Note: Descriptions are shown in the official language in which they were submitted.
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
POLISHI1~G PADS WITH POLYMER FILLED FIBROUS WEB, AND
METHODS FOR FABRICATING AND USING SAME
Shyng-Tsong Chen, Scott Clayton Billings, Kenneth M. Davis, David S. Gilbride,
Oscar Kai Chi Hsu, Kenneth P. Rodbell, and Jean Vangsness
10~
FIELD OF THE INVENTION
The present invention relates to polishing pads. The polishing pads of the
present
invention are especially useful in chemical-mechanical planarization of
semiconductor
wafers. Specifically the invention relates to pads of increased stiffness to
prevent over
25 polishing, and increased hardness and thickness for greater useful life.
The present
invention is also applicable to the polishing of other surfaces, for example
optical glass
and CRT and flat panel display screens. The present invention further relates
to methods
for fabricating arid using the pads.
BACKGROUND OF INVENTION
For many years, optical lenses and semiconductor wafers have been polished by
0 chemical-mechanical means. More recently, this technique has been applied as
a means
of planarizing intermetal dielectric layers of silicon dioxide and for
removing portions of
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
conductive layers within integrated circuit devices as they are fabricated on
various
substrates. For example, a conformal layer of silicon dioxide rnay cover a
metal
interconnect such that the upper surface of the layer is characterized by a
series of
non-planar steps corresponding in height and width to the underlying metal
interconnects.
The rapid advances in semiconductor technology has seen the advent of very
large
scale integration (VLSI) and ultra large scale integration (ULSI) circuits
resulting in the
packing of very many more devices in smaller axeas on a semiconductor
substrate. The
greater device densities require greater degrees of planarity to permit the
higher resolution
lithographic processes required to form the greater number of devices having
smaller
features as incorporated in current designs. Moreover, copper, because of its
low
resistance, is increasingly being used as interconnects. Conventionally,
etching
techniques are used to planarize conductive (metal) and insulator surfaces.
However,
certain metals, desirable for their advantageous properties when used as
interconnects
(Au, Ag, Cu) are not readily amenable to etching, thus the need for chemical-
mechanical
polishing (CMP).
Typically, the various metal interconnects are formed through lithographic or
damascene processes. The damascene technique is described in U.S. Patent
Number
4,789,648, to Chow, et al. assigned to the assignee of the present invention,
the entire
contents of which are incorporated herein by reference. ~ For example, in a
lithographic
process, a first blanlcet metal layer is deposited on a first insulating
layer, following which
electrical lines are formed by subtractive etching through a first mask. A
second
insulating layer is placed over the f rst metallized layer, and holes are
patterned into the
second insulating layer using a second mask. Metal columns or plugs are formed
by
filling the holes with metal. A second blanket metal layer is formed over the
second
insulating layer, the plugs electrically connecting the first and second metal
layers. The
second metal layer is masked and etched to form a second set of electrical
lines. This
process is repeated as required to generate the desired device.
_2_
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
Presently, VLSI uses aluminum for the wiring and tungsten for the plugs
because
of their susceptibility to etching. However, the resistivity of copper is
superior to either
aluminum or tungsten, making its use desirable, but copper does not have
desirable
properties with respect to etching.
Variations in the heights of the upper surface of the intermetal dielectric
layer
have several undesirable characteristics. The optical resolution of subsequent
photolithographic processing steps may be degraded by non-planar dielectric
surfaces.
Loss of optical resolution lowers the resolution at which lines may be
printed. Moreover,
where the step height is large, the coverage of a second metal layer over the
dielectric
layer may be incomplete, leading to open circuits.
In view of these problems, methods have been evolved to planarize the upper
surfaces of the metal and dielectric layers. One such technique is chemical-
mechanical
polishing (CMP) using an abrasive polishing agent worked by a rotating
polishing pad. A
chemical-mechanical polishing method is described in U.S. Patent 4,944,836,
Beyex, et
al., assigned to the assignee of the present invention, the entire contents of
which are
incorporated herein by reference. Conventional polishing pads are made of a
relatively
soft and flexible material, such as nonwoven fibers interconnected together by
a relatively
small amount of a polyurethane adhesive binder, or may be laminated layers
with
variations of physical properties throughout the thickness of the pad.
