Mechanisms of the reactive- and chemical-sputter deposition of TiO2 from Ti and TiC targets in mixed Ar+O2 discharges

Abstract

Polycrystalline TiO₂ films with a rutile crystal structure have been grown on glass substrates at a temperature of 250 °C by rf sputtering from either Ti or TiC targets in mixed Ar+O₂ discharges at a total pressure of 15 mTorr (2 Pa) and a peak‐to‐peak target voltage of −1.9 kV. In the case of the Ti target, film growth occurred by normal reactive sputtering in which oxidation of the target competes with the physical sputtering of titanium. The relative rates of these processes established critical oxygen concentrations f_O2 in the discharge of 0.6 and 6 mole%, respectively, at which the film deposition rate began to decrease from that observed for Ti sputtered in pure Ar and reached that of a TiO₂ target sputtered in O₂. TiO₂ films with a (110) preferred orientation were obtained at f_O2?6 mole% by the reaction of sputtered titanium with oxygen at the surface of the growing film. TiO₂ films were also grown using the TiC target with f_O2?7 mole%. Mass‐spectrometer sampling of the discharge during film growth showed a large concentration of CO which we believe derived primarily from chemical sputtering of the TiC target by oxygen. In this process the oxygen reacts with the target producing volatile CO, which is desorbed, and a nonvolatile titanium oxide. Ion bombardment from the discharge enhances the evolution of CO and simultaneously removes titanium by physical sputtering. The rate‐limiting step in the erosion of the TiC target is the physical sputtering of titanium. The critical value of f_O2 necessary to obtain TiO₂ films was also the value at which the CO concentration in the discharge saturated at a maximum and the film deposition rate saturated at a minimum. However, the f_O2 value at which the TiC‐target sputtering rate just began to decrease from that of TiC sputtered in pure Ar towards that of TiO₂ sputtered in oxygen was ∼1.3 mole%. This was considerably larger than the equivalent value found with the Ti target since additional oxygen was consumed in the reaction with carbon which occurred preferentially to the titanium oxide formation. In all cases, TiO₂ films grown from the TiC target at f_O2?7 mole% were found to be identical, as judged by x‐ray diffraction, Auger spectroscopy, and tribological measurements, with those grown by pure reactive sputtering from the Ti target.