Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-12-14
2004-12-21
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S121000, C349S128000
Reexamination Certificate
active
06833893
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an OCB (Optically Compensated Bend)-type liquid crystal display device, and particularly relates to an OCB-type crystal display device with high serviceability, which inhibits disturbance of the orientation surface due to a circumferential electric field or irregularity of the orientation plane.
2. Background Art
Liquid crystal display devices have rapidly become prevalent because they can easily display a large volume of information, displacing CRT type display devices.
Conventionally, the twisted nematic mode (hereinafter, called TN mode) has been mainly used, in which, the liquid crystal molecules are inserted between the upper and lower plates and are twist oriented by rotation of the molecular axis direction (hereinafter, called a director) by 90 degrees in plan view, and images are displayed by rotating the director in the vertical direction with respect to the substrates by a vertical electric field.
However, this TN mode has the problem that the angle of visibility is narrow. Accordingly, the TN mode has the problem not only in that images are not visible from an angle, but also in that, when the display screen becomes large as a result of progress in manufacturing large size screen, the clearness of images becomes different in the center and at edges of the screen, which results in non-homogeneous visibility. Although a technique to expand the angle of visibility by addition of a phase compensating plate is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 6-75116, it is difficult to completely compensate for the twist structure of the TN mode liquid crystal, and there is still demand for a solution of this problem.
In order to solve the problem of the small angle of visibility, attention has been given to a system, in which a phase compensating plate is applied to each bend oriented cell. The system in which the phase compensating plate is combined with the bend oriented cell is called OCB (optically compensated bend, or optically compensated birefringence, and the quick response speed of OCB devices is attracting considerable attention. An explanation of OCB devices is as follows.
The OCB device has a structure in which bend oriented cells and phase compensating plates for compensating the phase of the liquid crystal layer are arranged between two substrates. There are various types of OCB devices that use various types of phase compensating plates. One type of OCB devices uses a phase compensating plate having a negative birefringence, the other device uses a biaxial phase compensating plate disclosed by Kuo in SID '94 Digest, and still another device uses a pair of upper and lower phase compensating plates having negative birefringence with a hybrid arrangement, which is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 10-197862.
The liquid crystal cell used in the OCB devices is formed by inserting liquid crystal between two substrates, the opposing surfaces of which are orientation treated in one direction, and the liquid crystal Lc is oriented at the interface of the substrate surfaces with a tilt angle range (&thgr;,−&thgr;). When no electric field is applied to the liquid crystal cell in the above state, the tilt angle decreases to zero towards the center of the liquid cell. The tilt angle is reduced towards the center and the tile angle becomes zero at the center of the cell gap, where the liquid crystal molecules are oriented in parallel to both upper and lower substrates (this state is called a spray orientation). If the liquid crystal molecules are arranged in the above-described orientation, the desired wider angle of visibility is not obtainable. In the OCB mode, it is necessary for the liquid crystal by, for example, the application of a high voltage to the cell gap, to be rearranged in an arcuate structure (this state is called a bend arrangement), wherein the liquid crystal molecules at the center of the cell gap stand up and the tilt angle of the liquid crystal molecules at the interface with the substrate becomes zero.
In the OCB mode, the display is operated by controlling the tilt angle of liquid crystal molecules not located at the center of the cell gap, while the liquid crystal molecules at the center of the cell gap are maintained in the standing state. For example, when the phase compensating plate is arranged to conform with a dark representation of the display when the tilt angle of the liquid crystal molecules not located at the gap center is small (reclined), while the liquid crystal molecules at the gap center are standing as shown in
FIG. 17A
, the bright representation is obtained when the tilt angle of the liquid crystal molecules not located at the gap center is large (standing) while the liquid crystal molecules at the gap center are also standing as shown in FIG.
17
B.
However, problems arise when the bend-type orientation is applied to active matrix liquid crystal display devices.
The first problem is that the liquid crystal molecules oriented in the bend-type orientation is liable to be affected by nearby electric fields parasitically generated between the pixel electrodes and wiring. For example, when a potential difference is generated between a pixel electrode on a particular pixel region and the signal line adjacent to the pixel electrode on a active matrix substrate, this potential difference generates an electric field in the liquid crystal layer parallel to the substrate, which causes the liquid crystal molecules in the region where the liquid crystal orientation is affected by the electric field to rotate and the liquid crystal in this region in the bend-type orientation is converted into a twist-type orientation. When the liquid crystal is converted to a twist orientation, standing of the liquid crystal molecules is suppressed and it becomes difficult to rearrange them in the bend orientation. Since the desired birefringence cannot be obtained in the area where the bend orientation is deformed, the image quality declines. When an orientation core is formed, even locally, in the pixel area in question, this orientation tendency propagates due to the liquid crystal characteristics, which also causes degradation of the image quality by disturbing the liquid crystal arrangement.
The other problem of the bend type orientation is that the liquid crystal molecules near the substrate are liable to be affected by irregularity of the substrate surface. In general, on the substrate surface, a number of irregular portions are present due to thin film transistors or protective insulating layers formed on the substrate by repeated lamination and etching. The liquid crystal layers tend to orient along the slopes of the irregularity, and the tilt angles of some liquid crystal molecules turn to the opposite direction to the normal orientation, which is called the reversed tilt phenomenon, and thus the bend orientation is disturbed.
As described above, in order to apply the bend type orientation mode to the active matrix liquid crystal display, disturbance of the bend type liquid crystal orientation caused by the nearby electric field or the irregularity of the substrate surface must be securely excluded.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a liquid crystal display, in which the bend-type liquid crystal orientation mode is not disturbed by a horizontal electric field or irregularities of the substrate surface.
The present invention provides a liquid crystal display device defining a rectangular pixel region defined by a plurality of scanning lines arranged in parallel and a plurality of signal lines crossing the scanning lines, in which a liquid crystal is inserted between two facing substrates comprised of an active matrix substrate including pixel electrodes and thin film transistors and the other one is a transparent substrate including common electrodes, and the opposing surfaces of the active matrix substrate and the transpar
Akkapeddi P. R.
Kim Robert H.
NEC LCD Technologies Ltd.
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