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BACKGROUND OF THE INVENTION
This invention relates to an etching method for forming deep grooves in a
semiconductor substrate by reactive ion etching.
In general, wet etching using an etching liquid has been widely used for
producing semiconductor circuits and the like for many years. However, wet
etching has many disadvantages such as the occurrence of undercutting. As
the requirement of much finer circuit patterns increases with improved
integrated degrees and the like, so-called dry etching methods without
using any etching agent have been proposed. Among them, the reactive ion
etching method has been of particularly note. This method uses parallel
plate electrodes and accomplishes simultaneous use of physical etching
such as sputtering or the like and chemical etching by a chemical
reaction.
With this method, the parallel plate electrodes are arranged in a reaction
vessel, and after a substrate to be etched has been located on one of the
electrodes, an etching gas under a predetermined pressure is introduced
into the reaction vessel. A predetermined high frequency electric power is
then applied to the parallel plate electrodes in the etching gas
atmosphere to produce a plasma in the reaction vessel. The resulting
physical and chemical reactions etch the substrate.
In etching a silicon substrate by the reactive ion etching method of this
kind in general, various gases are used for this purpose. In most cases,
the gases are fluorinated hydrocarbon compounds such as CF.sub.4,
CHF.sub.3, other hydrocarbon fluoride compounds and the like, gases
containing chlorine, and gases containing bromine.
However, etching using a gaseous fluorine compound is likely to cause
undercutting resulting in a problem in processing accuracy. On the other
hand, etching using a chlorine-containing gas has less chance of side
etching in comparison with that using a gaseous fluorine compound;
however, there is a tendency in using a chlorine-containing gas for a
surface of a silicon substrate to be etched into a rough surface which
appears to be black. In etching using the bromine-containing gas,
moreover, side walls of grooves formed by the etching often leave
projections thereon to form rough surfaces.
Furthermore, in the respective etching methods using the above various
gases, side walls of etched grooves tend to be curved, thus deviating from
desired vertical surfaces, and sometimes fine grooves are unintentionally
formed in bottoms of the etched grooves. Etching using silicon
tetrachloride (SiCl.sub.4) and oxygen (O.sub.2) exhibits a high
selectivity for silicon dioxide which is usually used as masks and
achieves comparatively good etched configurations in comparison with the
etching methods using the other gases. However, reaction products
consisting of silicon and oxygen compounds formed in etching produce a
white powder which contaminates the reaction vessel and the substrates.
Moreover, the white powder acts as if it were a mask at locations to be
etched, so that portions of a substrate to be etched remain unetched even
after an etching step. A substrate is etched faster at a portion
surrounding the white powder than at a portion in direct contact with the
white powder, so that grooves are formed in bottoms of etched grooves or
the bottoms are uneven or appear to be black. Furthermore, in etching
using silicon tetrachloride, the etched configuration is likely to be
detrimentally affected by residual gases in the reaction vessel, with the
result that the etched configuration tends to be unstable.
With the above etching methods using the various kinds of gases, grooves
not having undersized curved portions, i.e., "anisotropic etching" may be
temporarily accomplished by properly adjusting various etching conditions
such as the flow rate ratio (a ratio of gas to the total volumetric flow
rate). However, setting the etching conditions is very delicate, so that
even if the etching is effected with the same conditions, the etching is
not reproducible.
SUMMARY OF THE INVENTION
It is the object of the invention to provide an improved etching method
which solves the problems of the prior art above described and prevents
the formation of white powder accompanied with the etching, prevents
incomplete etching and a black appearance of a silicon substrate, and is
reproducible in performing anisotropic etching.
In order to accomplish this object, in accordance with the invention, a
method of etching a substrate in a vessel is carried out by introducing
into the vessel an etching gas which contains a gaseous chloride of
silicon and a nitrogen-containing gas and converting the resulting etching
gas into a plasma.
