Optical properties of fluorinated silicon oxide and organic spin-on-glass films for thin-film optical waveguides

Optical properties of fluorinated silicon oxide (SiOF) and organic spin-on-glass (SOG) films in optical waveguide structures are investigated. The SiOF film for a core layer in an optical waveguide was formed at 23°C by a liquid-phase deposition (LPD) technique using a supersaturated hydrofluosilicic acid (H₂SiF₆) aqueous solution. The deposition rate using aluminum (Al) for supersaturation in LPD was around 42-50 nm/h. A siloxane-based SOG film was also evaluated for a cladding layer. Although the weak absorption peaks for fundamental vibration of the Si-OH bond in the Fourier transform infrared spectra are observed for thick LPD-SiOF films, the absorption bands for the H-OH bond are under the detection limit. The absorption peaks of Si-OH and H-OH bonds for the organic SOG films were negligibly small. The deviation ranges of refractive indices with thickness difference for the LPD-SiOF and organic SOG films were lower than 0.005 (<0.35%) and 0.01 (<0.71%), respectively, in the wavelengths ranging from 400 to 860 nm. The dispersion of the refractive index measured by spectroellipsometry agreed well with fitting results for both films in the measurement wavelengths ranging from 400 to 860 nm. In the measurement wavelength region, the organic SOG film kept lower refractive indices than the LPD-SiOF film. Any absorption due to water was not observed for the transmittance spectra in the wavelengths ranging from 350 to 2500 nm.