U.S. patent application number 09/878156 was filed with the patent office on 2002-02-28 for transmission line based inductively coupled plasma source with stable impedance.
Invention is credited to Khater, Marwan H., Overzet, Lawrence J..
Application Number | 20020023899 09/878156 |
Document ID | / |
Family ID | 26921776 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020023899 |
Kind Code |
A1 |
Khater, Marwan H. ; et
al. |
February 28, 2002 |
Transmission line based inductively coupled plasma source with
stable impedance
Abstract
A properly designed and positioned Faraday shield/dielectric
spacer/source-coil assembly is used to nearly fix the input
impedance of an Inductively Coupled Plasma (ICP) source-coil,
making a variable matching network almost unnecessary, and allowing
for pulsed plasma processing with very little reflected power.
Further, the nearly constant input-impedance also means a nearly
constant standing wave pattern on the ICP source-coil and constant
power deposition symmetry as well as plasma uniformity independent
of RF power, gas pressure and gas composition. This is not possible
without a properly designed and positioned Faraday shield because
the source-coil impedance is coupled to that of the plasma and
changes significantly with the plasma conditions. The ICP
source-coil/dielectric spacer/Faraday shield assembly can then be
designed to optimize the symmetry of the plasma generation
independent of plasma conditions by varying the source coil
structure, dielectric spacer material, dielectric spacer structure,
and Faraday shield structure. An appropriately positioned aperture
in the Faraday shield can allow enough capacitive coupling between
the vacuum and ICP source coil to ignite the plasma while
preventing any significant capacitive coupling during the
subsequent high-density ICP phase.
Inventors: |
Khater, Marwan H.;
(Poughkeepsie, NY) ; Overzet, Lawrence J.; (Plano,
TX) |
Correspondence
Address: |
Groover & Associates P.C.
Suite 230
17000 Preston Road
Dallas
TX
75248
US
|
Family ID: |
26921776 |
Appl. No.: |
09/878156 |
Filed: |
June 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60227804 |
Aug 25, 2000 |
|
|
|
Current U.S.
Class: |
219/121.41 ;
118/723I; 219/121.43 |
Current CPC
Class: |
H01J 37/321
20130101 |
Class at
Publication: |
219/121.41 ;
219/121.43; 118/723.00I; 156/345 |
International
Class: |
B23K 010/00 |
Claims
What is claimed is:
1. A method for plasma processing, comprising the actions of: a)
driving a coil with an RF power source to thereby initiate a glow
discharge in a process chamber, using capacitive coupling; b) and
thereafter inductively coupling power into said glow discharge from
said coil; wherein the impedance of said coil does not change by
more than 3:1 between said steps a) and b).
2. The method of claim 1, wherein said dielectric layer consists of
at least one solid body.
3. The method of claim 1, wherein said coil is electromagnetically
coupled to said chamber through a dielectric layer of nonuniform
thickness, through a Faraday shield, and through a vacuum-sealed
dielectric chamber wall.
4. An inductively-coupled-plasma reactor system, comprising: an RF
power source; a driver coil which is inductively coupled to a
process chamber; and an impedance-matching stage connecting said
power source and said coil; wherein no component of said system
provides impedance matching over a range of impedance magnitudes of
more than 3:1.
5. The system of claim 4, wherein said coil is electromagnetically
coupled to said chamber through a dielectric layer of nonuniform
thickness and through a Faraday shield.
6. The system of claim 4, wherein said coil is electromagnetically
coupled to said chamber through a dielectric layer of nonuniform
thickness, through a Faraday shield, and through a vacuum-sealed
dielectric chamber wall.
7. A plasma processing station, comprising: a chamber; and a coil
which is electromagnetically coupled to said chamber through a
Faraday shield and through a nonuniform dielectric layer; wherein
the nonuniformity of said dielectric layer is positioned to
increase the net uniformity of plasma generated in said
chamber.
8. The station of claim 7, wherein said Faraday shield is also
separated from the interior of said chamber by a dielectric
vacuum-sealed wall.
9. The station of claim 7, wherein said dielectric layer consists
of at least one solid body.
10. The station of claim 7, wherein said Faraday shield includes an
aperture under a portion of said coil which does not include any
voltage node on said coil.
11. The station of claim 7, wherein said dielectric layer has
nonuniform thickness.
12. The station of claim 7, wherein the nonuniformity of said
dielectric layer includes cutouts.
13. The station of claim 7, wherein said dielectric layer includes
multiple different materials.
14. A method for plasma processing, comprising the actions of:
driving a coil with an RF power source through a matching network
to symmetrically energize a plasma which provides an electrical
load to said coil, while stabilizing the input impedance of said
coil, independently of said matching network, to thereby maintain
the input impedance of said coil independent of variations in the
conditions of said plasma and maintain symmetry in the energizing
of said plasma independent of variations in the conditions of said
plasma.
15. The method of claim 14, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness and through a Faraday shield.
16. A plasma source structure, comprising in combination a Faraday
shield and driver coil and dielectric, which are aligned, for a
known standing-wave condition of said coil, such that nonuniformity
of current magnitude on said coil is compensated by nonuniform
geometry of said coil and/or said dielectric and/or said Faraday
shield, to provide improved uniformity of power deposition into the
plasma.
17. The source structure of claim 16, wherein said coil has an
approximately planar geometry.
18. The source structure of claim 16, wherein said coil has a
Khater/Overzet/Cherrington geometry.
19. The source structure of claim 16, wherein said dielectric has a
nonuniform thickness.
20. The source structure of claim 16, wherein said coil is
electromagnetically coupled to said chamber through said
dielectric, through said Faraday shield, and through a
vacuum-sealed dielectric chamber wall.
21. A method for plasma processing, comprising the actions of:
driving a coil, which is electromagnetically coupled to a process
chamber through a Faraday shield and through a nonuniform
dielectric layer, with RF power, to thereby energize a glow
discharge in the process chamber; wherein the nonuniform thickness
of said dielectric layer is positioned to increase the net
uniformity of plasma generated in the chamber.
22. The method of claim 21, wherein said Faraday shield is also
separated from the interior of said chamber by a vacuum-sealed
dielectric wall.
23. The method of claim 21, wherein said coil has an approximately
planar geometry.
24. A plasma processing station, comprising: a chamber; and a coil
which is electromagnetically coupled to said chamber and is
operatively connected to be driven by an RF power supply at a
frequency which induces at least one current node on said coil.
