U.S. patent application number 10/792462 was filed with the patent office on 2005-09-08 for inductively coupled plasma source using induced eddy currents.
Invention is credited to Jewett, Russell F. JR., Scholl, Richard A..
Application Number | 20050194099 10/792462 |
Document ID | / |
Family ID | 34911859 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194099 |
Kind Code |
A1 |
Jewett, Russell F. JR. ; et
al. |
September 8, 2005 |
Inductively coupled plasma source using induced eddy currents
Abstract
Methods and apparatus are provided for generating an inductively
coupled plasma using induced eddy currents. An inductively coupled
plasma source of the invention generally comprises a body
constructed substantially of a conductive material interrupted by
at least one dielectric gap. Radio frequency power is coupled from
a current carrier into the conductive body. The one or more
dielectric interruptions in the conductive body are disposed so as
to cause eddy currents to circulate about portions of the body and
thereby couple RF power into a plasma in proximity to the
conductive body. By utilizing induced eddy currents to couple power
into a plasma, the invention allows for substantial bodies of
conductive materials, such as structural metals, to be interposed
between the induction coils that receive power from a power
generator and the plasma.
Inventors: |
Jewett, Russell F. JR.;
(Fort Collins, CO) ; Scholl, Richard A.; (Fort
Collins, CO) |
Correspondence
Address: |
JOHN D. PIRNOT
ADVANCED ENERGY INDUSTRIES, INC.
1625 SHARP POINT DR.
FORT COLLINS
CO
80525
US
|
Family ID: |
34911859 |
Appl. No.: |
10/792462 |
Filed: |
March 3, 2004 |
Current U.S.
Class: |
156/345.48 ;
216/68; 427/255.28 |
Current CPC
Class: |
H05H 1/46 20130101; H05H
1/4652 20210501; H01J 37/32458 20130101; H01J 37/321 20130101; H01J
37/32807 20130101 |
Class at
Publication: |
156/345.48 ;
216/068; 427/255.28 |
International
Class: |
C23F 001/00 |
Claims
What is claimed is:
1. A plasma source apparatus, comprising: a) a substantially
conductive body comprising one or more conductive segments
interrupted by at least one dielectric break; b) a current carrier
adjacent to the substantially conductive body; and c) a power
supply that furnishes alternating current power to the current
carrier, the current carrier inducing eddy currents within the one
or more conductive segments, the eddy currents coupling power into
a plasma adjacent to the substantially conductive body.
2. The apparatus of claim 1 wherein the substantially conductive
body forms at least a portion of a plasma chamber that
substantially confines the plasma.
3. The apparatus of claim 2 wherein the plasma chamber is
substantially cylindrical and the at least one dielectric break
comprises one or more grooves that separate at least a portion of
the chamber into the conductive segments.
4. The apparatus of claim 3 wherein the conductive segments are
longitudinally aligned, and wherein the current carrier is an
induction coil disposed coaxially about the plasma chamber.
5. The apparatus of claim 3 wherein the one or more grooves are
covered by gastight dielectric seals.
6. The apparatus of claim 2 wherein the plasma chamber is a
substantially cylindrical body formed by longitudinal alignment of
the conductive segments, and wherein the current carrier is an
induction coil disposed coaxially about the plasma chamber.
7. The apparatus of claim 6 wherein the at least one dielectric
break comprises an insulating layer disposed upon mating surfaces
of the conductive segments.
8. The apparatus of claim 7 wherein the insulating layer results
from an anodization treatment of one or more of the mating
surfaces.
9. The apparatus of claim 7 wherein the insulating layer comprises
a dielectric adhesive.
10. The apparatus of claim 1 wherein one or more cooling channels
is disposed within at least one of the conductive segments.
11. The apparatus of claim 1 wherein the current carrier is
disposed within a hollow region of the substantially conductive
body.
12. The apparatus of claim 1, further comprising at least one inlet
for a gas to enter the plasma chamber.
13. The apparatus of claim 1 wherein the dielectric break
interrupts a wall of a cavity in at least one of the one or more
conductive segments.
14. A plasma processing system, comprising: a) a substantially
conductive body comprising one or more conductive segments
interrupted by at least one dielectric break; b) a current carrier
adjacent to the substantially conductive body; and c) a power
supply that furnishes alternating current power to the current
carrier, the current carrier inducing eddy currents within the one
or more conductive segments, the eddy currents coupling power into
a plasma adjacent to the substantially conductive body.
