within 1 ppm. Usually the exact elemental composition of the detected ions can be derived. FT LMMS costs about the same as TOF LMMS but the cost per analysis is much lower and the operational procedures are less straightforward.

21.3 Analytical features Table 2 1-1 characterizes LMMS instruments in terms of analytical performance. The larger spot in FT LMMS is sufficient for many material science problems, but is too large for biomedical applications at the sub-cellular level. Also, the larger spot compensates for the lower transmission and detection efficiency. As a result, FT LMMS attains sensitivity levels similar to those of TOF LMMS, except for elemental ions, because of their high energy spread. TOF LMMS produces full mass spectra, of which only the mass resolution limits the m/z range, while the sensitivity in


357

FT LMMS is maximized in the high-resolution mode, permitting the monitoring from one to a few selected ions. Registration of full mass spectra requires accumulation of different shots. Concerning the information depth, LMMS selectively detects the surface constituents but monolayers are not sufficient. Real depth-profiling experiments are hindered by the discrete nature of the laser interactions, making the erosion rate less reliable than, for instance, continuous sputtering. However, laser drilling remains a useful method for exposing underlying material for analysis. Quantitation in LMMS is difficult because the ionization yield depends strongly on the local energy regime. The latter depends on the power density applied but also on a variety of material properties such as, for instance, sample reflectivity, UV absorbance, thermal conductivity, etc. These features are most often ill-defined and vary from spot to spot in heterogeneous specimens. Table 21-1. Analytical characteristics of LMMS. TOF LMMS Geometry Transmission Reflection 0.5 1 Minimum spot diameter (pm) Information depth 10-50 nm Crater depth 1 Pm 100-200 nm Mass accuracy nominal 500 Mass resolution Sensitivity ( I ) Element ions 106- 1o7 (2) Not reported Organic ions 10'- 1o8 4x 1 O9 mlz range H-unlimited Dynamic range 102-1 o3 3) neutrals in sample calculated according to Simons (1988) for 100 ions detected.

FT LMMS Reflection 5

< 50 nm 100 nm

0.1-1 ppm > los 108-109 6x108

15-15,000 102-103

21.4 Diagnostic information in LMMS The composition of the sample is derived by deductive reasoning, not by fingerprinting. Usually even comparison with reference spectra is not required. Fig. 21-2 illustrates how inorganic speciation is performed by FT LMMS. Already the low m/z signals give useful hints about the class and composition of the analyte. Moreover, the high m/z range normally contains intense peaks from cluster ions consisting of the intact molecules and one of the stable charged species detected in the low m/z range. Simple subtraction yields the MW of the analyte. Even the speciation of closely related molecules such as sodium nitrate and nitrite is feasible. In such cases however, relative signal intensities must be considered. The speciation capabilities of LMMS are convincingly demonstrated by Fig. 2 1-3 which illustrates the negative ion mass spectrum of Cr203 mixed with Na2S04 (Hachimi et al, 1995). The peak pattern observed contains the series of M,O, type ions, which are typical of the oxide, to which the signals from Na2S04, i.e. SO;, SOi


M,O,M+

@--n

or

MO-

MnO,,,.MO

L

+

MnOm.O *

MnOm.MnO~

7

I

I

NO;

I

M ~ N O.M

nitrite I nitrate

NOj

so; so;

sulphate

Y

+ or

M~NO~.NO'

+

___ ~ $ 0 4 . or~ M,SO~.MSO;

* Distinction between nitrate and nimte depends on the ratios N O i / NO,'

and M,NO,.NO,

/ M,NO,.NO,'.

Fig. 21-2. Flow chart illustrating how speciation of an inorganic compound can be achieved from FT LMMS signals.

I

IaSO4'

L

Fig. 21-3. Negative-ion mass spectrum from a mixture of Cr203and Na2S04recorded by TOF LMMS. (Reprinted from Hachimi et al. (1999, with the permission of Elsevier Science B.V.).

2 1 Laser-microprobemass spectrometry

359

carnidazole

50

100

150

200

250

m/Z

Fig. 21-4. Positive- and negative-ion mass spectra of camidazole N-oxide (MW 260), recorded by TOF LMMS. (Reprinted from Van Vaeck et al. (1988) with the permission of John Wiley & Sons).

and NaSOi, are simply added. Only the relatively small signals NaCrOi and NaCr04.CrOi arise fiom cluster ions containing building blocks of both constituents. However, identification of the individual analytes remains quite straightforward. The same applies to organic mixtures (Van Vaeck et al., 1990). The detailed structural information from organic molecules is exemplified by camidazole N-oxide in Fig. 21-4 (Van Vaeck et al., 1988). N-oxides are major metabolites of various drugs but are found to give little information in conventional MS. The MW information is indicated by the (M-H)- ions at m/z 259. The parent peak at m/z 242 cor-


responds to the (M-H20)'. radical fragment. The loss of water from the molecule is expected because it makes possible the extension of the aromatic system towards the amido-thio-ester functional group. The signal at m/z 243 due to (M-H-0)- indicates the N-oxide function, while the peak at m/z 97, not found in the original, non-oxygenated drug, reflects the presence of the oxide functional group on the aromatic ring. Using background knowledge about ion formation and fragmentation makes structural identification fairly straightforward. The soft ionisation of extremely labile organics under conditions of high power-density issues from the ultra-fast energy input making it possible to avoid thermal destruction. This capability is strikingly demonstrated by the study of explosives. Ions and neutrals are released so fast that detonation of the explosives does not occur (Tang el al., 1987).

21.5 Problem-solving in science and industry 21.5.1 Bio-engineering problems in human vein grafts Although biomedical investigations at the sub-cellular level require better lateral resolution, FT LMMS remains a method of choice because of its direct speciation capabilities, for instance, in the study of calcium deposits in the thickened intima of sub-occluded human saphenous vein grafts (Bakker et al., 1996). According to the element ratios determined by analytical electron microscopy (AEM), hydroxy-apatite was expected. However, FT LMMS indicated the presence of free or bound Ca, but not in the form of an oxysalt. Apparently, the characteristic X-rays for Ca and P observed in AEM originate from different molecules. This clearly illustrates the merits of detecting molecular species instead of element ratios.

21.5.2 Faulty floppy discs The quality of a floppy disc depends on the homogeneous dispersion of the magnetic elements in the organic poly(ethyleneterephtha1ate) (PET) matrix. Fig. 2 1-5 illustrates typical FT LMMS results. Most spots produce intense signals from iron at m/z 56 and from PET at m/z 105 and 149. The assignment of the minor signals is discussed elsewhere (Struyf et al., 1994). Other regions, however, show the dispersion of Mn (m/z 55) and no Fe in the organic matrix. Finally, areas occur where only the Fe signal is detected. This example also demonstrates the capability of FT LMMS to close the traditional gap between organic and inorganic micro analysis.

2 I Laser-microprobemass spectrometry

361

100

50

0

1 , 70

40

I00

j 1

108

' I '166 I ' I ' I '1a0 I '

130

I

'

t

'220 I '

zs0 I

I ' I '

'

m/z

K+

50

0

100

Mn+

105

55

bl,,

1

40

' I

' I '

!Id.. I

70

' I

II,l.

149 ,

I

' I ' I ' I ' I '

198

I

, I

' I ' I ' I ' I ' I ' I ' I ' I ' l ' l ' l ' l '

130

n/ L

160

190

220

2s0

56

sa

0

Fig. 21-5. Typical positive-ion mass spectra recorded by FT LMMS of a floppy disc. The spectra are from different regions in the PET layer of the disc. (Reprinted ftom Struyf el al. (1994) with the permission of John Wiley and Sons).


21.5.3 Multi-technique analysis of coatings on neo-ceramics The gain in information obtained from FT LMMS compared with TOF LMMS is illustrated by the analysis of a B4C neo-ceramic coated with Sic. The bulk composition is primarily assessed by analysis of the mass spectra of the negative ions, while the positive ions characterize essentially the surface contamination, Table 21-2. Assignment of the common positive ions of B4C with a Sic coating, recorded by TOF and FT LMMS. d Z

39 40 41 48 53

TOF LMMS Assignment

K', BCO' or C3H3+ Ca+or Sic'

4 1 ~ +19

, SiCor CHSi'

57

C4+ C,OH', BC2.H20f,29SiC; or SiC2H+ SiOCHt, CaOH', B3C' or

65 67

SiC3H+,%+ or 29SiC3' NaSiO', SiCBO' or C2B02+

73 79 83

B3Cat C3B0; or H3Si03+ SiCBO;, NaSi0; or BC;

88

SiOB: or SiO.Si0'

Measured d Z

FT LMMS Element composition K' LiO2+ Cat

Absolute m/z ermr (PPm) 0.2 0.5 0.5 I .5 0.9 1.o

38.9632 39.0053 39.9621 40.9613 47.9474 52.9843

Ti' SiC2H+

56.9648

CaOH'

1.o

64.9699 66.961 1 66.9663 73.0284 79.0542 82.9351 82.9612 82.9805 87.9052

CaC2H' NaSiO' CaBO' C~HSO; CsH; KSiO' CaB0; C2H30Ca' Sr+

1.1

4 1 ~ +

1.2 1.2 0.3 0.0 0.0 0.3 0.6 0.9

Table 21-2 compares the best possible interpretation of the detected ions, feasible on the basis of low- and high-mass resolution data. Identification in TOF LMMS must be based on nominal values and characteristic isotope patterns of B, K, Si and C. Digitalization errors reduce the precision in the analysis of peak intensities in TOF LMMS, so that the interpretation of complex signal patterns becomes difficult. In contrast, FT LMMS permits the unambiguous identification of the ions, including unexpected contaminants. The signal at m/z 73 must be attributed to organic ions, while the peak at m/z 88 corresponds to Sr'. Because FT LMMS also detects the ions formed after the laser pulse, additional ions above m/z 90 occur. According to Table 21-3, most signals point to the presence of phthalates, at least in part as Ca salts. The contamination layer also explains the rather confusing results in dynamic SIMS. The Si/C ratio is low in the beginning, then rises sharply and finally follows a gradually decreasing profile as expected for an element forming a surface coating.


363

Table 21-3. Assignment of the positive ions of B4C with an SIC coating, detected with FT LMMS. m/Z

88.9781 91.0542 98.9753 100.937 105.033 105.001 105.070 104.952 110.975 1 1 1.012 113.027 124.991 125.027 137.905 148.991 149.027 149.023 151.007 151.073 160.991 167.074 169.090

Assignment NaB02.Na' C7H7+ C2H302Ca+ CaSi02H' C7H@+ C5H5Ca+ CsH; NaB02K+ C3H302Cat C4H70Ca' C4H90Ca' C4H50ZCa+ C5H90Ca+ Ba' C6H502Caf C7H90Ca' CsH~03+ C6H702Ca+ C7HI2O2Naf C7H502Ca+ CsH150Ca' CsHI70Caf

Absolute m h error (ppm) 0.1 0.1

0.6 0.2 0.1 0.1

0.2 0.1

0.6 0.2 0.5 0.5 0.5 0.5 0.9

1.O 0.8 0.8 0.1 0.5

0.4 0.4

21.5.4 Identification of oil residues in the manufacture of rolled aluminium The surface composition determines the suitability of rolled metals for further coating steps. A rolling machine necessitates the use of lubricating emulsions which should not contaminate the processed metal. Whenever this occurs, a micro analytical technique is required to identify microscopic residues responsible for the contamination. Fig. 21-6 shows the FT LMMS identification of an additive from the lubricating emulsion on aluminium. Apart from the K+ from surface contamination and the A13+ ions from the substrate, the main positive-ion signals are readily associated with the triethanolamine (TEA) oleate, which actually consists of the protonated TEA cation and an oleate anion, forming an ion pair. However, information on the dehydrated molecules only is obtained from the ions at m/z 452, 438, 436 and 414. It is not yet known whether the loss of water occurs in the instrument or in the sample itself. The oleate group is characterized by m/z 265 and 309, while the TEA group gives rise to the signals below m/z 200. Note also the detection of ions incorporating A13+ at m/z 438, 347 and 174. In contrast to the ions containing a neutral molecule combined with K', Na' or possibly Al', which may result from the ionization process itself, the ions containing A13+point to specific products, present as such in the sample and formed by interaction between the organic molecule and the substrate. This type of information on direct specificity cannot be obtained by other techniques but becomes essential to the

364 Part 3: Chemical bonding and molecular composition 39

132

1I

I

00

28

m/z

m/z

(52

309

m/i

m/t

436

265

Fig. 21-6. FT LMMS analysis of a triethanolamine oleate additive of the lubricating emulsion on rolled aluminium : (a) shows the positive-ion mass spectrum, (b) the structural assignment of the major diagnostic ions as derived from exact mass measurements within 1 ppm.


365

surface properties of aluminium and its use in further processing or manufacturing steps.

21.5.5 Miscellaneous applications of material characterization 21.5.5.1 Diffusion Under well controlled conditions, quantitation in LMMS is possible, as highlighted by the study of lateral diffusion of B in TaSi2 and CoSiz as a function of the annealing temperatures (Heimbrook et al., 1989). Nevertheless, the strength of LMMS resides in its capability to handle the most diverse samples and to yield in a short time indications on the possible cause of practical problems. 21.5.5.2 Local heterogeneities Difficult problems concerning polymers are often caused by local heterogeneities due to poor dispersion of ingredients. TOF LMMS proved capable of identifying accelerator agglomerates in rubber (Holtkamp et al., 1991) and titanium oxide inclusions in poly(viny1 chloride) (Southon et al., 1984). LMMS does not generate extensive oligomer distributions as static SIMS does, but still provides sufficient information to identify a poly(methy1 methacrylate) inclusion in an injection-moulded polycarbonate as a result of the traces left from previous runs on the same machine (Holtkamp et al., 1991). 21.5.5.3 Contaminants A major concern in industrial processing is surface contamination. Examples include the tracing of residues on integrated circuits, due to adhesives used to mount the devices (Heinen and Holm, 1984), of submicron polystyrene particles on a silicon wafer (Thompson et al., 1989) and of K, Sn, Pb contaminants in the upper and subsurface layers of integrated circuits (Muller et al., 1989). The black instead of gold appearance of hardened steels produced by vacuum deposition of titanium nitride has been shown to be related to K+ and Ca’ contamination on the substrate (Southon et al., 1985). The cause of a paint defect was revealed by laser drilling: between the base coat and the lacquer layer was a polyester fibre from a spraying glove (Holtkamp et al., 1991). Adhesion problems in the manufacture of microelectronics due to residues of photoresist on the surface were investigated (Odom and Schueler, 1990). Also the presence of residual mobile ions in the sub-surface layer could be identified as the reason for leakage of Schottky barrier diodes (Daniel et al., 1988). 21.5.5.4 Metallurgical applications Applications to metallurgical problems include study of the annealing colours that appear when stainless steel is heated in the presence of oxygen. TOF LMMS permitted detection of boron at the 1% level in the sub-surface layers, which was infeasible by Auger electron spectroscopy (Heinen et al., 1984). (Boron can cause ‘catastrophic’

366 Part 3 : Chemical bonding and molecular composition

oxidation at high temperature in the presence of oxygen.) TOF LMMS is frequently used for light-element analysis, that is, the detection of hydrogen in metal alloys down to 1-10 ppm (Dingle et al., 1982; Ruckman et al., 1984; Kohler et al., 1986) and for the study of copper-beryllium alloys (Dingle et al., 1981). TOF LMMS has been used to characterize the presence of metal boride particles in nickel superalloys (Southon et al., 1984) as well as the formation of borosilicate precipitates upon brazing of Oxide-dispersion strengthened superalloys in gas-turbine engines (Kohler et al., 1986), the fine-grain-boundary MgH2 precipitates at the fracture surface of a Magnox alloy (Ruckman et al., 1984), and the segregation of lithium and other alloying elements in gas-atomized, rapidly solidified aluminium powder (Kohler et al., 1986). (These alloys are of importance in the aerospace industry due to their high strength-to-weight ratios.) 21.5.5.5 Geological and mineralogical applications

TOF LMMS has been used to characterize clay minerals and assess their adsorptive properties using pyridine as probing molecule (Akyiiz et al., 1985). Polyaromatic hydrocarbons have been detected in interplanetary dust using direct laser ionization together with post-ionization (Radicati di Brozolo et al., 1989). The method has also been used to obtain information on the microchemical nature of inorganic (minerals) and organic components (macerals) of coal, which is essential for the understanding of the coalification process (Morelli, 1990).

21.5.5.6 Forensic applications Less academic but more dramatic is a case study of dyed cloth fibres (Schmidt and Brinkman, 1989). Individual jeans fibres were investigated by TOF LMMS and the detected peak patterns compared. Specifically, signals corresponding to indigo dyes and fabric softeners used after washing were obtained. In this way, the fibre found on the body of a victim was revealed to correspond to those of the defendant's trousers. The closely agreeing pattern was used as legal evidence, which means that LMMS increases the challenge of achieving a perfect murder.

21.6 Conclusion The combination of focused laser irradiation of solids with MS has proved to be a powerful method for the local analysis of organic and/or inorganic constituents by a single instrument with minimal sample preparation. Especially the newer breed of instruments makes possible high mass resolution of mass spectrometric features and thereby increases its application to practical problem solving, where the wealth of information can be effectively exploited for qualitative identification. The examples discussed demonstrate that LMMS well deserves an appropriate place among the current techniques for the local and surface analysis of solids.


367

References Adams F., Van Vaeck L., Eds. (1986), "Proceedings of the Third International Laser Microprobe Mass Spectrometry Workshop, 26-27 August, 1986, Antwerp, Belgium", 224 pp. Akyiiz S., De Waele J.K., Akyiiz T., Adams F.C. (1985), "Laser microprobe mass analysis of pyridine adsorbed on some clay minerals from Turkey" J. Incl. Phenom., 3, 125-133. Asamoto B., Ed. (l99l), "Analytical applications of Fourier transform ion cyclotron resonance mass spectrometry", VCH Publishers, Inc., New York, 306 pages. Bakker A., Van Vaeck L., Jacob W. (1996), "Applications of laser microprobe mass spectrometry in biology and medicine", Scanning Microscopy International. Brenna J.T., Creasy W.R., McBain W., Soria C. (1988), "Nd:YAG laser microprobe system for Fouriertransform ion cyclotron resonance mass spectrometry" Rev. Sci. Instrum., 59, 873-879. Daniel W.M., Delorenzo D.J., Wilson H.R. (1988), Microbeam Analysis - 1988, Newbury D.E., Ed., San Francisco Press, Inc.: San Francisco, pp. 365-366. Dingle T., Griffiths B.W., Ruckman J.C. (1981), "LIMA - a laser induced ion mass analyser", Vacuum, 31, 571-577. Dingle T., Griffiths B.W., Ruckman J.C., Evans C.A.Jr., (1982), "Microbeam Analysis -1982", Heinrich K.F.J., Ed., San Francisco Press, Inc.: San Francisco, pp. 365-368. Hachimi A,, Poitevin E., Krier G., Muller J.F., Ruiz-Lopez M.F. (1995), Int. J. Mass Spectrom. Ion Processes, 144,23-45. Heimbrook L.A., Moyers K.W., Hillenius S.J. (1989), Microbeam Analysis - 1989, Russell P.E., Ed., San Francisco Press, Inc.: San Francisco, pp. 335-336. Heinen H.J., Holm R. (1984), Scann. Electron Microsc., 3, 1129-1138. Heinen H.J., Holm R., Storp S. (1984), Fresenius' Z. Anal. Chem., 319,606-610. Hillenkamp F., Kaufmann R., Eds. (1981), Fresenius' Z. Anal. Chem., 308, Springer-Verlag: BerlidJ. F. Bergmann: Munchen, 198 I , pp. 193-320. Holtkamp D., Bayer G., Holm R. (1991), Mikrochim. Acta, 1,245-260. Kaufmann R., Spengler B. (l992), "Proceedings of the 50th Annual Meeting of the Electron Microscopy Society of America", Bailey G.W., Bentley J., Small J.A., Eds., San Francisco Press, Inc.: San Francisco, pp. 1558-1559. Kaufmann R., Spengler B. (1 992), "Proceedings of the 50th Annual Meeting of the Electron Microscopy Society of America", Bailey G.W., Bentley J., Small J.A., Eds., San Francisco Press, Inc.: San Francisco, pp. 1598-1599. Kohler V.L., Harris A., Wallach E.R. (1986), Microbeam Analysis - 1986, Romig A.D. Jr, Chalmers W.F., Eds. San Francisco Press Inc., San Francisco, pp.467-470. Laukien F., Alleman M., Bischofberger P., Grossman P., Kellerhals H., Kofel P. (1987), "Fourier transform mass spectrometry: evolution, innovation and applications", Buchanan M.V., Ed., ACS, Washington, p. 8 1-99. Marshall A.G., Verdun F.R., (1990), "Fourier transforms in NMR, optical, and mass spectrometry: a user's handbook", Elsevier Science Publishers B.V., Amsterdam, The Netherlands, 450 pages. Morelli J.J. (1990), "Lasers and Mass Spectrometry", Lubman D.M., Ed., Oxford University Press: New York, Oxford, pp. 138-156. Muller J.F., Pelletier M., Krier G., Weil D., Campana J. (1989), Microbeam Analysis - 1989, Russell P.E., Ed., San Francisco Press, Inc.: San Francisco, pp. 31 1-316. Odom R.W., Schueler B. (1990), Lasers and Mass Spectrometry, Lubman D.M., Ed., Oxford University Press: New York, Oxford, pp. 103-137. Pelletier M., Krier G., Muller J.F., Weil D., Johnston M. (1988), "Laser microprobe Fourier transform mass spectrometry" Rapid Commun. Mass Spectrom., 2, 146-150. Radicati di Brozolo F., Meeker G.P., Fleming R.H. (1989), Microbeam Analysis - 1989, Russell P.E., Ed., San Francisco Press, Inc.: San Francisco, pp. 370-372. Ruckman J.C., Davey A.R., Clarke N.S. (1984), "Laser-induced ion mass analysis: a novel technique for solid-state examination" Vacuum, 34, 9 1 1-924.

368 Part 3: Chemical bonding and molecular composition Russell P.E., Ed. (1989), Microbeam Analysis - 1989, San Francisco Press, Inc.: San Francisco, 608 pp, Schmidt P.F., Brinkman B. (1989), Microbeam Analysis - 1989, Russell P.E., Ed., San Francisco Press, Inc.: San Francisco, pp. 330-332. Seydel U., Lindner B., Eds. (1983), "Proceedings of the LAMMA-Workshop, 1-2 September, 1983, Borstel, Germany", 159 pp. Simons D.S. ( 1 988), "Laser microprobe mass spectrometry : description and selected applications", Appl. Surf. Sci., 34, 103-1 17. Southon M.J., Witt M.C., Harris A., Wallach E.R., Myatt J. (1984), "Laser-microprobe mass analysis of surface layers and bulk solids" Vacuum, 34,903-909. Southon M.J., Harris A., Kohler V., Mullock S.J., Wallach E.R. (1985), Microbeam Analysis - 1985, Armstrong J.T., Ed., San Francisco Press, Inc.: San Francisco, pp. 310-314. Struyf H., Van Roy W., Van Vaeck L., Van Grieken R., Gijbels R., Caravatti P. (1993), "Laser microprobe Fourier transform mass spectrometer with external ion source for organic and inorganic microanalysis" Anal. Chim. Acta, 283, 139-151. Struyf H., Van Roy W., Van Vaeck L., Van Grieken R., Caravatti P. (1994), "The Fourier-transform laser microprobe mass spectrometer with external ion source as a tool for inorganic micro-analysis'' Rapid Commun. Mass Spectrom., 8.32-39. lang T.B., Chaudhri M.M., Rees C.S., Mullock S.J. (1987), "Decomposition of solid explosives by laser irradiation: a mass spectrometric study" J. Mater. Sci., 22, 1037-1044. Thompson S.P., Dingle T., Griffiths B.W. (1989), Microbeam Analysis - 1989, Russell P.E., Ed., San Francisco Press, Inc.: San Francisco, pp. 3 19-322 Van Vaeck L., Van Espen P., Gijbels R., Lauwers W. (1988), "Structural characterization of drugs and oxygenated metabolites by laser microprobe mass spectrometry" Biomed. Environ. Mass Spectrom., 16, 121-130. Van Vaeck L., Bennett J., Lauwers W., Vertes A., Gijbels R. (1990), "Laser microprobe mass spectrometry : possibilities and limitations" Mikrochim. Acta, 111, 283-303. Van Vaeck L., Van Roy W., Struyf H., Adams F., Caravatti P. (1993), "Development of a laser microprobe Fourier transform mass spectrometer with external ion source" Rapid Commun. Mass Spectrom., 7,323-33 1 . Van Vaeck L., Struyf H., Van Roy W., Adams F. (1994a), "Organic and inorganic analysis with laser microprobe mass spectrometry. Part 1: instrumentation and methodology" Mass Spectrom. Rev., 13, 189-208. Van Vaeck L., Struyf H., Van Roy W., Adams F. (1994b), "Organic and inorganic analysis with laser microprobe mass spectrometry. Part 2: applications" Mass Spectrom. Rev., 13,209-232.

Acknowledgements Luc Van Vaeck and Wim Van Roy are indebted to the National Science Foundation of Belgium (N.F.W.O.) as a research director and assistant respectively. Herbert Struyf acknowledges financial support from the Impulse Programme in Marine Sciences backed by the Belgian State-Prime Minister Services-Science Policy Office (Contract MS/06/050). Katrien Poels acknowledges support fiom the Belgian Program on Inter university Attraction Pole, initiated by the Belgian State-Prime Minister's Office-Science Policy Programming (IUAP contract #48). This work is sponsored by DPWB in the Inter university Attraction Pole program.



22 Fourier-transform infrared spectroscopy J.O. Leppinen

22.1 Introduction Infrared spectroscopy is one of the most powerful analytical tools for elucidation of the molecular structures of inorganic and organic compounds. It has a very wide range of applications from identification of organic compounds to material science in the semiconductor industry. Infrared spectroscopy is based on absorption of IR radiation causing transition in the sample from one vibrational state to another, higher energy, vibrational state. Consequently, the infrared spectrum is a set of absorption bands whose intensity and frequency provides information of structure and bonding in the molecule. FTIR instruments employ interferometer techniques in the collection of spectral information and the spectrum is calculated as an inverse Fourier-Transform of the interferogram. Infrared spectroscopy has long been applied to characterization of surfaces but its success had been rather limited until the advent of Fourier Transform instruments in the late 1960s. The major disadvantage of infrared spectroscopy in surface analysis has been its low sensitivity compared to high vacuum spectroscopic techniques such as XPS, LEED, EELS and SIMS. By the development of Fourier Transform Infrared (FTIR) instrumentation the sensitivity has been markedly improved and the range of applications of infrared spectroscopy in surface analysis has been dramatically increased. Recent improvements of FTIR instruments have made possible the effective use of specialized IR techniques that are suitable for surface analysis such as reflection-absorption spectroscopy and diffuse reflectance spectroscopy. Today ordinary commercial FTIR instruments developed for routine analytical work can be successfully applied to surface characterization by using accessories which provide optical conditions for the interaction between infrared radiation and the surfaces. FTIR spectroscopy is non-destructive, structurally specific and, unlike the high vacuum techniques, FTIR is able to provide spectral information about solid/liquid interfaces in situ. This chapter presents different experimental techniques and gives a few examples to illustrate the performance of FTIR spectroscopy in surface analysis. General principles of infrared spectroscopy or the operation of FTIR spectrometers are not presented in this chapter but reader is advised to consult a variety of textbooks available illustrating these topics in detail (Barrow, 1973; Colthup et al., 1975; Griffiths and Haseth, 1986).


22.2 Experimental techniques for surface analysis Today a variety specialized techniques is available for surface analysis employing FTIR spectrometers. As a rule, surface analysis with FTIR spectroscopy is based on standard commercial instruments with which, special attachments can be employed for monitoring the surfaces on different types of sample. Because absorption signals of the to lo-’ A are typically detected in surface analysis, special sensitivity order requirements must be set for FTIR instruments applied to surface analysis. The simplest technique used for surface analysis is transmission. Reflection techniques such as internal reflection (IRS, ATR), external reflection (ERS, IRRAS) and diffuse reflection (DRIFT) have been successfully applied to surface analysis for a variety of applications (Bell, 1987). Photoacoustic spectroscopy (PAS) can also be used for certain surface-analytical problems for powders. This presentation focuses on transmission and reflection techniques in the characterization of solid surfaces. The special emphasis is on monitoring of adsorption on solids and characterization of thin films.

22.2.1 Transmission spectroscopy Transmission spectroscopy is the conventional IR technique for routine characterization of almost any type of material. It has been applied to surface analysis since the 1950s (Eischens and Pliskin, 1958). A wide variety of sampling methods is available in transmission spectroscopy and a suitable one for the sample can be selected. The simplest technique for characterization of surfaces or adsorbed molecules on fine powders is to mix the sample with KBr (NaCl or BaF2) powder and press a disc transparent to the IR beam. IR radiation passes through the disc interacting with the molecules adsorbed on the solid surfaces (Fig. 22-1).

THIN FILM

A

PARAFFIN OIL WITH SAMPLE

B

Fig. 22-1, Sample-preparation techniques in transmission spectroscopy.

C

22 Fourier-transform infrared spectroscopy

37 1

A typical KBr to sample ratio is 1:200. The powder sample can also be mixed with paraffin oil (Nujol) and mounted between two KE3r discs. A disadvantage in preparing the sample as a KBr disc is possible alteration of the adsorbed layer, e.g. due to ion exchange. With paraffin oil, ion exchange is not expected but the oil may cause changes in the adsorption layer. These transmission techniques are typical ex situ techniques where the sample is transferred from conditions of adsorption (solid/solution, solidlgas) to a foreign environment occurring in the sample compartment of an FTIR instrument. A typical form of sample for studying thin films with transmission spectroscopy is a very thin transparent disc or a film deposited on a supporting disc such as silicon. Thin-film work is typically done on films with a thickness of the order of one micron. When the film itself is studied, the spectral information varies with film thickness (Back, 1991). For very thin films the infrared spectrum emphasises the surface bonds whereas for thicker films bulk bonding predominates in the spectrum. Adsorbed molecules can be studied in a transmission configuration by depositing the substrate on transparent supporting material. In this case the surface area of the deposited substrate is critical because the coverage of adsorbing molecules is usually very small. The substrate on which adsorption takes place has typically a microcrystalline structure with a surface area much greater than the geometric area of the disc. In catalysis research, the metallic substrate is usually deposited on a 0.1-0.25 mm aluminium oxide disc. It is also possible to utilize a fine metal net, e.g. made of platinum, with a surface area high enough for adsorption studies. In catalysis research, in situ transmission measurements can be used to study adsorption from the gas phase by use of disc samples such as described above. In situ transmission studies for solidlsolution interfaces are not common. However, special cell designs where a thin liquid layer is placed between two transparent windows allows the monitoring of reactions on window surfaces in situ.

22.2.2 Internal reflection spectroscopy The internal reflection technique is based on total internal reflection of IR radiation at the interface of an IR-transparent internal reflection element (IRE) (Fig. 22-2). The element must be optically denser than the sample, i.e. the refractive index of the element, n2, must be greater than the refractive index of the sample, nl, (Harrick, 1979). For total internal reflection the angle of incidence must exceed the critical angle, 8,, according to Snell’s law: sin(8,)

= n2/n1

(22-1)

In the internal reflection mode an exponentially decaying evanescent field is set up and a fraction of radiation passes through the boundary of the surface of the element. The depth of penetration, d,, characterized by the distance required for the electric field amplitude to decrease to l/e of its value and is given by

372 Part 3 : Chemical bonding and molecular composition

d,

= h/[27cnl(sin28-

n2l2)"1

(22-2)

where h is the wavelength of the incident radiation and n21=n2/nl. According to eq. 22-2, the depth of penetration depends on the wavelength. This means that absorption bands at longer wavelength are intensified in the internal reflection spectra of thick samples. With very thin sample films the effective pathlength depends only on the angle of incidence and the ratio of refractive indices. Typical materials for internal reflection elements (Burrows, 1992) are listed in Table 22- 1.

I

EVANESCENT FIELD

k/A\"....I.

\

..................................

0 i..

01 ...!

INTERNAL REFLECTION ELEMENT (IRE)

\

Fig. 22-2. Schematic representation of multiple internal reflection in a parallelepiped-shaped IRE (Reproduced from Young and Miller, 1993 by permission of AIME).

The effective pathlength can be increased by using the multiple internal reflection (MIR) technique in which several reflections take place in the internal reflection elements. This principle is applied practically in all internal reflection accessories. The attenuated total reflectance (ATR) technique is based on multiple reflection in the element against which the sample is placed. In surface analysis of powders the sample is simply pressed against the element. Wet samples can also be measured in the form of filter cakes or as a paste. Thin films deposited on an internal reflection element can be effectively studied by multiple internal reflection techniques. The largest number of studies of inorganic thin films is for silicon but other inorganic and organic thin layers have also been studied. The outer surface of the reflection element can also be studied using multiple internal reflection. The surface can be chemically 'reactive' or 'inert'. Materials such as calcium carbonate, calcium fluoride and zinc sulphide, have been used as reactive surfaces in chemisorption or redox reactions.


373

The ATR technique presents a number of benefits which makes it superior to transmission techniques in surface analysis. 1) Spectra of strongly absorbing solids can be recorded; 2) molecular orientation can be determined by using a polarized beam; 3) highly scattering substances can be studied; 4) in situ characterization of solid-liquid interfaces is possible. Table 22-1. Materials used for IR internal reflection elements.

Refractive index Useful range (cm-') Comments 1.7 45000-2200 H, I 30000-1400 1.68 9000-1500,300-60 3.42 1.4 30000-1400 CaFz 2.22 14000-1000 ZnS 5000-800 4.0 Ge 10000-800 2.8 As2Se3 3.14 10000-700 GaAs 2.42 20000-500 ZnSe 2.00 25000-500 AgCl 2.4 10000-500 a-Se 20000-400 2.2 AgBr 10000-400 2.65 CdTe 1.72 20000-300 CSI 15000-250 2.35 KRS-5 2000-400 4.8. 5.3 Te 17000-200 2.7 ZnTe Diamond 2.4 10000-2600, 1700-100 H, I H=hard, S=soft, B=brittle, I=chemically inert, WS=water soluble Material A1203

22.2.3 External reflection spectroscopy Infrared external reflection spectroscopy (IR-ERS) has emerged an important tool for monitoring molecular structures and bonding at chemically modified surfaces. IR-ERS is generally based on single external reflection of IR radiation at the surface of metal or metal film. The advances made in FTIR instrumentation have markedly reduced the sensitivity limitations associated with external reflection spectroscopy. The basis for applying IR-ERS to the characterization of surfaces was developed by Greenler (1 966) and Francis and Ellison (1 959). These studies defined the dependence of a reflection spectrum on the optical functions of the absorbate and the metal substrate and on the angle of incidence and polarization of the incident light. External reflection spectroscopy requires a highly reflecting surface usually made of metal. IR radiation impinges at certain angle on the sample deposited on the metal. The electromagnetic radiation interacts with the sample and hits the reflective surface followed by a repeated interaction with the sample and reflection (Golden et al., 1984) (Fig. 22-3). This method is also called reflection-absorption spectroscopy (IRRAS) or


grazing angle reflection. The term reflection-absorption spectroscopy is often associated with external spectroscopy using a polarized IR beam.

I

c'

7

31,bate

a

b

A 0

Wavelength

Fig. 22-3. The classical three-phase model (a) for an absorbate layer on metal to calculate the relative absorption A defined in (b). (From Hayden (1987) by permission of Plenum Press).

When light is reflected from a metal surface, the electric vector of the incident radiation experiences a phase shift that depends on the angle of incidence and the state of polarizsation (Fig. 22-4). The parallel component, p, is defined as the component whose electric vector is in the plane of reflection. The perpendicular component, s, is perpendicular to the plane of reflection. For all angles of incidence, the phase change for the perpendicular component is nearly 180' and the reflected vectors sum up to almost zero at the surface, resulting in only a very small net electric field parallel to the surface. At small angles of incidence the phase change for the parallel component is nearly zero and again the


375

incident and reflected vectors nearly cancel. At larger angles of incidence, however, the vector sum of the incident and reflected electric vectors results in an electric field at the surface with a substantial component perpendicular to the surface normal. From the above it follows that only the parallel component of the incident radiation can be absorbed by a surface layer and the maximum absorption occurs at near grazing incidence.

Fig. 22-4. Illustration of the phase change upon reflection at a metal surface for the components of incident radiation polarized perpendicular or parallel to the plane of reflection. (From Golden et al. (1 984) by permission of American Chemical Society).

For metallic substrates, the optimum angle of incidence in IRRAS is near grazing incidence. Only the parallel component, p, of the incident radiation will then be absorbed. Vibrational modes of the molecule which oscillate perpendicular to the surface consequently have greatest probability of absorption. The modes which oscillate parallel to the surface will not be detected at all (surface selection rule). This polarization dependence can be exploited to determine the orientation of surface structures on metals. For non-metallic substrates such as semiconductors (Porter, 1988) the maximum sensitivity is obtained at angles of 60 to 80' for p-polarization and close to normal for s-polarization. In contrast with metallic substrates, vibrational modes from all three directions can be obtained with non-metallic substrates. At metal surfaces the electric field is enhanced several times at grazing angle of incidence while no


enhancement effects have been observed with non-metallic samples (Mielczarski, 1993). Uniformed reduction of the electric field is found for all angles of incidence. 22.2.3.1 Multiple external reflection In the multiple external reflection technique the IR beam is reflected several times between two parallel reflecting plates and spectral information characterizes the surface and adsorbates. Multiple external reflection techniques are typically used for surface characterization of metals, especially before the advent of FTIR spectrometers. In the multiple reflection technique, the pathlength and the number of absorbate molecules interacting with the IR beam can be markedly increased and consequently the intensity of the absorption signal is higher than for single reflection. Because improved experimental sensitivity obtained with modern set-ups allows monitoring of solidgas solidlvacuum interfaces, the use of the multiple reflection principle has diminished over that for single reflection. Due to the energy losses in each reflection and difficulty in optimizing the optical set-up, the signal-to-noise ratio in the multiple reflection technique is not expected to be better than for single reflection (Greenler, 1975).

22.2.4 Diffuse reflectance When IR radiation falls onto a sample surface several processes can occur: it can be absorbed, reflected or penetrate the sample before being scattered (Firth, 1988). Diffuse reflectance spectroscopy is based on the latter effect. The accessory shown schematically in Fig. 22-5 focuses the IR beam onto the solid sample and collects the diffusively scattered radiation before returning the radiation to the detector. The technique involves little sample preparation and the undesired effects of pressing the sample into a disc, or the disturbing effects of solvents or mulling agents, can be avoided. INPUT ELLIPSOID

OUTPUT ELLIPSOID

SAMPLE CUP Fig. 22-5. Schematic representation of a diffuse reflectance accessory. (Reproduced by permission of Spectra-TechCorporation).


377

In conventional diffuse reflectance measurement of powders the sample is mixed with potassium chloride or potassium bromide. The mixing ratio is typically 90-95% diluent and 5-10% sample. The mixture is placed in a sample cup and levelled off. In special cases samples can be measured without dilution but the quality of the spectra is usually worse than with dilution. A special sample preparation technique for diffuse reflection which can improve the sensitivity involves deposition of the sample as a thin layer on the surface of the non-absorbing matrix. An interesting sample-preparation technique is based on the use of a special silicon carbide paper on which the sample is rubbed until a small amount of sample is smeared on the surface of the paper. Diffuse reflectance is a powerful tool in studying of all types of solid sample. It is able to handle a wide range of samples including powders, crystals, solids with rough surfaces, gemstones, minerals, plastics and fibres. Diffuse reflectance is one of the techniques which has clearly benefited from FTIR instrumentation. With the advent of FTIR spectrometry and improved attachment design the diffuse reflectance technique has become practical. Diffuse reflectance spectra of pure samples, without dilution, tend to have very intense absorptions with little difference in intensity between absorptions designated as weak or strong in transmission spectra. This effect is removed by dilution the sample with non-absorbing material. Kubelka and Munk (1 93 1) developed the general theories for diffuse reflectance and Kortum et al. (1963) extended these theories for dilute samples in non-absorbing matrices. According to the studies above, the quantity (1-R)*/2R where R is the ratio of sample to reference reflectance, is linearly related to concentration provided that scattering effects are constant. In quantitative diffuse reflectance spectroscopy it is important that sample preparation and particle size are as consistent as possible. In surface analysis, however, the relationship of band intensity to concentration is more complex than for the study of adlayers of solid particles. The application of diffuse reflectance spectroscopy to surface analysis is mainly in characterization of adlayers on powders and on the rough surfaces of solids. The applicability of diffuse reflectance to surface analysis is wider than that of transmission techniques because of less destructive sample preparation and greater sensitivity. The requirements in regard to high surface area are essentially same in both transmission and diffuse reflectance techniques. Compared with transmission spectroscopy of powder surfaces there is an obvious greater sensitivity. This is predominantly due to beam pathlength in diffuse reflectance. The scattering, especially at high frequencies, observed in transmission measurements does not markedly interfere with diffuse reflectance measurements.

22.3 Surface characterization of different types of samples This section discusses the characterization and sample preparation for different types of surface. Two main sample types are powders and large solid surfaces which can be prepared by polishing or depositing techniques. Examples are given of the adsorption of carbon monoxide on metals and reactions products of xanthate on sul-

378

Part 3: Chemical bonding and molecular composition

phide mineral and metal surfaces. The suitability of the surface characterization techniques for in situ measurements are discussed and the principles of spectroelectrochemistry outlined.

22.3.1 Powder samples Infrared spectroscopy is a very useful technique for characterization of functional groups on the surface of powdered solids and their interactions with absorbate molecules, The earliest studies of surface interactions using infrared spectroscopy took place in the 1940s. The first application of infrared spectroscopy was adsorption processes at the solidlgas interface. Phenomena at the solid/liquid interface received less attention until developments in IR instrumentation and accessories for sample preparation. Transmission, internal reflectance and diffuse reflectance are the most widely used techniques for studying powder surfaces. Photoacoustic spectroscopy can also be used in certain conditions but this technique is outside the scope of this presentation. 22.3.1.1 Solid/gas interface In transmission techniques for powder samples, not only is infrared radiation absorbed by the bulk sample and surface species, but scattering of radiation also occurs. The scattering is reduced for smaller powder particles. The small size of particles is also advantageous because small particles have a large surface area which improves the possibilities of detecting the surface groups of adsorbed species. With modern FTIR instruments adsorption phenomena on powders with surface areas smaller than 1 m2 can be easily studied. From the point of view of scattering, best results are achieved for particle size below 5 pm (Bell, 1987). Ex situ transmission studies generally involve sample preparation in a separate chamber or reaction vessel followed by transfer of the sample to the spectrometer. The disadvantage in these techniques is that the adsorption reactions must be irreversible, otherwise alteration of the sample may occur and the spectral information obtained does not represent the original state of the surface. Contamination of the sample may also occur in transferring the sample, e.g. by adsorption of gases or by oxidation due to atmospheric oxygen. The majority of infrared transmission studies of high-surface-area powder surfaces have employed a technique in which the adsorbent material is compacted in a suitable die into a self-supporting disc. For catalyst samples the disc typically contains 10 to 50 mg cmT2sample and the thickness is about 100 pm. The disc is introduced into a vacuum cell in which it can be treated with vapours of absorbate molecules either at low or high temperatures. For these set-ups a large number of cell designs has been described in the literature. The cell is usually a small-volume enclosure with IRtransparent windows, passages for gas inlet and outlet and heater for the sample. A wide range of materials (see Table 22-1) can be used as IR-transparent windows. The selection of a particular material depends on the spectral range to be used, resistance against moisture, pressure strength and cost.


379

A technique based on species adsorbed on supported metals has been extensively applied to metals. The metal is usually dispersed on a finely divided oxide support, e.g. Si02, A1203, Ti02 and MgO. This technique involves impregnation of the supporting material with, for example, a solution of a metal salt which is evaporated to dryness and the precursor is converted to metal by reduction with hydrogen. Typical metal content in the support is about 10%. Very thin layers of catalyst powders can also be deposited on a supporting disc by spraying a suspension of the catalyst in a volatile solvent onto a heated IR-transparent window. A number of spectroscopic studies of CO adsorbed on supported metal catalysts has been presented in the literature using both dispersive and FT instruments. Cant and Bell (1982) presented in their hydrogenation studies the chemisorption of two CO isotopes on ruthenium supported by silica (Fig. 22-6). The chemisorbed CO exchanged rapidly with gas phase CO which was seen as the replacement of Cl60 adsorbed on RdSiOz by C"0.

Frequency (cm-3 Fig. 22-6. Spectra showing replacement of CI6O adsorbed on Ru/SiOz in the presence of hydrogen. (a) CL60alone; (b) 8 s after switching to C'*O; (c) 150 s after switching to C"0. (From Grifiths and Haseth (1986) by permission of John Wiley & Sons).


Diffuse reflectance has been successfully applied to in situ studies of the solidgas interface of metals and metal oxides. Today commercially manufactured diffuse reflectance cells are available for catalytic studies in gaseous atmospheres at temperatures up to 500 OC. The applications of internal reflection techniques to solid/gas interfaces are rather few because of the low energy throughput and the long optical pathlength in the solid. 22.3.1.2 Solid/liquid interfaces

FTIR characterization of solidniquid interfaces on powder samples is more demanding than for solidlgas interfaces. The simplest sample-preparation technique suitable for irreversible surface reactions and for stable surface layers is the KBr disc technique for powder samples after treating in solution. This technique involves removing the sample from the reaction conditions in solution followed by drying, making it a typical ex situ technique. Characterization of solid/solution interfaces on powder surfaces in situ can be achieved using the ATR technique. The sample treated in aqueous solution is transferred as a slurry or as a paste onto an ATR reflection element and the spectral information is collected from the surfaces of particles surrounded by the appropriate solution (Strojek et ul., 1983). With this technique the effect of solution conditions such as pH and concentrations of various ions and molecules has been studied successfully. In a special cell design the sample slurry can be transferred into the ATR cell without exposing the sample to atmospheric air (Leppinen et al., 1988) (Fig. 22-7).

L

8

Fig, 22-7. Schematic representation of an ATR cell used for studies of wet powders. ( 1 ) germanium element; (2) base plate; (3) cell body; (4) O-ring; (5) filter paper; (6) perforated movable plate; (7) screw; (8) sample slurry. (From Leppinen et al. (1988) by permission of Elsevier Science Publishers B.V.).


38 1

The difficulty in ATR techniques is the selection of the infrared-transparentmaterial for the reflection element. Most IR-transparent materials are not durable against aqueous solutions. The optical material should also be inert with regard to the adsorption of molecules of interest and hard enough for pressing the powder sample against the element. Germanium meets many of these requirements for a spectral range from about 5000 to 800 cm". Zinc selenide has a wider useful spectral range (5000 - 500 cm-') but it is not chemically as inert as germanium. Silicon is hard and inert but the frequency range is severely limited (9000-1500; 300-60 cm-I). Chemisorption of ethylxanthate C2Hs-O-CS2- on lead sulphide was studied (Leppinen and Rastas, 1986) with the in situ ATR technique presented above. The spectra revealed that ethylxanthate is attached to lead sulphide in the form of lead ethylxanthate surface compound. The sensitivity for xanthate adsorption measurements on high-surface-area PbS allowed monitoring of the adsorbed species at coverages of 2% of the theoretical monolayer (Fig. 22-8).

Q

b C

1400

1300

1200

1100

1000

WAVENUMBER, cm-1

goo

Fig. 22-8. FTIR spectra of lead sulphide at different coverages of ethyl xanthate: (a) O=O; (b) 8=0.09; (c) 0=0.32;. (d) e=o.58; (e) e = i . i ; (f) 8=2.1; (g) precipitated lead . . ethylxanthate. (From Leppinen et a/. (1988) by permission of Elsevier Science Publishers B.V.)


22.3.2 Surfaces of polished samples and thin films Thin films have been extensively studied by transmission spectroscopy, internal reflectance and external reflectance techniques. Several methods are also available where thin films serve as substrate for adsorption processes. This section focuses on the adsorption processes rather than on the structure of the film itself. Transmission spectroscopy is the simplest overall technique which has also been widely applied to thin film studies. The limitations for characterization of thin films are that the material must be transparent to infrared or sufficiently thin so that the radiation is not lost by absorption or scattering. The films are typically deposited on polished discs of KBr, NaCl, BaF2 or on silicon wafers. Transmission studies can be routinely carried out on one- to a few-micron films deposited on silicon. A great volume of infrared spectroscopic work on analysis of inorganic films is available for silicon and silicon-containing films. Films of carbon, aluminium oxide and metal oxides such as nickel oxide, have also been extensively studied. Internal reflection spectroscopy has also been applied to studies of semiconductor films. Semiconductor layers such as GaAs grown by metal-organic vapour phase epitaxy on GaAs substrates have been studied on Ge internal reflection elements. Films of Si02 on Si have been probed using a similar configuration. Significant enhancement of the electric field in the film region was observed with films having lower refractive index than the internal reflection element. Olsen and Shimura (1988) observed that the field was enhanced by a factor of 3 for the s-polarized beam and by more than two orders of magnitude for the p-polarized beam. Configurations in which the semiconducting material itself is used as an internal reflection element have also been successfully applied to semiconductor studies. This technique allows monitoring the surfaces in situ in different atmospheres and also under liquid conditions. 22.3.2.1 Adsorption on solid surfaces Transmission and internal and external reflection techniques have been extensively applied to studies of adsorption processes on polished solid substrates and thin films deposited on different kinds of solid surface. For small-area samples the transmission technique is not as useful as for the high-surface-area powder samples described earlier in this chapter. On the other hand, multiple internal reflection techniques (ATR) and external reflection techniques (IRRAS) are very usefid for investigation of such samples. Application of transmission methods for in situ surface analysis on polished solids is limited to materials transparent to IR radiation. In the case of interfaces between a solid and an aqueous solution, only window materials which are insoluble or slightly soluble can be employed. An example of in situ investigation is adsorption of ethyl xanthate on zinc sulphide presented by Richardson and Termes (1986). The cell is made of two parallel zinc sulphide windows and the aqueous ethyl xanthate solution is placed in the compartment between the discs. With this cell design the adsorption of


3 83

ethyl xanthate on zinc sulphide surfaces could be investigated under different solution conditions. Adsorption on thin films deposited on internal reflection elements such as germanium IRE have been studied (see Fig. 22-10). The advantage in this technique is that the interfaces can be studied in situ. Because the beam has to penetrate the film to reach the interfaces (solidgas, solidliquid) the film thickness is critical due to the strong infrared absorption, especially in multiple reflection measurements on metals. Depending on the material and the optical arrangement, typical film thickness for films deposited on IRE range from a few nanometers to tens of nanometers. The difficulty in this technique is the deposition of the film on the IREs. Consequently, only a limited range of materials can be studied with this technique. The majority of studies has been focused on metals (Fe, Au, Cu, Pt, Cd) deposited by vacuum evaporation on germanium and other IRE materials. Carbon, metal oxides and metal sulphides as well as polymer films deposited directly on IREs or metals films on IREs have also been studied. Internal reflection techniques are useful in characterization of solid/liquid interfaces. By using films on a suitable internal reflection element such as Ge, in situ infrared studies can be carried out under different solution conditions, e.g. as a function of pH or concentration of absorbate. The problem involved in such measurements is that the deposited layer must be very thin yet compact enough to represent the behaviour of the material to be investigated and not a composite of the element and the IRE. The contact between the IRE and the film must be sufficiently durable so that the film will not be detached under exposure to the solution. Characterization of absorbates with internal reflection spectroscopy, using the IRE itself as the surface, provides interesting information on solid-solution interfaces in situ. The IRE can be inert or reactive in surface reactions such as oxidation or adsorption from solution. Young and Miller (1992), presented an interesting system monitoring the absorption of oleate on calcite and fluorite IREs. The sorption of ethyl xanthate and other adsorbents on germanium has been presented in numerous studies. Oxidation and hydrogen passivation reactions on Si have been studied using Si as the IRE and solvent evaporation and oxidation on GaAs. Electrochemical control has been associated with some of the studies on reactive IREs. Submonolayer sensitivity can be easily achieved using these in situ ATR techniques External reflection (ERS, IRRAS) is perhaps the most useful technique for studying adsorption processes on solid samples prepared by polishing or deposition of film. Sensitivity limitations due to the small number of absorbate molecules for interaction with the IR beam have been to a large extent removed by recent developments in FTIR instrumentation. Numerous significant areas are currently being investigated with external reflection techniques: adhesion, electrocatalysis, corrosion and adsorption. Langmuir-Blodgett monolayers of cadmium arachidate (CdAA) deposited on evaporated silver substrates were presented by Golden et al. (1984). Fig. 22-9 shows the IRRAS spectrum of one monolayer of CdAA where symmetric and asymmetric carboxylate stretching bands at wavenumbers 1439 and 1560 cm-' can be observed. A new


band, possibly due to water adsorbed from the air at wavenumber 1595 cm-' is also evident.

I AR,

IR,

Fig. 22-9. FT-IRRAS spectrum of one monolayer of cadmium arachidate on silver at 4 cm" wavenumber resolution. (From Golden et al. (1984),by permission of American Chemical Society.)

Most external reflection measurements on surfaces are typically ex situ where the spectroscopic information is collected under conditions different from those under which the surface reactions occur. Solidgas interfaces can be studied in situ using an experimental set-up essentially similar to that used for transmission and diffuse reflectance studies, provided that the optical arrangement allows the use of an optimum angle of incidence. In situ studies at solid-liquid interfaces are more difficult than studies at solidgas interfaces because the IR beam has to penetrate the liquid film between the window and the reflecting surface (see Fig. 22-12). The full benefits of the


385

improved sensitivity gained by grazing angle of incidence cannot be achieved in in situ studies of solid/liquid interfaces due to the optical properties of the window and the liquid film.

22.3.3 Spectroelectrochemistry FTIR spectroscopy of surfaces during electrochemical processes has become increasingly important in monitoring processes at electrode/electrolyte interfaces (Korzeniewski and Pons, 1987). Compared with high-vacuum surface-characterization techniques the advantage of FTIR spectroscopy is that it provides information in situ about the properties of electrode surface and the double layer. It can also be used to characterize the absorbate species and the coverage of the electrode surface. Several sophisticated methods have been developed for in situ analysis of electrode surfaces including internal and external reflection spectroscopic methods. This section outlines briefly the experimental systems used in spectroelectrochemisty and gives an example of the performance of the external reflection technique in monitoring surface reactions under electrochemical control. Spectroelectrochemistry investigations employing internal reflection spectroscopy (Neugebauer et al., 1981) are based on deposition of the electrode material on an internal reflectance element or using the element itself as an electrode (Fig. 22-10). CONNECTION TO REFERENCE ELECTRODE IR B

COUNTER ELECTRODE DEPOSITED ELECTRODE LAYER

IRE

IR BEAM

Fig. 22-10. Schematic diagram of an electrochemical cell used for internal reflection. (Reproduced from Neugebauer et al. (1981), by permission of Elsevier Sequoia S.A., Lausanne.)

Spectroelectrochemical studies with internal reflection spectroscopy have been carried out for numerous materials deposited on IRES including metals, metal sulphides and conducting polymers. A number of non-conducting IRE materials has, also been successfully used in spectroelectrochemistry. The deposition technique suffers from difficulties associated with the contact between the deposited layer and the IRE. On the other hand, the range of materials which can be deposited by vacuum or other techniques is limited. For spectroelectrochemical studies of minerals (Leppinen et al., 1989) which cannot be deposited stoichiometrically, a special cell was designed


Solution Inlet

-Luggin

rence Electrode Capillary

Working Electrode , , Holder ,

Solution Outlet Cell Body

ounter Electrode

Fig. 22-1 I . Schematic representation of the electrochemical ATR cell constructed for in sifu FTIR measurements. (From Leppinen er al. (1989) by permission of Elsevier Science Publishers B.V.)

Luggin capillary

Gasket

Working electrode

Counter electrode

Fig. 22-12. Spectroelectrochemical external reflectance cell for in situ measurements. (From Talonen et al. (1991), by permission of John Wiley & Sons Ltd..)

(Fig. 22-1 1). In this cell the electrode was contacted with the IRE just collecting the spectral data. External reflection spectroscopic cells for spectroelectrochemistry usually consist of three-electrode microcells (Bewick et al., 1984; Pons et al., 1984)

22 Fourier-transform infi-ared spectroscopy

3 87

with an optical window transparent to the IR beam (Fig. 22-12). Because the ppolarised radiation is the only component which has significant amplitude at the electrode surface, it alone carries vibrational information of the surface species. Vibrational modes of the molecule which oscillate perpendicular to the surface will have the greatest possibility of absorption. Thus, the relative intensities of the absorption bands can provide information about the orientations of the molecules at the electrode surface. Since s-polarised radiation has virtually no electric field strength at the surface of a metal electrode this radiation contains no useful information about the adsorbing species. However, species in the solution are randomly oriented and can interact with each polarization. If the difference spectrum is calculated by subtracting the difference between p- and s-polarization the difference is only contributed to by the adsorbed species and thus distinction can be made between the adsorbed and solution species. The signal-to-noise ratio can be improved by using polarization modulation of the reflection absorption spectroscopy, which is based on modulation of polarization at high frequencies. Kunimatsu and Golden (1985) used a double modulation technique at 78 kHz frequency. The sensitivity ofreflection-absorption spectroscopy on FTIR instruments can by improved by Subtractively Normalized Interfacial Fourier Transform Infrared Spectroscopy (SNIFTIRS) (Beden and Lamy, 1988). This technique is based on collecting successive series of interferograms at two potential limits El and E2 which are selected according to the electrochemical behaviour of the system studied. EI is the reference potential and E2 the potential of interest at which the electrochemical process occurs. If RI and R2 are the reflectivities measured at El and E2, the relative change of reflectivity A M 3 is

AlUR = (R2 - Ri)/R = (R~/RI) -1

(22-3)

Sundholm and Talonen (1 995) studied the adsorption of ethyl xanthate on a silver electrode in situ. They used a spectroelectrochemical cell to monitor the chemical species on silver at different potentials. Fig. 22-13 shows that in the region where a pre-wave exists in a voltammogram, at -0.03 V, only chemisorbed xanthate adsorbed on the surface (1220 cm"). At higher potentials (0.03 V) the formation of silver xanthate is evident, as is indicated by new absorption peaks at 1200, 1188, 1143, 1114, 1032, 1018 and 999 cm-'. This study clearly shows that a monolayer of xanthate on a metal electrode is different from bulk silver xanthate. The significant difference between the spectra of the two xanthate species is probably due to the orientation of ethyl xanthate molecules in the chemisorbed layer. Although reflection-absorption spectroscopy in spectroelectrochemistry is a very promising technique, it still suffers from several experimental difficulties. The masstransfer properties and uneven electrochemical conditions can be problems in the thinlayer cell. The concentration of absorbate can also vary greatly due to the adsorptioddesorption processes in the small-volume cell. These difficulties can, however, be avoided with a flow-through cell design. Depending on the absorbate, electrolyte and


the electrode material, the signal-to-noise ratio in spectroelectrochemistry using IRRAS techniques can be sufficiently good to permit monitoring electrode processes at submonolayer and monolayer coverages. Efforts are still needed to increase the sensitivity.

1400

1300

1200 1100 Wavenurnber/cm-'

1000

Fig. 22-13. FTIR spectra of ethyl xanthate on a silver electrode. Borate buffer pH 9.2, KEX lo4 mol I-', (a) 0.03 V; (b) 0.03 V (250 pC (c) 0.03 V (330 pC cm-2);(d) 0.04 V (520 pC ern-'); (e) 0.07 V (1.3 mC ern-') (4 transmission spectrum of AgEX. (From Sundholm and Talonen, 1995, by permission of Elsevier Science S.A.)

References Back D.M. (1991), Phys. Thin Films, 15,265-312. Barrow G.M. (1973), Physical Chemistry. McGraw-Hill Kogakusha. Beden B., Lamy C. (1988), Spectroelectrochemistry, Theory and Practice: Gale R.J. (Ed.). Plenum Press, 1988, pp. 189-210. Bell A.T. (1 980), Vibrational Spectroscopies for Absorbes Species: Comstock M.J. (Ed.). American Chemical Society, 1980, pp. 14-35. Bell A.T. (1987), Vibrational Spectroscopy of Molecules on Surfaces: Yates J.T. and Madey T.E. (Eds), Plenum Press, pp. 105-134. Bell R.J.( 1972), Introductory Fourier Transform Spectroscopy, Academic Press. Bewick A., Kunimatsu K., Pons B.S., Russell J.W. (1984), J. Electroanal. Chem., 160,47. Burrows V.A. (1992), Solid State Electronics, 35,231-238. CantN.W., Bell A.T. (1982), J. Catal., 73,257.


