Laser Processing and Chemistry

656 Pages·1996·24.333 MB·English

by  Professor Dr. Dieter Bäuerle (auth.)

Laser Processing and Chemistry
Springer-Verlag Berlin Heidelberg GmbH
Dieter Bauerle
Laser Processing
and Chemistry
Second Edition
With 262 Figures
Springer
Professor Dr. Dieter Bauerle
Institut fUr Angewandte Physik
Johannes-Kepler-Universitiit Linz
A-4040 Linz, Austria
Title of the first edition: Chemical Processing with Lasers,
Springer Series in Material Sciences, Vol. 1
ISBN 978-3-662-03255-8 ISBN 978-3-662-03253-4 (eBook)
DOI 10.1007/978-3-662-03253-4
Library of Congress Cataloging-in-Publication Data. Biiuerle, D. (Dieter), 1940- Laser proces
sing and chemistryjDieter Biiuerle. p. cm. Includes bibliographical references. ISBN 3-540-
60541-X (hardcover:alk. paper) 1. Laser-Industrial applications. 2. Surfaces-Effect of radiation
on. 3. Materials-Effect of radiation on. 4. Surface chemistry-effect of radiation on. I. Title.
TA1677.B39 1996 660'.293'028-dc20 96-95
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© Springer-Verlag Berlin Heidelberg 1986, 1996
Originally published by Springer-Verlag Berlin Heidelberg New York in 1996
Softcover reprint of the hardcover 2nd edition 1996
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For Anne, Christoph, and Horst
Preface
Materials processing with lasers is an expanding field which is
captivating the attention of scientists, engineers, and manufacturers
alike. The aspect of most interest to scientists is the basic interac
tion mechanisms between the intense light of a laser and materials
exposed to a chemically reactive or nonreactive surrounding me
dium. Engineers and manufacturers see in the laser a tool which
will not only make manufacturing cheaper, faster, cleaner, and
more accurate but also open up entirely new technologies and
manufacturing methods that are simply not available using stan
dard techniques. The most established applications are laser ma
chining (cutting, drilling, shaping) and laser welding. Increasingly,
however, lasers are also being used for surface hardening, an
nealing, and glazing. Laser chemical processing (micro-patterning
and extended-area processing by laser-induced etching, material
deposition, chemical transformation, etc.) has actual and potential
applications in micromechanics, metallurgy, integrated optics, semi
conductor manufacture, and chemical engineering.
This book concentrates on various aspects of laser-matter inter
actions, in particular with regard to laser material processing.
Special attention is given to laser-induced chemical reactions at
gas-, liquid-, and solid-solid interfaces. The intention is to give
scientists, engineers, and manufacturers an overview of the extent
to which new developments in laser processing are understood at
present, of the various new possibilities, and of the limitations of
laser techniques. Students may prefer to read the book selectively,
not troubling themselves unduly with detailed calculations or des
criptions of single processes.
The book is divided into six parts, each of which consists in turn
of several chapters. The main symbols, conversion factors, abbrevi
ations, acronyms, and mathematical functions and relations used
throughout the text are listed in Appendices A.1-A.3. The different
materials investigated are listed in Appendices B.1-B.1O. These give
readers a quick and comprehensive overview of the "state of the
art" and direct them to the original literature of a particular area of
interest. Tables I-V are intended to encourage the reader to use the
VIII Preface
formulas presented for rapid estimation of various quantities. An
extensive subject index can be found at the end of the book.
I wish to thank my students and all my staff for valuable dis
cussions and critical reading of various parts of the manuscript. I
am deeply indebted to Dr. N. Arnold and Prof. B. Luk'yanchuk for
many suggestions for improvements to the manuscript. Last but
not least, I wish to express my deep gratitude to my secretary,
Dipl.-Ing. Irmengard Haslinger, for her tireless assistance in writing
this book.
Linz, July 1995 Dieter Bauerle
Contents
Part I: Overview and Fundamentals 1
1 Introduction 3
