3094
Langmuir 2003, 19, 3094-3097
Multilayer Line Micropatterning Using Convective Self-Assembly in Microfluidic Channels Hongseok Jang, Sangcheol Kim, and Kookheon Char* School of Chemical Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-gu, Seoul 151-744, Korea Received November 1, 2002. In Final Form: January 15, 2003 A new approach to create layer-by-layer assembled multilayer ultrathin films with well-defined micropatterns in a short process time is introduced. To achieve such micropatterns with high line resolution in organic multilayer films, microfluidic channels were combined with the convective self-assembly process employing both hydrogen bonding and electrostatic intermolecular interactions. The channels were initially filled with polymer solution by capillary pressure, and the residual solution was then removed by the spinning process. Micropatterns with clear line boundaries were obtained, and small ridges were also observed at the edges of the patterned lines.
1. Introduction Since the ionic layer-by-layer assembly technique was first introduced for the fabrication of polyelectrolyte multilayers,1 this self-assembling technique has been extended to conducting polymer composites,2 nonlinear optical dyes,3 and the assembly of biomolecular systems.4 Furthermore, other driving forces such as hydrogen and chemical bonding for the self-assembly have been explored.5 For the application of organic or organic/inorganic hybrid multilayer thin films to high performance devices, the internal structure of the films should be on the degree of high molecular order and the ability to pattern with at least micrometer scale features on the films is also required. Up until now, several methods for multilayer patterning have been reported.6-11 In these methods, however, chemically patterned templates should be prepared on the substrates because the adsorption process is based on self-diffusion and rearrangement due to the interactions between adsorbing molecules and templates. To achieve selective deposition, alternating regions of chemically different functionalities should be introduced on a surface: one region promotes the adsorption and the other effectively resists the adsorption of polyions on the surface. According to the recent work demonstrated by Cho et al., multilayer films with highly ordered internal structure could be easily prepared by the spin self-assembly (SA) process although it is much simpler and faster in preparation than the conventional dip self-assembly * To whom correspondence should be addressed. E-mail: khchar@ plaza.snu.ac.kr. (1) Decher, G. Science 1997, 277, 1232. (2) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985. (3) Lenahan, K. M.; Wang, Y. X.; Liu, Y. J.; Claus, R. O.; Heflin, J. R.; Marciu, D.; Figura, C. Adv. Mater. 1998, 10, 853. (4) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (5) Wang, L.; Wang, Z.; Zhang, X.; Chi, L.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (6) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862. (7) Hammond, P. T.; Whitesides, G. M. Macromolecules 1995, 28, 7569. (8) Clark, S. L.; Hammond, P. T. Adv. Mater. 1998, 10, 1515. (9) Ghosh, P.; Lackowski, W. M.; Crooks, R. M. Macromolecules 2001, 34, 1230. (10) Gao, M.; Sun, J.; Dulkeith, E.; Gaponik, N.; Lemmer, U.; Feldmann, J. Langmuir 2002, 18, 4098. (11) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100.
process.12 In this letter, we describe a new approach to produce well-defined micropatterns of multilayer ultrathin films in a short process time. Microfluidic channels were used to achieve such micropatterns with high line resolution, in combination with the convective SA process (i.e., the self-assembling process in the presence of external flow) employing both electrostatic and hydrogen bonding intermolecular interactions. We also demonstrate that the pattern size of the multilayer films could be reduced to a submicrometer. 2. Experimental Section Poly(4-vinylpyridine) (PVP, Mw ) 60 000), poly(acrylic acid) (PAA, Mv ∼ 450 000), poly(diallyldimethylammonium chloride) (PDAC, Mw ) 100 000-200 000), and poly(sodium 4-styrenesulfonate) (PSS, Mw ) 70 000) were purchased from Aldrich and used without further purification. All the polymer solutions were 10 mM (repeat unit basis) in Milli-Q water, absolute ethanol, and N,N-dimethylformamide (DMF). The COOH-terminated selfassembled monolayer (SAM) substrates were prepared by immersing gold-coated silicon wafers into 5 mM 16-mercaptohexadecanoic acid (Aldrich) in absolute ethanol for 30 min. The multilayer films were analyzed by atomic force microscopy (AFM) (Digital Instruments, Nanoscope IIIa).
