Langmuir 2003, 19, 10957-10961
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Block Copolymer Thermoplastic Elastomers for Microcontact Printing David Trimbach,*,† Kirill Feldman,‡ Nicholas D. Spencer,‡ Dirk J. Broer,†,§ and Cees W. M. Bastiaansen†,| Department of Polymer Technology, Eindhoven University of Technology, Helix STO 0.26, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Department of Materials, ETH-Zu¨ rich, ETH Zentrum, CH-8092, Zu¨ rich, Switzerland, Philips Research Laboratories, Professor Holstlaan 4 (WB 72), 5656 AA Eindhoven, The Netherlands, and Dutch Polymer Institute, Eindhoven University of Technology, STO 0.22, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received July 6, 2003. In Final Form: October 13, 2003 Two commercial thermoplastic block copolymer elastomers with a high stiffness were investigated as stamp materials for microcontact printing and compared to conventional poly(dimethylsiloxane) (PDMS). Stamps with a relief structure were produced by hot embossing techniques, utilizing the thermoplastic properties of these rubberlike block copolymers. It is shown that the stamps based on these copolymers are able to transfer a thiol ink to a gold substrate. After printing, the thiol ink acts as an etch resist, which indicates that a coherent self-assembled monolayer is formed. Like PDMS stamps, specific copolymer stamps can be used for repeated printing without re-inking. Moreover, the higher stiffness of the thermoplastic stamps increases the load above which structural collapse occurs by a factor of 10-15 in comparison to that of identical PDMS stamps, which is potentially useful in the reproduction of structures which are sensitive to sagging, buckling, or pairing. An example is presented of relief structures, which are accurately reproduced with the thermoplastic elastomers, in contrast to identical chemically crosslinked PDMS stamps.
Introduction Soft lithographic techniques, such as microcontact printing, light-coupling mask lithography, and microfluidics, have been investigated extensively for creating (sub)micrometer structures.1,2 In these techniques, soft elastomeric stamps with relief structures are used, which are able to conform to an inorganic substrate. These techniques rely on a locally well-defined and intimate contact between an elastomeric stamp and a substrate, known as conformal contact. In microcontact printing, the relief structure of a stamp is used to locally transfer an “ink” to a substrate, where it forms a self-assembled monolayer (SAM). This process is schematically depicted in Figure 1. A wide range of inks and inorganic substrates have been investigated, including (i) alkanethiols on silver and gold3 and (ii) alkyltrichlorosilanes on hydroxylterminated oxides, such as SiO2, Al2O3, and indium tin oxide (ITO).4 Actual printing has almost exclusively been performed with poly(dimethylsiloxane) (PDMS) stamps. However, rather limited studies were also reported on the use of polyurethanes, polyimides, and Novolac resins as stamp materials for printing thiols.5,6 * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Polymer Technology, Eindhoven University of Technology. ‡ Department of Materials, ETH-Zu ¨ rich. § Philips Research Laboratories. | Dutch Polymer Institute, Eindhoven University of Technology. (1) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (2) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 697. (3) Delamarche, E.; Michel, B.; Biebuyck, H.; Gerber, C. Adv. Mater. 1996, 8, 719. (4) Ulman, A. Introduction to thin organic films: From LangmuirBlodgett to self-assembly; Academic Press: Boston, 1991. (5) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550.
Figure 1. Schematic representation of the microcontact printing process. The inking solution is applied to a structured stamp (1), and the solvent is evaporated (2). The stamp is then brought into contact with the gold-coated substrate (3), and a monolayer is formed by self-assembly (4). This monolayer acts as a resist to etching (5).
PDMS is an extremely soft polymer and therefore not suited for the printing of features below 250 nm, due to stamp deformation. Several studies have been performed to enhance resistance against mechanical deformation of PDMS stamps during the printing process. For instance, Rogers et al. synthesized a photocurable PDMS for the fabrication of submicron structured stamps with a reduced shrinkage and a slightly higher modulus.8 These stamps were used to microcontact print 250 nm structures. It was shown by Michel et al. that regularly spaced 80 nm posts were accurately reproduced using PDMS grades with an even higher stiffness as a stamp material.2 Unfortunately, these high-modulus PDMS stamps have a low mechanical strength, which complicates the fabrication and handling of the stamps.7 Whitesides et al. investigated the printing behavior of composite PDMS stamps consist(6) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 14, 2002. (7) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (8) Choi, K. M.; Rogers, J. A. J. Am. Chem. Soc. 2003, 125, 4060.
