AVS 56th International Symposium & Exhibition
    Plasma Science and Technology Monday Sessions
       Session PS2+PV-MoM

Invited Paper PS2+PV-MoM9
Hydrogen-dominated Plasma, Due to Silane Depletion, for Microcrystalline Silicon Deposition

Monday, November 9, 2009, 11:00 am, Room A8

Session: Plasma Processing for Photovoltaics
Presenter: A.A. Howling, EPFL Lausanne, Switzerland
Authors: A.A. Howling, EPFL Lausanne, Switzerland
R. Sobbia, EPFL Lausanne, Switzerland
Ch. Hollenstein, EPFL Lausanne, Switzerland
Correspondent: Click to Email
Plasma conditions for microcrystalline silicon deposition generally require a high flux of atomic hydrogen, relative to SiHx radicals, on the growing film. The necessary dominant partial pressure of hydrogen in the plasma is conventionally obtained by hydrogen dilution of silane in the flow inlet. However, a hydrogen-dominated plasma environment can also be obtained due to plasma depletion of the silane in the gas mixture, even up to the limit of pure silane inlet flow, provided that the silane depletion is strong enough. At first sight, it may seem surprising that the composition of a strongly-depleted pure-silane plasma consists principally of molecular hydrogen, without significant contribution from the partial pressure of silane radicals. The aim here is to bring some physical understanding by means of a zero-dimensional, analytical plasma chemistry model. The model is appropriate for uniform, large-area showerhead reactors as shown by comparison with results of three-dimensional numerical simulations. The SiHx densities remain very low because of their rapid diffusion and surface reactivity, contributing to film growth which is the desired scenario for efficient silane utilization. Significant SiHx densities due to poor design of reactor and gas flow, on the other hand, would result in powder formation wasting silane. Conversely, hydrogen atoms are not deposited, but associate on the film surface and re-appear as molecular hydrogen in the plasma. Therefore, in the limit of extremely high silane depletion fraction (>99%), the silane density falls below the low SiHx densities, but only the H radical can eventually reach significant concentrations in the hydrogen-dominated plasma.