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Fabrication of polymer photonic crystal slabs using nanoimprint lithography
Current Applied Physics 6S1 (2006) e8–e11 www.elsevier.com/locate/cap www.kps.or.kr
Fabrication of polymer photonic crystal slabs using nanoimprint lithography Choon-Gi Choi a
a,*
, Chul-Sik Kee b, Helmut Schift
c
Basic Research Laboratory, Electronics and Telecommunications Research Institut (ETRI), Daejeon 305-700, Republic of Korea b Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea c Laboratory of Micro and Nanotechnology, Paul Scherrer Institut (PSI), CH-5232, Villigen PSI, Switzerland Received 15 July 2005 Available online 5 April 2006
Abstract We applied the thermal nanoimprint lithography to replicate two-dimensional photonic crystal slabs having line defected waveguides with a triangular array of air-holes on polymers. The photonic band gap structures, calculated by the plane wave expansion technique, show that 2-D polystyrene photonic crystal slabs have a complete small photonic band gaps between 0.49 and 0.51 for the TE-like modes. The fabrication process of 2-D polystyrene photonic crystal slabs is reported. The dimensions of the fabricated line defected waveguide slab structures with a triangular array of air-holes correspond well to those of the proposed structures. The nanoimprinted polymer photonic crystal slabs will be attractive candidates for the implementations of ultra-compact low cost photonic crystal integrated circuits. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.16.Nd; 42.82.Cr; 42.70.Qs Keywords: Thermal nanoimprint lithography; Polymer photonic crystals; Slab waveguides
1. Introduction Photonic crystals (PC) are periodic dielectric structures of two or more materials with index contrast. The periodic dielectric structures produce interesting properties such as photonic band gap (PBG), where the propagation of photons within a certain range of frequencies is forbidden, and defect mode where the states in the PBG introduced by local defects in the photonic crystals is allowed [1]. Waveguides (cavities) can be produced by introducing line (point) defects in photonic crystal structures, which behave like channels where the light is very efficiently guided. Furthermore, we can control the frequency of a defect mode by changing the size or the shape of defect. These adjustable defect modes have played key roles in the applications of photonic crystals in optical micro-cavities and waveguides *
Corresponding author. Tel.: +82 42 860 6834; fax: +82 42 860 6248. E-mail address: [email protected] (C.-G. Choi).
1567-1739/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2006.01.003
[2]. Of various structures that have been proposed for photonic crystals, photonic crystal slabs with two-dimensional (2-D) periodic patterns in a transparent dielectric layer have been intensively investigated, since they can be fabricated at telecommunication wavelengths using semiconductor technologies and give an opportunity to control the propagation of light in planes [3]. By incorporating structural defects such as point or line defects into photonic crystal slabs, various functional nanophotonic devices can be implemented. Thus they are expected to serve as ultracompact integrated optical devices such as photonic crystal integrated circuits (PCIC), to be the counterpart of integrated electronic circuits. Photonic crystal devices with 1D, 2-D and 3-D structures have been fabricated by wet/ dry etching, wafer bonding, self-assembled method and electron-beam lithography [4]. Among these 3-type structures, 2-D photonic crystals can be exploited to realize micro-cavity lasers and linear waveguides to direct the electromagnetic wave towards specific regions, thus allowing
C.-G. Choi et al. / Current Applied Physics 6S1 (2006) e8–e11
high integration in planar photonic crystal integrated circuits [5,6]. Recently, nanoimprint lithography (NIL) is standing in the spotlight of an emerging technique for next-generation lithography because of their producibility of nano-scale features with a cost-effective and high throughput [7]. Among the available materials for photonic applications, such as InP, GaAs, SOI and polymers, the polymers have attracted a great interest, due to their low temperature fabrication, good mass production possibilities with low processing cost, easy functionalization and possibility to tune their optical properties [8,9]. Compared to semiconductor materials such as InP, GaAs and SOI, the low refractive index contract based polymer photonic crystal waveguide can allow larger optical mode volume and facilitate the coupling to the standard single-mode optical fibers [10]. In this paper, we present the design and the fabrication of 2-D polymer photonic crystal slabs having line defected waveguides with a triangular array of air-holes obtained by using a thermal nanoimprint lithography. The high refractive index of polystyrene (PS) combined with the good optical properties and the possibility of the thermal imprinting makes this polymer ideal for the realization of high-resolution photonic crystal slabs. 2. Photonoic band gap calculation 2-D photonic crystal slabs typically consist of a periodic array of air-holes in a dielectric medium. Before the fabrication of the 2-D polymer photonic crystal slab with a triangular array of air-holes, the structure was carefully designed and calculated, as shown in Fig. 1. In the vertical direction, light is confined to the slab due to total internal reflection, and in the lateral direction, light is controlled by the means of distributed Bragg reflection due to the presence of the 2-D photonic crystal slab. The photonic crystal structure consists of a periodic arrangement of air-holes imprinted into a polymer thin film suspended so that it is surrounded by air on both substrate and superstate sides. In this work, we consider polystyrene as a suitable polymer because it can be used as a thermal imprintable material with its low glass transition temperature (Tg) of 90 °C and its high refractive index (n) of 1.59. The plane wave expansion method was used to calculate the photonic band structures of the polymer slab with a triangular array of
Fig. 1. Schematic of the 2-D polymeric photonic crystal slab consisting of line defected waveguides with a triangular array of air-holes.
