In order to reactive ion etch (in a commercially available hex system) a feature in silicon dioxide leaving a vertical sidewall profile with no change in the resultant linewidth relative to the photoresist mask, it was found necessary to utilize a bilevel masking structure with polysilicon acting as the nonerodable hard mask. This hard mask prevents tapering and linewidth changes due to lateral photoresist erosion during the oxide etch. In addition, the etch was performed under conditions producing very little fluorocarbon polymer deposition to prevent lateral mask growth during the etch, and at very low pressure (≈5 mTorr) to reduce off-angle ion scatter-induced etching due to collisions in the plasma sheath. In a nonpolymer-depositing mode, the lack of sidewall passivation was found to promote lateral etching of the oxide at pressures where this scattering occurs. The same phenomena were found to be present when etching nonphotomasked oxide sidewall spacers. Etching spacers in a very nonpolymerizing chemistry at normal operating pressures (≈50 mTorr) caused significant lateral erosion of the spacers. Use of a low pressure, nonpolymerizing chemistry, or a higher pressure, polymerizing process gave spacer widths consistent with the deposited thickness of the spacer material.