Effective surface oxidation of polymer replica molds for nanoimprint lithography
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NANO EXPRESS
Open Access
Effective surface oxidation of polymer replica molds for nanoimprint lithography Ilhwan Ryu, Dajung Hong and Sanggyu Yim*
Abstract In nanoimprint lithography, a surface oxidation process is needed to produce an effective poly(dimethylsiloxane) coating that can be used as an anti-adhesive surface of template molds. However, the conventional photooxidation technique or acidic oxidative treatment cannot be easily applied to polymer molds with nanostructures since surface etching by UV radiation or strong acids significantly damages the surface nanostructures in a short space of time. In this study, we developed a basic oxidative treatment method and consequently, an effective generation of hydroxyl groups on a nanostructured surface of polymer replica molds. The surface morphologies and water contact angles of the polymer molds indicate that this new method is relatively nondestructive and more efficient than conventional oxidation treatments. Introduction Recently, nanoimprint lithography [NIL] has attracted increasing attention as a facile technique for patterning polymer nanostructures [1-3]. The principle of NIL is very simple and described in detail elsewhere [1]. A hard mold with nanoscale surface-relief features is pressed onto a polymer cast at controlled temperature and pressure, which creates replica patterns on the polymer surface. Mold materials normally used for NIL include silicon, silicon dioxide, silicon nitride, or metals such as nickel, and the surface nanostructures are typically fabricated using various lithographic, electrochemical, and etching techniques [1,4-6]. While these conventional inorganic molds are thermally and mechanically stable [7], they often easily break due to their stiffness when pressed or removed. The large mismatch of thermal expansion between stiff inorganic molds and polymeric films is also problematic. For these reasons, several attempts have been made to use soft and flexible molds made from polymeric materials [8]. Various elastomeric polymers such as poly(dimethylsiloxane) [PDMS] were used for this purpose [9-11]. However, due to the innate softness of these materials with low elastic modulus, e.g., 2 to 4 MPa for PDMS, the molds tended to deform when pressure was applied, and hence, these materials were not suitable for * Correspondence: [email protected] Department of Chemistry, Kookmin University, Seoul, 136-702, South Korea
imprinting nanoscale features. Stiffer polymeric molds with a higher mechanical strength such as urethane[12,13] and epoxide-based [14,15] polymer molds were therefore introduced. For example, the Norland Optical Adhesives (NOA63, Norland Products, Cranbury, NJ, USA), a urethane-based UV-curable polymer, is a plausible candidate due to its good mechanical properties and high Young’s modulus (approximately 1, 655 MPa) [16]. The urethane- and epoxide-based polymers, however, possess high surface energies, leading to strong adhesion of the molds to the imprinted surface. Consequently, the mold surface must be coated wit
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