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COMMUNICATION Patterned Nanostructures B. H. Kim, D. O. Shin, S.-J. Jeong, C. M. Koo, S. C. Jeon, W. J. Hwang, S. Lee, M. G. Lee, S. O. Kim*..............................2303–2307 Hierarchical Self-Assembly of Block Copolymers for Lithography-Free Nanopatterning
Hierarchical self-assembly of block copolymers has been achieved by two steps of sequential ordering processes, consisting of self-organized micropatterning from a dewetting block polymer solution and thermal annealing. The self-organized micropattern induces the spontaneous alignment of self-assembled lamellae (see figure), which is successfully applied for a lithography-free, ultra-large-scale nanopatterning.
Hierarchical Self-Assembly of Block Copolymers for Lithography-Free Nanopatterning** By Bong Hoon Kim, Dong Ok Shin, Seong-Jun Jeong, Chong Min Koo, Sang Chul Jeon, Wook Jung Hwang, Sumi Lee, Moon Gyu Lee, and Sang Ouk Kim*
Hierarchical self-assembly is the ultimate fabrication process for complex nanostructures and ensures molecular-level pattern precision and parallel processing.[1–5] Natural building blocks, such as proteins, DNAs, and phospholipids, undergo various levels of intra- and intermolecular assembly having multiple length scales. Their complex functionalities and high selectivity are largely owing to the resultant hierarchical structures. Furthermore, the intimately interacting multiscale orderings of a hierarchical assembly may provide an opportunity for directing over the diverse length scales simultaneously. However, developing a hierarchical self-assembly for large-scale nanofabrication processes is still in its infancy.[6,7] Block copolymers are self-assembling materials composed of covalently linked macromolecular blocks. Owing to their diverse chemical functionalities and precise tunability over the shape and size of the self-assembled nanostructures, block copolymers have been extensively utilized for nanofabrication.[8,9] However, the spontaneously assembled morphology of block copolymers consists of randomly oriented nanodomains with a large number of defects. For diverse
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Prof. S. O. Kim, B. H. Kim, D. O. Shin, S.-J. Jeong Department of Materials Science and Engineering KAIST Institute for the Nanocentury Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305-701 (Korea) E-mail:
[email protected] Dr. C. M. Koo Hybrid Materials Research Center Korea Institute of Science and Technology (KIST) Cheongryang Seoul P.O. BOX 131 (Korea) S. C. Jeon, Dr. W. J. Hwang National Nanofab Center (NNFC) 373-1 Guseong-dong Yuseong-gu Daejeon 305-701 (Korea) Dr. S. Lee, M. G. Lee Samsung Advanced Institute of Technology (SAIT) Mt. 14-1 Nongseo-dong Giheung-gu Yongin-si Gyunggi-do 446-712 (Korea)
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We thank Sung Soon Bai for providing technical support for SEM characterization. This work was supported by the Samsung Advanced Institute of Technology (SAIT), the Korea Research Foundation (KRF-2005-003-D00085), the second stage of the Brain Korea 21 Project, the Korea Science & Engineering Foundation (KOSEF) (R01-2005-000-10456-0), the Korean Ministry of Science and Technology, and the Fundamental R&D Program for Core Technology of Materials funded by the Korean Ministry of Commerce, Industry and Energy. Supporting Information is available online from Wiley InterScience or from the author.
