We have developed a semiempirical feature scale model of Si etching in SF6 plasma. which incorporates the addition of small amounts of O in the discharge coming from the etching of the oxide mask and quartz window. The degrees of freedom in the model are reduced by using information from plasma diagnostics and previously published data to estimate the ion flux, the ion energy and angle distributions, and the relative F and O fluxes. Experimentally inaccessible parameters such as the F sticking coefficient, chemical etch rate constant, and the ion-enhanced etch yield are determined,by matching simulated feature profiles with those obtained from carefully designed etching experiments. Excellent agreement between experiments and simulations is obtained. (C) 2005 American Vacuum Society.