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Claims  |
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What is claimed is:
1. Plasma production apparatus comprising:
a substantially cylindrical plasma chamber having a longitudinal axis, said
plasma chamber having a prescribed gas therein;
magnetic field generating means for generating a magnetic field with an
associated electron cyclotron frequency .omega..sub.cc and magnetic lines
of force that axially traverse said plasma chamber; and
resonant excitation means for exciting a whistler wave having a frequency
.omega. in said plasma chamber using resonant electromagnetic energy,
where .omega..sub.cc <.omega., said whistler wave ionizing said prescribed
gas, whereby ionized particles exist in said plasma chamber having a
collisional frequency associated therewith, said ionized particles
comprising a plasma;
said resonant excitation means comprising a resonant cavity positioned
adjacent a first end of said plasma chamber, said resonant cavity being
substantially axially aligned with said longitudinal axis so that
electromagnetic energy resonating in said resonant cavity axially couples
into the plasma within said plasma chamber.
2. The plasma production apparatus as set forth in claim 1 further
including plasma delivery means for controlling the delivery of plasma
from said plasma chamber to a workpiece.
3. The plasma production apparatus as set forth in claim 2 wherein said
workpiece is positioned adjacent a second end of said plasma chamber, said
plasma delivery means directing plasma from said plasma chamber to said
workpiece, whereby plasma processing may be carried out on said workpiece.
4. Plasma production apparatus comprising:
a substantially cylindrical plasma chamber having a longitudinal axis, said
plasma chamber having a prescribed gas therein;
magnetic field generating means for generating a magnetic field with an
associated electron cyclotron frequency .omega..sub.cc and magnetic lines
of force that axially traverse said plasma chamber; and
resonant excitation means for exciting a whistler wave having a frequency
.omega. in said plasma chamber using resonant electromagnetic energy,
where .omega..sub.cc <.omega., said whistler wave ionizing said prescribed
gas, said resonant excitation means comprising:
a multiplicity of annular conductive rings within said plasma chamber, said
rings being centered about and uniformly spaced along the longitudinal
axis of the plasma chamber so as to form a slow wave structure inside of
said plasma chamber, and
means for causing an electrical current to flow through said conductive
rings so as to excite resonant electromagnetic energy that is radially
coupled into said plasma, said radially coupled electromagnetic energy
exciting said whistler wave within said plasma chamber;
whereby ionized particles exist in said plasma chamber having a collisional
frequency associated therewith, said ionized particles comprising the
plasma.
5. The plasma production apparatus as set forth in claim 4 wherein the
electrical current flowing in said conductive rings excites said whistler
wave using a TE.sub.01 mode.
6. The plasma production apparatus as set forth in claim 4 wherein the
electrical current flowing in said conductive rings excites said whistler
wave using a TE.sub.11 mode.
7. Apparatus for exciting a gas to produce a laser comprising:
a substantially cylindrical plasma chamber having a longitudinal axis, said
plasma chamber having a prescribed gas therein;
magnetic field generating means for generating a mangetic field with an
associated electron cyclotron frequency .omega..sub.cc and magnetic lines
of force that axially traverse said plasma chamber; and
resonant excitation means for exciting a whistler wave having a frequency
.omega. in said plasma chamber using resonant electromagnetic energy,
where .omega..sub.cc <.omega., said whistler wave ionizing said prescribed
gas;
said plasma chamber being closed at both ends;
said closed plasma chamber and the plasma confined therein comprising part
of an optical system that allows coherent light to reflect between two
reflective surfaces, one placed at each end of said plasma chamber;
said resonant excitation means including a resonant cavity that surrounds
at least a portion of said plasma chamber;
said resonant excitation means including means for exciting electromagnetic
energy within said resonant cavity that is radially coupled into said
plasma chamber so as to excite said whistler wave, said whistler wave, in
turn, ionizing said prescribed gas and producing coherent light that
reflects between said two reflective surfaces, thereby producing the
laser.
