Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-10-12
2003-12-02
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06657694
ABSTRACT:
RELATED APPLICATION
This application claims the benefit of Korean Patent Application No. 2000-60450, filed on Oct. 13, 2000, under 35 U.S.C. §119, the entirety of which is hereby incorporated by reference.
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. Description of Related Art
Recently, light and thin liquid crystal display (LCD) devices with low power consumption are used in office automation equipment, video devices, and the like. Such LCDs typically use an optical anisotropy and spontaneous polarization of a liquid crystal (LC). The liquid crystal has thin and long liquid crystal molecules, which cause a directional alignment of the liquid crystal molecules. At this point, an alignment direction of the liquid crystal molecules is controlled by applying an electric field to the liquid crystal molecules. When the alignment direction of the liquid crystal molecules are properly adjusted, light is refracted along the alignment direction of the liquid crystal molecules to display image data. Of particular interest is an active matrix (AM) LCD, in which a plurality of thin film transistors and pixel electrodes are arranged in matrix array, because of its high resolution and superiority in displaying moving pictures. Driving methods for such LCDs typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode. A TN liquid crystal panel has high transmittance and aperture ratio. In addition, since the common electrode on the upper substrate serves as a ground, static electricity is prevented from destroying the liquid crystal panel.
Although TN LCDs and STN LCDs, which have the same structure, have been put to practical use, they have a drawback in that they have a very narrow viewing angle. In order to avoid the problem of narrow viewing angle, IPS LCD devices have been proposed. IPS LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. The IPS LCD device has advantages in contrast ratio, gray inversion, and color shift that are related to the viewing angle.
FIG. 1A
is a detailed plan view showing a unit pixel region
10
of a typical IPS-LCD device. In addition, a cross-sectional view taken along a line “B—B” in
FIG. 1A
is illustrated in FIG.
1
B.
On the surface of a transparent substrate
1
a
adjacent to the liquid crystal layer, a scan signal line
2
made of, for example, aluminum (Al) is formed extending along the x-direction, as shown in FIG.
1
A. In addition, a reference signal line
4
, also known as a common line, is formed extending along the x-direction, close to the scan signal line
2
on the +y-direction side thereof. The reference signal line
4
is also made of, for example, Al. A region surrounded by the scan signal line
2
, the reference signal line
4
, and video signal lines
3
constitutes the unit pixel region
10
.
In addition, the unit pixel region
10
includes a reference electrode
14
formed by the reference signal line
4
, and another reference electrode
14
formed adjacent to the scan signal line
2
. The pair of horizontally extending reference electrodes
14
are positioned adjacent to one of a pair of video signal lines
3
(on the right side of FIG.
1
A), and are electrically connected to each other through a conductive layer
14
a
, which is formed simultaneously with the reference electrodes
14
.
In the structure described above, the reference electrodes
14
form a pair extending in the direction parallel to the scan signal line
2
. In other words, the reference electrodes
14
form a strip extending in a direction perpendicular to the video signal lines
3
.
As shown in
FIGS. 1A and 1B
, a first insulating layer
11
made of, for example, silicon nitride is formed on the surface of the lower substrate
1
a
on which the scan signal lines
2
are formed, thereby overlying the scan signal line
2
, the reference signal lines
4
, and the reference electrodes
14
. The first insulating layer
11
functions as (a) an inter-layer insulating film for insulating the scan signal line
2
and the reference signal line
4
from the video signal lines
3
, (b) as a gate-insulating layer for a region in which a thin film transistor (TFT) is formed, and (c) as a dielectric film for a region in which a capacitor “Cstg” is formed. The TFT includes a drain electrode
3
a
and a source electrode
15
a
. A semiconductor layer
12
for the TFT is formed near a crossing point of the gate and data lines (scan signal lines and video signal lines)
2
and
3
. A first polarization layer
18
is formed on the other surface of the lower substrate
1
a.
On the first insulating layer
11
, a display electrode
15
is formed parallel with the reference electrode
14
. One end portion of the display electrode
15
is electrically connected to the conductive layer
14
a
, and the other end portion thereof is electrically connected to the source electrode
15
a
. Still on the first insulating layer
11
, a first planar layer
16
is formed to cover the display electrode
15
. A first alignment layer
17
is formed on the first planar layer
16
.
Under an upper substrate
1
b
, a black matrix
30
is disposed. A color filter
25
is formed to close an opening in the black matrix
30
. A second planar layer
27
is placed to cover the color filter
25
and the black matrix
30
. A second alignment layer
28
is placed under the surface of the second planar layer
27
facing the liquid crystal layer LC.
The color filter
25
is formed to define three unit pixel regions adjacent to and extending along the video signal line
3
and to position a red (R) filter, a green (G) filter, and a blue (B) filter, for example, from the top of the three unit pixel regions. The three unit pixel regions constitute one pixel region for color display.
A second polarization layer
29
is also arranged on the surface of the upper substrate
1
b
that is opposite to the surface of the upper substrate
1
b
adjacent to the liquid crystal layer LC, on which various layers are formed as described above.
It will be understood that in
FIG. 1B
, a voltage applied between the reference electrodes
14
and the display electrode
15
causes an electric field “E” to be generated in the liquid crystal layer LC in parallel with the respective surfaces of the lower and upper substrates
1
a
and
1
b
. This is why the illustrated structure is referred to as the in-plane switching LCD device, as mentioned above.
With reference to
FIGS. 2
,
3
A, and
3
B, operation modes of a typical IPS LCD device are explained in detail.
FIG. 2
is a conceptual cross-sectional view illustrating the operation of the typical IPS LCD device. As shown, 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. On the first substrate
1
a
, pixel and common electrodes
15
and
14
are disposed. 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 molecules LCM. The liquid crystal molecules LCM have a negative dielectric anisotropy, and thus are aligned parallel to the electric field “E”. The pixel electrode
15
and common electrode
14
are the display electrode
15
and reference electrode
14
of
FIG. 1B
, respectively.
FIGS. 3A and 3B
illustrate operation modes for the typical IPS-LCD device shown in FIG.
2
Ko Doo-Hyun
Lee Jun-Ho
Birch & Stewart Kolasch & Birch, LLP
Kim Robert H.
LG.Philips LCD Co. , Ltd.
Schechter Andrew
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