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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plasma processing or treatment apparatus
for an object such as a semiconductor wafer.
2. Related Art
A plasma processing or treatment apparatus is configured such that a plasma
is generated by vacuum-charging the interior of a processing vessel in
which exists a processing gas, and that plasma is used to implement a
predetermined treatment on an object such as a semiconductor wafer. In the
prior art, such plasma treatment apparatuses are widely used throughout
the semiconductor fabrication process, in steps such as in sputtering,
ashing, CVD, and etching.
In a typical plasma treatment apparatus, a pair of horizontal, flat
electrodes that face each other act as the plasma generation source. In
this type of apparatus, the relationship of the electrode configuration
means that a comparatively high gas pressure of, for example, several
hundreds of mmTorr is used, and thus problems can occur such as ions
within the plasma impacting against the electrodes and causing sputtering
of the electrodes, which will lead to impurities being generated from the
electrodes. For that reason, this type of apparatus is difficult to adapt
to cope with recent ultra-fine processing techniques at a half-micron and
greater. That is why various plasma treatment apparatuses are currently
being developed to generate high-density plasmas under hard vacuum
conditions. Examples of these plasma treatment apparatuses include a
magnetron style that uses orthogonal electromagnetic fields, a
radio-frequency inductively coupled style that uses radio-frequency
electromagnetic energy, and, most recently, an apparatus that uses
helicon, or helical waves generated from electromagnetic waves propagated
parallel to a magnetic field.
A radio-frequency inductively coupled type of plasma treatment apparatus is
provided with: a processing vessel that has a gas supply portion for
supplying a processing gas and a gas exhaust portion for exhausting the
gas after the treatment, and which maintains a predetermined degree of
vacuum therein; a plasma generation means arranged on an upper surface of
the processing vessel; and a holder device that holds a semiconductor
wafer on which a predetermined treatment is to be performed in a plasma
produced by the plasma generation means. The processing vessel is provided
with a main body formed from an electrically conductive material such as
stainless steel, with an upper portion formed from an insulating material
that is transparent to radio-frequency waves, such as quartz, mounted on
an upper end of the main body. Each of the above mentioned gas supply
portion and gas exhaust portion is attached to the peripheral surface of
the main body. The plasma generation means is provided with, for example,
a one-turn antenna and a radio-frequency power source connected to this
antenna by a matching circuit, with the configuration being such that
radio-frequency electric power from the radio-frequency power source is
supplied to the antenna through the matching circuit, radio-frequency
waves are generated within the processing vessel from the antenna, and the
process gas is activated by this electromagnetic energy to cause the
generation of a plasma. The holder device is configured such that another
radio-frequency power source is connected thereto with a blocking
capacitor therebetween, radio-frequency electric power is supplied from
this radio-frequency power source, and a self-bias potential is maintained
with respect to the plasma's potential, via the blocking capacitor. When
the above mentioned plasma treatment apparatus is used to physically etch
a semiconductor wafer using argon, the pressure of the argon within the
processing vessel is first adjusted to 10 mmTorr, with the semiconductor
wafer held on the holder device. Next, when predetermined electric power
from the radio-frequency power source is supplied to the antenna via the
matching circuit, radio-frequency waves are generated within the
processing vessel from the antenna, this electromagnetic energy excites
the argon to form a plasma, and thus a high-density plasma of, for
example, the order of 10.sup.11 cm.sup.-3 is generated. At the same time,
since 100-kHz radio-frequency electric power is supplied through the
blocking capacitor from the radio-frequency power source in the holder
device, an ion sheath is formed between the holder device and the plasma,
and argon ions from the plasma impact against the semiconductor wafer to
perform the predetermined etching.
The previously mentioned prior art helical wave plasma treatment apparatus
is provided with: a processing vessel having a gas supply means for
supplying a process gas and a gas exhaust means for exhausting gases after
the treatment, and is maintained at a predetermined degree of vacuum; a
helical wave plasma generation means that surrounds an application portion
forming part of the processing vessel; and a holder device for holding a
semiconductor wafer that is to be subjected to a predetermined treatment
by the helical wave plasma generated from the process gas by the action of
the helical wave plasma generation means. This application portion is
formed of an insulating material that is transparent to electromagnetic
waves, such as quartz, and the above described main body connected to a
lower end aperture of the application portion is formed of an electrically
conductive material such as stainless steel. The helical wave plasma
generation means is also provided with an antenna surrounding the outer
peripheral surface of the application portion and an electromagnetic coil
also surrounding the application portion, but further out from the
antenna. The configuration is such that electromagnetic waves from the
antenna propagate parallel to the magnetic field shaped by the
electromagnetic coil; the electromagnetic waves are subjected to the
action of the magnetic field as they propagate through the plasma, and
thus generate helical waves; and a high-density helicon wave plasma is
generated by these helical waves.
