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Effect of anti-reflective nano-patterns on LED package
Current Applied Physics 13 (2013) S93eS97
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of anti-reflective nano-patterns on LED package Ju-Hyeon Shin a, Hak-Jong Choi a, Kang-Soo Han a, Seunghyun Ra b, Kyung-Woo Choi c, Heon Lee a, * a
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea AMD Lab/Corporate R&D Institute, Samsung Electro-Mechanics, Suwon 443-742, Republic of Korea c Korea Institute of Nuclear Safety, Daejeon 305-600, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2012 Received in revised form 2 January 2013 Accepted 5 January 2013 Available online 1 February 2013
Many recent studies have focused on enhancing the efficiency of optical devices such as light-emitting diodes (LEDs). However, optical device efficiency decreases when generated light passes through the LED packaging material. Herein, we developed a technique to improve the efficiency of LED packages and the external efficiency of the optical devices, which were packaged. Specifically, anti-reflection patterns consisting of moth-eye structures were used to prevent internal reflection from the surface of the LED package. These nanosized conical structures were fabricated with nano-imprint lithography, which is a next-generation lithography technology. Fine nanosized moth-eye patterns were formed on the surface of an LED package, increasing its efficiency by 3.27%. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nano-imprint lithography Silicone LED package Anti-reflective nano-structure Moth-eye structure
1. Introduction
2. Experimental procedures
Silicone material has recently replaced conventional organic polymers for LED packaging because it is very stable under ultraviolet (UV) radiation, high temperatures, and high humidity [1]. However, internal reflection from the silicone surface reduces the optical output of LED devices and consequently reduces their efficiency. In this study, anti-reflective nano-patterns were fabricated on the silicone surface to reduce internal reflection from the surface [2]. To fabricate these patterns, UV curing based on nano-imprint lithography (NIL) was used [3,4]. NIL is one of the most promising next-generation technologies [5e10]. Unlike conventional lithography techniques, NIL can fabricate nanosized patterns on a large area with high throughput and low production cost [11,12]. The moth-eye structures used in this study as an anti-reflection layer have a conical shape that is smaller than the wavelength of light [13,14]. These structures gradually change the effective refractive index from that of silicone to that of air. Consequently, the transmittance of the cured silicone/glass substrate and the LED package increases because the internal reflection is reduced by the moth-eye structures [15,16]. To fabricate moth-eye structures on cured silicone, NIP-K28 resin composed of acrylate materials was used [17].
Fig. 1 shows the scheme for preparing the cured silicone/glass substrate and the imprinting mold with the moth-eye patterns. As shown in Fig. 1(a), silicone was spin-coated onto the glass substrate and heated for 1 h at 200 C. This silicone material was cured through a thermoset process. The imprint mold was then fabricated from a urethane acrylate (UA) resist, which has the property of being both rigid and flexible [18]. The UA resist was spin-coated onto a polyethylene terephthalate (PET) film, and this UA resist/PET film was placed over a metal master stamp with the anti-reflection structures. Next, the UA resist was converted into polyurethane acrylate (PUA) by UV exposure for 30 s. Finally, the PUA/PET imprint mold was detached from the metal master mold. Moth-eye patterns were fabricated on the cured silicone/glass substrate using a UV NIL process, as illustrated schematically in Fig. 2. First, UV-curable resin (NIP-K28 resin composed of acrylate materials [17]) was dropped onto the cured silicone/glass substrate, and the PUA/PET imprint mold was placed over the substrate. To force the resin into the moth-eye patterns, the PUA/PET imprinting mold was compressed at 30 bar for 10 min and then exposed to UV for 30 s to cure the resin. Finally, the PUA/PET imprinting mold was detached from the patterned silicone/glass substrate.
* Corresponding author. E-mail address: [email protected] (H. Lee). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.01.014
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J.-H. Shin et al. / Current Applied Physics 13 (2013) S93eS97
Fig. 1. Scheme of preparing (a) cured silicone/glass substrate and (b) PUA/PET mold with anti-reflection structures.
Fig. 2. Process of fabricating moth-eye patterns on cured silicone/glass substrate.
Fig. 3. Process of fabricating moth-eye patterns on cured silicone/LED package.
