pubs.acs.org/Langmuir © 2009 American Chemical Society
Photoreactive Chemisorbed Monolayer Suppressing Polymer Dewetting in Thermal Nanoimprint Lithography Hirokazu Oda,†,‡ Tomoyuki Ohtake,§ Toshiaki Takaoka,§ and Masaru Nakagawa*,† †
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan, ‡Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan, and §Tsukuba Corporate Research Lab., NOF Corporation, 5-10 Toukoudai, Tsukuba, Ibaraki 300-2635, Japan Received March 14, 2009. Revised Manuscript Received May 11, 2009
We describe reactive-monolayer-assisted thermal nanoimprint lithography. The reactive monolayer inducing the graft reaction with thermoplastic poly(styrene) by ultraviolet light exposure was formed from 4-((10-mercaptodecyl)oxy)benzophenone on a gold thin film. The photochemical graft reaction suppressed the thermally induced dewetting of a poly(styrene) thin film on the modified gold surface. As a result, the poly(styrene) thin film used as a resist layer for wet etching could be patterned by thermal nanoimprint lithography, and 100-nm-scale patterns of a gold thin film could be prepared simply by wet etching.
The thermal nanoimprint lithography1-4 reported by Chou et al. in 19955 is considered to be a cost-effective, high-throughput mass production method in nanofabrication. Compared with UV-nanoimprint lithography with a liquid photopolymer cured by ultraviolet light (UV) irradiation,3,4,6-8 thermal nanoimprint lithography with a viscoelastic thermoplastic polymer has some issues that should be settled. The first is the longer process period due to a thermal cycle that requires heating and cooling and includes polymer viscoelasticity. The second is the difference in pattern size between a template mold and a transferred polymer due to thermal shrinkage on cooling arising from a hot emboss process.9 The third is the formation of defects arising from dewetting and capillary action. *Tel and Fax: +81-22-217-5668. E-mail:
[email protected]. (1) Zankovych, S.; Hoffmann, T.; Seekamp, J.; Bruch, J. U.; Torres, C. M. S. Nanotechnology 2001, 12, 91–95. (2) Hirai, Y.; Tanaka, Y. J. Photopolym. Sci. Technol. 2002, 15, 475–480. (3) Hirai, Y. J. Photopolym. Sci. Technol. 2005, 18, 551–558. (4) Reuther, F. J. Photopolym. Sci. Technol. 2005, 18, 525–530. (5) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114– 3116. (6) Fukuhara, M.; Ono, H.; Hirasawa, T.; Otaguchi, M.; Sakai, N.; Mizuno, J.; Shoji, S. J. Photopolym. Sci. Technol. 2007, 20, 549–554. (7) Goto, H.; Hagiwara, A.; Ishibashi, K.; Kokubo, M.; Okuyama, H.; Fukuyama, S. J. Photopolym. Sci. Technol. 2007, 20, 559–562. (8) Kurihara, M. J. Photopolym. Sci. Technol. 2007, 20, 563–567. (9) Heyderman, L. J.; Schift, H.; David, C.; Gobrecht, J.; Schweizer, T. Microelectron. Eng. 2000, 54, 229–245. (10) Reiter, G. Langmuir 1993, 9, 1344–1351. (11) Reiter, G. Phys. Rev. Lett. 1992, 68, 75–78. (12) Stange, T. G.; Evans, D. F.; Hendrickson, W. A. Langmuir 1997, 13, 4459– 4465. (13) Chaix, N.; Landis, S.; Hermelin, D.; Leveder, T.; Perret, C.; Delaye, V.; Gourgon, C. J. Vac. Sci. Technol., B 2006, 24, 3011–3015. (14) Landis, S.; Chaix, N.; Hermelin, D.; Leveder, T.; Gourgon, C. Microelectron. Eng. 2007, 84, 940–944. (15) Chaix, N.; Gourgon, C.; Landis, S.; Perret, C.; Fink, M.; Reuther, F.; Mecerreyes, D. Nanotechnology 2006, 17, 4082–4087. (16) Lazzarino, F.; Gourgon, C.; Schiavone, P.; Perret, C. Mold Deformation in Nanoimprint Lithography; A V S American Institute of Physics: 2004; pp 33183322. (17) Reiter, G. Macromolecules 1994, 27, 3046–3052. (18) Reiter, G.; Auroy, P.; Auvray, L. Macromolecules 1996, 29, 2150–2157. (19) Cole, D. H.; Shull, K. R.; Baldo, P.; Rehn, L. Macromolecules 1999, 32, 771–779. (20) Karapanagiotis, I.; Evans, D. F.; Gerberich, W. W. Colloids Surf., A 2002, 207, 59–67.
