Methylated Silica Surfaces Having Tapered Nipple- Dimple Nano-pillar Morphologies as Robust Broad- Angle and Broadband Anti-Reflection Coatings
Résumé
In this work, mechanically, chemically and thermally resistant broad-band and broad-angle anti-reflection coatings were prepared on 10 cm diameter glass substrates combining sol-gel deposition with nano-imprint lithography. The coatings are composed of water-repellent methylated silica (Si 4 O 7 Me 2) and exhibit a transverse refractive index gradient created by tapered, nipple-dimple, sub-wavelength nano-structures, featuring a record vertical aspect ratio of ~1.7. The structure is composed of hexagonal arrays of nano-pillars (~200 nm height; ~120 nm width) and holes (~50 nm depth, ~100 nm width) with 270 nm pitch. The corresponding effective refractive index is between 1.2 and 1.26 depending on the fabrication conditions. Total transmission for double-face nano-imprint wafers reaches 96~97% in the visible range, it is limited by specular reflection and mostly by the intrinsic diffusion of the glass substrate. The anti-reflective effect is effective up to ~60 degrees incidence angle. We address the robustness of the inorganic-based coating in various realistic and extreme conditions comparing them to the organic Perfluoropolyether (PFPE) counterpart (master reference). The sol-gel system is extremely stable at high temperature (up to 600°C, against 200°C for the polymer reference). Both systems showed excellent chemical stability, except in strongly alkaline conditions. The inorganic nano-structure showed abrasion resistance more than two orders of magnitude superior to the polymer one, with less than 20% loss of anti-reflective performance after 2000 rubbing cycles under ~2 Ncm-2 pressure. This difference springs from the large elastic modulus of the sol-gel material combined with an excellent adhesion to the substrate and to the specific nipple-dimple conformation. The presence of holes allows maintaining a refractive index gradient profile even after tearing-out part of the nano-pillars population. Our results are relevant to applications where transparent windows with broad-band and broad angle transmission are needed, such as protective glasses on photovoltaic cells or C-MOS cameras.
Origine : Fichiers éditeurs autorisés sur une archive ouverte
Loading...