U.S. patent application number 10/648742 was filed with the patent office on 2004-04-08 for composite noble metal wire.
Invention is credited to Seuntjens, Jeffrey Michael.
Application Number | 20040065468 10/648742 |
Document ID | / |
Family ID | 22551286 |
Filed Date | 2004-04-08 |
United States Patent
Application |
20040065468 |
Kind Code |
A1 |
Seuntjens, Jeffrey Michael |
April 8, 2004 |
Composite noble metal wire
Abstract
A method of forming a micron-dimensioned wire having a gold
alloy annulus surrounding an electrically-conductive non-noble
metal core.
Inventors: |
Seuntjens, Jeffrey Michael;
(Singapore, SG) |
Correspondence
Address: |
Synnestvedt & Lechner LLP
2600 Aramark Tower
1101 Market Street
Philadelphia
PA
19107-2950
US
|
Family ID: |
22551286 |
Appl. No.: |
10/648742 |
Filed: |
August 26, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10648742 |
Aug 26, 2003 |
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09154417 |
Sep 16, 1998 |
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6610930 |
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Current U.S.
Class: |
174/94R |
Current CPC
Class: |
H01L 2224/45147
20130101; H01L 2224/45572 20130101; H01L 2224/4847 20130101; H01L
2924/01004 20130101; H01L 2924/20751 20130101; B21C 37/042
20130101; H01L 2924/01027 20130101; H01L 2924/20757 20130101; H01L
2924/01028 20130101; H01L 2224/45639 20130101; H01L 2924/20755
20130101; H01L 2924/01032 20130101; H01L 2924/20753 20130101; H01L
2924/01029 20130101; H01L 2224/4312 20130101; H01L 2924/01079
20130101; H01L 2224/45644 20130101; H01L 2924/20756 20130101; H01L
2224/456 20130101; H01L 2224/48455 20130101; H01L 2224/45664
20130101; H01L 2924/0102 20130101; H01L 2924/01202 20130101; H01L
24/48 20130101; H01L 2224/45669 20130101; H01L 2924/20752 20130101;
H01L 24/45 20130101; H01L 2224/45144 20130101; H01L 2224/45565
20130101; H01L 2924/00011 20130101; H01L 2224/45 20130101; H01L
2224/43848 20130101; H01L 2924/20754 20130101; H01L 24/43 20130101;
H01L 2924/01078 20130101; H01L 2924/01082 20130101; H01L 2224/48247
20130101; H01L 2924/01005 20130101; H01L 2924/01047 20130101; H01L
2924/01204 20130101; H01L 2224/43 20130101; H01B 1/02 20130101;
H01L 2224/4321 20130101; H01L 2224/45015 20130101; H01L 2224/45644
20130101; H01L 2924/0102 20130101; H01L 2224/45644 20130101; H01L
2924/01004 20130101; H01L 2224/45565 20130101; H01L 2224/45147
20130101; H01L 2224/45644 20130101; H01L 2224/45644 20130101; H01L
2924/01202 20130101; H01L 2224/45015 20130101; H01L 2924/20757
20130101; H01L 2224/45015 20130101; H01L 2924/20756 20130101; H01L
2224/45015 20130101; H01L 2924/20755 20130101; H01L 2224/45015
20130101; H01L 2924/20754 20130101; H01L 2224/45015 20130101; H01L
2924/20753 20130101; H01L 2224/45015 20130101; H01L 2924/20752
20130101; H01L 2224/45015 20130101; H01L 2924/20751 20130101; H01L
2224/45144 20130101; H01L 2924/01004 20130101; H01L 2224/45144
20130101; H01L 2924/0102 20130101; H01L 2224/45144 20130101; H01L
2924/013 20130101; H01L 2924/00 20130101; H01L 2224/43848 20130101;
H01L 2924/00 20130101; H01L 2224/45669 20130101; H01L 2924/00014
20130101; H01L 2224/45664 20130101; H01L 2924/00014 20130101; H01L
2224/45639 20130101; H01L 2924/00014 20130101; H01L 2224/45147
20130101; H01L 2924/00014 20130101; H01L 2224/43848 20130101; H01L
2924/20106 20130101; H01L 2224/43848 20130101; H01L 2924/20107
20130101; H01L 2224/43848 20130101; H01L 2924/20108 20130101; H01L
2224/43848 20130101; H01L 2924/20109 20130101; H01L 2224/43848
20130101; H01L 2924/2011 20130101; H01L 2224/43848 20130101; H01L
2924/20111 20130101; H01L 2224/45644 20130101; H01L 2924/013
20130101; H01L 2924/01004 20130101; H01L 2924/0102 20130101; H01L
2924/00011 20130101; H01L 2924/01006 20130101; H01L 2224/45
20130101; H01L 2924/00012 20130101; H01L 2224/43 20130101; H01L
