Optically Selective Microlens Photomasks Using Self-Assembled Smectic Liquid Crystal Defect Arrays

Optically Selective Microlens Photomasks Using Self-Assembled Smectic Liquid Crystal Defect Arrays By Yun Ho Kim, Jeong-Oen Lee, Hyeon Su Jeong, Jung Hyun Kim, Eun Kyung Yoon, Dong Ki Yoon, Jun-Bo Yoon, and Hee-Tae Jung* Microlens photolithographic fabrication using self-assembled materials has attracted considerable attention in recent years because the techniques involved are very simple, inexpensive, and provide a route to the fabrication of large-area patterns. [1,2] Several materials, which include colloids, hydrogels, and liquid crystals (LCs), have been used to fabricate self-assembled microlens arrays for photolithographic use. [3 7] Colloidal microspheres, 3 mm in diameter, were embedded in a transparent polymer membrane. These spheres acted as lenses to reduce centimeter-scale images to micrometer-scale images in the image plane, providing a simple way to produce spontaneous assembly over a large area, with the appropriate feature size, using microlens arrays optimized for visible light. [8] Biomimetic hydrogels were also used to fabricate microlens arrays for photolithography. [9,10] These systems adopted a conventional microlens system morphology, containing spherical or hemispherical geometric shapes with a homogeneous refractive index. A more advanced microlens system has used LCs for the fabrication of active devices, because the molecular orientations within a LC can be easily controlled by an external electric field. A radial distribution of the refractive index can be attained through application of an axially distributed electric field. [11 13] However, previous LC-based microlens systems required complex thick LC cell structures to control the molecular orientations of the nematic LC. Such cell structures included circular hole-patterned electrodes and polymer stabilizers. In this Communication, we report a new type of microstructure for the fabrication of optically selective microlens arrays. This system uses a periodic toric focal conic domain (TFCD) of smectic LCs as a photomask, combining two imaging elements, microlens arrays and clear windows. The shape and focusing mechanism of the TFCD microlens photomasks are very different from those of conventional microlens photomasks, which use spherical (or hemispherical) structures with homogeneous refractive indices. TFCD microlens photomasks have [*] Prof. H.-T. Jung, Y. H. Kim, H. S. Jeong, J. H. Kim, E. K. Yoon, Dr. D. K. Yoon Department of Chemical and Biomolecular Eng. (BK-21) Korea Advanced Institute of Science and Technology Daejeon 305-701 (Korea) E-mail: heetae@kaist.ac.kr Prof. J.-B. Yoon, J.-O. Lee School of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology Daejeon 305-701 (Korea) DOI: 10.1002/adma.200903728 several remarkable features. First, the periodic toroid-shaped holes of the TFCD structure act as microlenses due to the intrinsic molecular orientations of each TFCD, which can focus illuminated light. The flat regions between the toroidal holes act as clear windows and do not scatter light. Second, this system uses the advantages of a graded refractive index in LCs as well as periodic microscale arrays. The ordered TFCD structures are generated through the control of the molecular orientations in the LCs on surface modified substrates. [14,15] The light passing through a TFCD is refracted and focused to the center of the TFCD by the graded refractive index according to the intrinsic LC molecular orientations in a TFCD. Therefore, LC-based TFCD microlenses are optically selective for the direction of polarization of the transmitted light, when used as a photomask. Accordingly, one can obtain a variety of microscale patterns with controlled domain sizes, geometries, and symmetries, by simply adjusting the illumination dose (intensity), the size of the TFCD photomask, the tone of the photoresist, and the direction of polarization of the illuminating light source. To generate the TFCD structure, we used a simple rodlike smectic LC material containing a rigid biphenyl core and a semifluorinated tail group, which was prepared by alkylation of ethyl 4-hydroxyphenylbenzoate with 1H,1H,2H,2H,3H,3H,4H, 4H-perfluorododecyl bromide (Fig. 1a). [14 16] As reported previously, this material consistently yielded a hexagonal highly ordered structure of TFCDs on the surface of a treated glass substrate. [14,15] Upon cooling ( 1 8C min 1 ) from the isotropic to the S m A phase, ordered TFCD domain arrays were generated over large areas. Because the small LC components had a high mobility and responded rapidly in the smectic phase, the fabrication of TFCD microlens arrays was very fast and simple relative to other soft self-assembly building blocks. We found that the generation of a uniform TFCD large-scale array on a glass substrate required only a few seconds. Figure 1b shows representative polarized optical microscopy (POM) images of the TFCD domains of smectic LCs on a flat