U.S. patent application number 13/718389 was filed with the patent office on 2013-09-12 for methods and apparatus for recovery of silicon and silicon carbide from spent wafer-sawing slurry.
This patent application is currently assigned to IOSIL ENERGY CORPORATION. The applicant listed for this patent is IOSIL ENERGY CORPORATION. Invention is credited to John Allan Fallavollita.
Application Number | 20130236387 13/718389 |
Document ID | / |
Family ID | 41162660 |
Filed Date | 2013-09-12 |
United States Patent
Application |
20130236387 |
Kind Code |
A1 |
Fallavollita; John Allan |
September 12, 2013 |
Methods and Apparatus for Recovery of Silicon and Silicon Carbide
from Spent Wafer-Sawing Slurry
Abstract
Methods, systems, and apparatus are disclosed herein for
recovery of high-purity silicon, silicon carbide and PEG from a
slurry produced during a wafer cutting process. A
silicon-containing material can be processed for production of a
silicon-rich composition. Silicon carbide and PEG recovered from
the silicon-containing material can be used to form a wafer-saw
cutting fluid. The silicon-rich composition can be reacted with
iodine containing compounds that can be purified and/or used to
form deposited silicon of high purity. The produced silicon can be
used in the photovoltaic industry or semiconductor industry.
Inventors: |
Fallavollita; John Allan;
(EDMONTON, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IOSIL ENERGY CORPORATION |
Groveport |
OH |
US |
|
|
Assignee: |
IOSIL ENERGY CORPORATION
Groveport
OH
|
Family ID: |
41162660 |
Appl. No.: |
13/718389 |
Filed: |
December 18, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13175802 |
Jul 1, 2011 |
8354088 |
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13718389 |
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12865989 |
Nov 24, 2010 |
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PCT/US09/40261 |
Apr 10, 2009 |
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13175802 |
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61044342 |
Apr 11, 2008 |
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61148033 |
Jan 28, 2009 |
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Current U.S.
Class: |
423/349 |
Current CPC
Class: |
Y02P 70/10 20151101;
B28D 5/007 20130101; B24B 57/00 20130101; C01B 33/027 20130101;
Y02P 70/179 20151101 |
Class at
Publication: |
423/349 |
International
Class: |
C01B 33/027 20060101
C01B033/027 |
Claims
1. A process for recovering silicon comprising: (a) separating
iron-containing particles from a cutting slurry using at least one
physical separation device, thereby producing a slurry product; (b)
removing liquid from said slurry product, thereby producing a
powder mixture of silicon carbide and silicon; (c) providing said
powder mixture to a first vessel containing silicon tetra-iodide,
thereby producing a vapor comprising silicon di-iodide; and (d)
providing said vapor comprising silicon di-iodide to a second
vessel, wherein said deposited silicon is formed from the silicon
di-iodide.
2. The process of claim 1 further comprising purifying and
recycling remaining silicon tetra-iodide from said second
vessel.
3. The process of claim 1 further comprising e) recovering said
iron-containing particles from step a).
4. The process of claim 1 further comprising f) adding a carrier
gas to said first vessel in step d) to adjust the flow rate of the
vapor-gas mixture.
5. The process of claim 1 further comprising recovering silicon
carbide particles from said first vessel.
6. The process of claim 1 further comprising recovering at least
one of glycol, oil or water from said slurry product.
7.-27. (canceled)
28. A method for forming silicon, comprising: (a) providing iodine
and a mixture of silicon carbide and silicon to a first vessel to
form a vapor comprising silicon tetra-iodide and/or silicon
di-iodide in the first vessel, wherein said mixture is recovered
from silicon saw kerf; (b) providing said vapor comprising silicon
tetra-iodide and/or silicon di-iodide to a second vessel; and (c)
forming, under vacuum, silicon and iodine (I.sub.2) from the
silicon tetra-iodide and/or silicon di-iodide in the second
vessel.
29. The method of claim 28, wherein the pressure of said second
vessel is substantially less than atmospheric pressure.
30. The method of claim 28, wherein the first vessel is operated at
a temperature from about 600.degree. C. to 900.degree. C.
31. The method of claim 28, wherein the second vessel is operated
at a temperature from about 900.degree. C. to 1300.degree. C.
32. The method of claim 28, further comprising purifying and
recycling remaining silicon tetra-iodide from said second
vessel.
33. The method of claim 28, further comprising recovering silicon
carbide particles from said first vessel.
34. The method of claim 28, wherein said silicon in (c) has a
purity of at least about 99.9999%.
35. The method of claim 28, wherein, in (c), said silicon is
deposited onto granules in said second vessel.
36. The method of claim 28, further comprising, prior to (a):
removing iron-containing particles from said silicon saw kerf to
produce a slurry product; and removing liquid from said slurry
product to produce said mixture of silicon carbide and silicon.
37. The method of claim 36, further comprising recovering said
iron-containing particles.
38. The method of claim 28, further comprising recovering silicon
carbide particles from said first vessel.
39. The method of claim 28, further comprising recovering at least
one of glycol, oil or water from said slurry product.
Description
CROSS-REFERENCE
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/175,802, filed Jul. 1, 2011, which is a
continuation of U.S. patent application Ser. No. 12/865, 989, filed
Nov. 24, 2010, which is national stage entry of PCT/US2009/40261,
filed Apr. 10, 2009, which claims priority to U.S. Provisional
Application No. 61/044, 342, filed Apr. 11, 2008 and U.S.
