MECHANICAL ASPECTS OF THE MATERIAL REMOVAL MECHANISM IN
CHEMICAL MECHANICAL POLISHING (CMP)
by
Yongsik Moon
B.E. (Chung-Ang University) 1994
M.S. (University of California, Berkeley) 1996
A dissertation submitted in partial satisfaction of the
requirements for the degree of
Doctor of Philosophy
in
Engineering-Mechanical Engineering
in the
GRADUATE DIVISION
of the
UNIVERSITY OF CALIFORNIA, BERKELEY
Committee in charge:
Professor David A. Dornfeld, Chair
Professor Paul K. Wright
Professor Nathan W. Cheung
Fall 1999
The dissertation of Yongsik Moon is approved:
University of California, Berkeley
Fall 1999
MECHANICAL ASPECTS OF THE MATERIAL REMOVAL MECHANISM IN
CHEMICAL MECHANICAL POLISHING (CMP)
Copyright 1999
by
Yongsik Moon
Abstract
Mechanical Aspects of the Material Removal Mechanism in Chemical
Mechanical Polishing (CMP)
by
Yongsik Moon
Doctor of Philosophy in Mechanical Engineering
University of California, Berkeley
Professor David A. Dornfeld, Chair
The chemical mechanical polishing (CMP) process has become a primary
planarization technique required for the manufacture of advanced integrated circuit (IC)
devices. As the feature size of IC chips shrinks down to 0.18 µm and below, the role of
CMP as a robust planarization process becomes increasingly important.
In this dissertation, mechanical aspects of the material removal mechanism in CMP
are investigated analytically and experimentally as part of a fundamental study on CMP.
The role of consumables, which include the polishing pad and abrasive particles in the
slurry, in CMP performance is evaluated, and tribological characteristics (lubrication,
friction, and wear) of CMP are also analyzed. To evaluate the role of slurry, the influence
of chemistry of the slurry on the mechanical aspects of the material removal mechanism is
examined. The mechanical and chemical contributions to material removal are studied to
determine the key mechanism of material removal in CMP.
1
From this study, it is shown that the material removal rate of the wafers is inversely
proportional to the pad density and proportional to the pad compressibility and surface
roughness. This is due to the fact that the material removal for a wafer in CMP is closely
related to the actual pad contact area of the wafer. This is an indication of the probability
of abrasives contacting the wafer surface. The material removal rate of silicon wafers is
also proportional to the friction force between the workpiece and the polishing pad.
The effect of the slurry film thickness variation on CMP performance, defined in
terms of material removal, planarization, surface defects, and surface roughness, is
significant. From the Stribeck curve, the lubrication condition under the wafer in CMP is
closer to boundary or elasto-hydrodynamic lubrication than to hydrodynamic lubrication.
It is also shown that the chemically reacted silicon layer causes the extension of the
brittle/ductile transition depth, and the brittle cutting behavior transition point becomes
less distinctive. The chemically reacted ‘ductile’ layer is proposed to be the origin of the
scratch/defect-free surface after CMP.
The material removal of CMP is more than the sum of the removal due to the
mechanical and the chemical polishing effects. The test results verify the important effect
of the combination of chemical and mechanical action to achieve high material removal in
CMP. This supports the theory that there is a synergy effect that amplifies material
removal only when the chemical and mechanical components act concurrently in CMP.
2
Dedicated to my parents and my wife.
iii
TABLE OF CONTENTS
DEDICATION
iii
TABLE OF CONTENTS
iv
LIST OF FIGURES
ix
LIST OF TABLES
xv
ACKNOWLEDGEMENTS
xvi
CHAPTER 1
INTRODUCTION
1
1.1
Overview
1
1.2
Scope of dissertation
3
BACKGROUND ON CHEMICAL MECHANICAL POLISHING
8
CHAPTER 2
(CMP)
2.1
Introduction
2.2
History of CMP
10
2.3
Critical issues in CMP
11
2.3.1
Critical components in CMP
11
2.3.2
Major process outputs in CMP
14
2.4
8
Review: material removal mechanism and process model of CMP
15
2.4.1
15
Material removal mechanism and process model at the
wafer scale
iv
2.4.2
Material removal mechanism and process model at the
19
feature scale
2.4.3
Material removal mechanism and process model at the
23
particle scale
2.5
Process monitoring in CMP
25
2.6
Post-CMP process
26
MECHANICAL PROPERTIES OF THE POLISHING PAD AND
41
CHAPTER 3
ITS RELATIONSHIP TO PROCESS PERFORMANCE IN
CHEMICAL MECHANICAL POLISHING (CMP)
3.1
Introduction
41
3.2
Polishing pad in CMP
44
3.3
Preston’s wear equation
45
3.4
Experiments
48
3.5
Results and discussion
50
3.5.1
50
Relationship between the material removal rate and the
properties of the polishing pad
3.5.2
Relationship between the friction force and coefficient
54
and the properties of the polishing pad
3.5.3
Relationship between the material removal rate and the
55
friction force and coefficient
3.5.4
Relationship between the Preston’s coefficient and the
v
57
friction coefficient
3.6
CHAPTER 4
Summary
59
THE EFFECT OF SLURRY FILM THICKNESS VARIATION IN
76
CHEMICAL MECHANICAL POLISHING (CMP)
4.1
Introduction
76
4.2
Wafer-pad contact modes
79
4.3
Hydrodynamic effect and lubrication theory
82
4.4
Experiments
87
4.5
Results and discussion
88
4.5.1
88
Effect of slurry film thickness variation on friction force,
friction coefficient, and wafer-pad contact mode.
4.5.2
Effect of slurry film thickness variation on the wafer
91
planarization.
4.5.3
Effect of slurry film thickness variation on the wafer surface
92
roughness and defects.
4.5.4
Effect of slurry film thickness variation on the material
94
removal and its mechanism.
4.6
CHAPTER 5
Summary
97
STUDY OF SLURRY CHEMICAL INFLUENCE IN
vi
122
friction coefficient
3.6
CHAPTER 4
Summary
59
THE EFFECT OF SLURRY FILM THICKNESS VARIATION IN
76
CHEMICAL MECHANICAL POLISHING (CMP)
4.1
Introduction
76
4.2
Wafer-pad contact modes
79
4.3
Hydrodynamic effect and lubrication theory
82
4.4
Experiments
87
4.5
Results and discussion
88
4.5.1
88
Effect of slurry film thickness variation on friction force,
friction coefficient, and wafer-pad contact mode.
4.5.2
Effect of slurry film thickness variation on the wafer
91
planarization.
4.5.3
Effect of slurry film thickness variation on the wafer surface
92
roughness and defects.
4.5.4
Effect of slurry film thickness variation on the material
94
removal and its mechanism.
4.6
CHAPTER 5
Summary
97
STUDY OF SLURRY CHEMICAL INFLUENCE IN
vi
122
DUCTILE/BRITTLE TRANSITION DEPTH IN CHEMICAL
MECHANICAL POLISHING (CMP) USING SCANNING
ELECTRON MICROSCOPE (SEM) AND ACOUSTIC EMISSION
(AE) SENSOR
5.1
Introduction
122
5.2
Ductile/brittle-cutting regime in diamond turning of brittle materials 125
5.3
Process monitoring using acoustic emission (AE)
126
5.4
Experiments
127
5.5
Results and discussion
129
5.5.1
SEM analysis
129
5.5.2
Acoustic emission (AE) monitoring of scratching test
131
5.5.3
Ductile/brittle transition in scratches on the untreated and
133
the chemically treated silicon wafer
5.6
CHAPTER 6
Summary
134
IDENTIFICATION OF THE MECHANICAL ASPECTS OF
150
MATERIAL REMOVAL MECHANISMS IN CHEMICAL
MECHANICAL POLISHING (CMP)
6.1
Introduction
150
6.2
Experiments
153
6.2.1
Preparation of Slurry
153
6.2.2
Polishing
154
vii
6.3
Results and discussion
155
6.4
Summary
160
CONCLUSIONS
172
CHAPTER 7
REFERENCES
179
viii
LIST OF FIGURES
CHAPTER 1
Fig. 1.1
Planarization Potential Solutions.
CHAPTER 2
Fig. 2.1
Difficulty in the fabrication of integrated circuit (IC) chip.
Fig. 2.2
A general schematic of the CMP process.
Fig. 2.3
Kinematic variables in the CMP process.
Fig. 2.4
Silicon oxide abrasive particles from Nalco2352 silicon polishing slurry.
Fig. 2.5
Side view ((a) x20 and (b) x110) and top view ((c) x200 and (d) x400) of
IC1000, polishing pad.
Fig. 2.6
Top view ((a) x40 and (b) x200) and oblique view ((c) x66 and (d) x300) of a
fixed abrasive pad.
Fig. 2.7
A (a) new and (b, c, d) used IC1000 polishing pad without conditioning.
Fig. 2.8
49-point inspection to measure the with-in-wafer-non-uniformity (WIWNU).
Fig. 2.9
Physical description of the polishing process in Warnock’s model.
Fig. 2.10
Arbitrary surface indicating the meaning of S, A, and K factors in Warnock’s
model.
Fig. 2.11
Schematic of pad deformation.
Fig. 2.12
Definitions of variables used in Stine model.
Fig. 2.13
Schematic of the abrasive in contact with the wafer and the pad surfaces.
ix
Fig. 2.14
A schematic of each polishing stage during CMP.
CHAPTER 3
Fig. 3.1
Oblique view of the polishing pads of (a) SUBA500, (b) IC60, (c) UR100
from the Rodel Inc.
Fig. 3.2
Cross-sectional view of (a) SUBA500, (b) IC60, and (c) UR100 polishing
pads at two magnifications (x20 and x66).
Fig. 3.3
Top view of (a) SUBA500, (b) IC60, and (c) UR100 polishing pads at two
magnifications (x40 and x200).
Fig. 3.4
Schematic of experimental setup.
Fig. 3.5
The material removal rate for each polishing pad.
Fig. 3.6
Material removal rate variation with polishing pad density and compressibility.
Fig. 3.7
Material removal rate variation with polishing pad roughness.
Fig. 3.8
Friction force and coefficient variation on the wafer for each polishing pad.
Fig. 3.9
Friction force variation with polishing pad density and compressibility.
Fig. 3.10
Friction force variation with polishing pad roughness.
Fig. 3.11
Material removal rate variation with (a) friction force and (b) coefficient.
Fig. 3.12
Material removal per sliding distance with friction coefficient.
Fig. 3.13
Preston’s coefficient variation during CMP process.
Fig. 3.14
Preston’s coefficient variation with friction coefficient.
CHAPTER 4
x
Fig. 4.1
Material removal mechanism in lapping and CMP processes.
Fig. 4.2
A cross-section of a UR100 polishing pad.
Fig. 4.3
Illustration of pad/wafer interaction in CMP.
Fig. 4.4
Definition of slurry film thickness.
Fig. 4.5
Stribeck curve.
Fig. 4.6
Lubrication under a wafer during CMP process.
Fig. 4.7
Slurry film thickness variation with the wafer velocity.
Fig. 4.8
Friction force variation between the wafer and the polishing pad with abrasive
slurry.
Fig. 4.9
Friction force variation between the wafer and the polishing pad without
abrasive slurry.
Fig. 4.10
Friction coefficient variation between the wafer and the polishing pad with
abrasive slurry.
Fig. 4.11
Friction force variation between the wafer and the polishing pad without
abrasive slurry.
Fig. 4.12
Two regimes in the Stribeck curve (regime I: commercial CMP condition,
regime II: CMP condition used in this research).
Fig. 4.13
Planarization of wafer by (a) a small slurry film and (b) a large slurry film.
Fig. 4.14
Surface roughness of oxide surface in 1µm x 1µm area of (a) surface before
CMP, (b) surface polished with small slurry film thickness, and (c) surface
polished with large slurry film thickness.
Fig. 4.15.
Surface roughness variation with slurry film thickness.
xi
Fig. 4.16
Silicon wafer sliding on the polishing pad for a certain distance with (a) a
small slurry film and (b) a large slurry film due to the velocity difference.
Fig. 4.17
Material removal per sliding distance with velocity.
Fig. 4.18
A proposed schematic of material removal in CMP for the slurry film
thickness.
Fig. 4.19
Mechanical and chemical removal for slurry film thickness.
CHAPTER 5
Fig. 5.1
Chemical treatment of silicon wafer.
Fig. 5.2
Experimental setup.
Fig. 5.3
Scratching test on chemically treated and untreated area of silicon wafer.
Fig. 5.4
Top view of initiation points of micro-scratches on (a) (b) chemically
untreated and (c) (d) treated areas of silicon wafer (tool radius = 48 µm, tilt
angle = 0.5 degree).
Fig. 5.5
Oblique view of initiation points of micro-scratches on (a) (b) chemically
untreated and (c) (d) treated areas of silicon wafer (tool radius = 48 µm, tilt
angle = 0.5 degree).
Fig. 5.6
Brittle/ductile cutting regimes of micro-scratches on (a) chemically untreated
and (b) treated areas of silicon wafer (tool radius = 48 µm, tilt angle = 0.5
degree).
Fig. 5.7
Extension of ductile/brittle cutting regime of micro-scratches (tool radius = 48
µm, tilt angle = 0.5 degree).
xii
Fig. 5.8
Micro-scratches on silicon wafer at specific locations with the same cutting
width on (a, c, e, g, i, k) untreated and (b, d, f, h, j, l) chemically treated areas
(tool radius = 350 µm, tilt angle = 0.05 degree).
Fig. 5.9
AE raw signal from the scratch on the normal area of the silicon wafer (tool
radius=350µm, tilt angle=0.05 degree).
Fig. 5.10
AE rms signal calculated from AE raw signal from the scratch on the normal
area of the silicon wafer (tool radius=350µm, tilt angle=0.05 degree).
Fig. 5.11
AE raw signal from the scratch on the chemically treated area of the silicon
wafer (tool radius=350µm, tilt angle=0.05 degree).
Fig. 5.12
AE rms signal calculated from AE raw signal from the scratch on the
chemically treated area of the silicon wafer (tool radius=350µm, tilt
angle=0.05 degree).
Fig. 5.13
Extension of ductile/brittle cutting regime of micro-scratches (tool radius =
350 µm, tilt angle = 0.05 degree).
Fig. 5.14
Ductile/brittle transition in scratches on (a) the normal area and (b) the
chemically treated area of silicon wafer.
CHAPTER 6
Fig. 6.1
Silica abrasives in Nalco2352 silicon wafer polishing slurry, (a)(b) x45,000
and (c) x200,000 magnification respectively.
Fig. 6.2
Preparation of abrasive-less and chemical-less slurries.
Fig. 6.3
Silica abrasives sampled from (a) the normal and (b) the chemical-less slurries.
xiii
Fig. 6.4
Material removal per sliding distance of the chemical, the mechanical, and the
normal polishing.
Fig. 6.5
SEM pictures of silicon wafer surfaces (a) before CMP, (b) after normal
CMP, (c) after chemical polishing, and (d) after mechanical polishing.
Fig. 6.6
PSG oxide wafer surfaces before ((a) x45,000, (b) x85,000) and after ((c)
x45,000, (d) x85,000) CMP.
Fig. 6.7
Irregular silica abrasives sampled from the used silicon wafer CMP slurry.
Fig. 6.8
Schematic of the material removal mechanism in CMP.
Fig. 6.9
Schematic of (a) the mechanical and (b) the chemical polishing action.
Fig. 6.10
Mechanical and chemical effect on the material removal in CMP.
xiv
LIST OF TABLES
CHAPTER 3
Table 3.1
Material Properties of SUBA500, IC60, and UR100 (Rodel).
CHAPTER 4
Table 4.1
Process parameters for the experiments on the effect of slurry film thickness
on CMP performance.
Table 4.2
Process parameters and Hersey numbers of experimental setups of the LMA
machine and the Cybeq 3000, a commercial CMP tool.
Table 4.3
A list of data for the planarization and surface roughness testing.
xv
ACKNOWLEDGEMENTS
First and foremost, I would like to express my sincere gratitude for my research
advisor, Professor David A. Dornfeld for his enlightening guidance and tremendous
support. His exceptional supervision made it possible for me to complete my study and
to reach this highlight of my life. I will never be able to thank him enough for helping to
hone my intellect and shape my character. I would like to thank Professor Paul K.
Wright for his invaluable support and guidance as a member of my qualifying exam and
dissertation committee as well as an admirable mentor. I would also like to thank
Professor Nathan W. Cheung for his intellectual discussions and insightful comments on
my dissertation.
I would like to thank Professor Richard E. Barlow for giving me
precious help on my research and qualifying examination. I also thank Professor C. K.
Hari Dharan and Professor Lisa Pruitt for their generosity and guidance in my qualifying
examination.
I am also grateful for the guidance by Professor Ömer Savas as an
academic advisor when I started my graduate program here at Berkeley five years ago.
I would like to thank Dr. Jeffrey W. Carr at Lawrence Livermore National
Laboratory (LLNL) for his generous support and valuable comments on my research. I
was lucky to know such a knowledgeable person during my academic career.
Mr.
Norman J. Brown, a former researcher at LLNL has been a wonderful teacher especially
for the empirical study of my research as a knowledgeable specialist in optics polishing.
The former and present graduate students in the Laboratory for Manufacturing
Automation (LMA) have been a great help for me to complete my study. I have really
xvi
enjoyed the time working and having fun together. I would like to thank Chih-Hsing
Chu, Dan Abels, Jianfeng Luo, Jinsoo Kim, Sangkee Min, and Yoon Lee. I would
especially like to thank Andrew Kunung Chang and Kori Bevans for their great
proofreading of my dissertation. My former labmates, Carsten Unger, Henning Dechow,
Dr. Ilwhan Park, Dr. Jay Daniel, Dr. Jianshe Tang, Dr. Seong Hwan Lee, Dr. Yohichi
Nakao and Dr. Xuemei Chen have been a great source of enjoyment and assistance. Also
I feel very fortunate to have had Dr. Eungsug Lee at Korea Institute of Machinery &
Materials for his various supports for my study. A former classmate and a close friend,
Professor Dongsik Kim at University of Texas, Austin has helped me whenever I have
had difficulties in my research.
No words would be enough to express my sincere appreciation for my parents and
their endless encouragement and support. They have always given me unconditional love
and care. I am also indebted to my dearly loving family including my two brothers,
Yongmin and Yongsang and their family, Hyunjung, and Suejung for their continuous
support.
Last, but not least, special appreciation and gratitude is given to my wife, Soyoung
for her endless love, support, understanding, and patience throughout my study. I cannot
imagine finishing my study without her precious support and love through my graduate
program.
xvii
Support for my research was provided by the California semiconductor industry, the
University of California UC-SMART program under contract 97-01, and the National
Science Foundation through award NSF DMI-9813039.
xviii
CHAPTER 1
INTRODUCTION
1.1 Overview
According to the Semiconductor Industry Association (SIA) roadmap, the design
rule (line width) of integrated circuit (IC) devices in production was 0.25 µm in 1997, and
extensive resources are being invested into research and development to realize 0.18 µmand even 0.15 µm-generation, IC technology in production [SIA, 1997]. Substantial
technical innovations in photolithography and interconnection technologies are among the
efforts being achieved to reach this sub-micron goal. Photolithography includes all the
processes used to transfer a pattern from a mask to the wafer surface [Jaeger, 1989]. The
pattern is transferred to a light-sensitive material, called photoresist, coated on the wafer
surface by a form of radiation such as ultraviolet light, electron beam, or X-ray. As the
feature size of the IC chip shrinks, the wavelength of light needs to be shorter, the depth
of focus of lithography tools decreases, and, thus, the topography of the wafer surface
becomes a severe barrier in focusing. The increasing number of multi-layer films in IC
chips brings numerous technological challenges in interconnection. The major challenges
encountered in interconnection technology are associated with the material changes (from
SiO2 and Al to low-k dielectric and Cu) and the requirement of new process architectures
(such as damascene process) [Wang, 1995].
1
A planarization process becomes increasingly important to assure circuit
performance and reliability as the number of multi-layer films required in very/ultra large
scale integration (VLSI/ULSI) technology increases, and the wavelength of light in
photolithography gets shorter. A new technique is also required in IC fabrication to build
up the new process architecture and to manage new materials which are difficult to
chemically etch. This is where chemical mechanical polishing, called CMP, process comes
in.
Chemical Mechanical Polishing has become one of the most widely used
planarization techniques in interlevel dielectric (ILD) planarization, shallow trench
isolation (STI), and metal damascene processes. It is one of the key fabrication processes
in the manufacture of advanced IC devices. As indicated in the SIA roadmap, more
research is needed especially on the fundamentals of CMP, Fig. 1.1 [SIA, 1997].
As a fundamental study on CMP, mechanical aspects of the material removal
mechanism in CMP are investigated analytically and experimentally. Among the many
important variables, the role of consumables (polishing pad and abrasive particles in the
slurry, for example) in CMP performance is evaluated, and tribological characteristics
(lubrication, friction, and wear) featured in CMP are also analyzed. To evaluate the role
of the slurry, the influence of chemistry on mechanical removal in material removal
mechanism is examined. The mechanical and chemical contributions to material removal
are studied to determine the key mechanism of material removal in CMP.
2
CMP is classified as a loose abrasive machining process among manufacturing
processes. Abrasive machining processes such as grinding, lapping, and polishing have
been used as finishing processes for thousands of years. The need to manufacture optical
instruments (e.g. microscopes, telescopes, etc.) became a driving force to trigger the
heavy use of loose abrasive machining in manufacturing optics. As the semiconductor
industry necessitated an advanced planarization method, the polishing process tailored for
IC manufacturing became the process of choice for wafer planarization. As in traditional
abrasive machining processes, the mechanical aspects of the material removal mechanism
in CMP are important and should be considered as a leading mechanism of material
removal.
1.2 Scope of dissertation
This dissertation is composed of the following three main parts:
(1) Background on chemical mechanical polishing (Chapter 2)
(2) Material removal mechanism at the wafer scale – mechanical properties of the
polishing pad and its relationship to process performance/effect of slurry film thickness in
CMP (Chapters 3 and 4)
(3) Material removal mechanism at the particle scale – slurry chemical influence on
ductile/brittle transition depth using acoustic emission feedback/identification of the
mechanical and chemical removal mechanism in CMP (Chapters 5 and 6)
3
In Chapter 2, background on the CMP process is introduced. The history of CMP is
reviewed, and the critical elements and the major outputs are explained. As the critical
variables, the role of pressure on the wafer and relative velocity between the wafer and the
pad in CMP is examined. The consumables, abrasive slurry and polishing pad, are chosen,
and their roles are identified.
The significance of pad conditioning is briefly stated.
Material removal rate and planarization are defined as the major outputs.
A review of process models and material removal mechanisms in CMP is presented
and the aspects of process monitoring and the post-CMP process are described.
In Chapter 3, the mechanical, material, and geometrical properties of polishing pads
typically utilized in the CMP process are examined. Next, the relationship between the
properties of the polishing pad and CMP performance is identified empirically.
Tribological aspects, such as friction and wear, of the CMP process are studied and
related to the physical description of the interfacial behavior between the silicon wafer and
the polishing pad. Finally, integration of the tribological aspects into a model for the CMP
process is accomplished by using the relationship between Preston’s coefficient and the
friction coefficient.
In Chapter 4, the effect of slurry film thickness variation on CMP performance
defined as material removal, planarization, surface defects, and surface roughness is
investigated. The friction force variation with velocity is correlated to the slurry film
4
thickness under the wafer surface. Based upon the experimental results, the lubrication
condition under the wafer in CMP is defined, and the contact mode between the wafer and
the pad surfaces is estimated. The effect of slurry film thickness on material removal
mechanism in CMP is proposed using the experimental and analytical results.
The
dependency of slurry film thickness between the wafer and the pad on velocity and Hersey
number are identified. The necessity of modifying Preston’s equation is proposed using
the influence of relative velocity on material removal per sliding distance. A possible need
to optimize the slurry film thickness to balance the mechanical with the chemical removal
in CMP is suggested.
In Chapter 5, to identify the effect of slurry chemistry on the mechanical removal,
the influence of the slurry chemical on the extension of ductile/brittle transition depth in
silicon is investigated by using scanning electron microscope (SEM) analysis and by
monitoring acoustic emission (AE) signals during a diamond cutting test. The AE raw and
the AE rms signal are used to monitor the ductile-brittle cutting regimes and transition in
scratching of silicon wafers. The variation of wafer surface property following chemical
treatment is examined and a possible extension of ductile regime machining was identified.
The role of a chemically reacted layer on the origin of the scratch/defect-free surface after
CMP is also inspected.
In Chapter 6, the mechanical and chemical contributions to material removal in CMP
are investigated independently by using ‘chemical-less’ and ‘abrasive-less’ slurries. In
5
order to investigate the role of the abrasive and the chemical independently, a commercial
slurry was separated to produce chemical-less and abrasive-less slurries. These, along
with the original slurry, are used in a series of polishing experiments. Material removal in
each of the three experiments is recorded and SEM analysis of the wafer surface and the
abrasive particles is performed before and after polishing. The significance of the role of
abrasives in material removal is identified. A material removal mechanism on the basis of
interaction between abrasives and the wafer surface is proposed.
The major conclusions of the study are detailed in Chapter 7.
6
First Year of IC Production
1997
250 nm
1999
180 nm
2001
150 nm
2003
130 nm
2006
100 nm
2009
70 nm
2012
50 nm
STI CMP SLURRIES AND CLEANS
CMP OF LOW k DIELECTRICS ON
ETCHED Al
Al CMP
Cu CMP
CMP AND CLEAN FUNDAMENTALS
PLANARIZATION ALTERNATIVES TO
CMP
Reserach Required
Development Underway
Fig. 1.1 Planarization Potential Solutions.
7
Qualification/Pre-Production
CHAPTER 2
BACKGROUND ON CHEMICAL MECHANICAL POLISHING (CMP)
2.1 Introduction
Recently, there has been increasing demand for highly integrated, high performance
integrated circuit (IC) chips.
To meet this demand the IC fabrication industry first
attempted to shrink the device dimensions, and then attempted to increase the number of
metal/dielectric layers on the IC chip. However, when these two strategies were used in
the IC fabrication processes, they resulted in severely uneven IC structures, which also
brought about the additional problem in the photolithography step because it was difficult
to focus on the highly non-planar structures, Fig. 2.1. Therefore, a new planarization
technique was needed, and chemical mechanical polishing (CMP) emerged as the process
of choice for microelectronic devices meeting the stringent critical dimension of the IC
design rule and multilevel interconnection technology.
CMP is a polishing process with more chemical reaction than conventional polishing
processes and is customized for use as a planarization method in IC fabrication. A general
schematic of the CMP process is shown in Fig. 2.2. The elements shown are the carrier
which holds the wafer, the polishing pad on the polishing plate, and the slurry feeder
supplying the abrasive slurry during the process. The carrier, in general, rotates about its
8
axis at the same time the polishing plate rotates with a minor oscillation. The abrasive
slurry is supplied to the pad surface through the slurry feeder. Inside the carrier, there is a
carrier film which holds the wafer using the effects of surface tension and capillary force.
The polishing pad and the carrier film are typically made of a polymer such as
polyurethane and polyester.
Despite its increased use, achieving global planarization, controlling the material
removal rate, and developing a new CMP process recipe (the make-up of the slurry, pad,
and specific conditions) for a new product are still difficult as the critical dimension of the
IC chip becomes smaller. Mainly these are due to the absence of an adequate and robust
process model, reliable process monitoring techniques, fundamental knowledge of the
material removal (mechanical and chemical elements) mechanism, and the effect of
consumables (polishing pad and abrasive slurry).
In this chapter, background on CMP is presented by reviewing the history of CMP,
the critical components in the process, the major outputs, the process models, and material
removal mechanism proposed to date. The aspects of process monitoring in CMP and the
post-CMP process are also described.
9
2.2 History of CMP
Chemical mechanical polishing (CMP) has been utilized in the semiconductor
industry from the early 1950’s as a method to prepare the silicon wafer substrate for
fabrication of IC chips [Bonora, 1977; Wolf, 1986]. As the demand for high density/high
performance IC chips increased, a new planarization technique was needed. In the mid
1980’s, CMP first began to be applied to IC manufacturing as a global planarization
method of the device wafer by IBM [Burggraaf, 1995] replacing traditional planarization
methods such as reactive ion etching (RIE). It was introduced as a new planarization
technique in IC manufacturing to the semiconductor industry in the late 1980’s, and the
first technical paper on the application of CMP for 16 Mb DRAM technology was
published in 1989 by Davari [Davari, 1989].
Initially, CMP was used for the planarization of interlevel dielectrics (ILD) such as
silicon oxide film.
But its application has spread widely among the IC fabrication
processes including shallow trench isolation (STI), damascene process, planarization of
low-k dielectric and copper film, and even micro-machining of micro-electro-mechanical
system (MEMS) [Sivaram, 1992; O’Mara, 1994; Sethuraman, 1996; Yasseen, 1997].
The CMP process has become the process of choice for planarizing the IC wafer in
current semiconductor manufacturing, and the number of its applications will increase as
more stringent design rules in circuit design are required for high performance/high density
IC chips.
10
2.3 Critical issues in CMP
Critical components which control CMP performance are addressed here, and the
principal process outputs are described.
2.3.1 Critical components in CMP
Pressure is required to remove material from the wafer surface.
It provides
mechanical action by abrasive particles in the material removal mechanism and controls the
characteristics of lubrication under the wafer surface. Pressure is defined as a load per
unit area, which means the pressure is uniform if the load is uniform. The actual pressure
applied on the wafer surface, in general, is not uniform due to the relative motion between
the wafer and the pad, the hydrodynamic effect by the slurry, and the visco-elastic
behavior of the polishing pad. To apply a uniform pressure on the wafer surface, various
methods have been proposed [Yamada, 1993; Hayashi, 1996; Hansen, 1996].
Velocities of the wafer and the pad (as well as the normal pressure on the wafer)
control material removal from the wafer and transport abrasive slurry to the wafer surface.
The kinematics of the relative motion between the pad and the wafer in CMP were
analyzed by using vector calculation [Brown, 1990; Hocheng, 1997]. Fig. 2.3 shows the
major variables in calculating the relative velocity of the wafer. The oscillation of the
wafer carrier was ignored in this calculation. The relative velocity, U, of an arbitrary
11
point, P, on the wafer can be expressed as a function of radius, R of the wafer and angle,
Θ.
U (R, Θ) = R 2 ⋅ (V p − Vc ) + V p ⋅ D 2 + 2 ⋅ R ⋅ (V p − Vc )⋅V p ⋅ D ⋅ cos Θ ,
2
2
(2.1)
where Vp is the angular velocity of the pad, Vc is the angular velocity of the carrier, and D
is the distance between the center of the platen and the center of the wafer. The angular
velocity of the carrier, Vc, is constant, so the time-averaged relative velocity of the wafer
is the same as the averaged relative velocity with respect to the angle, Θ, from 0 to 2π.
Therefore, the time-averaged relative velocity is
U (R ) =
U (R ) =
1
2π
∫
2π
0
1
2π
∫
2π
0
U (R, Θ)dΘ
(2.2)
R 2 ⋅ (V p − Vc ) + V p ⋅ D 2 + 2 ⋅ R ⋅ (V p − Vc )⋅ V p ⋅ D ⋅ cos ΘdΘ . (2.3)
2
2
Therefore, the rotation between the wafer and the pad becomes synchronous and the
relative velocity of the wafer is constant when the angular velocities of the pad and the
carrier are the same. The synchronized velocity condition is essential for uniform material
removal at any point on the wafer surface.
The abrasive slurry is a mixture of abrasive particles, a specially developed chemical
solution, and de-ionized (DI) water. The abrasives, generally, are silica particles with a
mean diameter less than 100 nm, Fig. 2.4. The chemical solution in the abrasive slurry is
selected depending on the material to be polished to achieve the selectivity in the material
removal between two materials. The slurry chemicals include: a buffering agent to control
12
pH of the slurry, an oxidizer, and a complexing agent to control the solubility of the wafer
surface [Steigerwald, 1997].
The polishing pad is generally made from cast polyurethane with a cellular structure,
or urethane coated polyester felt [Jairath, 1994]. A polishing pad, IC1000 from Rodel,
Inc. is shown in Fig. 2.5. During the process, the polishing pad helps distribute fresh
slurry to the wafer surface and provides contact between the abrasive particles and the
silicon wafer, causing wear from the wafer surface.
The mechanical, material, and
geometrical properties of the polishing pad determine the material removal and
planarization of the wafer and, ultimately, govern CMP performance. Recently, a fixed
abrasive type of polishing pad was introduced by 3M, Fig. 2.6. The fixed abrasive pad has
abrasives (such as cerium oxide) embedded on the pad surface and no abrasive slurry
(except DI water) is required during the polishing.
The conditioning of the polishing pad is essential in CMP due to the mechanical and
material degradation of the pad and the ‘glazing’ effect, Fig. 2.7, on the pad surface during
polishing. This is similar to the wheel dressing process in grinding. This degradation and
glazing phenomena are the cause of the decrease and instability of the material removal
rate during CMP. Conditioning is performed by abrading the pad surface with a diamondgrit wheel. It removes the glazing of the abrasive particles and recovers the initial surface
roughness of the pad.
13
2.3.2 Major process outputs in CMP
The material removal rate, generally measured in angstroms per minute, is an
important parameter in deciding the production capability of CMP applied in IC
fabrication. The material removal rate is dependent upon the pressure, velocity, polishing
pad property, and abrasive slurry in CMP. Typical material removal rates of CMP used in
IC production ranges from 1000 to 2000 Þ/min.
Planarization is one of the critical elements required to meet the stringent design
rules (sub-0.35 µm) of very large scale integration (VLSI) chips. Planarization of the
wafer is measured using a 49-point inspection, Fig. 2.8 [Fury, 1995].
For the
quantification of planarization, a standard method, called With-In-Wafer-Non-Uniformity
(WIWNU) is used. If an oxide wafer is polished, the WIWNU is defined as,
 oxide thickness i +1 - oxide thickness i

