The Art of Analog Layout

Preface to the Second Edition xvii

Preface to the First Edition xix

Acknowledgments xxi

1 Device Physics 1

1.1 Semiconductors 1

1.1.1. Generation and Recombination 4

1.1.2. Extrinsic Semiconductors 6

1.1.3. Diffusion and Drift 9

1.2 PN Junctions 11

1.2.1. Depletion Regions 11

1.2.2. PN Diodes 13

1.2.3. Schottky Diodes 16

1.2.4. Zener Diodes 18

1.2.5. Ohmic Contacts 19

1.3 Bipolar Junction Transistors 21

1.3.1. Beta 23

1.3.2. I-V Characteristics 24

1.4 MOS Transistors 25

1.4.1. Threshold Voltage 27

1.4.2. I-V Characteristics 29

1.5 JFET Transistors 32

1.6 Summary 34

1.7 Exercises 35

2 Semiconductor Fabrication 37

2.1 Silicon Manufacture 37

2.1.1. Crystal Growth 38

2.1.2. Wafer Manufacturing 39

2.1.3. The Crystal Structure of Silicon 39

2.2 Photolithography 41

2.2.1. Photoresists 41

2.2.2. Photomasks and Reticles 42

2.2.3. Patterning 43

2.3 Oxide Growth and Removal 43

2.3.1. Oxide Growth and Deposition 44

2.3.2. Oxide Removal 45

2.3.3. Other Effects of Oxide Growth and Removal 47

2.3.4. Local Oxidation of Silicon (LOCOS) 49

2.4 Diffusion and Ion Implantation 50

2.4.1. Diffusion 51

2.4.2. Other Effects of Diffusion 53

2.4.3. Ion Implantation 55

2.5 Silicon Deposition and Etching 57

2.5.1. Epitaxy 57

2.5.2. Polysilicon Deposition 59

2.5.3. Dielectric Isolation 60

2.6 Metallization 62

2.6.1. Deposition and Removal of Aluminum 63

2.6.2. Refractory Barrier Metal 65

2.6.3. Silicidation 67

2.6.4. Interlevel Oxide, Interlevel Nitride, and Protective Overcoat 69

2.6.5. Copper Metallization 71

2.7 Assembly 73

2.7.1. Mount and Bond 74

2.7.2. Packaging 77

2.8 Summary 78

2.9 Exercises 78

3 Representative Processes 80

3.1 Standard Bipolar 81

3.1.1. Essential Features 81

3.1.2. Fabrication Sequence 82

Starting Material 82

N-Buried Layer 82

Epitaxial Growth 83

Isolation Diffusion 83

83

Base Implant 84

Emitter Diffusion 84

Contact 85

Metallization 85

Protective Overcoat 86

3.1.3. Available Devices 86

NPN Transistors 86

PNP Transistors 88

Resistors 90

Capacitors 92

3.1.4. Process Extensions 93

Up-Down Isolation 93

Double-Level Metal 94

Schottky Diodes 94

High-Sheet Resistors 94

Super-Beta Transistors 96

3.2 Polysilicon-Gate CMOS 96

3.2.1. Essential Features 97

3.2.2. Fabrication Sequence 98

Starting Material 98

Epitaxial Growth 98

N-Well Diffusion 98

Inverse Moat 99

Channel Stop Implants 100

LOCOS Processing and Dummy Gate Oxidation 100

Threshold Adjust 101

Deep-N+

Polysilicon Deposition and Patterning 102

Source/Drain Implants 102

Contacts 103

Metallization 103

Protective Overcoat 103

3.2.3. Available Devices 104

NMOS Transistors 104

PMOS Transistors 106

Substrate PNP Transistors 107

Resistors 107

Capacitors 109

3.2.4. Process Extensions 109

Double-Level Metal 110

Shallow Trench Isolation 110

Silicidation 111

Lightly Doped Drain (LDD) Transistors 112

Extended-Drain, High-Voltage Transistors 113

3.3 Analog BiCMOS 114

3.3.1. Essential Features 115

3.3.2. Fabrication Sequence 116

Starting Material 116

N-Buried Layer 116

Epitaxial Growth 117

N-Well Diffusion and 117

Base Implant 118

Inverse Moat 118

Channel Stop Implants 119

LOCOS Processing and Dummy Gate Oxidation 119

Threshold Adjust 119

Polysilicon Deposition and Pattern 120

Source/Drain Implants 120

Metallization and Protective Overcoat 120

Process Comparison 121

3.3.3. Available Devices 121

NPN Transistors 121

PNP Transistors 123

Resistors 125

3.3.4. Process Extensions 125

Advanced Metal Systems 126

Dielectric Isolation 126

3.4 Summary 130

