Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-09-22
2004-07-13
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S126000, C349S044000
Reexamination Certificate
active
06762809
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to the technical field of an electro-optical device such as a liquid-crystal display device, and a method for manufacturing the electro-optical device. More particularly, the present invention relates to the technical field on an electro-optical device and a method for manufacturing the electro-optical device, which is appropriate for use in liquid-crystal display devices employing a TN (Twisted Nematic) liquid crystal, in particular, a thin-film transistor (hereinafter referred as TFT) active-matrix liquid-crystal display device, which adopts an alternating drive method in which the polarities of the potentials applied to adjacent pixel electrodes are periodically alternated every pixel row or every pixel column, so that the potentials applied to adjacent pixel electrodes in a row direction or in a column direction are inverted in polarity.
2. Description of Related Art
Electro-optical devices, such as liquid-crystal display devices, typically include an electro-optical material such as a liquid crystal interposed between a pair of substrates, and the alignment state of the electro-optical material is controlled by the property of the electro-optical material and an alignment layer formed on the substrate on its surface facing the electro-optical material. If there is a step in the surface of the alignment layer, an orientation defect occurs in the electro-optical material depending on the magnitude of the step. If such an orientation defect occurs, proper driving of the electro-optical material in that portion becomes difficult, and the contrast ratio of the device drops due to a visible defect in the electro-optical device. Since a TFT active-matrix electro-optical device includes, on a TFT array substrate, TFTs in many locations thereof for controlling and switching a variety of lines such as scanning lines, data lines, and capacitance lines, and includes pixel electrodes, a step inevitably occurs in the surface of an alignment layer in accordance with the presence of lines and elements, if no planarizing process is performed.
Conventionally, the portion of the substrate suffering from such a step is aligned with the spacing between adjacent pixel electrodes, and a light-shielding layer covers the portion of the step so that the portion of the electro-optical material suffering from the orientation defect may remain hidden or may not contribute to display light.
The electro-optical device of this sort typically adopts an alternating drive method in which the polarity of a potential applied to the pixel electrodes is alternated at a predetermined pattern to prevent degray scale of the electro-optical material as a result of the application of a direct current, and to control cross talk and flickering of a display screen image.
A 1H alternating drive method is relatively easy to control and presents a high-quality image display. During the presentation of a video signal of one frame or one field, the pixel electrodes arranged on an odd row are driven by a positive potential, while the pixel electrodes arranged on an even row are driven by a negative potential. During the presentation of a video signal of a next frame or a next field, conversely, the pixel electrodes arranged on the even row are driven by a positive potential while the pixel electrodes arranged on the odd row are driven by a negative potential, and at the same time, the potential polarity is alternated every row in the period of frame or field.
A 1S alternating drive method is also easy to control and presents a high-quality image display. The pixel electrodes on the same column are driven by the same polarity potential while the potential polarity is alternated every column in the period of frame or field.
SUMMARY OF THE INVENTION
The technique to cover the above-referenced step portion with the light-shielding layer narrows the aperture of the pixel depending on the size of the step portion, and cannot meet the basic requirement in the technical field of the electro-optical device that the aperture ratio of the pixel be increased in a limited image display area to present a brighter image. The number of lines and TFTs per unit area increases as the pixel pitch becomes fine for high-definition video presentation. Since there is a limitation to the miniaturization of the lines and the TFTs, the ratio of the step portion to the image display area becomes relatively high, and the problem of the step portion becomes serious as high-definition design is promoted in the electro-optical device.
In accordance with the above-referenced technique for planarizing the interlayer insulator beneath the pixel electrodes, no particular problem will be presented when adjacent pixel electrodes are of the same potential in a TFT array substrate. When the potentials (the potentials applied to the pixel electrodes adjacent in the column direction in the 1H alternating drive method, and the potentials applied to the pixel electrodes adjacent in the row direction in the 1S alternating drive method) are opposite in polarity as in the above-referenced 1H alternating drive method or 1S alternating drive method, the gap between the pixel electrode and the counter electrode becomes wider at the edge of the pixel electrode over the line and the TFT when the planarizing process is performed than when no planarizing process is performed. A transverse electric field taking place between the adjacent pixel electrodes (specifically, an electric field in parallel with the surface of the substrate or slant electric field having a component in parallel with the surface of the substrate) relatively intensifies. If such a transverse electric field is applied to the electro-optical material, which is expected to work under a longitudinal electric field present between the pixel electrodes and the counter electrode (i.e., an electric field perpendicular to the surface of the substrate), a disclination takes place in the electro-optical material, visible defect occurs there, and the contrast ratio drops. Although the area of the transverse electric field can be covered with the light-shielding layer, the aperture area of the pixel is reduced along with an increase in the area of the transverse electric field. As the distance between the adjacent pixel electrodes shrinks with a fine pixel pitch, the transverse electric field intensifies, and these become more problematic as high-definition design is promoted in the electro-optical device.
The present invention has been developed in view of at least the above problems. It is an object of the present invention to provide an electro-optical device, such as a liquid-crystal display device, which presents a high aperture ratio of pixel and displays a high-contrast-ratio, bright and high-quality image by controlling an orientation defect resulting from a step portion in the surface of a substrate in contact with an electro-optical material, such as a liquid crystal, while by keeping the aperture area of each pixel from narrowing as much as possible.
An electro-optical device of a first exemplary embodiment of the present invention includes a first substrate having a first alignment layer that has been subjected to a rubbing process; a second substrate opposed to the first substrate, having a second alignment layer that has been subjected to a rubbing process; an electro-optical material interposed between the first substrate and the second substrate; a step portion, formed on at least one of the first alignment layer of the first substrate and the second alignment layer of the second substrate, and downwardly rubbed in the direction of the rubbing process; and a light-shielding layer formed in an area facing the step portion that is downwardly rubbed in the direction of the rubbing process, on at least one of the first substrate and the second substrate.
The study carried out by the inventors of this invention reveals that an orientation defect of an electro-optical material due to a step is substantially more pronounced when
Oliff & Berridg,e PLC
Seiko Epson Corporation
Ton Toan
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