Method of forming electrode of semiconductor device

After a silicon layer is selectively grown on that part of a silicon substrate surface on which an electrode is to be formed, the silicon layer is reacted with a refractory metal so as to form the electrode made of a metal silicide layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of forming the electrode of a 
semiconductor device, and more particularly to a method which can form a 
good electrode on a silicon wafer. 
2. Description of the Prior Art 
As is well known, the electrodes of various semiconductor devices are 
formed principally by employing an Al alloy as an electrode material or by 
producing metal silicide directly on a silicon substrate. 
The method employing the Al alloy as the electrode material has the merit 
that the process is simple and that the manufacture is easy. Since, 
however, the Al alloy is readily alloyed with Si at a low temperature of 
about 500.degree. C., Al penetrates into the Si substrate on account of 
the solid-phase diffusing reaction between Si and Al. In case a diffusion 
layer in the Si substrate is thin, the Al having penetrated pierces the 
diffusion layer easily, resulting in the disadvantages that the junction 
shorting takes place and that the heat resistance is conspicuously low. 
The method which forms the electrode by forming metal silicide directly on 
the Si substrate, is more excellent in the heat resistance than the method 
which employs the Al alloy. 
In forming the electrode, however, the metal silicide is formed by the 
solid phase reaction between a metal and the Si substrate, so that when it 
is intended to form a thick metal silicide layer, the metal silicide in a 
lower part thereof enters the Si substrate into the state in which the 
interface between the metal silicide and the corresponding part of the Si 
substrate lies within the original Si substrate. 
Within the Si substrate, regions opposite in the conductivity type to the 
Si substrate are usually formed. Therefore, when the interface is formed 
within the Si substrate, a junction is feared to be ruined by the metal 
silicide, so that the thickness of the metal silicide cannot be made very 
great. 
In order to solve this problem, there has been proposed a method in which a 
polycrystalline Si film heavily doped with an impurity is formed on the Si 
substrate in advance, and a metal film is deposited thereon to react it 
with the polycrystalline Si film, thereby to form the metal silicide. 
Owing to the presence of the polycrystalline Si film, this method can 
prevent the junction within the substrate from being ruined. Since, 
however, a polycrystalline Si film needs to be formed in parts to become 
contact holes by the use of the photoetching process, the method is 
unsuitable for the formation of a semiconductor device of high packaging 
density having a very fine structure. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve the problems involved in the 
prior-art methods of forming electrodes, and to provide a method of 
forming the electrode of a semiconductor device which ensures a sufficient 
heat resistance of the electrode, which is not feared to ruin a junction 
within a substrate and which is also applicable to the manufacture of an 
integrated circuit of high packaging density. 
In order to accomplish the object, according to the present invention, a Si 
layer is formed on that selected part of the surface of a Si substrate on 
which an electrode is to be formed, and a metal film is deposited onto the 
Si layer so as to form metal silicide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1a, an impurity-doped layer 2 is formed in the surface 
region of a single-crystal Si substrate 1 by a well-known expedient such 
as thermal diffusion and ion implantation. 
Using a well-known process such as thermal oxidation and chemical vapor 
deposition (CVD), a SiO.sub.2 film 3 is deposited on the whole surface of 
the Si substrate. Thereafter, the part of the SiO.sub.2 film overlying the 
impurity-doped layer 2 is selectively removed by the well-known 
photoetching so as to form a contact hole 4. 
In an example, the deposition of the SiO.sub.2 film 3 and the formation of 
the contact hole 4 were carried out after the impurity-doped layer 2 had 
been formed. Needless to say, however, the impurity-doped layer 2 may well 
be formed after the deposition of the SiO.sub.2 film 3 and the formation 
of the contact hole 4 have been performed. 
As shown in FIG. 1b, using the well-known vapor epitaxial growth which 
employs SiCl.sub.4 and H.sub.2, a Si epitaxial layer 5 is selectively 
grown on the exposed surface of the Si substrate 1. Thereafter, a 
refractory metal film 6 of Mo, Ta, Ti, W or the like is deposited on the 
whole surface of the resultant substrate. Two or more of such metals may 
well be jointly used, or an alloy consisting of such metals may well be 
used. 
Subsequently, a heat treatment is conducted at about 500.degree. C. or 
above in a non-oxidizing atmosphere. Then, a solid phase reaction takes 
place between the refractory metal layer 6 and the Si epitaxial layer 5, 
and a metal silicide layer 8 is formed as shown in FIG. 1c. 
The temperature of the heat treatment for forming the metal silicide layer 
8 should preferably be about 500.degree. C.-1000.degree. C. At a 
temperature below about 500.degree. C., the metal silicide is difficult to 
be formed, and at a temperature above about 1000.degree. C., the 
impurity-doped layer 2 spreads due to thermal diffusion and a pn-junction 
7 deepens. The growth rate of the metal silicide depends greatly upon the 
atmosphere of the heat treatment. For example, the growth rate of the 
metal silicide is markedly higher with the heat treatment in a hydrogen 
atmosphere than with the heat treatment in a nitrogen atmosphere. 
