Beam effects, surface topography, and depth profiling in surface analysis

Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis

METHODS OF SURFACE CHARACTERIZATION Series Editors: Cedric J. Powell, National Institute of Standards and Technology, Gaithersburg, Maryland Alvin W. Czanderna, National Renewable Energy Laboratory, Golden, Colorado David M. Hercules, Vanderbilt University. Nashville, Tennessee Theodore E. Madey, Rutgers, The State University of New Jersey, Piscataway, New Jersey John T. Yates, Jr., University ofPittsburgh, Pittsburgh, Pennsylvania

Volume 1

VIBRATIONAL SPECTROSCOPIES OF MOLECULES ON SURFACES Edited by John T. Yates, Jr., and Theodore E. Madey

Volume 2

ION SPECTROSCOPIES FOR SURFACE ANALYSIS Edited by A. W. Czanderna and David M. Hercules

Volume 3

SURFACE INFRARED AND RAMAN SPECTROSCOPY Methods and Applications W. Suëtaka with the assistance of John T. Yates, Jr.

Volume 4

SPECIMEN HANDLING, PREPARATION, AND TREATMENTS IN SURFACE CHARACTERIZATION Edited by Alvin W. Czanderna, Cedric J. Powell, and Theodore E.Madey

Volume 5

BEAM EFFECTS, SURFACE TOPOGRAPHY, AND DEPTH PROFILING IN SURFACE ANALYSIS Edited by Alvin W. Czanderna, Theodore E. Madey, and Cedric J. Powell

A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately upon publication. Volumes are billed only upon actual shipment. For further information please contact the publisher.

Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis Edited by

Alvin W. Czanderna National Renewable Energy Laboratory Golden, Colorado

Theodore E. Madey Rutgers, The State University of New Jersey Piscataway, New Jersey

and

Cedric J. Powell National Institute of Standards and Technology Gaithersburg, Maryland

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

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Contributors Peter R. Boudewijn, Philips Research Laboratories, 5656 AA Eindoven, The Netherlands Alvin W. Czanderna, National Renewable Energy Laboratory, Golden, CO 80401 Linda S. Dake, National Renewable Energy Laboratory, Golden, CO 80401 John A. Dagata, National Institute of Standards and Technology, Gaithersburg, MD 20899 Andrew S. D’Souza, Pennsylvania State University, University Park, PA 16802 Kiyoshi Iizuka, Nikon Corporation, Tsukuba, Ibaraki, 300-26, Japan

David E. King, National Renewable Energy Laboratory, Golden, CO 80401 Carlo G. Pantano, Pennsylvania State University, University Park, PA 16802 J. Roland Pitts, National Renewable Energy Laboratory, Golden, CO 80401 Alan M. Then, Pennsylvania State University, University Park, PA 16802 John H. Thomas III, 3-M Corporate Research Laboratory, 3-M Co., St. Paul, MN 55144

Theodore V. Vorburger, National Institute of Standards and Technology, Gaithersburg, MD 20899 Helmut W. Werner, Philips Research Laboratories, 5656 AA, Einhoven, The Netherlands and Technical University, Vienna Austria Günter Wilkening, Physikalisch-Technische Bundestalt, 38023 Braunschweig, Germany

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Foreword Many books are available that detail the basic principles of the different methods of surface characterization. On the other hand, the scientific literature provides a resource of how individual pieces of research are conducted by particular laboratories. Between these two extremes the literature is thin but it is here that the present volume comfortably sits. Both the newcomer and the more mature scientist will find in these chapters a wealth of detail as well as advice and general guidance of

the principal phenomena relevant to the study of real samples. In the analysis of samples, practical analysts have fairly simple models of how

everything works. Superimposed on this ideal world is an understanding of how the parameters of the measurement method, the instrumentation, and the characteristics of the sample distort this ideal world into something less precise, less controlled, and less understood. The guidance given in these chapters allows the scientist to understand how to obtain the most precise and understood measurements that are currently possible and, where there are inevitable problems, to have clear guidance as the extent of the problem and its likely behavior. In many instances, when first considering a sample we wish to know something of the surface topography. Chapter 4 provides a full review of the possibilities from the finest level used in metrological laboratories to the practical requirements of engineers. Modern, functional scanned probes are discussed together with their limitations. Traceability and calibration are an important interest of the authors and here clear, practical advice is given. More traditional and optical methods are also covered. Following a study of the physical characterization of the surface, the surface may be analyzed by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), or secondary ion mass spectrometry (SIMS). Both AES and XPS may damage the surface being studied. AES is more damaging than XPS but since XPS is often used to study more delicate materials, the problems require understanding for both techniques. Chapters 1 and 2 cover these aspects clearly and in depth, with many excellent examples for different classes of materials. Electron stimulated adsorption and desorption are covered for AES and in sufficient detail vii

