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The ESD Handbook
The ESD Handbook

Steven H. Voldman
This edition first published 2021
© 2021 John Wiley & Sons Ltd.

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Library of Congress Cataloging-in-Publication Data

Names: Voldman, Steven H., author.

Title: The ESD handbook / Steven H. Voldman.
Description: First edition. | Hoboken, NJ : Wiley, [2021] | Includes
bibliographical references and index.
Identifiers: LCCN 2020025380 (print) | LCCN 2020025381 (ebook) | ISBN
9781119965176 (hardback) | ISBN 9781119233107 (adobe pdf) | ISBN
9781119233138 (epub)
Subjects: LCSH: Electric discharges. | Semiconductors–Protection. |
Breakdown (Electricity)
Classification: LCC QC585.7.E43 V65 2021 (print) | LCC QC585.7.E43
(ebook) | DDC 537.5/2–dc23
LC record available at https://lccn.loc.gov/2020025380
LC ebook record available at https://lccn.loc.gov/2020025381

Cover Design: Wiley

Cover Image: © Lehrer/Shutterstock

Set in 9.5/12.5pt STIXTwoText by SPi Global, Chennai, India

10 9 8 7 6 5 4 3 2 1
 To My Wife
Betsy H. Brown
 vii

Contents

About the Author xxxvii

Acknowledgements xxxix

1 ESD, EOS, EMI, EMC, and Latchup 1

1.1 Electrostatic Discharge (ESD) 1
1.1.1 What Do You Mean by the Term “Electrostatic Discharge”? 1
1.2 Human Body Model (HBM) 2
1.2.1 Why Do We Have a Human Body Model? 2
1.2.1.1 What Does it Characterize? 2
1.3 Machine Model (MM) 3
1.3.1 What is the Purpose of the Machine Model? 3
1.3.1.1 How is it Different from the Human Body Model? 3
1.4 Cassette Model 3
1.4.1 Why Do We Have a Cassette Model? What Does it Represent? 3
1.5 Charged Device Model (CDM) 4
1.5.1 What is the Charged Device Model? 4
1.5.1.1 Why is it Important? 4
1.6 Transmission Line Pulse (TLP) 5
1.6.1 Why was the TLP Model Introduced? 5
1.6.1.1 Why is it So Valuable for Circuit Designers and ESD Engineers? 5
1.7 Very Fast Transmission Line Pulse (VF-TLP) 8
1.7.1 Why Do We Need to Evaluate VF-TLP? 8
1.8 Electrical Overstress (EOS) 8
1.9 Electrical Overstress (EOS) 8
1.9.1 How is EOS Differ from ESD? 8
1.10 EOS Sources – Lightning 9
1.11 EOS Sources – Electromagnetic Pulse (EMP) 9
1.12 EOS Sources – Machinery 10
1.13 EOS Sources – Power Distribution 10
1.14 EOS Sources – Switches, Relays, and Coils 10
1.15 EOS Design Flow and Product Definition 10
1.15.1 How Do You Add EOS to the Product Design Flow? 10
1.16 EOS Sources – Design Issues 11
viii Contents

1.17 Electromagnetic Interference (EMI) 12

1.17.1 What Type of Products are Sensitive to EMI? 12
1.18 Electromagnetic Compatibility (EMC) 13
1.19 Latchup 13
1.19.1 What is Latchup? Why is it Important? 13
Questions and Answers 14
1.20 Summary and Closing Comments 15
References 15

2 ESD in Manufacturing 21
2.1 Flooring 21
2.1.1 Question: Why is Flooring an ESD Issue? 21
2.2 Work Surfaces 21
2.2.1 Why are Worksurfaces an ESD Issue? 21
2.3 Garments 22
2.3.1 Question: Do Garments Play a Role in Charging of Products? 22
2.4 Wrist Straps 22
2.4.1 Why are Wrist Straps Required in a Manufacturing Environment? 22
2.5 Shoes – Footwear 22
2.5.1 How Do Shoes Influence Tribocharging? 22
2.6 Ionization 23
2.6.1 What is the Role of Ionization in Manufacturing? 23
2.7 Clean Rooms 24
2.7.1 Seating 24
2.7.1.1 Why are Chairs a Charging Issue? 24
2.8 Carts 26
2.8.1 Why are Carts an ESD Issue? 26
2.9 Shipping Tubes 26
2.9.1 Why are Shipping Tubes a Problem? 26
2.10 Trays 27
2.10.1 What Application are Sensitive to ESD in Trays? 27
2.11 Measurements 27
2.11.1 Packaging and Shipping 27
2.11.2 ESD Identification 27
2.12 Verification 28
2.12.1 ESD Program Management – Twelve Steps to Building an ESD Strategy 28
2.13 Audit 28
2.13.1 ESD Program Auditing 28
2.14 Triboelectric Charging – How Does it Happen? 29
2.15 Conductors, Semiconductors, and Insulators 30
2.16 Static Dissipative Materials 30
2.17 ESD and Materials 31
2.18 Electrification and Coulomb’s Law 31
2.18.1 Electrification by Friction 32
2.18.2 Electrification by Induction 32
 Contents ix

2.18.3 Electrification by Conduction 32

2.19 Electromagnetism and Electrodynamics 33
2.20 Electrical Breakdown 33
2.20.1 Electrostatic Discharge and Breakdown 33
2.20.2 Breakdown and Paschen’s Law 33
2.20.3 Breakdown and Townsend 34
2.20.4 Breakdown and Toepler’s Law 34
2.20.5 Avalanche Breakdown 35
2.21 Electro-Quasistatics and Magnetoquasistatics 36
2.22 Electrodynamics and Maxwell’s Equations 36
2.23 Electrostatic Discharge (ESD) 36
2.24 Electromagnetic Compatibility (EMC) 37
2.25 Electromagnetic Interference (EMI) 37
2.26 Fundamentals of Manufacturing and Electrostatics 37
2.27 Materials, Tooling, Human Factors, and Electrostatic Discharge 38
2.28 Materials and Human-induced Electric Fields 39
2.29 Manufacturing Environment and Tooling 39
2.30 Manufacturing Equipment and ESD Manufacturing Problems 39
2.31 Manufacturing Materials 39
2.32 Measurement and Test Equipment 40
2.33 Manufacturing Testing for Compliance 41
2.34 Grounding and Bonding Systems 42
2.35 Work Surfaces 42
2.36 Wrist Straps 43
2.37 Constant Monitors 43
2.38 Footwear 43
2.39 Floors 44
2.40 Personnel Grounding with Garments 44
2.41 Garments 44
2.42 Air Ionization 44
2.43 Seating 45
2.44 Packaging and Shipping 46
2.45 Trays 46
2.46 ESD Identification 46
2.47 ESD Program Auditing 46
2.48 ESD On-Chip Protection 47
2.49 ESD, EOS, EMI, EMC, and Latchup 47
2.49.1 ESD 47
2.49.2 EOS 48
2.49.3 EMI 48
2.49.4 EMC 48
2.50 Manufacturing Electrical Overstress (EOS) 48
2.50.1 Manufacturing EOS Sources – Machinery 49
2.50.2 Manufacturing EOS Sources – Power Distribution 49
2.50.3 Manufacturing EOS Sources – Switches, Relays and Coils 49
x Contents

2.51 EMI 50
2.52 EMC 50
2.53 Summary and Closing Comments 50
References 50

3 ESD Standards 55
3.1 Factory – Flooring 55
3.1.1 Factory – Worksurfaces 55
3.1.2 Factory – Ionization 55
3.1.3 Factory – Garments 55
3.1.4 Factory – Wrist Straps 55
3.1.5 Factory – Grounding 56
3.2 Factory – Resistance Measurement of Materials 56
3.2.1 Components – HBM 56
3.2.2 Components – MM 56
3.2.3 Components – CDM 57
3.2.4 Components – SDM 57
3.2.5 Components – TLP 57
3.2.6 Components – VF-TLP 57
3.2.7 Systems – IEC 61000-4-2 58
3.2.8 Systems – Cable Discharge Event (CDE) 58
3.2.9 Components – HMM 58
3.3 JEDEC 58
3.4 International Electro-Technical Commission (IEC) 59
3.5 IEEE 59
3.6 Department of Defense (DOD) 59
3.7 Military Standards 59
3.8 SAE 60
3.9 Summary and Closing Comments 60
Questions and Answers 60
References 61

4 ESD Testing 65
4.1 Electrostatic Discharge (ESD) Testing 65
4.2 ESD Models 65
4.2.1 Human Body Model (HBM) 66
4.2.2 HBM Pulse Waveform 67
4.2.3 HBM Equivalent Circuit Model 67
4.2.4 HBM Tester Source 67
4.3 HBM Test System 69
4.4 HBM Two-pin Test System 69
4.5 Machine Model (MM) 69
4.5.1 MM Equivalent Circuit 69
4.5.2 MM Pulse Waveform 70
4.5.3 ESD MM Tester Source 70
 Contents xi

4.6 Small Charge Model (SCM) 70

4.7 Small Charge Model Source 71
4.7.1 Charged Device Model 71
4.8 CDM Pulse Waveform 72
4.8.1 CDM Commercial Tester 72
4.8.2 CDM Equivalent Circuit 72
4.8.3 CDM Equivalent Circuit with Tester Chassis 74
4.8.4 Human Metal Model (HMM) 74
4.8.5 HMM Pulse Waveform 75
4.8.6 HMM Pulse Waveform Equation 76
4.9 HMM Equivalent Circuit 77
4.10 HMM Test Equipment 77
4.11 HMM Test Configuration 78
4.11.1 HMM Horizontal Configuration 78
4.11.2 HMM Vertical Configuration 78
4.12 HMM Fixture Board 78
4.13 Transmission Line Pulse (TLP) 82
4.14 TLP Test Systems 84
4.14.1 Very Fast Transmission Line Pulse (VF-TLP) 84
4.14.2 VF-TLP Pulse Waveform 84
4.15 IEC 61000-4-2 87
4.15.1 IEC 61000-4-2 Air Discharge 88
4.15.2 IEC 61000-4-2 Direct Contact Discharge 88
4.15.3 IEC 61000-4-2 Pulse Waveform 88
4.15.4 IEC 61000-4-2 Pulse Waveform Equation 89
4.16 Equivalent Circuit 89
4.17 Test Equipment 89
4.18 Cable Discharge Event (CDE) 90
4.18.1 CDE – Charging, Discharging, and Pulse Waveform 92
4.18.2 Charging Process 92
4.18.3 Discharging Process 92
4.19 CDE Pulse Waveform 93
4.20 Equivalent Circuit 93
4.21 Commercial Test Systems 94
4.22 Systems Electromagnetic Interference (EMI) 95
4.23 Electromagnetic Compatibility (EMC) 95
4.24 Electrical Overstress (EOS) 95
4.25 Latchup 95
4.26 Electrical Overstress (EOS) 95
4.27 EOS Sources – Lightning 96
4.28 EOS Sources – Electromagnetic Pulse (EMP) 97
4.29 Electromagnetic Compatibility 97
4.30 Summary and Closing Comments 100
References 100
xii Contents