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Colthup N.B., Lawrence H.D., Wiberlay S.E. (1975), Introduction to Infrared and Raman Spectroscopy. Academic Press. Eischens R.P., Pliskin W.A. (1958), Adv.Catal, 10, 1. Firth S. (1988), International Laboratory, October, 26. Francis S.A., Ellison A.H. (1959), J. Opt. SOC.Am., 49, 131. Golden W.G., Saperstein D.D., Severson M.W., Overend J. (1984), J. Phys. Chem., 88,574. Greenler R.G. (1 966), J. Chem. Phys., 44, 3 10. Greenler R.G. (1975), J. Vac. Sci. Technol. 12, 1410. Griftiths P.R., Haseth J.H. (1986), Fourier Transform Infrared Spectrometry. John Wiley & Sons. Harrick N.J. (1979), Internal Reflection Spectroscopy. Harrick Scientific Corporation. Kortum M.P., Braun W., Herzog G . (1963), Angew. Chem., 2,333. Korzeniewski C., Pons S. (1987), Prog. Anal. Spectrosc., 10, 1. Kubelka P., Munk F. (1931), Z. Tech. Phys., 12, 593. Kunimatsu K., Seki H. Golden W.G., Gordon J.G., Philpott M.R. (1985), Surface Sci., 158, 596. Leppinen J.O., Basilio C.I., Yoon R.-H. (1989), Int. J. Miner. Proc., 26, 259. Leppinen J.O., Basilio C.J., Yoon R.-H. (1988), Colloids and Surfaces, 32, 113. Leppinen J.O., Rastas J.K. (1988), Colloids and Surfaces, 20,22 1. Mielczarski J.A. (1993), J. Phys. Chem., 97,2649. Olsen J.E., Shimura F. (1988), Appl. Phys. Lett., 53, 1934. Pons S., Davidson T., Bewick A. (1984), J. Electroanal. Chem., 160,63. Porter M.D.(1988), Analytical Chemistry, 60, 1143A. Richardson N.V., Sheppard N. (1987), Vibrational Spectroscopy of Molecules on Surfaces: Yates J.T. and Madey T. (Eds). Plenum Press, pp. 1-48. Strojek J.W., Mielczarski J., Nowak P. (1983), Advances in Colloid and Interface Science, 19, 309. Sundholm G., Talonen P. (1994), J. Electroanal. Chem., 377,91. Sundholm G., Talonen P. (1995), J. Electroanal. Chem., 380,261. Termes S.C., Richardson P.E. (1986), US. Bureau of Mines Report of Investigations 9019. Young C.A., Miller J.D. (1993), presented at AIME 122nd Annual Meeting, Reno, USA.



23 Raman spectroscopy E.J. Samuelsen

23.1 Introduction Raman spectroscopy was discovered in 1928 by C.V. Raman as a signal of shifted frequency when monochromatic light is scattered by solid materials like quartz. This phenomenon of inelastic light scattering long remained an interesting and much studied field in an academic context, but as a practical tool was not much utilized in research until the advent of the laser after 1960. In fact, the laser beams are ideally suited for Raman spectroscopy: to a very high degree they are usually only one single wavelength; they are very unidirectional; and their intensity can be heightened; furthermore the laser beams can easily be focused on the micrometer scale if required. Raman spectroscopy deals with vibrational phenomena in materials, just like infrared (IR) spectroscopy but with somewhat different selection rules. Lattice vibrational modes in crystals as well as molecular vibrations in gases, liquids and solids and on surfaces may be studied. The Raman effect does not depend strongly on the wavelength or frequency of the laser light being used. The resonance Raman effect may in certain cases be utilized to enhance the Raman signal. In such cases, particular laser frequencies are chosen that nearly coincide with electronic excitation frequencies of the sample. When a beam of monochromatic light of frequency 00 hits a sample, most of the beam intensity will either be transmitted or be elastically scattered. However, a minor fraction, of the order of 10-5-10-6, will interact with the vibrational quanta inelastically so that the vibrational quantum energy, Am,, is either added to the photon energy of the beam (‘Anti-Stokes process’) or subtracted from the photon energy (‘Stokes process’). Thus, by analysing the frequency spectrum w of the scattered light beam, an unshifted strong peak at the frequency 00 of the incident beam will be observed to dominate, but on either side of this frequency weak but distinct peaks at frequencies o = 00 k 60 will be seen. Due to energy conservation, 60 then corresponds to the frequency ovof one of the vibrational modes of the sample. The Ruman effect, is a second-order effect in the interaction between beam and matter. Its strength is determined by the derivative of the molecular polarizability of the sample with respect to the vibrational co-ordinates. (For comparison, the IR effect is determined by the derivative of the dipole moment.) Therefore, a given vibration may be observable by Raman spectroscopy and not by IR, and vice versa. This is always the case for species with inversion centre of symmetry, like COZor C2H4. It is said that the two methods are complementary. It is also interesting that molecules with a large dipole moment like H2O give very strong IR signals, but are poor Raman scatterers. Raman spectroscopy, therefore, is advantageous for studies of molecules in

23 Raman spectroscopy

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aqueous environments, where IR signals from water would swamp the molecular spectra. Since optical or near-optical laser frequencies are used, it is possible to use glass apparatus in Raman investigations.

23.2 Classification and applications Surface Raman may be classified into three types.

23.2.1 Classical Raman ‘Classical Raman ’ is not particularly surface-sensitive,but may in favourable cases give signals from adsorbed gases or vapour molecules on surfaces. For instance COz molecules tend to be adsorbed by metal surfaces and may relatively easily be observed by the 2200 cm-’ vibrational line. Monolayers of pyridine on smooth silver surfaces give just discernable signals at 992 and 1030 cm-’. Similar observations can be made on Au, Cu and Ni surfaces. The signals may sometimes be enhanced by the use of multiple reflection techniques (‘Attenuated Total Reflection’ ATR). This type of Raman Surface spectroscopy is not used very much.

23.2.2 Surface-enhanced Raman spectroscopy ‘Surface Enhanced Raman Spectroscopy’ (SERS) makes use of a very strong (103-106) enhancement effect discovered in about 1974 from molecules on electrolytically roughened metal surfaces. Subsequent work showed that the enhancement is related to the presence of strongly curved surfaces, like micro-droplets or micro-rills. The effect is not a resonance Raman effect, but is believed to be of electromagnetic origin. The effect is strongest on silver surfaces, but has been reported to appear also on surfaces of copper, gold, aluminium, sodium and lithium. Both simple molecules like oxygen, nitrogen, carbon monoxide and water, and organic molecules like alkanes, benzene and pyridine and more complicated ones like phthalocyanines can be involved. Studies of SERS have had considerable impact on questions related to tribology, catalysis and electrochemistry.

23.2.3 Surface-scanning methods ‘Surface-Scanning Methods’ make use of the fact that the laser beams can be focused to micrometer size. When the surface is scanned with such fine beams and detection of the scattered light is pre-set to specific frequency shifts only, the distribution of specific materials on the surface can be mapped. For instance, corrosion of iron surfaces may lead to both iron hydroxides, a or y haematite or magnetite. Each of them gives their characteristic Raman spectra, which are used for the mapping of their distri-


bution. It is expected that Raman micro-imaging will gain importance in corrosion and catalysis studies in the future.

23.3 Instrumentation There has been considerable development Raman instrumentation in recent years. The first laser Raman instruments were of the dispersive type, with large double or triple monochromators in order to reject the elastically scattered light, using precisionruled optical gratings. Such instruments were not well suited for practical analytic purposes, and IR-spectroscopy continued to be the prime laboratory tool for vibrational spectroscopy. During the 1980s, Raman spectrometers using near-infrared wavelengths, like the YAG-laser (wavelength 1064 nm), were developed and Fourier-Transform (FT) spectrometers were successfully introduced. Through this development, data collection became much more rapid and at the same time, background scattering originating from fluorescence from the sample could be strongly reduced. In recent years, new technical developments like three-dimensional optical gratings and two-dimensional detectors (Charge Coupled Device (CCD) cameras) have led to some renaissance of dispersive methods. By also introducing fibre beam guides, very rugged and versatile Raman instruments are now available for routine use and for in situ applications. Combined instruments that combine both spectroscopy and imaging are commercially available.

Further reading Pockrand 1. (1984), Surface Enhanced Raman Vibrational Studies at Solid/Gas Interfaces Springer Tracts in odern Physics 104, by G.Hohler (Editor) Springer-Verlag Berlin Heidelberg NewYork Tokyo. Hendra P., Mould H. (1988), FT-Raman spectroscopy as a routine analytical tool International Laboratory, Sept.

Commercial instruments Jobin Yvon Microprobe MOLE Perkin Elmer Near-Infiared FT-Raman System 1990 Renishaw plc Raman Imaging Microscope 1992 Spex Raman-500 1992



24 Mossbauer spectroscopy R. Wappling

24.1 Introduction Mossbauer spectroscopy has established itself as a technique which can provide valuable information in many areas of science, among them the study of thin layers, and an added advantage is that these layers do not have to be located close to the surface. The importance of Mossbauer spectroscopy as a means by which a variety of problems in a number of different scientific disciplines may be examined is already well covered in a number of texts (e.g., Cohen, 1976, 1980; Gonser, 1975, 1981; Giitlich et al., 1978; Long, 1984, 1987; Dickson and Berry, 1986; Long and Grandjean, 1989) and no attempt is made here to cover the details of the method. The text provides an introduction to Mossbauer spectroscopy and the various factors determining Mossbauer spectra, and the applications of the method to surfaces and thin layers are outlined. Finally, the applicability in selected research areas is surveyed by means of examples.

24.2 The Mossbauer effect For many years it was recognized that the gamma rays emitted when radioactive nuclei in excited states decay might excite other stable nuclei of the same isotope, thereby giving rise to nuclear resonant absorption and fluorescence. However, initial attempts to detect these resonant processes were unsuccessful mainly because the nuclear recoil, which accompanies both the emission and absorption of the gamma ray, was not taken into consideration. In a low-temperature experiment, Mossbauer discovered that a nucleus in a solid can sometimes emit and absorb gamma rays with essentially zero recoil energy, in this way making the resonance experiment possible (Mossbauer, 1958). In a solid matrix, the nucleus is bound in the lattice which leads to a certain probability that the recoil energy is taken up by the lattice (crystallite) as a whole. Since the recoil energy (the part of the excitation energy that is needed to fulfil momentum conservation) is inversely proportional to the mass of the recoiling system, it can be ignored if the mass of the crystallite replaces the nuclear mass and, thus, the gamma ray may be emitted with the full excitation energy. The probability of such a recoil-free event decreases, however, exponentially with the square of the energy of the nuclear gamma ray so that the observation of the Mossbauer effect is restricted to isotopes with low-level nuclear excited states and, thus, to a limited number of elements. A table of the different isotopes in which the Mossbauer effect has been observed is given in Fig. 24- 1.


.

Fig. 24-1. The periodic table of Mossbauer elements (the open squares).

As stated above, 57Feis the most frequently studied isotope.

24.3 Sensitivity and information depth The most usual experimental arrangement for Mossbauer spectroscopy involves a standardized radioactive source containing the Mossbauer isotope in an excited state and an absorber consisting of the material to be investigated with the same isotope in its ground state. In the normal transmission experiment the gamma rays emitted by the source pass through the absorber, where they may be partially absorbed, and are then counted in a suitable detector. In order to investigate the energy levels of the Mossbauer nucleus in the absorber it is necessary to modify the energy of the gamma rays emitted by the source so that they can have the correct energy for resonant absorption. This is usually accomplished by moving the source relative to the absorber which remains stationary hence giving the gamma rays an energy shift as a result of the Doppler effect. A Mossbauer spectrum is, therefore, a recording of gamma ray counts against the velocity of the movement of the source with respect to the absorber. The radioactive source nuclei are usually embedded in a matrix which provides the necessary solid environment as well as giving the simplest possible hyperfine interaction between these nuclei and their environment. For the simplest case in which both source and absorber contain the Mossbauer isotope in the same cubic environment, the spectrum consists of a single absorption line centred at zero velocity, while different hyperfine interactions will affect the spectral shape and lead to the shifting of lines and/or the lifting of degeneracies resulting in several absorption lines. A schematic presentation of a Mossbauer experiment is given in Fig. 24-2.

24 Mossbauer spectroscopy 395

-

I36 keV

4-

57Fe

91%

14.4 keV Mbssbauer

gamma ray

t

Mbssbauer gamma ray emitter nucleus

velocity ground state I absorber nucleiis

absorber transducer

t

w

feedback amplifier

A waveform

I

-

generator

I

data acquisition system

*

c l amplifier

le I

single channel analyser

output

Fig. 24-2. A schematic presentation of a Massbauer experiment. The decay of the parent activity (top), emission and absorption (middle) and the spectrometer (bottom).


It is often important to be able to change the sample environment and the most common environments used are furnaces, cryostats (in many cases a prerequisite for observing the Mossbauer effect at all) and applied magnetic fields. It should also be emphasized that the method is truly non-destructive which means that one can observe changes due to subsequent sample treatments, etc. A Mossbauer spectrum is derived over several hours or even days in order to reach the required statistical accuracy, and the long measuring times are obvious drawbacks when investigating surfaces since the surfaces might not be stable for the whole measuring period. The standard transmission geometry in which the absorber contains the sample under investigation and the Mossbauer gamma rays are detected after they have penetrated the sample, giving a spectrum consisting of absorption lines, is complemented by the detection of scattered radiation, the re-emitted radiation, i.e., gamma rays or X-rays and conversion electrons emitted in the de-excitation of the nuclei in the sample, giving a spectrum consisting of emission lines. It is also possible to carry out Mossbauer measurements in which the sample under investigation forms the radioactive source, thus allowing measurements involving elements which form the parent nuclei in a Mossbauer decay scheme, increasing the sensitivity to that particular element considerably. In the study of surfaces, the lower penetration depth of the conversion electrons allows for increased surface sensitivity. In the case of 57Fe,the ‘information depth’ is reduced from the 10-pm range to the 100 nm range by detection of the conversion electrons. The latter value is, however, still very large for surface and thin-layer studies and more refined methods need to be used to reduce the information depth further, It is also possible to record simultaneously information from the transmission (TMS) and backscattering geometry. In the back-scattering geometry one can record using both conversion electron Mossbauer spectroscopy (CEMS) and conversion X-ray Mossbauer spectroscopy (CXMS); the latter records the X-ray(s) emitted when the ‘hole’ left by the conversion electron is filled. An experimental set-up for simultaneous CEMS, CXMS and TMS measurements has been described (Schaaf et al., 1991) and a combined CEMS and CXMS detector has also been used (Kamzin and Grigorev, 1990). Very-low-energy electrons (515 eV) with high intensity are produced directly by Mossbauer absorption and conversion in the case of 57Fe.These electrons are more surface-sensitive due to their shorter attenuation length compared with the ‘normal’ 7 keV K-conversion electrons of S7Feand have been used to increase the surface sensitivity (Klingelhofer et al., 1992; Sinor et al., 1993). It should be emphasized that the electron transport at these energies is not well known (Liljequist and Lang, 1990) so it is difficult to make reliable estimates of the actual information depth. It is possible to obtain proper surface sensitivity in a Mossbauer experiment using total reflection of the gamma rays. This had already been demonstrated in the sixties (Wagner, 1968) and a more recent study was the investigation of sputtered iron films (Isaenko, 1994). The method has been described in detail from both experimental and theoretical points of view (Irkaev et al., 1993a, b) but is severely limited with respect to intensity due to the small useful solid angle and is of use only in extreme cases. An

24 Mlissbauer spectroscopy 397

alternative way of increasing the depth selectivity, and a method that has increased in use in recent years, is the selective doping of the Mossbauer isotope into the sample. This is illustrated below in connection with studies of polymer/metal systems and metallic multilayers. In a source experiment the information depth is directly related to how the activity is distributed in the sample; if the activity is confined to a monolayer, the information comes exclusively from that monolayer. The sensitivity is extremely high in source experiments, since the decay of every excited nucleus can, at least in principle, be detected, and a typical value is 1 0-4of a monolayer. The main drawback of the source experiments is, obviously, the necessity to work with radioactive materials in sample preparation, and relatively few laboratories are equipped for this.

24.4 Experimental A Mossbauer spectrum is characterized by the number, shape, position and relative intensity of the various resonance lines. Apart from time-dependent effects, these features result from the various hyperfine interactions, shifts, splittings and relative intensities, and the lattice vibration of the Mossbauer nuclei, ‘absolute’ intensities. It is difficult to derive element concentrations in a sample from Mossbauer spectroscopy but the relative concentrations of different chemical forms involving the Mossbauer nuclide can be determined with reasonable accuracy.

24.4.1 Isomer shift The isomer shift of the absorption lines in the Mossbauer spectrum, also sometimes known as the chemical shift or the centre shift, is a result of the Coulombic or electric monopole interaction between the nuclear charge distribution and the electronic charge density at the nucleus. This shift arises because of the difference in nuclear volume between the ground and excited states involved in the Mossbauer transition, and the difference between the electron densities at the Mossbauer nuclei in different materials. From an applications point of view, interest lies in the latter (atomic) part whereas the first (nuclear) part determines the sensitivity. (There are Mossbauer isotopes in which the nuclear part is so small that no isomer shift can be detected.) The isomer shift determines the position of the centre of gravity of the Mossbauer. spectrum. It is not an absolute quantity since it represents the difference between the interactions in the source and in the absorber and in order to make comparisons of the isomer shifts obtained from different absorbers the isomer shift data are generally expressed relative to a standard absorber, which is also used to determine the zero of the velocity axis of the spectrum. For 57Fe,metallic iron is used. Thermal vibration of the nuclei will also shift the gamma-ray energy as a result of the relative second-order Doppler effect. As these thermal vibrations are temperature-dependent the temperature of both the source and absorber should be considered when quoting and comparing isomer shifts.


The isomer shift is an important means by which atomic oxidation states, which are sometimes difficult to determine by other techniques, can be directly investigated. This is the basis of much of the work.within mineralogy and, in particular, the study of in the weathering of minerals.

24.4.2 Quadrupole splitting Nuclei in states with a nuclear angular momentum quantum number I 2 1 have nonspherical charge distributions which are characterized by a nuclear quadrupole moment. If this quadrupole moment experiences an asymmetric electric field, characterized by the tensor called the electric-field gradient (EFG), the energy of the nuclear levels varies depending on the relative orientation of the quadrupole moment with respect to the principal axis of the electric-field gradient. In the case of 57Fe, the excited state has I=3/2, and in the presence of a non-zero electric field gradient, it splits into two substates characterized by m=*3/2 and m=*1/2 (i.e., the Hamiltonian generates eigenvalues proportional to m2). This situation leads to a two-line spectrum, with the two lines separated by the quadrupole splitting, usually denoted A. For higher nuclear spins, .several spectral lines are expected, but it is the EFG that contains the relevant information in the present context. The EFG results from different contributions. One arises from the vaIence electrons of the Mossbauer atom and is associated with asymmetries in the electronic distribution due to partly filled shells. Another contribution is from the lattice, and arises from the asymmetric arrangement of the ligand atoms in non-cubic lattices. The quadrupole splitting observed reflects the symmetry of the bonding environment and the local structure in the vicinity of the Mossbauer atom.

24.4.3 Magnetic splitting When a nucleus is placed in a magnetic field, there is a magnetic dipole interaction between any nuclear magnetic moment and the magnetic field. This interaction completely lifts the degeneracy of the nuclear states and, in the case of 57Fe, the ground state with I = 'h splits into two substates and the excited state with I = 3/2 splits into four substates. The selection rule which states that the magnetic quantum number can, in the decay, make a maximum change of one unit Am=O, *l, appropriate for magnetic dipole radiation, leads to six possible transitions and hence a Mossbauer spectrum with six absorption lines. Since the splitting of the spectral lines is directly proportional to the magnetic field experienced by the nucleus, Mossbauer spectroscopy provides an effective means by which this field can be measured and correlated to the magnetic moment on the iron atom in question. The transition probabilities between the nuclear substates affect the intensities of the lines in the Mossbauer spectrum and can, therefore, give information on the orientation of the magnetic field at the nucleus and, hence, the orientation of the atomic magnetic moment.


100.0

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Fig. 24-3. Characteristic Mossbauer absorption spectra depicting, from the top, isomer shift only, isomer shift and quadrupole splitting, a purely magnetic interaction and the combination of all three in the presence of a preferred orientation of the sample. (Lines 2 and 5 are more intense than in the random situation above indicating that the magnetic moments are confined to the absorber plane.) For a scattering experiment the line positions are not randoin, rather the intensity increases at resonance.


-6

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Velocity

(mm/sl

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i 6

Fig. 24-4. Mossbauer spectra of an a) as deposited in the amorphous state and b) after crystallization as a result of annealing.

The effects of the magnetic and electric quadrupole interactions are usually more complex when they are present simultaneously (since they have different quantization systems), and the observed spectrum is then strongly dependent on their relative magnitudes and orientations. The combined interactions contain information about the relative orientations of the magnetic moments and the principal axes of the EFG, which are directly related to the crystal symmetry, and, hence, information also about the magnetic structure. The characteristic spectral features of the different interactions are depicted in Fig. 24-3. In general, it is expected that a spectrum contains all the different hyperfine parameters and often this can be for more than one crystal-chemical environment. It might also be important to remember that the magnetic dipole and electric quadrupole interactions have different quantization systems; the first is diagonal in the magnetic quantum number m and the second is diagonal in m2. This can necessitate diagonalization of the total Hamiltonian when the two interactions are of similar strength, and leads, among other things, to two further Mossbauer lines in the spectrum,

24 Mbssbauer spectroscopy 401

The Mossbauer spectrum is not always well resolved, as is illustrated in Fig. 24-4 for an amorphous metal (Bjarman and Wappling, 1983). As a result, the analysis of the resulting spectra is made using rather general computer codes (Jernberg and Sundqvist, 1983) that are run on minicomputers or, with their ever-increasing power, PCs. The development of the software took place in research laboratories, but some of the analysis programs are commercially available. Summing up the characteristics of Mossbauer spectroscopy, its unique properties lie in the wealth of information that is provided in each spectrum: The information is element-specific (actually even isotope-specific) so one can study one element at the time. The hyperfine interactions allow the nucleus under investigation to report on its surroundings; one can identify chemical species (fingerprinting) or the physical state, e.g. magnetic ordering. The line intensities give information on the abundance of a specific species and on the bonding strength in the solid. The relative line intensities give information on directional properties of the material - if there is a preferred orientation of the crystallites in the sample or about the orientation of ordered magnetic moments in space. The probing depth can be varied to increase the sensitivity to particular parts of the sample. The limitations are that only a small number of elements can be investigated (and some of those only at low temperatures) and that in some cases the resolution might not be sufficient.

24.5 Applications of Mossbauer spectroscopy to surfaces, thin films and multilayers The applications are, in general, investigation of the physical, chemical or technological properties of the investigated material or a combination of several of these properties. A selection of examples is, necessarily, subjective since the application area of Mossbauer spectroscopy is so large, even given the restrictions to ‘applied surface research’.

24.5.1 Chemical applications Many chemical applications of Mossbauer spectroscopy exploit the sensitivity of the technique to changes in electron density at the nucleus through the isomer shift. The chemical uses of quadrupole splitting data for the examination of the site symmetry of an atom and the electronic arrangement around the nucleus is also a valuable application. This ‘chemical’ information is utilized in most applications of the method and we will start with some examples of interest mainly in chemistry, but which also have pos-


sible implications in other areas. Mossbauer spectroscopy is a valuable tool in standard chemical research but in order to illustrate the specific advantages of the method we choose in the following some more exotic examples. When a material is rapidly quenched from the melt, there is a certain probability that there will not be sufficient time for the atoms to diffuse to their equilibrium positions and a metallic glass can be formed. In this metastable state, due to the random variation in local surroundings, there is a fairly continuous distribution in hyperfine parameters. This results in the broad absorption lines shown in Fig. 24-4. In the experiment (Bjarman and Wappling, 1983), iron was evaporated onto an aluminium foil and a glass substrate. Due to the good thermal conductivity of aluminium, an amorphous state (a metallic glass) was obtained on this substrate, whereas on the glass substrate the cooling rate was sufficiently low to allow crystallization. Fig. 24-2 is also a good illustration of the difference between a metallic glass and the same material after crystallization, from the Miissbauer spectroscopy point of view. In the crystallized state, one can detect additional absorption lines due to the presence of an iron-carbon compound; it was found that the glassy state had been stabilized by carbon from the atmosphere in the oil diffusion-pumped vacuum chamber used. This can also serve as an illustration of the result on the Mossbauer spectra of surface annealing using laser pulses, an area where MS has made valuable contributions (Gauzzi ef al., 1992; Valiev et al., 1992). 1 . 0 4 1~O6

I

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-5

0 VELOCITY M M / S

5

103

t k b?

z z -

W k

102 1.01 1

.oo

0.99

0.98

-10

10

Fig. 24-5. M6ssbauer spectrum of photodeposited iron after high-temperature consolidation:

In the study of iron photodeposited in porous Vycor glasses (first undertaken by Sunil ef al., 1993), the products derived from the UV photolysis of Fe(CO)S, phy-


sisorbed onto the glass, are examined by X-ray microprobe analysis, Mossbauer spectroscopy, and extended X-ray absorption fine-structure spectroscopy (EXAFS). Although the photolysis and subsequent heating is carried out in air, the spectroscopic data reveal two surprisingly different forms of iron. Isomer shift, pre-edge, and EXAFS data indicate that one product is similar to a-Fe203 and consists of an Fe3+ion octahedrally surrounded by six oxygen atoms at a distance of 1.8 A. The second compound, which comprises approximately 50% of the reaction product, is mainly elemental iron, in which a central Fe atom is surrounded by approximately eight other Fe atoms at a distance of 2 A. X-ray microprobe analysis shows that aggregation occurs during photolysis due to the diffusion of Fe(C0)j from the interior into exterior photodepleted volumes of glass. Heating has little effect on product ratio or distribution, but consolidating the glass at 1200 OC leads to further aggregation and formation of magnetically ordered particles that exhibit magnetic hyperfine fields of 370 and 425 kG (Fig. 24-5). The corresponding values for bulk iron and a-FezO3 are 330 and 515 kG and the observed difference is interpreted as due to the formation of silica bonds and inhomogeneities. This study is based on fingerprinting and the ability of the radiation to penetrate substantial amounts of matter; of the different methods used, Mossbauer spectroscopy is the only one able to detect the transition to small magnetic particles.

24.5.2 Small particles and catalysis Catalysis is a multi-disciplinary area of science and within this large field a particularly successful application of Mossbauer spectroscopy has involved the in situ study of solids which catalyse gaseous reactions. Since the catalytic activity takes place on the surface, one of the main aims has been to maximize the surface area and Mossbauer spectroscopy has become one of the standard tools in the study of small particles. The development of special cells in which these catalysts may be studied under conditions that closely resemble their actual working environment is an important feature of this work. In many of the Mossbauer spectroscopic studies of catalysts, the data have been used in conjunction with data from other techniques to elucidate fundamental properties of both the bulk and surface which influence catalytic performance; Mossbauer spectroscopy is routinely used in industrial laboratories for product and process control. The pioneering work in this area was done by a group at the Danish Technical University and the reader is referred to one of several reviews on this particular area (e.g.,van der Kraan et al., 1989). The Mossbauer method has, in recent years, been utilized quite frequently in studies of ultrafine particles in general, and some examples from this research area are illustrative of its potential for controlling preparation methods as well as in determining the physical properties of the particles. A detailed study of the magnetic properties of carbon-supported metallic iron particles with an average diameter of 3.7 nm in the temperature range 5 K-305 K and with external magnetic fields up to 4 T revealed a number of features ( B ~ d k e et r al., 1992, 1993). Depending on the preparation conditions, various amounts of amorphous Fe-C


particles are formed in parallel with the iron particles. The temperature above which the particle shows loss of magnetic ordering due to each particle being a magnetic domain is called the superparamagnetic blocking temperature. At this temperature the thermal energy exceeds the magnetic energy and the small magnetic domains due to each particle becomes unstable. The supermagnetic temperature for a-Fe particles is about 70 K, but after oxidation it decreases to about 50 K, indicating that the magnetic anisotropy energy constant of the oxidised particles is significantly lower than that of the metallic particles. The temperature-dependence of the spectral area is similar in the metallic and the oxidized states. In both cases, the vibrational modes, which Mossbauer spectroscopy probes (determining the intensity of the resonance lines) in the recoil free fraction, are dominated by particle vibrations. In a separate study (B~rdkeret al., 1994), the surface region was selectively oxidised and by recording the difference between the signal from the particles before and after oxidation (Fig. 24-6) the authors were able to isolate the signal from the surface layer and found this to be a new oxide in which the iron atoms were ferromagnetically coupled to the interior of the particles.

z

-12 -8

-4 0 4 8 Velocity (mms")

I

12

Fig. 24-6. MBssbauer spectra (a) for untreated iron nanoparticles, (b) after surface oxidation and (c) for the difference corresponding to the new surface oxide.

Transition metal oxides are used in magnetic recording media and, since this is a commercially important area, the details of the physical and chemical states have often been investigated. In these studies Mossbauer spectroscopy has made very valuable


contributions because of it microscopic nature. The spin canting, surface magnetisation, and finite-size effects in y-Fe203 particles have been investigated (Parker et al., 1993). Small y-FezO3 acicular particles (250 A by 2000 A) with and without surface coating of s7Fe were examined by Mossbauer spectroscopy in large longitudinal applied fields. As had been previously seen, the Fe moments were not aligned in large fields. However, the authors found that, in contrast with previous results, the canting is a finite-size effect rather than a surface property.

24.5.3 Buried layers and interfaces As mentioned before, Mossbauer spectroscopy is based on the detection of a variation of the counting rate for rather energetic gamma radiation transmitted through or scattering by a sample. This means that it is possible to obtain information from the interior of a sample although depth sensitivity can still be retained either by using the depth-selective mode based on detection of conversion electrons or by appropriate preparation of the sample. In the latter case, which has become increasingly popular in recent years, one introduces the Mossbauer-sensitive isotope (e.g. 57Fe)at a particular position in the sample that is to be investigated; in the interior, on the surface or at an interface. This is of particular interest when selective studies are to be made and means that the method is particularly suitable for studies of interactions taking place at an interface that is located well below the surface of a material. In the following the use of this specific property of the Mossbauer method to study the effects of heavy ion irradiation on the interface between a metal and a polymer (Wappling, 1993) will be discussed. The metallization of polymers makes new application areas for this class of materials possible, in such diverse fields as in microelectronics, optics and the automotive industry. In this connection the adhesion between the polymer and the metal is of the utmost importance and, since this is an area where detailed understanding is lacking, a number of studies, using different experimental methods, has been performed in recent years. Iron films were evaporated onto different substrates to thicknesses of 25-100 A using s7Feof enrichment 96-99% in a tungsten crucible. Without breaking the vacuum, the Mossbauer-sensitive 57Fe layer was subsequently covered by the evaporation of a further protective layer of 150 A of 56Fe. The resulting samples were irradiated in a large-area ion implanter at a tandem accelerator using different ions of energies in the range 16-48 MeV to a maximum dose of 2 - 5 ~ 1 0 ions ' ~ cm-2 at low beam-currents to prevent excessive heating of the samples. Typical Mossbauer spectra for a PVC substrate are displayed in Fig. 24-7. The films as deposited show the six-line pattern typical of iron metal. This indicates that there was no appreciable oxidation during the evaporation process and that the cooling rate was slow enough to prevent the occurrence of any amorphous phase. There is also some additional absorption at low velocities amounting to a few percent of the total spectrum. This is attributed to iron forming chemical bonds with impurities in the interface region. The effect of the irradiation (in this case with 48 MeV Br8+ ions) is the formation of two distinct new


patterns at the same time as the signal for a-Fe is reduced. The isomer shift and the quadrupole splitting of these two new patterns resemble those of one Fez+and one Fe3+ species. It should be stressed that the irradiation ions, in general, penetrate the complete sample and will not contribute to any further element being present and one can only expect elements that are either constituents of the substrate used or found in the evaporation atmosphere. For PVC, one can expect to find the elements H, C, CI and 0. The Fe2+ component in the spectrum after irradiation has an isomer shift (relative to a-Fe) of 1.2 mm s-' indicating that it corresponds to a chloride and, since the quadrupole splitting is 2.3 mm s-', the best agreement is obtained for FeC12x2Hz0 which has an isomer shift of 1.03 mm s-' and a quadruple splitting of 2.5 mm s-'. (An exact agreement is not to be expected since one cannot expect well-crystallized compounds in the radiation-damaged regions.) As regards the Fe3+ component of the spectrum after irradiation, there is number of possibilities, also involving ternary and higher compounds. Considering only binary compounds and noting that iron hydrides are not stable under the experimental conditions, the ferric iron compound is identified as an iron-carbon complex.

4

h

Velocity mm/s

c)

Fig. 24-7. Mossbauer spectra for an iron film at the interface between iron and a PVC foil. As deposited (a) and after irradiation by 16 MeV S ions to a dose of 10" (b) and ions cm-*(c),


24.5.4 Tribology The technique has been applied to studies of the effects of various chemical surface treatments of metals, treatments to reduce wear, for instance. The results of an early study of different nitrogen, carbon and/or sulphur treatments of a low alloy steel (Bustad et al., 1980) clearly revealed the conversion of the surface layers to different wear-resisting compounds and can also be used to compare, the results of wet-chemical and often hazardous methods with more environment-friendly ones. Thus, characterization of laser-nitrided iron and sputtered iron nitride films has been made (Schaaf et aE., 1995) and the authors found that the nitrogen is dissolved in y-Fe to well above the solubility limit, leading to a large amount of retained austenite. The films produced by sputtering could be resolved in terms of different iron sites, enabling accurate calculation of the nitrogen content. Another ‘new’ method in the surface treatment of steels is laser surface alloying. Amongst other applications, it has been used to form a composite material, steel plus Tic, by injecting particles of TiC into a molten surface layer (Ariely et al., 1995). The Mossbauer results reveal ternary Fe-TiC phases and a correlation was found between the substrate’s composition and microstructures, and the different phases present. The actual nature of the wear debris and the resulting changes at the surface of a material exposed to wear have also been investigated and for a low-alloy steel surface the formation of different oxides and oxihydroxides could easily be followed (Bustad et al., 1980).

24.5.5 Corrosion The technique has been applied to studies of the oxidation and corrosion of iron alloys in a variety of gaseous atmospheres and one example is the study of atmospheric corrosion of steel (Singh et al., 1985). In a more recent study, this time dealing with wet corrosion, the recording of Mossbauer spectra at various temperatures was useful in determining the chemical species produced on the surface (Maeda et al., 1992).

24.5.6 Magnetic thin films and multilayers As an example of the very large applications area that deals with magnetic materials, magnetic thin films and multilayers will be considered. Magnetic properties of ultrathin Fe and Fe60Au40 alloy films on Au(ll1) were studied by SQUID magnetometry and conversion electron Mossbauer spectroscopy. In order to get information on the influence of interdiffusion, iron films with thin alloy zones at the interfaces with Au were prepared by co-evaporation of iron and gold, and compared with iron films with presumably sharp interfaces. It was found that the presence of a 0.5 ML alloy zone (mass coverage in monolayer) reduces the effective magnetic interface anisotropy field and affects the growth mode of a subsequently deposited iron film such that the film is more sensitive to annealing (Brockmann et al. 1993). As is often


observed in thin films, the magnetic moments and hyperfine fields are significantly enhanced in Fe/Au( 111) (tFe 1.4 ML) and Fe60Au40 films, compared with bulk Fe. In a selective study of interface effects, Mossbauer spectroscopy was performed on three (FesAg40)2~multilayers grown in the (1 00) direction by molecular beam epitaxy (MBE) (Keavney et al., 1993). All of the Fe bilayer components in each sample were selectively loaded with two monolayers of 57Fe:at the Fe-on-Ag interfaces of one sample, at the Ag-on-Fe interfaces of another, and at the Fe bilayer centres of the third. In this way, selective information that is difficult to obtain with other methods was derived and the authors had to invoke three different iron environments in order to interpret the data. The same system was also studied by Heinrich and cd-workers (Shurer et uf.,1993) using one monolayer of 57Fe as a probe layer. The Mossbauer study shows that the Fe/Ag interface consists of atomic terraces, one atomic layer in height (Fig. 24-8). Different Fe sites can be distinguished at the Fe/Ag interface. The average size of the terraces can be determined from the relative intensity of the components in the Mossbauer spectrum. By growing the sample at higher temperatures, the terrace size can be increased significantly. However, at this temperature, the 57Fediffuses into the underlying Fe layers.

‘9

!2

b -6

-4

-2

0

VELOCITY (mm/s)

2

4

6

Fig. 24-8. Mossbauer spectra of one monolayer of S7Feat the Fe/Ag interfaces in a metallic multilayer. The different subspectra are due to different local surroundings at the interface.


References Ariely S., Bamberger M., Hugel H., Schaaf P. (1999, J. Mat. Sci. 30, 1849. Bjarman S., WBppling R. (1983), J. Magn. Magn. Mat. 40 219. Brockmann M., Pfau L., Lugert G., Bayreuther G. (1993), in Jonker, B.T et al. (Eds.) Magnetic Ultrathin Films, Multilayers and Surfaces, Mater. Res. SOC.Philadelphia p. 685. Bustad J., Ericsson T., Karner W., Wlppling R. (1980), Le Vide, les Couches Minces 201 Suppl. 471. Bsdker F., M0rup S., Linderoth S. (1994), Phys Rev. Lett. 72 282. Bsdker F., Mmup S . , Oxborrow C.A., Linderoth S., Madsen M.B., Niemantsverdriet J.W. (1992), J. Magn. Magn. Mat. 104-107 1695. Bsdker F., M0rup S., Oxborrow C.A., Linderoth S., Madsen M.B., Niemantsverdriet J.W. (1993), J. Phys.: Condensed Matter 4 6555. Cohen R.L. (Ed.) (1976), Mossbauer Spectroscopy I, Academic Press NY. Cohen R.L. (Ed.) (1980), Mossbauer Spectroscopy 11, Academic Press NY. Dickson D.P.E., Berry F.J. (1986), Mossbauer Spectroscopy, Cambridge University Press Cambridge F. E. Wagner 1968 Z.Physik210, 361. Gauzzi F., Principi G., Verdini B. (1992), Hyperfine Interactions 69, 545. Gonser U. (Ed.) (1 9751, Mossbauer Spectroscopy, Springer Heidelberg. Gonser U. (Ed.) (198 l), Mossbauer Spectroscopy 11, Springer Heidelberg. Gutlich P., Link R., Trautwein A. (1978), Mossbauer Spectroscopy and Transition Metal Chemistry, Springer Heidelberg. Irkaev S.M., Andreeva M.A., Semenov V.G., Belozerskii G.N., Grishin O.V. (1993a), Nucl. Instr. Meth. B74, 545. lrkaev S.M., Andreeva M.A., Semenov V.G., Belozerskii G.N., Grishin O.V. (1993b), Nucl. Instr. Meth. B74, 554. Jernberg P., Sundqvist T. (1983), A versatile Mossbauer analysis program, Uppsala University, Department of Physics, Report UUIP 1090 (Unpublished). Kamzin AS., Grigorev L.A. (1 990), Instr. Exp. Techn. 33, 3 14. Keavney D.J., Wieczorek M.D., Storm D.F., Walker J.C. (1993), J. Magn. Magn. Mat. 121 49. Klingelhofer G., Imkeller U., Kankeleit E., Stahl €3. ( I 992), Hyperfine Interactions 69, 8 19. Liljequist D., Lang H. (1990), Nucl. Instr. Meth. B52,79. Long G.J., Grandjean F. (Eds) (1989), Mossbauer Spectro. Appl. to Inorganic Chem. Vol3, Plenum NY. Long G.J. (Ed.) (1984), Mossbauer Spectroscopy Applied to Inorganic Chemistry Vol 1, Plenum NY. Long G.J. (Ed.) (1987), Mossbauer Spectroscopy Applied to Inorganic Chemistry Vol2, Plenum NY. Maeda Y . , Matsuo Y., Sugihara S., Momoshima N., Takashima Y. (1992), Corrosion Science 33 1557. Parker F.T., Foster M.W., Margulies D.T., Berkowitz A.E. (1993), Phys. Rev. B47 7885. Isaenco S.A., Chumakov A.I., Shinkarev S.I. (1994) Phys. Lett A186, 274. Schaaf P., Illgner C., Niederdrenk M., Lieb K.P. (l995), Hyperfine Interactions 95 199. Schaaf P., Kramer A., Blaes L., Wagner G., Aubertin F., Gonser U. (1991), Nucl. Instr. Meth. B53, 184. Schurer P.J., Celinski Z., Heinrich B. (1993), Phys. Rev. B48 2577. Singh A.K., Ericsson T., Hlggstrom L. (1985), Corrosion Science 25 931. Sinor T.W., Standifird J.D., Taylor K.N., Hong C., Carroll J.J., Collins C.B. (1993), Rev. Sci. Instr. 64, 2570. Sunil D., Sokolov J., Rafailovich M.H., Kotyuzhanskii B., Gafney H.D., Wilkens B.J., Hanson A.L. (1993), J. Appl. Phys. 74 3768. Valiev R., Bochkov V., Bashkirov Sh., Romanov E., Chistjakov V. (1992), Hyperfine Interact. 69 589. van der Kraan A.M., Ramselaar W.L.T.M., de Beer V.H.J. (1989) in Long, G. J. and Grandjean, F. (Eds) Mossbauer Spectroscopy Applied to Inorganic Chemistry Vol3, Plenum NY. Wlppling R. (l993), Nucl. Instr. Meth. B76 22.



25 Laminate analysis by chemometrics A.A. Christy, F.O. Libnau and O.M. Kvalheim

25.1 Introduction Detailed analyses of macro- and micro-sized surface areas of materials reveal a lot of information to scientists regarding the physical state and chemical composition of the surface. The analytical techniques (for example XPS, SIMS, EPMA etc.) can reveal the physical properties or the chemical composition of the surface. Surface problems can be very different. However, analyses of a micro surface (for example a size of 1Ox 10 pm’) are always useful to the scientist. Infrared spectrometry is a valuable technique for giving molecular information about a substance. Application of this technique effectively to surface problems had to wait until the development of FTIR spectrometers and sensitive detectors such as mercury-cadmium-telluride (MCT) detectors. Analysis of a surface at the micro surface level needs precision focusing of the infrared radiation on a micro surface area of a few square micrometers. This type of analysis is possible with purpose-built microscopes using cassagrain objectives as accessories to FTIR spectrometers. The technique is called infrared microspectroscopy. Infrared microspectrometry has established its effectiveness and usefulness in analysing micro samples. Several hundreds of applications using infrared microspectrometry can be found in the literature. Analysis of surfaces using micro sampling techniques produces a wealth of data for a macro sample and the comparison between them collectively is difficult if not impossible. Presentation of these data requires some special presentation techniques. For example three dimensional and contour plots of the data acquired on the surface. Similarly, analysis of surfaces using infrared spectrometry or rather microspectrometry produces several hundred infrared spectra of micro surfaces. The enormous number of spectra acquired over a wide range of wavenumbers, require intelligent presentation techniques in order to evaluate different chemical aspects of the surface. The data may contain information about specific chemical structure, for example a particular functional group. The past decade has witnessed a rapid development of different chemometric techniques for different applications in chemistry (Kvalhejm and Liang, 1992; Liang et al., 1992; Toft and Kvalheim, 1993; Liang and Kvalheim, 1993; Maeder and Zuberbuehler, 1986; Karjalainen, 1989). Most of these techniques have provided new insights, interpretations and even solutions to problems that have been uncertain and discussed over several decades. In this chapter the aim is to discuss analysis of a multilayer laminate sample using infrared microspectrometry and chemometric techniques.

25 Laminate analysis by chemometrics

4 11

25.2 FTIR microscope and redundant aperturing Modem FTIR microscopes are made to perform the following functions: a) irradiation of a micro sample with infrared radiation, b) collection of the radiation emerging from the sample, c) imaging the collected radiation on to a detector (usually mercurycadmium-telluride detectors) that is connected to the microscope and FT-IR instrument, and d) allowing the user to select and define the area he/she wants to measure. FTIR microscopes employ reflecting mirrors (Cassegrainian optics) for the optical elements instead of the lenses used in visible light microscopes. The aperture placed before the cassagrainian objective gives better sample definition. In transmission mode the infrared radiation is transmitted through the sample and condensed by a cassagrainian condenser and imaged through another aperture. This aperture blocks diffracted light that has strayed from the adjacent sample area. This technique is called redundant aperturing. Modem infrared microscopes are capable of measuring infrared spectra of surfaces by transmission as well as reflection techniques. When it comes to surface analysis one could easily understand that the reflection techniques are the preferred techniques. Transmission techniques are suitable only when very thin layers are available for analysis. There are three different external reflectance techniques possible with infrared microscopy, namely, specular reflection, reflection-absorption and diffuse reflection. These three techniques vary in the way the irradiated infrared radiation on the surface is reflected from the surface.

25.3 Analysis of a multi-layered laminate 25.3.1 Problem description The analysis of multilayer laminates by infrared microspectrometry has become one of the success stories of infrared microspectrometry. Application of chemometric techniques to the infrared microspectrometric data can reveal the spectra of individual layers and their concentration profiles. Furthermore, the chemical changes taking place at the interfacial regions can also be detected and their chemical information can be extracted in the form of the layer's infrared spectrum. Chemical changes over a period of time can be monitored by comparing the infrared spectra of the layers at regular intervals. This will help the industry in determining the life-span of the laminate.

25.3.2 Experimental 25.3.2.1 Sample and spectral measurements The multilayer laminate sample was prepared by cutting cross-section 5 pm thick using a microtome (Reichert-Jung, model 2050 -Leica). The sample was then mounted

4 12 Part 3: Chemical bonding and molecular composition

between NaCl windows in a compression cell (Spectra-Tech, Inc.). A small crystal of KBr was also placed in the same cell and this was used for collecting the background spectrum. All together 51 spectra of the laminate sample were collected at intervals of 2 pm, with a 12x100 pm2 sample area defined by redundant aperturing technique. A total of 256 scans were co-added at a resolution of 8 cm-'. The data were subjected to multiple component analysis using alternating leastsquares regression (ALS). 25.3.2.2 Multi-component analysis If N spectra at different positions on the laminate are acquired at M wavenumbers, they define a two-way data matrix X of size N by M. By assuming additivity of the spectra in different molecular environments the matrix X can be decomposed as shown by eq. 25- 1: A

X = C S +~ E

= ~ C ~ S +: i=l

E

(25-1)

In eq. 25-1, C is the concentration matrix of dimension N x A and ST, of dimension A x M, is the spectral matrix. The experimental noise is expressed by the matrix E. The superscript 'T' implies transposition of a column vector into a row vector. By applying eq. 25-1, we assume that each measured spectrum is the sum of contributions from A pure species with concentrations defined by the concentration profiles {c,, i = 1, 2, ..., A) and spectra by the spectral profiles {Si, i = 1,2, ..., A ) . If there is deviation from linearity caused by interactions between the different species, one has to include a set of factors that models the non-linearity caused by the interactions. A

X = CS" + E = ~

I=I

A'

C , S :

+ CC{S!~

(25-2)

1=I

The number of factors (A') needed to model the interactions, is dependent upon spectral similarity, similarity of spectral changes due to the interactions, as well as the noise level in the data-set. Both additive and multiplicative errors like baseline shift and intensity variation due to, e.g.,different path length for each spectrum, contribute to the noise. The successful resolution of X into the pure concentration and spectral profiles depends on the ability to correctly determine the number of underlying chemical species inducing the observed spectral variation (i.e. determination of chemical rank, A), and to identify selective spectral and/or concentration regions for the chemical components. Selective regions are of crucial importance in order to achieve optimum or unique resolution (Kvalheim and Liang, 1992; Liang et al., 1992). Without deviations from linearity, the chemical rank can be expressed as the number of latent variables necessary to reconstruct the data matrix within the noise. With interactions present, the correct discrimination of chemical components and interaction


4 13

factors might be a cumbersome task. It is guided by a priori knowledge of the system under investigation and by the observed variation in the measured spectra. Decomposition of isolated parts of the spectra (e.g. Eigenstructure Tracking Analysis, ETA (Toft and Kvalheim, 1993)) combined with spectral correlation can give sufficient information for successful resolution. 25.3.2.3 Resolution by alternating least squares regression (ALS) combined with selective information If one lacks selective information for the different components, and stripping (Liang and Kvalheim, 1993) does not succeed, one might have to combine the selective information at hand with an iterative approach such as ALS (Maeder and Zuberbuehler, 1986; Karjalainen, 1989). With the constraint of non-negative concentration and spectral profiles (local minima constraints, specific for the different pure spectra are more efficient) and no constraints on the interaction factors, the iterative process of calculating the least squares estimate of the spectral profiles in S* is given by S*T=(C*TC*)-'C*TX

(25-3)

and the estimate of the concentration profiles in C* by

c*= XS*(S*TS*)-'

(25-4)

For every cycle, negative intensities are set to zero. Prior to entering into a new cycle, the concentration profiles are normalized to sum to one as shown below. If one has any selective information regarding spectra or concentration profiles, the calculated values are substituted by the selective information. The relative concentrations for each component can be calculated by taking into account that they should sum to one (25-5)

and the constants necessary to scale concentration profiles are found by least squares as b=(CTC)-'CT1

(25-6)

25.3.3 Results and discussion 25.3.3.1 The infrared spectra of the laminate sample A stack plot of the infrared spectra of the laminate sample is shown in Fig. 25-1. Inspection of the stack plot shows that there are three different chemical species in the laminate. However, the components arising from interactions and other underlying components are difficult to visualize in the data set.


/

I

2.500

u

0)

-50.0

2.000

15.0

C

1

I

3500

~

I

3000

I

2500

I

I

2000

I

I

1500

I

1000

I

I

I

~

Wavenumber cm-I Fig. 25-1. A stack plot showing the infrared microspectroscopic spectra as scanned across the cross section of the laminate sample.

25.3.3.2 Identification of selective regions An example of a local decomposition analysis is shown in Fig. 25-2 for the wavenumber region 36 17-9 15 cm-' . The eigenvalues (log10 due to the large differences in size) are obtained from ETA in the step direction (concentration direction) with a window size of three. The evolving pattern of the eigenvalues indicates that there are mainly two components present. Furthermore, ETA indicates that the first few spectra are from the same component and hence a selective region is clearly indicated in that region. There is also further indication of selective regions in the vicinity of step numbers 3 1 and 5 1, (not necessarily belonging to the same component). The spectral


-4 0

5

10

15

20

25

30

35

40

45

415

50

Step-number Fig. 25-2. A plot showing the eigenvalues from a local decomposition analysis using ETA technique.

dissimilarity indicates that the above mentioned three selective regions belong to three different chemical components. This leaves us with an idea that the system may contain three components. However, the ETA plot obtained for the wavenumber region 1841-1668 cm-' (Fig. 25-3a) shows that the first eigenvaiue has a maximum at step number 20 which probably corresponds to yet another component. The spectral profiles in this stepregion have no counterparts in the spectra at step numbers 1, 3 1 or 5 1. Step number 5 1 does not appear in Figs. 25-2,25-3a and 25-3b.


-0

I

0

5

10

15

20

25

I

30

35

40

45

50

Step-number Fig. 25-3. Results from ETA analysis of the raw (3a) and second derivative (3b) spectral profiles in the wavenumber regions 2227-2054 (----)and 3999-3829 (ooooo) cm-'.

25 Laminate analysis by chernometrics

417

This is because when a window size of 3 data points is used ETA produces 49 points. Then the first point is plotted at step number 2 and the last is plotted at step number 50. The evaluation was made with reference to the noise level in the spectra obtained by ETA analysis of the raw and second derivative data profiles in the wavenumber regions 2227-2054 cm-l (---) and 3999-3829 cm-' (0000). The noise level in the regions are indicated in Figs. 25-3a and b. Further analysis of spectral intensity at 1471 cm-' and 2853 cm-' indicates that the spectral intensities with step number are not equal at these two wavenumbers. At 1471 cm-l the absorbance reaches a plateau ranging from spectrum 26 to 37, while at 2853 cm-' the absorbance goes through a distinct maximum at spectrum 32. This indicates the possibility of an underlying component with spectral features similar to the component present at step number 3 1. Alternating regression with four real and two interaction factors indicated that one of the interaction factors actually is a real chemical factor and support the above statement. Feeding the selective information into the alternating regression with five real and two interaction factors gave the result shown in Figs. 25-4 to 25-9. To test the solution the output was run using the same procedure, but without exchanging the recalculated spectra with the selective information. As can be seen in Figs. 25-4 to 25-9 there is good agreement between the two solutions. Cross-contamination between the real and the interaction factors due to correlation in spectral and/or concentration direction is a complicating obstacle and this possibility should be kept in mind when interpreting the resolved spectral and concentration profiles.

25.3.3.3 Chemical identification Fig. 25-4 shows the concentration profiles of the chemical components shown in Figs. 25-6 to 25-8. The manufacturer revealed the identity of the components in the laminate as poly(viny1 chloride) (PVC), polyethylene (PE) and polyvinyl dichloride (PVDC). Identification of chemical components was done by matching the spectrum with library spectra. The component number 1 is polyvinyl chloride (the spectrum indicates that this may be a carbonated poly(viny1 chloride)), component number 4 is polyethylene and component number 5 is poly(viny1 dichloride). The components number 2 and 3 resolved in our analysis are real components and their identities are yet to be revealed. Component number two may be a plasticizer, used as an adhesive in the production of packaging material, that was not mentioned by the manufacturer. This component has an infrared spectrum that resembles poly(viny1 acetate). Component number 3 is probably a product of interaction between the plasticizer and polyethylene. A plot of the concentration profiles of the chemical components of the laminate is shown in Fig. 25-4. One can easily evaluate the thickness of the layers from the profile map. The poly(viny1 chloride) layer has a thickness of approximately 40 pm, The overlap of component number 1 and 2 starts at approximately 30 pm. Component 2 which we concluded was a plasticizer has a distribution of 20 pm. The polyethylene has a distribution of approximately 34 pm. However, pure polyethylene comprises only


approximately 4 pm. The rest is a mixture of component number 3 and polyethylene. The poly(viny1 dichloride) has a thickness of approximately 24 pm and only approximately 4 pm is pure poly(viny1 dichloride).

I

0

S

10

15

20

25

30

35

40

45

SO

Step-num ber Fig. 25-4. Concentration profiles of resolved components: 1 and 2 (---)are shown in a. 3 (- - -), 4 and 5 (-----)are shown in (b). The results of both approaches are shown together.

25 Laminate analysis by chemometrics 0.41

0.:

,

b

I

a

0.;

0. I

c -0.I

-0.2

-0.3

-0.4

I

5

10

I

15

20

25

30

Step-number Fig. 25-5. Plots of sensitivity factors.

35

40

45

50

4 19


Y h . I

v)

C

QI

Y

E:

I

im

3500

3000

2500

2000

I500

Wavenumber em-* Fig. 25-6. Resolved infrared spectra of components number 1 (a) and number 2 (b). The resolved components from both techniques are plotted together. Spectra are nearly identical in both approaches.

25 Laminate analysis by chemometrics 1

421

I

1

b

2.2

’ I .5

I

0.5 Y

. I

in

E:

a

W w S

I

I0.8 -

0.60.4 -

4000

3500

3000

2500

2000

I500

1000

Wavenumber cm-1 Fig. 25-7. Resolved infrared spectra of components 3 (a) and 4 (b). The resolved components from both techniques are plotted together. Spectra are nearly identical in both approaches.

422 Part 3: Chemical bonding and molecular composition Corriporierit # 5 B

I

I

I

I

3500

3000

2500

2000

0.6 0.5 -

+.

c1

;*

s

c

0.4 -

.c)

0.3 -

4Ooo

I500

I000

Waveniimher cm-I Fig. 25-8. Resolved infrared spectra of component 5 . The resolved components from both techniques are plotted together. Spectra are nearly identical in both approaches.


423

3

0.ost

-0.3 -0.4

-

-0.5 -

-

-0.6

4000

3500

I(

3000

2500

2000

W a v c1111m bc r c m - 1

Fig. 25-9. A plot of the sensitivity factors against wavenumber.

I500

1000


References Karjalainen E.J. (1989), Chemometrics and Intelligent Laboratory Systems, 7, 32. Kvalheim O.M., (1987), Chemometrics and Intelligent Laboratory Systems, 2, 283. Kvalheim O.M., Liang Y.-Z. (1992), Analytical Chemistry, 64, 936. Kvalheim O.M., Karstang T.V. (1987), Chemomet. & Int. Lab. Syst. 2, 235. Liang Y.-Z., Kvalheim O.M. (1993), Chemometrics and Intelligent Laboratory Systems, 20, 115. Liang Y.-Z., Kvalheim O.M., Keller H.R., Massart D.L., Kiechle P., Emi F. (1992), Analytical Chemistry, 64, 946. Maeder M., Zuberbuehler A.D. (1986), Analytica Chimica Acta, 181,287. Toft J., Kvalheim O.M. (1993), Chemometrics and Intelligent Laboratory Systems, 19,65.

Acknowledgements Elsevier Science B.V., Amsterdam, The Netherlands is thanked for their kind permission to reprint text and reproduce Figs. from Chemomet. Int. Lab. Sys., 23, (1994) 197-204. The authors would like to thank Dr. John Reffner, Research Director, Spectra-Tech, USA for providing the infrared microspectroscopic data on the laminate analysis and some valuable discussion on the resolution of the spectra and their concentration profiles. Technical assistance of Steinar Vatne is highly appreciated.



Part 4: Crystallography and structure Crystal surfaces Crystalline materials are characterized by the long-range periodic arrangements of their constituent atoms or molecules: the bulk crystal is built up of a semi-infinite number of identical, three-dimensional unit cells. All the bulk properties of the material are determined by the content of the unit cells combined with the three-dimensional periodicity. The electronic band structure derives all its features from these two components, giving in its turn rise to the optical, electronic and magnetic properties of a material. For surface-specific properties the surface structure and periodicity plays a similar role. A surface differs from the bulk by the fact that the periodic unit cell stacking is finite in one of the three directions. This allows the near-surface unit cells to modify their geometric arrangements to give rise to a specific surface structure, differing somewhat from that of the bulk. Surface crystallography is primarily defined as the description of the atomic or molecular arrangements of the last few atomic or molecular layers near the surface or interface of a crystalline material. In these layers, the interatomic distances may relax to a varying extent, and atoms of the outermost surface layer may develop new bonding features by pairing off ‘dangling bonds’, giving rise to the phenomenon of ‘reconstruction’. Surface crystallography is studied by methods employing surface diffraction methods, which is the main theme of this part of the book. Surface adsorption Surface crystallography also includes the study of surface adsorption. Depending on temperature and pressure, gas atoms can be adsorbed by crystalline surfaces, and the adsorbed atoms can form a variety of phases and structures. Low temperatures and high coverage (approaching mono-layers) may give rise to ordered ad-atom structures, which may be commensurate or incommensurate with the structure of the crystal surface. Reduced coverage or increased temperature may show a transition between phases and may eventually lead to disordered, ‘liquid’ phases. Related to ad-atom surface structures is the phenomenon of atomic intercalation in layered materials such as graphite. Both of these phenomena can be studied by X-ray and neutron diffraction, whereas electron diffraction methods are less well suited. Deposited layers Layers of deposited materials of thicknesses larger than monolayers on top of a substrate material are of great interest for many purposes. Examples range from LangmuirBlodgett films, via multilayer, quantum well, semiconductor structures and biological membranes to oil and polymer-based anti-corrosion layers and painted surfaces. It is

426 Part 4: Crystallography and structure

often of great interest to know the molecular arrangement and long-range or shortrange order and to relate them to the properties of the layers. Diffraction and reflectivity techniques for studying such thin films in the presence of the substrate material are similar to those used for regular surface structure problems, mostly with X-ray and neutron beams.

Surface morphology The surface morphology is often as important as the crystallographic surface structure for the properties of a material. A good example is the efficiency of a heterogeneous catalytic surface, where the micro-roughness is believed to be crucial. Recent development has enabled information of this sort to be extracted from studies of X-ray surface diffraction as well as from work on X-ray and neutron reflectivity. Detailed analyses are required of the ‘surface truncation rods’ of scattered X-ray intensities for various diffraction angles along the normal to the surface. Similarly, the reflectivity observations of X-rays or neutrons over several orders of magnitude can be used. These studies are performed with the same instruments as in normal, surface crystallographic studies.

