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Thermal imprint lithography using sub-micron sized nickel template coated with thin SiO2 layer
Microelectronic Engineering 84 (2007) 1003–1006 www.elsevier.com/locate/mee
Thermal imprint lithography using sub-micron sized nickel template coated with thin SiO2 layer Kyeong-Jae Byeon, Ki-Yeon Yang, Heon Lee
*
Department of Materials Science and Engineering, Korea University, Anam-dong 5-1, Sungbuk-Ku, Seoul 136-701, Republic of Korea Available online 1 February 2007
Abstract Nickel template is very suitable for thermal imprinting process since it has high mechanical durability and can easily be duplicated using electroforming technique. However nickel has a poor anti-sticking property; in addition, proper and stable releasing layer on nickel surface is not yet available. In this study, thin layer of SiO2 film was deposited on nickel surface and silane based hydrophobic self-assembled monolayer (SAM) was formed on SiO2 film, coated on nickel. Since the silane based SAM layer can be stably formed on SiO2 layer coated nickel template, it can be used anti-sticking layer for thermal imprint process using thermoplastic polymer resin or thermally curable prepolymer resin. 2007 Elsevier B.V. All rights reserved. Keywords: Nanoimprint lithography; Nickel template; Silane based SAM; Anti-sticking coating
1. Introduction Recently, nanoimprint lithography (NIL) has attracted as an effective nano-patterning technique, due to its low cost, high throughput and high resolution patterning capability. Thus, NIL has been intensively studied in order to be used for various applications such as bio devices [1], MEMS/NEMS devices [2], optics components [3], microfluidic devices [4] and so on. In order to reduce temperature and pressure of imprinting process, liquid phase prepolymer resin has been introduced in thermally curing nanoimprint lithography, instead of thermoplastic polymer such as poly (methyl methacrylate) (PMMA) [5,6]. In prepolymer based imprinting process, silicon or fused silica template, fabricated by ebeam or deep UV lithography and reactive ion etching (RIE) process, is generally used as an imprint template. However, they are difficult to fabricate and can easily be damaged during imprinting process due to their brittleness
*
Corresponding author. E-mail address: [email protected] (H. Lee).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.102
and poor mechanical property. Thus, new imprint template with excellent mechanical property should be introduced. Nickel template has high mechanical strength and toughness. Moreover, it can easily be duplicated with low cost, using electroforming technique. However, imprint defects can easily be generated when nickel template is used because nickel itself has a poor anti-sticking property. To avoid this, proper releasing layer has to be formed on nickel template. 2. Experiments Nickel template was made from the fused silica master template, which contains patterns of 200 nm to 2 lm in size and 350 nm in height. By combining hot-embossing and electroforming processes, nickel replica can easily be fabricated [7]. A schematic diagram of overall duplication process of nickel replica is shown in Fig. 1. At first, PMMA resist was spin-coated on silicon (1 0 0) substrate and prebaked at 130 C for 5 min to remove solvent. Then hotembossing process was done on the PMMA coated silicon substrate with master template at 170 C and 50 atm for 5 min. On hot-embossed PMMA patterns, 20 nm thin nickel layer was deposited as a seed layer for electroform-
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K.-J. Byeon et al. / Microelectronic Engineering 84 (2007) 1003–1006
1. Heating (170˚C) and pressing (50atm, 5min) Fused silica master
2. Imprinting
3. Detachment of fused silica master
Spin-coated PMMA layer Si substrate 4. Deposition of metal seed layer (Ni) 20nm on PMMA layer
5. Electroforming of nickel
Nickel seed layer
6. Detachment of duplicated nickel template Nickel
Fig. 1. Duplication process of nickel template.