5 ESD Device Physics 117

5.1 Electro-thermal Instability 117
5.2 Stable System 118
5.3 Unstable System 118
5.4 Differential Relation of Voltage and Current 120
5.5 Time Constant Hierarchy 121
5.6 Thermal Physics Time Constants 121
5.7 Adiabatic, Thermal Diffusion Time Scale and Steady State 121
5.8 Electro-quasistatic and Magnetoquasistatics 122
5.9 Electrical Instability 124
5.9.1 Thermal Time Constant Approach 124
5.9.1.1 Heat Capacity 124
5.9.1.2 Thermal Diffusion 124
5.9.1.3 Heat Transport Equation 124
5.10 Thermal Physics Time Constants 125
5.11 Adiabatic, Thermal Diffusion Time Scale and Steady State 126
5.12 Electrical Instability and Breakdown 126
5.12.1 Electrical Instability 126
5.13 Spatial Instability and Electro-thermal Current Constriction 127
5.14 Equipotential Surface 127
5.15 Heat Flow 128
5.16 Conservation of Heat 128
5.17 Electric Potential and Temperature Gradient 128
5.17.1 Maximum Temperature and Minimum Potential 128
5.18 Electric Energy, Resistivity, and Thermal Conductivity 129
5.18.1 Intrinsic Temperature 129
5.18.2 Radius of Current Constriction 130
5.18.3 Differential Potential and Differential Thermal Potential 130
5.18.4 Resistance Reduction and Current Constriction 130
5.18.5 Contact Radius and Contour Relationship 131
5.18.6 Current Constriction Relationship 131
5.18.7 Intrinsic Temperature 131
5.19 Breakdown 131
5.19.1 Paschen’s Breakdown Theory 131
5.19.2 Townsend’s Concept 132
5.19.3 Toepler’s Law 132
5.19.4 Avalanche Breakdown 133
5.19.4.1 Energy Transfer from Electric Field to Carriers 133
5.19.4.2 Energy Balance Relationship 133
5.19.4.3 Impact Ionization Coefficient 133
5.19.4.4 Impact Ionization Mean Free Path and Optical Generation Mean Free
Path 134
5.19.4.5 Impact Ionization Coefficient 134
5.19.5 Breakdown in Air 135
5.20 Electron Current Continuity Relationship 136
5.20.1 Time-dependent Electron Population 136
 Contents xiii

5.20.2 Electron Density 137

5.21 Air Breakdown and Peak Currents 138
5.22 Electro-thermal Instability 139
5.23 Mathematical Methods – Green’s Function and Method of Images 141
5.23.1 Case of a Parallelepiped in an Infinite Medium 142
5.24 Mathematical Methods – Green’s Function and Method of Images 143
5.24.1 Case of a Parallelepiped in an Infinite Medium 144
5.24.2 Case of the Semi-infinite Domain 145
5.25 Mathematical Methods – Integral Transforms of the Heat Conduction
Equation 148
5.26 Flux Potential Transfer Relations Matrix Methodology 152
5.27 Heat Equation Variable Conductivity 154
5.28 Mathematical Methods – Boltzmann Transformation 156
5.29 Mathematical Methods – The Duhamel Formulation 158
5.30 Spherical Source Tasca Model 160
5.31 Wunsch–Bell Model 163
5.32 The Smith and Littau Model 166
5.33 The Arkihpov–Astvatsaturyan–Godovosyn–Rudenko Model 168
5.34 The Vlasov–Sinkevitch Model 169
5.35 The Dwyer, Franklin and Campbell Model 169
5.36 Negative Differential Resistor and Resistor Ballasting 174
5.37 Ash Model – Nonlinear Failure Power Thresholds 176
5.38 Statistical Models for ESD Prediction 178
5.39 Summary and Closing Comments 180
References 180

6 ESD Events and Protection Circuits 189

6.1 Human Body Model (HBM) 189
6.1.1 HBM Pulse Waveform 189
6.1.2 HBM Equivalent Circuit Model 189
6.1.3 HBM Tester Source 189
6.1.4 HBM Failure 190
6.2 Machine Model (MM) 191
6.2.1 MM Equivalent Circuit 191
6.2.2 MM Pulse Waveform 192
6.2.3 ESD MM Tester Source 192
6.2.4 Machine Model Example of MM Failure 193
6.3 Charged Device Model 193
6.3.1 CDM Pulse Waveform 193
6.3.2 CDM Commercial Tester 193
6.3.3 CDM Equivalent Circuit 196
6.3.4 CDM Equivalent Circuit with Tester Chassis 196
6.3.5 CDM Failure Mechanism 196
6.4 Human Metal Model (HMM) 197
6.4.1 HMM Pulse Waveform 198
6.4.2 HMM Pulse Waveform Equation 198
xiv Contents

6.4.3 Equivalent Circuit 200

6.4.4 HMM Test Equipment 200
6.4.5 HMM Test Configuration 201
6.4.5.1 Horizontal Configuration 201
6.4.5.2 Vertical Configuration 202
6.4.6 HMM Fixture Board 202
6.5 IEC 61000-4-2 History 204
6.5.1 IEC 61000-4-2 Scope 204
6.5.2 IEC 61000-4-2 Purpose 204
6.5.2.1 Air Discharge 205
6.5.2.2 Direct Contact Discharge 205
6.5.2.3 Pulse Waveform 205
6.5.2.4 Pulse Waveform Equation 206
6.5.3 Equivalent Circuit 207
6.5.4 Test Equipment 207
6.5.5 Test Configuration 208
6.5.6 IEC 61000-4-2 ESD Protection Circuitry 208
6.5.7 ESD Guns 209
6.6 IEC 61000-4-5 209
6.6.1 History 209
6.6.2 IEC 61000-4-5 Scope 210
6.6.3 IEC 61000-4-5 Purpose 210
6.6.4 IEC 61000-4-5 Pulse Waveform 210
6.6.5 IEC 61000-4-5 Test Equipment 211
6.6.6 IEC 61000-4-5 Test Sequence and Procedure 212
6.6.7 Failure Mechanisms 213
6.6.8 IEC 61000-4-5 ESD Current Paths 213
6.6.9 ESD Protection Circuit Solutions 213
6.7 Cable Discharge Event (CDE) 213
6.7.1 Cable Discharge Event History 213
6.8 CDM Scope 215
6.8.1 CDM Purpose 215
6.8.2 Cable Discharge Event – Charging, Discharging, and Pulse Waveform 216
6.8.3 CDE Charging Process 216
6.8.4 CDE Discharging Process 217
6.8.5 CDE Pulse Waveform 217
6.8.6 CDE Equivalent Circuit 217
6.8.7 CDE Test Equipment 218
6.8.7.1 Commercial Test Systems 218
References 219

7 ESD Failure Mechanism 235

7.1 Tables of CMOS ESD Failure Mechanisms 235
7.2 LOCOS Isolation-Defined CMOS 235
7.3 LOCOS-bound Thick Oxide MOSFET 241
 Contents xv

7.4 LOCOS-Bound Structures 242

7.4.1 LOCOS-bound P+/N-well Junction Diode 242
7.4.2 LOCOS-bound N+/P- Substrate Junction Diode 244
7.4.3 LOCOS-bound N-Well/P- Substrate Junction Diode 244
7.4.4 LOCOS-bound Lateral N-well to N-well 244
7.4.5 LOCOS-bound lateral N+ to N-well 245
7.5 Shallow Trench Isolation (STI) 245
7.6 STI Pull-down ESD Failure Mechanism 245
7.7 STI Pull-Down and Gate Wrap-Around 246
7.7.1 Silicide and Diodes 247
7.7.2 Non-silicided Diode Structures 247
7.8 MOSFETs 247
7.8.1 N-channel MOSFET 247
7.8.2 N-channel Multi-finger MOSFET 250
7.9 LOCOS-bound Thick Oxide MOSFET 252
7.9.1 Cascode Series N-channel MOSFET 252
7.9.2 P-channel MOSFET 252
7.9.3 P-channel Multi-finger MOSFET 253
7.9.4 Tungsten Silicide Gate MOSFET 253
7.9.5 Polysilicon Silicide Gate MOSFET 254
7.9.6 Metal Gate/High K Dielectric MOSFET 254
7.10 Bipolar Transistor Devices 254
7.10.1 LOCOS-bound Lateral PNP Bipolar 254
7.10.2 Polysilicon-defined Devices 254
7.10.3 Polysilicon-bound Gated Diode 255
7.10.4 Lateral Diode with Block Mask 255
7.10.5 Resistors 256
7.10.6 Diffused Resistors 256
7.10.7 N-well Resistors 256
7.10.8 Buried Resistors 258
7.11 Silicide Blocked N-diffusion Resistors 259
7.12 Silicon Germanium ESD Failure Mechanisms 259
7.13 Silicon Germanium Carbon ESD Failure Mechanisms 259
7.14 Gallium Arsenide Technology ESD Failure Mechanisms 260
7.15 Indium Gallium Arsenide ESD Failure Mechanisms 261
7.15.1 Magnetic Recording 261
7.15.2 FinFET Transistors 262
7.16 Micro Electromechanical (MEM) Systems 263
7.17 Micro-mirror Array Failures 265
7.17.1 Manufacturing Failure 265
7.17.2 EOS Failure Mechanisms 265
7.17.3 EOS Failure Mechanisms – Semiconductor Process – Application
Mismatch 267
7.17.4 EOS Failure Mechanisms – Bond Wire Failures 267
7.17.5 EOS Failure Mechanisms – External Load to Chip Failures 268
xvi Contents

