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Fabrication of nano-sized metal patterns on flexible polyethylene-terephthalate substrate using bi-layer nanoimprint lithography
Thin Solid Films 517 (2009) 4104–4107
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Fabrication of nano-sized metal patterns on flexible polyethylene-terephthalate substrate using bi-layer nanoimprint lithography Seon Yong Hwang a, Ho Yong Jung a, Jun-Ho Jeong b, Heon Lee a,⁎ a b
Department of Materials Science and Engineering, Korea University, Seoul, 136-701, Korea Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials, Yuseong-gu Daejeon, 305-343, Korea
a r t i c l e
i n f o
Available online 8 February 2009 Keywords: Metal nano-pattern Flexible substrate PET (polyethylene-terephthalate) PVA (poly-vinyl alcohol) Bi-layer imprint
a b s t r a c t Polymer films are widely used as a substrate for displays and for solar cells since they are cheap, transparent and flexible, and their material properties are easy to design. Polyethylene-terephthalate (PET) is especially useful for various applications requiring transparency, flexibility and good thermal and chemical resistance. In this study, nano-sized metal patterns were fabricated on flexible PET film by using nanoimprint lithography (NIL). Water-soluble poly-vinyl alcohol (PVA) resin was used as a planarization and sacrificial layer for the liftoff process, as it does not damage the PET films and can easily be etched off by using oxygen plasma. NIL was used to fabricate the nano-sized patterns on the non-planar or flexible substrate. Finally, a nano-sized metal pattern was successfully formed by depositing the metal layer over the imprinted resist patterns and applying the lift-off process, which is economic and environmentally friendly, to the PET films. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polymer films are used as a substrate for transparent-flexible displays and for organic electronic devices. Being transparent, lightweight, flexible and low-cost, and having easily designed material properties, such polymer materials, especially polyethylene-terephthalate (PET), can be used as a substrate for various devices requiring transparency, flexibility and good mechanical strength. [1–4] However, economic and environmentally friendly process technology for mass production in industrial applications is not yet available. Also, due to its flexibility, optical focusing over polymer films is extremely difficult and micro-to-nano-sized patterns cannot therefore be fabricated using the conventional photolithography process [5]. Nanoimprint lithography (NIL) can be used to fabricate nano-scale patterns on non-planar or flexible polymeric substrates. Due to its relative simplicity and economy, it has been developed for many fields including flexible LCD, OLED, PoRAM devices and organic solar cell [6–10]. Patterning methods using NIL can be categorized into direct etching and lift-off processes.[11] Compared with the former, the latter can be used to produce various material patterns, especially for non-etchable materials. However, rabbit-ear-shaped defects are often observed in the lift-off process, due to the sidewall deposition.[12] In order to minimize such defects, a bi-layer imprint process, which uses an underlayer for planarization and sacrificial etching and a top layer
⁎ Corresponding author. E-mail address: [email protected] (H. Lee). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.164
for pattern fabrication, is introduced. An organic solvent is used to liftoff the sacrificial layer but this can damage the polymeric substrate. Hence, the selection of polymeric substrates is restricted to avoid the damage. In this paper, an environmentally friendly process is used to develop a nano-sized pattern by using bi-layer NIL and water-soluble polymer on flexible substrate. Poly-vinyl alcohol (PVA) was chosen as an underlayer because it does not react with the substrate and can be effectively removed by oxygen plasma etching. After coating the PVA layer on the PET films, liquid-phase UV imprint resin was dispensed and the resist pattern was fabricated by UV-NIL process.[13–14] Controlling the shape of the undercut structure necessitated oxygen plasma processing and a highly etch-resistant imprint resin. Thus, methacryloxypropyl terminated polydimethylsiloxanes (M-PDMS) were added to UV-curable resin to increase the etch resistance to oxygen plasma. After oxygen reactive ion etching (RIE) was used to etch the PVA underlayer and to control the undercut shape, almost any kind of metal layer could be deposited over the imprinted resist patterns. Finally, nano-sized patterns of metal layer were formed over the flexible polymer film substrate by lift-off processing in water, without any degradation of the polymer substrate. 2. Experimental The content of the UV-curable imprint resin used in this study is summarized in Table 1. Benzylmethacrylate was selected as a base monomer, and Irgacure 184 (Ciba-Geigy Co) was used as a UV-radical generator. In order to enhance the etch resistance to oxygen plasma, M-PDMS was added up to 15 wt.%.
