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Claims  |
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What is claimed is:
1. A plasma treating method comprising a step of supplying a treating gas
into a treating chamber and evacuating to a predetermined pressure, the
treating chamber including parallel plate electrodes comprised of an anode
and a cathode opposed to each other; a step of applying an electric power
to the cathode as one of the parallel plate electrodes to thereby generate
an electric field between said parallel plate electrodes; a step of
generating a magnetic field by providing magnetic field generating means
including a first magnetic pole which has substantially a shape of a disc,
and a second magnetic pole which has a ring shape, which has a plurality
opposite to that of said first magnetic pole and which surrounds an outer
circumference of said first magnetic pole, in proximity thereto, the first
and second magnetic poles including N-poles and S-poles facing the cathode
with the anode between the N-poles and S-poles and the cathode, the
N-poles and S-poles being in proximity to each other such that the lengths
of lines of magnetic force extending at right angles to the electric
field, in proximity to the cathode, are relatively short as compared to
that of lines of magnetic force extending substantially parallel to the
electric field, so as to restrict cycloidal motion of electrons, and to
cause mainly a cyclotronic motion of electrons toward said anode side to
occur; and a step of treating a specimen arranged at said cathode by using
plasma generated by the cyclotronic motion of electrons.
2. A plasma treating method as defined in claim 1, wherein said plasma is
formed in a circular form with respect to a surface of the specimen.
3. A plasma treating method as defined in claim 2, wherein the circular
plasma is eccentrically rotated with respect to the specimen.
4. A plasma treating method as defined in claim 1, wherein the magnetic
field generating means has a length that is greater than a diameter of
said specimen.
5. A plasma treating method as defined in claim 4, wherein said specimen is
a semiconductor wafer.
6. A plasma treating apparatus comprising a treating chamber, the treating
chamber including parallel plate electrodes comprised of an anode and a
cathode opposed to each other; a gas supplying means for supplying a
treating gas into the treating chamber; an evacuating means for evacuating
said treating chamber to a predetermined pressure; a power supply
connected to said cathode, so as to generate an electric field between the
anode and cathode; and a magnetic field generating means for generating a
magnetic field starting from a surface of said anode toward said cathode
and returning to said anode, wherein the magnetic field generating means
includes a first magnetic pole which has substantially a shape of a disc,
and a second magnetic pole, which has a ring shape, which has a polarity
opposite to that of said first magnetic pole and which surrounds an outer
circumference of said first magnetic pole, in proximity thereto, the
magnetic field generating means including N-poles and S-poles facing the
cathode with the anode between the N-poles and S-poles and the cathode,
the N-poles and S-poles being arranged in proximity to each other such
that lines of magnetic force extending at right angles to the electric
field, in proximity to the cathode, are relatively short as compared to
that of lines of magnetic force extending substantially parallel to the
electric field, so as to restrict cycloidal motion of electrons and to
cause mainly a cyclotronic motion of electrons toward said anode side to
occur.
7. A plasma treating apparatus as defined in claim 6, wherein said magnetic
field generating means is rotatable eccentrically with respect to a
specimen disposed on said cathode.
8. A plasma treating apparatus as defined in claim 6, wherein said cathode
is a specimen holder, for holding the specimen to be treated in the plasma
treating apparatus, the specimen to be held on the surface of the cathode
facing the anode.
9. A plasma treating method comprising a step of supplying a treating gas
into a treating chamber and evacuating to a predetermined pressure, the
treating chamber having included therein opposed parallel plate electrodes
including an anode and a cathode; a step of applying electrical power to
the cathode as one of the opposing parallel plate electrodes and
generating an electric field between said parallel plate electrodes; a
step of generating a magnetic field between said parallel plate electrodes
so as to generate a plasma between said parallel plate electrodes, the
step of generating a magnetic field being performed by providing N-poles
and S-poles facing the cathode with the anode between the N-poles and
S-poles and the cathode, the magnetic field being generated by a first
magnetic pole which has substantially a shape of a disc, and a second
magnetic pole having a ring shape and a polarity opposite that of the
first magnetic pole, so as to provide the N- and S- poles, the second
magnetic pole surrounding the first magnetic pole, the inner diameter of
the second magnetic pole being equal to the diameter of the first magnetic
pole, such that a length of lines of magnetic force extending at right
angles to the electric field, in the proximity of the cathode, is
relatively short as compared to that of lines of magnetic force parallel
to the electric field, so as to restrict cycloidal motion of electrons and
to cause mainly a cyclotronic motion of electrons toward the anode side to
occur; and a step of treating a specimen arranged on said cathode using
said plasma.
