Plasma Diagnostics. Discharge Parameters and Chemistry

458 Pages·1989·9.193 MB·English

by  Orlando Auciello and Daniel L. Flamm (Eds.)

Plasma-Materials Interactions
A Series Edited by
Orlando Auciello Daniel L. Flamm
Microelectronics Center of AT&T Bell Laboratories
North Carolina and Murray Hill, New Jersey
North Carolina State University
Research Triangle Park, North Carolina
Advisory Board
J. L. Cecchi W. O. Hofer
Plasma Physics Laboratory Institut fiir Grenzflachenforschung
Princeton University und Vakuumphysik
Princeton, New Jersey Julich, Federal Republic of Germany
A. E. deVries N. Itoh
FOM-Instituut voor Department of Crystalline
Atoom-En Molecuulfysica Materials Science
A msterdam- Watergraafasmeer Nagoya University
The Netherlands Nagoya, Japan
H. F. Winters G. M. McCracken
IBM, Almaden Research Center Culham Laboratory
San Jose, California Abingdon, Oxfordshire
United Kingdom
A list of titles in this series appears at the end of this volume.
Plasma Diagnostics
Volume 1
Discharge Parameters and Chemistry
Edited by
Orlando Auciello
Microelectronics Center of
North Carolina and
North Carolina State University
Research Triangle Park, North Carolina
Daniel L. Flamm
A T&T Bell Laboratories
Murray Hill, New Jersey
®
ACADEMIC PRESS, INC.
Harcourt Brace Jovanovich, Publishers
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Copyright © 1989 by Academic Press, Inc.
All rights reserved.
No part of this publication may be reproduced or
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or mechanical, including photocopy, recording, or
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permission in writing from the publisher.
ACADEMIC PRESS, INC.
1250 Sixth Avenue, San Diego, CA 92101
United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24-28 Oval Road, London NW1 7DX
Library of Congress Cataloging-in-Publication Data
Plasma diagnostics.
(Plasma-materials interactions)
Includes bibliographies and index.
Contents: v. 1. Discharge parameters and chemistry
—v. 2. Surface analysis and interactions.
1. Plasma diagnostics. I. Auciello, Orlando,
Date- II. Flamm, Daniel L. III. Series.
QC718.5.D5P54 1988 530.41 87-35161
ISBN 0-12-067635-4 (v. 1)
ISBN 0-12-067636-2 (v. 2)
Printed in the United States of America
89 90 91 92 9 8 7 6 5 4 3 2 1
Contributors
Numbers in parentheses refer to the pages on which the authors'
contributions begin.
J. M. COOK (313), AT&T Bell Laboratories, Murray Hill, New
Jersey 07974
J. F. COUDERT (349), Laboratoire Ceramiques Nouvelles,
UA 320 CNRS, University of Limoges, Limoges, France
V. M DONNELLY (1), AT&T Bell Laboratories, Murray Hill, New
Jersey 07974
H. F. DYLLA (185), Plasma Physics Laboratory, Princeton
University, Princeton, New Jersey 08544
P. FAUCHAIS (349), Laboratoire Ceramiques Nouvelles, UA 320
CNRS, University of Limoges, Limoges, France
NOAH HERSHKOWITZ (114), Nuclear Engineering and Engineering
Physics Department, University of Wisconsin-Madison, Madison,
Wisconsin 53706
H. MEUTH (239), Department of Electrical Engineering and
Computer Science, University of California, Berkeley,
California 94720
TERRY A. MILLER (313), Department of Chemistry, Ohio State
University, Columbus, Ohio 43210
E. SEVILLANO (239), MIT Plasma Fusion Center, Cambridge,
Massachusetts 02139
M. VARDELLE (349), Laboratoire Ceramiques Nouvelles,
UA 320 CNRS, University of Limoges, Limoges, France
M. J. VASILE (185), AT&T Bell Laboratories, Murray Hill, New
Jersey 07974
JOHN F. WAYMOUTH (47), GTE Lighting Products, Danvers,
Massachusetts 02254
ix
Preface
The study of plasma-material interactions has evolved into an important
and dynamic field of research. An understanding of the basic physical and
chemical processes underlying these interactions is vital to the development
of microelectronics, surface modification, fusion, space, and other key
technologies of our age. Plasma processing is a critical technology for
leading-edge microelectronics. For example, ultra large scale integrated
circuits (ULSI) cannot be manufactured without plasma-assisted etching
and plasma chemical vapor deposition. Similarly, the various plasma-
surface phenomena—physical sputtering, chemical etching, particle trap­
ping in solid walls etc.—must be understood and controlled to achieve
self-sustained fusion reactions in future commercial power plants. Plasma
interactions with surfaces of spaceships can produce harmful degradation,
such as the undesirable etching of thermal blankets observed in the cargo
bay of the Space Shuttle. These effects could jeopardize long-term missions
in space. All of these problems are now being investigated by scientists and
engineers around the world.
