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BACKGROFUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to an improved plasma generating apparatus
and method which uses magnets to establish a multicusp static magnetic
field around the plasma region which is inside an insulated chamber. In
particular, the present invention relates to an apparatus and method which
uses the magnets to aid in confining the plasma in the chamber and also
produces electron cyclotron resonance (ECR) zones within the chamber which
impart energy to the electrons in the plasma.
(2) Prior Art
The present invention relates to an improvement on the plasma generating
apparatus described in U.S. Pat. No. 4,507,588 by some of the inventors
herein. This patent disclosed some confining magnetic field configurations
without detail as to the mounting of the magnets. The radial positioning
of the elongate magnets on the sliding short was effective in confining
the charged particles, particularly the electrons; however, electron
cyclotron resonance (ECR) was not discussed.
OBJECTS
It is therefore an object of the present invention to provide a plasma
generating apparatus which includes improved means for mounting of magnets
for confining the plasma. Further it is an object of the present invention
to provide a plasma generating apparatus which uses electron cyclotron
resonance (ECR) to impart energy to the plasma at very low pressures
(.ltoreq.10 microns). These and other objects will become increasingly
apparent by reference to the following description and the drawings.
IN THE DRAWINGS
FIG. 1 is a front partial cross-sectional view of the preferred plasma
generating apparatus particularly illustrating the positioning of the
magnets 34 and 35 in relation to a dish or chamber 15 to provide magnetic
cusps 16a inside the dish.
FIG. 2 is a plan cross-sectional view along line 2--2 of FIG. 1.
FIG. 3 is a front partial cross-sectional view of a modified plasma
generating apparatus of the present invention wherein additional magnets
38 are provided around the dish 15.
FIG. 4 is a plan cross-sectional view along line 4--4 of FIG. 3.
FIG. 5 is a cross-sectional view of a sliding short 12 wherein the magnets
35a are mounted on the short.
FIG. 6 is a front partial cross-sectional view of the sliding short 12b and
cylinder 10b wherein the magnets 38a are mounted in the cylinder 10b.
FIG. 7 is a front partial cross-sectional view of a modified plasma
generating apparatus wherein magnets 47 surround an opening 48 in the
plasma region 16d which acts as a magnetic nozzle directing electrons and
ions out of the plasma region.
FIG. 8 is a plan view along line 8--8 of FIG. 7.
FIG. 9 is a front cross-sectional view of a modified base 30b for a plasma
generating apparatus wherein shielded magnets 45a are provided inside the
plasma region 16e.
FIG. 10 is a plan cross-sectional view along line 10--10 of FIG. 9.
FIG. 11 is an equivalent electrical circuit diagram which approximates the
circuit elements of the plasma generating apparatus which is presented for
the purpose of describing the operation of the apparatus.
GENERAL DESCRIPTION
The present invention relates to a plasma generating apparatus including a
plasma source employing a radio frequency, including UHF or microwave,
wave coupler of a non-magnetic metal in the shape of a hollow cavity which
can be excited in one or more TE or TM modes of resonance, including an
electrically insulated chamber having a central longitudinal axis and
mounted in the coupler, including a gas supply means for providing a gas
which is ionized to form the plasma in the chamber, including a moveable
plate means of a non-magnetic metal in the cavity mounted perpendicular to
the axis of the chamber and moveable towards and away from the chamber as
a sliding short, including a moveable probe connected to and extending
inside the coupler for coupling the radio frequency waves to the coupler,
wherein movement of the moveable plate means and the probe in the coupler
achieves the selected TM or TE mode of resonance of the radio frequency
wave in the coupler and varies the resonance of the mode and wherein the
radio frequency wave applied to the coupler creates and maintains the
plasma at reduced pressures in the shape of an elongate plasma disk
perpendicular to and surrounding the central longitudinal axis in the
chamber, the improvement which comprises: a plurality of first magnets
mounted on the apparatus around the longitudinal axis of the chamber on a
ring of high permeability magnetic material so as to create magnetic cusps
in the chamber which aid in confining the plasma in the chamber; and
second magnets mounted on a sheet of high permeability magnetic material
in the apparatus so as to provide magnetic cusps inside the chamber which
aid in confining the plasma in the chamber.
