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Claims  |
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What is claimed is:
1. A plasma source for generating a plasma, comprising:
a chamber operable to confine the plasma and process gases; a coil operable
to generate a whistler wave in said chamber, said coil comprising a
plurality of coils located inside of said chamber and positioned
substantially parallel to sidewalls of said chamber, said plurality of
coils shaped to match the contour of said chamber and connected by a
connecting coil traversing a vertical axis of said chamber;
at least one set of electromagnets located outside of said chamber, said
electromagnets operable to define a preferred propagation direction of the
whistler wave in said chamber;
said coil further operable to inductively couple RF power to the whistler
wave to excite the whistler wave and to transfer a sufficient amount of
energy to the process gases in said chamber to induce a plasma state in
the process gases.
2. The plasma source of claim 1, wherein said coil is further operable to
resonantly inductive couple the RF power to the whistler wave to excite
the whistler wave.
3. The plasma source of claim 1, wherein said coil is located inside of
said chamber and positioned substantially parallel to sidewalls of said
chamber.
4. The plasma source of claim 1, wherein said coil further comprises:
a first coil routed outside said chamber along a bottom of said chamber;
and
a plurality of coils located outside of said chamber and substantially
parallel to sidewalls of said chamber, said plurality of coils located
between said electromagnets and said chamber.
5. The plasma source of claim 1, wherein said RF power is selected to be at
a frequency between the ion and electron cyclotron frequencies of the
process gases, the ion and electron cyclotron frequencies being less than
the electron plasma frequency of the process gases.
6. The plasma source of claim 1, wherein said coil is operable to provide
antennas of varying lengths for resonant inductive coupling of the RF
power to the whistler wave over a range of conditions impacting the
resonant inductive coupling of the RF power to the whistler wave, said
coil further operable to generate time-varying magnetic fields which
sustain the plasma state of the process gases.
7. The plasma source of claim 1, further comprising a plurality of
permanent multipolar magnets located outside of said chamber, said magnets
operable to establish a magnetic field along the surface of said chamber
for confining the plasma.
8. The plasma source of claim 1, wherein said coil further comprises a
passage for circulating coolant through said coil so as to cool said coil.
9. The plasma source of claim 1, wherein said coil is segmented with
capacitors placed between adjacent coils operable to reduce the impedance
of said coil.
10. A chamber operable for performing a plasma based process, the chamber
containing process gases and securing a semiconductor wafer relative to a
plasma source, comprising:
a plurality of permanent multipolar magnets located outside of the chamber,
said magnets operable to establish a magnetic field along the surface of
the chamber for confining a plasma;
a coil operable to generate a whistler wave in the chamber;
a set of electromagnets located outside of the chamber, said electromagnets
operable to define a preferred propagation direction of the whistler wave
in the chamber; and
said coil further operable to inductively couple RF power to the whistler
wave to excite the whistler wave and transfer a sufficient amount of
energy to the process gases to induce a plasma state in the process gases.
11. A method of inducing a plasma state in process gases in a chamber,
comprising the steps of:
generating a whistler wave with a coil, said coil including a first coil
outside the chamber and along a bottom of the chamber, and a plurality of
coils outside the chamber and along and substantially parallel to
sidewalls of the chamber;
generating a variable static electromagnetic field defining a preferred
propagation direction of the whistler wave in the chamber; and
exciting the whistler wave by inductively coupling RF power to the whistler
wave with the coil, the excited wave transferring sufficient energy to the
process gases to induce the plasma state in the process gases.
12. The method of claim 11, further comprising the step of generating
time-varying electromagnetic fields with the coil to sustain the plasma
state in the process gases.
13. The method of claim 11, wherein said generating a variable static
electromagnetic field step is accomplished with at least one set of
electromagnets located outside the chamber.
14. The method of claim 11, further comprising the step of generating a
magnetic field along the surface of the chamber for confining the plasma.
15. The method of claim 11, wherein said exciting step is accomplished by
resonant inductive coupling of the RF power to the whistler wave with the
coil.
16. The method of claim 11, further comprising the steps of:
selecting a frequency of the RF power to be between ion and electron
cyclotron frequencies of the process gases; and
selecting the ion and electron cyclotron frequencies to be less than an
electron plasma frequency of the process gases.
17. The method of claim 11, further comprising the steps of locating a
plurality of coils inside the chamber and substantially parallel to
sidewalls of the chamber, and connecting the plurality of coils with a
connector coil traversing a vertical axis of the chamber.