WIWNU(%) = Max 
× 100 ,
 mean of total oxide thicknesse s

(2.4)
where i=0,1,.., n-1, n. (n = the number of points). Eq. (2.4) indicates that planarization is
better with lower WIWNU values.
It was found that the surface roughness is proportional to the mean abrasive particle
size and the polishing pressure [Cook, 1990]. Surface roughness may depend on the
balance between the mechanical and the chemical removal in the CMP process
14
[Steigerwald, 1997].
Typical surface roughness of the wafer surface after CMP is
approximately in the range of 1 to 5 Þ root mean square (RMS) in 1µm x 1µm area.
2.4 Review: material removal mechanism and process model of CMP
In this section, the current material removal mechanism and process model will be
reviewed from three scales: wafer scale, feature scale, and particle scale [Runnel, 1994].
2.4.1 Material removal mechanism and process model at the wafer scale
A tribological model, Preston’s equation, has become a basic model for the CMP
process [Preston, 1927]. The Preston model predicts that the volumetric removal rate at a
point P on a workpiece is proportional to the normal load and the relative velocity.
dh( x )
dL( x ) ds( x )
|P = C
| ,
dt
dA
dt P
(2.5)
where h(x) is depth of wear, A is contact area, L(x) is total normal load, C is Preston’s
coefficient, s(x) is sliding distance, and t is processing time.
Preston’s coefficient, C, is a proportionality constant which depends on the
properties of the polishing pad, the abrasive particles, and slurry as well as the material
properties of the workpiece. Preston’s coefficient, therefore, closely depends on the
process conditions and contains all the unknown factors that cannot be explained using
15
only velocity and pressure. Since the Preston model was developed for glass/optics
polishing before the concept of CMP was proposed, it does not include the complicated
physical and chemical phenomena (such as hydrodynamic effect, electrochemical process,
and contact mechanics between the wafer and the pad, etc.) occurring in CMP. The
Preston model, however, has been adopted as a basic models of CMP and shows the
general dependence of velocity and pressure on the wear rate of the workpiece. Recently,
the suitability of the Preston’s equation was examined [Tseng, 1997] and a modified
Preston’s equation was proposed based upon the combined solid and fluid mechanics.
5
6
1
2
Material removal rate = M ( P,V ) ⋅ P ⋅ V ,
(2.6)
where P is pressure, V is velocity, and M(P, V) is a function of P and V. A velocity model
based on the kinematics of wafer-pad relative motion in a actual CMP process was
proposed [Hocheng, 1997].
Runnels et al. [Runnels, 1994] first considered the hydrodynamic effect of the slurry
film in CMP. There are three contact modes for any solid-solid interface which includes
lubrication and relative motion: direct contact, semi-direct contact, and hydroplane sliding
contact mode. In direct contact mode, the solid-solid contact supports the load between
two surfaces. In semi-direct contact mode, the lubrication between two surfaces partially
supports the load between two surfaces while some solid-solid contact still remains. In
hydroplane sliding contact mode, only the hydrodynamic lubrication film supports the load
between two surfaces. In Runnels’ model, the hydroplane sliding contact mode was
assumed to be a physical phenomenon occurring in CMP. Under the assumption that the
16
slurry exhibits Newtonian behavior and the pad and the wafer are rigid and flat, the
behavior of the slurry film was explained using Navier-Stokes equations for
incompressible Newtonian flow. The slurry film thickness was proportional to the speed
and viscosity, and the importance of wafer curvature on the slurry film thickness was
demonstrated. Based upon the slurry flow behavior, the normal directional erosion rate of
the wafer surface, Vn, was proposed as a function of the time-dependent tangential and
normal contact stresses, σt and σn.
Vn = f (σ t (t ), σ n (t )).
(2.7)
If the erosion rate in the normal direction is assumed as,
Vn = C ⋅ σ t ,
2
(2.8)
and by using the following relationships,
σt ≈
h∝
µ ⋅U
,
h
(2.9)
µ ⋅U
,
P⋅ A
(2.10)
where µ is viscosity, U is relative velocity, P is pressure, and A is wafer area, Vn becomes


Vn ∝ C  µ



2


U 
,
µ ⋅U 

P⋅ A 
Vn ∝ µ ⋅ A ⋅ U ⋅ P .
The Eq. (2.12) is identical to the form of Preston’s equation.
17
(2.11)
(2.12)
In Runnels’ study, the deflection of the polishing pad at the edges of the wafer and
the stress distribution from the wafer-pad contact were modeled and compared with the
experimental result [Runnels, 1993]. For the model of material removal rate (MRR), the
vector-valued shearing stress, ||S||, on the wafer surface replaced the relative velocity in
Preston’s model.
MRR = k ⋅ P ⋅ S .
(2.13)
As the shear stress is closely related to the physics of wear and erosion, shearing
stress was adopted.
In a recent study [Runnels, 1998] using a wafer-scale
phenomenological modeling, the Preston model was combined with an automatic model
validation algorithm and the combined model was implemented in CMP modeling software
environment called Plane-View.
A pad-bending model has been proposed based upon the assumption that the pad
behaves similar to a beam bending from the contact stress of the pad [Sivaram, 1992].
Using beam theory, the deflection in the vertical direction is
υ (x ) =
ω0
⋅ ( x 4 − 2 ⋅ l ⋅ x 3 + l 3 ⋅ x) ,
24 ⋅ E ⋅ I
(2.14)
where ωo is a normal load, E is elastic modulus, I is moment of inertia, and l is the length
of beam. Since the pad deflection has a great influence on the wafer planarization, Eq.
(2.14) was proposed as a method to measure the planarization of the wafer after CMP.
.
18
As mentioned earlier, the contact mode between the wafer and the pad surfaces has
been argued in the research on process modeling and material removal mechanism.
According to the different contact modes, separate process models have been proposed.
In hydroplane sliding contact mode, an erosion model based upon slurry-shear erosion and
hydrodynamic lubrication theory has been proposed [Runnels, 1994]. In direct contact
mode, a contact stress model and a pad-bending model have been proposed [Sivaram,
1992; Runnel, 1993]. It was found that in this semi-direct contact mode, none of the
models from the direct contact and hydroplane sliding contact modes was in good
agreement with the actual behavior of the CMP process [Bhushan, 1996].
2.4.2 Material removal mechanism and process model at the feature scale
A two-stage model relating the polishing time with the degree of non-planarity was
proposed by Burke [Burke, 1991]. The first stage consists of an analytical model based
upon the closed solution of an ordinary differential equation (ODE), and the second stage
includes a more complicated model adapting the polishing rate to the actual non-planarity
after iterations. In his model, Do is the percent polishing rate of low areas to that of a
blank wafer, and So is the initial step height related with Do. For Do < 0.3 and a constant
polishing rate U for ‘up’ areas, the polishing rate for ‘down’ areas is

S
D = 1 − (1 − Do ) ⋅
So

19

 ⋅ U ,

(2.15)
where S is the step-height and So is the step height value when D is Do. By using the
governing equation for the change in step height with time, t, the final equation of the
actual step height is
 U ⋅t ⋅(1− Do ) 