When this helicon wave plasma treatment apparatus is being used to
physically etch a semiconductor wafer using argon, argon is first supplied
from the gas supply means into the processing vessel and its pressure is
adjusted to 10 mmTorr, with the semiconductor wafer held on the holder
device. In this state, radio-frequency electric power is applied to the
antenna, the generation of electromagnetic waves proceeding from the
antenna in the axial direction within the processing vessel causes
electrons to absorb energy from these electromagnetic waves, these
electrons collide with the argon gas, ionizable energy is imparted to the
argon atoms, and this generates the high-density plasma. At the same time,
a magnetic field is shaped parallel to the direction of progress of the
electromagnetic waves within the processing vessel by the electromagnetic
coil. Thus, low-frequency helical waves from the electromagnetic waves are
propagated through the plasma by the action of this magnetic field, and,
during this time, the electrons within the plasm are accelerated to
increase the density of the plasma. At the same time, radio-frequency
electric power is supplied via the blocking capacitor by the
radio-frequency power source of the holder device, so that the holder
device becomes self-biased as described above, and the potential
difference with respect to the plasma's potential causes argon ions to
impact against the semiconductor wafer so that the semiconductor wafer is
etched thereby. Since this plasma treatment can be implemented by causing
the generation of helical waves within even a comparatively weak magnetic
field, it is possible to soften the effects of the magnetic field in
comparison with electronic cyclotron resonance (ECR) plasma treatment that
uses a strong magnetic field.
A high-density plasma of the order of 10.sup.11 cm.sup.-3 can be obtained
with the above described prior art plasma treatment apparatus that uses
inductively coupled radio-frequency waves, but if an attempt is made to
increase the power supplied to the antenna to further increase the density
of the plasma (for example, if the supplied power is increased to 600 W),
the plasma density does become approximately uniform within a central
portion within a radius of 50 mm from the center of the processing vessel,
but as the power is increased further to 800 W and then 1 kW to increase
the plasma density even further, as will be shown later in a graph, the
plasma density in the central portion drops, there is an increasing
tendency for the plasma density to increase with distance from the center
in a certain direction, and thus it becomes more difficult to obtain an
uniform high-density plasma as the plasma density increases, raising
concern that it is becoming more difficult to implement uniform plasma
treatment. This tendency becomes more and more obvious as the diameters of
semiconductor wafers increase, and it is becoming a big problem concerning
plasma treatment as diameters become increasingly larger in the future.
If the plasma densities obtained by plasma treatment apparatuses are up to
the order of 10.sup.11 cm.sup.-3, a self-bias potential of several tens of
volts to several hundred volts can be obtained by just the application of
power of several tens of Watts to the holder device, but when it comes to
the above described higher plasma densities of the order or 10.sup.11
cm.sup.-3 or higher, conventional radio-frequency bias circuitry cannot
achieve a self-bias potential of about several tens of watts, even if
power is supplied at several hundred watts, so that concern has been
raised that it has become impossible to implement such plasma treatment.
Further, when a prior art helical wave plasma treatment apparatus is used,
plasma treatment can be implemented even with an ECR plasma in a weak
magnetic field. However, if, for example, the magnetic field is further
weakened in order to lessen the effects of the magnetic field, etching
rates gradually decrease with distance from the antenna, etching at the
central portion of the semiconductor wafer is delayed the most, and thus a
problem occurs in that it becomes difficult to implement uniform etching
(plasma treatment) over the entire surface. In general, trends such as the
recent increase in diameter of semiconductor wafers are becoming more
obvious, and film thicknesses have also become even thinner. Therefore it
is becoming impossible to use a prior art helicon wave plasma treatment
apparatus to treat objects under weak magnetic field conditions.
