pubs.acs.org/Langmuir © 2009 American Chemical Society
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Replica Mold for Nanoimprint Lithography from a Novel Hybrid Resin Bong Kuk Lee,†, Lan-Young Hong,‡, Hea Yeon Lee,*,†,§ Dong-Pyo Kim,*,‡ and Tomoji Kawai*,†,§ †
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The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, ‡Department of Fine Chemical Engineering and Chemistry, 220 Kung-dong, Yusung-gu, Daejeon 305-764, Korea, and §Division of Quantum Phases & Devices, School of Physics, Konkuk University, Seoul 143-701, Korea. These authors contributed equally to this work Received April 6, 2009. Revised Manuscript Received August 5, 2009 The use of durable replica molds with high feature resolution has been proposed as an inexpensive and convenient route for manufacturing nanostructured materials. A simple and fast duplication method, involving the use of a master mold to create durable polymer replicas as imprinting molds, has been demonstrated using both UV- and thermal nanoimprinting lithography (NIL). To obtain a high-durability replicating material, a dual UV/thermal-curable, organic-inorganic hybrid resin was synthesized using a sol-gel-based combinatorial method. The cross-linked hybrid resin exhibited high transparency to UV light and resistance to organic solvents. Molds made of this material showed good mechanical properties (Young’s modulus=1.76 GPa) and gas permeability. The low viscosity of the hybrid resin (∼ 29 cP) allowed it to be easily transferred to relief nanostructures on transparent glass substrates using UV-NIL at room temperature and low pressure (0.2 MPa) over a relatively short time (80 s). A low surface energy release agent was successfully coated onto the hybrid mold surface without destroying the imprinted nanostructures, even after O2 plasma treatment. Nanostructures with feature sizes down to 80 nm were successfully reproduced using these molds in both UVand thermal-NIL processes. After repeating 10 imprinting cycles at relatively high temperature and pressure, no detectable collapse or contamination of the replica surface was observed. These results indicate that the hybrid molds could tolerate repeated UV- and thermal-NIL processes.
1. Introduction Recently, several techniques have been developed to fabricate nanoscale structures based on physical contact. These techniques may be divided into two categories: molding and imprinting. Molding methods include replica molding,1 microtransfer molding,2 micromolding in capillaries,3 and solvent-assisted micromolding.4 Imprinting techniques include thermal nanoimprint lithography (NIL),5,6 ultraviolet (UV)-NIL,7 and step and flash imprint lithography (SFIL).8,9 NIL, initially proposed and developed by the Chou group,5,6 has emerged as one of the most promising technologies for high-resolution nanoscale patterning. This technique consists of a thermoplastic embossed with a hard mold under pressure at a temperature above the glass transition. Different types of NIL, such as UV-NIL7 and SFIL8,9 have been developed as high-throughput, low-cost approaches to generate relief patterns over large areas. SFIL is a low-temperature, low-pressure UV-NIL process. These methods are capable *Corresponding author. (H.Y.L.) E-mail:
[email protected]; tel: þ81-6-6879-8447; fax: þ81-6-6875-2440. (T.K.) E-mail: kawai@sanken. osaka-u.ac.jp; tel: þ81-6-6879-8447; fax: þ81-6-6875-2440. (D.-P.K.) E-mail:
[email protected]; tel: þ82-42-821-6695; fax: þ82-42-823-6665. (1) Xia, Y.; Kim, E.; Zhao, X. M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347–349. (2) Zhao, X. M.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1996, 8, 837–840. (3) Kim, E.; Xia, X.; Whitesides, G. M. Nature 1995, 376, 581-584. (4) Kim, E.; Xia, Y.; Zhao, X. M.; Whitesides, G. M. Adv. Mater. 1997, 9, 651-654. (5) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114-3116. (6) Chou, S. Y.; Krauss, P. R.; Renstrom, J. P. Science 1996, 272, 85-87. (7) Haisma, J.; Verheijen, M.; Vandenheuvel, K.; Vandenberg, J. J. Vac. Sci. Technol. B 1996, 14, 4124-4128. (8) Ruchhoeft, P.; Colburn, M.; Choi, B.; Nounu, H.; Johnson, S.; Bailey, T.; Damle, S.; Stewart, M.; Ekerdt, J.; Sreenivasan, S. V. J. Vac. Sci. Technol. B 1999, 17, 2965- 2969. (9) Colburn, M.; Johnson, S.; Stewart, M.; Damle, S.; Bailey, T. C.; Choi, B.; Wedlake, M.; Michaelson, T.; Sreenivasan, S. V.; Ekerdt, J. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3676, 379-389.
