Scanning Electron Microscopy and. X-Ray Microanalysis SECOND EDITION. A Text for Biologists, Materials Scientists, and Geologists

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1 Scanning Electron Microscopy and X-Ray Microanalysis A Text for Biologists, Materials Scientists, and Geologists SECOND EDITION

2 Scanning Electron Microscopy and X-Ray Microanalysis A Text for Biologists, Materials Scientists, and Geologists SECOND EDITION Joseph I. Goldstein Lehigh University Bethlehem, Pennsylvania Dale E. Newbury National Institute of Standards and Technology Gaithersburg, Maryland Patrick Echlin Uf!iversity of Cambridge Cambridge, England David C. Joy University of Tennessee Knoxville, Tennessee A. D. Romig, Jr. Sandia National Laboratories Albuquerque, New Mexico Charles E. Lyman Lehigh University Bethlehem, Pennsylvania Charles Fiori National Institute of Standards and Technology Gaithersburg, Maryland Eric LHshin General Electric Corporate Research and Development Schenectady, New York PLENUM PRESS. NEW YORK AND LONDON

3 Library of Congress Cataloglng-In-Publicatlon Data ScannIng electron microscopy and x-ray microanalysis biologists, materials SCIentIsts, and geologists! I. Goldst~In,.. ret 21.). -- 2n~ ed. p. c~. Includes bibliographical references and index. ISBN a te>1 for.joseph 1. ScannIng electron microscopy. 2. X-ray microanalysis. 1. GoldsteIn,.Joseph, OH212.S3S '.S 25--oc20 92~9S40 CIP ISBN-13: e-isbn-13: DOl: e 1992, 1981 Plenum Press, New York Softcover reprint of the hardcover 2nd edition 1992 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N,Y, All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

4 Preface In the last decade, since the publication of the first edition of Scanning Electron Microscopy and X-ray Microanalysis, there has been a great expansion in the capabilities of the basic SEM and EPMA. Highresolution imaging has been developed with the aid of an extensive range of field emission gun (FEG) microscopes. The magnification ranges of these instruments now overlap those of the transmission electron microscope. Low-voltage microscopy using the FEG now allows for the observation of noncoated samples. In addition, advances in the development of x-ray wavelength and energy dispersive spectrometers allow for the measurement of low-energy x-rays, particularly from the light elements (B, C, N, 0). In the area of x-ray microanalysis, great advances have been made, particularly with the "phi rho z" [Ij)(pz)] technique for solid samples, and with other quantitation methods for thin films, particles, rough surfaces, and the light elements. In addition, x-ray imaging has advanced from the conventional technique of "dot mapping" to the method of quantitative compositional imaging. Beyond this, new software has allowed the development of much more meaningful displays for both imaging and quantitative analysis results and the capability for integrating the data to obtain specific information such as precipitate size, chemical analysis in designated areas or along specific directions, and local chemical inhomogeneities. During these 10 years we have taught over 1500 students in our Lehigh SEM short course in basic SEM and x-ray microanalysis and have updated our notes to the point that the instructors felt that a completely rewritten book was necessary. In this book we have incorporated information about the new capabilities listed above and added new material on specimen preparation for polymers, a growing area for the use of the SEM. On the other hand, we have retained the features of the First Edition, including the same general chapter headings that have been so well accepted. The authors have noticed that there are generally two groups of students who use this textbook and who attend our course, the real introductory or novice student and the experienced student who is looking to sharpen his or her basic skills and to delve into the newer v