10.1021/la035214j CCC: $25.00 © 2003 American Chemical Society Published on Web 11/14/2003
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ing of a normal PDMS bulk with a high-modulus PDMS surface layer on top with a relief structure.5 This improves the handling of the stamps although the stamps are more complicated to fabricate. Using these composite stamps, they were able to reproduce regularly spaced features as small as 30 nm, albeit by soft lithographic techniques other than microcontact printing. Apart from chemically cross-linked rubbers such as PDMS, an enormous variety of other polymeric elastomers is available that derive their elastomeric character from, for instance, self-aggregation of thermoreversible, nanosized structures within the bulk of the polymers. Typically, these elastomers are classified as thermoplastic elastomers. This class of materials includes di-, tri-, and multiblock copolymers9 and highly branched, semicrystalline polyolefins.10,11 The properties of these elastomers with respect to stiffness can be tuned, to a large extent, by a proper selection of their chemical structure, composition, and processing conditions while preserving their intrinsic toughness. In a recent paper,11 Csucs et al. reported the use of polyolefins for the printing of protein patterns. In the present paper, the thiol printing behavior of two wellknown, commercial block copolymers is investigated. Previously, it has been shown that relief structures can be produced in these elastomeric block copolymers via hot embossing techniques.12 Here, a study is presented in which it is attempted to exploit the higher stiffness of these thermoplastic elastomers in the microcontact printing of thiols using specific stamp geometries. Special attention is devoted to the printing of micron-sized relief structures that are vulnerable to sagging. Experimental Section Materials. PDMS (Sylgard 184, Dow Corning, Midland, MI) was used as received. Poly(styrene-block-butadiene-blockstyrene) (SBS, Kraton D1102, Sepulchre, Brussels, Belgium) granules and poly(styrene-block-ethylene-co-butylene-blockstyrene) (SEBS, Kraton G1652, Sepulchre, Brussels, Belgium) powders were dissolved in toluene to prepare a 10 wt % solution. This solution was centrifuged to clarify the solution by sinking the inorganic fillers. The clear solution was decanted, and the solvent was evaporated at ambient conditions for at least 24 h. The remaining solvent was removed by drying the films in a vacuum oven at 70 °C for 24 h. (Tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (ABCR, Karlsruhe, Germany) was used as received, to fluorinate the surfaces of the silicon masters, improving the release of the stamps during replication. Poly(tetrafluoroethylene-co-hexafluoropropylene) (Teflon FEP 100 Dupont, Luxembourg, Luxembourg) granules were compression molded at 330 °C to prepare films. Ethanol (99.9%, Biosolve, Valkenwaard, The Netherlands), hexadecanethiol (96%, SigmaAldrich), NaCN (97%, Sigma-Aldrich), K3FeCN6 (98%, SigmaAldrich), and KOH (85%, Sigma-Aldrich) were used as received. Masters. The silicon masters were obtained from Mikromasch (TGG01, TGT01, Tallinn, Estonia). For fluorination, the surface of the silicon master was first oxidized for 5 min using an oxygen plasma at a pressure of 0.6 mbar and 50 W, using an Emitech K1050X plasma asher (Emitech, Ashford, U.K.). Then the masters were fluorinated by placing them for 1 h in a desiccator containing a drop of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (TTTS) at a pressure of 100 mbar. Finally, the master was taken out of the desiccator and dried for 1 h at 80 °C in a vacuum. (9) Hamley, I. W. The physics of block copolymers; Oxford Science: New York, 1998. (10) Koch, K.; DeGroot, J.; Lasticova, J. Metallocene-based polyolefins; John Wiley & Sons: Chichester, 2000; Vol. 2, p 205. (11) Csucs, G.; Ku¨nzler, T.; Feldman, K.; Robin, F.; Spencer, N. D. Langmuir 2003, 19, 6104. (12) Fichet, G.; Stutzmann, N.; Muir, B. V. O.; Huck, W. T. S. Adv. Mater. 2002, 14, 47.