10. A plasma treating method as defined in claim 9, wherein said plasma is
formed in a circular form with respect to said specimen surface.
11. A plasma treating method as defined in claim 10, wherein said circular
plasma is eccentrically rotated with respect to said specimen.
12. A plasma treating method as defined in claim 9, wherein the first and
second magnetic poles are in contact with each other.
13. A plasma treating apparatus comprising a gas supplying means for
supplying a treating gas into a treating chamber; an evacuating means for
evacuating said treating chamber to a predetermined pressure; parallel
plate electrodes composed of a cathode and an anode arranged opposite to
each other inside said treating chamber; a power supply connected to said
cathode so as to generate an electric field between the anode and cathode;
and a magnetic field generating means for generating a magnetic field
between said parallel plate electrodes, wherein said magnetic field
generating means includes N-poles and S-poles facing the cathode with the
anode between the N- and S- poles and the cathode, the magnetic field
generating means being composed of a first magnetic pole which has
substantially a shape of a disc and a second magnetic pole having a ring
shape and a polarity opposite that of the first magnetic pole, so as to
provide the N- and S- poles, the second magnetic pole surrounding the
first magnetic pole, the inner diameter of the second magnetic pole being
equal to the diameter of the first magnetic pole, such that a length of
lines of magnetic force extending at right angles to the electric field,
in the proximity of the cathode, is relatively short as compared to that
of lines of magnetic force parallel to the electric field, so as to
restrict cycloidal motion of electrons and to cause mainly a cycloidal
motion of electrons toward said anode side to occur.
14. A plasma treating apparatus as defined in claim 13, wherein said
cathode is adapted to have a specimen arranged thereon, and wherein said
magnetic field generating means is eccentrically rotated with respect to
the specimen arranged on said cathode.
15. A plasma treating apparatus as defined in claim 13, wherein the first
and second magnetic poles are in contact with each other.
16. A plasma treating apparatus comprising a gas supplying means for
supplying a treating gas into the treating chamber; an evacuating means
for evacuating said treating chamber to a predetermined pressure; parallel
plate electrodes composed of a cathode and an anode arranged opposed to
each other inside said treating chamber; a power supply connected to said
cathode so as to generate an electric field between the anode and cathode;
and a magnetic field generating means for generating a magnetic field
between said parallel plate electrodes, having the anode between said
magnetic field generating means and the cathode, wherein said magnetic
field generating means is eccentrically rotated with respect to a specimen
arranged on said cathode, the rotation center of said magnetic field
generating means being at the center of said specimen; and wherein the
magnetic field generating means is composed of a first magnetic pole which
has substantially a shape of a disc, and a second magnetic pole, which has
a ring shape and which has a polarity opposite to that of said first
magnetic pole and surrounds an outer circumference of said first magnetic
pole, in proximity thereto. |
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Claims |
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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 treating method and apparatus
therefor, and more particularly to a plasma treating method and apparatus
therefor which is suitable for the treatment of a specimen such as the
substrate of a semiconductor device (hereinafter termed as `wafer`) with a
plasma.
2. Description of the Prior Art
As a conventional plasma treating apparatus such that a magnetic field is
introduced to improve the efficiency of generating a plasma and increase a
treating rate, there is well known one by which the treatment is performed
particularly with a magnetron discharge generated.
Most of the voltage inside a sheath to be formed on the electrodes for a
glow discharge to be generated by using a direct current or a
radio-frequency power supply turns out to have an electric field formed
perpendicularly to the electrodes. A magnetron discharge is generated by
imposing to such an electric field a magnetic field intersecting the
electric field orthogonally. Inside the sheath with the electric and
magnetic fields intersecting orthogonally, electrons perform a cycloidal
motion and the motion of electrons along the direction of the electric
field is restricted to a narrow range.
