Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing... – Injecting liquid crystal
Reexamination Certificate
2000-11-02
2003-08-26
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
Injecting liquid crystal
Reexamination Certificate
active
06611314
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1999-50992, filed on Nov. 17, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to manufacturing liquid crystal display (LCD) devices, and more particularly, to a method and apparatus for injecting liquid crystal materials into liquid crystal display panels.
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 switching elements, such as thin film transistors (TFTs), and pixel electrodes.
As the present invention relates to manufacturing liquid crystal display panels, a brief explanation about conventional liquid crystal display manufacturing processes will be helpful. 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 the seal such that the common electrodes of the upper substrate and the pixel electrodes of the lower substrate are opposed to each other, and such that liquid crystal cells are formed. Liquid crystal material is then injected into those cells 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 via the pixel and common electrodes. By controlling the electric fields desired characters or images can be displayed.
While fabricating the various components of a liquid crystal display, such as the thin film transistors or the color filters, typically require numerous manufacturing steps, the overall fabrication process is relatively straightforward.
FIG. 1
illustrates a typical liquid crystal panel manufacturing process in some detail. An initial step, st
1
, is to form an array matrix of thin film transistors and pixel electrodes over an array (lower) substrate.
The next step, st
2
, is to form an orientation film over the lower substrate. This involves uniformly 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 orientate the film. A typical orientation film is an organic thin film such as a polyamide thin film.
The third step, st
3
, is to produce a seal pattern on the lower substrate. When the upper and lower substrates are attached, the seal patterns form cell spaces 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 plastic and a screen-print technology are conventionally used to fabricate the seal pattern.
The fourth step, st
4
, is to spray spacers over the lower substrate. The spacers have a definite size and act to maintain a precise and uniform space between the upper and the lower substrates. Accordingly, the spacers are placed with a uniform density on the lower substrate using either a wet spray method, in which case the spacers are mixed in an alcohol and then sprayed, or a dry spray method in which only the spacers are sprayed. The dry spray method itself is divided into a static electric spray method that uses static electricity and into a non-electric spray method that uses gas pressure. Since static electricity can be harmful to the liquid crystal, the non-electric spray method is widely used.
The next step, st
5
, is to aligned and attach the upper and lower substrates together, and to attach color filters to the upper substrate and the lower substrate. The aligning margin, which is less than a few micrometers, is important. If the upper and lower substrates are aligned and attached beyond the aligning margin, light leaks away such that the liquid crystal cell cannot adequately performed its function.
In the sixth step, st
6
, the liquid crystal element fabricated through the first five steps is cut into individual liquid crystal cells. Conventionally, a liquid crystal material was injected into the space between the upper and the lower substrates before the cutting into individual liquid crystal cells. But, as displays become larger, the liquid crystal cells are usually cut first and then the liquid crystal material is injected. The process of cutting typically includes scribing using a diamond pen to form cutting lines on a substrate, and a breaking step that separates the substrate along the scribed lines.
The seventh step, st
7
, is to actually inject liquid crystal material into the individual liquid crystal cells. Since each individual liquid crystal cell is a few square centimeters in area, but has only a few micrometer gap between plates, a vacuum injection method is effectively and widely used. Generally, injecting the liquid crystal material into the cells takes the longest manufacturing time. Thus, for manufacturing efficiency, it is important to have optimum conditions for vacuum injection.
FIG. 2
shows a conventional vacuum injection process for injecting 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 is located over a vessel
8
that contains the liquid crystal material
10
. During operation, suction removes air from the vacuum apparatus
6
to create a strong vacuum.
In practice it is possible for small air bubbles in the liquid crystal material
10
to gradually add together to form a larger air bubble. Such air bubbles can cause problems. Accordingly, before injection, the liquid crystal material should be left under a vacuum of a few mTorr for a sufficient time that the air bubbles in the liquid crystal material
10
are removed. Conventionally, the vessel
6
containing the liquid crystal material
10
and the liquid crystal cell
2
are all left under this vacuum condition.
One method of injecting the liquid crystal material into a 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 involves touching (slightly dipping) only the injection hole
4
to the liquid crystal material. Still referring to
FIG. 2
, in the touch method, 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
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. Later, nitrogen gas is introduced into the vacuum apparatus
6
. The 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
2
with respect to time. During period “A”, a vacuum condition is being formed. At the end of period A the injection hole
4
is dipped into the vessel
8
containing the liquid crystal material
10
. During period “B”, the liquid crystal molecules are pressure injected into the liquid crystal cell.
After injection of the liquid crystal material is complete, the injection hole
4
is sealed with an epoxy-based sealant that is applied through a dispenser.
FIG. 4
illustrates a method of measuring the viscosity of the liquid crystal. A liquid crystal material having one of the orientations “n
1
”, “n
2
”, or “n
3
” is interposed between a fixed substrate
15
and a movable substrate
20
. The movable substrate
20
is then moved parallel to the fixed substrate
15
Choi Su-Seok
Kim Yong-Bum
Dudek James
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
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