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Nanoimprint lithography with a soft roller and focused UV light for flexible substrates
Microelectronic Engineering 98 (2012) 279–283
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanoimprint lithography with a soft roller and focused UV light for flexible substrates HyungJun Lim a, GeeHong Kim a, Kee-Bong Choi a, Mira Jeong a, JiHyeong Ryu b, JaeJong Lee a,b,⇑ a b
Nano Convergence and Manufacturing Systems Research Division, Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, South Korea Department of Nano Mechatronics, University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-807, South Korea
a r t i c l e
i n f o
Article history: Available online 9 June 2012 Keywords: Nanoimprint lithography Roller Ultraviolet light Flexible substrate
a b s t r a c t This paper presents a nanoimprint lithography system for flexible substrates. With this system, a flexible substrate is pressed on a stamp with a low pressing load, a narrow contact area, and a focused ultraviolet (UV) light. The system efficiently transfers a pattern from the stamp to the substrate. A pressing roller is made of a hard material wrapped with silicone rubber. The roller ensures the flexible substrate is in contact with the patterned surface of the stamp. A heavy pressing load widens the contact area of the substrate and the stamp. The focused line beam of UV light, which is smaller than the contact area, cures the resist between the substrate and stamp. The contact length and pressure can be calculated in simulations on the basis of the pressing load and the thickness of the silicone rubber. The simulation results show the stress and strain of all components, such as the pressing roller, the silicone rubber, the substrate, and the stamp. Some of the experiments confirm that the nanoimprint lithography system can be used to transfer nanopatterns and micropatterns from a hard stamp to a flexible film substrate. The system can be applied to the fabrication of flexible electronic devices for applications pertaining to energy, biology, and the environment. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Nanoimprint lithography (NIL) was developed so that the nanopatterns of a stamp could be transferred to a substrate in a quick, exquisite, and economical manner. Ultraviolet NIL (UV-NIL) is more advantageous than thermal NIL because it does not requires a heavy load to achieve a high level of pressure. Moreover, the resist can be cured without any time-consuming heating and cooling processes. On the other hand, roller NIL (RNIL) provides conformal contact of the substrate and stamp for a continuous NIL process [1]. RNIL is a promising technology because it can continually produce the patterning of a long, flexible substrate with high throughput. Recently, a UV-RNIL system was studied; it appears to have a wider variety of features than a flat UV-NIL system [2–6]. The system has two important requirements: contact must be maintained between the roll stamp and the flexible substrate; and the resist on the substrate must be cured during the process. Many other studies have suggested using a substrate that wraps around part of the roll stamp; in that case, an installed UV lamp cures the resist while the substrate is in contact with the stamp. This setup makes these ⇑ Corresponding author at: Nano Convergence and Manufacturing Systems Research Division, Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, South Korea. Tel.: +82 42 868 7145; fax: +82 42 868 7721. E-mail address: [email protected] (J. Lee).
systems somewhat complicated. The rollers rotate a few tens of degrees when in contact with the substrate, so the contact condition must be maintained to ensure the stamp and substrate do not slip during the movement. Reducing the contact area and processing time can therefore be a practical way of preventing potential patterning errors. This paper is focused on the design, simulation, implementation, and process of a new concept of a UV-RNIL system that can use a flat stamp or a roll stamp. The roller and plate ensure a narrow contact area between the substrate and the stamp; and the focused line beam of UV light then instantaneously cures the resist.
2. Soft roller NIL 2.1. System design Fig. 1 shows a conceptual drawing of the proposed UV-RNIL system, which incorporates a flat stamp and a flexible substrate. The flexible substrate is supplied from the right side and withdrawn to the left. The substrate is guided by two guide rollers and wraps around the press roller. Because the supply and withdrawal part of the substrate are parallel, there is no variation in the overall length of the substrate while the press roller rolls on the plate stamp from the left to the right. The stamp is fixed on the top surface of the window.
0167-9317/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.04.030
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Fig. 1. Conceptual drawing of the UV-RNIL system for use with a flexible substrate.