25. The station of claim 24, wherein said coil has an approximately
planar geometry.
26. The station of claim 24, wherein said coil has a
Khater/Overzet/Cherrington geometry.
27. The station of claim 24, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness and through a Faraday shield.
28. A method for plasma processing, comprising the actions of:
driving a coil which is electromagnetically coupled to a process
chamber, with RF power at a frequency which induces at least one
current node on said coil, to thereby energize a glow discharge in
the process chamber.
29. The method of claim 28, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness and through a Faraday shield.
30. The method of claim 28, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness, through a Faraday shield, and
through a dielectric chamber wall.
31. A method for plasma processing, comprising the actions of:
initiating a glow discharge in a process chamber, using capacitive
coupling, through at least one aperture in a Faraday shield, to a
coil which is connected to an RF power source; and thereafter
inductively coupling power into said glow discharge from said coil
through said Faraday shield, while said glow discharge blocks said
capacitive coupling through said aperture.
32. The method of claim 31, wherein said Faraday shield also
includes anti-eddy-current cutouts.
33. The method of claim 31, wherein said aperture of said Faraday
shield is located under a portion of said coil which does not
include any voltage node.
34. The method of claim 31, wherein said coil has an approximately
Khater/Overzet/Cherrington geometry.
35. The method of claim 31, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness and also through said Faraday
shield.
36. The method of claim 31, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness, through said Faraday shield, and
through a vacuum-sealed dielectric chamber wall.
37. A method for pulsed plasma processing, comprising the actions
of, at each pulse: igniting a plasma in a chamber, using capacitive
coupling, through at least one aperture in a Faraday shield, to a
coil which is connected to an RF power source; and thereafter
inductively driving said plasma using said coil, while said plasma
blocks said capacitive coupling through said aperture.
38. The method of claim 37, further comprising the action, after
each pulse, of allowing a delay which is longer than the free
electron lifetime before repeating said action a).
39. The method of claim 37, wherein said Faraday shield also
includes anti-eddy-current cutouts.
40. The method of claim 37, wherein said Faraday shield includes an
aperture under a portion of said coil which does not include any
voltage node.
41. The method of claim 37, wherein said coil has an approximately
Khater/Overzet/Cherrington geometry.
42. An inductively-coupled-plasma reactor system, comprising: an RF
power source; an impedance-stabilized driver coil which is
inductively coupled to a process chamber through a Faraday shield,
and connected to receive pulsed power from said RF power source;
and an automatic impedance-matching stage, connecting said power
source and said coil, which has a response time slower than the
time between pulses of said pulsed power.
43. The system of claim 42, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness and through a Faraday shield.
44. The system of claim 42, wherein said coil is
electromagnetically coupled to said chamber through a dielectric
layer of nonuniform thickness, through a Faraday shield, and
through a vacuum-sealed dielectric chamber wall.
Description
CROSS-REFERENCE TO OTHER APPLICATION
[0001] This application claims priority from U.S. provisional
application No. 60/227,804 filed Aug. 25, 2000, which is hereby
incorporated by reference.
BACKGROUND AND SUMMARY OF THE INVENTION
[0002] The present invention relates to plasma-processing sources,
systems, and methods, and particularly to inductively-coupled
plasma source architectures.
[0003] Standing waves unavoidably develop on inductively coupled
plasma (ICP) sources because they are mismatched transmission line
systems. In addition, the source electrical properties are coupled
to those of the plasma, since the source-plasma system behaves
similar to a transformer. As a result, the source input impedance
and the RF wavelength on the source can be substantially affected
by the very plasma conditions it generates. This can also influence
the symmetry of the source electromagnetic fields and plasma
generation, which in turn will influence the uniformity of the
plasma and the ion flux to the wafer surface.
[0004] Inductively coupled plasmas behave like an air-core
transformer with the inductive source-coil as the primary circuit
and the plasma as the secondary (single current loop) circuit. The
coil impedance is coupled to that of the plasma and changes with
the plasma conductivity, which determines the plasma resistance and
reactance, causing changes in the electrical characteristics of the
inductive coil. The effect of plasma loading on the coil's voltage,
current and phase shift in argon discharges has been studied using
transformer theory. (See Piejak 1992, Godyak 1994, Gudmundsson
1997, Gudmundsson 1998, Fayoumi 1997, and Fayoumi 1998, cited
below.) Changes in the electrical characteristics of the coil due
to plasma loading affect its electromagnetic fields, which largely
determine the plasma generation symmetry and process uniformity.
Understanding the interaction between the coil's fields and the
plasma is essential for inductive source design and scaling in
order to optimize plasma process uniformity. Gudmundsson et al.
(Gudmundsson 1998) modeled and measured the changes in the
source-coil's resistance and reactance at 13.56 MHz caused by
plasma loading. El-Fayoumi et al. (Fayoumi 1997, Fayoumi 1998)
measured the current induced in argon plasmas generated with a low
frequency ICP source-coil. They calculated the plasma resistance
and inductance from the induced plasma current and studied their
effects on the coupling constant with the coil and its electrical
properties.
[0005] Most studies have considered the coil's voltage and current
to be spatially averaged and did not take into account the effect
of plasma loading on the standing wave pattern that unavoidably
develops on ICP sources. Transmission line properties of an ICP
source result in voltage and current standing waves along its
length. The variations in current with position lead to asymmetries
in the induced electro-magnetic fields, which in turn can lead to
asymmetries in the power deposition, plasma generation and
non-uniformity in the processing. (Jaeger 1995, Kushner 1996, Lamm
1997) A three-dimensional model by Kushner et al. (Kushner 1996)
showed that the transmission line properties of the coil should
influence the power deposition symmetry as well as the ion flux
uniformity to the wafer surface. They examined the effect of
capacitive termination impedance and coil geometry on the standing
wave pattern and power deposition symmetry. In a related study,
Lamm 1997, an ICP was modeled as a uniform transmission line
system. Lamm made measurements of the standing wave for different
source geometries and powers from which he derived analytical
expressions for the spatial variations of the voltage and current
along the coil length. More recently, Wu et al. (Wu 2000)
investigated the influence of source configuration and standing
wave effects on argon discharge density profiles generated with a
large area ICP source. They modeled the inductive discharge as a
lossy transmission line system and applied a transformer model to
study the electrical properties of the system. In addition to a
matching network, they used a tuning network to launch a traveling
wave or a wave with a desired standing wave ratio along the source
length. Their experiments showed that the source configuration and
standing wave ratio could strongly influence the plasma density
profile. Changes in the standing wave pattern on a new ICP source
design caused by changes in plasma loading for argon and chlorine
discharges have been reported recently by the inventors (Khater
2000, Khater 2001). The voltage and current variations along the
coil's length, as well as the phase difference between them, are
determined by the coil's characteristic impedance. Since plasma
loading changes the coil's characteristic impedance, the standing
wave pattern will also change depending on the plasma conditions.
