CMP has been described qualitatively in terms of alternating cycles of chemical formation and mechanical removal of a thin layer on the wafer surface. A quantitative model of CMP has been developed2-7 which is based on mechanisms for surface kinetics, treating mechanical removal as one step in the mechanism. This model has been used successfully to explain removal rates for tungsten and thermal oxide CMP. In particular, for tungsten CMP the removal rate increases steeply with increasing oxidizer concentration at low concentrations, and then approaches an asymptotic maximum removal rate at high concentrations. The model explains this by starting with the assumption that mechanical abrasion removes only tungsten oxide but not tungsten metal. It then focuses on the fraction of wafer surface covered by a tungsten oxide layer. At low oxidizer concentrations, the oxide formation rate is small compared the removal rate, so only a small fraction of the surface is oxidized and the removal rate is small. At high oxidizer concentrations, the oxide formation rate is large compared to the removal rate, so most of the surface is oxidized and the removal rate is large. Increasing the oxidizer concentration in the high oxidizer concentration region does not significantly increase the surface fraction of tungsten oxide, and the removal rate approaches a constant value.