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Microlens array diffuser for a light-emitting diode backlight system Sung-Il Chang and Jun-Bo Yoon Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea
Hongki Kim, Jin-Jong Kim, Baik-Kyu Lee, and Dong Ho Shin Optical Module Laboratory, Central R&D Institute, Samsung Electro-Mechanics, 314 Maetan3-dong, Yeongtong-gu, Suwon, Gyunggi-do 443-743, Korea Received May 17, 2006; revised July 16, 2006; accepted July 20, 2006; posted August 7, 2006 (Doc. ID 71066); published September 25, 2006 Microlens array (MLA) diffusers for light-emitting diode (LED) backlight systems have been developed. A high fill-factor photoresist mold for the MLA was fabricated using three-dimensional diffuser lithography, and the patterns were transferred to a nickel master mold for UV-curable polymer replication. The fabricated microlens had various paraboloidal profiles, and its aspect ratio was controlled from 1.0 to 2.1. The MLA diffuser showed a batwing radiation pattern with a radiation angle of 150°. The fabricated MLA diffuser may greatly enhance the color-mixing characteristics of LED backlight systems and help reduce the number of LEDs required. © 2006 Optical Society of America OCIS codes: 230.3990, 230.4000, 350.3950.
Diffusers are used widely in backlight systems of various liquid-crystal display (LCD) devices such as mobile phones, monitors, and televisions. Their primary role is to spread the incident light from sources over a wide angle to prevent light sources from being seen directly by viewers and to keep the brightness uniform over the entire display area. Recently, direct-type backlight systems with redgreen-blue (RGB) light-emitting diodes (LEDs) have been developed for LCD televisions to enhance color reproducibility. In the LED backlight system, a diffuser with a large radiation angle is necessary for effective color mixing and a reduction in the number of LEDs.1 Furthermore, a batwing radiation pattern produced by the diffuser is required to eliminate hot spots on the screen that result from the LED arrays. Such a specific radiation pattern is not attainable with conventional opal-coated and sandblasted diffusers that have Lambertian and Gaussian radiation patterns, respectively. Microlens arrays (MLAs) are one of the most promising solutions for customized tailoring of the characteristics of diffusers.1,2 By controlling the profile of the microlens and the arrangement of the MLA, the diffusing characteristics of the MLA can be adjusted. However, it is not easy to control the profile of the microlens using conventional fabrication technologies such as photoresist reflow,3,4 microjet technology,5 and polymeric micromolding of isotropically etched substrates,6 which can produce only spherical curvatures. Furthermore, the radiation angles of the MLAs fabricated by these conventional methods are limited due to their low aspect ratios. Holographic technology is one of the most promising methods to produce aspherical curvatures, except that it requires an expensive and complex holography system.7 Another important requirement for a MLA diffuser is a high fill factor. If there is no 0146-9592/06/203016-3/$15.00
high fill factor, the light passing through the flat region between microlenses will not be diffused and will form hot spots on the screen. In this Letter, we propose a fabrication method for a high fill-factor MLA with controllable paraboloidal profiles. Then, we present a MLA diffuser with a batwing radiation pattern and a large radiation angle of 150°. The proposed fabrication method of the MLA dif-
Fig. 1. (Color online) Fabrication process of a MLA diffuser: (a) UV exposure on the photoresist through a diffuser and photomask, (b) UV flood exposure, (c) fabricated photoresist mold after development, (d) fabrication of a copper intermediate mold, (e) nickel master mold made by electroplating, (f) plastic replication from the nickel master mold. © 2006 Optical Society of America
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separated the solidified polymer MLA from the master mold [Fig. 1(f)]. Figure 2 shows scanning electron microscopy photographs of the fabricated plastic MLAs with various heights from 10 to 21 m. The pitch of the MLA was 10 m, and the microlenses were arranged in a hexagonal array; the area of the MLA diffuser was 1 cm by 1 cm. The gap between the plastic microlenses increased to 400 nm during the several replication steps. However, the fill factor of the MLA remained at more than 92%, and it can be improved further by optimization of the replication processes. In general, the profile of the microlens is modeled as Fig. 2. Scanning electron microscopy photographs of the fabricated plastic MLAs with a pitch of 10 m and heights of (a) 10 m, (b) 12 m, (c) 17 m, and (d) 21 m.
