Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-12-15
2002-12-31
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S139000, C257S072000
Reexamination Certificate
active
06501519
ABSTRACT:
CROSS REFERENCE
This application claims the benefit of Korean Patent Application No. 1999-59600, filed on Dec. 21, 1999, under 35 U.S.C. §119, the entirety of which is hereby incorporated by reference.
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 transflective LCD device.
2. Description of Related Art
In general, liquid crystal displays are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an internal or external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light device. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed therebetween. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFTs) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the backlight device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive LCD device has a transmittance of about 7.4% as seen in FIG.
1
.
FIG. 1
shows a transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive 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. However, this presents a problem in that the battery can not be used for a lengthy period of time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is light and easy to carry. Also, the reflective LCD device is superior in aperture ratio to the transmissive LCD device.
FIG. 2
shows a cross-section of a typical reflective LCD device. As shown in
FIG. 2
, the reflective LCD device includes upper and lower substrates
8
and
10
with a liquid crystal layer
12
interposed. The upper substrate
8
includes color filter layers
4
a,
4
b
and
4
c
(e.g., red, green, and blue) and a common electrode
6
. The lower substrate
10
includes a switching element (not shown) and a reflective electrode
2
.
Ambient light
100
passes through the upper substrate
8
and the liquid crystal layer
12
and is reflected on the reflective electrode
2
. When electrical signals are applied to the reflective electrode
2
by the switching element, the phase of the liquid crystal layer
12
varies. Then, reflected light is colored by the color filter layers
4
a,
4
b
and
4
c
and displayed in the form of images.
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from the light outdoors. Even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk).
In order to overcome the problems described above, a transflective LCD device has been developed.
FIG. 3
shows a conventional transflective LCD device. As shown in
FIG. 3
, the conventional transflective LCD device includes a gate line arranged in a transverse direction and a gate electrode
52
extended from the gate line
50
. A data line is formed in the direction perpendicular to the gate line
50
. A source electrode
62
extended from the data line
60
is overlapped with the gate electrode
52
.
A drain electrode
64
is formed spaced apart from the source electrode
62
. The drain electrode
64
contacts the pixel portions
68
and
70
formed of different materials, via a contact hole
66
. The pixel portions have a reflective electrode
68
of substantially non transparent material and a pixel electrode
70
of transparent conducting material. The reflective electrode
68
includes a transmitting hole
72
, which can have a rectangular shape. The pixel electrode
70
is larger than the transmitting hole
72
of the reflective electrode
68
.
FIGS. 4A
to
4
D illustrate manufacturing process in cross section taken along line IV—IV of FIG.
3
.
FIG. 4A
shows a gate electrode
52
on the substrate
1
. The gate electrode
52
is made of a material chosen from tungsten(W), Chrome(Cr), or aluminum alloy.
FIG. 4B
shows a gate insulation layer
80
and the semiconductor layer
82
and source and drain electrodes
62
and
64
stacked in this order.
FIG. 4C
shows a protection layer
84
on the source and drain electrodes
62
and
64
. The protection layer
84
has a drain contact hole
66
at a corresponding position of the drain electrode
64
. The protection layer is made of a material chosen from silicon nitride, silicon oxide, and so on. The pixel electrode
70
is formed on the protection layer
84
. The pixel electrode
70
has indium tin oxide and contacts the drain electrode
64
via the drain contact hole
66
.
FIG. 4D
shows formation of a reflective electrode
68
. An insulation layer
86
of benzocyclobutene (BCB) is deposited on the pixel electrode
70
and patterned to expose a portion of the pixel electrode
70
near the drain contact hole
66
. Afterwards, the reflective electrode
68
is formed on the insulation layer
86
.
FIG. 5
schematically shows a transflective LCD device in cross section. The portion of the transmitting hole, the pixel electrode and the reflective electrode are emphasized in the drawing.
The transflective LCD device in
FIG. 5
is operable in transmissive and reflective modes. First, in reflective mode, the incident light
110
from the upper substrate
106
is reflected on the reflective electrode
68
and directed toward the upper substrate
106
. At this time, when electrical signals are applied to the reflective electrode
68
by the switching element (not shown), phase of the liquid crystal layer
100
varies and thus the reflected light is colored by the color filter
104
and displayed in the form of images.
Further, in transmissive mode, light
112
generated from the backlight device
102
passes through portions of the pixel electrode
70
corresponding to the transmitting holes
72
. When the electrical signals are applied to the pixel electrode
70
by the switching element (not shown), phase of the liquid crystal layer
100
varies. Thus, the light
112
passing through the liquid crystal layer
100
is colored by the color filter
104
and displayed in the form of images.
As described above, since the transflective LCD device has both transmissive and reflective modes, the transflective LCD device can be used without regard to the time of day (e.g., noon or dusk). It also has the advantage that it can be used for a long time by consuming low power.
FIG. 6
is an enlarged view of “A” portion of FIG.
5
. Distance between the upper surface of the pixel electrode
70
and the upper surface of the reflective electrode
68
is designated as “d”, which is caused mainly by the insulation layer
86
. Since an equipotential surface is formed along surfaces of the electrodes, distortion occurs in the electric field at the interface portion “F” of the two electrodes
68
and
70
.
FIG. 7
illustrates a simulation graph showing equipotential lines and the direction of liquid crystal molecules in case of adopting the insulation layer
86
of 2 &mgr;m. The simulation result reveals that the electric fields are much in disarray near the interface between the reflective and pixel electrodes
68
and
70
. The liquid crystal molecules in the transmitting hole
72
do not have a uniform arrangement direction
85
, which is mainly affected by the equipotential line
67
. In a case of dark lighting conditions, since the arrangement of the liquid crystal molecules in the transmitting hole or portion
72
does not have symmetry and uniformity, the light fr
Baek Heume-Il
Ha Kyoung-Su
Kim Ik-Soo
Kim Yong-Beom
Akkapeddi P. R.
Birch & Stewart Kolasch & Birch, LLP
LG. Philips LCD Co. Ltd.
Sikes William L.
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