7.17.6 EOS Failure Mechanisms – Printed Circuit Board (PCB) to Chip Failures 268
7.17.7 EOS Failure Mechanisms – Reverse Insertion 268
7.18 EOS Bond Pad and Interconnect Failure 269
7.18.1 EOS Failure – Packaging Failure 269
7.18.1.1 Electrical Overstress – Packaging Ablation 269
7.19 Summary and Closing Comments 272
References 273

8 ESD Design Synthesis 281

8.1 ESD Design Synthesis and Architecture Flow 281
8.1.1 Fundamental Concepts of ESD Design 281
8.1.2 Top-down ESD Design 285
8.1.3 Bottom-up ESD Design 285
8.1.4 Top-down ESD Design – Memory Semiconductor Chips 285
8.1.5 Top-down ESD Design – ASIC Design System 286
8.2 ESD Design – the Signal Path and the Alternate Current Path 287
8.3 ESD Electrical Circuit and Schematic Architecture Concepts 289
8.4 The Ideal ESD Network 289
8.4.1 Ideal ESD Networks and the Current-Voltage D.C. Design Window 289
8.4.2 The ESD Design Window 290
8.4.3 The Ideal ESD Networks in the Frequency Domain Design Window 291
8.5 Mapping Semiconductor Chips and ESD Designs 293
8.6 Mapping across Semiconductor Fabricators 294
8.7 ESD Design Mapping across Technology Generations 295
8.7.1 Mapping from Bipolar Technology to CMOS Technology 296
8.7.2 Mapping from Digital CMOS Technology to Mixed Signal Analog-Digital CMOS
Technology 297
8.7.3 Mapping from Bulk CMOS Technology to Silicon On Insulator (SOI) 297
8.7.4 ESD Design – Mapping CMOS to RF CMOS Technology 298
8.7.4.1 ESD Chip Architecture, and ESD Test Standards 299
8.7.4.2 ESD Chip Architecture, and ESD Testing 299
8.7.4.3 ESD Chip Architecture, and ESD Alternative Current Paths 300
8.7.4.4 ESD Circuits, I/O, and Cores 300
8.7.4.5 ESD Signal Pin Circuits 300
8.7.4.6 ESD Power Clamp Networks 302
8.7.4.7 ESD Rail-to-Rail Circuits 303
8.7.4.8 ESD Design and Noise 304
8.7.4.9 ESD Internal Signal Path ESD Networks 305
8.7.5 Cross Domain ESD Networks 306
8.8 ESD Networks, Sequencing, and Chip Architecture 306
8.8.1 ESD Design Synthesis – Latchup-free ESD Networks 307
8.8.1.1 ESD Design Concepts – Buffering – Inter-device 309
8.8.1.2 ESD Design Concepts – Ballasting – Inter-Device 309
8.8.1.3 ESD Design Concepts – Ballasting – Intra-device 311
8.8.1.4 ESD Design Concepts – Distributed Load Techniques 311
 Contents xvii

8.8.1.5 ESD Design Concepts – Dummy Circuits 312

8.8.1.6 ESD Design Concepts – Power Supply Decoupling 313
8.8.1.7 ESD Design Concepts – Feedback Loop Decoupling 313
8.9 ESD Layout and Floorplan-related Concepts 314
8.9.1 Design Symmetry 314
8.9.2 Design Segmentation 315
8.9.2.1 ESD Design Concepts – Utilization of Empty Space 317
8.9.2.2 ESD Design Synthesis – Across Chip Line Width Variation (ACLV) 317
8.9.2.3 ESD Design Concepts – Dummy Shapes 318
8.9.2.4 ESD Design Concepts – Dummy Masks 319
8.9.2.5 ESD Design Concepts – Adjacency 320
8.9.2.6 ESD Design Concepts – Analog Circuit Techniques 321
8.9.2.7 ESD Design Concepts – Wire Bonds 322
8.9.2.8 ESD Design Rules 322
8.9.2.9 ESD Design Rule Check (DRC) 322
8.9.2.10 ESD Layout Versus Schematic (LVS) 322
8.9.3 Electrical Resistance Checking (ERC) 323
8.10 ESD Architecture and Floor-planning 323
8.10.1 ESD Design Floorplan 323
8.10.2 Peripheral I/O Design 324
8.10.2.1 Pad Limited Peripheral I/O Design Architecture 324
8.10.2.2 Pad Limited Peripheral I/O Design Architecture – Staggered I/O 326
8.10.3 Core Limited Peripheral I/O Design Architecture 327
8.10.4 Lumped ESD Power Clamp in Peripheral I/O Design Architecture 328
8.10.5 Lumped ESD Power Clamps in Peripheral I/O Design Architecture in the
Semiconductor Chip Corners 328
8.10.6 Lumped ESD Power Clamp in Peripheral I/O Design Architecture – Power
Pads 328
8.10.7 Lumped ESD Power Clamp in Peripheral I/O Design
Architecture – Master/Slave ESD Power Clamp System 329
8.10.7.1 Array I/O 330
8.10.7.2 Array I/O – Off Chip Driver (OCD) Banks 331
8.10.7.3 Array I/O Nibble Architecture 332
8.10.8 Array I/O Pair Architecture 333
8.10.9 Array I/O – Fully Distributed 334
8.10.10 ESD Architecture – Dummy Bus Architectures 338
8.10.11 ESD Architecture – Dummy VDD Bus 338
8.10.12 ESD Architecture – Dummy Ground (VSS ) Bus 339
8.10.12.1 Native Voltage Power Supply Architecture 340
8.10.12.2 Single Power Supply Architecture 340
8.10.12.3 Mixed Voltage Architecture 340
8.10.12.4 Mixed Signal Architecture 345
8.10.12.5 Mixed-system Architecture – Digital and Analog CMOS 345
8.11 Digital and Analog CMOS Architecture 347
8.12 Digital and Analog Floorplan – Placement of Analog Circuits 348
xviii Contents

8.13 Mixed-signal Architecture – Digital, Analog, and RF Architecture 350

8.14 Summary and Closing Comments 351
Questions 351
References 352

9 On-chip ESD Protection Circuits – Input Circuitry 363

9.1 Receivers and ESD 363
9.2 Receivers and Receiver Delay Time 363
9.3 ESD Loading Effect on Receiver Performance 364
9.4 Receivers and HBM 365
9.5 Receivers and CDM 366
9.6 Receivers and Receiver Evolution 368
9.7 Receiver Circuits with Half-pass Transmission Gate 368
9.8 Receiver with Full-pass Transmission Gate 371
9.9 Receiver, Half-pass Transmission Gate, and Keeper Network 373
9.10 Receiver, Half-pass Transmission Gate, and the Modified Keeper Network 377
9.11 Receiver Circuits with Pseudo-zero VT Half-pass Transmission Gates 379
9.12 Receiver with Zero VT Transmission Gate 381
9.13 Receiver Circuits with Bleed Transistors 383
9.14 Receiver Circuits with Test Functions 384
9.15 Receiver with Schmitt Trigger Feedback Network 385
9.16 Bipolar Transistor Receivers 389
9.16.1 Bipolar Single-ended Receiver Circuits 389
9.16.2 Differential Receivers 390
9.16.3 Signal Differential Receiver 391
9.16.4 Signal CMOS Differential Receivers 391
9.16.5 Signal Bipolar Differential Receivers 391
9.17 CMOS Differential Receiver with Analog Layout Concepts 397
9.18 CMOS Differential Receiver Capacitance Loading 398
9.19 CMOS Differential Receiver ESD Mismatch 398
9.20 Analog Differential Pair ESD Signal Pin Matching with Common Well
Layout 400
9.21 Analog Differential Pair Common Centroid Design Layout – Signal-Pin to
Signal-Pin and Parasitic ESD Elements 403
9.22 Off-chip Drivers (OCD) 405
9.23 Off-chip Driver I/O Standards and ESD 407
9.24 Off-chip Driver (OCD) ESD Design Basics 408
9.24.1 OCD: CMOS Asymmetric Pull-up/Pull-down 408
9.24.2 OCD: CMOS Symmetric Pull-up/Pull-down 410
9.24.3 OCD: Gunning Transceiver Logic (GTL) 411
9.24.4 OCD: High-speed Transceiver Logic (HSTL) 412
9.24.5 OCD: Stub Series Terminated Logic (SSTL) 413
9.25 Off-chip Drivers (OCD): Mixed Voltage Interface 414
9.26 Off-chip Drivers (OCD): Self-bias Well OCD Networks 414
9.27 Self-bias Well Off-chip Driver (OCD) Networks 415
 Contents xix

9.28 ESD Protection Networks for Self-bias Well OCD Networks 417
9.29 Programmable Impedance Off-chip Driver (OCD) Network 418
9.30 ESD Input Protection Networks for Programmable Impedance Off-chip
Drivers 422
9.31 Universal Off-chip Drivers 423
9.32 Gate Array Off-chip Driver Design 423
9.32.1 Gate Array Off-chip Driver ESD Design Practices 423
9.32.2 Gate Array OCD Design – Usage of Unused Elements 423
9.33 Gate Array OCD Design – Impedance Matching of Unused Elements 425
9.34 OCD ESD Design – Power Rails Over Multi-finger MOSFETs 426
9.35 Off-chip Driver: Gate-modulated MOSFET ESD Network 427
9.36 Off-chip Driver Simplified Gate Modulated Network 428
9.37 Off-chip Drivers ESD Design: Integration of Coupling and Ballasting
Techniques 428
9.38 Ballasting and Coupling 429
9.39 MOSFET Source-initiated Gate-bootstrapped Resistor Ballasted Multi-finger
MOSFET with Diode 429
9.40 MOSFET Source-initiated Gate-bootstrapped Resistor Ballasted Multi-finger
MOSFET with a MOSFET 430
9.41 Gate-coupled Domino Resistor-ballasted MOSFET 431
9.42 Substrate-modulated Resistor Ballasted MOSFET 433
9.43 Summary and Closing Comments 434
Problems 435
References 437

