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					        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 [1]. 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 [13] 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 [10]. 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 [7].
   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.
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