As a result, the plasma generation symmetry and uniformity for a
fixed ICP source geometry changes as the plasma conditions are
varied. This effect should be considered in the design of ICP
sources as they are scaled to large sizes for processing large area
substrates.
[0006] A Faraday shield can be used to minimize these deleterious
effects if properly designed and positioned. To date, Faraday
shields have been used simply to decrease capacitive coupling
between the source and the plasma and reduce sputtering of the
dielectric window. Faraday shields have been used in this fashion
for at least several decades. A dielectric spacer is placed between
the source and the Faraday shield to provide electrical insulation.
In most cases, air is chosen to be the dielectric because air has
the lowest relative permittivity and results in the smallest
standing-wave variation on the source. The present application
teaches that a "source-coil/dielectricspacer/Faraday shield"
assembly acts as a transmission line with a nearly fixed
characteristic impedance and standing wave pattern on the
source-coil. In this manner, the source impedance is made stable
regardless of plasma conditions since the Faraday shield decouples
the source-coil electrical properties from those of the plasma. The
key to designing this ICP source-coil/dielectric spacer/Faraday
shield assembly is to ensure that the impedance between the shield
and ICP source-coil dominates over the impedance between the ICP
source-coil and plasma. When this is the case, changes in the
plasma characteristics can cause little or no variation in the
total ICP source-coil impedance and therefore become negligible. As
a result, the standing wave pattern on the ICP source-coil becomes
constant, as does the input impedance and plasma generation
symmetry.
[0007] Such an assembly has important implications for plasma
system design and optimization. For example, the use of this type
assembly allows any ICP source to be impedance matched by a nearly
fixed matching circuit. The possibility of a fixed matching
condition will allow much simpler plasma control in addition to
easily allowing for pulsed plasma processing with very little
reflected power. This has been demonstrated experimentally (Khater
2001). In addition, once the source geometry is optimized for
symmetric electromagnetic fields and plasma uniformity with a fixed
standing wave pattern, it should stay uniform regardless of the
plasma conditions. Optimizing the structure of the dielectric
spacer (materials, shape) and Faraday shield structure in addition
to the source-coil geometry is important in optimizing the
electromagnetic field symmetry.
[0008] Finally, a calibrated aperture in the center, at the edge,
or at some other location in the Faraday shield can be designed to
allow a small amount of capacitive coupling to the plasma for
striking the discharge. Once a high-density plasma forms, it will
expel this capacitive coupled field and result in an inductively
coupled plasma. Consequently, the source will both strike reliably
and result in very little window sputtering or other deleterious
effects. In addition, the Faraday shield/dielectric
spacer/source-coil assembly will still prevent the plasma from
changing the source-coil standing wave pattern, input impedance,
and fields symmetries.
[0009] Transmission Line Based Inductively Coupled Plasma Source
with Stable Impedance
[0010] The present inventors have realized that the Faraday
shield/dielectric spacer/source-coil assembly provides a
fundamental change in the electrical characteristics of the coil
which drives the plasma, and that this change permits new
techniques for operating an inductively-coupled plasma reactor.
Without a Faraday shield, the RF behavior of the coil is determined
by the state of the chamber's interior, which varies dynamically.
The complex impedance of the coil changes dramatically when the
plasma is ignited, but also is dependent on other factors, such as
pressure, which affect the electron density of the plasma. Since
the coil is electrically coupled to the plasma, changes in the
electron density of the plasma also change the complex impedance of
the coil.
[0011] With the Faraday shield, capacitive coupling between the
coil and the plasma is largely removed. The present inventors have
realized that this makes the coil's complex impedance much more
independent of changes in the electron density of the plasma, and
that this is very beneficial in optimizing the uniformity and
controllability of the plasma source. Conventional ICP systems must
allow for a large shift in complex impedance. One result of this is
that conventional systems must use automatic matching networks
which can adapt to large changes in the magnitude of impedance,
e.g. over a range of ten to one.
[0012] The electrical behavior of an inductive source-coil is that
of a transmission line, which forms the primary of a transformer.
The transformer's secondary is the loop of current that flows in
the plasma. Conventional wisdom is that current nodes must
generally be avoided on the source-coil. A current node on a
transmission line will result in the appearance of voltage
antinodes, i.e. locations where the voltage has a much larger
magnitude than at other parts of the transmission line. This can
result in increased erosion of the dielectric shield at such
points. (A "node," analogously, is a location where the current or
voltage is lower than at adjacent positions.) Moreover, the current
distribution will be very non-uniform under such conditions, and
this can result in hot spots, at unpredictable locations in the
plasma, which cause non-uniformities in the wafer processing. The
present inventors have realized that the use of a three-dimensional
source-coil design coupled with the Faraday shield can allow one to
circumvent conventional wisdom. One can place current nodes on the
source-coil without causing hot spots and still produce symmetric,
uniform plasma. The ability to have current nodes on the
source-coil, however, allows one to produce uniform plasma over
much larger areas.
[0013] In one class of embodiments, the decoupling effect of the
Faraday shield is used to permit operation of the coil in resonant
or near-resonant conditions. Since the coil is decoupled from the
variations in the plasma electron density, the location of voltage
and current antinodes is less likely to shift unpredictably.
Moreover, since the current distribution in the coil is now more
predictable, the geometry of the coil can be modified to increase
the uniformity of power deposited into the plasma.
[0014] In a further class of embodiments this idea is taken even
farther, and the coil, supporting dielectric, and Faraday shield
are all jointly optimized for plasma uniformity.