e9
Fig. 2. Calculated diagram of photonic band structure for the TE-like modes of polystyrene photonic crystal slab in air with a triangular array of air-holes when the slab thickness of 1.2a and the hole radius (R) of 0.34a, where a is the lattice constant, was performed by plane wave expansion method.
air-holes. The transmission spectra were simulated using the 3-D finite difference time domain (FDTD) technique. Fig. 2 represents the photonic band structure for TE-like modes of the photonic crystal slab in polystyrene assuming the slab thickness (t) of 1.2a and the hole radius (R) of 0.34a, where a is the lattice constant. The calculation of the 2-D photonic band structure was performed by plane wave expansion method. The frequency is normalized to 2pc/a, where c is the light speed in vacuum. The solid line is the light line and thus the gray region denotes radiation modes that can be coupled with free space modes. The inset indicates the points of high symmetry in the first Brillouin zone of the triangular lattice. We can see that the polystyrene photonic crystal slab can exhibit a complete inplane PEG between 0.49 and 0.51. To confirm the narrow band gap, we have also calculated the transmission spectra for the TE-like modes propagating along CK and CM directions. Fig. 3 shows the simulated transmission spectra for the TE-like modes propagating along CK (solid line) and CM
Fig. 3. Simulated transmission spectra for the TE-like modes propagating along CK (solid line) and CM (dashed line) directions in the polystyrene photonic crystal slab in air with a triangular array of air-holes. The inset in the left corner shows the values of lattice constant, a = 755 nm, the hole radius, 2r = 527 nm, and the thickness of slab, h = 930. The stop band range where the transmittance is lower than 20 dB is from 1490 to 1580 nm.
e10
C.-G. Choi et al. / Current Applied Physics 6S1 (2006) e8–e11
(dashed line) directions assuming that the lattice constant of the triangular array is 755 nm and thus the hole diameter (2R) of 527 nm (0.68a) and the slab thickness of the polystyrene with 930 nm (1.2a). We can see the common stop band in the transmission spectra along the two directions. The stop band range, where the transmittance is lower than 20 dB is from 1500 to 1580 nm. This agrees well with the band gap range between 1519 and 1581 nm. 3. Photonic crystal slab fabrication A large-area and uniform silicon stamp with circular dot structures was fabricated in a silicon wafer by using electron-beam lithography and ICP etching in order to replicate 2-D photonic crystal slabs. The electron-beam exposures were carried out with a Gaussian vector scan on an electron-beam writer system (LEICA EBPG 5000 plus) at 100 kV acceleration voltage. ZEP520 was applied as an electron-beam resist. ZEP520 is a positive resist and is suitable to achieve high-resolution patterns as small as 50 nm with low energy of 1 keV exposures. ZEP520 was diluted using a thinner, spin-coated on a 4 in. (100) wafer with 4000 rpm for a duration of 45 s, and then baked at 190 °C for 2 min to achieve a desired film thickness of ˚ . The exposed beam energy was 100 keV and the 670 A selected beam current was 4.8 nA. Using the same design patterns, an exposure dose was implemented starting at 300 lC/cm2 using an increasing rate of 20% at each new exposure in order to find out the optimal writing condition. The resist patterns were developed using a developer for 1 min and rinsed for 10 s. The sizes of the developed patterns with air-hole structures in a developed resist were measured using an in-line critical dimension-scanning electron microscope (CD-SEM). After the CD-SEM measurements, a lift-off process was applied due to the high-aspect ratio dot structures of more than 1.5, which cannot be achieved with ZEP520 resist, and used to open windows of patterned structures in the silicon substrate, followed by the ICP etching to transfer the pattern into the silicon substrate. Chromium (Cr) with a thickness of 20 nm was deposited with a metal