 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a plasma etching apparatus used in
micropattering in fabrication of semiconductor devices.
In micropatterning in fabrication of semiconductor devices, plasma etching
using a high-frequency glow discharge of a reactive gas is mainly used. In
particular, in the process requiring high-precision control of dimensions
and shapes, reactive ion etching (RIE) is mainly used wherein a sample
(semiconductor wafer) is placed on a cathode electrode applied with
high-frequency power and directional etching is performed with accelerated
ions in a direction perpendicular to an electrode surface upon biasing of
the cathode electrode to a negative voltage.
RIE etching characteristics change depending on types and amounts of
radicals produced in a gaseous phase and the energy and density of ions
incident on the cathode electrode which supports an object to be etched.
An etching rate in normal conditions is a maximum of about several tens of
nanometers/min. When high-frequency power is increased to increase the
etching rate, the amount of radicals and an ion current density are
increased, and ion energy is also increased, thus degrading selectivity
with respect to an etching mask and an underlying layer and hence damaging
semiconductor crystals. In order to increase the throughput, a large
apparatus capable of processing a large number of semiconductor wafers is
required. RIE poses a problem of etching shape errors caused by disturbing
ions produced upon collision between accelerated ions and the neutral gas
in an ion sheath on the cathode electrode when micropatterning is further
advanced.
In order to solve the above problems, a so-called magnetron etching
apparatus is proposed to increase the throughput by applying a magnetic
field combined with a high-frequency electric field, thereby generating a
plasma. An example of the magnetron etching apparatus is described in U.S.
Pat. No. 4,422,896 issued to Walter H. Class et al. A similar magnetron
etching apparatus is also described in Japanese Patent Laid-Open (Kokai)
No. 57-159026 by Haruo Okano et al. In these apparatuses, electrons are
drifted in a direction perpendicular to the electric and magnetic fields
upon application of a magnetic field in a direction perpendicular to the
high-frequency electric field. As a result, collision of the electrons
with the gas is activated, and a discharge plasma density is increased.
Since the density of an ion current supplied to the sample on the cathode
electrode is increased, the etching rate can be increased to about ten
times as compared with the conventional RIE apparatus. Therefore, the
magnetron etching apparatus can have a sufficiently high throughput even
if one substrate is etched, thereby obtaining a compact apparatus.
Another conventional etching apparatus using a coil as a magnetic field
applying means is also known. An example of this etching apparatus is
described in a Paper "SiO.sub.2 High-Speed Etching", Kyungshik Kim et al.,
7th Dry Process Symposium, p. 95, 1985. A magnetron etching apparatus
using a coil is also described in Japanese Patent Laid-Open (Kokai) No.
63-17530 by Owen Wilkinson.
As described in Japanese Patent Publication No. 12346 and a paper
"Double-source Excited Reactive Ion Etching and Its Application to
Submicron Trench Etching", Masaaki Sato and Yoshinobu Arita, Extended
Abstracts of the 18th (1986 International) Conference on Solid State
Devices and Materials, Tokyo 233 (1986), which describe triode plasma
etching apparatuses for independently controlling the ion energy, the
current density, and the radial concentrations, an additional cathode is
arranged to oppose a cathode electrode on which an object to be etched is
placed, and a grid comprising a mesh or perforated plate is arranged as a
common anode electrode between the cathode electrodes.
FIG. 7 is a schematic view for explaining the triode plasma etching
apparatus. Reference numeral 21 denotes a vacuum chamber; 9, a vacuum pump
connected to the vacuum chamber 21; 8, a gas supply system connected to
the vacuum chamber 21; 1 and 2, first and second cathode electrodes
arranged in the vacuum chamber 21; 6 and 7, high-frequency power sources
respectively connected to the cathode electrodes 1 and 2; 5, blocking
capacitors arranged between the high-frequency power source 6 and the
cathode electrode 1 and between the high-frequency power source 7 and the
cathode electrode 2, respectively; 3, a grid arranged between the cathode
electrodes 1 and 2; and 4, an object to be etched, which is placed on the
cathode electrode 1. In the triode plasma etching apparatus, a discharge
area opposite to the cathode electrode 2 is divided by the grid 3 from a
discharge area opposite to the cathode electrode 1 on which the object to
be etched is placed. Upon application of high-frequency power to the
cathode electrode 2, decomposition and ionization of the gas are
accelerated to increase the density of active radicals and a plasma
density of the discharge area of the cathode electrode 1 which supports
the object through the grid 3. Therefore, the amount of ion current
supplied to the object and the amount of active radicals can be increased.
