Deposition of low residual stress SiO2 films by Hot-Wire CVD processes with a high rate for optical applications
Volker Sittinger, Markus Höfer, Tino Harig, Madeleine Justianto, Hendrik Thiem, Michael Vergöhl, Lothar Schäfer
Fraunhofer Institute for Surface Engineering and Thin Films IST,
Bienroder Weg 54 E, 38108 Braunschweig, Germany
Using a large scale Hot-Wire activated CVD in-line deposition system with maximum deposition areas of 500 x 600 mm2 SiOx films were deposited from gas mixtures of SiH4 and O2 at pressures between 0.5 and 5 Pa and at substrate temperatures from 100 °C up to 400 °C. Activation of the gas atmosphere was achieved via tungsten wires with diameters of different diameters at temperatures from 1900 °C up to 2100 °C. Keeping the silane gas flow at constant levels between 50 sccm and 150 sccm the ratio of oxygen gas flow to silane was varied from 0 to about 170 % in order to vary the stoichiometry of the deposited films. Optical transmission was measured on Schott B270 and quartz glass substrates. Further characterizations of the film properties were done by spectroscopy ellipsometry, SEM, AFM, X-ray microanalysis, grazing incidence X-ray reflectometry. Film stress was determined by curvature measurements of coated ultra-thin glass. Starting at oxygen fractions O2/SiH4 < 100 % increase of oxygen gas flow at first leads to an increase of transparency. After passing through 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. Location of the optima for low film stress and high transparency were investigated in detail as a function of the deposition parameters by using design of experiments. Highly transparent stoichiometric SiO2 films with residual compressive stresses of about ‑150 MPa were deposited. With this development antireflective coatings based on Si3N4-SiO2 layers on 10 x 10 cm² were deposited to demonstrate the potential for optical applications.