1. Field of the Invention
The present invention relates to a liquid crystal display element using ferroelectric liquid crystal and relates to a manufacturing method thereof.
2. Description of the Prior Art
In recent years, an opportunity to handle a lot of and various information by individuals is increasing rapidly with liberalization of telecommunication, and thus personal data processing apparatuses such as notebook-sized personal computers have spread remarkably. Moreover, because of the spread of multimedia, the contents of information to be handled are not only simple character information but also image information of high quality such as pictures.
Under such circumstances, display devices as data output apparatuses, more specifically flat panel displays are very important. Moreover, among the flat panel displays, liquid crystal displays by which an image of high accuracy and high contrast ratio can be obtained are used widely as display devices.
As the characteristics of the liquid crystal display used as a flat panel display for the data processing apparatus, the following characteristics (a) through (e) are given:
(a) a high information content display such as XGA (Extended Graphics Array) and SXGA (Super Extended Graphics Array) whose information contact is the greatest in the flat panel display is possible;
(b) a level of the back ground xe2x80x9cblackxe2x80x9d is excellent by using a polarization film of crossed Nicols, and a high contrast ratio of 200:1 can be obtained;
(c) a problem of gray scale display remains, but a moving picture can be displayed on a level equivalent to VGA (Video Graphics Array);
(d) in the flat panel display, display with comparatively low power is possible; and
(e) product results in notebook-sized computers, etc. are achieved, and the great variety of peripheral members such as driver LSIs can be obtained.
However, with diversification of information to be handled, there arise technical problems of how to display an image of high quality such as a photograph on a liquid crystal display. As the technical problems of the conventional liquid crystal displays for attaining excellent functions, the following problems (A) through (D) are given:
(A) a viewing angle is small, so in order to enlarge the viewing angle, another functions such as an aperture ratio and producibility should be lowered;
(B) a response speed is slow, particularly the response speed of gray shades display is slow, and image quality of moving picture display is lowered remarkably;
(C) since the response of the moving picture display is slow, use efficiency of backlight is lowered, and thus luminance of a screen is lowered; and
(D) an adjusting range of color balance which is indispensable for television display is remarkably narrow, namely, the adjustment is actually impossible.
There exist some possible methods which cope with the aforementioned technical problems of the liquid crystal display. The method which is very likely to solve all the problems (A) through (D) at the same time is a surface stabilized ferroelectric liquid crystal (SSFLC) display which was suggested by Noel A. Clark and Sven T. Lagerwall in Applied Physics Letter (Vol. 36, pp.899) in 1980.
SSFLC has a wide viewing angle in principle, and the contrast ratio does not actually depend on the viewing angle. Moreover, since SSFLC responds at a high speed to moving picture display, the image quality is not lowered. Further, according to the report in SID (Society for Information Display) (SID 96 DIGEST pp.157) by M. D. Wand et al. in 1996, color display by time domain makes it possible to adjust a color balance on a level equivalent to or higher than that of the conventional CRT (Cathode Ray Tube).
As mentioned above, SSFLC is very likely to improve the functions of a liquid crystal display, but it has a problem of gray shades display which is the most important factor for higher image quality. Since SSFLC provides binary display in principle, the dot matrix method, the dither method or the like have been used for the gray shades display. However, the gray shades display with half tone is indispensable for high-quality image display in multimedia, particularly for moving picture display. The half tone display in a high information content liquid crystal display such as XGA is possible by the control of the light intensity utilizing mildness of a voltage-light intensity curve and the control of time in the time domain color display. Since both of the aforementioned methods could not be applied to the conventional SSFLC technique, although the SSFLC display has a good possibility, it has not been put to practical use.
As a method which overcomes these defects of SSFLC, J. S. Patel suggested a ferroelectric liquid crystal display having a structure in which a layer structure of SSFLC is continuously twisted in Applied Physics Letter (Vol. 60, pp.280) in 1992. Since this display method has a structure in which a liquid crystal molecular axis is continuously twisted, the liquid crystal molecules are continuously displaced according to the applied electric field intensity, and the half tone display is possible in principle.
However, practically it is almost impossible that the ferroelectric liquid crystal is orientated while being continuously twisted. Even if the ferroelectric liquid crystal can be orientated temporarily, the twisted structure cannot be held stably for a long time. This is because since the ferroelectric liquid crystal has a layer structure, it cannot oppose a strong force to form a layer structure only by the stabilization of an interface on the surfaces of upper and lower substrates as suggested by J. S. Patel. Therefore, if the twisted structure of the ferroelectric liquid crystal which was suggested by J. S. Patel can be realized by any new method, the SSFLC display which can provide the half tone display is possible, and thus the SSFLC display can respond to the requirement for flat panel display of high definition.
It is a main object of the present invention to provide a ferroelectric liquid crystal display element which is capable of moving picture display and half tone display at a high speed with a wide viewing angle, and relates to a manufacturing method thereof.
In the ferroelectric liquid crystal display element of the present invention, when a photopolymerization compound (molecular weight: not less than 300) having at least two ultraviolet light sensitive portions in its molecular structure is mixed with ferroelectric liquid crystal and an exposure to ultraviolet light is performed, a continuous structure is formed over almost whole length between two glass substrates, and a continuous twisted structure is formed between the glass substrates in the state of liquid crystal molecular orientation. Before the exposure to ultraviolet light, the photopolymerization compound does not exhibit satisfactory compatibility with the ferroelectric liquid crystal to be mixed, but exhibits phase separation in the mixed state. The ferroelectric liquid crystal has a bookshelf layer structure.
Since the ferroelectric liquid crystal display element of the present invention has such a structure, a viewing angle is wide, and high quality display by high-speed moving picture display and half tone display becomes possible. Particularly, when the ferroelectric liquid crystal display element of the present invention is driven by an active element such as TFT (Thin Film Transistor), a moving picture of high quality which has not been obtained before can be displayed on a liquid crystal display.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.