The plasma treating apparatus for the treatment by generating such a
magnetron discharge has the following drawbacks.
A first drawback consists in uniformity of the distribution of a treating
rate over a full area on the wafer. The intensity of a magnetron discharge
depends upon a vector product of an electric field vector E and a magnetic
field vector B , 1 .times. 1, and a magnitude of the magnetic field vector
1 should be set so that an amplitude in the direction of the electric
field when electrons perform the cycloidal motion will be substantially as
large as the thickness of the sheath. But, in fact, a scattering of
electrons as the result of the collisions between electrons and molecules
inside the sheath prevents accomplishment of providing a sufficient effect
in converting into a plasma, and, then, a much higher intensity of the
magnetic field than should be necessary at such a time will be required.
However, it is impossible to obtain such a magnetic field having
uniformity through a full area inside the sheath on the wafer, and,
consequently, the state of the plasma on the wafer will be uneven, and, as
illustrated in FIG. 10, the etching rate becomes locally so high in the
portion where a magnetron discharge 61 is generated. Thus, there is a
drawback of being unable to have a uniform distribution of the etching
rate over the full area on the wafer.
As a method in the attempts of resolving such a drawback and accomplishing
a uniform treatment, there is one which is disclosed in U.S. Pat. No.
4,526,643, wherein a plurality of magnets is arranged as to form an
endless track and moved in one direction along the endless track, or one
which is disclosed in U.S. Pat. No. 4,552,639, wherein the magnetic source
consisting of a plurality of permanent magnets radially oriented is
eccentrically rotated.
Notwithstanding, there is remained a second drawback that is damaging of
the semiconductor device by charged particles, especially by ions. The
efficiency of utilization of the applied electric power is so high in the
magnetron discharge that the discharge can be maintained by a low voltage,
and, consequently, the voltage in the sheath may be made low. Hence, an
injection energy of ions in their injection onto the wafer becomes low,
and, accordingly, the resultant damaging is said to be low. But, as
described in Japanese Patent Publication No. 9394/1986, if an aluminum
material is etched, there occurs an objectionable problem that part of the
patterns of aluminum, which is covered with a resisting mask and on which
etching is not normally done, is destroyed together with the resisting
mask.
Generally, the aluminum material is to have layers formed on the Si wafer
via SiO.sub.2 of an insulation layer. In the magnetron discharge, a
potential difference inside the sheath is so high as some hundreds of
volt, and a kinetic energy of electrons performing the cycloidal motion
inside the sheath reaches more than 100 eV, which is a relatively high
level of energy with reference to the level of 10-20 eV for the primary
ionization. Owing to this, in the magnetron discharge, electrons having
such a high level of energy collide with the molecules of a treating gas
so frequently that the association of the molecules will be dissolved.
When BCl.sub.3 is used as a treating gas, for example, it is ionized into
molecules such as BCl.sub.2.sup.+ in case of the reactive ion etching
(termed hereinafter as `RIE`) while the same is dissociated into atoms
such as B.sup.+ +3Cl.sup.+, generating a large number of ions, in case of
the magnetron discharge. Hence, in the magnetron discharge, the quantity
of ions to be injected on the wafer will become large and they are charged
up on the photoresist, and this is considered to destroy part of the
aluminum patterns together with the resist. In this way, in the magnetron
discharge, ions as dissociated in the form of atoms are generated in large
quantities and those ions are injected on the wafer. Therefore, the ions
having a greater injection energy and a larger quantity of electric
charge, in comparison with the quantity of molecules required for the
reaction, will be imposed on the wafer, and this gives rise to a problem
of damaging the wafer electrically as well as physically.
Thus, in the magnetron discharge, there has been the drawback of damaging
the wafer even after the above referred means for accomplishing good
uniformity of the treatment have been put in practice, with a difficulty
of achieving a uniform treatment without damaging the wafer.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a plasma treating method
and apparatus therefor, by which it is made possible to facilitate a high
rate and uniform treatment without damaging the specimen.
