A Beginners' Guide to Scanning Electron Microscopy (eBook)

Anwar Ul-Hamid received his B.Sc. in Metallurgical Engineering and Materials Science at the University of Engineering & Technology in Lahore, Pakistan in 1991. He received his Ph.D,. for Oxidation of High Temperature alloys/Analytical Electron Microscopy at the Department of Materials Science & Metallurgy at the University of Cambridge in 1996.He has published over 70 peer-reviewed papers in journals and proceedings, and is currently Coordinator of the Materials Characterization Laboratory (MCL)/Research Institute at King Fahd University of Petroleum & Minerals, in Dhahran, Saudi Arabia.

Preface 7
Contents 9
Abbreviations 16
Symbols List 18
1: Introduction 22
1.1 What Is the SEM 22
1.2 Image Resolution in the SEM 22
1.3 Image Formation in the SEM 25
1.4 Information Obtained Using the SEM 25
1.5 Strengths and Limitations of the SEM 29
1.6 Brief History of the SEM Development 32
References 35
2: Components of the SEM 36
2.1 Electron Column 36
2.1.1 Electron Gun 38
2.1.1.1 Current Density and Brightness of the Electron Source 38
2.1.1.2 Size of the Electron Source 39
2.1.1.3 Stability of the Electron Source 41
2.1.1.4 Energy Spread of Electrons 41
2.2 Thermionic Emission Electron Guns 41
2.2.1 Tungsten Filament Gun 43
2.2.1.1 Material 43
2.2.1.2 Working Principle 44
2.2.1.3 Role of Self-Regulating Bias Resistor 45
2.2.1.4 Saturation 47
2.2.1.5 Advantages/Drawbacks 48
2.2.1.6 Service Lifetime 48
2.2.2 Lanthanum Hexaboride (LaB6) Emitter 49
2.2.2.1 Material 49
2.2.2.2 Tip Design 50
2.2.2.3 Advantages/Drawbacks 50
2.3 Field Emission Electron Guns 51
2.3.1 Working Principle 51
2.3.2 Advantages/Drawbacks 52
2.3.3 Cold Field Emitter (Cold FEG) 53
2.3.3.1 Working Principle 53
2.3.3.2 Service Lifetime 54
2.3.3.3 Advantages/Drawbacks 55
2.3.4 Schottky Field Emitter 55
2.3.5 Recent Advances 57
2.4 Electromagnetic Lenses 58
2.4.1 Condenser Lens 60
2.4.2 Apertures 62
2.4.3 Objective Lens 62
2.4.3.1 Pinhole Lens 62
2.4.3.2 Immersion Lens 63
2.4.3.3 Snorkel Lens 63
2.4.4 Lens Aberrations 65
2.4.4.1 Spherical Aberration 65
2.4.4.2 Chromatic Aberration 66
2.4.4.3 Diffraction at Aperture 68
2.4.4.4 Astigmatism 69
2.4.4.5 Effective Probe Diameter 71
2.4.5 Scan Coils 73
2.4.6 Magnification 74
2.5 Specimen Chamber 77
2.5.1 Specimen Stage 77
2.5.2 Infrared Camera 79
2.6 Detectors 80
2.6.1 Everhart-Thornley Detector 80
2.6.1.1 Working Principle 80
2.6.1.2 Efficiency of Signal Collection 83
2.6.1.3 Types of Signals Collected 84
2.6.1.4 Advantages of E-T Detector 85
2.6.2 Through-the-Lens (TTL) Detector 85
2.6.3 Backscattered Electron Detector 87
2.6.3.1 Working Principle 87
2.6.3.2 Advantages/Drawbacks 90
2.6.3.3 Scintillator BSE Detector 92
2.6.3.4 Channel Plate Detector 92
2.7 Miscellaneous Components 92
2.7.1 Computer Control System 92
2.7.2 Vacuum System 93
2.7.3 High-Voltage Power Supply (HT Tank) 95
2.7.4 Water Chiller 95
2.7.5 Heater 96