Trimbach et al. Table 1. The Young’s Modulus and Elongation at Break for the Elastomers material
Young’s modulus (MPa)
elongation at break (%)
PDMS PDMS (stiffened) SBS SEBS
1.8 1014 14.2 46.7
150 brittle5 880 500
Preparation of Stamps. Flat stamps were prepared by using a polished silicon wafer as a master. Stamps with a grating structure were prepared using a structured silicon master (TGG01). Stamps with a spikelike relief structure were prepared using a poly(tetrafluoroethylene-co-hexafluoropropylene) (Teflon FEP 100 Dupont) intermediate inverse replica of the master (TGT01). This intermediate copy in FEP was produced via hot embossing at 330 °C at a load of 200 g for 5 min.12 This FEP copy was then used as a master. PDMS stamps were prepared by cast molding. This was done by mixing the base and curing agent in a 10:1 ratio, pouring the mixture onto the master, and curing for 24 h at 70 °C. The thermoplastic elastomers were hot-embossed by compression molding in a special machine in which an accurate load and temperature can be applied (Tribotrack, Daca Instruments, Goleta, CA). Hot embossing of SBS was performed at 140 °C with a load of 200 g for 5 min, using either a polished silicon wafer, a structured silicon master, or a FEP intermediate. Hot embossing of SEBS was performed in a similar manner at 200 °C. Microcontact Printing Procedure. Gold substrates were prepared using a silicon wafer (Topsil, Frederikssund, Denmark) on which a chromium primer layer ((10 nm) was sputtered followed by a gold top layer ((100 nm), using an Emitech K575 XD Turbo Sputter Coater (Emitech), at 50 mA for 30 s and 75 mA for 3 min, respectively. Microcontact printing was performed using a 1 mM hexadecanethiol solution in ethanol. The stamp was soaked in the solution for 30 s and then dried in a nitrogen flow. Next, the stamp was brought into contact with the gold substrate for 30 s and then removed. Afterward, the gold substrate was rinsed with ethanol. Etching of the patterned gold was performed using an aqueous cyanide etch13 (1 M KOH, 0.1 M NaCN, and 0.01 M K3Fe(CN)6) at 0 °C for about 1 min. Analyses. Tensile tests were performed at room temperature with an Instron tensile tester (model 4411, Canton, MA) using dumbbell-shaped specimens with a gauge length and width of 12 and 2 mm, respectively. The crosshead speed was 10 mm/ min. Contact angles were measured on a Kru¨ss drop shape analysis system (DSA10, Hamburg, Germany), using the advancing drop method. Scanning probe microscope measurements were performed on a Nanoscope Dimension 5000 with a Nanoscope III controller (Digital Instruments, Santa Barbara, CA), using scan rates between 0.5 and 2 Hz. Scanning electron micrographs were recorded using a Philips XL30 ESEM-FEG (Eindhoven, The Netherlands) in high-vacuum mode, at an operating voltage between 2 and 10 kV. Optical microscopy was performed on a Zeiss Axioplan 2 equipped with a high-resolution CCD detector.
Results and Discussion Two thermoplastic materials were selected for this study: SBS and SEBS. These triblock copolymers derive their elastomeric character from their two-phase nanostructure comprising hard polystyrene domains, which act as physical cross-links in a soft, rubbery phase.9 In Table 1, the Young’s modulus and elongation at break of these elastomers are listed and compared to those of conventional PDMS and a special PDMS with enhanced stiffness.14 It is shown that the stiffness of the triblock copolymers is high in comparison to that of PDMS. Moreover, the block copolymers possess a high strain at (13) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (14) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310.
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Figure 2. The advancing contact angle of water on gold with printed hexadecanethiol as a function of the printing time for unstructured PDMS (circles) and SEBS (triangles) stamps.
