Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-01-20
2001-04-17
Malinowski, Walter J. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06219123
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a rubbing method, which can eliminate striped mottles appearing in half-tone display or near at a threshold voltage, hereby realizing quality display, and the present invention also relates to a liquid crystal display (LCD) processed by this method.
BACKGROUND OF THE INVENTION
It is a general tendency of the recent LCD market that a display capacity (a number of pixels) is increased in step with enlarging a screen size. To obtain a uniform display quality all over a large screen, how to process an orientation is a critical factor. An LCD has been oriented through rubbing a surface of polyimide orientation film with a rubbing buff. This film is made from thermesetting polyamic acid, and the rubbing buff is made of a cloth on which short fibers are transplanted. This orientation method requires simple process, a little time and reasonable cost.
This method, however, entails scrapes on the orientation film due to mechanical contacts between the rubbing-buff-tips and surface of the film. Strength of these mechanical contacts and a degree of scraping are greater at level differences on a substrate than other places, and particularly greater at edge portions thereon. The scrapes remaining on the surface sometimes produce some influence.
The striped mottles produced by an orientation and appearing on a color filter surface of a substrate, in particular, have not been eliminated although a rubbing condition has been changed. One example of the condition changes is that the buffing face is pressed vertically to the orientation film surface so that rubbing density can increase.
Striped mottles appearing on a moving direction can be illustrated in
FIG. 7
in which a thin disc represents a rubbing roller because the thin disc contacts a substrate surface at only one point for making a description simple. Thin disc
21
contacts substrate
7
atone point and rolls on shaft
19
b
of rubbing roller
19
in the direction indicated by arrow mark “b”. Solid line
20
showing a moving direction (arrow mark “a”) of glass substrate
7
and another solid line
20
a
showing a rotary direction projected on the surface of glass substrate produce an angle &thgr;. A prior art disclosed in the Japanese Patent Application Unexamined Publication No. H02-22624 teaches that the angle &thgr; is adjusted within the range of ±1°-45° so that the striped mottles can be eliminated.
The orientation process discussed above, however, is easily affected by the level differences on the surface of substrate because the thin disc contacts the surface at only one point. In the present market trend of increasing a display capacity, in particular, a transparent-conductive-film (ITO=indium-tin-oxide) is thickened in order to lower a resistance of ITO for overcoming cross-talk and variation of threshold values due to attenuation by an electrode. When an electrode pattern is formed on such a thickened ITO, grooves having level differences of several thousand angstroms appear linearly aligned. In the prior art where the thin disc having only one contact point with the film, the conventional rubbing buff of which fiber direction is the same as a rolling direction of the rubbing roller produces striped mottles due to these grooves.
The grooves produce influence at every rotation of the rubbing roller, and the influence is effected in series because of continuous rolling, whereby the striped mottles can be recognized while the LCD is operated. The mottles thus produced on LCD
24
are shown in FIGS.
11
. There are various types of mottles relative to places and widths such as “a1” parallel with a moving direction of the substrate, or wider (belt-like) mottles “a2”.
SUMMARY OF THE INVENTION
The present invention aims to provide an LCD orientation method that eliminates striped mottles to realize quality display and an LCD that is processed by the same method. The striped mottles are observed when the LCD is operated in half-tone display and also at around the threshold voltage.
The orientation method of the present invention is performed in the following order.
(a) Prepare orientation films both on an upper and a lower substrate of LCD.
(b) Prepare a cylindrical rubbing roller around which a rubbing buff having short fibers transplanted thereon is wrapped.
(c) Rub the orientation film by rolling this roller such that a shaft of the roller diagonally crosses with the moving direction of the substrate. The rubbing buff is affixed on the roller such that the fiber axial direction crosses diagonally with the roller rolling direction.
(d) A first rubbing is conducted within a crossing angle produced by the respective normal orientations of the upper and lower substrates, and in the direction within 5-30° with regard to the direction of normal orientation.
(e) A second rubbing determining the orientation direction is conducted. The method discussed above can prevent the striped mottles from appearing, and thereby obtaining quality display all over the LCD.
The fiber axial direction of the rubbing buff with regard to a rotary direction of the rubbing roller produces an angle &agr;. Then, the angle &agr; satisfies the following relation so that the striped mottles appear discontinuously and are not recognized:
D<(60×S/N) tan &agr;
where: N=rotating speed of the roller (rpm),
S=moving speed of a stage (mm/sec)
D=width of ITO electrode (mm)
Preferably, the orientation film is made of polyimide system material and has a pretilt angle of 3° or more.
REFERENCES:
patent: 5398127 (1995-03-01), Kubota et al.
patent: 5568296 (1996-10-01), Kodera et al.
patent: 64-44416 (1989-02-01), None
patent: 2-22624 (1990-01-01), None
patent: 5-45654 (1993-02-01), None
patent: 3032820 (1996-10-01), None
patent: 9-73087 (1997-03-01), None
patent: 10-123524 (1998-05-01), None
Matsukawa Hideki
Mochizuki Hideaki
Naito Yoshikatsu
Yamada Satoshi
Malinowski Walter J.
Matsushita Electric - Industrial Co., Ltd.
Parkhurst & Wendel L.L.P.
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