Provisional Application No. 61/148,033, filed Jan. 28, 2009, which
are all incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to methods and systems for recovering
silicon and silicon-containing compounds from spent slurry that is
generated during wafer cutting or sawing operations in the
microelectronic (ME) and photovoltaic (PV) industries. The
invention relates to methods and systems that can produce a variety
of useful products, including granular silicon products of high or
increased purity. The products, such as granular silicon products
of high or increased purity, can be suitable for multi-crystalline
ingot casting or replacing electronic grade silicon (EG-Si) for
single-crystal production in PV applications, and/or
high-efficiency single-crystal solar cells. Other products can
include fine silicon carbide abrasive powders and an associated
carrier liquid for reuse in the wafer sawing process.
BACKGROUND OF THE INVENTION
[0003] The market demand for solar energy collection systems in the
form of photovoltaic (PV) cells is growing in excess of 25% per
year globally due to factors including higher oil prices and
government policies addressing such environmental issues as global
warming. The dominant substrate material for PV is silicon, which
accounts for about 90% of installed commercial units at the present
time. A serious shortcoming in the silicon-based PV value chain,
however, is that there is a loss of around 40-50% of the silicon
during the wafer cutting process. This situation also exists in the
interconnected microelectronics (ME) silicon value chain.
[0004] The current process for developing a PV cell is a multi-step
chain of value-added activities, transforming basic silicon into a
power-generating device. With each step, silicon is refined and
shaped to enable placement into a solar cell. However, this value
chain is not without inefficiencies. During the critical step where
silicon ingots are sawed into thin wafers, roughly 40% of the
original ingot ends up as spent (or waste) kerf slurry resulting
from the most prevalent steel-wire-saw technology using SiC powder
in polyethylene glycol (PEG 200).
[0005] The spent slurry product from the wafer cutting process
generally consists of very fine solid particles within a liquid
phase. The solid particles are irregular shaped and consist mostly
of silicon carbide of between 15-20 micrometers effective diameter.
The remaining particles are from the steel wire saw and silicon
wafer. The steel particles may be associated with the silicon
carbide particles and are generally less than 2-4 micrometers in
effective diameter. The silicon particles are generally free of
silicon carbide and in the particle size range of 1-2 micrometers.
During the wire sawing operation the silicon carbide starting
material is slightly abraded and smaller particles in the range of
5-10 micrometers are formed over time.
[0006] Therefore, while a raw material silicon shortage exists
today for the PV industry that is driving prices toward the
electronic-grade silicon (EG-Si) level, about half of all silicon
produced for the ME and PV industries is being landfilled.
[0007] Although the silicon particles lost during this step are of
the same purity as the original ingot, there exist no commercially
viable technologies to recover and reuse this silicon. The main
reason for this state of art is that the spent slurry can be a very
complex, colloidal mixture of extremely small particles in the
range of 0.1 to 30 .mu.m--with the silicon portion being less than
about 2-5 .mu.m in effective diameter (comparable to the size of
bacteria). Efforts to physically separate these silicon particles
from the mixture are severely hampered by wire-saw particle
impurities (mostly iron, copper, and zinc) that prevent the
attainment of the original ingot purity. Even if it were possible
to completely remove the wire-saw particles from the slurry by
physical means, the remaining ultrafine silicon powder is both
dangerous to handle (due to potential dust explosions) and
extremely difficult to melt using conventional furnace
technology.
[0008] The effect of this market need on the overall economics of
the PV industry is significant. It has been well-documented that
the solar industry has suffered from a major silicon feedstock
shortage since 2005..sup.1 During these past 4 years, more than 40%
of the >100,000 tonnes of silicon produced during this timeframe
was discarded due to the inability to recycle polysilicon. This
inefficient use of a critical PV cell building block resulted in a
cumulative economic loss to the solar industry of at least $2
Billion over the period 2005-2008..sup.2 Moreover, given that the
cost of silicon feedstock comprises almost 20% of a PV cell's total
cost.sup.3, discarding approximately 40% of the feedstock has been
an important contributor to the economics preventing grid-parity
and broader adoption of PV cells. .sup.1Travis Bradford,
"Polysilicon: Supply, Demand & Implications for the PV
Industry," Greentech InDetail, (Jun. 25, 2008) [Prometheus
Institute], Pg. 24..sup.2During 2005-2008 period, average
polysilicon production for PV was 25K tonnes/yr., and average
contract price was $50/kg..sup.3Bradford, "Polysilicon: Supply,
Demand & Implications for the PV Industry,"," Greentech
InDetail, (Jun. 25, 2008) [Prometheus Institute], Pg. 29.
[0009] Therefore, there is a need to recover silicon in a form and
purity suitable for reuse within the silicon-wafer based PV
industry. U.S. Provisional Patent Application No. 61/044,342, filed
Apr. 11, 2008, incorporated herein by reference in its entirety,
describes a multistep process for recovering silicon granules from
spent wafer-sawing operations. The process described therein can
include a 2-stage iodine-catalysed reaction sequence that can
operate at temperatures between 800-1300.degree. C. to produce a
granular silicon product. The purity of silicon recovered can reach
99.9999 wt % (i.e., 6 nines or 6N) and possibly higher levels under
certain operating conditions.
[0010] However, for the highest efficiency PV cells in use today it
may be preferable to utilize a higher-purity silicon. For instance,
it may be desirable to obtain a silicon purity of 8N (i.e.,
99.999999 wt %).
[0011] Therefore, there remains a need in the art for commercial
operations that can efficiently separate the silicon particles from
the remainder of the slurry mixture. Furthermore, there exists a
need for ways of converting these fine silicon particles into a
useable form for application in the commercial production of
semiconductor devices such as photovoltaic solar cells. Also, there
remains a need in the art for commercial operations that can
recover and/or purify silicon to increased purities (e.g., 8N) from
various sources, such as the spent wafer-sawing slurry produced in
the PV and ME industries.