Unfortunately, scientific and technical information in these diverse fields
is often published in journals aimed at a narrow specialized audience. One
of the chief goals of this series on "Plasma-Materials Interactions", which
we are now initiating, is to provide an interdisciplinary forum. We hope to
disseminate knowledge of basic and applied physicochemical processes and
plasma-processing art to the global community. The series is structured to
make this information readily accessible to scientists, engineers, students
and technical personnel in universities, industry, and national laboratories.
We consider plasma-materials interactions to be one of the pivotal fields of
research that will contribute to the technological revolution now under way.
Therefore, we hope that this series will encourage the pursuit of new ideas
and expand the horizons of science and technology in allied interdisci­
plinary fields.
Diagnostics and characterization techniques are prerequisites for under­
standing plasmas and solid surfaces exposed to plasmas. Unfortunately the
XI
xii Preface
necessary know-how is scattered throughout the literature, often in a form
that is difficult to use. Consequently, we begin this series with an authorita­
tive and up-to-date treatment of plasma and surface diagnostics written for
an interdisciplinary audience. The authors are renowned specialists who
explain how to set up, make, and interpret measurements and how to assess
the validity of diagnostic data and detect complications. Finally, they
present the theoretical background necessary to understand each technique
with references to recent literature. Because the material is fairly compre­
hensive, the book is divided into two volumes.
Volume 1 contains seven chapters on the important diagnostic techniques
for plasmas and details their use in particular applications. This part
includes (1) optical diagnostics for low-pressure plasmas and plasma
processing, (2) plasma diagnostics for electrical discharge light sources,
(3) Langmuir probes, (4) mass spectroscopy of plasmas, (5) microwave
diagnostics, (6) paramagnetic resonance diagnostics, and (7) diagnostics in
thermal plasma processing.
Volume 2 covers diagnostics of surfaces exposed to plasmas and includes
chapters on (1) quartz crystal microbalances for studies of plasma-surface
interactions, (2) elemental analysis of treated surfaces by electron and ion
spectroscopies, (3) spectroscopic ellipsometry in plasma processing, (4) ion
beam analysis of plasma-exposed surfaces (Rutherford backscattering, elas­
tic recoil detection, particle-induced X-ray emission and nuclear reaction
analysis), (5) the interpretation of plasma probe data in fusion experiments,
and (6) non-destructive photoacoustic and photothermal techniques for the
analysis of plasma-exposed surfaces.
We hope that these, and subsequent books in this series, will be valuable
to experts and newcomers alike. "Plasma-Materials Interactions" volumes
on plasma etching technology and on plasma deposition and etching of
polymers are now in press. We would welcome your suggestions for future
volumes.
