Surface and Thin Film Analysis

Edited by Gernot Friedbacher and Henning Bubert Surface and Thin Film Analysis

Related Titles Watts, J. F., Wolstenholme, J.

An Introduction to SIMS for Surface and Thin Film Analysis 2011 ISBN: 978-0-470-09132-6

Guo, J. (ed.).

X-Rays in Nanoscience Spectroscopy, Spectromicroscopy, and Scattering Techniques 2010 ISBN: 978-3-527-32288-6

Birkholz, M.

Thin Film Analysis by X-Ray Scattering 2006 ISBN: 978-3-527-31052-4

Bordo, V. G., Rubahn, H.-G.

Optics and Spectroscopy at Surfaces and Interfaces 2005 ISBN: 978-3-527-40560-2

Edited by Gernot Friedbacher and Henning Bubert

Surface and Thin Film Analysis A Compendium of Principles, Instrumentation, and Applications

Second, Completely Revised and Enlarged Edition

The Editors Prof. Dr. Gernot Friedbacher Institute of Chemical Technology and Analytics Getreidemarkt 9 /164 1060 Vienna Austria Dr. Henning Bubert Augsburger Weg 51 59439 Holzwickede Germany

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2011 Wiley-VCH Verlag & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition Toppan Best-set Premedia Ltd., Hong Kong Printing and Binding Cover Design Adam Design, Weinheim Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-32047-9

V

Contents Preface to the First Edition XVII Preface to the Second Edition XIX List of Contributors XXI

1

Introduction 1 John C. Rivière and Henning Bubert

Part One 2 2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.3 2.4 2.4.1 2.4.2 2.4.3 2.5 2.6 2.6.1 2.6.2 2.6.3 2.6.4 2.6.5 2.6.6

Electron Detection 7

X-Ray Photoelectron Spectroscopy (XPS) 9 Henning Bubert, John C. Rivière, and Wolfgang S.M. Werner Principles 9 Instrumentation 12 Vacuum Requirements 12 X-Ray Sources 13 Synchrotron Radiation 16 Electron Energy Analyzers 16 Spatial Resolution 18 Spectral Information and Chemical Shifts 19 Quantification, Depth Profiling, and Imaging 21 Quantification 21 Depth Profiling 23 Imaging 26 The Auger Parameter 27 Applications 28 Catalysis 28 Polymers 30 Corrosion and Passivation 31 Adhesion 32 Superconductors 34 Semiconductors 35

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Contents

2.7

Ultraviolet Photoelectron Spectroscopy (UPS) 38 References 39

3

Auger Electron Spectroscopy (AES) 43 Henning Bubert, John C. Rivière, and Wolfgang S.M. Werner Principles 43 Instrumentation 44 Vacuum Requirements 44 Electron Sources 44 Electron-Energy Analyzers 45 Spectral Information 47 Quantification and Depth Profiling 51 Quantification 51 Depth Profiling 53 Applications 54 Grain Boundary Segregation 54 Semiconductor Technology 56 Thin Films and Interfaces 58 Surface Segregation 58 Scanning Auger Microscopy (SAM) 61 References 64

3.1 3.2 3.2.1 3.2.2 3.2.3 3.3 3.4 3.4.1 3.4.2 3.5 3.5.1 3.5.2 3.5.3 3.5.4 3.6

4

4.1 4.2 4.3 4.3.1 4.3.2 4.3.3 4.4 4.5 4.6

5 5.1 5.2 5.3 5.3.1 5.3.2 5.3.3 5.4

Electron Energy-Loss Spectroscopy (EELS) and Energy-Filtering Transmission Electron Microscopy (EFTEM) 67 Reinhard Schneider Principles 68 Instrumentation 70 Qualitative Spectral Information 72 Low-Loss Excitations 74 Ionization Losses 77 Fine Structures 79 Quantification 83 Imaging of Element Distribution 85 Summary 88 References 89 Low-Energy Electron Diffraction (LEED) 93 Georg Held Principles and History 93 Instrumentation 94 Qualitative Information 96 LEED Pattern 96 Spot Profile Analysis 100 Applications and Restrictions 100 Quantitative Structural Information 101

Contents

5.4.1 5.4.2 5.4.3 5.4.4 5.5 5.5.1 5.5.2

Principles 101 Experimental Techniques 102 Computer Programs 104 Applications and Restrictions 105 Low-Energy Electron Microscopy 106 Principles of Operation 106 Applications and Restrictions 108 References 108

6

Other Electron-Detecting Techniques 111 John C. Rivière Ion (Excited) Auger Electron Spectroscopy (IAES) 111 Ion Neutralization Spectroscopy (INS) 111 Inelastic Electron Tunneling Spectroscopy (IETS) 112 Reference 113

6.1 6.2 6.3

Part Two 7 7.1 7.2 7.2.1 7.2.2 7.2.2.1 7.2.2.2 7.3 7.4 7.5 7.5.1 7.5.2 7.5.3 7.5.4 7.5.5 7.5.6 7.5.7

8 8.1 8.1.1 8.1.2 8.1.3

Ion Detection 115

Static Secondary Ion Mass Spectrometry (SSIMS) 117 Heinrich F. Arlinghaus Principles 117 Instrumentation 119 Ion Sources 119 Mass Analyzers 120 Quadrupole Mass Spectrometers 120 Time-of-Flight Mass Spectrometry (TOF-MS) 121 Quantification 123 Spectral Information 125 Applications 127 Oxide Films 128 Interfaces 128 Polymers 131 Biosensors 133 Surface Reactions 134 Imaging 135 Ultra-Shallow Depth Profiling 137 References 138 Dynamic Secondary Ion Mass Spectrometry (SIMS) 141 Herbert Hutter Principles 141 Compensation of Preferential Sputtering 141 Atomic Mixing 142 Implantation of Primary Ions 142

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Contents

8.1.4 8.1.5 8.1.6 8.2 8.2.1 8.2.1.1 8.2.2 8.2.2.1 8.2.2.2 8.3 8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.4.4.1 8.4.4.2 8.4.4.3 8.5 8.6 8.6.1 8.6.2 8.7 8.7.1 8.7.2 8.8 8.9 8.9.1 8.9.2 8.9.3

