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Surface-Charge Lithography for Direct PDMS Micro-Patterning Simonetta Grilli,* Veronica Vespini, and Pietro Ferraro Istituto Nazionale di Ottica Applicata del CNR (CNR-INOA), Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy ReceiVed September 18, 2008. ReVised Manuscript ReceiVed October 13, 2008 Direct patterning of PDMS films is achieved by modulating the wettability of polar dielectric substrates. Periodic array structures of microbumps can be madeup by functionalizing periodically poled lithium niobate crystals. The modulation of surface wettability is obtained through the spatial distribution of the surface electric charges generated by the pyroelectric effect under electrode-less configuration. An appropriate thermal treatment of the substrates assures both the wettability patterning and the fast cross-linking of the PDMS film.
Introduction Polydimethylsiloxane (PDMS) is an elastomer material widely used in different fields of application, such as micro/nanofluidics,1,2 electrical insulation, micro/nanoelectromechanical (MEMS/NEMS) devices,3 soft lithography,4 quantum dots,5 and charge patterning in thin-film electrects.6 All of these applications make use of existing or emerging technologies where patterned nanostructures are of central importance. In fact, the ability to fabricate on the nanometer scale assures the miniaturization of functional devices, which always means more components per chip, faster response, lower cost, lower power consumption, and higher performance. In almost all applications of nanostructures, the fabrication process represents one of the most significant challenges to their realization. Among the various strategies for fabricating patterned nanostructures, self-assembly may provide a route to certain types of structures. Self-assembly consists basically of the spontaneous organization of specific subunits, such as molecules or meso-scale objects, which then aggregate into stable and well-defined structures based on noncovalent interactions. The characteristics of the subunits, in terms of topographies, shapes, surface functionalities, and electrical potentials, guide the assembly process. The final structure is obtained by reaching the equilibrium condition, which minimizes the free energy of the system. The PDMS material offers many advantages7 for the fabrication of patterned structures. It is optically transparent, electrically insulating, mechanically elastic, and gas-permeable. PDMS is also biocompatible,8 thus finding application in the field of bioengineering where the position of cells on a substrate is important for different purposes. These include biosensor fabrication for drug toxicity and environmental monitoring,9 tissue engineering,10 patterning of active proteins,11
patterning of animal cells,12 and basic biology studies where the role of cell adhesion, shape, proliferation, and differentiation are studied as a function of cell-cell and cell-extracellular matrix interactions.13 The ability to pattern PDMS reliably in the form of both thick substrates and thin membranes or films is critical to expanding the scope of its applications, especially in the fields of microfluidics and bioengineering. Unfortunately, the PDMS material is not photodefinable, and therefore, it cannot be simply spin-coated and patterned like photosensitive resists. Elastomers are often fabricated as bulk materials via molding.14 A simple PDMS pattern reported in the literature consists of a perforated membrane,15 which was used as a mask to pattern proteins. Ryu et al.16 reported PDMS patterning by pouring it onto a substrate with predefined patterns and removing the PDMS in excess by traversing it with a blade. Pawlowski et al.17 used a similar method to create a PDMS mask that was used to fabricate structures in glass by powder blasting. Garra et al.18 also attempted to pattern PDMS by both wet chemical etching and dry (plasma) etching, but the patterned PDMS had very high surface roughness, which is not desirable for some applications. Childs et al.19 developed “decal transfer microlithography” where patterned PDMS is added to a substrate. This technique is reliable only for smaller features, and the torn surface is very rough. In selective pattern release, the PDMS is spin-casted on a master, followed by treatment to create a nonstick surface. The support PDMS layer is then cast onto the pattern and cured. The pattern is removed from the master and bonded to a substrate. The handle PDMS is then peeled off, leaving behind the patterned PDMS. Decal transfer lithography has been applied for patterning on nonplanar substrates20 and for large-area patterning of coinage metal thin films.21 The implementation of alternative approaches for direct patterning of thin PDMS films is still attracting interest in the
* E-mail address:
[email protected]. (1) Jo, B.; Van Lerberghe, L.; Motsegood, K. M.; Beebe, D. J. J. Microelectromech. Syst. 2000, 9, 76. (2) Jeong, K.-H.; Liu, G. L.; Chronis, N.; Lee, L. P. Opt. Expr. 2004, 12, 2494. (3) Sulchek, T.; Hsieh, R.; Adams, J. D.; Minne, S. C.; Quate, C. F.; Adderton, D. M. ReV. Chil. Pediatr. ReV. Scientific Inst. 2000, 71, 2097. (4) Xia, Y.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, 153. (5) Bodas, D.; Khan-Malek, C. Sens. Actuators B 2007, 128, 168. (6) Jacobs, H. O.; Whitesides, G. M. Science 2001, 291, 1763. (7) Effenhauser, C. S.; Bruin, G. J.; Paulus, A.; Ehrat, M. Anal. Chem. 1997, 69, 3451. (8) Peterson, S. L.; McDonald, A.; Gourley, P. L.; Sasaki, D. Y. J. Biomed. Mater. Res. 2005, 72A, 10. (9) Morefield, S. I.; Keefer, E. W.; Chapman, K. D.; Gross, G. W. Biosens. Bioelectron. 2000, 15, 383. (10) Andersson, H.; van den Berg, A. Lab Chip 2004, 4, 98. (11) Atsuta, K.; Noji, H.; Takeuchi, S. Lab Chip 2004, 4, 333.
