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

Despite considerable study, the mechanisms underlying the process of bias e nhanced 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 th e generation of oriented nuclei. Spatially resolved optical emission spectroscopy (OES), photography and ele ctrical measurements have been used to study this region in detail for H-2/ CH4/Ar, H-2/Ar and purr Ar microwave plasmas. For a clean tungsten substrat e the secondary plasma appears once the bias exceeds a value of similar to 100 V: interestingly this coincides with threshold bias for significant enh anced nucleation. OES actinometry shows that axial profiles of relative ato mic hydrogen concentration and the electron temperature are peaked 1.8 mm a bove the substrate. The emission intensity profiles show strong similaritie s with the conditions existing close to the cathode of a DC glow plasma; el ectric field strengths an of similar magnitude. It is suggested that the se condary glow seen in biased microwave plasmas can be understood as a DC glo w plasma which is generated by secondary electrons arising from ion bombard ment of the substrate. These are then strongly accelerated in the sheath fi elds, leading to high inelastic process rates directly above the growth sub strate. (C) 2000 Elsevier Science S.A. All rights reserved.