U.S. patent application number 10/460620 was filed with the patent office on 2003-11-13 for slurry and method for chemical mechanical polishing of copper.
Invention is credited to Cadien, Kenneth C., Feller, A. Daniel, Miller, Anne E..
Application Number | 20030211745 10/460620 |
Document ID | / |
Family ID | 24873393 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030211745 |
Kind Code |
A1 |
Miller, Anne E. ; et
al. |
November 13, 2003 |
Slurry and method for chemical mechanical polishing of copper
Abstract
A copper polish slurry, useful in the manufacture of integrated
circuits generally, and for chemical mechanical polishing of copper
and copper diffusion barriers particularly, may be formed by
combining a chelating, organic acid buffer system such as citric
acid and potassium citrate; and an abrasive, such as for example
colloidal silica. Alternative copper polish slurries, in accordance
with the present invention, may be formed by further combining an
oxidizer, such as hydrogen peroxide, and/or a corrosion inhibitor
such as benzotriazole. Advantageous properties of slurries in
accordance with the present invention include the enhancement of Cu
removal rates to >3000 angstroms per minute. This high polish
rate is achieved while maintaining local pH stability and
substantially reducing global and local corrosion as compared to
prior art copper polish slurries. Local pH stability provides for
reduced within-wafer non-uniformity and reduced corrosion defects.
Furthermore, copper diffusion barriers such as tantalum or tantalum
nitride may also be polished with such slurries wherein the
oxidizer is not included.
Inventors: |
Miller, Anne E.; (Portland,
OR) ; Feller, A. Daniel; (Portland, OR) ;
Cadien, Kenneth C.; (Portland, OR) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD, SEVENTH FLOOR
LOS ANGELES
CA
90025
US
|
Family ID: |
24873393 |
Appl. No.: |
10/460620 |
Filed: |
June 11, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10460620 |
Jun 11, 2003 |
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10193794 |
Jul 11, 2002 |
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10193794 |
Jul 11, 2002 |
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09715282 |
Nov 16, 2000 |
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Current U.S.
Class: |
438/692 ;
257/E21.304; 257/E21.583 |
Current CPC
Class: |
C09G 1/02 20130101; H01L
21/7684 20130101; C23F 3/00 20130101 |
Class at
Publication: |
438/692 |
International
Class: |
H01L 021/302; H01L
021/461 |
Claims
What is claimed is:
1. A method of forming copper interconnect, comprising: forming a
copper diffusion barrier layer in at least a damascene structure;
forming a copper layer over the barrier layer; removing a portion
of the copper layer by chemical mechanical polishing with a slurry
comprising a chelating organic acid buffer system, colloidal
silica, and an oxidizer.
2. The method of claim 1, wherein the oxidizer comprises hydrogen
peroxide.
3. The method of claim 2, wherein the chelating organic acid buffer
system comprises citric acid and potassium citrate.
4. The method of claim 3, wherein the slurry further comprises a
corrosion inhibitor.
5. The method of claim 4, wherein the corrosion inhibitor comprises
benzotriazole.
6. A method of forming copper interconnect, comprising: forming a
barrier layer over a substrate having at least one trench therein;
forming a copper seed layer on the surface of the barrier layer;
forming a copper layer over the barrier and seed layers; removing a
portion of the copper layer by chemical mechanical polishing with a
first slurry comprising a chelating organic acid buffer system,
colloidal silica, and an oxidizer; and removing at least a portion
of the barrier layer by chemical mechanical polishing with a second
slurry comprising a chelating organic acid buffer system, and
colloidal silica; wherein the second slurry is formed without the
oxidizer.
7. The method of claim 6, wherein the barrier layer comprises
tantalum.
8. The method of claim 7, wherein the chelating organic acid buffer
system comprises citric acid and potassium citrate.
9. The method of claim 8, wherein the oxidizer comprises hydrogen
peroxide.
10. The method of claim 9, wherein the first slurry further
comprises a corrosion inhibitor.
11. The method of claim 10, wherein the first slurry has a pH in
the range of 3 to 6, and the corrosion inhibitor comprises
benzotriazole.
