Low-temperature sputtering of crystalline $TiO2$ films

This article reports on the investigation of reactive magnetron sputtering of transparent, crystalline titanium dioxide films. The aim of this investigation is to determine a minimum substrate surface temperature $Tsurf$ necessary to form crystalline $TiO2$ films with anatase structure. Films were prepared by dc pulsed reactive magnetron sputtering using a dual magnetron operating in bipolar mode and equipped with Ti(99.5) and ceramic $Ti5O9$ targets. The films were deposited on unheated glass substrates and their structure was characterized by x-ray diffraction and surface morphology by atomic force microscopy. Special attention is devoted to the measurement of $Tsurf$ using thermostrips pasted to the glass substrate. It was found that (1) $Tsurf$ is considerably higher (approximately by $100°C$ or more) than the substrate temperature $Ts$ measured by the thermocouple incorporated into the substrate holder and (2) $Tsurf$ strongly depends on the substrate-to-target distance $ds-t$⁠, the magnetron target power loading, and the thermal conductivity of the target and its cooling. The main result of this study is the finding that (1) the crystallization of sputtered $TiO2$ films depends not only on $Tsurf$ but also on the total pressure $pT$ of sputtering gas $(Ar+O2)$⁠, partial pressure of oxygen $pO2$⁠, the film deposition rate $aD$⁠, and the film thickness $h$⁠, (2) crystalline $TiO2$ films with well developed anatase structure can be formed at $Tsurf=160°C$ and low values of $aD≈5nm∕min$⁠, (3) the crystalline structure of $TiO2$ film gradually changes from (i) anatase through (ii) $anatase+rutile$ mixture, and (iii) pure rutile to x-ray amorphous structure at $Tsurf=160°C$ and $pT=0.75Pa$ when $pO2$ decreases and $aD$ increases above $5nm∕min$⁠, and (4) crystallinity of the $TiO2$ films decreases with decreasing $h$ and $Tsurf$⁠. Interrelationships between the structure of $TiO2$ film, its roughness, $Tsurf$⁠, and $aD$ are discussed in detail. Trends of next development are briefly outlined.