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Foreword

to alert users of other techniques, where an electron beam is added for charge neutralization or stabilization, to the importance of the problem. The greater part of surface analysis deals with composition-depth profile measurements. Either by using an addition ion gun or by using the probe ion gun, the surface is removed layer by layer by sputtering while surface analysis measurements are conducted. This provides the composition-depth profile. Unfortunately, the measured profile is rarely a perfect representation of the original sample. As the sputtering proceeds, compositional changes occur, elements are redistributed and recombined, and the surface topography changes. By understanding these phenomena the correct instrumental parameters may be chosen to minimize these unwanted effects. Thus, today, we see an increasing amount of work showing depth resolutions at the level of better than 1% of the depth sputtered for depths in the range 100 nm to 1000 nm. The sputtering condition has many degrees of freedom, from ion species and energy to current density and angle of incidence. Varying these parameters leads to many interesting effects detailed in Chapters 3 and 5 with many examples and much good advice on what should and should not be done to obtain meaningful results. A major problem for many practical analysts is the development of surface topography during sputtering. An understanding of this problem has been greatly assisted by the development of scanned probes already discussed in Chapter 4. In all chapters, the reader will find a clear exposition of the phenomena needed to understand the material response with equations kept to a minimum. All chapters have a very full set of figures so that the importance and behavior of the item being discussed can be properly appreciated. As is well known, a picture is worth ten thousand words1 but a diagram may be assimilated in the time taken to read only twenty words. For the reader to obtain more details, very extensive bibliographies are provided. For many of the phenomena, there is a broad understanding of what is going on but there is not always sufficient knowledge of the required parameters to provide simple rules or predictive behavior. In these cases, one must resort to descriptions of observed behavior, classified into whatever groupings are possible. Where this is appropriate, extensive examples of behavior are presented so that the analyst can appreciate what has happened or is likely to happen even if he or she cannot obtain the ideal result! (1) F R Barnard, Printer’s Ink, 10 March 1927

Martin P. Seah National Physical Laboratory Teddington, United Kingdom

About the Series Many techniques are now being used to characterize the chemical and physical properties of surfaces with an emphasis on surface composition, and improvements in these techniques are reported each year. While most of the techniques are generally relatively simple in concept, their successful application involves the use of complex instrumentation, adequate understanding of the physical principles, avoidance of many problems, identification of possible artifacts, and careful analysis of the data. We have designed this series to assist newcomers to the field of surface characterization although we hope that the series will also be of value to more experienced workers. The approach is pedagogical or tutorial. Our main objective is to describe the principles, techniques, and methods that are considered important for surface characterization, with emphasis on how important surface characterization measurements are made and how to ensure that the measurements and interpretations are satisfactory to the greatest extent possible. At this time, we have planned six volumes, but others may follow. The first volume provides a description of methods for vibrational spectroscopy of molecules on surfaces. Most of these techniques are still under active development; commercial instrumentation is not yet available for some techniques, but this situation could change in the next few years. The current state-of-the-art of each technique is described as are the relative capabilities. An important component of the first volume is a summary of the relevant theory. The second volume contains descriptions of ion spectroscopies for surface analysis and is the first of two volumes on the techniques and methods of electron and ion spectroscopies that are in widespread use for surface analysis. These two volumes address techniques for which commercial instrumentation is available. The books are intended to fill the gap between a manufacturer’s handbook and review articles that highlight the latest scientific developments. The third volume is concerned with infrared (IR) and Raman spectroscopies that provide detailed molecular-level information about the species present on surfaces in ultrahigh vacuum as well as in various absorbing media. This book is ix

