Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1997-06-17
2001-06-26
Wu, Shean C. (Department: 1756)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S106000, C349S138000, C349S139000, C349S147000, C349S148000, C349S184000, C428S001100
Reexamination Certificate
active
06252641
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a liquid crystal device used as, e.g., terminal displays for computers, various flat panel displays for word processors, typewriters and television receivers, view finders for video cameras, light valves for projectors, and light valves for liquid crystal printers. The present invention also relates to a liquid crystal apparatus using the liquid crystal device.
The most popular and extensively used display may be CRTs (cathode ray tubes) which have been widely used for displaying motion (moving) pictures of television and video tape recorders or as monitor displays for personal computers. Based on the operation characteristic, the CRT is accompanied with difficulties such that the recognizability of a static image is lowered due to flickering and scanning fringes caused by an insufficient resolution, and the fluorescent member is deteriorated due to burning. Further, it has been recently found that electromagnetic wave emitted from CRTs can adversely affect human bodies and health of VDT (video display terminal) operators. Further, the CRT structurally has a large rearward space behind the display surface, so that the space economization in offices and at home may be obstructed thereby.
As a type of device solving such problems of the CRT, there has been known a liquid crystal device, including a type using a twisted nematic (TN) liquid crystal as disclosed by M. Schadt and W. Helfrich, “Applied Physics Letters”, Vol. 18, No. 4 (Feb. 17, 1971), pp. 127-128.
The liquid crystal device using a TN-liquid crystal includes a simple matrix-type liquid crystal device which is advantageous from a viewpoint of production cost. This type of liquid crystal device is however accompanied with a problem that it is liable to cause crosstalk when driven in a multiplex manner by using an electrode matrix of a high pixel density, and therefore the number of pixels is restricted.
In contrast with such a simple matrix-type liquid crystal device, a TFT-type liquid crystal device has been developed in recent years, wherein each pixel is provided with and driven with a TFT (thin film transistor). As a result, the problems of crosstalk and response speed can be solved but, on the other hand, a larger area device of the type poses an extreme difficulty in industrial production thereof without inferior pixels. Further, even if such production is possible, the production cost can be increased enormously.
For providing improvements to the above-mentioned difficulties of the conventional types of liquid crystal devices, a liquid crystal device of the type which controls transmission of light in combination with a polarizing device by utilizing a refractive index anisotropy of ferroelectric (or chiral smectic) liquid crystal molecules, has been proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application (JP-A) 56-107216, U.S. Pat. No. 4,367,924). The ferroelectric liquid crystal generally has chiral smectic C phase (SmC*) or H phase (SmH*) in a specific temperature range and, in the phase, shows a property of assuming either one of a first optically stable state and a second optically stable state in response to an electric field applied thereto and maintaining such a 'state in the absence of an electric field, namely bistability, and also have a very quick response speed because it causes inversion switching based on its spontaneous polarization. Thus, the ferroelectric liquid crystal develops bistable states showing a memory characteristic and further has an excellent viewing angle characteristic. Accordingly, the ferroelectric liquid crystal is considered to be suitable for constituting a display device or a light valve of a high speed, a high resolution and a large area. Further, an anti-ferroelectric liquid crystal device using a chiral smectic liquid crystal showing three stable states has been proposed recently by Chandani, Takezoe, et al (Japanese Journal of Applied Physics, Vol. 27 (1988), pp. L729-L732).
Such a chiral smectic liquid crystal is accompanied with problems, such as the occurrence of zigzag-shaped alignment defects and twisting of liquid crystal molecules between a pair of substrates (called “splay alignment”) leading to a lowering in contrast (as described in, e.g., “Structures and Properties of Ferroelectric Liquid Crystals” authored by Atsuo Fukuda and Hideo Takezoe; Corona Sha Publishing Co. Ltd., (1990)). The defects are considered to be attributable to a layer structure of a chiral smectic liquid crystal including two types of chevron structures between a pair of substrates.
A known method of solving the above problem is to provide the liquid crystal molecules with a pretilt angle, thereby uniformizing the chevron layer structure in one direction so that the twisting state (splay alignment state) of liquid crystal molecules between the pair of substrate is made more unstable than a uniform alignment state in respect of elastic energy.
Another method is to form a liquid crystal layer structure not of the bent chevron structure but of a bookshelf structure wherein smectic layers are little inclined but aligned generally in parallel or a structure close thereto, thereby removing the zigzag defects and realizing the uniform alignment to provide a high contrast (as disclosed in, e.g., “Future Liquid Crystal Display and Its Materials” edited by Atsuo Fukuda; K.K. C.M.C., (1992)). In a method of realizing a bookshelf layer structure, a naphthalene-based liquid crystal material is used. In this case, however, the resultant tilt angle is on the order of 10 deg. and is much smaller than 22.5 deg. which theoretically provides a maximum transmittance, so that the device can exhibit only a low transmittance. Another method is to apply an external electric field to a liquid crystal device containing a liquid crystal in a chevron structure to induce a bookshelf structure, but the resultant structure is rather unstable against an external stimulation, such as a temperature fluctuation.
As a liquid crystal material providing a bookshelf structure or a structure close thereto, a mesomorphic compound having a perfluoroalkyl ether terminal chain (U.S. Pat. No. 5,262,082), a liquid crystal composition containing such a mesomorphic compound (Marc D. Raddiffe et al., The 4th International Ferroelectric Liquid Crystal Conference, p-46 (1993)), etc., have been proposed. By using such a liquid crystal material, it is possible to provide a bookshelf structure or a similar structure having a small layer inclination angle with an optimum tilt angle without using an external field, such as an electric field.
However, such a liquid crystal material does not generally assume cholesteric phase, thus being liable to fail in providing a sufficiently good alignment state.
A liquid crystal device comprises a pair of electrode plats each including a substrate and at least an electrode formed thereon, and a liquid crystal disposed between the electrode plates. The electrode generally comprises transparent electrodes of, e.g., ITO (indium tin oxide). Such transparent electrodes are required to have a high transmittance of light and a low (electrical) resistivity. If the light transmittance of the transparent electrodes, etc. is low, a resultant contrast is undesirably lowered since the liquid crystal device is used as, e.g., displays or optical shutters by controlling the light transmittance. In addition the liquid crystal device may be considered as an electrical circuit providing a capacitive load since the device is used by applying an electric field to a liquid crystal disposed between a pair of electrode plates. Accordingly, a problem of voltage waveform deformation (distortion) due to electrical signal delay with respect to the transparent electrodes cannot be neglected.
However, the transparent electrodes of ITO generally have a fairly high resistivity (sheet resistance=20-400 ohm; volume resistivity=200×10
−8
-4000×10
−8
ohm.m) when compared with a metallic material (e.g., volume resis
Kamio Masaru
Katakura Kazunori
Matsuo Yuji
Okada Shinjiro
Sato Koichi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Wu Shean C.
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