Effect of ion energies on the film properties of titanium dioxides synthesized via plasma enhanced atomic layer deposition

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A plasma enhanced atomic layer deposition process for synthesizing titanium dioxide (TiO2) films, which allows the film properties to be modified by tuning the ion energies of the discharges, was performed. The films were deposited via the oxidation of titanium tetrachloride in a typical capacitively coupled radio frequency (CCRF) discharge in argon/oxygen mixtures at a low temperature of 100 °C, resulting in all the films synthesized being amorphous. The energy distributions of ions hitting on the lower electrode (wafer surface) in the CCRF discharges were varied by tuning the impedance of the lower electrode, aimed at controlling the film properties. The wet etching rate of TiO2 films shows a clear correlation with the mean ion energy, 〈εi〉, i.e., a higher 〈εi〉 realizes a higher value of the wet etching rate. The variation of the film properties is explained by a change in the balance between the ion bombardment and the oxidation; the former is greatly affected by 〈εi〉, whereas the latter is mainly determined by radio frequency (RF) power. In a high 〈εi〉 condition, the bombardment of high energetic ions is pronounced, resulting in the formation of fine pores in the films, which was confirmed by physical analyses such as positron annihilation spectroscopy. In this study, the dependence of CCRF discharges as well as the film properties on RF power is also discussed.