Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-10-29
2004-11-02
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000
Reexamination Certificate
active
06812987
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an in-plane switching liquid display (IPS-LCD), more particularly, the present invention relates to an IPS-LCD with a compensation structure for critical dimension (CD) variation.
2. Description of the Related Art
Liquid crystal displays (LCDs) may be classified by the orientation of the liquid crystal molecules between the spaced glass substrates. In a conventional twisted nematic LCD (TN-LCD), the liquid crystal molecules are twisted between the two substrates. In contrast, in an in-plane switching LCD (IPS-LCD), common electrodes and pixel electrodes are formed on a lower glass substrate (TFT substrate) and an in-plane electrode field therebetween is generated to rearrange the liquid crystal molecules along the electrode field. Accordingly, the IPS-LCD has been used or suggested for improving drawbacks of the conventional TN-LCD, such as a very narrow viewing angle and a low contrast ratio.
FIGS. 1A and 1B
are sectional diagrams of a conventional IPS-LCD, in which
FIG. 1A
shows the alignment of the liquid crystal molecules in an off state and
FIG. 1B
shows the alignment of the liquid crystal molecules in an on state. The IPS-LCD has a lower glass substrate
10
, an upper glass substrate
12
, and a liquid crystal layer
14
disposed in a spacing between the two parallel glass substrates
10
and
12
. On the lower glass substrate
10
, serving as a TFT substrate, a plurality of strip-shaped common electrodes
16
is patterned on the lower glass substrate
10
, an insulating layer
18
is deposited on the common electrodes
16
and the lower glass substrate
10
, and a plurality of strip-shaped pixel electrodes
20
is patterned on the insulating layer
18
.
As shown in
FIG. 1A
, before an external voltage is applied to the IPS-LCD, the negative liquid crystal molecules
14
A are aligned in a direction parallel to the lower glass substrate
10
. As shown in
FIG. 1B
, when an external voltage is applied to the IPS-LCD, an in-plane electric field is generated between the common electrode
16
and the pixel electrode
20
, resulting in a rotation of the liquid crystal molecules
14
B toward the in-plane electric field.
Generally, the common electrode
16
and the pixel electrode
20
are formed on the same or different layers and arranged apart from each other by a predetermined distance, known as “spacing”. For example,
FIG. 2
shows a cross-section of a glass substrate having common electrodes and pixel electrodes thereon. The common electrodes
16
and the pixel electrodes
20
have a width of about 4.0 &mgr;m. The common electrodes
16
in the edge have a width of about 8.0 &mgr;m. Each spacing between a respective common electrode
16
and a respective pixel electrode
20
is about 9.0 &mgr;m in the same pixel and the adjacent pixel.
However, critical dimension (CD) variation is easily generated during formation of the common electrodes
16
and the pixel electrodes
20
caused by many parameters such as different substrate flatness, different resist thickness, and different etching recipe.
FIG. 3
is a top view showing muras on an IPS-LCD panel caused by CD variation at area B. The IPS-LCD panel
100
having area A and area B is disposed in an outer frame
102
. A plurality of muras
104
, curved spots, are generated on the panel
100
caused by localized CD variation.
Next,
FIG. 4
shows a more detailed diagram to explain muras caused by CD variation and shows a pixel array including area A and area B having CD variation according to the prior art.
As shown in area A of
FIG. 4
, the pixel array comprises a plurality of small rectangles having the same numeral (
10
.
00
). Each small rectangle denotes one unit pixel that has parallel pixel electrodes
20
and parallel common electrodes
16
positioned such that a respective pixel electrode
20
is disposed adjacent and parallel to a respective common electrode
16
. The numeral (
10
.
00
) in one small rectangle represents the spacing between any adjacent common electrode
16
and pixel electrode
20
. The spacing between any adjacent common electrode
16
and pixel electrode
20
in the same pixel is equal to that of the adjacent pixel. For example, the spacing between any adjacent common electrode
16
and pixel electrode
20
is 10.00 &mgr;m in the pixel
30
.
Turning now to area B of
FIG. 4
, area B shows a pixel array, having spacing CD variation of about 0.30 &mgr;m. The pixel array comprises a plurality of small rectangles having numeral (
10
.
30
) respectively. Each small rectangle denotes one unit pixel that has parallel pixel electrodes
22
and parallel common electrodes
28
positioned such that a respective pixel electrode
22
is disposed adjacent and parallel to a respective common electrode
28
. The numeral (
10
.
30
) in one small rectangle represents the spacing between any adjacent common electrode
28
and pixel electrode
22
. The spacing between any adjacent common electrode
28
and pixel electrode
22
in the same pixel is equal to that of the adjacent pixel. For example, the spacing between any adjacent common electrode
28
and pixel electrode
22
is 10.30 &mgr;m in the pixel
40
.
FIG. 5
is a three-dimensional diagram showing transmittance difference between area A and area B according to the prior art. In
FIG. 5
, Z-axle represents transmittance (%), X-axle and Y-axle mean pixel unit of the pixel array of
FIG. 4
including area A and area B.
FIG. 5
shows obvious transmittance difference between area A and area B so that an observer can perceive the apparent luminance difference.
Therefore, improved IPS-LCD panels formed on an active matrix substrate with a compensation structure for CD variation are needed.
SUMMARY OF THE INVENTION
In view of the above disadvantages, an object of the invention is to provide an IPS-LCD with a compensation structure for CD variation. According to the IPS-LCD, the transmittance difference between the two pixels can be reduced.
In accordance with one aspect of the invention, there is provided an IPS-LCD with a compensation structure for CD variation. The IPS-LCD comprises a first substrate; a first pixel, formed on the first substrate, having first parallel pixel electrodes and first parallel common electrodes positioned such that a respective pixel electrode is disposed adjacent and parallel to a respective common electrode; and a second pixel adjacent to the first pixel, wherein the second pixel has second parallel pixel electrodes and second parallel common electrodes positioned such that a respective pixel electrode is disposed adjacent and parallel to a respective common electrode. This LCD is characterized in that each spacing between any adjacent first common electrode and first pixel electrode is equal and has a first distance, each spacing between any adjacent second common electrode and second pixel electrode is equal and has a second distance different from the first distance. Furthermore, the IPS-LCD comprises a second substrate being opposed to the first substrate and a liquid crystal material being interposed between the first substrate and the second substrate.
In accordance with another aspect of the invention, the difference between the first distance and the second distance is preferably about 0.25* X &mgr;m, X=1, 2, 3, or 4. That is to say, the difference between the first distance and the second distance is 0.25, 0.50, 0.75 or 1.00 &mgr;m.
Furthermore, the first parallel pixel electrodes and the first parallel common electrodes are separately formed on the different layers. Otherwise, the first parallel pixel electrodes and the first parallel common electrodes can be formed on the same layer.
Furthermore, the first distance and the second distance can be about 10.00 &mgr;m to 11.30 &mgr;m, for example 10.00 &mgr;m, 10.25 &mgr;m, 10.50 &mgr;m, 10.75 &mgr;m, or 11.00 &mgr;m.
In accordance with a further aspect of the invention, there is provided an IPS-LCD with a compensation structure for CD variation. The IPS-LCD comprises a pluralit
Chowdhury Tarifur R.
Hannstar Display Corp.
Rabin & Berdo P.C.
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