Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-02-01
2003-11-04
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S111000, C349S113000
Reexamination Certificate
active
06642978
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-5074 filed on Feb. 2, 2000, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) panel, and more particularly, to a liquid crystal display panel and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for reducing an injection time in injecting a liquid crystal into the liquid crystal display panel.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) panel has upper and lower substrates and an interposed liquid crystal layer. The upper substrate usually includes common electrodes while the lower substrate includes pixel electrodes and switching elements, such as thin film transistors (TFTs). Since the present invention relates to manufacturing a liquid crystal display device, a brief description regarding the conventional liquid crystal display manufacturing processes will be helpful to fully understand the present invention. Common electrodes and pixel electrodes are formed on upper and lower substrates, respectively. A seal is then formed on the lower substrate. The upper and lower substrates are then bonded together using a sealing material such that the common electrodes of the upper substrate and the pixel electrodes of the lower substrate face against each other. Thus, a liquid crystal cell is formed therein. A liquid crystal material is then injected into the cell through injection holes. The injection holes are then sealed. Finally, polarizing films are attached to the outer surfaces of the upper and lower substrates.
In operation, the light passing through the liquid crystal cells is controlled by electric fields that are applied through the pixel and common electrodes. By controlling the electric fields, desired characters or images can be displayed on the panel.
While fabricating various components for a liquid crystal display, such as thin film transistors or color filters, typically require numerous manufacturing steps, the overall fabrication process is relatively straightforward.
FIG. 1
illustrates a typical liquid crystal cell manufacturing process. In the initial step (st
1
), an array of thin film transistors and pixel electrodes are formed over an array or TFT substrate (rear substrate).
The next step (st
2
) forms an orientation film over the lower substrate. This includes the processes of depositing a polymer thin film over the lower substrate and then uniformly rubbing the polymer thin film with a fabric. The rubbing process involves rubbing the surface of the polymer thin film so as to orient or align the film. A typical orientation film is an organic thin film such as a polyimide thin film.
The third step (st
3
) produces a seal pattern on the lower substrate. When the upper and lower substrates are attached, the seal patterns form a cell gap that will receive a liquid crystal material. The seal pattern will also prevents the interposed liquid crystal material from leaking out of the completed liquid crystal cell. A thermosetting resin and a screen-print technology are conventionally used to form the seal pattern.
In the fourth step (st
4
), spacers are sprayed over the lower substrate. The spacers have a predetermined size and act to maintain a precise and uniform space between the upper and lower substrates. Accordingly, the spacers are placed with a uniform density on the lower substrate using either a wet spray method or a dry spray method. In the wet spray method, the spacers are mixed in an alcohol and sprayed on the substrates. The dry spray method includes only the step of spraying a spacer. The dry spray method is further classified into a static electric spray method using static electricity and a non-electric spray method utilizing a gas pressure. Since a static electricity can be harmful to the liquid crystal, the non-electric spray method is more widely used in fabricating the LCD panel.
The next step (st
5
) aligns and attaches the color filter substrate (or upper substrate) and the TFT substrate (or lower substrate) together. An aligning margin should be less than a few micrometers. If the upper and lower substrates are aligned and attached with a margin greater than the aligning margin, a display quality is deteriorated by light leakage during the operation of the liquid crystal display panel.
In the sixth step (st
6
), the liquid crystal cells fabricated in the previous five steps are cut into an individual liquid crystal cell. Conventionally, a liquid crystal material was injected into the space between the upper and lower substrates before the substrates were cut into the individual liquid crystal cell. As a display size becomes larger, a liquid crystal material is infected into the cell after the liquid crystal display panel is cut into a unit cell. The processes of cutting the cells typically includes the step of scribing using a diamond pen to form cutting lines on the substrate, and the step of breaking separates the substrate along the scribed lines.
In the seventh step (st
7
), a liquid crystal material is infected into the individual liquid crystal cells. Since each individual liquid crystal cell occupies hundred square centimeters in area while it has only a few micrometer gap between the substrates, a vacuum injection method is effectively and widely used in injecting a liquid crystal. Generally, injecting a liquid crystal material into the cells takes the longest manufacturing time in the fabrication process. Thus, for manufacturing efficiency, it is important to have optimum conditions for a vacuum injection.
FIG. 2
shows a conventional vacuum injection process for injecting a liquid crystal material into the liquid crystal cell. To inject the liquid crystal material, a liquid crystal cell
2
having an injection hole
4
is placed inside a vacuum apparatus
6
. The liquid crystal cell is located over a vessel
8
that contains the liquid crystal material
10
. During operation, a suction process removes air from the vacuum apparatus
6
to create a vacuum condition.
In practice, it is possible to add small air bubbles in the liquid crystal material
10
to form larger air bubbles. However, such air bubbles can cause problems. Accordingly, before injection, the liquid crystal material
10
should be left under a vacuum condition of a few mTorr for a sufficient time so that the air bubbles in the liquid crystal material
10
are removed. Conventionally, the vessel
8
containing the liquid crystal material
10
and the liquid crystal cell
2
are left under this vacuum condition.
One method of injecting the liquid crystal material into the liquid crystal cell is to dip the liquid crystal cell into the tray containing the liquid crystal material. However, the dipping method consumes too much of the liquid crystal material. Another method includes touching (i.e., slightly dipping) only the injection hole
4
to the liquid crystal material. The touching method as shown in
FIG. 2
, after the air in the liquid crystal cell
2
and the liquid crystal material
10
is removed, the injection hole
4
is slightly dipped into the vessel
8
containing the liquid crystal material
10
. At first, the liquid crystal material
10
is injected into the liquid crystal cell
2
by capillary forces. Thereafter, a nitrogen gas is introduced into the vacuum apparatus
6
. A difference in pressure between the interior and exterior of the liquid crystal cell
2
forces liquid crystal material
10
into the liquid crystal cell
2
.
FIG. 3
is a graph illustrating the pressure in the vacuum apparatus
6
with respect to time. During the period “A”, a vacuum condition is generated. At the end of the period “A” the injection hole
4
is dipped into the vessel
8
containing the liquid crystal material
10
. During the period “B”, the liquid crystal material
10
is injected into the liquid crystal cell by a pressure difference. After the injection of the liquid
Kim Robert H.
LG. Philips LCD Co. Ltd.
Morgan & Lewis & Bockius, LLP
Nguyen Hoan
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