6604 DOI: 10.1021/la900902f
Many researchers have studied the formation of defects in thermal nanoimprint lithography.10-23 The dewetting of thermoplastic polymers is often induced on a substrate surface above the glass-transition temperature because the thin film prepared by spin-coating is metastable.10 The dewetting starts from nucleation. Holes grow from film rupture. The film is eventually transformed into droplets.10,12 Among thermoplastic polymers, poly(methyl methacrylate) (PMMA) and polycarbonate (PC) rather than poly(styrene) (PS) are preferably used in thermal nanoimprint lithography because polymer polarity induces stronger physisorption to a substrate surface. Although the choice of a high-molecular-weight thermoplastic polymer is suitable for dewetting suppression, a longer period is required for film patterning. The capillary action of fluid polymers between the surfaces of a mold and a substrate also causes the formation of defects, which are named capillary bridges.13-16 The partial filling of the polymer occurs in the region of crenellated patterns, in particular, when the film is thinner than the depth of a template mold. The capillary action is easily generated for polar polymers showing stronger physisorption to mold and substrate surfaces. One of the authors has studied photoresponsive monolayers, which can guide the movement of liquid droplets24 and control the alignment direction of liquid-crystalline molecules.25 Dewetting of the liquid droplets arises from an imbalance of surface tensions among the substrate, liquid, and air. We hit upon the idea that if a fluid polymer was anchored to a substrate surface by covalent bonds, the surface tension of the substrate would be almost identical to that of the fluid polymer. We noticed two articles. One is the photochemical attachment of polymer films to solid surfaces via monolayers of benzophenone derivatives reported by Ru¨he et al.26 Another is the fabrication of 3D nanostructures (21) Xu, L.; Shi, T. F.; An, L. J. J. Chem. Phys. 2008, 129, 044904.1–044904.7. (22) Al Akhrass, S.; Ostaci, R.-V.; Grohens, Y.; Drockenmuller, E.; Reiter, G. Langmuir 2008, 24, 1884–1890. (23) Feng, Y.; Karim, A.; Weiss, R. A.; Douglas, J. F.; Han, C. C. Macromolecules 1998, 31, 484–493. (24) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624–1626. (25) Oh, S. K.; Nakagawa, M.; Ichimura, K. J. Mater. Chem. 2002, 12, 2262– 2269. (26) Prucker, O.; Naumann, C. A.; Ruehe, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc. 1999, 121, 8766–8770.
Published on Web 05/21/2009
Langmuir 2009, 25(12), 6604–6606
Letter
Figure 1. Schematic illustration showing the preparation of a PS film on (a) bare, (b) unexposed monolayer-modified, and (c) UV-exposed monolayer-modified Au substrates.
