Multi-domain liquid crystal display device

Details Multi-domain liquid crystal display device Multi-domain liquid crystal display device

2000-12-28

2003-12-30

Dudek, James (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

Reexamination Certificate

active

06671020

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 1999-67947, filed on Dec. 31, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a multi-domain liquid crystal display device in which a common auxiliary electrode is formed around and in a pixel region on a same layer as a gate line, and at least one or more electric field induction windows and dielectric structures are formed in the pixel region.
2. Discussion of the Related Art
Among flat-panel displays enjoying image quality equivalent to the image quality offered by a cathode ray tube (CRT) display, 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 widely used equipment such as personal computers, word processors, office automation equipment, and home electrical appliances, including portable television sets. The market for such equipment, using TFT-LCDs, is expected to expand. Accordingly, there is demand for further improvement in image quality. A description will be made by using a TFT-LCD as an example. However, the present invention is not limited to a 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 produce image 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 advanced extraordinarily 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.
FIGS. 1A
to
1
C are diagrams for explaining this problem.
In the Figures, reference numerals
11
and
12
indicate substrates and reference numberal
14
indicates liquid crystal.
FIG. 1A
shows a state of white display to which no voltage is applied and liquid crystal molecules are aligned in the same direction with a slight inclination (about 1° to 5°). For convenience, throughout the figures, liquid crystal molecules are illustrated as in FIG.
1
A. In this white display state, light is seen as nearly white in any azimuth. Moreover, as shown in
FIG. 1C
, in the state in which a voltage is applied, intermediate liquid crystal molecules except those located near the alignment films (not shown), over the substrates, are aligned in a vertical direction. Incident linearly-polarized light is therefore seen as black but not twisted. At this time, light obliquely incident on an LCD screen (panel) has a direction of polarization that is twisted to some extent, because the light passes obliquely through the liquid crystal molecules that are aligned in the vertical direction. The light is therefore seen as halftone (gray) but not perfect black. As shown in
FIG. 1B
, in the state in which an intermediate voltage lower than the voltage applied in the state shown in
FIG. 1C
is applied, the liquid crystal molecules near the alignment films are aligned in a horizontal direction but the liquid crystal molecules in the middle parts of cells erect themselves halfway. The birefringent property of the liquid crystal is lost to some extent. This causes transmittance to deteriorate and brings about halftone (gray) display. However, this effect occurs only for light incident perpendicularly on the liquid-crystal panel. Obliquely incident light is seen differently, that is, light is seen differently depending on whether it is viewed from the left or right side of the drawing. As illustrated, the liquid crystal molecules are aligned mutually parallel relative to light propagating from right below to left above. The liquid crystal hardly exerts a birefringent effect. Therefore, when the panel is viewed from left, it appears black. By contrast, the liquid crystal molecules are aligned vertically relative to light propagating from below to right above. The liquid crystal exerts a great birefringent effect relative to incident light, and the incident light is twisted. This results in nearly white display. Thus, the most critical drawback of the TN LCD is that the display state varies depending on the viewing angle.
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 crystal molecules inside pixels to a plurality of mutually different directions. Generally, the orientation direction of the liquid crystal molecules (pre-tilt angles) is restricted by the direction of a rubbing treatment applied to the alignment film on the surfaces of the substrates as the liquid crystal molecule contact the alignment film. The rubbing treatment is a process, during which the surface of the alignment film is rubbed in one direction by a cloth such as rayon. The liquid crystal molecules are orientated in the rubbing direction. Therefore, viewing angle performance can be improved by making the rubbing direction different inside the pixels.
FIGS. 2A
to
2
C show a method of making the rubbing direction different inside the pixels. As shown in this drawing, an alignment film
22
is formed on a glass substrate
16
(whose electrodes, etc., are omitted from the drawing). This alignment film
22
is then bought into contact with a rotating rubbing roll
201
, which rotates in a first direction, to perform the rubbing treatment in one direction. Next, a photo-resist is applied to the alignment film
22
, and a predetermined pattern is exposed and developed by photolithography. As a result, a layer
202
of the photo-resist, which is patterned, is formed as shown in FIG.
2
B. Next, the alignment film
22
is brought into contact with a rubbing roll
201
, which rotates in a second direction opposite to the first direction so that only the open portions of the pattern are rubbed. In this way, a plurality of regions that are subjected to the rubbing treatment in different directions are formed within the pixel, and the multiple orientation directions of the liquid crystal are formed in the pixel. Incidentally, the rubbing treatment can be done in arbitrarily different directions when the alignment film
22
is rotated relative to the rubbing roll
201
.
In the process described above, there are some problems creating boundaries for the different orientation directions of the liquid crystal molecules for improving the viewing angle performance in a vertical alignment (VA) LCD.
It is desirable to improve a viewing angle characteristic of a VA liquid crystal display, and to create a VA liquid crystal display exhibiting a viewing angle characteristic that is as good or better than the one exhibited by in-plane switching mode LCDs, while permitting the same contrast and operating speed as the conventional liquid crystal displays.
In the VA mode employing a conventional vertical alignment film and using a negative liquid crystal as a liquid crystal material, a domain regulating means is included for regulating the orientation of a liquid crystal in which liquid crystal molecules are aligned obliquely when a voltage is applied so that the orientation will include a plurality of directions within each pixel. The domain regulating means is provided on at least one of the substrates. Further, at least one of domain regulating means has inclined surfaces (slopes). The inclined surfaces include surfaces which are almost vertical to the substrates. Rubbing need not be performed on the vertical alignment film.
In the VA-LCD device, when no voltage is applied, in almost all regions of the liquid crystal other than

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