© Copyright 2002 American Chemical Society
SEPTEMBER 3, 2002 VOLUME 18, NUMBER 18
Letters Micropatterning of Block Copolymer Solutions Tao Deng, Yung-Hoon Ha, Joy Y. Cheng, C. A. Ross, and Edwin L. Thomas* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 13, 2002. In Final Form: July 1, 2002 Submicron patterns of block copolymers were generated using an elastomeric mold against solutions of block copolymers. To prevent the swelling of the elastomeric mold by the solvent used in the block copolymer solution, a thin layer of amorphous fluorinated polymer was coated on the surface of the mold. The comparison of patterns generated using an unmodified mold with those generated using a modified mold is also reported. Coupled with their microphase separation at nanometer scale, these block copolymer patterns can be applied in nanofabrication as nanotemplates.
Introduction Block copolymers can self-assemble into periodic 1-D, 2-D, and 3-D structures with periodicity on the order of 10-100 nm.1 The interesting size range of these periodic structures, and the tunability of their sizes, morphologies, and even chemical and physical properties make block copolymers attractive in a number of areas of technology.2-9 For example, several research groups have reported using * To whom correspondence should be addressed. (1) Thomas, E. L.; Lescanec, R. L. Philos. Trans. R. Soc. London, A 1994, 348, 149. (2) Harrison, C.; Park, M.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. J. Vac. Sci. Technol. B 1998, 16, 544. (3) Koslowski, B.; Strobel, S.; Herzog, P.; Heinz, B.; Boyen, H. G.; Notz, R.; Ziemann, P.; Spatz, J. P.; Moller, M. J. Appl. Phys. 2000, 87, 7533. (4) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K. W.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (5) Li, R. R.; Dapkus, P. D.; Thompson, M. E.; Jeong, W. G.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Appl. Phys. Lett. 2000, 76, 1689. (6) Cheng, J. Y.; Ross, C. A.; Chan, V. Z. H.; Thomas, E. L.; Lammertink, R. G. H.; Vancso, G. J. Adv. Mater. 2001, 13, 1174. (7) Fink, Y.; Urbas, A. M.; Bawendi, B. G.; Joannopoulos, J. D.; Thomas, E. L. J. Lightwave Technol. 1999, 17, 1963. (8) Urbas, A. M.; Sharp, R.; Fink, Y.; Thomas, E. L.; Xenidou, M.; Fetters, L. J. Adv. Mater. 2000, 12, 812. (9) Edrington, A. C.; Urbas, A. M.; DeRege, P.; Chen, C. X.; Swager, T. M.; Hadjichristidis, N.; Xenidou, M.; Fetters, L. J.; Joannopoulos, J. D.; Fink, Y.; Thomas, E. L. Adv. Mater. 2001, 13, 421.
thin films of block copolymers as nanotemplates for the generation of nanostructures in cobalt, diamond, gallium arsenide, germanium, silicon, and silicon nitride.2-6 With proper periodicity and dielectric contrast, block copolymers can also serve as building blocks for microphotonic devices.7-9 In most of the applications reported so far, block copolymers are either used as spun cast or simple (quiescent) cast films. These films are continuous and cover the whole substrate. For the fabrication of practical devices at the micro- or nanoscale, block copolymers need to be patterned so that the desired block copolymer domains only appear at the places where they are needed. Currently there are several approaches for patterning block copolymers. Among these approaches, graphoepitaxy is a method that can generate micropatterns of block copolymer on a substrate that is prepatterned using conventional photolithography or e-beam lithography.10,11 Another possibility is to combine directional solidification and graphoepitaxy to create two types of microdomain patterns within a prepatterned structure with the ability to control the orientation of the microdomain patterns via the temperature gradient.12 The topographic features on (10) Segalman, R. A.; Yokoyama, H.; Kramer, E. J. Adv. Mater. 2001, 13, 1152. (11) Cheng, J. Y.; Ross, C. A.; Thomas, E. L.; Smith, H. I.; Vaneso, G. J. Unpublished results, 2002.