Crater Bottom Roughening 142 Sputter-Induced Roughness 142 Charging Effects 142 Instrumentation 143 Ion Sources 143 Duoplasmatron 144 Mass Analyzer 144 Magnetic Sector Field 144 Detector 145 Spectral Information 146 Quantification 147 Relative Sensitivity Factors 147 Implantation Standards 147 Metal Ceside (MCs+) Ions 148 Theoretical Models 148 Electron Tunneling Model 148 Broken Bond Model 148 Local Thermodynamic Equilibrium LTE 148 Mass Spectra 149 Depth Profiles 149 Dual-Beam Technique for TOF-SIMS Instruments Molecular Depth Profiles 152 Imaging 152 Scanning SIMS 152 Direct Imaging Mode 153 Three-Dimensional (3-D)-SIMS 154 Applications 156 Implantation Profiles 156 Layer Analysis 157 3-D Trace Element Distribution 158 References 159

9

Electron-Impact (EI) Secondary Neutral Mass Spectrometry (SNMS) 161 Michael Kopnarski and Holger Jenett Introduction 161 General Principles of SNMS 162 Postionization via Electron Impact 163 Suppression of Residual Gas and Secondary Ions Instrumentation and Methods 166 Electron Beam SNMS 166 Plasma SNMS 167 Spectral Information and Quantification 170 Element Depth Profiling 172

9.1 9.2 9.2.1 9.2.2 9.3 9.3.1 9.3.2 9.4 9.5

152

164

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9.6

Applications 174 References 175

10

Laser Secondary Neutral Mass Spectrometry (Laser-SNMS) Heinrich F. Arlinghaus Principles 179 Nonresonant Laser-SNMS 179 Resonant Laser-SNMS 179 Experimental Set-Up 180 Ionization Schemes 181 Instrumentation 182 Spectral Information 183 Quantification 183 Applications 184 Nonresonant Laser-SNMS 184 Resonant Laser-SNMS 186 References 189

10.1 10.1.1 10.1.2 10.1.3 10.1.4 10.2 10.3 10.4 10.5 10.5.1 10.5.2

11 11.1 11.2 11.3 11.4 11.5 11.6 11.6.1 11.6.2 11.6.3 11.6.4 11.7 11.8

12 12.1 12.2 12.3 12.3.1 12.3.2 12.4 12.5

Rutherford Backscattering Spectroscopy (RBS) Leopold Palmetshofer Introduction 191 Principles 191 Instrumentation 194 Spectral Information 194 Quantification 196 Figures of Merit 197 Mass Resolution 197 Sensitivity 198 Depth Resolution 198 Accuracy 198 Applications 198 Related Techniques 201 References 201 Low-Energy Ion Scattering (LEIS) Peter Bauer Principles 203 Instrumentation 206 LEIS Information 208 Energy Information 208 Yield Information 208 Quantification 211 Applications of LEIS 211 References 214

203

191

179

IX

X

Contents

13 13.1 13.2 13.3 13.4 13.5 13.6 13.7

14 14.1 14.2 14.3 14.4

15 15.1 15.2 15.2.1 15.2.2 15.2.3 15.3 15.3.1 15.3.1.1 15.3.1.2 15.3.2 15.3.2.1 15.3.2.2 15.3.2.3

16 16.1 16.1.1 16.1.2 16.2 16.3

Elastic Recoil Detection Analysis (ERDA) Oswald Benka Introduction 217 Fundamentals 218 Particle Identification Methods 220 Equipment 222 Data Analysis 223 Sensitivity and Depth Resolution 223 Applications 224 References 226

217

Nuclear Reaction Analysis (NRA) 229 Oswald Benka Introduction 229 Principles 231 Equipment and Depth Resolution 232 Applications 234 References 236 Field Ion Microscopy (FIM) and Atom Probe (AP) 237 Yuri Suchorski and Wolfgang Drachsel Introduction 237 Principles and Instrumentation 239 Field Ion Microscopy 239 Time-of-Flight Atom Probe Techniques 242 Field Ion Appearance Energy Spectroscopy 246 Applications 248 FIM Applications 248 FIM in Catalysis 248 Fluctuation-Induced Effects 249 Applications of AP Techniques 252 Applications of TOF-AP Techniques 252 PFDMS Applications 254 FIAES Applications 255 References 257 Other Ion-Detecting Techniques 261 John C. Rivière Desorption Methods 261 Electron-Stimulated Desorption (ESD) and ESD Ion Angular Distribution (ESDIAD) 261 Thermal Desorption Spectroscopy (TDS) 262 Glow-Discharge Mass Spectroscopy (GD-MS) 263 Fast-Atom Bombardment Mass Spectroscopy (FABMS) 263 References 264

Contents

Part Three 17 17.1 17.2 17.3 17.4 17.5 17.5.1 17.5.2 17.5.2.1 17.5.2.2

17.5.2.3 17.5.2.4

18 18.1 18.2 18.3 18.4 18.5 18.6

19

19.1 19.1.1 19.1.2 19.1.3 19.1.4 19.2 19.2.1 19.2.2 19.3 19.3.1 19.3.2 19.3.3

Photon Detection 265

Total-Reflection X-Ray Fluorescence (TXRF) Analysis 267 Laszlo Fabry, Siegfried Pahlke, and Burkhard Beckhoff Principles 267 Instrumentation 269 Spectral Information 275 Quantification 276 Applications 277 Particulate and Film-Type Surface Contamination 277 Semiconductors 278 Synchrotron Radiation-Based Techniques 280 Depth Profiling by TXRF and by Grazing Incidence XRF (GIXRF) for the Characterization of Nanolayers and Ultra-Shallow Junctions 283 Vapor-Phase Decomposition (VPD) and Droplet Collection 285 Vapor-Phase Treatment (VPT) and Total Reflection X-Ray Fluorescence Analysis 287 References 288 Energy-Dispersive X-Ray Spectroscopy (EDXS) 293 Reinhard Schneider Principles 293 Practical Aspects of X-Ray Microanalysis and Instrumentation Qualitative Spectral Information 303 Quantification 304 Imaging of Element Distribution 306 Summary 308 References 309 Grazing Incidence X-Ray Methods for Near-Surface Structural Studies 311 P. Neil Gibson Principles 311 The Grazing Incidence X-Ray Geometry 312 Grazing Incidence X-Ray Reflectivity (GXRR) 314 Glancing Angle X-Ray Diffraction 314 ReflEXAFS 316 Experimental Techniques and Data Analysis 317 Grazing Incidence X-Ray Reflectivity (GXRR) 318 Grazing Incidence Asymmetric Bragg (GIAB) Diffraction Applications 321 Grazing Incidence X-Ray Reflectivity (GXRR) 321 Grazing Incidence Asymmetric Bragg (GIAB) Diffraction Grazing Incidence X-Ray Scattering (GIXS) 324