Orlando Auciello
Daniel L. Flamm
January, 1989
1 Optical Diagnostic Techniques for
Low Pressure Plasmas and
Plasma Processing
V. M. Donnelly
AT&T Bell Laboratories
Murray Hill, New Jersey
I. Introduction 1
II. Plasma-Induced Optical Emission 3
III. Absorption Spectroscopy 15
A. Ultraviolet-Visible Absorption 15
B. Infrared Absorption 17
IV. Laser Techniques 19
A. Laser-Induced Fluorescence (LIF). 19
B. Raman Scattering 28
C. Coherent Anti-Stokes Raman Spectroscopy (CARS) 29
D. Tunable Infrared Laser Absorption 30
E. Optogalvanic Effects 34
V. Optical Techniques for Plasma-Surface Interactions 35
A. Laser Interferometry 36
B. Ellipsometry 37
C. Second Harmonic Generation (SHG) 39
D. Laser Raman Scattering From Surfaces 40
E. Surface Absorption and Photoacoustic Spectroscopy 41
VI. Summary 42
VII. Acknowledgments 43
References 43
I. Introduction
Advances in microelectronics technology over the last two decades have
exceeded even the most optimistic expectations. The rapid progress in the
manufacturing of complex devices such as dynamic random access memory
integrated circuits has been made possible, in part, by advances in process­
ing and process control. Low pressure glow discharges are used in many of
PLASMA DIAGNOSTICS 1 Copyright © 1989 by Academic Press, Inc.
Discharge Parameters and Chemistry All rights of reproduction in any form reserved.
ISBN 0-12-067635-4
2 V. M. Donnelly
these steps (Flamm, Donnelly, and Ibbotson, 1984). For example, radio-
frequency plasmas in SiH /N 0 and SiH /NH are used to deposit Si0
4 2 4 3 2
and silicon nitride thin films for applications including gate-oxides, device
isolation and encapsulation, diffusion masks, and etch masks. Oxygen
atoms formed in 0 discharges are employed to clean wafer surfaces and
2
remove photoresists. However, plasma processing has had the largest im­
pact in silicon integrated circuit technology by making it possible to etch
submicron-sized features with vertical side walls in silicon, metals, and
insulators. The precise control of both etching and deposition processes
often requires some in situ diagnostic probe to measure the rates or to
detect the endpoint when a film has been etched through to expose a
sensitive, underlying material. Several optical techniques have been success­
fully applied to this task.
Empirical optimization of plasma parameters (gas composition, pressure,
power, etc.) has produced many successful etching and deposition "recipes."
However, with the advent of increasingly sophisticated diagnostic tech­
niques and computer modeling, it is now possible to obtain very detailed
information on the chemistry and physics of these processing plasmas and
their interactions with surfaces. Of all the diagnostics, optical techniques
have yielded the most detailed information. With the availability of multi­
channel detectors, narrow bandwidth, tunable lasers, Fourier transform IR,
and other state-of-the-art equipment, many studies have already revealed
details of ion dynamics, radical formation and loss processes, and electron
impact excitation mechanisms.
More fundamental studies (Donnelly, Ibbotson, and Flamm, 1984) de­
signed to elucidate mechanisms can be placed in two categories. Since the
plasma is a very complex chemical environment, one ex situ technique is to
isolate part of the etching or deposition process from the discharge.
Discharge flow tube (Donnelly and Flamm, 1980; Flamm, et al., 1981;
Mucha et al, 1981; Ryan and Plumb, 1984; Danner and Hess, 1986) and
molecular beam (Coburn, 1985) experiments have determined many of the
reactions of both neutral and charged species with other gas-phase species
as well as with materials exposed to the discharge. A second, in situ
approach employs discharge conditions comparable to those in the plasma
processing environment but with equipment designed to be amenable to
various diagnostic probes. While these studies are often carried out under
less well controlled conditions, they are nonetheless extremely valuable
because they provide information more closely related to actual processing
conditions.
Optical probes are particularly well suited for in situ plasma diagnostic
studies because they are nonintrusive, species-selective, and can yield both
space- and time-resolved information. This paper will review relevant
Optical Diagnostic Techniques 3
optical diagnostic techniques and cite examples of applications to process
monitoring and fundamental studies. While many of the techniques de­
scribed can be applied to other types of plasmas (e.g., thermal plasmas and
microwave discharges), most of the examples are of low pressure, radio
frequency discharges of the type used in microelectronics materials process­
ing.
II. Plasma-Induced Optical Emission
Optical emission eminating from gas discharges has long been used as a
qualitative diagnostic of plasma physics and chemistry. The segmenting of
emission into bright and dark regions provides information on the motions
of ions and electrons in response to the spatially varying electric fields,
while the identification of the spectrally resolved features establish the
presence of radicals and ions formed by reactions in the discharge.
