Inorganic-organic hybrid material was synthesized by the sol-gel process. The control of the refractive index was achieved by varying the amount of phenyltrimethoxysilane as a refractive index modifier. Thick layers of hybrid material with 22㎛ thickness were fabricated by a single spin-coating without producing any crack at lower temperature. However, the heat-treatment of the hybrid film above 200°C induced significant cracks on the surface. The refractive index variations and structural changes as a function of heat-treatment condition were analyzed using Fourier transform infrared spectrometer.