Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1996-10-09
2001-01-16
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000, C349S110000, C349S151000
Reexamination Certificate
active
06175395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device.
2. Description of Related Art
Active matrix liquid crystal display devices are known as being effective for high-quality display. They are constructed such that thin-film transistors are formed on a transparent substrate (usually a glass or quartz substrate) for respective pixels. Each thin-film transistor controls charge that enters or exits from an electrode (pixel electrode) of the associated pixel. The active matrix liquid crystal display, devices require circuits (peripheral circuits) for driving the thin-film transistors for the respective pixels. In general, the peripheral circuits are constructed as an external IC circuit called a driver IC.
In an advanced version, the peripheral circuits formed by using thin-film transistors are integrated on the substrate. Providing a unified structure in which the pixel region and the peripheral circuit regions are integrated on the same substrate, this configuration facilitates the use of a liquid crystal panel.
As an example of application of the above liquid crystal panel, a projection-type liquid crystal display apparatus will be described below.
A first method of performing color display is to form color filters of R (red), G (green), and B (blue) in a liquid crystal panel. A second method is to prepare a plurality of panels and combine images formed by those panels. In recent years, with an increasing need for large screen display, the second method is used more frequently to implement a projection-type display apparatus, because in the first method the substrate size needs to be increased and hence it is difficult to manufacture a panel. The second method is disclosed in Japanese Utility Model Laid-Open No. 58-111580.
In the second method, to combine images, the consistency of optical axes is important. Conventionally, liquid crystal panels are arranged independently and the modulating of optical axes is performed by adjusting the position and orientation of each panel in a subtle manner. However, this is not preferable because it causes a cost increase and complicates the structure of the apparatus. There is known a further technique in which the same images are superimposed on each other to increase the screen size or the brightness. However, this technique has a problem of cost increase because it complicates the apparatus structure.
To solve the above problems, attempts have been made to integrate the three panels into a single panel. In this case, it is basically sufficient to generate a set of images corresponding to three colors of R, G and B. The brightness can be increased by generating two or more sets of images corresponding to R, G and B.
In this type of configuration, in forming peripheral driver circuit regions, it has been attempted to locate peripheral circuits that should be integrated at a high density at positions as close to the center of a substrate as possible, to increase a final production yield.
However, the above conventional liquid crystal display devices have two problems described below.
The first problem is as follows. A black matrix which is made of a reflective metal such as Cr and occupies a large area of a display screen is form ed on the inside surface of a upper transparent glass substrate that is located on the display screen side. External light is reflected by the black matrix and comes out of the display screen. This lowers the contrast of a displayed image and hence makes it less visible, that is, lowers the display quality.
The second problem relates to a case where a black matrix is formed on an opposed substrate. In this case, as shown in
FIG. 11A
, a black matrix
1
is so formed as to overlap with ITO pixel electrodes
2
by 5-7 &mgr;m in consideration of the bonding accuracy of the TFT substrate and the opposed substrate. Thus, the size of opening portions is restricted. In this case, to increase the brightness of the display device, it is necessary to employ a brighter back light, resulting in an increase in power consumption.
FIG. 11A
shows how the black matrix
1
on the opposed substrate and the ITO pixel electrodes
2
overlap with each other. Reference numerals
3
-
5
denote a signal line, a TFT, and a scanning line, respectively.
SUMMARY OF THE INVENTION
To solve the above two problems, an object of the present invention is to form a black matrix on TFTs of a d river circuit. This configuration has an advantage that the overlapping width can be reduced to about 2 &mgr;m because of improved bonding accuracy that is obtained by forming the black matrix and the ITO pixel electrodes on the same substrate.
This advantage will be described with reference to
11
B.
FIG. 11B
shows how they overlap with each other in a case where the black matrix
1
is formed on the TFT substrate. While in the former case (
FIG. 11A
) the aperture ratio is about 15% (overlapping width: 7 &mgr;m), in the latter case (
FIG. 11B
) it is greatly increased to about 40% (overlapping width: 2 &mgr;m).
On the other hand, in the above-mentioned configuration in which the opposed substrate is made large enough to be opposed to the driver circuits and the driver circuits are provided in the liquid crystal region, the driver circuit regions and the pixel region come close to each other, which requires light shielding even in the driver circuit regions.
Where the black matrix for light shielding of the pixel region is formed on the substrate on which TFTs are formed and is also used for light shielding of the driver circuits to satisfy the above requirement, there has occurred a problem that the capacitance of an interlayer insulating film between TFTs of the driver circuits and the black matrix is not negligible though the shielding itself does not cause any problem.
If the interlayer insulating film is a 3,000-Å-thick silicon nitride film, it has a unit area capacitance of 2.50×10
−16
F/&mgr;m
2
. For example, if a clock line or the like of a driver circuit has a wiring line of 100
m in width and 50,000 &mgr;m in length, a capacitance formed by this wiring line of the driver circuit and the black matrix amounts to 1.25×10
−9
F. In this case, if it is assumed that the wiring line of the driver circuit has a sheet resistance of 0.2 &OHgr;/&mgr;m
2
, its delay time amounts to 1.25×10
−7
sec, which will cause a problem when the wiring line is driven at several megahertz. The circuit characteristics are more important in the driver circuits than in the pixel TFTs. Therefore, it is necessary to reduce the capacitance of the interlayer insulating film formed between TFTs of the driver circuits and the black matrix.
It is practiced to form only a black matrix
16
for a pixel region
14
on a TFT substrate
11
so as to be adjacent to ITO electrodes
17
and form a black matrix
18
for driver circuit regions
13
on an opposed substrate
12
, as shown in FIG.
12
. However, although this configuration increases the aperture ratio, the number of manufacturing steps increases because of the need of forming the black matrix
16
and
18
on both of the TFT substrate
11
and the opposed substrate
12
. In
FIG. 12
, reference numerals
15
and
19
respectively denote an aluminum wiring line and color filters of R, G and B.
It is now desired to provide a liquid crystal display device which enables light shielding of driver circuit regions without increasing the number of manufacturing steps.
Another object of the invention is to prevent a capacitance from occurring in an interlayer insulating film formed between TFTs of a driver circuit and a black matrix, to reduce, in turn, the delay time of the driver circuit, to thereby produce high-resolution images.
To attain the above objects, according to the invention, there is provided an active matrix liquid crystal display device comprising: a first insulating substrate comprising: a pixel region in which a plurality of pixels having respective thin-film transistors are arranged in matrix
Nishi Takeshi
Yamazaki Shunpei
Fish & Richardson P.C.
Ngo Julie
Semiconductor Energy Laboratory Co. Ltd
Sikes William L.
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