Role of energy in low-temperature high-rate formation of hydrophilic TiO2 thin films using pulsed magnetron sputtering

The article reports on low-temperature high-rate deposition of hydrophilic TiO2 thin films using dc pulse dual magnetron (DM) sputtering in an Ar+O2 mixture on unheated glass substrates. DM is operated in a bipolar asymmetric mode and is equipped with Ti (99.5%) targets 50 mm in diameter. Main attention is concentrated on the investigation of the effect of an energy delivered to the TiO2-x film, growing on unheated and heated substrates, by the ion bombardment on its structure. The effect of the length and shape of the pulse on the structure and deposition rate aD of the film is investigated in detail. It is shown that (1) the shortening of the pulse length to about ∼2 μs strongly enhances the ion bombardment of electrically insulating film which makes it possible to form nanocrystalline strongly hydrophilic TiO2 films at low (∼100 deg. C) values of the substrate temperature Ts and (2) the increase of a repetition frequency fr of pulses results in a strong increase of aD; aD increases almost two times when fr is increased from 100 to 300 kHz. It was found that (1) hydrophilic TiO2 films several hundred nanometers thick are crystalline and can be sputtered on unheated glass substrates with a high deposition rate aD=80 nm/min at substrate surface temperatures Tsurf≥180 deg. C when fr=350 kHz is used and (2) hydrophilic TiO2-x films sputtered at low values of the substrate surface temperature Tsurf≅100 deg. C exhibit nanocrystalline structure and can be formed if much lower deposition rates aD≤5 nm/min are used. Correlations between the hydrophilicity of TiO2-x film, its structure, Tsurf, process parameters, and the film deposition rate aD are given. A summary of the present state of knowledge in this field is presented