ing process by e-beam evaporation. Then, nickel electroforming was followed with condition of 0.2–0.5 A of current, 4.0 of pH, 94 g/L of nickel concentration and 16AH (amp · hour) of plating rate. As a result, sub-micron sized surface cavities were completely filled and surface protrusions of master template were successfully duplicated on 300 lm thick nickel replica. Anti-sticking treatment of template was done by silane based SAM coating, using 0.1% of (heptadecafluoro1,1,2,2-tetra-hydrodecyl) trichlorosilane, CF3(CF2)5(CH2)2SiCl3 diluted in n-hexane solution [8]. Prior to the SAM coating, the template surface was activated by UVozone treatment. Thermally curable prepolymer imprint resin used in this study was formulated by mixing perfluorinated acrylate
monomers and thermal-radical generator (t-butyl peroxy 2-ethylhexanoate) [5]. Typical conditions of Imprinting process using thermally curable resin was 120 C and 30 atm for 5 min. High pressure was applied in order to remove the residual layer [5]. 3. Results and discussion In Fig. 2, SEM micrographs of fused silica master and nickel replica were shown. Sub-micron sized protrusion patterns of fused silica master were faithfully duplicated to electroformed nickel replica. Since silane based SAM layer can be stably bonded with SiO2 surface, but is easily degraded on nickel surface during thermal imprinting process, [9] 10 15 nm thin SiO2
Fig. 2. SEM micrographs of (a) sub-micron sized surface protrusion patterns of fused silica mater template and (b) the same structures of nickel replica, duplicated by hot-embossing and electroforming processes.
K.-J. Byeon et al. / Microelectronic Engineering 84 (2007) 1003–1006
layer was deposited on nickel template using Plasma Enhanced Chemical Vapor Deposition (PECVD) process. Then solution based SAM coating process was followed. The stability of silane based SAM layer on SiO2 coated nickel template was evaluated by measuring the contact angle of DI water droplet. First contact angle of fresh SAM coated nickel template was measured, then the nickel template was immediately socked in acetone solution at 40 C for 2 h with sonication. Then, a contact angle was measured again and imprinting process, which uses thermally curable prepolymer resin, was done using the same nickel template. Imprinting condition was 120 C and 30 atm for 5 min. After imprinting process, a contact angle of nickel template was re-measured. As shown in Fig. 3, contact angle value for all cases was about 120 and this implies that the anti-sticking property of nickel template
1005
was not degraded during dipping process in acetone solution and thermal imprinting process due to stable and strong bonding between SAM layer and SiO2 surface. The stability of silane based SAM anti-sticking layer on SiO2 coated nickel template has been demonstrated by repeated hot-embossing process on sticky epoxy thermoset polymer, which requires higher temperature (>140 C) and longer heating time (90 min) [10]. In that experiment, five consecutive hot-embossing processes were successfully demonstrated using silane based SAM layer deposited SiO2 coated nickel template. SEM micrographs of resist patterns imprinted using SiO2 coated nickel template is shown in Fig. 4a. Surface protrusion patterns of nickel replica were transferred to the resin layer on silicon (1 0 0) substrate with high fidelity without any noticeable imprint defects. Cross-sectional
Fig. 3. Contact angle measurement of DI water droplet on SiO2 coated nickel template after (a) SAM coating, (b) acetone dipping for 2 h and (c) thermal imprinting process at 120 C and 30 atm for 5 min. A contact angle of nearly 120 was maintained.
Fig. 4. (a) SEM micrographs of imprinted resist pattern using SiO2 coated nickel template and (b) cross-sectional SEM micrograph of imprinted resist pattern.
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K.-J. Byeon et al. / Microelectronic Engineering 84 (2007) 1003–1006
SEM micrograph in Fig. 4b also confirms high fidelity pattern transferring with near zero-residual layer imprinting.
the Ministry of Science and Technology (MOST) and the basic research program of the Korea Science and Engineering Foundation (Grant No. R01-2006-000-10904-0).