319

323

295

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19.3.4

ReflEXAFS 325 References 326

20

Glow Discharge Optical Emission Spectroscopy (GD-OES) Volker Hoffmann and Alfred Quentmeier Principles 329 Instrumentation 330 Glow Discharge Sources 330 Spectrometer 334 Signal Acquisition 334 Spectral Information 335 Quantification 336 Depth Profiling 337 Applications 339 dc GD Sources 340 rf GD Sources 340 References 342

20.1 20.2 20.2.1 20.2.2 20.2.3 20.3 20.4 20.5 20.6 20.6.1 20.6.2

21 21.1 21.2 21.2.1 21.2.2 21.3 21.4 21.5

22 22.1 22.2 22.3 22.4 22.5

23 23.1 23.2 23.3 23.3.1 23.3.2

Surface Analysis by Laser Ablation 345 Roland Hergenröder and Michail Bolshov Introduction 345 Instrumentation 346 Types of Laser 346 Different Schemes of Laser Ablation 347 Depth Profiling 348 Near-Field Ablation 354 Conclusion 354 References 355 Ion Beam Spectrochemical Analysis (IBSCA) Volker Rupertus Principles 357 Instrumentation 358 Spectral and Analytical Information 360 Quantitative Analysis by IBSCA 361 Applications 363 References 366

357

Reflection Absorption IR Spectroscopy (RAIRS) 367 Karsten Hinrichs Instrumentation 367 Principles 368 Applications 369 RAIRS 369 ATR and SEIRA 372

329

Contents

23.4

Related Techniques 374 References 374

24

Surface Raman Spectroscopy 377 Wieland Hill and Bernhard Lendl Principles 377 Surface-Enhanced Raman Scattering (SERS) 378 Instrumentation 380 Spectral Information 382 Quantification 383 Applications 383 Unenhanced Raman Spectroscopy at Smooth Surfaces 383 Porous Materials 385 Surface-Enhanced Raman Spectroscopy (SERS) 386 Near-Field Raman Spectroscopy 387 Nonlinear Optical Spectroscopy 387 Sum Frequency Generation (SFG) Spectroscopy 387 Coherent Anti-Stokes Raman Scattering (CARS) 389 Stimulated Femtosecond Raman Scattering (SFRS) 389 Spatially Offset Raman Spectroscopy (SORS) 390 References 390

24.1 24.2 24.3 24.4 24.5 24.6 24.6.1 24.6.2 24.6.3 24.6.4 24.7 24.7.1 24.7.2 24.7.3 24.7.4

25 25.1 25.2 25.3 25.3.1 25.3.2

26 26.1 26.2 26.2.1 26.2.2 26.3 26.4 26.4.1 26.4.1.1 26.4.1.2 26.4.1.3 26.4.1.4

UV-VIS-IR Ellipsometry (ELL) 393 Bernd Gruska and Karsten Hinrichs Principles 393 Instrumentation 395 Applications 398 UV-Vis-NIR Spectral Region 398 Infrared Ellipsometry 400 References 405 Sum Frequency Generation (SFG) Spectroscopy 407 Günther Rupprechter and Athula Bandara Introduction to SFG Spectroscopy 407 SFG Theory 410 SFG Signal Intensity and Lineshape 412 Determining the Number Density of Molecules from SFG Signal Intensity 413 SFG Instrumentation and Operation Modes 414 Applications of SFG Spectroscopy and Selected Case Studies 417 SFG Spectroscopy on Solid Surfaces and Solid–Gas Interfaces 417 SFG Spectroscopy under UHV Conditions 417 Polarization-Dependent SFG Spectroscopy 419 SFG Spectroscopy under Near-Atmospheric Gas Pressure 420 SFG Spectroscopy on Supported Metal Nanoparticles 421

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Contents

26.4.1.5 Time-Resolved (Pump-Probe) and Broadband SFG Spectroscopy 423 26.4.1.6 SFG Spectroscopy on Colloidal Nanoparticles and Powder Materials 427 26.4.2 SFG Spectroscopy on Solid–Liquid Interfaces 428 26.4.3 SFG Spectroscopy on Polymer and Biomaterial Interfaces 428 26.4.4 SFG Spectroscopy at Liquid–Gas and Liquid–Liquid Interfaces 429 26.5 Conclusion 430 References 430 27 27.1 27.1.1 27.2

Other Photon-Detecting Techniques 437 John C. Rivière Appearance Potential Methods 437 Soft X-Ray Appearance Potential Spectroscopy (SXAPS) 437 Inverse Photoemission Spectroscopy (IPES) and Bremsstrahlung Isochromat Spectroscopy (BIS) 437

Part Four

Scanning Probe Microscopy 439

28

Introduction 441 Gernot Friedbacher References 442

29

Atomic Force Microscopy (AFM) 443 Gernot Friedbacher Principles 443 Further Modes of AFM Operation 446 Friction Force Microscopy (FFM) 446 Young’s Modulus Microscopy (YMM) or Force Modulation Microscopy (FMM) 447 Phase Imaging 447 Force–Distance Curve Measurements 447 Pulsed Force Mode AFM 448 Harmonic Imaging and Torsional Resonance Mode 449 Instrumentation 452 Applications 455 References 462

29.1 29.2 29.2.1 29.2.2 29.2.3 29.2.4 29.2.5 29.2.6 29.3 29.4

30 30.1 30.2 30.3 30.4

Scanning Tunneling Microscopy (STM) 465 Gernot Friedbacher Principles 465 Instrumentation 467 Lateral and Spectroscopic Information 468 Applications 470 References 479

Contents

31

Scanning Near-Field Optical Microscopy (SNOM) 481 Marc Richter and Volker Deckert 31.1 Introduction 481 31.2 Instrumentation and Operation 482 31.2.1 Basic Set-Up 482 31.2.2 Variations of SNOM 483 31.2.3 Scanning and Feedback Techniques 484 31.2.4 Tip Fabrication 485 31.2.4.1 Taper Formation 486 31.2.4.2 Coating Deposition and Aperture Formation 486 31.2.4.3 Advanced Tip Fabrication 487 31.3 SNOM Applications 488 31.3.1 Fluorescence 488 31.3.2 Near-Field Raman Spectroscopy 490 31.3.3 SNOM-IR-Spectroscopy 492 31.4 Outlook 493 References 493

