Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-06-26
2003-11-25
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S113000, C349S110000
Reexamination Certificate
active
06654076
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transflective liquid crystal display and method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a high contrast ratio.
2. Discussion of the Related Art
The cathode-ray tube (CRT) was developed and is mainly used for display systems. However, flat panel displays are beginning to be incorporated into display systems because of their small dimension, low weight and low power consumption. Presently, thin film transistor-liquid crystal displays (TFT-LCD) having a high resolution are being developed.
In general, Liquid Crystal Display (LCD) devices have various advantages in that, for example, they are relatively thin and require low power for operation, when compared to CRT display devices. Therefore, such LCD devices are good candidates to replace CRT display devices and have been a matter of great interest in a variety of technical fields.
Liquid crystal displays are classified into transmission types and reflection types depending on whether an internal or external light source is used. The transmission type has a liquid crystal display panel that does not itself emit light, and has a backlight as a light-illuminating section.
The backlight is disposed at the rear or one side of the panel. The liquid crystal panel controls the amount of the light, which is generated from the backlight and passes through the liquid crystal panel, in order to implement an image display. In other words, the light from the backlight selectively passes through the LCD panel and the LCD displays images according to the arrangement of the liquid crystal molecules. However, the backlight of the transmission type LCD consumes 50% or more of the total power consumed by the LCD device. Providing a backlight therefore increases power consumption.
In order to overcome the above problem, a reflection type LCD has been selected for portable information apparatuses that are often used outdoors or carried with users. Such a reflection type LCD is provided with a reflector formed on one of a pair of substrates. Thus, ambient light is reflected from the surface of the reflector. The reflection type LCD using the reflection of ambient light is disadvantageous in that a visibility of the display is extremely poor when surrounding environment is dark.
To overcome the problems described above, a construction which realizes both a transmissive mode display and a reflective mode display in one liquid crystal display device has been proposed. This is called a transflective liquid crystal display device. The transflective liquid crystal display (LCD) device alternatively acts as a transmissive LCD device and a reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can be operated in bright ambient light and has a low power consumption.
The conventional transflective liquid crystal display device adopts a normally white mode in which the transflective device displays a white color when a signal is not applied. However, since the transflective liquid crystal display device is generally designed concentrating on the reflective mode, only about 50% of the light generated from the backlight device can pass through the liquid crystal display panel when the signal is not applied. Accordingly, the transflective LCD device often produces a gray color in operating.
To overcome the gray color problem, the transflective liquid crystal display device has different liquid crystal cell gaps between in the reflective portion and in the transmissive portion.
FIG. 1
is a schematic cross-sectional view of a conventional transflective LCD device having a transmissive portion and a reflective portion.
In
FIG. 1
, the transflective LCD device divided into the transmissive portion A and the reflective portion B, and includes lower and upper substrates
10
and
60
. A liquid crystal layer
100
having optical anisotropy is interposed between the lower and upper substrates
10
and
60
.
The lower substrate
10
includes a first passivation layer
20
on its surface facing into the upper substrate
60
. The first passivation layer
20
is made of an organic material and has a first transmitting hole
22
corresponding to the transmissive portion A. A transparent electrode
30
of transparent conductive material is disposed on the first passivation layer
20
. A second passivation layer
40
and a reflective electrode
50
are sequentially formed on the transparent electrode
30
. As shown in
FIG. 1
, the reflective electrode
50
corresponds to the reflective portion B and has a second transmitting hole
52
that exposes the second passivation layer
40
in the transmissive portion A. Although not shown in
FIG. 1
, a thin film transistor (TFT) is formed on the lower substrate
10
and electrically connected to both the transparent electrode
30
and the reflective electrode
50
.
The upper substrate
60
includes a color filter layer
61
on its surface facing into the lower substrate
10
. A common electrode
62
is formed on the surface of the color filter layer
61
facing toward the lower substrate
10
.
On the exterior surfaces of the lower and upper substrates
10
and
60
, lower and upper retardation films
71
and
72
are disposed, respectively. Since the lower and upper retardation films
71
and
72
have a phase difference &lgr;/4 (&lgr;=550 nm), they change the polarization state of the incident light. Namely, the lower and upper retardation films
71
and
72
convert the linearly polarized light into the right- or left-handed circularly polarized light, and they also convert the right- or left-handed circularly polarized light into the linearly polarized light of which polarization direction may be 45° or 135°. A lower polarizer
81
is disposed on the rear surface of the lower retardation film
71
, and an upper polarizer
82
is disposed on the front surface of the upper retardation film
72
. Optical axis of the upper polarizer
82
is perpendicular to that of the lower polarizer
81
. A backlight device
90
that emits an artificial light is adjacent to the lower polarizer
81
. Light generated from the backlight device
90
is used as a light source in the transmissive mode of the LCD device.
The liquid crystals interposed between the lower and the upper substrates
10
and
60
have a positive dielectric anisotropy such that the liquid crystal molecules are aligned parallel with the applied electric field. An optical retardation (&Dgr;n·d) of the liquid crystal layer
100
depends on refractive-index anisotropy and thickness of the liquid crystal layer
100
. Therefore, the liquid crystal layer
100
has different cell gaps between in the transmissive portion A and in the reflective portion B. The first transmitting hole
22
of the first passivation layer
20
allows the liquid crystal layer
100
of the transmissive portion A to be thicker than that of the reflective portion B, and makes the brightness uniform in all over the LCD device. Advisably, the thickness of the liquid crystal layer
100
in the transmissive portion A is twice as large as that in the reflective portion B.
The liquid crystal display shown in
FIG. 1
includes the organic passivation layer that has the opening therein to make the different cell gaps. Thus, it is possible for the LCD device to obtain a uniform transmissivity whether it is operating in the transmissive mode or in the reflective mode. The polarization state of the light passing through the LCD panel shown in
FIG. 1
is illustrated with reference to
FIGS. 2 and 3
.
From the point of the optical axis, the X-Y-Z coordinates are defined as illustrated in FIG.
1
. The Z-axis is a progressing direction of the light, and the X-Y plane is parallel with the lower and upper substrates
10
and
60
. From the observer's viewpoint at the bottom of the liquid crystal display devic
Ha Kyoung-Su
Kim Dong-Guk
Kim Woong-Kwon
Duong Tai
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
Ton Toan
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