Facile Approach to the Fabrication of a Micropattern Possessing

Nov 8, 2007 - On the basis of the combined technologies of photolithography and reaction-induced phase separation (RIPS), a facile approach has been ...
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Langmuir 2007, 23, 12663-12668

12663

Facile Approach to the Fabrication of a Micropattern Possessing Nanoscale Substructure Qiang Ji, Xuesong Jiang,* and Jie Yin* School of Chemistry & Chemical Technology, State Key Laboratory for Composite Materials, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed May 17, 2007. In Final Form: September 28, 2007 On the basis of the combined technologies of photolithography and reaction-induced phase separation (RIPS), a facile approach has been successfully developed for the fabrication of a micropattern possessing nanoscale substructure on the thin film surface. This approach involves three steps. In the first step, a thin film was prepared by spin coating from a solution of a commercial random copolymer, polystyrene-r-poly(methyl methacrylate) (PS-r-PMMA) and a commercial crosslinker, trimethylolpropane triacrylate (TMPTA). In the second step, photolithograph was performed with the thin film using a 250 W high-pressure mercury lamp to produce the micropattern. Finally, the resulting micropattern was annealed at 200 °C for a certain time, and reaction-induced phase separation occurred. After soaking in chloroform for 4 h, nanoscale substructure was obtained. The whole processes were traced by atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS), and Fourier transform infrared (FTIR) spectroscopy, and the results supported the proposed structure.

Introduction

* Corresponding authors. Tel: +86-21-54743268. Fax: +86-2154747445. E-mail: [email protected], [email protected].

the diffraction of exposed light, it is difficult to obtain nanoscale structure, which is the main drawback of photolithography. Generally, nanopatterns can be fabricated through the selfassembly of phase-separated block copolymers, which can form predetermined periodic structure with a high degree of registry and regularity on the nanoscale of 10-100 nm. The self-assembly of block copolymers has been well studied, and many different nanoscale structures have been generated.11,16-18 However, in comparison with the synthesis of homopolymers and random copolymers, it is generally complex and time consuming to prepare block polymers with exact compositions, which makes this technique difficult for large-scale production. Therefore, it is desired to develop a facile, effective approach to the production of nanoscale structure. Recently, reaction-induced phase separation (RIPS) has been identified in many polymer blends that can form different morphologies inside the matrix.19-28 Here, RIPS occurs as the entropic contribution to the Gibbs free energy becomes smaller, which finally results in a positive Gibbs free energy because of the curing reaction.21 Most studies in this area focus on the control of morphology inside the matrix to enhance the performance of materials.19,21,28,29 Unlike self-assembly, which

(1) Geissler, M.; Xia, Y. N. AdV. Mater. 2004, 16, 1249. (2) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823. (3) Henzie, J.; Barton, J. E.; Stender, C. L.; Odom, T. W. Acc. Chem. ReV. 2006, 39, 249. (4) Lachish-Zalait, A.; Zbaida, D.; Klein, E.; Elbaum, M. AdV. Funct. Mater. 2001, 11, 28. (5) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30. (6) Salaita, K.; Amarnath, A.; Maspoch, D.; Higgins, T. B.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 11283. (7) Choi, S. J.; Yoo, P. J.; Baek, S. J.; Kim, T. W.; Lee, H. H. J. Am. Chem. Soc. 2004, 126, 7744. (8) Xu, H.; Hong, R.; Lu, T. X.; Uzun, O.; Rotello, V. M. J. Am. Chem. Soc. 2006, 128, 3162. (9) Park, K. S.; Seo, E. K.; Do, Y. R.; Kim, K.; Sung, M. M. J. Am. Chem. Soc. 2006, 128, 858. (10) Cyr, P. W.; Rider, D. A.; Kulbaba, K.; Manners, I. Macromolecules 2004, 37, 3959. (11) Guo, S. W.; Rzayev, J.; Bailey, T. S.; Zalusky, A. S.; Olayo-Valles, R.; Hillmyer, M. A. Chem. Mater. 2006, 18, 1719. (12) Aizawa, M.; Buriak, J. M. J. Am. Chem. Soc. 2006, 128, 5877. (13) Xia, Y. N.; Odom, T. W.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 7288. (14) Toepke, M. W.; Kenis, P. J. A. J. Am. Chem. Soc. 2005, 127, 7674.

