Theoretical Analysis of Chemical Vapor Deposition of Ceramics in an Impinging Jet Reactor

Abstract

A mathematical model for chemical vapor deposition in an impinging jet reactor is extended to consider growth of materials that are nonstoichiometric or that contain multiple solid phases. The model treats the fluid flow of the reactant gas mixture, multicomponent heat and mass transfer, and simultaneous gas-phase and surface reactions. For a given system, insight into the rate-limiting steps can be obtained by identifying a set of reaction rate constants that gives a match between theoretical results and experimental data. The calculations also provide quantitative information on the relationships between operating conditions, deposition rate, and deposit composition. Results for production of boron carbide from BCI,, CH,, and Hz are presented as an example. Under typical processing conditions, deposition of B,C is found to be controlled primarily by the rate of CH, decomposition and the extent of surface coverage by CH,. Theoretical calculations are presented for different sets of operating conditions to illustrate the predictive capability of the model. Also, the sensitivity of results to various reaction mechanisms and to values for solid-phase activities is evaluated. Analogies are drawn between the boron carbide process and deposition of other ceramics in which a carboncontaining reactant is used.

Potential Surfaces for Unimolecular and Bimolecular Gas Phase Reactions of BHmCln Calculated at the G2 Level of Theory

“… a At 0 K, MP2 values without ZPE, G2 values include ZPE. b Compare with ref , .9 kcal/mol calculated by MRCI/pVTZ+sp. c Second-order saddle point. …”

“…Although there have been a number of publications examining the chemistry and kinetics of CVD film formation from BCl 3 , the validity of these analyses is uncertain because of a lack of accurate, relevant experimental or theoretical data on the decomposition of BCl 3 . For example, one of the most careful and detailed studies of boron carbide formation with CVD techniques modeled the chemistry and the fluid mechanics in a BCl 3 /CH 4 /H 2 stagnation flow system with just two gas phase and two gas−surface reactions However, because of a lack of experimental or theoretical data for these species, there is in fact no evidence that any of these global reactions play a role in the formation of boron carbide. CVD reaction schemes leading to formation of boron nitride from BCl 3 are of necessity similarly crude.…”

Chemical Vapor Deposition of Silicon from Disilane under Reduced Pressure in a Circular Impinging Jet Reactor: Simulation and Experiments

“…Nevertheless, an approximation such as the following one can be used when a small range of pressure is investigated kj = A3PBiT c~ exp (-Eaj/RT) [14] and for the corresponding reverse reaction k_~ = k/Kq [15] where A t and E~j are supposed to be constant, but differ from the classical A-factor and activation energy, the latter being defined as -R 0 in k/O(1/T). The uncertainty on the kinetic constants results from the complexity of the mechanism.…”

“…Accordingly, the classical A-factor as well as the activation energy of the Arrhenius expression of a reaction vary with both pressure and temperature. Nevertheless, an approximation such as the following one can be used when a small range of pressure is investigated kj = A3PBiT c~ exp (-Eaj/RT) [14] and for the corresponding reverse reaction k_~ = k/Kq [15] where A t and E~j are supposed to be constant, but differ from the classical A-factor and activation energy, the latter being defined as -R 0 in k/O(1/T). The corresponding parameters for the forward reactions are listed in Table II.…”