Spatially resolved EELS fine structures at a SiO2/TiO2 interface

SiO2/TiO2 multilayers stacks used in optical coatings have been studied by Electron Energy Loss Spectroscopy (EELS). The line-spectrum mode has been used: the incident electron probe of the STEM is scanned under digital control on the specimen surface in the direction perpendicular to the layers, while the whole spectrum is acquired. The selected energy range contains Ti L(23) and O K edges, in order to study the evolution of the fine structures visible on these edges. Every experimental spectrum is then fitted with a linear combination of the two reference spectra (TiO2 and SiO2) extracted from the same sequence. It is possible to identify in the neighbourhood of the interface some EELS fine structures which cannot be fitted to a combination of reference spectra, but are representative of hybrid environments, such as Si-O-Ti in the present study. A quantitative analysis of the changes in different fine structures on the titanium and oxygen edges enables to clearly discriminate different levels of interdiffusion at the boundary.