5 vi PREFACE techniques. Therefore, we have decided to highlight in the left margin the material which is essentially basic and should be read by every student who is a novice in the field. We have also added a new introductory chapter on quantitative x-ray microanalysis of bulk samples which will serve as a beginning for those readers interested in quantitation but overwhelmed at first by the physics and the mathematical expressions. This introductory chapter is descriptive in nature with a minimum of equations and should help those readers who want to understand the basic features of the quantitative analysis approach. The authors wish to thank their many colleagues who have contributed to this volume by allowing us to use material from their research, by their criticism of drafts of the chapters, and by their general support. One of the authors (J. I. G.) wishes to acknowledge the research support and encouragement from the Extraterrestrial Materials Program of the National Aeronautics and Space Administration. Special thanks go to Ms. Sharon Coe for her efforts with the manuscript, to Dr. John Friel of Princeton Gamma Tech and Dr. Bill Bastin of the Technical University of Eindhoven for their contributions to the chapters on quantitative x-ray microanalysis, and to Dr. David Williams of Lehigh University for continuous and helpful advice as the textbook was developed. J. I. Goldstein D. E. Newbury

6 Contents 1. Introduction Evolution of the Scanning Electron Microscope Evolution of the Electron Probe Microanalyzer Outline of This Book Electron Optics How the SEM Works Functions of the SEM Subsystems Why Learn about Electron Optics? 24 Electron Guns Thermionic Electron Emission Conventional Triode Electron Guns Brightness Tungsten Hairpin Electron Gun Lanthanum Hexaboride (LaB6) Electron Guns Field Emission Electron Guns Electron Lenses Properties of Magnetic Lenses Lenses in SEMs Producing Minimum Spot Size Lens Aberrations Electron Probe Diameter versus Electron Probe Current Calculation of d min and i max Comparison of Electron Sources Measurement of Microscope Parameters Summary of SEM Microscopy Modes Electron-Specimen Interactions Introduction Electron Scattering Elastic Scattering Inelastic Scattering 73 vii

7 viii 3.3. Interaction Volume Experimental Evidence 79 CONTENTS Monte Carlo Electron-Trajectory Simulation Influence of Beam Energy on Interaction Volume Influence of Atomic Number on Interaction Volume Influence of Specimen Surface Tilt on Interaction Volume Measures of Interaction Volume-Electron Range Bethe Range Kanaya-Okayama Range Range for a Tilted Specimen Comparison of Ranges Range at Low Beam Energy Signals from Elastic Scattering Backscattered Electrons Atomic Number Dependence Beam-Energy Dependence Tilt Dependence Angular Distribution Energy Distribution Lateral Spatial Distribution Sampling Depth of Backscattered Electrons Signals from Inelastic Scattering Secondary Electrons Definition and Origin Energy Distribution Specimen Composition Dependence Beam-Energy Dependence Specimen Tilt Dependence Angular Distribution of Secondary 111 Electrons Range and Escape Depth of Secondary Electrons Relative Contributions of SET and SEll X-Rays Continuum X-Ray Production Inner-Shell Ionization X-Ray Absorption X-Ray Fluorescence Auger Electrons Cathodoluminescence Specimen Heating Summary Image Formation and Interpretation Introduction The Basic SEM Imaging Process 150

8 4.2.l. Scanning Action. 150 ix Image Construction (Mapping) Line Scans. 153 CONTENTS Image (Area) Scanning Digital Imaging: Collection and Display Magnification Picture Element (Pixel) Size Low-Magnification Operation Depth of Field (Focus) Image Distortions l. Projection Distortion: Gnomonic Projection Projection Distortion: Image Foreshortening of Tilted Objects Corrections for Tilted Flat Surfaces Scan Distortion: Pathological Moire Effects Detectors l. Electron Detectors Everhart-Thornley Detector Dedicated Backscattered-Electron Detectors Specimen Current (The Specimen As Detector) Cathodoluminescence Detector Image Contrast at Low Magnification «1O,OOOx) Contrast Compositional (Atomic Number) Contrast Compositional Contrast with Backscattered Electrons Compositional Contrast with Secondary Electrons Compositional Contrast with Specimen Current Topographic Contrast Origin Topographic Contrast with the Everhart-Thornley Detector Light-Optical Analogy Topographic Contrast with Other Detectors Separation of Contrast Components Other Contrast Mechanisms Image Quality High-Resolution Microscopy: Intermediate (1O, ,OOOx) and High Magnification (> 100,000 x ) Electron-Specimen Interactions in High- Resolution Microscopy Backscattered Electrons Secondary Electrons High-Resolution Imaging at High Voltage High-Resolution Imaging at Low Voltage 226