3.5 Exercises 131

4 Failure Mechanisms 133

4.1 Electrical Overstress 133

4.1.1. Electrostatic Discharge (ESD) 134

Effects 135

Preventative Measures 135

4.1.2. Electromigration 136

Effects 136

Preventative Measures 137

Deep-N+

4.1.3. Dielectric Breakdown 138

Effects 138

Preventative Measures 139

4.1.4. The Antenna Effect 141

Effects 141

Preventative Measures 142

4.2 Contamination 143

4.2.1. Dry Corrosion 144

Effects 144

Preventative Measures 145

4.2.2. Mobile Ion Contamination 145

Effects 145

Preventative Measures 146

4.3 Surface Effects 148

4.3.1. Hot Carrier Injection 148

Effects 148

Preventative Measures 150

4.3.2. Zener Walkout 151

Effects 151

Preventative Measures 152

4.3.3. Avalanche-Induced Beta Degradation 153

Effects 153

Preventative Measures 154

4.3.4. Negative Bias Temperature Instability 154

Effects 155

Preventative Measures 155

4.3.5. Parasitic Channels and Charge Spreading 156

Effects 156

Preventative Measures (Standard Bipolar) 159

Preventative Measures (CMOS and BiCMOS) 162

4.4 Parasitics 164

4.4.1. Substrate Debiasing 165

Effects 166

Preventative Measures 167

4.4.2. Minority-Carrier Injection 169

Effects 169

Preventative Measures (Substrate Injection) 172

Preventative Measures (Cross-Injection) 178

4.4.3. Substrate Influence 180

Effects 180

Preventative Measures 180

4.5 Summary 183

4.6 Exercises 183

5 Resistors 185

5.1 Resistivity and Sheet Resistance 185

5.2 Resistor Layout 187

5.3 Resistor Variability 191

5.3.1. Process Variation 191

5.3.2. Temperature Variation 192

5.3.3. Nonlinearity 193

5.3.4. Contact Resistance 196

5.4 Resistor Parasitics 197

5.5 Comparison of Available Resistors 200

5.5.1. Base Resistors 200

5.5.2. Emitter Resistors 201

5.5.3. Base Pinch Resistors 202

5.5.4. High-Sheet Resistors 202

5.5.5. Epi Pinch Resistors 205

5.5.6. Metal Resistors 206

5.5.7. Poly Resistors 208

5.5.8. NSD and PSD Resistors 211

5.5.9. N-Well Resistors 211

5.5.10. Thin-Film Resistors 212

5.6 Adjusting Resistor Values 213

5.6.1. Tweaking Resistors 213

Sliding Contacts 214

Sliding Heads 215

Trombone Slides 215

Metal Options 215

5.6.2. Trimming Resistors 216

Fuses 216

Zener Zaps 219

EPROM Trims 221

Laser Trims 222

5.7 Summary 223

5.8 Exercises 224

6 Capacitors and Inductors 226

6.1 Capacitance 226

6.1.1. Capacitor Variability 232

Process Variation 232

Voltage Modulation and Temperature Variation 233

6.1.2. Capacitor Parasitics 235

6.1.3. Comparison of Available Capacitors 237

Base-Emitter Junction Capacitors 237

MOS Capacitors 239

Poly-Poly Capacitors 241

Stack Capacitors 243

Lateral Flux Capacitors 245

High-Permittivity Capacitors 246

6.2 Inductance 246

6.2.1. Inductor Parasitics 248

6.2.2. Inductor Construction 250

Guidelines for Integrating Inductors 251

6.3 Summary 252

6.4 Exercises 253

7 Matching of Resistors and Capacitors 254

7.1 Measuring Mismatch 254

7.2 Causes of Mismatch 257

7.2.1. Random Variation 257

Capacitors 258

Resistors 258

7.2.2. Process Biases 260

7.2.3. Interconnection Parasitics 261

7.2.4. Pattern Shift 263

7.2.5. Etch Rate Variations 265

7.2.6. Photolithographic Effects 267

7.2.7. Diffusion Interactions 268

7.2.8. Hydrogenation 270

7.2.9. Mechanical Stress and Package Shift 271

7.2.10. Stress Gradients 274

Piezoresistivity 274

Gradients and Centroids 275

Common-Centroid Layout 277

Location and Orientation 281

7.2.11. Temperature Gradients and Thermoelectrics 283

Thermal Gradients 285

Thermoelectric Effects 287

7.2.12. Electrostatic Interactions 288

Voltage Modulation 288

