A secondary glow region is commonly observed between the main plasma ball and the substrate during biased enhanced nucleation (BEN) of diamond films grown using the process of microwave plasma-enhanced chemical vapour deposition. Under conditions of very careful optimization BEN can allow (1 0 0) oriented polycrystalline diamond films to be grown on (1 0 0) silicon substrate; such films are industrial important. Recently it has been suggested that this secondary plasma plays a key role in nucleation density enhancement and orientation effects of the BEN process. However to-date this region of the plasma has received little study. We have used spatially resolved optical emission spectroscopy (OES), optical photography and electrical measurements as diagnostics to qualitatively investigate this region. The measurements show that the appearance of the secondary glow exhibits threshold behaviour which correlates with the I-V measurements carried out during BEN. OES measurements show that the glow region is located about 2 mm above the substrate surface and suggest that it can act as a strong source of atomic hydrogen supporting its role as a region below which strong etching occurs as suggested in recent models of the BEN process.
