Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-12-29
2002-06-11
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S141000
Reexamination Certificate
active
06404470
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a liquid crystal display having high aperture ratio and high transmittance, more particularly to a liquid crystal display having high aperture ratio and high transmittance capable of preventing the aperture ratio from deterioration that is originated from a black matrix.
2. Description of the Related Art
An active matrix liquid crystal display has an advantage of excellent response characteristic and is appropriate for high number of pixels. Therefore, high quality and large size of display devices comparable to the cathode ray tube(“CRT”), is realized.
The active matrix liquid crystal display has employed as its operation mode mainly a twist nematic(TN) mode and a super twist nematic(STN) mode, both to which an electric field disposed vertical to substrates is applied. The TN mode and STN mode liquid crystal displays have been commercially used, however they have narrow viewing angle characteristics. According to this disadvantage, an in-plane switching(“IPS”) mode liquid crystal display has been proposed to improve the narrow viewing angle characteristics of TN mode and STN mode liquid crystal displays.
Although not shown, in the IPS mode liquid crystal display, a pixel electrode and a counter electrode to drive liquid crystal molecules are formed on the same substrate in parallel to each other, also there is formed an electric field which is in-plane to the substrate surface. The IPS mode liquid crystal display has an advantage of wider viewing angle than that of the TN mode or the STN mode liquid crystal displays, however, improving the transmittance is limited since the pixel and the counter electrodes are made of opaque metal layers.
Accordingly, in order to overcome those limitations of high aperture ratio and high transmittance in the IPS mode liquid crystal display, a fringe field switching(“FFS”) mode liquid crystal display has been proposed. The FFS mode liquid crystal display is named as “liquid crystal display having high aperture ratio and high transmittance”.
In the FFS mode liquid crystal display, a pixel electrode and a counter electrode are made of transparent metal layers, and then the aperture ratio thereof is more improved compared to that of the IPS mode liquid crystal display. Further, since a distance between the pixel electrode and the counter electrode is narrower than that of the upper and lower substrates, a fringe field is formed at upper portions of the counter and the pixel electrodes when the FFS mode liquid crystal display is driven. Then, the transmittance thereof is more improved compared to that of the IPS mode liquid crystal display since liquid crystal molecules arranged over those electrodes are all driven.
FIG. 1
is a cross-sectional view showing an outline of the conventional FFS mode liquid crystal display. As shown in the drawing, a first transparent insulating substrate
1
and a second transparent insulating substrate
11
are opposed with a selected distance, and a liquid crystal layer
18
is interposed between the first transparent insulating substrate
1
and the second transparent insulating substrate
11
. Herein, the distance between the first transparent insulating substrate
1
and the second transparent insulating substrate
11
is referred to as a cell gap d.
A counter electrode
3
is formed on the first transparent insulating substrate
1
. The counter electrode
3
is made of a transparent metal layer such as ITO, and the counter electrode
3
to be described afterward in detail has a comb shape including a plurality of first branches
3
a.
A gate insulating layer
5
is formed on the first transparent insulating substrate
1
so as to cover the counter electrode
3
. A pixel electrode
7
is formed on the gate insulating layer
5
. The pixel electrode
7
is made of a transparent metal layer such as ITO. Similar to the counter electrode
3
, the pixel electrode
7
to be described afterward in detail has a comb shape including a plurality of second branches
7
a.
The second branches
7
a
are disposed between the first branches
3
a
of the counter electrode
3
. The reference symbol l means a distance between the branch
3
a
of the counter electrode
3
and the branch
7
a
of the pixel electrode
7
, and herein the reference symbol l is smaller than the cell gap d i.e. the distance between the first transparent insulating substrate
1
and the second transparent insulating substrate
11
.
Color filers
14
of red, green and blue are formed on the second transparent insulating substrate
11
, and a black matrix for color separation is formed between the color filters
14
. The black matrix
12
is made of an opaque material so as to prevent light leakage.
A first alignment layer
9
and a second alignment layer
16
are formed at the uppermost portions of the first and the second transparent insulating substrates
1
,
11
respectively so that those alignment layers
9
,
16
function to align the liquid crystal molecules previous to forming electric field. Those alignment layers
9
,
16
have low pretilt angle of approximately below 2°, and they are rubbed under anti-parallel state so as to make a selected angle of degrees with respect to the direction of the electric field.
FIG. 2
is a plane view showing a conventional FFS mode liquid crystal display. As shown in the drawing, a gate bus line
2
and a data bus line
4
are disposed cross to each other to define a pixel region. A thin film transistor
10
, hereinafter TFT is disposed at a portion of intersection of the gate bus line
2
and the data bus line
4
. A counter electrode
3
is disposed within the pixel region defined by the gate bus line
2
and the data bus line
4
. The counter electrode
3
includes a plurality of first branches
3
a
and a bar
3
b
connecting one ends of the first branches
3
a.
A common signal line
100
is made of an opaque material of high conductivity so as to transmit a common signal to the counter electrode disposed at each pixel region without signal delay. The common signal line
100
includes a second branch
100
a
disposed between the data bus line
4
and the first branch
3
a
at both edges of the counter electrode
3
, and the common signal line
100
is contacted with the bar
3
b
of the counter electrode
3
. The second branch
100
a
serves to block the light leakage at a space between the first branch
3
a
at both edges of the counter electrode
3
and the data bus line
4
.
A pixel electrode
7
is disposed over the counter electrode
3
. The pixel electrode
7
includes a plurality of third branches
7
a,
a first bar
7
b
connecting one ends of the third branches
7
a
and a second bar
7
c
connecting the other ends of the third branches
7
a.
The third branches
7
a
are disposed between the first branches
3
a
of the counter electrode
3
. A black matrix
12
is disposed to prevent light leakage. Reference symbol BM
1
means an inner boundary of the black matrix
12
and BM
2
means an outer boundary of the black matrix
12
.
In the FFS mode liquid crystal display having the foregoing constructions, according to influence of the alignment layers, the liquid crystal molecules are arranged almost parallel with the substrate surface before an electric field is formed between the counter and the pixel electrodes. On the other hand, when the electric field is formed between the counter and the pixel electrodes, the electric field is formed as a fringe field having a vertical component. Then, the liquid crystal molecules are twisted such that the optical axes thereof are aligned in a direction parallel to or perpendicular to the direction of the electric field, thereby leaking light.
The FFS mode liquid crystal display obtains high aperture ratio since the counter and the pixel electrodes are made of a transparent conductive material, and the FFS mode liquid crystal display obtains high transmittance since the liquid crystal molecules over the liquid crystal display are all driven owing to the narrow distance betwee
Kim Hyang Yul
Lee Seok Lyul
Lee Seung Hee
Hyundai Display Technology Inc.
Litovchenko V
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