10 On-Chip ESD Protection Circuits – ESD Power Clamps 441

10.1 ESD Power Clamps 441
10.2 ESD Power Clamp Design Practices 441
10.3 Current Loops 442
10.4 Impedance 442
10.5 Segmentation 443
10.6 Voltage Limitations 443
10.7 Latchup 443
10.8 ESD Power Clamp Circuits 444
10.9 Classification of ESD Power Clamps 444
10.10 Master-Slave ESD Power Clamps 445
10.11 Trigger Networks 445
10.12 ESD Power Clamp Characteristics and Issues 445
10.13 Design Synthesis of ESD Power Clamp – Key Design Parameters 446
10.14 Design Synthesis of ESD Power Clamps Trigger Networks 446
10.15 Transient Response Frequency Trigger Element and the ESD Frequency
Window 446
10.16 ESD Power Clamp Frequency Design Window 447
10.17 Design Synthesis of ESD Power Clamp – Voltage Triggered ESD Trigger
Elements 448
xx Contents

10.18 Design Synthesis of ESD Power Clamp – The ESD Power Clamp Shunting
Element 449
10.19 ESD Power Clamp Trigger Condition vs. Shunt Failure 450
10.20 ESD Clamp Element – Width Scaling 450
10.21 ESD Clamp Element – On-resistance 450
10.22 ESD Clamp Element – Safe Operating Area (SOA) 451
10.23 ESD Power Clamp Issues 451
10.24 ESD Power Clamp Issues – Power-up and Power-down 451
10.25 ESD Power Clamp Issues – False Triggering 452
10.26 ESD Power Clamp Issues – Pre-charging 452
10.27 ESD Power Clamp Issues – Post-charging 452
10.28 ESD Power Clamp Design 453
10.28.1 Native Power Supply RC-Triggered MOSFET ESD Power Clamp 453
10.28.2 Non-Native Power Supply RC-triggered MOSFET ESD Power Clamp 453
10.28.3 ESD Power Clamp Networks with Improved Inverter Stage Feedback 454
10.28.4 CMOS RC-trigger Clamp with CMOS PFET Half-latch Keeper Feedback 454
10.28.5 CMOS RC-trigger Clamp with CMOS PFET Full-latch Keeper Feedback 454
10.29 ESD Power Clamp Design Synthesis – Forward Bias Triggered ESD Power
Clamps 456
10.29.1 ESD Power Clamp Design Synthesis – IEC 61000-4-2 Responsive ESD Power
Clamps 456
10.29.2 ESD Power Clamp Design Synthesis – Pre-charging and Post-charging
Insensitive ESD Power Clamps 457
10.29.3 Master/Slave ESD Power Clamp Systems 457
10.30 Series Stacked RC-triggered ESD Power Clamps 459
10.30.1 ESD Power Clamps – Triple Well Series Diodes as Core Clamps 459
10.31 Triple Well Diode String ESD Power Clamp 463
10.31.1 Triple Well ESD Power Clamp Network with Independent N-Band Voltage
Bias 463
10.32 Bipolar ESD Power Clamps 464
10.32.1 Bipolar Voltage-Triggered ESD Power Clamps 464
10.32.2 Bipolar ESD Power Clamp – Zener Breakdown Voltage-Triggered 465
10.32.3 Bipolar ESD Power Clamp – BVCEO Voltage Triggered ESD Power Clamp 466
10.32.4 The Johnson Limit Relationship 466
10.33 ESD Power Clamp Design Synthesis – Bipolar ESD Power Clamps 469
10.33.1 Mixed Voltage Interface Forward-bias Voltage and BVCEO-Breakdown
Synthesized Bipolar ESD Power Clamps 471
10.33.2 Ultra-low-voltage Forward-biased Voltage-trigger BiCMOS ESD Power
Clamp 476
10.34 Bipolar ESD Power Clamps with Frequency Trigger Elements:
Capacitance-triggered 480
10.35 Silicon Controlled Rectifier Power Clamps 481
10.35.1 ESD Silicon Controlled Rectifier (SCR) Circuits 481
10.35.2 Uni-Directional Silicon Controlled Rectifier (SCR) 481
10.35.3 Bi-directional Silicon Controlled Rectifier (SCR) ESD Power Clamps 482
 Contents xxi

10.35.4 Medium Level SCR (MLSCR) ESD Power Clamps 482

10.35.5 Low Voltage Triggered SCR (LVTSCR) ESD Power Clamps 483
Problems 483
References 486

11 ESD Architecture and Floor Planning 491

11.1 ESD Design Floor Plan 491
11.2 Peripheral I/O Design 492
11.3 Pad Limited Peripheral I/O Design Architecture 493
11.4 Pad Limited Peripheral I/O Design Architecture – Staggered I/O 493
11.5 Core Limited Peripheral I/O Design Architecture 495
11.6 Lumped ESD Power Clamp in Peripheral I/O Design Architecture 496
11.7 Lumped ESD Power Clamp in Peripheral I/O Design Architecture in the
Semiconductor Chip Corners 496
11.8 Lumped ESD Power Clamp in Peripheral I/O Design Architecture – Power
Pads 497
11.9 Lumped ESD Power Clamp in Peripheral I/O Design Architecture –
Master/Slave ESD Power Clamp System 498
11.10 Array I/O 498
11.10.1 Array I/O – Off Chip Driver (OCD) Banks 499
11.11 Array I/O Nibble Architecture 501
11.12 Array I/O Pair Architecture 503
11.13 Array I/O – Fully Distributed 504
11.14 ESD Architecture – Dummy Bus Architecture 507
11.15 ESD Architecture – Dummy VDD Bus 507
11.16 ESD Architecture – Dummy Ground (VSS) Bus 508
11.17 Native Voltage Power Supply Architecture 508
11.18 Single Power Supply Architecture 509
11.19 Mixed Voltage Architecture 509
11.20 Mixed Voltage Architecture – Single Power Supply 509
11.21 Mixed Voltage Architecture – Dual Power Supply 511
11.22 Mixed Signal Architecture 514
11.22.1 Mixed Signal Architecture – CMOS 514
11.22.2 Mixed System Architecture – Digital and Analog CMOS 514
11.22.3 Digital and Analog CMOS Architecture 514
11.23 Digital and Analog Floor Plan – Placement of Analog Circuits 515
11.24 Mixed Signal Architecture – Digital, Analog, and RF Architecture 518
11.25 ESD Power Grid Design 519
11.25.1 ESD Power Grid 519
11.25.2 ESD Power Grid – Key ESD Design Parameters 519
11.25.3 Power Grid Layout Design 519
11.25.4 Power Grid Design – Slotting of Power Grid 519
11.25.5 Power Grid Design – Segmentation of Power Grids 520
11.25.6 Power Grid Design – Chip Corners 521
11.25.7 Power Grid Design – Stacking of Metal Levels 522
11.25.8 Power Grid Design – Wiring Bays and Weaved Power Bus Designs 523
xxii Contents

11.25.9 ESD Specification Power Grid Considerations 523

11.25.10 CDM Specification Power Grid and Interconnect Design Considerations 523
11.25.11 HMM and IEC Specification Power Grid and Interconnect Design
Considerations 524
11.25.12 Semiconductor Chip Guard Ring Seal 524
11.26 I/O to Core Guard Rings 525
11.26.1 I/O to I/O Guard Rings 526
11.27 Within I/O Guard Rings 527
11.27.1 Within I/O Cell Guard Ring 527
11.28 ESD-to-I/O Off-Chip Driver (OCD) Guard Ring 527
11.28.1 ESD Signal Pin Guard Rings 528
11.28.2 ESD Signal Pin Guard Rings and Dual-diode ESD Network 529
11.28.3 Mixed Signal Guard Rings – Digital to Analog 531
11.28.4 Mixed Voltage Guard Rings – High Voltage to Low Voltage 531
11.28.5 High Voltage Guard Rings 532
11.28.6 Passive and Active Guard Rings 533
11.28.7 Passive Guard Rings 534
11.28.8 Active Guard Rings 534
11.28.9 Trench Guard Rings 535
11.28.10 Through Silicon Via (TSV) Guard Rings 536
11.28.11 Guard Ring Design Rule Checking (DRC) 537
11.28.12 Internal Latchup and Guard Ring Design Rules 537
11.28.13 External Latchup Guard Ring Design Rules 538
11.29 Guard Rings and Computer Aided Design (CAD) Methods 539
11.29.1 Built-in Guard Rings 539
11.29.2 Guard Ring Parameterized Cells (Pcell) 539
11.29.3 Post-processing Methodology of Guard Ring Modification 540
11.30 Summary and Closing Comments 541
References 541

12 ESD Digital Design 551

12.1 Fundamental Concepts of ESD Design 551
12.2 Concepts of ESD Digital Design 551
12.3 Device Response to External Events 552
12.4 Alternative Current Loops 553
12.4.1 Switches 553
12.4.2 Decoupling of Current Paths 553
12.5 Decoupling of Feedback Loops 554
12.6 Decoupling of Power Rails 554
12.7 Local and Global Distribution 554
12.8 Usage of Parasitic Elements 555
12.8.1 Buffering 555
12.8.2 Ballasting 555
12.9 Unused Section of a Semiconductor Device, Circuit, or Chip Function 556
 Contents xxiii