Appendices 499 Appendix A Summary and Comparison of Techniques 501 Appendix B Surface and Thin-Film Analytical Equipment Suppliers 507

Index

519

XV

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Preface to the First Edition The surface of a solid interacts with its environment. It may be changed by the surrounding medium either unintentionally (for example, by corrosion) or intentionally due to technological demands. Intentional changes are made in order to refine or protect surfaces, that is, to generate new surface properties. Such surface changes can be made, for instance, by ion implantation, deposition of thin films, epitaxially grown layers, and other procedures. In all these cases, it is necessary to analyze the surface, the layer or system of layers, the grain boundaries, or other interfaces in order to control the process which finally meets the technological requirements for a purposefully changed surface. A wealth of analytical methods is available to the analyst, and the choice of the method appropriate for the solution of his problem requires a basic knowledge on the methods, techniques, and procedures of surface and thin film analysis. Therefore, the goal of this book is to give the analyst – whether a newcomer wishing to acquaint with new methods or a materials analyst seeking information on methods that are not available in his own laboratory – a clue about the principles, instrumentation, and applications of the methods, techniques, and procedures of surface and thin film analysis. The first step into this direction was the chapter Surface and Thin Film Analysis of Ullmann’s Encyclopedia of Industrial Chemistry (Vol. B6, Wiley-VCH, Weinheim 2002), in which practitioners give a brief outline of various important methods. The present book is based on that chapter. It has essentially been extended by new sections dealing with electron energy loss spectroscopy (EELS), low-energy electron diffraction (LEED), elastic recoil detection analysis (ERDA), nuclear reaction analysis (NRA), energy dispersive X-ray spectroscopy (EDXS), X-ray diffraction (XRD), surface analysis by laser ablation (LA), and ion-beam spectrochemical analysis (IBSCA). Thus, the book now comprises the most important methods and should help the analyst to make decisions on the proper choice of methods for a given problem. Except for atomic force microscopy (AFM) and scanning tunneling microscopy (STM), microscopic methods, as essential as they are for the characterization of surfaces, are only briefly discussed when combined with a spectroscopic method. Methods of only limited importance for the solution of very special problems, or without availability of commercial equipment, are not considered or

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Preface to the First Edition

only briefly mentioned in the sections entitled Other Electron/Ion/Photon Detecting Techniques. Furthermore, the objective was not to issue a voluminous book, but a clearly arranged one outlining the basic principles and major applications of important methods of surface and thin film analysis. For more detailed information on any of these topics, the reader is referred to the special literature given in the references. The editors are gratefully indebted to all contributors who were ready to redirect time from their research, educational, and private activities in order to contribute to this book. They also wish to thank Mrs Silke Kittel for her tireless help in developing our editorial ideas. Autumn 2001

Henning Bubert Holger Jenett

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Preface to the Second Edition The first edition of this book was very well received on the market and, after becoming “out-of-print”, a variety of ideas was discussed to produce a second edition. It became clear to us very quickly that, instead of an unchanged reprint of the first edition, the opportunity should be taken to update the information in the book and to add new chapters based on feedback from our readers. Fortunately, all authors of the first edition immediately supported this idea, though some were no longer available to actively contribute to the revisions due to changes in their professional careers. Almost all chapters of this book have been thoroughly revised, taking into consideration new developments on the described methods as well as valuable feedback from the First Edition. Although a complete collection of surface analytical techniques would be beyond the scope of a compendium such as this, new chapters on field ion microscopy (FIM) and atom probe (AP), sum frequency generation (SFG), and scanning near-field optical microscopy (SNOM) have been added. With regard to Appendix B the point must be addressed that, due to a rapidly changing market that is characterized by the frequent takeover of one company (or of their subsidiaries) by another, it became rather difficult to produce a compilation that was fully consistent with regard to the names of brands, branches, and company owners. However, the given internet addresses should serve to guide readers to the desired information and contacts to their local distributors. The editors would like to thank all authors for revising and updating their chapters from the First Edition of the book, and all new authors for writing the new chapters and for revising some of the chapters already in existence. To those authors who were unable to revise their chapters themselves, we are certainly indebted that they agreed to a revision of their chapters by new authors. Without this consent between “old” and “new” authors the revision of this book would not have been possible. Finally, we would like to thank Dr. Manfred Köhl and Mrs. Lesley Belfit from Wiley-VCH for their continued support to move this book project forward, as well as Mrs. Bernadette Cabo for the helpful and pleasant communication during the production process. April 2011

Gernot Friedbacher Henning Bubert

XXI

List of Contributors Heinrich F. Arlinghaus Westfälische Wilhelms-Universität Münster Physikalisches Institut Wilhelm-Klemm-Str. 10 48149 Münster Germany

Oswald Benka Johannes Kepler Universität Linz Institut für Experimentalphysik Altenbergerstr. 69 4040 Linz Austria

Athula Bandara University of Peradeniya Department of Chemistry Peradeniya (20400) Sri Lanka

Michail Bolshov Russian Academy of Sciences Institute of Spectroscopy Fizicheskaja street 5 142092 Troitsk, Moscow Region Russia

Peter Bauer Johannes Kepler Universität Linz Institut für Experimentalphysik Altenbergerstr. 69 4020 Linz Austria

Henning Bubert Leibniz-Institut für Analytische Wissenschaften - ISAS - e.V. Otto-Hahn-Str. 6b 44227 Dortmund Germany

Burkhard Beckhoff Physikalisch-Technische Bundesanstalt (PTB) X-ray Spectrometry Abbestr. 2-12 10587 Berlin Germany

Volker Deckert Friedrich Schiller Universität Jena Institut für Physikalische Chemie Helmholtzweg 4 07743 Jena Germany and IPHT Institut für Photonische Technologien e.V. Albert-Einstein-Str. 9 07745 Jena Germany

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List of Contributors

Wolfgang Drachsel Technische Universität Wien Institut für Materialchemie Getreidemarkt 9 1060 Wien Austria

