Effect of ion bombarding energies on photocatalytic TiO2 films growing in a pulsed dual magnetron discharge

Photocatalytic crystalline TiO2 films were deposited by a pulsed dc dual magnetron system. The depositions were performed using two unbalanced magnetrons with planar titanium targets of 50 mm diameter in Ar+O2 gas mixtures at a total pressure of 0.9 Pa with oxygen partial pressures ranging from 0.2 to 0.9 Pa. The maximum substrate surface temperature was 160 °C. Both magnetrons operated in the same asymmetric bipolar mode at the repetition frequencies of 100 and 350 kHz with a fixed 50% duty cycle and the average target power densities of 52–74 W cm−2 in the negative voltage phase of the pulses, but the magnetron operations were shifted by a half of the period. Time-averaged energy-resolved mass spectroscopy was performed at a substrate position located 100 mm from the targets. The measured structure of the ion energy distributions was correlated with the distinct pulse phases of the magnetron discharges. A decrease in the energy delivered by fast ions (E≥10 eV) to the unit volume of the growing films, together with possible effects of plasma-chemical processes, during the depositions at the oxygen partial pressures of 0.5–0.75 Pa and the repetition frequency of 350 kHz resulted in a strong predominance of the highly photoactive crystalline anatase phase in the TiO2 films.