 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a dry etching method and, more particularly, to a
method employed during etching for forming a connection hole in a silicon
compound insulating film on an Al based interconnection layer for
preventing the underlying Al-based interconnection layer from being
sputtered off and re-deposited on the sidewall surface of the connection
hole.
In a semiconductor device with a high integration degree and a high device
density, such as VLSIs or ULSIs of recent origin, the proportion of the
interconnection on a device chip has become higher. In order to prevent
the resulting increase in the chip size, a multilevel interconnection
process has become an indispensable technique. In the multilevel
interconnection process, it is necessary to bore a through-hole in an
interlayer insulating film between an upper interconnection layer and a
lower interconnection layer as a via-hole for establishing electrical
connection between the two layers.
As a material for the interlayer insulating film, a silicon oxide
(SiO.sub.x) based material is employed. The etching of an SiC.sub.x based
material layer is generally carried out under conditions of producing a
high incident ion energy for severing its strong Si-O bonds. That is, the
etching mechanism of the SiO.sub.2 material layer is comparable to a
physical process, such as sputtering, rather than a chemical process, such
as radical reaction.
Meanwhile, with an etching process accompanied by strong ion impact, the
problem of the lowering of underground selectivity is presented
inevitably. Above all, if a layer of the interconnection material
susceptible to sputtering, such as the layer of the Al-based material, is
present in the multilevel interconnection structure as an underlying layer
for the insulating film, the surface of the interconnection material layer
is sputtered and reduced in film thickness. Besides, products of the
sputtering tend to be re-deposited on the inner wall surface of the
via-hole to produce various problems.
The manner of the re-deposition in case the interconnection material layer
is an Al-based material is explained by referring to FIGS. 1A to 1C. FIG.
1A shows a resist mask 13 formed on an SiO.sub.2 interlayer insulating
film 12 deposited on an Al-based interconnection layer 11. An opening 14
is formed in the resist mask 13 in accordance with a hole pattern.
It is now assumed that the SiO.sub.2 interlayer insulating film 12 is
etched under this condition to form a via-hole 15. The etching is
performed in general under a higher incident energy condition and the
underlying Al-based interconnection layer 11 is a layer of a material
having a high sputtering rate. Consequently, the slightest overetching
results in the exposed surface of the Al-based Interconnection layer 11
being sputtered, with the sputtered product being deposited on the
sidewall surface of the via-hole 15 to form a re-deposited layer 16.
The re-deposited layer 16 is very difficult to remove and, even after
removing the resist pattern 14 by ashing, the layer 16 is left in a state
of being projected from an opening end of the via-hole 15, as shown in
FIG. 1C. If a wafer is observed from its upper surface with an electron
microscope, the re-deposited layer 16 looks like a royal crown, so that it
is termed an aluminum crown.
The re-deposited layer 16, if peeled off or destroyed only partially,
becomes a source of dust. Besides, if the layer 16 is protruded more or
less from the upper most surface of the interlayer insulating film 12, an
overlying layer tends to be affected in coverage to lower the yield of the
semiconductor device significantly.
For preventing the underlying layer from being sputtered off as described
above, a variety of methods have hitherto been proposed as
countermeasures. One of these methods is to adopt an operating condition
including a low self-bias potential Y.sub.dc, while another method
consists in adding a compound capable of etching the underlying layer of
the interconnection material during overetching to the etching gas. Still
another method consists in using a tapered cross-sectional shape of the
via-hole.
Of these, the method of using a tapered cross-sectional shape of the
via-hole is discussed in detail in Extended Abstract of 1990 Dry Process
Symposium, pages 105 to 109, title number V-3. The etching of the
SiO.sub.2 interlayer insulating film is performed using a CHF.sub.3 gas as
the wafer is cooled to a temperature of approximately -5.degree. C. In
other words, the etching proceeds as the effective mask width is
perpetually increased by the deposition of an excess carbonaceous polymer,
so that the via-hole presents an inclined sidewall surface. Since the
sidewall is inclined in this manner, it becomes possible for the ions to
be incident on the inclined surface, so that, even when the sputtering
product derived from the underlying Al-based interconnection layer is
re-deposited on the surface, it can be removed instantly. On the other
hand, since the particles of the sputtering product are incident on such
inclined surface at a small angle of incidence, the re-deposition itself
is hardly produced.
