Effective surface oxidation of polymer replica molds for nanoimprint lithography
© Ryu et al; licensee Springer. 2012
Received: 9 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
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.
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 . 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 , 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 . 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 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) . 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 with an anti-adhesive layer. Recently Kim et al. introduced the PDMS coating technology onto various hard and soft molds including these stiffer polymers. The PDMS-coated molds showed good surface properties, i.e., low surface energy and low adhesion properties, like normal PDMS molds [17, 18]. It was previously reported that to create a strong and highly stable PDMS coating, the oxidized polymer surface must be treated with 3-aminopropyltriethoxysilane [APTES] before PDMS deposition. The hydroxyl groups on the oxidized polymer surface can bind strongly with APTES by silanization, and subsequent PDMS deposition forms strong covalent bonds between the aminosilane (APTES)-treated surface and monoglycidyl ether-terminated PDMS through epoxy-amine chemistry [17, 18]. A well-established technique for surface oxidation and consequent generation of terminal hydroxyl groups for various semiconductors is the piranha soak (sulfuric acid and hydrogen peroxide mixed solution) . This approach, however, cannot be applied to polymer surfaces since most polymeric materials are highly vulnerable to strong acids. Photooxidation using UV-oxygen treatment has been reported as an alternative [17, 18]. However, this approach is also destructive [19, 20], and the polymer surfaces are rapidly etched before the formation of surface hydroxyl groups is optimized. In this study, we developed a relatively nondestructive oxidation approach using a mixed solution of ammonium hydroxide and hydrogen peroxide for the generation of hydroxyl groups on NOA63 polymer surfaces and compared the effectiveness of this method with that of previously reported photooxidation approaches.
Results and discussion
In conclusion, a novel surface oxidation method using a basic oxidative solution was successfully developed for the generation of hydroxyl groups on a nanostructured NOA63 polymer surface. In comparison with the previously reported UV photooxidation method, this new method is relatively nondestructive and more effective based on changes in the surface morphology and contact angle.
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0013057).
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