Fabrication of pure silica films for planar optical waveguides using colloidal suspensions

In this work, colloidal suspensions are used as a precursor to the formation of layers of optical quality glass on silicon substrates. The colloidal suspensions were made by dispersing a high concentration of fumed silica (particle diameter of 40 ± 30 nm) in water with tetramethylammonium hydroxide (TMAH) as a base to stabilize the suspension. Spin-coating was used as the film deposition method to prepare porous silica films. These films were then consolidated at 1300 °C in an atmosphere of helium and oxygen. Dense silica films with thicknesses up to 1.4 μm could be made this way from a single spin-coating step. The refractive index, as measured by reflectometry, of the densified film at a wavelength of 632.8 nm is 1.4577 ± 0.0005. This is comparable to that of a thermal oxide on a silicon wafer confirming that the film is fully densified. Also, fully dense, defect-free films with thicknesses in the order of 10 μm were fabricated with multiple spin-coating steps to meet the dimensional requirements of planar optical waveguides.