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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device and a method of
manufacturing the same. Particularly, the present invention is directed to
a method of forming an insulating film for isolating a wiring layer.
2. Description of the Related Art
An insulating film for electrically isolating an element wiring is used in
a semiconductor device. It was customary in the past to use as an
insulating film a SiO.sub.2 film formed by thermal oxidation of a Si
substrate or a SiO.sub.2 film formed by chemical vapor deposition under a
reduced pressure or atmospheric pressure using gaseous materials such as
silane and tetraethoxy silane (TEOS). For insulation of, particularly, Al
wirings, used is a SiO.sub.2 film formed by plasma CVD using TEOS and
O.sub.2 because the SiO.sub.2 film can be formed under such a low
temperature as about 400.degree. C.
In recent years, delay of signal transmission, which accompanies the
miniaturization of the element, has come to be worried about. To be more
specific, the interval between two adjacent wirings is shortened in
accordance with miniaturization of the element, leading to an increased
capacitance between the wirings and, thus, to the delay in the signal
transmission. The delayed signal transmission obstructs a high operating
speed of the semiconductor device so as to give rise to one of the causes
of inhibiting the performance improvement of the semiconductor device. It
follows that it is important to diminish the dielectric constant of the
insulating film interposed between the two adjacent wirings.
The SiO.sub.2 film formed by the conventional plasma CVD method has been
found to have a relative dielectric constant of 4.0 to 5.0. This makes it
interesting to introduce F into SiO.sub.2 in an attempt to lower the
dielectric constant.
For example, it is described in Published Unexamined Japanese Patent
Application No. 2-77127 that F is introduced into SiO.sub.2 by means of
ion implantation so as to lower the dielectric constant of SiO.sub.2. In
this method, however, it is necessary to set the dose of F at 1
.times.10.sup.19 atoms cm.sup.-3 or more, giving rise to the problem that
the ion implantation takes a long time. In addition, it is necessary to
apply a heat treatment at such a high temperature as at least 600.degree.
C. in order to stabilize F within the SiO.sub.2 layer. It follows that the
SiO.sub.2 layer formed by the method disclosed in this prior art cannot be
used for the electric isolation of Al wirings.
A CVD method under room temperature, which uses FSi(OC.sub.2 H.sub.5).sub.3
and H.sub.2 O, is reported in, for example, "T. Homma et al., IEEE IEDM,
pp. 289 (1991)". In this method, however, it is difficult to control the F
concentration in SiO.sub.2. In addition, a serious difficulty is brought
about that the formed SiO.sub.2 film is highly hygroscopic.
Also known is a method in which an aqueous solution of boric acid is added
to a supersaturated aqueous solution of H.sub.2 SiF.sub.6 so as to utilize
the reaction given below for forming a SiO.sub.2 film:
H.sub.2 SiF.sub.6 +2H.sub.2 O.fwdarw.SiO.sub.2 +6HF
It is reported in Published Unexamined Japanese Patent Application No.
3-97247 that 5 at % of F is contained in the SiO.sub.2 film formed by the
method noted above, with the result that the relative dielectric constant
of the SiO.sub.2 film is rendered smaller than 3.9, which is the specific
dielectric constant of a SiO.sub.2 film formed by thermal oxidation. In
this method, however, it is difficult to control the F concentration in
the SiO.sub.2 film. In addition, the growing rate of the SiO.sub.2 film is
as low as about 1 nm/min.
Further, a method of improving the step coverage of a SiO.sub.2 film by
using tetraethoxy silane (TEOS), O.sub.2 and NF.sub.3 as source gases is
disclosed in "Proc. 2nd Int. ULSI Science and Tech. Symp. ECS Proc.
(1989)", though the dielectric constant and hygroscopic property of the
SiO.sub.2 film are not referred to at all in this publication.
As described above, where the interval between two adjacent wirings is
diminished in accordance with miniaturization of the element, the
capacitance between these wirings is increased so as to bring about the
problem that the signal transmission is delayed. To overcome the
difficulty, it is proposed to introduce F into the SiO.sub.2 insulating
film so as to lower the dielectric constant of the insulating film. In the
prior art, however, it is difficult to control accurately the F
concentration in the SiO.sub.2 film. It is also difficult to enable the
F-containing SiO.sub.2 film to be less hygroscopic.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of forming a
SiO.sub.2 film which has a dielectric constant smaller than that of the
SiO.sub.2 film formed by the conventional plasma CVD method and is less
hygroscopic.
According to the present invention, there is provided a method of
manufacturing a semiconductor device, in which a silicon oxide film
containing fluorine, said film acting as an insulating film for
electrically isolating conductive layers included in a semiconductor
device, is formed by a plasma CVD method using an organic silane gas
containing fluorine.