PEI-coated glass substrate and reveals the formation of highly ordered periodic TFCDs over a large area. Each small circular domain corresponds to a single TFCD. Close inspection of the POM images of the film formed by the LC revealed that the TFCD were identical in size and were present in a hexagonal array, a characteristic typical of S m A phases under surface anisotropy conditions. [17] Each TFCD produced a characteristic Maltese cross pattern ( microlens region), indicating that the projection of the director field onto the plane of the substrate was radial within the area bounded by the circular base of the TFCD. Outside the circular base (the window region), the molecules were vertically aligned to the 2416 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 2416 2420

Figure 1. a) Molecular structure and thermal transition of the smectic LC material. b) The polarized optical microscope texture of a hexagonally ordered TFCD array on the glass substrate. Each TFCD produces a characteristic Maltese cross patter indicating that the projection of the director field onto the plane of the substrate is radial within the area bounded by the circular basis of the TFCD. c) Schematic model of a hexagonally ordered TFCD array on the Si wafer. The small red rods represent each smectic molecule within the TFCD. substrate (homeotropic alignment between the TFCDs) and these regions appeared dark under POM. [18] Figure 1c shows a schematic model of the TFCD array, hexagonally ordered from the glass to the air LC interface, in which small red rods indicate each LC mesogenic unit. The refractive index of the smectic LC TFCD could be altered according to the LC molecular orientations in the TFCD microlenses. We employed the TFCD array as a photomask. Figure 2a schematically illustrates the generation of lithographic patterns, in which a photoresist (PR) film was illuminated through a fabricated self-assembled organic TFCD photomask. PR films, 2 mm thick, (AZ4330, positive tone and AZ5214, negative tone, Clariant Co. Ltd.) were spin-coated onto a bare silicon substrate. Figure 2. a) Schematic presentation of a photolithographic experiment with the TFCD microlens mask. The light passing through a TFCD is refracted and focused to the center of the TFCD by the graded refractive index according to the intrinsic LC molecular orientations in a TFCD. b) Schematic presentation of focusing mechanism of TFCD microlens. The small red rods represent each smectic molecule within the TFCD. c) AFM data of the TFCD photomask show a window and lens region. The illumination light is directly passing through the window region or is focusing to one spot in lens region. In this system, each TFCD had the ability to focus light. The effective refractive index could be volumetrically altered according to the intrinsic LC molecular orientations in a TFCD, which is increasing to the center region of TFCD. The incident light passing through a TFCD is refracted and focused to the center of the TFCD. The two components of each TFCD structure can be distinguished: The window region consists of a parallel smectic layered structure, in which the smectic molecules are perpendicularly aligned with respect to the substrate, thus the director field of LC molecules is exactly same to the direction of the illuminating UV light (l ¼ 365 nm). The other structure is a lens region, the central part of each TFCD, in which the orientations of smectic molecules vary with the curvature of the smectic layers in the TFCD. [19] Light passed directly through the window region and focused with a specific focal length in the lens region. [12] We anticipate that the unique combination of functional elements, the window and lens elements, in a single TFCD photomask structure, will permit variation in the patterns projected onto a PR film due to a different illumination dose. Both optical elements in the TFCD array were characterized by atomic force microscopy (AFM) surface topology imaging, as shown in Figure 2c. To demonstrate the utility of the TFCD photomask, we fabricated microstructures on a PR film by illumination through the TFCD photomask. The illumination dose was fixed to the sensitivity threshold of the PR material, (I I th ¼ 10 mj cm 2 ). The PR film appeared to be selectively exposed under each TFCD photomask as a result of the lensing properties. Figure 3 shows the possible optical pathways through the TFCD photomasks and scanning electron microscopy (SEM) images of the resulting complex patterns generated on the PR film using TFCDs of varying size. The TFCD size (or radius), r, is defined as half of the center-to-center distance between neighboring TFCDs. The size of the TFCD photomask was easily tuned between 2 and 6 mmby varying the LC film thickness, h. The TFCD radius was nearly equal to the LC film thickness in these regions, (r h). [20,21] Because the focal length changed with the domain size of the TFCDs, the size and shape of the features in the PR film could be effectively controlled by the domain