sputtering method to be used as a mask layer for the ICP etching process. The e-beam resist lift-off was performed using ZDMAC for 20 min with ultrasonic. After the ICP etching, the Cr mask layer was removed in Cr etchant. The diameters and structures of the fabricated dots were repeatedly examined in a scanning electron microscopy. Fig. 4 shows the scanning electron microscopy image of the silicon stamp with triangular array of circular rods fabricated using the e-beam lithography and the ICP etching. The diameter of rod is about 520 nm and the lattice constant about 800 nm. The height of rod is usually lower than 950 nm. These measured values correspond approximately to the dimensions of the design parameters. Before imprinting, the silicon stamp was pretreated by a liquid phase deposition of 1H,1H,2H,2H-perfluorooctyltrichlorosilane forming a self-assembled monolayer
Fig. 4. Scanning electron microscopy image of the silicon stamp having a triangular array of circular rods with a line defected waveguide structure fabricated by using e-beam lithography and subsequent inductively coupled plasma etching.
(SAM) based anti-sticking layer on the surfaces to enforce the easy releasing of the stamp from the imprinted polymers. The water contact angles on the surface of stamps were changed from 45° to more than 100°, and then the hydrophilic surface changed to a hydrophobic surface. We applied the thermal nanoimprint lithography process to replicate 2-D periodic patterns on polystyrene thin films. A 13% solution of polystyrene (Mw = 125 K) diluted in toluene was spun on silicon wafer to produce a uniform 1000 nm thick player and cured at 180 °C for 10 min before
Fig. 5. SEM images of the fabricated large-area 2-D polystyrene photonic crystal slabs with a triangular array of air-holes using the thermal nanoimprint lithography. Polystyrene photonic crystal slabs having a line defected waveguide structure with a triangular array of air-holes consisting of holes with a diameter of 515 nm and depth of 850 nm (aspect ratio of 1.65) were replicated by the silicon stamp.
C.-G. Choi et al. / Current Applied Physics 6S1 (2006) e8–e11
applying an imprinting. The mold and the prebaked polystyrene coated substrate brought into contact by applying 30 bar using air-compressed press. The polystyrene film was imprinted for 10 min at 180 °C and after each imprinting the quality of stamp and substrate was inspected by using an optical microscope. We imprinted the silicon stamp with the rod height of 950 nm in polystyrene film on the silicon substrate to get the triangular array of air-holes using the thermal nanoimprint lithography process. Large-area polystyrene photonic crystal slabs having a line defected waveguide structure with a triangular array of air-holes were uniformly replicated by the silicon stamp, as shown in Fig. 5. The diameter of imprinted holes is about 515 nm and the lattice constant about 780 nm. The averaged depth of imprinted holes observed by the scanning electron microscopy is about 850 nm, which is a little bit lower than the height of the stamp. Large-area patterns of the 2-D photonic crystal slab have been successfully replicated on the polystyrene. The dimensions of the fabricated line defected waveguide slab structures with a triangular array of air-holes correspond well to those of the proposed structures. The fabrication and optical characterization of an air-suspended membrane-type 2-D photonic crystal slab will be realized in the near future after selectively removing the underlying substrate. 4. Conclusions We have reported on the design and the fabrication of 2D polymer photonic crystal slabs consisting of a line defected waveguide with a triangular array of air-holes using the thermal nanoimprint lithography. The calculation of photonic band gap structures shows that 2-D polystyrene photonic crystal slabs have a complete small photonic band gaps between 0.49 and 0.51 for the TE-like
e11
