Scanning Electron Microscopy and X-Ray Microanalysis. 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

2 Scanning Electron Microscopy and X-Ray Microanalysis A Text for Biologists, Materials Scientists, and Geologists Joseph I. Goldstein Lehigh University Bethlehem, Pennsylvania Dale E. Newbury National Bureau of Standards Washington, D.C. Patrick Echlin University of Cambridge Cambridge, England David C. Joy Bell Laboratories Murray Hill, New Jersey Charles Fiori National Institutes of Health Bethesda, Maryland Eric Lifshin General Electric Corporate Research and Development Schenectady, New York PLENUM PRESS NEW YORK AND LONDON

3 Main entry under title: Library of Congress Cataloging in Publication Data Scanning electron microscopy and X-ray microanalysis. Bibliography: p. Includes index. 1. Scanning electron microscope. 2. X-ray microanalysis. I. Goldstein, Joseph, QH212.S3S29 502'8' AACR2 ISBN-13: e-isbn-13: DOl: / First Printing-November 1981 Second Printing - April Plenum Press, New York Softcover reprint of the hardcover 1 st edition 1981 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 This book has evolved by processes of selection and expansion from its predecessor, Practical Scanning Electron Microscopy (PSEM), published by Plenum Press in The interaction of the authors with students at the Short Course on Scanning Electron Microscopy and X-Ray Microanalysis held annually at Lehigh University has helped greatly in developing this textbook. The material has been chosen to provide a student with a general introduction to the techniques of scanning electron microscopy and x-ray microanalysis suitable for application in such fields as biology, geology, solid state physics, and materials science. Following the format of PSEM, this book gives the student a basic knowledge of (1) the user-controlled functions of the electron optics of the scanning electron microscope and electron microprobe, (2) the characteristics of electron-beam-sample interactions, (3) image formation and interpretation, (4) x-ray spectrometry, and (5) quantitative x-ray microanalysis. Each of these topics has been updated and in most cases expanded over the material presented in PSEM in order to give the reader sufficient coverage to understand these topics and apply the information in the laboratory. Throughout the text, we have attempted to emphasize practical aspects of the techniques, describing those instrument parameters which the microscopist can and must manipulate to obtain optimum information from the specimen. Certain areas in particular have been expanded in response to their increasing importance in the SEM field. Thus energy-dispersive x-ray spectrometry, which has undergone a tremendous surge in growth, is treated in substantial detail. Moreover, we have come to realize the importance of developing careful procedures for qualitative x-ray microanalysis, that is, the identification of the elemental constituents present in a sample; suitable procedures for both energydispersive and wavelength-dispersive x-ray spectrometry are described. v

5 vi PREFACE The most conspicuous addition to the book is the material on biological specimen preparation and coating. Because of the great difficulties in properly preparing a biological sample for SEM examination and analysis, this topic has been considered in detail. It should be recognized that this material is of value not only to biologists, but also to many nonbiological disciplines in which fragile samples, often containing water or other fluids, must be prepared for the SEM. These include polymers, pigments, corrosion products, textiles, and many others. For the convenience of readers who are confronted with a need for numerical information on important parameters for SEM and x-ray microanalysis calculations, we have included a data base of frequently used information, including x-ray energies of principal lines, mass absorption coefficients, backscattering factors, and others. Some material from PSEM has been deleted in preparing this book. Some chapters, including "Contrast Mechanisms of Special Interest in Materials Science" and "Ion Microprobe Mass Analysis," have been removecl The~e tonic~ and several others will be nresented in a comnanion

6 Contents 1. Introduction Evolution of the Scanning Electron Microscope 1.2. Evolution of the Electron Probe Microanalyzer 1.3. Outline of This Book Electron Optics Electron Guns Thermionic Emission Tungsten Cathode The Lanthanum Hexaboride (LaB6) Cathode Field Emission Gun Electron Lenses General Properties of Magnetic Lenses Production of Minimum Spot Size Aberrations in the Electron Optical Column 2.3. Electron Probe Diameter, dp' vs. Electron Probe Current i Calculation of d min and i max Measurement of Microscope Parameters (dp' i, a) High-Resolution Scanning Electron Microscopy. 3. Electron-Beam-Speclmen Interactions 3.1. Introduction Scattering Elastic Scattering Inelastic Scattering Interaction Volume Experimental Evidence Monte Carlo Calculations 3.4. Backscattered Electrons Atomic Number Dependence Energy Dependence Tilt Dependence Angular Distribution vii

7 viii CONTENTS Energy Distribution Spatial Distribution Sampling Depth Signals from Inelastic Scattering Secondary Electrons X-Rays Auger Electrons Cathodoluminescence Summary I8 1I Image Formation In the Scanning Electron Microscope Introduction The Basic SEM Imaging Process Scanning Action Image Construction (Mapping) Magnification Picture Element (Picture Point) Depth of Field Image Distortions Stereomicroscopy Detectors Electron Detectors Cathodoluminescence Detectors The Roles of Specimen and Detector in Contrast Formation Contrast Atomic Number (Compositional) Contrast (Backscattered Electron Signal) Compositional Contrast (Secondary-Electron Signal) Contrast Components Topographic Contrast Image Quality Signal Quality and Contrast Information Strategy in SEM Imaging Resolution Limitations Signal Processing for the Display of Contrast Information The Visibility Problem Signal Processing Techniques Combinations of Detectors Beam Energy Effects Summary 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 Artifacts of the Detection Process..226

