In Situ Friction Measurements in Chemical Mechanical Planarization

1
In Situ Friction Measurements in Chemical
Mechanical Planarization

• Jim Vlahakis
• PhD. Candidate
• Tufts University
• 20 February 2006

1
2
Introduction

• Experimental setup
• Equipment
• Data generation
• Data analysis
• Results Discussion
• Coefficient of Friction (COF)
• Frequency Analysis
• Sources of Error
• Final thoughts

3
Experimental Setup

• Must accommodate our DELIF experiments
• transparent wafer
• 91 water diluted slurry to avoid polishing
• Framework supports optics

• Process parameters must be modified to account
  for laboratory scaling
• Wafer size 3 dia.
• Default ? 60rpm
• Flow rate 50cc/min

4
Equipment

• Motor ½ hp Dayton
• Wafer transparent BK7
• Table 136kgs, steel
• Platen 12 diameter
• Force table AMTI
• Polisher Struers RotoPol31

5
Equipment - Issues

• Alignment of polisher and force table
• Mechanical isolation
• Support frame
• Alignment of wafer drive belt

• In our setup, Fz, also includes the weight of any
  fluid in the system
• Platen runout can influence Fz

6
Equipment - Force Table

• Decomposes the loading into orthogonal components
  (forces and moments),
• Accuracy
• 355 bits/lb in x and y
• 710 bits/pound in z

7
Equipment - Polisher

• Struers RotoPol 31 table top polisher.
• Rests directly on top of the force platform
• Real time measurements of the wafer/pad
  interaction forces
• Fz process downforce
• Fx, Fy - friction due to polishing
• Custom LabView software allows us to select a
  rotation rate from 0 100rpm

8
Equipment - Wafer

• Transparent glass BK7 wafer
• Wafer concavity mates with drive shaft
• Drive plate (red plastic) ensures positive
  engagement with wafer drive pins
• Decent amount of play allows the wafer some
  freedom of movement

9
Data Generation

• Custom LabView software controls force table,
  digital amplifier and I/O settings
• Front panel seems complicated but is pretty
  straightforward
• Most settings are set and forget

10
Data Analysis

• Accuracy Issues
• .007N/bit in x and y
• .097N/bit in z
• Force table/polisher alignment

• Data format - 6 columns, tab delimited
• Each column represents one component (Fx, Fy,
  etc.)
• Sampling rate 2kHz
• Each data run 20sec
• Data file sizes up to tens of megabytes (ie
  manageable)

11
Results Discussion Coefficient of Friction

• Ungrooved FX9 pad

12
Results Discussion Coefficient of Friction

• Circular grooved FX9 pad

13
Results Discussion Coefficient of Friction

• xy grooved IC1000 pad

14
Results Discussion Coefficient of Friction

• xy grooved IC1000 pad low slurry flow rate

15
Results Discussion Coefficient of Friction

• For unvented pad
• Larger spread in instantaneous COF, ranging from
  0.0 to 3.0
• Indicates the lubrication regime is alternating
  from hydrodynamic to boundary lubrication
• Larger average COF and larger variation in COF
• Higher velocity decreases COF slightly
• For vented pads
• Smaller spreads in COF and smaller average COF
• Indicates more consistent lubrication regime
• Venting seems to moderate the changes in COF
• high Fz-30rpm-IC1000 dataset seems to show some
  sort of resonance effect

16
Results Discussion Frequency Analysis

• Examine the downforce frequency spectrum
• Which frequencies contribute the most
• Can we learn anything about the various polishing
  parameters based on the frequency signature

17
Results Discussion Frequency Analysis
Ungrooved FX9 pad
18
Results Discussion Frequency Analysis
Circular grooved FX9 pad
19
Results Discussion Frequency Analysis
xy grooved IC1000 pad
20
Results Discussion Frequency Analysis
xy grooved IC1000 pad low slurry flow rate
21
Results Discussion Frequency Analysis

• Features at 120Hz/240Hz/360Hz are grounding
  issues. Must be filtered out in the future.
• Resonant case (highFz-30rpm-IC1000 pad) shows a
  strong peak at 190Hz. May be related to pads
  natural frequency
• Which features are important? What scale should
  we be looking at?

22
Sources of Error

• Mechanical Issues
• Isolation from external inputs
• Bearing runout, unbalanced rotating components
• Electronic Issues
• Noise from other equipment
• Appropriate sampling rates
• Appropriate filtering

23
Final Thoughts

• What, exactly, do we want to learn?
• How to identify failure modes
• A polishing end point
• Correlate removal rates with COF
• What are the relevant variables?
• Which regions of parameter space do we want to
  explore?
• What is the best way to present this data?
• Thanks to
• Intel Cabot for their sponsorship
• Our advisors Vin Manno Chris Rogers
• Fellow researchers at U. of Arizona
• Howard Stone at Harvard and Gareth McKinley at MIT