U.S. patent application number 11/583033 was filed with the patent office on 2007-08-09 for plasma source assembly and method of manufacture.
This patent application is currently assigned to TOKYO ELECTRON LIMITED. Invention is credited to Steven T. Fink.
Application Number | 20070181064 11/583033 |
Document ID | / |
Family ID | 32179606 |
Filed Date | 2007-08-09 |
United States Patent
Application |
20070181064 |
Kind Code |
A1 |
Fink; Steven T. |
August 9, 2007 |
Plasma source assembly and method of manufacture
Abstract
A plasma source assembly including an outer shield, a dielectric
chamber wall, and a helical coil provided between the outer shield
and the dielectric chamber wall. The plasma source assembly also
includes a coil support assembly configured to facilitate
repeatable performance of the helical coil. Preferably, the
assembly includes a plenum cooling plate that is configured to
supply cooling fluid to a first cooling rod provided within a
resonator cavity defined by the chamber wall and the outer shield,
and receive cooling fluid from a second cooling rod provided within
the resonator cavity. The assembly preferably also includes a
spacer provided between the first cooling rod and the second
cooling rod, and coil insulators having holes configured to receive
the helical coil.
Inventors: |
Fink; Steven T.; (Mesa,
AZ) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOKYO ELECTRON LIMITED
Minato-ku
JP
|
Family ID: |
32179606 |
Appl. No.: |
11/583033 |
Filed: |
October 19, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10601590 |
Jun 24, 2003 |
7163603 |
|
|
11583033 |
Oct 19, 2006 |
|
|
|
60390361 |
Jun 24, 2002 |
|
|
|
Current U.S.
Class: |
118/723I ;
156/345.48 |
Current CPC
Class: |
H01J 37/321
20130101 |
Class at
Publication: |
118/723.00I ;
156/345.48 |
International
Class: |
C23F 1/00 20060101
C23F001/00; C23C 16/00 20060101 C23C016/00 |
Claims
1. A method of manufacturing a plasma processing system, said
method comprising the steps of: providing a process chamber;
providing a chuck assembly within the process chamber; providing a
gas inject assembly opposite the chuck assembly; and mounting the
gas inject assembly to the process chamber using an outer shield,
the outer shield comprising a plurality of plates.
2. The method according to claim 1, wherein the plurality of plates
are stacked and detachably joined to one another, and further
comprising the step of providing at least one sealing member
between adjacent plates of the plurality of plates.
3. The method according to claim 1, further comprising the steps
of: providing a dielectric chamber wall, wherein the dielectric
chamber wall and the plurality of plates define a resonator cavity;
and providing a helical coil within the resonator cavity.
4. The method according to claim 3, further comprising the step of
tuning the helical coil to a predetermined frequency.
5. The method according to claim 3, further comprising the steps
of: attaching a brass plug to the resonator cavity using a high
temperature soldering process; and attaching the brass plug to the
helical coil using a low temperature soldering process.
6. The method according to claim 3, further comprising the step of
providing a plenum cooling plate defining a manifold configured to
supply cooling fluid to the resonator cavity and the gas inject
assembly.
7. The method according to claim 6, further comprising the step of
providing the gas inject assembly between the dielectric chamber
wall and the plenum cooling plate.
8. The method according to claim 6, further comprising the steps
of: providing a first cooling rod within the resonator cavity
radially outside the helical coil; supplying cooling fluid to the
first cooling rod via the plenum cooling plate; providing an outlet
hole on the first cooling rod that is configured to discharge the
cooling fluid in a circumferential direction within the resonator
cavity; providing a second cooling rod within the resonator cavity
radially inside the helical coil; and receiving cooling fluid in
the plenum cooling plate via the second cooling rod.
9. A method of manufacturing a plasma source assembly, said method
comprising the steps of: providing an outer shield; providing a
dielectric chamber wall; and mounting a helical coil between the
outer shield and the dielectric chamber wall using a coil support
means for facilitating repeatable performance of the helical
coil.
10. The method according to claim 9, further comprising the step of
tuning the helical coil to a predetermined frequency.
