Langmuir 2008, 24, 5459-5463
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On the Mechanism of Low-Pressure Imprint Lithography: Capillarity vs Viscous Flow Dahl-Young Khang and Hong H. Lee* School of Chemical Engineering, Seoul National UniVersity, Seoul, 151–742, Korea ReceiVed October 5, 2007. ReVised Manuscript ReceiVed February 26, 2008 Dominant mechanisms in low-pressure imprint lithography processes have been identified for the regimes that are definable in terms of applied pressure, temperature, and mold material characteristics. Capillarity is found to be the dominant mechanism at high temperature and low pressure when stiff, hard molds are used. In the case of flexible thin-film (∼20 µm) molds, both the capillarity and the viscous flow are involved. Both mechanisms are operative in the initial stage of the imprinting, but the capillarity takes over as time progresses.
Introduction Nanoimprint lithography or hot-embossing1 is a simple, lowcost, and high-resolution patterning method. Typically, a hard mold with a protruding/recessed pattern on its surface is pressed against a polymer layer that is coated onto a substrate. For the hot molding, the imprinting process is usually carried out at high temperature (above the glass transition temperature of polymer, 100-250 °C) and high pressure (40-100 bar). The technique has been shown to have a resolution down to ∼7nm2 and large area pattern fidelity.3,4 The method has been applied to the fabrication of various devices, including quantized magnetic disks,5 micro-optics,6,7 compact disks,8 photodetectors,9 microfluidics and MEMS devices,10 field effect transistor (FET),11 etc. There are, however, very few studies on the detailed mechanism of the imprint process, and even then only on that of the hotembossing,12,13 where a squeezed viscous flow of polymer is the obvious dominant mechanism. In this work, an elaborate study is carried out to find the mechanisms of a novel imprint process, imprinting at a low (2-3 bar) pressure and/or with a flexible thin-film mold. As the imprinting pressure is lowered, the dominant mechanism has been found to change from the squeezed viscous flow induced by the high pressure to a capillarity phenomenon (capillary rise or filling). In what follows, the boundary between high and low pressure will be taken as ∼10 bar. The temperature range is divided into a high-temperature region, where the temperature is ∼50 °C above the glass transition temperature (Tg) of the polymer being patterned, and a low* To whom correspondence should be addressed. E-mail: honghlee@ snu.ac.kr. Tel:+82 2 880 7403. Fax: +82 2 878 5043. (1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114. (b) Science, 1996, 272, 85. (2) Chou, S. Y.; Krauss, P. R.; Zhang, W.; Guo, L.; Zhuang, L. J. Vac. Sci. Technol. 1997, B15, 2897. (3) Khang, D.-Y.; Lee, H. H. Appl. Phys. Lett. 1999, 75, 2599. (4) Li, M.; Chen, L.; Zhang, W.; Chou, S. Y. Nanotechnology 2003, 14, 33. (5) Chou, S. Y. Proc. IEEE 1997, 85, 652. (6) Seekamp, J.; Zankovych, S.; Helfer, A. H.; Maury, P.; Sotomayor Torres, C. M.; Bottger, G.; Liguda, C.; Eich, M.; Heidar, B.; Montelius, L.; Ahopelto, J. Nanotechnology 2002, 13, 581. (7) Moon, S. D.; Lee, N. S.; Kang, S. I. J. Micromech. Microeng. 2003, 13, 98. (8) Krauss, P. R.; Chou, S. Y. Appl. Phys. Lett. 1997, 71, 3174. (9) Yu, Z.; Schablitsky, S. J.; Chou, S. Y. Appl. Phys. Lett. 1999, 74, 2381. (10) Becker, H.; Gartner, C. ReV. Mol. Biotech. 2001, 82, 89. (11) Guo, L.; Krauss, P. R.; Chou, S. Y. Appl. Phys. Lett. 1997, 71, 1881. (12) Hydermann, L. J.; Schift, H.; David, C.; Grobrecht, J.; Schweizer, T. Microelect. Eng. 2000, 54, 229. (13) Scheer, H.-C.; Schultz, H. Microelect. Eng. 2001, 56, 311.
