Transflective LCD Using Multi-Functional PDMS Substrate Made by Replica Molding Hi-Jung Kim, Yeun-Tae Kim, Jong-Ho Hong, and Sin-Doo Lee* School of Electrical Engineering #032, Seoul National University, Kwanak P.O. Box 34, Seoul 151-600, Korea We reported on a transflective LCD using PDMS substrate that constitutes the dual cell gap structure and spacer. Since PDMS substrate can be fabricated easily by the replica molding technique and is thinner and lighter than the glass substrate, our transflective LCD shows higher productivity and portability than conventional devices. Moreover electro- optic characteristics of transmissive and reflective regions in our transflective LCD are well-matched due to relevant design configurations found by a computer simulation. This work is expected to contribute to the simplification of the fabrication process in mobile LCD applications. 1. Introduction electrode angle, and adding in-cell retarder [6-9]. This type also has a weak point because of Recently, as entering the era of mobile complexity of additional process. And the key issue communication, demands for displays that have of fabricating transflective LCD is matching the high portability and superior performance at both electro-optic (EO) characteristics of transmissive indoor and outdoor are rapidly increasing. Liquid and reflective regions each other. But existing crystal displays (LCD) have played an important methods for matching are so difficult and complex. role in the display industry during the past days In this study, we proposed novel dual cell gap more than 10 years. The mainstream of LCD is transflective LCD that can be produced easily by a transmissive LCD that has backlight as light source simple method. The upper substrate made of PDMS behind the LCD panel. This conventional was produced by replica molding technique in transmissive LCD has superior performance at order to reduce complexities of the process. And indoor environment in which there is weak light, matching the optical performance between but a transflective LCD consists of two subpixels transmission and reflection regions was with the transmissive and reflective regions, so it accomplished easily by adjusting electrode distance shows good readability under both strong and weak of two regions. Moreover we also achieved the low lighting conditions . Moreover, transflective weight and thickness of the device by using PDMS LCD has some superior characteristics such as high substrate instead of glass substrate. These results of portability and low power consumption. our study will be able to contribute to the future Transflective LCDs are divided into two types, works of mobile display industry. single cell gap type and dual cell gap type due to compensation method of light path difference. Dual 2. Simulation and Experiment cell gap type uses a method that makes different cell gap between transmissive region and reflective Figure 1(a) shows the schematic diagram of our region [2-3]. This type shows good optical transflective LCD. The LC cell is separated performance but has a weak point that it is difficult transmissive and reflective regions among optical to fabricate dual structure [4, 5]. Single cell gap components such as crossed polarizers and λ/4 type uses a method that light path difference is plates. Liquid crystals in the LC cell are vertically compensated by delivering additional process to aligned (VA) under no applied voltage, so no reflective region such as hybrid alignment, different retardation arises in both regions and dark state is observed. In contrast, when enough voltage is Before fabricating actual device, we performed applied transmissive and reflective regions show simulation to find the most appropriate design retardation of λ/2 and λ/4, respectively. So the LC specification that the EO characteristics of two cell turns to bright state. regions are well-matched. Simulation program Figure 1(b) shows the 3-D diagram of the LC cell. made using C++ calculates LC director and voltage The upper substrate made of PDMS is constitutes profile in the LC cell by relaxation method  and the dual structure of transmissive and reflective is available to set parameters such as cell gap, regions and the spacer that maintains the cell gap electrode width, distance between electrodes, and uniformly. This simple structure of PDMS substrate LC properties like dielectric constant or refractive can be fabricated so easily with a mold made by index. LC properties in this simulation followed replica molding technique. And since PDMS those of ZLI-2293 (Merck). substrate is thinner and lighter than glass substrate, The threshold voltage at which the LC cell turns our device has enhanced portability compared with from dark state to bright state is dependent on the conventional devices. Moreover our device is not distance between electrodes and cell gap. By required to coat vertical alignment layer on the performing simulation, we found that the ratio upper substrate due to self-VA characteristic of between cell gap and distance between electrodes PDMS . The bottom substrate on which in- determinates threshold voltage that stands for EO plane electrode is set and VA layer is coated is characteristics. So we applied the same ratio to two made of glass. When voltage is applied, electric regions. Cell gap and distance between electrodes field between electrodes reorients vertically-aligned of transmissive region are 2.4 μm and 20 μm, liquid crystal molecules along the direction of field respectively, and