Table Of ContentS I L I C I D ES
F OR V L SI A P P L I C A T I O NS
S. P. Murarka
Bell Telephone Laboratories
Murray Hill, New Jersey
1983
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Library of Congress Cataloging in Publication Data
Murarka, S. P.
Silicides for VLSI applications.
Includes index.
I. Silicides. 2. Electronics—Materials.
3. Integrated circuits—Very large scale integration.
I. Title. II. Title: Silicides for V.L.S.I,
applications. Date.
TK7871.15.S54M87 1983 621.381'73 82-16414
ISBN 0-12-511220-3
PRINTED I N TH E UNITED STATE S O F AMERIC A
83 84 85 9 8 7 6 5 4 3 2 1
To Mother
Preface
The idea of writing Silicides for VLSI Applications was the result of per
suasive comments made by H. J. Levinstein and S. M. Sze, both of Bell
Laboratories. Dr. Levinstein had reviewed a chapter I wrote for "Applied
Solid State Science, Supplement 2C, Silicon Integrated Circuits," and felt that
the material was of sufficient interest to be expanded into book length. Dr. Sze
also expressed the thought that a book on this subject would be particularly
appropriate at the present time. Although silicides have been investigated for
one reason or another since the beginning of this century, this will be the first
book devoted to them.
In recent years the interest in silicides has increased considerably because of
their potential usefulness as low-resistivity contacts, and gate and intercon
nection metallization in silicon-integrated circuits. The evolution of very large
scale integration (VLSI) necessitated a closer look at transition metal silicides,
that is, their thermodynamic, electrical, and mechanical properties, and their
stability at high temperatures. This book thus evolved from the continued
efforts of my colleagues and myself to examine and understand the properties
and to determine the usefulness of various transition metal silicides. Silicides
for VLSI Applications addresses the applicability of silicides in high-density
silicon-integrated circuits. It is written for practicing device engineers,
materials scientists, and newcomers, including students of microelectronics
technology. It does not attempt to teach the reader how to fabricate silicon-
integrated circuits using silicides. This book assumes that the reader already
has a knowledge of integrated-circuit fabrication. The book simply presents
the latest up-to-date information on silicides and describes their applicability
to VLSI processes.
To some extent, my own interests and preferences have determined the
contents of this book. However, the chapters are organized in groups that can
be read in whatever order is suitable to serve the reader's particular needs.
Chapter I discusses the use of silicides both (a) in MOS devices for which the
reduction in the dimensions are leading to increased RC values at the gate
ix
χ PREFACE
level, and (b) as Schottky barrier devices and junction contacts for which very
low contact-resistance metallization has become necessary. Topics therefore
include MOS devices and RC time constants, the Schottky barrier height and
empirical relations, and contact resistance.
Chapter II is devoted to properties of silicides. It describes their resistivity,
crystal structure, stress, and chemical reactivity. Techniques for routinely
measuring thin-film resistivity and stress are also discussed.
Chapters III and IV describe the thermodynamic and kinetic factors that
govern both the compound formation in metal-silicon systems and the
stability of these compounds. Chapter IV also considers the role of impurities
in intermetallic compound formation and makes comparisons between the
various experimental techniques for routine silicide formation.
Chapter V is concerned with the oxidation, oxidation mechanism, and oxi
dation stability of silicides. It shows that most silicides can be oxidized under
controlled oxidizing conditions to form an insulating layer with reasonable
dielectric breakdown properties. Chapter VI examines the integrated-circuit
fabrication that uses silicide for metallization. The discussion on silicide
etching lacks details because published data are unavailable. Chapter VII
describes shallow silicide contacts and methods of making them, epitaxial
silicides, and predicts novel (integrated-circuit) applications of silicides.