18. The method of claim 11, wherein said exciting the wave step further
comprises the coil providing antennas of varying lengths for resonant
inductive coupling the RF power to the whistler wave over a range of
conditions.
19. A plasma source for generating a plasma, comprising:
a chamber operable to confine the plasma and process gases;
a coil operable to generate a whistler wave in said chamber, said coil
including;
a first coil routed outside said chamber along a bottom of said chamber;
and
a plurality of coils located outside of said chamber and substantially
parallel to sidewalls of said chamber, said plurality of coils located
between said electromagnets and said chamber;
at least one set of electromagnets located outside of said chamber, said
electromagnets operable to define a preferred propagation direction of the
whistler wave in said chamber;
said coil further operable to inductively couple RF power to the whistler
wave to excite the whistler wave and to transfer a sufficient amount of
energy to the process gases in said chamber to induce a plasma state in
the process gases.
20. The plasma source of claim 19 wherein said coil is further operable to
resonantly inductive couple the RF power to the whistler wave to excite
the whistler wave.
21. The plasma source of claim 19 wherein said coil includes a plurality of
coils located inside of said chamber and positioned substantially parallel
to sidewalls of said chamber, said plurality of coils shaped to match the
contour of said chamber and connected by a connecting coil traversing a
vertical axis of said chamber.
22. The plasma source of claim 19 wherein said RF power is selected to be
at a frequency between the ion and electron cyclotron frequencies of the
process gases, the ion and electron cyclotron frequencies being less than
the electron plasma frequency of the process gases.
23. The plasma source of claim 19 wherein said coil is operable to provide
antennas of varying lengths for resonant inductive coupling of the RF
power to the whistler wave over a range of conditions impacting the
resonant inductive coupling of the RF power to the whistler wave, said
coil further operable to generate time-varying magnetic fields which
sustain the plasma state of the process gases.
24. The plasma source of claim 19 further comprising a plurality of
permanent multipolar magnets located outside of said chamber, said magnets
operable to establish a magnetic field along the surface of said chamber
for confining the plasma.
25. The plasma source of claim 19 wherein said coil further comprises a
passage for circulating coolant through said coil so as to cool said coil.
26. The plasma source of claim 19 wherein said coil is segmented with
capacitors placed between adjacent coils operable to reduce the impedance
of said coil.
27. A plasma source for generating a plasma, comprising:
a chamber operable to confine the plasma and process gases;
a coil operable to generate a whistler wave in said chamber;
at least one set of electromagnets located outside of said chamber, said
electromagnets operable to define a preferred propagation direction of the
whistler wave in said chamber;
a plurality of permanent multipolar magnets located outside of said
chamber, said magnets operable to establish a magnetic field along the
surface of said chamber for confining the plasma.
said coil further operable to inductively couple RF power to the whistler
wave to excite the whistler wave and to transfer a sufficient amount of
energy to the process gases in said chamber to induce a plasma state in
the process gases.
28. The plasma source of claim 27 wherein said coil is further operable to
resonantly inductive couple the RF power to the whistler wave to excite
the whistler wave.
29. The plasma source of claim 27 wherein said coil is located inside of
said chamber and positioned substantially parallel to sidewalls of said
chamber.
30. The plasma source of claim 27 wherein said coil comprises a plurality
of coils located inside of said chamber and positioned substantially
parallel to sidewalls of said chamber, said plurality of coils shaped to
match the contour of said chamber and connected by a connecting coil
traversing a vertical axis of said chamber.
31. The plasma source of claim 27 wherein said coil further comprises:
a first coil routed outside said chamber along a bottom of said chamber;
and
a plurality of coils located outside of said chamber and substantially
parallel to sidewalls of said chamber, said plurality of coils located
between said electromagnets and said chamber.
32. The plasma source of claim 27 wherein said RF power is selected to be
at a frequency between the ion and electron cyclotron frequencies of the
process gases, the ion and electron cyclotron frequencies being less than
the electron plasma frequency of the process gases.
33. The plasma source of claim 27 wherein said coil is operable to provide
antennas of varying lengths for resonant inductive coupling of the RF
power to the whistler wave over a range of conditions impacting the
resonant inductive coupling of the RF power to the whistler wave, said
coil further operable to generate time-varying magnetic fields which
sustain the plasma state of the process gases.
