Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-05-16
2004-10-05
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S152000, C349S043000, C349S143000
Reexamination Certificate
active
06801288
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to an active matrix type of liquid crystal display device in which a switching element is disposed in each pixel.
2. Description of the Related Art
Liquid crystal display devices are widely used as display devices for various kinds of image display apparatuses. A typical liquid crystal display device includes liquid crystal compounds having liquid crystal molecules which are sealed in the space between two opposite insulating substrates (hereinafter also referred to simply as substrates) at least one of which is transparent, and electrode-selecting pixels formed on either or both of the inside surfaces (main surfaces) of the two insulating substrates. The liquid crystal display device controls the alignment of the liquid crystal molecules by an electric field formed between the pixels, thereby turning on or off transmitted light or reflected light to display an image.
An active matrix type of liquid crystal display device which controls its image display operation by means of switching elements represented by thin film transistors (TFTs) is widely used as a display device for a display terminal such as a computer because of its advantages of small size, light weight and high image quality.
As described above, the active matrix type of liquid crystal device includes the switching elements disposed in the respective pixels, and has a first electrode (pixel electrode) whose potential is controlled by such a switching element, and a second electrode (counter electrode) whose potential variation is small compared to the first electrode. The active matrix type of liquid crystal display device displays an image by controlling the slate of alignment of the liquid crystal molecules which constitute the liquid crystal compounds by means of an electric field generated between these electrodes.
The active matrix liquid crystal display device is classified into two kinds according to the direction of the lines of electric force which is applied to the liquid crystal molecules sealed between the two substrates by the electrodes.
One of the two kinds is a so-called vertical electric field type which has electrodes disposed opposite to each other with a layer of liquid crystal compounds (hereinafter also referred to simply as a liquid crystal layer) being interposed therebetween, and forms an electric field between these electrodes in a vertical direction with respect to the substrate surfaces (refer to, for example, Japanese Patent Laid-Open No. 257142/1993 as well as the corresponding U.S. Pat. No. 5,432,626).
The other kind is a type which has electrodes shifted in position with respect to the liquid crystal layer, and forms an electric field in a direction parallel to the substrate surfaces (refer to, for example, Japanese Patent No. 2,708,098 and U.S. Pat. No. 5,754,266).
The latter type is called an in-plane switching (IPS) type or a lateral electric field type because the alignment of the liquid crystal molecules depends on the lines of electric force generated in the direction of the substrate surfaces.
In one IPS type of liquid crystal display device, as disclosed in the above-cited U.S. Pat. No. 5,754,266, the aforesaid two kinds of electrodes which generate an electric field to control the alignment of liquid crystal molecules are formed on a substrate (TFT substrate) on which thin film transistors (TFTs) are provided. In another IPS type of liquid crystal display device, as disclosed in U.S. Pat. No. 5,598,285, one of the first and second electrodes is formed on a TFT substrate and the other is formed on the other substrate.
In any of the constructions, each of the first and second electrodes has a region which is not opposite to the other, and care is taken that a conductive thin film is not formed particularly on the main surface of the first electrode that is opposite the region (i.e., a surface opposite to the liquid crystal layer). This art is described in Japanese Patent Laid-Open No. 191994/1995 or the like which is the art of reducing the resistance value of a light shielding film, i.e., a black matrix material.
SUMMARY OF THE INVENTION
For such an IPS type of liquid crystal display device, the present inventors formed disconnection testing terminals for gate lines for transmitting control signals to the gate electrodes of thin film transistors (field-effect transistors), by using a conductive material which was called ITO (Indium-Tin-Oxide) in which 1-5 weight % of tin oxide (SnO
2
) was added to indium oxide (In
2
O
3
).
This material has the advantages of less contamination of a liquid crystal layer due to its constituent elements and less deterioration due to oxidation or the like than metal materials used for related-art test terminals. In addition, the present inventors formed data lines each of which supplied a video signal to the corresponding one of the gate lines and to either one of the source and drain electrodes of the corresponding one of the aforesaid transistors, by using a metal material which was higher in conductivity than ITO.
Each of the test terminals made of ITO was formed in such a manner that a hole (or an opening: a contact hole) is formed to extend through at least two kinds of insulating films, i.e., a gate insulating film which covers the gate lines, and a protective film (called a passivation film) which covers the gate insulating film and the data lines, and the at least two kinds of insulating films are kept in contact with the gate line in the exterior of the hole and are led to the upper portion of the protective film.
In addition, a predetermined space must be set between a portion to which is applied a sealing material for forming a region in which to seal the liquid crystal layer and a display region (a region which actually contributes to image display).
Under such conditions, to meet a demand for narrowing a picture frame, i.e., narrowing the area of a peripheral portion relative to the display region, the test terminals made of the above-described ITO were provided in the liquid crystal sealing region. Accordingly, the test terminals made of the above-described ITO are disposed at locations near the liquid crystal layer.
However, when a liquid crystal display device having the test terminal made of such ITO is activated, the following problem (a first subject) occurs: that is to say, unexpected light leaks occur at pixels positioned at an end of the display region, i.e., even during the state of black display, stripes (bright lines) of a display color occur at the pixels positioned at the end of the display region.
In addition, to stabilize the potential of the second electrode (counter electrode), each counter voltage signal line for distributing a voltage to the counter electrodes of the corresponding pixels is required to conduct to a common line (common bus line) disposed at an end of the substrate. Since such counter voltage signal lines are formed on the main surface of the substrate that is opposite to the liquid crystal layer, it is necessary that as in the case of the counter electrodes, a gate insulating film, a protective film or an overcoat film be formed on the counter voltage signal lines, and furthermore, an alignment film be formed at positions where the counter voltage signal lines are in contact with the liquid crystal layer. Accordingly, there is a need for the step of covering the counter voltage signal lines with an insulating film.
However, during the formation of the insulating film particularly by CVD (Chemical Vapor Deposition) using plasma, if electric charges generated in the counter voltage signal lines flow into and are stored in the common bus line during the growth of the insulating film, an unexpected electric discharge occurs between this common bus line and the plasma or a plasma-generating electrode.
If a current which is thought of as due to this electric discharge returns to a counter voltage signal line and the resultant electric power exceeds the allowab
Ashizawa Keiichirou
Hikiba Masayuki
Ota Masuyuki
A. Marquez, Esq. Juan Carlos
Duong Thoi V.
Fisher Esq. Stanley P.
Hitachi , Ltd.
Kim Robert H.
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