The thickness of the metal silicide layer 8 can be controlled to a desired 
value by changing the conditions of the heat treatment. In the example in 
which the heat treatment was conducted at 700.degree. C., in the hydrogen 
atmosphere for 1 hour, a tungsten silicide layer 8 was about 500 nm thick. 
Even when the entire Si epitaxial layer 5 reacts with the refractory metal 
film 6, it is only required that the produced metal silicide film 8 does 
not pierce the impurity-doped layer 2 to destroy the junction 7. 
Therefore, when the thickness of the Si epitaxial layer 5 is made great to 
some extent beforehand (roughly, greater than the thickness of the metal 
silicide layer to be formed), the formation of the metal silicide layer 8 
can be performed with a sufficient allowance. 
More specifically, when the Si epitaxial layer 5 is formed to be somewhat 
thick in advance, this layer 5 is not fully reacted in the formation of 
the metal silicide but is left unreacted to some degree as illustrated in 
FIG. 1c. Therefore, the pn-junction 7 is not feared to be spoilt by the 
metal silicide layer 8, and a contact having a high reliability can be 
formed very easily. 
In the embodiment, the Si epitaxial layer 5 is formed on the impurity-doped 
layer 2. Since, however, the present invention requires only the formation 
of the metal silicide through the reaction with the refractory metal, it 
is possible to employ, not only the Si epitaxial layer, but also a 
polycrystalline silicone layer or an amorphous silicon layer. In this 
regard, however, it is difficult to form the amorphous silicon layer on 
the selected part of the impurity-doped layer 2 without using a mask or 
the like. In contrast, the Si epitaxial or the polycrystalline silicon 
layer can be selectively grown on the impurity-doped layer 2 and is 
therefore favorable in practical use. 
After the metal silicide layer 8 has been formed, the refractory metal film 
6 is etched and removed. In this case, only the unreacted refractory metal 
film 6 can be selectively etched and removed by employing an etchant with 
which the etching rate of the refractory metal is sufficiently greater 
than that of the metal silicide. 
By way of example, in a case where Mo is employed as the refractory metal 
and where Mo silicide is formed, only the Mo film is selectively removed 
with the Mo silicide layer left behind by employing a phosphoric 
acid-based solution as the etchant. In a case where W is employed as the 
refractory metal to form W silicide, only the W film is selectively 
removed with the W silicide layer left behind by employing a hydrogen 
peroxide liquid as the etchant. 
Next, a wiring metal film of, e.g., Al is deposited on the whole surface of 
the resultant substrate and thereafter has its unnecessary parts removed 
by the well-known photoetching. Then, a wiring layer 9 which is 
electrically connected with the metal silicide layer 8 is formed as shown 
in FIG. 1d. 
The metal silicide layer formed in the present invention can be used as the 
contact electrode with the impurity-doped layer within the Si substrate, 
the electrode of a Schottky diode, etc. In the case of using the metal 
silicide layer as the electrode of the Schottky diode, it is desirable to 
react the entire Si epitaxial layer with the metal silicide and to hold 
the metal silicide layer in direct contact and the silicon semiconductor 
substrate. 
In the above embodiment, the metal silicide layer is formed, the refractory 
metal layer is thereafter removed, and the wiring material layer made of 
Al or the like is deposited so as to form the wiring. 
It is a matter of course, however, that the present invention is not 
restricted to such steps. By processing the unreacted refractory metal 
layer into a desired shape by the photoetching, it can be used as the 
wiring of any of various semiconductor devices, the gate electrode of a 
MOS field effect transistor, etc. In the cases of employing the refractory 
metal as the wiring of the various semiconductor devices, the gate 
electrode of the MOS field effect transistor, etc., a metal silicide layer 
can also be formed on a contact hole by a heat treatment after the 
refractory metal has been processed into the desired shape by the 
photoetching. 
The metal silicide layer formed in the present invention can endure a heat 
treatment at temperatures up to about 1,200.degree. C. 
Accordingly, when the temperatures of various heat treatments which are 
conducted after the formation of the electrode are below about 
1,100.degree. C., the electrode is not feared to ruin, and almost all the 
ordinary silicon processes can be performed without hindrance. 
Moreover, when the wiring is formed of the unreacted part of the refractory 
metal used for forming the metal silicide, the heat resistance is very 
excellent, and the specific resistance is as low as at most 
2.times.10.sup.-5 .OMEGA..cm. Therefore, the wiring is very suitable for 
various semiconductor integrated circuits. 
In addition, according to the present invention, Si is selectively grown on 
the contact hole and is reacted with the refractory metal. Therefore, the 
metal silicide layer is formed in a self-alignment fashion, and the method 
can be very easily applied to the formation of a large-scale integrated 
circuit having a very fine structure. The invention has such numerous 
merits.