SUMMARY OF THE INVENTION
[0012] The invention provides methods, systems, and apparatus for
generating and/or recovering one or more silicon-containing
products from spent silicon wafer wire sawing slurry. Varying
grades of high or increased purity silicon (i.e., up to 7N to 10N
and higher) can be produced at high throughputs and low or
competitive cost with the processes and apparatus disclosed herein.
Various aspects of the invention described herein may be applied to
any of the particular applications set forth below or for any other
types of silicon purifying applications. The invention may be
applied as a standalone system or method, or as part of an
integrated silicon product manufacturing process. It shall be
understood that different aspects of the invention can be
appreciated individually, collectively, or in combination with each
other.
[0013] Disclosed herein are methods and apparatus for the
production of polycrystalline silicon granules and recovery of
silicon carbide particles from various sources. The various sources
can be spent slurry such as those generated during wire-sawing
processes used in the microelectronics (ME) and photovoltaic (PV)
industries.
[0014] Some embodiments of the invention provide purification
systems and methods that can perform one or more of the following:
(1) separation of wire-saw steel particles from the slurry using
one or more series of physical separation devices (for example,
magnets or electromagnets); (2) recovery of the slurry product and
subsequent removal of the liquid phase (either glycol-water or oil)
to produce a moist fine powder mixture of silicon carbide and
silicon; (3) complete removal of the remaining moisture or oil
using a liquid-to-gas phase separation of the powder mixture that
can form a dry mixture of silicon and silicon carbide; (4)
subjecting a dry mixture of silicon and silicon carbide to a high
temperature reactor containing pure silicon tetra-iodide to produce
a vapor containing silicon di-iodide; (5) separating the silicon
carbide by gravity or filter devices from the vapor stream thus
created; (6) conducting the vapor phase to a second vessel that is
preferably a fluidized bed with substantially lower temperature and
depositing pure silicon onto granules in said reactor; and (7)
recycling and purifying silicon tetra-iodide in a distillation
column or other device to remove any impurities. These methods and
processes can be combined, switched, or modified with any other
methods or processes described herein for the recovery and
production of silicon, silicon carbide, and PEG from a wafer-sawing
process. The methods and processes for recovering silicon, silicon
carbide, and PEG can be implemented in any order.
[0015] Other embodiments of the invention provide purification
systems and methods, which can be used to produce high purity
silicon, that can perform one or more of the following: (1)
separation of large silicon carbide particles (e.g., particles
greater than 5 micrometers in effective diameter) by means of
gravity separation methods with or without the aid of centrifugal
forces of different magnitude (e.g., settling tank, clarifier,
hydro-cyclone, centrifuge, filter, and hydraulic classifier that
uses additional convective flow to effect separation); (2) removal
of wire-saw steel particles from the slurry using one or more
series of magnetic separation devices (e.g., magnets or
electromagnets); (3) performing leaching, e.g., by reacting the
steel-depleted slurry with an acidic solution so as to further
reduce the content of steel; (4) removal of a liquid phase (e.g.,
PEG) to produce a moist fine powder mixture (e.g., less than 5
percent liquid) of enriched-silicon with only minor amounts of
steel and smaller-sized (e.g., less than 5 micrometers in effective
diameter) silicon carbide particles; (5) drying of the remaining
solids to effect virtually complete removal of liquid; (6)
subjecting said dry mixture of enriched-silicon to a heated reactor
containing pure iodine vapor at between about 600-800.degree. C. to
produce a vapor containing mostly silicon tetra-iodide and only a
very small amount of impurity iodides; (7) cooling the vapor phase
and conducting it to a purification unit that is preferably a
distillation column to remove impurities in the silicon
tetra-iodide; (8) collecting the purified silicon tetra-iodide and
then subjecting it to temperatures in the range of about
800-1300.degree. C. in a fluidized bed operating under vacuum
whereupon the silicon tetra-iodide is decomposed into pure silicon
and iodide vapor; and (9) recycling the iodine vapor to the
process. These methods and processes can be combined, switched, or
modified with any other methods or processes described herein for
the recovery and production of silicon, silicon carbide, and PEG
from a wafer-sawing process. The methods and processes for
recovering silicon, silicon carbide, and PEG can be implemented in
any order.
[0016] Other goals and advantages of the invention will be further
appreciated and understood when considered in conjunction with the
following description and accompanying drawings. While the
following description may contain specific details describing
particular embodiments of the invention, this should not be
construed as limitations to the scope of the invention but rather
as an exemplification of preferable embodiments. For each aspect of
the invention, many variations are possible as suggested herein
that are known to those of ordinary skill in the art. A variety of
changes and modifications can be made within the scope of the
invention without departing from the spirit thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The novel features of the invention are set forth with
particularity in the appended claims. A better understanding of the
features and advantages of the present invention will be obtained
by reference to the following detailed description that sets forth
illustrative embodiments, in which the principles of the invention
are utilized, and the accompanying drawings of which:
[0018] FIG. 1 is a schematic diagram of the apparatus illustrating
the flow of materials for the commercial production of silicon and
recovery of silicon carbide from a spent wafer cutting slurry.
[0019] FIG. 2 is a schematic diagram of the apparatus illustrating
how large silicon carbide particles can be recovered in a discrete
step within the process described in FIG. 1.
[0020] FIG. 3 is a schematic diagram of an exemplary apparatus
provided in accordance with the invention illustrating the flow of
materials for the recovery of silicon, silicon carbide and PEG
(polyethylene glycol) from a spent wafer cutting slurry and the
production of high or increased purity silicon.
