Practical Scanning Electron Microscopy

In the spring of 1963, a well-known research institute made a market survey to assess how many scanning electron microscopes might be sold in the United States. They predicted that three to five might be sold in the first year a commercial SEM was available, and that ten instruments would saturate the marketplace. In 1964, the Cambridge Instruments Stereoscan was introduced into the United States and, in the following decade, over 1200 scanning electron microscopes were sold in the U. S. alone, representing an investment conservatively estimated at $50,000- $100,000 each. Why were the market surveyers wrongil Perhaps because they asked the wrong persons, such as electron microscopists who were using the highly developed transmission electron microscopes of the day, with resolutions from 5-10 A. These scientists could see little application for a microscope that was useful for looking at surfaces with a resolution of only (then) about 200 A. Since that time, many scientists have learned to appreciate that information content in an image may be of more importance than resolution per se. The SEM, with its large depth of field and easily that often require little or no sample prepara­ interpreted images of samples tion for viewing, is capable of providing significant information about rough samples at magnifications ranging from 50 X to 100,000 X. This range overlaps considerably with the light microscope at the low end, and with the electron microscope at the high end.

I Introduction.- I. Evolution of the Scanning Electron Microscope.- II. Evolution of the Electron Probe Microanalyzer.- III. Combination SEM-EPMA.- IV. Outline and Purpose of This Book.- References.- Bibliography of Texts and Monographs in SEM and EPMA.- II Electron Optics.- I. Electron Guns.- A. Tungsten Filament Cathode.- B. LaB6 Rod Cathode.- C. Field Emission Gun.- II. Electron Lenses.- A. General Properties of Magnetic Lenses.- B. Production of Minimum Spot Size.- C. Aberrations in the Electron Optical Column.- D. Design of the Final Lens.- III. Electron Probe Diameter dp vs. Electron Probe Current i.- A. Calculation of dmin and imax.- B. High-Resolution Scanning Electron Microscopy.- IV. Depth of Field.- References.- III Electron Beam-Specimen Interaction.- I. Electron Scattering in Solids.- II. Electron Range and Spatial Distribution of the Primary Electron Beam.- III. Emitted Electrons—Backscattered Electrons.- IV. Emitted Electrons—Low-Energy Electrons.- V. X-Rays.- A. X-Ray Production.- B. X-Ray Absorption.- C. Depth of X-Ray Production.- VI. Auger Electrons.- VII. Summary—Range and Spatial Resolution.- References.- IV Image Formation in the Scanning Electron Microscope.- I. The SEM Imaging Process.- II. Signal Detectors.- A. Everhart-Thornley Detector.- B. Solid State Detector.- C. Specimen Current Detection.- D. Detection of Electromagnetic Radiation.- III. Contrast Formation.- A. Atomic Number Contrast.- B. Topographic Contrast.- C. Interpretation of Topographic Images.- IV. Signal Characteristics and Image Quality.- V. Resolution Limitations in the SEM.- A. Signal Limitations.- B. Resolution Limitation due to Beam-Specimen Interactions.- VI. Signal Processing.- A. Black Level Suppression.- B. Nonlinear Amplification (Gamma).- C. Signal Differentiation.- D. Y-Modulation.- VII. Image Defects.- VIII. Electron Penetration Effects in Images.- References.- V Contrast Mechanisms of Special Interest In Materials Science.- I. Introduction.- II. Electron Channeling Contrast.- A. Introduction.- B. The Mechanism of Electron Channeling Contrast.- C. The Electron Channeling Pattern.- D. Electron Optical and Signal Processing Conditions.- E. Electron Channeling Contrast from Polycrystals.- F. Selected Area Channeling Patterns (SACP).- G. Crystal Perfection Effects on Electron Channeling Patterns.- III. Magnetic Contrast in the SEM.- A. Introduction.- B. Classes of Magnetic Materials.- C. Type I Magnetic Contrast.- D. Type II Magnetic Contrast.- IV. Voltage Contrast.- V. Electron-Beam-Induced Current (EBIC).- VI. Cathodoluminescence.- References.- VI Specimen Preparation, Special Techniques, and Applications of the Scanning Electron Microscope.- I. Specimen Preparation for Materials Examination in the SEM.- II. Stereomicroscopy.- III. Dynamic Experiments in the SEM.- IV. Applications of the SEM.- A. Examination of Fractured Polycrystalline Iron.- B. Failure Analysis of a Composite Consisting of Borsic (Boron-Silicon Carbide) Rods in a Matrix of Plasma-Sprayed Aluminum.