12. A slurry produced by the process comprising: combining citric
acid, potassium citrate, silica, hydrogen peroxide, and
benzotriazole.
13. The slurry produced by the process of claim 12, wherein a
concentration of citric acid is approximately 3 g/l, a
concentration of potassium citrate is approximately 3 g/l, a
concentration of silica is approximately 5 wt. %, a concentration
of hydrogen peroxide is approximately 3 wt. %, and a concentration
of benzotriazole is approximately 0.015 molar.
14. The slurry produced by the process of claim 13, further
comprising combining the citric acid, potassium citrate, silica,
hydrogen peroxide, and benzotriazole with water.
15. A slurry, comprising: approximately 3 grams/liter of citric
acid; approximately 3 grams/liter of potassium citrate;
approximately 5 wt. % silica; approximately 3 wt. % hydrogen
peroxide; approximately 0.015 molar benzotriazole; and the mixture
and reaction products thereof.
16. The slurry of claim 15, wherein the slurry has a pH in the
range of 3 to 6.
17. A slurry formed by the process of combining a organic acid, an
organic acid salt; approximately 5 wt. % silica; approximately 3
wt. % hydrogen peroxide; and approximately 0.015 molar
benzotriazole.
18. The slurry of claim 17, wherein the organic acid comprises
acetic acid.
19. The slurry of claim 18, wherein the organic acid salt comprises
potassium acetate.
20. The slurry of claim 17, wherein the organic acid comprises 3
grams/liter of citric acid, and the organic acid salt comprises 3
grams/liter of potassium citrate.
21. A slurry for polishing copper diffusion barriers, comprising:
approximately 3 grams/liter of citric acid; approximately 3
grams/liter of potassium citrate; approximately 5 wt. % silica;
approximately 0.015 molar benzotriazole; and the mixture and
reaction products thereof.
22. The slurry of claim 21, wherein the copper diffusion barriers
comprise tantalum.
23. The slurry of claim 21, wherein the slurry has a pH in the
range of 3 to 6.
24. A slurry for polishing barriers comprised of tantalum,
comprising: organic acid, an organic acid salt, an abrasive, a
corrosion inhibitor, and the mixture and reaction products thereof,
and wherein no oxidizer is included.
25. The slurry of claim 24, wherein the organic acid comprise
citric acid.
26. The slurry of claim 24, wherein the corrosion inhibitor
comprises benzotriazole, and wherein the slurry has a pH in the
range of 3 to 6.
27. The slurry of claim 25, wherein the organic acid salt comprises
potassium citrate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to the field of
chemical mechanical polishing (CMP), and more specifically, to
methods and chemistries for providing increased metal polish
rates.
[0003] 2. Background
[0004] Advances in semiconductor manufacturing technology have led
to the development of integrated circuits having multiple levels of
interconnect. In such an integrated circuit, patterned conductive
material on one interconnect level is electrically insulated from
patterned conductive material on another interconnect level by
films of material such as, for example, silicon dioxide. These
conductive materials are typically a metal or metal alloy.
Connections between the conductive material at the various
interconnect levels are made by forming openings in the insulating
layers and providing an electrically conductive structure such that
the patterned conductive material from different interconnect
levels are brought into electrical contact with each other. These
electrically conductive structures are often referred to as
contacts or vias.
[0005] Other advances in semiconductor manufacturing technology
have lead to the integration of millions of transistors, each
capable of switching at high speed. A consequence of incorporating
so many fast switching transistors into an integrated circuit is an
increase in power consumption during operation. One technique for
increasing speed while reducing power consumption is to replace the
traditional aluminum and aluminum alloy interconnects found on
integrated circuits with a metal such as copper, which offers lower
electrical resistance. Those skilled in the electrical arts will
appreciate that by reducing resistance, electrical signals may
propagate more quickly through the interconnect pathways on an
integrated circuit. Furthermore, because the resistance of copper
is significantly less than that of aluminum, the cross-sectional
area of a copper interconnect line, as compared to an aluminum
interconnect line, may be made smaller without incurring increased
signal propagation delays based on the resistance of the
interconnect. Additionally, because the capacitance between two
electrical nodes is a function of the overlap area between those
nodes, using a smaller copper interconnect line results in a
decrease in parasitic capacitance. In this way, replacing aluminum
based interconnects with copper based interconnects provides,
depending on the dimensions chosen, reduced resistance, reduced
capacitance, or both.