34. The plasma source of claim 27 wherein said coil further comprises a
passage for circulating coolant through said coil so as to cool said coil.
35. The plasma source of claim 27 wherein said coil is segmented with
capacitors placed between adjacent coils operable to reduce the impedance
of said coil.
36. A plasma source for generating a plasma, comprising:
a chamber operable to confine the plasma and process gases;
a coil operable to generate a whistler wave in said chamber, said coil
being segmented with capacitors placed between adjacent coils operable to
reduce the impedance of said coil;
at least one set of electromagnets located outside of said chamber, said
electromagnets operable to define a preferred propagation direction of the
whistler wave in said chamber;
said coil further operable to inductively couple RF power to the whistler
wave to excite the whistler wave and to transfer a sufficient amount of
energy to the process gases in said chamber to induce a plasma state in
the process gases.
37. The plasma source of claim 36 wherein said coil is further operable to
resonantly inductive couple the RF power to the whistler wave to excite
the whistler wave.
38. The plasma source of claim 36 wherein said coil is located inside of
said chamber and positioned substantially parallel to sidewalls of said
chamber.
39. The plasma source of claim 36 wherein said coil comprises a plurality
of coils located inside of said chamber and positioned substantially
parallel to sidewalls of said chamber, said plurality of coils shaped to
match the contour of said chamber and connected by a connecting coil
traversing a vertical axis of said chamber.
40. The plasma source of claim 36 wherein said coil further comprises:
a first coil routed outside said chamber along a bottom of said chamber;
and
a plurality of coils located outside of said chamber and substantially
parallel to sidewalls of said chamber, said plurality of coils located
between said electromagnets and said chamber.
41. The plasma source of claim 36 wherein said RF power is selected to be
at a frequency between the ion and electron cyclotron frequencies of the
process gases, the ion and electron cyclotron frequencies being less than
the electron plasma frequency of the process gases.
42. The plasma source of claim 36 wherein said coil is operable to provide
antennas of varying lengths for resonant inductive coupling of the RF
power to the whistler wave over a range of conditions impacting the
resonant inductive coupling of the RF power to the whistler wave, said
coil further operable to generate time-varying magnetic fields which
sustain the plasma state of the process gases.
43. The plasma source of claim 36 further comprising a plurality of
permanent multipolar magnets located outside of said chamber, said magnets
operable to establish a magnetic field along the surface of said chamber
for confining the plasma.
44. The plasma source of claim 36 wherein said coil further comprises a
passage for circulating coolant through said coil so as to cool said coil.
45. A method of inducing a plasma state in process gases in a chamber,
comprising the steps of:
generating a whistler wave with a coil, said coil including a plurality of
coils located inside the chamber and located substantially parallel to
sidewalls of the chamber;
connecting the plurality of coils with a connector coil traversing a
vertical axis of the chamber;
generating a variable static electromagnetic field defining a preferred
propagation direction of the whistler wave in the chamber; and
exciting the whistler wave by inductively coupling RF power to the whistler
wave with the coil, the excited wave transferring sufficient energy to the
process gases to induce the plasma state in the process gases.
46. The method of claim 45 further comprising the step of generating
time-varying electromagnetic fields with the coil to sustain the plasma
state in the process gases.
47. The method of claim 45 wherein said generating a variable static
electromagnetic field step is accomplished with at least one set of
electromagnets located outside the chamber.
48. The method of claim 45 further comprising the step of generating a
magnetic field along the surface of the chamber for confining the plasma.
49. The method of claim 45 wherein said exciting step is accomplished by
resonant inductive coupling of the RF power to the whistler wave with the
coil.
50. The method of claim 45 further comprising the steps of:
selecting a frequency of the RF power to be between ion and electron
cyclotron frequencies of the process gases; and
selecting the ion and electron cyclotron frequencies to be less than an
electron plasma frequency of the process gases.
51. The method of claim 45 further comprising the step of locating a first
coil outside the chamber and along a bottom of the chamber, and a
plurality of coils outside the chamber and along and substantially
parallel to sidewalls of the chamber.
52. The method of claim 45 wherein said exciting the wave step further
comprises the coil providing antennas of varying lengths for resonant
inductive coupling the RF power to the whistler wave over a range of
conditions impacting the resonant inductive coupling of the RF power to
the whistler wave. |
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Claims |
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Description |
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CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to application Ser. No. 07/868,818 filed Apr.
