Article pubs.acs.org/Langmuir
Hierarchical Micro/Nano Structures by Combined Self-Organized Dewetting and Photopatterning of Photoresist Thin Films Priyanka Sachan, Manish Kulkarni, and Ashutosh Sharma* Department of Chemical Engineering & Center for Nanosciences, Indian Institute of Technology Kanpur, Kanpur, India 208016 ABSTRACT: Photoresists are the materials of choice for micro/nanopatterning and device fabrication but are rarely used as a self-assembly material. We report for the first time a novel interplay of self-assembly and photolithography for fabrication of hierarchical and ordered micro/nano structures. We create self-organized structures by the intensified dewetting of unstable thin (∼10 nm to 1 μm) photoresist films by annealing them in an optimal solvent and nonsolvent liquid mixture that allows spontaneous dewetting to form micro/nano smooth dome-like structures. The density, size (∼100 nm to millimeters), and curvature/contact angle of the dome/droplet structures are controlled by the film thickness, composition of the dewetting liquid, and time of annealing. Ordered dewetted structures are obtained simply by creating spatial variation of viscosity by ultraviolet exposure or by photopatterning before dewetting. Further, the structures thus fabricated are readily photopatterned again on the finer length scales after dewetting. We illustrate the approach by fabricating several threedimensional structures of varying complexity with secondary and tertiary features.
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INTRODUCTION Photoresists are most commonly used for fabricating a variety of micro/nanopatterned templates. Their ability to undergo chemical changes upon ultraviolet (UV)/e-beam exposure makes them highly useful in various applications in which microstructures are required for further processing. Because these resists are specifically synthesized for explicit patterning purposes, it is understandable that much work has been done on their pattern generation via optical methods and related applications. They are stable at high temperatures relative to other soft materials, compatible with a variety of commonly used chemicals, and thus of much use in microelectronics, sensors, biodetection, etc.1−6 However, photoresist materials are almost exclusively used in top-down methods such as photolithography with little efforts devoted to leveraging their possible self-assembly properties, such as spontaneous dewetting of thin polymer films to form small scale structures. The objective of this work is to develop an efficient method of selforganized dewetting of photoresist films together with a pre- or post-UV/e-beam processing, thus combining the top-down and bottom-up methods for the creation of ordered, hierarchical micro/nano structures. Photoresists are tailored to be sensitive to radiation of a particular wavelength and specialized optics, placing a restriction on their patterning by any other wavelength source or method. This, in turn, restricts the pattern area, shapes, and size domains that can be attained with a given photoresist. Further, while the structures with sharp edges are readily produced by conventional photopatterning, smooth surfaces such as those in microlenses are not directly obtained. In this work, we propose and demonstrate a new method of photoresist patterning to produce highly smooth, high-contact angle domes of a photoresist, Shipley, using the concept of self© 2015 American Chemical Society
assembly. The key ingredient of this approach is the spontaneous breakup and dewetting of a spin-coated thin film under specific conditions that allow rapid polymer mobility and its reorganization. Previously, suppression of dewetting in conventional polymers via cross-linking7 and dewetting of a photoresist on a PDMS substrate have been reported.8 Electric field-induced patterning of photoresists has also been explored as an alternative route to achieving microstructures in thin photoresist films.9 Dewetting of polymer thin films by thermal and solvent vapor annealing is a well-explored area.10−18 Recently, a new protocol in which the air−polymer interface is replaced by a liquid−polymer interface by immersion of the polymer film in a liquid mixture of an appropriate solvent and nonsolvent was described.19−21 Upon being exposed to a good solvent, polymer chains become mobile, thus bringing the glass transition temperature below room temperature.22−25 We first determine here a solvent−nonsolvent mixture that engenders self-organized dewetting of a positive tone photoresist, Shipley, which has seen widespread use in patterning applications. We show that perfectly ordered nano/micro domes can be produced by dewetting the thin films of Shipley (20 nm to 2 μm thickness), which can further be subjected to conventional photolithography to create multiscaled structures that mimic the hierarchical structures found in nature.2,26−30 Because of the positive tone of Shipley toward UV light, it is possible to expose it repeatedly to obtain multilevel/ordered patterning. This combination of self-assembly with developmental lithography uniquely produces convex nano/micro-sized spheres with controllable high contact angles and periodicity over a large Received: August 10, 2015 Revised: October 11, 2015 Published: October 21, 2015 12505
DOI: 10.1021/acs.langmuir.5b02977 Langmuir 2015, 31, 12505−12511
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Langmuir area with minimal effort and without involving complex micromachining. With a simple PDMS replication, these textures can be transferred onto any other desired soft material, as well. Although homopolymers such as PS and PMMA have been chemically patterned using UV and other surface energy modification techniques, they are mostly reorganizational. This work provides a missing link between patterning a polymer film via both reorganization (dewetting) and selective removal (developing).
