Control of abnormal grain inclusions in the nanocrystalline diamond film deposited by hot filament CVD

Formation of abnormal grain inclusions in nanocrystalline diamond films deposited by hot filament CVD (HFCVD) was investigated. The phenomenon was attributed to two different origins: an intrinsic and an extrinsic one. The inclusions due to the intrinsic origin could be either avoided or weakened by controlling chamber pressure, CH₄/N₂ concentrations in H₂, and by positive substrate bias. The extrinsic origin for the abnormal grains was found to be the contamination from the alumina insulation tubes for the thermocouple placed near the substrate, which were degraded by the extended exposure to the high temperature and strongly reducing atmosphere.