In a preferred embodiment of the invention the nitrogen-containing gas is
nitrogen gas and/or a gaseous nitrogen compound.
In another embodiment, the nitrogen-containing gas is a mixture of nitrogen
gas and a chlorine-containing gas.
In a further embodiment of the invention, the nitrogen-containing gas is a
nitrogen compound gas and a chlorine-containing gas.
With these features according to the invention, in etching a silicon
substrate masked by silicon dioxide, the etching is performed by
converting a gaseous mixture of a gaseous chloride of silicon and a
nitrogen-containing gas as an etching gas into a plasma.
In other words, by converting the etching gas into a plasma, ions and
radicals are produced. The ions impinge against the substrate as a
sputtering action, while a reaction between the radicals and the silicon
substrate produces a volatile substance. The etching is accomplished by
the sputtering action and the production of the volatile substance. In
this case, because the etching gas does not contain oxygen according to
the invention, the white powder as the reaction product of the silicon
chloride and oxygen is not produced. Moreover, the silicon chloride gas
and the nitrogen-containing gas used as an etching gas achieves
anisotropic etching which is superior in reproducibility and increases
both the speed and selectivity of etching.
In this etching process, nitrogen does not serve as a diluent gas, but
enters into a chemical reaction. For example, silicon tetrachloride
(SiCl.sub.4) and nitrogen (N.sub.2) react to form SiN.sub.x and Cl.sub.2.
Under certain conditions, a SiN.sub.x film may be deposited or formed on
the side walls of the grooves being formed in a substrate by the etching
and protects the side wall from an etching reaction, thus achieving the
anisotropic etching of the silicon substrate.
In order that the invention may be more clearly understood, preferred
embodiments will be described, by way of example, with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view illustrating an arrangement of a silicon substrate and a
mask;
FIG. 2 is a diagram showing the relationship between the nitrogen flow rate
ratio and etching speed;
FIG. 3 is a diagram showing the relationship between the nitrogen flow rate
ratio and etching selectivity;
FIG. 4 is a sectional view illustrating a configuration of an etched groove
at a nitrogen flow rate ratio of 5%;
FIG. 5 is a sectional view illustrating a configuration of an etched groove
at a nitrogen flow rate ratio of 30% and a chlorine flow rate of 30%;
FIG. 6 is a sectional view illustrating a configuration of an etched groove
at a nitrogen flow rate ratio of 60%;
FIG. 7 is a diagram showing the relationship between chlorine flow rate
ratio and etching speed;
FIG. 8 is a diagram showing the relationship between a chlorine flow rate
ratio and etching selectivity.
FIG. 9 is a sectional view illustrating the configuration of an etched
groove formed at a nitrogen flow rate of 0%;
FIG. 10 is a sectional view illustrating the configuration of an etched
groove formed at a nitrogen flow rate ratio of 25%;
FIG. 11 is a sectional view illustrating the configuration of an etched
groove at a nitrogen flow rate ratio of 50%;
FIG. 12 is a sectional view illustrating the configuration of an etched
groove at a nitrogen flow rate ratio of 75%;
FIG. 13 is a section view illustrating the configuration of an etched
groove at a nitrogen flow rate of 0% and an argon flow rate ratio of 50%.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The nitrogen-containing gas provides advantages even in concentrations as
low as 5% by volume or lower. The concentration of nitrogen-containing gas
in the etching gas is preferably in the range from about 10 volume percent
to about 55 volume percent. The optimum concentration of
nitrogen-containing gas depends to some degree upon the concentration of
additional gases in the gaseous mixture such as, for example, chlorine.
For example, a preferred concentration is about 30% nitrogen in a mixture
which also containing about 30% chlorine. This mixture achieves a high
selectivity, a high etching rate, and good anisotropy. In the absence of
significant amounts of chlorine gas in the mixture, a concentration of
nitrogen in the range of about 45% to about 55% is most preferred.
A more detailed description of the apparatus, materials, a method of
etching is set forth below.
