Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-02-25
2003-10-07
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S129000, C349S130000
Reexamination Certificate
active
06630975
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device having a wide viewing angle characteristic suitable for use in a liquid crystal display apparatus having a flat display such as a personal digital assistant, a personal computer, a word processor, an amusement apparatus, an educational apparatus, a television set, or the like, which may be viewed by a large number of people at the same time, and in a display board, a window, a door, a wall, or the like, utilizing a shutter effect.
2. Description of the Related Art
Various types of liquid crystal display devices have been used in the art, including TN (Twisted Nematic) and STN (Super Twisted Nematic) type liquid crystal display devices in which a voltage is applied through the liquid crystal layer to untwist the twisted liquid crystal molecules so as to obtain brightness/darkness, and other types of liquid crystal display devices in which a voltage is applied to change the orientation of the liquid crystal molecules from the initial orientation to another orientation so as to impart a change in birefringence to incident light, thereby obtaining brightness/darkness or color display.
However, these conventional liquid crystal display devices typically have a narrow viewing angle. Therefore, various techniques have been developed in order to increase the viewing angle.
Methods for increasing the viewing angle of a liquid crystal display device include a method in which the liquid crystal molecules are moved in a direction substantially parallel to the surface of the substrate, and a method in which each pixel is divided into a plurality of regions having different orientations with the movement of the liquid crystal molecules being perpendicular to the surface of the substrate. A typical example of the former type of method is an IPS (In-Plane-Switching) mode. As examples of the latter type of method, various liquid crystal display modes have been proposed in the art, including a wide viewing angle liquid crystal display mode in which Np (Nematic Positive) type liquid crystal molecules are horizontally oriented in axial symmetry (Japanese Laid-Open Publication No. 7-120728), another wide viewing angle liquid crystal display mode in which vertically aligned Nn (Nematic Negative) type liquid crystal molecules are horizontally oriented in axial symmetry upon application of a voltage (Japanese Laid-Open Publication No. 10-186330), another wide viewing angle liquid crystal display mode in which vertically aligned Nn (Nematic Negative) type liquid crystal molecules are oriented in a time division manner by controlling an electric field applied therethrough (Japanese Laid-Open Publication No. 7-64089), and another wide viewing angle liquid crystal display mode in which Np (Nematic Positive) type liquid crystal molecules are horizontally oriented while generally dividing each pixel into four regions (AM-LCD '96, P. 185 (1996)).
Among others listed above, Japanese Laid-Open Publication No. 7-120728 discloses a wide viewing angle liquid crystal display mode called “axially symmetric aligned microcell mode (or Np-ASM mode)”, where Np type liquid crystal molecules are oriented in axial symmetry in each pixel. In a display device of this mode, each pixel is divided into a plurality of liquid crystal regions each of which is substantially surrounded by polymer walls formed by phase separation from a mixture of a liquid crystal material and a photocurable resin. The liquid crystal molecules in each pixel are oriented in axial symmetry. According to this technique, a normally white display is produced by applying a voltage through liquid crystal molecules in an axially symmetric orientation so as to orient the liquid crystal molecules perpendicular to the substrate. The technique disclosed in Japanese Laid-Open Publication No. 10-186330 employs an Nn-ASM display mode with an Nn type liquid crystal material, and produces a normally black display. The liquid crystal molecules are oriented perpendicular to the substrate in the absence of an applied voltage, and when a saturation voltage is applied, the liquid crystal molecules are oriented in the respective liquid crystal regions in a pixel each of which is substantially surrounded by polymer walls so that the liquid crystal molecules are oriented in axial symmetry for each pixel. Japanese Laid-Open Publication No. 7-311383 discloses a technique in which the liquid crystal orientation is divided into four directions by providing an orientation controlling slope using an orientation controlling layer on each substrate.
Japanese Laid-Open Publication No. 7-64089 discloses a technique in which an orientation controlling electrode is provided between pixels for applying a driving voltage which is higher or lower than that of all the other transparent electrodes. An orientation controlling window is provided in the transparent electrode in the form of a gap in the electrode. This structure allows for adjustment of the electric field through the liquid crystal layer, thereby controlling the orientation of the liquid crystal molecules. The gaps in the transparent electrode are patterned in a “x” shape on one of a pair of substrates. On the other substrate, the orientation controlling electrodes are provided in a lattice pattern. By superimposing these patterns on each other, the electric field applied through the liquid crystal layer is bent, thereby realizing a “4-division ECB” where the liquid crystal orientation is divided into four directions. However, when these patterns are used, a disclination line occurs when the voltage is turned ON/OFF, thereby affecting the voltage response characteristics and thus deteriorating the display quality. Moreover, in this conventional technique, the liquid crystal material does not contain a chiral agent, and the color shift has a substantial cell thickness dependency in a 4-division mode, thereby requiring a high precision in controlling the cell thickness. Therefore, it is very difficult to obtain a uniform display quality for a large area display device.
In the liquid crystal display device of the Np-ASM mode, which employs a normally white mode, the light-blocking portion of the BM (black matrix) needs to have a large area in order to properly prevent light leakage when the voltage is OFF. Moreover, the production of an ASM mode display device has been relatively difficult because it involves a phase separation process, which requires a precise temperature control. Furthermore, both the Np-ASM mode and the Nn-ASM mode commonly suffer from the following problems:
{circle around (1)} The production cost is high due to the use of a photocurable resin.
{circle around (2)} The number of production steps is increased and the production process is complicated, thereby increasing the production cost because of the step of irradiating the liquid crystal molecules with UV light with the liquid crystal molecules being orientated in axial symmetry by an application of a voltage so as to cure the photocurable resin, thereby providing an orientation stabilizing layer, and the step of patterning the polymer walls by photolithography. Moreover, according to this method, dust is likely to attach to the substrate, thereby increasing the defect rate due to a defective liquid crystal orientation.
{circle around (3)} When fixing the orientation of the liquid crystal molecules, the UV light used to irradiate the liquid crystal molecules decomposes the liquid crystal material, the polymer wall material and the orientation film material, thereby lowering the voltage retention and thus lowering the reliability of the display (e.g., an image burn phenomenon may occur).
When the orientation stabilizing layer is not provided, the tilt direction of the liquid crystal molecules will not be well stabilized. Then, the response speed of the display device is reduced, whereby a stable ASM orientation may not be obtained upon driving the device, resulting in non-uniformity in displayed images.
SUMMARY OF THE INVENTION
Accord
Chung David
Kim Robert H.
Nixon & Vanderhye PC
Sharp Kabushiki Kaisha
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