Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-02-23
2002-01-08
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S143000
Reexamination Certificate
active
06337726
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an array substrate used for a liquid crystal display element, and particularly for a liquid crystal display element of in-plane switching type, a liquid crystal display element provided with the array substrate, and a method of manufacturing an array substrate.
In recent years, a display element having a large capacity and a high density which can be used for TV display and graphic display has been eagerly developed and practiced as a display element using liquid crystal. In particular, development and commercial production has been widely made of a liquid crystal display element of an active matrix type capable of displaying an image at a high contrast ratio without cross-talk.
Also, in recent years, a wider view angle has been required for a liquid crystal display element aimed for monitor use, and various techniques for wide view angle has been developed. Particular attention is being paid to a so-called IPS (In-Plane Switching) method in which display pixel electrodes and opposite electrodes are formed on one same substrate and liquid crystal is made respond by an electric field generated substantially in parallel to the substrate.
As a liquid crystal display element of an active matrix type adopting the IPS method, there is proposed a display element in which the display pixel electrodes and opposite electrodes are respectively made of display signal line layers and scanning signal line layers, a supplemental capacity is formed on opposite signal lines, and a thin film transistor (hereinafter referred to as only TFT) of a stagger type is used as a switching element.
Specifically, according to this liquid crystal display element, a MoW film is formed at 200 angstrom on an insulating substrate, scanning signal lines including gate electrodes and opposite signal lines parallel thereto are processed by photoetching thereafter, and the opposite electrodes extending perpendicularly from the opposite signal lines are processed into a predetermined shape. Next, pattern inspection of scanning signal lines is carried out. Thereafter, an insulating film made of SiO with a thickness of 3000 angstrom and i-type amorphous silicon (hereinafter referred to as a-Si) film with a thickness of 500 angstrom serving as a semiconductor layer constituting channel regions for the TFTs are each formed on the entire surface of the substrate by a CVD (Chemical Vapor Deposition) method.
Subsequently, an etching protection film made of SiN for protecting channels of the TFTs is formed at 2000 angstrom also by the CVD method, and thereafter, only the protection film is processed into a predetermined shape by photoetching. Further, an n+ type a-Si film is formed at 500 angstrom by the CVD method, and then, the i-type a-Si film and the n+ type a-Si film are processed into a predetermined shape by photoetching. Subsequently, power supply electrodes for the scanning signal lines and opposite signal lines are processed into a predetermined shape by photoetching.
Next, a Al film is formed at 3000 angstrom by a sputtering, and thereafter, display signal lines, source and drain electrodes of the TFTs, display pixel electrodes, supplemental capacity electrodes, power supply lines of the opposite signal lines, and the n+ type a-Si film between the source and drain electrodes are processed into predetermined shapes. In this case, the display pixel electrodes are arranged in parallel with the opposite electrodes.
Then, a protection film made of SiN is formed at 2000 angstrom by the CVD method and are processed into predetermined shape. A substrate for a liquid crystal display element of an active matrix type (hereinafter referred to as an array substrate) is thus prepared. This array substrate and an opposite substrate made of an insulating substrate are adhered together on each other with a predetermined gap, and a liquid crystal layer is sealed between these substrates, thus completing a liquid crystal display element of an active matrix type.
In the above-mentioned liquid crystal display element of an active matrix type adopting the IPS method, each pixel is constituted by a plurality of apertures interposed between a plurality of substantially linear display pixel electrodes and a plurality of substantially linear opposite electrodes, which are formed of different layers on the same substrate, by means of independent photoetching steps. Therefore, there is a case that the distance between the display pixel electrodes and the opposite electrodes is not uniform in the pixels due to misalignment of patterns during exposure.
For example, if each pixel region includes two display pixel electrodes parallel to each other and one opposite electrode arranged in parallel between the display pixel electrodes, the distances between the opposite electrode and the display pixel electrodes do not become uniform due to misalignment during exposure. In this case, the electric field generated between both electrodes is strong at the portion where the distance between both electrodes is short than at the portion where the distance between both electrodes is wide. Consequently, the responsibility of liquid crystal differs between both portions so that the luminance does not become uniform in one same pixel. This ununiformity in the pixel deteriorates the display quality of the screen and is observed as roughness with eyes.
Meanwhile, in consideration of the responsibility of liquid crystal, the electric field generated between the display electrodes and the opposite electrode should desirably be perfectly parallel to the surface of the array substrate. In practice, however, the electric field becomes arc-shaped due to electrode end effects. Therefore, the effective horizontal electric field between the electrodes is weaker and the responsibility of liquid crystal is lowered than in the case where the electric field is perfectly parallel to the substrate.
Narrowing of the distance between both electrodes can be considered to be a method of compensating the weakening of the electric field. In this case, the numerical aperture is lowered so that the permeability is deteriorated. If the back-light is strengthened to compensate this deterioration, increase of the power consumption is caused undesirably.
Increase of the voltage applied to both electrodes can be considered as another method. This means increase of the drive voltage and leads to increase of the power consumption.
Also, this kind of liquid crystal display element comprises an opposite substrate opposed to the array substrate. A black matrix as a light shielding layer, a color filter, and the like are formed on the opposite substrate. In general, the black matrix is arranged such that the opening portions thereof are opposed to the pixel region of the array substrate, respectively, and the periphery of each opening is layered over the opposed electrode.
However, if the peripheral edges of the openings of the black matrix are positioned between the opposite electrodes and the pixel electrodes due to a relative positional offset between the array substrate and the opposite substrate during assembly, the aperture ratio of the liquid crystal display element is lowered so that the screen luminance is lowered. Inversely, if the peripheral edges of the openings of the black matrix are positioned between the opposite electrodes and the display signal lines, light leaks so that roughness appears on the screen and the contrast is lowered, resulting in deterioration of the image quality. As a method of preventing this positional offset of the black matrix, the width of the opposite electrodes may be thickened. In this case, however, the aperture ratio is lowered.
BRIEF SUMMARY OF THE INVENTION
The present invention has been contrived in consideration of the above circumstances and its object is to provide an array substrate for a liquid crystal element, which is capable of improving the image quality and the response of liquid crystal without increasing the power consumption, a liquid crystal display element
Kamimura Takaaki
Kawano Hideo
Kabushiki Kaisha Toshiba
Nguyen Dung
Pillsbury & Winthrop LLP
Sikes William L.
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