15. The system of claim 14, further comprising a plasma chamber
that substantially contains the plasma.
16. The system of claim 15 wherein the substantially conductive
body forms at least one portion of the plasma chamber.
17. The system of claim 16 wherein the plasma chamber is a
substantially cylindrical body formed by longitudinal alignment of
the conductive segments, and wherein the current carrier is an
induction coil disposed coaxially about the plasma chamber.
18. The system of claim 16 wherein the substantially conductive
body is a planar disk formed by radial dispersal of the conductive
segments, and wherein the current carrier is a helical induction
coil disposed adjacent to the planar disk.
19. The system of claim 16 wherein the substantially conductive
body is a conformal dome formed by radial dispersal of the
conductive segments, and wherein the current carrier is a helical
induction coil disposed about the conformal dome.
20. The system of claim 15, further comprising a first gas inlet
for injection of a processing gas into the plasma chamber.
21. The system of claim 20, further comprising a second gas inlet
for injection of a precursor gas into the plasma.
22. A method of plasma processing, comprising: a) providing a
substantially conductive body comprised of one or more conductive
segments interrupted by at least one dielectric break; b) inducing
eddy currents in the conductive segments by furnishing alternating
current power to a current carrier disposed adjacent to the
substantially conductive body; and c) coupling power into a plasma
adjacent to the substantially conductive body using the induced
eddy currents.
23. The method of claim 22 wherein the plasma is substantially
contained within a plasma chamber.
24. The method of claim 23 wherein the substantially conductive
body forms at least one portion of the plasma chamber.
25. The method of claim 22, further comprising the step of
dissociating a feed gas in the plasma.
26. The method of claim 22, further comprising the step of abating
a feed gas in the plasma.
27. The method of claim 22, further comprising the step of etching
material from a workpiece using the plasma.
28. The method of claim 22, further comprising the step of
depositing material from the plasma upon a workpiece.
29. The method of claim 23, further comprising the steps of
injecting a feed gas into the plasma chamber to form an activated
gas; injecting a precursor gas into the plasma, the precursor gas
reacting with the activated gas to form a vapor deposition
compound; and depositing the vapor deposition compound upon a
workpiece.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to plasma processing
sources, and more particularly to apparatus and methods for
inductively coupled plasma processing.
[0003] 2. Brief Description of the Prior Art
[0004] Inductively coupled plasma sources in a variety of
configurations are employed in a broad range of industrial
applications. Inductively coupled plasma processing chambers are
used abundantly for modifying the surface properties of materials,
as for example in the manufacture of modern integrated circuits.
Inductively coupled plasma sources may also operate as remote
sources of activated gas species for downstream processing
operations, or as abatement devices for treatment of toxic or
environmentally harmful materials.
[0005] In one form of well known inductively coupled plasma source,
radio frequency (RF) power is coupled from inductive coils into a
plasma contained within a dielectric enclosure. For example, the
source may comprise a cylindrical dielectric discharge tube wrapped
by an inductive source coil. When energized by an RF power
generator, the source operates like an air core transformer with
the inductive coil as the primary circuit and the plasma within the
tube as the secondary circuit. Alternatively, induction coils may
be disposed in a planar or conformal helix configuration adjacent
to a dielectric discharge chamber for coupling of RF power into a
plasma contained within the chamber.
[0006] The use of dielectric chamber materials to separate
induction coils from the plasma discharge body can significantly
limit the scale and operational range of an inductively coupled
plasma source. Structural dielectric materials, such as quartz or
sapphire, typically suffer from mechanical and thermal constraints
when used in high power density and chemically reactive
applications. The need to extract and dissipate thermal energy
transferred from the plasma to the chamber walls is also more
challenging when the chamber is constructed of dielectric
materials. Cooling mechanisms such as forced air or circulating
fluids are not only complicated and expensive to implement, but
also typically result in reduced coupling efficiency of power to
the plasma. Moreover, electrostatic coupling between the induction
coils and the plasma can result in localized ion bombardment of the
chamber walls, which not only exacerbates the problem of chamber
heat extraction but may over time impair the structural integrity
of the chamber itself. Faraday shielding can be employed to
decrease the capacitive coupling between the source coils and the
plasma, thereby reducing ion sputtering of the chamber walls. A
Faraday shield or cage employed for this purpose is typically
designed so as to suppress or minimize eddy currents within the
shield.