(15) Revzin, A.; Russell, R. J.; Yadavalli, V. K.; Koh, W. G.; Deister, C.; Hile, D. D.; Mellott, M. B.; Pishko, M. V. Langmuir 2001, 17, 5440. (16) Morikawa, Y.; Nagano, S.; Watanabe, K.; Kamata, K.; Iyoda, T.; Seki, T. AdV. Mater. 2006, 18, 883. (17) Albercht, K.; Mourran, A.; Moeller, M. AdV. Polym. Sci. 2006, 200, 57. (18) Goren, M.; Lennox, R. B. Nano Lett. 2001, 1, 735. (19) Inoue, T. Prog. Polym. Sci. 1995, 20, 119. (20) Williams, R. J. J.; Rozenberg, B. A.; Pascault, J. P. AdV. Polym. Sci. 1997, 128, 95. (21) Goossens, S.; Goderis, B.; Groeninckx, G. Macromolecules 2006, 39, 2953. (22) Meng, F. L.; Zheng, S. X.; Li, H. Q.; Liang, Q.; Liu, T. X. Macromolecules 2006, 39, 5072. (23) Murata, K.; Sachin, J.; Etori, H.; Anazawa, T. Polymer 2002, 43, 2845. (24) Tran-Cong, Q.; Harada, A. Phys. ReV. Lett. 1996, 76, 1162. (25) An, N. R.; Yang, Y. M.; Dong, L. S. Macromolecules 2007, 40, 306. (26) Okada, M.; Inoue, G.; Ikegami, T.; Kimura, K.; Furukawa, H. Polymer 2004, 45, 4315. (27) Ishii, Y.; Ryan, A. J. Macromolecules 2000, 33, 158. (28) Nakanishi, H.; Satoh, M.; Norisuye, T.; Miyata, Q. T. C. Macromolecules 2006, 39, 9456. (29) Gan, W. J.; Yu, Y. F.; Wang, M. H.; Tao, Q. S.; Li, S. J. Macromolecules 2003, 36, 7746.

In the past decades, patterning has attracted much attention because of both fundamental research interests and wide industrial applications ranging from the production of integrated circuits and display units to the fabrication of microelectromechanical systems (MEMS), miniaturized sensors, and biochips1-3 To satisfy the growing demands for complicated micropatterns and nanopatterns as well as low cost and simple processing techniques, continuous advances in methods of fabrication of the various patterns have been developed. These methods mainly include direct writing such as laser-induced chemical vapor deposition (LCVD) and dip-pen nanolithography (DPN), embossing, microcontact printing (µCP), edge lithography, photolithography, and self-assembly.4-12 Among these methods, photolithography is one of the best-established technologies for micropatterning and has found wide applications in the microelectronic industry because of advantages such as large-scale production and simple processing.1,13-15 However, because of the wavelength limit and

10.1021/la7014176 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/08/2007

12664 Langmuir, Vol. 23, No. 25, 2007 Scheme 1. Processes Involved in the Preparation of Microcomples and Nanocomplex Patternsa

a (a, b) Exposure through a mask; (b, c) immersion in solvent to remove the unexposed area; (c, d) annealing at 200 °C; and (d, e) immersion in solvent to remove the copolymer microphase.

