The effect of added O2 on the etching of SiO2 sidewalls in a CF4 plasma was examined using a transformer-coupled plasma etcher, for two cases when the sidewall was either affected or unaffected by particles emitted from the bottom SiO2 surface. The deposition rate on the sidewall decreased in proportion to the amount of added O2. This occurred because the increased amount of F radicals enhanced the re-etching of SiO2, which is present beneath the surface polymer layer, and of etch products, which are redeposited from the bottom. The surface polymer layer on the sidewall, affected by particles emitted from the bottom, became thin and smooth as a result of the reaction with O radicals. The chemical etch rate of the sidewall, unaffected by energetic ions and bottom-emitted particles, and the bottom etch rates were the highest for oxygen concentrations of 10% and 20%, respectively. These concentrations were lower than an O2 concentration of 30%, which yields the maximum concentration of F radicals. The mismatch in the O2 concentrations arises because the O2 concentration required to obtain the maximum etch rate on the sidewall or bottom surface is determined by competitive reactions among F, CF2, O radicals, and incident ions. © 2003 American Vacuum Society.