The organic silane gas containing F, which is used in this method, has a
Si--F bond. Such an organic silane gas is represented by formula:
FSi(OR).sub.3, where R is alkyl group. For example, FSi(OC.sub.2
H.sub.5).sub.3 or FSi(OCH.sub.3).sub.3 is used as a desirable organic
silane gas. The organic silane gas containing F can be used singly or in
combination with at least one of gases selected from the group consisting
of an oxidizing gas and another gaseous compound containing F.
The oxidizing gas used in this method includes, for example, O.sub.2 and
N.sub.2 O. P The other gaseous compound containing F includes, for
example, NF.sub.3, CF.sub.4, ClF.sub.3, C.sub.2 F.sub.6, SiF.sub.4,
SiH.sub.3 F, SiH.sub.2 F.sub.2 and SiHF.sub.3.
In this method, it is desirable to use a plasma generated by using a
plurality of high frequency powers differing from each other in frequency.
Two high frequency powers, for example, having a frequency of 13.56 MHz
and another frequency not higher than 1 MHz, respectively, can be used
preferably.
According to the present invention, there is provided other method of
manufacturing a semiconductor device, in which a silicon oxide film
containing fluorine and at least one of nitrogen and carbon, said film
acting as an insulating film for electrically isolating conductive layers
included in a semiconductor device, is formed by a CVD method using a
source gas containing fluorine and at least one of nitrogen and carbon.
In this method, at least one gas selected from a group consisting of, for
example, HSi(N(CH.sub.2).sub.2).sub.3, Si(N(CH.sub.3).sub.2).sub.4,
(CH.sub.3).sub.3 SiN.sub.3, NH.sub.3, N.sub.2, NO and N.sub.2 O as a
source gas containing N. A FSi(N(Ch.sub.3).sub.2).sub.3 gas may be used as
a source gas containing F and N. The source gas selected from
above-mentioned organic silane gas containing F, oxidizing gas and another
gaseous compound containing F may be used with the source gas containing
N.
It is desirable for the silicon oxide film formed in this method to have a
F concentration of at least 3 atomic % and a N concentration of at least 1
atomic %.
According to the present invention, there is provided another method of
manufacturing a semiconductor device, in which a silicon oxide film
containing fluorine, said film acting as an insulating film for
electrically isolating conductive layers included in a semiconductor
device, is formed by a plasma CVD method using a source gas containing at
least silicon, oxygen and fluorine, under the conditions that the
relationship between the gas pressure P (Torr) and the ion energy E (eV)
satisfies formula A given below:
P.gtoreq.5.times.10.sup.-4,P.ltoreq.10.sup.-1 .times.10.sup.-E/45(A)
and the relationship between the ion energy E (eV) and the plasma density D
(/cm.sup.3) satisfies the formula B given below:
P.gtoreq.2.times.10.sup.11 .times.10.sup.-E/45,10.ltoreq.E (B)
In this method, a magnetron plasma CVD, a helicon wave plasma CVD or an
electron beam excited plasma CVD, which can provide a high plasma density,
is used as a desirable plasma CVD method. The ion energy is preferably set
to 100 eV or less.
According to this method, there is provided a semiconductor device, which
comprises a silicon oxide film acting as an insulating film for
electrically isolating conductive layers included in the semiconductor
device, said silicon oxide film having a fluorine concentration of at
least 1 at % and a Si dangling bond density of 10.sup.17 cm.sup.-3 or
less. The Si dangling bond density is preferably 10.sup.14 cm.sup.-3 or
less.
The SiO.sub.2 film formed by the method of the present invention has a low
dielectric constant. It is considered reasonable to understand that, if
Si--F bond is formed as a result of F addition to SiO.sub.2, the Si--O
network structure is broken to lower the density, leading to a smaller
dielectric constant. It follows that the capacitance between adjacent
wirings can be lowered, making it possible to suppress the delay in the
signal transmission and, thus, to achieve a high operating speed of the
element. What should also be noted is that, in the method of the present
invention, the F concentration in the SiO.sub.2 film can be easily
controlled by controlling the flow rate of the source gas.
It is considered reasonable to understand that, where the SiO.sub.2 film
contains both F and N, a Si--F bond and a Si--N bond are formed together,
with the result that a high density portion and a low density portion are
formed together in the SiO.sub.2 film. It follows that the dielectric
constant of the SiO.sub.2 film is lowered, making it possible to form an
insulating film smaller in moisture absorption. The similar effect can be
obtained where C is contained in .place of N contained in the SiO.sub.2
film.
In forming a SiO.sub.2 film containing F, it is desirable to apply a
plurality of high frequency powers differing from each other in frequency
to an organic silane containing F, which is used as a source gas, so as to
generate a plasma. In this case, the F concentration can be increased by
lowering the RF power, with the result that the gate breakage caused by
charged particles is unlikely to take place. In addition, the formed
SiO.sub.2 film is enabled to be much lower in its moisture absorption.
