INSTANTANEOUS IN-SITU IMAGING OF SLURRY 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/min  9:1 dilution  0.5 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 = 3.4  0.3  Blue square = 4.2  0.3mm  FX9 Pad (from profilometer) = 4.3  0.3mm

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)

Results 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 wafer  Correlate downforce with surface roughness Refine calibration method to determine absolute thickness  Roughness reported here is a relative measurement