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EXPERIMENTAL DETAILS
Shipley photoresist (MICROPOSIT, S1800) and its thinner used in this work were procured from Microposit (Shipley 1818). Chemicals acetone and methyl ethyl ketone (MEK) were procured from SigmaAldrich and were HPLC grade. Prior to dewetting, all processing was done in an UV protective fumehood to prevent any changes in the photoresist due to UV exposure. Films of different thicknesses were spin coated at 3000 rpm for 30 s on thoroughly cleaned glass surfaces, then soft baked on a hot plate for 60 s at 90 °C, and then annealed at 60 °C for 1 h to minimize solvents as well as residual stress. The film thickness was measured using an ellipsometer (EP3, Nanofilm) and varied from 20 nm to 2.2 μm. Dewetting of these films was performed by using an optimized solvent−nonsolvent mixture containing methyl ethyl ketone (MEK), acetone, and water in a 14:6:80 ratio. Samples of Shipley films were immersed in this dewetting solvent (DS) for a few minutes (2−5 min). In situ dewetting dynamics was studied using a large working distance objective-fitted Leica microscope. The experimental protocol is shown in Figure 1 in its entirety. Thermal annealing-induced dewetting of the photoresist films in vacuum was also attempted for comparison with the liquid-annealed dewetting. For the thermal dewetting trials, films were kept in a vacuum oven at 170 °C for extended periods of time. Photoresist films were exposed to the UV light using a maskless photolithography system (Intelligent Micro Patterning) with a 434 nm wavelength filter. The exposure time varied from 0.5 to 1.5 s depending upon the film thickness and pattern size. UV exposure created a spatial variation in the polymer viscosity. After UV exposure, instead of conventional post baking, developing, and selective polymer dissolution, the samples were directly immersed in the dewetting mixture to create microdroplets. These samples containing microdroplets can then be exposed to UV radiation to create secondary and tertiary patterns on top of the droplets. These secondary and tertiary patterns are selectively washed in the commercially available developer of Shipley, resulting in arrays of microdroplets having multiple patterns written on them. Images of dewetted samples were taken using an optical microscope (Leica) and a field emission scanning electron microscope (FESEM) (Zeiss, Supra 40VP).
Figure 1. Experimental protocol for creating hierarchical, threedimensional microstructures in the Shipley resist. Dark lines in panel A are UV exposure lines on the Shipley film that, upon dewetting, create arrays of ordered microdroplets in panel B. Further exposing these droplets to a UV source and developing them subsequently give us textured arrays of microdroplets as shown in panel C.