4. Summary References Imprint template must have high mechanical strength in order to transfer the nano-sized surface protrusion patterns. Nickel is very suitable material for imprinting template due to its strong durability and mechanical strength. Moreover, it can easily be duplicated with low cost using conventional electroforming technique. In this experiment, thin SiO2 film was deposited on nickel template and SAM anti-sticking layer could be stably formed on SiO2 surface. As the result, thermally curable prepolymer based imprinting process was successfully done on silicon substrate using SiO2 coated nickel template and patterns of nickel template were transferred to resin layer on substrate without any defects. Acknowledgements This study was supported by the Nano/Bio Science and Technology Program (M10536090001-05N3609-00110) of
[1] A. Pepin, P. Youinou, V. Studer, A. Lebib, Y. Chen, Microelectron. Eng. 61–62 (2002) 927. [2] Xiqiu Fan, Honghai Zhang, Sheng Liu, Xiaofeng Hu, Ke Jia, Microelectron. J. 37 (2006) 121. [3] M. Li, H. Tan, L. Chen, J. Wang, S.Y. Chou, J. Vac. Sci. Technol. B 21 (2003) 660. [4] C.A. Mills, E. Martinez, F. Bessueille, G. Villanueva, J. Bausells, J. Samitier, A. Errachid, Microelectron. Eng. 78–79 (2005) 695. [5] H. Lee, G.-Y. Jung, Microelectron. Eng. 77 (2005) 168–174. [6] H. Lee, S.-H. Hong, K.-Y. Yang, K.-W. Choi, Microelectron. Eng. 83 (2006) 323–327. [7] L.J. Heyderman, H. Schift, C. David, B. Ketterer, M. Auf der Maur, J. Gobrecht, Microelectron. Eng. 57–58 (2001) 375–380. [8] G.-Y. Jung, S. Ganapathiappan, X. Li, D.A.A. Ohlberg, D.L. Olynick, Y. Chen, W.M. Tong, R.S. Williams, Appl. Phys. A 78 (2004) 1169. [9] K.-J. Byeon, S.-H. Hong, K.-Y. Yang, D.-K. Kim, H. Lee, Mater. Sci. Forum 539–543 (2007) 968–973. [10] K.-J. Byeon, S.-H. Hong, K.-Y. Yang, S.-H. Ra, J.-H. Ahn, H. Lee, Sol. St. Phenom. 124–126 (2007) 147–151.
Thermal imprint lithography using sub-micron sized nickel template coated with thin SiO2 layer Kyeong-Jae Byeon, Ki-Yeon Yang, Heon Lee
*
Department of Materials Science and Engineering, Korea University, Anam-dong 5-1, Sungbuk-Ku, Seoul 136-701, Republic of Korea Available online 1 February 2007
Abstract Nickel template is very suitable for thermal imprinting process since it has high mechanical durability and can easily be duplicated using electroforming technique. However nickel has a poor anti-sticking property; in addition, proper and stable releasing layer on nickel surface is not yet available. In this study, thin layer of SiO2 film was deposited on nickel surface and silane based hydrophobic self-assembled monolayer (SAM) was formed on SiO2 film, coated on nickel. Since the silane based SAM layer can be stably formed on SiO2 layer coated nickel template, it can be used anti-sticking layer for thermal imprint process using thermoplastic polymer resin or thermally curable prepolymer resin. 2007 Elsevier B.V. All rights reserved. Keywords: Nanoimprint lithography; Nickel template; Silane based SAM; Anti-sticking coating
1. Introduction Recently, nanoimprint lithography (NIL) has attracted as an effective nano-patterning technique, due to its low cost, high throughput and high resolution patterning capability. Thus, NIL has been intensively studied in order to be used for various applications such as bio devices [1], MEMS/NEMS devices [2], optics components [3], microfluidic devices [4] and so on. In order to reduce temperature and pressure of imprinting process, liquid phase prepolymer resin has been introduced in thermally curing nanoimprint lithography, instead of thermoplastic polymer such as poly (methyl methacrylate) (PMMA) [5,6]. In prepolymer based imprinting process, silicon or fused silica template, fabricated by ebeam or deep UV lithography and reactive ion etching (RIE) process, is generally used as an imprint template. However, they are difficult to fabricate and can easily be damaged during imprinting process due to their brittleness
