Submitted to the Journal of the Electrochemical Society
In-Situ Measurement of Pressure and Friction During CMP of
Contoured Wafers
A.M. Scarfob, V.P. Mannoa, C.B. Rogersb, S. Anjurc and M. Moinpourc
a – Corresponding Author, Department of Mechanical Engineering, Tufts University, Medford, MA 02155,
617-627-2548, FAX-627-3819, [email protected]
b - Corresponding Author, Department of Mechanical Engineering, Tufts University, Medford, MA
c – Cabot Microelectronics, Aurora, IL
d – Intel Corp., Santa Clara, CA
Abstract
In this paper we document in-situ fluid film pressure and interfacial friction measurements during
Chemical Mechanical Planarization (CMP) over a range of applied loads and relative pad/wafer
velocities. The slurry film pressure beneath contoured test wafers is measured using a novel experimental
setup that enables dynamic data collection. The friction data have a repeatability of approximately 10%.
The uncertainly of the pressure measurements and the computed down forces are ±20.7 kPa (±3 psi) and
20%, respectively. The data indicate that wafer shape, specifically global curvature, is a significant factor
in determining the lubrication regime during CMP. Full hydrodynamic lubrication, in which the slurry
fluid film supports the entire applied load, was not realized for CMP of either concave (center high) or
convex (center low) wafers. The data for concave wafers show that –6% to 37% of the applied load is
supported by the slurry film, where the negative sign indicates suction conditions that were obtained at the
lowest applied load condition. CMP of convex wafers is found to operate closer to full hydrodynamic
lubrication, with the fluid layer supporting 36% to 64% of the applied downforce. In all cases, the
measured friction coefficient decreased as the support of the fluid layer increased (higher positive
pressures). CMP of concave wafers is more sensitive to changes in applied downforce, while the convex
wafer type was most affected by changes in the wafer/pad rotation speed, which in turn determines effective
slurry film velocity beneath the wafer. Overall, the CMP conditions seen in these scaled experiments
operate primarily in the partial lubrication regime shifting closer to hydrodynamic lubrication for convex
wafers at the high load, high speed conditions.
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Submitted to the Journal of the Electrochemical Society
1. Introduction
Chemical Mechanical Planarization (CMP) is an integral step in the manufacture of multilevel
integrated circuits. In oxide CMP, a rotating silicon wafer substrate is pressed against a rotating polishing
pad as chemically active, abrasive slurry is continually injected at the wafer/pad interface. CMP is superior
to other planarization methods due to its ability to achieve high levels of both local and global planarity
across the wafer surface. Due to its widespread use, a thorough understanding of the relationship between
CMP process conditions and the physics of material removal mechanisms is desirable. Much of the
uncertainty in obtaining this correlation relates to characterizing the tribology dynamics and lubrication
regimes that obtain for various CMP operating conditions.
CMP lubrication regime depends largely on how the applied load is distributed between the fluid
film layer and the pad asperities. In hydrodynamic lubrication, the fluid film is continuous with negligible
pad-wafer contact. As such, the fluid pressure balances the entire applied load. Material is removed in the
hydrodynamic regime through a combination of fluid induced erosion and slurry particle abrasion. At the
other extreme, boundary lubrication, the fluid film layer is thinner and there is significant solid-solid
contact between the wafer and pad, especially the pad asperities. In this case, material removal is due
primarily to the mechanical abrasion of the asperities against the wafer surface. The abrasion is enhanced
by chemical activation of the wafer surface and silica slurry particles entrapped in the pad asperity
interstices. The consensus hypothesis is that most practical oxide CMP operates somewhere in between
these two extremes in what is known as the partial lubrication regime.
