ABSTRACTWe have successfully used an ultrathin (20-50 Å) native oxide layer on the surface of InP as an etch mask for transferring patterns onto the substrate. The oxide mask is grown in situ in O2 atmosphere, and the mask pattern is created by locally removing the oxide with a focused ion beam. Depending on the thickness of the mask, the required ion dose varies from 2×1014 to 2×1015 Ga/cm2. C12 etches the exposed areas selectively. Features as deep as 3 microns have been produced with such an ultrathin mask. High quality InGaAs and InP epitaxial layers have been overgrown on such patterned substrate. We have studied the formation and desorption of the oxide mask with Auger analysis. We also demonstrate that the secondary charged particle emission from a substrate during ion exposure provides a useful signal for the determination of the required dose.