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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dry etching method applicable to the
production of semiconductor devices, and more particularly to a method of
selectively dry-etching silicon nitride layers and silicon oxide layers.
2. Description of the Related Art
A typical silicon semiconductor device has its layers insulated by a film
of silicon compound, especially silicon oxide (SiO.sub.x, where x is
typically 2). The SiO.sub.x interlayer insulating film undergoes dry
etching, which is a fully-developed technology that has been in use for
mass production since the advent of 64 k DRAM.
Dry etching resorts to an etching gas composed mainly of fluorocarbon
compound such as CHF.sub.3, CF.sub.4 /H.sub.2 mixture, CF.sub.4 /O.sub.2
mixture, and C.sub.2 F.sub.6 /CHF.sub.3 mixture, which offers the
following advantages.
(a) The fluorocarbon compound contains carbon atoms which form on the
surface of the SiO.sub.x layer, the C-O bond having a high interatomic
bond energy, thereby breaking or weakening the Si-O bond.
(b) The fluorocarbon compound forms CF.sub.x * radicals (where x is
typically 3), which are the major etchant for the SiO.sub.2 layer.
(c) The fluorocarbon compound provides good selectivity for the resist mask
and the underlying layer, with a minimum deposition of a carbon polymer,
if the C/F ratio in the etching reaction system is properly controlled.
(The underlying layer denotes silicon layers such as silicon substrate,
polysilicon layer, and polyside film.)
The silicon semiconductor device also has its layers insulated by a film of
silicon nitride (Si.sub.x N.sub.y, where especially x=3 and y=4). The
Si.sub.x N.sub.y layer also undergoes dry etching using an etchant which
has basically the same composition as that used for the SiO.sub.x layer.
While the etching of SiO.sub.x layers resorts to the ion-assisted
reaction, the etching of Si.sub.x N.sub.y layers resorts to the radical
reaction in which F* radicals play an important role. In addition, the
latter is faster than the former. This can be expected based on the
varying interatomic bond energies given below.
553 kcal/mol for the Si--F bond
465 kcal/mol for the Si--O bond
440 kcal/mol for the Si--N bond
(taken from "Handbook of Chemistry and Physics", 69th ed. (1988), edited by
R. C. Weast, published by CRC Press, Florida, U.S.)
The production of silicon semiconductor devices involves several steps for
the highly-selective etching of SiO.sub.x layers and Si.sub.x N.sub.y
layers. For example, the Si.sub.x N.sub.y layer on the SiO.sub.x layer
undergoes etching for the patterning to define the element separating
regions by the LOCOS (local oxidation of silicon) method. This etching
needs to have an especially high selectivity under the condition that the
pad oxide film (SiO.sub.2 layer) is made to be thin to minimize the bird's
beak length.
On the other hand, as a result of recent devices becoming smaller and more
complex than before, there has occurred an instance where it is necessary
to carry out selective etching for the SiO.sub.x layer on the Si.sub.x
N.sub.y layer as an etching stop layer to prevent etching damage. For
example, recent devices have a thin Si.sub.x N.sub.y layer formed on the
substrate surface to relieve the substrate from etching damage in the case
of over-etching. They also have a gate insulating film of ONO structure
(SiO.sub.x layer / Si.sub.x N.sub.y layer / SiO.sub.x layer), or they have
an Si.sub.x N.sub.y layer laminated on the surface of the gate electrode.
In these cases, it is necessary that the etching of the SiO.sub.x layer
stop with certainty when it reaches the surface of the Si.sub.x N.sub.y
layer.
For the highly-selective etching to be applied to layers formed on top of
the other, it is desirable that they differ in the interatomic bond energy
to some extent. Unfortunately, the SiO.sub.x layer and Si.sub.x N.sub.y
layer have an Si--O bond and Si--N bond, respectively, whose interatomic
bond energies are close to each other. Therefore, it is basically
difficult to perform highly-selective etching on them.