[0007] In another form of inductively coupled plasma source, RF
power is coupled from inductive coils through a high permeability
core material to a ring discharge plasma. In this configuration,
the source operates as a magnetic core transformer with the ring
plasma acting as a single-turn secondary circuit. The ring plasma
discharge may be confined within a chamber of closed-path topology,
such as a torus, as described for example in U.S. Pat. Nos.
3,500,118 and 4,431,898. The discharge chamber may be comprised of
a dielectric material to ensure that currents are coupled into the
plasma rather than within the body of the chamber itself. The
chamber may, however, be comprised substantially of a conductive
material provided that at least one insulating gap or break is
provided along the major circumference of the torus to prevent the
chamber itself from acting as a short-circuited turn, as described
for example in U.S. Pat. No. 3,109,801. By permitting use of a
nearly all-metal chamber, which may be fluid cooled, issues of
thermal management are simplified. As a result, magnetic core
inductively coupled plasma sources are useful for generating
charged particles and chemically active species at relatively high
densities and power levels. A topologically toroidal plasma source
is a complex apparatus, however, that does not lend itself to
simple design and manufacturing for commercial applications.
Moreover, the performance a toroidal source is limited by the
quality, expense, and ability to cool the high permeability ferrite
materials that must typically be employed for operation with RF
power sources in medium to high frequency ranges.
[0008] It would be desirable to construct an inductively coupled
plasma source having a relatively simple configuration, such as a
discharge tube, but without the attendant disadvantages of a plasma
tube or chamber constructed substantially of dielectric materials.
It would be further desirable if the plasma source were not
dependent for its operation upon expensive ferrite transformer
materials.
SUMMARY OF THE INVENTION
[0009] This invention provides methods and apparatus for creating
an inductively coupled plasma using induced eddy currents. The
invention generally comprises a body constructed substantially of a
conductive material interrupted by at least one dielectric break.
Alternating current power is inductively coupled from a current
carrier, such as an induction coil, into the conductive body. The
dielectric gap or gaps in the conductive body are disposed so as to
cause eddy currents to circulate about portions of the conductive
body and thereby couple RF power into an adjacent plasma.
[0010] In one embodiment of the invention, a plasma chamber
comprises conductive segments aligned longitudinally to form a
hollow tube, and separated by dielectric breaks or gaps. An
induction coil is disposed coaxially about the outer perimeter of
the chamber formed of the conductive segments. A power supply
provides alternating current to the induction coil, which creates
alternating magnetic fields in the space occupied by the chamber.
Because of the dielectric separation between the conductive chamber
segments, the alternating magnetic fields induce eddy currents that
circulate radially along the surfaces of the individual segments,
which are thick relative to the surface current skin depth. Net
alternating currents are thereby induced along the interior
conductive surfaces of the discharge tube. These net currents in
turn couple power into a plasma contained within the hollow
interior portion of the chamber.
[0011] By utilizing induced eddy currents to couple power into a
plasma, the invention allows for substantial bodies of conductive
materials to be interposed between the induction coils that receive
power from a power generator and the plasma. Thus, in one
embodiment of the invention, an inductively coupled plasma source
may be constructed in the form of a simple linear or solenoidal
discharge tube, but wherein the tube is composed almost entirely of
a conductive material such as a metal. The use of a nearly
all-metal plasma chamber can have many advantages, including
simplified manufacturability and thermal management. A plasma
chamber that is substantially conductive also largely avoids the
problem of ion bombardment of the chamber walls by reducing or
eliminating capacitive coupling between the induction coils and the
plasma. As a result, an inductively coupled plasma source of the
invention has enhanced performance and durability compared to
sources that rely substantially upon structural dielectric
materials for confinement of the plasma.
[0012] In one embodiment of the invention, conductive chamber
segments are separated by air gaps. Depending upon the application,
a dielectric window material may also be provided between chamber
segments in order to maintain vacuum integrity or to confine the
plasma. In another embodiment, the conductive chamber segments are
constructed so that adjoining surfaces of the segments mate flush
with each other. An insulating coating or treatment, such as an
anodization layer, is applied to the adjoining surfaces.
