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Current Applied Physics 13 (2013) S93eS97

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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,*

aDepartment of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of KoreabAMD Lab/Corporate R&D Institute, Samsung Electro-Mechanics, Suwon 443-742, Republic of KoreacKorea Institute of Nuclear Safety, Daejeon 305-600, Republic of Korea

a r t i c l e i n f o

Article history:Received 12 November 2012Received in revised form2 January 2013Accepted 5 January 2013Available online 1 February 2013

Keywords:Nano-imprint lithographySiliconeLED packageAnti-reflective nano-structureMoth-eye structure

* Corresponding author.E-mail address: heonlee@korea.ac.kr (H. Lee).

1567-1739/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2013.01.014

a b s t r a c t

Many recent studies have focused on enhancing the efficiency of optical devices such as light-emittingdiodes (LEDs). However, optical device efficiency decreases when generated light passes through theLED packaging material. Herein, we developed a technique to improve the efficiency of LED packages andthe external efficiency of the optical devices, which were packaged. Specifically, anti-reflection patternsconsisting of moth-eye structures were used to prevent internal reflection from the surface of the LEDpackage. These nanosized conical structures were fabricated with nano-imprint lithography, which isa next-generation lithography technology. Fine nanosized moth-eye patterns were formed on the surfaceof an LED package, increasing its efficiency by 3.27%.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Silicone material has recently replaced conventional organicpolymers for LEDpackaging 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 out-put of LED devices and consequently reduces their efficiency.

In this study, anti-reflective nano-patterns were fabricated onthe silicone surface to reduce internal reflection from the surface[2]. To fabricate these patterns, UV curing based on nano-imprintlithography (NIL) was used [3,4]. NIL is one of the most promisingnext-generation technologies [5e10]. Unlike conventional lithog-raphy techniques, NIL can fabricate nanosized patterns on a largearea with high throughput and low production cost [11,12].

The moth-eye structures used in this study as an anti-reflectionlayer have a conical shape that is smaller than thewavelength of light[13,14]. These structures gradually change the effective refractiveindex from that of silicone to that of air. Consequently, the trans-mittance of the cured silicone/glass substrate and the LED packageincreases because the internal reflection is reduced by the moth-eyestructures [15,16]. To fabricatemoth-eye structures on cured silicone,NIP-K28 resin composed of acrylate materials was used [17].

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2. Experimental procedures

Fig. 1 shows the scheme for preparing the cured silicone/glasssubstrate and the imprinting mold with the moth-eye patterns.As shown in Fig. 1(a), silicone was spin-coated onto the glasssubstrate and heated for 1 h at 200 �C. This silicone material wascured through a thermoset process. The imprint mold was thenfabricated from a urethane acrylate (UA) resist, which has theproperty of being both rigid and flexible [18]. The UA resist wasspin-coated onto a polyethylene terephthalate (PET) film, andthis UA resist/PET film was placed over a metal master stampwith the anti-reflection structures. Next, the UA resist was con-verted into polyurethane acrylate (PUA) by UV exposure for 30 s.Finally, the PUA/PET imprint mold was detached from the metalmaster mold.

Moth-eye patterns were fabricated on the cured silicone/glasssubstrate using a UV NIL process, as illustrated schematically inFig. 2. First, UV-curable resin (NIP-K28 resin composed of acrylatematerials [17]) was dropped onto the cured silicone/glass sub-strate, and the PUA/PET imprint mold was placed over the sub-strate. To force the resin into the moth-eye patterns, the PUA/PETimprinting mold was compressed at 30 bar for 10 min and thenexposed to UV for 30 s to cure the resin. Finally, the PUA/PETimprinting mold was detached from the patterned silicone/glasssubstrate.

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.

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Moth-eye patternswere also fabricated on an LED package usingUV NIL as shown in Fig. 3. UV-curable resin was dropped onto anLED package. Then, the fabricated PUA/PET moth-eye imprint moldwas placed over the resin. To force the resin into the patterns, themold and LED package were compressed at 25 bar for 10 min andexposed to UV for 30 s to cure the resin. Finally, the mold wasdetached from the LED package.

