Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-12-06
2004-03-23
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S136000
Reexamination Certificate
active
06710832
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a high response speed liquid crystal display having a wide viewing angle.
BACKGROUND ART
One known type of liquid crystal displays which have been put to practical use is Twisted Nematic (TN) Mode liquid crystal displays using a nematic liquid crystal. This mode, however, has drawbacks such as low response speed and narrow viewing angles. Ferroelectric Liquid Crystals (FLC) are known as a display mode having high response speed and wide viewing angles but suffer from serious problems in shock resistance and temperature characteristics. The Polymer Dispersed Liquid Crystal Mode, which makes use of light scattering, is a rubbingless alignment display mode, yet it still needs improvements in viewing angles and its response is not fast enough.
To overcome their drawbacks, there has been proposed the Optically Compensated Bend (OCB) Mode as a display mode having fast response and wide viewing angles (Japanese Patent Publication (KOKAI) No. 7-84254 (1995)).
FIG. 21
shows a conceptual structural sectional view of the pixel region of an OCB mode liquid crystal display.
As shown in
FIG. 21
, this liquid crystal display of the OCB mode has alignment layers
19
which are oriented in a certain direction; a liquid crystal cell
14
in which a bend alignment or a bend alignment including a twisted alignment is generated at the center of the cell
14
by voltage application; and a phase compensator
3
for optical phase compensation for achieving low voltage actuation and enlarged viewing angles. Regarding performance, the OCB mode can provide an active matrix type liquid crystal display having a wide viewing angle and high response speed. In addition, it has the high potential of finding wide applications as a transmissive or reflective type liquid crystal display.
The above liquid crystal display is formed as follows. The alignment layers
19
are attached to the inner surface of an array substrate
6
on which switching elements
20
or the like each connected to an pixel electrode
18
for actuating a pixel region is placed and to the inner surface of an opposed substrate
5
having an posed electrode
17
thereon. These alignment layers
19
have undergone alignment treatment so as to be parallel to each other and have pretilt angles which are about several to ten degrees, being opposite to each other in a positive
egative sense. A nematic liquid crystal having positive dielectric anisotropy is inserted between the alignment layers
19
to form a liquid crystal layer
12
. Accordingly, there is formed a spray alignment
11
composed of an alignment region in which liquid crystal molecules are diagonally spread in a vertically symmetrical manner when no voltage is applied.
Then, a voltage higher than the critical transition voltage is applied across the above electrodes, whereby transition is caused to form the bend alignment
13
composed of an alignment region in which the liquid crystal molecules at the center are raised upright or the liquid crystal molecules including twisted alignment are raised in a twisted manner, as shown in FIG.
21
(
b
) and this region is extended.
At least one polarizer and at least one phase compensator are disposed on the outer sides of the upper and lower substrates, extending in a specified axial direction. Specifically, there are provided polarizers
1
,
2
and the phase compensator
3
for optically compensating for the bend alignment cell and lowering voltage in order to attain sufficient contrast and an increased view angle.
After the transition of the whole pixels, driving signal voltage is varied to change the degree of the bend alignment state of the liquid crystal molecules, thereby changing the phase difference to be utilized for motion display. Therefore, in the liquid crystal display of the OCB mode, it is necessary to develop transition cores for transition from the spray alignment to the bend alignment to bring the pixels into an uniform bend alignment state and to cause such transition throughout the TFT panel pixel region without fail. In reality, it is not easy to develop transition cores without fail.
When observing the above pixel region with a microscope, it has been found that transition cores develop around spacers which are provided so as to scatter for gap formation or that transition cores develop around source lines and gate lines wired along the pixel electrodes, but developing positions are not fixed. In some cases, transition cores are not developed at all. In this case, the resultant panel has alignment defects and visible defects.
As an attempt to promote the above transition, there has been proposed a method in which a voltage higher than the critical transition voltage is applied across the opposed electrode and pixel electrodes of the liquid crystal cell. However, the applied voltage and the time required for voltage application, which are necessary for the transition, vary considerably depending on liquid crystal materials. With some liquid crystal materials, a time ranging from tens of seconds to several minutes is required for applying about 20 V across the facing electrodes in order to cause a transition in a TFT liquid crystal panel of the OCB mode. This case is not practical not only in view of the wait time taken for starting displaying but also in view of the power consumption of the liquid crystal display and the reliability of the driving unit.
In such circumstances, there have been strong demands to a technique for enabling a reliable transition of alignment within all of hundreds of thousands of pixels in the display panel of an OCB mode liquid crystal display.
Disclosure of the Invention
A series of inventions has been made taking the present status of the technology described above into account. The prime object of the inventions is therefore to provide a liquid crystal display which has no display defects and high response speed and is therefore suitable for displaying moving pictures and which also has a wide viewing angle, since it can cause alignment transition with substantial reliability and complete it in an extremely short time. More particularly, the object of the inventions is to provide a bend alignment type liquid crystal display which has high response speed and is therefore suitable for displaying moving pictures, since it can cause bend alignment transition with substantial reliability and complete it in an extremely short time.
The series of inventions is based on the same or similar concepts, but embodied by different examples. In the present specification, these inventions are classified according to analogy into a first invention group, a second invention group, a third invention group, a fourth invention group and a fifth invention group. The contents of each group (i.e., each invention group) will be hereinafter described in order.
First Invention Group
The first invention group is associated with a liquid crystal display wherein where the alignment state of a liquid crystal to which no voltage is applied is defined as an alignment state
1
and the alignment state of the liquid crystal used for performing displaying is defined as an alignment state
2
, the alignment state
1
differs from the alignment state
2
; and wherein for easy, reliable transition from the alignment state
1
to the alignment state
2
, the unevenness of a surface of a substrate is flattened and an interface of a liquid crystal layer is flattened.
In liquid crystal displays having a conventional active matrix substrate, significant irregularities are usually present on the substrates. This is due to the process in which the top layer, that is, an insulating layer having the greatest level difference is removed to expose the pixel electrodes. Level differences are also caused by source wiring and gate wiring positioned around the pixels. Thus, the presence of level differences is usual for active matrix substrates.
In conventionally used TN type liquid crystal displays, the presence of level differences causes alignment disturbance at their positions, resulti
Hattori Katsuji
Ishihara Shoichi
Nakao Kenji
Tanaka Yoshinori
Uemura Tsuyoshi
Chung David
Matsushita Electric - Industrial Co., Ltd.
Parker Kenneth
Parkhurst & Wendel L.L.P.
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