ABSTRACT
Titanium dioxide (TiO2) film is extensively used as a high index film material in optical coating. It is a perfect couple with SiO2 which is commonly used as a low index film material to make thin filmmultilayer dielectric coatings consisting of alternating layers of high and low refractive index materials (S. J. Chang et al. 2008).Titanium dioxide films can be prepared by most reactive coating techniques such as evaporation, solgel, sputtering, and plasma-enhanced chemical vapor deposition. In the case of evaporation, preferably electron beam evaporation, which is one of the most traditional methods because of high efficiency and low cost, is extensively used in scientific researches as well as in practical production. When TiO2 films are prepared from TiO2 starting material with the electronic beam evaporation method, the film is loose with a lower refractive index than the refractive index of the bulk (C. Yang, H et al. 2008,E. D. Palik et al. 1991). According to the literatures,Duyar et al. reported that it is quite difficult to obtain films with reproducible and stable optical properties, in particular, for materials that exist in a number of different stoichiometric forms of the system Ti-O. Whenever TiO, Ti2O3 or TiO2 is evaporated, the vapor may consist of variousTi-O combinations, which change as the evaporation continues. The refractive index of the deposited film, thus, is not constant because most Ti-O materials evaporate noncongruently. If, however, Ti3O5 is evaporated as the starting composition, the only one kinde of titanium species in the vapour is TiO, and the oxygen content in
the vapour remains constant (O. Duyar et al. 2008), (E. Ritter et al. 2003). Furthermore, the refractive index is higher than TiO2 starting material due to the higher packing density (H. Selhofer et al. 2002). Many researchers have reported the deposition of TiO2 films from Ti3O5 starting material by electron beam evaporation. However, TiO2 films are loosely packed that is caused by low molecular mobility. It indicates that the refractive index is lower than the bulk (H. Selhofer et al. 2002, F. X. Wang et al. 2007). Therefore, it is necessary for annealing to increase molecular mobility, make them more bulk-like. During annealing, thermal energy provides the mobility of the molecules, so the loose film become slimmer and increases the refractive index as the temperature increases. Many papers have been reported that TiO2 films from Ti3O5 starting material were annealed at rather low temperature. For example, Chen et al. reported that TiO2 films were deposited from Ti3O5 starting material by ion-assisted deposition.The refractive index with the substrate temperature of 150◦C increased from 100◦C to 200◦C during annealing, and declined as the annealing temperature from 200◦C to 300◦C (H. C. Chen et al. 2006). Jaing et al. reported that TiO2 films were deposited from Ti3O5 starting material by ion-assisted deposition. They found that the refractive index increased with the annealing temperature from 350◦C to 450◦C, they thought that the film became denser as the annealing temperature above 350◦C because the refractive index showed the densification of the film (C.C. Jaing et al. 2009). According to the open literature, little further researches were devoted to discuss the
fluctuation of refractive index and the phenomenon of cutoff wavelength shift to longer wavelength when TiO2 films from Ti3O5 starting material were annealed at above 400◦C. The main aims of this research were to investigate the effect of annealing temperature on the theoretically explained the reason for cutoff wavelength shift to long wavelength.
