Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-06-26
2003-10-14
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000, C349S038000
Reexamination Certificate
active
06633357
ABSTRACT:
This application claims the benefit of Korean Application No. P2000-35650, filed Jun. 27, 2000, which is hereby incorporated by reference as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a multi-domain liquid crystal display device and method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for minimizing a disclination area, thereby improving picture quality.
2. Discussion of the Related Art
As shown in
FIG. 1
, a liquid crystal display (LCD) includes a first substrate
11
, a second substrate
21
, and a liquid crystal sealed between the first and second substrates
11
and
21
according to the related art. More specifically, the first substrate
11
has R, G, B color filter
13
for representing colors, a black matrix
15
for blocking light from being transmitted to the portion other than pixel regions of the second substrate, and a common electrode
17
for applying a common voltage Vcom to a panel, which are formed thereon.
The second substrate
21
has gate lines
23
and data lines
25
arranged thereon in matrix to define the pixel regions. Each pixel region includes a thin film transistor (TFT) and a pixel electrode.
Currently, one of the most widely used the liquid crystal displays is a twisted nematic (TN) mode LCD. The TN-mode LCD is constructed in a manner that electrodes are respectively formed on the two substrates and liquid crystal molecules filled between them are twisted in a spiral shape, parallel to the substrates and having a predetermined pitch.
In this structure, a voltage is applied to the electrodes to drive a liquid crystal director. However, the TN-mode LCD has poor contrast because light is not completely blocked in an OFF-state. Furthermore, the TN-mode LCD generates a gray inversion so that a contrast ratio varies with angles to invert luminance of medium gray, thereby causing a difficulty in obtaining stabilized images. Moreover, the TN-mode LCD does not have satisfactory viewing angle.
A variety of research has been conducted for solving the narrow viewing angle problem of the LCDs. The research includes a film-compensated mode for compensating a viewing angle with a compensation film, a multi-domain mode in which pixels are divided into multiple domains and each domain has a different main viewing angle direction to compensate the viewing angle, an in-plane switching mode in which two electrodes are placed on the same substrate to generate a horizontal electric field, and an OCB (optically compensated birefringence) mode.
Meanwhile, a vertical alignment (VA) mode LCD uses a negative liquid crystal so that dielectric constant anisotropy is negative and a vertical alignment film. In a VA LCD, the longer sides of the liquid crystal molecules are arranged perpendicular to the plane of the alignment film when no voltage is applied, and a polarizing axis of a polarizer attached onto the substrate is located perpendicular to the longer sides of the liquid crystal molecules, to represent normally black mode.
On the other hand, when a voltage is applied to the LCD, the longer sides of the molecules are moved from the direction perpendicular to the alignment film plane toward the alignment film plane to transmit the light according to the characteristic that the negative liquid crystal molecules are orientated and inclined with respect to the electric field.
The VA-mode LCD is superior to the TN-mode LCD in terms of a contrast ratio, a response time, and so on. Furthermore, in case where a direction in which the liquid crystal molecules fall is divided into a predetermined number of multiple directions and a compensated film is employed, a viewing angle can be effectively realized.
Moreover, there have been recently proposed PVA (patterned vertical alignment) and MVA (multi-domain vertical alignment) in which structures such as side electrodes and ribs or slits are formed on the substrate to distort the electric field applied to the liquid crystal layer, instead of alignment treatment, thereby locating the liquid crystal molecular director in a desired direction.
FIGS. 2A
to
2
C are cross-sectional views for explaining problems of the TN LCD while
FIGS. 3A
to
3
C are cross-sectional views for explaining an alignment division according to a rubbing process. Although the TN LCD among TFT LCDs has advantages of excellent contrast and satisfactory color reproducibility, it has a disadvantage of a narrow viewing angle.
Referring to
FIG. 2A
, in a normally white mode TN LCD, liquid crystal molecules
114
are aligned in the same direction with a slight inclination (about 1 to 5°) when no voltage is applied between two substrates
112
and
113
of the LCD. In this state, light is seen nearly white in any azimuth. In case of application of a voltage higher than a threshold value, as shown in
FIG. 2C
, intermediate liquid crystal molecules
114
, except for those located near the substrates
112
and
113
, are aligned in a vertical direction. Incident linearly polarized light is therefore seen blocked, but not twisted. At this time, light obliquely incident on a screen (panel) has the direction of polarization thereof twisted to some extent because it passes obliquely through the liquid crystal molecules
114
aligned in the vertical direction. The light is therefore seen halftone (gray) but not perfect black.
As shown in
FIG. 2B
, in the state in which an intermediate voltage lower than the voltage applied in the state shown in
FIG. 2C
is applied, the liquid crystal molecules
114
near the alignment layers are aligned in a horizontal direction, but the liquid crystal molecules
114
in the middle parts erect themselves halfway. The birefringent characteristic of the liquid crystal is lost to some extent. This causes transmittance to deteriorate and cause halftone (gray).
However, this refers only to the light incident perpendicularly on the liquid crystal panel. The obliquely incident light is seen differently, that is, light is seen differently depending on whether it is seen from the left or right side of the drawing. As illustrated, the liquid crystal molecules
114
are aligned mutually parallel relative to the light propagating from the light below to left above.
The liquid crystal hardly exerts a birefringence effect. Therefore, when the panel is seen from the left side, it is seen black. By contrast, the liquid crystal molecules
114
are aligned vertically relative to light propagating from light below to right above. The liquid crystal exerts a great birefringence 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 a display state varies with a viewing angle.
It is known that a viewing angle of the liquid crystal display device (LCD) in the TN mode can be improved by setting 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 (pretilt angles) which keep in 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 process in which the surface of the alignment film is rubbed in one direction by a cloth such as rayon, so that the liquid crystal molecules are orientated in the rubbing direction. Therefore, a viewing angle can be improved by making a different rubbing direction inside the pixels.
FIGS. 3A
to
3
C show a method of making a different rubbing direction inside pixels. As shown in the drawing, an alignment film
122
is formed on a glass substrate
116
. For simplicity, electrodes and other elements are omitted from the drawing. The alignment film
122
is then bought into contact with a rubbing roll
201
to perform a rubbing treatment in one direction.
Next, a photoresist is applied to
Ko Doo Hyun
Lee Joun Ho
Chung David
LG. Philips LCD Co. Ltd.
Parker Kenneth
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