U.S. patent application number 09/909872 was filed with the patent office on 2001-11-08 for plasma processing apparatus and method.
Invention is credited to Arai, Masatsugu, Doi, Akira, Edamura, Manabu, Kanai, Saburo, Kazumi, Hideyuki, Maeda, Kenji, Nishio, Ryoji, Tetsuka, Tsutomu, Tsubone, Tsunehiko, Yoshioka, Ken.
Application Number | 20010037861 09/909872 |
Document ID | / |
Family ID | 18070782 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010037861 |
Kind Code |
A1 |
Kazumi, Hideyuki ; et
al. |
November 8, 2001 |
Plasma processing apparatus and method
Abstract
A plasma is generated by feeding an antenna with radio-frequency
electric power generated by a radio-frequency power source, and one
end of the antenna is grounded to the earth through a capacitor of
variable capacitance. A Faraday shield is electrically isolated
from the earth, and the capacitance of the variable capacitor is
determined to be such a value that the voltage at the two ends of
the antenna may be equal in absolute values and inverted to reduce
the partial removal of the wall after the plasma ignition. At the
time of igniting the plasma, the capacitance of the capacitor is
adjusted to a larger or smaller value than that minimizing the
damage of the wall.
Inventors: |
Kazumi, Hideyuki;
(Hitachi-shi, JP) ; Tetsuka, Tsutomu;
(Ibaraki-ken, JP) ; Nishio, Ryoji; (Mito-shi,
JP) ; Arai, Masatsugu; (Kudamatsu-shi, JP) ;
Yoshioka, Ken; (Hikari-shi, JP) ; Tsubone,
Tsunehiko; (Hikari-shi, JP) ; Doi, Akira;
(Ibaraki-ken, JP) ; Edamura, Manabu; (Ibaraki-ken,
JP) ; Maeda, Kenji; (Matsudo-shi, JP) ; Kanai,
Saburo; (Hikari-shi, JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
18070782 |
Appl. No.: |
09/909872 |
Filed: |
July 23, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09909872 |
Jul 23, 2001 |
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09732307 |
Dec 8, 2000 |
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09732307 |
Dec 8, 2000 |
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08979949 |
Nov 26, 1997 |
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6180019 |
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Current U.S.
Class: |
156/345.48 ;
118/723I |
Current CPC
Class: |
H01J 37/32009 20130101;
H01J 37/321 20130101; H01J 37/32174 20130101 |
Class at
Publication: |
156/345 ;
118/723.00I |
International
Class: |
H01L 021/3065 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 1996 |
JP |
JPA 08-315885 |
Claims
What is claimed is:
1. A plasma processing apparatus comprising: an antenna for
generating an electric field in a plasma generating portion; a
radio-frequency power source for supplying radio-frequency electric
power to said antenna; a vacuum chamber enclosing the plasma
generating portion to establish a vacuum therein; a Faraday shield
provided around said plasma generating portion; a gas supply unit
for supplying gas into said vacuum chamber; a sample stage on which
an object to be processed is placed; and a radio-frequency power
source for applying a radio-frequency electric field to said sample
stage, such that a plasma is generated by accelerating electrons
and ionizing the gas by collision with the electric field generated
by said antenna, so as to process said object, wherein the vacuum
chamber has an upper face and a lower face, and wherein the upper
face of said vacuum chamber has a smaller area than that of the
lower face, and the upper face is flat.
2. A plasma processing apparatus according to claim 1, wherein the
upper face of the vacuum chamber is circular in plan view, the
vacuum chamber has side faces joining the upper and lower faces,
and an angle contained between the side faces joining the lower
face and the upper face and the normal to the upper face is not
less than 5 degrees.
3. A plasma processing apparatus according to claim 1, wherein a
ratio of (a) a distance from the object to be processed to the
upper face, to (b) a radius of the upper face of the vacuum
chamber, is not more than 1.
4. A plasma processing apparatus according to claim 1, wherein a
magnetic field generating means is provided outside the vacuum
chamber.
5. A plasma processing apparatus according to claim 1, wherein a
plate made of a conductor or a semiconductor is placed on an inner
side of the upper face of the vacuum chamber.
6. A plasma processing apparatus according to claim 5, wherein a
radio-frequency power source is applied to said plate so as to
apply radio-frequency waves to said plate.
7. A plasma processing apparatus according to claim 5, wherein a DC
voltage source is applied to said plate so as to supply DC voltage
to said plate.
8. A plasma processing apparatus according to claim 5, wherein said
plate is grounded.
9. A plasma processing apparatus according to claim 1, wherein the
upper face of the vacuum chamber is flat, and wherein the upper
face of said vacuum chamber has a smaller area than that of the
lower face.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a plasma processing
apparatus for surface treatment to etch a substrate or to form a
thin film with a plasma by supplying a radio-frequency electric
field to an antenna, generating an electric field, and thereby
generating a plasma by the electric field, and a method of using
this apparatus. More particularly, the invention relates to a
semiconductor processing apparatus for processing a semiconductor
device, and a method of using this apparatus.