Most of the subject matter of this book has previously been available only in
the form of research papers and review articles. I have not attempted to refer
to all the published papers. The reader may find it advantageous to refer to the
references listed. I have tried to give a reasonably complete picture of the
present knowledge of silicides. The book inevitably emphasizes the research
and development work done at Bell Laboratories. I have freely included my
own opinions, many of which have not been proven and may change in the
future. I hope that the knowledge gained from this book will be of value to
process engineers, materials scientists, students, and anyone who is interested
in silicides.
Acknowledgments
I have been very fortunate to be working at Bell Laboratories and inter
acting with many talented people, notable among them are H. J. Levinstein,
A. K. Sinha, D. B. Fraser, C. C. Chang, and Τ. T. Sheng who through their
direct or indirect participation have immensely contributed to building the
state of knowledge presented in this book. I am greatly indebted to them for
their contribution. I am grateful to M. P. Lepselter for his encouragement and
to several other colleagues with whom I discussed several of the topics. It is a
pleasure to thank the management of the Bell Laboratories for providing such
an environment and support.
Many thanks are due and sincerely given to Geraldine Moore and Susan
Crisman of the Text Processing Center for typing the manuscript, Jean G.
Chee for excellent editing.
I am grateful to the many authors whose papers I have followed closely in
various parts of the book and who allowed me to use their work in this book. I
am thankful to the American Institute of Physics, American Physical Society,
Institute of Electrical and Electronics Engineers, The Electrochemical Society,
Inc., General Electric Company, Japanese Journal of Applied Physics,
Pergamon Press, Inc., John Wiley and Sons, Inc., and Solid State Technology
for permission to use copyrighted materials.
Most of all I want to thank my wife Saroj and sons Sumeet and Amal for the
love, understanding, patience, and impatience that made the preparation of
this book possible.
xi
Chapter I
Introduction
A. Overview 2
B. MOS Devices—RC Delay and Speed at the Gate
and Interconnection Level 9
C. Schottky Barrier Devices 14
1. The Schottky Barrier Height 14
2. Devices 23
D. Contact Metallization and Contact Resistance 24
1
2 INTRODUCTION
A. OVERVIEW
Metal suicides have attracted scientific curiosity and attention since
Moissan developed the electric furnace (1). He was possibly the first
to systematically prepare various silicides at about the turn of the
century. Investigations concerning silicides (2—19) can be grouped in
the following major categories: (a) studies stimulated by the high-
temperature stability of many refractory silicides, (b) studies aimed at
understanding the physical properties of silicides in terms of the
electronic and crystal structure of the elements and compounds,
(c) studies of silicides as Schottky barriers and ohmic contacts in the
integrated-circuit technology, and finally and more recently, (d) studies
of silicides as the low-resistivity metallization for gates and
interconnects.
The primary thrust of very large scale integration (VLSI) has
resulted in devices that are smaller (large packing density and hence
increased complexity on the chip) and faster and that consume less
power. The continued evolution of smaller and smaller devices has
aroused a renewed interest in the development of new metallization
schemes for low-resistivity gates, interconnections, and ohmic
contacts. This interest in new metallization arose because as the
device sizes are scaled down, the linewidth gets narrower and the sheet
resistance contribution to the RC delay increases (Section B). With
the currently available polysilicon sheet resistance of 30 to 60 Ω/D,
the advantages of further scaling are offset by the interconnect
resistance at the gate level. Table I lists the properties that disqualify
most metals, for one reason or another, as direct replacements for
polysilicon. Aluminum (20), tungsten (21,22), and and molybdenum
(21,22) are notable among the metals proposed for gate and
interconnect metallization. The use of aluminum, however, requires
all postgate processing of the devices to be limited to very low
temperatures, preferably below 500°C. The use of the refractory
metals tungsten and molybdenum requires complete passivation of
these metals from oxidizing ambients (Chapters III and V), deposition
by means that will not lead to unwanted traps in the gate oxide, and
reliable etching of the metals for pattern generation. The uncertainties
associated with the stability of these metal films have led to a search
for alternatives.
