On the balance between silylene and silyl radicals in rf glow discharges in silane: The effect on deposition rates of a-Si:H

Abstract

Plasma enhanced chemical vapor deposition (PECVD) of amorphous hydrogenated silicon (a-Si:H) is typically performed in low-pressure (≤0.5 Torr) radio frequency (rf) discharges in gas mixtures containing silane (SiH4). The initiating step is electron impact dissociation of silane, whose products are primarily the silylene (SiH2) and silyl (SiH3) radicals. The ratio of SiH2 to SiH3 in the plasma is important because these radicals incorporate differently into the a-Si:H film; and therefore the characteristics of the film are a function of the ratio [SiH3]/[SiH2]. The initial silane dissociation step is followed by a series of hydrogen abstraction, silylene insertion, and silyl association reactions which subsequently alter this ratio. The branching ratios for electron impact dissociation of silane, as well as the rate constants for the subsequent reactions, have not been measured or are uncertain. Using a model for a rf discharge in silane gas mixtures, the effects of branching ratios for silane dissociation, and rate constants for key reactions are investigated. We find that in order for SiH3 to be the dominant radical, as currently thought, its branching ratio from electron impact dissociation of SiH4 must exceed 0.75, and the yield for H atoms in the branch for SiH2 must also exceed 0.75. The deposition rate of a-Si:H is then controlled by the yield of H atoms and the subsequent generation of radicals by hydrogen abstraction from SiH4. These results require that the silyl association reaction has a rate constant ≤10−11 cm3 s−1, and that the rate constant for insertion of SiH2 into silane exceeds 10−11 cm3 s−1.