Thin film transistor and method of fabricating the same

Details Thin film transistor and method of fabricating the same Thin film transistor and method of fabricating the same

2002-05-15

2004-07-06

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

On insulating substrate or layer

C438S151000, C438S157000, C438S166000, C438S384000, C257S066000

Reexamination Certificate

active

06759283

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a thin film transistor and a method of fabricating the same, and more particularly to a thin film transistor having a reverse-stagger structure and being capable of reducing off-leakage current, and a method of fabricating such a thin film transistor.
2. Description of the Related Art
There has been developed an active matrix type liquid crystal display device including a thin film transistor (TFT) as a switching device for a liquid crystal display. An active matrix type liquid crystal display device is designed to generally include an active matrix substrate having a gate wiring, a drain wiring, a thin film transistor, a pixel electrode and so on, an opposing substrate having a color filter, a black matrix layer and so on, and a liquid crystal layer sandwiched between the active matrix substrate and the opposing substrate. In an active matrix type liquid crystal display device, a direction in which liquid crystal molecules are oriented is rotated in accordance with a voltage applied either across electrodes arranged on both an active matrix substrate and an opposing substrate or between electrodes arranged in an active matrix substrate, to thereby control a light passing through a liquid crystal layer in each of pixels for displaying desired images on a display screen.
On the assumption that a semiconductor layer is formed above an active matrix substrate, a thin film transistor is grouped into a forward-stagger type one in which a gate electrode is formed above the semiconductor layer and source/drain electrodes are formed below the semiconductor layer, and a reverse-stagger type one in which a gate electrode is formed below the semiconductor layer and source/drain electrodes are formed above the semiconductor layer. A thin film transistor is generally designed to have a reverse-stagger structure.
FIG. 1
is a cross-sectional view of a conventional active matrix type liquid crystal display device having a reverse-stagger structure.
With reference to
FIG. 1
, in the conventional active matrix substrate, a gate electrode
2
a
is formed on a glass substrate
1
, and a gate insulating film
3
is formed on the glass substrate
1
, covering the gate electrode
2
a
therewith. On the gate insulating film
3
are formed an island-shaped amorphous silicon (hereinafter, referred to simply as “a-Si”) layer
4
a
which will make a semiconductor layer in a thin film transistor, and an n
+
a-Si layer
4
b
containing much n-type impurity. The a-Si layer
4
a
and the n
+
a-Si layer
4
b
are partially removed to define a channel
4
d.
A drain electrode
5
a
and a source electrode
5
b
are formed on the n
+
a-Si layer
4
b
around the channel
4
d.
A passivation insulating film
7
is formed on the gate insulating film
3
, covering the drain electrode
5
a
and the source electrode
5
b
therewith, in order to planarize a surface of the active matrix substrate.
The passivation insulating film
7
is partially removed above the source electrode
5
b
to define a contact
6
. A pixel electrode
8
comprised of an electrically conductive transparent film such as a film composed of indium tin oxide (ITO) is formed in the contact
6
and each of pixels. An alignment film
9
is formed covering the pixel electrode
8
and the passivation insulating film
7
therewith.
Though not illustrated, red, green and blue color filters are formed on a glass substrate in each of pixels in the opposing substrate. An overcoating layer is formed on the color filters, and a transparent electrode composed of ITO is formed on the overcoating layer. Similarly to the active matrix substrate, an alignment film is formed on the transparent electrode in facing relation to the alignment film
9
of the active matrix substrate. The alignment films of both the active matrix substrate and opposing substrate are oriented into a predetermined direction.
The active matrix substrate and the opposing substrate are fixed to each other with spacers being sandwiched therebetween to define a gap therebetween. Liquid crystal is introduced into the gap.
In order to ensure desired switching characteristic in an active matrix type liquid crystal display device, it is generally important that an ON current, that is, a current running across source/drain electrodes when a gate is on is relatively high, and an OFF current, that is, a current running across source/drain electrodes when a gate is off is relatively small.
However, the conventional active matrix type liquid crystal display device having such a structure as mentioned above is accompanied with a problem that an off-leakage current is produced in a back-channel due to charging up in the spacers located above a thin film transistor and/or the alignment film
9
, and causes malfunction in the thin film transistor with the result of display defect.
In order to prevent an off-leakage current to be produced in a back-channel, Japanese Patent No. 2621619 and Japanese Patent Publication No. 6-9246 have suggested an active matrix type liquid crystal display device in which an inactive layer is formed at a surface of an amorphous silicon layer which defines a channel.
Hereinbelow is explained the suggested active matrix type liquid crystal display device with reference to
FIGS. 2A
to
2
C and
3
.
FIGS. 2A
to
2
C are cross-sectional views of the active matrix substrate suggested in Japanese Patent No. 2621619, illustrating respective steps of a method of fabricating the same.
The active matrix substrate suggested in Japanese Patent No. 2621619 is fabricated as follows.
First, as illustrated in
FIG. 2A
, a metal film such as a chromium film is formed on an electrically insulating transparent substrate
19
. Then, the metal film is patterned by photolithography and etching into a gate electrode
2
a.
Then, as illustrated in
FIG. 2B
, a gate insulating film
3
is formed on the electrically insulating transparent substrate
19
, covering the gate electrode
2
a
therewith. Then, a semiconductor layer
20
is formed on the gate insulating film
3
.
Then, as illustrated in
FIG. 2C
, the semiconductor layer
20
is exposed to hydrogen plasma
21
to thereby inactivate the semiconductor layer
20
at its surface.
Then, though not illustrated, a protection film is formed over the semiconductor layer
20
, and there are formed source/drain electrodes which make electrical contact with the semiconductor layer
20
through contact holes formed around a channel. Then, a second protection film is formed covering a resultant therewith.
As mentioned above, in the method of fabricating an active matrix type liquid crystal display device disclosed in Japanese Patent No. 2621619, the semiconductor layer
20
is exposed to the hydrogen plasma
21
before the formation of a protection film for protecting a thin film transistor, to thereby increase a surface level at an interface between the semiconductor layer
20
and the protection layer. As a result, a back-channel in the thin film transistor is inactivated, and resultingly, a leakage current running through the back-channel while the thin film transistor is off can be reduced.
FIG. 3
is a cross-sectional view of an active matrix substrate suggested in the above-mentioned Japanese Patent Publication No. 6-9246.
In the suggested active matrix substrate, a gate electrode
2
a
composed of NiCr is formed on a glass substrate
1
. A gate insulating film
3
is formed on the glass substrate
1
, covering the gate electrode
2
a
therewith. An amorphous silicon layer
4
a
and an n
+
amorphous silicon layer
4
b
are formed on the gate insulating film
3
. A back-channel is formed by partially removing both the n
+
amorphous silicon layer
4
b
and the amorphous silicon layer
4
a
by dry etching. The amorphous silicon layer
4
a
is formed at a surface thereof with a modified layer
22
containing oxygen, carbon and other element, by exposing the amorphous silicon layer
4
a
to plasma in a gas atmosphere in which at least one of nit

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