Friction

Nano Research
Vol. 5, No. 12, December 2012 ⅢI
ISSN 2223-7690
Vol. 1, No. 4, December 2013

Contents

Review
Scratch formation and its mechanism in chemical mechanical planarization (CMP) / 279–305
Tae-Young KWON, Manivannan RAMACHANDRAN, Jin-Goo PARK

Chemical mechanical polishing: Theory and experiment / 306–326

Dewen ZHAO, Xinchun LU

Research Article

Y2O3 nanosheets as slurry abrasives for chemical-mechanical planarization of copper / 327–332

Xingliang HE, Yunyun CHEN, Huijia ZHAO, Haoming SUN, Xinchun LU, Hong LIANG

Towards a unified classification of wear / 333–340

Michael VARENBERG

Mechanical and tribological properties of epoxy matrix composites modified with microencapsulated
mixture of wax lubricant and multi-walled carbon nanotubes / 341–349
Nay Win KHUN, He ZHANG, Jinglei YANG, Erjia LIU

Hydrophobic, mechanical, and tribological properties of fluorine incorporated hydrogenated

fullerene-like carbon films / 350–358
Li QIANG, Bin ZHANG, Kaixiong GAO, Zhenbin GONG, Junyan ZHANG

Abrasive-free polishing of hard disk substrate with H2O2-C4H10O2-Na2S2O5 slurry / 359–366

Weitao ZHANG, Hong LEI

Erratum

Erratum to: Green tribology: Fundamentals and future development / 367

Friction 1(4): 279–305 (2013)
DOI 10.1007/s40544-013-0026-y ISSN 2223-7690
REVIEW ARTICLE

Scratch formation and its mechanism in chemical mechanical

planarization (CMP)
Tae-Young KWON, Manivannan RAMACHANDRAN, Jin-Goo PARK*
Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea
Received: 14 June 2013 / Revised: 09 August 2013 / Accepted: 01 September 2013
© The author(s) 2013. This article is published with open access at Springerlink.com

Abstract: Chemical mechanical planarization (CMP) has become one of the most critical processes in
semiconductor device fabrication to achieve global planarization. To achieve an efficient global planarization
for device node dimensions of less than 32 nm, a comprehensive understanding of the physical, chemical, and
tribo-mechanical/chemical action at the interface between the pad and wafer in the presence of a slurry medium is
essential. During the CMP process, some issues such as film delamination, scratching, dishing, erosion, and
corrosion can generate defects which can adversely affect the yield and reliability. In this article, an overview of
material removal mechanism of CMP process, investigation of the scratch formation behavior based on polishing
process conditions and consumables, scratch formation mechanism and the scratch inspection tools were
extensively reviewed. The advantages of adopting the filtration unit and the jet spraying of water to reduce the
scratch formation have been reviewed. The current research trends in the scratch formation, based on modeling
perspective were also discussed.

Keywords: Chemical mechanical planarization (CMP); defects, scratch; post-CMP cleaning; defect source

1 Introduction in poor step coverage and contact interruption. In

order to improve the planarity, various planarization
Recent advances in integrated circuit (IC) technology techniques were considered, such as thermal reflow
have led to a significant increase in the number of the of borophosphosilicate glass (BPSG), reactive ion-etch
active components with a significant decrease in feature back, spin etch planarization, spin on deposition (SOD)
dimensions. This has resulted in the development of and others [1]. However, these techniques are extremely
high performance IC chips. As the critical features of limited in achieving a global planarization suitable
semiconductor devices have decreased to nanoscale for submicron devices. On the other hand, chemical
dimensions and additional levels are implemented mechanical planarization (CMP) is a unique technique
leading to multilevel-interconnection, the required that can provide excellent local and global planarity
degree of planarization has become more challenging. for ultra large scale integrated (ULSI) applications.
Moreover, continuous improvement is required for Figure 1 shows the planarization length of various
smaller technology nodes. As the device feature size methods used for removing the excess material.
decreases, it becomes very challenging to achieve Initially, the CMP process was pioneered by IBM in
high resolution on a non-planarized surface using 1980s [3, 4]. The CMP process became prominent due to
lithography because of the depth of focus requirement advantages such as global planarization, fewer defects,
in optical systems. Rough and irregular surfaces induce better step coverage, suitable for various materials,
variation in the photo resist thickness, which results and simplicity [1, 5]. The advantages of CMP are
tabulated in Table 1. CMP has been developed for
dielectric planarization applications. CMP is also used
* Corresponding author: Jin-Goo PARK.
E-mail: jgpark@hanyang.ac.kr to remove bulk dielectric films on the surface to isolate
280 Friction 1(4): 279–305 (2013)

Fig. 1 Planarization lengths of various planarization methods [2].

Fig. 2 Schematic diagram and consumables of CMP process.
the active devices on silicon substrates and to remove
the bulk metal films from the wafer surface to form Silica and ceria are the most commonly used abrasives
metal interconnection plugs or lines in dielectric films particles. The abrasive particles used are in the
[2, 6]. Due to an increase in the number of transistors on nanometer range. The nature of the abrasive particles
IC chips of dynamic random access memory (DRAM) and their size distribution plays an important role
and logic devices, new interconnect materials are in material removal during the CMP process [9, 10].
essential to satisfy the higher performance requirements. Additives added to the slurry play different roles
CMP is a global planarization process in which the during oxide and metal CMP. In general, metal CMP
wafer surface is planarized using the synergistic effect slurry contains more chemical additives when com-
of chemical and mechanical actions. During the CMP pared to an oxide CMP slurry. A metal CMP slurry
process, the wafer surface moves across a polishing contains oxidizing agents, complexing agents, corrosion
pad under a down pressure in the presence of a slurry. inhibitors, dispersion agents, and pH adjustors. The
There are many consumables for the CMP process, such CMP slurry is delivered to the polishing pad using a
as the slurry, polishing pad, and diamond conditioners pump. A rotating polishing pad transports the slurry
[3, 5, 7, 8]. CMP involves a complex interaction between to the wafer surface [11]. Contact area is provided
the wafer surface and the consumables. Figure 2 shows between the abrasive-pad and the abrasive-wafer
a schematic diagram of the CMP process and highlights interfaces [12, 13]. The structure of the polishing pad
the consumables. The type of slurry to be used depends and its properties are important in determining the
on the material surface, which, in turn, is related to the removal rate and planarization efficiency [7]. The
chemical and mechanical properties of wafer material. polishing pad has numerous micro pores and grooves

Table 1 Advantages of the CMP process (Reproduced from Ref. [1], with permission from Elsevier).
Advantages Remarks
Planarization Achieves global planarization
Planarize different materials Wide range of wafer surfaces can be planarized
Planarize multimaterial surfaces Useful for planarizing multiple materials during the same polishing step
Reduces severe topography to allow fabrication with tighter design rules and additional
Reduce severe topography
interconnection levels
Provides an alternate means of patterning metal, eliminating the need to plasma etch,
Alternative method of metal patterning
difficult to etch metals and alloys
Improved metal step coverage Improves metal step coverage due to reduction in topography
Increased IC reliability Contributes to increasing IC reliability, speed, yield (lower defect density)
Reduced defects CMP is a subtractive process and can remove surface defects
No hazardous gases Does not use hazardous gas, which is common in dry etch process
Friction 1(4): 279–305 (2013) 281

for delivery of the slurry [14]. Hence, the mechanisms Removal rate has a non-zero intercept at both zero
for CMP are lubrication behavior and abrasion, such velocity and pressure and has a greater dependence on
as direct contact between the wafer and polishing the velocity compared to the pressure. Thus, Luo et al.
pad (two body abrasion) and contact between the [18] proposed a modified Preston equation as follows:
wafer, the pad, and the abrasive in the presence of a
MRR  K(P  P0 )(V  V0 ) or MRR
slurry film occurring in the asperity region (three body (2)
abrasion) [14]. The role of the diamond conditioner is  KPV  aP  bV  Rc
to excise the pad surface in order to maintain its
where, P0, V0, a, b, and Rc are constants. However,
roughness against the plastic deformation and to
Eq. (2) predicts that the removal rate increases with
prevent glazing due to the accumulation of polishing
the pressure even at zero velocity, which was not
residues in the pad pores [8, 15, 16]. This review article
consistent with their experimental data.
is divided into the following sections: Section 2,
The final, modified form of the equation, according
modeling of CMP; Section 3, scratch issues in CMP
to Luo et al. [18] is given as follows
process; Section 4, scratch inspection tools; Section 5,
scratch formation source; and Section 6, scratch MRR= (KP  B)V  Rc (3)
formation mechanism. This review focuses on the
where K, B, and Rc are constants and were obtained
latest developments and current status of research
by a least squares procedure. The Preston coefficient
on CMP scratches and possible solution to avoid the
and other constants can be obtained from experimental
scratches and outline the scopes for future research.
data.
Cook [19] developed a MRR model based on
2 Modeling of chemical mechanical Hertzian elastic penetration of a spherical particle
planarization with pressure in which the interaction between the
abrasive particle and wafer surface occurs. Also, Liu
The mechanism of CMP based on the mechanical et al. [20] proposed a model which is based on a
interactions between the wafer, pad, and abrasive statistical method and elastic theory to describe the
particle has been studied by several groups. The MRR mechanism of silicon wafer surface during
most fundamental and basic material removal model the CMP process. In this model, the parameters of
in CMP is the Preston model, which is applicable for removal rate are hardness of wafer film and pad, and
glass polishing [17]. This equation states that the Young’s modulus of abrasive and film material. The
material removal rate (MRR) is directly proportional advantage of Cook’s and Liu’s MRR model, based on
to the pressure and relative velocity as follows: Hertzian contact, is the importance given to the role
MRR  Kp  P  V (1) and interactions of the consumable.
Runnels [21] proposed a model by considering the
where MRR is the material removal rate in m/min, P slurry fluid film. The importance of wafer curvature,
is the down pressure in N/m2, V is the relative velocity slurry viscosity, and thickness of the fluid film was
between the pad and wafer in m/min and Kp is the described in the model. The stresses induced by the
Preston coefficient in m2/N. The Preston coefficient flowing slurry on feature surfaces were computed and
depends on various factors that can affect the removal used in erosion models that empirically incorporated
rate such as friction force, chemical reaction, heating the fracture mechanics and chemistry. Tseng and
and so on. This is an empirical equation for under- Wang [22] proposed a MRR model for the CMP
standing mechanical action during the CMP process, process through the combination of solid and fluid
which shows the linear dependency. mechanics. This model is given by MRR = MP 5/6V 1/2 ,
However, MRR is not zero for some materials, even where M is a constant associated with material pro-
when P and V are zero. Such behavior is most com- perties such as abrasive concentration and chemical
monly seen in metal CMP. Hence, a modified Preston’s processes during CMP. Also, this model was obtained
equation was proposed based on the Cu CMP [18]. using a non-linear relationship between the material
282 Friction 1(4): 279–305 (2013)

removal and relative velocity. This might be due to

the contribution of velocity to the slurry flow instead
of a sliding of abrasives. Zhang et al. [23] proposed an
equation MRR = K(PV)1/2 which included the effects
of polishing pressure and platen speed on particle
penetration depth in the CMP process. This equation
was derived based on the surface plastic deformation,
the pad-wafer partial contact, and particle adhesion
theory. Abrasive particle-surface interactions were
analyzed and material removal by adhesive and
abrasive removal mechanisms during CMP process
were extensively studied by Ahmadi and Xia [24].
The material removal rate was found to be related to
the distribution of pad asperities. A linear dependence
was obtained when the pad asperities have a random
distribution, while a sub linear dependence was
observed when the pad asperities have a wavy
distribution.
Fig. 3 Schematic diagram showing the polishing mechanism and
During the CMP process, the removal rate was
the criterion for material removal [27].
affected by the pad surface properties. For example,
MRR increases with the pad surface roughness [25]. particles are rolling against the wafer surface under a
Yu et al. [26] considered the effect of pad surface pressure lower than the threshold value, the removal
roughness and the interaction between the pad and rate will be negligible. The removal rate was found to
wafer with the contact area. Their results showed that be significant only if the abrasive particles held by the
the real pressure is induced by the contact area, and pad were sliding against the wafer surface. In other
moderately depends on the applied pressure. Also, the words, removal rate was found to be negligible, if the
ratio of real contact area was smaller than the nominal applied pressure is less than the minimum threshold
contact area and is proportional to the down pressure. pressure.
The physical CMP model, which includes the effects Luo and Dornfeld [28] investigated the abrasion
of polishing pad roughness and dynamic interaction mechanism in solid−solid contact mode of the CMP
between the pad and wafer, is based on the asperity process based on the assumptions of plastic contact
theory. Zhao and Shi [27] also proposed a model over wafer-abrasive and pad-abrasive interfaces.
based on wafer-asperity contact. The polishing pressure Figure 4 shows the two contact modes of the CMP
dependence of MRR for the CMP with a soft pad was process: the hydro-dynamical contact mode and the
found to be sub-linear. Also, abrasive particles can solid−solid contact mode. The Luo and Dornfeld model
demonstrate a threshold pressure during CMP pro- combined the process parameters including pressure
cesses, which might have played a critical role in MRR. and velocity in addition to other properties such as pad
Furthermore, the contact area between the asperity and wafer hardness, pad roughness, abrasive particle
and the wafer is given by A ∝ P2/3 based on Hertzian size, morphology and its distribution in the same
elastic contact theory. Finally, the modified MRR equation to predict the MRR. The material removal rate
equation is given as MRR = K(V)(P2/3 – Pth2/3) at P ≥ Pth, can be predicted by MRR = ρw NVolremoved + C0, where
and MRR = 0 at P < Pth, where Pth is the threshold ρw is the density of wafer material, N is the number
pressure, and K(V) is a function of relative velocity (V) of active abrasive particles, Volremoved is the volume of
and other CMP parameters. Figure 3 shows a schematic material removed by a single abrasive per unit time,
diagram showing the polishing mechanism and the and C0 is the material removal due to chemical etching.
criterion for material removal. When the abrasive Also, they suggest that two-body abrasion between
Friction 1(4): 279–305 (2013) 283

of surface being polished. This may be attributed to

the effects of various chemicals and abrasive particles
as well as the pressure exerted on the wafer surface
[7, 29]. Defects typically formed during the CMP
process include organic residues [29], water marks [30],
particle adherence and impingement [31], corrosion
pit, and scratches [30, 31]. However, the removal of
organic residues and water mark formation are trivial
in oxide CMP, but other types of defects, such as
scratch formation, are critical, as they affect the yield
and reliability of the devices [32]. Table 2 shows the
CMP process induced defects and their specific effects
on the replacement metal gate (RMG) process [33].
Scratches are one of the most commonly generated
defects during the CMP process. It was found that CMP
scratches could cause an initial failure as well as long
term reliability failure [34]. The failure mechanism in
the shallow trench isolation (STI), inter-level dielectric
Fig. 4 Two contact modes of CMP: (a) hydro-dynamical contact (ILD), and poly-Si CMP processes is very similar in
mode and (b) solid–solid contact mode [28].
nature. Scratches cannot be detected after CMP, but are
usually identified after etching using the HF solution
the wafer and an abrasive particle mainly affects the [33]. The periodic arc scars generated on brittle
material removal when compared to three body materials such as oxide, BPSG, and poly-Si are called
abrasion. chatter mark-type scratches [33, 35]. Figure 5 shows
some examples of chatter mark scratches after STI
CMP. Scratch shape is influenced by the mechanical
3 Scratch issues in CMP process
properties of the material. A wide variety of scratches
In the manufacturing of IC chips, the wafer is polished are formed on a metal surface like Cu, which is
several times using the CMP process. CMP has been shown in Fig. 6.
applied for polishing various types of surfaces, Surface defects by CMP have been continuously
including oxides, Cu, W and others [7]. However, reduced by the development of abrasive particles and
several defects induced by CMP depend on the type slurries, polishing pads, diamond conditioners and

Table 2 Potential causes of CMP defects and possible solutions [33].

Defect mode Potential causes Impact to device Potential solutions
· Slurry/pad residue · Shorting/opens · Cleaner tooling
Particles · Polish byproducts · Pattern distortion · Clean chemistries
· Pad conditioning
· Large/hard foreign particles
Macro scratches · Pattern removal over multiple die · Pad cleaning
on polish pad · Environment
· Slurry agglomeration · Slurry filters
Micro scratches · Shorting/opens
· Pad asperities · Pad/pad conditioning
Corrosion (metal · Slurry chemistry · Passivating films,
· Opens, Reliability
CMP) · Clean chemistry · Chemistry optimization
· Weak adhesion · Shorting/opens · Improve adhesion
Film delamination · CMP shear force · Device parametrics · Low pressure CMP
· Cleaner tooling
· Inadequate cleaning · Shorting/opens
Organic residue · Slurry optimization
· Residual slurry components · Disturbed patterning of next layer
· Clean chemistries
284 Friction 1(4): 279–305 (2013)

so on. However, as the scale of integration is reduced,

strict control of surface defects, such as scratches, is
required according to the International Technology
Roadmap for Semiconductors (ITRS) (Table 3) [36].

4 Scratch inspection tools

As the application of the CMP process increases,
various unpredicted defects occur. However, those
defects cannot be easily detected after CMP, and the
shape of such scratches depends on the source. Various
contaminated particles and defects on the wafers
were identified and characterized by means of optical
microscope, surface scanning inspection, scanning
electron microscopy (SEM), and atomic force micros-
copy [37, 38]. In particular, the inspection tools that
use the light scattering behavior have been used for
monitoring the scratches. Some instruments such as
Fig. 5 Chatter mark scratches observed in STI CMP [33]. confocal review stations (CRS) [39], advanced inspection
tools (ATI) [40], and optical surface analyzers (OSAs)
[41] are used in the industry. The optical inspection
system usually uses a bright and dark field system. In
the case of bright-field systems, both the scattered light
and reflected light are collected through the same
aperture to obtain an image. However, a dark field
system collects selectively the scattered light and not
the reflected light within the collection angle [42].

5 Scratch formation sources

5.1 High particle concentration and agglomerated

particles

Fig. 6 Various scratches formed in Cu CMP [33]. In the CMP process, several possible reasons for scratch
formation have been proposed in the literature [43−74]

Table 3 Critical scratch length and number on ITRS 2010 [36].

STI CMP technology requirements Scratches
Year of production DRAM 1/2 pitch (nm) Wafer diameter (mm) Critical scratch length, Critical scratch count,
(contacted) sc (nm) spw (wafer−1)
2012 36 300 17.9 40.1
2013 32 300 15.9 40.1
2014 28 450 15.9 150.5
2015 25 450 12.6 104.6
2016 22.6 450 11.3 104.6
2017 20.0 450 10.0 104.6
2018 17.9 450 8.9 104.6
Friction 1(4): 279–305 (2013) 285

and can be broadly classified into process conditions with a POU filter. Based on their results, the defects
(down pressure, velocity, etc.) based scratches and were remarkably reduced after installation of the POU
consumables (slurries include abrasive particles, pads, filter. Also, they showed that the slurry filter plays an
conditioners, etc) based scratches. CMP consumables important role in the determination of pad lifetime.
can cause surface scratches due to particle agglomera- The effect of a high spray bar (HSB) method, i.e.,
tion, release of diamonds from the conditioner, or pad de-ionized water (DIW) with high pressure during
debris. Several reports discussing the effects of these CMP was evaluated. High spray bar can prevent the
factors on scratch formation have been published accumulation of large particles on the pad. As a result,
[44−74]. Lin et al. [75] evaluated the number of scratches the defect density was significantly reduced when
formed during CMP on various film surfaces in the compared with an un-installed high spray method.
manufacturing of DRAM devices. The micro-scratch Figure 9 [43] shows the defect density trend obtained
number on the SiN cap layer was much lower, which with and without a high pressure DI water spray bar
might be due to the higher hardness. Also, they during CMP.
optimized the film thickness of filled oxide and SiN Teo et al. [44] characterized the scratches generated
cap layer to reduce micro-scratches, based on the during Cu CMP as a function of process pressure
difference in material hardness. and velocity with different abrasive particles. In their
Typically, a CMP process consists of chemical results, scratches generated on the Cu surface were
and mechanical interactions between the wafer and classified into two types, long scratches and triangular
polishing pad with a slurry. The mechanical action scratches. A likely cause for a long scratch is that
is attributed to the abrasive particle and polishing abrasive particles become embedded in the polishing
pad interactions. Hence, scratches resulting from pad during the polishing process. On the other hand,
mechanical polishing are inevitable. The abrasive a possible cause for triangle scratches could be due to
particle size distribution influences the number and freely suspended abrasive particles being driven onto
size of the active abrasives [76]. Seo and Kim focused the Cu surface. Also, it was found that deeper scratches
on micro-scratch generation caused by agglomerated were detected when larger and harder abrasive particles,
particles, which are solidified and attached in the like alumina particles, were used for Cu CMP.
pipeline of a slurry supply system [30, 40, 43]. They Also, it was noted that the occurrence of scratches
evaluated the effect of abrasive particle size distribu- can increase due to the agglomeration of the abrasive
tion and controlled the large particle concentration by particles. Flushing the stagnant slurry in the slurry pipe
installing a point of use (POU) slurry filter. Figure 7
line might remove the agglomerated abrasive particles.
shows a schematic diagram of the CMP tool with a
For example, the flushing procedure effectively reduced
POU filter. Figure 8 shows a comparison of defect
scratch generation (Fig. 10).
densities as a function of number of wafers polished
Ahn et al. [45] evaluated the surface roughness of Al
after CMP performed using the optimum conditions
of a silica based slurry and compared these with the
conventional alumina based slurry. The agglomeration
of particles induced by zeta-potential and oxide layer
thickness of Al, which are a function of pH, could
also affect the surface roughness. Also, the surface
roughness of Al increased with an increase in abrasive
concentration. The reason for this seems to be that
friction was more severe at high abrasive concentrations.
Kim et al. [46] focused on controlling the agglomeration
of ceria particles using the organic additives and pH
Fig. 7 Schematic diagram of the CMP tool with a POU filter adjusters to reduce micro-scratches. Remsen et al. [47]
and high pressure spray bar (HSB) of DI water [30].
used a dual-sensor single particle optical sensing (SPOS)
286 Friction 1(4): 279–305 (2013)

Fig. 8 Defect density as a function of polished wafer counts (a) without filter and (b) with 0.5 μm filter [30].

Fig. 9 Defect density trend (a) with pre-wet flow rate of 700 ml/min and without the high spray bar of DI water and (b) pre-wet flow
rate of 200 ml/min and high spray bar of DI water (Reproduced from Ref. [43], with permission from Elsevier).

CMP were established. Figure 11 shows the correlation,

which is linear when considering values of LPC over
0.469 μm. Also, an example of LPC levels of filtered
slurries (A, C, D, E, F, and G) with scratch count results
is shown in Fig. 12.
Several researchers used modified abrasive particles
to reduce the surface defects such as scratches [48−52].
Generally, a mixed abrasive slurry and various
dispersants were used for the development of fine
slurries [49, 50]. Coutinho et al. [48] synthesized
composite particles containing ceria nanoparticles
dispersed within cross-linked, polymeric microspheres
formed by copolymerization of N-isopropylacrylamide
Fig. 10 Effect of flushing slurry line [44]. (NIPAM) with 3-(trimethoxysilyl)propyl methacrylate
(MPS), which can used as novel abrasive particles for
analysis method to quantify the large particle con- CMP. As a result, surfaces polished using composite
centration (LPC). Also, the correlation between LPC particles showed lower topographical variations and
in fumed silica slurries and scratch formation during surface roughness than surfaces polished using ceria
Friction 1(4): 279–305 (2013) 287

Fig. 13 Surface roughness of the polished wafer (Reproduced

Fig. 11 Correlation between scratch counts and LPC determined from Ref. [48], with permission from Elsevier).
for particles with diameter ≥ 0.469 μm (Reprinted with permission
from Ref. [47]. Copyright 2006, The Electrochemical Society).
with a higher hardness generated more and deeper
scratches. The -alumina/silica core−shell particles were
prepared by mixing 0.2 mol/L Na2SiO3 and 1 wt%
H2SO4 solutions with an -alumina dispersion and
simultaneously stirring at the reaction temperature.
The pH of the mixture was maintained between 9
and 10. Synthesized alumina/silica core−shell abrasives
were characterized using Fourier transform infrared
(FTIR) spectrocopy, X-ray photoelectron spectroscopy
(XPS), secondary ion mass spectroscopy (SIMS), and
a zeta potential analyzer. Figure 15 shows the SEM
image of alumina particles before and after coating.
When a composite abrasive-based slurry was used
for the polishing, surface roughness was significantly
decreased; the optical microscope images of disk
substrate are shown in Fig. 16.
Fig. 12 Expansion of the low scratch count region of the
correlation between scratch counts and LPC determined for
On the other hand, novel polymer-core silica-shell
particles with diameter ≥ 0.68 μm (Reprinted with permission composites were proposed by Armini et al. [52, 53].
from Ref. [47]. Copyright 2006, The Electrochemical Society). Polymethyl methacrylate (PMMA)-based terpolymer
particles (diameter 350 nm) were coated with colloidal
nanoparticles (Fig. 13). Also, optical microscopy images silica particles. The coating was performed either
of post-CMP oxide surfaces are shown in Fig. 14. by creating chemical bonds using a silane coupling
Commercial ceria particles resulted in severe scratches agent (composite A) or by adjusting the pH to form
on the oxide surface when compared to the composite electrostatic attractive interactions between the core
ceria particles. and the shell (composite B). They focused on tuning
Furthermore, some researchers have proposed the mechanical properties of the polymer core by
surface modified abrasive particles for CMP slurry varying its synthesis parameters. The major advantage
formulations [51−54]. Lei and Zhang [51] used of the silica coating is that it can be easily modified in
alumina/silica core−shell abrasive particles to get a terms of its surface chemistry and morphology. Also,
uniform surface with fewer scratches. Alumina particles composite particles are aimed at improving the CMP
288 Friction 1(4): 279–305 (2013)

Fig. 14 Optical microscopy images of silicon dioxide films polished with slurry containing (a) 0.5 wt% composite particles, (b) 0.5 wt%
CeO2 nanoparticles, and (c) 0.25 wt% CeO2 nanoparticles (Reproduced from Ref. [48], with permission from Elsevier).