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About the Series

written for those who are not spectroscopists but who are beginning to use IR or Raman spectroscopy to investigate surfaces of practical materials. The fourth volume is about techniques for specimen handling, preparation, and treatments in surface analysis and other surface studies. It provides a compilation of methods that have proven useful for those engaged in surface science, applied surface science, thin film deposition, surface analysis, and related areas. This fifth volume provides descriptions of common artifacts and problems associated with the bombardment of solid surfaces by photons, electrons, and ions. Descriptions are also given of methods for characterizing the surface topography

and of sputter depth profiling for measuring the near-surface composition of a solid. Surface-characterization measurements are being used increasingly in diverse areas of science and technology. We are confident that this series will be useful in ensuring that these measurements can be made as efficiently and reliably as possible. Comments on the series are welcomed, as are suggestions for volumes on additional topics. C.J. Powell Gaithersburg, Maryland A.W. Czanderna Golden, Colorado D.M. Hercules Nashville, Tennessee T.E. Madey Piscataway, New Jersey J.T. Yates, Jr. Pittsburgh, Pennsylvania

Preface The determination of surface composition is essential in many areas of science and technology and many approaches may be used for this purpose. Each method has particular strengths and limitations that often are directly connected to the physical processes involved. Typically, atoms on the surface and in the near-surface region may be excited by photons, electrons, ions, or neutral species and the detected particles are usually ions, electrons, or photons that have been emitted, ejected, or scattered. A major part of any surface analysis involves using photon, electron, and

ion beams that can damage the sample being studied but the resulting artifacts are rarely discussed in detail in the literature. The purpose of this book is to present a discussion of the damage and artifacts resulting from the beams used in surface compositional analysis or for sputter depth profiling. Characterization of the resulting surface topography is usually necessary for assessing the extent of beam damage, but is also important for reasons given below. Each of the five chapters in this volume contains extensive references to prior work and sufficient figures to illustrate the concepts being described. Tables are used to summarize information about a concept, technique, method, or process. The first chapter deals with photon beam damage in the surface and near-surface of solids, and especially the damage from X-rays used in X-ray photoelectron spectroscopy. Electrostatic charging of samples in photoemission experiments, calibration of the binding energy scale, use of the Auger parameter, and X-ray absorption and damage are emphasized. In the second chapter, the fundamentals of electronic-excitation processes including electron-beam interactions with solid surfaces, electron stimulated desorption or adsorption, decomposition or oxidation of surface layers, and the thresholds for electron-beam damage are discussed. Charging and electromigration in insulators, electron-beam-induced heating, and electron-beam effects during Auger electron spectroscopy surface analysis of several classes of solids are also considered. The third chapter is concerned with ion-beam-bombardment effects of solid surfaces at energies used for sputter depth profiling. The topics discussed include xi

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Preface

the fundamentals of ion-beam/solid interactions such as penetration, implantation, atomic mixing, and sputtering; structural changes that range from bond stretching to amorphizing the solid and the effect of the changes on the properties of the solid; ion-beam-induced topography from microscopic to macroscopic dimensions; compositional changes from differential sputtering and other chemical effects in organic, alloy, semiconductor, metal oxide and other compound materials; and the effects of the sample, ion beam, and instrument used on depth resolution. The fourth chapter is an overview of profiling methods used for the characterization of surface topography. The results obtainable with profiling instruments, a description of stylus-type instruments, methods for optical profiling techniques, types of scanned probe microscopy instruments, applications of the instruments, and intercomparisons among them are presented. A major focus in this chapter is

on scanned probe microscopy (SPM) and includes discussions of instrument calibration, characterization of displacement-type instruments, force-based SPM, methods based on probe-sample interactions, and applications of SPM in industries ranging from data storage to electrochemical science. The fifth chapter provides an overview of sputter depth profiling (SDP) for near-surface compositional analysis. Sputter depth profiling is compared with other destructive and non-destructive methods of depth profiling. The physical basis of sputtering, data for sputtering yields, determination of information depth, and experimental aspects of SDP are presented and followed by a discussion of the analysis of SDP, artifacts, and limitations on the obtainable depth resolution. Finally, application of SDP to a variety of materials and thin-film multilayers and the interpretation of the results obtained are discussed. The editors are deeply grateful to the authors whose work made this book possible, and for taking the time from their active research programs to prepare their contributions. Alvin W. Czanderna Golden, Colorado Theodore E. Madey Piscataway, New Jersey Cedric J. Powell Gaithersburg, Maryland

Contents 1.