Also, a silicon oxide film containing fluorine is formed by a plasma CVD
method under the conditions that the relationship between the gas pressure
P (Torr) and the ion energy E (eV) satisfies formula A given below:
P.gtoreq.5.times.10.sup.-4,P.ltoreq.10.sup.-1 .times.10.sup.-E/45(A)
and the relationship between the ion energy E (eV) and the plasma density D
(/cm.sup.3) satisfies the formula B given below:
D.gtoreq.2.times.10.sup.11 .times.10.sup.-E/45,10.ltoreq.E (B)
The particular method of the present invention permits the formed SiO.sub.2
film containing F to be less likely to absorb moisture. It should be noted
that a large number of active F radicals and C radicals are present in the
plasma formed under the conditions described above. In this case, the
density of the Si dangling bonds within the silicon oxide film, said Si
dangling bond providing the reaction site with water, is decreased to
10.sup.17 cm.sup.-3 or less, leading to the low moisture absorption noted
above. This effect can be observed for a silicon oxide film having a
fluorine concentration of 1 at % or more, particularly from 3 to 8 at %.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the construction of a plasma CVD apparatus used in Example 1;
FIGS. 2A to 2C are cross sectional views showing the steps of manufacturing
a semiconductor device in Example 1;
FIG. 3 shows an infrared absorption spectrum of the SiO.sub.2 film
containing a Si--F bond formed in Example 1;
FIG. 4 is a graph showing the relationship between the flow rate of a
NF.sub.3 gas and the Si--F absorption peak area observed at a wave number
of 940 cm.sup.-1 ;
FIG. 5 is a graph showing the relationship between the F concentration in
the SiO.sub.2 film and the relative dielectric constant of the SiO.sub.2
film;
FIG. 6 is a graph showing the relationship between the F concentration in
the SiO.sub.2 film and the hygroscopicity of the SiO.sub.2 film;
FIG. 7 is a graph showing the relationship between the F concentration in
the SiO.sub.2 film and the leakage current in the case where a constant
voltage is applied across a MOS capacitor;
FIGS. 8A to 8D are cross sectional views showing the steps of manufacturing
a semiconductor device in Example 2 of the present invention;
FIGS. 9A to 9C are graphs each showing the properties of the SiO.sub.2 film
formed in Example 3 of the present invention;
FIG. 10 shows the construction of a deposition apparatus used in Example 4
of the present invention;
FIGS. 11A and 11B are cross sectional views showing the steps of
manufacturing a semiconductor device in Example 4 of the present invention
FIG. 12 shows the construction of a deposition apparatus used in Example 5
of the present invention;
FIGS. 13A and 13B are graphs each showing the properties of the SiO.sub.2
film formed in Example 5 of the present invention;
FIG. 14 shows the construction of a deposition apparatus used in Example 6;
FIG. 15 is a graph showing the relationship between the F concentration in
the SiO.sub.2 film and the hygroscopicity of the SiO.sub.2 film;
FIG. 16 a graph showing the relationship between the power density and the
F concentration in the SiO.sub.2 film;
FIG. 17 shows the construction of a deposition apparatus used in Example 7
of the present invention;
FIG. 18 is an oblique view showing the discharge antenna included in the
deposition apparatus used in Example 7 of the present invention;
FIG. 19 is a graph showing the relationship between the F concentration in
the SiO.sub.2 film formed in Example 7 of the present invention and the
hygroscopicity of the SiO.sub.2 film;
FIG. 20 shows the construction of a deposition apparatus used in Example 8
of the present invention;
FIG. 21 shows the construction of a deposition apparatus used in Example 9
of the present invention;
FIG. 22 is a graph showing the relationship between the F concentration in
the SiO.sub.2 film and the relative dielectric constant of the SiO.sub.2
film;
FIG. 23 is a graph showing the conditions in terms of the relationship
between the ion energy and the plasma density for obtaining a SiO.sub.2
film which is low in its hygroscopicity;
FIG. 24 is a graph showing the conditions in terms of the relationship
between the ion energy and the pressure for obtaining a SiO.sub.2 film
which is low in its hygroscopicity;
FIG. 25 shows the Raman spectrums of the SiO.sub.2 films formed in Examples
7 to 9 of the present invention;
FIG. 26 is a graph showing the Si dangling bond density and the
hygroscopicity of the SiO.sub.2 film, which are changed depending on the
method employed for forming the SiO.sub.2 film; and
FIG. 27 is a graph showing the relationship among the F concentration in
the SiO.sub.2 film, the Si dangling bond density, and the hygroscopicity
of the SiO.sub.2 film in respect of the SiO.sub.2 films formed in Examples
7 to 9 of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Let us describe some Examples of the present invention.
EXAMPLE 1
FIG. 1 shows a parallel plate type plasma CVD apparatus used in Example 1.
