A Users Guide to Vacuum Technology [4 ed.] 9781394174133, 9781394174140, 9781394174225

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A Users Guide to Vacuum Technology

A Users Guide to Vacuum Technology Fourth Edition

John F. O’Hanlon

Emeritus Professor of Electrical and Computer Engineering University of Arizona Tucson, Arizona, USA

Timothy A. Gessert

Gessert Consulting, LLC Conifer, Colorado, USA

Copyright © 2024 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., 111 River Street, Hoboken, NJ 07030, (201) 748-­6011, fax (201) 748-­6008, or online at http://www.wiley.com/go/permission. Trademarks: Wiley and the Wiley logo are trademarks or registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries and may not be used without written permission. All other trademarks are the property of their respective owners. John Wiley & Sons, Inc. is not associated with any product or vendor mentioned in this book. Limit of Liability/Disclaimer of Warranty While the publisher and author have used their best efforts in preparing this book, they make no representations 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. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors 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 or for technical support, please contact our Customer Care Department within the United States at (800) 762-­2974, outside the United States 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 may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. Library of Congress Cataloging-­in-­Publication Data Names: O’Hanlon, John F., 1937– author. | Gessert, Timothy A., author. Title: A users guide to vacuum technology / John F. O’Hanlon, Emeritus   Professor of Electrical and Computer Engineering, University of Arizona, Tucson,   Arizona, USA, Timothy A. Gessert, Gessert Consulting, LLC, Conifer, Colorado, USA. Description: 4th edition. | Hoboken, New Jersey : John Wiley & Sons, Inc.,   [2024] | Includes index. Identifiers: LCCN 2023024446 (print) | LCCN 2023024447 (ebook) | ISBN   9781394174133 (hardback) | ISBN 9781394174140 (adobe pdf) | ISBN   9781394174225 (epub) Subjects: LCSH: Vacuum technology–Handbooks, manuals, etc. Classification: LCC TJ940 .O37 2024 (print) | LCC TJ940 (ebook) | DDC   621.5/5–dc23/eng/20230630 LC record available at https://lccn.loc.gov/2023024446 LC ebook record available at https://lccn.loc.gov/2023024447 Cover image: Wiley Cover design: Courtesy of NASA Set in 9.5/12.5pt STIXTwoText by Straive, Pondicherry, India

­For Jean, Carol, Paul, and Amanda and For Janet, Rachael, Kathryn, and Benjamin

vii

Contents Preface  xvii Symbols  xix Part I  Its Basis  1 1 1.1

Vacuum Technology  3 Units of Measurement  8 References  9

2 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.1.6 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.3 2.3.1 2.3.2

Gas Properties  11 Kinetic Picture of a Gas  11 Velocity Distribution  12 Energy Distribution  13 Mean Free Path  14 Particle Flux  15 Monolayer Formation Time  15 Pressure  16 Gas Laws  16 Boyle’s Law  17 Amontons’ Law  17 Charles’ Law  18 Dalton’s Law  18 Avogadro’s Law  18 Graham’s Law  19 Elementary Gas Transport Phenomena  19 Viscosity  19 Thermal Conductivity  22

viii

Contents

2.3.3 2.3.4

Diffusion  23 Thermal Transpiration  24 References  25

3 3.1 3.2 3.3 3.3.1 3.3.2 3.3.3 3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.4.1 3.4.4.2 3.4.5 3.5

Gas Flow  27 Flow Regimes  27 Flow Concepts  29 Continuum Flow  31 Orifice  32 Long Round Tube  34 Short Round Tube  36 Molecular Flow  37 Orifice  38 Long Round Tube  39 Short Round Tube  39 Irregular Structures  41 Analytical Solutions  42 Statistical Solutions  43 Components in Parallel and Series  43 Models Spanning Molecular and Viscous Flow  53 References  55

4 4.1 4.2 4.2.1 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.5 4.5.1

Gas Release from Solids  59 Vaporization  59 Diffusion  60 Reduction of Outdiffusion by Vacuum Baking  62 Thermal Desorption  63 Zero Order  63 First Order  64 Second Order  65 Desorption from Real Surfaces  67 Outgassing Measurements  67 Outgassing Models  69 Reduction by Baking  69 Stimulated Desorption  71 Electron-Stimulated Desorption  71 Ion-Stimulated Desorption  71 Stimulated Chemical Reactions  72 Photo Desorption  72 Permeation  73 Atomic and Molecular Permeation  73