Multilayer pads
generally have a flexible top polishing layer backed by a layer of stiffer
material.
The CMP art combines the chemical conversion of a surface layer to be removed,
with the mechanical removal of the conversion product. Ideally, the conversion
product
is soft, facilitating high polishing rates. CMP pads must resolve two
constraints relevant
to the present invention. The surface in contact with the substrate to be
polished must be
resilient. Of particular relevance to the present invention is the problem of
local over
polishing, also known as "dishing", resulting from too flexible a pad. This is
one of the
key problems encountered during CMP of metal substrates. Also, an increased
number
-3-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
and density of defects in the polished surface may be caused by frayed and
loose fibers
that develop as conventional fibrous pads become worn. Such defects correlate
with low
yields of product.
Some of the most commonly used polishing pads for manufacturing
semiconductor chips are a very soft foam pad, or a soft nonwoven fiber pad. An
advantage of a soft polishing pad is low defect density on the polished wafer
and good
within-wafer uniformity. However, soft CMP pads suffer from very short pad
life
requiring replacement after polishing about 50 wafers, and excessive dishing
of the
polished wafer because of the pad softness. Also, for a metal damascene CMP
process, a
soft pad usually causes much more dishing compared with a hard pad.
It is generally known that prevention of dishing requires a stiffer pad. Thus,
a
hard polishing pad usually has better planarization capability than a soft
pad. However,
the defects count is much higher than with the soft pad and the within-wafer
uniformity is
usually much worse. In addition, hard pads may be conditionable, which means
that the
pad surface condition can be regenerated using a diamond disk or an abrasive
roller to
recondition the pad surface by removing worn areas and embedded debris. This
reconditioning capability means that a hard pad may last much longer than a
soft pad.
Such reconditioning ih situ also means that polishing tool down time for pad
replacement
is greatly reduced.
Currently, these problems are handled using mufti-step techniques wherein
initial
polishing is effected at a high rate using one set of pads and abrasive
compounds,
followed by a second polishing step using a second set of pads and abrasive
compounds
differently optimized in comparison to the first set. This is a time consuming
process
and, moreover, it also suffers from high defect densities due to the use of
two different
pads. For Cu planarization, CMP pads are critical, and are as important as the
abrasive
slurry. Fibrous pads of the prior art have been too soft to obtain good
planarization.
Stacked nonwoven fiber and other types of pads have previously been tried in
an attempt
-4-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
to obtain better CMP performance. However, thin (5 to 20 mils thick) pads of
nonwoven
fibers bound with polyurethane are not sufficiently durable and do not long
survive the
CMP process.
Accordingly, the need exists for improved fibrous polishing pads. A high
quality
CMP pad should meet the following requirements: produce extremely low defects
counts
on polished surfaces, cause extremely small dishing and extremely low erosion
of
polished surfaces, and have a long pad life extendible by reconditioning. None
of the
existing prior art CMP pads can meet all of these requirements, which are
needed for the
future generation of CMP processes. A riew type of CMP pad is therefore needed
to meet
these requirements.
SUMMARY OF INVENTION
The present invention addresses problems in the prior art and provides a
relatively
thick, stiff and hard pad comprising nonwoven fibers embedded in a polymer
matrix. A
nonwoven fiber mat is filled substantially completely with reactants for
producing the
polymer matrix before those reactants are fully cured. During curing, there
may be some
shrinkage producing voids in the matrix as the reactants are converted to the
final hard
polymer. However, the resulting fiber and polymer composite is sufficiently
hard to be
compatible with current and future CMP process chemistry, and is conditionable
after use
by grinding (dressing) with a diamond containing abrasive dislc or roller to
regenerate the
working surface of the pad. The pad thickness may also be greater than
previously used,
which together with pad reconditionability, means that the pad life is
significantly longer,
such as polishing 500 to 1,000 wafers before pad replacement becomes
necessazy.
Applications are envisioned in the semiconductor and optical industries.
