1. Scratch Testing of Silicon Wafers for Surface Characterization
SFR Workshop
November 8, 2000
Edward I. Hwang, David A. Dornfeld
Berkeley, CA
2001 Goal: To build integrated CMP model for basic mechanical and chemical elements by 09/30/2001
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2. Motivation
Wafer surface characterization is important to understand and model the material removal mechanism in CMP
- Scratch testing supports the identification and verification of surface characteristics of the wafer representative of the CMP process
- Scratch testing can give insight on the stress levels occurring during the CMP Process
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3. plastically compressed network
Actual CMP Situations
Cross Section View
bulk
not affected by the process
polishing pad
Si wafer
layer 2(order of 20 nm )
plastically compressed network
– higher density
layer 1(order of a few nm )
highly hydrated, loosely bound network
– lower density
Trogolo et al “Near Surface Modification of Silica Structure Induced by Chemical/Mechanical Polishing”, J. Materials Science 29 (1994) pp
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4. Experimental Setup Workpiece: Silicon wafer <100> p-type
Pre-CMP Wafers & Post-CMP Wafers
Diamond tool: Nose radius: 48μm
Feed rate: V=399μm/s
Tilt angles: 0.06 degrees.
Acoustic emission sensor: DECI Pico-Z AE sensor
Data collection: 50kHz sampling rate
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5. AE signals are proportional to the depth of cut in
Layers vs. AE Signals (1)
Pre-CMP Wafers
AE signals are proportional to the depth of cut in
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6. Layers vs. AE Signals (2) Post-CMP Wafers
0.00
0.05
0.10
0.15
0.20
0.25
0.30
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
time(s)
AE Raw Signals (volts)
Air-cut + Layer 1
Layer 2
Bulk
0.35
Unlike the pre-CMP wafers, post-CMP wafers show discontinuous transitions in the AE signal due to penetration of Layer 2.
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7. SEM Pictures & AE Raw Signal Distribution
Layer 2 is more ductile than Bulk
Air-Cut + Layer 1
Layer 2
Bulk
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8. Results
Observation of distinct signal changes for transitions between Layer 1  Layer 2  bulk supports surface characterization
Signal for Layer 2 is observed up to 20 nm depth of cut
Highly compressed Layer 2 is more ductile than bulk :
- Plastic deformation dominates the material removal mechanism in this regime and should relate to removal rate during CMP
SEM images support the verification of the multi-layered wafer surface
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9. 2002 & 2003 Goals
Develop comprehensive chemical and mechanical model. Perform experimental and metrological validation, by 9/30/2003.
Replicate the scratch testing with AFM machine in order to be closer to actual CMP situations
Quantify the wafer surface characteristics in CMP
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