Photon Beam Damage and Charging at Solid Surfaces John H. Thomas III 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Electrostatic Charging of Samples in Photoemission Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Electrostatic Surface Charging . . . . . . . . . . . . . . . . 2.2. Differential Surface Charging . . . . . . . . . . . . . . . . 2.3. Lateral and In-Depth Charge Effects . . . . . . . . . . . . 2.4. Small-Spot Analysis Charging Effects . . . . . . . . . . . 3. Energy Scale Calibration . . . . . . . . . . . . . . . . . . . . . . 4. The Auger Parameter: Charge-Independent Chemical Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Photon Damage . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Photon Absorption Processes . . . . . . . . . . . . . . . . 5.2. Radiation Damage to Inorganic Materials . . . . . . . . . 5.3. Photon Damage to Polymers . . . . . . . . . . . . . . . . 6. Closing Comments . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.

1

2 2

8 10 13 13 18 20 20 25 29 34

35

Electron Beam Damage at Solid Surfaces Carlo G. Pantano, Andrew S. D’Souza and Alan M. Then 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Electronic Excitation Processes . . . . . . . . . . . . . 2.1.1. Electron-Stimulated Desorption . . . . . . . . 2.1.2. Electron-Stimulated Adsorption . . . . . . . . 2.1.3. Decomposition of Surface Layers and Thin Films . . . . . . . . . . . . . . . . . . . . xiii

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39 41 41 41 45

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49

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2.1.4. Oxidation of Surface Layers and Thin Films . . . 2.1.5. Thresholds for Sample Damage Resulting from Electronic Excitation . . . . . . . . . . . . . . . . 2.2. Charging Insulators . . . . . . . . . . . . . . . . . . . . 2.3. Electromigration in Insulators . . . . . . . . . . . . . . . 2.4. Electron-Beam-Induced Heating . . . . . . . . . . . . . . . 3. Electron Beam Effects in Auger Surface Analyses . . . . . . . . . 3.1. Physical Effects . . . . . . . . . . . . . . . . . . . . . . 3.2. Contaminated, Oxidized, or Coated Surfaces . . . . . . . 3.3. Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Sputter Depth Profiles . . . . . . . . . . . . . . . . . . . 3.6. Microanalyses . . . . . . . . . . . . . . . . . . . . . . 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Review of Beam Damage in Electron Microscopy . . . . 5.2. General Discussions of Electron Beam Damage in Surface Analysis . . . . . . . . . . . . . . . . . . . . . . 5.3. Fundamentals of Electron-Stimulated Desorption . . . . . 5.4. Studies of Electron Beam Interactions at Solid Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Charging of Insulators Resulting from Electron Beam Irradiation . . . . . . . . . . . . . . . . . . . . . . 5.6. Electron Beam Damage in Glasses . . . . . . . . . . . . . 5.7. Electron Beam Damage during Surface Analysis . . . . . 3.

53 54 58 60 64 65 65 69 74 74 80 83 85 87 87 87 87 88 92 93 94

Ion Beam Bombardment Effects on Solid Surfaces at Energies Used for Sputter Depth Profiling L. S. Dake, D. E. King, J. R. Pitts, and A. W. Czanderna 1.

2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Ion Beams and Solids: Topics Not Covered . . . . . . . . 1.3. Definitions and Nomenclature . . . . . . . . . . . . . . 1.4. Overview of Ion–Surface and Ion–Solid Interactions . . Ion Beam–Solid Interactions. . . . . . . . . . . . . . . . . . . . 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Ion Beam . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Reflection/Backscattering . . . . . . . . . . . . 2.2.2. Penetration and Trapping . . . . . . . . . . . . 2.3. Ion–Substrate Interactions . . . . . . . . . . . . . . . . .

. . . . . . . . . . .

97 97 102

103 105 108 108 110 110 111 116

Contents

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2.4.