Fig. 15 SEM image of alumina particles (a) before and (b) after coating (Reproduced from Ref. [51], with permission from Elsevier).

Fig. 16 Optical microscope images of disk substrates polished in slurries containing different abrasives (a) before polishing (200×),
(b) polished using pure alumina slurry (200×) and (c) polished using composite abrasive (with 10 wt% coating) slurry (200×) (Reproduced
from Ref. [51], with permission from Elsevier).

process of soft materials due to the cushion-like effect oxide thickness loss after 1 min of CMP using different
arising from the elastic properties of the core, which abrasive particles. For the silica abrasive, thickness
allow the composites to easily adapt to the pad loss decreased with increasing particle size. In the case
asperities (Fig. 17). of two composite particles, total defect counts were
Oxide removal rate and scratch generation were different. Composite B particles are spherical in shape
evaluated using four types of abrasive particles (30 and are more similar to the colloidal silica particle.
and 90 nm colloidal silica particles, 350 nm polymer Also, the larger size of colloidal silica shows a higher
particles, composite A and B). Figure 18 shows the number of defects level than the smaller size of
Friction 1(4): 279–305 (2013) 289

colloidal silica. The interaction force and composite

particle morphology were also described in other works
[53, 54]. Based on the average pull-off force vs. pH
plot, qualitative agreement between the measured
adhesion forces and the material removal rate was
reached [53]. Furthermore, the depth of the scratch
increased with increasing abrasive size of fumed
silica abrasive. Overall, fewer and shallower scratches
were detected for composite B particle with a colloidal
silica shell compared with only colloidal silica due to
the effect of the elasticity of the polymer core [54].
As mentioned earlier, slurry is one of the major
consumables for the CMP process. Slurry consists of
Fig. 17 Schematic diagram depicting μ-scale phenomena occurring fine abrasives which act as a source for scratch
during CMP. SEM images of (a) composite A and (b) composite generation. A typical CMP slurry consists of abrasives,
B abrasives (Reprinted with permission from Ref. [52]. Copyright
additives, and a pH buffing agent. The slurry distri-
2007, The Electrochemical Society).
bution system consists of a slurry tank, distribution
pumps, a pressure gauge, a flow meter, and a
pressurized air supply outlet/inlet [55]. A schematic
representation of the slurry distribution system is
shown in Fig. 19. Stability of the slurry is critical in
the CMP process. During pumping and mixing of the
slurry, particles tend to agglomerate due to the pH
shock, the dilution effect or the temperature change.
pH shock may be due to the dilution effect or mixing
effect caused by the additives (as in the case of a
two component slurry) [56]. Stress-induced particle
agglomeration has already been extensively studied
[55, 57, 58].
Stress-induced particle agglomeration can be
explained by the Smoluchowski theory based model,

Fig. 18 (a) Thickness loss vs. abrasive type and (b) total defect Fig. 19 Schematic illustration of the slurry distribution system
count vs. abrasive type after oxide CMP at pH 10 [52]. [55].
290 Friction 1(4): 279–305 (2013)

which considered the shear flow and the electrostatic are greater than the repulsive inter-particle force,
interaction between particles. It was assumed that particle agglomeration occurs. The degree of particle
particle collisions were binary and proportional to the agglomeration is influenced by the slurry properties
particle concentration. Chang et al. [55] simulated the (e.g., interparticle forces), external shear stress (i.e.,
aggregation rate of k-fold aggregates, dNk/dt, which type of pump), and the number of turnovers of the
is given by the time evolution of the cluster size slurry. They found that a magnetically levitated
aggregates, i and j-fold. centrifugal pump resulted in lower stress effects on
particle agglomeration and did not increase the
l  k -1 
dN k 1
dt

2
 (k
i jk
ij /Wi j )N i N j  N k  (kki /Wki )N i
k 1
(4) concentration of oversized particles, as shown in
Fig. 20 [55]. Also, the defectivity was evaluated using
the low-k dielectric CMP. Optical microscopy images
4 of the low-k dielectric film are shown in Fig. 21.
kij  G( ai  a j ) (5)
3
5.2 Pad surface properties and pad debris
where the aggregation constant, kij, is a function of
the shear rate (G) and particle size (a). The stability CMP is a complex interaction process between the
ratio (W) is the ratio of the rapid aggregation rate wafer surface and the consumables. The CMP polishing
without electrostatic interaction to the slow aggregation pad is an important consumable among all other
rate in the presence of electrostatic interactions consumables, and has a dominating effect on the
between particles. According to this model, the material removal rate [59]. The structure and material
shear flow causes particles to approach each other properties determine the material removal rate and
during slurry delivery. When van der Waals forces planarization ability [1, 60]. Usually, the polishing

Fig. 20 Cumulative concentration vs. particle size at 0, 250, and 500 turnovers for (a) bellows, (b) diaphragm, and (c) magnetically
levitated centrifugal pump system (Reprinted with permission from Ref. [55]. Copyright 2009, The Electrochemical Society).

Fig. 21 Optical microscopy images of BD1 wafers polished by circulated slurries using (a) bellows, (b) diaphragm and (c) magnetically
levitated centrifugal pump system (Reprinted with permission from Ref. [55]. Copyright 2009, The Electrochemical Society).
Friction 1(4): 279–305 (2013) 291

pad contains both pores and grooves, which help for ratio (percentage of scratches/defective die, i.e., the
better planarization [61, 62]. The pores of a pad act number of scratches formed on 100 defective dies) and
as a lake, store the slurry particles, and enhance the removal rate during the STI CMP process. Scratch
contact time between slurry particles and the wafer. formation was found to be higher in the contact regime
Grooves provide a channel for efficient and uniform and lower in the lubricating regime. The contact regime
slurry distribution across the pad surface to the wafer exists when the pad contains only grooves [63], and
surface. These parameters determine the slurry tran- the lubricating regime exists when the pad contains
sportation and contact area at the pad/wafer interface pores [64]. Optimum conditions were obtained in the
[1, 5, 7, 59, 62]. Choi et al. [62] studied the synergistic presence of a lubricating regime with fewer scratch
role of pores and grooves of a pad in forming the sources present on the pad [62]. Also, the presence of
scratches (especially chatter mark scratches) using grooves helps to discharge most of the scratch sources
three types of pads. Pad with only pores, only grooves, generated during the process away from the wafer–
and both pores and grooves were investigated to pad contact [65].
understand its effect on scratch formation. Figure 22 Both the structure of polishing pads, such as pores
shows the SEM images of scratch shapes formed on and grooves, and the hardness of the pad affect the
the STI patterned wafers polished using three types MRR and generate the scratches. Hsien et al. [66]
of pads. Different types of pads generated different reported scratch generation by comparing the hard
types of scratches. Pad-3 (containing both pores and and soft pads. It was reported that the soft pad with
grooves) generated short chatter mark-shaped scratches lower pressure generated fewer scratches [66, 67].
compared with the other types of pads. Furthermore, Eusner et al. [68] quantitatively analyzed
Figure 23 shows the effect of pad type on scratch the topography and material properties of fresh and

Fig. 22 SEM images of scratches formed on STI-patterned wafers after CMP using pads with (a) only grooves (pad-1), (b) only pores (pad-2),
and (c) pores and grooves (pad-3) [62].

Fig. 23 (a) Scratch ratio on the STI-patterned wafer, and (b) MRR of blanket oxide wafer with ceria slurry as a function of pad type [63].
292 Friction 1(4): 279–305 (2013)

broken-in pads to correlate with scratch generation on

Cu CMP. The hardness and modulus of the pad were
measured, and the change in pad asperity radius of
curvature was measured during pad break-in with a
blanket Cu wafer in the slurry. It was found that the
average pad modulus decreased from 0.66 to 0.34 GPa
and the average pad hardness decreased from 0.05
to 0.03 GPa through pad break-in. In contrast, the
average pad asperity radius of curvature increased
from 16 to 93 μm as a result of pad break-in, which
induced a reduction in severe scratch formation.
Scratches were detected using an optical scanning Fig. 24 SEM micrographs (top) and schematics (bottom) of (a)
porous pads and (b) solid pads (Reproduced from Ref. [70], with
method after polishing using the fresh and broken-in
permission from Elsevier).
pad with only water. The reason for using water was
to isolate the scratches generated by the pad from the
slurry particles. Also, the critical pad asperity radius
of curvature was based on asperity deformation (i.e.,
elastic or plastic).
During the CMP process, the pad surface can
undergo plastic deformation and the surface becomes
smoother as the pores are filled with the pad materials
[15]. Using a glazed pad causes the removal rate to
drop significantly [69]. Polishing pads were conditioned
with a diamond conditioner to provide consistent
performance and to prevent the glazing effect. Usually,
diamond grits used for pad conditioning are attached
to an alloy substrate using electrochemical deposition
methods [8]. Yang et al. [70, 71] investigated the CMP Fig. 25 Scratch level on STI patterned wafers generated by porous
process based on material removal rate and scratch and solid pads with 180 μm diamond conditioner (Reproduced from
Ref. [70], with permission from Elsevier).
defects by studying the pad interaction and conditioner
effect using two types of polishing pads: a porous
diamond size of conditioner on the removal rate and
pad and a solid pad with micro holes (Fig. 24). When
scratch generation. It was found that the micro holes
a solid pad with micro holes was used with a fumed in the pad acted as a defect source or coarse particle
silica slurry and a 180 μm diamond grit conditioner, reservoir to prevent micro scratching during the
the material removal rate decreased by approximately process [71]. They reported optimized results of solid
10% compared with the porous pad. However, the pads with micro holes using the hole depth control
scratch defects were reduced when compared with procedure to reduce the defects.
the porous pad which is shown in Fig. 25. In order to As mentioned earlier, pad debris can be generated
increase the removal rate obtained using a solid pad due to tearing of the pad by the conditioner. Prasad
with micro holes to a level comparable to a regular et al. [72] studied the generation of pad debris and its
porous pad, various diamond conditioners with characterization. They reported that pad debris could
diamond size ranging from 70 to 130 μm were adopted. act as a main scratch source, resulting in scratches with
Also, pad surface roughness and contact area were several size ranges with irregular shapes, mostly in
analyzed to understand the removal rate and the agglomerated form. It was also proposed that the
scratch generation. Figure 26 shows the effect of surface properties were changed by their adsorption
Friction 1(4): 279–305 (2013) 293

Fig. 27 SEM image and EDX analysis of (a) fresh pad, (b) pad
debris with only DI water, and (c) pad debris with silica slurry [72].

Fig. 26 The effect of diamond size on (a) removal rate and (b)
scratch generation (Reproduced from Ref. [70], with permission
from Elsevier).

with abrasive particles. Figure 27 shows FESEM images

of fresh pad particles and pad debris generated using
DI water and silica abrasive particles. Park’s group
[73] also investigated the scratch number using the
three different scratch source (vis., pad debris, dried
particles, and diamond particles) on scratch formation
comprehensively with their classification. Figure 28
shows the material removal rate and generated scratch
number as a function of scratch source. A small
amount of impurity in slurry did not affect the MRR.
However, scratch number was affected by the kind of
scratch sources. Figure 29 shows the distribution of
scratchess formed by adding different scratch sources
during polishing. Borken chatter type of scratches
was easily formed when dry slurry paritcles were Fig. 28 (a) Material removal rate, non-uniformity and (b) the
added but group chatter when pad debris were added. variation of number of scratches formed with addition of different
Yang et al. [74] measured the pad surface hardening scratch sources [73].
294 Friction 1(4): 279–305 (2013)

Fig. 30 A plot of the dependence of light-point defect counts,

measured with a Tencor 6220 on oxide wafers, as a function of
PSM vacuum level. A reduction of nearly 50% was observed as a
PSM vacuum [77].

6 Scratch formation mechanism

Brittle fracture can occur by three basic types of static
indentations: Hertzian cracks, radial cracks, and lateral
cracks (Fig. 31) [78−80]. Hertzian cracks are cone cracks
that are created from a spherical indenter. Radial
cracks are semi-circular cracks perpendicular to the
glass surface from a sharp indenter, and lateral cracks
Fig. 29 (a) Effect of addition of pad debris, dried slurry particle are cracks that run generally parallel to the glass
and (b) diamond particles on distribution of scratch shapes formed surface, which are also typically created by a sharp
on oxide surface after CMP process with silica slurry [73]. indenter. Suratwala et al. [78, 79] measured the
distribution and characteristics of surface crack (sub-
phenomenon based on force–distance (F–D) curves. surface damage) formation during grinding on fused
It was found that the interaction between abrasive silica glass using a surface taper polishing technique.
particle and polyurethane pad under tribo-mechanical The observed surface cracks were characterized as
action could change the pad surface hardness. Benner near-surface lateral- and deeper trailing indent-type
et al. [77] used a vacuum cleaner to remove the pad fractures. They showed that only a small fraction of
debris and agglomerated large particles from the pad; the abrasive particles are being mechanically loaded
they dubbed this process the pad surface manager and causing fracture, and most likely it is the larger
(PSM). Figure 30 contains a plot of light-point defects particles in the abrasive particle size distribution that
measured using a Tencor 6220 on polished oxide bear the higher loads. Surface damage depth increased
wafers using different levels of PSM vacuum. The data with load and with a small amount of larger con-
were normalized to that observed without vacuum. taminant particles, which is based on the brittle facture
As the PSM vacuum level was increased, CMP models (Fig. 32) [78]. Also, the surface damage depth
induced wafer defects decreased. Approximately a distribution has been related to the length distribution
50% reduction in light-point defects was observed to gain insight in effective size distribution of particles
using the PSM technique. participating in the fracture. Figure 33 shows the
various types of scratches that were observed as a result
Friction 1(4): 279–305 (2013) 295

Fig. 32 (a) Lateral crack depth as a function of load1/2 and (b)

Hertzian cone depth and radial crack depth as a function of
load2/3 [78].

Fig. 31 Schematic illustrations of the fracture geometry of the

idealized fractures created by static indentation: (a) Hertzian cone
crack from a blunt indenter, (b) radial or median cracks from a
sharp indenter, and (c) lateral crack from a sharp indenter [78].

of addition of rogue particles [81]. These scratches

Fig. 33 Categories of different types of scratches observed in
were classified into three basic categories: (1) plastic fused silica sample (Reproduced from Ref. [29], with permission
scratches that show no brittle fracture, (2) brittle from Elsevier).
scratches, which only have cracks (trailing indent or
lateral) and (3) mixed scratches that contain both Furthermore, Ring et al. [29] reviewed the mechanical
plastic modification and cracks. properties and fracture mechanics of materials in order
296 Friction 1(4): 279–305 (2013)

to understand the surface damage caused during and particles impurities, which were not spherical
CMP. The resulting failure was predicted by various but angular in shape. Also, the distribution of radii of
mechanical wear (or scratching) equations depending curvature for the point of the impurity particle in
upon the assumption of plastic deformation or brittle contact with the wafer surface was considered. Hence,
fracture (Fig. 34). The wear rate goes from reasonably the plastic deformation scratch depth is given by
low rates for plastic wear to rates with higher orders of
 L E 
12

b   N 2   cot  
13
magnitude for brittle fracture. The wear rate transition (9)
occurs at a threshold normal load, i.e.,  H 
Here, E’ is the relative modulus of elasticity and φ is
LNc ~ 2  10 5 KIc4 H 3 (6)
the angle between opposite edges of the indenter. The
where H is the hardness of the surface being damaged depth of the radial cracks, gives the scratch depth for
and KIc is its fracture toughness. In the case of plastic brittle fracture as follows:
deformation, the differential volume, dV, of material 23
  E  1 2  L  23
removed per unit length, dx, of the scratch depends c  CR   r    N  cot    (10)
upon the load of the abrasive point normal to the   H   KIc  
surface, LN, and the mechanical properties of the where r is a dimensionless constant. There is a transi-
materials comprising the surface as follows: tion between plastic and brittle fracture scratching
dV dx ~(LN H ) (7) that takes place as the load is increased. Therefore,
when the load on an impurity is less than LNc, plastic
This equation assumes that the abrasive point is harder deformation will take place; when the load on an
than the material comprising the surface. In the case impurity particle is greater than LNc, brittle fracture
of brittle fracture, the fracture wear rate could be will take place.
represented as follows: Particle impurities are forced by pad asperities to
be in contact with the wafer surface. The asperities
dV dx ~(E H )4 5 KIc-1 2 H -5 8 L9N8 (8)
press the impurity particles into the wafer surface,
where E is Young’s modulus. Ring et al. considered creating a normal load that allows the depth of the
each of these scratching particles to be attached to the surface damage to be predicted using Greenwood and
tip of an asperity or, if larger than an asperity, to be Williamson’s [82, 83] and Yu’s theories [26]. Figure 35
pressed into the pad to determine the depth distri- shows the size distribution of scratches produced
bution of the scratches due to both abrasive particles by the impurity particles. The deepest scratches were
formed by the large impurity particles and the po-
pulation of scratches decreased as the scratch depth
increases for a given size of particle impurities.
Saka et al. [84] estimated the scratch formation at
lower and upper-bound loads based on contact
mechanics models. Additionally, the width and depth
of scratches are dependent on process parameters
such as particle size, abrasive volume fraction, and
mechanical and geometric properties of the pad and
surface coatings. In their study, interactions between
the Al2O3 abrasive particles and the Cu/low-k surface
were described. They assumed that the Young’s
modulus and hardness of abrasive particles are greater
Fig. 34 Schematic of (a) plastic deformation and (b) brittle than the coated films. Particles were assumed to be
fracture (Reprinted with permission from Ref. [29]. Copyright spherical and rigid with smooth and sufficiently thick.
2007, The Electrochemical Society). The radius of the contact on the coated film at yield
Friction 1(4): 279–305 (2013) 297

Fig. 36 Schematic of a hard particle indenting a soft coating at

the onset of yielding [84].

During polishing under full-contact mode, abrasive

particles sticking to the wafer were pressed, which
is shown in Fig. 37. The hardness of coated film (Hc)
[84, 87], is given by

RUB RUB
Hc   (14)
A π 2
ac
2
where PUB is the applied load, A is the projected con-
tact area, and ac is the semi-width of a scratch. Based on
the geometry of the scratch, the relation between the
depth of the scratch (δc) and the semi-width is given by
Fig. 35 Size distribution of scratches produced in (a) ILD and
(b) copper by particle impurities (Reprinted with permission from
c
2
1  ac 
Ref. [29]. Copyright 2007, The Electrochemical Society).     c  ac  (15)
R 2 R 
(aY,c), the depth of the indentation in the film at yield
load (δY,c), the yield load (PY,c) as a function of the
particle radius (R) and the mechanical properties of the
coating were represented based on Hertzian analysis
and the Tresca criterion for yielding as follows [84−86]:

π Hc
aY,c  R (11)
4 Ec

π 2 H c2
 Y,c  R (12)
16 Ec2

π 3 H c3 2
PY,c  R (13)
48 Ec2

where Ec and Hc are the Young’s modulus and hardness

of the coated film, respectively. Figure 36 shows the
geometry of the contact. Fig. 37 A hard particle scratching a soft coating [84].
298 Friction 1(4): 279–305 (2013)

For a fully plastic contact, the semi-width and Chandra et al. [89] proposed a multi-scale model
the depth of a scratch, and the upper-bound load, encompassing the pad response and slurry behavior
respectively, are to predict the scratch propensity in CMP. The pad
response delineates the interplay between the local
12
 2P  particle-level deformation and the cell-level bending
ac   UB  (16)
 πH c  of the pad. Although the agglomeration process is
traditionally classified into two separate regimes,
PUB diffusion-limited agglomeration (DLA) and reaction-
c  (17)
πRH c limited agglomeration (RLA), DLA occurs near the
iso-electric point of the slurry particles, while RLA
PUB  π c RH c  c  ac ≪ R  (18) occurs when the pH of the slurry is away from the
iso-electric point [89]. For the general case, the
Figure 38 shows the normalized experimental load agglomeration process can be modeled using the
versus the normalized scratch depth. The solid line Smoluchowski rate equation [89, 90], which gives the
represents the normalized upper-bound load. Therefore, time rate of change of the number of particle clusters
all the points on the graph should be below the line of with volume M, N(M), as follows:
the upper bound load according to Eq. (18). In Fig. 38,
d 1 M -1
all the points were below the solid line; therefore, the N ( M )   a (M , K )N (M -K )N(K )
load per particle can be related to the scratch width dt 2 K 1

(19)
and depth, according to Eq. (18). Based on the above   a (M ,K )N (M )N (K )
modeling and experimental results, multi-particle K 1

contact behavior and the effect of pad asperity geometry The agglomeration kernel, a(M,K), is the rate at which
for the initiation of scratches were analyzed [84, 88]. clusters of volume M agglomerate with particles of
The various regimes of scratching by polishing pads volume K. It has been shown that most agglomeration
in CMP have been delineated by contact mechanics results from smaller particles sticking themselves onto
based theoretical. a larger cluster [91]. The spatial distribution of the
MRR is also affected by pad wear, which takes place
mainly at the asperity level. The probability density
function of the asperity height z at any time t is given
as follows:

4C E*  s 
d
dt
 (z ,t )  a
3π z
 
z-d(t ) (z ,t ) (20)

where Ca is the pad wear rate coefficient, E* is the

effective modulus of the pad and  s is pad asperity
tip curvature. Also, the calculation of scratch depth
involves two random variables, pad asperity height (z)
and effective particle cluster radius (X). The two
variables are independent and the scratch depth W(i,j)
due to the jth particle under the ith asperity is given by
[89, 91]