As compared with the counterelectrode type plasma etching apparatus, the
etching rate can be increased to 2 to 4 times. In addition, the width of
the ion sheath formed on the surface of the object to be etched is
decreased with an increase in ion current density. Therefore, disturbance
of the ions caused by collision with gas molecules in the ion sheath can
be suppressed. Therefore, etching in a depth about 10 times the width of
0.25 .mu.m of a submicron area can be performed with high precision.
In the conventional magnetron RIE apparatus using a permanent magnet, the
magnetic field generated by the permanent magnet is fixed, and flexibility
of etching conditions is degraded. For example, the ion energy and its
current density cannot be independently controlled. In addition,
high-precision etching uniformity control cannot be performed. In the
apparatus using the coil, although flexibility of the etching conditions
can be improved as compared with the apparatus using a permanent magnet,
it is difficult to independently control the ion energy and its current
density.
In various types of magnetron etching apparatuses in which magnetic fields
are changed as a function of time, e.g., a coil or a permanent magnet is
physically moved, or a current supplied to a coil is changed as a function
of time, a change in magnetic field is slower than a change in
high-frequency electric field as a function of time. This causes
variations in ion energy and its directivity, thus causing element damage
and degradation in etching shapes.
In the conventional triode etching apparatus, flexibility of the etching
conditions is increased, and the ion energy and its current density can be
independently controlled. In addition, discharge variations as a function
of time can be eliminated, so that damage and degradation of etching
shapes can be eliminated. However, the throughput of the triode etching
apparatus is not sufficient as compared with the magnetron etching
apparatus due to the following reason. The plasma generated in the
discharge area of the cathode electrode 2 is diffused to the discharge
area of the cathode electrode 1, and a large amount of the plasma
generated in the discharge area of the cathode electrode 2 is not guided
to the object 4 but is diffused to be recombined. Therefore, even if the
high-frequency power applied to the cathode electrode 2 is increased, an
increase in density of the ion current supplied to the object 4 is
saturated. In addition, nonuniform etching also occurs by the influence of
the grid 3.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a plasma etching
apparatus wherein an etching rate can be increased while good etching
characteristics are maintained.
It is another object of the present invention to provide a plasma etching
apparatus wherein uniformity of a plasma potential on a surface of a first
cathode electrode can be improved as compared with a conventional etching
apparatus.
It is still another object of the present invention to provide a plasma
etching apparatus wherein a density of an ion current supplied to an
object to be etched can be further increased.
In order to achieve the above object of the present invention, there is a
plasma etching apparatus comprising an anode electrode arranged to oppose
a first cathode electrode and connected to a constant voltage, magnetic
field applying means for applying a magnetic field whose lines of magnetic
force pass near the first cathode electrode and a second cathode
electrode, the second cathode electrode being an annular electrode to
surround the first cathode electrode, and high-frequency power sources
respectively connected to the first and second cathode electrodes.
In a means for enhancing interference between discharges generated by the
high-frequency voltage, frequencies of high-frequency voltages applied
from the high-frequency power sources to the cathode electrodes are set to
be equal to each other, and phases of the high-frequency voltages applied
to the first and second cathode electrodes are set to be variable.
In this case, the phase of the high-frequency voltage applied to the second
cathode electrode advances first cathode electrode by 0 to 1/3 the
wavelength.
The magnetic field applying means comprises a first magnetic field
generating coil arranged on a side of the first cathode electrode which is
opposite to a side on which the second cathode electrode is preset, a
second magnetic field generating coil arranged on a side of the second
cathode electrode which is opposite to a side on which the first cathode
electrode is present, and coil power sources for supplying opposite
currents to the first and second magnetic field generating coils, wherein
the currents are controlled such that the lines of magnetic force are
almost parallel to the first cathode electrode.