Contents

4.5.2 4.5.3 4.6

Dissociative Permeation  74 Permeation and Outgassing Units  75 Pressure Limitations During Pumping  76 References  78 Part II  Measurement  81

5 5.1 5.1.1 5.1.2 5.2 5.2.1 5.2.1.1 5.2.1.2 5.2.1.3 5.2.2 5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.3.4

Pressure Gauges  83 Direct Reading Gauges  83 Diaphragm and Bourdon Gauges  84 Capacitance Manometer  85 Indirect Reading Gauges  88 Thermal Conductivity Gauges  88 Pirani Gauge  90 Thermocouple Gauge  91 Stability and Calibration  92 Spinning Rotor Gauge  93 Ionization Gauges  95 Hot Cathode Gauges  95 Hot Cathode Gauge Errors  100 Cold Cathode Gauge  103 Gauge Calibration  105 References  105

6 6.1 6.2 6.3 6.4 6.4.1 6.4.2 6.4.3

Flow Meters  109 Molar Flow, Mass Flow, and Throughput  109 Rotameters and Chokes  111 Differential Pressure Devices  112 Thermal Mass Flow Technique  114 Mass Flow Meter  114 Mass Flow Controller  117 Mass Flow Meter Calibration  119 References  119

7 7.1 7.2 7.3 7.3.1 7.3.2

Pumping Speed  121 Definition  121 Mechanical Pump Speed Measurements  122 High Vacuum Pump Speed Measurements  123 Methods  123 Gas and Pump Dependence  124

ix

x

Contents

7.3.3 7.3.4

Approximate Speed Measurements  125 Errors  125 References  127

8 8.1 8.1.1 8.1.1.1 8.1.1.2 8.1.2 8.1.2.1 8.1.2.2 8.1.2.3 8.1.3 8.1.3.1 8.1.3.2 8.2 8.2.1 8.2.1.1 8.2.1.2 8.2.2 8.2.2.1 8.2.2.2 8.3 8.4

Residual Gas Analyzers  129 Instrument Description  129 Ion Sources  131 Open Ion Sources  131 Closed Ion Sources  133 Mass Filters  134 Magnetic Sector  134 RF Quadrupole  135 Resolving Power  138 Detectors  138 Discrete Dynode Electron Multiplier  139 Continuous Dynode Electron Multiplier  140 Installation and Operation  142 Operation at High Vacuum  142 Sensor Mounting  142 Stability  143 Operation at Medium and Low Vacuum  145 Differentially Pumped Analysis  145 Miniature Quadrupoles  148 Calibration  148 Choosing an Instrument  149 References  150

9 9.1 9.1.1 9.1.2 9.1.3 9.1.4 9.1.5 9.2 9.3 9.3.1 9.3.2

Interpretation of RGA Data  153 Cracking Patterns  153 Dissociative Ionization  153 Isotopes  154 Multiple Ionization  154 Combined Effects  154 Ion–Molecule Reactions  157 Qualitative Analysis  158 Quantitative Analysis  163 Isolated Spectra  164 Overlapping Spectra  165 References  169

Contents

Part III  Production  171 10 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9

Mechanical Pumps  173 Rotary Vane  173 Lobe  177 Claw  180 Multistage Lobe  182 Scroll  184 Screw  185 Diaphragm  185 Reciprocating Piston  187 Mechanical Pump Operation  189 References  189

11 11.1 11.2 11.2.1 11.2.2 11.2.3 11.3 11.4 11.5

Turbomolecular Pumps  191 Pumping Mechanism  191 Speed–Compression Relations  192 Maximum Compression  193 Maximum Speed  195 General Relation  197 Ultimate Pressure  198 Turbomolecular Pump Designs  199 Turbo-Drag Pumps  201 References  203

12 12.1 12.2 12.3 12.4

Diffusion Pumps  205 Pumping Mechanism  205 Speed–Throughput Characteristics  207 Boiler Heating Effects  211 Backstreaming, Baffles, and Traps  212 References  215

