NANO LETTERS
Low-Pressure Nanoimprint Lithography Dahl-Young Khang, Hyewon Kang, Tae-Il Kim, and Hong H. Lee*
2004 Vol. 4, No. 4 633-637
School of Chemical Engineering, Seoul National UniVersity, Seoul, 151-742, Korea Received January 19, 2004; Revised Manuscript Received February 24, 2004
ABSTRACT A low pressure (2∼3 bar) nanoimprint lithography technique is developed that utilizes a thin fluoropolymer film (∼100 µm) mold. The flexible film mold allows imprinting of submicron pattern features at such a low pressure primarily due to “sequential” imprinting made possible by the mold flexibility and the conformal contact made between the film mold and the substrate. The surface energy of the fluoropolymer mold material is low enough that no mold surface treatment is needed for clean demolding. Easy replication of the film mold by a simple solvent casting is another advantage of the proposed method. Also the nanoscale (100 bar). After the compression and demolding (detachment of mold), there remains the “negative” of the mold pattern imprinted into the polymer layer. The polymer layer with this thickness contrast can be further processed by reactive ion etching, for instance, to finally realize the desired polymer pattern formation on the substrate. The technique has been shown to have a resolution2 down to ∼7 nm and large area pattern fidelity.3 The method has been applied to fabrication of various devices, including quantized magnetic disk,4 microoptics,5 compact disk,6 photodetector,7 microfluidics and microelectromechanical system (MEMS) devices,8 field effect transistors,9 etc. Imprinting with a hard, stiff mold (typically SiO2/Si) has the key advantage of high resolution. However, there are some disadvantages as well. First, the high temperature and pressure required may lead to thermal cycling and also to fracture of mold and/or substrate. Second, mold surface treatment is necessary for clean release of the mold from the patterned polymer surface after the imprinting, typically with a self-assembled monolayer (SAM)10 or fluoropolymer deposition.11 Third, there is a mass transport problem for large pattern/spacing, which requires optimization of process variables such as polymer thickness, temperature, pressure, and time for the given mold pattern.12 This mass transport limitation is especially problematic for patterns having a variable fill factor (fractional area occupied by printed * Corresponding author. E-mail:
[email protected]. Tel: 82-2-8807403. Fax: 82-2-878-5043. 10.1021/nl049887d CCC: $27.50 Published on Web 03/16/2004
© 2004 American Chemical Society
pattern). To overcome or relieve these problems, various modifications of the imprinting method have been proposed, such as room-temperature imprinting,13 step-and-flash imprint lithography (SFIL)14 with photocuring of pre-polymer through a transparent quartz mold, and reversal imprinting15 of casting polymer on mold surface and then transfer-bonding the molded polymer onto the polymer layer that is prepared on a substrate surface. In this letter, we demonstrate a low pressure (2∼3 bar) nanoimprint lithography (LP-NIL) using a flexible film (∼100 µm>) mold made of fluoropolymer material. This low-pressure process eliminates the problem of substrate fracture, and the low surface energy (15.6 dyn/cm)16 of the fluoropolymer mold material removes the mold surface treatment step that is required in the conventional imprinting process for easy demolding. In addition, the flexible nature of the film mold makes it much easier to demold from the polymer surface than the hard, stiff mold. In the case of the stiff mold, the whole of the mold is removed from the polymer surface, which involves separation from a large contact area. On the other hand, the peeling-off of the flexible mold allows separation from a small contact area, since only the contact area being peeled off is involved in the removal process. A distinct advantage with the film mold is that it can be replicated easily and rapidly by a solvent-casting method and that any material can be used for the master mold, including inorganics and organics with the exception of fluorinated organics. The fluoropolymer material chosen for the mold is amorphous Teflon, Dupont Teflon AF 2400, that is a copolymer of 2,2-bistrifluoromethyl-4,5-difluoro-1,3,-dioxole (PDD) and tetrafluoroethylene (TFE).16 The AF 2400 fluoropolymer powder was dissolved in perfluorinated solvent (FC-77, 3M) at room temperature. The solubility of the fluoropolymer is about 2 wt % maximum in the solvent. The