Liquid surfaces The structure of liquid surfaces can also be studied by the surface diffraction of X-rays and neutrons. Liquid crystals are liquids consisting of long molecules that in certain temperature regions show the phenomenon of spontaneous molecular orientation. Surfaces and interfaces can strongly influence the ordering. For instance, it has been observed, by means of diffraction studies, that nematic liquid crystals have a thin layer at the air-liquid interface where the ordering is smectic. Langmuir films are mono-molecular layers of organic materials like fatty acids on top of water surfaces. Very interesting ordering schemes and phases have been revealed by surface X-ray diffraction of Langmuir films as a function of temperature and lateral pressure.

Electron scattering Surface diffraction studies are performed with both electron, X-ray and neutron beams. Electrons at low energy, as used in LEED (‘Low Energy Electron Diffraction’), are specifically surface-sensitive because their penetration depth in condensed matter is limited to one or two atomic layers. LEED is being used extensively for characterizing the surface geometry of clean crystal surfaces and ordered adsorbates. The other electron-scattering method, W E E D (‘Reflection High Energy Electron Diffraction’), has a larger penetration depth, and reflection geometry is used to control this parameter. Electron-scattering methods are hampered by two limiting factors: 0 electron beams can be used only under high vacuum and 0 the interpretation of the scattered intensities is not straightforward because a multiple-event ‘dynamic’ scattering model has to be used.

Overview

427

On the other hand electron-scattering investigations can be performed with smallscale laboratory equipment available commercially. Electron microscopy Transmission Electron Microscopy (TEM) used for surface or interface studies requires considerable sample handling, because only ultrathin samples (< 100 nm) can be used with TEM. After proper cutting, polishing and ion etching the samples normal to the surface Zayer to be studied, very detailed information can be obtained on length scales spanning from 0.3 nm to several pm both by imaging and diffraction. The versatility of TEM instruments can be used to furnish valuable additional analytical information about the surface layers. Use of TEM is limited to materials that can endure both the rather rough sample treatment and the strong radiation environment in the high electron beam. X-ray scattering X-rays are not particularly surface-sensitive, but, as for WEED, the penetration depth can be controlled by having the beam impinge at grazing incidence. In contrast with electron-scattering methods, X-ray methods do not require vacuum conditions. X-rays are thus well suited for studies of all types of surface problems, be it in vacuum or not. Because the interaction between X-rays and matter is weak, the interpretation of observed intensities is simplified by the use of a single-event, ‘kinematic’ scattering model. X-ray studies of surfaces require extraordinarily strong beams with ultra-high collimation and position precision. Although feasibility experiments can be performed with laboratory, rotating anode sources, full-scale experiments can be conductedat synchrotron sources only, the most modem (1994) being the European Synchrotron Radiation Facility (ESRF) in Grenoble. Neutron scattering Neutron beams have been used for surface studies in favourable cases, and for studies of intercalation compounds. Neutron beams are used for reflectivity studies. Neutron beams are available only at research nuclear reactor installations, of which there are several national and international sources, one being the Institute LaueLangevin (ILL) in Grenoble. Charged particle interaction Beams of high-energy charged particles can also be used for surface studies of crystalline targets by use of the channelling effect. Channelling occurs when the ion beam is incident within a certain critical angel from a crystal axis or plane. Channelling has become a major technique complementary to the techniques of X-ray and electron diffraction. The instrumentation required is an accelerator which produces beams at energies in the keV-MeV range. The technique provides information about localization of foreign atoms in crystals, and finds application, e.g., in semi-conductor technology.



26 Studies of surface structures by X-ray diffraction and reflectometry E.J. Samuelsen

26.1 Introduction The application of X-ray-scattering methods to the study of surface structures represents a relatively recent development, made possible by the advent of powerful X-ray sources. Because the number of ‘surface’ atoms is low compared to the number in the ‘bulk’, conventional, stationary, laboratory X-ray generators are unable to provide sufficiently strong surface signals. Even the much more intense rotating-anode sources are only just strong enough to enable experiments on favourable surfaces. It is only by the use of X-rays from storage rings (‘synchrotron radiation’) that surface X-ray studies can be performed to take full advantage of the method, thus complementing the already long-established electron diffraction methods LEED and WEED. In fact X-ray methods have definite advantages over electron-diffraction methods. This is related to the strength of the interaction between beam and material: the X-ray photon interaction is dipolar in nature; such interaction is thus so weak that it can be described as a single-stage process (‘kinematic interaction’), and for kinematic interaction the mathematical relation between the observable intensities and the structural parameters is well established. This favourable situation should be contrasted with that of the interaction of the electron beam with material: one is now dealing with the much stronger coulombic interaction, which requires a multi-stage process (‘dynamic interaction’), generally not easily expressible in closed form. This fact has long hampered the quantitative application of electron beam methods for determination of structure. On the other hand, for exactly the same reason electron scattering methods are, generally speaking, much more surface-specific than X-ray methods, and with the latter special approaches are required to distinguish genuine surface effects from bulk effects. X-rays can be utilized for the study of single crystal surfaces (i.e. surface reconstruction and relaxation), adsorbed layers on crystal surfaces, and for the study of liquid surfaces and interfaces and membranes. For a recent, extensive overview of surface crystallography see Robinson (1991). As will be shown in the following sections both X-ray diffraction (XRD) studies as well as X-ray reflectivity studies are becoming increasingly important (see for instance Als-Nielsen (I 99 1) or Sauvage-Simkin (1994)). Neutron beams have also been applied for certain surface diffraction and reflectivity studies (Thomas, 1994).

26 Studies of surface structures by X-ray difiaction and reflectometry

429

26.1.1 Surface crystallography For three-dimensional crystals the diffraction of radiation can conveniently be described with the help of reciprocal space (Fig. 26-la). In the following it will be assumed that the reader is familiar with this formalism. (Various introductory textbooks may be consulted, e.g. Glasser (1977), Stout and Jensen (1989))

+

Fig. 26-1. (a) Three-dimensional crystal. Left: Part of crystal lattice with unit cell vectors g, b, c . Right:

-

corresponding reciprocal lattice with unit vectors 8*,b*, -d *.

t"

z,

+

Fig. 26-1, (b) Two-dimensional crystal. Left: the crystal lattice with lattice vectors b and the surface normal fi , Right: corresponding reciprocal lattice showing the reciprocal rods, lattice vectors

i*,G*,li* .


A three-dimensional bulk crystal is characterized by its (quasi-)infinite translational symmetry in three independent crystallographic lattice directions. A surface is distinctly different by the lack of translational symmetry. in one of the three directions, namely the one along the normal to the surface. In general a ‘surface’ consists of several atomic layers near the interface. In comparison with the bulk crystal structure these atomic layers may be distorted (‘reconstructed’) to a varying degree, depending on their distance from the interface. Nevertheless a ‘surface’ retains its translational symmetry in the lateral directions and thus to a good approximation may be termed a ‘two-dimensional lattice’. For single atomic layers the two-dimensionality is obvious. The reciprocal lattice of a two-dimensional crystal possesses no structure in the direction normal to the surface, because no interference can exist along that direction. In fact the reciprocal points corresponding to the periodicity in the two, in-plane directions will extend into reciprocal rods in the direction normal to the surface (Fig. 26-lb). In other words in a diffraction experiment it is only the component of the radiation wave vectors in the surface plane which takes part in the diffraction interference. These surface reciprocal rods and the corresponding crystal truncation rods play a very central role in the terminology and interpretation of surface X-ray diffraction.

26.1.2 Diffraction in two and three dimensions In elementary textbooks on (kinematic) diffraction theory (James, 1948; Kakudo and Kasai, 1972) it is shown that the scattered intensity, Is, from a crystalline sample consisting of N = N I N ~ N unit ~ cells with Nj units of length a, in the jth direction (j=1,2,3 or x,y,z) can be expressed by: 1, = CIF(Q]21ntlInt21nt~

(26- 1)

Here C is a scaling factor, F(Q) is the structure factor for the unit cell, and Int, are the Laue interference functions for each of the three crystallographic directions: Intj

= [sin(N,a,Q,

/2)/sin(a,Q, / 2 ) r

(26-2)

Q is the jth component of the scattering vector:

Q = I;, -I;i

c,

(26-3)

c,

where and are the wave vectors of the scattered and the incident beams, respectively. The length of the wave vector can be expressed in terms of half the total scattering angle 28 by: Q=4n sine/h

(26-4)

26 Studies of surface structures by X-ray dimaction and reflectometry

43 1

where h is the wavelength. Int, are sharply peaked functions of Q when Nj is large, because the peak-width is inversely proportional to Nj. For bulk samples Nj are large numbers and Int, are finite only when the Bragg (or Laue) relations are fulfilled: alQl=2xh; azQ2=2zk; a3Q3=2zl

(26-5)

(h, k, 1 are integers, called ‘reflection indices’) giving an intensity expression: I, = C.IF(Q]2(N1N2N3)2

(26-6)

For a single layer a similar expression holds, taking N3=l: (26-7) For the latter case there is no interference along Q3 (except a possible, slowly varying effect in F(Q)), corresponding to the reciprocal rods. What, then, is the situation for dvfiaction at a surface? Apparently, for 2x. 2x. 271. and Q2 = -integer, when also Q3 = --.integers the bulk Bragg Q1 = -integer a1 a2 a3 conditions (eq. 26-5) are fulfilled. However, if the former two conditions are met, but: 4 3 #

2n:

-. integers a3

the presence of the surface can be emphasized by introducing a weak, layer-dependent attenuation factor exp(-pa3n3), where n3 is the number of the atomic layer below the surface (the uppermost surface layer being number 1). Then the layers closest to the surface will contribute most to the scattering, and the sin(N3a3Q3/2) squared will be averaged over the resolution width DQ to give %. One obtains an intensity expression valid for scattering at wave vector transfer between the bulk Bragg peaks, depending on Q3 as (Robinson, 1991): I,”,

Cx =

x

N: x N: x 1

4sin2(a3Q3 12)

(26-8)

Eq. 26-8 describes what is known as ‘crystal truncation rods’ in reciprocal space. They form a natural continuation between the sharp reciprocal ‘dots’ of a three-dimensional, bulk lattice, and the constant, reciprocal rods of a two-dimensional lattice. (See Fig. 26-2.) It should be noticed, however, that specific surface effects, such as reconstruction of the outer atomic layers, are not involved in the truncation rods, but will give additional effects like two-dimensional superlattice reflections corresponding to unit cell enlargments, also indicated in Fig. 26-2.


-

Fig. 26-2. Surface of a crystal. Left: four unit cells of the top-most layer, unit cell vectors g, b, and the normal fi; a possible reconstruction by systematic shift along k‘i is indicated by arrows. Right: corresponding reciprocal lattice indicating ‘surface truncation rods’ at reciprocal lattice vectors corresponding to ?iand b *, and superlattice, two-dimensional rods at k 2 * , corresponding to the reconstruction period.

*

-

It can be shown (Robinson, 1991) that for rough surfaces a correction factor of the form: (1 - K)2 (1 - 2Kcos(ajQ3)+ K2]

(26-9)

can be introduced into eq. 26-8. The parameter K ranges from K=O for ideally flat surfaces to K=l for very rough surfaces (Robinson, 1991).

26.1.3 Integrated intensities The scattered intensity always has to be observed with an instrument with finite resolution because of the finite collimation of the incident and the scattered beams. This resolution function can be characterized by three parameters DQ,, DQ2 and DQ3 whose typical values lie in the range 0.001 to 0.05 A-’ for synchrotron radiation beams. Since the Bragg peaks are very sharp for bulk samples (the widths exhibit a InUjdependence), an actual observation automatically records the integrated intensity:

Iint = C x IF(Q]’

x

N I x Nz x N j

(26-10)

26 Studies of surface structures by X-ray diffraction and reflectometry

433

For a single layer, however, the integrated intensity reads: Iint.~ayer= C x

2

IF(Q1 x N I x N2 x DQ3

(26-1 1 j

that is, the intensity is determined by the instrumental width for the direction normal to the layer. In fact DQ3 samples a fraction of the infinite reciprocal rod from the twodimensional lattice. A resolution correction similar to eq. 26-1 1 also applies for studies of ‘truncation rods’ from crystal surfaces. The way the instrumental correction enters into eq. 26-1 1 should be borne in mind when bulk and surface intensities are compared or normalized to each other.

26.1.4 Determination of surface scattering The surface structure of a crystalline sample involves the upper few atomic layers. It is essential that the scattering from this part of the sample can be distinguished from the bulk scattering. A very important means of achieving this is by grazing angle incidence, where the X-ray beam impinges on the surface close to the conditions for total reflection: n = 1- ph2ro /27t

(26- 12j

The real part of the refractive index, n, for X-rays is slightly below unity, h is the radiation wavelength, ro the classical electron radius and p the electron density of the sample. In optics courses we learn that the surface reflectivity RF is determined by the Fresnel laws (Born and Wolf, 1970):

with a the angle between the surface and the direction of incidence (Fig. 26-3). There exists a critical angle, a,,given by: cos a,= n

(26- 14)

For grazing angles, a,below this value total-reflection will occur. For such situations the radiation will not penetrate into the bulk sample, but will interact only with the surface region. Typical values of a, are of the order of a few tenths of a degree. The penetration depth decreases exponentially with a - a,(Born and Wolf, 1970). By using subcritical incidence angles one can avoid or reduce scattering events from the bulk, and thus greatly reduce background problems. However, the method requires flat, extended surfaces. The crystallographic diffraction data are collected by lateral scans parallel to the surface, indicated by the angle -20 in Fig. 26-3. A corresponding illustration with the Ewald sphere in reciprocal space is shown in Fig. 26-4. The scat-


tering from a two-dimensional rod is indicated by the thick region where the rod cuts the surface of the Ewald sphere. With a two-dimensional stationary detector one can image the whole rod system by rotation of the sample around a vertical axis. The measurements are made by means of various types of diffraction instrument, four- or five-circle instruments being frequently used. For grazing incidence a very well collimated incident beam is required, normally from Si double monochromator. Details of experimental techniques can be found elsewhere (Robinson, 1991).

\

\

\

Fig. 26-3. Incident

(k,) and scattered beams (Es and ispec) at the surface: for diffraction studies the

angle of incidence, a,!,is kept below the critical angle, a,, and the scattered intensity is recorded at various a, and 20 with a, also small (-1'). For reflectivity studies specularly reflected intensity is recorded as a function of a,= a, with 29 = 0, thus giving R(Q), Q = 471sina,/h.

435

26 Studies of surface structures by X-ray dieaction and reflectometry

-

Fig. 26-4. Two-dimensional diffraction geometry in reciprocal space. The incident wave vector k i is

(cs)

where a reciprocal rod cuts directed at the origin 0, and scattering will take place in the direction through the surface of the Ewald sphere (indicated by the dark region). The observed fraction of the rod is determined by the vertical instrumental resolution DQ3 (dimension of thick region along fi).

26.1.5 Determination of surface structure As in the case of the determination of bulk structure the structural details of a surface can be deduced from the structure factors F(0). From intensity observations of a

set of truncation rods, the moduli of structure factors IF(Q] can be determined. To this end data-reduction procedures, similar to those known from bulk structure determinations, are required (Robinson, 1991) (including corrections for polarization, Lorentzfactor and instrumental resolution). As many intensity reflection data as possible are


collected to give a set of observed moduli of structure factors, from which the direct space structure is to be derived. The normal problems associated with determinations of three-dimensional structure are also encountered here, in particular the ‘phase problem’. These problems can be handled by similarly well-established tools: trial-and-error from analogous structures; direct methods; anomalous scattering, etc. (Robinson, 1991).

26.1.6 Reflectivity studies Whereas diffraction studies may throw light on the atomic arrangement of the surfaces of crystalline materials, reflectivity studies can yield valuable information not only about the surface profile of crystalline materials, but also about surfaces with noncrystalline character, such as liquid surfaces or membranes (Als-Nielsen, 1991). The Fresnet reflectivity expression for RF of eq. 26- 13 is macroscopic in origin, and presupposes an abrupt termination at the interface. For the case of small incident angles a, eq. 26- 13 can be rewritten as: (26-1 5 ) for a > a,. For the case a >> a , one easily finds that RF(a)has the form:

R F ( ~=) (a,/2a)4

(26-16)

It is also customary in studies on specular reflectivity to introduce the scattering wave vector Q analogous to eq. 26-4 where a is substituted for 6.Then for the reflectivity function one can write: Real surfaces normally do not terminate abruptly, but are described by a surfaceden-

sify profzle. Information on the profile can be found by detailed analysis of the wave-

vector-dependence of the reflectivity. The wave vector (Q-)dependence of the reflectivity can be derived from the fact that the refractive index, n, is determined by the electron density p. If the electron density varies with the co-ordinate z relative to the surface, p(z), then the X-ray scattering from various parts of the sample will interfere to give a reflectivity which is dependent on the wave-vector (Als-Nielsen, 1991): (26- 17)

26 Studies of surface structures by X-ray difllaction and reflectometry

437

Applied to an abrupt surface eq. 26-17 reproduces eq. 26-16, because for such cases dp/dz is a delta function at z = 0. The importance of eq. 26-17 is that the profile p(z) can be obtained from observed deviations from the Fresnel reflectivity. In practice, model functions of p(z) are introduced into eq. 26-17. For example one may argue that a surface relaxation combined with surface roughness could be described by means of a Gaussian form of dp(z)/dz. Such a model predicts: R(Q)/RF(Q) = nxo2xexp(-(a2Q2))

(26-1 8)

where (J denotes the thickness of the surface profile. Eq. 26-18 can be used for any kind of single surface, including liquid surfaces. Eq. 26-17 can also be used to describe thin layers deposited on surfaces. Examples of such films include soap or oil films on solids, oil or lipid films on water, Langmuir or Langmuir-Blodget films, thin-layer deposits such as polymers on substrates, etc. The layers are modelled by appropriate p(z)functions, for instance, rectangular functions rounded by Gaussian edges. Because of interferences the reflectivity of multilayer syxtems will show patterns of maxima and minima. Reflectivities by X-rays are normally recorded over several orders of magnitude, as illustrated by examples in the following paragraphs. Strong X-ray sources and well collimated beams are required, for instance, synchrotron X-rays. The principle of neutron reflectivity studies is the same as for X-rays, but since the radiation sources available are not as bright as X-rays, neutron studies are not used to the same extent.

26.1.7 X-ray studies of surfaces in practice In many cases general-purpose beam lines at synchrotron X-ray sources can be utilized. An important example is Beamline no.9 (BL9), at insertion device no 10 (IDlO), known by the name ‘TROIKA’ at the European Synchrotron Radiation Facility (ESRF). The requirements that must be met are a well-collimated monochromatic incident beam, and great freedom of sample orientation and translation. For liquid samples the vertical slope of the incident and scattered beams must be adjustable. Most synchrotron radiation laboratories offer special beam lines for surface studies, often well equipped with auxiliary tools such as surface sputtering, cleaning and polishing facilities and a LEED incorporated in the sample chamber. The ESRF BL7 at insertion device no 3 (ID3), ‘Surface diffraction’ can be taken as an example. The beam line has one station for ultra-high-vacuum studies, available since 1994, and a station at atmospheric pressure for liquid and membrane studies, available since 1996. A sketch of the UHV diffraction station and a photo are shown in Fig. 26-5. A monochromatic beam in the energy range 5-30 keV, with a beam size of typically 0.08x0.03 mm, enters the sample compartment from the rear (normal to the plane of the figures). The station is equipped with a 6-axis diffractometer, and a sample stage


Fig. 26-5. Schematic view of the surface diffraction equipment of the UHV station at beam line 7 at the ESRF. The monochromatic beam enters from the rear (normal to the plane of the figures). Legend: 1-sample position. 2-detector. 3-beryllium window. 4-electron analyser. 5-preparation chamber. 6-differential pump feed-through. 7-ion pump. 8-transfer rod for samples. 9, 10 and 1 1-sample orientation circles. 12-detector circle. 13, 14 and 15-tables for support and translations of installation. 16-ion pump.

for cryogenic and high temperature measurements. W E E D experiments can be performed simultaneously with the X-ray studies. X-ray studies of surfaces can be technically quite demanding and require an experienced experimenter. At synchrotron installations the required expertise is available. The interested user should contact the synchrotron institutions well in advance for assistance with the preparation of proposals for experiments.

26.1.8 Examples of X-ray studies of surfaces In Table 26-1 we list some questions in the field of surfacelinterface research that can be addressed by X-ray methods.


439

In the following we will describe some examples of applications of surface X-ray scattering. Table 26-1. Surface X-ray methods for various purposes. Method Purpose Crystalline solid surfaces Adsorbed molecules Thin films on solid surfaces (-m) Langmuir-Blodgett films Oil, soap Thick films on solid substrates (-IOOnm) Multilayer structures Liquid surfaces Liquid crystals Thin layers on liquids Soap, oil Langmuir layers

Lateral diffiaction

Truncation rods

Reflectivity

Lateral crystallography Reconstruction Lateral ordeddisorder Lateral order

Perpendicular relaxation

Interfacial profile (Coverage) Profile and thickness

Lateral order, orientation

Profile and thickness

Crystallinity, structure, orientation; anchoring Lateral order

Thickness

Interfacial order

Superlattice structure

Thickness Interface width and profile Profile and thickness

Oxygen chernisorbed on copper This problem is closely related to both corrosion and heterogeneous catalysis, and it has been intensively studied by various surface methods, including X-ray diffraction. In the presence of oxygen Cu(ll0) surfaces readily form a 2 x 1-superstructure, as evidenced by the Observation of half-index surface reflections (Half-index relative to the indexing allowed for the bulk f.c.c. structure of Cu). Several models have been proposed for the oxidized surface layers, one involving extra rows of Cu atoms along the direction of reconstruction, located on top of the oxygen-containinglayer. Feidenhans’l et al. (1990) performed a detailed study of the crystal truncation rods of the (1 10) oxidized surface. This comprised the observation of altogether 41 reflection rods with half-integer indices (super reflection rods) and 14 integer index rods. In Fig. 26-6 these published observations for scans along the normal to the surface are shown through two reciprocal lattice points (1 10 and 11 1 in cubic indices). In the figure various calculated curves, corresponding to various models of the surface are shown, including a model for an abruptly-terminated Cu bulk crystal. The observations are in excellent agreement with the ‘extra-row model’, giving a fitted distance between the oxygen-containing layer and the extra Cu row layer. One should have in mind that the fit includes all the 55 truncation rods observed, and thus it represents an example of a determination of ‘surface crystallography’, including both the lateral 2x 1, reconstructed structure and the vertical ‘relaxation’ related to the chemisorption.


0.0

0.5

1.0 0.0

L (units of (1 10)buj,J

0.5

1.o

Fig. 26-6. Two crystal truncation rods observed for an oxidized Cu(l10)-surface (Feidenhans'l et al., 1990). The rods run through the bulk reciprocal lattice points 110 (left) and 11 I, with the index 1 denoting the vertical wave vector transfer Q, = 2n I ( d 2 a,-")*l. Dashed-dotted curve: model of the abruptly terminated bulk Cu. Dotted curve: as above, but half-filled top-most layer. Dashed curve: extra row model, with oxygen layer 0.034 nm above. Solid curve: best fit model: extra row with the oxygen layer 0.034 nm below.

Molecular-beam epitaxy Molecular-beam epitaxy (MBE) is a technique much employed for layer-by-layer fabrication of semiconductor superlattice structures. The growth kinetics can be studied by surface reflectivity techniques with electron or X-ray beams. In Fig. 26-7 we reproduce the time evolution of the reflectivities observed during epitaxial growth of germanium (1 1l),as reported by van der Veen (1 990).The angle of incidence (beta) is kept constant at values such that destructive interference would OCCUT between reflection from neighbouring molecular layers. As is seen in the figure, the reflectivities of both radiation types oscillate as the deposition proceeds, with a period of about 2000 s. The behaviour strongly supports a model in which the growth proceeds by the successive formation of terraces in molecular steps, as indicated in Fig. 26-7a, with a

26 Studies of surface structures by X-ray dieaction and reflectometry

441

growth time of 2000 s per layer. The detailed shape of the peak for the X-ray case was explained by inclusion of some surface roughness. As is seen in the figure, the X-ray case shows a stronger contrast compared to the electron beam case, probably related to multiple scattering events with electrons.

0.0

0.2

0.4

06

0.8

Deposition time ( lo4 seconds)

10

Fig. 26-7. Time-dependence of surface reflectivities of electrons (WEED) and X-rays on the germanium (1 1 ])-surface during MBE growth at 200 "C (van der Veen,1990). (a) Illustration of reflection from the two levels at a step. (b) Observed reflectivties.


Growth of overlayer As an example of studies of time evolution during deposition of overlayers on semiconductors we show in Fig. 26-8 X-ray reflectivity observed during exposure of a Si( 111) surface to In vapour (Finney et al., 1992). 1000

800 i?

e

5

600

0 v

P 400 v)

c

-c 0)

-Id

200

0 In deposition time (mins)

Fig. 26-8. Time evolution of the reflectivity of indium deposition on a silicon (1 11) surface at 350 "C (Finney et al. 1992) at a fixed vertical wave vector transfer Q,=2n/(.\/2as,)x2.1. The fitted model (solid curve) is valid during the deposition of the two first indium monolayers.

The instrument is kept stationary to observe the reflectivity at a fixed wave vector transfer Q near a bulk reflection (Q=(0,2.1,2.1)). The pointed peak observed after 15 min is interpreted as being associated with the growth of the first monolayer of indium. From that point the reflectivity decreases, because the growth of the second monolayer, at a certain interlayer distance, d, gives rise to an increased, destructive interference between the layers as the second layer approaches completion. Further growth is less ordered. The formation of steps and islands of irregular thicknesses gives rise to time-independent reflectivity after about 30 minutes. By model fitting the authors were able to derive possible values of the interlayer distance, d (d=0.27 nm).


443

1 oo ............................

1 o-2

v

CI)

......................

...........................

0

J

....................................

0.02

0.1 15

0.21

0.305

0.4

Fig. 26-9. Reflectivity from an annealed deposit of polystyrene-poly(buty1methylacrylate) copolymer on silicon (bottom) and a micrograph picture of the surface (top). The curve is a fit to a two-layer model, corresponding to two thicknesses, 28.8 and 58.0 nm. (Courtesy of G. Griibel, ESRF, Grenoble, 1995, to be published).


Thin layers of polymer blends In a study of a symmetric polystyrene/poly(butyl methacrylate) (PSPBMA) diblock copolymer deposited as a thin layer on a silicon substrate the reflectivity curve in Fig. 26-9 (bottom) was obtained after heat treatment (Vignaud ef al., 1995). The observed reflectivity spans seven orders of magnitude and shows a set of welldeveloped interference fringes as a function of the scattering vector Q. The interference pattern can be analysed in terms of two independent periods, corresponding to two layers of thicknesses 28.8 and 58.0 nm. After annealing, the film consists of holes 29.2 nm deep on top of a flat surface 58.0 nm thick. Fig. 26-8 (top) shows the surface of the sample, where the holes appear as light patches on the dark background. The behaviour is related to a phase separation of the PS and the PBMA polymers. Before annealing only one single interference pattern was observed, originating from a flat film of disordered copolymer.

References Als-Nielsen J. (1991), Handbook of Synchrotron Radiation. Brown, G. and Moncton, D.E. (Eds.) Elsevier, Vol. 3 pp. 472-503. Born M., Wolf E. (1970), ‘Principle of Optics’ Pergamon Press, 4th Ed. Feidenhans’l R., Grey F., Johnson R.L., Mochrie S.G.J., BohrJ., Nielsen M. (1990), Phys. Rev.B41 5420-5423. Finney M.S., Norris C., Howes P.B., Vlieg E. (1992), Surf. Sci. 277 330-336. Glasser L.S. (1 977), ‘Crystallography and its applications’. van Nostrand Reinbold (UK). James R.W. (1948), ‘The Optical Principles of the Diffraction of X-Rays’. G. Bell & Sons Ltd (London). Kakudo M., Kasai N. (l972), ‘X-Ray Diffraction by Polymers’, Elsevier. Robinson I.K. (1991), Handbook on Synchrotron Radiation, Brown G., Moncton D.E. (Eds.) Elsevier Science Publishers B.V. Vol. 3. pp. 221 - 266. Sauvage-Simkin M. (1 994), ‘Neutron and Synchrotron Radiation for Condensed Matter Studies’ HERCULES. Editions de Physique & Springer Verlag. Vol I1 p. 51 - 73. Stout G.H., Jensen L.H. (1989), ‘X-Ray Structure Determination. A practical guide’. John Wiley & Sons, 2nd Ed. Thomas R.K. (1994), ‘Neutron and Synchrotron Radiation for Condensed Matter Studies’ HERCULES. Editions de Physique & Springer Verlag. Vol11. p. 75 - 95. van der Veen J.F. (19901, Vacuum 41 1781-1782. Vignaud G., Griibel G., Legrand J.G., Gibaud A,, Auserre D. (to be published).



27 Transmission electron microscopy and diffraction E. Johnson

27.1 Introduction This chapter reviews transmission electron microscopy and diffraction in materials science. Emphasis is placed on conventional applications of electron microscopy to crystalline samples and on diffraction using both parallel-beam and convergent-beam techniques. The modern transmission electron microscope operating at 200-400 kV with a resolution between 0.1 and 0.2 nm is a very sophisticated and complex instrument. Its function is, however, strictly related to the wave-optical principles of optical microscopy developed in the 19th century, substituting electrons for light and replacing the glass lenses with axially symmetric magnetic fields. Virtually any material that can be prepared in sufficiently thin sections, usually less than 10-100 nm thick, can in principle be used as a sample, although, of course, they will be of widely varying quality. Conductive samples, metals and semiconductors, are often very stable even under exposure to the electron beam, particularly if displacement damage can be minimized. In general therefore, they provide excellent samples for both conventional and highresolution microscopy. The unique duality of the modern electron microscope, which allows it to be used either as an imaging instrument or as a diffiaction instrument, makes it particularly useful for studies of crystalline materials, providing simultaneous information on microstructure down to the atomic level and crystallography from areas as small as a few nm in size. The transmission electron microscope (TEM) is an optical analogue of the ordinary light microscope where an electron beam replaces the light, and magnetic fields replace the glass lenses. The standard microscopes used in physics and materials sciences typically operate with acceleration voltages of 200 or 300 kV producing electrons with wavelengths of 2.51 and 1.97 pm respectively. For all types of microscope based on optical principles the resolution is governed by a compromise between lens aberrations and diffraction of the limiting aperture. In light microscopes advanced lens designs can completely suppress spherical and chromatic aberrations. These microscopes are operated with very large opening angles, and the resolution is therefore close to the ideal value of h/2, where 3L is the wavelength of the light. The magnetic lenses used in electron microscopes, on the other hand, exhibit significant spherical and chromatic aberrations. These lenses are thus operated with very small opening angles - less than 1" - and the point resolution that can be achieved with a high-performance microscope is now less than 0.2 nm. Using microscopes equipped with 200 or 300 kV field-emission guns and coupled to very sophisticated image analysis systems that record series of images using a CCD camera, it is possible to extend the resolution down to the image infor-


mation limit determined by chromatic aberrations, which on such instruments can be as low as 0.11 nm (Coene et al., 1994). The best performance to date was recently achieved with the 1250 kV atomic resolution microscope installed at The Max-PlanckInstitut fiir Metallforschung in Stuttgart, which has a point resolution of 0.105 nm and pn image information limit around 0.08 to 0.09 nm (Phillipp, 1994). The transmission electron microscope is an extremely versatile instrument that can be used on virtually any kind of sample provided can be made sufficiently thin to allow the electrons to pass through without any significant energy losses. This condition in general requires samples that are less than a few hundred nanometers thick, and for microscopy at atomic resolution the samples should be less than 10 nm in thickness. The desired microstructural information can be extracted from the samples only if they are also able to sustain the intense electron radiation (p radiation) and the energy deposited by ionization and direct displacements for at least the time it takes to find a suitable area in the sample, to orient it properly, and to expose the required number of micrographs. The rate of energy deposition due to ionisation by the electron beam is extremely high, typically -10’ Gy s-l, which is comparable to the energy density developed very close to the centre of a moderately sized nuclear explosion (Grubb and Keller, 1975). Direct electron-atom ‘knock-on’ displacement damage occurs at a much lower rate, but is nevertheless a problem, particularly for microscopy at higher voltages under conditions where the same area is observed for long times such as in situ microscopy or high resolution microscopy. The versatility of the transmission electron microscope is fully demonstrated by the fact that it is a key instrument in many research areas such as materials science, physics, chemistry, biology, medicine, mineralogy, environmental sciences, etc. Besides providing direct microstructural information down to atomic level at the resolution limit, it is also a diffraction instrument capable of producing diffraction patterns from small well-defined areas of the samples. Furthermore, by equipping the electron microscope with suitable detectors it is also possible to measure analytical information associated with the various types of excitation that are inevitably induced by the incident electron beam. The most commonly used techniques are X-ray fluorescence spectroscopy and energy-loss spectroscopy, both of which can be carried out quantitatively using beam spots ranging from a few hundred nm to a few nm in size, under transmission conditions, scanning-transmission conditions or in imaging mode. It is the fact that it produces images from the same area of a sample with microstructural information and diffraction patterns with crystallographic information, and combines these with the analytical information available, that is perhaps the most powerful asset of the present day electron microscope.

27.2 Image formation The Abbe principle of image formation originally developed for light microscopes is also valid for image formation in the transmission electron microscope (Reimer,

27 Transmission electron microscopy and difffaction 447

1989; Cherns, 1988). The electron beam incident on the sample, which is considered to be monochromatic and parallel, can be represented by a plane wave:

w = \VO

(27- 1)

exp(ik-r)

where yo is the incident amplitude, k is the wave vector of the incident electron beam with wavelength X(k=27tlh), and r is the position vector. During penetration of the sample the electrons are scattered elastically into different angles without loss of kinetic energy. Rays with equal scattering angles from different parts of the sample will intersect in the back focal plane of the objective lens where they form the diffraction pattern (Fig. 27-1).

Fig. 27-1. Image formation in the transmission electron microscope. The dimaction pattern of the sample is formed in the back focal plane of the objective lens, where the objective aperture is used to select a subset of the diffracted electrons to form the image.

The diffraction pattern F(q) is the Fourier transform of the electron wave function w,( r) at the exit surface of the sample F(q) = F{Ws (r)>

(27-2)

where F is the Fourier transform operator and q = k' - k, is the scattering vector for electrons with incident wave vector k scattered into a direction with wavevector k'. The image of a point in the sample is then formed by summing the wave contributions from a preselected part of this q-plane at the corresponding image point (Fig. 27-1). The wave function w,, (r') describing the electron distribution in the image with position vector r' can then be expressed as the inverse Fourier transform of F(q): Wm

(4= F-'{M(q)F(q)}

(27-3)


The integration in eq. 27-3 is limited to a subset of the available distribution of spatial frequencies by incorporating an aperture function M(q) having the values M(q)=l for the subset of q-vectors lying inside the aperture, and M(q)=O for all other q-vectors. In practice, this is done by insertion of an aperture at the back focal plane of the objective lens, so that only electrons passing through the aperture will contribute to the image, Higher resolution requires a larger spread in spatial frequencies and hence a larger aperture. A bright-field image is formed by paraxial rays alone. The objective aperture is centred around the optical axis and dark image features then correspond to areas in the sample where strong electron scattering has taken place - due to large thickness, to the presence of atoms with high atomic numbers, or - for crystalline samples - to the Bragg diffraction being strong. A dark-field image, on the other hand, is formed by using only electrons that are scattered within a given off-axis solid angle. The objective aperture in this case is displaced away from the optical axis and the image is, in principle, complementary to the bright-field image. If the entire electron distribution in the back focal plane of the objective aperture is used to form the image, taking appropriate account of phase shifts induced by differences in optical path lengths, the image will be a true magnified representation of the sample. For such aberration-free imaging there will be no further phase shifts, the integration in eq. 27-3 will hence be taken over all spatial frequencies, q, in the back focal plane of the objective lens (M(q)=l for all q-values), and the resolution of the image will in principle be extended to the diffraction limit. Lens aberrations and defocusing add extra phase shifts to the electron distribution in the back focal plane which set an and the corresponding spatial frequency (q,,,,) upper limit to the scattering angle (em,) which in turn carry useful image information (Spence, 1988). Theoretically this is done by incorporating an instrument-dependent contrast-transfer function T(q) combining the effects of lens defocus, diffraction on the objective aperture, and spherical and chromatic aberrations in eq. 27-3. (27-4) In practice, this limitation on spatial frequencies corresponds to using an objective aperture that is sufficiently large to allow all spatial frequencies up to qm, to contribute to the image. Electrons scattered at larger angles are redundant and if they are allowed to contribute to the image they will only increase the background intensity. In contrast to the light microscope where the diffraction pattern is formed inside the microscope tube, and is never observed in practice, an extremely powerful asset of the transmission electron microscope is that it can also be used as an electron diffraction instrument. Because the lenses are magnetic fields their strengths can be varied continuously by varying the lens currents. Therefore, recording the diffraction pattern requires only that the strength of the first intermediate lens - the lens following the objective lens - is altered so as to image the back focal plane of the objective lens rather than the surface of the sample from which the electrons exit (Fig. 27-1). Diffraction can then be carried out in two different modes. In selected area diffraction (SAD) an aperture in the image plane of the first intermediate lens is used to select a small,

27 Transmission electron microscopy and difiaction 449

well-defined area of the sample with a lower size limit determined by spherical aberration to be about 200 nm, from which the diffraction pattern is recorded using nearparallel illumination. In nano diffraction a beam spot of a few nanometers in size is focused on the sample and the diffraction pattern is recorded from that area alone. If the divergence of the incident beam is increased to a value comparable with or larger than the Bragg angles for the diffraction spots in question, individual diffraction spots will show complicated contrast features which can yield detailed information about the orientation of the sample and its thickness. The technique is then called convergentbeam electron diffraction (CBED), and it is well-known for providing high-precision structural data, and information on the three-dimensional crystal symmetry properties of the samples.

27.3 Electron diffraction When an electron beam is incident on a crystal, diffraction from different sets of lattice planes will occur when Bragg’s law 2d sin 8 = nh;n E N

(27-5)

is fulfilled, where d is the planar spacing of the set of diffracting planes, 8 is the angle of incidence, h is the wavelength, and N is the set of positive integers. In three dimensions Bragg’s law is replaced by the Laue conditions stating that diffraction can only take place when the scattering vector Ak is a reciprocal lattice vector G: Ak = k’ - k = Gandlk(= Ik’l

(27-6)

where k and k‘ are the wave vectors of the incident and diffracted beams, respectively (Hirsch et al., 1977). Geometrically, the Laue conditions can be illustrated by the Ewald sphere construction, where a sphere with radius k is drawn through a section of the reciprocal lattice (Fig. 27-2). If the sphere is drawn with the k vector passing through the origin of the reciprocal lattice, diffraction will occur whenever the sphere intersects another reciprocal lattice point which then becomes the termination point for the k‘ wave vector. This construction is geometrically equivalent to Bragg’s law and, ideally, diffraction will occur only when the Laue conditions are fulfilled. In electron diffraction, however, the crystal is usually very thin along the beam direction in comparison with its lateral extent. Therefore, the reciprocal lattice points which constitute the Fourier transform of the real lattice are correspondingly extended parallel to the beam direction, thus reducing on the restrictiveness of the Laue conditions (Hirsch et al., 1977; Reimer, 1989). Because the radius of the Ewald sphere is much larger than the length of the reciprocal lattice vectors, i.e., (kl >> [GI, an electron diffraction pattern will therefore most often be made up of a regular array of spots that decrease in intensity with increasing scattering angle. Geometrically the diffraction pattern is often


tion illustrating the Laue conditions that dimaction takes place whenever the change in the wave vector Ak equals a reciprocal lattice vector g.

Ak=g .........m.......

described as a planar section through the reciprocal lattice - an approximation which makes geometrical interpretations very easy. For crystals with relatively large planar spacings along the incident beam direction, the reciprocal lattice spacing becomes small and the Ewald sphere will intersect the upper layers of the reciprocal lattice within the angular scattering range accessible with the microscope (Fig. 27-3).

0th -

1st 2nd -

order Laue zones

Fig. 27-3. Schematic Ewald sphere construction showing diffraction effects from upper layers in the reciprocal lattice corresponding to Laue zones of different orders in the diffraction patterns.

Such upper layer diffraction effects, appearing as sets of diffraction spots located in rings at larger angles, are called Laue zones. The central diffraction pattern originating from the intersection of the Ewald sphere with the layer of the reciprocal lattice containing the origin is called the 0th order Laue zone, and higher order Laue zones are named according to the layers from which they originate.

27 Transmission electron microscopy and diMaction 45 1

It is not easy to obtain high-precision quantitative structural information from selected area electron diffraction (SAD) analysis alone. The precision is only rarely better than about 1% unless the sample contains an international calibration standard (Hirsch et al., 1977), and use of X-ray diffraction analysis with its higher precision is in most cases more appropriate. However, under circumstances where the sample contains only very small amounts of micrometer or nanometer sized crystalline phases, application of X-ray diffraction may be precluded, and electron diffraction can be the only feasible alternative (Sundberg, 1994). This strategy, which is not only applicable to diffraction but also to imaging of microstructures, has been utilized with great success in studies of materials such as ceramics and minerals where localised compositional variations are accompanied by the existence of several polytypes and polymorphs (Wenk, 1976; McLaren, 1991). Fig. 27-4 shows a SAD pattern and a bright-field micrograph from an aluminium sample ion-implanted with thallium. Thallium is insoluble in aluminium and spontaneously forms inclusions a few nanometers in size during the implantation.

Fig. 27-4. Selected area diffraction (SAD) pattern (a) and bright-field micrograph (b) &om an (01 I) aluminium grain with nanosized thallium inclusions. The double array of spots in the SAD pattern indicates that the inclusions have a metastable fcc structure, and that they grow in a cubelcube orientation with the matrix.

When the inclusions are smaller than about 10 nm they have a fcc structure and grow in topotaxy with the aluminium matrix in a cubekube orientation (Johnson et al., 1993). The fcc thallium phase is only stable at pressures above 4 GPa (Tonkov, 1992), while the stable low pressure phase is hcp. The lattice parameter for the fcc thallium phase in the inclusions was obtained by electron diffraction, using aluminium as the internal standard, and was found to be 0.484 f 0.002 nm, indicating that the inclusions are under low pressure and hence in a metastable configuration (Johnson, 1993). This was verified by X-ray diffraction in a high-pressure experiment on thallium where the pressure corresponding to the lattice parameter of the inclusions was found to be as low as 0.24 GPa (Staun Olsen et al., 1994). The precipitates, which are imaged in moirC contrast, have a strict cuboctahedral morphology with large atomically smooth (1 11) facets and smaller less regular (001) facets. The moirk images arise as an effect


of the near-match between planes in the two structures for each of the four thallium planes and five aluminium planes respectively. The possibility of carrying out in situ experiments in the electron microscope utilizing both diffraction and image information is a further strong asset of the instrument (Ross, 1994). In situ heating experiments, in particular, are relatively easy to conduct using commercially available heating stages. Fig. 27-5 shows SAD patterns and the associated bright-field micrograph from a rapidly quenched A190Y 10 alloy that has been heated to 460 K in the microscope (Li et al., 1992). The as-quenched alloy is an amorphous metallic glass which produces a characteristic diffraction pattern with two or three broad diffuse rings (Fig. 27-5a). During annealing, crystallisation of the alloy takes place by initial formation of an fcc A1-Y solid solution characterized in the diffraction pattern by a set of sharp rings. The simultaneous occurrence of the diffuse diffraction rings from the glass phase and the sharp rings from the fcc phase shows that the first recrystallization stage is only partial, in accordance with the transmission electron microscope image (Fig. 27-5c).

Fig. 27-5. SAD patterns and bright-field micrograph from a rapidly solidified A19DYloalloy, annealed in situ in the microscope. The diffise rings in the SAD pattern ffom the as-quenched alloy (a) indicate that the structure is initially amorphous. Annealing to 160 "C induces a partial crystallization with formation of an fcc aluminium solid solution, identified ffom the SAD pattern (b) and shown in the micrograph (c).

Detailed structural analysis by SAD has up till now usually been precluded by 1) the strong interaction between the electrons and the sample leading to dominating many-

27 Transmission electron microscopy and diffraction 453

beam dynamical diffraction effects that are difficult to handle theoretically, 2) the difficulty of measuring intensities over large dynamic ranges using photographic films, and 3) bending of the thin film samples across the sampling area. The theoretical problems have now mostly been overcome by modern computing techniques (Kastner, 1993). With the introduction of CCD cameras and image plate systems (Daberkow et al., 1991; Ogura et al., 1994) facilities for absolute intensity measurements are now available that make it possible to carry out structure determinations by electron diffraction on a quantitative basis. Access to microscopes with nanometer sized beamspots has largely solved the problems of sample bending and, when used in combination with large incident-beam convergence angles, the CBED techniques have opened up a whole new research area for electron diffraction analysis (Eades, 1989; Spence and Zuo, 1992). In CBED the diffraction spots are extended to discs with an angular width that is determined by the divergence of the incident beam (Fig. 27-6).

Diffraction discs

Fig. 27-6. Schematic illustration of the formation of a convergent beam diffraction pattern (CBED). The angular width of the diffraction discs is equal to the opening angle of the incident beam. The incident beam direction P corresponds to the positions P' in the diffraction discs.

The intensity variations across the diffraction discs are then practically equivalent to the rocking-curve distributions familiar from X-ray diffraction measurements. One of the simplest applications is the use of CBED for thickness determination. When a systematic row of diffraction spots is excited so that only one set of crystal planes is diffracting, the CBED diffraction spots consist of intensity fringes resembling the intensity distributions of rocking curves or bend contours (Fig. 27-7). The positions of the fringes depend on the thickness of the sample, so for crystals oriented under so-called two-beam conditions - where one transmitted and one diffracted beam are strongly excited - it is possible to determine the thickness of the sample and to assess the extinction distance. The extinction distance is an electron diffraction parameter that is

454

Part 4: Crystallography and structure

inversely proportional to the structure factor of the corresponding sets of planes (Kelly et al., 1975; Spence and Zuo, 1992). Using many-beam theory, numerical optimization processes and structure-factor refinement it is possible given the best circumstances to obtain precise values for the structure factors from a CBED analysis with a precision comparable to that achievable in X-ray diffraction (See Spence and Zuo, 1992 and references therein).

Fig. 27-7. CBED pattern from aluminium in the symmetry orientation taken with a systematic set of (002) diffraction spots. The parallel fringes in the diffraction discs correspond to rocking curves in X-ray diffraction. Fitting the intensity profiles of the fringes to calculated curves provides information on the local crystal thickness and the structure factors. The inclined lines traversing the dimaction discs are the axial parts of first-order HOLZ lines.

The frequent appearance in the CBED diffraction discs of dark lines is another feature related to the large convergence angle. The lines are the convergent-beam kinematical equivalents of Kikuchi lines, originating from diffraction of electrons first scattered inelastically with very little energy loss, and normally seen in SAD patterns from thick undeformed crystals. The lines in the CBED patterns are associated with diffraction spots lying in the first or higher order Laue zones (Fig. 27-3) and are therefore called HOLZ lines. Due to their origin the HOLZ lines display the three-dimensional symmetry of the crystals in contrast with the normal SAD pattern which only shows the symmetry of the Oth-order Laue zone. They always occur in pairs with one dark line lying close to the centre of the diffraction pattern and the other bright line close to the diffraction spot in the higher order Laue zone from which the pair originates (Fig. 27-8). The exact positions of the HOLZ lines and the detailed intensity distributions in the diffraction discs are extremely sensitive to the acceleration voltage and the local composition and strain of the sample. They can therefore be used both for precise structure determinations (Zuo and Spence, 1991), for structure-factor refinements (Bird and Saunders, 1992) and for precise determinations of local lattice strains induced either externally or internally by defects (Duan et al., 1994).


Fig. 27-8. CBED pattern ffom (1 11) silicon showing the diffraction discs of the 0th order (ZOLZ) and 1st order (FOLZ) Laue zones.

27.4 Diffraction contrast In electron microscopy of crystalline materials under diffraction contrast conditions, a single dominant diffracted beam is generally selected by the objective aperture to form either a bright-field or a dark-field image. In this case the contrast in the image originates from changes in the amplitude of the electron wave function contributing to the image (Hirsch et al., 1977). This imaging mode is usually applied to studies of contrast either from perfect crystals or from extended crystal defect structures characterized by long-range strain fields. In either case, the experimental observations should be related to theoretical two-beam or many-beam contrast calculations (Hirsch et al. , 1977; Metherell, 1976; Kastner, 1993) or to image simulations (Head et al., 1973; Rasmussen and Carter, 1991). The efficiency of electron diffraction close to the Bragg condition is very sensitive to the exact crystal orientation. Contrast from crystal defects then arises as a result of local tilts of crystal planes towards or away from the Bragg conditions due to the strain fields associated with the defects (Hirsch et al., 1977; Edington, 1976). The image-width of a defect will therefore be associated with the lateral extension of the strain field. Techniques for imaging defects will in most cases be related to two-beam diffraction conditions where the crystal is oriented so that a single set of crystal planes with diffiaction vector G is close to the Bragg condition. Images of dislocations formed under these conditions either in bright-field or in dark-field are typically 10 to 20 nm across, and they can be located on either side of the positions of

456 Part 4: Crystallographyand structure

the dislocations (Hirsch et al., 1977). In the weak-beam technique where a weakly excited first order Bragg spot is used for dark-field imaging, defect contrast is enhanced on a dark background and dislocation images become much narrower -1 -2 nm (Cockayne, 1972). Both strong-beam and weak-beam techniques may be used to analyse crystal defect structures using either various analytical contrast criteria or image matching methods based on computer-simulated images calculated from two-beam or many-beam theory (Satoh et al., 1994). Earlier, image matching between sets of micrographs taken under well-defined diffraction conditions and sets of corresponding computer-simulated images was always done on a qualitative basis by visual inspection (Head el al., 1973). However, attempts are now being made to match experimental and calculated image intensities quantitatively, either by fitting image details to approximate analytical expressions or by use of advanced statistical correlation methods (Rasmussen et al., 1991). This type of approach will become much more important in the years to come due to the rapid development of electron microscopes instrumented with CCD cameras and image plate systems capable of recording image intensities linearly with dynamic ranges spanning six to seven orders of magnitude. A micrograph with dislocation loops produced in an (001) aluminium TEM sample by ion implantation of 30-keV hydrogen under conditions where the hydrogen ions penetrate the thin foil is shown in Fig. 27-9. The loops are formed by agglomeration of radiation induced point defects and have a mean size of about 50 nm. They are inclined with respect to the electron beam and can barely be resolved into arc segments. The larger somewhat jagged dislocation lines surrounded by zones denuded of small loops are formed by merging and subsequent climb-induced sweeping up of smaller loops (Johnson .and Ytterhus, 1973).

Fig. 27-9. Bright-field micrograph of irradiation induced inclined dislocation loops in H’-irradiated aluminium. The image width of the dislocations is around 10 to 20 nm. The contrast at the most steeply inclined parts of the loops is reduced giving the small loops a typical ‘coffee-bean’ like appearance.


Fig. 27-10 shows strong-beam and weak-beam images of the same area of an aluminium sample containing a surface layer about 100 nm thick with a dense distribution of small indium inclusions made by ion-implantation of about 2 at.% indium into pure aluminium. This process causes heavy radiation damage leading to formation of complicated dislocation tangles. In the strong-beam image (Fig. 27-1Oa) taken with the electron beam nearly parallel with a set of { 11l} planes the dislocation structure is difficult to resolve, and the broad image features obscure details of the images of the inclusions. In the same area taken under weak-beam dark-field conditions (Fig. 27-lob) the dislocation images become much narrower and the contrast is much more distinct on the dark background. It is then not only possible to resolve details of the dislocation structures, but it can also be seen that some of the inclusions are attached to the dislocations where they have grown larger than the average size by pipe diffusion of indium atoms along the dislocation cores (Johnson et al., 1992).

Fig. 27-10. Bright-field (a) and weak-beam dark-field (b) micrographs from aluminium implanted with indium. Under weak-beam conditions the image width of the dislocation lines is reduced to 1 to 2 nm and it is possible to see indium inclusions in moire contrast that are attached to the dislocation lines.

Planar defects such as stacking faults, domain boundaries and grain boundaries have no perceptible strain field. Instead, contrast from planar defects is caused by an abrupt phase shift of the diffracted beams traversing the defect planes. In most cases the phase shifrs are large, leading to a modulation of the transmitted and diffracted intensities. Hence the changes in intensity will depend sensitively on the values of the phase shifts, and for inclined planar defects in thicker crystals the contrast will often be in the form of fringes somewhat similar to thickness fringes (Van Tendeloo and Amelinckx, 1976; Edington, 1976). Fig. 27-11 shows an inclined stacking fault in a 304-type stainless


steel lying on a { 111] plane. The difference in contrast of the outer fringes between the bright-field (Fig. 27-lla) and the dark-field (Fig. 27-llb) images is due to dynamic diffraction effects and can be used to distinguish between the top and the bottom parts of the stacking fault and hence to determine the sense of its inclination (Hirsch et al., 1977). The side of the fault where the fringes are similar corresponds to the intersection of the fault with the top surface of the sample where the electron beam enters the thin foil.

Fig. 27-11. Bright-field (a) and darkfield (b) images of a stacking fault in 304 stainless steel. The fault, lying on an inclined { 111) plane, is imaged as a set of closely spaced fringes. By comparison of the outer fringes in the two micrographs it is possible to distinguish between top (T) and bottom (B), and to determine the sense of the inclination of the fault.

Fig. 27-12. Irregular incoherent 8 precipitates in rapidly solidified A1-2.5%Cu alloy annealed to 300 "C in situ in the microscope. The contrast of the precipitates is largely due to the heavier mass of the 8 phase (Al2Cu).


Contrast from precipitates or second phase particles can be extremely complicated due to contributions from strain contrast, interface contrast and mass-absorption contrast originating from differences in composition between precipitates and matrix. Fig. 27-12 shows a micrograph from an A1-2.5%Cu alloy made by rapid quenching. During subsequent in situ annealing in the microscope to 300 "C incoherent 8 precipitates nucleate and grow on dislocations inside the aluminium grains (Li et al., 1993). In most cases transmission electron microscopy is carried out in plan view, i.e. under conditions where the electron beam is nearly perpendicular to the sample plane containing the features of interest. With the development of more sophisticated sample-preparation techniques it is now possible to prepare cross-section samples based on ion-beam thinning of a variety of materials. Gluing or squeezing pieces of material together combined with ion-beam thinning open up the possibility of edge-on imaging of surfaces or interfaces. Preparation of cross-section samples has as yet mostly been applied to semiconductor and ceramic materials. These materials respond very differently from the way metals do to the ion-induced radiation damage. However, new developments in ion-thinning technology using low-energy, low-angle beams appear to be more applicable to metals (Barna, 1992). Traditionally, cross-section samples of metals bonded together by electroplating have been made from metals that display good adhesion during the electroplating process. Fig. 27- 13 shows a cross-section sample of a copper crystal ion-implanted with lead. During the implantation process the heavy lead atoms will sputter copper atoms very efficiently off the implanted surface, which for a sufficiently high fluence develops a distinct crystallographic surface topography that can be assessed in detail from cross-section samples only (SarholtKristensen et al., 1991).

Fig. 27-13. Cross-section electron micrograph of high-fluence lead-implanted copper showing implantation-induced development of a coarse surface topography. The sample is made by electroplating a pure copper layer directly onto the implanted surface, followed by cross-sectioning by spark machining, cutting of 3 mm discs, and electrolytical thinning of the discs to perforation at the interface. (By courtesy of Nucl. Instrum. Meth. B.)


27.5 High-resolution electron microscopy In high resolution imaging several diffracted beams are recombined by interference to form the resulting image. To achieve optimum resolution and unambiguous image information, the samples must be so thin that the interaction with the incident electron beam only alters the phase while the amplitude remains constant. For a sample to behave in this way - as a pure phase object - it must be thin, usually less than 10 nm. The wave function at the exit surface of the crystal w,( r) for incident unit amplitude may then be calculated from the crystal potential, projected to the exit surface plane (x,y) of the crystal as w s (x, Y)

= exp(-

i@(X, Y>>

(27-7)

where = nt/hE, and t, h and E are crystal thickness, electron wavelength and accelerating potential respectively (Cherns, 1988; Spence, 1988). The 90" phase change indicated by the imaginary unit i in (eq. 27-7) is an inherent effect of the scattering process which will minimize the contrast. To achieve optimum contrast it is necessary to induce a further 90" phase shift by a small defocus similar to that obtained with a h/4 plate in optical-phase contrast microscopy (Reimer, 1989). A full wave-optical analysis, incorporating aberration effects through the contrast transfer function T(q), gives optimum imaging conditions with maximum positive phase contrast. In this case atom columns will be dark at the so-called Scherzer focus Aoptgiven by: Aopt = - , / f C s h

(27-8)

where C, is the spherical aberration coefficient for the objective lens, and the minus sign indicates underfocus. The corresponding opening angle for the objective aperture then gives a convenient measure for the point resolution dmin of the microscope: dmin= 0.66C:'4h3'4

(27-9)

Image details with variable contrast can be recorded at smaller distances than dmin by changing the defocus for transfer of narrow bands of higher spatial frequencies until the contrast transfer function is finally cut off at the information limit set by chromatic aberration. Fig. 27-14 shows an atomic resolution micrograph of a lead inclusion about 5 nm in size embedded in aluminium. The inclusion which has cuboctahedral shape is viewed along an direction where large { 111} and smaller { 00 1} facets are seen edgeon. The superimposed moire effect which complicates the contrast considerably is induced by lattice mismatch of 22% between the two structures (Johnson e l al., 1992; Xiao et al., 1995). At the atomic resolution level the smoothness of the { 111} interfaces is distinct, supporting earlier results obtained indirectly from X-ray diffraction (Gribzk et al., 1992). Free lead crystallites at the edges of the thin aluminium foils sometimes contain internal defects in the form of thin twin lamellae (Fig. 27-15) that


have never previously been seen in lead inclusions embedded in the aluminium (Xiao et al., 1995).

Fig. 27-14. Atomic resolution micrograph of 50-nm lead inclusion in aluminium. The (01 1) lattice image of the inclusion is complicated lYom the overlapping moire contrast, cf Figs. 27-4 and 27-10.

Fig. 27-15. Atomic-resolution image of a (01 1) lead crystallite at the very edge of an aluminium foil. The strong contrast of the lead crystallite is due to the higher scattering strength of the heavier lead atoms. The arrows indicate three-layer thick twin lamellae on one of the edge-on { 11 I} planes.


Fig. 27-16. Atomic-resolution image of a nanosized diamond-cubic germanium precipitate in (01 1) aluminium. The arrows indicate an edge-on twin plane in the precipitate where the bottom half grows in cubelcube orientation with the matrix. The line extending above the precipitate along the { 1 1 1) plane (F) is part of a Frank dislocation loop induced during observation by radiation damage from the 800 keV electron beam. (By courtesy of S.-Q. Xiao.)

Fig. 27-17. Atomic resolution micrograph of an edge-on interface between MoSiz (m) and MojSiS viewed along the [OOI] matrix direction. The interface is characterized by a regular array of interface dislocations (arrows). (By courtesy of S.-Q. Xiao).

Internal twinning in nanosized precipitates is a fairly common phenomenon that can also be seen in embedded crystallites. Fig. 27-14 shows a germanium precipitate in aluminium growing in a cubekube orientation with respect to the matrix. The germanium precipitate is separated into two halves by an edge-on twin boundary on the { 111} plane parallel to its long axis. The lower part of the precipitate displaying regular moirC contrast follows the cubekube orientation with the aluminium matrix, while the upper twin-related part shows a much more complicated contrast.


The accessible resolution of modern atomic resolution microscopy provides a unique tool for studying grain boundaries and precipitates and their interfaces in a variety of materials (Duly el al., 1995). Fig. 27-17 from a MoSi2 crystal shows an edge-on view along the [OOI] matrix direction of the interface between MosSis precipitate formed by Si depletion and the MoSi2 matrix (Maloy et al., 1993). Both crystals have body-centred tetragonal structure with nearly coincident (33O)mawix and (1 1O)precipitate reflections (the misfit is 0.18 %) and the interface can be seen to consist of an array of misfit dislocations with a spacing given by 12 lattice fringes in the MoSiz matrix.