E*  s
W (i , j)  z-d(t )X(i , j ) (21)
Hπ
Fig. 38 Normalized experimental load versus the normalized
scratch depth (Reproduced from Ref. [84], with permission from Using the above equations, the cumulative density
Elsevier). function of the scratch depth can be calculated. The
Friction 1(4): 279–305 (2013) 299

probability per active particle, P(W  ω), a scratch of the probability density of scratch depth, which was
depth W, which is less than a prescribed threshold ω, simulated from the proposed equations. It was
will be created and is given by observed that the scratch depth increased while scratch
frequency decreased for harder pads as well as for
Xmax w2 H 2
P(W ≤ w)    f z (z)f x (x)dzdx (22) softer wafer surfaces.
0 0
Typically, chatter mark-type scratches, which have
The model predictions were compared with the a repetitive C-shaped crack, were generated in inter-
experimental results in Fig. 39. The maximum scratch level dielectric (ILD) materials (Fig. 40). In this image,
depth predicted by the model was much lower. This the cracks are larger at one end and smaller at the other
discrepancy was thought to be caused by inaccuracies end of the repetitive line. Furthermore, the repetitive
in the assumed initial particle distribution in the slurry. C-shaped surface showed damage that is tens of nm
This might be due to the contamination of the slurry deep with some individual cracks that were deeper
with a very low percentage of relatively large particles. than others, in atomic force microscope (AFM) images.
Additionally, the model was adopted as a function Ring et al. [29] explained this phenomenon based on
of pad modulus and wafer surface hardness. The bouncing particle model. The springiness of the pad
scratch depth was affected by pad modulus, and hence causes the particle to bounce against the wafer surface.
Bouncing may be initiated by a particle impurity that
is sliding across the surface of the wafer. After the first
bounce, the particles have sufficient force to indent
the surface of the wafer. This force is supplied by
the elastic properties of the pad when the particle is
pushed into it and then rebounds. The frequency of
bounces can be determined by the simple physics of a
mass (the particle) on a spring (the pad). The governing
equation is given by

d2 x
F  k1 x  m (23)
dt 2

where F is the force supplied by elastic property of

Fig. 39 Normalized experimental load versus the normalized the pad, k1 is the spring constant of the pad, m is the
scratch depth (Reproduced from Ref. [89], with permission from mass of the particle and x is the vertical distance that
Elsevier). the particle moves into the pad during rebound. The

Fig. 40 Chatter surface damage showing repetitive, 40-nm-deep indentations in the wafer surface (Reprinted with permission from
Ref. [29]. Copyright 2007, The Electrochemical Society).
300 Friction 1(4): 279–305 (2013)

solution to the above equation is given by spring constant k of the model connecting the step
motor (moving at a constant speed V0) to the slider
x  Asin( t   0 ) (24) can be obtained from the slope of the horizontal force
versus time curve (Fig. 42) during the sticking stage.
where A is the amplitude, which is given by
The total mass of the slider and sample is m. L is the
normal load applied to the specimen and x is the real
 
2

A  x02   0  (25) scratch distance moved by the indenter. The force

 
balance in the sliding direction is given by
where x0 is the initial displacement of the particle in
k(V0 t  x)  f  mx
 (27)
the pad and v0 is the initial vertical velocity of the
particle. The angular frequency, ω (and frequency, f)
During scratching, the horizontal force is measured
for a mass on a spring are given by
by k(V0t− x), where k(V0t − x) is the real extension of
the spring being stretched, f is the force needed to
2π k1
  2πf  (26) plastically deform the material in front of the indenter.
T m
A saw-tooth wave form characteristic of stick-slip
where T is the period of oscillation. behavior is shown in Fig. 42. It was observed that the
During the oxide CMP, even more chatter mark- scratching motion was preceded by jerks instead of
type scratches are formed on the wafer surface [90]. a smooth path. In their result, it was reported that,
However, the explanation of chatter mark scratch during slip, the indenter velocity started from zero,
generation using only basic contact theory is not easy. increased to a maximum and then decreased to zero
Stick-slip phenomena between two sliding surfaces again. The scratch groove made during slip showed a
are commonly observed in a wide range of length non-uniform depth, which increased with decreasing
scales from atomic to macroscopic [73, 91, 92]. Gao et of scratch velocity. Although the scratch velocity and
al. [92, 93] developed an empirical equation describing groove depth changed markedly during slip stage,
the stick-slip friction as a function of humidity, speed, the scratch force remained almost constant for most
and applied load. Zhang and Li [94] proposed that of the scratch distance.
the normal load is the main contributing factor in the Kim et al. [95] also studied the generation of chatter
scratch force, rather than the driving speed during mark scratches and proposed the controlling parameters
stick-slip, and proposed a micromechanical model for chatter mark scratching. Based on the force balance
to describe the slip process. Figure 41 shows a simple in the sliding direction, stick-slip friction was used
model of that proposed scratch system. The effective in the model. The distance between chatter marks

Fig. 41 A simple model of the scratch system (Reproduced from Fig. 42 The horizontal force measured by the load cell, k(V0t–x)
Ref. [94], with permission from Elsevier). (Reproduced from Ref. [94], with permission from Elsevier).
Friction 1(4): 279–305 (2013) 301

was predicted by controlling the applied velocity to permits any use, distribution, and reproduction in any
characterize the chatter scratch formation. Thus, the medium, provided the original author(s) and source
particle position from the starting point increased are credited.
with increased oscillatory motion and sliding time
(or distance).
References
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7 Concluding Remarks
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[7] Li Y. Microelectronic Applications of Chemical Mechanical
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Planarization. New Jersey (USA): John Wiley Sons, 2007.
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Jin-Goo PARK. He received PhD directors of Micro Biochip Center and Nano-bio
degree in materials science and Electronic Materials and Processing Lab. (NEMPL,
engineering from University of www.nempl.net). His research interests include
Arizona in 1993. From 1992 to 1994, wafer cleanings and chemical mechanical polishing
he was with Texas Instruments, as well as nano-bio MEMS. He is now president of
Dallas, TX, where he was Korea CMPUGM (www.cmpugm.com) and a
responsible for microcontamination founder and president of International Conference on
control in semiconductor wet processing and DMD Planarization/CMP Technology (ICPT) which is the
development. In 1994, he joined Hanyang University largest CMP conference in the world. He is also a
at Ansan, where he is now a professor in the founder of Korea Surface Cleaning Users Group
Department of Materials Engineering as well as Meeting (www.scugm.com).

Tae-Young KWON. He received and Ph.D. student at the same university. He has
his Bachelor degree in Department recently obtained his Ph.D. degree in Department of
of Materials Engineering in 2006 Materials Engineering at Hanyang University. His
from Hanyang University, Ansan, research interests include Chemical Mechanical
Korea. After then, he was a M.S. Planarization process and its tribology.
Friction 1(4): 306–326 (2013)
DOI 10.1007/s40544-013-0035-x ISSN 2223-7690
REVIEW ARTICLE

Chemical mechanical polishing: Theory and experiment

Dewen ZHAO, Xinchun LU*
State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China
Received: 25 October 2013 / Accepted: 24 November 2013
© The author(s) 2013. This article is published with open access at Springerlink.com

Abstract: For several decades, chemical mechanical polishing (CMP) has been the most widely used planarization
method in integrated circuits manufacturing. The final polishing results are affected by many factors related to
the carrier structure, the polishing pad, the slurry, and the process parameters. As both chemical and mechanical
actions affect the effectiveness of CMP, and these actions are themselves affected by many factors, the CMP
mechanism is complex and has been a hot research area for many years. This review provides a basic description
of the development, challenges, and key technologies associated with CMP. We summarize theoretical CMP
models from the perspectives of kinematics, empirical, its mechanism (from the viewpoint of the atomic scale,
particle scale, and wafer scale), and its chemical–mechanical synergy. Experimental approaches to the CMP
mechanism of material removal and planarization are further discussed from the viewpoint of the particle wear
effect, chemical–mechanical synergy, and wafer–pad interfacial interaction.

Keywords: chemical mechanical polishing (CMP); CMP model; planarization mechanism; wafer–pad interaction;
uniformity

1 Introduction In chip manufacturing, the front-end process

fabricates the circuit elements, while the back-end
The chemical mechanical polishing/planarization (CMP) process wires these elements within an integrated
process was developed at IBM and was first used in circuit. Both the front-end and the back-end processes
oxide polishing in 1986, and in tungsten polishing need the CMP process to produce a flat structure. To
in 1988. After several decades of development, it has accommodate the improvements of decreased feature
become accepted worldwide as a mainstream process
size and increased device speed, chip interconnects,
in the fabrication of planar film. Using CMP, planar,
which function as back end of the line (BEOL)
smooth, and damage-free surface can be obtained.
processes, have become as important as the front end
By definition, CMP is a process whereby both
of the line (FEOL) processes [2, 3]. CMP is one of the
chemical and mechanical actions complement each
most important processes in the BEOL processes [4].
other to improve the material removal rate (MRR).
Figure 1 shows the section view of Intel’s 65 nm
CMP can produce both global and local planar sur-
technology silicon back-end interconnect stack with 8
faces to the wafer by micro, nano, or atomic material
metal layers [5]. With CMP process, the interconnect
removal, so as to satisfy the planarity constraint
imposed by current advanced lithography processes materials can be stacked layer upon layer.
[1]. Over the past few decades, CMP has emerged as a In keeping with Moore’s law, the IC manufacture
necessary planarization process in the manufacture of process has for many years seen the developing of
integrated circuits (IC) products because of its effective small feature size, increased wafer size, and higher
performance in thinning and flattening thin films. integration. Presently, 300 mm wafers are widely used,
and 450 mm wafers are expected to emerge in several
* Corresponding author: Xinchun LU. years, while the interconnections have exceeded 10
E-mail: xclu@tsinghua.edu.cn
Friction 1(4): 306–326 (2013) 307

2 Basics of CMP

2.1 Principle of CMP

There are four types of commercially available CMP

equipments that are most representative and most
widely used in industry (see Fig. 2): (a) a rotary-type
polisher with a wafer carrier that has a reciprocation
motion along the platen diameter; (b) a rotary-type
polisher with a carrier that has an oscillation motion;
(c) an orbital-type polisher with the platen that has an
orbital rotation; (d) a linear-type polisher that has a
linear motion belt as the polishing pad.
For the typical rotary type CMP tool, the platen and
the wafer carrier rotate in the same direction, while
the wafer carrier reciprocates synchronously along the
radial direction of the platen. The wafer is held in a
rotating carrier, as shown in Fig. 3(a). The carrier has
a membrane that applies the downforce on the wafer
back, and a retaining ring around the outside of the
wafer to keep the wafer in the carrier. A polishing
pad is mounted on the rotating platen. The surface
of the wafer being polished is pressed against the
Fig. 1 Section view of Intel’s 65 nm technology silicon back-end
interconnect stack (adapted from Intel Developer Forum 2009 [5]).

levels. Therefore, CMP faces many challenges that

need to be overcome, such as the need to provide
nano level planarity and sub-nano level roughness to
wafer surfaces, while avoiding surface and subsurface
damage, which has almost reached the limit in surface
manufacturing.
To improve the CMP technique, two aspects of the
mechanism must first be investigated. On one hand,
we need to understand the micro/nano/atomic scale
material removal mechanism caused by the synergetic
effects of chemical and mechanical actions. On the
other hand, for large-dimension wafers, we need to
know how to achieve a global planar surface by local
material removal.
In this paper, we review the main factors, key
challenges, and technologies of CMP. Theoretical
models will be introduced from the viewpoint of the
atomic scale, particle scale, and wafer scale. In addition,
Fig. 2 Schematic of different types of CMP equipment: (a) rotary
we will review experimental studies regarding its
type, reciprocation mode, (b) rotary type, oscillation mode, (c)
mechanism and process. orbital type, and (d) linear type.
308 Friction 1(4): 306–326 (2013)

the SEM image of the pad top surface, respectively.

The chemical reaction softens the deposited film
surface to enable it to be a more easily removed layer.
From the combination of the chemical actions of the
chemicals and the mechanical actions of the particles,
micro material removal takes place, enabling surface
finishing to be realized [6].

2.2 Main factors

The MRR, the non-uniformity, and the surface quality

are the main results which indicate the machine’s
efficiency and surface quality. Factors that are related
to the wafer–pad interaction can affect the polishing
Fig. 3 Schematic of CMP equipment and wafer–pad interactions: results. The major factors include machine structures
(a) CMP equipment, (b) wafer–pad interactions, (c) details of (e.g., carrier structure), process parameters (e.g., down-
particle–film interactions, and (d) SEM image of pad top surface. force and kinematic parameters), and consumables
(e.g., slurry and pad), as shown in Fig. 4. These input
polishing pad. The motions of the carrier and the variables affect the wafer pad interaction, including
platen generate the relative motion for the polishing. the pressure/stress distribution, the slurry film
A slurry containing particles and chemical solutions distribution, the sliding distance distribution, and the
is delivered on the pad as the abrasive. Figures 3(b), temperature distribution. The final polishing results
3(c), and 3(d) give a detailed schematic diagram of are determined by the synergetic action of the above
wafer–pad interactions, particle–film interactions, and process parameters [7].

Fig. 4 Schematic of CMP factors affecting the final profile.

Friction 1(4): 306–326 (2013) 309

2.2.1 Carrier structure the corresponding process control method, a marked

improvement in the global uniformity of the wafer
Previous wafer carriers use a fixed rigid packing plate
after CMP can be realized.
and a fixed retaining ring to grip the wafer and to
apply the polishing pressure [8], as shown in Fig. 5(a). 2.2.2 Polishing pad
Because the ring cannot be applied to a separate
The polishing pads are usually made of porous
pressure to accommodate the wafer contact pressure,
polyurethane, with a filler material added to modify
the wafer edge has a large edge exclusion due to the
pad hardness [4]. The hardness of the pad is one of
edge effect. To improve the uniformity of the wafer
its most important properties, and can affect both the
contact stress, a flexible membrane is used to load
MRR and uniformity. Both the hard pad and soft pad
the wafer and to apply a soft load on the wafer’s
are needed for different film materials and different
back surface. In addition, a floating ring which can
process steps. Soft pads, such as Suba and Politex,
be separately loaded is used as the retaining ring, as
and hard pads, such as IC1000 and IC1010, are most
shown in Fig. 5(b). The retaining ring can effectively
widely used in IC manufacturing. The details of pad
shift the stress concentration near the wafer edge to
top in Fig. 3(d) give the SEM image of an IC1000 pad.
the surface of the retaining ring. Usually, a relatively
The bulk materials and the surface are full of
larger pressure is applied to the retaining ring to
micropores, which are useful for storing the slurry
ensure that the wafer has a uniform contact stress; as
and the abrasive particles in the slurry, and they
a result, good uniformity and smaller edge exclusion
survive the aggressive slurry chemistries.
can be realized for the wafer [9, 10].
Due to mechanical loads and chemical reactions at
However, when the wafer diameter increases to
the pad surface, physical properties of a CMP pad,
300 mm or larger, a uniform load pressure cannot
such as the elastic modulus, compressibility, hardness,
produce a uniform contact pressure. Besides, the wafer
and surface roughness, are expected to vary during
may have an incoming surface topography. Therefore,
CMP [12−14]. These changes may have important
to improve the uniformity of the CMP for a large-size
effects on the overall CMP process. Therefore, a pad
wafer, a novel multizone carrier is developed, and is
conditioner is used to introduce a pad conditioning
widely used in today’s industrialized CMP equipment,
process that can generate new asperities on the pad
as shown schematically in Fig. 5(c). The multizone
surface to maintain the pad performance (see Fig. 3(a)).
carrier has a multizone membrane for the application
With the excepting of the mechanical properties of the
of individual pressures to different eccentric zones
pad, grooves on the pad comprise another important
and the retaining ring [11]. Using this technique and
factor for the pad, and are used for slurry transfer and
for removing the polishing debris. A reasonable groove
design may result in good polishing results [15].

2.2.3 Slurry

Slurry is the most complex consumable of CMP.

The slurry is a stable mixture of abrasive materials
dispersed in DI wafer with other chemicals, such as
oxidant, inhibitor, surfactant, and bases to provide an
acid or alkaline pH. Particles such as SiO2, CeO2, and
Al2O3, with the average particle size ranging from 10
to 100 nanometers, can be used as the abrasive. The
chemical elements, particles size and concentration,
as well as the pH value of the solution can affect the
Fig. 5 Schematic section view of wafer carrier: (a) hard plate
MRR, uniformity, and surface quality. Especially, the
carrier with no ring pressure, (b) flexible membrane carrier with
interaction and balance of the oxidant, inhibitor, and
ring pressure, and (c) multizone carrier.
310 Friction 1(4): 306–326 (2013)

complexing agent can significantly affect the polishing and the 22 nm node needs 13 layers. The nonuniformity
results [16, 17]. will accumulate when the number of interconnect
layers increases, which may introduce additional
2.2.4 Process parameters
challenges to the CMP process.
As shown in Fig. 4, the removal rate profile is codeter- To increase the production efficiency and to reduce
mined by the wafer–pad interfacial parameters of the chip cost, the wafer dimension has been increased
pressure distribution, sliding distance distribution, from 200 mm (8 inches) to 300 mm (12 inches), and
temperature distribution, and slurry distribution. Many subsequently toward 450 mm. The semiconductor
process parameters, such as the downforce (including industry has effectively adapted its CMP technology
the zone pressure and the ring pressure), the kinematic for the 300 mm wafer. For large-diameter wafers, the
parameters (including the carrier/platen speed and realization of global planarity across the whole wafer
reciprocating motion parameters), the slurry (including will also be a major challenge for CMP.
its flow rate, pH value, and particle parameters), and 2.3.2 Low-k material
the pad (including its hardness, groove form, and
conditioning parameters), can affect the final polishing To reduce the RC delay of the device, copper
results by modifying above interfacial parameters at interconnects have been introduced to replace Al
the wafer–pad interface. interconnects, and the damascene process has been
introduced. Ultra low-k materials will be used as
2.3 Development trend and main challenges interlayer dielectrics to further decrease the RC
delay. According to the ITRS roadmap 2012, materials
2.3.1 Feature size and wafer dimension
with a dielectric constant of 2.2 will be integrated
With the development of different technique, inte- into the IC by the year 2019 (Table 1). However, the
grated circuits have trended toward having smaller low-k dielectrics are soft and weak relative to the
size, higher integration, and lower price. As a result, metal material. Both of the single and dual damascene
several new challenges have emerged for the CMP structures comprising ultra low-k materials are more
process. Base on the International Technology Roadmap prone to buckling and crushing failures. The difference
for Semiconductors (ITRS 2012 [18]), both STI CMP between the mechanical property and polishing rates
and interconnect CMP are being developed toward of copper and the low-k materials will significantly
sub-22 nm node (see Table 1).
Table 1 Interconnect CMP demand from ITRS 2012 [18].
The ITRS 2012 predicts that by 2015, the half pitch
of Metal 1 will be below 22 nm, and will be further Interlevel metal
Metal 1 wiring Number
insulator effective
reduced to 14 nm by 2019. However, as the feature Year half-pitch of metal
dielectric constant,
size decreased, the focus depth of the lithography (nm) levels
k
is shortened accordingly. The nonuniformity of the
2012 32 12 2.82–3.16
wafer surface will therefore result in a nonuniform
lithography width, subsequently leading to chip 2013 27 13 2.55–3.00
failure. 2014 24 13 2.55–3.00
For the ultra-large scale integrated-circuit (ULSI), 2015 21 13 2.55–3.00
the number of transistors that are fitted on a single
2016 19 13 2.40–2.78
chip has exceeded 1 billion. Multi-lever interconnects
2017 17 14 2.40–2.78
are introduced to improve the connection efficiency.
With the increasing number of transistors per chip, 2018 15 14 2.40–2.78
the number of interconnect layers also increases. For 2019 13 14 2.15–2.46
the 65 nm node, there are 9−10 layers, and when the 2020 12 14 2.15–2.47
feature size is below 45 nm, the number of interconnect
2025 7 16 1.60–2.00
layers exceeds 10, while the 32 nm node needs 12 layers,
Friction 1(4): 306–326 (2013) 311

affect post-CMP surface planarity and surface quality.

New technologies and processes, such as stress free
CMP and low downforce CMP, are therefore demanded
to be developed to address these problems [19].

2.4 Key technology of CMP

2.4.1 Pressure control

As the original surface profile of wafers produced

from the electrochemical plating (ECP) process is not Fig. 6 Schematic of endpoint detection.
sufficiently planar, traditional one-zone CMP cannot
control the profile (especially at the wafer edge)
for different incoming wafers. Thus, a new type of
3 Kinematics and stress simulation for
multizone CMP was developed, and is expected to
improve the uniformity and to provide a wider CMP
processing window. Unlike the typical single-zone
3.1 Kinematic simulation
configuration, the wafer carrier is divided into multiple
zones in the radial position, and different pressures The kinematic aspect is the most basic uniformity
can be applied to each zone individually (see Fig. 5(c)). factor that affects the final polishing results [25−28].
Using this technique, the within-wafer nonuniformity The relative motion between the wafer and the pad is
(WIWNU) can be significantly improved. produced by the three basic motions of the carrier and
Further, using the multizone carrier, a closed-loop the platen. The relative velocity of the pad at one point
zone pressure control technology was developed in relative to the wafer is given by Eq. (1) [29]
AMAT’s machine based on In Situ Profile Control
(ISPC™) using next generation polishing heads. Using v  p  ( e  r )  (w  r  vR )
(1)
the real-time profile adjustments technique, the ISPC  p  e  (p  w )  r  vR
system can significantly improve the post-polish
within-wafer and wafer-to-wafer non-uniformity. The where p and w represent the angular speed of the
zone-to-zone range was improved from 1300 Å open- platen and the wafer carrier, respectively, vR represents
loop to 70 Å with ISPC control for ILD0 CMP, and the translational velocity of the wafer carrier, and e is
from 870 Å open-loop to 200 Å with ISPC control for the center distance between the platen and the wafer
STI CMP [20]. carrier.
By calculating velocity integral during the entire
2.4.2 Endpoint detection
polishing time, the sliding distance of each point of
In-line monitoring and automatic endpoint detection the wafer can be given as follows:
of CMP can provide information regarding the film
t
thickness, surface profile, and the time at which S(r , )   v(t )|( r , ) dt (2)
0
the film will be fully removed [21]. It offers many
advantages to the manufacturing process such as Kinematic analysis reveals that the basic kinematic
improved process yields, reduced product variability, parameters significantly affect the velocity distribution,
closer conformance to target requirements, and higher the sliding distance distribution, and the nonuniformity
throughput. The optical method [22, 23], eddy current [7, 30−33]. Zhao et al. [29] found that the intrinsic
method [21], and motor current detecting [24] are relations, especially the coupling relations among the
most widely used as the in-line monitoring methods basic motions, i.e., the rotary speed ratio of the wafer
for the endpoint, and Fig. 6 shows the schematic to the pad α and the period ratio of the reciprocating
configurations of these endpoint detection methods. motion of the wafer to the rotary motion of the platen
312 Friction 1(4): 306–326 (2013)

kT0, significantly affect the uniformity of the sliding

distance of the wafer relative to the pad, and the
distribution of the particle sliding trajectories [34].
For better uniformity, the speed ratio should be close
to 1 (but should not equal to 1), and the reciprocating
motion of the carrier is necessary.

3.2 Contact stress analysis

The contact stress at the wafer–pad interface largely

represents the mechanical action and significantly
affects the material removal. Researchers have studied
the contact stress distribution of CMP based on a two-
dimensional axisymmetric quasi-static finite element Fig. 8 Profile of material removal rate of oxide (Reproduced
model, as shown in Fig. 7(a). The wafer is loaded by from Ref. [35], by permission of The Electrochemical Society).
the carrier through a flexible carrier film. Early finite
element analysis (FEA) calculation of the interfacial Compared to the FEA results, Fu et al. [41] gave
von Mises stress of CMP found that the wafer edge an approximate analytical solution to the two-body
has a stress concentration, and the stress distribution interaction problem. The model reveals that uniform
corresponds to the profile of the oxide removal rate pressure on the wafer backside will still result in a
(see Fig. 8) [8, 35]. The elastic modules of the pad non-uniform contact stress and edge effect.
and the carrier film have obvious effects on the stress The retaining ring plays an important role in CMP,
distribution [36−39]. Besides, the parameters of the and should be considered in the FEA model (see
wafer, such as wafer dimension, wafer thickness, and Fig. 7(b)). The ring gap and ring pressure both affect
surface curvature may affect the contact stress [40] the contact stress (especially the contact stress at the
wafer edge). The peak value of the von Mises stress
can be decreased by increasing the ratio of the ring
load [10]. Using a suitable ring pressure and ring gap,
a more uniform contact stress can be obtained relative
to the case of no ring pressure [42].
For an actual multizone wafer carrier, the back
pressure is divided into several individual zones (see
Fig. 7(c)). Wang et al. [11] investigated the contact
stress of the multizone carrier, and found that both
the contact stress and the MRR of the wafer can be
adjusted by varying the applied load at the zones
and the retaining ring in multizone CMP. The contact
stress at one zone was strongly related to the applied
pressure of the loading zone and was slightly affected
by the adjacent zones. Figure 9 gives one example of
the zone pressure loading effect (Fig. 9(a)) and its effect
on the MRR (Fig. 9(b)) when a larger or small pressure
Fig. 7 Two-dimensional axisymmetric finite element model: (a) is applied to zone 2, respectively. The MRR profile of
without retaining ring, (b) with retaining ring, and (c) with the wafer exhibited the same trend as the contact stress
multizone carrier film. on the wafer surface [11].
Friction 1(4): 306–326 (2013) 313

experimental data used an empirical equation to

evaluate the effect of the macroscopical polishing
variables on the MRR. The most famous model is the
Preston equation [28], which describes the linear
relationship between the MRR and the product of the
downforce P and the relative velocity V, as shown
in Eq. (3).