Charge Spreading 292

Dielectric Polarization 293

Dielectric Relaxation 294

7.3 Rules for Device Matching 295

7.3.1. Rules for Resistor Matching 296

7.3.2. Rules for Capacitor Matching 300

7.4 Summary 303

7.5 Exercises 304

8 Bipolar Transistors 306

8.1 Topics in Bipolar Transistor Operation 306

8.1.1. Beta Rolloff 308

8.1.2. Avalanche Breakdown 308

8.1.3. Thermal Runaway and Secondary Breakdown 310

8.1.4. Saturation in NPN Transistors 312

8.1.5. Saturation in Lateral PNP Transistors 315

8.1.6. Parasitics of Bipolar Transistors 318

8.2 Standard Bipolar Small-Signal Transistors 320

8.2.1. The Standard Bipolar NPN Transistor 320

Construction of Small-Signal NPN Transistors 322

8.2.2. The Standard Bipolar Substrate PNP Transistor 326

Construction of Small-Signal Substrate PNP Transistors 328

8.2.3. The Standard Bipolar Lateral PNP Transistor 330

Construction of Small-Signal Lateral PNP Transistors 332

8.2.4. High-Voltage Bipolar Transistors 337

8.2.5. Super-Beta NPN Transistors 340

8.3 CMOS and BiCMOS Small-Signal Bipolar Transistors 341

8.3.1. CMOS PNP Transistors 341

8.3.2. Shallow-Well Transistors 345

8.3.3. Analog BiCMOS Bipolar Transistors 347

8.3.4. Fast Bipolar Transistors 349

8.3.5. Polysilicon-Emitter Transistors 351

8.3.6. Oxide-Isolated Transistors 354

8.3.7. Silicon-Germanium Transistors 356

8.4 Summary 358

8.5 Exercises 358

9 Applications of Bipolar Transistors 360

9.1 Power Bipolar Transistors 361

9.1.1. Failure Mechanisms of NPN Power Transistors 362

Emitter Debiasing 362

Thermal Runaway and Secondary Breakdown 364

Kirk Effect 366

9.1.2. Layout of Power NPN Transistors 368

The Interdigitated-Emitter Transistor 369

The Wide-Emitter Narrow-Contact Transistor 371

The Christmas-Tree Device 372

The Cruciform-Emitter Transistor 373

Power Transistor Layout in Analog BiCMOS 374

Selecting a Power Transistor Layout 376

9.1.3. Power PNP Transistors 376

9.1.4. Saturation Detection and Limiting 378

9.2 Matching Bipolar Transistors 381

9.2.1. Random Variations 382

9.2.2. Emitter Degeneration 384

9.2.3. NBL Shadow 386

9.2.4. Thermal Gradients 387

9.2.5. Stress Gradients 391

9.2.6. Filler-Induced Stress 393

9.2.7. Other Causes of Systomatic Mismatch 395

9.3 Rules for Bipolar Transistor Matching 396

9.3.1. Rules for Matching Vertical Transistors 397

9.3.2. Rules for Matching Lateral Transistors 402

9.4 Summary 402

9.5 Exercises 403

10 Diodes 406

10.1 Diodes in Standard Bipolar 406

10.1.1. Diode-Connected Transistors 406

10.1.2. Zener Diodes 409

Surface Zener Diodes 410

Buried Zeners 412

10.1.3. Schottky Diodes 415

10.1.4. Power Diodes 420

10.2 Diodes in CMOS and BiCMOS Processes 422

10.2.1. CMOS Junction Diodes 422

10.2.2. CMOS and BiCMOS Schottky Diodes 423

10.3 Matching Diodes 425

10.3.1. Matching PN Junction Diodes 425

10.3.2. Matching Zener Diodes 426

10.3.3. Matching Schottky Diodes 428

10.4 Summary 428

10.5 Exercises 429

11 Field-Effect Transistors 430

11.1 Topics in MOS Transistor Operation 431

11.1.1. Modeling the MOS Transistor 431

Device Transconductance 432

Threshold Voltage 434

11.1.2. Parasitics of MOS Transistors 438

Breakdown Mechanisms 440

CMOS Latchup 442

Leakage Mechanisms 443

11.2 Constructing CMOS Transistors 446

11.2.1. Coding the MOS Transistor 447

Width and Length 448

11.2.2. N-Well and P-Well Processes 449

11.2.3. Channel Stop Implants 452

11.2.4. Threshold Adjust Implants 453

11.2.5. Scaling the Transistor 456

11.2.6. Variant Structures 459

Serpentine Transistors 461

Annular Transistors 462

11.2.7. Backgate Contacts 464

