A theory of plasma-assisted chemical vapor transport processes

Abstract

A theory is presented for plasma-assisted chemical vapor transport (PACVT) processes, applicable to reactive ion or plasma etching, plasma-assisted chemical vapor deposition and, in the absence of a plasma, low-pressure thermally driven chemical vapor transport processes. The central results of this theory are the steady-state or nonequilibrium gas-phase chemical concentrations in the vicinity of the reacting surfaces and the rate at which the etch/deposit solid is transported into or out of the reactor volume and/or between reactor surfaces as a function of reactor geometry, (ion-bombardment-enhanced) surface reaction rates, diffusion, and flow rates. Taking the rate-limiting chemical reactions to be heterogeneous leads to a transport equation that predicts behavior for such processes that is essentially in agreement with experiment. Instead of depending on the homogeneous reaction kinetics, this behavior stems from the competition between surface reaction rates, diffusion, and flow rates, yielding a dependence on reactive surface to volume ratios or reactor loading. In contrast to the preceding loading equation, this one is applicable to the entire range of flow for which the transport rate depends on reactor loading, while the reciprocal of the etch rate still exhibits the linear dependence on loading obtained earlier. Also, the theory clarifies some previously observed, but inadequately explained, phenomena unique to PACVT, such as increased flow rate sensitivity for more exoenergetic reactions, flow-dependent ultrahigh etch selectivity between conducting and insulating films, and etch pattern sidewall deposition for endoenergetic reactions.