Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2003-07-02
2004-09-28
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S042000, C349S139000, C349S162000, C257S059000, C257S072000
Reexamination Certificate
active
06798466
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a liquid crystal display (LCD) device, and to a method for fabricating the LCD device.
2. Description of the Related Art
The LCD device has been used as a display device of a data processing apparatus, such as a desktop computer, a notebook computer, and as a display terminal of a television.
FIG. 1
shows an array formed on one substrate of a conventional LCD device. In
FIG. 1
, a plurality of gate lines, a plurality of data lines, which are arranged in a perpendicular direction to the gate lines, and a plurality of pixels, each of which is formed at an intersection of the gate line and the data line, are formed on one glass substrate, for example, a lower glass substrate
2
shown in FIG.
2
. Only four data lines (e.g., D
1
through D
4
), and only four gate lines (e.g., G
1
through G
4
) are shown in FIG.
1
. The pixel includes a thin film transistor (TFT)
7
and a capacitor
8
. The gate line is connected to a gate of the TFT
7
, the data line is connected to a drain of the TFT, a source of the TFT is connected to one terminal of the capacitor, and the other terminal of the capacitor is connected to a reference potential.
A data line driver is connected to the data lines to apply the data pulses to the data lines, and a gate line driver is connected to the gate lines to sequentially apply the gate pulses to the gate lines. During the application of the gate pulse to one gate line, such as the gate line G
1
, the data line driver applies the data pulses to the data lines to display the image.
FIG. 2
shows a cross sectional structure of the conventional LCD device
1
along dotted line
2
A-
2
B in FIG.
1
. The conventional LCD device
1
includes a lower glass substrate
2
, an upper glass substrate
3
, a lower polarizer plate
4
, an upper polarizer plate
5
and a backlight device
6
. The data lines D
1
through D
4
and the gate lines G
1
through G
4
are formed on the lower glass substrate
2
, but the gate lines are not shown in FIG.
2
. The upper terminals or a display electrode of the capacitor
8
, which is made of Indium Tin Oxide (ITO) layers
9
, is formed in an area between the data lines. Passivation layers
10
are formed to cover the data lines and to isolate the ITO layers from the data lines.
An alignment layer
11
is formed to cover the entire structure. Black matrices
12
are formed on the upper glass substrate
3
to face the data lines, respectively. Red, green and blue color filters
13
are formed to face the ITO layers
9
, respectively. An insulating layer
14
is formed on the color filters
13
to provide a flat surface. The ITO layer
15
, which is called as a common electrode for operating as the lower electrode of the capacitor
8
shown in
FIG. 1
, is formed on the insulating layer
14
. An alignment layer
16
is formed on the ITO layer
15
. A twisted nematic liquid crystal is sandwiched between the lower alignment layer
11
and the upper alignment layer
16
. A length L
1
represents an aperture size defined by the adjacent black matrices
12
, and a length L
2
represents an overlap of the black matrix
12
and the ITO layer
9
.
FIG. 3
shows a normally white mode operation of the LCD device
1
. The backlight device
6
generates a white light. The polarizer plate
4
has a polarizing plane as shown by the vertical lines and passes the light parallel to the polarizing plane. The alignment layer
11
is rubbed in the vertical direction, the alignment layer
16
is rubbed in the horizontal direction, and the polarizer plate
5
has a polarizing plane in the horizontal direction. It is noted that the ITO layers
9
and
15
are not shown to simplify the drawing. FIG.
3
(A) shows the case in which the voltage is not applied across the pixel electrode (e.g., the ITO layer
9
), and the common electrode (e.g., the ITO layer
15
through the TFT
7
), shown in
FIG. 1
, so that liquid crystal molecules
17
is twisted by 90 degrees between the pixel electrode
9
and the common electrode
15
. In this case, the polarized light passing through the polarizer plate
4
is rotated by the 90 degrees through the twisted liquid crystal molecules
17
, and passes through the polarizer plate
5
, so that the white image is displayed.
When the voltage is applied across the ITO layer
9
and the ITO layer
15
, the liquid crystal molecules
17
are aligned along the electric field, as shown in FIG.
3
(B), so that the polarized light from the polarizer plate
4
passes through the liquid crystal molecules
17
without being rotated, whereby the polarized light is shut off by the polarizing plate
5
, and the black image is displayed. In this manner, the operation mode, in which the white image is displayed when the voltage is not applied across the pixel electrode
9
and the common electrode
15
, is called as the normally white mode.
However, the following problems are found in the conventional LCD device.
FIG. 4
shows a first problem in the conventional LCD device due to a dot defect or a line defect. The dot defect means that the pixel, for example the pixel P
11
, becomes inoperative since the gate electrode of the TFT is cut at a portion A. In the conventional technology, an additional connection B is formed to directly connect the data line D
1
to the display electrode of the pixel P
11
. This technology, however, causes the following additional problem.
When the gate pulse is applied to the gate line G
1
to activate the pixels connected to the gate line G
1
, the pixel P
11
is applied with the data pulse on the data line D
1
. In this case, the pixel P
11
display the correct image since the data line D
1
is connected to the pixel P
11
through the connection B. But, when the pixel P
11
displays the white image, as shown in FIG.
3
(A) and the pixel P
31
displays the black image, as shown in FIG.
3
(B), the data pulse for displaying the black image on the pixel P
31
is also applied to the pixel P
11
through to the direct connection B between the data line D
1
and the pixel P
11
, so that the pixel P
11
displays the black image or the wrong image.
The line defect means that the gate line, for example the gate line G
1
, is cut at a portion C so that the horizontal pixels succeeding to the pixel P
12
always display the white image, or that the data line, for example the data line D
2
is cut at a portion D so that the vertical pixels succeeding to the pixel P
22
always display the white image. It has been difficult to repair the line defect in the conventional art.
A second problem in the conventional LCD device is that when it is desired to realize a high resolution image display, it is necessary to increase the size of the glass substrates for the following reasons. The increase of the resolution has been accomplished by increasing the number of pixels. The increase of the pixels means the increase of the number of data and gate lines which dissipate a large area on the glass substrate, so that the aperture size, through which the light passes, becomes small, and the displayed image becomes dark. To solve the problem of the dark image, the size of the glass substrates is increased, whereby the aperture size is increased. But, the increased size of the glass substrates causes a new problem in that the length of the data line and the gate lines is increased, so that a voltage drop along the data line and the gate line is increased, whereby luminance of each pixel along the data line and the gate line is gradually decreased. To solve the gradual decrease of the luminance, a cross sectional area of the data line and the gate line must be increased, or the data line and the gate line made of a high electrically conductive material must be used. These technical changes, however, require a development of a new fabrication process.
A third problem in the conventional LCD device is achieving a wide viewing angle with a good image quality. To realize the wide viewing angle, a technology called an In-Plane switching (IPS) mode had
Kitahara Hiroaki
Takeichi Masatomo
International Business Machines - Corporation
McGinn & Gibb PLLC
Nguyen Hoan C.
Ton Toan
Trepp, Esq. Robert M.
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