In this paper we present the development of a technique for anisotropically etching silicon on GeSi. Wet chemical etching exhibits a selectivity of nearly 40:1 but is completely isotropic and requires the use of a hard mask. The fluorine plasma process which has been reported in the literature as having high selectivity was nonreproducible as a result of significant polymeric deposition on the chamber surfaces. Furthermore the etch residue was not easily removed in an O2 plasma and led to highly resistive contacts. Reactive ion etch processes using BC13/C12 and SiCl4 were found to anisotropically etch the silicon layer, but had poor selectively to GeSi. By combining a reactive ion etch at 300 W forward power and 20 mTorr in SiCl4, with a wet chemical dip, an adequate process for selectively patterning submicron features was obtained.