Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2002-06-26
2004-03-09
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S090000
Reexamination Certificate
active
06703997
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an electrooptical apparatus such as an active matrix liquid crystal apparatus driven by thin-film transistors (hereinafter also referred to as TFTs), and also to a method of producing such an electrooptical apparatus. More particularly, the present invention relates to an electrooptical apparatus such as a liquid crystal apparatus suitable for use in a liquid crystal projector, and including peripheral circuits such as a data line driving circuit and a scanning line driving circuit, wherein a light blocking film is disposed under each TFT. The present invention further relates to a method of producing such an electrooptical apparatus.
2. Description of Related Art
In conventional liquid crystal apparatus having built-in peripheral circuits, peripheral circuits such as a data line driving circuit, a scanning line driving circuit, and a sampling circuit are formed on a TFT array substrate, which is one of two substrates between which a liquid crystal is disposed. In general, these peripheral circuits are produced using the same production process as that for producing TFTs for switching an image signal applied to pixel electrodes provided in respective pixels (hereinafter such TFTs will also be referred to as pixel TFTs) so as to achieve a high production efficiency.
On the TFT array substrate, there are provided a great number of data lines and scanning lines in an image displaying area in contact with the liquid crystal, wherein these data lines and scanning lines cross each other at different layer levels. Input/output interconnections connected to peripheral circuits are disposed in a sealing area, outside the image displaying area and in contact with a sealing material for enclosing a liquid crystal, and in a peripheral area outside the sealing area. More specifically, leading interconnections which extend from the data lines, the scanning lines, and the capacitance lines, and which serve as the input/output interconnections for the peripheral circuits, are disposed in the sealing area and image signal lines, control signal lines, power supply lines, clock signal lines, and other signal lines, connected to external input terminals, and are disposed in the peripheral area.
In particular, in a liquid crystal apparatus having a sampling circuit as a peripheral circuit, when an image signal is supplied to an image signal line via an external input terminal, the image signal is sampled, from one data line to another, by sampling switches of the sampling circuit, in response to a sampling circuit driving signal output with predetermined timing from the data line driving circuit.
Because the image signal line is a signal line used to supply the image signal itself, which defines the voltage applied to the liquid crystal, it is extremely important that the image signal line has a low electric resistance and a small time constant to prevent degradation in the picture quality. To this end, the image signal line is generally formed of a thin metal film such as an aluminum film which has the lowest resistivity of all thin films used in the active matrix TFT liquid crystal apparatus, and which is also used to form data lines.
On the other hand, there are no currently available techniques which can be used to form the scanning lines of a thin metal film or a thin metal silicide film, because existing techniques would be subject to the problem that the scanning lines would peel off during a high-temperature process performed after the formation of the scanning lines. For the above reason, the scanning lines are generally formed of a thin polysilicon film. The interconnection formed of a thin polysilicon film has as high a resistance as 200 times that of an interconnection formed of a thin metal film, and thus has a correspondingly high time constant. For the above reason, if the image signal line was formed of a thin polysilicon film, degradation in the image quality would occur depending on the resistance and the time constant of the interconnection employed as the image signal line. In practice, to avoid the above problem, the image signal line is formed of a thin metal film, as described above.
In the above-described type liquid crystal apparatus including the peripheral circuit, if there is only one image signal line, it is possible to form the image signal line with an interconnection formed of a thin metal film at the same single layer level (produced in the same process) over the entire path from an external input terminal disposed at an end of a substrate to respective sampling switches of the sampling circuit. However, in the case where a plurality of image signal lines are required to transmit image signals expanded into a plurality of phases so as to handle high-frequency driving operation in the liquid crystal apparatus, or in the case where a plurality of image signal lines are required to handle respective colors of an RGB color image signal, at least one image signal line has to cross another image signal line somewhere in the path to sampling switches. That is, it is impossible to produce interconnections for all image signal lines using only a thin metal film at the same layer level.
Therefore, a relay interconnection is formed using a polysilicon film at a different layer level separated by an interlayer insulating film from the thin metal film. More specifically, one interconnection includes a first interconnection (main interconnection) made of a thin metal film and disposed at a crossing point. Another interconnection includes a second interconnection (relay interconnection) made of a thin polysilicon film extending at a different level and crossing below or above the first interconnection via an interlayer insulating film, wherein the second interconnection is electrically connected to a part of the interconnection made of a thin metal film via contact holes formed, at both sides of the crossing point, in the interlayer insulating film.
If only a part, which crosses another interconnection, is constructed in the form of a relay interconnection made of a thin polysilicon film, and the other parts are constructed in the form of main interconnections made of a thin metal film which are connected to each other via the relay interconnection as described above, then the relay interconnection made of the thin polysilicon film has a very small length, and thus the existence of the relay line made of the thin polysilicon film does not cause the image signal line to have so significant increases in the overall resistance and time constant that can cause a problem to occur during practical applications.
When a liquid crystal apparatus of the above-described type is used as a light valve in a liquid crystal projector or the like, a projection light ray is generally incident on an opposite substrate opposing, via a liquid crystal layer, a TFT array substrate. If the channel region made of an a-Si (amorphous silicon) film or a p-Si (polysilicon) film of TFTs is exposed to the projection light, a photocurrent is generated in the channel region by the photoelectric conversion effect, and the transistor characteristic of the TFTs are degraded. To avoid the above problem, a light blocking film, called a black matrix or a black mask, made of a metal material such as Cr (chromium), or a black resin is formed on the opposite substrate so that each TFT is covered with the light blocking film. This light blocking film defines opening areas (through which the projection light ray is allowed to pass) of the respective pixels. That is, the light blocking film serves not only to prevent the p-Si layer of TFTs from being exposed to light, but also to improve the contrast and prevent colors from mixing.
In the case of the liquid crystal apparatus for use as a light valve, in particular when a normal stagger type or coplanar type a-Si or p-Si TFT with the top gate structure (in which the gate electrode is disposed above the channel on the TFT array substrate) is employed, it is required to prevent the channels of
Alphonse Fritz
Saras Steven
Seiko Epson Corporation
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