As shown in the drawing, a gas within a chamber 11 is discharge to the
outside by a pump 12. Reaction gases are introduced into the chamber 11
through quartz nozzles 13. Parallel plate type electrodes 14 and 15 are
arranged in parallel within the chamber 11. A high frequency power source
16 is connected to the electrode 14 via a matching box, with the electrode
15 being connected to a ground potential point. A Si substrate 10 is
mounted on the electrode 15.
FIGS. 2A to 2C are cross sectional views showing as an example the process
of forming an interlayer insulating film using TEOS, O.sub.2 and NF.sub.3
gases as the source gases.
In the first step, the Si substrate 10 is mounted on the electrode 15,
followed by heating the substrate 10 to 400.degree. C. with a resistance
heater. Under this condition, a tetraethoxy silane (TEOS) gas, an O.sub.2
gas and a NF.sub.3 gas are simultaneously introduced into the chamber 11
at flow rates of 50 sccm, 500 sccm and 0 to 500 sccm, respectively, so as
to set up a pressure of 5 Torr within the chamber 11. Also, 13.56 MHz of
RF power is supplied to the electrode 14 so as to cause discharge and,
thus, to form a SiO.sub.2 film 22 having a thickness of 500 nm on the Si
substrate 21, as shown in FIG. 2A.
In the next step, an Al film is formed in a thickness of 400 nm by means of
a DC magnetron sputtering, followed by patterning the Al film to form a
first Al wiring 23 having a width of 500 nm and a height of 400 nm, as
shown in FIG. 2B. Then, a SiO.sub.2 film 24 is formed in a thickness of
800 nm as shown in FIG. 2C. The SiO.sub.2 film 24 is formed as in the
formation of the SiO.sub.2 film 22. Further, an Al film having a thickness
of 400 nm is formed to cover the second SiO.sub.2 film 24, followed by
patterning the Al film to form a second Al wiring 25, as in the formation
of the first Al wiring 23. Finally, a SiO.sub.2 film 26 having a thickness
of 800 nm is formed to cover the second Al wiring 25 as in the formation
of the SiO.sub.2 film 22 or 24.
FIG. 3 shows an infrared absorption spectrum of the SiO.sub.2 film formed
under the condition that the NF.sub.3 flow rate was set at 150 sccm. As
shown in FIG. 3, peaks derived from the Si--0 bond are found at the wave
numbers of 1080 cm.sup.-1, 810 cm.sup.31 1 and 450 cm.sup.-1. Also found
at the wave number of about 940 cm.sup.-1 is a peak derived from Si--F
bond. Clearly, the infrared absorption spectrum shown in FIG. 3 indicates
the formation of a SiO.sub.2 film having a Si--F bond.
FIG. 4 shows the relationship between the flow rate of the NF.sub.3 gas and
the Si--F absorption peak area observed at a wave number of 940 cm.sup.-1
in respect of the SiO.sub.2 films formed by changing the NF.sub.3 gas flow
rates in various fashions. It is clearly seen that the Si--F bond within
the SiO.sub.2 film is increased with increase in the flow rate of the
NF.sub.3 gas. Measured was the fluorine amount within the SiO.sub.2 film
formed by changing the NF.sub.3 gas flow rate in various fashions. The F
concentration was found to be about 2 atomic % where the flow rate of the
NF.sub.3 gas was set at 50 sccm, about 3 atomic % where the flow rate of
the NF.sub.3 gas was set at 100 sccm, about 4 atomic % where the flow rate
of the NF.sub.3 gas was set at 150 sccm, and about 5 atomic % where the
flow rate of the NF.sub. 3 gas was set at 200 sccm.
Further, measured were C-V characteristics of a MOS capacitor consisting of
a SiO.sub.2 film formed by changing the flow rate of the NF.sub.3 gas in
various fashions and an Al film patterned to have an area of about 0.1
mm.sup.2 so as to obtain the relative dielectric constant of the SiO.sub.2
film. FIG. 5 shows the relationship between the F concentration in the
SiO.sub.2 film and the relative dielectric constant of the SiO.sub.2 film.
It is clearly seen that the fluorine introduction into the SiO.sub.2 film
permits lowering the dielectric constant of the SiO.sub.2 film. It should
be noted, however, that the hygroscopicity of the SiO.sub.2 film is
sharply increased with increase in the F concentration in the SiO.sub.2
film, as shown in FIG. 6.
FIG. 7 shows the relationship between the F concentration in the SiO.sub.2
film and the leakage current which takes place when a predetermined
voltage (electric field intensity of 3 MV/cm) is applied across the MOS
capacitor.
As described above, where the F concentration is not high than 5 atomic %,
the dielectric constant of the SiO.sub.2 film is lowered so as to suppress
the current leakage.
An additional experiment was conducted to form a SiO.sub.2 film by the
method equal to that described above, except that SiH.sub.4, N.sub.2 O and
NF.sub.3 used as source gases were introduced into the chamber 11 at flow
rates of 50 sccm, 500 sccm and 0 to 500 sccm, respectively, in place of
TEOS, O.sub.2 and NF.sub.3 used in the experiment described previously,
and that the film-forming pressure was set at 1 Torr. It has been
confirmed that, where the F concentration in the SiO.sub.2 film is not
higher than 5 atomic %, the dielectric constant of the SiO.sub.2 film is
lowered and the leak current is suppressed.
