Method for fabricating semiconductor device

Method for fabricating a semiconductor device, is disclosed, in which a grain size is made coarse for forming a thin film with a low resistance, including the steps of (1) depositing an insulating film on a substrate, (2) depositing a silicon layer on the insulating film, (3) depositing an amorphous metal nitride film on the silicon layer, and (4) heat treating the amorphous metal nitride film to alter into a crystalline pure metal film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor device, and more 
particularly, to a method for fabricating a semiconductor device, in which 
a grain size is made coarse for forming a thin film with a low resistance. 
2. Discussion of the Related Art 
As semiconductor device packing advances, widths of lines therein are in 
general reduced, with increased resistance that causes problems, such as 
slowing down a device operation speed. To solve the problem of an 
increased sheet resistivity, a thickness of the line may be increased 
while reducing a width of the line, which again causes problems of 
complicated fabrication process and low yield due to difficulty in gap 
filling in deposition of an interlayer insulating film between the lines 
having a greater aspect ratio, with a high possibility of void formation. 
In order to solve these problems, a refractory metal silicide, such as 
tungsten silicide Wsix, titanium silicide TiSix, or cobalt silicide CoSix 
or the like is formed on a polysilicon layer in a background art, for 
preventing increased resistivity. Even though the formation of the 
refractory metal silicide may improve a resistivity and a step coverage to 
some extent, an improved method for forming a polycide has been in need. 
A background art method for fabricating a semiconductor device will be 
explained with reference to the attached drawings. FIGS. 1a.about.1c 
illustrate sections showing the steps of a background art method for 
fabricating a semiconductor device. 
Referring to FIG. 1a, the background art method for fabricating a 
semiconductor device starts with forming a silicon oxide film SiO.sub.2 12 
on a semiconductor substrate 11, for use as a gate insulating film. Then, 
a polysilicon layer 13, a tungsten nitride film 14, and a pure tungsten 
film 15 are formed on the silicon nitride film 12 in succession, for use 
as a gate electrode. The tungsten nitride film 14 is formed very thin, and 
the tungsten film 15 is reactive sputtered. The tungsten nitride film 14 
is formed for preventing reaction between the tungsten film 15 and the 
polysilicon layer 13, that forms tungsten silicide at an interface of the 
tungsten film 15 and the polysilicon layer 13. The tungsten silicide at 
the interface of the tungsten film 15 and the polysilicon layer 13 
increases a sheet resistivity. Then, a photoresist film 16 is coated on 
the tungsten film 15, subjected to exposure and development, to pattern 
the photoresist film 16 to define a gate region. As shown in FIG. 1b, the 
patterned photoresist film 16 is used as a mask in selectively removing 
the tungsten film 15, the tungsten nitride film 14, the polysilicon layer 
13, and the silicon oxide film 12, to form a gate electrode 18 and a gate 
insulating film 12a. As shown in FIG. 1c the photoresist film 16 is 
removed, the gate electrode 18 is used as a mask in lightly doping the 
semiconductor substrate 11, and insulating sidewalls 19 are formed at both 
sides of the gate electrode 18. The gate electrode 18 and the insulating 
sidewalls 19 are used as a mask in doping the semiconductor substrate 11 
heavily, to form source/drain impurity regions 17 in surfaces of the 
semiconductor substrate 11 on both sides of the gate electrode 18. 
However, the background art method for fabricating a semiconductor device 
has the following problems. 
First, the additional tungsten thin film formation process makes the 
fabrication process complicated, with a reduction of productivity. 
Second, formation of an even and thin, to a few tens of .ANG., tungsten 
nitride film is difficult. 
Third, the low temperature approx. 300.degree. C. in formation of the 
tungsten thin film hampers formation of the tungsten thin film with coarse 
grains because diffusion of tungsten atoms should be accompanied for 
growth of crystal grains in the tungsten thin film. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to a method for fabricating 
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 fabricating a 
semiconductor device, in which crystal grain is made coarse for forming a 
thin film with a low resistance. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure particularly pointed out in the written 
description and claims hereof as well as the appended drawings. 
To achieve these and other advantages and in accordance with the purpose of 
the present invention, as embodied and broadly described, the method for 
fabricating a semiconductor device includes the steps of (1) depositing an 
insulating film on a substrate, (2) depositing a silicon layer on the 
insulating film, (3) depositing an amorphous metal nitride film on the 
silicon layer, and (4) heat treating the amorphous metal nitride film to 
alter into a crystalline pure metal film. 
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. FIGS. 2a.about.2d illustrate sections showing the steps of a 
method for fabricating a semiconductor device in accordance with a first 
preferred embodiment of the present invention. 