The magnetic field applying means comprises a first magnetic field
generating coil located opposite to the anode electrode and near the first
cathode electrode, a second magnetic field generating coil arranged to be
surrounded by the second cathode electrode, a third magnetic field
generating coil arranged to surround the second cathode electrode, and
coil power sources for supplying currents having the same direction to the
first and third magnetic field generating coils and a current to the
second magnetic field generating coil in a direction opposite to the first
and third magnetic field generating coils. Alternatively, the magnetic
field applying means comprises a first magnetic field generating coil
located opposite to the anode electrode and near the first cathode
electrode, a second magnetic field generating coil arranged to surround
the first magnetic field generating coil and having a diameter larger than
that of the first magnetic field generating coil, a third magnetic field
generating coil arranged to be surrounded by the second cathode electrode,
a fourth magnetic field generating coil arranged to surround the second
cathode electrode, and coil power sources for supplying currents having
the same direction to the first and fourth magnetic field generating coils
and currents to the second and third magnetic field generating coils in a
direction opposite to the first and fourth magnetic field generating
coils, wherein the currents of the power sources are controlled such that
the lines of magnetic force are almost parallel to the surfaces of the
first and second cathode electrodes.
The anode electrode opposite to the first cathode electrode is connected to
a constant voltage, and the second cathode electrode is the annular
electrode which surrounds the first cathode electrode. The magnetic field
applying means is arranged to apply magnetic fields whose lines of
magnetic force pass near the first and second cathode electrodes, so that
the density of a plasma enclosed by the magnetic fields is increased. In
addition, since electrons are moved along the lines of magnetic force,
plasma interference by the high-frequency voltages applied to the first
and second cathodes is increased. Since the anode electrode connected to
the constant voltage is present opposite to the first cathode electrode,
uniformity of the potential of the plasma enclosed between the first
cathode electrode and the anode electrode is improved as compared with the
conventional apparatus. The grid used in the conventional triode RIE
apparatus to assure uniformity of the plasma potential can be omitted. For
this reason, the plasma generated by the high-frequency power applied to
the second cathode electrode is effectively guided near the first cathode
electrode, and the density of the ion current applied to the object to be
etched can be increased. In addition, since the grid is not used, slight
nonuniformity of the plasma density which is caused by the presence of the
grid can be eliminated.
Since the frequencies of the high-frequency voltages applied from the
high-frequency power sources to the cathode electrodes are set to be equal
to each other, the variations in high-frequency voltages as a function of
time, which are applied to the first and second cathode electrodes, can be
eliminated. The phase of the high-frequency voltage applied to the second
cathode electrode is advanced from that applied to the first cathode
electrode by 0 to 1/3 the wavelength, so that the density of the plasma
near the object to be etched can be increased, and the density of the ion
current supplied to the object to be etched can be increased.
At this time, the magnetic field applying means comprises the first
magnetic field generating coil arranged on a side of the first cathode
electrode which is opposite to the side on which the second cathode
electrode is present, the second magnetic field generating coil arranged
on a side of the second cathode electrode which is opposite to the side on
which the first cathode electrode is present, and the coil power sources
for supplying currents in the opposite directions to the first and second
magnetic field generating means, so that the lines of magnetic force are
almost parallel to the surface of the first cathode electrode. The
direction of the magnetic flux on the electrode surface is parallel to the
direction of the electric field, so that the electrons are subjected to
drift motion to turn above the electrode. Collision between the electrons
and the gas is enhanced to increase the plasma density. In addition, since
the lines of magnetic force pass in the radial direction of the object to
be etched, mobility of electrons in the radial direction can be enhanced.
Therefore, uniformity of the plasma density on the object to be etched can
be improved.
Further, in this case, the magnetic field applying means comprises the
first magnetic field generating coil arranged opposite to the anode
electrode and near the first cathode electrode, the second magnetic field
generating coil arranged to be surrounded by the second cathode electrode,
the third magnetic field generating coil arranged to surround the second
cathode electrode, and the coil power sources for supplying the currents
having the same direction to the first and third magnetic field generating
coils and a current to the second magnetic field generating coil in a
direction opposite to that supplied to the first and third magnetic field
generating coils, wherein the currents are supplied such that the lines of
magnetic force almost parallel to the first cathode electrode are also
almost parallel to the second cathode electrode. The direction of the
magnetic flux is perpendicular to the direction of the electric field on
the first and second cathode electrodes. The electrons are subjected to
drift movement to turn above both the electrodes, and collision between
the electrons and the gas is enhanced. Therefore, the plasma density is
increased. Since the electrons are moved along the lines of magnetic
force, plasma interference by the high-frequency voltages applied to the
first and second cathode electrodes is increased. The phase of the
high-frequency voltage applied to the second cathode electrode is advanced
from that applied to the first cathode electrode by 0 to 1/3 the
wavelength, so that the plasma density near the object to be etched is
increased, and the density of the ion current supplied to the object to be
etched is increased. Furthermore, since the lines of magnetic force pass
in the radial direction of the object to be etched, the electrons can be
easily moved in the radial direction. Therefore, the uniformity of the
plasma density on the object to be etched can be improved.
