One-dimensional modelling of a capacitively coupled rf plasma in silane/helium, including small concentrations of O-2 and N-2

摘要

Functional coating deposition using plasma is broadly used in industrial application working at a pressure ranging from low pressure discharges (a few Pascals) to atmospheric plasmas. The active gas (silane is selected for this study) is often diluted in a gas that helps in stabilizing the discharge like helium or argon. In addition, the discharge can be polluted by uncontrolled external gas source like air or oxygen coming from water adsorbed in reactor walls. In this paper, we study the interactions taking place within the bulk of a capacitively coupled plasma and study the impact of these reactions on the flux of species moving towards the substrate and so the impact on the composition of deposited film. A one-dimensional fluid model is presented for the modelling of radio frequency capacitively coupled plasmas in a mixture of silane/helium, including small concentrations Of O-2 and N-2. In total, 48 different species (electrons, ions, neutrals, radicals and excited species) are considered in the model. After a sensitivity study, 27 electron-neutral and 76 chemical reactions (i.e. ion-neutral and neutral-neutral reactions) were maintained in the fluid model. The fluid model itself consists of a set of mass balance equations (i.e. one for every species), the electron energy equation and the Poisson equation. The reaction rate coefficients of the electron-neutral reactions, as a function of average electron energy, are obtained from a Boltzmann model. The reaction rate coefficients of the ion-neutral and neutral-neutral reactions are assumed to be constant. It is found that helium does not affect the silane plasma chemistry drastically. The incorporation of small amounts of air (containing about 82% N-2 and 18% O-2) in a silane/helium plasma, however, influences the plasma chemistry to a large extent. A large number of nitrogen species (i.e. N-2, N, N-2(+)), and species containing oxygen (i.e. SiH3O, SiO, OH and others), are present in the discharge at relatively high densities (i.e. of the order of 10(14)-10(17) m(-3)). [References: 60]