Roll-to-roll deposition of permeation barrier layers using a rotatable dual magnetron sputtering system

High performance permeation barrier coatings are usually multilayer stacks consisting of inorganic and organic layers. Besides the water vapor transmission rate the optical properties of such layers have a high importance. This study focuses on the deposition of silicon oxynitride as the inorganic layer. This material attracts a widespread interest due to its varying refractive index depending on its oxygen and nitrogen content. The experiments were carried out in a roll-to-roll coating machine using a rotatable magnetron system. The layers were deposited by reactive pulsed magnetron sputtering using targets of one meter length. It was established that the ratio of oxygen and nitrogen in the layer was not only determined by the reactive gas mixture but also by the chosen set point of the closed loop control. Both the Secondary Ion Mass Spectrometry and the Glow Discharge Optical Emission Spectroscopy (GDOS) measurements revealed a vertically different composition of the SiOxNy layers. This result was attributed to the fact that the substrate passes zones of different plasma density during the deposition process. A model for the description of the optical properties was developed on the basis of these observations. A five layer system was introduced in place of the SiOxNy single layer. These five sub-layers differed in their dispersion relation in accordance with the results of the GDOS measurement. The model provided an excellent description of the measured transmittance spectra.