Roland Hergenröder Leibniz-Institut für Analytische Wissenschaften - ISAS - e.V. Otto-Hahn-Str. 6b 44227 Dortmund Germany

Laszlo Fabry Wacker Chemie AG Johannes-Hess-Str. 24 84489 Burghausen Germany

Wieland Hill LIOS Technology GmbH Schanzenstr. 39 51063 Köln Germany

Gernot Friedbacher Technische Universität Wien Institut für Chemische Technologien und Analytik Getreidemarkt 9/164-IAC 1060 Wien Austria

Karsten Hinrichs Leibniz-Institut für Analytische Wissenschaften - ISAS - e.V. Department Berlin Albert-Einstein-Str. 9 12489 Berlin Germany

P. Neil Gibson European Commission – Joint Research Centre Institute for Health and Consumer Protection TP 500 21027 Ispra, VA Italy

Volker Hoffmann Leibniz-Institut für Festkörper- und Werkstoffforschung IFW Institut für Komplexe Materialien Helmholtzstr. 20 01069 Dresden Germany

Bernd Gruska SENTECH Instruments GmbH Schwarzschildstr. 2 12489 Berlin Germany

Herbert Hutter Technische Universität Wien Institut für Chemische Technologien und Analytik Getreidemarkt 9/164-IAC 1060 Wien Austria

Georg Held University of Reading Department of Chemistry Whiteknights P.O. Box 224 Reading, Berkshire RG6 6AD UK

Holger Jenett Albrecht-Dürer-Gymnasium Heinitzstr. 73 58097 Hagen Germany

List of Contributors

Michael Kopnarski Institut für Oberflächen- und Schichtanalytik IFOS GmbH Trippstadter Str. 120 67663 Kaiserslautern Germany Bernhard Lendl Technische Universität Wien Institut für Chemische Technologien und Analytik Getreidemarkt 9/164-UPA 1060 Wien Austria Siegfried Pahlke Analytical Consulting Keltenstr. 7 84375 Kirchdorf am Inn Germany Leopold Palmetshofer Johannes Kepler Universität Linz Institut für Halbleiter- und Festkörperphysik Altenbergerstr. 69 4040 Linz Austria Alfred Quentmeier Zum Paradies 2a 34516 Vöhl Germany Marc Richter IPHT Institut für Photonische Technologien e.V. Albert-Einstein-Str. 9 07745 Jena Germany

John C. Rivière Oxford University Begbroke Science Park (OUBSP) Department of Materials Sandy Lane Yarnton Kidlington OX5 1PF UK Volker Rupertus SCHOTT AG Corporate Research & Technology Development Process Technology and Characterization Hattenbergstr. 10 55122 Mainz Germany Günther Rupprechter Technische Universität Wien Institut für Materialchemie Getreidemarkt 9 1060 Wien Austria Reinhard Schneider Karlsruher Institut für Technologie (KIT) Laboratorium für Elektronenmikroskopie Engesserstr. 7 76131 Karlsruhe Germany Yuri Suchorski Technische Universität Wien Institut für Materialchemie Getreidemarkt 9 1060 Wien Austria

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Wolfgang S.M. Werner Technische Universität Wien Institut für Angewandte Physik Wiedner Hauptstr. 8 1040 Wien Austria

1

1 Introduction John C. Rivière and Henning Bubert

Wherever the properties of a solid surface are important, it is also important to have the means to measure those properties. The surfaces of solids play an overriding part in a remarkably large number of processes, phenomena, and materials of technological importance. These include: catalysis; corrosion, passivation, and rusting; adhesion; tribology, friction, and wear; brittle fracture of metals and ceramics; microelectronics; composites; surface treatments of polymers and plastics; protective coatings; superconductors; and solid-surface reactions of all types with gases, liquids, or other solids. The surfaces in question are not always external; processes occurring at inner surfaces such as interfaces and grain boundaries are often just as critical to the behavior of the material. In all of the above examples, the nature of a process or of the behavior of a material can be understood completely only if information about both the surface composition (i.e., the types of atoms present and their concentrations) and the surface chemistry (i.e., the chemical states of the atoms) is available. Furthermore, knowledge of the arrangement of surface atoms (i.e., the surface structure) is also necessary. First of all, what is meant by a solid surface? Ideally, the surface should be defined as the plane at which the solid terminates – that is, the last atom layer before the adjacent phase (vacuum, vapor, liquid, or another solid) begins. Unfortunately such a definition is impractical, because the effect of termination extends into the solid beyond the outermost atom layer. Indeed, the current definition is based on that knowledge, and the surface is thus regarded as consisting of that number of atom layers over which the effect of termination of the solid decays until bulk properties are reached. In practice, this decay distance is of the order of 5–20 nm. By a fortunate coincidence, the depth into the solid from which information is provided by the techniques described here matches the above definition of a surface in many cases. These techniques are, therefore, surface-specific; in other words, the information they provide comes only from that very shallow depth of a few atom layers. Other techniques can be surface-sensitive, in that they would normally be regarded as techniques for bulk analysis, but have sufficient sensitivity for certain elements that can be analyzed only if they are present on the surface. Surface and Thin Film Analysis: A Compendium of Principles, Instrumentation, and Applications, Second Edition. Edited by Gernot Friedbacher, Henning Bubert. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