modes. Patterns of the 2-D photonic crystal slab with the same dimensions as in the simulation have been successfully replicated on the polystyrene. The nanoimprint lithography can provide a promising opportunity to rapidly and inexpensively implement large-area, ultra-compact photonic crystal circuits using relatively simple steps in the fabrication process. Acknowledgement This work was financially supported by Ministry of Science and Technology through Korea–Swiss International Cooperative Research Program and 21st Century Frontier Research Program (Center for Nanoscale Mechatronics & Manufacturing). References [1] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Photonic Crystals: Molding the Flow of Light, Princeton University Press, Princeton, 1995. [2] S. Fan, P.R. Villeneuve, J.D. Joannopoulos, H.A. Haus, Phys. Rev. Lett. 80 (1998) 960. [3] S. Noda, M. Imada, M. Okano, S. Ogawa, M. Mochizuki, A. Chutinan, IEEE J. Auantum Electron. 38 (2002) 726. [4] E. Chomski, G.A. Ozin, Adv. Mater. 12 (14) (2000) 1071. [5] C.J.M. Smith, R.M. De La Rue, M. Rattier, S. Olivier, H. Benisty, C. Weisbush, T.F. Krauss, R. Huodre´, U. Osterle, Appl. Phys. Lett. 78 (2001) 1487. [6] J.D. Joannopoulos, P.R. Villeneuve, S. Fan, Nature 386 (1997) 143. [7] S.Y. Chou, P.R. Krauss, W. Zhang, L. Guo, L. Zhuang, J. Vac. Sci. Technol. B 15 (6) (1997) 2897. [8] M. De Vittorio, M.T. Todaro, T. Stomeo, R. Cingolani, D. Cojoc, E. Di Fabrizio, Microelectron. Eng. 73–74 (2004) 388. [9] H. Schift, S. Park, B. Jung, C.-G. Choi, C.-S. Kee, S.-P. Han, K.-B. Yoon, J. Gobrecht, Nanotechnology 16 (2005) S261. [10] C. Liguda, G. Bottger, A. Kuligk, R. Blum, M. Eich, H. Roth, J. Kunert, W. Morgenroth, H. Eisner, H.G. Meyer, Appl. Phys. Lett. 78 (2001) 2434.
Fabrication of polymer photonic crystal slabs using nanoimprint lithography Choon-Gi Choi a
a,*
, Chul-Sik Kee b, Helmut Schift
c
Basic Research Laboratory, Electronics and Telecommunications Research Institut (ETRI), Daejeon 305-700, Republic of Korea b Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea c Laboratory of Micro and Nanotechnology, Paul Scherrer Institut (PSI), CH-5232, Villigen PSI, Switzerland Received 15 July 2005 Available online 5 April 2006
Abstract We applied the thermal nanoimprint lithography to replicate two-dimensional photonic crystal slabs having line defected waveguides with a triangular array of air-holes on polymers. The photonic band gap structures, calculated by the plane wave expansion technique, show that 2-D polystyrene photonic crystal slabs have a complete small photonic band gaps between 0.49 and 0.51 for the TE-like modes. The fabrication process of 2-D polystyrene photonic crystal slabs is reported. The dimensions of the fabricated line defected waveguide slab structures with a triangular array of air-holes correspond well to those of the proposed structures. The nanoimprinted polymer photonic crystal slabs will be attractive candidates for the implementations of ultra-compact low cost photonic crystal integrated circuits. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.16.Nd; 42.82.Cr; 42.70.Qs Keywords: Thermal nanoimprint lithography; Polymer photonic crystals; Slab waveguides
1. Introduction Photonic crystals (PC) are periodic dielectric structures of two or more materials with index contrast. The periodic dielectric structures produce interesting properties such as photonic band gap (PBG), where the propagation of photons within a certain range of frequencies is forbidden, and defect mode where the states in the PBG introduced by local defects in the photonic crystals is allowed [1]. Waveguides (cavities) can be produced by introducing line (point) defects in photonic crystal structures, which behave like channels where the light is very efficiently guided. Furthermore, we can control the frequency of a defect mode by changing the size or the shape of defect. These adjustable defect modes have played key roles in the applications of photonic crystals in optical micro-cavities and waveguides *
Corresponding author. Tel.: +82 42 860 6834; fax: +82 42 860 6248. E-mail address: [email protected] (C.-G. Choi).