Fig. 6. Change of transmittance by moth-eye patterns.

3. Results and discussion

The fabricated moth-eye patterns on the cured silicone/glasssubstrate were measured by scanning electron microscopy (SEM).As shown in Fig. 4, fine-textured patterns were fabricated on a largearea with a width of 150e200 nm. Because the patterns hada conical shape, it was hard to resolve their shape using SEM.Moreover, it was difficult to measure their height because theywere fabricated on elastic cured silicone. Therefore, the overallshape and height of the fabricated patterns were measured byatomic force microscopy (AFM).

Fig. 5 shows AFM images of the fabricated moth-eye patterns. Itwas confirmed that the patterns were fabricated very finely anduniformly and shaped as cones with widths of 250e400 nm andheights of 130e140 nm.

To confirm the changes in the internal reflection due to themoth-eye patterns, the transmittance of the moth-eye-patternedcured silicone/glass substrate was measured. As shown in Fig. 6,

Fig. 4. SEM images of fabrica

Fig. 5. AFM images of fabrica

the transmittance was 3% higher than that of an unpatternedsubstrate. This result implies that the refractive index changedgradually from that of the air to those of the UV-curable resin andcured silicone/glass substrate. Finally, the internal reflection fromthe surface of the cured silicone/glass substrate was decreased by

ted moth-eye patterns.

ted moth-eye patterns.

Fig. 7. AFM images of fabricated moth-eye patterns.

Fig. 8. Change of power & lumen by moth-eye patterns.

J.-H. Shin et al. / Current Applied Physics 13 (2013) S93eS97S96

the fabricated moth-eye patterns, thereby increasing the trans-mittance of the substrate (Fig. 7).

The surface of the moth-eye patterned LED package was alsomeasured using AFM. Compared to the moth-eye patterns fab-ricated on the cured silicone/glass substrate, the heights of thecones were slightly different and not as uniform, owing to thecurved surface of the LED package. However, the patterns wereformedwell on the overall area and hadwidths of 250e350 nm andheights of 50e120 nm (Fig. 8).

Finally, the power (W) and luminous flux (lm) weremeasured at0.09 A and 0.18 A to compare the efficiencies of the patterned andunpatterned LED packages. Moreover, the moth-eye-patternedsamples were measured twice to increase the reliability of the re-sults. The power of the unpatterned LED package was 0.978 W at0.09 A and 1.856Wat 0.18 A. In the case of moth-eye patterning, thepower of the LED package in the first measurement was 1.010 W at0.09 A and 1.909 W at 0.18 A, and in the second measurement, it

was 1.009Wat 0.09 A and 1.914W at 0.18 A. On average, the powerincreased by 3.00% at 0.09 A and by 3.22% at 0.18 A. The luminousflux of the unpatterned LED package was 358.4 lm at 0.09 A and678.4 lm at 0.18 A. After moth-eye patterns were formed on the LEDpackage, the luminous flux increased to 369.4 lm at 0.09 A and696.8 lm at 0.18 A in the first test, and to 368.9 lm at 0.09 A and698.9 lm at 0.18 A in the second test. Consequently, the averageluminous 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 efficiencyof LED packages. Our technique could be used to enhance theexternal efficiency of LED packages using an anti-reflection layer.We used NIL to fabricate a moth-eye pattern as the anti-reflectionlayer. These patterns were fabricated on a cured silicone/glasssubstrate and an LED package. Moth-eye patterns were formed very

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finely on the cured silicone/glass substrate (250e400 nmwidth and130e140 nm height) and LED package (250e350 nm width and50e120 nm height). The transmittance of the cured silicone/glasssubstrate with the moth-eye pattern was increased by 2%e3%. Thepower of the LED package was enhanced by 3.00% at 0.09 A and by3.22% at 0.18 A, and the average luminous flux was increased by3.00% at 0.09 A and by 2.87% at 0.18 A.

Acknowledgments

This work was supported by Manpower Development Programfor Energy & Resources funded by the Ministry of Knowledge andEconomy (MKE) and by Basic Science Research Program throughthe National Research Foundation of Korea (NRF) funded by theMinistry of Education, Science and Technology (2012-0002363).

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