using reactive polymer nanosheets.27 In both articles, the benzophenone derivatives bearing a photoreactive benzophenone moiety and a trialkoxysilyl group are used, which anchor hydrocarbon polymers to silica surfaces by the photoinduced graft reaction of the benzophenone moiety. Here, we investigated thermally induced dewetting of a poly(styrene) thin film on UV-exposed and unexposed gold (Au) surfaces modified with a monolayer of a thiol-containing benzophenone derivative. We demonstrated the effect of the photoinduced graft reaction, as illustrated in Figure 1, on polymer dewetting. Polymer dewetting could be suppressed on the UVexposed monolayer-modified Au surface. As a result, 100-nmscale patterns of the polymer thin film could be obtained by thermal nanoimprint lithography, leading to the fabrication of fine patterns of Au thin films by simple cost-effective wet etching. 4-((10-Mercaptodecyl)oxy)benzophenone used as an adsorbate was synthesized by the reaction of 4-hydroxybenzophene with 1,10-dibromodecane, followed by the reaction of 4-((10bromodecyl)oxy)benzophenone with thiourea and hydrolysis with sodium hydroxide. A cleaned Au substrate (100 nm Au/ 10 nm Cr on a silicon wafer with a native oxide layer; 1.5 1.5 cm2) was immersed in a 0.1 mmol dm-3 ethanol solution containing the adsorbate for 24 h and rinsed with ethanol. The thus-obtained Au substrate exhibiting a sessile contact angle of 81.9 ( 0.2° for water was used as a monolayer-modified Au substrate. A poly(styrene) (PS) thin film of 0.1 μm thickness was spin coated onto the modified Au substrate with a toluene solution of PS (weight-average molecular weight (Mw) 45 000 g/mol). UV-light irradiation with an intensity of 120 mW cm-2 monitored at 365 nm was carried out for 28 min to the side of the PS thin film. Figure 2 shows the optical microscope images of PS thin films on Au substrates after annealing at 200 °C for 10 min, with results for bare, unexposed monolayer-modified, and UV-exposed monolayer-modified Au substrates indicated in Figure 2a-c, respectively. Hole defects of a PS film induced by dewetting were observed on the bare and unexposed modified Au substrates, whereas such hole defects were not observed at all on the UVexposed modified Au substrate. Holes started to be generated at 120 °C on the bare Au substrate and at 140 °C on the unexposed substrate above the glass-transition temperature of 92 °C. According to the result reported by Ru¨he et al.,26 a PS layer of approximately 5 nm thickness was considered to be anchored to (27) Kado, Y.; Mitsuishi, M.; Miyashita, T. Adv. Mater. 2005, 17, 1857–1861.
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the monolayer by the graft reaction of the benzophenone moiety with PS by UV exposure. As a result of the graft reaction, the surface tension of the monolayer surface will be changed to that of a PS surface, which is almost identical to that of fluid PS. Because the PS thin film exhibited absorption bands at wavelengths smaller than approximately 320 nm, PS was not directly related to the absorption of UVlight at 365 nm that was used for UV exposure. Actually, the weight-average molecular weight and polydispersity of PS analyzed by gel permeation chromatography were not changed after UV exposure. The suppression of thermally induced dewetting was attributable to the photoinduced graft reaction of the benzophenone moiety with PS. Thermal nanoimprint lithography (TNI) was conducted at 120 °C under the applied pressure of 5 MPa. A silicon mold treated with an antisticking layer formed from Daikin OptoolDSX was used. The thermal cycle composed heating to 120 °C for 60 s, steadily increasing applied pressure to 5 MPa for 60 s, maintaining the temperature and pressure for 300 s, cooling to the initial temperature of 30 °C for 60 s, and demolding for 60 s. Figure 3a-c show the scanning electron microscope (SEM) images of the PS thin films on three kinds of Au substrates after thermal nanoimprint lithography by the mold with 500 nm line-and-space patterns. The results on the bare, unexposed modified, and UV-exposed modified Au substrates are shown in Figure 3a-c, respectively. The dark regions correspond to convex thick PS patterns, and the bright regions correspond to concave thin PS patterns owing to secondary electron generating from Au. As shown in Figure 3a, many defects of partial filling in the crenellated pattern region and capillary bridge in the plain region were generated in the PS thin film owing to the dewetting and capillary action of PS. Defect generation was minimal on the unexposed monolayer-modified Au substrate as shown in Figure 3b. This is probably because the surface tension of the monolayer-modified surface approached that of fluid PS. Interestingly, defects of the PS thin film were hardly induced on the UV-exposed monolayer-modified Au substrate after thermal nanoimprint lithography. It was confirmed that the photoinduced graft reaction resulted in the suppression of defect generation. Because the graft reaction occurs near the adsorbate monolayer, the bulk of the PS thin film maintains the physical properties and is readily transformed to fine crenellated patterns by thermal nanoimprint lithography. After the transformation of the PS thin film, oxygen dry etching was carried out with a UV/O3 cleaner to remove a PS residual layer in concave regions of the PS patterns. The patterned PS was used as a resist material for subsequent wet etching of a 100-nm-thick Au film with a 10-nm-thick Cr adhesion layer. Wet etching was conducted at 20 °C for 30 s with the aqueous etchant containing 16 mmol dm-3 potassium iodide (KI) and 5.2 mmol dm-3 iodine (I2) in deionized water.28 Figure 3d-f shows the SEM images of Au thin film patterns after wet etching, followed by the removal of PS by rinses with acetone and the removal of the adsorbate monolayer with a UV/O3 cleaner, with the results for the bare, unexposed modified, and UV-exposed modified Au substrates shown in Figure 3d-f, respectively. The dark region and bright region correspond to the Au surface and the Cr surface, respectively. Defects of the Au film patterns were almost entirely derived from those of the resist PS patterns. It is worth noting here that unexpected Au etching occurred under the PS line patterns. Because the PS resist layer is not anchored by covalent bonds in the case of the unexposed monolayer-modified Au substrate, the etchant solution will penetrate readily between (28) Eidelloth, W.; Sandstrom, R. L. Appl. Phys. Lett. 1991, 59, 1632–1634.