Mixing and Implantation of Material . . . . . . . . . . . . 2.4.1. Ballistic Mixing . . . . . . . . . . . . . . . . . . . 2.4.2. Diffusional Mixing Processes . . . . . . . . . . . . 2.5. Removal of Material . . . . . . . . . . . . . . . . . . . . 2.5.1. Physical Sputtering . . . . . . . . . . . . . . . . . 2.5.2. Sputter Yields . . . . . . . . . . . . . . . . . . . . 2.5.3. Differential Sputtering ............... 2.6. Altered Layer (Zone of Mixing) . . . . . . . . . . . . . . 3. Structural Changes Resulting from Ion Beam Bombardment. . . 3.1. Bond Stretching, Bond Breaking, and Surface Reconstruction from Ion Beam Bombardment . . . . . . 3.2. Structural Changes as Nanotopography from Ion Beam Bombardment . . . . . . . . . . . . . . . . . . . . . . . 3.3. Surface Defect Formation from Ion Beam Bombardment . . . . . . . . . . . . . . . . . . . . . . . 3.4. Damage Depth and Defect Density from Ion Beam Bombardment . . . . . . . . . . . . . . . . . . . . . . . 3.5. Enhanced Diffusion and Changes in Electrical Properties from Ion Beam Bombardment . . . . . . . . . 4. Physical Effects: Ion-Beam-Induced Topography . . . . . . . . . 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Mechanisms for Topography Development . . . . . . . . . 4.3. Microscopic and Macroscopic Roughness . . . . . . . . . 4.4. Etch Pits . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Pyramids . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Whiskers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8. Ripples and Corrugation . . . . . . . . . . . . . . . . . . 4.9. Sputter-Induced Recrystallization . . . . . . . . . . . . . . 4.10. Coalescence to Form Islands . . . . . . . . . . . . . . . . 4.11. Swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.12. Smoothing . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13. Miscellaneous Results . . . . . . . . . . . . . . . . . . . . 4.13.1. Topographical Differences in the Same Sample . . . . . . . . . . . . . . . . . . . 4.13.2. Topography of Kapton® and Teflon® after Atom Beam Bombardment . . . . . . . . . . . . 4.13.3. Sputtering with Non-Noble-Gas Ions . . . . . . . 4.13.4. Annealing Sputter Damage . . . . . . . . . . . . 4.14. Concluding Remarks . . . . . . . . . . . . . . . . . . . . 5. Compositional Changes and Chemical Effects . . . . . . . . . . 5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .

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118 118 122 123 124 125 131 133 139

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140

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143

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149

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154

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163 168 168 169 173 176 181 182 189 189 193 193 194 194 197

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198 198 201 201 201 201

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5.2. 5.3.

Organic Materials . . . . . . . . . . . . . . . . . . . . . . Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1. Ternary Alloys . . . . . . . . . . . . . . . . . . . 5.4. Semiconductors . . . . . . . . . . . . . . . . . . . . . . . 5.5. Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1. Simple Metal Oxides . . . . . . . . . . . . . . . . 5.5.2. Complex Oxides: Perovskites . . . . . . . . . . . 5.5.3. Complex Oxides: Glasses . . . . . . . . . . . . . 5.6. Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7. Calculations and Simulations . . . . . . . . . . . . . . . . 6. Depth Resolution: Sample, Beam, and Instrumental Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Nature and Condition of the Sample . . . . . . . . . . . . . 6.3. Ion Beam Bombardment Effects . . . . . . . . . . . . . . . 6.4. Instrumental Effects . . . . . . . . . . . . . . . . . . . . . 6.5. Ultimate and Practical Limits on . . . . . . . . . . . . . . 7. Combined Beam Effects . . . . . . . . . . . . . . . . . . . . . . 8. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Summary and Concluding Remarks . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 1. Acronyms and Abbreviations . . . . . . . . . . . . . . . 4.