1567-1739/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2006.01.003
[2]. Of various structures that have been proposed for photonic crystals, photonic crystal slabs with two-dimensional (2-D) periodic patterns in a transparent dielectric layer have been intensively investigated, since they can be fabricated at telecommunication wavelengths using semiconductor technologies and give an opportunity to control the propagation of light in planes [3]. By incorporating structural defects such as point or line defects into photonic crystal slabs, various functional nanophotonic devices can be implemented. Thus they are expected to serve as ultracompact integrated optical devices such as photonic crystal integrated circuits (PCIC), to be the counterpart of integrated electronic circuits. Photonic crystal devices with 1D, 2-D and 3-D structures have been fabricated by wet/ dry etching, wafer bonding, self-assembled method and electron-beam lithography [4]. Among these 3-type structures, 2-D photonic crystals can be exploited to realize micro-cavity lasers and linear waveguides to direct the electromagnetic wave towards specific regions, thus allowing
C.-G. Choi et al. / Current Applied Physics 6S1 (2006) e8–e11
high integration in planar photonic crystal integrated circuits [5,6]. Recently, nanoimprint lithography (NIL) is standing in the spotlight of an emerging technique for next-generation lithography because of their producibility of nano-scale features with a cost-effective and high throughput [7]. Among the available materials for photonic applications, such as InP, GaAs, SOI and polymers, the polymers have attracted a great interest, due to their low temperature fabrication, good mass production possibilities with low processing cost, easy functionalization and possibility to tune their optical properties [8,9]. Compared to semiconductor materials such as InP, GaAs and SOI, the low refractive index contract based polymer photonic crystal waveguide can allow larger optical mode volume and facilitate the coupling to the standard single-mode optical fibers [10]. In this paper, we present the design and the fabrication of 2-D polymer photonic crystal slabs having line defected waveguides with a triangular array of air-holes obtained by using a thermal nanoimprint lithography. The high refractive index of polystyrene (PS) combined with the good optical properties and the possibility of the thermal imprinting makes this polymer ideal for the realization of high-resolution photonic crystal slabs. 2. Photonoic band gap calculation 2-D photonic crystal slabs typically consist of a periodic array of air-holes in a dielectric medium. Before the fabrication of the 2-D polymer photonic crystal slab with a triangular array of air-holes, the structure was carefully designed and calculated, as shown in Fig. 1. In the vertical direction, light is confined to the slab due to total internal reflection, and in the lateral direction, light is controlled by the means of distributed Bragg reflection due to the presence of the 2-D photonic crystal slab. The photonic crystal structure consists of a periodic arrangement of air-holes imprinted into a polymer thin film suspended so that it is surrounded by air on both substrate and superstate sides. In this work, we consider polystyrene as a suitable polymer because it can be used as a thermal imprintable material with its low glass transition temperature (Tg) of 90 °C and its high refractive index (n) of 1.59. The plane wave expansion method was used to calculate the photonic band structures of the polymer slab with a triangular array of
Fig. 1. Schematic of the 2-D polymeric photonic crystal slab consisting of line defected waveguides with a triangular array of air-holes.
e9
Fig. 2. Calculated diagram of photonic band structure for the TE-like modes of polystyrene photonic crystal slab in air with a triangular array of air-holes when the slab thickness of 1.2a and the hole radius (R) of 0.34a, where a is the lattice constant, was performed by plane wave expansion method.
air-holes. The transmission spectra were simulated using the 3-D finite difference time domain (FDTD) technique. Fig. 2 represents the photonic band structure for TE-like modes of the photonic crystal slab in polystyrene assuming the slab thickness (t) of 1.2a and the hole radius (R) of 0.34a, where a is the lattice constant. The calculation of the 2-D photonic band structure was performed by plane wave expansion method. The frequency is normalized to 2pc/a, where c is the light speed in vacuum. The solid line is the light line and thus the gray region denotes radiation modes that can be coupled with free space modes. The inset indicates the points of high symmetry in the first Brillouin zone of the triangular lattice. We can see that the polystyrene photonic crystal slab can exhibit a complete inplane PEG between 0.49 and 0.51. To confirm the narrow band gap, we have also calculated the transmission spectra for the TE-like modes propagating along CK and CM directions. Fig. 3 shows the simulated transmission spectra for the TE-like modes propagating along CK (solid line) and CM
Fig. 3. Simulated transmission spectra for the TE-like modes propagating along CK (solid line) and CM (dashed line) directions in the polystyrene photonic crystal slab in air with a triangular array of air-holes. The inset in the left corner shows the values of lattice constant, a = 755 nm, the hole radius, 2r = 527 nm, and the thickness of slab, h = 930. The stop band range where the transmittance is lower than 20 dB is from 1490 to 1580 nm.
