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Claims  |
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What is claimed is:
1. Plasma production apparatus comprising:
a conductive cylindrical chamber having a first cross-sectional area and a
first longitudinal axis;
power input means for injecting microwave power into said cylindrical
chamber;
resonating means for causing said microwave power to resonate within said
cylindrical chamber, said resonating means comprising a resonant cavity
positioned within the cylindrical chamber, said resonant cavity having a
conductive end plate at one end thereof and a conductive plunger at the
other end thereof;
a non-conducting plasma formation tube filled with a prescribed gas having
a second cross sectional area less than said first cross-sectional area,
said plasma formation tube having a second longitudinal axis and being
positioned within said cylindrical chamber so that said second
longitudinal axis is substantially coincident with said first longitudinal
axis;
concentrating means for concentrating the resonating microwave power, which
initially fills the first cross-sectional area, down to the second
cross-sectional area of said plasma formation tube, thereby providing an
increased power density in said second cross-sectional area;
means for drawing said microwave power at said increased power density into
said plasma formation tube as a whistler wave, said whistler wave ionizing
said prescribed gas, thereby forming a plasma within said plasma formation
tube; and
means for preventing the formation of plasma in the cylindrical chamber
outside of said plasma formation tube.
2. The plasma production apparatus as set forth in claim 1 wherein said
means for propagating the microwave power through the plasma formation
tube includes magnetic field generating means for generating a magnetic
field with an associated electron cyclotron frequency .omega..sub.ce, and
magnetic field lines that are substantially parallel to said first and
second longitudinal axes, and further wherein said microwave power has a
frequency .omega. associated therewith, where .omega..sub.ce is greater
than .omega..
3. The plasma production apparatus as set forth in claim 2 wherein said
plasma formation tube passes through a center of said conductive end
plate, with a major portion of said plasma formation tube protruding into
said resonant cavity.
4. The plasma production apparatus as set forth in claim 3 wherein said
concentrating means includes means for initiating the formation of a
plasma within said plasma formation tube; said plasma, once initiated,
comprising a dominant power loss for the resonating microwave power within
said resonant cavity; which power loss draws additional microwave power
into said plasma formation tube, which additional microwave power
effectively concentrates additional power within the second
cross-sectional area of said plasma formation tube, thereby providing an
increased power density in the plasma formation tube, which increased
power density sustains the formation of a plasma within said plasma
formation tube having an increased density.
5. The plasma production apparatus as set forth in claim 4 wherein said
plasma formation tube is in fluid communication with the first end of said
cylindrical chamber on a side of said conductive end plate that is not
within said resonant cavity, and wherein said cylindrical chamber includes
an inlet port through which said prescribed gas may flow and a pump port
to which a pump may be attached, said inlet and pump ports allowing said
prescribed gas to enter said plasma formation tube and be maintained at a
prescribed pressure.
6. The plasma production apparatus as set forth in claim 2 further
including a conductive iris plate spaced inwardly within the cylindrical
chamber and apart from said conductive plunger; said conductive iris being
positioned within about a distance .+-.d1 from a tip of said plasma
formation tube, said distance d1 being approximately equal to a diameter
d2 of said plasma formation tube; said conductive iris having an opening
therein through which the resonating microwave power may evanescently
penetrate into said plasma formation tube to initiate the plasma; a
remainder of the plasma formation tube being located outside of said
resonant cavity but still within said cylindrical chamber.
7. The plasma production apparatus as set forth in claim 6 wherein the
opening of said conductive iris has a diameter of at least approximately
d2, whereby the tip of said plasma formation tube may fit inside of said
opening as it is positioned within the distance .+-.dl of said conductive
iris.
8. The plasma production apparatus as set forth in claim 7 wherein said
plasma formation tube comprises a fused quartz tube coaxially mounted
within said cylindrical chamber, an end of said tube opposite the tip
positioned within a distance .+-.d1 of said conductive iris being in fluid
communication with a source of said prescribed gas and a pump means for
maintaining a prescribed pressure of said gas, whereby said fused quartz
tube may be filled with said prescribed gas at said prescribed pressure.
9. The plasma production apparatus as set forth in claim 1 further
including cooling means for cooling said plasma formation tube.
10. The plasma production apparatus as set forth in claim 1 wherein said
means for preventing the formation of plasma in the cylindrical chamber
outside of said plasma formation tube comprises means for maintaining the
area of said cylindrical chamber that lies outside of said plasma
formation tube at atmospheric pressure.
11. The plasma production apparatus as set forth in claim 1 wherein said
means for preventing the formation of plasma in the cylindrical chamber
outside of said plasma formation tube comprises means for evacuating the
area of said cylindrical chamber that lies outside of said plasma
formation tube.