Thus, spatially selective UV exposure of the photoresist modulates the viscosity locally and allows spatially variable dewetting leading to ordered structure formation. Here, the photoresist film is liquefied by immersing it in a mixture of good solvents and a nonsolvent. Acetone and MEK are good solvents of Shipley and readily diffuse in the film and, at the same time, reduce its interfacial tension by ∼2 orders of magnitude, which minimizes the stabilizing influence of surface tension in pattern formation.14,19,21,34 The added nonsolvent, water, which is also the majority phase in the annealing liquid, prevents polymer dissolution. The presence of a liquid mixture (which has good affinity and wetting for the substrate) also increases the contact angle of the polymer on the substrate, thus increasing the aspect ratio of droplets compared to that of dewetting in vacuum. The evolving structures in the photoresist can be arrested and made permanent at any stage of evolution by removing the sample from the annealing mixture and gently drying it in a stream of air. Spatiotemporal Evolution of Structures. All films regardless of their thickness show qualitatively similar dewetting behavior when immersed in a dewetting solution. The appearance and growth of holes in a 280 ± 5 nm Shipley film on a Si substrate were recorded for the first 3 min (Figure 2). Upon immersion in dewetting media, solvents MEK and acetone diffuse into the polymer matrix and lower its viscosity. The rate of increase in the area of a random hole slows with time. Droplets are formed either by the residual photoresist
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RESULTS AND DISCUSSION Thin liquid films on nonwettable surfaces are inherently unstable or metastable because the free energy of the droplet configuration is lower than that of the film. Instability and dewetting of the thin film are initiated by the intersurface attractive forces such as van der Waals, electrostatic, etc., that become stronger with a local reduction in film thickness. Thus, any preexisting inhomogeneities or thermal fluctuations are able to amplify and lead to dewetting.31−33 In unstable thin liquid films, dewetting engendered by the growth of the surface instability leads to the formation of randomly located holes, but with a mean spacing (length scale of instability) determined by the film thickness and interfacial tension.10,11,19,21,33 The holes grow and coalesce to form liquid ribbons and droplets. A smaller thickness and interfacial tension engender finer structures with smaller spacing. The kinetics of dewetting depends additionally on the liquid viscosity with the appearance and growth of holes being faster at the lower-viscosity domains. 12506
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considerably above the Tg for more than 70 h were still found to be stable, which rules out thermally induced dewetting in this case. Length Scale of Dewetting. The characteristic dewetting length scale of a thin film is related to its thickness by a power law variation in cases in which thermal annealing is used.10,12 As discussed, however, the photoresist films were stable against thermal annealing (at 170 °C for 72 h) well beyond their glass transition temperature (∼135 °C). In contrast, the surface instability manifested promptly upon immersion in the solvent mixture. The composition of this solution was optimized on the basis of the coupled requirement of sufficient mobility (engendered by the presence of a good solvent’s diffusion into film) and the absence of polymer film dissolution and detachment from the substrate. This is ensured by keeping the proportion of solvents (acetone and MEK) much lower than that of the nonsolvent water. Over prolonged periods of time (>100 h), no significant loss of material was observed with the mixture composition used, a 7:3:40 MEK/acetone/water mixture. Figure 3 shows the variations of the mean interdroplet distance and drop diameter with film thickness. This figure also shows large area views of the dewetted structures and the shape of a typical droplet. For thin viscoelastic films, dewetting wavelength λ (defined as the average distance between two holes) is dependent on the destabilizing intersurface interactions such as the van der Waals and electrostatic forces:
Figure 2. Micrographs of ∼284 nm Shipley film dewetting in a solvent−water mixture as time advances (inset showing the duration of immersion). Samples were imaged live under the dewetting solution (the intact film shows a slightly red hue; Shipley droplets that accumulated are deeper red, while the exposed substrate is bright white). Each micrograph is 372 μm × 279 μm in size.
