In Situ Measurements of the Coefficient of Friction and Drag
Forces during Chemical Mechanical Planarization
*,
James Vlahakis Caprice Gray*, Chris Barns+, Moinpour Monsour+, Sriram Anjur`, Ara
Philliposian^, Vincent Manno*, and Chris Rogers*
*
Tufts University, Medford, MA
Intel Corporation, Santa Clara, CA
`Cabot Microelectronics, Aurora, IL
^University of Arizona, Tucson, AZ
+
Principal Contact:
James Vlahakis, Tufts University
Email: James.Vlahakis@tufts.edu
Abstract
The semiconductor industry utilizes chemical mechanical planarization (CMP) as an essential
component in wafer processing. We employ a new technique for determining the friction forces
that develop during CMP using an AMTI MC12 force platform sensor and custom LabView
software, allowing in situ measurements of interfacial drag forces and the coefficient of friction
(COF). Spectrum analysis of the drag forces and coefficient of friction reveal interesting features
that vary depending upon rotational speed, pad type, and process downforce. The essential
elements of CMP are the polishing pad, the wafer, and the slurry. The friction measurements
result from the interaction of all three elements, both through direct loading and shear loading.
Introduction
The semiconductor industry utilizes chemical mechanical planarization (CMP) as an essential
component in wafer processing. As microelectronic feature sizes continue to shrink, the ability to
planarize various substrates over wide ranging length scales becomes an essential element of the
manufacturing process. In order to gain some insight and understanding into the material
removal mechanisms and planarization performance, we examine the forces generated during
CMP. These interfacial shear forces have a direct influence on material removal rates,
temperature variations, and failure modes. Consequently, the investigation of these forces as a
function of these, and other, parameters has been an area of keen interest to researchers in the last
few years.
Shear force measurements present several advantages. Primarily, these interfacial forces are
measured in situ, rather than through indirect means. This gives a clearer picture of the
mechanisms involved. Our technique allows us to sample the forces at sampling frequencies of
2kHz. Most previous studies examined time-averaged, mean shear force. These time averaging
techniques do not capture the interfacial forces generated by stick/slip phenomena and other
vibrational modes. In addition, the instantaneous, in situ shear force measurements allow us to
determine the instantaneous, in situ coefficient of friction (COF).
We describe a technique for measuring interfacial forces and determining the COF. In addition,
we explore how variations in slurry flow, process downforce, and polishing pad effect the data.
Our technique utilizes an AMTI 3 axis force platform, a Struers ½ scale laboratory polisher, and
custom LabView software.
Experimental Overview
Various polishing parameters are adjusted to account for laboratory research scaling and the
optical measurements required for our concurrent DELIF experiments.[1], [2] It is necessary to
replace the traditional opaque silicon wafer with a transparent BK-7 glass wafer to enable our
DELIF measurements. The slurry used during conditioning is a 9:1 water dilution of Cabot
Microelectronics’ Cab-O-Sperse SC-1 3.1 wt% abrasive. The dilution of the slurry is necessary
to maintain a consistent shape of the BK-7 wafer during the polishing run. A magnetic stir bar
constantly stirs the slurry in order to discourage aggregation of the silica nano-particles and a
peristaltic pump delivers the slurry to the pad at a rate of 50 cc per minute. The laboratory CMP
equipment is scaled down from an industrial CMP system by a factor of 2. A summary of these
adjustments are listed in Table 1.
Table 1. Industrial to Laboratory Scaling Adjustments.
Adjustment
Parameter Held Constant
Wafer Diameter
(Pad area)/ (Wafer area)
Pad and Wafer
(Relative Industrial Linear Velocity)/
Rotation Velocity
(Relative Laboratory Linear Velocity)
Slurry Flow Rate
Flow rate/[(Pad Area)*(wafer
position)*(pad rotation velocity)]
Lab Scale Dimensions
3”
60 RPM
50cc/min
AMTI Force Platform [2]
The force platform measures the forces on the surface of the platform and decomposes the
loading into orthogonal component forces and moments – Fx, Fy, Fz, Mx, My, and Mz. A right
handed coordinate system is utilized with the positive z-axis pointing downwards. The platform
surface measures 12” x 12” consists of 7075-T6 aluminum and is equipped with numerous
threaded inserts to facilitate attachment to various experimental set-ups.
The transducer incorporates strain gages and four precision elements in a patented design to
isolate and measure applied forces and moments. Multiple channel bridge excitation and signal
amplification are provided by an AMTI amplifier. We utilize our own custom LabView software
to control the force platform/amplifier settings and collect data. See Figure 1.
