Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-03-07
2003-08-05
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S129000
Reexamination Certificate
active
06603524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device used for display sections of computers and OA apparatuses and the like. More specifically, the present invention relates to a liquid crystal display device which has excellent display characteristics and a high aperture ratio.
2. Description of the Related Art
Conventionally, a liquid crystal display device using an active matrix substrate is known as a display device for computers and OA apparatuses. An example of such a liquid crystal display device using an active matrix substrate is shown in FIG.
26
. The active matrix substrate in this example has thin film transistors (hereinbelow referred to as TFTs) as switching elements.
Referring to
FIG. 26
, TFTs
106
and pixel capacitors
108
are formed in a matrix on a substrate made of glass or the like. A gate electrode of each TFT
106
is connected to a corresponding gate signal line
104
, so that the TFT
106
is switched on and off in response to a signal input into the gate electrode via the gate signal line
104
. A source electrode of the TFT
106
is connected to a corresponding source signal line
102
, so that a video signal is input into the TFT
106
. A drain electrode of the TFT
106
is connected to a pixel electrode and one terminal of the corresponding pixel capacitor
108
. The other terminal of the pixel capacitor
108
is connected to a corresponding pixel capacitor line
110
and also connected to a counter electrode provided on a substrate facing the active matrix substrate.
FIG. 27
is a plan view of such an active matrix substrate,
FIG. 28
is a sectional view taken along line
28
—
28
of
FIG. 27
, and
FIG. 29
is a sectional view taken along line
29
—
29
of FIG.
27
.
Referring to
FIGS. 27 and 28
, each pixel of the liquid crystal display device includes the TFT
106
(see FIG.
26
), an extended drain electrode
125
, a storage capacitor electrode
126
, and a pixel electrode
140
. Referring to
FIG. 29
, for each pixel, the gate signal line
104
together with the gate electrode, a gate insulating film
103
, a semiconductor layer
134
, a channel protection layer
128
, an n
+
-Si layer
130
and an ITO (indium tin oxide) film
132
which together constitute the source and drain electrodes, the source signal line
102
made of a metal layer, an interlayer insulating film
136
, and the pixel electrode
140
made of a transparent conductive layer are formed in this order on a transparent insulating substrate
120
, to form the active matrix substrate. The pixel electrode
140
is connected to the drain electrode of the TFT
106
via a contact hole
142
(see
FIG. 28
) formed through the interlayer insulating film
136
.
FIGS. 28 and 29
also show a substrate
122
provided to face the active matrix substrate with a liquid crystal layer
112
interposed therebetween.
In the active matrix substrate with the above configuration, the interlayer insulating film
136
is formed between the gate signal line
104
or the source signal line
102
and the pixel electrode
140
. This allows the periphery of the pixel electrode
140
to overlap the signal lines
102
and
104
. As a result, a liquid crystal display device with a high aperture ratio can be obtained. Moreover, the overlapping pixel electrode
140
shields an electric field generated due to the potential at the signal lines, effectively suppressing failure in the orientation of liquid crystal molecules.
Referring to
FIGS. 28 and 29
, a light-shading layer
144
, and color layers
146
exhibiting red, blue, or green constituting a color filter are formed on the substrate
122
facing the active matrix substrate with the liquid crystal layer
112
therebetween. A counter electrode
148
and an alignment film
150
are formed in this order on the color filter. Another alignment film
150
is formed on the surface of the active matrix substrate in contact with the liquid crystal layer
112
.
FIG. 30A
is an enlarged plan view of a portion of
FIG. 27
where the gate signal line
104
and the source signal line
102
cross each other.
FIG. 30B
is a sectional view taken along line
30
B—
30
B of
FIG. 30A
, showing the overlap portions of the pixel electrodes
140
on the source signal line
102
.
Referring to
FIG. 30A
, the vertically adjacent pixel electrodes
140
overlap the corresponding gate signal line
104
by overlap widths dg1 and dg2, while the horizontally adjacent pixel electrodes
140
overlap the corresponding source signal line
102
by overlap widths ds1 and ds2. These overlap widths are generally determined in consideration of the processing precision of the gate signal lines
104
and the source signal lines
102
which serve as light-shading films, the overlap precision of the pixel electrodes
140
on the gate signal lines
104
and the source signal lines
102
, and the processing precision of the pixel electrodes
140
. Conventionally, the pixel electrodes
140
overlap the gate signal lines
104
and the source signal lines
102
so that the overlap widths dg1 and dg2 are equal to each other and the overlap widths ds1 and ds2 are equal to each other.
The liquid crystal display device where the pixel electrodes overlap the signal lines as described above causes no problem as far as it is driven by a flame inversion driving method. However, when such a liquid crystal display device is driven by a gate line inversion driving method, a source line inversion driving method, or a dot inversion driving method, the following problem arises. That is, referring now to
FIG. 30B
, the orientation of liquid crystal molecules
152
a
is disturbed due to an electric field generated between the adjacent pixel electrodes, generating a reverse tilt domain having liquid crystal molecules
152
b
which have a reverse pretilt angle, i.e., are oriented in the opposite direction of an orientation direction D
1
(see
FIG. 30A
) of the liquid crystal molecules
152
a
. The generation of such a reverse tilt domain causes light leakage and thus eminently degrades the display characteristics of the resultant liquid crystal display device.
In order to prevent light leakage of the liquid crystal display device due to the disturbance of the orientation of liquid crystal molecules, increasing the overlap widths of the pixel electrodes on the gate signal lines and the source signal lines is known. Increasing the overlap widths, however, causes another problem of increasing the occupation of the light-shading portions in the liquid crystal display device and thus decreasing the aperture ratio.
Also known is a liquid crystal display device where each pixel is divided into two portions having different orientation directions D
2
and D
2
of liquid crystal molecules as shown in
FIGS. 31A
to
31
C.
FIG. 31A
is a plan view of a portion of such a liquid crystal display device where a gate signal line
104
and a source signal line
102
cross each other.
FIG. 31B
is a sectional view taken along line
31
B—
31
B of
FIG. 31A
, and
FIG. 31C
is a sectional view taken along line
31
C—
31
C of FIG.
31
A.
In such a liquid crystal display device, also, pixel electrodes conventionally overlap signal lines so that overlap widths dg1 and dg2 are equal to each other and overlap widths. ds1 and ds2 are equal to each other as shown in FIG.
31
A. This causes no problem as far as the liquid crystal display device is driven by a flame inversion driving method. However, as in the above case, when it is driven by a gate line inversion driving method, a source line inversion driving method, or a dot inversion driving method, the following problem arises. That is, the orientation of liquid crystal molecules
152
a
are disturbed due to an electric field generated between the adjacent pixel electrodes, generating a reverse tilt domain having liquid crystal molecules
152
b
which have a reverse pretilt angle as shown in
FIGS. 31B and 31C
. This causes light leakage and thus eminently degrades the display characteristics of the resultant
Ohgami Hiroyuki
Sakuhana Yoshikazu
Shimada Takayuki
Shimada Yoshinori
Nixon & Vanderhye P.C.
Parker Kenneth
Sharp Kabushiki Kaisha
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