Download 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
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A User's Guide to
Vacuum Technology
Third Edition
This Page Intentionally Left Blank
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.
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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
~
ITS BASIS
1. Vacuum Technology
1.1 Units of Measurement 6
References 8
2. Gas Properties
2.1 Kinetic Picture of a Gas 9
2.1.1 Velocity Distribution 10
2.1.2 Energy Distribution 11
2.1.3 MeanFreePath 12
2.1.4 ParticleFlux 13
2.1.5 Monolayer Formation Time 14
2.1.6 Pressure 14
2.2 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
2.3 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
3. 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
3.4.5 Combining Molecular Conductances 39
Parallel Conductances 39
Series Conductances 39
Exit and Entrance Effects 44
Series Calculations 45
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
4.2.1 Reduction of Outdiffision by Vacuum Baking 60
4.3 Thermal Desorption 61
4.3.1 Desorption Without Readsorption 62
Zero-Order Desorption 62
First-Order Desorption 62
Second-Order Desorption 63
4.3.2 Desorption from Real Surfaces 65
Outgassing Measurements 65
Outgassing Models 67
Reduction of Outgassing by Baking 68
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 Permeation 71
4.5.1 Molecular Permeation 71
4.5.2 Dissociative Permeation 73
4.5.3 Permeation and Outgassing Units 73
4.6 Pressure Limits 74
References 77
Problems 77
57
xi
MEASUREMENT
5. Pressure Gauges
5.1 Direct-Reading Gauges 81
5.1.1 Diaphragm and Bourdon Gauges 82
5.1.2 Capacitance Manometers 83
5.2 Indirect-Reading Gauges 87
5.2.1 Thermal Conductivity Gauges 87
PiraniGauge 88
Thermocouple Gauge 91
Stability and Calibration 92
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 Molar Flow, Mass Flow, and Throughput 109
6.2 Rotameters and Chokes 112
6.3 Differential Pressure Techniques 114
6.4 Thermal Mass Flow Meter Technique 115
6.4.1 Mass Flow Meter 115
6.4.2 Mass Flow Controller 120
6.4.3 Mass Flow Meter Calibration 120
References 121
Problems 121
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
8.1.2 Mass Filters 139
Magnetic Sector 139
RFQuadrupole 141
Resolving Power 145
8.1.3 Detectors 145
Discrete Dynode Electron Multiplier 147
Continuous Dynode Electron Multiplier 148
8.2 Installation and Operation 150
8.2.1 High Vacuum Operation 150
Mounting 150
Stability 151
8.2.2 Medium and Low Vacuum Sampling 153
Differentially Pumped Sampling 153
Miniature Quadrupoles 156
8.3 RGA Calibration 156
8.4 RGA Selection 158
References 159
Problems 160
133
9. Interpretation of RGA Data
9.1 Cracking Patterns 161
9.1.1 Dissociative Ionization 161
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
161
PRODUCTION
10. Mechanical Pumps
10.1 RotaryVanePump 183
10.2 Rotary Piston Pump 187
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
11.1 Pumping Mechanism 201
11.2 Speed-compression Relations 203
11.2.1 Maximum Compression Ratio 203
11-2.2 Maximum Speed 206
11.2.3 General Relation 207
11.3 Ultimate Pressure 209
11-4 Turbomolecular Pump Designs 2 10
11.5 Turbomolecular Drag Pumps 213
References 2 14
Problems 215
201
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
217
13. Pump Fluids
13.1 Fluid Properties 229
13.1.1 Vapor Pressure 229
13.2.2 Other Properties 233
13.2 Pump Fluid Types 234
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
13.3.1 Rotary Vane, Piston, and Lobe Pumps 238
13.3.2 Turbomolecular Pumps 240
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
247
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 Medium Vacuum Sorption Pumps 276
15.4.2 High Vacuum Gas Refrigerator Pumps 279
15.4.3 High Vacuum Liquid Pumps 283
References 284
Problems 286
263
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
289
17. Joints, Seals, and Valves
17.1 Permanent Joints 313
17.1.1 Welding 3 14
17.1.2 Soldering and Brazing 3 18
313
xv
17.1.3 Joining Glasses and Ceramics 319
17.2 Demountable Joints 321
17.2.1 Elastomer Seals 322
17.2.2 Metal Gaskets 328
17.3 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
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 Lubrication Techniques 349
18.3.1 Liquid Lubrication 349
18.3.2 Grease Lubrication 352
18.3.3 Dry Lubrication 353
References 35 5
Problems 356
345
SYSTEMS
19. Rough Vacuum Pumping
19.1 PumpingRate 360
19.1.1 PumpSize 360
19.1.2 Aerosol Formation 362
19.2 Crossover 365
19.2.1 Oil Backstreaming 366
19.2.2 Overload Criteria 369
Diffusion Pumps 369
Turbomolecular Pumps 37 1
Cryogenic Pumps 373
IonPumps 374
References 375
Problems 376
359
20. High Vacuum Systems
20.1 Diffusion-Pumped Systems 379
