Ultraviolet Laser Technology and Applications

350 Pages·1995·18.063 MB·English

by  David L. Elliott (Auth.)

ULTRAVIOLET
LASER
TECHNOLOGY
AND APPLICATIONS
David J. Elliott
Excimer Laser Systems
Wayland, Massachusetts
San Diego New York Boston London Sydney Tokyo Toronto
This book is printed on acid-free paper, fe)
Copyright © 1995 by ACADEMIC PRESS, INC.
All Rights Reserved.
No part of this publication may be reproduced or transmitted in any form or by any
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Academic Press, Inc.
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United Kingdom Edition published by
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Library of Congress Catalog!ng-in-Publication Data
Elliott, David, date.
Ultraviolet laser technology and applications / by David Elliott
p. cm.
Includes index.
ISBN 0-12-237070-8 (alk. paper)
1. Lasers. 2. Ultraviolel radiation. I. Title.
TA1677.E55 1995
621.36'6--dc20 95-20628
PRINTED IN THE UNITED STATES OF AMERICA
95 96 97 98 99 00 EB 9 8 7 6 5 4 3 2 1
Preface
Ultraviolet laser technology is evolving from a research phase to commercial
use in a wide variety of fields. Practical applications for UV laser energy
have emerged in medicine, semiconductor processing, optical communica
tions, micromachining, and other fields. Commercial deep-UV laser delivery
systems are being manufactured to perform a variety of commercial prod
ucts.
Increased activity and growth in UV technology are results of laser re
search technology being reduced to practice. The unique interaction mecha
nisms of UV laser energy with a wide variety of materials have been studied
worldwide in many R&D labs, and published literature on ultraviolet laser
phenomenon is rapidly expanding. UV laser technology provides unique
functional features that include high resolution, athermal processing, and
precise microstructuring and micropatterning capabilities. These practical
capabilities have been recognized are now in demand in several fields.
Technical progress in UV optics, coatings, and UV laser source technolo
gies has made it possible for the technology to expand to provide useful
products. A summary of the fields where UV laser technology is being ap-
XI
plied, specific applications, and key properties are listed below.
Field Application Key Property
Medicine Corneal Milling Athermal Reactions Surgery
Microsurgery Sharp, Precise Cuts
Semiconductors Photolithography Submicron Resolution
Packaging Direct Structuring of
Metals and Plastics
Cleaning Ablation of Particle
and Film Contamination
Optical
Communication Waveguide Precision Polymer
Fabrication Ablation and Micro-
cutting
Fiber Networks Microstructuring of
Glass and Plastic
Materials
Research Mass Spec Ablation Reactions
Magnetics/Circuits
Micromachining Miniature Flow Direct Structuring
Motors/Devices by Ablation
The overall aim of this book is to provide a practical, "hands-on" refer
ence source on the basic technical areas and applications of UV laser technol
ogy. This book is intended for researchers, technologists, process engineers,
students, business managers, and newcomers to the field who require a basic
introduction to the field. Each chapter deals with a different aspect of UV
laser technology, beginning with UV light itself, moving through the optics,
sources, and systems, followed by detailed descriptions of applications in
various fields.
Chapter 1, "Ultraviolet Light," deals with the nature of UV energy and
UV sources. Chapter 2, "Ablation", discusses the phenomenon of ablation,
physics of the many reactions involved in ablation, and the by-products.
Chapter 3, "The Excimer Laser," describes the primary light source for in
tense deep-UV radiation. Chapter 4, "Materials Research," reviews primary
areas of UV laser research, such as spectroscopy, UV polymer research, and
Xll
UV biology. Chapter 5, "UV Optics and Coatings," covers the main optical
materials and coatings for UV laser beam delivery. Chapter 6, "UV Laser
Cleaning," deals with surface treatment using UV energy and chemical inter
actions. Chapter 7, "Annealing and Planarization," discusses the uses of UV
laser energy for surface melting and planarizing in semiconductor technology;
microlithography, another microelectronic application, is treated separately
in Chapter 8. Chapter 8 on "Deep-UV Microlithography" describes the use
of UV lasers and optics in a wafer stepper system used for imaging advance
IC devices; Chapter 9, "Micromachining," explains the use of UV lasers for
machining small structures in various materials; and Chapter 10 reviews
medical applications of UV laser technology.