[0002] In a semiconductor processing apparatus for generating a
plasma by induction by feeding an electric current to a coil-shaped
antenna, there is a problem that a vacuum chamber wall made of an
non-conductive material and enclosing a plasma generating unit so
as to establish a vacuum atmosphere is partly removed by the
plasma. In order to solve this problem, there has been conceived a
method using a field called the "Faraday shield", as disclosed in
Japanese Patent Laid-Open No. 502971/1993. If the Faraday shield is
used, however, the plasma ignitability is so deteriorated that the
plasma is not ignited unless a voltage as high as tens of KV is
applied to the feeding portion of the coil-shaped antenna. This
apparatus may fail with a high possibility by the discharge between
the antenna and a conductive structure nearby. In order to prevent
this discharge, an additional structure is needed to insulate the
antenna from the existing structure, causing the apparatus to be
complicated.
[0003] When a Faraday shield is used to reduce the partial removal
of the wall, foreign matters are liable to adhere to the wall and
to appear if its sticking rate to the wall from the plasma is
accelerated. Therefore the partial removal of the wall must be
adjusted according to the process.
[0004] The plasma density distribution is determined mainly by the
generation rate distribution and by the state of transportation of
ions and electrons. In the absence of an external magnetic field,
the transportation of the plasma diffuses isotropically in every
direction. At this time, electrons instantly escape and tend to
reach the wall of the vacuum chamber because the mass is no more
than {fraction (1/1,000)} of that of an ion, but they are repelled
by the sheath (ion sheath) formed in the vicinity of the wall. As a
result, a quasi-neutral condition of the electron and ion densities
is always met in the plasma, so that both the ions and electrons
are bipolarly diffused toward the wall. At this time, the potential
of the plasma takes on its maximum where the plasma density, i.e.,
the ion density, is the maximum. This potential is termed the
plasma potential Vp, approximately expressed by
Vp.apprxeq.Te.times.ln(mi/me), where Te, mi and me are the electron
temperature, the mass of an ion, and the mass of an electron,
respectively. In the plasma, the potential distribution is
determined by the potential Vp and the wall potential (ordinally at
0 V), so that the density distribution is correspondingly
determined. Since, in this case, the plasma is confined by the
electrostatic field established by itself, the density distribution
is determined by the shape of the apparatus, the place where the
induced electric field takes on the maximum, and the ratio of the
generation ratio/the bipolar diffusion flux.
[0005] When the coil is wound by several turns on the vacuum
chamber, for example, the magnetic flux generated by the coil takes
on the maximum at the central portion so that the induced electric
field takes on the maximum at the central portion. Moreover, the
induced electric field cannot penetrate deeper than about the skin
depth, e.g., 1 cm, so that both the ionization factor and the
dissociation factor take on their maximums at the radially central
portion (in the direction of arrow r, e.g., in FIG. 21(a)) and just
below the dielectric member (in the direction of arrow z, e.g., in
FIG. 21(a)). After this, the plasma diffuses towards the wafer side
(downstream side). In the case of an ordinary chamber having a
cylindrical shape, therefore, the plasma density is the maximum at
the central portion in the direction of arrow r, and the degree of
central concentration rises downstream so that the plasma density
becomes nonuniform in the region where the wafer is placed.
SUMMARY OF THE INVENTION
[0006] A first object of the invention is to control the removal
extent of the vacuum chamber wall around the plasma generating
portion by the plasma. A second object of the invention is to
improve the plasma ignitability.
[0007] A third object is to realize a uniform plasma of high
density. This object is particularly desired in processing large
semiconductor wafers (e.g., large-size semiconductor wafers of 300
mm).
[0008] In order to achieve the above-specified objects, according
to the invention, there is provided a plasma processing apparatus
comprising an antenna (coil) for generating an electric field in a
plasma generating portion, a radio-frequency power source for
supplying radio-frequency electric power to said antenna, a vacuum
chamber enclosing the plasma generating portion to establish a
vacuum atmosphere therein, a Faraday shield provided around said
plasma generating portion (e.g., around the vacuum chamber), a gas
supply unit for supplying gas into said vacuum chamber, a sample
stage on which an object to be processed is placed, within the
vacuum chamber, and a radio-frequency power source for applying a
radio-frequency electric field to said sample stage, a plasma being
generated by accelerating electrons and ionizing them by collision
with the electric field generated by said antenna, and thereby
processing said object; characterized in that a load is provided in
the earth portion of said antenna, the average potential of said
antenna is adjusted so as to improve the ignitability at a plasma
ignition time, and the load is adjusted after the plasma is
produced so that the average potential of said antenna may be close
to that of the earth, and the removed amount of the wall of said
vacuum chamber after the plasma generation may be small. The
above-specified objects are also achieved, according to the present
invention, by a method of operation of this apparatus whereby the
load provided in the earth portion of the antenna is adjusted (that
is, the voltage on the ends of the antenna (coil) is controlled)
such that ignitability of the plasma at the time of plasma ignition
is facilitated, and is then again adjusted to be close to that of
the ground to limit the amount of chamber wall removed (e.g.,
etched) by the plasma.
[0009] Here, the phenomenon that the average potential of the
antenna comes close to that of the earth means that the potentials
30a and 30b of FIG. 4 are mutually opposite in phase but
substantially equal to each other, that is, Va.apprxeq.-Vb.
[0010] As another technique and structure to achieve the
above-specified objects, the Faraday shield can be provided with at
least one switch. When igniting the plasma at a plasma ignition
time, the at least one switch is positioned such that the Faraday
shield is held in a floating state, to facilitate ignition of the
plasma. Thereafter, the at least one switch is thrown to ground the
Faraday shield, so as to protect the wall of the plasma chamber
from removal by the plasma.
