Deposition-rate reduction through improper substrate-to-electrode attachment in very-high-frequency deposition of a-Si:H

We have tracked down one of the major causes for nonuniformities in film thickness in large-area deposition of hydrogenated amorphous silicon, a-Si:H. To simulate improper substrate-to-electrode attachment we deliberately introduced a gap behind the substrate. The rf-excitation-frequency dependence of the influence of this gap on the deposition rate is presented. We show that a local small gap behind the glass has a detrimental effect on the local deposition rate, and therefore on the uniformity of the films. For example, at a frequency of 60 MHz typically the reduction of the deposition rate amounts to 25% when a gap of 1 mm is present. To explain the observed effects the plasma-sheath dynamics are considered. The relations between the dc self-bias voltage, the amplitude of the applied rf voltage, and the deposition rate are determined experimentally. A theoretical model that explains the reduction of the deposition rate is presented. We conclude from the model that the ion density in the sheath is independent of the excitation frequency.