Carbon thin films produced by electron beam sublimation deposition and solvent liftoff have been used as reactive ion etch masks for etching deep trenches on InP. Using a Cl-2:HBr:BCl3:Ar-based plasma in a reactive ion etcher, 15-mu m-wide trenches were etched to a depth of 18 mu m. The etch rate selectivity between InP and the carbon mask is better than 100:1. The resulting sidewall profiles are highly anisotropic due to the low sputter rate of carbon.