Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing... – Aligning liquid crystal with means other than alignment layer
Reexamination Certificate
2001-06-29
2004-04-20
Ngo, Julie-Huyen L. (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
Aligning liquid crystal with means other than alignment layer
C349S175000, C349S115000, C349S098000
Reexamination Certificate
active
06724459
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Patent Application No. 2000-62803, filed on Oct. 25, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE RELATED ART
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a reflective LCD device including a cholesteric liquid crystal (CLC) color filter.
2. Description of Related Art
In general, a liquid crystal display (LCD) device employing a thin film transistor (TFT) as a switching element is typically called a thin film transistor-liquid crystal display (a TFT-LCD) device. The TFT-LCD has a great advantage in displaying colored images.
The TFT-LCD is generally comprised of upper and lower substrates and an interposed liquid crystal layer therebetween. The upper and lower substrates are respectively referred to as a color filter substrate and a TFT array substrate. Further, as a light source, the TFT-LCD also includes a backlight device under the lower substrate such that the light from this backlight device passes through the upper and lower substrates and is used for displaying images. However, only about 7% of the light generated from the backlight device pass through the pair of substrates. For this reason, the TFT-LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device.
To solve these problems, a reflective LCD device has been researched and developed. Since the reflective LCD device is operated using ambient light other than an internal light source, such as a backlight device, battery life can be increased resulting in longer use times. Namely, only the drive circuitry that drives the liquid crystal uses the battery power in the reflective TFT-LCD device. Therefore, it is adopted for such application as the notebook computer and PDA (Personal Digital Assistant).
For the reflective LCD device, a reflector or/and a reflective electrode is arranged in a pixel region where the transparent electrode would be formed in a transmissive LCD device. In other words, the reflective LCD device is driven using the light reflected from the reflective electrode or/and the reflector. However, the reflective LCD device is low in brightness due to the fact that the reflective LCD device uses the ambient light and the brightness depends on this ambient light from surroundings. One of the reasons for the low brightness is that the ambient light passes twice through the color filter. Due to the reflection on the reflector, the incident light from the outside passes the color filter and then is reflected from the reflector. Then, it is directed toward the color filter again and used for displaying the images. Therefore, most of the light is absorbed by the color filter, thereby decreasing the brightness.
In order to overcome above-mentioned problem, it is essential to raise the transmittance of the color filter. Further, to get an excellent transmittance, the color filter ought to have low color purity. However, there is a limitation of lowering the color purity.
Accordingly, for greater characteristics (such as brightness) of the reflective LCD device, a cholesteric liquid crystal (CLC) has been developed, which selectively transmits or reflects light while acting as a color filter. If the CLC color filter is used in the reflective LCD device, it is possible to omit the reflector from the reflective LCD device, thereby simplifying the manufacturing process. Furthermore, it has advantages of increasing color purity and contrast ratio.
The CLC has a helical shape and the pitch of the CLC is controllable. Therefore, it is possible that the CLC color filter can selectively transmit and/or reflect light. In other words, as well known, all objects have their intrinsic wavelength, and the color that an observer recognizes is the wavelength of the light reflected from or transmitted through the object. The visible spectrum of light is about 400 nm to 700 nm. The visible light region can be broadly divided into red, green, and blue regions. The wavelength of the red visible light region is about 660 nm, that of green is about 530 nm, and that of blue is about 470 nm. Due to the pitch of the liquid crystal, the CLC color filter can selectively transmit or reflect the light having the intrinsic wavelength of the color corresponding to each pixel thereby clearly displaying the colors of red (R), green (G) and blue (B) with high purity. In order to provide a precise color, a plurality of the CLC color filters can be arranged to display the color more clearly than the conventional color filter. Further, the CLC color filter selectively reflects or transmits the right- or left-handed circularly polarized light. Thus, it can transmit a large amount of light, compared to the conventional color filter.
FIG. 1
is a schematic cross-sectional view illustrating a display area of a reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter. As shown, a reflective LCD device
50
includes upper and lower substrates
10
and
30
and an interposed liquid crystal layer
20
therebetween. The upper and lower substrates
10
and
30
are a transparent material such as glass. On the surface facing the lower substrate
30
, the upper substrate
10
includes a transparent common electrode
12
that induces voltage to the liquid crystal layer
20
.
Still referring to
FIG. 1
, on the surface facing the upper substrate
10
, the lower substrate
30
includes an alignment layer
36
, a CLC color filter
38
formed on the alignment layer
36
, and a transparent pixel electrode
48
for applying voltage to the liquid crystal layer
20
on the CLC color filter
38
. On the other surface, the lower substrate
30
includes a light absorption layer
40
. The light absorption layer
40
is made of a material that greatly absorbs light to absorb the light passing through the CLC color filter
38
.
In the above-mentioned structure of the reflective LCD device
50
shown in
FIG. 1
, the external ambient light is selectively reflected by or transmitted through the CLC color filter
38
as described before. Some portion of the ambient light passing through the CLC color filter
38
is absorbed by the light absorption layer
40
. And some portion of the light having a certain wavelength is reflected by the CLC color filter
38
to display a color. Therefore, a reflector is not required.
FIGS. 2A
to
2
D are perspective views illustrating manufacturing process steps of the reflective LCD device of FIG.
1
.
Referring to
FIG. 2A
, the alignment layer
36
is formed on the transparent lower substrate
36
. The alignment layer
36
is necessary for allowing a cholesteric liquid crystal, which will be formed in a later step, to align in a particular direction relative to the light reflection or transmission. The alignment layer
36
is usually formed of polyimide-based polymer that aligns the liquid crystal in one direction. In general, the polyimide-based polymer has advantages of good adhesiveness to the liquid crystal and provides sufficient liquid crystal alignment.
Now, referring to
FIG. 2B
, the alignment layer
36
formed on the lower substrate
30
is rubbed in a designated direction.
The rubbing method is generally classified into a method in which the substrate itself is rubbed by a fabric or a rubber including: a method of rubbing an inorganic substance that is formed on the substrate; a method of rubbing a polyimide-based polymer that is formed on the substrate; and a method of rubbing a polymeric material that has a similar chemical structure as the liquid crystal. Here, the method of rubbing a polyimide-based polymer is employed.
FIG. 2C
shows a manufacturing step of forming the CLC color filter
38
on the alignment layer
36
.
First, the cholesteric liquid crystal (CLC) is coated on the alignment layer
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge
Ngo Julie-Huyen L.
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