Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-05-21
2003-12-09
Kim, Robert (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06661492
ABSTRACT:
This application claims the benefit of Korean Patent Applications No. 2000-27850 filed on May 23, 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 device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment orientation of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Because incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, image data is displayed.
Liquid crystal is classified into positive liquid crystal and negative liquid crystal, depending on the electrical properties of the liquid crystal. The positive liquid crystal has a positive dielectric anisotropy such that long axes of liquid crystal molecules are aligned parallel to an electric field. Whereas, the negative liquid crystal has a negative dielectric anisotropy such that long axes of liquid crystal molecules are aligned perpendicular to an electric field.
By now, active matrix LCDs, in which the thin film transistors and the pixel electrodes are arranged in the form of a matrix, are widely used because of their high resolution and superiority in displaying moving video data.
FIG. 1
is a cross-sectional view illustrating a typical twisted nematic (TN) LCD panel. As shown in
FIG. 1
, the TN LCD panel has lower and upper substrates
2
and
4
and an interposed liquid crystal layer
10
. The lower substrate
2
includes a first transparent substrate
1
a
and a thin film transistor (“TFT”) “S”. The TFT “S” is used as a switching element to change orientation of the liquid crystal molecules. The lower substrate
2
further includes a pixel electrode
15
that applies an electric field to the liquid crystal layer
10
in accordance with signals applied by the TFT “S”. The upper substrate
4
has a second transparent substrate
1
b
, a color filter
8
on the second transparent substrate
1
b
, and a common electrode
14
on the color filter
8
. The color filter
8
implements color for the LCD panel. The common electrode
14
serves as another electrode for applying a voltage to the liquid crystal layer
10
. The pixel electrode
15
is arranged over a pixel region “P,” i.e., a display area. A transparent conductive material like indium tin oxide (ITO) having superior light transmittance is used for the pixel electrode
15
. Further, to prevent leakage of the liquid crystal layer
10
between the lower and upper substrates
2
and
4
, those substrates are sealed by a sealant
6
.
As described above, because the pixel and common electrodes
15
and
14
of the conventional TN LCD panel are positioned on the lower and upper substrates
2
and
4
, respectively, the electric field induced therebetween is perpendicular to the lower and upper substrates
1
a
and
1
b
. The above-mentioned liquid crystal display device has advantages of high transmittance and aperture ratio, and further, since the common electrode on the upper substrate serves as an electrical ground, the liquid crystal is protected from a static electricity.
However, the above-mentioned operation mode of the TN LCD panel has a disadvantage of a narrow viewing angle. To overcome the above-mentioned problem, an in-plane switching (IPS) LCD panel was developed. The IPS LCD panel implements a parallel electric field that is parallel to the substrates, which is different from the TN or STN (super twisted nematic) LCD panel. A detailed explanation about operation modes of a typical IPS LCD panel will be provided with reference to
FIGS. 2
,
3
A,
3
B,
4
A and
4
B.
As shown in
FIG. 2
, first and second substrates
1
a
and
1
b
are spaced apart from each other, and a liquid crystal “LC” is interposed therebetween. The first and second substrates
1
a
and
1
b
are called an array substrate and a color filter substrate, respectively. Pixel and common electrodes
15
and
14
are disposed on the first substrate
1
a
. The pixel and common electrodes
15
and
14
are parallel with and spaced apart from each other. On a surface of the second substrate
1
b
, a color filter
25
is disposed opposing the first substrate
1
a
. The pixel and common electrodes
15
and
14
apply an electric field “E” to the liquid crystal “LC”, then it is aligned parallel to the electric field “E”.
FIGS. 3A and 3B
conceptually illustrate “off state” operation modes for a typical IPS LCD device. In off state, the long axes of the LC molecules “LC” maintain a definite angle with respect to a line that is perpendicular to the pixel and common electrodes
15
and
14
. The pixel and common electrode
15
and
14
are parallel with each other. Herein, the angle difference is 45 degrees, for example.
FIGS. 4A and 4B
conceptually illustrate “on state” operation modes for the typical IPS LCD device. In an on state, an in-plane electric field “E”, which is parallel with the surface of the first substrate
1
a
, is generated between the pixel and common electrodes
15
and
14
. The reason is that the pixel electrode
15
and common electrode
14
are formed together on the first substrate
1
a
. Then, the LC molecules “LC” are twisted such that the long axes thereof coincide with the electric field direction. Thereby, the LC molecules “LC” are aligned such that the long axes thereof are perpendicular to the pixel and common electrodes
15
and
14
.
By the above-mentioned operation modes and with additional parts such as polarizers and alignment layers, the IPS LCD device displays images. The IPS LCD device has wide viewing angle and low color dispersion. Specifically, the viewing angle of the IPS LCD device is about 70 degrees in direction of up, down, right, and left. In addition, the fabricating processes of this IPS LCD device are simpler than other various LCD devices. However, because the pixel and common electrodes are disposed on the same plane of the lower substrate, the transmittance and aperture ratio are low. In addition, the IPS LCD device has disadvantages of a relatively slow response time and a relatively small alignment margin. Because of the small alignment margin, the IPS LCD device needs a uniform cell gap.
The IPS LCD device has the above-mentioned advantages and disadvantages. Users may or may not select an IPS LCD device depending on the intended use.
Now, with reference to
FIGS. 5
, and
6
A to
6
D, a fabricating process for a conventional IPS LCD device is provided.
FIG. 5
is a plan view illustrating a unit pixel region “P” of a conventional IPS LCD device. As shown, a gate line
50
and a common line
54
are arranged parallel to each other, and a data line
60
is arranged perpendicular to the gate and common lines
50
and
54
. Near a cross point of the gate and data lines
50
and
60
, a gate electrode
52
and a source electrode
62
are disposed. The gate and source electrodes
52
and
62
integrally communicate with the gate line
50
and the data line
60
, respectively. The source electrode
62
overlaps a portion of the gate electrode
52
. In addition, a drain electrode
64
is disposed opposite to the source electrode
62
with an interval therebetween.
A plurality of common electrodes
54
a
are disposed perpendicular to the common line
54
and connected to the common electrode. The plurality of common electrode
54
a
are spaced apart from each other with an equal interv
DiGrazio Jeanne A.
Kim Robert
LG.Philips LCD Co. , Ltd.
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