pubs.acs.org/Langmuir © 2009 American Chemical Society
Effects of Mold Rising Angle and Polymer Concentration in Solvent-Assisted Molding Sung-Hoon Lee, Hong-Nam Kim, Rho-Kyun Kwak, and Kahp Y. Suh* School of Mechanical and Aerospace Engineering, World Class University Program on Multiscale Mechanical Design, Seoul National University, Seoul, 151-742, Korea Received August 29, 2009. Revised Manuscript Received September 17, 2009 We investigated the capillary rise of a thin polymer solution in a simple soft lithographic technique termed “solventassisted molding (SAMo)” by using various mold rising angles and polymer concentrations in a good solvent. For patterning and mold materials, poly(methyl methacrylate) (PMMA, Mw = 120 000) in toluene and poly(dimethyl siloxane) (PDMS) were mostly used. It was found that the capillary rise takes place in two steps: (i) a low-viscosity polymer solution rapidly rises into the cavity (1 cm) was used for the same purpose. The assembly was then allowed to remain undisturbed for a period of time (4-5 h) until the solvent completely evaporated. After complete solidification of the polymer, the PDMS mold was peeled off from the assembly, resulting in a molded microstructure. Scanning Electron Microscopy (SEM). High-resolution SEM images were obtained using a HITACHI S-4800 microscope (Hitachi, Japan) operating at an accelerating voltage of 5 kV. To avoid charging effects, substrates were sputter-coated with Au to the thickness of 20 nm prior to measurement. Measurement of Advancing Contact Angle (CA). The advancing CAs of polymer solution on flat PDMS surface were measured by increasing the volume of the drop (captive drop method) with a CA analyzer (Drop Shape Analysis System DSA100, Kruss, Germany).
Preparation of Masters with Different Rising Angles. A silicon master with vertical trench was prepared by standard photolithography and deep reactive ion etching (RIE). First, a sulfuric peroxide mixture (SPM) cleaned silicon wafer (100) was spin-coated with a positive photoresist (PR) (AZ 7220). The coated silicon wafer was soft-baked for several minutes, with a brief relaxation time. The wafer was then exposed under the mask using a photoaligner (EVG 6200). After development, the wafer was placed on a hot plate for several minutes (hard baking), followed by a short relaxation time. Subsequently, the exposed wafer was dry-etched using an advanced oxide etcher (MACS AOE, STS), resulting in a silicon master with vertical trench after additional stripping of the residual PR layer. A polymer master with acute angled trench was fabricated using wet etching and replica molding. After developing the exposed PR, the silicon wafer was wet-etched with 25 wt % tetramethylammonium hydroxide (TMAH) solution, resulting in a slope angle of 54.7°. Then, the silicon master was replicated using UV-curable poly(urethane acrylate) (PUA, 311RM, Minuta Tech).30 After drop-dispensing of a PUA precursor onto the silicon master, a transparent poly(ethylene terephthalate) (PET) film of 120 μm thickness was placed on the liquid precursor followed by UV (λ = 250-400 nm) exposure for a few tens of seconds. After the UV curing, the mold was peeled off from the master, leaving behind a PUA master with acute angled trench. A silicon/glass composite master with obtuse angled trench was prepared by “double-sided etching” technique. This method is based on the sequential etching of both sides of silicon wafer. After developing the exposed PR, the frontside of a double-sided polished silicon wafer was wet-etched with 25 wt % TMAH solution. To facilitate back-side etching, the front-side-etched silicon wafer was assembled with soda-lime glass by anodic bonding, which rendered irreversible hermetic sealing (EV501 bonder). Subsequently, the backside of assembly was dry-etched until the wetetched pattern was exposed. After the process, the silicon/glass composite master with an obtuse angled trench was obtained with an angle of 125.3°. Fabrication of Polymeric Microstructures by SAMo. The PDMS (Sylgard 184, Dow Corning) mold was prepared by casting PDMS precursor (with 10 wt % curing agent) against the masters fabricated above, resulting in a complementary relief structure. For the patterning material, four polymer solutions of PMMA (Mw = 120 000) were prepared with different