-5-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
The present invention also relates to a method of making the above disclosed
pads. In particular, the method comprises pressing the reactants into the
interstices of a
fibrous mat in a mold and then, when the interstices are substantially full,
curing the
reactants to produce the above disclosed polishing pad. Both heat and pressure
are
applied to cure the precursor system within the fibrous mat in the mold. After
curing and
removal from the mold, the pad may be buffed with an abrasive disk or roller
to remove a
skin-like covering and to fracture a surface portion of the polymer to form a
thin
polishing surface Iayer of free fibers, segments of which remain embedded in
the adjacent
composite body.
Still other objects and advantages of the present invention will become
readily
apparent to those skilled in the art from the following detailed description,
wherein is
shown and described preferred embodiments of the invention, simply by way of
illustration of the best mode contemplated for carrying out the invention. As
will be
realized by the skilled person, the invention is capable of other and
different
embodiments, and its details are capable of modifications in various obvious
respects,
without departing from the invention. Accordingly, the description is to be
regarded as
illustrative in nature and not as restrictive.
DESCRIPTION OF DRAWINGS
The invention may be further understood by reference to the detailed
description
below taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a photograph of a portion of the polishing surface of a used pad of
the
invention taken at a magnification of 100 times;
Fig. 2 is a photograph of the polishing surface of the used pad of Fig. 2
talcen at a
greater magnification of 500 times, and with the image focused on a fiber
layer above the
surface of the hard polymer and fiber body; and
-6-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
Fig. 3 is a photograph of the polishing surface of the used pad of Fig. 2
taken at a
greater magnification of 500 times, and with the image focused on the surface
of the hard
polymer and fiber body such that the fiber layer above this surface is out of
focus.
BEST AND VARIOUS MODES FOR CARRYING OUT THE INVENTION
Typical materials suitable as a first fiber group are Rayon, polycarbonate,
polyamide, polyphenylene sulfide, polyimide, Aramide fibers including Nomex
and
Kevlar, polyvinylchloride, Hemp, and combinations of these fibers. Typical
materials
suitable as a second fiber group are polyester, polypropylene, Nylon, acrylic,
and
polyethylene, and combinations of these fibers. The listed fibers are meant to
be
illustrative of the types that may be used, but the invention is not thereby
limited to the
enumerated types. The fibers of the first group are preferred because they
provide pads
having a higher hardness than the fibers of the second group. Combinations of
the fibers
of the first and second groups are also possible. The fibers and matrix
polymers together
typically have a hardness of about 30 Shore D to about 100 Shore D, and
preferably about
40 Shore D to about 80 Shore D, and more preferably about 50 Shore D to about
70
Shore D, as measured by Durometer Hardness test method ASTM D2240.
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
The fibers are preferably in the form of a web or mat, but may be individual
fibers
which are mixed with polymer precursors or to which polymer precursors are
added. The
fiber web may be a loose pile of fibers or may be formed by any well known
nonwoven or
woven production techniques, such as needle-ptmching, hydroentangling,
chemical
bonding, air-through bonding, weaving, knitting, felting or the like. The
fiber mat alone
preferably has a Durometer hardness from about 10 to about 90 Shore A,
preferably from
about 30 to about 70 Shore A, as measured by the aforesaid test method. The
web of
fibers, before impregnation with the polymer reactants (precursors),
preferably has a
thickness in the range of about 5 to about 130 mils, more preferably about 15
to about
100 mils, and most preferably about 50 to about 100 mils. During the molding
process,
these thicknesses may be reduced by about 10 to about 20%. The thickness of a
new
molded pad is preferably in the range from about 10 mils to, about 100 mils.
The pad is
sufficiently strong and cohesive to be used and reconditioned down to a
thiclcness of
about 5 mils.
_g_
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
The fiber mat is embedded in a matrix of a polymeric material. Examples of
suitable matrix materials are polyurethanes including polyester and polyether
urethanes,
polycarbonates, polyacrylates including polymethylmethacrylate (PMMA),
polyaramides,
thermosetting polymers such as epoxies and derivatives of epoxies, and
combinations of
these polymers. The chemical-physical properties, hence the polishing
performance, of
the fiber and polymer composite are governed by the types and sizes of the
fibers, the
types and hardness of the polymers, the fiber to polymer ratio, the friability
of the
polymers, and the local and global distribution of the polymer matrix within
the fiber mat.
