Method for forming silicon nitride film

Silicon nitride films are formed by controlling the internal stress more precisely than conventional methods without varying its optical properties, mechanical strength, composition and density. The film is formed by sputtering, using an inert gas or a mixture of an inert gas and nitrogen, onto a substrate while keeping the substrate temperature within a given temperature range according to the pressure of the sputtering gas or gas mixture, the two being interrelated, thus carefully and precisely controlling the internal stress of the film formed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for forming a silicon nitride 
film used as, for example, a protective film for semiconductor chips or 
memory disks and an X-ray transmission film. More particularly, the 
present invention relates to a method for forming a silicon nitride films 
with a highly controlled internal stress. 
2. Description of the Prior Art 
Conventionally, the CVD method and the sputtering method have often been 
used for formation of a silicon nitride film. The silicon nitride film 
formed by the CVD method, however, has had the following drawbacks. 
Firstly, there are used, as the raw material gases, a silicon compound 
such as a silicon hydride (e.g. silane SiH.sub.4), a silicon fluoride 
(e.g. SiF.sub.4) or a silicon chloride (e.g. SiCl.sub.4), ammonia 
(NH.sub.3) and nitrogen (N.sub.2); that is, the easily decomposable raw 
material gases contain not only silicon and nitrogen which are constituent 
elements of silicon nitride (Si.sub.x N.sub.y) but also other elements; 
consequently, the silicon nitride films formed by the CVD method 
inevitably contains impurities in principle. The silicon nitride films 
containing impurities besides silicon and nitrogen largely vary in 
internal stress depending upon the content of the impurities. In order to 
precisely control the impurities content in the silicon nitride films, it 
is necessary to always make the film formation (deposition) conditions 
constant; that is, in the thermal CVD method, for example, it is required 
to always make constant the deposition temperature, gas composition, gas 
flow rate and gas pressure. In the plasma CVD method, not only the 
deposition temperature, gas composition, gas flow rate and gas pressure 
but also the plasma state must be made constant. To always keep these 
parameters constant is extremely difficult and the impurities content in 
the silicon nitride cannot be kept constant. Thus, the precise control of 
internal stress of silicon nitride films has been impossible in the CVD 
method. Secondly, the impurities in the silicon nitride films 
significantly reduce the chemical stability of the films. For example, in 
the substrate dissolution step in the production of an X-ray lithography 
mask using a silicon nitride film as an X-ray transmission film, the 
partial dissolution of the silicon nitride film takes place, thereby 
allowing the film to have flaws. Further, the impurities in silicon 
nitride films are easily eliminated by the application of an ionizing 
radiation, thereby causing a change in the composition, optical 
transparency and physical properties of the film. 
Meanwhile, the conventional sputtering method for forming silicon nitride 
films has had the following problems. Firstly, in the conventional 
sputtering method, the internal stress of the silicon nitride films is 
controlled by the pressure of the sputtering gas used. In this case, the 
precise control of the internal stress is impossible because the internal 
stress is greatly changed even by the slight change of the gas pressure, 
and the control of the internal stress has been possible only in the order 
of, for example, about 10x10.sup.8 dyn/cm.sup.2. Secondly, the gas 
pressure must be fairly large (at least 10 Pa) in order for the internal 
stress to be a tensile stress; use of a large gas pressure incurs trapping 
of impurities (e.g. hydrogen, oxygen) in silicon nitride; these impurities 
significantly reduce the chemical stability of the silicon nitride film 
obtained. For example, in the substrate dissolution step in the production 
of an X-ray lithography mask using a silicon nitride film as an X-ray 
transmission film, partial dissolution of the silicon nitride film takes 
place, thereby allowing the film to have flaws. Further, the impurities in 
silicon nitride films are easily eliminated by the application of an 
ionizing radiation, thereby causing a change in the composition, optical 
transparency and physical properties of the films. 
As stated above, in conventional methods for forming a silicon nitride 
film, it has been very difficult to control the internal stress of the 
film. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method for forming a 
silicon nitride film with a highly controlled internal stress. 
Other objects will be apparent from the following description and drawings. 
The present invention resides in a method for forming a silicon nitride 
film which comprises depositing a silicon nitride film on a substrate by a 
sputtering method using, as a sputtering gas, an inert gas or a mixed gas 
of an inert gas and nitrogen gas, said method further comprising, during 
the deposition of said silicon nitride film, keeping the substrate 
temperature at a given temperature range appropriate for the pressure of 
the sputtering gas to control the internal stress of the film formed.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention utilizes the fact that in depositing a silicon 
nitride film on a substrate by a sputtering method using, as a sputtering 
gas, an inert gas or a mixed gas of an inert gas and nitrogen gas, the 
internal stress of the film formed can be controlled by keeping the 
substrate temperature at a given temperature range appropriate for the 
pressure of the sputtering gas used. 
