Reactive ion etching of polycrystalline diamond films has been performed in an electron cyclotron resonance SF6 plasma by applying negative substrate bias (V-dc) The structure and composition at the surface were examined as a function of V-dc by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The SEM images revealed that the etching proceeded almost uniformly rather than preferentially over the surface, and the morphology after the etching was independent of V-dc The bulk plasmon loss features in the C ls spectra by angle-resolved XPS revealed the evolution of an amorphous layer over a diamond film surface with increasing V-dc The residual sulfur and fluorine concentrations were found to be high in the interior of the film or in the underlying diamond, ranging up to 0.6 and 1.3% for sulfur and fluorine, respectively, and rather low in the amorphous overlayer. The depth variation of their concentrations was interpreted in terms of subsurface dynamic effects based on the incorporation and displacement of atoms by penetrating energetic ions. (C) 2001 The Electrochemical Society. All rights reserved.