We have investigated the etching characteristics of InP etched with aCH3Cl/Ar/H2gas mixture with electron-cyclotron-resonance excitation and compared them with those for aCH4/Ar/H2gas mixture. Some advantages ofCH3Cl/Ar/H2over theCH4/Ar/H2are found. A smooth etched surface without etch residue is obtained at 120 °C and above. Etch residue, which is drop shaped and originates from the preferential desorption of phosphorus, is inevitably generated when theCH4/Ar/H2is used. The etch rate (15–25 nm/min at the temperatures of 120–175 °C) is larger than inCH4/Ar/H2etching. This etch rate is controllable for fine structure fabrication, such as the grating in a semiconductor laser diode, and successful fabrication of a grating with 150 nm pitch and 30 nm depth is demonstrated.