Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-07-01
2004-03-09
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C257S059000
Reexamination Certificate
active
06704069
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to liquid crystal display devices and more particularly to a liquid crystal display device having a thin-film transistor (TFT).
Liquid crystal display devices are used extensively in information processing apparatuses such as a computer as a compact display device consuming little electric power.
In order to realize a high-quality color representation, recent liquid crystal display devices tend to use a so-called active-matrix driving method, in which each of the pixel electrodes in the liquid crystal display device is turned on and off by a corresponding TFT that is provided on a glass substrate constituting the liquid crystal display device in correspondence to the pixel electrode.
FIG. 1
shows the construction of a conventional active-matrix type liquid crystal display device
10
.
Referring to
FIG. 1
, the liquid crystal display device
10
includes a TFT glass substrate
11
carrying thereon a number of TFTs and corresponding transparent pixel electrodes, and a glass substrate
12
is provided on the TFT substrate
11
so as to face the TFT substrate
11
with a gap formed therebetween. The gap thus formed is filled by a liquid crystal layer
1
in the state that the liquid crystal layer
1
is confined between the TFT substrate
11
and the opposing substrate
12
by a seal member not illustrated.
In the conventional liquid crystal display device
10
of the foregoing construction, the direction of the liquid crystal molecules in the liquid crystal layer
1
is selectively modified by applying a drive voltage to a selected pixel electrode via a corresponding TFT.
Further, it should be noted that the liquid crystal display device
10
includes a pair of polarizers at respective outer sides of the glass substrates
11
and
12
in the crossed Nicol state, and the glass substrates
11
and
12
further carry molecular alignment films on the respective interior sides thereof in contact with the liquid crystal layer
1
.
FIG. 2
shows a part of the TFT substrate
11
in an enlarged scale.
Referring to
FIG. 2
, the TFT substrate
11
carries thereon a number of pad electrodes
11
A for receiving a scanning signal and a number of scanning electrodes
11
a
each extending from a corresponding pad electrode
11
A in a first direction. Further, the TFT substrate
11
carries thereon a number of pad electrodes
11
B for receiving an image signal and a number of signal electrodes
11
b
each extending from a corresponding pad electrode
11
B in a second direction generally perpendicular to the first direction. Further, in correspondence to each intersection of a scanning electrode
11
a
and a signal electrode
11
b
, there is provided a TFT
11
C and a corresponding transparent pixel electrode
11
D.
In the liquid crystal display device
10
of the foregoing construction, one of the scanning electrodes
11
a
is selected by selectively supplying a scanning signal to the corresponding electrode pad
11
A. Further, a signal electrode
11
b
is selected by supplying an image signal to the corresponding electrode pad
11
B. Thereby, the image signal is forwarded to the corresponding transparent pixel electrode
11
D via the TFT
11
C.
FIG. 3
shows the construction of a conventional TFT
11
C.
Referring to
FIG. 3
, the TFT
11
C is constructed on a glass substrate
21
corresponding to the TFT substrate
11
of FIG.
1
and includes a gate electrode
22
formed on the glass substrate
21
in electrical connection to the scanning electrode
11
a
, wherein a gate insulation film
23
provided on the glass substrate
21
covers the gate electrode
22
. Further, an amorphous silicon pattern
24
is provided on the gate insulation film
23
so as to cover the gate electrode
22
. Typically, the gate electrode
22
is formed of an Al—Nd alloy or an Al—Sc alloy.
It should be noted that the foregoing amorphous silicon pattern
24
constitutes the active region of the TFT
11
C and is covered by a channel protection pattern
25
of SiN in the part corresponding to the channel region of the TFT
11
C located immediately above the gate electrode
22
.
On the amorphous silicon pattern
24
, there are provided a pair of amorphous silicon patterns
26
A and
26
B of the n
+
-type at both lateral sides of the channel protection pattern
25
, and the amorphous silicon pattern
26
A carries thereon a Ti layer
27
a
, an Al layer
27
b
and a Ti layer
27
c
consecutively, wherein the layers
27
a
-
27
c
constitute an ohmic electrode
27
A connected to the signal electrode
11
b
. Similarly, the amorphous silicon pattern
26
B carries thereon a Ti layer
27
d
, an Al layer
27
e
and a Ti layer
27
f
consecutively, wherein the layers
27
d
-
27
f
constitute an ohmic electrode
27
B.
It should be noted that the ohmic electrodes
27
A and
27
B are covered by a protective film
28
of SiN, and a transparent pixel electrode
29
of In
2
SnO
5
(ITO) is provided on the protective film
28
, wherein the pixel electrode
29
makes a contact with the uppermost Ti layer
27
f
of the ohmic electrode
27
B via a contact hole formed in the protective film
28
.
In the TFT
11
C having such a construction, it should be noted that the conduction between the ohmic electrode
27
A and the ohmic electrode
27
B via the channel region formed in the amorphous silicon pattern
24
is controlled in response to the scanning signal supplied to the gate electrode, and the pixel electrode
29
corresponding to the TFT
11
C thus turned on is selectively activated by the image signal supplied to the ohmic electrode
27
A.
It should be noted that the fabrication process of the TFT
11
C of
FIG. 3
includes the steps of consecutively depositing, on an amorphous silicon layer constituting the amorphous silicon patterns
26
A and
26
B, a Ti layer corresponding to the Ti layers
27
a
and
27
b
, an Al layer corresponding to the Al layers
27
b
and
27
e
, and a Ti layer corresponding to the Ti layers
27
c
and
27
f
, followed by a patterning process conducted on the layered structure thus obtained by a dry etching process while using an etching mask. The dry etching process may be conducted typically by an RIE (reactive ion etching) process that uses a mixture of Cl
2
and BCl
3
as an etching gas. As a result of the dry etching process, the foregoing amorphous silicon patterns
26
A,
26
B and the electrode patterns
27
A and
27
B are patterned on the amorphous silicon pattern
24
substantially simultaneously.
In such a fabrication process of the TFT
11
C, it should be noted that the Al pattern
27
b
or
27
e
may experience a selective lateral etching at the exposed edge part of the electrode patterns
27
A and
27
B as indicated in FIG.
3
. When such a selective lateral etching occurs in the Al patterns
27
b
and
27
e
, there inevitably occurs a problem of overhang formation at the edge part of the ohmic electrode
27
A or
27
B, wherein the existence of such an overhang structure may induce the problem of failure of electrical connection in the patterns connected to the ohmic electrode
27
A or
27
B. For example, the electrical connection of the pixel electrode
29
to the ohmic electrode
27
B may suffer from such a failure at the receded side edge of the Al pattern
27
e.
While it is possible to suppress the overhang formation in the foregoing dry etching process by enhancing the anisotropy of the etching process, such a highly anisotropic dry etching process is also disadvantageous in eliminating the electrical connection failure, as a vertical side edge of the ohmic electrodes
27
A and
27
B, formed as a result of the highly anisotropic dry etching process, tends to induce a poor step coverage in the conductor pattern such as the pixel electrode
29
extending across the vertical side edge.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful thin-film transistor, liquid crystal display device using such a thin-film transistor and a fabrication process thereof wherei
Fujitsu Display Technologies Corporation
Greer Burns & Crain Ltd.
Ton Toan
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