Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-04-05
2002-05-14
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S110000, C349S111000
Reexamination Certificate
active
06388721
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a technique which is suitably adapted for production of a substrate for a liquid crystal device, and a liquid crystal device and projection type display device based on the use thereof. This invention relates more particularly to a light shielding structure of the substrate for the liquid crystal device which is used as a pixel switching element of a thin film transistor (to be abbreviated as TFT hereinafter).
PRIOR ART
Conventionally, a liquid crystal device is put into practice where pixel electrodes have been arranged in the form of a matrix on a glass substrate, and TFTs made of an amorphous silicon film or a polysilicon film have been prepared in correspondence with each pixel electrode, and which is so constructed as to drive a liquid crystal by applying a voltage through the TFT to each pixel electrode.
Among such liquid crystal devices, one incorporating a polysilicon film of which it is possible to assemble peripheral driving circuits such as a shift register or the like on the same substrate through the same process, allows a high density integration of circuit elements and attracts general attention.
With the liquid crystal device incorporating TFTs, the top of a TFT for driving a pixel electrode (to be referred to as a pixel TFT hereinafter) is covered by a light shielding film such as a chromium film called a black matrix (or a black stripe), which is placed on the opposite substrate. This is to prevent the channel region of the TFT from being exposed to direct light which would otherwise cause a leakage current. However, a leakage current caused by exposure of the TFT to stray light may arise as a result of light reflected from a polarizer placed on the back surface of the liquid crystal device, not to mention the adverse effects due to incident light itself.
To minimize such leakage current due to reflective light, an invention is proposed in which the back surface of the TFT is also covered by a light shielding film (Japanese Patent Publication, No. Hei 3-52611). If the light shielding film is placed on the back surface of the TFT such that it exceeds in size the opening of the black matrix placed on the opposite substrate, incident light strikes directly on the light shielding film, and light reflected therefrom illuminates the channel region of the TFT, which may cause it to generate a leakage current. This is because, when a process necessary for the placement of a light shielding film on the back surface of the TFT is put into practice, precise alignment of a black matrix placed on the opposite matrix with a pixel region placed on the substrate for the liquid crystal device is difficult, and thus incident light through the opposite substrate directly impinges and is reflected on the part of light shielding film that exceeds in size the opening of the black matrix As a result, the channel region of TFT is illuminated, causing the leakage current to flow. Particularly when alignment of the light shielding layer placed on the substrate for the liquid crystal device with the black matrix takes place with a large error, light reflected from the surface of light shielding film increases considerably, and, as the channel region is illumined by this reflective light, a leakage current from the TFT is increased, resulting in a degraded display as a result of flaws such as cross-talks or the like.
The object of this invention is to provide a technique which, when applied to a liquid crystal device, can minimize a leakage current generated from a TFT exposed to light. Another object of this invention is to provide a technique which can minimize a leakage current from a TFT exposed to light, without resorting to a black matrix placed an the opposite substrate.
DISCLOSURE OF THE INVENTION
To achieve the above objects, this invention is characterized by providing a substrate for a liquid crystal device as described in claim
1
comprising:
a plurality of data lines formed on the substrate;
a plurality of scan lines crossing the plurality of data lines;
a plurality of thin film transistors connected to the plurality of data lines and scan lines; and
a plurality of pixel electrodes connected to the plurality of thin film transistors; wherein:
a first light shielding film formed at least below a channel region of the thin film transistor, and the junctions between the channel region and source/drain regions; and
a second light shielding film formed above the channel region and the junctions between the channel region and the source/drain regions.
According to the substrate for a liquid crystal device, light impinging from above on the channel region and on the junctions between the channel region and the source/drain regions is shielded by the first shielding film, and light impinging from below is blocked by the second light shielding film. Through this arrangement, a leakage current which would otherwise be generated in the TFT exposed to light can be stably reduced.
The substrate for the liquid crystal device as described is characterized in that the first light shielding film may be a metal film selected from the group consisting of a tungsten film, titanium film, chromium film, tantalum film and molybdenum film, or an alloy film thereof.
According to the substrate for the liquid crystal device, when a metal film or a metal alloy film which is highly impenetrable to light and highly electrically conductive is used as a first light shielding film, it effectively acts as a light shielding film against reflective light from the back surface of the substrate for the liquid crystal device, and protects the channel region and the junctions between the channel region and the source/drain regions from exposure to light.
The substrate for the liquid crystal device as described is characterized in that a first lead extending from the first light shielding film is electrically connected to a constant potential line outside a pixel display region.
According to the substrate for the liquid crystal device, when the first light shielding film is formed in a floating state below the channel region of the TFT, irregular potential differences are generated between different terminals of the TFT, which may affect the TFT's performance. As a measure against such inconvenience, the first light shielding film must be stabilized at a specific potential level. This is the reason why the first lead extending from the first light shielding film is connected to a line having a constant potential such as a ground potential, outside a display region. This measure serves for inhibiting generation of potential differences among different terminals of the TFT, thus preventing alteration of TFT performance and occurrence of degraded display quality.
The substrate for the liquid crystal device as described is characterized in that the first lead extending from the first light shielding film is formed along and beneath the scan line.
According to the substrate for the liquid crystal device, the first lead extending from the first light shielding film is formed along and below the scan line. Through this arrangement it is possible for the lead to run without encroaching the aperture of the pixel. However, the first light shielding film is placed below the scan line and is positioned with respect to the side of scan line close to the aperture area of the pixel in such a way as to prohibit the direct impingement of incident light on the surface of first light shielding film.
The substrate for the liquid crystal device as described is characterized in that a width of the first lead extending from the first light shielding film is less than a width of the scan line formed above it.
The substrate for the liquid crystal device as described is characterized in that the first lead extending from the first light shielding film is covered by the scan line formed above it.
According to the substrate for the liquid crystal device as described, the scan line can prevent the first lead extending from the first light shielding film from being directly exposed t
Chowdhury Tarifur R.
Oliff & Berridg,e PLC
Seiko Epson Corporation
Sikes William L.
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