12. Plasma production apparatus comprising:
a conductive substantially cylindrical chamber having a first length and
diameter and a first longitudinal axis;
means for injecting microwave power into a first end of said cylindrical
chamber;
a conductive end plate disposed near a second end of said cylindrical
chamber, said conductive end plate and cylindrical chamber comprising a
resonant cavity wherein said microwave power resonates;
a quartz tube having a second length and diameter less than the first
length and diameter, respectively, of said cylindrical chamber; said
quartz tube being positioned inside of said cylindrical chamber so that a
second longitudinal axis of said quartz tube is substantially coaxial with
the first longitudinal axis of said cylindrical chamber; a first end of
said quartz tube being disposed near the first end of said cylindrical
chamber;
means for filling said quartz tube with a prescribed gas at a prescribed
pressure;
said resonating microwave power being both inside and outside said quartz
tube;
the microwave power inside of said quartz tube ionizing said prescribed gas
to initiate the formation of a plasma within said quartz tube; and
wherein said plasma, once initiated, represents a dominant power loss for
the resonating microwave power, thereby drawing additional microwave power
into said quartz tube, which additional microwave power further serves to
feed and sustain the formation of the plasma within said quartz tube;
whereby plasma is produced within said quartz tube.
13. The plasma production apparatus as set forth in claim 12 further
including means for exciting a mode of said microwave power that causes a
whistler wave to propagate longitudinally through said cylindrical
chamber, including said quartz tube.
14. The plasma production apparatus as set forth in claim 13 wherein said
first end of said cylindrical chamber includes an adjustable plunger that
may be used to tune said resonant cavity.
15. Plasma production apparatus comprising:
a conductive substantially cylindrical chamber having a first length, a
first diameter, and a first longitudinal axis;
means for injecting microwave power into a first end of said cylindrical
chamber;
a conductive iris end plate positioned within said cylindrical chamber and
spaced apart from said first end, a region of said cylindrical chamber
bounded by said first end and said conductive iris end plate comprising a
resonant cavity wherein said microwave power resonates, said conductive
iris end plate having an aperture of a second diameter therein;
a quartz tube having a second length less than the first length, a closed
end and an open end, and a diameter that is approximately the same as said
second diameter, said quartz tube being positioned inside of said
cylindrical chamber so that the closed end of said quartz tube is within a
distance of about .+-.d1 of said conductive iris end plate;
means for filling said quartz tube with a prescribed gas at a prescribed
pressure;
said resonating microwave power evanescently penetrating the closed end of
the quartz tube, said microwave power thus penetrating said quartz tube
ionizing said prescribed gas to initiate the formation of a plasma within
said quartz tube; and
wherein said plasma, once initiated, represents a dominant power loss for
the resonating microwave power, thereby drawing additional microwave power
into said quartz tube, which additional microwave power further serves to
sustain the formation of the plasma within said quartz tube;
whereby plasma is produced within said quartz tube.
16. The plasma production apparatus as set forth in claim 15 wherein said
first end of said cylindrical chamber includes an adjustable plunger that
may be used to tune said resonant cavity.
17. Apparatus for producing high density plasma comprising:
a plasma formation tube having a first cross-sectional area;
means for filling said plasma formation tube with a prescribed gas at a
prescribed pressure;
a resonant cavity having a second cross-sectional area, said second
cross-sectional area being greater than said first cross-sectional area;
means for injecting microwave energy into said resonant cavity and causing
said microwave energy to resonate, said resonating microwave energy having
a power density associated therewith that is a function of said second
cross-sectional area;
means for concentrating the resonating microwave energy from said second
cross-sectional area to said first cross-sectional area, thereby
increasing the power density associated with the microwave energy; and
means for launching the concentrated microwave energy into the plasma
formation tube as a whistler wave;
said whistler wave causing said prescribed gas to ionize, thereby forming a
plasma.
18. The plasma production apparatus of claim 18 wherein said plasma
formation tube is coaxially positioned inside of said resonant cavity.
19. The plasma production apparatus of claim 18 wherein said plasma
formation tube has a closed end, and further wherein said plasma formation
tube is axially aligned with said resonant cavity, with the closed end of
said plasma formation tube being positioned a distance .+-.d1 from one end
of said resonant cavity, where d1 is approximately equal to a diameter d2
associated with said second cross-sectional area.