8. The apparatus as set forth in claim 7 wherein said resonant cavity
comprises a slotted resonant cavity surrounding said plasma chamber, said
slotted resonant cavity having an inner wall at a first radius from said
longitudinal axis, said slotted resonant cavity having periodically
slotted gaps, the spacing of said gaps being determined by the wavelength
of the excited whistler wave, and a solid outer wall spaced at a second
radius from said longitudinal axis, said second radius being greater than
said first radius.
9. Plasma production apparatus comprising:
a cylindrical cavity having a prescribed gas therein;
a magnetic field generator that generates a magnetic field having magnetic
lines of force that axially traverse said cylindrical cavity; and
an electromagnetic launcher that generates and launches resonant
electromagnetic energy into the gas held in said cylindrical cavity, said
electromagnetic energy exciting a whistler wave that axially propagates
through said cylindrical cavity following said magnetic lines of force and
ionizes said prescribed gas, thereby producing a plasma;
said electromagnetic launcher comprising:
a resonant cavity positioned adjacent a first end of said cylindrical
cavity, said resonant cavity being substantially axially aligned with a
longitudinal axis of said cylindrical cavity, and
means for exciting the resonant electromagnetic energy in said resonant
cavity, said resonant electromagnetic energy being coupled axially into
said cylindrical cavity.
10. Plasma production apparatus comprising:
a cylindrical cavity having a prescribed gas therein;
a magnetic field generator that generates a magnetic field having magnetic
lines of force that axially traverse said cylindrical cavity; and
an electromagnetic launcher that generates and launches electromagnetic
energy into the gas held in said cylindrical cavity, said electromagnetic
energy exciting a whistler wave that axially propagates through said
cylindrical cavity following said magnetic lines of force and ionizing
said prescribed gas, thereby producing a plasma;
said electromagnetic launcher comprising
a multiplicity of annular conductive rings within said cylindrical cavity,
said rings being centered about and uniformly spaced along a longitudinal
axis of the cylindrical cavity so as to form a slow wave structure inside
of said cylindrical cavity, and
means for exciting an electrical current in said conductive rings so as to
excite resonant electromagnetic energy within said cylindrical cavity,
said resonant electromagnetic energy being coupled radially into the gas
contained within said cylindrical cavity.
11. A method of producing plasma comprising:
(a) selectively injecting a specified gas into a cylindrical cavity, said
cylindrical cavity having a longitudinal axis;
(b) generating a magnetic field having an associated electron cyclotron
frequency .omega..sub.cc and magnetic lines of force that are
substantially parallel to said longitudinal axis; and
(c) generating resonant electromagnetic energy in a resonant cavity, said
resonant cavity being positioned adjacent said cylindrical cavity; and
(d) exciting a whistler wave in said cylindrical cavity by coupling the
resonant electromagnetic energy from said resonant cavity into said
cylindrical cavity, said whistler wave propagating through said
cylindrical cavity following said magnetic lines of force and ionizing
said prescribed gas, thereby producing a plasma, said whistler wave having
a frequency .omega., where .omega..sub.cc >.omega..
12. The method as set forth in claim 11 wherein step (c) further includes
axially coupling the resonant electromagnetic energy into said cylindrical
cavity.
13. The method as set forth in claim 11 wherein step (c) further includes
radially coupling the resonant electromagnetic energy into said
cylindrical cavity.
14. The method as set forth in claim 11 wherein said plasma is a high
density plasma. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for producing high
density plasma for use with plasma processing and other applications, such
as high power lasers. More particularly, the invention relates to plasma
production apparatus and methods that axially or radially excite whistler
waves in a cylindrical plasma imbedded in a high magnetic field.
A plasma is an ionized gas. Because a plasma is a gas, it exhibits fluid
characteristics that allow it to fill a desired space, assume a specific
shape, or otherwise be formed for desired purposes. Because a plasma is an
ionized gas, it is electrically conductive, meaning that electrical
currents can flow therethrough, and the plasma can be controlled and
managed to a certain extent through the application of magnetic and
electric fields. Because a plasma is ionized, the ionized atoms and atomic
particles therein may be chemically active or energetic, and can thereby
also be used to trigger or promote a desired chemical reaction or physical
process, e.g., the removal of material, as is done in plasma etching.