Several investigations have focused on better understanding the behavior of the fluid film layer
during CMP. Levert et al. [1] studied the role that surface properties play in defining the lubrication regime
during CMP. They found that, for an acrylic and semi-permeable pad, the film layer thickness increased
with increasing relative wafer/pad speed and applied load. However, no significant slurry film developed
for the permeable pad sample. In another study, Shan et al. [2] collected pressure data between a rotating,
soft polyurethane pad and a stationary opposed steel disk to determine how the applied force is distributed
at the interface. They found that the leading (i.e. slurry injection side) two-thirds of the wafer experienced
sub-ambient pressure, while positive load-bearing pressures were only obtained in the vicinity of the wafer
trailing edge. Bullen et al. [3] used a pressure measurement system that was a prototype of that used in this
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work. They collected data during static (wafer not rotating) and dynamic (wafer rotating) CMP. Although
trends were consistent with Shan et al., Bullen’s results show higher pressures for the static cases. As such,
they conclude that the static method is not a good approximation of the actual CMP process.
Our group has focused on exploiting a combination of optical and ancillary experimental
techniques to measure directly the slurry film during CMP. The primary optical technique used is known as
Dual Emission Laser Induced Fluorescence or DELIF [4]. Using DELIF, Lu et al. [5] measured the slurry
fluid film thickness at various applied loads, wafer/pad relative velocities, and wafer curvatures. Wafer
curvature, which is characterized qualitatively as convex (wafer edges curving away from pad surface) or
concave, is determined by the relative orientation of the wafer and pad surfaces. As the current DELIF
implementation requires transparent wafers, BK7 glass wafers are used as surrogates for silicon wafers. Lu
et al. found that, for convex wafers, the fluid layer thickens with increasing speed and decreasing
downforce, resulting in a decrease in friction. The data were opposite for concave wafers; the film actually
grew thicker in the area of inquiry as the downforce increased. Note that since the slurry region near the
outer edges of the wafer is unavailable for measurement, no comment on the overall film thickness
behavior can be made.
In this paper, we expand on the earlier work of Lu et al. In this study, fluid film pressure and
interfacial friction measurements were collected during dynamic CMP using contoured Plexiglas wafers.
As is described in the next section, Plexiglas is used rather than BK7 glass due to the wafer machining
required for the pressure data collection. Each wafer is classified based upon its shape and data are reported
over a range of applied downforces and relative pad/wafer rotation speeds. The net force on the wafer is
compared with the applied force to determine the percentage of the load supported by the fluid film. In
addition, the interfacial drag data are analyzed to elicit how certain process parameters change the degree of
contact between the pad and wafer.
2. Experimental Setup
Figure 1 is a schematic diagram of the test setup. The rig is a Struer’s Roto Pol tabletop polisher,
built on a 1:2 scale based on a SpeedFam IPEC 472 rotary polishing tool. A 30.5 cm (12 inch) diameter
removable aluminum platen serves as the pad carrier. A modified drill press, which also acts as the wafer
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carrier, is to allow variable downforce and wafer rotation speeds. A weighted traverse mounted atop the
drill press provides variable downforce ranging up to 50 kPa (7 psi). Wafer rotational speeds can be set
from 0–120 rpm ± 1 rpm. The pressure measurement hardware is also mounted concentrically on the drill
press spindle.
The polisher is positioned on a friction force detector consisting of two parallel plates separated by
a pair of linear rails that allow movement in only one direction (tangential to the polisher motion). A load
cell is mounted between the plates to detect any motion of the polishing pad relative to the wafer. Since the
drill press is bolted to the floor, the relative motion of the polisher gives a measure of the friction between
the pad and wafer. Freudenberg FX-9 polyurethane polishing pads, which are relatively hard with closed
cellular structure, are used in the data reported. In-situ pad conditioning is employed using a 5.1 cm (2
inch) diameter 163 µm diamond grit wafer, which rotates and oscillates across the pad to prevent pad
glazing and to keep pad surface conditions relatively constant.
A custom pressure measurement device was developed for this study to enable in situ dynamic
fluid film pressure measurements (see Figure 2). More details on the design, fabrication and operation of
the pressure measurement system are available in Scarfo [6]. The device consists of three basic assemblies,
each performing a specific function: pressure detection, position sensing, and data transfer. Pressure
detection is achieved using Omega PX-203 series transducers. Each transducer has a 1 ms response time
and can measure pressure ranging from 0-207 kPA or 0-30 psia providing a quantitative measure of suction
force. Incorporating all factors of the measurement uncertainty is ± 21 kPa or ± 0.3 psi. The transducers are
connected to the wafers via Tygon tubing. Special adapters enable removal of trapped air in the tubing
extending from each transducer to the wafer in order to reduce artificial damping of the pressure
fluctuations.
