The Effect of Wafer Shape on Lubrication Regimes in Chemical

Mechanical Planarization

Researcher: Joseph LuPrincipal Investigator: Chris Rogers

Corporate Sponsors: Cabot CorporationIntel CorporationFreudenberg Nonwovens VEECO Insturments

Outline

• Advantages of chemical mechanical planarization• 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• Effects of wafer curvature on fluid pressure distribution• Summary and conclusions

Rotary CMP Polisher

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

1 10 100 1,000 100,000

1.0

0.1

0.01

0.001

Mixed Lubrication

Boundary Lubrication

Turbulence

ZN/P

Coe

ffic

ien

t of

Fri

ctio

n

Full Fluid Film Lubrication0.0001C

oeff

icie

nt

of F

rict

ion

ZN/P

Z= Viscosity (poise) N= Speed (RPM) P= Pressure (Psi)

Lubrication Regimes

Pressure Measurements

Polishing Pad

Wafer

Gimbal

Chuck

ManifoldTygonTubing

Rotating Platform

PressureTransducer

PowerSource

Pick-upArm

SlidingContact

Plugs

- Locations of 7 pressure taps onwafer

Non-Rotating Wafer

Convex Concave

-60 RPM platen speed -3 Psi Downforce

Fluid Inlet Fluid

Inlet

High Pressure

Low Pressure

Rotating Wafer

Convex Concave

-60 RPM platen speed-3 Psi Downforce

% Wafer Radius % Wafer Radius

Pre

ssu

re (

Psi

g)

Pre

ssu

re (

Psi

g)

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 lubrication regime may be characterized by the

coeff. of friction and slurry thickness data

• Slurry film thickness is not independent of the polishing pad’s response to process parameters

• There are significant pressure differences between different wafer shapes

Conclusions & Future Work

• Lubrication regime = f (slurry film thickness, friction, fluid pressure)– Convexities = +pressure = hydrodynamic lift

– Concavities = suction = asperity contact

• Slurry thickness, friction, and fluid pressure are correlated– The understanding of the relationship of these parameters can improve the

control of the planarization process

• Examine changes in slurry thickness and friction of a polishing wafer as it changes shape

• Examine localized feature scale effects - ‘hot spots’

Acknowledgements

• Cabot Corporation– Frank Kaufman

• Intel Corporation– Mansour Moinpour, Ara Philipossian

• Tufts University– Chris Rogers, Vincent Manno, Alicia Scarfo

http:\\www.tuftl.tufts.edu

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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