Preparation of a gradient SiO2 antireflective coating by a co-sputtering method using a dual rotatable magnetron system

Current standard antireflective coatings are layer stacks of alternating high and low refractive transparent dielectric layers. An alternative possibility for getting antireflective surfaces is to apply a gradient layer for a smooth change of the refractive index from the substrate to the surrounding medium (air). To achieve this, a gradient layer of SiO2 and aluminum doped zinc oxide (AZO) was prepared onto glass substrates by a co-sputtering method using a dual magnetron arrangement with two rotatable targets. After the film deposition the AZO was removed by an etching step with hydrochloric acid. The result of this preparation method was a gradient layer of SiO2 with good antireflective properties. The antireflective behavior was observed in the range of 300 nm to 1100 nm. The visible reflectance is decreased by 3 % and the solar reflectance is decreased by 2 % as well, resulting in an appropriate increase of the transmittance. The films had a maximum roughness Rt in the range of 60 nm. A scrubbing brush test showed a mechanical stability as good as an (HL)2 antireflective multilayer system consisting of SiO2 and TiO2. The described deposition technique was exemplary and used for the deposition of rough silica films and has the potential to be used for any kind of material to obtain rough surfaces.