Controlling the plasma chemistry of silicon nitride and oxide deposition from silane

Abstract

We have studied the chemistry of glow discharge plasma-enhanced chemical vapor deposition (PECVD) by using vibrating quartz crystal deposition rate monitoring at 200–300 °C and line-of-sight mass spectrometry of orifice-sampled reactive neutral species. In the deposition of dielectric films from silane plus a large excess of oxidant (NH3, N2, or N2O), the key process factor is the ratio of plasma power to silane supply rate. When enough of the oxidant is activated by the plasma, it completely converts the silane to films which have no excess Si and no Si–H bonding. The critical ratio can be detected by the disappearance of Si2H6 byproduct or by the presence of excess activated oxidant. Nitride deposited from N2 is electrically leaky due to porous microstructure even when deposited using excess activated oxidant. Conversely, nitride deposited from NH3 is nonporous, and when deposited using excess activated oxidant it has a surprisingly low electron trapping rate which is at least as low as that achievable in PECVD oxide. Deposition using N2 involves no formation of gas-phase precursor molecules; but using NH3, •Si(NH2)3, and Si(NH2)4 were the dominant precursors. SiO2 deposition from N2O involved a minor contribution from Si(OH)4. High reactant partial pressure or excess rf power can lead to particle formation and deposition rate loss, especially in SiO2 deposition, due to gas-phase reaction among precursors. Partial pressure reduction either by He dilution or by total pressure reduction suppressed these problems with equal effectiveness in SiO2 deposition. No other effects of He could be detected either in the plasma chemistry or in the bulk film trapping rate.