2924/00012 20130101; H01L 2224/45144 20130101; H01L 2924/00
20130101; H01L 2224/45644 20130101; H01L 2924/01204 20130101; H01L
2224/45644 20130101; H01L 2924/01201 20130101; H01L 2224/45644
20130101; H01L 2924/01049 20130101; H01L 2224/45644 20130101; H01L
2924/01032 20130101; H01L 2224/45644 20130101; H01L 2924/013
20130101; H01L 2924/00013 20130101 |
Class at
Publication: |
174/094.00R |
International
Class: |
H02G 003/06 |
Claims
1. A method of forming a composite wire having a diameter greater
than about 15 microns and less than about 100 microns and
consisting of a gold alloy annulus surrounding a wire core
comprising an electrically-conductive non-noble metal, said method
comprising: assembling a composite billet consisting essentially of
a core of said non-noble metal, an intermediate layer of said gold
alloy and an outer metal layer; extruding said composite billet
with force to form a composite rod comprising corresponding core,
intermediate and outer metal layers, wherein the core fraction
measured by cross-sectional area of the cylinder defined by the
core and intermediate layer of said rod is essentially the same as
the corresponding core fraction of said billet; removing said outer
metal layer of said composite rod; drawing said composite rod to
form a first composite wire having a diameter between about 0.5 and
about 5 mm and a core fraction essentially the same as said core
fraction of said composite rod; and drawing said first composite
wire to form a second composite wire having a diameter between
about 15 and about 100 microns and a core fraction essentially the
same as said core fraction of said first composite wire.
2. The method of claim 1, wherein said core metal comprises
copper.
3. The method of claim 2, wherein said core metal consists
essentially of copper.
4. The method of claim 1, wherein said gold alloy is at least 99%
gold.
5. The method of claim 4, wherein said gold alloy comprises gold
doped with less than 30 ppm calcium, less than 20 ppm beryllium and
less than 50 ppm other elements.
6. The method of claim 5, wherein said gold alloy comprises gold
doped with less than 10 ppm of beryllium and less than 10 ppm of
calcium.
7. The method of claim 1, wherein said core metal and said gold
alloy have melting temperatures within a range of 5.degree. C.
8. The method of claim 1, wherein said billet has a core fraction
between about 25 and about 95% by cross-sectional area.
9. The method of claim 1, wherein said billet is preheated to a
temperature between about 200 and about 700.degree. C. prior to
extrusion.
10. The method of claim 1, wherein said billet is extruded with a
force between about 50 and about 200 kg/mm.sup.2.
11. A composite wire, having a micron-dimensioned diameter, formed
by the method of claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a Divisional of U.S. patent
application Ser. No. 09/154,417 filed Sep. 16, 1998, now U.S. Pat.
No. 6,610,930, the disclosure of which is incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to composite noble metal wires
having an electrically-conductive, non-noble metal-containing core
clad with a noble metal annulus. In particular, the present
invention relates to composite wires formed by co-extrusion of the
non-noble metal-containing core material and the noble metal. The
present invention further relates to methods of forming composite
wires in which a core containing a non-noble metal is clad with a
noble metal annulus by co-extrusion of the non-noble metal core
material with noble metal.
[0003] Advances in semiconductor packaging towards finer pitches,
longer spans, and lower packaging costs are not adequately met by
present gold bonding wire technology. The wire modulus, cost and
strength requirements dictate the use of a more complex wire
material than universally accepted 4N gold alloy.
