Manufacturing method of liquid crystal element for injecting...

Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing... – Injecting liquid crystal

Reexamination Certificate

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C349S190000, C349S153000, C349S154000

Reexamination Certificate

active

06288766

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a manufacturing method of a liquid crystal element, and a liquid crystal injecting device used in manufacturing the liquid crystal element.
BACKGROUND OF THE INVENTION
A liquid crystal element is conventionally produced by the following steps of: forming electrodes, alignment films, etc., and spacers when needed, on each of two substrates made of glass or the like; producing a cell by laminating these substrates in parallel with each other while keeping a minute space therebetween; and injecting liquid crystal into the space between the substrates.
Before these substrates are laminated, as shown in
FIG. 12
, a sealing agent
54
is applied around the circumference of a liquid crystal filling area
53
in at least one of the substrates (herein, substrate
52
) except portions
51
which will be used as an inlet through which the liquid crystal is injected.
A conventional liquid crystal injecting device for injecting the liquid crystal into the cell and a conventional liquid crystal injecting method are shown in FIG.
13
. More specifically, a liquid crystal reservoir
62
withholding the liquid crystal and a cell
63
are provided in a pressure reducing bath
61
. After an internal pressure of the pressure reducing bath.
61
is reduced, the liquid crystal is supplied to an inlet of the cell
63
by dipping the inlet into the liquid crystal reservoir
62
, for example. Then, the liquid crystal is injected into the cell
63
by a difference in pressure between the interior and exterior of the cell
63
given by increasing the internal pressure of the pressure reducing bath
61
as high as or higher than atmospheric pressure.
Another example of a conventional liquid crystal injecting device and a conventional liquid crystal injecting method are shown in FIG.
14
. More specifically, an LC (Liquid Crystal) inlet
72
and an air outlet
73
are made through one of the opposing substrates which form the cell (herein, substrate
71
). After displacing air in the cell through the LC inlet
72
and air outlet
73
by pressure reducing devices
77
and
78
which are equipped with vacuum pumps
75
and
76
, respectively, liquid crystal
74
is injected into the cell through the LC inlet
72
.
In case of injecting liquid crystal having a smectic phase, and therefore a high viscosity, such as ferroelectric liquid crystal, the liquid crystal is generally heated before the injection, so that the liquid crystal becomes less viscous and more fluid when injected. This process demands three following conditions:
(1) the cell of the liquid crystal element should be filled with the liquid crystal without leaving any unfilled area;
(2) the substrates forming the cell of the liquid crystal element should not be deformed, and the cell of the liquid crystal element should not be damaged; and
(3) each step in the injecting process should not take long.
The first condition is required because if an unfilled area is left in the resulting liquid crystal element, not only the outward appearance is deteriorated, but also an operating life of the liquid crystal is shortened as the liquid crystal element is repeatedly driven.
The second condition is required for the following reason. That is, if the cell of the liquid crystal element is deformed, a cell gap between the substrates forming the cell varies, in response to which a threshold voltage is varied, the electrodes formed on the opposing substrates develop a short circuit, the alignment state of the liquid crystal is changed, etc., thereby making it impossible to attain satisfactory display quality. In particular, in case of the ferroelectric liquid crystal or anti-ferroelectric liquid crystal, since the cell gap is generally as small as 2 &mgr;m, the aforementioned inconveniences can readily occur with a very small variance. Moreover, when the substrate is deformed considerably, there arises another problem that the cell forming the liquid crystal element is readily damaged.
The third condition is required for the following reason. That is, as previously mentioned, since the liquid crystal injecting process involves the vacuuming step and heating step, if this process takes too long, some particular components contained in the liquid crystal may evaporate and the composition of the liquid crystal may be changed, or the liquid crystal may be deteriorated by heat. Further, the longer the liquid crystal injecting process, the lower the producing efficiency, thereby causing another problem that the manufacturing costs are undesirably increased.
To satisfy these conditions, many patent applications relating to the injecting method of the smectic liquid crystal have been filed, and some of them have been granted patents.
For example, Japanese Patent No. 18007010 discloses a liquid crystal injecting method. According to this injecting method, a cell applied with the smectic liquid crystal near its opening is placed in a pressure-resistant vessel, and air in the pressure-resistant vessel is displaced. Then, the cell is heated, so that the opening of the cell is closed with the heated and thus having become fluid smectic liquid crystal. Subsequently, reducing of the internal pressure of the vessel is stopped.
However, according to the injecting method of the above Japanese Patent, air in the cell is not readily displaced at the very initial stage of the air-displacing process, and for this reason, a difference in pressure between the interior and exterior of the cell is so small that the cell may be broken during the liquid crystal injecting step. This problem occurs more frequently with the cells of a larger size. This problem may be eliminated by reducing an internal pressure of the vessel more slowly. However, from the standpoint of satisfying the condition of shortening the injecting process, this solution is not preferable because a displacement rate decreases with increasing sizes of the cells.
In addition, the cell is heated while the ferroelectric liquid crystal is injected therein. However, if the injection method of the above Japanese Patent is adopted in this case, the substrates forming the cell may be deformed by heat, and the cell readily warps, and possibly, to the extent that it is broken. This problem also occurs more frequently with the cells of a larger size.
Moreover, since air in the cell is not displaced satisfactory, air left in the cell may be compressed when the liquid crystal is injected, and left as bubbles in the end. When air in the cell is not displaced satisfactory, a non-polymerized monomer of a high molecular film and moisture contained in color filters provided on the substrates forming the cell are left in the cell in the form of a gas, thereby making it impossible to inject the liquid crystal into the cell to its full.
Further, the smectic liquid crystal increases its volume (expands) when heated during the injecting step, and reduces its volume (contracts) when cooled later. Thus, even when the smectic liquid crystal is injected fully into the cell at the injecting step, cavitation may occur due to the volume loss (contraction) of the smectic liquid crystal in the cooling step, thereby causing deficient injection of the liquid crystal.
A method disclosed in Japanese Laid-open Patent Application No. 297386/1993 (Japanese Official Gazette, Tokukaihei No. 5-297386) is suitably used to cut the injection time shorter and eliminate residual bubbles. According to this method, at least one injection hole and at least one air-releasing hole are provided at the circumference of the cell, and air in the cell is displaced through these holes, after which the liquid crystal is injected into the cell through the injection hole while air is kept released from the air-releasing hole.
Also, a method disclosed in Japanese Laid-open Patent Application No. 220550/1996 (Japanese Official Gazette, Tokukaihei No. 8-220550) is suitably used to eliminate the cavitation. According to this method, after the phase of the smectic liquid crystal which has been injected into the cell is restored t

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