THERMODYNAMICS AND KINETICS OF GAS-PHASE REACTIONS IN THE TI-CL-H SYSTEM

A mechanism and associated rate constants for the gas-phase chemical r eactions that occur during the chemical vapor deposition (CVD) of tita nium from titanium tetrachloride (TiCl4)/hydrogen mixtures is presente d. TiCl4 is the most widely used inorganic precursor employed in the C VD of titanium-containing materials, such as titanium nitride, titaniu m carbide, and titanium diboride. Although the thermodynamic propertie s of this compound and its subchlorides TiCl4 (n = 2-4) are fairly wel l known, their gas-phase decomposition kinetics have not been characte rized. In this work, rate constants for the unimolecular decomposition reactions of the titanium chlorides were predicted using Rice-Ramsber ger-Kassel-Marcus theory for unimolecular reaction, while the rate con stants for bimolecular reactions between TiCl4 species and hydrogen at oms were estimated using an empirical correlation. Calculations at the rmodynamic equilibrium over a range of temperatures and total pressure s characteristic of Ti CVD conditions are presented first. The time-de pendent evolution of the gas-phase composition is then simulated using the proposed mechanism. The results show that at 1500 K complete equi librium is reached in approximately 2 s at any total pressure in the r ange 0.01-101 kPa (1-760 Torr). The approach to equilibrium is much sl ower 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 equilibri um predictions of gas-phase concentrations should be a good approximat ion at 1500 K, unless very short residence times and low pressures are involved. In contrast, equilibrium calculations do not accurately ref lect the gas-phase composition at 1000 K.