2.7.6 Anti-vibration Platform 96
References 97
3: Contrast Formation in the SEM 98
3.1 Image Formation 98
3.1.1 Digital Imaging 99
3.1.2 Relationship Between Picture Element and Pixel 101
3.1.3 Signal-to-Noise Ratio (SNR) 103
3.1.4 Contrast Formation 106
3.2 Beam-Specimen Interaction 107
3.2.1 Atom Model 107
3.2.2 Elastic Scattering 108
3.2.3 Inelastic Scattering 109
3.2.4 Effect of Electron Scattering 110
3.2.5 Interaction Volume 111
3.2.5.1 Effect of Beam Energy on Interaction Volume 111
3.2.5.2 Effect of Atomic Number on Interaction Volume 112
3.2.5.3 Effect of Tilt on Interaction Volume 114
3.2.6 Electron Range 114
3.3 Origin of Backscattered and Secondary Electrons 116
3.3.1 Origin of Backscattered Electrons (BSE) 116
3.3.2 Origin of Secondary Electrons (SE) 116
3.4 Types of Contrast 118
3.4.1 Compositional or Atomic Number (Z) Contrast (Backscattered Electron Imaging) 118
3.4.1.1 Yield of Backscattered Electrons 118
3.4.1.2 Energy Distribution of BSE Yield 118
3.4.1.3 Effect of Beam Energy on BSE Yield 119
3.4.1.4 Effect of Atomic Number on BSE Yield 119
3.4.1.5 Effect of Tilt on BSE Yield 122
3.4.1.6 Effect of Crystal Structure on BSE Yield 122
3.4.1.7 Directional Dependence of BSE Yield 122
3.4.1.8 Collection Efficiency of the BSE Detector 125
3.4.1.9 Spatial Distribution of BSE 126
3.4.1.10 Formation of Compositional or Z Contrast with BSE 127
3.4.1.11 Spatial Resolution of BSE Images 131
3.4.1.12 Applications of Backscattered Electron Imaging 132
3.4.1.13 Limitations of Backscattered Electron Imaging 133
3.4.2 Topographic Contrast (Secondary Electron Imaging) 134
3.4.2.1 Secondary Electron Yield 134
3.4.2.2 Escape Depth of SE 134
3.4.2.3 Energy Distribution of SE 135
3.4.2.4 Types of SE Signal (SE1, SE2, SE3, SE4) 136
3.4.2.5 Effect of Beam Energy on SE Yield 139
3.4.2.6 Effect of Atomic Number on SE Yield 141
3.4.2.7 Effect of Tilt on SE Yield 141
3.4.2.8 Directional Dependence of SE Yield 143
3.4.2.9 Formation of Topographic Contrast with SE 143
E-T Detector 143
Factors Affecting Topographic Contrast 144
Edge Effect 145
Spherical Particles 147
Non-regular Specimens 147
Effect of Lateral Placement of E-T Detector 147
References 148
4: Imaging with the SEM 150
4.1 Resolution 150
4.1.1 Criteria of Spatial Resolution Limit 152
4.1.1.1 Rayleigh Criterion 152
4.1.1.2 Sparrow Criterion 153
4.1.1.3 Schuster´s Criterion 153
4.1.1.4 Houston Criterion 153
4.1.1.5 Buxton Criterion 153
4.1.1.6 Edge Resolution 153
4.1.1.7 Radial Intensity Distribution 153
4.1.1.8 Maximum Spatial Frequency 154
4.1.2 Imaging Parameters That Control the Spatial Resolution 155
4.1.2.1 Probe Size 155
4.1.2.2 Beam Current 156
4.1.2.3 Convergence Angle of the Probe 156
4.1.2.4 Accelerating Voltage 157
4.1.3 Guidelines for High-Resolution Imaging 159
4.1.4 Factors that Limit Spatial Resolution 161
4.2 Depth of Field 162
4.3 Influence of Operational Parameters on SEM Images 167
4.3.1 Effect of Accelerating Voltage (Beam Energy) 167