10.1021/la020446o CCC: $22.00 © 2002 American Chemical Society Published on Web 08/09/2002
6720
Langmuir, Vol. 18, No. 18, 2002
the prepatterned substrate offer boundaries for guiding the packing of the block copolymer microdomains. One possible drawback to the graphoepitaxy approach is the coexistence of the topographic features with the block copolymer domain patterns. These topographic features may interfere with the subsequent use of the block copolymer films in fabricating actual devices; removal of these features may sometimes be necessary. Embossing is another promising approach for patterning block copolymers. Ober et al. recently used patterned silicon masters to fabricate a negative replica into the surface of poly(styrene-b-ethylene-b-butylene-b-styrene) (SEBS) block copolymers.13 In their experiments, they carefully chose solvents and melt-release agents to optimize the interfacial energy for complete liftoff of the master without damaging the patterned block copolymer surface. Huck et al. also developed a solid-state embossing method for generating surface relief micropatterns in poly(styrene-b-isoprene-b-styrene) (SIS) block copolymers.14 They employed poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP, also a hard, relatively high modulus material) masters instead of silicon masters. Hard masters, however, cannot have complete conformal contact with the substrate, especially for substrates with uneven surfaces. Extra care also needs to be taken to avoid difficulties during the separation of the hard master from the patterned block copolymer. The required conditions of high pressure and elevated temperature may also make embossing undesirable for certain applications. Here we report the use of soft lithography for patterning block copolymers. Soft lithography has been established as a useful tool in patterning polymeric, biologic, and other soft materials.15 In soft lithography, the patterning master is usually an elastomeric poly(dimethylsiloxane) (PDMS) mold. Generally, solutions that are patternable by PDMS molds involve only solvents that do not swell PDMS, such as water- or ethanol-based solutions. For many common block copolymer systems, toluene, a PDMS-swelling solvent, is the typical solvent employed. The swelling of PDMS by toluene either makes the block copolymer solution unpatternable or results in poor patterns. To pattern such polymer systems in a controllable fashion and to generate desirable structures, one has to prevent the swelling of PDMS by the solvent. In this paper we show that coating of a PDMS mold with a fluorinated polymer eliminates the swelling of PDMS by toluene during the patterning and results in ordered block copolymer patterns. Results and Discussion Figure 1 illustrates the process for patterning block copolymers. The PDMS molds we used were fabricated by casting a PDMS precursor (Sylgard 184, Dow Corning Corp) on a photoresist master. The photoresist master was patterned by standard photolithography, and we fluorinated the surface of the photoresist using (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (United Chemical Technologies, Inc.) before casting the PDMS precursor on the master. After the PDMS precursor was cured at 60 °C for 2 h, we separated the mold from the (12) Park, C.; Cheng, J. Y.; Fasolka, M. J.; Mayes, A. M.; Ross, C. A.; Thomas, E. L. Appl. Phys. Lett. 2001, 79, 848. (13) Schmaljohann, D.; Griesebock, B.; Lee, H. J.; Li, X.; Ober, C. Presented at the American Chemical Society 220th National Meeting, 2000. (14) Fichet, G.; Stutzmann, N.; Muir, B. V. O.; Huck, W. T. S. Adv. Mater. 2002, 14, 47. (15) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550.
Letters
Figure 1. Patterning block copolymers using the PDMS mold coated with Teflon AF: (a) uncoated PDMS mold; (b) PDMS mold coated with a layer of Teflon AF; (c) block copolymer solution molded using the PDMS mold;(d) block copolymer separated from the PDMS mold after the solvent fully evaporated (typically 30-60 min).
photoresist master and spin coated the mold with an amorphous fluoropolymer (Teflon AF, Dupont) 0.05 wt % solution in FC-75 (3M) at 2K-6K rpm for 1 min. After the PDMS mold dried at room temperature for 1 h, Teflon AF formed a surface layer 20-60 nm thick with surface roughness < 5 nm. The block copolymer formed a negative replica of the PDMS mold after molding for ∼30-60 min.15 The mold coated with Teflon AF can be used for up to five times without noticeable degrading of the coating. Moreover, the mold can be simply recoated with Teflon AF after cleaning in FC-75 and ethanol. Teflon AF coating prevented the swelling and deformation of the PDMS mold by toluene. The ability of the Teflon AF coating to protect the PDMS is demonstrated by placing droplets of toluene (Sigma-Aldrich) with diameters between 500 µm and 1 mm on top of PDMS molds using a micropipet. A CCD camera (NEC, Model Nx18A, Cambridge Instruments) captured the images of droplets and PDMS molds at different stages of solvent evaporation. Figure 2a-c shows images of a toluene droplet on a bare PDMS mold, and Figure 2d-f depicts the evaporation of a toluene droplet on a Teflon AF coated PDMS mold. In Figure 2a, toluene wetted the PDMS and formed a nonspherical droplet on the surface of PDMS. During this stage, the evaporation of toluene and the swelling of PDMS by toluene occurred simultaneously. After 90 s, the height of the toluene droplet was significantly reduced whereas its diameter only decreased slightly (Figure 2b). The swelling may pin the boundary of the droplet at the toluene/PDMS interface. After 3 min, the toluene droplet totally disappeared while the PDMS surface had deformed into a bump because of the swelling by toluene. Compared to the nonspherical toluene droplet on the bare PDMS mold, the one on the PDMS mold coated with Teflon AF was more spherical-like because of the dewetting of PDMS on the Teflon AF surface (Figure 2d). Here the change of the droplet shape only occurred because of the evaporation of toluene. Since there was no PDMS swelling, the droplet had almost isotropic shrinkage at the toluene/air interface. After 90 s, the droplet became much smaller than the original droplet (Figure 2e). After 3 min, the droplet completely evaporated and there was no local deformation of PDMS observed (Figure 2f). The AFM (Nanoscope III, Multimode; Digital Instruments) images in Figure 3 compare the block copolymer
Letters
Langmuir, Vol. 18, No. 18, 2002 6721
Figure 2. (a) Toluene droplet on top of a bare PDMS mold. (b) Image after 90 s. As the toluene evaporated, it also swelled the PDMS. The diameter of the droplet only slightly reduced while the height of the droplet decreased dramatically. (c) Image after 3 min: the toluene droplet disappeared, and the PDMS mold surface deformed into a bump. (d) Toluene droplet on top of a PDMS mold coated with Teflon AF. (e) Image after 90 s: toluene evaporated and the droplet shrank in size. (f) Image after 3 min: the toluene droplet disappeared and the PDMS mold surface was unchanged.