[0011] As still another technique and structure to achieve the
above-specified objects, the load can be provided in the earth
portion of the antenna and a switch or switches can be provided for
the Faraday shield. By adjusting the load and positioning the
switch as described in the preceding paragraphs, ignition of the
plasma is facilitated and removal of the wall in the plasma chamber
is avoided.
[0012] Means for solving the above-specified problems will be
described with reference to FIG. 2. FIG. 2 shows an ordinary
induction type plasma generating apparatus. With this apparatus,
the methods for reducing the partial removal of the vacuum chamber
wall around the plasma generating portion by the plasma and for
improving the ignitability of the plasma are examined by changing
the way of grounding the Faraday shield and the antenna to the
earth.
[0013] In this apparatus, a mixed gas of a chlorine gas and a boron
trichloride gas is supplied into a vacuum chamber 2 made of
alumina, by the gas supply unit 4. The gas is ionized to produce a
plasma 6 with the electric field which is generated by a
coil-shaped antenna 1 of two turns wound around the vacuum chamber
2. After this plasma production, the gas is discharged to the
outside of the vacuum chamber by a discharge unit 7. The electric
field for producing the plasma is generated by feeding the antenna
1 with radio-frequency electric power of 13.56 MHz generated by a
radio-frequency power source 10. In order to suppress the
reflection of the electric power, an impedance matching unit 3 is
used to match the impedance of the antenna 1 with the output
impedance of the radio-frequency power source 10. The impedance
matching unit is one using two capacitors of variable capacitance,
generally called an "inverted L type". The other end of the antenna
is grounded through a capacitor 9 to the earth, and a switch 21 is
provided for shorting the capacitor 9. In order to prevent the
vacuum chamber 2 from being etched by the plasma 6, moreover, a
Faraday shield 8 is interposed between the antenna 1 and the vacuum
chamber 2. By turning on/off a plurality of switches 22, the
Faraday shield can be brought into either the grounded state or the
ungrounded state. FIG. 3 is a perspective view showing the state
that the Faraday shield is installed. This Faraday shield 8 is
provided with a slit 14 for transmitting the inductive electric
field 15a generated by the coil-shaped antenna 1, into the vacuum
chamber but intercepting a capacitive electric field 15b. The
plasma is ignited mainly with the capacitive electric field 15b.
When the Faraday shield is grounded to the earth, however, the
capacitive electric field from the antenna is hardly transmitted
into the vacuum chamber, thereby deteriorating the ignitability of
the plasma. When the Faraday shield is not grounded to the earth,
the antenna and the Faraday shield are capacitively coupled to
bring the potential of the Faraday shield close to the average
potential of the antenna. Thus, it is considered that the
capacitive electric field is established between the Faraday shield
8 and an electrode 5, and hence the ignitability of the plasma is
not deteriorated so much.
[0014] The capacitive electric field 15b is normal to the wall of
the vacuum chamber 2, so that the charged particles in the plasma
are accelerated to impinge upon and damage the wall. Light 16
emitted from the plasma was observed with a spectroscope 20, and
the removal of the wall was measured by observing the light
emission strength of aluminum in the plasma as the wall aluminum
was removed.
[0015] First of all, here will be described a method for optimizing
the capacitance of the capacitor 9 connected to the earth portion
of the antenna in the experimental apparatus shown in FIG. 2 so
that the removal of the wall may be reduced. In the following, the
conduction state between the two ends of the switch will be
referred to as "on", and the cut-off state will be referred to as
"off". With the switch 21 being off, that is, with the capacitor 9
being not shorted, here will be described the optimum value of the
magnitude of the capacitance of the capacitor 9. The experimental
apparatus of FIG. 2 can be shown as an equivalent circuit in FIG.
4.
[0016] Then, the antenna 1 acts as the primary coil of a
transformer, and the plasma 6 acts as the secondary coil of the
same. The antenna 1 and the plasma 6 are coupled capacitively, and
their capacitance is shown by capacitors 31a and 31b. The
capacitance C of the capacitor 9 is determined so that a relation
of Va=-Vb always holds between the potential Va at the position of
the point 30a on the circuit and the potential Vb at the position
of the point 30b when the antenna has an inductance L. When this
condition is satisfied, the potentials to be applied to the two
ends of the capacitors 31a and 31b are minimized, minimizing the
wall damage. FIG. 5 further simplifies the circuit of FIG. 4,
namely the antenna and the plasma are combined together as an
element 17 having one combined impedance. The impedance of the
element was experimentally determined to be Z1=2.4+141j(.OMEGA.),
where j is a complex number. This measurement of the impedance can
be simply executed by measuring the electric current flowing
through an object to be measured, and the voltages at the two ends
of the object. The capacitor 9 has an impedance Z2=-(1-.omega.C)j ,
where .omega. is the angular frequency corresponding to 13.56 MHz.
For Va=-Vb, the relation between the impedances Z1, Z2 is
(Z1+Z2):Z2=1:-1 since the real part of Z1 is so small that it can
be ignored. The calculated electric capacitance of the capacitor 9
is about 150 pF, therefore, the relation Va=-Vb holds. FIG. 6
illustrates the results of calculation of the amplitudes of the
potentials at the point 30a (the dotted curve) and the point 30b
(the solid curve). The graph shows the capacitance of the capacitor
9 as the abscissa, and the amplitudes of the generated potentials
as the ordinates. As a result, the generated potentials were
mutually equal in the vicinity of the capacitance of 150 pF of the
capacitor 9, the phases of the oscillating voltages at that time
were shifted by 180 degrees, and the relation of Va=-Vb was
satisfied. This makes it possible to determine by the method thus
far described such a capacitance of the capacitor to be connected
to the earth side of the antenna that the damage of the wall is
minimized.