A glossary of terms and a reading bibliography are included at the end
of each chapter.
Xlll
Acknowledgments
I wish to thank a number of people who have contributed in different ways
to the writing of this book, namely: Bernhard Piwczyk, whose vision and
enthusiasm drew me into the field; Dr. Herbert Pummer, Jack Andrellos,
Dr. Gerard Zaal, and Dr. Dirk Basting for introducing me to excimer
lasers at Lambda Physik; Dr. Uday K. Sengupta, Bob Akins, Dan Wilson,
and others at Cymer Laser Technologies for guidance and training on litho
graphic lasers; Glen P. Callahan and Dr. Bruce Flint of Acton Research for
information on UV coatings; Dr. R. Srinivasan for information on ablation;
Dr. Daniel J. Ehrlich of MIT, Lincoln Labs, for references to his pioneering
work in the field; Dr. David C. Shaver of MIT, Lincoln Labs, for technical
information and assistance in developing a deep-UV lithography tool; Dr.
Bruce W. Smith of RIT for providing our first excimer laser; Dr. Richard
F. Hollman, while Chief Scientist at Excimer Laser Systems and later at
UV Tech Systems Inc., for many hours of cogitation and effort on deep-
UV optical systems; Frank M. Yans and Dan K. Singer for working closely
with me in the development of new deep-UV surface treatment technology
at UVTech; Mr. George D. Whitten for continued friendship and inspira
tion; John Frakenthaler for help in computer searching, Amanda Peacock
and Deanna Sklenak for their dilligent effort in preparing the manuscript;
and the staff at Academic Press for their editing, technical assistance, and
patience.
I also wish to thank the many authors, technicians, process engineers,
managers, and other professionals in the field of UV laser technology whose
data, papers, illustrations, references, and helpful discussions made the writ
ing of this book possible.
Boston, March 1995
David J. Elliott
xv
Chapter 1
Ultraviolet Light
1.1 Introduction
Ultraviolet light has become more important in recent years as the various
technologies necessary to provide practical UV laser imaging and beam de
livery systems have progressed, especially the light sources. Historically, UV
light has been available only from relatively low power lamps, thereby re
stricting the usefulness of the technology. The discovery and development
of the excimer laser in the 1980s made possible, for the first time, the avail
ability of intense ultraviolet light. Researchers explored and uncovered the
unique properties of this new light source. As various phenomena involving
UV energy and material interactions were discovered and optimized, practi
cal applications emerged. In this chapter, we discuss the UV spectrum, early
UV lamp sources, laser physics and operating principles, and development
leading to the discovery of the UV laser.
1.2 The Ultraviolet Spectrum
The "ultraviolet" is a small portion of the electromagnetic spectrum, yet
within its boundaries there are three distinct regions (near, mid, and far
UV), each with particular significance in terms of applications and proper
ties. Ultraviolet light is roughly defined as that portion (400- to 100-nm
wavelengths) of the electromagnetic spectrum between longer wavelength
visible light (400-700 nm) and shorter wavelength x-ray (10-100 nm) energy.
Figure 1.1 shows the ultraviolet, visible, and infrared spectrums, indicating
the various emission lines (1).
1
2 CHAPTER 1. ULTRAVIOLET LIGHT
1
- NR EINAFDR ~A RE MID-IDN F' RAR- EFAR-D I-NFRARE
(lun)i Kta
J__J L _l J !_ .J L L
CW Dey
40- 100m00 n
(tun) able
CO O 2 C
5-7> |i« » 10.8M»
1 NdG: Y'A (9-11.8H
10m64 n
15m7 / n (132) 0nm
Figure 1.1: Laser Spectrum
1.2.1 Far or Vacuum UV
The shortest wavelength portion of the ultraviolet region of the electromag
netic spectrum is the "far UV" or "vacuum UV" (VUV), which is roughly
the region from 100 to 200 nm. At these very shortest of the UV wave
lengths, air becomes opaque, requiring that experiments be performed in
a vacuum (or inert gas) so that the air does not absorb all the UV light.
In commercial UV optical delivery systems, far or vacuum UV wavelengths
must be contained in inert gas (argon, nitrogen) -purged beam containment
tubes. If this is not provided, considerable energy losses will occur from air
molecules absorbing the UV photons.
