Table Of ContentSpringer Senes in Materials Science 3
Edited by Aram Mooradian
In
Springer Senes Materials Science
Editors: Aram Mooradian Morton B. Panish
Volume 1 Chemical Processing with Lasers
By D. Bauerle
Volume 2 Laser-Beam Interactions with Materials
Physical Principles and Applications
By M. von Allmen
Volume 3 Laser Processing of Thin Films and Microstructures
Oxidation, Deposition and Etching of Insulators
By I. W. Boyd
Volume 4 Microclusters
Editors: S. Sugano, Y. Nishina, and S. Ohnishi
Ian W. Boyd
Laser Processing
of Thin Films
and Microstructures
Oxidation, Deposition and Etching
of Insulators
With 77 Figures
Springer-Verlag Berlin Heidelberg New York
London Paris Tokyo
Dr. Ian W. Boyd
Department of Electronic and Electrical Engineering,
University College London, Torrington Place, London WCIE 7JE, UK
Series Editors:
Dr. Aram Mooradian
Leader of the Quantum Electronics Group, MIT,
Lincoln Laboratory, P.O. Box 73, Lexington, MA 02173, USA
Dr. Morton B. Panish
AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974, USA
ISBN-13 :978-3-642-83138-6 e-ISBN-13 :978-3-642-83136-2
DOl: 10.1007/978-3-642-83136-2
Library of Congress Cataloging-in-Publication Data. Boyd, Ian W., 1958-Laser processing of thin films and
microstructures; oxidation, deposition, and etching of insulators. (Springer series in materials science; 3).
1. Lasers-Industrial applications. I. Title. II. Series.
TA1677.B69 1987 621.36'6 87-20660
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© Springer-Verlag Berlin Heidelberg 1987
Softcover reprint of the hardcover 1st edition 1987
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Preface
This text aims at providing a comprehensive and up to date
treatment of the new and rapidly expanding field of laser pro
cessing of thin films, particularly, though by no means exclu
sively, of recent progress in the dielectrics area. The volume
covers all the major aspects of laser processing technology in
general, from the background and history to its many potential
applications, and from the theory to the necessary experimental
considerations. It highlights and compares the vast array of
processing conditions now available with intense photon beams,
as well as the properties of the films and microstructures pro
duced. Separate chapters deal with the fundamentals of laser
interactions with matter, and with experimental considerations.
Detailed consideration is also given to film deposition, nuclea
tion and growth, oxidation and annealing, as well as selective
and localized. etching and ablation, not only in terms of the
various photon-induced processes, but also with respect to
traditional as well as other competing new technologies.
This monogr,aph came to be written upon the invitation of the
publisher and is based on a variety of lectures and review
papers given on the subject over the past few years. It was
decided to avoid producing a volume containing mainly computer
ized tables and bibliographic listings devoid of the necessary
description and discussion. Thus, the book is intended not only
as a snapshot review but also as an introductory text for those
new to the field. It is written for engineers and scientists
alike, from graduate to senior professional level. The subject
v
is approached on a pedagogical level, rather than as a heavily
specialized high level review. By its nature, the laser pro
cessing field is multidisciplinary, drawing widely from materi
als and laser science, from electronic engineering and inor
ganic chemistry. Therefore an attempt has been made to reduce
the amount of technological jargon and technical terms, and
necessary chemical formulae are defined where they are first
introduced.
Many people have contributed to the production of this book.
Firstly, I would like to thank Dr. H. Lotsch of Springer-Verlag
for his patient assistance and invaluable advice over many
months of preparation. I am also indebted to many of my colle
agues in the field of laser processing for all the valuable
discussions over the past years, and for their kindness in pro
viding original figures and references from their work. Special
thanks go to those who habitually include a thorough reference
list in their papers, and to those who have written review art
icles .... Without these, this book would have taken much longer
to write. I am also grateful to Lindsey Gall for typing earlier
drafts of the manuscript, to Bridget Bradley for making many
last minute adjustments and updates, and also to Betty Smith
for the necessary secretarial back-up.