requires the use of block copolymers, RIPS technology can yield microscopic phase separation using various low-cost commercial monomers, homopolymers, or random copolymers. Unfortunately, however, it is rather difficult to obtain nanoscale structures using this technique.19,20,25-28 In this article, we develop a facile method to fabricate micropatterns possessing nanoscale substructure based on the combined technologies of photolithography and reaction-induced phase separation. The thin film consists of two components: a cross linker and a polymer. Considering that commercial materials are of special interest in industrial applications, here we chose a commercial trifunctional monomer, trimethylolpropane triacrylate (TMPTA), as the cross linker and a commercial random copolymer, polystyrene-r-poly (methyl methacrylate) (PS-rPMMA), as the matrix. As shown in Scheme 1, our approach involves three steps. In the first step, TMPTA and PS-r-PMMA are spin coated onto a glass or silicon substrate to form a thin film (∼120 nm). In the second step (processes a-c), photolithography was performed with the thin film to yield the micropattern. Finally (processes d and e), the resulting micropattern was annealed at 200 °C for a certain period of time, and a PS-r-PMMA protrusion formed because of reaction-induced phase separation, which generated nanoscale substructure after the protrusion was removed by soaking in chloroform. Experimental Section Materials. Polystyrene-r-poly(methyl methacrylate) (PS-rPMMA, Mw ) 1.5 × 104, PS content ) 60 wt %) was purchased from Aldrich. Trimethylolpropane triacrylate (TMPTA) was purchased from Nantong Litian Chemical Company. 2,2-Bis-(2chlorophenyl)-4,4,5,5-tetraphenyl-1,2-biimidazole(o-Cl-HABI) was purchased from Shanghai Lucky Biological and Chemical Technology. 2-Mercaptobenzoxazole (MBO) was purchased from J&K Chemical. Other chemicals are of analytical grade except as noted. Substrates (glass quartz or silicon wafer) were prepared by immersion in a 30:70 H2O2 (30%)/H2SO4 (98%) mixture for 1 h at 80 °C (piranha etch).

Ji et al. Fabrication of Micropatterns and Nanopatterns. TMPTA, PSr-PMMA, and photoinitatior were dissolved in chloroform at a total concentration of 1 wt %. The weight ratios of TMPTA to PS-rPMMA are 0.6:1 except where otherewise noted. The photoinitiator system is composed of o-Cl-HABI and MBO (1:1 molar ratio), and the weight fraction is about 5% to TMPTA. The polymer solutions were filtered through a 0.45 µm Millipore membrane before use. Films were prepared via spin coating on clean glass or silicon substrates, followed by drying at 80 °C for 30 min and finally exposing to a 250 W high-pressure mercury lamp with a UV intensity of 1.5 mW/cm2 for 30 min. After immersing in chloroform for 3 min to remove the unexposed area, the micropattern was generated. The thickness of the film is about 120 nm, and the films were further annealed at 200 °C for different times and immersed in chloroform for 4 h to fabricate the nanostructure. The whole process, including film preparation, photolithography, and annealing, was carried out in air. Measurement. FTIR spectra were recorded on a Perkin-Elmer Paragon 1000 FTIR spectrometer. The surface morphologies of samples were acquired in taping mode on an AFM (Nanoscope III, Digital instruments). XPS experiments were carried out on a PHI5000C ESCA system (Perkin-Elmer) with Al Ka radiation (hν ) 1486.6 eV). In general, the X-ray anode was run at 250 W, and high voltage was maintained at 14.0 kV with a detection angle at 54°. The pass energy was fixed at 46.95 eV to ensure sufficient sensitivity. The base pressure of the analyzer chamber was about 5 × 10-8 Pa. The sample was pressed into a self-supported disk (10 × 10 mm2) and mounted on a sample holder and then transferred to the analyzer chamber. The entire spectral range (0-1200 eV) and the narrow spectra of all of the elements at very high resolution were recorded. Binding energies were calibrated by using containment carbon (C 1s ) 284.6 eV). All of the C 1s peaks were calibrated to the standard binding-energy shifts. The data analysis was carried out by using the RBD AugerScan 3.21 software provided by RBD Enterprises or XPS Peak 4.1 provided by Raymund W. M. Kwok).

