Cl-2-based plasmas for etching GaAs, AlGaAs, and GaP have been examined as a function of gas additive (Ar, N-2, or H-2), radio frequency (RF) and microwave power, plasma composition, mask material, and process pressure. In a load-locked reactor, smooth etched surface morphologies were obtained over basically all conditions investigated, with typical root-mean-square roughness of less than or equal to 1.5 nm measured by atomic force microscopy. The etch rates for all three materials increase with RF power (ion energy), microwave power (ion current), Cl-2 percentage, and pressure, with controlled rates of similar to 0.4 mu m/min at a condition of 2Cl(2)/13Ar, 850 W microwave power, 1.5 mTorr, and 100 to 150 W of RF power. Operating under electron cyclotron resonance conditions where the ion density is greater than or equal to 5 x 10(11) cm(-3) (measured by microwave reflection interferometry) produces rapid degradation of photoresist, and more robust mask materials such as SiNX, SiO2, or W are necessary.