size in the TFCD photomask. TFCDs with large feature sizes (4.5 mm < r < 6 mm) generated from thick LC films (4.5 mm < h L < 6 mm) produced a disklike microstructure with small holes at the center, as shown in Figure 3a. This type of feature resulted from the perfect transmission of the illuminating light in the window region, with focusing in the core lens regions. Several focal lengths resulted from the smectic molecular orientations. Because the focal lengths of the TFCD microlenses (2 mm) were small relative to the LC film thickness (5 mm), the illuminating light could not be focused at a single point on the PR film. The refracted light rays passing through the focal point crossed, generating disklike microstructures, as shown in Figure 3b. However, a small portion of the light in the lens region focused into the center and generated a shallow hole structure (200 nm in depth) at the core of the microstructure at low illumination doses. When the radii of TFCD (3 mm < r < 4.5 mm) microlenses in the photomask were gradually decreased to within a thickness range of 3 mm < h M < 4.5 mm, the illuminating light focused better at the core region, as shown in Figure 3d. The shape of the microstructure changed from a disklike to a conical structure. Adv. Mater. 2010, 22, 2416 2420 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2417

Figure 3. Schematic optical pathway through the various-sized TFCD microlens photomask and the SEM results of generated photoresist patterns as function of the TFCD photomask size. The size and shape of microstructures are changed by the radius, r, of TFCD photomask, which is easily controlled by smectic liquid crystal film thickness, h: a c) Large-sized TFCD (4.5 mm < r < 6.0 mm) is producing a hexagonally packed disklike microstructure; d f) medium-sized TFCD (3.0 mm < r < 4.5 mm) is generating hexagonally networked microstructure; g i) small-sized isolated TFCD (2.0 mm < r < 3.0 mm) is showing perfect ring structure. Structures in (b,e,h) resulted from positive-tone resists and structure in (c,f,i) from a negative-tone resist. The insets of (b,c,e,f) show magnified views of the generated structures. The scales bars are 20 mm (b,c,e,f; insets: 5 mm) and 2 mm (h,i). The pattern sizes decreased with decreasing TFCD microlens size in the photomask. Because the area of the window region was relatively small compared to the large TFCD array, a hexagonal network structure was generated on the substrate, as shown in Figure 3e. The neighboring regions ( window regions) between the TFCD domains were selectively etched away due to the large illumination dose. Six small hole patterns were observed around each domain with a diameter of 1 mm, and the border between domains remained on the substrate surface. Finally, isolated TFCDs (2 mm < r < 3 mm) were generated in very thin films (2 mm < h S < 3 mm), Figure 3g. These TFCDs acted as perfect microlenses. All of the light illuminating the lens region was focused to a single spot in the PR film. Because the focusing region was illuminated with a high illumination dose, we observed perfect ring patterns from the isolated TFCD photomask with small radii (2 mm < r < 3 mm) and small line widths (1 mm), Figure 3h. The use of a negative tone resist, AZ5214 (Fig. 3c, 3f, and 3i), yielded structures that were perfectly inverted with respect to those produced by the positive-tone PR. By controlling the illumination dose and focal length of the TFCD photomask, 3D microstructures could be formed on the PR film. This mask was similar to grayscale masks in that it permitted the fine adjustment of the intensity of transmitted light for the generation of 3D microstructures. [22,23] In our system, the illuminated light was refracted at a variety of focal lengths according to the molecular orientations in the TFCD. As a result, the TFCD structures efficiently created a distribution of transmitted intensity at the PR films. To realize an optically selective TFCD photomask, we used polarized illuminating light for photolithography by inserting a polarizer above the TFCD photomask, Figure 4a. Instead of using an optically isotropic material, as used in conventional microlenses, the TFCD microlenses possessed intrinsic LC molecular orientations along the geometric curvature of the smectic layer in the TFCD, as shown in Figure 1c. The TFCD microlenses could selectively pass or block the illumination light, depending on the direction of polarization, Figure 4a. [11,12] This occurred even in the absence of optical selectivity in the window region containing homeotropic molecular alignment. It is important to note that the large birefringence of LC molecules conveyed switching characteristics to the microlens photomask. Thus, the illumination dose applied to the PR film was not uniform. As a result, the microstructures differed depending on whether they were created by polarized or unpolarized illuminating light. All experimental conditions (illumination wavelength, dose, etc.) were the same as those given in Figure 2a. The polarization direction of the illumination light was easily controlled by the rotation of the polarizer. SEM images showed that anisotropic 2418 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 2416 2420

