Clusters in A Silane Glow Discharge: Mechanism of Their Formation and How to Avoid Them

The formation of clusters in glow discharge plasma processing is of great concern with respect to the production yield. Their appearance, trapping and transport in silane plasmas have been the subject of several publications, but little is known about the mechanism of their formation and growth and how to avoid them in intense discharges used for high rate deposition of amorphous silicon. We present mass spectrometric and light scattering data and theoretical modelling which show that the formation of clusters in a clean silane discharge (total impurity ≈ 10 ppm) is due to a sequential growth of higher silanes SinH2n+2 with a strong, catastrophic-like onset starting from pentasilane. A selfconsistent mechanistic model will be presented together with a discussion of alternative ionic mechanisms. Several possibilities of how to avoid the cluster formation at high deposition rates of a-Si will be discussed and documented.