Fig. 2. Conceptual drawing of the contacting parts before and after the pressing.
The system uses a UV light source, as with other systems, but the UV light is focused in the shape of a narrow line. While pressing the stamp onto the substrate, the press roller rolls on the stamp and the UV light source follows the center of the press roller to cure the resist line by line. The stamp and the window must be transparent so that the UV light can reach the resist between the substrate and the stamp. The contact area between the stamp and the substrate is also narrow but can be made wider than the length of the focused line beam of UV light. Because the resist should spread throughout the stamp pattern before it is cured, it must be pressed between the stamp and the substrate under certain press conditions when exposed to UV light. This requirement implies that the contact length between the stamp and the substrate must be larger than the length of the UV light. Otherwise, the UV light is illuminated before or after the substrate is in contact with the stamp. If cured before contact, the resist may not fill in the space of the stamp. In contrast, if the resist is cured after contact, the UV efficiency deteriorates. This behavior implies that the press roller should roll slowly. Thus, the contact length and the position of the UV light source must be controlled throughout the entire process. 2.2. Soft roller As shown in Fig. 2, silicone rubber is wrapped around the press roller as a means of controlling the contact length. The status of the contacting parts, before and after the pressing, is illustrated in the left and right diagrams, respectively. Because the roller is made of metal and the stamp and window are made of glass or quartz, the silicone rubber is severely deformed. The contact length can be determined in terms of the thickness of the silicone rubber and the pressure of the pressing load. In a simulation, several design parameters were fixed. The diameter of the press roller was 100 mm, and the roller was made
Fig. 3. Stress distribution at the contact region between the flexible substrate and the stamp under a load of (a) 98 N, (b) 196 N, and (c) 294 N.
of an aluminum alloy. The thickness of the silicone rubber was 1.5 mm. Polycarbonate (PC) film (0.2 mm thick) was used as the flexible substrate. The press roller and the PC film were both 150 mm wide. The simulation was performed under the following assumptions. (1) The roller has a shape of a perfect cylinder. (2) The roller only moves down to the stamp until it reaches a certain pressing force. (3) There are no frictions between the surfaces. (4) The tension of the flexible substrate is neglected because it is much smaller than the pressing force. Fig. 3 shows the stress distributions with respect to the pressing load by simulations. As the pressing load increases from 98 to 294 N, the normal stress becomes higher and the contact length becomes wider. The calculated contact pressure distributions are shown in Fig. 4. The graph shows that the average pressure increases as the load becomes greater. The contact pressure at the center is
H. Lim et al. / Microelectronic Engineering 98 (2012) 279–283
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Fig. 8. Photo image of the UV-RNIL system including the press roller, the UV source, and a flexible substrate. Fig. 4. Contact pressure distribution between the flexible substrate and the stamp.
Fig. 5. Contact pressure at the center of the contact area with respect to the pressing load.
Fig. 6. Contact length of the substrate and the stamp with respect to the pressing load.
Fig. 9. SEM images: (a) nanodots on a transparent flat stamp; (b) nanodots transferred onto the flexible substrate.
Fig. 7. The measured power of the UV light along the center line of the roller. (The measurement was done at the top surface of the quartz window.)
proportional to the load, and its proportionality coefficient is about 1.3 10 3 MPa/N, as shown in Fig. 5. The contact length also becomes larger as the load is increased. However, as shown in Fig. 6, the contact length tends to be proportional to the square root of the pressing load. The length is calculated by measuring the width of the side peaks of the contact pressure distribution
graph of Fig. 4 for each load case. In contrast to the general tendency of the contact pressure distribution of a soft roll and a hard plate, the pressure increases sharply at both ends of contact. This phenomenon is caused by the material properties of the substrate and the silicone rubber. The material stress is much higher at the vicinity of both ends of contact because the polycarbonate substrate has a larger modulus of elasticity than the silicone rubber and because the radius of curvature of the substrate reaches its smallest level on both sides. Without the polycarbonate substrate, the peak of the contact pressure distribution for both sides disappears. However, this system uses the contact area whenever the contact pressure distribution is almost flat. For the case of a 294 N load, the length of the stable pressure distribution is about 3 mm.
