Etching of InP using an electron cyclotron resonance source in a Cl2/Ar plasma has been investigated for the fabrication of microcavity laser structures. Fast, smooth, and reproducible etching was achieved with etch rates of 2.14 μm/min. It was found that the etch profile depended on the Cl2 concentration in the Cl2/Ar plasma. Vertical etch profile was obtained for Cl2 concentration ≤15%. Hard-baked photoresist and Ni have been used as etch masks. Typical etch rate of hard-baked photoresist was 0.10 μm/min and etch selectivity between InP and photoresist was 21. Typical etch rate of Ni was 13 nm/min with a selectivity of 165. The etched InP surface was smoother for samples with photoresist mask. For samples with Ni mask, the etched surface became rougher when narrower or deeper trenches were etched. With optimized etch conditions, high aspect ratio microstructures in InP that were 0.75 μm wide and 10.7 μm tall were fabricated with nearly vertical profile and smooth etched surface using photoresist mask. Similarly, arrays of microcavities of 1 μm in diameter and 4.5 μm tall have been formed. © 1997 American Vacuum Society.