However, the above-mentioned countermeasures are not without problems.
First, the method of lowering the self-bias potential V.sub.dc consists in
lowering the incident ion energy to prevent incidental removal of the
underlying interconnection layer. However, with the dry etching of recent
origin, the prevalent concept is to achieve substantial anisotropy using a
low pressure discharge plasma. As compared to the ion density in the
conventional RF plasma, the ion density in the low pressure discharge
plasma tends to be decreased because the ion density is acutely lowered in
the RF plasma with decrease in the gas pressure. Consequently, this method
is not effective to achieve a practically useful etch rate or throughput
with the layer of the silicon compound for which an etching mechanism is
based essentially on an ion-assisted reaction. While it is possible to
accelerate the ions intentionally by increasing the input power or the
substrate bias, the substrate tends to be damaged by the high energies
afforded to the ions in this manner.
With the method of using a gas capable of etching the underlying
interconnection material layer during overetching, it is possible to
prevent the re-deposition. However, since the layer of the interconnection
material is removed simultaneously, the aspect ratio of the via-hole is
increased so that difficulties are raised in the subsequent plugging of
the via-hole. In extreme cases, the layer of the interconnection material
may be removed and eventually lost.
On the other hand, with the technique of providing a tapered
cross-sectional shape of the via-hole, an excess amount of the
carbonaceous polymer needs to be generated for achieving a significant
taper, so that there is the risk of the particle level becoming
undesirable. There is also raised another problem that the contact
resistance between the layer of the electrically conductive material
buried in the via-hole and the underlying layer of the interconnection
material is increased because the bottom surface of the via-hole becomes
narrower than the opening area in the mask.
Consequently, one has to make an extremely difficult selection of adopting
an anisotropic cross-sectional shape of the via-hole and of achieving a
practically useful etch rate while preventing wasteful etching,
re-deposition or damage done to the underlying interconnection layer.
For overcoming the above-mentioned difficulties, what may be demanded most
strongly of the low-pressure discharge plasma includes an improved
ionization ratio and controllability of the incidention energy.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome the
above-mentioned difficulties and to provide an etching method whereby a
layer of a silicon compound may be etched on an Al-based interconnection
layer with high anisotropy, high etch rate, high selectivity and low
damage.
With the dry etching method of the present invention, selective etching of
a silicon compound layer stacked on an Al-based interconnection layer is
carried out in an etching chamber capable of generating a plasma with an
ion density not less than 10.sup.11 ions/cm.sup.3 with the use of an
etching gas consisting mainly of a hydrocarbon compound under an incident
ion energy condition in which at least a part of a layer of a reaction
product produced on an exposed surface of the Al-based interconnection
layer is left.
Examples of the plasma having the ion density not less than 10.sup.11
ions/cm.sup.3 include an ECR plasma, helicon-wave plasma, inductive
coupling plasma (ICP), a hollow anode plasma and helical resonator plasma.
With the method of the present invention, the layer of the reaction product
is removed after the end of the etching with the use of an etching gas
containing a chlorine compound.
With the method of the present invention, after removing the layer of the
reaction product using the etching gas containing the chlorine compound as
described above, the residual chlorine is removed simultaneously with the
resist mask employed for the previous etching.
With the method of the present invention, ashing is carried out under high
vacuum in continuation to removal of the reaction product layer for
removing the residual chlorine simultaneously with the resist mask
employed for the previous etching.
Also, with the method of the present invention, plasma processing is
carried out under high vacuum in continuation to removal of the reaction
product layer using a gas containing a hydrogen atom containing compound
in its molecule for removing the residual chlorine as the chlorine is
reacted with hydrogen-based chemical species.
