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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 treatment apparatus, and more
particularly to a plasma treatment apparatus utilizing the electron
cyclotron resonance (referred to as ECR hereinafter) discharge.
2. Description of the Background Art
In manufacturing of a semiconductor device such as an IC (Integrated
Circuit), treatment such as formation of thin film and etching are applied
on the surface of a semiconductor substrate (wafer). Lately, a plasma
treatment apparatus using plasma generated by ECR discharge has been
developed and put into practical use as a apparatus for processing such a
semiconductor substrate. A structure of a conventional plasma treatment
apparatus using plasma generated by ECR discharge will be described below.
FIG. 12 is a sectional view schematically showing a structure of a
conventional plasma treatment apparatus. Referring to FIG. 12, a plasma
treatment apparatus 310 includes a reaction chamber 101, an
electromagnetic coil 103, a microwave generating source 104, a waveguide
105, a microwave introducing window 106, a pipe 107, an exhaust hole 108,
and a sample table 109.
One end of waveguide 105 is attached to the upper portion of reaction
chamber 101. Waveguide 105 and reaction chamber 101 are separated by
microwave introducing window 106. The other end of waveguide 105 is
attached to microwave generating source 104.
Electromagnetic coil 103 is provided to surround the periphery of reaction
chamber 101. Pipe 107 which supplies a reactive gas is provided at the
upper portion of reaction chamber 101. Exhaust hole 108 is provided at the
bottom of reaction chamber 101. A pump (not shown) is attached to exhaust
hole 108. Sample table 109 is attached inside reaction chamber 101. A
wafer 120 can be placed on sample table 109.
A conventional plasma treatment method will be described.
Referring to FIG. 12, a residual gas in reaction chamber 101 is well
exhausted from exhaust hole 108. Then a reactive gas is introduced into
reaction chamber 101 through pipe 107. While the reactive gas is
introduced, a portion of the reactive gas is exhausted through exhaust
hole 108, so that gas pressure in reaction chamber 101 will be kept at a
predetermined value.
Microwave is then generated from microwave generating source 104. The
microwave will be introduced into reaction chamber 101 through waveguide
105 and microwave introducing window 106. Meanwhile, a magnetic field,
which gradually attenuates from the upper portion of reaction chamber 101
to wafer 120, will be formed in reaction chamber 101 by rendering
electromagnetic coil 103 conductive. Plasma is produced by the
electromagnetic field and the microwave. Electrons in the plasma move
around a magnetic line of force by the Lorentz's force due to a magnetic
field. By adjusting the strength of the magnetic field so that a frequency
of the circular motion and the frequency of the microwave will be
coincided (that is, adjusting magnetic flux density B such that it
satisfies f=.vertline.q.vertline..multidot.B/2 .pi.m for the microwave
frequency f), energy of the microwave will be converted effectively into
kinetic energy of electrons by resonance absorption. This is referred to
as electron cyclotron resonance. Usually, the microwave having the
frequency of 2.45 GHz, which is a frequency used in the industry, is often
used, while the resonance magnetic field of 875 Gauss is often used.
Electrons in the reactive gas in reaction chamber 101 are accelerated by
absorbing the microwave energy and move circularly at a high speed. The
electrons making circular motion at a high speed collide with the reactive
gas molecule, so that the reactive gas in reaction chamber 101 will be
ionized at a high ionization ratio for generating a high density gas
plasma. The electrons in the plasma are restricted by the magnetic field
and move spirally around a magnetic line of force: its momentum will be
changed in the direction of the magnetic line of force by the attenuating
field, and they travel to wafer 120.
An electric field, so called an ion sheath electric field, which is
perpendicular to the surface of wafer 120, will be generated at the
surface of wafer 120 by the travel of the electrons. In the ion sheath
electric field, plasma side is made positive and the surface of wafer 120
side is made negative. Reactive ions in the plasma, which are positive
ions, are accelerated in the direction to wafer 120 by the ion sheath
electric field. The reactive ions are incident upon the surface of wafer
120, and thus, treatment such as etching will be effected on the surface
of wafer 120 with these ions.
