Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-09-10
2003-07-15
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S138000, C349S187000, C257S059000
Reexamination Certificate
active
06593977
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display apparatus, particularly, to an active matrix liquid crystal display apparatus and IC (Integrated Circuit) cards.
Currently, an active matrix liquid crystal display apparatus using active elements, represented by TFT (Thin Film Transistor) devices, have been widely used as monitors for personal computers and work stations, in view of their high image quality, which is comparable to a CRT (Cathode Ray Tube), their low power consumption, which is less than a CRT, and saving space. However, an active matrix liquid crystal display apparatus is expensive in comparison with a CRT, and the lowering of its price is required in order to provide for wider use of this apparatus.
As one of the methods for lowering its price, application of an organic thin film transistor (organic TFT), which can be manufactured readily, to the active element has been proposed. However, both a plasma Chemical Vapor Deposition (CVD) apparatus for manufacturing insulating layers and semiconductor layers of the current amorphous silicone TFT and a spattering apparatus for manufacturing electrodes are very expensive. Furthermore, the CVD method requires a high temperature in the range of 230-350° C. for forming films, and the throughput of the apparatus is low, because the apparatus requires frequent maintenance, such as cleaning and the like. On the other hand, an application apparatus and a vapor deposition apparatus for manufacturing an organic TFT is cheap in comparison with the cost of a CVD apparatus and a spattering apparatus. The temperature for forming films using the above apparatus is lower, and their maintenance is easier in comparison with the CVD apparatus and the spattering apparatus. Therefore, a significant decrease in production cost can be realized by using an organic TFT for a liquid crystal display apparatus.
Generally, an organic TFT is composed of a glass substrate, gate electrodes, a gate insulating film, source electrodes, drain electrodes, and an organic semiconductor film. The amount of charge at a boundary between the gate insulating film and the organic semiconductor can be altered from excessive to deficient, or vice versa, by varying the voltage applied to the gate electrode. Switching is performed by varying the value of drain current flowing through the source electrode/the organic semiconductor/the drain electrode.
A method for manufacturing an organic TFT using a sexithiophene oligomer vapor deposition film as the organic semiconductor film was disclosed in JP-A-8-228035 (1996). In (Y-Y, Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson; IEEE Transactions on Electron Devices, Vol. 44, No. 8, p 1325-1331 (1997)), a method of manufacturing a high performance organic TFT using pentacene vapor deposition film as the organic semiconductor film was disclosed. JP-A-8-191162 (1996) disclosed a method for manufacturing an organic TFT using organic materials for the semiconductor film, source electrodes, drain electrodes, and gate electrodes, wherein the gate insulating film is made of an insulating polymer having cyano-groups.
In accordance with JP-A-8-228035 (1996) and JPA-10-125924 (1998), the organic semiconductor films were formed by vapor deposition methods, but pattern formation of the semiconductor was not described. For instance, when the pattern is formed using a metallic mask made of molybdenum, the minimum size of the pattern becomes approximately 100 &mgr;m, which is larger than a pixel size (10×30 &mgr;m
2
) in the current liquid crystal display apparatus. When conventional lithography (photolithography) is used, deterioration by carrier injection into the semiconductor layer and the like in an annealing process for removing a polar solvent or a solvent used for resist materials becomes a concern. A two terminal electron element having an organic semiconductor layer, wherein a pattern was formed between two parallel electrodes on a substrate, was disclosed in JP-A-2-239663 (1990). In accordance with JP-A-2239663 (1990), a patterned insulating film, wherein electrode portions were removed, was formed on a lower electrode, and an organic semiconductor film having the same size as the lower electrode was formed using the lower electrode. However, the material was restricted to an organic material which would be an electron donor, such as tetracyanoquinonedimethane and the like, and other materials can not be used. Furthermore, the technique can not be applied to three terminal elements, such as a TFT and the like.
SUMMARY OF THE INVENTION
With respect to a conventional organic TFT, the organic semiconductor film can not be manufactured finely, and the area of the organic semiconductor film is larger than the gate electrodes. Therefore, an off-current caused by wraparound is increased. On account of the large area of the organic semiconductor film, the organic semiconductor film can not be covered by a light shielding layer, and off current with carriers generated by photo excitation is increased. As a result, an on/off ratio indicating the performance of the switching element is decreased. If the on/off ratio is decreased, for instance, when the device is used in a liquid crystal display active element, the applied voltage to the liquid crystal is decreased because the current flows even in an off condition, and the holding characteristics are deteriorated.
In accordance with a decrease in the off-current, the amount of gate voltage variation (S-value) necessary for increasing the drain current by one order, which is an index indicating the speed of the switching action, is increased, and the TFT characteristics are deteriorated.
If the above organic TFT is used for the active elements in the liquid crystal display apparatus, writing into a liquid crystal pixel may be produced by an adjacent signal line and lowering of the contrast results, because a TFT is composed of an organic semiconductor film between the drain electrode and the adjacent signal line.
One of the objects of the present invention is to provide a method of forming patterns, which makes it possible to avoid deterioration the TFT characteristics of the organic TFT element and a lowering of the contrast in the liquid crystal display apparatus due to the influence of an adjacent signal line.
In an organic thin film transistor comprising a substrate, gate electrodes, gate insulating layers, source electrodes, drain electrodes, and an organic semiconductor layer, the above object can be realized by making channel regions of the organic semiconductor film formed as patterns of the same size as the gate electrodes via the insulating layer which is formed as a pattern between the gate insulating layer and the semiconductor layer.
In accordance with the present invention, a photosensitive insulating film can be used instead of the insulating layer.
The present invention is characterized by use of such an organic thin film transistor as an active element in an active matrix liquid crystal display apparatus.
The organic TFT referred to here comprises conductive gate electrodes, gate insulating layers, source electrodes and drain electrodes which are arranged relative to each other horizontally with an interval between them, and an organic semiconductor layer. The organic TFT operates with either of an accumulating condition and a vacant condition depending on the polarity of the voltage applied to the gate electrodes.
The gate electrode relating to the present invention is arranged in the region between the source electrode and the drain electrode, and above or beneath the region having a longitudinal direction of the source/drain electrode as one side. The size of the gate electrode is desirably in the range from 1.1 times to 1.2 times of respective sides of the above regions in consideration of the position adjustment. As the material of the electrode, an organic material, such as polyaniline, polythiophene, and the like, or a conductive ink, which can be readily formed in the shape of the electrode by a coating method, is
Ando Masahiko
Ishihara Shingo
Wakagi Masatoshi
Antonelli Terry Stout & Kraus LLP
Chowdhury Tarifur R.
Hitachi , Ltd.
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