A fast thermal-curing nanoimprint resist based on cationic polymerizable epoxysiloxane
© Ge and Xia; licensee Springer. 2012
Received: 9 May 2012
Accepted: 25 June 2012
Published: 9 July 2012
We synthesized a series of epoxysiloxane oligomers with controllable viscosity and polarity and developed upon them a thermal-curable nanoimprint resist that was cross-linked in air at 110°C within 30 s if preexposed to UV light. The oligomers were designed and synthesized via hydrosilylation of 4-vinyl-cyclohexane-1,2-epoxide with poly(methylhydrosiloxane) with tunable viscosity, polarity, and cross-linking density. The resist exhibits excellent chemical and physical properties such as insensitivity toward oxygen, strong mechanical strength, and high etching resistance. Using this resist, nanoscale patterns of different geometries with feature sizes as small as 30 nm were fabricated via a nanoimprint process based on UV-assisted thermal curing. The curing time for the resist was on the order of 10 s at a moderate temperature with the help of UV light preexposure. This fast thermal curing speed was attributed to the large number of active cations generated upon UV exposure that facilitated the thermal polymerization process.
KeywordsNanoimprint Epoxysiloxane Transfer layer UV-assisted thermal curing Cationic polymerization
Resist is one of the key components for nanoimprint lithography (NIL) in addition to the imprint mold, the imprint machine, and imprint processes[1, 2]. Depending on the process, nanoimprint resists can be classified into two basic categories: thermoplastic polymers or thermal curable materials[4, 5] for thermal-NIL, and UV-curable monomers or oligomers for UV-NIL[6–9]. The most commonly used NIL resists in earlier years were thermal plastic polymers, which required high pressure and high temperature so that the mold can be pressed into the molten resist film. Curing of thermal curable resists, although it requires lower pressure, is a time-consuming process due to the low speed of thermal-initiated polymerizations. Recently, UV-NIL has been developed for device fabrication, and it allows for imprinting at a low pressure and room temperature[6, 10, 11]. The process involves pressing a transparent mold into a low viscous photo-curable liquid thin film on a substrate and then solidifying the liquid materials via a UV light irradiation. The liquid resist can automatically fill the recess portions of the features on the mold due to capillary effect. An ideal UV resist usually has low viscosity, low surface tension, good adhesion to the substrate, fast cross-linking speed, high mechanical strength, and high etching resistance after cross-linking.
The fundamental ingredients of a UV resist include a UV-curable matrix, a photo-initiator, and other additives such as plasticizer, curing accelerator, photo-sensitizer, flow and leveling agent, fluorinated surfactant, and so on. Acrylate-based resins have been widely used in various UV-curable material systems as the backbone because of their great reactivity with a wide choice of acrylated monomers and oligomers[12, 13]. However, a significant drawback of acrylated materials is that free radical polymerization is strongly inhibited by oxygen scavenging of the free radicals. This prevents UV-curing imprint process from being operated in ambient air environment, increasing the cost of equipment. To address this issue, materials such as vinyl ethers and epoxides based on cationic polymerization were used for UV-curable nanoimprint resists, which effectively cross-link upon UV exposure in the presence of air. However, vinyl ether resins are costly because of a narrow choice of commercially available monomers and oligomers. The cationic polymerization rate of epoxy groups is low, and the conversion rate is less than 10% when the temperature is lower than 50°C[17, 18]. Recently, new epoxy-based materials were developed for imprint resists[19, 20]; the mechanism of the polymerization is not fully revealed. Furthermore, these resists can only be cross-linked by UV light, so a transparent mold or substrate is required. On the other hand, epoxy-based negative photoresist, such as SU-8, was used for combined UV and thermal NIL, but it required very high imprint pressure (as high as 30 bar) to press the mold into the molten resist[21–23].