INCORPORATION BY REFERENCE
[0021] All publications and patent applications mentioned in this
specification are herein incorporated by reference to the same
extent as if each individual publication or patent application was
specifically and individually indicated to be incorporated by
reference.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The invention provides methods and systems for recovering
silicon, silicon carbide, and cutting fluids, for producing high
purity silicon from spent slurry generated in various industrial
processes. In particular, the invention may be applied to spent
slurry from wafer-cutting operations in the microelectronics and
photovoltaic industries.
[0023] In an embodiment, the invention further provides processes
that are scalable to commercial capacity (for example, 50-5,000 or
500-5,000 tonne per year) for producing silicon suitable for use in
the photovoltaic industry.
[0024] In an aspect of the invention, economical, high through-put
methods of depositing pure silicon granules are provided that are
useful for applications in the continuous processes of leading PV
manufacturers using string ribbon or spherical cells.
[0025] In an aspect, an apparatus is disclosed that produces pure
granular silicon feedstock and silicon carbide powder. In an
embodiment, an apparatus comprises a system for recovering the
slurry liquid medium for reuse in the wafer cutting process.
[0026] In another aspect, a system, method or apparatus of the
invention can recover at least 60%, 70%, 80%, 85%, 90%, 95%, or 99%
of the silicon contained in a silicon-cutting slurry or a waste
slurry from an ingot cutting process. In an embodiment, 90% or more
of the silicon is recovered. The silicon can have a purity of at
least or at least about 99.9999%, 99.99999%, 99.999999%,
99.9999999%, 99.99999999%, or 99.999999999%. In other words, the
silicon may have a purity up to or greater than 6N, 7N, 8N, 9N,
10N, or 11N.
[0027] As shown in FIG. 1, the systems, methods, and apparatus of
the invention can comprise one or more separation steps (between
streams 1 and 7) configured to process a silicon-containing input
material (stream 1) to a silicon-rich stream (stream 7). These
separation steps can include any of the following separations:
magnetic, solid/liquid, solid/gas, gas/liquid, density,
sedimentation velocity, drying, or leaching. As shown in FIG. 1,
these separation steps can be used to recover or remove metals,
silicon carbides, liquids, for example PEG, water, or oil, from the
silicon-containing input material in various output streams, e.g.,
streams 2, 4, and 6. These streams can be metals-rich streams
(stream 2 in FIG. 1) and silicon carbide-rich streams (stream 3a
and 7a in FIG. 2).
[0028] The silicon-rich stream can be processed using any of the
systems, methods, or apparatus described herein. In some
embodiments of the invention, the silicon-rich stream can be
reacted with silicon tetra-iodide to produce a silicon di-iodide
rich stream in a first reactor (Silicon Reactor 1 in FIG. 1). The
silicon di-iodide rich stream can then be used to form deposited
silicon in a second reactor (Silicon Reactor 2 in FIG. 1).
[0029] In other embodiments of the invention, the silicon-rich
stream can be reacted with iodine to produce a silicon tetra-iodide
rich stream in a first reactor (Reactor 1 in FIG. 3). The silicon
tetra-iodide rich stream can be purified to form a high-purity
silicon tetra-iodide rich stream. For example, the silicon
tetra-iodide rich stream can be purified using a using a
distillation process (Distillation in FIG. 3). The high-purity
silicon tetra-iodide rich stream can be used to form deposited
silicon by reacting the purified to form silicon di-iodide in a
second reactor (Reactor 2 in FIG. 3).
[0030] Recovery of High Purity Silicon
[0031] Some aspects of the invention provide methods of producing
pure granular silicon feedstock by continuously feeding a
spent-slurry from a wafer cutting process into a first unit wherein
the steel particles from the slurry are substantially removed from
the slurry by means of a physical separator. In an embodiment, the
physical separator is a magnetic separator. The physical separator
can be any system that exploits the physical property difference
between iron-containing materials (for example, steel) and the
other slurry components. The iron-containing particles can be sent
to a recycling facility for substantial recovery of steel. The
iron-free slurry can be subjected to liquid-solid separation steps
that remove the liquid phase (either glycol-water or oil) for reuse
in the wafer cutting process. Iron-free slurry can refer to slurry
that has been subjected to a physical separator for removing
iron-containing particles. Iron-free slurry can also refer to
slurry that is completely, substantially, mostly, or somewhat free
of iron-containing particles.
[0032] A moist powder product from a method described herein can be
dried to remove all of the remaining liquid (for example, glycol,
water or oil). A drying step generally utilizes a moderate heating
of the material and/or reduction in pressure to effect the desired
removal of adhering liquid. In some embodiments, a dry powder
product comprises silicon and silicon carbide particles of sizes
ranging from about 1 to 20 micrometers.
[0033] In one embodiment of the invention, the powder mixture is
subjected to a temperature of about 1250.degree. C. and a gas phase
including some silicon tetra-iodide vapor. The powder mixture can
be subjected to temperatures of a range of about 1000 to about
1500.degree. C., wherein the silicon portion may be in either a
solid or a liquid form. Given enough residence time (for example,
about 1 minute) the silicon powder reacts with the iodide vapor to
produce substantial quantities of silicon di-iodide in the vapor
phase. For example, a residence time can be from about 5 seconds to
about 10 minutes. In an embodiment where a process is carried out
in a series of reactors including a cyclone or a porous ceramic
filter, silicon carbide particles typically are removed from the
process. In another embodiment, silicon di-iodide vapor is
transported to another reactor that is held at a temperature around
700-1000.degree. C. In this vessel, for example a fluidized bed
containing silicon seed particles such as vessels disclosed in
co-owned U.S. patent application Ser. No. 11/893,980, which is
incorporated by reference herein in its entirety, the silicon
di-iodide can be substantially converted back to silicon with a
purity similar to or about matching that of the original silicon
ingot used in the wafer cutting process. Any remaining silicon
tetra-iodide vapor can be therefore re-circulated in the process.