[0004] Future bonding wire may need to conform to requirements of
approximately 50 microns pitch at an approximately 5000 micron
span, at a wire diameter of 20 microns. Bonding wire sag and sway
are concerns in such a configuration. The sway deflection of one
wire relative to another must be limited to about 30 microns. The
relative strain required to cause a short between adjacent wires is
less than 0.005%, which is an elastic strain. 4N gold alloys have
an increased modulus over pure gold, but gold-based bonding wire
alloys are not expected to have a sufficient modulus for such
future requirements.
[0005] Copper is an ideal bonding wire in terms of modulus,
resistivity, density and cost. However, oxidation concerns and
higher bonding costs have prevented copper from becoming a common
bonding wire material.
[0006] U.S. Pat. No. 5,097,100 discloses a noble metal-plated
copper wire. A drawn copper wire having a diameter of from about 44
to 56 microns is electrolytically plated with gold, the surface of
which may be cold-drawn to harden the gold layer.
[0007] However, it is not possible to uniformly plate a gold layer
of adequate purity at a reasonable cost. The gold may not
adequately adhere to a copper core by following the disclosure of
U.S. Pat. No. 5,097,100. The other metal coating deposition
techniques disclosed by this patent, including electroless plating,
vapor deposition, sputtering, dipping, and the like are problematic
for the same reasons. Furthermore, none of these techniques can
coat a copper wire core with a 4N gold alloy bonding wire
sheath.
[0008] While U.S. Pat. No. 5,097,100 discloses that the copper and
gold may be co-drawn, there is no teaching, let alone a working
example, of how this may be accomplished with micron dimensioned
wires. There remains a need for a composite gold-clad copper wire
that is capable of meeting the anticipated future performance
requirements of the semiconductor industry at a reasonable
cost.
SUMMARY OF THE INVENTION
[0009] This need is met by the present invention. It has now been
discovered that composite wire having a non-noble metal core of
consistent diameter with a noble metal layer of uniform thickness
firmly adhered thereto may be economically produced by forming the
noble metal layer on a non-noble metal core before the wire is
drawn.
[0010] Therefore, according to one aspect of the present invention,
a composite wire is provided consisting essentially of a wire core
containing an electrically-conductive non-noble metal, and a noble
metal annulus metallurgically bonded to the wire core.
[0011] Copper is the preferred non-noble metal, and a wire core
consisting essentially of copper is most preferred. The noble metal
is preferably gold, and more preferably gold having a purity
greater than 90%. The purity is preferably greater than 99% and
even more preferably greater than 99.99%. Preferably, the gold is a
gold alloy in which the gold is doped to obtain sound deformation
of the gold/copper composite as it is drawn, and good bonding
properties for the composite wire, such as, for example, gold doped
with less than 30 ppm of calcium, less than 20 ppm of beryllium,
and less than 50 ppm of other elements. A particularly preferred
alloy is 4N gold.
[0012] The present invention provides a method by which a non-noble
metal wire may first be coated with a noble metal and then drawn to
micron dimensions rather than attempting to form a layer of noble
metal on a micron-dimensioned wire. Therefore, according to another
aspect of the present invention, a method is provided for forming a
micron-dimensioned composite wire consisting essentially of a
conductive wire core containing a non-noble metal and a noble metal
annulus metallurgically bonded thereto, wherein the method
includes:
[0013] providing a first composite wire having a diameter between
about 0.5 and about 5 millimeters, wherein the first composite wire
consists essentially of a core containing a non-noble metal, and a
noble metal annulus metallurgically bonded to the core; and
[0014] drawing the first composite wire to form a second composite
wire having a diameter between about 15 and about 75 microns, so
that the core fraction measured by cross-sectional area of the
second composite wire is essentially the same as the core fraction
of the first composite wire.
[0015] The first composite wire is drawn from a composite rod
produced by co-extrusion of a noble metal billet having a non-noble
metal core material thereby metallurgically bonding the noble metal
and core metal layers. The composite wire having a diameter of 20
microns is drawn from a composite wire having millimeter
dimensions, which in turn is formed from a composite cylindrical
rod formed by extrusion of a composite billet, with the relative
cross-section of the composite core and noble metal layer remaining
unchanged from the billet to the rod to the wire. This permits
direct control of the core fraction of the nominally 20 micron
diameter composite wire to a degree heretofore unknown. Therefore,
according to another aspect of the present invention, a composite
wire is provided, having a micron-dimensioned diameter, prepared by
the method of the present invention.