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3. Implementation and experiments 3.1. UV light source The NIL system requires a focused line beam of UV light. Because a UV lamp and its optical parts are generally optimized for a flat lithography system, the system may require complex optical design. Moreover, if a UV lamp is used, there may be some difficulty in constructing a compact UV module than can follow the center position of the press roller. The cooling of the UV lamp is another issue of implementation. The window, which is an essential component that holds the stamp, must be thick enough to withstand the pressing force. As shown in Fig. 2, the UV source is set apart from the contact area because of the window. Therefore, if the shape of the UV light is formed by using a mask with a narrow slit, the light is spread by diffraction and scattering when it reaches the resist. Thus, for the system of this study, the combination of a light emitting diode (LED) array and a cylindrical lens is suitable. The UV light from
the LED array is designed to have a line shape with a minimum size at the contact area. The diffraction from the window and the stamp should be considered to design the cylindrical lens. The area of the focused UV beam on the top surface of the stamp was designed to be 2 mm by 200 mm. The gap between the LEDs should be optimized so that a uniform line beam can be obtained with a minimum number of LED sources. The gap between the LEDs can be widened by using fewer LEDs. In this case, the intensity of the focused line beam fluctuates in the direction of its width. However, the use of many LEDs is uneconomical. Since the system used 34 LEDs with 8 mm gap, the total length of the LED array is 264 mm. As one can see in Fig. 7, the average power of the UV beam along the center line in the width direction was measured as 26.4 mW/cm2 with a standard deviation of 1.0 mW/cm2. The fluctuation of overall UV power is because of the deviation of each LED’s power. Despite four more LEDs on each side, the guaranteed width was only 190 mm. Fig. 8 shows the main parts of the UVRNIL system, including the UV light source, the press roller, and the flexible substrate.
Fig. 10. WSI images: (a) microlines on the transparent flat stamp; (b) microlines transferred onto the flexible substrate.
H. Lim et al. / Microelectronic Engineering 98 (2012) 279–283
3.2. Experimental parameters and results The experiment on the nanopattern and micropattern transfer was conducted under the same conditions as the simulations. Silicone rubber was wrapped around a press roller with a diameter of 100 mm, and polycarbonate film (0.2 mm thick) was installed as a flexible substrate. Nanopatterns and micropatterns of polyurethaneacrylate (PUA) with a 6 in. glass wafer were used as a transparent stamp. The stamp contains micropatterned lines and nanopatterned dots. The microlines have a width, depth, and pitch of 4 lm, 110 nm, and 16 lm, respectively. The nanodots have a diameter of 140 nm and a pitch of 1 lm. The pressing load was at 294 N. The results of the simulation show that a load of 294 N produces a contact length of about 4 mm. Because the focused length of the UV light source is 2 mm, the contact length should be more than 3 mm on account of the 1 mm margin. Under these conditions, the pressure in the middle of the contact length was 0.44 MPa. After the press roller brings the stamp and substrate into contact, the center of the roller moves from left to right at a velocity of 1.67 mm/s (or 100 mm/min). The velocity of the motion is an important parameter for the pattern transfer. An excessively fast speed may cause premature curing; an excessively slow speed may cause the resist to be cured before the substrate comes into contact with the stamp. Even though the system utilizes the focused line beam, the UV light tends to suffer some leakage because of the scattering and reflection from the surfaces of the stamp and the window. Therefore, the velocity of the press roller in the horizontal direction should be optimized by experiments. The resist for the experiments was UV-curable Ormostamp (micro resist technology GmbH). The resist is dropped on the patterned surface of the stamp and then transferred to the surface of the flexible substrate during the RNIL process. The nanopatterns and micropatterns of the stamp and substrate are shown in Fig. 9 and Fig. 10, respectively. The micropatterns were measured by means of white-light scanning interferometry (WSI) so that the depth of the transferred patterns could be compared. Three-
283
