Optical grade SiO2 films prepared by HWCVD

Using a large-scale HWCVD in-line deposition system with maximum deposition areas of 500 × 600 mm2 SiOx films were deposited from gas mixtures of SiH4 and O2 at pressures between 1.5 and 3.5 Pa with substrate temperatures of 140 up to 360 °C. Activation of the gas atmosphere was achieved via tungsten wires at temperatures of 1900 up to 2100 °C. Keeping the silane gas flow at constant levels between 50 and 150 sccm the ratio of oxygen gas flow to silane was varied from 0 to ~ 170% in order to vary the stoichiometry of the deposited films. Optical transmission was measured on Schott B270 and quartz glass substrates. Starting at oxygen ratios O2/SiH4 < 100% an increase of oxygen gas flow at first leads to an increase of transparency. After passing through transparency maxima at O2/SiH4 ratios of about 110-140%, further increase of oxygen gas flow leads to a decrease of transparency due to incorporation of partly vaporized tungsten wires. Process conditions for the optima of transparency and other film properties were investigated in detail as a function of the deposition parameters by using design of experiments. Due to a lack of ion-bombardment by HWCVD process film damage with high stress levels could be prevented. After optimization of the process parameters, highly transparent stoichiometric SiO2 films with residual compressive stresses of about 150 MPa with high deposition rates (a > 2 nm/s) were deposited. With this development, antireflective coatings based on Si3N4-SiO2 layers on 10 × 10 cm2 were both deposited by in-line HWCVD processes to demonstrate the potential for optical applications for the first time.