e10
C.-G. Choi et al. / Current Applied Physics 6S1 (2006) e8–e11
(dashed line) directions assuming that the lattice constant of the triangular array is 755 nm and thus the hole diameter (2R) of 527 nm (0.68a) and the slab thickness of the polystyrene with 930 nm (1.2a). We can see the common stop band in the transmission spectra along the two directions. The stop band range, where the transmittance is lower than 20 dB is from 1500 to 1580 nm. This agrees well with the band gap range between 1519 and 1581 nm. 3. Photonic crystal slab fabrication A large-area and uniform silicon stamp with circular dot structures was fabricated in a silicon wafer by using electron-beam lithography and ICP etching in order to replicate 2-D photonic crystal slabs. The electron-beam exposures were carried out with a Gaussian vector scan on an electron-beam writer system (LEICA EBPG 5000 plus) at 100 kV acceleration voltage. ZEP520 was applied as an electron-beam resist. ZEP520 is a positive resist and is suitable to achieve high-resolution patterns as small as 50 nm with low energy of 1 keV exposures. ZEP520 was diluted using a thinner, spin-coated on a 4 in. (100) wafer with 4000 rpm for a duration of 45 s, and then baked at 190 °C for 2 min to achieve a desired film thickness of ˚ . The exposed beam energy was 100 keV and the 670 A selected beam current was 4.8 nA. Using the same design patterns, an exposure dose was implemented starting at 300 lC/cm2 using an increasing rate of 20% at each new exposure in order to find out the optimal writing condition. The resist patterns were developed using a developer for 1 min and rinsed for 10 s. The sizes of the developed patterns with air-hole structures in a developed resist were measured using an in-line critical dimension-scanning electron microscope (CD-SEM). After the CD-SEM measurements, a lift-off process was applied due to the high-aspect ratio dot structures of more than 1.5, which cannot be achieved with ZEP520 resist, and used to open windows of patterned structures in the silicon substrate, followed by the ICP etching to transfer the pattern into the silicon substrate. Chromium (Cr) with a thickness of 20 nm was deposited with a metal sputtering method to be used as a mask layer for the ICP etching process. The e-beam resist lift-off was performed using ZDMAC for 20 min with ultrasonic. After the ICP etching, the Cr mask layer was removed in Cr etchant. The diameters and structures of the fabricated dots were repeatedly examined in a scanning electron microscopy. Fig. 4 shows the scanning electron microscopy image of the silicon stamp with triangular array of circular rods fabricated using the e-beam lithography and the ICP etching. The diameter of rod is about 520 nm and the lattice constant about 800 nm. The height of rod is usually lower than 950 nm. These measured values correspond approximately to the dimensions of the design parameters. Before imprinting, the silicon stamp was pretreated by a liquid phase deposition of 1H,1H,2H,2H-perfluorooctyltrichlorosilane forming a self-assembled monolayer
Fig. 4. Scanning electron microscopy image of the silicon stamp having a triangular array of circular rods with a line defected waveguide structure fabricated by using e-beam lithography and subsequent inductively coupled plasma etching.
(SAM) based anti-sticking layer on the surfaces to enforce the easy releasing of the stamp from the imprinted polymers. The water contact angles on the surface of stamps were changed from 45° to more than 100°, and then the hydrophilic surface changed to a hydrophobic surface. We applied the thermal nanoimprint lithography process to replicate 2-D periodic patterns on polystyrene thin films. A 13% solution of polystyrene (Mw = 125 K) diluted in toluene was spun on silicon wafer to produce a uniform 1000 nm thick player and cured at 180 °C for 10 min before
Fig. 5. SEM images of the fabricated large-area 2-D polystyrene photonic crystal slabs with a triangular array of air-holes using the thermal nanoimprint lithography. Polystyrene photonic crystal slabs having a line defected waveguide structure with a triangular array of air-holes consisting of holes with a diameter of 515 nm and depth of 850 nm (aspect ratio of 1.65) were replicated by the silicon stamp.
