U.S. patent application number 09/740189 was filed with the patent office on 2001-06-07 for electrochemical cobalt silicide liner for metal contact fills and damascene processes.
Invention is credited to Hu, Yongjun Jeff, Li, Li.
Application Number | 20010003063 09/740189 |
Document ID | / |
Family ID | 23128165 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010003063 |
Kind Code |
A1 |
Hu, Yongjun Jeff ; et
al. |
June 7, 2001 |
Electrochemical cobalt silicide liner for metal contact fills and
damascene processes
Abstract
A liner material and method of use is disclosed. The method
includes depositing a silicon layer into a deep void, such as a via
or trench, and physical vapor depositing a cobalt seed layer onto
the silicon. A supplemental cobalt layer is electroplated over the
seed layer. The structure is then annealed, forming cobalt silicide
(CoSi.sub.x). The layer can be made very thin, facilitating further
filling the via with highly conductive metals. Advantageously, the
layer is devoid of oxygen and nitrogen, and thus allows low
temperature metal reflows in filling the via. The liner material
has particular utility in a variety of integrated circuit
metallization processes, such as damascene and dual damascene
processes.
Inventors: |
Hu, Yongjun Jeff; (Boise,
ID) ; Li, Li; (Meridian, ID) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
23128165 |
Appl. No.: |
09/740189 |
Filed: |
December 19, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09740189 |
Dec 19, 2000 |
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09293212 |
Apr 16, 1999 |
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6194315 |
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Current U.S.
Class: |
438/683 ;
257/E21.584 |
Current CPC
Class: |
H01L 21/76855 20130101;
H01L 21/76868 20130101; H01L 21/76873 20130101; H01L 21/76843
20130101 |
Class at
Publication: |
438/683 |
International
Class: |
H01L 021/44 |
Claims
We claim:
1. A method of lining a hole in an integrated circuit, comprising:
depositing a silicon layer into a hole in an interlevel dielectric
of a substrate assembly, the hole comprising sidewalls and a floor;
depositing a cobalt seed layer onto the silicon layer within the
hole; electroplating a supplemental cobalt layer onto the cobalt
seed layer; and reacting the supplemental and seed cobalt layers
with the silicon layer to form a cobalt silicide liner along the
hole sidewalls and floor.
2. The method of claim 1, wherein the silicon layer comprises
undoped silicon.
3. The method of claim 1, wherein the silicon layer comprises
amorphous silicon.
4. The method of claim 3, wherein the silicon layer has a thickness
between about 50 .ANG. and 500 .ANG..
5. The method of claim 1, further comprising removing an unreacted
portion of the supplemental cobalt layer after reacting the
supplemental and seed cobalt layers with the silicon layer.
6. The method of claim 1, wherein depositing the cobalt seed layer
comprises a physical vapor deposition.
7. The method of claim 6, wherein the cobalt seed layer has a
thickness between about 5 .ANG. and 150 .ANG..
8. The method of claim 1, wherein the cobalt seed layer has a
thickness between about 50 .ANG. and 100 .ANG..
9. The method of claim 1, wherein the seed layer is
noncontiguous.
10. The method of claim 1, wherein depositing the cobalt seed layer
comprises sputtering a cobalt target.
11. The method of claim 1, wherein the hole comprises a contact via
through the interlevel dielectric.
12. The method of claim 11, wherein the via extends downwardly from
the floor of a trench.
13. The method of claim 1, wherein electroplating the supplemental
cobalt layer comprises immersing the via in a solution of cobalt
and hydrochloric acid.
14. The method of claim 1, wherein the supplemental cobalt layer at
least fills the hole.
15. The method of claim 1, wherein the cobalt silicide liner has a
thickness of less than about 300 .ANG..
16. The method of claim 15, wherein the cobalt suicide liner has a
thickness between about 150 .ANG. and 200 .ANG..
17. The method of claim 1, wherein the hole has an aspect via
greater than about 2.
18. The method of claim 17, wherein the hole has an aspect via
greater than about 8.
19. The method of claim 18, wherein the hole has an aspect via
greater than about 10.