S.Y. Hwang et al. / Thin Solid Films 517 (2009) 4104–4107
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Table 1 Composition of UV-curable monomer-based resin. Reagent
Wt.% of resin
Benzylmethacrylate Methacryloxypropyl terminated polydimethylsiloxanes Irgacure 184 (UV radical generator)
82% 15% 3%
In this experiment, a quartz master template was fabricated by using deep-ultraviolet photolithography and RIE. The master template had a period of 800 nm and a 380 nm-sized, photonic crystal dot pattern. For smooth detachment of the master quartz template after NIL, a hydrophobic, antistiction, self-assembled monolayer (SAM) was coated on the master quartz template. The SAM coating was done by dipping the quartz template into normal hexane solution containing 0.1% of heptadeca-fluoro-1,1,2,2-tetra-hydrodecyl trichlorosilane for 10 min. Fig. 1 shows the overall process flow of the bi-layer NIL process using a UV-curable, monomer-based, imprint resin and a PVA underlayer. A 300 nm-thick PVA underlayer was coated on a PET film to which a 30 nm-thick SiO2 passivation layer had been deposited. After coating of the PVA, annealing was performed at 100 °C for 5 min to remove the water solvent from the PVA layer. The UV-curable monomer resin was then dispensed onto the substrate and the imprinting process was undertaken for 10 min at an increased pressure of up to 25 bars in order to transfer the high fidelity pattern. After the imprint resist pattern was formed on the PVA layer by UV curing, the underlying PVA layer was etched and the shape of its undercut structure was controlled by oxygen RIE. Finally, a metal film was deposited on the resist patterns and nano-sized metal patterns were obtained after lift-off processing in water. 3. Result and discussion Fig. 2 (a) to (c) show the pattern structures of the master template, and top and cross-sectional views of the imprinted resist pattern on the PVA-coated Si substrate, respectively. Since it is extremely difficult to observe the cross-sectional view of resist patterns on PET films, a Si wafer was chosen as the substrate to investigate the imprint process. According to Fig. 2 (c), the master template patterns were faithfully transferred to the substrate without any noticeable residual layer. In order to improve the etch resistance of UV-curable imprint resin against oxygen plasma, M-PDMS was added up to 15 wt.%. When exposed to oxygen plasma, PDMS-based materials are generally converted to a SiOx compound that protects the polymer resin from the oxygen plasma.[15] However, the polymerization of an imprint
Fig. 2. SEM micrographs of a) master template, b) imprinted resist pattern, and c) crosssectional view of the imprinted resist pattern.
Fig. 1. Overall procedure of bi-layer imprint and lift-off processing.
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resin containing more than 15 wt.% of M-PDMS is difficult. Therefore, in this experiment, the M-PDMS concentration was fixed to 15 wt.%. By applying the lift-off process using UV NIL, the nano-sized pattern of the Si master template was faithfully transferred to the imprint resist patterns. The PVA underlayer was then etched by oxygen plasma. As shown in Fig. 3, the undercut structure of the PVA underlayer was formed by oxygen RIE under the conditions of 150 W of power, 40 mTorr of pressure and 40 sccm of flow rate for 120 s. The undercut structure is essential to obtain lifted-off metal nano-patterns without rabbit-ear-shaped defects. Scanning electron microscopy (SEM) micrographs of the lifted-off Au metal patterns are shown in Fig. 4. Fig. 4(a) and (b) show the lifted-off Au metal patterns on the Si wafer substrate and Fig. 4(c) that on the flexible PET film. The same experimental procedure was applied to the PET film substrate. The PVA could be coated thickly, and was easily etched-off by oxygen RIE. PVA film is usually applied in the fabrication process for conventional displays. Likewise, the resin pattern, with its high etch resistance, can be used to fabricate the undercut shape of the PVA underlayer. This enables the PVA underlayer to planarize the nonuniform substrate and modify the substrate surface to improve adhesion with the resin, thereby reducing the imprint defects. PVA provides the further advantages of facilitating the complete removal of imprint resin by using water and the fabrication of an undercut structure to improve the lift-off process. Therefore, this method can be applied to any devices that require a flexible polymeric substrate. In addition, the PVA's water solubility avoids the need for an organic solvent that can limit the selection of polymeric substrates. 4. Summary We have developed an economic and environmentally friendly liftoff process using PVA as an underlayer on a flexible polymeric substrate. The PVA's water solubility allows the lift-off process to be carried out with water and the polymeric substrate is not damaged during the lift-off process. Au metal patterns with a height of 200 nm were successfully fabricated using bi-layer NIL. The lift-off process using M-PDMS-containing resin and a PVA bi-layer structure proved to be more effective than that using the single-layer resin pattern. In the bi-layer NIL process, the etch resistance of the resin is one of the most important issues. To fabricate the metal dot patterns using UV-NIL, the imprint resin was modified by M-PDMS addition. Since the M-PDMS-containing resin has a high etch resistance against oxygen plasma, high aspect ratio PVA patterns with an undercut structure could be fabricated. The properties of these patterns enabled high fidelity metal patterns to be formed and rabbit-ear-shaped defects effectively prevented. Consequently, high fidelity metal
Fig. 4. SEM micrographs of lift-off patterns: a) tilted-view of Au metal pattern on Si substrate, b) top-view of Au metal pattern on Si substrate, and c) top-view of Au metal pattern on PET films. Nano-sized original pattern was successfully transferred to metal patterns.
