We report the development of very high etch rates (>4 mu m/min), for InP via hole processes. These processes were developed in an electron cyclotron resonance system using a Cl-2/Ar plasma without heating the sample. The InP etch rates increased as a function of Cl-2 percentage in the Cl-2/Ar mixture, rf power, or microwave power. Via holes, with depths of 100 mu m, suitable for monolithic microwave- integrated circuits applications, have been achieved at etch rates as high as 4 mu m/min. To the best of our knowledge, this is the highest etch rate ever reported in InP for via hole applications.