So

 −
S
= e
So
.
(2.16)
Warnock’s model [Warnock, 1991] is based on the physical description of the waferpad contact, Fig. 2.9. In this model, the pad deformation determines the horizontal length
scale and the pad surface roughness determines the vertical length scale. Based upon this
physical description, the polishing rate, Pi at a point, i on the wafer is
Pi =
K i ⋅ Ai
,
Si
(2.16)
where Ki is the kinetic factor (horizontal component), Ai is an accelerating factor (higher
points on the wafer), and Si is the shading factor (lower points on the wafer). The
meaning of the three coefficients is illustrated in Fig. 2.10. Si is high and Pi is low in lower
regions and Ai is high and Pi is high in higher regions. Ki is calculated by an effective
vertical component of the horizontal polishing rate and depends on the slope of the
surface. Under the assumption that the polishing rate is linearly proportional to the
pressure, Si is
Si = e
 ∆zi

 z
 o




,
(2.17)
where zo is a scaling factor for the vertical length scale and ∆z i is dependent upon how
much the surrounding topography protrudes above point i. Ai is determined by the
20
iterative process from Si, and Ki is 1+Ko tanαi , where Ko is a model parameter and αi is a
local angle between the horizontal and the polished surface. The Warnock model was
improved by Runnels [Runnels, 1995] by allowing closer fits to experimental data and
non-uniform characteristics of the wafer features after CMP.
Patrick [Patrick, 1991] proposed in that the pressure increases at the leading edge of
an IC feature on a wafer which travels relative to the polishing pad, Fig. 2.11. The
polishing pad is compressed, and the pressure increase at the leading edge of the feature
results in planarization developing from the edge toward the center of the feature. The
diminished pressure due to the loss of pad contact occurs at the inside corner. It was
proposed that the length from the base of the feature to the point at which the feature
separates with the pad depends on the characteristics of the pad being used, including the
dynamic response of the pad to deformation and the feature height. Thus, this physical
analysis indicates that closely spaced features will wear more slowly than widely spaced
features. This model explained the phenomena of superior material removal at the edges
of the feature obtained from the experimental result.
Ouma et al. proposed a two-stage CMP model considering the wafer-scale variation,
within-die pattern dependence, and their interaction [Ouma, 1997]. For the local-density
based model, a pattern dependent model, which is based on Preston’s equation, proposed
by Stine was adopted [Stine, 1997], Fig. 2.12.
21

 K ⋅t 


z = z 0 − 
z = g (x , y , K ) = 
 ρ 0 ( x, y ) 
 z = z − z − K ⋅ t + ρ (x , y ) ⋅ z

0
1
0
1
K ⋅ t < ρ 0 ⋅ z1
(2.18)
K ⋅ t > ρ 0 ⋅ z1
 ρ (x , y )
ρ ( x, y, z ) =  0
 1
z > z 0 − z1
z < z 0 − z1 .
(2.19)
In Eq. (2.18) and (2.19), z is the oxide thickness, K is the polishing rate of the blank
wafer, ρ0(x, y) is the local pattern density, and t is time. For the wafer-level modeling, a
series of experiments using different down forces, table speeds, carrier speeds, and back
pressures were conducted, and the removal rate was measured at a minimum of 121 sites
on a blank wafer. A second order model of the polishing rate, R(x, y), is defined as,
R( x, y ) = a + bx + cy + dxy + ex 2 + fy 2 ,
(2.20)
where a, b, c, d, e, and f are model coefficients and x and y are spatial coordinates on the
wafer. The coefficients are determined by a multiple regression using experimental data.
The average K in the local density based model for each die, i is determined as,
Ki =
1
Ai
∫∫ R(x, y )dxdy ,
(2.21)
where Ai is the area of each die. Thus, the combined wafer/die mode for the CMP process
predicts a pattern effect of any layout and the polishing characteristics of any wafer.
2.4.3 Material removal mechanism and process model at the particle scale
22
Yu et al. related the asperity of the polishing pad with the material removal rate in
his model [Yu, 1993]. To simulate the distribution of the asperities of the polishing pad, a
Gaussian distribution was adopted.
Based upon Preston’s equation, the Preston
coefficient was divided into three factors: a constant determined by the roughness and
elasticity of the polishing pad, the effect of surface chemistry and abrasion by the slurry,
and the contact area by the pad asperities.
A model based on elastic theory and statistical method was proposed to explain the
wear mechanism by abrasive particles by Liu et al. [Liu, 1996]. It was assumed that the
depth of penetration of the abrasive particles into the pad surface is greater than that into
the wafer surface, and the particles are in direct contact with the wafer surface during
polishing, Fig. 2.13. The depth of penetration, H, is determined by the hardness of the
abrasive and the wafer surface. By considering the duration of the penetration and the
deformation of the surface, the material removal rate (MRR) is defined as,
MRR =
3

HV w
2⋅R
⋅ H 2 ⋅V ⋅ 
 HV + HV
3
p
w


,


(2.22)
where R is the radius of the abrasive particle, V is the relative velocity, HVw and HVp are
Vickers’ hardness numbers for the wafer surface and the pad, respectively. The depth of
penetration is calculated using Hertz’s contact theory.
  E + Ew
H = C ⋅   a
  Ea ⋅ E w
23
2
3
 F 
 ,
 ⋅

 R
(2.23)
where Ea and Ew are Young’s moduli for the abrasive particle and the wafer surface,
respectively, and C is a constant. By substituting Eq. (2.23) into Eq. (2.22), the material
removal rate is defined as,

HV w
MRR = C ⋅ A ⋅ V ⋅ 
 HV + HV
p
w

  Ea + E w
⋅
  E ⋅E
  a w
 F 
 ⋅   ,
  A
(2.24)
where A is the area of the wafer, and F is the normal force. The constant C includes the
effect of the slurry chemical under the assumption that the chemical and the mechanical
actions are independent.
The chemical interaction between the abrasive and the oxide surface was well
defined by Cook [Cook, 1990]. In his glass polishing model, the factors determining the
rate of mass transport during glass polishing are defined as the rate of water diffusion into
the glass surface, the dissolution of the glass under the applied load, the adsorption rate of
the dissolved material onto the abrasive surface, the re-deposition of the dissolved material
onto the surface of the workpiece, and the aqueous corrosion between particle impacts.
He also considered the material removal process as a plowing process by abrasive particles
traveling across the wafer surface. Hertzian contact was assumed to be an indentation
process by the abrasives and its contact stress was calculated from the theory of elasticity.
The electrochemical effect and material removal mechanism in metal CMP were
proposed by Kaufman and Sainio [Kaufman, 1991; Sainio, 1996]. In metal CMP, the
chemical action by the slurry chemical dissolves the metal surface and forms a passivating
24
film preventing the isotropic chemical etching process on the wafer surface. By the
mechanical action of the abrasive particles and the polishing pad, the passivated film is
removed, achieving a degree of global planarization that is unmatched by the chemical
etching process. In general, the dissolution rate of the metal surface was found to be two
orders of magnitude lower than the polishing rate.
2.5 Process monitoring in CMP
In CMP, the material removal rate and the planarization of the wafer vary with time
during each wafer polishing and from wafer to wafer polishing, and, thus, it is quite
difficult to predict and control the CMP performance in-situ. To control the polishing
performance in-situ, a variety of process monitoring methods have been adopted
[Fukuroda, 1995; Dishon, 1997; Bibby, 1998; Tang, 1998].
After the polishing process removes a certain thickness of layer from the surface, it is
necessary to terminate the process at the desired end-point of the layer to prevent overpolishing, Fig. 2.14. Since there are too many variables which will have an influence on
the material removal rate and the planarization in CMP, it is necessary to adopt a robust
process monitoring technique to control the process performance. In predicting CMP
processes, time is generally the only process variable in measuring the end-point.
Numerous techniques have been proposed for in-situ end-point detection for use in CMP
25
[Bibby, 1998]. Among them are optical, electrical, and acoustic sensing methods. The
primary techniques used in optical sensing are the interferometry, reflectance, and spectral
reflectivity. Electrical sensing uses conductivity and impedance monitoring, or motor
current measurement for friction monitoring.
Acoustic methods measure the unique
acoustic wave emitted from the different layers being polished.
2.6 Post-CMP process
Since the CMP process is a ‘dirty’ process, it is necessary to clean the wafer with a
post-CMP process after each polishing to minimize surface contamination on the wafer
surface. Most contaminations originate from residual abrasive particles or slurry chemical
on the wafer surface. In general, mechanical and chemical methods are combined and
used as cleaning techniques in CMP.
In mechanical cleaning, brush scrubbing,
hydrodynamic jets, and ultrasonic or megasonic cleaning methods are commonly used.
For chemical cleaning, HF and NH4OH solutions mixed with DI water are utilized.
26
Photolithography
Out-of-focus
New planarization technique
required
Fig. 2.1 Difficulty in the fabrication of integrated circuit (IC) chip.
27
Fig. 2.2 A general schematic of the CMP process.
28
Fig. 2.3 Kinematic variables in the CMP process.
29
Fig. 2.4 Silicon oxide abrasive particles from Nalco2352 silicon polishing slurry.
30
(a)
(b)
(c)
(d)
Fig. 2.5 Side view ((a) x20 and (b) x110) and top view ((c) x200 and (d) x400) of
IC1000, polishing pad.
31
(a)
(b)
(c)
(d)
Fig. 2.6 Top view ((a) x40 and (b) x200) and oblique view ((c) x66 and (d) x300) of a
fixed abrasive pad.
32
(a)
(b)
(c)
(d)
Fig. 2.7 A (a) new and (b, c, d) used IC1000 polishing pad without conditioning.
33
Fig. 2.8 49-point inspection to measure the with-in-wafer-non-uniformity (WIWNU).
34
Fig. 2.9 Physical description of the polishing process in Warnock’s model.
35
Fig. 2.10 Arbitrary surface indicating the meaning of S, A, and K factors in Warnock’s
model.
36
Fig. 2.11 Schematic of pad deformation.
37
Fig. 2.12 Definition of variables used in Stine model.
38
Fig. 2.13 Schematic of the abrasive in contact with the wafer and the pad surfaces.
39
Fig. 2.14 A schematic of each polishing stage during CMP.
40
CHAPTER 3
MECHANICAL PROPERTIES OF THE POLISHING PAD AND ITS
RELATIONSHIP TO PROCESS PERFORMANCE IN CHEMICAL
MECHANICAL POLISHING (CMP)
3.1 Introduction
In CMP, the polishing pad is one of the most critical elements having a direct
impact on CMP performance (normally defined as material removal rate and
planarization). The pad is typically manufactured from cast polyurethane with a cellular
structure, or urethane coated polyester felt [Jairath, 1994], which is then sliced to the
proper thickness. During the process, the polishing pad serves as a medium to provide
contact between the abrasive particles and the silicon wafer, and also influences the
degree of the wafer planarization. The nature of contact between the polishing pad and
the workpiece during the CMP process has been determined to be strongly correlated to
the mechanical, material, and geometrical properties of the polishing pad that ultimately
govern CMP performance.
The material removal rate of silicon and oxide CMP is inversely proportional to the
density and shear modulus of the polishing pad [Bajaj, 1994; Li, 1995, Beeler, 1999]. A
low-density pad tends to have a higher initial material removal rate than a high-density
pad. The polishing rate of silicon wafer by the low-density pad, however, drops much
41
faster compared to that of the high-density pad because deterioration of the mechanical
properties of the low-density pad occurs faster. Hence, the high-density polishing pad
normally has a more consistent material removal rate during CMP processing.
The
consistency of the material removal rate is also known to be dependent upon the cellular
structure (open-cell or closed-cell) of the pad [Anjur, 1998]. The material removal rate in
CMP also decreases as the period of usage, the pad soaking time in water, and the pad
surface temperature increase during CMP [Desai, 1994; Li, 1995].
The planarization efficiency in CMP has a close relationship with the pad properties
as well. Generally a stiffer or harder pad has better planarization [Singer, 1994] than
softer ones. The planarization is also controlled by the pad thickness and the use of a
subpad which is generally softer than the regular pad [Devriendt, 1999]. A number of
studies using the Finite Element Method (FEM) have been done to investigate the effect
of stress distribution on the wafer as applied by the polishing pad on the efficiency of
planarization [Murthy, 1997; Wang, 1997; Guo, 1998; Sasaki, 1998; Tseng, 1998]. The
origin of the irregular planarization of the wafer in CMP has been explained by the nonuniformity of Von Mises, normal, and shear stresses applied on the workpiece by the
polishing pad.
The pad conditioning step is another important issue in CMP. Due to mechanical
and material degradation, and more critically the ‘glazing’ effect, the polishing pad needs
to be conditioned regularly in order to keep the process performance constant. Glazing
occurs when the pores of the pad become clogged with abrasive particles from the slurry.
42
This phenomenon prevents uniform transport of the slurry across the wafer.
Conditioning is usually done by abrading the surface of the pad with a diamond coated
wheel, which recovers the original surface and the initial surface roughness of the pad. It
has been observed that the material removal rate dramatically decreases with time
without pad conditioning until it reaches a low steady-state value in CMP [Li, 1995;
Stein, 1996]. The polishing pad profile after conditioning has a critical effect on the
wafer uniformity after CMP [Freeman, 1996; Mullany, 1999]. The overall polishing pad
lifetime can be extended by increasing the wafer backpressure during CMP processing
[Janzen, 1996].
In this chapter, the mechanical, material, and geometrical properties of polishing
pads typically utilized in CMP are examined.
Next, the relationship between the
properties of the polishing pad and the CMP performance is identified by an empirical
method. Tribological aspects such as friction and wear occurring in the CMP process are
studied and related to the physical description on the interfacial behavior between the
silicon wafer and the polishing pad. Finally, an integration of the tribological aspects into
a model of the CMP process is accomplished by using the relationship between Preston’s
coefficient and the friction coefficient between pad and wafer surfaces.
43
3.2 Polishing pad in CMP
Polyurethane is one of the engineered polymers and major material comprising the
polishing pad [Fried, 1995]. It has high strength, good resistance to chemical, and high
abrasion resistance.
It is normally created by the step-growth polymerization of
diisocyanates with dihydroxyl compounds [Jairath, 1994]. The foam structure in the
polishing pad is made by a gaseous phase dispersion in the polymer possibly
accomplished by a direct gas injection under pressure or by the addition of certain
chemical agents which decompose into gas and other by-products [McCrum, 1997].
The structure of the polishing pad is basically an open cell structure and the
mechanical properties of the cellular solids or foams are related to the properties of the
cell wall material and to the cell geometry [Ashby, 1983].
Three categories of polyurethane polishing pads are commonly used in the CMP
process:
(1) impregnated felt substrated-coagulated urethane in a fiber matrix, (2)
microporous polyurethane polishing material and (3) napped poromerics-porous urethane
layers on supporting substrates.
The first type is used for stock removal or rough
polishing. Increased removal rates and extended pad life are achieved because of the
improved pad porosity and reduced compressibility due to a special impregnation
technique. The second type is designed to minimize scratching and provide optimum
polishing performance such as global planarization. The third type is adopted to remove
only small amounts of surface irregularities and is most often used in the final polishing
step.
44
In the experiment, SUBA500, IC60, and UR100 polishing pads from Rodel were
used, representing type I, type II, and type III of pad, Fig. 3.1, Fig. 3.2, and Fig. 3.3,
respectively.
3.3 Preston’s wear equation
The basis for the CMP process model which will be developed here is Preston's wear
equation [Preston, 1927]. Preston’s equation states that the volumetric removal rate on a
workpiece, due to the relative motion between surfaces, is proportional to the bearing
load and the relative velocity. The equation has been accepted and utilized as a basic
mathematical model for the CMP process, and has the form
h& = C ⋅ P ⋅ v ,
(3.1)
where P is pressure, v is relative velocity, h& is average height removed/unit time, and C
is Preston’s coefficient.
Preston’s coefficient, C, is simply a proportionality constant and, so far, it has not
been fully determined which physical variables can affect its value.
What has been
determined is that C depends on the properties of the polishing pad and the abrasive
slurry as well as the material properties of the workpiece. Thus, C is highly dependent
upon the different operating conditions used in the specific CMP process and it is unique
for each process.
We can rewrite Eq. (3.1) as [Brown, 1990],
45
∆h
L ∆s
=C⋅ ⋅
A ∆t
∆t
→ ∆h = C ⋅
L
⋅ ∆s
A
(3.2)
(3.3)
→ ∆h ⋅ A = C ⋅ L ⋅ ∆s
(3.4)
→ ∆V = C ⋅ L ⋅ ∆s,
(3.5)
where h is height removed, A is apparent contact area between workpiece and pad, L is
contact load, V is volume of material removed, s is sliding distance, P is pressure on
workpiece, and t is time.
By multiplying Eq. (3.5) by the density of the workpiece, the mass removed can be
obtained.
ρ ⋅ ∆V = ρ ⋅ C ⋅ L ⋅ ∆s
(3.6)
→ ∆m = ρ ⋅ C ⋅ L ⋅ ∆s ,
(3.7)
where ρ is density of workpiece and m is mass.
Therefore, Preston’s coefficient can be determined from Eq. (3.7),
C=
∆m
,
ρ ⋅ L ⋅ ∆s
(3.8)
The energy input, ∆Q , should be proportional to the volume of material removed.
∆Q = k1 ⋅ ∆V ,
(3.9)
where k1 is constant and ∆Q is energy input.
The energy input is also proportional to the work done by friction, which is the
product of the friction force, F and the displacement, ∆s ,
∆Q = k 2 ⋅ F ⋅ ∆s ,
46
(3.10)
where k2 is constant and F is friction force between workpiece and polishing pad.
Therefore,
k 1 ⋅ ∆V = k 2 ⋅ F ⋅ ∆s ,
(3.11)
k1 ⋅ C ⋅ L ⋅ ∆s = k 2 ⋅ F ⋅ ∆s ,
(3.12)
k1
⋅C ⋅ L .
k2
(3.13)
By using Eq. (3.5),
→F =
For Coulomb friction, which is assumed in this case, the friction force is defined as
the friction coefficient, µ, times the normal load, L,
F = µ ⋅L.
(3.14)
Therefore, we can obtain a mathematical relationship between Preston’s coefficient
and the friction coefficient as,
k1
⋅C ⋅ L = µ ⋅ L
k2
(3.15)
k1
⋅µ
k2
(3.16)
→ C = α ⋅ µ,
(3.17)
→C =
where α is constant of proportionality and µ is friction coefficient.
Therefore, theoretically, Preston’s coefficient should be proportional to the friction
coefficient.
This relationship needs to be evaluated and verified experimentally.
Preston’s coefficient in Eq. (3.17) can be obtained by Eq. (3.8) when the material
removal is obtained experimentally. The friction coefficient in Eq. (3.17) is calculated
from the friction force measured in-situ during the CMP process.
47
3.4 Experiments
CMP experiments were conducted to study the effects of the properties of the
polishing pad (density, compressibility, and surface roughness) on the material removal
rate in CMP. Also the friction force between the silicon wafer and the polishing pad was
measured in process and correlated with the properties of the polishing pad and the CMP
performance. By using the friction force data, the relationship between the Preston’s
coefficient and the friction coefficient was estimated and integrated in Preston’s equation.
The CMP machine used in this experiment is a laboratory-scale polishing tool,
which was originally designed as an optics polishing machine at Lawrence Livermore
National Laboratory. The advantage of this machine is that the relative velocity between
the wafer and the polishing pad is always constant due to the translational motion of the
carrier. The carrier rotates with respect to the center of the polishing pad, not to the
center of the wafer. Hence, the material removal rate, theoretically, is constant for any
location of the wafer. In addition, a load cell (333.75N (=75 lb) maximum load) which is
able to measure the friction force between the polishing pad and the workpiece is
mounted on the polishing plate. The load cell was connected to an oscilloscope and a PC
with a Gagescope data acquisition system for data collection.
Polished bare silicon wafers (<100> orientation and P-type) were used as the
workpiece. The slurry was Nalco 2352, a commercial colloidal silica polishing slurry
with a mean abrasive particle diameter of 70-90 nm. The dilution ratio was 10:1 (de48
ionized (DI) water:slurry) which contains 4.5 % abrasives by weight in the slurry. The
abrasive slurry (continuously stirred by a magnetic mixer) is supplied to the pad surface
through a peristaltic pump with a 30 ~ 150 ml/min flow rate. The relative speed of the
workpiece was 5 cm/s. The normal load on the wafer was 45N which corresponds to a
pressure of 5.7 kPa. The polishing time for each silicon wafer was 5 hours. The CMP
process was stopped every hour to measure the mass of workpiece. Before the mass
measurement, the silicon wafer was cleaned first with an ultrasonic cleaner and air-dryer,
and its mass measured using a Sartorius electric balance with 0.1 mg resolution. A Zygo
Maxim3D laser interferometer with 0.1 angstrom resolution was used to measure the
final surface roughness.
Before each test, the wafer flatness was inspected using a
Taylor-Hobson profilometer to exclude the geometric effect on the material removal rate
and the friction force.
The signal output from the load cell was amplified and measured every 10 minutes
using a PC with a Gagescope data acquisition system at a 100Hz sampling rate. The
signal was calibrated to convert it to an equivalent friction force.
From Eq. (3.8),
Preston’s coefficient was calculated and compared with the measured friction coefficient.
All friction coefficients were sampled every 10 minutes for each hour and averaged to get
a medium value. In other words, if µ1, µ2, ..., µ6 are the friction coefficients sampled at
ten minute intervals for an hour, the medium value µ will be (µ1+µ2+...+µ6)/6.
A schematic of experimental setup is shown in Fig. 3.4. The material properties of
each pad are given in Table 1.
49
3.5 Results and discussion
3.5.1 Relationship between the material removal rate and the properties of the
polishing pad
The material removal rate for each silicon wafer polished using the three polishing
pads ranged from 100 to 340 Þ/min, Fig. 3.5. The UR100 pad resulted in the maximum
material removal rate and the IC60 had the minimum removal rate under the same
process parameters including relative velocity and normal pressure. During the process,
the material removal rate was nearly constant within a 15% variation. Since all the
controllable process parameters except the polishing pad were fixed in this experiment, it
is strongly believed that the discrepancy in material removal rates of each silicon wafer
results from the difference of the mechanical, material, or geometrical properties of the
polishing pads. In this study, among the properties of the pad, density, compressibility,
and surface roughness were chosen for analysis.
The material removal rate of silicon wafer by each pad was correlated to the pad
density, compressibility, and surface roughness. The material removal rate of silicon
wafer was inversely proportional to the pad density and proportional to the pad
compressibility and surface roughness although the relationship was not exactly linear,
Fig. 3.6 and Fig. 3.7.
The relationship between the material removal rate and the properties of the
polishing pad suggests that the material removal of silicon wafer in CMP is closely
50
related to the actual pad contact area to the wafer, which is an indication of the
probability for abrasives to contact the wafer surface. With higher surface roughnesses of
the pad, there is more contact between the pad and the wafer. If it is assumed that the
abrasive particles trapped between the polishing pad asperity and the wafer surface cause
material removal, the more pad area contact with wafer indicates the more material
removal rate.
The restriction of the slurry flow due to the pad surface roughness can be another
factor to control the actual contact area between the polishing pad and the wafer surface.
A polishing pad with high surface roughness can prevent the free flow of the abrasive
slurry and, thus, the formation of the slurry film between the wafer and the pad as the
wafer slides over the pad. A thin slurry film induced by a rough pad surface results in
more pad contact with the wafer surface. A smooth pad, however, promotes the slurry
flow and aids the formation of a slurry film between the wafer and the pad, resulting in
less contact between the wafer and the pad. The decreased contact area between the
wafer and the pad leads to less material removal.
These experiments verified that the material removal rate is inversely proportional
to the pad density and proportional to the compressibility of the pad.
Using the
relationship of the pad density with its mechanical properties, it is believed that the
material removal rate is inversely proportional to the square root of the elastic modulus
and shear modulus of the pad [Bajaj, 1994].
Material removal rate ∝
51
1
ρ pad
(3.18)
2
 ρ pad
E ∝
ρ
 polymer




