Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-12-17
2002-06-25
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S038000, C349S054000
Reexamination Certificate
active
06411347
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) having a storage capacitor and a method of manufacturing thereof. More specifically, the present invention relates to a structure and method for improving the image quality of an LCD by reducing the fluctuations in the capacitance of a storage capacitor of an LCD.
2. Discussion of the Related the Art
A thin film transistor (TFT) LCD device includes TFTs for use as switching devices, capacitors being defined by liquid crystals disposed between upper and lower plate electrodes, subsidiary capacitors, gate lines, and data lines.
For driving a TFT-LCD, a signal voltage is applied to a gate electrode, the TFT then turns on so that a data signal including image data is transmitted to the liquid crystals through the TFT. The liquid crystals that are within the electrode plates of the capacitors are charged. Ideally, the total electric charge that is stored in the liquid crystals remains constant until the next signal is applied.
However, the liquid crystal voltage varies due to the existence of various sources of capacitance and the voltage varies by an amount &Dgr;V, which is expressed by the following approximate formula: &Dgr;V=Cgd*Vg/(Cgd+CLC+Csto), where &Dgr;V is the maximum amount of variation of liquid crystal voltage, Cgd is parasitic capacitance due to the overlap between the gate and drain electrodes, CLC is liquid crystal voltage, Csto is the capacitance of a storage capacitor, and Vg is the voltage of the gate electrode. The existence of &Dgr;V causes distortion in the liquid crystal voltage and is the primary reason for flicker in images produced on the LCD device. To decrease &Dgr;V, it is preferable to increase the capacitance of the storage capacitor Csto.
FIG. 1
shows an arrangement of an LCD device having a gate storage capacitor. Referring to
FIG. 1
, a pixel is defined on a substrate (not shown) by the intersection of the gate lines
10
L and
11
L and the data line
12
L. A gate electrode
11
G is connected to the gate line
11
L, a source electrode
12
S is connected to the data line
12
L, and a drain electrode
12
D is arranged so as to oppose the source electrode
12
S. An active layer
15
is overlapped with the above-mentioned three electrodes and constitutes a TFT for use as a switching device. A pixel electrode
17
is connected to the drain electrode
12
D and covers the pixel area.
A portion of the (n−1)th gate line
10
L and a portion of the nth pixel electrode
17
are overlapped and define a storage capacitor. In general, a gate insulating layer and a passivation layer defines a dielectric layer of the storage capacitor and the gate line and the pixel electrode define the electrodes of the storage capacitor.
But, when a subsidiary electrode is connected to the pixel electrode and disposed on the gate insulating layer as shown in
FIG. 2
, the gate insulating layer functions as a dielectric layer since the subsidiary electrode and gate line also define electrodes of a storage capacitor. In the above-described case, the capacitance can be increased since it is possible to reduce the thickness of the dielectric layer.
FIG. 2
shows a layout view of a storage capacitor in an LCD device according to a related art.
FIG. 3
shows a cross-sectional view of a storage capacitor in an LCD device according to a related art. Referring to
FIGS. 2 and 3
, a gate line
21
L including a gate electrode
21
G is provided on a substrate
200
, and a gate insulating layer
22
is provided on an exposed surface of the substrate including the gate line
21
L. A subsidiary electrode
23
, which is preferably made of a metal that is used to form the source/drain electrode, is defined on the gate insulating layer
22
. A passivation layer
24
covers the subsidiary electrode
23
, and a contact hole (not shown in
FIG. 2
) is defined on the passivation layer
24
and exposes a portion of the subsidiary electrode
23
. A pixel electrode
25
is connected to the subsidiary electrode
23
through the contact hole that is defined on the passivation layer
24
.
In the above-described structure, storage capacitance of a storage capacitor is provided by the subsidiary electrode
23
, a portion of the gate line
21
L that is overlapped with the subsidiary electrode
23
, a portion of the pixel electrode
25
which is not overlapped with the subsidiary electrode
23
, and a portion of the gate line
21
L that is overlapped with the pixel electrode which portion is not overlapped with the subsidiary electrode.
For the sake of explanation, the term “storage capacitor” in this specification is defined as the subsidiary electrode
23
and a portion of the gate line
21
L which is overlapped with the subsidiary electrode in the present specification.
The capacitance of a storage capacitor is expressed as ‘C∝A/d’, where A is the overlapped area between the subsidiary electrode and the gate line, which are two electrodes of the storage capacitor, and d is the thickness of the gate insulating layer, which defines the dielectric layer between the electrodes. Unfortunately, it is difficult to increase the area of the subsidiary electrode due to the structural limitations of the LCD of the related art. Thus, in the related art, it is very difficult to increase the capacitance of the storage capacitor. Accordingly, the conventional LCD device has very poor image quality due to flickering and other image defects.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a storage capacitor in an LCD device that improves the image quality by increasing the capacitance of the storage capacitor to reduce the fluctuations in the liquid crystal voltage.
Further, preferred embodiments of the present invention provide a storage capacitor in an LCD device that improves the image quality by increasing the capacitance of the storage capacitor by increasing the total exposed surface of the gate line. The total exposed surface of the gate line can be increased by increasing the total number of lateral surfaces in the gate line. The total number of lateral surfaces of the gate line can be increased by one or more open portions in the gate line.
Further, preferred embodiments of the present invention provide a storage capacitor in an LCD device that improves the image quality by increasing the capacitance of the storage capacitor by increasing the area of overlap between the gate line and subsidiary electrode.
A preferred embodiment of the present invention includes a substrate, a gate line on the substrate and including at least one open portion on a surface of the gate line, the open portion defining additional lateral surfaces of the gate line, a gate insulating layer covering the gate line, and a storage capacitor electrode on the gate insulating layer wherein the storage capacitor electrode is overlapped with the gate line.
In another preferred embodiment of the present invention, a method of manufacturing an LCD device includes the steps of providing a substrate, forming a gate line having at least one of open portion on a surface of the gate line, forming a gate insulating layer covering the gate line, and forming a storage capacitor electrode on the gate insulating layer to be overlapped with the gate line.
Therefore, preferred embodiments of the present invention improve the image quality of an LCD display device by reducing flickering and other image defects that are caused by fluctuations in the liquid crystal voltage by increasing the capacitance of a storage capacitor of the LCD display device.
Other elements, features, details and advantages of the present invention will become apparent from the detailed description of preferred embodiments of the present invention in conjunction with the attached drawings.
REFERENCES:
patent: 5162931 (1992-11-01), Holmberg
patent: 5831708 (1998-11-01), Hiraishi et al.
patent: 6285418 (2001-09-01), Ko et al.
patent: 3-240263 (1991-10-01), None
patent
Chung In-Jae
Ko Tae-Woon
Park Sung-Il
Birch & Stewart Kolasch & Birch, LLP
Chung David
LG. Philips LCD Co. Ltd.
Parker Kenneth
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