- Email: [email protected]
Formation of nanoimprinting mould through use of nanosphere lithography
ARTICLE IN PRESS
Journal of Crystal Growth 288 (2006) 200–204 www.elsevier.com/locate/jcrysgro
Formation of nanoimprinting mould through use of nanosphere lithography Benzhong Wanga,, Wei Zhaoa, Ao Chenb, Soo-Jin Chuaa,b a
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore b Centre for Optoelectronics, National University of Singapore, Singapore 119260, Singapore
Abstract Two-dimensional (2D) photonic crystal (PC) fabrication is generally carried out by electron beam lithography (EBL). This technique is very expensive and has a low throughput because it works serially. In this paper, we report an inexpensive, fast and simple nanofabrication technique for creating nanostructures based on nanoimprint lithography (NIL) and nanosphere lithography (NSL) techniques. A monolayer of self-assembled polystyrene colloidal particle is used as a mask for dry etching of SiO2 to create periodic ordered nano-plates on glass. Then the nanopatterns are transferred by imprinting onto a polymer film or poly (methyl methacrylate) (PMMA)-coated Si substrate. Preliminary results show that this technique is a promising way to fabricate 2D photonic crystals. r 2005 Elsevier B.V. All rights reserved. PACS: 81.16.Dn; 81.16.Nd; 81.16.Rf Keywords: A1. Nanostructures; B2. Colloidal crystals; B2. Dielectric materials; B2. Nanoimprint lithography; B2. Nanosphere lithography; B2. Photonic crystals
Two-dimensional (2D)-ordered nanostructures have received much attention [1–6] in recent years due to their unique properties and potential applications in photonic crystals (PCs) [2], microelectronics and optoelectronics [3]. To create a large-area with high-resolution nanostructures, various techniques have been developed such as electronbeam lithography (EBL) [7], nanoimprint lithography (NIL) [8] and the so-called nanosphere lithography (NSL). Among them, EBL is a well-known technique for creating nanostructures. It has the advantages of precise control of nanostructure dimension and with the flexibility of creating various shapes. However, EBL is also a high cost and low throughput technique because it works serially. There exists a demand for a lithography method that can cope with the predicted linewidth reduction and the requirement of being low cost. NIL has the potential of producing nanopatterns over a large area employing a single lithographic step. With this technique, successful pattern transfer of nanostructures has been demonstrated Corresponding author.
E-mail address: [email protected] (B. Wang). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.051
for up to 6 cm wafer [9] and individual feature size down to 6–8 nm has been reported [10]. However, in most cases, the mould used for NIL is fabricated by EBL, an expensive and low throughput method, which limits the accessibility of the nanoimprint technique. Fabricating moulds in an economical way for use in nanoimprinting is still a big challenge. It has demonstrated that single layer or double layer of self-assembled nanosphere arrays can be used as masks to create a periodic array of nanostructures on a substrate [11–16]. This is a very simple, low cost, and high throughput method with a limited flexibility to control structure features. Therefore, the cost of NIL should be reduced further if NSL is used to fabricate the nanoimprinting moulds. In this paper, we report the experimental results of nanoimprinting by using moulds fabricated with NSL. Our approach of producing moulds for nanoimprinting consists of 4 steps: (1), self-assembling hexagonal closepacked (hcp) single layer of polystyrene (PS) spheres on the surface of SiO2 deposited on a glass substrate; (2), reducing the diameters of the PS spheres by reactive ions etching
ARTICLE IN PRESS B. Wang et al. / Journal of Crystal Growth 288 (2006) 200–204
(RIE) with oxygen; (3), dry etching the SiO2 layer by using the PS spheres to form SiO2 nanoplates; (4), finally, removing the PS spheres to form a nanoimprinting mould. In a method previously report [17], metal was deposited through the voids between the closely packed sphere arrays. In our method, close-packed monolayer of PS spheres, diameters have been reduced by dry etching, forms a mask for forming the SiO2 nanoplates. Our approach not only is simple, but, more importantly, allows flexibility for sufficient size control, e.g. lattice constant a and radius r of the nanoplates, which are more important parameters of photonic band gap structures. The detailed procedure for fabricating a large area of periodic nanoplate arrays, which serves as a mould for NIL is outlined in Fig. 1. A glass sheet coated with a 500 nm thick SiO2 film by PECVD is used as a substrate. The monodispersed PS spheres (Duke Scientific Corporation) are spin-coated on the SiO2 to form a large area of closepacked structure on the surface. PS spheres with diameters of 600 and 900 nm are used to demonstrate this technique.