The conventional plasma treatment apparatus 310 using such ECR discharge
has such characteristics that the microwave energy are absorbed
effectively in electrons, and that electrons are hard to dissipate in the
radial direction of reaction chamber 101 due to the magnetic field.
Accordingly, a high density plasma can be produced even in a low gas
pressure environment where it is difficult to maintain plasma. Thus, the
conventional plasma treatment apparatus 310 using ECR discharge is broadly
used currently.
Generally, however, the velocity of the thermal motion of electrons in the
plasma produced in reaction chamber 101 is sufficiently higher than that
of ions. In other words, mobility of electrons in the plasma is greater
than ions. Thus the electrons reach the inner wall of reaction chamber 101
earlier than ions, and vanish. Accordingly, in the center of reaction
chamber 101 (a dash-dotted line c--c), relatively large number of ions are
left behind, and the plasma potential at ECR plane 130 in the radial
direction of reaction chamber 101 will be as shown in FIG. 13, where ECR
plane means a plane having a magnetic flux density B which satisfies
f=.vertline.q.vertline..multidot.B/2 .pi.m for the microwave frequency f,
while q denotes the amount of electron charge, and m denotes electron
mass.
Referring to FIG. 13, abscissa denotes a position of reaction chamber 101
in radial direction, and ordinate denotes plasma potential. The plasma
potential becomes higher positive potential toward the direction of the
arrow. The plasma potential indicates the highest positive potential in
the center of reaction chamber 101 because relatively large number of ions
having positive charges are left behind, and the potential becomes lower
toward the inner wall of reaction chamber 101. Meanwhile, an ion sheath
region is formed in the vicinity of the inner wall of reaction chamber 101
since the electromobility of electrons is greater than ions, thus the
plasma potential in this region drops abruptly. The plasma potential at
ECR plane 130 is thus become non-uniform. The above plasma potential being
non-uniform in the radial direction of reaction chamber 101 is described
in Howe: J.A.P. 24 (1953) 892.
According to the Boltzman relationship, a distribution of electron density
n.sub.e in the radial direction of reaction chamber 101 is represented as:
n.sub.e (x)=n.sub.e (0) exp (e.phi.(x)/k.sub.B T.sub.e)
where x: distance from the center of reaction chamber, n.sub.e (x):
electron density at point x, .phi. (x): a potential at point x
(.phi.(0)=0), k.sub.B : Boltzman constant, T.sub.e : electron temperature,
and e: charge of one electron.
According to energy conservation equation and its subsequent equation, a
distribution of ion density n.sub.i in the radial direction of reaction
chamber 101 is represented as:
##EQU1##
where M: ion mass and v: average velocity of ions.
Accordingly, when the distribution of plasma potential in the radial
direction of reaction chamber 101 is such as shown in FIG. 13, both the
distribution of electron density (n.sub.e) and the distribution of ion
density (n.sub.i) are decreased from the center to the inner wall of
reaction chamber 101 as shown in FIG. 14.
In conventional plasma treatment apparatus 310 utilizing ECR discharge
shown in FIG. 12, the distributions of plasma potential, electron density
and ion density in the radial direction of reaction chamber 101 are not
uniform. Thus, conventional plasma treatment apparatus 310 has the
following disadvantages.
FIG. 15A is a schematic diagram showing magnetic lines of force developed
within the reaction chamber. FIGS. 15B and 15C are schematic diagrams
respectively showing electrons at P2 and Q2 in FIG. 15A. Referring to FIG.
15A, a magnetic line of force 140 which diverges from the upper portion of
reaction chamber 101 to wafer 120 is formed in reaction chamber 101 by
rendering an electromagnetic coil conductive (not shown). Electrons in
plasma in reaction chamber 101 travel along magnetic line of force 140
from the upper portion of reaction chamber 101 to wafer 120. The electron
density in the ECR plane in reaction chamber 101 is high in the center of
reaction chamber 101, while it is low at the periphery. Thus, in the ECR
plane there are more electrons in the center of the reaction chamber 101
than at the periphery. Thus, there are more electrons which travel along
magnetic line of force 140p formed in the center of reaction chamber 101
than those which travel along magnetic line of force 140q formed at the
periphery. In other words, as shown in FIGS. 15B and 15C, the number of
electrons traveling along magnetic line of force 140p at P.sub.2 is
greater than the number of electrons traveling along magnetic line of
force 140q at Q.sub.2.