[0016] In other words, the desired core fraction, for example, for
a 20 micron diameter composite wire, is produced by a composite
billet having the same relative fraction of core material. By
constructing a billet having the fraction of core material desired
for the composite wire end product, a micron dimensional composite
wire is obtained having the desired fraction of core material.
[0017] The composite wires of the present invention possess the
desired modulus, strength and conductivities required for
semiconductor packaging, and at the same time provide a cost
advantage. Therefore, according to another aspect of the present
invention, a semiconductor package is provided having at least one
lead bonded to the second composite wire of the present invention.
Composite wires having diameters as small as 25 microns have been
wedge-bonded without disrupting the continuity of the noble metal
outer layer, which is necessary in order to avoid oxidation of the
non-noble metal core.
[0018] The composite wires of the present invention may be employed
in other end-use applications for fine wire. Such applications
include, but are not limited to, wires or cabled wires for jewelry,
cathodic protection, or for harsh environments. The foregoing and
other objects, features, and advantages of the present invention
are more readily apparent from the detailed description of the
preferred embodiments set forth below, taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a cross-sectional view of a composite wire in
accordance with one embodiment of the invention;
[0020] FIG. 2 is a side, cross-sectional view of a composite billet
in accordance with one embodiment of the invention, which is
extruded to form the composite rod from which the composite wires
of the present invention are drawn;
[0021] FIG. 3 is perspective view of a semiconductor package in
accordance with one embodiment of the invention, depicting a lead
bonded to a composite wire of the invention;
[0022] FIG. 4 is a longitudinal, cross-sectional SEM micrograph of
a composite gold wire in accordance with one embodiment of the
invention, wedge-bonded to a lead of a semiconductor package;
and
[0023] FIG. 5 is a comparison of the elongation vs. break load
properties of a composite gold wire in accordance with one
embodiment of the present invention to AW-14 gold wire (a type of
4N gold alloy).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] A composite wire, in accordance with one embodiment of the
present invention, is depicted in FIG. 1, in which core 12
containing a non-noble metal is metallurgically bonded to noble
metal annulus 14. (Noble metals are defined as metals that do not
oxidize by heating in air, and, in addition to gold, include
platinum, palladium, silver and the like. R represents the radius
of the wire 10 formed by the core 12 and annulus 14, while r
represents the radius of core 12. The amount of the core material
12 relative to annulus 14 and wire 10 is expressed as the "core
fraction" measured by cross-sectional area, and this is a function
of the ratio of r to R (r/R)
[0025] For purposes of the present invention, a metallurgical bond
is defined as a bond formed by the application of heat and
pressure. The amount of heat and pressure to employ depends upon
the non-noble core metal or metal alloy and the noble metal or
metal alloy annulus materials employed, and can be readily
determined by one ordinarily skilled in metallurgy without undue
experimentation. For example, for a copper or copper alloy core and
a gold or gold alloy annulus a temperature greater than about
200.degree. C. and a pressure greater than about 50 kg/mm.sup.2
should be employed.
[0026] The wire is formed by drawing a composite rod formed by
extruding the composite billet 20 of FIG. 2. For example, a
core-metal cylinder 22 containing copper is encased in gold in the
form of a sleeve or wrapped sheets forming intermediate layer 24.
This assembly is placed in a copper extrusion can 26 with end caps
28 and 30, and the resulting billet 20 is welded, evacuated, and
sealed. The billet is preheated to a temperature between about
200.degree. and about 700.degree. C., and more preferably to a
temperature between about 400.degree. and about 500.degree. C., and
extruded by direct extrusion with a force/unit area between about
50 and about 200 kg/mm.sup.2, and more preferably between about 100
and about 150 kg/mm.sup.2, to form an extruded composite
cylindrical rod having a diameter suitable for wire-drawing.