Silicon tetra-iodide vapor can also be periodically cleaned of any
impurities by distillation and/or other methods including a
solvent. In some embodiments of the invention, the silicon
tetra-iodide vapor is continuously purified using a distillation
process or other separation process to increase the purity of the
deposited silicon up to or greater than 6N, 7N, 8N, 9N, 10N, or
11N.
[0034] High Purity Silicon Iodides
[0035] In some embodiments of the invention, silicon and other
materials from the spent slurry are reacted with iodine to form
silicon tetra-iodide and other iodides. The iodides, including the
silicon tetra-iodide, can be separated from other iodides by a
variety of separation processes such as distillation, membrane
separations, chromatography, and other methods known to one skilled
in the art. In some embodiments of the invention, the silicon
tetra-iodide can be separated from other components using one or
more separation processes, including distillation-based,
temperature-based, or phase-based (e.g. solid/liquid, liquid/gas,
solid/liquid/gas, and/or solid/gas) separation processes at
low-pressure or vacuum. Crystallization, precipitation, and other
methods known to one skilled in the art can be used to separate or
increase the purity of the silicon tetra-iodide. The rate,
pressure, and temperature of the separation processes can be
optimized to increase the purity of the silicon tetra-iodide and/or
reduce corrosion or deterioration of the apparatus for performing
the separation processes. The purity of the silicon tetra-iodide
recovered after the one or more separation processes can be at
least or at least about 70, 80, 90, 95, 97, 99, 99.9%, 99.99%,
99.999%, 99.9999%, 99.99999%, 99.999999%, 99.9999999%, or
99.99999999%. The lifetime for the apparatus provided herein for
performing the one or more separation processes, e.g., a
distillation column or any other distillation device used for
separation silicon tetra-iodide, can be extended for decades or
extended periods of time. This can be performed by optimization of
the one or more separation processes.
[0036] Examples of a distillation process used to increase the
purity of silicon tetra-iodide are described in U.S. Pat. No.
6,712,908, herein incorporated by reference in its entirety.
Briefly, SiI.sub.4 can be separated from other iodides using a
distillation process. The other iodides can include BI.sub.3,
PI.sub.3, CI.sub.4, FeI.sub.2, and AlI.sub.3. FeI.sub.2 and
AlI.sub.3 may be separated from BI.sub.3, PI.sub.3, CI.sub.4, and
SiI.sub.4 in a vaporization step due to the lower relative vapor
pressure of FeI.sub.2 and AlI.sub.3. Once vaporized, SiI.sub.4 may
condense at a higher temperature than BI.sub.3 and PI.sub.3, and at
a lower temperature than CI.sub.4.
[0037] Recovery of SiC
[0038] In another embodiment of the invention the silicon carbide
particles in the slurry are separated into two fractions, one
containing mostly larger particles (for example, about 10-20
micrometer particles) and the other containing the fraction of
silicon carbide particles that are produced during wire cutting and
possessing a smaller particle size (for example, about 1-10
micrometers). For example, a separation step can be implemented
using a hydro-cyclone either after the physical separation step
(for example, using a magnet) or in an air-cyclone of appropriate
geometry after the drying step. Either type of cyclone is capable
of effectively separating most of the large sized silicon carbide
particles. An advantage of removing the large silicon carbide
particles before the high-temperature reaction steps is that less
heat input is required for the overall process.
[0039] In an alternative embodiment, if a spent slurry entering the
process description above is already in a dry condition and mostly
free of the liquid phase, a liquid with low oxygen concentration
and mechanical stirring devices may be used to create the slurry
that is treated according to the methods previously described.
[0040] Alternative Inputs
[0041] In yet another alternative embodiment, the input raw
material may be the waste from a slurry recovery process, wherein
the composition is mostly devoid of large silicon carbide
particles. This material may contain large amounts of steel and
silicon along with small amounts of small-diameter silicon carbide
particles and glycol, water or oil. This raw material would be
treated according to the methods described herein, however, in most
cases the removal of the large particle silicon carbide fraction is
not required.
[0042] Methods and Systems for Recovery of Silicon and a Wafer-Saw
Cutting Fluid
[0043] FIG. 1 illustrates a schematic diagram of the flow of
materials for the commercial production of silicon and recovery of
silicon carbide from a spent wafer cutting slurry of an apparatus.
It shall be understood that any one or more of the processes shown
in FIG. 1 can be implemented in any order and combinations thereof.
Referring generally to the example methods, apparatus, and systems
of FIG. 1, the spent slurry 1 from a wafer cutting operation is
added to a stirred tank wherein a liquid solution containing water
is added to create an appropriate viscosity for subsequent
processing. Mechanical energy through stirring and/or vibration is
used to adequately disperse particles in the stirred tank. The
dispersed slurry is then transported to a high gradient magnetic
separator or similar device that exploits substantial differences
in the physical properties of the steel particles wherein the
iron-containing particles are effectively removed and conducted to
a waste recycle stream 2. The iron-free slurry 3 is then pumped
into a liquid/solid separator. This unit may consist of a filter
press, centrifuge, hydro-cyclone or other solid-liquid separation
device that can operate with a particle size of between 1-20
micrometers.