For example, employing a larger fiber diameter (thus with fewer fibers for a
given density
of the fiber mat) and the use of a high fiber: polymer ratio will result in a
pad structure
having a lower overall density and surface hardness, and a higher
compressibility.
Conversely, employing a smaller fiber diameter, a lower fiber: polymer ratio,
and harder
polymer types will result in a pad structure having higher density, lower
compressibility
and higher surface hardness. A solid polymer is preferred over a porous
polymer for the
matrix. If the matrix polymer is porous, it is preferable that the pore sizes
be in the range
of 5-100 microns, more preferably 5-50 microns, to achieve the desired
hardness. If the
polymer matrix is porous, uniform porosity and a higher density yields pads
with better
polishing uniformity, less dishing, and a higher polishing rate. This permits
greater
process throughput and greater product yields.
The pads of the present invention typically comprise about 30 to about 70
percent
by weight and preferably about 40 to about 60 percent by weight of the fibers
and
correspondingly typically about 70 to about 30 percent by weight and
preferably about 60
to about 40 percent by weight of the polymeric matrix. The percentages of the
fibers and
polymeric matrix are based upon the total of the fibers and polymeric matrix
in the pad.
-9-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
The pads of the present invention preferably have densities of about 0.5 g/cc
to
about 1.1 g/cc, and the fiber mats from which the pads are made preferably
have densities
of about 0.15 g/cc to about 0.9 g/cc. To ensure the desired hardness of the
pad, the fiber
mat comprises a relatively loose network of fibers and this network is
substantially
completely filled with the precursor reactants for forming the polymer matrix
in which
the fiber mat becomes embedded after the reactants are cured. The cured
polymer
preferably forms a relatively hard but friable matrix. Following curing, the
molded pad is
conditioned by buffing with a diamond disk or opposing inline abrasive rollers
to remove
a skin-like polymer surface and expose about a 1 to 2 mil thickness of the
fiber mat,
which thereby creates about a 1 to 2 mil thick fiber surface layer containing
fibers that are
partially free. The creation of this surface layer results from the friable
nature of the
cured polymer matrix. In other words, the strength of the fiber is stronger
than the filler
or matrix material such that, during buffing, the filler material is removed
at the surface
while the surface fibers remain attached to the main body or backing layer of
the fiber and
polymer composite. Thus, after buffing, a small thickness or depth of surface
polymer is
removed to leave a thin surface layer of free fibers, segments of at least a
portion of
which remain embedded in the adjacent composite body of polymer and fibers, as
can be
seen in Figs. 1, 2, and 3. During CMP processes, this fibrous polishing
surface helps to
reduce up to or more than about 90% of the defects count caused by using a
conventional
hard pad. In addition, the solid matrix formed by the polymer densely filling
the fiber
mat makes the pad up to 50% harder than the hardest conventional CMP pad
presently on
the market.
-10-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
Accordingly, the thin fibrous surface layer of the preferred pad of the
present
invention significantly reduces the defects count of the wafers polished
therewith, and the
hard backing body or layer beneath.the fibrous surface Iayer results in much
less dishing
of the polished wafer surface. As a result, metal dishing can be minimized to
less than
about 0.04% of the size of the metal features on the wafer. In addition,
erosion of the
wafer surface is very small so as to be negligible.
In addition, the pad surface can be reconditioned after polishing one or more
wafers to maintain a high performance Ievel. This makes the pad service life
much
longer (potentially over 1,000 wafers) than conventional soft fiber-based
pads. The
conditioning process can actually recreate the thin (about 1 to 2 mils)
fibrous surface
layer which continues to help reduce the defects count, while the underlying
hard fiber
and polymer body sufficiently fixes and supports the fiber layer to reduce the
dishing
phenomenon.
The pads may have multiple layers, as described in U.S. Patent Application
Serial
No. 09/599,514, to allow for independent optimization of pad stiffness and
hardness in
independent layers. A bottom support layer imparts mechanical stiffness to the
pad. The
stiffness of the bottom support layer is preferably optimized in relation to
the malleability
of the material comprising the surface to be worked. The top working layer,
the body of
which carries and which includes the thin surface layer of free fibers, is
preferably
optimized with respect both to the properties of the surface to be polished,
and with
respect to the chemical properties of the abrasive mixture used in the CMP
process.