1.1 Conventional Laser Processing 4
1.2 Laser Chemical Processing ... 7
2 Thermal, Photophysical, and Photochemical Processes . .. 13
2.1 Excitation Mechanisms, Relaxation Times . 13
2.2 The Heat Equation ... . . . . . . . 19
2.2.1 The Source Term ............ 19
2.2.2 Dimensionality of Heat Flow ..... 21
2.2.3 Kirchhoff and Crank Transforms ... 21
2.2.4 Phase Changes .............. 22
2.2.5 Limits of Validity ........ 23
2.3 Selective Excitations of Molecules .. 25
2.3.1 Electronic Excitations ...... 26
2.3.2 Infrared Vibrational Excitations 29
2.4 Surface Excitations ..... 34
2.4.1 External Photoeffect ....... 34
2.4.2 Internal Photoeffect ....... 35
2.4.3 Electromagnetic Field Enhancement,
Catalytic Effects ... 37
2.4.4 Adsorbed Molecules ........ 37
3 Reaction Kinetics and Transport of Species 39
3.1 Photothermal Reactions .... 41
3.2 Photochemical Reactions . . . . 43
. 3.3 The Concentration of Species . 44
3.3.1 Basic Equations . . . . . . 45
3.3.2 Dependence of Coefficients on Temperature
and Concentration .. . . . . . . 49
3.4 Heterogeneous Reactions . . . . . . . . 51
3.4.1 Stationary Equations . . . . . . . 52
x Contents
3.4.2 Transport Limitations 53
3.4.3 Dynamic Solutions 57
3.4.4 Heterogeneous Versus
Homogeneous Activation 58
3.5 Combined Heterogeneous
and Homogeneous Reactions . 59
3.5.1 The Boundary-Value Problem 59
3.5.2 Approximate Solutions .... 60
3.6 Homogeneous Photochemical Activation 61
4 Nucleation and Cluster Formation . 62
4.1 Homogeneous Processes .... 62
4.1.1 Classical Kinetics 63
4.1.2 Droplets within a Laser Beam 65
4.1.3 Transport of Clusters, Thermophoresis,
Chemophoresis ....... . 68
4.2 Heterogeneous Processes .... . 69
4.2.1 Nucleation in Laser-CVD . 69
4.2.2 Coalescence ......... . 71
4.2.3 Liquid-Solid and Solid-Solid Interfaces . 72
5 Lasers, Experimental Aspects, Spatial Confinement 73
5.1 Lasers 73
5.1.1 CW Lasers, Gaussian Beams ...... 73
5.1.2 Pulsed and High-Power CW Lasers . 75
5.2 Experimental Aspects ............. 76
5.2.1 Microprocessing ............. 77
5.2.2 The Reaction Chamber; Typical Setup 79
5.2.3 Large-Area Processing 80
5.2.4 Substrates ......... 81
5.3 Confinement of the Excitation 82
5.3.1 The Thermal Field .. . . 83
5.3.2 Non-thermal Substrate Excitations 83
5.3.3 Gas-, Liquid- and Adsorbed-Phase Excitations . 83
5.3.4 Plasma Formation 84
5.3.5 Material Damages 84
5.3.6 Nonlinearities 84
Part II: Temperature Distributions and Surface Melting ... 89
6 General Solutions of the Heat Equation 91
6.1 The Boundary-Value Problem ... 91
6.1.1 The Attenuation Function J(z) 92
Contents XI
6.1.2 Boundary and Initial Conditions 93
6.2 Analytical Solutions 96
6.3 Pulse Shapes ................ 98
6.4 Beam Shapes ................ 101
6.5 Characteristics of Temperature Distributions . 102
6.6 Numerical Techniques .............. 104
7 Semi-infinite Substrates .. 106
7.1 The Center Temperature Rise ..... . 106
7.2 Stationary Solutions
for Temperature-Independent Parameters . 108
7.2.1 Surface Absorption 109
7.2.2 Finite Absorption .......... . 110
7.3 Stationary Solutions
for Temperature-Dependent Parameters 113
7.4 Scanned CW Laser ........... . 115
7.5 Pulsed-Laser Irradiation ........ . 117
7.5.1 Gaussian Intensity Profile ... . 117
7.5.2 Uniform Irradiation ....... . 119
7.6 Dynamic Solutions
for Temperature-Dependent Parameters 120
8 Infinite Slabs 124
8.1 Strong Absorption ...... 124
8.1.1 Thermally Thin Film . 124
8.1.2 Scanned CW Laser ... 125
8.2 The Influence of Interferences 128
8.3 Coupling of Optical and Thermal Properties . 130
8.4 Average Temperature Distributions ...... 131
9 Non-uniform Media ........... . 132
9.1 Continuous Changes in Optical Properties .. 132
9.2 Absorption of Light in Multilayer Structures . 133
9.2.1 Thin Films ....... 134
9.2.2 Two-Layer Structures .............. 136
9.2.3 Three-Layer Systems ............... 137
9.3 Temperature Distributions for Large-Area Irradiation 137
9.3.1 Stationary Solutions for Thin Films ...... 137
9.3.2 Dynamic Solutions ................ 139
9.4 Temperature Distributions for Focused Irradiation 140
9.4.1 Strong Film Absorption ...... 141
9.4.2 Finite Film Absorption ...... 142
9.5 The Ambient Medium .......... 144