Results and Discussion The strategy for the fabrication of a micropattern possessing nanoscale substructure is illustrated in Scheme 1. The micropattern of the thin film was formed through photolithography (processes a-c). The film was irradiated by UV light through the mask. In the exposed area, the photointiatior system generated radicals, which initiated TMPTA to form an insoluble cross-linked network. The unexposed part did not undergo a cross-linking reaction and therefore could be readily removed by immersing the film in chloroform (redissolution). The remaining part (exposed part) forms the desired micropattern (shape of the mask). It should be noted that there is still a considerable amount of the unreacted acrylate group even in the exposed part because the curing reaction caused the solidification of the TMPTA phase and the mobility of TMPTA decreased drastically, which prevented further reaction of the acrylate group. The resulting micropattern was then annealed at 200 °C for a different period of time. At high temperature, TMPTA underwent a further curing reaction to form a more compact cross-linked network, and part of PS-r-PMMA was “extruded” to the film surface as a result of phase separation (process d). After the protrusion of PS-rPMMA was removed through redissolution in chloroform, nanoscale substructure was obtained on the surface of the crosslinked network (process e). Figure 1 shows the AFM image of the thin film after photolithography and annealing at 200 °C for 12 h before immersion in chloroform (process d). The protrusion on the thin film surface is about 45 nm in height and 1 µm in diameter. Because of thermal cross linking of the residual acrylate groups at 200 °C, the cross-linking density of the TMPTA network increased in the annealing process, which induced phase

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Figure 1. AFM image after photolithography and annealing at 200 °C for 12 h. The scan size is 50 µm × 50 µm × 200 nm.

Figure 2. AFM images in the exposure area after annealing at 200 °C for different periods of time: (a, d) 3 h; (b, e) 12 h; and (c, f) 24 h. The scan size is 10 µm × 10 µm × 50 nm.

separation between the cross-linked TMPTA network and random copolymer PS-r-PMMA because of the decrease in the Gibbs free energy of mixing. Because of the very fast cross-linking reaction of TMPTA and the relatively low mobility of PS-rPMMA, further phase separation inside the thin film is suppressed by the formation of a more compact network. As a result, PSr-PMMA has no choice but to be “extruded” to the surface to form the protrusion. Figure 2 shows the AFM topographical images in the exposed area after annealing at 200 °C for different periods of time. With the increase in annealing time, the size of the protrusion became larger, indicating that more PS-r-PMMA was extruded from inside to the surface. We also found that the morphology of the sample that was annealed for 48 h is almost the same as that for the sample that was annealed for 24 h. This indicated that the phase separation lasts for 24 h and reaches equilibrium thereafter. To understand when the phase separation between the TMPTA network and PS-r-PMMA started, AFM images of the thin film

before and after exposure to UV light were studied (Figure 3). From Figure 3a (topography) and b (phase), we can see that before exposure PS-r-PMMA and TMPTA can form a homogeneous film and no phase separation appears. After exposure to UV light, the photocuring of TMPTA induced phase separation as a result of the decrease in the entropic contribution to the free energy of mixing (Figure 3c,d). However, the surface was still quite smooth, with a root-mean-squared roughness of about 0.7 nm. This may be ascribed to the occurrence of vitrification of the system as a consequence of the curing reaction of TMPTA, which restricted the mobility of PS-r-PMMA and “froze” the phase separation at room temperature. The curing reaction was traced by FTIR spectra. As shown in Figure 4, the intensity of the absorption band at 1642 cm-1 (stretching vibration of CdC) decreased after exposure of UV light, indicating that part of the acrylate groups were consumed by cross linking. Because the carbonyl group remained unchanged before and after UV exposure, the absorption band of the carbonyl

