A User's Guide to Vacuum Technology

Third Edition

A User's Guide to Vacuum Technology

Third Edition

John F. O'Hanlon Professor Emeritus of Electrical and Computer Engineering

The University of Arizona

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright 0 2003 by John Wiley & Sons, Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada.

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Library of Congress Cataloging-In-Publication Data is available: 0-471 -27052-0

Printed in the United States of America.

1 0 9 8 7 6 5 4 3 2 1

For Jean, Carol, Paul, and Amanda

Preface

This book is intended for the vacuum system user-the university student, technician, engineer, manager, or scientist-who wishes a fundamental understanding of modern vacuum technology and a user’s perspective of modern laboratory and industrial vacuum technology.

Vacuum technology is largely secondary; it forms part of other technologies that are central to analysis, research, development, and manufacturing. It is used to provide a process environment. Many advances in vacuum technique have resulted from the demands of other technologies, although scientists and engineers have studied vacuum for its own sake. The average user is process-oriented and becomes immersed in vacuum technique only when problems develop with a process or when new equipment purchases become necessary.

A User’s Guide to Vacuum Technology, 3rd Edition focuses on the operation, understanding, and selection of equipment for processes used in semiconductor, optics, and related technologies. It emphasizes subjects not adequately covered elsewhere, while avoiding in-depth treatments of topics interesting only to the designer or curator. Residual gas analysis is an important topic whose treatment differs from the usual explanation of mass filter theory. Components such as the turbomolecular and helium gas refrigerator cryogenic pumps are now widely used but not well understood. The discussion of gauges, pumps, and materials is a prelude to the central discussion of systems. System designs are grouped according to their function. Current designs are either single-chamber or multichamber; the details of each design are determined by the requirements of an industrial or research application.

In this edition, the discussion of gauges, pumps, and materials has been updated, where relevant, to reflect changes in practice. Spinning rotor gauges are no longer a laboratory curiosity. Ultrahigh vacuum gauges, though limited in their availability, will be a necessity in next-generation production deposition systems. Ultraclean, low dead volume metrology and valves, along with superior materials and cleaning techniques, have made contamination-free manufacturing a reality.

Ultraclean vacuum, once the domain of the researcher, is now routinely used for high-volume production of semiconductor chips and storage

vii

viii PREFACE

media. However, methodologies for reaching low pressures in a clean manner have changed significantly. No longer are single-chamber systems baked for twenty- four hours. Rather, cassette-based loadhnload chambers serve as high-volume interfaces between atmosphere and ultraclean process chambers. These chambers, which can be accessed in serial or random order, are only exposed to atmosphere during maintenance.

Large, efficient multichamber medium and highvacuum systems are used in high-speed coating of numerous consumer products such as window glass, solar cells, video tape, printer paper, eyeglass lenses, automobile headlamps, plastic films and security devices.

The gap in knowledge and training between those who manufacture and those who use vacuum equipment continues to widen. It is from this perspective that the previous edition of this book has been revised. Important formulas have been denoted with a b for emphasis. Easy questions have been emphasized with a 'f.

Thanks are due to countless researchers who, individually and collaboratively, have advanced this field by creative solutions to real problems; I also thank Dr. Bruce Kendall for his insightful comments and thoughtful review.

J . F. O'Hanlon

Tucson, Arizona

Contents ~

1.

2.

3.

ITS BASIS

Vacuum Technology 1.1 Units of Measurement 6 References 8

Gas Properties 2.1

2.2

2.3

Kinetic Picture of a Gas 9 2.1.1 Velocity Distribution 10 2.1.2 Energy Distribution 1 1 2.1.3 MeanFreePath 12 2.1.4 ParticleFlux 13 2.1.5 Monolayer Formation Time 14 2.1.6 Pressure 14 Gas Laws 15 2.2.1 Boyle'sLaw 15 2.2.2 Amonton's Law 16 2.2.3 Charles' Law 16 2.2.4 Dalton's Law 16 2.2.5 Avogadro's Law 16 2.2.6 Graham'sLaw 17 Elementary Gas Transport Phenomena 18 2.3.1 Viscosity 18 2.3.2 Thermal Conductivity 20 2.3.3 Diffusion 21 2.3.4 Thermal Transpiration 22

