SCANNING ELECTRON MICROSCOPY AND X-RAY MICROANALYSIS

Transcription

1 SCANNING ELECTRON MICROSCOPY AND X-RAY MICROANALYSIS Robert Edward Lee Electron Microscopy Center Department of Anatomy and Neurobiology Colorado State University P T R Prentice Hall, Englewood Cliffs, New Jersey 07632

2 Acknowledgments xiii Chapter 1 The Scanning Electron Microscope 1 HOW THE SCANNING ELECTRON MICROSCOPE PRODUCES AN IMAGE 2 CHARACTERISTICS OF THE IMAGE PRODUCED BY THE SCANNING ELECTRON MICROSCOPE 7 DEVELOPMENT OF THE SCANNING ELECTRON MICROSCOPE 7 THE MODERN SCANNING ELECTRON MICROSCOPE 13 Chapter 2 Electron Emission 16 TYPES OF ELECTRON EMISSION 16 THERMIONIC EMISSION 19 v

3 vi Contents ELECTRON GUNS 21 ELECTRON GUN WITH THERMIONIC EMISSION FROM A TUNGSTEN CATHODE 22 Wehnelt Cylinder, 27 Operation of the Self-biased Gun, 29 Distance between the Tungsten Cathode and the Wehnelt Cylinder, 34 Failure of the Tungsten Filament, 35 Brightness of Illumination, 37 Effect on the Electron Beam of Biasing the Wehnelt Cylinder, 38 Accelerating Voltage, 39 Pointed Tungsten Filaments, 40 LANTHANUM HEXABORIDE CATHODE 40 FIELD EMISSION 44 Chapter 3 Lenses and Magnetism 50 PATH OF RADIATION THROUGH LENSES 50 CREATION OF MAGNETIC FIELDS 53 MAGNETIC FIELDS CREATED BY AN ELECTRICAL CURRENT 55 PATH OF AN ELECTRON THROUGH AN ELECTROMAGNET 61 VECTOR FORCES IN THE CORE OF AN ELECTROMAGNETIC LENS 63 DESIGN OF ELECTROMAGNETIC LENSES 72 Chapter 4 Lens Aberrations 75 SPHERICAL ABERRATION 75 CHROMATIC ABERRATION 81 ASTIGMATISM 84 Causes of Astigmatism, 87 Stigmators, 88

4 Chapter 5 Assembled Column of the Scanning Electron Microscope 93 vii ELECTRON GUN 94 ANODE 95 BEAM (SHIFT AND TILT) DEFLECTOR COILS 95 LENSES AND APERTURES 100 Demagnification of the Electron Beam to Produce the Final Probe Diameter, 100 Working Distance, 102 Design of the Objective Lens, 104 Demagnification and the Amount of Current in the Electron Beam, 105 FOCUSING 109 APERTURES 112 OBJECTIVE STIGMATOR 115 SCANNING COILS 115 Chapter 6 Electron Beam-Specimen Interactions 119 TYPES OF SIGNALS PRODUCED 119 ATOMIC STRUCTURE IN METALS AND INSULATORS 120 ELASTIC VERSUS INELASTIC SCATTERING 122 YIELD OF SECONDARY AND BACKSCATTERED ELECTRONS IN METALS AND INSULATORS 123 INFLUENCE OF ATOMIC NUMBER OF THE SPECIMEN ON ELASTIC AND INELASTIC EVENTS 124 DIMENSIONS OF THE SPECIMEN INTERACTION VOLUME 125 DIRECTION OF ELECTRONS ESCAPING FROM A SPECIMEN 126

5 viii Contents ELASTIC SCATTERING PRODUCES BACKSCATTERED ELECTRONS 128 INELASTIC SCATTERING PRODUCES SECONDARY ELECTRONS, AUGER ELECTRONS, X-RAYS, CATHODOLUMINESCENCE, AND HEAT 130 X-Rays, 134 Auger Electrons, 142 Cathodoluminescence and Phonons, 145 ESCAPE DEPTHS OF SIGNALS FROM A SPECIMEN 147 Chapter 7 Detectors 149 SCINTILLATOR-PHOTOMULTIPLIER SYSTEMS (EVERHART-THORNLEY DETECTOR) 150 Faraday Cage, 150 Scintillator, 151 Light Guide Pipe, 151 Photomultiplier, 152 Modification of the Everhart-Thornley Detector, 154 TAKE-OFF ANGLE AND SOLID ANGLE OF COLLECTION 155 MAXIMIZING THE COLLECTION OF BACKSCATTERED ELECTRONS 156 Multiple Detectors, 157 Reversing the Specimen Current, 158 SPECIMEN CURRENT USED AS A DETECTOR 158 AUGER ELECTRON DETECTORS 160 CATHODOLUMINESCENCE DETECTORS 163 Chapter 8 Image Reconstruction 166 IMAGE CONSTRUCTION IN TELEVISION 166

