Jl. Castillo et De. Rosner, ROLE OF HIGH ACTIVATION-ENERGY HOMOGENEOUS CHEMICAL-REACTIONS IN AFFECTING CVD-RATES AND DEPOSIT QUALITY FOR HEATED SURFACES, Chemical Engineering Science, 51(24), 1996, pp. 5325-5340
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