1.2.2 Deep-UV
The "deep-UV" portion of the ultraviolet spectrum is the region from ap
proximately 180 to 280 nm, so-called because it is the deepest area of the
ultraviolet where practical UV imaging is routinely done. It is also the deep
est part of the UV spectrum where work can be done at atmosphere without
side efforts.
1.2.3 Mid-UV
The "mid-UV" is the region of the ultraviolet spectrum from 280 to 315 nm,
so-called because it is midway between deep-UV and near-UV.
1.3. HISTORICAL DEVELOPMENT OF THE LASER 3
1.2.4 Near-UV
The "near-UV" is the region from 315 to 400 nm, so-called because it is
nearest to the visible portion of the electromagnetic spectrum.
1.3 Historical Development of the Laser
Before UV lasers, there were long wavelength, red lasers. In this section,
we will discuss the origins of the UV laser back to the initial concepts of
amplification of wave energy. The origins of the laser (Light Amplification
by Stimulated Emission of Radiation) are traced back to the early 1900s
when Albert Einstein, Niels Bohr, Max Plank, and Ernest Rutherford were
developing theories about the nature and behavior of matter.
Rutherford formed the idea of the atom composed of a nucleus with or
biting electrons. The electron orbits occurred at various energy levels. Plank
developed the idea of electromagnetic waves as the form taken by radiant
energy, and further specified that each frequency had a fixed quantum or
amount of energy.
Niels Bohr described the phenomenon of fluorescence or spontaneous
emission. This occurs when an atomic electron drops from a high energy
level to an unoccupied lower energy level. When this happens, a quantum
of light is emitted spontaneously.
Einstein theorized that other forms of emission were possible and formu
lated the idea of stimulated emission. Einstein predicted that when energy
was applied to atoms, the response would be emission of energy.
These scientific theories remained to be proven in the lab, and during
World War II, another step was taken to identify what became known as
"lasing action" or lasers. High frequency radiowave and microwave oscilla
tors were developed that could generate electromagnetic waves of very high
frequencies and short wavelengths. The RADAR (Radio Detection And
Ranging) was a useful result of this work. The Microwave Amplification by
the Stimulated Emission of Radiation (MASER) was also developed before
the laser and proved the theory of population inversion.
An analogy of how light behaves in a laser can be taken from how sound
behaves in an amplifier. If a loudspeaker is placed too close to a micro
phone, the sound from the speaker is reamplified through the microphone
and produces a howl. The howl is caused by sound waves, placed at closely
spaced intervals, reinforcing each other by being "in phase." At a certain
level, an oscillation begins which produces the howl (3). In the following
4 CHAPTER 1. ULTRAVIOLET LIGHT
section, we will explain how this same principle works with light to produce
a laser beam.
1.4 How a Laser Works
Lasers are possible because of the way light interacts with atoms and mole
cules. To describe how a laser works, we will first review some basic aspects
of light/matter interaction. The electrons in an atom or molecule exist in
very specific energy levels, called "states." Each atom possesses electrons
that are characteristic to the specific element, or combination of elements
(molecules) represented.
1.4.1 Energy State Transitions
When an electron moves from one state to a lower energy level, or state,
the atom gives up the excess energy as a "photon" of electromagnetic ra
diation (either light or x-rays). The amount of energy carried away equals
the difference in the energy between the original higher state and the new
lower state. In molecules, where these movements (transitions) between
states involve motions of entire atoms rather than single electrons, the same
phenomenon generally produces lower-energy photons (infrared light).
1.4.2 High and Low Energy Photons
The energy carried by a photon is determined by how rapidly the light waves
in it oscillate, and this oscillation is measured either as the frequency (num
ber of oscillations per second) or wavelength (distance that the waves move
during one oscillation). Wavelength and frequency are related to each other
by the speed at which the photons (waves) move—the speed of light. The
energy of a photon (or, equivalently, the frequency or wavelength) deter
mines its color. Blue light photons have more energy-a higher frequency
and a shorter wavelength-than red light photons.
1.4.3 Spontaneous Emission
An electron in a particular state can also absorb a photon that has an energy
equal to the difTerence between that state and a higher one. As a result, it
can jump to the higher one (called an excited state), where it stays for a
period of time before giving up energy by radiating a photon of the same or