20. A method of forming a protective cobalt silicide layer in an
integrated circuit, the method comprising: providing an undoped,
amorphous silicon layer; depositing a cobalt seed layer over the
silicon layer by physical vapor deposition; electroplating an
additional cobalt layer over the cobalt seed layer; and reacting
the cobalt with the silicon layer.
21. The method of claim 20, wherein depositing the cobalt seed
layer comprises sputtering a pure cobalt target.
22. A protective liner interposed between a highly conducting metal
element in an integrated circuit and an interlevel dielectric, the
liner comprising a CoSi.sub.x layer having a thickness of less than
about 300 .ANG..
23. The liner of claim 20, wherein the CoSi.sub.x layer has a
thickness of between about 150 .ANG. and 200 .ANG..
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates in general to protective metal
silicides for use with integrated circuits and methods of making
the same, and in particular to silicide liners between a via wall
and a metal contact fill.
[0003] 2. Description of the Related Art
[0004] When fabricating integrated circuits (IC), layers of
insulating, conducting and semiconducting materials are deposited
and patterned in sequence. Contact vias or holes are commonly
formed in insulating materials known as interlevel dielectrics
(ILDs). The vias are then filled with conductive material, thereby
interconnecting electrical devices and wiring at various levels.
Similarly, damascene processing involve etching trenches in
insulating layers in a desired pattern for a wiring layer. These
trenches are then filled with conductive material to produce the
integrated wires. Where contact vias, extending downwardly from the
bottom of the trenches, are simultaneously filled, the process is
known as dual damascene.
[0005] Conductive elements, such as gates, capacitors, contacts,
runners and wiring layers, must each be electrically isolated from
one another for proper IC operation. In addition to interlevel
dielectrics surrounding contacts, care must be taken to avoid
conductive diffusion and spiking, which can cause undesired shorts
between devices and contacts. Protective liners are often formed
between via or trench walls and metals in a substrate assembly, to
aid in confining deposited material within the via or trench walls.
Liners are practically required for certain severe metal deposition
processes, such as hot metal reflow and forcefill, particularly in
damascene and dual damascene interconnect applications. Protective
layers are similarly applied to transistor active areas and other
circuit elements to which contacts are formed.
[0006] Candidate materials for protective layers should demonstrate
good adhesion with materials on either side, such as via walls and
metal fillers. Processes should be available for depositing the
material with good step coverage into deep, high-aspect ratio vias
or trenches. Perhaps most importantly, the liner should serve as an
effective diffusion barrier. Typically, liners have been formed of
metal nitrides, such as TiN, for which chemical vapor deposition
(CVD) processes have been developed. As is known in the art, CVD is
particularly well adapted to conformally depositing into deep vias
and trenches.
[0007] Continued miniaturization of integrated circuits, in pursuit
of faster and more efficient circuit operation, results in contact
vias having ever higher aspect ratios (defined as the ratio of
height to width of the via). Continued scaling of critical device
dimensions leads to more narrow contacts, while contact height
cannot be proportionately decreased. ILDs must be maintained at a
adequate thickness to avoid short circuits and interlevel
capacitance, which tends to tie up electrical carriers and slow
signal propagation speed. Accordingly, the aspect ratios of contact
vias and trenches inevitably increase as circuit designs are scaled
down. As is known in the art, high aspect ratio vias and trenches
are very difficult to fill conformally, that is, without forming
keyholes which can adversely affect conductivity of the
contacts.
[0008] A conformal liner effectively further increases the aspect
ratio, by reducing the narrow width of the via without a
proportionate reduction in height. With ever smaller available
volume within contact vias, it is desirable to provide thinner via
liners, which would not only facilitate filling the via, but would
also leave more room for more highly conductive filler metals.
Thinning the liner, however, generally reduces the liner's
effectiveness in performing its general function of protecting
against metal diffusion or spiking, due to the risk of incomplete
via wall coverage and the ability of metals and contaminants to
more easily diffuse through thin liners.
[0009] Conventional liner materials and processes for forming them
have been found unsatisfactory for advanced generation fabrication
technology, which dictates extremely high aspect ratios and
attending harsh metallization processes.
[0010] Accordingly, there is a need for improved processes and
materials for protective liners in contact vias and runner
trenches. Desirably, such processes should also be compatible with
conventional fabrication techniques, and thereby easily integrated
with existing technology.
