In-plane switching liquid crystal display

Details In-plane switching liquid crystal display In-plane switching liquid crystal display

1999-11-09

2002-05-14

Ton, Toan (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S123000

Reexamination Certificate

active

06388727

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter referred to as an LCD) in particular, to an in plane switching (hereinafter referred to as an IPS) LCD and a manufacturing method thereof.
2. Description of the Related Art
Liquid crystal displays are one of the most highlighted flat panel displays, which use electro-optical properties of liquid crystal materials. In view of driving method, the LCDs are classified into simple matrix type and active matrix type.
An active matrix type LCD has a plurality of switching elements having non-linear characteristics, and pixels of the LCD are controlled by the switching elements. Examples of the switching elements are three-terminal thin film transistors (hereinafter referred to as TFTs) and two-terminal thin film diodes such as metal-insulator-metal (MIM) devices.
A commonly used TFT LCD is comprised of a substrate having pixel electrodes, an opposite substrate having a common electrode, and a liquid crystal material therebetween. When the pixel electrodes and the common electrode are applied with voltages, the molecules of the liquid crystal material change their orientations in response to the electric field due to the potential difference between the pixel electrode and the common electrode. Furthermore, a twisted-nematic (herein after referred to as a TN) mode where the molecular director of the liquid crystal is twisted on going from one substrate to the other substrate is generally used.
However, the LCD, in particular the TN LCD, has a narrow viewing angle and a contrast dependent on the viewing angle. In addition, it has a problem that the number of the process steps are too many since the electrodes are formed in each substrate and the two substrates have a short point for applying voltages into the common electrode.
In order to overcome these problems, IPS LCDs are suggested.
An IPS LCD has pixel and common electrodes formed in only one of the two substrates, and the voltage difference of the pixel electrodes and the common electrode produces substantially horizontal electric fields. One of the IPS LCDs is disclosed in European patent application No. 93307154.0.
Now, conventional IPS LCDs are described with reference to attached figures.
FIGS. 1 and 2
are layout views of IPS LCDs disclosed in the paper entitled “Development of Super-TFT-LCDs with In-Plane Switching Display Mode” (M. Ohta et al.) of ASIA DISPLAY '95 pp. 707-710.
Fist, an LCD shown in
FIG. 1
is described.
A gate line
1
is formed on a glass substrate (not shown) in a transverse direction, and a data line
11
crossing the gate line
1
is formed in a longitudinal direction. A common electrode line
2
made of the same material as the gate line
1
is arranged so as to be parallel to the gate line
1
. A longitudinal branch
3
of the common electrode line
2
goes towards the gate line
1
from the common line electrode
2
, and its transverse branch
4
connected to the end of the longitudinal branch
3
is arranged so as to be parallel to the gate line
1
. Near the cross point of the gate line
1
and the data line
11
, a TFT is formed. A gate electrode
5
and a source electrode
13
of the TFT are a portion of the gate line
1
and of the data line
11
, respectively, and a drain electrode
12
of the TFT is opposite the source electrode
13
. The drain electrode
12
is made of the same material as the data line
11
, and there is a semiconductor layer
20
between the gate electrode
5
and the source and the drain electrodes
13
and
12
. The drain electrode
12
extends to form a pixel electrode
16
,
17
,
18
and
19
. The pixel electrode
16
has a rectangular shape having two longitudinal sides
16
and
17
and two transverse sides
18
and
19
. The transverse sides
18
and
19
are overlapped with the common electrode line
2
and its transverse branch
4
, respectively. These overlapped structures function as storage capacitors. The longitudinal branch
3
of the common electrode line
2
is arranged between the two longitudinal sides
16
and
17
of the pixel electrode, and the potential difference between the longitudinal sides
16
and
17
and the longitudinal branch controls performance of the liquid crystal molecules.
Next, an LCD shown in
FIG. 2
is described, where the shape of the branches of the common electrode line is replaced with the shape of a pixel electrode in comparison with the LCD shown in FIG.
1
.
A common electrode line
2
has two longitudinal branches
6
and
7
and one transverse branch
8
connecting the longitudinal branches
6
and
7
, and thus the common electrode line
2
along with its branches
6
,
7
and
8
forms a rectangle. On the other hand, a pixel electrode
13
,
14
and
15
extended from a drain electrode
12
is comprised of two transverse portions
14
and
15
and a longitudinal portion
13
connecting the midpoints of the transverse portions
14
and
15
. The transverse portions
14
and
15
are overlapped with the common electrode line
2
and its transverse branch
8
, and these overlapped structures serve as storage capacitors. The longitudinal portion
13
is arranged between the two longitudinal branches
6
and
7
of the common electrode line
2
, and the potential difference between the longitudinal branches
16
and
17
and the longitudinal portion
13
adjusts performance of the liquid crystal molecules.
It is disclosed in the above-described paper that these LCDs have improved viewing characteristics.
However the paper lodges some issues.
First, the liquid crystal molecules should be shielded from the electric fields produced from the data line.
The longitudinal side
16
of the pixel electrode shown in FIG.
1
and the longitudinal branch
6
of the common electrode shown in
FIG. 2
, both adjacent to the data line
11
, serve as shields from the electric field of the data line
11
. Since the pixel electrode is floated when the TFT is in off state, its potential is easily disturbed by the potential of outside. On the other hand, the common electrode line and its branches are not easily disturbed since they are constantly applied with voltages.
Accordingly, the LCD shown in
FIG. 2
effectively shields the electric fields from the data line.
Second, aperture ratio should be considered since all the electrodes are formed in a substrate.
The aperture ratio is determined by the effective display area, that is, the area surrounded by the pixel electrodes
16
,
17
,
18
and
19
shown in
FIG. 1
or the area enclosed by the common electrode line
2
and its branches
6
,
7
and
8
shown in FIG.
2
. In case of the LCD shown in
FIG. 1
, the longitudinal side
16
of the pixel electrode adjacent to the data line
11
should be separated from the data line
11
by predetermined distance in order to prevent their short-circuiting since the pixel electrode
16
,
17
,
18
and
19
and the data line
11
are made of the same layer. On the other hand, concerning the LCD shown in
FIG. 2
, since the common electrode line
2
and its branches
6
,
7
and
8
are insulated from the data line
11
via an insulating layer (not shown), it is permissible that they become considerably closer.
Therefore, the LCD shown in
FIG. 2
is effective to obtain large aperture ratio.
However, these conventional IPS LCDs have a disadvantage that they cannot sufficiently shield the electric fields from the data line. Further, there are disadvantages that the capacitance of a parasitic capacitor comprised of a gate and a source of a TFT is not uniform between pixels when misalign occurs, and the on current of the TFT is low.
SUMMARY OF THE INVENTION
An object of the present invention is to shield an electric field of a data line effectively in an IPS LCD. Another object of the present invention is to provide TFTs having high on currents. Another objects of the present invention are to increase aperture ratio and to obtain uniform parasitic capacitance pixel by pixel.
In order to achieve these objects, an in-plane swit

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