Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing... – Defect correction or compensation
Reexamination Certificate
1999-07-22
2001-04-03
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
Defect correction or compensation
C349S123000
Reexamination Certificate
active
06211939
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a liquid crystal display, and more particularly to a liquid crystal display which is capable of preventing display defect caused by steps formed on inner surfaces of substrates. The invention relates also to a method of preventing display defect caused by alignment defect in a liquid crystal display.
2. Description of the Related Art
There has been widely used a twist nematic (TN) type liquid crystal display.
FIG. 1
illustrates a structure of a conventional liquid crystal display. The illustrated liquid crystal display is comprised of a lower substrate
5
, an upper substrate (not illustrated) joined to the lower substrate
5
with a certain space being kept therebetween, and a liquid crystal layer
1
sandwiched between the lower substrate
5
and the upper substrate.
As illustrated in
FIG. 1
, an interlayer insulating film
4
, a pixel electrode film
3
, and an insulating film
2
are deposited on the lower substrate
5
in this order.
There are formed a lot of steps on inner surfaces of the lower and upper substrates with which the liquid crystal layer
1
makes contact. In particular, steps of the insulating film
2
formed on an inner surface of the lower substrate
5
cause reverse tilt and reverse twist which often extend to a display area of a liquid crystal display to thereby cause display defect with the result of deterioration in quality of images.
Since alignment treatment such as rubbing is conducted along steps formed on inner surfaces of the substrates, the liquid crystal layer
1
is put in reverse tilt condition in an inclined portion B which is shaded in a direction A in which alignment treatment is conducted. As a result, there occurs such display defect as mentioned above.
A lot of solutions to such a problem have been suggested so far.
The first solution is to form a film on inner surfaces of substrates by overcoating to thereby eliminate steps or flatten inner surfaces.
However, this solution is accompanied with a problem of deterioration in lead time caused by additional fabrication steps, and a problem of an increase in fabrication cost.
The second solution is to increase a pre-tilt angle &thgr;).
However, it is generally difficult to make a pre-tilt angle &thgr; greater than an angle &phgr; of steps. Hence, it is quite difficult or almost impossible to prevent occurrence of the area B in which alignment is defective.
In addition, as a liquid crystal display has been fabricated smaller in size, an interval M between adjacent steps has become smaller. Hence, it is no longer possible to prevent occurrence of the area B only by making a pre-tilt angle &thgr; greater.
Japanese Unexamined Patent Publication No. 5-265046 has suggested a liquid crystal display in which a side by which a pre-tilt angle of liquid crystal molecules can be obtained, among adjacent two sides of a pixel electrode, and the other side by which a pre-tilt angle in a reverse direction can be obtained are arranged in certain positions to thereby prevent alignment defect in liquid crystal.
Japanese Unexamined Patent Publication No. 7-64096 has suggested a liquid crystal display in which a pre-tilt direction is controlled so that a direction in which liquid crystal molecules are oriented by electric field running in a thickness-wise direction of a liquid crystal layer is common to a direction in which liquid crystal molecules are oriented by electric field, to thereby prevent deterioration in display quality.
However, the liquid crystal displays suggested in the above-mentioned Publications attempt to prevent alignment defect in liquid crystal by controlling a pre-tilt angle as a sole parameter. Hence, for the same reason as mentioned earlier, the suggested liquid crystal displays could not prevent occurrence of the area B.
SUMMARY OF THE INVENTION
In view of the above-mentioned problem, it is an object of the present invention to provide a liquid crystal display which is capable of preventing display defect caused by steps formed on inner surfaces of substrates. It is also an object of the present invention to provide a method of preventing display defect caused by alignment defect in a liquid crystal display.
In one aspect, there is provided a liquid crystal display including (a) a liquid crystal layer, (b) substrates sandwiching the liquid crystal layer therebetween, and (c) pixels arranged in a matrix on at least one of the substrates, display defect caused by alignment defect being prevented by controlling parameters including (a) an angle formed by steps formed on an inner surface of the substrates with which the liquid crystal layer makes contact, (b) an interval between the steps, (c) a height of the steps, and (d) a pre-tilt angle of liquid crystal in the liquid crystal layer.
In the liquid crystal display in accordance with the present invention, there are specified parameters with which display defect occurs due to steps formed on inner surfaces of upper and lower substrates, and such parameters are appropriately controlled to thereby prevent occurrence of display defect.
That is, the inventors have studied mechanism by which display defect occurred, and found out that the parameters with which display defect occurred due to steps formed on inner surfaces of upper and lower substrates were as follows: (a) an angle formed by steps formed on an inner surface of the substrates with which the liquid crystal layer makes contact; (b) an interval between the steps; (c) a height of the steps; and (d) a pre-tilt angle of liquid crystal in the liquid crystal layer. The liquid crystal display in accordance with the present invention controls these four parameters to thereby prevent display defect caused by alignment defect.
There is further provided a liquid crystal display including (a) a liquid crystal layer, (b) substrates sandwiching the liquid crystal layer therebetween, and (c) pixels arranged in a matrix on at least one of the substrates, an angle &phgr; formed by steps on formed on an inner surface of the substrates with which the liquid crystal layer makes contact, an interval M between the steps, a height H of the steps, and a pre-tilt angle &thgr; of liquid crystal in the liquid crystal layer, being controlled so that the following equation is established.
(
H/M
)×(&phgr;/&thgr;)≦0.018 (A)
In the liquid crystal display, there are specified parameters with which display defect occurs due to steps formed on inner surfaces of upper and lower substrates, and there is also specified a relation among the parameters, and such parameters are appropriately controlled to thereby prevent occurrence of display defect.
In accordance with the study of the inventors, the parameters with which display defect occurs due to steps formed on inner surfaces of upper and lower substrates are as follows: (a) an angle formed by steps formed on an inner surface of the substrates with which the liquid crystal layer makes contact; (b) an interval between the steps; (c) a height of the steps; and (d) a pre-tilt angle of liquid crystal in the liquid crystal layer. The liquid crystal display in accordance with the present invention controls these four parameters in accordance with the above-mentioned equation (A), to thereby prevent display defect caused by alignment defect.
A function of the liquid crystal display in accordance with the present invention is explained hereinbelow with reference to FIG.
1
.
As mentioned earlier, steps of the insulating film
2
formed on an inner surface of the lower substrate
5
cause the area B above the steps, in which alignment of liquid crystal is defective. In the area B that is shaded in a direction A in which alignment treatment such as rubbing is carried out, liquid crystals are oriented in accordance with inclination of a step. Since a step angle &phgr; is generally greater than a pre-tilt angle &thgr; of liquid crystal in the liquid crystal layer
1
, the liquid crystal located above the step is apparently in reverse tilt condition.
Accordingly, liquid crystals
Kobayashi Kazumi
Satou Masaki
Dudek James A.
NEC Corporation
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