Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-11-15
2003-04-29
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S110000, C349S039000, C349S160000
Reexamination Certificate
active
06556265
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention belongs to the technical field of a liquid crystal device according to the active-matrix driving method by thin film transistor (hereafter referred to as “TFT” whenever appropriate) driving, and particularly belongs to the technical field of liquid crystal devices of the format wherein a light-shielding film is provided to the lower side of TFTs, and used for liquid crystal projectors and the like.
2. Description of Related Art
Conventionally, in cases wherein such liquid crystal devices are used in liquid crystal projectors and the like as light valves, generally, projected light is cast in from the side of an opposite substrate positioned in a manner opposing a TFT array substrate across a liquid crystal layer. Now, in the event that projected light is cast into a channel area comprised of an a-Si (amorphous silicone) film or a p-Si (poly-silicone) film of a TFT, a photo-current is undesirably generated in this area due to photo-electric conversion effects, and the transistor properties of the TFT deteriorate. Accordingly, generally a light-shielding film formed of a metal material such as Cr (chromium), resin black, or the like is formed at each position facing each TFT on the opposite substrate. The light-shielding films serve to shield light to the opening area (namely, the area transmitting projected light) of each pixel by defining the opening area, and also having the functions of improving contrast, preventing mixing of color materials, and so forth, in addition to shielding light from the p-Si layer of the TFT.
In this type of liquid crystal device, particularly in the event that positive stagger type or coplanar type, a-Si or p-Si TFTs having a top-gate structure (namely, a structure wherein the gate electrode is provided above the channel on the TFT array substrate) are used, there is the need to prevent part of the projected light being cast into the TFT channel from the side of the TFT array substrate as returning light from the projection optical system within the liquid crystal projector. In the same way, there is the need to prevent reflected light from the surface of the TFT array substrate at the time the projected light passes through, or part of projected light cast from other liquid crystal devices passing through the projection optical system in the event that a plurality of liquid crystal devices are combined for color, being cast into the TFT channel from the side of the TFT array substrate as returning light. To this end, Japanese Unexamined Patent Publication No. 9-127497, Japanese Examined Patent Publication No. 3-52611, Japanese Unexamined Patent Publication No. 3-125123, Japanese Unexamined Patent Publication No. 8-171101, and so forth propose a liquid crystal device wherein a light-shielding film is formed of a non-transparent metal with a high melting point at a position facing a TFT on the TFT array substrate formed of a quartz substrate or the like (namely, to the lower side of the TFT).
On the other hand, with such liquid crystal devices, a method is generally used wherein pixel electrodes are provided with a storage capacitor in order to extend the amount of time that voltage is maintained to the pixel electrodes with regard to the time wherein image signals are supplied to the pixel electrodes by setting the TFTs in a conducting state by means of applying scanning signals to the gate electrodes, namely, so that the liquid crystal driving voltage can be applied in a sufficient amount of time even in the event that the duty ratio is small. In such a case, a portion of capacitance lines formed parallel to scanning lines generally comprise other storage capacitor electrodes.
SUMMARY OF THE INVENTION
There is a strong general request for improvement of image quality for liquid crystal devices, and it is important to raise the driving frequencies of liquid crystal devices to this end.
However, in order to provide a storage capacitor to the pixel electrodes as described above, in the event that a high-temperature process including a process wherein the substrate temperature is subjected to a high temperature such as 900 degrees or the like, capacitance lines including the one storage capacitor electrode are formed of a poly-silicone film as with the scanning lines, so lowering resistance is difficult as compared with wirings formed of low-resistance metal film such as Al as data lines, for example. Accordingly, the resistance and time-constant of the capacitance lines increases, and the potential of the capacitance lines fluctuates in the capacitance lines wired in a manner intersecting underneath a plurality of data lines, owing to capacitance coupling with each data line, resulting in a problem that the image quality deteriorates due to sideways cross-talk, ghosting, and the like.
More specifically, as shown in
FIG. 20
, in the event that an image
801
drawn with a gray background and a highly-contrasted black portion is to be displayed, in the case that image signals of a voltage (here, a voltage corresponding with black) partially differing from the image signal voltage (here, a voltage corresponding with gray) provided to other pixels in a pixel line along a scanning line are provided, writing is performed to each pixel in the pixel line before the potential fluctuation of the capacitance line owing to such capacitance coupling stabilizes. Accordingly, in the actually displayed image
802
, there is insufficient voltage at the pixels to the right and left of the pixel to which the image signal of a partially different voltage to be displayed black is provided, resulting in a phenomena wherein the entire line to be displayed as gray turns whitish, namely, sideways cross-talk, ghosting, or the like, is generated.
In this case, particularly, the closer the point wherein image signals of a voltage partially different to be displayed black are provided is to the end of completing writing to each scanning line, namely, the closer the pixel to be displayed black is to the end of a line opposite to the side from which scanning signals are supplied in the event that scanning signals are supplied to one scanning line from the right or the left, or the closer to the center the pixel is in the case that scanning signals are supplied from both ends, the more writing is performed to each pixel in the pixel line before the potential fluctuation of the capacitance line owing to such capacitance coupling stabilizes, so sideways cross-talk, ghosting, or the like, is generated more easily.
Such sideways cross-talk, ghosting, or the like, is generated more easily in the event that the driving frequency increases with liquid crystal devices of types such as so-called XGA, SXGA, and so forth, since the time constant of the capacitance line becomes relatively large. Further, in the event of performing pre-charging wherein pre-charging signals of a certain voltage level are each supplied to data lines before the image signals so that image signal voltage can be written to the data lines with a small load, there is the need to secure a horizontal retrace line period of a certain length for pre-charging, so time for the potential fluctuation of the capacitance line owing to capacitance coupling to stabilize cannot be sufficiently secured following providing the image signal with a partially different voltage at a point close to the point of completing writing in each scanning line. Accordingly, there is a problem in that it is difficult to present the above-described sideways cross-talk, ghosting, or the like, when performing pre-charging.
In order to solve the problem of such sideways cross-talk, ghosting, or the like, the data line inversion driving method wherein the polarity of driving voltage applied to the liquid crystal is inverted for each data line (
1
S inversion driving method) and the dot inversion driving method wherein each dot is inverted are effective, but these methods generate intense generation of disclination (defective alignment) of the liquid crystal following the data
Oliff & Berridg,e PLC
Schechter Andrew
Seiko Epson Corporation
Ton Toan
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