Research Paper: X-ray Photoelectron Spectroscopy of TiO2 Optical Thin Films at Managed Annealing Process

Abstract

Titanium dioxide (TiO2) is widely used in optical coating technology because of its stability and high index of refraction. The homogeneity of the chemical composition (stoichiometry) of the optical thin films plays an important role in the quality of the layers. Increasing the homogeneity increases the transmission coefficient and the laser-induced damage threshold and decreases the absorption coefficient. To increase the homogeneity of the layers, various methods are used during and after the process of film coating. Annealing (heating the deposited films up to a certain temperature) is a common technique. After the coating process, TiO2 thin film is uniformly heated to 400 °C to obtain the homogenization of the stoichiometry. In this research, the dependence of the homogeneity of the stoichiometry of TiO2 optical thin films on the rate of heat treatment (a gradual increase of heat treatment temperature over time) is investigated by X-ray Photoelectron Spectroscopy (XPS) of the samples. So, the 400 nm thick TiO2 layers are coated on the BK7 optical glass, utilizing an electron gun. Then, each sample is subjected to heat treatment at different rates. The data obtained from the XPS analysis show that the samples subjected to slow heat treatment (2.2 °C per minute, up to a final temperature of 400 °C) have a more homogeneous stoichiometry and contain higher amounts of TiO2. Based on the results, the laser-induced damage threshold of laser mirrors can be improved in the nanosecond and femtosecond regimes of radiation.

Keywords

• thin film
• x-ray photoelectron spectroscopy
• heat treatment
• stoichiometry