MRR = kPV (3)

where k is an empirical constant based on experi-

mental data. P, V, and MRR have an average value.
The Preston equation mainly considers the mechanical
action and the MRR. Therefore, it has some limitations.
In fact, P and V may have a nonlinear relationship
with MRR under some conditions. Tseng and Wang
[43] re-examined the pressure and speed dependences
on the removal rate, and conducted a more precise
Preston equation:

MRR = kP5/6V 1/2 (4)

The V 1/2 term indicates a much weaker dependence of

the removal rate on the speed V. A higher speed may
be considered to imply a larger centrifugal force for
the slurry and a larger hydrodynamic pressure at the
wafer–pad interface [44]. Therefore, the MRR may
Fig. 9 Zone pressure loading effect and its effect on the MRR not always increase linearly with the speed.
when a larger or small pressure is applied to zone 2, respectively: Then, modified Preston equations in the form of
(a) The contact stress, and (b) MRR profiles (Reprinted from Ref. MRR = kPαV β were proposed. Unfortunately, each
[11], Copyright 2011, with permission from Elsevier).
equation has limitations because they are empirical
equations that are based on limited experimental data.
4 Modeling of CMP A more accurate local relevant expression for the MRR
is more reasonable [45]:
The mechanism and modeling of CMP have been
an attractive area of research for many years. Early MRR(x, y) = kP(x, y)V(x, y) (5)
CMP models were empirically summarized from the
Using Eq. (5), the MRR of one point on the wafer
industrial production. In addition, some theoretical
surface can be achieved by calculating the integral of
models that considered the mechanical action from the
P and V during the whole polishing time.
viewpoint of the contact mechanism, fluid mechanism,
or both of them were developed. Further, as the 4.2 Modeling from perspective of mechanism
chemical action is an indispensable component in the
CMP process, the models that considered the chemical 4.2.1 Model based on contact mechanism
action were also developed. The most important elements that contribute to material
4.1 Empirical model removal during CMP include the abrasive particles,
slurry chemicals, and polishing pad. The abrasive–
The early CMP model which was derived from the wafer interaction, chemical–wafer interaction, and
314 Friction 1(4): 306–326 (2013)

wafer–pad interaction all play the important roles in Zhao’s model gives the wear volume of the wafer
CMP. The contact mechanism model ignores the fluid by a single particle as
action. The downforce applied on the polishing pad
is assumed to be carried by the solid–solid contact of G  K SVt (6)
the wafer surface, i.e., the abrasive–wafer interaction where K is the wear constant, ΔS is the cross section
and asperity–wafer interaction. The interactions consist area of the worn groove, V is the relative velocity
of three different models based on the dimensions between the wafer and the pad, and t is the polishing
[46, 47], namely the particle scale model, asperity scale time. The pad properties affect the contact status of the
model, and wafer scale model, as shown in Fig. 10. particles, and should be considered in the model [50].
The particle scale model and asperity scale model are Shi et al. [51] and Wang et al. [52] compared the
the bases used to access the wafer scale model. different contact statuses for the soft pad and hard
(a) Particle scale model pad (see Fig. 12). For the hard pad (Fig. 12(a)), the
The particle scale model evaluates the indentation particles make contact with the wafer surface, while
depth and the wear volume of the particle. A single the pad asperities do not; for the soft pad (Fig. 12(b)),
particle wear model was proposed by Zhao et al. the particles are embedded in the pad asperities, and
[48, 49], as shown in Fig. 11. Because the pad is much both the particles and the pad asperities make contact
softer than the hard particles, the particle will be with the wafer surface. Therefore, the removal rate
indented into the pad. The indentation depth and model is quite different for the soft pad and hard pad.
section area of a single particle can be calculated based The relationship between the removal rate and the
on the theory of contact mechanics in conjunction particle size was further developed [53].
with the force equilibrium. (b) Asperity scale model
In the asperity scale model, one or more particles
are trapped at the wafer–asperity interface. Only
the particles embedded in the asperity contribute to
material removal in CMP, and they can therefore be
defined as active particles [54]. The asperity defor-
mation and contact area are calculated to evaluate the

Fig. 10 CMP model at different scales: (a) wafer scale, (b) asperity
scale, and (c) particle scale (Reproduced from Ref. [6], by
permission of The Electrochemical Society).

Fig. 12 Contact status of (a) hard pad, and (b) soft pad (Reprinted
Fig. 11 Single particle contact model (Reprinted from Ref. [48], from Ref. [51], with kind permission from Springer Science +
Copyright 2002, with permission from Elsevier). Business Media).
Friction 1(4): 306–326 (2013) 315

number of active particles, and to further evaluate the of the model is determined by the above assumptions.
MRR. Zhao et al. [49] studied the contact model of a In fact, the actual contact ratio is very small (<1%)
single asperity for elastic, plastic, and elastic-plastic [54, 57].
statuses. Their results reveal that the pad property and
4.2.2 Model considering fluid mechanism
topography have an important effect on the efficiency
on the material removal. Fluid lubrication plays an important role in the
(c) Wafer scale model wafer–pad interactions. The fluid force can support a
The atomic scale model and asperity scale model are part of the downforce. Assume that s is the complex
both local models. In order to obtain the MRR model roughness and h is the fluid film thickness. Based
across the entire wafer surface, it is necessary to on lubrication theory, if h >> s, full film lubrication is
expand the local models to the wafer scale. The wafer generated and Reynolds equation can be used to solve
scale model uses a mathematical statistical method to the fluid pressure, while if h ≈ s, mixed lubrication is
calculate the actual contact area across the wafer and generated and the roughness of the surface cannot
to evaluate the number of active particles. Using the be ignored. Some researchers have used simplified
particle scale model as the element, the global MRR lubrication models and the Reynolds equation to solve
model can be obtained. the fluid pressure for CMP.
The pad asperity is randomly distributed, as shown The full film CMP lubrication model was first
in the left figure of Fig. 13. The right figure of Fig. 13 introduced to CMP and assumes that the wafer has
gives a description of the probability density distri- been absolutely separated by a slurry film. The most
bution of the pad height. A classic probability statistical simplified CMP lubrication model ignored the defor-
model for the rough surface, G-W model [55], is mation of the wafer and the pad (as shown in Fig. 14(a)).
selected to evaluate the actual contact area between Based on the cylindrical coordinate Reynolds equation
the wafer and the pad. and the equations for the force and torque, the fluid
pressure of the slurry film was calculated using

A  N   ( z  d)  ( z)dz (7) numerical methods. The results suggested a positive
d
pressure, with the center pressure being much larger
where N is the total number of asperity,  (z) is the than the pressure at the edge [58−60]. Sundararajan
probability density distribution function of the pad et al. [61] further considered the deformation of the
asperity height, β is the characteristic length scale for wafer in the model, as shown in Fig. 14(b). Thakurta
the roughness of the pad surface, z is the pad height, et al. [14] further considered the deformation of the
and d is the distance to the mean line of pad surface. pad, as shown in Fig. 14(c). Also, a positive pressure
The number of active particles is evaluated base on was obtained.
several hypotheses [50]. Zhao’s model [48] assumes Actually, the pad surface is not flat, but has a
that the particles in the contact area have the same specific roughness and micropores. The pad surface
face density with the slurry, while Jeng’s model [56] profile will affect the lubrication, especially when the
assumes that particles with the same number of that in roughness is comparable to the film thickness. Kim et
the slurry with the volume of the compress asperities al. [62] and Ng et al. [63] added the pad roughness
were trapped at the wafer–pad interface. The precision to the model and introduced the flow factor to the
average Reynolds equation. This kind of model is
close to the actual condition, however, the pad profile
is difficult to model.
For general CMP, the asperity/particle must be in
contact with the wafer. Therefore, the mixed lubrication
model is more suitable for CMP [64]. Tichy et al. [65]
simulated the regular distribution of the pad asperities,
as shown in Fig. 15(a). Tsai et al. [66] assumed that a
Fig. 13 Probability density distribution of the pad height.
316 Friction 1(4): 306–326 (2013)

Fig. 14 Lubrication models of CMP: (a) rigid pad/wafer, (b) considering wafer deformation (Reproduced from Ref. [61], by permission
of The Electrochemical Society), and (c) considering wafer and pad deformation (Reprinted from Ref. [14], Copyright 2000, with
permission from Elsevier).

part of the wafer is in contact with the pad, while a the wafer, the friction torque will drag the leading edge
part of the wafer has a hydrodynamic lubrication down toward the pad, and the wafer’s leading edge
with the pad, as shown in Fig. 15(b). Using the mixed has a much tenser contact with the pad. Therefore,
lubrication model of CMP, the fluid pressure, the fluid a suction pressure is formed in the leading region
film thickness, and the contact ratio can be obtained. of the wafer owing to a diverging clearance [65]. In
The relative motion is another important factor the above models, the simplification of the carrier
that affects the lubrication during CMP. The friction structure, especially the retaining ring, may obviously
torque at the interface produced by the relative affect the contact feature of the wafer [9, 10, 67], which
motion will cause the wafer to lean and change the may further affect the slurry flow and the lubrication
wafer orientation. As a result, the contact force will behavior between the wafer and pad. It is desired that
be nonuniform. If there is no retaining ring around more practical model considering the carrier structure
and loading characteristic will be developed.

4.3 Molecular dynamics simulation of CMP: atomic

view

To study the physical process of material removal by

abrasive particles during CMP on an atomic scale,
molecular dynamic (MD) simulations were widely
used to analyze the material removal process caused
by the silica cluster on the silicon substrate under
different conditions. The extruding effect, the sliding
effect, and the rolling effect were all found to affect
material removal and surface polishing [68−74].
A basic silica cluster impact simulation was carried
out in dry conditions by Chen et al. [71]. When a silica
cluster impacts on the crystal silicon substrate with a
suitable velocity and incidence angle, the silicon surface
is extruded (as shown in Fig. 16) due to the combined
effects of thermal spread, phase transformation, and
Fig. 15 Mixed lubrication model of CMP: (a) Tichy’s model crystallographic slip, with the thermal spread being
(Reproduced from Ref. [65], by permission of The Electrochemical the most significant. A higher impacting speed results
Society), and (b) Tsai’s model [66]. in a larger extrusion of the substrate.
Friction 1(4): 306–326 (2013) 317

Fig. 17 MD simulation results of the silica particle rolling

process under an external downforce of 5 nN and a lateral driving
force of 10 nN (Reprinted with permission from Ref. [73].
Fig. 16 Section view of atoms after normal impacting by 5184
Copyright 2011, American Institute of Physics).
cluster at different impact velocities: (a) 2,500 m/s, (b) 4,313 m/s,
(c) 6,000 m/s, and (d) enlarged drawing of an extrusion after the
impact in Fig. (c) (Reprinted from Ref. [71], Copyright 2011,
with permission from Elsevier).

During CMP, the wafer surface is exposed in the

slurry. Therefore, the particle–wafer interaction takes
place in wet conditions. The surface damage in the
wet condition was further simulated using the MD
method for comparison with the dry condition [69, 70].
The damage to the substrate after the dry impact is
more severe than that after the wet impact under the
same other conditions, and it is especially obvious for
large incidence angles. The water film will affect the Fig. 18 Material removal characteristics in abrasive rolling
energy transfer process for the wet impact as compared process: (a) atom–atom interactions between the atoms of the
to the dry impact. silicon substrate and the silica particle, (b) atomic vacancies on
During CMP, the particle clamped between the the silicon substrate after rolling, and (c) silica particle after
rolling across the silicon substrate (Reprinted with permission
wafer and the pad may slide and roll when the pad
from Ref. [73]. Copyright 2011, American Institute of Physics).
moves relative to the wafer. Using the MD method,
the sliding effect was investigated by Han et al. [75],
and the silica particle formed stronger atom–atom
and the abrasive rolling effect on the material removal
interactions. As the silica particle rolled forward,
and the surface finish in the CMP process was studied
some of the Si–Si bonds on the substrate surface were
by Si et al. [73]. In Si’s model, an external downforce
broken and the Si atoms were dragged out from their
was applied to the particle on the substrate, and drove
original positions and adhered to the silica particle,
the particle to roll forward under a lateral driving
as shown in Fig. 7(c).
force. Their results show that the silica particle will
From the above discussion, we propose that abrasive
roll across the silicon substrate. Meanwhile, some
extruding, sliding, and rolling play important roles in
atoms of the substrate are dragged out and adhered
material removal in the abrasive CMP of the silicon
to the silica particle, leaving some atomic vacancies
substrate. If the chemicals were considered in the MD
on the substrate surface, as shown in Fig. 17. As a
model, the simulation results could be closer to those
result, a high quality surface can be obtained.
of the actual CMP process.
Si et al. [73] further described the material removal
mechanism. During the rolling process, the material 4.4 Modeling of chemical-mechanism synergy
was mainly removed by adhering wear. As shown during CMP
in Fig. 18(a), under the external down force and the
driving force, some atoms of the silicon substrate The mechanical models are not sufficiently accurate
318 Friction 1(4): 306–326 (2013)

as they ignored the chemical action, which is an pure wear and pure corrosion, respectively; rc-w and
important part of CMP. Luo and Dornfeld [50] give a rw-c represent the part of corrosion-induced wear,
general model which considers the chemical corrosion: and the part of wear-induced corrosion, respectively.
Therefore, in Li’s model, rc-w and rw-c gives the
MRR   w NVremoved  C0 (8)
synergism of the wear and corrosion, which results in
where  w is the density of the wafer, N is the number the greatest material removal during CMP.
of the active particles, Vremoved is the removal rate of a Based on the mechanical model, the real wafer–pad
single particle, and C0 represents the removal rate contact area can thus be evaluated. By multiplying the
caused by chemical corrosion. This model considers number of active particles with the removal volume
both the mechanical action and chemical action in of a single particle, Li gives an expression for the MRR
CMP. However, it is not accurate to use a constant to due to abrasive wear. When the film on the wafer
describe the corrosion. surface is removed by particles, a fresh wafer surface
In another general accepted model, a thin film is is exposed, which promotes the disolution of the
generated on the wafer surface, which is soft and copper. As a result, the anodic current subsequently
can be easily removed. The film is removed by the increases due to the enhanced dissolution of the wafer
mechanical action of the particle. The film generation surface. Hence, the MRR due to corrosion during CMP
and removal are parts of a dynamic process. When can be calculated by Faraday’s law. Finally, Li gives
the growth rate and the removal rate attain some the total MRR as follows:
equilibrium, the best polishing results are obtained
C1
[76, 77]. MRR  rwc  rcc  ( h 2  C2C3 h)P02/ 3 v  C3 i0 (12)
C02 R2
In fact, the chemical action and mechanical action
have a synergistic effect, in which they are both pro- Li’s model not only quantifies the chemical mechanical
moted. Li et al. [78, 79] considered the interaction of synergy, but also isolates each component’s con-
mechanical part and chemical part in their model. Based tribution to the MRR. Li’s model reveals that major
on the corrosion and wear theory, a mathematical factors affecting the material removal include the
material removal model incorporating both chemical process parameters, properties of the pad, particle,
and mechanical effects during CMP was proposed. and slurry (pH, concentration).
During CMP, the slurry has an (electro-) chemical In order to assess the relative importance of
erosion effect on the wafer surface, and the particles mechanical wear and chemical corrosion to the MRR
also have a mechanical abrasive wear effect on the during CMP, Li gives a parameter of the mechanical-
wafer surface. The synergistic effect of the (electro-) to-chemical ratio (rwc/rcc).
chemical corrosion effect and the mechanical abrasive
rwc
wear effect result in a high efficiency MRR and good  RP1/ 3 (13)
surface quality to CMP. The CMP system is similar to rcc
a corrosion-wear system. Li et al. [79] gives a synergy where R is the particle size. Equation (13) indicates
model which expresses the total MRR using the that the mechanical-to-chemical ratio increases linearly
mechanical component rwc, and chemical component rcc: with particle size, and that an increase in the applied
pressure will enhance the mechanical effect.
MRR  rwc  rcc (9)
Figures 19(a) and 19(b) show the corrosion–wear
where maps [79] from Li’s model according to the applied
pressure and the particle size, respectively. These
rwc  rw  rc-w (10)
maps reveal that the chemical–mechanical synergy
and dominates the material removal during CMP. As the
applied pressure and particle size increase, there is the
rcc  rc  rw-c (11)
appearance of a transition mechanism from corrosion-
where, rw and rc represent the removal rate due to induced wear to wear-induced corrosion [79].
Friction 1(4): 306–326 (2013) 319

Fig. 20 AFM of the scratched area morphology of copper samples

after exposure to different solutions at pH 4 for 8 min: (a) virgin
Cu, (b) solution No. 1 containing 5 wt% H2O2, (c) solution No. 2
containing 5 wt% H2O2 and 1 wt% glycine, and (d) solution
No. 3 containing 5 wt% H2O2, 1 wt% glycine, and 0.1 wt% BTA
(Reprinted from Ref. [81], with kind permission from Springer
Science + Business Media).

value of the slurry. The scratched depth of all of the

etched Cu samples was greater than that of the virgin
Cu sample. For solution No. 1 containing 5 wt% H2O2,
the scratched depth was about two times greater than
Fig. 19 Corrosion–wear mechanism regime map with chemical that of the virgin Cu. For solution No. 2, the combination
corrosion vs. mechanical wear: (a) mechanism of different of H2O2 and glycine greatly increased the scratched
applied pressures, and (b) mechanism of different particle sizes depth. However, for solution No. 3 with a further
(Reproduced from Ref. [79], by permission of The Electrochemical addition of BTA, the scratched depth was lower than
Society).
that of solution No. 2, which suggested that BTA not
only inhibited the chemical dissolution of copper, but
5 Experimental study of CMP also inhibited the mechanical removal of copper.

5.1 Nano-scale material removal experiments 5.2 Material removal regime of CMP

Atomic Force Microscope (AFM) has been widely used Luo and Dornfeld [82] have given a map of material
to study the effects of the particle wear effect and the removal regions according to the abrasive weight
effect of slurries on the mechanical removal of the concentration. It is also important to give the material
surface layer. Yu et al. [80] found that the tribochemical removal regime from the aspect of the slurry chemical
wear of the silicon surface occurred for the SiO2 tips property.
and single-crystalline silicon wear pair, even at contact To determine the material removal regime of copper
pressures that are much lower than the hardness. The CMP from the perspective of the roles of chemical
surface topography of an etched Cu sample with or corrosion, abrasive wear, and their synergistic effects on
without probe scratching can be investigated by AFM. the material removal, Li et al. [78] used electrochemical
Liao’s [81] comparative AFM scratch tests for copper analysis and a nano-scratching method to investigate
samples after exposure to different solutions (see the MRR and surface quality after CMP with slurries
Fig. 20) revealed that the MRR and surface roughness having different pH values. They calculated the
are significantly influenced by the chemicals and pH mechanical–chemical removal rate ratio based on the
320 Friction 1(4): 306–326 (2013)

experimental data, and finally constructed a removal wafer carrier [84–89]. Their experiments found that a
mechanism map for copper CMP depending on the large negative pressure region occupying more than
pH values, as shown in Fig. 21. The pure chemical 70% of the contact area between the disk and the pad
effect accounts for almost all of the material removal existed near the leading edge of the disk. However,
at pH 3.0 and 10.0, indicating that the chemical as the rigid disk is quite different from the wafer with
corrosion effect plays a dominant role during the CMP respect to its bending property, the results may be
process; in the alkaline slurry, the wear–corrosion quite different from those for real situations.
effect predominates in the material removal at pH To study the fluid lubrication behavior during an
values of 8.0 and 9.0, while the copper removal actual CMP process which uses the multizone carrier
mechanism transfers to corrosion–wear action in the and the retaining ring, Zhao et al. [90–92] developed a
acidic slurry from pH 4.0 to 6.0. The wear-induced novel in-situ fluid pressure and wafer status measure-
corrosion effect resulted in a majority of the material ment system, which uses an array of pressure sensors
removal from a pH of 7.0 to 9.0, and a good surface to measure the fluid pressure, and an array of
quality was obtained. Li’s results provide strategies distance sensors to monitor the wafer status. The in
for realizing the process optimization of CMP. situ measurement system was integrated in a 12-inch
CMP equipment. The schematic section view of the
5.3 In situ study of fluid lubrication behavior during integrated measurement system is shown in Fig. 22.
CMP Zhao’s fluid pressure measurements revealed the
presence of a small negative pressure region at the
The slurry plays an important role at the wafer–pad
leading edge, while the positive pressure is dominant
interface during CMP. The particles and chemicals
(see Fig. 23), which is quite different from the test
are brought to the interface with the slurry flow [83];
results obtained from the simplified CMP test tool.
the slurry can build a lubrication film and decrease
The fluid pressure can support 10%–30% of the
the friction force, and the fluid pressure can bear some
downforce depending on the downforce [44]. Wafer
of the downforce, thus causing wafer to have a flexible
bending/orientation measurements reveal a micron
landing on the pad.
level wafer bending and a slight wafer pitch angle
To experimentally determine the fluid behavior at
during the dynamic polishing process, both of which
the wafer–pad interface, several fluid pressure mapping
increase linearly with the downforce.
studies were performed on the simplified experimental
setups of CMP, using a disk to simulate the wafer and

Fig. 21 Li’s material removal mechanism map for copper CMP Fig. 22 Schematic of in situ measurement system of CMP
(Reprinted from Ref. [78], with kind permission from Springer (Reprinted from Ref. [92], Copyright 2013, with permission from
Science+Business Media). Elsevier).
Friction 1(4): 306–326 (2013) 321

5.4 Process capability

Figure 25 gives one optimized process results of

the MRR profile using a five-zone wafer carrier. The
platen/carrier speed is 90/87 rpm and the slurry flow
rate is 300 mL/min. The pressure applied to zones 1–5
and retaining ring (see Fig. 5(c)) are 1.0, 1.0, 1.1, 2.1,
3.3, and 3.6 psi, respectively. The average MRR is
close to 5,000 Å/min (in fact, the MRR can increases
to 6,000 –7,000 Å/min when the downforce increases
to 2 psi), the standard deviation (STD) of the MRR is
74 Å/min, and the nonuniformity is 1.49%. After CMP,
the surface roughness is easily to be decreased to
sub-nanometer. The recent reported results show that,
using an optimized silicon slurry and an optimized
polishing process, the minimum surface roughness
after CMP can achieve 0.05 nm (Ra, measured by AFM)
Fig. 23 Fluid pressure distribution across the 12-inch wafer at [93]. The process potentiality is still developing toward
0.5 psi downforce, 80/80 rpm carrier/platen speed, and 250 mL/min to the unknown ultimate.
slurry flow rate (Reproduced from Ref. [90], by permission of The
Electrochemical Society).

Zhao et al. [91] gave a reasonable explanation from

the viewpoint of the wafer–pad contact status and
contact stress, as shown in Fig. 24. The convexly bent
and trailing edge pitched wafer produce a convergent-
dominated wedged gap between the wafer and the
pad, and generate a positive dominated fluid pressure.
The edge stress concentration effect causes a small
negative pressure at the leading edge. Fig. 25 MRR profile at downforce of about 1psi, platen/carrier
speed of 90/87 rpm, and slurry flow rate of 300 mL/min.