Attempts have been made to establish a technique for such selective
etching. There are some reports on the method of etching an Si.sub.x
N.sub.y layer on an SiO.sub.x layer. In fact, the present inventors have
disclosed in Japanese Patent Laid-open No. 142744/1986 a technique which
employs as an etching gas a mixture composed of a fluorocarbon gas (such
as CH.sub.2 F.sub.2 having a low C/F atomic ratio) and 30-70 mol% of
CO.sub.2. A fluorocarbon gas of low C/F ratio forms CF.sub.x..sup.+
(especially x=3) as an etchant for the SiO.sub.x only through
recombination of F*. If this system is supplied with a large amount of CO*
which captures F* and removes it in the form of COF, the formation of
CF.sub.x.sup.+ decreases and hence the etching rate for the SiO.sub.2
layer decreases. On the other hand, since the Si.sub.x N.sub.y layer
undergoes etching by F*, the etching rate for the Si.sub.x N.sub.y layer
remains almost unchanged even though the amount of CF.sub.x.sup. +
decreases due to the addition of copious amounts of CO.sub.2. The
consequence is the selectivity for the two layers.
Another etching technique is reported in "Proceedings of Symposium on Dry
Process", vol. 88, No. 7, pp. 86-94 (1987). This technique is
characterized by feeding a chemical dry-etching apparatus with NF.sub.3
and Cl.sub.2 and carrying out etching for the Si.sub.x N.sub.y layer on
the SiO.sub.x layer by utilizing FCl which is formed in the gas phase by
microwave discharge. The fact that 55% of the Si--O bond energy is ionic
whereas 30% of the Si--N bond energy is ionic suggests that the chemical
bond in the Si.sub.x N.sub.y layer is similar to the chemical bond
(covalent bond) in single-crystal silicon. Therefore, the Si.sub.x N.sub.y
layer is subject to etching by F* and Cl* radicals dissociated from FCl,
whereas the SiO.sub.x layer is immune to etching by these radicals. This
is the reason for the high selectivity.
As mentioned above, there are reports on several techniques of selective
etching of the Si.sub.x N.sub.y layer on the SiO.sub.x layer. These
techniques are a natural consequence of the fact that etching of the
Si.sub.x N.sub.y layer by radical reaction is necessarily decelerated as
it reaches the SiO.sub.x layer. A disadvantage of these conventional
techniques is that the process employing FCl (or the radical reaction)
involves inherent difficulties with anisotropic etching.
By contrast, only a few techniques have been disclosed on selective etching
of the SiO.sub.x layer on the Si.sub.x N.sub.y layer because it is more
difficult to establish a desired selectivity than in the case where the
two layers are reversed. The reason for this is that etching of the
SiO.sub.x layer by an ion-assisted reaction inevitably forms radicals in
the reaction system and these radicals accelerate the etching rate when
the underlying layer (or the Si.sub.x N.sub.y layer) is exposed.
Recent technical advancement has, however, achieved this object, namely by
use of a new plasma source which generates a high-density plasma with a
smaller amount of radicals. An example is reported in Proceedings of the
43rd Symposium on Semiconductors and Integrated Circuits Technology, p. 54
(1992). According to this report, etching is carried out by means of
induction coupled plasma (ICP) of C.sub.2 F.sub.6 (hexafluoroethane) gas
for the SiO.sub.x layer (formed by the TEOS-CVD process) on the Si.sub.3
N.sub.4 layer (formed by LP-CVD process), so as to make a connecting hole
which partly overlaps with the gate electrode. Etching in the reported
technique occurs presumably due to CF.sup.+ formed from C.sub.2 F.sub.6 by
dissociation in the high-density plasma. The high selectivity stems from
the fact that etching deposits a fluorocarbon polymer of a low C/F ratio
and the carbon atoms in the polymer combine more readily with the oxygen
atoms in SiO.sub.x than with the nitrogen atoms in Si.sub.x N.sub.y, with
the result that they are removed from the surface of the SiO.sub.x layer
but they accumulate on the surface of the Si.sub.x N.sub.y layer.
The foregoing technique seems to be promising but has the disadvantage that
it lacks a stable selectivity. For example, it is reported that the
selectivity is infinite for the flat part but is 20 or above for the
corner part. Presumably, the fluctuation in the selectivity is due to F*
radicals, resulting from an extremely dissociated C.sub.2 F.sub.6 .