If etching of a semiconductor wafer is done using a high-density plasma
with a prior art helical wave plasma treatment apparatus, there is a
problem in that only the peripheral parts of the semiconductor wafer that
are outside a high-density region of the plasma are etched, virtually no
etching occurs in the central portion within this region, and also there
is a problem that the etching rate is not constant over all positions on
of the semiconductor wafer.
SUMMARY OF THE INVENTION
The present invention was devised in order to solve the above described
problems and has as its object the provision of a plasma treatment
apparatus that creates a uniform high-density plasma wherein the plasma
density does not drop at the central portion of the processing vessel, to
enable uniform, rapid processing of a large-diameter object.
A further object of the present invention is to provide a plasma treatment
apparatus that causes the generation of helical waves under weak magnetic
field conditions, to create a uniform high-density plasma throughout and
thus enable uniform, rapid processing of a large-diameter object.
A yet further object of the present invention is to provide a plasma
treatment apparatus that can sufficiently ensure and maintain a self-bias
potential at a low level of supplied power, even in a high-density plasma
on the order of 10.sup.11 cm.sup.-3.
A still further object of the present invention is to provide a plasma
treatment apparatus that can perform uniform, rapid treatment throughout,
even for the treatment of a large-diameter object, by using a helical wave
plasma.
To achieve the above objectives, the present invention provides a plasma
treatment apparatus comprising: a processing vessel capable of being
maintained at a predetermined degree of vacuum; gas supply means for
supplying a process gas into the processing vessel; gas exhaust means for
exhausting gases from within the processing vessel after treatment is
completed; plasma generation means provided in the processing vessel, to
generate radio-frequency waves within the processing vessel to cause
generation of a plasma in the process gas; and holder means provided in
the processing vessel to hold an object to be subjected to a predetermined
treatment by the plasma. The gas supply means comprises: a plurality of
first gas supply means arranged equidistantly in the peripheral direction
around the processing vessel; and second gas supply means arranged at the
center of an upper surface of the processing vessel. The gas exhaust means
comprises a plurality of gas exhaust means arranged equidistantly in the
peripheral direction around the processing vessel.
In this plasma treatment apparatus, the holder means is provided with a
capacitor that maintains a bias potential, and this capacitor has a
capacitance that is at least 10 times that of an ion sheath formed in the
vicinity of the holder means.
The present invention also provides a plasma treatment apparatus
comprising: a processing vessel capable of being maintained at a
predetermined degree of vacuum; gas supply means for supplying a process
gas into the processing vessel; gas exhaust means for exhausting gases
from within the processing vessel after treatment is completed; an
application portion provided in an upper portion of the processing vessel;
helical wave plasma generation means provided surrounding the application
portion and causing the generation of a helical wave plasma within the
processing vessel; holder means provided within the processing vessel to
hold an object to be subjected to a predetermined treatment by the helical
wave plasma; and second plasma generation means provided above the
application portion, to generate radio-frequency waves toward the interior
of the application portion and thus increase the density of the plasma.
The present invention still further provides a plasma treatment apparatus
comprising: a processing vessel capable of being maintained at a
predetermined degree of vacuum; gas supply means for supplying a process
gas into the processing vessel; gas exhaust means for exhausting gases
from within the processing vessel after treatment is completed; holder
means for holding an object in the treatment vessel; and helical plasma
generation means provided in a side portion of the processing vessel in
such a manner as to cause a helical wave plasma to be generated within the
process gas, in a flat plate-shaped region interposed within a gap with
respect to an object held on the holder means. The helical plasma
generation means comprises: an electromagnetic wave generation means for
generating electromagnetic waves parallel to the object on the holder
means; and a rectangular magnetic field shaping means for shaping a
rectangular magnetic field in the same direction as the electromagnetic
waves from the electromagnetic wave generation means, to cause the
generation of a helical wave plasma from the electromagnetic waves.
Preferred embodiments of the present invention will be described in more
detail below, with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a radio-frequency inductively coupled type of
plasma treatment apparatus in accordance with one embodiment of the
present invention;
FIG. 2 is a vertical section through the plasma treatment apparatus of FIG.
1;
FIG. 3 is a graph evaluating the effects on plasma density within the
processing vessel of using different methods to supply the process gas
from a first gas supply portion of the plasma treatment apparatus of FIG.