Most known applications for using plasma are significantly enhanced if the
density of the plasma can be increased and maintained. Disadvantageously,
most known techniques for making and maintaining a plasma do not result in
a high density plasma. Hence, there is a need in the art for high density
plasma production techniques.
There are several ways in which a plasma can be made. One of the most
effective ways to make a plasma is to inject microwave energy into a gas.
The energy associated with the microwave signal ionizes molecules and
atoms in the gas, thereby forming the plasma Unfortunately, there is a
limit to how dense the plasma can become As the plasma begins to form and
become more dense, for example, it also becomes more conductive and starts
to appear as an electrical short. Such an electrical short can reflect the
microwave signal out of the plasma. Thus, the microwave energy may only be
able to penetrate into the plasma a short distance before it is reflected
out of the plasma. For this reason, the prior art teaches limiting the
thickness of the plasma into which the microwave energy is injected. See,
e.g., U.S. Pat. No. 4,507,588 (Asmussen et al.); U.S. Pat. No. 4, 585,668
(Asmussen et al.); U.S. Pat. No. 4,691,662 (Roppel et al.); and U.S. Pat.
No. 4,727,293 (Asmussen et al.); wherein the plasma is confined to a very
shallow disk.
Unfortunately, a shallow plasma disk is of limited utility for many plasma
processing applications. There are at least two reasons for this. First,
the "loss rate" of the plasma in a shallow disk may be higher than the
loss rate for a "long" or "deep" plasma. (The "loss rate" of a plasma is
the rate at which the plasma is lost either through the ions and electrons
in the plasma recombining to form neutral molecules and atoms in the gas
or through the ions and electrons hitting the walls of the containment
vessel. In the formation of a plasma, an equilibrium point is thus reached
where the ion production rate equals the ion loss rate. The loss rate may
depend on such factors as the surface to volume ratio.) Second, a shallow
plasma disk does not generally provide a sufficient volume of plasma for
efficient use in downstream processing applications. Downstream processing
applications preferably position the microwave plasma formation apart from
the location where the plasma is used. See, e.g., Plasma Processing of
Materials, p. 31 National Research Council, (National Academy Press,
Washington, D.C., 1991). It would thus be desirable for the plasma volume
positioned upstream from the location where the plasma is used to be a
relatively large volume, such as a "long" or "deep" plasma cylinder, or
equivalent large volume, rather than a relatively small volume, such as a
shallow plasma disk. What is needed, therefore, is a technique that allows
a microwave signal to be injected into a plasma volume without having the
microwave signal reflected back out of the plasma due to the plasma's
conductivity, thereby allowing a "deeper" or "longer" plasma volume, and
thus a potentially larger plasma volume, to be formed and maintained at a
location upstream from the location where the plasma is to be used.
In order to prevent the plasma from shorting out, it is known in the art to
immerse the plasma in a strong magnetic field. The strong magnetic field,
in general, makes it more difficult for the charged particles within the
plasma to cross the magnetic field lines, and thus prevents the charged
particles from shorting out. Hence, by orienting the microwave electric
field used to create the plasma so that it is perpendicular to the
magnetic field in which the plasma is immersed, it is possible to prevent
the shorting of the plasma, and thereby improve the density limit of the
plasma. U.S. Pat. No. 4,101,411 (Suzuki et al.); U.S. Pat. No. 4,401,054
(Matsuo et al.); U.S. Pat. No. 4,810,935 (Boswell); and U.S. Pat. No.
4,876,983 (Fukuda et al.) are all examples of prior art apparatus and
devices that utilize microwaves and a magnetic field for various plasma
processing operations.
However, even when a magnetic field is used to prevent the plasma from
shorting, the injected microwave signal is still subject to damping, and
such damping imposes a further density limit on the plasma. What is
needed, therefore, is a technique for injecting microwaves into a plasma
while increasing the density limit imposed by the damping of the microwave
signal.