The magnetic field applying means comprises a first magnetic field
generating coil located opposite to the anode electrode and near the first
cathode electrode, a second magnetic field generating coil arranged to
surround the first magnetic field generating coil and having a diameter
larger than that of the first magnetic field generating coil, a third
magnetic field generating coil arranged to be surrounded by the second
cathode electrode, a fourth magnetic field generating coil arranged to
surround the second cathode electrode, and coil power sources for
supplying currents having the same direction to the first and fourth
magnetic field generating coils and currents to the second and third
magnetic field generating coils in a direction opposite to the first and
fourth magnetic field generating coils, wherein the currents of the power
sources are controlled such that the lines of magnetic force almost
parallel to the first cathode electrode are also almost parallel to the
second cathode electrode. The same effect as in the arrangement having
three coils is obtained in this apparatus. However, the arrangement having
four coils can set so that the lines of magnetic force are perfectly
parallel to the first and second cathode electrodes as compared with the
arrangement having three coils, and the plasma density can be further
uniformed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing a plasma etching apparatus according to
an embodiment of the present invention;
FIG. 2 is a graph showing etching uniformity and currents supplied to coils
10 and 11;
FIG. 3 is a waveform chart showing a waveform of a high-frequency voltage
(Va) applied to an ion sheath on a cathode electrode 1, a waveform of a
high-frequency voltage (Vc) applied to a cathode electrode 2, a waveform
of a difference (Vb) between a potential of a plasma near the ion sheath
on the cathode electrode 1 and a potential of a plasma near the ion sheath
on the cathode electrode 2 when phases between the cathode electrodes 1
and 2 are shifted from each other;
FIG. 4 is a schematic view showing a plasma etching apparatus according to
another embodiment of the present invention;
FIG. 5 is a schematic view showing a plasma etching apparatus according to
still another embodiment of the present invention;
FIGS. 6a, b and c are views showing lines of magnetic force when the
numbers of coils is two, three, and four coils; and
FIG. 7 is a schematic view showing a conventional triode plasma etching
apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a plasma etching apparatus according to an embodiment of the
present invention. Referring to FIG. 1, reference numeral 10 denotes a
first magnetic field generating coil arranged on a side of a first cathode
electrode 1 which is opposite to a side on which a second cathode
electrode 20 is located; 11, a second magnetic field generating coil
arranged on a side of the cathode electrode 20 which is opposite to the
side on which the cathode electrode 1 is located; and 12 and 13, coil
power sources for supplying currents to the magnetic field generating
coils 10 and 11. The magnetic field generating coils 10 and 11 and the
coil power sources 12 and 13 constitute a magnetic field applying means.
Reference numerals 16 and 17 denote high-frequency amplifiers connected to
the cathode electrodes 1 and 20, respectively; 18, an oscillator connected
to the high-frequency amplifiers 16 and 17; and 19, a phase controller
arranged between the oscillator 18 and the high-frequency amplifier 17.
The high-frequency amplifiers 16 and 17, the oscillator 18, and the phase
controller 19 constitute high-frequency power sources. Reference numeral
22 denotes a grounded anode electrode opposite to the cathode electrode 1;
and 20, a second cathode electrode, i.e., an annular electrode arranged
between the cathode electrode 1 and the anode electrode 22. In this
embodiment, the second cathode electrode 20 comprises a cylindrical body.
Reference numeral 23 denotes an insulating material for insulating the
cathode electrodes 1 and 20 from a ground potential. The second cathode
and the coils are almost concentric with each other.