J.-H. Shin et al. / Current Applied Physics 13 (2013) S93eS97
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Moth-eye patterns were also fabricated on an LED package using UV NIL as shown in Fig. 3. UV-curable resin was dropped onto an LED package. Then, the fabricated PUA/PET moth-eye imprint mold was placed over the resin. To force the resin into the patterns, the mold and LED package were compressed at 25 bar for 10 min and exposed to UV for 30 s to cure the resin. Finally, the mold was detached from the LED package. 3. Results and discussion The fabricated moth-eye patterns on the cured silicone/glass substrate were measured by scanning electron microscopy (SEM). As shown in Fig. 4, fine-textured patterns were fabricated on a large area with a width of 150e200 nm. Because the patterns had a conical shape, it was hard to resolve their shape using SEM. Moreover, it was difficult to measure their height because they were fabricated on elastic cured silicone. Therefore, the overall shape and height of the fabricated patterns were measured by atomic force microscopy (AFM). Fig. 5 shows AFM images of the fabricated moth-eye patterns. It was confirmed that the patterns were fabricated very finely and uniformly and shaped as cones with widths of 250e400 nm and heights of 130e140 nm. To confirm the changes in the internal reflection due to the moth-eye patterns, the transmittance of the moth-eye-patterned cured silicone/glass substrate was measured. As shown in Fig. 6,
Fig. 6. Change of transmittance by moth-eye patterns.
the transmittance was 3% higher than that of an unpatterned substrate. This result implies that the refractive index changed gradually from that of the air to those of the UV-curable resin and cured silicone/glass substrate. Finally, the internal reflection from the surface of the cured silicone/glass substrate was decreased by
Fig. 4. SEM images of fabricated moth-eye patterns.
Fig. 5. AFM images of fabricated moth-eye patterns.
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Fig. 7. AFM images of fabricated moth-eye patterns.
Fig. 8. Change of power & lumen by moth-eye patterns.
the fabricated moth-eye patterns, thereby increasing the transmittance of the substrate (Fig. 7). The surface of the moth-eye patterned LED package was also measured using AFM. Compared to the moth-eye patterns fabricated on the cured silicone/glass substrate, the heights of the cones were slightly different and not as uniform, owing to the curved surface of the LED package. However, the patterns were formed well on the overall area and had widths of 250e350 nm and heights of 50e120 nm (Fig. 8). Finally, the power (W) and luminous flux (lm) were measured at 0.09 A and 0.18 A to compare the efficiencies of the patterned and unpatterned LED packages. Moreover, the moth-eye-patterned samples were measured twice to increase the reliability of the results. The power of the unpatterned LED package was 0.978 W at 0.09 A and 1.856 W at 0.18 A. In the case of moth-eye patterning, the power of the LED package in the first measurement was 1.010 W at 0.09 A and 1.909 W at 0.18 A, and in the second measurement, it
was 1.009 W at 0.09 A and 1.914 W at 0.18 A. On average, the power increased by 3.00% at 0.09 A and by 3.22% at 0.18 A. The luminous flux of the unpatterned LED package was 358.4 lm at 0.09 A and 678.4 lm at 0.18 A. After moth-eye patterns were formed on the LED package, the luminous flux increased to 369.4 lm at 0.09 A and 696.8 lm at 0.18 A in the first test, and to 368.9 lm at 0.09 A and 698.9 lm at 0.18 A in the second test. Consequently, the average luminous flux increased by 3.00% at 0.09 A and by 2.87% at 0.18 A. 4. Conclusion In this study, we developed a method to increase the efficiency of LED packages. Our technique could be used to enhance the external efficiency of LED packages using an anti-reflection layer. We used NIL to fabricate a moth-eye pattern as the anti-reflection layer. These patterns were fabricated on a cured silicone/glass substrate and an LED package. Moth-eye patterns were formed very
J.-H. Shin et al. / Current Applied Physics 13 (2013) S93eS97
finely on the cured silicone/glass substrate (250e400 nm width and 130e140 nm height) and LED package (250e350 nm width and 50e120 nm height). The transmittance of the cured silicone/glass substrate with the moth-eye pattern was increased by 2%e3%. The power of the LED package was enhanced by 3.00% at 0.09 A and by 3.22% at 0.18 A, and the average luminous flux was increased by 3.00% at 0.09 A and by 2.87% at 0.18 A. Acknowledgments This work was supported by Manpower Development Program for Energy & Resources funded by the Ministry of Knowledge and Economy (MKE) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0002363). References [1] A.W. Norris, M. Bahadur, M. Yoshitake, Proceedings of SPIE 5941 (2005) 594115. [2] K.H. Hong, J.L. Lee, Electronic Materials Letters 7 (2011) 77e91.