15, 1992, by Ajit Pramod Paranjpe, and assigned to Texas Instruments
Incorporated, entitled "Plasma Source and Method of Manufacturing",
(TI-15886) now U.S. Pat. No. 5,231,334, issued Jul. 27, 1993.
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to the field of generating plasma by
radio frequency induction, and more particularly to an improved method and
apparatus for generating plasma by radio frequency resonant induction
coupling.
BACKGROUND OF THE INVENTION
Inductively coupled plasmas ("ICPs") generated with radio frequency ("RF")
waves having a frequency generally between 1 MHz and 100 MHz are capable
of providing charged particle (electron and ion) concentrations in excess
of 10.sup.11 cm.sup.-3 and ion currents to wafer substrates in excess of 5
mA/cm.sup.2. The ICP source is thus competitive with electron cyclotron
resonance ("ECR") plasma sources for semiconductor manufacturing processes
requiring plasma generation. Semiconductor manufacturing processes that
make use of plasmas include dry etching, plasma enhanced deposition, dry
cleaning of wafers, and applications requiring the generation of
ultraviolet (UV) light.
Inductively coupled RF plasma sources have advantages over both
capacitively coupled RF plasma sources and ECR plasma sources. In contrast
to capacitively coupled RF plasmas, inductively coupled RF plasmas have
substantially lower intrinsic plasma potentials (<50 V) and achieve a
substantially higher ionization efficiency (>5%). Also, the intrinsic
plasma potential is relatively independent of the RF power. The low
intrinsic plasma potential is useful in applications where high ion
energies cannot be tolerated, such as in dry etching where high ion
energies can damage the devices on the wafer.
In ECR plasma sources, the plasma ions are produced by electron bombardment
in a discharge chamber, and directed towards the surface using magnetic
and/or electric fields. As in the case of ECR systems, the ion energy of
an inductively coupled RF plasma can be varied independently of the plasma
density by biasing the integrated circuit wafer with a separate RF or DC
power supply. For an ECR plasma source, the pressure at which the plasma
may be effectively generated is also a concern. An ECR source is most
effective at pressures below 1 mTorr, which is too low for most
semiconductor process applications. The ICP source, however, has the
advantage of operating over a pressure range that is more compatible with
semiconductor process requirements (1 mTorr to 50 mTorr). Since the
operating pressure is higher, the pumping requirements for a given gas
flow rate are more modest for the ICP source. In addition, the ICP source
can provide a larger diameter (15 cm to 30 cm), homogeneous plasma, in a
compact design, and at substantially lower cost than an ECR source.
One type of plasma source employing RF induction coupling couples energy
into the plasma through whistler or helicon waves. This type of generator
is called a helicon plasma source. In the presence of a magnetic field
ranging from 100 G to 1 kG directed along the axis of the source, a
whistler wave can be excited by applying an RF voltage to a loop antenna
located around the source cavity. Although these axial magnetic fields are
generally weaker than the magnetic fields employed in ECR sources, the
plasma is non-uniform across the diameter of the source. Thus, a wafer
undergoing a plasma process must be located away or "downstream" of the
source, in a region where the plasma is sufficiently uniform. This
requires the input power of the source to be increased to maintain a
sufficient plasma density (i.e., electron and ion concentration) at the
downstream position. Also, large solenoidal coils are required to generate
the axial magnetic field. These features increase source cost and
complexity.
A second type of plasma source differs from the generic whistler wave or
helicon source by omitting the axial magnetic field. The wafer undergoing
a plasma process can therefore be placed within the plasma generation
region. Even though the peak plasma densities (5.times.10.sup.11
cm.sup.-3) for such a source are about an order of magnitude lower than
those for the whistler wave source, the proximity of the wafer to the
plasma generation region in the source ensures that processing rates are
comparable. Wafer etch rates of over 1 .mu.m/min are possible for many
materials of interest. This source is simpler, more compact, and cheaper
than the helicon plasma source.