Typically, the support Iayer(s) has stiffer fibers and is thicker than the
Iayer carrying the
free fibers used as the polishing surface, and is typically about 55% to about
90% of the
total thickness of the pad.
-11-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
As indicated above, stacked nonwoven and other types of fibrous pads have been
tried in the past in an attempt to obtain better CMP performance. However,
thin (5 to 15
mil thick) fibrous pads are not sufficiently durable and do not survive the
CMP process.
In the present invention, a single body polishing pad or the working body of a
mufti-layer
pad can be buffed down to 5 mils while still maintaining structural integrity
during the
CMP process. In either form, the free fiber layer provides a scratch-free
polishing surface
and the hard underlying body reduces the excessive dishing which usually
occurs during
CMP with softer pads. Thus the invention allows for independent control of the
optimal
properties to prevent over polishing, for compatibility with the substrate to
be polished,
and for compatibility with the polishing compound.
According to the present invention, the fibers may be precoated with the same
or a
different polymer prior to being embedded in the matrix polymer. Examples of
polymers
suitable for precoating the fibers are copolymers of styrene and an acrylate
or
methacrylate such as ethyl or methyl acrylate or methacryl~te; acrylonitrile
rubbers; and
butadiene-styrene rubbers, polyurethanes, fluorocarbons, and epoxy resins.
The precoating may help maintain the stability of the free fibers by enhancing
adhesion of segments of these fibers to the polymer matrix and can be used in
amounts of
about 10 to about 90% by weight and preferably about 15 to 50% by weight based
upon
the total weight of the fibers and precoating.
-12-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
The pads of the present invention can be fabricated by forming a loose fibrous
web or mat of one or more layers of nonwoven fibers, followed by applying a
precoating,
when used, to the loose fibrous mat such as by spraying, and then curing the
precoat. In
the alternative, each of the fiber layers can separately be precoated and then
stacked upon
each other, followed by partially curing the precoat such as to the B-stage.
At this stage,
the fibrous mat structure is then embedded into the matrix. This can be
accomplished by
placing the mat into a pad-shaped mold and applying an unreacted viscous
polymer
precursor system on top of the mat, such as an isocyanate system known as
ADIPRENE
from Uniroyal or AIRFLEX from Air Products. The mold is then closed and
sufficient
differential pressure is applied for causing the polymer precursors to
substantially
completely fill in the spaces (interstices) between the fibers and thereby
embed them in an
essentially continuous polymer matrix. As an alternative to pressurizing the
mold, a
vacuum, such as about minus 10 prig, may be used to pull the polymeric
reactants
(precursors) into the fibrous mat.
During or after this "fill" stage, the mold is heated to affect either a
partial or a
final cure of the matrix polymer. The curing of the matrix polymer is
typically performed
at temperatures of about 60° to about 250°F, preferably about
100°F to about 180°F; a
pressure of about 1 psig to about 200 psig preferably about 10 psig to about
150 psig,
more preferably about 50 psig to about 75 psig; for about 5 to about 24 hours.
Where the
pad is removed from the mold after only partial curing of the polymer, a final
cure may be
affected at ambient pressure in an oven or the like, the time and temperature
of this cure
depending on the polymer and extent of the partial cure.
-13-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
Whereas composite fiber and polymer pads of the prior art used just enough
polymer to bind together the nonwoven fibers of a mat, the present invention
substantially
completely fills the interstices of the fiber mat with the reactants for the
polymer, such as
an isocyanate system for polyurethane, to provide an extremely hard polymer
matrix with
embedded fibers. The pads of the invention also may be made of one or more
such hard
layers of fiber and polymer composite.
The fibers of the mat used have fiber diameters preferably in the range of
about 15
microns to about 70 microns, more preferably about 20 microns to about 50
microns, and
most preferably about 25 microns.
Because of the unusually hard matrix of the pad, it may be relatively
inflexible.
Therefore, after molding has been completed, the pad may be provided with
holes to
increase its flexibility. Where holes are used to increase pad flexibility,
they preferably
pass all the way through the pad from the working side to the mounting side,
and the size
of the holes are preferably in the range of 1/16 inch to 1/4 inch in diameter,
with the 1/4
inch holes being preferably spaced 1/2 inch apart and the 1/16 holes being
preferably
spaced about 1/4 inch apart.