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of creating nanopatterns with sub-30-nm features over large areas.5-9 NIL has been employed in numerous fields, including electronics,10-12 photonics,13-15 magnetic devices,16-18 and biological applications.19-21 However, since NIL is based on the mechanical molding of polymer materials, some basic problems still remain, such as the requirement of an expensive master mold, which is often deformed and contaminated by the imprinting process. As a result of the relatively long processing times and repeated use in low-pressure22 and room-temperature imprinting,23 pattern collapse has been observed when the imprinted features were fine and dense with a high aspect ratio.24,25 There is therefore a need for (10) Guo, L. J. ; Krauss, P. R. ; Chou. S. Y. Appl. Phys. Lett. 1997, 71, 1881-1883. (11) McAlpine, M. C.; Friedman, R. S.; Lieber, C. M. Nano Lett. 2003, 3, 443-445. (12) Li, D.; Guo, L. J. Appl. Phys. Lett. 2006, 88, 63513-1-63513-3. (13) Guo, L. J.; Cheng, X.; Chao, C. Y. J. Mod. Opt. 2002, 49, 663-673. (14) Pisignano, D.; Persano, L.; Raganato, M. F.; Visconti, P.; Cingolani, R.; Barbarella, G.; Favaretto, L.; Gigli. G. Adv. Mater. 2004, 16, 525-529. (15) Ahn, S. W.; Lee, K. D.; Kim, J. S.; Kim, S. H.; Park, J. D.; Lee, S. H.; Yoon, P. W. Nanotechnology 2005, 16, 1874-1877. (16) Wu, W.; Cui, B.; Sun, X. Y.; Zhang, W.; Zhuang, L.; Kong, L. S.; Chou, S. Y. J. Vac. Sci. Technol. B 1998, 16, 3825-3829. (17) Chen, Y.; Lebib, A.; Li, S. P.; Natali, M.; Peyrade, D.; Cambril, E. Microelectron. Eng. 2001, 57-58, 405-410. (18) McClelland, G. M.; Hart, M. W.; Rettner, C. T.; Best, M. E.; Carter, K. R.; Terris, B. D. Appl. Phys. Lett. 2002, 81, 1483-1485. (19) Hoff, J. D.; Cheng, L. J.; Meyhofer, E.; Guo, L. J.; Hunt, A. J. Nano Lett. 2004, 4, 853-857. (20) Lee, B. K.; Lee, H. Y.; Kim, P.; Suh, K. Y.; Kawai, T. Lab Chip, 2009, 9, 132-139. (21) Lee, B. K.; Lee, H. Y.; Kim, P.; Suh, K. Y.; Seo, J. H.; Cha, H. J.; Kawai, T. Small 2008, 4, 342-348. (22) Khang, D. Y.; Kang, H.; Kim, T. I.; Lee, H. H. Nano Lett. 2004, 4, 633-637. (23) Lu, Y.; Chen, X.; Hu, W.; Lu, N.; Sun, J.; Shen, J. Langmuir 2007, 23, 3254-3259. (24) Becker, H.; Heim, U. Sens. Actuators A: Phys. 2000, A83, 130-135. (25) Gourgon, C.; Perret, C.; Micouin, G.; Lazzarino, F.; Tortai, J. H.; Joubert, O. J. Vac. Sci. Technol. B 2003, 21, 98-105.