The present invention is characterized, by providing a plasma treating
apparatus comprising a gas supplying means for supplying a treating gas
into a treating chamber, an evacuating means for evacuating the treating
chamber to a predetermined pressure, parallel plate electrodes arranged
opposite to each other in the treating chamber, with a cathode as one of
the parallel plate electrodes to be disposed to be opposed to each other
inside the treating chamber, an anode as the other of said parallel plate
electrodes, a power supply to be connected to the cathode, so as to
generate an electric field between the anode and cathode and a magnetic
field generating means for generating a magnetic field starting from a
surface of the anode toward the cathode and returning to the anode,
wherein the N- and S- poles of the magnetic field generating means face
the cathode, with the anode therebetween, and are arranged in proximity to
each other such that the magnetic field has relatively short lines of
magnetic force crossing at a right angle with the electric field as
compared to the length of the lines of magnetic force substantially
parallel with the electric field, so as to make a cycloidal motion of
electrons restricted and to cause a cyclotronic motion of electrons toward
the anode side to occur in large quantities and its treating method,
applied as a plasma treating method, comprising a step of supplying the
treating gas into the treating chamber having parallel plate electrodes of
anode and cathode oppositely faced to each other and evacuating to the
predetermined pressure, a step of applying an electric power to the
cathode as one of the parallel plate electrodes being opposed to each
other inside the treating chamber to thereby generate an electric field
between the parallel plates of electrodes, a step of generating the
magnetic field starting from the surface of the anode of the parallel
plate electrodes toward the cathode, and returning to the anode, the
magnetic field being generated by providing N- and S- poles of the
magnetic field generating means to face the cathode, with the anode
between the N- and S- poles and the cathode, and with the N- and S-poles
in proximity to each other such that the magnetic field has relatively
short lines of magnetic force crossing at a right angle with the electric
field as compared to the length of the lines of magnetic force
substantially parallel with the electric field, in the proximity to the
cathode, so as to make the cycloidal motion of electrons restricted and to
cause the cyclotronic motion of electrons toward the anode side to occur
in large quantities, and a step of treating a specimen arranged on the
cathode with plasma generated by the cyclotronic motions of the electrons.
Thus, it is made possible to facilitate a high rate and uniform treatment
without damaging the specimen.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical sectional view of the plasma treating apparatus
illustrating one embodiment according to the present invention.
FIG. 2 is a plan view of a magnet element seen along 1--1 in FIG. 1.
FIG. 3 is a vertical sectional view seen along 2--2 in FIG. 2.
FIG. 4 is a plan view of a magnet element used in experiments.
FIG. 5 is a vertical sectional view seen along 3--3 in FIG. 4.
FIG. 6 is a diagram illustrating the state of a plasma generated in
experiments using the magnet element in FIG. 4.
FIG. 7 is a curve showing the distribution of an etching rate relative to
the state of the plasma in FIG. 6.
FIG. 8 is a diagram illustrating a potential difference of the component of
direct current between the electrodes relative to the state of discharge
in FIG. 6.
FIG. 9 is a diagram illustrating the process of generating a plasma in the
state of discharge in FIG. 6.
FIG. 10 is a diagram illustrating the state of discharge and the
distribution of an etching rate when a magnetron discharge is generated by
providing the magnet element of FIG. 4 on the cathode side.
FIG. 11 is a diagram illustrating the distribution of an etching rate and
the distribution of a magnetic flux density when the distance between the
magnetic pole face of the magnet element and the cathode relative to FIG.
10 is extended.
FIG. 12 is a curve showing the distribution of an etching rate when the
magnet element in FIG. 2 is used.
FIG. 13 is a curve showing the distribution of an etching rate when the
magnet element in the condition of FIG. 12 is eccentrically rotated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
One embodiment of the present invention will be described with reference to
FIGS. 1 through 13.