[0044] As illustrated in the exemplary embodiment of FIG. 1, the
liquid stream 4 passing through the liquid/solid separator is
collected and later recombined with large silicon carbide particles
to form a fresh wire-saw cutting fluid for the wafer cutting
operation. The iron-free solid particles stream 5 obtained from the
liquid/solid separator are conveyed via a screw feeder or similar
device and dried by increasing the temperature and/or decreasing
the pressure in this unit thereby volatilizing the remaining liquid
phase. The collected liquid stream 6 is transported into a
collection vessel and can later be recombined with large silicon
carbide particles to form fresh wire-saw slurry for wafer cutting
operations. The dried particles stream 7 consisting of silicon and
silicon carbide is then injected into a gas-vapor stream through a
pressure-sealed valve into Silicon Reactor 1. The gas-vapor stream
typically consists of a carrier gas and silicon tetra-iodide in
varying volume ratios. The residence time of the particles and
vapor in this unit is generally less than 1 minute and the
temperature is kept above about 1100.degree. C., and preferably
between 1250-1500.degree. C.
[0045] Also in the example of FIG. 1, in Silicon Reactor 1 the
silicon particles react completely to form silicon di-iodide in the
gas-vapor phase. A cyclone or similar solid-gas separator can be
added as part of this system and can allow for the capture and
removal of silicon carbide particles in stream 8. The gas-vapor is
transported via stream 9 to the entrance of Silicon Reactor 2 which
consists of a fluidized bed or similar contacting device. The
associated silicon carbide particles entering Reactor 1 do not
generally react with silicon tetra-iodide. To avoid carry-over of
silicon carbide particles into stream 9, a ceramic filter may also
be added in-line to accomplish a final removal of this solid
material.
[0046] As an example, the gas-vapor stream 9 is injected either
into the dense phase of a fluidized bed (Silicon Reactor 2) or in
the entrance to the distributor plate of said fluidized bed as
illustrated in FIG. 1. Reactor 2 is maintained at a constant
temperature throughout its volume in the range of 700-1000.degree.
C. In this example, the silicon di-iodide vapor is preferably
deposited onto the particulate phase of the fluidized bed that
consists of silicon seed material. As the bed particles of silicon
grow into granules of about 0.5-10 millimeters (for example, 5 mm)
they are removed from the bed by appropriate mechanical means and
enter stream 11. The silicon granules are then cooled down to room
temperature and form the saleable product. The gas-vapor phase 10
that exits the fluidized bed can be recycled back to Silicon
Reactor 1. After many operation cycles of this type there can be a
tendency for impurity buildup in the gas-vapor phase; therefore,
some of the recycle stream 10 can be sent to a purification unit
that performs distillation and/or solvent extraction of the
impurities in the silicon tetra-iodide.
[0047] Methods and Systems with Reduced Heat Demand
[0048] Another exemplary embodiment of the invention as shown in
FIG. 2 illustrates the flow of materials for the recovery of large
silicon carbide particles from a process, such as the process
previously described. FIG. 2 shows an exemplary variation of the
process that is designed to improve or reduce the heat
requirements. As silicon carbide particles participate in the
Silicon Reactor 1 shown in FIG. 1, there can be a greater demand
for heat as these particles are brought to the operating
temperatures of 1100-1500.degree. C. Furthermore, since the silicon
carbide does not measurably react with the silicon tetra-iodide
vapor then it effectively acts as a "dead-load" in this unit. The
modifications to FIG. 1 shown in streams 3a and 7a can be used to
effectively reduce the heat demand on the process, if required.
[0049] In stream 3a a majority of the large silicon carbide
particles are removed with a hydrocyclone while in stream 7a these
particles are removed with an air-cyclone. Either of the cyclone
systems can be effective although the air-cyclone may be more
efficient at removing very small particles due to the larger
density difference between fluid and particles.
Example 1
[0050] Two replicate experiments were performed in a bench-scale
process system. The conditions used with apparatus (Reactor 1 and
Reactor 2) in FIGS. 1 and 2 for Runs S1-31-08-11-14 and
S1-31-08-12-05 are listed in Table 1 below.
TABLE-US-00001 TABLE 1 Run S1-31-08-11-14 Run S1-31-08-12-05 Raw
materials used in High purity iodine High purity iodine SiI4
production and silicon wafer and silicon wafer pieces pieces Si
Source material Treated industrial Treated industrial (Reactor 1)
kerf beads kerf beads Deposition zone bed Quartz slides Quartz
slides material (Reactor 2) Reactor 1 (.degree. C.) 1200 1200
Reactor 2 (.degree. C.) 900 900 Carrier gas Argon Argon
[0051] Kerf raw material from an industrial source was subjected to
a series of steps including magnetic separation, leaching,
solid/liquid separation, and drying. Table 2 shows a comparison of
the composition difference between Stream 1 and 7 in FIG. 1 using
GDMS analysis. The Boron and Phosphorous composition of silicon
product is given in Table 3. Finally, a representative sample of
the shape and size of silicon granules for run S1-31-08-12-05 is
shown in FIG. 3.