DOI: 10.1021/la900902f
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Figure 2. Optical microscope images of PS thin films on (a) bare, (b) unexposed monolayer-modified, and (c) UV-exposed monolayermodified Au substrates after annealing at 200 °C for 10 min.
Figure 4. SEM images of (a) a patterned PS thin film and (b) an Au thin film after Au and Cr wet etching. The patterning was conducted with the mold having 200 nm convex lines, spatially separated by 100, 200, and 500 nm concave spaces. Figure 3. SEM images of (a-c) PS thin films after thermal nanoimprint lithography using a 500 nm line-and-space mold and (d-f ) Au thin films after wet etching using the PS pattern as a resist and the removal of PS and adsorbate monolayer. Three kinds of Au substrates (100 nm Au/10 nm Cr/Si) were used: (a, d) a bare Au substrate, (b, e) an unexposed monolayer-modified Au substrate, and (c, f ) a UV-exposed monolayer-modified Au substrate. The UV exposure was carried out at an exposure dose of 200 J cm-2 monitored at 365 nm.
the Au and PS layers. However, fine Au etching was induced in the case of the UV-exposed monolayer-modified Au substrate. It was confirmed that the UV exposure causing the graft reaction with PS was important for the thermal nanoimprint lithography to obtain fine patterns of the PS and Au thin films. Figure 4a,b shows the SEM images of a nanoimprinted PS thin film on a UV-exposed monolayer-modified Au substrate (25 nm Au/ 20 nm Cr/ Si) and the Au thin film after dry etching, Au and Cr wet etching, and the removal of the PS resist and the adsorbate monolayer, respectively. A mold having 200 nm convex line patterns with concave intervals of 100, 200, and 500 nm in sequence was used. An Au thin film patterned with line widths of approximately 100, 200, 500 nm could be prepared by additional Cr wet etching with an aqueous etchant containing nitric acid. Thermal nanoimprint lithography with the photoreactive monolayer was suitable for the nanofabrication of thermoplastic PS resists and metal Au thin films on the 100 nm scale.
6606 DOI: 10.1021/la900902f
In summary, we reported that the adsorbate monolayer of 4-((10-mercaptodecyl)oxy)benzophenone formed on a Au substrate showed the dewetting suppression of PS thin films by UV exposure. The photoinduced graft reaction enabled us to fabricate patterned thin films of PS and Au by thermal nanoimprint lithography, followed by simple wet etching. We would like to name the nanofabrication technique reactive-monolayerassisted thermal nanoimprint lithography. In industry, the microfabrication of Cu and Au film patterns has been currently achieved by photolithography and wet etching from the standpoint of high-throughput and cost performance. Reactive-monolayer-assisted thermal nanoimprint lithography will open opportunities for researchers to access the nanofabrication of metallic materials. Application to other metal thin films is under investigation. Acknowledgment. We gratefully acknowledge Professor Shinji Matsui and Dr. Ken-ichiro Nakamatsu for assistance with the fabrication of the silicon molds in the Matsui Laboratory of University of Hyogo. Supporting Information Available: Full experimental details for the adsorbate synthesis and apparatus, and results for the optical microscope images of PS thin films after annealing at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2009, 25(12), 6604–6606