379
N
i
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 Ion-Pumped Systems 391
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 High Vacuum Chambers 397
20.5.1 Managing Water Vapor
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 Sputter-Ion, TSP, and NEG Pumps 406
21.2 Ultraclean Chambers 407
2 1.2.1 Chamber Materials and Components 407
2 1.2.2 Chamber Pumping 409
21.2.3 Pressure Measurement 412
References 4 12
Problems 413
403
22. High Flow Systems
22.1 Mechanically Pumped Systems 4 17
22.2 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
415
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
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 Performance 450
24.2.1 Sensitivity 450
24.2.2 Response Time 452
24.2.3 Sampling Pressurized Chambers 453
24.3 Leak-Hunting Techniques 453
References 457
Problems 457
447
Symbols
459
APPENDIXES
A. Units and Constants
A. 1 Physical Constants 463
463
B. Gas Properties
B. 1 Mean Free Paths of Gases as a Function of Pressure 466
B.2 Physical Properties of Gases and Vapors at T = 0°C 467
B.3 Cryogenic Properties of Gases 468
B.4 Gas Conductance and Flow Formulas 469
B.5 Vapor Pressure Curves of Common Gases 475
B.6 Appearances of Discharges in Gases and Vapors at
Low Pressures 477
466
A.2
A.3
SIBaseUnits 463
Conversion Factors 464
C. Material Properties
478
C. 1 Outgassing Rates of Vacuum Baked Metals 478
C.2 Outgassing Rates of Unbaked Metals 479
C.3 Outgassing Rates of Unbaked Ceramics and Glasses 480
C.4 Outgassing Rates of Elastomers 480
C.5 Permeability of Polymeric Materials 481
C.6 Vapor Pressure Curves of Solid and Liquid Elements 482
C.7 Outgassing Rates of Polymers 485
C.8 Austenitic Stainless Steels 486
xviii
D. Isotopic Abundances
488
E. Cracking Patterns
E. 1 Cracking Patterns of Pump Fluids 492
E.2 Cracking Patterns of Gases 494
E.3 Cracking Patterns of Common Vapors 495
E.4 Cracking Patterns of Common Solvents 496
E.5 Cracking Patterns of Semiconductor Dopants 497
492
F. Pump Fluid Properties
F. 1 Compatibility of Elastomers and Pump Fluids 498
F.2 Vapor Pressures of Mechanical Pump Fluids 499
F.3 Vapor Pressure of Diffusion Pump Fluids 500
F.4 Kinematic Viscosity of Pump Fluids 501
F.5 Kinematic Viscosity Conversion Factors 502
References 503
498
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
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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
VACUUM TECHNOLOGY
4
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
groupThe background pressure must be reduced to the very high vacuum
range for electron microscopy, mass spectroscopy, crystal growth, and xray 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, thinTable 1.1 Vacuum Ranges
Degree of Vacuum
Low
Medium
High
Very high
Ultrahigh
Extreme ultrahigh
Pressure Range
(Pa)”
lo5 >
3 . 3 ~ 1 01~
lo-’ 1
104 2
10-’O
2
P
P
P
P
P
> P
> 3.3~10~
> lo-’
> lo4
>
>
lo7
io-*O
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.
” 101323.3Pa = 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 highspeed 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
VACUUM TECHNOL4)CY
6
Table 1.2 Components of Dry Atmospheric Air
Constituent
N2
0 2
co*
Ar
Ne
He
Kr
Xe
H2
cH4
N20
Content
(vol. %)
78.084 f 0.004
20.946 f 0.002
0.037
0.934 f 0.001
(PPm)
18.18 50.04
5.245 0.004
1.14 f 0.01
0.087 f 0.001
0.5
2.
0.5 f 0.1
Pressure
(Pa)
79,117
21,223
37.5
946.357
1.842
0.51
0.1 16
0.009
0.051
0.203
0.051
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.
Carbon dioxide data from Mama Kea, Hawaii, 2000. Data since 1955 are
available as: http://stratus.mlo.hawaii.govhjects/GASES/co2graph.htrn.
a
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 HZwith 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
-*OL
-;-$ -4 1 Loglo
-A -I
-; d 1’ ; A
Pressure
7
4
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.
VACUUMTECHNOLOGY
8
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, 11 8 (1984).
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 (1990).
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 molecules,
0~~
whereas at a pressure of
Pa, a very high
'~
Indeed, any volume and pressure
vacuum, it contains 2 . 5 ~ 1 0 molecules.
normally used in the laboratory will contain a large number of molecules.
(ii) 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 1~0-9m at atmospheric pressure
(1O5 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
GAS PROPERTIES
10
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
dn _
- 2N
_
-
dv
m
z1l2( 2 k T )
312
2 -rnv2/(2kT)
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.
v'(s)
112
0
500
1000
Velocity (m/s)
1500
Fig. 2.1 Relative velocity distribution of air at O"C, 25"C, and 400°C.
b (2.2)
2000