fluoropolymer solution was poured into a Petri dish that contains the master mold for the replication. The solvent does not dissolve other materials (inorganics, metals, and even organics, except for fluorinated materials), such that it can be applied to almost any material of choice for the master mold. In fact, we could replicate the fluoropolymer film mold from a master mold made of inorganics such as SiO2/Si, of photoresist patterned on Si, even of organic polymers (polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(dimethylsiloxane) (PDMS)) patterned by other methods, including imprint lithography and soft lithography. After the solvent evaporates completely (cf. the boiling point of FC77 is 97 °C), the dried film was peeled off from the master mold. No damage on the master mold was observed and the master was repeatedly used for mold replication. When the film is too thin, say less than ∼50 µm thick, however, the film was prone to breakage during the peeling-off. Typical size of the film mold used in this work was ∼1 cm × 1 cm, and the thickness was about 100 µm. The film mold can easily be manipulated by mechanical tweezers. Prior to spin-coating a silicon substrate with a polymer solution, the substrate was cleaned by nitrogen blowing, ulrtasonification for 5 min each in trichloroethylene and acetone and then in isopropyl alcohol, rinsing by deionized water and blow-drying by nitrogen. The polymer used was polystyrene (Mn ) 100 000 g/mol, polydispersity ) 2, LG Chemicals, Korea), and the concentration of polymer in toluene was typically in the range between 2 wt % and 10 wt %. After spin-coating at 3000 rpm for 20 s, the substrate with the polymer film was baked in a vacuum oven at a temperature between 160 and 200 °C for 60 min to fully remove any residual solvent in the film. Figure 1 shows the schematic experimental procedure for LP-NIL process. The PS/Si substrate with a film mold was placed on a hot stage. After the stage temperature reached the set point, a rounded-bottom roller that was made of PDMS (Sylgard 184, Dow Corning) was used to uniformly contact the mold with the polymer. This step was found to be critical to expel entrapped air and thus achieve conformal contact between the film mold and the substrate. When the imprinting was carried out without this rolling contact, there remained unpatterned regions on the polymer surface due to the entrapped air bubbles. The rolling contact should be initiated at the center region and then rolled outward for efficient removal of the entrapped air. Then we simply put a stainless steel block weighing 2∼3 kg on top of a flat PDMS block, which was inserted between the weighing block and the mold. After maintaining the high temperature (above Tg of PS (100 °C)), typically 160 °C, for 3∼10 min, the stage temperature was allowed to cool to ∼30-40 °C. The film mold was then peeled off the polymer surface. No adhered polymer was found on the mold surface. This film mold was reused many times, only needing nitrogen blowing each time it is reused to remove particles attached from the laboratory environment. As mentioned above, inertness of the solvent permits easy film mold replication by the solvent casting with various types of master mold materials. Shown in Figure 2 are the 634
Figure 1. Schematic illustration of the low-pressure nanoimprint lithography (LP-NIL).
scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of polymer surfaces imprinted with the film mold prepared. Figure 2a shows the SEM image of a ∼150 nm line-and-space pattern imprinted with the film mold replicated from a photoresist patterned on an Si substrate. Shown in Figure 2b are ∼1 µm wide line features patterned using the film mold replicated from a patterned PDMS surface, which had been prepared from a patterned SiO2/Si substrate. Note that a protruding (recessed) pattern feature on the master mold surface is replicated on the film mold as a recessed (protruding) pattern by direct casting. If the original pattern is to be replicated, the replica-molding method17can be used with a PDMS replica of the master mold. Shown in Figure 2c are the perspective and sectionalprofile AFM images of a sample imprinted with the film mold replicated from a typical SiO2/Si mold. To check the uniformity of the pattern imprinted by the LP-NIL process, we measured pattern depth uniformity over a sample surface. Figure 3a shows the schematic layout of measured points (dark rectangles) on the sample surface. It is noted that the whole sample surface was full of test dot patterns, although measurements were made only at these four positions. The measurements at these four positions yielded uniform depth, hardly distinguishable from one another. Thus, only representative 2-dimensional (Figure 3b), 3-dimensional (Figure 3c), and sectional profile (Figure 3d) images are shown. The pattern depth uniformity was very Nano Lett., Vol. 4, No. 4, 2004
Figure 2. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of polymer surface patterned by LP-NIL process, using film molds prepared by solvent casting from the master molds made of (a) photoresist patterned on Si (∼150 nm line and space pattern), (b) patterned poly(dimethylsiloxane) (PDMS) (∼1 µm wide lines), and (c) patterned SiO2/Si substrate (∼5 µm line and space), respectively. The scale bars in (a) and (b) are, 2 µm and 20 µm, respectively.
