Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-08-08
2003-10-21
Chowdhury, Tarifur R. (Department: 7871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S111000, C349S043000, C349S044000, C349S039000
Reexamination Certificate
active
06636284
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an active-matrix type electro-optical device and, more particularly, to an electro-optical device of the type that includes pixel switching thin-film transistors (TFTs) within a laminate structure on a substrate.
2. Description of Related Art
If incident light rays enter the channel region of a pixel switching TFT arranged for each pixel in a TFT active-matrix type electro-optical device, a current is generated through photoexcitation, thereby changing characteristics of the TFT. Particularly, in an electro-optical device being used as a light valve in a projector, it is important to block incident light entering the channel region of the TFT and the peripheral area thereof because of the high intensity of the incident light. This can be accomplished by a light shield layer, arranged on a counter substrate and defining the aperture area of each pixel, or a data line fabricated of a metal film such as Al (aluminum) and extending over the TFT blocks light entering the channel region and the peripheral area thereof. Japanese Unexamined Patent Application Publication No. 9-33944 discloses a technique which reduces light entering the channel region using a light shield layer fabricated of a-Si (amorphous silicon) having a large refractive index. Furthermore, a light shield layer fabricated of a refractory metal, for example, is arranged on the TFT array substrate in a position facing the pixel switching TFT (i.e., beneath the TFT). The light shield layer mounted beneath the TFT prevents a rear surface reflection from the TFT array substrate, or prevents projection light coming in from another electro-optical device and penetrating a prism from entering the TFT of the electro-optic device when a plurality of liquid crystal devices are combined to form an optical system via the prism or the like.
SUMMARY OF THE INVENTION
In the conventional art, each of the above light shield techniques suffer from one or more of the following problems. The light shield layers arranged on the counter substrate and the TFT array substrate are not sufficient to block light entering the channel region of the thin-film transistor and the peripheral area of the thin-film transistor. Additionally, the light shield layer and the channel region are substantially spaced from each other in terms of interlayer distance, in 3-dimensional terms sandwiching a liquid-crystal layer, thee electrodes, and interlayer insulators therebetween. This structure fails to block light obliquely entering between the light shield layer and the channel region. In a compact electro-optical device used as a light valve in a projector, incident light from a light source is condensed, and a component of incident light entering at an oblique angle with respect to the electro-optical device is too large to be neglected. Insufficient light blocking against the obliquely entering light presents practical problems.
For example, light enters into the electro-optical device through an area having no light shield layer and is reflected from the light shield layer and the internal surface of the data line (i.e., the surface of the data line facing the channel region). Such light may be further reflected from the internal surfaces of the light shield layer and the data line, thereby becoming multiple reflections. The reflected light and resulting multiple reflections may reach the channel region of the TFT. In the technique of using the data line as the light shield layer, the data line is formed in a stripe and extends in a direction perpendicular to the scanning line in a plan view, and an interlayer insulator laminated between the data line and the channel region must be thick enough to reduce the adverse effect of capacitive coupling therebetween to a negligible level. Accordingly, in this arrangement, it can be difficult to assure sufficient light blocking.
In accordance with the technique disclosed by Japanese Unexamined Patent Application Publication No. 9-33944, an a-Si layer is deposited on a gate electrode, and the lamination of a relatively thick interlayer insulator between the gate electrode and the a-Si layer is required to reduce the adverse effect of capacitive coupling therebetween. As a result, the additional lamination of the a-Si layer and the interlayer insulator enlarges the laminate structure of the device, and it is still difficult to fully block obliquely entering light and internal reflections. To satisfy current consumers' demand for a higher quality display, high definition and high pixel pitch are required of the electro-optical device. As a higher definition display and a finer pixel pitch are introduced, the above-described light shield techniques have even more difficulty with assuring sufficient light blocking, and changes in the TFT transistor characteristics cause flickering, degrading the image quality of the electro-optical device.
To increase light resistance, the expansion of the formation area of the light shield layer has been contemplated. However, the expansion of the formation area of the light shield layer makes it difficult to heighten the aperture ratio of each pixel in an attempt to improve image brightness.
The light shield performance of the light shield layers formed on a counter substrate and a TFT array substrate is still not satisfactory. For example, the light shield layer having a light transmittance ratio of 0.1 to 0.01% is fabricated of Ti (titanium) or WSi (tungsten silicide). In the technique of using the data line as a light shield layer on the TFT array substrate, the light shield layer having a light transmittance ratio of 0.01% or so is typically fabricated of Al. Light from a light source of a projector is typically 10 M luxes or so. According to the study carried out by the inventors of this invention, the TFTs cause a photo-leakage current of 5E-11 [A] under light of 1000 luxes. As the electro-optical device has a higher definition and a finer pixel pitch to satisfy the consumers' demand, a photo-leakage current is generated even by a low level of light in the above-reference light shields. Specifically, a change in the transistor characteristics visibly degrades an image on a screen.
A preventive step of thickening the light shield layer and the data line for higher light shield capability has also been contemplated. However, if these layers are thickened, the laminate structure of the substrate suffers from stress, and a diversity of problems are then expected including a warp of the substrate, an increase of processing time for film formation and etching steps.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-33944, the light transmittance ratio of an a-Si layer is substantially higher than that of Ti, WSi, Al, etc. In the electro-optical device incorporating higher definition and finer pixel pitch designs, a light shield layer fabricated of a-Si cannot block light from a light source in a projector, in particular.
The present invention has been developed in view of the above problems, and it is a first object of the present invention to provide an electro-optical device which presents excellent light resistance, features a high aperture ratio in each pixel, and displays a high-quality image.
It is another object of the present invention to provide an electro-optical device which presents excellent light resistance with a thickness increase of a light shield layer controlled and displays a high-quality image.
To achieve the above objects, a first electro-optical device of the present invention can include a pair of substrates, an electro-optical material interposed between the pair of substrates, a plurality of pixel electrodes arranged in a matrix on one of the substrates, thin-film transistors respectively electrically connected to the pixel electrodes, an upper light shield layer having a crossing portion above the thin-film transistor on the one substrate, a lower light shield layer having a crossing portion beneath the thin-film t
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