SUMMARY OF THE INVENTION
The present invention was completed in view of the forgoing. Accordingly,
it is an object of the present invention to provide a dry-etching method
that can be applied to the SiO.sub.x layer with a stable, high selectivity
for the Si.sub.x N.sub.y layer.
The present invention is embodied in a dry-etching method which comprises
performing etching on an SiO.sub.x layer formed on an Si.sub.x N.sub.y
layer with a plasma of etching gas composed mainly of a fluorocarbon
compound represented by the formula of C.sub.x F.sub.y (where
y.ltoreq.x+2, x and y being natural numbers), said plasma being generated
in an etching apparatus capable of generating a plasma having an ion
density higher than 10.sup.11 /cm.sup.3.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a)-1(d) is a series of schematic sectional views showing how a
contact hole is made by the sequential steps of the method of the present
invention. FIG. 1(a) shows the step in which a resist mask is formed on an
interlayer insulating film of SiO.sub.2. FIG. 1(b) shows the step in which
etching of the interlayer insulating film of SiO.sub.2 ceases as it
reaches the underlying layer of Si.sub.x N.sub.y. FIG. 1(c) shows the step
in which the resist mask is removed by ashing. FIG. 1(d) shows the step in
which the underlying film of Si.sub.3 N.sub.4 in the contact hole is
selectively removed.
FIGS. 2(a)-2(b) is a series of schematic sectional views showing how a
self-aligned contact for SRAM is made by the sequential steps of the
method of the present invention. FIG. 2(a) shows the step in which an
interlayer insulating film of SiO.sub.2 is formed (which covers two gate
electrodes having thereon an etch stop layer of Si.sub.x N.sub.y) and a
resist mask is formed. FIG. 2(b) shows the step in which side walls are
formed and a contact hole is formed.
FIG. 3 is a schematic sectional view showing a contact hole for SRAM formed
by the self-alignment technique in Comparative Example. It is to be noted
that the gate electrode and the side wall are attacked owing to the low
selectivity for the Si.sub.x N.sub.Y etch stop layer.
DETAILED DESCRIPTION OF THE INVENTION
By the term, "high-density plasma" as it is used in the present invention
is meant a plasma in which collisions between electrons and gaseous atoms
take place more frequently than in a plasma of the conventional type which
is induced by glow discharge which occurs upon application of RF power
across two parallel flat electrodes or by microwave discharge resulting
from microwave supplied through a waveguide. By contrast, the high-density
plasma is generated by electron cyclotron resonance which is based on the
mutual action of the microwave and the magnetic field, or by the microwave
propagation mode in the magnetic field (which is referred to as the
whistler mode). The high-density plasma has a high ion density owing to a
high degree of dissociation of the gas.
Examples of the plasma having an ion density higher than 10.sup.11
/cm.sup.3 include ECR plasma, helicon wave plasma, inductively coupled
plasma (ICP), transformer coupled plasma (TCP), hollow anode type plasma,
and helical resonator plasma.
The above-mentioned fluorocarbon compound is an unsaturated compound as its
general formula suggests. It may have a linear or cyclic carbon skeleton.
With a large number of carbon atoms, a compound of linear structure
necessarily has successive multiple bonds or conjugated multiple bonds,
and a compound of cyclic structure has conjugated multiple bonds or takes
on the condensed ring structure, polycyclic structure, spiral structure,
or ring assembly structure.
Examples of the fluorocarbon compound that meets the requirements include
tetrafluoroethylene (C.sub.2 F.sub.4), hexafluorobutadiene (C.sub.4
F.sub.6), tetrafluorocyclopropene (c-C.sub.3 F.sub.4),
hexafluorocyclobutene (c-C.sub.4 F.sub.6), hexafluorobenzene (C.sub.6
F.sub.6), octafluorocycloheptatriene (c-C.sub.7 F.sub.8), and
octafluorobicyclo[2.2.1]heptadiene (C.sub.7 C.sub.8).
