Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-02-02
2002-08-06
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000, C349S138000
Reexamination Certificate
active
06429907
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a liquid crystal display which has a high aperture ratio to improve light efficiency and more specifically, the present invention relates to a structure and method for overcoming the deficiencies associated with increasing the aperture ratio by reducing the excited interference voltage.
2. Discussion of Related Art
In a liquid crystal display (LCD), thin film transistors (TFTs) act as switching devices for driving devices of a LCD. Along with pixel electrodes that transmit or reflect light, the TFTs and the pixel electrodes are arranged in matrix of rows and columns. Each of the TFTs and pixel electrodes constitute a basic unit of a LCD.
In an LCD, a plurality of unit pixels consisting of a TFT used as a switching device and a pixel electrode are arranged on a lower substrate and defines an array. An array consisting of unit pixels includes at least one gate line and one data line that cross each other. Gate electrodes of the TFTs are connected to each other and constitute at least one gate line for transferring signals to either a column or row, and source electrodes of the TFTs are connected to each other and constitute at least one data line that carries signals to a row or column.
On an upper substrate, a black matrix is formed such that it corresponds to the array of the lower substrate but without the pixel electrodes. In the black matrix of the upper substrate, common electrodes for applying common voltage and color filters that correspond to the pixel electrodes of the lower substrate are provided. The upper and lower substrates are then bonded together. Next, liquid crystals are injected into the space between the upper and lower substrates.
In forming the black matrix on the upper substrate of the LCD having the above-described structure, misalignment between the TFTs of the lower substrate and the black matrix of the upper substrate may occur when the upper and lower substrates are put together. Thus, a sufficient misalignment margin is required to compensate for the possible misalignment. However, the aperture ratio is decreased when the misalignment margin is introduced.
Accordingly, a method for improving the aperture ratio, by forming a narrow matrix that overlaps the data line with the pixel electrodes with the use of an organic insulating layer having a low dielectric constant has been proposed.
To drive the LCD with the above-described structure, a signal voltage is applied to the gate which turns on a selected TFT. During this time, a data signal with image information passes through the TFT to apply an electric field to the liquid crystals.
However, a part of the voltage applied to the liquid crystals varies because various sources of parasitic capacitance exist in a LCD. Thus, a pixel voltage Vp varies depending on the amount of the excited interference voltage &Dgr;V. The excited interference voltage &Dgr;V driven by dot inversion is expressed approximately by the following formula. &Dgr;V=(C
d1
*V
d1
+C
d2
*V
d2
)/(Cst+C
LC
+C
d1
+C
d2
), where C
d1
is parasitic storage capacitance generated from an Nth data line that is overlapped with a pixel electrode, C
d2
is parasitic storage capacitance generated from a (N+1)th data line that is overlapped with a pixel electrode, C
st
is storage capacitance of a pixel electrode, C
LC
is parasitic storage capacitance generated from a transparent electrode of the upper and lower substrates between which liquid crystals are placed, V
d1
is voltage applied to the Nth data line, and V
d2
is voltage applied to the (N+1)th data line.
The excited interference voltage &Dgr;V distorts the voltage applied to the liquid crystals, which cause flickering, crosstalk, and residual image. To fix this, the difference between C
d1
and C
d2
should be decreased in order to compensate for the parasitic storage capacitance by reducing the excited interference voltage &Dgr;V as well as by increasing C
st
.
FIG. 1
is a layout of a LCD according to the related art. Referring to
FIG. 1
, a gate line
23
and a data line
17
are provided on a substrate
11
. The gate line
23
and the data line
17
cross each other, which defines a pixel. A gate electrode
23
G is connected to the gate line
23
, a source electrode
17
S is connected to the data line
17
, and a drain electrode
17
D is arranged to face with the source electrode
17
S.
An active layer
16
on the gate electrode
23
G is overlapped with the source and drain electrodes
17
S and
17
D. The gate electrode
23
G, source and drain electrodes
17
S and
17
D, and active layer
16
constitute a TFT for use as a switching device.
A redundancy layer
15
, which should be formed right after the active layer
16
is disposed on the gate electrode
23
G, is covered by the data line
17
. A pixel electrode
21
is then formed on the whole surface of the pixel area. The pixel electrode
21
is connected to the drain electrode
17
D through a contact hole
25
. A portion of the gate line
23
that is adjacent to the pixel electrode
21
is overlapped, thus constituting a storage capacitor in the pixel area.
In order to increase the capacitance of a storage capacitor, a subsidiary electrode
27
that is connected electrically to the pixel electrode
17
may be provided between a gate insulating layer and a protecting layer through a second contact hole
29
. The subsidiary electrode
27
and the gate line
23
are used as an electrode of the storage capacitor. Thus, the capacitance of the storage capacitor is increased as the gate insulating layer becomes a dielectric layer so that the thickness is reduced.
FIG. 2
shows a cross-section view of the layout in
FIG. 1
bisected along the cutting line A—A. Referring to
FIG. 2
, a redundancy layer
15
and the data line
17
are disposed on a substrate
11
such that a gate insulating layer
13
is provided in between the substrate
11
and the redundancy layers
15
and the data line
17
. Note that the data line
17
covers the redundancy layer
15
. A protecting layer
19
is then disposed on the gate insulating layer
13
and covers the data line
17
. In the above-described case, the protecting layer
19
is made of an organic insulator having a low dielectric constant. A pixel electrode
21
is defined on the protecting layer
19
such that a portion of the protecting layer
19
that corresponds to the redundancy layer
15
is exposed. In the above-described case, the pixel electrode
21
overlaps with the redundancy layer
15
only partially, thus improving the aperture ratio.
The thickness of the portion of the protecting layer
19
under the Nth data line
17
that is overlapped by the pixel electrode
21
is d
11
, while that of the portion of the protecting layer
19
under the (N+1)th data line
17
that is overlapped by the pixel electrode
21
is d
12
. When the protecting layer
19
is made of an organic insulator, the surface of the protecting layer
19
is even because of the excellent flow characteristic of the organic insulator. Therefore, the thickness d
11
is about the same as the thickness d
12
.
However, the overlapped areas between the pixel electrode and the adjacent data lines differs in area in the LCD. In other words, the overlapped area between the pixel electrode and the data line where the TFT is provided is less than the overlapped area between the pixel electrode and the data line where the TFT is not provided. Thus, the parasitic storage capacitance will vary even if the thickness of the protecting layers under the data lines that are overlapped with the pixel electrode is exactly the same. Therefore, the excited interference voltage &Dgr;V is significant in the LCD and will cause flickering, crosstalk, and residual images because of the different parasitic storage capacitances in the pixel area.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide an LCD that prevents poor image quality due to flickering,
Chung Jae-Young
Park Sung-Il
Birch & Stewart Kolasch & Birch, LLP
Duong Tai V.
LG Philips LCD Co., Ltd.
Sikes William L.
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