(12) De Silva, M. N.; Desai, R.; Odde, D. J. Biomed. MicrodeV. 2004, 6, 219. (13) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Science 1997, 276, 1425. (14) Papra, A.; Bernard, A.; Juncker, D.; Larsen, N. B.; Michel, B.; Delamarche, E. Langmuir 2001, 17, 4090. (15) Ostuni, E.; Kane, R.; Chen, C. S.; Ingber, D. E.; Whitesides, G. M. Langmuir 2000, 16, 7811. (16) Ryu, K. S.; Wang, X.; Shaikh, K.; Liu, C. J. Microelectromech. Syst. 2004, 13, 568. (17) Pawlowski, A.; Sayah, A.; Gijs, M. J. Microelectromech. Syst. 2005, 14, 619. (18) Garra, J.; Long, T.; Currie, J.; Schneider, T.; White, R.; Paranjape, M. J. Vac. Sci. Technol., A 2002, 20, 975. (19) Childs, W. R.; Nuzzo, R. G. J. Am. Chem. Soc. 2002, 124, 13583. (20) Childs, W. R.; Nuzzo, R. G. AdV. Mater. 2004, 16, 1323. (21) Childs, W. R.; Nuzzo, R. G. Langmuir 2005, 21, 195.
10.1021/la803046j CCC: $40.75 2008 American Chemical Society Published on Web 11/06/2008
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Figure 1. (a) Optical microscope image of the periodically poled LN sample; (b) schematic view of the process steps.
field of research, and recently, a so-called “bond-detach” method has been proposed for nanolithography patterning.22 In this paper, we propose a different approach for direct PDMS patterning. A lithography-based technique for realizing periodic patterning of thin-film PDMS structures onto lithium niobate (LN) substrates is presented and discussed. By exploiting surface functionalities and electrical potentials generated by the pyroelectric effect, the PDMS film is microstructured according to the wettability variation of the LN substrate. The appropriate thermal treatment applied to the crystal induces the cross-linking of the PDMS film, leading to a stable and reliable PDMS pattern. The technique presented here takes advantage of the pyroelectric effect to induce electrostatic fields able to guide self-assembly, while the periodicity of the structures is achieved by using the periodic reversed domains fabricated in congruent z-cut LN wafers.
