APPLIED PHYSICS LETTERS
`
`VOLUME 73, NUMBER 20
`
`16 NOVEMBER 1998
`
`Electro-optic characteristics and switching principle of a nematic
`liquid crystal cell controlled by fringe-field switching
`S. H. Lee,a) S. L. Lee, and H. Y. Kim
`TFT Process Development Department, LCD Division, Hyundai Electronics Ind., San 136-1, Ami-ri,
`Bubal-eup, Ichon-si, Kyungki-do, 467-701, Korea
`共Received 7 July 1998; accepted for publication 22 September 1998兲
`
`We have fabricated a nematic liquid crystal cell associated with a homogeneously aligned to twisted
`transition of a liquid crystal director. In the absence of an electric field, the liquid crystal molecule
`is homogeneously aligned under the crossed polarizers, and thus the cell appears to be black. When
`a fringe field induced by interdigital electrodes is applied, liquid crystal molecules rotate in plane
`even above electrodes and thus the cell transmits light. The device exhibits a high transmittance
`ratio as well as a wide viewing angle, which solves a long standing problem of low transmittance
`existing in the conventional in-plane switching mode. We show that the distance between electrodes
`smaller than the width of an electrode and cell gap is required for generating fringe field with
`applied voltage and rotating molecules above electrodes. We also investigate the mechanism of
`fringe-field switching and dependence of electro-optic effect on different cell conditions and
`dielectric anisotropy of liquid crystal. © 1998 American Institute of Physics.
`关S0003-6951共98兲04346-0兴
`
`Flat panel displays are extensively being studied for an
`application to personal portable computers and the replace-
`ment of Cathode-ray-tube 共CRT兲 displays at present. Among
`twisted nematic liquid crystal displays 共TNLCDs兲
`them,
`have been mainly used for notebook computers in spite of
`their narrow viewing angle characteristics. Since LCDs uti-
`lize liquid crystals, organic chemicals with anisotropic struc-
`ture, their electro-optic characteristics intrinsically have a
`viewing angle dependency. Recently, new concepts of LCDs
`to minimize the viewing angle dependency such as in-plane
`switching 共IPS兲 mode,1–4 the vertical alignment 共VA兲 with
`negative5 and positive dielectric anisotropy of LC6,7 and ho-
`meotropic to multi-domainlike 共HMD兲 VA8,9 have been sug-
`gested. The IPS mode shows wide viewing angle character-
`istics comparable to the CRT displays due to in-plane
`rotation of liquid crystal molecules between electrodes.
`However, the liquid crystal above the electrodes does not
`twist with an applied field so that the aperture ratio, i.e., the
`area where the light can be transmitted, is low compared
`with that of the twist nematic mode. The VA modes also
`have demerit in transmittance with similar reasons. Such low
`transmittance of the liquid crystal cell needs backlight with
`high luminance, resulting in high power consumption.
`In this letter, we suggest a nematic liquid crystal cell
`associated with a homogeneously aligned to twisted transi-
`tion of the liquid crystal director, induced by fringe-field
`switching 共FFS兲. This device shows a very wide viewing
`angle the same as the IPS mode but the liquid crystal mol-
`ecules even above electrodes can rotate, giving rise to high
`transmission of the incident light.
`Figure 1 shows the cell structure with equipotential lines
`generated by interdigital electrodes when the distance 共l兲 be-
`tween electrodes is smaller and larger than the cell gap 共d兲.
`In the conventional IPS mode, the horizontal component of
`
`a兲Electronic mail: lsh1@hei.co.kr
`
`an electric field is dominant in the range between electrodes
`for l⬎d, as shown in Fig. 1. However, in the FFS mode
`where l⬍d, the vertical as well as horizontal components of
`an electric field exist above electrodes. In the IPS and FFS
`cells, the liquid crystal is homogeneously aligned throughout
`the cell gap by antiparallel rubbing of polyimides. The po-
`larizer was oriented along the rubbing direction at the bottom
`plate and the analyzer perpendicular to the polarizer. In the
`IPS cell, the normalized transmission of light is
`
`T/To⫽sin 2共2␺兲sin2共␲d⌬n/␭ 兲 ,
`
`where ␺is an angle between the crossed polarizers and the
`liquid crystal director, ⌬n is the birefringence of liquid crys-
`tal medium, and ␭ is the wavelength of the incident light.
`Therefore, in the voltage-off state, the ␺is zero and the cell
`appears to be black. With bias voltage larger than Freeder-
`
`FIG. 1. 共a兲 Schematic drawing of cell structures with equipotential lines
`generated by interdigital electrodes for IPS and FFS cells and 共b兲 top view
`of interdigital electrodes with rubbing direction.