According to the present invention, a plasma of etching gas composed mainly
of fluorocarbon compound represented by the formula of C.sub.x F.sub.y
(where y.ltoreq.x+2, x and y are natural numbers) is generated in an
etching apparatus capable of generating a plasma having an ion density
higher than 10.sup.11 /cm.sup.3, and etching is performed on an SiO.sub.x
layer using a patterned Si.sub.x N.sub.y layer as a mask.
In a preferred embodiment of the present invention, the above-mentioned
fluorocarbon compound is hexafluorobenzene.
The etching gas of fluorocarbon compound, which is in the form of
high-density plasma having an ion density higher than 10.sup.11 /cm.sup.3,
contains a large amount of F.sub.x.sup.+ ions (where x is 1 in most cases)
because of dissociation which takes place more readily than in the case of
conventional RF plasma even under a low pressure. These ions take part in
the etching of the silicon compound layer at a practical etching rate.
The formation of CF.sub.x.sup.+ ions might be accompanied by the formation
of F* radicals. In such a case, the selectivity for the Si.sub.x N.sub.y
layer becomes poor. To avoid this, it is necessary to use a fluorocarbon
compound having a high C/F ratio (in which there are less fluorine atoms
relative to carbon atoms). The fluorocarbon compound used in the present
invention contains more carbon atoms than fluorine atoms by 2 at the most.
Therefore, it may be assumed that, in the simplest case, one fluorocarbon
molecule will form 2 F* radicals at the most when it forms x CF.sup.+
ions.
Since the present invention employs such a fluorocarbon compound for the
etching of the SiO.sub.x layer on the Si.sub.x N.sub.y layer, there is no
possibility that the Si.sub.x N.sub.y layer is exposed to F* radicals in
high concentration. This is the reason why a high selectivity is achieved
for the underlying Si.sub.x N.sub.y layer.
This principle holds true also in the case where a patterned Si.sub.x
N.sub.y layer is used as an etching mask for an underlying SiO.sub.x
layer. In this case, too, a high mask selectivity is attained.
In the present invention, it is recommended that hexafluorobenzene (C.sub.6
F.sub.6) be used as the above-mentioned fluorocarbon compound because of
its stability, availability, and high C/F ratio (which is 1). Attempts to
use C.sub.6 F.sub.6 for the etching of the SiO.sub.x layer have not been
successful because, when used alone, it gives rise to a large amount of
CF.sub.3.sup.+ and CF.sub.2.sup.+ ions which in turn form a polymer to
interfere with etching, as described in Japanese Patent Publication No.
60938/1989. According to the disclosure, this problem is solved by mixing
C.sub.6 F.sub.6 with an equal amount of CF.sub.4 to inhibit the formation
of carbon polymer.
In the present invention, there is no possibility of carbon polymer
excessively accumulating, because C.sub.6 F.sub.6 is dissociated into
CF.sup.+ ions in the high-density plasma. Even in the case where each atom
gives rise to 6 CF.sup.+ ions through the cleavage of all of the
carbon-carbon bonds, theoretically none of F* radicals is formed. This
leads to a high selectivity for the Si.sub.x N.sub.y layer. The fact that
highly selective etching can be achieved with a single gas in the present
invention is advantageous from the standpoint of stability and
controllability.
The invention will be described in more detail with reference to the
following examples.
EXAMPLE 1
This example demonstrates how a contact hole is made by etching an
SiO.sub.2 interlayer insulating film, with an Si.sub.3 N.sub.4 film
underlying, using C.sub.6 F.sub.6 gas and a magnetically enhanced
microwave plasma etching system.
On a silicon substrate (1), with an impurity diffusion region (2) therein,
formed were a 10-nm thick Si.sub.3 N.sub.4 underlying film (3) by LP-CVD
and subsequently a 1000-nm thick SiO.sub.2 interlayer insulating film (4)
by atmospheric CVD. The latter film was coated with a positive-type
novolak photoresist ("TSMR-V3" from Tokyo Ouka Kogyo Co., Ltd.), which
subsequently underwent ion-beam lithography and alkali development to be
made into a resist mask (5) having an opening (6) of 0.35 .mu.m in
diameter. (FIG. 1(a))
The wafer was placed on the wafer holder electrode in the
magnetically-enhanced microwave plasma etching system. Etching was
performed on the SiO.sub.2 interlayer insulating film (4) under the
following conditions.