SUMMARY OF THE INVENTION
[0011] Briefly stated, the present invention provides a thin cobalt
suicide layer and a method of forming such a layer as a liner
within a high aspect ratio hole.
[0012] In accordance with one aspect of the invention, a method is
provided for lining a hole, such as a via or a trench, in an
integrated circuit. The method includes depositing a silicon layer
into the hole. A cobalt seed layer is deposited onto the silicon
layer within the hole, and a supplemental cobalt layer is
electroplated onto the cobalt seed layer. Thereafter, the cobalt
layers are reacted with the silicon layer to form a cobalt silicide
liner along the hole sidewalls and floor.
[0013] Advantageously, the process creates a liner which can be
used with a via having a high aspect ratio. Also, the liner is
readily integrated with existing metallization technology, and
particularly with newer hot metal and forcefill applications.
[0014] In accordance with another aspect of the present invention,
a protective liner is provided between a highly conducting metal
element in an integrated circuit and an interlevel dielectric. The
liner includes a CoSi.sub.x layer with a thickness of less than
about 300 .ANG..
[0015] In accordance with another aspect of the invention, a method
is disclosed for forming a protective cobalt silicide layer in an
integrated circuit. The method includes providing an undoped,
amorphous silicon layer. A cobalt seed layer is deposited over the
silicon layer by physical vapor deposition, and an additional
cobalt layer electroplated over the cobalt seed layer. The cobalt
is then reacted with the silicon layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] These and other aspects, features and advantages of this
invention will be apparent from the detailed description of the
preferred embodiment and the accompanying drawings, which are
intended to illustrate and not to limit the invention. Like
reference numerals are employed to designate like parts throughout
the figures, wherein:
[0017] FIG. 1 is a flow diagram illustrating the basic steps of a
process for forming a via or trench liner, in accordance with a
preferred embodiment of the present invention;
[0018] FIG. 2 is a partial elevational cross-section of a partially
fabricated integrated circuit or substrate assembly, showing a
conventional interlevel dielectric and a via therethrough, exposing
a conductive circuit element beneath the via;
[0019] FIG. 3 shows the substrate assembly of FIG. 2 after
deposition of a conformal silicon layer;
[0020] FIG. 4 shows the substrate assembly of FIG. 3 after
deposition of a cobalt seed layer into the via and over the silicon
layer;
[0021] FIG. 5 shows the substrate assembly of FIG. 4 after
electroplating of a fuller cobalt layer onto the seed layer;
[0022] FIG. 6 shows the substrate assembly of FIG. 5 after an
anneal step, forming a cobalt silicide liner;
[0023] FIG. 7 shows the substrate assembly of FIG. 6 after excess
elemental cobalt has been removed; and
[0024] FIG. 8 shows the liner of FIG. 7 after the via has been
filled with a highly conductive metal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] The preferred embodiments are illustrated in the context of
an integrated contact for electrically connecting a lower
conductive circuit element to an upper wire or runner in an
integrated circuit. The disclosed processes and materials have
particular utility in the context of damascene and dual damascene
metallization. The skilled artisan will readily appreciate,
however, that the materials and methods disclosed herein will have
application in a number of other contexts where thin conductive
liners are desirable in high aspect ratio trenches or vias.
[0026] Conventional via or trench liners comprise metal nitrides,
and most typically titanium nitride (TiN), for which effective CVD
processes are known. Metal suicides are also employed in addition
to or in place of metal nitrides, for more effective adhesion to
insulating material of the via or trench sidewalls, and for lower
contact resistivity with underlying circuit elements. Conventional
metal nitrides and silicides, however, each demonstrate grain sizes
of at least 200-300 .ANG.. Nitridation of metal silicides to form
metal silicon nitrides (e.g., TiSi.sub.xN.sub.y, TaSi.sub.xN.sub.y,
WSi.sub.xN.sub.y) can reduce grain size from 30-40 nm (300-400
.ANG.) to the nanometer or even amorphous range. Nevertheless, for
effective liner function, conventional nitride or silicide liners
need to be greater than about 500 .ANG., particularly for newer
copper damascene, hot metal reflow, and metal forcefill processes.