6 Conclusions
For several decades, chemical mechanical polishing
(CMP) has been developed from both a theoretical
and technical perspective. The mechanism of CMP is
shown based on theoretical modeling and experimental
verification, but it still requires further development.
The following conclusions have been made from this
review.
(1) The reduction in the feature size of IC products,
the increase in wafer dimensions, and the use of low-k
Fig. 24 Schematic of wafer–pad interaction and fluid lubrication materials all result in further challenges to CMP. More
(Reprinted from Ref. [91], Copyright 2013, with permission from precision technologies, such as the pressure control
Elsevier). technology and the end point detecting technology,
322 Friction 1(4): 306–326 (2013)

are significant for CMP process control. (Grant No. 51021064), and the National Natural Science
(2) CMP is a complex mechanism. Many factors Foundation of China (Grant No. 51305227). The authors
related to the carrier structure, the polishing pad, the would like to thank Enago (www.enago.cn) for the
slurry, and the process parameters may affect the final English language review.
polishing results. The wafer–pad interfacial status,
including the pressure/stress distribution, the slurry Open Access: This article is distributed under the terms
film distribution, the sliding distance distribution, and of the Creative Commons Attribution License which
the temperature distribution play important roles in permits any use, distribution, and reproduction in any
determining the final polishing results. medium, provided the original author(s) and source
(3) The kinematics and the contact stress are the most are credited.
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Xinchun LU. He received the BS and micro-nano tribology of the surface and interface, and
MS degrees in material science and equipment and processes of chemical mechanical
engineering from Jilin University polishing. He is the author or coauthor of over 100
of Technology, Changchun, China, journal publications and conference proceedings
in 1988 and 1991, respectively, and papers. He holds over 16 patents in the area of CMP
the PhD degree in the same field equipment.
from the Institute of Metal Research, Prof. Lu was the recipient of the Trans-Century
Chinese Academy of Sciences, in 1994. Training Program of the National Ministry of Educa-
He is a chair professor of Changjiang Scholars in the tion, and the National Science Found for Distinguished
Department of Precision Instruments and Mechanology Young Scholars of China. He has received numerous
of Tsinghua University, China, and is a member of national awards, including the Award for National
the international executive committee of ICPT. His Science Development (grade two), and the Science &
current areas of research include micro-nano fabrica- Technology Advancement Award (grade one), from
tion technology, the theory and applications of the the National Ministry of Education.

Dewen ZHAO. He received the BS Dissertation Award in 2013. Dr. Zhao is currently a
degree in mechanical engineering postdoctoral research fellow at Tsinghua University,
from Huazhong University of Science Beijing, China. He has more than 10 papers indexed
and Technology, Wuhan, China, in by SCI, and 9 authorized national invention patents.
2007, and the PhD degree in mech- His major research areas include chemical mechanical
anical engineering from Tsinghua polishing equipment and principles, tribology, and
University, Beijing, China in 2012. process monitoring.
He received the Bronze Medal of HIWIN Doctoral
Friction 1(4): 327–332 (2013)
DOI 10.1007/s40544-013-0017-z ISSN 2223-7690
RESEARCH ARTICLE

Y2O3 nanosheets as slurry abrasives for chemical-mechanical

planarization of copper
Xingliang HE1, Yunyun CHEN1,2, Huijia ZHAO3, Haoming SUN3, Xinchun LU3, Hong LIANG1,2,*
1
Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA
2
Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3123, USA
3
Department of Precision Instruments and Mechanology, Tsinghua University, Beijing 100084, China
Received:13 March 2013 / Revised: 22 May 2013 / Accepted: 30 May 2013
© The author(s) 2013. This article is published with open access at Springerlink.com

Abstract: Continued reduction in feature dimension in integrated circuits demands high degree of flatness after
chemical mechanical polishing. Here we report using new yttrium oxide (Y2O3) nanosheets as slurry abrasives
for chemical-mechanical planarization (CMP) of copper. Results showed that the global planarization was
improved by 30% using a slurry containing Y2O3 nanosheets in comparison with a standard industrial slurry.
During CMP, the two-dimensional square shaped Y2O3 nanosheet is believed to induce the low friction, the better
rheological performance, and the laminar flow leading to the decrease in the within-wafer-non-uniformity,
surface roughness, as well as dishing. The application of the two-dimensional nanosheets as abrasive in CMP
would increase the manufacturing yield of integrated circuits.

Keywords: Y2O3 nanosheets; chemical-mechanical planarization (CMP); nanoabrasives; slurry flow; wafer-pad
contact

1 Introduction We have recently reported that using boron oxide

nanoparticles (NPs) the materials removal rate was
Chemical mechanical planarization (CMP) has been increased during copper (Cu) CMP [11]. In this study,
used as a major process step for manufacturing inte- we focus on mechanisms of planarization. We report
grated circuits in last three decades [1]. Significant a new CMP slurry containing yttrium oxide (Y2O3)
effort has been made in developing new and effective nanosheets (NS) as abrasives leading to improvement
slurries [2, 3]. To date, global planarization remains in promising efficiency. For comparison, planarization
to be a major concern [4], particularly for patterned processes using a commercial colloidal silica (SiO2)
wafers where the metal/dielectric density differs across slurry were studied. The mechanism of planarization
the wafer. The limitation of ion and slurry transfer was studied via friction performances and dynamic
is one of the key factors affecting planarization. The behaviors under a fluid shear. Two-dimensional Y2O3
planarization is characterized by the within-wafer- NS abrasives prompt an alternative solution to improve
non-uniformity (WIWNU) [5, 6]. Previous studies in the wafer planarization during CMP.
this regard have been focused on optimization of
polishing parameters and utilization of corrosion
inhibitors [7–10]. It is always desirable to develop a
2 Experimental
slurry that improves the slurry transport and contact
2.1 Materials
between the polishing pad and the wafer surface.
The citric acid, benzotriazole (BTA), and hydrogen
* Corresponding author: Hong LIANG. peroxide (H2O2) used in this study were purchased
E-mail: hliang@tamu.edu from Sigma-Aldrich (USA) and were used without
328 Friction 1(4): 327–332 (2013)

further purification. A home-made abrasive, Y2O3 NS 500 s–1. In rheological experiments, three different con-
(~16 nm thick and >200 nm side, see Fig. S1 in Electronic centrations were selected for slurries, 0.3 wt%, 3 wt%,
Supplementary Material) was used to prepare CMP and 10 wt% in DI water. During the measurement,
slurry. The Y2O3 NS was synthesized via a hydrother- a stainless steel parallel spindle (Ø 25 mm) rotated
mal method, and those results will be reported while the lower Peltier plate was stationary. The gap
elsewhere. The home-made slurry was composed of (500 μm) between parallel plates was filled with slurries,
citric acid (0.01 M), BTA (0.05 wt%), H2O2 (3 vol%), and the temperature was maintained at 25 °C.
Y2O3 NS abrasive (3 wt%), and deionized (DI) water. The averaged thickness of the Cu film was measured
A commercial SiO2 slurry (~Ø 35 nm, Fujimi Corpora- using a table top four point probe (CDE ResMap 273)
tion) was used as-received for comparison in CMP. choosing 80 spots along the diameter of each wafer.
Another SiO2 NPs filtered from a commercial slurry The percentage ratio of the standard deviation of
(~Ø 35 nm, Cabot Electronics co.) with the same thickness relative to the averaged value was used
particle size and shape were used in friction and to calculate the WIWNU [12–14]. A surface profile
rheological experiments. Unwanted chemicals in the topography system (KLA-Tencor HRP-350) was used
slurry were removed by filtering and rinsing with DI to measure the surface roughness and the Cu dishing
water for three times. The thoroughly rinsed SiO2 NPs on Si wafers. Results of the WIWNU, the surface
were collected after drying at 40 °C for 24 h for future roughness, and the Cu dishing were presented
friction and rheological experiments. statistically.
Cu film (2 μm thick) coated silicon (Si) wafers
(Ø 300 mm) were used as target substrates for CMP
3 Results and discussion
experiments. These wafers were then CMPed with an
IKONICTM polishing pad (Rohm & Haas). The comparison of WIWNU before and after CMP
experiments in different slurries is shown in Fig. 1.
2.2 CMP experiment and Characterizations
The trend in the WIWNU after CMP is indicated by
All polishing experiments were conducted using a arrows. It is interesting to see that the WIWNU is
universal CMP tester. Polishing was conducted for reduced by 30% using the Y2O3 slurry. Using the
1 min. Wafers were placed face-down onto the commercial SiO2 slurry, on the contrary, it shows an
polishing pad. The applied pressure was 1 psi increase in the WIWNU by 48%. Meanwhile, the wafer
(6894.757 Pa), and rotation speeds of the pad and the polished using the Y2O3 slurry also has better surface
wafer were maintained at 79 rpm and 76 rpm, quality. As shown in Fig. 2, wafers polished using the
respectively. The speeds were kept close to each other Y2O3 slurry have lower arithmetic averaged surface
for good uniformity in wafer planarization. Each roughness than that polished with the SiO2 slurry. To
slurry was used to polish four wafers. understand the effects of abrasives on WIWNU and
Frictional behaviors and rheological properties of the surface roughness, frictional and rheological results
slurry were examined. In order to solely investigate are shown in Figs. 3 and 4, respectively. In Fig. 3, it is
the frictional behaviors and rheological properties of observed that the Y2O3 slurry has lower friction
SiO2 NP and Y2O3 NS, the measurements were con- coefficient than the SiO2 slurry. In Figs. 4(a) and 4(b),
ducted in DI water. Friction experiments of Cu wafers it is clear that the SiO2 slurry with higher concentration
were carried out using a tribometer (CSM Instruments). has the larger slope in shear stress-shear rate plots.
IC1000 polishing pads (Rohm & Haas) with SiO2 With the increase in SiO2 concentration, the slurry
(3 wt%) and Y2O3 (3 wt%) slurries were used in friction becomes more viscous. Viscosity is directly related to
experiments. Friction coefficients were recorded during the friction and mass transfer among fluid layers [15].
each test for 60 cycles (20 mm per cycle, 20 mm/s) with The change in slope of the shear stress-rate plots
an applied pressure of 80 kPa. An AR-G2 rheometer implies movement of one fluid layer respect to another
(TA Instruments) was used to measure the change with significant mass transfer. This is the evidence of
of shear stress with shear rate ranging from 30 s–1 to a turbulent flow [16]. With the same concentration,
Friction 1(4): 327–332 (2013) 329

affected by the addition of Y2O3 NS. The unchanged

slope of the shear stress-rate plots indicates the
movement of one fluid layer past another with little
matter transfer. This is the evidence of a laminar flow
[17]. It is concluded from rheological measurements
that SiO2 NPs increases the viscosity of slurries while
Y2O3 NS shows no effects.
Based on frictional behaviors and rheological
properties of slurries, mechanisms in reduction of
Fig. 1 Changes of WIWNU before (black) and after (gray) WIWNU are proposed in schemes illustrated in
CMP using different slurries. Fig. 5. When wafer is polished using the SiO2 slurry,
spherical NPs (see inset of Fig. 5(a)) can embed in the
wafer and abrade it through particle-wafer contact
mode (Fig. 5(a)) [18, 19]. Such abrasion through
3-body and 2-body wear is believed to be responsible
for materials removal in CMP. On the contrary, when
square Y2O3 NS (see inset of Fig. 5(b)) is used, it enables
them to have larger contact area. The increased contact
leads to a uniform distribution of the down force and
the reduced contact pressure. When the applied pres-
sure is low, a fluid film will be able to form between
the pad and wafer (Fig. 5(b)) [20, 21]. As a result, the
uniformed contact and improved slurry transport
Fig. 2 The arithmetic averaged surface roughness of wafers that lead to more effective lubrication [22, 23]. This is con-
are polished using different slurries. firmed by the friction results. Accordingly, polishing
under lubricative condition can reduce the WIWNU
after CMP [24]. In addition, when slurries entered the
interface between the pad and wafer, Y2O3 NS can be
deemed as parallel layers whereas SiO2 NPs distribute
chaotically and stochastically. As demonstrated by
rheological experiments (Fig. 4), a laminar flow and a
turbulent flow were believed to form in Y2O3 and
SiO2 slurries, respectively. A laminar slurry flow that
has low viscosity with little flow fluctuation leads to
uniform distributions of relative velocity and abrasive
movement trajectories [5, 25]. In such the WIWNU is
decreased.
In microelectronic devices, an important factor to
planarize a wafer is elimination of Cu dishing [26, 27].
Fig. 3 Results of friction between the Cu film and the polishing Results of Cu dishing in our CMP are shown in Fig. 6.
pad in SiO2 (black, top) and Y2O3 (red, bottom) slurries. Wafers polished with the Y2O3 slurry obtained less
dishing than that polished with the SiO2 slurry. During
the shear stress in SiO2 slurry changes at a faster rate CMP, the protruded areas were polished while the low
against the shear rate than the Y2O3 slurry, as shown areas were passivated resulting in a smooth surface
in Fig. 4(c). It is interesting to observe in Fig. 4(d) that [18, 19]. Localized pad deformation occurs and has
the ratio of shear stress to shear rate in water is not been reported to be an important reason causing metal
330 Friction 1(4): 327–332 (2013)

Fig. 4 Results of rheological measurements: (a) The comparison of shear stress-shear rate plots in different slurries with different
abrasive concentrations; (b) variation of shear stress to shear rate in SiO2 slurries with different concentrations; (c) the clear comparison
of shear stress-shear rate plots in different slurries with the same abrasive concentration (3 wt%); (d) variation of shear stress to shear
rate in Y2O3 slurries with different concentrations.

Fig. 6 The Cu dishing in wafers that are polished using different

slurries.

dishing [26, 28, 29]. In the current work, however,

Y2O3 NS has larger contact area than SiO2 NPs. The
down force distributes uniformly in the contact area.
The low area undertakes a comparable pressure to
that protruded area experiences. A uniform pressure
distribution is beneficial for reduction in dishing [26].
In addition, dishing can be reduced through gentle
Fig. 5 Schematic representations of abrasion modes using the com- contacts of pad through Y2O3 NS to wafer, which is
mercial SiO2 NPs (inset) slurry (a) and the Y2O3 NS (inset) slurry (b). similar to soft landing in abrasive free polishing [30–32].
Friction 1(4): 327–332 (2013) 331

The CMP conducted using the Y2O3 slurry obtained [3] Zantye P B, Kumar A, Sikder A K. Chemical mechanical
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reduction model for chemical mechanical planarization

Xingliang HE. He is a PhD candidate and Chinese Academy of Sciences, China, in 2006 and
at Department of Mechanical 2010, respectively. His current research area includes
Engineering, Texas A&M University, synthesis and characterizations of nanomaterials for
USA. He received his BS and MS semiconductor processing, lubrication, and wear &
degrees in Materials Physics and corrosion resistance.
Chemistry from Yunnan University,

Hong LIANG. Professor of Depart- and industry. She is a fellow of American Society of
ment of Mechanical Engineering Mechanical Engineers (ASME) and a fellow of Society
and Materials Science Engineering, of Tribologists and Lubrication Engineers (STLE). She
at Texas A&M University, USA. Dr. has maintained long-standing interests and activity
Liang has extensive experience in in tribology, surface science, chemical-mechanical
academia, government laboratories, planarization, and nanomanufacturing.
Friction 1(4): 333–340 (2013)
DOI 10.1007/s40544-013-0027-x ISSN 2223-7690
RESEARCH ARTICLE

Towards a unified classification of wear

Michael VARENBERG*
Department of Mechanical Engineering, Technion – IIT, Haifa 32000, Israel
Received: 12 July 2013 / Revised: 03 September 2013 / Accepted: 26 September 2013
© The author(s) 2013. This article is published with open access at Springerlink.com

Abstract: Since the beginning of the systematic study of wear, many classification schemes have been devised.
However, though covering the whole field in sum, they stay only loosely connected to each other and do not
build a complete general picture. To this end, here we try to combine and integrate existing approaches into a
general simple scheme unifying known wear types into a consistent system. The suggested scheme is based on
three classifying criterions answering the questions “why”, “how” and “where” and defining a 3-D space filled
with the known wear types. The system can be used in teaching to introduce students to such complex
phenomena as wear and also in engineering practice to guide wear mitigation initiatives.

Keywords: relative motion; energy dissipation; surface disturbance; surface state; surface damage

1 Introduction for various interpretations.

Classification suggested by Siebel in 1938 relied on
The origin of analysis lays in identifying similarities the type of relative motion as a classifying criterion
in a diversity of things and processes we deal with [4]. The distinguished classes of wear were related to
and arranging entities into classes of similar items. (1) dry sliding, (2) lubricated sliding, (3) dry rolling,
Without first bringing order into any field we work (4) lubricated rolling, (5) oscillating, (6) solid particles
in, it is nearly impossible to understand anything or motion, and (7) fluids motion. Classification suggested
make any statement about it. It is for this reason that by Archard and Hirst in 1956 relied on the scale of
classification is one of the most important methods of surface damage as a classifying criterion [8]. The
science [1]. distinguished classes of wear were (1) mild wear and
Though first recorded observations of wear date (2) severe wear related to localization of surface damage
back to the 1st century BCE [2], apart from the work within the layers of different chemical composition,
of Leonardo da Vinci (circa 1493) that remained lost outer protective and inner bulk ones, respectively.
in libraries until 1967 [3], the systematic studies of Classification suggested by Burwell in 1957 relied on
wear have started far more recently [4, 5]. It was the type of wear mechanism as a classifying criterion
the requirement for increased accuracy and smaller [9]. The distinguished classes of wear were related to
clearances needed for successful operation of early (1) adhesive, (2) abrasive, (3) corrosive, and (4) surface
twentieth-century machinery that have led to the fatigue mechanisms, and (5) other minor wear types,
growing interest in wear studies, which were further such as erosion and impact chipping. Classification
supported by the advent of modern imaging techniques suggested by Kostetskii et al. in 1976 relied on the
having an adequate resolution [6, 7]. Since then, many reliability of surface performance and the nature of
classification schemes have been devised, partly due interaction processes as two classifying criterions [10, 11].
to the accumulation of knowledge and partly due to The distinguished classes of wear were (1) acceptable
the complexity of wear processes leaving much room wear consisting of (a) normal mechanochemical
oxidative, (b) normal mechanochemical non-oxidative,
* Corresponding author: Michael VARENBERG. and (c) mechanochemical form of abrasive wear, and
E-mail: michaelv@technion.ac.il
334 Friction 1(4): 333–340 (2013)

(2) unacceptable damage consisting of (a) seizure, Table 1 Normalized classifying criterions used in key classification
schemes.
(b) fretting damage, (c) mechanical form of abrasive
Year Author(s) Classifying criterion(s)
wear, (d) rolling fatigue (pitting), and (e) other forms
of damage, such as corrosion, erosion, cavitation, and 1938 Siebel 1. Relative motion
crushing. Classification suggested by Czichos in 1978 1956 Archard & Hirst 1. Damage severity
integrated some of the previous approaches and relied 1957 Burwell 1. Damage mechanism
on the type of relative motion, the interacting elements, 1976 Kostetskii et al. 1. Damage severity
and the dominant wear mechanism as three classifying 2. Damage mechanism
criterions [12]. The distinguished classes of wear 1978 Czichos 1. Relative motion
were (1) sliding wear, (2) rolling wear, (3) impact wear, 2. Interacting elements
(4) fretting wear, (5) cavitation wear, and (6) fluid 3. Damage mechanism
erosion ordered into a table of six rows representing 1987 Lim & Ashby 1. Interaction mechanism
the relative motion types grouped by the interacting 2. Damage severity
elements and four columns representing the main
wear mechanisms able to act in various combinations determines the interacting elements, (2) mechanism
within each of the six classes of wear. Classification of what happens to the surface, when interaction
suggested by Lim and Ashby in 1987 relied on the mechanism refers to the process and damage mech-
mechanism of surface interaction as a classifying anism refers to the result, and (3) damage severity. It
criterion [13]. The distinguished classes of wear were is easy to assume that the generalized classification
(1) seizure, (2) melt-dominated wear, (3) oxidation- of wear should also rest on the system of three
dominated wear, and (4) plasticity-dominated wear, independent axes. Supported by this assumption and
while the last two groups were additionally subdivided based on previous studies, we will now proceed to
into (a) mild and (b) severe wear subclasses. the following in an attempt to derive all classifiers
To the best of my judgement, these schemes make a from the common source.
list of the most important approaches to classification Wear is defined as the damage to a solid surface,
of wear. However, though covering the whole field in generally involving progressive loss of material, due
sum, they stay only loosely connected to each other to relative motion between that surface and a
and do not build a complete general picture. In trying contacting substance or substances [14]. Based on this
to introduce students to such complex phenomena simple definition, we can recognise three classifying
as wear when teaching undergraduate course on criterions according to which the system has to be
tribology, it became clear to me that there is a need to characterized. These are the answers to the following
devise a basic classification, which may present the questions: (1) Why does it happen? (2) How does it
state of the art before entering microscopic or even happen? and (3) Where does it happen? To make
nanoscale origins of wear. To this end, the goal of this the picture complete, it is probably worth adding
paper is to review, combine and integrate the existing that the other interrogative words used in gathering
approaches into a general scheme unifying known wear information seem not relevant, as the answers are
types into a consistent system. The target audience is known (who–wear process, what–damages the surface,
scholars who study, teach or start practicing solving when–continuously).
the wear-related problems.
2.1 Why?

2 Definitions The question “why” determines the reason, which is

explicitly specified in the above definition as a relative
Systematizing the wear classification schemes deve- motion. Clearly, the type of relative motion will serve
loped so far, we can normalize the used classifying us as a first classifying criterion.
criterions according to Table 1. Presented in this Analysing relative motion, we can distinguish
way, they allow us to see that there are only three between the following five types. (1) Fretting, which,
independent ones: (1) relative motion, which also according to a less known (but more accurate than
Friction 1(4): 333–340 (2013) 335

classic) definition [15], is the relative cyclic motion inconsistency between, say, the coefficients of friction
between two solid bodies, having a non-uniform and the coefficients of wear. Analysing this list, we
distribution of local relative displacement at their come to the conclusion that the wear-related energy
contact. This type of motion is directly connected to losses are pooled from (a) generation of defects, leading
preliminary displacement [16], which always takes to internal material changes, and (b) generation of
place before gross sliding occurs. (2) Sliding, the relative heat, leading to increase in temperature activating
motion in the tangential plane of contact between interactions with external agents. Both items can be
two solid bodies [14]. To distinguish it from fretting, traced further, to let us distinguish between the
it is worth adding that sliding is the uniform relative following four processes to be united under the name
motion, which means that it is possible to neglect the of surface disturbance. (1) Storage of defects, which can
differences in distribution of local relative displace- appear or move to, and pile up at certain characteristic
ment at the contact zone. (3) Rolling, the relative motion locations. (2) Motion of defects, which can come and
between two non-conforming solid bodies whose leave, passing through a material volume under
surface velocities in the nominal contact location consideration. (3) Chemical interactions, which consist
are identical in magnitude, direction, and sense [14]. of reactions with active environmental elements to
(4) Impact, the relative cyclic motion between two solid form secondary surface films. (4) Physical interactions,
bodies that come in and out of contact. (5) Flow, the which consist of such processes as ablation, adsorption,
relative motion between a solid body and a fluid. and diffusion that remove existing or bring new
elements from and to the system.
2.2 How?
2.3 Where?
The question “how” illuminates the mechanism, which
can also be deduced from the above definition. The The question “where” defines the significance, which is
surface under consideration interacts with “a contacting related to the scale of the problem that may be clearly
substance or substances”, which results in external recognized on either macroscopic, or microscopic, or
forces exerted on it. Given the presence of relative nano level as surface colour, reflectivity, texture,
motion, these forces act through certain distances so integrity, homogeneity, etc. “Solid surface” is not merely
mechanical work is performed on the surface, and the an interface between the body and the outside world,
latter accumulates energy that has to be dissipated. but rather a complex layered system [18], whose
The amount of energy involved in this process actually behaviour is altered depending on what layers are
determines the form of surface damage [11], allowing involved in the processes of energy dissipation. Hence,
us to define the second classifying criterion based on a distinction in the scale of surface damage can be
energy dissipation. used as a third classifying criterion.
Examining the processes of conversion and Reviewing the scale of surface damage, we can
dissipation of mechanical energy taking place within recognise the following two types. (1) Normal state,
the topmost surface layers, we can list the following which is characterized by localization of damage
“losses”. The energy is expended on generation of within the outer (protective) surface layers due to the
structural defects (dislocations, stacking faults, cracks, dynamic equilibrium between the processes of surface
vacancies, misplacements, stripe patterns, etc.), stored destruction and formation of secondary surface films
as a result of elastic strains, emitted in the form of driven by chemical reaction with active environmental
phonons (acoustic waves and sound), photons (tribo- elements. (2) Pathological state, which is characterized
luminescence) and electrons (exo-electrons, Kramer by insufficient regeneration of disrupted protective
effect), and transformed into heat [17]. Interestingly, surface layers, resulting in that the “relative motion
all these processes constitute the ultimate origin of between that surface and a contacting substance or
friction [17], though not all of them give rise to substances” is accommodated within the deeper (bulk)
wear, which may probably explain the well-known layers and the basis material is torn [19].
336 Friction 1(4): 333–340 (2013)

3 Classification common example, and the tribological interaction

between the “surface and a contacting substance or
Now, having three independent groups of answers to substances” that removes the reaction products from
the above classifying questions, we define a 3-D space that surface. Interestingly, the latter process bears
described by 5 types of relative motion, 4 mechanisms the name of the most broadly defined sense, as any
of surface disturbance, and 2 surface states (Fig. 1) interaction taking place in a contact can be called
and will fill this space with the known wear types. tribological. Indeed, though speaking about the same
Obviously, different wear types can be superimposed, type of wear, different authors [5, 10, 11, 21−23] indicate
so, in order to map them unambiguously, certain different modes of surface destruction, mentioning
simplification is inevitable. To this end, here we will use
fatigue, abrasion, adhesion, erosion, melting, and
the approach based on dominant and accompanying
plastic deformation as possible mechanisms. This
processes [10], the essence of which is as follows.
means that tribo-chemical wear is not limited to any
Depending on loading conditions, environment and
particular mechanism of surface destruction, but can
materials involved, different mechanical, physical and
be run by every one of them. It can be interpreted in
chemical processes may take place simultaneously
such a way that if the wear process is localized within
on friction surfaces [20]. The processes that have the
the chemically formed secondary surface structures
greatest impact on friction and surface damage are
capable of continuous self-regeneration, such as oxides,
called dominant. Together with dominant processes
for instance, the actual reason and mechanism of
there are accompanying processes, whose effect on
surface destruction are much less important. Only if
friction and wear can be neglected to a first appro-
the basis material below the secondary structures is
ximation. Clearly, changes in working conditions
torn, the surface degrades to the pathological state
may lead to transition from one dominant process to
and there is a need to find out what mechanism is
another. In developing this classification, only the
responsible for the damage. Along this line of thought,
dominant processes with no regard to their determining
we will put the tribo-chemical wear into the above-
conditions will be considered.
defined 5 × 4 × 2 wear space in that way it occupies the
Let us start with (1) Tribo-chemical wear. As follows
whole 5 × 4 slice of the normal surface state (Fig. 2).
from its name, this type of wear combines two pro-
Though this broad definition may seem not having
cesses, namely, the reaction with chemically active
enough resolving power or much less likely to satisfy,
environmental elements, with oxygen being the most
for instance, fretting damage, still and all, it looks
consistent and leaves space for future refinements.
Left with the 5 × 4 slice of the pathological surface
state, we will fill the vacant places by arranging the
remaining wear types as shown in Fig. 3. There are
eleven additional wear types to be categorized, with
some of them being further subdivided into smaller
subgroups.
(2) Fretting fatigue and (3) Fretting wear, which appear
in fretting, originate from vibration or temperature
changes in a nominally motionless contact. Damaged
surfaces exhibit no signs of sliding direction, large
amounts of powder oxide debris coloured differently
than usual rust, and fatigue cracks initiated in fretted
area [24]. These two types of damage commonly coexist,
though, usually depending on operating fretting regime
Fig. 1 3-D classification space defined by 5 types of relative
(partial or gross slip), one of them always dominates
motion, 4 mechanisms of surface disturbance and 2 surface states.
[25]. If fretting fatigue is the dominant form of damage,
Friction 1(4): 333–340 (2013) 337

Fig. 2 Normal wear determined by localization of damage within the self-regenerating secondary protective layers.

