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 11/8/2000

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 11/8/2000

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. 4554 - 4558 11/8/2000

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 11/8/2000

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 11/8/2000

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. 11/8/2000

SEM Pictures & AE Raw Signal Distribution Layer 2 is more ductile than Bulk Air-Cut + Layer 1 Layer 2 Bulk 11/8/2000

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 11/8/2000

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 11/8/2000