11. The method according to claim 9, wherein the dielectric chamber
wall and the plurality of plates define a resonator cavity, and
wherein the helical coil is provided within the resonator cavity,
further comprising the step of securing the helical coil within the
resonator cavity.
12. The method according to claim 11, further comprising the step
of supplying cooling fluid to the resonator cavity using a plenum
cooling plate defining a manifold.
13. The method according to claim 12, wherein: the plenum cooling
plate is configured to supply cooling fluid to a first cooling rod
provided within the resonator cavity; the first cooling rod is
provided radially outside the helical coil; and the first cooling
rod has at least one outlet hole configured to discharge the
cooling fluid in a circumferential direction within the resonator
cavity.
14. The method according to claim 13, wherein: the plenum cooling
plate is configured to receive cooling fluid from a second cooling
rod provided within the resonator cavity; the second cooling rod is
provided radially inside the helical coil; and the second cooling
rod has at least one inlet hole configured to receive the cooling
fluid from within the resonator cavity.
15. The method according to claim 14, further comprising the steps
of: providing a spacer between the first cooling rod and the second
cooling rod; and providing coil insulators abutting the spacer and
between the first cooling rod and the second cooling rod, wherein
the coil insulators have holes configured to receive the helical
coil.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 10/601,590, filed Jun. 24, 2003, which is a
non-provisional application claiming priority under 35 USC .sctn.
119(e) of U.S. Application No. 60,390,361, filed on Jun. 24, 2002.
This application is related to U.S. Application Nos. 60/291,337,
filed on May 17, 2001, and 09/774,182, filed on Feb. 5, 2001, now
U.S. Pat. No. 6,491,742, issued Dec. 10, 2002. The entire contents
of each of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to manufacturing of
semiconductor integrated circuits.
[0004] 2. Discussion of the Background
[0005] Manufacturers of semiconductor integrated circuits are faced
with intense competitive pressure to improve their products and
processes used to fabricate the products. The manufacturers have a
large business motivation to lower production costs by improving
product throughput, quality and complexity. Additionally,
manufacturers have a need for repeatability and consistency in the
assembly and functioning of semiconductor fabrication equipment.
Accordingly, semiconductor manufacturers strive to formulate a low
cost way to manufacture high quality process equipment.
[0006] One goal of semiconductor manufacturers is to improve tool
performance at a low cost. Another goal is to make process
equipment function the same regardless of particular hardware
sampled. The company that can enhance tool performance without
increasing tool cost is in a position to increase profit margins.
In cyclical industries such as the semiconductor capital equipment
industry, increased profit margins can have a dramatic impact on
market penetration.
[0007] For many years Inductively Coupled Plasma (ICP) sources have
been used in a variety of applications. Most recently, low pressure
(<100 mTorr) ICP sources have been used in wafer production
where plasmas are required to deliver high densities of ions,
electrons and radicals with high uniformity over wafer diameters of
200 mm and larger. These plasma sources need to deliver ions that
are uniform in density and energy distribution while keeping ion
and electron energy very low.
[0008] The Electrostatically Shielded Radio Frequency (ESRF) plasma
source is a type of ICP source which is particularly useful in
applications where substrate materials are susceptible to damage
from high energy plasma ions or electrons, uncontrolled bias
voltages and thermal fluxes. ESRF sources feature pure inductive
coupling with reduced capacitive coupling. The radio frequency (RF)
power produces only plasma density and induces very little voltage
on the plasma. This inductive coupling is sufficiently devoid of
capacitive coupling so that the plasma does not search for counter
electrodes. The plasma remains mainly within the process
(dielectric) chamber at all powers and pressures.
[0009] The main components of an ESRF ICP processing system are
depicted in the generic FIG. 1. The ESRF ICP processing system 10
includes a process chamber 20 with a wafer and chuck assembly 30
provided therein. A gas inject assembly 40 is provided opposite the
wafer and chuck assembly 30. A plasma region or area 22 is provided
adjacent a dielectric chamber wall 60 in between the wafer and
chuck assembly 30 and the gas inject assembly 40.
