Aluminum oxides ($Al_2O_3$) have been deposited by a chemical vapor deposition (CVD) technique using $AlCl_3$, $CO_2$ and $H_2$ gas mixture onto TiN-coated cemented carbide substrates. In this work, the effects of deposition time, total flow rate, deposition temperature and partial pressures of reactants on the deposition rate of $Al_2O_3$ and the final structure of $Al_2O_3$ film are investigated. Especially, the effect of the supersaturation of reactants ($H_2O$ or $AlCl_3$) on the final structure of $Al_2O_3$ film is studied.
The supersaturation of reactant is calculated by assuming chemical equilibria.
From the experimental results it is found that the formation reaction of $Al_2O_3$ is a thermally activated one and that is limited by surface reaction.
The morphology of $Al_2O-3$ film changes from a fine structure to a coarse one as the deposition temperature is increased, and the crystallographic structure of $Al_2O_3$ changes from random orientation to <1014> preferred orientation with increasing the deposition temperature.
Deposition rate is increased with $AlCl_3$ partial pressure at lower $AlCl_3$ partial pressure (＜1 torr), but that is increased with $AlCl_3$ partial pressure at higher $AlCl_3$ partial pressure (＞1 torr) and a maximum deposition rate is obtained about 1% $AlCl_3$ of total flow. The deposition rate as a function of $H_2$ partial pressure shows a maximum between 50 torr and 65 torr of $H_2$ partial pressure, which is related to the formation reaction of $H_2O$. From the experimental results, it is found that the reaction of hydrolysis of $AlCl_3$ is kinetically more favorable than that of $H_2O$ formation, and the deposition rate of $Al_2O_3$ is, therefore, mainly controlled by the rate of the reaction of $H_2O$ formation. The crystal size of $Al_2O_3$ is decreased with increasing the $H_2O$ supersaturation at constant deposition temperature. The supersaturation of $AlCl_3$, however, does not affect the crystal size of $Al_2O_3$.
The empirical equation of the deposition rate of $Al_2O_3$ formation is as follows;
$γ{Al_2O_3} = \acute{Rc}ㆍ{P}_{H_2}^{\frac{1}{2}}ㆍ{P}_{CO_2}^{\frac{1}{2}} exp (-\frac{35000}{RT})$.