*
Corresponding author. E-mail address: [email protected] (H. Lee).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.102
and poor mechanical property. Thus, new imprint template with excellent mechanical property should be introduced. Nickel template has high mechanical strength and toughness. Moreover, it can easily be duplicated with low cost, using electroforming technique. However, imprint defects can easily be generated when nickel template is used because nickel itself has a poor anti-sticking property. To avoid this, proper releasing layer has to be formed on nickel template. 2. Experiments Nickel template was made from the fused silica master template, which contains patterns of 200 nm to 2 lm in size and 350 nm in height. By combining hot-embossing and electroforming processes, nickel replica can easily be fabricated [7]. A schematic diagram of overall duplication process of nickel replica is shown in Fig. 1. At first, PMMA resist was spin-coated on silicon (1 0 0) substrate and prebaked at 130 C for 5 min to remove solvent. Then hotembossing process was done on the PMMA coated silicon substrate with master template at 170 C and 50 atm for 5 min. On hot-embossed PMMA patterns, 20 nm thin nickel layer was deposited as a seed layer for electroform-
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K.-J. Byeon et al. / Microelectronic Engineering 84 (2007) 1003–1006
1. Heating (170˚C) and pressing (50atm, 5min) Fused silica master
2. Imprinting
3. Detachment of fused silica master
Spin-coated PMMA layer Si substrate 4. Deposition of metal seed layer (Ni) 20nm on PMMA layer
5. Electroforming of nickel
Nickel seed layer
6. Detachment of duplicated nickel template Nickel
Fig. 1. Duplication process of nickel template.
ing process by e-beam evaporation. Then, nickel electroforming was followed with condition of 0.2–0.5 A of current, 4.0 of pH, 94 g/L of nickel concentration and 16AH (amp · hour) of plating rate. As a result, sub-micron sized surface cavities were completely filled and surface protrusions of master template were successfully duplicated on 300 lm thick nickel replica. Anti-sticking treatment of template was done by silane based SAM coating, using 0.1% of (heptadecafluoro1,1,2,2-tetra-hydrodecyl) trichlorosilane, CF3(CF2)5(CH2)2SiCl3 diluted in n-hexane solution [8]. Prior to the SAM coating, the template surface was activated by UVozone treatment. Thermally curable prepolymer imprint resin used in this study was formulated by mixing perfluorinated acrylate
monomers and thermal-radical generator (t-butyl peroxy 2-ethylhexanoate) [5]. Typical conditions of Imprinting process using thermally curable resin was 120 C and 30 atm for 5 min. High pressure was applied in order to remove the residual layer [5]. 3. Results and discussion In Fig. 2, SEM micrographs of fused silica master and nickel replica were shown. Sub-micron sized protrusion patterns of fused silica master were faithfully duplicated to electroformed nickel replica. Since silane based SAM layer can be stably bonded with SiO2 surface, but is easily degraded on nickel surface during thermal imprinting process, [9] 10 15 nm thin SiO2
Fig. 2. SEM micrographs of (a) sub-micron sized surface protrusion patterns of fused silica mater template and (b) the same structures of nickel replica, duplicated by hot-embossing and electroforming processes.
