A process is provided for etching a silicon based material in a substrate, such as a photomask, to form features with straight sidewalls, flat bottoms, and high profile angles between the sidewalls and bottom, and minimizing the formation of polymer deposits on the substrate. In the etching process, the substrate is positioned in a processing chamber, a processing gas comprising a fluorocarbon, which advantageously is a hydrogen free fluorocarbon, is introduced into the processing chamber, wherein the substrate is maintained at a reduced temperature, and the processing gas is excited into a plasma state at a reduced power level to etch the silicon based material of the substrate. The processing gas may further comprise an inert gas, such as argon.