27.6 Conclusion In the context of surface characterization, transmission electron microscopy and diffraction are ‘crude’ techniques that in general are neither surface-specific nor quantitative, and distinct surface information can only be obtained in special modes such as reflection high-energy electron diffraction andlor imaging, or by dark-field microscopy using diffraction spots originating from the surfaces for imaging. The typical thickness of a transmission sample can vary from a few atomic layers to several pm and depth information is not easily accessible. Nevertheless, the transmission electron microscope is a powerful instrument that, with its very high spatial resolution and ability to combine crystallographic and microstructural information, can provide unique information on surface and near-surface properties of materials. Future standards for electron microscopy are now being set by the further development of instruments equipped with field-emission guns able to obtain more coherent illumination, to be used for example in electro-holography, in the application of energy filtering systems both in imaging and diffraction modes, and in the strong emphasis on digital-image-recording using faster large-area high-resolution devices.

References Barna A., (1992), Materials Research Society Symposium Proceedings, Anderson R., Tracy B., Bravman T., (Eds), Pittsburgh, Materials Research Society, Vol. 254, pp. 3 - 22. Bird M., Saunders M. (1992), Acta Cryst. A , 48,555. Cherns D. (1988), Analytical Techniques for Thin Films, Tu K.N., Rosenberg R. (Eds), San Diego: Academic Press Inc., pp.297-335. Cockayne D.J.H. (1972), Z. Naturforsch. a, 27a, 452. Coene W.M.J., Thust A., Op de Beeck M., Van Dijck D. (1994), Proceedings of the 13th International Congress on Electron Microscopy, Jouffrey B., Colliex C. (Eds), Les Ulis, Les Edition de Physique, 1994; VOI. 1, pp. 461 - 462. Daberkow I., Henmann K.H., Liu L., Rau W.D. (1991), Ultramicroscopy, 38,215. DuanX.F., Cherns D., Steeds J.W. (1994), Philos. Mag. A, 70, 1091. Duly D., Zhang D.-Z., Audier M. (1999, Philos. Mag. A, 71, 187. Eades J.A. (Ed.) (1989), J. Elec. Micr. Technique, Vol. 13, no. 1,2. Edington J. W. (1976), Practical Electron Microscopy in Materials Science, London, The MacMillan Press Ltd and Philips Technical Library.

464 Part 4: Crystallography and structure Grubb D.T., Keller A. (1975), Proceedings of the Fifth European Conference on Electron Microscopy, Manchester, p. 554. GrAbzek L., Bohr J., Andersen H.H., Johansen A., Johnson E., Sarholt-Kristensen L., Robinson 1.K. (1992), Phys. Rev. B, 45,2628. Head A.K., Humble P., Clarebrough L.M., Morton A.J., Fonvood C.T. (1973), Computed Electron Micrographs and Defect Identification, Amsterdam, North-Holland Publ. Co. Hirsch P.B., Howie A., Nicholson R.B., Pashley D.W., Whelan M.J. (1977), Electron Microscopy of Thin Crystals, Florida, Robert E. Krieger Publ. Co. Johnson E., Hjemsted K., Schmidt B., Bourdelle K.K., Johansen A., Andersen H.H., Sarholt-Kristensen L. (l992), Materials Research Society Symposium Proceedings, Was G.S., Rehn L.E., Follstaedt D.M., (Eds), Pittsburgh, Materials Research Society, Vol. 235, pp. 485 - 490. Johnson E., Johansen A., ThoR N.B., Andersen H.H., Sarholt-Kristensen L. (l993), Philos. Mag. A, 68, 131. Johnson E., Ytterhus J.A. (1973), Philos. Mag. 28,489. Kastner G. (1 993), Many-Beam Electron Diffraction Related to Electron Microscope Diffraction Contrast, Berlin, Akademie Verlag, GmbH. Kelly P.M., Jostson A,, Blake R.G., Napier J.G. (1975), Phys. Stat. Sol. (a), 31, 771. Li Q., Johnson E., Madsen M.B., Johansen A., Sarholt-Kristensen L. (1992), Philos. Mag. B, 66,427. Li Q., Johnson E., Johansen A., Sarholt-Kristensen L. (1993), J. Mater. Sci., 28, 691. McLaren A.C. (l99l), Transmission Electron Microscopy of Minerals and Rocks, Cambridge, Cambridge University Press. Maloy S.A., Xiao S.-Q., Heuer A.H., Garrett J. (1993), J. Mater. Res., 8, 1079. Metherell A.J.F. (l976), Electron Microscopy of Materials Science, Ruedl E., Valdrk U. (Eds), Brussels, Commission of The European Communities, Vol. 11, pp. 397 - 552. Ogura N., Yoshida K., Kojima Y., Saito H. (1994), Proceedings of the 13th International Congress on Electron Microscopy, JouMey B., Colliex C. (Eds), Les Ulis, Les Edition de Physique, 1994; Vol. 1, pp. 219-220. Phillipp F., HBschen R., MBbus G., Osaki M., RUhle M. (1994), JEOL News, 3 1E, No. I , 2. Rasmussen D.R., Carter C.B. (1 99 1), J. Elec. Micr. Technique, 18,429. Rasmussen D.R., McKernan S, Carter C.B. (1991), Philos. Mag. A , 63, 1299. Reimer L. ( 1 989), Transmission Electron Microscopy. Physics of Image Formation and Microanalysis, Berlin, Springer-Verlag. Ross F.M. (Ed.) (1994), MRS Bulletin, XIX, no. 6. Sarholt-Kristensen L., Johansen A., Johnson E., Stenstrup S., Chechenin N., Chernysh V.S. (1991), Nucl. Instrum. Meth. B59/60, 85. Satoh Y., Taoka H., Kojima S . , Yoshie T., Kiritani M. (l994), Philos. Mag. A, 70, 869. Spence J.C.H. (l988), Experimental High-Resolution Electron Microscopy. New York, Oxford University Press. Spence J.C.H., Zuo J.M. (1992), Electron Microdifftaction, New York, Plenum Press. Staun Olsen J., Genvard L., Steenstrup S., Johnson E. (1994), J. Appl. Cryst., 27, 1002. Sundberg M. (1994), Proceedings of the 13th International Congress on Electron Microscopy, Jouffrey B., Colliex C. (Eds), Les Ulis, Les Edition de Physique, 1994; Vol. 1, pp. 955-958. Tonkov E.Yu (l992), High Pressure Phase Transformations. A Handbook, Vol. 2, Philadelphia, Gordon and Breach, pp. 683-687. Van Tendeloo G., Amelinckx. (l976), Electron Microscopy of Materials Science, Ruedl E., Valdrk U. (Eds.), Brussels, Commission of The European Communities, Vol. 111, pp. 775 - 826. Wenk H.-R. (ed.) (1 976), Electron Microscopy in Mineralogy, Berlin, Springer-Verlag. Xiao S.-Q., Johnson E., Hinderberger S., Johansen A., Bourdelle K.K., Dahmen U. (1995), J. Microscopy, 180,6 1. Zuo J.M., Spence J.C.H. (1991), Ultramicroscopy, 35, 185.



28 Electrons for surface diffraction, imaging and vibrational spectroscopy C. Nyberg

28.1 Introduction The study of the interaction of electrons with surfaces yields much information about the surface region. Electrons are diffracted by the periodic lattice which makes up the surface. Low electron energy gives high surface sensitivity due to the short mean free path of the electrons. For high electron energy the surface specificity may be maintained if the electrons are arranged at a grazing incidence. Electrons also scatter inelastically at the surface. The resulting energy losses to which the electrons are subjected are associated with excitations of the characteristic vibrational modes of the surface. The electrons may also be used, analogous to light, for forming a real image of the surface. This chapter mainly deals with Low-Energy Electron Diffraction (LEED) and High-Resolution Electron-Energy-Loss Spectroscopy (HREELS). The related techniques, Reflection High-Energy Electron Diffraction (NEED), Low-Energy Electron Microscopy (LEEM) and Photo-Emission Electron Microscopy (PEEM), are also briefly discussed.

28.2 Low-energy electron diffraction One of the cornerstones in surface science is the ability to determine the geometric structure in the surface region; one of the most important tools for this purpose is low energy electron diffraction (LEED). About half of the known surface structures have been determined by this technique. Electrons are diffracted by the periodic surface lattice and the diffraction intensities contain, in principle, all the information that is necessary to describe the structure.

28.2.1 The experimental arrangement A typical experimental arrangement is shown in Fig. 28-1. The electron gun usually consists of a thermally heated cathode which emits electrons with an energy spread of about 0.5 eV. The electrons are focused so that the spot size is about 1 mm at the sample. The electron energy may be varied between 20 and 500 eV by changing the potential of the gun relative to the sample, which is kept at ground potential. The crystal is mounted on a manipulator which allows the angle of incidence of the electrons to be varied. The diffracted beams emerging from the crystal are detected by a fluorescent screen. A number of highly transparent grids are placed in


front of the screen. The grid closest to the sample is maintained at ground potential to provide a field-free region between sample and grid. The other grids act as a high-pass filter for the electrons: typically only electrons which have lost less than 1 eV of energy are transmitted. Finally a potential difference of about 5 kV is applied between the last grid and the screen. Window

n

Screen

nJ-= Camera

i

Cornparision

Trial structure model

t

.

:Computer

I

-v

culve

Fig. 28-1. Schematic drawing of the LEED equipment.

The low energy of the electrons ensures that the probing depth is 5- 10 A. The electron current at the sample can easily be kept below 1 pA and even below 1 nA when the screen is replaced by a position-sensitive detector (digital LEED). The coherence length of the electron beam is typically about 100 A. This quantity is important when the diffraction pattern is evaluated because it determines the area of the surface which scatters coherently, i.e. where amplitudes of the scattered electron waves can be added. For more details about the experimental equipment see Ertl and Kuppers (1985), van Hove ef al. (1 986), and Woodruff and Delchar (1986).

28.2.2 Data acquisition The diffraction pattern can, of course, be observed by directly viewing the screen and can be documented by taking a photograph of the screen. It is important to observe the pattern at different electron energies and angles of incidence. The diffraction intensity is more difficult to measure. There are three main measuring devices: the Faraday cup, the spot photometer and the video camera. The Faraday cup consists of a small movable detector which can be positioned to collect the electrons in one diffracted beam at a time. Obviously the Faraday cup must be situated

28 Electrons for surface diffraction, imaging and vibrational spectroscopy467

inside the vacuum system. A spot photometer is used to monitor the intensity of the diffracted beams from outside the vacuum system. This is done by monitoring the intensity of light emission from the fluorescent screen. In modem instruments a video camera is used to view the diffraction spots from the outside; The Faraday cup and the spot photometer are only found in older equipment. To make a detailed structural analysis the intensity of the diffraction spots must be measured as the incoming electron energy is varied (Wcurves). Since the diffraction angle varies with electron energy the Faraday cup and the spot photometer have to follow the diffraction beam. This is usually done automatically. The video camera offers a more efficient way of recording the IN curves. The video camera can be connected to a video recorder. The data are subsequently fed into a computer and a whole set of W curves can be determined. Today commercial video LEED systems are available and constitute an important part of an effective quantitative structural analysis.

28.2.3 Analysis of data The perfect surface is characterized by translational symmetry operations and pointgroup symmetry operations. Taken together these form the 17 space groups that are used to describe two-dimensional structures. The space group operations limit the possible geometry of the surface to five Bravais lattices which, together with a basis, can be used to describe the atomic geometry of the surface (van Hove et al., 1986). The goal is now to determine the geometry from the diffraction data. The experimental data to be analysed consist of a diffraction pattern and W curves for different diffraction spots. We first discuss what kind of information can be extracted from the LEED pattern. All diffraction phenomena are conveniently described in reciprocal space. For a twodimensional lattice the reciprocal space consists of a set of rods perpendicular to the plane of the two-dimensional lattice. For normal incidence the LEED pattern is an image of a cut, in reciprocal space, perpendicular to the rods. First one tries to find a unit cell which builds the pattern. This unit cell is then transformed to real space. A word of caution is appropriate here: the image of the pattern is always larger or equal to the image of the surface structure. This complication appears because the surface may consist of several domains, which are translated (by a non-lattice translational vector) or rotated (by an angle which is not symmetrical to the lattice) with respect to each other. If the coherence-width of the electron beam is smaller than the domain sizes diffraction patterns from several domains will be superimposed on the screen. An example of the existence of two types of domain is shown in Fig. 28-2. The figure shows the diffraction pattern observed when CO is adsorbed on a Pd(100) surface to a coverage of 0.5 monolayers. For normal incidence several spots are ‘missing’ making it hard to find a unit cell. However, when the sample is tilted the ‘missing’ spots appear and the unit cell is evident. The appearance of a rectangular cell means that two types of domain may grow on the surface and the LEED pattern is a superposition of the diffraction from both. The disappearance of certain spots for nor-

468 Part 4:Crystallography and structure

ma1 incidence is characteristic of a special symmetry of the structure: glide plane symmetry. This symmetry operation is composed of a mirror reflection in the glide plane and subsequent translation by half a lattice parameter parallel to the glide plane. The real space structure is also shown in the figure. It is not possible to determine the coordination of the CO molecules to the substrate atoms from the geometry of the diffraction pattern alone. In this particular case the co-ordination follows conclusively from the observation of glide planes (missing spots) and the CO molecules must be situated at bridge sites (van Hove et al., 1986). (2,/2xJ2)R45"

0

0

0 0

+

0

+

0

*--+-.O

0

6 - - 00

0

Substrate spots

0

Intense extra spots

+

0

+ . . ; - . -6

0 0

0

Extra spots missing a t normal incidence

Fig. 28-2. The LEED pattern from the structure obtained when half a monolayer of CO is adsorbed on Pd( 100) is shown in the left part of the figure. The unit cells from two domains are indicated. The right part of the figure shows the corresponding real space structure (only one domain shown).

The spot profile, intensity versus position on the screen, can also give important information without any complicated calculations. For example, the splitting of all spots indicates that the surface is stepped, and elongation of spots in a certain direction can be due to bad order in that direction. A more detailed spot-profile analysis can be performed on special dedicated instruments, Spot-Profile-Analysed LEED (SPALEED) (Henzler, 1993). Many important parameters of growing films can be derived from SPALEED using a kinematic approximation for the scattering. The determination of the positions of the atoms within the unit cell constitutes the real problem in LEED. The kinematic theory of diffraction, where each particle scatters only once in the sample, cannot be used for a quantitative evaluation. The strong interaction between impinging electrons and atoms in the surface implies that multiple

28 Electrons for surface diffraction, imaging and vibrational spectroscopy 469

scattering effects are very important and therefore a dynamic theory is required. So far no inversion scheme has been developed to directly transform the diffraction data into a real space structure. To perform a calculation of the diffraction phenomena one needs to know the potential from which the electrons are scattered. Usually a muffin-tin type of potential is used. The next step is to make a clever guess (hints are provided by other surface spectroscopies) at a trial structure and perform a calculation of the corresponding diffraction intensities. Comparison of the calculated intensities with the measured intensities is accomplished by using ‘reliability factors’, R, which measure the overall quality of agreement between the two curves. The R-factor varies from 0 to 1,O corresponding to perfect agreement and 1 no agreement. As an example an R-factor of 0.25 is considered to indicate good agreement. The trial structure may be improved by varying the input parameters. Since there are many parameters to explore and a full calculation has to be made for each parameter change the time taken to perform this trial-and-error search grows exponentially with the size of the system. Accordingly this procedure is applicable when there are just a few non-equivalent atoms in the unit cell. CO adsorbed on Pd(l00), discussed in the previous section, has been analysed in this way. The CO molecules are found to be adsorbed, with the C atom towards the surface and the molecular axis perpendicular to the surface. The Pd-CO bond length is 1.93 A. Recently there have been important advances in the development of effective schemes for calculating with LEED. The tensor-LEED (TLEED) theory uses the first trial calculation to make a perturbation expansion, with the original structure as a reference structure. Since much of the computational work has already been done the perturbation can be performed very rapidly. Thus the vicinity of a given structure can be explored at low computational cost. In the linear-LEED (LLEED) theory the changes in the diffraction intensities are linearly dependent on the displacement of the atoms from their positions in a reference structure. Holographic methods have also been tested and have shown promising results. For details of LEED theory see Pendry (1 993) and van Hove et al. (1 993). The precision in the atomic positions is 0.02-0.2 A. Positions parallel to the surface are more uncertain than positions perpendicular to the surface. The LEED technique has been used to determine the structure of single crystal surfaces. Both relaxation, where the lattice plane spacing perpendicular to the surface is different from the spacing deep in the bulk, and reconstruction, where the atoms in the surface region are displaced laterally compared with an ideal bulk termination, have been investigated. The formation of ordered adsorbed layers of atoms or molecules can be followed and the adsorption site and bond lengths can be determined. The temperature dependence of diffraction spot intensity and profile can be used to determine the diffusion constant for species on the surface. Disordered overlayers (lattice gas) may be studied by monitoring the intensity change in the background as a function of the electron energy. This is called Diffuse-LEED (DLEED). TLEED computational schemes have been used for data evaluation. The technique works well when only one type of site is present (Saldin et al., 1985).


28.3 Reflection high-energy electron diffraction It is also possible to conduct surface diffraction studies using high-energy electrons. When the electron energy is in the range 10-50 keV Reflection High-Energy Electron Diffraction (WEED) is used. To achieve surface sensitivity (a few atomic layers) the electron beam impinges on the surface with grazing incidence.

28.3.1 The experimental arrangement A typical experimental set-up is shown schematically in Fig. 28-3. The electron gun is essentially of the same type as used for LEED. The electrons hit the surface at a grazing angle of a few degrees, and the diffraction pattern from electrons scattered into grazing angles is imaged using a fluorescent screen. Note the grazing incident-exit configuration allows excellent accessibility to the sample surface permitting the simultaneous use of other surface techniques.

n

Screen

Electrongun

Sample

Fig. 28-3. A schematic drawing of the M E E D equipment.

28.3.2 Data acquisition and analysis The diffraction pattern, which consists of a number of streaks, now represents an image of a cut in reciprocal space parallel to the reciprocal rods, while in LEED the cut was perpendicular to the rods. This means that the sample must be rotated around the surface normal in order to determine the symmetry of the surface unit mesh. Intensity measurements, like the W curves in LEED, would in W E E D correspond to the intensity variation along a diffraction streak. The intensity versus angle of detection (rocking curves) also contains information about the surface structure. Together with theoretical simulations the data are used for studying the growth process in MBE (Molecular Beam Epitaxy) (Beeby, 1993). W E E D has, however, so far mainly been used for monitoring the number of layers deposited during molecular-beam epitaxy (MBE) (Dabiran et al., 1993). The oscillations in the intensity of the diffracted beams are monitored during the layer by layer

28 Electrons for surface diffraction, imaging and vibrational spectroscopy 47 1

deposition of material. Every time the intensity goes through a maximum, one layer has been deposited. A qualitative explanation of the oscillations is that the surface passes from smooth to rough to smooth. The grazing incidence makes this technique very sensitive to surface morphology. For instance, if there are protrusions at the surface a diffraction pattern characteristic of a three-dimensional crystal will be superimposed on the screen.

28.4 High-resolution electron energy-loss spectroscopy When the electrons are scattered in the surface region some will suffer characteristic energy losses due to such events as excitation of electrons or creation of phonons. In HREELS the phonon losses are measured (Ibach and Mills, 1982). Both substrate phonon losses and losses associated with an adsorbate layer can be studied. This type of measurement yields important information about the local structure and the interatomic interactions.

28.4.1 The experimental arrangement A schematic experimental set-up is shown in Fig. 28-4. Since the losses to be determined have energies in the range 0-500 meV the incoming electrons have to be well monochromated; this is accomplished by the monochromator. The electrons scatter from the surface and the energy of the outgoing electrons is measured by the analyser. Usually the angle of incidence can be varied and the angular dependence of the loss spectrum of the scattered electrons can be monitored. The overall energy resolution is in the range 1-10 meV. The angular resolution is a few degrees. The energy of the incoming electrons is usually 1-10 eV. In the case of phonon dispersion measurements energies in the range hundreds of eV are used. The incident beam current is in the range 1-100 pA and the beam focused on a spot of area a few square mm. This makes the electron-induced effects on the sample surface very small.

Sample

Fig. 28-4. A schematic drawing of the HREELS equipment.


28.4.2 Data acquisition and analysis The electron energy-loss spectrum is measured. To determine the characteristic energy losses the spectrum is recorded for specularly reflected electrons. Occasionally some losses are forbidden by symmetry in the specular direction and require offspecular direction spectra to be identified. When possible, spectra for different isotopes of the species under study are recorded. This enables the determination of the isotope shifts of the vibrational modes. To determine the dispersion relations for phonons, offspecular spectra are recorded for a set of detection angles. The first step is to identify the possible vibrational modes, substrate and adsorbate modes, which may be responsible for the observed losses. Often simple observations are sufficient to make an assignment. For a single atomic adsorbate there are three degrees of freedom and accordingly three vibrational modes are possible: one mode where the atom vibrates perpendicularly to the surface and two modes where the motion is parallel to the surface (Ertl and Kiippers, 1985). It is now appropriate to look into the scattering process. Two types of scattering mechanism can be identified: dipole scattering, where the scattering is associated with the long-range part of the Coulomb potential, and impact scattering, where the scattering is due to the short-range part of the potential. Dipole scattering peaks in the specular direction whereas impact scattering is more isotropic. Dipole-excited modes obey the surface-selection rule, which states that the vibration must have a non-vanishing dynamic dipole moment perpendicular to the surface. Only the vibrational mode perpendicular to the surface will have a dynamic dipole moment perpendicular to the surface. This allows a distinction to be made between parallel and perpendicular modes. There will be a rather sharp decrease in loss intensity as the detector is moved away from the specular direction if the corresponding mode is dipole-excited. The vibrational energies are estimated for different adsorption sites. A lattice dynamical calculation to determine the adsorption site might be used for this purpose. In the case of molecules additional losses occur due to internal modes. It is now convenient to make a symmetry analysis of the displacements connected to the possible modes of the adsorbed molecule (Richardson and Sheppard, 1987). Modes that are totally symmetric are dipole-allowed. Fig. 28-5 shows an example where CO is adsorbed on Pd(100) (Uvdal et al., 1988). EEL spectra are shown for two different coverages (0.5 and 0.67 monolayers) in Fig. 28-5a and the corresponding structures are shown in Fig. 28-5b. In the case of 0.5 monolayers coverage the unit cell contains two CO molecules, i.e. four atoms (compare with LEED, Section 28.2, this chapter). Thus there must be 12 modes for the CO molecules in the unit cell. The types of displacements involved are schematically shown in Fig. 28-6. The vibrations perpendicular to the surface are the whole molecule moving relative to the surface (hindered translation) and the internal stretch mode of the molecule. For displacements parallel to the surface there are hindered translations and hindered rotations (or bending). There are two totally symmetric modes: the hindered translation mode perpendicular to the surface, corresponding to the loss at 43 meV, and the internal stretch mode corresponding to the loss at 242 meV. If the molecular axis is tilted away from the surface normal the sym-

28 Electrons for surface difiaction, imaging and vibrational spectroscopy 473

metry will be lowered and some of the hindered rotations may become totally asymmetric. The EEL spectrum thus gives information about the presence or absence of a certain bond in the molecule (e.g. the C - 0 bond, showing that CO is undissociated) and the orientation of the molecule.

Pd (100) 80K W

L

I-

3 W IY

v

1-

2L2

>

!= m

$

0.5-

I-

z 0-

I

0

8=

1

1

50 100 150 200 ENERGY LOSS (mevl

I

0.5

t

I

250

8=

0.67

Fig. 28-5. EEL spectra for CO adsorbed on Pd(100) at two coverages, 0.5 and 0.67 monolayers, are shown in a) and the corresponding surface structure models are depicted in b).


Substrate surface Fig. 28-6. Schematic diagram of possible vibrational displacements of the CO molecule.

For 0.67-monolayer CO coverage, two low-energy losses are observed indicating that the molecular axis may have tilted away from the surface normal during compression of the CO adlayer. However, now the unit cell contains four molecules and the total number of modes is 24; symmetry analysis shows that six modes are totally symmetric, i.e. dipole active, see Fig. 28-7.

Fig. 28-7. The surface structure model for 0.67 monolayers of CO adsorbed on Pd(100) is shown in (a) and the totally symmetric adsorbate vibrations are shown in (b). The mode based on C - 0 stretching vibrations as well as that based on Pd-C stretching is represented by b(i). Similarly, b(ii) and b(iii) each represent two modes based on hindered rotations and translations with atomic displacements parallel to the surface.

Each of Figs. 28-7b (i)-(iii) represent two vibrational modes. Fig. 28-7b (i) shows the internal C - 0 stretch and the motion of the whole CO molecule against the surface. In Figs. 28-7b (ii) and 28-7b (iii) one mode corresponds to the motion of both the carbon and oxygen atoms, on a given CO molecule, in the same direction, as in a hindered translation, and one mode corresponds to the motion of carbon and oxygen atoms in

28 Electrons for surface diffraction, imaging and vibrational spectroscopy475

opposite directions, as in a hindered rotation. Four of these modes (representing hindered rotations and translations) are not dipole-active if only the local site symmetry is considered. This shows that it is very important to consider the full symmetry when the unit cell contains more than one adsorbate molecule. The hindered translations are expected to have a very low energy and cannot be observed. The loss at 50 meV is assigned to a hindered rotation. The intensity of the loss is comparable with that of the symmetric Pd-C stretch loss indicating that the molecule is tilted. Tilting will give added intensity to the bending vibration, since the mode is now dipole-active even in a local site symmetry analysis. From the structure model (see Fig. 28-5b) it may also be expected that the non-uniform adsorbate-adsorbate interaction will induce tilting to reduce repulsion by the nearest neighbour. It is suggested that the vibration pattern shown in Fig. 28-7b (ii) describes the hindered rotation mode observed at 50 meV, since the molecules most probably are tilted out of the bridge plane. The adsorption site is often inferred from a comparison with infrared-absorption data of organometallic complexes (e.g. transition metal carbonyls in the case of CO on metals). However, recent investigations have shown that this may give erroneous site assignments (Schindler et al., 1993). For larger molecules isotopic substitution is very useful for identifying the vibrational modes, and this in turn makes possible the determination of the orientation of the molecules at the surface. Negative-ion resonance scattering may occur in the case of adsorbed molecules. For a very short time the electron populates an electron affinity level of the molecule and may thereby excite a vibrational mode of the molecule. The cross-section is high for resonance scattering which makes it easier to observe overtones. The observation of both fundamental and overtone vibrations allows the bond strength to be determined. Phonon dispersion relations are obtained by plotting the energy of a phonon mode for different parallel momentums (kit), determined by the scattering angle. Specula scattering corresponds to kll=0. Dispersion relations have been determined for both substrate phonons and overlayer phonons (Lehwald et al., 1986). More detailed information about the interatomic force field is obtained than that available from the theoretical simulations. This is of importance for understanding applications such as surface reconstructions. HREELS has also frequently been used for studying chemical reactions at surfaces. The intensity of a loss peak in the EEL spectrum, used as a fingerprint of a certain molecule present on the surface, is monitored as a function of some relevant parameter such as temperature. The dissociation of NO on a Ni surface, when the surface temperature is increased, has been gauged by observing the decrease in the intensity of the N - 0 stretch loss and the simultaneous appearance of losses characteristic of adsorbed atomic oxygen and atomic nitrogen.

28.5 Low-energy electron microscope Electrons reflecting from the surface may also be used to create a real image of the surface. This is accomplished in the low energy electron microscope (LEEM). The


large field of view and the rapid image acquisition makes it ideal for studying surface processes (Bauer, 1990).

28.5.1 The experimental arrangement A schematic drawing of the experimental arrangement is shown in Fig. 28-8. The electrons from the electron gun are accelerated to an energy of about 20 keV and subsequently pass through a magnetic prism, which is used to separate the incoming and outgoing electrons. When the electrons pass the objective lens they are decelerated and hit the sample at normal incidence with an energy of a few tens of eV. The reflected electrons move back through the objective lens and are accelerated. After passing the magnetic prism the electrons enter a lens system which is used to form an image on the screen. The lateral resolution is about 10-20 nm and the field of view is several pm. The objective lens can also be used for emission microscopy, i.e. the electrons emitted from the surface are used to produce an image of the surface. Here photoelectron emission microscopy (PEEM) is only mentioned. To perform PEEM a photon source such as an UV-lamp, which illuminates the surface at grazing incidence, is added to the equipment, see Fig. 28-8.

Sample

Fig. 28-8. Schematic drawing of the LEEM equipment.

28.5.2 Data analysis The contrast in the image is mainly due to different parts of the surface having different lattice orientations or lattice constants (diffraction contrast). Interference contrast may also be important. For example, the path difference for waves reflected from terraces surrounding a step will cause interference effects and provide atomic depth reso-

28 Electrons for surface diffraction, imaging and vibrational spectroscopy 477

lution. The LEED pattern from the surface may also be observed. The diffraction pattern appears in the back focal plane of the objective lens so if the succeeding lenses are set to image this plane the pattern is observed on the screen. LEEM has been used to study surface processes like surface segregation and epitaxial growth. The contrast in PEEM is usually due to variations of the work function across the surface. Examples of applications are the study of decoration of steps and the oxidation of CO on a Pt surface (Rotemund et al., 1991).

References Bauer E. (1990), in: Chemistry and Physics ofSolid Surfaces VIII, Vanselow, R., Howe, R. (Eds.). Berlin: Springer. Beeby J.L. (1993), Surf Sci., 298,307. Dabiran A.M., Nair S.K., He H.D., Chen K.M., Cohen P.I. (1993), SurJ Sci., 298, 384. Ertl G., Kuppers J. (1985), Low Energy Electrons undSurjace Chemistry. Weinheim: VCH. Henzler M. (1993), SurJ Sci., 298, 367. Ibach H., Mills D.L. (1982), Electron Energy Loss Spectroscopy and Surface Vibrations. New York: Academic Press. Lehwald S., Rocca M., Ibach H. (1986), . I Electron Spectr. Rel. Phenom., 38,29. Pendry J.B. (1993), Surf: Sci. Rep., 19, 87. Richardson N.V., Sheppard N. (1987), in: Vibrational Spectroscopy of Molecules on Surfaces:: Yates Jr. J.T., Madey T.E. (Eds.). New York: Plenum Press, 1987. Rotermund H.H., Engel W., Jakubith S., von Oertzen A,, Ertl G. (1991), Ultramicroscopy, 36, 164. Saldin D.K., Pendry J.B., van Hove M.A., Somorjai G.A. (1985), Phys. Rev., B 31, 1216. Schindler K.M., et al. (1993), J. Electron Spectr. Rel. Phenom., 64/65,75. Uvdal P., Karlsson P.-A., Nyberg C., Andersson S., Richardson N.V. (1988), Surface Sci., 202, 167. van Hove M.A., Moritz W., Over H., Rous P.J., Wander A., Barbieri A. Materer N., Starke U., Somorjai G.A. ( 1993), SurJ Science Rep., 19, I9 1. van Hove M.A., Weinberg W.H., Chan C.-M. (1986), Low-Energv Electron Diffraction. Berlin: Springer Verlag. Woodruff D.P., Delchar T.A. (1986), Modern Techniques of Surface Science. Cambridge: Cambridge University Press.



29 Channelling R. Hellborg

29.1 Introduction When a high-energy beam of charged particles is incident upon a target a wide variety of processes occurs: energy-loss processes, secondary-electron emission, Rutherford back-scattering (RBS), nuclear reactions, X-ray production, etc. Some of these processes can be used for surface characterization and are described in other chapters of this book. The cross-sections involved in collisions with individual atoms for these different processes depend on the impact parameters. For a homogeneous and isotropic target material the distribution of the impact parameters is independent of the relative orientation of the beam direction and the target. Therefore the observed yield of interaction processes between beam and target are also independent of orientation. However, for a monocrystalline target the situation has been found to be quite different. The distribution of impact parameters and therefore the yields of closeencounter processes are very strongly dependent on the relative orientation of the beam and the target. This effect, first demonstrated about thirty years ago, is called channelling (Morgan, 1973; Williams and Elliman, 1989). The channelling effect of ions at MeV energies can be well described within the framework of classical mechanics. If a charged particle enters a surface of a monocrystalline target at a direction of movement close to a major crystalline direction (axial or planar), the particle will have a high probability of suffering very small-angle scattering as it passes through the first plane of atoms in the crystal. The probability is also high that this first deflection will be so small that the particle will suffer similar smallangle scattering in the next atomic plane. Because of the ordered structure of the target, such a particle will undergo a correlated series of gentle, small-angle collisions. This results in a collective steering effect which causes such particles to follow trajectories which oscillate in the open channels between atomic rows or planes. Under such conditions, the yield of close-encounter processes (such as RBS, nuclear reaction, etc.) is considerably reduced. Channelling occurs when ions are incident within a certain critical angle from a crystal axis or plane. For low-energy (tens of keV) heavy ions the critical angle can be as large as 10" for close-packed axes, whereas for high-energy, light ions (such as protons or He-ions at energies of a few MeV) critical angles are below 1'. The instrumentation needed is an accelerator producing beams at energies in the keV-MeV range. This is coupled to an experimental beam line, consisting of a beamcollimating system, capable of producing a beam with an angular resolution of a few hundredths of a degree, an experimental chamber under high vacuum and a goniometer

29 Channelling

479

for precise manipulation of the single-crystal target. The goniometer should preferably be computer-controlled. The careful orientation of the target by means of the beam is often time consuming, sometimes representing a major fraction of the time taken for the whole measurement. It can be a rather time-consuming process to carefully orient the target with the help of the beam. This process can easily take several tens of minutes per sample. This orientation time is, therefore, often a large part of the total measuring time. The cost per sample can therefore be several times the cost for other types of nuclear methods. The sensitive depth is dependent on the type of close encounter processes used. As an example, to detect Rutherford-back-scattered light particles the information depth is about 5 pm with a resolution of about 20 nm. A channelled ion penetrates further into a crystal because of the smaller energy loss and the fewer large energy transfers than occur in the random case. This reduced probability of interaction can be exploited to investigate the structure of crystals, surfaces and interfaces. A variety of practical uses of the channelling effect have been demonstrated but most applications have concentrated on the determination of the near-surface crystal disorder or location of atoms (dopant or impurity) using light ions at MeV energies. Studies of surfaces and epitaxial layers, crystallography (i.e. simple use of the effect for crystal orientation) or the more complex problem of determining the structure are other examples of the use of the effect. An analogous process, called blocking, can influence scattered ions or reaction products which travel towards a detector along a major crystal axis or plane. When an ion undergoes a major interaction with a lattice atom, the scattered ion or reaction products are blocked from travelling along close-packed directions by the presence of neighbouring atoms and their shadow cones in these directions. For example, blocking can be used for locating foreign atoms in the lattice and for measurement of nuclear lifetimes (Andersen et al., 1975). The choice of a particular ion and energy depends upon the interaction product to be detected, but most channelling experiments involve IU3S of MeV He' or H+ ions. Channelling has become a mature technique complementary to the techniques of X-ray and electron diffraction. This is demonstrated by the number of research papers in which channelling is used see, for example, the proceedings from the latest conferences in the series Ion Beam Analysis (Gyulai et al., 1994) and Atomic Collisions in Solids (Zinke-Allmang et al., 1994).

29.2 Basic principles 29.2.1 Continuum potential model The motion of a channelled particle can be described by a simple classical picture. The interaction between the particle and atomic rows (axial channelling) or planes


Axial channelinq

Plan ar c hanne ling

Fig. 29-1. Ion scattering from a row and a plane in the continuum potential model.

@lanar channelling) can be considered to take the form of a sequence of ion-atom collisions, as illustrated in Fig. 29- 1. The continuum model of channelling assumes that ion-row or ion-plane scattering can be approximated by scattering from an entire row or plane of atoms in which the averaging of the ion-atom potentials smears out the discrete nature of the many individual collisions. The discrete nature of the atoms can be assumed to be unimportant since each steering collision is an average of many individual collisions. The continuum potential for axial channelling is given by: (29-1)

where V is the interatomic potential and d, r and z are the lattice distance, the distance between the ion and the row and the position along the row, respectively, and UR(r) is cylindrically symmetric about the atom row (see the upper part of Fig. 29-1). The corresponding potential for planar channelling is given by:

29 Channelling

48 1

(29-2) where dl and d2 are the lattice distances in the two dimensions of the plane considered, and Up(r) is translationally symmetric in the plane (see the lower part of Fig. 29-1). If the Lindhard potential (Lindhard, 1965), given by:

is used, eq. 29-1 can be solved analytically. For axial channelling this will give: (29-3) where Z1 and Z2 are the atomic numbers of the incident ion and the target atom respectively, e is the electronic charge, C is an adjustable parameter taken to be 43 and a is the Thomas-Fermi screening distance, given by:

where

is the Bohr radius.

29.2.2 Channel trajectories The plane normal to the channel is referred to as the transverse plane and the ion energy associated with motion in this plane is referred to as transverse energy. This energy, ET, consists of two components, a kinetic term and a potential term, and can be written: ET = EY2+U(r)

(29-4)

where E is the ion energy, Y is the angle between the direction of travel and the atomic row and U(r) is the potential observed by the ion at r. When the transverse energy is comparable in magnitude to the potential energy confining it to the channel, it can approach the row or plane closely and the discrete nature of the rows and planes becomes evident. This means that individual collisions between the ions and the atoms can take place. There exists a critical approach distance, rc, to the atomic planes or rows within which individual collisions are probable. This is connected with a critical angle for channelling, Yc,given by:

EY‘,~= U(rc)

(29-5)


These limiting conditions have been discussed in detail by Lindhard (1965). For the interaction potential given by eq. 29-3 Lindhard found that the continuum approximation holds for angles: (29-6) provided the projectile energy fulfils the condition: E>

2Z1Z2e2d2 a2

(29-7)

The existence of a critical angle Y chas a direct physical implication in that a beam of ions entering a crystal is split into one fraction of particles having an incidence angle to a low index direction less than Y cand one fraction having an angle bigger than Yc.The former group is steered (channelled) away from close encounter processes (i.e. they will constitute an aligned or channelled beam, whilst the latter can undergo uncorrelated, large-angle collisions with the crystal atoms (i.e. they will constitute a random or dechannelled beam). In the continuum model the lattice atoms are assumed to be fixed in their lattice position. Actually they are, of course, subject to thermal vibrations about these positions. Including the thermal displacements leads to broadening of the continuum potential, eqs. 29-1 and 29-2, and increased thickness of the continuum atomic row and plane. The existence of thermal vibration violates the assumption of a conserved transverse energy ET. The average value of ET therefore increases with increasing penetration depth due to multiple scattering. Eventually some ions acquire sufficient. transverse energy to surmount the potential barrier of the channel and can undergo smallimpact-parameter processes and escape into the crystal as part of the random beam component. These ions are said to be dechannelled. The gradual increase in dechannelling with depth in a perfect crystal is not often important. However, the rate of dechannelling is significantly enhanced by the presence of crystal defects and is a useful measure of crystal disorder.

29.2.3 The yield curve The most striking feature of channelling is the reduction in the probability of different physical processes that need close encounters between the ion and the lattice atoms. This leads to suppression of the yield of the physical process if observed in the channel direction, see Fig. 29-2. Away from this direction the yield increases due to increased dechannelling and eventually approaches the value for random incidence where the lattice structure does not play any role. For values of Y between aligned and random incidence the yield overshoots the random value (seen as a shoulder). For these directions dechannelling dominates and practically every projectile suffers large-angle

29 Channelling

483

Fig. 29-2. The angular yield, x(Y), as a function of the angle, Y, between the ion beam and the axial channel. xmln and Y are described in the text.

scattering. The most pronounced angular distribution curve is measured close to the surface. At increased depths multiple scattering tends to smooth out the effect and the minimum yield Xmin (measured when the beam and the row or plane are aligned) increases, the shoulders decrease and the half-angle Y I D(measured where the yield is midway between minimum and random) decreases. In the continuum model the minimum yield for axial channelling is given by: (29-8) where N is the atomic density, p l is the mean-square thermal amplitude perpendicular to the channel. Eq. 29-8 implies that an area of n( p: +a2) is ascribed to each surface atom and a close-encounterprocess takes place whenever an ion hits such an area, while the ions that miss it become channelled. For axial channelling the halfangle Y1/2is closely related to the critical angle Yc: YJ%= C Y C

(29-9)

484 Part 4:Crystallography and structure

The proportionality factor C contains the temperature-dependence. From experiments and computer simulations it has been found that C is approximately unity. The energy loss of ions with energies of some MeV is caused mainly by interactions with target electrons. As channelled ions are confined to a region well away from the rows of atoms, they encounter a lower than average electron density and consequently experience a reduced rate of energy loss. This can have implications for channelling analysis since it may limit the accuracy with which depth scales can be determined.

29.2.4 Processes influenced by the channelling effect As has been described above, any process that requires a close encounter between the ion and the lattice atoms can be used to demonstrate the channelling effect. Below some different processes will be briefly mentioned. For more detailed descriptions refer to literature on channelling (e.g. Morgan, 1973; Williams and Elliman, 1989). Penetration. One of the first demonstrations of channelling was in a penetration experiment by Nelson and Thompson (1963) for 75 keV Ht and He' ions incident on Au foil 3000 A thick. Sharp maxima in the transmitted ion current were observed when the ions entered the crystal close to a channel direction. Ranges. As was discussed above, channelled ions experience an anomalously low rate of energy loss. Therefore the ranges of the ions in the crystalline material will be anomalously large. In fact, this process led initially to the first observations of channelling (Davies et al., 1960a, Piercy et al., 1963; Lutz and Sizmann, 1963). Since then this topic-has been studied intensively particularly with respect to applications in ion implantation in semiconductor materials. This is extensively described in Dearnaley et al. (1973) and Ziegler (1 992), amongst others. Close-encounter processes. Close-encounter processes require the ion to approach the material nucleus at a distance roughly lo4 times the channel-width. Therefore a channelled particle might be expected to be less likely to initiate such a process. The first direct evidence of the strong influence of channelling on RBS yields was found by Nelson and Thompson (1963). They observed sharp minima in the scattering of 50 keV Hf, Hef, Nef and Xe' ions from a Cu crystal whenever the direction of the incident beam was along a channelling direction. Since then RBS has become the most commonly used tool in channelling studies. It has the advantages of high cross-sections, small impact parameters and the availability of a wide variety of ion species and energies. Similar effects are seen when a nuclear reaction yield is measured. A typical experimental arrangement for RBS and nuclear reaction yield measurements is illustrated in Fig. 29-3. In fact a whole range of experiments, where some measured rate depends on close collisions, shows channelling effects. Examples are the rate of secondary-electron emission, sputtering, radiation damage and characteristic X-ray production (PIXE).

29 Channelling

- 0.3

485

m

COOLED TUBES SLIT 6 1.8 mm

I

SLIT

10.5 mm

I BEAM

I

CRYSTAL

from VdO-acc.

DETECTOR

Fig. 29-3. Example of the experimental arrangement for RBS and nuclear reaction yield measurements (from Hellborg, 197 I).

29.3 Application of the channelling effect to surface characterization Some examples of the extensive range of application of the channelling effect which have been employed in materials and surface characterization are given in Table 29-1. More detailed descriptions of these applications can be found in the literature on channelling, e.g. Ziegler (1992), Dearnaley et al. (1 973), Williams and Elliman (1989). Table 29-1. Examples of the application of ion channelling to surface characterization Surfaces and interfaces: Atomic arrangements, adsorbed atoms Texturing, background reduction Thin films: Crystal growth, strained superlattices Epitaxial layers: Precise atom location, defect trapping of impurities, solid solubility in semiForeign atoms: conductors and metals Dislocations, stacking faults, twins, point defects Defects: Radiation damage: Semiconductors, metals, insulators

The relative magnitude of direct scattering to dechannelling processes depends on the nature and concentration of defects. For example interstitial atoms, stacking faults and grain or twin boundaries, which introduce atoms directly into the path of the channelled ion, result directly in scattering events with small impact parameters. The scattering yield will be directly proportional to the concentration of defects at a given depth. Defects such as dislocations give rise to lattice distortions which have the effect of increasing the transverse energy of the channelled ions. This will result in an increased dechannelling rate. Thus the dechannelled fraction at a given depth is related


to the total number of defects encounted in reaching that depth. A few examples of investigations of crystal disorder will be outlined briefly below.

29.3.1 Foreign-atom location Atoms which are displaced significantly from lattice rows or planes at some depth in a crystal give rise to an enhancement in the interaction yield under channelling conditions. This produces an additional peak in the energy spectrum of interaction products at an energy corresponding to the depth of the disordered layer. The number of displaced atoms is proportional to the area under the peak in the spectrum. The fraction of atoms shadowed along a given crystallographic direction is not necessarily the same as the fraction that occupies substitutional lattice sites. Shadowed atoms may occupy sites between atoms constituting the rows or planes of the axial or planar channel. Such atoms will be shadowed when viewed along one direction but will be visible along other directions. The precise location of impurity atoms must therefore involve measurements along different crystallographic directions, as outlined in Fig. 29-4.

0 Substitutional impurity-/ X

Facecentred interstitial

Y,

0 Body-centred interstitial

Y Y C P\P P

T\Y\P 0

0

O

@

U

o

o

u

o

0

0

0

0

1

t

Direction 1

\+

'Direction

Random

Surface

O

0

ttt

'1 Substitutional

0 Host atom

Directions 1 and 2

t

1

Surface

2

*AXI

Depth

Fig. 29-4. Principles involved in foreign-atom location. (From Williams and Elliman, 1989.)

-

29 Channelling

487

29.3.2 Combination of different close-encounter processes Nuclear reactions provide a very selective mode of analysis for specific elements within a material, especially for light elements in a heavier substrate. When nuclear reaction analysis is combined with, for example, RJ3S in a single crystal it is possible to obtain information concerning impurities, sublattices, etc. Fig. 29-3 illustrates the experimental arrangement for a study of a compound material (CaF2). The Ca-sublattice was studied by RBS and the F-sublattice by the nuclear reaction 19F(p,ay)'60 (Hellborg, 1971). The crystal was irradiated by 1.4 MeV protons and the back-scattered protons were detected by an annular surface barrier detector mounted coaxially with the incident beam. Gamma rays from the nuclear reaction were detected by an NaI(T1) scintillator. The RBS and the nuclear reaction yields are shown in Fig. 29-5 for angular scans through some axes. The angular dips are different depending on the composition along the different axes. Another example is a study of the crystalline order and disorder in GaN (Linden and Hellborg, 1992). Thin epitaxial layers of GaN grown on sapphire were investigated using channelling of 1.3 MeV deuterons. FU3S and the nuclear reaction %(d,~x)'~C were used to investigate the gallium and nitrogen sublattices independently. The disturbance found in the GaN layer seems to be a displacement of the crystal atoms along the direction rather than perpendicular to it. The nitrogen atoms seem to be more influenced than the gallium atoms.

< 100 >

1.0

-

0,s

-

3

:as

- 1.0 - as

*4C 9

- a8

-

9.. I a2

-

- a4

-

- 0.2 -LO -as

o

0.1 LO

-1.0 -as o as 1.0 1111 ANOLE (OEOREESI

-1.0 -8s

o

as

to

Fig. 29-5. RBS (0) and nuclear-reaction yields (A) for angular scans through some different axes for a compound material (CaF2). The proton energy was 874 keV and the zone of interaction was close to the surface (from Hellborg, 1971).


29.3.3 Structural anomalies in high-temperature superconductors Investigations on high-temperature superconductor (HTSC) oxides indicate that there exists a relation between superconductivity and structure. In a recent experiment (Turos et al., 1994) channelling analyses were performed at different temperatures for an a-axis Yba2Cu302 layer on a SrTi03 substrate. Angular scans across the c-axis obtained for the Tc-, Y - , Cu- and 0-sublattices. were measured with He-ions with energies between 2 and 3 MeV at various temperatures below and above T,. Random and aligned spectra are shown in Fig. 29-6. 308 MeV tie on YBaCuO a-Axis Thin Film at T = 80 K Minimum Veld

250

Chonnel No

350

450

Fig. 29-6. Random and aligned RBS spectra obtained for 3.08-MeV He ions incident on a thin YBazCu307film oriented along the a-axis(from Turos et al., 1994).

Values of the critical angle close to those expected for pure Debye behaviour were observed in the Ba- and Y-sublattices, while an anomalous increase of about 6% occurred in the Cu-sublattice. The increase of the critical angle for oxygen was even greater and amounted to 8%. In order to check whether such behaviour may be due to the special vibration mode of the apical oxygen atoms (i.e. those oxygen atoms forming the apex in the crystal structure) the critical angle was also measured for thin films oriented along the a-axis. Along this direction the measured angular scan did not reveal any anomalous increase of the critical angle resulting from cooling below T, either for Cu or for the other metal sublattices. In contrast, a clearly anomalous increase of the critical angle was observed for the oxygen sublattice. This is demonstrated in Fig. 29-7. The explanation suggested for these experimental results is that anomalous changes in the oxygen sublattice take place upon cooling below T,. On the other hand, such anomalous behaviour was also observed for the Cu-sublattice, although, in the c-axis

29 Channelling

489

direction only. This may be due to strong anisotropy of Cu atom vibrations (preferentially along the a-axis) or it may be a consequence of strong vibrations of the apex oxygen atoms. In the latter case the apparent Cu anomaly can be observed for the c-axis, which is a mixed row, but not for the pure a-axis.

*

"M 08

b {l

Fig. 29-7. axial channelling scans for a thin film of EuBaCuO oriented along the a-axis, measured in the different sublattices: (a) EuBa, (b) Cu and (c) 0 below (50 K) and above (70 K) T, (65 K). The lines are spline fits through the data points which serve as guides to the eye (from Turos et al.,, 1994).

29.3.4 Analysis of strain in superlattices Strain in epitaxially grown thin films has been analysed in a number of experiments by the channelling technique. These layers, known as strained-layer superlattices (SLS), consist of alternating layers of epitaxially grown materials with lattice mismatches of the order 0.5 to 5%. By keeping the layers sufficiently thin, or the mismatch sufficiently small, commensurate growth can be achieved without the presence of misfit dislocations or other defects. Typical layer thicknesses are limited to a few tens of nm for a lattice mismatch of about 1%. In the semiconducting, strained-layered superlattices attention has been especially focused on the new electronic and optical properties which can be achieved in these composite structures, due both to quantum-well effects and the presence of strain in the layers. The change in crystal direction at each interface in the strained-layer superlattices is illustrated in Fig. 29-8.

490 Part 4: Crystallography and structure 0)

STRAINED-LAYER SUPERLATTICE

j..

110

.XI8

100

A118

b) DECHANNELING

a

30.6 n m ~ L A Y E R

I n

CHAMMELED

1.4

1.3

.!,

7

1.6

1.6

P 1.8

1.1

t

E w n w (wv)

c ) ANGULAR SCANS

i i -

0.0

'

.-.

LL-

R

$/Z

I

43

44

48

48

47

Fig. 29-8. (a) Schematic structural diagram of a Strained-Layer Superlattice (SLS). (b) Axial dechannelling with a 2-MeV He-beam along the [ I 101 and [IOO] directions in an I Q , ~ ~ G ~ , ~ A ~ / G ~ A ~ , 36.5/36.5 nm SLS. (c) Angular [I 101 scans in an IQ, lSG&85As/GaAs,38/38 nm SLS with GaAs, which has the smaller lattice constant, (a, material) corresponding to the top layer (from Picraux et al., 1986).

29 Channelling

491

This figure has been taken from an overview article in this field (Picraux et al., 1986). Under epitaxial growth the in-plane lattice constant remains the same, giving rise to alternating compressive and tensile stresses in the layers. The sensitivity of the channelling effect is due to the fact that the tilt angle at each interface is of the order of the critical angle.

29.3.5 Microprobe-channelling analysis Ion beam surface analysis is normally performed using an ion beam of 1 mm diameter and therefore these techniques are restricted to use on large, uniform samples. By employing an ion lens it is possible to focus the ion beam to produce probing beams with lateral extensions of less than 10 pm to carry out the analysis on microscopic areas within a feasible time. However, for many ion-lens systems (often called nuclear microprobes) at present in use the convergence of the beam is too large for channelling of light ions at MeV energies. It is, therefore, necessary to use a modified ion-optical system to apply the channelling technique in microprobe analysis. The full-beam convergence of the probe has to be Iimited to 0.2-0.3" and the angle of incidence of the deflected beam to the undeflected beam direction has also to be limited to around 0.1" for a 1-mm deflection. A practical beam fulfilling these conditions can have a diameter of some tens of pm without significant dechannelling. The technique of microprobe-channelling analysis has been applied mostly to semiconductor structures. Dopant profiles in polycrystalline resistors have been measured, as well as micro-alloying in metal-GaAs contacts and damage/atom location in individual, polycrystalline, silicon grains (Brown et al., 1985). Hg(&d,Te epitaxial layers on GaAs substrates grown by Metal Organic Chemical Vapour Deposition (MOCVD) display growth defects resembling pyramidal, faceted hillocks which appear to originate from defects originally present on the substrate. The technique of microprobe channelling has allowed these hillocks (typically about 20 pm square for 2 pm thick epitaxial layers) to be imaged (Jamieson et al., 1992).

References Andersen J.U. et al. (1975), Nuclear Phys., A241,317. Brown R.A. et al. (l985), Proc. Mat. Res. SOC., 48,403. Davies J.A. et al. (1960a), Can. J. Chem., 38, 1526. Davies J.A. efal. (1960b), Can J. Chem., 38,1535. Deamaley G. et al. (eds.) (1973), in: Ion Implantation. North Holland Publ., Amsterdam, The Netherlands. Gyulai J. et al. (eds.) (1 994), Proc. Eleventh Int. ConJ on Ion Beam Analysis, Balatonfired, Hungary. 1993, Nucl. Instr. Meth. B85. Hellborg R. (1971), Physica Scripta, 3,279. Jamieson D.N. et al. (1992), Proc. Mat. Res. SOC.,238,253. Linden M., Hellborg R. (1992), Nucl. Instr. Meth. B68, 170. Lindhard J. (1965), Mat. Fys. Medd. Dan. Vid. Selsk., 34, 1 .

492 Part 4: Crystallography and structure Lutz H.O., Sizmann R. (1 963), Phys. Letters, 5, 1 13. Morgan D.V. (ed.) (1 973), Channeling-Theory, Observation and Applications. J. Wiley and Sons. Nelson R.S., Thompson M.W. (1963), Phil. Mag., 8, 1677. Picraux S.T. et al. (1986), Nucl. lnstr. Meth., B15, 306. Piercy G.R. er al. (1963), Rev. Letters, 10, 399. Turos A. et al. (1994), Nucl. Instr. Meth., B85,448. Williams J.S.,Elliman R.G. (1989), in: Ion Beams for Materials Analysis. Bird J. R. and Williams J. S. (Eds.), Academic Press. Ziegler J.F. (ed.) (1992), in: Ion Implantation Technology. North Holland Publ., Amsterdam, The Netherlands. Zinke-Allmang M. el al. (eds.) (1994), Proc. Fifreenth Int. Con$ on Atomic Collisions in Solids, London, Ontario, Canada, 1993. Nucl. Instr.Meth., B90.



Part 5: Surface films The sensitivity of optical techniques for the characterization of surfaces has been recognized since the pioneering studies of Paul Drude and Lord Rayleigh in the 19th century. These methods do not require a vacuum environment and are also capable of providing information about buried interfaces and bulk properties. The intrinsic simplicity and versatility of existing techniques make them suitable for a variety of analytical problems and sample categories. There exists today an abundance of methods and it is occasionally difficult to make the right choice. This abundance is illustrated in Fig. 1, which illustrates the processes that can take place when photons interact with a surface. INCIDENT WAVE

SPECULA R R E F L E C T I O N

BRILLOUIN. RAMAN SECOND HARMONIC

~ R F A CW EAVES

PHOTOELECTRONS

4

TRANSMISSION xACOUSTIC EMISSION

Fig. 1. Principal excitation processes for surface-optical properties.

Assume that an optical beam is directed at the sample to be characterized. There are several response channels that correspond to various physical processes: the photons can be reflected specularly or diffusely, absorbed, cause emission of photoelectrons and so on. Reflected and transmitted photons can be detected directly, while absorbed photons heat the sample and can then be measured indirectly by monitoring the temperature increase. The light can also generate surface electromagnetic waves that can be detected. All channels, if monitored, contain information about the sample, and we therefore have available a multitude of techniques for the study of surface properties. The frequency of the reflected light may be shifted due to Raman or Brillouin scattering processes. The presence of a surface may lead to the generation of second or higher harmonic photons. These second and higher order channels also contain information about the sample. Only second harmonic and sum frequency generation is dealt with in this chapter, while the Raman technique is briefly discussed in Part 3. Raman spectroscopy in general has limited surface sensitivity. Questions which are often asked and which can be answered by means of optical techniques include: 0 What are the bulk dielectric constants of the material ?

494 Part 5: Surface films 0

0

0 0

0 0 0

Is there a film on the surface? What is the film thickness and what are its dielectric constants ? What is the chemistry of the surface; how and where are adsorbates bound to a particular surface ? Are there any surface states at the surface of the sample ? What is the topography of the surface ? Is the sample homogeneous or inhomogeneous ? Is the sample, or the surface layer, anisotropic ? What is the full symmetry of the surface ?

The first-order optical techniques are in general not surface-specific,but high absolute accuracy allows one to extract information on the submonolayer scale from a technique which is inherently bulk-sensitive. The second harmonic generation, on the other hand, may have a high, built-in, surface specificity. In the bulk of a centro-symmetric crystal the lowest order, non-linear polarization density is of magnetic dipole and electric quadrupole symmetry. At the surface of such a crystal, the inversion symmetry is broken, however, and photons of twice the energy of the incoming photons are generated. All these photons come from the outermost atomic layer, and the second harmonic generation or sum frequency generation provides us with a powerful tool to study, for example, the symmetry of the surface. Optical properties also depend upon the local topography of the surface, topographical structures with Fourier coefficients at the wave-vector of the light lead to diffuse light scattering and reduction of the specular reflectivity. The specular reflectivity is also influenced by surface structures with wave-vectors along the surface which are larger than the surface wave-vector of the light. The treatment is restricted to rms roughness, which is small compared to the wavelength of the light. With these restrictions the specular intensity is only weakly perturbed by surface roughness. A further limitation is that the treatment deals only with the near ultraviolet (UV), visible (VIS) and infrared (IR) regime. Bombarding a surface with electrons or ions is an alternative way of obtaining information about a surface. These techniques are discussed in separate chapters. All electron- and ion-based techniques have an inherent, high surface-sensitivity. On the negative side it is noted that all these methods require ultrahigh vacuum, and the energy resolution is generally inferior to that of optical techniques, The accepted view is that optical and electrodion-based techniques are complementary. In general, a number of techniques is necessary for complete understanding of the properties of a specific surface. This part is also devoted to a description of thin films in terms of their mechanical, electrical, magnetic and thermal behaviour. This is important in today’s technology since thin films or layers constitute an essential part in the manufacture of semiconductors and optical devices. Since the properties of films depend on the method of formation, basic production methods frequently used in industry, such as sputtering and evaporation, are outlined.



30 Optical characterization of surfaces by linear spectroscopy J. Bremer. 0. Hunderi and E. Wold

30.1 Introduction The first-order interaction of light with matter is one of the principal tools for obtaining information about electronic and vibrational properties of solids. However, the study of optical properties has also proven to be practical for characterizing surface films, multilayer samples, powders and heterogeneous multiphase samples. This review consists of three main parts. In Section 30.2 we give a summary of the theoretical background of the optical properties of materials. This includes the transfer matrix method for the calculation of the optical properties of a surface covered with a multilayer film system, and effective medium models for the description of heterogeneous multiphase samples. In Section 30.3 we give an extensive summary of the main techniques for measuring the optical properties of various sample types and finally in Section 30.4 we give a more detailed account of the most surface-sensitive of optical techniques, namely ellipsometry. The present review deals only with linear processes. Second and higher order processes are treated separately.

30.2 Theoretical background 30.2.1 Electromagnetic boundary conditions In optical surface spectroscopy the signal depends on both components of the complex permittivity E(a)=El+iEZ (Born and Wolf, 1975). The refractive index, N=n+ik, is given by the relationship N2=&.The optical constants k and n are the extinction coefficient and refractive index.