An SiO.sub.2 film formed on a single-crystal Si is etched by the plasma
etching apparatus of FIG. 1 by using an organic resist film pattern as a
mask. A vacuum chamber 21 is evacuated by a vacuum pump 9, and a reactive
gas such as CHF.sub.3 gas is supplied from a gas supply system 8 to the
vacuum chamber 21 at a flow rate of 50 SCCM to set a pressure of the
vacuum chamber to be 0.8 Pa. At the same time, currents of 9,000 A turn
and 4,000 A turn are supplied from the coil power sources 12 and 13 to the
magnetic field generating coils 10 and 11, respectively. Therefore, a
magnetic field having a magnetic flux density of several tens to several
hundreds of gauss on the cathode electrode 1 and lines of magnetic force
which are almost parallel to the cathode electrode 1 and pass near the
cathode electrode 20 is applied. High-frequency powers of 150 W and 200 W
having frequencies of 13.56 MHz are applied from the high-frequency
amplifiers 16 and 17 to the cathode electrodes 1 and 20. The reactive gas
is decomposed and ionized to generate a plasma, thereby etching an object
4 to be etched.
In this case, an electric field formed upon application of the
high-frequency power to the cathode electrode 1 is almost perpendicular to
the surface of the cathode electrode 1 near this electrode. Since the
magnetic field has a component parallel to the cathode electrode 1,
electrons are subjected to cyclotron movement around the lines of magnetic
force and are drifted in a direction, i.e., the ExB direction,
perpendicular to the electric field E and the magnetic flux density B.
This direction is a circularly turning direction along the surface of the
cathode electrode 1. Therefore, collision between the electrons and the
gas is enhanced immediately above the surface of the cathode electrode 1,
thereby increasing the plasma density. In a conventional triode RIE
apparatus, when a magnetic field is applied to a sample, a plasma
potential is set to be a larger negative potential toward the center of
the cathode electrode 1 in the absence of the grid 3, and it is therefore
difficult to assure etching uniformity. However, in the plasma etching
apparatus of this embodiment, since the anode electrode 22 is arranged
opposite to the cathode electrode 1 and the anode electrode is located in
a direction in which mobility of electrons which are easily moved in the
direction of the lines of magnetic force is increased. Therefore, the
high-frequency current is supplied between the cathode electrode 1 and the
anode electrode 22, and uniformity of the plasma potential on the surface
of the cathode electrode 1 can be improved. For this reason, the grid can
be omitted, and slight etching nonuniformity can be eliminated. In
addition, loss of the plasma density upon insertion of the grid can also
be eliminated. Even if the high-frequency powers applied to the cathode
electrodes are equal to each other, the plasma density can be increased,
and the density of the ion current supplied to the object 4 can be
increased. The etching rate can be increased, and variations in ion
incidence direction can be perfectly suppressed. In addition, ion energy
can be reduced, and damage to the object 4 can be minimized.
According to experiments of the present inventors, when a magnetic field
was not applied in etching using the plasma etching apparatus shown in
FIG. 1, an etching rate was about 80 nm/min. However, when the magnetic
field was applied in etching, the etching rate was increased to about 500
nm/min. A relationship between the currents supplied to the coils 10 and
11 and etching uniformity is shown in FIG. 2. The lines of magnetic force
were radially spread from the center of the cathode electrode 1 upon
current control, and the density of electrons in the radial direction of
the electrode could be uniformed. Therefore, etching uniformity could fall
within the range of .+-.2% on a 10-cm wafer.
