Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing... – Sealing of liquid crystal
Reexamination Certificate
2001-01-31
2004-10-12
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
Sealing of liquid crystal
C349S153000, C349S189000
Reexamination Certificate
active
06803986
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-4912, filed on Feb. 1, 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 method of fabricating liquid crystal display panel for increasing yield as well as decreasing an injection time of the liquid crystal.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) panel has upper and lower substrates and an interposed liquid crystal layer. More specifically, the upper substrate includes common electrodes, while the lower substrate includes switching elements, such as thin film transistors (TFTs), and pixel electrodes.
As the present invention relates to manufacturing liquid crystal display panel, a brief explanation about 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 so that the common electrodes of the upper substrate and the pixel electrodes of the lower substrate are facing to each other. Thereafter, liquid crystal panels are completed. A liquid crystal material is injected between the substrates 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 operating the liquid crystal display panel, light passing through the liquid crystal display panel is controlled by electric fields. The electric fields are applied through the pixel and common electrodes. By controlling the electric fields, desired characters or images are displayed on the panel.
A fabrication process of the various components of a liquid crystal display, such as the thin film transistors or the color filters, typically requires numerous manufacturing steps. However, the overall fabrication process is relatively straightforward.
FIG. 1
illustrates a typical liquid crystal display panel fabrication process. In the initial step (st
1
) an array of thin film transistors and pixel electrodes are formed on an array or TFT (lower) substrate.
In the next step (st
2
), an orientation film is formed on the lower substrate. This step includes uniformly depositing a polymer thin film on the lower substrate and uniformly running the polymer thin film with a fabric. The rubbing process includes 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 cell gaps that will receive the 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 fabricate the seal pattern.
In the fourth step (st
4
), spacers are sprayed on 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 alcohol and then sprayed. Only the spacers are sprayed in the dry spray method. The dry spray method is classified into a static electric spray method and a non-electric spray method. The static electric spray method uses static electricity while the non-electric spray method uses gas pressure. Since static electricity can be harmful to the liquid crystal, the non-electric spray method is more widely used.
In the next step (st
5
), the color filter substrate (upper substrate) and the TFT substrate (lower substrate) are aligned and attached to each other. An aligning error margin in this case is less than a few micrometers. If the upper and lower substrates are aligned and attached with an aligning margin larger than the error margin, a display quality is deteriorated due to light leakage during the operation of the liquid crystal display cell.
In the sixth step (st
6
), the liquid crystal element fabricated in the above five steps is cut into individual liquid crystal cells. Conventionally, a liquid crystal material was injected into the space between the upper and lower substrates before the substrates were cut into individual liquid crystal cells. However, as displays become larger, the liquid crystal cells are usually cut first and then the liquid crystal material is injected. The process of cutting includes the step of scribing by using a diamond pen to form cutting lines on a substrate and the step of breaking that separates the substrate along the scribed lines.
In the seventh step (st
7
), the liquid crystal material is injected into the individual liquid crystal cells. Since each individual liquid crystal cell has many hundred square centimeters in area while it has only a few micrometers gap between the substrates, a vacuum injection method is effectively and widely used. Generally, injecting the liquid crystal material into the cells takes the longest time among the fabrication processes. Thus, for manufacturing efficiency, it is important to have an optimum condition for the vacuum injection.
FIG. 2
shows a conventional vacuum injection process for injecting a liquid crystal material into a 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
2
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
, thereby forming a high vacuum condition.
In practice, small air bubbles in the liquid crystal material
10
may be gradually added to form larger air bubbles. Such air bubbles however may cause problems. Accordingly, before the 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, both 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 a 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 waists too much of the liquid crystal material. Another method is touching (slightly dipping) only the injection hole
4
to the liquid crystal material. Referring to
FIG. 2
, after air in the liquid crystal cell
2
and in the liquid crystal material
10
has been removed, the injection hole
4
is slightly dipped into the vessel
6
containing the liquid crystal material
10
. In this process, the liquid crystal material
10
is injected into the liquid crystal cell
2
by capillary forces. A nitrogen gas is then introduced into the vacuum apparatus
6
. A difference in pressure between the interior and exterior of the liquid crystal cell
2
forces the liquid crystal material
10
into the liquid crystal cell
2
.
FIG. 3
is a graph illustrating a pressure in the vacuum apparatus
6
with respect to time. During the period “A”, a vacuum condition is formed. 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 infected by pressure into the liquid crystal cell. After the injection of the liquid crystal material is complete, the injection hole
4
is sealed with an epoxy-based sealant that is applied by a dispenser
Kim Robert H.
LG.Philips LCD Co. , Ltd.
Morgan & Lewis & Bockius, LLP
Nguyen Hoan C.
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