Sol-Gel Synthesis of thin Films for use as Planar Waveguides
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SOL-GEL SYNTHESIS OF THIN FILMS FOR USE AS PLANAR WAVEGUIDES* G.W. DALE, H.H. FOX, BRIAN J.J. ZELINSKI** and LAURA WELLER-BROPHY*** **Materials Science and Engineering Department, Arizona Materials Laboratory, University of Arizona, Tucson, Arizona 85721 ***Optical Data Storage Center, Optical Sciences Center, University of Arizona, Tucson, Arizona 85721
SiO2 -TiO 2 sol-gel solutions and synthesis techniques have been developed to produce thin films of proper index, thickness and quality for use as planar waveguides. The composition of the synthesized films ranged from 50 to 68 mole % TiO2 and produced layers with indices ranging from 1.7 to 1.8. Final film thicknesses ranged from 1000 to 3000 A and varied as a function of the coating chemistry and conditions. Also, changes in film shrinkage and index with heat treatment were determined and compared to literature data on similar films used as antireflective coatings. Finally, confirmation of the waveguiding qualities of these films was made through prism coupler measurements conducted by the University of Arizona Optical Sciences Center.
INTRODUCTION Sol-gel technology has led to the development of gel forming solutions that are deposited on substrates, gelled, and then densified. These solutions, particularly those containing silica or titania precursors, have been used in a number of applications. Spin-on dielectric films of silica and titania have been developed to create an insulating layer of inorganic oxides on semiconductors such as GaAs and InP [1]. Antireflective coatings from Si02 -TiO2 have also been fabricated [2,3,4]. These methods involved tailoring the refractive index by varying the relative amounts of SiO2 and TiO2 in the resulting film. Amorphous Si0 2 has an index of refraction of approximately 1.46, while TiO2 has an index of refraction over 2.6. The thickness of the spun-on film depends on the spin speed, solution concentration, and solution chemistry. Thus, by tailoring the chemistry of the precursor solutions, films of desired index of refraction and thickness can be made. Dielectric films may also be used as planar optical waveguides [5,6,7,8]. Unlike other approaches to waveguide synthesis, the sol-gel method permits the embossing of a master grating into the undensified waveguide for generation of integrated optic surface-relief gratings [5-8]. These gratings can be used as input and output couplers as well as Bragg reflectors. Roncone [9] et al. of the Optical Sciences Center at the University of Arizona have generated SiO2 -TiO 2 waveguides from commercial solutions with losses less than 1 dB/cm. They have also embossed surface features into the films of a depth and quality suitable for use in optical components. In the present paper, the sol-gel approach is used to produce Si0 2 -TiO 2 coatings for waveguide applications. The solutions, which are stabilized to hydrolysis using complexing agents [10,11], yield films of excellent homogeneity. The effect of composition on refractive index of films from the complexed solution is compared
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