12.10 Unused Corners 556

12.11 Unused White Space 556
12.12 Impedance Matching Between Floating and Non-floating Networks 556
12.13 Unconnected Structures 557
12.13.1 Utilization of Dummy Structures and Dummy Circuits 557
12.13.2 Non-scalable Source Events 557
12.13.3 Area Efficiency 557
12.14 Symmetry 557
12.15 Design Synthesis 557
12.15.1 Synthesis and Architecture of a Semiconductor Chip for ESD Protection 557
12.15.2 Electrical and Spatial Connectivity 558
12.15.3 Electrical Connectivity 558
12.15.4 Thermal Connectivity 559
12.15.5 Spatial Connectivity 559
12.16 ESD, Latchup, and Noise 559
12.16.1 Noise 560
12.16.2 Latchup 561
12.16.3 Interface Circuits and ESD Elements 561
12.16.4 ESD Power Clamp Networks 564
12.16.5 Placement of ESD Power Clamps 566
12.16.6 ESD Rail-to-Rail Networks 568
12.16.7 Placement of ESD Rail-to-Rail Networks 570
12.16.8 Peripheral and Array I/O 570
12.16.9 Guard Rings 572
12.16.10 Pads, Floating Pads, and No Connect Pads 573
12.17 Structures Under Bond Pads 574
12.18 Summary and Closing Comments 575
References 576

13 ESD Analog Design 583

13.1 Analog Design: Local Matching 583
13.2 Analog Design: Global Matching 583
13.3 Symmetry 584
13.3.1 Layout Design Symmetry 584
13.4 Analog Design – Local Matching 584
13.5 Analog Design – Global Matching 584
13.5.1 Design Orientation 585
13.5.2 Symmetry and Matching 585
13.5.3 Layout Design Symmetry 585
13.5.4 Thermal Symmetry 585
13.6 Common Centroid Design 586
13.7 Common Centroid Arrays 586
13.7.1 One-axis Common Centroid Design 586
13.7.2 Two-axis Common Centroid Design 586
xxiv Contents

13.8 Interdigitation Design 586

13.9 Common Centroid and Interdigitation Design 587
13.9.1 Linewidth Control 588
13.9.2 Analog Design – Across Chip Line Width Variation (ACLV) 590
13.9.3 Passive Element Design 591
13.9.4 Resistor Element Design 591
13.9.5 Resistor Element Design: Dogbone Layout 591
13.9.6 Resistor Design – Analog Interdigitated Layout 592
13.10 Dummy Resistor Layout 593
13.11 Thermoelectric Cancelation Layout 593
13.12 Electrostatic Shield 593
13.13 Interdigitated Resistors and ESD Parasitics 594
13.14 Capacitor Element Design 595
13.15 Inductor Element Design 596
13.15.1 Quality Factor 597
13.16 ESD Failure in Inductors 597
13.17 Inductor Physical Variables 598
13.18 Inductor Element Design 599
13.19 Diode Design 599
13.19.1 Circular Diode Designs 600
13.19.2 Octagonal Diode Design 600
13.19.3 MOSFET Design 601
13.19.4 Multi-finger MOSFET with Dummy Fingers 601
13.20 Analog ESD Circuits 602
13.20.1 Analog ESD Devices and Circuits 602
13.20.2 Analog ESD Diodes 602
13.20.3 Analog Dual Diode and Series Diodes 602
13.20.4 Analog ESD: Dual Diode – Resistor 602
13.20.5 Dual Diode – Resistor – Dual Diode 605
13.20.6 Dual-diode Resistor – Grounded Gate MOSFET 606
13.20.6.1 Back-to-Back Diode Strings 606
13.21 ESD MOSFETs 607
13.21.1 Grounded Gate MOSFET 608
13.21.2 ESD Power Clamps – RC Triggered MOSFET 609
13.22 Receivers 609
13.22.1 Signal Bipolar Differential Receivers 610
13.23 CMOS Differential Receiver with Analog Layout Concepts 614
13.23.1 CMOS Differential Receiver Capacitance Loading 616
13.23.2 CMOS Differential Receiver ESD Mismatch 616
13.23.3 Analog Differential Pair ESD Signal Pin Matching with Common Well
Layout 617
13.24 Analog Differential Pair Common Centroid Design Layout – Signal-pin to
Signal-pin and Parasitic ESD Elements 620
13.25 Summary and Closing Comments 624
References 624
 Contents xxv

14 ESD RF Design 629

14.1 Fundamental Concepts of ESD Design 629
14.2 Fundamental Concepts of RF ESD Design 632
14.3 RF CMOS Input Circuits 637
14.3.1 RF CMOS ESD Diode Networks 637
14.3.2 RF CMOS Diode String ESD Network 641
14.3.3 RF CMOS – Diode-inductor ESD Networks 643
14.3.4 RF Inductor-diode ESD Networks 645
14.3.5 RF Diode-inductor ESD Networks 646
14.4 RF CMOS Impedance Isolation LC Resonator ESD Networks 647
14.4.1 RF CMOS LC-diode ESD Networks 647
14.4.2 RF CMOS Diode-LC ESD Networks 648
14.5 RF CMOS LC-diode Networks Experimental Results 648
14.6 RF CMOS LNA ESD Design – Low Resistance ESD Inductor and ESD Diode
Clamping Elements in Π-configuration 650
14.7 RF CMOS T-coil Inductor ESD Input Network 653
14.8 RF CMOS Distributed ESD Networks 655
14.9 RF CMOS Distributed ESD-RF Networks 656
14.10 RF CMOS Distributed RF-ESD Networks Using Series Inductors and
Dual-diode Shunts 656
14.11 RF CMOS Distributed RF-ESD Networks Using Series Inductors and MOSFET
Parallel Shunts 659
14.12 RF CMOS Distributed ESD Networks – Transmission Lines and Co-planar
Waveguides 661
14.13 RF CMOS – ESD and RF LDMOS Power Technology 663
14.14 Summary and Closing Comments 666
References 666

15 ESD Power Electronics Design 681

15.1 Reliability Technology Scaling and the Reliability Bathtub Curve 681
15.1.1 Reliability Design Box 681
15.1.2 Application Voltage and Voltage Metrics – Trigger Voltage, and Absolute
Maximum Voltage 682
15.1.3 Safe Operating Area (SOA) 683
15.1.4 Electrical Safe Operating Area (E-SOA) 683
15.1.5 Thermal Safe Operating Area (T-SOA) 684
15.1.6 Transient Safe Operating Area 685
15.2 Input Circuitry 686
15.2.1 ESD Input Circuits 686
15.2.2 Analog Input Circuit Protection 686
15.2.3 High-voltage Analog Input Circuit Protection 686
15.2.4 Analog Input High-voltage Grounded Gate NMOS (GGNMOS) 686
15.2.5 Two-stage High-voltage Analog Input Circuit Protection 687
15.2.6 Analog ESD Output Circuits 687
15.2.7 Analog ESD Output Networks and Distinctions 688
xxvi Contents

15.2.8 Analog Open Drain ESD Output Networks 689

15.2.9 Analog ESD Ground-to-Ground Networks 689
15.2.10 Back-to-Back CMOS Diode String 690
15.2.11 HV GGNMOS Diode-configured Ground-to-Ground Network 690
15.2.12 ESD Silicon-controlled Rectifier Circuits 690
15.2.13 Unidirectional Silicon-controlled Rectifier (SCR) 690
15.2.14 Bi-directional Silicon-controlled Rectifier (SCR) 691
15.2.15 Medium-level Silicon-controlled Rectifier (MLSCR) 691
15.2.16 Low-voltage Triggered SCR (LVTSCR) 692
15.2.17 Analog and Power Technology with ESD Circuit Integration 693
15.2.18 Analog ESD – Isolated and Non-isolated Designs 693
15.2.19 Integrated Body Ties 693
15.2.20 Self-PROTECTING vs Non-self Protecting Designs 693
15.2.21 Lateral Diffused MOS (LDMOS) Circuits 693
15.2.22 LOCOS-defined LDMOS 694
15.2.23 RESURF Transistor 694
15.2.24 Advantages and Disadvantages of LOCOS-defined LDMOS Transistors 694
15.2.25 Shallow Trench Isolation (STI)-defined LDMOS 695
15.2.26 Shallow Trench Isolation (STI)-defined Isolated LDMOS 695
15.2.27 LDMOS Layout – Circular Design 696
15.2.28 LDMOS Transmission Line Pulse (TLP) I-V Characteristic 696
15.2.29 Drain-extended MOS (DeMOS) Circuits 697
15.2.30 DeNMOS 697
15.2.31 DeNMOS-SCR Transistor 698
15.2.32 Ultra-high Voltage LDMOS (UHV-LDMOS) Circuits 699
15.2.33 Ultra-high Voltage LDMOS (UHV-LDMOS) 699
15.2.34 UHV-LDMOS Layout – Circular Design 699
15.2.35 Ultra-high Voltage LDMOS (UHV-LDMOS) SCR 699
15.3 Summary and Closing Comments 702
References 702

16 ESD in Advanced CMOS 709

16.1 Interconnects and ESD 709
16.2 Aluminum Interconnects 710
16.3 Interconnects – Vias 714
16.3.1 Tapered Aluminum Via 714
16.4 Interconnects – Wiring 715
16.4.1 Titanium Interconnects Ti/Al/Ti 715
16.4.1.1 Copper Interconnects 715
16.5 Junctions 719
16.5.1 Abrupt Junctions 719
16.5.2 Low-doped Drains and ESD 720
16.5.3 Extension Implants 721
16.5.4 Gate Structures 722
 Contents xxvii