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`0003-6951/98/73(20)/2881/3/$15.00
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`2881
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`© 1998 American Institute of Physics
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`Appl. Phys. Lett., Vol. 73, No. 20, 16 November 1998
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`Lee, Lee, and Kim
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`icksz transition threshold, Vth, the LC molecules are twisted
`along 共perpendicular to兲 the field direction when a liquid
`crystal with positive 共negative兲 dielectric anisotropy is
`present in the cell and thus the value of ␺starts to increase,
`giving rise to transmission of the incident light. In the IPS
`cell, the electric field lines parallel to the substrate in the area
`between electrodes mainly exist so that only twist deforma-
`tion of liquid crystal director in between them occurs, giving
`rise to transmission of the incident light. However, the liquid
`crystal molecules above electrodes do not experience twist
`deformation due to equipotential surface of electrodes them-
`selves. Consequently the light can be transmitted only in the
`area between electrodes, resulting in low transmittance com-
`pared with that of TN mode. In the FFS cell, l is smaller than
`d and w so that the electric field parallel to the substrate
`cannot be formed but instead the electric field lines of para-
`boliclike form are formed in the whole area. In other words,
`such field lines having vertical components as well as hori-
`zontal ones exist near bottom substrates throughout the cell
`and the dielectric torque exists on the liquid crystal medium
`in the whole area, resulting in light transmission.
`We have constructed several cells to investigate the
`electro-optic characteristics of the IPS and FFS cells, varying
`the cell gap, the distance between electrodes, and the liquid
`crystals.
`For the IPS cell, the indium tin oxide 共ITO兲 with 400 Å
`was deposited on the bottom glass substrate, and the inter-
`digital electrodes with a distance of 10 ␮m between elec-
`trodes and an electrode width of 6 ␮m were patterned
`through the photolithographic process. For the FFS cells, the
`two ITO layers with passivation layer, SiO2 共1500 Å兲, be-
`tween them exists as a counter and a pixel electrode. The top
`ITO layer with an electrode width of 3 ␮m was patterned and
`the bottom ITO was patterned allowing the distance between
`the top and bottom electrodes to be varied as 0, 1, and 4 ␮m.
`The top glass substrate has no electrode on it for all cells.
`The alignment layer from the Japan Synthetic Rubber Co.
`共AL-1051兲 was coated on both substrates and the rubbing
`was done in antiparallel directions. The rubbing directions
`共␣兲 were 12° and 78° with respect to the electrode directions,
`which were in optimal range for maximum transmission. For
`the cells with rubbing directions of 12° and 78°, the liquid
`crystal with positive and negative dielectric anisotropy was
`filled, respectively. The pretilt angle generated by the rub-
`bing is 2°. Two glass substrates were then assembled to give
`a cell gap of 4.0 ␮m. The liquid crystals with positive dielec-
`tric anisotropy (⌬n⫽0.074 at ␭⫽589 nm, ⌬⑀⫽8.0) and
`negative dielectric anisotropy (⌬n⫽0.074 at ␭⫽589 nm,
`⌬⑀⫽⫺3.8) from Merck Co. were used and filled at room
`temperatures.
`For electro-optic measurement, the Halogen lamp was
`used as a light source and a square wave, 60 Hz voltage
`source from the function generator was applied to the sample
`cell. The light passed through the cell was detected to the
`photomultiplier tube.
`Figure 2 shows the transmitted light intensity as a func-
`tion of the applied voltage for the IPS cell with positive LC,
`and several FFS cells with negative LC. For the cells with l
`of 0 and 1 ␮m, the light transmission starts to occur at the
`applied voltage of 1.3 V, an the transmission becomes almost
`
`FIG. 2. Voltage-dependent transmission curves for the IPS and several FFS
`cells when negative LCs are used.
`
`saturated at 7 V. Furthermore, the maximum-transmitted in-
`tensity is higher than that of the cell with l of 4 ␮m though
`the transmission-saturation voltage is the same. This indi-
`cates that when l is smaller than d with a thin electrode
`width, the fringe field exists and, consequently, the interac-
`tion between the liquid crystals and the horizontal compo-
`nent of fringe field causes twist deformation of the liquid
`crystal director, giving rise to light transmittance in whole
`area. However, when l is larger than the electrode width, the
`liquid crystal molecules above the electrodes experience less
`twist deformation than those existing in the area between the
`electrodes, due to the weaker potential difference above the
`electrodes, resulting in less light transmittance. When l be-
`comes larger than the cell gap for w of 6 ␮m, i.e., the value
`of l/d ratio greater than 1, which is the condition for the
`conventional IPS mode, the intensity of the transmitted light
`rapidly decreases. This indicates that the width of electrodes
`less than 5 ␮m and the distance between electrodes less than
`2 ␮m give rise to effective results in the viewpoint of trans-
`mittance. Figure 3 shows the voltage-dependent transmission
`curve when different types of liquid crystals are used. When
`the distance between electrodes is less than 1 ␮m, the trans-
`mittance depends on the type of dielectric anisotropy of liq-
`uid crystals. The negative type of liquid crystal gives rise to
`better
`transmission than the positive type though the
`transmission-saturation voltage for the positive liquid crystal
`is about 2.5 V lower than that of the negative one. When a
`negative liquid crystal is used, the interaction between the
`director and the horizontal component of the fringe field
`causes twist deformation, resulting in light
`transmission.
