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The Effect of Wafer Shape in Chemical Mechanical
Planarization
Researcher: Joseph LuPrincipal Investigator: Chris Rogers
Corporate Sponsors: Cabot CorporationIntel CorporationFreudenberg Nonwovens VEECO Insturments
Outline
• 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
Polishing Platform
100 RPMStruers RotoPol-31
Drill Press
Weighted Traverse
Two Aligned 12 BitCamera
Three Way Solenoid Valve
Tagged Slurry
Slurry
Color Separation
Detection
Ratio
Calibration
Ratio Calibration Measurement of passive scalar
DELIF Technique
Wafer- Pad Interaction
Pad Asperities
Wafer
Pad
10- 20 Microns
Friction Measurements
Coeff. of Friction =Friction Force
Downforce
(Fdrag )
Convex vs Concave Wafers
• Wafers used are typically ~ 5 m convex or concave• Glass (BK-7) windows
– 0.5 in thick, 3 in diameter
wafer
Polishing Pad
wafer
Polishing Pad
Convex Wafer Concave Wafer
Slurry Thickness vs. Pad Speed
• Increasing pad speed = Increasing slurry thickness
• Repeatable and consistent data
Convex Wafer
Coefficient of Friction vs. Pad Speed
• Increasing pad speed = Decreasing friction
• Repeatable and consistent data
Convex Wafer
Wafer Shape & Pad Speed Effects
Convex Wafer Concave Wafer
Speed -> Slurry Thickness -> Coeff. Of friction
Speed -> Slurry Thickness -> Coeff. Of friction
Wafer Shape & Downforce Effects
Convex Wafer Concave Wafer
Downforce -> Slurry Thickness -> Coeff. of Friction
Downforce -> Slurry Thickness -> -- Coeff. of Friction
Wafer Angle of Attack
• Convex wafer AOA much greater than Concave wafer AOA
• Very small AOA for concave wafer
• Measurement error ~0.003 AOA may support thicker
fluid film
Pad
slurryVpad
Angle
Summary
• Clear difference in slurry film thickness and coeff. of friction trends between convex and concave wafers– Convex wafers seem to be able to support a thicker slurry
layer than a concave wafer– Pad - wafer interaction may be characterized by coeff. of
friction and slurry thickness data
• Slurry film thickness is not independent of the polishing pad’s response to process parameters
• There is measurable pressure differences between different wafer shapes
Conclusions & Future Work
• Lubrication regime is a function of slurry film thickness and friction– Convexities = hydrodynamic lift
– Concavities = asperity contact
• Slurry thickness and friction are correlated– based on future work, friction can be used as end point detector in
the planarization process
• Examine changes in slurry thickness and friction of a polishing wafer as it changes shape
• Examine localized frictional effects - ‘hot spots’
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