203 206 210 211 213 213 217 218 221 226 227 227 227 231 239 247 249 251 252 255 273

Characterization of Surface Topography T. V. Vorburger, J. A. Dagata, G. Wilkening, and K. Iizuka 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Results Obtainable with Profiling Instruments . . . . . . . . . . . 2.1. Profile Recordings and Dimensional Measurement . . . . . 2.2. Surface Statistics . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Surface Parameters . . . . . . . . . . . . . . . . . . 2.2.2. Statistical Functions . . . . . . . . . . . . . . . . 2.2.3. Other Statistical Descriptors . . . . . . . . . . . . 2.3. Bandwidth Limits . . . . . . . . . . . . . . . . . . . . . . 3. Stylus Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Height Resolution and Range . . . . . . . . . . . . . . . . 3.2. Lateral Resolution and Range . . . . . . . . . . . . . . . . 3.3. Stylus Load and Surface Deformation . . . . . . . . . . . . 3.4. Other Distortions . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . .

275 279 279 281 282 285 290 290 291 291 293 296 297 299

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3.6. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 300 3.7. Area Profiling with Stylus Instruments . . . . . . . . . . . 300 4. Optical Profiling Techniques . . . . . . . . . . . . . . . . . . . 302 5. Scanned Probe Microscopy . . . . . . . . . . . . . . . . . . . . 307 5.1. Short History of Scanned Probe Microscopy . . . . . . . . 307 5.2. Calibration and Characterization . . . . . . . . . . . . . . 311 5.2.1. Instruments for Displacement Calibration . . . . . 311 5.2.2. Calibration Specimens for Displacement . . . . . . 314 5.2.3. Instruments for Critical Dimensions and High Resolution . . . . . . . . . . . . . . . . . . . 315 5.2.4. Specimens for Critical Dimensions . . . . . . . . . 318 5.3. Other Types of Scanned Probe Microscopes . . . . . . . . 319 5.3.1. Force-Based Methods (Mechanical) . . . . . . . . 321 5.3.2. Methods Based on Other Probe-Sample Interactions . . . . . . . . . . . . . . . . . . . . . 323 5.4. Applications of SPM Measurements . . . . . . . . . . . . . . . 325 5.4.1. Data Storage Industries . . . . . . . . . . . . . . . 325 5.4.2. Microelectronics Industries . . . . . . . . . . . . . 326 5.4.3. Polymers and Coatings Industries . . . . . . . . . 327 5.4.4. Optical Element Industries . . . . . . . . . . . . . 327 5.4.5. Mechanical Parts Industries and Materials Science . . . . . . . . . . . . . . . . . . . . . . . 328 5.4.6. Electrochemical Science . . . . . . . . . . . . . . 329 5.5. Future Directions: Techniques and Instrumentation . . . . 330 6. Intercomparisons . . . . . . . . . . . . . . . . . . . . . . . . . . 334 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 5.

Depth Profiling Using Sputtering Methods H. W. Werner and P. R. Boudewijn 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Principle of Sputter Depth Profiling . . . . . . . . . . . . . 1.2. Methods for Sputter Depth Profiling . . . . . . . . . . . . 1.3. Different Modes of Sputter Depth Profiling . . . . . . . . 1.3.1. Planar Sputter Depth Profiling . . . . . . . . . . . 1.3.2. Crater-Wall, Tapered-Section, or Angular-Mapping Depth Profiling . . . . . . . . . . . . . . . . . . . 1.4. Comparison of Sputter Depth Profiling with Other Methods . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1. Consumptive Methods for Depth Profiling . . . . .

355 357 359 360 360 362 362 362

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1.4.2.

