Characterization of reactively sputtered permeation barrier materials on polymer substrates

Transparent permeation barrier layers are not only used for food packaging but are also needed to encapsulate flexible electronic devices. Magnetron sputtering allows the deposition of high quality oxide barrier layers with a low water vapor and oxygen permeation. This paper compares different metal oxide layers which are deposited onto a polyethylene terephthalate (PET) film using a reactive dual magnetron sputtering process. The oxides of aluminum, silicon, titanium, zinc and a zinc-tin alloy are compared regarding their permeation barrier, structural and surface properties to determine the relationship between the layer structure and the gas permeation. Thereby, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the morphology and surface structure of the layers and X-ray diffraction analysis (XRD) was used to determine the solid state phase. Cross-section images taken with SEM show a very compact structure for both aluminum oxide and zinc-tin oxide layers. These materials also have the lowest water vapor permeation compared to all other materials. Zinc oxide and titanium oxide layers both exhibit a columnar structure. Zinc oxide is polycrystalline and has a surprising low water vapor and oxygen permeation. In contrast to that the amorphous titanium oxide layers show a high water vapor and oxygen permeation which is not decreasing with an increasing layer thickness above 40 nm.