16.5.4.1 Salicides and ESD 722

16.5.5 Salicide Resistance Model 723
16.6 Titanium Silicide 725
16.6.1 Molybdenum and Titanium Salicide 729
16.6.2 Cobalt Silicides 730
16.7 Shallow Trench Isolation 731
16.7.1 Isolation Structures and ESD 731
16.7.2 LOCOS Isolation 731
16.8 LOCOS-bound ESD Structures 734
16.9 LOCOS-bound p+/n-well Junction Diodes 734
16.10 LOCOS-bound n+ Junction Diodes 736
16.11 LOCOS-bound n-well/Substrate Diodes 737
16.12 LOCOS-bound Lateral N-Well to N-Well Bipolar ESD Element 738
16.13 LOCOS-bound Lateral N+ to N-well Bipolar ESD Element 738
16.14 LOCOS-bound Lateral pnp Bipolar ESD Element 739
16.15 LOCOS-bound Thick Oxide MOSFET ESD Element 739
16.16 Shallow Trench Isolation 739
16.16.1 Shallow Trench Isolation Pull-down 740
16.17 STI-bound ESD Structures 741
16.17.1 STI-bound p+/N-well Junctions 741
16.17.2 STI-bound N+ Junction Diodes 744
16.17.3 STI-bound N-well/Substrate Diodes 745
16.17.3.1 Substrates 745
16.17.4 Substrate P++ with Epitaxial Region 745
16.18 Substrate Modeling – Electrical and Thermal Discretization 746
16.19 Heavily Doped Substrates 750
16.19.1 Substrates: Heavily Doped Substrates 750
16.19.2 P-substrate Doping Scaling 751
16.19.2.1 Substrates: Low-doped Substrates 751
16.19.3 Diffused Wells 752
16.19.4 Diffused Well Vertical Profile 753
16.19.5 Retrograde and Vertically Modulated Wells 755
16.19.6 Retrograde and Vertically Modulated Wells 756
16.19.7 Retrograde Well Substrate Modulation 760
16.20 Retrograde Wells and ESD Scaling 766
16.20.1 Sub-collectors 769
16.20.2 Ballast Resistors 772
16.21 Triple Well and Isolated MOSFET CMOS 775
16.21.1 Deep Trench Isolation 776
16.21.2 Deep Trench as Guard Rings 777
16.21.3 Deep Trench and Latchup 778
16.21.4 Deep Trench and ESD Structures 778
16.22 Summary and Closing Comments 779
References 779
Another random document with
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 CHAPTER XXV
A DIFFERENCE OF OPINION

Betty was whipping mayonnaise in the kitchen when a voice hailed

the Diamond Bar K ranch in general.
“Hello the house!”
Through the window she saw a rider on a horse, and a moment later
her brain localized him as a neighborhood boy who had recently
joined the forest rangers. She went to the door, sleeves still rolled
back to the elbows of the firm satiny arms.
“Hoo-hoo!” she called, flinging a small hand wildly above her head in
greeting. “Hoo-hoo, Billy boy!”
He turned, caught sight of her, and at once began to smile. It was
noticeable that when Betty laughed, as she frequently did for no
good reason at all except a general state of well-being, others were
likely to join in her happiness.
“Oh, there you are,” he said, and at once descended.
“Umpha! Here I am, but I won’t be long. I’m making salad dressing.
Come in to the kitchen if you like. I’ll give you a cookie. Just out o’
the oven.”
“Listens good to Billy,” he said, and stayed not on the order of his
coming.
She found him a plate of cookies and a stool. “Sit there. And tell me
what’s new in the hills. Did you pass the dam as you came down?
And what d’you know about the tunnel?”
The ranger stopped a cookie halfway to his mouth. “Say, that fellow
—the one drivin’ the tunnel—he’s been shot.”
“What!”
“Last night—at Don Black’s cabin.”
A cold hand laid itself on her heart and stopped its beating. “You
mean—on purpose?”
He nodded. “Shot through the window at dark.”
“Mr. Hollister—that who you mean?”
“Yep. That’s what he calls himself now. Jones it was at first.”
“Is he—hurt badly?”
“I’ll say so. In the side—internal injuries. Outa his head when I was
there this mo’ning.”
“What does the doctor say?”
“He ain’t seen him yet. On the way up now. I ’phoned down from
Meagher’s ranch. He’d ought to pass here soon.”
“But why didn’t they get the doctor sooner? What were they thinking
about?” she cried.
“Nobody with him but Don Black. He couldn’t leave him alone, he
claims. Lucky I dropped in when I did.”
Impulsively Betty made up her mind. “I’m going to him. You’ll have to
take me, Billy.”
“You!” exclaimed the ranger. “What’s the big idea, Betty?”
“Dad’s gone to Denver to the stock show. I’m going to look after him.
That’s what Dad would want if he were here. Some one’s got to
nurse him. What other woman can go in on snowshoes and do it?”
“Does he have to have a woman nurse? Can’t Mr. Merrick send a
man up there to look after him?”
“Don’t argue, Billy boy,” she told him. “You see how it is. They don’t
even get a doctor to him for fifteen or sixteen hours. By this time he
may be—” She stopped and bit her lip to check a sudden swell of
emotion that choked up her throat.
Bridget came into the kitchen. Betty’s announcement was both a
decision and an appeal. “Mr. Hollister’s been hurt—shot—up in the
hills. I’m going up to Justin to make him take me to him.”
“Is he hur-rt bad?” asked the buxom housekeeper.
“Yes. I don’t know. Billy thinks so. If I hurry I can get there before
night.”
Bridget hesitated. “I was thinkin’ it might be better for me to go,
dearie. You know how folks talk.”
“Oh, talk!” Betty was explosively impatient. She always was when
anybody interfered with one of her enthusiasms. “Of course, if you
could go. But you’d never get in through the snow. And what could
they say—except that I went to save a man’s life if I could?”
“Mr. Merrick might not like it.”
“Of course he’d like it.” The girl was nobly indignant for her fiancé.
“Why wouldn’t he like it? It’s just what he’d want me to do.” Under
the brown bloom of her cheeks was the peach glow of excitement.
Bridget had traveled some distance on the journey of life, and she
had her own opinion about that. Merrick, if she guessed him at all
correctly, was a possessive man. He could appreciate Betty’s valiant
eagerness when it went out to him, but he would be likely to resent
her generous giving of herself to another. He did not belong to the
type of lover that recognizes the right of a sweetheart or a wife to
express herself in her own way. She was pledged to him. Her
vocation and avocation in life were to be his wife.
But Bridget was wise in her generation. She knew that Betty was of
the temperament that had to learn from experience. She asked how
they would travel to the dam.
“On horseback—if we can get through. The road’s not broken yet
probably after yesterday’s storm. We’ll start right away. I can’t get
Justin on the ’phone. The wire must be down.”
The ranger saddled for her and they took the road. Betty carried with
her a small emergency kit of medical supplies.
Travel back and forth had broken the road in the valley. It was not
until the riders struck the hill trail that they had to buck drifts. It was
slow, wearing work, and, by the time they came in sight of the dam,
Betty’s watch told her that it was two o’clock.
Merrick saw them coming down the long white slope and wondered
what travelers had business urgent enough to bring them through
heavy drifts to the isolated camp. As soon as he recognized Betty,
he went to meet her. Billy rode on down to the tents. He knew when
he was not needed.
Rich color glowed in her cheeks, excitement sparkled in her eyes.
“What in the world are you doing here?” Merrick asked.
She was the least bit dashed by his manner. It suggested censure,
implied that her adventure—whatever the cause of it—was a bit of
headstrong folly. Did he think it was a girl’s place to stay at home in
weather like this? Did he think that she was unmaidenly, had bucked
miles of snowdrifts because she could not stay away from him?
“Have you heard about Mr. Hollister? He’s been hurt—shot.”
“Shot?”
“Last night. At Black’s cabin.”
“Who shot him?”
“I don’t know. He’s pretty bad, Billy says.”
“Doctor seen him yet?”
“He’s on the way now. I want you to take me to him, Justin.”
“Take you? What for?”
“To nurse him.”
He smiled, the superior smile of one prepared to argue away the
foolish fancies of a girl.
“Is your father home yet?”
“No. He’ll be back to-morrow. Why?”
“Because, dear girl, you can’t go farther. In the first place, it’s not
necessary. I’ll do all that can be done for Hollister. The trip from here
won’t be a picnic.”
“I’ve brought my skis. I can get in all right,” she protested eagerly.
“I grant that. But there’s no need for you to go. You’d far better not.
It’s not quite—” He stopped in mid-sentence, with an expressive lift
of the shoulders.
“Not quite proper. I didn’t expect you to say that, Justin,” she
reproached. “After what he did for us.”
“He did only what any self-respecting man would do.”
Her smile coaxed him. “Well, I want to do only what any self-
respecting woman would do. Surely it’ll be all right if you go along.”
How could he tell her that he knew no other unmarried woman of her
age, outside of professional nurses, who would consider such a thing
for the sake of a comparative stranger? How could he make her see
that Black’s cabin was no place for a young girl to stay? He was
exasperated at her persistence. It offended his amour propre. Why
all this discussion about one of his employees who had been a tramp
only a few months since?
Merrick shook his head. His lips smiled, but there was no smile in his
eyes. “You’re a very impulsive and very generous young woman. But
if you were a little older you would see—”
She broke impatiently into his argument. “Don’t you see how I feel,
Justin? I’ve got to do what I can for him. We’re not in a city where we
can ring up for a trained nurse. I’m the only available woman that
can get in to him. Why did I take my Red Cross training if I’m not to
help those who are sick?”
“Can’t you trust me to look out for him?”
“Of course I can. That’s not the point. There’s so much in nursing.
Any doctor will tell you so. Maybe he needs expert care. I really can
nurse. I’ve done it all my life.”
“You don’t expect to nurse everybody in the county that falls sick, do
you? Don’t you see, dear girl, that Black’s shack is no place for
you?”
“Why isn’t it? I’m a ranchman’s daughter. It doesn’t shock or offend
me to see things that might distress a city girl.” She cast about in her
mind for another way to put it. “I remember my mother leaving us
once for days to look after a homesteader who had been hurt ’way
up on Rabbit Ear Creek. Why, that’s what all the women on the
frontier did.”
“The frontier days are past,” he said. “And that’s beside the point,
anyhow. I’ll have him well looked after. You needn’t worry about
that.”
“But I would,” she urged. “I’d worry a lot. I want to go myself, Justin—
to make sure it’s all right and that everything’s being done for him
that can be. You think it’s just foolishness in me, but it isn’t.”
She put her hand shyly on his sleeve. The gesture was an appeal for
understanding of the impulse that was urgent in her. If he could only
sympathize with it and acknowledge its obligation.
“I think it’s neither necessary nor wise. It’s my duty, not yours, to
have him nursed properly. I’ll not shirk it.” He spoke with the finality
of a dominant man who has made up his mind.
Betty felt thrown back on herself. She was disappointed in him and
her feelings were hurt. Why must he be so obtuse, so correct and
formal? Why couldn’t he see that she had to go? After all, a decision
as to what course she would follow lay with her and not with him. He
had no right to assume otherwise. She was determined to go,
anyhow, but she would not quarrel with him.
“When are you going up to Black’s?” she asked.
“At once.”
“Do take me, please.”
He shook his head. “It isn’t best, dear girl.”
In her heart flamed smokily rebellious fires. “Then I’ll go with Billy.”
He interpreted the words as a challenge. Their eyes met in a long,
steady look. Each measured the strength of the other. It was the first
time they had come into open conflict.
“I wouldn’t do that, Betty,” he said quietly.
“You don’t know how I feel about it. You won’t understand.” Her voice
shook with emotion. “I’ve got to go.”
Merrick knew that he could prevent the ranger from guiding Betty to
the gulch where the wounded man was, but it was possible to pay
too great a price for victory. He yielded, grudgingly.
“I’ll take you. After you’ve seen Hollister, you can give us directions
for nursing him. I should think the doctor ought to know, but, if you
haven’t confidence in him, you can see to it yourself.”
Betty found no pleasure now in her desire to help. Justin’s opposition
had taken all the joy out of it. Nor did his surrender give her any
gratification. He had not yielded because he appreciated the validity
of her purpose, but because he had chosen to avoid an open
breach. She felt a thousand miles away from him in spirit.
“Thank you,” she said formally, choking down a lump in her throat.
 CHAPTER XXVI
BLACK IS SURPRISED