8 CONTENTS Ix The Main Amplifier and Pulse Pileup Rejection Artifacts from the Detector Environment The Multichannel Analyzer Summary of EDS Operation and Artifacts Comparison of Wavelength-Dispersive Spectrometers with Energy-Dispersive Spectrometers 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 Qualitative X-Ray Analysis Introduction EDS Qualitative Analysis X-Ray Lines 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 X-Ray Scanning Quantitative X-Ray Microanalysis Introduction ZAF Technique Introduction The Absorption Factor, A The Atomic Number Factor, Z The Characteristic Fluorescence Correction, F The Continuum Fluorescence Correction Summary Discussion of the ZAF Method The Empirical Method Quantitative Analysis with Nonnormal Electron Beam Incidence Analysis of Particles and Rough Surfaces Geometric Effects Compensating for Geometric Effects Summary Analysis of Thin Films and Foils Thin Foils Thin Films on Substrates Quantitative Analysis of Biological Material Introduction Mass Loss and Artifacts during Analysis Bulk Samples Thick Sections on Bulk Substrates. 369

9 x CONTENTS Thin Samples The Continuum Method Thick Specimens on Very Thin Supports Microdroplets Standards Conclusion Appendix A: Continuum Method. 381 Appendix B: Worked Examples of Quantitative Analysis of Biological Material. 387 Notation Practical Techniques of X-Ray Analysis General Considerations of Data Handling Background Shape Background Modeling Background Filtering Peak Overlap Linearity Goodness of Fit The Linear Methods The Nonlinear Methods Error Estimation Dead-Time Correction Example of Quantitative Analysis Precision and Sensitivity in X-Ray Analysis Statistical Basis for Calculating Precision and Sensitivity Sample Homogeneity Analytical Sensitivity Trace Element Analysis Light Element Analysis Materials Specimen Preparation for SEM and X-Ray Microanalysis Metals and Ceramics Scanning Electron Microscopy X-Ray Microanalysis Particles and Fibers Hydrous Materials Soils and Clays Polymers Coating Techniques for SEM and Microanalysis Introduction Specimen Characteristics Alternatives to Coating Thin-Film Technology Thermal Evaporation High-Vacuum Evaporation Low-Vacuum Evaporation.479

10 CONTENTS Sputter Coating Ion Beam Sputtering Diode or Direct Current Sputtering Cool Diode Sputtering Sputtering Techniques Choice of Target Coating Thickness Advantages of Sputter Coating Artifacts Associated with Sputter Coating Specialized Coating Methods High-Resolution Coating Low-Temperature Coating Determination of Coating Thickness Estimation of Coating Thickness Measurement during Coating Measurement after Coating Removing Coating Layers 11. Preparation of Biological Samples for Scanning Electron Microscopy Introduction Compromising the Microscope Environmental Stages Nonoptimal Microscope Performance Compromising the Specimen Correlative Microscopy Specimen Selection Specimen Cleaning Specimen Stabilization Exposure of Internal Surfaces Localizing Areas of Known Physiological Activity Specimen Dehydration Specimen Supports Specimen Conductivity Heavy Metal Impregnation Interpretation and Artifacts. 12. Preparation of Biological Samples for X-Ray Microanalysis Introduction The Nature and Enormity of the Problem Applications of X-Ray Microanalysis Types of X-Ray Analytical Investigations Types of Biological Specimens Strategy Criteria for Satisfactory Specimen Preparation Ambient Temperature Preparative Procedures Before Fixation Fixation Histochemical Techniques Precipitation Techniques. xl

11 xii CONTENTS Dehydration Embedding Sectioning and Fracturing Specimen Supports Specimen Staining Specimen Coating Low-Temperature Preparative Procedures Specimen Pretreatment Freezing Procedures Movement of Elements within a Given Cellular Compartment Postfreezing Procedures Frozen-Hydrated and Partially Frozen-Hydrated Material Freeze Drying Freeze Substitution Sectioning Fracturing Specimen Handling Microincineration Applications of the SEM and EPMA to Solid Samples and Biological Materials Study of Aluminum-Iron Electrical Junctions Study of Deformation in Situ in the Scanning Electron Microscope Analysis of Phases in Raney Nickel Alloy Quantitative Analysis of a New Mineral, Sinoite Determination of the Equilibrium Phase Diagram for the Fe-Ni-C System Study of Lunar Metal Particle 63344, Observation of Soft Plant Tissue with a High Water Content Study of Multicellular Soft Plant Tissue with High Water Content Examination of Single-Celled, Soft Animal Tissue with High Water Content Observation of Hard Plant Tissue with a Low Water Content Study of Single-Celled Plant Tissue with a Hard Outer Covering and Relatively Low Internal Water Content Examination of Medium Soft Animal Tissue with a High Water Content Study of Single-Celled Animal Tissue of High Water Content. 14. Data Base Table Atomic Number, Atomic Weight, and Density of Metals Table Common Oxides of the Elements Table Mass Absorption Coefficients for Ka Lines.620 Table Mass Absorption Coefficients for La Lines.622 Table Selected Mass Absorption Coefficients Table K Series X-Ray Wavelengths and Energies.641 Table L Series X-Ray Wavelengths and Energies..642

12 CONTENTS xiii Table M Series X-Ray Wavelengths and Energies Table Fitting Parameters for Duncumb-Reed Backscattering Correction Factor R Table J Values and Fluorescent Yield, w, Values Table Important Properties of Selected Coating Elements. 646 REFERENCES. 649 INDEX..665

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

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