Two sources of damping have been identified in the prior art. The first is
collisional damping, caused by collisions between electrons associated
with the injected microwave energy and the ions and neutral gas molecules
present in the plasma. The more dense the ions or molecules in the plasma,
the more collisions that occur, and the more difficult it is for the wave
to penetrate further into the plasma. Collisional damping is believed to
be the factor that has heretofore limited the available plasma density in
the prior art devices. See, e.g., U.S. Pat. No. 4,990,229 (Campbell et
al.), where the use of an excitation frequency of 13.56 MHz for the
microwave energy creates a collision frequency on the order of
2.5.times.10.sup.8 sec.sup.-1. Such a collision frequency corresponds to a
plasma density of about 10.sup.19 m.sup.-3 (10.sup.13 cm.sup.-3). It would
be desirable if a plasma density greater than 10.sup.13 cm.sup.-3 could be
achieved.
The second source of damping is collisionless damping, also known as Landau
damping. Landau damping results when the particles in the plasma have a
velocity nearly equal to the phase velocity of the microwave signal
injected into the plasma. The theory is that because the particles in the
plasma travel with the microwave signal, they do not see a rapidly
fluctuating electric field, and hence can effectively exchange energy with
the microwave signal. Further, although there are electrons in the plasma
that travel faster and slower than the microwave signal, the distribution
of electrons is such that there are more slow electrons than fast
electrons. Hence, there are more particles taking energy from the
microwave signal than adding to it, and the microwave signal becomes
quickly damped. Landau damping is best controlled by assuring that the
phase velocity of the injected microwave signal is sufficiently larger
than the thermal velocity of the particles in the plasma.
It is known in the art to use a so called "whistler wave", also known as a
helicon wave, in a plasma producing apparatus. See, e.g., U.S. Pat. No.
4,990,229 (Campbell, et al.). A whistler wave propagates along the
magnetic field lines. Its frequency should be much less than the electron
cyclotron frequency, .omega..sub.ce. (The electron cyclotron frequency,
.omega..sub.ce, is equal to eB/mc where e and m are the electron charge
and mass, respectively; B is the magnetic field strength; and c is the
speed of light.) In order to excite the desired whistler wave in the
plasma, Campbell, et al. show particular types of antenna configurations
used to surround the plasma chamber of a given plasma processing device.
These antenna configurations are determined by the frequency of the rf
excitation that is used, which Campbell, et al. teach, must be a low
frequency, e.g. 13.56 MHz. Collisional damping thus remains the limiting
factor for configurations such as those shown in Campbell et al. Hence,
what is needed is a means of exciting plasma, e.g., by using whistler mode
microwave signals, in a way that increases the density limit caused by
collisional damping.
The present invention advantageously addresses the above and other needs.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided
apparatus that creates a high density plasma in a long cylindrical cavity.
As used herein, the term "high density plasma" refers to a plasma having a
density in excess of about 10.sup.12 cm.sup.-3. The cylindrical cavity,
and hence the plasma, is imbedded in a high magnetic field, with magnetic
lines of force passing axially (longitudinally) through the cavity. In one
embodiment, electromagnetic radiation is coupled axially into the
cylindrical cavity using a resonant cavity in order to excite a whistler
wave in the cylindrical cavity, and hence in the plasma. In another
embodiment, electromagnetic radiation is coupled radially into the
cylindrical cavity using a slow wave structure in order to excite the
whistler wave in the plasma. In either embodiment, the plasma is
advantageously created without using electrodes; and the excitation of the
whistler wave is achieved at a high Q value, thereby allowing radio
frequency (rf) power to be transmitted into the plasma at a good
efficiency. For purposes of the present application, the definition of "Q"
is that 1/Q is proportional to the fraction of energy lost per cycle of
oscillation. Thus, if Q is large, a larger amount of energy may be stored
in the resonant cavity. By "high Q" value, it is meant that the Q of the
resonant circuit or cavity in the absence of a plasma must be high enough
so that the dominant power loss will be to the plasma.
In accordance with another aspect of the invention, various geometries are
provided for coupling energy between the resonant cavity and the plasma.
Such varied geometries advantageously allow the invention to be used for
numerous applications, for example, plasma processing applications, such
as plasma etching, stripping or deposition; high power laser excitation
applications; ion source applications; or sputtering gun applications.
