Thickness-dependent crystallinity of sputter-deposited titania

In this study, we sputter deposited titania films of thickness ranging from 256 to 705 nm on unheated substrates and studied changes in phase constituency as a function of film thickness. X-ray diffraction, and infrared spectroscopy were used for post-deposition analysis. The results show that the thinnest films consisted of anatase, rutile, and an amorphous structure. As film thickness increased, anatase formed at the expense of the amorphous constituent, whereas the amount of rutile per unit film volume remained constant. We hypothesized that if the thickness-related crystallographic changes were caused by bulk annealing effects due to in situ plasma heating during deposition, then it should be possible to reproduce these changes, ex situ, by furnace annealing. The thinnest films were annealed at three temperatures: 300, 500, and 700 °C. There was no change in phase composition at 300 °C. Metallurgical recovery of both crystalline phases occurred at 500 °C. At 700 °C, rutile grew at the expense of anatase, which completely disappeared. The results showed that post-deposition annealing caused crystallographic changes that were inconsistent with the thickness-dependent crystallographic changes in the as-grown films. We suggested that the latter was caused by enhanced surface, not bulk diffusion, possibly due to an increase in substrate temperature during deposition.