We have investigated the etching characteristics of InP etched with a CH3Cl/Ar/H-2 gas mixture with electron-cyclotron-resonance excitation and compared them with those for a CH4/Ar/H-2 gas mixture. Some advantages of CH3Cl/Ar/H-2 over the CH4/Ar/H-2 are found. A smooth etched surface without etch residue is obtained at 120 degrees C and above. Etch residue, which is drop shaped and originates from the preferential desorption of phosphorus, is inevitably generated when the CH4/Ar/H-2 is used. The etch rate (15-25 nm/min at the temperatures of 120-175 degrees C) is larger than in CH4/Ar/H-2 etching. 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.