Alternatively, a dielectric adhesive or filler is disposed between
the adjoining surfaces of the conductive segments. When the
conductive segments are assembled, the resulting discharge chamber
is a nearly seamless and unitary metallic article that has embedded
within it the dielectric breaks needed for formation of induced
eddy currents within the chamber body.
[0013] In forming a plasma chamber of conductive segments, the
dielectric breaks between segments may extend along the entire
length of the chamber. The chamber may also be formed by joining
the segments at their longitudinal ends using caps or rings of
dielectric material. Alternatively, the conductive segments may be
joined at their longitudinal ends with a conductive material.
Although this provides a leakage current path that reduces the
power coupled from the induction coils into the plasma, power loss
may be minimized by making the path of the leakage current
substantially longer than that of the eddy currents.
[0014] Conductive chamber segments may be configured to form a
plasma chamber having any cross-sectional shape, including circular
or rectangular. Conductive segments may also be disposed in other
configurations in accordance with the present invention so as to
couple RF energy into a plasma by means of eddy currents induced
within the segments. In one embodiment, a planar fixture comprised
of radially disposed conductive segments separated by dielectric
gaps is provided between a helical induction coil and a plasma. In
another embodiment, radially disposed conductive segments form a
conformal dome between an induction coil and a plasma. In
appropriate configurations, plasma chambers of the invention are
suitable for use in numerous plasma processing applications
including inline abatement, dissociation, or processing of working
gases; remote production of activated gases for downstream
processing; plasma modification of surface properties of a
workpiece; glass cleaning, etching, or coating; physical or
chemical vapor deposition of materials upon a process substrate;
etching, coating, stripping or ashing of a substrate surface, as in
production of integrated circuit wafers or memory disks; and the
like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 illustrates an inductively coupled plasma source in
accordance with one embodiment of the invention.
[0016] FIG. 2 is an orthographic view of the plasma discharge tube
of the embodiment depicted in FIG. 1.
[0017] FIG. 3 is a cross-sectional view of the plasma discharge
tube of the embodiment depicted in FIG. 1.
[0018] FIGS. 4a, 4b, and 4c illustrate the plasma discharge tube of
a further embodiment of the invention.
[0019] FIG. 5 illustrates an inductively coupled plasma source
adapted for use in a chemical vapor deposition (CVD) application in
accordance with a further embodiment of the invention.
[0020] FIGS. 6a and 6b illustrate inductively coupled plasma
chambers in accordance with further embodiments of the
invention.
[0021] FIG. 7 illustrates an inductively coupled plasma source
having an external plasma discharge in accordance with another
embodiment of the invention.
[0022] FIG. 8 illustrates an alternative embodiment of the
invention having an external plasma discharge.
DETAILED DESCRIPTION
[0023] FIG. 1 illustrates an inductively coupled plasma source 10
in accordance with one embodiment of the invention. An RF power
source 12 furnishes alternating current to induction coils 14
disposed coaxially about a substantially metallic plasma discharge
tube 16 containing a plasma within. As illustrated in the
embodiment of FIG. 1, plasma discharge tube 16 is configured as a
hollow cylinder open at both ends 18 to allow for gas inlet and
exhaust, as for example in an inline gas processing application.
Alternatively, the plasma tube may be configured as a sealed vacuum
chamber having metered inlet and exhaust ports for feed and
processing gases. Although not shown, the apparatus may also
comprise impedance matching elements or circuitry disposed between
RF power source 12 and induction coils 14, as well as measurement
and feedback circuitry to regulate operation of the device. Also
not shown are other features that may typically be included in a
plasma processing system such as vacuum pumping manifolds, gas
delivery connections or manifolds, fluid cooling apparatus, plasma
ignition electrodes or other devices, and mechanisms for workpiece
mounting, transfer, or electrical biasing.
[0024] FIGS. 2 and 3 represent orthographic and cross-sectional
views, respectively, of the plasma discharge tube 16 of FIG. 1. In
this embodiment, plasma discharge tube 16 is formed of a metal
cylinder having longitudinal grooves 22 through the body of the
cylinder. A gastight dielectric seal comprising gas seal 24 and
dielectric cover 26 is disposed across each groove 22 in order to
preserve the gas confinement integrity of the discharge tube 16.