The apparatus required for this technique is one of the least complicated.
It requires a monochromator to disperse the plasma emission, a set of
optics to image light from the discharge onto the detector, and a photode-
tector to measure the dispersed fluorescence. A 0.25 m focal length
monochromator can provide ~ 0.05 nm resolution, which is sufficient for
most studies. When resolution of atomic fine structure, molecular rotational
levels, or detailed line shape measurements are necessary, a long focal
length monochromator (e.g., 1 m focal length, ~ 0.01 nm resolution) or
Fabre-Perot interferometer (~ 0.0005 nm resolution) can be used. The
Fabre-Perot interferometer must be used in conjunction with a monochro­
mator, since the scanning range of an interferometer is typically less than
1 nm.
Monochromatic light emerging from the exit sht of the monochromator
can be detected with a photomultiplier tube used in an analog (current
measuring) or digital (photon counting) mode. Alternatively, the exit sht
can be replaced with an optical multichannel analyzer (OMA) placed in the
image plane of the exit sht. The advantage of an OMA is that a portion of
the emission spectrum (defined by the monochromator dispersion) can be
recorded simultaneously without scanning the monochromator. The most
sensitive OMAs have a quantum efficiency nearly as good as photomulti­
plier tubes and so can provide much faster data collection. Most species of
interest emit in the spectral region between 200 and 900 nm, where GaAs
and S-20 detectors have adequate sensitivity. Both shorter and longer
wavelength detectors and optics are available. Below - 190 nm, vacuum
enclosures are required to prevent absorption by 0 . Hence, studies in the
2
region are less common. Longer wavelength (> 900 nm) emissions from
4 V. M. Donnelly
electronic transitions are relatively weak and less common and hence not as
useful for most applications. IR emission from vibrationally excited levels
can provide useful information. However, signals are many orders of
magnitude weaker than visible-UV emission, while blackbody background
emission requires all surfaces "seen" by the IR-detector to be cooled,
preferably to liquid nitrogen temperature. Consequently, very few studies
have been published in the IR region.
Light from the discharge can be imaged onto the monochromator en­
trance slit with UV-grade fused silica lenses and UV-coated aluminum
mirrors for best average response between 200 and 900 nm. Care should be
taken in determining what region of the discharge is imaged onto the slit,
since this carries added information and is important in comparing studies
under various operating conditions and, in particular, in different reactor
geometries.
Spectrally resolved optical emission has been reported for a variety of
discharges, under conditions employed in both etching and deposition. For
example, emission from atomic fluorine is observed from CF /0 dis­
4 2
charges (Harshberger et al., 1977; Flamm, 1978; Donnelly et al., 1984).
Intensity increases with 0 content (up to - 50% 0 ) and decreases with
2 2
the area of silicon exposed to the plasma. From these observations one
concludes that oxygen consumes CF^. species to liberate F-atoms, which in
turn react with silicon to form volatile products.
Plasma-induced emission can arise from electron impact excitation,
A + e-+A* + e, (1)
electron impact dissociation,
AB + e ->A* + B + e, (2)
or an ion impact process,
A++ e{ + M) ->,4*( + M), (3)
where A and B are atoms or molecules, * indicates the excited, emitting
species, and e{ + M) may be a neutral species, a negative ion, an electron
plus a third body, or a surface. Each of these processes has been found to
occur in low pressure glow discharges. For example, reaction (1) is responsi­
ble for emission from excited F-atoms in CF /0 discharges (Gottscho and
4 2
Donnelly, 1984), and also from Cl in Cl discharges under most conditions
2
(Gottscho and Donnelly, 1984), while process (2) has been observed by
time-resolved emission in the momentary cathode sheath of a radio-
frequency Cl discharge (Gottscho and Donnelly, 1984; Donnelly et al.,
2
1985; Flamm and Donnelly, 1986). Excitation of H-atoms in a H -DC
2
discharge has been ascribed in part to reaction (3) (Capelli et al., 1985;
Benesch and Li, 1985).