For generating the plasma, the collision between electrons and gas atoms is
indispensable. The high density plasma employed in the present invention
is such a plasma in which contrivances have been made to increase the
number of times of such collision as compared to the plasma known
heretofore. The plasma known heretofore is excited by applying an RF power
across parallel flat electrode plates for producing glow discharge or by
supplying the micro-wave to a wave guide for generating microwave
discharge. With the high density plasma, gas dissociation is promoted to
achieve a high ion density by taking advantage of electron cyclotron
resonance based on the interaction between the micro-wave electric field
and the magnetic field or a micro-wave propagation mode in the magnetic
field known as the Whistler mode.
With the high density plasma employed in the present invention, it is
necessary for the incident ion energy to be controllable. To this end, it
is desirable that the high density plasma be a so-called remote plasma,
that is a plasma of the type in which plasma generation by the electric
discharge and the control of the incident ion energy may be performed
independently of each other.
If the high density plasma with an ion density of not less than 10.sup.11
ions/cm.sup.3 is generated using an etching gas consisting mainly of the
fluorocarbon compound, dissociation of the fluorocarbon compound proceeds
more outstandingly than in the conventional RF plasma, even under a
reduced pressure, such that a large quantity of CF.sub.x.sup.+, mainly
with x=1, are generated highly efficiently. The layer of the silicon
compound is etched at a practically useful rate by being assisted by the
abundant supply of these ions.
If the underlying Al-based interconnection layer is exposed with the
progress of etching, the etching is terminated at this time point, due to
coating of the exposed surface of the Al-based interconnection layer with
a layer of a low vapor pressure reaction product containing at least
AlF.sub.x, with typically x=3, as has been confirmed experimentally.
Besides, since the incident ion energy is optimized with the present
method for minimizing the sputtering rate of the reaction product layer,
the latter may be used as a surface protection film for the underlying
Al-based interconnection layer. Consequently, even when collectively
forming a number of connection holes of different depths, the underlying
Al-based interconnection layer may be protected from excess overetching
even for the connection holes of shallow depths, thus preventing the
occurrences of so-called aluminum crowns.
Meanwhile, the reaction product layer may be easily removed in the form of
AlCl.sub.x by employing an etching gas containing a chlorine compound.
However, if the chlorine compound is employed, chlorine is necessarily left
in the reaction system after removal of the reaction layer. Such residual
chlorine tends to cause the after-corrosion of the Al-based
interconnection layer when the wafer is contacted with the moisture on
being exposed to atmospheric air. Consequently, it is necessary to remove
the residual chlorine by a process consecutive to the etching without
allowing the as-etched wafer to be exposed to atmosphere.
For removing the residual chlorine, the present invention provides two
methods, namely the method of ashing the resist mask employed as an
etching mask, and the method of plasma processing by a compound containing
a hydrogen atom in its molecule.
With the ashing method, the resist pattern containing a large quantity of
occluded chlorine is removed for significantly reducing the amount of the
residual chlorine on the wafer. On the other hand, with the plasma
processing method, hydrogen-based chemical species, such as H*, generated
from the compound containing a hydrogen atom in its molecule, are reacted
with the residual chlorine, so that the residual chlorine is removed
promptly in the form of hydrogen chloride (HCl).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a cross-sectional view for illustrating the problem met in the
formation of a via-hole and showing the state in which an SiO.sub.2
interlayer insulating film and a resist mask are formed step by step on an
Al-based interconnection layer.
FIG. 1B is a cross-sectional view showing the state in which the surface of
the Al-based interconnection layer is sputtered during overetching to form
a re-deposited layer.
FIG. 1C is a cross-sectional view showing the state in which the
re-deposited layer is left after removal of the resist pattern.