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Figure 3. AFM images: (a) topography, (b) phase contrast image before exposure, (c) topography, and (d) phase contrast image after exposure. The scan size is 2 µm × 2 µm × 50 nm.

group (1720 cm-1) can be used as the internal standard. By calculating the intensity ratios between the bands at 1642 and 1720 cm-1, it is estimated that only about 30% of the acrylate groups underwent the cross linking reaction during exposure to UV light. The absorption band at 1642 cm-1 almost disappeared after annealing at 200 °C for 1 h. This is attributed to the further cross-linking reaction of acrylate groups at high temperature. By annealing at 200 °C, which is higher than the Tg of PS-r-PMMA and the network of TMPTA, both the curing reaction and phase separation continued because of the high mobility of the whole system. It is interesting that although the curing reaction was almost complete after 1 h of annealing, phase separation continued for 24 h (Figures 2 and 4). Figure 5 shows the AFM image of the annealed thin film after soaking in chloroform (stage e). It can be seen that the PS-rPMMA protrusion could be readily removed by immersing (redissolving) the thin film in chloroform for 4 h, and a large number of nanoholes of about 5 nm depth and 55 nm diameter appeared in the exposure area. However, the thickness of the thin film hardly changed. The nanostructure between the TMPTA network and random copolymer PS-r-PMMA under the surface of the thin film is obviously different from common microphase

Ji et al.

separation between the cross-linked network and the homopolymer or random copolymer induced by reaction.19,20,25-27 Although the film is thin enough (less than 250 nm) to suppress the development of surface-directed spinodal decomposition waves,30,31 Karim and Okada’s research32-34 indicated that it is difficult to obtain nanoscale structure by using only commercial polymers such as homopolymers and random copolymers. Several factors may be responsible for the formation of microscopic protrusions on the thin film surface and nanoscale substructure under the thin film surface. As a trifunctional cross linker, TMPTA can form a very compact network after curing. Because the curing reaction of TMPTA is much faster than phase separation in the process of annealing at 200 °C (Figures 2 and 4), the compact cross-linked network of TMPTA prevents the further growth of PS-r-PMMA nanostructure inside the thin film, resulting in PSr-PMMA aggregation only on the nanoscale. Because of the thermodynamic instability of the system, part of PS-r-PMMA is extruded from the bulk to the thin film, which may aggregate on the surface to form a microscopic protrusion. Figure 6 shows the XPS spectra of the film surface treated at different stages: after photolithography only (process c), after photolithography and annealing at 200 °C for 12 h (process d), and after development in chloroform for 4 h (process e). Figure 6 displays strong signals related to C 1s and O 1s as well as Si 2p from the substrate. It can be seen that the carbon content significantly increased after the thin film was annealed at 200 °C. Because PS-r-PMMA has a significantly higher carbon content (79.7%) than TMPTA (60.0%), the increase in the percentage of carbon atoms due to the annealing treatment should be attributed to PS-r-PMMA extruded from the bulk to the surface of the thin film. After the film was soaked in chloroform for 4 h (process e), the carbon content decreased from 76.1 to 71.5%. This suggests that most of the PS-r-PMMA protrusion on the surface was removed by redissolution. A detailed analysis of the high-resolution XPS C 1s spectra (shown on the right-hand side of Figure 6) also confirmed the changes in the thin film surface composition from process c to e. C 1s core-level scans clearly demonstrate three distinct features corresponding to C-C/CH, C-O, and OdC-O functional groups in order of increasing energy. They can be assigned to the aliphatic or aromatic ether and ester carbons in PS-r-PMMA and TMPTA. The intensities of the C-C/C-H peaks in PS-r-PMMA and TMPTA are 88.2 and 60.0%, respectively. From process c to d, the content of

Figure 4. FTIR spectra of the thin film after exposure and annealing at 200 °C for different periods of time.

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Figure 5. AFM image after annealing at 200 °C for 12 h and developing in chloroform for 4 h. The scan size is 2 µm × 2 µm × 40 nm.

