Thermodynamics and Kinetics of Gas‐Phase Reactions in the Ti‐Cl‐H System

Abstract

A mechanism and associated rate constants for the gas‐phase chemical reactions that occur during the chemical vapor deposition (CVD) of titanium from titanium tetrachloride /hydrogen mixtures is presented. is the most widely used inorganic precursor employed in the CVD of titanium‐containing materials, such as titanium nitride, titanium carbide, and titanium diboride. Although the thermodynamic properties of this compound and its subchlorides (n = 2–4) are fairly well known, their gas‐phase decomposition kinetics have not been characterized. In this work, rate constants for the unimolecular decomposition reactions of the titanium chlorides were predicted using Rice‐Ramsberger‐Kassel‐Marcus theory for unimolecular reaction, while the rate constants for bimolecular reactions between species and hydrogen atoms were estimated using an empirical correlation. Calculations at thermodynamic equilibrium over a range of temperatures and total pressures characteristic of Ti CVD conditions are presented first. The time‐dependent evolution of the gas‐phase composition is then simulated using the proposed mechanism. The results show that at 1500 K complete equilibrium is reached in approximately 2 s at any total pressure in the range 0.01–101 kPa (1–760 Torr). The approach to equilibrium is much slower at 1000 K, with more than 300 s required to reach equilibrium at 101 kPa. Due to the pressure dependence of the unimolecular reactions in the mechanism these times are significantly shorter at atmospheric pressure than at reduced pressures. The results suggest that equilibrium predictions of gas‐phase concentrations should be a good approximation at 1500 K, unless very short residence times and low pressures are involved. In contrast, equilibrium calculations do not accurately reflect the gas‐phase composition at 1000 K.