I am also indebted to colleagues at the Heriot-Watt Univer
sity and ,Hughes Microelectronics Ltd. in Scotland, and at the
North Texas State University, for stimulating discussions and
collaborations, and more recently my colleagues in the depart
ment of Electronic and Electrical Engineering at University
College London for their generous support and encouragement
over the past two years. Finally, I would like to thank my
wife, Ann, for her help during the preparation of this mono
graph, and also for her support and great patience in tolerat
ing the eccentricities and odd hours of an absentminded and
somewhat preoccupied husband over the past few years.
London, J'uly 1987 l.W. Boyd
VI
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1
1.1 Historical Background ............................ 1
1.2 Advantages of Laser Technology. . . . . . . . . . . . . . . . . . .. 5
1.3 Requirements for Laser Processing ................. 9
1.4 OUtline ....................................... 13
2. Interaction and Kinetics .......................... 15
2.1 Laser Excitation of Matter ...................... 15
2.2 Laser Excitation of the Gas Species ............... 17
2.2.1 Selective Vibrational Excitation ............ 18
2.2.2 Selective Electronic Excitation ............. 22
2.3 Interaction of Laser Radiation with Solids ......... 26
2.3.1 Metals .....................•............. 26
2.3.2 Insulators and Semiconductors .............. 32
2.3.3 Non linear Optical Absorption ............... 47
2.3.4 Plasma Formation .......................... 48
2.4 Interactions with Surfaces and Adsorbates ......... 49
2.4.1 Adsorbates ............................... 49
2.4.2 Desorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 51
2.4.3 Adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 57
2.5 Laser-Induced Heating ........................... 64
2.5.1 Thermalization ............................ 64
2.5.2 Heating Models ............................ 66
2.5.3 Impurity Incorporation ..................... 82
2.6 Nucleation and Growth. .. . . . .. . . . . . . .. . . . . . . . . . .. 85
2.7 Chemical Reactions and Growth Rates .............. 89
2.7.1 Low Intensity Levels....................... 90
2.7.2 High Intensity Levels ...................... 93
2.7.3 cW Laser Controlled Reaction Rates .......... 97
3. Experimental Considerations ..................... 100
3.1 Properties of Laser Beams . . . . . . . . . . . . . . . . . . . .. 100
3.2 Spatial Resolution. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 107
3.3 Modes of Laser Processing ...................... 111
3.3.1 Geometrical Configurations ................ 111
3.3.2 Pattern Generation ....................... 118
3.3.3 Process Uniformity and Reproducibility ...... 122
3.3.4 Beam Profile Measurements ................. 127
3.4 The Choice of Laser ............................ 128
VII
4. Laser-Assisted Oxidation and lIitridation .......... 134
4.1 Oxidation. . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 134
4.2 Background and Theory ..................•... 135
4.3 Metal Oxidation ............................ 148
4.4 Silicon Oxidation .............................. 154
4.4.1 Alternative Processing Techniques •..•..••.. 155
4.4.2 Laser-induced Growth ..•.•.........•...... 157
4.4.3 Mechanisms of Photonically Enhanced Oxidation 164
4.4.4 Rapid Thermal Oxidation •.•...•.••.......•. 171
4.5 Oxidation of Compound Semiconductors ...•........ 180
4.6 Nitridation .•....•................•..•.••..... 184
4.6.1 Thermal Nitridation. . . . .. . .. . .. . .•. . . . . . .. 184
4.6.2 Laser Nitridation. . . . . • . • . . . • . . . . . . • . . . . .• 186
4.7 Laser Curing .•..••.......•...•..•...•.....•... 188
5. Passivation by Laser Annealing and Melting. ... . . •. 190
5.1 Modes of Laser Annealing ......•....•......•.... 190
5.2 Laser Annealing in Oxygen. . . . . . . . . . . . . . . . . . • . . .. 193
5.3 Oxygen Implantation. . • • . • • . . . . . . • . . . . . . . . . . . . .• 203
5.4 Nitrogen Implantation ..............•........... 204
5.5 Impurity Effects .....•............•..•..••...• 205
5.6 Laser Cleaning of Surfaces .••.•....••.•..•..... 206
6. Laser-Induced Deposition ..............•.......... 209
6.1 Background .........................•......... 209
6.2 Metal Oxides .......•..•....................... 213
6.3 Silicon Oxide ................................. 223
6.4 Silicon Nitride ................................ 229
