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 T-surf 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 T-surf using thermostrips pasted to the glass substrate. It was found that (1) T-surf is considerably higher (approximately by 100 degrees C or more) than the substrate temperature T-s measured by the thermocouple incorporated into the substrate holder and (2) T-surf strongly depends on the substrate-to-target distance d(s-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 T-surf but also on the total pressure PT of sputtering gas (Ar+O-2), partial pressure of oxygen p(O2), the film deposition rate a(D), and the film thickness h, (2) crystalline TiO2 films with well developed anatase structure can be formed at T-surf = 160 degrees C and low values of a(D) approximate to 5 nm/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 T-surf = 160 degrees C and p(T)=0.75 Pa when p(O2) decreases and aD increases above 5 nm/min, and (4) crystallinity of the TiO2 films decreases with decreasing h and T-surf. Interrelationships between the structure of TiO2 film, its roughness, T-surf, and a(D) are discussed in detail. Trends of next development are briefly outlined. (c) 2006 American Vacuum Society.