Fig. 1. Schematic illustrations of the procedure for fabricating moulds of nanoimprinting by NSL: (a) arrangement of hexagonal closely packed PS spheres on a SiO2 film deposited by PECVD on a glass substrate as template; (b) thinning the template by reactive ion etching (RIE) with oxygen; (c) etching the SiO2 film through the PS sphere arrays to form SiO2 nanoplates which serves as a mould for nanoimprinting; and (d) removing the PS template.
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Depending on the concentration of the PS particle in solution and the spin speed, monolayer or bilayer of PS spheres can be obtained [17–19]. The concentration of the purchased suspension of PS particle with mean diameter 900 and 600 nm 76 nm was 10% w/w, and was used as received. Spin coating of the PS spheres was carried out at the speed of 800 rpm. At this condition, most area of the substrate was covered with a single layer of spheres. Diameters of the 2D hcp arranged PS spheres were then reduced by dry etching with oxygen RIE as shown in Fig. 1(b) at the following conditions: O2 flow, 20 sccm, RF power, 200 W, chamber pressure, 8 Torr. The etching process allows the ratio of diameter of sphere inter-particle distance to be varied. This ratio plays an important role in determining the optical properties of the nanostructures [20]. The PS spheres with reduced diameter serves as masks to create the nanoplates on the SiO2 film. Inductive coupled plasma (ICP) with mixed CF4 and O2 was used to etch the SiO2 film at the following conditions: CF4 flow, 20 sccm, O2 flow, 10 sccm, RF power, 100 W, ICP power, 500 W, chamber pressure, 8 Torr. Finally, the wafer with 2D hcp-ordered SiO2 nanoplates was cleaned by ultrasonic agitation in toluene solution to remove the PS material and a mould with large area of nanoplate arrays was obtained. The nano-feature on the mould is then transferred to other substrates. In our experiments, both poly (ethylene terephthalate) (PET)- and poly (methyl methacrylate) (PMMA)-coated Si substrates are used. A schematic of the imprint process is depicted in Fig. 2. Before imprinting, the SiO2 mould was treated with 1H,1H,2H,2H-perfluorodecyl-trichlorosilane (FDTS, 96%, Lancaster) to form an anti-stick layer on the mould surface. Our experimental
Fig. 2. Schematic illustrations of the procedure of imprinting: (a) spincoating a PMMA layer over Si substrate; (b) imprinting the SiO2 mould on the PMMA layer under an extra pressure and a temperature above the glass transition temperature (Tg) of the polymer and (c) releasing the mould at a temperature below Tg of the polymer, leaving negative pattern of the mould to the polymer layer.
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B. Wang et al. / Journal of Crystal Growth 288 (2006) 200–204
Fig. 3. (a) Photograph, (b) micro-photograph and (c) SEM images showing periodic ordered monolayer of PS spheres with 600 nm diameter arranged on the SiO2/glass substrate.
results indicate that the surface treatment process is necessary to release the mould after imprinting. A 5 wt% solution of PMMA in toluene was spin coated on a Si substrate at a speed of 2000 rpm for 30 s to form a polymer layer with thickness of 300 nm. The mould was then imprinted on the polymer layer at a temperature of 180 1C and at a pressure of 4 MPa for 5 min. The pressure was then released after the temperature dropped below the glass transition temperature of PMMA, and a negative pattern was thus formed on the PMMA layer with the greatest fidelity. Such a pattern in PMMA can be further transferred to the underlying substrate by standard selective etching process. The SiO2 mould can be used repeatedly without further surface treatment. Fig. 3(a) shows a photograph of the hcp-ordered singlelayered arrays of the PS spheres on a SiO2/Si substrate. The color of the array is changed by changing the observation angle with respect to the light source. The change in coloring is a result of optical diffraction from an ordered array of dielectric spheres deposited on a SiO2 film. Homogeneous color of the array demonstrates the uniformity, which is also confirmed by a micrograph as shown in Fig. 3(b). As seen in the image, a uniform arrangement of the spheres with single domain is observed in a large area although line and point defects were also observed. Fig. 3(c) shows a SEM image of the array, revealing that the arrangement of the PS spheres is hexagonally close-packed. The lattice constant is 615 nm which corresponds to the diameter of the PS spheres used here. Several vacancies and dislocations mainly due to spheres with larger or smaller diameters are observed. The orientation of the crystal lattice, however, was not affected by these defects.
