Unimolecular Decomposition of Phosphine in Chemical Vapor Deposition Reactors

Abstract

Unimolecular decomposition of a source species is often the first step in the sequence of reactions in the gas phase and has been found to be of primary kinetic significance in CVD systems. Here, we compute the rate constants for phosphine decomposition under both polysilicon and epitaxial silicon deposition conditions; rate constant values are provided for decomposition at 900 and 1300 K in a pressure range of 0.2–760 Torr. Rate constants are calculated using a master equation formalism with an exponential model for the collisional energy transfer. A canonical variational approach is used to locate the transition state of the H atom elimination channel. It is shown that the hydrogen molecule elimination channel dominates the hydrogen atom removal channel under all conditions studied. Significantly, our rate constant values are about three orders of magnitude lower than the upper bound estimates, obtained as strong collision values multiplied by a collision efficiency factor, reported in the literature. These differences could be accounted for by our accurate estimation of the critical energy of the hydrogen molecule elimination channel. The contribution of the unimolecular decomposition step to the species mass balance equation for phosphine is determined by calculating the appropriate dimensionless parameter. Based on the magnitude of this dimensionless parameter, we find that the unimolecular decomposition step has to be taken into account in the phosphine balance equation for pressures greater than about 10 Torr at 1300 K.