Introduction

Silicon dioxide is the most widely used low-index material for oxide multilayer stacks, in particular with TiO₂. Silica coatings are truly amorphous, with a refractive index of approximately 1.44-1.46 in the visible [1, 2]. The glassy nature of silica coatings is particularly obvious in cross-sectional transmission electron micrographs of TiO₂/SiO₂ multilayers, in which the columnar structure of the titania contrasts sharply to the silica [3]. The films are transparent from 0.2 to 9 μm [1]. Silica films exhibit compressive stress, which compensates well in multilayers with the tensile stress usually present in requiring an electron beam to evaporate silica starting material. The films have reported packing densities of 0.88-0.95 for 50°C substrates, increasing to 0.98-0.99 at 250°C [5, 6]. SiO₂ coatings can also be deposited by resistively evaporating SiO in an oxygen atmosphere. Films deposited in this way can contain a mixture of SiO, Si₂O₃, and SiO₂ phases [7]. Care must therefore be taken to completely oxidize the growing film. Ion-assisted deposition with an oxygen ion beam can improve the stoichiometry of these films [8]. The position of the 1000 cm⁻¹ Si-O stretch absorption band can be used to determine the phase of the coating [9]. The band shifts from 960 cm⁻¹ for SiO to 1085 cm⁻¹ for SiO to 1085 cm⁻¹ for SiO₂.