In the method of the present invention, it is possible to use organic
silane gases such as HSi(OC.sub.2 H.sub.5).sub.3 and H.sub.2 Si(C.sub.4
H.sub.9).sub.2 in place of TEOS. Further, it is possible to use gaseous
F-containing compounds such as CF.sub.4, ClF.sub.3 and SiF.sub.4 in place
of NF.sub.3.
An additional experiment was conducted to form a SiO.sub.2 film by the
method equal to that described above, except that FSi(OC.sub.2
H.sub.5).sub.3 and O.sub.2 used as source gases were introduced into the
chamber 11 at flow rates of 50 sccm and 500 sccm, respectively, in place
of TEOS, O.sub.2 and NF.sub.3 used in the experiment described previously,
and that the film-forming pressure was set at 1 Torr. It has been
confirmed that the F concentration in the SiO.sub.2 film was 5 atomic %,
that the relative dielectric constant of the SiO.sub.2 film was about 3.4,
and that the leakage current was much suppressed. In this case, it is also
possible to control the F concentration in the SiO.sub.2 film by
controlling the O.sub.2 flow rate or the discharge power.
A similar effect can also be obtained in the case of using a mixed gas
comprising an organic silane gas containing no fluorine and an organic
silane gas containing fluorine, for example, FSi(OC.sub.2
H.sub.5).sub.2,O.sub.2 and TEOS. In this case, the F concentration in the
SiO.sub.2 film can be controlled without difficulty by changing the ratio
of the flow rate of FSi(OC.sub.2 H.sub.5).sub.2 to the flow rate of TEOS.
It is also possible to form a SiO.sub.2 film having a Si--F bond by using
a FSi(OC.sub.2 H.sub.5).sub.2 gas alone or both FSi(OC.sub.2
H.sub.5).sub.2 gas without using the oxidizing agent of O.sub.2. Further,
it is possible to use F-containing gaseous inorganic silane compounds such
as SiH.sub.3 F, SiH.sub.2 F.sub.2 and SiHF.sub.3 in place of the
FSi(OC.sub.2 H.sub.5).sub.2 gas.
EXAMPLE 2
The present invention covers the case where a P-containing SiO.sub.2 film
and a SiO.sub.2 film which does not contain F are laminated one upon the
other as shown in FIG. 8. An insulating film of such a laminate structure
permits markedly suppressing the moisture absorption so as to improve the
reliability of the metal wiring.
As shown in FIG. 8A, a BPSG (borophosphosilicate glass) film 82 having a
thickness of 800 nm is formed first on a Si substrate 81, followed by
forming an Al wiring 83 having a width of 500 nm and a height of 400 nm on
the BPSG film 82.
As shown in FIG. 8B, a SiO.sub.2 film 84, which does not contain fluorine,
is formed in a thickness of 100 nm to cover the Al wiring 83 and the
exposed surface of the BPSG film 82 by using TEOS and O.sub.2 as source
gases. Further, a SiO.sub.2 film 85 containing F is formed in a thickness
of 500 nm to cover the SiO.sub.2 film 84 by using TEOS, O.sub.2 and
NF.sub.3 as source gases, as in Example 1, followed by further forming a
SiO.sub.2 film 86, which does not contain F, in a thickness of 100 nm to
cover the SiO.sub.2 film 85 using TEOS and O.sub.2 as source gases.
In the next step, the SiO.sub.2 film 86 is coated with photoresist,
followed by exposure to light and, then, development. Further, a hole 87
is formed in the laminate structure of the SiO.sub.2 films positioned
above the Al wiring 83 as shown in FIG. 8C.
Further, the hole 87 is filled with a tungsten layer 88 by means of a
selective CVD method using WF.sub.6 and SiH.sub.4, followed by forming an
Al film by a sputtering method and subsequently patterning the Al film so
as to form an Al wiring 89. After formation of the Al wiring 89, a
SiO.sub.2 film 810 which does not contain fluorine is formed in a
thickness of 100 nm, followed by forming a SiO.sub.2 film 811 containing
fluorine in a thickness of 500 nm on the SiO.sub.2 film 810 and
subsequently forming a SiO.sub.2 film 812 which does not contain fluorine
in a thickness of 100 nm on the SiO.sub.2 film 811, as in the step shown
in FIG. 8B.
It should be noted that the SiO.sub.2 film, which does not contain
fluorine, is lower in its hygroscopicity than the SiO.sub.2 film
containing fluorine. It follows that the metal wiring included in the
semiconductor device shown in FIG. 8D is unlikely to contact the moisture
absorbed by the SiO.sub.2 film.