9 x CONTENTS Resolution Improvements: The Secondary Electron Signal Image Interpretation at High Resolution Image Processing for the Display of Contrast Information The Visibility Problem Analog Signal Processing Display of Weak Contrast (Differential Amplification) Enhancement of a Selected Contrast Range (Gamma Processing) Enhancement of Selected Spatial Frequencies (Derivative Processing) Signal Mixing Contrast Reversal Y-Modulation Digital Image Processing Real Time Digital Imaging Off-Line Digital Image Processing Digital Imaging for Minimum-Dose Microscopy Defects of the SEM Imaging Process Contamination Charging Incipient Charging Severe Charging Special Topics in SEM Imaging SEM at Elevated Pressures (Environmental SEM) The Vacuum Environment Detectors for Elevated-Pressure Microscopy Contrast in Elevated-Pressure Microscopy Resolution Benefits of SEM at Elevated Pressures Stereo Microscopy Qualitative Stereo Microscopy Quantitative Stereo Microscopy STEM in SEM Developing a Comprehensive Imaging Strategy X-Ray Spectral Measurement: WDS and EDS Introduction Wavelength-Dispersive Spectrometer Basic Design The X-Ray Detector Detector Electronics Energy-Dispersive X-Ray Spectrometer Operating Principles The Detection Process Charge-to-Voltage Conversion

10 Pulse-Shaping Linear Amplifier and Pileup Rejection Circuitry The Computer X-Ray Analyzer Artifacts of the Detection Process Peak Broadening Peak Distortion Silicon X-Ray Escape Peaks Absorption Edges Internal Fluorescence Peak of Silicon Artifacts from the Detector Environment Microphony Ground Loops Ice-Oil Accumulation Sensitivity to Stray Radiation Summary of EDS Operation and Artifacts 5.4. Comparison of WDS and EDS Geometrical Collection Efficiency Quantum Efficiency Resolution Spectral Acceptance Range Maximum Count Rate Minimum Probe Size Speed of Analysis Spectral Artifacts.. Appendix: Initial Detector Setup and Testing xi CONTENTS 6. Qualitative X-Ray Analysis Introduction EDS Qualitative Analysis X-Ray Lines Guidelines for EDS Qualitative Analysis General Guidelines for EDS Qualitative Analysis Specific Guidelines for EDS Qualitative Analysis Pathological Overlaps in EDS Qualitative Analysis Examples of EDS Qualitative Analysis WDS Qualitative Analysis Measurement of X-Ray Lines Guidelines for WDS Qualitative Analysis Automatic Qualitative EDS Analysis X-Ray Peak and Background Measurements General Considerations for X-Ray Data Handling Background Correction Background Correction for EDS Background Modeling Background Filtering

11 xii CONTENTS Background Correction for WDS Interpolation Substitute Material Method 7.3. Peak Overlap Correction EDS Peak Overlap Correction Linearity Goodness of Fit The Linear Methods The Nonlinear Methods Error Estimation WDS Peak-Overlap Correction Quantitative X-Ray Analysis: The Basics Introduction Advantages of Quantitative X-Ray Microanalysis in the SEM/EPMA Quantitative Analysis Procedures The Approach to X-Ray Quantitation: The Need for Matrix Corrections The Physical Origin of Matrix Effects X-Ray Production Effect of Atomic Number X-Ray Generation with Depth, cp(pz) ZAF Factors in Microanalysis Atomic Number Effect X-Ray Absorption Effect X-Ray Fluorescence Types of Matrix Correction Schemes Caveats Quantitative X-Ray Analysis: Theory and Practice Introduction ZAF Technique Introduction The Atomic Number Correction Z The Absorption Correction A Formulation Expressions for f(x) Practical Considerations Calculations of the Absorption Factor A The Characteristic Fluorescence Correction F The Continuum Fluorescence Correction Summary Discussion of the ZAF Method cp(pz) Technique Introduction The cp(pz) Curves Definition Measurement of cp(pz) Curves. 439