Further the present invention relates to a plasma generating apparatus
which comprises: a plasma source employing a radio frequency, including
UHF or microwave, wave coupler which is metallic and non-magnetic and in
the shape of a hollow cavity which can be excited in one or more TE or TM
modes of resonance; an electrically insulated chamber having a central
longitudinal axis and mounted in the coupler; gas supply means for
providing a gas which is ionized to form the plasma in the insulated
chamber; a movable probe connected to and extending inside the coupler for
coupling the radio frequency waves to the coupler; a plurality of first
magnets mounted around the longitudinal axis of the chamber on a ring of
high permeability magnetic material so as to create magnetic cusps in the
chamber which aid in confining the plasma in the chamber; movable metal
plate means as a sliding short in the cavity which is non-magnetic mounted
perpendicular to the axis and movable towards and away from the chamber;
second magnets mounted on a sheet of high permeability magnetic material
and on the plate means so as to provide magnetic cusps inside the chamber
which aid in confining the plasma in the chamber; and wherein movement of
the plate means and the probe in the coupler achieves the selected TE or
TM mode of resonance of the radio frequency wave in the coupler and varies
the resonance of the mode and wherein the radio frequency wave applied to
the coupler creates and maintains the plasma at reduced pressures in the
shape of an elongate plasma disk perpendicular to and surrounding the
central longitudinal axis in the chamber which is confined in the chamber
by the magnetic cusps.
Further still the present invention relates to a method for forming a
plasma which comprises: a plasma generating apparatus including a plasma
source employing a radio frequency, including UHF or microwave, wave
coupler of a non-magnetic metal in the shape of a hollow cavity which can
be excited in one or more TE or TM modes of resonance, including an
electrically insulated chamber having a central longitudinal axis and
mounted in the coupler, including a gas supply means for providing a gas
which is ionized to form the plasma in the chamber, including a moveable
plate means of a non-magnetic metal in the cavity mounted perpendicular to
the axis of the chamber and moveable towards and away from the chamber as
a sliding short, including a moveable probe connected to and extending
inside the coupler for coupling the radio frequency waves to the coupler,
wherein movement of the moveable plate means and the probe in the coupler
achieves the selected TM or TE mode of resonance of the radio frequency
wave in the coupler and varies the resonance of the mode and wherein the
radio frequency wave applied to the coupler creates and maintains the
plasma at reduced pressures in the shape of an elongate plasma disk
perpendicular to and surrounding the central longitudinal axis in the
chamber, the improvement which comprises: a plurality of first magnets
mounted on the apparatus around the longitudinal axis of the chamber on a
ring of high permeability magnetic material so as to create magnetic cusps
in the chamber which aid in confining the plasma in the chamber; and
second magnets mounted on a sheet of high permeability magnetic material
so as to provide magnetic cusps inside the chamber which aid in confining
the plasma in the chamber; and forming the plasma disk in the chamber
confined by the magnetic cusp.
Preferably the present invention utilizes rare earth magnets with a field
strength between about 0.01 and 0.5 Tesla. Superconducting magnets can
also be used to produce even higher magnetic field strengths. In this
later case, the applied magnetic fields can be varied (by varying coil
currents) to provide an optimum magnetic field geometry. The magnetic
field strength is preferably:
B.gtoreq..omega..sup.m e/e
.omega.=2.pi.f and f is the cavity excitation frequency. m.sub.e is the
mass of the electron and e is the magnitude of the charge of the electron.
While the apparatus described in U.S. Pat. No. 4,507,588 works without an
applied static magnetic field, the addition of a magnetic field is a
variation of the technology that has advantages for many potential
applications. These advantages are: (1) a reduction of charged particle
diffusion losses resulting in higher discharge efficiencies, (2) providing
ECR zones in the discharge volume thereby enhancing electromagnetic
coupling to the discharge at low pressures (<10 microns) and low input gas
flow rates, (3) control of discharge uniformity by spatially adjusting the
ECR zones, (4) creating a group of high energy electrons yielding
different plasma chemistry than microwave discharges without ECR magnetic
fields and (5) providing a method for discharge ignition at low pressures,
i.e., electron cyclotron resonance breakdown. This is an important feature
of the present invention which allows easy starting of the plasma
discharge at low pressures.