FIG. 1 provides combination of a typical RIE apparatus for treating a
specimen, i.e. the wafer, with a plasma with the magnetic field generating
means according to the present invention. The treating chamber 1 which has
a gas-introducing inlet 8 and a gas-exhausting outlet 9 is maintained at a
constant low pressure atmosphere by introducing a treating gas via a
diffusion plate 7 and exhausting at the same time. The cathode 3, on which
side the wafer 12 (6 inch is used in this case) is mounted, is connected
to a radio-frequency power supply 6 via a coupling capacitor 4 and a
matching box 5. The cathode 3 is installed in the treating chamber 1 by
means of a supporting material 14. The anode 2, the diffusion plate 7 and
the treating chamber 1 are electrically grounded. The magnet element 10 is
installed on a motor 11 via a supporting plate 13 and is disposed on the
anode 2 at the counter-cathode 3 side thereof.
The magnet element 10 consists of permanent magnets 21, 22 and a yoke plate
23 as illustrated in FIGS. 2 and 3. The permanent magnets 21, 22 in this
case are made of samarium-cobalt (Sm - Co) and formed by bonding small
pieces of magnet as magnetized to have a magnetic flux density of 8,500
gauss. Surrounding the disc-like permanent magnet 21 having a diameter of
80 mm, there is arranged the ring-like permanent magnet 22 having an
internal diameter of 80 mm and an external diameter of 180 mm, with their
poles to be opposite to each other. As can be appreciated from these
magnet dimensions, and as seen in FIGS. 2 and 3, the magnets 21 and 22
contact each other. These permanent magnets are arranged to be installed
on the yoke plate 23 made of soft steel and having a diameter of 190 mm.
The magnet element 10 and the motor 11 are arranged to be located
nonaxisymmetrically with respect to each other, and the magnet element 10
is made rotatable eccentrically. That is, the magnet element 10, for
generating a magnetic field between the anode 2 and cathode 3, has the
anode 2 between the magnet element 10 and the cathode 3, wherein the
magnet element 10 is eccentrically rotated with respect to the specimen,
e.g. water 12, arranged on the cathode 3. The rotation center of the
magnet element 10 is at the center of the specimen, and the magnetic
element 10 is composed of a first magnetic pole which has substantially a
shape of a disk, and a second magnetic pole, which has a ringed shape, and
which has a polarity opposite to that of said first magnetic pole and
surrounds an outer circumference of the first magnetic pole m in proximity
thereto.
According to the above described arrangement of the apparatus, a
fundamental technology to be attempted by the present invention will be
set forth prior to describing the function and the effect of the present
invention.
Instead of the magnet element 10 in the apparatus with the above described
arrangement, there is set on the anode side a magnet element 30 as
illustrated in FIGS. 4 and 5, in which permanent magnets 31, 32 having a
width of 10 mm in this case are adjoined with a distance of 20 mm between
the centers thereof and installed on the yoke plate 33. In this
arrangement, the wafer 12 of 6 inch having a layer of SiO.sub.2 formed on
the surface thereof is mounted on the cathode 3, a 30 SCCM of CF.sub.4 gas
as a treating gas is supplied to the treating chamber 1, with the pressure
inside the treating chamber 1 being held at 70 m Torr, and a 400 W
radio-frequency power of 13.56 MHz is applied by the radio-frequency power
supply 6 thereby causing to generate a plasma inside the treating chamber
1 so as to etch the layer of SiO.sub.2. In the same arrangement, the
distance between the cathode 3 and the magnetic pole face of the magnet
element 30 is 40 mm, and a magnetic flux density at the magnetic pole face
of the permanent magnets 31, 32 is 8,500 gauss. In this way, the N- and S-
poles have magnetizing directions in parallel with the electric field, in
parallel with each other, and are arranged in proximity to each other.
By applying the radio-frequency power, a glow discharge is generated
between the electrodes consisting of the anode 2 and the cathode 3, and,
when on such a glow discharge a magnetic field by the magnet element 30 is
imposed, a Lorentz's force is acted on secondary electrons to be derived
from the cathode 3 in the portion of a shower-like magnetic field
spreading from the anode 2 into the cathode 3, as shown in FIG. 6. Then, a
cyclotronic motion is caused to occur in large quantities, in which an
intensive plasma region 41 is generated.
It is the fundamental technology of the present invention that such an
intensive plasma region 41 is caused to be generated by the cyclotronic
motion of electrons.