References 23 Problems 24

Gas Flow 3.1 Flow Regimes 25 3.2 Throughput, Mass Flow, and Conductance 27 3.3 ContinuumFlow 28

3.3.1 Orifices 29 3.3.2 Long Round Tubes 30

25

X

3.3.3 Short Round Tubes 32 3.4 Molecular Flow 32

3.4.1 Orifices 33 3.4.2 Long Round Tubes 34 3.4.3 Short Round Tubes 34 3.4.4 Other Short Structure Solutions 36

Analytical Solutions 37 Monte Car10 Technique 38

Parallel Conductances 39 Series Conductances 39 Exit and Entrance Effects 44 Series Calculations 45

3.4.5 Combining Molecular Conductances 39

3.5 The Transition Region 49 3.6 Models Spanning Several Pressure Regions 50 3.7 Summary of Flow Regimes 51 References 52 Problems 53

4. Gas Release from Solids 4.1 Vaporization 57 4.2 Diffbsion 58

4.3 Thermal Desorption 61 4.2.1

4.3.1 Desorption Without Readsorption 62

Reduction of Outdiffision by Vacuum Baking 60

Zero-Order Desorption 62 First-Order Desorption 62 Second-Order Desorption 63 Desorption from Real Surfaces 65 Outgassing Measurements 65 Outgassing Models 67 Reduction of Outgassing by Baking 68

4.3.2

4.4 Stimulated Desorption 70 4.4.1 Electron-Stimulated Desorption 70 4.4.2 Ion-Stimulated Desorption 70 4.4.3 Stimulated Chemical Reactions 70 4.4.4 Photodesorption 71

4.5.1 Molecular Permeation 71 4.5.2 Dissociative Permeation 73 4.5.3

4.6 Pressure Limits 74 References 77 Problems 77

4.5 Permeation 71

Permeation and Outgassing Units 73

57

MEASUREMENT

5. Pressure Gauges 5.1 Direct-Reading Gauges 8 1

5.1.1 5.1.2 Capacitance Manometers 83

5.2.1 Thermal Conductivity Gauges 87 PiraniGauge 88 Thermocouple Gauge 91 Stability and Calibration 92

Diaphragm and Bourdon Gauges 82

5.2 Indirect-Reading Gauges 87

5.2.2 Spinning Rotor Gauge 92 5.2.3 Ionization Gauges 94

Hot Cathode Gauges 94 Hot Cathode Gauge Errors 100 Cold Cathode Gauge 103 Gauge Calibration 104

References 105 Problems 106

6. Flow Meters 6.1 6.2 Rotameters and Chokes 112 6.3 Differential Pressure Techniques 114 6.4 Thermal Mass Flow Meter Technique 1 15

6.4.1 Mass Flow Meter 115 6.4.2 Mass Flow Controller 120 6.4.3 Mass Flow Meter Calibration 120

Molar Flow, Mass Flow, and Throughput 109

References 12 1 Problems 121

xi

81

109

7. Pumping Speed 123 7.1 Pumping Speed 123 7.2 Mechanical Pumps 124 7.3 High Vacuum Pumps 125

7.3.1 Measurement Techniques 125 Pump Dependence 126 Measurement of Water Vapor Pumping Speed 126 Pumping Speed at the Chamber 127

7.3.2 Measurement Error 128 References 130 Problems 130

XU

8. Residual Gas Analyzers 8.1 Instrument Description 133

8.1.1 Ion Sources 134 Open Ion Sources 135 Closed Ion Sources 136

Magnetic Sector 139 RFQuadrupole 141 Resolving Power 145

Discrete Dynode Electron Multiplier 147 Continuous Dynode Electron Multiplier 148

8.1.2 Mass Filters 139

8.1.3 Detectors 145

8.2 Installation and Operation 150 8.2.1 High Vacuum Operation 150

Mounting 150 Stability 151 Medium and Low Vacuum Sampling 153 Differentially Pumped Sampling 153 Miniature Quadrupoles 156

8.2.2

8.3 RGA Calibration 156 8.4 RGA Selection 158

References 159 Problems 160

9. Interpretation of RGA Data 9.1 Cracking Patterns 161

9.1.1 Dissociative Ionization 16 1 9.1.2 Isotopes 162 9.1.3 Multiple Ionization 163 9.1.4 Combined Effects 163 9.1.5 Ion Molecule Reactions 165

