The effect of substrate bias voltage was investigated for bias‐enhanced diamond nucleation pretreatments of diamond thin films in a microwave plasma chemical vapor deposition reactor. A critical bias voltage of approximately −200 V was observed for nucleation density enhancement from ∼104cm−2to ∼1010cm−2. Furthermore, the nucleation density under bias conditions was five orders of magnitude lower for a small silicon region electrically isolated from the otherwise negatively biased silicon substrate. These results confirm that bombardment of the substrate by energetic cations plays a significant role in the diamond nucleation mechanism during bias pretreatments. ©1995 American Institute of Physics.