Optical inhomogeneous gradient films have been produced by reactive pulse magnetron sputtering of a silicon target in a variable mixture of the reactive gases oxygen and nitrogen. The presented coating system allows the automatic deposition of the gradient films with predefined, freely varying refractive index at a deposition rate of 1 nm/sec uniformly across 8” diameter.
This system has been used for deposition of antireflective coatings, rugate filters, edge filters and dichroic filters composed of SixOyNz films with both gradient and sharp transitions between oxide and nitride. The deposited antireflective coatings consist of one gradient and one homogeneous layer and have a reflectivity of less than 0.5% in the wavelength range between 440 and 620 nm. Thermal substrate load during deposition of these coating is considerably lower than for deposition of conventional multilayer of silicon and titanium oxide. An example of a deposited simple narrow band rugate filter has 45 cycles of sinusoidal refractive index modulations between 1.67 and 1.95, resulting in a reflectivity of > 99.9% at 550 nm with a half-power bandwidth of 42.5nm and an average transmission of 95% in the remaining visual spectral range. The produced edge and dichroic filters consist of quarter-wave oxide and nitride layers. All films have been deposited at one sputtering station without interruption of the plasma thus guaranteeing a very stable, reproducible and highly efficient process.