An introduction to surface analysis by XPS and AES

An Introduction to Surface Analysis by XPS and AES

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An Introduction to Surface Analysis by XPS and AES

John F. Watts University of Surrey, UK

John Wolstenholme Thermo VG Scientific, East Grinstead, UK

WILEY

Copyright © 2003 by John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester. West Sussex PO19 8SQ, England National International

01243 779777 (+44) 1243 779777

E-mail (for orders and customer service enquiries): [email protected] Visit our Home Page on http://www.wiley.co.uk or http://www.wiley.com All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London, UK W1P 9 HE. without the permission in writing of the Publisher. Other Wiley Editorial Offices John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012, USA Wiley-VCH Verlag GmbH, Pappelallee 3, D-69469 Weinheim, Germany John Wiley & Sons (Australia) Ltd, 33 Park Road, Milton. Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark. Singapore 0512 John Wiley & Sons (Canada) Ltd, 22 Worcester Road, Rexdale, Ontario. M9W 1L1. Canada

Library of Congress Cataloging-in-Publication Data Watts, John F. An introduction to surface analysis by XPS and AES/John F. Watts. John Wolstenholme. p. cm. Includes bibliographical references and index. ISBN 0-470-84712-3 (cloth : alk. paper) — ISBN 0-470-84713-1 (pbk. : alk. paper) 1. Surfaces (Technology)—Analysis. 2. Electron spectroscopy. I. Wolstenholme, John. II. Title. TP156.S95W373 2003 620'.44 — dc21 2002153114 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0-470 84712 3 (Hardback) 0-470 84713 1 (Paperback) Typeset in 10.5/13pt Sabon by Thomson Press (India) Ltd., Chennai Printed and bound in Great Britain by TJ International Ltd, Padstow, Cornwall 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.

Contents Preface Acknowledgements

ix xi

1 Electron Spectroscopy: Some Basic Concepts

1

1.1 Analysis of Surfaces 1.2 Notation 1.2.1 Spectroscopists' notation 1.2.2 X-ray notation 1.3 X-ray Photoelectron Spectroscopy (XPS) 1.4 Auger Electron Spectroscopy (AES) 1.5 Scanning Auger Microscopy (SAM) 1.6 The Depth of Analysis in Electron Spectroscopy 1.7 Comparison of XPS and AES/SAM 1.8 The Availability of Surface Analytical Equipment

1 3 3 5 5 7 10 11 13 14

2 Electron Spectrometer Design 2.1 2.2 2.3

2.4 2.5

2.6

2.7

The Vacuum System The Sample X-ray Sources for XPS 2.3.1 The twin anode X-ray source 2.3.2 X-ray monochromators 2.3.3 Charge compensation The Electron Gun for AES 2.4.1 Electron sources Analysers for Electron Spectroscopy 2.5.1 The cylindrical mirror analyser 2.5.2 The hemispherical sector analyser Detectors 2.6.1 Channel electron multipliers 2.6.2 Channel plates Small Area XPS 2.7.1 Lens-defined small area XPS 2.7.2 Source-defined small area analysis

17 17 19 22 22 24 28 28 29 35 35 37 45 45 47 47 48 49

vi

CONTENTS

2.8

XPS Imaging and Mapping 2.8.1 Serial acquisition 2.8.2 Parallel acquisition 2.9 Lateral Resolution in Small Area XPS 2.10 Angle Resolved XPS

3 The Electron Spectrum: Qualitative and Quantitative Interpretation 3.1

3.2

3.3

Qualitative Analysis 3.1.1 Unwanted features in electron spectra 3.1.2 Data acquisition Chemical State Information 3.2.1 X-ray photoelectron spectroscopy 3.2.2 Electron induced Auger electron spectroscopy 3.2.3 The Auger parameter 3.2.4 Chemical state plots 3.2.5 Shake-up satellites 3.2.6 Multiplet splitting 3.2.7 Plasmons Quantitative Analysis 3.3.1 Factors affecting the quantification of electron spectra 3.3.2 Quantification in XPS 3.3.3 Quantification in AES