The position sensing system consists of a Hall switch magnetic sensor and a magnet mounted on
the side of the rotating base. The Hall switch, which produces an electrical signal due to the voltage
changes when the embedded magnet passes in its vicinity, is secured to the stationary portion of the
polisher. Since both the pad and wafer rotate during CMP, data and power leads are secured to a slip ring,
mounted concentrically on the rotating bit. The signals are passed over sliding metal contacts in the slip
ring and are wired directly to the computer. Computer-based data acquisition software is used to record the
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pressure and the position signals simultaneously. Data post-processing allows the pressure data to be
correlated to the angular position in the rotation.
Typically, BK7 glass is used in our DELIF set up. However, BK7 glass is brittle and difficult to
machine without inducing cracks. Instead, Plexiglas is employed as the wafer material. The Plexiglas
wafers are tapped in six locations positioned 60° apart, spaced linearly from 1.3 to 3.3 cm from the wafer
center (see Figure 3). These taps are needed to allow pressure communication from the wafer/pad/slurry
interface region of interest to the corotating pressure transducers. Each tap consists of a small hole
extending approximately halfway though the wafer thickness. A larger tapped hole mates to meet this
smaller hole. Barbed tubing connectors are screwed to enable tubing attachment. The current
implementation of the pressure rig only allows three simultaneous pressure measurements at one time.
Therefore, a data run is conducted in two sequential measurement periods. After half of the data are
acquired, the device is removed and reconfigured to allow sampling at pressure tap numbers 4, 5, and 6
after the data acquisition from tap numbers 1, 2 and 3 is completed.
As wafer shape affects the lubrication properties during CMP, each of the 25 test wafers were
scanned using a VEECO Dektak V 200-Si stylus profilometer. Two perpendicular line scans crossing the
wafer center are used to classify the wafer shape. Since the wafers were manufactured from bulk Plexiglas
1.27 cm (0.5 inch) sheet stock, surface irregularities exist in the machined surfaces. Wafers are classified
into one of three categories: convex, indeterminate or flat, or concave. As depicted in Figure 4, a convex
wafer is bowed outward toward the pad while the opposite is the case for concave wafers. In this
investigation, 5 wafers are classified as concave or type A, with a center to edge bow ranging from 5-15
µm. 10 wafers fall into the indeterminate category (type B), neither clearly convex nor concave or close to
flat, and 10 wafers are designated as convex or type C, with 5-15 µm center to edge convexity. In practice,
CMP slurries include suspensions of small silica particles that enhance material removal. Also, slurry
particles clog the small holes in the tapped wafers. As this investigation required non-varying wafer
curvature, commercial slurries are not useful since they would slowly polish the Plexiglas and change the
curvature. Instead, NaOH buffered water (pH~11) is used as the polishing “slurry”.
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3.
Results and Discussion
Each wafer data set consists of a series of ring pressure measurements taken at the six radial
positions depicted in Figure 3. Data for a total of 25 rotations were collected for each wafer tested at each
of the six operating conditions. The data acquisition system was programmed to sample 1800 points per
rotation with every five sequential data samples averaged to yield a new of 360 data. The data for each of
the three wafer curvature categories are then averaged to yield the results presented herein. Pressure
measurements are normalized by the applied downforce and are plotted versus the normalized wafer radial
position. Friction measurements are also reported for all wafers at each operating condition. These data are
also segregated and averaged based on wafer curvature classification.