13 13.1 13.1.1 13.1.2 13.2

Getter and Ion Pumps  217 Getter Pumps  217 Titanium Sublimation  218 Non-evaporable Getters  223 Ion Pumps  224 References  229

xi

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Contents

14 14.1 14.2 14.3 14.4 14.4.1 14.4.2 14.4.3

Cryogenic Pumps  233 Pumping Mechanisms  234 Speed, Pressure, and Saturation  237 Cooling Methods  241 Cryopump Characteristics  245 Sorption Pumps  246 Gas Refrigerator Pumps  249 Liquid Cryogen Pumps  253 References  253



Part IV  Materials  257

15 15.1 15.1.1 15.1.2 15.1.3 15.1.3.1 15.1.3.2 15.1.4 15.2 15.3

Materials in Vacuum  259 Metals  260 Vaporization  260 Permeability  260 Outgassing  261 Dissolved Gas  262 Surface and Near-Surface Gas  264 Structural Metals  269 Glasses and Ceramics  272 Polymers  277 References  281

16 16.1 16.1.1 16.1.2 16.1.3 16.2 16.2.1 16.2.2 16.3 16.3.1 16.3.2 16.3.3 16.3.4

Joints Seals and Valves  285 Permanent Joints  285 Welding  286 Soldering and Brazing  290 Joining Glasses and Ceramics  291 Demountable Joints  293 Elastomer Seals  294 Metal Gaskets  300 Valves and Motion Feedthroughs  302 Small Valves  302 Large Valves  304 Special-Purpose Valves  307 Motion Feedthroughs  308 References  313

Contents

17 17.1 17.1.1 17.1.1.1 17.1.1.2 17.1.2 17.1.2.1 17.1.2.2 17.1.2.3 17.1.2.4 17.1.2.5 17.1.3 17.1.3.1 17.1.3.2 17.1.3.3 17.1.4 17.2 17.2.1 17.2.1.1 17.2.1.2 17.2.1.3 17.2.2 17.2.2.1 17.2.2.2 17.2.2.3

Pump Fluids and Lubricants  315 Pump Fluids  315 Fluid Properties  315 Vapor Pressure  316 Other Characteristics  319 Fluid Types  319 Mineral Oils  320 Esters  321 Silicones  321 Ethers  322 Fluorochemicals  322 Selecting Fluids  323 Rotary, Vane, and Lobe Pump Fluids  323 Turbo Pump Fluids  325 Diffusion Pump Fluids  325 Reclamation  328 Lubricants  328 Lubricant Properties  329 Absolute Viscosity  330 Kinematic Viscosity  331 Viscosity Index  332 Selecting Lubricants  333 Liquid  333 Grease  334 Solid Film  336 References  338



Part V  Systems  341

18 18.1 18.1.1 18.1.2 18.2 18.2.1 18.2.2 18.2.2.1 18.2.2.2

Rough Vacuum Pumping  343 Exhaust Rate  344 Pump Size  344 Aerosol Formation  346 Crossover  350 Minimum Crossover Pressure  351 Maximum Crossover Pressure  354 Diffusion  354 Turbo  357

xiii

xiv

Contents

18.2.2.3 Cryo  357 18.2.2.4 Sputter-Ion  360 References  362 19 19.1 19.1.1 19.1.2 19.2 19.2.1 19.2.2 19.3 19.3.1 19.3.2 19.4 19.4.1 19.4.2 19.4.3 19.5 19.5.1

High Vacuum Systems  365 Diffusion-Pumped Systems  365 Operating Modes  368 Operating Issues  369 Turbo-Pumped Systems  371 Operating Modes  374 Operating Issues  375 Sputter-Ion-Pumped Systems  376 Operating Modes  377 Operating Issues  379 Cryo-Pumped Systems  379 Operating Modes  380 Regeneration  380 Operating Issues  382 High Vacuum Chambers  383 Managing Water Vapor  384 References  386

20 20.1 20.1.1 20.1.2 20.1.3 20.1.4 20.2 20.3

Ultraclean Vacuum Systems  387 Ultraclean Pumps  389 Dry Roughing Pumps  390 Turbopumps  390 Cryopumps  390 Sputter-Ion, TSP, and NEG Pumps  391 Ultraclean Chamber Materials and Components  392 Ultraclean System Pumping and Pressure Measurement  394 References  398

21 21.1 21.1.1 21.1.2 21.1.2.1 21.1.2.2 21.1.2.3 21.2 21.2.1

Controlling Contamination in Vacuum Systems  401 Defining Contamination in a Vacuum Environment  401 Establishing Control of Vacuum Contamination  401 Types of Vacuum Contamination  402 Particle Contamination  403 Gas Contamination  409 Film Contamination  410 Pump Contamination  411 Low/Rough and Medium Vacuum Pump Contamination  411