Thus, the number of electrons traveling along magnetic line of force 140 in
the center of reaction chamber 101 is not equal to that at the periphery.
Also, there are more electrons incident on unit area on the surface of
wafer 120 (i.e. the electron current density) in the center of wafer 120
than at the periphery.
Particularly when a film to be etched on wafer 120 is not conductive, the
surface of wafer 120 will be charged not uniformly because of the
non-uniformity of the number of electrons which are incident on the
surface of wafer 120.
FIG. 16 is a schematic sectional diagram of the wafer showing the surface
of the wafer being charged up not uniformly. Referring to FIG. 16, wafer
120 includes a substrate 121, a film to be etched 122 which is deposited
on the surface of substrate 121, and a resist pattern 123. Resist pattern
123 is patterned to have a desired form, and thus an exposed surface 122a
in the center (a dash-dotted line c--c) of the reaction chamber and an
exposed surface 122b at the periphery are exposed respectively from resist
pattern 123. As described above, there are more electrons incident at
exposed surface 122a than at exposed surface 122b, since the electron
current density becomes heavier toward the center of wafer 120. Thus,
exposed surface 122a will be charged up deeply negative than the exposed
surface 122b.
Generally, ions 182 in the plasma will be incident on the surface of wafer
120 until the surface of wafer 120 is in electrically steady state, in
other words, the electron current density and the ion current density
become equal. Accordingly, the number of ions 182 which are incident at
exposed surface 122a is greater than ions 182 which are incident at
exposed surface 122b. Etching rate will be higher at exposed surface 122a
than at exposed surface 122b. As a result, the amount of etching is
relatively large at exposed surface 122a in the center of wafer 120 (a
dash-dotted line c--c), while the amount of etching is relatively small at
exposed surface 122b at the periphery as shown in FIG. 17. Thus, when the
distribution of electron density becomes non-uniform, a film to be etched
in the wafer may not possibly be etched uniformly.
Also, if the area of exposed surface 122a or the like is large, the amount
of etching at the center (line c--c) is large and the amount of etching is
small in the periphery even within one exposed surface 122a as shown in
FIG. 18, possibly causing non-uniform etching.
Non-uniformity of etching might occur also when the film to be etched is
conductive. FIG. 19 is a schematic sectional diagram of the wafer showing
that non-uniformity of etching occurs when the film to be etched is
conductive. Referring to FIG. 19, wafer 120 includes a substrate 124, an
insulating film 125 which is formed at the surface of substrate 124, a
conductive film 126, and a resist pattern 127 which is formed at the
surface of conductive film 126. Resist pattern 127 is patterned to have a
desired form and conductive film 126 will be etched using resist pattern
127 as a mask. During etching, the electron current density is higher in
the center of the reaction chamber (a dash-dotted line c--c) than at its
periphery. However, when the film to be etched is conductive, such as in
the case of polycrystalline silicon, electrons will move in the conductive
film 126. Conductive film 126 is thus charged up negatively on the entire
surface of 120 uniformly. Accordingly, the number of ions incident on each
exposed surface of conductive film 126 will be equal, resulting in uniform
amount of etching.
However, if the conductive film 126 is overetched, underlying insulating
film 125 formed of silicon oxide film or the like will be exposed. As
insulating film 125 is not conductive, non-uniformity of etching might
occur as described above in which the amount of etching is large at
exposed surface 125a in the center of the reaction chamber (a dash-dotted
line c--c) while the amount of etching of exposed surface 125b as the
periphery is small.
As described above, etching on the surface of the wafer might be
non-uniform when the electron density in the ECR plane becomes
non-uniform.
Further, when the distribution of the electron density in the ECR plane is
non-uniform, local deviation of the distribution of the electron density
will occur as shown at B of the FIG. 20, which is so called drift
unstability. This is described, e.g. in Motohiko Tanaka and Takaharu
Nishikawa, "Physics of High Temperature Plasma" (Maruzen). When the drift
unstability occurs, random movement of ions will be enhanced because ions
in the plasma move along the local deviation of the electron density.
