A multipolar, tuned cavity, electron cyclotron resonance (ECR) source is applied to hydrocarbon plasma etching of InP. Various ECR modes are created at different cavity dimensions and etch properties have been studied as a function of this parameter and ECR power. These etch characteristics are correlated with in situ optical emission spectroscopy, Langmuir probe, and secondary ion mass spectrometry diagnostics. Relative concentrations of plasma species and, therefore, etch characteristics, are highly dependent on specific cavity modes and ECR power. Plasma diagnostics show clearly that rough morphologies are associated with large concentrations of hydrogen and argon ions and that these are produced for all modes at high power and for efficiently coupled modes at lour power Finally, a threefold increase in etch rate with improved surface morphology has been achieved at 160 W ECR power by increasing the relative flow of methane in the gas mixture.