Figure 1. Low-pressure imprinting schemes for hard or film molds. “P” means pressurization with a stainless steel weighting block. The roller contact was done with a cylindrical PDMS roller for ∼10 s.
temperature region, where the temperature is well below Tg or around room temperature. Imprinting at the high temperature and pressure has the advantages of high resolution and exact replication of mold pattern features (high pattern fidelity).1–4 In this case, the dominant mechanism is obviously the viscous flow of polymer melt into the surrounding mold cavity squeezed by the protruding parts of the mold surface.12,13 On the other hand, the patterning mechanism is different for the room-temperature imprinting in which the pressure is high but the temperature is low.14 In this process, the dried, glassy polymer layer that is prepared onto a substrate is directly pressed with a hard mold such as a SiO2/Si wafer at room temperature without any heating. The advantages of the method lie in largearea patterning capability (unlimited area in principle) with stepand-repeat scheme and generation of more complex pattern by multiply imprinting on the same substrate surface. The mechanism involved in this method is the compaction of the free volume (14) Khang, D.-Y.; Yoon, H. S.; Lee, H. H. AdV. Mater. 2001, 13, 749.
10.1021/la703084p CCC: $40.75 2008 American Chemical Society Published on Web 04/16/2008
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inherent in an amorphous solid polymer film and the plastic deformation of polymer above its yield stress.14,15 The purpose of this work is to elucidate the dominant mechanisms in low-pressure imprint processes. More specifically, the role or importance of capillarity in imprinting process, compared to viscous flow, is the main focus here. For that purpose, we concentrate on the imprinting process at low pressure and high temperature, with a flexible film mold or a hard one. Due to the high temperature applied, the polymer being patterned is in a melt state and it flows when a pressure is applied. If the imprint pressure is low enough, however, it is expected that the squeezed viscous flow of polymer melt would be minimized, and thus, we may easily identify the effect of capillarity on the imprinting process. Also, the effect of mold characteristics, i.e., hard or flexible, on the imprint mechanism in the low-pressure regime has been studied, and indeed, we find differences in the dominant mechanism of imprint depending on the mold characteristics.
Experimental Section Hard Mold for Low-Pressure Imprinting. The hard mold used was SiO2/Si wafer that was prepared by conventional photolithography. The thermally grown SiO2(500nm) on Si(100) was patterned by photolithography, where the pattern height was the same as the thickness of SiO2 layer. It has protruding pattern features, with various shapes (lines-and-spaces, squares, characters, etc.), sizes (from sub-micrometer to hundreds of micrometers), and pattern densities. The size of SiO2/Si hard mold was typically ∼2 × 2 cm2 and was used for imprinting without any surface treatment on it. Film Mold for Low-Pressure Imprinting. The film mold was solution cast from a master mold of SiO2/Si that had been fabricated by photolithography. The replicated film mold has features of ∼150 nm high. It has dense arrays of protruding discs, where the disk diameter and spacing were designed to be 400 nm. The surface treatment of the master mold was unnecessary because the replicated film material is Teflon that has very low surface energy. The overall size of final film mold (∼1 × 1 cm2) was slightly smaller than that of master mold (∼1.5 × 1.5 cm2) due to the edge-trimming of films after casting. The fluoropolymer powder, Teflon AF-2400, was purchased from Dupont and dissolved in FC-77 solvent (3 M). The solution was poured into Petri dish in which the master mold to be replicated is placed. After the solvent evaporates completely at roomtemperature overnight (cf. boiling point of FC-77 is ∼100 °C), there remained a transparent, continuous film that was peeled off the master substrate manually using mechanical tweezers. The thickness of the film mold was controlled by varying the solution concentration or the total quantity of solution poured onto the master mold. The thickness of cast film mold was measured using a digital caliper and was controlled in the range of 20-100 µm. It was found to be difficult to handle the film mold if it is thinner than ∼20 µm due to curling and/or electrostatic interaction of the film mold with other surfaces, as well as film rupture during peeling off. Polymer Layer for Imprinting. Polystyrene (PS, Mn ) 100 000 g/mol, polydispersity (PI) ) 2, LG Chemicals, Korea) was used as the patterning layer with toluene as the solvent. Prior to spin-coating, the Si substrate was cleaned by nitrogen blowing, ultrasonication for 5 min in trichloroethylene (TCE) and acetone and then in isopropylalcohol (IPA), rinsing with deionized (DI) water and blowdrying with nitrogen. The concentration of the polymer solution in toluene was 10% polymer by weight. After the spin-coating, the PS-coated Si substrate was annealed in vacuum at 150 °C, >1 h. The film thickness, measured by atomic force microscopy (AFM) was ∼550 nm. Low-Pressure Imprinting. For the low-pressure imprinting with hard SiO2/Si mold, the PS-coated substrate was placed on a hot plate, and the mold was placed on it with the patterned surface face down and with a slight pressing (0.5-3 kg of stainless steel block). (15) Elias, H.-G. Macromolecules; Plenum Press: New York, 1984; Vol. 1.