1;
FIG. 4 is a graph of the relationship between the electric power applied to
the holder device of the plasma treatment apparatus of FIG. 1 and the
self-bias potential;
FIG. 5 is a vertical section through a helical wave plasma treatment
apparatus of another embodiment of the present invention;
FIG. 6 is a graph comparing etching rates of the plasma treatment apparatus
of FIG. 5 and a prior art plasma treatment apparatus of the same type;
FIG. 7 is a sectional view of a central portion of a plasma treatment
apparatus of a parallel flat-electrode configuration;
FIG. 8 is a partial cross-sectional view of an upper electrode shown in
FIG. 7, with the right half in section;
FIG. 9 is a graph showing the relationship between position in the radial
direction on a semiconductor wafer and electron density in the plasma at
that position, when the power applied to the antenna of a prior art
radio-frequency inductively coupled type of plasma treatment apparatus is
varied;
FIG. 10 is a graph of the relationship between electric power applied to a
holder device and self-bias potential, in a prior art plasma treatment
apparatus;
FIG. 11 is a vertical sectional view through a helical wave plasma
treatment apparatus in accordance with a third embodiment of the present
invention; and
FIG. 12 is a graph of the relationship between distance from the center of
the semiconductor wafer and etching rate during processing using the
plasma treatment apparatus of FIG. 11.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of a radio-frequency inductively coupled type of plasma
treatment apparatus in accordance with the present invention is shown in
FIGS. 1 and 2. As shown in the plan view of FIG. 1, this plasma treatment
apparatus has a processing vessel 3, with first gas supply pipes 1A, 1B,
1C, and 1D for supplying a process gas G into the processing vessel 3
being provided in a radial fashion at constant peripheral spacing at a
plurality of places (four in the example shown in these figures) around
the outer periphery of the processing vessel 3. As can be seen from FIG.
2, the first gas supply pipes 1A to 1D are provided toward the upper end
of the processing vessel 3. Gas exhaust structures, e.g. pipes 2 for
exhausting gases from the interior of the processing vessel 3 are also
provided at constant peripheral spacing at a plurality of places (four in
the example shown in these figures) around the outer periphery of the
processing vessel 3. As shown in FIG. 2, the gas exhaust pipes 2 are
provided toward the lower end of the processing vessel 3, and the
positions of the gas exhaust pipes 2 in the peripheral direction around
the processing vessel 3 are such that each one is between neighboring
first gas supply pipes 1A, 1B, 1C, and 1D, as shown in FIG. 1.
A plasma generation device 4 for generating radio-frequency within the
processing vessel 3 to cause the generation of a high-density plasma in
the process gas G is provided above the processing vessel 3. A known
holder device 5 for holding on the upper surface thereof a semiconductor
wafer W as an object to be treated is provided in the interior of the
processing vessel 3. The holder device 5 is connected to a radio-frequency
power source 7 with a blocking capacitor 6 therebetween, in such a manner
that a radio-frequency voltage is applied to the holder device 5. A
coolant supply pipe 8A that supplies a coolant into the interior of the
holder device 5 to cool it in a known manner is connected to the holder
device 5, and a coolant exhaust pipe 8B for exhausting the coolant after
this cooling is also provided.
The plasma generation device 4 has a one-turn loop antenna 4A of a
configuration such that it generates radio-frequency waves in the process
gas G within the processing vessel 3 by the action of a matching circuit
9B and a radio-frequency power source 9C of known types that are connected
to the antenna, and the electromagnetic energy of these radio-frequency
waves turn the process gas G into a plasma to generate a high-density
plasma of the order of 10.sup.11 cm.sup.-3. All of the first gas supply
pipes 1A to 1D, gas exhaust pipes 2, processing vessel 3, and holder
device 5 are formed of an electrically conductive material such as
stainless steel, a central portion 3A of an upper surface of the
processing vessel 3 at which the loop antenna 4A is arranged is formed of
an insulating material such as quartz or alumina, and a peripheral portion
3B thereof is formed of the same electrically conductive material as the
processing vessel 3.
In addition to the first gas supply pipes 1A to 1D, a second gas supply
pipe 1E is arranged at a place in the center of the upper surface of the
processing vessel 3 as another gas supply means, so that process gas G can
be supplied uniformly within the processing vessel 3 by this second gas
supply pipe 1E and the radially arranged first gas supply pipes 1A to 1D.