BRIEF DESCRIPTION OF THE DRAWING
[0015] The disclosed inventions will be described with reference to
the accompanying drawings, which show important sample embodiments
of the invention and which are incorporated in the specification
hereof by reference, wherein:
[0016] FIG. 1 shows a transmission line inductive plasma source:
the left side (part a) shows an exploded view of major components
of the source, and the right side (part b) shows a complete
plasma-processing reactor which includes this source.
[0017] FIGS. 2(a) through 2(f) show six sample embodiments where a
transmission line inductive plasma source 130 is combined with
different examples of dielectric spacer 140 cross-section for a
primarily planar source configuration.
[0018] FIGS. 3(a) through 3(d) show four sample embodiments where
the dielectric spacer 140 is implemented with different materials
as part of its structure. Although only three dielectrics are
delineated here, those skilled in the art will recognize that any
number of different materials can be used. Although the dielectric
spacer is drawn for the primarily planar configurations of the FIG.
2 embodiments, those skilled in the art will realize that the same
kinds of variations can be applied to the cylindrical and domed
assemblies of the FIG. 4 embodiments as well.
[0019] FIGS. 4(a) and 4(b) show sample embodiments of a
transmission line inductive plasma source with different examples
of dielectric spacer cross-section for a primarily cylindrical
source, and FIG. 4(c) shows an example of the dielectric spacer
cross-section for a domed source. Those skilled in the art will
realize that all the variation possibilities exemplified in the
various FIG. 2 and FIG. 3 embodiments will also apply to the FIG. 4
embodiments.
[0020] FIGS. 5(a) through 5(d) show several different examples of
the Faraday shield that can allow enough capacitive coupling for
plasma ignition while preserving the transmission line source
impedance as a near constant for a planar configuration.
[0021] FIGS. 6(a) through 6(d) show several different examples of a
Faraday shield that can allow enough capacitive coupling for plasma
ignition while preserving the transmission line source impedance as
a near constant for a substantially cylindrical configuration.
[0022] FIGS. 7(a) and 7(b) schematically show how capacitive
coupling can be important when the plasma density is small (during
plasma ignition) and become negligible when the inductive coupling
to the plasma becomes dominant. In FIG. 7(a), with no plasma
present, fields penetrate into the chamber 110. In FIG. 7(b), with
plasma present, the fields are largely excluded from the chamber
and nearly all the potential drop occurs within the dielectric
window 114.
[0023] FIG. 8(a) shows a loss-less transmission line circuit model,
consisting of a distributed series inductance and a distributed
shunt capacitance, for the source assembly. FIGS. 8(b) through 8(e)
show how this model corresponds to the various physical elements of
the plasma reactor, in various configurations and conditions. The
shunt capacitance is determined by the source-coil assembly and
plasma conductivity. It becomes larger when the plasma is present
without a Faraday shield but is largely unaffected by the presence
of plasma when the Faraday shield is present.
[0024] FIGS. 9(a) and 9(b) show Output-current to Input-Current
magnitude Ratios (OICRs) as a function of ICP source power for (a)
chlorine and (b) argon plasmas, WITHOUT the Faraday
shield/dielectric spacer assembled to the source coil.
[0025] FIG. 10 shows the Output-current to Input-Current magnitude
Ratio (OICR) as a function of ICP source power for chlorine and
argon plasmas with the Faraday shield/dielectric spacer assembled
to the source coil.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The numerous innovative teachings of the present application
will be described with particular reference to the presently
preferred embodiment (by way of example, and not of
limitation).
[0027] FIG. 1 shows a transmission line inductive plasma source
(TLICP): the left side (part a) shows an exploded view of major
components of the source, and the right side (part b) shows a
complete plasma-processing reactor which includes this source.
Major components shown include the inductive source-coil 130,
dielectric spacer 140 and Faraday shield 150.
[0028] The TLICP source coil 130 is shown with a complex geometry
like that described in U.S. Pat. No. 6,028,285 of Khater, Overzet
and Cherrington. The illustrated source-coil design consists of two
layers of loops separated by a few centimeters. The bottom layer
loops are complete circles and are shown as thin lines. The top
layer loops are 3/4 of a circle (except for the outer most loop
which is a full circle), and are shown as thick lines. Plasmas in a
wide variety of gases and mixtures have been generated at pressures
between 1-500 mTorr and powers up to 1000 W using this source
(Khater 2000, Khater 2001).
[0029] The dielectric spacer 140 can consist of any number of
materials including but not limited to carbon based materials like
Teflon.RTM., ceramics like alumina, air or other gases/vapors, and
dielectric liquids. The primary purpose of the dielectric insulator
to date has been to prevent arcing from the source coil to the
Faraday shield 150. In the present invention, the dielectric is
structured to produce uniform plasma generation by controlling the
transmission-line characteristic-impedance along the source-coil in
addition to acting as simple insulation.
[0030] The Faraday shield 150 generally consists of a good
conductor with anti-eddy-current slots 152. The slots 152 can vary
in number and dimension, but it has been found that 16 slots in a
circular arrangement of {fraction (1/16)}" width works well for
primarily planar source-coils up to 10" in diameter. The number of
slots generally lies between 4 and 64. The slot width generally
lies between {fraction (1/64)}" and 1/4".
[0031] Part (b) of FIG. 1 shows the TLICP source assembly (130, 140
and 150) installed on a plasma-processing reactor. (The reactor
shown is simply a generic reactor, and many other reactor
configurations can be used.) The TLICP source generates plasma in
the plasma region 110. It has an RF power source 126 connected to
it through a matching network 120 consisting of primarily reactive
components 122 and 124. In this example the reactive elements shown
(122 and 124) are both variable capacitances (as is customary for
Inductively Coupled Plasma sources), but fixed capacitances and/or
fixed or variable inductances and transformers can all be used.
Typical frequencies of the RF power lie between about 100 kHz and
100 MHz. The assembly sits on a dielectric window 114, which allows
the electric and magnetic fields produced by the TLICP source
assembly to enter the plasma region 110 while also providing a
vacuum seal. A gas inlet 112 allows calibrated amounts of gas to
enter the plasma region 110. The plasma region 110 is bounded by a
vacuum vessel 102 and by a chuck assembly 104 holding a work piece
106 for processing using either a clamp 108 or some other
mechanism. (Other mechanisms might include electrostatic clamping
or gravity.) The chuck assembly can have power applied to it.
Either DC power or RF power 126' can be applied to the chuck
assembly through a matching network, a simple capacitance 116, or a
direct connection.
