Sintering anomaly in silica-titania sol-gel films

Sol-gel processing has been successfully applied to the preparation of silica-titania planar waveguides for integrated optics. In order to achieve low loss guides, proper densification is essential. A series of single layer silica-titania films were deposited by spin-coating on Si substrates from precursor sols prepared by mixing tetraethylorthosilicate (TEOS) with titanium tetra-iso-propoxide (TPOT). The thermal densification behavior of these thin gel films was followed by measuring their refractive index, by ellipsometry, as a function of the temperature, between 300-900 degreesC and the heating time; an anomalous sintering behavior was found, consisting of a rapid initial rise in film density (calculated from the refractive index) at any heat-treatment temperature, followed by a pronounced minimum at times of the order of 3 minutes, after which the density slowly increased until the maximum plateau value, reached at each temperature. In order to explain the above sintering anomaly, a detailed compositional and structural characterization was performed on the films by infrared spectroscopy (IR) and X-ray photoemission spectroscopy (XPS). A redistribution of the Ti4+ ions within the film thickness was detected, which may in part explain the observed anomaly and has implications regarding the possible occurrence of phase separation during film sintering.