Contents

21.2.1.1 21.2.1.2 21.2.2 21.2.2.1 21.2.2.2 21.2.2.3 21.2.2.4 21.3 21.3.1 21.3.2 21.4 21.5 21.6 21.6.1 21.6.2 21.6.3 21.7 21.7.1 21.7.2 21.8 21.8.1 21.8.2 21.8.3 21.9 21.9.1 21.10 21.10.1 21.10.2

Fluid-Sealed Mechanical Pumps  412 Dry Mechanical Pumps  413 High and UHV Vacuum Pump Contamination  415 Diffusion Pumps  416 Turbo- and Turbo-Drag Pumps  417 Cryopumps  418 Sputter-Ion and Titanium-Sublimination Pumps  419 Evacuation Contamination  420 Particle Sources  420 Remediation Methods  421 Venting Contamination  422 Internal Components, Mechanisms, and Bearings  423 Machining Contamination  426 Cutting, Milling, and Turning  426 Grinding and Polishing  427 Welding  428 Process-Related Sources  429 Deposition Sources  429 Leak Detection  430 Lubrication Contamination  432 Liquid Lubricants  432 Solid Lubricants  433 Lamellar, Polymer, and Suspension Lubricants  434 Vacuum System and Component Cleaning  434 Designing a Cleaning Process  435 Review of Clean Room Environments for Vacuum Systems  436 The Cleanroom Environment  438 Using Vacuum Systems in a Cleanroom Environment  438 References  442

22 22.1 22.2 22.2.1 22.2.2 22.2.3

High Flow Systems  445 Mechanically Pumped Systems  447 Throttled High Vacuum Systems  449 Chamber Designs  449 Turbo Pumped  451 Cryo Pumped  455 References  459

23 23.1 23.2 23.2.1

Multichambered Systems  461 Flexible Substrates  462 Rigid Substrates  465 Inline Systems  465

xv

xvi

Contents

23.2.2 23.3

Cluster Systems  469 Analytical Instruments  472 References  472

24 24.1 24.1.1 24.1.2 24.2 24.2.1 24.2.2 24.2.3 24.2.4 24.3 24.4

Leak Detection  475 Mass Spectrometer Leak Detectors  476 Forward Flow  476 Counter flow  477 Performance  478 Sensitivity  478 Response Time  480 Testing Pressurized Chambers  481 Calibration  482 Leak Hunting Techniques  483 Leak Detecting with Hydrogen Tracer Gas  486 References  487 Part VI  Appendices  489 Appendix A  Appendix B  Appendix C  Appendix D  Appendix E  Appendix F  Index  543

Units and Constants  491 Gas Properties  495 Material Properties  509 Isotopes  519 Cracking Patterns  525 Pump Fluid Properties  535

xvii

Preface A Users Guide to Vacuum Technology, Fourth Edition, focuses on the operation, understanding, and selection of equipment for processes used in semiconductor, optics, renewable energy, and related emerging technologies. It emphasizes subjects not adequately covered elsewhere, while avoiding in-­depth treatments of topics of interest only to the vacuum system designer or vacuum historian. The discussions of gauges, pumps, and materials present a required prelude to the later discussion of fully integrated vacuum systems. System design options are grouped according to their function and include both single-­ and multichamber systems and how details of each design are determined by specific requirements of a production or research application. During the twenty years since the publication of the third edition, the needs of vacuum technology users have evolved considerably. For example, in 2003, when the third edition was published, the minimum feature width for a typical semiconductor fabrication facility was on the order of 200 nm: the “200-­nm node.” Approximately ten years later in 2013, production at the 20-­nm node was becoming available, and its related lithography tools began to require UV exposure in vacuum because any gas ambient detrimentally absorbed or scattered UV light. Presently, the 2-­nm node is being tested for advanced integrated circuit processes, and their (ultra-­) UV light sources require even more advanced vacuum systems, as well as related equipment with increasingly tightened specification regarding particle, film, and gas-­phase contamination. Few (if any) historic vacuum textbooks include these topics to the extent required by today’s technologists. The past two decades have also featured an unprecedented increase in the use of sophisticated vacuum-­based processes for mass producing consumer products, such as low-­cost eyeglass reflective coatings, durable cookware coatings, secure bank notes, RFID tags, and coated plastic films. Since the publication of the third edition, the authors of this book have collectively taught several thousands of students at academic institutions, in high-­technology companies, and at professional society meetings. Through their experience,