detaching from the receding rim front or by coalescence of the rim boundaries. Upon attaining a final spatial architecture in 2− 3 min, these droplets undergo a curvature increment because of to their large equilibrium contact angle in the liquid mixture, which allows creation of micro/nano spheres with tunable contact angles as large as ∼140°, depending on the immersion time. The contact angle of the polymer droplets increases with time in the liquid medium because the initial angle at the time of drop formation is smaller than the equilibrium contact angle, which is large because of the higher affinity of the liquid medium for Si compared to that of the polymer. It may be noted that the shape tuning in a wide range of contact angles cannot be achieved by thermally induced or solvent vaporinduced dewetting as the contact angles in air are much smaller. In fact, the photoresist films when thermally annealed
⎛ ∂Φ ⎞ ⎟ λ 2 = 8πγ 2/⎜ − ⎝ ∂h ⎠
where Φ is the destabilizing potential per unit volume, h is the film thickness, and γ is the interfacial tension between the film and dewetting media. It has been shown previously that this thermodynamic result gives a power law dependence of λ on film thickness.12,14,20,35 On the basis of the best fit of the data on the dewetting length scales, the wavelength of dewetting is related to the film thickness by λ ∝ h1.13±0.06. Naturally, the mean diameter of the
Figure 3. (A and B) Optical micrographs of 375 ± 12 and 165 ± 5 nm Shipley films dewetted under a solvent−water mixture. (C) Magnified transverse FESEM image of a droplet obtained from dewetting an ∼28 nm film. Scale bars are 50 μm in panels A and B and 200 nm in panel C. (D) Dependence of the dewetting length scale on film thickness on a log−log scale. 12507
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In case of relatively thicker films (>1 μm), some of the dewetted structures are large enough (≤1 mm) for the gravity (buoyancy) to overcome adhesion, and as a result, the largecontact angle droplets detach and float to the liquid surface. To avoid the loss of material by dissolution, the dewetting solution was diluted to up to 95% water after the initial dewetting stages were complete. This procedure removes much of the trapped solvent from the film and makes the polymer droplets more solid and hence less mobile and more stable on the substrate. After incubation for ∼5 min, samples were taken out and dried gently without disrupting the dewetted spheres. Multiscale Ordered Micro/Nano Structures by Combined Photoexposure and Dewetting of Films. Shipley films can be patterned using maskless photolithography (434 nm exposure filter). The exposure time varies from 0.25 to 1.5 s depending on the film thickness. A unique advantage of Shipley (or any other positive resist) is that, unlike the negative resist, the unexposed part after the film has been developed remains un-cross-linked and is almost identical to a nonpatterned film. Thus, it can be exposed further to the solvent mixture to generate a combination of lithographic and self-assembled hierarchical patterns. If the UV-exposed pattern length scale is close to the instability wavelength, the drop distribution is expected to show good alignment and order. If the two length scales are widely different, a disordered dewetting takes place as observed for non-photopatterned films discussed above. Interestingly, we observe that patterns up to 10 times smaller than the natural dewetting length scale could be prepared in prepatterned Shipley films. For example, an ∼300 nm thick film upon dewetting produces a mean droplet distance of ∼32 μm, while a grid exposure of 3 μm on the same film results in a rectangular array of dewetted droplets with a 3 μm periodicity. Similarly, patterns larger than the natural dewetting length scale can also be dewetted faithfully with pre-exposures. This broadening of dewetting length scales has also been previously noticed for dewetting of patterned (physical or chemical) PS and PMMA films.19,20,37 Also, a variety of fractal and complex patterns could be created and dewetted to obtain structures of mixed length scales. In Figure 5, a few such UV-written and then dewetted patterns are shown, indicating the efficacy of this method in creating complex fractal and Fibonacci arrangements. Significantly, using the UV prepatterning followed by dewetting, drops with a variety of diameters and volumes can be obtained for a film with a constant thickness (Figure 5, right bottom corner), whereas unpatterned films yield drops of similar length scales and volumes. Exposing Shipley to UV radiation generates a localized viscosity contrast