Small particle growth in silane radio-frequency discharges

Abstract

Particle size and number density are measured versus discharge operating time (t) in pure silane, capacitively coupled rf discharges. Discharge conditions are those typically used to produce amorphous silicon devices, except the temperature is 300 K. Particle radii (Rp) from 4 to 25 nm are observed, where Rp is determined from the rate of particle diffusive loss in the plasma afterglow. The scattered light intensity then yields the particle number density. We generally observe a Rp(t) consistent with initial rapid growth to Rp∼2 nm, followed by a constant dRp/dt at a rate consistent with growth due to SiH3 reactions. However, at higher pressures and film growth rates, or rf voltage, the particle growth rate exceeds that due to SiH3, indicating additional growth mechanisms. Particle density increases orders of magnitude when pressure and rf voltage increase by small amounts. Measurement of the particle spatial distribution in the afterglow, when most particles are neutral, shows that they have sufficient sticking probability on the upper electrode to yield essentially equal deposition on both electrodes.