Processing of SiO 2 -TiO 2 Thin Film Waveguides
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PROCESSING OF SiO2 -TiO2 THIN FILM WAVEGUIDES L. Weisenbach*, T.L. Davis%, and B.J.J. Zelinski* R.L. Roncone", and L.A. Weller-Brophy** Materials Laboratories; "*Optical Sciences Center; University of Arizona, Tucson, AZ 85721
"Arizona
ABSTRACT The shrinkage behavior and changes in refractive index of sol-gel derived SiO2 TiO2 thin films, with a composition of 50 mole %, were investigated. Two regions of shrinkage rate were observed, a rapid initial stage and a slower second stage. The refractive index of the film was found to be dependent upon the heat treatment. Samples prebaked to 100*C and processed at temperatures below 500*C were found to have lower indices than films processed without prebaking. Films heated to 700°C contained small crystallites of anatase uniformly distributed throughout the film. INTRODUCTION Thin films of SiO2-TiO2 have been studied previously as anti-reflectance coatings and planar waveguides. In 1979, Yoldas presented a method to synthesize uniform solutions of high titania content by compensating for the difference in 2 hydrolysis rates of the componentsi. In 1981, Brinker et al. investigated SiO2 -TiO2 sol-gel films for use as anti-reflection layers. Lukosz et al. have used commercially processed solution to produce embossed gratings in thin SiO2 3 TiO2 films with losses of < 1 db/cm in the waveguides. Recent work on this system in our laboratories has focused upon producing embossible, low-loss waveguide structures using solution chemistry techniques. 4 A preliminary study by Dale et al. investigated the effect of organic additive content and processing on the index and shrinkage of SiO2 -TiO2 waveguides. Roncone et al.--' studied the effect of processing on the embossibility of Si0 2 TiO2 waveguides, for the purpose of producing passive optical devices, such as focusing grating couplers. The purpose of this work is to investigate the densification behavior of Si0 2 -TiO2 thin films and characterize the change and reproducibility of refractive index and thickness with processing. EXPERIMENTAL An acid catalyzed solution of TEOS was prepared with a water:TEOS mole ratio of 2:1 and mixed for 30 minutes. Titanium tetrabutoxide was reacted with a complexing agent, 2,4-pentanedione (acetylacetone, acac), in a molar ratio of 1:2 and stirred for 20 minutes. The two solutions were mixed together in a molar ratio of 1:1 (SiO2 -TiO2 ) and stirred for four days. This solution was diluted 1:1 by volume with distilled isopropanol prior to spinning. Thin films were deposited onto a single crystal Si wafers by flooding the substrate with solution, and then spinning at 3000 rpm for 30 seconds using a Headway Spin Coatera. The coated substrate was then pre-baked for 1.5 minutes 0 at 100 C prior to an isothermal heat treatment at higher temperatures. Thickness measurements were made on samples pre-heated to 100'C using a stylus
aPhoto-Resist
Spinner, model ECI01,
Headway Research,
Inc.,
Mat. Res. Soc. Symp. Proc. Vol. 180. Q1990 Materials Research Society
Garland, TX.
378
profilometerb, and
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