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 fi lm thickness in large-area deposition of hydrogenated amorphous silico n, a-Si:H. To simulate improper substrate-to-electrode attachment we d eliberately introduced a gap behind the substrate. The rf-excitation-f requency dependence of the influence of this gap on the deposition rat e 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 typical ly 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 dyna mics are considered. The relations between the de self-bias voltage, t he amplitude of the applied rf voltage, and the deposition rate are de termined experimentally. A theoretical. model that explains the reduct ion of the deposition rate is presented. We conclude from the model th at the ion density in the sheath is independent of the excitation freq uency. (C) 1996 American Institute of Physics.