Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing... – Injecting liquid crystal
Reexamination Certificate
2000-12-13
2003-10-28
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
Injecting liquid crystal
C349S187000
Reexamination Certificate
active
06639647
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a manufacturing method of a liquid crystal display element with improved display quality as a result of prevention of decomposition or degradation of the liquid crystal, and also relates to a manufacturing device for use with the method.
BACKGROUND OF THE INVENTION
A typical liquid crystal display element is manufactured in the following manner. Transparent electrodes are formed on the inner sides of a pair of transparent substrates. Then the transparent substrates are subjected to an alignment process. Sealing material is applied around display areas of the transparent substrates. Using the sealing material, the substrates are subsequently combined to form a liquid crystal cell, with an injection opening being left open to allow the charging (injection) of liquid crystal through it. Liquid crystal is then charged in the liquid crystal cell through the injection opening. After the charging, the injection opening is closed using a sealant made of a UV-curing resin composition. To finish the manufacture, ultraviolet light is projected to the sealant to cure it.
Various methods have been suggested to improve the display quality of a liquid crystal display element manufactured by the above manufacturing processes. For example, Japanese Laid-Open Patent Application No. 60-170830/1985 (Tokukaisho 60-170830; published on Sep. 4, 1985) discloses a method to manufacture a highly reliable liquid crystal display element by preventing the formation of foam inside the liquid crystal cell. According to the disclosure, the foam formation is prevented by a heating and evacuation process in which the liquid crystal cell, before being filled with liquid crystal, is heat-dried to remove residual water content inside the cell and evacuated to 10
−6
Pa to 10
−7
Pa (10
−5
Torr to 10
−6
Torr).
More specifically, as shown in the flow chart of
FIG. 6
, in the “heating” step, the transparent substrate is heated at 130° C. using a vacuum oven or the like. Water content is then removed by evacuating the cell to about 10
−6
Pa in the “evacuation” step. In the next step of “air introduction,” the air is introduced into a chamber of the slowly cooled vacuum oven to restore normal pressure. The transparent substrate is removed from the chamber to form transparent electrodes on it in the “transparent electrode formation” step. A pair of transparent substrates are subjected to alignment, and sealing material is applied around the display areas of the pair of transparent substrates in the “liquid crystal cell fabrication” step. In the following step of “cutting,” liquid crystal cells, which are formed as a single piece by combining the two substrates using the sealing material with an injection opening being left open to allow the charging of liquid crystal, are now cut into many individual cells or a cell cluster in which cells are lined up end to end.
In the “heating” step which immediately follows the cutting, the individual liquid crystal cell is heated at 130° C. using a vacuum oven. Water content is then removed by evacuating the cell to about 10
−6
Pa in the “evacuation” step. In the next step of “air introduction,” the air is introduced into a chamber of the slowly cooled vacuum oven to restore normal pressure. The liquid crystal cell is then removed from the chamber. Thereafter, liquid crystal is charged through the injection opening in the “injection” step. To complete the fabrication of the liquid crystal display element, the injection opening is sealed with a sealant, which is then exposed to ultraviolet light to cure in the “sealing” step.
When molecules in the liquid crystal absorb ultraviolet light in a particular wavelength region, they become optically excited and highly active, causing radical reactions and optical reactions. The excited liquid crystal molecules decompose or degrade (as a result of oxidization) due to interaction with the material composing the alignment film and reactions with foreign objects (for example, oxygen, water content, acid compounds such as nitrogen oxides and sulfur oxides, and gaseous and other molecules in solvents used in the manufacturing processes of the liquid crystal display element) that are found in the liquid crystal cell. To address these problems, for example, Japanese Laid-Open Patent Application No. 11-2825/1999 (Tokukaihei 11-2825; published on Jan. 6, 1999) discloses a method to manufacture a liquid crystal display device with excellent display quality through the prevention of decomposition and degradation of the injected liquid crystal. According to the disclosure, 300 nm or shorter wavelength light (electromagnetic waves) is removed from the ultraviolet light emitted from a light source, such as a high pressure mercury lamp or a metal halide lamp, and the remaining light is projected to cure the sealant.
A vacuum impregnation technique is typically used in the “injection” step described in the foregoing manufacturing processes of liquid crystal display element to charge liquid crystal to a liquid crystal cell. The device to implement such a vacuum impregnation technique is disclosed, for example, in Japanese Laid-Open Patent Application No. 11-287998/1999 (Tokukaihei 11-287998; published on Oct. 19, 1999). According to the disclosure, a liquid crystal injection device is constituted by three separate chambers: a preliminary heating chamber for performing preliminary heating on a liquid crystal cell, a preliminary defoam chamber for performing preliminary defoaming on liquid crystal, and a vacuum chamber. In the device, a liquid crystal cell and liquid crystal are introduced to the vacuum chamber after undergoing preliminary heating and preliminary defoaming respectively, so as to charge the liquid crystal to the liquid crystal cell.
However, according to the disclosure in Japanese Laid-Open Patent Application No. 60-170830/1985 mentioned earlier, the liquid crystal cell is heat-dried and evacuated before the depressurization is terminated to revert to normal pressure. Consequently, the air is allowed to enter the liquid crystal cell. Oxygen, water content, acid compounds such as nitrogen oxides and sulfur oxides including sulfurous compounds, and gaseous and other molecules in solvents used in the manufacturing processes of the liquid crystal display element, which are found in the air, are attracted to the alignment film inside the liquid crystal cell. The attracted molecules adversely affect the liquid crystal, causing it to change undesirably over time. The resultant liquid crystal display element fails to deliver a satisfactory level of display quality.
In addition, a general purpose vacuum pump is not powerful enough to evacuate the cell to 10
−6
Pa to 10
−7
Pa, i.e., is not sufficient to achieve an extremely high degree of vacuum. Therefore, performing steps in such a high degree of vacuum requires a special vacuum pump, such as a diffusion pump, turbo molecular pump, cryopump, or sputter ion pump, which is capable of creating an extremely high degree of vacuum.
The concentration extinction coefficient k of a liquid crystal is given by the equation:
k
=[log(1
/T
)]/
C
where T is the light transmittance in percentage points (%) and C is the concentration in grams per liter (g/L).
FIG. 7
shows that when the incident light wavelength exceeds about 330 nm (ultraviolet region of the spectrum), the concentration extinction coefficient k of a typical liquid crystal equals 0. Typical liquid crystal molecules have a light absorption threshold wavelength at about 330 nm; they absorb ultraviolet light in a 330 nm or shorter wavelength region and, in particular, absorb ultraviolet light in a 320 nm or shorter wavelength region extremely well. Meanwhile, the high pressure mercury lamp and the metal halide lamp, which provide popular light sources, emits ultraviolet light with a line spectrum at 313 nm. Therefore, the method disclosed in Japanese Laid-Open Patent Application No. 11-2825/1999 still falls short of completel
Conlin David G.
Duong Thoi V.
Konieczny J. Mark
Sharp Kabushiki Kaisha
Ton Toan
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