Experimental Section Both sides polished and 500-µm-thick LN crystals (from Crystal Technology, Inc.) were subject to standard electric field poling23,24 in order to achieve a square array of hexagonal reversed domains. The samples were first resist patterned (photoresist Shipley S1813J2, around 1.3 µm thick) by conventional mask lithography in order to achieve a square array of circular resist openings. The subsequent application of high-voltage pulses allowed fabrication of the periodic domain reversed sample. The period of the structure was around 200 µm along both x and y crystal axis directions, and Figure 1a shows the optical microscopy image of the sample just after poling. Figure 1b shows schematically the process steps implemented for direct patterning of PDMS films. A layer of PDMS polymer solution (Dow Corning Sylgard 184, 10:1 mixing ratio base to curing agent) was spun onto the original z- face of the PPLN substrate at 6000 RPM for 2 min. The PDMS-coated sample was then placed onto a hotplate at a temperature of 170 °C for 30 s, thus inducing rapid heating of the sample. Theoretical Basis. The PDMS patterning is related to the spatial wettability variation of the LN substrate under the pyroelectric effect, as already investigated and demonstrated in a previous paper by the same authors,25 where thin films of oily substances were patterned onto PPLN crystals through the electrowetting process induced by the pyroelectricity. The pyroelectricity exhibited at room temperature by LN26 makes the spontaneous polarization Ps change according to ∆Pi ) pi∆T where Pi is the coefficient of the polarization vector, pi is the pyroelectric coefficient, and ∆T is the temperature variation. At equilibrium, the whole Ps in the crystal is fully screened by the (22) Thangawng, A. L.; Swartz, M. A.; Glucksberg, M. R.; Ruoff, R. S. Small 2007, 3, 132. (23) Yamada, M.; Nada, N.; Saitoh, M.; Watanabe, K. Appl. Phys. Lett. 1993, 62, 435. (24) Grilli, S.; Paturzo, M.; Miccio, L.; Ferraro, P. Meas. Sci. Technol. 2008, 19, 074008. (25) Ferraro, P.; Grilli, S.; Miccio, L.; Vespini, V. Appl. Phys. Lett. 2008, 92, 213107. (26) Bourim, E. M.; Moon, C.-W.; Lee, S.-W.; Yoo, I. K. Phys. B: Condens. Matter 2006, 383, 171.
Figure 2. (a) Schematic view of the surface charge formation under thermal treatment; (b) simulation of the electric potential distribution generated by the pyroelectric charges.
external screening charge σ, and no electric field exists. The polarization intensity decreases under heating and increases under cooling, thus causing an excess or lack of surface charge, respectively, as shown schematically in Figure 2a. These surface charges generate high electric field distributions, which have been investigated and simulated already by the authors in ref 25. For the sake of clarity, the main simulation results obtained therein are reported in this paper. In particular, Figure 2b shows the side view of the sample with the numerical simulation of the electric potential distribution induced into the liquid by the pyroelectric charges. The hexagonal reversed domains correspond to the narrower regions into the scheme of the crystal cross section. It is important to note that the potential is distributed according to the reversed domain pattern with higher intensities corresponding to the regions outside the hexagonal domains. Consequently, the surface tension at the PDMS-crystal interface varies according to an electrowetting-like effect, where the electric fields are generated by the uncompensated surface charges produced during the thermal treatment. In fact, it is well-known that, in the general case of a sessile droplet, the surface tensions at the solid-liquid γsl, solid-gas γsg, and liquid-gas γlg interfaces are described by the one-dimensional Young equation
γsl + γlg cos ϑ ) γsg
(1)
where ϑ corresponds to the contact angle of the droplet. The charges at the solid-liquid interface reduce the surface tension according to the Lippman equation27
γsl(V) ) γsl0 -
1 2 cV 2
(2)
where γsl0 corresponds to zero charge condition, V is the electric potential, and c is the capacitance per unit area, assuming that the charge layer can be modeled as a symmetric Helmholtz capacitor.28 Even though the surface here was not a metal and the liquid was not an electrolyte, as assumed by the double charge model,29 a similar model still describes the effect in case of dielectric surfaces.29
Results and Discussion Figure 3a,b shows the optical microscope images of the cured PDMS layer pattern, under bight field and crossed polarizers, respectively. The PDMS layer clearly experienced a surface profile modification during the polymer state according to the underneath domain geometry. Successively, the progressive heating of the sample made the cross-linking of PDMS occur soon after the profile formation, thus freezing the periodic array of PDMS microbumps with intermediate wells. The same experiment was performed by coating the original z+ face of the PPLN sample, and the same results were obtained. The surface charge in excess, produced during heating, is no longer attracted by the crystal polarization charge. Consequently, an electric potential arises and modulates the surface tension at the PDMS-crystal interface, through the electrical interaction with the dipole molecules into (27) Lippmann, M. G. Ann. Chim. Phys. 1875, 5, 494. (28) Colgate, E.; Matsumoto, H. J. Vac. Sci. Technol., A 1990, 8, 3625. (29) Mugele, F.; Baret, J.-C. J. Phys.: Condens. Matter 2005, 17, R705.
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Figure 3. Optical microscopy image of the PDMS periodic pattern (a) under bright field and (b) crossed polarizers vision.