patterns were fabricated by UV-NIL and lift-off processing, using water-soluble PVA as an underlayer. Acknowledgment This work was supported by Industry and Energy and the Ministry of Knowledge Economy (MKE) and the Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology, and this work was financially supported by the Government and the Korea Sanhak Foundation. References
Fig. 3. SEM micrograph of a cross-sectional view of the resist pattern after oxygen plasma etching (150 W, 40 sccm, 40 mTorr, 120 s).
[1] S. Logothetidis, Thin Solid Films 482 (2005) 9. [2] C.D. Sheraw, L. Zhou, J.R. Huang, D.J. Gundlach, T.N. Jackson, M.G. Kane, I.G. Hill, M.S. Hammond, J. Campi, B.K. Greening, J. Francl, J. West, Appl. Phys. Lett. 80 (2002) 1088. [3] H. Lee, S.H. Hong, K.Y. Yang, K.W. Choi, Appl. Phys. Lett. 88 (2006) 143112.
S.Y. Hwang et al. / Thin Solid Films 517 (2009) 4104–4107 [4] H. Lee, S.H. Hong, K.Y. Yang, K.W. Choi, Microelectron. Eng. 83 (2006) 323. [5] T.-W. Lee, S. Jeon, J. Maria, J. Zaumseil, J.W.P. Hsu, J.A. Rogers, Adv. Funct. Mater. 15 (2005) 1435. [6] J.H. Lee, K.Y. Yang, S.H. Hong, H. Lee, K.W. Choi, Microelectron. Eng. 85 (2008) 710. [7] X. Cheng, Y.T. Hong, J. Kanicki, L. Jay Guo, J. Vac. Sci. Technol., B 20 (6) (2002) 2877. [8] B. Chandar Shekar, J.Y. Lee, S.W. Rhee, Korean J. Chem. Eng. 21 (1) (2004) 267. [9] S.W. Ahn, K.D. Lee, J.S. Kim, S.H. Kim, J.D. Park, S.H. Lee, P.W. Yoon, Nanotechnology 16 (2005) 1874. [10] M.S. Kim, J.S. Kim, J.C. Cho, M. Shtein, L. Jay Guo, J.S. Kim, Appl. Phys. Lett. 90 (2007) 123113.
