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 de 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. (C) 1996 American Institute of Physics.