30.2.1.1 Amplitude and phase Consider an incoming wave that is partially reflected and transmitted at a planar interface (Jackson, 1982). The angle of incidence is 0 and the intensity of the outgoing wave is proportional to the incoming intensity and to the squared modulus of the Fresnel reflectivity:

R = Irl

2

(30-1)

496 Part 5 : Surface films

Propagation of waves in a non-magnetic medium is governed by the wave impedance Z = Z a with ZO= (~o/Eo)’. The reflection amplitude at the interface between two media can be written as: r =

z’z - z ; Zl,

+

Zi

(30-2)

Where the surface impedance is given by Z‘=Zcose and Z’=Z/cose depending on whether the electric field amplitude is parallel with (p case), or perpendicular to (s case) the plane of incidence. The transmission amplitude is:

(30-3) Since the refractive index is complex the phase difference A between the p and s component can have any value. It is normal to write the ratio p between the reflection amplitudes as:

(30-4) The two angles w and A are important parameters for the determination of the optical properties. At the limit of zero absorption the angular variation of Rs and Rp depends on whether n is smaller or larger than unity. At the Brewster angle OB = tan% the ppolarized intensity is transmitted through the surface, and Rp=O. This behaviour is accompanied by a sharp change in the phase angle. Similar curves can be plotted for the n>D. For a summary of effective medium theories, the reader is referred to Bergmann and Stroud (1 992) and to TheiJ3 (1993).

30.2.2 Surface films Optical properties of bulk samples are often masked by films or multilayers at the surface. Such layers may grow naturally as in oxidation processes, but can also be produced intentionally for various purposes. The propagation of the field vector through an optical system can be described within a matrix formalism where each slab of material is represented by a matrix. Thus the problem of analysing complex samples is reduced to determining the appropriate matrix elements of the transfer matrices T (Azzam and Bashars, 1977). For a single film with thickness d the reflection coefficient can be written as: r=

rol + r12e+i2b 1+ rolr12ei2b

(30-5)

where p and s labels have been suppressed. Here ro1 and r12 are the reflection coefficients at the front and back interfaces, respectively, of the film, and P=27cd (E-sin28)’”lh where d is the thickness of the film and h is the wavelength of the light. This equation has many applications in fields such as optical coatings, electrochemistry, electronics, metallurgy, etc. The reason for this is, that by observing phase and amplitude, the thickness of the film can be followed as function of time. Alternatively, when the thickness is known, the expression enables calculation of the permittivity, by measuring phase and amplitude.

30.3 Measurement strategies Various optical methods for studying surfaces are discussed. The technique to choose depends upon what information about the surface is sought, the quality, size and shape of the sample, the roughness of the surface, and so on. Only the three main categories of first-order spectroscopy are dealt with here. Higher-order spectroscopy is treated elsewhere in this book. Class I: The detected photons come from an external light source. This class includes the conventional methods reflection spectroscopy, transmission spectroscopy and ellipsometry.

498 Part 5: Surface films

Class IZ: These methods are based on the measurement of light emitted from the sample itself. A fundamental relationship in spectroscopy states that the absorption, A = 1 - R, of a surface is equal to the emission coefficient, e. Class IZZ: This class is based on indirect measurement of absorption. The absorption may lead to a temperature increase, which in turn leads to increased IR emission and thermal expansion, modified electrical conductivity, acoustic emission, etc. Monitoring these quantities is an indirect method for the measurement of optical absorption. All three classes are included in Fig. 30-1, which illustrates schematically what happens when photons interact with a sample. The information contained in some of these channels is often equivalent to each other. This is, for example, the case for specular reflection, transmission and indirect measurement of the absorption. Other channels contain new or additional information. Such is true of second-order processes like Raman and Brillouin scattering.

CLASS A1

CLASS A11

CLASS A111

THEREVIAL ACOUSTIC

Fig. 30-1. Schematic picture of the main classes of optical spectroscopy.

30 Optical characterization of surfaces by linear spectroscopy 499

As illustrated in Fig. 30-2, extra information may be obtained by exposing the sample to external ‘forces’ like heat, stress, electric or magnetic fields. If the external perturbation is periodic, lock-in techniques can be used to monitor very small changes in reflectance. EXTERNAL PERTURBATION

Fig. 30-2. Schematic picture of modulation spectroscopy.

30.3.1 Class I systems 30.3.1.1 Source and chromatization For spectroscopic purposes the sources of importance are: thermal sources, e.g. globars, halogen bulbs, high pressure gashapour sources e.g. Hg, Xe, Ar, H and tuneable lasers. For additional information the reader is referred to Pederotti and Pederotti (1 987). The generalized optical system in Fig. 30-1 includes a unit we called a chromatiser, i.e. a unit which directly or after processing allows one to separate the response into spectral components. This can be obtained by use of dispersive systems (prism and grating monochromators), by interference systems (interference filters, wedge filters and Fabry-Perot filters) or by Fourier-transform systems. The main questions when choosing a method for chromatization are spectral resolution and signalhoise ratio. For a dispersive system the main factor determining the throughput is the properties of the grating and the f-number of the monochromator. The circular symmetry of the entrance and exit stops makes use of a larger source area in Fourier-transform spectrometers than in dispersive systems. This leads to larger throughput. The multiplex advantage means that several spectral components are measured simultaneously, this reduces the measuring time in certain cases. The general conclusion to be drawn is that Fourier transform methods are advantageous in the IR where there are limitations due to detector-noise, while a dispersive system, or a scanning Fabry Perot interferometer is preferred in the visible where photon-noise often causes impediments. The various detectors available are not discussed. The interested reader is referred to one of the many books on the subject (Boyd, 1983).


30.3.1.2 Layout of spectrophotometers The main challenge in spectrophotometry is to create ‘reference’ data which allow one to measure the absolute value of the reflectivity or transmittivity of a sample. Many ingenious designs have been proposed for eliminating energy-dependent source intensity I(h) and detector detectivity. Most of these are of double beam design; the same light intensity is sent along two optical paths to the detector. The optical paths are as identical as possible, except that one path includes the sample. All these designs rely on pairs of mirrors being identical. Signal processing allows one to obtain absolute values for the reflectivity or transmittivity of a sample. To the best of the authors’ knowledge only one design avoids the condition of identical mirrors (Hunderi, 1972).

M2

ROTATING SAMPLE WHEEL DETECTOR

OPE

SOURCE

M1

PLE

@)

Fig. 30-3. Two-beam reflectometer.

As an example of a typical design, Fig. 30-3 shows a very simple automatic reflectometer, originally designed by Beaglehole (1 968). Light from the monochromator, or a tuneable light source, is focused onto the sample. The sample is mounted in a rotating wheel as shown in Fig. 30-3b. When the sample is in the light path, the light is reflected from the sample and is focused onto the detector, via the toroidal mirror M1 and a flat mirror M4. When the light hits the opening in the rotating wheel it is directed to the detector via the corresponding mirrors M2 and M3. Provided these are identical pairs of mirrors, the ratio of the first harmonic signal, as measured by a lock-in amplifier to the dc level is:

30 Optical characterization of surfaces by linear spectroscopy

S=K-

1-R 1+R

501

(30-6)

Finding the constant K is simple; by blocking the sample light path one simulates a sample with R=O. If the sample is partially transparent the system can be used to measure the absorption, A=l-(R+T), of the sample, and if we block the R path of the system we can measure the transmission. Errors stem mainly from the difference in reflectance of the pairs of mirrors in the system, beam inversion and misalignment so that the beams do not hit the same area on the detector. If the sample cannot be rotated, it may be interchanged with one of the fixed mirrors in the system. The layout in Fig. 30-3 must then be changed so that the relevant pairs of mirrors have the same angles of incidence. This sub section ends with a comment on ellipsometry which is covered in more detail in Section 30.4. An ellipsometer can be considered to be a two-beam instrument where the s-polarization corresponds to the ‘reference’ beam and the p-polarization corresponds to the ‘sample’ beam. Due to the perfect match of the two channels the error sources above are avoided, and the sensitivity is increased considerably.

30.3.1.3 Sample requirements All the techniques discussed so far require mirror-like surfaces. In practice, surfaces often have considerable roughness. In such cases photoacoustic and photothermal techniques may be preferred. These are techniques where the optical absorption is measured indirectly by monitoring the temperature increase in the sample. In addition methods known under acronyms such as IRS (internal reflection spectroscopy), ATR (attenuated total-reflection spectroscopy), FTS (frustrated total reflection spectroscopy) and SPW (surface plasmon-wave spectroscopy) are examples of techniques where the quality of the sample surface is of less importance, and where small spectral features are enhanced (see Palik and Holm, 1978). Fig. 30-4 shows three examples of such spectroscopes. In (a) a sample is brought into contact with a prism under a configuration where there is total internal reflection in the absence of a sample. If the sample is absorbing or if it increases the critical angle above the angle at which the measurements are performed a strong reduction in the reflectivity can be seen. The sensitivity is high since it is above or near the critical angle. Opening up a gap (b) between the sample and the prism enables the excitation of surface polaritons, or surface plasmons if the sample is metallic. The propagation of surface plasmons is sensitive to the presence of surface films and adsorbates on surfaces. In (c) we increase the sensitivity through multiple reflections.


(4

ib)

SAMPLE

SAMPLE

I

SAMPLE

I

Fig. 30-4. Various internal reflection arrangements.

30.3.2 Class 11: Emission spectroscopy In this class no light source is required and the surface does not need to be of optical quality. One simply records the spectral distribution of light emitted from the sample. From the relation e=A (emission=absorption) one calculates the optical absorption. The disadvantage of emission spectroscopy, is that the emitted intensity decreases rapidly with decreasing temperature, and this puts a lower limit on the temperature at which this technique can be used. Allara et al. (1984) have succeeded in obtaining an emission spectrum from a single monolayer of PNBA at room temperature. A practical lower limit for the technique is in the 500 K range in the infrared spectral range and even higher if one is interested in the visible range. Chiang et al. (1 984) obtained the emission spectrum corresponding to the C-Ni vibration of CO on Ni. However, in order to overcome the thermal noise problem the whole Fourier transform spectrometer had to be cooled to 70 K.

30.3.3 Class 111: Indirect methods All the techniques of class I11 in Fig. 30-1 are based upon the existence of a number of channels for de-excitation after photon absorption. In addition, some of the methods also provide information about other physical properties. De-excitation leads to a temperature increase which again causes: thermal expansion, change in electrical conduc-

30 Optical characterization of surfaces by linear spectroscopy 503

tivity, acoustic emission, surface acoustic waves, etc. The temperature rise can also couple to the surrounding gas and thereby increase the infrared radiation from the sample. A number of methods based on these effects has been developed over the last two decades; see Table 30-1 for a summary. The last three techniques in this table are used in the study of semiconductors,but are outside the scope of this review. 30.3.3.1 Calorimetry The first technique in Table 30-1 measures the absorption by recording the temperature increase of the sample by means of a thermometerholometer. This technique is useful with two types of material: (a) ones with high reflection coefficient R+l, A+O and (b) in transparent ones with low absorption coefficient, such as optical fibres. The limit (a) was studied by Hunderi (1972) who investigated absorption in metals and alloys. The temperature rise of the sample caused by optical absorption was measured by a Ga-doped Ge bolometer glued to the back of the sample. The sensitivity of such a bolometer is strongly temperature-dependent and the doping is generally chosen so as to optimize the system at liquid He temperatures. Table 30-1. Indirect methods for measurement of optical absorption. Name Calorimetry

Detection principle Temperature increase in the sample is measured with a contact thermometer Temperature increase is measured through the increased IR Photothermal radiation. Spectroscopy Temperature increase in the sample is measured by detecting Photoacoustic acoustic waves in the surrounding gas. spectroscopy Temperature increase is measured through effects caused by Photothermal thermal expansion of the sample or thermal lensing in the air Deflection above the sample. spectroscopy Photo-conductivity The optical absorption in semiconductors is measured through the increase in electrical conductivity . The bandgap in semiconductors is measured by means of the Photore-emitted photons. Luminescence Photo-luminescence States above the bandgap are studied by means of the intensity of re-emitted photons. Excitation Spectroscopy

Detector Bolometer IR detector

Microphone Position sensitive optical detector

Photodetector Photodetector

30.3.3.2 Photo thermal spectroscopy In photo thermal spectroscopy the absorption is measured through the increase in thermal radiation caused by the temperature increase. A typical system is shown in Fig. 30-5. Light is absorbed by the sample. This gives rise to a temperature increase, AT, at the surface. The thermal emission is given by Stefan-Boltzmann's law. A temperature increase AT leads to an increase in emission given by:

AW(T) = 4eaT3 AT

(30-7)


Here e is the effective emission coefficient. For small temperature increases the radiated intensity is proportional to the temperature increase in the sample. AT depends on the optical absorption, but also on the thermal conductivity of the sample. Phototherma1 spectroscopy has been used successfully to record optical spectra of complex objects ranging from powders of inorganic materials to ‘living’ organic materials such as the leaf of a plant (Kanstad and Nordal, 1981). In the latter case the resulting spectrum exhibited characteristic features expected from green plants. The situation is simple for metals where the absorption of light takes place near the surface. The detected signal is then proportional to the absorption coefficient, A. For a weak absorbing insulator with low reflection from the surface the situation is more complex. One observes saturation effects if the absorption length is much smaller than the thermal diffusion length. There is no spectral structure in the radiated intensity. This is in a way analogous to using a sample which is too thick, in a transmission experiment where all the light is absorbed in the sample. In a transmission experiment the sample thickness should be of the order o f t = l/a,where a,=471k/h, is the optical absorption coefficient. From the point of view of optics, the dependence upon thermal conductivity is a disadvantage; on the other hand, if the optical properties can be found by other means, we have a way of studying the thermal properties of the samples. Subsurface cracks and flaws also affect the thermal properties and the technique can therefore be used for detecting hidden flaws in a material.

ELECTRONICS

IR DETECTOR FILTER

FOCUSING OPTICS CHOPPER

LIGHT SOURCE

1 -

SAMPLE

Fig. 30-5. Photothermal spectroscopy system.

30.3.3.3 Photoacoustic spectroscopy Photoacoustic spectroscopy is an old technique which is based on the so-called optoacoustic effect. This effect is observed when gas enclosed in a small cell is illumi-


505

nated with chopped light. If the gas absorbs the light, periodic temperature increases arise, which in turn lead to periodic pressure increases, and these can be heard. By replacing the ear with microphones, one has a sensitive spectroscopic technique, both for optical absorption in gases, and solid samples. In the latter case, a solid sample located in a small cell is heated by the absorbed light, the sample in turn heats the gas and the ensuing pressure fluctuations are detected by the microphone (Rosencwaig, 1975). A schematic illustration of a photoacoustic system is shown in Fig. 30-6. This technique has many similar features to the photothermal technique. It can, for example, be used as a non-destructive method to study hidden flaws in materials.

PHOTOACOUSTIC CELL

Fig. 30-6. Photoacoustic spectrometer.

30.3.3.4 Photothermal beam-deflection spectroscopy Two beam-deflection methods have been developed. The first is based on local thermal expansion as a result of the local heating, due to optical absorption. A secondary laser beam will be deflected by this thermoelastic protrusion as shown in Fig. 30-7a. The other beam-deflection technique works as follows: the heating of a sample due to optical absorption increases the temperature of the gas above the surface. We then get a thermal lensing (Fig. 30-7b) or a thermal prismatic effect. A secondary laser beam just above and parallel to the surface is deflected by this effect. The deflection can be measured and is a measure of the surface temperature. The sensitivities of the two methods are comparable (Olmstead et al., 1985). The techniques discussed above can also be applied to microscopy. For example, the thermal propagation allows one to test a sample for hidden flaws. Furthermore, by varying the chopping frequency, one can probe different sub-surface depths. This information is contained in the phase of the signal. We can build up two images of a


surface, an amplitude map and a phase map. Sub-surface flaws are more easily detected in a phase map than in an amplitude map. The two images may be quite different as illustrated in Fig. 30-8 (Thomas et al., 1982). The figure shows thermal wave microscopy images of an integrated circuit, with the magnitude image to the right and the phase image to the left. When raster scanning to build an image, it is important to realize that the phothermal magnitude image is a superposition of three images: the image of the optical absorption, the image of the infrared emission and the thermal wave image. The photothermal phase angle image is a thermal wave image only, and depends entirely upon the thermal properties near the surface. It is, therefore, better suited for non-destructive testing purposes than the magnitude image.

I

POSITION

"HEAT1

Cllr?PE d""1,LL

PROBING BEAM

f

\

TUNABLE LIGHT

\

I

;" BEAM

THERMOELASTIC

(b)

(a)

Fig. 30-7. Photothermal-deflection spectroscopic system (a), and details of the thennoelastic protrusion and thermal lensing (b).

A

Y

of an integrated circuit showing both signal magnitude, A, and phase, j.


507

30.3.4 Modulation spectroscopy The starting point for the use of modulation spectroscopy can be twofold: it is either a wish to enhance otherwise weak structures on a large background, or a wish to study the effect of an external perturbation on the sample. The simplest form of modulation spectroscopy is wavelength modulation, the purpose of which is to enhance weak spectral features. Consider a reflection spectrum R(h)consisting of a weak absorption structure on an otherwise large and monotonous background. If the wavelength varies in time, as h(t) = h, + Ahsinwt, the corresponding reflectivity becomes first order as: dR R(h,t)= R(ho)+ -Ah-sinot dh

(30-8)

A Fourier analysis of the signal by a PC or a lock-in amplifier tuned to the frequency w gives a signal -dWdh. This can be amplified and the structure is enhanced relative to the background. The signal will also depend on higher derivatives, but this leads only to small corrections in the lock-in case. A number of parameters can be modulated (Pollak and Glembocki, 1988); Table 30-2 gives a summary of the various techniques. Table 30-2. Summary of modulation spectroscopies. Name Wavelength-modulation

Modulation h=h,+ Ahsinot

Angle of incidence modulation

€I=€I,+ABsinot

Ellipsomehy Electroreflectance(long. And transv.)

Polarization of incident or reflected light Electric field in sample surface: E=E, + AEsinot

Thermoreflectance

T=T,+ ATsinwt

Piezoreflectance

P=P,+ APsinwt

Magnetoreflectance

H=H,+AHsinwt

Photomodulation

Signal

dR -

d3L dR, dA - dR, ~ d0 ' d0 'd0 +I

andA

dR dE dR dT dR

Modulates with light of wavelength h,, measures with light of wavelength h2

The primary aim of modulation spectroscopy is to obtain more detailed information about the electronic structure of the sample and to see features which are not directly visible in the ordinary spectra.

-


30.4 Ellipsometry As a more detailed example of how to implement and perform optical experiments we discuss in this section the method of ellipsometry. Ellipsometry is a natural choice because it is the most versatile of the methods discussed so far, and because instnunentation and principles of data treatment apply to many other optical arrangements. An extensive treatment of ellipsometry is given by Azzam and Bashara (1 977). Ellipsometers are instruments characterized by having polarizers at both the entrance and exit optics as shown in Fig. 30-9. The major advantage is the ability to determine simultaneously the reflection ratio Irp/rsl and the phase-shift A in eq. 30-4. This not only supplies more information about the sample, it also enhances the sensitivity considerably. For this reason ellipsometry is routinely used to determine the permittivity of materials and the accurate thicknesses and properties of thin films, to monitor chemical reactions and sample growth, and to study sub-monolayer coverage of adsorbates in catalysis.

SAMPLE

Fig. 30-9. A standard set-up in ellipsometric experiments. The polarization state of the light is indicated.

The light-beam from the source ideally has random polarization. After passing the polarizer the light-beam is linearly polarized and after passing the sample it is generally elliptical. The polarization ellipse is analysed by recording the detector intensity with the second polarizer, the analyser, set to different orientations. Used at normal incidence an ellipsometer measures the difference in optical properties along different directions on the surface. This is known as reflection anisotropy spectroscopy ( U S ) and is used to study the anisotropy of surfaces, for example the anisotropy caused by surface reconstructionson GaAs.

30.4.1 Instrumentation In the NIR-VIS-UV range xenon, deuterium, or halogen lamps serve as high-intensity wide-range sources and prism- or grating-monochromators are used to disperse the


509

light. The monochromator may be placed between the source and the polarizer, or between the analyser and the detector, depending on the measuring strategy to be used. The latter configuration suppresses stray light and enables the use of array detectors for high-speed measurements. In single-wavelength ellipsometry a laser is used as a source and there is no need for monochromators or focusing optics. Prism polarizers (GlanThompson, Glan-Foucault, Rochon) with extinction ratios of the order of 1O5 produce practically, completely linearly polarised light, which is essential for the quality of the measurement. Retarders, or photoelastic modulators (PEMs) (compensators, and quarter-wave-plates) are optical elements that introduce phase-shifts between the p- and scomponents of the field. They improve the measurements, but are not always practical in spectroscopy, since satisfactory wide-range retarders are not available. A PEM is a birefringent crystal in which the refractive index is modified when applying a voltage. It can thus be used for fast modulation of the polarisation state of the incoming light. Photomultipliers, photodiodes, and photoresistors are used as detectors, depending on the wavelength-range of interest. A general problem in the IR-range is the low intensity. The problem is reduced by using a Fourier-transform spectrometer. The signal-to-noise ratio can be further improved by purging the equipment at MIR-energies or using vacuum at FIR-energies. A critical problem for IR ellipsometry is the lack of good polarizers. Wire-grid polarizer are used with extinction ratios of the order of lo2. Such low values mean that the polarization-dependence of the optical elements, is not eliminated by a fixed polariser. As a result the data treatment is more intricate (Wold and Bremer, 1994). Detectors for infrared radiation are pyroelectric detectors (e.g. DTGS), nitrogen-cooled semiconductor alloys (e.g. MCT, InSb) and Si-bolometers. 30.4.1.1 Measuring techniques Today ellipsometers are highly automated. The most favourable configuration includes a compensator in a fixed polarizer-sample-rotating compensator-fixed analyser configuration (RCE) (Aspnes, 1985). With this configuration both the orientation of the polarization ellipse, and the amount of unpolarized light can be determined, and at the same time any polarization dependence of source- or detector-system is eliminated. Most spectroscopic ellipsometers utilize a configuration without a compensator, where either the polariser (RPE) or the analyser (RAE) is rotated. The advantage of this configuration is the simplicity. The drawbacks are the sensitivity to polarization-dependent components, and the incapability to determine usefulness and unpolarized light. A PEM can be inserted between the polariser and the sample to modulate the light, with this configuration there are no movable parts and this enables high-speed measurements with sampling rates in the 50 kHz range. Table 30-3 summarises the various techniques.


Table 30-3. Summary of main ellipsometric methods. Name RCE

Principle Rotating compensator ellipsometry

Comments Adv: Determines both the orientation and degree of polarization; polarization dependencies suppressed

RAE

Rotating analyser ellipsometry

Adv: Simple instrumentation. Source polarization suppressed. Wide spectral range

Rotating polariser ellipsometry

Like RAE but sensitive to source polarisation, and not to detector properties

RPE

Disadv: No wide range compensators for spectroscopy

Disadv: Cannot determine the orientation and degree of polarization; sensitive to detector polarization dependence

Ellipsometer Adv: Fast measurements; insensitive to source and detector properties with photoelastic Disadv: Cannot determine the orientation and degree of polarization; high modulator cost; Linearity problem RAS

Reflection anisotropy spectroscopy

Ellipsometry at normal incidence; measures optical anisotropy in the sample surface plane

30.4.1.2 Rotating-analyser ellipsometry RAE is usually preferred if the monochromator is placed between the source and the polarizer, or if the source itself shows polarization-dependence.W E is preferred if the monochromator is placed between analyser and the detector, or if the detector is polarisation-dependent. The intensity, which depends on the polarizer and analyser angles P and A and the sample properties is characterized by the angles y~ and A from eq. 30-4 (Aspnes and Studna, 1975):

I=Io (1 + cos2Pcos2A - a(cos2P + cos2A) + psin2Psin2A)

(30-9)

where

p = sin2yrcos A =

2,/%

cos A

(30-10)

Rs + R ,

When the transmission axes of the polarizers are parallel with the p-direction, P=O" and A=O". The optimum measuring condition occurs when the difference between Rs and Rp is large, since this gives good 'contrast' in the measurement. The angle o f incidence should, therefore, be close to the Brewster or pseudo-Brewster angle. If P=45", eq. 30-9 becomes:

I = I, (1 - a cos2A+ p sin2A)

(30-11)

30 Optical characterization of surfaces by linear spectroscopy 5 1 1

It is sufficient to determine a and p to calculate p. The analysis can be done by digitizing the detector signal by means of an analogue-to-digital converter (ADC), and the coefficients of eq. 30-1 1 can be calculated from a numerical analysis of the datapoints. The above applies to systems based on dispersive monochromators, where the measurements are made on a wavelength-by-wavelength basis. If an FTIR-based system is used, however, the response at all wavelengths is measured simultaneously. In this case the spectra recorded at equally spaced analyser settings must be stored for later numerical Fourier transformation. If the source and/or the detector optics contain polarizing elements, this gives rise to fourth order terms in eq. 30-1 1. The polarizing properties can be described by two parameters, a2 and b2, which express the dichroism and the main axis orientation of the element. Eq. 30-1 1 changes to (Bremer et al., 1992; Wold and Bremer, 1994):

I = I,(l+:(-aa2

+pb2)+(-a+a2)cos2A+(p+b2)sin2A

+ +(- aa2 - pb2)cos 4A + 3 (- ab2 + pa2)sin 4A)

(30-12)

By measuring the Fourier coefficients of eq. 30-12 both the polarizing characteristics of the source or detector optics (a2 and bz), and the sample parameters ( a and p) can be calculated.

30.4.2 Data treatment 30.4.2.1 Calculating sample parameters The reflectivity coefficients rp and rs for any sample can be calculated if the optical constants, layer thicknesses and the angle of incidence are known. Unfortunately, the converse statement is not true. Only in the case of isotropic bulk samples can the permittivity be calculated: (30-13) where E~ is the permittivity of the ambient. In all other cases one has to follow the scheme outlined in Fig. 30-10. The experimentally determined yr and A values are compared with a sample model. The input parameters are iteratively changed to reduce the discrepancy between calculated and experimental angles. The permittivities ‘determined’ by the experiment are those which, within the model, reproduce the experimental ellipsometric angles. An ellipsometric experiment gives only two independent parameters, yr and A. When inverting data a larger number of quantities is needed if the sample is complex. This under-determination problem can, in principle, be overcome by performing measurements at several angles of incidence or at different sample orientations. For this rea-

5 12 Part 5 : Surface films

son variable angle of incidence spectroscopic ellipsometry (VASE) is widely used. Alternatively, the sample may be immersed in media of different refractive indices, or measurements on series of samples may be made. Increasing the number of equations and data points is advantageous, but does not always give the extra information required, because the equations linking the sample parameters and measured quantities are non-linear. Even for an over-determined system there is no unique solution. For example, a film thickness d always appears as a product with the refractive index, Nd. A series of combinations of N and d values may then produce the same y- and Avalues.

E',W, d;....

correction

Optical model

I

Final model,

I

Fig. 30-10. A schematic diagram of data analysis in ellipsometry. Measurement, modelling and computation are of equal importance in the study.

In order to iterate towards better input parameters it is convenient to define the function F(x):

(30- 14) which sums up the discrepancies between all calculated and measured angles. The sum covers all different experimental points at a given wavelength (i.e. different angles of incidence, ambients, samples, etc.). The vector x denotes all input parameters, and 0, is the variance of the input parameters. There are several ways to iterate towards a minimum in F(x). Efficient methods usually apply some steepest descent approach where one steps in the opposite direction to the gradient of F(x) in solution space until a minimum value is found. Equations similar to eq. 30-14 may be based on the difference between experimental and calculated pseudo-permittivities, or other experimental parameters.


5 13

30.4.2.2 Film-thickness measurement The process of determining thickness from measured data is non-trivial. For a film (1) on a substrate (2) in an medium (0) eq. 30-5 gives: (30-15) where r,jP,Eis the Fresnel coefficient for reflection at the interface between media i and j for p,s-polarized light. If the permittivity of the substrate and the film material is known the thickness can be calculated directly by rewriting eq. 30-15 as a quadratic equation in exp(2iP) (McCrackin, 1964). From the two complex solutions the one, which yields a thickness with a zero (or the smallest) imaginary part should be chosen, since the thickness is a real entity. If the permittivity of the film is real, both the thickness and the permittivity of the film can be determined by calculating the thickness of several films with different hypothetical real permittivities. The film permittivity is the one which leads to the smallest errors in eq. 30-14. For small film thicknesses, i.e. d/h is identical to ~ 2This . follows from the fact that the minima appear when 2P=m2x in which case the film is ‘invisible’ and eq. 30-15 yields p=po2. Secondly, the upper envelope function goes through the points where 2P=m(2n+l), i.e. where exp(2iP)=-l. The corresponding formula for p from eq. 30-15 does not depend on d, and allows for the calculation of E , . Thirdly, d may be determined from the fringe period.

30.4.3 Representative examples The first example to be discussed illustrates a classical method for obtaining both the film thickness and film growth rate. The example is taken from a study of thermal oxidation of metallic glasses (Hunderi and Bergersen, 1982). Assume that we follow a film growth in situ and that the film permittivity does not change during growth. It is assumed that the dielectric constants of the clean surface are measured before film growth starts. It is useful to plot contours of film dielectric constants that satisfy the measured A values, with film thickness as a variable parameter along the contour. Fig. 30-1l a shows such contours for five y~ and A values. All curves intersect at a fixpoint, which thus uniquely determines the permittivity of the corrosion film. The film thickness can be read off as parameters along the curves (not shown in the graph). The contours in Fig. 30-1 la represent very thin films; with thicker films the contours would intersect at larger angles and the accuracy in the determination of the film dielectric constants would improve. The thickness of the corrosion film determined in this way as a function of time at various temperatures is shown in Fig. 30-1 lb. The second ellipsometry example, shown in Fig. 30-12, illustrates how additional information can be obtained by the method of ‘physical constraints’ (Aspnes et al., 1984). The structure to be analysed consisted of a c-Si substrate, approximately 100 nm of thermally grown oxide, and approximately 530 nm of deposited p-Si. The dielectric function of the p-Si layer was not known and had to be determined along with the two thicknesses. Since the deposited layer was polycrystalline, the presence of a rough top surface had to be assumed. The top curve in Fig. 30-12 shows the imaginary part of the dielectric constants of the deposited p-Si layer as a function of photon energy for the layer thicknesses indicated. All other values of the thicknesses resulted

w,

30 Optical characterization of surfaces by linear spectroscopy 5 15

Eo'

02 CAIL 70

-

60

-

50

-

40

-

30

-

20

-

10

-

0

Fig. 30-1 1. Contours of dielectric constants satisfying measured ellipsometric parameters after various exposure times (a) and film thickness in dry air against time for various temperatures (b).

5 16 Part 5 : Surface films

L

A

p-si

0.4

0.2

0

0

"

1.5

2 .o E (cv)

Fig. 30-12. The imaginary part of the dielectric function of p-Si for the choice of thicknesses indicated. The data are plotted against photon energy.

2 .S

I

25.88 A Oxide 437 A G o b

I

GaAs Substrate F i n a l model f o r s a m p l e

V A S E model f o r sample Y2352

Y.2352

0

Wovslsnpch,

h

3500

I

4500

I

5500

Wovslsqth,

A

I

I

6500

7500

Fig. 30-13. Nominal sample structure (top left), final model (top right) and comparison of measured and calculated w and A values plotted against wavelength.


5 17

in interference artefacts as shown in the lower curves. Notice that including a top rough surface was essential for removing the artefacts. The third example is a VASE analysis of a complex multilayer GaAs/AlGaAs multilayer structure (Merkel et al., 1988). The sample contains a single quantum well on a GaAs/AlGaAs superlattice. In the modelling of this structure the superlattice was modelled as a single bulk AlGaAs layer of unknown composition and thickness. The nominal sample structure used in the modelling is depicted at the left in Fig. 30-13. The thickness and composition of the superlattice equivalent layer, as well as the other layer thicknesses were solved by regression analysis. The best fit to the measured tp and A values and the corresponding best fit structure is also shown in Fig. 30-13. This example shows that spectroscopic ellipsometry can enable accurate non-destructive characterization of rather complex structures.

30.5 Summary and recommendations Optical techniques can be used to characterize a variety of samples. The type of technique to be used depends upon the sample, its structure, and the information sought. Well-defined mirror-like surfaces with or without surface films, for example, are best studied by means of ellipsometry. Rough surfaces and powders are better studied by diffuse light scattering, which is the subject of a separate chapter in this book, and by indirect methods such as photoacoustic and photothermal spectroscopy. In Table 30-4 we have briefly summarized which technique to use under a variety of circumstances. Table 30-4. Summary of recommended methods. Sample type

Problem

Polished bulk sample or evaporated film Thin film on substrate

Bulk dielectric constants

Multilayer structures Thin film on transparent substrate Rough metallic film or powder sample Organic samples Strongly absorbing samples Samples of low optical absorption Rough metal surface

Film thickness or dielectric constants of film Layer thicknesses and dielectric constants Dielectric constants Optical absorption Optical absorption Optical absorption Optical absorption Surface roughness

Recommended Technique Ellipsometry Ellipsometry Ellipsometry Double beam reflectometer or ellipsometer Photoacoustic or photothermal spectroscopy Photoacoustic or photothermal spectroscopy Attenuated total reflection Photothermal beam deflection spectroscopy Light-scattering interferometry, or speckle techniques

5 18 Part 5: Surface films

Optical techniques are non-invasive and non-destructive, do not require vacuum ambients and are also capable of providing information about buried interfaces and bulk properties. They are useful when studying the chemistry and structure of the surface, and relatively simple modelling allows one to determine, for example, the thickness of surface films. The intrinsic simplicity and versatility of existing methods make them suitable for a variety of analytical problems and sample categories.

References Allara D.L. (1980), ACS Symposium Series 137, 33-47: Bell A.T. and Hair M.L. (Eds.), ACS, Washington. Aspnes D.E., Studna A.A. (1975), Appl. Optics, 14,220-228. Aspnes D.E., Studna A.A., Kinsbron E. (1984), Phys. Rev. B29,768-779. Aspnes D.E. (l985), Handbook of Optical Constants of Solick 1 : Palik, E. D. (Ed.) Academic Press, Orlando,1985. Azzam R.M.A., Bashara N.M. (1 977), Ellipsometry and Polarized Light. North-Holland, Amsterdam. Beaglehole D. (1968), Applied Optics 7,22 18-2220. Bergman D.J., Stroud D. (1992), Solid State Physics, Ehrenreich H. and Tumbull D. (Eds) Boston Academic Press, Vol. 46. Born M., Wolf E. (l975), Principles of Optics. Pergamon, Oxford. Boyd R.W. (1983), Radiometry and the Detection of Optical Radiation. John Wiley and Sons, New York. Bracewell R.N. (1986), The Fourier Transform and its Applications. Mc Graw, New York. Bremer J., Hunderi O., Kong Fanping, Skauli T., Wold E. (1992), Appl. Opfics31,471-478. Chiang S., Tobin R.G., Richards P.L., Thiel P.A. (1984), Phys. Rev. Leu. 52,648 - 651. Hunderi 0. (1971), Rev. Sci. Instr. 42, 1596-1599. Hunderi 0. (1972), Applied Optics 1 1 , 1435-1436. Hunderi 0. (l976), Surface Sci. 57,45 1-459. Hunderi O., Bergersen R. (1982), Corr. Sci. 22, 135. Jackson J.D. (l982), Classical Electrodynamics. Wiley, New York. Kanstad S.O., Nordal E. (1981), Appl. Phys. Lett. 38,486-488. McCrackin F.L., Colson J.P. (1964), in: Symp. Proc. on Ellipsometry in the Measurement of Suflaces and Films, Passaglia, E. , Stromberg, R. R. and Kruger, J. (Eds.) National Bureau of Standards Miscellaneous Publications, 1964; 256,61-82. Merkel K.G., Snyder P.G., Wollam J.A., Alterovitz S.A. (1988), SPIE Vol 946 Spectroscopic Characterization Techniquesfor Semiconductor Technology, 105-1 1 1. Olmstead M.A., Amer N.M., Kohn S. (1985), Appl. Phys. 32, 141-154. Palik D.E., Holm R.T. (1978), Optical Engineering 17, 512-524. Palik E.D. (l985), Handbook of Optical Constants of Solids I. Academic Press, Orlando. Palik E.D. (l991), Handbook of Optical Constants ofsolids II. Academic Press. Pederotti F.L., Pederotti L.S. (1987), Introduction to Optics. Prentic Hall, New Jersey. Rosencwaig A. (1975), Physics Today 28,23-30 TheiR W. (l993), Solid State Physics, Vol. 33, R. Helbig (Vieweg, Braunscweifliesbaden), 149-176. Thomas R.L., Favro L.D., Griece K.R., Ingelhart L.J., Kuo P.K., Lotha J., Busse G. (1 982), Proc. IEEE Ultrasonics Symposium, 586-590. Wold E., Bremer J. (1994), Appl. Opt. 33,5982-5993.



31 Optical second-harmonic and sum-frequency generation J.F. McGilp

31.1 Introduction Optical electromagnetic (EM) radiation generally penetrates a considerable distance into condensed matter, which makes the isolation of a surface or interface contribution difficult. However, deeper understanding of the underlying physics of the optical response, combined with advances in instrumentation, have allowed this contribution to be identified (McGilp, 1995). Symmetry differences between the bulk and interface, and interface electronic and vibrational resonances, have proved to be particularly important. Optical techniques offer significant advantages over normal surface probes: the material damage and contamination associated with particle beams is eliminated; all pressure ranges are accessible; insulators can be studied without charging effects; buried interfaces are accessible owing to the large penetration depth of the optical radiation. Optical techniques also offer micron lateral resolution and femtosecond temporal resolution. The interaction of an EM field of optical frequency, o,with condensed matter can be described in terms of the polarization amplitude, P(0,20,. ..), induced by the field, E(0): P(o,20,

...I

...)= E~~X("(CO).E(CO)+ x(~)(w,~w):E*(w)+

(31-1)

where x(~)is the ith-order dielectric susceptibility tensor describing the material response. The intensity of the EM radiation depends on the square of the polarization amplitude. The first term on the right-hand side of the equation, depending linearly on the field, describes the linear optical response, which is exploited in techniques like spectroscopic ellipsometry and reflection-anisotropy spectroscopy. The non-linear terms become significant at high EM field strengths, and the second term in eq.31-1, which has a polarization dependent on the square of the EM field, describes the lowestorder non-linear response responsible for second-harmonic generation (SHG). SHG and other three-wave mixing phenomena are potentially surface-sensitive at nondestructive power densities (Shen, 1984). This is most easily seen for centrosymmetric materials, such as metals and elemental semiconductors, where bulk electric dipole transitions are forbidden by parity, leaving only the much smaller magnetic dipole and electric quadrupole contributions. At a surface or interface, the bulk symmetry is broken and electric dipole transitions become possible. Cross-sections for three-wave mixing events are small (typically one signal photon per 10'3-10'7incident photons) and, with large field gradients normal to the surface, higher order SH signals can be


generated in the bulk of crystals which are comparable in size to the interface signal. However, where an interface resonance can be probed, the electric dipole contribution from the interface can dominate the SH response. This spectroscopic aspect of threewave mixing is increasingly being used in interface characterization. In the early 1980s, Shen’s group at Berkeley established the potential of SHG as a surface probe (Shen, 1985). The first UHV study, where conventional surface probes were used to characterize the surface, showed that the adsorption of 0 and CO damped the SHG signal from Rh(l1 l), while the adsorption of Na enhanced it (Tom et al., 1984). The following year Heinz and co-workers showed that the azimuthal dependence of the SHG signal was sensitive to the symmetry change between the (2x1) and the (7x7) reconstructions of the Si( 1 11) surface (Heinz et al., 1985). Later, McGilp and Yeh used the Si( 111)-Au system to show that SHG could provide structural information about buried metal-semiconductor interfaces (McGilp and Yeh, 1986). New techniques like these have to be tested on well-characterized interfaces, and SHG studies of this type were important in showing interface sensitivity at the monolayer (ML) level. Reviews of SHG applied to thin films, surfaces and interfaces are available (Richmond et al., 1988; Heinz, 1991), and an overview of optical techniques for interface characterization has been published (McGilp et al., 1995). The phenomenological theory is outlined below, because a basic understanding of theory is necessary in order to decide on appropriate experimental configurations for these non-linear techniques. This is followed by selected examples, which mainly concern semiconductor interfaces, as most work has been in this area.

31.2 Theory of the non-linear optical response Calculation of the surface non-linear optical response from first principles is difficult, but the phenomenology is well known and allows symmetry arguments to be used to interpret experimental data. In the most general second order non-linear response of a system, three-wave mixing occurs in which two incident fields of frequency W I and 02 combine to produce a third field of frequency 013, where 013=01

+OZ

(3 1-2)

The four possible combinations and degeneracies in eq. 3 1-2 can produce SHG, optical rectification, sum-frequency generation (SFG) and difference frequency generation (Hopf and Stegeman, 1986). It is important to note that these processes are coherent, producing radiating fields which have well defined directions. For example, SHG from a surface in the usual reflection geometry-in-vacuum emerges along the path of the primary reflected beam, which simplifies detection. This coherent property enables SHG and SFG to be used as surface probes, even though the cross-section for the process is very small. The phenomenology of surface SHG and SFG from cubic centrosymmetric crystals will only be outlined here: details may be found elsewhere (Heinz, 1991; Sipe et al.,

3 1 Optical second-harmonic and sum-frequency generation 52 1

1987). In the bulk of centrosymmetric crystals the electric dipole term is zero, and the lowest order non-linear polarization density is of magnetic dipole and electric quadrupole symmetry. Surface SHG can also arise from these small, higher order terms, due to the large field gradients normal to the surface, but there is also a much larger electric dipole term, because the inversion symmetry is broken at the surface: (31-3) where x : ~ is the second-order susceptibility tensor component reflecting the structure and symmetry properties of the surface or interface. Eq. 3 1-3 shows that the bulk electric dipole contribution from a centrosymmetric material is forbidden. Under a parity operation (r + -r), the sign of an EM field changes, but the sign of a bulk centrosymmetric tensor component does not. The tensor has 18 independent components, but any symmetry elements present in the surface will reduce this number substantially. Choice of experimental geometry and polarisation vectors, based on the point-group symmetry of the surface, can then be used to isolate individual tensor components, and structural information can be obtained. Symmetry analysis is a powerful tool here, and it should be noted that the symmetry rules for the linear and non-linear optical response are different. Sipe et al. (1987) have tabulated expressions for the total SH fields from the (OOl), (1 10) and (1 1 1) faces of cubic centrosymmetric crystals. Carehl use of experimental geometry can simplify the information obtained. Experiments at normal incidence, for example, provide in-plane symmetry information about the interface (Heinz et al., 1985; McGilp and Yeh, 1986). For a crystalline solid with an interface in the xy-plane, and normally-incident radiation linearly polarized at an angle, cp, to the x-axis of the crystal, the SHG intensity polarized along the x- and yaxes is given by: I ',"(cp)

- lxixxcos

2

cp + xiyysin2 cp + xiv sin 2 4

2

~ z y ~ ( c p ) -I x k cos2 cp+ x h sin2 cp+ xhxysin2cp12

(31-4) (3 1-5)

Symmetry simplifies these expressions. The components xyx, yyy, yxx are zero for the l m symmetry characteristic of stepped surfaces; additionally, for the 3m symmetry of (1 1 1 ) surfaces, xxx = -xyy = -yxy; for 2mm and 4mm symmetries (characteristic of (110) and (100) surfaces), all in-plane components are zero, which means that SHG cannot be detected using this experimental geometry. Off-normal excitation brings the z components into play, making the choice of experimental geometry and light polarization even more important, particularly with lower symmetry interfaces where there may be many non-zero components. Experiments under ambient conditions typically involve sample rotation plots, where the m-polarized SH intensity, I z ( w ) , for an npolarized pump beam, is measured as a function of w , the azimuthal angle between


the k-axis of the crystal and the plane of incidence (the laboratory xz-plane) (Fig. 31-1).

’y

q x

,

I I

Fig. 3 1-1. SHG geometry in reflection from an interface. The incoming linear field, at incidence angle, $o, is polarized at an angle, a, to the plane of incidence (the laboratory xz-plane), which lies at an azimuthal angle, yi, to the 6-axis of the sample: p-polarization has a = O”, s-polarization has a = 90°, q-polarization has a = 45”. The s- and p-polarized components of the outgoing SH field are usually detected.

Polarizations of high symmetry with respect to the optical plane, particularly s- and p-polarizations, are used. For example, with a (1 11) surface of 3m symmetry, such as Si(111)-7x7, the surface SH intensities are given by:

I:?(v)- Iflx:,

S~~(~V)E~(@)~I~

(31-6)

(31-7)

where f, are Fresnel factors, the x-axis is parallel to , and the z-axis is along the surface normal. The role of the Fresnel factors has been reviewed by Heinz (1991). Two-phase or three-phase models, analogous to those used to describe the linear optical response, are used to determine the EM field amplitudes. The effect of local fields on the SH response provides an additional complication: quantitative measurements of this effect on well-characterized liquid crystal monolayers have been reported (Tang et al., 1993a, b). In general, both the susceptibility components and the Fresnel factors may be complex, The interpretation of the SH response may also be complicated by contributions from different tensor components (see eqs. 3 1-8and 3 1-9). The pp-configuration is the most difficult, but is often chosen precisely because the many z-dependent components contributing are expected to produce a large SH signal. Eqs. 31-6 and 3 1-7 show, how-

3 1 Optical second-harmonic and sum-kequency generation 523

ever, that xsxxxcan be measured independently, and eq. 31-8 shows that, by choosing w = 30" and an s-input/p-output configuration, the xSw component can also be measured independently. This type of approach is particularly useful for coverage-dependent studies. Sample rotation studies can be difficult, however, in vacuum systems because of the long optical path lengths which place severe restrictions on the degree of sample precession which can be tolerated. An alternative approach is to rotate the input polarization angle instead of the sample. The plane of incidence is typically aligned along a symmetry line in the surface plane, and the experiment is repeated for other, inequivalent, symmetry lines (Power et al., 1995). -The phase of the SH signal can be used to detect the presence of electronic resonances near either the excitation frequency or the SH frequency, where a full spectroscopic SHG study (Section 4) is not possible. Methods of measuring the SH phase by interferometry are well-known, and recently have been used under UHV conditions (Kelly et al., 1992). The phase angle, 4, of the complex surface tensor component, in Xsijk' I xsijkI eio,can be related to the phase of the SH intensity, measured interferometrically. Off resonance, 4 = 0" or 1 80", and XSijk is real, with sign *l . For cp = 90" or 270°, xS,jkis exactly on resonance and is purely imaginary. For intermediate values of the phase, xsvk is complex, and there are resonances nearby. However, the phase shift depends on both the energy and the width of the resonance, and at least two phase measurements at different wavelengths near resonance have to be made to enable these parameters to be determined quantitatively.

31.3 Experiment Pulsed laser systems, where the wavelengths of interest lie between 500 nm and 2500 nm, are the main excitation sources for three-wave mixing experiments in semiconductors. Surface damage considerations, together with the advantages of simple gating electronics, favour pulsed sources over continuous-wave laser excitation. A typical experiment might use nanosecond pulses of a few mJ energy, into a beam diameter of a few mm and with an energy density kept below a few kJ m-' to avoid any laser-induced desorption or damage effects. Q-switched Nd:YAG lasers have been widely used at 1064 nm excitation and, frequency-doubled, at 532 nm, while dye, Tisapphire and optical parametric oscillator (OPO) systems are beginning to be used for wavelength-dependent studies. A typical UHV experimental configuration for SHG at oblique incidence is shown in Fig. 31-2. Glan-Taylor prisms, together with a half-wave-plate or double Fresnel rhombus are used for polarization selection. It is very important to remove, using optical filters, any SHG signal generated by these optical components in the input line. The SHG signal, detected by a photomultiplier tube, is small, but the pulsed laser system allows time-gating of the signal to be used to enhance the signal-to-noise ratio. A gated integrator, gated photon counter, or digital storage oscilloscope is typically used. The


UHV Chamber Half-Wave Plate

artz Plate & Filter in sifu

Nd:YAG

E

n

A-D Converter

Monochromator

Fig. 3 1-2. Schematic diagram of an oblique-incidence UHV experiment for measuring both the intensity and phase of a SH signal. (From Kelly et al., 1992.)

energy meter, used to monitor fluctuations in beam energy, can be replaced by a second SHG line which uses a bulk SHG signal to provide pulse-by-pulse normalization of the SHG signal. Such systems have a remarkable discriminating power of 10l8 between primary and SH photons. The SH intensity can be calibrated by inserting an x-cut quartz plate in the input beam and observing the Maker fringes produced by SHG in the bulk of the quartz (Shen, 1984). The phase of the SH signal can be measured by non-linear interferometry. A quartz plate is inserted in the output line and traversed along the beam, while the SH intensity, from the superimposed sample and quartz signals, is monitored. The different dispersion in air of the fundamental and SH frequencies is used to produce the variation in optical path length. If a second quartz plate is rotated into the output beam within the UHV chamber then, by comparing the phase difference between the two quartz plates, the phase shift arising from the UHV window can be eliminated and the absolute phase of the system measured (Kelly et al., 1992).

3 1 Optical second-harmonic and sum-frequency generation 525

31.4 Examples of the application of SHG to well characterized surfaces and interfaces 31.4.1 Clean Si surfaces Many in situ UHV studies have now been performed on clean Si surfaces under UHV conditions. Strong SH signals are observed from Si(ll1)-7x7, Si(lll)-2xl and Si( 100)-2x1 surfaces using 1064-nm excitation, with the surface dipolar response (eq. 31-3) being much larger than the higher order bulk response. This is a difficult region for spectroscopic studies using pulsed lasers, and phase measurements have been of particular value here. As discussed in Section 1 , azimuthal studies of rotational anisotropy at, or near, normal incidence can be used to probe in-plane symmetry. This approach works particularly well for the (1 1 1) surface of elemental semiconductors. The first such study of well-characterized surfaces was reported by Heinz et al. (1985). The SH signal, using 1064-nm excitation, from Si( 1 1 1)-2x 1 and Si( 11 1)-7x7 surfaces is shown in Fig. 3 1-3, and is clearly sensitive to the symmetry change between the two reconstructions, the solid lines being fits to eqs. 31-4 and 31-5. This work was important because it showed, in a particularly clear way, that surface information was available from SHG. Spectroscopic studies in this region have only recently become possible with the development of reliable OPO systems, but McGilp, Rasing and co-workers made absolute phase measurements in UHV to show that the Si(ll1)-7x7 surface has a nearby resonance, for 1064-nm (1.17-eV) excitation (Kelly et al., 1992). In a later study, Power and McGilp (1993) used the adsorption of Sb atoms at very low coverages to show that the resonance was due to electronic states associated with the Si adatoms of the dimer-adatom-stacking fault (DAS) structure (Takayanagi et al., 1985). This was confirmed in a recent study which shows the potential of spectroscopic SHG; Pedersen and Morgen (1995) used an OPO to scan the pump energy from 1 .O eV to 1.8 eV, and found three distinct resonance peaks. The temperature-dependence of the SH signal in the region of the Si(ll1)-7x7 -+ 1x1 order-disorder phase transition has also been studied, and provides evidence supporting the adatom gas model for the 1x1 phase (Hofer et al., 1995; Suzuki et al., 1995). The presence of steps on semiconductor surfaces can affect both the electronic structure and the mechanism of semiconductor growth. Characterizing step structure is difficult, and vicinal surfaces, prepared by cutting a few degrees away from the low Miller index plane of interest, are often used to increase the step density. (Cutting to expose a low Miller index plane produces a singular surface.) A number of studies of clean, vicinal surfaces of Si(ll1) and Si(OO1) has been reported showing that the SH response can be very sensitive to step structure (McGilp and Yeh, 1986; Hollering et al., 1991; Lupke et al., 1994; Power et al., 1995).

526

Part 5 : Surface films

Fig. 31-3. SH intensity, polarized along the 412> and azimuths, from Si( 11 1)-2x 1 and Si( 1 1 1)-7x7 surfaces, as a function of the polarization of the normally incident pump beam. (From Heinz et ai., 1985.)

31.4.2 Elemental semiconductor-adsorbate systems The Si( 111)-Au system was used for the first characterization of buried metal-semiconductor interfaces by SHG (McGilp and Yeh, 1986). In the normal incidence study results from Si(l11) and Si(OO1) interfaces were compared and it was concluded that the SHG signal originated from ordered structures at the buried interface between the Si(ll1) substrate and the Au layer. After room-temperature deposition, annealing produces Si(ll1)-5x2-Au, Si(ll1)-.\/3x.\/3-Au and Si(ll1)-6x6-Au structures, as coverage increases to 1.2 ML. The SH response in this coverage regime has been followed and shows a distinct maximum of the xSm response on completion of the (5x2) structure, at 0.5 ML, for 1064-nm excitation (O'Mahony et al., 1992). Spectroscopic ellipsometry (SE) results from the same system show a much smaller, but reproducible, change in the pseudo-dielectric function at this coverage (OMahony et al., 1993). The advantage of SHG, where symmetry suppresses the bulk signal, over the linear optical SE technique, which relies on detecting changes in a bulk-dominated response, is clearly seen in this example.

3 1 Optical second-harmonic and sum-frequency generation 527

The effect of Ba deposition on the SH response of Si(OO1)-2x 1 has been measured, and provides a good example of the detailed information available when individual tensor components are identified by suitable choice of polarization and azimuth (Hollering et al., 1990). By 4 ML coverage, the out-of-plane components have increased by an order of magnitude, but the in-plane tensor components are unchanged. The results were interpreted in terms of a free electron Drude-like bulk Ba model, where only out-of-plane components of the fields contribute.

~. 150

150 *O

8

so e 0

3

h

N

75

0

8

-

e

v

2

U

&O

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-75

0 00 0

U \

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v)

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9 0 0.00

% bJ aJ

I

0.50

Ga coverage /

1

ML

,L 0

.oo

Fig. 3 1-4. SH intensity ( 0 ) and phase shift ( 0 ) for s-polarised input and p-polarised output, as a b c t i o n of Ga coverage on Si(l1 I), for 1064-nmexcitation at y~ = 30" to the azimuth. (From Kelly et al., 1992.)

The Si( 1 1 1)-Ga system has also been studied. Fig. 3 1-4 shows the intensity variation and the phase shift, relative to the Si( 1 1 1)-7x7 value, for the xsm component as a function of Ga coverage, using 1064-nm excitation. The absolute values of the phase (see above) provided direct evidence that the SHG response from Si(ll1)-7x7 is near resonance, for 1064-nm excitation. The Si(llI)-d3xd3-Ga structure, which is complete at the maximum intensity position in Fig. 31-4, was shown to be even closer to resonance. This contrasts with the off-resonance behaviour observed for this system using 634-nm excitation. The electronic structure of Si(l1 I)-d3xd3-Ga, determined by angle-resolved photoemission and inverse photoemission, indicates that suitable energy


levels are available. The effect of Sb deposition on singular Si(ll1)-7x7 and Si(OO1)2x1 has been characterized up to 1 ML coverage (Power and McGilp, 1993, 1994). Quite different behaviour is observed for the (001) and (111) surfaces, and between 1064-nm and 634-nm excitation. Turning to gaseous adsorbates, both the desorption kinetics and the surface diffusion of hydrogen on singular Si(lll)-7x7 have been investigated using SHG (Reider et al., 1991a, b). In the former case, isothermal desorption at low coverages was studied by using the high sensitivity of the SH response of Si(l11)-7x7 to adsorbates when using near-resonance 1064-nm excitation (see above). Different hydrogen adsorption sites on Si(ll1)-7x7 were identified. In the second study, the diffraction of the SH signal from a submonolayer grating of adsorbed hydrogen, produced by laserinduced desorption, was followed as a function of temperature. This technique was first demonstrated by Shen and co-workers for the diffusion of CO on Ni( 111) (Zhu et al., 1988). The advantage of this technique is its high sensitivity, with surface diffusivities 4O-I4 cm2 s-' being measurable. In a recent study, phonon-assisted sticking of molecular hydrogen on Si(ll1)-7x7 has also been identified (Bratu and Hofer, 1995). Isothermal desorption of hydrogen from singular Si(OO1)-2x1 at low coverages has been studied by SHG (Hofer et al., 1992). The kinetics of oxygen dissociation on Si( 1 11)-7x7 has also been investigated (Bratu et al., 1994). The technologically important Si/SiOz interface provides a final example in this section. Shen and co-workers, in an early study, showed that surface and bulk contributions from native-oxide-covered Si(ll1) and Si(OO1) wafers could be identified (Tom et al., 1983). The formation of the oxide interface removes the surface-state resonances responsible for the enhanced signal when 1064-nm excitation is used. The surface and bulk, higher-order, contributions are comparable in size, and detailed studies are required to separate them. Indeed, for some terms, exact separation is not possible. Vicinal Si(1 1 1)/Si02 interfaces, where the xsxxxcomponent is large, have been used to help quantify surface and bulk contributions (van Hasselt et al., 1990). Recently, detailed studies by van Driel and co-workers of vicinal Si/SiO2 interfaces, with oxide produced by both dry and wet oxidation, and using 765-nm excitation from a modelocked Tixapphire laser, have been reported (Lupke et al., 1994). These studies show the potential of the new pulsed laser sources now available. Similar studies have been performed, using 1053-nm excitation, on chemically-modified vicinal Si( 1 1 1) interfaces; these reveal a correlation between SH response and the density of interface traps (Emmerichs et al., 1993). The SH response from the Si/SiOz system is also sensitive to static electric fields.at the interface (electric-field-induced SHG) and is inhomogeneous (Kulyuk et al., 1991; Aktsipetrov et al., 1994). Information on the technologically important metal-oxidesemiconductor (MOS) structure has been obtained. The Si( 1 1 1)/Si02/electrolyte interface (Fischer el al., 1994) and the Si( IIO)/Si02 interface (Malliaras et al., 1993), have also been studied recently. The former is a good example of solid-liquid interface characterization by SHG.

3 1 Optical second-harmonic and sum-kequency generation 529

31.5 Examples of spectroscopic SHG and SFG The use of spectroscopic three-wave mixing (SHG and SFG) for interface characterization is now feasible because reliable, pulsed OPO and Ti:sapphire systems are being marketed. The first spectroscopic SHG and SFG study of a buried semiconductor interface was of the CaFz/Si(11 1) interface through 50 nm of CaF2 (Heinz et al., 1989), where resonant three-wave mixing was used to determine an interface state band gap. This is an excellent example of the unique information provided by optical probes. Conventional surface probes cannot access the buried interface, and will also produce charging effects in the insulator.

-

20 ,2 -.

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0 0 =- I 0? ,c a .*lr

.

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/'?

(c)

'

- h . " ' I

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.-

. (a - .'

-

(e)

'

'

. . (f)' 7

1

Fig. 31-5. SFG (a) and SHG (b)-(0 spectra of differently oxidized (a)-(d),

Another material system with resonances accessible by dye laser was studied by Yodh and co-workers. In an elegant series of experiments, the buried ZnSe/GaAs(OOl) interface was probed and, by combining spectroscopic SHG, SFG and photoinduced band bending, a resonance associated with a quantum well state at the buried interface was identified (Yeganeh et al., 1992a). Sensitivity to interfacial electronic traps, lattice relaxation and buried interface reconstruction have also been demonstrated in this


interesting system (Yeganeh et al., 1992b). The effect of the depletion layer electric field (band bending) on SHG from GaAs(OO1) has also been studied (Qi et al., 1992). Daum and co-workers, using an OPO system, have found a resonance just below the direct optical gap of Si (Daum et al., 1993). Fig. 31-5 shows the SFG and SHG resonance behaviour of oxidized, clean and hydrogen-terminated singular Si(OO1) samples. Similar results were found for Si(ll1) samples. The response is dominated by the xS, component, and the resonance was ascribed to direct transitions between valence and conduction band states in a few monolayers of strained Si at the surface or interface. This resonance has been studied on singular and vicinal Si(OO1) and Si(OO1)-Sb (McGilp et al., 1994; Power et al., 1995). Fig. 3 1-6 shows a dramatic switch of SH intensity from the y-azimuth ([i101direction) of Si(OO1)-1 x2 to the x-azimuth ([ 1 101 direction) of vicinal Si(OOl)-lxl-Sb. The l m mirror plane perpendicular to the step edges has been lost. This is due to a reconstruction at the step edge on Sb adsorption which, on annealing to higher temperatures, forces the formation of the opposite domain, Si(001)-2xl-Sb, terrace structure. The absence of the l m mirror plane shows that the resonance response is dominated by the local reconstruction of surface steps.

