Array substrate for use in LCD device

Details Array substrate for use in LCD device Array substrate for use in LCD device

2000-12-15

2003-10-07

Ton, Toan (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S054000, C349S192000, C349S187000, C257S059000

Reexamination Certificate

active

06630976

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an array substrate of the LCD device having thin film transistors.
2. Description of Related Art
In general, a liquid crystal display (LCD) device displays an image using a plurality of pixels. The LCD device having a thin film transistor (TFT) as a switching element is typically called a thin film transistor liquid crystal display (TFT-LCD) device.
A typical liquid crystal display device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in arrangement resulting from their thin and long shapes. The arrangement direction of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, if electric fields are applied to the liquid crystal molecules, the arrangement of the liquid crystal molecules changes. Since incident light is refracted according to the arrangement direction of the liquid crystal molecules due to the optical anisotropy of the arranged liquid crystal molecules, image data can be displayed.
By now, an active matrix LCD (AM-LCD) that the thin film transistors and the pixel electrodes are arranged in the form of a matrix is receiving a great deal of attention due to its high resolution and superiority in displaying moving video data.
FIG. 1
is a cross-sectional view illustrating a conventional liquid crystal display (LCD) panel. As shown in
FIG. 1
, the LCD panel
20
has lower and upper substrates
2
and
4
with a liquid crystal layer
10
interposed between the lower and upper substrates
2
and
4
. The lower substrate
2
has the TFT “S” as a switching element to change an orientation of the LC molecules and includes a pixel electrode
14
to apply a voltage to the LC layer
10
according to signals of the TFT “S”. The upper substrate
4
has a color filter
8
for implementing colors and a common electrode
12
on the color filter
8
. The common electrode
12
serves as an electrode for applying a voltage to the LC layer
10
. The pixel electrode
14
is arranged over a pixel portion “P”, i.e., a display area. Further, to prevent leakage of the LC injected into a space between the two substrates
2
and
4
, the two substrates
2
and
4
are sealed by a sealant
6
.
FIG. 2
is a plan view illustrating an array substrate. A gate line
22
is arranged in a transverse direction and a data line
24
is arranged in a longitudinal direction perpendicular to the gate line
22
such that pixel region having pixel electrode
14
is defined by the gate line
22
and the data line
24
.
In the AM-LCD, the switching element (TFT “S”) applying the voltage to the LC layer
10
(see
FIG. 1
) is formed nearby the cross point of the gate line
22
and the data line
24
. The TFT “S” has a gate electrode
26
extended from the gate line
22
, a source electrode
28
extended from the data line
24
and a drain electrode
30
electrically contacting the pixel electrode
14
via contact hole (not shown). When the gate electrode
26
of the TFT receives gate signals, in the ON-state, the data line
24
transmits data signals to the pixel electrode
14
. On the other hand, when the gate electrode
26
is in the OFF-state, data signals are not transmitted to the pixel electrode
14
. In general, a manufacturing process depends on the materials of the elements used, and on the intended design. For example, the resistivity of the material used in the gate line and the data line determines the picture quality in a large LCD panel (over 18 inches) and in an LCD panel having a high resolution. In the case of these LCD panels, the material such as Aluminum (Al) or Al-alloy is used for the gate line and data line.
FIGS. 3
a
to
3
d
are cross-sectional views illustrating process steps for fabricating a conventional array substrate for the active matrix LCD device.
An inverted staggered type TFT is generally used for an LCD device due to the simple structure and superior efficiency. The inverted staggered type TFT includes a back channel etched type (EB) and an etch stopper type (ES). The manufacturing method of the back channel etched type TFT will be explained hereinafter.
First, a first metal layer is deposited on a substrate
1
by a sputtering process after a cleaning process which enhances adhesion between the substrate
1
and a metal layer and removes organic materials and alien substances from the substrate.
FIG. 3
a
shows a step for forming the gate line
22
, the gate electrode
26
and a capacitor electrode
32
by patterning the first metal layer using a first mask. As a metal for the gate electrode
26
, Aluminum is mainly used so as to reduce the RC delay owing to its low resistance. However, pure Aluminum is weak in acidity and may result in line defects by a formation of a hillock during a high temperature process, so Aluminum alloy and multi-layered Aluminum are used.
Referring to
FIG. 3
b
, the gate insulation layer
34
is formed on the entire surface of the substrate
1
, while covering the gate line
22
and the gate and capacitor electrodes
26
and
32
. Then, a pure amorphous silicon (a-Si:H) layer and a doped amorphous silicon (n
+
a-Si:H) layer are formed in series on the gate insulation layer
34
. As shown in
FIG. 3
b
, an active layer
36
and an ohmic contact layer
38
are formed by patterning the silicon layers. The ohmic contact layer
38
reduces contact resistance between the active layer
36
and an electrode that will be formed later.
As depicted in
FIG. 3
c
, the data line
24
and the source and drain electrodes
42
and
44
are formed by depositing, and then patterning, a second metal layer. A metal for the source and drain electrodes
42
and
44
may be selected from Cr, Mo, or the like. The portion of the ohmic contact layer
38
on the active layer
36
is etched using the source and drain electrodes
42
and
44
as a mask. If the ohmic contact layer
38
between the source and drain electrodes
42
and
44
is not removed, serious problems such as deterioration of electrical characteristics and efficiencies can be caused in the TFT “S” (see FIG.
2
). Etching the portion of the ohmic contact layer
38
over the gate electrode
26
requires special attention. While etching the ohmic contact layer
38
, the active layer
36
is over-etched by 50-100 nm due to the fact that the active layer
36
and the ohmic contact layer
38
have the same etch selectivity. This is because etching uniformity directly affects the electrical characteristics of the TFT.
As shown in
FIG. 3
d
, a protection layer
46
is formed on the source and drain electrodes
42
and
44
in order to protect the active layer
36
by depositing, and then patterning, an insulating material. Due to an unstable energy state of the active layer
36
and an alien substances generated during the etching process (which affect electrical characteristics of the TFT), the protection layer
46
is usually made of a material selected from inorganic materials such as SiN
x
and SiO
2
, or organic materials such as BCB (benzocyclobutene). In addition, the protection layer
46
is used as a material having high light transmittance, humidity resistance and durability, in order to protect the channel area of the TFT and major portions of a pixel region from possible humidity and scratch (occurring during later process steps).
A contact hole
45
is formed in the protection layer
46
to expose the portion of the rain electrode
44
.
FIG. 3
d
also shows a step of forming a pixel electrode
40
by depositing, and then patterning, a transparent conducting oxide (TCO) layer. Indium tin oxide (ITO) is usually employed for the transparent conducting oxide layer. The pixel electrode
40
makes electrical contact with the drain electrode
44
via the contact hole
45
and overlaps the capacitor electrode
32
to form a storage capacitor.
In the above-mentioned process, the ga

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