Abstract: Method and apparatus for masked etching of a polysilicon surface layer or film to expose a dielectric underlying layer or film on a semiconductor material using ion bombardment from an ionized mixture of a fluorine based gas with a chlorine or bromine containing gas. A particularly useful gas is a mixture of sulfur hexafluoride and Freon 115 gases (C.sub.2 ClF.sub.5). The mixture of gases achieves the result of highly selective etching through the polysilicon film without significantly attacking the underlying dielectric film and without significant undercutting in the polysilicon film or etching of the masking layer.