In the present method, the substrate temperature can be controlled 
precisely and the change of the internal stress of the silicon nitride 
film as a function of the change of the substrate temperature is small; 
therefore, the internal stress of the film can be controlled precisely. 
Further, the change of the substrate temperature causes no change in 
properties of the silicon nitride film other than internal stress, such as 
refractive index, composition, visible light transmission and the like. It 
is preferred, however, that film deposition be effected at a low 
sputtering gas pressure in order to prevent the trapping of impurities in 
the film and at a high substrate temperature in order to obtain a film of 
high chemical stability. 
The present invention is illustrated in more detail by way of Example. 
However, the present invention is in no way restricted to the Example. 
EXAMPLE 
A silicon nitride film of 2 .mu.m in thickness was deposited on a silicon 
substrate by a rf magnetron sputtering method. The sputtering target was a 
single crystal silicon target. The sputtering gas was a mixed gas of Ar 
gas (an inert gas) and N.sub.2 gas. The flow rate of this mixed gas was 
constant (Ar gas flow rate=12.0 sccm (cc at standard condition/min), 
N.sub.2 gas flow rate=4.0 sccm, ratio of N.sub.2 gas to total gas flow 
rate (N.sub.2 /(Ar+N.sub.2))=0.25). The internal stress of silicon nitride 
film was controlled by the control of the substrate temperature. In this 
Example, the control of the substrate temperature was made by fixing an 
electric heating wire to a plate for holding the substrate so as to ensure 
uniform heating of the substrate and supplying an electric current to the 
heating wire. The rf power was constant at 16.72 W/cm.sup.2 but there were 
employed various sputtering gas pressures, i.e. 0.4 Pa, 0.5 Pa, 0.6 Pa, 
0.65 Pa, 0.7 Pa, 0.8 Pa and 1.0 Pa. 
The change of internal stress of silicon nitride film by the change of 
substrate temperature is shown in FIG. 1. Incidentally, the internal 
stress was measured by the Newton rings method. 
In the case of sputtering gas pressure=0.4 Pa, the internal stress was a 
compressive stress at a substrate temperature of 100.degree. C.; and the 
compressive stress increased gradually as the substrate temperature was 
increased. In the case of sputtering gas temperature =0.5 Pa, the internal 
stress was a tensile stress at a substrate temperature of 100.degree. C.; 
the tensile stress decreased gradually as the substrate temperature was 
increased: and the internal stress transform from a tensile stress to a 
compressive stress at a substrate temperature of 290.degree. C. The 
compressive stress showed a gradual increase with the further increase of 
the substrate temperature. In this case of 0.5 Pa, since the internal 
stress changes linearly with the change of the substrate temperature, the 
internal stress can be controlled at a precision of 0.5x10.sup.8 
dyn/cm.sup.2 for a substrate temperature change of 10.degree. C. In the 
case of sputtering gas pressure=0.6 Pa, the internal stress was a tensile 
stress at a substrate temperature of 100.degree. C.; the tensile stress 
increased gradually as the substrate temperature was increased and became 
a maximum at a substrate temperature of 200.degree. C; and with the 
further increase of the substrate temperature, the tensile stress 
decreased gradually. In the case of sputtering gas pressure=0.8 Pa, the 
internal stress was a tensile stress at a substrate temperature of 
100.degree. C.; and the tensile stress increased gradually as the 
substrate temperature was increased. As seen from the above, the change of 
internal stress as a function of the change of substrate temperature 
differs according to the pressure level of the sputtering gas employed. As 
is clear from FIG. 1, however, the change of internal stress as a function 
of the change of substrate temperature is very small when the sputtering 
gas pressure, is less than 1.0 Pa. Accordingly, the control of internal 
stress by the change of substrate temperature provides an excellent method 
for the control of internal stress. Moreover, the internal stress obtained 
by this method has an excellent reproducibility. 
As is clear from FIG. 1, when the sputtering gas pressure is 1.0 Pa, the 
internal stress changes more diamatically as a function of the change of 
substrate temperature than when the sputtering gas pressure is less than 
1.0 Pa, making the control of internal stress by the control of substrate 
temperature more difficult. Nevertheless, the above control of internal 
stress by the control of substrate temperature is far superior to the 
conventional methods for the control of internal stress by, for example, 
sputtering gas pressure alone. 