Referring to FIG. 2a, the steps of the method for fabricating a 
semiconductor device in accordance with a first preferred embodiment of 
the present invention starts with deposition of a silicon oxide SiO.sub.2 
film 22 on a semiconductor substrate 21 to a thickness of approx. 65 .ANG. 
by chemical vapor deposition or thermal oxidation, for use as a gate 
insulating film. A polysilicon layer 23 is chemical vapor deposited on the 
silicon oxide film 22 to a thickness of approx. 800.about.1200 
.ANG.(preferably 1000 .ANG.). Then, an amorphous metal nitride film 24(for 
example, a tungsten nitride film or a molybdenum nitride film) is 
deposited on the polysilicon layer 23 to a thickness of approx. 
600.about.1500 .ANG.(preferably 1000 .ANG.). The tungsten nitride film or 
the molybdenum nitride film is formed such that a nitrogen content therein 
is below 10%.about.70% of an amorphous metal and the film is amorphous. 
Though the tungsten nitride film or the molybdenum nitride film can be the 
purer as the nitrogen content is the less, the possibility of a suicide 
formation between the polysilicon and the metal nitride becomes the higher 
as the nitrogen content is the less. As shown in FIG. 2b, the 
semiconductor substrate 21 having the tungsten amorphous metal nitride 
film 24 formed thereon is heat treated at a temperature ranging 
900.degree. C..about.1410.degree. C., or at 400.degree. 
C..about.600.degree. C. for the first time and at 900.degree. 
C..about.1410.degree. C. for the second time. The heat treatment may be 
conducted in a gas ambient containing hydrogen H.sub.2, nitrogen N.sub.2, 
and argon Ar, or any one of the gases, but under a vacuum to an extent the 
amorphous metal nitride film 24 is not oxidized. That is, the pure 
tungsten(or molybdenum) or amorphous metal nitride film is susceptible to 
oxidation even with a very small amount of oxygen present in a chamber, an 
adequate vacuum is required. The reason that the first heat treatment is 
conducted at 400.about.600.degree. C. when the amorphous metal nitride 
film 24 is a tungsten nitride film is for diffusing nitrogen in the 
tungsten nitride film to outside of the film as far as the tungsten 
nitride film is maintained to be amorphous, and the reason that the second 
heat treatment is conducted at 900.degree. C..about.1410.degree. C. is 
that the tungsten nitride film has either a co-existence state of a 
quasistable tungsten nitride(W.sub.2 N) 24a state and a crystalline pure 
tungsten film(.alpha.-W) state or a state in which all the tungsten 
nitride film 24 is altered into a crystalline pure tungsten film 24b, for 
diffusing an excessive nitrogen to outside of the film with easy. That is, 
the amorphous metal nitride film 24 is decomposed into tungsten(or 
molybdenum) and nitrogen by the heat treatment, and altered into a 
crystalline pure tungsten film(or pure molybdenum film) 24b starting from 
a surface as a heat treatment time period is elapsed. In this instance, 
portions of the tungsten nitride film(or molybdenum nitride film) 24a 
which are not decomposed yet act as diffusion barriers between the 
polysilicon layer 23 and the crystalline tungsten film(or molybdenum film) 
24a. And, even though all the tungsten nitride film(or molybdenum nitride 
film) is altered into pure tungsten film(or molybdenum film) 24b, the 
excessive nitrogen from the tungsten nitride film(or molybdenum nitride 
film) is segregated between the polysilicon layer 23 and the crystalline 
pure tungsten film(or molybdenum film), and acts as diffusion barriers. 
And, a grain size of the tungsten(or molybdenum) becomes greater while the 
pure tungsten film(or molybdenum film) is formed by the heat treatment. As 
shown in FIG. 2c, the crystalline pure tungsten film(or molybdenum film), 
the tungsten nitride film(or molybdenum nitride film) 24a, the polysilicon 
layer 23, and the first oxide film 22 are subjected to anisotropic 
etching, to form a gate electrode 28 and a gate insulting film 22a. As 
shown in FIG. 2d, the gate electrode 28 is used as a mask in lightly 
doping surfaces of the semiconductor substrate 21, and insulating film 
sidewalls 29 are formed at both sides of the gate electrode 28 and the 
gate insulating film 22a. Then, the gate electrode 28 and the insulating 
film sidewalls 29 are used as a mask in heavily doping the semiconductor 
substrate 21, to form source/drain impurity regions 27. 
FIGS. 3a.about.3e illustrate sections showing the steps of a method for 
fabricating a semiconductor device in accordance with a second preferred 
embodiment of the present invention. 
Referring to FIG. 3a, the steps of a method for fabricating a semiconductor 
device in accordance with a second preferred embodiment of the present 
invention starts with forming a silicon oxide film 22 on a semiconductor 
substrate 21, for use as a gate insulting film. And, a polysilicon layer 
23, a first tungsten nitride film Wnx 25a, and a second tungsten nitride 
film 25b are deposited in succession on the silicon oxide film 22, for use 
as a gate electrode. In this instance, the first tungsten nitride film 25a 
is formed to have an amorphous lattice with approx. 20.about.50% of a 
nitrogen atom concentration, and the second tungsten nitride film 25b is 
to have an amorphous lattice with a nitrogen atom concentration below 20%. 
