Journal of The Electrochemical Society, 158 (4) H401-H404 (2011)
0013-4651/2011/158(4)/H401/4/$28.00 The Electrochemical Society
H401
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Surface Shaping Using Chemical Mechanical Polishing
Purushottam Kumar,*,z Seung-Young Son, Aniruddh Khanna, Jaeseok Lee, and
Rajiv K. Singh**
Materials Science and Engineering, University of Florida, 100 Rhines Hall, Gainesville, Florida 32611, USA
In this study we have investigated the phenomena of edge rounding in chemical mechanical polishing (CMP) of patterned surfaces
for its potential for creation of different topographies. Hexagonal array of SiO2 cylindrical pillars with 20 mm diameter, 1 mm spac-
ing and ꢀ1 mm height was polished under different conditions to form rounded surfaces. The effect of CMP variables: pad and
pressure were studied by polishing these patterned wafers with Politex and IC1000/Suba IV stacked pad at different pressures.
CMP with soft Politex pad led to formation of spherical curvature with radius of curvature dependent on pressure and polishing du-
ration. Radius of curvature as small as 300 mm was obtained at 2.5 psi after polishing for 2 min using Politex pad. The surface evo-
lution dynamics during polishing has been discussed based on contact mechanical model for CMP.
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2011 The Electrochemical Society. [DOI: 10.1149/1.3547710] All rights reserved.
Manuscript submitted August 18, 2010; revised manuscript received December 13, 2010. Published February 23, 2011.
In the last couple of decades, chemical mechanical polishing
(CMP) has grown from a glass polishing technology to a standard IC
fabrication technique. Chemical mechanical polishing ensured the
miniaturization of integrated circuits by providing an appropriate cop-
per removal technique and also providing flatter wafer surfaces for
next generation lithographic tools. CMP is used in both front-end and
back-end processes, in shallow trench isolation (STI), interlevel
dielectric planarization, local tungsten interconnects, and copper dam-
ascene.1 It is finding application in wafer planarization of nonsilicon
semiconductor materials, e.g., wide bandgap semiconductors like SiC,
GaN for providing damage free substrates.2–4 Research and develop-
ment in CMP has almost totally been toward achieving a better local
and global wafer planarity, lower defectivity, and damage-free surface,
which are the fundamental needs of the semiconductor industry.
Owing to this, chemical mechanical polishing has become synony-
mous with chemical mechanical planarization. Nonplanarizing phe-
nomenon, such as dishing and edge rounding, are categorized as
defects and efforts have been made to reduce or eliminate these
defects. In our recent study, we reported the use of CMP for fabrica-
tion of microlens arrays.5 Microlenses are used in numerous applica-
tions in optical and optoelectronic systems, e.g., in detector and sensor
arrays, optical fiber interconnects, switches, amplifiers, isolators, mul-
tiplexers, attenuators, and imaging flat panel display systems and pho-
tocopiers.6 This inherent nonplanarizing nature has the possibility of
creating curved surfaces, which would open a new avenue for chemi-
cal mechanical polishing, that of a surface shaping process.
The material removal rate during CMP depends on the applied
pressure, linear velocity, the characteristic of the polishing medium
(pad and slurry) and the wafer material. Among these, applied pressure
and properties of the pad are the parameters which affect the contact
pressure during CMP. Material removal at any location in the wafer is
directly proportional to the contact pressure it experiences.7 Contact
pressure is uniform for a featureless flat wafer, whereas for a wafer
with high and low elevation features it varies along the wafer.8–12 Pol-
ishing pad and wafer when brought together under an applied pressure
leads to deformation of the pad along the wafer features. The local de-
formation of the pad determines the local contact pressure and hence
the local removal rate. It has been observed that the initial removal
rate at the edges/corners of a step feature during chemical mechanical
polishing is high leading to rounding of edges. Patrick et al. observed
the polishing rate of a step feature (rate of height reduction) to progres-
sively decrease during a CMP process.8 The decrease in polishing rate
was more prominent at the edge of the step and reduced toward the
center. It can be inferred that as the corners were progressively
rounded, the pressure at the edges decreased, leading to decrease in
the polishing rate. The contact pressure was dynamic and changed
continuously during polishing with change in the topography of the
surface. Chekina et al.9 and Saxena et al.,10 based on two-dimensional
contact mechanical model for CMP of a step feature, predicted a
reduction in variation in contact pressure with the progression of
CMP, until a steady state was reached. The surface shape reached
equilibrium in their model where further polishing did not change the
surface topography.
It is generally agreed that the above phenomena is due to the
conformal nature of the polishing pad, and that softer pads lead to
more edge rounding than harder pads.8,9 In literature, various other
reasons have been attributed to higher polish rate at the edge of a
step feature. Runnels, on his flow based feature scale model rea-
soned the rounding to occur because of high stress generated at the
corners by the flowing slurry.11 Patrick et al. attributed it to the pres-
sure enhancement at the leading edges of the step feature due to the
relative motion of the wafer and pad.8 Chekina et al. based on con-
tact mechanics model suggested that the high initial contact pressure
at the corners due to bending of the pad causes edge rounding.9 In
this work we have studied surface shaping using edge rounding phe-
nomena and the effect of CMP variables on the dynamics of surface
evolution. Nonplanarizing aspects of CMP, steady state as well as
nonsteady state, can be used to shape surfaces with applications in
optoelectronics, tribology, etc.
Experimental
A micron thick plasma enhanced chemical vapor deposition
(PECVD) silicon oxide was deposited on 1 in.2 coupons of soda
lime glass. Silicon dioxide was deposited by flowing nitrous oxide
(N2O) and silane (2% SiH4/N2) gases at a flow rate of 1420 and 400
sccm, respectively. The glass coupons were heated to 300ꢁC while
the chamber pressure was maintained around 550 mTorr. Power was
kept at 60 W at a frequency of 187 kHz for formation of low stress
silicon oxide. The SiO2 layer was patterned using Microposit S1813
photoresist and standard UV photolithography using a Karl Suss
MA6 Aligner with a 365 nm Hg i-line radiation. The pattern con-
sisted of hexagonal arrangement of circular dots of 20 mm diameter
and 1 mm spacing. The samples were etched in 15 sccm SF6 and
5 sccm Ar in a Uniaxis Shuttlelock RIE-ICP reactor under 100 W
DC and 600 W RF power to form cylindrical pillar-type patterns.
Struers Co. TegraPol-35 with TegraForce-5 tabletop polisher was
used for polishing the samples at a down pressure of 2.5, 3.6, 5.4,
and 8.2 psi. The platen and head were rotating in the same direction
at 70 and 30 rpm, respectively. The relative linear velocity between
the pad and the wafer was ꢀ0.7 m/s. Two types of pads: Politex
(soft) pad and IC1000/Suba IV (hard) stacked pad supplied by Rodel
Inc. were used for polishing. SiO2 slurry with 5 wt. % loading and
pH 4 made from dilution of Levasil 50cK with nominal particle size
of 80 nm was used for polishing. Slurry flow rate of 50 mL/min was
used during CMP. The surface was characterized using a Veeco
Dimension 3100 atomic force microscopy using contact mode.
*
**
Electrochemical Society Active Member.
Electrochemical Society Fellow.
z E-mail: purukr@ufl.edu