Microstructure and composition of composite SiO2/TiO2 thin films

Abstract

The microstructure and composition of the SiO2/TiO2 analog (codeposited) and digital (thin layer pairs) gradient-index films were examined by transmission electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, scatterometry, and in situ ellipsometry. Both analog and digital structures were amorphous as-grown. The SiO2 was incompletely oxidized at some of the interfaces in the layered structures and in the codeposited films, indicated competition from the TiO2 for available oxygen during growth. Digital structures with thin (65 Å) layers remained well defined after annealing at 900 °C, but their order was completely destroyed by 1100 °C. Structures with thick (500 Å) layers remained intact up to 1100 °C, with the TiO2 crystallizing throughout the layer width. Both anatase and rutile TiO2 crystallites were present in the layered and codeposited films after a high-temperature anneal (T≳650 °C), while only the anatase phase was observed for 300 °C<T<650 °C. The optical scatter of the digital films increased with increasing annealing temperature and layer thickness. TiO2 precipitated out of the codeposited films after a high-temperature anneal; however, the scatter of these films at 0.633 μm was several orders of magnitude lower than that of the annealed layered films. Codeposited films consisting of greater than ∼60% TiO2 displayed form birefringence in the as-grown state.