Dry etching of InP using a Cl-2/Ar plasma generated by an electron cyclotron resonance source has been studied. A fast InP etch rate (2.7 mu m/min) with vertical profile and smooth surface morphology was achieved at room temperature for via holes that are 30 mu m wide and 110 mu m deep. InP etch rate has been found to increase with microwave power, temperature, RF power, and pressure. The results indicate that high temperature is not necessary to etch InP using Cl-2/Ar when the reactive species densities are high at high microwave power. Compared to other compound semiconductors, InP has the highest etch rate, followed by GaAs, GaInAs, GaAlAs, and AlInAs. In situ monitoring using mass spectrometry was used to provide precise control of the etch initiation time and the etch depth. Etch initiation time was found to decrease with ion energy. Since devices based on via holes usually have Ti/Au metallization layer on the front side, the (PCl2+)-P-101 and (TiCl3+)-Ti-155 signals were used for end point detection. Etching can be controlled to stop with <50 nm Ti removed after reaching the bottom of the InP via holes.