K.-J. Byeon et al. / Microelectronic Engineering 84 (2007) 1003–1006
layer was deposited on nickel template using Plasma Enhanced Chemical Vapor Deposition (PECVD) process. Then solution based SAM coating process was followed. The stability of silane based SAM layer on SiO2 coated nickel template was evaluated by measuring the contact angle of DI water droplet. First contact angle of fresh SAM coated nickel template was measured, then the nickel template was immediately socked in acetone solution at 40 C for 2 h with sonication. Then, a contact angle was measured again and imprinting process, which uses thermally curable prepolymer resin, was done using the same nickel template. Imprinting condition was 120 C and 30 atm for 5 min. After imprinting process, a contact angle of nickel template was re-measured. As shown in Fig. 3, contact angle value for all cases was about 120 and this implies that the anti-sticking property of nickel template
1005
was not degraded during dipping process in acetone solution and thermal imprinting process due to stable and strong bonding between SAM layer and SiO2 surface. The stability of silane based SAM anti-sticking layer on SiO2 coated nickel template has been demonstrated by repeated hot-embossing process on sticky epoxy thermoset polymer, which requires higher temperature (>140 C) and longer heating time (90 min) [10]. In that experiment, five consecutive hot-embossing processes were successfully demonstrated using silane based SAM layer deposited SiO2 coated nickel template. SEM micrographs of resist patterns imprinted using SiO2 coated nickel template is shown in Fig. 4a. Surface protrusion patterns of nickel replica were transferred to the resin layer on silicon (1 0 0) substrate with high fidelity without any noticeable imprint defects. Cross-sectional
Fig. 3. Contact angle measurement of DI water droplet on SiO2 coated nickel template after (a) SAM coating, (b) acetone dipping for 2 h and (c) thermal imprinting process at 120 C and 30 atm for 5 min. A contact angle of nearly 120 was maintained.
Fig. 4. (a) SEM micrographs of imprinted resist pattern using SiO2 coated nickel template and (b) cross-sectional SEM micrograph of imprinted resist pattern.
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K.-J. Byeon et al. / Microelectronic Engineering 84 (2007) 1003–1006
SEM micrograph in Fig. 4b also confirms high fidelity pattern transferring with near zero-residual layer imprinting.
the Ministry of Science and Technology (MOST) and the basic research program of the Korea Science and Engineering Foundation (Grant No. R01-2006-000-10904-0).
4. Summary References Imprint template must have high mechanical strength in order to transfer the nano-sized surface protrusion patterns. Nickel is very suitable material for imprinting template due to its strong durability and mechanical strength. Moreover, it can easily be duplicated with low cost using conventional electroforming technique. In this experiment, thin SiO2 film was deposited on nickel template and SAM anti-sticking layer could be stably formed on SiO2 surface. As the result, thermally curable prepolymer based imprinting process was successfully done on silicon substrate using SiO2 coated nickel template and patterns of nickel template were transferred to resin layer on substrate without any defects. Acknowledgements This study was supported by the Nano/Bio Science and Technology Program (M10536090001-05N3609-00110) of
[1] A. Pepin, P. Youinou, V. Studer, A. Lebib, Y. Chen, Microelectron. Eng. 61–62 (2002) 927. [2] Xiqiu Fan, Honghai Zhang, Sheng Liu, Xiaofeng Hu, Ke Jia, Microelectron. J. 37 (2006) 121. [3] M. Li, H. Tan, L. Chen, J. Wang, S.Y. Chou, J. Vac. Sci. Technol. B 21 (2003) 660. [4] C.A. Mills, E. Martinez, F. Bessueille, G. Villanueva, J. Bausells, J. Samitier, A. Errachid, Microelectron. Eng. 78–79 (2005) 695. [5] H. Lee, G.-Y. Jung, Microelectron. Eng. 77 (2005) 168–174. [6] H. Lee, S.-H. Hong, K.-Y. Yang, K.-W. Choi, Microelectron. Eng. 83 (2006) 323–327. [7] L.J. Heyderman, H. Schift, C. David, B. Ketterer, M. Auf der Maur, J. Gobrecht, Microelectron. Eng. 57–58 (2001) 375–380. [8] G.-Y. Jung, S. Ganapathiappan, X. Li, D.A.A. Ohlberg, D.L. Olynick, Y. Chen, W.M. Tong, R.S. Williams, Appl. Phys. A 78 (2004) 1169. [9] K.-J. Byeon, S.-H. Hong, K.-Y. Yang, D.-K. Kim, H. Lee, Mater. Sci. Forum 539–543 (2007) 968–973. [10] K.-J. Byeon, S.-H. Hong, K.-Y. Yang, S.-H. Ra, J.-H. Ahn, H. Lee, Sol. St. Phenom. 124–126 (2007) 147–151.