The longitudinal grooves 22 thus divide the walls of plasma
discharge tube 16 into longitudinally aligned conductive segments
28 interrupted by dielectric breaks.
[0025] Alternating current 32 applied to induction coils 14 causes
time-varying magnetic fields to develop in the space occupied by
the chamber 16. Conductive chamber segments 28 are of a thickness
that is greater than the skin depth as determined by the material
properties of the segments 28 and the operating frequency of the RF
power source 12. Eddy currents 34 thus develop that circulate
radially along the surfaces of each conductive chamber segment 28.
As a result, a virtual current loop 36 is established along the
interior conductive surfaces of the chamber 16. The virtual current
loop 36 further creates time-varying magnetic fields in the
interior plasma containment portion of chamber 16, inducing
currents within and thereby coupling power into the plasma 50.
[0026] Only one dielectric gap 22 need be provided in order to
create the eddy currents within the conductive chamber body needed
to couple power into the plasma within. In principle, the chamber
may be comprised of any number of conductive segments 28 separated
by dielectric gaps, provided that the resulting segments are of
sufficiently substantial dimension to carry the required eddy
currents and create the virtual current loop 36. The conductive
segments 28 may be comprised of a common structural metal such as
aluminum or stainless steel, or any other conductive material
suitable to the thermal and chemical environments of a particular
plasma processing application. Preferably, each conductive segment
28 is also sufficiently substantial to have embedded within it one
or more cooling channels 40 through which cooling fluids may
circulate, while retaining such structural properties as may be
required of the segment. Fittings 42 may be provided for connection
of the cooling channels 40 to a source of chilled water or other
cooling fluid (not shown) for thermal management of the plasma
source apparatus.
[0027] Dielectric gaps 22 need only be of sufficient width and
dielectric strength to resist the peak-to-peak breakdown voltages
that develop across conductive segments 28 upon application of RF
power to the induction coils 14. In the embodiment of FIG. 2, the
dielectric gaps 22 do not extend the entire length of the discharge
tube 16. As a result, a leakage current path exists that reduces
the power coupled from the induction coils into the plasma. This
power loss may be minimized to an acceptable level by making the
discharge tube 16 substantially greater in overall length than the
region occupied by induction coils 14, thus making the path of the
leakage current substantially longer than that of the eddy currents
that couple power into the plasma. Alternatively, the leakage
current may be reduced or eliminated by forming one or more of the
dielectric gaps of a structural insulating material that extends
the length of the chamber, or by joining conductive segments at
their longitudinal ends using caps or rings of a structural
dielectric material.
[0028] By transferring the RF power furnished to induction coils 14
into a virtual current loop within the plasma discharge tube, the
electromagnetic fields applied to the plasma are concentrated and
coupling of power to the plasma is improved. Due also to the
enhanced durability and thermal properties of a nearly all-metal
plasma chamber, significantly greater power densities can be
realized with a plasma source of the invention as compared with a
conventional discharge tube apparatus of similar scale.
[0029] FIGS. 4a, 4b, and 4c illustrate a plasma discharge tube in
accordance with another embodiment of the invention. Conductive
discharge tube segments 128 comprise mating surfaces 122 treated
with an electrically insulating layer 124. The insulating layers
124 may be provided by anodization or similar treatment of the
conductive surface, or by application of a dielectric coating
material such as an epoxy adhesive. As shown in FIGS. 4b and 4c,
conductive segments 128 assemble to form a hollow cylindrical
discharge tube 120 having embedded longitudinal dielectric
interruptions 126 and cooling channels 140. Mating surfaces 122 may
be made optically flat so that additional gas sealing between
segments 128 is not required. Alternatively, gas sealing may be
accomplished through use of a dielectric filler or adhesive between
segments, such a high temperature epoxy resin or refractory ceramic
paste.
[0030] When alternating current 132 is applied to induction coils
114, induced eddy currents 134 develop within conductive chamber
segments 128 and create virtual current loop 136. The virtual
current loop 136 induces currents within a plasma 150 contained
within the hollow portion of discharge chamber 120.