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C.sub.6 F.sub.6 gas flow rate
20 SCCM
Gas pressure 0.65 Pa
Microwave power 1500 W (2.45 GHz)
RF bias power 200 W (800 kHz)
Temperature of wafer holding electrode
20.degree. C.
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The great microwave power is for ECR discharge which promotes the
dissociation of C.sub.6 F.sub.6 and hence produces a high-density plasma
having a high ion density in the order of 10.sup.11 /cm.sup.3. The etching
of the SiO.sub.2 interlayer insulating film (4) proceeded rapidly owing to
CF.sup.+ ions produced in large quantities in the high-density plasma. The
selectivity for the Si.sub.3 N.sub.4 underlying film (3) was as high as
about 30, because there no excess F* radicals existed in the plasma when
it was exposed. The selectivity for the resist mask (5) was also good.
As a result of etching, a contact hole (7) having an anisotropic shape was
obtained, as shown in FIG. 1(b). The Si.sub.3 N.sub.4 underlying film (3)
exposed at the bottom of the contact hole remained intact.
The etching caused a slight deposition of carbon polymer (not shown). This
carbon polymer contributed to the protection of the resist mask (5), the
side wall of the contact hole (7), and the exposed surface of the Si.sub.3
N.sub.4 underlying film (3). In the etching region of the SiO.sub.2
interlayer insulating film (4), the carbon polymer was eventually removed
by combustion by oxygen atoms sputtered out of that region. The deposition
of the carbon polymer was not so heavy as to interfere with etching,
unlike that encountered in the conventional method.
The wafer was transferred to a plasma ashing apparatus, in which ashing
with oxygen was carried out in the usual way to remove the resist mask
(5), as shown in FIG. 1(c). During ashing, the carbon polymer (not shown),
which contributed to the protection of surfaces and side walls, was also
removed.
Finally, the wafer was dipped in a hot aqueous solution of phosphoric acid
to decompose and remove the Si.sub.3 N.sub.4 underlying film (3) exposed
at the bottom of the contact hole (4a), as shown in FIG. 1(d).
Thus, the contact hole (7) having a good anisotropic shape was formed
without the possibility of causing particle contamination and damage to
the impurity diffusion region (2).
EXAMPLE 2
This example demonstrates how etching is performed on an SiO.sub.2
interlayer insulating film using C.sub.6 F.sub.6 gas and an ICP etching
system to make connection by the self-alignment technique between the gate
electrode and the memory node of a TFT as load for SRAM. The process will
be explained with reference to FIG. 2.
The wafer used for etching is constructed as shown in FIG. 2(a). It is
composed of a silicon substrate (11) and a gate oxide film (13) formed
thereon by surface oxidation. It has two gate electrodes (16) of a driver
transistor, with an Si.sub.3 N.sub.4 etching stop layer (17) patterned
thereon so as to protect them from the subsequent etching. The gate
electrode (16) is a polyside film composed of a polysilicon layer (14) and
a tungsten silicide (WSi.sub.x) layer (15) formed on top of the other. On
both sides of the gate electrode (16) are SiO.sub.2 side walls (18) which
were formed by the etching back process. In the silicon substrate (11) is
an impurity diffusion region (12) of the LDD structure which was formed by
performing ion implantation twice using the gate electrode (16) and side
wall (18) as the mask.
The wafer is entirely covered with an SiO.sub.2 interlayer insulating film
(19) formed by the CVD process. On the SiO.sub.2 interlayer insulating
film (19) is patterned a resist mask (20). The resist mask (20) partly
covers each of the electrodes (16) and has an opening (21) between the
electrodes (16). Etching on the SiO.sub.2 interlayer insulating film (19)
is performed in this opening (21) to make a contact hole reaching the
impurity diffusion region.
The wafer was placed in an ICP etching system, and etching was performed on
the SiO.sub.2 interlayer insulating film (19) under the following
conditions.
