Influence of a DC grid on silane r.f. plasma properties

Abstract

In this work we show that it is possible to control the plasma regime in the region close to the substrate in r.f. silane discharges. The PECVD reactor works in a modified triode configuration, where the control over the plasma regime is performed by polarising a grid electrode, placed close to the r.f. electrode, with a DC power source. Besides that, the DC grid allows also to control the energy of the ion bombardment, because the plasma potential will be a function of the voltage ( V pol) applied to the DC grid. The silane plasma was characterised with a Langmuir probe and an impedance probe. We were able to identify three plasma regimes in the region close to the substrate: γ′ regime for V pol <0 V ; γ′– α regime for 0 V<V pol <40 V ; and α regime for V pol >40 V . The γ′ regime is associated with a high concentration of dust particles in plasma and high electron energy (≈8 eV), while the α regime is associated with a free dust plasma and low electron energy (≈2 eV). The intermediate regime, γ′– α, is characterised by the presence of smaller particles (≈2–5 nm) that can be beneficial for the film's properties.