31.6 SFG for vibrational spectroscopy of surfaces The use of SFG to determine whether an electronic resonance is associated with the hndamental or SH frequency has been mentioned above. However, the most important development in SFG has been infra-red-(1R)-visible SFG, which allows vibrational spectroscopy of surfaces and interfaces to be performed by resonant IR excitation (Guyot-Sionnest et al., 1987; Zhu et al., 1987; Harris et al., 1987; Guyot-Sionnest et al., 1988). In this type of experiment, tunable IR radiation is mixed with visible radiation to obtain a visible SF signal which shows resonant structure at vibrational mode frequencies. As well as the inherent surface-sensitivity due to symmetry, this technique has the advantage, over conventional IR spectroscopy, of producing an easily detected visible signal. Hydrogen on Si surfaces has been studied in some detail. Picosecond SFG of Si( 1 1 1)-1x 1-H, as a function of temperature, revealed a lifetime of 0.8*0.1 ns for the Si-H stretch vibration (Chabal et al., 1991). SFG is a coherent process and this allows defacing of the transient polarization of the vibration to be followed (GuyotSionnest, 1991a, b). Hydrogen-terminated vicinal Si(ll1) surfaces have also been studied (Morin et al., 1992; Kuhnke et al., 1994). Measurements of the excited-state lifetimes of the Si-H stretch vibration show that the steps play an important role in vibrational energy transfer at the surface.

31.7 Semiconductor growth There have only been a few examples, so far, of semiconductor growth being characterized using non-linear optical techniques. Heinz and co-workers, in an early study

3 1 Optical second-harmonic and sum-frequency generation 53 1

0.35

-x-azimuth

0.30

(c) Si(00 I )-2x 1 -S b

0.25

0.20 0.15

0.10

0.05 n c m

.d

C

1

0.0

0.40

J ex-azimuth

1

(b) Si(OOl)-lxl-Sb

0.30 0.20 0.10

0.0 0.20

-

0.15

-

o.,o -

-+ x-azimuth

-A-

- y-azimuth

I

c’

(a) Si(OO1)-1x2

I

A

‘1 \ \

\

.

Fig. 3 1-6. Variation of q-ids-out SH intensity with SH energy, at the resonance maxima, for vicinal (a) Si(OOl)-Ix2, (b) Si(OOl)-lxl-Sb, and (c) Si(001)-2xI-Sb. Note the large in-plane anisotropy in (a) and (b), and the switch of intensity between azimuths. (From Power et al., 1995.)

using 1064-nm excitation, showed that Si deposition on Si(ll1)-7x7 at room temperature produced a decrease in the SH intensity, while the signal was constant under epitaxial growth conditions at 650°C (Heinz et al., 1987). Under these experimental conditions, the SH signal is sensitive to disorder in the surface layers.


The characterization of compound semiconductor growth is more complicated because the zinc blend structure is eccentric and can produce a bulk dipole SH response. However, Shen and co-workers showed that, by appropriate choice of polarizations and substrate orientation, the bulk dipole response can be suppressed, while allowing many of the surface tensor components to be determined (Stehlin et al., 1988). In an molecular beam epitaxy (MBE) growth study, rotational anisotropy has been observed which differs between the GaAs (001)-2x 1 and the GaAs(OO1)-4x6 reconstructions (Yamada and Kimura, 1993).

31.8 Magnetic thin films As most bulk ferromagnets are centrosymmetric, any change in the SH or SF response from such magnetic thin films, when the magnetisation direction is changed, must originate from the surface or interface. This magnetic component of the nonlinear response was identified in studies of the clean Fe(ll0) surface (Reif et al., 1991). The result excited considerable interest and, following the prediction of a giant non-linear Kerr rotation in low-dimensional magnetic structures (Pustogowa et al., 1994), a rotation of 22" was observed in the SH response at 385 nm from a 2-nm Fe layer on a Si(OO1) substrate, capped by a 2-nm Cr layer (Koopmans et al., 1995). This is almost three orders of magnitude larger than the linear Kerr effect from this sample. Interface magnetism has also been detected in the SH response from ultrathin Co/Cu films (Wierenga et al., 1995). Magnetization-induced SHG is an important tool for studying interface magnetism, the understanding of which may lead to the development of new, ultrathin layer, magnetic materials.

References Aktsipetrov O.A., Fedyanin A.A., Golovkina V.N., Murzina T.V. (1994), Opt. Lett., 19, I. Bratu P., HBfer U. (1995), Phy. Rev. Lett., 74, 1625. Bratu P., Kompa K.L., H6fer U. (1994), Phys. Rev., B 49, 14070. Chabal Y.J., Dumas P., Guyot-Sionnest P., Higashi G.S. (1991), Surf. Sci., 242, 524. Daum W., Krause H.-J., Reichel U., Ibach H. (1993), Phys. Rev. Lett., 71, 1234. Emmerichs U., Meyer C., Bakker H.J., Kurz H., Bjorkman C.H., Shearon Jr. C.E., Ma Y., Yasuda T., Jing Z., Lucovsky G., Whitten J.L. (1993), Phys. Rev., B 50,5506. Fischer P.R., Daschbach J.L., Richmond G.L. (1994), Chem. Phys. Lett., 218,200. Guyot-SioMest P. ( I 991a), Phys. Rev. Lett., 66, 1489. Guyot-Sionnest P. (1991b), Phys. Rev. Lett., 67,2323. Guyot-Sionnest P. (1993), J. Electron Spectrosc. Relat. Phenom., 64/65, 1. Guyot-Sionnest P., Hunt J.H., Shen Y.R. (1987), Phys. Rev. Lett., 59, 159. Guyot-Sionnest P., Superfine R., Hunt J.H., Shen Y.R. (1988), Chem. Phys. Lett., 144, 1. Harris A.L., Chidsey C.E.D., Levinos N.J., Loiacono D.N. (1987), Chem. Phys. Lett., 141,350. Heinz T.F. (1991), in: Ponath H.-E. and Stegeman G. I. (Eds.), Non-linear Surface Electromagnetic Phenomena, Amsterdam, North-Holland, 1991, p. 353. Heinz T.F., Himpsel F.J., Palange E., Burstein E. (1989), Phys. Rev. Lett., 63,644. Heinz T.F., Loy M.M.T., Iyer S.S. (1987), Mater. Res. SOC.Symp. Proc., 55,697.

3 1 Optical second-harmonic and sum-frequency generation 533 Heinz T.F., Loy M.M.T., Thompson W.A. (1985), Phys. Rev. Lett., 54,63. Hofer U., Li L., Ratzlaff G.A., Heinz T.F. (1 9 9 3 , Phy. Rev., B 52,5264. Hofer U., Li L., Heinz T.F., (1 992), Phys. Rev., B 45,9485. Hollering R.W.J., Dijkkamp D., Elswijk H.B. (1991), Surf. Sci., 243, 121. Hollering R.W.J., Dijkkamp D., Lindelauf H.W.L., van der Heide P.A.M., Krijn M.P.C.M. (1990), J. Vac. Sci. Technol., A 8, 3997. HopfF.A., Stegeman (3.1. (1986), Applied classical electrodynamics, Vols 1 and 2. New York: Wiley. Kelly P.V., OMahony J.D., McGilp J.F., Rasing Th. (l992), Surf. Sci., 269/270,849. Koopmans B., Koerkamp M.G., Rasing Th., van den Berg H. (1995) Phy. Rev. Lett., 74,3692. Kuhnke K., Harris A.L., Chabal Y.J., Jakob P., Morin M. (1994), J. Chem. Phys., 100,6896. Kulyuk L.L., Shutov D.A., Strumban E.E., Aktsipetrov O.A. (1 99 l), J. Opt. SOC. Am., B 8, 1766. Lupke G., Bottomley D.J., van Driel H.M. (1994), J. Opt. SOC.Am., B 11,33. Malliaras G.G., Wierenga G.G., Rasing Th. (1993), Surf. Sci., 287/288, 703. McGilp J.F. ( 1 995), Prog. Surf. Sci., 49, I. McGilp J.F., Yeh Y. (1986), Solid State Commun., 59,91. McGilp J.F., Cavanagh M., Power J.R., O'Mahony J.D. (1994), Appl. Phys., A 59,401. McGilp J.F., Weaire D., Patterson C.H. (1995), Epioptics: Linear and Non-linear Optical Spectroscopy of Surfaces and Interfaces. Berlin: Springer-Verlag. Morin M., Jakob P., Levinos N.J., Chabal Y.J., Harris A.L. (1992), J. Chem. Phys., 96,6203. OMahony J.D., Kelly P.V., McGilp J.F. (1992), Appl. Surf. Sci., 56-8,449. OMahony J.D., McGilp J.F., Verbruggen M.H.W., Flipse C.F.J. (1993), Surf. Sci., 287/288,713. Pedersen K., Morgen P. (1 9 9 3 , Phys. Rev., B 52, R2277. Power J.R., McGilp J.F. (1993), Surf. Sci., 287/288, 708. Power J.R., McGilp J.F. (1994), Surf. Sci., 307/309, 1066. Power J.R., OMahony J.D., Chandola S., McGilp J.F. (1995), Phys. Rev. Lett., 75, 1138. Pustogowa U., Hiibner W., Bennemann K.H. (1994), Phys. Rev., B 49, 1003 1 . Qi J., Yeganeh M.S., Koltover I., Yodh A.G., Theis W.M. (1992), Phys. Rev. Lett., 71,633. Reider G.A., Hdfer U., Heinz T.F. (1991a), J. Chem. Phys., 94,4080. Reider G.A., Hdfer U., Heinz T.F. (1 99 1b), Phys. Rev. Lett., 66, 1994. Reif J., Zink J.C., Schneider C.M., Kirschner J. (1991), Phy. Rev. Lett., 67,2878. Richmond G.L., Robinson J.M., Shannon V.L. (1988), Prog. Surf. Sci., 28, 1. Shen Y.R. (1984),The Principles of Non-linear Optics. New York: Wiley. Shen Y.R. (1985), J. Vac. Sci. Technol., B 3, 1464. Sipe J.E., Moss D.J., van Driel H.M. (1987), Phys. Rev., B 35, 1129. Stehlin T., Feller M., Guyot-Sionnest P., Shen Y.R. (1988), Opt. Lett., 13,389. Suzuki T., Mikami A., Uehara K., Aouo M. (1999, Surf. Sci. Lett., 323, L293. Takayanagi K., Tanishiro Y., Takahashi M., Takahashi S.J. ( I 985), Vac. Sci. Technol., A 3, 1502. Tang, Z.-R., Cavanagh, M., and McGilp, J. F. (1993a), J. Phys.:Condens.Matter, 5, 3791; (1993b), 5, 7903. Tom H.W.K., Heinz T.F., Shen Y.R. (1983), Phys. Rev. Lett., 51,1983. Tom H.W.K., Mate C.M., Zhu X.D., Crowell J.E., Heinz T.F., Somorjai G.A., Shen Y.R. (1984), Phys. Rev. Lett., 52, 348. van Hasselt C.W., Verheijen M.A., Rasing Th. (1 990), Phys. Rev., B 42,9263. Wierenga H.A., de Jong W., Prins M.W.J., Rasing Th., Vollmer R., Kirilyuk A., Schwabe H., Kirschner J. (1995), Phy. Rev. Lett.,74, 1462. Yamada C., Kimura T. (1993), Phys. Rev. Lett., 70,2344. Yeganeh M.S., Qi J., Yodh A.G., Tamargo M.C. (1992a), Phys. Rev. Lett., 68,3761. Yeganeh M.S., Qi J., Yodh A.G., Tamargo M.C. (1992b), Phys. Rev. Lett., 69,3579. Zhu X.D., Rasing Th., Shen Y.R. (1988), Phys. Rev. Lett., 61,2883. ZhuX.D., Suhr H., Shen Y.R. (1987), Phys. Rev., B 35,3047.



32 Electrical and magnetic properties of thin films S. Hellstrom

32.1 Introduction The electrical and magnetic behaviour of thin films is of great importance for applications in electronics. In solid-statephysics, a theory was developed for the transport of charge carriers, electrons and ‘holes’ and their relation to an electric field. The wellknown Ohm’s law, for metals, V = RxI

(32-1)

where V = voltage, I = current and R = resistance, is today formulated as, i = oxE

(32-2)

i = current density Nm2, o = electrical conductivity ANm and E = electrical field Vlm. If v is the speed of charge q, and n the concentration of carriers with this charge, the current density can be written i = nxqxv

(32-3)

A useful parameter in the theory, is the mobility p, V

p=E

(32-4)

Thin films are often used in the study of superconduction, i.e. the loss of resistivity at very low temperatures. An example is the Josephson effect, that is, the conduction, by the tunnelling phenomenon, between two superconductors separated by a thin insulating layer. The various effects referred to in this section are described, e.g.,by Ohring (1992). Another mechanism is the Schottky effect. This can account for the conduction through thin film contacts with an electrical barrier denoted the ‘Schottky barrier’. In insulators and dielectrics, electrical traps are used to explain the conduction as in the case of Pool-Frenkel conduction or the ”hopping” model. The electron band theory of semiconductors has also been applied. The electrical breakdown phenomena caused by intense electrical fields are important. Beside the ferromagnetic films that are used as memories, magnetic interaction with non-magnetic thin films occurs if there is an external magnetic field with a component perpendicular to the surface of the film. The magnetic field will exert forces on moving

32 Electrical and magnetic properties of thin films

535

electrons in the films and, in the case of semiconducting films, the positive holes, giving the carriers a lateral force component. This is known as the Hall effect: F = (E+vxB)xq

(32-5)

where F = force from electric and magnetic field, q = charge (negative for electrons, positive for holes), E = electric field, B = magnetic field and v = speed of carrier. As there can be no lateral motion a lateral component of the electric field is established to compensate for the magnetic force. The net force will be zero. By measuring the lateral voltage drop it is possible to determine the hole density. Various examples of applications in thin film technology are given elsewhere, e.g.,Nguyen et al. (1 995).

32.2 Electrical conduction in metallic films For thin films in general, there are several factors that influence the electric conduction mechanisms compared with bulk materials. The structure of the film can vary according to the deposition conditions. There are size effects such as scattering and tunnelling. In particular, discontinuities in the films give rise to different conduction mechanisms. The measurement of their resistivity is mandatory for characterization of the films. A typical parameter, sheet resistance Rsq, is defined as the resistance of a 'square' of the material dependent on the thickness. Rsq will be independent of the size of the square:

(32-6) where 1 = length, d = thickness and p = specific resistivity of the materials.

32.3 Almost pure metallic films As for bulk material, the thin film resistivity is composed of three components:

(32-7) is predominant at 300 K and varies with temperature, Pimp is impurity dependent, has its origin in defects of the films. Eq. 32-7 is called Mathiessen's rule (Ohring, 1992). The defects are often in the form of vacancies. The value of Pimp + Pdef is called the residual resistivity. At low temperatures (-4.2 K), the thermal part is much less than the residual part, which is temperature independent. By comparing the value of the total resistivity p at room temperature (300 K) and at a low temperature (-4.2 K), an estimate of the ratio between the thermal part and residual part of the resistivity can be obtained. Pth

Pdef


The conduction in metals is performed by electrons. According to quantum theory, they can also be treated as waves and pass through a lattice attenuated only by scattering, the origin of resistivity. The length between two scattering points is called the mean free path. As the atoms of the metal lattice have thermal vibrations, the electron will interact with the lattice atoms. This is explained by the phonon theory. The temperature dependence can be seen from the free mean path at different temperatures: Metal

cu

Ag

1-200 "C I2965 (2425

0 "C 421 515

100 "C

294 405

A A

Impurity atoms will cause scattering of the electrons as well as vacancies and interstitials. The study of the resistivity of alloys confirms this kind of scattering but the contribution of scattering from dislocations is small. Finally, two more scattering modes will be mentioned. Scattering at the surface of the metal, the electron can reflect in a way similar to a photon at a mirror. The corresponding resistivity will then be the same as in the bulk, and no film thickness effect is present. If the electron at the surface undergoes diffuse scattering (reflection) with a random angle the resistivity is increased. Interaction with surface is a size effect as the thickness of film is less than the free mean path. Grain boundaries between crystals of different orientation are scattering points for the electrons. If the crystal size is smaller then the mean free path of the electrons, the resistivity increases. Thin film resistors that have been doped, for instance in reactive sputtering, for example tantalum nitride, will exhibit non-linear conducting behaviour to some extent. Hellstrom (1976) proved the possibility of determining the temperature coefficient of tantalum nitride resistors by non-linearity measurements. It is difficult to get strong support for some special conduction mechanisms from experimental results. Thin film conductors, frequently used in electronic devices, show corrosional effects when exposed to moisture. Aluminium is sensitive in this respect. Tantalumnitride resistors are protected by a thin oxide layer. Electromigration starts at rather high current densities (-10' A cmS2)and means transport of atoms. It can end with an interruption of the conductor. Small admixtures of Cu (-2-5 %) and or Si (1-2%) in the metal will serve as a barrier to the moving of grain boundaries counteracting electromigration. The mechanisms of electromigration are related to the ratio of grain size and width of the conductors. Aluminium stripes in micro circuits are often of about 1 pm thickness and width less than 1 pm. A temperature rise accelerates the electromigration rate. Because of the good thermal contact between the A1 conductors and the substrate, rather high currents can be tolerated before heating of the conductor becomes dangerous. There is a momentum transfer from the electrons to the atoms of the metal. Current and material flux are about proportional at a fixed temperature. The result is formation of voids and hillocks (material


537

concentration points). The following formula for electromigration was derived by Black (1 974):

Ji

tSo= Bj-"ekxT

where tso = mean time to failure (MTF), B = process-dependent constant, n = 2 (can vary) and Ea = activation energy. Ea is variable, often 0.5-0.6 eV. For large grain Al, Eaz0.73 eV and for a single crystal, Eazl.3 eV.

32.4 Superconduction and tunnelling The fact that many metals and alloys loose their resistivity at very low temperatures (-4.2 K), i.e. become superconducting,has been known for a long time. The discovery was made with Hg by Onnes (191 1). The discovery in recent years that some ceramics become superconducting at higher temperatures (100 K and more) has started new research with superconductors.

I

\

T,

c

T

Fig. 32-1. Temperature-magnetic field characteristics for a superconductor.

The critical magnetic field H, is the field strength necessary to 'quench' the superconducting state at a certain temperature. The critical temperature T, is the highest temperature at which a certain conductor can turn superconducting. As to thin films, the Josephson junction is referred to Josephson (1964). The theoretical interpretation is based on the theory of Bardeen, Cooper and Schieffer about the creation of electron pairs (Lynton, 1969). Their theory gives a correct description of magnetic and thermal behaviour in superconduction. The so called London-London equation is another approach that, for instance, accounts for the low-frequency magnetic properties. A brief account for the Josephson junction is given below. The concept of tunnelling is according to quantum theory, that an electron can pass through a potential barrier of finite height, VO,and width, W. Fig. 32-2 illustrates this.


_I 1

.

V, = barrier height w = barrier width E = electron energy less than V,e

y = wave function

Fig. 32-2. Tunnelling of an electron with energy less than the barrier height energy. This is explained by quantum-mechanical wave theory.

The Josephson junction is based on the Josephson effect. This implies that supercurrents can tunnel through a junction. Thus, Cooper pairs (BCS-theory) can tunnel like electrons between two superconductors separated by an insulator (-50 A thick). A schematic configuration is shown in Fig. 32-3,

Insulator

Fig. 32-3. Energy band for transition between two superconductors with different critical temperatures

separated by an insulator.

There are two branches for the tunnelling process, one normal and the Josephson tunnelling branch. As the Josephson currents are extremely sensitive to magnetic fields, it is easy to shift between the two branches by means of an H-field, and memory application is possible. Thin films are very much used, as earlier, for the study of superconductingeffects.


539

32.5 Conduction in insulating films The energy-band structure used for semiconductors is not easily applicable to insulators. A very wide bandgap will limit the filling of the conduction band (-9 eV for Si02). Other approaches are more common. The charging of the insulator builds up a barrier that limits conduction. Metal contacts on an insulator injects carriers. Even for injected carriers, there is a limitation on transport through the bulk. Some mechanisms for charge transport in insulators will be mentioned. Recent investigations with amorphons thin films is reported (Devine et al., 1995).

32.5.1 Schottky emission A contact barrier between a metal and a semiconductor or insulator is denoted a Schottky barrier. The Schottky emission through the barrier is written: J=AxT2exp(-qcpe/kT) where v e is the effective barrier height, A is the Richardson constant used in thermionic emission, and qqe can be of the order of 0.4 to 4 eV depending on the metalinsulator combination. A1 on Si02 gives 3.3 eV and A1 on Si 0.6 eV. Electrons emitted will overcome the small energy barrier and enter the empty conduction band of the insulator (Rhoderick, 1978).

32.5.2 Tunnelling The process of tunnelling is described above. This can also occur between islands in discontinuous metal films if the distance between them is small enough. The electron in an insulator can penetrate at constant energy into the vacant conduction band states instead of surmounting the barrier energy, if the electric field E is strong enough. Also, in the case of a thin insulator (-30 A), tunnelling can occur between two metal layers.

32.5.3 Space-charge-limited conduction At a high injection rate from a contact, the transport of the carrier is too slow and a cloud of charge develops that counteracts further injection. At low injection rates, Ohm’s law is valid. Carriers can also be removed by empty states. If the distribution of traps is uniform in the band gap, a thermally activated current flows: E2 -a%T J e - xe d where E = electric field, d = thickness and a = constant. Thin polymer films have been proven to obey this relationship.


32.5.4 Ionic conduction At high temperature and with thick insulators, ionic rather then electron conduction takes place. When ions diffuse they transport a charge, but they need high activation energies to give the nearest neighbour a jump. As an important example of this kind of conduction, the migration of sodium ions in SiO2 can be mentioned.

32.5.5 Poole-Frenkel emission An internal emission can transfer charges from potential traps at impurity levels to the conduction band of an insulator. If the trap is a donor with Coulomb potential, an applied electric field will lower the potential and promote detrapping. This is the Poole-Frenkel effect. Electrons first neutralizing the trap will jump to the next trap and so on giving a net flow of electrons. Electrets are thin polymer foils charged with electrons sticking in traps. Investigations of Teflon foils with accelerated detrapping at elevated temperatures gave results that could be fitted with Poole-Frenkel emission. Hellstrom derived from approximate Poole-Frenkel theory a relationship from which a fair estimate of ionization energy and filled-trap densities can be made (Wesemeyer and Hellstrom, 1978). Careful experimental work and proper electret material is necessary. Fig. 32-4 gives the trap potentials for the Poole-Frenkel effect.

I

Coulomb potential

t

X

I

Poole-Frenkeleffect

b X

Fig. 32-4. Coulomb potential (a) a = Poole-Frenkel constant. Poole-Frenkel effect potential (b) a=

e J z , where e = electron charge, E = permittivity of material, E~= permittivity of vacuum,

and 0 = trap potential.

At some sufficient field strength E, the Poole-Frenkel effect will give rise to field emission of the electrons. There are of course other approaches to conduction mechanisms in electrets as for instance that described by Perlman ef al. (1 973). A special group of polymers are the so-called electroactive polymers. These are conductive, owing to admixtures or binding properties. Antistatic polymer materials can be prepared by UV or electron beam irradiation. Polyacetylene is a well known electroactive polymer.


541

32.5.6 Electrical breakdown in insulation Insulators and dielectrics have a limit resistance to electric fields. At a certain field strength a breakdown (dielectric breakdown) will occur. The value depends upon the quality of the material. In electronic devices, thin silicon oxide or nitride layers are used, especially in MOS structures. Thin layers (100-1 000 A) are produced in different processes. Tests have shown that for silicon oxide of ordinary quality the limit for breakdown is about 5 MV cm-' field strength. The value depends on the thickness, probably because of different structure and purity achieved. Surface states are of great importance, as proved by Wolters and van der Schoot (1985) and other investigators. 40

t

7

-

Relative 30-number of failures 20-10-

: MVIcm

Fig. 32-5. Electrical field stress-screening of SiOJayers. sec

Cumulative percent failures Fig. 32-6. Time-dependent dielectric breakdown (TDDB) for SiOz layers at room temperature. Stress field about 5 MV. Curves drawn from common results given in literature.

If many samples of layers from the same deposition process are measured, there is generally a distribution around some mean value for the breakdown values. Temperature can also have some influence. In so-called accelerated tests of insulation layers, both field strength and temperature are increased. Such tests can be used to screen out


weak samples. In Figs. 32-5 and 32-6 some test diagrams are shown as an illustration of results obtainable for silicon oxide layers.

32.6 Magnetic properties of thin films 32.6.1 Magnetic behaviour Ferromagnetic films are made of elements, alloys or compounds exhibiting ferromagnetism. Fe, Ni and Co are elements forming alloys, for example, Fe-Ni, Co-Ni. Compounds are, for instance, EuS, EuI2 and CrB. Oxides such as strontium ferrite and nickel-zinc ferrite exhibit ferromagnetism. The theory for magnetic films is the same as for bulk material except for some size effects. The origin of magnetism is assumed to be orbiting and spinning atomic electrons with quantized angular momentum, and quantum theory is applied. When most of the moments exhibited align in parallel over the whole space in a material and produce a strong total moment of magnetization, M, ferromagnetism is observed. As magnetization theory is well developed for bulk materials it will not be repeated here only the hysteresis loop is given to demonstrate hard and soft magnets. Ferromagnetic materials that are not magnets can be magnetized by an external magnetizing force, H, created by an electrical current. The formation of domains in a material means that there are areas with different directions of the magnetization moment vector M. The narrow transition part between domains is denoted the ‘domain wall’. A domain within which M is saturated is denoted a ferromagnetic domain. M, (saturated M) M, = remanent value of M M, =wail coercive force, a material constant

H

t

Hard magnetic material

Soft magnetic material

Fig. 32-7. Form of hysteresis loop for hard and soft magnetic materials.

M will decrease with increasing temperature and above a critical temperature, T , named the ‘Curie temperature’, M vanishes.


543

The vector M shows anisotropy both in bulk and thin film materials. Anisotropy for M means that it is not isotropically distributed but aligned with certain directions in the lattice. Typical magnetic soft materials are: Permalloy (-78% Ni-22% Fe), and the amorphous materials CoZr, FegoB20 and Fe7zSilgCl0. Some hard magnetic materials are: Co-Re, Co-Pt, F304, yFezO,(Co), GdCo. Hard magnetic materials are used for permanent magnets and magnetic recording devices. Soft magnetic materials, that are magnetised easily and shift direction of magnetisation easily have found applications as computer memories and heads for recording. The magnetic field represents energy. In the interaction with an external field, H, the energy density is expressed by EH= -HxM.

32.6.2 Size effects There is some controversy about size effects in thin films. Although looked upon as simple, because surface states of ferromagnetic films are not constrained as in the interior of the material, the question is how thin films can remain ferromagnetic. The two theories used, spin wave and molecular field, both describe the films as being paramagnetic rather than ferromagnetic at absolute zero, the array being a twodimensional network of ferromagnetic atoms. Quantum theory work with these problems has led to a conclusion of enhancement of the magnetic moment per atom in the top layers. This is due to spin densities in the ground state in the case of Fe-Ni for films consisting of only a few atomic layers in comparison with layers in the bulk.

32.6.3 Memory applications The discovery that magnetic films deposited when surrounded by a magnetic field had a square hysteresis loop was important. This implies that the film can occur in two states. Switching time is about s, but metallurgical problems shortened the application period. Another magnetic film approach for memory application became more successful, magnetic ”bubble” memories. A special form of thin magnetic films (garnets) with cylindrical domains (‘bubbles’) could be used. Generation, movement and detection of the bubbles are utilized in the processing of information. The behaviour of the domain walls is important. There are Bloch walls and Nbel walls (Ohring, 1992). Magnetization in the Bloch wall centre can point both upward and downward. The Nbel wall appears in very thin films and the direction of magnetization rotates around an axis perpendicular to the film plane. Domain walls are often around 1000 A. The magnetization M will cause a ferromagnetic crystal to exhibit spontaneous strain, which depends on the direction of M relative to crystal axis. The phenomenon is called magnetostriction.


References Black J.R. (1974), 12th Annual Rehab. Phys. Symp, 1974, pp. 142-149. Devine R.A.B. el al. (eds) (1995), Amorphous insulating thin films 11, Proc. Europ. Mat. SOC.Spring Meeting 1994, J non - crystalline solids vol. 187, pp 1-9,49-85,425-434. Hellstrum S., Wesemeyer H. (1976), Vacuum and Thin Film Technology, Ed. Yanvood J, Proc. of an IntSymp, University of Uppsala, Sweden, p. 339, Published 1978. Josephson B.D. (1964), Rev. Mod.Physics, Vol. 36, p 216. Lynton E.A. (1969), Superconductivity, Methuen and Co Ltd. Maissel L.I., Glang R. (1970), Handbook of Thin Film Technology, Mc Graw-Hill Book Company. Nguyen T.D., Lairson B.M., Clemens B.M., Shin S-C, Sat0 K. (eds) (1995), Structure and properties of multilayered thin films, Mat. Res. SOC.Symp. Proc. vol382, p. 225-276. Ohring M. (1992), The Materials Science of Thin Films, Academic Press Inc. Harcourt Brace Jovanovich, Publishers. Onnes H.K. (191 l), Communications from the Physical Laboratory at Leyden, Netherlands, 191 1. Perlman M.M. (1973), Electret Charge Storage and Transport in Dielectrics, Int. Conf. On Electrets, charge Storage and Transport in Dielectrics 2, Ed. Perlman M M: Miami Beach, Princeton, N.J.: Electrochemical SOC.,Dielectrics and Insulation Division, cop. 1973. Rhoderick E.H. (1 978), Metal-Semiconductor Contacts, Oxford University Press, Oxford. Clarendon Press, Oxford. Wesemeyer H., HellstrBm S. (1978), Improvements of a Proposed Theory. Ericsson Technics No. 2, p. 108, Stockholm. Wolters D.R., van der Schoot J.J. (1985), Dielectric breakdown in MOS devices, Part 1-3 Philips J of R, Vol. 40, No. 3.



33 Measurement and properties of thin films S. Hellstrom

33.1 Introduction New methods in the preparation of thin films are established and old techniques improved parallel with the development of measuring equipment. The coating of surfaces and applications in microelectronics and optical devices are very important for the industry. Properties are closely linked with preparation; in this chapter the topic is presented as follows. For thin film deposition different methods are available: Evaporation, sputtering, electrodeposition, chemical vapour deposition, anodization and polymerisation. Preparation and control process: I. Evaporation of different materials in high vacuum: A. Source shape B. Shape of evaporated bundle and distribution at target 11. Sputtering method A. Sputtering mechanism B. Sputtering conditions 111. Measurement of film thickness Additional information related to properties and applications is given elsewhere (e.g. Auciello and Engemann, 1993; Katz et al., 1992; Benkreira, 1993).

33.2 Evaporation method The process of evaporation consists of a complete cycle of vaporization, transit and condensation of the material upon a substrate. This usually takes place in a vacuum chamber (-lom6mbar or less). A vacuum sufficient to suppress residual gases that can contaminate the deposition is important. Today, there exists a variety of vacuum systems producing a suitable environment for evaporation. Fig. 33-1 depicts a common evaporation assembly. The essential parts are: the vacuum enclosure, a vapour source to carry the material to be evaporated and a heater. The substrates to be coated by deposition of the evaporated material are placed above the source.

546 Part 5 : Surface films Bell jar Substrate holder and heater Substrates

,,

Molecules from the evaporant

\

\ \

I

I '\

1

I

I

\ I / U/

-

Vapor source

/'

I

1

.Baffle

I

t

Fig. 33-1. Evaporation system.

The following relationships for gases are essential: -PV =N kT

(33-1)

where N = number of molecules, V = volume, P = pressure, T = absolute temperature and k = Bolzmann's constant. mbar, N N = 3.2~10"~ m and ' ~only 1 ~ 1 0 ' of '~ Even at high vacuum levels, the volume is occupied by the vapour atoms/molecules. The speed of molecules is described by the Maxwell-Bolzmann law of distribution: -.-=1 dN

N dv

I:;-{

4 ( __ m ) ' v exp ,,h 2kT

__

=(F)x (3%

(33-2)

The most probable speed is that for which eq. 33-2 is maximum: vm

=1.289x104

-

cm/s

(33-3)

correspondingly there are average and root-mean-square speeds, va and vr, respectively: vm = 1 . . 4 5 5 ~ 1 T 0 ~% ( ~cm/s ) (33-4) 2 3v2m v, =2

33 Measurement and properties of thin films 547

These speeds are rather high. For air with M=29 we get at 25 OC,i.e. 298 K: va = 47Ods Another important parameter for the molecules of a gas is the mean free path. It depends on the number of collisions a molecule undergoes and is represented as: 0.225 L = ( x f i x n x do2)-' = ndo2

(33-5)

where L = vaz, z = average time between two collisions, L depends on the density and diameter of the molecules, n = density and do = 'equivalent' molecular diameter assuming hard elastic spheres. Some common values of do: Gas

4 (4

2.2 ' 2.1 '

He

H2

0 2

Air H20

3.6 . lo-* 3 . 7 , loA8 4.7.

5x

cm ,where p = pressure in mbar P i f P = lOOOmbar(1 atm) L = 6 7 n m ( z = 1 . 4 ~ 1 0 - ' ~ s ) ifP = lo4 L = 51 cm (z = 1.8~10"s) ifP L = 5100 cm (z = 1.8x10-' s) Thus for air at 25 "C: L =

~

At the pressures used for evaporation, L is always much larger than the distance between the source and the deposition substrate, so the collision rate will be low.

33.3 Vaporization process The source carrier can be seen in Fig. 33-1. Heating of this will cause the evaporation of the source material, when the heat necessary for the evaporation of the specific material (the evaporant) is evolved and the necessary temperature reached. The heat is mainly the energy required to overcome the attraction between the atoms in the solid or liquid phase. The released atoms possess a kinetic energy which on average is 3/2 kT/atom, only a small percentage of the necessary heat of evaporation (- 0.2 eV). The values are for transition from liquid to gas except for Cr which sublimes. All materials, solid or liquid, are in equilibrium with the gaseous phase of the material exerting a vapour pressure P,.

548 Part 5: Surface films Metal Al Cr

cu

Au Ni

W

Heat of vaporization (cal mole-’)* 67.580-0.20 T - 1 . 6 1 ~ l O ~ ~ T ~ 89.440 + 0.20 T - 1.48x103TZ 80.070 - 2.53 T 88.280 - 2.00 T 95.820 - 2.84 T 202.900 - 0.68 T - 0.433x1O3TZ

*) The heat energy in joules mole-’ IS obtained by multiplying the given expressions by the factor 4.186.

Thermodynamically, it can be found for this pressure: dP” -dT

-

AH, T(vg -v,)

where AH, = heat of vaporisation V, = specific volume of the gas phase and VI = specific volume of liquid phase. Because Vg >> VI and the ideal gas law is valid at low ROT . pressures i.e. v g = ~. Pv dP, dT

-

-AHv AH,P, or d(lnP,) -Ro d&) ROT* ~

This means that if In P, is plotted as a function of 1/T, the slope will be -AH,/Ro, making the measurement of AH, possible. Vapour pressure vs temperature curves for many materials can be found in the literature. It is to be noted that a small change in temperature produces a large change in evaporation rate. Proper control of temperature is mandatory to master the evaporation rate.

33.4 Deposition of evaporant on the substrate (film carrier) The emitted evaporant is condensed on the substrate surface. Many parameters influence film formation. The condition of the surface, adsorbed contaminants for instance, are important in film growth. The initial evaporant deposit influences subsequent film formation. The film-to-substrate adherence must be sufficient to ensure that the film sticks to the substrate during subsequent handling, measuring, etc. This demands a very clean substrate surface in the deposition process. An atom in the vapour that hits the substrate can undergo different processes such as reflection, physical adsorption with no potential barrier (involving van der Waal’s forces), chemisorption (with forces of the same nature as normal chemical bonds, a potential barrier must often be overcome), association with other atoms already adsorbed. Atoms can migrate over the surface or change from physical to chemical adsorption. Migration is promoted by elevated temperature. The substrate temperature is important to the structure of the deposited film.


Extremely clean surfaces can be obtained by use of single crystals. Other substrate materials are polycrystalline or amorphous in structure. Hydrocarbons from the pump oil can interfere with film formation. Initial growth has been described by Maissel and Glang (1970).

33.5 Types of evaporation sources (carriers) Temperatures in the range 1000-2000 "C are needed for the evaporation of most materials. The heating of the sources is generally accomplished by resistive heating. A filament or a tungsten boat will often do for the purpose. Electron bombardment of the evaporant has enabled evaporation of high-meltingpoint materials. The source is heated directly and the carrier can be cooled. Highfrequency induction and radiation are other means of producing the heat for evaporation. Fig. 33-2 gives examples of heating arrangements. Multistrand wire

(a) Helix

(c)Boat

(b)Conical basket

Fig. 33-2. Common filament and foil evaporation sources.

The source for sublimation of dielectrics is more complex. If the deposited film is to stick to the substrate, there must be some coupling between film atoms and the top layer of the substrate. The strength of these usually physical adsorption bonds determines the adhesive properties. Glass substrates are common. Chemically more active metals result in better adhesion than others. Descriptions of evaporants of different types of material, metals and alloys as well as insulators and dielectrics, can be found in the literature.

33.6 Silicon monoxide The monoxide of silicon (SiO) is an important dielectric, but is also easy to evaporate. SiO is usually chemically inert to environmental conditions. Reactions producing SiO include: Si02+ Si -+ 2 SiO SiO2+H2 + SiO+H20 Si02+C + SiO+CO


When evaporating SiO a ‘chimney’ is inserted between the source and the substrate to prevent the evaporant from sticking to the walls. The temperature of the source can be controlled by a thermocouple welded to it. Temperatures less than -1250 “C promote the reaction: 2 SiO + SiOz+ Si resulting in a condensed film of the mixture Si, SiO and SiOz which is under tensile stress and porous in nature. As other oxides of Si can appear, it is most correct to write SiO, with x varying.

33.7 Geometrical evaporation conditions The general arrangements, of the evaporation process is apparent from Fig. 33-1. Some rules governing the evaporation and deposition process related to geometrical configuration will be pointed out. The evaporant carrier is considered as a point or as a small plane source in the following.

33.7.1 Point source A point source is defined as a sphere emitting isotropically on evaporation. The evaporant will condense on any surface (substrate) in its way, see Fig. 33-1. The mass of the source is denoted ml. The amount dmz arriving at a substrate surface dS2 is proportional to the solid angle dSZ subtended by the surface and to ml:

dmz = C x m l x d Q

Fig. 33-3. Effect of receiving angle on the deposit thickness. Note: The small source may be either a point or plane source. The angle 8 is the angle between the normal to the plane source and the line between the source and the receiving surface. The angle p is the angle between the normal to the surface and the line between the source and the receiving surface.

33 Measurement and properties of thin films 55 1

If the total substrate is a sphere concentric with the source, C can be evaluated by integrating over this sphere: n

jdm2 = Cml j(2xsin 0d0) e=o

m2 = Cm14x

For complete evaporation m2=m1 and C= '/4 x (Berry et al., 1968).

33.7.2 Small plane source It can be shown that the amount of material evaporated from a plane source at angle 0 is proportional to cos 0 (0 as in Fig. 33-3). Thus, for a small plane source: dm2 = Cml cos0 x dR =

Cml cos6cospdS2 r2

In the case of a plane source, emission occurs only inside a solid angle of 2x (hemisphere) and integration of dm2 gives:

mlcosOxcos~xdS~ (small plane source). xxr2 For a homogenous deposition, a density p can be introduced and the thickness, t, calculated:

Thus dmz =

dm2 = ptdS2 and t=-

t=

ml cosp 4xpr

(point source)

ml cos0cosp (small plane source) apr


33.8 Sputtering technique Another way of depositing materials as thin films on substrates is by sputtering. This implies the bombardment of a solid by ions, e.g. positive gas ions, resulting in ejection of atoms from the target. Any surface near the target can be used to receive these ejected atoms. Sputtering is used very much in industry. Sputtering as a phenomenon has been known since the last century. Its application for the depositing of films is more recent and new research in sputtering has concentrated on determining the relationship between film properties and deposition parameters. The usual arrangement for sputtering is a chamber with two parallel electrodes filled with a suitable gas in a pressure range from a few mbar to several hundred mbar. An electric field between the electrodes will ionize the gas and the accelerated ions hit the cathode making atoms escape from the cathodic material, as shown schematically in Fig. 33-4. ,gas molecules

from cathode

Fig. 33-4. Sputtering arrangement.

There are two main theories for this sputtering process. The first was proposed in 1891 by Crookes and later, 1926, by von Hippel (Berry et al., 1968). This states that spots of the cathodic material are heated to a high temperature by the ion bombardment, causing evaporation of the material. Thus, the sputtering rate should be a function of the heat of sublimation of the target and the energy of the impacting ions. The second theory has the momentum transferred from ions to sputtered material atoms as its basis. Atoms are ejected by the energy transferred during bombardment. It was also observed that the emission did not follow a cosine law, but became strongest in the direction of closed packing in the crystals. The sputtering yield was also proved to depend not only on the energy of the bombarding ions, but also on their mass, and an energy level of the ions must be reached for sputtering to occur. There is also a decrease of yield at high energies because the ions penetrate deeper into the target beneath the surface. Co-sputtering of two materials is possible (e.g. Ta + Al). For dielectrics, an ACtechnique can be used. In magnetron sputtering, both electrical and magnetic fields are used. It is often used in industry because of the high deposition rate attainable.


33.9 Thickness measurement techniques The following procedures are commonly applied (Berry et al., 1968; Bengtsson, 1994). 1. A common method is to weight the deposit film and calculate the thickness from the density and the surface area of the deposition. 2. Fringes produced by interference between light reflected from the substrate and a reference flat, can be used as Multiple Beam Interferometry. These interference fringes are sharp because of multiple reflections and can be used to determine the film thickness from steps at the film edge, see Fig. 33-5. 3. Back-scattering of 0-particles is another possibility. High-energy 0-particles (electrons) hit the surface and the back-scattering rate is measured. From that, the thickness can be calculated if the rate is calibrated for known sample thicknesses. 4. A direct way is to use a stylus. It implies the measurement of a film-substrate step mechanically. Attached to a lever and fulcrum, the stylus can be made to traverse a film substrate step and the movement is recorded after amplification. 5 . By directing X-rays onto the film surface secondary X-rays are excited (fluorescence). From the intensity of the secondary beam, the thickness is determined. Calibration is needed. 6. Ellipsometry is a sophisticated method for high-precision thickness measurements of transparent films. The refractive index of the film material is also obtained. Another name of the technique is polarimetry and polarization spectroscopy. Extremely thin films can be measured (cf Chapter 30). The method is based on the determination of the change in the state of polarisation .of the light on reflection from the substrate. The state is obtained from the relative amplitude of the parallel (pp) and perpendicular (ps) components of radiation and their phase difference Ap-As. When reflected from a surface, bare or film-covered, the ratio of the two amplitudes, pp/psand the phase difference, Ap-As, undergo changes that depend on the optical constants of the substrate, n3 and k3, the incident angle 8,, the optical constants of the film, n2 and k2, and its thickness d. If the optical constants of the substrate are known and the film material non-absorbing (k2=0), the only unknowns in the equations describing the state of polarization will be refractive index n2 and the thickness d of the transparent film. Thus in principle, with a complete knowledge of the state of polarization of the incident and reflected light, the refractive index n2 and thickness can be calculated. Below are given some comments about the different methods. 7. For a method employing glow discharge see Bengtson (1 994).

554 Part 5: Surface films Incident Reference flat

Multiple reflections

Substrate Schematic of multiple beam interferometry

h Fringe spacing =?when the incident light is normal h = 5460Afor Hg green line

Fig. 33-5.Fringes produced by multiple beam interferometry across a film substrate step.

33.9.1 Method of weighing If A is the surface area, p the density, t the thickness and m2 the mass of the deposited film, one can write: &xAxp = m2, i.e. & = mdAp

m2 is obtained by weighting, p and t are unknown. Assume bulk density = p~ giving an equivalent thickness te = m2/Ap~.If the error in the surface area A is negligible, the relative error in te will be:

m2 can be measured within f 2 pg precision. An aluminium film of 10 cm2 having g cm" at 20 "C is: p ~ 2.699 = t, = 100k7.4A

Factors limiting the accuracy of the method are absorption of water vapour, difference from bulk density p~ and non-uniform thickness.


33.9.2 Method of interferometry This is the most accurate method available for thickness measurements; the resolution is f5 A perpendicularly to the substrate surface. These measurements are performed according to two possibilities. In the first, so-called Fizeau fringes of equal thickness are produced by a monochromatic light source. A sharp step is produced between the substrate surface and that of the film. The fringe spacings and fringe displacement across the step are measured and used to calculate the film thickness. In the second, a white-light source produces fringes of equal chromatic order. The procedure is the same as in the first case, but the parallel incident light is white with light-waves of different wavelengths, h. The wavelengths of the reflected light are measured with a spectroscope. When a step is present on the substrate, the distance between the substrate and the reference flat will change by an amount equal to the film thickness. If hl and ho are the wavelengths that give constructive interference at the substrate and film surfaces, respectively, the relationship between film thickness t, hl and ho will be: 2t = a(h1 - 10)= a(Ah1) where ‘a’ is the chromatic order of any given line. The order, ‘a’ can be obtained from measurements of two adjacent fringes, because ah, = (a+l)xh2. Thus a = h2/(hl-h2)xhl and 12 are measured with a spectroscope. From the preceding ‘relationship,t is given by:

For films thinner than a few hundred angstroms, the second method is preferred since it is capable of a higher resolving power.

33.9.3 Method with radioactive sources Calibration is made by measuring a film of the investigated material of known thickness. A radioactive source with high energy P-particles is used. The number of back-scattered particles depends on the energy of the P-particles (a P-emitting radioactive source has an energy spectrum with a characteristic maximum value), the atomic number, Z, of the film material and the direction angle of back-scattering. This method is easy to apply. The need for calibration is, however, a disadvantage. The minimum film surface for this method is about 2.5 mm diameter. Multi-layer or alloy films cannot easily be measured.

556 Part 5 : Surface films Substrate Film

Platform with aperture Back-scattered beta particles Geiger-Muller counting tube Radioactive source holder Fig. 33-6. Experimental arrangement of beta particle back-scattering thickness measuring apparatus.

The accuracy of this method will vary much due to the radioactive source applied, i.e. max I3 energy and source strength. The latter depends upon the resolution of the counter. If a proportional counter is used instead of a Geiger-Muller counter, the counting accuracy is increased because of better resolution; this shorter measuring times possible or higher statistical accuracy. The back-scattering properties of the substrates is important. The back-scattered fraction of B particles increases with the atomic number Z of the substrate, but is also a function of the maximum energy of the I3 particles.

33.9.4 Mechanical method There are commercial stylus instruments available. The diamond stylus has a tip radius of 2 . 5 ~ 1 0 cm - ~ and exerts a force of about 10” N on the sample to be measured. The passage of a film-substrate step is necessary. The electrical signal from the movement when passing the step is recorded on rectilinear paper. A profile graph of the cross-section of the step is produced; this includes surface irregularities. Errors are a combination of machine errors, those due to the width of the recording line and reader errors in determining the step height. Background vibrations must be reduced. A one-sigma limit of about k40 A is obtainable. This method compares well with Multiple Beam Interferometry.

33.9.5 Method of X-rays The intensity of the secondary X-rays depends on the atomic number Z of the film material and its mass. Here, as in Section 33.9.3, back-scattering, there is a maximum thickness that can be measured due to total absorption of the backscattered X-rays: Advantages of this method are independence of substrate material in most cases and no comparison with standards necessary for obtaining absolute values of mass per unit area.

33 Measurement and properties of thin films 557 Detector

Analyzing

Primary x-ray

x-rays

Fig. 33-7. Schematic representation of an X-ray spectrograph.

There is a disadvantage because thickness is not obtained directly unless calibration against standards are performed or an assumption of film density made. The equipment, an X-ray spectrograph, is also rather expensive.

33.10 Different deposition techniques 33.10.1 Chemical-vapour deposition A method used a great deal for production of optoelectronic components utilizes a gas compound containing the element to be deposited. The gas is decomposed by high temperatures or by adding a reducing agent such as hydrogen. This is called vapour plating. Atmospheric or reduced pressure is used. A rather high melting temperate of the material to be coated is necessary. The gas is obtained by evaporation of the compound or by letting a reactive gas pass over the material creating a volatile compound. The process is different from the evaporation method described above. A gas flow passes continuously over the substrate. Some of the chemical reactions used can illustrate the process:

+ Ge 2 GeI2 SiC14+2H2 Sic14 + 2 H20

I2

j

GeI2 (formation) Ge + Ge14 (deposition) Si+4HC1 Si02 + 4 HC1

--j

j

-+

33.10.2 Anodization A special case of electrolysis, anodization, is the formation of an oxide on a metallic anode surface with the evolution of hydrogen at the cathode


M + nH20 -+ MO, + 2nH++ 2ne (anode) 2ne + 2nH20 -+ nH2 + 2nOK (cathode) A typical reaction is: -+ Ta205 + 10Hf + 1Oe2Ta + 5H20 an alternative way of writing the equation is: 2Ta + 1 0 0 K -+ Ta2O5 + 5H20 + 10eAs the oxide layer is formed, the ions have to penetrate it to react with Ta. This will limit the layer thickness according to applied voltage. The stability of the oxide layer can be optimized by controlof the voltage. Both cations and anions are able to penetrate the oxide with reaction at the anode, inside the layer or at its surface (Hellstrom and Wesemeyer, 1976). Not all metals can be anodized. The pH of the electrolyte must be correct for A1 or the oxide will redissolve. An empirical relationship between the current and the electrical field was derived by Gunterschultze and Betz (1934):

where j = current density, E = field strength and a, b are constants; ‘a’ is in the range 1O-20- 10”’ A cmV2and ‘b’ is in the range 10-6-10-7cm V‘ = 1 E-’.

33.10.3 Polymerization Insulating films can be prepared by polymerization of monomers. The monomer is evaporated and when condensing on a relatively cool substrate will polymerize thermally. Irradiation of a monomer by an electron-beam or ultraviolet light is another method. Photoresists used in the formation of patterns for electronic circuits are irradiated in this way through a mask to give insoluble polymer areas. Glow discharge is also used for polymerization.

33.1 1 Properties The importance of thin film properties are very much related to their applications. Adhesion to substrate, tensile strength, temperature endurance and chemical resistivity should be the main parameters in the choice of film composition.

33.11.1 General properties In the case of evaporation and sputtering, the temperature of the substrate will very much influence the structural composition, grain size, etc., of the film. Gas pressure and deposition rate have a similar effect. The thickness of the films can be regulated, for instance, by means of a quartz crystal oscillator inside the evaporation vessel. The


frequency of the oscillator will depend on the mass density of the evaporant deposited on the crystal surface (calibration needed). Film properties change with thickness. Cleanness of substrate and its flatness must be controlled.

33.11.2 Contacts Some metals adhere sufficiently to glass substrates only with difficulty. Gold, for instance, which is used extensively as a contact material in electronic circuits, is not easily attached to glass or refractory metals (e.g. Ta); a thin layer of NiCr is first applied, some hundreds of nm thick. Onto this layer, a relatively thick layer of gold (-1-2 pm) is deposited giving a good electrical contact. Another problem is interdiffusion between two metal layers. A barrier metal can be used to separate the two metals, blocking the interdiffusion.

33.11.3 Capacitors Thin films that are to be anodized to the oxide are prepared with a suitable lattice to facilitate the oxidization. A sparse lattice is preferable. In the case of tantalum, an a-b.c.c. lattice is used for resistors, but for capacitors (oxide-layer performed), the f.c.c. is better but more difficult to obtain.

33.11.4 Resistors, conductors Materials such as Ta, Al, W and NiCr are used to form conductors for electric circuits. A1 and NiCr are evaporated and Ta, W sputtered. A1 is used in ICs, NiCr in hybrid circuits as conductors. Ta in the b.c.c. form appears in resistors, which can be anodized to the desired resistance value. When using NZ as a reactive agent in the sputtering process, the resistor is doped with nitrogen. The temperature coefficient changes with the doping concentration and can adopt negative values. In a circuit with tantalum resistors and capacitors, the positive temperature coefficient of the capacitors can be compensated for by the negative coefficient of the resistors, making RC values in the circuit almost temperature-independent. Aluminium conductors are in general doped with copper and/or silicon to inhibit electromigration.

33.1 1.5 Epitaxy To achieve films of single crystals the method of epitaxial growth was developed. In some electronic components materials with an almost perfect crystal structure are needed. Lasers are a good example (Ga A1 As). In an epitaxial deposition the lattice of the condensed atoms combines with the lattice of the substrate atoms to give a continuous interface. Misfit can occur if the difference between the two lattice constants is too large.


Substrate

ml 1

1

Matched

Strained

1

lTfFFl Relaxed

Fig. 33-8. Overview of epitaxial growth.

Fig. 33-8 gives an idea of substrate-single crystal layer formation. If substrate and deposit are of the same materials, e.g. silicon, one has homoepitaxy. In the more common case, heteroepitaxy, the two materials are different. In homoepitaxy, the lattices are matched very well with no strain at the interface. Small differences in lattice constants make heteroepitaxy similar to homoepitaxy. substantial differences will cause a mismatch. Then, either edge dislocations (see Fig. 33-8, case 3) are formed to relax the strain, or the two lattices deform to accommodate each other (case 2). This happens when the two materials have the same crystal structure. The relaxation phenomenon with formation of defects will prevail during following process steps. In all cases described above, the chemistry of film and substrate and the differences between the coefficients of thermal expansion will have a strong effect on the electronic properties and crystal perfection at the interface. This is crucial when composing electronic devices with different layers, see Fig. 33-9. The epitaxial layers can easily be doped to furnish semiconductivity. (Examples of dopants: P, As, Sb (donors); Zn, In, Ga (acceptors)). Optoelectronic devices are often of the 111-V combination (chemical groups 111- and V- of the periodic table). There are also 11-VI components. Epitaxial film deposition can be achieved by LPE, CVD and MBE. LPE = Liquid Phase Epitaxy (not yet described) involves the formation of a crystalline film by precipitation from a supersaturated melt onto the substrate; it is both a template for epitaxial growth and a physical support for the heterostructure. Multiple heterostructures such as GaAs-AlGaAs can also be obtained by this method. Chemical vapour deposition (CVD), described above, is another method. ZnS on a GaAs- or Gap-substrate, and CdS on GaAs-substrate are examples of CVD-grown epitaxy. To achieve lasers with long life times it was soon realised that almost perfect single crystal structures were necessary. MBE (= Molecular Beam Epitaxy) was developed to attain this goal. The process is, in brief: evaporation with high precision in ultrahigh vacuum (-lo-'' mbar) produces molecular beams of the element to be grown and of dopants. Reaction of these atoms and molecules with a single-crystal substrate gives

33 Measurement and properties of thin films 56 1

the desired layer. A very high-precision fabrication of semiconductor heterostructures is attained. Sharp physical contours of the films can be obtained. For GaAs materials thicknesses from about 1 pm down to a monolayer are possible (Mathew, 1975). The word ‘superlattice’ has been coined for structures composed of periodically alternating single-crystal film layers.

4002 n-GaAs (Si) AuGe

Fig. 33-9. A double heterstructure laser (AIGaAs - GaAs).

33.1 1.6 Mechanical properties In analysing mechanical properties of these films, the material’s structure is of predominant importance as it is for bulk materials. The Newtonian law of action and reaction is valid. An outer force, e.g. the bending of the film, will cause stress inside the film. Depending of the film structure, the electronic properties of the lattice will change. In the case of semiconductor composition, contraction and dilation, when shear stress is distorting the atomic layers, result in strain. Dislocations, point defects and interstitials are known to reduce mechanical resistance. Annealing either thermally or by irradiation, is often used to recover the materials from disturbances. Interfaces can be improved by the latter type of annealing. Oxides can also be applied on such metal surfaces as protective layers.

33.11.7 Tensile strength Measurements of the stress-strain properties of thin films have revealed that they are strong compared with hard-drawn materials; strengths 2- 10 times higher were found. Electron microscope observations of defects have been performed during stress tests. Polycrystal films are in general stronger than single-crystal films. Different methods have been used for measuring tensile strength. In general, the elongation is recorded when small loads are applied. An electromagnet can produce the strain, elongation being observed through a microscope. The strain can also be sensed using interferrometric methods. In these cases, the film must be separated from the substrate, a severe drawback. The change of spacing in a thin film lattice can be measured by X-ray and electronic diffraction techniques.


33.11.8 Thermal stress Because of the usually high temperatures used in the preparation of thin films, thermal stresses must be considered. Cooling from high to normal temperature introduces stress in the contracting film. Hooke's law will give the stress, cr, as function of elastic modulus, E, relative elongation: ly,=-

lo x AT

K-' (1 = length):

G = Exa(T-To)

As the thermal parameters of film and substrate are usually different, a thermal mismatch force will appear to make the strains equal for film and substrate. For evaporated film, the thermal stress depends on several variables, the nature of the combination film-substrate, deposition parameters and film thickness; thus, results from different laboratories can vary substantially. Stress in sputtered films depends on the nature of the plasma environment and effects from the active gas. Generalizations are difficult to make. The temperature of the substrate can cause compression of the films if it is low. Theories about intrinsic stress have been developed.

References Auciello O., Engemann J. (1993), Multicomponent and multilayered thin films for advanced microtechnologies: techniques, fundamentals and devices, Kluwer Academics, Dorndrecht, Holland. Avishay K. (ed) (l992), Materials Research Society, symposia proceedings, p 20. Bengtson A. (1 994), Spectrochimica Acta, Vol49B, No 4, pp 4 1 1-429. Benkreira H. (ed) (1993), Thin film coating, The Royal SOC.of Chem., Cambridge, p.199. Berry R.W., Hall P.M., Harris M.T. (1968), Thin Film Technology, D. Van Nostrand Company, Inc., Princeton New Jersey. Gihtherschuetze A,, Betz H. (1934), Zeitschrift fir Physik, Vol. 92, pp 367-374. Hellstr6m S., Wesemeyer H. (l976), Nonlinearity Measurements of Thin Films in Vacuum and Thin Film Technology, (Yanvood J, ed) Proc. of an IntSymp, University of Uppsala, Sweden, 1976, Published 1978. Katz ef al. (eds) (1 992), Advanced metallization and processing for semiconductor devices and circuits, Proc. Mat. Res. SOC.Symp. Proc., p. 260. Maissel L.I., Glang R. (1970), Handbook of Thin Film Technology. Mc Graw-Hill Book Company. Mathew J.W. (1975), Epitaxial Growth. Ed. Mathews J W, Academic Press, New York, 1975. Ohring M. (1992), The Materials Science of Thin Films. Academic Press Inc. Harcourt Brace Jovanovich. Publishers.