Fig. 4. SEM image of SiO2 nanostructures created by ICP etching through the masks formed by PS spheres. The inset is a high magnification SEM image of the same sample shows detailed structural features.
Fig. 4 shows a SEM image of the SiO2 nanostructures fabricated through the PS monolayer template as described in Fig. 1. The diameters of the PS spheres were first reduced. The resulting spheres act as masks for the etching of the SiO2. As seen in Fig. 4, the patterns of PS spheres were transferred to the SiO2 film. The mean diameter of the SiO2 nanoplates is about 300 nm separated by a gap of 150 nm. The height of the pillars is estimated to be 350 nm by cross-section view of SEM. The images of the negative pattern transferred to polymer layer were shown in Fig. 5. When a polymer sheet served as the substrate, nano-patterns were successfully
ARTICLE IN PRESS B. Wang et al. / Journal of Crystal Growth 288 (2006) 200–204
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Fig. 5. Photographic images of the imprinting results: (a) structures created on a polymer sheet; (b) structures created on a PMMA film by the mould fabricated through the PS spheres with 900 nm diameter; (c) structures created on a PMMA film by the mould fabricated through the PS spheres with 600 nm diameter.
Fig. 6. (a) and (b) top view of SEM images showing the imprinting results created on a PMMA film by the mould fabricated through the PS spheres with 600 nm diameter.
transferred to the substrate directly without any additional steps. Fig. 5(a) shows the patterns transferred to a poly (ethylene terephthalate) (PET) by NIL. In the case of PMMA-coated Si substrate, the patterns were fully transferred to the PMMA layer. Fig. 5(b) shows the patterns transferred to a PMMA layer from SiO2 moulds fabricated by using the PS spheres with diameters of 900 nm, while Fig. 5(c) shows the patterns transferred to PMMA layer from the mould fabricated by using the PS spheres with diameters of 600 nm. In both cases, patterns from the mould were transferred with fidelity and uniformly over large areas. The SEM images shown in Fig. 6 reveal the details of the imprinted structures. The low magnification image (Fig. 6(a)) shows the uniformity of the patterns transferred from the mould. Fig. 6(b) shows the top view in a higher magnification. It has been noted that the surface of the sidewall in each individual circular well is rough. The roughness is believed to be first generated during the ICP etching process, in which arrays of nanoplates with rough sidewalls were observed as seen in Fig. 4. The roughness of the mould was then transferred to the PMMA layer during the imprinting process. Bilayer of the colloidal crystals also can be used to fabricate the moulds for nanoimprinting. However, the structures of the SiO2 created through bilayer colloidal crystals are totally different with that created through monolayer colloidal crystals. The self-assembled structures of bilayer PS spheres have openings directly to substrates. Nanohole arrays of SiO2 can be created by ICP etching through the openings. Fig. 7 shows the imprinting results using such SiO2 nanohole structures. Uniform nano-pillar
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In conclusion, NSL is used to fabricate hexagonal closed-packed nanostructures on a SiO2 layer deposited on Si substrate. The 2D periodic-ordered nanostructure is then used as a mould to successfully transfer to a polymer sheets or a PMMA film coated on a Si substrate by imprinting. Our approach opens a promising technique to lower the cost of creating 2D periodic-ordered nanostructures by nanoimprinting.
References
Fig. 7. (a) Low magnification and (b) high magnification SEM images showing the imprinting results created on a PMMA film by the mould fabricated through the bilayered PS spheres with diameter of 900 nm.
arrays with diameter of about 200 nm were successfully obtained by the imprint technique through the mould fabricated by 900 nm spheres.