those of reflective region are 1.2 and retardation arises. (VA-IPS mode [11, 12]) μm and 10 μm, respectively. And Electrode width is 10 μm in both regions. In conclusion, we could match EO characteristics of two regions that are represented by threshold voltage without complex process such as applying dual driving circuits or hybrid alignment. Actual device was made according to design configurations from simulation. At first we fabricated a mold that has dual structure by photo lithography with positive photoresist (AZ1512, Clariant Corp.). And PDMS (Sylgard 184, Dow Corning Corp.) was poured on the mold for replica molding. This PDMS poured on the mold became an upper substrate by being cured at 150℃ during about 15 minutes and separated from the mold. The size of PDMS upper substrate is 3.5cmⅹ3.5cm. A bottom substrate was fabricated by patterning in-plane electrode on the glass substrate having the same area with PDMS upper substrate. Electrode was patterned by lift-off technique with aluminum. And the VA layer (AL60101, JSR) was coated on the whole area of the bottom substrate under 3000 rpm and 30 seconds. After the liquid crystal (ZLI- 2293, Merck) was set on the bottom substrate uniformly, by combining PDMS upper substrate and glass bottom substrate each other the whole device was completed. Figure 1. Concept of our transflective LCD: (a) the schematic diagram of our transflective LCD, (b) the 3-D diagram of 3 Result and Discussion whole LC cell except polarizers and λ/4 plates. Figure 2 shows microscopic textures of LC cell 0V 10V 15V 20V Figure 2. Microscopic textures of LC cell under increasing voltage between 0V and 20V. There are only transmissive Figure 3. The V-T characteristics of LC cell. The EO characteristics without reflective mirror in these figures for characteristics that is represented by threshold voltage of two confirming operation principles two regions at the same time. regions are well-matched each other. Simulation result is also well-matched with experimental result. by a potical microscope under crossed polarizers. The optic axis of the crossed polarizers was placed at angle of 45° with respect to the direction of in- plane electrodes. And for confirming operation principles of two regions at the same time, we observed only transmissive characteristics of two regions by removing the reflective mirror in reflective region. Since the LC molecules are vertically aligned by alignment layer under the no applied voltage, the dark state is obtained in both two regions. As increasing applied voltage, LC molecules between electrodes are reoriented along the direction of electric field. So retardation arises and LC cell turns to bright state. Figure 3 shows voltage - transmittance and reflectance (V-T) characteristics of LC cell. Threshold voltages of transmissive and reflective region are well-matched each other, and these experimental results are also well-matched with simulation result. And Figure 4 shows response time (R-T) characteristics of LC cell. The rising time (10% to 90%) and falling time (90% to 10%) of transmissive region are 7.49 ms and 5.43 ms, respectively. and those of reflective region are 6.67 ms and 3.07 ms, respectively. So it has been confirmed that the response speeds of both regions are fast enough to apply video applications . Since PDMS substrate is thinner and lighter than the glass substrate, our LC cell also shows superior performance in portability that is imperative to Figure 4. The R-T characteristics of the LC cell. The straight mobile applications. We compared the thickness line represents applied voltage, and the spotted line and weight of PDMS substrates that is used in our represents normalized transmittance or reflectance. The experiment with those of glass substrates having operation of the LC cell shows response times of ms order. the same area. Thickness and weight of PDMS substrate are 200 μm and 0.128 g, respectively. References Comparably the thickness and weight of the glass substrates are 700 μm and 0.761 g, respectively.  X. Zhu, Z. Ge, S. -T. Wu, et al., 2005, Our cell in which PDMS substrate is used as an IEEE/OSA Journal of Display Technology, vol. 1, upper substrate has thickness of 900 μm and weight No. 1, 15. of 0.889 g while conventional devices that use only  C. –L. Yang, Jpn. J. Appl. Phys., 2004, Part 1, the glass substrate as an upper and bottom substrate vol. 43, 4273. has a thickness of 1400 μm and weight of 1.522 g.  G. S. Lee, J. C. Kim and T. –H. Yoon, 2006, Compared with the value of the conventional Jpn. J. Appl. Phys., Part 1, vol. 45, 8769. device, thickness and weight of our cell are 64.3%  J. Kim, D. –W. Kim, and S. –D. Lee, et al, 2004, and 58.4%, respectively. Due to enhanced Jpn. J. Appl. Phys., Part 1, vol. 43, L1369. portability controlling factors such as thinness and  Y. J. Lim, J. H. Song and S. H. Lee, 2005, Jpn. lightness, our cell can be applied to emerging mobile applications. J. Appl. Phys., Part 1, vol. 44, L3080.  C. –J. Yu, D. –W. Kim, and S. –D. Lee, 2004, Appl. Phys. Lett., vol. 85, No. 22, 5146. 4. Conclusion  Y. –W. Lim, J. Kim, and S.-D. Lee, 2006, Mol. . Cryst. Liq. Cryst., vol. 458, 45. We proposed a novel transflective LCD device  J. H. Song and S. H. Lee, 2004, Jpn. J. Appl. that can be produced easily due to its simple Phys., Part 1, vol. 43, L1130. structure and has good optical performances. The  Y. –J. Lee, T. –H. Lee, and J. –H. Kim, 2006, upper substrate made of PDMS was produced by Jpn. J. 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