A User's Guide to Vacuum Technology

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

This Page Intentionally Left Blank


Third Edition



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.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 11 1 River Street, Hoboken, NJ 07030, (201) 748-601 1, fax (201) 748-6008, e-mail: [email protected].

Limit of LiabilityiDisclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representation or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

For general information on our other products and services please contact our Customer Care Department within the U.S. at 877-762-2974, outside the U.S. at 317-572-3993 or fax 317-572-4002.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print, however, may not be available in electronic format.

Library of Congress Cataloging-in-Publication Data:

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


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


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


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


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


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


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


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


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


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

Its Basis

An understanding of how vacuum components and systems h c t i o n begins with an understanding of the behavior of gases at low pressures. Chapter 1 discusses the nature of vacuum technology. Chapter 2 reviews basic gas properties. Chapter 3 describes the flow of gases at reduced pressures, and Chapter 4 discusses how gas is evolved fiom the surfaces of materials. Together, these chapters form the basis of vacuum technology.

1


CHAPTER 1

Vacuum Technology

Torricelli is credited with the conceptual understanding of the vacuum within a mercury column by 1643. It is written that his good friend Viviani actually performed the first experiment, perhaps as early as 1644 [1,2]. His discovery was followed in 1650 by Otto von Guericke’s piston vacuum pump. Interest in vacuum remained at a low level for more than 200 years, when a period of rapid discovery began with McLeod’s invention of the compression gauge. In 1905 Gaede, a prolific inventor, designed a rotary pump sealed with mercury. The thermal conductivity gauge, diffusion pump, ion gauge, and ion pump soon followed, along with processes for liquefaction of helium and refinement of organic pumping fluids. They formed the basis of a technology that has made possible everything from light bulbs to space simulation. The significant discoveries of this early period of vacuum science and technology have been summarized in a series of historical review papers [2-71.

A vacuum is a space from which air or other gas has been removed. All gas cannot be removed. The amount removed depends on the application, and is done for many reasons. At atmospheric pressure molecules constantly bombard surfaces. These molecules can bounce from surfaces, attach themselves to surfaces, or perhaps chemically react with surfaces. Air or other surrounding gas quickly contaminates a cleaned surface. A clean surface-for example, a freshly cleaved crystal-will remain clean in an ultrahigh vacuum chamber for long periods of time, because the rate of molecular bombardment is low.

Molecules are crowded closely together at atmospheric pressure and travel in every direction much like people in a crowded plaza. It is impossible for a molecule to travel &om one wall of a chamber to another without colliding with many molecules. By reducing the pressure to a suitably low value, a molecule from one wall can travel to another without a collision. Many effects become possible if molecules can travel long distances between collisions. Metals can be evaporated from a pure source without reacting in transit. Molecules or atoms can be accelerated to a high

3

4 VACUUM TECHNOLOGY

energy and sputter away, or be implanted in the bombarded surface. Electrons or ions can be scattered from surfaces and be collected. The energy changes they undergo on scattering or release from a surface can be used to probe or analyze the surface or underlying layers. For convenience the subatmospheric pressure scale has been divided into several ranges. Table 1.1 lists these ranges.

The required vacuum level depends on the application. Epitaxial growth of semiconductor films (reduced pressure epitaxy) and laser etching of metals are two processes that are performed in the low vacuum range. Sputtering, plasma etching and deposition, low-pressure chemical vapor deposition, ion plating, and gas filling of encapsulated heat transfer modules are examples of processes performed in the medium vacuum range.

Pressures in the high vacuum range are needed for the manufacture of traditional low- and high-tech devices such as microwave, power, cathode ray and photomultiplier tubes, light bulbs, architectural and automotive glazing, decorative packaging, degassing of metals, vapor deposition, and ion implantation. A number of medium technology applications including medical, microwave susceptors, electrostatic dissipation films, and aseptic packaging use films fabricated in a vacuum environment [8]. Retail security, bank note security, and laser and inkjet paper have joined this group-

The background pressure must be reduced to the very high vacuum range for electron microscopy, mass spectroscopy, crystal growth, and x- ray and electron beam lithography, and storage media production. For ease of reading, we call the very high vacuum region “high vacuum” and call the pumps “high vacuum pumps.”

Pressures in the ultrahigh vacuum range were formerly the domain of the surface analyst, materials researcher, or accelerator technologist. Critical high-volume production applications, such as semiconductor devices, thin-

Table 1.1 Vacuum Ranges

Pressure Range Degree of Vacuum (Pa)”

Low lo5 > P > 3 . 3 ~ 1 0 ~ Medium 3 . 3 ~ 1 0 ~ 1 P > lo-’ High lo-’ 1 P > lo4 Very high 104 2 P > l o 7 Ultrahigh 2 P > i o - * O Extreme ultrahigh 10-’O > P

Suurce: Reprinted with permission h m D i c t i o q for Vacuum Science and Technology, M. Kaminsky and J. M. Iafferty, Eds., American Vacuum Society, New York, 1980. ” 101323.3 Pa = 1 atmosphere.

