Aluminum oxide films were deposited on silicon substrates at low deposition temperatures (150-300°C) by plasma enhanced chemical vapor deposition (PECVD) using trimethylaluminm (TMA), $N_2O$ and He gases. The deposition characteristics of the films such as deposition rate, thickness uniformity and step coverage were studied. The film properties including chemical composition, microstructure, refractive index, stress, etching rate, resistivity and breakdown strength were also investigated.
The deposition characteristic and film properties are strongly affected by the process parameters such as deposition temperature, reactant gas composition, RF power, and system pressure.
The deposition rates of aluminum oxide films deposited at 150-170°C are very high reaching 320-400A/min. On the other hand, above 200°C the deposition rates are nearly constant to 160A/min. The films deposited at 150°C have an amorphous structure, and those above 250°C have an extremely fine $\gamma$-alumina polycrystalline structure. Aluminum hydroxide and $H_2O$ molecules are detected in as-deposited film and their concentration increases as the deposition temperature decreases. The temperature dependence of the deposition rate and impurities content is explained by the change of the adsorption type and desorption rate of reactant gases with temperature.
The film deposited at 250 °C shows a very smooth and defect free surface. And the step coverage of the deposited film is fairly conformal. The film has compressive stress in the range of 1-3×$10^9$ dyne/$cm^2$. The heat treatment of the deposited film reduces the film thickness and increases the refractive index. The wet etching rate of the film in 5% HF solution is very high, whereas the plasma etching rate using $CF_4$ gas is nearly zero. The electrical properties of the aluminum oxide films were studied by measuring the leakage current at do voltages. The resistivity and the breakdown strength of the films deposited under the typical deposition condition are $10^{14}Ω$cm and 6 MV/cm, respectively. These values indicate that the aluminum oxide films are good as the insulating dielectric film.
From these results, it is believed that the aluminum oxide film deposited by PECVD using TMA source is very promising for the further use in VLSI and microelectronics as etch barrier, interlevel dielectric, and passivation buffer layer.
