Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-04-27
2003-12-02
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S129000, C349S130000
Reexamination Certificate
active
06657695
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a vertical alignment liquid crystal display which has a structure capable of dividing a pixel area into multiple domains with different orientation directions of liquid crystal molecules.
(b) Description of the Related Art
Generally, liquid crystal displays have a structure where a liquid crystal is sandwiched between two substrates, and an electric field is applied to the liquid crystal to control the amount of light transmission.
In the usual twisted nematic (TN) liquid crystal displays, the liquid crystal molecules injected into the gap between the two substrates are oriented parallel to the substrates, and spirally twisted with a predetermined pitch. The long axis (usually called the “director”) of the liquid crystal molecules continuously varies in orientation direction, and the viewing angle characteristics depend upon such orientation directions of the liquid crystal molecules.
However, in the TN liquid crystal display, light is not completely blocked at an off state in the normally black mode so that a poor contrast ratio results. The contrast ratio is altered depending upon the viewing angle, and there is a half tone of brightness difference depending on the viewing angle so that stable picture images cannot be obtained. Furthermore, the picture qualities at side edge portions of the screen are not symmetrical to each other with respect to the middle portion. These all result in poor viewing angle characteristics.
However, the vertical alignment liquid crystal displays where the liquid crystal molecules are vertically aligned in the absence of a voltage but twisted in various directions with the voltage applied exhibit excellence in various aspects, such as contrast ratio and response speed compared to the TN liquid crystal displays. Furthermore, when a compensation film is used to divide the twisting of the liquid crystal molecules in various predetermined directions, a wide viewing angle can be effectively obtained.
Recently, a technique for forming an alignment control member such as a pyramid-shaped protrusion on the substrates, a technique for forming an opening pattern at the transparent electrodes, and a technique for forming a protrusion pattern on one of the substrates while forming an opening pattern at the other substrate have been proposed as methods to control the orientation directions of the liquid crystal molecules. The protrusion or opening pattern is designed to achieve four domain divisions in the orientation direction of the liquid crystal molecules at which the efficiency of light usage becomes highest.
FIGS. 1A and 1B
are cross sectional views of a liquid crystal display according to a prior art where the orientation states of liquid crystal molecules are illustrated in the absence and presence of the voltage application.
As shown in the drawings, a transparent pixel electrode
11
is formed at a bottom substrate
10
, and a first opening portion
1
is formed at the pixel electrode
11
. A top substrate
20
facing the bottom substrate
10
is provided with a transparent common electrode
21
. A second opening portion
2
is formed at the common electrode
21
. The bottom and top substrates
10
and
20
are arranged such that the first opening portion
1
is displaced with respect to the second opening portion
2
. Negative dielectric anisotropy liquid crystal molecules
30
are injected into the gap between the bottom and top substrates
10
and
20
.
As shown in
FIG. 1A
, the liquid crystal molecules
30
are oriented perpendicular to the substrates
10
and
20
in the absence of the voltage application.
As shown in
FIG. 1B
, when voltage is applied to the pixel electrode
11
and the common electrode
21
, most of the regions at the pixel area are under the influence of an electric field normal to the substrates
10
and
20
, but the regions adjacent to the opening portions
1
and
2
are under a fringe field beginning from the edges of the opening portions
1
and
2
and focused onto the common electrode
21
and the pixel electrode
11
, respectively. As the negative dielectric anisotropy liquid crystal molecules
30
are inclined to orient in a direction normal to that of the electric field, the long axes of the liquid crystal molecules adjacent to the opening portions
1
and
2
are twisted while being tilted with respect to the substrates
10
and
20
. In this case, two side regions of either of the opening portions
1
and
2
where the orienting directions of the liquid crystal molecules
30
are opposite to each other are present so that the optical characteristics of the two side regions are compensated, resulting in a wide viewing angle.
FIGS. 2A and 2B
are cross sectional views of a liquid crystal display according to another prior art where the orientation states of liquid crystal molecules are illustrated in the absence and presence of voltage application.
As shown in
FIGS. 2A and 2B
, a pixel electrode
12
based on a transparent conductive material such as indium tin oxide is formed at a bottom substrate
10
, and a pyramid-shaped first protrusion
13
and a vertical alignment film (not shown) are sequentially formed on the pixel electrode
12
. A transparent common electrode
22
is formed at a top substrate
20
, and a second pyramid-shaped protrusion
23
and a vertical alignment film (not shown) are sequentially formed at the common electrode
22
. Negative dielectric anisotropy liquid crystal molecules
30
are injected into the gap between the vertical alignment films of the bottom and top substrates
10
and
20
.
As shown in
FIG. 2A
, in the absence of voltage application, most of the liquid crystal molecules
30
are oriented perpendicular to the vertical alignment films, but the liquid crystal molecules
30
positioned close to the protrusions
13
and
23
are tilted with respect to the vertical alignment films at predetermined angles.
As shown in
FIG. 2B
, when voltage is applied to the pixel and common electrodes
12
and
22
, the liquid crystal molecules are twisted in a direction parallel to the substrates
10
and
20
. As the liquid crystal molecules
30
positioned close to the protrusion
13
are tilted in the opposite directions with respect to the region of the protrusion
13
in the absence of the voltage application, the twisting directions thereof are also opposite to each other with the voltage applied. Therefore, two side regions of the protrusion
13
where the twisting directions of the liquid crystal molecules are opposite to each other are present so that the optical characteristics of the two regions are compensated, resulting in a wide viewing angle. In addition, disclination regions where the orientation directions of the liquid crystal molecules
30
are disorderly altered are focused at the regions of the protrusions
13
and
23
so that a black matrix for shielding the disclination regions can be formed in a predetermined manner.
However, in order to fabricate the above-described liquid crystal displays, additional processes for forming the protrusions
13
and
14
or the opening portions
1
and
2
must be performed.
On the one hand, in the case of the liquid crystal display shown in
FIGS. 1A and 1B
, a wet etching process for forming the opening portion
2
at the ITO-based common electrode
21
of the top substrate
20
should be provided. Furthermore, in order to prevent the color filter from being contaminated or damaged due to the etching solution, a protective layer of organic or inorganic materials should be coated onto the color filter before the ITO processing.
On the other hand, in the case of the liquid crystal display shown in
FIGS. 2A and 2B
, before the formation of the protrusions
13
and
23
, separate organic layers should be coated onto the pixel electrode
12
and the common electrode
22
, and etched.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a vertical alignment liquid crystal
Kim Sang-Soo
Kwag Jin-Oh
Song Jang-Kun
McGuireWoods LLP
Qi Mike
Ton Toan
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