The gas exhaust pipes 2 are arranged equidistantly along the periphery of
the lower peripheral surface of the processing vessel 3, so that the
product gases within the processing vessel 3 after the treatment can be
exhausted through all these gas exhaust pipes 2 to the exterior without
any unevenness.
The present inventors measured the distribution of electron density Ne
generated within the processing vessel 3 when the process gas G was
supplied from the gas supply pipes 1A to 1E by different methods. The
results of these measurements are shown in FIG. 3. For example, a
distribution curve A in this figure was obtained as the results of
supplying process gas G by opening only the first gas supply pipe 1A,
leaving all of the other first gas supply pipes 1B to 1D and the second
gas supply pipe 1E closed, then measuring the electron density Ne in the
diametric direction across the processing vessel 3 from the first gas
supply pipe 1C to the first gas supply pipe 1A. It can be seen from curve
A that the electron density Ne is at a maximum in the vicinity of the
first gas supply pipe 1A and gradually decreases with distance from the
first gas supply pipe 1A.
The first gas supply pipe 1B was then opened alone, process gas G was
supplied with all of the other first gas supply pipes and the second gas
supply pipe 1E being closed, and a distribution curve B was obtained as
the results of measuring the electron density Ne in the diametric
direction across the processing vessel 3 from the first gas supply pipe 1B
to the first gas supply pipe 1D. It can be seen from curve B that the
electron density Ne is at a maximum in the vicinity of the first gas
supply pipe 1B and gradually decreases with distance from the first gas
supply pipe 1B, in a similar manner to that described above.
The second gas supply pipe 1E alone was then closed, with all of the first
gas supply pipes 1A to 1D being open, and a distribution curve C was
obtained as the results of measuring the electron density Ne in the
diametric direction across the processing vessel 3 from the first gas
supply pipe 1B to the first gas supply pipe 1D. It can be seen from curve
C that the electron density Ne is somewhat lower at the periphery than at
the central portion of the processing vessel 3, so that a uniform electron
density Ne can not be obtained by simply supplying the process gas G
radially from positions that are equidistantly spaced around the
processing vessel 3.
Next, the process gas G was supplied from all of the gas supply pipes 1A to
1E and a distribution curve D was obtained thereby, from which it can be
seen that the electron density Ne within the processing vessel 3 can be
made approximately uniform. From the above results, it is clear that
supplying the process gas G uniformly from the periphery and upper surface
of the processing vessel 3 enables a uniform electron density Ne within
the processing vessel 3, which means that a uniform high-density plasma
can be achieved. In contrast, if the supply of process gas G is
unbalanced, as it is in the radio-frequency inductively-coupled plasma
treatment apparatus of the prior art, even if various techniques are
devoted to the plasma generation means, the electron density Ne will be
lower at the central portion, and rise toward the left side as seen in
FIG. 9, so it is clear that there is a limit to the uniformity of the
electron density.
The present invention provides uniform supply of the process gas G from
around the periphery of the processing vessel 3 by the first gas supply
pipes 1A to 1D, and also provides a downward supply of the process gas G
from the center of the top surface of the processing vessel 3 by the
second gas supply pipe 1E. Therefore, the supply of process gas G from
above can compensate in portions where the process gas G supplied from the
first gas supply pipes 1A to 1D cannot scatter sufficiently, the
concentration of the process gas G within the processing vessel 3 can be
rapidly made even, this enables the achievement of a uniform high-density
plasma within the processing vessel 3, and thus treatment can be performed
over the entire surface of the semiconductor wafer both uniformly and
rapidly. In accordance with this embodiment of the present invention,
since a one-turn loop antenna 4A is used as the plasma generation device
4, the second gas supply pipe 1E can be mounted in the central portion
thereof so that uniform plasma treatment can be implemented without any
drop in the plasma density at the center, even with a semiconductor wafer
W of a larger diameter as described above.