3. Results and Discussion A schematic of the micropatterning procedure of multilayer films is illustrated in Figure 1. The patterning is performed in two steps: polymer solution is first allowed to fill the microfluidic channels, which are formed by contact between an elastomeric mold and a hydrophilic substrate, by capillary pressure. Filling the channel with a polymer solution also enables the polymer to adsorb onto the surface, and the removal of residual polymer solution is then carried out by the spinning process. These two steps are repeated for a predetermined number of bilayers. When a polymer solution fills a microfluidic channel with a rectangular cross section, the time τ taken to travel the channel with a length L is given by the following equation:13 (12) Cho, J.-H.; Char, K.-H.; Hong, J.-D.; Lee, K.-B. Adv. Mater. 2001, 13, 1076. (13) Delamarche, E.; Bernard, A.; Schmid, H.; Bietsch, A.; Michel, B.; Biebuyck, H. J. Am. Chem. Soc. 1998, 120, 500.
10.1021/la026788b CCC: $25.00 © 2003 American Chemical Society Published on Web 03/11/2003
Letters
Langmuir, Vol. 19, No. 8, 2003 3095
Figure 1. Schematic of micropatterning of multilayer films using the convective self-assembly process.
τ)
ηL2 2∆PCg
where η is the viscosity of the polymer solution, ∆P is the pressure drop across the channel, and Cg is the geometric factor of the capillary channel. Generally, we note that the channel is filled in a short time as far as the solvent preferentially wets the channel surface. For the case of DMF as a solvent, the time taken for complete filling was observed to be within 3 s, which is in agreement with the time estimated by assuming ηDMF ) 0.794 × 10-3 kg/(m s), ∆PDMF ≈ 48 kPa, and Cg ≈ 1.4 × 10-13 m2 (10 µm width, 1.2 µm height, 5 mm length channel). The multilayers in the microfluidic channels are obtained by the convective SA method through the adsorption and rearrangement of polymer chains onto a substrate and the desorption of weakly bound chains in the presence of a shear field. In the spin SA process without mold confinement, there are several driving forces such as centrifugal force, viscous force, and air shear force. Because the removal of the polymer solution in the microfluidic channels is restricted by the confined geometry, these forces are dependent on the position of the mold on a substrate. Typically, in the present work, the mold is located near the rim of the SAMcoated wafer and the channel direction is aligned in parallel with the radial direction. However, these constraints on the location and the orientation of the microfluidic channels are not strict. We believe that
multilayer patterns with other geometries can be realized by using sophisticated microfluidic devices.18 By using the microfluidic channels, the patterning of multilayer films with alternating PVP and PAA layers was performed. The AFM topography images of 5, 8, and 20 bilayers consisting of PVP and PAA are shown in Figure 2a-c. Micropatterns with clear edge boundaries were obtained, and small ridges were observed at the edges of the patterned lines. This patterning method was applied to large area patterning in a cm2 scale and is quite reproducible. We investigated the patterning method in detail for different bilayer numbers (over 20), polymer solution pairs based on different intermolecular interactions, and different substrates. Preliminary results using a bare Si wafer and a quartz slide are similar (data not shown). In the case of the microfluidic channels employed in the present work, the ridge formation is due to the wetting of a solvent on the poly(dimethylsiloxane) (PDMS) walls.14 When ethanol with good wettability on the PDMS walls is used as a solvent, the height of ridges is even increased 7 times. This is consistent with the fact that the contact angle of ethanol on PDMS is 31°, much lower than that of DMF (i.e., 63°).15 For the (PVP/PAA)5, (PVP/PAA)8, and (PVP/PAA)20 multilayers, the feature heights were found to be about 12.1, 17.3, and 48.2 nm, respectively. (14) Furuki, M.; Kameoka, J.; Craighead, H. G.; Isaacson, M. S. Sens. Actuators, B 2001, 79, 63. (15) Jackman, R. J.; Duffy, D. C.; Ostuni, E.; Willmore, N. D.; Whitesides, G. M. Anal. Chem. 1998, 70, 2280.
3096
Langmuir, Vol. 19, No. 8, 2003
Letters
Figure 2. AFM images of patterned multilayer films of (a) (PVP/PAA)5, (b) (PVP/PAA)8, and (c) (PVP/PAA)20 in micrometer scale. (d) Thickness as a function of the number of bilayers for (i) unpatterned films and (ii) patterned films.
