Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-10-12
2002-05-21
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C349S040000
Reexamination Certificate
active
06392622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to active-matrix substrates, electro-optical devices, methods for manufacturing active-matrix substrates, and electronic equipment. More specifically, it relates to an electrostatic-destruction prevention technology suited to manufacturing of an active-matrix substrate of a type in which pixel electrodes are driven by poly-silicon thin-film transistors (thin film transistor is hereinafter referred to as TFT) formed on an insulating substrate.
2. Background of the Related Art
Among various liquid crystal panels, an active-matrix liquid crystal panel is formed, for example, by sequentially and selectively forming a semiconductor layer, an insulating layer, and a conductive layer on a large substrate, such as a glass substrate, to form a plurality of panel areas provided with active elements, passive elements, electrodes, and other components, and by cutting these panel areas from the large substrate. This active-matrix substrate is used for an electro-optical device. Specifically, it is used in an electro-optical device in which a liquid crystal is sandwiched by the active-matrix substrate and an opposing substrate. In the active-matrix substrate, a number of pixels are formed in a matrix and they form a pixel section. In the pixel section, thin-film transistors (TFTs) are formed and a voltage is applied to pixel electrodes through the TFTs.
In such an active matrix substrate, when poly-silicon (Poly-Si) is used as a semiconductor material to form the TFTs, since transistors and other devices constituting peripheral circuits such as a shift register and a driving circuit can be formed in the same process, high integration is enabled.
In such an active matrix substrate, when poly-silicon TFTs are formed as transistors, since the active-matrix substrate can be formed in a low-temperature process, it is advantageous that a glass substrate made from silica glass or non-alkaline glass can be used as an insulating substrate.
Since the glass substrate is likely to be charged, however, when static electricity is discharged from the charged substrate, TFTs and other devices working as active elements may be destroyed (hereinafter called electrostatic destruction) by static electricity.
In an active-matrix substrate, an alignment layer is formed on the glass substrate on which active elements, passive elements, and electrodes are formed, to align liquid-crystal molecules in a prescribed direction. In a rubbing process for the alignment layer, however, the substrate is charged with high-voltage static electricity generated by the rubbing. When static electricity is discharged from the charged substrate, the TFTs and other devices working as active elements may be electrostatically destroyed.
More specifically, an organic high-polymer film made from polyimide resin or the like is formed on the glass substrate on which the active elements and other devices are formed, and the rubbing process is applied to a surface of this resin film to align the liquid-crystal molecules, in which the surface is rubbed with textile fabrics made from fiber, such as rayon and nylon, in a constant direction at a prescribed load. In this process, friction between the resin film and the fiber generates high-voltage static electricity. This static electricity charges the substrate itself, or is discharged over insulation to electrostatically destroy semiconductor devices, such as the TFTs formed on the substrate.
SUMMARY OF THE INVENTION
According to knowledge which the inventors of the present application obtained, since poly-silicon TFTs and other devices formed in the low-temperature process at a maximum process temperature of about 400 to 600° C. have an extremely low dielectric strength, they are likely to be electrostatically destroyed. In some cases, they may cause a serious problem almost identical to a fatal error, in which the entire driving circuit does not function.
Accordingly, an object of the present invention is to provide an active-matrix substrate, an electro-optical device, and a method for manufacturing an active-matrix substrate, which provide a structure that can effectively prevent TFTs and other devices formed on a substrate from being destroyed by static electricity generated for some reason or by a rubbing process for a liquid-crystal alignment layer.
To achieve the foregoing object, according to the present invention, an active-matrix substrate, on which are formed a pixel section provided with a pixel electrode and a switching element connected to the pixel electrode, a peripheral circuit disposed around the pixel section that controls the switching element, and an external-connection terminal electrically connected to the peripheral circuit, is characterized in that an antistatic conductive layer is formed at least at a part of the area on the substrate excluding the pixel section.
In the present invention, the antistatic conductive layer collects static electricity generated when a rubbing process is applied to a polyimide film formed on the active matrix substrate or the like to change it to a liquid crystal alignment layer, and the charges are dispersed. Therefore, the substrate itself is prevented from being charged. Active elements and other elements formed in the peripheral circuit and other circuits are prevented from being electrostatically destroyed during discharging. Therefore, TFTs formed in a low-temperature process, which are not immune to static electricity, can be used as active elements. In addition, since the antistatic conductive layer serves as a large-capacitance bypass capacitor (“pass cap”) when an electro-optical device is operated, it contributes to providing lower noise and lower EMI. Therefore, higher image quality and higher resolution are implemented in the electro-optical device.
In the present invention, it is preferred that the antistatic conductive layer be formed in the area on the substrate excluding the pixel section, only at the upper layer sides of a no-wiring section, where wiring is not formed, of an area where wiring is formed to which a DC voltage is applied, and of an area where wiring is formed to which a DC voltage is applied when an image is displayed. With this configuration, even when an antistatic conductive layer is formed, the capacitive load of the driving circuit does not increase. Therefore, since a signal transmitted through the wiring is not delayed, transistors are prevented from being electrostatically destroyed while enabling a high-speed operation.
In the present invention, it is preferred that the antistatic conductive layer be formed such that it is exposed on a surface of the substrate. With this configuration, the antistatic conductive layer positively collects static electricity generated in the rubbing process to disperse it. Therefore, the substrate itself is prevented from being charged, and active elements and other elements are prevented from being electrostatically destroyed during discharging.
In the present invention, it is preferred that the antistatic conductive layer be formed at least at the outer peripheral edge of the substrate. In other words, it is preferred that, after the pixel section, the peripheral circuit, the terminal section, and the antistatic conductive layer are formed in each of a plurality of panel areas, each of which is cut from a large substrate as the active-matrix substrate, the antistatic conductive layer be formed so as to cross over the boundary of adjacent panel areas. With this configuration, since a potential difference between panel areas is eliminated and the same potential plane can be extended, problems caused by static electricity are more positively prevented from occurring.
In the present invention, it is preferred that external-connection terminals be electrically connected through an electrostatic protection circuit in which two sets of diode chains are disposed in reverse directions to each other. It is also preferred that each external-connection terminal and the antistatic conductive lay
Eisen Alexander
Oliff & Berridg,e PLC
Seiko Epson Corporation
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