FIG. 21 shows behavior of ions in the reaction chamber when drift
unstability occurs. Referring to FIG. 21, when the random movement of ions
182 is enhanced, the number of ions proceeding into an ion sheath region
150 with a small approach angle .theta. is increased. The travel direction
of ions within ion sheath region 150 is determined by the sum of vectors
of ions at the time of approaching into ion sheath region 150 and vectors
of the ion sheath electric field E. Thus, when the approach angle .theta.
is small, ions are hardly incident vertically on the surface of wafer 120.
FIG. 22 is an enlarged partial sectional view of the wafer schematically
showing ions incident on the wafer. Referring to FIG. 22, wafer 120
includes a film to be etched 128 and a resist pattern 129 which is formed
on film to be etched 128. Resist pattern 129 is patterned to have a
desired form, and underlying film to be etched 128 will be etched using
resist pattern 129 as a mask. During the etching, if an ion 182 is not
incident vertically on the surface of wafer 120, then ion 182 will collide
against the sidewall of a groove 128a. The sidewall of groove 128a will be
etched and removed as shown by a dotted-line 128b accordingly, so that
highly anisotropic etching will be difficult.
As described above, there was a problem where the anisotropic property of
etching might be impaired because of unstability of drift due to a
non-uniformity of the electron density.
Generally, most ions move along the flow of electrons traveling along the
magnetic line of force. If the ion density in the ECR plane is not uniform
as shown in FIG. 14, then the number of ions incident on wafer 120 is
considered not to uniform, either. Referring to FIG. 12, the number of
ions incident at the center of wafer 120 will be greater than the number
of ions incident on the periphery of wafer 120, because the ion density in
the center of reaction chamber 101 (a dash-dotted line c--c) is relatively
large, while that is relatively small at the periphery of reaction chamber
101. Thus, the etching of wafer 120 will not be carried out uniformly.
As described above, there was a problem of non-uniform etching when the ion
density in the ECR plane becomes not uniform.
When the distribution of the plasma potential becomes non-uniform as shown
in FIG. 13, anisotropic property of the etching might be impaired, which
will be described in detail below.
FIG. 23A is a schematic sectional view of the reaction chamber showing the
reduction of anisotropic property of the etching when the distribution of
the plasma potential is not uniform. FIG. 23B shows the distribution of
the plasma potential in the plane indicated by a dotted-line 131 in FIG.
23A. Mainly referring to FIG. 23A, in the conventional plasma treatment
apparatus, the plasma potential in the radial direction of reaction
chamber 101 becomes non-uniform in general, due to the difference in
mobility of electrons as well as ions. In other words, the plasma
potential becomes non-uniform also in 131 plane 131 of the radial
direction in addition to ECR plane 130. Ions in the plasma have positive
charges, so that they tend to move to where the plasma potential is low.
Thus, ion 182 which travels from the upper portion of reaction chamber 101
to wafer 120 will move from the center of reaction chamber 101 to the
periphery. As a result, the approach angle .theta. of ion 182 which
proceeds into ion sheath region 150 is reduced, so that its anisotropic
property might be decreased as described above.
As described above, non-uniformity of the plasma potential might decrease
the anisotropic property of etching.
SUMMARY OF THE INVENTION
An object of the present invention is to apply a good plasma treatment over
a semiconductor substrate such as a wafer, a surface layer thereof and the
like.
Another object of the present invention is to improve etching uniformity on
a semiconductor substrate such as a wafer, a surface layer thereof and the
like.
Still another object of the present invention is to improve anisotropic
property of etching over a semiconductor substrate such as a wafer, a
surface layer thereof and the like.
A further object of the present invention is to improve anisotropy and
uniformity of etching on a semiconductor substrate such as a wafer, a
surface layer thereof and the like by eliminating non-uniformity of
distribution of plasma potential, electron density, and ion density in a
reaction chamber.
In order to accomplish above objects, a plasma treatment apparatus
according to one aspect of the present invention performs a prescribed
treatment at a main surface of a substrate by generating plasma through
electron cyclotron resonance discharge above the main surface of the
substrate which is placed in a reaction chamber, and is provided with a
peripheral wall portion which surrounds the plasma, which portion includes
a first peripheral wall portion disposed such that it can apply a first
voltage and a second peripheral wall portion which is insulated from the
first peripheral wall portion and disposed such that it can apply the
second voltage different from the first voltage; and the apparatus is
provided with power supply for applying the first and the second voltages
to the first and the second peripheral wall portions, respectively.