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[3] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Applied Physics Letters 67 (1995) 3114e 3116. [4] H. Lee, K.Y. Jung, Japanese Journal of Applied Physics 43 (2004) 8369e8373. [5] C. Wagner, N. Harned, Nature Photonics 4 (2010) 24e26. [6] S.R. Kim, J.W. Jung, T.S. Lee, J.H. Jeong, S.Y. Lee, S.M. Yang, J.R. Jeong, Electronic Materials Letters 8 (2012) 71e74. [7] J.Y. Shim, K.H. Baek, K.S. Park, H.S. Shin, K.S. No, K. Lee, L.M. Do, Journal of Nanoscience and Nanotechnology 10 (2010) 3628e3630. [8] J. Choi, M.H. Chung, K.Y. Dong, E.M. Park, D.J. Ham, Y. Park, I.S. Song, J.J. Pak, B.K. Ju, Journal of Nanoscience and Nanotechnology 11 (2011) 778e781. [9] Y.K. Su, J.J. Chen, C.L. Lin, S.M. Chen, W.L. Li, C.C. Kao, Japanese Journal of Applied Physics 47 (2008) 6706e6708. [10] T. Ogino, S. Nishimura, J. Shirakashi, Japanese Journal of Applied Physics 47 (2008) 712. [11] M. Colburn, S. Johnson, M. Stewart, S. Damle, T. Vailey, B. Choi, M. Wedlake, T. Michaelson, S.V. Sreenivasan, J. Ekerdt, C.G. Willson, Proceedings of SPIE 3676 (1999) 379. [12] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Science 272 (1996) 85e87. [13] C.G. Bernhard, Endeavour 26 (1967) 79. [14] P.B. Clapham, M.C. Hutley, Nature 244 (1973) 281e282. [15] W.H. Miller, Ocular optical filtering, in: H. Autrum (Ed.), Handbook of Sensory Physiology, vol. VII/6A, Springer, Berlin, 1979, pp. 69e143. [16] K.S. Han, H.J. Lee, D.H. Lee, H. Lee, Solar Energy Materials & Solar Cells 93 (2009) 1214e1217. [17] N. Yoshinobu, H. Atsushi, G. Takumi, Y. Naoki, I. Takeo, Composite Interfaces 14 (2007) 117e130. [18] D. Suh, S.J. Choi, H.H. Lee, Advanced Materials 17 (2005) 1554e1560.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of anti-reflective nano-patterns on LED package Ju-Hyeon Shin a, Hak-Jong Choi a, Kang-Soo Han a, Seunghyun Ra b, Kyung-Woo Choi c, Heon Lee a, * a
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea AMD Lab/Corporate R&D Institute, Samsung Electro-Mechanics, Suwon 443-742, Republic of Korea c Korea Institute of Nuclear Safety, Daejeon 305-600, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2012 Received in revised form 2 January 2013 Accepted 5 January 2013 Available online 1 February 2013
Many recent studies have focused on enhancing the efficiency of optical devices such as light-emitting diodes (LEDs). However, optical device efficiency decreases when generated light passes through the LED packaging material. Herein, we developed a technique to improve the efficiency of LED packages and the external efficiency of the optical devices, which were packaged. Specifically, anti-reflection patterns consisting of moth-eye structures were used to prevent internal reflection from the surface of the LED package. These nanosized conical structures were fabricated with nano-imprint lithography, which is a next-generation lithography technology. Fine nanosized moth-eye patterns were formed on the surface of an LED package, increasing its efficiency by 3.27%. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nano-imprint lithography Silicone LED package Anti-reflective nano-structure Moth-eye structure
1. Introduction
2. Experimental procedures
Silicone material has recently replaced conventional organic polymers for LED packaging because it is very stable under ultraviolet (UV) radiation, high temperatures, and high humidity [1]. However, internal reflection from the silicone surface reduces the optical output of LED devices and consequently reduces their efficiency. In this study, anti-reflective nano-patterns were fabricated on the silicone surface to reduce internal reflection from the surface [2]. To fabricate these patterns, UV curing based on nano-imprint lithography (NIL) was used [3,4]. NIL is one of the most promising next-generation technologies [5e10]. Unlike conventional lithography techniques, NIL can fabricate nanosized patterns on a large area with high throughput and low production cost [11,12]. The moth-eye structures used in this study as an anti-reflection layer have a conical shape that is smaller than the wavelength of light [13,14]. These structures gradually change the effective refractive index from that of silicone to that of air. Consequently, the transmittance of the cured silicone/glass substrate and the LED package increases because the internal reflection is reduced by the moth-eye structures [15,16]. To fabricate moth-eye structures on cured silicone, NIP-K28 resin composed of acrylate materials was used [17].