Figure 3. Electron micrograph of an etched gold film after microcontact printing with a structured SEBS stamp.
break, which illustrates their high toughness, and this is despite their high modulus (compared to high-modulus PDMS). In a first set of experiments, the transfer of the hexadecanethiol ink from the stamps to the substrate was investigated. Flat stamps without a relief structure were used, and the thiol ink was printed on a freshly deposited
material
spacing (µm)
height (µm)
silicon master PDMS stamp SBS stamp SEBS stamp
3.0 3.0 3.0 3.0
1.90 1.19 1.36 1.36
gold surface. Contact angle measurements were performed, and it was found that the advancing contact angle increases in a similar manner for both PDMS and SEBS (Figure 2), which indicates that the ink is transferred to the gold surface at a comparable rate. In a subsequent experiment, a patterned stamp of SEBS was used for microcontact printing on a freshly deposited gold substrate, which was etched afterward using an aqueous cyanide etch. In Figure 3, an electron micrograph of an etched pattern is shown. This illustrates that the printed hexadecanethiol acts as an etch-resist. Apparently, a well-defined self-assembled monolayer is formed, which is an efficient barrier for the etchant. Repeated printing experiments, without re-inking, were performed using all polymeric materials. With SEBS stamps, similar results were obtained as with PDMS stamps; that is, reproducible results were obtained even after 10 successive prints. Apparently, SEBS absorbs and releases the thiol ink in a similar manner as PDMS. In the case of SBS, repeated printing without re-inking proved to be problematic. After the first printing, the quality of the structures etched in gold deteriorates rapidly, and after the third printing these structures disappear altogether. Therefore, it seems that SBS does not significantly absorb and release the ink molecules. A specific relief structure was selected (Figure 4a) for a first comparison of the deformation behavior of PDMS, SBS, and SEBS stamps. In Figure 4b-d, scanning probe micrographs are shown of the replicas of the master in PDMS, SBS, and SEBS. In Table 2, the height and pitch of the master and the replicas are compared. The experimental data suggest that PDMS and SBS are replicated somewhat more accurately than SEBS. However, the actual stamp production was not optimized extensively and a more detailed study on reproduction
Figure 4. (a) Scanning probe micrograph of the silicon master. Reproductions of the master in (b) PDMS, (c) SBS, and (d) SEBS.
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Figure 5. SEM micrographs of line patterns printed on gold prior to etching, microcontact printed with (a) PDMS (the inset shows the zero load image), (b) SBS, and (c) SEBS at a load of 1 N. The dark areas correspond to the printed SAM, and the light areas to the unprinted gold. The scale bar equals 3 µm in all images.
issues is required. Nevertheless, these stamps are sufficiently identical to make a further comparison between the different stamp materials possible. Subsequently, the stamps described above were used for microcontact printing and the printed patterns were investigated with scanning electron microscopy (SEM). In Figure 5, typical micrographs are shown of the printed patterns after microcontact printing and prior to etching. With PDMS stamps, a homogeneously printed pattern is obtained at zero load (inset, Figure 5a), and this is in contrast to SBS and SEBS (not shown in Figure 5). In the latter two cases, extremely inhomogeneous printed patterns are obtained at zero load, which indicates that conformal contact requires a load during printing due to the increased stiffness of these materials. Subsequently, printing was performed at a load of approximately 1 N to make a direct comparison between PDMS, SBS, and SEBS possible. The micrographs illustrate that the printed line width is large in the case of PDMS (2.6 µm at 1.08 N, Figure 5a) in comparison to SBS (1.2 µm at 1.06 N, Figure 4b) and SEBS (1.0 µm at 1.14 N, Figure 4c). Apparently, the stamp deformation during printing is less in the case of SBS and SEBS, in comparison to PDMS, which is a direct consequence of the increased stiffness of these materials. Next, microcontact printed patterns were investigated after etching. In Figure 6a, the line width of the printed patterns is compared prior to and after etching. The experimental data are identical, within experimental error, which further illustrates that it is indeed possible to evaluate the line width of a patterned SAM, prior to etching, using SEM. In Figure 6b, the width of the printed lines, after etching, is plotted versus the applied weight for different stamp materials. Again, it is observed that the line width decreases with increasing stamp stiffness. The curve in these lines is as expected, because as the line width increases, proportionally more force is required, due to the shape of the relief structure. Moreover, it is also observed that the load at which complete collapse of the stamp occurs increases from 1 N for PDMS to 10 and 15 N for SBS and SEBS, respectively. This shows that failure mechanisms associated with the use of PDMS stamps, such as sagging, collapse, and pairing are reduced with these stiffer thermoplastic elastomers. To exploit the benefits of these new materials further, an additional set of experiments was performed with stamps with a spikelike relief structure (Figure 7a). The PDMS, SBS, and SEBS stamps (Figure 7b-d) were prepared using a FEP intermediate copy, as described in the Experimental Section. In Table 3, the dimensions of the master are compared with the dimensions of the stamps. Again, it is observed that the relief structures in PDMS are lower in height. Upon microcontact printing with PDMS, structural collapse is observed, even at a zero applied load. The microstructure is absent, and a
Figure 6. (a) Line width of the printed patterns prior to (open squares) and after etching (filled squares) as a function of the applied load (SBS stamp). The top left inset shows an electron micrograph of a patterned SAM printed on gold using SBS, prior to etching. The dark areas correspond to the printed SAM, and the light areas to the unprotected gold. The bottom right inset shows a similar sample, after etching. The scale bars represents 3 µm in both images. (b) Line width after etching as a function of the applied load for PDMS (circles), SBS (squares), and SEBS stamps (diamonds).