Fig. 3 Pathological wear types determined by relative motion and surface disturbance.

then the main mechanism of surface disturbance is (7) Abrasive wear, which appears in sliding, results
the storage of defects. If fretting wear is the dominant from scratching by hard particles trapped by or pro-
form of damage, then the other three mechanisms of tuberances projecting from the mating surface and is
surface disturbance act simultaneously promoting each characterized by the presence of parallel scratches in
other. Interestingly, depending on loading conditions the sliding direction. Interestingly, only a very small
and materials involved, different mechanisms may fraction of the contacting particles or protuberances
become more pronounced, which reflects in different may contribute to pure mechanical chip cutting
surface behaviour [26]. [29, 30], while the rest is only capable of deforming
(4) Fatigue wear, (5) Pitting and (6) Impact wear, which the surface. Deformation results in generation of
appear in sliding, rolling, and impact, respectively, numerous defects providing passageways for easy
result in abrupt surface destruction due to sub-surface diffusion of active atoms, such as oxygen, into the
cracks propagated by stress cycling [5, 27, 28]. Damaged lower surface layers, which change their mechanical
surfaces exhibit shallow or deep craters (pits) with properties due to chemical reactions further accelerated
sharp walls. Obviously, we will associate these types of by heating. It is known, for instance, that abrasive
wear with the storage of defects as the main mechanism wear of metals decreases significantly if oxygen is
of surface disturbance. removed from the surface environment [31]. Thus,
338 Friction 1(4): 333–340 (2013)

it seems that not only the direct cutting but also the and vibratory cavitation erosion [36]. Worn surfaces
ploughing action may contribute to the surface exhibit deep pits that are often getting larger towards
destruction if due to increased chemical activity of the the inside. Liquid-impact erosion is associated with
surface the width of the brittle outer layers becomes cyclic deformation, making it a fatigue-based process,
larger. Hence, in light of the above, we will associate which allows us to connect it to the storage of defects
abrasive wear with two simultaneously acting mech- as the main mechanism of surface disturbance.
anisms of surface disturbance, namely, motion of (11) Solid-particle erosion, which appears in flow,
defects and chemical interaction. Abrasive wear can results from ploughing or cutting by hard particles
also be further subdivided into 2- and 3-body abrasion, entrained in a flowing liquid or gas and is characterized
with larger relative contribution of the chemical by the presence of random impact sites with raised
interaction mechanism in the latter case. crater rims. In contrast to 3-body abrasive wear, where
(8) Solid-particle crushing, which appears in rolling the volume of the worn material depends on the normal
or impact, results from indentation of hard particles load and the sliding distance in solid-particle erosion,
trapped between the contacting surfaces and is the wear volume depends on the mass of particles
characterized by the presence of dent cavities of and the velocity at which they strike the surface [37].
random orientation. This type of damage is also Similar to abrasive wear and solid-particle crushing,
referred to as impact-abrasion [5, 32]. Similar to chemical processes accelerated by mechanical activation
abrasive wear, hard particles deform and activate the also play an important role in solid-particle erosion.
surface, which leads to formation of brittle secondary For example, it was demonstrated that, on one hand,
structures of significantly increased width and their the oxidation rates under erosion conditions are
subsequent destruction by other particles. It was found, dramatically higher than static oxidation rates [38],
for instance, that the presence of solid particles in while, on the other hand, the erosion rate is higher
lubricated rolling has led to about 60% less wear when under conditions of larger thickness of the oxide scale
the tests were performed in argon and about 40% less [39]. This allows us to associate solid-particle erosion
wear when anti-oxidant additive was used [33]. This with motion of defects and chemical interaction as
allows us to associate solid-particle crushing with well.
motion of defects and chemical interaction as well. (12) Ablation erosion, which appears in flow, results
(9) Adhesive wear, which appears mainly in sliding, from the heating of a surface induced by high-speed
but can also be present in rolling and impact, results passage of gas or electric discharges. These processes
from solid-state welding of contacting surfaces and are known by the names of gas erosion [40] and
subsequent destruction of the junctions formed spark erosion [41], respectively. Worn surfaces exhibit
[34, 35]. Damaged surfaces exhibit clear signs of random depressions and channels with scalloped
material transfer. Based on that the tendency of edges. Clearly, we will associate this type of wear
contacting surfaces to adhere arises from the attractive with physical interaction as the main mechanism of
forces between the surface atoms of the two materials, surface disturbance.
we will associate this type of wear with the physical
interaction as the main mechanism of surface
4 Discussion
disturbance.
(10) Liquid-impact erosion, which appears in flow, The suggested classification scheme seems to har-
results from repeated impacts induced by liquid monize the wear processes, while covering the whole
droplets impinging the surface or liquid jets hitting field without leaving any wear type outside, which
the surface due to the near-surface collapse of vapor creates a coherent view of the problem. Another
bubbles. The former process is known by the name question is whether the system can also be used in
of liquid-droplet erosion and the latter process is engineering practice to guide wear mitigation. And the
known by the name of cavitation erosion [5], with the answer is yes. However, its use is not in determining
latter being further subdivided into hydrodynamic the wear types that can be identified based on analysis
Friction 1(4): 333–340 (2013) 339

of contact conditions, examination of damaged surface interrogative words “why”, “how” and “where”.
and/or studies of wear debris, but rather in recognizing (2) A concept of surface disturbance mechanisms
the mechanisms of surface disturbance, which have suitable for description of various wear types is
to be fought in order to solve for wear problems. suggested based on analysis of wear-related energy
As a famous example, we can discuss adhesive and losses.
abrasive wear in the presence of lubrication or even (3) Known wear types seem to fit the suggested
humid air, which are long known to reduce the former scheme.
[34] and increase the latter [42] when much reactive (4) The scheme can be useful in engineering practice
fluid is used (e.g., water is replaced with oil) or just as to guide wear mitigation initiatives.
a result of increase in humidity. The more reactive is
the environment, the thicker are the secondary surface
Acknowledgements
layers. In the case of adhesive wear, where physical
interaction is the main mechanism of surface distur- I thank Grigory Halperin, Izhak Etsion, and Yuri
bance, thicker passive secondary structures separate Kligerman for helpful discussion.
better between active bulk layers and, hence, reduce
the interaction leading to lower wear. In the case of Open Access: This article is distributed under the terms
abrasive wear, where motion of defects and chemical of the Creative Commons Attribution License which
interaction are the main mechanisms of surface permits any use, distribution, and reproduction in any
disturbance, both thicker and thinner secondary medium, provided the original author(s) and source
structure patches may be fractured and removed in a are credited.
single contacting event due to their brittleness and/or
stress concentration at the boundary. However, the
wear rate will be obviously larger in the former case, References
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Michael VARENBERG. He received where he is currently an assistant professor and the

his PhD degree in 2004 from Technion head of Shamban & Microsystems Tribology Labs. His
– Israel Institute of Technology. In research interests are in friction and wear of engineering
2007 he joined the Department of surfaces, micro/nano tribology, bionic tribology, tri-
Mechanical Engineering at Technion, bological instrumentation, and contact mechanics.
Friction 1(4): 341–349 (2013)
DOI 10.1007/s40544-013-0028-9 ISSN 2223-7690
RESEARCH ARTICLE

Mechanical and tribological properties of epoxy matrix

composites modified with microencapsulated mixture of
wax lubricant and multi-walled carbon nanotubes
Nay Win KHUN, He ZHANG, Jinglei YANG*, Erjia LIU*
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
Received: 19 June 2013 / Revised: 01 August 2013 / Accepted: 09 October 2013
© The author(s) 2013. This article is published with open access at Springerlink.com

Abstract: The mechanical and tribological properties of epoxy composites modified with microencapsulated wax
lubricant and multi-walled carbon nanotubes (MWCNTs) were investigated. The increased soft microcapsules
embedded in the epoxy matrices were responsible for the reduced micro-hardness and Young’s modulus of
the epoxy composites. It was found that the friction of the epoxy composites greatly decreased with increased
microcapsule content due to combined lubricating effects of the both wax lubricant and MWCNTs. As a result,
the wear of the epoxy composites apparently decreased with increased microcapsule content.

Keywords: epoxy composite; microcapsule; wax lubricant; MWCNT; tribology

1 Introduction microencapsulated lubricant in epoxy matrices could

overcome the above mentioned problems because the
Fibre reinforcements and solid or liquid lubricants sustainably released lubricant from broken micro-
are frequently applied to improve the wear resistance capsules during sliding lubricated rubbing surfaces.
of polymers. Fibre reinforcements, such as carbon and Recently, Khun et al. [11] reported that silicone
glass fibres, in polymer matrices enhance the wear composite coatings modified with microencapsulated
resistance of the polymer composites (PCs) by im- wax lubricant exhibited significantly lower friction
proving their mechanical strength [1−4]. Solid lubricants, coefficients compared to pure silicone coatings. Though
such as graphite and polytetrafluoroethylene (PTFE), the friction coefficient of epoxy composites apparently
reduce the wear of PCs by developing a transfer film decreases with increased microcapsule content, the
between two rubbing surfaces [5−7]. However, solid mechanical strength of the composites significantly
additives cannot always give desired tribological pro- decreases due to the increased content of soft micro-
perties of PCs. Wang and co-workers [8] reported capsules in the hard epoxy matrices [10]. Therefore, it
that the use of liquid paraffin during tribological test is a great challenge to keep the low friction of epoxy
improved the wear resistance of epoxy composites composites when a small amount of microcapsules
by reducing the friction of the composites. Although is introduced into the composites. It is reported that
a liquid lubricant can improve the tribological per- carbon fillers, such as carbon nanotubes (CNTs), in
formance of polymers, a degradation of materials polymer matrices can reduce the friction of the PCs via
caused by absorption and osmosis of the lubricant their solid lubricating and free-rolling effects [12−19].
into them may limit the application of materials It is therefore expected that incorporation of wax
[8, 9]. Guo et al. [10] reported that incorporation of lubricant mixed with carbon fillers in microcapsules
would improve the tribological performance of
* Corresponding author: Jinglei YANG, Erjia LIU. PCs. An understanding of a correlation between
E-mail: MJLYang@ntu.edu.sg; MEJLiu@ntu.edu.sg the microcapsule content in epoxy composites and
342 Friction 1(4): 341–349 (2013)

their tribological properties is essential for successful stirring rate of 500 rpm (Caframo, Model: BDC6015),
tribological applications. 3.17 g of an aqueous solution containing 37 wt%
In this study, a wax lubricant mixed with multi- formaldehyde was dropped into the emulsion. After
walled carbon nanotubes (MWCNTs) was microen- the final mixture was heated to 55 °C at a heating rate
capsulated through in-situ polymerization. The of 35 °C/h and agitated for 4 h, the microencapsulation
synthesized microcapsules were incorporated in epoxy process was stopped. The microcapsules were separated
matrix to form a new type of epoxy based composite. under vacuum with a coarse-fritted filter, which were
The microcapsule content was varied from 0.5 to 5 wt% then rinsed with DI water and dried for 24 h at RT.
to investigate the tribological properties of the epoxy Epoxy resin and hardener were mixed with a weight
composites with respect to microcapsule content. ratio of 3:1. The epoxy and composites with different
microcapsule contents from 0.5 to 5 wt% were moulded
and fully cured at RT.
2 Experimental details
2.2 Characterization
2.1 Materials
The root-mean-square surface roughnesses (Rq) of the
The urea (CO(NH2)2), formaldehyde (CH2O, PUF),
samples were measured using surface profilometry
ammonium chloride (NH4Cl), and sodium hydroxide
(Talyscan 150) with a diamond stylus of 4 μm in
(NaOH) were purchased from Sigma-Aldrich. The
diameter in a scan size of 2 mm × 2 mm. Five measure-
surfactant, ethylene maleic anhydride copolymer
ments on each sample were carried out to get an
(EMA), was purchased from MP Biomedicals. The
average Rq value.
MWCNTs were supplied by Chengdu Organic
The surface morphology of the samples was studied
Chemicals Co., Ltd. The diameter, length, number
using scanning electron microscopy (SEM, JEOL-
of walls, and bulk density of the MWCNTs according
JSM-5600LV).
to the manufacturer’s specifications were 5−20 nm,
The hardnesses and Young’s moduli of the samples
1−10 μm, 3−15, and 140−230 kg·m–3, respectively. The
were measured using a microindenter (MHT, CSM)
wax lubricant (Episol B2531, C14-C20) was ordered
from EP chem. International Pte. Ltd. Epoxy resin with a pyramidal shaped diamond tip of 20 μm in
(Epocote 1008 Part A) and hardener (Part B) both diameter. The indentation test was performed in a
were ordered from Shell AG. load control mode with a total load of 3 N. In each
The 1 wt% MWCNTs were dispersed into the wax indentation test, the loading and unloading rates and
lubricant by a two-step method: Sonication (Misonix, dwelling time at the peak load were 6 N/min, 6 N/min
Model: Sonicator 3000) in an ice-bath under power 30 and 5 s, respectively. The hardness and Young’s
watts for 10 min followed by homogenization (Ika, modulus of the samples were derived using Oliver &
Model: T18 basic Ultra-Turrax) for another 10 min at Pharr’s method and average values were taken from
10,000 rpm. Microcapsules were prepared by in-situ eight indentation measurements carried out at different
polymerization in an oil-in-water emulsion [20]. Under locations on each sample [21].
room temperature (RT) (about 22–24 °C), 50 mL The tribological properties of the samples were
deionized (DI) water, 1.25 g urea, 0.125 g NH4Cl, investigated using a ball-on-disc microtribometer (CSM)
0.125 g resorcinol (C6H6O2) and 12.5 mL of an aqueous operated in rotary mode at RT. Three tests were
solution containing 2.5 wt% ethylene maleic anhydride conducted on each sample to get average tribological
copolymer (EMA) were added into a 250 mL beaker results. In a test, a steel ball (Cr6) of 6 mm in diameter
placed in a temperature-controlled water-bath. The was rotated on a sample in a circular path of 2 mm in
pH of the mixture was adjusted to 3.5 by adding radius for about 20,000 laps at a sliding speed of
NaOH. After that, 15 mL of the MWCNT dispersed 2 cm/s under a normal load of 2 N. The wear tracks
wax lubricant was slowly poured into the mixture. on the samples were then measured using white light
When the mixture was emulsified for 10 min at a confocal imaging profilometry (Nikon L150).
Friction 1(4): 341–349 (2013) 343

3 Results and discussion the epoxy composite with the incorporation of 5 wt%
microcapsules, as shown in Fig. 2(b), indicates the
Figure 1(a) shows the SEM micrograph of the micro- improved uniformity of the epoxy composite. Although
capsules containing wax lubricant and MWCNTs, in the slightly protruded microcapsules above the surface
which the sizes of the microcapsules range from about
of the epoxy composite (5 wt% microcapsules) are
150 to 300 μm in diameter. As shown in Fig. 1(b), the
found in Fig. 2(b), its lower Rq value (~1.9 μm) than
cotton-like features observed on the rough surface of
that (~3.1 μm) of the epoxy is mainly attributed to
the microcapsule result from the dangling MWCNTs
the suppression of the pin holes on the surface of the
in the shell wall and the precipitation of urea-
composite.
formaldehyde (UF) nanoparticles [20]. In Fig. 1(c),
Figure 3 shows the hardnesses and Young’s moduli
the SEM micrograph of an artificially crushed
microcapsule clearly indicates a core-shell structure of the epoxy and composites with different microcapsule
of the microcapsule [22]. The MWCNTs are evidently contents. The hardness and Young’s modulus of
found and fairly dispersed on the interior surface of the epoxy matrix are about 198.5 MPa and 3.4 GPa,
the microcapsule with some agglomeration, as shown respectively. However, the increased content of micro-
in Fig. 1(d). capsules in the epoxy composites from 0.5 to 5 wt%
Figure 2 presents the surface topographies of the decreases the hardness and Young’s modulus of the
epoxy and epoxy composite with 5 wt% microcapsules. composites from about 177.8 MPa and 2.2 GPa to
In Fig. 2(a), pin holes can be clearly seen on the surface about 113 MPa and 1.5 GPa, respectively, which is a
of the epoxy, which may result from degassing or direct consequence of the lower hardness and elastic
rapid curing of the epoxy. However, the apparently modulus of the microcapsules with respect to those
lessened formation of the pin holes on the surface of of the epoxy matrix [23, 24].

Fig. 1 SEM micrographs of (a) microcapsules containing wax lubricant and MWCNTs, (b) an enlarged view of a microcapsule, (c) a
crushed microcapsule, and (d) an interior surface of a crushed microcapsule.
344 Friction 1(4): 341–349 (2013)

Fig. 2 Surface topographies of (a) epoxy and (b) epoxy composite with 5 wt% microcapsules measured using surface profilometry.

up to 10,000 laps and then further slightly increases

with increased laps till 20,000 laps. In the running-in
period, the increased wear of the rubbing surfaces
increases the friction coefficient of the epoxy by
promoting a contact area between the steel ball and
epoxy. Furthermore, the prolonged rubbing of the
steel ball on the epoxy detaches wear debris from the
epoxy surface so that the existence of irregular shaped
wear debris at the polymer/metal interface as third
bodies gives rise to the higher friction than the initial
Fig. 3 Hardnesses and Young’s moduli of epoxy composites as friction via third body abrasive wear [25]. As a result,
a function of microcapsule content. the friction coefficient of the epoxy further increases
with increased laps after 10,000 as shown in Fig. 4(a).
The tribological properties of the epoxy and epoxy However, the incorporation of 0.5 wt% microcapsules
composites with different microcapsule contents were greatly lowers the friction coefficient of the epoxy
investigated by sliding against the 6 mm Cr6 steel composite throughout the wear test although the
balls for about 20,000 laps under a normal load of 2 N. increased friction coefficient with increased laps is still
Figure 4(a) shows the friction coefficients of the epoxy observed in the running-in period. During the sliding,
and epoxy composites as a function of the number of the released lubricant from broken microcapsules
laps. The friction coefficient of the epoxy dramatically effectively lubricates the rubbing surfaces, lessens the
increases to about 0.63 after 2,400 laps, becomes stable direct solid−solid contact between the steel ball and
Friction 1(4): 341–349 (2013) 345

composite, reduces the wear of the rubbing surfaces, Figure 4(b) shows that the mean friction coefficient
and lessens the interactions between the rubbing of the epoxy composites significantly decreases
surfaces and wear particles. In addition, the MWCNTs from about 0.27 to 0.049 with increased microcapsule
incorporated in the embedded microcapsules in the content from 0.5 to 5 wt% as the friction coefficients
epoxy composite can be released during the wear of of the epoxy composites are apparently lower than
the composite and transferred to the interface between that (about 0.54) of the epoxy. It is clear that the
the steel ball and composite. Thus, the MWCNTs increased microcapsule content promotes the combined
released serve as a solid lubricant to reduce the friction lubricating effects of the wax lubricant and MWCNTs
of the composite [15−19]. Moreover, the MWCNTs on and the free-rolling effect of the MWCTNs. Therefore,
the surface can not only serve as spacers to prevent the the microcapsule incorporated epoxy composites
direct contact between the steel ball and composite exhibit the much lower friction than the epoxy as
but also slide or roll between the rubbing surfaces [26]. the higher microcapsule content gives rise to the
Therefore, the combined lubricating effects of the lower friction of the epoxy composites. Guo et al. [10]
both wax lubricant and MWCNTs greatly reduce the reported that the incorporation of 10 wt% micro-
friction of the epoxy composite even with only 0.5 wt% capsules containing lubricant oil in an epoxy matrix
microcapsules. The increased microcapsule content could give the friction coefficient of about 0.14
in the epoxy composite through 5 wt% significantly measured using a block-on-ring apparatus. In this
shortens the running-in period and results in the study, the friction coefficient of the epoxy composite
consistently lowered friction coefficient of the com- with 5 wt% microcapsules containing both wax
posites as shown in Fig. 4(a). Since a smoother surface lubricant and MWCNTs measured using a ball-on-
can result in a lower friction, the reduced surface disc apparatus is about 0.049, indicating that the
roughness of the epoxy composites with increased co-incorporation of the wax lubricant and MWCNTs
microcapsule content (Fig. 2) should be correlated to in the microcapsules would give rise to the better
the decreased friction of the composites [27−31]. frictional performance of the composite.
Figure 5 shows the wear widths and depths of the
epoxy and epoxy composites with different micro-
capsule contents. It is found that the wear width and
depth of the epoxy are about 300.2 μm and 2.1 μm,
respectively. The incorporation of 0.5 wt% micro-
capsules in the epoxy matrix significantly decreases
the wear width and depth of the composite to about
233.5 μm and 1.6 μm, respectively. The wear width
and depth of the epoxy composites further decrease
from about 193.5 μm and 0.9 μm to about 68 μm and

Fig. 4 Friction coefficients of epoxy composites as functions of Fig. 5 Wear widths and depths of epoxy composites as a function
(a) the number of laps and (b) microcapsule content. of microcapsule content.
346 Friction 1(4): 341–349 (2013)

0.3 μm, respectively, with increased microcapsule clearly seen in the centre of the wear track where the
content from 2.5 to 5 wt%, indicating that the released most severe wear of the epoxy surface occurs as shown
lubricant effectively reduces the wear of the epoxy in Fig. 6(b) [32, 33].
composites during the sliding. The incorporation of 5 wt% microcapsules in the
Figure 6(a) shows the surface morphology of the epoxy greatly reduces the wear of the composite so the
worn epoxy after sliding against a steel ball for about wear of the composite is not as severe as that of the
20,000 laps under a normal load of 2 N. The wear track epoxy, as shown in Fig. 6(c). In addition, micro-cracks
on the surface of the epoxy shows that the rubbing and wave features are not apparently found on the
of the steel ball on the epoxy during the wear test surface of the epoxy composite with 5 wt% micro-
generates the abrasive wear of the epoxy as shown in capsules because the self-lubricating of the composite
Fig. 6(a). The wear track looks like a fish backbone surface effectively suppresses the surface fatigue
and some tiny cracks along the track can be found. wear. In Fig. 6(c), the surrounding areas of the broken
The cross-points of the cracks are probably near the microcapsules on the wear track of the epoxy com-
centre of the wear track and the cracks are convex posite are apparently contaminated by the released
with an angle of about 120° to the sliding direction. wax lubricant, which confirms that the wear of the
This arises from repeated stress concentration occurred composite surface during the sliding results in the
in front of the steel ball during the repeated sliding of breakage of the microcapsules and the subsequent
the steel ball on the epoxy surface. In addition, the release of the wax lubricant for self-lubricating.
repeated sliding causes surface fatigue that in turn Figure 6(d) shows a view of a broken microcapsule
initiates minute cracks perpendicular to the sliding on the wear track of the epoxy composite with 5 wt%
direction and propagates the cracks into the subsurface microcapsules, from which it can be seen that the
of the epoxy [32, 33]. The formation of a network of breakage of the microcapsule leaves a single hole on
micro-cracks creates micro-wave features that can be the surface. In addition, the debris produced by the

Fig. 6 SEM micrographs showing surface morphologies of worn (a and b) epoxy and (c and d) epoxy composite with 5 wt% microcapsules
at different magnifications.
Friction 1(4): 341–349 (2013) 347

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Nay Win KHUN. He received his from the Nanyang Technological University, Singapore.
MS degree in “Mechanics and Proc- His research interests include thin films and coatings,
essing of Materials” in 2006 and his composite materials, corrosion, tribology and surface
PhD degree in “Thin Films Physics and interface.
and Electrochemistry” in 2011, both
Friction 1(4): 341–349 (2013) 349

Jinglei YANG. He received his Aerospace Engineering at Nanyang Technological

master degree in solid mechanics University in 2008. His current position is an assistant
from the University of Science and professor. His research areas cover multifunctional
Technology of China, and PhD self-healing polymers and coatings, FRP composites
degree from the University of and nanocomposites and their mechanical, dynamic,
Kaiserslautern, Germany. He joined and tribological performances, and encapsulated
the School of Mechanical and phase change materials for green building.