[0027] The extruded rod is cropped, cleaned, and drawn by
conventional single die drawing to form a composite wire having a
diameter between about 0.5 and about 5 millimeters, and preferably
less than about 3 millimeters. The outer layer formed by the
extrusion can is removed, preferably by etching, resulting in a
coil of a gold-clad composite wire having a copper core, which is
further drawn to a diameter less than 100 microns and preferably
between about 15 and about 75, microns by standard bonding wire
process technology. The core fraction by cross-sectional area is
relatively unchanged from the original composite billet, so that
the core fraction of the wire product is controlled by the billet
design.
[0028] Preferred billets are between 25 and 100 mm diameter, which
permits economical extrusion. The relative sizes of the core, noble
metal layer and outer layer scale with the billet size, i.e., the
dimensions are selected to obtain the core fraction desired for the
composite wire to be produced. The extrusion can is about 10 to 20%
of the entire billet cross section. The cylinder within that can
defined by the non-noble metal core and the intermediate noble
metal layer has a core fraction of between about 25 and about 95%
by cross-sectional area, and most preferably has a core fraction of
between about 50 to about 90%.
[0029] The extrusion reduction ratio (cross-sectional area of the
billet divided by the cross-sectional area of the extruded rod) is
preferably between about 10 and about 100, and most preferably
between about 15 and about 50. The cylindrical rods extruded from
the billet therefore have a diameters between about 2 and about 25
millimeters. The cylindrical rods are preferably extruded to
diameters between about 4 and about 20 millimeters. The rods will
have the same core fraction as the billets from which they are
extruded.
[0030] To meet the anticipated future modulus requirements of the
semiconductor industry, the non-noble metal 12 or wire 10 formed
from core cylinder 22 of billet 20 preferably has an elastic
modulus greater than about 95 GPa. Suitable core materials thus
include the metals copper, nickel, and the like, and alloys
thereof. The core material more preferably is a metal or metal
alloy with high conductivity and high drawability. Thus, the core
material is most preferably copper or a copper alloy, which also
possess a significant cost advantage.
[0031] The core material for wedge bonding is preferably
oxygen-free, high purity copper (OFHC). For ball bonding, the core
material preferably has a melting point within 5C..degree. of the
melting, point of the annulus metal. For a gold or a gold alloy
annulus, the core material is preferably a copper alloy with this
melting point. More preferably, the copper alloy will also have
improved oxidation resistance over that of pure copper. Preferred
gold-clad, copper-cored composite wires have a resistivity between
about 1.70 and about 2.00 .mu.Ohm-cm, a modulus between about 95
and about 120 GPa, and a composite density between about 9.0 and
about 15.0 g/cc. Each characteristic represents an improvement over
4N gold wire.
[0032] As noted above, the noble metal forming the annulus 14
metallurgically bonded to the non-noble metal-containing core 12 is
preferably gold having a purity of at least 90%, preferably at
least 99%, and most preferably at least 99.99%. The gold is
preferably an alloy that is doped to obtain sound deformation of
the composite and good bonding properties for the composite wire. A
preferred gold alloy is doped with less than 30 ppm of calcium,
less than 20 ppm of beryllium and less than 50 ppm of other
elements. Gold alloys containing less than 10 ppm of calcium and
less than 10 ppm of beryllium are even more preferred. A
particularly preferred gold alloy is 4N gold, and a 4N gold
nominally containing 7.5 ppm beryllium, 6.5 ppm calcium and less
than 30 ppm of other elements is most preferred.
[0033] The composite wires of the present invention may be bonded
to the leads of semiconductor packages by essentially conventional
techniques. FIG. 3 depicts a semiconductor package 40 in which
leads 42a, 42b, 42c, etc. are bonded to wires 10a, 10b, 10c, etc.
by wedge bonds 44a, 44b, 44c, etc. A cut away view of wire 10b
depicts case 12b surrounded by annulus 14b.
[0034] The present invention thus provides a composite bonding wire
with a higher modulus, higher strength, and higher conductivity
than standard 4N gold alloy bonding wire. The composite bonding
wire noble metal content is nominally half that of conventional
wire, so that the composite wire is significantly less expensive
than the equivalent size 4N gold alloy wire, yet the composite wire
maintains the standard 4N gold alloy bonding characteristics.