The plasma treatment apparatus is structured such that different potentials
are applied respectively to the first and the second peripheral wall
portions by the power supply. Accordingly, it becomes possible to control
the movement of electrons and ions near the first and the second
peripheral wall portions in a reaction chamber by applying a potential
difference to the first and the second peripheral wall portions.
Therefore, for example, the electron density near the peripheral wall
portion in the distribution of electron density in the radial direction of
the reaction chamber can be increased, and thus non-uniformity in the
distribution of the electron density in the radial direction of the
reaction chamber can be avoided. Therefore, the distribution of the
electron density or the ion density can be made uniform, and accordingly
the distribution of the plasma potential can be made uniform. Thus,
etching can be accomplished uniformly and .with a high anisotropic
property.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view schematically showing a structure of a plasma
treatment apparatus according to a first embodiment of the present
invention.
FIG. 2 is a schematic sectional view of the reaction chamber showing the
interior of the reaction chamber when the plasma treatment apparatus is in
operation.
FIG. 3 is an enlarged view of R.sub.1 in FIG. 2 showing the movement of
electrons when a negative voltage is applied to the first portion.
FIG. 4 shows a distribution of electron density in the ECR plane acquiring
to the first embodiment of the present invention.
FIG. 5A is a schematic diagram showing electrons at P.sub.1 of the FIG. 2.
FIG. 5B is a schematic diagram showing electrons at Q.sub.1 of the FIG. 2.
FIG. 6 is a schematic sectional view of a wafer showing the surface of the
film to be etched being uniformly charged up.
FIG. 7 is a schematic sectional view of the wafer showing etching of the
film to be etched when the surface of the film to be etched is uniformly
charged up.
FIG. 8 is an enlarged sectional view schematically showing exposed surface
22a of FIG. 7 after etching.
FIG. 9 is a sectional view schematically showing a structure of a plasma
treatment apparatus according to a second embodiment of the present
invention.
FIG. 10 is a sectional view schematically showing a structure of a plasma
treatment apparatus according to a third embodiment of the present
invention.
FIG. 11A is an enlarged view schematically showing R.sub.2 of FIG. 10 when
the plasma treatment apparatus according to the third embodiment of the
present invention is in operation.
FIG. 11B shows the plasma potential in the ECR plane of the plasma
treatment apparatus according to the third embodiment of the present
invention.
FIG. 12 is a sectional view schematically showing a structure of a
conventional plasma treatment apparatus.
FIG. 13 shows the distribution of the plasma potential in the ECR plane of
the conventional plasma treatment apparatus.
FIG. 14 shows the distributions of the electron density (N.sub.e) and the
ion density (n.sub.i) in the ECR plane of the conventional plasma
treatment apparatus.
FIG. 15A schematically shows a magnetic line of force generated in the
reaction chamber of the plasma treatment apparatus shown in FIG. 12.
FIG. 15B schematically shows electrons at P.sub.2 in FIG. 15A.
FIG. 15C schematically shows electrons at Q.sub.2 in FIG. 15A.
FIG. 16 is a schematic sectional view of the wafer showing the surface of
the film to be etched being charged up not uniformly.
FIG. 17 is a schematic sectional view of the wafer showing etching of the
film to be etched when the film to be etched is charged up not uniformly.
FIG. 18 is an enlarged sectional view schematically showing the shape of
exposed surface 122a in FIG. 17 after etching.
FIG. 19 is a schematic sectional view of the wafer showing non-uniformity
of etching caused when the film to be etched is conductive.
FIG. 20 shows drift unstability caused when the distribution of the
electron density becomes non-uniform in the conventional plasma treatment
apparatus.
FIG. 21 shows behavior of ions in the reaction chamber when the drift
unstability occurs.
FIG. 22 is an enlarged partial sectional view of the wafer schematically
showing ions being incident on the wafer.
