ROLE OF HIGH ACTIVATION-ENERGY HOMOGENEOUS CHEMICAL-REACTIONS IN AFFECTING CVD-RATES AND DEPOSIT QUALITY FOR HEATED SURFACES

We present here a rational but simple asymptotic theory for the influe nce of homogeneous reagent consumption on heated surface chemical vapo r deposition (CVD) rates, exploiting the fact that in most cases the o verall activation energy of the homogeneous chemical reaction is large enough to confine reaction effects to a thin chemical sublayer embedd ed within the ordinary diffusion boundary layer. Explicit analytical r esults are obtained and illustrated for cases in which both heterogene ous and homogeneous kinetics can be represented by power-law/Arrhenius functions. Because of the large molecular weight disparities and high -temperature gradients prevailing in many CVD-systems we also allow fo r the Soret reduction of dilute reagent Fick transport to the hot surf ace. The resulting closed-form rate expressions provide rational quant itative criteria for 'vapor-phase ignition' (VPI) in terms of the CVD- system parameters and the presumed 'known' chemical kinetic parameters characterizing the vapor reactants. Conversely, armed with such a the ory, one can use experimentally observed VPI conditions to infer the e ffective homogeneous kinetic parameters for the system in question-inf ormation not always: independently available. In effect, one uses the gaseous boundary layer as a 'flow reactor', with the CVD-surface as a detector (albeit imperfect) of the surviving reactant. Our general res ults can be applied to specific film systems of current interest, with our present emphasis being the CVD of TiO2(s) [from TiCl4 or Ti(OC3H7 )(4)(g) + O-2]. Copyright (C) 1996 Published by Elsevier Science Ltd