Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-05-31
2002-05-21
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S143000
Reexamination Certificate
active
06392731
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an active matrix type liquid crystal display device which adopts an In-plane switching system mode as a display mode having a wide viewing angle.
2. Description of the Related Art
The In-plane switching (hereinafter also called IPS) mode which is one of display modes of an active matrix type liquid crystal display device has extremely wide viewing angle characteristics compared with the twisted nematic (hereinafter also called TN) mode which is currently the main stream. The IPS mode can realize a contrast ratio of not less than 10 at a viewing angle of 80° in upward and downward directions as well as in left and right directions without using a particular optical compensation film. Accordingly, it is expected that the IPS mode will be popularly used by display devices having wide screens such as OA monitors and televisions.
FIGS.
3
(
a
) and
3
(
b
) are schematic views showing the constitution and the operation principle of the conventional typical liquid crystal display device adopting the IPS mode, wherein FIG.
3
(
a
) shows an OFF state and FIG.
3
(
b
) shows an ON state. In the drawing, numeral
2
indicates comb-like electrodes (only central portions of a pair of comb-like electrodes are shown), numeral
3
indicates liquid crystal molecules, numeral
5
indicates an electrode substrate, numeral
6
indicates a counter substrate, numeral
7
indicates an alignment layer, numeral
8
a
indicates a polarization film disposed at the counter substrate side, numeral
8
b
indicates a polarization film disposed at the electrode substrate side, numeral
9
a
indicates a light transmission axis of the polarization film
8
a
, numeral
9
b
indicates a light transmission axis of the polarization film
8
b
, numeral
10
indicates a backlight, and numeral
11
indicates a direction of an applied electric field. Besides the above-mentioned constitution, between the alignment layer
7
and the electrode substrate
5
as well as between the alignment layer
7
and the comb-like electrodes
2
, insulation films may be interposed. Further, the comb-like electrodes
2
may be formed on the counter substrate side. As the material of the comb-like electrodes, an opaque material such as chromium or aluminum is used. This is because, the liquid crystal molecules rise up on the electrode due to the electric field in the vertical direction and hence, when the transparent material is used, the viewing angle characteristics are deteriorated.
Here, an alignment layer treatment such as rubbing is applied to the surface of the alignment layer such that the alignment direction makes an angle of &thgr;
1
relative to the long side direction of the comb-like electrodes and hence, the liquid crystal molecules in the OFF state where the voltage is not applied take a homogeneously aligned state and such an angle &thgr;
1
is set to approximately 5° in the drawing. Further, the liquid crystal is tilted at approximately 3° in an up and down direction toward the counter substrate
6
side and this tilting is called a pretilt angle. Further, the polarization film
8
a
has its light transmission axis arranged in parallel with the alignment direction of the liquid crystal, while the polarization film
8
b
is arranged such that the film
8
b
and
8
a
become crossed nicols. That is, in FIG.
3
(
a
), the transmission axis
9
a
of the polarization film
8
a
becomes equal to the direction of the rubbing treatment.
In the ON state shown in FIG.
3
(
b
), the twist deformation is induced in the liquid crystal layer due to the transverse electric field
11
applied perpendicular to the long side direction of the comb-like electrodes. Due to the birefringence effect of the liquid crystal layer caused by the twist deformation, the liquid crystal layer wholly or partially transmits the light and the transmittance thereof is controlled by the magnitude of the twisting of the liquid crystal.
The detail of such an operation principle is, for example, described in the paper “Liquid Crystals 22(4), pp. 379-390(1997)” written by M. Oh-e et al. Further, although the drawing shows the case that the Np type liquid crystal is used, there may be a case that the Nn type liquid crystal is used. Compared to the case which uses the Np type liquid crystal, in the case which uses the Nn type liquid crystal, the alignment layer treatment is performed in a 90-degrees rotated direction.
However, in the conventional active matrix type liquid crystal display device which adopts the IPS mode as the display mode, since the electric field is applied radially to the distal ends of the comb-like electrodes, there has been a problem that a reverse twisted region where the liquid crystal is twisted in the reverse direction is partially formed and hence, a disclination occurs in the periphery of the region.
FIG. 4
is a view which schematically shows the comb-like electrodes used in the liquid crystal display device which adopts the IPS mode. In the drawing, numeral
4
indicates the alignment treatment direction, numerals
11
a
,
11
b
,
11
c
indicate the direction of electric field, and numeral
13
indicates the disclination. In the central portions of the comb-like electrodes
2
, as depicted by the direction
11
c
of electric field, the electric field is applied in the direction substantially perpendicular to the long side direction of the comb-like electrodes and hence, no reverse twist occurs. At the distal ends of the comb-like electrodes
2
, however, as depicted by the directions
11
a
,
11
b
of electric field, the electric field is applied radially and hence the reverse twisting region
12
is formed by the electric field having the direction
11
a
of the electric field. Although the electric field having the direction
11
b
of the electric field is applied radially, as shown in the drawing, since the applying direction is aligned with the rubbing direction, such an application of electric field does not form the reverse twisting region.
Further, as shown in
FIG. 4
, in the conventional IPS system, the homogeneous alignment layer treatment is performed in the direction which makes &thgr;
2
relative to the long sides of the comb-like electrodes to control the twisting direction of the liquid crystal layer. However, there has been a problem that in case such homogeneous alignment layer treatment is performed, the viewing angle characteristics such as transmittance, contrast and the like do not become symmetrical in up and down directions as well as in left and right directions.
FIG. 5
shows the viewing angle dependency of the contrast in case the direction of the alignment layer treatment is set to 10°, wherein numeral
20
indicates equi-contrast lines of the contrast ratio 10 and numeral
21
indicates equi-contrast lines of the contrast ratio 15. As can be understood from the drawing, are rotated by the angle &thgr;
3
(=10°) and hence, the equi-contrast lines are asymmetrical in up and down directions as well as in left and right directions. In the conventional liquid crystal display device which uses the comb-like electrodes, the viewing angle characteristics are not less than 80° in an up and down direction as well as in a left and right direction and the contrast ratio is not less than 10. However, if the equi-contrast lines are symmetric in an up and down direction as well as in a left and right direction, substantial viewing angle characteristics as the display device are further enhanced.
In addition, in case the homogeneous alignment layer treatment is performed in the proper alignment layer treatment direction with the use of the comb-like electrodes shown in
FIG. 4
, the liquid crystal molecules are twisted in one direction and hence, it gives rise to a phenomenon that the color is changed corresponding to the direction of viewing angle.
FIG. 6
is a schematic view showing the change of color depending on the viewing angle, wherein numeral
30
indicates th
Fuchigami Hiroyuki
Kurata Tetsuyuki
Nishioka Takahiro
Saito Tadashi
Satake Tetsuya
Chung David
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Parker Kenneth
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