[0017] Next, with the capacitance of the capacitor 9 fixed at 150
pF in FIG. 2, the removed amount of the wall and the plasma
ignitability were examined, as tabulated in FIG. 15, when the
switches 21 and 22 are turned on or off. The wall removal is found
to be great when the switch 21 is on and the switch 22 is off.
Under this condition, the plasma ignitability is excellent. Under
the other conditions, however, the wall removal can be reduced, but
the plasma ignitability is low. Therefore, it has been found that
the condition for little wall removal and for excellent plasma
ignitability is not present in this system. However, these two
purposes can be achieved by operating either the switch 21 or the
switch 22 so as to reduce the wall damage after the plasma was
ignited under the condition that the switch 21 is on and the switch
22 is off at the ignition time. Here, it is better to use only the
switch 21 for the simplification of the apparatus structure. This
is partly because the potential of the Faraday shield has to be
lowered to zero as much as possible so as to reduce the wall damage
by using the switch 22, and consequently the switch 22 has to be
provided in plurality, and partly because the Faraday shield has to
be grounded with the shortest distance to the earth so that the
plural switches 22 have to be provided just near the antenna and
the Faraday shield. If the plural switches are arranged for those
necessities at the portion adjacent to the antenna and the Faraday
shield, the result is a complicated structure. This complicated
structure can be avoided with respect to the switch 21 because only
one switch 21 is connected to the capacitor 9 side which is
provided at a considerable distance from the antenna.
[0018] The off state of the switch 21 is the state that the
capacitor of 150 pF is connected between the antenna and the earth,
and the on state of the switch 21 is identical to the state that
the capacitance of the capacitor 9 is increased to infinity in a
radio-frequency band of HF or VHF. This means that the wall removal
increases more as the capacitance of the capacitor 9 is raised to a
higher level from 150 pF. The wall removal also increases even if
the capacitance of the capacitor 9 is lowered from 150 pF. Thus,
the wall removal can be controlled by varying the capacitance of
the capacitor 9.
[0019] In an apparatus shown in FIG. 7, the capacitance of the
capacitor 9 connected to the earth side of the antenna 1 is
variable, so that the wall removal by the plasma can be reduced by
varying the capacitance of the capacitor 9. Moreover, the plasma
ignitability can be drastically improved by making the capacitance
of the capacitor 9 far larger or smaller than 150 pF at the time of
igniting the plasma.
[0020] By adjusting the capacitance of the capacitor connected to
the earth side of the antenna, as described above, the removed
amount of the wall by the plasma can be reduced to achieve the
first object of the invention. At the plasma ignition time,
moreover, the capacitance of the capacitor connected to the earth
side of the antenna can be changed to establish an excellent
ignitable state, thereby achieving the second object of the
invention.
[0021] Here will be examined a method for generating a uniform
plasma. When the coil-shaped antenna is placed on the upper face of
the vacuum chamber, the induced electric field is generated at the
central portion, even if the diameter of the antenna is varied to
vary the intensity of the induced electric field in the radial
direction, so that the plasma density distribution is nonuniformly
concentrated at the center. This tendency of concentration of the
plasma density at the center is not varied even if a plurality of
antennas are arranged to vary the distance between each antenna and
the dielectric member. FIG. 21(b) illustrates one example of the
calculation of the plasma density distribution when the antenna is
placed on the vacuum chamber like FIG. 21(a). From this
calculation, when the ratio of the apparatus height H to the radius
R (the aspect ratio) is as large as H/R=20/25, as illustrated in
FIG. 21(b), the plasma density at the place, just below the antenna
(z=2 cm), where the antenna is present takes on its maximum and
increases in its absolute value (z=10 cm) downstream (in the
direction where the value z increases) but is small just above the
substrate. It is then found that the plasma density is nonuniform.
When viewed in the z direction, the density takes on its maximum at
the apparatus center z=10 cm. When the aspect ratio is reduced, as
illustrated in FIG. 21(c), the density distribution is
substantially identical to that of FIG. 21(b), but the distribution
just above the substrate is gentler than that of (b) and is
concentrated at the center.
[0022] The plasma density distribution is determined by the
boundary condition that the plasma density is zero on the vacuum
chamber wall and by the generation rate distribution, i.e., the
antenna position. Even if the antenna position is changed, as
illustrated in FIG. 21(d ), and if a plurality of antennas are
placed to change the power distribution, the shape of the density
distribution remains unchanged. When the coil is provided on the
upper face, the induced electric field generated by the antenna
takes on its maximum just below the antenna, so that a centrally
concentrated distribution is always established on the
downstream.