For plasma processing applications, such as plasma etching, stripping or
deposition, the invention provides the requisite coupling between a
resonant cavity and a plasma column. A strong magnetic field is axially
applied to the plasma column. The magnetic field has an electron cyclotron
frequency .omega..sub.ce associated therewith (.omega..sub.ce =eB/mc, as
previously described). A whistler wave having a sufficiently high
frequency, .omega., e.g., .omega./2.pi.=2.45 GHz, is excited in the plasma
column. Advantageously, the use of the whistler wave at such a high
frequency increases the limiting collision frequency, thereby increasing
the achievable plasma density. However, the use of higher frequencies and
densities requires a totally different excitation geometry from that used
in the prior art. The present invention advantageously provides such
different excitation geometries. In a preferred geometry, for example,
axial coupling is provided between a resonant cavity and the plasma
column, with the resonant cavity being positioned at one end of the plasma
column. In an alternative geometry, multiple loop structures are placed
within the plasma chamber in order to radially excite the desired whistler
wave in the plasma. Advantageously, such geometries yield plasma densities
well in excess of those achievable using prior art devices. Further, the
use of the resonant cavity allows for a high Q-value, thereby providing
for the efficient coupling of rf energy into the plasma.
For high power laser applications, an axial excitation is inconvenient
because it interferes with the optical system, which optical system
typically utilizes two facing mirrors between which the lasing medium
resonates. Hence, the present invention provides that the whistler wave is
excited with a radial excitation geometry that comprises a microwave
cavity with periodically slotted gaps that surround a cylindrical plasma
chamber. The spacing of the gaps is determined by the wavelength of the
excited mode.
For high density ion source applications, accelerating grids are placed at
the end of the plasma column in either a radially or axially excited
system to create a high density ion beam.
For sputtering gun applications, a sputtering target is placed in front of
the grids of a high density ion source. The ion beam may then be directed
towards the target, thereby creating a high density sputtering apparatus.
It should thus be apparent that it is a feature of the invention to provide
apparatus and methods for producing dense plasma usable in a wide variety
of applications, such as plasma processing, e.g., plasma etching,
stripping or deposition; lasers; ion sources; or sputtering guns.
It is a further feature of the invention to provide configurations or
geometries wherein microwave or other rf energy can be transmitted to
penetrate into a gas confined within a suitable "long" cavity while
allowing a high density plasma to be formed within the cavity. In
particular, it is a feature of the invention to provide such apparatus and
geometries wherein high frequency microwave energy, e.g., greater than 1
Ghz, may be coupled into the plasma so as to excite a whistler wave
therein, which high frequency whistler wave advantageously allows for a
much higher plasma density.
It is an additional feature of the invention to provide a technique for
axially or radially exciting high frequency whistler waves in a
cylindrical plasma imbedded in a high magnetic field.
It is yet a further feature of the invention to provide a means for
exciting plasma without using electrodes.
It is still an additional feature of the invention to provide a means of
exciting a whistler wave in a cylindrical plasma through the use of a
resonant cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features and advantages of the present
invention will be more apparent from the following more particular
description thereof, presented in conjunction with the following drawings
and Appendix wherein:
FIG. 1 illustrates the interface between a microwave cavity and a plasma,
and schematically depicts how an input wave excites both slow and fast
waves in the plasma, as well as reflects from the plasma interface;
FIG. 2A shows a preferred configuration for axially coupling rf energy into
a plasma chamber so as to excite a whistler wave therein;
FIG. 2B similarly shows a preferred configuration for radially coupling rf
energy into a plasma chamber so as to excite a whistler wave therein;
FIG. 3 diagrammatically illustrates a plasma processing device made in
accordance with the present invention utilizing the axial coupling shown
in FIG. 2A;
FIG. 4A shows a perpendicular cross section of a TE.sub.01 slow wave
structure that may be used to radially excite a whistler mode in a plasma
processing device in accordance with the present invention;
FIG. 4B shows a longitudinal cross section of the structure of FIG. 4A;
FIG. 4C depicts a cross sectional profile of the axial magnetic field
component associated with the structure of FIG. 4A;
FIG. 5A shows a perpendicular cross section of a TE.sub.11 slow wave
structure that may be used to radially excite a whistler mode in a plasma
processing device in accordance with the present invention;
FIG. 5B shows a longitudinal cross section of the structure of FIG. 5A; and
FIG. 6 diagrammatically illustrates a slotted resonant cavity used to
radially excite a whistler wave in a high power gas laser.