[0032] FIGS. 2(a) through 2(f) show six sample embodiments where a
transmission line inductive plasma source 130 is combined with
different examples of dielectric spacer 140 cross-section for a
primarily planar source configuration. In various embodiments the
source-coil 130 either lies on top of, partially inside of and/or
completely inside of a dielectric spacer 140, which in turn lies on
top of a Faraday shield 150. The dielectric spacer 140 can be made
of any number of materials including but not limited to
Teflon.RTM., alumina, air or other gases/vapors, and dielectric
liquids. The dielectric spacer can be a uniform layer of thickness
"t" as shown in FIGS. 2(a) and 2(b). It can also have a wide
variety of structural elements including a varying thickness (FIG.
2(c)); a central opening (circular, rectangular, etc.) of major
dimension x shown in FIG. 2(d); openings of major dimension y
placed either under a source-coil element or between source-coil
elements shown in FIG. 2(e); recesses shown in FIG. 2(e); and
stacks of materials or combinations of the above shown in FIG.
2(f). In addition, the structural elements can include a ground
shield 252 placed in close proximity to a section of the
source-coil with a dielectric spacer 242 between the ground shield
252 and source-coil shown in FIG. 2(d) and 2(f). The structure of
the dielectrics (140, 242) and coil 130 are designed together to
optimize the desired properties of the full assembly.
[0033] FIGS. 3(a) through 3(d) show top views of four sample
embodiments where the dielectric spacer 140 is implemented with
different materials as part of its structure. Although only three
dielectrics are delineated here, those skilled in the art will
recognize that any number of different materials can be used.
Although the dielectric spacer is drawn for the primarily planar
configurations of the FIG. 2 embodiments, those skilled in the art
will realize that the same kinds of variations can be applied to
the cylindrical and domed assemblies of the FIG. 4 embodiments as
well.
[0034] In the various FIG. 3 embodiments, several modifications of
the dielectric spacer 140 are shown for a primarily planar source
coil configuration. Multiple dielectric materials can be used to
optimize the transmission line properties of the TLICP assembly.
Those skilled in the art will recognize that a central dielectric
304 with different permittivity from the remaining material of the
dielectric spacer 302 can prove beneficial for striking plasma as
one example. Either the central dielectric 304 or the circular
dielectric 302 can also be a material with limited conductivity. In
a second example, an arc of the circular dielectric spacer 140 can
be made from another dielectric material 306 in order to optimize
plasma generation uniformity. In another example, support
structures for the coil can be made of one dielectric material 308
while the remainder of the spacer is made of another 302. This
includes dielectric support structures 308 in air 302 but is not
limited to such a choice of the materials. Another example of a
complex interleaving and stacking of multiple dielectric materials
302, 310, and 312 is shown in FIG. 3(d). The structure of the
dielectric and coil are designed together to optimize the desired
properties of the system.
[0035] FIGS. 4(a) and 4(b) show sample embodiments of a
transmission line inductive plasma source with different examples
of dielectric spacer cross-section for a primarily cylindrical
source, and FIG. 4(c) shows an example of the dielectric spacer
cross-section for a domed source. Those skilled in the art will
realize that all the variation possibilities exemplified in the
various FIG. 2 and FIG. 3 embodiments will also apply to the FIG. 4
embodiments, and all of the examples used to describe the
possibilities for a primarily planar configuration can also be
applied to cylindrical, domed, conical and hemispherical
configurations as well. Those skilled in the art will recognize
that there may be slight differences between each configuration
caused by the change of symmetry for each coordinate system
(Cartesian (x-y) to polar (r-theta) to cylindrical (z-theta) to
spherical (theta-phi)), but that the concepts demonstrated for the
Cartesian and polar coordinates also apply for other coordinate
systems.
[0036] FIGS. 5(a) through 5(d) show several different examples of
the Faraday shield 150 that can allow enough capacitive coupling
for plasma ignition while maintaining the transmission line source
impedance nearly constant. The FIG. 5 diagrams are for a primarily
planar configuration, but the concepts demonstrated in these
embodiments can easily be applied to other coordinate systems by
those skilled in the art. The Faraday shield 150 has radial anti
eddy-current slots 152 cut into it to match the radial symmetry
assumed for the primarily planar source coil 130 of FIG. 1. The
slots are cut substantially perpendicular to the direction of
current flow in the source-coil, and prevent the Faraday shield
from blocking the RF electromagnetic fields generated by those
currents. One can also envision a non-uniform density of anti
eddy-current slots 510, e.g. as shown in FIG. 5(c), in order to
improve the electromagnetic field symmetry of a non-uniform
source-coil.
[0037] A central opening 502 can also be made that allows the
plasma to ignite by stray "capacitive" electric fields. These
fields are generated by the large voltages on the source-coil,
which the Faraday shield would ordinarily block from the plasma
region 110 were not the central opening 502 present. Other kinds of
openings can also be used to allow the plasma to strike, e.g. as
shown in FIG. 5(b), (c) and (d): for example a radial arc 504, a
rectangular opening 506 or a circular opening 508 can be used. The
rectangular opening shown in FIG. 5(c) is merely a limited
expansion of one of the slots 152. In addition, multiple
combinations of these openings can be used in concert. The
distinguishing characteristic of these openings is that they allow
the plasma source-coil to capacitively ignite the plasma while
maintaining the source-coil impedance as a near constant. As such,
they will be placed near regions of the source-coil at large
voltages with respect to ground rather than only in regions where
the source-coil has near ground voltages. Openings may also be
placed in proximity to large permittivity dielectrics that will
help to reliably ignite the plasma by causing a larger fraction of
the voltage drop to occur in the plasma chamber 110. These openings
are generally expected to have a static size so that the
source-coil input impedance remains stable, but dynamically
variable slot sizes can be used as well.
[0038] FIGS. 6(a) through 6(d) show several different examples of a
Faraday shield that can allow enough capacitive coupling for plasma
ignition while preserving the transmission line source impedance as
a near constant for a substantially cylindrical configuration. For
example, an aperture 602 can be placed between anti eddy-current
slots 652 to allow plasma ignition. An expanded slot 604 can do the
same thing. The expanded slot or aperture can have almost any
shape, but is shown as rectangular in 602, rounded rectangular in
604 and circular in 606. The central opening 502 in FIG. 5
corresponds to the central aperture 608 in FIG. 6, and the
reduction in anti eddy-current slots 510 in FIG. 5 corresponds to
the reduction of slots 610 in FIG. 6.
