We have developed a dry etching technique in which the chemically reactive species and the energetic ions are independently controlled. This technique, which we call ion beam assisted etching (IBAE), is accomplished by impinging a chemically reactive gas from a jet and an ion beam from a Kaufman ion source upon the sample. It is based upon the work of Coburn e t a l., 1 and differs fundamentally from reactive ion etching (RIE), which relies upon a plasma to produce both the chemically active species and the energic ions. In IBAE the partial pressure of the reactive gas can be as high as several Pa (tens of microns) at the sample surface while the chamber pressure can easily be maintained at about 1×10−2 Pa (10−4 Torr). Although a variety of reactive gases and sample materials have been characterized, the presentation will concentrate on using Cl2 as a reactive gas and an argon ion beam. The anisotropic character of this etching technique has been found to depend on the angles that the gas jet and the ion beam make with the surface of the sample. Etching rates as high as a few micrometers per minute have been obtained with GaAs and Al samples and anisotropicetching aspect ratios as large as 50 were easily obtained with GaAs. The etched surface of the GaAs was also electrically characterized in two ways: first, by fabricating Schottky diodes on it and measuring the ideality factor, and secondly, by growing VPE GaAs on the etched surface and profiling the traps at the interface between the etched surface and the VPE GaAs using deep level transient spectroscopy(DLTS). The quality of the etched surface was found to improve as the reactive gas flux varied from about 1×1017 to 5×1018 atoms cm−2 s−1. Measured ideality factors of the Schottky diodes fabricated on GaAsetched under high reactive gas flux conditions were equivalent to diodes fabricated on virgin GaAs. The density of traps at the interface between the etchedGaAs and the VPE GaAs was less than 1×1015 cm−3 which is low enough for many GaAs device applications. The independent parameter control afforded by this technique has enabled us to achieve desirable anisotropicetching characteristics while producing etchedGaAs surfaces with acceptable electrical quality.