VLSI metallization

480 Pages·1987·36.532 MB·English

by  Norman G Einspruch, Simon S Cohen, Gennady Sh Gildenblat

VLSI Electronics
Microstructure Science
A Treatise Edited by
Norman G. Einspruch
VLSI Electronics
Microstructure Science
Volume 15
VLSI Metallization
Edited by
Norman G. Einspruch
College of Engineering
University of Miami
Coral Gables, Florida
Simon S. Cohen
Lincoln Laboratory
Massachusetts Institute of Technology
Lexington, Massachusetts
Gennady Sh. Gildenblat
Center for Electronic Materials and Devices
and Department of Electrical Engineering
The Pennsylvania State University
University Park, Pennsylvania
19,7 (g)
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Library of Congress Cataloging in Publication Data
VLSI metallization.
(VLSI electronics; v. 15)
Includes index.
1. Integrated circuits-Very large scale integration-
Design and construction. 2. Metallic films. I. Einspruch,
NormanG. II. Cohen, Simon S. III. Gildenblat, Gennady
Sh. IV. Series.
TK7874.V56 vol. 15 621.38173 s 86-28744
[621.38173]
ISBN 0-12-234115-5 (alk. paper)
PRINTED IN THE UNITED STATES OF AMERICA
87 88 89 90 9 8 7 6 5 4 3 2 1
Preface
The limits of integrated circuit technology have been constantly chal­
lenged by the continued quest for higher degrees of complexity. Scaling
down to VLSI and beyond by setting more aggressive design rules has
become the general trend in recent years. The main driving forces for this
are the undeniable advantages offered by scaling down minimum feature
size. These include the possibility of fabricating new classes of devices and
the all-important factors of increased circuit density and reduced fabrica­
tion costs. The concentrated research and development efforts made since
the beginning of this decade are thus resulting in more complex circuits
that are denser, faster, consume less power, and cost less per unit of
operation.
New processing technologies have been the goal of this intense research
activity. For these new technologies, processing equipment and techniques
had to be developed, and new materials had to be introduced. All of these
new developments have brought the technology to the current l-//m level
of integration. Continued efforts are being invested in extending the suc­
cess achieved so far to the submicrometer level of integration. Documenta­
tion of the research and development activity is essential for keeping pace
with the ever-changing nature of this field. Therefore, the purpose of this
volume is to summarize the recent developments and trends in one of the
critical aspects of VLSI technology.
Metallization plays a central role in the fabrication of integrated circuits.
In VLSI, new problems have arisen that require the development of several
different approaches to device metallization. The high density of fine lines
has put stringent requirements on the photolithographic and etching pro­
cesses. In addition, small geometric features have posed new reliability
concerns. Also, the shallow junctions encountered in VLSI devices reflect
on contact-resistance scaling and add reliability hazards. These and related
issues have to be addressed for a reliable VLSI process to be developed. The
present volume is dedicated to the various issues and problems related to
IX
X Preface
VLSI metallization. It details the available solutions and discusses emerg­
ing trends.
The two introductory chapters by Cohen (Chapter 1) and Singh (Chapter
2) are meant to serve as general references for the electrical and metallurgi­
cal properties of thin conducting films. These are followed by a sequence of
review chapters that discuss the various aspects of VLSI metallization. The
order of presentation has been chosen to follow the common processing
sequence. In Chapter 3, Skelly, Lu, and Woodruff discuss some relevant
metal deposition techniques, and in Chapter 4 Gorowitz, Saia, and Balch
discuss methods of VLSI lithography and etching. Conducting films are
first deposited at the gate definition step; therefore, the issues related to
gate metallization are discussed next in Chapter 5 by Sabnis. In Chapter 6,
Gildenblat and Cohen discuss contact metallization, and Chapter 7 by
Sabnis is devoted to multilevel metallization schemes. These schemes have
been developed to answer the needs for higher densities and device speed,
as well as higher reliability. Long-time reliability is also the subject of the
chapter by Schwarz (8), who discusses the issues of contact and intercon­
nect electromigration.
For many years silicon has been the main semiconductor material of the
integrated circuit industry. However, special needs and applications re­
quire the use of other semiconductors. Among these, GaAs is by far the
most widely used. The technology of GaAs is markedly different from that
of silicon. The subject of GaAs metallization is discussed in Chapter 9 by
Welch, Nelson, Shen, and Venkataraman.
The present volume concludes with a general discussion of the functions
of interconnect systems in VLSI given by Ferry, Grondin, and Porod in
Chapter 10.
The selection of the material in this volume was made to be useful for
materials scientists, processing and design engineers, and device physicists
alike. We expect that it will be found helpful by students of these disci­
plines.
