Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-27
2001-10-09
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S143000, C349S043000
Reexamination Certificate
active
06300996
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device. More particularly, it relates to a liquid crystal display device in which it is possible to improve orientation characteristics of a liquid crystal material sealed in a space between a pair of substrates facing each other.
DESCRIPTION OF THE RELATED ART
In a conventional liquid crystal display device, a twisted nematic system is widely used. This system has a drawback that the direction of orientation of liquid crystal molecules of an intermediate layer of the liquid crystal layers at the time of voltage application is uniform in a pixel such that the color tone becomes different depending on different angles of view. As means for improving this viewing angle characteristics, there is known an orientation split type liquid crystal display device. In this type of the liquid crystal display device, each pixel is split into plural regions and the direction of orientation of liquid crystal molecules in the intermediate layer in each region is caused to differ to effect compensation of the viewing angle characteristics of the respective regions one another to realize wide viewing angle characteristics.
JP Patent Kokai JP-A-4-223436 discloses a liquid crystal display wherein orientation processing is applied to a TFT substrate so as to align the liquid crystal molecules along either gate lines or drain lines in order to suppress disclination of the liquid crystal molecules. The gate lines are disposed in right angles to the drain angles.
A liquid crystal display device of the orientation splitting type is described in JP Patent Kokai JP-A-10-20323.
FIG. 10
shows a cross-sectional side view of essential portions of the liquid crystal display device described in this publication. This liquid crystal display device includes a plurality of gate line
22
and a plurality of drain line
23
, extending in the row and column directions on a substrate
31
, respectively, as shown in FIG.
7
. In each intersection of the gate line
22
and the drain line
23
, there are arranged a switching device, made up of a gate electrode
33
, a gate insulating film
34
, a semiconductor layer
35
, a drain electrode
36
and a source electrode
37
, and a pixel electrode
38
electrically connected to the source electrode
37
to apply a voltage across the liquid crystal material. The voltage applied across the pixel electrode
38
is controlled by the switching device. The pixel electrode
38
is provided with an opening (slit)
39
operating as the boundary of the orientation-split region. A control electrode
11
is disposed as an underlayer for the opening
39
. On the opposite side, there is provided a substrate
32
on which is arranged a common electrode
12
for applying a common voltage across each pixel electrode
38
. There is no necessity of providing the common electrode
12
with an opening. As uppermost layers of the substrates
31
,
32
, there are formed orientation films
13
,
14
, respectively, for retaining a liquid crystal molecule
15
sandwiched in-between.
If a voltage is applied across the control electrode
11
, there is generated an obliquely extending electrical field ranging from an end of the opening
39
to an end of the pixel electrode
38
. The liquid crystal molecule
15
is split in orientation by oblique electrical fields of different orientations produced with the opening
39
as a boundary.
FIG. 10
shows the direction of orientation of the liquid crystal molecule
15
mainly present in a mid layer of the liquid crystal layer. By illumination of UV rays, a minor amount of a UV curable monomer or oligomer previously mixed into a liquid crystal material is polymerized to a polymer
16
. The polymer
16
follows the direction of orientation of the liquid crystal molecule at the time of illumination of the UV rays and is fixed in its configuration when the voltage ceases to be applied across the control electrode
11
. Also, since the polymer
16
is present in a minor amount, the liquid crystal molecule is controlled only in the rising direction at the time of voltage application and is changed in its gradient corresponding to the applied voltage. Therefore, it is unnecessary to apply the voltage across the control electrode
11
at the time of driving, such that display is possible only by voltage application across the pixel electrode
38
.
FIGS. 7 and 8
show the plan views of essential portions of the liquid crystal display device shown in FIG.
10
and the state of splitting of the orientation in the liquid crystal display device of
FIG. 7
, respectively.
Referring to
FIG. 7
by way of example, the opening
39
and the control electrodes
11
, arranged as an underlayer below the opening
39
, are of a structure in which a pair of oppositely disposed Y letters (Y-bottom being connected to each other) are arranged in an up-and-down direction in the drawing sheet. This splits the pixel electrode
38
in four segmented regions
38
A to
38
D. If this structure is used in executing the orientation splitting process described above, the splitting configuration shown in
FIG. 8
is achieved. If the orientation films
13
,
14
are previously processed with orientation processing, such as with rubbing, a liquid crystal molecule
18
on the side of the pixel electrode and a liquid crystal molecule
19
on the side of the control electrode are oriented at an angle of approximately 45° with respect to the gate lines
22
and the drain lines
23
extending in the orthogonal directions shown in FIG.
7
. The directions of orientation of the liquid crystal molecules
18
,
19
are set so as to be substantially perpendicular to each other. Since the rising direction of the liquid crystal molecule
20
of each intermediate layer present in each of the split four regions
38
A to
38
D differ from one another, the viewing angle characteristics compensate one another, thus realizing wide viewing angle characteristics.
SUMMARY OF THE DISCLOSURE
However, the following problems have been encountered in the course of the investigations toward the present invention. Namely, the above-described orientation splitting type liquid crystal display device has a drawback that minute domains with different directions of orientation tend to be produced in each of the split regions.
FIG. 9
shows a plan view of the defective orientation domains in the above-described conventional liquid crystal display device. The reason of generation of the defective orientation domains
28
resides in the transverse electrical field generated from the gate lines
22
and the drain lines
23
(FIG.
7
). The directions of the transverse electrical fields Ef
1
, Ef
2
from the respective lines
22
,
23
are perpendicular to the extending directions A, B of the lines
22
,
23
. By these transverse electrical fields, the direction of orientation of the intermediate layer of the liquid crystal molecule
20
is forced to be parallel to the direction of the transverse electrical fields Ef
1
, Ef
2
, respectively.
Thus, during display driving, with the voltage being applied across the pixel electrode
38
, the liquid crystal molecules
20
of the intermediate layer are forced to be substantially perpendicular to the drain lines
23
, under the effect of the transverse electrical field Ef
2
from the drain lines
23
(FIG.
7
), as shown in
FIG. 9
, so that there are produced domains of defective orientation
28
having the distorted orientation different from that in other regions. If the domains of defective orientation
28
are produced, the area ratio of the respective regions obtained on orientation splitting (splitting ratio) is partialized (inbalanced) to certain part of the entire regions thus giving rise to fluctuations in the pixel-wise gradation observed mainly on changing the angle of view insofar as the display is concerned.
There is disclosed in JP Patent Kokai JP-A-8-76125 an orientation splitting type liquid crystal display device having an opening (slit) in a common electrode. This liquid crystal display d
Hirai Yoshihiko
Ishii Toshiya
Kobayashi Kazumi
Matsuyama Hiroaki
Murai Hideya
Hutchins, Wheeler & Dittmar
NEC Coporation
Ngo Julie
Sikes William L.
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