Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-08-10
2004-01-27
Bella, Matthew C. (Department: 2676)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S096000, C345S094000, C349S107000, C349S166000
Reexamination Certificate
active
06683592
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. More particularly, the present invention relates to the technical field of an electro-optical device, such as 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 voltages applied to adjacent pixel electrodes are periodically alternated for every pixel row or every pixel column, so that the voltages 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, include an electro-optical material, such as a liquid crystal, interposed between a pair of substrates. 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 (in other words, if there is a step in the surface of the pixel electrode beneath the alignment layer or in the surface of an interlayer insulator serving a substrate for the pixel electrode), an orientation defect (a disclination) 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 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 capacitive lines, and pixel electrodes, a step inevitably occurs in the surface of an alignment layer in accordance with the presence of the 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 called a black mask or a black matrix covers the portion of the step (i.e., the spacing between the pixel electrodes) so that the portion of the electro-optical material suffering from the orientation defect may remain hidden or may not contribute to display light.
Techniques for planarizing the surface of a substrate beneath the pixel electrode has been developed, in which an interlayer insulator beneath the pixel electrode is fabricated of a planarized film, such as an organic SOG (Spin On Glass) film, so that a step resulting from the presence of lines and TFTs may not be created.
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 degradation of the electro-optical material as a result of the application of a direct current voltage and to control a 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, wherein 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 polarity relative to the potential of an opposing electrode, while the pixel electrodes arranged on an even row are driven by an negative polarity relative to the potential of the opposing electrode, and 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 polarity while the pixel electrodes arranged on the odd row are driven by a negative polarity (in other words, the pixel electrodes on the same row are driven by the same polarity potential and the potential polarity is alternated every row with the period of frame or field). A 1S alternating drive method is also easy to control and presents a high-quality image display, wherein the pixel electrodes on the same column are driven by the same polarity potential while the potential polarity is alternated every column with the period of frame or field.
SUMMARY OF THE INVENTION
The technique to cover the above-referenced step 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 polarity in a TFT array substrate. When the phases of the voltages (the voltages applied to the pixel electrodes adjacent in the column direction in the 1H alternating drive method, and the voltages 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 opposing 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 an 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 opposing electrode (i.e., an electric field perpendicular to the surface of the substrate), an orientation defect 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 of the pixel is reduced with 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 more 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 at least provide an electro-optical device such as a liquid-crystal display device, which may present a high aperture ratio of pixel while displaying a high-contrast-ratio, bright and high-quality image, by generally reducing an orientation defect resulting from a step in the surface of a substrate in contact with an electro-optical material, such as a liquid crystal, and an orientation defect resulting from a transverse electric field.
An electro-optical device of an exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes, a second substrate having an opposing electrode facing the pixel electrodes, and an electro-optical material interposed between the first substrate and the second substrate, wherein the thickness of the electro-optical material between adjacent pixel electrodes which are driven by mutually opposite
Bella Matthew C.
Nguyen Hau
Oliff & Berridg,e PLC
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