Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2004-03-11
2004-11-30
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S093000
Reexamination Certificate
active
06825892
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device to be used for television and other display apparatuses, to a method of fabricating the same and, more particularly, to a liquid crystal display device that uses a liquid crystal material containing a photosensitive material and a method of fabricating the same.
2. Description of the Related Art
A liquid crystal display device is a display device that comprises a liquid crystal sealed between two opposing substrates and that uses electrical stimulus for optical switching by exploiting the electro-optical anisotropy of a liquid crystal. Utilizing the refractive index anisotropy that the liquid crystal possesses, the brightness of the light transmitted by the liquid crystal panel is controlled by applying a voltage to the liquid crystal and thereby reorienting the axis of the refractive index anisotropy.
In such a liquid crystal display device, it is extremely important to control the alignment of liquid crystal molecules when no voltage is applied to the liquid crystal. If the initial alignment is not stable, when a voltage is applied to the liquid crystal, the liquid crystal molecules do not align in a predictable manner, resulting in an inability to control the refractive index. Various techniques have been developed to control the alignment of liquid crystal molecules, representative examples including a technique that controls the initially formed angle (pretilt angle) between the alignment film and the liquid crystal and a technique that controls the horizontal electric field formed between the bus line and the pixel electrode.
The same can be said of a display device that uses a liquid crystal material containing a photosensitive material; specifically, in a liquid crystal display mode in which the initial alignment is controlled by radiation of light in the presence of an applied voltage, the voltage application method during the radiation becomes important. The reason is that, if the magnitude of the applied voltage differs, a change will occur in the initially formed pretilt angle, resulting in a change in transmittance characteristics.
In connection with a first aspect of the invention, techniques called passive matrix driving and active matrix driving have usually been used to drive liquid crystals; nowadays, with an increasing demand for higher resolution, the active matrix display mode that uses thin-film transistors (TFTS) is the dominant liquid crystal display mode. In a liquid crystal display having such TFTs, when radiating light onto the liquid crystal while applying a voltage to it, it is usually practiced to expose the liquid crystal to light radiation while applying a TFT ON voltage to each gate bus line and a desired voltage to each data bus line, as shown in
FIGS. 1 and 2
.
However, when such a liquid crystal exposure method is employed, if there is a line defect due to a bus line break or short, as shown in
FIG. 3
, the liquid crystal will be exposed to light when the liquid crystal in the affected area cannot be driven, and a pretilt angle different from that in other areas will be formed in this defect area, resulting in the problem that the brightness in this area differs from the brightness in other areas.
Or, in the TFT channel ON state, a shift in the TFT threshold value can occur due to exposure to ultraviolet radiation, as shown in
FIG. 4
, resulting in the problem that the region where the TFTS can be driven stably shifts from the desired region.
On the other hand, in connection with a second aspect of the invention, displays using the TN mode have been the predominant type of active matrix liquid crystal display, but this type of display has had the shortcoming that the viewing angle is narrow. Nowadays, a technique called the MVA mode or a technique called the IPS mode is employed to achieve a wide viewing angle liquid crystal panel.
In the IPS mode, liquid crystal molecules are switched in the horizontal plane by using comb-shaped electrodes, but a strong backlight is required because the comb-shaped electrodes significantly reduce the numerical aperture. In the MVA mode, liquid crystal molecules are aligned vertically to the substrates, and the alignment of the liquid crystal molecules is controlled by the use of protrusions or slits formed in a transparent electrode (for example, an ITO electrode). The decrease in the effective numerical aperture due to the protrusions or slits used in MVA is not so large as that caused by the comb-electrodes in IPS, but compared with TN mode displays, the light transmittance of the liquid crystal panel is low, and it has not been possible to employ MVA for notebook computers that require low power consumption.
When fine slits are formed in the ITO electrode, the liquid crystal molecules tilt parallel to the fine slits, but in two different directions. If the fine slits are sufficiently long, liquid crystal molecules located farther from a structure such as a bank that defines the direction in which the liquid crystal molecules tilt are caused to tilt randomly in two directions upon application of a voltage. However, the liquid crystal molecules located at the boundary between the liquid crystal molecules caused to tilt in different directions, cannot tilt in either direction, resulting in the formation of a dark area such as that shown in FIG.
29
. Further, in a structure where the liquid crystal molecules are caused to tilt in two different directions in order to improve viewing angle, if there are liquid crystal molecules that are caused to tilt in the opposite direction, as shown in
FIG. 29
, the viewing angle characteristics degrade.
In connection with a third aspect of the invention, in an LCD (MVA-LCD) in which an N-type liquid crystal is aligned vertically and in which, upon application of a voltage, the molecules of the liquid crystal are caused to tilt in a number of predefined directions by using alignment protrusions or electrode slits, the liquid crystal molecules are almost completely vertically aligned in the absence of an applied voltage, but are caused to tilt in the various predefined directions when a voltage is applied. The tilt directions of the liquid crystal molecules are controlled so that they always make an angle of 45° to the polarizer absorption axis, but the liquid crystal molecules as a continuum can tilt in a direction intermediate between them. Furthermore, areas where the tilt direction of the liquid crystal molecules is displaced from the predefined direction inevitably exist because of the effects of the horizontal electric field, etc. at the time of driving or irregularities in the structure. In normally black displays where the polarizers are arranged in a crossed Nicol configuration, this means that dark areas appear when the display is driven in the white display state, and the screen brightness thus decreases. To address this problem, in a liquid crystal display device constructed by sandwiching between two substrates a liquid crystal composition containing a photopolymerizable or thermally polymerizable component, there is employed a technique that polymerizes the polymerizable component while applying a voltage, thereby defining the direction in which the liquid crystal molecules tilt in the presence of an applied voltage.
With this technique, however, if the polymerization is insufficient, image sticking can occur. This is believed to occur because the rigidity of the polymerized polymer is insufficient and deformation occurs due to the realignment of the liquid crystal molecules at the time of voltage application. On the other hand, to sufficiently polymerize the polymer, the duration of light radiation must be increased, but in that case, takt time at the time of volume production becomes a problem.
In connection with a fourth aspect of the invention, conventional liquid crystal display devices predominantly use the TN mode in which horizontally aligned liquid crystal molecules are twisted between the top and bottom substrates, but gray-sca
Inoue Hiroyasu
Nagaoka Kenichi
Nakahata Yuji
Taniguchi Yoji
Fujitsu Display Technologies Corporation
Greer Burns & Crain Ltd.
Ton Toan
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