The present invention relates to a liquid crystal display (LCD), or more particularly, to a technology for realizing orientation division for a vertically-aligned (VA) LCD.
Among flat-panel displays enjoying image quality equivalent of the one offered by the CRT, it is a liquid crystal display (LCD) that has been most widely adopted nowadays. In particular, a thin-film transistor (TFT) type LCD (TFT LCD) has been adapted to public welfare-related equipment such as a personal computer, word processor, and OA equipment, and home electric appliances including a portable television set, and expected to further expand its market. Accordingly, there is a demand for further improvement of image quality. A description will be made by taking the TFT LCD for instance. However, the present invention is not limited to the TFT LCD but can apply to a simple matrix LCD, a plasma addressing type LCD and so forth. Generally, the present invention is applicable to LCDs which include liquid crystal sandwiched between a pair of substrates on which electrodes are respectively formed and carry out displays by applying voltage between the electrodes.
Currently, a mode most widely adopted for the TFT LCD is a normally-white mode that is implemented in a twisted nematic (TN) LCD. The technology of manufacturing the TN TFT LCD has outstandingly advanced in recent years. Contrast and color reproducibility provided by the TN TFT LCD have surpassed those offered by the CRT. However, the TN LCD has a critical drawback of a narrow viewing angle range. This poses a problem that the application of the TN LCD is limited.
In an effort to solve these problems, Japanese Examined Patent Publication Nos. 53-48452 and 1-120528 have proposed an LCD adopting a mode referred to as an IPS mode.
However, the IPS mode suffers from slow switching. At present, when a motion picture representing a fast motion is displayed, drawbacks including a drawback that an image streams take place. In an actual panel, therefore, for improving the response speed, the alignment film is not rubbed parallel to the electrodes but rubbed in a direction shifted by about 15°. However, even when the direction of rubbing is thus shifted, since the response time permitted by the IPS mode is twice longer than the one permitted by the TN mode, the response speed is very low. Moreover, when rubbing is carried out in the direction shifted by about 15°, a viewing angle characteristic of a panel does not become uniform between the right and left sides of the panel. Gray-scale reversal occurs relative to a specified viewing angle.
As mentioned above, the IPS mode that has been proposed as an alternative for solving the problem on the viewing angle characteristic of the TN mode has the problem that the characteristics offered by the IPS mode other than the viewing angle characteristic are insufficient. A vertically-aligned (VA) mode using a vertical alignment film has been proposed. The VA mode does not use a rotary polarization effect which is used in the TN mode, but uses a birefringent (double refraction) effect. The VA mode is a mode using a negative liquid crystal material and vertical alignment film. When no voltage is applied, liquid crystalline molecules are aligned in a vertical direction and black display appears. When a predetermined voltage is applied, the liquid crystalline molecules are aligned in a horizontal direction and white display appears. A contrast in display offered by the VA mode is higher than that offered by the TN mode. A response speed is also higher, and an excellent viewing angle characteristic is provided for white display and black display. The VA mode is therefore attracting attention as a novel mode for a liquid crystal display.
However, the VA mode has the same problem as the TN mode concerning halftone display, that is, a problem that the light intensity of display varies depending on the viewing angle. The VA mode provides a much higher contrast than the TN mode and is superior to the TN mode in terms of a viewing angle characteristic concerning a viewing angle or a viewing angle characteristic, because even when no voltage is applied, liquid crystalline molecules near an alignment film are aligned nearly vertically. However, the VA mode is inferior to the IPS mode in terms of the viewing angle characteristic.
It is known that viewing angle performance of a liquid crystal display device (LCD) in the TN mode can be improved by setting the orientation directions of the liquid crystalline molecules inside pixels to a plurality of mutually different directions. Generally, the orientation direction of the liquid crystalline molecules (pre-tilt angles) which keep contact with a substrate surface in the TN mode are restricted by the direction of a rubbing treatment applied to the alignment film. The rubbing treatment is a processing which rubs the surface of the alignment film in one direction by a cloth such as rayon, and the liquid crystalline molecules are orientated in the rubbing direction. Therefore, viewing angle performance can be improved by making the rubbing direction different inside the pixels.
Though the rubbing treatment has gained a wide application, it is the treatment that rubbs and consequently, damages, the surface of the alignment film and involves the problem that dust is likely to occur.
A method which forms a concavo-convex pattern on an electrode is known as another method of restricting the pre-tilt angle of the liquid crystalline molecules in the TN mode. The liquid crystalline molecules in the proximity of the electrodes are orientated along the surface having the concavo-convex pattern.
It is known that viewing angle performance of a liquid crystal display device in the VA mode can be improved by setting the orientation directions of the liquid crystalline molecules inside pixels to a plurality of mutually different directions. Japanese Unexamined Patent Publication (Kokai) No. 6-301036 discloses a LCD in which apertures are provided on a counter electrode. Each aperture faces a center of a pixel electrode and oblique electric fields are generated at a center of each pixel. The orientation directions of the liquid crystalline molecules inside each pixel are divided into two or four directions due to the oblique electric fields. However, the LCD disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-301036 has a problem that its response (switching) speed is not enough, particularly, a response speed for transition from a state in which no voltage is applied to a state in which a voltage is applied is slow. A cause of his problem is presumed that no oblique electric field exists when no voltage is applied between the electrodes. Further, because a length of each area having continuously oriented liquid crystalline molecules in each pixel is a half of a pixel size, a time for all liquid crystalline molecules in each area to be oriented in one direction becomes long.
Further, Japanese Unexamined Patent Publication (Kokai) No. 7-199193 discloses a VA LCD in which slopes having different directions are provided on electrodes and the orientation directions of the liquid crystalline molecules inside each pixel are divided. However, according to the disclosed constitutions, the vertical alignment film formed on the slopes are rubbed, therefore, the VA LCD disclosed in Japanese Unexamined Patent Publication (Kokai) No-7-199193 also has the above-mentioned problem that dust is likely to occur. Further, according to the disclosed constitutions, the size of the slopes is a half of the pixel, therefore, all liquid crystalline molecules faces the slopes are inclined, a good black display cannot be obtained. This causes a reduction of contrast. Further, inclination angles of the slopes are small because two or four slopes are provided across each pixel. It is found that the gentle slopes cannot fully define the orientation directions of the liquid crystalline molecules. In order to realize steep slopes, it is necessary to increase a thickness of a structure having slopes. However, when the thickness of the structure becomes large, charges accumulated on the structure becomes large. This causes a phenomenon that the liquid crystalline molecules do not change their orientations when a voltage is applied due to the accumulated charges. This phenomenon is so-called a burn.
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