EXAMPLE 3
In the present invention, it is possible to form an interlayer insulating
film for a multi-layer wiring as in FIG. 2 by using a parallel plate type
plasma CVD apparatus as shown in FIG. 1. Used in this example as source
gases are a HSi(N(CH.sub.3).sub.2).sub.3 gas, a FSi(OC.sub.2
H.sub.5).sub.3 gas and an O.sub.2 gas.
In the first step, the substrate 10 is mounted on the electrode 15, and the
substrate 10 is heated to 400.degree. C. by a resistance heater. Then, a
HSi(N(CH.sub.3).sub.2).sub.3 gas, a FSi(OC.sub.2 H.sub.5).sub.3 gas and an
O.sub.2 gas, which are used as source gases, are introduced into the
reaction chamber 11 at flow rates of 50 sccm, 500 sccm and 0 to 300 sccm,
respectively. Also, the film-forming pressure is set at 5 Torr. Under this
condition, an RF power of 13.56 MHz is applied to the electrode 14 so as
to convert the source gases into a plasma and, thus, to form a SiO.sub.2
film containing F and N on the Si substrate 21, as shown in FIG. 2A. The
SiO.sub.2 film is formed in a thickness of 500 nm in this step.
In the next step, an Al film is formed in a thickness of 400 nm by means of
a DC magnetron sputtering method, followed by patterning the Al film so as
to form a first Al wiring 23 having a width of 500 nm and a height of 400
nm, as shown in FIG. 2B. Further, a SiO.sub.2 film is formed in a
thickness of 800 nm by using source gases equal to those used in the
previous step, as shown in FIG. 2C. Still further, an Al film is formed in
a thickness of 400 nm, followed by patterning the Al film to form a second
Al wiring 25 as in the formation of the first Al wiring 23. Finally, a
SiO.sub.2 film 26 is formed in a thickness of 800 nm using the same source
gases.
FIG. 9A is a graph showing the relationship between the flow rate of the
HSi(N(CH.sub.3).sub.2).sub.3 gas and the N and F concentrations in the
SiO.sub.2 film. It is seen that the N concentration is increased with
increase in the flow rate of the HSi(N(CH.sub.3).sub.2).sub.3 gas.
However, the F concentration remains constant regardless of the flow rate
of the HSi(N(CH.sub.3).sub.2).sub.3.
FIG. 9B is a graph showing the relationship between the flow rate of the
HSi(N(CH.sub.3).sub.2).sub.3 gas and the relative dielectric constant of
the SiO.sub.2 film. It is seen that the relative dielectric constant of
the SiO.sub.2 film is 3.4 where the the flow rate of the
HSi(N(CH.sub.3).sub.2).sub.3 gas is 0 sccm. However, the density of the
SiO.sub.2 film is gradually increased with increase in the N
concentration, leading an increase in the relative dielectric constant of
the SiO.sub.2 film.
FIG. 9C is a graph showing the relationship between the flow rate of the
HSi(N(CH.sub.3).sub.2).sub.3 gas and the hygroscopicity of the SiO.sub.2
film. It is seen that the nitrogen introduction into the SiO.sub.2 film
permits lowering the hygroscopicity of the SiO.sub.2 film.
As apparent from FIGS. 9A to 9C, a SiO.sub.2 film containing F and N, which
has a relative dielectric constant of 3.5 and is low in hygroscopicity,
can be formed by setting the flow rate of the HSi(N(CH.sub.3).sub.2).sub.3
gas at 100 sccm.
It is possible to use organic silane gases containing nitrogen such as
Si(N(CH.sub.3).sub.2).sub.4 and (CH.sub.3).sub.3 SiN.sub.3 in place of the
HSi(N(CH.sub.3).sub.2).sub.3 gas. It is also possible to use inorganic
silane gases containing fluorine such as SiH.sub.3 F, SiH.sub.2 F.sub.2,
SiHF.sub.3 and SiF.sub.4 in place of the organic silane gas such as
FSi(OC.sub.2 H.sub.5).sub.3. Further, other oxidizing agents such as
N.sub.2 O and O.sub.3 can be used in place of the O.sub.2 gas.
An additional experiment was conducted to form a SiO.sub.2 film by the
method equal to that described above in the temperature and pressure
conditions, except that NH.sub.3 gas and FSi(OC.sub.2 H.sub.5).sub.3 gas,
which were used as source gases were introduced into the chamber 11 at the
flow rate of 50 sccm for each of these source gases, in place of
HSi(N(CH.sub.3).sub.2).sub.3 gas and the FSi(OC.sub.2 H.sub.5).sub.3 gas
used in the experiment described previously. It has been confirmed that it
is possible to form a SiO.sub.2 film containing both F and N, which has a
relative dielectric constant of 3.5 and is low in its hygroscopicity.