The design approach for the microwave plasma/ion sources described here is
similar to that described in earlier U.S. Pat. No. 4,507,588. A microwave
discharge is created in a disk shaped discharge plasma region which is
separated from the applicator (cavity) aperture (or antenna) by a quartz
confining enclosure or disk. The applicator is in the shape of a hollow,
cylindrical cavity which focuses and matches the microwave energy into the
plasma region utilizing single or controlled multimode electromagnetic
excitation and "internal cavity" matching. This apparatus can be used as a
broad-beam ion source or as a plasma source for materials processing.
SPECIFIC DESCRIPTION
FIGS. 1 and 2 show the preferred improved plasma generating apparatus of
the present invention. The basic construction of the apparatus without the
magnet mountings shown is described in U.S. Pat. No. 4,507,588. It will be
appreciated that various non magnetic materials can be used in the
construction of the apparatus, such as copper, brass, aluminum, silver,
gold, platinum, non-magnetic stainless steel and the like.
The apparatus includes copper or brass cylinder 10 forming the microwave
cavity 11 with a copper or brass sliding short 12 for adjusting the length
of the cavity 11. Silver plated copper brushes 13 electrically contact the
cylinder 10. The brushes 13 are provided entirely around the circumference
of the sliding short 12; however, in FIG. 2 only 4 are shown. Moveable
excitation probe 14 provides impedence tuning of the microwave energy in
the cavity 11. The probe 14 is mounted in cavity 11 by brass or copper
conduit 21. Radial penetration of the probe 14 into the cavity 11 varies
the coupling to the plasma in the cavity 11. Sliding short 12 is moved
back and forth in cavity 11 to aid in tuning the microwave by rods 22
using conventional adjustment means (not shown) such as described in U.S.
Pat. No. 4,507,588.
A quartz dish or chamber 15 shaped like a petri dish or round bottle bottom
defines the plasma region 16 along with a stainless steel base 30 and
holder 39. The holder 39 can have an electrical bias (not shown), which
can be D.C. or R.F. to attract ions from the plasma. Gas is fed by tube 19
to annular ring 18 and then flows into the plasma region 16. Optionally a
cooling line 42 is provided which cools the base 30. The cylinder 10
slides onto the base 30 and is held in place on base 30 by copper or brass
ring 10a secured to the cylinder 10. Sliding silver plated copper brushes
32 mounted on a brass ring 31 contact the cylinder 10 to provide good
electrical contact. The ring 10a is held in place on base 30 by copper or
brass bolts 33. This construction allows the base 30 and dish 15 to be
removed from the cylinder 10. The basic device operates without magnets as
described in U.S. Pat. No. 4,507,588.
In the improved plasma apparatus, the dish 15 and plasma region 16 are
surrounded by magnets on three sides. In the preferred embodiment, eight
(8) or more equally spaced magnets 34 surround the dish 15 around axis
a--a. A second set of magnets 35 is mounted on sliding short 12 by means
of a thin aluminum cup 12a. The combination of the magnets 34 and 35
provide interconnected magnetic field cusps 16a in the plasma region 16 of
the dish 15 as shown in FIG. 1. The magnets 34 and 35 reduce particle
diffusion losses from region 16 inside the dish 15. The magnetic field
strength decreases as the longitudinal axis a--a and center of the plasma
region 16 is approached because of the positioning of the magnets 34 and
35.
The magnets 34 are mounted on a high magnetic permeability (iron) ring 37
around the ring 31 and held in place by magnetic attraction. The iron ring
37 is secured to brass ring 31 such as by soldering. The ring 37 partially
surrounds the magnets 34 in an "L" shape so that the magnetic cusps 16a
extends into dish 16 and then terminates at the bottom leg of the "L". The
magnets 35 are held in place on a high magnetic permeability (iron)
circular plate 36. The plate 36 is fastened to sliding short 12.
