Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-02-09
2002-12-03
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S043000, C349S138000, C349S187000
Reexamination Certificate
active
06490019
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-6224, filed on Feb. 10, 2000 under 35 U.S.C. §119, the entirety of each 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 reflective LCD device.
2. Description of Related Art
Recently, liquid crystal display (LCD) devices with light, thin, and low power consumption characteristics have been used in office automation equipment and video units and the like. Such LCDs typically uses an optical anisotropy of a liquid crystal (LC). The LC has thin and long LC molecules, which causes an orientational alignment of the LC molecules. Therefore, alignment direction of the LC molecules is controlled by applying an electric field to the LC molecules. When the alignment direction of the LC molecules are properly adjusted, the LC is aligned and light is refracted along the alignment direction of the LC molecules to display image data.
By now, an active matrix (AM) LCD where a plurality of thin film transistors (TFTs) and pixel electrodes are arranged in shape of an array matrix is most focused on because of its high resolution and superiority in displaying moving pictures. When each TFT serves to switch a corresponding pixel, the switched pixel transmits an incident light. Since an amorphous silicon layer is relatively easy formed on a large, inexpensive glass substrate, an amorphous silicon thin film transistor (a-Si:H TFT) is widely used.
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. 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
, which 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 has a problem 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 pixel of a typical reflective LCD device in plane view. As shown in
FIG. 2
, Nth gate line
8
and (N−1)th gate line
6
are transversely formed on an insulated substrate (reference
1
of FIG.
3
A), while Nth data line
2
and (N+1)th data line
4
are formed perpendicular to the gate lines
6
and
8
. Near a cross point of the gate and data lines
2
and
8
, a gate electrode
18
is formed, and a source electrode
12
overlaps the gate electrode
18
. Opposite to the source electrode
12
, a drain electrode
14
is formed. The drain electrode
14
electrically contacts a reflective electrode
10
via a drain contact hole
16
, which is a through hole formed over the drain electrode
14
.
The reflective electrode
10
, which is formed over a pixel region defined by the gate and data lines, has a plurality of concave portions
20
on itself. The concave portions
20
enhance a reflective area of the reflective electrode
10
and prevent a mirror effect of the reflective electrode
10
. An opaque metal is conventionally selected for the reflective electrode
10
to reflect an incident light. When light is incident to the reflective electrode
10
, the concave portions
20
induce a diffused reflection of the incident light such that the mirror effect of the reflective electrode
10
decreases.
Now, with reference to
FIGS. 3A
to
3
D, a fabricating process for the typical reflective LCD device shown in
FIG. 2
will be explained in detail.
At first, a first metal layer is deposited on an insulated substrate
1
of FIG.
3
A and patterned to form the gate electrode
18
and gate lines (reference
2
and
4
of FIG.
2
). The first metal layer for the gate electrode
18
is selected from a group consisting of chromium (Cr), molybdenum (Mo), aluminum (Al), titanium (Ti), tin (Sb), tungsten (W), and copper (Cu). A sputtering process is used for depositing the first metal layer.
Next, as shown in
FIG. 2B
, a gate-insulating layer
30
formed to cover the gate electrode
18
and the gate lines on the lower substrate
1
. Silicon nitride (SiN
x
) or silicon oxide (SiO
2
) is selected for the gate-insulating layer
30
. Thereafter, a silicon layer is deposited and patterned on the gate-insulating layer
30
to form a silicon island
32
, which serves as an active layer for a thin film transistor. On the silicon island
32
, the source and drain electrodes
12
and
14
are formed.
Next, as shown in
FIG. 3C
, a passivation layer
34
is formed to cover the source electrode
12
and drain electrode
14
such that the source electrode
12
, the drain electrode
14
and the silicon island
32
are protected from other electrodes, which will be formed in later steps. Over the drain electrode
14
, the drain contact hole
16
is formed through the passivation layer
34
. The passivation layer
34
also has a plurality of concave portions
20
a, which correspond to the concave portions
20
of the reflective electrode
10
. Silicon nitride (SiN
x
) or silicon oxide (SiO
2
) is selected for the passivation layer
34
like the gate-insulating layer
30
.
Next, as shown in
FIG. 3D
, the reflective electrode
10
is formed on the passivation layer
34
. As explained previously, the reflective electrode
10
electrically contacts the drain electrode
14
via the drain contact hole
16
, and the concave portions
20
of the reflective electrode
10
induce the diffused reflection of an incident light.
FIG. 4
is a partial sectional view of the typical reflective LCD device shown in FIG.
2
. As shown, an upper substrate
44
having an insulated substrate
1
, a color filter layer
46
and a common electrode
48
is spaced apart from a lower substrate
40
that is formed via the above-explained process. In the spacing between the upper and lower substrates
44
and
40
, a liquid crystal layer
42
is interposed. As mentioned previously, the typical reflective LCD device uses an ambient light incident from an external light source. The reflective electrode
10
of the reflective LCD device serves to reflect the incident light.
FIG. 5
illustrates a problem of the conventional reflective LCD device described in FIG.
4
. As shown, the concave portion
20
conventionally has a cylindrical shape with a depth “d
1
” measured from a surface of the substrate (reference
1
of FIG.
4
). Since edges of the concave portion
20
are sharply stepped, open lines
22
occur in the reflective electrode
10
around the edges of the concave portion
20
. Further, due to a cylindrical shape of the concave portion
20
, an incident ray
24
a
incident onto a bottom surface of the concave portion
20
and a reflected ray
24
b
reflected from the bottom surface have a very narrow range of incident angles. Therefore, though the concave portion
20
prevents the mirror effect of the reflect
Lee Jae-Gu
Lim Byoung-Ho
Chowdhury Tarifur R.
LG. Philips LCD Co. LTD
Morgan & Lewis & Bockius, LLP
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