Figure 3. Uniformity of LP-NIL process: (a) schematic layout of measured points on the PS/Si sample surface (note that the whole area was full of dot patterns), and representative AFM images of (b) 2-dimensional, (c) 3-dimensional, and (d) sectional profile views, respectively. The pattern depth variation at the four measured points is within 147 nm ( 3 nm.
good, i.e., 147 nm ( 3 nm, which was obtained from the AFM sectional images at the four positions. Some comments are in order here as to why high pressure (>100 bar) is necessary in the conventional nanoimprint Nano Lett., Vol. 4, No. 4, 2004
lithography with a stiff mold. In this case, it is not easy to achieve the conformal contact between the mold and the substrate at low pressure or without pressurization due to small surface roughness, conformal in the sense that all the 635
protruding parts of the mold are in contact with the underlying surface.18 Further, even if the conformal contact could be achieved by some means, all the protruding pattern features on the mold surface should penetrate into the polymer melt simultaneously, due to the stiff nature of the mold material itself. This requirement results in a high resistance to imprinting because of a large amount of polymer mass that needs to be displaced all over the sample surface, which can only be overcome with a high pressure or temperature. Therefore, a low-pressure imprinting would be possible if a conformal contact can be made between the mold and the substrate and if a sequential, not parallel, imprinting of pattern features on the mold surface is assured by some means. One way to achieve the conformal contact and the sequential imprinting is to use a flexible film mold, as demonstrated in this work. The conformal contact and the sequential imprinting made possible by the use of a flexible film mold are the key to this low-pressure imprinting method. The mass transport problem, however, cannot be completely solved by the flexible film mold, especially for large pattern or spacing (> 200 µm). Nevertheless, the mass transport problem can significantly be reduced with the film mold. When the film mold is pressed, the contact between the mold and the substrate occurs not only at the protruding surface of the patterns on the mold but also at the midpoint between two protruding surfaces due to the flexural deformation of the film mold.19 This additional midpoint contact enhances the mass transport of the polymer melt by the corresponding decrease in the distance the underlying polymer mass has to cover. The midpoint acts as a source for generating a squeezing flow of the melt by the applied load. As the polymer mass fills the void space in the mold, the deformed film mold regains its original flat shape, if the distribution of the applied load is uniform. Another feature of the LP-NIL process is that the sample is heated only from below and not between heated press platens as in an ordinary imprinting process. Shown in Figure 4 are the AFM images of a master mold (Figure 4a) and the corresponding pattern imprinted on polymer surface (Figure 4b), respectively. The master mold was prepared by depositing ZnO nanoparticles on Si substrate.20 Due to the particle nature of the pattern, it was not possible to compare the feature dimension directly in both images. Rather, we used average grain size and surface roughness analysis supported by AFM software. The average grain size and RMS (root-mean-square) surface roughness were 5.85 nm and 1.85 nm, respectively, for the master mold, while those values were 6.59 nm and 1.57 nm, respectively, for the imprinted pattern. On the other hand, these nanoscale features could not be replicated onto PDMS (Sylgard 184, Dow Corning) at all. This preliminary result is an indication that nanoscale features could be replicated and patterned with a solvent-cast thin film fluoropolymer mold by this LPNIL process.21 In summary, we have demonstrated nanoimprint lithography at low pressure (2∼3 bar) using a fluoropolymer film mold. The flexible film mold contacts the polymer surface 636
Figure 4. Nanoscale pattering capability of LP-NIL process with a solvent-cast thin fluoropolymer film mold: (a) 3D AFM image of master mold prepared by ZnO nanoparticle deposition on Si substrate, and (b) 3D AFM image of imprinted polymer surface. (The z-scale in both cases is 30 nm.)