One end of the dish 15 adjacent the holder 39 is free of magnets. In this
region, grids (not shown) or an article 100 to be treated is mounted on
holder 39 as described in Ser. No. 641,190, filed Aug. 16, 1984. Gases
pass out the opening 41 in tube 40 from the plasma region 16. The magnets
34 and 35 thus surround the dish 15. The plasma region 16 is surrounded by
the interconnected magnetic cusps 16a.
FIGS. 3 and 4 show a variation of the device of FIGS. 1 and 2 wherein the
dish 15a is taller along the axis a--a. In this embodiment, additional
magnets 38 are mounted on high magnetic permeability (iron) ring 44
secured to cylinder 10. This allows the magnetic cusps 16b to join
together as shown by FIGS. 3 and 4 in a manner similar to that shown in
FIG. 1. The remaining construction of FIGS. 3 and 4 is otherwise identical
to that of FIGS. 1 and 2.
In the following description, letters beside the reference numbers are used
for elements functionally identical to those of FIGS. 1 to 4. Where the
function is the same, they are not necessarily redescribed.
FIGS. 5 and 7 show a modified brass or copper sliding short 12b with a
recess 12c which supports magnets 35a on a thin portion 12d. The magnets
35a are positioned on a circular iron plate 36a. In this modification, the
thin portion 12d allows the magnetic cusps 16c to form inside the dish
15c. FIG. 6 shows a variation wherein the magnets 38a are mounted in slots
10c in cylinder 10b with a thin portion 10d which allows the magnetic
cusps (not shown) to penetrate the dish (not shown). High magnetic
permeability ring 37a holds the magnets 38a in place on the cylinder 10b.
Thus there are a number of ways of mounting the magnets 34, 35, 35a, 38
and 38a on the cylinder 10, 10a or 10b and sliding short 12, 12a or 12b.
The magnets 35 do not have to be mounted on the cylinder 10 or short 12
and can be independently mounted in or outside of the cavity 11. So long
as the cavity is constructed of a nonmagnetic material, the magnetic cusps
will penetrate the dish 15, 15a or 15b confining the plasma region 16 or
16d if they are sufficiently strong and properly positioned.
FIGS. 7 and 8 also show the use of magnets 45 located in the plasma region
16d inside the dish 15 and also show magnets 47 surrounding an opening 48
in a non-magnetic plate 46. The magnets 47 are sealed from the plasma by a
covering (not shown) and are secured to an iron ring 47a. In this
configuration, the magnets 45 and 47 produce a reduced magnetic field
region in opening 48 allowing the high energy electrons to pass through
the opening 48. The reduced field region in the opening 48 thus allows
electrons to be accelerated from the plasma 16d and the ions are pulled
along by electrostatic forces through the opening 48. This provides thrust
such as for an ion engine or it can be used for ion treatment.
FIGS. 9 and 10 show a dish 15d defining a plasma region 16e. Magnets 45a
are provided around and in the region 16e and are encased in a thin
stainless steel shield 50 to prevent exposure to the plasma in the region
16e. A voltage biased grid 51 is mounted on a ring insulator 52 to isolate
the grid 51. The grid 51 attracts ions from the plasma region 16e. The
plasma apparatus of FIGS. 9 and 10 otherwise has the same construction as
FIG. 1 and 2 where the letters inside the numbers have been used to
identify functionally identical elements.
FIG. 11 shows the equivalent electrical circuit of the cavity 11 and plasma
region 16 in dish 15. This is discussed more fully hereinafter.
One end of the disk shaped discharge region 16 is free of magnets. It is in
this region where the grid 51 (FIG. 9) for ion extraction or the
processing of material 100 on plate 39 (FIG. 1) is located. The type,
positions and connections of the grids/or processing plate are as
described in U.S. Pat. No. 4,507,588 and application Ser. No. 641,190. It
is important to note however, that the static magnetic field at the grid
51 or plate 39 locations is made very low (<50 gauss to zero) by placing a
lip on the ring 37. This "shorts out" the magnetic field from the adjacent
strong magnets 34. The array of alternating poles of magnets 35 also
produce little static magnetic field in the location of the grids 51 or
plate 39.