See, e.g., R. Iggulden et al., "Dual Damascene Aluminum for 1-Gbit
DRAMs," SOLID STATE TECHNOLOGY (November 1998), p. 37; Z. Hong et
al., "High Pressure Aluminum-plug Interconnects with Improved
Electromigration by Microstructural Modification," VMIC PROCEEDINGS
(Jun. 18-20, 1996), p. 449. Such liners occupy a considerable
portion of vias in current and future generation circuit designs,
making it difficult, if not impossible, to fill the lined vias with
highly conductive metals.
[0027] Moreover, nitrogen or oxygen content in liners
disadvantageously affects subsequent metallization processes.
Aluminum, for example, effectively fills high aspect ratios when
deposited slowly at high temperatures (e.g., about 450.degree. C.)
in hot metal reflow processes. High nitrogen or oxygen content,
however, such as in metal nitride or metal silicon nitride liners,
raises the reflow temperature considerably, increasing costs and
risking thermally induced damage to lower integrated
structures.
[0028] Cobalt silicide (CoSi.sub.x) has the potential to serve as
an effective via or trench liner due to several advantageous
characteristics. As a fine grain material, with grain sizes as low
as 50 .ANG. to 70 .ANG., even very thin CoSi.sub.x layers form
effective diffusion barriers, adequate to contain even
fast-diffusing elements such as copper. Additionally, CoSi.sub.x
can be formed with low oxygen and nitrogen content, facilitating
subsequent lower temperature metal fills.
[0029] Unfortunately, satisfactory chemical vapor deposition (CVD)
techniques have yet to be developed for CoSi.sub.x. Unlike other
metal halides, cobalt chlorides and cobalt fluorides are relatively
nonvolatile, making deposition of CoSi.sub.x difficult with
conventional CVD methods. Accordingly, conformal deposition
techniques are required before CoSi.sub.x can serve as a realistic
liner for present technology metallization.
[0030] FIG. 1 schematically illustrates a process flow for forming
a conductive liner in accordance with a preferred embodiment of the
invention. As shown, the process begins with formation of a contact
via through an insulating layer. It will be understood that the
same process may be applied to trenches, such as in damascene
metallization process flows. The via is then lined with silicon and
a thin cobalt seed layer applied to the lining silicon. A thicker
layer of cobalt can then be electroplated onto the structure,
followed by silicidation anneal. The underlying silicon is consumed
in the process, to leave a cobalt silicide layer lining the
insulating walls of the via. Excess elemental cobalt is then
selectively removed from over the silicide, and the
CoSi.sub.x-lined via is filled with metal to complete the
contact.
[0031] The process will now be described in detail with reference
to FIGS. 2-8.
[0032] A partially fabricated integrated circuit or substrate
assembly 10 is shown in FIG. 2. The structure is formed above a
substrate (not shown), which may comprise a single-crystal wafer or
other semiconductive layer in which active or operable portions of
electrical devices are formed. An interlevel dielectric (ILD) 12 is
formed above the substrate. Typical ILD materials include oxides
formed from tetraethylorthosilicate (TEOS), borophosphosilicate
glass (BPSG), polyamide, etc., and the illustrated ILD 12 comprises
BPSG. The ILD 12 has a thickness adequate to electrically insulate
underlying conductors from overlying conductors, which depends upon
circuit design and operational parameters. In the illustrated
embodiment, where the substrate assembly represents a 64 Mbit
dynamic random access memory (DRAM) circuit, the ILD 12 is
preferably between about 0.40 .mu.m and 0.60 .mu.m.
[0033] A contact via or hole 14 is etched through the ILD 12 to
expose an underlying conductive circuit element. In the illustrated
embodiment, the via 14 is narrow due to circuit design constraints.
Preferably, the via 14 has a width of less than about 0.25 .mu.m,
more preferably less than about 0.20 .mu.m, resulting in aspect
ratios greater than about 0.5, preferably greater than about 8, and
more preferably greater than about 10. Conventional
photolithographic techniques may be employed to define the via 14,
and anisotropic etching (e.g., reactive ion etching) is preferred
for producing vertical via sidewalls.