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cheap replica molds with high resolution and reliable mechanical properties. A thermally and chemically stable hard mold retains fine nanoscale features with minimal local deformation, and the use of inexpensive precursors would provide an economic solution. Several studies have focused on the fabrication of replica molds for NIL and soft lithographies. In general, molds for these processes are required to have high mechanical strength, low surface energy, and high solvent resistance. Given these requirements, soft polymer molds made from poly(dimethylsiloxane) (PDMS) are not optimal for nanopatterns with high-density features and sub-100-nm resolution due to its low tensile modulus (approximately 1.8 MPa for Sylgard 184 PDMS or 8.2 MPa for hard PDMS).26 Several polymer materials have been used as an alternative low-cost replica mold, including urethane-based UVcurable polymers,27-29 amorphous fluoropolymer,22,30 photocurable fluoropolymer,31-33 photocurable fluorinated organic-inorganic hybrid materials,34 and photocurable organosilicon prepolymer.35 Polyurethane-based UV-curable polymers, such as MINS101m (Minuta Technology)27 and NOA 63 (Norland Products, Inc.),29 have been used for imprinting mold materials because they both exhibit a high Young’s modulus (∼1.7 and 1.655 GPa, respectively). However, to use these materials, it is necessary to increase the temperature of the mold to 70 °C before separation in order to decrease adhesion between the polymer mold and the master.28 An amorphous fluoropolymer (Dupont Teflon AF 2400) with a high tensile modulus (1.6 GPa) was successfully used in low-pressure NIL for patterning sub-100-nm features.22,30 In this case, however, high pressure (150 MPa) and temperature (300 °C) were required because of the intrinsic properties of the polymer.30 A photocurable fluoropolymer and a fluorinated organic-inorganic hybrid material were also used to fabricate a replica mold, but were not suitable for high-pressure imprinting as a result of low mechanical strength (tensile modulus of 3.932 and 40 MPa,34 respectively). Successful molds boasting defect-free replication have been fabricated using expensive, fluorine-containing materials with low surface energies, and a cross-linked organosilicon replica with 30-nm features was fabricated by UV-NIL and used in both low-pressure (0.2 MPa) UVNIL and medium-pressure (190 psi) thermal-NIL.35 The surface of the cross-linked organosilicon replica could be modified with a release agent without destruction of the nanostructure. However, the mechanical properties of the replica were not reported, and may limit the durability of the replica mold in high-pressure NIL applications. This study demonstrates facile fabrication of a polymer replica mold that exhibits high durability against chemical, thermal, and mechanical stresses, from a master mold at low pressure and room temperature. A dual UV/thermal-curable organic-inorganic (26) Choi, K. M.; Rogers, J. A. J. Am. Chem. Soc. 2003, 125, 4060-4061. (27) Kim, Y. S.; Lee, H. H.; Hammond, P. T. Nanotechnology 2003, 14, 1140-1144. (28) Kim, Y. S.; Lee, N. Y.; Lim, J. R.; Lee, M. J.; Park, S. Chem. Mater. 2005, 17, 5867-5870. (29) Lee, N. Y.; Lim, J. R.; Lee, M. J.; Park, S.; Kim, Y. S. Langmuir 2006, 22, 7689-7694. (30) Khang, D. Y.; Lee, H. H. Langmuir 2004, 20, 2445-2448. (31) Rolland, J. P.; Hagberg, E. C.; Denison, G. M.; Carter, K. R.; DeSimone, J. M. Angew. Chem., Int. Ed. 2004, 43, 5796-5799. (32) Rolland, J. P.; Van Dam, R. M.; Schorzman, D. A.; Quake, S. R.; DeSimone, J. M. J. Am. Chem. Soc. 2004, 126, 2322-2323. (33) Truong, T. T.; Lin, R.; Jeon, S.; Lee, H. H.; Maria, J.; Gaur, A.; Hua, F.; Meinel, I.; Rogers, J. A. Langmuir 2007, 23, 2898-2905. (34) Kim, W. S.; Choi, D. G.; Bae, B. S. Nanotechnology 2006, 17, 3319-3324. (35) Ge, H.; Wu, W.; Li, Z.; Jung, G. Y.; Olynick, D.; Chen, Y.; Liddle, J. A.; Wang, S. Y.; Williams, R. S. Nano Lett. 2005, 5, 179-182.
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hybrid material with a low viscosity was developed for fast replicating via UV-NIL. The organic-inorganic mesoporous resin exhibited excellent mechanical and optical properties, and a surface that could be chemically modified by a release agent in order to lower the surface energy. Relief nanostructures, originating from the master, were finely and efficiently transferred to the replicating resin via UV- and thermal-NIL processes with low adhesion.