Figure 1 – Force Platform
CMP Polisher
We employ a Struers RotoPol 31 as our table top polisher. It rests directly on top of the force
platform and allows real time measurements of the forces of the wafer and pad interacting,
whether in the Z (applied pressure) or the X and Y directions (friction due to polishing). The
polisher's platen is fitted with 12 inch diameter polishing pads. Again, our experiments require a
3 inch by ½ inch thick BK-7 glass wafer in place of the industry standard silicon wafer. The
wafer's applied pressure and rotation are controlled by an aluminum shaft pressing the wafer into
the pad. The shaft is held in place by two linear bearings that allow the shaft to rotate as well as
slide to apply pressure. A 1/2 hp Dayton motor rotates the shaft, and a series of weights can be
utilized to vary the process downforce. The shaft and motor are held above the polisher by a
custom 80/20 extruded aluminum support frame. The entire structure is mounted on a 136kg
solid steel isolation table. Figure 2 summarizes the experimental setup.
Motor
Wafer
Platen
Steel Table
RotoPol-31
Force Table
Figure 2 - Experimental setup
The experimental work presented here builds on CMP research conducted at Tufts since 1995. If
the reader desires a more detailed description of the experimental set-up, please refer to [3]
In the experiments under consideration, we varied several parameters;
1. Downforces - 2.5psi (low Fz) and 5psi (high Fz)
2. Polishing speeds - 30rpm and 60rpm
3. Polishing pads – an ungrooved FX9, an FXP with circular grooves and an IC1000 with
xy grooves.
4. In addition, the IC1000 was tested for both normal and low slurry flow rate conditions
Each pad was conditioning for at least 20 minutes before data logging. We sampled forces and
moments at a rate of 2kHz. In addition, a 500Hz Bessel filter was employed to reduce aliasing.
The standard slurry flow rate was maintained at 50cc/min. In the low slurry flow rate case, we set
the rate to 25cc/min.
Results and Discussion
Data set summary;




Data Set 1 – ungrooved Freudenberg FX9 pad
Data Set 2 – circular grooved Freudenberg FX9 pad
Data Set 3 – xy grooved Rodell IC1000 pad
Data Set 4 – xy grooved Rodell IC1000 pad at a lower slurry flow rate
Overall, the COF values are in agreement with previous data collected here at Tufts.[4]. The
ungrooved FX9 exhibits much higher coefficient of friction (COF) values, and much higher
standard deviations, than the grooved pads. In fact, even though the average COF values are
~0.8, instantaneous COF values can range has high as 3.0 and as low as 0.0. These widely
disparate values indicate different lubrication regimes – lower values are clearly hydrodynamic,
while higher values could be interpreted as a mixed or boundary lubrication regime.
The grooved pads at the standard slurry flow rate exhibit some common features, the most
prominent being much smaller COF’s, varying from 0.36 to 0.47. In addition, the COF standard
deviations are much smaller than the ungrooved pad, varying from 0.05 to 0.08. It appears the
venting allowed by the grooved pads serves to limit the swings between different lubrication
regimes that we saw in the ungrooved pad.
The low slurry flow rate appears to have some features of both the grooved and ungrooved data.
First of all, we see greater COF’s, 0.42 to 0.62, than in the grooved pads. However, we do not
see large, instantaneous COF values that approach 3.0 as we did in the ungrooved pad. In
addition, the standard deviations in the COF data, 0.04 to 0.10, are similar to the grooved pads,
indicating that, once again, the venting serves to “damp out” radical variations in the COF. In the
meantime, the relative lack of slurry slightly increases the average COF in the low downforce
cases. In the high downforce cases, the average COF’s are essentially the same.
Future Work
We plan to continue exploring various parameter regimes using our existing experimental setup.
In addition, we hope to apply other analytical techniques to the data, specifically, frequency
analysis.
References
1. C. Gray, et al, Instantaneous, High Resolution, In-situ Imaging of Slurry Film Thickness
During CMP, 2004 CMP-MIC Symposium paper
2. D. Apone, Instantaneous Fluid Film Imaging in CMP, Master’s Thesis, Tufts University,
April 2005
3. Force Platform specifications provided by AMTI, Watertown Massachusetts
4. C. Gray, et al, In-Situ Friction and Pad Topography Measurements During CMP, Spring
2004 Materials Research Society Symposium paper
5. J. Lu, et al, Measurements of Slurry Film Thickness and Wafer Drag During CMP,
Journal of the Electrochemical Society, 151 (4) 2004
Data Set 1 – Ungrooved FX9 pad
Data Set 2 – circular grooved FX9 pad
Data Set 3 – xy grooved IC1000 pad
Data Set 4 – xy grooved pad, low slurry flow rate