Reactive ion etching of silicon in an RF parallel plate system, using SF6/O-2/CHF3, plasmas has been studied. Etching behavior was found to be a function of loading, the cathode material, and the mask material, Good results with respect to reproducibility and uniformity have been obtained by using silicon as the cathode material and silicon dioxide as the masking material for mask designs where most of the surface is etched. Etch rate, selectivity, anisotropy : and self-bias voltage have been examined as a function of SF6 flow, O-2 flow, CHF3 flow, pressure, and the RF power, using response surface methodology, in order to optimize anisotropic etching conditions. The effects of the variables on the measured responses are discussed. The anisotropic etch mechanism is based on ion-enhanced inhibitor etching. SF6 provides the reactive neutral etching species, O-2 supplies the inhibitor film forming species, and SF6 and CHF3 generate ion species that suppress the formation of the inhibitor film at horizontal surfaces. Anisotropic etching of high aspect ratio structures with smooth etch surfaces has been achieved. The technique is applied to the fabrication of three-dimensional micromechanical structures.