Appendix A provides a mathematical analysis of the use of a whistler wave
to produce a high density plasma.
Corresponding reference characters indicate corresponding components
throughout the several views of the drawings.
DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best mode presently contemplated for
carrying out the invention. This description is not to be taken in a
limiting sense, but is made merely for the purpose of describing the
general principles of the invention. The scope of the invention should be
determined with reference to the claims.
As indicated above, the present invention relates to the production of a
high density plasma by axially or radially exciting whistler waves (one
form of rf energy) in a cylindrical plasma imbedded in a high magnetic
field. The strength of the magnetic field is sufficiently high so that the
electron cyclotron frequency is several times greater than the wave
frequency. In general, two conditions must be met in order to produce a
high density plasma. First, a wave of rf energy must be transmitted so as
to propagate into the plasma, thereby causing the requisite discharge to
occur that creates and maintains the plasma. Second, once transmitted, the
wave must not damp too quickly. A whistler wave advantageously does not
have a high density cutoff associated therewith. Hence, such wave can
easily penetrate into the plasma to sustain the requisite discharge,
thereby fulfilling the first condition. The second condition means that
the rf frequency must be higher than the electron collision frequency with
the plasma ions and the neutral gas molecules, and the wave must not
experience excessive collisionless damping Hence, the second condition may
be satisfied largely through the proper selection of the rf frequency for
the particular application at hand.
As will be evident from the description that follows, the present invention
provides specific geometries that may be used to excite rf energy in the
form of a whistler wave in a deep or long plasma cavity, thereby promoting
the formation and maintenance of high density plasma within such cavity.
Both the geometry of the launcher and the geometry of the plasma cavity
are important. Such geometries will vary somewhat depending upon the
particular application for which the plasma is being used.
Before describing some of the preferred geometries associated with the
invention, it will be helpful to first present a brief overview of some
basic concepts applicable to the coupling of rf energy into a plasma
chamber. The description presented below in connection with FIGS. 1, 2A
and 2B is intended to provide such a brief overview. A mathematical
analysis of such coupling principles may be found in Appendix A, attached
hereto and incorporated herein by reference. A good overview of the role
plasma processing now plays, and will play in the years to come, may be
found in Plasma Processing of Materials, Scientific Opportunities and
Technological Challenges, National Research Council (National Academy
Press, Washington, D.C. 1991).
Referring first to FIG. 1, a plasma chamber 20 is shown adjacent a
microwave cavity 22. An interface 24 separates the microwave cavity 22
from the plasma chamber 20. The interface 24 is located (assuming an
appropriate coordinate system having a z-direction that is horizontal for
the orientation shown in FIG. 1) at z=0. In general, the plasma chamber 20
is cylindrically shaped, having a longitudinal axis 26 passing through the
center thereof in the z-direction, thereby allowing the formation of a
"deep" or "long" plasma (as compared to a shallow or thin disk-shaped
plasma, as described in the prior art). A deep or long plasma is important
because it provides a larger plasma mass having a lower loss rate, thereby
giving rise to a higher density plasma. In addition, a deep or long plasma
better provides for the possibility of downstream plasma processing. Note
that as shown in FIG. 1, the microwave cavity 22 is aligned with the
longitudinal axis 26 of the plasma chamber.