Erjia LIU. He received his bachelor engineering from Catholic University of Leuven. He
degree in materials engineering joined the School of Mechanical and Aerospace
from Harbin University of Science Engineering at Nanyang Technological University in
& Technology, master degree in 1999. His current position is an associate professor.
materials engineering from Harbin His research interests include thin films and coatings,
Institute of Technology, and PhD carbon based materials, nanocomposites, nanotribology,
degree in metallurgy and materials and electrochemistry.
Friction 1(4): 350–358 (2013)
DOI 10.1007/s40544-013-0031-1 ISSN 2223-7690
RESEARCH ARTICLE

Hydrophobic, mechanical, and tribological properties of fluorine

incorporated hydrogenated fullerene-like carbon films
Li QIANG, Bin ZHANG, Kaixiong GAO, Zhenbin GONG, Junyan ZHANG*
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Received: 20 June 2013 / Revised: 07 September 2013 / Accepted: 25 October 2013
© The author(s) 2013. This article is published with open access at Springerlink.com

Abstract: Fluorine-incorporated hydrogenated fullerene-like nanostructure amorphous carbon films (F-FLC)

were synthesized by employing the direct current plasma enhanced chemical vapor deposition (dc-PECVD)
technique using a mixture of methane (CH4), tetra-fluoromethane (CF4), and hydrogen (H2) as the working
gases. The effect of the fluorine content on the bonding structure, surface roughness, hydrophobic, mechanical,
and tribological properties of the films was systematically investigated using Fourier transform infrared
spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman analysis, atomic force microscope (AFM),
contact angle goniometer, nano-indenter, and reciprocating ball-on-disc tester, respectively. The fluorine content
in the films increased from 0 to 2.1 at.% as the CF4 gas flow ratio increased from 0 to 3 sccm, and incorporated
fluorine atoms existed in the form of C–FX (X = 1, 2, 3) bonds in the film. The fullerene nanostructure embedded
in the hydrogenated amorphous carbon films was confirmed by Raman analysis. The water contact angle was
significantly increased because of fluorine doping, which indicates that the hydrophobicity of the carbon films
could be adjusted to some extent by the fluorine doping. The hardness and elastic modulus of the films
remained relatively high (22 GPa) as the fluorine content increased. Furthermore, the friction coefficient of the
carbon films was significantly reduced and the wear resistance was enhanced by fluorine doping.

Keywords: fullerene-like carbon film; fluorine doping; hydrophobicity; mechanical properties; tribological
performance

1 Introduction attributed to the unique fullerene-like nanostructure

embedded in the amorphous diamond-like carbon
Hydrogenated diamond-like carbon (DLC) films have matrix [5, 6]. However, fullerene-like nanostructure
attracted significant interest in the area of science carbon (FLC) films with high sp2 hybrid carbon
because of their extraordinary properties such as high content showed excellent mechanical properties. This
mechanical hardness, low friction coefficient, and high is due mainly to the presence of the curvature in an
wear resistance [1–4]. These properties make them all-sp2 three-dimensional network, as with a “molecule
excellent candidates for a wide range of applications. spring” fixed in the amorphous carbon matrix [7, 8].
Generally, the mechanical properties are mainly This also indicates that the sp3 hybrid carbon content
determined by the sp3 hybrid carbon content, which is not the only factor that determines the mechanical
contributes to higher mechanical hardness. It should properties of the film, as the microstructure also
be noted that our previous studies have reported significantly affects the mechanical properties.
films with a hardness of 20.9 GPa and elastic recovery From the above discussion, fullerene-like nano-
as high as 84%, and these excellent properties were structure carbon films have promising and valuable
prospect as protective coating. Furthermore, it is
* Corresponding author: Junyan ZHANG. well known that the incorporation of non-metallic
E-mail: zhangjunyan@licp.cas.cn elements, including N [9], Si [10, 11], and F [12–14], can
Friction 1(4): 350–358 (2013) 351

significantly improve and modify the performances min at a discharge voltage of −800 V. During this time,
of films. Among them, fluorine incorporated DLC the working pressure was kept at 4.8 Pa. The working
films have attracted significant interest due to their gas of the deposition was composed of 300 sccm Ar,
superior low friction coefficient, low surface energy, low 10 sccm CH4, and 20 sccm H2. The diameters of the
internal stress, particularly excellent biocompatibility, upper electrode and the substrate holder were 300
and hydrophobic properties. Yu et al. [15] found that and 200 mm, respectively, and the distance between
the F content rapidly increased with the introduction the two electrodes was 5 cm. A negative pulsed voltage
of CF4, leading to a sharp reduction in the surface of 1,000 V and a duty-cycle of 80% were applied to the
energy of the film. Marciano [16] also reported that substrate, and the working pressure was kept at 15 Pa.
F-DLC films presented lower stress and surface free The deposition time was set as 120 min. The fluorine
energy as their F content increased. In addition, content was controlled by varying the CF4 gas flow to
according to Refs. [17, 18], the biocompatibility and 0, 1 sccm, 1.5 sccm, and 3 sccm. The substrate is not
hydrophobic properties can be effectively improved heated, while the temperature was unintentionally
by the incorporation of fluorine. Recently, a number increased to around 100℃ because of the plasma
of deposition technologies have been proposed for bombardment during the deposition process. After
the synthesis of F-FLC films [19–21]; the common processing, the samples were cooled down inside the
deposition technique involves the use of a pulsed chamber.
direct current plasma enhanced chemical vapor
deposition (dc-PECVD) system, which has several 2.2 Sample characterization
advantages such as low film stress, low deposition
temperature, and good uniformity on a large-area The thickness of the films was measured using cross-
substrate. section scanning electron microscope (SEM) images,
Considering the complexity and diversity of and the deposition rate can be obtained by the ratio
amorphous carbon film, in our present work, we of the thickness to the deposition time. The bonding
synthesized fluorine-incorporated hydrogenated structure and the chemical state were characterized
fullerene-like amorphous carbon films (F-FLC) by using Fourier Transform Infrared Spectroscopy
employing the pulsed dc-PECVD technique using a (FTIR) in the range of 400–4,000 cm–1, with a 2 cm–1
mixture of methane (CH4), tetra- fluoromethane (CF4), resolution and 32 scan times at room temperature.
and hydrogen (H2) gases. The aim of this work is to The microstructure of the films was characterized by
study the effect of the F content on the properties of VG ESCALAB 210 type X-ray photoelectron spectros-
FLC film, and to determine whether the hydrophobic, copy (XPS), with Al Kα radiation (photo energy
mechanical, and tribological properties of the FLC film 1476.6 eV) as the excitation source and using a Raman
can be significantly improved by fluorine doping. spectrometer (Jobin Yvon T64000) in back-scattering
configuration with laser excitation at a wavelength of
514.5 nm, which is over the 800–2,000 cm–1 wavenumber
2 Experimental details range. The surface topography was observed by a
Smart SPM type atomic force microscope (AFM,
2.1 Film deposition
AIST-NT Co, USA), and the relative roughness factor
All of the films were prepared on a Si substrate (Ra) was calculated by the analysis of a Nanoscope III
(n-100) using the dc-PECVD technique. The substrate 5.12r2 program. The hydrophobicity examination
was sequentially ultrasonic cleaned in ethanol and of the film was performed using purified water by
acetone for 10 min to remove surface stains, then employing the sessile drop method with a DSA100-
dried in the ambient atmosphere and placed into the type (KRUSS Co., Germany) contact angle goniometer,
chamber. Prior to the deposition, the vacuum chamber which has an accuracy of 2°. For each sample, 5
was evacuated up to 1.0 × 10–3 Pa, and then 300 sccm different surface location measurements were evaluated
argon was introduced into the chamber. The entire Si to obtain average values, and all measurements were
substrate was first cleaned by argon discharge for 30 reported as the mean of 5 replicates to obtain more
352 Friction 1(4): 350–358 (2013)

reliable data. The hardness and elastic recovery were the CF4 gas flow increases from 0 to 1 sccm, the
measured by a nanoindenter (Nano indenter Ⅱ, MTS. fluorine content increases from 0 (FLC) to 0.85 at.%
Co., USA) with a maximum indentation depth of (F-FLC1). As the CF4 gas flow increases further, the
50 nm. The tribological properties were tested using fluorine content increases from 1.21 at.% (F-FLC2) to
a UMT-2MT ball-on-plate reciprocating tribo-tester, 2.1 at.% (F-FLC3). The result implies that the fluorine
which slides at 25°C and at a relative humidity (RH) of content can be adjusted by varying the CF4 gas flow,
15%–19%. As the mating material, Al2O3 balls ( 5 mm) which is consistent with the results of many studies
were ultrasonic cleaned with acetone before each test. [22–24]. Furthermore, the deposition rate of the films
All of the measurements were performed at a sliding still increased slightly from 3.3 nm/min to 3.9 nm/min
velocity of 15 cm/s and with a 30 N load. The sliding as the CF4 gas flow increased from 0 to 3 sccm.
distance was 90 m. The specific friction coefficient was Figure 2(a) shows the FTIR spectrum of the films.
calculated by averaging the data of at least 5 individual The C−F vibrational modes for the wavenumber range
operations. After friction, the wear morphologies of of 400–2,200 cm−1 are listed in Table 1 [25, 26]. The
mating balls and scars of the films were observed by broad peak at 2,950 cm−1 for the CHn group and the
SEM. The wear volume was calculated by measuring bond at 1,600 cm−1 of C=C stretching are so weak that
the wear scars of the substrate with a three-dimensional it is not easily found. This indicates that there is little
profilometer, and then the specific wear rates (k) of hydrogen in the films. Besides, the FTIR spectrum
the films were obtained from Eq. (1) as follows can be divided into two groups: the absorption peak
located at 600 cm−1, which is associated with the CF2
k = V /( D·L) (1) wagging mode (Group Ⅰ), and a broad bond observed
where V is the wear volume of the samples, D is the in the range of 980–1,500 cm−1 (Group Ⅱ). The peak
sliding distance, and L is the sliding load.

3 Results and discussion

3.1 Fluorine content, deposition rate, and fluorine

atoms bond state

Figure 1 depicts the dependence of the fluorine content

and deposition rate of the films on the CF4 gas flow.
It is clear that the fluorine content increases with the
increase of the CF4 gas flow (confirmed by XPS). As

Fig. 1 The dependence of the fluorine content and the deposition Fig. 2 (a) the FTIR spectrum and (b) the XPS F1s peak of the
rate on the CF4 gas flow. film as a function of the fluorine content.
Friction 1(4): 350–358 (2013) 353

Table 1 FTIR assignment for different vibrational modes of

F-DLC films.
Group Wavenumber (cm−1) Vibrational modes
Ⅰ 650 CF2 wagging
Ⅱ 980 CF3
1030 CF
1050 CF in CF2
1160 CF2 symmetric stretch
1220 CF2 asymmetric stretch
1340 CF stretch
1450 CF2 asymmetric stretching

near 1,000 cm−1 is attributed to the C−F bond and the

sharp peak at 1,100 cm−1 is due to CF2. Moreover, the
XPS F 1s spectra of the films with different fluorine
contents are shown in Fig. 2(b). The intensity of the
F 1s peak clearly increased and the peak position
shifted slightly toward a higher binding energy with
an increase in the fluorine content from 686.5 eV for
the F-FLC1 film to 686.6 and 686.8 eV for the F-FLC2
and F-FLC3 films, respectively. The F1s spectra could
be divided by the Gaussian fitting into two peaks,
centered at 686.4 eV and 687.8 eV, related to the C–F Fig. 3 The Raman spectra of (a) the convention DLC film and
and C–F2 bonding, respectively. The higher F content (b) the FLC films.
in the film clearly leads to a higher C–F2 content,
resulting in a higher binding energy shift. The results only in aromatic rings, appeared at approximately
from the FTIR and XPS F1s analysis suggested that 1,380 cm–1 [27]. However, in addition to the D and G
the incorporated fluorine atoms exist mainly in the peaks, two additional Gaussian peaks centered at
form of the C–F, C–F2, and C–F3 bonds in the film. approximately 1,230 cm–1 and 1,490 cm–1 were also
observed, as shown in Fig. 3(b), which is consistent
3.2 Raman analysis with our previous results [5, 28] that indicated that
these two peaks originated from the seven- and five-
Raman spectroscopy, which is a popular, effective,
number carbon rings of curved graphite, fullerene, or
and non-destructive tool that is used to distinguish
onion.
different bonding types, was used to probe the
bonding structure of the carbon films. Figure 3 displays 3.3 Surface roughness and hydrophobicity
the Raman spectra of the conventional amorphous
carbon film and the FLC films. A broad asymmetric The relative roughness factor (Ra) roughness of the
Raman bond, which represents the typical features film surface was obtained using the smart SPM-type
of the conventional DLC films, could be observed in AFM technique, as shown in Fig. 4. Table 2 summarizes
the wavenumber range of 800–2,000 cm–1 (Fig. 3(a)). the specific Ra values as a function of the fluorine
Normally, the Raman spectrum can be fitted into two content. For a fluorine content of 0, the film has a
Gaussian peaks: (1) a relatively sharp peak (so-called flat and smooth surface and the Ra value is 0.37 nm.
G peak), which originated from all pairs of sp2-C When the fluorine content increased up to 2.1 at.%,
atoms in both rings and chains, is located at about the surface of the film becomes rougher and many
1,580 cm–1, and (2) a shoulder peak (D bond), which protuberances appear, and the Ra value also increases
is assigned to the breathing mode of sp2-C atoms significantly from 0.37 nm to 1.07 nm. The effects of the
354 Friction 1(4): 350–358 (2013)

Fig. 4 AFM images of the film with a fluorine content of (a) 0 and (b) 2.1 at.%.

Table 2 The surface roughness (Ra) of the films as a function angle is 65.5° for the FLC film, and quickly increases
of the fluorine content. to 78.2° for the F-FLC1 film. Eventually, the water
Samples Ra roughness (nm) contact angle reaches 90.1° for the F-FLC3 film. This
FLC 0.37 suggests that the increase in the water contact angle
F-FLC (F = 0.85 at.%) 0.59 is due mainly to the incorporation of F, which is
F-FLC (F = 1.21 at.%) 0.72 consistent with Ref. [30]. It is clear that the increase in
F-FLC (F = 2.10 at.%) 1.07 the water contact angle is due not only to the presence
of CFX bonds, but also to the surface roughness.
ions’ bombardment may contribute to the increased Previous studies [31, 32] have shown that the surface
surface roughness [29]. With the increase of the roughness can significantly affect the hydrophobicity
fluorine content, an increasing number of F+ ions of the films, as a smoother surface will result in a
bombarded the film surface, which would promote smaller water contact angle. As the fluorine content
increased surface roughness. increases, the surface roughness increases, as shown in
The hydrophobicity of the films was determined by Table 2, leading to the increased water contact angle.
the water contact angle. Figure 5 shows the variation
of the water contact angles as a function of the fluorine 3.4 Mechanical and tribological properties
content. Clearly, the water contact angle continuously
Figure 6 illustrates the hardness and elastic modulus
increases with the fluorine content. The water contact
of the films as a function of the fluorine content. The
lowest hardness of about 21.2 GPa and elastic modulus
of around 179 GPa were observed for the FLC film.
As the fluorine content increases, the hardness and
elastic modulus increase to 22.6 GPa and 184.1 GPa
for the F-FLC1 film, respectively. Ultimately, for the
F-FLC3 film, the hardness and elastic modulus increase
to 23.1 GPa and 191 GPa, respectively. Generally, the
hardness and elastic modulus of the film will decrease
due to the fluorine doping, which, because it is a
termination radical, could disrupt the continuity of
the C−C network [33]. However, in this study, the
hardness and elastic modulus monotonously increase
with the increasing fluorine content. The bombardment
effect of the F+ ions should be considered because the
Fig. 5 Water contact angles for the film with a fluorine content F+ ions bombardment results in not only an increase in
of (a) 0, (b) 0.85 at.%, (c) 1.21 at.%, and (d) 2.1 at.%. the film density, but also the increase of compressive
Friction 1(4): 350–358 (2013) 355

Fig. 6 The elastic modulus and hardness of the F-FLC films as

a function of the fluorine content.

stress in the film. The increased hardness and elastic

modulus may be due to a combination of the increases
in both the density and the compressive stress.
The friction and wear property of the films were
measured on the reciprocating ball-on-disc tester. The
friction coefficient curves of the films as a function
of the fluorine content are shown in Fig. 7(a), and
the steady state friction coefficients are also given in
Fig. 7(b). It can be observed that the FLC film presents
a high friction coefficient of around 0.029. When the
fluorine content increases to 0.85 at.%, the friction
coefficient sharply decreases to 0.019. When the fluorine
content then increases to 2.1 at.%, the lowest friction
Fig. 7 Friction coefficient of the F-FLC films as a function of
coefficient of about 0.011 was obtained. Figure 8 shows the fluorine content.
the wear surface and the corresponding wear scar of
the matching ball. There is some obvious wear debris
scattered on both sides of the wear track for the FLC
film, while no obvious wear was found, as shown in
Fig. 8(b). Moreover, the corresponding wear scar of the
matching ball for the FLC film is also obviously larger
than that of the F-FLC3 film (2.1 at.%), which implies
that the F-FLC film possesses more excellent wear
resistance compared with the FLC film. The specific
wear rate of the F-FLC3 film is 5.3 × 10−9 mm3/Nm,
much lower than that of the FLC film, which is around
15 × 10−9 mm3/Nm. There is usually a close relationship
between the tribological and mechanical properties,
and the outstanding wear resistance of the F-FLC3 film
may be attributed to its higher hardness and elastic
Fig. 8 Wear surface of the films with the fluorine content of (a)
modulus. This result indicates that a much better wear 0 and (b) 2.1 at.% and the corresponding wear scar (c) and (d) of
resistance could be obtained by fluorine doping. the contact balls.
356 Friction 1(4): 350–358 (2013)

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358 Friction 1(4): 350–358 (2013)

Li QIANG. He received his Bachelor Lanzhou University of Technology. Then, he joined

degree in chemistry in 1999 from Lanzhou Institute of Chemical Physics, Chinese
Northwest University for Nation- Academy of Sciences. His research interests include
alities, Lanzhou, China, and MS in carbon thin film as solid lubricant and the application
chemical engineering in 2012 from in engine.

Junyan ZHANG. He received his University of California at Berkeley, the University of

Bachelor degree in chemistry in 1990 Alabama, and Rice University from 2000 to 2005. In
from Lanzhou University, MS in 1997 2007, he spent 3 months in Argonne National Lab as
and PhD in physical chemistry in a guest scientist. He is now a professor of Lanzhou
1999, both from Lanzhou institute Institute of Chemical Physics, Chinese Academy of
of Chemical Physics, Chinese Sciences. His current research concerns nanostructured
Academy of Sciences. He then did carbon films as solid lubrication films and super low
his postdoctoral researches at the friction behaviors and mechanisms.
Friction 1(4): 359–366 (2013)
DOI 10.1007/s40544-013-0032-0 ISSN 2223-7690
RESEARCH ARTICLE

Abrasive-free polishing of hard disk substrate with

H2O2-C4H10O2-Na2S2O5 slurry
Weitao ZHANG, Hong LEI*
Research Center of Nano-Science and Nano-Technology, Shanghai University, Shanghai 200444, China
Received: 29 August 2013 / Revised: 16 October 2013 / Accepted: 25 October 2013
© The author(s) 2013. This article is published with open access at Springerlink.com

Abstract: The effect of tert-butyl hydroperoxide-sodium pyrosulfite ((CH3)3COOH-Na2S2O5) as an initiator system

in H2O2-based slurry was investigated for the abrasive-free polishing (AFP) of a hard disk substrate. The polishing
results show that the H2O2-C4H10O2-Na2S2O5 slurry exhibits a material removal rate (MRR) that is nearly 5 times
higher than that of the H2O2 slurry in the AFP of the hard disk substrate. In addition, the surface polished by
the slurry containing the initiator exhibits a lower surface roughness and has fewer nano-asperity peaks than
that of the H2O2 slurry. Further, we investigate the polishing mechanism of H2O2-C4H10O2-Na2S2O5 slurry.
Electron spin-resonance spectroscopy and auger electron spectrometer analyses show that the oxidizing ability
of the H2O2-C4H10O2-Na2S2O5 slurry is much greater than that of the H2O2 slurry. The results of potentiodynamic
polarization measurements show that the hard disk substrate in the H2O2-C4H10O2-Na2S2O5 slurry can be rapidly
etched, and electrochemical impedance spectroscopy analysis indicates that the oxide film of the hard disk
substrate formed in the H2O2-C4H10O2-Na2S2O5 slurry may be loose, and can be removed easily during polishing.
The better oxidizing and etching ability of H2O2-C4H10O2-Na2S2O5 slurry leads to a higher MRR in AFP for hard
disk substrates.

