Scanning Auger Electron Microscopy
Scanning Auger Electron Microscopy Edited by M. Prutton and M. El Gomati # 2006 John Wiley & Sons, Ltd. ISBN: 0-470-86677-2
Scanning Auger Electron Microscopy Martin Prutton University of York, York, UK
Mohamed M. El Gomati University of York, York, UK
Copyright ß 2006
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2005031938
British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN-13 978-0-470-86677-1 (HB) ISBN-10 0-470-86677-2 (HB) Typeset in 10/12pt Sabon by Thomson Press (India) Limited, New Delhi Printed and bound in Spain by Grafos S.A., Barcelona This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production.
To Aisha and in loving memory of Elsie who both believed that MULSAM was a concept worth supporting. You did so with care, encouragement and love. We could not have spent so much time, energy and effort on it without your support.
Contents List of Contributors
ix
Preface
xi
Acknowledgments
xv
1. Introduction M.M. El Gomati and M. Prutton
1
2. The Auger Process J.A.D. Matthew
15
3. Instrumentation M.M. El Gomati and M. Prutton
45
4. The Spatial Resolution M.M. El Gomati
125
5. Forming an Auger Image M.M. El Gomati and M. Prutton
165
6. Image Processing and Interpretation M. Prutton
201
7. Quantification of Auger Images M. Prutton
245
8. Applications: Materials Science R.K. Wild
259
9. Applications: Semiconductor Manufacturing C.F.H. Gondran
295
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CONTENTS
10. Concluding Remarks M.M. El Gomati and M. Prutton
341
Author Index
351
Subject Index
359
List of Contributors M. M. El Gomati, Department of Electronics, University of York, Heslington, York, YO10 5DD, UK C. F. H. Gondran Process Characterization Laboratory, ATDF Inc. [A subsidiary of SEMATECH], 2706 Montopolis Drive, Austin, TX 78741, USA J. A. D. Matthew Department of Physics, University of York, Heslington, York, YO10 5DD, UK M. Prutton Department of Physics, University of York, Heslington, York, YO10 5DD, UK R. K. Wild Interface Analysis Centre, Oldbury House, 121 St Michaels Hill, Bristol BS2 8BS, UK
Preface Activity in surface science underwent an enormous explosion in the mid1960s when ultra-high vacuum (UHV) technology became commercially available and so it became possible to clean a surface and maintain sufficient cleanliness for a time adequate to make a measurement of some kind. The activity started with measurements of work functions and of the diffraction of low energy electrons but soon expanded to the development and use of many spectroscopies that yielded information about the electronic and vibrational states of atoms at or near the free surface. Applications to the early stages of chemical reactions and to the metallurgical properties of materials were very prominent at this stage. Thus, the state of a nominally clean surface was of interest as were the effects of various surface contaminants upon its properties and the subsequent reaction with the components of a gaseous atmosphere or a beam of some other kind of atoms. Once the electron spectroscopies had been applied to these problems there was considerable progress in the development of the understanding of surface processes. These spectroscopies included X-ray photoelectron spectroscopies and Auger electron spectroscopy that were sensitive to the number and kind of atoms that were present right at the surface of the sample. However, practical surfaces were unlikely to consist of a clean, flat, arrangement of atoms and the spatial resolution of these spectroscopies was so poor that the information so gleaned required the preparation of such perfect surfaces or was an integration over many differently oriented crystallographic grains or even different materials. Nevertheless, enormous progress was made in revealing the nature of the free surface of a solid and many surprising phenomena were discovered. Electron stimulated Auger electron spectroscopy was a good candidate for a microscopy because the beam of incident electrons upon the sample could be focused onto the surface and could be scanned across the surface. These two properties were critical in the conversion of a spectroscopy to a microscopy that could provide maps or images of the distribution of a selected element in the surface of a sample. All that was
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PREFACE
required was a scanning electron microscope that provided a UHV environment and was equipped with an electron energy analyser to select a particular Auger electron energy from the distribution of electron energies emitted from the sample. This apparently simple statement is quite deceptive. This book sets out to describe the considerations required for this development to be brought to reality and then to illustrate the application of scanning Auger electron microscopy to the surfaces of semiconducting device structures and to systems in materials science. It is now a mature methodology that is used in commercial instruments developed in Europe, Japan and the USA. Naturally, the complexity of these instruments means that they are expensive and so unlikely to be found in the corner of any laboratory! Notwithstanding this expense they have provided information crucial, for instance, to the successful production of integrated circuits and to the oxidation of the superalloys, and so the reliability of the jet engines of the aeroplanes that we all use. The editors (and authors) of this book have been contributing to the development and the applications of this microscopy since the early 1970s. They have built, developed and used three complete Auger electron microscopes. The first was a UHV scanning electron microscope (HB200) from Vacuum Generators Ltd and funded by the R.W. Paul Instrument Fund of the Royal Society that we adapted by adding a concentric hemispherical analyser for electron energy analysis. This instrument proved, to our satisfaction, that Auger electron imaging was feasible and delivered our first results. It also showed that