FIGS. 2A to 2D are cross-sectional views showing the dry etching method of
the present invention, as applied to the formation of the via-hole, step
by step, where FIG. 2A shows the state in which the SiO.sub.2 interlayer
insulating film and the resist mask are formed step by step on an Al-1%Si
layer, FIG. 2B shows the state in which just-etching of the SiO.sub.2
interlayer insulating film has come to an end, FIG. 2C shows the state in
which a reaction product layer has been formed on an exposed surface of
the Al-1%Si layer, and FIG. 2D shows the state in which chlorine is left
as the reactive layer is removed.
FIG. 3 is a schematic cross-sectional view showing the state in which the
resist mask and residual chlorine have been removed from the state shown
in FIG. 2D.
FIG. 4 is a schematic cross-sectional view showing the state in which the
residual chlorine has been removed by plasma processing employing a
compound containing a hydrogen atom.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be hereinafter explained with reference to
several illustrative Examples.
EXAMPLE 1
The present Example is directed to a process of forming a via-hole in an
interlayer insulating film on an Al-1% Si layer using a magnetic
micro-wave plasma etching device. In the above process, two-step etching
of the interlayer insulating film is carried out using a c-C.sub.4 F.sub.8
/CH.sub.2 F.sub.2 mixed gas system and a c-C.sub.4 F.sub.8 sole gas
system. Subsequently, a layer of a reaction product formed on an exposed
surface of the Al-1% Si layer is removed by plasma processing employing
Cl.sub.2, and a resist ashing -residual chlorine removing step is carried
out in an in-line ashing device connected under high vacuum to the
micro-wave plasma etching device. The process is hereinafter explained by
referring to FIGS. 2A to 2D and FIG. 3.
FIG. 2A shows a wafer employed as an etching sample in the present Example
1. This wafer comprises an Al-1% Si layer 1, an SiO.sub.2 interlayer
insulating film 2 thereon, having a thickness of approximately 0.6 .mu.m,
and a resist mask 3 thereon patterned to a pre-set shape. The resist mask
3 has a thickness of approximately 1.0 .mu.m, while an opening 4, formed
in accordance with a hole pattern, is approximately 0.4 .mu.m in diameter.
The above-described wafer was set on a magnetic micro-wave plasma etching
device to perform two-stage etching of the SiO.sub.2 interlayer insulating
film 2. The two-stage etching means an etching method in which the etching
process is divided into a just-etching partial process until substantial
exposure of an underlying layer and an overetching partial process until
complete exposure of the underlying layer, with the etching conditions
being switched between these two partial processes.
As an example, the SiO.sub.2 interlayer insulating film 2 was just-etched
under the following conditions.
______________________________________
c-C.sub.4 F.sub.8 flow rate:
20 SCCM
CH.sub.2 F.sub.2 flow rate:
10 SCCM
gas pressure: 0.25 Pa
micro-wave power: 1200 W (2.45 GHz)
RF bias power 300 W (800 kHz)
electrode temperature
-50.degree. C. (alcoholic
cooling medium was
used)
______________________________________
It is noted that c-C.sub.4 F.sub.8 is a fluorocarbon compound having a
higher C/F ratio of the molecule (ratio of the number of C atoms to that
of F atoms) and capable of yielding a large amount of CF.sub.x.sup.+,
mainly with x=2, when allowed to stand in a conventional magnetron RIE
device having a plasma ion density on the order of 10.sup.10
ions/cm.sup.3. However with the magnetic micro-wave plasma device employed
in the present Example, gas dissociation proceeds further such that a
high-density ECR plasma is formed, the ion density of which is on the
order of 10.sup.11 ions/cm.sup.3.
However, the predominant chemical species generated in the ECR plasma are
CF.sup.+ dissociated further from CF.sub.x.sup.+, with the amount of F*
being increased concomitantly. Thus, CH.sub.2 F.sub.2 is added to the
reaction system with a view to supplying H* to the plasma for capturing
excess F*. Besides CH.sub.2 F.sub.2 tends to deposit a carbonaceous
polymer. Consequently, with the above gas system, resist selectivity is
improved based on the carbonaceous polymer deposition effect and the F*
decreasing effect.
