Imprint lithography for curved cross-sectional structure using replicated Ni mold

Imprint lithography for curved cross-sectional structure using replicated Ni mold Yoshihiko Hirai, a) Satoshi Harada, Hisao Kikuta, and Yoshio Tanaka Mechanical System Engineering, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Masato Okano Factory for Advanced Optical Technology, Osaka Science and Technology Center, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan Satoshi Isaka and Michio Kobayasi Hikifune Co. Ltd., 4-2-12 Higashiyotsugi, Katsushika-ku, Tokyo 124-0014, Japan Received 28 May 2002; accepted 26 August 2002 Fabrication of patterns with curved cross-sectional profiles for diffractive optical elements are demonstrated by imprint lithography using replicated Ni mold. The master mold pattern is fabricated by an electron beam lithography, where dosage distribution is automatically optimized by a computer aided design CAD system taking the proximity effects and resist development process into account. Utilizing the CAD system, a modulated pitched resist pattern with polynomial shaped cross-sectional profiles are successfully obtained by poly methylmethacrylate PMMA on Si substrate. Using the PMMA resist as a master pattern, replicated mold is fabricated by Ni electroforming followed by Ni electroless plating. Fine and curved cross-sectional patterns are successfully transferred to a resist on Si substrate by imprint lithography using the Ni replicated mold. In the same way, fabrication of sine waved and circular curved structures are demonstrated. This method is exceedingly useful for fabrication of integrated diffractive optical elements with a cross-sectional profile. 2002 American Vacuum Society. DOI: 10.1116/1.1515305 I. INTRODUCTION Imprint lithography is one of the promising technologies for fabricating fine and integrated pattern by low production cost. 1 On the other hand, one of the difficulties is that the resolution is restricted by the mold pattern. However, it is possible to transfer cross-sectional profiles to a polymer, once a mold is fabricated having objective cross-sectional structures such as curved cross-sectional profiles for optical elements. 2 Several methods have been approached using semiconductor lithography technique such as gray level mask 3 in photolithography or dose modulated electron beam lithography 4 6 to fabricate curved cross-sectional patterns or saw-teeth like patterns for diffractive optical elements. In those works, metal substrates or quarts substrate are mostly dry etched after these lithography processes to transfer the resist patterns to the substrate materials, which causes topological errors in the dry etching process. In addition, it is very difficult to design an exposure dose profile to obtain arbitrary cross-sectional profiles in electron beam lithography for a fine pattern. In this article, curved cross-sectional structures, whose master pattern is formed by optimized dose electron beam lithography, are demonstrated using imprint lithography. The schematic diagram of the proposed method is shown in Fig. 1. First, a curved cross-sectional master resist pattern is fabricated by automatically dose optimized electron beam exposure. Next, the master resist pattern is replicated by Ni electroforming to fabricate replicated mold for imprint a Electronic mail: hirai@mecha.osakafu-u.ac.jp lithography. Using the replicated mold, the master pattern is transferred by imprint lithography. Based on this method, fabrication of chirped diffraction grating, which has parabolic curved cross sectional profile with 2.0 m pitched pattern, is demonstrated by imprint lithography using replicated Ni mold. II. MASTER PATTERN FABRICATION BY ELECTRON BEAM LITHOGRAPHY Electron beam lithography is one of the most practical methods to fabricate various cross-sectional resist profiles because the exposure dose distribution is easily modulated by conventional electron beam exposure system. However, designing the dose distribution to obtain arbitrary crosssectional profile is very difficult due to the proximity effect. 7 Daschner et al. proposed novel proximity effect correction method to obtain arbitrary cross-sectional profiles for optical elements. 8 They utilize the double-gaussian approximation for energy intensity distribution 9 to calculate absorbed energy in the resist and the resist contrast characteristics to estimate residual resist thickness after developments. To obtain an arbitrary profile, exposure dose distribution is modulated by iteration process but the procedure is not presented in detail. This correction method is useful to fabricate the blazed grating with a period much longer than the wavelength of light. However, it is no longer valid for shortperiod patterns because the electron scattering and the time evolution of the resist development process are not precisely considered. 2867 J. Vac. Sci. Technol. B 20 6, NovÕDec 2002 1071-1023Õ2002Õ20 6 Õ2867Õ5Õ$19.00 2002 American Vacuum Society 2867