analog detection of the electron current leaving the analyser was not a practical way forward and that field emission electron sources were a subject for research in their own right! The second instrument was developed with electron counting techniques for measuring the current leaving the analyser and Schottky field emission sources in the gun for the electron column. This instrument was much more successful and we were able to use it for the study of the surfaces of nickel superalloys and for the identification of artefacts that can occur at the sharp edges of structures on top of a surface. Attempts were started at this stage to acquire quantitative images of the distribution of a given element on a surface. This appeared feasible because of successful attempts to analyse the amount of material on a surface using Auger electron spectroscopy carried out by ourselves and others (see Chapters 2 and 3). The difficulties of this image quantification led us to consider the simultaneous acquisition of other types of electrons and photons from
PREFACE
xiii
the sample in order to try to disentangle the structure of the surface region from the yield of each kind of signal being detected. This led to the invention and development of the third microscope, the multispectral Auger microscope (MULSAM), that was equipped with detectors for simultaneous measurement of images from an area of interest using scanning electron microscopy, energetic back-scattered electrons, loss electrons and characteristic X-rays. This instrument has been used for a wide variety of applications, some of which are described in this book. It may be clear from this description that the design, building and development of these three instruments has been a huge task that has occupied the time of many people. We are very pleased that we are able to acknowledge their various contributions in the subsequent section. Spending all this time on instrument development has meant that the editors felt it unwise to attempt to review other areas of the subject. Accordingly, we are very pleased that Jim Matthew, Bob Wild and Carolyn Gondran were able to contribute Chapters 2, 8 and 9 from their areas of expertise – contributions that reflect the value of their work in this field. The material in this book is intended as a guide to the subject of Auger electron microscopy and so it is hoped that it will be of interest to researchers in this field as well as to others who wish to discover what can be achieved with this technique and what are its limitations. Thus it is hoped that it will be useful to analysts working with scanning Auger electron microscopes, who are hard pressed to hurry up and measure many samples and so have little time to work on other aspects of the behaviour of their instrument or the problems that they may, perhaps unwittingly, encounter. M. M. El Gomati M. Prutton
Acknowledgments The editors (and authors) have many people to thank for the help they have been given in the building, development and use of the MULSAM instrument at York. Building an entire electron microscope from scratch is a far from minor undertaking especially when it has to be a UHV instrument with energy analysis and it is entirely computer controlled. There were times when we wished that we had not committed ourselves to the job! The first and most important acknowledgment is to Oliver Heavens who backed us from the paper study stage through all the trials and tribulations with humour, a deep interest and, most importantly, money. In spite of an absurdly heavy workload when he was head of department, he managed to come by at least once a week to see how we were progressing and to bring along some champagne at times of success. Thank you very much Oliver. The project started with the design and build of a fast concentric hemispherical analyser which was funded by the R.W. Paul Fund of The Royal Society. The Royal Society Assessors for this work were Peter Duncumb and Reg Garton who came not to criticise but to offer their experience and help. This was invaluable and an important contributor to the speed of progress. Peter Bassett worked on this development which successfully began the whole set of moves to build, develop and use a scanning Auger electron microscope. In fact, three scanning Auger electron microscopes were constructed and a number of postdoctoral fellows and research students were involved. The first instrument was an analog realisation to which Ray Browning and the late Dave Peacock were crucial contributors. They quickly demonstrated that although the Auger electrons could be detected in this way it was the wrong way to go about it. Dave and Ray stayed on and were joined by Peter Kenny and Chris Walker (at different times) to build a digital instrument. Peter Kenny worked for 6 years on the software for the microscope (writing some 200 000 lines of
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software which worked first time) with occasional inputs from Martin Prutton and Mohamed El Gomati and from Chris Walker and Dave Peacock. This was an exciting and productive time helped by the fact that we were joined by Ron Roberts from the University of Newcastle, New South Wales, Australia, who brought many skills in experimental techniques and electronics to the project. Ron has made repeated visits to work with us and has been an invaluable help. The final instrument to be built was MULSAM. This is the most complex instrument and took the efforts of many individuals. Matt Wenham contributed to the software with multivariate statistics, John Greenwood and Ian Barkshire contributed to the hardware and software designs and execution in many important ways. David Wilkinson extended the use of an energy dispersive X-ray detector to work simultaneously with the various electron detectors. Support for this work relied on help generously given by Cedric Powell of NIST and Martin Seah of NPL. There were many contributions from D. Phil. students including Ahmed Assa’d, Taib Bakoush, Martin Crone, Barack