Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2003-02-24
2004-05-25
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06741305
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color display device such as a color liquid crystal display device. A color display device of the present invention can be used in a direct-view type of liquid crystal display such as a liquid crystal color television, or a projection type of liquid crystal display such as a liquid crystal projector.
2. Description of the Background Art
An example of a color display device is a color liquid crystal display device including an active matrix substrate on which a plurality of active elements such as TFTs are formed, a color filter substrate on which color filter layers of different colors and a common electrode are deposited in this order, and a liquid crystal layer interposed between the substrates. The active matrix substrate includes a plurality of pixel electrodes arranged in a matrix pattern, and the plurality of pixel electrodes define a display area of the display device. Moreover, the active matrix substrate includes scanning signal lines and video signal lines, which are connected respectively to the plurality of pixel electrodes via TFTs. The scanning signal lines and the video signal lines extend out of the display area into an area outside the display area. The area outside the display area is called “frame area” or “outer frame area”.
Elements formed in the frame area of the active matrix substrate include a scanning driver connected to the scanning signal lines, a data driver connected to the video signal lines, signal input terminals connected to these drivers, a plurality of common transition electrodes for supplying a voltage from the signal input terminals to the common electrode on the color filter substrate. The common transition electrodes are connected to the common electrode in the frame area via a conductive paste.
However, the common electrode is made of a transparent conductive material such as ITO, and thus has a greater electric resistance than that of an electrode made of a metal material such as aluminum. Therefore, a brightness gradient, a display non-uniformity, or the like, may occur due to, for example, a delay in the polarity inversion signal, and these problems are more likely to occur in large liquid crystal display devices.
In order to prevent the delay in the polarity inversion signal, it is necessary to reduce the resistance of the common electrode. One possible way to reduce the resistance of the common electrode is to increase the thickness of the common electrode. With such an approach, although the resistance is reduced, the tact time increases. Therefore, the production efficiency decreases, and the production cost increases as the tact time increases. In addition, since the transparent electrode is not completely transparent, the transmittance of the entire screen decreases as the thickness thereof increases.
Another possible way is to increase the number of contact points between the common electrode and the conductive paste. With such an approach, however, the tact time, and thus the production cost, increases, and the number of common transition electrodes formed on the active matrix substrate also increases. In recent years, there is a strong demand for reducing the frame area of display devices, i.e., a demand for increasing, as much as possible, the display area observed by the user while reducing, as much as possible, the frame area around the display area, which does not contribute to the display. When the number of contact points between the common electrode and the conductive paste is increased, the frame area increases, thereby failing to meet the demand for display devices with a reduced frame area. Therefore, increasing the thickness of the common electrode leads to other problems, and the maximum number of common transition electrodes is limited.
The present inventors have obtained the following findings during the development of a color display device that allows for both a reduction in the resistance of the common electrode and a reduction in the frame area.
In the frame area of the color filter substrate, a black matrix (also called “black mask”) is formed for the purpose of improving the contrast and the display quality. The black matrix is provided for expressing the same black color as that in a black display, and is formed by patterning a black paste layer or a metal layer. Moreover, Japanese Laid-Open Patent Publication No. 2000-249823 and Japanese Laid-Open Patent Publication No. 2000-352939, for example, disclose forming color filter layers of different colors used in the display area so that color filter layers of at least two colors extend in the frame area in order to reduce the cost by eliminating the black matrix production step.
FIG. 7A
is a diagram illustrating an important part of a color filter disclosed in Japanese Laid-Open Patent Publication No. 2000-249823.
FIG. 7B
is a cross-sectional view taken along line a-b in
FIG. 7A
, and
FIG. 7C
is a cross-sectional view taken along line c-d in FIG.
7
A. In a display area
14
a,
color filter layers
17
,
18
,
19
of three primary colors (RGB) are provided in a stripe arrangement. Moreover, in a frame area
14
b
, the red (R) and blue (B) color filter layers
17
and
18
are provided also in a stripe arrangement.
When the color filter layers are formed by a DFL (Dry Film Lamination) method, dry films of different colors are, one by one, pressed against the substrate while being heated. Since a dry film is typically about 1 &mgr;m to 2 &mgr;m thick, bubbles may occur between a new dry film and an already-formed dry film pattern depending on the resiliency of the dry film to be pressed. In view of this, a gap of 2 &mgr;m or more is provided between dry films of different colors, and a black matrix
16
is formed in the gap. Moreover, the black matrix
16
is patterned so that it also extends along the boundary between the display area
14
a
and the frame area
14
b.
However, it has been found that the color filter has the following problem. As illustrated in
FIG. 7C
, there is a step between the color filter layer and the black matrix
16
due to the presence of the black matrix
16
between the display area
14
a
and the frame area
14
b.
Therefore, the thickness of the common electrode
11
formed on the color filter layers is non-uniform around the step, thereby increasing the resistance of the common electrode
11
between the display area
14
a
and the frame area
14
b.
This phenomenon occurs not only when color filters are provided in the frame area, but also when the entire frame area is covered with a black matrix.
Moreover, also in the stripe arrangement in the frame area
14
b,
the thickness of the common electrode
11
is non-uniform, thereby increasing the resistance of the common electrode
11
, due to a step between the colored layers
17
and
18
and the black matrix
16
, as illustrated in FIG.
7
B. Therefore, in the frame area, particularly in a common transition area where a conductive paste is applied, a potential non-uniformity occurs, thereby lowering the display quality.
Even if only the color filter layers of different colors are provided, without providing the black matrix
16
, in the frame area
14
b
, a dry film being pressed may overlap with the already-formed dry film pattern to cause a step, thereby increasing the resistance of the common electrode
11
. In addition, the color filters may overlap with one another, thereby varying the tint due to color mixing.
Furthermore, the present inventors found that the phenomenon is more pronounced when the color filter layer has an irregular surface configuration.
SUMMARY OF THE INVENTION
A color display device according to a first aspect of the present invention includes: a first substrate and a second substrate opposing each other via a display medium layer interposed therebetween; a plurality of pixel electrodes arranged in a matrix pattern on one side of the first substrate that is closer to the display medium layer; a common electrode formed on one side of the second substrate
Nakagawa Hidetoshi
Nishino Takahiro
Dudek James A.
Nixon & Vanderhye P.C.
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