PHOTOINDUCED STRUCTURAL RELAXATION AND DENSIFICATION IN SOL-GEL-DERIVED NANOCOMPOSITE THIN FILMS : IMPLICATIONS FOR INTEGRATED OPTICS DEVICE FABRICATI ON

The solution sol-gel method is used to produce thin films of photosensitive hybrid organic-inorganic glass on silicon. Glasses consisted of photoinitiator, methacryloxypropyltrimethoxysilane, methacrylic acid, and zirconium oxide. Clear, low optical loss films are obtained, indicating nanophase homogeneity in the samples. The nanocomposite films are shown to be suitable for fabricating optical components such as ridge wave guides and Bragg diffraction gratings. The increase in the refractive index of the glass relative to the surrounding material during photolithographic processing is identified as a key material parameter in device fabrication. Accordingly, electronic and vibrational spectroscopy are used to provide insight into the structural changes that occur when glasses are irradiated with continuous narrow band 4.9 eV and pulsed 6.4 eV light. Arguments are advanced, linking the changes in refractive index to collateral densification leading to volume compaction of the silicate network during organic free-radical polymerization. This was shown by following the time evolution of relevant IR absorption bands. Free silanol and unreacted methoxysilane are consumed in the process. Matrix densification is indicated by shifts to low wave number in the transverse optical phonon mode associated with decreasing Si-O-Si bond angles of the antisymmetric stretching vibration (compression). Growth in the Si-O-Si framework is observed through increased intensity in this IR absorption. Similar behaviour is observed for films irradiated with 6.4 eV light from an excimer laser. A phase mask in combination with pulsed 6.4 eV light is used to inscribe a 1.5 mm, high-reflectivity polarization-independent Bragg grating into a ridge wave guide. The high reflectivity is thought to arise from a periodic modulation of the volume compaction of the matrix. Overall, the organic component of the glass confers unique properties on the material that allow it to be densified even with 4.9 eV light. By comparison, sol-gel silica with no organic component must be densified at nearly twice the photon energy.