PlanarizationJoseph Lu, Vincent P. Manno* & Chris Rogers
Department of Mechanical EngineeringTufts University
Medford, MA
May 2001
Lubrication of Curved Wafers During Chemical Mechanical
Outline
• Need to understand chemical mechanical planarization (CMP)
• Laboratory scale CMP setup– Slurry film thickness measurement technique– Friction measurement technique
• Define wafer shapes• Effects of wafer curvature on slurry film thickness
and coefficient of friction• Summary and conclusions
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Other Participants
Mansour Moinpour - Intel Corporation
Ara Philipossian – University of Arizona
Sam Anjur, Jim Dirksen – Cabot Corporation
Edward Chan, Jesse Cornely, Irene Li, Alicia Scarfo - Tufts University
Freudenberg Nonwovens
VEECO Insturments
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Rotary CMP Polisher
Wafer
Pad
10- 20 Microns
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Pad Asperities
4
Polishing Platform
Vibration Isolation Table
Tabletop Polisher
Polisher HeadDyed SlurrySolution
Variable Downforce (2-8 psi)
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CameraSystem
UV Lamp
Peristaltic Pump
Wafer (60 rpm)Polishing Pad(30-300 rpm)
FrictionTable
5
Consumables
• Cab-o-sil SC-1 Slurry (Cabot Microelectronics)– 13.4% wt abrasive concentration (2.09 cps)– 3.1% wt abrasive concentration (1.13 cps)
• Freudenberg FX-9 pads (50 mil)– specially dyed
• Other pads in study, not emphasized– Rodel IC-1000 – Rodel Polytex Supreme
• BK-7 Glass
Slurry solutions
Polishing pads Wafers
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DELIF Technique
I1fI2f
Excitation Color Detection Ratio Absolute calibrationof passive scalar
Rat
io
Passive ScalarDyed fluid
Fluid film thickness measurements• No incident light issues• One dye absorbs the other
proportional to path length• 1 µm relative accuracy• 5 µm absolute accuracy
J. Coppeta, J. Elec. Chem. Soc., 2000
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Friction Measurements
Vibration Isolation Table
Tabletop Polisher
Slider Table
Load Cell
Friction force (Ff)Coefficient of friction (Cf) = Downforce
Calibration error < 8% Cf < 0.01
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Wafer Shapes
Convex Wafer
Polishing Pad
Concave Wafer
Polishing Pad
• Wafers used are typically ~ 5 µm convex or concave• Glass (BK-7) windows
– 0.5 in thick, 3 in diameter
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Convex WaferFilm Thickness (h) & Cf vs. Downforce (Fdf ) & Pad Speed (U)
U ➟ h ➟ Cf
Fdf ➟ h ➟ Cf
-Consistent data (independent of slurry abrasive concentration)
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Concave WaferFilm Thickness (h) & Cf vs. Downforce (Fdf ) & Pad Speed (U)
U ➟ h ➟ Cf
Fdf ➟ h ~➟ Cf
-Strong dependence on slurry concentration (polishing causes wafer shape change)
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Wafer Shape TransitionConcave ➠ Convex
concave convex
• 4 psi downforce, 60 rpm pad speed• Edge-fast polishing system• Speed of transition
• Influence of wafer shape established
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LubricationFf
Coefficient of friction (Cf) = • Bulk parameter
Fdf
hminFilm parameter (Λ) = • rms of unloaded surface
(rmsw2 + rmsp
2)1/2
µ U rw3
Sommerfeld no. = • “Newtonian”• U = relative linear velocity• Geometric scaling (rw)
Ff h2
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Lubrication Regime
B. Hamrock, Fundamentals of Fluid Film Lubrication, 1991
• Λ does not represent a dynamic parameter• FX-9, IC-1000 and Polytex pads• Concave ➾ convex = partial ➾ hydrodynamic
lubrication regime
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Wafer Performance
• Different performance for different wafer shapes• Cf much more sensitive to shape with greater concavity
– Different lubrication regimes– Stick-slip phenomena
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Stick-slip
• Intense fluctuations in friction force• Predominantly present with concave wafers• Higher frequencies do not shift with pad speed ~> 10 Hz• Power of higher frequencies reflect intense fluctuations
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Stick-slip of Convex Wafer
• Ff low• Stick-slip low
– with Fdf
– with U• Hydrodynamic lift
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Stick-slip of Concave Wafer
• Ff high• Stick-slip low
– with Fdf
– with U & Ff with U• Supportive observation of wafer suction
(Levert, Danyluk & Tichy)
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Slip Resistance
Convex Wafer
Polishing Pad
Concave Wafer
Polishing PadContact Pressure, pc
Slip
Res
istan
ce, s
Amonton’s laws α pc
Slip resistance atlow contactpressures
Slip resistance at high contact pressures
S*
L. Anand, Computational Mechanics, 1993
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Slurry Abrasive ConcentrationAlso Important
Polishing pad
Wafer
Slurry particles in solution
up
High abrasiveconcentration
Polishing padSlurry particles in solution
Wafer
up
Low abrasiveconcentration
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Summary• Clear difference in slurry film thickness and coeff. of friction
trends between convex and concave wafers– Convex wafers support thicker slurry layers than concave wafers
• Degree of wafer curvature affects lubrication regime and operational performance
• Stick-slip is seen to be greater with concave wafers and lower abrasive loading– Increased contact pressures
• Slurry abrasive concentration affects stick-slip
• Polishing pad conformity eases contact pressures & increases uniform distribution
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Conclusions & Future Work
• CMP operates predominantly in the partial (mixed) lubrication regime– Convex wafers operate closer to hydrodynamic lubrication
• Measured average friction forces may not be a good indicator of material removal– High levels of stick-slip may lead to higher wafer non uniformities
• Slurry thickness, friction are correlated to abrasive concentration, and wafer curvature in addition to standard process parameters– Further understanding of the relationship of these parameters can improve
the control and precision of the planarization process
• Higher order wafer curvature and local topographies• Stick-slip on more pads and with other abrasives
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Visit our web site at
http:\\www.tuftl.tufts.edu
Convex wafer
Before experiment After experiment
• Polished with 13.4% wt slurry (3:2)
Convex wafer
Before experiment After experiment
• Polished with 3.1% wt slurry (9:1)
Concave wafer
Before experiment After experiment
• Polished with 13.4% wt slurry (3:2)
Concave wafer
Before experiment After experiment
• Polished with 3.1% wt slurry (9:1)
Pad Properties
Pad Microstructures
Polytex Supreme
IC-1000/SUBA IV stack