Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2002-01-03
2003-03-18
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S592000, C438S655000, C438S630000, C438S683000
Reexamination Certificate
active
06534400
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for depositing a tungsten suicide layer, and more particularly to a method for depositing a tungsten silicide layer on a polysilicon layer using dichlorosilane gas as a silane source gas so as to prevent void generation on an interfacial surface formed between the polysilicon layer and the tungsten silicide layer.
2. Description of the Related Art
Generally, in a semiconductor memory device such as a DRAM (dynamic random access memory), a multi-layer structure consisting of a polysilicon layer and a tungsten silicide layer is widely used as a conductive layer so as to improve the electrical conductivity of a word line by combining a high resistance characteristic of the polysilicon layer and a low resistance characteristic of the tungsten silicide layer.
The deposition of the tungsten silicide layer is generally carried out by a CVD (chemical vapor deposition) process, wherein hexafluoride (WF
6
) gas is deoxidized using monosilane (SiH4) gas, hydrogen (H2) gas or silicon (Si).
According to the conventional method, the density of fluorine atoms accumulated on the tungsten silicide layer exceeds 1.0×10
20
atoms/cc when the tungsten silicide layer is deposited on the polysilicon layer by deoxidizing the hexafluoride (WF
6
) gas using the monosilane (SiH
4
) gas.
The high-density fluorine atoms accumulated in the tungsten silicide layer causes the diffusion of boron (B) thereby lowering the device characteristics. Particularly, in the case of a gate electrode, the threshold voltage of a transistor is shifted and the thickness of a gate oxide layer increases.
In addition, step coverage and an adhesive feature of the deposited tungsten silicide layer are poor. In order to solve the above problems, a post annealing process is carried out. However, the tungsten silicide layer can be cracked and delaminated while the post annealing process is being carried out.
Recently, dichlorosilane (SiH
2
Cl
2
; DCS) gas is widely used as a deoxidizing gas for depositing the tungsten silicide layer so as to solve the problems caused by the monosilane gas.
If the WF
6
gas is deoxidized using the DSC gas, the density of the fluorine atoms accumulated in the tungsten silicide layer is reduced by 1.0×10
3
times as compared with the density when the monosilane (SiH
4
) gas is used as the deoxidizing gas. Further, the step coverage and the adhesive characteristic against the polysilicon layer can be improved.
However, though the characteristics of the tungsten silicide layer can be improved, if the tungsten silicide layer is used for the gate electrode having a multi-layer structure of polysilicon/tungsten silicide which requires a re-oxidation process after a patterning process is performed, voids are created on the polysilicon layer while the above processes are being carried out. The voids deteriorate the reliability of the device.
FIG. 1
is a view showing a structure of a gate electrode layer of DRAM device. In order to fabricate the gate electrode layer, a gate oxide film
12
, a polysilicon layer
14
, a tungsten silicide layer
16
, a nitride film
18
and an oxide film
20
are sequentially stacked on a substrate
10
and then the stacked layers are patterned. The polysilicon layer
14
and the tungsten silicide layer
16
are used as a gate conductive layer and the nitride film and the oxide film are used as a mask layer.
After forming the gate electrode layer, a re-oxidation process is carried out. At this time, chlorine atoms contained in the dichlorosilane gas are accumulated on the polysilicon layer
14
. When the re-oxidation process is carried out, silicon diffuses from the polysilicon layer
14
into the tungsten silicide layer
16
so that voids
22
are created. The chlorine accumulated on the polysilicon layer facilitates the creation of the voids
22
.
In order to reduce the creation of the voids
22
, as shown in
FIG. 1
, a thermal oxidation process was carried out with respect to the monocrystalline silicon wafer
10
, so that the oxide film
12
having a thickness of 100 Å was formed. Then, the doped polysilicon layer
14
having a thickness of 1,000 Å was formed by means of an LPCVD process. After that, the tungsten silicide layer
16
was formed on the polysilicon layer
14
by introducing a mixing gas of the WF
6
gas and the DCS gas. Then, the nitride film
18
and the oxide film having a thickness of 3,000 Å were formed so as to be used as the mask layer.
After that, the gate electrode layer was defined by coating a photoresist film on the resultant structure, exposing the photoresist film and then developing the exposed photoresist film. The gate electrode layer was patterned by etching a lower layers of the resultant structure through a dry etching process.
Then, as shown in
FIG. 2
, the oxidation process was carried out for 60 to 100 minutes in an O
2
atmosphere at a temperature of 850° C. As a result, a thermal oxide film
24
was formed at a sidewall of the gate electrode layer.
Then, as shown in
FIG. 3
, the nitride layer
18
and the oxide layer
20
as mask layers were etched using a wet etching process. After that, as shown in
FIG. 4
, the tungsten silicide layer
16
was selectively etched.
When the surface of the exposed polysilicon layer
14
was observed using analyzing apparatus such as SEM/TEM/AFM, the voids
22
were detected.
As mentioned above, when the tungsten silicide layer is deposited according to the conventional method, a great amount of dichlorosilane is introduced into the process chamber at an early stage so that chlorine atoms are accumulated on the polysilicon layer
14
. Therefore, when the re-oxidation process is carried out, silicon diffuses through the tungsten silicide layer so that voids
22
are created. The chlorine atoms accumulated on the polysilicon layer
14
facilitate the creation of the voids
22
.
The voids
22
created on the polysilicon layer
14
deteriorate the reliability of the device and cause the failure of the device thereby lowering the yield of the device.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems of the prior art. It is an object of the present invention to provide a method for depositing a tungsten suicide layer. The tungsten silicide layer is deposited by deoxidizing a WF
6
gas using a DSC gas on a polysilicon layer. The surface of the polysilicon layer is pre-treated using a hydrogen compound gas including any one selected from the group consisting of group III elements and group V elements of the periodic table of the elements before the tungsten silicide layer is deposited, thereby preventing voids from being created at the polysilicon layer.
In accordance with the invention, there is provided a method for depositing a tungsten silicide layer on a substrate coated with a polysilicon layer in a CVD process chamber. A surface of the polysilicon layer is pre-treated by introducing into the CVD process chamber a hydrogen compound gas including elements selected from group III elements or group V elements of the periodic table. Then, the tungsten silicide layer is deposited on the polysilicon layer by introducing a silane source gas and a tungsten source gas into the CVD process chamber.
For example, the hydrogen compound gas may be PH
3
(phosphine), B
2
H
6
(diborane), AsH
3
, or a mixture thereof. A hydrogen compound gas made of elements selected form group V elements is used for NMOS and a hydrogen compound gas made of elements selected form group III elements is used for PMOS.
As a silane source gas, a dichlorosilane gas (SiH
2
Cl
2
) may be used. As a tungsten source gas, a tungsten hexafluoride (WF
6
) gas may be used.
Since the hydrogen compound gas including elements selected from group III elements or group V elements of the periodic table is absorbed on the surface of the polysilicon layer and the DCS gas is reacted with the absorbed hydrogen compound at the surface of the polysilicon layer, fluorine atoms are not accumulated
Ahn Jae Young
Kang Man Sug
Kim Hee Seok
Lee Woo Sung
Jr. Carl Whitehead
Mills & Onello LLP
Pham Thanhha
Samsung Electronics Co,. Ltd.
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