Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2000-10-05
2002-09-17
Whitehead, Jr., Carl (Department: 2822)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S625000, C438S627000, C438S643000, C438S648000, C438S656000, C438S688000
Reexamination Certificate
active
06451690
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of forming an electrode structure including a lower layer of polysilicon or amorphous silicon and an upper layer of a high-melting-point metal, and a method of fabricating a semiconductor device including a gate electrode formed from the electrode structure.
In a conventional MOS transistor, a gate electrode is formed from a polysilicon film. In accordance with improvement of LSIs in refinement and high speed operation, there are increasing demands for lowering the resistance of the gate electrode of a MOS transistor.
Therefore, as technique to lower the resistance of the gate electrode, a polymetal gate electrode having a laminated structure including a lower polysilicon film and an upper high-melting-point metal film has been proposed to be used as the gate electrode, and a tungsten film has been proposed as the upper high-melting-point metal film. By using a tungsten film as the upper high-melting-point metal film, the resistance value of the gate electrode can be lowered.
It is necessary to form a barrier film of tungsten nitride (WN
x
) or titanium nitride (TiN) between a polysilicon film and a tungsten film in order to prevent an impurity (such as B, P and As) introduced into the polysilicon film from diffusing into the tungsten film (as disclosed in, for example, Japanese Laid-Open Patent Publication No. 11-261059 or 7-235542).
FIG.
12
(
a
) is a sectional view of an electrode structure of a first conventional example. As is shown in FIG.
12
(
a
), a gate electrode is formed on a semiconductor substrate
1
with a gate insulating film
2
sandwiched therebetween, and the gate electrode includes a polysilicon film
3
, a barrier film
4
A of tungsten nitride (WN
x
) and a tungsten film
5
successively formed upward.
FIG.
12
(
b
) is a sectional view of an electrode structure of a second conventional example. As is shown in FIG.
12
(
b
), a gate electrode is formed on a semiconductor substrate
1
with a gate insulating film
2
sandwiched therebetween, and the gate electrode includes a polysilicon film
3
, a barrier film
4
B of titanium nitride (TiN) and a tungsten film
5
successively formed upward.
In the electrode structure of the first conventional example, a heat treatment conducted in a later procedure evaporates nitrogen included in the barrier film
4
A of tungsten nitride, so that the barrier film
4
A can be changed into the tungsten film
5
. In addition, nitrogen included in the barrier film
4
A reacts with silicon included in the polysilicon film
3
, so that a reaction layer
6
of silicon nitride (SiN) having a very large resistance value can be formed between the polysilicon film
3
and the tungsten film
5
as is shown in FIG.
12
(
c
). As a result, the resistance value of the gate electrode is disadvantageously increased.
Therefore, Japanese Laid-Open Patent Publication No. 7-235542 describes that the sheet resistance of the reaction layer
6
can be reduced to lower the resistance value of the gate electrode by setting the surface density of nitrogen included in the reaction layer
6
of silicon nitride to a predetermined value or less.
The present inventors have found, however, that the resistance value of the gate electrode cannot be lowered even by setting the surface density of nitrogen included in the reaction layer
6
to the predetermined value or less in the electrode structure of the first conventional example.
Therefore, the reason why the resistance value of the gate electrode cannot be lowered in the first example has variously examined to find the following: When the thickness of the barrier film
4
A is reduced to approximately 0.1 through 1.0 nm in order to reduce the surface density of nitrogen included in the reaction layer
6
, the barrier film
4
A cannot exhibit the barrier function. Accordingly, tungsten silicide (WSi
x
) is formed, so that the resistance value of the gate electrode cannot be lowered. When the thickness of the barrier film
4
A is increased to exceed 1.0 nm, although the barrier film
4
A can exhibit the barrier function, the reaction layer
6
of silicon nitride having a very large resistance value is formed between the polysilicon film
3
and the tungsten film
5
. Accordingly, the interface resistance value between the polysilicon film
3
and the tungsten film
5
is increased.
As another problem, since a tungsten nitride film is poor at heat resistance, a great deal of nitrogen included in the tungsten nitride film can be diffused through a heat treatment conducted at a temperature of 750° C. or more, so that the tungsten nitride film can be changed into a tungsten film.
In the case where a barrier film of titanium nitride is used as in the second conventional method, the interface resistance value between the polysilicon film
3
and the tungsten film
5
is increased owing to the reaction layer
6
of silicon nitride having a very large resistance value formed between the polysilicon film and the tungsten film for the reason described below.
First, as is shown in FIG.
13
(
a
), a polysilicon film
3
is formed on a semiconductor substrate
1
with a gate insulating film
2
sandwiched therebetween. The polysilicon film
3
is doped with a p-type impurity such as boron when a p-type gate electrode is to be formed, and is doped with an n-type impurity such as phosphorus when an n-type gate electrode is to be formed. Next, in order to deposit a titanium nitride film
4
B on the polysilicon film
3
, the semiconductor substrate
1
is loaded within a chamber where a titanium target
7
including titanium as a main component is placed, and a mixed gas including argon and nitrogen is introduced into the chamber and discharge is caused within the chamber. In this manner, plasma of the argon gas and the nitrogen gas is generated, so that a reaction layer
6
of silicon nitride can be formed in a surface portion of polysilicon film
3
through a reaction between nitrogen ions of the plasma and silicon of the polysilicon film
3
. Furthermore, the titanium target
7
is nitrided so as to form a titanium nitride film
8
thereon, and titanium nitride is sputtered out from the titanium nitride film
8
. As a result, the barrier film
4
B of titanium nitride is formed on the reaction layer
6
as is shown in FIG.
13
(
b
).
Then, the semiconductor substrate
1
is transferred to a chamber where a tungsten target
9
including tungsten as a main component is placed, and an argon gas is introduced into the chamber and discharge is caused within the chamber. In this manner, plasma of the argon gas is generated, so that tungsten can be sputtered out from the tungsten target
9
through sputtering of argon ions included in the plasma. The sputtered tungsten is deposited on the surface of the titanium nitride film
4
B, and thus, a tungsten film
5
is formed on the titanium nitride film
4
B as is shown in FIG.
13
(
c
).
Next, an impurity layer to be formed into a source or drain of a MOS transistor is formed in the semiconductor substrate
1
, and a heat treatment is carried out for activating the impurity layer at a temperature of, for example, 750° C. or more. In this manner, as is shown in FIG.
14
(
a
), excessive nitrogen included in the barrier film
4
B is diffused into an upper portion of the polysilicon film
3
. As a result, the reaction layer
6
of titanium nitride is increased in its thickness as is shown in FIG.
14
(
b
).
Moreover, the present inventors have examined the relationship between the temperature of the heat treatment and the interface resistance of the barrier film after the heat treatment.
FIG. 15
shows the relationship between the temperature (°C.) of the heat treatment and the interface resistance (R
c
) between the polysilicon film and the high-melting-point metal film after the heat treatment. In
FIG. 15
, a symbol &Circlesolid; indicates the result obtained when a barrier film of tungsten nitride (WN
x
) is formed on an n-type polysilicon film (indicated as NPS); a symbol ◯ indicates the result obtai
Matsumoto Michikazu
Sengoku Naohisa
Duong Khanh B.
Jr. Carl Whitehead
Matsushita Electronics Corporation
Studebaker Donald R.
LandOfFree
Method of forming electrode structure and method of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of forming electrode structure and method of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of forming electrode structure and method of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2838193