[0023] In the case of an arrangement in which the antenna is wound
horizontally around the vacuum chamber, the induced electric field
takes on its maximum on the side face of the chamber. A sheath is
formed on the side face of the chamber, so that the plasma density
takes on its maximum slightly inside the sheath, at the place the
closest to the antenna. As shown in a horizontal section at this
time, the potential is higher at the sheath end than at the wall
and than at the plasma center so that the plasma is transported to
the two sides from the sheath. Simultaneously with this, the plasma
flows downstream from that position, and hence the density
distribution is uniform in a portion in a horizontal section, at a
distance in the z direction from the highest density portion. In
the case of a cylindrical apparatus, for example, a concave
distribution may be established in the vicinity of the wafer for a
small H/R ratio, and a convex distribution may be established for a
sufficiently large H/R ratio where H is the height of the apparatus
and R is the radius thereof, so that the plasma density
distribution can be controlled to some extent (refer to FIGS. 22(a)
and 22(b)). The dominant factors at this time are the shape of the
apparatus, i.e., the ratio H/R. When the antenna is provided on the
side face, however, the plasma density is lowered by the reduction
in the coupling efficiency due to the large coupling area of the
antenna and the plasma and by the large loss of the plasma because
the region where the density is a maximum is near to the side face
wall. If the supplied power and the vacuum chamber size are the
same, the plasma density of this case is lower than that of the
aforementioned case in which the antenna is provided on the upper
face. This raises a problem that the processing speed of the object
to be processed is low.
[0024] As thus far described, the plasma density distribution of
the inductively coupled plasma varies with the apparatus shape and
the antenna arrangement, but the third object of the invention is
achieved by such a construction where the upper face of the vacuum
chamber has a smaller area than that of the lower face, and the
upper face is flat. Thus, the apparatus of the present invention
can be used to process large-sized semiconductor wafers discussed
previously.
[0025] In the plasma processing apparatus, preferably, the angle
between the edge at which the lower face and the upper face
intersect and the normal of the upper face is not less than 5
degrees.
[0026] In the plasma processing apparatus, more preferably, the
ratio of the apparatus height (the distance from the object to be
processed to the upper face) to the radius is not more than 1.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows a structure of a first embodiment of the
invention.
[0028] FIG. 2 shows a structure of an experimental system used for
verifying the invention.
[0029] FIG. 3 shows the state that a Faraday shield is mounted.
[0030] FIG. 4 shows an equivalent circuit diagram of the
experimental system used for verifying the invention.
[0031] FIG. 5 shows an equivalent circuit diagram of an
experimental system used for verifying the invention.
[0032] FIG. 6 shows a graph illustrating the amplitude of a
potential established between the two ends of an antenna.
[0033] FIG. 7 shows a diagram of an experimental system used for
verifying the invention.
[0034] FIG. 8 shows a structure of a second embodiment of the
invention.
[0035] FIG. 9 shows a structure of a third embodiment of the
invention.
[0036] FIG. 10 shows a fourth embodiment of the invention.
[0037] FIG. 11 shows a structure of a fifth embodiment of the
invention.
[0038] FIG. 12 shows a structure of a sixth embodiment of the
invention.
[0039] FIG. 13 shows a structure of a seventh embodiment of the
invention.
[0040] FIG. 14 shows a perspective view of a plasma processing
apparatus indicating the flow of eddy current in the seventh
embodiment of the invention.
[0041] FIG. 15 shows a table indicating the switches 21 and 22, the
removed amount of the wall of a vacuum chamber and RF powers
necessary to ignite a plasma.
[0042] FIG. 16 shows a plasma processing apparatus of an eighth
embodiment of the invention.
[0043] FIGS. 17(a) and 17(b) show a plasma processing apparatus of
a ninth embodiment of the invention.
[0044] FIG. 18 shows a plasma processing apparatus of a tenth
embodiment of the invention.
[0045] FIG. 19 shows a plasma processing apparatus of an eleventh
embodiment of the invention.
[0046] FIG. 20 shows a plasma processing apparatus of a twelfth
embodiment of the invention.
[0047] FIGS. 21(b)-21(d) show the plasma density distribution when
the antenna is placed on the upper face of the plasma processing
apparatus as shown in FIG. 21(a).
[0048] FIG. 22(b) shows the distribution of ion current incident on
the wafer when the antenna is placed on the side face of the plasma
processing apparatus, as shown in FIG. 22(a).
[0049] FIGS. 23(a) and 23(b) show schematic diagrams illustrating a
principle of the invention.
[0050] FIGS. 24(b) and 24(c) show diagrams illustrating the
distribution of ion current incident on the wafer in the case of
the invention, wherein the apparatus is schematically illustrated
in FIG. 24(a).
[0051] FIG. 25 a diagram illustrating the effects of the fourth
embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0052] Embodiments of the present invention will be described in
the following. These embodiments are only illustrative of the
present invention, the present invention being defined by the
appended claims.
[0053] FIG. 1 shows a first embodiment of the semiconductor
processing apparatus according to the present invention. In the
present apparatus, a material gas of, e.g., oxygen, chlorine, boron
trichloride or the like to be used for processing a semiconductor
is supplied into a vacuum chamber by a gas supply unit 4 and is
ionized to generate a plasma 6 with the electric field which is
generated by a coil-shaped antenna 1. After this plasma generation,
the gas is discharged to the outside of the vacuum chamber by a
discharge unit 7. The plasma generating electric field is generated
by supplying the antenna 1 with radio-frequency electric power
which is generated by a radio-frequency power source 10 of 13.56
MHz, 27.12 MHz, 40.68 MHz or the like. In order to suppress the
reflection of the electric power, however, an impedance matching
unit 3 is employed to match the impedance of the antenna 1 with the
output impedance of the radio-frequency power source 10. The
impedance matching unit is of a so-called inverted L type.
Depending on the frequency or the structure of the antenna,
however, it is necessary to employ an impedance matching unit by
which matching is easy. The other end of the antenna 1 is grounded
to the earth through a capacitor 9 having a variable capacitance.