NORMAN G. EINSPRUCH
SIMON S. COHEN
GENNADY SH. GILDENBLAT
VLSI ELECTRONICS: MICROSTRUCTURE SCIENCE, VOL. 15
Chapter I
Electrical Properties of Thin
Conducting Films
SIMON S. COHEN
Lincoln Laboratory
Massachusetts Institute of Technology
Lexington, Massachusetts 02173
I. Introduction 1
II. The Physics of Transport Phenomena in Thin Films 4
A. The Causes of Resistivity 4
B. Conductivity of Continuous Films 5
C. Conductivity of a Continuous Line 11
D. Temperature Coefficient of Resistivity 19
E. Electrical Transport in Discontinuous Films and Lines 21
III. Electromigration in Thin Metal Lines 25
IV. The Interconnect Impedance 27
A. The Resistance 28
B. Distributed RC Interconnect Lines 31
V. Test Structures 36
A. Sheet Resistivity and Linewidth 36
B. Interconnect Electromigration 37
References 38
I. INTRODUCTION
Thin film interconnections are an integral part of integrated circuits
(ICs). They are the means by which active regions in a device communi­
cate among themselves and single devices communicate with each other
and with the outside world. Owing to these varied tasks it is possible,
indeed advisable, to have more than one level of interconnection networks
1
Copyright © 1987 by Academic Press, Inc.
All rights of reproduction in any form reserved.
2 Simon S. Cohen
employed in a given IC. A multilevel interconnect system is recommended
for achieving higher speed and greater density, for minimizing the effects of
certain parasitic phenomena, and for increasing the process reliability.
The reliability of thin film interconnects, in particular, impacts critically
on the successful implementation of all circuit types. In fact, it would seem
that a multilevel interconnect scheme could actually result in additional
concerns due to the added processing steps. While indeed a yield issue in
current VLSI technologies, it turns out that in multilevel metallization
schemes the reliability is improved. This is owing to the relaxation of
certain stringent requirements regarding the geometrical features and the
metallurgical properties of the thin metal interconnect system.
The basic electrical property of an interconnect line is its conductivity.
Electrical conductivity of an ideal continuous thin metal film is different
than that of the bulk material. The film's resistivity is usually higher due to
an enhanced surface scattering of electrons. The increase in resistivity is
even more pronouned in discontinuous films. Such films are usually made
of many tiny grains separated by grain boundaries that present an impede-
ment to current flow. The barrier at the grain boundaries is further in­
creased by the presence of adsorbed contaminants. If the separation region
between grains were allowed to exceed a certain critical value, a dis­
connected grain structure would result, leading to a phase transition
from a conducting to a nonconducting film. This situation is particularly
severe in those systems in which oxidation of the grain boundaries can
take place.
The separation between adjacent grains in a discontinuous film can also
grow under bias conditions. This phenomenon, known as electromigra-
tion, can eventually lead to large enough voids that would result in a
disconnected film and, hence, in a total failure of the device. Electromigra-
tion is particularly pronounced in ductile metallic films, notably those
made of aluminum and some of its alloys. The mechanism responsible for
this detrimental phenomenon is discussed in Section III.
The electrical properties of thin metallic films are thus seen to be inti­
mately dependent on their metallurgical structure, in addition to their
dependence on geometrical features. Understanding the process by which
thin films form is, therefore, essential for the evaluation of their electrical
properties.
According to the nucleation theory [1], the formation of a continuous
thin film follows several stages. First, the impinging atoms on the substrate
get adsorbed on the surface and form patches of a monolayer. These then
grow to form critical nuclei that develop into clusters of atoms. In the next
stage the clusters coalesce to form islands of thin metallic material. The
1. Electrical Properties of Thin Conducting Films 3
islands continue to grow until large enough to form a "connected network"
characterized by conductive channels that surround holes of exposed sub­
strate. This is the stage referred to as the discontinuous film. A discontin­
uous film, unlike a disconnected one, is capable of current conduction,
although its conductivity is usually only a fraction of that realized in the
bulk. The transport phenomena in such a stochastic-geometry-disordered-
metallic system can be dealt with by an interesting theoretical technique
called the percolation theory. We shall discuss it, along with other theoreti­
cal results, in Section II.
The final stage in the film growth involves filling up the open holes by
secondary nucleation of the channels to result in a continuous film. We
note, however, that a truly continuous film, meaning a monocrystalline
structure, is seldom realized. Grain boundaries will usually be present.
Even if such a continuous film were formed it would most probably
develop various imperfections during subsequent heat treatments. This is
due to differences in the thermal expansion characteristics between the
film and the underlying substrate. Other causes of imperfection are the
tendency of thin films to reproduce substrate features, and the need to
accommodate the usual lattice misfit between the film material and the
substrate [2].
The basic requirement on the electrical properties of a thin metallic film
is that it would be of low resistivity and be capable of handling reasonably
high current densities. A low resistivity is particularly important in order to
assure minimal power dissipation in the interconnect system and a small
propagation delay along the runs. The propagation delay is proportional to
the film resistivity, inversely proportional to the film thickness, and varies
as the square of the line length. Hence, it is clear that whenever possible
thicker and shorter metal runs should be employed (as we shall see in
Section II, a thicker film is in addition of a lower resistivity thereby leading
further to minimization of the delay time). This goal is at least partially
achieved by utilizing multilevel metallization scheme. The propagation
delay is also affected by bends in the thin film that increase the line
resistance, and by inductive and capacitive phenomena. These effects will
be discussed in some detail in Sections IV.