because of chain scission, which is the reason for its selective removal (or developing) in a developer solution. When UV-exposed Shipley films are immersed in the dewetting solution instead of a developing solution, the UV-exposed regions of lower viscosity rupture faster. The different rates of dewetting introduced by the gradients of viscosity allow the formation of patterns based on the UV exposure pattern. Theoretically, the dewetting time scale of thin viscoelastic films is found to be related to its shear modulus, μ, and viscosity, η.31,38 Because the rupture occurs preferentially at exposed regions, dewetted droplets form at the center of unexposed regions. This behavior is found to be similar to dewetting of a positive tone e-beam resist, PMMA, upon exposure to an e-beam.37 Thus, lowering the viscosity, by photoexposure-induced chain scission in Shipley films, leads to confined film rupture and hence controls the placement of
dewetted droplets (for a particular thickness) is related to the corresponding wavelength by an overall mass balance (here, d ∝ h0.80±0.07). This dependence is found to be similar to the dependence previously observed in liquid immersion dewetting of polystyrene thin films.19 Experimental scaling of the dewetting diameter obtained from a destabilizing electrostatic van der Waals potential under water (Φ ∼ h−3) for PS is established to be d ∝ h1.17 and in vacuum d ∝ h1.49, which was observed theoretically in previous works, as well.10,19,20,35,36 The length scales of structures for Shipley are also found to be dependent on the composition of the dewetting solution used. It is observed that the dewetting solution containing a smaller fraction of solvents results in a reduced wavelength of dewetting. Figure 4 indicates the logarithmic length scales of a
Figure 4. Four micrographs are dewetting length scales of an ∼270 nm Shipley film dewetted in different solvent fractions (fraction given in insets) of a solvent−water mixture (solvent being the combination of MEK and acetone in a constant ratio of 7:3 as mixed in water). Each micrograph is 372 μm × 279 μm in size. The graph shows increased structure size and spacing with an increased fraction of the good solvents.
270 ± 5 nm Shipley film upon exposure to different compositions of a dewetting solution. This observation suggests that the relative rate of propagation of a hole front as opposed to the rate of eruption of a new hole slows upon exposure to a more dilute solution. The rate of eruption remains almost constant, while the rate of growth depends strongly on the proportion of the good solvent present (slower kinetics for less solvent). This means that for a smaller solvent fraction, more holes erupt in the time it takes for a film to grow holes, resulting in more nodes per unit area, which in turn translates into more droplets per unit area. This observation can be explained largely on the basis of the decrease in polymer mobility (increased viscosity). 12508
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Figure 5. Top two FESEM images that show an ∼30 nm thick dewetted film, without and with UV patterning (both scale bars of 2 μm). The rest are optical microscope images of a 165 ± 5 nm Shipley film prepatterned with maskless lithography and then dewetted to produce microdome arrays in various fractal arrangements. All scale bars in the last four optical images are 80 μm (the smallest feature visible in the last sample is 2 μm because of optical limitations).
Figure 6. Hierarchical patterns written on dewetted microdomes. (A) Optical images of a 165 ± 5 nm Shipley film prepatterned with maskless lithography, dewetted, and further patterned with UV lithography. (B−D) The 8 μm lines and/or 2 μm grid written on dewetted domes. (E and F) Magnified top surface of microdomes with 2 μm holes written on 8 μm channels and 15 μm holes, respectively, written on top of the dewetted droplets. All scale bars are 10 μm.
dewetted droplets upon completion of the dewetting process. A few interesting geometries and arrangements of dewetted droplets are shown in Figure 5. It may be noted that the difference in dewetting and rupture kinetics of exposed/unexposed films is the governing mechanism of the patterned dewetting. If there is no local differential in viscosity, the entire film dewets uniformly over the entire substrate, irrespective of the intensity of the spatially uniform exposure. Hierarchical Patterning of Dewetted Structures. As discussed above, a distinct advantage of a Shipley film is the ability to pattern and dewet photoresist structures in a desired sequence. Polymers like PS and PMMA can also be