[0006] As noted above, copper has electrical advantages, such as
lower resistance per cross-sectional area, the ability to provide
for reduced parasitic capacitance, and greater immunity to
electromigration. For all these reasons, manufacturers of
integrated circuits find it desirable to include copper in their
products.
[0007] While advantageous electrically, copper is difficult to
integrate into the process of making integrated circuits. As is
known in this field, copper can adversely affect the performance of
metal oxide semiconductor (MOS) field effect transistors (FETs) if
the copper is allowed to migrate, or diffuse, into the transistor
areas of an integrated circuit. Therefore copper diffusion barriers
must be used to isolate copper metal from those transistor areas.
Additionally, unlike aluminum based metal interconnect systems
which are formed by subtractive etch processes, copper
interconnects are typically formed by damascene metal processes.
Such processes are also sometimes referred to as inlaid metal
processes. In a damascene process, trenches are formed in a first
layer, and a metal layer is formed over the first layer including
the trenches. Excess metal is then polished off, leaving individual
interconnect lines in the trenches. The removal of excess copper is
typically accomplished by chemical mechanical polishing. Although
there are many known variations of the damascene method of
metallization, the most common method for removing the excess
copper is by CMP.
[0008] Accordingly, there is a need for CMP methods, materials, and
apparatus to polish conductive materials such as copper.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic cross-sectional view of a copper
damascene structure. This structure represents a post-plating,
pre-polishing state of fabrication.
[0010] FIG. 2 is a flowchart showing the operations in a process of
forming a slurry in accordance with the present invention
[0011] FIG. 3 is a flowchart showing the operations in a process of
polishing a thin film in accordance with the present invention.
[0012] FIG. 4 is a flowchart showing the operations in a process of
polishing a thin film in accordance with the present invention.
DETAILED DESCRIPTION
[0013] Methods and slurries for the chemical-mechanical polishing
of copper are described. In the following description numerous
specific details are set forth to provide an understanding of the
present invention. It will be apparent, however, to those skilled
in the art and having the benefit of this disclosure, that the
present invention may be practiced with apparatus and processes
that vary from those of the illustrative examples provided
herein.
[0014] Terminology
[0015] The terms, chip, integrated circuit, monolithic device,
semiconductor device or component, microelectronic device or
component, and similar terms and expressions, are often used
interchangeably in this field. The present invention is applicable
to all the above as they are generally understood in the field.
[0016] RPM (also rpm) refers to revolutions per minute.
[0017] Reference herein to "one embodiment", "an embodiment", or
similar formulations, means that a particular feature, structure,
or characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
the appearances of such phrases or formulations herein are not
necessarily all referring to the same embodiment. Furthermore,
various particular features, structures, or characteristics may be
combined in any suitable manner in one or more embodiments.
[0018] Overview
[0019] Polishing of copper metal layers in connection with the
formation of conductive interconnect lines for integrated circuits
is becoming more important for the semiconductor industry. Unlike
aluminum metallization, which is typically formed on integrated
circuits by subtractive metal etch, copper interconnect lines are
typically formed by way of a damascene, or inlaid, metal process.
Such a process requires the removal, typically by chemical
mechanical polishing, of the excess copper.
[0020] Several prior art slurries for chemical mechanical polishing
of copper have had problems associated with them. For example, one
such prior art slurry, based on a hard abrasive such as
Al.sub.2O.sub.3, tended to cause excessive scratching and had an
unpleasant odor. In another prior art example, a copper polish
slurry contained propianic acid and a silica abrasive but had
unsatisfactory characteristics with respect to corrosion,
scratching, and odor.
[0021] An exemplary copper polish slurry, in accordance with the
present invention, may be formed by combining a chelating, organic
acid buffer system such as citric acid and potassium citrate; and
an abrasive, such as for example colloidal silica and an oxidizer,
such as hydrogen peroxide (H.sub.2O.sub.2) Alternative copper
polish slurries, in accordance with the present invention, may be
formed by further combining a corrosion inhibitor such as
benzotriazole (BTA).
