Optical properties and mechanical stress in SiO2/Nb2O5 multilayers

Abstract The aim of this work is the formation of stress-controlled reflection layers which are suitable for application on micromechanical elements without bending them out of shape. Thin films of Nb 2 O 5 and SiO 2 have been deposited by magnetron sputtering of an oxide target (SiO 2 ) and reactive sputtering of a niobium target in an oxygen containing atmosphere, respectively. Silicon and niobium oxide had been selected on the basis of preliminary experiments out of nine different thin film oxides since they yielded sufficiently low absorption coefficients of magnetron sputtered films. Both optical parameters and film stress have been investigated in dependence on deposition parameters. In particular, by varying the substrate bias voltage (for Nb 2 O 5 ) and the sputtering pressure (for SiO 2 ) the film stress could be influenced to a large extent without deterioration of the optical properties. In Nb 2 O 5 , the stress could be varied between compressive and zero stress, whereas in SiO 2 compressive stress was always obtained. Hence, a complete stress-compensation in the multilayers necessitates the application of an additional underlying metal layer having tensile stress. We found magnetron-sputtered chromium films most suitable for that purpose since they provided the highest line stress in comparison to their additional mass per area. To demonstrate our approach we present as an example a layer stack of six SiO 2 and six Nb 2 O 5 films which were designed for maximum reflection at the wavelengths 446, 532 and 629 nm. The film stack was formed after a chromium film (nominal thickness 143 nm) was deposited onto a thin (thickness 30 μm) single-crystal silicon mirror plate. Reflectivity at the wavelengths given above was between 96 and 98%. The multilayer was highly stress-compensated with a typical residual bow of 0.4 μm for a 3.5×3.5 mm 2 mirror.