FIG. 23A is a schematic sectional view of the reaction chamber showing that
anisotropy of etching is decreased when the distribution of the plasma
potential is non-uniform.
FIG. 23B shows the distribution of the plasma potential in the surface
shown by a dotted-line 131 in FIG. 23A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A plasma treatment apparatus according to the present invention will be
described with reference to the accompanying drawings.
A structure of a plasma treatment apparatus according to a first embodiment
of the present invention will be described.
Referring to FIG. 1, a plasma treatment apparatus 10 includes a reaction
chamber 1, power supply 2a, an electromagnetic coil 3, a microwave
generating source 4, a waveguide 5, a microwave introducing window 6, a
pipe 7, an exhaust hole 8 and a sample table 9.
One end of waveguide 5 is attached to the upper portion of reaction chamber
1. Waveguide 5 and reaction chamber 1 are separated by microwave
introducing window 6. The other end of waveguide 5 is attached to
microwave generating source 4. Electromagnetic coil 3 is provided in the
periphery so as to surround reaction chamber 1. Pipe 7 which supplies a
reactive gas is provided at the upper portion of reaction chamber 1.
Exhaust hole 8 is provided on the bottom of reaction chamber 1. Sample
table 9 is attached to the interior of reaction chamber 1 so that a wafer
20 can be placed.
Reaction chamber 1 includes a first conductive portion 1a, a second
conductive portion 1b, and an electrically insulating portion 1c. First
portion 1a is provided in the vicinity of the ECR plane 30, and negative
voltage can be applied thereto by way of power supply 2a. Other than first
portion 1a, reaction chamber 1 includes second portion 1b which is
insulated from first portion 1a by insulating portion 1c. Second portion
1b is connected to the ground. Thus, first portion 1a as well as second
portion 1b are structured such that a predetermined potential difference
is provided from power supply 2a.
An operation of the plasma treatment apparatus according to the first
embodiment of the present invention will be described below.
Referring to FIG. 1, a residual gas within reaction chamber 1 is
sufficiently exhausted from exhaust hole 8. Then, a reactive gas is
introduced into reaction chamber 1 through pipe 7. While the reactive gas
is introduced, a portion of the reactive gas is exhausted from exhaust
hole 8, thus keeping gas pressure within reaction chamber 1 at a
predetermined value.
Thereafter a microwave is generated from microwave generating source 4. The
microwave is introduced into reaction chamber 1 through waveguide 5 and
microwave introducing window 6. By rendering conductive electromagnetic
coil 3, a magnetic field which gradually attenuates from the upper portion
of reaction chamber 1 to wafer 20 is formed in reaction chamber 1. When
the microwave and the magnetic field satisfy the above mentioned condition
of cyclotron resonance, ECR discharge will occur in the reactive gas in
reaction chamber 1. Negative potential, for example, about -10.about.-100
V will be applied to first portion 1a in reaction chamber 1 by way of
power supply 2a, and thus the potential difference is provided between
first conductive portion 1a and second conductive portion 1b.
Referring to FIG. 2, a magnetic line of force 40 which diverges from the
upper portion of reaction chamber 1 to wafer 20 within reaction chamber 1
is formed by rendering conductive the electromagnetic coil. Electrons move
along this magnetic line of force 40. However, if negative voltage is
applied to first portion 1a by power supply 2a, movement of the electrons
in the vicinity of first portion 1a will change.
Referring to FIG. 3, electrons possess negative charges. When negative
voltage is applied to first portion 1a, the electrons which have moved
along magnetic line of force 40a in the direction of arrow A receive an
opposite force of magnetic line of force 40a in the direction of arrow B
due to the potential of first portion 1a. Thus, the electrons do not reach
the inner wall of reaction chamber 1, but rather stop in the vicinity of
an end of ion sheath region 60 (a dash-dotted line D--D). Accordingly, the
number of electrons near the inner wall of the reaction chamber 1 will be
increased in ECR plane 30, so that the distribution of the electron
density in ECR plane will be uniform as shown in FIG. 4.