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[11] K.Y. Yang, S.H. Hong, H. Lee, J.W. Choi, Mater. Sci. Forum. 510–511 (2006) 446. [12] G.Y. Jung, W. Wu, S. Ganapathiappan, D.A.A. Ohlberg, M. Saif Islam, X. Li, D.L. Olynick, H. Lee, Y. Chen, S.Y. Wang, W.M. Tong, R.S. Williams, J. Appl. Physi., A. 81 (2005) 1331. [13] H. Lee, G.Y. Jung, Microelectron. Eng. 77 (2005) 42. [14] K. Nakamatsu, K. Tone, S. Matsui, Jpn. J. Appl. Phys. 44 (2005) 8186. [15] H. Ge, W. Wu, Z. Li, G.Y. Jung, D. Olynick, Y. Chen, J. Alexander Liddle, S.Y. Wang, R. Stanley Williams, Nano Lett. 5 (2005) 179.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Fabrication of nano-sized metal patterns on flexible polyethylene-terephthalate substrate using bi-layer nanoimprint lithography Seon Yong Hwang a, Ho Yong Jung a, Jun-Ho Jeong b, Heon Lee a,⁎ a b
Department of Materials Science and Engineering, Korea University, Seoul, 136-701, Korea Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials, Yuseong-gu Daejeon, 305-343, Korea
a r t i c l e
i n f o
Available online 8 February 2009 Keywords: Metal nano-pattern Flexible substrate PET (polyethylene-terephthalate) PVA (poly-vinyl alcohol) Bi-layer imprint
a b s t r a c t Polymer films are widely used as a substrate for displays and for solar cells since they are cheap, transparent and flexible, and their material properties are easy to design. Polyethylene-terephthalate (PET) is especially useful for various applications requiring transparency, flexibility and good thermal and chemical resistance. In this study, nano-sized metal patterns were fabricated on flexible PET film by using nanoimprint lithography (NIL). Water-soluble poly-vinyl alcohol (PVA) resin was used as a planarization and sacrificial layer for the liftoff process, as it does not damage the PET films and can easily be etched off by using oxygen plasma. NIL was used to fabricate the nano-sized patterns on the non-planar or flexible substrate. Finally, a nano-sized metal pattern was successfully formed by depositing the metal layer over the imprinted resist patterns and applying the lift-off process, which is economic and environmentally friendly, to the PET films. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polymer films are used as a substrate for transparent-flexible displays and for organic electronic devices. Being transparent, lightweight, flexible and low-cost, and having easily designed material properties, such polymer materials, especially polyethylene-terephthalate (PET), can be used as a substrate for various devices requiring transparency, flexibility and good mechanical strength. [1–4] However, economic and environmentally friendly process technology for mass production in industrial applications is not yet available. Also, due to its flexibility, optical focusing over polymer films is extremely difficult and micro-to-nano-sized patterns cannot therefore be fabricated using the conventional photolithography process [5]. Nanoimprint lithography (NIL) can be used to fabricate nano-scale patterns on non-planar or flexible polymeric substrates. Due to its relative simplicity and economy, it has been developed for many fields including flexible LCD, OLED, PoRAM devices and organic solar cell [6–10]. Patterning methods using NIL can be categorized into direct etching and lift-off processes.[11] Compared with the former, the latter can be used to produce various material patterns, especially for non-etchable materials. However, rabbit-ear-shaped defects are often observed in the lift-off process, due to the sidewall deposition.[12] In order to minimize such defects, a bi-layer imprint process, which uses an underlayer for planarization and sacrificial etching and a top layer
⁎ Corresponding author. E-mail address: [email protected] (H. Lee). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.164
for pattern fabrication, is introduced. An organic solvent is used to liftoff the sacrificial layer but this can damage the polymeric substrate. Hence, the selection of polymeric substrates is restricted to avoid the damage. In this paper, an environmentally friendly process is used to develop a nano-sized pattern by using bi-layer NIL and water-soluble polymer on flexible substrate. Poly-vinyl alcohol (PVA) was chosen as an underlayer because it does not react with the substrate and can be effectively removed by oxygen plasma etching. After coating the PVA layer on the PET films, liquid-phase UV imprint resin was dispensed and the resist pattern was fabricated by UV-NIL process.[13–14] Controlling the shape of the undercut structure necessitated oxygen plasma processing and a highly etch-resistant imprint resin. Thus, methacryloxypropyl terminated polydimethylsiloxanes (M-PDMS) were added to UV-curable resin to increase the etch resistance to oxygen plasma. After oxygen reactive ion etching (RIE) was used to etch the PVA underlayer and to control the undercut shape, almost any kind of metal layer could be deposited over the imprinted resist patterns. Finally, nano-sized patterns of metal layer were formed over the flexible polymer film substrate by lift-off processing in water, without any degradation of the polymer substrate. 2. Experimental The content of the UV-curable imprint resin used in this study is summarized in Table 1. Benzylmethacrylate was selected as a base monomer, and Irgacure 184 (Ciba-Geigy Co) was used as a UV-radical generator. In order to enhance the etch resistance to oxygen plasma, M-PDMS was added up to 15 wt.%.