conformally and permits sequential imprinting, thus enabling such a low-pressure process. The low surface energy and flexible nature of the fluoropolymer film mold material is advantageous for cleanly removing the mold after the imprinting. Easy replication of the working mold from a master mold, by simple solvent casting, is another benefit of the method. Acknowledgment. The authors are grateful to Mr. JungHwan Kang and Shin Jeong FC (authorized distributor of 3M Korea Ltd., www.fluorochemical.com) for their kind supply of FC-77 solvent. This work was supported in part by Brain Korea 21 Project. References (1) (a) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1996, 67, 3114. (b) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (c) Sotomayor Torres C. M.; Zankovych S.; Seekamp J.; Kam, A. P.; Clavijo Cedeno, C.; Hoffmann, J.; Ahopelto, J.; Pfeiffer, K.; Bleidiessel, G.; Gruetzner, G.; Maximov, M. V.; Heidari, B. Mater. Sci. Eng. C 2003, 23, 23. (2) Chou, S. Y.; Krauss, P. R.; Zhang W.; Guo, L.; Zhang, L. J. Vac. Sci. Technol. B 1998, 16, 2897. Nano Lett., Vol. 4, No. 4, 2004
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(18) (a) Ferguson, G. S.; Chaudhury, M. K.; Sigal, G. B.; Whitesides, G. M. Science 1991, 253, 776. (b) Kim, C.; Burrows, P. E.; Forrest, S. R. Science 2000, 288, 831. (c) Loo, Y.-L.; Willet, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654. (d) Kim, C.; Shtein, M.; Forrest, S. R. Appl. Phys. Lett. 2002, 80, 4051. (e) Loo, Y.-L.; Willet, R. L.; Baldwin, K. W.; Rogers, J. A. Appl. Phys. Lett. 2002, 81, 562. (f) Matsui, S.; Igaku, Y.; Ishigaki, H.; Fujita, J.; Ishida, M.; Ochiai, Y.; Namatsu, N.; Komuro, M. J Vac. Sci. Technol. B 2003, 21, 688. (g) Loo, Y.-L.; Lang, D. V.; Rogers, J. A.; Hsu, J. W. P. Nano Lett. 2003, 3, 913. (h) Zaumseil, J.; Meitle, M. A.; Hsu, J. W. P.; Acharya, B. R.; Baldwin, K. W.; Loo, Y.-L.; Rogers, J. A. Nano Lett. 2003, 3, 1223. (19) The flexural rigidity (D) is defined as D ) Et3/[12(1 - V2)], where E is Young’s modulus [E(Si) ∼ 60 GPa, E(film) ∼ 1 GPa], t is the thickness [t(Si) ∼ 0.7 mm, t(film) ∼ 0.1 mm], and V is the Poisson ration [V(Si) ∼ 0.23, V(film) ∼ 0.4], respectively. These values yield, D(Si)/D(film) ∼ 10-4, showing that the film is more flexible than the typical hard mold material of silicon wafer by 4 orders of magnitude. (20) The ZnO template was kindly provided by professor B. H. Sohn at Pohang University of Science and Technology, Korea. (21) This preliminary result was obtained without any optimization of process conditions. For Figure 4b, the imprint was carried out at 170 °C, 2-3 bar for 60 min, where the rather long imprint time was deliberately chosen to rule out the effect of unoptimized process conditions on the nanoscale patterning capability of the thin film Teflon mold.
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