It is not in youth to be long cast down for the troubles of a stranger,

even one who has very greatly engaged the sympathy. In spite of
Betty’s anxiety about the wounded man, her resilient spirits had sent
her eagerly upon this adventure.
She would see Justin. He would approve her plans with enthusiasm.
Together they would ski across the white wastes, they two alone in a
vast world of mysterious stillness. The thin clear air of the high
Rockies would carry their resonant voices like the chimes of bells.
Silences would be significant, laughter the symbol of happy
comradeship. For the first time they would come glowing through
difficulties, perhaps dangers, conquered side by side. And at the end
of the journey waited for them service, that which gave their joyous
enterprise the value of an obligation.
And it was not at all like that—not a bit as she had day-dreamed it on
the ride to Sweetwater Dam. The joy was struck dead in her heart.
Miserably she realized that Justin could not understand. The ardent
fire that burned in her soul seemed only mushy sentiment to him, on
a par with the hysteria that made silly women send flowers to brutal
murderers they did not know.
The bars were up between them. The hard look in his eyes meant
anger. There would be no expression of it in temper. He was too self-
contained for that. None the less it was anger. The reflection of it
gleamed out from under her own dark lashes. She told herself she
hated the narrowness in him that made him hold so rigidly to the
well-ordered, the conventional thing. Why couldn’t he see that there
was an imperative on her to live? Well, she would show him.
Probably he thought that in every clash of will she ought to yield. He
could learn his lesson just as well now as later.
She held her head high, but there was a leaden weight in her bosom
that made her want to sob.
Often she had been proud of his tremendous driving power, the force
that made of him a sixty-horse-power man. She resented it fiercely
to-day. He was traveling just a little too fast for her, so that she could
hardly keep up with him. But she would have fallen in her tracks
rather than ask him to go slower.
Once the slither of his runners stopped. “Am I going too fast?” he
asked coldly.
“Not at all,” she answered stubbornly.
He struck out again. They were climbing a long slope that ended in a
fringe of timber. At the top he waited, watching her as she labored up
heavily. The look he gave her when she reached him said, “I told you
so.”
Before them lay a valley, beyond which was another crest of pines.
“How far now?” Betty asked, panting from the climb.
“Just beyond that ridge.”
“That all?” she said indifferently. “Thought it was a long way.”
“We’ll coast into the valley,” he replied curtly.
She watched him gliding into the dip of the slope. He was not an
expert on runners as her father was, but he had learned the trick of
the thing pretty well. It was in line with his thoroughness not to be a
novice long at anything he set out to master.
Betty shot down after him, gathering impetus as she went. She was
watching the path ahead, and it was not till she was close upon him
that she saw Merrick had fallen. She swerved to the left, flinging out
her arms to prevent herself from going down. Unsteadily she
teetered for a moment, but righted herself with an effort and kept
going till she reached the bottom.
Merrick was on his feet when she turned.
“Anything wrong?” she called.
“One of my skis broken.”
She went back to him. “How did it happen?”
“Dipped into a rock under the snow.” His voice was sullen. Like many
men who do well whatever they undertake, he resented any mishap
due to lack of his own skill. His sense of superiority would have been
satisfied if the accident had befallen her instead of him.
Betty did not smile, but, nevertheless, she was maliciously pleased.
It would bring him down a peg, anyhow.
“What’ll you do?” she asked.
“I suppose I can hobble along somehow. Perhaps I’d better take your
skis and hurry on. I could borrow a pair at the cabin and come back
for you. Yes, I think that would be better.”
She shook her head. “No, I’ll go on and send Mr. Black with a pair.
I’d rather not wait here in the cold. I’ll not be long. You can keep
moving.”
This did not suit Merrick at all. He did not want to be regarded as an
incompetent who had bogged down in the snow. It hurt his pride that
Betty should go on and send back help to him, especially when they
felt criss-cross toward each other.
“I’d rather you didn’t,” he said. “You don’t know who is at the cabin.
That tramp Cig may be there—or Prowers. They’re dangerous, both
of them. Yesterday they tried to blow up the men working on the
tunnel.”
“You can lend me your revolver, then, if you like. But I’m not afraid.
Mr. Black wouldn’t let them hurt me even if they wanted to.”
“It’s not very cold. I’d be back in a little while. And, as you say, you
could keep moving.”
“No, I’m going on,” she answered, and her quiet voice told him she
had made up her mind.
He unbuckled his belt and handed it to her. “You’ll be safer with that
.38,” he said. What he thought is not of record.
“Thanks.” Betty’s little smile, with its hint of sarcasm, suggested that
there was not the least need of the revolver; if she wore it, the only
reason was to humor his vanity and let him feel that he was
protecting her.
She crossed the valley and climbed the ridge. From the farther side
of it she looked down upon a log cabin of two rooms, a small stable,
and a corral. They nestled in a draw at her feet, so close that a man
could have thrown a stone almost to the fence. The hillside was
rough with stones. With Justin’s mishap in mind, she felt her way
down carefully.
Smoke poured out of the chimney and polluted the pure light air. No
need of seeing the fire inside to know that the wood was resinous fir.
Betty knocked on the door.
It opened. Black stood on the threshold looking down at her in
ludicrous amazement. She had taken off her coat and was carrying
it. Against a background of white she bloomed vivid as a poinsettia in
her old-rose sweater and jaunty tam. The cold crisp air had whipped
the scarlet into her lips, the pink into her cheeks.
“What in—Mexico!” he exclaimed.
“How’s Mr. Hollister?”
“A mighty sick man. Howcome you here, miss?”
The sound of a querulous voice came from within. “Tell you I don’t
want the stuff. How many times I got to say it?”
“I’ve come to nurse him. Billy brought us word. Father wasn’t home
—nor Lon. So Mr. Merrick brought me.”
“Merrick,” he repeated.
“He’s over the hill, a ways back. Broke a ski. He’d like you to take
him a pair. I’ll look after Mr. Hollister.”
As she followed the lank range rider into the cabin, she pulled off her
gauntlets. Her cold fingers fumbled with the ski ties.
“Lemme do that,” Black said, and dropped on a knee to help.
“I guess you can do it quicker.” She looked at the patient and let her
voice fall as she asked a question. “Is he delirious?”
“Crazy as a hydryphoby skunk.” He repeated what he had said
before. “A mighty sick man, looks like.”
Betty looked into the hot, fevered face of the man tossing on the bed.
From her medicine kit she took a thermometer. His fever was high.
She prepared medicine and coaxed him to swallow it.
“Where is he wounded?” she asked.
“In the side.”
“Did you wash out the wound and bind it up?”
“Yes’m. I’ve took care of fellows shot up before.”
“Bleed much?”
“Right smart. Did you hear when Doc Rayburn was comin’?”
“He’s on the way.” She found cold water and bathed the burning
face.
“Wisht he’d hustle along,” the range rider said uneasily.
“He won’t be long.” With a flare of anger she turned on Black. “Who
shot him?”
“I dunno. He was shot through the window whilst he was ondressin’
for bed. We come together from the old Thorwaldson cabin a while
before.”
“Did that Cig do it?”
“Might have, at that.” Black was putting on his webs. “Reckon I’ll drift
back an’ pick up yore friend Merrick.”
“Yes,” she said absently. “It was that tramp Cig or Jake Prowers,
one.”
“Yore guess is as good as mine,” he said, buttoning to the neck a
leather coat.
“Can’t we have more light in here? It’s dark. If you’d draw back that
window curtain—”
“Then Mr. Bushwhacker would get a chanct for another shot,” he
said dryly. “No, I reckon we’ll leave the curtain where it’s at.”
Her big startled eyes held fascinated to his. “You don’t think they’d
shoot him again now.”
“Mebbeso. My notion is better not give ’em a show to get at him. You
keep the door closed. I’ll not be long. I see you got a gun.”
There was something significant in the way he said it. Her heart
began to beat fast.
“You don’t think—?”
“No, I don’t. If I did, I’d stay right here. Not a chanct in a hundred.
How far back’s yore friend?”
“Less than a mile.”
“Well, he’s likely been movin’ right along. When I reach the ridge, I’ll
give him the high sign an’ leave the skis stickin’ up in the snow
there.”
“Yes.” And, as he was leaving, “Don’t be long,” she begged.
“Don’t you be scared, miss. Them sidewinders don’t come out in the
open an’ do their wolf-killin’. An’ I won’t be gone but a li’l’ while. If
anything worries you, bang away with that .38 an’ I’ll come a-
runnin’.”
He closed the door after him. From behind the curtain she watched
him begin the ascent. Then she went back to her patient and bathed
his hot hands. Betty echoed the wish of the range rider that the
doctor would come. What could be keeping him? From the Diamond
Bar K ranch to Wild Horse was only a few miles. He must have
started before she did. It would not be long now.
In spite of a two days’ growth of beard, the young fellow on the bed
looked very boyish. She gently brushed back the curls matted on the
damp forehead. He was rambling again in desultory speech.
“A cup o’ cold water—cold lemonade. Happy days, she says. No
trouble friendship won’t lighten, she says, with that game smile
lighting up her face. Little thoroughbred.”
A warm wave of exultant emotion beat through her blood. It reached
her face in a glow of delicate beauty that transformed her.
“You dear boy!” she cried softly, and her eyes were shining stars of
tender light.
 CHAPTER XXVII
THE MAN WITH THE BLEACHED BLUE EYES