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DOI: 10.1002/adma.200702285
advanced applications, such as optical elements,[10] sensor arrays,[11] or nanofluidics,[12] control over the lateral orientation and orderings of these nanodomains is required. To date, various methods such as the application of external fields,[13–15] templated assembly upon prepatterned surfaces,[16–20] and directional solidification[21,22] have been developed for the well-ordered nanoscale morphologies of block copolymers. However, those approaches require complicated facilities and multistep processing for applying external fields or lithographic prepatterning, thereby practically limiting the application to an arbitrarily large area. We introduce hierarchical self-assembly of block copolymers as a lithography-free, ultra-large-scale nanopatterning process. As schematically described in Figure 1, the hierarchical assembly was derived from two levels of spontaneous orderings. The large-scale ordering was defined by the periodic thickness modulation of a block-copolymer film, which was self-organized from the receding contact line of an evaporating block-copolymer solution upon a substrate. Its length scale was tunable over a scale of tens of micrometers by adjusting the processing parameters. The small-scale ordering corresponds to the self-assembled nanostructure of a block copolymer. Its length scale was on a scale of tens of nanometers, as determined by the molecular weight of the block copolymer. Unlike ordinary hierarchical assembly, where a small-scale structure determines the further organization into a larger structure, the large-scale structure directs the small-scale ordering in our approach. This offers a unique opportunity to attain control over nanoscale morphology by manipulating a microscale structure. A symmetric polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA, molecular weight: 104 kg mol 1, lamellar spacing: 48 nm) diblock copolymer was dissolved in toluene and used for the hierarchical assembly. A micropatterned film of the block copolymer was prepared from the polymer solution confined between a glass plate and silicon substrate, as described in Figure 1a. Upon the slow movement of the upper glass plate, the polymer film, having a periodic thickness modulation, was self-organized from the receding edge of the polymer solution.[23,24] Because of the evaporation of volatile toluene at the solution front, polymer residue was deposited at the edge of the polymer solution and temporarily retarded the movement of the solution edge such that the receding velocity of solution edge was periodically varied. The resultant polymer film left on the silicon substrate exhibited thickness variation
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orientation was orthogonal to that of the microscale pattern (Supporting Information, Fig. S1). Figure 2b shows a hierarchically ordered morphology extending over a broad field of 0.4 mm 0.05 mm. A close examination in a magnified image (Fig. 2c) demonstrates a remarkably high degree of ordering of nanoscale morphology in the well-aligned region. This clearly contrasts with the morphology at the randomly aligned region with a distinctive boundary between the two areas. The morphology inside a hierarchically ordered film was investigated by crosssectional SEM imaging, as presented in Figure 3. The width of the hill-shaped thickness modulation was about 75 mm and well-aligned lamellae extended over 65 mm. The corresponding aspect ratio of polymeric nanowires exceeded 2600 (Supporting Information, Fig. S2). Because the underlying silicon surface was neutrally treated for PS-b-PMMA, the lamellae were oriented perpendicularly to the surface throughout the film thickness.[25] The film thickness at the highest region was about 1.4 mm, where the aspect ratio of the lamellar morphology in the surface-normal direction was over 55. Since neither an external field nor a Figure 1. Experimental procedure for the hierarchical nanopatterning process of a block copo- lithographic prepatterning process was lymer. a) The micropatterned block-copolymer film was spontaneously organized from the applied to the block-copolymer film, the evaporating block-copolymer solution. b,c) The subsequent annealing at high temperature led observed lamellar alignment was hardly to the spontaneous alignment of nanoscale lamellar morphology along the thickness gradient of anticipated. In order to attain a detailed the patterned film. understanding of this unique behavior, the reflecting the periodic velocity variation of solution edge. Note morphological evolution was characterized that the surface of the underlying substrate was neutrally while varying the microscale morphology of a film and thermal modified for the PS and PMMA blocks to induce lamellae of a annealing conditions. As expected, a block-copolymer film PS-b-PMMA block copolymer that are perpendicularly having a uniform thickness did not show any preferential oriented to the surface.[25] Figure 2a presents the height alignment of lamellae (Supporting Information, Fig. S3). The profile of a micropatterned film along the direction that the lamellae became macroscopically aligned provided that a depositing solution had propagated. It shows a periodic thickness modulation was introduced by micropatterning. In thickness variation having a sinusoidal shape. The characterthe early stage of thermal annealing, the morphology of a istic length scales of the periodic pattern were tunable by thickness-modulated film consisted of randomly oriented varying the polymer concentration in the solution or the lamellae throughout the whole film plane, signifying that no receding velocity of the contact line.