9.2 Qualitative Analysis 166 9.3 Quantitative Analysis 172

9.3.1 Isolated Spectra 172 9.3.2 Overlapping Spectra 173

References 177 Problems 178

PRODUCTION

10. Mechanical Pumps 10.1 RotaryVanePump 183 10.2 Rotary Piston Pump 187

133

161

183

10.3 Lobe Pump 189 10.4 ClawPump 193 10.5 Scroll Pump 194 10.6 Screw Pump 195 10.7 Diaphragm Pump 196 10.8 Mechanical Pump Operation 198 References 199 Problems 199

11. Turbomolecular Pumps 1 1.1 Pumping Mechanism 201 1 1.2 Speed-compression Relations 203

1 1.2.1 Maximum Compression Ratio 203 11 -2.2 Maximum Speed 206 1 1.2.3 General Relation 207

Turbomolecular Pump Designs 2 10 Turbomolecular Drag Pumps 213

1 1.3 Ultimate Pressure 209 1 1 -4 1 1.5 References 2 14 Problems 215

12. Diffusion Pumps 12.1 Pumping Mechanism 2 17 12.2 Speed-Throughput Characteristics 2 19 12.3 Boiler Heating Effects 223 12.4 Backstreaming, Baffles, and Traps 224 References 227 Problems 228

13. Pump Fluids 13.1 Fluid Properties 229

13.1.1 Vapor Pressure 229 13.2.2 Other Properties 233

13.2.1 Mineral Oils 234 13.2.2 Synthetic Fluids 235

Esters 236 Silicones 236 Ethers 237 Fluorochemicals 237

13.3 Fluid Selection 238 Rotary Vane, Piston, and Lobe Pumps 238

13.2 Pump Fluid Types 234

13.3.1 13.3.2 Turbomolecular Pumps 240

201

217

229

13.3.3 Diffusion Pumps 24 1 13.4 Reclamation 244 References 244 Problems 245

14, Getter and Ion Pumps 14.1 Getter Pumps 247

14.1.1 Titanium Sublimation Pumps 248 14.1.2 Nonevaporable Getter 258

14.2 IonPumps 256 References 260 Problems 261

15. Cryogenic Pumps 15.1 Pumping Mechanisms 264 15.2 Speed, Pressure, and Saturation 267 15.3 Refiigeration Techniques 271 15.4 Cryogenic Pump Characteristics 276

15.4.1 15.4.2 15.4.3

Medium Vacuum Sorption Pumps 276 High Vacuum Gas Refrigerator Pumps 279 High Vacuum Liquid Pumps 283

References 284 Problems 286

MATERIALS

16. Materials in Vacuum 16.1 Metals 290

16.1.1 Vaporization 290 16.1.2 Permeability 290 16.1.3 Outgassing 291

Dissolved Gas 292 Surface and Near-Surface Gas 295

16.1.4 Structural Metals 299 16.2 Glasses and Ceramics 300 16.3 Polymers 306 References 309 Problems 311

17. Joints, Seals, and Valves 17.1 Permanent Joints 313

17.1.1 Welding 3 14 17.1.2 Soldering and Brazing 3 18

247

263

289

313

xv

17.1.3

17.2.1 Elastomer Seals 322 17.2.2 Metal Gaskets 328 Valves and Motion Feedthroughs 329 17.3.1 Small Valves 330 17.3.2 Large Valves 332 17.3.3 Special Purpose Valves 335 17.3.4 Motion Feedthroughs 337