[0022] Advantageous properties of slurries in accordance with the
present invention include the enhancement of Cu removal rates to
>3000 angstroms per minute. Additionally, this high polish rate
is achieved while maintaining local pH stability and substantially
reducing global and local corrosion as compared to prior art copper
polish slurries. Those skilled in the art will appreciate that
local pH stability provides for reduced within-wafer non-uniformity
and reduced corrosion defects.
[0023] The Slurry
[0024] Slurries, in accordance with the present invention, include
a buffer system to increase the polish rate of a metal CMP system.
These slurries are formed by combining a chelating organic acid
buffer system such as citric acid and potassium citrate, with an
abrasive such as colloidal silica. If the metal to be polished is
copper or a copper alloy, then an oxidizer such as hydrogen
peroxide should be combined with the slurry mixture. It will be
appreciated by those skilled in the art that combining such
ingredients may be done in any appropriate container, and may
include mixing. Furthermore these ingredients may be combined
outside of a container, such as, for example on a polishing pad.
Alternative inventive slurries may be formed by further combining
the above with a corrosion inhibitor such as benzotriazole. Such
slurries are particularly useful for polishing copper, and copper
diffusion barriers.
[0025] An exemplary slurry, in accordance with the present
invention, for chemical mechanical polishing, has a pH of
approximately 3.8, and includes a SiO.sub.2 abrasive, a
H.sub.2O.sub.2 oxidizer, a benzotriazole corrosion inhibitor, and a
citric acid/potassium citrate buffer system. These ingredients are
combined, typically with water, to form the slurry. Those skilled
in the art will appreciate that the slurry is a mixture of these
ingredients, that various chemical reactions may occur amongst the
ingredients, and that the slurry may contain various mixture and
reaction products of the ingredients, including, but not limited
to, complexes and disassociated ionic species. In other words, the
slurry that results from combining, or mixing the ingredients, will
contain at equilibrium, or at such other conditions as it may be
subjected to, chemical constituents that arise by virtue of the
combination of the ingredients in accordance with the present
invention. It is noted that slurries in accordance with the present
invention may have a pH in the range of 3 to 6.
[0026] In one particular illustrative slurry, the citric
acid/potassium citrate buffer system is provided by including in
the slurry mixture approximately 3 g/l of citric acid and
approximately 3 g/l of potassium citrate.
[0027] An abrasive suitable for use in the embodiments of the
present invention is a precipitated SiO.sub.2. Precipitated
SiO.sub.2 is sometimes referred to in this industry as colloidal,
although this term, i.e., colloidal, is not a technically accurate
designation for this material. The illustrative slurry may contain
5 wt. % silica such as Klebesol 1498-50 (available from Rodel,
Inc., 3804 East Watkins Street, Phoenix, Ariz. 85034).
[0028] The illustrative slurry may further be formed from combining
hydrogen peroxide with the slurry mixture such that this oxidizer
comprises 3 wt. %. Benzotriazole may be combined with the slurry
mixture as the corrosion inhibitor. In the illustrative embodiment,
the slurry mixture includes 0.015M benzotriazole.
[0029] Method
[0030] In an embodiment of the present invention, a copper
damascene structure is polished to form individual interconnects.
FIG. 1 shows a copper damascene structure prior to the removal of
the excess copper and copper diffusion barrier layer. An interlayer
dielectric (ILD) layer is patterned to form ILD 102 on a surface of
a wafer as illustrated in the figure. ILD 102 has a thickness
represented by TILD in FIG. 1. A copper diffusion barrier 104 is
formed over the exposed surfaces of the wafer and ILD 102. Various
materials may be used as the copper diffusion barrier. Tantalum and
tantalum-nitride may each be used as copper diffusion barriers.
Typically, a copper seed layer is then formed on copper diffusion
barrier 104. A complete copper layer 106 is then formed, typically
by plating, over diffusion barrier 104. That portion of the copper
that is above the top surface 103 of ILD 102 is considered to be
excess. It can be seen by inspection of FIG. 1 that removal of the
excess copper will result in the formation of two separate
conductive interconnect structures.
