Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2003-07-09
2004-12-28
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000
Reexamination Certificate
active
06836302
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an active matrix substrate, an electro-optical device and electronic equipment. More specifically, the invention relates to a construction of an active matrix substrate ideally used with a liquid crystal light valve mounted on a projection display unit.
2. Description of Related Art
A related art liquid crystal light valve is an optical modulating device to be mounted on a projection display unit, such as a liquid crystal projector. The liquid crystal light valve is primarily constructed of a pair of substrates that are disposed to oppose each other with a liquid crystal layer sandwiched therebetween, and electrodes to apply voltages to the liquid crystal layer. Normally, in the related art, the liquid crystal light valve uses an active matrix liquid crystal cell, and it is advantageous to achieve higher definition of images.
As the methods of driving a liquid crystal light valve, reversal drive methods, including dot reversal, line reversal and field reversal, have been used in the related art to reduce or prevent seizure or deterioration of liquid crystal.
Each of the above inversion drive methods has advantages and disadvantages. In the case of the dot reversal or the line reversal, voltages of opposite polarities are applied to the pixel electrodes of adjoining dots, so that a horizontal electric field is generated between the adjoining dots, and light leakage attributable to disclination caused by the horizontal electric field may occur. As mentioned above, since higher definition is required of liquid crystal light valves, the leakage of light leads to deteriorated contrast or aperture ratio, which is a major cause of degraded display quality. From this viewpoint, it is required to adopt a field reversal drive system free of the occurrence of horizontal electric fields.
SUMMARY OF THE INVENTION
However, in the related art, it has been impossible to adopt the field reversal drive method in the construction of a related art active matrix substrate, because of the following reason.
In the field reversal drive, when attention is focused on, for example, a single data line, image signals (voltages) of the same polarity are written in a certain one field with respect to all dots to which a signal from the foregoing data line is supplied. Then, the moment the next field is reached, the polarity of an image signal to be supplied to the foregoing data line is reversed. At this time, on the scanning line side, if scanning is carried out from the top to bottom of a display region, the image signal supplied to the foregoing data line is immediately written at upper dots in the display region. On the other hand, lower dots remain in a prolonged state where pixel electrodes retain an image signal written in the preceding filed, while an image signal of the opposite polarity from that is applied to the data line. During this period of time, coupling between the pixel electrodes and the data line takes place, thus posing a problem in that the potentials of the pixel electrodes vary due to the influences of the data line at the lower dots of the display region, resulting in deteriorated display quality.
The present invention addresses the above and/or other problems, and provides an electro-optical device, such as a liquid crystal device, that permits improved display quality to be achieved by adopting field reversal drive, an active matrix substrate used therewith, and electric equipment.
To this end, an active matrix substrate in accordance with the present invention includes: a substrate body equipped with a plurality of data lines and a plurality of scanning lines provided such that they cross each other, a plurality of thin-film transistors electrically connected to these data lines and scanning lines, and a plurality of pixel electrodes electrically connected to the plurality of thin-film transistors, respectively. Gate electrodes constituting the thin-film transistors and the scanning lines are formed in separate layers, and electrically connected via contact holes penetrating an interlayer insulating film between the gate electrodes and the scanning lines. A layer constituting the scanning lines is positioned above a layer constituting the data lines but under a layer constituting the pixel electrodes, and a pattern of the scanning lines, a pattern of the data lines and a pattern of the pixel electrodes are partly overlapped in a top plan view.
More specifically, in the active matrix substrate in accordance with the present invention, the gate electrodes constituting thin-film transistors (hereinafter “TFTs”) are not formed integrally with the scanning lines. Instead, the gate electrodes are independently formed using a different layer from that of the scanning lines, and the gate electrodes and the scanning lines are electrically connected via a contact hole. In the sectional structure, the layer constituting the scanning lines is positioned between the layer constituting the data lines and the layer constituting the pixel electrodes, and the pattern of the scanning lines partly overlaps the pattern of the data lines and the pattern of the pixel electrodes in a top plan view. Structurally, therefore, the portion wherein the scanning lines overlap with the data lines and the pixel electrodes functions as a shielding layer to block the coupling between the pixel electrodes and the data lines described with regard to the related art. This reduces or minimizes the chance of variations occurring in the potentials of pixel electrodes caused by the influences of the data lines at any location in the display region. Thus, an electro-optical device employing the active matrix substrate permits the field reversal drive. The use of the field reversal drive makes it possible to obtain an electro-optical device, such as a liquid crystal device, with, for example, a higher contrast and a higher aperture ratio. Moreover, since the scanning lines required for the active matrix substrate are used as the shielding layer, no separate pattern functioning solely as the shielding layer is added, so that the pattern construction will not be particularly complicated.
In the active matrix substrate in accordance with the present invention, the scanning lines are preferably formed of a material including a metal. Further preferably, the data lines are also formed of a material including a metal. The term “a material including a metal” means that the layer may be formed of a single metal layer or a laminated film containing a metal film.
For instance, a liquid crystal device mounted as an optical modulating device on a projection display unit is irradiated with far more intense light than that applied to a direct view liquid crystal display equipped with, for example, a backlight. At this time, when light is applied to a TFT provided as a pixel switching element, light leakage current passes between source and drain regions, leading to a problem in which the characteristics of the TFT deteriorate or the TFT malfunctions in an extreme case. For this reason, the related art creates a light shielding film on the active matrix substrate. A case may exist where the light shielding film is formed separately from various wires, or scanning lines are formed of a material, e.g., WSi (tungsten silicide) exhibiting high light shielding performance, to use them as the scanning lines serving also as a light shielding film in order to simplify the substrate configuration. However, especially in the latter case, the material, such as WSi, exhibits good light shielding property, while it disadvantageously has a high sheet resistance, approximately 5&OHgr;, leading to a problem of degraded display quality due to signal delay in the scanning lines. In addition, the light shielding film is disposed only in one direction, resulting in inadequate restraint of light leakage current.
In contrast to the above, in the active matrix substrate according to the present invention, if the scanning lines are formed of a material, including a metal, such
Dudek James A.
Oliff & Berridg,e PLC
Seiko Epson Corporation
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