2868 Hirai et al.: Imprint lithography 2868 FIG. 1. Schematic diagram of the proposed method to obtain curved crosssectional structure by imprint lithography: a electron beam exposure by modulated dose distribution; b resist development to obtain master pattern by PMMA; c replicated mold fabrication of the master pattern by the nanoforming process; and d imprint lithography by the replicated mold. To solve the problem, we have developed advanced proximity effect correction system based on precious evaluation of the energy absorption and development process as shown in Fig. 2. 10 A Monte Carlo method 11 is applied to estimate the space distribution of the absorbed energy in the resist. Then, the resist development simulation 12 is performed to obtain time evolutions of the cross-sectional profile of the resist by development process. Based on the simulations, the exposure dose at the ith point i,n 1 is optimized to i,n by the following equation in the nth iteration process: Ē m t i,n i,n 1, 1 Ē where Ē t is the mean energy density from the initial resist surface to the target position in the resist and Ē is to the predicted resist position by the simulation, respectively. The order m is usually 0.5 1.0. Figure 3 shows the optimized results for a chirped diffraction grating. The target profile is chirped grating with 2.0 m in minimum period, 1.7 m in height and 100 m in width. The electron beam energy is 50 KeV and the resist is 2.0 m poly methyl methacrylate PMMA OEBR-1000 on Si. FIG. 2. Schematic diagram of the dose optimization process in electron beam lithography: a black diagram of the optimization process and b dose collection for target cross-sectional profile. The optimized dose distribution and the predicted resist cross sectional profile after development are shown in Figs. 3 a and 3 b. The error between the target profile and the predicted profile is shown in Fig. 3 c. The error becomes large at the steep edges. The mean error is around 90 nm. Figure 4 shows simulation and experimental result of the resist cross sectional profiles by the optimized dose exposure. Figure 4 a shows the time evolution of the resist crosssectional profiles during development process. The development is proportionally proceeds to the development time. As the development process is fairly stable, the cross-sectional profile could be controlled by the development time. Figure 4 b show the experimental result of the resist cross-sectional profile after 120 s development. Curved cross-sectional structure is successfully obtained by the optimized dose exposure and development time. J. Vac. Sci. Technol. B, Vol. 20, No. 6, NovÕDec 2002

2869 Hirai et al.: Imprint lithography 2869 FIG. 3. Dose optimization results for a chirped pattern: a optimized dose profile; b predicted resist profile after development by the optimized exposure; and c simulated errors of the resist profile compared to the target profile. It is confirmed that the dose optimization system is fairly effective for fine cross-sectional patterns. FIG. 4. PMMA master pattern fabrication by dose optimized electron beam exposure 50 KeV, 2.0 m PMMA on Si : a simulation result for time evolution of the resist development process and b cross-sectional SEM photograph of the resist tilt angle 70 ). FIG. 5. Schematic diagram of the replication process by combination of Ni electroless plating and Ni electroforming: a Ni electroless plating for the PMMA master pattern by Ni/B solution; b Ni electroforming by Ni sulfamate electrolyte using plus power supply; and c releasing from the master substrate by organic solvent. III. REPLICATED MOLD FABRICATION BY NANO FORMING Using the dose optimization system, curved-cross sectional structure is successfully obtained. The PMMA is effective material for an optical element but it is costly and time consuming for industrial fabrication using the electron beam exposure. To reduce the cost, we demonstrated fabrication of a Si mold using PMMA as an etching mask for Si substrate. 13 However, it is very hard to transfer the PMMA pattern to the substrate by the dry etching process because lateral etching for the PMMA causes fatal error in the pattern transfer process. One possible way for precious transformation of the master PMMA pattern is replication by the nanoforming process, which is a combination of electroless plating for insulation material and electric forming. Figure 5 shows a schematic diagram of the nanoforming process. After a surface cleaning process of the PMMA master pattern, electroless plating is performed by a commercially available Ni/B solution at 80 C for several minutes to grow the Ni thin conductive layer around 200 nm as shown in Fig. 5 a. Next, electroforming is carried out using a solution of nickel sulfamate electrolyte at 50 C as shown in Fig. 5 b. The current density is initially 0.5 and up to 5.0 A/dm 2 to reduce residual stress. The Ni replica plate is grown to a thickness of around 4.0 mm, which takes several days. At present, the residual stress has not been evaluated quantitatively. But the surface flatness is about less than 30 nm in a 0.43 mm square area by the optical interferometer Zygo observation. It seems to be flat enough for this application. After Ni electroforming, the master PMMA is removed by an organic solvent as shown in Fig. 5 c. Using this process, the PMMA master pattern is preciously replicated without fatal dimension errors. Figure 6 a shows a photograph of the replicated Ni mold by scanning electron microscopy SEM. Figure 6 b shows the scanning ion microscopy SIM of the cross section of the replicated Ni mold after focused ion beam FIB sputter- JVST B-Microelectronics and Nanometer Structures