Figure 4. a) Schematic presentation of a photolithographic experiment using polarized exposure light. The illuminated light through optically selective TFCD photomask is focused with an anisotropic pattern following as the polarizing direction. b c) The SEM results of photoresist patterns generated using large-sized TFCD microlens arrays (4.5 mm < r < 6.0 mm) from positive-tone resist (b), and negative-tone resist (c) (158 tilted image). The inset in each SEM image shows a magnified view of the photoresist pattern. The black parallel arrows in (b) and (c) indicate the direction of the polarized light. Scale bars: 10 mm (b,c). microstructures on both positive-tone, Figure 4b, and negativetone, Figure 4c, PR films were obtained under polarized illumination. The black parallel arrows indicate the direction of polarized light. These SEM images were obtained from a large TFCD photomask (4.5 mm < r < 6 mm), which was identical to the photomask described previously in Figure 3b and 3c. Instead of isotropic disklike structures, as shown in Figure 3b, a fan-shaped structure with two-fold symmetry formed on the positive PR, Figure 4b, because the parallel-polarized light passed and focused only in the parallel direction of the TFCD microlens photomask. However, in the flat window regions between the TFCD microlenses, the LC molecules were aligned normal to the substrate (homeotropic alignment) and did not exhibit birefringence in the xy plane. This implied that the polarized illumination was not effective in providing anisotropic microstructures in these window regions. When a negative-tone PR was used, Figure 4c, we observed that the structures were inverted relative to the structures that developed on the positive-tone PR. In summary, we have introduced the concept of photolithography using a liquid crystal defect (TFCDs) array of smectic LCs as an optically selective photomask based on a new type of LC microlens system. Unlike conventional microlens systems that use geometric factors, such as spherical or hemispherical shapes with homogeneous refractive index, the TFCD photomask, acting as a microlens, combines two imaging elements, microlens arrays and clear windows, in a single structure. Moreover, a variety of complex patterns, with controlled size, geometry, topography, and symmetry, could be generated by controlling (i) the difference between illumination doses reaching the photoresist film through the window or lens regions; (ii) the domain size of the TFCD photomask; (iii) the tone of the photoresist, which yields inverted structures in either positive or negative tone resists; and (iv) the polarization of the illuminating light, because the optical selectivity of the TFCD microlens photomask is sensitive to the polarization of the illumination light. Thus, this method is cost-effective and amenable to mass production, and it could potentially lead to the use of LC defects as microlens photomasks. Experimental Photolithography: After spin-coating with a thick AZ4330 positive-tone photoresist (Clariant Co. Ltd.) layer at 2000 rpm for 2 s and rotating at 4700 rpm for 30 s to achieve a thickness of 2 mm on the Si wafer substrate, followed by soft baking at 90 8C for 5 min, the photoresist film was exposed to 365-nm UV light through a TFCD microlens photomask. The TFCD mask fabricated on the glass substrate was then placed directly on the photoresist film with a small pressure (220 N m 2 ) achieved by the application of weights to yield conformal contact. The illumination dose was fixed at the threshold of the photoresist, 100 mj cm 2. Experiments using the AZ5214 negative-tone photoresist used samples prepared as described above. The AZ5214 film was fixed at exactly the same thickness, 2 mm, as the AZ4330 film. After exposure, the photoresist films were developed for 60 s using a commercially available developing agent, AZ300MIF (Clariant Co. Ltd.). Surface Topological Measurements: SEM (Sirion FE-SEM, FEI, NNFC in KAIST) images were obtained by collecting the secondary electrons produced by bombarding the sample under an acceleration voltage between 5 and 10 kv. Surface-topological measurements were performed under ambient conditions using a commercial AFM (SPA400, Seiko, Japan) equipped with a 100 mm 100 mm scan head. All substrates were imaged in contact mode under atmospheric conditions using standard Si 3 N 4 cantilevers with a nominal spring constant of 0.08 m 1. Acknowledgements This work was supported by the National Research Laboratory Program of the Korea Science and Engineering Foundation (KOSEF) and the Basic Research Program (R01-2007-000-20655-0). Received: October 31, 2009 Revised: December 1, 2009 Published online: April 7, 2010 [1] S. Yang, G. Chen, M. Megens, C. K. 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