Sputter process with time-variant reactive gas mixture for the deposition of optical multilayer and gradient layer systems

Magnetron sputtering of a silicon target in a time-variant mixture of the reactive gases oxygen and nitrogen allows the deposition of optical multilayer and gradient layer systems of silicon oxinitride at one stationary sputtering station. In this paper the processes within the sputter discharge and the properties of the growing film during the change of the reactive gas composition are investigated using optical in- situ monitoring, optical plasma emission spectroscopy and plasma impedance monitoring. A time delay between the change to the reactive gas composition and the resulting change to the film composition was observed. The time delay is longer for the transition from oxide to nitride deposition then vice versa. This asymmetry is attributed to the different affinity of nitrogen and oxygen to the silicon target. Examples of deposited antireflective coatings as well as rugate filters based on silicon oxinitride multilayer and gradient layer designs are given.