Graded refractive index layer systems for antireflective coatings and rugate filters deposited by reactive pulse magnetron sputtering

Reactive pulse magnetron sputtering enables the long term stable deposition of compound films by sputtering of a metal target in a mixture of inert and reactive gas. It also allows depositing ternary compounds such as SixNyOz or AlxNyOz using mixtures of different reactive gases such as oxygen and nitrogen. Gradient layers with varying composition and refractive index in growth direction can be obtained by changing the gas mixing ratio during the deposition. In the paper the application of this new technology to fabricate antireflective (AR) coatings and rugate filters is reported. The possibility of choosing intermediate refractive indices gives a new degree of freedom for the design of an AR coating with a simple and robust design. The reflectivity of the deposited AR coatings, consisting of a twofold gradient SixNyOz film, is less than 0.5% in the wavelength range between 440 and 620 nm. Rugate filters composed of a film with sinusoidal oscillating refractive index have been produced at high precision by reactive sputtering in a varying reactive gas mixture, using appropriate process control. An example of a deposited rugate filter has a reflectivity >99.9% at 550 nm with a bandwidth of 60 nm. Both AR coatings and rugate filters are produced by stationary sputtering at only one deposition station without interruption of the plasma process. This guarantees a very stable and efficient process. The applied hardware allows coating of 8 inch substrates with film thickness uniformity of ±1%.