Figure 6. XPS survey spectra (left) and C 1s high-resolution spectra of the thin film (top to bottom) after photolithography, after annealing at 200 °C for 12 h, and after developing in chloroform for 4 h.

C-C/C-H increases from 76.9 to 86.9%, which is quite close to that of PS-r-PMMA. This indicates that most of the surface

is covered with the PS-r-PMMA protrusion in stage d, which is consistent with the AFM results shown in Figure 2. For the

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Figure 7. AFM image of the thin film containing a higher TMPTA content (1:1 TMPTA/PS-r-PMMA) after annealing at 200 °C for 12 h and developing in chloroform for 4 h. The scan size is 2 µm × 2 µm × 100 nm.

sample in stage e, the content of C-C/C-H decreased from 86.9 to 81.6% because of the removal of most of the PS-r-PMMA protrusion. To investigate the effect of cross-linking density on the formation of nanoscale substructure, the thin film having a higher content of TMPTA cross linker (1:1 TMPTA/PS-r-PMMA by weight) was prepared and treated under the same conditions as that of the foregoing sample. As shown in Figure 7, a nanocolumn of about 45 nm in height and 55 nm in diameter appeared on the surface, which is different from the nanohole structure shown in Figure 5. A higher content of TMPTA resulted in a higher cross-linking density and thus a more compact cross-linked network after the annealing treatment. This caused more PSr-PMMA be extruded from the bulk to the surface of the thin film, which in turn aggregated to form a PS-r-PMMA protrusion that is larger than that of the sample with lower TMPTA content. After the PS-r-PMMA protrusion was removed, a nanocolumn was generated. This indicates that different nanoscale substructures can be obtained by controlling the cross-linking density of the thin film. (30) Krausch, G.; Dai, C. A.; Kramer, E. J.; Marko, J. F.; Bates, F. S. Macromolecules 1993, 26, 5566. (31) Sung, L.; Karim, A.; Douglas, J. F.; Han, C. C. Phys. ReV. Lett. 1996, 76, 436. 8 (32) Karim, A.; Slawecki, T. M.; Kumar, S. K.; Douglas, J. F.; Satija, S. K.; Han, C. C.; Russell, T. P.; Liu, Y.; Overney, R.; Sokolov, J.; Rafailovich, M. H. Macromolecules 1998, 31, 857. (33) Wang, X.; Okada, M.; Han, C. C. Macromolecules 2006, 39, 5127. (34) Okada, M.; Inoue, G.; Ikegami, T.; Kimura, K.; Furukawa, H. Polymer 2004, 45, 4315.

Conclusions A facile approach has been successfully developed for the fabrication of a micropattern possessing nanoscale substructure based on the combination of photolithography and RIPS technologies. Upon annealing treatment at 200 °C, part of the PS-r-PMMA was extruded from the bulk to the surface of the thin film to form protrusion. After the annealed sample was soaked in chloroform, nanoscale substructure formed as a result of the removal of PS-r-PMMA. The curing reaction and the changes in morphology and surface composition were studied by FTIR, AFM, and XPS analyses. The formulation of the thin film greatly influenced the nanoscale substructure, i.e., a lower content (0.6:1 TMPTA/PS-r-PMMA) of cross linker resulted in nanoholes, and a higher content (1:1 TMPTA/PS-r-PMMA) of cross linker gave nanocolumns. Systematic studies are in progress on the relationship between the formation of patterns and many factors such as polymer and cross-linker structures, polymer/ cross-linker miscibility, composition, film thickness, and crosslinking intensity. The present approach may find potential applications in the fabrication of nanoporous membranes and nanopatterns for templates. Acknowledgment. We thank the Science & Technology Commission of the Shanghai Municipal Government (no. 06JC14041) for financial support. Supporting Information Available: AFM images after annealing and photolithography. This material is available free of charge via the Internet at http://pubs.acs.org. LA7014176