Reactive ion etching (RIE) of GaSb samples was investigated under electron cyclotron resonance (ECR) conditions using CH4/ H-2/ Ar gas mixtures. Different etching parameters [ e. g., microwave and radio-frequency (rf) power levels, gas ratios, chamber pressure, etc.] were systematically varied to observe their effects on the etch characteristics. For microwave powers up to similar to 200 W the etch rate was found to increase with increasing microwave power. Above 200 W the etch rate appears to saturate for lower rf power levels ( 100 W ) but at higher rf power ( 275 W ) the etch rate continues to increase linearly with microwave power over the range investigated. Gas ratios of CH4/ H-2/ Ar of 4: 16: 7.6 and 8: 12: 25 were used with the latter ratio yielding higher etch rates. The etch rate decreased with increasing chamber pressure over the range of 2 - 20 mTorr. Etching of the SiO2 mask was observed and the corresponding GaSb etch rates were adjusted to account for this mask erosion. Excellent feature geometries were observed with vertical sidewalls and a smooth surface morphology. (c) 2007 The Electrochemical Society.