Nonconsumptive Methods and Modes for Depth Profiling . . . . . . . . . . . . . . . . . . . 2. Physical Basis of the Sputtering Process . . . . . . . . . . . . . . 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Theory of the Sputtering Process . . . . . . . . . . . . . . 2.2.1. Binary Collision Theory . . . . . . . . . . . . . . 2.2.2. Classification of Sputtering Events . . . . . . . . 2.2.3. Sputtering from Linear Collision Cascades . . . . 2.3. Sputtering Yields (Experimental) . . . . . . . . . . . . . . 2.4. Information Depth . . . . . . . . . . . . . . . . . . . . . . 2.5. Processes Related to Sputtering . . . . . . . . . . . . . . . . 3. Experimental Aspects . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Ion Beam Sources . . . . . . . . . . . . . . . . . . . . . . 3.3. Time Needed to Obtain a Depth Profile . . . . . . . . . . . 4. Analysis of Sputter Depth Profiles . . . . . . . . . . . . . . . . . . 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Conversion of the Sputter Depth Profile of an Element X, I(X, t), into a Concentration Depth Profile c(z) . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Determination of the Depth Scale . . . . . . . . . 4.2.2. Conversion of the Measured Signal I into an Elemental Concentration c . . . . . . . . . . . . . 4.3. Artifacts in Sputter Depth Profiles . . . . . . . . . . . . . . 4.3.1. Artifacts Related to the Interaction Process between Energetic Projectiles and the Solid . . . . 4.3.2. Artifacts Related to the Properties of the Sample . 4.3.3. Artifacts Related to Instrumental Parameters . . . 4.4. Evaluation of a Measured Depth Profile . . . . . . . . . . . . 4.4.1. Depth Resolution . . . . . . . . . . . . . . . . . . . 4.4.2. Detection Limit for a Given Element . . . . . . . 5. Application of Sputter Depth Profiling to Various Thin-Film Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Stable Isotope Tracers . . . . . . . . . . . . . . . . . . . . 5.2. Thin-Film Interdiffusion and Multilayer Analysis . . . . . . 5.3. Corrosion and Oxidation . . . . . . . . . . . . . . . . . . . . 5.4. Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Polymer–Metal Interfaces . . . . . . . . . . . . . . . . . . 5.6. Insulating Materials . . . . . . . . . . . . . . . . . . . . . 5.7. Semiconductor Materials . . . . . . . . . . . . . . . . . . 5.8. Miscellaneous Applications . . . . . . . . . . . . . . . . . 5.9. Newcomers to Sputter-Depth-Profiling Techniques . . . . .

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6. Summary and Future Prospects . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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

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Photon Beam Damage and Charging at Solid Surfaces John H. Thomas III

1. Introduction Particle-excited surface spectroscopies such as Rutherford backscattering (RBS), secondary-ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES) produce data that may be strongly affected by the interaction of a particle (ion, electron, etc.) with the solid surface of the sample. As described in other chapters in this book, some of these effects can be large and totally obscure the information. Others can be handled with proper mathematical algorithms or instrumentally.(1) Photon-excited spectroscopies [X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS)] were naturally assumed to be the least

“damaging” or perturbing of the available surface methods to the spectroscopist.(2–7) This assumption is mostly true in applications to inorganic material systems. However, when X-ray excited spectroscopy is applied to organics, and to some inorganic polymeric materials, photon absorption can result in materials modification and, in some cases, decomposition. X-ray- or ultraviolet-induced changes are likely to be more common than the literature leads one to believe. Photons used in the production of photoelectrons in insulators or semiconductors cause other effects, such as surface charging and differential charging.(3,8,9) These phenomena can ultimately cause considerable difficulty in data interpretation. This chapter is concerned with these problems, namely how they affect data acquisition, and which

John H. Thomas III • 3M Corporate Research Laboratory, St. Paul, MN 55144. Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis, edited by Czanderna et al. Plenum Press, New York, 1998 1

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methods reduce their effect. Because photons are generally encountered only in XPS, UPS, and synchrotron-excited photoemission, this section is oriented toward understanding problems that can occur in the surface or near-surface regions.

2. Electrostatic Charging of Samples in Photoemission Experiments 2.1. Electrostatic Surface Charging Figure 1 is a block diagram of a typical photoemission experiment. Soft X-ray photons of energy irradiate a material surface, with fluxes ranging from to and produce a flux of photoemitted electrons that, given enough energy, will leave the material surface. Photoelectrons, emitted into the analysis vacuum chamber, are ultimately collected and their kinetic energy analyzed

to determine the chemical elements of a surface of unknown composition. Most photoemission experiments are concerned with accurately measuring the kinetic energy of the emitted photoelectrons that escape without energy loss. Kinetic energy, is commonly measured to an accuracy of 0.1 eV in 1000 eV or 1 part in and is converted to a relative electron binding energy, by using accurate elemental standards of known binding energy to determine the energy scale. Modern methods also include using a field emission electron source powered from an accurate voltage source that can be used to flood the entrance slit of the electron spectrometer. The electron energy is accurately adjusted and used to adjust the spectrometer power supplies for calibration purposes. Binding energy is referenced to the Fermi level of a solid conductive material and is corrected for the spectrometer