With the above-described just-etching system, the RF power density is set
to a lower value so that the incident ion energy is weakened to a
necessary minimum value. However, the etching proceeded anisotropically
and at a practically useful rate by CF.sup.+ yielded at a high density.
The just-etching was terminated directly before exposure of the underlying
Al-1%Si layer 1, as shown in FIG. 2B. Thus a via-hole was formed only
halfway.
Then, for removing a residual portion 2a of the interlayer insulating film
2, an overetching was performed under the following typical conditions:
______________________________________
c-C.sub.4 F.sub.8 flow rate:
30 SCCM
gas pressure: 0.25 Pa
micro-wave power: 1200 W (2.45 GHz)
RF bias power: 220 W (800 kHz)
electrode temperature:
-50.degree. C. (alcoholic
cooling medium was
used)
______________________________________
Since CH.sub.2 F.sub.2 is eliminated from the gas composition during the
overetching process for lowering the C/F ratio in the etching reaction
system, the amount of F* yielded in the plasma was increased as compared
with that for the just-etching process. Thus, when the Al-1% Si layer is
exposed on the bottom surface of the via-hole 5, as shown in FIG. 2C, a
layer of the reaction product 6 was generated quickly on the exposed
surface. This layer of the reaction product 6 exhibited high resistance
against attack by ions or radicals because of the low vapor pressure and
the low sputtering rate under the above-mentioned etching conditions.
Consequently, the surface of the Al-1% Si layer 1 was effectively
protected during the overetching.
However, the layer of the reaction product 6, if left as it is, tends to
increase the contact pressure. Thus the layer 6 was removed under the
following typical conditions:
______________________________________
Cl.sub.2 low rate: 100 SCCM
gas pressure: 2.0 Pa
micro-wave power: 1200 W (2.45 GHz)
RF bias power: 90 W (800 kHz)
electrode temperature
-10.degree. C. (alcoholic
cooling medium was
used)
______________________________________
By this etching, the layer of the reaction product 6 was removed, as shown
in FIG. 2D. Since chlorine-based chemical species are used at this time as
an etchant, there is the possibility of the Al-1% Si layer 1 being removed
in a minor quantity in the form of AlCl.sub.x. However, since this etching
was carried out for a shorter time under conditions of extremely weak
incident ion energies, the subsequent process of plugging the via-hole 5
was not affected significantly. Rather, the merit of preventing the
formation of Al crowns by the presence of chlorine-based chemical species
is outstanding.
Meanwhile, chlorine was left on the wafer surface on termination of
etching.
The wafer was then transferred to an in-line ashing device, connected to
the magnetic micro-wave plasma etching device via a vacuum load-lock
device, for ashing the resist mask 3 under the following typical
conditions:
______________________________________
O.sub.2 flow rate: 100 SCCM
gas pressure: 5.0 Pa
RF bias power: 0 W
ashing time: 120 sec
______________________________________
In the present Example, the wafer was not opened to atmosphere on
termination of etching of the SiO.sub.2 interlayer insulating film 2. Thus
the wafer was transferred into the ashing device with substantially no
water adsorbed on its surface. The major portion of the residual chlorine
on the wafer was removed with the removal of the resist mask 3.
After the ashing, the wafer was tentatively allowed to stand in atmospheric
air. After 72 hours, no post-corrosion was observed to be produced.
EXAMPLE 2
In the present Example, an SiO.sub.2 interlayer insulating film was etched
in one step for formation of a via-hole, using c-C.sub.4 F.sub.8. After
the layer of a reaction product formed on an exposed surface of the Al-1%
Si layer was etched off using a BCl.sub.3 /Cl.sub.2 mixed gas, residual
chlorine was removed by plasma processing using an H.sub.2 gas. This
process is explained by referring to FIGS. 2A, 2C, 2D and 4.