2

1 Introduction

Why should surfaces be so important? The answer is twofold. First, the properties of surface atoms are usually different from those of the same atoms in the bulk; and second, because in any interaction of a solid with another phase the surface atoms are the first to be encountered. Even at the surface of a perfect single crystal the surface atoms behave differently from those in the bulk, simply because they do not have the same number of nearest neighbors; their electronic distributions are altered, and hence their reactivity. Their structural arrangement is often also different. When the surface of a polycrystalline or glassy multielemental solid is considered – such as that of an alloy or a chemical compound – the situation can be very complex. The processes of preparation or fabrication can produce a material, the surface composition of which is quite different from that of the bulk, in terms of both constituent and impurity elements. Subsequent treatment (e.g., thermal and chemical) will almost certainly change the surface composition to something different again. The surface is highly unlikely to be smooth, and roughness at both the micro and macro level can be present, leading to the likelihood that many surface atoms will be situated at corners and edges and on protuberances (i.e., in positions of increased reactivity). Surfaces exposed to the atmosphere, which include many of those of technological interest, will acquire a contaminant layer that is one to two atom layers thick, containing principally carbon and oxygen but also other impurities present in the local environment. Atmospheric exposure might also cause oxidation. Because of all these possibilities, the surface region must be considered as a separate entity, effectively a separate quasi-two-dimensional (2-D) phase overlaying the normal bulk phase. Analysis of the properties of such a quasi phase necessitates the use of techniques in which the information provided originates only or largely within the phase – that is, the surface-specific techniques described in this volume. Nearly all these techniques involve interrogation of the surface with a particle probe. The function of the probe is to excite surface atoms into states giving rise to the emission of one or more of a variety of secondary particles such as electrons, photons, ions, and neutrals. Since the primary particles used in the probing beam can also be electrons or photons, or ions or neutrals, many separate techniques are possible, each based on a different primary–secondary particle combination. Most of these possibilities have now been established, but in fact not all the resulting techniques are of general application – some due to the restricted or specialized nature of the information obtained, and others due to difficult experimental requirements. In this book, therefore, most space is devoted to those surface analytical techniques that are widely applied and readily available commercially, whereas much briefer descriptions are provided of some others, the use of which is less common but which – under appropriate circumstances, particularly in basic research – can provide vital information. Since the various types of particle can appear in both primary excitation and secondary emission, most authors and reviewers have found it convenient to group the techniques in a matrix, in which the rows refer to the nature of the exciting particle and the columns to the nature of the emitted particle. Such a matrix of techniques is provided in Table 1.1, which uses widely accepted acronyms. The

1 Introduction Table 1.1 Surface-specific analytical techniques using particle or photon excitation. The acronyms (see Listing 1.1) printed in bold are those used for methods discussed in more detail in this book.

Excitationa)

Detection

Ions, neutrals, A+, A−, A0

−

Electrons, e e−

AES EELS EFTEM LEED

SAM

A+, A−, A0

ESD

ESDIAD

hv

EDXS SXAPS IPES

Photons, hv

IAES INS

XPS

UPS

TXRF XRD LA RAIRS SHG ELL SNOM

LIBS SERS SFG

RHEED SIMS GDMS RBS ERDA

SNMS FABMS LEIS NRA

GD-OES IBSCA BIS

a) Some of the techniques in Table 1.1 have angle-resolved variants, with the prefix AR (e.g., ARUPS), or use Fourier-transform methods, with the prefix FT (e.g., FT-RAIRS). Table 1.2

Surface-specific analytical techniques using non-particle excitation.

Detection

Excitation Heat, kT

High electrical field, F

A+

TDS

FIM

A−

TDS

e− (Displacement)

Mechanical force

AP

IETS STM, STS AFM

meanings of the acronyms, together with some of the alternatives that have appeared in the literature, are provided in Listing 1.1. A few techniques cannot be classified according to the nature of the exciting particle, because they do not employ primary particles but depend instead on the application either of heat or a high electric field. These techniques are listed in Table 1.2.

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1 Introduction

Listing 1.1. Meanings of the surface analysis acronyms, and their alternatives, that appear in Tables 1.1. and 1.2. 1.

Electron Excitation AES, Auger electron spectroscopy BIS, Bremsstrahlung isochromat spectroscopy (or ILS, ionization loss spectroscopy) EDXS, Energy-dispersive X-ray spectroscopy EELS, Electron energy loss spectroscopy EFTEM, Energy-filtered transmission electron microscopy ESD, Electron-stimulated desorption (or EID, electron-induced desorption) ESDIAD, Electron-stimulated desorption ion angular distribution IPES, Inverse photoemission spectroscopy LEED, Low-energy electron diffraction RHEED, Reflection high-energy electron diffraction SXAPS, Soft X-ray appearance potential spectroscopy (or APS, appearance potential spectroscopy) SAM, Scanning Auger microscopy

2. Ion Excitation ERDA, Elastic recoil detection analysis GDMS, Glow discharge mass spectrometry GD-OES, Glow discharge optical emission spectroscopy IAES, Ion (excited) Auger electron spectroscopy IBSCA, Ion beam spectrochemical analysis (or SCANIIR, surface composition by analysis of neutral and ion impact radiation or BLE, bombardmentinduced light emission) INS, Ion neutralization spectroscopy LEIS, Low-energy ion scattering (or ISS, Ion-scattering spectroscopy) NRA, Nuclear reaction analysis RBS, Rutherford back-scattering spectroscopy (or HEIS, high-energy ion scattering) SIMS, Secondary-ion mass spectrometry (SSIMS, static secondary-ion mass spectrometry) (DSIMS, dynamic secondary-ion mass spectrometry)

1 Introduction

SNMS, Secondary neutral mass spectrometry 3. Photon Excitation ELL, Ellipsometry LA, Laser ablation LIBS, Laser-induced breakdown spectroscopy (or LIPS, Laser-induced plasma spectroscopy) RAIRS, Reflection–absorption infrared spectroscopy (or IRRAS, infrared reflection–absorption spectroscopy, or IRAS, infrared absorption spectroscopy, or ERIRS, external reflection infrared spectroscopy) SERS, Surface-enhanced Raman scattering SFG, Sum frequency generation SHG, (optical) Second harmonic generation SNOM, Scanning near-field optical microscopy TXRF, Total reflection X-ray fluorescence analysis UPS, Ultraviolet photoelectron spectroscopy XPS, X-ray photoelectron spectroscopy (or ESCA, electron spectroscopy for chemical analysis) XRD, X-ray diffraction 4. Neutral Excitation FABMS, Fast-atom bombardment mass spectrometry 5. Thermal Excitation TDS, Thermal desorption spectroscopy 6. High-Field Excitation AP, Atom probe FIM, Field ion microscopy IETS, Inelastic electron tunneling spectroscopy STM, Scanning tunneling microscopy STS, Scanning tunneling spectroscopy 7. Mechanical Force AFM, Atomic force microscopy

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Part One Electron Detection

9

2 X-Ray Photoelectron Spectroscopy (XPS) Henning Bubert, John C. Rivière, and Wolfgang S.M. Werner

X-ray photoelectron spectroscopy (XPS) is one of the most widely used surface analytical techniques, and is therefore described here in more detail than any of the other techniques. At its inception by Siegbahn and coworkers [1], it was called ESCA (electron spectroscopy for chemical analysis), but the name ESCA is now considered too general, as many surface electron spectroscopies exist, and the name given to each one must be precise. Nevertheless, the name ESCA is still used in many places, particularly in industrial laboratories and their publications. Briefly, the reasons for the popularity of XPS are the exceptional combination of compositional and chemical information that it provides, its ease of operation, and the ready availability of commercial equipment.

