Qualitative Prediction of SiO2 Removal Rates during Chemical Mechanical Polishing

A two-dimensional, finite element wafer scale model for chemical mechanical polishing (CMP) is described. The model is solved using the commercial software package ANSYS (Version 5.5. ANSYS Inc.). The proposed model shows good qualitative agreement between observed CMP nonuniformities obtained on Applied Materials' Mirra polisher and the distribution of calculated von Mises stresses on the wafer surface. The paper also considers three different polishing parameters that influence the polishing nonuniformity on the Mirra: (i) the effect of the distance between the retaining ring and wafer, known as the gap, (ii) the effect of varying retaining ring pressure at a constant wafer down force, and (iii) the effect of pad/wafer relative velocity. The results suggest that improved wafer uniformity is achieved when the gap between the ring and wafer is maximized or completely eliminated, when the ring pressure is close to the carrier pressure, and when the pad/wafer relative velocity is high. The model results also indicate that all three of the effects mentioned above must be considered in an optimization process.