Joining Glasses and Ceramics 3 19 17.2 Demountable Joints 321

17.3

References 34 1 Problems 342

18. Lubrication 18.1 Lubrication Processes 345 18.2 Rheology 347

18.2.1 Absolute Viscosity 347 18.2.2 Kinematic Viscosity 348 18.2.3 Viscosity Index 348

18.3.1 Liquid Lubrication 349 18.3.2 Grease Lubrication 352 18.3.3 Dry Lubrication 353

18.3 Lubrication Techniques 349

References 3 5 5 Problems 356

SYSTEMS

19. Rough Vacuum Pumping 19.1 PumpingRate 360

19.1.1 PumpSize 360 19.1.2 Aerosol Formation 362

19.2.1 Oil Backstreaming 366 19.2.2 Overload Criteria 369

Diffusion Pumps 369 Turbomolecular Pumps 3 7 1 Cryogenic Pumps 373 IonPumps 374

19.2 Crossover 365

References 375 Problems 376

20. High Vacuum Systems 20.1 Diffusion-Pumped Systems 379

345

359

379

Ni

20.1.1 System Operation 382 20.1.2 Operating Concerns 383

20.2 Turbomolecular-Pumped Systems 385 20.2.1 System Operation 388 20.2.2 Operating Concerns 389

20.3.1 System Operation 391 20.3.2 Operating Concerns 393

20.4 Cryogenic-Pumped Systems 394 20.4.1 System Operation 394 20.4.2 Regeneration 394 20.4.3 Operating Concerns 396

20.5.1 Managing Water Vapor

20.3 Ion-Pumped Systems 391

20.5 High Vacuum Chambers 397

References 400 Problems 400

21. Ultraclean Vacuum Systems 21.1 Ultraclean Pumps 405

2 1.1.1 Turbomolecular Pumps 405 2 1.1.2 Cryogenic Pumps 406 2 1.1.3

2 1.2.1 2 1.2.2 Chamber Pumping 409 21.2.3 Pressure Measurement 412

Sputter-Ion, TSP, and NEG Pumps 406

Chamber Materials and Components 407 21.2 Ultraclean Chambers 407

References 4 12 Problems 413

22. High Flow Systems 22.1 22.2

Mechanically Pumped Systems 4 17 Throttled High Vacuum Systems 419 22.2.1 Process Chambers 419 22.2.2 TurboPumped 421 22.2.3 CryoPumped 424

References 429 Problems 429

23. Multichamber Systems 23.1 Flexible Substrates 432 23.2 Rigid Substrates 434

23.2.1 Inline Systems 435 23.2.2 Cluster Systems 440

403

415

431

xvii

23.3 Instrumentation Systems 443 References 444 Problems 444

24. Leak Detection 24.1 Instruments 448

24.1.1 Forward-Flow Leak Detector 448 24.1.2 Counter-Flow Leak Detector 449

24.2.1 Sensitivity 450 24.2.2 Response Time 452 24.2.3 Sampling Pressurized Chambers 453

24.2 Performance 450

24.3 Leak-Hunting Techniques 453 References 457 Problems 457

Symbols

APPENDIXES

A. Units and Constants A. 1 Physical Constants 463 A.2 SIBaseUnits 463 A.3 Conversion Factors 464

B. Gas Properties B. 1 B.2 B.3 B.4 B.5 B.6

Mean Free Paths of Gases as a Function of Pressure 466 Physical Properties of Gases and Vapors at T = 0°C 467 Cryogenic Properties of Gases 468 Gas Conductance and Flow Formulas 469 Vapor Pressure Curves of Common Gases 475 Appearances of Discharges in Gases and Vapors at Low Pressures 477

C. Material Properties C. 1 C.2 C.3 C.4 C.5 C.6 C.7 C.8 Austenitic Stainless Steels 486

Outgassing Rates of Vacuum Baked Metals 478 Outgassing Rates of Unbaked Metals 479 Outgassing Rates of Unbaked Ceramics and Glasses 480 Outgassing Rates of Elastomers 480 Permeability of Polymeric Materials 481 Vapor Pressure Curves of Solid and Liquid Elements 482 Outgassing Rates of Polymers 485

447

459

463

466

478

xviii

D. Isotopic Abundances

E. Cracking Patterns E. 1 E.2 E.3 E.4 E.5

Cracking Patterns of Pump Fluids 492 Cracking Patterns of Gases 494 Cracking Patterns of Common Vapors 495 Cracking Patterns of Common Solvents 496 Cracking Patterns of Semiconductor Dopants 497

488

492

F. Pump Fluid Properties 498 F. 1 F.2 F.3 F.4 F.5 References 503

Compatibility of Elastomers and Pump Fluids 498 Vapor Pressures of Mechanical Pump Fluids 499 Vapor Pressure of Diffusion Pump Fluids 500 Kinematic Viscosity of Pump Fluids 501 Kinematic Viscosity Conversion Factors 502

Index 505