One version of this type of induction plasma source employs a multi-turn
pancake coil located along the top surface of a cylindrical vacuum
chamber. A quartz vacuum window, typically 0.5 in. thick, isolates the
coil from the chamber. When the coil is powered by an RF source, large
currents circulate in the coils. These currents induce intense electric
fields inside the chamber that sustain the plasma state. The time-varying
magnetic and electric fields generated by the pancake coil are
proportional to the coil current, and increase in proportion to the square
of the number of coil turns and the coil diameter. The uniformity of the
induced electric field from a pancake coil improves with increasing coil
diameter and the number of coil turns. However, the inductance of the coil
is also proportional to the square of the number of coil turns. This
implies that the voltage drop across the coil increases with an increasing
number of coil turns for a fixed coil current. As an example, the voltage
drop across a 5 .mu.H coil for an RMS current of 20 A at 13.56 MHz is 8.5
kV. Such a high voltage is a hazard and results in capacitive energy
coupling between the coil and the plasma. Capacitive coupling is
undesirable because the intrinsic plasma potential increases dramatically
if a significant amount of energy is transferred via capacitive coupling.
These issues constrain the number of coil turns to about three in these RF
plasma sources with multi-turn pancake coils located along the top surface
of the source.
SUMMARY OF THE INVENTION
Therefore a need has arisen for an RF plasma source that combines the
advantages of the helicon plasma source and the ICP plasma source,
minimizes the number of system components, efficiently uses output power,
provides good plasma uniformity, and maintains coil voltages at safe
levels. In accordance with the present invention, an apparatus and method
for generating a plasma are provided which substantially eliminate or
reduce disadvantages and problems associated with existing plasma sources.
The present invention is an inductively coupled plasma (ICP) source
comprising an apparatus for generating a plasma. The source includes a
vacuum chamber for containing the plasma. Outside the chamber may be
included a plurality of permanent multipolar magnets which can be used to
establish a cusp magnetic field to reduce plasma losses to the chamber
wall, enhance plasma density, and extend operation of the plasma source to
lower pressures. The source includes at least one set of electromagnets
located outside the chamber for generating a variable static magnetic
field defining a preferred propagation direction for the whistler wave.
Also included are coils that serve as an antenna for resonant inductive
coupling of RF power to generate and excite a whistler wave. The excited
whistler wave transfers sufficient energy to induce and sustain a plasma
within the process gases. The coil also generates time varying standing
wave electromagnetic fields. These fields enhance the plasma density in
the plasma source. The at least one coil is located either inside or
external to the chamber of the plasma source of the present invention.
More specifically, to achieve whistler wave coupling the frequency of the
RF power used to excite the whistler wave should be chosen to be between
the ion and electron cyclotron frequencies of the process gases, and the
cyclotron frequency should be less than the electron plasma frequency of
the process gases. The RF power source, in the method and apparatus of the
present invention, can provide either transverse electromagnetic (TE) mode
waves, transverse magnetic (TM) mode waves, or mixed TE and TM mode waves
for establishing the whistler wave.
A technical advantage of the present invention is that it can be integrated
into existing semiconductor processing equipment. For example, dry etch
chambers which require a plasma to perform etching can be fitted with the
plasma source of the present invention. An integrated dry etch/plasma
source chamber provides enhanced etch capabilities over wet-etching
systems.
An important technical advantage of the disclosed invention is that the
coil configuration can generate more intense fields when compared to
single turn coils. The multiple turn coil configuration of the present
invention gives the technical advantage of effectively providing a range
of antenna lengths thereby improving resonant inductive coupling
efficiency over a range of plasma discharge conditions. Off-resonance
induction coupling is not very efficient; thus, providing antennas of
varying lengths can widen the operating range of the plasma generator.
Also, antennas of different lengths can lead to the technical advantage of
multi-mode operation and more homogeneous plasmas.
Another technical advantage is that the electromagnetic fields generated in
the chamber of the plasma source of the present invention are uniform and
concentrated in the chamber. The present invention provides the technical
advantage of containing the electromagnetic field generated entirely
within the process chamber. This eliminates eddy current heating of metal
surfaces outside the chamber, and leads to more efficient plasma
generation. Also, the multi-polar magnetic confinement provided with the
coils enhances plasma densities for pressures less than 50 mTorr.
Another technical advantage of the present invention is that if the plasma
discharge is operated in a regime in which resonant induction coupling to
the whistler wave does not occur, then the electromagnetic fields induced
by the multiple close-packed coils sustain the plasma through inductive
coupling only. Thus, it should be possible to obtain and maintain a plasma
in the plasma source of the present invention under conditions where
resonant induction coupling is not efficient nor possible.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the
advantages thereof, reference is now made to the following descriptions
taken in conjunction with the accompanying drawings, in which:
FIG. 1 depicts a partially cross-sectional, partially schematic diagram of
the plasma source with coils located inside the chamber;
FIG. 2 depicts a top plan view of the plasma source in FIG. 1;
FIG. 3 depicts the magnetic field due to current in a length of wire;
FIG. 4 depicts a coil consisting of four loops;
FIG. 5 depicts a cross-sectional view of FIG. 2 along plan I--I';
FIG. 6 depicts a partially cross-sectional, partially schematic diagram of
a second embodiment of the plasma source with the coils located outside
the source chamber;
FIG. 7 depicts a top-down plan view of the plasma source depicted in FIG.