xviii

Preface

the authors have acquired an unusually diverse and unique exposure to industries involved in present vacuum technology processes, future directions, and the related problems they are facing. Much of this experience has been incorporated into this fourth edition, with the goal of assisting users with insight needed for success in both their present and future activities. Although it is expected that academic students will continue to find this book a valuable reference in their pursuit of advanced degrees, the primary audience of this fourth edition is expected to be vacuum technologists and scientists already working in vacuum technology. However, it is expected that initiatives to expand semiconductor production and developments related to quantum computing both will require the type of advanced guidance presented in this book. Finally, vacuum users in other technology fields are also expected to find this book a valuable resource, e.g., space simulation, fusion research, renewable energy, and medical devices. In addition to including new requirements and related equipment changes within these technology sectors, another enhancement in this fourth edition includes expanded discussions on vacuum technology Best Practices. This type of general guidance would have been acquired historically through mentoring by experienced colleagues; however, the authors have seen rapid developments in many high-­technology sectors, as well as frequent career changes or added management responsibilities, have left many vacuum technologists in greater need of this type of reliable yet succinct guidance. It is hoped that this edition of A Users Guide to Vacuum Technology can fill some of the education gap resulting from this loss of historic “mentoring,” as well as assist senior technologists in appreciating some of the more advanced vacuum concepts and descriptions. The authors thank countless personal colleagues, students, and other researchers who over many years have provided numerous questions and practical solutions to the vacuum topics that have been included in this book. At the risk of many unintentional omissions, the authors would like to particularly thank Bruce Kendal for many discussions that continue to remain highly relevant to this book, Frank Zimone for the idea of incorporating “Best Practices,” and Howard Patton for the original development of the AVS Short Course, Controlling Contamination in Vacuum Systems, on which Chapter 21 of this fourth edition is broadly based. John F. O’Hanlon     Timothy A. Gessert Tucson, Arizona, USA   Conifer, Colorado, USA

xix

Symbols

Symbol

Quantity

Units

A

Area

m2

B

Magnetic field strength

T (tesla)

C

Conductance (gas)

L/s

C′

vena contracta

D

Diffusion constant

m2/s

Eo

Heat transfer

J-­s−1-­m−2

F

Force

N (newton)

G

Electron multiplier gain

H

Heat flow

K

Compression ratio (gas)

Kp

Permeability constant (gas)

J/s m2/s

Kn

Knudsen’s number

KR

Radiant heat conductivity

J-­s−1-­m−1-­K−1

KT

Thermal conductivity

J-­s−1-­m−1-­K−1

M

Molecular weight

N

Number of molecules

No

Avogadro’s number

(kg-­mol)−1

P

Pressure

Pa (pascal)

Q

Gas flow

Pa-­m3/s

R

Gas constant

J-­(kg-­mol)−1-­s−1

R

Reynolds’ number

S

Pumping speed

L/s

S′

Gauge sensitivity

Pa−1 (Continued)

xx

Symbols

Symbol

Quantity

SC

Critical saturation ratio

Units

T

Absolute temperature

K

U

Average gas stream velocity

m/s

U

Mach number

V

Volume

m3

Va

Acceleration potential

V

Vb

Linear blade velocity

m/s

Vo

Normal specific volume of an ideal gas

m3/(kg-­mol)