[0031] FIG. 5 illustrates an embodiment of the invention adapted
for use in a chemical vapor deposition (CVD) application. Plasma
chamber 516 is a conductive hollow body having one or more feed gas
inlets 530 at one end 518 of the body and a substantially open
discharge region at opposing end 520. Also provided near the
discharge end of plasma chamber are ports 532 for one or more
precursor gases 534 to be injected into the process zone. The
cross-sectional aspect ratio of plasma chamber 516 is optimized for
dispersal of CVD reaction precursors in the vicinity of a
translating workpiece 536.
[0032] A plurality of longitudinal grooves 522 is provided through
the conductive body of plasma chamber 516, creating a series of
longitudinally aligned conductive segments 528 separated by
dielectric breaks. If needed, dielectric covers and gas seals may
be provided across the grooves 522. Disposed about the chamber body
are induction coils 514 oriented transversely to the conductive
segments 528. When energized by RF current, the induction coils
induce eddy currents in the conductive segments, which in turn
couple RF power into a plasma 550 contained within the hollow
plasma chamber 516. As an example, the plasma source of this
embodiment may be used to generate a plasma from an oxygen feed gas
injected at first gas inlets 530. A silane or other silicon-bearing
precursor is injected into the plasma 550 at second inlets 532
where it dissociates and reacts to form a Si.sub.XO.sub.Y compound,
such as SiO.sub.2, which is deposited as a solid film upon the
translating substrate 536.
[0033] In accordance with alternative embodiments of the invention,
an inductively coupled plasma is generated by inducing eddy
currents in conductive bodies that form only a portion of a plasma
confinement chamber, or that are ancillary to the chamber. In FIG.
6a, plasma processing chamber 602 is an enclosed cylinder
containing a workpiece (not shown). Disposed atop processing
chamber 602 is a conductive disk 604 having a plurality of radial
grooves 606, creating an array of radially disposed conductive
segments 608. Adjacent to conductive disk 604 are helical induction
coils 610. When energized by RF current, the induction coils 610
induce eddy currents in the conductive segments 608, which in turn
couple RF power into a plasma contained within processing chamber
602 and that acts upon the workpiece. The same principle is
illustrated in the embodiment of FIG. 6b, wherein radially disposed
conductive segments form a conformal dome between a helical
induction coil and a plasma.
[0034] FIG. 7 illustrates an embodiment of the invention that
generates an external inductively coupled plasma. A substantially
conductive body is a hollow cylindrical tube that comprises
longitudinally aligned conductive segments 728 interrupted by
dielectric breaks 722. Disposed within the conductive body are
induction coils 714 wound transversely to the conductive segments
728. A flux concentrating magnetic material (not shown) such as a
ferrite core may be disposed within induction coils 714 to enhance
magnetic fields generated by the coils. When energized by RF
current, induction coils 714 induce eddy currents 734 in the
conductive segments and create virtual current loop 736 external to
the cylindrical tube. The virtual current loop 736 induces currents
within a coaxial plasma 750 external to the cylindrical tube.
Plasma 750 may be provided as an exposed external discharge, or
alternatively may be confined within an outer coaxial enclosure
(not shown). If a confined plasma is to be subatmospheric, gastight
dielectric windows 724 may also be added to seal dielectric breaks
722.
[0035] An alternative embodiment of the invention that generates an
external inductively coupled plasma is illustrated in FIG. 8.
Conductive body 820 is disposed adjacent to a current carrier 814.
In cross section, conductive body 820 is formed so as to have a
conductive portion 828 that surrounds a hollow cavity with a wall
that is interrupted by a dielectric air gap 822. When current
carrier 814 is energized by RF current, eddy currents 834 are
induced in conductive portion 828 and create virtual current loop
836. The virtual current loop 836 induces currents within a plasma
850 in the hollow interior cavity of conductive body 820. Due to
the position of air gap 822, however, the plasma 850 is not
confined within conductive body 820 but may appear as an external
discharge. In the embodiment of FIG. 8, conductive body 820 is
disposed as a body of revolution about current carrier 814,
resulting in coaxial ring plasma discharge 850.
[0036] Although there is illustrated and described herein specific
structure and details of operation, it is to be understood that
these descriptions are exemplary and that alternative embodiments
and equivalents may be readily made by those skilled in the art
without departing from the spirit and the scope of this invention.
Accordingly, the invention is intended to embrace all such
alternatives and equivalents that fall within the spirit and scope
of the appended claims.
* * * * *