Model of particle growth in silane discharges

Abstract

The growth of silicon particles in the neutral plasma region of pure silane, rf capacitively coupled, steady-state discharges is calculated with a homogeneous, plasma-chemistry model. Plasma conditions are typical of those used in hydrogenated amorphous silicon $(a-\mathrm{S}\mathrm{i}:\mathrm{H})$ device production. ${\mathrm{SiH}}_3$ and ${\mathrm{SiH}}_3^{\mathrm{-}}$ grow into particles by the step-by-step addition of silicon atoms, primarily due to reactions with ${\mathrm{SiH}}_3.$ Attrition of growing ${\mathrm{Si}}_x{\mathrm{H}}_m^z$ radicals and ions with z charges, which are “particles” for large x, occurs by diffusion of neutral and positively charged radicals to the electrodes. Rate coefficients for electron, ion, radical, and silane collisions with the ${\mathrm{Si}}_x{\mathrm{H}}_m^z$ for $x=1\mathrm{}–{10}^5$ are estimated from detailed considerations of the literature and relevant physics. Self-consistent anion, cation ${(n}_{+}),$ and electron ${(n}_e)$ densities and charge fluxes are used, and charge neutrality is maintained. Typically $n_{+}{/n}_e\cong 100,$ which causes a large fraction of neutral particles and thereby a major particle flux into the growing $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ film. The density of visible particles $(x>\mathrm{}{10}^4)$ varies many orders of magnitude with relatively minor changes in discharge power, pressure, and electrode gap. This parameter dependence agrees with experiment, and by adjusting collision parameters within a reasonable range the calculated particle densities can be brought into exact agreement with experiment. An additional result of the model, which has not yet been detected, is that ${\mathrm{Si}}_x{\mathrm{H}}_m$ clusters with $3<x<\mathrm{}30\mathrm{}$ are continuously deposited into growing films, and for typical conditions yield a very significant fraction (1–10 %) of total film growth.