Khang and Lee For the uniformity of the applied pressure, we used a ∼3-mm-thick, flat polydimethylsiloxane (PDMS) block as a deformable buffer layer between the weighting block and the mold. After the sample was heated to 160 °C with a heating rate of 90 °C/min, the sample was maintained for a while (5-30 min) at the high temperature for imprinting and then was cooled down to room temperature. To identify the dominant mechanism in the low-pressure/hightemperature imprinting with a flexible film mold, three sets of experiments were carried out. First, the flexible film mold was brought into contact with the polymer layer on the substrate that is placed on a hot plate at 160 °C by rolling a PDMS rolling pin on the back of the mold film and then the mold was immediately removed, the contact time being less than 10 s (Set I). In the second set, the roller-contacted mold was left on the polymer surface for 10 or 20 min before removing the mold, which is essentially an annealing at 160 °C for the period of time. While no pressure was applied in the first two sets, a weighting metal block was placed on the rollercontacted film mold and annealed for 10 or 20 min in the third set of experiments. Figure 1 shows these four different process sequences schematically. Measurements. The patterned polymer surface was characterized by various imaging techniques: optical microscopy (Olympus BX60), AFM (Veeco, Dimension 3000) in tapping mode, and scanning electron microscopy (SEM; Phillips XL30FEG).
Results and Discussions Imprinting at high temperature and low pressure with a hard mold leads to a very different situation compared to the conventional imprinting at high temperature and pressure, as shown in Figure 2. Interestingly, the protruding pattern features did not penetrate (or imprint) into the polymer layer at all, as can be seen in Figure 2a. Instead, the polymer climbed up the walls of two protruding strips on the mold, the locations of which are the dark strips in Figure 2a and the centers of the two centerrecessed columns in the sectional AFM image in Figure 2a. Because of the depletion of the polymer that has risen along the walls, trenches form around each protruding strip, as shown in Figure 2a. The three-dimensional AFM image of Figure 2a that is shown in Figure 2b also reveals the capillary rise around the two walls of the protruding strips. Had a high pressure been used, the protruding strips on the mold would have penetrated into the polymer. Because of the low pressure applied, however, the polymer simply climbed up the walls by capillarity to minimize the surface energy.16,17 The same phenomenon was previously observed with negative (recessed) molds.18 The simplified schematic drawing of this capillary rise phenomenon is shown in Figure 2c for a mold having protruding line features. The tilted SEM image in Figure 2d shows the pattern resulting when a mold with a protruding rectangular stripe and two alphabet letters was used. The recessed rectangular stripe between the inner and outer rectangular fences is where the rectangular stripe on the mold was in contact with the polymer surface. Similar SEM images of the patterning results with a line-shaped mold are shown in Figure 2e and f; when the spacing between protruding lines is far enough, the gaps between the lines are not filled completely, as shown in Figure 2e. When the spacing is rather small as in Figure 2f, the gap are filled completely with polymers. The holelike features on filled polymer lines in Figure 2f, which is ∼200 nm deep, are believed due to the dewetting of polymer in confined geometry.19 Much more is involved when a flexible thin film is used as a mold in this low-pressure/high-temperature imprinting (16) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley-Interscience: New York, 1997. (17) Quere, D.; Di Meglio, J.-M. AdV. Colloid Interface Sci. 1994, 48, 141. (18) Khang, D.-Y.; Lee, H. H. AdV. Mater. 2004, 16, 176. (19) Suh, K. Y.; Lee, H. H. J. Chem. Phys. 2001, 115, 8204.