[0035] The following non-limiting example set forth hereinbelow
illustrates certain aspects of the invention. All parts and
percentages are by weight unless otherwise noted, and all
temperatures are in degree Celsius.
EXAMPLE
[0036] 800 g AW-14 (American Fine Wire, Ltd., Willow Grove, Pa.), a
4N gold alloy containing less than 10 ppm of Ca and Be, and less
than 20 ppm each of In and Ge was cast into a 28 mm diameter mold.
The casting process was a conventional batch casting consisting of
melting the alloy in a graphite crucible and pouring the melt into
a cylindrical graphite mold.
[0037] The resulting gold ingot was bored to form an 18 mm inside
diameter (ID) center hole and machined to 25 mm outside diameter
(OD). The resulting tube was machined to 76 mm length. A cylinder
of OFHC grade copper was machined into a cylinder of 18 mm OD and
76 mm length. The copper cylinder fit inside the gold alloy tube
with a tolerance of less than 0.1 mm.
[0038] A sleeve of OFHC copper was made with an ID of 25 mm, an OD
of 28 mm, and a length of about 85 mm. Billet end caps were
machined to fit the ends of the copper sleeve.
[0039] The billet caps were then electron-beam welded to seal the
billet. The billet was preheated for one hour at 450.degree. C. The
heated billet was placed into a 50 ton extrusion press which was
also preheated to 450.degree. C. The billet was extruded to 6.4 mm
diameter at a nominal run force of 48 tons.
[0040] The extrudate was cleaned with an abrasive pad and washed in
water. The billet nose and tail were cropped off, and samples were
taken. The resulting rod was drawn to 1 mm diameter by conventional
single-die drawing. The resulting wire was placed in 50% nitric
acid in water to chemically remove the copper sheath originating
from the extrusion can. The etched wire was rinsed with water, then
with alcohol.
[0041] The wire was further etched in aqua regia (1 part nitric
acid, 3 parts hydrochloric acid, and 4 parts water) for about ten
seconds to remove any gold-copper compounds on the surface of the
wire. The resulting wire was drawn to nominally 25 micron diameter
using a standard 8 to 12% reduction die schedule on standard
multi-die drawing machines using oil-in-water emulsion lubricant.
Wire drawability was excellent, with lengths greater than 5
kilometers drawn without breaking.
[0042] The elongation and break-load properties of the composite
wire were measured. As depicted in FIG. 5, the 24.8 micron diameter
composite wire was about 20% stronger than AW-14 gold alloy for
elongations above 2% (bonding wire specifications are >2%
elongation for most applications), breaking at about 14 g@4%
elongation. The copper core was very uniform along the axis of the
wire. At the final wire diameter of 24.8 microns, the standard
deviation of the copper core cross section was only 0.26%.
[0043] The modulus of the composite wire was about 108 GPa when
annealed, about 26% higher than AW-14. The resistivity of the
composite wire was 2.0 micro-ohms-cm, which is about 12% lower than
that of AW-14. Measurement of the resistivity versus time and
temperature of the composite wire shows negligible resistivity
increase up to 500 hours at temperatures less than or equal to
200.degree. C.
[0044] Initial wedge bonding trials on the 24.8 micron composite
wires showed strong bonding. A SEM micrograph cross-section of a
semiconductor package wedge-bonded with the composite wire is shown
in FIG. 4. Continuity of the gold sheath within the wedge bond is
maintained.
[0045] The present invention thus provides a strong, flexible
composite wire suitable for bonding wire applications having a
non-noble metal core ensheathed in a uniform,
metallurgically-bonded noble metal annulus. By using copper or
copper alloy as the core material, a composite wire is obtained
having optimum modulus, strength and conductivity, as well as
significantly reduced cost.
[0046] The foregoing example and description of the preferred
embodiments should be taken as illustrating, rather than as
limiting, the present invention as defined by the claims. Numerous
variations and combinations of the features set forth above can be
utilized without departing from the presently-claimed invention.
Such variations should not be regarded as a departure from the
spirit and scope of the invention, and are intended to be included
within the scope of the following claims.
* * * * *