2870 Hirai et al.: Imprint lithography 2870 FIG. 7. Cross-sectional profiles of the replicated Ni mold after FIB sputtering tilt angle 45 ): a sine waved curve and b circular-like curve. FIG. 6. Cross-sectional profiles of the Ni replicated mold and imprint result for the chirped pattern: a SEM image of the replicated Ni mold tilt angle 70 ); b SIM image of the cross-sectional profile after FIB sputtering of the replicated Ni mold tilt angle 45 ); and c SEM image of the imprinted pattern to PMMA film using replicated Ni mold tilt angle 70 ). The minimum period of the chirped pattern is 2.0 m. ing. The master pattern is successfully replicated by the Ni electroless plating and electroforming. Finally, a single molecular fluoropolymer layer is coated on the Ni surface using perfluoropolyether with methoxy hydrolysable silane-coupling agents to prevent adhesion problems in imprint lithography. 14 Using methoxy hydrolysable agents, the Ni surface is successfully covered with fluoropolymer without any erosion. molds. The imprint pressures are 90 MPa at 170 C. Curved fine patterns are successfully transferred to the PMMA films. As demonstrated above, curved structures are successfully obtained without any additional process such as multistep development or dry etching, once the mold is fabricated. This method is exceedingly useful for mass production of unusually shaped patterns. V. CONCLUSIONS To obtain a curved cross-sectional structure by imprint lithography, an automatic dose optimization system is utilized to fabricate the master resist pattern using electron beam lithography. Using the system, a PMMA master pattern IV. IMPRINT EXPERIMENTS Using the Ni replicated mold with curved cross-sectional structure, imprint lithography is carried out to a PMMA thin film on Si substrate. A 2.0 m thick PMMA is spun coated on a Si substrate. The molds are pressed into the PMMA film at 170 C at 20 MPa for 5 min and released at 60 C. Figure 6 c shows the SEM photograph of the imprint result. The pattern size is 2.0 m in a minimum period, 100 m in width, and 2.5 mm in length. A fine pattern with a curved structure is successfully transferred to the PMMA surface using the replicated Ni mold. In the same way, imprint lithography for a sine waved curve and a circular-like curve is demonstrated using a dose optimization system and replicated Ni molds. Figure 7 shows the cross-sectional profiles of the replicated Ni molds. The period of the patterns are 4.0 m. Fine and smooth crosssectional profiles are successfully obtained for curved structures. Figure 8 shows cross-sectional profiles of the imprinted results to 2.0 m PMMA on Si using the replicated FIG. 8. Cross-sectional profiles of the imprinted pattern using replicated Ni molds tile angle 70, 2.0 m PMMA on Si, 90 MPa, 170 C): a sine waved curve and b circular-like curve. J. Vac. Sci. Technol. B, Vol. 20, No. 6, NovÕDec 2002

2871 Hirai et al.: Imprint lithography 2871 with curved cross-sectional pattern is successfully obtained on the Si substrate. The master pattern is replicated by the combination of Ni electroless plating and Ni electroforming. Using the replaced Ni mold, imprint lithography for a chirped optical grating pattern with 2.0 m in the minimum period, 1.5 m in height, and sine waved and circular-like patterns are successfully demonstrated. This method is a promising lithographic method to obtain cross-sectional profiles for optical devices or microfluid structures for biochemical devices. ACKNOWLEDGMENTS The authors thank Dr. K. Murata at Osaka Prefecture University and Nalux Corporation for their helpful discussions and supports. 1 S. Y. Chou, P. R. Krauss, and P. J. Renstrom, Appl. Phys. Lett. 67, 3114 1995. 2 H. Herzing, Micro-optics Taylor and Francis, London, 1997. 3 W. Daschner, P. Long, M. Larsson, and S. Lee, J. Vac. Sci. Technol. B 13, 2729 1995. 4 E. Kley, Microelectron. Eng. 34, 243 1997. 5 M. Ekberg, F. Nikolajeff, M. Larsson, and S. Hard, Appl. Opt. 33, 103 1994. 6 M. Okano, T. Yotsuya, Y. Hirai, H. Kikuta, and K. Yamamoto, Proc. SPIE 4440, 268 2001. 7 D. Kyser and N. Viswanathan, J. Vac. Sci. Technol. 12, 1305 1970. 8 W. Daschner, M. Larsson, and S. Lee, Appl. Opt. 34, 2534 1995. 9 T. Chang, J. Vac. Sci. Technol. 12, 1271 1970. 10 Y. Hirai, H. Kikuta, M. Okano, T. Yotsuya, and K. Yamamoto, Jpn. J. Appl. Phys., Part 1 39, 6831 2000. 11 D. Kyser and K. Murata, IBM J. Res. Dev. 18, 352 1974. 12 F. Dill, A. R. Neureuther, J. A. Tuttle, and E. J. Walker, IEEE Trans. Electron Devices ED-22, 456 1970. 13 Y. Hirai, M. Okano, H. Okuno, H. Toyota, T. Yotsuya, H. Kikuta, and Y. Tanaka, Proc. SPIE 4440, 228 2001. 14 Y. Hirai, S. Yoshida, A. Okamoto, Y. Tanaka, M. Endo, S. Irie, H. Nakagawa, and M. Sasago, J. Photopolym. Sci. Technol. 14, 457 2001. JVST B-Microelectronics and Nanometer Structures