Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1997-11-24
2001-04-03
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S066000, C257S072000, C257S348000, C257S349000, C257S350000, C257S351000, C257S352000, C257S353000, C257S354000, C257S355000, C257S336000, C257S344000
Reexamination Certificate
active
06211553
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a thin-film transistor, a method for manufacturing the same, and a liquid crystal display device using the transistor.
Liquid crystal display devices have wide utility as display devices capable of realizing lightweight, miniaturization, and thinning. Among them, twisted nematic mode (TN mode) active matrix liquid crystal devices are widely known as a display device whose drive voltage is low with an attendant low power consumption and which is high in contrast and is able to provide high-quality images.
FIG. 7
shows a typical prior art thin-film transistor having a top gate structure provided in this type of liquid crystal device. A thin-film transistor A of the prior art is arranged such that it includes an island-shaped semiconductor unit
2
on a transparent substrate
1
, an insulating layer
3
formed on the substrate
1
to cover the semiconductor unit
2
therewith, a source electrode
4
and a drain electrode
5
connected via the insulating layer
3
to the semiconductor unit
2
, an insulating layer
6
to cover them as shown, and a pixel electrode
7
formed on the insulating layer
6
.
The semiconductor unit
2
is constituted of a channel formation portion
8
at the center thereof, and a source region
9
and a drain region
10
formed to sandwich the channel formation portion
8
therebetween from opposite sides thereof. The source electrode
4
is connected to the source region
9
and the drain electrode
5
is connected to the drain region
10
, and the pixel electrode
5
is connected to the drain electrode
5
. A gate electrode
12
is formed within the insulating layer
3
above the channel formation portion
8
via a gate insulating layer
11
.
In the structure depicted in
FIG. 7
, the semiconductor unit
2
is generally constituted of an amorphous silicon or a polysilicon, the source electrode
4
and the drain electrode
5
are constituted of conductive metal materials, and the pixel electrode
7
is constituted of a transparent conductive film such as ITO (indium tin oxide). The thin film transistor A of this example has a structure such that a load in the channel formation portion
8
is controlled by the action of electric field generated by the gate electrode
12
to thereby obtain the operation as a switch.
In the structure depicted in
FIG. 7
wherein the pixel electrode
7
is indirectly connected to the drain region
10
via the drain electrode
5
, an insulating layer
6
is freshly formed after the formation of the drain electrode
5
. This essentially requires the formation of a contact hole in the insulating layer
6
, after which the pixel electrode
7
has to be formed, thus presenting the problem that the manufacturing process becomes complicated. Especially, there arises the problem that for the formation of the contact hole in the insulating layer
6
, an additional mask is required in a photolithographic step.
It may occur to one that using the structure shown in
FIG. 8
, a connection terminal
7
A of the pixel electrode
7
is connected directly to the drain region
10
to make a contact. In this connection, however, the structure of
FIG. 8
has the problem that any good contact is not possible owing to the reason set out below.
The semiconductor unit
2
is constituted of an amorphous silicon film or a polysilicon film, and the film is doped with ions to form n
+
layers thereby forming the source region
9
and the drain region
10
. These source and drain regions
9
,
10
, respectively, have a relatively great specific resistance of about 10
−2
to 10
−3
&OHgr;×cm. If the ITO pixel electrode
7
is formed directly on the regions
9
,
10
, there arises the problem that the electric resistance at the connections becomes great.
To avoid this, it is usual in prior art to diffuse a silicide of an element such as Cr, Ta, W or the like into the upper portions of the source and drain regions
9
,
10
to form a thin silicide layer whose specific resistance is low (e.g. a specific resistance of about 10
−4
&OHgr;×cm), ensuring the connections through the silicide layer. However, where the contact holes are formed in the insulating layer
3
for the formation of the connection terminal
7
A, a problem is involved in that when over-etched, the silicide layers beneath the contact holes are etched and thus disappear. Thus, it becomes difficult to make good contacts ensuring low resistance connections. Moreover, Al is known as a material for interconnection which is low in specific resistance and which is unlikely to cause the delay of signals. Al is also known as an element which is difficult to make good contact with the ITO pixel electrode
7
, thus presenting the problem that this material cannot be adopted in the structure shown in FIG.
8
.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a thin-film transistor which can overcome the problems of the prior art counterparts and wherein the interconnection resistance between a drain region and a drain electrode and the interconnection resistance between a source region and a source electrode are, respectively, lessened to ensure good contacts thereat, along with the good contact between the drain electrode and a pixel electrode and the good contact between the source interconnection and the source electrode.
It is another object of the invention to provide a method for making such a thin-film transistor of the type mentioned above wherein the transistor can be made according to a process having a reduced number of steps than in prior processes, with a reduced number of masks.
It is another object of the invention to provide a method for making a thin-film transistor wherein the thin-film transistor which has a reduced leakage current at the Off time and which is improved in reliability can be readily made without changing the manufacturing conditions from those conditions of prior art.
It is another object of the invention to provide a liquid crystal display device which comprises a thin-film transistor of the type mentioned above.
According to one embodiment of the invention, there is provided a thin-film transistor which comprises a silicon semiconductor unit formed on a substrate and having a channel formation portion sandwiched between a source region and a drain region at opposite sides of the channel formation portion, a gate electrode formed on the channel formation portion via a gate insulating layer, a pixel electrode formed on the substrate, kept away from the semiconductor unit and made of ITO, a drain electrode electrically connecting the pixel electrode with the drain region, a source interconnection formed in spaced relation with the source region, and a source electrode electrically connecting the source interconnection and the source region, wherein the source electrode and the drain electrode are, respectively, formed of a metal which is more unlikely to be oxidized than tin and is capable of alloying with silicon, and a connection between the drain electrode and the drain region and a connection between the source electrode and the source region is formed of a silicide of the metal and silicon.
In the practice of the invention, if the drain electrode or the source electrode is made of any of Cr, Mo, Ta and W, a low resistance connection with a transparent pixel electrode made of ITO can be realized, ensuring good contact.
Further, if a source interconnection is made of Al, Al makes a good contact with any element selected from Cr, Mo, Ta and W which constitutes the source electrode, so that good connection with the source electrode is ensured as having a low resistance. In addition, any delay of signals inputted to the source interconnection is not caused.
According to another embodiment of the invention, there is also provided a method for making a thin-film semiconductor, which comprises the steps of forming, on a substrate, a semiconductor layer and a transparent pixel electrode made of ITO, which are kept away from each other, forming a gate
Abraham Fetsum
Brinks Hofer Gilson & Lione
L. G. Philips LCD Co., Ltd.
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