Liquid crystal display element and manufacturing method thereof

Details Liquid crystal display element and manufacturing method thereof Liquid crystal display element and manufacturing method thereof

1999-04-21

2002-10-08

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S123000

Reexamination Certificate

active

06462796

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a ferroelectric liquid crystal display element which realizes analog-type gray scale display, and a method for producing the same.
BACKGROUND OF THE INVENTION
Ferroelectric liquid crystal was first synthesized by R. B. Meyer in 1975. Thereafter, it was confirmed by N. A. Clark and S. T. Lagerwall that in the case where a gap between substrates provided vis-a-vis is narrowed, ferroelectric liquid crystal exhibits bistability.
Further, a variety of researches have been done on ferroelectric liquid crystal to seek for application of its utility, and consequently various display modes have been contrived. A typical one among them is a display mode in which liquid crystal material has positive dielectric anisotropy and is aligned in a C
1
orientation. The C
1
orientation refers to an alignment wherein liquid crystal molecules
22
are tilted at wider angles with respect to smectic layers
21
which are bent to form a chevron structure, as shown in FIG.
2
(
a
).
A mode generally called &tgr;-Vmin mode, in which liquid crystal material has negative dielectric anisotropy and is aligned in a C
2
orientation, is also typical. The C
2
orientation refers to an orientation, as shown in FIG.
2
(
b
), wherein the liquid crystal molecules
22
are tilted at narrower angles with respect to the smectic layers
21
which are bent to form the chevron structure. This mode is characterized in that due to the C
2
orientation, high response is easily realized as compared with the case of the C
1
orientation, and that since the liquid crystal material has negative dielectric anisotropy, a memory angle is increased by a bias voltage.
In the foregoing two modes, since the liquid crystal molecules
22
basically exhibit binary bistability, only gray scale display with two gray levels (black and white) is obtained in display in the case where either of the modes is applied to a display device. Therefore, as to such a conventional display device, to increase the number of gray levels in gray scale was an object to be achieved. By using the frame division driving scheme, the pixel division scheme, or the like, the number of gray levels in gray scale can be increased to some extent, but to further increase the number of gray levels in gray scale is hardly possible because of limitations of the liquid crystal material and the device structure. Particularly to achieve the gray scale of 256 gray levels per one color of RGB which is required for full color display is extremely impossible.
Therefore, realization of the analog-type gray scale display in a ferroelectric liquid crystal display element has been demanded. To meet this demand, the following, for example, have conventionally been proposed: a method wherein analog-type gray scale display is realized by generating domains using as nuclei fine particles mixed in liquid crystal (see the Japanese Publication for Laid-Open Patent Application No. 194693/1994 (Tokukaihei 6-194693), date of publication: Jul. 15, 1994); and a method wherein analog-type gray scale display is realized by generating domains using polymers dispersed in liquid crystal (see the Japanese Publication for Laid-Open Patent Application No. 236830/1997 (Tokukaihei 9-236830), date of publication: Sep. 9, 1997, which corresponds to the U.S. patent application Ser. No. 09/066215).
In the case where these methods are applied to analog-type gray scale display, however, there arises a problem of display history that display of a current frame is affected by a state of a display of a previous frame. More specifically, in the case where signals of black data are continuously applied to pixels and thereafter signals of half tones are continuously applied to pixels, the intensity of light transmitted through pixels, supposing that display history would not exist, immediately changes from a black level to a half-tone level as shown in FIG.
15
(
a
). In contrast, since display history exists in practice, as shown in FIG.
15
(
b
), the intensity of the transmitted light does not immediately change to a half-tone level after application of half-tone data and gradually changes to reach the half-tone level after several frames.
Conventionally, such a scheme as using a conductive alignment film or shortening each frame time of a driving signal (The 22nd Japanese Liquid Crystal Conference, pp.191-192) has been preferably applied to solve the foregoing problem. There has been also proposed a scheme of giving conductivity to an alignment film by mixing a conductive substance therein (Japanese Journal of Applied Physics, Vol. 28, No. 1 January 1989, pp. L116-L118, date of publication: Dec. 15, 1988). This scheme, however, is used for enhancing the bistability of liquid crystal.
Incidentally, the following description will explain behaviors of ferroelectric liquid crystal in the case where an ionic substance is mixed in the ferroelectric liquid crystal.
Usually, mixture of an ionic substance in ferroelectric liquid crystal adversely affects characteristics of ferroelectric liquid crystal. For example, a burn-in phenomenon occurring to a display screen, and a uni-stable state, i.e., a state in which only one state of bistable states is stabilized, are induced. This is caused by accumulation of ions in liquid crystal. As a method for preventing such adverse influences of ions, there have been proposed a method of causing an alignment film to attract ionic substances (see the Japanese Publication for Laid-Open Patent Application No. 245962/1996 (Tokukaihei 8-245962), date of publication: Sep. 24, 1996) and a method of injecting ion-removed liquid crystal into an empty cell (see the Japanese Publication for Laid-Open Patent Application No. 181508/1995 (Tokukaihei 7-181508), date of publication: Jul. 21, 1995).
In some cases, ions enhance characteristics of ferroelectric liquid crystal. For example, in the case where ferroelectric liquid crystal is driven by TFTs (thin film transistors), mixture of ions produces an electrical double layer in the ferroelectric liquid crystal, which, without raising a signal voltage, prevents a voltage applied to the liquid crystal from lowering (see the Japanese Publication for Laid-Open Patent Application No. 43643/1997 (Tokukaihei 9-43643), date of publication: Feb. 14, 1997). In this method, liquid crystal is injected into a cell after ionic substances are directly mixed in the liquid crystal.
Actually, however, the problem of display history cannot be sufficiently solved by using a conductive alignment film or by shortening a frame time of a driving signal. Besides, by the method of mixing ions into liquid crystal before injection of the liquid crystal, ions are not homogeneously dispersed in the liquid crystal due to a chromatography phenomenon, whereby domains also do not become homogeneous. Therefore, this method cannot be effective to solve the problem of display history.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a liquid crystal display element which is capable of forming fine domains in liquid crystal and drastically reducing influences of display history, as well as a method for producing the same.
To achieve the foregoing object, a liquid crystal display element of the present invention, which includes a pair of substrates with an insulating property on each of which electrodes are formed, an alignment film provided on at least one of the substrates so as to cover the electrodes thereon, and a liquid crystal layer made of a ferroelectric liquid crystal material sealed between the substrates, is characterized in that the alignment film contains a substance with ionic bond in the vicinity of the surface or inside thereof.
According to the foregoing arrangement, the alignment film contains the substance with ionic bond in the vicinity of its surface or inside. Therefore, ions are eluted from the alignment film into the liquid crystal layer, whereby ions are homogeneously dispersed in the liquid crystal layer. Therefore, during the half-tone display, fine domains are formed in th

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