Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1997-09-02
2002-08-20
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S117000
Reexamination Certificate
active
06437844
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to liquid crystal display devices well suited for use in computer displays, television receivers and other industrial products and to methods for fabricating them. More particularly, the invention pertains to light-transmissive type and light-reflective type liquid crystal display devices capable of providing rapid response and a wide range of viewing angles and to fabrication methods thereof.
(2) Description of the Related Art
There have been practically used twisted-nematic (TN) liquid crystal display devices incorporating a nematic liquid crystal. The TN mode, however, has the drawback of poor response. Another disadvantage of the TN mode is that viewing angles, that is, angles through which the viewer can see images properly are narrow. Concretely, when diagonally viewing images in a TN liquid crystal display device, brightness and contrast decrease and gray scale inversion occurs. For this reason, such TN mode is unacceptable for liquid crystal display systems that operate at high speed to provide animatic images or require good angular viewability when viewed in diagonal directions. Another known type of liquid crystal display devices is the Polymer Dispersed Liquid Crystal (PDLC) mode that utilizes the effect of light dispersion. This mode advantageously provides high brightness, because it does not require use of a polarizing plate. However, the response speed of the PDLC mode is as low as that of TN liquid crystal display devices. Additionally, the PDLC mode provides a wide range of viewing angles but the viewing angles of the PDLC mode cannot be controlled in principle by a phase compensating layer like the TN mode. There have been developed other types of liquid crystal display devices: Ferroelectric Liquid Crystal (FLC) and Anti-Ferroelectric Liquid Crystal (ALFC). These modes suffer from the critical problems of poor shock resistance and temperature characteristics and therefore have not been put to practical use.
In an attempt to overcome the foregoing problems, Optically Compensated Bend (OCB) liquid crystal display devices have been proposed, which exhibit extremely rapid response and a relatively wide range of viewing angles. One example of such devices is disclosed in Japanese Patent Laid-Open Publication No. 7-84254 (1995). One embodiment of the OCB liquid crystal display devices according to this publication is designed as shown in
FIG. 1
to have a liquid crystal cell
11
in which a liquid crystal
12
is enclosed between a pair of transparent substrates
13
,
14
and in which a pixel electrode
15
, a counter electrode
16
and alignment films
17
,
18
are formed on the transparent substrates
13
,
14
. The surfaces of the alignment films
17
,
18
are conditioned so as to form a bend alignment state in which liquid crystal molecules
12
a,
12
b
proximate to or contacting the alignment films
17
,
18
are symmetrically tilted as shown in FIG.
1
. More concretely, the surfaces of the alignment films
17
,
18
are rubbed in the same direction to form a pretilt angle ranging from several degrees to 10 degrees. The bend alignment state may include twist in the proximity of the centers of the transparent substrates
13
,
14
(i.e., liquid crystal molecules in the proximity of the centers are twisted so that they do not lie in the plane where the X and Z axes lie) depending on design conditions. Provided on both sides of the liquid crystal cell
11
are polarizing plates
19
,
20
. Sandwiched between the transparent substrate
14
and the polarizing plate
20
is a phase compensating layer
21
for optically compensating the director alignment of the liquid crystal
12
. In the above-described bend alignment state, the liquid crystal molecules change rapidly with a change in the driving voltage applied between the pixel electrode
15
and the counter electrode
16
, and consequently, fast response can be achieved. Such fast response due to the rapid molecular change can be obtained even when changing applied voltage between two levels corresponding to two halftones that have a slight difference in brightness. The symmetry of the bend alignment state increases the angular viewability in the plane where the X and Z axes lie so that e.g., a viewing angle of about ±50° can be achieved, whereas the phase compensating layer
21
increases angular viewability in the plane where the Y and Z axes lie so that e.g., a viewing angle of about ±40° can be achieved. Note that, in
FIG. 1
, the X and Y axes designate the transverse direction and vertical direction, respectively, of the display screen. The phase compensating layer
21
also contributes to a reduction in driving voltage.
The OCB liquid crystal display device presents a difficult problem. That is, the device requires formation of the bend alignment state prior to image displaying, which is unfavorable for the following reason. When no voltage is applied between the pixel electrode
15
and the counter electrode
16
, the bend alignment state is not formed but a splay alignment state P with the liquid crystal molecules arranged fanwise is created as shown in
FIG. 2
, even if the above surface treatment is applied to the alignment films
17
,
18
. Therefore, at the time such as when a power supply is turned on, the splay alignment state P should be changed to the bend alignment state Q by application of high electric energy. The transition from the splay alignment state P to the bend alignment state Q can be caused at relatively high speeds by applying a comparatively high voltage ranging from e.g., 10 V to 30 V between the pixel electrode
15
and the counter electrode
16
. However, it takes more than tens of minutes to cause the transition when applying a voltage (several volts) that is low enough to avoid excessive load on the driving ICs. In the worst case, such transition does not occur until after an elapse of more than one hour. This delay hinders practical use of the OCB liquid crystal display device.
As an attempt to solve the above problem, Japanese Patent Laid-Open Publication No. 9-96790 (1997) proposes a technique in which the twisted alignment of the liquid crystal molecules as seen in the TN mode is combined with the rising alignment (in which the liquid crystal molecules are aligned in a direction normal to the substrates) similar to that of the OCB mode. This technique is intended to solve the above problem by eliminating the need for formation of the bend alignment state and to achieve higher response speed than the TN mode by forming a director alignment similar to the bend alignment state. In reality, however, fast response can not be necessarily achieved even if a director alignment similar to the bend alignment state is formed.
Although the above prior art OCB liquid crystal display device succeeds in providing wide viewing angles to a certain extent, the device still have difficulty in largely increasing the viewing angle within the plane where the Y and Z axes lie (see
FIG. 1
) by the phase compensating layer
21
alone and therefore the viewing angle characteristics vary significantly according to viewing directions. Accordingly, the OCB liquid crystal display device leaves much to be desired in the viewing angle uniformity. As mentioned earlier, the viewing angle within the plane where the X and Z axes lie (
FIG. 1
) can be improved by the symmetry of the bend alignment state. In order to further increase the viewing angles not only in this direction but also in other directions, it is conceivable to use a biaxial phase compensating layer as the phase compensating layer
21
. However, the fabrication of such a phase compensating layer requires accurate control of the index of refraction in triaxial directions, so that where the OCB liquid crystal display device is applied to a large screen display system, it is extremely difficult to form such a compensating layer that possesses uniform properties throughout the display screen.
In many cases, the polarizing plates
19
,
20
are pl
Hattori Katsuji
Ishihara Shoichi
Yamazoe Hiroshi
Matsushita Electric - Industrial Co., Ltd.
Nguyen Dung
Parkhurst & Wendel L.L.P.
Sikes William L.
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