dimensional images show that the microlines on the substrate are the reverse of the stamp patterns. 4. Conclusions A UV-RNIL system for a flexible substrate was designed and implemented for this study. The use of a soft roller enabled the contact pressure between the substrate and the stamp to be controlled. A focused line beam of UV light was then illuminated on the contact area to instantaneously cure the resist. The simulation results were used to calculate the length of the contact area and the pressure distribution. Finally, the lithography system was verified by experiments on micropatterning and nanopatterning with few droplets of the resist. In future works, the process conditions will be optimized for various types of resists and substrates. The system also requires a reliable resist coating unit to get a uniform resist layer on a large area of the substrate. The proposed UV-RNIL system can be used to produce flexible nanopatterned devices for applications pertaining to energy, biology, and the environment. Acknowledgements This research was supported by the Cooperative Research Program of the Korea Research Council for Industrial Science and Technology (ISTK) and the 21st Century Frontier Research Program of Center for Nanoscale Mechatronics and Manufacturing funded by the Korean Ministry of Education, Science and Technology. References [1] Hua. Tan, Andrew. Gilbertson, Stephen.Y. Chou, J. Vac. Sci. Technol., B 16 (1998) 3926–3928. [2] Suho. Ahn, Joowon. Cha, Ho. Myung, Seok.-min. Kim, Shinill. Kang, Appl. Phys. Lett. 89 (2006) 213101. [3] Se Hyun. Ahn, L. Jay Guo, Adv. Mater. 20 (2008) 2044–2049. [4] Colin. Stuart, Yong. Chen, ACS Nano. 3 (2009) 2062–2064. [5] Se Hyun. Ahn, L. Jay Guo, ACS Nano 3 (2009) 2304–2310. [6] J. Han, S. Choi, J. Lim, B.S. Lee, S. Kang, J. Phys. D: Appl. Phys. 42 (2009) 115503– 115506.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanoimprint lithography with a soft roller and focused UV light for flexible substrates HyungJun Lim a, GeeHong Kim a, Kee-Bong Choi a, Mira Jeong a, JiHyeong Ryu b, JaeJong Lee a,b,⇑ a b
Nano Convergence and Manufacturing Systems Research Division, Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, South Korea Department of Nano Mechatronics, University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-807, South Korea
a r t i c l e
i n f o
Article history: Available online 9 June 2012 Keywords: Nanoimprint lithography Roller Ultraviolet light Flexible substrate
a b s t r a c t This paper presents a nanoimprint lithography system for flexible substrates. With this system, a flexible substrate is pressed on a stamp with a low pressing load, a narrow contact area, and a focused ultraviolet (UV) light. The system efficiently transfers a pattern from the stamp to the substrate. A pressing roller is made of a hard material wrapped with silicone rubber. The roller ensures the flexible substrate is in contact with the patterned surface of the stamp. A heavy pressing load widens the contact area of the substrate and the stamp. The focused line beam of UV light, which is smaller than the contact area, cures the resist between the substrate and stamp. The contact length and pressure can be calculated in simulations on the basis of the pressing load and the thickness of the silicone rubber. The simulation results show the stress and strain of all components, such as the pressing roller, the silicone rubber, the substrate, and the stamp. Some of the experiments confirm that the nanoimprint lithography system can be used to transfer nanopatterns and micropatterns from a hard stamp to a flexible film substrate. The system can be applied to the fabrication of flexible electronic devices for applications pertaining to energy, biology, and the environment. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Nanoimprint lithography (NIL) was developed so that the nanopatterns of a stamp could be transferred to a substrate in a quick, exquisite, and economical manner. Ultraviolet NIL (UV-NIL) is more advantageous than thermal NIL because it does not requires a heavy load to achieve a high level of pressure. Moreover, the resist can be cured without any time-consuming heating and cooling processes. On the other hand, roller NIL (RNIL) provides conformal contact of the substrate and stamp for a continuous NIL process [1]. RNIL is a promising technology because it can continually produce the patterning of a long, flexible substrate with high throughput. Recently, a UV-RNIL system was studied; it appears to have a wider variety of features than a flat UV-NIL system [2–6]. The system has two important requirements: contact must be maintained between the roll stamp and the flexible substrate; and the resist on the substrate must be cured during the process. Many other studies have suggested using a substrate that wraps around part of the roll stamp; in that case, an installed UV lamp cures the resist while the substrate is in contact with the stamp. This setup makes these ⇑ Corresponding author at: Nano Convergence and Manufacturing Systems Research Division, Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, South Korea. Tel.: +82 42 868 7145; fax: +82 42 868 7721. E-mail address: [email protected] (J. Lee).