In order to couple rf energy into the plasma chamber, an input wave W.sub.i
is excited in the microwave cavity 22, or equivalent rf energy source, so
as to propagate in the z-direction towards the plasma chamber 20. The
presence of a strong magnetic field B.sub.0, having lines of force that
are substantially parallel to the longitudinal axis 26, helps guide the
input wave W.sub.i in the desired direction, and further helps confine the
plasma within the plasma chamber 20. As explained more fully in Appendix
A, if a high density plasma is to be produced, it is important that the
electron cyclotron frequency (associated with the magnetic field B.sub.0)
be much larger than the wave frequency (associated with the whistler
wave), and that the wave frequency, in turn, be larger than the collision
frequency (associated with the creation and maintenance of the plasma).
The input wave W.sub.i, upon encountering the interface 24, excites a slow
wave W.sub.s and a fast wave W.sub.f in the plasma. The fast wave is cut
off at high density. The slow wave continues to propagate in the
z-direction. A portion of the wave W.sub.i is also reflected from the
interface as a wave W.sub.re. The present invention describes various
geometries suitable for directing the input wave W.sub.i into the plasma
chamber 20 so that the desired slow wave W.sub.s, or whistler wave, is
launched in the plasma chamber. The energy associated with the slow wave,
or whistler wave, is then efficiently transferred to the gas in the plasma
chamber 20 in order to ionize the gas, thereby forming the desired plasma.
The plasma thus formed is then available for use for a desired
application.
A preferred approach for launching or coupling rf energy into the plasma
chamber in order to excite the desired whistler wave is illustrated in
FIGS. 2A and 2B. In FIG. 2A, such launching involves axially coupling the
microwave energy into the plasma chamber in order to excite the desired
whistler wave. In FIG. 2B, such launching involves radially coupling the
microwave energy into the plasma chamber in order to excite the desired
whistler wave.
Referring to FIG. 2A, a resonant cavity 30 is positioned at one end of the
plasma chamber 20. Appropriate input power is directed into the resonant
cavity 30 so as to excite a resonant condition, which resonant condition
is manifest by the presence of a resonant signal. A resonant condition
advantageously assures that reflected energy is not wasted, but rather
goes back into the resonance for later use. This is especially true when a
high Q resonance exists. The resonant signal may comprise an appropriate
microwave signal, e.g., of the TE.sub.01 or TE.sub.11 modes, which when it
enters the plasma chamber 20 launches the desired whistler wave in the
plasma. The resonance of the cavity operates at a high Q value, thereby
allowing a portion 32 of the energy associated with the resonant signal to
be efficiently coupled into the plasma chamber 20. As further seen in FIG.
2A, a suitable magnetic field generating means 28, which may comprise a
coil wound around the plasma chamber 20, generates the requisite magnetic
field B.sub.0 needed to help sustain the production of the high density
plasma.
Similarly, referring to FIG. 2B, an annular resonant structure 31 is
positioned around the periphery of the plasma chamber 20. Appropriate
input power is directed into the resonant structure 31 so as to excite a
high Q resonant condition, which resonant condition is manifest by the
presence of a resonant signal. Such resonant signal may comprise an
appropriate microwave signal, e.g., of the TE.sub.01 or TE.sub.11 modes,
which excites the desired axially traveling whistler wave in the plasma.
The magnetic and electrical fields at the boundary of the plasma chamber
20 that are created by the resonant electrical current excite the desired
axially traveling whistler wave in the plasma. The resonance of the cavity
operates at a high Q value, thereby allowing a portion 33 of the energy
associated with the resonant signal to be efficiently coupled into the
plasma chamber 20. As further seen in FIG. 2B, a suitable magnetic field
generating means 29, which may comprise a coil wound around the plasma
chamber 20, generates the requisite magnetic field B.sub.0 needed to help
sustain the production of the high density plasma.
Turning next to FIG. 3, a diagrammatic illustration of a plasma processing
device 40 made in accordance with the present invention is shown. The
device 40 represents one particular geometry that may be used to axially
launch a whistler wave into a plasma chamber 42. As seen in FIG. 3, the
plasma chamber 42 comprises a cylindrical chamber 42 coupled to a suitable
source of gas 44 through an inlet valve 46. The side walls 48 of the
plasma chamber 42 may be made from any suitable material, and are
preferably simply an extension of the walls of a resonant cavity 30'. A
window 49, which may | | |
