Ultrahigh Vacuum Practice

Ultrahigh vacuum practice G. F. Weston, MSC, Finstp Philips Research Laboratories Butterworths London · Boston · Durban · Singapore · Sydney · Toronto · Wellington All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright holder, applications for which should be addressed to the Publishers. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature. This book is sold subject to the Standard Conditions of Sale of Net Books and may not be re-sold in the UK below the net price given by the Publishers in their current price list. First published 1985 © Butterworth & Co. (Publishers) Ltd 1985 British Library Cataloguing in Publication Data Weston, G.F. Ultrahigh vacuum practice. 1. Vacuum technology I. Title 621.55 TJ940 ISBN 0-408-01485-7 Library of Congress Cataloging in Publication Data Weston, G. F. (George Frederick) Ultrahigh vacuum practice. Includes bibliographies and index. 1. Vacuum technology. I. Title. TJ940.W47 1985 621.5'5 84-23885 ISBN 0-408-01485-7 Filmset by Mid-County Press, London SW15 Printed and bound in Great Britain at the University Press, Cambridge Preface In the 1950s vacuum science and technology took a great leap forward. For the first time, pressures below 10~5Pa (10~7torr) could be attained and measured and the term 'ultrahigh' vacuum was coined to cover this low- pressure regime. The initial studies created considerable interest with the result there was a huge upsurge of research and development work in the field. By the 1960s pressures down to 10~n Pa were being reached. Since most of the applications did not require pressures below 10 ~8 Pa, there seemed little need to push the technology any further and for the last twenty years there has been a period of consolidation. Ultrahigh vacuum components based on the designs of the 1950s and 1960s have been developed and engineered into reliable commercial products, so that ultrahigh vacuum can be exploited as a tool to provide the necessary environment for many research, development and production processes. In the 1960s a number of textbooks were published on vacuum technology. Most of them covered the entire vacuum pressure range with limited information on ultrahigh vacuum. The few devoted to ultrahigh vacuum alone, concentrated mainly on the physical principles involved. Since the 1960s there have been hardly any books published on the subject, so that textbook information, particularly on hardware, now tends to be rather dated. In an attempt to fill this gap the author wrote a series of articles for Vacuum in their Educational Series at the beginning of the 1980s, aimed at giving practical information on materials and components such as pumps, gauges, valves, etc. for ultrahigh vacuum applications. The articles were well received, judging by the request for reprints, and it was suggested by his colleagues that the author should gather this information together in a textbook which would then be more readily available to the non-specialist user, who would not be familiar with conference reports and journals on the subject. Thus this book was born. Ultrahigh Vacuum Practice is written mainly for the user or potential user of ultrahigh vacuum equipment, who is not a vacuum expert but is acquainted with general vacuum practice. However, in bringing the information together in one place with extensive references, it is hoped that even the expert vacuum engineer will find it a useful book to have on his shelf. It is basically a practical book, dealing with components suitable for ultrahigh vacuum applications, their theory of operation, their assembly and use and their performance and calibration. In order to obtain maximum performance from vacuum equipment, the users must often have a reasonable understanding of the physical principles involved, so that where necessary background theory has been included. Thus Chapter 1 covers the fundamental principles of vacuum such as adsorption and desorption of gases from surfaces, diffusion, pumping mechanism and gas flow. Chapter 2 discusses materials, their preparation and joining methods necessary for work in this field. In Chapter 3 the various pumps available are detailed with performance and calibration techniques. Chapters 4 and 5 deal with total pressure and partial pressure gauges respectively, whilst Chapter 6 gives details of valves, seals and other vacuum line components. The design of systems using the components described in the previous chapters is discussed in Chapter 7, including also the procedures for making best use of the equipment. Chapter 8 discusses the problems of leak detection and describes suitable leak detectors. The author gratefully acknowledges the cooperation and encouragement of his colleagues in the research laboratories. He is especially indebted to Dr L. G. Pittaway who read