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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a processing apparatus for treating a solid
surface using ions and, more particularly, to an apparatus for etching a
solid surface by the use of ions, radical atoms and/or radical molecules
in a plasma.
Recently, dry etching with employs an ion beam or ions, radical atoms and
radical molecules in a plasma has been increasingly utilized in technical
fields, such as the field of semiconductors, which require a very fine
etching technique of several microns or less. As compared with the former
method employing an ion beam, the latter method employing ions in a plasma
has a higher ion density and has the merit of a higher etching rate, so
that various studies have been made thereon. Among them, an expedient in
which the plasma is generated by a microwave discharge is especially
advantageous for several reasons, e.g. (1) since a discharge gas can break
down even under a low pressure [below 1 .times. 10.sup.-3 Torr], the
directions of ions become regular, (2) a plasma of especially high density
can be generated, and (3) owing to an electrodeless discharge, the life of
the apparatus is long, a chemical-active gas being usable. The details of
this expedient are described in Japanese patent laid open specification
No. 51-71597.
FIG. 1 shows the construction of a prior-art etching apparatus exploiting
the microwave discharge. Microwaves produced by a microwave generator 1
are radiated into a rectangular waveguide 2. At a position of about 1/4 of
a guide wavelength (.lambda..sub.g) from the terminal end of the
rectangular waveguide 2, one end of an inner conductor 4 of a coaxial
waveguide 3 protrudes into the waveguide 2, as shown in the figure.
Through an antenna thus constructed, microwaves are propagated from the
waveguide 2 to the coaxial waveguide 3. Further, an electric field of
microwaves is propagated into a discharge area 5 through an insulator 7 as
well as a device 6 for the coupling between the microwaves and the plasma.
Shown at 9 is magnetic field supplying means, which produces an external
magnetic field perpendicular to the aforecited microwave electric field.
The magnetic field gives rise to a microwave discharge jointly with the
microwave electric field. In general, the magnetic field to be produced by
the magnetic field supplying means is a mirror type magnetic field which
contains the plasma effectively. Numeral 8 designates a gas supply
conduit, from which a gas such as N.sub.2, Ar and O.sub.2 under 1 .times.
10.sup.-3 to 10.sup.-5 Torr is introduced. The gas is ionized by the
microwave discharge. Shown at 12 is a substrate to be etched, which is
maintained at a predetermined potential by a power source 13 or is
grounded. Owing to such a construction, an ion sheath is formed between
the substrate 12 and the plasma generated by the microwave discharge in
the magnetic field. The ions of the introduced gas pass through the ion
sheath, and impinge on the substrate 12 to etch it.
However, when the coaxial waveguide is employed as in this construction,
the inner conductor of the coaxial waveguide is held by the insulator and
is therefore thermally isolated from the other parts, so that the inner
conductor easily reaches a high temperature and is subject to thermal
destruction at the connecting portion with the insulator. Moreover, in
plasma etching apparatus the homogeneity of the plasma density
distribution in the radial direction must be good. In the construction
employing the coaxial waveguide, however, a part underneath the inner
conductor or the microwave - plasma coupling device and a part surrounding
the part exhibit large difference in plasma density, so that uniform
etching cannot be carried out.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved plasma etching
apparatus.
Another object of the invention is to provide a plasma etching apparatus
employing a microwave discharge which has solved the problems of the prior
art stated above.
