INSTANTANEOUS IN-SITU IMAGING OF SLURRY FILM THICKNESS DURING CMP

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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

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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

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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

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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

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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

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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.

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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
• 91 dilution
• 0.5 g/L Calcein

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Previous Work

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

• Film thickness increases as pad speed increases
• Inverse relationship for downforce and thickness

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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

2mm
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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

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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

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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.

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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

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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