[0034] The illustrated circuit element exposed by the etch
comprises a contact landing pad of an underlying conductive runner
or wiring layer 18. The conductive layer preferably comprises
copper or aluminum, though the skilled artisan will appreciate that
other conductive materials may be suitable, depending upon the
function and desired conductivity of the circuit element. The
illustrated embodiment preferably includes an antireflective layer
16 (e.g., TiN), through which the via 14 preferably extends.
[0035] With reference to FIG. 3, a silicon layer 20 is then
deposited over the substrate assembly 10 and into the sidewalls of
the via 14. Preferably, the silicon layer is conformally deposited
by low pressure chemical vapor deposition (LPCVD). Silicon source
gas, such as silane (SiH.sub.4), bubbled dichlorosilane (DCS) or
trichlorosilane (TCS), are introduced into a reaction chamber. The
silicon source gas reacts with the substrate assembly 10, which is
mounted and heated within the chamber, to leave silicon at the
surface. The illustrated silicon layer 20 is deposited at low
temperatures, preferably between about 450.degree. C. and
550.degree. C. and more preferably about 505-525.degree. C., such
that the silicon layer 20 is amorphous. The amorphous silicon layer
20 is thus adequately conductive for the process purposes, without
the need for complicated and limiting doping processes. The silicon
layer 20 is relatively thin, preferably between about 50 .ANG. and
500 .ANG., and more preferably less than about 300 .ANG., and most
preferably between about 150 .ANG. and 200 .ANG., depending upon
the desired thickness of the CoSi.sub.x liner to be formed.
[0036] Referring to FIG. 4, a cobalt seed layer 22 is deposited
onto the silicon layer 20. In accordance with the illustrated
embodiment, the seed layer 22 is very thin, preferably between
about 5 .ANG. to 150 .ANG., more preferably between about 50 .ANG.
and 100 .ANG., and need not be contiguous or fully cover the
silicon layer 20. Only a small amount of cobalt needs to reach into
the via 14, and particularly at the bottom of the via 14.
Advantageously, therefore, the cobalt can be deposited by
conventional physical vapor deposition.
[0037] In the preferred embodiment, cobalt deposition is
accomplished by sputtering a pure cobalt target. For example, such
sputtering may be carried out in an Endura 5500 PVD II.TM.
processing chamber, commercially available from Applied Materials
of Santa Clara, Calif. The illustrated cobalt layer 22 can be
formed by sputtering in the exemplary chamber with an RF power of
about 1 kW to 2 kW for about 8-12 seconds. The skilled artisan will
readily appreciate that other deposition techniques may be equally
viable.
[0038] Referring now to FIG. 5, after the seed layer 22 is
deposited onto the layer 20, a supplemental cobalt layer 24 is
deposited over the seed layer 22. Preferably, sufficient cobalt is
provided, in combination with the seed layer 22, to fully consume
the underlying silicon layer 20. Accordingly, in the illustrated
embodiment, at least about 100 .ANG. of cobalt is preferably
deposited into the via 14, and more preferably between about 200
.ANG. and 800 .ANG., with an exemplary target of about 500
.ANG..
[0039] Advantageously, the illustrated amorphous silicon layer 20
and cobalt seed layer 22 enable growth of the cobalt supplemental
layer 24 by electroplating. Because electroplating ensures good
coverage of all interior via 14 surfaces, and because the final
liner thickness is limited by the thickness of the silicon layer
20, there is no real upper limit to the thickness of the
supplemental layer 24. Accordingly, the supplemental layer 24 can
be deposited to completely fill or overfill the via 14.
[0040] In the illustrated embodiment, formation of the supplemental
layer 24 is achieved by immersing the substrate assembly 10 into a
solution of cobalt and hydrochloric acid. Both chlorine and cobalt
ionize in the solution, according to the following formula:
Co+2 HCl.fwdarw.Co.sup.2++2 Cl.sup.-+H.sub.2 (Eq. 1)
[0041] The cobalt ions receive electrons at the negatively biased
substrate assembly 10, leaving elemental cobalt over the seed layer
22. Desirably, the chlorine ions do not attack any of the exposed
materials, including silicon, silicon oxide, BPSG or other ILD
material.