 ρ pad
G∝
ρ
 polymer




(3.19)
2
→ Material removal rate ∝
(3.20)
1
1
,
,
E G
(3.21)
where ρpad is pad density, ρpolymer is polymer density, E is elastic modulus, and G is shear
modulus.
Pads with low density also have low elastic and shear moduli, E and G. This causes
larger deformation and higher compressibility of the pad surface. The large deformation
of the pad surface causes increased contact between the polishing pad surface and the
wafer. The increased pad contact due to the large deformation of pad will increase the
probability that the abrasive particles trapped between the wafer and the pad asperity
abrade more from the wafer surface.
Pad density and compressibility have a strong relationship with the pore size and
the number of pores. If a pad has a large pore size and a large number of pores, the pad
density will be low and the compressibility will be high. The low pad density results in
the low elastic and shear modulus, causing large deformation and higher compressibility
of pad. The pad density is inversely proportional to its compressibility and the effect of
compressibility was the opposite of that of the density effect.
There is no material removal without the abrasive slurry.
In other words, the
polishing pad itself cannot remove material. It transports and supplies the abrasive slurry
for the mechanical and chemical action.
52
The higher material removal also results from the enhanced abrasive slurry flow to
the wafer surface and from the ease of transport of the abraded material from the wafer
surface.
The foam-structured polishing pad aids both the chemical action as well as the
mechanical action. Although the pores cannot make the slurry reaction more active
chemically, they can play a role in helping to transport slurry to all parts of the wafer
uniformly. Following the mechanical action and the chemical action by the abrasive
slurry, the pores in the pad facilitate the transportation of the removed material from the
wafer surface. If pad materials do not have a sufficiently porous structure, the free flow
of slurry in and out of the pad would be impeded, and this would result in the decrease of
material removal from the wafer surface.
The fatigue behavior or mechanical deterioration of the polishing pad affects the
material removal of the wafer surface. The stability of the process output will change
when pad properties vary during processing. Pads with low density are more likely to
collapse under the applied load and immediately begin to lose their effectiveness. The
pad with high density can maintain its initial polishing rate longer due to its rigidity even
though its initial polishing rate is considerably lower than that of the low-density pad.
53
3.5.2 Relationship between the friction force and coefficient and the properties
of the polishing pad
Before considering the effect of the friction force on the workpiece, it was necessary
to consider the effect of surface roughness variation of the workpiece. The surface
roughness was measured before and after each CMP test for each silicon wafer. Before
CMP, the surface roughness of the three wafers was typically around 2nm Rms over a
235µm x 212µm area. After the CMP process, the surface roughness was measured
again to determine the change. The final surface roughness for the three polished wafers
was around 3nm Rms over a 235µm x 212µm area. Compared to the typical surface
roughness of a polishing pad(~ 100µm) [Stein, 1996], the effect of surface roughness
variation of silicon wafer on friction force was negligible.
The friction force variation applied on the wafer surface from the contact with the
polishing pads was measured during the polishing process, Fig. 3.8 (a). Using the friction
force data, the friction coefficient variation was calculated, Fig. 3.8 (b).
Under the
normal force of 45N, the friction force exerted by each pad ranged approximately from
15 to 40N, resulting in a range of the friction coefficient from 0.35 to 0.9.
The friction force on the silicon wafer polished by UR100 was the highest of the
polishing pads. The friction force by the IC60 pad was the lowest. This was also seen
from the variation of the friction coefficient under the constant normal load.
The relationship between the friction force and the properties of the polishing pad is
similar to what has been observed from the relationship between the material removal
54
rate and the pad properties. The friction force applied on the wafer during the process
was inversely proportional to the pad density and proportional to the pad compressibility
and the surface roughness. The results verify the variation of actual contact area of
polishing pad on the wafer. It is believed that the actual contact area between the
polishing pad and the wafer surface increases as the density of polishing pad decreases
and the compressibility and surface roughness increase. The increased actual pad contact
area on the wafer surface causes higher friction force under a constant normal force,
which was verified from the experimental results. In the relationship between the friction
force and the surface roughness, the slurry film under the wafer, which acts as a
lubrication film, also contributes in determining the friction force applied on the wafer.
For the smooth pad, due to the enhanced formation of the slurry film, the friction force
was minimized. In the relatively rough pad, since the creation of the slurry film is much
more difficult due to the hindrance of the slurry flow and the impeded formation of the
slurry film, the friction force between the wafer and the pad tends to be very large.
3.5.3 Relationship between the material removal rate and the friction force and
coefficient
The material removal rate of each silicon wafer was correlated with the friction
force applied on the wafer surface and the friction coefficient calculated from the friction
force.
The material removal rate was proportional to the friction force and coefficient
between the wafer and the pad, Fig 3.11. As the compressibility and surface roughness
55
of the pad increase, or the pad density decreases, the actual contact area between the
wafer and the pad increases, causing an increase in the friction coefficient between wafer
and pad surfaces. Under a constant normal force, the friction force applied on the wafer
surface increases due to the increase in the friction coefficient. Since the increase of the
actual contact area between the wafer and the pad also increases the material removal
rate, the friction force and coefficient will be proportional to the material removal rate.
By measuring the friction coefficient during the process, it is possible to estimate the
material removal of a workpiece given a set of experimental parameters such as chemical
composition of the slurry, abrasive particle size and material, and the surface material of
the workpiece. First, to ensure the material removal was independent from the relative
velocity of the workpiece, the material removal per sliding distance instead of material
removal rate was calculated. The material removal per sliding distance is correlated with
the friction coefficient obtained during the CMP process, Fig. 3.12.
A first order
interpolation was performed to calculate the proportionality constant for the relationship
between the material removal per sliding distance and friction coefficient.
∆h
= k ⋅ µ,
∆s
(3.22)
where k is proportionality constant which is 116.16 Þ/meter.
Slurry film thickness under the wafer is a function of relative velocity of the wafer,
normal pressure on the wafer, and viscosity of slurry, and the actual contact area is highly
dependent upon the slurry film thickness, the properties of the polishing pad (density,
compressibility, and surface roughness), and integrated circuit pattern density.
56
Therefore, assuming the friction coefficient is dependent upon the characteristics
(slurry film thickness and actual contact area under the wafer surface) of the interface
between the wafer and the pad, the material removal is predicted by monitoring the
variation of the friction coefficient given a chemical in the slurry, abrasive particle size
and material, and the material of the wafer surface. This is a promising in-situ method to
monitor the material removal in CMP process.
3.5.4 Relationship between the Preston’s coefficient and the friction coefficient
Preston’s coefficient was calculated from Eq. (3.8) and plotted versus time, Fig.
3.12. In Eq. (3.8), the material removal was taken from the experimental result. Similar
to the result of the material removal rate, the UR100 pad had the highest value and the
IC60 pad had the lowest.
As expected from the analytical results, Preston’s coefficient was linearly
proportional to the friction coefficient.
After plotting each data set, a first order
interpolation was performed to calculate α as 2.038×10-2 m2/N.
The experiments verified that Preston’s coefficient is linearly proportional to the
friction coefficient with a proportionality constant, α (=2.038×10-2 m2/N).
C =α ⋅µ
(3.17)
→ C = 2.038 × 10 −2 ⋅ µ .
(3.23)
Thus, a modified model for CMP process is proposed as
57
h& = C ⋅ P ⋅ v
(3.1)
h& = α ⋅ µ ⋅ P ⋅ v
(3.24)
h& = 2.038 × 10 −2 ⋅ µ ⋅ P ⋅ v.
(3.25)
For situations where the hardness of the workpiece, Pw, is less than 80% of the
hardness of abrasive, 0.8Pa, the abrasion equation [Rabinowicz, 1975] can be used to
determine the change in volume of an abraded workpiece, ∆V, as
∆V = tanθ
L
∆s ,
π ⋅ Pw
(3.26)
where tanθ is weighted average of the tanθ value and θ is angle of abrasive grain with
bearing surface.
If Eq. (3.26) is compared with Eq. (3.5) after plugging the Eq.(3.17) in,
∆V = tanθ
α ~
L
∆s = α ⋅ µ ⋅ L ⋅ ∆s
π ⋅ Pw
tanθ
.
Pw
(3.27)
(3.28)
Therefore, assuming the friction coefficient is constant, the constant α can be
expressed as a function of the material properties of workpiece such as hardness and the
mechanical properties of the abrasive slurry.
If we apply Eq. (3.17) for the volume removal, Eq. (3.5),
∆V = C ⋅ L ⋅ ∆s
(3.5)
→ ∆V = α ⋅ µ ⋅ L ⋅ ∆s .
(3.29)
58
By using the definition of the friction force and the work by the friction force, Eq.
(3.29) changes to
∆V = α ⋅ F ⋅ ∆s
(3.30)
→ ∆V = α ⋅ W friction .
(3.31)
Thus, the volumetric removal is linearly proportional to the friction force between
workpiece and polishing pad and also to the work by friction force, Wfriction, during CMP.
3.6 Summary
Experiments on the CMP process using silicon wafers and three different types of
polishing pads were conducted to study the effects of the properties of the polishing pad
(density, compressibility, and surface roughness) on the material removal rate and
friction force between the silicon wafer and the polishing pad. The friction force applied
on the wafer surface was also correlated with the material removal rate of the silicon
wafer in CMP.
The material removal rate of the silicon wafer is inversely proportional to the pad
density and proportional to the pad compressibility and surface roughness. It is believed
that this is due to the fact that the material removal for a silicon wafer in CMP is closely
related to the actual pad contact area to the wafer. This is an indication of the probability
of abrasives contacting the wafer surface. The friction force applied on the wafer during
the process was inversely proportional to the pad density and proportional to the pad
compressibility and the surface roughness since the actual pad contact area on the wafer
59
surface highly influences the friction force on the wafer surface. Thus, the material
removal rate is proportional to the friction force between the workpiece and the polishing
pad and also to the work done by the friction force on the workpiece for silicon CMP. By
using the friction force data, the relationship between the Preston’s coefficient and the
friction coefficient was estimated and integrated into Preston’s equation.
Preston’s
coefficient is linearly proportional to the friction coefficient during CMP. Therefore, the
material removal can be predicted by measuring the friction coefficient applied between
the wafer and the pad during the CMP process.
60
Table 3.1 Material Properties of SUBA500, IC60, and UR100 (Rodel).
COMPRESSIBILITY(%)
MEAN
THICKNESS(IN)[cm]
WEIGHT
DENSITY(g/cm3)
HARDNESS
(SHORE A)
SURFACE
ROUGHNESS(µm)-PV
SUBA500
7
0.05
[0.127]
IC60
4
0.05
[0.127]
UR100
14
0.059
[0.15]
0.37
0.685
0.35
65-78
52-60
Not available
133µm
58µm
186µm
61
(a)
(b)
(c)
Fig. 3.1 Oblique view of the polishing pads of (a) SUBA500, (b) IC60, (c) UR100
from the Rodel Inc.
62
(a)
(b)
(c)
Fig. 3.2 Cross-sectional view of (a) SUBA500, (b) IC60, and (c) UR100 polishing
pads at two magnifications (x20 and x66).
63
(a)
(b)
(c)
Fig. 3.3 Top view of (a) SUBA500, (b) IC60, and (c) UR100 polishing pads at
two magnifications (x40 and x200).
64
Fig. 3.4 Schematic of experimental setup.
65
Material removal rate(angstrom/min)
500
450
400
UR100
350
300
250
SUBA500
200
150
IC60
100
50
0
0
1
2
3
4
5
6
Time(hour)
Fig. 3.5 The material removal rate for each polishing pad.
66
Material removal rate(angstrom/min)
400
First order regression
UR100
350
300
250
SUBA500
200
150
100
IC60
50
0.3
0.4
0.5
0.6
0.7
Pad density(g/cm3)
(a) Material removal rate variation with polishing pad density
Material removal rate(angstrom/min)
400
UR100
350
300
SUBA500
250
200
150
100
IC60
First order regression
50
2
4
6
8
10
12
14
16
Compressibility(%)
(b) Material removal rate variation with polishing pad compressibility
Fig. 3.6 Material removal rate variation with polishing pad density and compressibility.
67
Material removal rate(angstrom/min)
400
UR100
350
300
SUBA500
250
200
150
IC60
100
First order regression
50
40
60
80 100 120 140 160 180 200
Pad roughness(mm)
Fig. 3.7 Material removal rate variation with polishing pad roughness.
68
50
45
UR100
Friction force(N)
40
35
30
SUBA500
25
20
IC60
15
10
5
First order regression
0
-1
0
1
2
3
4
5
6
Time(hour)
(a) Friction force variation on the wafer for each polishing pad
1.0
UR100
0.9
Friction coefficient
0.8
0.7
SUBA500
0.6
0.5
IC60
0.4
0.3
0.2
0.1
First order regression
0.0
-1
0
1
2
3
4
5
6
Time(hour)
(b) Friction coefficient variation on the wafer for each polishing pad
Fig. 3.8 Friction force and coefficient variation on the wafer for each polishing pad.
69
40
UR100
Friction force(N)
35
30
SUBA500
25
IC60
20
First order regression
15
0.3
0.4
0.5
0.6
0.7
Pad density(g/cm3)
(a) Friction force variation with polishing pad density
45
First order regression
Friction force(N)
40
UR100
35
SUBA500
30
25
IC60
20
15
2
4
6
8
10
12
14
16
Compressibility(%)
(b) Friction Force Variation with Polishing Pad Compressibility
Fig. 3.9 Friction force variation with polishing pad density and compressibility.
70
45
UR100
Friction force(N)
40
35
SUBA500
30
25
IC60
20
First order regression
15
40
60
80 100 120 140 160 180 200
Pad roughness(mm)
Fig. 3.10 Friction force variation with polishing pad roughness.
71
Material removal rate(angstrom/min)
400
UR100
350
300
SUBA500
250
200
150
IC60
100
50
First order regression
0
0
5 10 15 20 25 30 35 40 45 50
Friction force(N)
(a) Material removal rate variation with friction force
Material removal rate(angstrom/min)
350
UR100
300
250
SUBA500
200
150
IC60
100
50
First order regression
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Friction coefficient
(b) Material removal rate variation with friction coefficient
Fig. 3.11 Material removal rate variation with (a) friction force and (b) coefficient.
72
First order regression
Fig. 3.12 Material removal per sliding distance with friction coefficient.
73
Fig. 3.13 Preston’s coefficient variation during CMP process.
74
First order regression
Fig. 3.14 Preston’s coefficient variation with friction coefficient.
75
CHAPTER 4
THE EFFECT OF SLURRY FILM THICKNESS VARIATION IN CHEMICAL
MECHANICAL POLISHING (CMP)
4.1 Introduction
Chemical mechanical polishing (CMP) is an abrasive machining process in which
an aqueous abrasive slurry is used as a ‘cutting tool’ to remove material on a nano scale.
Unlike other abrasive machining processes such as lapping and grinding, CMP still has
an unclear description of the interaction of abrasives on the wafer surface with the surface
of the polishing pad. In the lapping process, the major material removal mechanisms are
micro-scratching or direct-indentation by abrasive particles on the workpiece [Phillips,
1977; Marshall, 1982; Golini, 1991]. Due to the relatively large size of abrasives and
hardness of the lapping plate, direct contact of abrasive particles on the surface of
workpiece is made [Izumitani, 1973; Chauhan, 1993; Buijs, 1993], Fig. 4.1. In CMP, the
material removal is created due to the contact between the wafer surface and the
polishing pad charged with abrasive slurry. The major material removal mechanism,
however, is uncertain due to the relatively small size of abrasives (less than 0.1µm), the
softness of the polishing pad, and the chemical reaction occurring on the wafer surface
due to the chemistry of the slurry.
To obtain a geometrical relationship between the pad asperity and the abrasives, a
cross-section of a UR100 polishing pad was inspected by using a scanning electron
76
microscope (SEM), Fig. 4.2. The pad has double layers where the top layer is made of
polyurethane and the bottom layer is urethane-impregnated polyester layer. The top
layer, which is in contact with the wafer surface during CMP, consists of end fibrils and
vertically oriented pores. The total height is approximately 400µm. The total thickness
of the pad is nearly 1500µm. The abrasives, which are normally less than 0.1µm, are
invisible at this scale. Therefore, it is improbable that the abrasives make indentations or
micro-scratches through direct contact supported by the polishing pad or the plate under
the pad.
Based upon the geometrical relationship between the wafer and the pad surface, the
microscopic view of the interface is schematically illustrated in Fig. 4.3. With applied
normal pressure, the wafer slides on the pad, which is charged with abrasive slurry.
According to the geometrical relationship illustrated, it is believed that the material
removal rate will be influenced by the actual pad contact area on the wafer surface since
the actual pad contact area on the wafer surface controls the number of active abrasives
which are in contact with the wafer surface and abrade material from the wafer surface.
Under a certain pad and slurry combination, the actual pad contact area, therefore, will
heavily depend on the wafer-pad contact mode or the slurry film thickness, which is
defined as the distance between the bottom of the wafer and the base surface of the
polishing pad in this study, Fig. 4.4. In this study, the effect of slurry film thickness
variation on CMP performance is investigated.
Understanding the wafer-pad contact mechanics is fundamental and crucial to
understanding the material removal mechanism of the CMP process as well as to better
77
control CMP performance. There exist quite a few factors which control the wafer-pad
contact mode. The surface roughness (or bearing ratio), density, compressibility of the
polishing pad and the slurry film thickness between the wafer and the pad which is
controlled by relative velocity, pressure, and viscosity of slurry are among them.
Given a certain consumable set (abrasive slurry and polishing pad), the major factor
which dominates the wafer-pad contact mode is slurry film thickness between the wafer
surface and the polishing pad. It has been acknowledged that understanding the behavior
of slurry film thickness in CMP is fundamental to the investigation of material removal
mechanisms and the development of a process model for CMP [Runnels, 1994].
The slurry viscosity, rotation speed, normal pressure, and wafer curvature are
critical variables determining the slurry film thickness in CMP [Runnel, 1994].
The
variation of slurry film thickness has been numerically calculated in terms of rotation
speed and slurry viscosity [Runnels, 1994; Ticky, 1998] and was measured in-situ by
using capacitance air gap probes [Levert, 1996].
A simplified two-dimensional
numerical model of slurry flow under a wafer surface has been developed to predict the
pressure and shear stress under a wafer [Rogers, 1998]. Wafer drag force and slurry film
thickness have been measured experimentally.
The characteristics of lubrication (boundary lubrication, elasto-hydrodynamic
lubrication, and hydrodynamic lubrication) were determined and shown to depend on the
lubrication film thickness [Williams, 1994]. In order to explain the behavior of the slurry
film during CMP, hydrodynamic lubrication theory has been adopted [Runnels, 1994].
The slurry layer thickness was assumed to be in the range from 10 to 50 µm depending
on the relative velocity of the wafer.
78
The wafer-pad contact mechanics will be considered in more detail in the following
section 4.2 using three wafer-pad contact modes. In addition, the concept of
hydrodynamic lubrication theory as applied to CMP will be explained.
4.2 Wafer-pad contact modes
When the wafer slides on the polishing pad during CMP, there exist three types of
contact modes between the wafer and the pad: direct, semi-direct, and hydroplane sliding
[Komvopoulos, 1990; Williams, 1994; Runnels, 1994; Bhushan, 1995]. When the wafer
rigidly contacts the pad surface with a high normal pressure and low relative velocity, the
effect of lubrication will be negligible and the friction force at the interface will be high.
This contact is known as direct contact. As the normal pressure decreases or the relative
velocity between the wafer and the pad increases, the thickness of lubrication increases
and the wafer will slide without much friction on the pad surface.
This is called
hydroplane sliding contact. Semi-direct contact is in the transition between direct and
hydroplane sliding contact.
A general relation of the friction coefficient and the lubrication film with viscosity,
velocity, and normal load has been characterized by the Stribeck curve [Hutchings, 1992;
William, 1994], Fig. 4.5.
At low viscosity, low velocity, and high pressure, the
lubrication film thickness is small and the state of contact of two surfaces is in direct
contact.
When two surfaces are in direct contact, the state of lubrication is called
boundary lubrication. At boundary lubrication, the friction force in the interface of two
79
surfaces is extremely high.
As the viscosity and velocity increase and the normal
pressure decreases, the lubrication film thickness between two surfaces increases and the
contact modes will change from direct contact to semi-direct and hydroplane sliding
contact. The lubrication in semi-direct and hydroplane sliding contact mode correspond
to the elasto-hydrodynamic and the hydrodynamic lubrication, respectively. The friction
coefficient will initially decrease until elasto-hydrodynamic lubrication is the dominant
lubrication; then it will start to increase after the lubrication between two surfaces reaches
a state of hydrodynamic lubrication.
The effect of three contact modes on CMP performance has been studied [Bhushan,
1995]. By controlling the thickness of slurry film between the wafer and the pad, it was
found that the material removal of an oxide wafer is independent of normal pressure,
relative velocity, and the pad surface properties. Three possible erosion models (padabrasion-based erosion, slurry-shear-based erosion, and pad-abrasion and slurry shear
combined erosion) were proposed according to the wafer-pad contact mode and a
phenomenological-physical hybrid model was developed to predict CMP performance
[Runnels, 1996].
It has been previously shown that the slurry film thickness increases with the
relative velocity of the wafer, slurry viscosity, and decreasing normal pressure on the
wafer [Hersey, 1966; Schliting. 1979; Runnels, 1994]. Thus, the Hersey number
measured for the process, defined as,
Hersey number =
viscosity ⋅ velocity
pressure
80
(4.1)
is an ideal indicator of slurry film thickness. Due to the fact that slurry film thickness
influences the characteristics of lubrication, boundary lubrication, elasto-hydrodynamic
lubrication, and hydrodynamic lubrication, the Hersey number is known to be a key
parameter that indicates the wafer-pad contact mode (i.e. direct contact, semi-direct
contact, and hydroplane sliding). Slurry film thickness is proportional to the square root
of the wafer velocity and the Hersey number if pressure and viscosity are kept constant in
the hydrodynamic lubrication regime [Runnels, 1994].
In this study, the effect of slurry film thickness variation on the CMP performance
is investigated. First, the slurry film thickness variation is studied using hydrodynamic
lubrication theory. By lubrication theory [Schlichting, 1979; White, 1986], the slurry
film thickness is calculated based on the conditions used in the CMP experiment
conducted in this study. The Hersey number [Hersey, 1966] is considered as a main
process variable to determine the slurry film thickness. Among the three parameters
(velocity, viscosity, and pressure) in the Hersey number, the relative velocity is the only
variable to be controlled in this study.
In addition, by measuring the friction force
variation between the silicon wafer and the polishing pad and referring to the Stribeck
curve [Williams, 1994], lubrication characteristics in CMP are determined. Next, the
effect of slurry film thickness variation on CMP performance (such as material removal,
planarization, surface defects, and surface roughness) is studied empirically. Based upon
the results of this study, the dependence of the material removal mechanism on slurry
film thickness is proposed.
81
4.3. Hydrodynamic effect and lubrication theory
In CMP, the abrasive slurry removes the material from the wafer surface. The fluid
in the slurry has many important functions in CMP. Besides direct chemical reaction, it
carries and distributes abrasive particles evenly on the wafer surface and helps the wafer
slide over the polishing pad without excessive friction. It also dissipates undesirable heat
generated between the wafer and the pad surface.
When the wafer moves on the
polishing pad within the abrasive slurry, it is lifted or hydroplaned, and slides with little
friction due to the thin fluid layer underneath it. In the thin fluid layer, the viscous force
is predominant, and high hydrodynamic pressure is generated. This high hydrodynamic
behavior is explained by lubrication theory.
As shown in Fig. 4.6, a silicon wafer slides over a pad surface with the inclination
of a positive angle, θ, and at the constant velocity, U. The pad surface is considered to be
flat here. The width of this wafer is assumed to be infinite, which simplifies the problem
to a two-dimensional problem.
The height profile of the wafer surface is h(x) and the length of the wafer is L. The
fluid layer under the wafer is thin, so the height h(x) is assumed to be much smaller than
the length of the wafer L. Therefore,
h( x )
<<1.
L
(4.2)
The Navier-Stokes equations for the two dimensional, Newtonian, and
incompressible flow with steady state conditions are
82
 ∂ 2u ∂ 2 u 
 ∂u
∂u 
∂p
+ ρ  u + v  = ρg x + µ  2 + 2 
∂y 
∂x
∂y 
 ∂x
 ∂x
(4.3)
.
 ∂ 2v ∂ 2v 
 ∂v
∂v 
∂p
+ ρ  u + v  = ρg y + µ  2 + 2  ,
∂y 
∂y
∂y 
 ∂x
 ∂x
(4.4)
where ρ: density of wafer, p: hydrodynamic pressure, u: flow velocity in xdirection, v: flow velocity in y-direction, gx, gy: acceleration of gravity in x and y axis,
and µ: viscosity.
In this fluid flow, the velocity in the y direction, v, is much less than that in the x
direction, u.
v<<u.
(4.5)
Therefore, after neglecting the effect of gravity, which is valid when fluid buoyancy
is not dominant,
(4.4):
∂p
∂p dp
= 0 → p = p( x ) →
=
∂y
∂x dx
(4.6)
(4.3):
 ∂ 2u ∂ 2 u 
dp
 ∂u 
+ ρ u  = µ  2 + 2  .
 ∂x 
∂y 
dx
 ∂x
(4.7)
Using dimensional analysis to compare the two terms on the right side of Eq. (4.7),
gives:
∂ 2u U
∂ 2u U
~ 2 ,
~
∂x 2
L
∂y 2 h 2
(4.8)
∂ 2u
U
2
2
∂x
L2  h 
~
=   << 1 .
U  L
∂ 2u
h2
∂y 2
(4.9)
83
Therefore,
∂ 2u
∂ 2u
<< 2 .
∂x 2
∂y
(4.10)
If we compare the viscous and inertia forces from Eq. (4.7) after examining Eq.
(4.10),
∂u
U
2
ρU
Inertia force
∂x
L ρUL  h 
~
=
=