6; and
FIG. 8 depicts a partially cross-sectional, partially schematic diagram of
the plasma source of FIG. 1 integrated into a semiconductor processing
chamber.
DETAILED DESCRIPTION OF THE INVENTION
The preferred embodiments of the present invention and their advantages are
best understood by referring to FIGS. 1-8, like numerals being used for
like and corresponding parts of the various drawings.
FIG. 1 depicts a partially cross-sectional, partially schematic diagram of
plasma source 10 employing coils 12 to inductively couple RF power to
process gases to induce a plasma state in the process gases. Plasma source
10 includes chamber 14 which is typically made of a suitable dielectric
material such as quartz so as to better contain the plasma. Chamber 14 is
also typically vacuum sealed. Chamber 14 has inlet 16 for introducing the
process gases into chamber 14. Inlet apertures 18 allow the process gases
to enter chamber 14 at a uniform controlled rate. Once the plasma state is
achieved in the process gases, the plasma is discharged from chamber 14
through an opening represented in FIG. 1 by end 19. Plasma source 10 of
FIG. 1 can be attached to any suitable semiconductor processing chamber
requiring a plasma for the process. Wafer etching and deposition chambers
are exemplary of semiconductor processing chambers that plasma source 10
could be attached to.
Chamber 14 may have within it a plurality of coils 12. Coils 12 are a
series of loop antennas arranged concentrically with the coil arrangement
and shaped to match the contour of chamber 14. Coils 12 may be made of any
suitable RF conductive material. Copper, aluminum, or copper clad tubing
of quarter-inch diameter have been shown to be suitable materials for the
manufacture of coils 12. In the embodiment of FIG. 1 coils 12 are located
inside of chamber 14 and are encased in dielectric coating 22 to prevent
their contamination. Quartz and epoxy encapsulants may be suitable
materials for dielectric coating 22. Coils 12 may be coupled together by
connecting line 24 running through the center of chamber 14. Connecting
line 24 is likewise encased in dielectric coating 22. Coils 12 may also be
arranged with water inlet 26 and water outlet 28. Water can be pumped
through coils 12 and connecting line 24 in order to maintain the
temperature of coils 12 and line 24 below the temperature at which the
coils or coating may be damaged. Coils 12 are also coupled to RF power
source 30 and ground 32. An intervening matching network (not explicitly
shown) is required to apply the RF power (1 MHz to 100 MHz) from RF power
source 30 to coils 12. In the embodiment of FIG. 1, RF power source 30
provides RF energy in the form of transverse electromagnetic (TE) mode
waves.
Outside of chamber 14 may be included multiple permanent magnets 34
arranged around the entire circumference of chamber 14 with like polarity
magnets facing each other, see FIG. 2. It should be noted that the
permanent magnets may be omitted without affecting the inventive concepts
of the present invention. Permanent magnets establishing a magnetic field
of 100-500 gauss at the surface of the chamber have been found to be
suitable. In the embodiment of plasma source 10 depicted in FIG. 1, a
single set of electromagnets 36 are located outside of chamber 14.
Solenoidal electromagnets establishing an electromagnetic field of
100-1000 gauss at the center of chamber 14 have been found to be suitable.
It is envisioned that the number and arrangement of permanent magnets 34
and solenoidal electromagnets 36 can be varied without affecting the
inventive concepts of the present invention.
FIG. 2 depicts a top plan view of plasma source 10 of FIG. 1. As can be
seen in this view, coils 12 are located inside of chamber 14 and are
encased with dielectric coating 22. Coils 12, while being formed
concentrically in a planar view (SEE FIG. 4), are also contoured to match
the circular shape of chamber 14. Therefore, the top plan view of FIG. 2
shows coils 12 in the form of circle 38. Coils 12 are in fact separated by
spaces 40 and 42. While the embodiment of FIGS. 1 and 2 is shown with two
coils 12, it is understood that the number of coils can be varied without
affecting the inventive concepts of the present invention.