directly modified in their viscosity (chemical modifications) via UV exposure and then dewetted, but the changes thus created are small and hence cannot be further manipulated for selective etching or removal,39−41 as in a photoresist. A Shipley resist can be modified to an extent to allow its development into a pattern by selective dissolution. Because Shipley is a positive tone photoresist, its chemical structure remains unaltered through the developing (in its commercial developer) or dewetting (in our dewetting solution), unless it is exposed to a UV light source. This allows us to pattern the dewetted micro/nano domes further with UV photopatterning. This dewetting-based technique results in various hierarchically patterned domes. A few such patterns are reported in Figure 6. In all of these cases, the first order of patterning (such as that shown in Figure 6) is conducted on Shipley films dewetted into three-dimensional (3D) micro/nano domes and then dried onto a hot plate for 1 h at 60 °C. The domes/droplets are then exposed to a UV pattern (8 μm width and pitch channels or 15 μm diameter holes, for example) and developed into the developer solution for 30 s. After the samples had been dried on the hot plate
again, a secondary pattern (such as a 2 μm square grid or 3 μm holes in Figure 6) is exposed on them and then developed. This procedure can be reiterated to obtain multilayer patterning on 3D microdomes that would be almost impossible to achieve with simple photolithography. Panels E and F of Figure 6 are two representative examples of secondary and tertiary patterns, respectively, written on top of the dewetted microdomes. Structures such as those shown in panel F can be extended further and further to obtain hierarchical assemblies similar to that observed in nature. It is important to point out that conventional photolithography is limited by the resolution of incident light. Thus, the smallest secondary and tertiary patterns on top of dewetted droplets (as shown in Figure 6) are ≥1 μm. However, Shipley is an excellent e-beam resist as well and responds well to e-beam exposures for positive resist patterning. The compatibility of dewetted Shipley droplets has been tested for e-beam lithography, making submicrometer secondary/tertiary patterns on curved resist droplets a reality. Submicrometer patterns such as lines and dots on polymer curved surfaces may find use in various applications with functional materials patterned on a multiscale.42−46 An attractive feature of a dewetting−patterning methodology of a photoresist is its ability to make micro/nano patterns over a much larger area and iterative self-assembly lithography patterning that allows fabrication of 3D patterns with dimensions different from those of the template mask. In conventional lithography, the pattern area is limited by mask area and developed pattern replicates the original mask dimensions. To demonstrate the ability of our patterning method to be used in large, roll-to-roll processing, a large area 12509
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with a variety of diameters and volumes can be obtained for a film with the same thickness as opposed to a monomodal distribution of drops obtained for unpatterned films. Also, the positive tone of the Shipley resist allows further patterning of dewetted structures to produce hierarchically patterned structures. Clearly, a similar solvent dewetting strategy can be extended to other photoresists such as SU-8 for other applications. This technique enlarges the scope of soft patterning by a versatile combination of the top-down and bottom-up strategies.
hexagonal grid was exposed by a collimated 365 nm UV source and dewetted to successfully produce patterns on an area of ∼10 cm2. As shown in Figure 7A, a developed pattern can have dimensions smaller than those of the original pattern (which can be controlled by film thickness and pattern confinements).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support of Department of Science and Technology, New Delhi, for the Centre for Nanosciences at IIT Kanpur, which provided access to various characterization tools like the FESEM, the atomic force microscope, and an optical microscope.
Figure 7. (A1−A3) Different magnification images of hierarchical patterns written on dewetted microdomes over a large area [3 cm in panel A1 as taken by a stereo zoom microscope; the same pattern is magnified by an optical microscope (A2) and by SEM (A3)]. (B1) PDMS replica of a patterned array of Shipley microdroplets. (B2) Height profile of the line drawn in panel B1. Scale bars of panels A3 and B1 are 5 and 10 μm, respectively.
