Manufacturing method of cholesteric liquid crystal color filter

Details Manufacturing method of cholesteric liquid crystal color filter Manufacturing method of cholesteric liquid crystal color filter

2001-12-26

2004-02-10

Jackson, Jerome (Department: 2875)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S093000, C349S094000, C349S124000

Reexamination Certificate

active

06690439

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 2000-81491, filed on Dec. 26, 2000 in Korea, which is hereby incorporated by reference as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a method of manufacturing a cholesteric liquid crystal (CLC) color filter. The CLC color filter is often used in flat panel displays such as liquid crystal display devices.
2. Discussion of the Related Art
Thin film transistor liquid crystal display (TFT-LCD) devices are commonly used for liquid crystal display devices because of its superior reproduction of color images and thin size. The conventional thin film transistor liquid crystal display device includes an upper substrate (a color filter substrate) and a lower substrate (an array substrate) facing each other. A back light unit is located under the lower substrate in the conventional thin film transistor liquid crystal display devices. Because only about 7% of light irradiated from the back light unit reaches the display screen through liquid crystal cells, the back light unit needs to be brighter to obtain higher brightness for the liquid crystal display, which leads to a higher power consumption. Accordingly, batteries having large capacitance and heavy weight have been used to supply enough power to the back light unit. However, these batteries are still limited in terms of their duration time. To overcome the above-described problems, reflective type liquid crystal display devices have been considered. Because the reflective type liquid crystal display devices use ambient light for a light source, the power consumption of the back light unit can be decreased dramatically. Accordingly, the reflective type liquid crystal display devices are usually used for portable electronic devices such as a personal digital assistant (PDA) that can be driven for long hours.
A pixel region of the reflective type liquid crystal display device is made of an opaque reflector or an opaque reflective electrode, whereas the pixel region of a transmissive type liquid crystal display device is made of a transparent electrode. However, because the reflective type liquid crystal display device uses the ambient light for the light source, the brightness of the display is very low. The ambient light passes through the color filter substrate and is reflected by the reflective electrode on the lower substrate. Thereafter, the light passes through the color filter substrate again to display color images. The ambient light loses most of its brightness during its double passage through the color filter substrate. The transmittance characteristics of the color filter should be improved to overcome the low brightness problem in the reflective type liquid crystal display device. To this end, color purity needs to be lowered to increase the transmittance of the color filter. However, there is a limit to increasing the brightness by lowering the color purity.
Liquid crystal display devices using cholesteric liquid crystal (CLC), which selectively reflects or transmits an incident light, have been developed to improve the liquid crystal display devices. Generally, liquid crystal molecules have liquid crystal phase depending on the structure and composition of the liquid crystal molecules. The liquid crystal phase is affected by temperature and concentration. Nematic liquid crystal which has liquid crystal molecules regularly aligned in a certain direction, has been researched and applied widely in the liquid crystal display field. The nematic liquid crystal is commonly applied to liquid crystal display devices. The cholesteric liquid crystal (CLC) has twisted molecular axes or twisted directors of nematic liquid crystal from mixing the nematic liquid crystal with molecules having chiral characteristic, which means that a molecular structure of the liquid crystal does not superimpose on its mirror image. Generally, the nematic liquid crystal phase has regularity in that the liquid crystal molecules are aligned in a certain direction. On the other hand, the cholesteric liquid crystal (CLC) has a layered structure and the liquid crystal molecules in every layer show similar regularity to that of the nematic liquid crystal. However, the alignment of the liquid crystal molecules of each layer rotates in a certain direction, which can be clockwise or counterclockwise, and thus causing a difference in the reflectance between layers. A color can be displayed by a reflection and an interference of light that are caused by the difference in the reflectance between the layers. The rotations of the cholesteric liquid crystal (CLC) molecules form a helical structure. The two most important characteristics in the helical structure of the cholesteric liquid crystal (CLC) are rotational direction and pitch, i.e., period for 360 degrees rotation of the liquid crystal molecules. That is, the pitch can be understood as a distance between the first cholesteric liquid crystal (CLC) layer and the last cholesteric liquid crystal (CLC) layer when the cholesteric liquid crystal (CLC) molecules in the first cholesteric liquid crystal (CLC) layer rotate 360 degrees. The pitch is a parameter that decides the hue of the cholesteric liquid crystal (CLC). That is, if the pitch is the same as the wavelength of red color, i.e., 650 mn, the cholesteric liquid crystal (CLC) reflects the red color observed in the front direction. If the light reflected from the cholesteric liquid crystal (CLC) is observed in an angle to the plane of the color filter substrate, all colors such as yellow, green and blue, for example, which are included in a region of visible light, can be seen depending on the viewing angle. If the cholesteric liquid crystal (CLC) is used for flat display devices, which use transmission and scattering phenomenon to display images, a color image can be displayed using reflection and scattering phenomenon of a particular color. Another important characteristic in the helical structure of the cholesteric liquid crystal (CLC) is the rotational direction of the CLC helix. The rotational direction of the CLC helix is an important factor for the polarization phenomenon. That is, the direction of a circular polarization of the reflected light depends on whether the helix structure of the cholesteric liquid crystal (CLC) is right-handed or left-handed. The right-handed cholesteric liquid crystal (CLC) reflects a right circular polarization that has a wavelength corresponding to the pitch of the right-handed cholesteric liquid crystal (CLC). Because the ambient light is a mixture of a right circular polarization and a left circular polarization, the right circular polarization or the left circular polarization can be extracted according to the structure of the cholesteric liquid crystal (CLC), i.e., a right handed helix or left-handed helix. Because polarization property, i.e., a linear polarization, is used in the conventional liquid crystal display devices, the degree of light utilization will be greatly improved using the cholesteric liquid crystal (CLC), and will result in an effective reduction of power consumption compared to the color filters including pigment or dye.
The conventional manufacturing method of a cholesteric liquid crystal (CLC) color filter will be described hereinafter with reference to drawings attached herein.
FIGS. 1A
to
1
D are cross-sectional views illustrating a conventional manufacturing sequence of a cholesteric liquid crystal (CLC) color filter.
In
FIG. 1A
, an alignment layer
10
is coated on a transparent substrate
1
. A polyimide-based resin is usually used as the material for the alignment layer because it has excellent alignment characteristics with various liquid crystal materials and is suitable for the liquid crystal material. The coated alignment layer
10
then undergoes a heat-curing process.
In
FIG. 1B
, the surface of the alignment layer is rubbed by a rubbing fabric. The surface of the cured alignment

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