We complete the present discussion of the electrical properties of thin
conducting films by describing some relevant test structures in Section V.
An exact evaluation of such parameters as the sheet resistivity, the line-
width, and mean time to failure has become of prime importance with the
advent of VLSI technology. The test structures discussed here, when com­
bined with various analytical techniques, enable a complete characteriza­
tion of the thin film properties.
4 Simon S. Cohen
II. THE PHYSICS OF TRANSPORT PHENOMENA IN THIN FILMS
This section is devoted to the discussion of relevant transport phenom­
ena in thin conducting films. We examine first the causes of resistance to
carrier transport in bulk metallic conductors. Transport phenomena in
thin conducting films and "wires" are then discussed. We distinguish
between the ideal continuous film case and that of the more realistic
discontinuous metal film. The case of a combined metal-insulator system
is also discussed along with the percolation theory.
A. The Causes of Resistivity
Electrical conduction in metals is owing to the motion of conduction-
band electrons under the influence of an external electric field. According
to quantum mechanics, "free" electrons should move through a perfect
crystalline lattice without being scattered. This means that the electrons
encounter no resistance and their mean free path is infinitely large. In real
metallic systems, however, several scattering mechanisms may be active,
giving rise to a finite mean free path and hence to a finite resistivity value.
At room temperature an average value for the mean free path in metals is
of the order of 10~5 cm or less. In pure metals at very low temperatures,
such as at the liquid-helium temperature, the mean free path may be as
large as 10~2cm.
Scattering of electrons can come as a result of interaction with phonons,
with impurities that disturb the translational symmetry of the lattice, and
with other lattice imperfections, such as dislocations. In addition, electron
scattering is enhanced in the vicinity of surfaces. In the case of the bulk,
this surface effect is of very limited consequences since the majority of the
free electrons are at distances, exceeding their mean free path, from the
surface. Since for thin films the surface to volume ratio is much higher
than it is for the bulk, surface scattering is a major factor affecting the
film's resistivity.
For a bulk material that contains n free electrons per unit of volume, the
conductivity is given by the following expression [3]
<T = q2nÀ /m*v (1)
B 0
where v is the electron velocity at the Fermi surface, λ the mean free path
0
in the bulk, q the electron's charge, and m* the effective mass of the
electron. As is seen in this relation, the higher the mean free path is, the
higher the bulk conductivity would be. In the derivation of this relation,
conductivity is assumed to be limited by electron-phonon scattering. In
1. Electrical Properties of Thin Conducting Films 5
fact the value of λ is affected by all the scattering processes just mentioned.
0
We note also that this simple relation is only correct for an isotropie,
monovalent metal that has a single conduction band [3]. For more com­
plicated metal systems it is only an approximation. Hence, it is customary
to generalize the theory and treat the conductivity as if consisting of two
terms—an ideal term, given by Eq. (1), and a residual resistance term.
Since the number of electrons, «, is given by
n = 8n/3(m*v/h)\ (2)
it follows that at relatively high temperatures the bulk conductivity is
proportional to T. [In Eq. (2) h is Planck's constant.] As the temperature is
lowered the mean free path increases, owing to the reduced phonon popu­
lation. However, the mean free path does not increase indefinitely as the
temperature keeps dropping. Its value tends to a residual value, which is
determined by the presence of various lattice imperfections. This residual
resistivity is formally expressed by the same expression given in Eq.(l) with
λ replaced by its residual value λ. It turns out that at ordinary tempera­
0
tures the residual resistivity may be neglected for pure enough metals. By
neglecting nonphonon scattering processes, it is found that at very low
temperatures σ α Τ5.
Β
B. Conductivity of Continuous Films
Until recently the notion of continuous film has been largely regarded as
a mere ideal concept. In recent years the realization of truly continuous
films having well-defined crystalline structure has been reported [4-6].
This progress has been made possible by the availability of advanced
deposition techniques, such as molecular-beam epitaxy (MBE). A continu­
ous film, free of such defects as grain boundaries, foreign inclusions, and
dislocations, lends itself to a careful evaluation of the scattering mecha­
nisms and the conductivity characteristics. When such results are made
available, existing theoretical results could be better examined.
An electron being scattered from a metallic surface may be either specu­
larly or nonspecularly (diffusely) reflected. In a nonspecular (inelastic)
collision the direction of the electron following the collision event is inde­
pendent of that prior to the collision. Pure specular (elastic) collisions
would be expected to occur with a perfectly plane surface. This is because
the wavelength of electrons in metals is only of the order of a typical lattice
spacing (i.e., ~5 A). On this scale most real surfaces are rather rough;
hence, at least a fraction of the scattering events is expected to be nonspec­
ular. The nonspecular reflections from the surface are those which influ-