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Figure 2. Capillary rise of polymer around the protruding pattern features on mold surface. The SiO2/Si hard mold was used for the imprinting at low pressure (2-3 bar) and high temperature (160 °C). (a) AFM top-view image of two parallel strips (∼4 µm wide, 50 µm long, and 500 nm high, 100 × 100 µm2 scan) with a sectional profile in the red line, and (b) three-dimensional perspective view of (a). In (c), a simplified drawing of capillary rise along a line-shaped mold feature is given (not to scale). (d) Tilted SEM image of the patterned surface using a mold with a rectangular stripe and two alphabet letters (scale bar ) 10 µm), and (inset) top view image of the same pattern by optical microscopy (OM) (scale bar ) 10 µm). (e) SEM image similar to (d), using a SiO2/Si mold having five protruding line features (scale bar ) 5 µm) and its magnified view in the inset (scale bar ) 1 µm). If these line features are close enough, the gaps between lines are filled completely, as shown in (f) (scale bar ) 2 µm).
process. A flexible-film mold allows for intimate or conformal contact with the polymer on a substrate that is difficult to achieve with a stiff, hard mold. The flexible nature also enables a sequential, not parallel, imprinting of the pattern features on the mold surface because the surface is imprinted in sequence, as opposed to the whole surface imprinting with a hard mold.20 The flexibility of the film mold can be ascertained quantitatively with the flexural rigidity,21 D, defined by D ≡ Et3/[12(1 - ν2)] where E is Young’s modulus (E(Si) ≈ 130 GPa, E(film) ≈ 1 GPa), t is the thickness (t(Si) ≈ 0.7 mm, t(film) ≈ 0.1 mm), and ν is the Poisson ratio (ν(Si) ≈ 0.23, ν (film) ≈ 0.4), respectively. These values yield, D(Si)/D(film) ≈ 104, showing that the film is more flexible than the typical hard-mold material of silicon wafer by 4 orders of magnitude. The results of the first set of experiments are shown in Figure 3. The mold pattern is arrays of protruding circular pillars, except for the places where the criss-crossing lines are present, which correspond to the light whitish stripes (lines) in Figure 3a. A number of patterns emerged. In the area designated as S in Figure 3a, a negative replica of the mold pattern resulted and is magnified in Figure 3b. In the area designated as L, the patterned surface shows a large-scale, vertical, and horizontal undulation upon which are dubbed with imprinted mold features; its magnified AFM micrograph is given in Figure 3c. Also, a region designated as F reveals fingering that has taken place, the magnification of which is shown in Figure 3d. The unpatterned oval shape in Figure 3a is the area where the entrapped air was present. The fingering and large-scale undulation are due to the compressed (20) Khang, D.-Y.; Kang, H.; Kim, T.-I.; Lee, H. H. Nano Lett. 2004, 4, 633. (21) Timoshenko, S.; Woinowsky-Krieger, S. Theory of Plates and Shells, 2nd ed.; McGraw-Hill: New York, 1959.
air entrapped during the roller contact. It is known for instance that a fingering instability22–24 sets in when a less viscous fluid such as air penetrates into a more viscous one. A remarkable reduction in the surface undulations, air trapping, and fingering results when annealed at a high temperature (160 °C) is shown in Figure 4a for the second set of experiments. When annealed for 10 min (Figure 4b), the pattern depth increased by 10-20 nm compared with that in set I where the depth was 20-30 nm. When annealed 10 min more (Figure 4c), the pattern depth more than doubled from ∼50 to ∼130 nm and the surface became flatter, as the sectional profiles in the insets show. The picture emerging here is that the annealing during the first 10 min has the effect of a global planarization of the undulations over the entire area (compared to set I, shown in Figure 3) while the additional annealing results in a significant increase in the pattern depth. Therefore, the mechanism in the high-temperature annealing is capillarity, which leads to an increased pattern depth by capillary filling of the mold cavity and to a global planarization by a capillary pressure (γ(∂2h/∂x2), γ is the surface tension and h is the surface profile)25 driven flattening that stabilizes the surface undulations. Shown in Figure 5 are the patterning results from the set III experiment. As a result of applying a pressure of about 0.6 bar, the entrapped air was eliminated and most of the fingering region disappeared, as shown in Figure 5a. With the pressurization, the pattern depth increased by 10-20nm compared with the set II (22) Bensimon, D.; Kadanoff, L. P.; Liang, S.; Shraiman, B. I.; Tang, C. ReV. Mod. Phys. 1986, 58, 977. (23) Schift, H.; Hydermann, L. J.; Auf der Maur, M.; Gobrecht, J. Nanotechnology 2001, 12, 173. (24) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2003, 44, 2379. (25) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084.
