Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2000-11-28
2003-03-11
Everhart, Caridad (Department: 2825)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S591000, C438S592000, C438S608000, C438S595000, C438S655000, C438S665000
Reexamination Certificate
active
06531394
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a gate electrode of a semiconductor device which improves thermal stability of a tungsten/polysilicon structure.
2. Background of Related Art
Generally, in order to reduce gate resistance in a process for forming a gate electrode of a semiconductor device, tungsten(W) having specific resistance order lower than WSi
x
is deposited on a polysilicon so that a gate electrode is formed. When tungsten is reacted with silicon at a temperature of 600° C. or greater, a silicide is formed. Accordingly, WN
x
is formed as a diffusion barrier layer between tungsten and silicon to form a gate electrode having W/WN
x
/polysilicon structure.
A related art method for forming a gate electrode of a semiconductor device will be described with reference to the accompanying drawings.
FIGS. 1
a
to
1
d
are sectional views of process steps showing a related art method for forming a gate electrode of a semiconductor device.
As shown in
FIG. 1
a
, field oxide films
12
are formed in a semiconductor substrate
11
at predetermined intervals, and then the field oxide films
12
are divided into an isolation region and an active region.
A first insulating film
13
for a gate oxide film is formed on the active region at a thickness of about 40 Å by thermal oxidation method.
As shown in
FIG. 1
b
, a polysilicon layer
14
is formed on an entire surface of the semiconductor substrate
11
at a thickness of about 1000 Å by low pressure chemical vapor deposition (LPCVD). N+ ions or P+ ions are then implanted into the polysilicon layer
14
. When the N+ ions or P+ ions are implanted into the polysilicon layer
14
, the polysilicon layer
14
is masked by a photoresist according to devices to be formed, so that the ions are implanted into a specific desired portion of the polysilicon layer
14
.
Subsequently, the polysilicon layer
14
is annealed for ten minutes at a temperature of 800° C. so that the implanted impurity ions (N+ or P+) are activated.
As shown in
FIG. 1
c
, the semiconductor substrate
11
is subsequently washed by an HF solution and then a WN
x
layer
15
is formed at a thickness of about 50 Å. A tungsten layer
16
is formed on the WN
x
layer
15
at a thickness of about 400 Å and a second insulating film
17
is formed on the tungsten layer
16
at a thickness of about 2000 Å.
Here, the WN
x
15
is used as a diffusion barrier between the
153
tungsten layer
16
and the polysilicon layer
14
. WN
x
and TiN are generally used as the diffusion barrier layer. At present, WN
x
is more frequently used as the diffusion barrier layer. This is because the grain size of tungsten is remarkably reduced, thereby increasing the resistance of pure tungsten two times or more than a W/Si structure in a case where tungsten is deposited on TiN by a sputtering method. This is also because TiN is oxidized during selective oxidation of silicon.
As shown in
FIG. 1
d
, a photoresist (not shown) is deposited on the second insulating film
17
and then patterned by an exposure and developing processes to define a gate electrode region. The second insulating film
17
, the tungsten layer
16
, the WN
x
layer
15
, the polysilicon layer
14
and the first insulating film
13
are selectively removed using the patterned photoresist as a mask to form a gate electrode
18
.
Subsequently, the sides of the gate electrode
18
are selectively oxidized to form a third insulating film on the entire surface including the gate electrode
18
. The third insulating film is then etched back to form insulating film sidewalls
19
at both sides of the gate electrode
18
.
However, the related art method for fabricating a semiconductor device has several problems.
The diffusion barrier layer, WN
x
decomposes into W and N
2
at a temperature of 800° C. or greater. Thus, a silicide may be formed at the boundary between the WN
x
and polysilicon
14
. In this case, WN
x
fails to act as a diffusion barrier at a temperature of 800° C. or greater, thereby reducing thermal stability of the structure in a high temperature process.
Furthermore, if WN
x
contains more than 10% nitrogen, the WN
x
is decomposed into W and N
2
, thereby forming pores in a grain boundary. As a result, when etching a gate, polysilicon is locally over-etched. This may degrade characteristics of the device.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method for forming a gate electrode of a semiconductor device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for forming a gate electrode of a semiconductor device in which a tungsten/polysilicon structure having excellent thermal stability and no pores is obtained.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for forming a gate electrode of a semiconductor device according to the present invention includes: forming a first insulating film, a polysilicon layer and a tungsten layer on a semiconductor substrate; adding oxygen to the tungsten layer; forming a second insulating film on the tungsten layer to which oxygen is added; and selectively removing the second insulating film, the tungsten layer, the polysilicon layer and the first insulating film to form a gate electrode.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
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Everhart Caridad
Luu Chuong Anh
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