INSTANTANEOUS IN-SITU IMAGING OFSLURRY FILM THICKNESS DURING CMP

Caprice Gray, Daniel Apone, Chris Rogers, Vincent P. Manno, Chris Barns, Mansour Moinpour, Sriram Anjur, Ara Philipossian

Motivation

Microelectronic devices continue to decrease in size; current features are routinely smaller than 100nm

The semiconductor industry requires a deeper understanding of the physical processes involved in CMP to help attain smoother surfaces

Using Dual Emission Laser Induced Fluorescence (DELIF) we can measure instantaneous fluid film thicknesses (and temperatures) during a polishing run

Here we look at how the pad conforms to features on a wafer

Polishing Setup

Struers RotoPol-31 table top polisher

Polisher sits atop a force transducer table capable of measuring down and shear forces during a polish

Optical Setup

Evolution VF 12 bit digital cameras Region of

Interrogation: 2mm by 3mm on the pad

355 nm Nd-YAG Laser provides excitation light Laser Pulse Length: 6ns

Dual Emission Laser Induced Fluorescence

Calcein, Coumarin in slurry solution UV light excites Coumarin Coumarin emission excites Calcein Each emission is captured by a camera Taking the ratio of the two emissions normalizes the image

by initial excitation intensity Images taken are 3 second temporal averages Note: pads must be dyed black to mute any fluorescence

DELIF with One Dye Natural pad fluorescence replaces

Coumarin; Laser replaces UV lamps Allows for non dyed pads Laser now excites pad Pad emission then excites Calcein in

slurry Since Laser is much more powerful than

UV lamps, we can now take instantaneous images, not 3 second averages as before.

Experimental Parameters Freudenberg FX9 Pad Wafer & Platen Rotation: 30 rpm

Relative Velocity: 0.34 m/s Downforce: 1.8 PSI Slurry

Flow Rate: 50 cc/min9:1 dilution0.5 g/L Calcein

Previous Work

Film thickness increases as pad speed increases

Inverse relationship for downforce and thickness

Film thickness are measured from the wafer surface down to some mean height within the pad

Calibration

These known heights allow for a calibration of intensity to fluid film thickness

The 27 micron deep well (b) is brighter than the 14 micron deep well (a), indicating more fluid

(b)(a)

2mm

Results Surface roughness calculations compare

single points in an image to a mean thickness value

Indicates the wafer is compressing the pad

Roughness of:Red square = 3.40.3Blue square = 4.20.3mm FX9 Pad (from profilometer) =

4.30.3mm

Results

22% of images of the 27 micron deep well show air bubbles

The roughness in the air bubble is between the roughness inside and outside of the well

(b)(a)

ResultsAverage Surface Roughness with Variable

Down-Force

4.24.8

4.4

0

1

2

3

4

5

6

0.4 1.8 3.2

Pressure (psi)

Su

rfa

ce

Ro

ug

hn

es

s (

um

)

Previous modeling research has shown that pressure varies locally beneath a wafer suggesting that we must interrogate many other regions before we can draw any significant conclusion about roughness variation with applied global down force.

Conclusion

This work supports the notion that the CMP polishing regime is in the partial lubrication regime Wafer is partly supported by asperities, partly by fluid

pressure If it were true hydrodynamic lubrication, the

roughness under the wells would be the same as the rest of the pad

The asperities are free to expand under the etched wells and do so

Future Work

Investigate larger region/multiple regions under the waferCorrelate downforce with surface roughness

Refine calibration method to determine absolute thicknessRoughness reported here is a relative

measurement