A still additional experiment was conducted to form a SiO.sub.2 film by the
method equal to that described above in the temperature and pressure
conditions, except that a HSi(N(CH.sub.3).sub.2).sub.3 gas and a NF.sub.3
gas, which were used as source gases in place of the
HSi(N(CH.sub.3).sub.2).sub.3 gas and the FSi(OC.sub.2 H.sub.5).sub.3 gas
used in the experiment described previously, were introduced into the
reaction chamber at the flow rates of 100 sccm and 50 sccm, respectively.
It has been confirmed that it is possible to form a SiO.sub.2 film
containing both F and N, which has a relative dielectric constant of 3.5
and is low in its hygroscopicity. Similar effects can also be obtained in
the cases where F-containing gaseous compounds such as CF.sub.4 and
ClF.sub.3 are used in place of the NF.sub.3 gas.
EXAMPLE 4
Used in this example is a hot wall type batch thermal CVD apparatus shown
in FIG. 10, and a SiO.sub.2 film is formed by using a NH.sub.3 gas, a
ClF.sub.3 gas, a SiH.sub.4 gas and an O.sub.2 gas as source gases.
As shown in FIG. 10, a discharge pump 42b is connected to a discharge port
42a of a quartz tube 41, and a resistance heater 43 is arranged to
surround the quarts tube 41. A quartz boat 45 is disposed within the
quartz tube 41, and a plurality of Si substrates 10 are arranged on the
quartz boat 45 in the flowing direction of the gases. The Si substrate 10
can be heated to 600.degree. C. to 700.degree. C. by the resistance heater
43. On the other hand, a plurality of quartz nozzles 46 for introducing
source gases into the quartz tube 41 are mounted on the inlet side of the
quartz tube 41 opposite to the discharge port 42a.
Let us describe how to form a thermal CVD oxide film on the gate electrode
by using the apparatus described above. FIGS. 11A and 11B are cross
sectional views showing the steps for forming a SiO.sub.2 film. In this
case, it is possible to form a SiO.sub.2 film containing F and N, which
has a relative dielectric constant of 3.5 and is low in its
hygroscopicity.
In the first step, an element isolation region 52 is formed on a Si
substrate 51, followed by forming a gate oxide film 53, a polycrystalline
silicon gate 54 and regions 55 doped with an impurity and subsequently
forming a SiO.sub.2 film 56 in a thickness of 300 nm to cover the entire
surface, as shown in FIG. 11A. In forming the SiO.sub.2 film 56, an
NH.sub.3 gas, a ClF.sub.3 gas, a SiH.sub.4 gas, and an O.sub.2 gas are
introduced into the reaction chamber at flow rates of 1000 sccm, 100 sccm,
500 sccm and 100 sccm, respectively, and the SiO.sub.2 film 56 is formed
at a temperature of 700.degree. C. and a pressure of 0.4 Torr.
In the next step, a BPSG film 57 is formed in a thickness of 500 nm as
shown in FIG. 11B. In this case, the BPSG film 57 is heated to 850.degree.
C. to bring about a melt re-flow, followed by forming another SiO.sub.2
film 58 as in the formation of the SiO.sub.2 film 56. In this embodiment,
it is possible to diminish the capacitance between the gate and an upper
wiring (not shown) so as to suppress the delay in the signal transmission.
It is possible to use other gaseous F-containing compounds such as NF.sub.3
and CF.sub.4 in place of the ClF.sub.3 gas. It is also possible to use
organic silane gases such as TEOS, HSi(OC.sub.2 H.sub.5).sub.3 and H.sub.2
Si(C.sub.4 H.sub.9).sub.2 in place of the SiH.sub.4 gas, with
substantially the same effect.
It is also possible to use a mixed gas consisting of an N-containing silane
gas, an F-containing silane gas and an O.sub.2 gas as an oxidizing gas in
place of the NH.sub.3 gas, ClF.sub.3 gas, SiH.sub.4 gas and O.sub.2 gas
referred to above, with substantially the same effect. The N-containing
silane gas noted above includes, for example, HSi(N(CH.sub.3).sub.2,
Si(N(CH.sub.3).sub.2).sub.4, and (CH.sub.3).sub.3 SiN.sub.3. On the other
hand, the F-containing silane gas noted above includes, for example,
SiH.sub.3 F, SiH.sub.2 F.sub.2, SiHF.sub.3, SiF.sub.4, and FSi(OC.sub.2
H.sub.5).sub.3.
An additional experiment was conducted. In this case, an NH.sub.3 gas, a
FSi(OC.sub.2 H.sub.5).sub.3 gas and an O.sub.2 gas were introduced into
the reaction chamber at the flow rate of 50 sccm for each of these gases,
and a SiO.sub.2 film was formed under the same temperature and pressure
conditions as above. The SiO.sub.2 film thus formed was found to contain
both F and N, to have a relative dielectric constant of 3.5, and to be
lower in its hygroscopicity.