4 Compositional Depth Profiling 4.1

4.2

4.3

4.4

Non-destructive Depth Profiling Methods 4.1.1 Angle resolved electron spectroscopy 4.1.1.1 Elastic scattering 4.1.1.2 Compositional depth profiles by ARXPS 4.1.1.3 Recent advances in ARXPS 4.1.2 Variation of analysis depth with electron kinetic energy Depth Profiling by Erosion with Noble Gas Ions 4.2.1 The sputtering process 4.2.2 Experimental method 4.2.3 Sputter yield and etch rate 4.2.4 Factors affecting the etch rate 4.2.5 Factors affecting the depth resolution 4.2.6 Calibration 4.2.7 Ion gun design Mechanical Sectioning 4.3.1 Angle lapping 4.3.2 Ball cratering Conclusions

49 50 51 54 56

59 59 60 62 64 64 66 67 69 71 71 73 73 74 75 76

79 79 79 86 87 89 91 93 93 94 96 97 99 103 104 107 107 107 110

CONTENTS

5 Applications of Electron Spectroscopy in Materials Science 5.1 5.2

5.3 5.4 5.5

5.6 5.7

Introduction Metallurgy 5.2.1 Grain-boundary segregation 5.2.2 Electronic structure of metallic alloys 5.2.3 Surface engineering Corrosion Science Ceramics and Catalysis Microelectronics and Semiconductor Materials 5.5.1 Mapping semiconductor devices using AES 5.5.2 Depth profiling of semiconductor materials 5.5.3 Ultra-thin layers studied by ARXPS Polymeric Materials Adhesion Science

6 Comparison of XPS and AES with Other Analytical Techniques 6.1 6.2 6.3 6.4 6.5

X-ray Analysis in the Electron Microscope Electron Analysis in the Electron Microscope Mass Spectrometry for Surface Analysis Ion Scattering Concluding Remarks

vii

113 113 113 114 120 124 131 139 143 143 146 148 149 157

165 167 170 172 178 182

Glossary

183

Bibliography

195

Appendices

203

Appendix 1: Auger Electron Energies Appendix 2: Table of Binding Energies Accessible with AlKc* Radiation

Index

203 204

207

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Preface When one of us (JFW) wrote an earlier introductory text in electron spectroscopy the aim was to fill a gap in the market of the time (1990) and produce an accessible text for undergraduates, first year postgraduates, and occasional industrial users of XPS and AES. In the intervening years the techniques have advanced in both the area of use and, particularly, in instrument design. In XPS X-ray monochromators are now becoming the norm and imaging has become commonplace. In AES, field emission sources are to be seen on high-performance systems. Against that backdrop it was clear that a new, broader introductory book was required which explored the basic principles and applications of the techniques, along with the emerging innovations in instrument design. We hope that this book has achieved that aim and will be of use to newcomers to the field, both as a supplement to undergraduate and masters level lectures, and as a stand-alone volume for private study. The reader should obtain a good working knowledge of the two techniques (although not, of course, of the operation of the spectrometers themselves) in order to be able to hold a meaningful dialogue with the provider of an XPS or AES service at, for example, a corporate research laboratory or service organization. Further information on all the topics can be found in the Bibliography and the titles of papers and so on have been included along with the more usual citations to guide such reading. The internet provides a valuable resource for those seeking guidance on XPS and AES and rather than attempt to be inclusive in our listing of such sites we merely refer readers to the UKSAF site (www.uksaf.org) and its myriad of links. Finally, we have both been somewhat perturbed by the degree of confusion and sometimes contradictory definitions regarding some of the terms used in electron spectroscopy. In an attempt to clarify the situation we have included a Glossary of the more common terms. This

x

PREFACE

has been taken from ISO 18115 and we thank ISO for permission to reproduce this from their original document. John F Watts John Wolstenholme Guildford Surrey UK East Grinstead West Sussex UK