These and other objects are accomplished by a plasma etching apparatus
having a microwave generator, a waveguide for propagating microwaves, a
discharge area, coupling means to introduce the microwave power into the
discharge area, magnetic field supplying means to supply an external
magnetic field to the discharge area, and a gas supply conduit for
introducing a discharge gas into the discharge area therefrom, whereby a
substrate surface is processed by ions, radical atoms and radical
molecules in a plasma generated in the discharge area, the plasma etching
apparatus comprising the improvement wherein a round waveguide is employed
as the microwave coupling means, the discharge area being provided within
the round waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatical sectional view of a prior-art etching apparatus;
FIG. 2 is a diagrammatical sectional view of an embodiment of the etching
apparatus according to the invention;
FIG. 3 is a graph for explaining effects of the etching apparatus according
to the invention; and
FIGS. 4 and 5 are diagrammatical sectional views of further embodiments of
the etching apparatus according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 2 is a view showing an embodiment of the invention. Microwaves
generated by a microwave generator 1 are radiated into a rectangular
waveguide 2. The microwaves are propagated into a round waveguide 11 which
is so connected that its axis lies at a position of 1/4 of the guide
wavelength from the terminal end of the rectangular waveguide 2. In order
to ensure a good propagation efficiency from the retangular waveguide 2 to
the round waveguide 11, the end of the waveguide 2 is cut obliquely
relative to the axis of the waveguide 11. The round waveguide 11 is
divided into a first section connected to waveguide 2 and a second section
forming a discharge area 5 by an insulator 7. Microwave power is
propagated into the discharge area 5 through the insulator 7. Numeral 8
designates a gas supply conduit, numeral 9 magnetic field supplying means,
and numeral 12 a substrate to be etched. The substrate 12 is grounded as
shown in the figure, or it is maintained at a predetermined potential. As
is apparent from the present embodiment, the invention employs the round
waveguide as the microwave introducing or coupling means. Unlike the
prior-art example, therefore, a thermally weak portion such as the inner
conductor of the coaxial waveguide is not included. Owing to this fact,
the apparatus is not destroyed even when high-power microwaves are
introduced. Further, owing to the absence of the inner conductor, the
concentration of electric fields does not occur, so that the homogeneity
of the density distribution of a plasma in the radial direction is
enhanced and that the uniformity of the etching of the substrate 12 is
enhanced.
FIG. 3 illustrates an experimental result obtained by employing the plasma
etching apparatus shown in FIG. 2. In FIG. 3, the abscissa represents the
distance from the center of the substrate, while the ordinate represents
the etched depth. Curve a indicates, for comparison's sake, the
performance of the prior-art apparatus employing the coaxial waveguide as
the microwave coupling means, and curve b indicates the performance of the
apparatus of the invention employing the round waveguide. In the
experiment, microwave power at 2.45 GHz and 200 Watts was introduced, the
introduced gas was Ar under 5 .times. 10.sup.-4 Torr, the substrate
voltage was -500 V, the intensity of the magnetic field was 1600 Gauss,
and the etching period was 1 minute. As is apparent from the Figure, the
diameter at which the etched depth becomes 90% of the central value is
approximately 20mm with the round waveguide, and approximately 10mm with
the coaxial waveguide, so that the uniformity is enhanced by a factor of
two or more. Although not shown in the drawing, when the coaxial waveguide
was employed, the etched proportion was lower at the center of the
substrate than at the surrounding part under some conditions. On the
average, the uniformity of the etching in the invention was enhanced by a
factor of approximately two over the prior art.
FIG. 4 shows another embodiment of the invention. The propagation of
microwaves up to the round waveguide is the same as in the embodiment of
FIG. 2. A discharge tube 10 made of an insulator such as quartz glass is
disposed within the round waveguide, and is filled with a discharge gas
(Ar, O.sub.2, N.sub.2, etc. are selected according to the purpose of use)
so as to produce a plasma. The function of the insulator tube 10 is to
prevent the contamination of the substrate 12 by impurities. That is, the
plasma produced by the microwave discharge is prevented from sputtering
the inner wall of the round waveguide 11, and substances such as the metal
constituting the inner wall are prevented from mingling into the
substrate. This combination of round waveguide and insulator tube is more
effective than the combination of the coaxial waveguide and the insulator
tube. In the latter case, when the introduced discharge gas is a gas such
as CF.sub.4 gas, which is liable to produce a chemically-active gas when
decomposed, the insulator tube around the inner conductor heated to a high
temperature and the chemically-active gas component produce local
reactions, and the insulator tube is destroyed. In actuality, therefore,
etching with CF.sub.4 or the like would be impossible. On the other hand,
in the case of the present embodiment, even when CF.sub.4 or the like is
employed, the insulator tube is not destroyed because no part is locally
heated. Accordingly, reactive gas etching employing a chemically-active
gas such as CF.sub.4, CCl.sub.4, and BCl.sub.3 is also possible.
