TiO2 | Titanium dioxide basic knowledge
TiO2 has a refractive index near 2.55 at 550 nm and a transmittance range from below 400 nm to 3 μm. It exists in three crystalline phases: anatase, brookite, and rutile. The rutile phase is transparent and birefringent, with indices of 2.9 and 2.6; while the anatase phase is less birefringent, with an index of about 2.5. Brookite is seldom observed in thin films.
TiO2 is one of the most extensively used high-index material for multilayer optical thin film device applications operating in the VIS and near-IR regions. This is because TiO2 has the highest known refractive index in the VIS region among materials in practical use, although the optical losses, microstructures, and chemical compositions of the films depend on the deposition conditions.
Electron beam evaporation is the most common method for the deposition of TiO2 thin films. Other starting materials such as TiO and Ti3O5 are also popular for fabricating TiO2 thin films.
The use of energetic ion-beam- and plasma-based processes produces denser and more amorphous TiO2 thin films than conventional electron beam evaporation processes. But using the use of ion assisted deposition (IAD) also has its drawbacks: First, the optical constants and film structure tend to be unstable. And second, the use of ion assistance often crystallizes the TiO2 film and creates columnar structures on its surface. This increases optical losses through, for example, increased absorption, scattering, and spectral shifting. Though IAD usually increases the refractive index and discourages crystalline growth, this does not always apply to TiO2, as explained above.
The microstructures of TiO2 thin films are strongly determined by the IAD parameters employed. Without a doubt, in optics industries, the most frequently used materials for optical multilayer thin films are TiO2 and SiO2. Shiftless thin film of TiO2 is expected for a long time.
Conventional physical vapor deposition (PVD) of TiO2 and SiO2 produces thin films with intrinsic characteristics, makes the development of a repeatable manufacturing process problematic. These problems include vacuum-to-air shifts for edge and bandpass filters; film porosity, resulting in moisture adsorption; nonuniform deposition and the resulting refractive index variation, which is strongly dependent upon the evaporation rate; oxidation at high substrate temperatures; and poor adherence to temperature-sensitive substrates. Ion-assisted deposition has been extensively researched with respect to the modification of these problematic film properties.
Thin films produced by conventional PVD or electron beam or thermal evaporation have columnar structures. Porosity, reduced packing density, and a lower refractive index result in the deterioration of the film performance in humid environments, primarily due to the adsorption of water vapor. In addition, the refractive indices of TiO2 vary with the titanium-oxygen phases of the starting material as well as with the conventional PVD process parameters.