12 Calculation of cj>(pz) Curves 440 xiii Atomic Number Correction Zi Absorption Correction Ai. 445 CONTENTS Summary Discussion of the cj>(pz) Method Quantitative Analysis with Nonnormal Electron-Beam Incidence Standardless Analysis The Biological or Polymer Specimen: Special Procedures The Characteristic Signal The Continuum Signal Derivation of the Hall Procedure in Terms of X-Ray Cross-Sections Error Analysis Bulk Targets and Analysis of a Minor Element Special Procedures for Geological Analysis Introduction Formulation of the Bence-Albee Procedure Application of Bence-Albee Procedure Specimen Conductivity Special Sample Analysis Introduction: The Analytical Total Films on Substrates Foils Particles and Rough Surfaces Mass Effect Absorption Effect Fluorescence Effect Compensating for Geometric Effects in Quantitative Analysis Precision and Sensitivity in X-Ray Analysis Statistical Basis for Calculating Precision and Sensitivity Sample Homogeneity Analytical Sensitivity Trace-Element Analysis Variance under Peak Overlap Conditions in the EDS Light-Element Analysis Introduction Operating Conditions for Light-Element Analysis X-Ray Spectrometers WDS EDS Chemical Bonding Shifts WDS EDS Standards for Light-Element Analysis Surface Contamination Measurement of Background Intensities WDS EDS Quantitation Procedures for the Light Elements. 517

13 xiv Appendix 9.1. Equations for the (1', (J, y, and rjj(o) Terms of the Packwood-Brown rjj(pz) Equation CONTENTS Appendix 9.2. Solutions for the Atomic Number and Absorption Corrections Compositional Imaging Introduction Analog X-Ray Area Scanning (Dot Mapping) Procedure Limitations and Artifacts Digital Compositional Mapping Principles Data Collection Dead-time Correction Defocusing or Decollimation Correction Background Correction Standardization (k-value) Matrix Correction Combined EDS-WDS Strategy Statistics in Compositional Mapping Advantages Specimen Preparation for Inorganic Materials: Microstructural and Microchemical Analysis Metals Specimen Preparation for Surface Topography Specimen Preparation for Microstructural and Microchemical Analysis Final Specimen Preparation for Microstructural Analysis Final Specimen Preparation for Microchemical Analysis Preparation of Standards for X-Ray Microanalysis Ceramics and Geological Specimens Initial Specimen Preparation Mounting Polishing Final Specimen Preparation Electronic Devices and Packages Initial Specimen Preparation Mounting Polishing Final Preparation Semiconductors Voltage Contrast Charge Collection Electron Channeling Sands, Soils, and Clays Particles and Fibers 564

14 Particle Substrates. 565 XV Bulk Substrates Thin-Foil Substrate 566 CONTENTS Transfer and Attachment of Particles to Substrates Abundant, Loose Particles Particle Transfer from a Filter Particles in a Solid Matrix Transfer of Individual Particles Sample Preparation for Biological. Organic. Polymeric. and Hydrated Materials Introduction Compromising the Electron-Beam Instrument Environmental Stages Environmental Microscopes Nonoptimal Microscope Performance Compromising the Sample Correlative Microscopy Techniques for Structural Studies Specimen Selection Specimen Cleaning Specimen Stabilization Specimen Dehydration Chemical Dehydration Critical-Point Drying Low-Temperature Drying Ambient-Temperature Sublimation Exposure of Internal Surfaces Sectioning Fracturing Replication Surface Etching Specimen Supports High-Resolution Scanning Microscopy Isolation of Object of Interest Stabilization and Conductive Staining Specimen Coating Specimen Preparation for Localization of Metabolic Activity and Chemical Specificity Introduction The Nature of the Problem The Form of the Substance Being Analyzed Precision of Analytical Investigation Types of Specimens Types of Instrumentation Types of Analytical Applications General Preparative Procedures Before Fixation Fixation Histochemical Techniques 619

