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Glow Discharge Processes

Glow Discharge Processes SPUTTERING AND PLASMA ETCHING

Brian Chapman

A WILEY-INTERSCIENCE PUBLICATION JOHN WILEY & SONS, New York Chichester Brisbane Toronto

Copyright 1980 by John Wiley & Sons, Inc.

Reproduction or translation of any part of this work beyond that permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permi:

QC702.7.P6C48 537..V2 80-17047 ISBN 0-471-07828-X

Printed in the United States of America

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Preface

This book is based on a series of seminars held in 1978 and 1979. The seminars were intended to give some more insight into several practical glow discharge processes that are being increasingly used, particularly in the semiconductor industry. I hope that the text will serve as a useful general introduction to some of the scientific principles involved in these processes.

Glow discharges, like so many topics in science, are incompletely understood. Results are often misinterpreted, contradictory, or irrelevant. Glow discharge science has its own sub-language of special terms, with names that are often misleading, and with meanings which cannot be assumed to be constant from author to author! One can easily understand the need for the precision of scien-tific writing and sympathise with the conditions, provisos and double negatives of the author who is taking care not to make any definite statement which might be wrong. This is probably as scientific literature must be when one is close to the borders of knowledge and ignorance, but it is rather daunting to a newcomer to that particular branch of science.

Many of you will have had the experience of wanting to learn something about a particular area of science you're not familiar with, and so you go along for advice to the chap in your company or university who is considered the local expert. More often than not you come away with a list of references, in just about all of which it is assumed that you know the subject pretty well! And this is a particular problem in multi-disciplinary subjects such as sputtering where you are as likely to meet some electrical engineering phase angles as you are some organic chemistry.

This book is trying to be an introductory book. It attempts to thread a path through all the basic material you need before you can read the much more erudite reviews on the subject. In an effort to spare readers from attacks of mental indigestion, I have selected those aspects which appear to be more useful for first-time acquaintance, and even these are dealt with too briefly.

This text was written for readers with a wide range of backgrounds, and the emphasis is on concepts rather than on rigorous detail. I have usually restricted discussions to the general application of an idea, and have often ignored excep-

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vi PREFACE

tions. So don' t take my arguments and results too literally or too seriously; they are meant to give a qualitative 'feel' for the technology, not to comprise a scholarly tome. It has been my experience, particularly whilst I was teaching at Imperial College, that the qualifications and detail are best left until later when the basic concept has been assimilated. For similar reasons, fine distinctions such as between torr and mmHg, have been ignored. After reading this book, I hope that readers will go on to learn more about the specific aspects of glow discharges that they are interested in, and then will be able to return and pick out all the faults in this book. Apart from the mechanical errors of text preparation, I'm sure there are also errors in my understanding. I would be obliged if readers would write to me care of the publishers, and advise me of such errors; we have a duty not to promote misunderstanding.

In a readable technical book of finite thickness, it's necessary to assume some knowledge of the reader. I've tried to assume only that which should be com-mon knowledge to science and engineering graduates. Further background and other items are discussed in the appendices.

A few words of confession about units. I have tried to use the cgs, imperial and hybrid units commonly encountered in the technology. Unfortunately, in moments of panic, I revert to the MKS system with which I learned. This only adds further confusion to an already tortured situation and I can only apologize and join the guilty. But be reasonable see it my way.

The author of a book is just one of a small army of people who contribute to the innumerable details of its production. In gratitude, first thoughts go to the many people who have established an understanding of plasma physics and glow discharge processes, and without whom this book would not exist. Thanks are due to authors and publishers for permission to use copyrighted material; formal acknowledgment follows.

Harold Winters, John Vossen, John Thornton and John Coburn (Harold is nearly always last, so this time I've inverted the usual order) kindly let me force them into reviewing my work. I thank them for their time and their many help-ful comments, and for their endurance in reading my first manuscript. Others of my colleagues have also been very patient with me.

Many folks, unfortunately too numerous to mention, worked very hard and co-operatively in the mechanical preparation of the book. I thank them all. Sadly, I would like to acknowledge particularly the contribution of the late Don Brown, who prepared most of the artwork.

Don Marchese, Frank Bresnock, Walt Koste, and several of my other friends helped me in more ways than they realise, and were always ready to lend an ear

PREFACE

to my complaints and frustrations. I look forward to reciprocating their kindnesses.