Part 6: Surface reactions This part deals with chemical properties, atom-to-atom binding and chemical changes that can be used to characterize a surface. Either the surface reacts in a chemical sense, e.g. it corrodes, or chemical reactions occur on it and by the very outcome of the reaction provide information about the surface underneath. Alternatively, physical forces of adhesion or adsorption, as the case may be, act and provide a tool for investigation of the surface. The chapter about surface energy deals with the basic concepts, such as wetting angles, adhesive forces and spreading of an adsorbed film. In all these cases, it is the minimization of the Gibbs free energy .of the surface that is the driving force. This factor is also of extreme importance in the preparation of small particles, where a certain critical radius sets the limit between nucleation and crystal growth. Adhesion can be used for the analysis of a surface, for instance, by pulling-off tests. For example, a layer of paint is applied to the surface and pulled off. Those parts of the surface that contain chemical units that interact with the paint will show up in such a test. In the description of surface reactivity, the ability of the surface to participate in chemical reactions is highlighted. The primary problem when dealing with the highly dispersed metal systems often used in chemistry (e.g. catalysis) is the determination of the totally available surface area. The determination of specific surface area with the Brunauer-Emmet-Teller (BET) technique is described. Application of the classical emanation method is also outlined. Titration techniques can be used to determine the number of adsorption sites on the metal that might be of chemical importance. One species (e.g. hydrogen) is adsorbed on the system to be investigated and another species (e.g.oxygen) is added to the point when no further reaction between the two species can be detected. In this way, a proper assessment can be made of on the number of reactive sites on the metal surface. Determination of the number of sites can also be performed by direct chemisorption, i.e., measuring the amount of substance needed to cover the surface by means of a suitable reagent. However, one has to know the number of surface atoms reacting with one atom or molecule of the species that is adsorbed. Such knowledge is not always at hand. Another commonly used technique is Temperature Programmed Desorption (TPD). By gradually increasing the temperature and detecting the desorbed species, one can distinguish between loosely bound species, desorbing at low temperature, and more tightly bound species that require a higher temperature to desorb. Consequently, a kind of mapping of the properties of the surface can be achieved. The corrosion of a surface is dependent on and thus a measure of the surface composition. It is obvious that the difference between stainless steel and a pure sheet of iron is determined by the chemical composition of the surfaces in the two cases. Corrosion is essentially an electrochemical process and it is therefore appropriate to study corrosion by electrical measurements. The simplest of these is the measurement of the

564 Part 6: Surface reactions

corrosion potential, i.e., the non-equilibrium potential difference between the metal and the electrolyte just outside the metal. For instance, a passivated metal might have attained a high corrosion potential because of the building up of an oxide surface layer. If the corroding solution attacks this layer a sudden drop in potential will indicate break-through. The technique reveals the condition of the surface. It is possible to detect material defects in surface layers. Another property of electricity is the current. The corrosion current can be meaningfully measured only when the anodic and cathodic parts can be separated. This technique is useful in testing dental materials. The third property of interest is the resistance of the corrosive circuit. This determines the relationship between potential and current (using polarization curves) and is built up from the charge-transfer resistance at the two electrodes (idealized) and the ionic resistance of the solution. By using ACimpedance spectroscopy, it is possible to separate these two parts of the resistance. This and similar techniques show promising possibilities in corrosion studies. Two fundamentally different nuclear methods may further elucidate corrosion processes. Nuclear Corrosion Monitoring (NCM) describes the use of radioactive nuclei for easy measurement of the dissolution of metals from an alloy surface under corroding conditions. This technique implies the formation of radioactive isotopes by irradiation of the sample in a neutron flux. It has the advantage that the atoms that are transformed are isotopic with the constituents, e.g., "Cr and 59Feare produced from the chromium and iron atoms of the sample. Thus, they are used to discover the corrosion of the same species in the surface. The disadvantage of this method is that the entire sample is irradiated and consequently personnel have to be protected from a higher radiation level than that emerging specifically from the surface of interest. In order to reduce the activity level, slices or foils can be activated. In this way, the total mass is reduced. The technique is useful in various industrial applications as well as in biomaterials testing. The other nuclear technique, Thin Layer Activation (TLA) utilizes a proton or deutron beam from an accelerator in the irradiation process. In this way, the radioactive nuclei appear only in a very thin layer of the specimen, thus making the technique more surface-specific. The new nuclei that are created, however, are not isotopic with the original ones; e.g., %o produced by irradiating iron with protons is not found in the same lattice position as an iron atom would be. However, the total structure of the sample surface might also be affected by the irradiation. One advantage of the method is that as only a limited, central part of the surface is irradiated, the detection of corrosion particles is not disturbed by an intensified corrosion rate at the edges of test coupons as is often the case when using more conventional techniques. The TLA technique is also used to advantage in wear testing, described in the section dealing with tribology.



34 Adhesion and surface energy R. Larsson

34.1 Introduction Adhesion means the attraction that binds one material to another. In contrast, cohesion is the attraction within one and the same body that keeps it together. Adhesion is brought about by intermolecular forces, rather than interatomic ones. The range of interaction is small, of the order of 10 A and therefore it is essential that the surface is wetted by the adhesive (the substance that is attracted). The surface energy is defined as the change in free energy of a material for a standard increase of surface area (1 cm2). For a liquid, with a deformable surface, the surface tension is defined as the force (dyne) acting at right angles to any line of 1 cm length on the surface. The work done in extending the surface by movement of this 1 cm line against the force over a distance of 1 cm is thus the surface energy in erg cmS2.It follows that the surface tension, g (dyne cm-'), and the surface energy is measured by the same number and have the same dimension; they are mathematically equivalent. For solid materials without or with only slightly deformable surfaces, these concepts become much more difficult to treat. A thorough and recent presentation of the thermodynamics of solid surfaces has been made by Rusanov (1996). In the border land between the liquid and the solid state, in the formation of solid particles from solution, surface energy plays an important role. As the hypothetically forming particle is increasing its surface energy with a term proportional to r2 (r = radius) and the free energy of formation is a negative quantity increasing as r3, there will be a certain critical size of the particle below which the formation process is not spontaneous. (Fig. 34-1). As the critical size is of the order of 30-100 atoms, it is necessary to provide 'seeds' of a size that is larger than the critical size (cf, e.g., Somorjai, 1981). For a liquid on the surface of a solid substance, the situation can be as depicted in Fig. 34-2. A molecule at the intersection of the surfaces between liquid, solid and vapour is affected by the forces representing the surface tensions denoted as yLv,ysL and ysv, respectively. When the forces are in equilibrium, a wetting angle O is defined as: YLVCOS

0 = ysv - YSL

For a complete wetting it is required that cos O approaches 1. It can be deduced that the work of adhesion is WA = yL"( 1

+ COSO)

and thus has a maximum for such adhesives that for a given surface give 0 = 0; the liquid spreads completely on the solid surface.


In practice, one often calls a surface wettable by a liquid when the angle 0 is < 90" and non-wettable when 0 is > 90'.

\

\

\

\

\a r3 \

\ I I

!I

Fig. 34-1. The free energy of a growing particle with radius r. The increase of surface energy is overcompensated by the decrease of the energy of formation at a certain critical radius. (Adopted from Somorjai , 1981.)

\

I I I

I

I

I I

\

YLV

LZiL Liquid drop

7% Smooth, rigid, impermeable solid

YSV

Fig. 3 2. Schematic picture of the interplay between the sdiagram tensions of a droplet on a solid material. ,,y, ,y, and y,, are the surface tensions of the liquid-vapor, solid-liquid and solid-vapor surfaces, respectively.

34 Adhesion and surface energy 567

34.2 Applications To turn to the problem of two materials joined together by a glue or similar substance, the interface between the glue and one of the materials can actually be an interface, i.e., a substantial region of material where the two materials mix with each other by a gradual change of properties. The same holds, of course, for the interface between the glue and the second material. Most measurements are concerned with the determination of the strength of the adhesive bond. Depending on the nature of what can be called the adherents (the materials to which the adhesive or glue adheres), different techniques are used. If the two adherents are strong and non-defonnable, a lap shear test is used (Fig. 34-3a). The area that is glued is usually standardized (to 3.23 cm2) and the results are expressed as pressure where the glue area is considered to be the one on which the measured force is applied. If one of the adherents is flexible, it is customary to use a peel test, of which one example is shown in Fig. 34-3b. In principle, the force necessary to open up the adhesive bond is measured. The result is expressed as Newtons per meter. I f the two adherents are nondeformable a mechanical fracture test is used (Fig. 34-3c). All measurements are performed with conventional tensile testing equipment. For results and typical data for common materials the reader is referred to a recent review by Pocius (1991).

Fig. 34-3. Test techniques for the adhesion bond: a) is a lap shear test, b) is a peel test, and c) is a mechanical fracture test.


34.3 Case study Another important problem is the study of an existing surface. One of the most obvious examples is the determination of the degree of oxidation on polymers, especially on polythene used in packaging industry. In order for printing inks to adhere to the surface, experience dictates that the surface must be more or less oxidized, i.e. show polar surface groups, such as carbonyls and carboxylates. As a typical Case Study a quality control procedure is presented here by courtesy of Tetra Pak Fibre Packaging Division; Tetra Pak Converting AB, Sweden, one of the major industrial producers of polymer/paper packaging materials.

Methods Four different tools or methods are used for measuring the degree of oxidation on polythene are used; ‘the red ink method’, a surface tension measuring device -‘the test pen’, FTIWATR and XPS. Of these it is the red ink method that is used routinely and will be outlined here. Equipment Special red ink, a hand proofer (roller device) to distribute the ink on the material to be tested, special grade of adhesive vinyl tape. Precautions Before sampling, make sure that the top of the test bench is clean. During the sampling and test performance, avoid touching the surface to be tested as surface contaminants can give false readings. The surface to be tested should also be kept free from dust and foreign particles. Performance Apply the red ink to the material by means of the proofer on the full length of the test sample (Fig. 34-4).Let the ink dry at room temperature. Carefully press a strip of tape on the sample with a spatula or similar tool and then peel off the tape from the packaging material (Fig. 34-5).The amount of ink remaining on the polymer surface measures the level of oxidation. Reading of results Judge the degree of oxidation as follows: I. Not oxidized. The surface is practically colour free. The tape is easy to tear off without resistance. Surface tension measured with test pen y I 33 dyn cm-’ 11. Evident oxidation. 15-50% of the surface covered by colour. Noticeable resistance when the tape is torn off, Surface tension 33 < y < 35 dyn cm-’. 111. Strongly oxidized. 50-100% of the surface covered by colour. Powerful resistance when the tape is tom off. Surface tension y 2 35 dyn cm-’.

34 Adhesion and surface energy

569

Fig. 34-4. The red ink is applied to the partially oxidized material.

Fig. 34-5. A strip of tape is pressed onto the sample and firmly pulled off.

References Pocius, A.V. (1991), Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition; Volume 1. Editors J.I. Kroschwitz and M Howe-Grant; John Wiley & Sons, New York; p. 445. Rusanov, A.I. (1996), Surface Science Reports 23, 173. Somorjai, G.A. (1981), "Chemistry in Two Dimensions : Surfaces", Cornell University Press, lthaca and London.



35 Surface reactivity R. Larsson

35.1 Introduction 35.1.1 Posing the problem It is generally recognized that chemical reactions on a surface, e.g. catalytic reactions, are governed by the properties of the surface. Therefore it is possible to state that a specific reaction, e.g. ammonia synthesis, proceeds faster on, say, a ruthenium surface than on a copper surface of the same area. The reaction is affected by the chemical properties of the surface and knowing these properties we can learn about the reaction. It has also been customary to ascribe to the surfaces of metal catalysts certain ‘ensembles, of atoms that are thought to be required and to have a necessary size for a certain reaction to proceed. But how easy is it to reverse the analysis? Is it possible from a knowledge of the ‘properties’ of the reaction to learn something about the surface? It is this question that we must address in this chapter. First of all we will define the ‘reaction’ rather broadly. A surface reaction is a chemical reaction at or on a surface. Hence one can include adsorption and desorption reactions. One might even extend the concept of “reaction” so as to include the physisorption processes.

35.1.2 What do we mean by surfaces? The surfaces we will discuss in this chapter are mainly those of small metallic particles frequently used as catalysts. The particles are often supported on a carrier. For example, platinum can be finely dispersed and deposited on alumina (written in shorthand as PdA1203). Problems that originate from the existence of such supported metal dispersions are: What is the specific area (m2 g-’) of the support? What is the specific area of the metal particles? What is the chemical composition of the system at a specified temperature, or at a specified reagent pressure? Indeed, from a chemist’s point of view, it is not so important to know about the area of the surface itself. Rather one should ask how many adsorption sites are present on the available surface, or how many sites for a specified reaction are available. The first of these questions (the specific area) is answered by measuring the physical adsorption of a suitable species. The term physisorption is often used to describe physical adsorption. One here has to take into consideration not only the adsorption in a monolayer, but also successive M h e r layers where the interaction between mole-

35 Surface reactivity

571

cules or atoms is more or less as it is in the condensed phase of the adsorbed species. From a knowledge of the number of molecules or atoms adsorbed in one layer and of the size of the molecule adsorbed, or rather its projected surface, one can deduce the area of the surface. The second question (the specific sites) can be approached from a knowledge of direct chemical bonding. Hence one studies the adsorption of a highly specific agent for which the stoichiometry of interaction is known. This can be, for example, hydrogen adsorbing on the surface of platinum particles forming a very distinct Pt-H bond. This type of adsorption is called chemisorption, to discriminate it from physical adsorption.

35.2 Physisorption Physisorption for the determination of specific area of a dispersed system is usually carried out using nitrogen or an inert gas as the adsorbate. The bonding of the atoms or molecules is governed by dispersion forces, or van der Waals forces. This means that the bonding is weak and not very specific. The measurements therefore have to be performed at a low temperature, e.g. at the temperature of liquid nitrogen, 77 K, so that the probe molecule will be adsorbed on all the surface sites available. The first formal description of physisorption was by Langmuir in his famous work in the 1920s.

r

n

W

Fig. 35-1 Schematic depiction of a simultaneous volumetric and gravimetric adsorption experiment. The volume (V) and pressure (p) give information about the amount of substance present (n), and hence the amount absorbed (n'). This last quantity can also be obtained fkom the difference in readings w and wo on the balance. (From Pure & Appl. Chem. 5 7 , 1985; 0 1985 IUPAC).

35.2.1 Isotherms When measurements of adsorption are made at a constant temperature and the amount of adsorbed material is plotted as a function of the partial pressure of the adsorbed species (or against its concentration, if adsorption occurs from solution) the


resulting curve is termed an isotherm. Fig. 35-1 gives a very schematic description of the basis of the experimental arrangement for adsorption measurements. The isotherm has often been expressed as an empirical analytical expression as, for instance in the empirical Freundlich equation which is of exponential form: na = kxp"" where na is the amount adsorbed, p is the pressure and k and m are parameters to be fitted to the experimental data. Langmuir (19 16) developed a dynamic model describing the equivalence between the rate of desorption and the rate of adsorption at equilibrium. Assuming a uniform surface and only one adsorption layer he found that: na/n: = kp/(l+ kp)

(35-1)

where na and p are as defined above and n is the amount needed to saturate one layer of adsorbate molecules on the surface. n a /II; expresses the fraction of the surface covered, or the coverage, which is often designated 0. This simple Langmuir isotherm is applicable to both chemisorption and physisorption, if the assumptions made are valid. Langmuir also extended his simple model to cases where the surface could be considered heterogeneous in the sense that it consisted of a set of i different adsorption sites with differing physical properties, reflected in differing values of the parameter k: (35-2) The adsorption isotherm that has been most widely used, and is also quite general, is that of Brunauer, Emmett and Teller (1938). The so-called BET method treats multilayer adsorption and is based on views similar to those used in the deduction of Langmuir's formula. Hence the rate of evaporation from the first layer is considered to be equal to the rate of adsorption to the solid adsorbing surface. Similarly the rate of evaporation from the second layer is equated to the rate of adsorption onto the first layer, and so forth for as many layers as is physically realistic. It is further assumed that the adsorption constant of the first interaction contains the heat of adsorption whereas the subsequent adsorption constant contains the heat of liquefaction of the adsorbed material. The formula that is thus deduced can be written as: (35-3) Here na is the amount adsorbed, n ;is the monolayer capacity, the amount of adsorbate needed to cover the surface with a complete monolayer of molecules, p/po is the relative pressure and po is the saturation pressure of the substance that is adsorbed at the temperature of adsorption measurement.


573

It can be seen from eq. 35-3 that there is a linear relationship between p/n" (p"-p) and p/p". Empirically this linearity holds true only for a limited range of p/po, between about 0.05 and 0.30. This will in many cases allow the determination of the monolayer capacity n ;. One should bear in mind, though, that the most accurate value of n r, is obtained for 0 = 1 . Sometimes a negative intercept results and this indicates that the BET technique is not applicable. In such cases it might be worth trying the simpler Langmuir approach. A practical guide to the use of the BET equation and to the various forms of isotherms is given by Gregg and Sing (1 982). Several other models for adsorption have been proposed and an up-to-date description of this field, especially for heterogeneous surfaces, has recently been published (Rudzinski and Everett, 1992).

35.2.2 Determination of surface area Given that the BET equation is applicable, it is possible to obtain n ; and use this quantity to determine the area, A,, of the surface exposed to the adsorbed substance. The calculation is based on a knowledge of the average area (molecular cross-sectional area) of the adsorbate molecule. If this is called am it holds that: A, =nkLam

(35-4)

where L is the Avogadro constant. Consequently, one can calculate the specific surface area by dividing A, by the mass of the sample. For the widely used adsorbent nitrogen the value a,,, (N2) = 0.162 nm2 at 77 K, is commonly used. However, this figure may vary somewhat from one surface to another. An authoritative prescription of surface area determinations has been given by the International Union of Pure and Applied Chemistry (IUPAC, 1985).

35.3 Titration techniques The most obvious application of chemical reactions for the characterization of surfaces is that of titration of the adsorbed species with another agent. The master model of titration methods is no doubt the use of the catalytic reaction between H2 and 0 2 . It was first introduced by Benson and Boudart (1965) in a study on the availability of platinum atoms (adsorption sites) on a platinum catalyst supported on alumina. They measured how much hydrogen was consumed when reacting with the platinum surface that had been fully oxidized to form a Pt 0 monolayer. This amount of hydrogen would give an indication of how much oxygen was present and thus, according to the assumptions made, how many platinum atoms were available at the surface. The authors assumed the following stoichiometry of the reaction: PtO + 312 H2(g) + PtH(surface)+ H~O(support)

(35-5)


35.3.1 Case study 1 This work is of methodological importance and we will give some detailed results. In order to ascertain the Pt - 0 stoichiometry Benson and Boudart first studied an unsupported platinum black preparation. After cleaning the surface by evacuation at room temperature for 30 min, followed by evacuation at 125 OC for the same time, the surface was treated with excess hydrogen at room temperature for 12 h and then at 125 OC for 30 min. After subsequent outgassing, the chemisorption of oxygen on the surface was measured at room temperature. The flat portion of the isotherm (between 100 and 650 Torr) indicated that an amount corresponding to 296 patoms O/g had been adsorbed. Following a similar outgassing the surface area was measured with the BET method using nitrogen. The result was 13.4 m2/g. Combining this figure with a value of 8.4 A' Pf' atom site (an average from diffraction studies on the 100, 1 10 and 11 1 planes) was obtained a surface site density of 266 pmol of sites/g. The O/Pt ratio was thus found to be 1.1. The small deviation from unity was assumed to be associated with the uncertainty in forming the average of the exposed crystal planes. Following this, Benson and Boudart ibid. studied a series of alumina-supported Pt catalysts, where the loading was varied from 0.05% Pt up to 3.5% Pt. They used the titration technique described above and, parallel to that, chemisorption of carbon monoxide. They showed that the titration technique was superior for small loadings of platinum because the correction for adsorption on the support (measured on a pure support sample) was very small for hydrogen'compared with the adsorption of carbon monoxide. Thus for small loadings the hydrogen titration method showed a proportionality between the consumed hydrogen and the amount of Pt present (1 9.5 pmol Hz/g and 0.35% Pt; 5.4 pmole Hz/g and 0.10% Pt ) indicating, inter alia, that the dispersion was almost 100%. Of even greater interest was their observation that for sufficiently high loadings (where also the chemisorption data were sufficiently accurate) the ratio of net consumed HZ to net consumed CO was constant, indicating that the method gave results which were consistent with each other. Using eq. 35-5, the stoichiometry in the hydrogen-oxygen titration can thus be summarized in the ratios 1:1:3, meaning one platinum site reacts with one oxygen which reacts with a total of 3 hydrogens. This relation was later disputed, but a thorough investigation by Menon and co-workers (Prasad ef al., 1978) confirmed that this was the actual stoichiometry.

35.3.2 Case study 2 These authors showed that the reason for previous ambiguity could be traced back to the instability of the surface of freshly prepared, highly dispersed, metal particles. They strongly advised that the chemisorption measurement, in this case the adsorption of hydrogen on a pure platinum surface (or the corresponding adsorption of oxygen), should not be performed unless a series of repeated oxygen - hydrogen titrations resulted in constant values of the respective consumption of gases (the 'titers'). In this


575

way it was found that the ratios of the resulting data for hydrogen chemisorption: oxygen chemisorption : hydrogen titration equalled 1:1:3. The technique used by Prasad et al. (1978) was a gas chromatographic pulse technique suggested by Free1 (1972) which is now frequently used. The principle is illustrated in Fig. 35-2. Pulses of the reacting gas are swept past the surface by a carrier gas and recorded by a detector. For those pulses that are consumed by the surface, no signals will result. When all sites have been reacted the gas emerges at the detector unchanged. Total uptake is the difference between the number of pulses injected into the reactor and the number reaching the detector.

CATALYST BED

’

Fig. 35-2. Pulse Chemisorption. The reactant is adsorbed in the first pulses and the detector gives a zero response. Eventually, pulses of the reactive gas arrive at the detector unchanged, indicating that all sites have been saturated. The total uptake is the difference between the number of pulses injected into the carrier gas and the number registered by the detector. (Reproduced with permission fkom Micromeritics Instrument Corporation.)

35.3.2.1 Other titration applications It is not necessary of course to use hydrogen and/or oxygen in order to perform a surface titration. The requirement is that a reagent is available that reacts with the species on the surface, the amount of which one wants to determine, Some examples of this will be given here. The first example concerns the distribution of hydrogen on a supported catalyst under such circumstances that ‘hydrogen spillover’ is expected. By this term is meant a storage of hydrogen in the support, promoted by the metal particles present on the support. The presence of the metal particles in direct physical contact with the support is a prerequisite for the spillover effect to be observed. The spillover is reversible so that the stored hydrogen can migrate back to the metal particle and partake in chemical reactions on the metal surface. The situation is depicted schematically in Fig. 35-3.


Fig. 35-3.Hydrogen spillover. Hydrogen atoms set free at the metal surface are spread over the surface of the support. The process is reversible. (From J. Org. Chem. 46, 1981,Q Am. Chem. SOC.).

35.3.3 Case study 3 In order to study the presence of hydrogen, a reagent that reacts with hydrogen must be chosen. In an investigation by Augustine and Warner (1981) 1-butene was used. It reacts with hydrogen on the metal surface to form butane C4Hs + H2 + C4Hio

(35-6)

15

Y

" l i

a 3

w z

Y

5

50

a Y CL

w a

2 L

w

4 z 2 m

'2

"

25

a w

"%

Fig. 4. 1 -butene titration on 5 mg of Pt/CPG (controlled pore glass). The %Do '0 points indicate the percentage of butane formed in every pulse. The pulse interval was 3.5 min. (After J. 300 Org. G e m . 46, 1981, Q Am. Chem. SOC.)

%

. * 100

200 MICROLlTEilS OF 1-EUTENE PbLSED


577

The technique was to give 1-butene in pulses to a carrier gas that passed the catalyst bed. The product was then measured by gas chromatography. Before the addition of 1-butene, the catalyst system Pt/support was treated for a considerable time with hydrogen. This procedure would result in the spillover of hydrogen to the support. The question posed was, how much. Fig. 35-4 presents a typical result. For every pulse the fraction of butane that was formed from the butane introduced is plotted. The amount of 1-butene consumed is determined as the area under the curve. For the specific example, 5 mg of a 4.9% Pt/support catalyst, this amounted to at least 140 pL butane and consequently 140 pL hydrogen or at least 6.2 pmol H2. The platinum in the 5 mg sample, however, corresponded to 1.2 pmol Pt. Even if the platinum was 100% dispersed, i.e. every atom was exposed, the ratio of adsorbed HZ to platinum could not be less than 5:l. It is not chemically possible that 1 platinum atom binds 10 H atoms. Therefore the result can be interpreted as a measure of the degree of spillover of hydrogen from the metal particle to the total surface of the supporting material. This example of the titration technique illustrates how a surface can be characterized by a specific chemical reaction.

35.4 Chemisorption In principle, the direct measurement of chemisorption should be a convenient method for the characterization of surfaces, especially the determination of the specific area of a metal dispersed on a supporting material. Those adsorbents that are most commonly used are H2, 02 or CO. These molecules react strongly with many metals and adsorb much less strongly on the materials used as support. Hence, with only a small correction, the amount of gas adsorbed could be translated to a surface area or to a number of sites, presuming that one knows the area that one atom or one complete molecule takes up on the surface, or how many atoms are adsorbed per metal site. In contrast to the case for physisorption, where a constant ratio of adsorbed molecules to the occupied area is determined by the requirements of close packing, the ratio between surface sites and adsorbed atoms (molecules) is not necessarily known. Only if different probes give the same results can one be satisfied that the assumptions are good. Of course, if the method is used routinely on very similar samples a comparison of relatively high accuracy can be achieved. Let us use as an example the adsorption of hydrogen on metals. Several possible arrangements are depicted in Fig. 35-5.If there were only one possibility, e.g. a 1 to 1 assignment, i.e. arrangement a), there would be an easy choice. One could just measure from the decrease of pressure of a defined volume how many hydrogen molecules had been adsorbed and hence obtain the available number of sites as twice that number. But suppose, instead, that the arrangement were like b) in Fig. 35-5 with two atoms of hydrogen at each site (2:l). Then the calculated number of sites would be only half the previous number calculated. Or suppose it to be as in case c) in Fig. 35-5. Where, for very low coverage at least, there is only one hydrogen atom for every three metal atoms (1:3). An average number of a 1:l ratio is often taken. However, a possible way of


solving this special problem would be to apply IR spectroscopy (Szilagyi, 1988). Using this method it can be seen that at least two bands are formed in the spectral range where the a) and b) species would absorb. This might mean that case b) is actually at hand, the two bands corresponding to the symmetric and asymmetric M-H stretchings. However, this is not certain. There might equally well be two slightly different sites of type a) with a slight difference of the force constant governing the vibration and thus yielding the two bands of slightly different frequency.

H

H C

Fig. 35-5. Schematic view of three different possibilities for hydrogen-metal atom interaction.

If one knew the detailed surface geometry for any crystal plane from diffraction measurements the number of adsorbed molecules could be translated into a surface area. Another alternative is to use a non-supported metal dispersion (e.g., platinum black) and perform a BET analysis of the surface and thereafter perform the chemisorption measurement. This gives an empirical calibration. The use of chemisorption measurements has been illustrated above in connection with the H2/02 titrations of Benson and Boudart (1 965). As a complementary method, these authors used the chemisorption of carbon monoxide and we have already said that they obtained a quite constant ratio of titrated Hz : chemisorbed CO. Actually the numerical value of this ratio was 2.0kO. 1. What does this mean? If one sticks to the view that 1 Pt is binding 1 H and that consequently 3 H are used for every Pt in the oxygen titration reaction, it follows that only 0.75 CO is bonded to every Pt site. This situation can be explained if not every CO is bonded in a 1:1 ratio to the available platinum atoms, but a considerable proportion is forming bridges between two Pt atoms or capping three Pt atoms. This will result in a lower ratio than 1:1 for CO adsorbed on the metal surface. This effect can also be followed by IR spectroscopy.


579

35.5 Temperature-programmed desorption A technique commonly used to characterize the presence of species on surfaces is TPD or temperature programmed desorption. As the name suggests it is the opposite to adsorption, the desorption of the substances adsorbed on the surface investigated. The principle is illustrated in Fig. 35-6. The temperature of the sample is gradually increased, often linearly with time. An inert gas stream is passed through the sample and carries with it those molecules that have been desorbed at the temperature prevailing at that time. The effluent is analysed by a detector and the reading gives an indication of how much desorbate is contained in the stream at that temperature. Hence it is possible to distinguish between loosely bound species, desorbing at a low temperature, and more tightly bound species that require a higher temperature to desorb.

Fig. 35-6. TPD of a catalyst containing three differently active metals (or containing three different species adsorbed to one and the same metal with different strength). For each temperature the desorbed species is carried to the detector by the flowing gas. (Reproduced with permission from Micromeritics Instrument Corporation.)

35.5.1 Case study 4 As a case study let us consider the careful investigations (Frennet and Wells, 1985) of hydrogen desorption on a special catalyst system denoted EUROPT-1. This is a 6.3 Pt/SiOz that has been characterized by a great many methods and by many laboratories in Europe. The reports on these investigations have been published (Bond and Wells, 1985) and can be used as a source of information on most methods of characterization of surfaces, especially of supported metal particles.


Fig. 35-7 illustrates typical results of TPD of hydrogen from the surface of this catalyst system (Frennet and Wells, 1985). The left hand side, part (a), represents results from one laboratory; the right hand side, part (b), gives results under somewhat differing conditions from another laboratory. Consider first case (a): Three curves are recorded. They represent from left to right, initial loading of hydrogen 200, 90 and 35 pmol H2/g. One clearly distinguishes three components in the desorptogram for the highest loading at about 200, 400 and 550 K, designated in the figure as peaks A, B and C. Peaks B and C are distinguishable also in the other two desorptograms. Taking case (b): The hydrogen loading was such that it was about 110 pmol H2/g. It is therefore reasonable that the appearance of the desorptogram is between the high and medium loaded samples of case (a), although more closely related to the medium case. Both peak B and peak C are observed (B' and C', respectively). The measurement was extended to temperatures > 800 K in this case and a fourth peak, called D, is observed at about 770 K. It should be noted that the first mentioned laboratory reports that for medium loaded samples 14 out of 90 pmol H2/g and 7 out of 110 pmol H2/g, respectively, remained on the samples at a temperature of 700 K. The second laboratory reports peak D to represent about 10% of the original load, indicating a good quantitative agreement.

Temperature / K

Fig. 35-7. TPD of hydrogen adsorbed on the catalyst EUROPT- 1, measured by two different laboratories (a) and (b) , respectively. Four intrinsically different species are characterized, A, B(B'), C(C'), and D. A is the most weakly bound species and D is the most strongly bound. (From Applied Catalysis, 18, 1985; 0 Elsevier Science Publishers, B.V.)

These four peaks must be related to four different types of hydrogen bonding on the surface. The identification is, however, not straightfoward without complementary knowledge. The interpretations of the authors are given here for the sake of informa-


581

tion and to illustrate the complexity that might prevail on a compounded surface system like this one. The species most strongly bound to the system (peak D) is regarded as spillover hydrogen. The next most strongly bonded species (peak C ) is considered to be associated with hydrogen set free by the reversal of a reaction that had taken place at the saturation of the system with hydrogen : 2 (Si-0-Pt) + HZ-+ 2 (Si-OH) + Pt - Pt

(35-7)

Peaks A and B are thought to represent hydrogen that is truly chemisorbed on the platinum surface. Peak B, that prevails at approximately room temperature, can be used to determine the number of available sites. Under the assumption that there is a 1:1 stoichiometry it was calculated that 65% of the platinum atoms were available. This is a measure of the dispersion that agreed well with what could be determined for the same preparation (EUROPT-1) by electron microscopy. Hence one can fairly safely assume that peak B corresponds to Pt-H on the metal surface, i.e. case a) of Fig. 35-5. Peak A then, that is the most loosely bound species, might relate to case b) of Fig. 35-5; in other words, there are some sites on the surface where two hydrogen atoms can be weakly bound.

35.5.2 Case study 5 Another application of temperature-programmed desorption is given by investigations on the potassium doping of iron surfaces for ammonia synthesis. We reproduce here some recent results (Fastrup, 1994).

"1

0.03-

-25 5 E

0.025 0.025-

i

0.02-

0.0150.010.005 0.005-

1

m 6 gbo &

0sbo0 6 s50 5 0 760 7

7 5 0 8800 0 0 8 850 50 750

temperature [I

b)

~

Fig. 40-13. SEM micrographs of diamond coatings worn against 6-pm diamond abrasives. a) Stress-free coating. b) Coating with 2 GPa biaxial compressive stress.

Fig. 40-14. Topographical AFM images and surface-profile cross-sections of the coatings in Fig. 40-1 5 . a) Stress-free coating. b) Coating with 2 GPa biaxial compressive stress.

40 Tribosurface properties

637

Recommended techniques for further studies XRD is recommended for a more accurate determination of the magnitude of the residual stress. However, MRS has the advantage of a considerably better spatial resolution. It is possible, for example, to measure the variations in residual stress of diamond films around a sharp edge of a metal cutting tool (Alahelisten, 1994).

40.5.3 EP additives in lubricants -

Characterization techniques used: SEM, EDS, AES, GDOES.

Objective To increase the understanding of the action of EP additives in lubricated sliding contact between steel surfaces. Test procedure Reciprocated sliding tests (50 Hz, 20 mm sliding distance) with cylinder-on-disk geometry were performed with two model oils based on a poly-alpha-olefine of 8 cSt viscosity and containing EP additives; 2% TNPS (di-tert-nonyl pentasulphide) and 0.7% ZDDT (zinc dialkyl dithio phosphate), respectively. The material in both test surfaces was ball-bearing steel (AISI 52100), heat-treated to 820 and 240 HV for the disk and cylinder, respectively. During the test, the normal load was continuously increased until the friction suddenly rose from about 0.1 to 0.5, indicating the onset of severe adhesive wear (seizure). Analytical procedure The topography and chemical composition of the test surfaces were analysed before and just after the onset of seizure. The presence of a solid film of chemical reaction

638 Part 7: Tribology

products was revealed by SEM for both types of additive. It was concluded that the onset of seizure is associated with a mechanical disruption of the film, see Fig. 40-15a. It was possible to obtain EDS spectra of the reaction films, see Fig. 40-15b. However, it is difficult to deduce the true chemical composition since the films are highly inhomogeneous in thickness and composition, and the signals from the light elements (0 and C ) cannot be quantified. In addition to the information from SEM, EDS confirms that the surface film is composed of elements from the oil additives and that the thickness is of the order of 0.1 mm. Semi-quantitative AES elemental depth profiles were obtained through the reaction films by argon sputtering, see Fig. 40-16. They clearly reveal the distribution of the elements taking part in the formation of the chemical film. GDOES depth profiles, showing the same type of composition averaged over an area of lox 10 mm2, are seen in Fig. 40-17. Information from AES and GDOES depth profiles of the types given in Figs. 40-16 and 40- 17, combined with detailed studies of the corresponding Auger peaks gave substantial information on the chemical constituents forming the reaction layers (Johansson, 1993). The two techniques are complementary in that AES is a microscopic, highresolution method, whereas GDOES gives information averaged over a relatively large area.

?

1

2

3

4

5 (kEV)

Fig. 40-15. a) Disruption of a solid reaction film on the cylinder during seizure with the ZDDP oil. b) EDS spectrum of the same surface. (Windowless X Ray detector, 10 keV electron beam.)

Recommended techniquesfor further studies SIMS and XPS could possibly add to information about chemical surface reactions.


639

100

80

70 b-

z

W u U W

I

a u H

T

a

I-

SPUTTERING

TIME (MIN.)

W z

I-

0

a W a

u H

a T a +

1

b)

OJ 0

4

8

SPUTTERING

16

12

TIME

(HIN.)

Fig. 40-16. Examples of AES depth profiles obtained at 10 keV and 2 prn spot size. a) Disc surface before seizure, ZDDP oil. b) Cylinder after seizure, TNPS oil.

640 Part 7: Tribology 1.0. .95.

.9&.

Full scale c.orresponds io ( J I ' ~ )

N

0

ZDDP

10

loo

100

i

L

C 3 0

5

-B0

a

.d

b)

Depth (nonometer)

Fig. 40-17. Examples of GDOES elemental depth profiles of the disc surface before seizure. a) ZDDP oil, b) TNPS oil.


641

40.5.4 Performance of ceramic materials in face seals Characterization techniques used: LOM, SEM, EDS, XRD, AES, TEM, XTEM and SAED. Application Hard, planar, ring-shaped disks are often used in rotating machinery as seals, e.g. to prevent leakage of water from the environment into vital parts of, for example, electric motors operating submersible pumps. To suppress abrasive wear from hard constituents in the surrounding medium, these mechanical face seals have to be made of relatively hard materials such as cemented carbides or ceramics; the sealing function requires that the flatness and smoothness is maintained to within less than 1 pm. The most common ceramics used in face seals are based on polycrystalline silicon carbide (Sic) or alumina (A1203).Since the face seal is a crucial component, there is a pressing need for improved materials for this application. Objective To evaluate a silicon-carbide-whiskers-reinforced (25 ~01%)alumina (WRA) by simulating a face seal action where two self-mated seal rings are rotated against each other. Analytical procedure The test was performed in air at room temperature (22 "C and 50 % RH), at a contact pressure of 0.05 MPa and a rotational speed of 1500 rpm (3.8 m s-'). Low magnification LOM of the worn WRA surface revealed a change in colour from bright green to dark greeddark grey. This indicates a change in the chemical composition of the worn surface. Also, the worn surface was rougher than the unworn one. However, it was not possible to use LOM for more detailed studies of the worn topography due to its limited depth of focus. SEM was used to study the worn surface in more detail. A large fraction of the surface area was found to be covered with a discontinuous tribofilm, 2-4 pm in thickness. The film was sheared in the sliding direction and the ductile appearance is reminiscent of adhesively worn metallic surfaces. Furthermore, cracks spaced at 50-100 pm had formed perpendicular to the sliding direction. Polished crosssections revealed the crack depth to be 5-10 pm, see Fig. 40-18. In order to study the tribofilms in more detail cross-sectional TEM was used. The tribofilm was found to have a relatively homogeneous and dense microstructure with relatively few pores and microcracks, see Fig. 40-19a. It consisted of compacted wear particles 20-200 nm in diameter, held together by an amorphous matrix. An analysis of the tribofilm by high resolution electron microscopy revealed the existence of three types of particle: mullite (A16Si20,3),corundum (a-Al203) and p-Sic, see Fig. 40-19b. The interface to the underlying WRA substrate is very sharp and without interfacial cracks or pores, see Fig. 40-20. The tendency of the whiskers in the wear surface to disintegrate by microfracture and gradual dissolution is evident in Fig. 40-21. A change in chemical compo-


sition was verified by EDS and AES. Large amounts of 0, Si and A1 were found in the tribofilm. By contrast, the C content was relatively low. The chemical shifts of the lower-energy peaks of A1 and Si in the Auger spectra indicate the presence of alumina, silicon dioxide (Si02) and S i c in small amounts. This is in good agreement with the results from the XRD and SAED analyses.

Fig. 40-18. Scanning electron micrograph of whiskers-reinforced alumina (WRA) exposed to sliding wear. A discontinuous tribofilm covers a large part of the surface, and superficial cracks extend to a depth of 5-10 pm.

Fig. 40-19. Planar transmission electron micrograph showing the structure of the tribofilm on the WRA.Ring pattern of the selected area indicates fine crystalline particles of mullite (Al6Si2OI3)and corundum (a-A1203) in an amorphous matrix.

Fig. 40-20. High resolution transmission electron micrograph of a crystalline corundum (a-A1203) particle in an amorphous matrix of the tribofilm on the WRA. The planar spacing of 0.348 nm between the (012) planes of the hexagonal structure is indicated.

Fig. 40-2 1. Cross-sectional transmission electron micrograph revealing a sharp interface between the tribofilm and the underlying bulk material. No voids or interfacial cracks are visible. The increase in the size of the particles towards the interface can also be noted.


643

Fig. 40-22. Planar transmission electron micrograph of the worn surface showing fracture (a) and gradual dissolution, at the arrow, (b) silicon carbide whiskers in the WRA.

Fig. 40-23. Auger electron spectrum of worn WRA. The chernica1,shifts of the A1 and Si peaks indicate the presence of A1203,Si02 and Sic.

Recommended techniquesfor further studies SIMS could possibly detect small amounts of elements in the sliding surfaces.


References Alahelisten A. (1994), Mechanical and Tribological Properties of Hot Flame Deposited Diamond Coatings, Uppsala University Dissertation, ISBN 91-554-3424-X, Paper 7. Blau P. (ed.) (1 992), ASM Handbook Vol 18, ASM International. Booser E.R. (ed.) (1983), CRC Handbook of Lubrication, Vols. 1-111, CRC Press. Bowden F.P., Tabor D. (1950), The Friction and Lubrication of Solids, Oxford Univ. Press, Oxford. Godfrey D. (1980), Wear Control Handbook, Peterson M.B., Winer W. (eds.), ASME, New York, 283. Hogmark S., Jacobson S. (1991), STLE Lubrication Engineering, 48,401. Hogmark S., Jacobson S., Vingsbo 0. (19801, Wear Control Handbook, Peterson M.B., Winer W. (eds.), ASME, New York, 176. Johansson E. ( 1 993), Surface Modification in Tribology, Uppsala University Dissertation, ISBN 91-5543061-9, Paper 5. Peterson M.B., Winer W.O. (eds.) (1980), Wear Control Handbook, ASME, New York.



41 Wear measurements using thin-layer activation J. Asher

41.1 Introduction Wear of components in a tribological system is conventionally monitored by weighing or by measuring the corresponding change in dimensions. This normally requires the disassembly of the system. On-line measurements make it possible to separate clearly the influence of dynamic variables, such as the effects of cold start in automobile engines, on wear rate, or the clear isolation of the effects of different loads and speeds. Additionally, it is particularly desirable to use a non-intrusive measurement technique that does not physically disturb the operating system or affect significantly the material properties of its components. The use of radioactive tracers produced within the material to be tested allows the removal of the material produced by wear to be monitored as it passes through the operating system, for example, the lubricant in an engine. These measurements can be made on-line, non-intrusively and without significantly affecting the wear properties of the material. Because of the very high sensitivity that can be achieved in radionuclide measurements, correspondingly high sensitivity can be achieved in the measurement of wear. Two methods are used to introduce radionuclide tracers into engineering components: neutron irradiation in a nuclear reactor and high-energy ion beam irradiation using an accelerator. In both cases, the sensitivity to wear is determined primarily by the concentration of radioactivity (normally expressed in units of Bequerels (Bq)) per unit mass of material within the wearing surface. The comparative benefits and disadvantages of these two methods of activation are summarized below: Neutron irradiation (by exposure to the neutron flux in a nuclear reactor) produces a uniform distribution of activity throughout the volume of small components and is relatively low in cost. Inevitably, the whole volume of the component has to be made radioactive to the concentration required to achieve sufficient wear sensitivity, giving a high total radioactivity at which significant handling, regulatory licensing and disposal problems arise and extensive radiation shielding is required to contain the hazard. Neutron activation, where it is still used, is therefore normally carried out only on small components such as piston rings, and not on major components such as engine blocks. Thin layer activation (using the ion beam from an accelerator) limits the production of the radiotracer to a defined area and close to the surface within the penetration depth of the beam, which typically reaches only a few tens to hundreds of micrometres. Total component activity levels are therefore modest, typically lower than of those


required by neutron activation to give the same concentration of tracer in the wearing material. Handling and safety procedures are therefore much simpler and licensing requirements are much easier to fulfil. The cost of ion-beam activation is normally higher than neutron irradiation, but that is usually more than compensated for by the lower costs arising from easier handling and reduced safety and licensing requirements. Neutron activation is now used only by a few developers for on-line wear measurements (see, for example, Perrin et d., 1995). TLA provides the engineer with a tool to make on-line measurements of material loss with a sensitivity that allows critical parameters affecting wear to be assessed in a period as short as a few hours. Typical sensitivity to material loss is in the range 1 nm to 1 pm, depending on the way in which the method is implemented and the nature of the measurements. The TLA method was developed in the USSR (Konstantinov and Krasnov, 1971), Germany (GervC, 1972) and the UK (Conlon, 1974). Both the technique and its applications have undergone considerable development since then (see, for example, Conlon, 1985) This chapter describes how TLA works, its advantages and limitations. Practical examples illustrate the range of its applications in the measurement of wear.

41.2 Principles of TLA In simple terms, TLA uses a radioactive tracer, generated to a specified depth close to the surface of the test component, as a marker with which the transport of material from the surface can be followed. Careful monitoring of the gamma radiation arising from the radioactivity of the test component and of any fluid system in which it operates provides a means of detecting and quantifying the loss of material. Because gamma radiation is penetrating, measurements can be made on-line to allow wear from the test component within the operating system to be quantified as it occurs.

41.2.1 The activation process Components are activated using a beam of high-energy ions from an accelerator, such as a Tandem Van de Graaff or a cyclotron. The energy of the beam has to be higher than the threshold energy for the nuclear reaction that produces the radioactive nuclide(s) required and is typically in the range 5-20 MeV. The nature of the nuclides produced depends on the material composition and the beam type. Typical examples are given in Table 41-1. A high-energy ion beam loses energy in a well-characterized way as it penetrates solid material. As it slows down, there is a small probability of a collision with the nucleus of an atom in the component, leading to a nuclear reaction that generates a radioactive nuclide. When the energy of the ion drops below the reaction threshold, no further production of radionuclides occws. The depth of the active layer is therefore restricted to a thin layer close to the surface of the material (Fig. 41-1). In general, the density of activity generated also diminishes with depth, but

41 Wear measurements using thin-layer activation

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in a well-defined and predictable way, between the surface and the 'cut-off depth. The higher the energy of the incident ion beam, the further it penetrates and the greater the depth of activation. Table 41-1. Elements commonly found in test materials, the nuclides that can be generated by TLA and their half-lives. The types of ion beam used to generate them are also listed. Element Iron Copper Titanium Nickel Chromium Lead

Nuclide(s) 'ko

Energy of main gamma ray (keV)

a47 57c0 121 1115 65Zn 984 48V 5 6 c ~ + 5 8 c ~847 + 8 1 I "Cr 320 206~i 803

* lonenergy

Half-life (days) 78 27 1 244 16 78 I 7 0 28 6.2

Ion beam Protons Deuterons Protons Protons Deuterons Protons Protons

1

,

deoth

Ion beam

Component surface

\

Activated layer

Fig. 41-1. Schematic diagram illustrating the thin-layer activation process. A beam of high energy ions enters the surface and loses energy as it penetrates into the material. An activated layer is formed by nuclear reactions with the substrate material, extending from the surface to the depth at which the beam energy drops below the reaction threshold energy.


The incident energy provides the primary means of controlling the activation depth.

A typical depth for the production of T o in steel with a 10 MeV proton beam is about

100 pm. If the angle, 0, between the ion beam and the surface is less than 90°, the depth of the layer into the surface, d,, will be reduced by a geometric factor: d, = d, sine

(41-1)

where d, is the depth at normal incidence (Fig. 41-2). A 10 MeV proton beam striking a steel component at an angle of 30" to the surface will generate a 50 pm layer of TO, equal in activity, but half the depth of that generated at 90". Thus, varying the angle of incidence provides a secondary control of the depth of activation. The component to be activated is set up in a well-defined position so that the beam strikes the surface at the required location. An ion beam can be focused to a spot as small as 1 mm* or can be scanned to cover an area of several cm'. The component itself can be scanned mechanically in the beam by translation or rotation, for further extension of the activated area. For example, a continuous band of activation can be generated around the inside of an engine cylinder, or the entire rubbing surface of a piston ring can be activated to a uniform depth.

Fig. 41-2. Reduction in the depth of activation as a result of the angle, 0, between the beam and the surface. The activation depth at normal incidence, d,, is reduced to a value d, = d"sin0.

41.2.2 Range of materials A wide range of materials used in engineering can be activated to generate nuclides suitable for TLA measurements of wear. These include all ferrous metals and most alloys and metals commonly used in bearings. Table 41- 1 shows some elements typical of these materials, the nuclides that can be generated and the ion beam used to generate them. The half-life of each nuclide, that is, the time taken for the activity of the nuclide to decrease to half its initial value, is also tabulated.

4 1 Wear measurements using thin-layer activation

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41.2.3 Working life of a TLA component The useful working life of a TLA component is governed by the half-life of the relevant nuclide. Typically, measurements can be extended to 3-6 half-lives, depending on details of the measurement system and the degree of sensitivity required. For examwill provide useful measurements for ple, a steel component, activated to produce TO, 6-12 months. For most measurements, it is not practical to operate with half-lives of much less than five days without very tight scheduling of the activation, delivery, installation and test operation. Activated area

I

Direct measurement

Indirect measurement

Fig. 41-3. The two methods of TLA measurement used to quantify wear. Direct measurement is based on the determination of the loss of activity from the test component due to wear; indirect measurement detects the increase in activity in the wear debris in the fluid environment.


41.2.4 Methods of measurement There are two principal methods of measurement, which offer different features and sensitivities, namely direct measurement of the decrease in the activity of the component and indirect measurement of the activity of the fluid circulating around the component (Fig. 41-3). In both cases, sodium iodide (NaI) gamma detectors are normally used to detect the gamma radiation emitted by the active component or active wear debris. These detectors are capable of operating in the temperature range of about 10-50 "C, but must be temperature-stabilized to prevent gain drifts which could give rise to systematic errors in the count rate. The direct measurement method is based on the use of a gamma detector to measure directly the activity of the component in the operating engine. Any decrease in activity, beyond that expected due to the half-life of the nuclide, can be attributed to material loss. A simple reference curve is used to relate the level of activity reduced by wear as a percentage of the initial activity, corrected for natural decay, to the amount of material lost. A typical example is shown in Fig. 41-4. Indirect measurements of the activity of wear debris, can for example, be made in the lubrication oil circuit or filter of an automobile engine. In this case, the activity level above natural background is used to determine the amount of wear debris present in the measured sample volume. A calibration is required to relate the measured signal to the amount of wear. It is an obvious requirement for both types of measurement that worn material must be removed from the vicinity of the test component, otherwise a change in activity will not be recorded. The direct measurement method works only with TLA. The change in total activity of a neutron-activated component is normally too small to detect; for 100 80

w-

0

s

20

0

0

20

40

60

80

100

120

140

160

Depth of wear (micrometers) Fig. 41-4. Curve relating the cumulative reduction in total activity, due to material loss, to the depth of wear in pm.The curve is for 56C0in steel, generated at a proton energy of I0 MeV at normal incidence.


65 1

example, if a 2-mm-thick piston ring loses 1pm by wear, then the change in total activity is only 0.005%. The indirect measurement method, however, can be used to achieve similar wear sensitivity with both activation routes.

41.2.5 Sensitivity Since the depth of the active layer is limited normally to a few tens of micrometres, the sensitivity of the TLA technique, which depends for its operation on detecting changes in activity due to wear, is usually in the sub-micrometre range. The inherent sensitivity of the direct measurement method can be increased by decreasing the depth of the activated layer, through a combination of reducing the beam energy to a value nearer the reaction threshold energy and by reducing the angle between the beam and the surface to concentrate the activity nearer the surface. In practice, both of these approaches have limits. By lowering the beam energy, both the activity per micrometre and the total activity are reduced for a given beam exposure, so that processing times are increased. If too shallow an angle of incidence is used, nonuniformity of the surface becomes more important and the alignment of the beam with the required location on the surface becomes more critical and more difficult to achieve. It is seldom practical to work with an angle of less than 10" to the surface,

41.2.6 Factors affecting sensitivity The sensitivity of direct measurements depends on the precision with which the gamma radiation can be measured reproducibly. The response of a gamma detector is inversely proportional to the square of the distance between the source and the detector. Therefore the gamma detector must be positively located at a well-defined, fixed distance from the source if reproducible measurements are required. Also, intervening material will absorb gamma radiation; dense materials of high atomic number absorb much more strongly than low-density materials. Any dynamic changes in intervening material during measurements will result in changes in absorption and therefore spurious changes in the signals. With typical detector configurations under working conditions it is difficult to achieve a precision of much better than 0.5%. For a layer of about 100 pm in depth, a sensitivity of about 0.5% of the depth, or 0.5 pm, is about the best that can be achieved routinely in a typical rig environment. A series of measurements will show a statistical scatter at about the level of 0.5%, which limits the point-to-point accuracy that can be seen, although longer-term trends can be extracted from an extended series of data collected under constant operating conditions. Systematic errors will arise from temperature changes affecting the gain of the detector and from vibration or temperature-related expansion of mounting arms for the detector, altering the distance between source and detector during a set of measurements. Care in the design of mounting and cooling arrangements is required to minimize these effects.


With indirect, debris-based measurements, the factors affecting sensitivity are quite different from those affecting direct measurements. The most important factors are the concentration of activity (Bq pm-’) within the wearing surface and the signal-to-background ratio which determines the limits of the measurement of very low levels of activity. The main source of background that has to be suppressed is that penetrating to the detector directly from the activated component. This can be reduced by a combination of distance and intervening shielding material, normally lead, several centimetres thick. In engine-based measurements, it is normally necessary to bring the oil flow loop outside the engine to achieve sufficient separation and to introduce effective intervening shielding material. In all working environments there is a background of gamma radiation from cosmic rays, natural radioactivity in the ground, building materials and even that occurring in the bodies of the human operators. The contribution from these can be reduced by enclosing the sampled volume of fluid and the detector within a shielded enclosure. The level of signal can be increased by increasing the volume of fluid being measured so that the detector is exposed to a greater fraction of the total debris, within the constraints of the system being tested. The signal can also be increased by increasing the activity density in the material being tested, although this is normally limited by a combination of cost and the general requirement to keep activity levels as low as reasonably practicable. Systematic error arises with indirect measurements if there is a variation in the collection and retention of debris in the fluid, for example, if wear particles are trapped in an oil gallery, or if the size of wear particles changes as a result of a change in operating conditions and the filter collects more or less of the debris.

41.2.7 Limitations One of the most important limitations of TLA is that it responds only to actual loss of material. In corrosive environments, it is possible to convert metal into corrosion products without the products being removed from the surface. Under these conditions, corrosion will not be detected by TLA. Also, TLA is unsuitable as a monitor of general condition. Wear will be detected by TLA only from an activated component. The area subject to wear has to be determined in advance and a representative part of that area must be activated. Therefore, TLA cannot be used to detect wear from unexpected sources. Alternative techniques, such as the analysis of spectroscopic wear particles, are more appropriate to these requirements.


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41.3 Typical applications TLA can be applied to a very wide range of measurements of wear, erosion and corrosion, including corrosion in chemical plant, oil production and process plant, erosion in multi-phase fluid systems, and wear and tribology research. The following sections provide illustrative examples of the application of TLA in various fields of wear testing and measurement.

41.3.1 Wear testing Various types of experimental rig are used to simulate different wear situations under controlled conditions. Examples include ball-on-plate and pin-on-disc testers (Fig. 41-5). Asher and Peacock (1982) used TLA to measure the very low levels of wear from the disc of a pin-on-disc wear tester lubricated by white spirit. Although the pin and the disc were both pure iron, it was shown that the wear of the disc was a factor of 30 lower than that predicted on the basis of equal volumetric wear, and that the ratio of disc wear to pin wear was dependent on the applied load - see Fig. 41-6. Although the total, accumulated wear in a sequence of tests was only about 1 pm, only just within the iimits of conventional surface profilometry, the greater TLA sensitivity, about 0.1 nm, made it possible to observe and quantify these subtle effects. Testing of this type has also been used to evaluate the effects of ion implantation on reducing the wear of pure iron (Goode et al., 1983). COUNTER B A L A N C E

I

Fig. 41-5. Typical arrangement for a pin-on-disc wear tester. The required load is applied via a balanced lever to the pin which bears down on the surface of the rotating disc. Friction is measured using a transducer to detect the horizontal thrust, while pin wear is determined by measuring the vertical movement of the lever.

654 Part 7: Tribology r

1.0

5

0.6

c

0.4

c

(a)

0.0

3-

"

Disc w e a r / p i n wear

I

X1/2

I

2.01

40

--r-----l-----r-----T---

1

0.0

j

(b) P i n wear r a t e

(c) F r i c t i o n

(d) Load

0.2

0.4

0.6

T o t a l p i n wear

0.8

1.0

1.2

(mrn)

Fig. 41 -6. Results from a pin-on-disc wear tester of varying pin load (d), on pin wear rate (b), friction (c) and the ratio of disc wear (in nm) to pin wear (in pm) (a). Both pin and disc were pure iron, lubricated by white spirit.

Childs and Kinsella (1981) studied the friction cutting of cast iron pressed against a rotating, water-cooled, mild steel disc. By using TLA on the disc and separately on the cast iron, they established transitions between mild and severe slitting under changing load conditions and also examined material transfer between the surfaces. Changes in the distribution of activity around the circumference of the disc were also examined to identify localized loss and transfer of material. Various groups have examined the wear of cutting tools. Bhattacharyya and Jetley (1979) used TLA to measure the performance of various cutting fluids on the wear of turning inserts. Amini and Winterton (1981) measured the wear of the cutting


655

edge/flank of twist drills and identified effects during the very early stages of flank wear. Good correlation was established with optical measurements of the wear. Ivkovic (1982) investigated the wear rates during metal cutting of twist drills, taps and mill cutters, and examined the influence of the cutting fluids on the wear performance.

41.3.2 Engine-wear By far the most common application of TLA is in the determination of the wear of critical automotive engine components. Many of these measurements are carried out for engine/component development, driven by the need to increase power efficiency, reduce weight or increase durability. Lubricant manufacturers use TLA as a routine tool in the development of new additives to operate at lower viscosity and higher stress in modem engines, to differentiate their products from those of competitors’ and to show that protection against wear can be demonstrated under a wide range of operating conditions. Fluid systems of plant or engines can be monitored to detect the activity of wear debris. In an operating engine, the activity in the oil and filter increases as wear produces debris; sensitivity to 10 nm (0.01 pm) is possible during on-line engine measurements. In off-line or laboratory measurements, sensitivity of 1 nm (0.001 pm) can be achieved. This sensitivity allows typical wear performance of engine components to be established in only a few hours of running, an order of magnitude less than that required to determine wear by conventional methods. In a typical lubricant test, the selected component is sent to an activation centre for treatment to generate the thin ‘marker layer’ close to the surface. The treated component is despatched to the client’s test centre where it is installed in an engine and operated using a reference oil. Wear measurements are made every few minutes until the component is ‘run in’ and wearing occurs at a steady-rate. The test oil is introduced and the engine is run until the new wear-rate is determined. The reference oil is then re-introduced to confirm that any change in wear rate reflects the oil change rather than some other factor. This cycle is repeated to measure the wear performance of each oil formulation under test. Fig. 41-7 shows an example of typical data, from a lubricant test in which cam wear was being monitored. Data obtained from direct and indirect (oil/filter) measurements were used to differentiate the relative wear performance of reference and test oils. Two lubricants were under test. Lubricant ‘A’ was a reference lubricant, with well-characterized wear performance, while lubricant ‘B’ was the test product. In a comparison of three 5-h runs, the difference in wear rate is clearly visible (A: 0.05 pm h-’ and B: 0.14 pm h-I), and more clearly so in the higher-sensitivity measurements of oil+filter activity. Confidence in the quantitative nature of the comparison is reinforced by the fact that the wear rate with lubricant ‘A’ is the same before and after running with lubricant ‘B’.


(a) Direct measurement

5

0

10

15

10

15

Time (hours)

(b) Indirect measurement (oil + filter) 2.5 cn

h

2

F2

. I -

A

2

Lubricant B

I-. - -I

.-0

E

v

9

1.5

1 0

5

Time (hours) Fig. 41-7. On-line data from a comparative test of two lubricants, A and B, showing a clear difference in wear-rate, established in only a few hours of running an automobile test engine. Measurements are shown (a) of the loss of activity measured from the component and (b) from the increase in activity of the lubricant.

This example demonstrates that the TLA technique can be used to complete a test in as little as 3-5 h compared to perhaps 200 h if a more conventional technique of wear measurement, such as surface profiling metrology, is used.


657

There are many examples in the literature of applications in engine-wear testing. Evans (1980) carried out a comparison of TLA and neutron activation for measurements of wear from cam followers. He established a good correlation between the two methods, but found that neutron activation gave consistently higher values for wear, a difference attributed to activated material being removed from surfaces other than the follower pad. Both methods were capable of giving significant measurements of wear well below the threshold capability of conventional measurement techniques, for the relatively low rates experienced for this type of component. He concluded that the greater selectivity and much lower level of radioactivity made TLA far more practical. The following two publications illustrate the types of measurement requirements posed by engine and lubricant design. Schneider et al. (1988) used TLA to investigate correlations between speed and power output with wear of chromium-plated piston rings on a Detroit Diesel engine, operating under a range of conditions. The effects of running-in and increases in wear rate with decreasing engine speed and increasing power output were quantified. It was also established that only about 10% of the wear debris was retained by the oil filters most wear particles are assumed to be too small to be collected by conventional automotive oil filters and remain in suspension in the oil flow. Ohmori et al. (1993) examined the effects of oil viscosity on engine wear, by activating the rubbing surfaces of test piston rings and the top nose profile of cam faces. Fired engine tests were carried out at full load and different rpm. The high sensitivity of TLA was exploited to show that low-viscosity oils may produce acceptable wear protection at moderate temperatures, but at high running temperatures, wear showed a significant increase.

41.3.3 Biomedical applications TLA has also proved useful in studying the wear-behaviour of materials in applications for biomedical engineering. Ethical considerations restrict the use of radioactive materials in humans for direct treatment or medical diagnosis, so the use of TLA in biomedical fields is limited to simulator tests, which also allow reproducible load cycling patterns to be investigated. Total hip joint replacement is probably the most widely undertaken orthopaedic procedure, but the orthroplastry does not generally have acceptable durability; wear performance is one of the critical problems. Hip replacement materials have been studied using TLA on wear testing rigs (Conlon, 1985). Measurements of wear debris provided a quantitative comparison of improvements to wear performance introduced by different surface treatments on TiVAl alloys bearing against ultra-high-molecular weight polyethylene. Nitrogen ion implantation was shown to reduce wear rates over those of untreated alloy test pieces by a factor of up to 400. No other technique is known to produce results of such high sensitivity over such a wide dynamic range. Neumann et al. (1990) have used TLA to examine the wear of CoCrMo alloy hip joints in a simulator and demonstrated a wear sensitivity of 0.1 pm. They also com-


pared the TLA results with results from a 3D co-ordinate measurement system. The latter gave a consistently higher value for the maximum wear; this was attributed to the fact that TLA averages over a larger area.

