Thin film growth on nanostructured polymer webs for anti-reflection purposes

Nanostructures offer an alternative way of imparting anti-reflection properties to a surface, but most of the structuring techniques are quite complex and cost-intensive. At the Fraunhofer-IOF in Jena, a method for generating nanostructures on a PMMA surface by using a simple plasma-etching procedure has been developed. In the work described herein, we have attempted to transfer this technique to polymer webs in a roll-to-roll process with the intention of creating a highly effective and low-cost anti-reflection surface. Important for prospective applications of the webs is the protection of the nanostructured polymer surface by a thin glass-like layer. The challenge of this task is to enhance the mechanical stability without deteriorating the anti-reflection effect at the same time. First experiments involving the deposition of SiO2 layers in a thickness range from 15 nm to 45 nm on nanostructured TAC samples showed noticeable differences in the spectral reflectances obtained. While for "bump-like" nanostructures the average residual reflection could be minimized with increasing thickness of the protective layers, the reflectance curves of "sponge-like" samples were simply shifted to longer wavelengths. This could be rationalized in terms of different growth patterns of the SiO2 coatings, which were identified from SEM micrographs. While on a "bump-like" nanostructure the film enwraps the bumps and boosts the material/air fill factor in a suitable range for anti-reflection, a coating on a "sponge-like" structure closes the voids and grows predominantly on top of the structure. On such a "sponge"-type structure, the abrasion resistance can be significantly increased, almost reaching the value of the untreated surface. (C) 2011 Elsevier B.V. All rights reserved.