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C.sub.6 F.sub.6 gas flow rate
20 SCCM
Gas pressure 0.65 Pa
RF source power 2500 W (2 kHz)
RF bias power 50 W (1.8 MHz)
Temperature of wafer holding electrode
0.degree. C.
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In this process, etching on the SiO.sub.2 interlayer insulating film (19)
was by CF.sup.+ ions contained in the high-density plasma (having an ion
density of the order of 10.sup.12 /cm.sup.3) produced by the ICP etching
system. Thus, a contact hole (22) was formed, with the SiO.sub.2
interlayer insulating film (19) and the SiO.sub.2 gate film (13) partly
removed and the side wall (18) covered with another side wall (19a), as
shown in FIG. 2(b).
In the course of etching, the Si.sub.3 N.sub.4 etching stop layer (17) was
exposed; however, it was left unetched owing to the high selectivity
resulting from the fact that the plasma contains only a small amount of F*
radicals because of the high dissociation of C.sub.6 F.sub.6 (as in the
case of Example 1). Thus, it was possible to perform etching on the
greatly stepped SiO.sub.2 interlayer insulating film (19) without causing
damage to the gate electrode (16).
Comparative Example
This comparative example demonstrates how etching is performed to make a
contact hole for SRAM by the self-alignment technique using C.sub.2
F.sub.6 gas and an ICP etching system as in Example 2.
The same wafer as shown in FIG. 2(a) was set in the ICP etching system.
Etching on the SiO.sub.2 interlayer insulating film (19) was performed
under the following conditions.
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C.sub.2 F.sub.6 gas flow rate
20 SCCM
Gas pressure 0.65 Pa
RF source power 2500 W (2 kHz)
RF bias power 50 W (1.8 MHz)
Temperature of wafer holding electrode
0.degree. C.
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In this process, not only CF.sup.+ ions but also a large amount of F*
radicals were formed by the high dissociation of C.sub.2 F.sub.6. The F*
radicals reduced the selectivity for the Si.sub.3 N.sub.4 etch stop layer
(17) which had been exposed during etching As a result, the Si.sub.3
N.sub.4 etch stop layer (17b) and the gate electrode (16) thereunder were
partly attacked and the side walls (18b, 19b) took on a poor shape.
Although the invention has been described with reference to the foregoing
two examples, it is to be understood that the invention is not limited to
the specific embodiments thereof. For example, the ECR plasma and ICP used
as a high-density plasma in Examples may be replaced by a hollow
anode-type plasma (having an ion density of 10.sup.12 /cm.sup.3) or a
helicon wave plasma (having an ion density of 10.sup.12 -10.sup.13
/cm.sup.3). In addition, the fluorocarbon compound C.sub.x F.sub.y as an
etching gas is not limited to C.sub.6 F.sub.6. It is possible to use any
compound so long as it meets the requirement (y.ltoreq.x+2) and is stable
and capable of being readily introduced in the form of gas into the
etching chamber.
The dry-etching method of the present invention may also be applied to the
etch back of the SiO.sub.2 interlayer insulating film on the three-layered
gate insulating film of the ONO structure (SiO.sub.x /Si.sub.x N.sub.y
/SiO.sub.x). The etch back to form side walls on the gate electrode stops
as it reaches the Si.sub.x N.sub.y film in the middle of the gate
insulating film, owing to the high selectivity. It is possible, of course,
to modify the etching conditions and etching system as required.
The foregoing demonstrates that the dry-etching method of the present
invention can employ as an etching gas a fluorocarbon compound having a
high C/F ratio by highly dissociating it in a high-density plasma. (Such a
fluorocarbon compound has never been used because it forms a large amount
of carbon polymer.) In addition, the fluorocarbon compound provides high
selectivity for the SiO.sub.x layer and the Si.sub.x N.sub.y layer because
it does not form excess F* radicals when it undergoes dissociation by
discharge. Therefore, the present invention will be suitably applied to
the production of sophisticated, highly-integrated, high-performance
semiconductor devices, and hence it is of great industrial value.
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