- C. Investigation of Returned Lunar Material—Glassy Spherules from Apollo 11.- D. Analysis of Corrosion Mechanism in a Steam Boiler Tube.- E. Rapid Phase Delineation in a Mineral Specimen.- F. Characterization of Wear Particles and Surface Degradation Produced by Wear in Bearing and Gear Tests.- G. Examination of Human Teeth.- H. Applications of Electron Channeling Contrast to Materials Problems.- I. Examination of Magnetically Written Information with Type I Magnetic Contrast.- J. Applications of Type II Magnetic Contrast.- K. Study of Reliability of Integrated Circuit Chips—Quality Control.- L. Observation of Ferroelectric Domains.- M. Electron-Beam Writing.- References.- VII X-Ray Spectral Measurement and Interpretation.- I. Introduction.- II. Crystal Spectrometers.- A. Basic Design.- B. The X-Ray Detector.- C. Detector Electronics.- III. Solid State X-Ray Detectors.- A. Operating Principles.- B. Pulse Pileup.- C. The Multichannel Analyzer.- IV. A Comparison of Crystal Spectrometers with Solid State X-Ray Detectors.- V. The Analysis of X-Ray Spectral Data.- A. General Considerations.- B. The Background Shape.- C. Characteristic X-Ray Production Efficiencies.- D. Indirect-to-Direct X-Ray Excitation Ratios.- References.- VIII Microanalysis of Thin Films and Fine Structure.- I. Introduction.- II. Factors Affecting X-Ray Spatial Resolution.- A. Probe Position and Stability.- B. Probe Current as a Function of Diameter.- C. Electron Beam Penetration and Scattering.- D. Indirect X-Ray Production.- III. Characterizing the X-Ray-Excited Volume.- A. Depth Distribution Profile.- B. Monte Carlo Calculations.- IV. Thin-Film Analysis.- V. Particles, Inclusions, and Fine Structures.- References.- IX Methods of Quantitative X-Ray Analysis Used in Electron Probe Microanalysis and Scanning Electron Microscopy.- I. Introduction.- II. The Absorption Factor kA.- A. Formulation.- B. Expression Used to Calculate $${f_p}$$.- C. Experimental Results and the $${f_p}$$ Expressions.- III. Atomic Number Correction kZ.- IV. The Characteristic Fluorescence Correction kF.- V. The Continuum Fluorescence Correction.- VI. Summary Discussion of the ZAF Method.- VII. The Empirical Method for Quantitative Analysis.- VIII. Comments on Analysis Involving Elements of Atomic Number of 11 or Less.- IX. Quantitative Analysis with Nonnormal Electron-Beam Incidence.- X. Analysis Involving Special Specimen Geometries.- XI. Discussion.- Appendix. The Analysis of an Iron-Silicon Alloy.- References.- X Computational Schemes for Quantitative X-Ray Analysis: On-Line Analysis with Small Computers.- I. Introduction.- II. Summary of Computational Schemes for Quantitative Analysis.- III. The FRAME Program.- A. Fundamentals of FRAME.- B. Limitations of FRAME.- C. Comparison of Results from FRAME and COR2.- D. Example Showing On-Line Analysis Using FRAME.- E. Conclusions.- IV. Data Reduction Based on the Hyperbolic Method.- V. Summary.- References.- XI Practical Aspects of X-Ray Microanalysis.- I. Grappling with the Unknown.- A. Elemental Identification.- B. Crystal Spectrometer Systems.- C. Elemental Distribution (X-Ray Area Scanning).- D. Applications of Elemental Identification and Distribution.- II. Specimen Preparation for Quantitative Analysis.- A. Surface Roughness and Polishing.- B. Choice of Coating Material.- C. Standards in Microprobe Analysis.- III. Applications Involving Compositional Analysis.- References.- XII Special Techniques in the X-Ray Analysis of Samples.- I. Light Element Analysis.- II. Precision and Sensitivity in X-Ray Analysis.- A. Statistical Basis for Calculating Precision and Sensitivity.- B. Sample Homogeneity.- C. Analytical Sensitivity.- D. Trace Element Analysis.- III. X-Ray Analysis at Interfaces.- A. Spatial Resolution.- B. X-Ray Absorption.- C. X-Ray Fluorescence.- IV. Soft X-Ray Emission Spectra.- V. Thin Films.- Appendix. Deconvolution Technique.- References.- XIII Biological Applications: Sample Preparation and Quantitation.- I. Sample Preparation.- A. Liquid Samples.- B. Thick Samples.- C. Particles.- D. Single Cells and Cell Organelles.- E. Sections.- F. Microincineration.- G. Coating.- II. Analysis.- A. Qualitative Analysis.- B. Quantitative Analysis.- III. Summary.- References.- XIV Ion Microprobe Mass Analysis.- I. Basic Concepts and Instrumentation.- II. Ion Microscope.- III. Ion Microprobe.- IV. Production of Ions.- V. Sputtering.- VI. Qualitative Analysis.- VII. Quantitative Analysis.- VIII. Dead-Time Losses.- IX. In-Depth Profiling.- X. Applications.- References.