`However, the angle between the director and the vertical
`component of the fringe field is 90°, and therefore the prob-
`ability of interaction is zero. That is, mainly twist deforma-
`tion does occur in all regions where the angle between the
`
`FIG. 3. Voltage-dependent transmission curve depending on different types
`of liquid crystals.
`
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`Appl. Phys. Lett., Vol. 73, No. 20, 16 November 1998
`
`Lee, Lee, and Kim
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`2883
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`as the conventional IPS cell. This result indirectly proves
`that the director mainly does twist deformation with the ap-
`plied field.
`In summary, the nematic LCD whose on and off states
`are controlled by fringe-field switching has been developed.
`In this device, the liquid crystal molecules do twist in the
`whole area with applied voltage when the distance between
`electrodes is smaller than the cell gap and electrode width.
`The device exhibits high transmission ratio and wide view-
`ing angle characteristics, and overcomes the limited applica-
`tion of the conventional IPS cell. We expect that this concept
`of device is applicable to wide viewing angle active-matrix
`LCDs for both monitor and notebook uses.
`
`The authors are deeply grateful to Director Y. K. Mun,
`Senior Researchers J. Y. Lee and W. G. Lee, our hard work-
`ing engineers for their full support of the project, and Pro-
`fessor Young Hee Lee for his valuable comments on this
`letter. The authors also thank Merck-Korea for their kind
`support of materials.
`
`1 R. A. Soref, J. Appl. Phys. 45, 5466 共1974兲.
`2 R. Kiefer, B. Weber, F. Windscheid, and G. Baur, Proceedings of the 12th
`International Display Research Conference 共Society for Information Dis-
`play and the Institute of Television Engineers of Japan, Hiroshima, 1992兲,
`p. 547.
`3 M. Oh-e, M. Ohta, S. Aratani, and K. Kondo, Proceedings of the 15th
`International Display Research Conference 共Society for Information Dis-
`play and the Institute of Television Engineers of Japan, Hamamatsu,
`1995兲, p. 577.
`4 K. Kondo, S. Matsuyama, N. Konishi, and H. Kawakami, Digest of Tech-
`nical Papers of 1998 Society for Information Display International Sym-
`posium 共Society for Information Display, Anaheim, 1998兲, p. 389.
`5 A. Takeda, S. Kataoka, T. Sasaki, H. Chida, H. Tsuda, K. Ohmuro, Y.
`Koike, T. Sasabayashi, and K. Okamoto, in Ref. 4, p. 1077.
`6 S. H. Lee, H. Y. Kim, I. C. Park, Y. H. Lee, B. G. Rho, J. S. Park, and H.
`S. Park, The 1st Korean Symposium on Information Display 共Korea Soci-
`ety for Information Display, Seoul, 1997兲, p. 23.
`7 S. H. Lee, H. Y. Kim, I. C. Park, B. G. Rho, J. S. Park, H. S. Park, and C.
`H. Lee, Appl. Phys. Lett. 71, 2851 共1997兲.
`8 S. H. Lee, H. Y. Kim, S. C. Kim, Y. H. Lee, I. C. Park, B. G. Rho, J. S.
`Park, C. H. Lee, H. G. Galabova, and D. W. Allender, in Ref. 4, p. 838.
`9 S. H. Lee, H. Y. Kim, Y. H. Lee, I. C. Park, B. G. Rho, H. G. Galabova,
`and D. W. Allender, Appl. Phys. Lett. 73, 470 共1997兲.
`
`FIG. 4. Iso-contrast plot of the cell with 0 ␮m oflfor a negative liquid
`crystal.
`
`LC molecules and the field takes a value intermediate be-
`tween 0° and 90°. When a positive liquid crystal is used, the
`director has a tendency to align along the field so that it does
`tilt up as well as twist by the fringe field. Therefore the
`transmittance is less than that of cells with negative LC.
`However, when the value of the l/d ratio greater than 1, the
`in-plane field mainly exists between electrodes so that the
`director does mainly twist deformation irrespective of the
`dielectric anisotropy of liquid crystal, giving rise to about the
`same transmission. The transmission-saturation voltage de-
`pends on the absolute value of dielectric anisotropy, i.e., in-
`versely proportional to root of ⌬⑀.3 This explains why it is
`lower for the positive type than for the negative one.
`In order to verify that the director does twist deformation
`by fringe-field switching, we measured viewing angle char-
`acteristics. Figure 4 shows the iso-contrast plot with the ap-
`plied voltage of 0 and 7 V for the off and on states, respec-
`tively, when 1 is 0 ␮m with the negative liquid crystal. The
`region where the contrast ratio greater than 10 exists over
`70° of polar angle in all directions is exactly the same result
`
`Page 3 of 3
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