- 14-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
The pads of the present invention are especially amenable to grooving to
provide a
grooved polishing pad that is capable of consistently forming uniformly
polished surfaces
on high quality wafers. The apparatus for grooving a pad may comprise a platen
with
positioning post for holding the pad in position for engagement by a router to
machine
grooves in the working surface of the pad. In order to precisely control the
depth of the
grooves as they are routed in the pad, a spacing mechanism may be used to
provide a
constant and precise separation between the working surface of the pad and the
chuclc for
holding and rotating the router. An apparatus of this type is described in
U.S. Patent
Application Serial No. 09/605,69, filed June 29, 2000, for a "Polishing Pad
Grooving
Method and Apparatus", the entire contents of this application being
incorporated herein
by reference. Whereas the fibers of prior art pads are often frayed by such
grooving
processes, the fibers of the present pads, whether precoated or not, do not
sustain
significant fraying during the grooving process.
The present pad design therefore offers a versatility of properties and
performance
required to give a high degree of planarization and global uniformity to a
variety of
polished substrates. The pads of the present invention can be used for
polishing
aluminum and aluminum alloys such as Al-Si and Al-Cu, Cu, Cu alloys, W, W
alloys, a
variety of adhesion and diffusion barriers such as Ti, Ti alloys, TiN, Ta, Ta
alloys, TaN,
Cr and the like, silicon oxide, polysilicon, silicon nitride, Au, Au alloys,
as well as other
metals and alloys, and glasses of various compositions.
The polishing slurries employed can be any of the known CMP slurries.
Particular
examples are alumina in deionized water, or an acidic composition having a pH
less than
3 obtained by the addition of hydrofluoric or nitric acid to the alumina and
water slurry;
and slurries with pH 3 or greater, including basic slurries having a pH above
7.
- IS -
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
An embodiment, suitable for the semiconductor industry, is a substantially
cylindrical pad having general dimensions such that it might be used in a
polishing
apparatus, for example in the equipment described in the IBM Technical
Disclosure
Bulletin, Vol. 15, No. 6, November 1972, pages 1760-1761, the entire contents
of which
are incorporated herein by reference.
As an alternative embodiment, the polishing apparatus includes a polishing
station
having a rotatable platen on which is mounted a polishing pad, such as
illustrated
diagrammatically in Fig. 14 of Provisional Application Serial no. 60/214,774,
referred to
above. The pad in this embodiment is preferably about 10 to about 36 inches,
more
preferably about 24 inches in diameter, the latter being capable of polishing
"eight-inch"
or "twelve-inch" semiconductor wafers. The platen typically rotates the pad at
speeds
from 30 to 200 revolutions per minute, though speeds less than and greater
than this range
may be used. Semiconductor wafers are typically mounted on a rotatable carrier
head
using a vacuum chuck. The head presses the wafer against the pad causing
polishing, for
example with 1 to 10, preferably ~ to 8 pounds per square inch pressure, but
greater or
lesser pressures could also be used. The rate of polishing is controlled by
the
composition of the slurry, the rotation rates of the head and platen, and the
contact
pressure.
Polishing tests on Cu revealed that pads of the present invention provided
excellent results that are not obtainable with currently available pads.
-16-
CA 02441419 2003-05-20
WO 02/100593 PCT/USO1/43208
The foregoing description of the invention illustrates and describes only the
preferred embodiments of the present invention. However, as mentioned above,
it is to
be understood that the invention is capable of being made and used in various
other
combinations, modifications, and environments, and is capable of being changed
or
modified within the scope of the inventive concept as expressed herein,
commensurate
with the above teachings and/or the skill or knowledge of persons skilled in
the relevant
art. The embodiments described hereinabove are further intended to explain the
best
modes known of practicing the invention and to enable others skilled in the
art to utilize
the invention in such, or other, embodiments and with the various
modifications required
by the particular applications or uses of the invention. Accordingly, the
description is not
intended to limit the invention to the form disclosed herein. Also, it is
intended that the
appended claims be construed to include alternative embodiments. .
-17-