Apparatus
Apparatus for carrying out the etching method according to the invention is
similar to those used in carrying out methods of the prior art, and
comprises a reaction vessel for accommodating a substrate, gas supply
means for supplying an etching gas to the reaction vessel,
plasma-producing means for forming a plasma from the etching gas
introduced into the reaction vessel and vacuum means for evacuating the
reaction vessel. The plasma-producing means comprises a pair of plate
electrodes arranged in opposition to each other in the reaction vessel and
a power source for applying electric voltage across the plate electrodes.
The application of the voltage to the electrodes causes an electric field
between the plate electrodes to produce a plasma from the etching gas
introduced into the reaction vessel.
Material for the Etching
Silicon substrates were used as substrates to be etched, and silicon
dioxide films, each formed with a circuit pattern were used as masks. As
shown in FIG. 1, a mask was provided on a surface of a silicon substrate
to be etched.
In etching, the silicon substrate was located on a side of one of the plate
electrodes (on a side of a high frequency power).
Etching Method
The etching method is similar to the prior art with the exception that the
composition of the etching gas is different from those of the prior art.
In other words, the silicon substrate covered by the mask as shown in FIG.
1 was arranged on one of the plate electrodes to which electric voltage
was applied, while an etching gas was introduced into the evacuated
reaction vessel to form plasma from the etching gas. In this manner, the
silicon substrate was etched by sputtering and chemical reaction of the
plasma.
Etching Gas
In a first embodiment, the gaseous mixture consists of silicon
tetrachloride (SiCl.sub.4) gas and nitrogen-containing gas (nitrogen gas
or nitrogen compound gas), while in a second embodiment, the gaseous
mixture consists of silicon tetrachloride (SiCl.sub.4), a
nitrogen-containing gas and chlorine (Cl.sub.2) gas. Concrete embodiments
of these two cases using the gaseous mixtures will be explained
hereinafter.
1. First embodiment
The etching performance on the substrate was studied with a variation of
the flow rate ratio of nitrogen (N.sub.2) in the gaseous mixture
consisting of silicon tetrachloride (SiCl.sub.4) gas and a
nitrogen-containing gas.
FIG. 2 illustrates a characteristic diagram showing the relationship
between etching speed and the nitrogen flow rate ratio. According to this
result, the etching speed is the maximum in the proximity of 30% of the
nitrogen flow rate ratio.
FIG. 3 illustrates the relationship between the nitrogen flow rate ratio
and selectivity (a ratio of the etching ratio of silicon to that of
silicon dioxide (SiO.sub.2)). In this case, the selectivity is also the
maximum in the proximity of 30% of the nitrogen flow rate ratio.
It is evident from the above results that when an etching gas consisting of
silicon tetrachloride (SiCl.sub.4) and a nitrogen-containing gas is used,
the etching speed becomes more than 1000 .ANG./min at 30% of the nitrogen
flow rate ratio and the selectivity to silicon dioxide of the mask is more
than 10 so that the ideal etching characteristics are obtained.
FIGS. 4 and 6 illustrate etched configurations for nitrogen flow rate
ratios of 5% and 60%, respectively. As can be seen from FIG. 4, side walls
of an etched groove 3 are curved when the nitrogen flow rate ratio is 5%.
As shown in FIG. 6, the etched groove is tapered when the nitrogen flow
rate is 60%.
When the nitrogen and chlorine flow rate ratios are each 30%, an ideal
anisotropic etched configuration is obtained as shown in FIG. 5.
Etched grooves formed in a substrate (wafer) were observed by a scanning
electron microscope to ascertain uniform depths of the grooves.
Experiments were carried out etching silicon wafers which were masked with
SiO.sub.2 using various gaseous mixtures. All conditions except the
composition of the gaseous mixture, such as pressure, net power, etch time
and distance between electrodes were maintained constant. The
cross-sectional shapes of the resulting etched grooves were established by
a scanning electron microscope and are as shown in FIGS. 9-13. The
composition of the gaseous mixture for each experiment is given in the
following Table:
TABLE
______________________________________
Corresponding
Gas Flow Rate Sccm
Run No.