[0042] Referring to FIG. 6, after electroplating, the substrate
assembly 10 is subjected to an anneal to react the silicon layer 20
with cobalt in the seed layer 22 and supplemental layer 24, forming
a cobalt silicide liner 26. Preferably, the anneal is conducted at
between about 450.degree. C. and 850.degree. C., and more
preferably between about 600.degree. C. and 650.degree. C., for
about 10-30 seconds, more preferably for about 20 seconds. The
cobalt silicide layer 26 desirably has the form CoSi.sub.x, where x
is less than 2. Desirably, this material exhibits a lattice
mismatch of less than about 3%. Accordingly, the silicide layer 26
is barely thicker than the silicon layer 20 consumed by the
reaction. In accordance with the illustrated embodiment, therefore,
the CoSi.sub.x layer 26 is preferably between about 50 .ANG. and
500 .ANG., and more preferably less than about 300 .ANG., and most
preferably between about 150 .ANG. and 200 .ANG.,
[0043] As noted, the growth of the silicide is limited by the
thickness of the silicon layer 20, such that a layer of unreacted
or excess cobalt 28 typically remains over the silicide layer 26.
As is well known in the art, the excess cobalt 28 can be removed by
a selective metal etch, the result of which is shown in FIG. 7. The
cobalt suicide layer 26 is left lining the walls of the via 14.
[0044] Referring to FIG. 8, after the etch forms the liner
boundaries, the via 14 can be filled with a conductive metal filler
30, completing the contact and providing sufficiently high
conductivity for signal propagation between the lower circuit
element 16 and upper levels of wiring. Most preferably, the
conductive metal filler 30 comprises copper, which is known to be
highly conductive and inexpensive. Advantageously, the cobalt
silicide liner 26 serves as a good barrier against copper
diffusion. Other suitable metals include aluminum, Al/Cu alloy,
Al/Ti alloy, Al/Si alloy, and Al/Ge alloy.
[0045] In recognition of the high aspect ratios of the via 14, the
metal filler 30 of the illustrated embodiment comprises a metal
deposited by hot metal reflow or forcefill processes. Hot metal
reflow involves slow deposition of metal at close to the metal
transition point (e.g., about 450.degree. C. for aluminum), and is
more fully described in R. Iggulden et al., "Dual Damascene
Aluminum for 1-Gbit DRAMs," SOLID STATE TECHNOLOGY (November 1998),
p. 37, the disclosure of which is incorporated herein by reference.
Forcefill applications involve extremely high pressures (e.g.,
200-300 atm.), literally forcing deposited metal into the via 14.
Forcefill processes are more fully described in Z. Hong et al.,
"High Pressure Aluminum-plug Interconnects with Improved
Electromigration by Microstructural Modification," VMIC PROCEEDINGS
(Jun. 18-20, 1996), p. 449, the disclosure of which is incorporated
herein by reference.
[0046] FIG. 8, it will be understood, is merely schematic. The
contact formation may be planarized leave a contact plug, or it may
be followed by photolithographic patterning and etching to define
wiring layers in the portions of the cobalt suicide layer 26 and
filler 30 above the ILD 12. More preferably, however, the inventive
process is applied to the high aspect ratio vias and trenches
employed in damascene or dual damascene constructions.
[0047] In accordance with dual damascene embodiments, the contact
described above and illustrated in FIG. 8 extends from the bottom
of a trench, which has been etched into an ILD in a desired wiring
pattern. Specifically, after the ILD is etched to form trenches,
the via 14 is etched from a trench floor down to a lower conductive
element. The effective aspect ratio of the via 14 is thus
particularly high.
[0048] The contact forming process described above is conducted on
the via 14 and the trench (not shown) formed above the via 14.
Thus, the cobalt silicide layer 26 lines both the via 14 and the
walls of the overlying trench, and the metal filler 30 fills, and
preferably overfills, both the via 14 and the overlying trench. The
metal is subsequently planarized or etched back so that the metal
26, 30 remains isolated in paths within the trenches and vias.
[0049] Although this invention has been described in terms of a
certain preferred embodiment and suggested possible modifications
thereto, other embodiments and modifications may suggest themselves
and be apparent to those of ordinary skill in the art are also
within the spirit and scope of this invention. Accordingly, the
scope of this invention is intended to be defined by the claims
which follow.
* * * * *