2. Experimental Methods Synthesis of Organic-Inorganic Hybrid Resin. The mesoporous inorganic material was prepared using a consecutive sol-gel-based reaction as described elsewhere.36 Tetraethylorthosilicate (TEOS) was prehydrolyzed in 100:50 (v:v) ethanol/water containing HCl at 60 °C. A nonionic triblock copolymer, Pluronic P123 (EO20PO70EO20), was used as a structure-directing agent and was dissolved in ethanol and added to the TEOS sols. The titania sols were prepared by mixing TiCl4 in 80:20 (v:v) water/ethanol at room temperature. SiO2-TiO2 mesoporous composite sols were prepared by mixing the silica and titania sols at various ratios, followed by stirring for 2 h. 4,5-Dihydroxy-1,3-benzenedisulfonic acid disodium salt (catecholic salt, (OH)2C6H2(SO3Na)2, Tiron, Aldrich) was subsequently added to the homogeneous SiO2-TiO2 mesoporous composite sols, and the solutions instantly turned a pale, transparent orange. A typical molar ratio for a composite sol was TEOS/TiCl4/catecholic salt=1:0.1:0.025. In order to find the optimized condition of chemical composition maintaining a homogeneous phase between organic and inorganic phases, we investigated various chemical compositions (see Supporting Information, Table S1). For the preparation of the organic-inorganic hybrid resins, the SiO2-TiO2 mesoporous composite sols were mixed with 3-(trimethoxysilyl)propyl methacrylate (TMSPM; Sigma-Aldrich, St. Louis, MO) as a silane coupling agent, and poly(ethylene glycol) dimethacrylate (PEG-DMA, MW =550; Sigma-Aldrich) as a cross-linker, followed by stirring for 3 h. UV-curing of the hybrid resin containing approximately 41 wt % Si was activated in the presence of 1 wt % 2,20 -dimethoxy-2-phenylacetophenone (DMPA; Sigma-Aldrich).
Measurements of Mechanical and Physical Properties.
Hybrid resin films approximately 3 μm thick and containing 1 wt % DMPA as a photoinitiator were deposited on Si wafers to remove the influence of the substrate. The hybrid resin was cured at 365 nm (200 mJ cm-2 UV dose) with a UV lamp (Toscure251; Toshiba, Tokyo, Japan) and baked on a hot plate at 120 °C for 1 h to fully complete the curing and condensation reactions. The hardness and Young’s modulus of the coated resins were measured with a commercial nanoindentation system (Nanoindenter XP; MTS Nano Instruments, Oak Ridge, TN) at room temperature. To minimize position-dependent factors, the data presented are the statistical mean values calculated from at least 10 measurements. The viscosity of the hybrid resin was determined at 25 °C with a Brookfield viscometer Model DV-II (Brookfield Engineering Laboratories, Inc., Stoughton, MA). Swelling Behavior in Various Solvents. Solvents used for comparing the rate of swelling were methanol, ethanol, dehydrated toluene, and tetrahydrofuran. Free-standing hybrid resins, cured by UV irradiation and baking, were allowed to swell in an excess of each solvent at room temperature. After 48 h, the samples were removed from the swelling medium, and the excess solution on the swollen sample was dried with filter paper. The swollen mass (Ws) was weighed, and the procedure was repeated until a constant weight was achieved. The samples were subsequently dried for 48 h in desiccators at room temperature, and their dry weights (Wd) were recorded.20 (36) Kim, D. P.; Hong, L. Y.; Won, J. H.; Park, Y. S.; Shin, C. K. International Patent WO 07/069867 A1.
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Figure 1. Synthesis of the UV-curable organic-inorganic hybrid resin. (R = CH3).
Figure 2. (a) Hardness and (b) Young’s modulus as a function of contact depth for the hybrid resin film.