The second tungsten nitride film 25b is almost crystalline with the 
nitrogen concentration ranges 10%.about.20%, and .beta.-tungsten 
crystalline when the nitrogen concentration is below 10%. Next, as shown 
in FIG. 3b, an entire surface of the second tungsten nitride film 25b is 
subjected to ion implantation of at least one of phosphorus(P), boron (B), 
and arsenic(As) ions, to break the lattice of the second tungsten nitride 
film 25b. That is, upon injection of impurity ions into the second 
tungsten nitride film 25b, the lattice of the second tungsten nitride film 
25b is broken, allowing to alter the second tungsten nitride film 25b into 
a pure tungsten film. In the meantime, an energy of the ion implantation 
may be adjusted to inject impurity ions, not only to the second tungsten 
nitride film 25b, but also to the first tungsten nitride film 25a. Then, 
as shown in FIG. 3c, the amorphous second tungsten nitride film 25b is 
heat treated at 600.about.1410.degree. C., to alter into a crystalline 
pure tungsten film 24b. In this instance, as the second tungsten nitride 
film is mostly composed of pure .alpha.-tungsten and excessive atoms, 
nitrogen therein is diffused to outside of the film. The .alpha.-tungsten 
crystalline lattice has a body-centered cubic lattice, a simple cubic 
lattice added with an atom at a center thereof. A grain size of the pure 
tungsten film becomes greater due to a latent heat discharged at 
alteration of the tungsten nitride film into the pure tungsten film 24b. 
Because the atoms ion injected into the second tungsten nitride film 25b 
can not make solid solution with the .alpha.-tungsten of the crystalline 
pure tungsten film 24b, the atoms are segregated at grain boundaries, 
filling vacancies and voids in the grain boundaries and increasing a 
density of the grain boundaries. In this instance, since the second 
tungsten nitride film 25b is altered into the pure tungsten film and the 
first tungsten nitride film 25b is altered into the pure tungsten film 24b 
as the heat treatment time period elapsed, the resistance of the second 
tungsten nitride film 25b is sharply dropped because the grain boundary 
density in the second tungsten nitride film is increased. As shown in FIG. 
3d, a photoresist film 26 is coated on the crystalline second tungsten 
nitride film 25b, and subjected to patterning by exposure and development, 
to define a gate electrode region. Then, the patterned photoresist film 27 
is used as a mask in removing the crystalline pure tungsten film 24a, the 
first tungsten nitride film 25a, the polysilicon layer 23, and the silicon 
oxide film 22 selectively, to form a gate electrode 28, and a gate 
insulating film 22a. As shown in FIG. 3e, the photoresist film 26 is 
removed, and the gate electrode 28 is used as a mask in lightly doping the 
semiconductor substrate 21. Insulating film sidewalls 29 are formed at 
sides of the gate electrode 28 and the gate insulating film 22a, and the 
gate electrode 28 and the insulating film sidewalls 29 are used as a mask 
in heavily doping the semiconductor substrate 21, to form source/drain 
impurity regions 27 in the semiconductor substrate 21 on both sides of the 
gate electrode 28. Also, in this second embodiment, a molybdenum nitride 
film may be formed instead of the tungsten nitride film, and heat treated, 
to form a pure molybdenum film. 
In the aforementioned second embodiment method for fabricating a 
semiconductor device of the present invention, by forming a metal nitride 
film with a higher nitrogen content on a polysilicon layer and a metal 
nitride film with a lower nitrogen content thereon, and subjecting to heat 
treatment, heat treatment at a low temperature is made possible, with a 
reduction of the heat treatment time period. 
As has been explained, the method for fabricating a semiconductor device of 
the present invention has the following advantages. 
First, the large grain size obtained by altering an amorphous tungsten 
nitride film(or molybdenum nitride film) to a pure crystalline tungsten 
film(or molybdenum film) can lower a resistance of a thin film. 
Second, the increased concentration of atoms at grain boundaries coming 
from segregation of phosphorus atoms thereto can lower resistance of a 
thin film. 
Third, as a diffusion barrier and a crystalline pure tungsten film(or 
molybdenum film) can be formed on the same time by heat treating a 
tungsten nitride film(or molybdenum nitride film) without deposition of a 
pure metal film(of tungsten or molybdenum), fabrication process can be 
simplified and a process cost can be reduced. 
Fourth, a heat treatment can be conducted within a shorter time period and 
at a lower temperature if two layers of tungsten nitride film(or 
molybdenum nitride film) of different nitrogen concentrations are formed 
in obtaining a pure tungsten film(or molybdenum film) by the heat 
treatment. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the method for fabricating a semiconductor 
device of the present invention without departing from the spirit or scope 
of the invention. Thus, it is intended that the present invention cover 
the modifications and variations of this invention provided they come 
within the scope of the appended claims and their equivalents.