The silicides have attracted attention because of their low and
metal-like resistivities (Chapter II) and their high temperature stability
(Chapters II, III, V, and VI). The use of silicides, with resistivity
OVERVIEW 3
TABLE I. Properties That Make Metals Unsuitable for VLSI Application
Undesirable property Metal
Low eutectic temperature (<800°C) Au, Pd, Ai, Mg
Medium eutectic temperature (800—1100°C) Ni, Pt, Ag, Cu
High diffusivity in silicon All
High oxidation rate, poor oxidation Refractory metals;
stability rare earths; Mg,
Fe, Cu, Ag
Low melting point Al, Mg
Interaction with substrate or poly-
silicon at temperature less Pt, Pd, Rh, V(?)
than 450°Ca Mo(?), Cr(?)
Interaction with substrate or poly-
silicon at temperatures up to 1000°C All
Interaction with SiO? Hf, Zr, Ti, Ta, Nb
V, Mg*, Al*
Poor chemical stability, especially Refractory metals,
in HF containing solutions
Fe, Co, Ni, Cu, Mg, Al
Poor etchability
Pt, Pd, Ni, Co, Au
Electro migration problems
Al
Contact spiking due to interdiffusion
Al
a Typical last high temperature in device fabrication.
^ Interact with Si0 to form metal oxide which (self) limits the
2
further interaction.
about one-tenth (or lower) of the polysilicon, will certainly improve
the speed of the circuits. Silicides are also attractive for gate and
interconnection metallization for the following reasons: expected
higher electromigration resistance (Chapter III), and the possibility of
4 INTRODUCTION
forming silicides directly on the polysilicon, thus preserving the basic
polysilicon MOS gate (Section B), while decreasing the resistance.
Scaling down the size of the device also means reduced junction
depths, which can lead to contact problems. In particular, shallow
junctions (Chapter VII) limit the use of aluminum due to its known
penetration in silicon. Forming silicides in the contact windows by
reaction between the silicon substrate and a thin metal layer offers a
possibility of forming contacts with lower contact resistances (Section
C). The possibility of using deposited silicides directly into contact
windows offers the advantage of preserving shallow junctions, which
may be penetrated by a conventional suicide formed by reacting metal
with the silicon (Chapter VII).
Lepselter (9) at Bell Laboratories pioneered the use of silicides as
Schottky barriers (Section C.l), and Kahng and Lepselter (10) were
the first to demonstrate the application. A large volume of papers on
the subject has been written in the last two decades; we refer the
reader to several review papers (8—13). The formation of silicides by
metallurgical interaction between pure metal film and silicon leads to
the most reliable and reproducible Schottky barriers, with barrier
heights ranging from 0.55 eV for ZrSi to 0.94 eV for IrSi on n-type
2 3
silicon (23). The properties are reliable and reproducible because
silicide formation by metal-Si interaction frees the silicide-silicon
interface of surface imperfections and contamination. Similarly, the
formation of the silicide-silicon contacts atomically cleans the
interface, thus avoiding the variability in contact properties that may
otherwise occur when the surface is contaminated or imperfect. In this
respect, PtSi has been the most useful silicide.
Besides the desired low resistivity, the usefulness of the silicide
metallization scheme depends on the ease with which the silicides can
be formed (Chapter IV) and patterned (Chapters II and VI) and on the
stability of the silicides throughout the device-processing and during
actual device usage (Chapter VI). Table II lists the desired properties
of a silicide for use in integrated circuits. Silicides used to produce
gates and interconnections must satisfy all these requirements but all
silicides do not have these characteristics. Silicides used for contacts,
however, do not have to meet all the requirements, since the contacts
are formed towards the end of the processing.
More than half of the elements in the periodic table react with
silicon to form one or more silicides. Figure 1 shows all these
silicides. Of all these, we are particularly interested in the silicides of