4.3.2 Effect of Probe Current/Spot Size 168
4.3.3 Effect of Working Distance 172
4.3.4 Effect of Objective Aperture 175
4.3.5 Effect of Specimen Tilt 178
4.3.6 Effect of Incorrect Column Alignment 180
4.4 Effects of Electron Beam on the Specimen Surface 181
4.4.1 Specimen Charging 181
4.4.1.1 Methods to Reduce Charge Buildup 184
4.4.2 Surface Contamination 187
4.4.3 Beam Damage 188
4.5 Influence of External Factors on SEM Imaging 189
4.5.1 Electromagnetic Interference 190
4.5.2 Floor Vibrations 190
4.5.3 Poor Microscope Maintenance 191
4.6 Summary of Operating Conditions and Their Effects 192
4.7 SEM Operation 193
4.7.1 Sample Handling 194
4.7.1.1 Sample Size 194
4.7.1.2 Sample Preparation 194
4.7.2 Sample Insertion 195
4.7.3 Image Acquisition 196
4.7.4 Microscope Alignment 197
4.7.5 Maintenance of the SEM 198
4.8 Safety Requirements 199
4.8.1 Radiation Safety 199
4.8.2 Safe Handling of the SEM and Related Equipment 200
4.8.3 Emergency 200
References 201
5: Specialized SEM Techniques 202
5.1 Imaging at Low Voltage 202
5.1.1 Electron Energy Filtering 203
5.1.1.1 E x B Filter 204
5.1.1.2 r-Filter 204
5.1.2 Detector Technology 204
5.1.2.1 Energy Selective Backscatter (EsB) Detector (Made by Zeiss) 204
5.1.2.2 Upper Electron Detector, UED (Made by JEOL Ltd) 204
5.1.2.3 Solid-State Backscattered Detector 207
5.1.3 Electron Beam Deceleration 207
5.1.4 Recent Developments 208
5.1.5 Applications 210
5.2 Imaging at Low Vacuum 210
5.2.1 Introduction 210
5.2.2 Brief History 211
5.2.3 Working Principle 211
5.2.4 Detector for Low Vacuum Mode 213
5.2.5 Gas Path Length 214
5.2.6 Applications 217
5.2.7 Latest Developments 218
5.3 Focused Ion Beam (FIB) 218
5.3.1 Introduction 218
5.3.2 Instrumentation 223
5.3.2.1 Ion Sources 223
5.3.2.2 Lens System 225
5.3.2.3 Stage 225
5.3.2.4 Detector 225
5.3.3 Ion-Solid Interactions 225
5.3.4 Ion Imaging 226
5.4 STEM-in-SEM 227
5.4.1 Working Principle 227
5.4.2 Advantages/Drawbacks 229
5.4.3 Applications 229
5.5 Electron Backscatter Diffraction (EBSD) 231
5.5.1 Brief History 232
5.5.2 Working Principle 232
5.5.3 Experimental Setup 234
5.5.4 Applications 236
5.6 Electron Beam Lithography 239
5.6.1 Introduction 239
5.6.2 Experimental Set-Up 240
5.6.3 Classification of E-beam Lithography Systems 241
5.6.4 Working Principle 242
5.6.4.1 Beam Deflection and Blanking 242
5.6.4.2 Pattern Design and Electron Beam Resist 242
5.6.4.3 Pattern Processing 243
5.6.5 Applications 243
5.7 Electron Beam-Induced Deposition (EBID) 245
5.7.1 Mechanism 245
5.7.2 Advantages/Disadvantages of EBID 246
5.7.3 Applications 246
5.8 Cathodoluminescence 247
5.8.1 Introduction 247
5.8.2 Instrumentation 248
5.8.3 Strengths and Limitations of SEM-CL 250
5.8.4 Applications 251
References 251
6: Characteristics of X-Rays 254
6.1 Atom Model 254
6.2 Production of X-Rays 255
6.2.1 Characteristic X-Rays 255
6.2.2 Continuous X-Rays 257
6.2.3 Duane-Hunt Limit 260
6.2.4 Kramer´s Law 260