[1] S.I. Matsushita, T. Miwa, D.A. Tryk, A. Fujishima, Langmuir 14 (1998) 6441. [2] A. Birner, U. Gruning, S. Ottow, A. Schneider, F. Mu¨ller, V. Lehmann, H. Fo¨ll, U. Go¨sele, Phys. Status Solidi A 165 (1998) 111. [3] M. Imada, S. Noda, A. Chutinan, T. Tokuda, M. Murata, G. Sasaki, Appl. Phys. Lett. 75 (1999) 316. [4] N.G.R. Broderick, G.W. Ross, H.L. Offerhaus, D.J. Richardson, D.C. Hanna, Phys. Rev. Lett. 84 (2000) 4345. [5] P.M. Tessier, O.D. Velev, A.T. Kalambur, A.M. Lenhoff, J.F. Rabolt, E.W. Kaler, Adv. Mater. 13 (2001) 396. [6] T. Tatsuma, A. Ikezawa, Y. Ohko, T. Miwa, T. Matsue, A. Fujishima, Adv. Mater. 12 (2000) 643. [7] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, Nature 391 (1998) 667. [8] X.D. Huang, L.R. Bao, X. Cheng, L.J. Guo, S.W. Pang, A.F. Yee, J. Vac. Sci. Technol. B 20 (2002) 2872. [9] B. Heidari, L. Montelius, I. Maximov, J. Vac. Sci. Technol. B 18 (2000) 3557. [10] S.Y. Chou, et al., J. Vac. Sci. Technol. B 15 (1997) 2897. [11] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [12] X. Chen, Z. Chen, N. Fu, G. Lu, B. Yang, Adv. Mater. 15 (2003) 1413. [13] U.C. Fischer, H.P. Zingsheim, J. Vac. Sci. Technol. B 19 (1981) 881. [14] F. Burmeister, C. Schafle, B. Keilhofer, C. Bechinger, J. Boneberg, P. Leiderer, Adv. Mater. 10 (1998) 495. [15] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 5599. [16] I.W. Hamley, Angew. Chem. Int. Ed. 42 (2003) 1692. [17] C.W. Kou, J.Y. Shiu, Y.H. Cho, P. Chen, Adv. Mater. 15 (2003) 1065. [18] H.W. Dechman, J.H. Dunsmuir, J. Vac. Sci. Technol. B 1 (1983) 1109. [19] D. Wang, H. Mohwald, Adv. Mater. 16 (2004) 244. [20] S.H. Fan, J.D. Joannopoulos, Phys. Rev. B 65 (2002) 235112.
Journal of Crystal Growth 288 (2006) 200–204 www.elsevier.com/locate/jcrysgro
Formation of nanoimprinting mould through use of nanosphere lithography Benzhong Wanga,, Wei Zhaoa, Ao Chenb, Soo-Jin Chuaa,b a
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore b Centre for Optoelectronics, National University of Singapore, Singapore 119260, Singapore
Abstract Two-dimensional (2D) photonic crystal (PC) fabrication is generally carried out by electron beam lithography (EBL). This technique is very expensive and has a low throughput because it works serially. In this paper, we report an inexpensive, fast and simple nanofabrication technique for creating nanostructures based on nanoimprint lithography (NIL) and nanosphere lithography (NSL) techniques. A monolayer of self-assembled polystyrene colloidal particle is used as a mask for dry etching of SiO2 to create periodic ordered nano-plates on glass. Then the nanopatterns are transferred by imprinting onto a polymer film or poly (methyl methacrylate) (PMMA)-coated Si substrate. Preliminary results show that this technique is a promising way to fabricate 2D photonic crystals. r 2005 Elsevier B.V. All rights reserved. PACS: 81.16.Dn; 81.16.Nd; 81.16.Rf Keywords: A1. Nanostructures; B2. Colloidal crystals; B2. Dielectric materials; B2. Nanoimprint lithography; B2. Nanosphere lithography; B2. Photonic crystals
Two-dimensional (2D)-ordered nanostructures have received much attention [1–6] in recent years due to their unique properties and potential applications in photonic crystals (PCs) [2], microelectronics and optoelectronics [3]. To create a large-area with high-resolution nanostructures, various techniques have been developed such as electronbeam lithography (EBL) [7], nanoimprint lithography (NIL) [8] and the so-called nanosphere lithography (NSL). Among them, EBL is a well-known technique for creating nanostructures. It has the advantages of precise control of nanostructure dimension and with the flexibility of creating various shapes. However, EBL is also a high cost and low throughput technique because it works serially. There exists a demand for a lithography method that can cope with the predicted linewidth reduction and the requirement of being low cost. NIL has the potential of producing nanopatterns over a large area employing a single lithographic step. With this technique, successful pattern transfer of nanostructures has been demonstrated Corresponding author.