The distribution of a potential difference of the direct current component
between the electrodes is, as illustrated in FIG. 8, divided into an anode
sheath portion 51 generated in proximity to the anode 2, which has a
potential gradient and a small potential difference, a cathode sheath
portion 53 generated in proximity to the cathode 3, which has a potential
gradient and a large potential difference, and a plasma portion 52
generated between the anode sheath portion 51 and the cathode sheath
portion 53, which has a small potential. Furthermore, as illustrated in
FIG. 6, by disposing the magnet element 30 on the anode side, a magnetic
field with a high intensity is generated in proximity to the anode 2 and
this intensity is gradually weakened toward to the cathode 3. It is
secondary electrons having a high energy to generate a high density plasma
by supplying an energy to the plasma portion 52, and this portion of the
magnetic field causes to change the motion of secondary electrons as
described hereinbelow and to transmit the energy had by the secondary
electrons to the molecules of a treating gas efficiently.
By the action of such a portion of the magnetic field, the secondary
electrons to be generated on the cathode 3 are so accelerated by a
self-bias voltage of the cathode sheath portion 53 as to turn to high
speed electrons and fly out to the anode 2 side in a direction
perpendicularly to the cathode 3. At this time, they approach to a portion
of the magnetic field which has a small component positioned to be in
parallel to the cathode 3. The magnetic field portion having a slight
parallel component means a magnetic field, such as shown in FIG. 6, which
is moved away from a point in parallel with the electrode near the
cathode, has a component in parallel with the electric field and is
directed toward the anode, and wherein lines of magnetic force crossing at
a right a angle with the electric field near the cathode are relatively
short, as compared to the length of the lines of magnetic force
substantially parallel with the electric field. This magnetic field does
not have a long portion of lines of magnetic force in parallel with the
cathode 3, but rather has lines of magnetic force in an elongated shape
such that the lines of magnetic force parallel to the cathode surface are
relatively short in length as compared to that in a direction parallel to
the electric field. secondary electrons are bestowed a Lorentz's force,
thereby the motion of electrons which has been perpendicular to the
cathode 3 is changed into a drifting motion toward the anode 2 while
performing the cyclotronic motion along the magnetic field. That is, as
shown in FIGS. 5 and 6, magnetic poles of permanent magnets are arranged
adjacent to each other, so that the magnetic field portion near the
cathode has relatively short lines of magnetic force at right angles with
the electric field as compared to the length of the lines of magnetic
force parallel to the electric field. That is, the magnetic field
intersecting substantially orthogonally with the electric field almost
disappears near the cathode 3, and even if cycloidal motion of electrons
is generated at a point where the electric field and magnetic field are
crossed at a right angle to each other, secondary electrons may strike
against treating gas molecules momentarily, change their directions, fly
away, and approach other magnetic fields having a component in parallel
with the electric field, resulting in producing a cyclotronic motion of
electrons along the magnetic field. Due to this fact, cycloidal motion
does not continue and a magnetron discharge under the cycloidal motion is
restricted, but a discharge having mainly a cyclotronic motion is carried
out.
Meanwhile, the secondary electrons collide with the molecules of a treating
gas, CF.sub.4 in this case, as illustrated in FIG. 9, when passing through
the plasma portion 52 shown in FIG. 8, by which part of a kinetic energy
is given to CF.sub.4 to ionize or excite it to produce CF.sub.3.sup.+ or
reactive species such as CF.sub.3 *. The secondary electrons are then
going to start flying out in a voluntary direction, but, by receiving the
Lorentz's force due to the magnetic field, they start to perform a new
cyclotronic motion and drift into the direction of the anode 2 along the
magnetic field. By this action, the frequency in which secondary electrons
and treating gas molecules strike each other is increased, and energy
included in the secondary electrons can be transferred to the molecules of
the treating gas with good efficiency above the wafer 12 so that a high
density plasma can be generated.
