Process control in sputtering of optical coatings
Reactive pulse magnetron sputtering processes are of increasing interest for the deposition of high-precision optical coatings of high density, low roughness and shift-free environmental stability. SiO2, Si3N4 and silicon oxynitride are attractive coating materials for interference filters produced by sputtering of a silicon target in a precisely controlled reactive gas mixture of oxygen and nitrogen. Reactive co-sputtering of different target materials such as silicon and tantalum in oxygen gas allows depositing SixTayOz layers with a refractive index determined by the power ratio of the two targets. Using nanoscale film growth control both technologies allow to achieve optical filter systems with a defined dependency of refractive index on film thickness. Such so called rugate filters can find application in spectroscopy, laser optics and solar concentrator systems. Furthermore antireflective coatings can be produced cost effective due to the uninterrupted coating process in one deposition step. New developments in reactive pulse magnetron sputtering have significantly improved process stability and reproducibility. The possibility of operating the magnetron system in unipolar, bipolar and pulse packet mode allows adjusting the energetic substrate bombardment in a wide range. This gives new technological degrees of freedom to optimize the deposition process and the film properties. Several aspects of process control are specified in this paper and completed by exemplary optical coatings and their characterization and environmental stability tests results as well.