E-mail address: [email protected] (B. Wang). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.051
for up to 6 cm wafer [9] and individual feature size down to 6–8 nm has been reported [10]. However, in most cases, the mould used for NIL is fabricated by EBL, an expensive and low throughput method, which limits the accessibility of the nanoimprint technique. Fabricating moulds in an economical way for use in nanoimprinting is still a big challenge. It has demonstrated that single layer or double layer of self-assembled nanosphere arrays can be used as masks to create a periodic array of nanostructures on a substrate [11–16]. This is a very simple, low cost, and high throughput method with a limited flexibility to control structure features. Therefore, the cost of NIL should be reduced further if NSL is used to fabricate the nanoimprinting moulds. In this paper, we report the experimental results of nanoimprinting by using moulds fabricated with NSL. Our approach of producing moulds for nanoimprinting consists of 4 steps: (1), self-assembling hexagonal closepacked (hcp) single layer of polystyrene (PS) spheres on the surface of SiO2 deposited on a glass substrate; (2), reducing the diameters of the PS spheres by reactive ions etching
ARTICLE IN PRESS B. Wang et al. / Journal of Crystal Growth 288 (2006) 200–204
(RIE) with oxygen; (3), dry etching the SiO2 layer by using the PS spheres to form SiO2 nanoplates; (4), finally, removing the PS spheres to form a nanoimprinting mould. In a method previously report [17], metal was deposited through the voids between the closely packed sphere arrays. In our method, close-packed monolayer of PS spheres, diameters have been reduced by dry etching, forms a mask for forming the SiO2 nanoplates. Our approach not only is simple, but, more importantly, allows flexibility for sufficient size control, e.g. lattice constant a and radius r of the nanoplates, which are more important parameters of photonic band gap structures. The detailed procedure for fabricating a large area of periodic nanoplate arrays, which serves as a mould for NIL is outlined in Fig. 1. A glass sheet coated with a 500 nm thick SiO2 film by PECVD is used as a substrate. The monodispersed PS spheres (Duke Scientific Corporation) are spin-coated on the SiO2 to form a large area of closepacked structure on the surface. PS spheres with diameters of 600 and 900 nm are used to demonstrate this technique.
Fig. 1. Schematic illustrations of the procedure for fabricating moulds of nanoimprinting by NSL: (a) arrangement of hexagonal closely packed PS spheres on a SiO2 film deposited by PECVD on a glass substrate as template; (b) thinning the template by reactive ion etching (RIE) with oxygen; (c) etching the SiO2 film through the PS sphere arrays to form SiO2 nanoplates which serves as a mould for nanoimprinting; and (d) removing the PS template.
201
Depending on the concentration of the PS particle in solution and the spin speed, monolayer or bilayer of PS spheres can be obtained [17–19]. The concentration of the purchased suspension of PS particle with mean diameter 900 and 600 nm 76 nm was 10% w/w, and was used as received. Spin coating of the PS spheres was carried out at the speed of 800 rpm. At this condition, most area of the substrate was covered with a single layer of spheres. Diameters of the 2D hcp arranged PS spheres were then reduced by dry etching with oxygen RIE as shown in Fig. 1(b) at the following conditions: O2 flow, 20 sccm, RF power, 200 W, chamber pressure, 8 Torr. The etching process allows the ratio of diameter of sphere inter-particle distance to be varied. This ratio plays an important role in determining the optical properties of the nanostructures [20]. The PS spheres with reduced diameter serves as masks to create the nanoplates on the SiO2 film. Inductive coupled plasma (ICP) with mixed CF4 and O2 was used to etch the SiO2 film at the following conditions: CF4 flow, 20 sccm, O2 flow, 10 sccm, RF power, 100 W, ICP power, 500 W, chamber pressure, 8 Torr. Finally, the wafer with 2D hcp-ordered SiO2 nanoplates was cleaned by ultrasonic agitation in toluene solution to remove the PS material and a mould with large area of nanoplate arrays was obtained. The nano-feature on the mould is then transferred to other substrates. In our experiments, both poly (ethylene terephthalate) (PET)- and poly (methyl methacrylate) (PMMA)-coated Si substrates are used. A schematic of the imprint process is depicted in Fig. 2. Before imprinting, the SiO2 mould was treated with 1H,1H,2H,2H-perfluorodecyl-trichlorosilane (FDTS, 96%, Lancaster) to form an anti-stick layer on the mould surface. Our experimental
Fig. 2. Schematic illustrations of the procedure of imprinting: (a) spincoating a PMMA layer over Si substrate; (b) imprinting the SiO2 mould on the PMMA layer under an extra pressure and a temperature above the glass transition temperature (Tg) of the polymer and (c) releasing the mould at a temperature below Tg of the polymer, leaving negative pattern of the mould to the polymer layer.