VACUUM TECHNOLOGY s

film media heads, and extreme UV lithography systems, require ultrahigh vacuum base pressures to improve yield by reducing gaseous impurity contamination. Additionally, processes carried out in these systems must be free of particle contamination, so we call them ultraclean vacuum systems.

A vacuum system is a combination of pumps, valves, and pipes, which creates a region of low pressure. It can be anything from a simple mechanical pump or aspirator for exhausting a vacuum storage container to a complex system such as an underground accelerator with miles of piping that is maintained at ultrahigh vacuum.

Removal of air at atmospheric pressure is usually done with a displacement pump. A displacement pump is one that removes the air from the chamber and expels it to the atmosphere. Rotary vane and piston pumps are examples of pumps used to exhaust gases at atmospheric pressure. Liquid nitrogen capture pumps or sorption pumps have also been designed for exhausting gases at atmospheric pressure. They are used only on small chambers because of their finite gas sorption.

Rotary vane, piston and sorption pumps have low-pressure limits in the range lO’’-lO” Pa. Pumps that will function in a rarefied atmosphere are required to operate below this pressure range. Several displacement and capture pumps can remove air at these low pressures. The diffusion pump was the first high vacuum pump. It is a displacement pump. Its outlet pressure is below atmosphere. The turbomolecular pump, a system of high- speed rotating turbine blades, can also pump gas at low pressures. The outlet pressures of these two pumps need to be kept in the range 0.5-50 Pa, so they must exhaust into a rotary vane or piston “backing” pump, or “fore” pump. If the diffision or turbomolecular pump exhaust gas flow would otherwise be too great, a lobe blower will be placed between the exhaust of the diffusion or turbomolecular pump and the inlet of the rotary pump to pump gas at an increased speed in this intermediate pressure region.

Capture pumps can effectively remove gas from a chamber at low pressure. They do so by freezing molecules on a wall (cryogenic pump), chemically reacting with the molecules (getter pump), or accelerating the molecules to a high velocity and burying them in a metal wall (ion pump). Capture pumps are more useful as high vacuum pumps than as atmospheric exhaust pumps because the number of molecules to be captured at high vacuum is less than the number removed during initial evacuation from atmosphere.

Air is the most important gas to understand, because it is in every vacuum system. It contains at least a dozen constituents, whose major constituents are described in Table 1.2. The differing ways in which pumps remove air, and gauges measure its pressure, can be understood in terms of the partial pressures of its components. The concentrations listed in Table 1.2 are those of dry atmospheric air at sea level (total pressure

6 VACUUM TECHNOL4)CY

Table 1.2 Components of Dry Atmospheric Air

Content Pressure Constituent (vol. %) (PPm) (Pa)

N2 0 2 co* Ar Ne He Kr Xe H2 cH4 N20

78.084 f 0.004 20.946 f 0.002 0.037 0.934 f 0.001

18.18 50.04 5.24 5 0.004 1.14 f 0.01 0.087 f 0.001 0.5 2. 0.5 f 0.1

79,117 21,223 37.5

946.357 1.842 0.51 0.1 16 0.009 0.051 0.203 0.05 1

Source: Reprinted with permission from The Handbook of Chemistry and Physics, 59th ed., R C. Weast, Ed., copyright 1978, The Chemical Rubber Publishing Co., CRC Press, Inc., West Palm Beach, FL 33409. a Carbon dioxide data from Mama Kea, Hawaii, 2000. Data since 1955 are available as: http://stratus.mlo.hawaii.govhjects/GASES/co2graph.htrn.

101,323.2 Pa or 760 Torr). The partial pressure of water vapor is not given in this table, because it constantly changes. At 20°C a relative humidity of 50% corresponds to a partial pressure of 1165 Pa (8.75 Torr), making it the third largest constituent of air. The total pressure changes rapidly with altitude, as shown in Fig. 1.1, whereas its proportions change slowly but significantly. In outer space the atmosphere is mainly HZ with some He [6].

In the pressure region below 10 Pa, gases evolving from material surfaces contribute more molecules per second to the total gas load than do the gases originally filling the chamber. The correct pump is not the only requirement needed to reach low pressures-the materials of construction, techniques for joining components, surface cleaning techniques, and operational procedures are all critically important. In the remaining chapters the pumps, gauges, and materials of construction and operational techniques are described in terms of fundamental gas behavior. The focus is on the understanding and operation of vacuum systems for a variety of technological applications.

1.1 UNITS OF MEASUREMENT

Units of measurement present problems in many disciplines and vacuum technology is no exception. The use of noncoherent vacuum units has been common in the US long after the adoption of System International.

1.1 UNITS OF MEASUREMENT 7

-*OL -; -$ -4 1 -A -I -; d 1’ ; A 4 Loglo Pressure

Fig. 1.1 Relation between the atmospheric pressure and the geometric altitude. Reprinted with permission h m The Handbook ofchemistry and Physics, 59th ed., R. C. Weast, Ed. copyright 1978, The Chemical Rubber Publishing Co., CRC Press, Inc., West Palm Beach, FL 33409.