systems somewhat complicated. The rollers rotate a few tens of degrees when in contact with the substrate, so the contact condition must be maintained to ensure the stamp and substrate do not slip during the movement. Reducing the contact area and processing time can therefore be a practical way of preventing potential patterning errors. This paper is focused on the design, simulation, implementation, and process of a new concept of a UV-RNIL system that can use a flat stamp or a roll stamp. The roller and plate ensure a narrow contact area between the substrate and the stamp; and the focused line beam of UV light then instantaneously cures the resist.
2. Soft roller NIL 2.1. System design Fig. 1 shows a conceptual drawing of the proposed UV-RNIL system, which incorporates a flat stamp and a flexible substrate. The flexible substrate is supplied from the right side and withdrawn to the left. The substrate is guided by two guide rollers and wraps around the press roller. Because the supply and withdrawal part of the substrate are parallel, there is no variation in the overall length of the substrate while the press roller rolls on the plate stamp from the left to the right. The stamp is fixed on the top surface of the window.
0167-9317/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.04.030
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H. Lim et al. / Microelectronic Engineering 98 (2012) 279–283
Fig. 1. Conceptual drawing of the UV-RNIL system for use with a flexible substrate.
Fig. 2. Conceptual drawing of the contacting parts before and after the pressing.
The system uses a UV light source, as with other systems, but the UV light is focused in the shape of a narrow line. While pressing the stamp onto the substrate, the press roller rolls on the stamp and the UV light source follows the center of the press roller to cure the resist line by line. The stamp and the window must be transparent so that the UV light can reach the resist between the substrate and the stamp. The contact area between the stamp and the substrate is also narrow but can be made wider than the length of the focused line beam of UV light. Because the resist should spread throughout the stamp pattern before it is cured, it must be pressed between the stamp and the substrate under certain press conditions when exposed to UV light. This requirement implies that the contact length between the stamp and the substrate must be larger than the length of the UV light. Otherwise, the UV light is illuminated before or after the substrate is in contact with the stamp. If cured before contact, the resist may not fill in the space of the stamp. In contrast, if the resist is cured after contact, the UV efficiency deteriorates. This behavior implies that the press roller should roll slowly. Thus, the contact length and the position of the UV light source must be controlled throughout the entire process. 2.2. Soft roller As shown in Fig. 2, silicone rubber is wrapped around the press roller as a means of controlling the contact length. The status of the contacting parts, before and after the pressing, is illustrated in the left and right diagrams, respectively. Because the roller is made of metal and the stamp and window are made of glass or quartz, the silicone rubber is severely deformed. The contact length can be determined in terms of the thickness of the silicone rubber and the pressure of the pressing load. In a simulation, several design parameters were fixed. The diameter of the press roller was 100 mm, and the roller was made
Fig. 3. Stress distribution at the contact region between the flexible substrate and the stamp under a load of (a) 98 N, (b) 196 N, and (c) 294 N.