[0039] FIGS. 7(a) and 7(b) schematically show how capacitive
coupling between the source-coil 130 and plasma region 110 can be
important when the plasma density is small (during plasma ignition)
and become negligible when the inductive coupling to the plasma
becomes dominant. In FIG. 7(a), with no plasma present, the voltage
on the source-coil establishes electric fields 702 which can
penetrate into the plasma region 110 through a calibrated aperture
(502 etc.) in the Faraday shield 150. With no plasma present, the
field penetration can be substantial because the conductivity of
the vacuum is low. This is illustrated in FIG. 7(a). Once the
plasma density becomes significant, the electrons of the plasma act
to expel all electric fields below the plasma frequency. As a
result (FIG. 7(b)), the electric fields established by the voltage
on the source-coil are also expelled from the plasma region and
pushed primarily inside the dielectric window 114.
[0040] FIG. 8(a) through 8(e) show (using transmission line
modelling) how the Faraday shield 150 acts to stabilize the
source-coil 130 input-impedance. The source-coil 130 acts as a
transmission-line when driven with RF currents. The characteristic
impedance of that source-coil transmission-line is unstable without
a Faraday shield because the plasma conductivity changes and the
proximity of the plasma to the source-coil changes. These have a
large influence on the characteristic impedance. The impedance is
stable with a Faraday shield because the Faraday shield has a fixed
conductivity and proximity to the source-coil. To illustrate these
effects, FIG. 8(a) shows an analog model of a loss-less
transmission-line. The model simulates transmission-line behavior
well and consists of series inductances and shunt capacitances. The
shunt capacitances in the model are determined in large part by the
capacitance between individual loops (wires) of the source-coil and
ground potential. Ground potential is almost invariably asserted at
the chamber walls 102, so FIG. 8(b) illustrates how one might
envision the capacitance of the transmission-line model to be
established by the chamber. This capacitance should be small since
the chamber is far from the source-coil loops. Since plasma has a
finite conductivity and acts to expel electric fields generated by
the coil, the formation of plasma in the chamber significantly
affects the capacitance between the source coil loops and ground.
This is illustrated in FIG. 8(c) where a capacitance between the
source coil loops and the plasma is placed in series with a
capacitance between the plasma and ground. The series combination
of these capacitances is significantly larger than the capacitance
without plasma and consequently changes the transmission-line
characteristics substantially. A Faraday shield 150 can stabilize
the transmission-line characteristics of the source-coil as shown
in FIGS. 8(d) and 8(e). The shunt capacitance of FIG. 8(a) is now
dominated by the capacitance between the source-coil and Faraday
shield instead of the chamber ground. The small capacitance to the
chamber ground can still exist through the Faraday shield aperture,
but adds an insignificant capacitance. When the plasma is started,
the large capacitance to the Faraday shield is unaffected and even
though the small capacitance to the chamber ground increases, it
does not cause any substantial change in the source-coil's
transmission-line characteristics.
[0041] FIGS. 9(a), 9(b), and 10 show data taken from Argon and
Chlorine plasmas illustrating the input-impedance stability
afforded by a properly designed source-coil 130/dielectric spacer
140/Faraday shield 150 assembly. The ICP source voltage and
currents at both the powered (input) and ground leads were measured
at different powers and pressures for argon and chlorine plasmas
without and with the Faraday shield. The voltage and currents were
similar for both argon and chlorine discharges under the same
conditions (Khater 2001). The voltage generally increased with ICP
source power as expected. It had a small dependence on pressure
with the highest value occurring at the lowest pressure (1 mTorr).
This is mainly due to an increase in the source current, which is
necessary to sustain the discharge as the pressure is decreased
(Piejak 1992, Godyak 1995). The impedance of the source remained
inductive and the phase difference between the current at the
powered lead of the source-coil, I.sub.Prms, and the voltage at the
powered lead ranged from 80.degree. to 90.degree. for different
plasma conditions. In addition, the current standing wave on the
source did not obtain a current node.
[0042] Since there was no current node on the source-coil and the
source current is largest at the grounded lead, I.sub.Grms, the
Output to Input Current Ratio (OICR) can be defined as:
OICR=.vertline.I.sub.Grms .vertline./I.sub.Prms.vertline.
[0043] The source-coil OICR for chlorine and argon plasmas without
the Faraday shield are shown in FIGS. 9(a) and 9(b) as a function
of ICP source power and pressure. The OICR increased by a factor as
large as 2 with source power and discharge pressure for both
chlorine and argon plasmas. Such increase in the OICR indicates a
change in the impedance of the source, which is coupled to changes
in the plasma parameters. FIGS. 9(a)/9(b) also show that the OICR
is dependent on the gas type, where the OICR for argon plasma is
smaller at lower pressures. Changes in the OICR result in changes
in the electromagnetic field profile generated by the source, which
affects the power deposition symmetry to the plasma as well as
plasma processing uniformity (Jaeger 1995, Kushner 1996, Khater
2000). As a result, it is difficult to stabilize the plasma
uniformity for a fixed ICP source geometry as the plasma conditions
are varied.
[0044] FIG. 10 shows the Output-current to Input-Current magnitude
Ratio (OICR) as a function of ICP source power for chlorine and
argon plasmas with the Faraday shield/dielectric spacer assembled
to the source coil. In these experimental results, the present
inventors installed a Faraday shield between the quartz window and
the ICP source. The shield and the source were separated by a
dielectric spacer made out of Teflon (see FIG. 1). In addition to
reducing capacitive coupling and window sputtering, the Faraday
shield acts as the second conductor in a transmission line system
with the source as the first conductor and the spacer as the
dielectric medium between them. In this manner the transmission
line parameters of the ICP source (as well as input impedance) are
fixed by the Faraday shield and are no longer dependent on plasma
conductivity variations. The transmission line parameters can be
further controlled by adjusting the dielectric spacer properties
and structure to optimize the power deposition symmetry and plasma
uniformity.
[0045] The RMS current at both leads with the Faraday shield
increased compared to that measured without the shield. This was
expected for equal source input powers. The grounded lead current
increased more than the powered lead current as well. This causes
an increased OICR and is mainly due to larger capacitance to the
shield. Despite the larger OICR with the Faraday shield, a properly
designed source can generate electromagnetic fields and plasmas of
high symmetry and uniformity. The OICR is plotted in FIG. 10 for
chlorine and argon plasmas at a variety of powers and pressures.