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Fig. 3. (a) Photograph, (b) micro-photograph and (c) SEM images showing periodic ordered monolayer of PS spheres with 600 nm diameter arranged on the SiO2/glass substrate.
results indicate that the surface treatment process is necessary to release the mould after imprinting. A 5 wt% solution of PMMA in toluene was spin coated on a Si substrate at a speed of 2000 rpm for 30 s to form a polymer layer with thickness of 300 nm. The mould was then imprinted on the polymer layer at a temperature of 180 1C and at a pressure of 4 MPa for 5 min. The pressure was then released after the temperature dropped below the glass transition temperature of PMMA, and a negative pattern was thus formed on the PMMA layer with the greatest fidelity. Such a pattern in PMMA can be further transferred to the underlying substrate by standard selective etching process. The SiO2 mould can be used repeatedly without further surface treatment. Fig. 3(a) shows a photograph of the hcp-ordered singlelayered arrays of the PS spheres on a SiO2/Si substrate. The color of the array is changed by changing the observation angle with respect to the light source. The change in coloring is a result of optical diffraction from an ordered array of dielectric spheres deposited on a SiO2 film. Homogeneous color of the array demonstrates the uniformity, which is also confirmed by a micrograph as shown in Fig. 3(b). As seen in the image, a uniform arrangement of the spheres with single domain is observed in a large area although line and point defects were also observed. Fig. 3(c) shows a SEM image of the array, revealing that the arrangement of the PS spheres is hexagonally close-packed. The lattice constant is 615 nm which corresponds to the diameter of the PS spheres used here. Several vacancies and dislocations mainly due to spheres with larger or smaller diameters are observed. The orientation of the crystal lattice, however, was not affected by these defects.
Fig. 4. SEM image of SiO2 nanostructures created by ICP etching through the masks formed by PS spheres. The inset is a high magnification SEM image of the same sample shows detailed structural features.
Fig. 4 shows a SEM image of the SiO2 nanostructures fabricated through the PS monolayer template as described in Fig. 1. The diameters of the PS spheres were first reduced. The resulting spheres act as masks for the etching of the SiO2. As seen in Fig. 4, the patterns of PS spheres were transferred to the SiO2 film. The mean diameter of the SiO2 nanoplates is about 300 nm separated by a gap of 150 nm. The height of the pillars is estimated to be 350 nm by cross-section view of SEM. The images of the negative pattern transferred to polymer layer were shown in Fig. 5. When a polymer sheet served as the substrate, nano-patterns were successfully
ARTICLE IN PRESS B. Wang et al. / Journal of Crystal Growth 288 (2006) 200–204
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Fig. 5. Photographic images of the imprinting results: (a) structures created on a polymer sheet; (b) structures created on a PMMA film by the mould fabricated through the PS spheres with 900 nm diameter; (c) structures created on a PMMA film by the mould fabricated through the PS spheres with 600 nm diameter.
Fig. 6. (a) and (b) top view of SEM images showing the imprinting results created on a PMMA film by the mould fabricated through the PS spheres with 600 nm diameter.