In the plasma etching apparatuses shown in FIG. 1, since the frequencies of
the high-frequency voltages applied to the cathode electrodes 1 and 20 are
equal to each other, an increase in maximum ion energy caused by a beat of
the two frequencies does not occur. In addition, the phase of the
high-frequency voltage applied to the cathode electrode 20 is shifted from
that applied to the cathode electrode 1, and therefore the interference
between the discharge cycles of the high-frequency voltages applied to the
cathode electrodes 1 and 20 is changed. The plasma density is changed
accordingly, and the plasma density distribution between the electrodes
can be changed. This phenomenon is caused by an effect of an electron flow
upon a change in potential distribution in the plasma and will be
described as follows. FIG. 3 shows a waveform of a high-frequency voltage
(Va) applied to the ion sheath on the cathode electrode 1, a waveform of a
high-frequency voltage (Vc) applied to the sheath of the cathode electrode
20, and a waveform of a difference (Vb) between the plasma potential near
the ion sheath on the cathode electrode 1 and the plasma potential near
the ion sheath on the cathode electrode 20. When no phase difference is
present (.theta.=0), no potential difference occurs (Vb=0). However, when
the phase of the high-frequency voltage applied to the cathode electrode
20 is advanced (.theta.=.pi./4), the phase of the difference Vb is close
to that of the voltage Va and is opposite to that of the voltage Vc. When
motion of electrons is taken into consideration and the voltage Va is
almost equal to the potential of the electrode, the difference Vb is
increased, and the electrons are concentrated on the cathode electrode 1,
thereby increasing the plasma density near the object to be etched. To the
contrary, when the phase of the high-frequency voltage applied to the
cathode electrode 20 is lagged (.theta.=-.pi./4), electrons are
concentrated near the cathode electrode 20, and the plasma density near
the object to be etched is decreased. Therefore, when the phase of the
high-frequency voltage applied to the cathode electrode 20 is slightly
advanced from the phase of the high-frequency voltage applied to the
cathode electrode 1, etching at a maximum ion current density and a
minimum ion energy can be performed. According to the experiments of the
present inventors, when the phase of the high-frequency voltage applied to
the cathode electrode 20 was advanced from that applied to the cathode
electrode 1 by 0 to +1/2 (inclusive) the wavelength, the electrons tended
to be concentrated on the cathode 1. However, since high-frequency
interference caused a decrease in plasma density upon an increase in phase
difference as a whole, the wavelength range in which an increase in plasma
density occurred as compared with no phase difference was given such that
the phase of the voltage applied to the cathode electrode 20 fell within
the range of 0 to +1/3 the wavelength. The plasma density was maximized by
the interfence between the two high-frequency powers in the range of +1/8
to +1/4 the wavelength. The phase different which causes the maximum
plasma density is changed upon a change in ratio of high-frequency powers
supplied to the two electrodes. More specifically, when the high-frequency
power supplied to the cathode electrode 20 is increased, the phase
different which causes the maximum plasma density is increased.
In the above embodiment, the cylindrical electrode is used as the second
cathode electrode 20. However, if the second cathode electrode 20 can
surround the cathode electrode 1, the shape of the second cathode
electrode 20 is not limited to the cylindrical shape, but can be a
polygonal hollow shape, an elliptical shape, or an incomplete annular
shape if the drift of electrons by E x B has a turning motion. In this
case, the same voltage need not be applied to the divided portions, and
the phases of these divided portions need not be equal to each other. In
the above embodiment, the pressure of the vacuum chamber 21 is set to be
0.8 Pa. However, the pressure in the vacuum chamber 21 may fall within the
range of 0.1 to 100 Pa. CHF.sub.3 is used as an etching gas. However,
other halogen-containing gases or oxygen-containing gases may be used and
other objects to be etched may be employed to obtain the same effect as
described above.
In the etching apparatus shown in FIG. 1, two coils are used to apply
magnetic fields. However, other means such as permanent magnets may be
used to produce a magnetic flux distribution for obtaining the same effect
as described above.
FIG. 4 shows another embodiment of the present invention. This embodiment
employs three coils. Reference numeral 24 denotes a first magnetic
generating coil arranged to oppose an anode electrode 22 and near a
cathode electrode 1; 25, a second magnetic field generating coil arranged
to be surrounded by a cathode electrode 20; 26, a third magnetic field
generating coil arranged to surround the cathode electrode 20; and 27, 28,
and 29, coil power sources for supplying currents to the magnetic field
generating coils 24, 25, and 26. The magnetic field generating coils 24,
25, and 26 and the coil power sources 27, 28, and 29 constitute a magnetic
field applying means. In this case, the currents having the same direction
are supplied to the magnetic field generating coils 24 and 26, and an
opposite current is supplied to the magnetic field generating coil 25.
FIG. 5 shows still another embodiment using four coils. Reference numeral
30 denotes a first magnetic field generating coil arranged opposite to an
anode electrode 22 and near a cathode electrode 1; 31, a second magnetic
field generating coil arranged to surround a cathode electrode 20 and
having a diameter larger than that of the first magnetic field generating
coil 30; 32, a third magnetic field generating coil arranged to be
surrounded by the cathode electrode 20; 33, a fourth magnetic field
generating coil arranged to surround the cathode electrode 20; and 34, 35,
36, and 37, coil power sources for supplying currents to the magnetic
field generating coils 30, 31, 32, and 33. The magnetic field generating
coils 30, 31, 32, and 33 and the coil power sources 34, 35, 36, and 37
constitute a magnetic field applying means. In this case, a current having
the same direction is supplied to the magnetic field generating coils 30
and 33, and a current having a direction opposite to that of the coils 30
and 33 is supplied to the magnetic field generating coils 31 and 32.