Finally, and most of all, to Carol and Toby. I needed to say nothing to them nor they to me. Say no more! Now we can get back to the plans, the 505 and the radio 'trolled robot.

New Fairfield, Connecticut

Brian Chapman

Reproduction and Copyright Acknowledgments

My thanks are due to the many authors and publishers who have permitted me to reproduce the figures and tables herein. Copyright of the authors or publishers, as relevant, is acknowledged. A list of publishers and related publications follows:

Academic Press (including Adv. Electronics & Electron Phys.) Airco Temescal American Chemical Society American Elsevier Publishing Co. American Institute of Mechanical Engineers (Trans. Met. Soc. AIME) American Institute of Physics (J. Chem. Phys., Physics of Fluids, J. Appl. Phys.,

Appl. Phys. Lett., Rev. Sci. Instrum., J. Vac. Sci. Tech.) American Physical Society (Phys. Rev.) Dover Publications Electrochemical Society (J. Electrochem. Soc.) Heinemann Educational Books IBM Corp. (IBM J. Res. Develop.) Institute of Electrical & Electronics Engineers (Proc. IEEE) Japanese J. Appl. Phys. Joint Institute for Laboratory Astrophysics (JILA) Litton Industries (General Mills Report) McGraw Hill Book Company M.I.T. Press North Holland Publishing Co. (Nucl. Instrum. & Methods) Optical Society of America (Optics & Spectroscopy) Oxford University Press Plenum Publishing Corp. RCA Corp. (RCA Review) Royal Society (Proc. Roy. Soc.) Societe Francaise du Vide (Le Vide, Les Couches Minces) Society for Applied Spectroscopy (Appl. Spectroscopy) Solid State Technology Standard Telecommunication Laboratories Taylor & Francis (Wykeham Publications) Tokuda Seisakusho John Wiley & Sons

ix

Contents

Chapter 1: Gases 1 Masses and Numbers of Atoms 1 Kinetic Energy and Temperature 1 Mean Speed c 2 Maxwell-Boltzmann Distribution 2 Pressure 4 Partial Pressures 6 Pressure Units 6 Avogadro's Laws 6 Number Density of Gases "7 Impingement Flux 8 Monolayer Formation Time 8 Mean Free Path 9 Probability of Collision 10 Collision Frequency 11 Energy Transfer in Binary Collisions 11 Gas Flow 12 Types of Gas Flow 12 Pumping Speed and Throughput 14 Measurement of Gas Flow Rate 16 Residence Time 16 Flow Velocity 16 Conductance 17 General References 19

Chapter 2: Gas Phase Collision Processes 21 Collision Cross Section 22 Elastic and Inelastic Collisions 23 The Main Collision Processes 25

Electron Volts 25 Elastic Collisions 26 Ionization 27 Excitation 31 Relaxation 34 Recombination 35

3 Body Collision 35 In a Two Stage Process 37 Radiative Recombination 37

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CONTENTS xm

Some Other Collision Processes 37 Dissociation 38 Electron Attachment . 38 Ion-Neutral Collisions 39

Charge Transfer 39 Ionization by Ion Impact 42 Ion Chemistry 42

Metastable Collisions 43 Metastable-Neutral Collisions 44 Metastable-Metastable Ionization 44 Electron-Metastable Ionization 44

Total Collision Cross-Section 45 Plasma 46 References 47

Chapte r 3; Plasmas 49 Electron and Ion Temperatures 49 Plasma Potential 51 Sheath Formation at a Floating Substrate ' 53 Deb ye Shielding 57 Probe Characteristics 60

Practical Complications 62 Positively Biased Probes 64

Sheath Formation and The Bohm Criterion . 65 The Floating Potential - Again 69

Plasma Oscillations 70 Electron Oscillations 70 Ion Oscillations 73

Ambipolar Diffusion 73 References 75

Chapter 4: DC Glow Discharges 77 Architecture of the Discharge 78 Maintenance of the Discharge 81 Secondary Electron Emission 82.

Electron Bombardment 82 Ion Bombardment 84 Neutral Bombardment 92 Photon Bombardment 93 Summary 93

The Cathode Region 95 Ionizati