20. A method for producing high density plasma comprising:
(a) filling a non-conductive plasma formation tube with a prescribed gas at
a prescribed pressure, said plasma formation tube having a first
cross-sectional area;
(b) injecting microwave power into a resonant cavity having a second
cross-sectional area, said second cross-sectional area being greater than
said first cross-sectional area, said microwave power having a power
density associated therewith;
(c) drawing the resonating microwave power from said second cross-sectional
area down to said first cross-sectional area, thereby increasing the power
density of the microwave power; and
(d) launching the concentrated microwave power into the plasma formation
tube as a whistler wave; said whistler wave causing said prescribed gas to
ionize, thereby forming a 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. Nos. 4,507,588 (Asmussen et al.); 4,585,668 (Asmussen et
al.); 4,691,662 (Roppel et al.); and 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
Materials, Scientific Opportunities and Technological Challenges, National
Research Council, p. 31 (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. Nos. 4,101,411 (Suzuki et al.); 4,401,054 (Matsuo et
al.); 4,810,935 (Boswell); and 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
The present invention provides 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 field lines 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 one 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.
In accordance with another aspect of the invention, alternate geometries
are provided that concentrate the microwave power available in the high-Q
resonant cavity into a smaller cross-sectional area. Such concentrated
microwave power thus provides an increased power density which can
advantageously be directed into a plasma column having a correspondingly
smaller cross-sectional area, causing a whistler wave to be excited
therein that produces a higher density plasma than could be produced
otherwise.
One geometry, for example, useful for producing high density plasma,
coaxially places a plasma formation tube inside of the resonant cavity.
The plasma formation tube comprises, e.g., a quartz tube having a
prescribed gas at a prescribed pressure therein. The resonant cavity
comprises a conductive cylindrical chamber, bounded at one end by an
adjustable conductive plunger, and at the other end by a conductive end
plate. The cylindrical chamber has a larger cross-sectional area than the
plasma formation tube. Microwave power at a high frequency, e.g., 2.45
GHz, is injected into the cylindrical chamber and allowed to resonate.
Such power has an initial power density that is a function of the
cross-sectional area of the cylindrical chamber. Some of the power begins
to ionize the prescribed gas in the plasma formation tube, thereby
starting the formation of a plasma. As the plasma forms, it becomes the
dominant power loss (load) for the resonating microwave power. Such power
loss draws additional microwave power into the plasma formation tube,
thereby effectively concentrating the microwave power within the narrower
cross-sectional area of the plasma formation tube, thus increasing the
power density. The resulting increased power density advantageously
sustains the formation of an increased density plasma within the plasma
formation tube.
An alternate geometry, also useful for promoting the formation of a high
density plasma in accordance with the present invention, coaxially aligns
a narrow plasma formation tube with an adjacent, axially aligned, resonant
cavity. The resonant cavity has a cross-sectional area that is larger than
the cross-sectional area of the plasma formation tube. Microwave power at
a high frequency, e.g., 2.45 GHz, is injected into the resonant cavity and
allowed to resonate. A tip of the plasma formation tube is positioned near
one end of the resonant cavity. Prior to plasma formation, the resonating
microwave power penetrates evanescently into just the tip portion of the
plasma formation tube. Such power starts the plasma formation process and
excites a whistler wave in the plasma formation tube. As the plasma forms,
it presents a significant power loss (load) to the resonating microwave
power in the resonant cavity, thereby drawing additional power into the
tip of the plasma formation tube. This action effectively concentrates the
power from the cross-sectional area of the resonate cavity to the smaller
cross-sectional area of the plasma formation tube. Such concentrated power
is then able to be carried further into the plasma formation tube by the
whistler wave, thereby creating a higher density plasma than could
otherwise be achieved.
Variations of the above two coaxial geometries provide useful apparatus for
a plasma applicator used, e.g., for ion implantation, ion beam milling,
ion-assisted deposition, or plasma etching devices and/or systems.
It is significant to note that the above two coaxial geometries are not
equivalent to simply using a smaller diameter conductive plasma formation
tube. For a given microwave frequency, the dimensions of the resonant
cavity cannot decrease the cross-sectional area, determined e.g., by the
diameter of the resonant cavity, below a cut-off cross-sectional area,
e.g, a cut-off diameter. The coaxial geometries of the present invention
advantageously allow the diameter of the plasma formation tube to be
smaller than the cut-off diameter of the resonant cavity, thereby allowing
increased power densities and hence an increased plasma density to be
achieved.
It is thus seen that one feature of the present invention, in accordance
with the above-described two geometries, is to provide plasma production
apparatus that produces high density plasma by concentrating resonant
microwave power from a relatively large cross-sectional area of a resonant
cavity down to a smaller cross-sectional area of a plasma formation tube.
Another feature of the invention is 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 fi | | |