There was not much she could do for him except bathe again his
face and hands. He asked for a drink, and Betty propped him up with
her arm while she held the tin cup to his lips. Exhausted by the effort,
he sank back to the pillow and panted. All the supple strength of his
splendid youth had been drained from him. The muscles were lax,
the movements of the body feeble.
Sunken eyes stared at her without recognition. “Sure I’ll take your
hand, and say ‘Thank you’ too. You’re the best little scout, the best
ever.”
She took the offered hand and pressed it gently. “Yes, but now you
must rest. You’ve been sick.”
“A Boche got me.” His wandering subconscious thoughts flowed into
other memories. “Zero hour, boys. Over the top and give ’em hell.”
Then, without any apparent break from one theme to another, his
thick voice fell to a cunning whisper. “There’s a joint on South Clark
Street where I can get it.”
Into his disjointed mutterings her name came at times, spoken
always with a respect that was almost reverence. And perhaps a
moment later his voice would ring out clear and crisp in directions to
the men working under him. Subjects merged into each other
inconsequently—long-forgotten episodes of school days, college
larks, murmured endearments to the mother who had died many
years since. Listening to him, Betty knew that she was hearing
revelations of a soul masculine but essentially clean.
A sound startled her, the click of the latch. She turned her head
swiftly as the door opened. Fear drenched her heart. The man on the
threshold was Prowers. He had come out of a strong white light and
at first could see nothing in the dark cabin.
Betty watched him as he stood there, his bleached blue eyes
blinking while they adjusted themselves to another focus.
“What do you want?” she asked sharply, the accent of alarm in her
voice.
“A woman, by jiminy by jinks!” The surprise in his squeaky voice was
pronounced. He moved forward to the bed. “Clint Reed’s girl. Where
you come from? How’d you get here?”
She had drawn back to the wall at the head of the bed in order to
keep a space between them. Her heart was racing furiously. His cold
eyes, with the knife-edge stab in them, held hers fast.
“I came in over the snow to nurse him.”
“Alone?”
“No. Mr. Merrick’s with me.”
“Where?”
“At the top of the hill. He broke a ski.”
“Where’s Don?”
“Gone to meet him. They’ll be here in a minute.”
A cunning, impish grin broke the lines of the man’s leathery face. He
remembered that he had come prepared to be surprised to hear of
Hollister’s wound. “Nurse who?” he asked suavely.
“Mr. Hollister, the engineer driving the tunnel.”
“Sick, is he?” He scarcely took the trouble to veil his rancorous
malice. It rode him, voice, manner, and mocking eye. His mouth was
a thin straight line, horribly cruel.
“Some one shot him—last night—through the window.” She knew
now that he had done it or had had it done. The sense of outrage, of
horror at his unhuman callousness, drove the fear out of her bosom.
Her eyes accused him, though her tongue made no charge.
“Shot him, by jiminy by jinks! Why, Daniels had ought to put the
fellow in the calaboose. Who did it?”
“I don’t know. Do you?” she flashed back.
His evil grin derided her. “How would I know, my dear?”
He drew up a chair and sat down. The girl did not move. Rigid and
watchful, she did not let her eye waver from him for an instant.
He nodded toward the delirious man. “Will he make it?”
“I don’t know.”
“Doc seen him yet?”
“No.”
“Glad I came. I can help nurse him.” He cut short a high cackle of
laughter to ask a question. “What’s yore gun for, dearie? You
wouldn’t throw it on poor Jake Prowers, would you?”
He was as deadly as dynamite, she thought, more treacherous than
a rattlesnake. She wanted to cry out her horror at him. To see him
sitting there, humped up like a spider, not three feet from the man he
had tried to murder, filled her with repulsion. There was more in her
feeling than that; a growing paralysis of terror lest he might reach out
and in a flash complete the homicide he had attempted.
She tried to reason this away. He dared not do it, with her here as a
witness, with two men drawing closer every minute. Don Black had
told her that he wouldn’t strike in the open, and the range rider had
known him more years than she had lived. But the doubt remained.
She did not know what he would do. Since she did not live in the
same world as he, it was not possible for her to follow his thought
processes.
Then, with no previous intimation that his delirium had dropped from
him, the wounded man startled Betty by asking a rational question.
“Did you come to see how good a job you’d done?” he said quietly to
Prowers.
The cowman shook his head, still with the Satanic grin. “No job of
mine, son. I’m thorough.”
“Your orders, but maybe not your hand,” Hollister insisted feebly.
Betty moved into his line of vision, and to his startled brain the
motion of her was like sweet unearthly music. He looked silently at
her for a long moment.
“Am I still out of my head?” he asked. “It’s not really you, is it?”
“Yes,” she said, very gently. “You mustn’t talk.”
“In Black’s cabin, aren’t we?”
“Yes.”
“Shot through the window, Black told me. Remember, if I don’t get
well, it was this man or Cig that did it.”
“I’ll remember,” she promised. “But you’re going to get well. Don’t
talk, please.”
“Just one thing. What are you doing here?”
“I came to look after you. Now that’s all—please.”
He said no more, in words. But the eyes of sick men are like those of
children. They tell the truth. From them is stripped the veil woven by
time and the complexities of life.
Sounds of voices on the hillside drifted to the cabin. Betty’s heart
leaped joyfully. Friends were at hand. It was too late now for Prowers
to do any harm even if it was in his mind.
The voices approached the cabin. The girl recognized that of
Merrick, strong and dominant and just a little heavy. She heard
Black’s drawling answer, without being able to distinguish the words.
The door opened. Four men came into the room. The two who
brought up the rear were Dr. Rayburn and Lon Forbes.
“Oh, Lon!” Betty cried, and went to him with a rush. “I’m awf’ly glad
you came.”
She clung to him, trembling, a sob in her throat.
The rawboned foreman patted her shoulder with a touch of
embarrassment. “There—there, honey, ’s all right. Why didn’t you
wait for old Lon instead o’ hoppin’ away like you done?”
Prowers tilted back his chair on two legs and chirped up with satiric
comment. “We got quite a nice party present. Any late arrivals not
yet heard from?”
Both Lon and Justin Merrick were taken aback. In the darkness they
had not yet recognized the little man.
The foreman spoke dryly. “Might ’a’ known it. Trouble and Jake
Prowers hunt in couples. Always did.”
“I could get a right good testimonial from Mr. Lon Forbes,” the
cowman said, with his high cackle of splenetic laughter. “Good old
Lon, downright an’ four-square, always a booster for me.”
Betty whispered. “He’s an awful man, Lon. I’m scared of him. I didn’t
know any minute what he was going to do. Oh, I am glad you came.”
“Same here,” Lon replied. “Don’t you be scared, Betty. He can’t do a
thing—not a thing.”
Merrick had been taking off his skis. He came up to Betty now. “Did
he annoy you—say anything or—?”
“No, Justin.” A shiver ran down her spine. “He just looked and
grinned. I wanted to scream. He shot Mr. Hollister. I know he did. Or
had it done by that Cig.”
“Yes. I don’t doubt that.”
The doctor, disencumbered of impedimenta of snowshoes and
wraps, fussed forward to the bedside. “Well, let’s see—let’s see
what’s wrong here.”
He examined the wound, effervesced protests and questions, and
prepared for business with the bustling air that characterized him.
“Outa the room now—all but Miss Reed and one o’ you men. Lon,
you’ll do.”
“I’ll stay,” announced Merrick with decision.
“All right. All right. I want some clean rags, Black. You got plenty of
hot water, I see. Clear out, boys.”
“You don’t need a good nurse, Doc?” Prowers asked, not without
satiric malice. He was playing with fire, and he knew it. Everybody in
the room suspected him of this crime. He felt a perverted enjoyment
in their hostility.
Black chose this moment to make his declaration of independence.
“I’d light a shuck outa here if I was you, Jake, an’ I wouldn’t come
back, seems to me.”
The cold, bleached eyes of the cowman narrowed. “You’re givin’ me
that advice as a friend, are you, Don?” he asked.
The range rider’s jaw stopped moving. In his cheek the tobacco quid
stuck out. His face, habitually set to the leathery imperturbability of
his calling, froze now to an expressionless mask.
“I’m sure givin’ you that advice,” he said evenly.
“I don’t hear so awful good, Don. As a friend, did you say?” The little
man cupped an ear with one hand in ironic mockery.
Black’s gaze was hard as gun-metal. “I said I’d hit the trail for home if
I was you, Jake, an’ I’d stay there for a spell with kinda low visibility
like they said in the war.”
“I getcha, Don.” Prowers shot a blast of cold lightning from under his
scant brows. “I can take a hint without waitin’ for a church to fall on
me. Rats an’ a sinkin’ ship, eh? You got a notion these fellows are
liable to win out on me, an’ you want to quit while the quittin’ is good.
I been wonderin’ for quite a while if you wasn’t yellow.”
“Don’t do that wonderin’ out loud, Jake,” the other warned quietly. “If
you do, you’ll sure enough find out.”
The little man laughed scornfully, met in turn defiantly the eyes of
Betty, Merrick, and Forbes, turned on his heel, and sauntered out.