[0031] An embodiment of the process of forming a slurry in
accordance with the present invention is illustrated in the flow
diagram of FIG. 2.
[0032] As shown in block 202 of FIG. 2, a chelating organic acid
buffer system and an abrasive are combined with water. In one
embodiment the chelating organic acid buffer system is citric acid
and potassium citrate, and the abrasive is colloidal silica. In
block 204 an oxidizer is combined with the previously described
mixture. In one embodiment the oxidizer is a low electrochemical
potential oxidizer such as hydrogen peroxide. In block 206 a
corrosion inhibitor is combined with the other ingredients
identified above. It will be understood by those skilled in the art
that specific order of introducing the ingredients to the slurry
mixture may be changed consistent with the present invention. The
present invention is not limited in terms of the order of combining
ingredients. For example, water and benzotriazole may be combined,
then the chelating buffer added, followed by an abrasive, and an
oxidizer.
[0033] An embodiment of the method of polishing a thin film on a
wafer, in accordance with the present invention, is described in
conjunction with FIG. 3.
[0034] As is well known, in a typical CMP system, a wafer is placed
face down on a rotating table covered with a polishing pad, which
has been coated with a slurry. A carrier, which may be attached to
a rotatable shaft, is used to apply a downward force against the
backside of the wafer. A retaining ring may be used to center the
wafer onto the carrier and to prevent the wafer from slipping
laterally. By applying the downward force, and rotating the wafer,
while simultaneously rotating a pad having slurry thereon, a
desired amount of material may be removed from the surface of a
thin film.
[0035] FIG. 3 shows a flow diagram of a process embodying the
present invention. At block 302, a slurry, having a chelating
organic acid buffer system in accordance with the present
invention, is prepared, delivered to, and dispensed onto, a
polishing pad. The slurry, as described above, may have a pH of
approximately 3.8. Then, as shown at block 304, a wafer with a
copper damascene structure formed thereon, is brought into contact
with the polishing pad. As shown at block 306 the copper damascene
structure is polished. Typical polishing conditions using an
orbital polisher (e.g., IPEC 576 Orbital Polisher from Speed-Fam
IPEC, 305 North 54.sup.th Street, Chandler, Ariz. 85226) are a down
force of approximately 3.75 psi, a spindle speed of approximately
310 rpm, a wafer rotational speed of approximately 19 rpm, a slurry
flow rate of approximately 130 ccm, and a delta P of 0.0 psi. Delta
P is the pressure difference exerted on the top and bottom of the
wafer and allows fine control of the rate at the edge of the wafer.
Stacked polishing pads such as the IC1000, with a Suba-4 sub-pad,
both made by Rodel, Inc. of 3804 East Watkins Street, Phoenix,
Ariz. 85034, may be used with the slurry to polish copper films.
Other commercially available polishing pads may be used with the
present invention, for example FX-9 pads available from Freudenberg
of Lowell, Mass.
[0036] Copper diffusion barriers, such as, for example, tantalum or
tantalum nitride, are also successfully polished with slurries and
polishing conditions in accordance with the present invention. In
particular, by leaving out the oxidizer-but including the chelating
organic acid buffer system tantalum based copper diffusion barriers
can be effectively polished.
[0037] A method of forming copper interconnect in accordance with
the present invention is described in conjunction with FIG. 4.
Referring to FIG. 4, a illustrative method includes forming a
copper diffusion barrier layer over a patterned ILD layer (402).
This ILD layer, patterned so as to have trenches and vias therein,
may be produced with any of the conventional methods of forming an
ILD for damascene metal processing. ILD layers may include any
suitable dielectric material, including but not limited to, silicon
oxide, fluorinedope silicon oxide, carbon-doped silicon oxide, and
ILD layers based on materials other than oxides of silicon, such
as, but not limited to organic polymers and porous inorganic
materials. In the illustrative embodiment of the present invention
a tantalum-based copper diffusion barrier is used. Such a barrier
layer may be made of tantalum or tantalum nitride. A copper seed
layer is then formed over the copper diffusion barrier layer (404).