2.1 Principles

The surface to be analyzed is irradiated with soft X-ray photons. When a photon of energy hν interacts with an electron in a level with binding energy EB (EB is the energy EK of the K-shell in Figure 2.1), the entire photon energy is transferred to the electron, with the result that a photoelectron is ejected with kinetic energy E kin = hv − EB − ΦS

(2.1)

where ΦS is a small, almost constant, work-function term. Obviously, hν must be greater than EB. The ejected electron may come from a core level or from the occupied portion of the valence band, but in XPS most attention is focused on the electrons in core levels. As no two elements share the same set of electronic binding energies, measurement of the photoelectron kinetic energies enables elemental analysis. In addition, Equation 2.1 indicates that any changes in EB are reflected in Ekin, which means that changes in the chemical environment of an atom can be followed by monitoring changes in the photoelectron energies, leading to the provision of chemical information. In principle, XPS can be used to analyze all elements in the Periodic Table; however, in general Surface and Thin Film Analysis: A Compendium of Principles, Instrumentation, and Applications, Second Edition. Edited by Gernot Friedbacher, Henning Bubert. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

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2 X-Ray Photoelectron Spectroscopy (XPS)

Figure 2.1 Schematic diagram of electron emission processes in solids. Left: Auger process; Right: photoelectron emission process. Electrons involved in the emission processes are indicated by open circles.

hydrogen and helium cannot be detected due to the low cross-section of interaction. Although XPS is concerned principally with photoelectrons and their kinetic energies, the ejection of electrons by other processes also occurs. An ejected photoelectron leaves behind a core hole in the atom. The sequence of events following the creation of the core hole is shown schematically in Figure 2.1. In the example, the hole has been created in the K shell, giving rise to a photoelectron whose kinetic energy would be (hν − EK). This hole can be filled, for example, by an electronic transition from the L1 shell (Figure 2.1, left). The energy EK − EL1 associated with the transition can then either be dissipated as a characteristic X-ray photon or given up to an electron in the same or a higher shell, in this example the L23 shell. The second of these possibilities is called the Auger process after its discoverer [2], and the resulting ejected electron is called an Auger electron and has an energy given by E kin (KL1L23 ) = EK − EL1 − EL23 − E inter (L1L23 ) + ER − ΦS

(2.2)

where Einter(L1L23) is the interaction energy between the holes in the L1 and L23 shells, and ER is the sum of the intra- and extra-atomic relaxation energies. X-ray photon emission (i.e., X-ray fluorescence) and Auger electron emission are obviously competing processes, but for the shallow core levels involved in XPS and Auger electron spectroscopy (AES) the Auger process is far more likely. Thus, in all X-ray photoelectron spectra, features appear due to both photoemission and Auger emission. In XPS, the Auger features can be useful but are not central to the technique, whereas in AES (see Chapter 3 and Equation 2.2) they form the basis of the technique. At this point, the nomenclature used in XPS and AES should be explained. In XPS, the spectroscopic notation is used, and in AES the X-ray notation. The two are equivalent, the different usages having arisen for historical reasons, but the differentiation is a convenient one. They are both based on the so-called j − j coupling

2.1 Principles Table 2.1

Spectroscopic and X-ray notations.

Quantum numbers n 1 2 2 2 3 3 3 3 3

l 0 0 1 1 0 1 1 2 2 etc.

j 1/2 1/2 1/2 3/2 1/2 1/2 3/2 3/2 5/2

Spectroscopic state

X-ray state

1s 2s 2p1/2 2p3/2 3s 3p1/2 3p3/2 3d3/2 3d5/2 etc.

K L1 L2 L3 M1 M2 M3 M4 M5 etc.

scheme describing the orbital motion of an electron around an atomic nucleus, in which the total angular momentum of an electron is found by summing vectorially the individual electron spin and angular momenta. Thus, if l is the electronic angular momentum quantum number and s the electronic spin momentum quantum number, the total angular momentum for each electron is given by j = l + s. Since l can take the values 0, 1, 2, 3, 4, … and s = ± 21 , clearly j = 21 , 23 , 25 , etc. The principal quantum number n can take values 1, 2, 3, 4, … . In spectroscopic notation, states with l = 0, 1, 2, 3, … are designated s, p, d, f, … , respectively, and the letter is preceded by the number n; the j values are then appended as suffixes. Therefore, one obtains 1s, 2s, 2p1/2, 2p3/2, 3s, 3p1/2, 3p3/2, etc. In X-ray notation, states with n = 1, 2, 3, 4, … are designated K, L, M, N, … , respectively, and states with various combinations of l = 0, 1, 2, 3, … and j = 21 , 23 , 25 are appended as the suffixes 1, 2, 3, 4, … . In this way, one arrives at K, L1, L2, L3, M1, M2, M3, etc. The equivalence of the two notations is set out in Table 2.1. In X-ray notation, the Auger transition shown in Figure 2.1 would therefore be labeled KL1L23. In this coupling scheme, six Auger transitions would be possible in the KLL series. Obviously, many other series are possible (e.g., KLM, LMM, MNN). These are discussed more fully in Chapter 3. The reasons why techniques such as XPS and AES, which involve measurement of the energies of ejected electrons, are so surface-specific, should be examined. An electron with kinetic energy E moving through a solid matrix M has a probability of traveling a certain distance before losing all or part of its energy as a result of an inelastic collision. Based on that probability, the average distance traveled before such a collision is known as the inelastic mean free path (IMFP) λM(E). The IMFP is a function only of M and of E. Figure 2.2 shows a compilation of measurements of λ made by Seah and Dench [3], in terms of atomic monolayers as a function of kinetic energy. Note that both λ and energy scales are logarithmic. The important consequence of the dependence of λ on kinetic energy is that in the ranges of secondary electron kinetic energies used in XPS and AES, the values of

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2 X-Ray Photoelectron Spectroscopy (XPS)

Figure 2.2 Compilation by Seah and Dench [3] of measurements of inelastic mean free path as a function of electron kinetic energy. The solid line is a least-squares fit.