Moreover, in the conventional RIE apparatus, the secondary electrons which
are emitted from the cathode 3, accelerated inside the cathode sheath and
have a high kinetic energy are apt to advance in a straight line toward
the anode 2, so that a time for the secondary electrons to stay in a space
is short, a moving distance of secondary electrons in a space becomes
short, a degree of frequency collision between the secondary electrons and
treating gas molecules is decreased, and an efficiency for applying an
energy with the treating gas molecules is decreased, resulting in that a
strong plasma is not generated. This high density plasma of the present
invention, i.e., a plasma mainly performing a cyclotronic motion,
generated by making cyclotronic motion of the secondary electrons with the
magnetic field and drifting them toward the anode 2 along the magnetic
field (named) "Electron Cyclotron Drifting Discharge" by the inventors and
termed hereinafter as `ECD discharge`), is suitable for treating the wafer
comparatively with the magnetron discharge.
Between the magnetic field and the electrons, there is the relationship
that the more intensive the magnetic field is, the smaller a radius of the
cyclotron motion of electrons and the lower a diffusion rate of electrons
will be. If the magnetic field is intensive, electrons are caused to stay
in the space long, and, hence, the frequency of collisions between
molecules and electrons in the space becomes high.
In case of the conventional, magnetron discharge, the secondary electrons
which have flown out from the cathode with a high kinetic energy are
caused to perform the cycloidal motion due to an intensive magnetic field
intersecting at a right angle with an electric field in proximity to the
cathode, this cycloidal motion is continued, and, as the result, ions
dissociated in the form of atoms are generated in large quantities.
Against this, in the ECD discharge, the magnetic field is composed to
become stronger from a location near the cathode toward the anode 2, and,
therefore, the frequency of collisions between the secondary electrons
generated from the cathode 3 and the molecules of a treating gas is
increased gradually toward the direction into the anode 2. Further, the
energy which the secondary electrons may have in proximity to the cathode
3 is no less than that in case of the magnetron discharge, which is
sufficiently high to be able to dissociate the molecules of the treating
gas, However, since the lines of magnetic force orthogonally intersecting
the electric field near the cathode are relatively short, the cycloidal
motion of the secondary electrons having a high energy is not occurring
and the cyclotronic motion is caused, by the magnetic field directed
toward the anode 2. Due to this, the quantity of ions as dissociated in
the form of atoms by collisions is extremely small in the vicinity of the
cathode 3 as compared with the case of the magnetron discharge, because of
low frequency of collisions between the secondary electrons and the
molecules of the treating gas. Subsequently, as they drift into the
direction of the anode 2 from in the vicinity of the cathode 3, the
magnetic field becomes intensive step by step and the radius of the
cyclotronic motion of secondary electrons becomes smaller and smaller.
Accordingly, the frequency of collisions between the secondary electrons
and the molecules of the treating gas becomes higher and higher gradually.
Thus, the energy had by the secondary electrons is transmitted to the
molecules little by little in a broad space above the wafer 12 spreading
from the cathode 3 to the anode 2, and molecular ions are generated in
large quantities and atomic ions in small quantities.
In this way, in the ECD discharge generated between the cathode 3 to the
anode 2 along the magnetic field, the quantity of molecular ions generated
is larger than that of atomic ions generated, and such generation occurs
in a broad space spreading along the magnetic field, as shown in FIG. 6.
By use of the above described ECD discharge, the etching rate will not
become so high locally as in the magnetron discharge, and it can be seen
that, as indicated in FIG. 7, there is extended an area which has a
considerably high etching rate, e.g. 500 nm/min. The plasma portion 41 in
FIG. 6 and the distribution diagram of etching rate in FIG. 7 are drawn to
correspond to each other. In these diagrams relative to the ECD discharge,
the quantity of the plasma to be generated in the portion where the
electric and magnetic fields intersect orthogonally is not very large, and
the etching rate becomes lower than the highest level. The highest level
of etching rate in the ECD discharge lies in the portions where the plasma
is generated in large quantities, namely, where the magnetic field is
concentrated substantially in parallel to the electric field, and, in this
case, where the permanent magnets 31, 32 are installed. In other words, a
plasma having a high degree of ECD discharge is generated below the
permanent magnets 31 and 32 emitting magnetic force lines which include
relative short lines of magnetic force crossing at a right angle with the
electric field as compared to the length of the lines of magnetic force in
parallel with the electric field.