SAM is formed over the entire gold surface (Figure 7e). With SBS and SEBS (Figure 7f,g) stamps, the pattern is transferred to the gold substrate without collapse of the structure. This demonstrates that with SEBS and SBS stamps structures can be printed which cannot be reproduced with PDMS due to sagging and/or collapse. The above-described results illustrate that highmodulus, tough, thermoplastic, elastomeric copolymers are useful as stamp materials in the reproduction of deformation-sensitive relief structures with microcontact printing. More specifically, these elastomers have a strongly reduced tendency for sagging and collapse in comparison to PDMS, and it is expected that the pairing of high-aspect-ratio relief structures is reduced. However, a few critical remarks concerning the results presented in this study are appropriate. First, the production of the stamps from thermoplastic elastomers is rather straightforward and damage to the masters or stamps during or
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Figure 7. (a) Scanning probe micrographs of the spikelike master, (b) the PDMS stamp, (c) the SBS stamp, and (d) the SEBS stamp. All divisions are 2 µm. Electron micrographs of etched gold, after microcontact printing with (e) PDMS, (f) SBS, and (g) SEBS stamps. The scale bar equals 2 µm. Table 3. Comparison of the Dimensions of the Various Replicas and the Master material
spacing (µm)
height (nm)
spike master PDMS stamp SBS stamp SEBS stamp
2.12 2.12 2.12 2.12
500 295 360 369
after molding is not observed (see also ref 12). Also, we recently demonstrated that stamps can be reproduced from highly cross-linked epoxy masters which further facilitates stamp production. In all cases, the reproduction of the stamps was somewhat inaccurate which, in our opinion, is predominantly related to the molding technique used in this study. For instance, it is expected that the accuracy of stamp production can be improved by applying a vacuum or by improving stamp/substrate alignment. Second, an often-cited advantage of PDMS is its ability to conform to curved or spherical substrates. It is expected that this is more problematic for stiff thermoplastic elastomers because a load is required to establish conformal contact. More importantly, PDMS is also extremely useful in other soft lithographic techniques such as microfluidics, micromolding, and light-coupling mask lithography. It is expected that a careful selection of thermoplastic elastomers with respect to properties such as swelling in
solvents, chemical resistance, transparency, and conformal sealing results in suitable stamp materials for these soft lithographic techniques. Conclusions Relief structures were produced in two block copolymer thermoplastic elastomers via hot embossing, and it is shown that these relief structures are suitable as stamps for microcontact printing. After printing on gold, a selfassembled monolayer of an alkanethiol is formed, which acts as a resist. In the case of SEBS, repeated printing without re-inking is possible, which is an additional benefit. The thermoplastic elastomers possess a high modulus and toughness in comparison to PDMS, and consequently, stamp deformation during printing is decreased, which results in a more accurate reproduction at a specific load. It is also shown that the enhanced stiffness of the elastomers is useful in the reproduction of demanding relief structures with extreme aspect ratios, which cannot be printed with PDMS. Acknowledgment. The authors acknowledge financial support from the TopNano21 program of the Council of the Swiss Federal Institutes of Technology. LA035214J