6 ix IMAGE CONSTRUCTION IN THE SCANNING ELECTRON MICROSCOPE 168 Line Scan, 170 MAGNIFICATION 176 RESOLUTION AND IMAGE QUALITY IN THE SCANNING ELECTRON MICROSCOPE 179 DEPTH OF FOCUS (FIELD) 184 CONTRAST 189 Topographic Contrast, 189 Atomic Number (Compositional) Contrast, 192 DISTORTIONS OF THE IMAGE 193 Projection Distortions, 193 Tilt Distortions, 194 Moire Effects, 196 THE HARD COPY 197 Photographing the Image, 197 Videoprinters, 204 STEREO PHOTOGRAPHS 207 Chapter 9 Image Processing 209 OPTICAL PHOTOGRAPHIC METHODS 209 ELECTRICAL ANALOG METHODS 209 Linear Amplification, 210 Nonlinear Amplification (Gamma), 212 Contrast Reversal, 213 Area Scan Using Y-modulated Raster Lines, 214 DIGITAL IMAGE PROCESSING 216 Image Processor, 216 Digital Computer, 219 Display and Recording Devices, 219 Storage Devices, 219 PROCESSING THE IMAGE 219 Point-processing Operations, 220 Examples of Point-processing Operations, 221 Spatial-processing Operations, 228

7 Geometric-processing Operations, 233 Multiple-image Operations, 237 Colorization of the Black and White Image, 237 Chapter 10 Vacuum 239 GASES AND VAPORS 240 Effect of Pressure and Temperature on a Gas, 240 Total Pressure and Partial Pressure, 241 Diffusion, 242 Flow of Gases, 242 Outgassing, 243 VACUUM PUMPS 243 Mechanical Rotary Oil Pump, 243 Diffusion Pumps, 247 Turbomolecular Pumps, 252 Ion-getter Pumps, 254 VACUUM GAUGES 259 Thermal Conductivity Vacuum Gauges, 259 Penning (Cold Cathode Ionization) Gauges, 263 SEALS 266 O-rings, 266 Metal Gaskets, 268 VALVES 269 Types of Valves Used in Electron 269 Microscopes, Chapter 11 Specimen Preparation 272 DRY SPECIMENS 272 SPECIMENS CONTAINING VOLATILE COMPONENTS 273 Chemical Fixation, 273 Dehydration, 285 Specimen Drying, 286 Fixation by Rapid Freezing Followed by Freeze Drying, 297 Freeze Drying, 307 Specimen Coating, 310

8 xi Chapter 12 X-ray Microanalysis 329 HISTORY OF X-RAYS 329 WAVELENGTH DISPERSIVE SPECTROSCOPY 331 ENERGY DISPERSIVE SPECTROSCOPY 343 Semiconductors, 344 Solid-state X-ray Detectors, 347 Preamplifier (Head Amplifier), 354 Cryostat, 355 Main Amplifier, 360 Multichannel Analyzer, 361 Summary of the Components of an Energy Dispersive Spectrometer, 364 Deadtime, 365 Artifacts Produced by the Energy Dispersive Spectrometer, 365 Detector Efficiency, 373 Advantages of Energy Dispersive Spectroscopy, 374 Disadvantages of Energy Dispersive Spectroscopy, 374 Operation of the Scanning Electron Microscope during Energy Dispersive Spectroscopy, 375 QUALITATIVE ANALYSIS 381 Calibration of the X-ray Microanalytical 362 Distinguishing X-ray Peaks, 383 X-ray Mapping, 388 QUANTITATIVE MICROANALYSIS 391 Unit, Acquisition of an X-ray Spectrum for Quantitative Microanalysis, 392 Processing of the X-ray Spectrum for Quantitative Microanalysis, 395 Analysis of the X-ray Spectrum, 396 Computer Programs Used to Quantify X-ray Spectra, 406 Appendix I Steradian Definition 409 Appendix SI Pressure Conversion Chart 411

9 xii Appendix III Appendix IV Contents Wavelengths, Energies, and Critical Excitation Energies of К X-ray Lines 412 Wavelengths and Energies of L Lines and Critical Excitation Energies of l\w Shell Electrons 413 Appendix V Wavelengths and Energies of M Lines 415 Appendix VI Principle Emission and Absorption Energies of Elements 416 Glossary 418 Bibliography 439 Index 443