With the previously described radio-frequency inductively-coupled plasma
treatment apparatus of the prior art, a high-density plasma of the order
of 10.sup.11 cm.sup.-3 can be obtained, as shown in FIG. 9, but if an
attempt is made to obtain an even denser plasma by increasing the power
supplied to the antenna 14A (for example, by increasing the power supplied
further to 600 W), the plasma density will still be substantially uniform
in a central portion within a radius of 50 mm from the center of the
processing vessel, as shown in FIG. 9. However, as the supplied power is
increased further to 800 W or 1 kW, it can be seen from this graph that
the plasma density at the central portion will drop as the plasma density
increases, and also the plasma in the leftward portion will tend to
increase in particular, so there is the problem that it becomes more
difficult to obtain a uniform high-density plasma as the density
increases, and thus it is difficult to implement uniform plasma treatment.
This tendency becomes more obvious as the diameters of the semiconductor
wafers increase, which is making it a big problem for plasma treatment of
even larger diameters in the future.
If the plasma density obtained by the plasma treatment apparatus is up to
the order of 10.sup.11 cm.sup.-3, the application of several tens of watts
of electric power to the holder device 5 will achieve a self-bias
potential -Vdc of several tens of volts to several hundred volts, but when
the plasma density reaches the above described order of 10.sup.11
cm.sup.31 3, a prior art radio-frequency bias circuit would only be able
to achieve a self-bias potential -Vdc of about several tens of volts, even
if power of several hundred watts is supplied, and thus there is the
problem that plasma treatment becomes impossible. The relationship between
supplied power and self-bias potential -Vdc is shown graphically in FIG.
10, and this relationship between the two factors will be described below
with reference to this figure. It can be seen from this figure that, if
the plasma density is of the order of 10.sup.11 cm.sup.-3, it becomes
increasingly difficult to achieve a self-bias potential at the holder
device 5, even if the power supplied to the holder device 5 is increased,
and when it exceeds 6.6.times.10.sup.11 cm.sup.-3, virtually no self-bias
potential is created which is a problem in that treatment of the
semiconductor wafer W cannot be done even if a high-density plasma could
be obtained by some means. Note that the data shown in FIG. 10 refers to a
self-bias potential -Vdc when different electric powers are supplied at
100 kHz when the capacitance of the blocking capacitor 6 is 3000 pF. This
self-bias potential -Vdc was measured between the holder device 5 and the
blocking capacitor 6.
To solve the above described problem, the capacitance of the blocking
capacitor 6 is set to at least 10 times that of the ion sheath between the
holder device 5 and the plasma, and is preferably set to at least 50 times
that value, even more preferably to at least 100 times. A capacitance of
less than 10 times that value raises concern in that it might not be
possible to create the self-bias potential -Vdc necessary for plasma
treatment, even with a large power for generating the high-density plasma
of, for example, 10.sup.11 cm.sup.-3. The relationship between supplied
power (at 100 kHz) and self-bias potential -Vdc when the plasma density is
varied as a parameter between 1.8 and 6.6.times.10.sup.11 cm.sup.-3, using
a 25 000-pF blocking capacitor 6 is shown in FIG. 4. It is clear from this
figure that a sufficient self-bias potential -Vdc can be obtained at a low
power of several tens of watts when the plasma density is
1.8.times.10.sup.11 cm.sup.-3. As a result of calculating the capacitance
of the ion sheath, we have determined that capacitance to be 637 pF.
Therefore, the capacitance 25,000 pF of the blocking capacitor 6 is
approximately 40 times that of the ion sheath. The capacitance of an ion
sheath can be calculated using the following equation for obtaining the
thickness of an ion sheath when general-purpose flat-plate electrodes are
used:
0.97=(yp-1/2).sup.3/2
where: yp=eVp/.kappa.Te and .eta.=d/.lambda.D, Vp is the electrode
potential measured in the negative direction and based on the plasma
potential, and .lambda.D=(E.kappa.Te/Ne e.sup.2).sup.1/2 is the length of
the device. Note that the capacitance of a prior art blocking capacitor is
usually about 3000 pF.
If, for example, the above equation is used to calculate the capacitance of
an ion sheath in argon for an electrode area (equivalent to the area of
the upper surface of the holder device 5) of 314 cm.sup.2 (for 8" wafers),
a value of 3.85 nF is obtained. Note that the electron temperature Te is 3
eV. In this case, if the capacitance of the blocking capacitor 6 is set to
100 times that of the ion sheath, 385 nF, a sufficient self-bias potential
can be ensured and maintained, even when the plasma density is of the
order of 10.sup.11 cm.sup.-3.
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