Date of Award

2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Systems Engineering

First Advisor

Li, Zhichao

Abstract

Chemical Mechanical Polishing (CMP) is a major manufacturing step extensively used to planarize and smooth silicon wafers upon which semiconductor devices are built. In CMP, the polishing pad surface is glazed by residues as the process progresses. Typically, a diamond disc conditioner is used to dress the pad to regenerate newer pad asperity and a desired surface profile in order to maintain favorable process conditions. Conditioner selection and the determination of the optimal conditioning parameter values to yield a desired pad surface still remain difficult problems. Various analytical process models have been proposed to predict the pad surface profile. However, not much work has been done concerning the incorporation of conditioner and pad design features in these analytical models. This research sought to address the concern about the lack of models that are reliable enough to be used for verification and optimization of the process. In this research, two kinematic models were developed to predict the pad surface profile due to conditioning. One model was developed using a surface element approach and the other by characterizing the diamond disc conditioning density distribution. Three metrics; Total Thickness Variation, Bow, and Non-Uniformity, were defined and utilized to evaluate the resulting pad surface profile characteristics. Experimental data confirmed that both models were able to simulate the kinematics of diamond disc pad conditioning and accurately predict the pad surface profile. However, a slightly skewed deviation of the simulation results corroborated the suspicion that, deformation of the microporous pad could affect the pad surface profile. Thus, a 2-D image processing procedure was developed to characterize the morphological and mechanical properties of microporous Class-III CMP pads. Pad characterization data was incorporated into a 2-D axisymmetric quasi-static finite element model to investigate effects of process parameters such as stack height, pad stiffness, and conditioning pressure on the pad deformation with enhanced fidelity. Simulation results were consistent with literature and showed that the pad profile was affected by deformation due to conditioning. Since the conditioner design also has a significant effect on the pad conditioning process, a new metric to evaluate the pad surface texture generated by a specific conditioner design was developed. The metric was applied in a genetic algorithm (GA) to optimize conditioner design parameters including geometric arrangement of diamonds, grit density and disc size. The GA model was able to find design parameter values that produced better CMP pad surface textures.

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