Gas Permeation Measurements. Gas permeation properties were measured by a gas permeability tester (BS Chem, GPA2001). The effective membrane area was 17.3 cm2. The operating pressures were 0.3 and 0.5 MPa for N2 and CO2, respectively. Membrane surfaces were observed by optical microscopy (SOMETECH, ICSL-305). Fabrication of the Hybrid Resin Replica Mold. UV-NIL was used to fabricate the hybrid mold. The transparent glass substrates were oxidized with UV-ozone for 1 h using an ozone cleaner (NL-UV253; Nippon Laser Denshi, Tokyo, Japan). The substrates were then spin-coated with a thin film (ca. 450 nm thickness) of 30 wt % hybrid resin diluted in ethanol containing 1 wt % DMPA at 3000 rpm for 20 s, followed by prebaking on a hot plate at 70 °C for 3 min to evaporate the solvent (Figure 3a, stage I). A positive (100 nm in height) and negative (188 nm in height) quartz masters, coated with 0.1 wt % Optool DSX (Daikin Industries, Osaka, Japan) as a release agent to prevent the adhesion of cured hybrid resin to the master mold, was imprinted at a pressure of 0.2 MPa at room temperature for 20 s under vacuum. The hybrid resin was cured at 365 nm (200 mJ cm-2 UV dose) for about 1 min while maintaining the pressure (Figure 3a, stage II) using a nanoimprint apparatus (NM-401; Meisyo Kiko, Hyogo, Japan) equipped with a UV lamp (Toscure251; Toshiba). The quartz master was then detached from the substrate (Figure 3a, stage III). After master separation, the substrates, now patterned with hybrid resin, were baked at 120 °C for 1 h to enhance the mechanical properties and solvent resistance. In a final step, the edges were trimmed from the hybrid resin replica mold, resulting in a negative of the master (Figure 3a, stage IV). NIL Processes with Hybrid Resin Replica Mold. Release agent (0.1 wt % Optool DSX) was coated onto the replica mold surface after oxidizing with an O2 plasma at 10 Pa for 20 s using a reactive ion etching system (RIE-10NR; Samco, Inc., Tokyo, 11770 DOI: 10.1021/la901203e
Figure 3. A schematic illustration of the experimental procedure. (a) The process flow of creating hybrid replica molds via UV-NIL: (I) spin-coating of the hybrid resin onto the glass substrate, (II) imprinting with a positive quartz mold and UV irradiation, (III) mold separation from the imprinted substrate, and (IV) edge cutting for removal of the nonimprinted component. The UV- or thermal-NIL method of replica molding is shown in panel b: (I) spin-coating of the polymer onto a silicon wafer and prebaking, (II) UV or thermal imprinting with a negative replica mold, and (III) mold separation from the imprinted substrate. Japan). This replica mold was used as a stamp for the UV- and thermal-NIL processes. A schematic of the nanoimprinting process is shown in Figure 3b. For UV-NIL, UV-curable hybrid resin was nanoimprinted with the replica mold on a silicon wafer. The imprinting conditions were the same as described above. For thermal-NIL, the silicon wafer was cleaned with an UV-ozone cleaner and spin-coated with a thin film of poly(methyl methacrylate) (PMMA; MicroChem Corp., Newton, MA), and baked at 80 °C for 10 min to evaporate the solvent (Figure 3b, stage I). PMMA was imprinted with the replica mold at 150 °C and 10 MPa for 5 min (Figure 3b, stage II). After cooling to room temperature, the replica mold was separated from the substrate (Figure 3b, stage III). Observation of Surface Pattern Morphology. Optical images of the hybrid resin mold were acquired with an Olympus BX51 inverted research microscope equipped with a high-resolution digital camera (DP70; Olympus, Tokyo, Japan). The patterned nanostructures were imaged with a Digital Instruments NanoScope III atomic force microscope (AFM; Veeco Instruments, Inc., Woodbury, NY) in tapping mode in air at ambient temperature. The scan rate was 0.5 Hz, and 512 lines were scanned per sample. Tapping-mode cantilevers (NCH-10 V) with a 38 N m-1 spring constant and a radius of curvature of 5-10 nm were obtained from Veeco. Data were processed using SPIP V3.3.7.0 software (Image Metrology, Lyngby, Denmark). The surface morphology of the hybrid mold was observed after the imprinting process with a field emission scanning electron microscope (FESEM, S-4300 type; Hitachi Co., Tokyo, Japan). To prevent charging, the samples were coated with a 3-nm platinum layer prior to analysis using a Quick Coater SC-701HMC (Sanyu Electron Co. Ltd., Tokyo, Japan). Langmuir 2009, 25(19), 11768–11776
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Table 1. Swelling Ratio of the Cured Hybrid Resin in Various Solvents solvent
swelling ratio [wt %]
methanol ethanol dehydrated toluene tetrahydrofuran
