Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-12-10
2004-11-23
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06822716
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to an in-plane switching liquid crystal display (IPS-LCD). In particular, the present invention relates to an IPS-LCD with an alignment free structure and a method of using back exposure to form the same.
2. Description of the Related Art
Liquid crystal displays (LCDs) may be classified by the orientation of the LC molecules between the spaced apart glass substrates. In a conventional twisted nematic LCD (TN-LCD), the LC 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 electric field therebetween is generated to rearrange the LC molecules along the electric 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.
In order to achieve a better result of the in-plane electric field, a comb-shaped electrode array is built in the IPS-LCD to solve the problems such as an insufficient aperture ratio and crosstalk produced between data lines and common electrodes.
FIGS. 1A and 1B
are sectional diagrams of a conventional IPS-LCD.
FIG. 1A
shows the alignment of the LC molecules at an off state, and
FIG. 1B
shows the alignment of the LC molecules at 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 space 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
arranged as a comb-shape structure 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
arranged as a comb-shape structure is patterned on the insulating layer
18
.
As shown in
FIG. 1A
, before an external voltage is applied to the IPS-LCD, the LC 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-plain electric field
22
is generated between the common electrode
16
and the pixel electrode
20
, resulting in a rotation of the LC molecules
14
B toward the in-plane electric field
22
.
Depending on the material and the structure design of the common electrode
16
and the pixel electrode
20
, the conventional comb-shaped electrode array is classified as three types.
FIGS. 2A
to
2
C are sectional diagrams showing three types of the common electrode
16
and the pixel electrode
20
in the conventional comb-shaped electrode array. In the first type, as shown in
FIG. 2A
, the common electrode
16
and the pixel electrode
20
are patterned on the same plane and made of a transparent conductive material, such as ITO or IZO. In the second type, as shown in
FIG. 2B
, the common electrode
16
made of a non-transparent conductive material, such as Al and MoW, is patterned on the lower glass substrate
10
followed by depositing the insulating layer
18
, and then the pixel electrode
20
made of a transparent conductive material, such as ITO or IZO, is patterned on the insulating layer
18
. In the third type, as shown in
FIG. 2C
, the common electrode
16
and the pixel electrode
20
are patterned on the same plane and made of a non-transparent conductive material, such as Al and MoW.
FIG. 3
is a simulation result of the optical characteristics of opaque electrodes (Al) and transparent electrodes (ITO). The transmittance is estimated 1.25 times when one of the pair electrodes is transparent, and 1.5 times when both of the pair electrodes are transparent respectively. The first type (
FIG. 2A
) can provide a greater luminance to the IPS-LCD than the second type (
FIG. 2B
) and the third type (FIG.
2
C). The first type (FIG.
2
A), however, provides a worsen view-angle characteristic than the second type and the third type. Also, the third type severely decreases the luminance of the IPS-LCD because most of the light is blocked by the non-transparent conductive material. Therefore, the second type (
FIG. 2B
) is the most common type used in the conventional comb-shaped electrode array.
FIGS. 4A
to
4
C show the response characteristics of each electrode type corresponding to the first type (FIG.
2
A), the second type (
FIG. 2B
) and the third type (FIG.
2
C), respectively. The frame frequency dependence on the flicker property are observed in the second type, but not observed in other types. This means that LC molecules move on transparent electrodes and the behavior influences that optical characteristics. Also, as shown in
FIG. 4B
, the luminance variation is found in the second type. This phenomenon is caused by flexo-electro polarity when a voltage is applied to the electrodes, resulting in the offset flicker.
FIG. 5A
is a top view showing an electrode array within a pixel area of an IPS-LCD according to the prior art, and
FIG. 5B
is a sectional view along line I—I of
FIG. 5A
showing the electrode array of the IPS-LCD according to the prior art. The conventional IPS-LCD
1
has a plurality of pixel areas arranging in a matrix form and constituted by a plurality of gate lines
2
and data lines
4
. Each pixel area comprises a TFT structure
6
, a comb-shaped common electrode structure
16
and a comb-shaped pixel electrode structure
20
. The comb-shaped common electrode structure
16
comprises a bar
16
a
and three teeth
16
b
, and the comb-shaped pixel electrode structure
20
comprises a bar
20
a
and two teeth
20
b
. By using the second type as shown in
FIG. 2B
, the teeth
16
b
and the teeth
20
a
of different transmittance materials are patterned on different planes.
However, since misalignment in the photolithography process is not easily controlled, it is possible to form different intervals between the common electrodes
16
and the pixel electrodes
20
on the electrode array, resulting in different capacitances and transmittances.
FIG. 6
is a simulation result of the misalignment effect. In this practical case, demerits such as trip mura, shot mura and flicker are commonly found in the conventional IPS-LCD.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an IPS-LCD with an alignment free structure and a method of using back exposure to form the same. This alignment free structure can solve the demerits of shot mura and flicker found in the prior art.
In each pixel area of the IPS-LCD with an alignment free structure, at least one floating metal layer is disposed between two common electrodes and patterned on the same plane with the common electrodes, and at least one pixel electrode is disposed between the two common electrodes and covers the floating metal layer. The center of the pixel electrode is aligned to the center of the floating metal layer, and each interval between two adjacent common electrode and pixel electrode is fixed at a constant.
A method of forming an in-plane switching liquid crystal display with an alignment free structure, comprises steps of: providing a glass substrate; forming a plurality of gate lines extending in a first direction on the glass substrate; forming a comb-shaped common electrode structure within each predetermined pixel area, wherein the comb-shaped common electrode structure comprises a common bus line parallel to the gate line and at least two common electrodes extending in a second direction that is perpendicular to the first direction; forming a floating metal pattern within each predetermined pixel area, wherein the floating metal pattern comprises at least one floating metal layer extending in the second direction between the two common electrodes; forming an insulating layer to cover the gate lines, the comb-shaped common electrode structure, the floating metal pattern and glass substr
Jen Tean-Sen
Lee Deuk Su
Hannstar Display Corp.
Kim Richard H
Thomas Kayden Horstemeyer & Risley
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