Between the antenna 1 and the vacuum chamber 2, there is interposed
a Faraday shield 8 for preventing the vacuum chamber 2 from being
adversely etched by the plasma 6. The Faraday shield is not
electrically grounded. As shown in FIG. 3, moreover, the Faraday
shield 8 has a slit perpendicular to the direction in which the
coil of the antenna is wound. A semiconductor wafer 13 to be
processed is placed on an electrode 5. In order to attract ions
existing in the plasma to the space above the wafer 13, an
oscillatory voltage is applied to the electrode 5 by a
radio-frequency power source 12. It is important that the
capacitance of the variable capacitor 9 be a value at which the
partial removal of the wall is minimized.
[0054] In the present embodiment, at the time of igniting the
plasma 6, the capacitance of the capacitor 9 is adjusted to a value
larger or smaller than that which minimizes the wall removal. The
capacitance is adjusted to about two times or one half as large as
the value which minimizes the wall removal, so that the plasma can
be ignited with the radio-frequency power of several tens of
watts.
[0055] After this plasma ignition, the capacitance of the capacitor
9 is brought closer to the value minimizing the damage so that the
scraping of the wall may be minimized. When the wall should be
partly removed to some extent from the standpoint of a foreign
substance, the capacitance of the capacitor 9 is determined to be a
value to cause desired removal. The optimum value has to be
determined by repeating the semiconductor process.
[0056] A second embodiment of the invention will be described with
reference to FIG. 8. The basic construction of the apparatus of the
present embodiment is identical to that of the first embodiment,
but what is different from the first embodiment is the structure of
the capacitor provided at the earth side of the antenna 1. In the
present embodiment, two capacitors, a capacitor 9a and a capacitor
9b, are connected in parallel with the earth side of the antenna 1.
Of these, the capacitor 9a is connected directly to the earth
whereas the capacitor 9b is connected through a switch 21 to the
earth.
[0057] When the capacitance of the capacitor 9a is adjusted to the
value to minimize the damage, the capacitance provided to the earth
side of the antenna 1 is increased by the amount corresponding to
the capacitance of the capacitor 9b by turning on the switch 21 at
the time of igniting the plasma, so that the plasma ignitability is
improved by making the capacitance of the capacitor 9b sufficiently
high. After the plasma ignition, the switch 21 is turned off to
minimize the removal of the wall. If the removal is desired to some
extent from the standpoint of foreign substance, as in the first
embodiment, the capacitance of the capacitor 9a may be adjusted to
a value for the desired wall plasma-etching.
[0058] A third embodiment of the invention will be described with
reference to FIG. 9. The basic construction of the apparatus of the
present embodiment is identical to that of the second embodiment,
but what is different from the second embodiment is the employment
of an inductor 19 in place of the capacitor of FIG. 8. When the
capacitor 9 has an capacitance C, the inductor 19 has an inductance
L and the radio-frequency power source 10 outputs radio-frequency
waves having an angular frequency .omega., the impedance Z between
the earth side of the antenna and the earth is expressed by
Z=-(1/.omega.C)j, when the switch 21 is off, and by Z=-(1/(
.omega.C-1/.omega.L))j when the switch 21 is on. When the
capacitance of the capacitor 9 is adjusted to minimize the wall
removal, with the switch 21 off, the value Z can be changed by
operating the switch 21 to improve the ignitability of the plasma.
At the time of igniting the plasma, therefore, the switch 21 is
turned on to ignite the plasma. After the plasma ignition, the
switch 21 is turned off to minimize the wall damage. If the wall
should be partly removed to some extent from the standpoint of
foreign matter, the capacitor 9 may be set to the value for the
desired wall scraping.
[0059] In the third embodiment, there has been described a method
of varying the impedance of the load connected between the antenna
and the earth, by combining the capacitor, the inductor and the
switch. By using means other than that of this embodiment for
varying the value of the impedance of the load, it is possible to
establish a state that an excellent plasma ignitability is achieved
and a state that a smaller wall removal is achieved.
[0060] A fourth embodiment of the invention will be described with
reference to FIG. 10. The basic construction of the apparatus of
the present embodiment is identical to those of the first, second
and third embodiments, but the difference of the present embodiment
is that the Faraday shield 8 made of a conductive material is
buried in the wall of the vacuum chamber 2 made of a non-conductive
material. As the material for the vacuum chamber 2, there is used
alumina or glass. Since a metal such as chromium or aluminum can be
easily fused to the alumina, a pattern thereof can be formed in the
aluminum. When glass is used, a metal foil can be buried in the
glass as in the defrosting heater of an automobile.
[0061] The advantages as obtained from the structure in which the
Faraday shield 8 is buried in the wall of the vacuum chamber 2, are
that the insulating structure can be eliminated from between the
antenna and the Faraday shield 8, and that the distance between the
vacuum chamber 2 and the antenna 1 can be reduced to make the
apparatus compact.
[0062] A fifth embodiment of the invention will be described with
reference to FIG. 11. The basic construction of the apparatus of
the present embodiment is identical to that of the fourth
embodiment, but the difference of the present embodiment is that a
wall surface of the vacuum chamber 2 made of a non-conductive
material is covered with a film made of a conductive material and
acting as the Faraday shield. In the present embodiment, as an
example, the internal side of the vacuum chamber on the plasma side
is coated with the conductive Faraday shield 8, but similar effects
can be attained even when the atmospheric side of the vacuum
chamber is coated with the Faraday shield 8.