Ions having positive charges are drawn to first portion 1a, since the
negative voltage is applied to first portion 1a. However, the number of
electrons increases in the vicinity of the end (a dash-dotted line D--D)
of ion sheath region 60 which is along the inner wall of reaction chamber
1 as shown in the FIG. 3, and also the mobility of ions is small, so that
ion decay will not increase. Accordingly, the ion density will not
decrease in ion sheath region 60 in reaction chamber 1. This indicates
that the negative potential applied to an electrode is cut off by the
electrons.
If the electron density goes high in the vicinity of the end of ion sheath
region 60 (a dash-dotted line D--D), these electrons receive the microwave
energy in ECR plane 30 and generate plasma. The plasma density near the
end of ion sheath region 60 arises, also-the number of ions increases
there, and thus the ion density can be made uniform. As a result, by
applying negative voltage to first portion 1a, uniformity of the electron
density is supposed to be improved without deteriorating the uniformity of
the ion density at ECR plane.
As shown in FIG. 4, when the distribution of the electron density in ECR
plane is made uniform, the number of electrons which travel along the
magnetic line of force becomes approximately equal in the center and at
the periphery of reaction chamber 1. In other words, referring to FIG. 2,
the number of electrons at P.sub.1 of magnetic line of force 40p and the
number of electrons at Q.sub.1 of magnetic line of force 40q will be
approximately equal as shown in FIGS. 5A and 5B.
Thus, when the number of electrons traveling along the magnetic line of
force in the center becomes approximately equal to that at the periphery,
the same number of electrons will be incident nearly uniformly on unit
area on the surface of the wafer. Even if the film to be etched is not
conductive, the surface of the wafer will be charged up uniformly.
Referring to FIG. 6, a wafer 20 includes a substrate 21, a film to be
etched 22 which is formed on the surface of substrate 21, and a resist
pattern 23. Resist pattern 23 is patterned to have a desired form, and
etching is effected on film to be etched 22 using resist pattern 23 as a
mask. During etching, the same number of electrons are incident on unit
area on the surface of wafer 20. Thereby, respective exposed surfaces. 22a
and 22b which are exposed from resist pattern 23 will be charged up
approximately uniformly with electrons 81. When the wafer surface is thus
charged up uniformly, approximately the same number of ions 82 are
incident on exposed surfaces 22a and 22b respectively. Therefore,
respective surfaces 22a and 22b are etched approximately uniformly as
shown in FIG. 7. It is understood that the etching uniformity of the wafer
is improved when the distribution of the electron density in the ECR plane
is uniform.
On one exposed surface 22a, etching can be done with high uniformity as
shown in FIG. 8, since exposed surface 22a is charged up uniformly.
Further, even if the conductive film, which is the film to be etched, is
overetched, each exposed surface will be etched approximately uniformly as
described above.
Because the distribution of the electron density in the ECR plane becomes
uniform as shown in FIG. 4, so called drift unstability will be hard to
occur. Accordingly, the random movement of ions due to the drift
unstability is restrained, thus preventing the accompanying decrease of
the etching anisotropy. It is understood that the improvement of the
etching anisotropy can be achieved by rendering the distribution of the
electron density uniform.
In a structure of the first embodiment of the present invention, negative
voltage is applied to first portion 1a while second portion 1b is
connected to the ground, thus providing the potential difference between
portions 1a and 1b. However, the present invention is not limited to this
structure, and the similar effect can be obtained by connecting first
portion 1a to the ground and applying positive voltage to second portion
1b so that the potential difference is provided between portions 1a and
1b.
A structure of the plasma treatment apparatus according to a second
embodiment of the present invention will be described.
Referring to FIG. 9, a plasma treatment apparatus 110 according to the
second embodiment of the present invention has an approximately the same
structure as plasma treatment apparatus 10 in the first embodiment except
power supply 2b which applies the potential difference between first
portion 1a and second portion 1b. Radio frequency (RF) power supply is
employed as power supply 2b. A voltage from the radio frequency power
supply is applied to first portion 1a. The remaining second portion 1b is
connected to the ground.
When the radio frequency potential, for example 13.56 MHz, is applied to
first portion 1a by power supply 2b, negative voltage will be generated in
a self-biasing manner in first portion 1a. This is the same situation
where the negative voltage is applied to first portion 1a. It can thus be
expected that approximately the same effect as is obtained in first plasma
treatment apparatus 10 can be obtained in plasma treatment apparatus 110
of the second embodiment.