The OICR is almost completely independent of the plasma conditions
when using the Faraday shield indicating that the source electrical
properties have been decoupled from the plasma and that its
impedance is stabilized. It is about the same for both argon and
chlorine plasmas, at pressures between 1 and 20 mTorr and RF powers
between 100 and 1000 Watts. It was observed that the phase
difference between the voltage and current at either lead of the
source remained nearly constant for all plasma conditions and that
the matching capacitors (122 and 124 in FIG. 1) required minimal
changes. All of these were taken as indications that the Faraday
shield stabilized the source-coil input-impedance and standing wave
pattern. As a result, the symmetry of the fields generated by the
source-coil assembly will also be stabilized when the shield is
used. There are some small variations in the OICR plotted in FIG.
10. It is expected that these were caused by a smaller interaction
between the source and the plasma through the center hole 502 in
the Faraday shield (FIG. 5(a)).
[0046] Definitions
[0047] Following are short definitions of the usual meanings of
some of the technical terms which are used in the present
application. (However, those of ordinary skill will recognize
whether the context requires a different meaning.) Additional
definitions can be found in the standard technical dictionaries and
journals.
[0048] Faraday shield: a conductive layer which serves to block
electro-static fields.
[0049] Impedance: a complex number which expresses both resistance
(the real part) and reactance (the imaginary part).
[0050] Transmission line: a conductor which is long enough, in
relation to the electrical wavelength it carries, that substantial
phase shifts occur within the length of the line. The electrical
parameters of the transmission line are expressed as distributed
resistance, inductance and capacitance (i.e. resistance etc. per
unit length). The distributed resistance, inductance and
capacitance will define a characteristic impedance of the
transmission line.
[0051] Impedance matching: In an RF circuit, power will not be
efficiently transmitted between elements having different
characteristic impedances. Where such an impedance mismatch
appears, some of the power will be reflected back to the source.
This reflected power will produce a standing wave, which may have
an undesirably high magnitude. The degree of mismatch can be
measured by Voltage Standing Wave Ratio (VSWR) or by current
standing wave ratio. To avoid or minimize the effects of mismatch,
it is common to use an impedance matching circuit, which transforms
the RF impedances to reduce or eliminate mismatch. In conventional
plasma processing systems, the impedance matching circuit operates
automatically, to track the wide variations in impedance caused by
changes in the plasma conditions.
[0052] Inductively Coupled Plasma (ICP): a type of plasma source
which uses predominantly inductive coupling (rather than capacitive
coupling) to electrically drive a plasma with RF power.
[0053] According to at least some disclosed embodiments, there is
provided: A method for plasma processing, comprising the actions
of: a) driving a coil with an RF power source to thereby initiate a
glow discharge in a process chamber, using capacitive coupling; b)
and thereafter inductively coupling power into said glow discharge
from said coil; wherein the impedance of said coil does not change
by more than 3:1 between said steps a) and b).
[0054] According to at least some disclosed embodiments, there is
provided: An inductively-coupled-plasma reactor system, comprising:
an RF power source; a driver coil which is inductively coupled to a
process chamber; and an impedance-matching stage connecting said
power source and said coil; wherein no component of said system
provides impedance matching over a range of impedance magnitudes of
more than 3:1.
[0055] According to at least some disclosed embodiments, there is
provided: A plasma processing station, comprising: a chamber; and a
coil which is electromagnetically coupled to said chamber through a
Faraday shield and through a nonuniform dielectric layer; wherein
the nonuniformity of said dielectric layer is positioned to
increase the net uniformity of plasma generated in said
chamber.
[0056] According to at least some disclosed embodiments, there is
provided: A method for plasma processing, comprising the actions
of: driving a coil with an RF power source through a matching
network to symmetrically energize a plasma which provides an
electrical load to said coil, while stabilizing the input impedance
of said coil, independently of said matching network, to thereby
maintain the input impedance of said coil independent of variations
in the conditions of said plasma and maintain symmetry in the
energizing of said plasma independent of variations in the
conditions of said plasma.
[0057] According to at least some disclosed embodiments, there is
provided: A plasma source structure, comprising in combination a
Faraday shield and driver coil and dielectric, which are aligned,
for a known standing-wave condition of said coil, such that
nonuniformity of current magnitude on said coil is compensated by
nonuniform geometry of said coil and/or said dielectric and/or said
Faraday shield, to provide improved uniformity of power deposition
into the plasma.
[0058] According to at least some disclosed embodiments, there is
provided: A method for plasma processing, comprising the actions
of: driving a coil, which is electromagnetically coupled to a
process chamber through a Faraday shield and through a nonuniform
dielectric layer, with RF power, to thereby energize a glow
discharge in the process chamber; wherein the nonuniform thickness
of said dielectric layer is positioned to increase the net
uniformity of plasma generated in the chamber.
[0059] According to at least some disclosed embodiments, there is
provided: A plasma processing station, comprising: a chamber; and a
coil which is electromagnetically coupled to said chamber and is
operatively connected to be driven by an RF power supply at a
frequency which induces at least one current node on said coil.
[0060] According to at least some disclosed embodiments, there is
provided: A method for plasma processing, comprising the actions
of: driving a coil which is electromagnetically coupled to a
process chamber, with RF power at a frequency which induces at
least one current node on said coil, to thereby energize a glow
discharge in the process chamber.
[0061] According to at least some disclosed embodiments, there is
provided: A method for plasma processing, comprising the actions
of: initiating a glow discharge in a process chamber, using
capacitive coupling, through at least one aperture in a Faraday
shield, to a coil which is connected to an RF power source; and
thereafter inductively coupling power into said glow discharge from
said coil through said Faraday shield, while said glow discharge
blocks said capacitive coupling through said aperture.
[0062] According to at least some disclosed embodiments, there is
provided: A method for pulsed plasma processing, comprising the
actions of, at each pulse: igniting a plasma in a chamber, using
capacitive coupling, through at least one aperture in a Faraday
shield, to a coil which is connected to an RF power source; and
thereafter inductively driving said plasma using said coil, while
said plasma blocks said capacitive coupling through said
aperture.
