Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-12-21
2004-06-29
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S184000, C349S172000
Reexamination Certificate
active
06757035
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1999-59598, filed on Dec. 21, 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
This invention relates to liquid crystal displays. More particularly, it relates to liquid crystal displays that use ferroelectric and anti-ferroelectric liquid crystals.
2. Discussion of the Related Art
A conventional liquid crystal display (LCD) includes a display panel having upper and lower substrates that are attached to each other, and an interposed liquid crystal, usually a nematic, a smetic, or a cholesteric liquid crystal. A liquid crystal display utilizes the electro-optic effects of the liquid crystal. The display panel is operationally divided into a plurality of liquid crystal cells. On the exterior surfaces of the upper and lower substrates, polarizers or retardation films are selectively attached.
A major design consideration of a liquid crystal cell is the characterisitic of the particular liquid crystal that is used. A good liquid crystal should have a fast response time, a good gray scale, and a wide viewing angle, all while operating at a low driving voltage. However, it is very difficult to find a liquid crystal that has all of those characteristics. Thus, various designs have been adopted for liquid crystal display devices.
Among the various types of liquid crystals displays, the 90° twist mode TN liquid crystal display has neither a wide viewing angle nor a fast response time. An in-plane switching (IPS) mode has a wide viewing angle, but a relatively slow response time that results in decreased luminance and poor moving image quality. To overcome the problem of a slow response time, various technologies have been proposed. For a fast response time, LTN (low twisted nematic) and OCB (optically compensated birefringence) modes have been studied. However, those technologies have not yet been able to provide the fast response time of a CRT.
FIG. 1
is a cross-sectional view illustrating a conventional TN-LCD panel. As shown in
FIG. 1
, the TN-LCD panel has lower and upper substrates
2
and
4
and an interposed liquid crystal layer
10
. The lower substrate
2
includes a substrate
1
having a TFT “S” that is used as a switching element that changes the orientation of the liquid crystal molecules. The TFT “S” includes a pixel electrode
14
that applies a voltage to the liquid crystal layer
10
in accordance with signals that are applied to the TFT “S”. The upper substrate
4
has a color filter
13
for implementing color, and a common electrode
12
on the color filter
8
. The common electrode
12
serves as an electrode for applying a voltage to the liquid crystal layer
10
. The pixel electrode
14
is arranged over a pixel portion “P,” i.e., a display area. Further, to prevent leakage of the liquid crystal layer
10
between the two substrates
2
and
4
, those substrates are sealed by a sealant
6
.
FIGS. 2A
to
2
C illustrate possible alignments of liquid crystal molecules in a liquid crystal layer. In a nematic liquid crystal, while each rod-like molecule fluctuates quite rapidly, as shown in
FIG. 2A
the molecules have a definite orientational order expressed by a unit vector “ñ” called a director. As shown in
FIG. 2B
, in a smetic liquid crystal the molecules positionably have a layered structure in which the molecular orientation is tilted perpendicular to the layers. As shown in
FIG. 2C
, in a cholesteric liquid crystal, the director ñ changes its orientation gradually along a helical axis. The helical axis coincides with the optical axis of this material. Among the different types of liquid crystals, the nematic liquid crystal is most widely used in liquid crystal display devices.
Liquid crystals for liquid crystal display devices should:
a) have a liquid crystal phase that extends from low to high temperatures, and thus can operate over a range of temperatures;
b) be chemically and optically stable over time;
c) have a low viscosity and a fast response time;
d) have highly ordered molecular alignments, and thus provide good contrast; and
e) have a large dielectric anisotropy and a low operating voltage.
The electro-optic effect enables electrical modulation of light by changing the alignment of the liquid crystal molecules using an applied electric field.
Among the various types of nematic liquid crystals, a twisted nematic (TN) liquid crystal and a super twisted nematic (STN) liquid crystal are often used. In a TN liquid crystal panel, a nematic liquid crystal is interposed between transparent lower and upper substrates. Those substrates induce a definite molecular arrangement such that a gradual rotation of the molecules occurs between the lower transparent substrate and the upper transparent substrate until a twist angle of 90 degrees is achieved. In an STN liquid crystal panel the angle of twist rotation is increased to 180 or to 360 degrees.
The basic configuration and operation of a twisted nematic liquid crystal display device will now be explained. As shown in
FIG. 3A
, opposed and spaced apart first and second polarizers
10
and
16
, respectively, have perpendicular first and second transmittance axis directions
40
and
42
. Between the two polarizers
10
and
16
are first and second transparent substrates
12
and
14
, which are also opposed to and spaced apart from each other. Spacers are used to maintain the cell gap between the substrates. For example, plastic balls or silica balls having a diameter of 4 to 5 micrometers can be sprayed on the first substrate.
Still referring to
FIG. 3A
, the first and the second transparent substrates
12
and
14
include first and second orientation films
20
and
22
, respectively, on their opposing surfaces. Between the first and second orientation films
20
and
22
is a positive TN liquid crystal
18
.
The positive TN liquid crystal has a characteristic that it arranges according to an applied electric field. The first and second polarizer
10
and
16
, respectively, transmit light that is parallel with their transmittance-axis directions
40
and
42
, but reflect or absorb light that is perpendicular to their transmittance-axis directions
40
and
42
.
The first and second orientation films
20
and
22
were previously rubbed in a proper direction with a fabric. This rubbing causes the positive TN liquid crystal molecules between the first and second transparent substrates
12
and
14
to become tilted several degrees. First and second rubbing directions
50
and
52
of the first and second orientation films
20
and
22
are, respectively, parallel with the transmittance-axis directions of the first and second polarizers
10
and
16
. With no electric field applied across the positive TN liquid crystal
18
, the orientation of the liquid crystal molecules twists between one substrate and the other at a definite angle, that angle being the twisted angle of the positive TN liquid crystal
18
.
During operation, a back light device
24
irradiates white light onto the first polarizer
10
. The first polarizer
10
transmits only the portion of the light that is parallel with the first transmittance-axis direction
40
. The result is a first linearly polarized light
26
that passes through the polarizer
10
. The first linearly polarized light
26
then passes into the positive TN liquid crystal
18
via the first transparent substrate
12
.
As the first polarized light
26
passes through the positive TN liquid crystal
18
, the first polarized light
26
changes its phase according to the twist alignment of the positive TN liquid crystal molecules. Accordingly, the first linearly polarized light
26
becomes an elliptically us (possibly circularly) polarized light
28
.
The elliptically polarized light
28
passes through the second transparent substrate
14
, and meets the second polarizer
16
. When the elliptically polarized light
28
passes through the second polarizer
16
, the second polarizer
16
transmits only
Choi Su-Seok
Choi Suk-Won
Dudek James A.
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
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