Figure 5 displays pressure data for each wafer type at an applied down load pressure of 27.6 kPa
(4 psi). Results differ significantly among the three wafer shapes. For the concave wafer type A,
subambient pressure is obtained near the middle portion of the wafer. Pressure seems to start out slightly
positive near the wafer edge, but a vacuum is created upon moving closer to the wafer center. The pressure
profile shifts slightly more positive as the relative speed is increased, but the effect is minimal. In
qualitative agreement with Levert et al. [1], only positive pressure is observed for convex wafer type C,
increasing parabolically toward the wafer center. Relative wafer speed changes have more of an effect in
the case of convex wafers, increasing the maximum pressure observed from 75% to 130% of the applied
load. The behavior of the indeterminate or near flat wafer type B falls between the behaviors of the other
two wafer shapes. The pressure is positive over all conditions for wafer type B and increases at the wafer
center. However, there is a slight dip in pressure upon moving inward from the outermost tap.
Analogous data for a higher applied load (41.4 kPa or 6 psi) are reported in Figure 6. The increase
in load seems to cause a transition from subambient to positive pressure for wafer type A over all speeds.
The maximum fluid pressure observed corresponded to nearly 80% of the applied load compared to a
maximum of 20% for the lower load condition. Hence, the fluid film never fully supported the applied load
under any of the conditions tested. The load not supported by the film is presumed to be supported through
wafer/pad contact. Although the slurry film pressure moved into the positive range, the effect of the
curvature is still reflected in the shape of the pressure profile. Upon moving in from the edge, the pressure
dips lower but then builds up again. The increase in load seems to result in a more classic parabolic
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pressure profile. The normalized pressure under wafer type C increased with applied load, with the
maximum local pressure at the highest speed reaching 170% of the average applied pressure, compared to
130% at the lower applied load condition.
The average normalized friction for each of the three wafer types is presented in Table 1 for all
six operating conditions. It is clear that the friction is greatest for the concave wafer type (A) and least for
the convex wafer type (C). Increasing the relative wafer/pad speed causes a reduction in the friction
coefficient for all wafer types. The increase in the applied pressure from 27.6 kPa to 41.4 kPa decreased
friction level for wafer type A, but has little influence on friction for the other two wafer shapes.
The net force exerted on each wafer was computed in order to represent the pressure data as a
single, integral parameter. The calculation procedure involved dividing the wafer surface area into a series
of six concentric rings, each corresponding to an annular region associated with each of the pressure tap
radial position. The measured fluid pressure at a particular tap location is then multiplied by its respective
area and the force subtotals are summed to yield a net opposing force exerted on the wafer. The results,
calculated as the % of applied load, are displayed in Table 2. Once again, the results differ among each
wafer type. Wafer type A seems to border on boundary lubrication at the lower applied load condition
despite increasing the speed. However, a clear transition occurs when the load is increased, causing the net
opposing fluid force to build to over 30% of the applied downforce. As the wafer shape transitions from
concave to convex (i.e. type A to type B to type C), the fluid film supports a greater percentage of the
applied load. However, fully hydrodynamic lubrication is not observed under any of the conditions tested,
with only 60-65% of the applied load borne by the fluid film at the data extrema.
Figures 7 and 8, which are complementary to the data reported in Table 2, illustrate the sensitivity
of each wafer type to changes in applied load and rotation speed. Wafer type A is most sensitive to the
changes in downforce. One possible explanation is that as the downforce is increased, the outer edge of the
concave wafer digs into the pad causing the pad to deform. The pad, in turn, conforms to the bowed area at
the wafer center, creating a converging gap. As a result, positive pressures develop. This transition is also
marked by a 26% reduction in the normalized friction force, often defined as the coefficient of friction or
COF (see Figure 8). As mentioned earlier, these observations must be assessed with the knowledge that the
outer regions of the slurry film are out of measurement range.
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Increasing wafer rotational speed has the greatest effect on the fluid flow under wafer type C.
Even at low speeds, a parabolic pressure profile develops due to the convex geometry of the flow passage.
As the relative velocity increases, the load carrying capacity of the fluid film also increases, causing the
lubrication regime to move closer to fully hydrodynamic. As expected, the friction coefficient drops (by
43%) under these conditions. Qualitatively, it seems that an increase in speed causes the convex wafer to
hydroplane on the continuous fluid layer.