[0063] According to at least some disclosed embodiments, there is
provided: An inductively-coupled-plasma reactor system, comprising:
an RF power source; an impedance-stabilized driver coil which is
inductively coupled to a process chamber through a Faraday shield,
and connected to receive pulsed power from said RF power source;
and an automatic impedance-matching stage, connecting said power
source and said coil, which has a response time slower than the
time between pulses of said pulsed power.
[0064] Modifications and Variations
[0065] As will be recognized by those skilled in the art, the
innovative concepts described in the present application can be
modified and varied over a tremendous range of applications, and
accordingly the scope of patented subject matter is not limited by
any of the specific exemplary teachings given.
[0066] Many modifications have been illustrated in the various
configurations illustrated in the subparts of FIGS. 2, 3, 4, 5, 6,
and 7. It will also be recognized that these modifications and
embodiments can be used in combination with each other, so that a
very large number of total possibilities have already been
indicated.
[0067] The TLICP source coil 130 is illustrated with a complex
Khater/Overzet/Cherrington geometry like that described in U.S.
Pat. No. 6,028,285, but many other source coil geometries are
possible. In particular, those skilled in the art will recognize
that all manner of two-dimensional geometries can be used
(including but not limited to planar geometries; e.g. concentric
circle or helical or multi-helix geometries), as well as
hemispherical geometries, domed geometries, and cylindrical
geometries can be used.
[0068] For another example, the preferred class of embodiments uses
an aperture in the shield to retain a small amount of capacitive
coupling for igniting the plasma. However, the aperture is not
necessary in all embodiments; for example, laser or microwave
ignition could be used instead.
[0069] In another alternative, part of the coil can be tapered line
if desired, to provide a graduated impedance transformation and
hence a graduated current.
[0070] In embodiments which use nonuniform dielectric (to tailor
the uniformity of the plasma power deposition profile), the
nonuniformity of the dielectric can be achieved not only by
nonuniform thickness, but also by different materials, including
materials of different permittivities, and also including vacuum,
air or other gasses, and even liquids (e.g. connected in a flow
through a heat exchanger).
[0071] The complete plasma processing stations using the present
invention do not have to be simple one-chamber systems like that
shown in the drawing. The disclosed inventions are also fully
applicable to and advantageous in multistation modules.
[0072] The disclosed inventions are not applicable only to plasma
etching methods and systems, but also to methods and systems for
plasma-assisted deposition, implantation, and other plasma
processes.
[0073] The disclosed inventions are particularly advantageous in
pulsed plasma systems. The disclosed impedance-stabilized
configurations are especially advantageous in such systems, since
the rapid changes in plasma state do not have to be tracked by
adaptations of the automatic impedance-matching network. Indeed,
the impedance-matching network can now be specified to have both a
more limited bandwidth and more limited range than was previously
possible.
[0074] The disclosed inventions are not applicable only to plasma
methods and systems used in microfabrication, but can also be
adapted to methods and systems for plasma-assisted processing of
large articles.
[0075] Those skilled in the art will understand that Maxwell's
equations imply that the magnitude of electromagnetic fields driven
by the coil will necessarily vary within the total driven volume.
Thus the uniformity or symmetry sought, in design and selection of
plasma sources, is typically a requirement of (e.g. for a planar
coil) circumferential (or "azimuthal") uniformity, while allowing
some known smooth gradation in the radial and axial directions. The
disclosed techniques for increasing uniformity can be used to
achieve various desired distributions of RF power deposition. For
example, depending on the relation between workpiece geometry and
source geometry, a system designer might wish to modify the radial
distribution of power density, while keeping the circumferential
distribution perfectly uniform. The disclosed optimizations can be
applied to such specifications if desired.
[0076] The following publications provide additional detail
regarding possible implementations of the disclosed embodiments,
and of modifications and variations thereof, and the predictable
results of such modifications, and are all hereby incorporated by
reference: R. Piejak, V. Godyak, and B. Alexandrovich, Plasma
Sources Sci. Technol. 1, 179 (1992); V. Godyak, R. Piejak, and B.
Alexandrovich, Plasma Sources Sci. Technol. 3, 169 (1994); J.
Gudmundsson, and M. Lieberman, Plasma Sources Sci. Technol. 6, 540
(1997); J. Gudmundsson, and M. Lieberman, Plasma Sources Sci.
Technol. 7, 83 (1998); I. El-Fayoumi and I. Jones, Plasma Sources
Sci. Technol. 6, 201 (1997); I. El-Fayoumi and I. Jones, Plasma
Sources Sci. Technol. 7, 179 (1998); E. Jaeger, L. Berry, J.
Tolliver, and D. Batchelor, Phys. Plasmas 2, 2597 (1995); M.
Kushner, W. Collison, M. Grapperhaus, J. Holland, and M. Barnes, J.
Appl. Phys. 80, 1337 (1996); A. Lamm, J. Vac. Sci. Technol. A 15,
2615 (1997); Y. Wu and M. Lieberman, Plasma Sources Sci. Technol.
9, 210 (2000); M. Khater and L. Overzet, Plasma Sources Sci.
Technol. 9, 545 (2000); M. Hopkins and W. Graham, Rev. Sci.
Instrum. 57, 2210 (1986); V. Godyak, R. Piejak, and B.
Alexandrovich, Plasma Sources Sci. Technol. 4, 332 (1995); S.
Shinohara, S. Takechi, and Y. Kawai, Jpn. J. Appl. Phys. 35, Part
1, 4503 (1996); M. Khater and L. Overzet, J. Vac. Sci. Technol A
19, 785 (2001); Grill, Cold Plasma in Materials Fabrication (1994);
Chapman, Glow Discharge Processes; Coburn, Plasma etching and
reactive ion etching (1982); Handbook of Plasma Processing
Technology (ed. Rossnagel); Lieberman, Principles of Plasma
Discharges and Materials Processing (1994); PLASMA PROCESSING (ed.
Dieleman et al. 1982); and Plasma Etching (Manos and Flamm, 1989).
All of these publications are hereby incorporated by reference.
[0077] None of the description in the present application should be
read as implying that any particular element, step, or function is
an essential element which must be included in the claim scope: THE
SCOPE OF PATENTED SUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED
CLAIMS. Moreover, none of these claims are intended to invoke
paragraph six of 35 USC section 112 unless the exact words "means
for" are followed by a participle.
* * * * *