6.5 Organic Polymer Formation ...................... 232
"1. Material Reaoval ...............•................ 236
7.1 Introduction and Background . . . . . . . . . . . . . . . . . . . .• 236
7.2 Etching ......•............................... 240
7.3 Ablation .••...............••...•..•.......... 252
7.4 Trimming ..•...•..........•.•.....•••.••••••.. 266
7.5 Cut'ting and Drilling •.......................... 268
8. Sua.aryand Conclusions ..•...................... 272
8.1 Properties and Applications. . . . . • . . . . . . . . • . . . . .. 272
8.2 Fut;ure Prospects ..•••••••••.••...•......•..•.. 279
8.3 Postscript •..•.........•...................... 286
References .•.....•.••.•.•..•.•...............•.... 287
Subject Index .•••.•..•..............•....•..•...... 315
VIII
1. Introduction
This introductory chapter is aimed at providing the reader with
the background as to why laser processing has attracted so
much attention throughout the world in recent years. After a
brief summary of the history of the field in relation to the
more traditional industrial applications of lasers in cutting,
drilling and welding, the special advantages afforded by laser
processing are discussed. Here, the unique properties of lasers
are introduced, and extended to show how special new processing
conditions can be achieved. In this respect, several areas of
potential application of these new processing procedures, par
ticularly in microfabrication technology, are introduced,
1.1 Historical Background
If one examines the present market-place closely, it is clear
that alongside research and development, the area of materials
processing is the largest application of lasers, regularly cap
turing some 20% of the dollar sales in recent years. Further
more, this field is strongly expected to continue increasing
very rapidly during the remainder of this century [1.1].
The reason for this optimistic forecast is the ever increas
ing demand from manufacturers for production techniques that
are cheaper and faster, or more precise than existing manufac
turing methods. Additionally, the laser offers a most unique
set of processing parameters that are not available with other
existing technologies, such that new structures and new materi
als can be formed. Also, in terms of application on the produc-
1
tion lines, they are completely compatible with the control
technology of existing modern digital electronics.
In the field of materials processing, lasers have already
found applications in welding, bonding, cutting, drilling, and
shaping of a wide variety of materials, including various
metals, ceramics and cloth [1.2-8]. There is now an additional
and increasing interest in the many real and potential applica
tions in areas of high technology such as micromechanics, in
tegrated optics, semiconductor device fabrication, and various
aspects of chemical processing [1.8-22].
Materials processing with lasers can be divided into two
categories. Firstly, the area of non-reactive processing, where
the energy from the laser beam serves to modify in some way
the structure of the workpiece. Phase changes, permanent or
otherwise, may be involved but basically there will be no new
compounds present in the final product. This includes tech
niques such as cutting, marking and hardening, as well as laser
annealing, where the original defective structure of a material
can be reorganised into an ordered crystalline form in periods
of the order of microseconds and less. The opposite phase
transformation, that from the crystal to an amorphous struc
ture, is also possible using laser radiation [1.23]. These truly
remarkable laser-induced phase transitions were the subject of
intense investigations worldwide in the mid-1910's and early
1980's in the area of laser interactions with semiconductors
[1.8-12]. In fact, the high level of activity in recent years in
this area.of laser interactions with and modifications of thin
films is a consequence of the publication of a series of con
trolled experiments in laser annealing during the 1910's by
several Soviet research groups [1.24-21]. Figure 1.1, for
example, shows how the initial attraction of laser annealing
has decreased somewhat over the 1980's from a peak around
1981. In contrast to this trend, there has been a corresponding
upsurge in interest in related areas of laser processing of the
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