4. Conclusion
The data reported in this paper add insight into the lubrication regime of the CMP process under
various operating conditions. In particular, the interplay of downforce, relative pad/wafer velocity and
global wafer contour is reported. The in-situ, concurrent fluid film pressure and integrated friction data are
useful not only for characterizing CMP operations but also as validation data for simulation model
developers. Certain data trends are noted by combining this pressure data with the measured friction
coefficients for each wafer type. Additional data are also available in the work of Scarfo [6]. In general, it
appears that all wafers tested operated in the partial lubrication regime over most conditions, with the fluid
film supporting a fraction of the applied load. At low loads, the concave wafer type exhibited suction and
high friction, most closely representative of the full boundary lubrication regime. However, as the load
increased, positive pressures developed and friction dropped as the process moved towards partial
lubrication. Only positive pressure was measured for the convex wafers, even at low applied load, low
speed conditions. This wafer type was more affected by increasing speed, which moved the lubrication
phenomena move closer to fully hydrodynamic conditions with the associated reduced friction. No fully
hydrodynamic tribological conditions were observed in any of the range of conditions tested.
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References:
1. J. Levert, F. Mess, L. Grote, M. Dmytrychenko, L. Cook, and S. Danyluk, Slurry film thickness
measurements in float and semi-permeable and permeable polishing pad geometries, Proceedings of the
International Tribology Conference; Yokahama, Japan (1995).
2. L. Shan, J. Levert, L. Meade, J. Tichy, and S. Danyluk, Journal of Tribology, 122(3), 539–543, (2000).
3. D. Bullen, A. Scarfo, A. Koch, D. Bramono, J. Coppeta, and L. Racz, Journal of the Electrochemical
Society, 147(7), 2741–2743, (2000).
4. J. R. Coppeta, Ph.D. thesis, Tufts University, (1999).
5. J. Lu, J. Coppeta, C. Rogers, L. Racz, A. Philipossian, and F. B. Kaufman, Materials Research Society
Proceedings, 613:E1.2, (2000).
6. A.M. Scarfo, MS thesis, Tufts University, (2003).
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Figure 1: CMP Polisher Setup
Variable Downforce (3-8 psi)
Polisher
H
ead
Wafer (45, 60, 75 rpm)
Polishing Pad
(30-300 rpm)
Pressure Device
Slurry
Solution
Pump
Tab leto
p Polish
er
Vibratio
n
Isolation
Tab le
Friction Table
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Figure 2: Pressure Measurement Device
Computer
Stabilization Pin
1-way Valve
Magnet
Slip Ring
Position Sensor
Transducers
Adapters
Plexiglas wafer
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Figure 3: Wafer Tap Configuration
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Figure 4: Wafer Shape Characterization (Type B pads are neither globally convex or
concave)
Convex-Type C
Pad
Concave-Type A
Pad
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Figure 5: Normalized Pressure vs. Radius at 27.6 kPa
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Figure 6: Normalized Pressure vs. Radius at 41.4 kPa
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Figure 7: Net change in % of Load Supported by Fluid Film
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Figure 8: % Reduction of COF
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Table 1: Normalized Friction Measurements (Measured Divided By Applied Force)
for all Conditions
.35 m/s
.47 m/s
.58 m/s
.35 m/s
.47 m/s
.58 m/s
27.6 kPa Downforce
Wafer A
Wafer B
Wafer C
31%
22%
17%
27%
18%
13%
24%
16%
10%
41.4 kPa Downforce
Wafer A
Wafer B
Wafer C
24%
19%
18%
19%
14%
13%
17%
12%
10%
Table 2: Percentage of Downforce Exerted on Wafer Surface
.35 m/s
.47 m/s
.58 m/s
.35 m/s
.47 m/s
.58 m/s
27.6 kPa Downforce
Wafer A
Wafer B
Wafer C
-6%
22%
36%
3%
33%
51%
5%
38%
57%
41.4 kPa Downforce
Wafer A
Wafer B
Wafer C
25%
32%
41%
31%
45%
55%
37%
51%
64%
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