Figure 3. AFM images of (a) a PDMS mold used and (b) a patterned multilayer film of (PVP/PAA)5 in submicrometer scale.
The surface roughnesses of the films are 4, 6, and 20 Å, respectively, which are much smaller than the film thickness. The average bilayer thickness is about 24 Å, and this thickness is lower than that of unpatterned spinassembled multilayer film (i.e., 32 Å) as shown in Figure 2d. For the deposition of both patterned and unpatterned multilayers, the convective self-assembly process was employed (i.e., the capillary flows and the spin removal of residual solution for patterned multilayers and the spinassembly for unpatterned multilayers, at 6000 rpm for 2 min) and there is one washing step with pure DMF solvent at the same condition. In spin-coating, in general, the final film thickness is strongly dependent on spin speed and solution viscosity. For a given spin speed, the film thickness drops rapidly at first due to the contributions from radial convective flow and then reduces slowly at
longer times. In the later stage, the thickness reduction is dominantly affected by the solvent evaporation.16,17 During the spinning process in microfluidic channels, however, the removal of excess polymer solution is severely retarded due to the capillary force in the channel. This slower drying process prevents the increase in the solution viscosity, and thus the convective loss is dominant during the entire spinning process. As a result, a film that is thinner than the unpatterned film is believed to be obtained. The method proposed here can easily control the pattern size of multilayer films by varying the channel size in the (16) Flack, W. W.; Soong, D. S.; Bell, A. T.; Hess, D. W. J. Appl. Phys. 1984, 56, 1199. (17) Lawrence, C. J. Phys. Fluids 1988, 31, 2786. (18) Thorsen, T.; Maerkl, S. J.; Quake, S. R. Science 2002, 298, 580.
Letters
Langmuir, Vol. 19, No. 8, 2003 3097
Figure 4. AFM (a) plane view image and (b) side view image of a patterned multilayer film of (PDAC/PSS)5 based on electrostatic interaction.
PDMS mold. Figure 3 shows the PDMS mold used for the submicrometer patterning and the pattern of (PVP/PAA)5 multilayer films. The multilayer pattern with round tips is due to the negative image of the PDMS mold employed. The patterning methods of multilayers previously reported were developed for specific interactions. In other words, a template to direct the deposition of ionic multilayers should be specifically modified for the patterning of multilayers based on hydrogen bonding or chemical bonding. However, our method of micropatterning with microfluidic channels combined with the convective SA process is not dependent on the specific intermolecular interactions forming multilayers as far as the channel is marginally wettable. PDAC and PSS were used in this study as the cationic and anionic polymers in aqueous solution, and a patterned multilayer was obtained with the same PDMS mold. Figure 4 shows the AFM image of the patterned (PDAC/PSS)5 film in aqueous solution. The feature height of the film is 7.0 nm, lower than that of (PVP/PAA)5, but no ridge is observed. We believe that the disappearance of the ridge at the edge boundary is related to the poor wettability of the water solvent on the PDMS mold (i.e., θPDMS ) 108°), but the water is still a allowed to flow within the microchannel due to the hydrophilic bottom surface.
4. Conclusion We have demonstrated the feasibility of a new patterning method of multilayer films, which has several attractive features: (1) this new patterning method is flexible in the choice of substrate, and a chemically patterned template is not required. In principle, any kind of substrate can be utilized if the microfluidic channels are marginally wettable with a given solvent. Furthermore, all the multilayer films based on hydrogen bonding, electrostatic attraction, and chemical bonding can be patterned. (2) It also allows us to precisely predict and control the bilayer thickness and the surface roughness by using the convective SA process. (3) Perfect selectivity is given by the physical confinement of the microfluidic channel. (4) Submicropatterning is also possible using a mold (i.e., barrier) with controlled geometric structure. (5) The deposition of multilayers is achieved with a small amount of solution in a fast process time. Acknowledgment. This work was financially supported by the National Research Laboratory Program (Grant M1-0104-00-0191) and funded in part by the Ministry of Education through the Brain Korea 21 Program at Seoul National University. We acknowledge Dr. Jinhan Cho and Mr. Bongwoo Ha for helpful discussions and Dr. Younsang Kim and Mr. Ohjoong Kwon for material preparations. LA026788B