The meter-kilogram-second (MKS) system was first introduced over a half-century ago; its use became commonplace only after a decade or more of classroom education by instructors committed to change. In a similar manner, those who teach vacuum technique will lead the way to routine use of SI units. Instruments are manufactured for use in a global economy and their readings can be displayed in several formats. The advantages of using a coherent unit system are manifold. Calculations become straightforward and logical and the chance for error is reduced. Incoherent units such as permeation constant, the volume of gas (at standard temperature and pressure) per material thickness per material area per sec pressure difference, are cumbersome. Additionally, these permeation units mask their relation to solubility and diffusion. Ultimately, SI units will be routinely used. To assist with this change, dual labels have been added throughout the text. Basic SI units for pressure (Pa), time (s) and length (m) will be assumed in all formulas, unless noted differently within a formula statement.

8 VACUUMTECHNOLOGY

REFERENCES

1. W. E. K. Middleton, The History of the Barometer, Johns Hopkins Press, Baltimore, 1964.

2. P. A. Redhead, Vacuum, 53, 137 (1999). 3. T. E. Madey, J. Vac. Sci. Technol. A, 2, 110 (1984). 4. M. H. Hablanian, J. Vac. Sci. Technol. A, 2, 1 1 8 (1 984). 5. J. H. Singleton, J. Vac. Sci. Technol. A, 2, 126 (1984). 6. P. A. Redhead, J. Vac. Sci. Technol. A, 2, 132 (1984). 7. T. E. Madey and W. C. Brown, Eds., History of Vacuum Science and Technology,

American Institute of Physics, New York, 1984. 8. P. R. Johansen, J. Vac. Sci. Technol. A, 8,2798 (1 990). 9. D. J. Santeler, et al., Vacuum Technology and Space Simulation, NASA SP 105,

National Aeronautics and Space Administration, Washington, DC, 1966, p. 34.

CHAPTER 2

Gas Properties

In this chapter we discuss the properties of gases at atmospheric and reduced pressures. The properties developed here are based on the kinetic picture of a gas. Kinetic theory has its limitations, but with it we are able to describe particle motion, pressure, effusion, viscosity, diffusion, thermal conductivity, and thermal transpiration of ideal gases. We will use these ideas as the starting point for discussing gas flow, gauges, pumps and systems.

2.1 KINETIC PICTURE OF A GAS

The kinetic picture of a gas is based on several assumptions. (i) The volume of gas under consideration contains a large number of molecules. A cubic meter of gas at a pressure of lo5 Pa and a temperature of 22°C contains 2 . 4 8 ~ 1 0 ~ ~ molecules, whereas at a pressure of Pa, a very high vacuum, it contains 2 . 5 ~ 1 0 ' ~ molecules. Indeed, any volume and pressure normally used in the laboratory will contain a large number of molecules. (i i) Adjacent molecules are separated by distances that are large compared with their individual diameters. If we could stop all molecules instantaneously and place them on the coordinates of a grid, the average spacing between them would be about 3 . 4 ~ 1 0-9 m at atmospheric pressure (1 O5 Pa). The diameter of most molecules is of order 2 4 x lo-'' m and their separation distances are -6-15 times their diameter at atmospheric pressures. For extremely low pressures, say Pa, the separation distance is about 3x10" m. (iii) Molecules are in a constant state of motion. All directions of motion are equally likely and all velocities are possible, although not equally probable. (iv) Molecules exert no force on one another except when they collide. If this is true, then molecules will be uniformly distributed throughout the volume and travel in straight lines until they collide with a wall or with one another.

Using these assumptions, many interesting properties of ideal gases have been derived. Some elementary properties are reviewed here.

9

10 GAS PROPERTIES

2.1.1 Velocity Distribution

As the individual molecules move about they collide with elastic collisions. Elastic collisions conserve energy, whereas the colliding particle's velocity is changed after each collision. We stated that all velocities are possible, but not with equal probability. The distribution of particle velocities calculated by Maxwell and Boltzmann is

312 dn - 2N m 2 -rnv2/(2kT) __- - dv z1l2 ( 2 k T )

rn is the particle mass and T is the Kelvin temperature. The relation between the Kelvin scale and the Celsius scale is T(K) = 273.16 + T C ) . In (2.1) N is the total number of particles, and k is Boltzmann's constant. Figure 2.1 illustrates (2.1) for nitrogen molecules (air) at three temperatures. It is a plot of the relative number of molecules between velocity v and v + dv. We see that there are no molecules with zero or infiite velocity, and that the peak or most probable velocity vp is a function of the average gas temperature. The particle velocity also depends on the molecular mass, the peak velocity can be expressed as vp = (2kT/m)". The arithmetic mean or average velocity v is useful when describing particle flow.

112 v'(s) b (2.2)

0 500 1000 1500 2000 Velocity (m/s)

Fig. 2.1 Relative velocity distribution of air at O"C, 25"C, and 400°C.

A User's Guide to Vacuum Technology

Documents