[24] In a typical sample, preferential lamellar alignment occurred during the preceding the pattern period was tunable over tens of micrometers and self-organized micropatterning process (Supporting Informathe film thickness at the highest region ranged from a tion, Fig. S4). As annealing proceeded, well-aligned lamellar few hundred nanometers to a few micrometers. After the domains were nucleated from the thick part of the film (Fig. self-organized micropatterning process, the block-copolymer 4a), gradually propagated following the thickness gradient film was substantially annealed at a high temperature. (Fig. 4b), and finally overwhelmed most of the patterned film When a micropatterned film was examined by field-emission (Fig. 4c). Such a directional domain growth could be caused by scanning electron microscopy (FE-SEM) after annealing, it the influence of film thickness upon the defect annihilation demonstrated a marvelous hierarchical morphology of microrate. Since lamellae were oriented perpendicularly to the scale stripes consisting of alternately well-aligned and substrate surface, the defect morphology at the film surface randomly aligned lamellae (Fig. 2b and c). The well-aligned penetrated throughout the film thickness. Therefore, the defect lamellae appeared at the thick part of the film, and their energy throughout the film thickness, which is given by the core
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Figure 2. Hierarchically organized PS-b-PMMA diblock-copolymer film, having both microscale and nanoscale orderings. The micropatterned block-copolymer film was spontaneously organized from the receding contact line of evaporating polymer solution with a substrate surface [24]. The depositing solution propagated along the x-direction and the resultant microscale stripes were extended in the y-direction. Subsequent annealing at a high temperature led to the self-alignment of lamellae in the x-direction. a) Height profile for a micropatterned blockcopolymer film. The thickness modulation had a sinusoidal shape, having a period of about 60 mm. b) Top-down scanning electron microscopy (SEM) image of a hierarchically organized block-copolymer film over a large area of 0.4 mm 0.05 mm. The periodically appearing dark regions correspond to the valley of the sinusoidal thickness modulation, where nanoscale morphology consists of randomly oriented lamellae. The periodically appearing bright regions correspond to the thick part of film where lamellae were well-aligned in the x-direction. c) High-resolution SEM images demonstrating the nanoscale ordering along a half pitch of microscale pattern. The highly ordered lamellar morphology in the well-aligned region clearly contrasts to the morphology in the randomly aligned region.
energy and the energy penalty for the morphology distortion around the core, was proportional to the film thickness.[26] The influence of film thickness upon the defect energy could impose the gradient in the defect annihilation rate along the thickness gradient (Supporting Information, Fig. S3).[27] As a consequence, the well-aligned lamellar domains were nucleated at the thick part of the film and propagated following the thickness gradient upon an isothermal annealing process. The lamellae became oriented along the thickness gradient upon the nucleation of well-ordered domains (Fig. 4a). The spontaneous lamellar alignment should involve the anisotropic mobility of defects. Since defects were annihilated through the
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interaction with oppositely signed defects,[26–28] the nucleation and growth of well-ordered domains was accompanied by the propagation of defects along the thickness gradient (Fig. 4a and b). Note that, because of the topological constraint of lamellar morphology, the defect mobility is higher in the lamellar parallel direction (climb) than in the lamellar perpendicular direction (glide).[28,29] The movement in the lamellar parallel direction requires the short-range diffusion of polymer molecules whereas movement in the lamellar perpendicular direction accompanies the sequential breakage and recombination of adjacent lamellae, which cause a high energy penalty. Because of the anisotropic mobility, the propagation of defects
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COMMUNICATION Figure 3. Tilted SEM images showing both the top surface and cross section of a PS-b-PMMA film that demonstrates thickness modulation. A block-copolymer film on a silicon substrate was cryofractured to reveal the cross section. The sample was tilted 458 during SEM observation to reveal both the top surface and cross section. The white scale bar corresponds to 5 mm. a) Tilted SEM image for a hill-shaped thickness modulation (the high-resolution image is presented in the Supporting Information, Fig. S2) b,c) SEM images for well-aligned lamellae that appeared where the PS-b-PMMA film had a sufficient thickness gradient. The lamellae were well-aligned along the thickness gradient and were observed on the top side of the film. In the cross section, lamellae were oriented perpendicularly to surface throughout the film thickness. d) The lamellar arrangement around the boundary between a well-aligned region and a randomly aligned region. The boundary is indicated with the white dotted line. Randomly aligned lamellae appeared where the PS-b-PMMA film had a modest thickness gradient. In the cross section, lamellae were oriented perpendicularly to the surface regardless of the film thickness.