of an aluminum alloy. The thickness of the silicone rubber was 1.5 mm. Polycarbonate (PC) film (0.2 mm thick) was used as the flexible substrate. The press roller and the PC film were both 150 mm wide. The simulation was performed under the following assumptions. (1) The roller has a shape of a perfect cylinder. (2) The roller only moves down to the stamp until it reaches a certain pressing force. (3) There are no frictions between the surfaces. (4) The tension of the flexible substrate is neglected because it is much smaller than the pressing force. Fig. 3 shows the stress distributions with respect to the pressing load by simulations. As the pressing load increases from 98 to 294 N, the normal stress becomes higher and the contact length becomes wider. The calculated contact pressure distributions are shown in Fig. 4. The graph shows that the average pressure increases as the load becomes greater. The contact pressure at the center is
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Fig. 8. Photo image of the UV-RNIL system including the press roller, the UV source, and a flexible substrate. Fig. 4. Contact pressure distribution between the flexible substrate and the stamp.
Fig. 5. Contact pressure at the center of the contact area with respect to the pressing load.
Fig. 6. Contact length of the substrate and the stamp with respect to the pressing load.
Fig. 9. SEM images: (a) nanodots on a transparent flat stamp; (b) nanodots transferred onto the flexible substrate.
Fig. 7. The measured power of the UV light along the center line of the roller. (The measurement was done at the top surface of the quartz window.)
proportional to the load, and its proportionality coefficient is about 1.3 10 3 MPa/N, as shown in Fig. 5. The contact length also becomes larger as the load is increased. However, as shown in Fig. 6, the contact length tends to be proportional to the square root of the pressing load. The length is calculated by measuring the width of the side peaks of the contact pressure distribution
graph of Fig. 4 for each load case. In contrast to the general tendency of the contact pressure distribution of a soft roll and a hard plate, the pressure increases sharply at both ends of contact. This phenomenon is caused by the material properties of the substrate and the silicone rubber. The material stress is much higher at the vicinity of both ends of contact because the polycarbonate substrate has a larger modulus of elasticity than the silicone rubber and because the radius of curvature of the substrate reaches its smallest level on both sides. Without the polycarbonate substrate, the peak of the contact pressure distribution for both sides disappears. However, this system uses the contact area whenever the contact pressure distribution is almost flat. For the case of a 294 N load, the length of the stable pressure distribution is about 3 mm.
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H. Lim et al. / Microelectronic Engineering 98 (2012) 279–283
3. Implementation and experiments 3.1. UV light source The NIL system requires a focused line beam of UV light. Because a UV lamp and its optical parts are generally optimized for a flat lithography system, the system may require complex optical design. Moreover, if a UV lamp is used, there may be some difficulty in constructing a compact UV module than can follow the center position of the press roller. The cooling of the UV lamp is another issue of implementation. The window, which is an essential component that holds the stamp, must be thick enough to withstand the pressing force. As shown in Fig. 2, the UV source is set apart from the contact area because of the window. Therefore, if the shape of the UV light is formed by using a mask with a narrow slit, the light is spread by diffraction and scattering when it reaches the resist. Thus, for the system of this study, the combination of a light emitting diode (LED) array and a cylindrical lens is suitable. The UV light from
the LED array is designed to have a line shape with a minimum size at the contact area. The diffraction from the window and the stamp should be considered to design the cylindrical lens. The area of the focused UV beam on the top surface of the stamp was designed to be 2 mm by 200 mm. The gap between the LEDs should be optimized so that a uniform line beam can be obtained with a minimum number of LED sources. The gap between the LEDs can be widened by using fewer LEDs. In this case, the intensity of the focused line beam fluctuates in the direction of its width. However, the use of many LEDs is uneconomical. Since the system used 34 LEDs with 8 mm gap, the total length of the LED array is 264 mm. As one can see in Fig. 7, the average power of the UV beam along the center line in the width direction was measured as 26.4 mW/cm2 with a standard deviation of 1.0 mW/cm2. The fluctuation of overall UV power is because of the deviation of each LED’s power. Despite four more LEDs on each side, the guaranteed width was only 190 mm. Fig. 8 shows the main parts of the UVRNIL system, including the UV light source, the press roller, and the flexible substrate.
