Identification of Nonlinear Viscoelasticity of Polishing Pad Using an On- Machine Compression Tester

This paper presents measurement of nonlinear viscoelasticity of a polishing pad and identification of nonlinear viscoelasticity parameters. Since the polishing pad consists of high polymer foam with rough asperity surface, it behaves as a nonlinear viscoelastic material against the wafer contact during polishing. Thus, mechanical properties of the polishing pad directly affect polishing performance, and identification of the nonlinear viscoelasticity of the polishing pad is significantly important especially to attain analysis of the polishing process. In order to measure the nonlinear elasticity of the polishing pad, an on-machine compression tester was developed in the present study. The developed compression tester can be attached to the polishing machine and high-speed compression can be attained by using PZT actuator. From measured stress-strain curve, nonlinear elasticity parameters are identified based on the proposed asperity layer model based on the traditional contact model. The viscosity parameter is also calculated by comparing the measured compression data at different compression speeds. By implementing compression tests, the nonlinear elasticity and the viscosity coefficient of the polishing pad were identified successfully. Soft EHL analysis of the CMP process was also conducted by using identified parameters. Calculated results indicated an importance of the consideration of the nonlinear viscoelastic properties. Keywords: CMP, viscoelasticity, nonlinear elasticity, polishing pad, identification, compression test