It is well known that at electron cyclotron resonance (ECR) energy can be
efficiently transferred from an electromagnetic field to electrons. In the
plasma region 16 this energy in turn is transferred to the electron gas
via electron-electron and electron heavy molecule collisions or to the
heavier ion and neutral gases via electron-ion and electron-neutral
collisions. ECR occurs when the exciting radian frequency .omega. equals
the cyclotron radian frequency .omega..sub.b.
.omega..sub.b =eB/m.sub.e
wherein
e is the magnitude of the charge of the electron
m.sub.e is the mass of the electron
B is the static magnetic field strength.
Expressed in terms of frequency
f.sub.b =28.times.10.sup.9 Hz/Tesla
Thus, for an exciting frequency of 2.45.times.10.sup.9 Hz the static
magnetic field required for ECR is approximately 0.0875 Tesla or 875
gauss. It is also noted that the average cyclotron radius r.sub.c for
electrons is given by
r.sub.c =V.sub.t /.omega..sub.b
where V.sub.t is the rms thermal velocity of the electron gas (V.sub.t
=3.89.times.10.sup.3 T.sub.e.sup.1/2 m/sec for electrons). Thus, for
plasmas where the electron temperature, T.sub.e, is 10.sup.5 .degree.K.
or less the cyclotron radius is less than one millimeter. Thus, electron
acceleration takes place in a small, thin layer (or volume) called an ECR
surface, around the magnets 34 and 35. These ECR zones can be observed in
the discharge as intensely glowing thin discharge layers resulting from
the gas excitation by the accelerated electrons.
If the magnets 34 and 35 are strong enough, the ECR zone (or surface)
occurs within the discharge region 16. The dotted lines 53 and 53a in FIG.
1 and 3 show examples of such ECR zones (or surfaces) located in the
plasma region of the quartz dish 15. As can be observed from these figures
these zones 53 and 53a trace out a three dimensional surface within the
region 16. Whenever an electron passes through this surface it experiences
an energy gain from the time varying electromagnetic field if the electric
field has a component perpendicular to the static magnetic field lines.
Thus, an electron will move through the discharge by reflecting from
magnetic cusp (16a or 16b) to magnetic cusp and gaining energy each time
it passes through an ECR surface. Most electrons then give up energy via
collisions throughout the discharge. The ECR zone positions can be varied
by increasing or decreasing the strength of the magnets 34, 35, 36 and 38.
Increasing the magnetic field strengths moves the ECR surface away from
the walls further into the center of the discharge. Decreasing the magnet
strengths moves the surface toward the walls of the dish 15 or 15a. It is
often desirable to have the ECR surface located entirely within the region
16 as shown in the Figures and not cutting through the quartz dish 15 or
15a.
This method of electron gas heating has been employed in other ion and
plasma sources (R. Geller and F. Gugliermotte, U.S. Pat. No. 4,417,178; T.
Consoli, L. Saint-Cloud, R. B. Clamart, R. Geller, A. Bernard, U.S. Pat.
No. 3,571,734; and R. Geller, IEEE Trans. Nucl. Sci. NS-23, 904 (1976)).
However, the present concept differs in cavity tuning, excitation, and
geometry, and is able to produce a large, magnetic field free plasma
surface in plasma region 16 for ion extraction or plasma processing.
Another important difference is that the discharge region and cavity 11
are separate, allowing each to be optimized individually. Thus, the
optimization of the discharge volume and shape, and discharge matching are
more independent. This produces a highly ionized discharge with densities
much greater than the critical density while using very low incident power
levels, typically not more than 100 to 200 watts.
Variations of the present technology are many. For example, the cusp
magnetic field strengths can be adjusted to provide: (1) ECR surfaces
surrounding the entire end and cylindrical side walls of the dish 15 as
shown in FIGS. 1 and 2, (2) an ECR surface in one region of the discharge
and just a confining magnetic fields in other regions. For instance, ECR
zones can be created in the top of the discharge region by magnets 35
while the magnets 34 and 38 provide a confining field. This also can be
reversed where magnets 35 produce the confining cusps fields 16b over the
top of the discharge and the ring magnets 34 create the ECR zones as well
as confining fields.
