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 degreesC in an atmosphere of helium and oxygen. Dense silica films with thicknesses up to 1.4 mum 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 mum were fabricated with multiple spin-coating steps to meet the dimensional requirements of planar optical waveguides. (C) 2004 Elsevier B.V. All rights reserved.