15 xvi Precipitation Techniques Dehydration. 621 CONTENTS Embedding Sectioning and Fracturing Specimen Supports Specimen Staining Localizing Regions of Biological Activity and Chemical Specificity Backscattered-Electron Cytochemical Methods Radioactive Labelling Methods Immunocytochemical Methods Criteria for Satisfactory Specimen Preparation Preparative Procedures for Organic Samples Such as Polymers, Plastics, and Paints Introduction Examination of the Surface of Polymers and Fibers Examination of the Interior of Polymers and Fibers Sectioning Polished Cut Surfaces Peelback Procedure Fracturing Replicas Surface Etching of Polymers High-Energy Beam Bombardment Argon Ion-Beam Etching Oxygen Plasma Etching Chemical Dissolution Chemical Attack Enzymatic Digestion Staining of Polymers and Plastics Osmium Tetroxide Chlorosulphonic Acid Phosphotungstic Acid Ruthenium Tetroxide Silver Salts Specialized Preparative Methods Low-Temperature Specimen Preparation for Structural and Analytical Studies Introduction Water: Properties Ice Rapid Cooling Procedures Cryosectioning Cryofracturing Freeze Drying Freeze Substitution and Low-Temperature Embedding Low-Temperature SEM Low-Temperature X-Ray Microanalysis Damage, Artifact, and Interpretation Introduction 658

16 Sample Damage during Preparation. 658 XV;; Specimen Damage during Examination and Analysis Observable Damage 659 CONTENTS Nonobservable Damage Artifacts Interpretation Specific Preparative Procedures: A Bibliography Coating and Conductivity Techniques for SEM and Microanalysis Introduction Specimen C.haracteristics Conductivity Thermal Damage Secondary- and Backscattered-Electron Emission X-Ray and Cathodoluminescence Emission Mechanical Stability Untreated Specimens Bulk Conductivity Staining Methods Specimen Mounting Procedures Thin-Film Methods Thermal Evaporation High-Vacuum Evaporation The Apparatus Choice of Evaporant Evaporation Techniques Artifacts Associated with Evaporative Coating Low-Vacuum Evaporation Sputter Coating Diode or Direct-Current Sputtering Plasma-Magnetron Sputtering Ion-Beam Sputtering Penning Sputtering Sputtering Techniques Choice of Target Material Coating Thickness Advantages of Sputter Coating Artifacts Associated with Sputter Coating Specialized Coating Methods High-Resolution Coating Coating Samples Maintained at Low Specimen Temperatures Coating Frozen-Hydrated Material Maintained at Low Temperatures Coating Techniques for X-Ray Microanalysis Determination of Coating Thickness Estimation of Coating Thickness Measurement during Coating Measurement after Coating Removing Coating Layers 736

17 xv;;; CONTENTS Artifacts Related to Coating and Bulk-Conductivity Procedures Conclusions Data Ba.e Table Atomic Number, Atomic Weight, and Density of Elements 741 Table Common Oxides of the Elements Table Mass Absorption Coefficients for Ka Lines 744 Table Mass Absorption Coefficients for La Lines 752 Table Mass Absorption Coefficients for Ma Lines 768 Table K Series X-Ray Wavelengths and Energies 778 Table L Series X-Ray Wavelengths and Energies 780 Table M Series X-Ray Wavelengths and Energies 781 Table J and Fluorescent Yield (w) by Atomic Number 782 Table Important Properties of Selected Coating Elements. 784 Reference Index

Scanning Electron Microscopy and X-Ray Microanalysis. A Text for Biologists, Materials Scientists, and Geologists

Scanning Electron Microscopy and X-Ray Microanalysis A Text for Biologists, Materials Scientists, and Geologists Scanning Electron Microscopy and X-Ray Microanalysis A Text for Biologists, Materials Scientists,