[0010] The plasma source is composed of several main elements and
is affixed to an opening of a suitable process chamber 20. A wafer
that is being processed is located on the chuck assembly 30. The
plasma source comprises a resonator chamber or cavity 72 bounded by
an outer shield or housing 50 and the dielectric chamber wall 60,
within which a helical coil 90 is mounted. The outer shield 50 and
the dielectric chamber wall 60 further define a fluid cooling area
70, within which the helical coil 90 is immersed. The dielectric
chamber wall 60 contains the plasma area 22 of the plasma source.
Furthermore, the dielectric chamber wall 60 has appropriate sealing
devices to seal cooling fluid within fluid cooling area 70 and
maintain the process pressure within plasma area 22 at appropriate
levels. Additionally, an electrostatic shield 80 is provided on an
outer surface of the dielectric chamber wall 60 in an interior of
the fluid cooling area 70.
[0011] In the construction of ESRF source assemblies, there are
several elements that are expensive to fabricate for various
reasons. The outer shield or housing can be the most expensive part
in the source. It can be fabricated from several aluminum parts and
subsequently furnace or dip brazed to form a singular assembly. The
interface of these parts must be machined to tolerances required in
the brazing process. Once machined, the parts are then assembled
utilizing an appropriate holding fixture and brazed using the
specified processes. Various machining operations must then be
performed on the resulting brazed assembly before it is ready for
use.
[0012] Another problem seen in ESRF plasma sources is the method
and repeatability of mounting the helical coil. In ESRF plasma
sources, particularly those sources comprising a quarter-wave or
half-wave resonant coil, the coil is tuned to a particular
frequency. In order to tune the helical coil to a particular
frequency, a labor intensive process of adjusting the length of the
coil is involved. Once the coil is tuned, changes in coil position
can adversely affect the tuning.
SUMMARY OF THE INVENTION
[0013] The present invention advantageously provides a plasma
source assembly including an outer shield, a dielectric chamber
wall, and a helical coil. The helical coil is advantageously
mounted within a cavity bounded by the outer shield and the
dielectric chamber wall.
[0014] It is an object of the invention to produce the outer shield
(housing) in a very cost effective manner that requires no special
processes or machining after original fabrication of the parts.
Such a source assembly configuration may allow quick changes and
modifications to the outer shield housing and electrostatic shield
using many original parts, without other special processes or
special tools.
[0015] In the preferred embodiment of the present invention, the
plasma source assembly further includes an electrostatic shield
provided outside the dielectric chamber wall, forming an interior
of the cavity. The plasma source assembly preferably includes a
plenum cooling plate defining a manifold configured to supply
cooling fluid to the cavity and a gas inject assembly attachable to
the outer shield.
[0016] The plasma source assembly preferably includes structure for
stacking and detachably joining a plurality of plates to form the
outer shield, and structure for constructing the gas inject
assembly and the dielectric chamber wall to be removable from the
plasma source assembly without using a tool. The preferred
embodiment of the present invention includes structure for
circulating cooling fluid throughout the cavity and the gas inject
assembly, and structure for removing bubbles from the cooling fluid
within the cavity.
[0017] It is another object of the present invention to provide a
coil support assembly and method that supports, separates, and
holds the helical coil in such manner that the plasma source only
needs to be tuned once. Such a manufacturing method makes helical
coil tuning repeatable even after complete disassembly and
subsequent reassembly of the entire plasma source assembly.
[0018] It is a further object to circulate cooling fluid throughout
the plasma source and the gas inject assembly in a way that
promotes efficient cooling, and also removes and discourages the
forming of any bubbles in the cooling fluid. Air bubbles,
especially bubbles located inside the resonator cavity, degrade the
insulating properties of the dielectric cooling fluid. Maintenance
and cleaning are needed to ensure that acceptable process
conditions are met. One aspect of maintenance and cleaning is the
removal of the dielectric chamber wall for wet cleaning. A goal of
maintenance and cleaning operations is short machine downtime.