First, a wafer shown in FIG. 2A was set on a magnetic micro-wave plasma
etching device for etching an SiO.sub.2 interlayer insulating film 2 under
the following typical conditions:
______________________________________
c-C.sub.4 F.sub.8 low rate:
30 SCCM
gas pressure: 0.25 Pa
micro-wave power: 1200 W (2.45 GHz)
RF bias power: 250 W (800 kHz)
electrode temperature:
-10.degree. C. (alcoholic
cooling medium was
used)
______________________________________
Since CH.sub.2 F.sub.2 is not added to the gas system, the amount of F*
yielded in the etching reaction system is more than that for Example 1,
such that the layer of the reaction product 6 could be formed as soon as
the Al-1% Si layer 1 was exposed. However, since the etching is carried
put in one step, the RF power was set to a lower value than in Example 1
for assuring practically useful selectivity. By this etching, the via-hole
5 having an anisotropic shape as shown in FIG. 2C was formed, and the
layer of the reaction product 6 was formed on its bottom surface.
Then, for removing the layer of the reaction product 6, etching was
performed under the following typical conditions:
______________________________________
BCl.sub.3 flow rate:
100 SCCM
Cl.sub.2 flow rate: 50 SCCM
gas pressure: 2.0 Pa
micro-wave power: 1200 W (2.45 GHz)
RF bias power: 50 W (800 kHz)
electrode temperature:
-10.degree. C. (alcoholic
cooling medium was
used)
______________________________________
By this etching, the layer of the reaction product 6 was removed quickly by
the contribution of the chlorine-based chemical species. Meanwhile, the
above gas composition is a composition widely known as an etching gas for
the Al-based material layer. Even when a native oxide film is produced on
the surface of the Al-1% Si layer 1 under the effect of the residual
oxygen, such native oxide film may be quickly removed based on the
reduction effect provided by BCl.sub.3.
Then, for removing residual oxygen generated with the removal of the layer
of the reaction product 6, plasma processing was carried out under the
following typical conditions:
______________________________________
H.sub.2 flow rate: 30 SCCM
gas pressure: 0.25 Pa
micro-wave power: 1200 W (2.45 GHz)
RF bias power: 20 W (800 kHz)
electrode temperature:
-10.degree. C. (alcoholic
cooling medium was
used)
plasma processing time
20 sec
______________________________________
With the plasma processing process, residual chlorine was removed in the
form of hydrogen chloride HCl by H* yielded from H.sub.2. The wafer, when
left to stand in atmosphere for 72 hours, was not susceptible to
after-corrosion.
Although the present invention has been described with reference to two
Examples, the present invention is not limited to these merely
illustrative Examples.
As for the Al-1% Si layer, employed as the Al-based interconnection layer
in the above Examples, an antireflection film, such as TiON film, may be
employed in conjunction with the Al-based interconnection layer, in
consideration that such anti reflection film is used in a majority of
cases in connection with the Al-based interconnection layer for improving
processing accuracy in photolithography.
Although the interlayer insulating film composed of SiO.sub.2 is given as
an illustrative example, an interlayer insulating film composed e.g. of
PSG, BSG, BPSG, AsSG, AsPSG or AsBSG may similarly be etched.
The chlorine-based compounds of Cl.sub.2 or BCl.sub.3, employed for etching
the layer of the reaction product, may be replaced by HCl, as an example.
As the hydrogen atom containing compound, employed for removing residual
chlorine, NH.sub.3 or various hydrocarbons, such as CH.sub.4, may also be
employed besides H.sub.2 mentioned above.
Although the ECR plasma is employed as the high-density plasma in the
above-described embodiments, the ion density on the order of 10.sup.12
/cm.sup.3 has been reported with a hollow anode type plasma, while the ion
density on the order of 10.sup.12 to 10.sup.13 ions/cm.sup.3 has similarly
been reported for a helicon wave plasma or TCP. Any of these plasmas may
be employed for the present invention.
Besides, the etching devices employed, etching conditions, sample wafer
composition or the plasma processing conditions may also be changed, if so
desired.
* * * * *
|
|
 |
|
 |
Description |
|