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Figure 3. Patterning results from the set I experiment. Images of the patterned polymer surface obtained by roller-contact of a thin (∼20 µm)-film mold with PS/Si substrate at 160 °C. (a) Unpatterned and fingering instability region due to entrapped air and close-up view of fingers (inset; scale bar ) 10 µm) obtained by OM, (b) plane and sectional (inset) images of patterned surface by AFM (scan size ) 10 × 10 µm2, z scale ) (50 nm), (c) patterned surface with large-scale undulation by AFM (scan size ) 20 × 20 µm2, z scale ) (250 nm), and (d) fingering instability by AFM (scan size ) 100 × 100 µm2, z scale ) (500 nm). The letters in (a) designate different regions: S, small-scale vertical surface undulation; L, large-scale vertical and lateral surface undulation; and F, fingered region.
Figure 4. Patterning results from the set II experiment. Images of the patterned polymer surface obtained after roller-contact of a thin (∼20 µm)-film mold with PS/Si substrate at 160 °C and annealing for 10 or 20 min at the temperature. (a) Large-area view by OM and magnification of fingered region (inset; scale bar ) 10 µm), (b) plane-view and sectional surface profile for the sample annealed for 10 min by AFM (scan size ) 10 × 10 µm2, z scale ) (50 nm), and (c) plane-view and sectional surface profile for the sample annealed for 20 min by AFM (scan size ) 10 × 10 µm2, z scale ) (150 nm). Note that, as the annealing time increases, the surface undulation decreases and the pattern depth almost doubles.
data in the first 10 min of annealing, say up to 70 nm, shown in Figure 5b. With an additional 10 min of annealing (Figure 5c),
the pattern depth increased to about 130 nm, which is comparable to the result in the set II where no pressure was applied. A flatter
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Figure 5. Patterning results from the set III experiment. Images of the patterned polymer surface obtained after roller-contact of a film (∼20 µm) mold with PS/Si substrate at 160 °C and pressing for 10 or 20 min at the temperature. (a) Large-area view by OM, (b) plane-view and sectional surface profile for a sample annealed for 10 min with a pressure of ∼0.6 bar (10 × 10 µm2 scan, z scale ) (50 nm), (c) plane-view and sectional surface profile for a sample annealed for 20 min with a pressure of ∼0.6 bar (10 × 10 µm2 scan, z scale ) (150 nm).
surface profile and a larger pattern depth during the first 10 min of annealing can only be due to a squeezed viscous flow caused by the pressurization. On the basis of the three sets of experiments, it can be concluded that the mechanism involved in the low-pressure/high-temperature imprinting with a flexible film mold is a combination of capillarity and squeezed viscous flow. In the initial stage of the imprinting, both mechanisms are operative. As time progresses, however, capillarity is the main mechanism.
Conclusions The mechanism in the low-pressure/high-temperature imprinting depends on the type of mold used. When a hard mold is used,
capillarity is the mechanism. In the case of a flexible film mold, however, both capillarity and viscous flow are responsible for the imprinting. As time increases, only capillarity is operative. An understanding of the mechanisms involved in various imprinting methods is valuable on its own. More important is the insight it can provide into possible modifications of the imprinting process that can be tailored for specific applications. Acknowledgment. This work was supported in part by the Brain Korea 21 project. LA703084P