<< 1 .
U
µ  L
Viscous force
∂ 2u
µ 2
µ 2
h
∂y
ρu
(4.11)
The inertia force can be ignored with respect to the viscous force in the fluid layer
considered. Therefore, the final Navier-Stoke’s equation is
dp
∂ 2u
=µ 2
dx
∂y .
(4.12)
The boundary conditions are
x= -L/2,
p = pa y = 0,
u = -U
x= L/2,
p = pa y = h(x), u = 0,
where pa is atmospheric pressure.
In order to get the velocity function, u, Eq. (4.12) can be integrated twice with
respect to y because the pressure is a function of x.
u=
1 dp 2
y + C1 y + C2 .
2 µ dx
(4.13)
After applying the boundary conditions for Eq. (4.13),
u=
1 dp 2 1 
1 dp 2 
y + U −
h  y −U.
h
2 µ dx
2 µ dx 
The integral form of mass conservation here is
84
(4.14)
∫
h( x )
0
udy = constant = Q ,
(4.15)
where Q = Volume flow rate
Therefore,
Q=∫
h( x )
0
udy = ∫
h( x )
0
 1 dp 2 1 
1 dp 2  

y + U −
h  y dy
h
2 µ dx  
 2 µ dx
Uh h 3 dp
=−
−
.
2 12 µ dx
(4.16)
(4.17)
If we rewrite Eq. (4.17) with respect to p(x),
dp
Q
 U
= −12 µ  2 + 3  .

2h
dx
h 
(4.18)
The right side is a function of only x and can be integrated with respect to x. After
using boundary conditions (x = -L/2, p = pa),
p = −6 µU ∫
x
L
−
2
x 1
1
2 dx − 12 µQ ∫ L 3 dx + pa .
− h
h
2
(4.19)
With the boundary conditions (x = L/2, p = pa),
1
Q=− U
2
where
L
2
L
−
2
L
2
L
−
2
∫
1
dx
h2
∫
1
dx
h3
h(x) = ax+b = (tan θ)x+ ha
ha = the averaged height
θ = the tilt angle
µ = viscosity of fluid.
85
,
(4.20)
If U = 10m/s, µ = 10-3(N S)/m2, L = 0.174m, θ = 0.01 degree, ha = 100µm, and pa =
1 atm = 105 Pa, the maximum pressure can increase to approximately 10 times the
atmospheric pressure.
By using force equilibrium between the normal force and the force by
hydrodynamic pressure created under the silicon wafer surface, the slurry film thickness
is calculated with respect to the velocity.
In order to obtain the force, P, created by hydrodynamic pressure, p, the
hydrodynamic pressure, Eq. (4.19) is integrated with respect to the diameter of the wafer,
L.
Given the viscosity, µ, wafer diameter, L, and atmospheric pressure, pa, the
hydrodynamic pressure force will be a function of velocity, U, averaged height, ha, and
tilt angle, θ.
L
L
x 1
x 1


P = ∫ 2L pdx = ∫ 2L  − 6µU ∫ L 2 dx − 12 µQ ∫ L 3 dx + p a dx = P(U , ha ,θ )
− h
− h
−
−