6.2.5 Implication of Continuous X-Rays 261
6.3 Orbital Transitions 263
6.3.1 Nomenclature Used for Orbital Transition 263
6.3.2 Energy of Orbital Transition 263
6.3.3 Moseley´s Law 265
6.3.4 Critical Excitation Energy (Excitation Potential) 265
6.3.5 Cross Section of Inner-Shell Ionization 267
6.3.6 Overvoltage 268
6.4 Properties of Emitted X-Rays 270
6.4.1 Excited X-Ray Lines 270
6.4.2 X-Ray Range 271
6.4.3 X-Ray Spatial Resolution 272
6.4.4 Depth Distribution Profile 274
6.4.5 Relationship Between Depth Distribution ?(?z) and Mass Depth (?z) 275
6.4.6 X-Ray Absorption (Mass Absorption Coefficient) 277
6.4.6.1 Mass Absorption Coefficient in a Single Element 280
6.4.6.2 Mass Absorption Coefficient in a Mixer of Elements 281
6.4.7 Secondary X-Ray Fluorescence 282
References 284
7: Microchemical Analysis in the SEM 286
7.1 Energy Dispersive X-Ray Spectroscopy (EDS) 286
7.1.1 Working Principle 288
7.1.2 Advantages/Drawbacks of EDS Detector 293
7.2 Qualitative EDS Analysis 293
7.2.1 Selection of Beam Voltage and Current 294
7.2.2 Peak Acquisition 294
7.2.3 Peak Identification 294
7.2.4 Peak to Background Ratios 296
7.2.5 Background Correction 296
7.2.6 Duration of EDS Analysis 296
7.2.7 Dead Time 297
7.2.8 Resolution of EDS Detector 297
7.2.9 Overlapping Peaks 299
7.3 Artifacts in EDS Analysis 299
7.3.1 Peak Distortion 299
7.3.2 Peak Broadening 299
7.3.3 Escape Peaks 302
7.3.4 Sum Peaks 303
7.3.5 The Internal Fluorescence Peak 304
7.4 Display of EDS Information 304
7.4.1 EDS Spectra 305
7.4.2 X-Ray Maps 305
7.4.3 Line Scans 306
7.5 Quantitative EDS Analysis 308
7.5.1 Introduction 308
7.5.2 EDS with Standards 310
7.5.2.1 Castaing´s First Approximation 310
7.5.2.2 Deviation from Castaing´s First Approximation 311
7.5.2.3 Matrix Effects 312
Atomic Number Effect 312
Absorption Effect 314
Fluorescence Effect 314
7.5.2.4 ZAF Iterative Process 315
7.5.2.5 Phi-Rho-Z Correction Method 316
7.5.3 Examples of ZAF Correction Method 316
7.5.3.1 Stainless Steel 317
7.6 Standardless EDS Analysis 317
7.6.1 First Principles Standardless Analysis 319
7.6.2 Fitted Standards Standardless Analysis 319
7.7 Low-Voltage EDS 320
7.8 Minimum Detectability Limit (MDL) 321
7.9 Wavelength Dispersive X-Ray Spectroscopy (WDS) 321
7.9.1 Instrumentation 321
7.9.2 Working Principle 322
7.9.3 Analytical Crystals 325
7.9.4 Detection of X-Rays 325
7.9.5 Advantages/Drawbacks of WDS Technique 326
7.9.5.1 Advantages 326
7.9.5.2 Disadvantages 326
7.9.6 Qualitative WDS Analysis 327
References 327
8: Sample Preparation 329
8.1 Metals, Alloys, and Ceramics 329
8.1.1 Sampling 329
8.1.2 Sectioning 330
8.1.3 Cleaning 330
8.1.4 Embedding and Mounting 332
8.1.5 Grinding, Lapping, and Polishing 332
8.1.6 Impregnation 335
8.1.7 Etching 335
8.1.8 Fixing 336
8.1.9 Fracturing 336
8.1.10 Coating Process 338
8.1.10.1 Sputter Coating 338
Advantages 339
Limitations 340
8.1.10.2 Metal Coating by Vacuum Evaporation 340
Advantages 341
Disadvantages 341
