Aluminum oxide films were deposited on silicon wafers by plasma enhanced chemical vapor deposition, using trimethylaluminum, $N_2O$ and He gases.
The chemical composition, states of functional groups and microstructure of the aluminum oxide films were investigated using FTIR, XPS, AES and TEM. Etch rates were measured and related to the microstructure of the films. It was found that carbon and hydrogen atoms are incorporated less at the higher deposition temperatures and are almost completely removed as gas phases, such as $CO_2$ and $H_2O$, by the post-deposition heat-treatment at 800℃ in an oxygen environment. Carbon atoms incorporated into the films are in the chemical form of $AlCH_3$ or AlCOOH, and the atomic concentration varies from 2% at 300℃ to 5% at 120℃. Hydrogen atoms are in the chemical form of Al-OH, and the atomic concentration estimated from the absorbance FTIR band of the O-H stretching mode varies from about 7% at 300℃ to about 28% at 120℃. The aluminum oxide films deposited at 300℃ have a microcrystalline structure of hydrogen stabilized $\gamma-Al_2O_3$ with an O/Al ratio of 1.6, whereas those deposited at 120℃ have an amorphous structure. Etching properties of the films were related to the change in the microstructure.
The OES(Optical Emission Spectroscopy) was used to identify the precursors of the reactant gases and the floating double probe was used to measure the electron temperature and the ion density in the actual deposition condition. $N_2O$ gases was found to be dissociated in the precursor form of NO, $N_2$ and O. Only the Al spectrum line was observed in the Trimethyle aluminum and He plasma. It was found that Trimethyle aluminum gas was more effective in cooling the electron temperature of He plasma than $N_2O$ gas was. The OES results of plasma diagnosis shows that the precursor concentration did not increase by simply increasing mole fraction of reactant gas because of the cooling effect of the reactant gases. From the result of double probe method, it was found that electron temperature was about 4000K and ion density was in the range of $2.5-8.0\times10^{15}$/m$^3$. Also, as the RF power increased, the ion density sensitively increased while the electron temperature was cooled down slightly.
The electric properties of aluminum oxide films was investigated by measuring the leakage current, the breakdown voltage and the flat band voltage through I-V and C-V characteristics curve using MOS structure. The leakage current and the breakdown voltage were sensitively affected by the deposition temperature and the RF power among various deposition parameters. Aluminum oxide films deposited at the condition of 300℃ and 20W had the properties that the breakdown voltage was about 6MV/cm and the leakage current was about $2\times10^{-12}$A/cm$^2$ at the electric field of 3MV/cm. The result of C-V measurement shows that the hysteresis existed with the sweeping direction and the flat band voltage shift had the dependence on the starting voltage. It was founded that the cause of the hysteresis is electron trap in the bulk of aluminum oxide film. The hysteresis could be removed by the heat treatment of 400℃ in $H_2$ environment.