transferred to the substrate directly without any additional steps. Fig. 5(a) shows the patterns transferred to a poly (ethylene terephthalate) (PET) by NIL. In the case of PMMA-coated Si substrate, the patterns were fully transferred to the PMMA layer. Fig. 5(b) shows the patterns transferred to a PMMA layer from SiO2 moulds fabricated by using the PS spheres with diameters of 900 nm, while Fig. 5(c) shows the patterns transferred to PMMA layer from the mould fabricated by using the PS spheres with diameters of 600 nm. In both cases, patterns from the mould were transferred with fidelity and uniformly over large areas. The SEM images shown in Fig. 6 reveal the details of the imprinted structures. The low magnification image (Fig. 6(a)) shows the uniformity of the patterns transferred from the mould. Fig. 6(b) shows the top view in a higher magnification. It has been noted that the surface of the sidewall in each individual circular well is rough. The roughness is believed to be first generated during the ICP etching process, in which arrays of nanoplates with rough sidewalls were observed as seen in Fig. 4. The roughness of the mould was then transferred to the PMMA layer during the imprinting process. Bilayer of the colloidal crystals also can be used to fabricate the moulds for nanoimprinting. However, the structures of the SiO2 created through bilayer colloidal crystals are totally different with that created through monolayer colloidal crystals. The self-assembled structures of bilayer PS spheres have openings directly to substrates. Nanohole arrays of SiO2 can be created by ICP etching through the openings. Fig. 7 shows the imprinting results using such SiO2 nanohole structures. Uniform nano-pillar
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In conclusion, NSL is used to fabricate hexagonal closed-packed nanostructures on a SiO2 layer deposited on Si substrate. The 2D periodic-ordered nanostructure is then used as a mould to successfully transfer to a polymer sheets or a PMMA film coated on a Si substrate by imprinting. Our approach opens a promising technique to lower the cost of creating 2D periodic-ordered nanostructures by nanoimprinting.
References
Fig. 7. (a) Low magnification and (b) high magnification SEM images showing the imprinting results created on a PMMA film by the mould fabricated through the bilayered PS spheres with diameter of 900 nm.
arrays with diameter of about 200 nm were successfully obtained by the imprint technique through the mould fabricated by 900 nm spheres.
[1] S.I. Matsushita, T. Miwa, D.A. Tryk, A. Fujishima, Langmuir 14 (1998) 6441. [2] A. Birner, U. Gruning, S. Ottow, A. Schneider, F. Mu¨ller, V. Lehmann, H. Fo¨ll, U. Go¨sele, Phys. Status Solidi A 165 (1998) 111. [3] M. Imada, S. Noda, A. Chutinan, T. Tokuda, M. Murata, G. Sasaki, Appl. Phys. Lett. 75 (1999) 316. [4] N.G.R. Broderick, G.W. Ross, H.L. Offerhaus, D.J. Richardson, D.C. Hanna, Phys. Rev. Lett. 84 (2000) 4345. [5] P.M. Tessier, O.D. Velev, A.T. Kalambur, A.M. Lenhoff, J.F. Rabolt, E.W. Kaler, Adv. Mater. 13 (2001) 396. [6] T. Tatsuma, A. Ikezawa, Y. Ohko, T. Miwa, T. Matsue, A. Fujishima, Adv. Mater. 12 (2000) 643. [7] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, Nature 391 (1998) 667. [8] X.D. Huang, L.R. Bao, X. Cheng, L.J. Guo, S.W. Pang, A.F. Yee, J. Vac. Sci. Technol. B 20 (2002) 2872. [9] B. Heidari, L. Montelius, I. Maximov, J. Vac. Sci. Technol. B 18 (2000) 3557. [10] S.Y. Chou, et al., J. Vac. Sci. Technol. B 15 (1997) 2897. [11] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [12] X. Chen, Z. Chen, N. Fu, G. Lu, B. Yang, Adv. Mater. 15 (2003) 1413. [13] U.C. Fischer, H.P. Zingsheim, J. Vac. Sci. Technol. B 19 (1981) 881. [14] F. Burmeister, C. Schafle, B. Keilhofer, C. Bechinger, J. Boneberg, P. Leiderer, Adv. Mater. 10 (1998) 495. [15] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 5599. [16] I.W. Hamley, Angew. Chem. Int. Ed. 42 (2003) 1692. [17] C.W. Kou, J.Y. Shiu, Y.H. Cho, P. Chen, Adv. Mater. 15 (2003) 1065. [18] H.W. Dechman, J.H. Dunsmuir, J. Vac. Sci. Technol. B 1 (1983) 1109. [19] D. Wang, H. Mohwald, Adv. Mater. 16 (2004) 244. [20] S.H. Fan, J.D. Joannopoulos, Phys. Rev. B 65 (2002) 235112.