Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-06-26
2004-09-07
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000, C349S143000
Reexamination Certificate
active
06788374
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-35649, filed on Jun. 27, 2000, 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 display device, and more particularly, to a multi-domain liquid crystal display (LCD) device and a method for fabricating the same.
2. Discussion of the Related Art
A Cathode Ray Tube (CRT) is one of display devices mainly used in monitors of information terminals and measuring instruments including telelvision. However, it is difficult for a CRT to be adapted to miniaturization and to have light weight due to its weight and size.
A liquid crystal display (LCD) device having a small size, light weight and low power consumption has been actively developed to substitute for such a CRT. Recently, the liquid crystal display devices can be configured as a flat panel display device. Thus, demand of the LCD device is increasing.
Such an LCD device is based on electric optical characteristic of a liquid crystal injected within a panel unlike a plasma display panel (PDP) or a field emission display (FED), the LCD device does not itself emit light. Accordingly, to view a picture displayed in an LCD, a separate light source, i.e., a back light assembly is required for uniformly irradiating light onto a display panel.
FIG. 1
shows a general LCD. Referring to
FIG. 1
, the LCD includes a first substrate
1
, a second substrate l
a
, and a liquid crystal (not represented) injected and sealed between the first and second substrates
1
and
1
a.
In more detail, on the first substrate
1
, a color filter layer
2
is formed to display color, a black matrix layer
3
is formed to prevent light from being transmitted to a portion other than a pixel region of the second substrate, a common electrode
4
is formed to apply a common voltage Vcom to the panel.
On the second substrate
1
a
, a gate line
5
and a data line
6
are arranged to cross each other, so that a pixel region is formed in a matrix arrangement. A thin film transistor (TFT) and a pixel electrode are formed in each pixel region.
Currently, one of the most widely used liquid crystal displays is a twisted nematic (TN) mode LCD. The TN-mode LCD is constructed in a manner such that electrodes are formed on each of the two substrates, respectively. And liquid crystal molecules interposed 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, it generates a gray inversion so that a contrast ratio varies with angle to invert luminance of medium gray, thereby causing difficulty in obtaining stabilized images. Moreover, the TN-mode LCD does not have satisfactory viewing angle characteristic.
A variety of research has been conducted in an attempt to solve the narrow viewing angle problem of the LCD. 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 having a negative dielectric constant anisotropy and a vertical alignment film. In this type of the 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 perpendicular to the longer sides of the liquid crystal molecules, to produce a normally black mode.
On the other hand, when 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, which are oriented and inclined with respect to the electric field.
The VA-mode LCD is superior to the TN-mode LCD in terms of contrast ratio, response time, and so on. Furthermore, in the 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 direction 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
14
are aligned in the same direction with a slight inclination (about 1 to 5°) when no voltage is applied between two substrates
12
and
13
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
14
except for those located near the substrates
12
and
13
are aligned in a vertical direction. Incident linearly polarized light is therefore 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
14
aligned in the vertical direction. The light is therefore seen halftone (gray), but not perfectly 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
14
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
14
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
14
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 ke
Ko Doo Hyun
Lee Joun Ho
Chowdhury Tarifur R.
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
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