2
2
2
2
(4.21)
The normal force, FN, is in a state of equilibrium with the hydrodynamic pressure
force, P. Assuming that the tilt angle, θ, remains constant during CMP, the velocity, U
and the averaged height, ha will be the variables of the function, P.
FN = P(U , ha )
(4.22)
A commercial mathematics application software, Mathematica, was utilized to
calculate the variation of the slurry film thickness with respect to the sliding velocity of
the wafer. The result is shown in Fig. 4.7. In this calculation, the parameters required in
the calculation were based upon the experimental setup utilized in this study to support
the experimental results.
In this calculation, the normal force was 100N, the slurry
86
viscosity, 10-3 (N S)/m2, the wafer diameter, 100mm, the wafer tilt angle, 0.001 degree,
and the atmospheric pressure, 105 Pa.
In Fig. 4.7, the x-axis is velocity ranging from 0 to 20 cm/s and the y-axis is slurry
film thickness ranging from 0 to 6 mm. The slurry film thickness ranged from 2.5 to 5.7
mm and increased with respect to the square root of the relative velocity of wafer, which
is in agreement with the previous work [Runnel, 1994; Modak, 1997]. The calculated
thickness is unrealistic, however, and this may be due to the oversimplification of the
wafer as an infinite plate and the surface of the polishing pad as perfectly flat.
4.4 Experiments
The objectives of this experiment are to measure the friction force variation which
is strictly correlated to the slurry film thickness and velocity and to identify the effect of
slurry film on CMP performance.
Performance is defined here as material removal,
planarization, surface defects, and surface roughness. A schematic of the experimental
setup was previously shown in Fig. 3.4. Workpieces and process parameters adopted for
this experiment are listed in Table 4.1. Polished bare silicon wafers (<100> and p-type)
were used in the friction force measurement and the material removal tests to eliminate
the effect of surface roughness of the workpiece on the friction and material removal.
The load cell was calibrated to measure an equivalent friction force, and the friction
coefficient was calculated based on that measured friction force. Friction coefficients
87
were sampled every 10 minutes for each hour and averaged to obtain a mean value at
each velocity.
For the material removal tests, the mass removed from the workpiece for each
velocity condition was measured every 10 minutes for an hour. For the evaluation of
planarization and surface roughness effects on slurry film thickness, only oxide wafers
with measured initial curvature and peak-to-valley height of less than 5 µm were chosen
as workpieces. After CMP, the oxide thickness was measured with a Nanospec film
thickness measuring system while the surface roughness and defects were measured
using an atomic force microscope(AFM) and a Zygo laser interferometer, respectively.
4.5. Results and discussion
4.5.1 Effect of slurry film thickness variation on friction force, friction
coefficient, and wafer-pad contact mode.
The friction force on the wafer surface during CMP was measured with respect to
velocity in the presence of the abrasive slurry, Fig. 4.8. In this experiment, 4 normal
loads ranging from 10 to 62 N were applied to the wafer during polishing. To verify the
influence of abrasive slurry on the friction force, the friction force was also measured
without slurry on the pad, Fig. 4.9. In Fig. 4.8 and 4.9, the x-axis is velocity ranging
from 0 to 20 cm/s and y-axis is friction force ranging from 0 to 40 N.
88
The friction force between the wafer and the pad in dry polishing had a slightly
negative slope with velocity, and the averaged difference between the initial and the final
friction was approximately 10% of the initial friction, Fig. 4.9. This result is identical to
results of previous research [Rabinowicz, 1995]. When abrasive slurry was used, the
overall absolute amount of the friction force decreased, and, more interestingly, the
friction force decreased dramatically with increasing velocity, i.e., increasing slurry film
thickness, Fig. 4.8. The averaged difference between the initial and the final friction was
almost 80% of the initial friction. Thus, it is evident that the abrasive slurry greatly
reduced friction between the wafer and the pad, and from the Stribeck curve, the slurry
film thickness between the wafer and the pad increased as the relative velocity increased.
Since the slurry viscosity and the normal load were kept constant for each data set, the
slurry film thickness also increases with the Hersey number. In the case of dry sliding,
there is no influence of the relative velocity of the wafer on the variation of the distance
between the wafer and the pad surface.
The friction coefficient was calculated from the friction force data, and its variation
was plotted with relative velocity, Fig. 4.10 and 4.11. In Fig. 4.10 and 4.11, the x-axis is
velocity ranging from 0 to 20 cm/s and y-axis is friction coefficient ranging from 0 to 1.
The friction coefficient in dry polishing ranged from 0.45 to 0.6 and slightly
decreased with respect to the relative velocity, Fig. 4.11. When abrasive slurry was
applied, the friction coefficient dramatically decreased to near zero as the wafer velocity
increased, Fig. 4.10.
89
According to the Stribeck curve, Fig. 4.5, the friction force between two surfaces in
direct or semi-direct contact initially decreases and reaches a minimum value just before
the lubrication between the two surfaces enters the hydrodynamic lubrication regime.
The minimum friction coefficient in the Stribeck curve is related to the fact that the
viscous shear losses in the lubrication film and the friction due to solid contact are both
relatively within a low range [Suh, 1986; Johnson, 1989; William, 1994]. As the Hersey
number continues to increase, the friction force starts to increase. From our experiments,
the friction force between the wafer and the pad decreased as the Hersey number
increased. Thus, it is believed that the lubrication condition under the wafer in CMP is
closer to the boundary lubrication or elasto-hydrodynamic lubrication, than to the
hydrodynamic lubrication.
This contradicts the general view that hydrodynamic
lubrication represents the behavior of the slurry film [Runnel, 1994]. The contact mode
between the wafer and the pad surfaces is believed to be in direct contact or semi-direct
contact, not in hydroplane sliding mode.
The Hersey number from our experimental setup and process parameters of a
commercial CMP machine [Hetherington, 1996] were calculated and compared to
identify the characteristics of lubrication and contact mode of a commercial CMP
process, Table 4.2. Since the standard normal pressure utilized in a commercial CMP
machine is much higher than that of our laboratory experiment, the commerciallyadopted Hersey number will be much lower than that used in our experiment. The slurry
film thickness determined in this study, therefore, is considerably greater than the slurry
film thickness commonly used in the commercial CMP process.
90
It has been
acknowedged that in boundary lubrication, the friction coefficient in contacting surfaces
may remain constant or increase depending on the roughness of each surface [Williams,
1994]. Thus, it is believed that the lubrication used in this experiment is primarily within
elasto-hydrodynamic lubrication and the contact mode is the semi-direct contact mode,
Fig. 4.12. From the calculated Hersey number, it is proposed that the lubrication under a
wafer in the actual CMP process is generally near the border of boundary and elastohydrodynamic lubrication or within boundary lubrication regime.
The contact mode
between the silicon wafer and the polishing pad surface is believed to be in direct or
semi-direct contact modes in a commercial CMP process.
4.5.2 Effect of slurry film thickness variation on the wafer planarization.
The oxide film thickness on the wafer was measured at twenty points on the
diameter parallel to the wafer primary flat to identify the effect of the slurry film
thickness on the planarization before and after CMP, Fig. 4.13. In Fig. 4.13, the x-axis is
the wafer diameter ranging from -60 to 60 mm and y-axis is oxide thickness ranging from
0 to 1200 nm. Maximum and a minimum wafer velocities (17.5 and 2.7 cm/s) of the
CMP machine used were chosen to simulate a large and a small slurry film thickness
under the wafer. The upper and lower graphs show the oxide film thickness of the wafer
before and after CMP, respectably. The original oxide thickness was 1000nm and the
final thickness ranged approximately from 910 to 970 nm. The oxide wafer surface
polished at low speed showed good planarization after CMP except for approximately
91
two data points from each edge. The oxide surface polished at high speed, however,
showed irregular surface characteristics. Therefore, the more the wafer contacts the pad
surface, the better the planarization will be. To quantify wafer planarization, a standard
parameter, With-In Wafer Non-Uniformity (WIWNU) was calculated. The WIWNU is
defined as the maximum of,
WIWNU(%) =
oxide thickness i +1 - oxide thickness i
mean of total oxide thicknesse s
× 100 ,
(4.23)
where i=0,1,.., n-1, n (n = the number of points).
Eq. (4.23) indicates that planarization is better with lower WIWNU values. From
the data obtained from the experiment, WIWNU of the oxide surface was calculated,
Table 4.3. The WIWNU of oxide wafer polished by a large slurry film was 5% while the
oxide surface polished by a small slurry film had 2.25 % WIWNU.
It is believed that this observed influence of slurry film thickness on the wafer
planarization is due to the wafer-pad contact mechanics. A more detailed description will
follow in section, 4.5.4.
4.5.3 Effect of slurry film thickness variation on the wafer surface roughness
and defects.
Surface roughness of the oxide wafer before and after CMP was measured along
with measurements of oxide thickness variation using an atomic force microscope
(AFM). Three measuring areas, 0.5µm x 0.5µm, 1µm x 1µm, and 5µm x 5µm, were
92
chosen and silicon wafers in three stages (before CMP, after CMP using a minimum
speed ( 2.7cm/s), and after CMP using a maximum speed (17.5 cm/s)) were tested.
Three AFM images of 1µm x 1µm area oxide surfaces are shown in Fig. 4.14. It is
seen that there are apparent differences in the oxide surface before, Fig. 4.14(a), and after
CMP, Fig. 4.14(b)(c). Compared to the oxide surface before CMP, the micro-texture of
the oxide surface changed from a fine to a coarse structure after CMP. The oxide surface
polished by a small slurry film exhibited a slightly rougher surface than that polished by a
large slurry film and several nano-scale scratches of approximately 100 nm in length
were observed on this rougher surface and considered as surface defects.
Surface roughness of oxide surfaces before and after CMP were calculated and
given according to two definitions, Ra. and Rms. in Table 4.3 and plotted with respect to
the slurry film thickness in Fig. 4.15. In the figure, the y-axis is surface roughness in the
nano-meter scale. The surface roughness of the oxide wafer, in general, increased after
CMP. The difference of surface roughness before and after CMP was maximum in the
5µm x 5µm area measurements. The surface roughness of the oxide wafer increased as
the slurry film thickness between the wafer and the polishing pad decreased as observed
in Fig. 4.15. It is believed that this result is caused by the dependence of the material
removal mechanism on the slurry film thickness. Additional explanation of the influence
of slurry film thickness on the wafer planarization will be provided in the following
section, 4.5.4.
As mentioned in the analysis on surface roughness variation, surface defects such as
nano-scratches on the wafer surface were more frequently observed on the oxide surface
93
polished using a small slurry film. The degree of scratching on the surface, however, was
not quantified in this study.
4.5.4 Effect of slurry film thickness variation on the material removal and its
mechanism.
In this study, the material removal per sliding distance of the wafer was measured
instead of the material removal rate with the wafer velocity. This measure was used in
order to compare the material removal of each wafer with respect to only the slurry film
thickness between the wafer and the polishing pad, Fig. 4.16. If the material removal rate
with the wafer velocity is used, the variation of the amount of wear from the wafer will
be the result of wafer sliding distance as well as from the slurry film thickness effects.
The material removal per sliding distance was correlated with the wafer velocity to
identify the dependency of material removal on the slurry film thickness, Fig. 4.17. In
this experiment, three normal loads, 44.7, 61.9, and 79.1 N were utilized. In Fig. 4.17,
the x-axis is velocity ranging from 0 to 20 cm/s and the y-axis is material removal per
sliding distance ranging from 0 to 250 Þ/m. The upper graph indicates the material
removal when the normal load is 79.1 N and the lower one is plotted for a 44.7 N normal
load. As indicated in Fig. 4.17, the material removal per sliding distance was relatively
high at low wafer speed. As the wafer sliding speed increased, the material removal per
sliding distance decreased and asymptotically approached a certain value in the range of
60 to 80 Þ/m. In other words, when the wafer slides more closely to the pad surface, the
94
material removal for a unit travel distance is aggressive and high. As the wafer starts to
glide over the pad surface with increasing slurry film thickness, the material removal for
a unit travel distance diminishes and becomes low.
From this study, a material removal mechanism of CMP with respect to the slurry
film thickness is proposed, Figs. 4.18 and 4.19.
When the wafer has a small slurry film under it due to a high pressure and a low
velocity, the actual contact area of the polishing pad on the wafer surface increases. The
increased pad contact will also increase the number of the active abrasive particles which
will actually contact and abrade the wafer surface. Due to the increase of contact of
abrasives on the wafer surface, the material removal is high and the mechanical removal
(nano-plowing and nano-scratching) is dominant compared to the chemical removal
(erosion).
The dominant mechanical removal increases the possibility of generating
surface defects such as nano-scratches on the wafer surface and, thus, a relatively rough
surface is generated, compared to a surface finished by a chemically dominant removal.
The intimate contact of the polishing pad on the wafer surface, however, creates a better
global planarization after CMP.
When the wafer has a large slurry film under it due to low pressure and high
velocity, the actual contact area of the polishing pad on the wafer surface decreases. The
decreased pad contact will also decrease the number of the active abrasive particles
which will actually contact and abrade the wafer surface. Due to the decrease of contact
95
of abrasives on the wafer surface, the material removal is low and the chemical removal
(erosion) by the slurry plus minor mechanical removal (minor nano-plowing and nanoscratching) becomes the dominant material removal mechanism. The dominant chemical
removal and reduced mechanical removal decreases the possibility of generating surface
defects such as nano-scratches on the wafer surface and, thus, a relatively smooth surface
is generated, compared to a surface finished by a mechanically dominant removal. It is
more difficult, however, to achieve global planarization since the chemically dominant
material removal is more isotropic in directionality, as in an etching process.
The evidence showing the dependence of material removal on the slurry film
thickness suggests the need to modify Preston’s equation for use as a practical CMP
model. This is because the material removal per sliding distance is considered to be
constant with velocity from the standard Preston’s equation. Furthermore, it may prove
useful to optimize the slurry film thickness to balance the mechanical with the chemical
removal to maximize the material removal and improve planarization (as measured by
WIWNU) in the CMP process.
Preston’s wear equation, predicting the material removal rate on a workpiece due to
the relative motion between surfaces, is proportional to the load on the workpiece and the
relative velocity. Preston’s equation, Eq. (3.1), can be rewritten as,
h& = C ⋅ P ⋅ v
(3.1)
→
∆h
∆s
= C⋅P⋅
∆t
∆t
(4.24)
→
∆h
= C ⋅ P,
∆s
(4.25)
96
where P is pressure, v is relative velocity, h& is average height removed/unit time, C is
Preston’s coefficient, s is sliding distance, and t is time.
The material removal per sliding distance, theoretically, is constant at any velocity
since there is no velocity term in Eq. (4.25). According to the experimental results, Fig.
4.17, the material removal per sliding distance clearly decreased with respect to the
velocity due to the increase of slurry film thickness. Preston’s equation has been widely
applied in the lapping process where the main material removal is caused by mechanical
action through direct contact (indentation, scratching, or plowing) of the abrasive
particles on the workpiece. In CMP, however, the application of Preston’s equation may
not be entirely reliable since the material removal mechanism is more complicated and
the roles of pressure and velocity differ from those in lapping. Since the dependence of
material removal on the slurry film thickness is not explained by Preston’s equation, it
needs modification for use as a practical model for CMP.
4.6 Summary
Since CMP uses an aqueous slurry, the lubrication effect of the slurry is an
important element in determining CMP performance. The effect of slurry film thickness
variation on CMP performance (defined here as material removal, planarization, surface
defects, and surface roughness) was investigated.
The friction force variation with
velocity was correlated to the slurry film thickness under the wafer surface.
97
The abrasive slurry greatly reduced the friction between the wafer and the pad, and
the friction force decreases as the slurry film thickness between the wafer and the pad
increases. Thus, it is believed that the lubrication condition under the wafer in CMP is
closer to boundary lubrication or elasto-hydrodynamic lubrication, than to hydrodynamic
lubrication. The contact mode between the wafer and the pad surfaces is believed to be
direct contact or semi-direct contact, but not hydroplane sliding mode.
When the wafer has a small slurry film under it due to high pressure and low
velocity, the actual contact area of the polishing pad on the wafer surface increases. The
increased pad contact will also increase the number of the active abrasive particles which
will actually contact and abrade the wafer surface. Due to the increase of contact of
abrasives on the wafer surface, the material removal is high and the mechanical removal
(nano-plowing and nano-scratching) is dominant compared to the chemical removal
(erosion).
The dominant mechanical removal increases the possibility of generating
surface defects such as nano-scratches on the wafer surface and, thus, a relatively rough
surface is generated, compared to a surface finished by a chemically dominant removal.
The intimate contact of the polishing pad on the wafer surface, however, creates a better
global planarization after CMP.
When the wafer has a large slurry film under it due to low pressure and high
velocity, the actual contact area of the polishing pad on the wafer surface decreases. The
decreased pad contact will also decrease the number of the active abrasive particles
which will actually contact and abrade the wafer surface. Due to the decrease of contact
of abrasives on the wafer surface, the material removal is low and the chemical removal
(erosion) by the slurry plus a minor mechanical removal (minor nano-plowing and nano98
scratching) becomes the dominant material removal mechanism. The dominant chemical
removal and reduced mechanical removal decreases the possibility of generating surface
defects such as nano-scratches on the wafer surface and, thus, a relatively smooth surface
is generated, compared to a surface finished by a mechanically dominant removal. It is
more difficult, however, to achieve global planarization since the chemically dominant
material removal is more isotropic in directionality, as in an etching process.
The slurry film thickness between the wafer and the pad is proportional to the
velocity of the wafer and the Hersey number, and the effect of slurry film thickness
variation on the CMP process performance (material removal, planarization, surface
defects and roughness) is significant. Since the dependence of material removal on the
slurry film thickness is not explained by Preston’s equation, it needs to modify to use as a
practical CMP model. Furthermore, it may prove useful to optimize the slurry film
thickness to balance the mechanical with the chemical removal to maximize the material
removal and improve planarization (as measured by WIWNU) in the CMP process.
99
Table 4.1 Process parameters for the experiments on the effect of slurry film thickness on
CMP performance.
Workpiece
Friction force measurement and Planarization, surface roughness,
material removal variation
and defects
4 oxide wafers
5 polished bare silicon wafers
(<100> and p-type)
(1 µm thick thermal oxide)
Polishing pad
Slurry
IC60
Nalco2352
ILD1300
Dilution ratio
(DI water:slurry)
10:1
Flow rate
50ml/min
Normal force
10-80N
80N
Velocity
3-18cm/s
3 and 18cm/s
Polishing time
10 minutes – 1 hour
10 and 20 minutes
100
Table 4.2 Process parameters and Hersey numbers of experimental setups of the LMA
machine and the Cybeq 3000, a commercial CMP tool.
The LMA machine
Cybeq 3900
0.127 kPa
51 kPa
Velocity
0.2 m/s
Platen/Head = 20/15 rpm
Averaged relative velocity
= 0.84 m/s
Viscosity
10-3 N s/m2
10-3 N s/m2
Hersey number
1.57‰10-6
1.65‰10-8
Normal pressure
101
Table 4.3 A list of data for the planarization and surface roughness testing.
Planarization
(WIWNU)
Surface roughness
(5µmx5µm)
Surface roughness
(1µmx1µm)
Surface roughness
(0.5µmx0.5µm)
Before CMP
V= 17.5cm/s
(large gap)
V= 2.7cm/s
(small gap)
Less than 1%
5%
2.25%
Rms= 0.173nm
Rms= 0.347nm
Rms= 0.381nm
Ra= 0.129nm
Ra= 0.271nm
Ra= 0.293nm
Rms= 0.153nm
Rms= 0.170nm
Rms= 0.202nm
Ra= 0.114nm
Ra= 0.133nm
Ra= 0.154nm
Rms= 0.123nm
Rms= 0.123nm
Rms= 0.131nm
Ra= 0.097nm
Ra= 0.093nm
Ra= 0.098nm
102
Abrasive particle
(10-150mm)
Abrasive slurry
Workpiece
Abrasive particle
(less than 0.1mm)
Workpiece
Polishing pad
Lapping plate
Polishing plate
Lapping process
CMP process
100mm
100mm
Soda lime glass surface
by lapping
Silicon wafer surface
by CMP
Fig. 4.1 Material removal mechanism in lapping and CMP processes.
103
End fibrils
100µ
µm
Vertically
oriented pores
400µm
Urethane
impregnated
polyester felt
1500µm
Fig. 4.2 A cross-section of a UR100 polishing pad.
104
Fig. 4.3 Illustration of pad/wafer interaction in CMP.
105
Fig. 4.4 Definition of slurry film thickness.
106
Film thickness
Friction coefficient
Direct
contact
Semi-direct
contact
Elastohydrodynamic
lubrication
Hydroplane
sliding
Hydrodynamic
lubrication
Boundary
lubrication
Hersey number(=
Viscosity ⋅Velocity
Pressure
Fig. 4.5 Stribeck curve.
107
)
Fig. 4.6 Lubrication under a wafer during CMP process.
108
Fig. 4.7 Slurry film thickness variation with the wafer velocity.
109
40
L= 10.3N
L= 27.5N
L= 44.7N
L= 61.9N
First order regression
35
Friction force(N)
30
25
20
15
10
5
0
-5
0
2
4
6
8
10
12
14
16
18
20
Velocity(cm/s)
Fig. 4.8 Friction force variation between the wafer and the polishing pad with abrasive
slurry.
110
40
L=
L=
L=
L=
First order regression
35
10.3N
27.5N
44.7N
61.9N
Friction force(N)
30
25
20
15
10
5
0
-5
0
2
4
6
8
10
12
14
16
18
20
Velocity(cm/s)
Fig. 4.9 Friction force variation between the wafer and the polishing pad without abrasive
slurry.
111
1.0
First order regression
0.9
L= 10.3N
L= 27.5N
L= 44.7N
L= 61.9N
Plot 3 Regr
Plot 3 Pred1
Friction coefficient
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
10
12
14
16
18
20
Velocity(cm/s)
Fig. 4.10 Friction coefficient variation between the wafer and the polishing pad with
abrasive slurry.
112
1.0
L= 61.9N
L= 44.7N
L= 27.5N
L= 10.3N
Plot 2 Upper Control
Plot 2 Mean
Plot 2 Lower Control
0.9
Friction coefficient
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
First order regression
0.0
0
2
4
6
8
10
12
14
16
18
20
Velocity(cm/s)
Fig. 4.11 Friction force variation between the wafer and the polishing pad without
abrasive slurry.
113
Friction coefficient
Film thickness
I II
Direct
contact
Semi-direct
contact
Elastohydrodynamic
lubrication
Hydroplane
sliding
Hydrodynamic
lubrication
Boundary
lubrication
Hersey number(=
Viscosity ⋅ Velocity
Pressure
)
Fig. 4.12 Two regimes in the Stribeck curve (regime I: commercial CMP condition,
regime II: CMP condition used in this research).
114
1200
1100
Oxide thickness(nm)
1000
900
800
700
600
500
400
300
Before CMP
After CMP
200
100
0
-60
-40
-20
0
20
40
60
Diameter(mm)
(a) Planarization with a small slurry film thickness
1200
1100
Oxide thickness(nm)
1000
900
800
700
600
500
400
300
Before CMP
After CMP
200
100
0
-60
-40
-20
0
20
40
60
Diameter(mm)
(b) Planarization with a large slurry film thickness
Fig. 4.13 Planarization of wafer by (a) a small slurry film and (b) a large slurry film.
115
(a)
(b)
(c)
Fig. 4.14 Surface roughness of oxide surface in 1µm x 1µm area of (a) surface before
CMP, (b) surface polished with small slurry film thickness, and (c) surface polished with
large slurry film thickness.
116
(a) Ra surface roughness variation
(b) Rms surface roughness variation
Fig. 4.15. Surface roughness variation with slurry film thickness.
117
(a)
(b)
Fig. 4.16 Silicon wafer sliding on the polishing pad for a certain distance with (a) a small
slurry film and (b) a large slurry film due to the velocity difference.
118
Material removal per sliding distance(A/m)
250
Second order regression
225
200
L = 44.7N
L = 61.9N
L = 79.1N
175
150
125
100
75
50
25
0
0
2
4
6
8
10
12
14
16
18
20
Velocity(cm/s)
Fig. 4.17 Material removal per sliding distance with velocity.
