Optically compensated splay mode LCD

Details Optically compensated splay mode LCD Optically compensated splay mode LCD

2000-06-29

2002-10-22

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S117000, C349S120000, C349S178000, C345S214000, C252S299010

Reexamination Certificate

active

06469762

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to liquid crystal displays(LCDS), more particularly to an optically compensated splay mode LCD having enhanced picture quality.
BACKGROUND OF THE INVENTION
Nevertheless its narrow viewing angle characteristic, the twisted nematic(TN) mode LCDs have been widely used for notebook computers. However, in order to substitute the cathode ray tube(CRT) in the monitor and television set market, it is a preconsideration of LCDs to have wide viewing angle characteristic.
As a recent development to improve the TN mode LCD's viewing angle characteristic, a technique to form a dual domain in a liquid crystal layer of a TN-LCD and an IPS-LCD have been suggested. Herein, followings are the technique to form the dual domain in the liquid crystal layer; (1) a multiple rubbing method; (2) a multiple alignment layer method; (3) an edge fringe field method; and (4) parallel fringe field method.
However, those methods require cumbersome manufacturing steps. For example, in the case of the multiple rubbing method, it requires more than one rubbing step and photolithography step for each panel or two substrates. In the case of the multiple alignment layer method, alignment layer patterning and etching steps for one substrate or two substrates are required. In the case of the parallel fringe field method, a patterning step for an ITO layer on a color filter is required. Those steps in the foregoing three methods include the further steps of coating, baking, patterning, developing and removing photoresist. Furthermore, the multiple rubbing method requires a rubbing step for an additional layer, the multiple alignment layer method requires a step of coating an additional layer, or the parallel fringe field method requires an etching method at the color filter portion. Therefore, the manufacturing process of the dual domain is more complicated than that of the conventional single domain, also costs a greater deal. Moreover, viewing angles in the multiple rubbing method are asymmetric.
Further, the IPS-LCD has great performance in the viewing angle characteristics, however its transmittance and aperture ratio are very low and the response time is very slow.
As a result, there has been suggested an optically compensated bend (OCB) mode LCDs compensating the birefringence of liquid crystal molecules so as to obtain the uniform viewing angle characteristic at all directions without requiring numbers of rubbing steps, as well as to the response time characteristic. (See reference: SID 93 Digest page. 277, “Wide-Viewing-Angle Display mode for the Active-Matrix LCD Using Bend-Alignment Liquid Crystal Cell, Y. Yamaguchi, T. Miyashita, T. Uchida).
FIGS. 1A
to
1
C are cross-sectional views for illustrating the constructions and the operation of a conventional OCB mode LCD.
As shown in
FIG. 1A
, a lower substrate
10
and an upper substrate
15
are opposed with intervening a selected distance therebetween. A liquid crystal layer
18
having a plurality of liquid crystal molecules
18
a
is sandwiched between the lower and the upper substrates
10
,
15
. Herein, the liquid crystal molecules
18
a
have, for example, the positive dielectric anisotropy. At inner surfaces of the lower and the upper substrates
10
,
15
, a pixel electrode
11
and a counter electrode
16
for driving the liquid crystal molecules
18
a
are formed respectively. Further, a first alignment layer
12
is disposed between the liquid crystal layer
18
and the lower substrate
10
including the pixel electrode
11
, a second alignment layer
17
is disposed between the liquid crystal layer
18
and the upper substrate
15
including the counter electrode
16
. The first and the second alignment layers
12
,
17
are homogeneous alignment layers having the pre-tilt angle of below 10° C., and they are rubbed in a direction parallel to each other. First and second polarizing plates
19
a,
19
b
are disposed at outer surfaces of the lower substrate
10
and the upper substrate
15
respectively. Herein, the polarizing axes of the first and the second polarizing plates
19
a,
19
b
are arranged to cross each other, and one of the polarizing axes forms a selected angle with respect to the rubbing direction, for example approximately 45° C. or 135° C.
Operation of the OCB mode LCD is as follows.
First of all, as shown in
FIG. 1A
, the liquid crystal molecules
18
a
are arranged in a splay type according to the influence of the first and the second alignment layers
12
,
17
when no voltage difference is occurred between the pixel electrode
11
and the counter electrode
16
. As a result, the polarizing state of an incident light passing through the liquid crystal layer is changed, and the light is leaked while passing the second polarizing plate
19
b.
Meanwhile, when a voltage above the critical voltage Vs is applied to the pixel electrode, the liquid crystal molecules
18
a
in a middle layer as shown in
FIGS. 1B and 1C
are arranged such that their long axes are almost parallel to an electric field. As a result, the liquid crystal molecule arrangement of the splay type is changed into a bend type. Herein,
FIG. 1B
illustrates the liquid crystal molecule arrangement when the critical voltage Vs is applied to the pixel electrode, and
FIG. 1C
illustrates the liquid crystal molecule arrangement when the voltage above the critical voltage Vs is applied to the pixel electrode.
When voltage is applied, the splay state of the liquid crystal molecule arrangement is changed into the bend state that long axes thereof are parallel to the electric field because an electric field force directly loaded on the middle layer are not bearable.
Furthermore, when the voltage above the critical voltage is applied to the pixel electrode, liquid crystal molecules
18
a
of not only in the middle layer but also around the alignment layers are arranged their long axes to be parallel to the electric field thereby reducing the amount of transmitted light and also becoming gradually dark state. At this time, the OCB mode LCD employs the maximum amount of light as the white state during the bend state, and the OCB mode LCD employs as the dark state the minimum amount of light when the voltage above the critical voltage is applied to the pixel electrode. That is, the voltage below the critical voltage is not used for the display mode.
Since the liquid crystal molecules
18
a
are arranged to form symmetries up and down with respect to the middle layer when the electric field is formed, the phase of the liquid crystal molecules is naturally compensated while light is passing through the lower substrate
10
to the upper substrate
15
. Furthermore, since the dark state is obtained when a predetermined critical voltage is applied to, the backflow is not occurred and relatively fast response time is obtained.
However, the OCB mode LCD has the following problems.
General LCDs include spherical or elliptic spacers(not shown) dispersed in the liquid crystal layer so as to maintain the cell gap. At this time, the liquid crystal
30
molecules in the conventional OCB mode LCDs are in the splay state and arranged along curved faces of the spaces before the electric field is applied. That is to say, as shown in
FIG. 2
, as the liquid crystal molecules
18
a
are arranged along the curved faces of the spacers in the splay state, the liquid crystal molecules in the spacer-disposed portions are arranged insecurely. Accordingly, the initial liquid crystal molecule arrangement is insecure, change of the splay state into the bend state is insecure when a high voltage above the critical voltage.
Furthermore, as shown in
FIG. 3
, the pixel electrode
11
is formed as a pattern and having a predetermined step difference, the first alignment layer
12
is formed on the pixel electrode
11
. As a result, the long axes of the liquid crystal molecules
18
a on the first driving electrode
11
are parallel to the first alignment layer
12
. However, the liquid crystal molecules
18
a
at the step difference portion of

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