Herein, we report on a new liquid NIL resist based on epoxysiloxane oligomers that can be thermally cured within a short period of time in air at a moderate temperature using a UV-assisted thermal curing process. We designed and synthesized these oligomers via hydrosilylation of 4-vinyl-cyclohexane-1,2-epoxide with Si-H group-functionalized siloxane precursor. The viscosity, cross-linking density, and polarity of epoxysiloxane were controlled on demand by employing siloxane precursors with varied molecular weights and Si-H functionality. We also developed a fast UV-assisted thermal imprint process that requires low pressure, moderate temperature without the demand for optically transparent mold and vacuum equipment. We fabricated nanostructures of different geometries and sizes with the resist and further demonstrated high aspect ratio pattern transfer by reactive ion etching (RIE).
Polymethylhydrosiloxane (PMHS) precursors with different Si-H functionality, (15% to 18% methylhydrosiloxane)-dimethylsiloxane copolymer, (50% to 55% methylhydrosiloxane)-dimethylsiloxane copolymer and polymethylhydrosiloxane terminated with trimethylsilyl, and p-(octyloxyphenyl)phenyliodonium hexafluoroantimonate (95%, photo-acid generator) were purchased from Gelest, Inc. (Morrisville, PA, USA). A 4-vinyl-1-cyclohexene 1,2-epoxide(VCHE) mixture of isomers, propylene glycol monomethyl ether acetate (PGMEA), and chlorotris(triphenylphosphine) rhodium(I) (Rh catalyst) were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA). Lift-off layer (LOL) 2000 was purchased from MicroChem Corp (Newton, MA, USA). All materials were used as purchased.
Synthesis and characterization
Imprinting and pattern transfer
Cross-linking degree characterization
Solubility measurements of the resist films under various curing conditions were taken to check if the resist film was fully cross-linked. The measurement was simplified by rinsing the resist films with acetone after curing. Once fully polymerized, the thin film is insoluble in acetone due to the formation of highly cross-linked polymer network. An uncured or partially cured film would be stripped or destroyed by acetone.
Results and discussion
Material properties of synthesized epoxysiloxanes and their mixtures
Si-H of PMHS (mol%)
Viscosity of PMHS (mPa s)
Viscosity of epoxysiloxane (mPa s)
Properties of cured films
Water contact angle (degrees)
Strain at peak (%)
Young's modulus (MPa)
50 to 55
Ingredients of a typical NIL resist used in this study and their functions
Silicon-containing oligomer; increases the viscosity and polarity of resist and provides high cross-linking density
Silicon-containing oligomer; provides oxygen RIE etching resistance and moderate cross-linking density
Silicon-containing oligomer; relieves mechanical properties as plasticizer
Low-viscosity diluents; improves resist flow for spin coating
Photo-initiator; generates cationic acids upon exposure to UV radiation
Finally, in contrast to the photo-initiated radical polymerization, photo-initiated cationic polymerization can be implemented in the presence of oxygen. The resist also has long shelf time due to the fact that the propagating polymer cations are not reacting with themselves. We have achieved similar imprint results with resists that were prepared a year earlier and have been kept in dark bottle at room temperature. This indicates that the shelf time of the resists is at least 1 year.
We have synthesized a series of epoxysiloxane oligomers with controllable viscosity and polarity and developed a fast curable nanoimprint resist based on them. The new imprint resist was thermally cured within 30 s at a moderate temperature when a preexposure to UV light was adopted. Various patterns with feature sizes ranging from 30 nm to 50 μm were faithfully duplicated onto the resist film during this UV-assisted thermal curing NIL that was carried out in ambient air environment. The short cross-linking time was attributed to the large amount of cations that accelerated the thermal polymerization process. The unique properties of the resist had enabled a novel imprint process that combines the advantages of both thermal and UV-curable NIL such as low pressure, short processing time with a wider choice of imprint mold materials.
This work was jointly supported by the National Natural Science Foundation of China (grant nos. 10874072 and 91023014) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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