λ are very small. In XPS, for example, typical energy ranges are 250–1500 eV, corresponding to a range of λ from about four to eight monolayers, while in AES, the energy range is typically 20 to 1000 eV, in which case λ would range from about two to six monolayers. What this means in practice is that if the photoelectron or the Auger electron is to escape into a vacuum and be detected, it must originate at or very near the surface of the solid. This is the reason why the electron spectroscopic techniques are surface-specific. Furthermore, the inelastic mean free path is of paramount importance for quantifying XPS-data. Presently, it is most commonly accepted that the calculations, and in particular the semiempirical formula for the IMFP, by Tanuma, Powell, and Penn, constitute a reliable source for this quantity [4].

2.2 Instrumentation 2.2.1 Vacuum Requirements

Electron spectroscopic techniques require a vacuum on the order of 10−8 Pa for their operation. This requirement arises from the extreme surface specificity of these techniques, as discussed in Section 2.1. With sampling depths of only a few atom layers, and elemental sensitivities down to 10−5 atom layers (i.e., one atom of a particular element in 105 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, for sufficient

2.2 Instrumentation

time to be available to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10−8 Pa or lower – that is, in the region of ultrahigh vacuum (UHV). The requirement for UHV conditions imposes restrictions on the types of material that can be used for the construction of surface analytical systems, or inside the systems, because UHV can be achieved only by accelerating the rate of removal of gas molecules from internal surfaces by raising the temperature of the entire system (i.e., by baking). Typical baking conditions are 150–200 °C for several hours. Inside the system, any material is permissible that does not produce volatile components either during normal operation or during baking. For example, brass (which contains the volatile metal zinc) cannot be used. The principal construction material is stainless steel, with mu-metal (76% Ni, 5% Cu, 2% Cr) used occasionally where magnetic screening is needed (e.g., around electron energy analyzers). For the same reasons, metal seals – not elastomers – are used for the demountable joints between individual components; the sealing material normally used is pure copper, although gold is sometimes employed. Other materials that may be used between ambient atmosphere and UHV are borosilicate glass or quartz for windows, and alumina for electrical insulation for current or voltage connections. 2.2.2 X-Ray Sources

The most important consideration in choosing an X-ray source for XPS is energy resolution. Equation 2.1 gives the relationship between the kinetic energy of the photoelectron, the energy of the X-ray photon, and the binding energy of the core electron. Since the energy spread – or linewidth – of an electron in a core level is very small, the linewidth of the photoelectron energy depends on the linewidth of the source, if no undue broadening is introduced instrumentally. In XPS, the analyst devotes much effort to extracting chemical information by means of detailed study of individual elemental photoelectron spectra. Such a study needs an energy resolution better than 1.0 eV, if subtle chemical effects are to be identified. Thus, the linewidth of the X-ray source should be significantly smaller than 1.0 eV, if the resolution required is not to be limited by the source itself. Other considerations are that the source material – which forms a target for high-energy electron bombardment leading to the production of X-rays – should be a good conductor to allow the rapid removal of heat, and it should also be compatible with UHV. Table 2.2 lists the energies and linewidths of the characteristic X-ray lines from a few possible candidate materials. In practice, MgKα and AlKα are the two used universally, due to their line energies and width and their simple use as anode materials. For the efficient production of X-rays by electron bombardment, exciting electron energies that are at least an order of magnitude higher than the line energies

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2 X-Ray Photoelectron Spectroscopy (XPS) Table 2.2 Energies and linewidths of some characteristic low-energy X-ray lines.

Line

Energy (eV)

Width (eV)

YMζ ZrMζ NbMζ MgKα AlKα SiKα YLα ZrLα

132.3 151.4 171.4 1253.6 1486.6 1739.5 1922.6 2042.4

0.47 0.77 1.21 0.70 0.85 1.00 1.50 1.70

Table 2.3 Satellite lines of magnesium and aluminum [5].

X-ray line

Kα′ Kα3 Kα4 Kα5 Kα6 Kβ

Mg

Al

Separation from Kα1,2 (eV)

Relative intensity, % (Kα1,2 = 100%)

Separation from Kα1,2 (eV)

Relative intensity, % (Kα1,2 = 100%)

4.5 8.4 10.0 17.3 20.5 48.0

1.0 9.2 5.1 0.8 0.5 2.0

5.6 9.6 11.5 19.8 23.4 70.0

1.0 7.8 3.3 0.4 0.3 2.0

must be used, so that in Mg and Al sources accelerating potentials of 15 kV are employed. Modern sources are designed with dual anodes; in this case, one anode face is coated with magnesium and the other with aluminum, and with two filaments, one for each face. Thus, a switch from one type of X-irradiation to the other can be made very quickly. To protect the sample from stray electrons from the anode, from heating effects, and from possible contamination by the source enclosure, a thin (ca. 2 μm) window of aluminum foil is interposed between the anode and the sample. For optimum X-ray photon flux on the surface (i.e., optimum sensitivity), the anode must be brought as close to the sample as possible, which means in practice a distance of ≈2 cm. The entire X-ray source is therefore retractable by means of a bellows and a screw mechanism. The X-radiation from magnesium and aluminum sources is quite complex. The principal Kα lines are in fact unresolved doublets and should correctly be labeled Kα1,2. Besides the Kα1,2 lines, a series of further lines – so-called “satellite lines” – also exist, of which the more important ones are Kα3,4. The energy separations of the satellite lines for Mg and Al and their intensities relative to Kα1,2 are listed in Table 2.3.

2.2 Instrumentation

Figure 2.3 Schematic of X-ray monochromatization to remove satellites (S), eliminate

Bremsstrahlung background (B), and separate the Al Kα1,2 doublet. X denotes the characteristic Al Kα only. Illustration courtesy of Kratos Analytical.

The removal of satellites, elimination of the Bremsstrahlung background, and separation of the Kα1,2 doublet can be achieved by monochromatization, as shown schematically in Figure 2.3. The X-ray source is positioned at one point on a spherical surface, called a Rowland sphere, and a quartz crystal is placed at another point. X-rays from the source are diffracted from the quartz, and by placing the sample at the correct point on the Rowland sphere, the Kα1 component can be selectively focused on it. Quartz is a very convenient diffracting medium for AlKα, because the spacing between the 1010 planes is exactly half the wavelength of the X-radiation. Since the width of the AlKα1 line is