W

Ho coefficient

a

Transmission probability

b

Turbopump blade chord length, or length dimension

c

Condensation coefficient

cp

Specific heat at constant pressure

J-­(kg-­mol)−1-­K−1

cv

Specific heat at constant volume

J-­(kg-­mol)−1-­K−1

m

d

Diameter dimension

m

do

Molecular diameter

m

d′

Average molecular spacing

m

e

Length dimension

m

ie

Emission current

A

ip

Plate current

A

k

Boltzmann constant

J/K

l

Length dimension

m

m

Mass

kg

n

Gas density

m−3

q

Outgassing rate

Pa-­m/s

qk

Permeation rate

Pa-­m/s

r

Radius

m

s

Turbomolecular pump blade spacing

m

sr

Turbomolecular pump blade speed ratio

u

Local gas stream velocity

m/s

v

Average particle velocity

m/s

w

Length dimension

m

Γ

Particle flux

m−2-­s−1

Symbols

Symbol

Quantity

Units

Δ

Free molecular heat conductivity

J-­s−1-­m−2-­ K−2-­Pa−1

α

Accommodation coefficient

β

Molecular slip constant

γ

Specific heat ratio cp/cv

δ

Kronecker delta function

ε

Emissivity

η

Dynamic viscosity

Pa-­s

λ

Mean free path

m

ξ

Volume to surface area ratio

π

Pi

ρ

Mass density

kg/m3

τ

Vacuum system time constant

s

φ

Angle

deg

ω

Angular frequency; (heat transfer rate)

rad/s; (m/s)

xxi

1

Part I Its Basis An understanding of how vacuum components and systems function 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 complexities of gas flow at near-­atmosphere and reduced pressures, and Chapter 4 discusses a most important topic: how gases evolve from and within material surfaces. Together, these chapters form the understanding of gauges, pumps, and systems that form the mainstay of vacuum technology as we know it today.

A Users Guide to Vacuum Technology, Fourth Edition. John F. O’Hanlon and Timothy A. Gessert. © 2024 John Wiley & Sons, Inc. Published 2024 by John Wiley & Sons, Inc.

3

1 Vacuum Technology Torricelli is credited with the conceptual understanding of the vacuum within a mercury column by the year 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 liquefying helium and refining organic pumping fluids. They formed the basis of a technology that has made possible everything from incandescent light bulbs to space exploration. The significant discoveries of this early period of vacuum science and technology have been summarized in a number of historical reviews [2,3,4,5,6,7]. The gaseous state can be divided into two fundamental regions. In one region, the distances between adjacent particles are exceedingly small compared to the size of the vessel in which they are contained. We call this the viscous state because gas properties are primarily determined by interactions between nearby particles. The rarefied gas state is a space in which molecules are widely spaced and rarely collide with one another. Instead, they collide with their confining walls. Figure 1.1 sketches this behavior. This is an extremely important distinction that will appear in many discussions throughout this material. A vacuum is a space from which air or other gas has been removed. Of course, it is impossible to remove all gas from a container. The amount removed depends on the application and is done for many reasons. At atmospheric pressure, molecules constantly bombard surfaces. They can bounce from surfaces, attach themselves to surfaces, and even chemically react with surfaces. Air or other surrounding gas can quickly contaminate a clean surface. A clean surface, e.g., a freshly cleaved crystal, A Users Guide to Vacuum Technology, Fourth Edition. John F. O’Hanlon and Timothy A. Gessert. © 2024 John Wiley & Sons, Inc. Published 2024 by John Wiley & Sons, Inc.

4

1  Vacuum Technology

Fig. 1.1  View of a viscous gas and a rarefied gas.

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 molecules to travel from one wall of a chamber to another without myriad collisions with others. By reducing the pressure to a suitably low value, molecules can travel from one wall to another without collision. Many things become possible if they can travel long distances without collisions. Metals can be evaporated from pure sources without reacting in transit. Molecules or atoms can be accelerated to a high energy and sputter away or be implanted in a surface. Electrons or ions can be scattered from surfaces and be collected. The energy changes they undergo on scattering or release from a surface are used to probe or analyze surfaces and underlying layers. For convenience the sub-­atmospheric pressure scale has been divided into several ranges that are listed in Table 1.1. The ranges in this table are not so arbitrary; rather, they are a concise statement of the materials, methods, and equipment necessary to achieve the degree of vacuum needed for a given vacuum process. The required degree of vacuum depends on the application. 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 low-­and high-­tech devices such as microwave, power, cathode ray and photomultiplier tubes, light bulbs, architectural and automotive glass, decorative packaging, and processes including 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

1  Vacuum Technology 

Table 1.1  ISO Definition of Vacuum Pressure Ranges and Descriptions The reasoning for the definition of the ranges is as follows (typical circumstances):

Pressure Ranges

Definition

Prevailing atm. pressure (31–110 kPa) to 100 Pa (232–825 to 0.75 Torr)

Low (rough) vacuum

Pressure can be achieved by simple materials (e.g., regular steel) and positive displacement vacuum pumps; viscous flow regime for gases