Thus, preferably the dielectric chamber wall (process tube) may be
removed and the inject assembly may be removed and/or replaced for
maintenance purposes quickly without using tools.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] A more complete appreciation of the invention and many of
the attendant advantages thereof will become readily apparent with
reference to the following detailed description, particularly when
considered in conjunction with the accompanying drawings, in
which:
[0020] FIG. 1 is a general section view of an electrostatically
shielded radio frequency (ESRF) inductively coupled plasma (ICP)
source;
[0021] FIG. 2 is a section view of an ESRF ICP source according to
the present invention;
[0022] FIG. 3 is a perspective view of an assembly including a
helical coil, cooling rods, and spacers according to the present
invention;
[0023] FIG. 4 is a side view of an assembly including a helical
coil, cooling rods, and spacers according to the present invention;
and
[0024] FIG. 5 is an exploded view of an assembly including a
helical coil, cooling rods, and spacers according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0025] FIG. 2 is a section view of an ESRF ICP source according to
one embodiment of the present invention. The present invention
provides an inexpensive, dielectric fluid cooled, efficient, ESRF
ICP plasma source that can easily be modified, remains tuned to a
particular frequency, and can easily and quickly be cleaned.
[0026] FIG. 2 depicts an ESRF ICP plasma source assembly 110 that
generally includes a process chamber 120 with a wafer and chuck
assembly 130 provided therein. A gas inject assembly 140 is
provided opposite the wafer and chuck assembly 130. A plasma region
or area 122 is provided adjacent a dielectric chamber wall 160 in
between the wafer and chuck assembly 130 and the gas inject
assembly 140. An outer shield or housing 150 is mounted between the
process chamber 120 and the gas inject assembly 140. The outer
shield 150 and the dielectric chamber wall 160 define a resonator
cavity 172 that bounds a fluid cooling area 170. An electrostatic
shield 180 is provided outside the dielectric chamber wall 160
forming an interior wall in the fluid cooling area 170. A coil 190
is provided within the resonator cavity 172 of the fluid cooling
area 170.
[0027] FIG. 2 depicts a plasma source assembly 110 having a
generally conical inner surface. The configuration of the plasma
source of the present invention is not restricted to a conical
inner shape. For example, the plasma source assembly can be
configured with a cylindrical, spherical, semi-spherical, or other
shape inner surface.
[0028] In the embodiment depicted in FIG. 2, the outer shield or
source housing 150 is comprised of three separate manufactured
plates 152, 154, 156. The plates of the housing 150 are made from
aluminum plate stock, although other metallic materials or alloy
materials can alternatively be used. The plates of the housing 150
can be formed of various thickness, depending upon source size and
process requirements.
[0029] The plates 152, 154, 156 are machined in a manner that, when
completed, they can be stacked and detachably joined or fastened
together with common hardware 155 as shown. O-ring seals 158 are
inserted during assembly to prevent leakage of dielectric cooling
fluid. Grounding devices 159 are also inserted between respective
plates during assembly. The grounding devices 159 ensure RF
grounding requirements are met. The plates 152, 154, 156 are also
constructed in such a manner that cooling rod-mounting features are
provided where necessary. The cooling rod-mounting features can
simply be through holes and/or blind counter bores as needed.
[0030] The embodiment depicted in FIG. 2 has three plates 152, 154,
156 that form the outer shield or source housing 150. The present
invention can be constructed having an outer shield formed with one
or two plates, or with four or more plates depending upon
individual plasma source configuration requirements. However, the
plates of the outer shield of the present invention do not require
brazing or post machining of parts in any of these configurations.
In an alternative embodiment, the outer shield or source housing
150 is formed from a rolled-ring forging as described in pending
U.S. Patent App. Ser. No. 60/291,337 (filed on May 17, 2001).
[0031] The electrostatic shield 180 is attached to the inner
diameters of the upper and lower plates of the source housing 150,
thereby forming an interior wall of the resonator cavity 172. The
electrostatic shield 180 has a number of slots positioned in a
predetermined arrangement. The electrostatic shield 180 is attached
to the cavity 172 using common hardware. Grounding features can be
utilized with the electrostatic shield 180 if desired. The
electrostatic shield 180 is preferably made from aluminum alloy
sheet stock, however alternative materials may be used and/or the
electrostatic shield 180 can be plated with other metallic
materials. The use of an electrostatic shield reduces the
capacitive coupling, thereby reducing the plasma potential and,
hence, permitting independent control of the ion density and the
ion energy. The ion density and the ion energy can be independently
controlled by adjusting the power to the coil and the power to the
substrate bias, respectively. Some capacitive coupling is desired
in order to improve the plasma starting characteristics.