119
(a)
(b)
Fig. 4.18 A proposed schematic of material removal in CMP for the slurry film thickness.
120
Material removal/sliding distance
Mechanical removal(major)
Chemical removal+Mechanical removal(minor)
Velocity(=slurry film thickness)
Fig. 4.19 Mechanical and chemical removal for slurry film thickness.
121
CHAPTER 5
STUDY OF SLURRY CHEMICAL INFLUENCE IN DUCTILE/BRITTLE
TRANSITION DEPTH IN CHEMICAL MECHANICAL POLISHING (CMP)
USING SCANNING ELECTRON MICROSCOPE (SEM) AND ACOUSTIC
EMISSION (AE) SENSOR
5.1 Introduction
Unlike other conventional abrasive machining processes (lapping and grinding),
CMP uses a special chemical slurry with added abrasive particles to enhance the material
removal rate, to achieve a better surface finish, and to increase the selectivity of the
material removal between two adjacent materials such as metal and oxide layers in IC
fabrication.
In general, the chemical in the abrasive slurry is selected according to the material
to be polished to achieve the necessary chemical interaction and compatibility with the
polishing surface. Typical constituents that are mixed with de-ionized (DI) water in the
slurry chemical include: a buffering agent, an oxidizer, and a complexing agent
[Steigerwald, 1997]. The buffering agent is added to keep the pH of the slurry constant
during the process, thereby preventing any unpredicted chemical reactions and reducing
the agglomeration of the abrasive particles and to increase the material removal rate from
the wafer surface.
The oxidizer plays a role in the reaction with metal surfaces to
increase the oxidation state of the metal through a reduction/oxidation process and to
122
promote dissolution of the metal surface. Complexing agents control the solubility of the
thin film to be polished and, thus, the balance between the mechanical removal and the
chemical removal, occurring during the process.
The role of slurry chemical in the chemical reaction of the wafer in CMP has been
identified. It was proposed that in oxide CMP, the water in the slurry diffuses into the
oxide film and breaks the silicon-oxygen bonds, which weakens the glass network and
leads to the dissolution of the network on the oxide surface [Izumitani, 1979; Cook, 1990;
Steigerwald, 1997]. The depth of water diffusion into the oxide film was also calculated
and predicted to be 0.5 ~ 12 nm [Cook, 1990]. In silicon polishing, the role of OH-,
which is normally provided by a KOH buffer in the colloidal silica slurry, has been
acknowledged to be significant in the silicon chemical etching during silicon polishing
[Pietsch, 1994; Pietsch, 1995].
The concentration of OH- determines the oxidation
reaction of the silicon surface and the oxidized surface, which is a silicon oxide, is
removed by the same chemical reaction proposed for oxide CMP. In metal CMP, it has
been acknowledged that the metal oxide film is created by an oxidizer in slurry chemical
and this film prevents further chemical etching by the slurry [Kaufman, 1991]. The
abraded material from the metal film is dissolved by the complexing agent in the slurry
chemical to prevent any redepositon on the wafer surface [Steigerwald, 1997].
As opposed to the known contributions of the chemical reactions, minimal
investigation has been done on the role of the mechanical action occurring in CMP.
123
It has been acknowledged that for glass polishing in the optics industry, the water in
the slurry compounds attacks the siloxane bonds (Si-O-Si) and causes the corrosion of
glass. This corrosion process of glass produces a silica rich gel film (= hydrated layer)
which is relatively easy to remove by the mechanical action of abrasives due to its
relative softness [Izumitani, 1971; Izumitani, 1973; Tomozawa, 1985; Brown, 1989;
Cook, 1990; Golini, 1991; Kirk, 1994]. It was reported the thickness of this hydrated
layer is of the order of 0.5 µm [Kirk, 1994]. It was also found that the polishing rate is
determined by a combination of the chemical durability of the glass and the hardness of
the hydrated layer [Izumitani, 1971].
As a part of a fundamental study on the material removal mechanism of CMP, the
slurry chemical influence on the change of material property of silicon wafer surface is
identified by examining an extension of the ductile/brittle transition depth of a silicon
wafer. When scratching a brittle material, both ductile and brittle modes of cutting are
observed with respect to the depth of cut [Blackley, 1991; Schinker, 1991; Daniel, 1996;
Kunz, 1996; Leung, 1998].
At shallow depths of cut, brittle materials behave in a
ductile-cutting mode. It is expected that the ductile-cutting regime can be extended by
property changes in the outer surface of silicon wafer. Acoustic emission (AE) has been
utilized as an indicator to identify the material-removal regime in glass and silicon
machining [Bifano, 1992; Dornfeld, 1993; Webster, 1994; Chang, 1995; Daniel, 1996;
Lee, 1998]. In this study, the AE raw and the AE rms signal are used to monitor the
ductile-brittle cutting regimes and transition in the scratching of silicon wafers.
124
5.2 Ductile/brittle-cutting regime in diamond turning of brittle materials
Single-point diamond turning has been carried out in machining brittle materials
(such as glass and silicon) to produce high quality optical surfaces without any postprocessing such as polishing [Blackley, 1991; Shigeru, 1996; Morris, 1996].
It has been reported that the crystalline characteristics of a brittle material can be
transformed to a ductile, metallic, or amorphous structure by large compressive stress (812 GPa) [Lambropoulos, 1998; Puttick, 1998, Kerstan, 1998].
This crystalline-
amorphous transition was verified by a study that showed the electrical conductivity of a
brittle material dramatically increases under a high hydrostatic pressure [Clarke, 1998].
In diamond turning of brittle materials, excessive hydrostatic compressive stress is
created underneath the diamond cutting edge, which enables plastic behavior and ductilemachining of brittle materials such as silicon. At a certain cutting speed, this ductilemode cutting is limited by a critical depth of cut.
The critical depth of cut is influenced by the properties of the workpiece material
and a model of this critical depth is defined as,
2
 E  K 
d c = 0.15  c  ,
 H  H 
(5.1)
where dc is the critical depth of cut, E is the elastic modulus, H is the hardness, and
Kc is the fracture toughness of the glass [Bifano, 1988; Bifano, 1991].
Beyond the critical depth of cut, the cutting mode changes to brittle-mode cutting
where median and lateral cracks start to propagate. Below a certain depth of cut, the
125
ductile/brittle-mode cutting transition depends on the cutting speed and tool geometry
[Kerstan, 1998].
The transition of ductile/brittle mode cutting of brittle materials has been monitored
using acoustic emission (AE) sensor [Bifano, 1992, Daniel, 1996; Lee, 1998]. In this
study, the influence of the slurry chemical on the extension of ductile/brittle transition
depth of silicon is investigated by using scanning electron microscope (SEM) analysis
and by monitoring acoustic emission signals generated during a diamond scratch test.
5.3 Process monitoring using acoustic emission (AE)
Acoustic emission (AE) refers to the elastic energy or waves which are abruptly
generated by materials being deformed and fractured [Miller, 1987]. Sources of AE range
from earthquakes and rock bursts in mines as natural sources to crack nucleation and
growth, moving dislocations, slip, twining, grain boundary sliding, and the fracture of
inclusions in materials. These elastic waves generated from these various sources travel
to the surface of the material and can be detected by a piezoelectric transducer, or AE
sensor. AE sensors generally consist of a disk of piezo material as a electro-mechanical
conversion device mounted in a metal housing and damping material filled inside of
metal housing [Miller, 1987].
Acoustic emission detection has been used as a nondestructive testing method since
its development in the 1950’s because it is sensitive enough to characterize material
126
behavior and to detect small changes in mechanical equipment. Acoustic emission, as
generally defined, propagates with frequencies from 100 kHz to 2 MHz which is beyond
most natural frequencies of machine structures [Bifano, 1992]. AE detection has been
used as an on-line monitoring method of various manufacturing (especially precision
manufacturing) processes, such as metal cutting, welding, diamond turning, burnishing,
and grinding [Dornfeld, 1980; Inasaki, 1985; Bifano, 1992]. In the burnishing process,
for example, it was found that there is a strong correlation between the characteristics of
the disk textures and the AE signal [Dornfeld, 1993]. Due to the requirements for high
sensitivity in machining of brittle materials, acoustic emission monitoring has been used
to detect the onset of brittle fracture in diamond turning of brittle materials [Bifano, 1992;
Daniel, 1996; Lee, 1998].
In this experiment, the AE raw and the AE rms signal are monitored to examine the
extension of the transition of ductile-brittle cutting regimes in the scratching of silicon
wafers from the influence of the slurry chemical used in CMP.
5.4 Experiments
Polished silicon wafers (<100>, p-type) were used as workpieces. For the chemical
treatment, half of a silicon wafer was placed in a CMP slurry for 30 minutes at 47oC, Fig.
5.1. The CMP slurry was simulated by diluting Nalco2352 silicon wafer polishing slurry
from Rodel with DI water using the same dilution ratio (DI water:slurry = 10:1) normally
127
adopted in production CMP processes. A series of scratching tests were performed using
a PNEUMO diamond turning machine, Fig. 5.2. During the scratching test, scratches
were made in the [001] direction in each chemically treated and untreated areas in order
to obtain constant cutting conditions for each scratch made by one stroke of diamond tool
(rake angle, crystallographic orientation of wafer, inclination angle, etc), Fig. 5.3.
Two diamond tools from Contour Fine Tooling, Inc. with a 48µm and a 350µm
nose radii were mounted in the tool holder and a 399µm/s feed rate was used. To impose
an increasing depth of cut during the scratch, two wafer adapters with tilt angles of 0.05
and 0.5 degrees were utilized. The top surface of the wafer adapters was diamond cut to
promote a flat contact surface with the wafer. Thirteen vacuum holes (0.0625 inch in
diameter) were drilled coinciding with the locations of the vacuum holes in the vacuum
chuck so that the wafer is stationary during the scratching test.
A DECI Pico-Z acoustic emission sensor attached to the wafer surface was used to
monitor the scratching tests for the 350 µm tool nose radius and 0.05 degree tilt angle.
The AE raw signal after pre-amplification (40db) and amplification (20db) was collected
using 500 kHz sampling rate and a 200kHz – 2 MHz bandwidth filter. The moving AE
rms signal was calculated using a 10ms window length.
examined using a Scanning Electron Microscope (SEM).
128
Individual scratches were
5.5 Results and discussion
5.5.1 SEM analysis
Each scratch was inspected using a scanning electron microscope (SEM) at
magnifications ranging from 600x to 20000x.
The top view of the initiation points of the micro-scratches on the chemically
untreated and treated areas (tool radius = 48µm, tilt angle = 0.5 degree) is shown in Fig.
5.4. In the scratch on the chemically untreated area of the silicon wafer, Fig. 5.4 (a)(c), it
was noticed that the formation of lateral cracks was more active and it often went beyond
the cutting edge. To find the ductile cutting regime, the starting points of the scratches
were inspected with a higher magnification (x6000), Fig. 5.4 (b)(d). Smooth and crackfree ductile-cutting regimes were noticed at the starting points of the scratches on both
chemically untreated and treated areas. The crack initiation started at a certain point for
both scratches.
To verify the extension of the ductile/brittle regime, the initiation points of microscratches were inspected in an oblique view, Fig. 5.5. It was apparent that the ductilecutting regime, which is the length from the initiation point to the location at which the
median/lateral cracks begin, was extended for the scratch on the area of chemical
treatment. In order to calculate the length of extended ductile cutting regime and the
critical depth of cut, dc, the cutting edge was marked on the top view of the initiation
point of the scratches, Fig. 5.6. The length of the ductile regime was approximately 9.2
µm for the scratch on the chemically untreated area and 11.7 µm on the scratch on the
129
chemically treated area. By using the inclination angle of the wafer adapter, the depth of
cut at which the brittle regime begins can be calculated for each scratch on the untreated
and the chemically treated areas of the silicon wafer, Fig. 5.7.
The ductile/brittle
transition depth (critical depth of cut) of the untreated area (dc1) was approximately 80
nm, while it was 102 nm for the chemically treated area (dc2). Therefore, from the SEM
analysis, it was found that the ductile regime was extended by 2.5 µm and the
ductile/brittle transition depth was extended by 22 nm on the silicon wafer after chemical
treatment
SEM analysis of the scratches also showed the change of surface property of the
silicon wafer before and after chemical treatment, Fig. 5.8. Scratches were made using a
350 µm tool radius and 0.05 degree of tilt angle from the wafer adapter and
magnifications from 2000x to 20000x were used for the SEM pictures. To exclude the
effect of the depth of cut on ductile-brittle transition study, locations with the same
cutting width along both the treated and untreated scratches were selected for SEM
analysis
Scratches on the chemical-treated area did not have distinct cutting paths and edges
compared to those on wafers without chemical treatment, Fig. 5.8(a)(b). In addition, the
edges of cracks on the chemical-treated areas were not distinct, Fig. 5.8(c, d, e, f). The
texture on the scratch surface on the chemical-treated area looked rough and coarse, Fig.
5.8(g, h, I, j, k, l). From the SEM analysis, it is believed that the surface property of the
silicon wafer changes from ‘brittle’ to ‘ductile’ after slurry chemical treatment.
130
It has been reported that an elastic contact regime exists before ductile mode cutting
in plunge cutting of brittle materials [Brinksmeier, 1996]. Similarly, it is believed that an
elastic contact regime exists before ductile mode cutting in this scratch test although no
trace of damage was observed in the SEM analysis.
5.5.2 Acoustic emission (AE) monitoring of scratch test
The AE raw signal was captured at a 500kHz sampling rate during the scratch tests
on both areas (with and without chemical treatment) of the silicon wafers, Fig. 5.9 and
5.11. The moving rms graphs were obtained by post-processing the raw AE signal, Fig.
5.10 and 5.12. The time window used for the moving rms was 0.01 seconds (5000
points).
The AE raw signal from the scratch on the untreated area clearly showed three
different cutting regimes: air-cut/elastic contact (noise signal before tool engagement),
ductile-mode (first burst emission), and brittle-mode cutting (second burst emission), Fig.
5.9. These three different cutting regimes were easier to observe from the AE rms signal,
Fig. 5.10. The AE rms signal increased at a certain rate (ductile-mode cutting) and
rapidly increased at a higher rate (brittle-mode cutting). The duration of the ductile-mode
cutting was approximately 0.019 seconds, which represents 7.6 µm in length. The AE
raw signal from the scratch on the chemically treated area also showed three different
cutting regimes, Fig. 5.11. The amplitude of the AE raw signal, however, gradually
131
increased at approximately 0.02 seconds without any burst emission in ductile-mode
cutting. A burst emission was clearly observed for the brittle-mode cutting. A gradual
transition from air-cut to ductile-mode cutting was also seen in the AE rms signal, Fig.
5.12. The AE rms signal shows a less distinct transition of ductile to brittle regime for
the chemically treated area. The duration of the ductile-mode cutting was approximately
0.024 seconds, which represents 9.6 µm in length.
The AE raw and the AE moving rms graphs showed the effect of increasing depth
of cut and three different cutting regimes: air-cut, ductile-mode, and brittle-mode cutting.
It was also seen that the ductile-cutting regime of the scratch on the area of the silicon
wafer after chemical treatment was extended.
By using the cutting speed and the inclination angle of the wafer adapter, the
distance from the tool engagement and the depth of cut at which the brittle regime begins
can be calculated for each scratch on the untreated and the chemically treated areas of the
silicon wafer, Fig. 5.13.
The ductile/brittle transition depth of the normal area of silicon wafer (dc1) was
around 6.7nm, while it was 8.4nm for the chemically treated area of silicon wafer (dc2).
Therefore, from the AE analysis, it was found that the ductile regime was extended by 3
µm and the ductile/brittle transition depth was extended by 1.5 nm on the silicon wafer
after chemical treatment in the scratch test using a 350 µm tool nose radius and 0.05
degree tilt angle.
132
5.5.3 Ductile/brittle transition in scratches on the untreated and the chemically
treated silicon wafer
It is believed from the SEM and AE monitoring analysis that the apparent
discrepancy in the ductile/brittle transition depth of silicon wafers before and after
chemical treatment is due to the ‘ductile’ layer on the silicon wafer surface created by the
slurry chemical reaction, Fig. 5.14. In addition, the chemically reacted layer (Lc) on the
silicon surface is proposed to be in the range from 1.7 to 22 nm thick which is the
difference of the ductile/brittle transition depths of the untreated silicon wafer (dc1) and
the chemically treated silicon wafer (dc2). This proposed thickness of chemically reacted
layer on silicon wafer in CMP falls within the same order of magnitude with the depth of
water diffusion into oxide film, 0.5 ~ 12 nm, proposed from the previous research [Cook,
1990].
It is believed that the discrepancy in the results of the extended ductile/brittle
transition depth (critical depth of cut) of the silicon wafers after chemical treatment from
SEM and AE analysis is due to the difference in tool geometry (nose radius).
Any deflection of the wafer/tool/fixture apparatus was not considered here.
However, both tests (with and without treatment) were run on the same wafer under
identical conditions, which should not affect the comparison.
133
5.6 Summary
The surface property of a silicon wafer following chemical treatment was changed
so that the ductile regime machining mode was extended. This was verified from the
SEM and the AE monitoring analysis.
The AE signal measured during scratching
indicated three unique cutting regimes: air-cut/elastic contact, ductile-mode, and brittlemode cutting.
In addition, this study clarified the slurry chemical influence on the
ductile/brittle transition depth. The chemically reacted silicon layer is believed to be in
the range from 1.7 to 22 nm thick which is the difference in the ductile/brittle transition
depths of the untreated silicon wafer (dc1) and the chemically treated silicon wafer (dc2).
The chemically reacted silicon layer is believed to be the cause of the extension of
the brittle/ductile transition depth and the brittle cutting behavior transition point
becomes less distinctive. The chemically reacted ‘ductile’ layer is proposed as the origin
of the scratch/defect-free surface after CMP.
134
Fig. 5.1 Chemical treatment of silicon wafer.
135
Fig. 5.2 Experimental setup.
136
Fig. 5.3 Scratching test on chemically treated and untreated area of silicon wafer.
137
(a)
(b)
(c)
(d)
Fig. 5.4 Top view of initiation points of micro-scratches on (a) (b) chemically untreated
and (c) (d) treated areas of silicon wafer (tool radius = 48 µm, tilt angle = 0.5 degree).
138
(a)
(b)
(c)
(d)
Fig. 5.5 Oblique view of initiation points of micro-scratches on (a) (b) chemically
untreated and (c) (d) treated areas of silicon wafer (tool radius = 48 µm, tilt angle = 0.5
degree).
139
Cutting edge
Brittle regime
Ductile regime
Cutting edge
Brittle regime
(a)
(b)
Ductile regime
Fig. 5.6 Brittle/ductile cutting regimes of micro-scratches on (a) chemically untreated and
(b) treated areas of silicon wafer (tool radius = 48 µm, tilt angle = 0.5 degree).
140
Fig. 5.7 Extension of ductile/brittle cutting regime of micro-scratches (tool radius = 48
µm, tilt angle = 0.5 degree).
141
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 5.8 Micro-scratches on silicon wafer at specific locations with the same cutting
width on (a, c, e, g, i, k) untreated and (b, d, f, h, j, l) chemically treated areas (tool radius
= 350 µm, tilt angle = 0.05 degree).
142
(g)
(h)
(i)
(j)
(k)
(l)
Fig. 5.8 (Continued) Micro-scratches on silicon wafer at specific locations with the same
cutting width on (a, c, e, g, i, k) untreated and (b, d, f, h, j, l) chemically treated areas
(tool radius = 350 µm, tilt angle = 0.05 degree).
143
Brittle-mode cutting
Air-cut/elastic contact
Ductile-mode cutting
Fig. 5.9 AE raw signal from the scratch on the normal area of the silicon wafer (tool
radius=350µm, tilt angle=0.05 degree).
144
Brittle-mode cutting
Air-cut/elastic contact
Ductile-mode cutting
Fig. 5.10 AE rms signal calculated from AE raw signal from the scratch on the normal
area of the silicon wafer (tool radius=350µm, tilt angle=0.05 degree).
145
Brittle-mode cutting
Air-cut/elastic contact
Ductile-mode cutting
Fig. 5.11 AE raw signal from the scratch on the chemically treated area of the silicon
wafer (tool radius=350µm, tilt angle=0.05 degree).
146
Brittle-mode cutting
Air-cut/elastic contact
Ductile-mode cutting
Fig. 5.12 AE rms signal calculated from AE raw signal from the scratch on the
chemically treated area of the silicon wafer (tool radius=350µm, tilt angle=0.05 degree).
147
Fig. 5.13 Extension of ductile/brittle cutting regime of micro-scratches (tool radius = 350
µm, tilt angle = 0.05 degree).
148
(a)
(b)
Fig. 5.14 Ductile/brittle transition in scratches on (a) the normal area and (b) the
chemically treated area of silicon wafer.
149
CHAPTER 6
IDENTIFICATION OF THE MECHANICAL ASPECTS OF MATERIAL
REMOVAL MECHANISMS IN CHEMICAL MECHANICAL POLISHING
(CMP)
6.1 Introduction
Chemical mechanical polishing (CMP) is a process where chemical and mechanical
effects combine and act together to remove material from a workpiece surface. Chemical
effects caused mainly by electrochemical phenomena are controlled by the slurry chemical
and the material of the wafer surface. In this study, the workpiece is a silicon wafer. The
formation of the passivating layer at the wafer surface by an oxidizer, dissolution of the
material on the wafer surface or the abraded materials from the wafer surface, and
agglomeration of the abrasive particles are influenced by the electrochemical phenomena
induced by the chemicals in the slurry and the material on the wafer surface [Philipossian,
1996; Steigerwald, 1997; Oliver, 1999].
Mechanical effects are associated with the physical interactions among the polishing
pad, abrasives, and wafer surface. The physical interactions are mainly determined by the
characteristics of lubrication controlled by slurry viscosity, relative velocity of the wafer,
and normal pressure applied on the wafer surface, the abrasive size and hardness, the pad
properties (surface roughness, density, compressibility), the IC pattern density, and the
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wafer curvature [Zhang, 1996; Coppeta, 1998, Guo, 1998; Zhao, 1999]. Compared to the
traditional abrasive machining processes such as grinding and lapping, the combined
effects of mechanical and chemical action and the mechanical and chemical contributions
to material removed in CMP are not well understood. In the lapping process, for example,
it is known that the material removal of the workpiece is mainly caused by mechanical
removal such as median/lateral crack, direct indentation, or micro-scratching by abrasives
[Golini, 1991; Buijs, 1993; Chauhan, 1993] and the slurry chemistry is only able to change
the surface property to a softer layer, causing increased material removal rate [Izumitani,
1971].
There is great difficulty in distinguishing the chemical aspect from the mechanical
aspect in CMP [Runnels, 1996]. The chemical effect was defined as the effect of changing
the mechanical properties of the wafer surface through the presence of the polishing
media. The mechanical effects are defined by a continuum concept of stress, mass and
momentum conservation, and fracturing [Runnels, 1996]. The problem is complicated by
the fact the slurry chemistry information is highly proprietary and not easily available from
the vendors.
It has been proposed that the mechanical and chemical contributions to the material
removal in CMP can be distinguished by differentiating the wafer-pad contact modes
[Bhushan, 1995].
In the direct contact mode, the removal mechanism is mainly
mechanical and the process follows Preston’s equation. As the contact mode becomes
semi-direct (meaning losing complete contact between the pad surface and wafer at higher
151
wafer-pad relative velocity), the chemical reaction on the surface increases due to the
increased supply of the chemical in the slurry and the mechanical removal is reduced.
Chapter 4 proposed that when there is direct or semi-direct contact between a silicon
wafer and the polishing pad, mechanical removal will be the dominant material removal
mechanism. Chemical reaction is believed to change wafer surface properties and enhance
abrasive mechanical removal. As the slurry film thickness between the wafer and the pad
increases (due, for example, to increased velocity), the mechanical removal decreases and
chemical removal becomes the dominant removal mechanism.
Investigation has shown that a transition from brittle to ductile behavior of the
material helps develop a smooth surface after CMP [Rajan, 1998]. It was suggested that