In operation of plasma source 10 depicted in FIGS. 1 and 2, a process gas,
e.g. argon or sulphur hexaflouride, are introduced into chamber 14 through
inlet 16 and inlet apertures 18. The arrangement of permanent magnets 34
with magnets of alternating polarity placed next to each other establish a
cusp magnetic field along the surface of chamber 14 which aids in
confinement of the plasma. Electromagnets 36 generate a variable static
magnetic field which defines the propagation direction for the whistler
wave generated by coils 12. Additional sets of electromagnets 36 can be
used to improve the uniformity of the axial magnetic field, although only
one set is shown in this embodiment. It may be necessary to use three or
more electromagnets to improve plasma uniformity.
In resonant induction plasma generators, the whistler wave in the cavity is
excited by resonant coupling when the length of an antenna located in or
around the cavity matches one-half the wavelength of the whistler wave. To
ensure that the dominant coupling of RF energy is to the whistler wave
through resonant induction, the optimum length of the antenna required for
maximum coupling is a function of the conditions in the plasma source. The
wavelength of the whistler wave can be represented by Equation (1):
##EQU1##
where: B.sub.o is the magnetic field in gauss; n.sub.e is the electron
concentration in 1/cm.sup.3 ; and
.function. is the frequency in Hz.
Thus, for an antenna of fixed length, resonant induction and excitation of
the whistler wave leading to plasma generation will occur only if the
variables in Equation (1) are such that the wavelength of the whistler
wave is equal to one-half of the length of the antenna in the cavity.
Since the wavelength .delta. of the whistler wave is dependent on the
magnetic field B.sub.o, the electron concentration n.sub.e, and frequency
.function. of the wave in the chamber, it will not always be possible to
achieve and maintain a plasma state in the process gases with a single
loop antenna.
In the embodiment of the present invention depicted in FIGS. 1 and 2, as
the magnetic field B.sub.o, electron concentration n.sub.e, and frequency
.function. change in chamber 14, thereby affecting the wavelength .delta.
of the whistler wave, coils 12 provide the multiple antenna lengths
necessary to match one-half the changing wavelength .delta. of the
whistler wave. Coils 12, therefore, effectively provide a range of antenna
lengths making it possible to achieve resonant induction excitation of the
whistler wave thereby improving coupling efficiency over a range of plasma
conditions.
Coils 12 also induce intense time-varying electromagnetic fields in chamber
14. Once the time-varying electromagnetic fields reach the necessary
threshold, the process gases are ionized and a plasma is established in
chamber 14. The antennas of varying lengths, e.g. coils, can also lead to
multi-mode whistler wave generation and a more homogeneous plasma.
Furthermore, in operation of plasma source 10 depicted in FIGS. 1 and 2,
if the discharge is operated in a region in which resonant induction does
not occur, then the intense time-varying electromagnetic fields inside
chamber 14 generated by multiple coils 12 will sustain the plasma. Thus,
it is possible to maintain the plasma state in the process gases even
under conditions that do not support whistler wave generation and resonant
induction. Also, the magnetic field B configuration due to coils 12 has
large tangential and radial components, that are desirable for exciting
the whistler mode.
Also identified in FIG. 2 is point 44 which will be used as a reference to
discuss the fields in plasma source 10. Point 44 should be viewed as any
unfixed point along plane 1--1' above or below connecting line 24, and
within the upper and lower boundaries of coils 12. The magnetic field B at
point 44 on plane 1--1' due to the current in the length l of wire 46 at a
distance R as depicted in FIG. 3 is given by Equation (2):
##EQU2##
where: I is the current in wire 46 R is the distance from wire 46
l is the length of wire 46
.mu..sub.o is the permeability of the medium in which the wave is traveling
Magnetic field B should be viewed as traveling on a circular path centered
around wire 46.
Each coil may be treated as a number of segments of wire attached end to
end. This concept is more clearly illustrated in FIG. 4. In coil 12
consisting of four loops as shown in FIG. 4, the coil is four lengths of
wire identified as l.sub.1, l.sub.2, l.sub.3, and l.sub.4. Therefore, the
magnetic field at point 44 in accordance with Equation (1) is given by
Equation (3):
##EQU3##
If l.sub.1 =1", l.sub.2 =3", l.sub.3 =5", l.sub.4 =7", .mu..sub.o
=4.pi..times.10.sup.-7 H/m, and if the current I in coil 12 is 40 amps,
then magnetic field B in chamber 14 is 7.5 gauss.
