Coating processes – Coating by vapor – gas – or smoke – Plural coatings applied by vapor – gas – or smoke
Reexamination Certificate
2000-09-13
2002-05-14
Zimmerman, John J. (Department: 1775)
Coating processes
Coating by vapor, gas, or smoke
Plural coatings applied by vapor, gas, or smoke
C438S656000, C438S685000
Reexamination Certificate
active
06387445
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a method for forming a tungsten layer, which is formed on the surface of an object to be processed, such as a semiconductor wafer, and a laminated structure of a tungsten layer.
BACKGROUND ART
In a typical process for producing a semiconductor integrated circuit, a metal or metal compound, such as W (tungsten), WSi (tungsten silicide), Ti (titanium), TiN (titanium nitride) or TiSi (titanium silicide), is deposited to form a thin film, in order to form a wiring pattern on a semiconductor wafer serving as an object to be processed or in order to fill in a recessed portion between wiring parts or the like.
As methods for forming a metal thin film of this type, there are known three methods, such as the H
2
(hydrogen) reduction process, the SiH
4
(silane) reduction process and the SiH
2
Cl
2
(dichlorosilane) reduction process. The SiH
2
Cl
2
reduction process is a method for forming a W or WSi (tungsten silicide) film at a high temperature of about 600° C. using, e.g., dichlorosilane, as a reducing gas in order to form a wiring pattern. The SiH
4
reduction process is a method for forming a W or WSi film at a low temperature of about 450° C., which is lower than that in the SiH
2
Cl
2
reduction process, using, e.g., silane, as a reducing gas in order to similarly form a wiring pattern.
The H
2
reduction process is a method for depositing a W film at a temperature of about 400° C. to about 450° C. using, e.g., hydrogen, as a reducing gas in order to fill a recessed portion in the surface of a wafer, such as a recessed portion between wiring parts.
In all of the above described cases, a gas, such as WF
6
(tungsten hexafluoride), is used as a material gas. An example of the formation of a tungsten layer of via-fill type and/or blanket type will be described. If a film is first intended to be formed by the CVD, the adhesion of the film is bad, so that an incubation time, in which the adhesion of the film does not occur, tends to increase.
Therefore, in order to prevent this, WF
6
gas serving as a material gas, and a reduction gas, such as silane or hydrogen gas, are first caused to flow little by little to allow a nuclear crystalline film of tungsten serving as a seed crystal to grow on the surface of a wafer. After this nuclear crystalline film growth step is carried out for a predetermined period of time, e.g., tens seconds, the material gas and the reducing gas are caused to flow in large quantities to allow a main tungsten film mainly including the nuclear crystalline film to grow at a high deposition rate. Thus, a tungsten layer having a desired thickness as a whole is obtained.
FIG. 10
schematically shows the sectional structure of the tungsten layer at this time. In
FIG. 10
, a nuclear crystalline film
2
of tungsten and a main tungsten film
4
are sequentially stacked on the surface of a semiconductor wafer
1
.
FIG. 11
is a graph showing the relationship between time and thickness of the film at the deposition step. As shown in
FIG. 11
, although a nuclear crystalline film forming step is carried out after a pretreatment, an incubation time T
1
, in which the film does not deposit for some time, exists after the nuclear crystalline film forming step starts. In addition, an incubation time T
2
, in which the film does not deposit for some time, also exists when a main tungsten film forming step starts after the nuclear crystalline film forming step. Then, (after a lapse of the incubation time T
2
) at the main tungsten film forming step, the adhesion of the film is carried out in large quantities.
By the way, the control of the thickness of a film to be deposited is generally carried out by controlling the gas flow rate and time. However, the incubation times T
1
and T
2
are considerably influenced by the state of the surface of the semiconductor wafer, e.g., the quality of an underlying film, such as a TiN film, when a tungsten film is deposited, so that it is difficult to artificially control the incubation times T
1
and T
2
.
Therefore, even if the target thickness of the nuclear crystalline film
2
is set to be, e.g., 500 angstroms, one incubation time T
1
varies, so that the thickness is, in fact, 600 angstroms or 400 angstroms as shown by the chain line in FIG.
11
. In addition, since the other incubation time T
2
also varies (not shown in FIG.
10
), the substantial deposition time for the main tungsten film also varies.
Thus, even if the thickness of the nuclear crystalline film
2
and the substantial deposition time for the main tungsten film vary, the finally required thickness of the tungsten layer is, e.g., about 8000 angstroms, which is far greater than the above described variation in thickness of the nuclear crystalline film
2
, e.g., 100 angstroms. In addition, since the variation in the incubation time T
2
is far shorter than T
1
, the influence of the tungsten layer on the whole film thickness is very small. Therefore, since the difference in thickness between wafers is not so great, there is no problem due to the above described variation in substantial deposition time.
However, the multilayering of semiconductor integrated circuits is recently advanced, so that the thickness of various laminated films is decreased. For example, in the case of the above described tungsten layer, although the conventional target thickness is about 8000 angstroms, the recent target thickness is about 1000 angstroms, which is ⅛ as large as the conventional target thickness, by the recent request of the decrease of the thickness.
In such a situation, if the same deposition method as conventional methods is adopted, there are the following problems. That is, as shown in
FIG. 12
, when a tungsten layer having a target thickness of 1000 angstroms is formed, if the variations in incubation times T
1
and T
2
cause the variation in thickness of the nuclear crystalline film in the range of, e.g., from about 400 to about 600 angstroms, as described above. Then, the thickness of the main tungsten film greatly reflects directly the variation of thickness of the nuclear crystalline film.
Taking the variation in incubation time T
2
into consideration, the target thickness (1000 angstroms) of the tungsten layer greatly varies by, e.g., ±10% (100 angstroms) or more. Therefore, the thickness is greatly different between wafers, so that there is a problem in that it is difficult to uniform the thickness of the film. In addition, by the existence of the incubation times T
1
and T
2
, in which no film deposits, it is difficult to shorten the time required to form the tungsten layer.
In this case, it is considered to shorten the time required to carry out the whole nuclear crystalline film deposition step. However, if the time required to carry out this step is excessively shortened, the growth of the nuclear crystalline film is insufficient, so that there is the possibility that the film can not be sufficiently grow at a main tungsten film deposition step which is an after process.
DISCLOSURE OF THE INVENTION
The present invention has been made by effectively solving the above described problems. It is an object of the present invention to provide a method for forming a tungsten layer and a laminated structure of a tungsten layer, which can remove any incubation times after the deposition of a nuclear crystalline film to enhance the whole deposition rate and to enhance the uniformity in thickness of films between objects to be processed.
After the inventor has diligently studied the deposition of tungsten films, the inventor has obtained knowledge that an incubation time, which has been taken immediately after a nuclear crystalline film forming step, can be removed by carrying out an intermediate tungsten film forming step, in which the concentration of fluorine (F) in a process gas is smaller than that at a main tungsten film forming step, immediately after the nuclear crystalline film forming step, so that the deposition rate can be greatly improved as a whole.
Therefore, according to one aspe
Aiba Yasushi
Koike Yukio
Smith , Gambrell & Russell, LLP
Zimmerman John J.
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