the material removal is increased by a combination of structural degradation of the wafer
surface by the slurry chemical and physical removal by abrasive particles. The role of the
slurry chemistry in the brittle/ductile transition of the wafer surface in CMP has also been
investigated in Chapter 5.
CMP employs abrasives and chemicals in slurry to polish the surface of the
workpiece. In this study, the mechanical and chemical contributions to material removal
in CMP are investigated independently by using ‘chemical-less’ and ‘abrasive-less’ slurries.
In order to investigate the role of the abrasive and the chemical independently, a
commercial slurry was separated to produce chemical-less and abrasive-less slurries.
These, along with the original slurry, were used in a series of polishing experiments.
152
Material removal in each of the three experiments was recorded and SEM analysis of the
wafer surface and the abrasive particles was performed before and after polishing.
6.2 Experiments
6.2.1 Preparation of Slurry
The abrasive-less and chemical-less slurries were prepared from a Nalco2352
colloidal silica polishing slurry with a mean abrasive particle diameter of 70-90 nm. The
silica abrasives were examined using a scanning electron microscope (SEM), Fig. 6.1.
The silica abrasives were spherical, and their sizes were evenly controlled.
The procedure to prepare abrasive-less and chemical-less slurries is illustrated in Fig.
6.2. The slurry was centrifuged for approximately twenty hours (in 400 ml batches) in a
Servall Superspeed centrifuge at 16,000 rpm. The chemicals were carefully decanted from
the centrifuge tube with a precision pipette and the remaining abrasives were removed and
dried thoroughly. The abrasive-less solution slurry was formulated using the extracted
slurry chemical and de-ionized water in a 1:15.1 volume ratio. The chemical-less slurry
was produced by combining the dried abrasive particles and de-ionized water in a 1:15.3
weight ratio. The pH of the chemical-less slurry was adjusted to that of the original slurry
using a chemical buffer.
These two ratios were determined to simulate the same
153
conditions (particle dispersion density and volume ratio of the chemical) used in the
standard slurry. To aid dispersion, the mixture was agitated in a specialized mixer and the
pH was adjusted to that of the original slurry (10-11) to hinder particle agglomeration.
6.2.2 Polishing
A schematic of the experimental setup is the same as the one used in chapter 3,
shown in Fig. 3.4 except for the fact that friction force was not measured in this study.
This experiment used a commercially available Rodel IC60 pad to polish a bare silicon
wafer (p-type, <100> orientation).
The slurry, which was continuously stirred by a
magnetic stirrer, was supplied to the pad surface using a peristaltic pump. Slurry delivery
rates ranged from 30-120ml/min depending on rotational velocity of the wafer, which
ranged from 2-16cm/s.
Three different types of slurries were used: 1) unaltered Nalco2352, 2) chemicalless, and 3) abrasive-less slurries as discussed above. The wafers were polished with a
normal force of 10kPa, which corresponds to a pressure of 1.46psi. Each wafer was
polished for 2-3 hours and material removal was measured on an electric balance to a
resolution of 0.1mg every 10 minutes.
During the experiment, the relative velocity of the silicon wafer and the polishing
pad was controlled to induce a variation of slurry film thickness. The material removal per
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sliding distance with respect to the velocity was calculated for the chemical and the
mechanical polishing experiments and compared with the material removal for standard
CMP.
Scanning Electron Microscope (SEM) analysis of the abrasives was conducted to
verify that the abrasives had not been damaged after being removed from suspension in the
chemical slurry and reconstituted in the de-ionized water. The waste slurry was also
observed with the SEM to investigate possible abrasive shape change and residual
particles in the slurry. The wafer surface before and after polishing was observed with the
SEM to help understand the material removal mechanism in both abrasive-less and
chemical-less polishing.
Finally, to compare the characterization of the wafer surface before and after CMP,
phosphorus silica glass (PSG) oxide wafers were fabricated by a chemical vapor
deposition (CVD) process and polished using ILD1300 oxide.
6.3 Results and discussion
The possible transformation of abrasive size or shape during the preparation of the
chemical-less slurries was inspected before conducting experiments since the variation in
geometrical properties of the abrasives also plays a role in material removal.
Silica
particles from the normal slurry and the chemical-less slurry were sampled and observed
155
under the SEM, Fig. 6.3. Although the particle size distribution was not quantified here, it
was observed that the centrifuging and agitating processes used in preparing the chemicalless slurry have no noticeable effect on the abrasive size and shape. Hence, it is believed
that the effect of the variation of particle size and shape on the material removal of each
polishing process due to the experimental procedure is negligible.
By using normal, chemical-less, and abrasive-less slurries, silicon wafer polishing
was conducted under identical process conditions such as velocity, normal pressure,
polishing pad, and slurry flow rate. The measured mass change of the silicon wafer and
the relative velocity were used to calculate the material removal per sliding distance, Fig.
6.4. In Fig. 6.4, the x-axis is velocity ranging from 0 to 20 cm/s and the y-axis is material
removal per sliding distance ranging from 0 to 250 Þ/m. The graphs from the tip to
bottom indicate the material removal per sliding distance of normal CMP, mechanical
polishing, and chemical polishing.
In standard CMP, the material removal per wafer sliding distance ranged from 75 to
210 Þ/m. At a low relative speed the material removal was aggressive and prominent due
to the increased contact of the polishing pad with the wafer. As the velocity increases, the
wafer-pad contact area decreases due to hydrodynamic effects and this, in turn, causes the
decrease in material removal. As the relative velocity increased, the material removal
asymptotically approached a value in the range of 75 to 80 Þ/m. The material removal in
chemical polishing using the abrasive-less slurry, however, was almost zero and had no
correlation with the relative velocity. In mechanical polishing using the chemical-less
156
slurry, the material removal ranged from 40 to 80 Þ/m, which is approximately 40-60% of
that of standard CMP depending on the relative velocity. The material removal per sliding
distance of mechanical polishing decreased slightly with increasing relative velocity.
The surface characteristics of silicon wafer specimens after each polishing process
were examined using SEM, Fig. 6.5.
From the SEM analysis, no noticeable differences on the wafer surfaces were
observed after normal, chemical, and mechanical polishing processes. This may indicate
that the material removal mechanism in mechanical polishing is similar to that in standard
CMP. Micro- or nano-scratches caused by abrasives were not observed.
To compare the characterization of the wafer surface before and after CMP,
phosphorus silica glass (PSG) oxide wafers were fabricated by a chemical vapor
deposition (CVD) process and polished using ILD1300 oxide under the identical process
conditions used in silicon wafer polishing. The SEM pictures of the oxide wafer surface
clearly show the change in the surface characteristics after CMP, Fig. 6.6. Compared to
the oxide surface fabricated by thermal oxidation process, the oxide film by CVD was
initially rough and had a certain micro-texture. It became smooth after CMP. Again, no
micro- or nano-scratches by abrasives were detected.
The waste slurry was also observed with the SEM to investigate possible abrasive
shape change and residual particles in the slurry. The abrasive particles in the “new” slurry
were well controlled and evenly distributed in size and shape. On the other hand, after
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SEM analysis, it was observed that the size and shape of abrasive particles in the “used”
slurry became relatively uneven and irregular, Fig. 6.7. This may be due to particle
fracture and breakage during polishing, the attachment of chemically reacted material from
the wafer surface to the abrasive particles, or the wear of the particles themselves during
the CMP.
From the experimental results, it can be observed that material removal in CMP is
not simply the sum of the removal due to the mechanical polishing and the chemical
polishing elements of the process. Enhanced material removal can only be obtained when
the chemical components (chemicals in slurry) and the mechanical components (abrasives
in slurry) combine and act together during the process. It is believed that the material
properties of the wafer surface are changed by chemical reaction. Due to the relative
softness of the pad, each abrasive particle can become momentarily embedded in the
polishing pad, acting like an abrasive in the grinding wheel in grinding process. Then, the
particles can abrade the wafer surface at the atomic/molecular scale in 2- or 3-body
abrasion, Fig. 6.8. The combination of chemical and mechanical effects creates a ‘synergy
effect’, which is proposed to be the material removal mechanism in the CMP process.
In chemical polishing, due to the absence of the ‘cutting tool’ (abrasives) in the
slurry, there is little material removed even though a chemically reacted wafer surface is
created, Fig. 6.9. In mechanical polishing due to the absence of the ‘softened’ (chemically
reacted) surface, the material removed is much lower than that in CMP using the regular
abrasive slurry which has chemical and abrasives components. Although the ‘cutting tool’
158
(abrasives) is present in the mechanical polishing, the materal removal was low since the
wafer surface was not softened.
Based on the results of these experiments, a more detailed explanation of the roles of
the chemical and mechanical actions in the presence of the pad surface in CMP is
proposed. The chemically reacted wafer surface is abraded on an atomic/molecular scale
by individual abrasives in the slurry or abrasives attached on the pad surface rolling or
sliding against the wafer surface. Each particle, acting as a fixed abrasive (similar to twobody interaction in typical abrasive processes such as grinding or tape burnishing), is
momentarily embedded in the surface elements or asperities of the polishing pad. As the
normal pressure increases, the particle is pressed more firmly into the wafer and the
number of abrasives in actual contact with the wafer surface increases due to the increased
pad contact area. At a given normal pressure, the decreased material removal per sliding
distance is due to the decreased wafer-pad contact area as the relative velocity increases.
At low relative velocities, the mechanical removal will be the dominant material removal
mechanism. As the velocity increases, this mechanical removal will decrease and the
chemical reaction will increase due to the increased slurry flow rate under the wafer.
Hence, at a certain slurry film thickness or a wafer-pad contact mode, the variation
of the amount of the material removal will be dominated by the chemical effect since the
thickness of the ‘softened’ layer will be influenced by the slurry chemical, Fig. 6.10. In the
presence of a certain slurry chemical, the trend that the material removal per distance
159
asymptotically approaches a constant value will be influenced by mechanical aspects in
CMP such as speed, pressure, abrasive geometry, or slurry viscosity. This is due to the
variation of the slurry film thickness or the wafer-pad contact mode.
6.4 Summary
The mechanical and chemical contributions to material removal in CMP were
investigated independently by using ‘chemical-less’ and ‘abrasive-less’ slurries. In order to
create both slurries, a commercial slurry, Nalco2352 colloidal silica polishing slurry with a
mean abrasive particle diameter of 70-90 nm, was centrifuged to extract the abrasives
from the chemical. Then the abrasives and the chemical were mixed separately with deionized water to create chemical-less and abrasive-less slurries. The experiment used a
commercially available Rodel IC60 pad to polish a bare silicon wafer (p-type, <100>
orientation) on a laboratory CMP machine.
In chemical polishing using abrasive-less slurry, the amount of material removal
decreased to almost zero. In mechanical polishing using chemical-less slurry, the material
removal was only 40-60 % of that of CMP using normal slurry. This indicates that the
material removal of the CMP process is more than the sum of the removal due to the
mechanical and the chemical polishing effects.
160
The test results verify the important effect of the combination of chemical and
mechanical action to achieve high material removal rates in CMP. Based on the analysis
of the abrasive particles before and after polishing with and without conventional slurry
chemicals, some additional insight into potential material removal mechanisms at the micro
level is gained. This supports the theory that there is a synergy effect that amplifies
material removal only when the chemical and mechanical components act concurrently in
CMP.
161
(a)
(b)
(c)
Fig. 6.1 Silica abrasives in Nalco2352 silicon wafer polishing slurry, (a)(b) x45,000 and (c)
x200,000 magnification respectively.
162
Abrasive slurry
Abrasive particles
DI water
‘Chemical-less slurry’
Chemical
DI water
‘Abrasive-less slurry’
Centrifuge
Fig. 6.2 Preparation of abrasive-less and chemical-less slurries.
163
(a)
(b)
Fig. 6.3 Silica abrasives sampled from (a) the normal and (b) the chemical-less slurries.
164
250
First and second order regression
225
200
175
150
125
100
CMP
75
50
Mechanical polishing
25
0
Chemical polishing
-25
0
2
4
6
8
10
12
14
16
18
20
Velocity(cm/s)
Fig. 6.4 Material removal per sliding distance of the chemical, the mechanical, and the
normal polishing.
165
(a)
(b)
(c)
(d)
166
Fig. 6.5 SEM pictures of silicon wafer surfaces (a) before CMP, (b) after normal CMP, (c)
after chemical polishing, and (d) after mechanical polishing.
167
(a)
(a)
(b)
(c)
(d)
Fig. 6.6 PSG oxide wafer surfaces before ((a) x45,000, (b) x85,000) and after ((c)
x45,000, (d) x85,000) CMP.
168
(a)
(b)
Fig. 6.7 Irregular silica abrasives sampled from the used silicon wafer CMP slurry.
169
Fig. 6.8 Schematic of the material removal mechanism in CMP.
170
(a)
Abrasive
particle
Silicon wafer
Polishing pad
Pad asperity
Silicon wafer
(b)
‘Softened’ surface by
chemical reaction
Pad asperity
Polishing pad
Fig. 6.9 Schematic of (a) the mechanical and (b) the chemical polishing action.
171
Fig. 6.10 Mechanical and chemical effect on the material removal in CMP.
172
CHAPTER 7
CONCLUSIONS
Mechanical aspects of the material removal mechanism in CMP were investigated
analytically and experimentally.
Among many variables, the role of consumables
(polishing pad and abrasive particles in slurry) in CMP performance was evaluated.
Tribological characteristics (lubrication, friction, and wear) observed in CMP were also
analyzed.
The influence of chemistry on the mechanical removal mechanism was
inspected, and the mechanical and chemical contributions to material removal in CMP
were studied.
CMP processing using silicon wafers and three different types of polishing pads was
conducted to study the effects of the properties of the polishing pad (density,
compressibility, and surface roughness) on the material removal rate and friction force
between the silicon wafer and the polishing pad. The friction force applied on the wafer
surface was also correlated with the material removal rate of the silicon wafer in CMP.
The material removal rate of silicon wafers is inversely proportional to the pad
density and proportional to the pad compressibility and surface roughness. This is due to
the fact that the material removal of silicon wafers in CMP is closely related to the actual
pad contact area of the wafer, which is an indication of the probability for abrasives to
contact the wafer surface. Pads with low density also have low elastic and shear moduli,
172
E and G. This causes larger deformation and higher compressibility of the pad surface.
The large deformation of the pad surface causes increased contact between the polishing
pad surface and the wafer. The increased pad contact due to the large deformation of the
pad will increase the probability that the abrasive particles trapped between the wafer and
the pad asperity abrade the wafer surface. The pad density is inversely proportional to its
compressibility and the effect of compressibility, was opposite that of the density effect.
With higher surface roughness of the pad, there is more contact between the pad and the
wafer.
The restriction of the slurry flow due to the pad surface roughness can be another
factor to control the actual contact area between the polishing pad and the wafer surface.
A polishing pad with high surface roughness can prevent the free flow of the abrasive
slurry and, thus, the formation of the slurry film between the wafer and the pad as the
wafer slides over the pad. A thin slurry film induced by a rough pad surface results in
more pad contact with the wafer surface.
The friction force applied on the wafer during the process was inversely proportional
to the pad density and proportional to the pad compressibility and the surface roughness
since the actual pad contact area on the wafer surface highly influences the friction force
applied on the wafer surface. Thus, the material removal rate is proportional to the
friction force between the workpiece and the polishing pad and also to the work done by
the friction force on the workpiece for the CMP process with silicon. By using the friction
force data, the relationship between Preston’s coefficient and the friction coefficient was
estimated and integrated in Preston’s equation.
173
Preston’s coefficient is linearly
proportional to the friction coefficient during CMP. Therefore, the material removal can
be predicted by measuring the friction coefficient applied between the wafer and the pad
during the CMP process.
Since CMP uses an aqueous slurry, the lubrication effect of the slurry is an important
element in determining process performance.
The effect of the slurry film thickness
variation on CMP performance, defined in terms of material removal, planarization,
surface defects, and surface roughness, was investigated. The friction force variation with
velocity was correlated to the slurry film thickness under the wafer surface.
The abrasive slurry greatly reduces friction between the wafer and the pad, and the
friction force decreases as the slurry film thickness between the wafer and the pad
increases. Thus, from the Stribeck curve, the lubrication condition under the wafer in
CMP is closer to boundary lubrication or elasto-hydrodynamic lubrication, than to
hydrodynamic lubrication. The contact mode between the wafer and pad surfaces is direct
contact or semi-direct contact, not hydroplane sliding mode.
When the wafer has a thin slurry film under it due to high pressure and low velocity,
the actual contact area of the polishing pad on the wafer surface increases. The increased
pad contact will also increase the number of active abrasive particles which actually
contact and abrade the wafer surface. Due to the increase of contact of abrasives on the
wafer surface, the material removal is high and mechanical removal (nano-plowing and
nano-scratching) is dominant compared to chemical removal (erosion). The mechanically
dominant removal increases the possibility of generating surface defects such as nano-
174
scratches on the wafer surface and, thus, a relatively rough surface is generated, compared
to a surface finished by a chemically dominant removal. The intimate contact of the
polishing pad on the wafer surface, however, creates a better globally planarized surface
after CMP.
When the wafer has a large slurry film under it due to low pressure and high
velocity, the actual contact area of the polishing pad on the wafer surface decreases. The
decreased pad contact will also decrease the number of active abrasive particles which
actually contact and abrade the wafer surface. Due to the decrease of contact of abrasives
on the wafer surface, the material removal is low and chemical removal (erosion) by the
slurry plus a minor mechanical removal (minor nano-plowing and nano-scratching)
becomes the dominant material removal mechanism. The dominant chemical removal and
minor mechanical removal combination decreases the possibility of generating surface
defects such as nano-scratches on the wafer surface and, thus, a relatively smooth surface
is generated, compared to a surface finished by a mechanically dominant removal. It is
more difficult, however, to achieve global planarization since the chemically dominant
material removal is more isotropic in directionality, as in an etching process.
The slurry film thickness between the wafer and the pad is proportional to the
velocity of the wafer and the Hersey number, and the effect of slurry film thickness
variation on the CMP process performance (material removal, planarization, surface
defects and roughness) is significant. Since the dependence of material removal on the
slurry film thickness is not explained by Preston’s equation, it needs to be modified to be
useful as a practical CMP model. Furthermore, it may prove useful to optimize the slurry
175
film thickness to balance the mechanical and the chemical removal effects to maximize the
material removal and improve planarization (as measured by WIWNU) in the CMP
process.
The chemical in the slurry plays a key role in the mechanical action as well as the
chemical action in material removal rate in CMP. To identify the effect of slurry chemical
on the mechanical removal, the influence of the slurry chemical on the extension of
ductile/brittle transition depth in silicon was investigated by using scanning electron
microscope (SEM) analysis and by monitoring acoustic emission signals during a diamond
cutting test.
The surface property of a silicon wafer following chemical treatment was changed so
that the ductile machining regime was extended. This was verified from the SEM and the
AE monitoring analysis. The AE signal measured during scratching indicated three unique
cutting regimes: air-cut, ductile-mode, and brittle-mode cutting. In addition, this study
clarified the slurry chemical influence on the ductile/brittle transition depth.
The
chemically reacted silicon layer is believed to be in the range from 1.7 to 22 nm thick
which is the difference of the ductile/brittle transition depths of the untreated silicon wafer
and the chemically treated silicon wafer. The chemically reacted silicon layer is the cause
of the extension of the brittle/ductile transition depth, and the brittle cutting behavior
transition point becomes less distinctive.
The chemically reacted ‘ductile’ layer is
proposed to be the origin of the scratch/defect-free surface after CMP.
176
CMP employs abrasives and chemicals in slurry to polish the wafer surface. Thus,
the mechanical action by abrasives and the chemical action by slurry chemical are
synchronized in removing materials during CMP.
The mechanical and chemical
contributions to material removal in CMP were investigated independently by using
‘chemical-less’ and ‘abrasive-less’ slurries. In order to investigate the role of the abrasive
and the chemical independently, a commercial slurry was separated to produce chemicalless and abrasive-less slurries. These, along with the original slurry, were used in a series
of polishing experiments.
In chemical polishing using abrasive-less slurry, the amount of material removal
decreases to almost zero. In mechanical polishing using chemical-less slurry, the material
removal is only 40-60 % of that of CMP using normal slurry. This indicates that the
material removal of the CMP process is more than the sum of the removal due to the
mechanical and the chemical polishing effects. The test results verify the important effect
of the combination of chemical and mechanical action to achieve high material removal
rates in CMP. Based on the analysis of the abrasive particles and the wafer surface before
and after polishing with and without conventional slurry chemicals, some additional insight
into potential material removal mechanisms at the micro level is gained. This supports the
theory that there is a synergy effect that amplifies material removal only when the chemical
and mechanical components act concurrently in CMP.
As a summary of the conclusions, it is proposed that the role of abrasive particles in
determining material removal in CMP is critical. Little or no material removal is achieved
177
in the absence of abrasives. The chemical in the slurry can increase the degree of material
removal, but cannot change the key mechanism of material removal in CMP. The amount
of abrasives in contact with wafer surface during CMP, therefore, determines the material
removal of the wafer. This abrasive amount is substantially influenced by the actual waferpad contact area. The actual contact area between the wafer and the pad is determined by
the slurry film thickness under the wafer, i.e., wafer-pad contact mode, and the mechanical
and material properties of the polishing pad. The pad properties as well as the wafer-pad
contact mode also determine the degree of wafer planarization and the characteristics
(surface roughness and defects) of the wafer surface after CMP.
The influence of the mechanical aspects on the material removal mechanism in CMP
was quite significant. To understand the exact material removal mechanism of the CMP
process and, eventually, develop a practical model to predict and control CMP process are
critical. The key for this knowledge lies in identifying the mechanical and chemical aspects,
and their interactions in the material removal mechanism.
178
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