| Nanoimprint lithography(NIL)has been demonstrated as a high-throughput and low cost lithography technology with sub-10 nm resolution.Various kinds of NIL techniques have been developed for different applications.Among them,soft UV-NIL which combines advantages of both UV-NIL to achieve a high resolution has been widely used as a simple and feasible method to pattern micro and nanostructures.The key component of this technology is a flexible or soft mold.The most commonly used mold(stamp)material in soft UV-NIL or soft lithography is the flexible polymer,poly(dimethylsiloxane)(PDMS),which allows an intimate physical contact with substrates without applying large external pressure.However,there are still shortcomings for PDMS as mold material.Its low modulus limits the achievable resolution.PDMS mold tends to absorb low viscous liquid resist during UV-imprinting process due to the intrinsic high permeability of PDMS for gases and organic molecules.Treating the PDMS surface by O2 plasma for coating a mold release layer of fluoroalkyltrichlorosilane can cause the spontaneous formation of disordered wavy patterns on it.The sticky property of the PDMS surface dueto the residual uncured prepolymer could increase the risk of adhesion of particles and other contaminants and require great care in handling.In contrast to thermoset elastomers based on cross-linking bonds such as PDMS,thermoplastic elastomers(TPE)are the other family of polymeric elastomers that combine the properties of rubber with the recyclability and thermal processing advantages of plastics.Poly(styrene-block-ethylene-co-butylene-block-styrene)(SEBS)is one of the most important TPE consisting of plastic blocks of polystyrene and rubber blocks of hydrogenated polybutadiene.Compared with PDMS,SEBS can also offer transparency and flexibility while it is relatively economical and easy to use in manufacture by extrusion,molding,thermoforming and heat welding.Herein,we propose to use commercial SEBS as a rapid prototyping alternative for applications in flexible nanoimprint mold.In this work,the thermal properties,mechanical properties,along with Ultraviolet-visible(UV-vis)transmission properties were first estimated,proving SEBS to be an excellent flexible mold material.The flexible SEBS molds were fabricated via a thermal imprint process.To protect the master mold,the SEBS pellets were first thermally pressed into a smooth sheet by sandwiching them between a pair of fluoroalkyltrichlorosilane coated silicon wafers.The blank SEBS sheet was further thermally imprinted by the master mold.The mold release agent,fluoroalkyltrichlorosilane,could not directly react with SEBS due to its chemical inertia.A 10 nm thick SiO2 layer was deposited on the patterned SEBS by a PECVD process.The mold release agent,fluoroalkyltrichlorosilane was able to covalently bind on its surface through the silanol groups on the SiO2.The soft UV-NIL process using the flexible mold was conducted manually with a broadband UV exposure.The homemade UV-curable resist was spin-coated onto a substrate.The pitch and diameter of the imprinted holes were identical to those of the master mold.To determine the durability of the SEBS mold,its imprint behavior was tested through repeated imprinting cycles.The SEBS mold preserved its imprint fidelity over 10 repeated imprint cycles.The blank SEBS sheet was also tried as an elastic support for hybrid nanoimprint mold instead of PDMS.The SEBS hybrid mold consist of a hard patterning layer on a 0.3 mm-thick SEBS support.The prepared blank SEBS was placed onto a film of a silicon containing UV-curable resist spin-coated onto a silicon wafer.After,the SEBS,absorbed with liquid UV-curable resist,was separated from the Si wafer and then carefully placed onto a master imprint mold which had been spin-coated with the same resist film.Then,the sample was exposed to UV radiation and separated from the master mold.With a brief exposure to an O2 plasma,the surface of the cured resist was oxidized to form inorganic silica and was then coated with a self-assembled monolayer of fluoroalkyltrichlorosilane by vapor phase deposition.The resulting hybrid mold combines the advantages of both a hard mold to achieve a high-resolution pattern transfer and a flexible mold to enable conformal contact to the substrate.A sub-50 nm resolution pattern transfer was got on a plant substrate.To demonstrate the utility and flexibility of the hybrid mold on SEBS,a microfiber with 7 ?m diameter tapered from a standard optical fiber was employed as a highly curved substrate.Gratings with 278 nm pitch were patterned on the microfiber using the SEBS hybrid mold through a double transfer UV imprint method with air pressure assistance.The UV-curable resist was spin coated on a silicon wafer and covered by the hybrid mold.Afterwards,the mold was removed from the wafer with some liquid resist adhered onto it,placed on the side of the optical fiber and UV cured by a mercury lamp under an air pressure.After UV-curing and mold separation,the grating patterns were imprinted onto the surface of the fiber.The imprinted grating patterns cover the top half of the cylindrical surface.The imprinting result on the cylindrical microfiber indicated that the mold had capability of patterning highly curved substrate. |
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