We have developed a semiempirical feature scale model of Si etching in SF6/O-2 plasma. The kinetic parameters in the model are determined by matching simulated profiles with experimentally observed feature profiles obtained at various pressures, rf-bias voltages, and O-2 mole fraction in the feed gas. The model parameters are further constrained by using information about the relative radical concentrations, ion flux, and ion energy obtained from plasma diagnostics. Excellent agreement between experiments and simulations is obtained. The combined experimental and simulation study reveals that chemical etching in the lateral direction is significantly reduced through competitive adsorption of O on the feature sidewalls and subsequent formation of a fluorinated oxide layer that passivates the sidewalls. The flux of F and SF, radicals is focused toward the feature bottom due to increased neutral reflection off the passivated sidewalls. The net result is enhanced etching in the vertical direction and improved feature anisotropy with decreasing F-to-O ratio (increasing O-2 fraction). However, too much O-2 addition eventually leads to the slowing down of the vertical etch rate as O adsorption on active surface sites dominates even at the feature bottom. (c) 2005 American Vacuum Society.