A plasma treatment apparatus according to a third embodiment of the present
invention will be described.
Referring to FIG. 10, a plasma treatment apparatus 210 in the third
embodiment of the present invention has approximately the same structure
as plasma treatment apparatus 10 in the first embodiment except power
supply 2c which applies the potential difference between first portion 1a
and second portion 1b. More particularly, in plasma treatment apparatus
210, positive potential is applied from power supply 2c to first portion
1a which is provided in the vicinity of ECR plane 30, while second portion
1b is connected to the ground.
An operation of the plasma treatment apparatus according to the third
embodiment of the present invention will be described.
Referring to FIG. 10, the residual gas within reaction chamber 1 is
exhausted sufficiently from exhaust hole 8. Then, the reactive gas is
introduced into the reaction chamber 1 through pipe 7. While the reactive
gas is introduced, a portion of the reactive gas is exhausted from exhaust
hole 8, thus keeping the gas pressure within reaction chamber 1 at a
predetermined value.
The microwave is then generated from microwave generating source 4. The
microwave is introduced into reaction chamber 1 through the waveguide 5
and microwave introducing window 6. Meanwhile, electromagnetic coil 3 is
made conductive. Thereby, a magnetic field which gradually attenuates from
the upper portion of reaction chamber 1 to wafer 20 is formed. As the
microwave and the magnetic field satisfy the condition of the cyclotron
resonance, the reactive gas within reactive container 1 will generate the
ECR discharge. Positive voltage, for example 10.about.100 V, is applied to
first portion 1a by power supply 2c, thus providing the potential
difference between first portion 1a and second portion 1b.
Referring to FIG. 11A, electrons in the plasma move along magnetic line of
force 40 which is formed by the electromagnetic coil (not shown). The
electrons having negative charge and traveling along magnetic line of
force 40c will be pulled in first portion 1a because the positive voltage
is applied to first portion 1a by the supply source 2c. Thus, more
electrons move at portion C in ECR plane 30 than at portion D along the
magnetic line of force 40c to first portion 1a. Accordingly, relatively
large negative charge is lost in ECR plane 30 at the periphery (i.e. near
portion C) of reaction chamber 1. Thus, the plasma potential in ECR plane
30 at the periphery of reaction chamber 1 is raised, and thus the plasma
potential in ECR plane 30 is made uniform, such that the electric field in
the radial direction of reaction chamber 1 is decreased.
By applying positive voltage to first portion 1a, it becomes possible to
make uniform the plasma potential in the radial direction of reaction
chamber 1 as well as ECR plane 30. Ions are thus prevented from moving
from the center to the periphery of reaction chamber 1 as they travel from
the upper portion of reaction chamber 1 to wafer 20 due to non-uniformity
of the electric field. As a result, an approach angle of ions when they
proceed into the ion sheath region becomes relatively large, and thus the
anisotropy could be improved.
However, when the positive voltage is applied to the first portion 1a, the
electron density at the periphery of reaction chamber 1 is supposed to be
decreased. Accordingly, the distribution of the electron density in the
radial direction of reaction chamber 1 becomes further non-uniform, the
drift unstability might be increased, and the accompanying anisotropy of
etching is decreased. Whether the etching anisotropy can be improved or
not may depend on the coordination of the magnetic field, the shape of
reaction chamber 1 and the like.
In plasma treatment apparatus 210 according to the third embodiment of the
present invention, positive voltage is applied to first portion 1a and the
ground potential is applied to second portion 1b, thus providing the
potential difference between portion 1a and portion 1b. Also, when the
ground potential is applied to first portion 1a and the negative voltage
is applied to second portion 1b, the potential difference can be provided
between first portion 1a and second portion 1b as above. It is understood
that approximately the same effect as described above can be obtained.
As described above, the plasma treatment apparatus according to the first,
the second and the third embodiments of the present invention have such
structure that the potential difference is provided between first portion
1a and second portion 1b by applying the positive, negative or radio
frequency (RF) voltage to first portion 1a, thus enabling the control of
the uniformity and anisotropy of etching.
In plasma treatment apparatu | | |
