Role of He gas mixture on the growth of anatase and rutile TiO2 films in RF magnetron sputtering

The effects of He addition on the growth of crystallized TiO2 films and the change in both the substrate-incident ion current and plasma parameters in rf magnetron sputtering were investigated. The growth of rutile TiO2 was achieved on a non-heated substrate in the case of a mixture of Ar (10sccm) and He (40sccm) at a rather high total pressure of 20mTorr. He (50sccm) sputtering brought about anatase TiO2 film growth, improving radial inhomogeneity. Mass-resolved analyses of the substrate-incident ion current showed a significant change in species, such as Ar+, O-2(+), O+ and Ti+, with increasing He. Studies using the Langmuir probe and gridded energy analyzer showed that high-energy electrons which contribute to excitation and ionization of reactant particles increased over the whole discharge space, while electron density decreased with increasing He. Metastable He atom densities were estimated by the optical absorption method and the effect of the Penning process was discussed quantitatively. We pointed out that the increase in high-energy electrons by He mixture is responsible for the change in the substrate-incident ion current and formation of anatase/rutile TiO2. The Penning effect caused by the metastable He atom was thought to be rather small in the present experimental condition.