fuser consists of three steps, as shown in Fig. 1: formation of a high fill-factor photoresist mold by use of three-dimensional (3-D) diffuser lithography,8 fabrication of a copper intermediate mold and a nickel master mold by electroplating, and plastic replication from the nickel master mold. During 3-D diffuser lithography, UV exposure of 4500 mJ/ cm2 on 80 m thick AZ9260 positive photoresist (Clariant Co., Ltd.) was performed through a sandblasted diffuser plate (F43-725; Edmund Optics Co., Ltd.) and a photomask [Fig. 1(a)]. We adjusted the flood exposure dose from 25 to 150 mJ/ cm2 to achieve aspect ratios of the microlens mold of 1.0 to 2.1 [Fig. 1(b)]. Because the exposed regions of the adjacent microlens patterns overlapped each other, sharp edges between the microlens molds were formed after development [Fig. 1(c)]. The radius of the curvature of the sharp apex and the surface root-mean-square (RMS) roughness of the photoresist mold were measured as 100 and 2.22 nm, respectively. Although the photoresist mold could be used as a master for the plastic replication, it was not suitable for repeated replication because of its fragility and degradation during the replication process. Instead, a rigid nickel master mold was developed for the plastic replication. A 300 nm thick copper seed layer was thermally evaporated on the photoresist mold, and the copper mold was electroplated up to 30 m [1200 mA, 2 h; Fig. 1(d)]. The photoresist mold was subsequently removed with acetone. After antiadhesion pretreatment of the copper surface, the nickel master mold was formed by electroplating (1500 mA, 2 h), and its thickness was 35 m [Fig. 1(e)]. During the replication process, the surface roughness of the nickel master mold was not degraded compared with that of the photoresist mold, and its RMS value was measured to be 2.26 nm. Since the nickel master was a positive replica of the photoresist mold, positive-relief MLA patterns could be obtained from the plastic replication of the nickel master mold. Finally, a UV-curable polymer (refractive index, 1.56; CCTech, Inc.) was poured onto the nickel master mold and covered with a polycarbonate plate. We exposed 1300 mJ/ cm2 of UV light for curing and
r2
1 h共r兲 =
R 冑1 + 关1 − 共K + 1兲r2/R2兴
,
共1兲
where h is the height of the microlens, r = 共x2 + y2兲1/2 is the distance from the surface of the curvature to the optical axis, R is the radius of the curvature at the vertex, and K is the aspherical constant9; r can be replaced with x for a two-dimensional profile of the microlens by setting y = 0. Assuming a parabolic curvature, K = −1 can be used. As a result, the parabolic profile can be expressed as 1 h共x兲 = −
2R
x2 + hm ,
共2兲
where hm is the maximum height of the microlens. The minus sign was added to present the positiverelief profile of the microlens. Comparisons between the actual profiles of the fabricated microlenses and the ideal parabolic profiles were performed, as shown in Fig. 3 (dotted and solid curves, respectively). The profiles of the fabricated microlenses fit very well with the ideal parabolic profiles. Although the RMS error between the actual and the ideal profiles increased to 0.5 m for the microlens with an aspect ratio over 1.7, the error might be reduced by taking the high-order terms of the parabolic profile formula into consideration. Finally, Fig. 4(a) shows a schematic view of the measurement setup for the radiation patterns of the fabricated MLA diffusers. A collimated red LED was used as a light source. The MLA was placed 500 m in front of the LED. The radiation patterns were
Fig. 3. Actual profile of the fabricated microlens (dotted curves) and the ideal parabolic profiles (solid curves) with aspect ratios of (a) 1.0, (b) 1.2, (c) 1.7, and (d) 2.1.
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measured by a goniometer; the photodetector was placed 30 cm from the MLA diffuser. Figures 4(b) and 4(c) show the radiation pattern of the MLA diffuser with an aspect ratio of 1.0 in the polar and Cartesian coordinates, respectively. The MLA diffuser had a very large full width at half-maximum radiation angle of 150°, as well as a batwing radiation pattern, which might be effective for color mixing and reducing the number of LEDs in RGB LED backlight systems. As the aspect ratio of the microlens increased, the intensity peak at the center of the radiation patterns was also enhanced due to the total internal reflection. Figure 4(d) provides an almost semi-Lambertian radiation pattern of the MLA diffuser with an aspect ratio of 1.2. The profile of the microlens can be controlled according to its applications and required radiation patterns. To summarize, a fabrication method for a high fillfactor MLA diffuser was proposed. By adjusting the UV flood exposure dose during the 3-D diffuser lithography, the paraboloidal shape of the microlenses could be controlled. A UV-curable polymer MLA diffuser was replicated from the nickel master mold and showed a batwing radiation pattern with a radiation angle of 150°. This MLA diffuser may be useful for RGB LED backlight systems and other applications that require various radiation characteristics in MLA diffusers. This work was supported by a Korea Research Foundation Grant (KRF-2004-041-D00278) and the Brain Korea 21 Project, School of Information Technology, Korea Advanced Institute of Science and Technol during 2006. S.-I. Chang’s e-mail address is
[email protected]. References
Fig. 4. (Color online) (a) Schematic view of the measurement setup for the radiation patterns of the MLA diffusers. The radiation pattern of the MLA diffuser with an aspect ratio of 1.0 is shown in (b) polar and (c) Cartesian coordinates. The radiation pattern of the red LED without the MLA diffuser is also shown in (b) and (c) by dashed–dotted curves. (d) Semi-Lambertian radiation pattern of the MLA diffuser with an aspect ratio of 1.2.
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