Losses in TiO2/SiO2 multilayer coatings

Abstract

This paper deals with optical losses in the coatings consisting of a combination of titanium dioxide (TiO2) and silicon dioxide (SiO2) layers evaporated by the ion beam assisted deposition (IBAD). This combination is commonly used for optical coatings as a standard choice for antireflective or any other optical filter in the visible and near IR range. Although the technology has been known for decades, we point out that some undescribed parasite losses can still appear and we show how to deal with them. In fact, in some cases, the losses made the target coating even inapplicable. In this paper we try to investigate the origin of the losses and we describe the deposition parameters which allow us to reduce or completely remove them. We determined whether the losses are proportional to the total thickness of the coating or to the number of layers. The influence of scattering was measured as well. Deposition parameters which were studied are the substrate temperature, discharge voltage of the assisting ion gun, oxygen flow of the assisting ion gun and the deposition rate, especially its starting curve. Influence of the post process annealing was studied as well. Starting curve of the deposition rate of SiO2 layer and the amount of oxygen flowing through the assisting ion gun were found as a crucial parameters.