Figure 4. Top-down SEM images of a PS-b-PMMA film observed at the various stages of well-aligned domain growth. a) The nucleation of aligned domains at the thick part of the PS-b-PMMA film. The preferential movement of the defects along the thickness gradient led to the spontaneous alignment of lamellae following the thickness gradient. b) Lamellar arrangement at the boundary between a propagating well-aligned domain and a randomly oriented domain observed at the intermediate stage of domain growth. The domain boundary indicated by the white dotted line was propagating along the thickness gradient. The major defect structure in the well-aligned domain was edge dislocation, oriented normally to the domain boundary. Disclinations as well as dislocations were present in the randomly aligned region. c) The tilted SEM image showing the cross-sectional morphology of a patterned film having a steep thickness gradient at its edge. Lamellae became well-aligned even in the vicinity of the film edge, provided that the film had a sufficient thickness gradient. The bare wafer surface was exposed on the right side of the image.
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Experimental A symmetric diblock copolymer, polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA, number-average molecular weight, Mn, of PS block: 52 kg mol 1, Mn of PMMA block: 52 kg mol 1, Polymer Source, Inc.), was dissolved in toluene with a concentration of approximately 1–4 wt%. The prepared solution was confined in the gap between a slide glass and a silicon wafer yielding a straight contact line of the polymer solution along the edge of the slide glass. The surface of the underlying substrate was neutrally treated to have identical interfacial tensions for PS and PMMA blocks. The glass plate located on top was mechanically drawn at a constant velocity (2–40 mm s 1). Since the polymer residue deposited at the evaporating front of the polymer solution temporarily pinned the receding contact line, the contact line underwent successive pinning and depinning to the substrate surface. As a consequence, the block-copolymer film, spontaneously organized from the receding contact line, had periodic thickness variation. The prepared micropatterned film was completely dried at room temperature and was sufficiently annealed at 190 8C. The resultant hierarchically nanopatterned film was characterized by using a 3D confocal measurement system (surf, Nano Focus) and FE-SEM (S4800, Hitachi). Received: September 8, 2007 Revised: November 2, 2007 Published online: May 26, 2008
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along a thickness gradient could raise the spontaneous alignment of lamellae in the same direction (Supporting Information, Fig. S5). Figure 4c shows the cross-sectional morphology of a patterned film at the edge of the patterned region. The bare wafer surface was exposed on the right side of the image. The well-aligned lamellae almost reached the edge of the patterned film. Lamellae became aligned even in the very thin part of the film, provided that the film had a sufficient thickness gradient. In summary, we have demonstrated a hierarchical nanopatterning process using self-assembling block copolymers. The microscale architecture in a block-copolymer film directed the spontaneous alignment of the nanoscale morphology through the directional growth of well-ordered domains. This facile and robust nanopatterning process employing the two levels of spontaneous orderings represents a versatile pathway to control nanoscale morphology by manipulating microscale architecture. Because of the simplicity of the microscale patterning process, our approach is readily applicable to an arbitrarily large area. Furthermore, the shape and characteristic length scales of a microscale pattern could be finely tuned in conjunction with other noninvasive patterning techniques such as imprinting or contact printing.[30] This could potentially allow for diversifying the shape and length scales of the hierarchical structures through all-parallel processing.
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