Fig. 1 shows the scheme for preparing the cured silicone/glass substrate and the imprinting mold with the moth-eye patterns. As shown in Fig. 1(a), silicone was spin-coated onto the glass substrate and heated for 1 h at 200 C. This silicone material was cured through a thermoset process. The imprint mold was then fabricated from a urethane acrylate (UA) resist, which has the property of being both rigid and flexible [18]. The UA resist was spin-coated onto a polyethylene terephthalate (PET) film, and this UA resist/PET film was placed over a metal master stamp with the anti-reflection structures. Next, the UA resist was converted into polyurethane acrylate (PUA) by UV exposure for 30 s. Finally, the PUA/PET imprint mold was detached from the metal master mold. Moth-eye patterns were fabricated on the cured silicone/glass substrate using a UV NIL process, as illustrated schematically in Fig. 2. First, UV-curable resin (NIP-K28 resin composed of acrylate materials [17]) was dropped onto the cured silicone/glass substrate, and the PUA/PET imprint mold was placed over the substrate. To force the resin into the moth-eye patterns, the PUA/PET imprinting mold was compressed at 30 bar for 10 min and then exposed to UV for 30 s to cure the resin. Finally, the PUA/PET imprinting mold was detached from the patterned silicone/glass substrate.
* Corresponding author. E-mail address: [email protected] (H. Lee). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.01.014
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Fig. 1. Scheme of preparing (a) cured silicone/glass substrate and (b) PUA/PET mold with anti-reflection structures.
Fig. 2. Process of fabricating moth-eye patterns on cured silicone/glass substrate.
Fig. 3. Process of fabricating moth-eye patterns on cured silicone/LED package.
J.-H. Shin et al. / Current Applied Physics 13 (2013) S93eS97
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Moth-eye patterns were also fabricated on an LED package using UV NIL as shown in Fig. 3. UV-curable resin was dropped onto an LED package. Then, the fabricated PUA/PET moth-eye imprint mold was placed over the resin. To force the resin into the patterns, the mold and LED package were compressed at 25 bar for 10 min and exposed to UV for 30 s to cure the resin. Finally, the mold was detached from the LED package. 3. Results and discussion The fabricated moth-eye patterns on the cured silicone/glass substrate were measured by scanning electron microscopy (SEM). As shown in Fig. 4, fine-textured patterns were fabricated on a large area with a width of 150e200 nm. Because the patterns had a conical shape, it was hard to resolve their shape using SEM. Moreover, it was difficult to measure their height because they were fabricated on elastic cured silicone. Therefore, the overall shape and height of the fabricated patterns were measured by atomic force microscopy (AFM). Fig. 5 shows AFM images of the fabricated moth-eye patterns. It was confirmed that the patterns were fabricated very finely and uniformly and shaped as cones with widths of 250e400 nm and heights of 130e140 nm. To confirm the changes in the internal reflection due to the moth-eye patterns, the transmittance of the moth-eye-patterned cured silicone/glass substrate was measured. As shown in Fig. 6,
Fig. 6. Change of transmittance by moth-eye patterns.
the transmittance was 3% higher than that of an unpatterned substrate. This result implies that the refractive index changed gradually from that of the air to those of the UV-curable resin and cured silicone/glass substrate. Finally, the internal reflection from the surface of the cured silicone/glass substrate was decreased by
Fig. 4. SEM images of fabricated moth-eye patterns.
Fig. 5. AFM images of fabricated moth-eye patterns.
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Fig. 7. AFM images of fabricated moth-eye patterns.
Fig. 8. Change of power & lumen by moth-eye patterns.
