A two-dimensional plasma etching model based on the SIMBAD Monte Carlo thin-film growth simulation program has been modified to include ion-enhanced etch processes. The modified program has been applied to a study of sidewall passivation in the etching of silicon in a CF4/O2 plasma. Experimentally measured mask undercut in the etching of 0.8-mum-wide lines was reduced from 150 angstrom/min to virtually zero with an increase in oxygen from 8% to 45% of total gas flow. The corresponding decrease in etch rate was from about 750 to 325 angstrom/min. Simulations account for this change based on competition between oxidation and flourine etching of the silicon surface. Reactive gases follow collisionless trajectories at the simulation scale and have a high probability of diffuse re-emission following contact with the surface. This simulation method extends current profile simulation capabilities for a well-known etching chemistry.