The magnetic field B from coils 12 in chamber 14 induces a time-varying
electric field E. It is possible to estimate the time varying electric
field E within chamber 14. Consider FIG. 5 depicting a closed loop 48
along plan I--I' within the boundaries of coils 12 in FIG. 2. The
relationship between the electric field E and magnetic field B along plan
I--I' of FIG. 2 can be depicted as shown in FIG. 5. Magnetic field B
within closed loop 48 at point 49 is shown transecting plan I--I'. The
time varying electric field E generated is tangential to the magnetic
field B and varies according to the distance r.sub.1 from point 49. The
time varying electric field E can be represented by Equations (5):
##EQU4##
where: .omega. is the radian frequency of the signal; B is the magnetic
field in gauss; and
r is the distance in inches from point 49.
For .omega.=2.pi.f=8.5.times.10.sup.2 radians per second; B=7.5 gauss and
r.sub.1 =1 inch, then electric field E is 8 volts per centimeter.
This value of electric field E is an underestimation, since the electric
field is known to be more intense along the inner surface of chamber 14
compared to the center of chamber 14. On average, it has been determined
that the electric field E is likely to be three times as intense along the
inner surface of chamber 14 over that at the center of chamber 14. Thus,
the electric field E is approximately 25 volts per centimeter along the
inner surface of chamber 14.
When using Argon as the process gas, the minimum electric field E required
to induce a plasma state in Argon is represented by Equation (6):
E/N=4.times.10.sup.-16 Vcm.sup.2 @ 50 mTorr (6)
where: N is the gas density of Argon, and for N=1.7.times.10.sup.15
/cm.sup.3 @ 50 mTorr
therefore, E=(E//N)N=0.67 V/cm
Therefore, applying Equation (6) the electric field E required to induce a
plasma state in the example above is 0.67 volts per centimeter. Since it
has been shown above that a current of 40 amps in coil 12 depicted in FIG.
4 results in a magnetic field B of 7.5 gauss, which in turn, from Equation
(5), results in an electric field E of 8 volts per centimer, then the
current required to generate an electric field E of 0.67 V/cm can be
calculated, ([(40 amps)(0.67 V/cm)]/(8 V/cm)). Thus, it is possible to
sustain an RF discharge in Argon with a minimum coil current of
approximately 3 amps.
For process gas sulphur hexaflouride (SF.sub.6), the minimum electric field
E required to induce a plasma state can be represented by Equation (7):
E/N=4.times.10.sup.-15 Vcm.sup.2 @ 50 mTorr (7)
where: N is the gas density of sulphur hexaflouride, and,
N=1.7.times.10.sup.15 /cm.sup.3 @ 50 mTorr
therefore:
N=(E/N)(N)=6.7 V/cm
Therefore, applying Equation (7), the electric field E required to induce a
plasma state in sulphur hexaflouride is 6.7 volts per centimeter. Thus,
the minimum coil current required to sustain an RF discharge in sulphur
hexaflouride is approximately 33 amps, ([(40 amps)(4.7 V/cm)]/(8 V/cm)).
The plasma density n.sub.e achievable under these conditions can also be
calculated. Assume the power provided by RF power source 30 is 500 watts.
It is known that the portion of the power used in ionization of the
process gas, i.e. ionization efficiency is 0.4, for an electric field E
equal to 8 volts per centimeter. The ionization threshold for Argon is 16
electronvolts, therefore the volumetric ionization rate can be represented
by Equations (8) and (9):
##EQU5##
The loss rate of the plasma can be represented by Equation (10):
##EQU6##
where: D.sub.e is the diffusion coefficient for the electrons and ions,
and
D.sub.e =7.times.10.sup.4 cm.sup.2 /s @ 50 mTorr
.LAMBDA. A is the scale length for diffusion, and
.LAMBDA.=5 cm
Therefore, the plasma density n.sub.e is 2.times.10.sup.12 /cm.sup.3. It is
recognized that the actual electron density is likely to be higher since
an underestimation of the electric field E has been used.
The embodiment shown in FIGS. 1 and 2 makes use of two coils 12 connected
together by connecting line 24. It is understood that a single coil 12
providing circle 38 without spaces 40 and 42 as well as a l | | |
