Model of particle growth in silane discharges

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-Si:H) device production. SiH3 and SiH3- grow into particles by the step-by-step addition of silicon atoms, primarily due to reactions with SiH3. Attrition of growing SixHmz 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 SixHmz for x = 1-10(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)congruent to 100, which causes a large fraction of neutral particles and thereby a major particle flux into the growing a-Si:H film. The density of visible particles (x > 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 SixHm clusters with 3<x<30 are continuously deposited into growing films, and for typical conditions yield a very significant fraction (1-10 %) of total film growth.