Acknowledgements There are many people who have influenced the development of this book: students, research workers, customers and potential customers, and many colleagues too numerous to mention, both at the University of Surrey and Thermo VG Scientific. At the University of Surrey the staff and students associated with The Surface Analysis Laboratory have provided a stimulating and exciting atmosphere in which to work. Professor Jim Castle has been an inspiration not only to the authors (and one in particular!), but to the entire applied electron spectroscopy community. We both wish him well in his retirement. In addition, Andy Brown and Steve Greaves must be thanked for the production of many of the spectra and other graphics used in the text. At Thermo VG Scientific, Kevin Robinson and Bryan Barnard have provided stimulating leadership in their respective fields, and have provided invaluable assistance in certain areas of the text. Present and former members of Thermo VG Scientific's Applications Laboratory are gratefully acknowledged for their assistance in providing data and valuable information for inclusion in this volume. Certain figures and data have been reproduced from other sources and we thank the copyright holders for their permission to do so. The cover design makes use of original computer graphics generated by Paul Belcher (Thermo VG Scientific).

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1

Electron Spectroscopy: Some Basic Concepts

1.1 Analysis of Surfaces All solid materials interact with their surroundings through their surfaces. The physical and chemical composition of these surfaces determines the nature of the interactions. Their surface chemistry will influence such factors as corrosion rates, catalytic activity, adhesive properties, wettability, contact potential, and failure mechanisms. Surfaces, therefore, influence many crucially important properties of the solid. Despite the undoubted importance of surfaces, only a very small proportion of the atoms of most solids are found at the surface. Consider, for example, a 1 cm cube of a typical transition metal (e.g., nickel). The cube contains about 1023 atoms of which about 1016 are at the surface. The proportion of surface atoms is therefore approximately 1 in 107 or 100 ppb. If we want to detect impurities at the nickel surface at a concentration of 1 per cent then we need to detect materials at a concentration level of 1 ppb within the cube. The exact proportion of atoms at the surface will depend upon the shape and surface roughness of the material as well as its composition. The above figures simply illustrate that a successful technique for analysing surfaces must have at least two characteristics. 1. It must be extremely sensitive. 2. It must be efficient at filtering out signal from the vast majority of the atoms present in the sample.

2

ELECTRON SPECTROSCOPY: SOME BASIC CONCEPTS

This book is largely concerned with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). As will be shown, both of these techniques have the required characteristics but, in addition, they can answer other important questions. 1. Which elements are present at the surface? 2. What chemical states of these elements are present? 3. How much of each chemical state of each element is present? 4. What is the spatial distribution of the materials in three dimensions? 5. If material is present as a thin film at the surface, (a) how thick is the film? (b) how uniform is the thickness? (c) how uniform is the chemical composition of the film? In electron spectroscopy we are concerned with the emission and energy analysis of low-energy electrons (generally in the range 20-2000 eV1). These electrons are liberated from the specimen being examined as a result of the photoemission process (in XPS) or the radiationless de-excitation of an ionized atom by the Auger emission process in AES and scanning Auger microscopy (SAM). In the simplest terms, an electron spectrometer consists of the sample under investigation, a source of primary radiation, and an electron energy analyser all contained within a vacuum chamber preferably operating in the ultra-high vacuum (UHV) regime. In practice, there will often be a secondary UHV chamber fitted with various sample preparation facilities and perhaps ancillary analytical facilities. A data system will be used for data acquisition and subsequent processing. The source of the primary radiation for the two methods is different: X-ray photoelectron spectroscopy makes use of soft X-rays, generally AlKa or MgKa, whereas AES and SAM rely on the use of an electron gun. The specification for electron guns used in Auger analysis varies tremendously, particularly as far as the spatial resolution is concerned which, for finely 1 Units: in electron spectroscopy, energies are expressed in the non-Si unit the electron volt. The conversion factor to the appropriate SI unit is 1 eV = 1.595 x 10 - 1 9 J.

NOTATION

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APPENDICES

Appendix 2: Table of Binding Energies Accessible with AlKa Radiation

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