Characterisation of the secondary glow region of a biased microwave plasma by optical emission spectroscopy

Abstract

Abstract Despite considerable study, the mechanisms underlying the process of bias enhanced nucleation (BEN) as applied to CVD diamond remain unclear. However, early in the bias process a bright disc-like secondary plasma appears just above the substrate surface and recent reports implicate this region in the generation of oriented nuclei. Spatially resolved optical emission spectroscopy (OES), photography and electrical measurements have been used to study this region in detail for H 2 /CH 4 /Ar, H 2 /Ar and pure Ar microwave plasmas. For a clean tungsten substrate the secondary plasma appears once the bias exceeds a value of ∼100 V; interestingly this coincides with threshold bias for significant enhanced nucleation. OES actinometry shows that axial profiles of relative atomic hydrogen concentration and the electron temperature are peaked 1.8 mm above the substrate. The emission intensity profiles show strong similarities with the conditions existing close to the cathode of a DC glow plasma; electric field strengths are of similar magnitude. It is suggested that the secondary glow seen in biased microwave plasmas can be understood as a DC glow plasma which is generated by secondary electrons arising from ion bombardment of the substrate. These are then strongly accelerated in the sheath fields, leading to high inelastic process rates directly above the growth substrate.