Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-02-08
2004-12-21
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S042000, C349S187000, C257S059000, C257S072000
Reexamination Certificate
active
06833896
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2001-6819, filed on Feb. 12, 2001, 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 liquid crystal display devices. More particularly it relates to liquid crystal display devices implenting in-plane switching (IPS) where an electric field to be applied to liquid crystals is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired light image can be produced.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin and have low power consumption characteristics. The typical liquid crystal display (LCD) panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs) and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
In a conventional LCD device, since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. However, the conventional LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to
FIGS. 1
,
2
A, and
2
B.
FIG. 1
is a schematic cross-sectional view illustrating a concept of a conventional IPS-LCD panel. As shown in
FIG. 1
, upper and lower substrates
10
and
20
are spaced apart from each other, and a liquid crystal layer
30
is interposed therebetween. The upper and lower substrates
10
and
20
are often referred to as an array substrate and a color filter substrate, respectively. On the lower substrate
20
are a common electrode
22
and a pixel electrode
24
. The common and pixel electrodes
22
and
24
are aligned parallel to each other. On a surface of the upper substrate
10
, a color filter layer (not shown) is commonly positioned between the pixel electrode
24
and the common electrode
22
of the lower substrate
20
. A voltage applied across the common and pixel electrodes
22
and
24
produces an electric field
26
through the liquid crystal
32
. The liquid crystal
32
has a positive dielectric anisotropy, and thus it aligns parallel to the electric field
26
.
FIGS. 2A and 2B
conceptually help illustrate the operation of a conventional IPS-LCD device. When no electric field is produced by the common and pixel electrodes
22
and
24
, i.e., off state, as shown in
FIG. 2A
, the longitudinal axes of the liquid crystal (LC) molecules
32
are parallel and form a definite angle with the common and pixel electrodes
22
and
24
. For example, the longitudinal axes of the LC molecules
32
are arranged parallel with both the common and pixel electrodes
22
and
24
.
On the contrary, when an electric voltage is applied to the common and pixel electrodes
22
and
24
, i.e., on state, as shown in
FIG. 2B
, because the common and pixel electrodes
22
and
24
are on the lower substrate
20
, an in-plane electric field
26
that is parallel to the surface of the lower substrate
20
is produced. Accordingly, the LC molecules
32
are re-arranged to bring their longitudinal axes into coincidence with the electric field. However, the first LC molecules
32
a
positioned corresponding to the common and pixel electrodes
22
and
24
do not change their orientation, while the second LC molecules
32
b
positioned between the common and pixel electrodes
22
and
24
are arranged perpendicular to the common and pixel electrodes
22
and
24
. Therefore, the result is a wide viewing angle that ranges from about 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD panel, for example.
FIG. 3
is a plan view illustrating one pixel of an array substrate according to a conventional IPS-LCD device. As shown, a gate line
66
is transversely arranged and a data line
70
is disposed substantially perpendicular to the gate line
66
. A pair of gate and data lines
66
and
70
define a pixel region on the array substrate. An island-shaped semiconductor layer
84
is positioned near the crossing of the gate and data lines
66
and
70
, thereby forming a thin film transistor (TFT) “T” with a gate electrode
80
, a source electrode
86
and a drain electrode
88
. The gate electrode
80
extends from the gate line
66
, and the source electrode
86
extends from the data line
70
. The drain electrode
88
is connected to a pixel connecting line
72
that connects a plurality of pixel electrodes
74
to each other. A common line
64
is spaced apart from the gate line
66
and disposed parallel with the gate line
66
. A plurality of common electrodes
62
protrude from the common line
64
and are disposed parallel to each of the pixel electrodes
74
, so that each common electrode
62
is spaced apart from the adjacent pixel electrodes
64
with a predetermined interval therebetween.
At the ends of the gate and data lines
66
and
70
, gate and data pads
67
and
71
are respectively positioned for a connection with the external driving circuits (not shown). A gate pad electrode
75
and a data pad electrode
77
are disposed on the gate pad
67
and the data pad
71
, respectively. A gate pad contact hole
83
and a data pad contact hole
73
are formed over the gate pad electrode
75
and the data pad electrode
77
, respectively, to expose those electrodes for the connection with the external driving circuits.
FIGS. 4A
to
4
G,
5
A to
5
G and
6
A to
6
G are cross-sectional views taken along line IV—IV, V—V and VI—VI of
FIG. 3
, respectively illustrating process steps of manufacturing the array substrate of a conventional IPS-LCD.
FIGS. 4A
to
4
G show the steps of formi
Chowdhury Tarifur R.
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
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