An anisotropic nonselective etch for GaAs/AlGaAs rib waveguides has been developed for use under electron cyclotron resonance conditions. The plasma chemistry features BCl3 to minimize AlGaAs oxidation effects and small additions of N-2 to induce sidewall protection when using photoresist masks. The fundamental mode attenuation hi GaAs/AlGaAs waveguides is sensitive to the choice of both plasma chemistry and masking material, but can be reduced to <1 dB cm(-1) for channel widths of 4-5 mu m.