Keywords: abrasive-free polishing; material removal rate; initiator; hard disk substrate

1 Introduction head-disk spacing has become narrower. It is believed

that a flying height as small as 2.5–3.5 nm is necessary
Chemical mechanical polishing (CMP) has been used for an areal recording density of 1 Tbit/in2, which is
as a global planarization technique since the 1990s, the next target in industry [3]. In this case, there is a
and it is currently widely used in the manufacture need to satisfy significantly higher requirements for
of ultra-precision surfaces, such as Si wafers and the the machining accuracy and the surface quality of
Damascus interconnection structures of Cu in integrated the heads and hard disk substrates. As a result, the
circuits, computer disks and heads, etc. [1, 2]. Due to CMP process for hard disk substrates should also be
the rapid development of information technology, the improved.
amount of information that is transmitted has also In the traditional CMP process, there are chemical
exponentially increased, leading to much higher storage reactions between the slurry and the material surface
requirements for computer disk systems, which is to be polished, which form a layer of oxidation film.
an important aspect of information processing. For Then, with the mechanical wear of the abrasive in the
example, the physical size of hard drives continues to slurry and the polishing pad, the oxidation film will
decrease, while the memory capacity of computer be removed, and the above behavior will happen
hard drives has increased rapidly, meaning that the again and again [4, 5]. Due to the combined chemical
and mechanical processes, the material surface can be
* Corresponding author: Hong LEI. planarized. However, this leads to depression, erosion,
E-mail: hong_lei2005@163.com and mechanical damage because of the abrasives [6−9].
360 Friction 1(4): 359–366 (2013)

Further, these damages are difficult to remove using agent added into deionizer (DI) water to obtain the H2O2
the CMP technique, resulting in faulty products. For slurry. 0.7 wt%–4.2 wt% (CH3)3COOH-Na2S2O5 was used
the above reasons, an alternative process is to reduce or as the initiator to obtain H2O2-C4H10O2-Na2S2O5 slurry.
eliminate the abrasives in the slurry. Compared with The molar ratio of (CH3)3COOH: Na2S2O5 was 1:1.
the CMP process, in the abrasive-free polishing (AFP)
process, the oxidation film is removed by the soft 2.2 Abrasive-free polishing tests
polishing pad without the abrasives. Therefore, it will
Polishing tests were conducted using a UNIPOL-1502
not cause defects in the AFP process. In addition, there
polishing equipment (Shenyang Kejing instrument,
is a simpler post-clean process for materials polished
Co. LTD, China). The down force was 0.80 psi and the
by AFP [10], which enables significant reductions in
plate rotating speed was 80 rpm. The polishing time
the cost of the product.
was 30 min. Work pieces were φ95 mm × 1.25 mm
In 2000, a Japanese researcher [10] developed a aluminum alloy disk substrates that were NiP plated;
completely abrasive-free process for Cu damascene the plated layer consists of about 85 wt% nickel and
metallization, which provided a very clean, scratch- 15 wt% phosphorus elements. The polishing pad was
free, and anticorrosive surface. Since then, AFP has a Rodel porous polyurethane pad. The supply rate of
attracted the interest of many researchers, and it has the slurry was 160 mL/min. After polishing, the hard
been successfully applied to the surface planarization disk substrates were washed by ultrasonic cleaning in
of materials such as Al metal film [11], GaN [12], Si, a cleaning solution containing 0.5 wt% surfactant in DI
and SiC [13]. To the best of our knowledge, there have water. Finally, they were dried by a multifunctional
been few reports of surface machining of hard disk drying system. The mass of the hard disk substrate
substrates that use the AFP process. was measured by an analytical balance both before
Oxidizers play a key role in the AFP process. Up to and after the AFP process.
the present, due to its very strong oxidation capacity,
and the fact that decomposed product is water (which 2.3 Observation of the surface morphology of the
is environmentally friendly), H2O2 has been chosen substrates
as the preferred oxidizer for use in the AFP process.
However, the decomposition energy of H2O2 is The surface roughness is a parameter that is most
54 kcal/mol, which causes it to decompose too slowly commonly used to evaluate the quality of a surface.
at room temperature to be completely effective. In our A white light interferometer (NanoMap WLI, Caep
previous studies, the use of Cu (Ⅱ) [14] or potassium Technology Corp., U.S.A) with a solution of 0.1 Å
was used to measure the average roughness (Ra) and
peroxydisulfate-sodium hydrogensulfite (K2S2O8-
morphology of the polished substrate surface. The
NaHSO3) as catalyzers [15] in the H2O2 slurry promoted
measurement area was 100 μm × 100 μm.
the decomposition of H2O2, and caused it to exhibit a
much higher material removal rate (MRR); a better 2.4 Electrochemical measurement of the substrates
substrate surface was obtained compared to that of
the H2O2 slurry under the same conditions after All of the electrochemical experiments were carried
the AFP process. In this paper, we investigated the out using a Solartron electrochemical workstation in
effect of tert-butyl hydroperoxide-sodium pyrosulfite a conventional three electrode system. The electrolyte
((CH3)3COOH-Na2S2O5) as the free radical initiator solutions were prepared from the slurries. One of the
system for H2O2 slurry on the hard disk substrate AFP. slurries was the H2O2 slurry, and the other was the
H2O2 slurry containing 3.5 wt% of initiator. The round
hard disk sheet sealed with epoxy was used as the
2 Experiment
working electrode, and its diameter was 10 mm. The
2.1 Preparation of the abrasive-free slurry counter electrode was a Pt electrode and the reference
electrode was a saturated calomel electrode (SCE). The
A series of abrasive-free slurries was prepared, and we potentiodynamic polarization plots were acquired by
used 5 wt% H2O2 as the oxidant and 6 wt% dispersing scanning the working electrode in the potential range
Friction 1(4): 359–366 (2013) 361

from −0.60 V to +0.20 V at a scan rate of 1 mV/s, and the Figure 1 clearly shows that as the concentration of
electrochemical impedance spectroscopy (EIS) spectrum the C4H10O2-Na2S2O5 initiator increases, the MRR
was acquired in the frequency range of 106 Hz to 0.1 Hz increases gradually, but it begins to decrease when
with a potential amplitude of 10 mV rms. the initiator’s concentration is above 3.5 wt%. The
maximum MRR (H2O2 slurry containing 3.5 wt% of
2.5 Electron spin-resonance spectroscopy (EPR) test
C4H10O2-Na2S2O5 initiator) is almost 5 times that of
of the slurries
the minimum MRR (H2O2 slurry), which indicates
The concentration of free radicals in the H2O2 slurry that the C4H10O2-Na2S2O5 initiator enhances the MRR
and the H2O2 slurry containing 3.5 wt% of initiator of the hard disk substrate in the H2O2-baesd slurry at
was measured by EMX EPR (Center field: 3518.07 G, the same polishing condition. It may be inferred that
scan width: 100 G, microwave power: 20 mW, scan the chemical effect of the slurry is improved
time: 5.24 s, scanning frequency: 6 times). Dimethyl dramatically by the C4H10O2-Na2S2O5 initiator.
In addition, the maximum MRR at 3.5 wt% of the
pyridine-N-oxide (DMPO) was added as trapping
initiator may be due to the moderate ratio of the
agent.
oxidant to the initiator, while the amount of initiator
2.6 Element examination of hard disk substrate is 3.5 wt% and the oxidant is 5 wt%. This means that
surface there is an ideal rate of oxidant decomposition due
to free radicals promoted by the initiator. In this
To study the chemical reaction between the slurry and situation, the oxidation reaction rate of the free radical
the disk substrate, we conducted a static immersion is much higher than its quenching rate, so the disk
test of the disk substrate in the slurries. One of the substrate has a maximal MRR. However, when the
slurries was the H2O2 slurry, and the other was the initiator concentration increases to 4.2 wt%, the de-
H2O2 slurry containing 3.5 wt% of initiator. After 48 h composition of the oxidant promoted by the initiator
immersion at room temperature, the disk substrates is too quick, and the quenching rate of the free radical
were cleaned and dried. Then, the contents of the may be higher than the oxidation reaction rate, so the
elements and their deep distribution in the polished disk substrate has a smaller MRR.
surfaces were analyzed using PHI 680-Auger electron Further, 3D images of the surface morphology of
spectroscopy (AES, beam voltage: 5 kV, beam current: the disk substrates are shown in Fig. 2, which clearly
10 nA, Ar ion beam: 2 kV 1 mm × 1 mm, sputter rate shows that the protuberance on the non-polished
(relative to SiO2) is 6 nm/min). substrate surface can be partly removed after polishing
with the H2O2 slurry. However, it still failed to achieve
an adequate global flat surface (Fig. 2(b)). However,
3 Results and Discussion

3.1 AFP performances of the slurries

MRR can be calculated as in Eq. (1).

4  10 9 m
MRR  (1)
d 2 t 

where: MRR- material removal rate, nm/min

m- mass of material removed, g
d- diameter of work piece, mm
t- polishing time, min (in the test, t = 30)
ρ- density, g/cm3 (plated NiP layer consisting of
about 85 wt% nickel and 15 wt% phosphorus elements, Fig. 1 The MRR of the disk substrates polished with the H2O2
ρ = 7.9) slurry added with different weights of the C4H10O2-Na2S2O5 initiator.
362 Friction 1(4): 359–366 (2013)

half of that of the non-polished surface (Ra = 33.4 nm).

In other words, the H2O2-C4H10O2-Na2S2O5 slurry can
obviously improve the surface planarization of hard
disk substrates.

3.2 Mechanism discussion

3.2.1 Electrochemical analysis of hard disk substrate

The electrochemical test can be carried out to investigate

the corrosion action of the hard disk substrate in AFP
slurry [16−18]. It can reflect the kinetic action and
thermodynamic action of the metal dissolution and
the surface film formation of metal in the AFP slurry,
which can be used to predict the MRR. Herein, the
potentiodynamic polarization plots were used to
analyze the corrosion of the hard disk substrate in
the slurries, as shown in Fig. 3. The corrosion current
(Icorr) values are estimated from these plots using the
Tafel extrapolation method.
In particular, by comparing the behavior of the
anodic polarization curve between the H2O2 slurry and
the H2O2-C4H10O2-Na2S2O5 slurry, the different slopes of
the anodic polarization curve can be observed. In the
H2O2-C4H10O2-Na2S2O5 slurry, the slope of the anodic
polarization is decreased. This indicates that the
corrosion reaction of the oxide film on the substrate
surface occurs easily in the H2O2-C4H10O2-Na2S2O5
slurry. Besides, The Icorr of the hard disk in the H2O2-
C4H10O2-Na2S2O5 slurry is 0.684 mA, which is much
larger than that in the H2O2 slurry (Icorr = 0.201 mA).
The increase of the corrosion current indicates that

Fig. 2 3D images of the disk substrate surfaces: (a) before

polishing, Ra = 33.4 nm; (b) polished with the H2O2 slurry, Ra =
25.6 nm; (c) polished with the H2O2-C4H10O2-Na2S2O5 slurry, Ra =
17.8 nm.

it’s the global flat surface (Fig. 2(c)) after polishing with
the H2O2-C4H10O2-Na2S2O5 slurry and the nano-asperity
peaks are hardly observed. Besides, the H2O2-C4H10O2- Fig. 3 Potentiodynamic polarization plots of hard disk substrates
Na2S2O5 slurry has a Ra of 17.8 nm, which is nearly in the H2O2 slurry and the H2O2-C4H10O2-Na2S2O5 slurry.
Friction 1(4): 359–366 (2013) 363

the C4H10O2-Na2S2O5 initiator promotes the corrosion 3.2.2 Comparison of the oxidizing ability of the slurries
of the disk surface in the kinetics. In this case, the hard
It is known that the oxidation performance of H2O2
disk substrate in the H2O2-C4H10O2-Na2S2O5 slurry
solution is very high because H2O2 can be decomposed
can be etched faster than that with the H2O2 slurry.
into hydroxyl free radicals, which shows a stronger
Therefore, the enhancement of the electrochemical
oxidizing ability. It may be inferred that the solution’s
corrosion can help to increase the MRR of the hard disk
oxidizing ability can be determined by the con-
substrate in AFP processes with the H2O2-C4H10O2-
centration of free radicals in it. To investigate the free
Na2S2O5 slurry.
radical’s change caused by the C4H10O2-Na2S2O5
In the previous CMP studies, He et al. [19] analyzed
initiator, we conducted EPR tests of the two slurries,
the oxide film thickness of Cu surfaces in different
as shown in Fig. 5.
media by using the impedance spectra, and reported
that the magnitude of the impedance can be used to There are four obvious spectral peaks in the EPR
characterize the oxide film formed on the Cu surface. spectrum of H2O2 slurry (Fig. 5(a)), and the relative
Here, the impedance spectra were used to characterize intensity of the peaks is 1:2:2:1. This indicates a typical
the oxide film of the hard disk substrate in the slurries. EPR spectrum of hydroxyl free radical [20]. The EPR
Fig. 4 shows the impedance spectra of the hard disk spectrum of the H2O2-C4H10O2-Na2S2O5 slurry is shown
substrate in the H2O2-C4H10O2-Na2S2O5 slurry (Fig. 4(b)) in Fig. 5(b), and shows that the four spectral peaks of
and the H2O2 slurry (Fig. 4(a)), respectively. All of the hydroxyl free radical in Fig. 5(a) also exist in the spectra
spectra show the same pattern, i.e., two linked of Fig. 5(b). In addition, the peak intensity of these
depressed semicircle. A comparison of the two spectra four spectral peaks of the spectrum in Fig. 5(b) is even
shows that the magnitude of the impedance at low 150 times that shown in Fig. 5(a). It can be concluded
frequencies is significantly smaller for the surface on that the concentration of hydroxyl free radicals in the
a hard disk substrate in the H2O2-C4H10O2-Na2S2O5 H2O2-C4H10O2-Na2S2O5 slurry is much larger than that
slurry. This indicates that the polarization resistance in the H2O2 slurry. The reason may be that the C4H10O2-
of the surface is effectively decreasing. Accordingly, it Na2S2O5 initiator can strongly induce the generation
can be inferred that the oxide film formed on the of the hydroxyl free radical (as seen in Eq. (2), Eq. (4),
substrate surface in the H2O2-C4H10O2-Na2S2O5 slurry and Eq. (5)). In Fig. 5(b), it is also noted that there are
may be very loose, and can be removed easily and also three small spectral peaks with an intensity of
quickly by the polishing pad in AFP. Consequently, the about 2.5 × 105, which may be alkoxy free radicals (as in
MRR of the hard disk substrate in the AFP process Eq. (2)). The possible reaction is shown as Eqs. (2)–(5).
with the H2O2-C4H10O2-Na2S2O5 slurry becomes large, The C4H10O2-Na2S2O5 initiator in the H2O2 slurry may
which is consistent with the results of polishing tests. greatly enhance the oxidation ability of the slurry since

Fig. 4 The EIS spectra of hard disk substrate in the H2O2 slurry (a) and the H2O2-C4H10O2-Na2S2O5 slurry (b).
364 Friction 1(4): 359–366 (2013)

Fig. 5 EPR analysis of two slurries: (a) the H2O2 slurry; (b) the H2O2-C4H10O2-Na2S2O5 slurry.

it can strongly induce the generation of the hydroxyl the atomic concentrations of the elements Ni and P
free radical and the alkoxy free radical. present a similar increasing trend, that is, it first
increases and then fluctuates slightly as the sputter
ROOH  RO· + ·OH (2) time increases. At the same time, the atomic con-
centrations of the elements O and C present a similar
HO–OH  HO· + ·OH (3)
decreasing trend and are then stabilized. In addition,
4ROOH + S2O52–  from the spectra of the two slurries before sputtering,
we found that the atomic concentration of the elements
4RO· + 2SO42– + 2H+ + H2O (R: (CH3)3C–) (4)
Ni, P, and O were 15 wt%–40 wt%, 0 wt%–10 wt%, and
RO·+ HO–OH  HO· + RO–OH (5) 20 wt%, respectively. In comparison with that consisting
of 85 wt% Ni and 15 wt% P for the plated NiP substrate,
Next, AES was conducted to observe the element the introduction of the element O and the reduction
components and their deep distribution in the surface in the atomic concentration of Ni and P imply that an
of hard disk substrate. Figure 6 shows the element oxidization reaction may occur between the substrate
components in the surface of the hard disk substrate and the two slurries.
after the immersion test in the H2O2 slurry (Fig. 6(a)) Further, the thickness of the oxide film formed on
and the H2O2-C4H10O2-Na2S2O5 slurry (Fig. 6(b)). A the substrate surface in the slurries can be estimated
comparison of the two slurries’ spectra shows that by the sputter time taken to remove O. The sputter

Fig. 6 AES analysis of the hard disk substrate soaked in (a) the H2O2 slurry and (b) the H2O2-C4H10O2-Na2S2O5 slurry.
Friction 1(4): 359–366 (2013) 365

time for the removal of O is 0.19 min and 1.07 min for Open Access: This article is distributed under the terms
the H2O2 slurry and the H2O2-C4H10O2-Na2S2O5 slurry, of the Creative Commons Attribution License which
respectively. The sputter rate (relative to SiO2) is permits any use, distribution, and reproduction in any
6 nm/min. Therefore, we can estimated that the medium, provided the original author(s) and source
thickness of the oxide film formed in the H2O2 slurry are credited.
and the H2O2-C4H10O2-Na2S2O5 slurry is about 1.14 nm
and 6.42 nm, respectively. This means that the oxidizing
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Weitao ZHANG. He received his China. Now, he is a Master candidate in the Research
Bachelor degree in material chemi- Center of Nano-Science and Nano-Technology,
stry in 2008 from Harbin Institute Shanghai University. His research interest is chemical
of Technology University, Harbin, mechanical polishing.

Hong LEI. He received his MS and University from 2003. His current position is a
PhD degrees in applied chemistry professor in the Research Center of Nano-Science and
from Huazhong University of Science Nano-Technology, Shanghai University. His research
and Technology in 1996 and 2001, interests include functional abrasives, CMP slurry,
respectively. He joined Shanghai and post-CMP cleaning.
Friction 1(4): 367 (2013)
DOI 10.1007/s40544-013-0036-9 ISSN 2223-7690
ERRATUM

Erratum to: Green tribology: Fundamentals and future

development
Si-wei ZHANG*
School of Mechanical and Storage and Transportation Engineering, China University of Petroleum, 20 Xueyuan Rd, PO Box 902, Beijing
100083, China
Received: 30 October 2012 / Revised: 23 Jaruary 2013 / Accepted: 20 May 2013

© The author(s) 2013. This article is published with open access at

Springerlink.com

Erratum to
Friction 1(2): 186–194 (2013)
DOI 10.1007/s40544-013-0012-4

The original version of this article unfortunately

contained three incorrect author lists. In the reference
list on page 193, instead of

[7] Nasonovsky M, Bhushan B. Green tribology. Phil Trans R

Soc A 368: 4675–4676 (2010)
[8] Nasonovsky M, Bhushan B. Towards the “green tribology”:
Biomimetic surfaces, biodegradable lubrication, and renewable
energy. In Proceedings of STLE/ASME International Joint
Tribology Conference, San Francisco, 2010.
[11] Nasonovsky M, Bhushan B (Eds). Green Tribology:
Biomimetics, Energy Conservation and Sustainability. Berlin:
Spring, 2012.

It should read

[7] Nosonovsky M, Bhushan B. Green tribology. Phil Trans R

Soc A 368: 4675–4676 (2010)
[8] Nosonovsky M, Bhushan B. Towards the “green tribology”:
Biomimetic surfaces, biodegradable lubrication, and renewable
energy. In Proceedings of STLE/ASME International Joint
Tribology Conference, San Francisco, 2010.
[11] Nosonovsky M, Bhushan B (Eds). Green Tribology:
Biomimetics, Energy Conservation and Sustainability. Berlin:
Spring, 2012.

The online version of the original article can be found at

http://dx.doi.org/10.1007/s40544-013-0012-4

* Corresponding author: Si-wei ZHANG.

E-mail: swzhang99@sina.com
Friction
ISSN 2223-7690 Friction
ISSN 2223-7690
Vol. 1, 2013

Total Contents

Vol. 1, No. 1

Review

Hydration lubrication / 1–23

Jacob KLEIN

Energy dissipation in atomic-scale friction / 24–40

Yuan-zhong HU, Tian-bao MA, Hui WANG

Method of reduction of dimensionality in contact and friction mechanics: A linkage between micro and
macro scales / 41–62
Valentin L. POPOV

Research Article

Frictional behavior of nanostructured carbon films / 63–71

Dongfeng DIAO, Chao WANG, Xue FAN

Adhesive and corrosive wear at microscales in different vapor environments / 72–80

Sihan SHEN, Yonggang MENG

Running-in process of Si–SiOx /SiO2 pair at nanoscale—Sharp drops in friction and wear rate during
initial cycles / 81–91
Lei CHEN, Seong H. KIM, Xiaodong WANG, Linmao QIAN

Spatial evolution of friction of a textured wafer surface / 92–97

Huaping XIAO, Ke WANG, Grant FOX, Michel BELIN, Julien FONTAINE, Hong LIANG

Vol. 1, No. 2
Guest editorial: Special issue on bio-tribology / 99
Zhongmin JIN, Ming ZHOU

Review

Bio-friction / 100–113
Zhongmin JIN, Duncan DOWSON
Friction ISSN 2223-7690

Recent advances in gecko adhesion and friction mechanisms and the development of gecko-inspired
dry adhesive surfaces / 114–129
Ming ZHOU, Noshir PESIKA, Hongbo ZENG, Yu TIAN, Jacob ISRAELACHVILI

Skin tribology: Science friction? / 130–142

E. VAN DER HEIDE, X. ZENG, M.A. MASEN

Research Article

Use of opposite frictional forces by animals to increase their attachment reliability during
movement / 143–149
Zhouyi WANG, Yi SONG, Zhendong DAI

Influence of synovia constituents on tribological behaviors of articular cartilage / 150–162

Teruo MURAKAMI, Seido YARIMITSU, Kazuhiro NAKASHIMA, Yoshinori SAWAE, Nobuo SAKAI

Potential hydrodynamic origin of frictional transients in sliding mesothelial tissues / 163–177

Stephen H. LORING, James P. BUTLER

Damage due to rolling in total knee replacement—The influence of tractive force / 178–185
Markus A. WIMMER, Lars BIRKEN, Kay SELLENSCHLOH, Erich SCHNEIDER

Short Communication

Green tribology: Fundamentals and future development / 186–194

Si-wei ZHANG

Vol. 1, No. 3

Review

Modeling of surface texturing in hydrodynamic lubrication / 195–209

Izhak ETSION

The friction of diamond-like carbon coatings in a water environment / 210–221

D. C. SUTTON, G. LIMBERT, D. STEWART, R. J. K. WOOD

Research Article

Lithium-based ionic liquids as novel lubricant additives for multiply alkylated cyclopentanes
(MACs) / 222–231
Zenghong SONG, Yongmin LIANG, Mingjin FAN, Feng ZHOU, Weimin LIU

Static/dynamic friction and wear of some selected polymeric materials for conformal tribo-pairs
under boundary lubrication conditions / 232–241
Daniel NILSSON, Braham PRAKASH
ISSN 2223-7690 Friction

Atomistic simulations of friction at an ice−ice interface / 242–251

N. SAMADASHVILI, B. REISCHL, T. HYNNINEN, T. ALA-NISSILÄ, A. S. FOSTER

Mechanism of friction reduction of unsaturated fatty acids as additives in diesel fuels / 252–258
Jean Michel MARTIN, Christine MATTA, Maria-Isabel De Barros BOUCHET, Cyrielle FOREST,
Thierry Le MOGNE, Thomas DUBOIS, Michael MAZARIN

Triboelectric behaviors of materials under high speeds and large currents / 259–270
Yongzhen ZHANG, Zhenghai YANG, Kexing Song, Xianjuan PANG, Bao SHANGGUAN

Effect of dicarboxylic acid esters on the lubricity of aviation kerosene for use in CI engines / 271–278
G. ANASTOPOULOS, S. KALLIGEROS, P. SCHINAS, F. ZANNIKOS

Vol. 1, No. 4

Review

Scratch formation and its mechanism in chemical mechanical planarization (CMP) / 279–305
Tae-Young KWON, Manivannan RAMACHANDRAN, Jin-Goo PARK

Chemical mechanical polishing: Theory and experiment / 306–326

Dewen ZHAO, Xinchun LU

Research Article

Y2O3 nanosheets as slurry abrasives for chemical-mechanical planarization of copper / 327–332

Xingliang HE, Yunyun CHEN, Huijia ZHAO, Haoming SUN, Xinchun LU, Hong LIANG

Towards a unified classification of wear / 333–340

Michael VARENBERG

Mechanical and tribological properties of epoxy matrix composites modified with microencapsulated
mixture of wax lubricant and multi-walled carbon nanotubes / 341–349
Nay Win KHUN, He ZHANG, Jinglei YANG, Erjia LIU

Hydrophobic, mechanical, and tribological properties of fluorine incorporated hydrogenated

fullerene-like carbon films / 350–358
Li QIANG, Bin ZHANG, Kaixiong GAO, Zhenbin GONG, Junyan ZHANG

Abrasive-free polishing of hard disk substrate with H2O2-C4H10O2-Na2S2O5 slurry / 359–366

Weitao ZHANG, Hong LEI

Erratum

Erratum to: Green tribology: Fundamentals and future development / 367