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Optical grade SiO₂ films prepared by HWCVD

: Sittinger, V.; Höfer, M.; Harig, T.; Justianto, M.; Thiem, H.; Vergöhl, M.; Schäfer, L.


Surface and coatings technology 336 (2018), S.61-66
ISSN: 0257-8972
Fraunhofer IST ()
hot wire chemical vapor deposition (HWCVD); silicon oxide; low stress; anti-reflective coating

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.