lubrication of curved wafers during chemical mechanical

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

Society of Tribologists and Lubrication Engineers - May 22, 2001 2

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


Rotary CMP Polisher

Wafer

Pad

10- 20 Microns

Society of Tribologists and Lubrication Engineers - May 22, 2001

Pad Asperities

4

Polishing Platform

Vibration Isolation Table

Tabletop Polisher

Polisher HeadDyed SlurrySolution

Variable Downforce (2-8 psi)

Society of Tribologists and Lubrication Engineers - May 22, 2001

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


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


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


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


Convex WaferFilm Thickness (h) & Cf vs. Downforce (Fdf ) & Pad Speed (U)

U ➟ h ➟ Cf

Fdf ➟ h ➟ Cf

-Consistent data (independent of slurry abrasive concentration)


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)


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


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


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


Wafer Performance

• Different performance for different wafer shapes• Cf much more sensitive to shape with greater concavity

– Different lubrication regimes– Stick-slip phenomena


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


Stick-slip of Convex Wafer

• Ff low• Stick-slip low

– with Fdf

– with U• Hydrodynamic lift


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)


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


Slurry Abrasive ConcentrationAlso Important

Polishing pad

Wafer

Slurry particles in solution

up

High abrasiveconcentration

Polishing padSlurry particles in solution

Wafer

up

Low abrasiveconcentration


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


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


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



Concave wafer



Pad Properties

Pad Microstructures

Polytex Supreme

IC-1000/SUBA IV stack

lubrication of curved wafers during chemical mechanical

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