Figure 5. Sequence of four optical microscope images acquired during sample cooling after heating at 100 °C for 30 s. Figure 4. Schematic view of (a) the microbump array and of (b) the profile along the black line corresponding to a single microbump.
the polymer solution, according to the eq 2. Therefore, the PDMS layer assumes a wave-like profile as shown in the schematic 3D and profile views in Figure 4a,b, as a result of the equilibrium condition between the surface tension and those electric forces. It is important to note that, even though not shown completely into the pictures, due to a limited field of view, the array of microbumps is extended homogeneously over areas around 1 cm2 large, giving about 700 holes-bumps structures. Different surface-charge lithography experiments were performed by varying both the temperature and the time of the thermal treatment, and the values at 170 °C and 30 s appeared to be the best settings for activating the pyroelectric effect, through the abrupt temperature increase of the substrate, as well as to ensure the cure of the PDMS layer with the modified profile. Another experiment was performed to study the behavior of the PDMS layer under non-cross-linking conditions. The PDMScoated PPLN crystal was subject to thermal treatment at 100 °C for 30 s, and Figure 5 shows a sequence of four optical microscopy images acquired soon after removal from the hot plate, namely, during cooling. The polymer solution appears to exhibit a twostep profile modulation with wettability reduction occurring first outside and then inside the hexagonal regions. The PDMS layer kept the geometry in Figure 5d for about 1 min and then relaxed back to the initial condition (flat surface). This experiment makes evident the transient behavior occurring in PDMS during the polymer state under the action of the pyroelectric effect. A wettability reduction occurs outside the hexagons during the temperature increase, and Vice Versa during cooling. Therefore, the geometry achieved by the cured PDMS (see Figure 3) in the previous experiment corresponds to the profile generated by the first step of the wettability modulation. It is important to note that the wettability patterning of LN was studied in ref 25 in the case of oily substances, which exhibit transient modifications of the surface profile. Those results demonstrated the proof of principle of the wettability modulation through the pyroelectric effect. In this paper, such wettability modulation is exploited in order to achieve direct and permanent patterning of PDMS layers for applications in the fields of microfluidics as well as in biomedicine and biology. The lithography method presented here appears to exhibit various
differences and advantages compared to the conventional PDMS patterning techniques. This method can be considered costeffective and relatively quick, since no sophisticated process steps, such as atmosphere and pressure control, or UV treatment are required by the procedure. In fact, nowadays, standard PPLN crystals are routinely fabricated through relatively cheap processes, while PDMS coating and thermal treatments can be performed by conventional spin-coaters and hot plates. Moreover, the PDMS patterning methods generally involve soft lithography making use of specific molds,4 which can in principle transfer surface imperfections onto the PDMS layer profile. Conversely, the unique feature of this lithography technique to modify the PDMS profile by a mold-free process based on self-assembly, which manipulates the LN wettability related to the polymer solution, enables the formation of smooth surfaces generated at the air-fluid interface. Wet chemical etching and dry (plasma) etching have been used to pattern PDMS,18 but the patterned PDMS had a very high surface roughness that is likely to prevent its use for some applications. Therefore, the PDMS patterns obtained here could be used also as alternative molds for nonconventional soft lithography applications. The extensive homogeneity of the PPLN crystals available nowadays, and the inherent flexibility in the choice of the desired geometries, makes this technique interesting for applications in the field of biology, such as for cell patterning, where high-throughput screens into single integrated microfluidic devices are desirable and the additional chemical patterning of the substrate could be avoided. Moreover, the possibility to pattern PDMS layers directly onto LN substrates could open the way for a variety of applications in microfluidics and biology, where the chemical stability and the specific properties (piezoelectricity; electro-optics; pyroelectricity; nonlinearity; etc.) of the bare LN regions could be exploited for innovative operational functions. Other possible applications of this technique include microlens arrays and liquidfilled microlens arrays for integrated optical devices.
Conclusions The direct patterning of PDMS thin films was shown to be possible by a relatively simple procedure consisting of a surfacecharge lithography process making use of the pyroelectric effect in PPLN crystals and capable of inducing self-assembly. Periodic square arrays of PDMS microbumps, with intermediate wells,
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have been fabricated by an appropriate thermal treatment providing both surface modification and thermal curing of the PDMS layer. This lithographic technique could be a quick and cheap alternative to the conventional PDMS patterning methods which generally use external forces, load/pressure, or sophisticated surface treatments. Since shorter period gratings can be fabricated in LN substrates, this technique is expected to provide PDMS
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patterns with improved resolution and higher aspect ratio, and further experiments are under investigation. Potential applications of the technique are in the field of biology and biomedicine for fabricating large areas of adhesion/growth site arrays for cells analysis. LA803046J