[0032] The plasma source assembly 110 depicted in FIG. 2 includes a
plenum cooling plate 200, which is detachably mounted to an upper
surface of the outer shield 150 whereby the gas inject assembly 140
and the electrostatic shield 180 are secured when the outer shield
150 is attached to the process chamber 120. The plenum cooling
plate 200 functions as a manifold that circulates cooling fluid,
which is preferably a dielectric fluid, to cool the source
resonator cavity 172 and the gas inject assembly 140. The plenum
cooling plate includes appropriate seals and grounding features. A
viewing window 202 is located between the gas inject assembly 140
and plenum cooling plate 200. The window 202 has appropriate vacuum
and fluid seals. A window may or may not be employed. The plenum
cooling plate 200 is preferably made from aluminum alloy plate
stock, however other alternative materials can be used.
[0033] The plenum cooling plate 200 supplies cooling fluid to one
or more supply cooling rods 210 located radially outside the
helical coil 190 in the resonator cavity 172, as depicted in FIGS.
2 and 3. The plenum cooling plate 200 has a supply inlet 204 that
receives cool dielectric cooling fluid and transfers the fluid via
a supply chamber 205 to the various supply cooling rods 210
distributed about the resonator cavity 172. The supply cooling rods
210 have holes 212 in sidewalls located so fluid is forced in a
circumferential direction inside the resonator cavity 172, as
generally depicted using dashed lines in FIG. 3.
[0034] The plenum cooling plate 200 receives cooling fluid from one
or more return cooling rods 220 located radially inside the helical
coil 190 in the resonator cavity 172, as depicted in FIGS. 2 and 3.
Cooling fluid is returned through the bottom of each tube 220
(depicted with a section removed in FIG. 3), each exiting to a
return chamber 207. Chamber 207 is connected to a return outlet 206
of the plasma cooling plate 200.
[0035] Cooling fluid also returns to the return chamber 207 through
several holes or return openings 209 in an uppermost part of the
resonator cavity 172. Air bubbles naturally rise to the highest
portions of the resonator cavity 172 as they are circulated by the
dielectric cooling fluid. As the bubbles reach the uppermost part
of the resonator cavity 172, the bubbles proceed through holes 209
connecting the resonator cavity 172 with the return chamber 207 in
the plenum cooling plate 200. Cooling fluid containing the bubbles
is then channeled to the gas inject assembly 140 via circulation
chambers 208 prior to exiting the plasma source assembly 110 via
the return outlet 206 and returning to a remote fluid cooling
assembly, where the bubbles are collected and removed. Higher power
settings are possible for plasma generated when air bubbles are
removed from the resonator cavity 172, thereby resulting in faster
etching times.
[0036] The cooling rods 210, 220 are arranged as depicted in FIGS.
2-5 to lock coil insulators 240 and insulating spacers 230 securely
in place, and hold the helical coil 190 in a predetermined position
based on its frequency-based tuning. The helical coil 190 extends
through the coil insulators 240, which maintain proper spacing of
the helical coil 190. The coil insulators 240 are stacked between
insulating spacers 230. The insulating spacers 230 maintain proper
spacing between the cooling rods 210, 220, and maintain the
location of the coil insulators 240. The cooling rods 210, 220,
coil insulators 240, and coil spacers 230 are preferably made of
dielectric material such as Teflon, Rexolite, or other similar
dielectric or ceramic material.
[0037] The gas inject assembly 140 is retained between the plenum
cooling plate 200 and the dielectric chamber wall 160. Fasteners
142 near a center of the gas inject assembly 140 retain the
assembly 140 to the plenum cooling plate 200. Other fasteners 142,
located on the outer periphery of the plenum cooling plate 200,
attach the plenum cooling plate 200 to the uppermost plate 156 of
the cavity. The fasteners 142 are preferably removable by hand,
thereby requiring no tools to retain the gas inject assembly 140.