FIG. SiCl.sub.4
N.sub.2
Ar % N Ar
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1 9 40 0 0 0 0
2 10 30 10 0 25 0
3 11 20 20 0 50 0
4 12 10 30 0 75 0
5 13 20 0 20 0 50
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A comparison of FIGS. 9, 11 and 13 show that while a 50% concentration of
nitrogen (FIG. 11) results in a highly anisotropic groove, a 50%
concentration of argon (FIG. 13) does not improve the anisotrophy over a
pure SiCl.sub.4 gas (FIG. 9) and appears to be deleterious.
2. Second embodiment
The mixture of gases consisting of silicon tetrachloride (SiCl.sub.4),
nitrogen gas and chlorine (Cl.sub.2) gas was used to etch silicon
substrates.
The chlorine gas served to increase the etching speed. FIG. 7 is a graph
illustrating the relationship between the etching speed and the chlorine
flow rate ratio. As seen from the graph shown in FIG. 7, the etching speed
is the maximum at 30% of the chlorine flow rate ratio.
FIG. 8 is a graph illustrating the relationship between the selectivity and
the chlorine flow rate ratio. In this case, likewise, the selectivity is
the maximum at 30% of the chlorine flow rate ratio. In the second
embodiment, the ideal etched configurations were obtained as shown in FIG.
5, when the nitrogen flow rate ratio and the chlorine flow rate ratio were
both 30%.
In this second embodiment, moreover, the etching speed could be increased
by adding chlorine gas to the etching gas. Under such circumstances,
superior etching characteristics were obtained such as etching speeds more
than 1500 .ANG./min and selectivity more than 15, while the ideal etched
configurations were obtained.
In the above first and second embodiments, no white powder from reaction
products occurred in etching, and therefore, very clear etched
configurations were obtained without any contamination of the silicon
substrates and insides of the reaction vessels. Moreover, there were no
defects such as unetched or insufficiently etched portions, uneven bottoms
of etched grooves, surfaces which appeared to be black and the like such
as would occur in the prior art. Moreover, residual etching gases had
little effect on the silicon substrates. In this manner, anisotropic
etched products which are superior in reproductivity were realized.
Although silicon tetrachloride (SiCl.sub.4) had been used as etching gases
in the above embodiments, it was only by way of example and other gaseous
chlorides of silicon, such as, for example, trichlorosilane (SiHCl.sub.3)
may be used for this purpose.
Either nitrogen gas or a gaseous nitrogen compound gas was used as the
nitrogen-containing gas in the above embodiment. In all cases using these
gases, respectively, the same good results were obtained. It was further
ascertained that the same good result was obtained by the use of a gaseous
mixture of nitrogen gas and a gaseous nitrogen compound as the
nitrogen-containing gas.
Moreover, although the reactive ion etching apparatus equipped with
opposite flat plate electrodes was used in the above embodiments, other
reactive ion etching systems may be used with the same good results. These
systems include hexagonal column electrode etching apparatus, an etching
system using electronic cyclotron resonance, a reactive ion beam etching
system and ion assist beam etching.
According to the present invention the etching gas consisting mainly of a
gaseous chloride of silicon and a nitrogen-containing gas is used to
obtain even etched surfaces without surface defects and unevenness and at
the same time prevent the formation of white powder thus avoiding silicon
substrates having unetched or insufficiently etched portions, or portions
which appeared to be black. Moreover, the etching method according to the
invention makes it possible to stably and easily achieve etching which is
superior in anisotropism by properly setting etching conditions.
It is further understood by those skilled in the art that the foregoing
description is that of preferred embodiments of the disclosed method and
that various changes and modifications may be made in the present
invention without departing from the spirit and scope thereof.
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