Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-06-04
2004-02-24
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000
Reexamination Certificate
active
06697141
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a liquid crystal display and a fabrication method thereof. More particularly, the present invention relates to a liquid crystal display which provides a wide viewing angle and a high-speed response and a fabrication method thereof.
BACKGROUND ART
First Background Art
FIGS.
23
(
a
)(
b
) are side cross-sectional views showing an operation of a liquid crystal in the conventional liquid crystal panel and FIGS.
23
(
c
)(
d
) are front views thereof. In
FIG. 23
, an active element is omitted. Here, part of one pixel is shown, although a plurality of pixels are formed by stripe-shaped electrodes.
FIG.
23
(
a
) is a side cross-sectional view showing a cell in the state in which no voltage is applied and FIG.
23
(
c
) is a front view thereof. Line-shaped electrodes
103
,
104
are formed on an inner surface of one of a pair of transparent substrates and an alignment control layer
106
is applied thereon and has been subjected to alignment treatment. A liquid crystal composition is interposed between the substrates. While no voltage is applied, bar-shaped liquid crystal molecules
105
are oriented to have a certain angle with respect to the longitudinal direction of a stripe-shaped Y electrode, i.e., 45 degrees ≦|&phgr;Lc|<90 degrees (&phgr;Lc: an angle with respect to a long axis (optical axis) of liquid crystal molecules in the vicinity of interfaces). Here, it is assumed that an orientation direction of the liquid crystal molecules on upper and lower interfaces is parallel, and dielectric anisotropy of the liquid crystal composition is positive.
Subsequently, when an electric field
109
is applied, as shown in FIGS.
23
(
b
), (
d
), the liquid crystal changes its direction according to the electric field. By placing a polarizer
102
at a predetermined angle
108
, light transmittance can be varied by application of a voltage. Thus, display with contrast is realized without a transparent electrode.
However, in such an In-Plane Switching type liquid crystal display, a response of a nematic liquid crystal to the electric field is slow. In addition, as shown in
FIG. 23
, since the electrode has a peculiar structure such as the strip shape and it is difficult that the electric field is applied to the liquid crystal, the response speed is low.
Rise time &tgr;rise and Fall time &tgr;fall of the liquid crystal of the In-Plane Switching type are represented by the following expressions disclosed in Japanese Laid-Open Patent Publication No. Hei. 7-225388:
&tgr;rise=&ggr;
1
/(&egr;
0
&Dgr;&egr;E
2
−&pgr;
2
K
2
/d
2
) (1)
&tgr;fall=&ggr;
1
d
2
/&pgr;
2
K
2
=&ggr;
1
/&egr;
0
&Dgr;&egr;E
2
(2)
where &ggr;
1
is viscosity coefficient, K
2
is elastic constant of twist, d is cell gap, &Dgr;&egr; is dielectric anisotropy, &egr;
0
is vacuum dielectric constant, E is electric field strength, and Ec is threshold electric field.
In order to achieve the high-speed response in the In-Plane Switching type liquid crystal display, according to the first and second expressions, the cell gap disreduced, a liquid crystal material with low viscosity coefficient and high dielectric constant (e.g., cyano-based liquid crystal material) is employed, or a drive voltage is increased to increase an electric field strength E.
However, the following problems remains unsolved in the above-described liquid crystal display.
(1) If the cell gap is reduced, then time required for filling the liquid crystal and hence, time for fabrication are increased. Also, uneveness due to a variation in gap precision is noticeably observed.
(2) When a cyano-based liquid crystal material is used instead of a fluorine-based liquid crystal material or an addition ratio thereof is increased, heat resistance and light resistance become unstable, which might bring about inferior display such as partial abnormality in contrast or flicker.
(3) If the drive voltage is increased, a consumed power is correspondingly increased. Besides, a drive IC which has been conventionally used cannot be used and there is a need for a dedicated drive ID.
(4) If the transparent electrode such as ITO is used as a pixel electrode or a common electrode for the purpose of improvement of the transmittance and response speed, it is necessary to form a layer with larger thickness. But, if the layer is formed so as to be thicker, transmittance is reduced and layer surface is roughened because fine crystals are deposited. For this reason, a light diffusing value is increased and light availability is reduced.
Second Background Art
FIG. 24
is a cross-sectional view showing an In-Plane Switching type liquid crystal display disclosed in Japanese laid-Open Patent Publication No. Hei. 9-236820. As defined herein, the In-Plane Switching is a method in which a pixel electrode and a counter electrode are both formed on an inner surface of one of transparent substrates, potential is given across the pixel electrode and the counter electrode on the same plane, and a lateral electric field parallel to planes in which the transparent substrates respectively exist is applied to the liquid crystal, thereby controlling arrangement of liquid crystal molecules. This method is directed to improving display viewing angle dependency of the display.
FIG.
24
(
a
) shows a cross section in the direction orthogonal to a source bas line (video signal line) and in the vertical direction (direction orthogonal to the substrate surface) of a portion including no semiconductor switching element mentioned later. FIG.
24
(
b
) shows a cross section of the portion including the semiconductor switching element. FIG.
24
(
c
) shows a cross section of the portion in the direction parallel to the source bas line and including the semiconductor switching element.
In
FIG. 24
,
201
a
denotes a lower transparent substrate and
201
b
denotes an upper transparent electrode.
202
denotes a counter electrode.
203
a
denotes a gate electrode.
204
denotes a source bas line.
205
denotes a pixel electrode and
205
a
denotes its extended end portion.
206
denotes a semiconductor switching element.
207
denotes a liquid crystal layer.
208
a
denotes a lower alignment layer and
208
b
denotes an upper alignment layer.
209
denotes a transparent insulating layer.
As shown in
FIG. 24
, in this liquid crystal display, the two transparent substrates
201
a,
201
b
are placed opposite to each other, the liquid crystal is filled between opposed surfaces thereof, and alignment layers in contact with the upper and lower surfaces of the liquid crystal layer are adapted to align the liquid crystal molecules to have a predetermined orientation, which has been conventionally adopted.
The characteristics of this display are as follows. A transparent insulating layer
209
is placed between the alignment layer
208
a
and the transparent substrate
201
a
on the side of the array substrate, i.e., the transparent substrate on which the electrode is formed, which corresponds to the substrate
201
a
in this display. The transparent insulating layer serves to insulate between the source bas line and the counter electrode and between the source bas line and the pixel electrode. In addition, the counter electrode and the source bas line can overlap with each other when seen from a user of this display (when the user of this display sees a display screen).
This constitution enables an area of a light-blocking portion due to the existence of the electrode to be reduced and an aperture ratio of the pixel portion to be increased. Consequently, luminance of the entire screen is improved.
However, the following problems exists in this liquid crystal display.
(1) When the pixel electrode and the counter electrode formed in the pixel portion are comprised of non-transparent conductive layers, light is not transmitted through these electrodes, thereby causing the aperture ratio to be decreased. On the other hand, even if the pixel electrode and the counter electrode are comprised of the transparent
Asada Satoshi
Inoue Kazuo
Kimura Masanori
Kumagawa Katsuhiko
Satani Hiroshi
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Rude Timothy
Ton Toan
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