S.Y. Hwang et al. / Thin Solid Films 517 (2009) 4104–4107
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Table 1 Composition of UV-curable monomer-based resin. Reagent
Wt.% of resin
Benzylmethacrylate Methacryloxypropyl terminated polydimethylsiloxanes Irgacure 184 (UV radical generator)
82% 15% 3%
In this experiment, a quartz master template was fabricated by using deep-ultraviolet photolithography and RIE. The master template had a period of 800 nm and a 380 nm-sized, photonic crystal dot pattern. For smooth detachment of the master quartz template after NIL, a hydrophobic, antistiction, self-assembled monolayer (SAM) was coated on the master quartz template. The SAM coating was done by dipping the quartz template into normal hexane solution containing 0.1% of heptadeca-fluoro-1,1,2,2-tetra-hydrodecyl trichlorosilane for 10 min. Fig. 1 shows the overall process flow of the bi-layer NIL process using a UV-curable, monomer-based, imprint resin and a PVA underlayer. A 300 nm-thick PVA underlayer was coated on a PET film to which a 30 nm-thick SiO2 passivation layer had been deposited. After coating of the PVA, annealing was performed at 100 °C for 5 min to remove the water solvent from the PVA layer. The UV-curable monomer resin was then dispensed onto the substrate and the imprinting process was undertaken for 10 min at an increased pressure of up to 25 bars in order to transfer the high fidelity pattern. After the imprint resist pattern was formed on the PVA layer by UV curing, the underlying PVA layer was etched and the shape of its undercut structure was controlled by oxygen RIE. Finally, a metal film was deposited on the resist patterns and nano-sized metal patterns were obtained after lift-off processing in water. 3. Result and discussion Fig. 2 (a) to (c) show the pattern structures of the master template, and top and cross-sectional views of the imprinted resist pattern on the PVA-coated Si substrate, respectively. Since it is extremely difficult to observe the cross-sectional view of resist patterns on PET films, a Si wafer was chosen as the substrate to investigate the imprint process. According to Fig. 2 (c), the master template patterns were faithfully transferred to the substrate without any noticeable residual layer. In order to improve the etch resistance of UV-curable imprint resin against oxygen plasma, M-PDMS was added up to 15 wt.%. When exposed to oxygen plasma, PDMS-based materials are generally converted to a SiOx compound that protects the polymer resin from the oxygen plasma.[15] However, the polymerization of an imprint
Fig. 2. SEM micrographs of a) master template, b) imprinted resist pattern, and c) crosssectional view of the imprinted resist pattern.
Fig. 1. Overall procedure of bi-layer imprint and lift-off processing.
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resin containing more than 15 wt.% of M-PDMS is difficult. Therefore, in this experiment, the M-PDMS concentration was fixed to 15 wt.%. By applying the lift-off process using UV NIL, the nano-sized pattern of the Si master template was faithfully transferred to the imprint resist patterns. The PVA underlayer was then etched by oxygen plasma. As shown in Fig. 3, the undercut structure of the PVA underlayer was formed by oxygen RIE under the conditions of 150 W of power, 40 mTorr of pressure and 40 sccm of flow rate for 120 s. The undercut structure is essential to obtain lifted-off metal nano-patterns without rabbit-ear-shaped defects. Scanning electron microscopy (SEM) micrographs of the lifted-off Au metal patterns are shown in Fig. 4. Fig. 4(a) and (b) show the lifted-off Au metal patterns on the Si wafer substrate and Fig. 4(c) that on the flexible PET film. The same experimental procedure was applied to the PET film substrate. The PVA could be coated thickly, and was easily etched-off by oxygen RIE. PVA film is usually applied in the fabrication process for conventional displays. Likewise, the resin pattern, with its high etch resistance, can be used to fabricate the undercut shape of the PVA underlayer. This enables the PVA underlayer to planarize the nonuniform substrate and modify the substrate surface to improve adhesion with the resin, thereby reducing the imprint defects. PVA provides the further advantages of facilitating the complete removal of imprint resin by using water and the fabrication of an undercut structure to improve the lift-off process. Therefore, this method can be applied to any devices that require a flexible polymeric substrate. In addition, the PVA's water solubility avoids the need for an organic solvent that can limit the selection of polymeric substrates. 4. Summary We have developed an economic and environmentally friendly liftoff process using PVA as an underlayer on a flexible polymeric substrate. The PVA's water solubility allows the lift-off process to be carried out with water and the polymeric substrate is not damaged during the lift-off process. Au metal patterns with a height of 200 nm were successfully fabricated using bi-layer NIL. The lift-off process using M-PDMS-containing resin and a PVA bi-layer structure proved to be more effective than that using the single-layer resin pattern. In the bi-layer NIL process, the etch resistance of the resin is one of the most important issues. To fabricate the metal dot patterns using UV-NIL, the imprint resin was modified by M-PDMS addition. Since the M-PDMS-containing resin has a high etch resistance against oxygen plasma, high aspect ratio PVA patterns with an undercut structure could be fabricated. The properties of these patterns enabled high fidelity metal patterns to be formed and rabbit-ear-shaped defects effectively prevented. Consequently, high fidelity metal
Fig. 4. SEM micrographs of lift-off patterns: a) tilted-view of Au metal pattern on Si substrate, b) top-view of Au metal pattern on Si substrate, and c) top-view of Au metal pattern on PET films. Nano-sized original pattern was successfully transferred to metal patterns.
