We examine methane-hydrogen-argon-based deep dry etching of InP for facet-quality sidewalk by reactive ion etching. A process is developed for etch depths as high as 5.8 mum. Masks studied include Ni, NiCr, Ti, SiO2, and Ti-SiO2. Sidewall roughness was estimated to be a few nn (based on high resolution scanning electron micrographs). This may be useful for fabrication of optical microelectromechanical systems, photonic wires, and photonic crystals in the InP material system.