TABLE-US-00002 TABLE 2 Stream 1 Stream 7 Concentration
Concentration Element [ppm wt] [ppm wt] B 2.5 1.5 P 16 5.9 Fe ~0.4
wt % 240 Co 0.49 0.28 Ni 20 12 Cu 100 3 Zn 60 0.4 As 0.36 0.2 Zr 12
9 Mo 1.7 0.82
TABLE-US-00003 TABLE 3 Run S1-31-08-11-14 Run S1-31-08-12-05 Boron
(ppmw) 0.40 0.45 Phosphorous (ppmw) 0.26 0.57
[0052] Methods and Systems for Recovery of High-Purity Silicon
[0053] FIG. 3 illustrates a schematic diagram of the flow of
materials for recovery of silicon, silicon carbide and/or
polyethylene glycol (PEG) from a spent wafer cutting slurry and
production of high purity silicon. The steps shown in FIG. 3
include solid-liquid separation, magnetic separation, filtration,
leaching, drying, Reactor 1 (which can be reaction with iodine),
distillation, and Reactor 2 (which can be deposition). One or more
of the steps may be used in the separation process. The order of
separation processes do not necessarily have to be in the order
shown in FIG. 3. The separation processes can be in any order. For
example, a magnetic separation step can be before or after a
solid-liquid separation step. The separation processes can be
supplemented by additional separation processes known to one
skilled in the art and/or one or more of the separation processes
can be omitted.
[0054] Referring generally to FIG. 3, the spent slurry 1 from a
wafer cutting operation may be added to a solid-liquid separator or
series of separators. An additional liquid or separating agent 2
can be PEG, a liquid matching that of the input slurry, or any
other liquid or slurry can be added to optimize, increase, or
decrease the percent solids of the input slurry. The percent of
solids in the spent slurry can be raised or lowered to affect the
separation rate or efficiency of the solid-liquid separator or
other separation steps. The input spent kerf slurry 1 that enters
this process stage can have the following composition: about 40%
PEG, about 50% SiC fines (about 5-30 .mu.m), about 5% steel fines
(about 0.1-5 .mu.m), and about 5% silicon fines (about 0.1-5
.mu.m). The solids can contain about 80% SiC (quasi-Gaussian
particle size distribution (psd) with a volume arithmetic mean of
about 10 .mu.m and standard deviation of about 5 .mu.m), about
5-10% brass-coated steel particles (irregular psd; about 0.1-2
.mu.m effective diameter); and about 10-15% silicon particles
(irregular psd; about 0.1-2 .mu.m effective diameter).
[0055] An objective for this process stage, i.e., the solid-liquid
separation, can be to remove as much of the SiC particles as
possible without losing substantial amounts of Si. At least two
types of technologies can achieve or approach this goal: (a)
settling technologies using primarily gravity (advanced classifier;
thickener) or centrifugal force (hydrocyclone, centrifuge); and (b)
filtration using filters (not with screens or cross-flow units).
The settling and filtration steps may be used in tandem to achieve
the separation goal. The targeted output may be a slurry that
contains about 10-20 wt % solids with solids composition of
approximately 20 wt % SiC (psd; about <7 .mu.m diameter), 20-30
wt % steel ((psd; about 0.1-2 .mu.m diameter), and 50-60 wt %
silicon ((psd; about 0.1-2 .mu.m diameter).
[0056] Centrifuges can be used to separate SiC using methods known
to one skilled in the art. Generally, methods have been developed
to recover as much SiC as possible without regard to any
entrainment of Si. Any Si or steel that ends up in the product may
be leached with acids and/or bases to remove these "contaminants"
in the recycle stream of SiC. Thus there has not been a concerted
effort to optimize the recovery of both SiC and Si. However, a
centrifuging step to separate SiC may be incorporated into a
process that may provide for recovery of both SiC and Si. The
methods and apparatus of the present invention provide for the
recovery of both SiC and Si.
[0057] With regard to settling methods, Stokes' Law indicates that
the terminal velocity of a particle in a fluid is proportional to
d.sup.2, where d is the effective particle diameter, and 1/.mu.,
where .mu. is the liquid viscosity. This means that the SiC
particles (50% of the total) of .gtoreq.10 .mu.m may have terminal
velocities that are between 50-400 times larger than those of steel
and Si particles. This advantage may be somewhat diminished by the
fact that the viscosity of a PEG solution is about 50 times greater
than water. Hence, even though very good particle separation can be
obtained, the process time may be too slow for commercial use. In
some embodiments of the invention, ultrasound or ultrasonic
frequencies can be used or applied to the separator to reduce
settling times. Other additional vibrational energy can be used to
facilitate separation of SiC particles from other particles. For
example, the solution containing SiC and PEG can be heated to
reduce the viscosity of the solution, thus reducing the settling
time or increasing the terminal velocity of settling particles.
[0058] Referring to FIG. 3, the products from this process stage,
i.e., solid-liquid separation, are SiC-enriched slurry 3, and a
SiC-depleted slurry 4. Uses for the SiC-enriched slurry are
discussed in greater detail below. The SiC-depleted slurry 4 may
then be transported to a magnetic separator or a similar device
that can exploit substantial differences in the magnetic
susceptibility or magnetic properties of the steel particles where
the iron-containing particles may be effectively or substantially
removed and conducted to a waste recycle stream 6. The SiC-depleted
slurry can have about 5-10% (total solid mass basis) of SiC.
[0059] The solids content in this stage, i.e., magnetic separation,
may be adjusted by adding 5 PEG or liquid matching the slurry
liquid so that an acceptable separation (i.e., >90%) of steel is
achieved. Stream 6 can then be subjected to a controlled oxidation
by dilution in water to produce hydrogen gas. This gas can be
stored or flared or burned for production of energy to the plant.
Alternatively, the hydrogen gas can be supplied to a fuel cell to
generate electricity. The electricity can be used by the plant,
stored, or transferred. Any silicon particles entrained in stream 6
can be recovered by leaching of the remaining steel in an aqueous
solution with organic and/or inorganic acids. Economic
considerations can determine if this practice is utilized.
[0060] The magnetic separation step may also yield silicon-rich
slurry. The silicon-rich slurry 7 is transported into a filtration
stage wherein 85-90% of the PEG can be removed in stream 9 without
significant Si loss. Cake filters with pore sizes of <1 .mu.m
can be used. Other types of filters known or later developed in the
art, such as rotary drum filters and/or pressure filters, can be
used. PEG that passes through the filters can form stream 9.