Results and Discussion Figure 1a shows a schematic illustration of the experimental procedure. A close examination of the process reveals that the molding typically occurs in two steps: (i) a low-viscosity polymer solution rapidly rises into the cavity within a short period of time (2 μL). It should be noted in this regard that most of the polymer solution is squeezed out from the mold while placing the mold (especially with a pressure), so that the initial amount of polymer solution would be the same. Therefore, the current structural phase diagram is quite general in describing the formation of various microstructures, provided that a sufficiently larger amount of polymer solution is given. In the case of partial filling (Figure 4d-f), the polymer solution stops rising at the middle of mold, owing to limited amount of polymer solution (mass depletion). In this process, the solvent is continuously absorbed into the mold after pinning of a meniscus, resulting in a partially filled structure. As expected, the thickness of residual layer is determined by the polymer concentration as well as the mold rising angle; an increase in the polymer concentration (red arrows) brings about the increase of residual layer to a different degree as affected by mold rising angle. To explore the generality and extend the applications of our study, the SAMo process was applied to different polymers and mold dimensions. Figure 5a,b shows the cross-sectional SEM images of molded microstructures using polystyrene (PS, Mw = 230 000) dissolved in toluene. Under the same experimental conditions with type III (5 wt % polymer concentration and 2.0 μL initial amount of drop-dispensed polymer solution), the completely but non-fully filled microstructures (hollow structures) were generated. In addition, the effect of solvent absorption in the SAMo process was investigated by molding solvent-free UV-curable PUA resin. In this experiment, the PUA resin was first drop-dispensed on PET film and then exposed to UV-irradiation after conformal contact of PDMS mold with Langmuir 2009, 25(20), 12024–12029
Lee et al.
Letter
for partial filling and 2.0 μL for complete filling) without any shape deformation in the molded structures. Furthermore, different mold dimensions were tested by using PDMS molds with 4 μm height and 7 μm width (Figure 5e, 1 order of magnitude smaller) and 120 μm height and 150 μm width (Figure 5f, 1 order of magnitude larger). As seen from the figures, the completely but non-fully filled microstructures were fabricated by adjusting the amount of drop-dispensed polymer solution (0.2 μL and 15 μL, respectively) under the same polymer concentration (5 wt %). On the basis of the above results, our findings would be applied to other polymers and mold dimensions with appropriate change of the polymer solution and the amount of solution.
Figure 5. Cross-sectional SEM images of molded microstructures with different polymers and mold dimensions. (a,b) Completely but non-fully filled microstructures (hollow structures) molded by PS solution (5 wt %, 2.0 μL). (c,d) Partially molded (c) and completely molded (d) microstructures molded by UV-curable PUA resin. 0.5 μL and 2.0 μL of polymer solution was used for each result, respectively. (e,f) Completely but non-fully filled microstructures (hollow structures) using PDMS molds with different dimension: (e) 4 μm height and 7 μm width and (f) 120 μm height and 150 μm width.
polymer layer. As shown in Figure 5c,d, the partially molded (c) and the completely molded (d) structures were obtained depending on the amount of drop-dispensed polymer solution (0.5 μL
Langmuir 2009, 25(20), 12024–12029
Conclusions We have presented the role of several experimental parameters such as mold rising angle, polymer concentration, and amount of polymer solution in the SAMo process. It was found that the capillary rise occurs in two steps (rapid capillary rise within 10 s þ continuous solvent absorption and evaporation), which was supported by a constant advancing CA of a PMMA solution on flat PDMS surface. By testing 24 combinations of the parameters, two structural phase diagrams have been constructed for complete and partial molding regimes, revealing that the geometrical features of molded microstructures can be tailored by controlling the aforementioned experimental conditions. In particular, five distinct microstructures were found under different conditions, such as completely molded (I), humped (II), completely molded but non-fully filled (III), partially filled (IV), and partially filled, meniscal (V) microstructures. It is envisioned that our study could be used as a guide for optimal design and process of the SAMo method. Acknowledgment. This work was supported by the Intelligent Microsystem Center (IMC; http://www.microsystem.re.kr), which carries out one of the 21st century’s Frontier R&D Projects and in part by the Grant-in-Aid for Next-Generation New Technology Development Programs (No.10030046) sponsored by the Korea Ministry of Knowledge Economy.
DOI: 10.1021/la903236d
12029