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3

contact potential (the difference in work function of the sample and spectrometer detector material). Numerous methods to mechanize binding-energy corrections have been developed and described in detail.(6,9,10) An understanding of the effects of photon irradiation on core-level and valence-level binding energies is very important in the interpretation of photoemission spectra, especially of poorly conducting materials. When photons create a photoelectron current from an insulating surface, a net positive charge is left behind. The positive charge accumulates because the rate of electron current flow into the insulator is insufficient to compensate the positive charge. Hnatowich et al.(11) were the first to investigate this phenomenon systematically as to how it relates to photoelectron emission spectral interpretation. Although it is not our intent to present a detailed account of the theory of

photoelectron production, a brief review is necessary to appreciate how the surface potential depends on this physical process. In general, the photoelectron current, I, can be represented by ( 1 , 4 , 1 2 – 1 5 )

where

is the number of A atoms, i is the core-level designation (e.g., 2p),

is the photoionization cross section of the i core level, is the angular dependence of the photoelectron yield, is the electron escape depth, M is the instrumental property correction, and is the X-ray photon flux. Because all the parameters affect the photoelectron current produced from an insulating (or any) surface, the magnitude of the surface charging potential, under a given set of X-ray irradiation conditions depends on the physics of photoelectron excitation in and from the sample surface. In addition to these parameters, the surface and bulk conductivity affect This effect is readily demonstrated by comparing the photoelectron spectrum from a conductive gold foil with one evaporated onto an insulator surface such as glass, as shown in Fig. 2. In thespectrum,thegrounded conductivegoldfoil peak is observed at 84.0 eV.(16) Under identical irradiation conditions, the peak of the insulated gold film is shifted to a lower kinetic energy by The shift in kinetic (or binding) energy indicates the average value of The value of varies in magnitude with the total X-ray flux which generates the photoelectron current. Surface charge buildup and subsequent peaks shift are larger when a monochromated X-ray source of photons is employed, such as those used in many modern analytical systems. Ebel and Ebel(17) showed that when a standard nonmonochromated X-ray source is used, this shift is partially compensated. Most nonmonochromated X-ray sources use an aluminum window to contain high-energy backscattered electrons generated at the anode and to act as an X-ray photon low-pass filter. A typical source is shown in Fig. 3. Varying the X-ray tube parameters (operating voltage, current, and window material) separates the effect

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of the window on surface charge compensation from other operating parameters.

Backscattered electrons from the X-ray tube anode excite low-energy secondary electrons from the window material (aluminum), and these electrons can flood the emitting sample surface. In addition, X-ray absorption in the window material also produces secondary electrons and primary fluorescent X-rays from the window. Figure 4 shows a typical spectrum of secondary electrons emitted from an aluminum window.(17) Ebel and Ebel(17) also demonstrated that the secondary electron

emission current is governed by the X-ray tube current and not the operating voltage. By applying Kirchhoff’s current law to the irradiated sample surface, one can readily determine if all currents are accurately known. Refer to Fig. 5. The current sum at the node is

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5

where and are the surface and bulk resistances of the irradiated sample, is the total photoelectron current leaving the surface, and is the total incident secondary-electron current from all sources, including the X-ray tube window area. The surface current, varies linearly with X-ray flux, as expected, and is material dependent. If the current leaving the sample surface is large enough, the surface charges positively; if the electron current from the X-ray tube window or a separate source of electrons such as a flood gun (low energy) is large enough, the surface can charge negatively. This produces peaks at a higher kinetic energy than

expected. Thus, the photoelectron peak energy depends on the surface potential, and the correct peak energy is obtained when is zero. Generally, the presence of the X-ray tube window and its low-energy secondaryelectron emission is advantageous. When monochromatic X-rays are employed,

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this form of surface charge compensation of insulating surfaces does not occur, and in cases of high-resistance insulators (e.g., Teflon) the surface potential can be 100 V or more, and a spectrum cannot be obtained without using an external source of low-energy electrons. This situation was typical of the Hewlett–Packard 5950 series of instruments where a well-designed electron flood source was used to compensate for surface charging. Positive surface charge can be compensated by using a low-energy (typically