Sintering anomaly in silica-titania sol-gel films

Sol-gel processing has been successfully applied to the preparation of sili ca-titania planar waveguides for integrated optics. In order to achieve low loss guides, proper densification is essential. A series of single layer s ilica-titania films were deposited by spin-coating on Si substrates from pr ecursor sols prepared by mixing tetraethylorthosilicate (TEOS) with titaniu m tetra-iso-propoxide (TPOT). The thermal densification behavior of these t hin gel films was followed by measuring their refractive index, by ellipsom etry, as a function of the temperature, between 300-900 degreesC and the he ating time; an anomalous sintering behavior was found, consisting of a rapi d initial rise in film density (calculated from the refractive index) at an y 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 a nd structural characterization was performed on the films by infrared spect roscopy (IR) and X-ray photoemission spectroscopy (XPS). A redistribution o f the Ti4+ ions within the film thickness was detected, which may in part e xplain the observed anomaly and has implications regarding the possible occ urrence of phase separation during film sintering.