We studied the magnetron ion etching of GaAs in SiCl4/Cl-2 discharges and sidewall passivation effect which closely relates to the anisotropic dry etching. The effects of a variety of process parameters on the final via hole profiles and morphologies were also examined. Furthermore, in order to determine the surface chemistry of the residues on GaAs sidewall in magnetron plasmas, surface characterization was undertaken using scanning electron microscopy, transmission electron microscopy combined with an energy dispersive x-ray spectrometer, and x-ray photoelectron spectroscopy. It was confirmed that the residues, namely the sidewall protection film, consists of double layers. The aluminum content of the underlayer is four times greater than that of the upper layer. The aluminum mainly results from sputtering of the alumina cathode covers. The formation of the sidewall protection film was identified as the key factor in controlling the via hole profile for GaAs device fabrication.