[0063] In the present embodiment, the plasma 6 comes in direct
contact with the Faraday shield 8, and therefore the wall of the
vacuum chamber 2 is partly removed by the plasma 6 at the slit
portions of the Faraday shield 8. In an oxide film etching process
using oxygen as the material gas, although depending upon the
process, excellent fusibility between alumina and aluminum is
utilized, realizing a construction in which an insulating material
is coated with a conductive material, by using a Faraday shield 8
of conductive aluminum and a vacuum chamber 2 of insulating
aluminum (e.g., alumina). In the case of the metal process in which
the material gas is chlorine or boron trichloride, the purpose can
be achieved by adopting alumina as the insulating material and SiC
as the conductive material. Many other combinations can be
conceived, and similar effects can be expected from any combination
if the combination brings about such performances that the coating
conductive material is hardly removed even if the temperature of
the vacuum chamber rises, and that both the insulating material and
the conductive material are hardly removed by the plasma.
[0064] A sixth embodiment of the invention will be described with
reference to FIG. 12. The basic construction of the apparatus of
the present embodiment is identical to those of the first, second
and third embodiments, but what is different from those embodiments
is that the Faraday shield 8 is grounded to the earth through a
resistor 18.
[0065] It is expected that a worker may frequently touch the
Faraday shield 8 at the time of reassembling the apparatus. A
mechanism is required for preventing the Faraday shield from being
charged at that time. In the present embodiment, a resistor 18 is
used to ground the Faraday shield to the earth. The resistance of
this resistor 18 has to be a higher impedance than that of the
capacitance between the Faraday shield 8 and the earth, at the
frequency of the radio-frequency power source 10 for generating the
plasma. For this necessity, if the grounded resistor 18 has a
resistance R and if the radio-frequency waves to be outputted by
the radio-frequency power source 10 have an angular frequency
.omega., the resistance R should satisfy R>1/.omega.C. In other
words, the Faraday shield and the earth are coupled to the load to
give a higher impedance than that of the capacitance between the
Faraday shield and the earth, at the high frequency for generating
the plasma, and the impedance of the load is low in direct current,
thereby preventing the Faraday shield from being charged at the end
of the operation.
[0066] A seventh embodiment of the invention will be described with
reference to FIG. 13. The basic construction of the apparatus of
the present embodiment is identical to that of the sixth
embodiment, but what is different from the sixth embodiment is that
the vacuum chamber 2 is made of a conductive material to produce
the effect of the Faraday shield.
[0067] Since the vacuum chamber also acting as the Faraday shield
cannot be provided with a slit to shut off the inductive electric
field, as has been described with reference to FIG. 3, the
inductive electric field has to be able to pass by adjusting the
thickness of the wall of the conductive vacuum chamber. Here will
be disclosed a structure in which the vacuum chamber is
electrically floated from the earth by an insulating flange 24.
[0068] In the case of the present embodiment, no work of providing
a Faraday shield around the vacuum chamber is required, improving
the workability. In the present embodiment, the circuit for
adjusting the average potential to a larger absolute value than
that at the vicinity of the earth or of the earth itself is
identical to that of the sixth embodiment.
[0069] FIG. 14 is a perspective view showing the behavior of eddy
current flowing in the vacuum chamber in the present embodiment.
The eddy current for preventing an inductive electric field 15a, as
described with reference to FIG. 3, from being transmitted into the
vacuum chamber 2 will flow in the circumferential direction
indicated by arrow 25, of the vacuum chamber 2 having a cylindrical
shape. If a relation R>.omega.L holds among the resistance R,
the inductance L in the path of the eddy current and the angular
frequency .omega. of the radio-frequency waves outputted from the
radio-frequency power source 10, the eddy current attenuation by
the resistor is increased and hence the inductive electric field is
transmitted into the vacuum chamber.
[0070] The vacuum chamber 2 has to be made of such a material that
it is hardly removed by the plasma, because it is directly exposed
to the plasma as in the fifth embodiment. Since the vacuum chamber
ordinarily has a wall as thick as about 2 cm, it may be made of a
material having an electrical resistivity of about 0.02 .OMEGA.m so
as to achieve such a skin thickness at a frequency of 13.56
MHz.
[0071] The vacuum chamber 2 is insulated from the earth by using
the insulating flange 24 and is equipped with the charge preventing
resistor 18 as in the sixth embodiment. The resistance of the
resistor 18 has to be a higher impedance than that between the
Faraday shield and the earth at the frequency of the
radio-frequency power source 10 for generating the plasma. In the
semiconductor processing, a bias voltage is applied to the
electrode 5 by the radio-frequency power source 12. If the plasma
is electrically floated from the earth, however, a high bias
voltage is not generated between the plasma and the electrode. In
order to prevent this, the plasma has to be brought as much as
possible into contact with the earth thereby to lower the potential
of the plasma. This lowering of the potential of the plasma can be
achieved by allowing the resistance of the resistor 18 to have a
lower impedance than that between the Faraday shield and the earth
in the frequency band of the radio-frequency power source 12.
[0072] The present embodiment is directed to an apparatus in which
the vacuum chamber is wholly made of the conductive material.
Similar effects could be achieved by eliminating the slits from the
Faraday shields of the foregoing embodiments and by adjusting the
thickness of the conductive material as in the present
embodiment.
[0073] The foregoing embodiments described comprise the vacuum
chamber 2 having a cylindrical shape. Even if the vacuum chamber 2
is given an incline side face, has a trapezoidal section and is
equipped with a coil and a Faraday shield, such a vacuum chamber 2
can be used in foregoing embodiments likewise.