patterns were fabricated by UV-NIL and lift-off processing, using water-soluble PVA as an underlayer. Acknowledgment This work was supported by Industry and Energy and the Ministry of Knowledge Economy (MKE) and the Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology, and this work was financially supported by the Government and the Korea Sanhak Foundation. References
Fig. 3. SEM micrograph of a cross-sectional view of the resist pattern after oxygen plasma etching (150 W, 40 sccm, 40 mTorr, 120 s).
[1] S. Logothetidis, Thin Solid Films 482 (2005) 9. [2] C.D. Sheraw, L. Zhou, J.R. Huang, D.J. Gundlach, T.N. Jackson, M.G. Kane, I.G. Hill, M.S. Hammond, J. Campi, B.K. Greening, J. Francl, J. West, Appl. Phys. Lett. 80 (2002) 1088. [3] H. Lee, S.H. Hong, K.Y. Yang, K.W. Choi, Appl. Phys. Lett. 88 (2006) 143112.
S.Y. Hwang et al. / Thin Solid Films 517 (2009) 4104–4107 [4] H. Lee, S.H. Hong, K.Y. Yang, K.W. Choi, Microelectron. Eng. 83 (2006) 323. [5] T.-W. Lee, S. Jeon, J. Maria, J. Zaumseil, J.W.P. Hsu, J.A. Rogers, Adv. Funct. Mater. 15 (2005) 1435. [6] J.H. Lee, K.Y. Yang, S.H. Hong, H. Lee, K.W. Choi, Microelectron. Eng. 85 (2008) 710. [7] X. Cheng, Y.T. Hong, J. Kanicki, L. Jay Guo, J. Vac. Sci. Technol., B 20 (6) (2002) 2877. [8] B. Chandar Shekar, J.Y. Lee, S.W. Rhee, Korean J. Chem. Eng. 21 (1) (2004) 267. [9] S.W. Ahn, K.D. Lee, J.S. Kim, S.H. Kim, J.D. Park, S.H. Lee, P.W. Yoon, Nanotechnology 16 (2005) 1874. [10] M.S. Kim, J.S. Kim, J.C. Cho, M. Shtein, L. Jay Guo, J.S. Kim, Appl. Phys. Lett. 90 (2007) 123113.
4107
[11] K.Y. Yang, S.H. Hong, H. Lee, J.W. Choi, Mater. Sci. Forum. 510–511 (2006) 446. [12] G.Y. Jung, W. Wu, S. Ganapathiappan, D.A.A. Ohlberg, M. Saif Islam, X. Li, D.L. Olynick, H. Lee, Y. Chen, S.Y. Wang, W.M. Tong, R.S. Williams, J. Appl. Physi., A. 81 (2005) 1331. [13] H. Lee, G.Y. Jung, Microelectron. Eng. 77 (2005) 42. [14] K. Nakamatsu, K. Tone, S. Matsui, Jpn. J. Appl. Phys. 44 (2005) 8186. [15] H. Ge, W. Wu, Z. Li, G.Y. Jung, D. Olynick, Y. Chen, J. Alexander Liddle, S.Y. Wang, R. Stanley Williams, Nano Lett. 5 (2005) 179.