The authors present the results and the optimization procedure for a time-multiplexed dry etching process to etch GaAs in an inductively coupled plasma reactive ion etching system. The gas feed chopping sequence employed a SiCl4 etch phase and an O-2 passivation phase. Care is taken not to intermix O-2 with SiCl4. The investigated structures consist of pillars, trenches, stripes, and holes, all with lateral structure size of 1 mu m or less. This feature size is interesting for diffractive elements and cavities in integrated mid-IR optoelectronics. They achieve an aspect ratio of 10 for holes, 17 for trenches, and 30 for stripes with a selectivity of 200:1 on open areas. The improvements in the sidewall morphology are related to the O-2 passivation step investigated by optical emission spectroscopy and energy dispersive x-ray analysis that reveals a Si-rich SiOX sidewall. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3225599]