We have studied the mechanism of increased etch rate induced in InP substrates by focused Ga ion implantation and Cl2etching. We cannot account for the depth of surface steps formed in this process with a purely kinetic mechanism. The preferential etching of implanted areas is attributed instead to local modification or removal of native oxides from the surface of InP. The thin oxide layer effectively protects the substrate and inhibits Cl2etching. Consistent with the thermodynamic prediction, a cleaned InP(100) surface is etched by Cl2(5×10−4Torr) with a rate of approximately 1000 A˚/min at 200 °C. Surface steps as deep as 3 &mgr;m have been reproducibly prepared using an oxide mask believed to be approximately 20 A˚ thick. In the etching process, any substrate damage caused by the Ga beam writing is completely removed. The oxide mask, which can be patterned on a very fine scale by energetic particle bombardment, provides a new avenue forinsituprocessing of InP.