the fabricated moth-eye patterns, thereby increasing the transmittance of the substrate (Fig. 7). The surface of the moth-eye patterned LED package was also measured using AFM. Compared to the moth-eye patterns fabricated on the cured silicone/glass substrate, the heights of the cones were slightly different and not as uniform, owing to the curved surface of the LED package. However, the patterns were formed well on the overall area and had widths of 250e350 nm and heights of 50e120 nm (Fig. 8). Finally, the power (W) and luminous flux (lm) were measured at 0.09 A and 0.18 A to compare the efficiencies of the patterned and unpatterned LED packages. Moreover, the moth-eye-patterned samples were measured twice to increase the reliability of the results. The power of the unpatterned LED package was 0.978 W at 0.09 A and 1.856 W at 0.18 A. In the case of moth-eye patterning, the power of the LED package in the first measurement was 1.010 W at 0.09 A and 1.909 W at 0.18 A, and in the second measurement, it
was 1.009 W at 0.09 A and 1.914 W at 0.18 A. On average, the power increased by 3.00% at 0.09 A and by 3.22% at 0.18 A. The luminous flux of the unpatterned LED package was 358.4 lm at 0.09 A and 678.4 lm at 0.18 A. After moth-eye patterns were formed on the LED package, the luminous flux increased to 369.4 lm at 0.09 A and 696.8 lm at 0.18 A in the first test, and to 368.9 lm at 0.09 A and 698.9 lm at 0.18 A in the second test. Consequently, the average luminous flux increased by 3.00% at 0.09 A and by 2.87% at 0.18 A. 4. Conclusion In this study, we developed a method to increase the efficiency of LED packages. Our technique could be used to enhance the external efficiency of LED packages using an anti-reflection layer. We used NIL to fabricate a moth-eye pattern as the anti-reflection layer. These patterns were fabricated on a cured silicone/glass substrate and an LED package. Moth-eye patterns were formed very
J.-H. Shin et al. / Current Applied Physics 13 (2013) S93eS97
finely on the cured silicone/glass substrate (250e400 nm width and 130e140 nm height) and LED package (250e350 nm width and 50e120 nm height). The transmittance of the cured silicone/glass substrate with the moth-eye pattern was increased by 2%e3%. The power of the LED package was enhanced by 3.00% at 0.09 A and by 3.22% at 0.18 A, and the average luminous flux was increased by 3.00% at 0.09 A and by 2.87% at 0.18 A. Acknowledgments This work was supported by Manpower Development Program for Energy & Resources funded by the Ministry of Knowledge and Economy (MKE) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0002363). References [1] A.W. Norris, M. Bahadur, M. Yoshitake, Proceedings of SPIE 5941 (2005) 594115. [2] K.H. Hong, J.L. Lee, Electronic Materials Letters 7 (2011) 77e91.
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[3] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Applied Physics Letters 67 (1995) 3114e 3116. [4] H. Lee, K.Y. Jung, Japanese Journal of Applied Physics 43 (2004) 8369e8373. [5] C. Wagner, N. Harned, Nature Photonics 4 (2010) 24e26. [6] S.R. Kim, J.W. Jung, T.S. Lee, J.H. Jeong, S.Y. Lee, S.M. Yang, J.R. Jeong, Electronic Materials Letters 8 (2012) 71e74. [7] J.Y. Shim, K.H. Baek, K.S. Park, H.S. Shin, K.S. No, K. Lee, L.M. Do, Journal of Nanoscience and Nanotechnology 10 (2010) 3628e3630. [8] J. Choi, M.H. Chung, K.Y. Dong, E.M. Park, D.J. Ham, Y. Park, I.S. Song, J.J. Pak, B.K. Ju, Journal of Nanoscience and Nanotechnology 11 (2011) 778e781. [9] Y.K. Su, J.J. Chen, C.L. Lin, S.M. Chen, W.L. Li, C.C. Kao, Japanese Journal of Applied Physics 47 (2008) 6706e6708. [10] T. Ogino, S. Nishimura, J. Shirakashi, Japanese Journal of Applied Physics 47 (2008) 712. [11] M. Colburn, S. Johnson, M. Stewart, S. Damle, T. Vailey, B. Choi, M. Wedlake, T. Michaelson, S.V. Sreenivasan, J. Ekerdt, C.G. Willson, Proceedings of SPIE 3676 (1999) 379. [12] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Science 272 (1996) 85e87. [13] C.G. Bernhard, Endeavour 26 (1967) 79. [14] P.B. Clapham, M.C. Hutley, Nature 244 (1973) 281e282. [15] W.H. Miller, Ocular optical filtering, in: H. Autrum (Ed.), Handbook of Sensory Physiology, vol. VII/6A, Springer, Berlin, 1979, pp. 69e143. [16] K.S. Han, H.J. Lee, D.H. Lee, H. Lee, Solar Energy Materials & Solar Cells 93 (2009) 1214e1217. [17] N. Yoshinobu, H. Atsushi, G. Takumi, Y. Naoki, I. Takeo, Composite Interfaces 14 (2007) 117e130. [18] D. Suh, S.J. Choi, H.H. Lee, Advanced Materials 17 (2005) 1554e1560.