Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-07-31
2003-05-20
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S046000, C349S110000
Reexamination Certificate
active
06567135
ABSTRACT:
CROSS REFERENCE
This application claims the benefit of Korean Patent Application No. 1999-31487, filed on Jul. 31, 1999, under 35 U.S.C. § 119, the entirety of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display device having a thin film transistor (TFT) and a method of manufacturing the same.
2. Description of Related Art
A typical liquid crystal display device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in arrangement resulting from their thin and long shapes. The arrangement direction of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the arrangement direction of the liquid crystal molecules is changed, the arrangement of the liquid crystal molecules also changes. Since Incident light is refracted to the arrangement direction of the liquid crystal molecules due to the optical anisotropy of the arranged liquid crystal molecules image data can be displayed.
By now, an active matrix LCD that the thin film transistors and the pixel electrodes are arranged in the form of a matrix is most attention-getting due to its high resolution and superiority in displaying moving video data.
FIG. 1
is a cross-sectional view illustrating a pixel of a conventional liquid crystal display device.
The liquid crystal display device
20
has a bottom substrate
2
and a top substrate
4
spaced apart from each other. The liquid crystal display device further includes a liquid crystal layer
10
is injected between the two opposite substrates
2
and
4
. The top substrate
4
has a color filter to display colors, and the bottom substrate
2
has switching elements such as thin film transistors (TFTs) that applies electrical signals to the liquid crystal layer
10
to change the arrangement direction of the liquid crystal molecules of the liquid crystal layer
10
. Each of the TFTs “S” has a gate electrode
30
, a source electrode
32
and a drain electrode
34
.
In detail, the top substrate
4
further includes a common electrode
12
covering the color filter layer
8
. The common electrode
12
plays a role of the first electrode to supply a voltage to the liquid crystal layer
10
. The bottom substrate
2
further includes a pixel electrode
14
. The pixel electrode
14
is electrically connected with the drain electrode
34
of the TFT “S”. The pixel electrode
14
receives electrical signals from the thin film transistor “S”, and plays a role of the second electrode to supply voltage to the liquid crystal layer
10
. A portion, on which the pixel electrodes
14
are formed, is defined as a pixel electrode portion “P”. In order to prevent leakage of the liquid crystal layer
10
disposed between the top substrate
4
and the bottom substrate
2
, edge portions of the top substrate
4
and the bottom substrate
2
are sealed by a sealant
6
.
Recently, as the display area of the liquid crystal display device becomes larger, the fabricating process of the bottom substrate
2
becomes complicated. That is to say, for a liquid crystal display device having over 12 inch display area, a step-and-repeat exposure technique is applied to fabricating the bottom substrate. The step-and-repeat exposure technique is to perform at least two exposing steps with the same patterned mask. The reason for the step and repeat technique to be applicable to the bottom substrate is that the patterns formed on the bottom substrate are repeats of the same form.
Referring to the
FIGS. 2 and 3
, a batch exposure technique and the step-and-repeat exposure technique are explained as follows. Referring to the
FIG. 2
showing a patterning mask, in the batch exposure technique, a display area A, data pad portions D and E and gate pad portions B and C surrounding the display area “A” are formed at one time with the patterning mask.
The batch exposure technique is just applicable to the bottom substrate of the liquid crystal display device having a less than 10 inch-sized display area. Namely, in case of the bottom substrate of the liquid crystal display device having a larger than 10 inch-sized display area, the batch exposure technique is useless due to the diffraction of light incident from an exposure apparatus.
Referring to the
FIG. 3
illustrating the step-and-repeat exposure technique, the display area is formed into a plurality of neighboring display exposure regions like A
1
, A
2
, . . . , A
9
, sequentially. Each of the display exposure regions has an identical image projected onto itself with a same display patterning mask.
By the same technique, the data pad portions are formed into a plurality of neighboring data exposure regions like D
1
, D
2
, D
3
and E
1
, E
2
E
3
having an identical image, sequentially. And the gate pad portions are formed into a plurality of neighboring gate exposure regions like B
1
, B
2
, B
3
, C
1
, C
2
and C
3
having an identical image, sequentially.
The above-mentioned step-and-repeat exposure technique is more widely used than the batch exposure technique as an exposure method.
But, to fabricate the liquid crystal display device using the step-and-repeat exposure technique may give rise to a serious degradation of image quality at the display area. The reason is that the step-and-repeat exposure technique needs at least over 40 processes of photolithography. Comparing with the step-and-repeat exposure technique, the batch technique needs at least just 5 processes of photolithography. Thus, no matter how accurate exposure equipment and arrangement apparatus are used for the step-and-repeat exposure technique, it may give rise to misalignment between the exposure regions.
For example, as shown in
FIG. 4
, the display exposure regions A
7
and A
8
, the display exposure region A
7
includes a pixel electrode
71
, a half of a data line
60
, and a half of a data line
61
. The display exposure region A
8
includes a pixel electrode
72
, a half of a data line
61
, and a half of a data line
62
. The display exposure regions A
7
and A
8
include the data line
61
in common, and are divided by an imaginary boundary line
50
. That is to say, the display exposure regions A
7
and A
8
differ in an exposed order with the imaginary boundary line
50
centering on between the display exposure regions A
7
and A
8
. That difference in the exposed order may bring out a difference in distances between the pixel electrodes
71
and
72
and the data lines
60
,
61
and
62
. Since the exposure equipment or the arrangement apparatus has an accuracy limitation of itself, misalignment between the exposure regions may occur. The misalignment may result in shift, rotation and distortion of the patterns, thereby causing defects such as disconnection of the wirings and differences in electrical properties between the exposure regions.
Namely, the distance between the pixel electrode
71
and the data line
60
is different from the distance between the pixel electrode
71
and the data line
61
. And, the distance between the pixel electrode
72
and the data line
61
is different from the distance between the pixel electrode
72
and the data line
62
. The pixel electrodes
71
and
72
are the pixel portions P
1
and P
2
, respectively.
In other words, fabricating the thin film transistor by the step-and-repeat exposure technique, it may bring about spotted effects near the boundaries of the neighboring display exposure regions resulted from the sudden difference in the distance between the pixel electrodes and the data lines at each exposure region.
In case of manufacturing the large-sized liquid crystal display device using the step-and-repeat exposure technique, driving the liquid crystal display device by a dot inversion method, it brings about the difference in parasitic capacitance Cdp between the data line and the right and left pixe
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
Nguyen Hoan
Ton Toan
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