Properties of TiOx coatings prepared by dc magnetron sputtering

TiO2 is frequently used in optical coatings as an inexpensive material with a particularly high refractive index. Often this high refractive index is only achieved if the substrate is heated to enable the deposition of crystalline films with rutile structure. In this study titanium oxide films have been deposited by dc reactive sputtering at room temperature. The effects of oxygen flow on structural and optical properties as well as deposition stress have been investigated. The stoichiometry of the films has been determined by Rutherford backscattering spectroscopy. All films deposited in the oxidic mode at 1 Pa have been found to be amorphous, while samples prepared at 0.75 Pa and high oxygen flow have been found to be crystalline. The optical analysis showed that the optical band gap of the films prepared at 1 Pa increases while the refractive index decreases with increasing O-2 flow. The variation of refractive index is related to the increase in porosity. The increase in porosity upon increasing oxygen flow has been calculated using effective medium theories. The crystalline film prepared at 0.75 Pa has a higher refractive index than the amorphous samples prepared at 1 Pa. Both the amorphous and crystalline titania films have compressive stresses of around 250 MPa.