Fig. 10. WSI images: (a) microlines on the transparent flat stamp; (b) microlines transferred onto the flexible substrate.
H. Lim et al. / Microelectronic Engineering 98 (2012) 279–283
3.2. Experimental parameters and results The experiment on the nanopattern and micropattern transfer was conducted under the same conditions as the simulations. Silicone rubber was wrapped around a press roller with a diameter of 100 mm, and polycarbonate film (0.2 mm thick) was installed as a flexible substrate. Nanopatterns and micropatterns of polyurethaneacrylate (PUA) with a 6 in. glass wafer were used as a transparent stamp. The stamp contains micropatterned lines and nanopatterned dots. The microlines have a width, depth, and pitch of 4 lm, 110 nm, and 16 lm, respectively. The nanodots have a diameter of 140 nm and a pitch of 1 lm. The pressing load was at 294 N. The results of the simulation show that a load of 294 N produces a contact length of about 4 mm. Because the focused length of the UV light source is 2 mm, the contact length should be more than 3 mm on account of the 1 mm margin. Under these conditions, the pressure in the middle of the contact length was 0.44 MPa. After the press roller brings the stamp and substrate into contact, the center of the roller moves from left to right at a velocity of 1.67 mm/s (or 100 mm/min). The velocity of the motion is an important parameter for the pattern transfer. An excessively fast speed may cause premature curing; an excessively slow speed may cause the resist to be cured before the substrate comes into contact with the stamp. Even though the system utilizes the focused line beam, the UV light tends to suffer some leakage because of the scattering and reflection from the surfaces of the stamp and the window. Therefore, the velocity of the press roller in the horizontal direction should be optimized by experiments. The resist for the experiments was UV-curable Ormostamp (micro resist technology GmbH). The resist is dropped on the patterned surface of the stamp and then transferred to the surface of the flexible substrate during the RNIL process. The nanopatterns and micropatterns of the stamp and substrate are shown in Fig. 9 and Fig. 10, respectively. The micropatterns were measured by means of white-light scanning interferometry (WSI) so that the depth of the transferred patterns could be compared. Three-
283
dimensional images show that the microlines on the substrate are the reverse of the stamp patterns. 4. Conclusions A UV-RNIL system for a flexible substrate was designed and implemented for this study. The use of a soft roller enabled the contact pressure between the substrate and the stamp to be controlled. A focused line beam of UV light was then illuminated on the contact area to instantaneously cure the resist. The simulation results were used to calculate the length of the contact area and the pressure distribution. Finally, the lithography system was verified by experiments on micropatterning and nanopatterning with few droplets of the resist. In future works, the process conditions will be optimized for various types of resists and substrates. The system also requires a reliable resist coating unit to get a uniform resist layer on a large area of the substrate. The proposed UV-RNIL system can be used to produce flexible nanopatterned devices for applications pertaining to energy, biology, and the environment. Acknowledgements This research was supported by the Cooperative Research Program of the Korea Research Council for Industrial Science and Technology (ISTK) and the 21st Century Frontier Research Program of Center for Nanoscale Mechatronics and Manufacturing funded by the Korean Ministry of Education, Science and Technology. References [1] Hua. Tan, Andrew. Gilbertson, Stephen.Y. Chou, J. Vac. Sci. Technol., B 16 (1998) 3926–3928. [2] Suho. Ahn, Joowon. Cha, Ho. Myung, Seok.-min. Kim, Shinill. Kang, Appl. Phys. Lett. 89 (2006) 213101. [3] Se Hyun. Ahn, L. Jay Guo, Adv. Mater. 20 (2008) 2044–2049. [4] Colin. Stuart, Yong. Chen, ACS Nano. 3 (2009) 2062–2064. [5] Se Hyun. Ahn, L. Jay Guo, ACS Nano 3 (2009) 2304–2310. [6] J. Han, S. Choi, J. Lim, B.S. Lee, S. Kang, J. Phys. D: Appl. Phys. 42 (2009) 115503– 115506.