[0074] An eighth embodiment of the invention will be described with
reference to FIG. 16. The basic apparatus construction of the
present embodiment is identical to those of the first, second and
third embodiments. What is different from the other embodiments is
that the area of the upper face 2a (far from the electrode 5 for
the object to be processed) of the vacuum chamber is smaller than
that of the lower face. Preferably, the upper face is flat. In the
invention thus constructed, the degree and position of the coupling
of the plasma and the antenna can be varied according to the
arrangement of the antenna, the number of turns of the antenna, the
distance between the antenna and the vacuum chamber, and so on.
When the number of turns of the coil of the antenna is one the
antenna is installed horizontally, for example, the coupling
position is varied as the antenna is moved vertically, as
illustrated in FIG. 23(a). When the number of turns of the coil is
more than one, the coupling state can be varied (FIG. 23(b))
according to the vertical position of the antenna and the distance
between each of the turns and the vacuum chamber. The antenna may
be moved upward, when the density at the central portion is
increased, and downward when the density is intended to have a
distribution where the density is high at the periphery. Thus, the
coupling position can be varied because the apparatus shape is
given a gradient by the large lower face area and the small upper
face area. In the case of an inductive coupling plasma, the
electrons/ions are isotropically diffused towards the chamber wall
by the bipolar diffusion so that their distributions are influenced
by the chamber shape. As a result, the plasma distribution is
easily flattened if the upper face is flat. The control of the
plasma density distribution is facilitated by the antenna
arrangement and the characteristic apparatus shape. Because of the
static field by the antenna 1, moreover, many foreign matters and
reaction products are produced in the vicinity of the antenna by
the interaction between the plasma and the vacuum chamber wall 2.
Because of the large area of the lower face, however, a path is
formed along the chamber wall and the discharge line 7 to allow the
gas to flow easily along the wall, so that the rate of flow towards
the wafer 13 to be processed can be reduced to realize a
satisfactory processing.
[0075] A ninth embodiment of the invention is shown in FIG. 17. The
basic apparatus construction of the present embodiment is identical
to that of the eighth embodiment. What is different from the other
embodiments is that the angle (see FIG. 18) between the edge where
the upper face 2a of the vacuum chamber 2 and the lower face 2b
thereof intersect (that is, the side of the vacuum chamber 2) and
the normal of the upper face is not less than 5 degrees. FIG. 24
shows the distribution of the density of the ion current incident
on the surface of the object when the shape of the vacuum chamber
is such that, for example, the ratio of the upper surface radius Ru
to the lower face radius Rd is 4:5. For the vacuum chamber height
H=13 cm, the ion current is flat up to .phi.=300 (r=15 cm). If the
height H is increased, the distribution is shown by a curve the
center of which is rather high. It has also been confirmed that the
curve is high at the periphery when the height H is decreased. If
tan.sup.-1{(Rd-Ru)/H}.gtoreq.5 degrees, it is possible to realize
the distributions which are flat and higher at the central portion
and at the periphery.
[0076] FIG. 18 shows a tenth embodiment of the invention. The basic
apparatus construction of the present embodiment is identical to
that of the eighth embodiment, but what is different from the other
embodiments is that the ratio H/R of the height H (i.e., the
distance from the electrode 5 to the upper face 2a) of the vacuum
chamber 2 to the radius R of the vacuum chamber 2 is H/R.ltoreq.1.
This relation is satisfied, for example, by the shape of the vacuum
chamber of FIG. 24(a).
[0077] FIG. 19 shows an eleventh embodiment of the invention. The
basic apparatus construction of the present embodiment is identical
to that of the eighth embodiment, but what is different from the
other embodiments is that magnetic field generating means 16 is
disposed outside the vacuum chamber 1. The plasma density
distribution just above the substrate in the presence of the
magnetic field is illustrated in FIG. 25. From the graph showing
the plasma density distribution, it is found that the plasma
density is higher in the periphery as the magnetic field is
increased. Thus, the magnetic field generating means acts as an
auxiliary one capable of controlling the distribution.
[0078] FIG. 20 shows a twelfth embodiment of the invention. The
basic apparatus construction of the present embodiment is identical
to that of the eighth embodiment, but what is different from the
other embodiment is that a plate 27 made of a conductor or a
semiconductor is placed on the face confronting the electrode 5 or
on the inner side of the upper face 2a of the vacuum chamber.
Moreover, radio-frequency voltage applying means 28 is preferably
connected with the plate 27 to apply radio-frequency waves. Instead
of the radio-frequency waves a pulsating DC voltage may be used.
Alternatively, the plate 27 may be grounded to the earth.
[0079] By employing the present embodiment, the partial removal of
the vacuum chamber wall enclosing the plasma generating portion by
the plasma can be controlled while improving the plasma
ignitability.
[0080] Moreover, by varying the degree and position of the coupling
of the plasma and the antenna according to the arrangement of the
antenna, the number of turns of the coil of the antenna, the
distance between the antenna and the vacuum chamber and so on, the
plasma distribution can be controlled to establish a uniform
plasma.
[0081] Many different embodiments of the present invention may be
constructed without departing from the spirit and scope of the
invention. It should be understood that the present invention is
not limited to the specific embodiments described in this
specification. To the contrary, the present invention is intended
to cover various modifications and equivalent arrangements included
within the spirit and scope of the claims.
* * * * *