References Amini E., Winterton R.H.S. (1981), Proc. Instn. Mech. Engrs. 195, 241. Asher J., Peacock A.T. (1982), Wear 81,275. Bhattacharyya S.K., Jetley S.K. ( I 979), Tribology International, 277. Conlon T.W. (1974), Wear29,69. Conlon T. W. ( 1 985), Contemporaty Physics 26, 52 1. Evans R. (1980), Wear 64,3 1 1. Gem6 A. (1 972), Kerntechnik 14,204. Goode P.D., Peacock A.T., Asher J . (1983), Nucl. lnstr. Meth 209,925. Ivkovic B. (1982), Tribology International 15( l), 3. Konstantinov LO., Krasnov N.N. (1971), J.Radioanal.Chem, 8,357. Neumann W . , Wolfi W., Heimgartner P., Streicher R.M. (1990), Nucl. Instrum. Meth. Phys. Rex B50,57. Ohmori T., Tohyama M., Yamamoto M., Akiyama K., Tasaka K., Yoshihara T. (1993), SAE Technical Paper 932782. Perrin K., Pandosh J., Searle A., Schaub H., Sprague S. (1995), SAE Technical Paper 952474. Schneider E.W., Blossfeld D.H., Balnaves M.A. (1988), SAE Technical Paper 880672.



Part 8: Life sciences Two current issues related to life sciences are described in the following part dealing with biomaterials and nuclear microscopy, respectively. On a global scale, more than one million ‘body replacements’, also denoted implants, and in a wider sense biomaterials, are inserted in the human body annually. Implants comprise hip joints, artificial knees, artifical teeth etc., which are mainly composed of various alloys such as steel, chromium-cobalt and titanium alloys. Polymers are widely used in applications such as intraocular lenses, vascular grafts and prosthetic devices. Surface properties of implants, which are closely linked to preparation and characterization, are central issues in biomaterial research and applications. In the achievement of the optimum preparation of biomaterial surfaces and to follow their interaction with the biological environment, i.e. with body fluids and tissues, several analytical techniques are used as valuable tools, elucidating various properties related to physical and chemical properties and biocompatibility. Techniques described in preceding parts (acronyms given here) such as XPS, AES, SEM, SIMS, FTIR, ellipsometry or STM and AFM all find valuable applications in the analytical assay of chemical bonding, microstructure or element distribution. Nuclear microscopy is based upon interactions between beam particles and atoms and nuclei of the irradiated specimen, followed by the assay of emitted characteristic X-rays and gamma-radiation.Several nuclear methods are exploited as analytical tools, such as PIXE, PIGE, RBS, etc. The technique allows two-dimensional mapping and a lateral resolution of less than 1 pm. Nuclear microscopy finds a wide range of applications in, medical, biological and environmental research. It is worth mentioning in this context that these methods have been applied in AIDS, and neurobiological research. Environmental mapping is further highlighted.



42 Biomaterials B. Kasemo and J. Lausmaa

42.1 The biomaterial-biosystem interface Imagine that a titanium dental implant has just been placed in its implantation site (Fig. 42-1). The screw shaped implant has been machined, cleaned in several baths and finally sterilized. The appearance of the surface in an SEM picture (Fig. 42-2) is a characteristic grooved pattern with many pits and elevations on the micrometer and submicrometer scale. An Auger-electron spectrum of the surface (Fig. 42-3) reveals that the surface is oxidized and covered by a thin oxide (usually -3-7 nm, depending on preparation conditions) whose composition is essentially TiO,. In addition, there are some trace impurities on the clinical implant (e.g. Si, S, C1, and Na) and an almost inevitable hydrocarbon contamination overlayer (about one molecular monolayer), due to ambient exposure and/or residues from the cleaning procedure. This is the surface that meets the biological system at the implant site. Here, the first encounter is with blood with its complex content of water, ions, and biomolecules, primarily proteins (Kasemo and Lausmaa, 1986, 1991, 1994). They will rapidly form a biofilm coating (Fig. 42-4) on the surface (on a time scale from fractions of seconds to a few seconds). This biofilm will subsequently change its composition, initially through protein exchange, and eventually also through the action of cells appearing on the scene. As the cells occupy the surface, they may deposit new proteins, or even form tissue deposits. In parallel with this process, the original, unperturbed tissue (bone in the present example) at some distance from the implant surface starts to grow through the normal healing process and eventually new tissue will surround the whole implant (Fig. 42-5). Provided the implant material were correctly chosen and the surgical procedures were correctly executed, a stable, bone-integrated implant is established that can now bear the load of a new set of artificial teeth (Brhemark and Zarb, 1985; Brhemark and Lausmaa, 1995; Adell and Lekholm, 1990). The above example could be extended to many other implants: in the blood stream, artificial hearts valves, stents, and artificial blood vessels encounter blood as the interacting tissue. Intraocular lenses in the eye encounter the eye's biological milieu. In artificial hip joints, there are interfaces with bone and with the synovial liquid of the joint, etc. In all these cases, the surface of a man-made material suddenly meets the living tissue of a human being. It is then not surprising that very different reactions may occur for a given implant site depending on the choice of material. Since the meeting place, on the molecular level, is at the surface of the implant, the surface properties are particularly important (Kasemo and Lausmaa, 1986, 1991, 1994).

42 Biomaterials

Ti implant

Titanium

661

-1 mm H

1

/

-1 pm

\

-1 nm

Titanium oxide

H

H

Biofluid

Fig. 42-1, Schematic illustration, at different levels of magnification, of a titanium dental implant placed in bone.

662 Part 8: Life sciences

Fig. 42-2. Scanning electron micrograph (secondary electrons) of titanium dental implant surface, showing the typical grooves resulting from machining of the material.

It is today realized that surface properties - and consequently surface preparation and characterization - are central issues in biomaterials research and applications (Kasemo and Lausmaa, 1986, 1991, 1994; Ratner, 1987, 1993; Baier and Meyer, 1988). It is the specific composition of the surface which determines how the first conditioning biofilm is formed - e.g. which are the proteins that bind and how strongly and how the subsequent reforming and eventual healing around the implant occurs. If the choice of material is right, a firm and safe integration between the tissue and the implant results, e.g. with titanium in bone. If the choice of material is unfortunate, bone does not form, but rather a capsule of connective tissue (Carlsson and Rostlund, 1986). In order to prepare biomaterial surfaces optimally and to follow their interaction with the biological environment, the whole arsenal of surface characterization tools come into play. For implant characterization before implantation, XPS, AES and SIMS are major analytical tools to determine the chemical composition of the surface. For characterization of the microstructure TEM, SEM and the scanning-probe techniques (STM, AFM) are frequently used. In order to investigate the interaction with the biological system, methods are needed to study water and biomolecules on surfaces. This calls for a variety of techniques such as FTIR, ellipsometry, thermal-desorption spectroscopy, XPS and the same scanning-probe techniques as mentioned above.

42 Biomaterials

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The intention of this article is to give a number of examples of how some of these and some other techniques have been used to characterize biomaterial surfaces or their interactions with the biosystem. The ambition is not to give a comprehensive review of the field, nor will the techniques be described in detail. The intention is rather to give a flavour of how surface-sensitive techniques are currently being used in biomaterials research.

Ti dental implant

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Kinetic energy (eV) Fig. 42-3. Auger electron survey spectrum from a titanium dental implant surface, showing that the surface consists of a titanium surface oxide, with a carbon contamination overlayer and some trace impurities (3.0 keV primary electrons, 250 nA primary beam current).


Cell membrane: lipid bilayer rc with functional biomolecules

Growth factors, enzymes, oxidants, etc.

bond 4gand) Fig. 42-4. Schematic illustration of the dynamic interaction between an implant and the biological host. The biomaterial surface initially becomes covered by an adsorbed, ‘conditioning layer’ consisting of water, ions and different biomolecules. The original surface properties of the biomaterial play a decisive role in the exact composition and structure of this conditioning layer. The composition of the layer changes with time as a result of exchange reactions, changes in the physiological conditions, and the action of cells approaching the surface. The behaviour of cells (adhesion, spreading, differentiation, activation, etc.) at the interface is influenced by their interaction with immobilized biomolecules at the surface and with possible products released from the surface, as well as by the topography of the surface. Different biologically active substances secreted by the cells in turn influence the behaviour and recruitment of other cells and the organization of the tissue which forms around the implanted material.

42 Biomaterials

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100pm

10 m m

Fig. 42-5. Schematic illustration of possible tissue responses to an implanted material. Note the quasilogarithmic length and time scales. Initially, there is relatively wide fluid space separating the implant from the tissue. Due to the surgical trauma, the latter may consist of a zone of damaged tissue. If the material and surgical procedures are correctly chosen, the following wound-healing process can lead to the formation of healthy tissue which grows close to the implant surface (tissue integration). If, on the other hand, the material is inappropriately chosen, formation of a fibrous capsule can occur, which prevents a stable anchorage of the implant. The latter case is obviously an unfavourable result in the case of load-bearing implants.

42.2 Spectroscopic characterization of biomaterial surfaces 42.2.1 XPS, AES, and SIMS analyses of titanium surfaces used in biomaterials research and applications In this section, some examples are given from surface analyses of different titanium surfaces used in biomaterials research and development. In addition to characterizing existing materials, surface analyses also play a key role in the development of model surfaces with systematically varied surface properties, which can then be used in various biological evaluation studies in vivo and in vitro. Titanium and its surface oxides constitute a suitable model system for this type of research, as will be exemplified below.

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42.2.1.1 Clinical titanium implants and electrochemically modified titanium

As mentioned already in the introductory section, machined titanium implant surfaces consist mainly of a thin layer of oxide which is covered by a hydrocarbon-dominated adsorption layer and trace amounts of other impurities (Lausmaa and Kasemo, 1990; Lausmaa, 1995). This information can be deduced from the AES survey spectrum shown in Fig. 42-3. The surface composition of different titanium surfaces is, however, strongly dependent on the surface preparation method (Lausmaa and Kasemo, 1990; Lausmaa, 1995; Smith and Pilliar, 1991).

I

1000

I

500

I

BINDING ENERGY (eV)

Fig. 42-6. XPS survey spectra of different titanium oxide surfaces: (A) Pure bulk TiOZ,(9)Clinical titanium dental implant, (C) Fluorine-contaminated titanium oxide surface, (D) Anodized in I M H2S04, (E) Machined Ti6AI4V.(Monochromatic Al Ka).

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To illustrate this a set of XPS survey spectra from titanium (oxide) surfaces prepared by different methods is shown in Fig. 42-6 (Kasemo and Lausmaa, 1986, 1988). All spectra are dominated by Ti, 0 and C, due to the surface oxide and carbonaceous contamination layer, but they differ with respect to which impurities are detected. The bulk Ti02 sample (Fig. 42-6a) was cleaned by Ar sputtering and annealing in 0 2 in a vacuum, and therefore shows only minor traces of impurities. This sample serves as a reference, especially regarding the intensities and energies of the Ti and 0 peaks (see below). Fig. 42-6b shows a spectrum from a titanium implant (Lausmaa and Mattsson, 1986). This particular surface was machined, whereby it was thermally oxidized, and then ultrasonically cleaned in different organic solvents, and finally sterilized by conventional autoclaving. The spectrum gives essentially the same information as the AES survey spectrum in Fig. 42-3. The surface consists of a Ti oxide, and surface contamination dominated by carbon and some trace amounts of inorganic impurities (in this case, C1, N and Ca). Depending on the details of the preparation procedure (cleaning solvents, sterilization method, packaging/storage material, etc.), the absolute carbon levels and exact composition of the surface contamination layer of titanium implants can vary over quite a wide range (Lausmaa and Kasemo, 1990; Lausmaa, 1995). The fluorine impurity on the sample in Fig. 42-6c is an unintentional impurity from the handling of this particular sample (Lausmaa and Kasemo, 1985). It is due to residues of a rinsing agent (SiNazF6) which was used when laundering the cloth used for wrapping the box in which the implant was placed during autoclaving. The fluorine contamination in this case catalyses accelerated growth of the titanium surface oxide during autoclaving, which in turn gives rise to a discoloration of the surface. This example underlines the need for carefully controlling the preparation and handling of titanium implants in practical situations. The anodic oxide sample (Fig. 42-6d) was made in a sulphuric acid electrolyte which is the source of the sulphur signal in this case (Lausmaa and Kasemo, 1990). The AES depth profiles from a similar sample (Fig. 42-7) show that the sulphur is incorporated throughout most of the oxide layer. Note also that the anodized sample has a much thicker oxide than the machined titanium surfaces. Interestingly, a chlorine signal is also seen located around the metaloxide interface. This is because the sample was electropolished in a perchloric-acidbased electrolyte prior to anodization. The observation that the chlorine is located around the metal-oxide interface after the formation of the anodic oxide strongly indicates that the anodic oxide has grown primarily by Ti ion diffusion from the metal to the electrolyte-oxide interface. By using different electrolytes or etching fluids, other impurities can (intentionally or unintentionally) become deposited on titanium surfaces, for example, P or F from phosphoric acid and hydrofluoric acid, respectively (Lausmaa, 1995; Lausmaa and Kasemo, 1990, 1988). Wet chemical surface treatments can thus be used intentionally to produce variations in the chemical composition of titanium surfaces. The electrochemically prepared Ti samples have been used in various biological evaluation studies, and results from animal experiments indicate that the rate of bone formation is somewhat higher for thick oxides and rough surfaces, as


compared to the machined titanium surfaces that are normally used in clinical practice (Larsson and Thomsen, 1994, 1996). Finally, Fig. 42-6e shows a survey spectrum from a machined Ti6A14V sample (Ask and Lausmaa, 1988-89). Surface oxides on alloys should not be expected to have a composition identical with that of the corresponding pure metal. Instead, they frequently contain oxides of the different alloying elements.

0

200

400

600

Sputtering time (s)

800

1000

Fig. 42-7. SIMS depth profiles from titanium sample anodized in 1 M HzS04 (10 keV Cs' primary ions, negative secondary ions).

6

3

Anodized Ti6A14V, 1M H,SO,, 4%

0

200 400 600 Sputtering time (s)

1

800

Fig. 42-8. SIMS depth profiles from anodized Ti6AI4Vsample (10 keV 0; primary ions, positive secondary ions).

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For this particular sample, this is manifested by the presence of A1 at the outermost surface. V is, however, usually not detected by XPS at the outermost surface of oxidized Ti6A14V samples. The SIMS depth profiles in Fig. 42-8, however, clearly demonstrate that V is present in the oxide layer. The depth distributions of Ti, Al, and V reflect the thermodynamics and kinetics of the oxidation process: The enrichment of A1 at the surface, the expense of the other elements, and the depletion of V, is in line with the free energies of formation for Al, Ti and V-oxides (Samsonov, 1973). More detailed information about the surface layer is obtained from high-resolution spectra of the detected elements. The Ti 2p peak shape and position (Fig. 42-9b) for machined titanium implant surfaces reveal that the oxide composition is mainly TiO, (Lausmaaand Kasemo, 1990; Lausmaa, 1995).

I

Ti2p

460

2p

3’2fi

464 460 456 Binding energy (eV)

452

Fig. 42-9. XPS Ti 2p spectra from: (A) TiOz reference sample (single crystal rutile (1 lo)), and (B) from a titanium dental implant surface (Monochromatic A1 Ku).

This is evident from the main contribution in the Ti 2p 312 peak, which is located at 459 eV in close agreement with the Ti02 reference (Fig. 42-9a). However, additional components in the spectra are frequently observed for machined surfaces, for example around 454 eV. This contribution is assigned to metallic titanium, and is due to photoelectrons originating from the metal underlying the surface oxide (Lausmaa and Kasemo, 1990; Lausmaa, 1995). Provided that the inelastic mean free path of the Ti 2p photoelectrons in the surface oxide is known, the intensity ratio between the metal and oxide contributions can be used to estimate the surface oxide thickness. For machined titanium surfaces, the oxide thicknesses normally lie within the range 2-6 nm (Lausmaa and Kasemo, 1990; Lausmaa, 1995). The chemical sensitivity of XPS can thus be used for measuring the thickness of oxides or other surface layers.

42.2.1.2 Glow-discharge plasma treated Ti In all cases discussed above, the surface oxide composition is essentially TiO2, and the surface contamination levels and compositions are difficult to control and can vary over quite a wide range. There is therefore a need for preparation methods which can be used to prepare other oxide compositions on Ti (e.g. suboxides, and nitrides or carbides) and which offer better control of surface contamination. Glow-discharge-


plasma-treatment is such a method, which is now well established in, for example, the field of microelectronics. The method is currently also receiving increasing attention in the field of biomaterials. In this section, some examples will be used to show how plasma preparation in combination with surface analysis can be used for advanced and controlled preparation of titanium surfaces (Aronsson, 1995; Aronsson and Lausmaa, 1996; Aronsson and Lausmaa, 1996). Fig. 42-10, shows XPS survey spectra from two titanium samples which have been subjected to two different glow-discharge-plasma-treatments (Aronsson and Lausmaa, 1996; Aronsson and Lausmaa, 1996).

1000

800

600 400 Binding energy (eV)

200

0

Fig. 42- 10. XPS survey spectra from glow-discharge-plasma-treated titanium surfaces: (A) cleaned in Ar plasma, (B) Ar-plasma cleaned, followed by oxidation in pure 02-plasrna.

Comparison of the conventionally prepared samples (Figs. 42-3 and 42-6) and the Ar-plasma-treated sample (Fig. 42-1Oa) clearly shows that the latter treatment has removed most of the traces from the previous preparation steps: The spectrum is dominated by Ti signals and contains only small signals from oxygen and carbon. This

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spectrum is representative of a nearly metallic Ti surface. The small oxygen and carbon signals are due to submonolayer amounts of molecules (H20, CO, hydrocarbons) adsorbed from the residual gas present in the vacuum chamber during transport from the plasma preparation site to the analysis position (Aronsson and Lausmaa, 1996). Provided that proper plasma process parameters are used, Ar plasma treatments can be used for controlled cleaning, i.e., removal of adsorbed contaminants and native oxide layers from the surface (Aronsson and Lausmaa, 1996). Alternatively, more gentle Ar plasmas can be used to remove the outermost surface contamination layer, without removing the oxide (Aronsson and Lausmaa, 1996). The Ar-plasma-cleaned surfaces serve as a reproducible and clean starting point for the formation of new surface layers which are prepared with a high degree of control in the vacuum system. Fig. 42-lob shows an XPS survey spectrum from a machined sample which was first Ar-plasma-cleaned and then reoxidized in an oxygen plasma (Aronsson and Lausmaa, 1996).

468 466 464 462 460 458 456 454 452 Binding Energy (eV)

Fig. 42-11. XPS Ti 2p spectra from (A) anodically oxidised titanium surface (TiOz), (B)the same surface after short Ar-plasma treatment.

The spectrum contains only Ti and oxygen signals and no impurity elements are detected. This example shows that atomically clean titanium oxide (TiO2) surfaces can be prepared in oxygen plasmas if the proper procedures are used. Similarly, by using other process gases, alternative surface layers, e.g. Ti nitrides or Ti carbides, can be prepared (Aronsson and Lausmaa, 1996). Finally, an example is shown of a more subtle surface modification (Aronsson and Lausmaa, 1996). Fig. 42-11A shows Ti 2p spectra from an anodically oxidized titanium sample. The Ti 2p 3/2 peak consists of a single sharp component centred around 459 eV, showing that the plasma oxide is stoichiometric TiO2. The spectrum in Fig. 42-1 1B is from the same sample, but after a short treatment in an Ar plasma. The Ti 2p


312 peak is now significantly broadened and it contains components in the region of 455-458 eV due to the suboxides T i 0 and Ti203 and defects (oxygen vacancies). Thus, plasma treatment can be used to produce surface oxides with various stoichiometries and defect densities on titanium (Aronsson and Lausmaa, 1996). Such modifications are particularly interesting since surface defects are known to play an important role in the chemical reactivity of surfaces.

42.3 Surface topography and morphology In addition to chemical composition, as revealed by the spectroscopic analyses discussed above, it is of general interest to characterize the structure of biomaterial surfaces at different levels of resolution. A thorough characterization of the structural features of biomaterials should include a variety of techniques, such as light microscopy, scanning electron microscopy and transmission electron microscopy, and the scanning probe techniques (STM and AFM). A few examples will be given below of how different surface treatments influence surface structure, and how the surface microarchitecture can be manufactured in a controlled way.

42.3.1 Surface topography It is increasingly being realized that surface topography plays an important role in implant-tissue interactions. It is now well established that microstructures on the cell size level, i.e. from a few to tens of micrometers, significantly affect cell behaviour on surfaces. It is, therefore, important to control the surface microarchitecture on this length scale. It is, for example, unlikely that the machined surface of Fig. 42-2 is optimum. However, the control of surface topography should not be restricted to the > 1 pm level. As Fig. 42-12 (log scale of different structures) illustrates, the dimensions of surface topographic features have their biological counterparts at all length scales from micrometers down to the atomic scale (Kasemo and Lausmaa, 1986; Kasemo and Lausmaa, 1991). Fig. 42-13a shows an AFM picture of a machined surface (Larsson and Thomsen, 1994). In comparison, Fig. 42-13b shows a similar picture of an electropolished surface (Larsson and Thomsen, 1994). In the latter case, a much smoother surface results, inducing significantly different biological response in bone, compared with the machined surface (Larsson and Thomsen, 1994, 1996). Anodization of electropolished titanium leads to a roughening of the surface, and a pitted appearance, as shown in Fig, 42-13c (Larsson and Thomsen, 1994, 1996). Plasma-treated Ti-implants have a very special appearance after heavy plasma etching as shown by the SEM image in Fig. 42-14 (Aronsson and Lausmaa, 1996). Obviously, the etching rate is very different at the grain boundaries, compared with that at the grain surfaces. However, if the plasma oxidation is performed gently, the surface oxide can be grown without affecting the microstructure (Aronsson and Lausmaa, 1996). These preparation-dependent variations in surface microstructure call for a more controlled technique to prepare the surface microarchitecture.

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t Dislocation

Fig. 42-12. Approximate sizes of structures at the interface between a titanium implant and the host tissue.


Fig. 42- 13. AFM images of (a) machined, (b) electropolished, and (c) electropolished and anodized titanium surfaces.

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Fig. 42-14. SEM image of plasma etched titanium surface.

42.3.2 Microfabricated surfaces It is generally desirable to control the microarchitecture of the implant surface in detail. The micro- and nanofabrication techniques developed primarily for microelectronics, i.e., for modern integrated circuits, offer a variety of such opportunities. A few examples will be given below. Fig. 42-15 shows a simple array of aluminium squares, 10x10 pm, deposited on titanium by microlithography. This surface was used to explore differences in the affinity of bacteria for Al, Ti and V surfaces (Gabriel and Gold, 1994).

Fig. 42- 15. AES element map of microfabricated sample with A1 squares on Ti surface.

Fig. 42-16 shows an example where elongated fibres of titanium, of 10 pm length and 0.5 pm width, have been deposited on a silica substrate. This structure could well be used to study cell behaviour on the micropatterned surface. However, its first use


was quite different; by an additional production step, called the lift-off technique (Kasemo and Gold, 1996; Gold and Nilsson, 1995), the titanium fibres are released from the surface, and free fibres are obtained as shown in the SEM picture in Fig. 42-17, where a titanium fibre is residing on a Millipore filter. These fibres are actually used to study how fibres affect the response of lung macrophages in a project which is aimed at understanding how asbestos and other fibres induce lung diseases (Kasemo and Gold, 1996; Gold and Nilsson, 1995). In this research, identically shaped fibres of Ti and Si02 are manufactured, and the cell responses to the different fibres are evaluated (Kasemo and Gold, 1996; Gold and Nilsson, 1995). This concept is currently being extended towards smaller structures on the 10 nm scale, where the biological targets are not cells but rather biological macromolecules.

Fig. 42-16. Microfabricated artificial titanium oxide fibres deposited on a silica substrate.

Fig. 42-17. Free titanium fibres deposited on a Millipore filter.

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42.3.3 Transmission electron microscopy Transmission electron microscopy (TEM) is an obvious tool for studies and control of the morphology and microstructure of surfaces. In connection with the different preparations of titanium surfaces, described in Section 42.2, TEM has been an invaluable method. A few examples will be given below. During the preparation of anodic oxides on titanium and titanium alloys, very interesting morphologies were observed (Kasemo and Lausmaa, 1988; Lausmaa and Mattsson, 1986; Lausmaa and Kasemo, 1988; Ask and Lausmaa, 1988-89; Mattsson and Rolander, 1985; RBdegran and Lausmaa, 1991; Ask and Rolander, 1990; Lausmaa and Ask, 1989). Fig. 42-1 8 shows the basic structures observed.

Fig. 42-19. TEM images of (a) thin (-10 m) and (b) thick (-40 nm) thermal oxide film on titanium


Fig. 42-1 8a is a TEM picture of the titanium metal, revealing its grain size distribution. Fig. 42-19b is a TEM picture of an anodic oxide layer (160 nm thick) grown on the metal. The anodically oxidized surfaces show a distinct mosaic pattern reflecting the oxide on individual grains of the polycrystalline metal (Mattsson and Rolander, 1985). Some areas of the anodic oxide are porous. For thicknesses above 50 nm, the anodic oxides are polycrystalline (Mattsson and Rolander, 1985). It is obvious that the oxide grows with a texture that mimics the underlying metal’s grain structure. Fig. 42-18c shows a similar TEM picture as in Fig. 42-18b, but for the oxide on a TibA14V sample. Due to the much more complex phase structure of this alloy, the oxide also takes a more complex structure. Scanning AES and SIMS analysis shows that the concentrations of the alloying elements vary laterally over the surface (Ask and Lausmaa, 1988-89; Ask and Rolander, 1990).

TI

Tissue + PMMA

de

Embed in PMMA -,

Implant

PMMA + t i s q

/

-

1

Electropolishing

Sectioning

Ti metal

Oxide

Refill with PMMA

to obtain TEM

Tissue + PMMA

Fig. 42-20. Schematic illustration of preparation technique for producing TEM samples with an intact titanium oxide-tissue interface. The implant is first removed with its surrounding tissue. After fixation and embedding the sample is connected to an electrolytic cell where the bulk metal is removed by electropolishing. The resulting cavity is filled with plastic and can then be sectioned by an ultramicrotome to produce sections that are thin enough for TEM analysis.

Thin thermal oxides show a very different morphology compared with the thick anodic oxides. The thinnest films show no contrast at all in TEM (Fig. 42-19a). In contrast to the anodic oxides, which show a diffraction pattern corresponding to anatase, the thin thermal oxides exhibit no diffraction pattern, indicating an amorphous or extremely microcrystalline structure. When the thermal oxidation is performed at higher temperature, producing thicker films, the texture of the underlying metal starts to be visible (Fig. 42-19b), and the film becomes more crystalline. The pictures in Figs. 42-18 - 42-19 were obtained with samples that were first oxidized on one side,

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and then jet-electropolished on the reverse to produce a self-supporting Ti02 window in the middle of the sample. An electropolishing method was later developed to enable TEM imaging of retrieved implants (Bjursten and Emanuelsson, 1990). This technique, which is summarized in Fig. 42-20 and the associated figure text, makes it possible to prepare biological samples containing both the surface oxide of the implant and the adhering biological tissue (Fig. 42-21). - . .. ......

-

-2

Fig. 42-21. TEM image of the interface between soft tissue and a titanium implant. The electron-dense, black line represents the oxide that was left after the sample preparation described in Fig. 42-20. On the tissue side of the interface, cells can be seen adjacent to, but never in direct contact with, the oxide surface.

References Adell R., Lekholrn U., Brfinemark P . 4 , Jemt T. (1990). Int. J. Oral Maxillofac. Implants, 5 : 347-359. Aronsson B.-O., Lausmaa J., Kasemo B. (1996a), XPS studies of glow discharge modified titanium surfaces. Appl. Surface Sci. (to be published). Aronsson B.-O., Lausmaa J., Kasemo B. (1996b), Glow discharge plasma treatment for surface cleaning and modification of metallic biomaterials. . I Biomed. Mafer. Res. (to be published). Aronsson B.-0. ( 1995), Preparation and characterization of glow discharge modf?ed titanium surfaces. PhD Thesis, Chalmers University of Technology and G6teborg University. Ask M., Lausmaa J., Kasemo B. (1988-89), Preparation and surface spectroscopic characterization of oxide films on Ti6A14V. Appl. &$ace Sci., 35: 283-301. Ask M., Rolander U., Lausmaa J., Kasemo B. (1990), Microstructure and morphology of surface oxide films on Ti6A14V. J Muter. Res., 5 : 1662-1667. Baier R.E., Meyer A.E. (l988), Implant surface preparation. Int. J Oral Maxillofac. Implants, 3: 9-20. Bjursten L.M., Emanuelsson L., Ericson L.E., Thomsen P., Lausmaa J., Mattsson L., Rolander U., Kasemo B. ( I 990), Method for ultrastructural studies of the intact tissue-metal interface. Biomaterials, 11: 596-601. Brlnemark P.-I., Lausmaa J., Ericson L., Thomsen P., Brhemark R., Skalak R. (1995), Fonseca RJ, Davis WH, eds. Reconstructive preprosthetic oral and maillofacial surgery., Philadelphia: W.B. Saunders Co., pp. 165-224. Brhemark P.-I., Zarb G.A., Albrektsson T. (1985), Eds., Tissue integratedprostheses: Osseointegration in clinical practice Chicago: Quintessence). Carlsson L., ROstlund T., Albrektsson B., Albrektsson T., Brfinemark P.4. (l986), Osseointegration of titanium implants. Acta Orthop. Scand., 57: 285-292. Gabriel B.L., Gold J., Gristina A.G., Kasemo B., Lausmaa J., Harrer C., Myrvik Q.N. (1994), SiteSpecific Adhesion of S. Epidermidis (RP12) in Ti-AI-V metal systems. Biomaterials, 15: 628-634. Gold J., Nilsson B., Kasemo B. (1995), Microfabricated metal and metal oxide fibres for biological applications. J. Vuc. Sci. Technol. A, 13: 2638.

680 Part 8: Life sciences Kasemo B., Gold J. (1996), Applications of surface science methods and microlithography to make and characterize samples for biological evaluation. In: Mohr U, eds. Correlations between in vifro and in vivo investigations in inhalation toxicology., Washington: ILSI Press, pp. (in press). Kasemo B., Lausmaa J. (1986a), Surface science aspects on inorganic biomaterials. CRC Crit. Rev. Biocomp., 2: 335-380. Kasemo B., Lausmaa J. (1 986b), Steenberghe DFv, eds. Tissue Integration in Oral and Maxillo-Facial Reconstruction., Amsterdam: Excerpta Medica, pp. 4 1-45. Kasemo B., Lausmaa J. (1988), Int. J. Oral & Maxillofacial Surg., 3: 247-259. Kasemo B., Lausmaa J. (199 l), Davies J.E., eds. The bone-biomaterials interface., Toronto: University of Toronto Press, pp. 19-32. Kasemo B., Lausmaa J. (1994), Materials Science and Engineering, C1: 115-1 19. King DA. (1975), Thermal desorption from metal surfaces: A review. Surface Science, 47: 384-402. Larsson C., Thomsen P., Aronsson B.-O., Rodahl M., Lausmaa J., Kasemo B., Ericson L.E. (1996), Bone response to surface modified titanium implants: Studies on the early tissue response to machined and electropolished implants with different oxide thicknesses. Biomaterials (in press). Larsson C., Thomsen P., Lausmaa J., Rodahl M., Kasemo B., Ericson L.E. (1994), Bone response to surface modified titanium implants: Studies on electropolished implants with different oxide thicknesses and morphology. Biomaterials, 15: 1062-1074. Lausmaa J., Ask M., Rolander U., Kasemo B. (1989), Hanker JS, Giammara BL, eds. Biomaterials and biomedical devices., vol. 110 Material Research Society, pp. 647-653. Lausmaa J., Kasemo B., Hansson S. (l985), Accelerated oxide growth on titanium implants during autoclaving caused by fluorine contamination. Biornaterials, 6: 23-27. Lausmaa J., Kasemo B., Mattsson H., Odelius H. (1990a), Multi-technique surface characterization of oxide films on electropolished and anodically oxidised titanium. Appl. Surface Sci., 45: 189-200. Lausmaa J., Kasemo B., Mattsson H. (l990b), Surface Spectroscopic Characterization of Titanium Implant Materials. Appl. Surface Sci., 44: 133-146. Lausmaa J., Kasemo B., Rolander U., Bjursten L.M., Ericson L.E., Rosander L., Thomsen P. (1988), Ratner BD, eds. Surface Characterization of Biomaterials., New York: Elsevier, pp. 161-1 74. Lausmaa J., Mattsson L., Rolander U., Kasemo B. (1986), Williams JM, Nichols MF, Zingg W, eds. Biomedical materials., vol. 55 Materials Research Society, pp. 35 1-359. Lausmaa J. (1995), submitted. Lafgren P., Lausmaa J., Krozer A., Kasemo B. (1995) Lafgren P., Lausmaa J., Krozer A., Kasemo B. (1994), Adsorption and coadsorption of water and amino acids on Pt( 11 1). In 41st National A VSSymposium, Denver CO. Mattsson L., Rolander U. (1985), Structure and Morphology of Anodic Oxide Films on Titanium. Chalmers University of Technology. GIPR-264, Pages. Ratner B.D. (1987), Biomaterial surfaces. J. Biomed. Mater. Res.: Appl. Biomater., 21(A1): 59-90. Ratner B.D. (1993), New ideas in biomaterials science - a path to engineered biomaterials. J. Biomed Mater. Res., 27: 837-850. Riidegran G . , Lausmaa J., Rolander U., Mattsson L., Kasemo B. (1991), Preparation of ultra-thin oxide windows on titanium for TEM analysis. J. El. Microscopy. Techn., 19: 99-106. Samsonov G.V. (1973), The oxide handbook New York: IFIPlenum. Sauerbrey G.Z. (1959), Vewendung von schwingquartzen zur wtigung dtinner schichten und zur microwagung. Z. Phys., 155: 206-222. Smith D.C., Pilliar R.M., Metson J.B., McIntyre N.S. (1991), Dental implant materials. 11. Preparative procedures and surface spectroscopic studies. J. Biomed. Muter. Res., 25: 1069-1084.



43 Biological nuclear microscopy U. Lindh

43.1 Introduction The history of nuclear microscopy began in 1970 when Cookson and Pilling first succeeded in focusing a proton beam down to 4 pm for analytical purposes. The performance characteristics and a few examples of two-dimensional elemental maps were published later (Cookson et al., 1972). It was a very timely event, because semiconductor X-ray detectors also became available at that time. Since its arrival the nuclear microscope has found applications in various disciplines including biology and medicine. The state-of-the-art nuclear microscope enables lateral resolution of less than 1 pm at a particle intensity corresponding to 100 PA. Modem nuclear microscopes accommodate several detection systems and current computer technology makes complex data handling easily manageable. Imaging has improved substantially in recent years although nuclear does not yet match electron microscopy in its lateral resolution.

X-ray photons

Nuclear reaction

STIM (dark ficblcl)

STIM (bright field)

Forwclrci sc;1ttered Secondary clcctrc)ns

1

~

1

~

Fig. 43-1. Reaction products that are used to collect information about the irradiated sample: IBIC = ionbeam-induced current, IBIL = ion-beam-induced light, STIM = scanning-transmission ion microscopy.

Nuclear microscopy is, as has already been mentioned, based upon a series of atomic and nuclear interactions between the ion beam particles and the atoms and nuclei of the irradiated sample (Fig. 43-1). The characteristic X-rays induced in the nuclear microscope are the major analytical tool and the high performance of this technique is one important reason for its development even though it is a technique very similar to electron microscopy in its analytical mode. Other atomic phenomena are ion-


beam-induced current (IBIC), probably of limited use in biology, and ion-beaminduced light (IBIL), not yet used in biology, although the similar cathodoluminescence has been used for some time in electron microscopy. Ion-induced Auger electrons are also used in some applications but have hitherto not found a use in biology. Most biological matter is composed of hydrogen, carbon, oxygen, and nitrogen. These elements are often of little interest to the biologist using nuclear microscopy. Nevertheless, they are easily assessed by back-scattering ( C , 0, N) and forward-scattering (H) spectroscopy, Their importance in biological nuclear microscopy is that they provide a useful way of determining mass loss during analysis. They also provide the basis for analytical models in standard-less elemental analysis (Grime and Dawson, 1994). An increasingly important analytical technique used in nuclear microscopy is scanning transmission ion microscopy (STIM). It was developed simultaneously by two groups in 1983 (Overley et al., 1983; Sealock et al., 1983). STIM makes possible the density mapping of thin specimens. The introduction of this technique has improved the capability of nuclear microscopy substantially. Earlier, it was necessary to work with both stained and unstained specimens. Surface characterization of biological samples most often involves sectioning of tissues or cells. When using nuclear microscopy with relatively low detection limits (in the order of 1 pg g-' dry weight), staining is not acceptable because of the risk of contamination, elemental redistribution and elemental loss. STIM has eliminated the use of the staining of thin samples where surface characterization is the objective. Another important feature of nuclear microscopy is that nuclear interaction phenomena can be exploited for elemental detection. The most popular interactions are particle-induced gamma rays (PIGE) and Rutherford back-scattering (RBS) together with forward scattering. PIGE are dealt with in the chapter by Malmqvist and RBS are dealt with in the chapter by Whitlow and Ostling. PIGE has, however, not found widespread applications in biology. It can be of interest for determination of the lighter biogenic elements such as carbon, nitrogen, and oxygen. An interesting feature of nuclear reactions is that in some cases they show resonances. This phenomenon makes it possible to study not only the concentrations of some elements but also their depth distributions. In an elemental characterization of dental enamel, it was shown that the depth distribution of fluorine could be determined with a resolution of less than 0.1 pm down to (Lindh and about 10 pm below the surface using the nuclear reaction 19F(p,ay)'60 Tveit, 1980). Sundqvist et al. (1976) showed that the depth distribution of nitrogen in single seeds could be determined using the nuclear reaction I4N(d,p)l5N.RBS and forward scattering is mostly used for the monitoring of mass loss during microbeam irradiation and not as much for detection of elements. As has been indicated, X-ray microanalysis in the scanning electron microscope is the predecessor of nuclear microscopy. It has been, and still is, widely used in biology. In fact, nuclear microscopy shares many of the ideas of electron microscopy, the difference being that focused beams of protons are used instead of electron beams. In this context it is interesting to draw the reader's attention to a comparison of X-ray microanalysis and nuclear microscopy by Pilsgird et al. (1994b). This group emphasized the study of ion fluxes in the cell types

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investigated. The authors show that nuclear microscopy and X-ray microanalysis are complementary and that, for some applications, nuclear microscopy is superior.

43.2 Sampling The objective of sampling and sample preparation is to preserve the integrity of the sample so that it is representative of the biological state that is under investigation. This process involves the choice of an adequate number of samples and of an appropriate preparation technique. For nuclear microscopy, in vivo investigations are out of reach in most cases. If the biological object is alive, biopsies of organs or samples of body fluids will be available and the practical considerations comprise sampling techniques, instruments and vessels to store the samples before sample preparation. If, for example, blood is to be sampled for separation of cells, the most critical point is the choice of vacuum tubes. Today, vacuum tubes are specially manufactured for trace-element analysis and no anticoagulant, which may be detrimental, needs to be added. It has been found that tubes with K3EDTA as an anticoagulant work well for analysis of isolated erythrocytes, thrombocytes, and neutrophil granulocytes (polymorphonuclear cells) with nuclear microscopy (Lindh and Johansson, 1987a). The major problem with these tubes is contamination with zinc, but investigation showed that the contribution was negligible at the detection limits achieved by nuclear microscopy. When biopsies are considered, the variety of sampling instruments is limited. Usually, a biopsy suited to trace element analysis will be taken by a fine needle. This will provide a ‘sausage’ of a few millimetres in length and about 1 millimetre in diameter. Parts of this sample can be rapidly quench-frozen and stored. It may be necessary to ‘trim’ the derived sample with quartz or other non-metal knives to remove parts that have been in contact with metal surgical instruments. Sampling from sacrificed experimental animals imposes other kinds of difficulties in that the organ of interest has to be rapidly excised from the animal and cut into appropriately sized pieces, protected from loss of moisture and then frozen. Even in this case, it may be necessary to ‘trim’ the specimens before sectioning.

43.2.1 Preparation In most applications nuclear microscopy requires a high vacuum (1O-’-l O-’ mbar). To conserve the living characteristics in the best possible way, the biochemical processes of the cells have to be terminated instantaneously and preserved in that state. This is necessary because the moment nourishment stops, degeneration starts. The elemental distribution can be preserved in a near lifelike condition by shock-freezing. For morphological studies the samples are commonly dehydrated at room temperature and conservation is usually achieved in solutions containing chemicals and in some cases heavy metals. Such procedures allow diffusion and frequently the chemicals them-


selves induce redistribution of elements. Also, exogenous elements may be introduced in this way. Freezing should occur as fast as possible (Echlin, 1992). Otherwise, ice crystals are generated, perturbing both morphology and elemental distribution. Biological samples are poor heat conductors and the samples, therefore, have to be as small as possible to maximize the surface-to-volume ratio. Only an outer surface layer of some tens of micrometers is frozen without the presence of disturbing ice crystals. Rapid freezing can be performed either by plunging the specimen into a liquid coolant, or by slamming the specimen against a metal mirror cooled to liquid nitrogen or liquid helium temperature. The relative merits of the two methods have been debated. The metal mirror provides better conduction for heat removal from the specimen, but poor contact between the specimen and metal surface may remove this advantage. Liquid coolants provide lower heat conduction but better contact, and are presently considered adequate for freezing of tissue samples for microanalysis. Common choices are liquid propane or fieon, cooled by liquid nitrogen. Heat transfer to the sample must be avoided after the freezing has been initiated. The samples are always handled with heat insulated tools. Once frozen, the samples are safely stored in liquid nitrogen. If successful freezing is achieved, the samples can be prepared in many ways depending on the requirements of the investigation and the instrumentation available. Poor freezing can never be compensated for by the subsequent preparation. The initial steps are indeed crucial.

43.2.2 Freeze-drying and freeze substitution Freeze-drying of freeze-fixed specimens has long been the preferred method of preparation because it best preserves morphology and elemental distribution. Analysis of the frozen hydrated specimen may be carried out and can provide some advantages: wet-weight concentrations can be measured, element dislocation which may occur during ice sublimation is avoided, loss of mass is almost negligible at low temperatures and the water content of subcellular compartments may be determined. The primary difficulties with this form of analysis are: to ensure no transfer of water from or onto the specimen before or during analysis, to inhibit ice recrystallization, to prevent contamination, to obtain suitable image contrast and to cope with increased background levels due to beam interactions with the ice. The latter difficulty makes this form of analysis unsuitable for trace-element microanalysis. In addition, analysis of frozen hydrated specimens demands complicated specimen stages. Freeze-drying, however, removes the ice by sublimation thereby improving image contrast over the frozen hydrated specimen, giving a better peak-to-background ratio in the X-ray spectra, and providing greater ease of handling at room temperature and generally better stability during analysis (Kirby and Legge, 1993). An alternative to sublimation is to dehydrate by substituting the water with an organic solvent. This technique is often called freeze substitution. In order not to distort morphology or elemental distribution, the substitution should be accomplished gently. Therefore, the solvent should not contain water initially and the water should be


685

replaced slowly (Echlin, 1992). The substitution is carried out at a low temperature to delay the diffusion rate. The water molecules in the sample are replaced by solvent molecules because the solvent volume is large compared to the sample volume. Ideally, the solvent molecules should be of the same size as the water molecules. Solvent polarity is also crucial so as not to distort the hydrated molecules. The preservation of molecular tertiary structure is particularly important when retention of immunological or enzyme activity is required. However, ions in the cytoplasm are also hydrated and movements can be induced by altered charge distributions. Because of the factors mentioned, different temperatures and different times are required depending on the solvent used. The thawing conditions are also important. It has been suggested that substitution can occur during thawing. Therefore, the temperature rise should be slow, 1 OC per hour. Having reached ambient temperature, the specimens are then embedded. Such samples are considered easier to cut than frozen tissue. Embedding can also be performed when the freeze-substituted samples are still at a low temperature. In this case methacrylate resins are polymerized using ultraviolet light. In a comparative study, PdsgArd et al. (1994a) showed that freeze-substitution using tetrahydrofuran was as efficient as cryosectioning followed by freeze-drying in preserving both morphology and elemental integrity as judged by the potassium-to-sodium and potassium-to-phosphorusratios. This is very promising, since the Ereeze-substitution followed by embedding and sectioning at room temperature is a much easier procedure than cryosectioning and freeze-drying. Cryosectioning with high-quality sections is an art and very few are skilled enough to perform it.

43.3 Nuclear microscopy of tissues Tissue samples may originate from experimental animals or from humans, in the latter case as a result of a biopsy or an autopsy. Nuclear microscopy has been used to characterize highly varied tissues. Often, one makes a distinction between hard, or mineralised, tissues and soft tissues. The former tissues are more difficult to handle if thin sections are required. However, in an investigation of the distribution of lead in femur samples from deceased workers in a smeltery, thin sections were obtained after embedding small samples with an epoxy resin (Lindh, 1983). Thick samples of bone tissue were also investigated for their lead distribution (Lindh et al., 1978; Lindh, 1980). In a study of lead distribution in the central nervous system following early lowdose exposure, it was shown that lead accumulated preferentially in the white matter although this is the minor part of the cerebellum (Lindh et al., 1989; Lindh, 1991). The cerebellum lies at the back of the skull behind the brain stem and under the great hemispheres of the cerebrum.


43.3.1 Protective effects of selenium Considerable evidence has accrued to show that an excess of one essential trace element may have a detrimental effect on one or more of the other essential elements, particularly if the latter are present at a minimum level. Furthermore, non-essential elements may have a profound effect on the absorption, metabolism and excretion of essential trace elements. In fact, the opposite can also be found in nature. Several studies have, for example, shown that selenium has a protective effect against mercury toxicity from both organic and inorganic compounds (Burk et al., 1977; Parizek and Ostadalova, 1977; Ganther et al., 1972; El-Begearmi et al., 1977; Johnson and Pond, 1974; Lindh and Johansson, 1987b). From the toxicological literature it is fairly well known what kind of effect could be expected after toxic exposure of experimental animals. For both inorganic mercury and cadmium, the critical organ is the kidney. However, the liver is also affected by both kinds of exposure. In the kidneys, the cells lining the proximal tubules are the critical cells. The corresponding cells in the liver are the periportal hepatocytes, parenchymal cells, and Kupffer cells. In addition, the literature provides information about the doses sufficient to produce damage. An attempt has been made to shed some light upon this kind of protective effect using nuclear microscopy (Lindh et al., 1996). A study was designed to use high enough exposure levels of cadmium and mercury to produce damage and to use supplementation of selenium in doses adjusted to what is thought to be a safe and adequate intake for humans. Sprague-Dawley rats were randomly allocated to six groups, one of which served as a control. The other groups were administered either salts of selenium, cadmium and mercury or combinations of selenium and cadmium and also selenium and mercury. The trace elements were administered intraperitoneally over a 30 day period. After that, the animals were sacrificed. Livers and kidneys were rapidly removed and coronal sections of the kidney and a slice from the left lobe of the liver were immediately quench-frozen in isopentane chilled with liquid nitrogen. The tissues were cryosectioned at 5-10 pm and freeze-dried. Adjacent sections were trapped onto microscope slides and stained with haematoxylin and eosin or toluidine blue to allow optical inspection and histopathological analysis. Since STIM was not available at that time, the stained sections were also used to localize regions of interest to aid nuclear microscopy of the unstained sections. Animals exposed only to mercury and cadmium were clearly affected and tissue damage was found as expected. The same parts of the tissues from animals in the combinationtreatment groups were inspected histopathologically and no signs at all of toxic effects could be detected in the cells; nor did the animals show signs of such effects. Analysis by nuclear microscopy showed the co-localization of selenium-cadmium and seleniummercury in those cells corresponding to the cells where toxicity was manifested in the animals solely exposed to cadmium and mercury. Fig. 43-2 shows two elemental maps of the same region of a kidney section from an animal treated with both selenium and cadmium. The map covers a transverse section of a proximal tubule with the lumen in the centre. Selenium and cadmium are obviously co-localized in the maps with three


687

distinctive spots. These are the nuclei of the three proximal tubule cells covered, possibly corresponding to the intranuclear refractive inclusions observed by light microscopy in the stained tissue. The concentrations of selenium and cadmium could suggest equimolarity. However, conclusions drawn from atomic ratios should be treated with care.

Selenium

Cadmium

Fig. 43-2. Elemental maps of selenium and cadmium covering a small section of a rat kidney. The rat had been given selenium and cadmium. The three hot spots correspond to three epithelial cells in a proximal tubule. The centre part is the lumen of the tubule.

Selenium

Mercury

Fig. 43-3. Elemental maps of selenium and mercury covering a small section of a rat kidney. The rat had been given selenium and mercury. The hot spot corresponds to a epithelial cell in a proximal tubule. The centre part is the lumen of the tubule.


The elemental maps in Fig. 43-3 show the distribution of mercury and selenium in a section of a kidney from an animal treated with selenium and mercury. In the scanned part of this section, part of a proximal tubule has been covered and one nucleus is visible. Here, the co-localization of the elements is obvious. In none the animals of the combination treatments could any disturbance of growth or effect on behaviour be observed. These results, besides the finding that there were no signs of toxicity in the kidneys or livers, are interpreted as demonstrating a clear protective effect from selenium. The protective effect is additionally manifested in a co-localization of selenium-cadmium and selenium-mercury in those cells where toxic effects would be expected. Studies are in progress to try to explain the molecular mechanisms behind this co-localization.

43.3.2 Dermatology and cardiology Nuclear microscopy of skin is by now an established method for the study of the elemental distribution of normal and pathological tissue. Emphasis has been put on the distributions of the electrolytes, phosphorus, sulphur and several trace elements. The quotient of sodium to potassium has been used as an indicator of cell or tissue damage. Normally, the concentration of potassium is kept high in the cells and low outside, whereas the opposite is true for sodium. The quotient may also be a marker of cell regeneration activity. Atopic dermatitis, a constitutional condition in 10-20% of the Swedish population, has previously been shown to induce abnormal production of barrier lipids. This partially explains the increased transepidermal water loss in the dry non-eczematous skin of atopics. Although immunologically well documented, the disorder is yet unexplained in many of its appearances. A group in Lund has recently carried out an investigation of pathological skin as compared with normal skin (Pallon et al., 1993). They observed that in the normal skin iron and zinc peak in the stratum germinatum and decrease towards the skin surface. The calcium level increases in the epidermis, which may reflect the influence of calcium on the differentiation process in the epidermis. Phosphorus peaks at about the same place as iron and zinc. The atopic skin showed distributions of sulphur, potassium and chlorine similar to those of the normal skin. However, the peaking of calcium is missing. Phosphorus was also at a higher level and the peak less pronounced in the atopic case. Copper displayed a sharp peak at the same location as the phosphorus peak. The concentration of iron was higher and the peak was broader, as was the case with zinc. The walls of arteries are composed of three coats and a hollow core, the lumen, through which the blood flows. The inner coat of an arterial wall, the intima, is composed of a lining of endothelium, in contact with the blood, and a layer of elastic tissue. The middle coat, or tunica media, is usually the thickest layer. It consists of elastic fibres and smooth muscle. The outer coat, the adventia, is composed primarily of elastic and collagenous fibres.


689

Atherosclerosis is the deposit of lipid in the intima of large and medium sized arteries to form plaques. The lipid deposits contain cholesterol. The fatty material may later be replaced by dense connective tissue and calcium deposits. Arteriosclerosis, the hardening of the arteries, is characterized by a thickening of the intima, making the tunica media less elastic. Fat gradually accumulates between the elastic and collagenous fibres to produce lesions that protrude into the lumen. The calcification may lead to cell death, in turn responsible for the disruption of the arterial wall and subsequent plaque formation. Not much is known about elemental involvement in atherosclerosis. There are indications of significant variations of potassium and calcium in blood serum. Nuclear microscopy has been used to show a clearly visible accumulation of calcium in the affected area accompanied by an increase in zinc concentration (Vis et al., 1982).

1200

Concentration (mg/kg d.w.)

1000 800

600 400

200 0

Normal

P S

Moderate

Advanced

CI K

Fig. 43-4. Elemental concentrations in arterial walls from three stages of atherosclerosis. (Adapted from Pallon ef al., 1993.)

Atherosclerosis was studied by Pallon et al. (1993). They found that the elemental concentrations decreased with progressing atherosclerosis (Fig. 43-4). In moderate atherosclerosis, the elemental concentrations were closer to those of advanced atherosclerosis than was expected from histopathology. In normal cases, or at very early stages of the disease, iron, copper, and zinc were homogeneously distributed. All elements were found concentrated at the arterial border. In deeper regions, roughly 40 pm away from


the arterial border, calcium, potassium, phosphorus and iron seemed to appear in granules, the diameters of which varied from 10 to 20 pm. In the advanced stage these granules disappeared almost completely. Instead, there was a massive calcified border, where the concentration of calcium reached 10000 pg g-' . The normal epithelial concentration was estimated at 500-900 pg g-'. The phosphorus concentration followed the calcium pattern, although there was a tendency for increased P/Ca ratios in advanced atherosclerosis. Iron, copper and zinc showed higher concentrations in the epithelium than in adjacent muscular tissue. In tissue damaged by arteriosclerosis, however, the concentrations of the trace elements dropped significantly. This might be expected, as the damaged arterial wall with dead cells showed very low metabolic activity. The granules found deeper in the arterial wall for normal and moderate cases were of two types. In the first type, having a size of about 10 pm, the concentrations of sulphur, chlorine, and potassium were slightly increased as compared with the adjacent tissue. On the other hand, the concentrations of phosphorus, calcium, and iron were more than twice as high as normal. In the second type of granule a much higher calcium concentration (>5000 pg g-') was found and an increase of phosphorus, copper and zinc. The conclusion was that these granules were probably plaques in a different stage of development (Pallon et al., 1993).

43.3.3 Mussel shells as environmental archives Mussels increase their shells by one increment each year. On the basis of the hypothesis that elements passing through the mussel with the water are incorporated according to the environmental concentrations, the annual increments should serve as environmental-chemical archives. Various bivalve species have been used as environmenta' indicators. However, mostly the soft parts have been used. The first attempt to employ nuclear microscopy on freshwater bivalves was presented by Carell et al. (1987). In this work, Margaritj e r a margaritifera, the pearl mussel, was chosen mainly because it is holarctic, it can grow very old (more than 200 years), and it can withstand quite low pH. This was a feasibility study, in which nuclear microscopy was combined with neutron-activation analysis to obtain trend values, and it showed that continuing the work was worthwhile. Later, nuclear microscopy was applied to another species (Anodoonra) with a much shorter life span (Lindh et a f . ,1988). The annual increments of mussel shells are often small, from some tens of micrometers to one millimetre depending on the growth rate. Using high-resolution nuclear microscopy to cover large areas in mussel shells would be impossibly time-consuming. To ensure that line scans are adequate, high-resolution elemental mapping was carried out on a mussel shell showing that large areas of three annual increments displayed only temporal, not lateral, variations (unpublished results). Temporal is to be interpreted as parallel to the growth direction and lateral perpendicular to it.


691

Differences between upstream and downstream localities in a watercourse may reveal suspect incidents between the two points. A polluting industry could be monitored using mussel shells. Owing to the pearl mussel’s life span, history could be traced back 150-200 years. One such attempt was made by Nystrom e l al. (1995b). They studied mussel shells from localities upstream and downstream of a paper mill that was known to have discharged significant amounts of mercury over 38 years. However, these authors did not find any differences in mussel shells taken from the different localities, and mercury was not detected at all. A factor to be considered in this context, however, is the biochemical properties of calcium carbonate and the interstitial glucoproteins. Differences in the affinity of certain metals to these matrices may have a great influence on the possibility of detection. In another study by the same group (Nystrom et al., 1995a), it was found that manganese showed a variation with peaks in the period May to June, correlating with high water temperature. These peaks also correlated with the oxygen concentration minima in the water. Strontium varied in an opposite phase to the manganese variation with strontium maxima co-located with manganese minima and vice versa. These studies have shown that with a good knowledge of shell growth and structure, mussel shells can be used to disclose environmental-chemical archives and probably other useful information.

43.4 Nuclear microscopy at the cellular level Biological cells have challenged nuclear microscopy since the coming of the nuclear microprobe. The first aim was to reach a resolution good enough to discriminate between elemental information from one cell to another, be it cell cultures or cells in an intact tissue section. The first spectrum of an individual cell was probably that obtained by Legge in 1974 (Legge and Mazzolini, 1980). Cookson and Pilling (1976) were the second to analyse a single cell with distribution information from 1975. Later, Lindh (1978) showed a spectrum from an in vucuo analysis of a single hamster V79cell performed in 1976. Glia and glioma cells from cultures were analysed individually and the groups were compared as to differences in elemental contents. Copper and zinc were at a higher concentration levels in the glioma (tumour) cells (Lindh, 1982). It was not until the spatial resolution reached below 5 pm that intracellular distribution studies became feasible.

43.4.1 AIDS research Never before has science been confronted with an epidemic in which the primary disease simply lowers the victim’s immunity, and the second unrelated disease produces the symptoms that may result in death. This primary disease, first recognised in June 1981, is called acquired immune deficiency syndrome (AIDS).


Nuclear microscopy has been successfully applied in the validation of organometallic and inorganic drugs against AIDS at the Micro Analytical Research Centre, Melbourne (Cholewa et al., 1993). A series of drugs has been extensively tested for toxicity and effectiveness. However, information on the relative penetration of the drugs into the cells and on the subsequent composition of the cells was lacking. The drugs were designed to inhibit the replication of HIV in T lymphocytes. A characteristic feature of these drugs is that they contain heavy metal atoms that are not normally found in cells. Lymphocytes were incubated with a growth medium containing the drug being tested at various sub-toxic levels. Extracellular traces of the drug were removed by repeated centrifugation in pure growth medium, and the medium was after that removed by resuspending the cells in a buffer of ammonium acetate. The lymphocytes, however, do not tolerate long exposure to this buffer. Consequently, the exposure to the medium was kept to less than 10 min. Droplets of the suspension were placed on nylon foils, immediately quench-frozen in isopentane chilled with liquid nitrogen and subsequently freeze-dried. This buffer is well suited to work with blood cells and the ammonium salt appears to evaporate without leaving any traces during freeze-drying. The features of nuclear microscopy were very elegantly exploited in that individual lymphocytes were scanned with 3 MeV protons at a resolution of 1 pm. To avoid radiation damage, the beam current was kept as low as 30 PA. The results indicated that the drug was incorporated into the cell and that the ratio of cobalt to tungsten remained unchanged. The intracellular distribution of phosphorus was used to trace the cell nucleus. This study was extended recently to inorganic anions and co-ordination complexes of chromium. V79 hamster lung cells and Vero cells (African green monkey kidney cell lining) were used to assess the uptake and distribution (Cholewa ef al., 1995).

43.4.2 Cellular neurobiology In a neoplastic state, cellular proliferation and growth occur without any continuing external stimuli. The term neoplasia therefore describes a state of poorly regulated cell division. In neoplastic cells, the mechanisms that control cellular proliferation and maturation are out of order. In the central nervous system the functioning cells of nervous tissue, the neurones, are not directly supported by connective-tissuecells or their intercellular substances, as occurs in the rest of the body. Cells derived from the same progenitor cells as neurones form the support. These cells are of a type termed glia or neuroglia because in their supportive role they seem to ‘glue’ neurones together by being present between neuronal cell bodies and fibres, holding them together. Among the tumours of the brain, the most common type is derived from the glial cells. These twnours are called gliomas. Several types of glioma tumours exist and some of them are being cultured at several laboratories. In Lund, human glia cells (U-622 CG) were taken from adult brain biopsies and a human malignant glioma cell


693

line (U-118 MG). Both cell lines were normally grown as monolayers in Eagle’s minimum essential medium supplemented with 10% bovine calf serum and antibiotics at 37 OC in a humid atmosphere containing 5% carbon dioxide. Elemental concentrations were studied in cultured glia and glioma cells and the conclusion was reached that both zinc and copper were more abundant in the tumourtransformed cells (Lindh, 1982). Recent work has concentrated on the intracellular distribution of trace elements, especially zinc and copper. The glia and glioma cells were grown in monolayers on stretched films of polypropylene (

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