FIG. 5 shows still another embodiment of the invention. A cylinder 10 which
is made of quartz glass containing very few impurities and which easily
propagates microwaves is placed in the round waveguide 11 so that the
microwaves may be propagated in a vacuum. A magnetic field, preferably a
mirror type static magnetic field, is superposed in a direction orthogonal
to the high-frequency electric field by the magnetic field coil 9, so that
electrons in the vacuum acquire energy from the microwave electric field
and conduct a spiral motion. The electrons collide with gaseous molecules
introduced from the gas supply conduit 8 and are ionized. Thus, a plasma
is produced. According to the present embodiment, where a magnetic field
of 1600 Gauss was applied with microwaves of 2.45 GHz, a plasma having an
electron temperature of about 6 eV and an electron density of about
10.sup.11 /cm.sup.3 was obtained with argon gas under 10.sup.-4 Torr. Even
when CF.sub.4 gas was used as the active gas, a plasma at substantially
the same degree was obtained. When the potential of the plasma is zero, an
object inserted in the plasma assumes a floating potential. The floating
potential V.sub.f is given approximately by:
V.sub.f = (kT.sub.e /e) ln .sqroot.m/M
where
k: Boltzmann's constant,
T.sub.e : plasma electron temperature,
e: electron charge,
m: mass of electron, and
M: ionic mass.
Here, the electron temperature of the plasma is about 6 eV by actual
measurement. Therefore, supposing F.sup.+ ions are the gas ions, the
floating potential becomes:
V.sub.f .apprxeq. -22V
that is, even when the substrate in the plasma is not intentionally given
an applied voltage, it acquires a potential of about -20 V, and the ions
are accelerated to about 20 eV and impinge onto the substrate. The current
density is one to two orders or more higher than in the case of the prior
art, and the processing speed does not decrease. According to the present
embodiment, where a silicon single crystal was etched with the CF.sub.4
gas without applying any voltage to the substrate, it could be processed
in about 0.1 .mu.m per minute, and where a negative potential of 500 V was
applied to the substrate with respect to the plasma potential, the
substrate could be processed at about 1 .mu.m per minute. In these cases,
the etching rate of photoresist employed for a mask exhibited a value one
order or more less than that of silicon, and the processed profile was
better than by the conventional RF (radio frequency) plasma etching. The
contamination of the substrate attributed to impurities was the same as in
the case of the plasma etching apparatus having its periphery covered with
quartz glass.
In this manner, there are obtained the benefits when a gas plasma highly
reactive to the material to be processed is generated in the magnetic
field and electric field and that the discharge area is provided in the
container which employs the insulator for the peripheral wall so as to
prevent the impurities from mingling due to the collision of the plasma,
the substrate to-be-processed being located in the discharge area.
While the invention has been described above with reference to the
arrangements illustrated in the drawings, the waveguide for propagating
microwaves to the round waveguide need not be rectangular, but a waveguide
of a different type, for example, a coaxial waveguide or a round waveguide
may be employed.
The coupling between the waveguide for propagating microwaves and the round
waveguide need not be orthogonal to each other as illustrated in the
drawings, but any shape for the coupling may be adopted insofar as the
microwaves are propagated.
Furthermore, since the insulator tube or cylinder needs only propagate the
microwaves and hold a vacuum, the material is not restricted to quartz
glass, but aluminous procelain etc, can also be used.
Furthermore, the outer wall of the discharge area 5 in the apparatus shown
in FIG. 2 may be coated with any of the insulators mentioned above, and
such apparatus achieves similar effects.
When etching a substrate with an introduced gas such as Ar, a predetermined
potential must be applied to the substrate, but when performing chemical
etching with a chemically-active gas such as CF.sub.4, the substrate may
be maintained at the floating potential.
As the active gas, C.sub.2 F.sub.6, C.sub.3 F.sub.8, CCl.sub.4, CF.sub.2
Cl.sub.2, or BCl.sub.3 may be used in addition to CF.sub.4.
The apparatus of the present invention described above in detail is free
from thermal or electrical destruction and has a good homogeneity of
etching as a plasma etching apparatus employing the microwave discharge,
and it is very effective in industry for surface processing and surface
treatment of semiconductors, etc.
While we have shown and described several embodiments in accordance with
the present invention, it is understood that the same is not limited
thereto but is susceptible to numerous changes and modifications as known
to a person skilled in the art, and we therefore do not wish to be limited
to the details shown and described herein but intend to cover all such
changes and modifications as are obvious to one of ordinary skill in the
art.
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