Subsequently, a copper layer is electroplated over the seed layer
(406). The excess portion of the copper layer (as described above
with reference to FIG. 1) is then removed by chemical mechanical
polishing (408) with a slurry that includes a chelating organic
acid buffer system and a low electrochemical potential oxidizer.
Such a slurry may contain a citric acid/potassium citrate chelating
organic acid buffer system, along with hydrogen peroxide as the
oxidizer. An abrasive such as silica is also included in the
slurry. A corrosion inhibitor such as benzotriazole may also be
included in the slurry. As the copper layer is removed the
underlying diffusion barrier layer becomes exposed. The excess
portion of the barrier layer, i.e., that portion over the top
surface of the ILD, is then removed (410). The slurry chemistry is
modified such that the oxidizer is left out for removing the excess
portion of the diffusion barrier layer. In other words, a first
slurry formulation is used when beginning to polish the copper
layer, but a second slurry formulation, similar to the first except
for the presence of the oxidizer, is then dispensed to polish the
underlying tantalum-based diffusion barrier layer.
[0038] With respect to the illustrative embodiment of FIG. 4,
copper polishing and barrier layer polishing may be performed on
the same pad or on different pads. In the either scenario, copper
is polished until a predetermined end point is reached, either by
timing the polish, by detecting a change in CMP motor current, or
by any other suitable method. If both layers are to be polished on
the same pad, the slurry chemistry is modified either by dispensing
a second slurry without the oxidizer, or by simply turning off the
oxidizer dispenser if this was being delivered directly to the
polishing pad. If each layer is to be polished on separate pads,
then when the desired endpoint is detected, the wafer may be moved
to a second pad to which the second slurry is delivered.
CONCLUSION
[0039] Embodiments of the present invention provide a slurry
suitable for chemical mechanical polishing of metals, such as, for
example, copper. Other embodiments of the present invention provide
methods for forming conductive interconnect lines in an integrated
circuit.
[0040] An advantage of some embodiments of the present invention is
that the chelating agent enhances the copper removal rate to
greater than 3000 angstroms per minute while using a low
electrochemical potential oxidizer such as hydrogen peroxide.
Compatibility with low electrochemical potential oxidizers reduces
the driving force for pitting and other forms of localized
corrosion. A further advantage of some embodiments of the present
invention is that in the presence of a citric acid buffer system,
the concentration of benzotriazole can be significantly increased
to control the static etch rate (sometimes referred to global
corrosion) without shutting down the polish rate.
[0041] A still further advantage of some embodiments of the present
invention is that the chelating, organic acid buffer enhances the
removal rate in the presence of a soft abrasive such as colloidal
SiO.sub.2.
[0042] A still further advantage of some embodiments of the present
invention is that the buffer system substantially ensures local pH
uniformity which, in turn, decreases within-wafer non-uniformity,
and also reduces local corrosion.
[0043] A still further advantage of some embodiments of the present
invention is that the chelating agent enhances the removal rate
over a wide pH range and can be used at high pH with a high pH
buffer system.
[0044] A still further advantage of some embodiments of the present
invention is that the chelating, organic acid buffer system, as
opposed to conventional slurries, has no substantial stability,
odor, health, or disposal issues associated therewith.
[0045] A still further advantage of some embodiments of the present
invention is that the ingredients of the slurry form a
cost-effective product.
[0046] A still further advantage of some embodiments of the present
invention is that an effective slurry for polishing Ta and TaN
(i.e., copper diffusion barriers), can be formed by combining the
chelating, organic acid buffer system with an abrasive and a
corrosion inhibitor, i.e. the oxidizer component need not be added
to the slurry.
[0047] It will be apparent to those skilled in the art that a
number of variations or modifications may be made to the
illustrative embodiments described above. For example, various
combinations, slurry pH, slurry delivery rate, pad rotation speed,
pad temperature, and so on, may be used within the scope of the
present invention.
[0048] Other modifications from the specifically described
apparatus, slurry, and process will be apparent to those skilled in
the art and having the benefit of this disclosure. Accordingly, it
is intended that all such modifications and alterations be
considered as within the spirit and scope of the invention as
defined by the subjoined claims.
* * * * *