8.1.10.3 Coating by Carbon Evaporation 341
8.1.10.4 Imaging of Coated Specimens 342
8.1.11 Marking Specimens 343
8.1.12 Specimen Handling and Storage 343
8.2 Geological Materials 344
8.2.1 Preliminary Preparation 344
8.2.2 Cleaning 344
8.2.3 Drying 345
8.2.4 Impregnation 345
8.2.5 Replicas and Casts 346
8.2.6 Rock Sample Cutting 346
8.2.7 Mounting the Sample into the SEM Holder 346
8.2.7.1 Using Stub 346
8.2.7.2 Embedding Media to the Sample 346
8.2.7.3 Grain Mounts 347
8.2.7.4 Mounting Standards 347
8.2.8 Polishing 347
8.2.9 Etching 348
8.2.10 Coating 348
8.3 Building Materials 349
8.3.1 Preparation of Cement Paste, Mortar, and Concrete Samples 349
8.3.1.1 Dry Potting 349
8.3.1.2 Wet Potting 349
8.3.2 Cutting and Grinding 350
8.3.3 Polishing 350
8.3.4 Impregnation Techniques 351
8.3.4.1 Epoxy Impregnation 351
8.3.4.2 Dye Impregnation Method 351
8.3.4.3 Impregnation by Wood´s Metal 351
8.3.4.4 High-Pressure Epoxy Impregnation Method 351
8.3.5 Drying the Specimen 352
8.3.6 Coating the Specimen 352
8.3.7 Cleaning the Surface of the Specimen 352
8.4 Polymers 352
8.4.1 Types of Polymers 353
8.4.1.1 Thermoplastics 353
8.4.1.2 Thermosets 354
8.4.1.3 Rubbers and Elastomers 354
8.4.2 Morphology of Polymers 354
8.4.2.1 Amorphous Polymers 354
8.4.2.2 Semicrystalline Polymers 355
8.4.3 Problems Associated with the SEM of Polymers 355
8.4.3.1 Radiation Sensitivity of Polymers 355
8.4.3.2 Low Contrast of Polymers 356
8.4.3.3 Charging 357
8.4.3.4 Degraded EDS or WDS Spectrum 357
8.4.4 General Aspects in Polymers Preparation for SEM 357
8.4.5 Sample Preparation Techniques for Polymers 358
8.4.5.1 Cutting and Sectioning 358
8.4.5.2 Microtomy of Polymers 358
8.4.5.3 Peel-Back Method 359
8.4.6 Devices Used in Microtomy 359
8.4.6.1 Microtome 359
8.4.6.2 Ultramicrotome 359
8.4.6.3 Cryo-microtome and Cryo-ultramicrotome 359
8.4.7 Sample Preparation Procedure for Polymers 360
8.4.7.1 Mounting of Polymer Samples 360
8.4.7.2 Grinding of Polymers 361
8.4.7.3 Polishing of Polymers 361
8.4.7.4 Etching of Polymers 362
Solvent and Chemical Etching 363
Acid Etching 363
Permanganate Etching 363
Plasma and Ion Etching 363
Focused Ion Beam Etching 363
Replication of Polymers 364
Staining of Polymers 365
Conductive Coatings 365
8.4.7.5 Cryogenic and Drying Methods 366
8.4.7.6 Simple Freezing Methods 366
8.4.7.7 Freeze-Drying 366
8.4.7.8 Critical Point Drying 367
8.4.7.9 Yielding and Fracture 367
8.5 Biological Materials 368
8.5.1 Fixation 369
8.5.1.1 Chemical Fixation 369
Formaldehyde (FA) 370
Glutaraldehyde (GA) 370
Osmium Tetroxide (OT) 370
Protein Cross-linking Reagents 371
8.5.1.2 Physical Fixation 371
8.5.2 Examples of Biological Sample Preparation 371
8.5.2.1 Bone Tissue 371
8.5.2.2 Heart Tissue 372
8.5.2.3 Stem Cells 372
8.5.2.4 Bacteria 373
8.5.2.5 Insect 374
References 378
Questions/Answers 380
Chapter 1 380
Chapter 2 382
Chapter 3 388
Chapter 4 397
Chapter 5 400
Chapter 6 405
Chapter 7 408
Chapter 8 412
Index 416