Other variations of this technology allow magnets 35a and 38a to be placed
outside the applicator electromagnetic excitation volume as is shown in
FIGS. 5 and 6. Also, the applicator sliding short 12 can be adjusted
separately from the end plate magnets 35.
Another variation of this technology, shown in FIGS. 7 and 8, encloses all
sides of the disk shaped discharge region 16e with magnetic cusps. As
shown, the base 30a is also enclosed with an array of permanent magnets 47
except for a central region where a magnet is removed to form the opening
48 (where the magnetic field is deliberately reduced). High energy
electrons accelerated via ECR in the discharge will escape through this
low magnetic field region of the opening 48 i.e., a magnetic nozzle is
produced. The electrons will in turn pull by electrostatic forces the
heavier lower energy ions along through the opening 48 of the nozzle
producing a neutral beam of charged particles. This neutral beam 49 can
then be used for material processing, space engine acceleration, and the
like. An advantage of this configuration is that the neutral, high density
beam can be produced without the need for the usual grid optics of FIG. 9.
An important feature of this ion and plasma source is its ability to match
(i.e., operate with zero or very little reflected power) the incident
microwave power into the low, variable pressure (.sup..about. 10 micron)
disk plasma region 16 for many different discharge conditions. Variable
cavity 11 length and variable coupling probe 14 tuning allow the discharge
to be matched over a wide range of discharge pressure, input powers, gas
flows and gas mixtures, etc. This match is accomplished using single mode
or controlled mutlimode excitation and hence is accomplished without
altering either the plasma shape or the applied electromagnetic field
patterns and without losing microwave power in external tuning stubs (not
shown). Increases in input power increase the electric and magnetic field
strengths, however, the geometry of the mode field patterns, remains
approximately constant throughout the tuning process keeping the geometry
of the fields exciting the plasma region 16, i.e., the electromagnetic
focus, constant. Preferably the microwave frequency is between about 400
MegaHertz and 10 GigaHertz.
The input impedance of a microwave cavity is given by
##EQU1##
where P.sub.t is the total input power coupled into the cavity 11 (which
includes metal wall losses as well as the power delivered to the
discharge), W.sub.m and W.sub.e are, respectively, the time-averaged
magnetic and electric energy stored in the cavity fields and
.vertline.I.sub.o .vertline. is the total input current on the coupling
probe. R.sub.in and jX.sub.in are the cavity 11 input resistance and
reactance and represent the complex load impedance as seen by the feed
transmission line.
At least two independent adjustments are required to match this load to a
transmission line. One adjustment must cancel the load reactance while the
other must adjust the load resistance to the characteristic impedance of
the feed transmission system. In the cavity 11 the continuously variable
probe 14 and end plate 12 tuning provide these two required variations,
and together with single mode excitation are able to cancel the discharge
reactance and adjust the discharge resistance to equal the characteristic
impedance of the feed transmission line.
The internal cavity 11 matching technique employed in the applicator can be
understood with the aid of the equivalent circuit shown in FIG. 11. This
is a standard circuit representation for a cavity 11 which is connected to
a feed waveguide or transmission line and is excited in the vicinity of a
single mode resonance. G.sub.c, L.sub.c and C.sub.c represent the
conductance, inductance and capacitance respectively of the excited mode
near resonance and the jX represents the reactive effect of the evanescent
modes far from resonance. The coupling probe 14 (or aperture) is
represented as the ideal transformer 60 of turns ratio M:L and coupling
reactance jX. Both circuit elements and the transformer 60 are drawn with
arrows to indicate their variability during the tuning process. At
resonance, the capacitive and inductive susceptance cancel resulting in a
pure conductive input admittance.
The discharge is ignited by first adjusting the probe 14 and cavity 11
length positions to excite a specific empty cavity 11 resonance and to
match the empty cavity 11 applicator to the input transmission system.
Microwave power is then applied, absorbed into the cavity 11 without
reflection and a discharge is ignited in plasma region 16 even with low
input powers of 10-20 W if the pressure in the disk discharge zone is
reduced to less than 10 Torr. The presence of the discharge then changes
L.sub.c, G.sub.c, and C.sub.c and adds an additional discharge conductance
G.sub.L and susc | | |
