Morphology and structure of TiO2 thin layers vs. thickness and substrate temperature

Abstract TiO 2 thin layers are often used in multilayer coatings for optical filters and this makes their study especially interesting. Different TiO 2 thin layers corresponding to different thicknesses and deposited at different temperatures have been prepared by vapour deposition onto amorphous substrates. Most of these layers are amorphous but small crystals of anatase and rutile can also exist. These crystals are more frequent (i) if the substrate temperature is raised (350 °C) and/or (ii) if the thickness of the thin layer is higher. The surface roughness of TiO 2 thin layers has also been investigated by electron microscopy by two different and complementary methods: direct imaging by transmission electron microscopy and carbon-platinum replica observation. The thin layer surfaces are rather irregular, and this practically independently of the film thickness and of the substrate temperature during the deposition. Surface irregularities have been measured to reach about 10 nm. They have been shown to be slightly reduced on increasing the thin layer thicknesses from 35 to 70 nm. Each layer is made of blocks about 40 nm in size and each block is made of smaller blocks (about ten times smaller). The origin of this phenomenon has been sought. The formation of the smaller blocks is in agreement with existing theories of nucleation and growth of amorphous thin films. An attempt to find the origin of the larger blocks has also been carried out. This could be related to the permanent chemical transformation occuring during the thin layer formation. Finally, we relate the optical properties of the TiO 2 thin layers to their morphology and structure.