11.3 Floating-Gate Transistors 467

11.3.1. Principles of Floating-Gate Transistor Operation 469

11.3.2. Single-Poly EEPROM Memory 472

11.4 The JFET Transistor 474

11.4.1. Modeling the JFET 474

11.4.2. JFET Layout 476

11.5 Summary 479

11.6 Exercises 479

12 Applications of MOS Transistors 482

12.1 Extended-Voltage Transistors 482

12.1.1. LDD and DDD Transistors 483

12.1.2. Extended-Drain Transistors 486

Extended-Drain NMOS Transistors 487

Extended-Drain PMOS Transistors 488

12.1.3. Multiple Gate Oxides 489

12.2 Power MOS Transistors 491

12.2.1. MOS Safe Operating Area 492

Electrical SOA 493

Electrothermal SOA 496

Rapid Transient Overload 497

12.2.2. Conventional MOS Power Transistors 498

The Rectangular Device 499

The Diagonal Device 500

Computation of 501 RM

Other Considerations 502

Nonconventional Structures 503

12.2.3. DMOS Transistors 505

The Lateral DMOS Transistor 506

RESURF Transistors 508

The DMOS NPN 510

12.3 MOS Transistor Matching 511

12.3.1. Geometric Effects 513

Gate Area 513

Gate Oxide Thickness 514

Channel Length Modulation 515

Orientation 515

12.3.2. Diffusion and Etch Effects 516

Polysilicon Etch Rate Variations 516

Diffusion Penetration of Polysilicon 517

Contacts Over Active Gate 518

Diffusions Near the Channel 518

PMOS versus NMOS Transistors 519

12.3.3. Hydrogenation 520

Fill Metal and MOS Matching 521

12.3.4. Thermal and Stress Effects 521

Oxide Thickness Gradients 522

Stress Gradients 522

Thermal Gradients 522

12.3.5. Common-Centroid Layout of MOS Transistors 523

12.4 Rules for MOS Transistor Matching 528

12.5 Summary 531

12.6 Exercises 531

13 Special Topics 534

13.1 Merged Devices 534

13.1.1. Flawed Device Mergers 535

13.1.2. Successful Device Mergers 539

13.1.3. Low-Risk Merged Devices 541

13.1.4. Medium-Risk Merged Devices 542

13.1.5. Devising New Merged Devices 544

13.1.6. The Role of Merged Devices in Analog BiCMOS 544

13.2 Guard Rings 545

13.2.1. Standard Bipolar Electron Guard Rings 546

13.2.2. Standard Bipolar Hole Guard Rings 547

13.2.3. Guard Rings in CMOS and BiCMOS Designs 548

13.3 Single-level Interconnection 551

13.3.1. Mock Layouts and Stick Diagrams 551

13.3.2. Techniques for Crossing Leads 553

13.3.3. Types of Tunnels 555

13.4 Constructing the Padring 557

13.4.1. Scribe Streets and Alignment Markers 557

13.4.2. Bondpads,Trimpads, and Testpads 558

13.5 ESD Structures 562

13.5.1. Zener Clamp 563

13.5.2. Two-Stage Zener Clamps 565

13.5.3. Buffered Zener Clamp 566

13.5.4. Clamp 568

13.5.5. Clamp 569

13.5.6. Antiparallel Diode Clamps 570

13.5.7. Grounded-Gate NMOS Clamps 570

13.5.8. CDM Clamps 572

13.5.9. Lateral SCR Clamps 573

13.5.10. Selecting ESD Structures 575

13.6 Exercises 578

14 Assembling the Die 581

14.1 Die Planning 581

14.1.1. Cell Area Estimation 582

Resistors 582

Capacitors 582

Vertical Bipolar Transistors 583

Lateral PNP Transistors 583

MOS Transistors 583

MOS Power Transistors 584

Computing Cell Area 584

14.1.2. Die Area Estimation 584

14.1.3. Gross Profit Margin 587

14.2 Floorplanning 588

14.3 Top-Level Interconnection 594

14.3.1. Principles of Channel Routing 594

14.3.2. Special Routing Techniques 596

Kelvin Connections 597

Noisy Signals and Sensitive Signals 598

14.3.3. Electromigration 600

14.3.4. Minimizing Stress Effects 603

14.4 Conclusion 604

14.5 Exercises 605

Appendices

A. Table of Acronyms Used in the Text 607

B. The Miller Indices of a Cubic Crystal 611

C. Sample Layout Rules 614

D. Mathematical Derivations 622

E. Sources for Layout Editor Software 627

Index 628