Kola, John Kudjoe, Dan Loveday, Phil Tenney, John Walton, Torquil Wells, Andy Gelsthorpe and Li Chen, all of whom helped to move the whole project forward. Of course, a big instrumental build of this kind also required the technical assistance of many people. Jack Dee worked as a technician looking after all three microscopes and his help was indispensable. It is a pleasure to be able to thank Colin Ovenden, David Coulthard and Mick Peters for a wide range of skills that helped to keep the instruments running. Staff in the mechanical workshop made beautiful jobs of the concentric hemispherical analyser, the specimen manipulator, the electron column and many other smaller components. They included John Eastwood, the late Lennie Jarvis, Leigh Crosby, Peter Durkin, Bob Easton, Brent Wilkinson, Ian Wright and Pete Turner. Staff in the electronics workshop made excellent high stability power supplies, beam scanning and stigmators as well as many smaller pieces of electronic hardware. They included the late Jim Scott, Steve Lawson, Simon Hart, Pete Turner and Bob Hide. The technical staff was as important to this project as the scientists. Help with electron optical design was generously given in the early stages of the project by Ken Smith and Eric Munro of the Engineering Department, University of Cambridge. We would also like to thank Tom Mulvey, Aston University, Birmingham, for his encouragement and advice from his wide experience of many different kinds of electron
ACKNOWLEDGMENTS
xvii
optical devices. Don Whitehead of the then VSW Ltd was extremely generous with financial support for electron optics for which we are most grateful. Collaboration with colleagues in the semiconductor fabrication industry was very important in demonstrating just what could be learned with an Auger microscope. Funding for the project came from the Alvey Project of the Department of Trade and Industry and from the Adequat Project of the European Union for which we are most grateful. In these projects we worked with Chris Hill, Pete Pearson, Peter Augustus and Kevin Stribley of Plessey, with Barry Lamb of Standard Telecommunications Ltd and Geoff Spiller and Chris Tuppen of British Telecom. We are most grateful to these collaborators and for the excellent samples that they made available to us. Carolyn Gondran would like to thank Mark Clark, Chris Sparks, Charlene Johnson but most especially Milt Godwin and Laurie Modrey for their many, many helpful comments and suggestions, and Marilyn Redmond and Bob Ruliffson from the SEMATECH library for assistance with reference material. She would also like to thank her family: Chris for moral support and her daughters for the endless distractions.
1 Introduction M. M. El Gomati and M. Prutton
The region near to the surface of a solid material can play importantroles in the properties of that solid. Should an atom or molecule arrive at such a surface, be it in vacuo, in air, in a liquid or in contact with the surface of a different material, then the crystallographic structure, the atomic type, the electronic structure, the vibrations of surface atoms and the bonding forces between the arrival and the surface may all affect what happens next. Thus, for example, the arrival may adhere to the solid surface or be scattered off of it, or the arrival may react with the surface forming a new compound locally. Should the temperature, the structure and the binding energies of the atoms in the surface have appropriate values then the arrival may diffuse into the solid or even cause atoms in the solid to diffuse out to the surface. For these reasons solid surfaces are important in many processes in a wide variety of different parts of science, including biology, chemistry, materials science and physics. Further, they are important in many areas of technology such as semiconductor device fabrication and characterisation, the design of catalysts to speed up chemical reactions, and the development of anti-corrosion layers on alloys and metals. The subject of surface science is thus very broad indeed, having scientific and commercial implications in the effects that it has on large industries. Introductions to the subject include books by Prutton1, Walls2, Woodruff and Delchar3 and Zangwill4. The whole area has been reviewed, for instance, by Duke5 and by Duke and Plummer6. What is meant by the surface of a solid? The answer to this question depends upon what surface properties are under investigation and what Scanning Auger Electron Microscopy Edited by M. Prutton and M. El Gomati # 2006 John Wiley & Sons, Ltd. ISBN: 0-470-86677-2
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INTRODUCTION
experimental techniques are being used for their measurement. The theoretical physicist may be interested in the wave functions of atoms in the outermost layer of the solid. Most extremely, interest may be on the wave functions and their properties in the region in a vacuum outside the solid surface. The experimental scientist may be measuring the properties of the topmost few atomic layers of the solid or the topmost few hundred layers depending upon the methods being used. Most experimental methods involve the bombardment of the surface under study by particles or photons and the detection of scattered particles or photons. If visible photons are incident and reflected photons are detected then the depth of the region of the solid being probed is of the order of the wavelength of the light being used – the information depth is of the order of many hundreds of nanometers. If energetic X-rays are incident and detected then this depth may be of the order of microns. If energetic X-rays are incident and photoelectrons are detected this depth can be as small as a fraction of a nanometer – only a few atom layers are being probed. A similar information depth is obtained when energetic electrons are incident and Auger electrons are emitted from the atoms in the solid. In this book the surface is taken to be the region of a solid within a depth of a few (