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REFERENCES
(1) Schiltz, A.; Paniez, P. J. In-Situ Determination of Photoresist Glass Transition Temperature by Wafer Curvature Measurement Techniques. Microelectron. Eng. 1995, 27, 413−416. (2) Wu, D.; Wang, J. N.; Wu, S. Z.; Chen, Q. D.; Zhao, S.; Zhang, H.; Sun, H. B.; Jiang, L. Three-Level Biomimetic Rice-Leaf Surfaces with Controllable Anisotropic Sliding. Adv. Funct. Mater. 2011, 21, 2927− 2932. (3) Goldberg-Oppenheimer, P.; Mahajan, S.; Steiner, U. Hierarchical Electrohydrodynamic Structures for Surface-Enhanced Raman Scattering. Adv. Mater. 2012, 24, OP175−OP180. (4) Radha, B.; Lim, S. H.; Saifullah, M. S. M.; Kulkarni, G. U. Metal Hierarchical Patterning by Direct Nanoimprint Lithography. Sci. Rep. 2013, 3, 1078. (5) Ami, Y.; Tachikawa, H.; Takano, N.; Miki, N. Formation of Polymer Microneedle Arrays Using Soft Lithography. J. Micro/ Nanolithogr., MEMS, MOEMS 2011, 10, 011503. (6) Jacot-Descombes, L.; Gullo, M. R.; Cadarso, V. J.; Brugger, J. Fabrication of Epoxy Spherical Microstructures by Controlled Dropon-Demand Inkjet Printing. J. Micromech. Microeng. 2012, 22, 074012. (7) Carroll, G. T.; Sojka, M. E.; Lei, X.; Turro, N. J.; Koberstein, J. T. Photoactive Additives for Cross-Linking Polymer Films: Inhibition of Dewetting in Thin Polymer Films. Langmuir 2006, 22, 7748−7754. (8) Chen, W.; Lam, R. H. W.; Fu, J. Photolithographic Surface Micromachining of Polydimethylsiloxane (Pdms). Lab Chip 2012, 12, 391−395. (9) Lyutakov, O.; Tuma, J.; Prajzler, V.; Huttel, I.; Hnatowicz, V.; Švorčík, V. Preparation of Rib Channel Waveguides on Polymer in Electric Field. Thin Solid Films 2010, 519, 1452−1457. (10) Reiter, G. Dewetting of Thin Polymer Films. Phys. Rev. Lett. 1992, 68, 75−78. (11) Redon, C.; Brzoska, J. B.; Brochard-Wyart, F. Dewetting and Slippage of Microscopic Polymer Films. Macromolecules 1994, 27, 468−471. (12) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Spinodal Dewetting of Thin Polymer Films. Phys. Rev. Lett. 1998, 81, 1251−1254. (13) Richardson, H.; Carelli, C.; Keddie, J. L.; Sferrazza, M. Structural Relaxation of Spin-Cast Glassy Polymer Thin Films as a Possible Factor in Dewetting. Eur. Phys. J. E: Soft Matter Biol. Phys. 2003, 12, 437−441.
Patterns created with a Shipley resist can be transferred onto a PDMS mold to replicate it on a different soft material, eliminating the need for a photosensitive material for replicating the patterns. As opposed to traditional polymers such as PS/PMMA, this material has a simple 3D patterning ability using these protocols. By using PDMS replication, these mixed length scale patterns (containing both micro- and nanostructures) can be simply transferred on any polymer using a host of soft lithographies, e.g., micromolding, capillary force lithography, or nano imprint lithography.47 In Figure 7B1, one such pattern was replicated on PDMS, which can be used to then create nano/micro lens arrays in other appropriate materials. Adsorption of functional materials on these textured microsites, doping of patterned curved surfaces, hierarchical patterning of functional soft materials using these resist microstructures as molds, etc., are a few possibilities to realize with this protocol.
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CONCLUSIONS A combination of photopatterning and self-organized dewetting of thin photoresist films under a liquid mixture of a good solvent and a nonsolvent offers new and creative avenues in multiscale patterning. Natural length scales of dewetting have been established for a range of film thicknesses. Aside from simple UV exposures followed by dewetting, physically confined patterns can also be dewetted by completely developing the exposed films prior to dewetting. Many structures such as fractal, Fibonacci, and other complex geometrical patterns containing regular arrangements of micro/nano droplets are demonstrated. Unlike conventionally dewetted polymers such as polystyrene, PMMA, etc., Shipley can be easily patterned upon being exposed to UV radiation to provide physically or chemically confined films, droplets, channels, etc. Additionally, we showed that by using the prepatterning followed by dewetting, drops 12510
DOI: 10.1021/acs.langmuir.5b02977 Langmuir 2015, 31, 12505−12511
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DOI: 10.1021/acs.langmuir.5b02977 Langmuir 2015, 31, 12505−12511