A still additional experiment was conducted. In this case, an
HSi(N(CH.sub.3).sub.2).sub.3 gas, a ClF.sub.3 gas and an O.sub.2 gas were
introduced into the reaction chamber at the flow rates of 100 sccm, 50
sccm, and 50 sccm, respectively, and a SiO.sub.2 film was formed under the
same temperature and pressure conditions as above. The SiO.sub.2 film thus
formed was found to contain both F and N, to have a relative dielectric
constant of 3.5, and to be lower in its hygroscopicity. It is also
possible to use gaseous F-containing compounds such as CF.sub.4 and
NF.sub.3 in place of the ClF.sub.3 gas noted above.
EXAMPLE 5
In the present invention, it is possible to use a cold wall type thermal
CVD apparatus of a batch system as shown in FIG. 12. Used in this example
as source gases are a NF.sub.3 gas, a TEOS gas and a N.sub.2 O gas.
As shown in FIG. 12, an ozonizer 62a serving to convert oxygen into ozone
by silent discharge is connected to a reaction chamber 61. An NF.sub.3 gas
and a N.sub.2 O gas are introduced into the ozonizer 62a, with the result
that oxygen in the N.sub.2 O gas is converted into ozone, which is
introduced into the reaction chamber 61 through a gas inlet pipe 62b. The
apparatus also comprises gas inlet pipes 62c and 62d serving to introduce
the FSi(OC.sub.2 H.sub.5).sub.3 gas and the HSi(N(CH.sub.3).sub.2).sub.3
gas into the reaction chamber 61 and a discharge pump 64. A sample holder
63 having a heater 63a buried therein is arranged within the reaction
chamber 61.
In forming a SiO.sub.2 film, an NF.sub.3 gas, a TEOS gas and a N.sub.2 O
gas are introduced into the reaction chamber 61 at the flow rates of 200
sccm, 100 sccm and 1000 sccm, respectively. Under this condition, the
substrate is heated by the heater 63a to 350.degree. C., and a
film-forming pressure is set at 5 Torr. The SiO.sub.2 film thus formed,
which contains both F and N, exhibits a re-flow shape, has a relative
dielectric constant of 3.5, and is low in its hygroscopicity. In this
case, it is possible to use other gaseous F-containing compounds such as
CF.sub.4 and ClF.sub.3 in place of the NF.sub.3 gas, with substantially
the same effect.
As a result of an extensive research, the present inventor has found that,
in Examples 3 to 5 described above, it is desirable to use a parallel
plate type plasma CVD apparatus as shown in FIG. 1 and to use as a source
gas a N-containing silane gas having a Si--N bond 10 in the molecule such
as HSi(N(CH.sub.3).sub.2).sub.3, Si(N(CH.sub.3).sub.2).sub.4 or
(CH.sub.3).sub.2 SiN.sub.3. It has been found that, in this case, it is
possible to form a SiO.sub.2 film having a low relative dielectric
constant and a much lower hygroscopicity. It is considered reasonable to
understand that the use of a source gas having a Si--N bond permits N to
remain in the formed SiO.sub.2 film with a high probability, even if
dissociation of the source gas is promoted by plasma.
FIGS. 13A and 13B are graphs each showing the properties of the SiO.sub.2
film in the case where HSi(N(CH.sub.3).sub.2).sub.3 is used as a silane
gas containing nitrogen. It has been found that, where it is intended to
obtain a SiO.sub.2 film having a fluorine concentration of, for example, 5
atomic % and a relative dielectric constant of 3.8 or less, it is
desirable to set the N concentration in the SiO.sub.2 film at 15 atomic %
or less, as shown in FIG. 13A. It has also been found that, where the N
concentration in the SiO.sub.2 film is at least 2.9 atomic %, it is
possible to obtain a SiO.sub.2 film free from moisture absorption.
In the present invention, it is possible to obtain a SiO.sub.2 film low in
hygroscopicity, if the N concentration in the silicon oxide film is at
least 1 atomic %. On the other hand, if the F concentration in the
SiO.sub.2 film is at least 3 atomic %, it is possible to form a SiO.sub.2
film having a low dielectric constant.
EXAMPLE 6
FIG. 14 schematically shows the construction of a parallel plate type
plasma CVD apparatus which permits excitation with two different
frequencies. As shown in the drawing, the gas within a chamber 11 is
discharged to the outside by a pump 12. On the other hand, reaction gases
are introduced into the chamber 11 through a plurality of quartz nozzles
13. Parallel plate type electrodes 14 and 15 are arranged in parallel
within the chamber 11. A high frequency power source 16 of 13.56 MHz is
connected to the electrode 14 via a matching box 17. A high frequency
power source 19 of 400 kHz is also connected to the electrode 14 via a low
pass filter 18. On the other hand, the electrode 15 is connected to the
ground potential point. Further, a Si substrate 10 is mounted to the
electrode 15.
The apparatus of the construction described above is used for forming a
SiO.sub.2 film as follows. In the first step, the Si substrate 10 is
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