The lines of magnetic force from the two coils (the embodiment in FIG. 1),
the three coils (the embodiment in FIG. 4), and the four coils (the
embodiment in FIG. 5) are shown in FIGS. 6(a), 6(b), and 6(c),
respectively. In the arrangement having two coils, the lines of magnetic
force passing parallel to the electrode surface near the cathode electrode
20 cross the cathode electrode 1. However, in the arrangement having three
or four coils, the lines of magnetic force passing parallel to the
electrode surface near the cathode electrode 20 can also be parallel to
the surface of the cathode electrode 1. In particular, in the arrangement
having the four coils, the values of currents supplied to the coils can be
optimized, and the parallel relationship between the lines of magnetic
force and the cathode electrode 1 can be further improved. When a magnetic
field whose lines of magnetic force passing parallel to the surfaces of
the first and second electrodes 1 and 20 is applied, electrons are
subjected to drift motion to turn almost parallel to the surface of the
cathode electrode 20, so that the discharge is of a magnetron type.
Therefore, collision between the electrons and the gas is enhanced to
increase the plasma density. In addition, the electrons are subjected to
the turning drift motion with respect to the center of the cathode
electrode 1, so that discharge on the surface of the cathode electrode 1
is of a magnetron type. Therefore, the plasma density can be increased.
Furthermore, since the lines of magnetic force pass near the cathode
electrodes 1 and 20, the interference between the discharge cycles by the
high-frequency voltages applied to the respective cathode electrodes can
be enhanced. When the phase of the high-frequency voltage applied to the
cathode electrode 20 is advanced from that applied to the cathode
electrode 1 by 0 to 1/3 the wavelength, the plasma density near the
cathode electrode 1 can be further increased. The density of the ion
current supplied to the object 4 to be etched can be increased
accordingly. The etching rate can therefore increased. According to the
experiments of the present inventors, etching uniformity is closely
associated with the parallel relationship between the lines of magnetic
force passing near the cathode electrode 1 and its surface. Etching
uniformity can be improved when the parallel relationship has higher
precision. Therefore, the etching conditions for better uniformity can be
easily obtained when the four coils which achieve the better parallel
relationship are used.
When the plasma etching apparatus according to the present invention is
used in the fabrication process of semiconductor devices, etching of
smaller micropatterning width than the widths obtained by the conventional
process can be performed with a high throughput. In addition, damage to
the semiconductor devices can be reduced, the manufacturing cost of
semiconductor devices can be reduced, and the performance of the
semiconductor devices can be remarkably improved.
The anode electrode opposite to the first cathode electrode is connected to
a constant voltage, and the second cathode electrode is the annular
electrode which surrounds the first cathode electrode. The magnetic field
applying means is arranged to apply magnetic fields whose lines of
magnetic force pass near the first and second cathode electrodes, so that
the density of a plasma enclosed by the magnetic fields is increased. In
addition, since electrons are moved along the lines of magnetic force,
plasma interference by the high-frequency voltages applied to the first
and second cathodes is increased. The plasma density can be increased.
Since the high-frequency power to be applied to the first cathode
electrode is increased while the ion energy is kept low, an etching rate
can be increased while good etching characteristics are maintained. Since
the anode electrode connected to the constant voltage is present opposite
to the first cathode electrode, uniformity of the potential of the plasma
enclosed between the first cathode electrode and the anode electrode is
improved as compared with the conventional apparatus. The grid used in the
conventional triode RIE apparatus to assure uniformity of the plasma
potential can be omitted. For this reason, the plasma generated by the
high-frequency power applied to the second cathode electrode is
effectively guided near the first cathode electrode, and the density of
the ion current applied to the object to be etched can be increased. In
addition, since the grid is not used, slight nonuniformity of the plasma
density which is caused by the presence of the grid can be eliminated.
Therefore, etching u | | |