Figure 3. (a) SIS line patterns generated by molding with a bare PDMS mold. The schematic at the bottom is a cross section profile of these lines. (b) SIS line patterns and a cross section profile generated by molding with a PDMS mold coated with Teflon AF.
patterns generated using a bare PDMS mold to the patterns generated using a PDMS mold coated with Teflon AF. The block copolymer system was a poly(styrene-bisoprene-b-styrene) (SIS) triblock copolymer (Dexco Polymers) solution in toluene (0.1% in w/w). The total molecular weight of the SIS triblock copolymer is about 102 kg/mol, and it contains about 29 wt % polystyrene. The PDMS mold pattern comprised lines 4.5 µm wide, with periodicity 7 µm and height 1 µm. Using the process described in Figure 1, we molded the block copolymer solution onto a cleaned glass slide using both a bare PDMS mold and a mold coated with Teflon AF. After molding and separation of the PDMS mold, the samples were annealed at 110 °C in a vacuum for 24 h. Using the plain PDMS mold generally resulted in no pattern or a poor pattern because of the swelling of PDMS. In a few areas there were some periodic line structures, but these lines were not uniform and had a variable profile (Figure 3a). In the case of the PDMS mold coated with Teflon AF, because of the dewetting of toluene to the surface of Teflon AF, and the low surface roughness, the block copolymer tended to form patterns with a uniform profile between the walls of the lines
(Figure 3b). Compared to patterning PDMS-compatible solutions using a plain PDMS mold, this approach has similar resolution in patterning solutions with PDMSswelling solvents.16 We also patterned block copolymer solutions to produce feature sizes ∼ 200 nm. A block copolymer solution of poly(styrene-b-ferrocenyldimethylsilane) (SFS) diblock copolymer solution in toluene (0.2% in w/w) was used to generate the patterns in Figure 4. The total molecular weight of the SFS diblock copolymer was ∼62 kg/mol with approximately 19 wt % polyferrocenyldimethysilane, which formed spherical microdomains inside the polystyrene matrix after microphase separation. The mold was fabricated by casting the PDMS precursor against a fluorinated silicon master. After it was coated with Teflon AF, the PDMS mold had lines with width 200 nm, periodicity 350 nm, and height 35 nm. During the patterning, the solution was molded against a silicon wafer primed with hexamethyldisilazane (HMDS; Sigma-Aldrich) and the samples were annealed in a vacuum at 140 (16) Rogers, J. A.; Bao, Z. N.; Raju, V. R. Appl. Phys. Lett. 1998, 72, 2716.
6722
Langmuir, Vol. 18, No. 18, 2002
Letters
Therm, Model 790; oxygen pressure, 5 mTorr; time, 20 s) was used to selectively etch away the polystyrene matrix, leaving spheres of iron-silicon oxide, approximately 30 nm in diameter. Figure 4a shows well-formed SFS lines, which are the replica of the PMDS mold, after molding. Figure 4b is the SFS lines after annealing and RIE. At present, the PFS spheres are not regularly positioned with respect to the edges of the lines and have smaller diameters in the vicinity of the edges. Incorporation of an external driving force for alignment may improve the ordering of the block copolymer within the patterned features. Conclusion We used a surface treatment to modify a PDMS mold for patterning block copolymer solutions with a solvent that normally swells PDMS. We coated the surface of PDMS with a Teflon AF layer, thus preventing the swelling of PDMS by the solvent. Using this approach, we have generated various block copolymer micropatterns including the formation of arrays of 200 nm wide block copolymer strips. This process offers not only the opportunity for patterning block copolymer solutions but also an approach for studying the phase behavior of block copolymers confined in boundaries with different shapes and dimensions. Moreover, the solvent resistant AF coating also enables us to use PDMS to pattern other materials that dissolve in solvents noncompatible with PDMS. Figure 4. (a) AFM (tapping mode) image of SFS line patterns generated using the coated PDMS mold: line width, 200 nm; periodicity, 350 nm; height, 35 nm. (b) Scanning electron micrographs of lines of spherical PFS domains after RIE of the SFS lines. The inset is a magnified view of the microdomains.
°C for 24 h after molding and separation of the PDMS mold from the substrate. Reactive ion etching (RIE; Plasma
Acknowledgment. Support from Draper Laboratory (Contract DL-H-539063) and the MIT Center for Materials Science and Engineering is gratefully acknowledged. We appreciate the help from E. Chan for the coating of Teflon AF, DuPont for supplying the solution, and Professor G. J. Vancso at University of Twente (The Netherlands) for supplying the SFS diblock copolymer. LA020446O