the manuscript and made many useful suggestions and corrected a number of errors. G.F.W. 1985 Chapter 1 Fundamentals of vacuum science and technology 1.1 Properties of vacuum Whenever the density of gas in a given volume is reduced below that corresponding to the density of atmospheric gas at ground level, a vacuum is obtained. The greater the reduction of the gas density, the better the vacuum. This reduction of density gives rise to a number of interesting properties which can be exploited in a variety of applications. For example, the chemical activity of atmospheric gas such as oxidation of metals is greatly reduced. Thus, under the protective environment of vacuum, reactive materials can be stored and their special properties utilized. If the density is low enough, surfaces can be kept clean for several hours without even a monolayer of gas depositing on them. Thus, atomically clean surfaces can be investigated and the effects of gas layers determined. The relative absence of molecules in a vacuum allows very small projectiles to pass unimpeded across large distances. This is of particular interest when dealing with charged particles such as electrons, ions and protons whose precise path in the absence of collisions can be controlled by electric and/or magnetic fields. Physical processes such as the passage of heat, of sound and indeed of the gas itself, which take place by interaction between the gas particles under atmospheric conditions, are greatly modified when the gas density is reduced, to the extent that interaction of the gas molecules is no longer the predominant mechanism of transport. The various properties and effects mentioned above obviously depend on the degree of vacuum achieved. Thus the density of the gas remaining in the space serves directly as a measure of the degree of vacuum. However, since historically following on the work of Boyle, the density of a gas was known to be directly proportional to the gas pressure, in practice the gas pressure has become the accepted measure of the degree of vacuum. Modern vacuum techniques can now produce a vacuum in which the pressure has been reduced by a factor of 1015 compared with atmospheric pressure. This is a very large range of vacuum and it is convenient to subdivide it. Figure 1.1 is a chart showing the generally accepted terms for the subdivisions, and the corresponding pressures expressed in Pascals*. Also * The Pascal is the special name for the Newton per square metre and is the SI unit for pressure. Previously mm of mercury or torr have been commonly used and often millibars are used instead of the SI units. A conversion table relating the various pressure units is given in Appendix 3 1 2 Fundamentals of vacuum science and technology Rough vacuum Medi urn vacuum High vacuum Ultra high vacuum 5 4 3 2 -1 -2 -3 -4 -5 -6 -8 -9 -10 10 10 10 10 10 1Pa 10 10 10 10 10 10 10 10 10 10 Pa Mechanical Electronic tubes CRT X-ray hoist Vacuum Photo emission forming Semiconductor Smelting processes processing Electron microscopy Space simulation ^ CD o. Degassing furnace Surface studies — u i_ Φ a Thin film processing O < ~ur uut? ULLtricruiurb Figure 1.1 Vacuum scale and applications included in Figure 1.1 are some examples of practical applications which utilize the special properties of a vacuum. The application to mechanical hoists arises not because of any special property of a vacuum, but merely because the pressure difference which exists across the walls of a vacuum container can give rise to large forces which can be harnessed in mechanical devices by proper design. The exploitation of the vacuum properties outlined above thus requires provision of the correct vacuum environment and this in turn requires employing the correct equipment in a satisfactorily designed system. To design and derive the optimum performance from a vacuum system, the engineer not only needs to know the performance of the equipment but also the factors which will affect it. It is not enough to know that a pump has a speed of say 10_1m3s_1 and can reach an ultimate pressure of 10~6 Pa. In a badly designed system the performance can fall below the optimum by more than an order of magnitude. Indeed, he needs to understand the fundamental principles of vacuum technology to get the ultimate from his equipment. This is particularly true in the ultrahigh vacuum region (pressures below 10 ~6 Pa) where, for example, the number of gas molecules adsorbed on the surface of the vacuum chamber can far exceed the number in the space itself. This chapter deals with these fundamental principles. The subject is covered fairly briefly as a background to the chapters that follow, and for a more detailed presentation the reader is referred to the excellent book by Redhead et al.1.

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