C.-G. Choi et al. / Current Applied Physics 6S1 (2006) e8–e11
applying an imprinting. The mold and the prebaked polystyrene coated substrate brought into contact by applying 30 bar using air-compressed press. The polystyrene film was imprinted for 10 min at 180 °C and after each imprinting the quality of stamp and substrate was inspected by using an optical microscope. We imprinted the silicon stamp with the rod height of 950 nm in polystyrene film on the silicon substrate to get the triangular array of air-holes using the thermal nanoimprint lithography process. Large-area polystyrene photonic crystal slabs having a line defected waveguide structure with a triangular array of air-holes were uniformly replicated by the silicon stamp, as shown in Fig. 5. The diameter of imprinted holes is about 515 nm and the lattice constant about 780 nm. The averaged depth of imprinted holes observed by the scanning electron microscopy is about 850 nm, which is a little bit lower than the height of the stamp. Large-area patterns of the 2-D photonic crystal slab have been successfully replicated on the polystyrene. The dimensions of the fabricated line defected waveguide slab structures with a triangular array of air-holes correspond well to those of the proposed structures. The fabrication and optical characterization of an air-suspended membrane-type 2-D photonic crystal slab will be realized in the near future after selectively removing the underlying substrate. 4. Conclusions We have reported on the design and the fabrication of 2D polymer photonic crystal slabs consisting of a line defected waveguide with a triangular array of air-holes using the thermal nanoimprint lithography. The calculation of photonic band gap structures shows that 2-D polystyrene photonic crystal slabs have a complete small photonic band gaps between 0.49 and 0.51 for the TE-like
e11
modes. Patterns of the 2-D photonic crystal slab with the same dimensions as in the simulation have been successfully replicated on the polystyrene. The nanoimprint lithography can provide a promising opportunity to rapidly and inexpensively implement large-area, ultra-compact photonic crystal circuits using relatively simple steps in the fabrication process. Acknowledgement This work was financially supported by Ministry of Science and Technology through Korea–Swiss International Cooperative Research Program and 21st Century Frontier Research Program (Center for Nanoscale Mechatronics & Manufacturing). References [1] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Photonic Crystals: Molding the Flow of Light, Princeton University Press, Princeton, 1995. [2] S. Fan, P.R. Villeneuve, J.D. Joannopoulos, H.A. Haus, Phys. Rev. Lett. 80 (1998) 960. [3] S. Noda, M. Imada, M. Okano, S. Ogawa, M. Mochizuki, A. Chutinan, IEEE J. Auantum Electron. 38 (2002) 726. [4] E. Chomski, G.A. Ozin, Adv. Mater. 12 (14) (2000) 1071. [5] C.J.M. Smith, R.M. De La Rue, M. Rattier, S. Olivier, H. Benisty, C. Weisbush, T.F. Krauss, R. Huodre´, U. Osterle, Appl. Phys. Lett. 78 (2001) 1487. [6] J.D. Joannopoulos, P.R. Villeneuve, S. Fan, Nature 386 (1997) 143. [7] S.Y. Chou, P.R. Krauss, W. Zhang, L. Guo, L. Zhuang, J. Vac. Sci. Technol. B 15 (6) (1997) 2897. [8] M. De Vittorio, M.T. Todaro, T. Stomeo, R. Cingolani, D. Cojoc, E. Di Fabrizio, Microelectron. Eng. 73–74 (2004) 388. [9] H. Schift, S. Park, B. Jung, C.-G. Choi, C.-S. Kee, S.-P. Han, K.-B. Yoon, J. Gobrecht, Nanotechnology 16 (2005) S261. [10] C. Liguda, G. Bottger, A. Kuligk, R. Blum, M. Eich, H. Roth, J. Kunert, W. Morgenroth, H. Eisner, H.G. Meyer, Appl. Phys. Lett. 78 (2001) 2434.