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A STUDY OF WEAR MECHANISMS FOR CHEMICAL MECHANICAL POLISHING
Chung, Liu La
The increasing complexity of devices and the emergence of deep submicrometer structures in integrated circuit (IC) industry require minimum surface topographies of wafer for lithography process. Chemical mechanical polishing (CMP) has been used in the planarization of wafer surfaces as, compared to other planarization processes, it can achieve global planarization. However, the process mechanism of CMP is not well understood yet. This thesis, from the mechanical viewpoint, investigates the fluid-based and abrasive-based wear mechanisms. Experiments are designed and conducted to verify the mechanisms and to understand the relationship between material removal and mechanical parameters.
Achievement of this study includes the following items. (1) Material on wafer surface cannot be removed by hydrodynamic effects without chemical reaction. (2) The relationship between the material removal rate (RR) and the applied polishing pressure is not linear. (3) Experimental data shows that polishing velocity has higher effect than polishing pressure on RR. That is, RR depends stronger on the shear stress than on the normal stress. (4) RR based on soft pad matched better with theoretical value. It is observed that abrasive-based wear mechanism is suitable for CMP with soft pad. (5) It is also observed from abrasive-free polishing that RR is not obvious when polishing pressure increases. This is caused by the decrease of slurry between the wafer and the pad due to the increasing pressure and thus reduces the chemical reaction. (6) Non-uniformity (NU) of wafer is highly affected by the back pressure and stiffness of pad. (7) Surface roughness increases as polishing pressure increases. The trend, however, is similar to the theoretical value of scratch.
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