Process gas is supplied to a gas plenum area 144, and from the gas
plenum area 144 the gas is manifolded to a multitude of gas inject
holes 146 located on a lower surface of the gas inject assembly
140. The gas inject assembly 140 is preferably made from aluminum
alloy plate stock, and can be subsequently processed using, for
example, an anodization process. Alternatively, the gas inject
assembly 140 can be formed using other materials and surface
treatments.
[0038] FIGS. 2, 3, 4 and 5 depict a helical coil 190. FIGS. 3
through 5 depict coil insulator geometry and locking features of
insulators, spacers and cooling rods. The coil insulators 240 of
the spacer 230 interlock with the cooling rods 210, 220 and space
the turns of the helical coil 190 at a correct distance from each
other as required by a particular process and design considerations
in order to achieve a desired resonance frequency. An upper end of
the coil 190 is affixed to the cavity 172. The method used to
attach the upper end of the coil 190 to the resonator cavity 172
can be mechanical, soldered, or welded, dependant on materials used
and functional requirements present in the design. In a preferred
method, the helical coil 190 is attached to a brass plug using a
low temperature soldering method. Prior to the low temperature
soldering step, the brass plug is soldered to the resonator cavity
172 using a high temperature soldering method. The soldering
process described above provides advantageous grounding of the
helical coil 190 to the resonator cavity 172. At a lower end of
coil 190 opposite the upper end, the helical coil is electrically
open. At the lower end, the helical coil 190 is terminated in a
rounded tip, which includes a port through to an inside of the
helical coil 190. Cooling fluid is forced through the inside of the
helical coil 190 from the supply inlet via a supply chamber 205 of
the plenum cooling plate 200. Fluid exits the coil 190 through the
tip and mixes with cooling fluid already circulating in the
resonator cavity 172. A tap 196 intersects the helical coil 190 at
a location along the coil 190. The tap 196 is a connection to an
external Fast Match Assembly (FMA). The Fast Match Assembly (not
shown) comprises an impedance match network for matching the output
impedance of a RF generator (not shown) to the input impedance of
the plasma source and plasma. The FMA incorporates automatic
control hardware and software for adjusting the impedance match
according to changes in the load (plasma) impedance. Impedance
match network design and the control thereof for plasma processing
operations are well known to those skilled in the art of plasma
source design and RF (radio frequency) electronics. Appropriate
insulators and seals are positioned as required to facilitate the
connection of the tap to the FMA at interface 241. The helical coil
190 is preferably made from copper tubing, however alternative
materials can be utilized. The helical coil 190 may or may not be
plated with some other metallic material. For example, as described
above, helical coil 190, configured to have a grounded end, an open
end opposite the grounded end and a tap location between the open
end and the grounded end, can be designed as a quarter wave or half
wave resonator. Helical resonators are well known to those of skill
in the art of plasma source design. In an alternate embodiment,
coil 190 comprises a tap location at a first end of coil 190 and a
grounded end at a second end of coil 190.
[0039] The dielectric chamber wall or process tube 160 is installed
in the assembly as depicted in FIG. 2. Appropriate seals 162, 166,
173, 174 and load bearing spacers 164, 168, 171 are utilized to
secure the process tube 160 in a correct position. The seals 162,
166, 173, 174 and load bearing spacers 164, 168, 171 can be
configured as described in U.S. Application Ser. No. 60/256,330,
which is incorporated herein by reference, or can be positioned as
separate parts as depicted in FIG. 2. The outer rim of the process
tube 160 has at least one dielectric pin 169 installed on the outer
surface, protruding through the outer shield or housing 150. The
dielectric pin(s) 169 retain the process tube 160 in position as
the plasma source is rotated on hinges from the process chamber 120
opening during maintenance events. The dielectric pin(s) 169 are
made from Teflon or other dielectric material and are removed, by
hand, to facilitate removal of the dielectric chamber wall 160 when
maintenance is necessary. The absence of mechanical fasteners or
other parts other than dielectric pins allows for very fast removal
and replacement of the dielectric chamber wall 160 when process
requirements deem a maintenance event necessary.
[0040] It should be noted that the exemplary embodiments depicted
and described herein set forth the preferred embodiments of the
present invention, and are not meant to limit the scope of the
claims hereto in any way.
[0041] Numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that, within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *