Reactive ion beam etching experiments have been performed on and polycrystalline Si using , , and . The etch rate for using ions generated from fluorocarbon gases is enhanced over the rate obtained using argon ions, whereas the etch rate for Si is suppressed. Etch rates for thermal, LPCVD, and “Nitrox” oxides, and phosphorus‐doped and undoped polycrystalline Si are compared. LPCVD etches slightly faster than the other types of oxides while doped and undoped polycrystalline Si show little difference in etch rate. etch rates of greater than 600 Å/min have been observed with selectivities over Si of 6:1. The anisotropy of this technique is demonstrated by the formation of 0.5 μm diam contact windows in a 1 μm thick layer of on single crystal Si.