The silicon nitride film formed by the above sputtering method can be used 
as an X-ray transmission film for an X-ray lithography mask. This X-ray 
transmission film preferably has an internal stress of 10.0x10.sup.8 
dyn/cm.sup.2 or less in terms of tensile stress. According to FIG. 1, the 
preferable substrate temperature range which enables the production of a 
silicon nitride film (an X-ray transmission film) having an internal 
tensile stress of 10.0x10.sup.8 dyn/cm2 or less is about 100.degree. C. to 
about 290.degree. C. in the case of sputtering gas pressure=0.5 Pa, about 
340.degree. C. to about 380.degree. C. in the case of sputtering gas 
pressure=0.6 Pa, and about 100.degree. C. to about 170.degree. C. in the 
case of sputtering gas pressure=0.8 Pa. Especially in the case of 
sputtering gas pressure=0.5 Pa, the internal stress can be controlled at 
5x10.sup.8 dyn/cm.sup.2 or less in terms of tensile stress over a wide 
substrate temperature range of 200-290.degree. C.; the change of internal 
stress as a function of the change of substrate temperature is linear; and 
the internal stress can be controlled at a precision of 0.5x10.sup.8 
dyn/cm.sup.2 for a substrate temperature change of 10.degree. C. Hence, 
the sputtering gas pressure is most preferably 0.5 Pa or its vicinity 
(e.g. 0.45-0.55 Pa) when the silicon nitride film formed is used as an 
X-ray transmission film. 
Shown in FIG. 2 is the change of refractive index of silicon nitride film 
as a function of the change of substrate temperature. As seen in FIG. 2, 
the refractive index of silicon nitride film has substantially no 
dependency on the substrate temperature and was 2.0 for both cases of 
sputtering gas pressure=0.5 Pa 0.6 Pa. In addition, these silicon nitride 
films were transparent in the visible region. 
The Fourier transform infrared absorption spectra of the above silicon 
nitride films confirmed that the films contained no impurities. Therefore, 
when the films are applied to an X-ray lithography mask, there occurs no 
mask strain, no compositional change, no reduction in optical transparency 
and no change in physical properties by the application of an ionizing 
radiation. 
When film formation was conducted at substrate temperatures of 200.degree. 
C. or more, the resulting films showed significant improvement of chemical 
stability as well as improvement of optical property and mechanical 
strength. The silicon nitride sample formed at a substrate temperature of 
100.degree. C. and the silicon nitride sample formed at a substrate 
temperature higher than 200.degree. C. were immersed in a 50% NaOH 
solution at 100.degree. C. for 3 hours; the former sample dissolved 
partially followed by film breakage while the latter sample saw no change. 
Hence, in order to obtain a silicon nitride film with satisfactory 
internal stress and chemical stability, the preferable substrate 
temperature is 200.degree. C. to 290.degree. C. in the case of 0.5 Pa and 
340.degree. C. to 380.degree. C. in the case of 0.6 Pa. 
In the above method for the control of internal stress of silicon nitride 
films by the control of substrate temperature, the properties of the 
formed films other than chemical stability, for example, the film 
composition and density, were confirmed to exhibit no change. 
The above Example can be modified as follows. 
In the above Example, the Si target was used as a sputtering target and the 
mixed gas of Ar gas and N.sub.2 gas was used as a sputtering gas. It is 
possible to use, as a sputtering target, a Si.sub.x N.sub.y target of 
desired composition and, as a sputtering gas, only an inert gas such as Ar 
gas or the like. It is also possible to use a substrate other than Si 
substrate, i.e. a SiO.sub.2 substrate (glass wafer). 
The present method for forming films silicon nitride film can control the 
internal stress of the formed silicon nitride more precisely than the 
conventional methods. 
Further, the present method can effect the precise control of the internal 
stress of the films without varying the optical properties, mechanical 
strength, composition and density. Accordingly, the present method has 
excellent practical uses. 
The silicon nitride films obtained by the present method are transparent in 
a visible region and have excellent chemical stability and mechanical 
strength, and therefore are suitable for use as an X-ray transmission film 
for X-ray lithography mask. 
The above-described embodiments are just an example of the present 
invention, and therefore, it will be apparent for those skilled in the art 
that many modifications and variations may be made without departing from 
the scope of the present invention.