Moreover, in the magnetic field by generating the ECD discharge, a large
portion of the magnetic field is utilized, and, therefore, the efficiency
of utilization is high differently from the magnetic field by generating
the magnetron discharge, in which only a portion of the magnetic field
where the electric and the magnetic fields intersect orthogonally is
utilized.
Then, it will be hereinbelow described that the improvement of etching rate
by such ECD discharge is not due to the conventional magnetron discharge.
In the conventional magnetron discharge, for example, a discharge can be
generated by setting the magnet element 30 in FIG. 4 on the cathode 3 side
at the counter-anode side thereof under the same etching conditions as the
ECD discharge is generated. FIG. 10 gives the state of discharge and the
distribution of etching rate in this instance, which indicates that, at a
portion of the magnetic field above the surface of the cathode 3 where a
field portion in parallel to the cathode 3 is most, there is generated a
focus discharge 61, and at this portion a tendency of the etching rate
shows to be remarkably high. Furthermore, an experiment conducted after
extending the distance (to 45 mm) between the cathode 3 and the magnet
element 30 set on the cathode 3 side and weakening the magnetic field in
the portion above the surface of the cathode 3, showed that there is
generated almost no magnetron discharge and almost no increase in the
etching rate as indicated in FIG. 11. Namely, it can be seen from FIGS. 10
and 11 that, in case the magnet element 30 is set on the cathode 3 side,
no ECD discharge is generated even after changing the distance between the
cathode 3 and the magnet element 30. From FIGS. 7 and 10, it can be seen
also that the etching rates within the treated surface of the wafer 12 for
the ECD discharge and the magnetron discharge are utterly different in
their distribution diagram. Besides, in a type that a magnetic field is
formed in parallel to the surface of the wafer (the surface of the cathode
side thereof) by setting the magnet element on the anode side, as
disclosed in U.S. Pat. No. 4,552,639, electrons perform essentially a
cycloidal motion, and, therefore no ECD discharge primarily caused by
cyclotronic motion of electrons, as in the present invention is generated.
It is the apparatus shown in FIG. 1 such as to be constructed to
incorporate the magnet element 10 in FIGS. 2 and 3 therein that the ECD
discharge invented by the inventors of the present invention is produced
to form a ring so as to attempt to have an intensive plasma generating
region spread over the wafer.
By using such an apparatus, for example, under the etching conditions as
above specified (the wafer 12 is 6 inch having a SiO.sub.2 layer formed on
the surface thereof, the supply rate of a treating gas CF.sub.4 is 30
SCCM, the pressure inside the treating chamber 1 is 70 m Torr, and the
radio-frequency power is 13.56 MHz, 400 W), etching was performed by
setting the distance between the electrodes to be 35 mm and the distance
between the cathode 3 and the magnetic pole face of the magnet element 10
to be 40 mm.
At first, when etching was done by setting the magnet element 10 stably and
keeping the centers of the magnet element 10 and the wafer 12 in
agreement, the distribution of etching rate showed a wave-like tendency
from the center toward the periphery of the wafer as in FIG. 12, where the
etching rate at the center of the wafer becomes low. Such a distribution
of the etching rate was found similar at any section taken radially to the
circumference of the wafer, and although this is more smoothed than the
distribution of the magnetron discharge it still can not achieve a
satisfactory uniformity.
Secondly, etching was carried out under the same conditions by providing
eccentricity in the arrangement of the magnet element 10 with respect to
the center of the wafer by 16.5 mm and rotating the magnet element 10
eccentrically on the center of the wafer 12. As the result of this
etching, the distribution of etching rate was, as indicated in FIG. 13,
substantially uniform from the center toward the periphery of the wafer
12. The above value of eccentricity can be determined for the optimum by
mapping transformation of the distribution of etching rate at a still
time.
In this case, if the distance between the magnetic pole face of the magnet
element 10 and the cathode 3 was made not more than 20 mm and as the
distance between the magnet element 10 and the wafer 12 is decreased, a
portion near the cathode 3 having the electric field and the magnetic
field orthogonally intersecting each other has a certain length, and at
the same time a strength of the m | | |