Washing fluid 8 may be added to adjust the cake properties to
enhance separation efficiencies. Examples of washing fluids can
include distilled deionized water and/or organic liquids, e.g.,
isopropyl alcohol. The output stream 10 from the filtration step
can be rich in Si and substantively depleted of steel.
[0061] As illustrated in the exemplary embodiment of FIG. 3, the
liquid stream 3 passing through the liquid/solid separator, which
can be SiC rich, can be collected and later recombined with
filtrate 9, which can be a PEG solution, to form wire-saw cutting
slurry for the wafer cutting operation. Alternatively, liquid
stream 3 can be further purified to increase the purity of the SiC
and/or filtrate 9 can be further purified to increase the purity of
the PEG solution prior to being combined. The combined PEG solution
and SiC particles can be recycled to a wafer cutting device.
[0062] Referring to FIG. 3, further removal of the steel in stream
10 may be achieved by subjecting the solids therein to an acid
leaching stage using inorganic and/or organic acids of appropriate
concentration delivered by stream 11. Examples of acids that can be
used include HCl with a calcium chloride catalyst, HNO.sub.3,
H.sub.2SO.sub.4, and oxalic acid. A liquor stream 12 containing
soluble, steel contaminants can be discarded and a solids-rich
stream 13 can be pumped to a drying stage. Alternatively, the acids
in the liquor stream can be neutralized or reacted to form useful
products. Solids can be precipitated from the liquor stream 12,
gathered, and sold as scrap metal to steel mini mills. In this
stage, i.e., drying stage, the slurry can be first filtered and
then vacuum dried at elevated temperature with a gas, e.g., an
inert gas like argon or hydrogen, blanket flow 14. The drying stage
can be maintained at a temperature up to about 400.degree. C.,
greater than about 400.degree. C., at least about 200.degree. C.,
or at least about 300.degree. C. The dried slurry can become dried
particles and exit the dryer through stream 16. The off gas 15 can
contain moisture from the slurry.
[0063] The dried particles stream 16, which can include mostly
silicon with small amounts of silicon carbide and steel, can then
be injected into a gas-vapor stream through a pressure-sealed valve
into Reactor 1. In some embodiments of the invention, stream 16 can
be processed by a dry cyclone to remove solid silicon carbide or
other solid materials.
[0064] Alternatively, stream 16 entering Reactor 1 can be
metallurgical grade silicon. For instance, a stream 16 may be
provided with silicon and other components that may or may not have
undergone the previous steps. For example, any silicon with
impurities may be provided to Reactor 1. The metallurgical grade
silicon can be pulverized or ground to a particle size to increase
the effective surface area for reactions in Reactor 1.
[0065] Referring generally to FIG. 3, the dried particle stream or
metallurgical grade silicon 16 can be reacted with a gas-vapor
stream 17 in Reactor 1. Gas-vapor stream 17 can include a carrier
gas and iodine in varying volume ratios. Reactor 1 can be any type
of dilute-phase reactor, such as for example a Fast Fluidized Bed
(FFBR), preferably. The FFBR can contain a mixture of solids--inert
particles of a specific size plus the injected stream 16. The
velocity of gas-vapor 17 can be adjusted to ensure that the inert
particles circulate as they are carried up the vertical tube
section, pass through the attached cyclone and return to the
distributor plate of the FFBR. The residence time of the particle
stream 16 can be determined by the efficiency of the cyclone and it
is generally known that some smaller particles (e.g., about <1
.mu.m) may be transported past the cyclone. These very small
particles can be trapped in a filter consisting of either an
electrostatic precipitator or porous solid-gas ceramic filter with
average pore size less than about 1 .mu.m. The temperature of
Reactor 1 can be generally kept between about 600-900.degree. C.
Alternatively, the reactor can be maintained at higher temperatures
or cycled through high temperatures.
[0066] In this manner, most of the iodine can be reacted with
silicon to produce silicon tetra-iodide (SiI.sub.4) or silicon
di-iodide (SiI.sub.2). In some embodiments of the invention, the
reaction conditions can be such that the majority of the silicon
iodides are in the form of silicon tetra-iodide. In some
embodiments of the invention, the reaction conditions in Reactor 1
can be such that up to about, about, or greater than about 10, 30,
50, 70, 90, 95, or 99% of the silicon iodides in the product stream
are in the form of silicon tetra-iodide. Referring to FIG. 3, any
impurities in the stream 16 can either be converted to their
corresponding iodide vapor or compounds of silicon, e.g.,
iron-silicide. The silicides and other compounds of silicon can be
retained in the FFBR and grow in size and may need to be withdrawn
after a convenient time as shown by stream 18. The silicides and
other compounds of silicon can be withdrawn continuously or
periodically. The corresponding iodide vapors are transported along
with SiI.sub.4 to the next process stage via stream 19. The process
stream 19 can contain un-reacted iodine vapor, SiI.sub.4, impurity
iodide vapors and some inert gas.
[0067] A distillation step may be provided for improving the purity
of process stream 19. The distillation step can be prior to a
deposition step for forming deposited silicon from a stream
containing silicon iodides. For example, the process stream mixture
19 can be distilled in a continuous fashion in a column with
sufficient theoretical plates to remove iodide and low boiling
point impurity iodides (such as BI.sub.3) at the reflux end of the
column and higher boiling point iodides (such as AlI.sub.3) at the
re-boiler. SiI.sub.4 can be recovered between those two levels of
the column. The column many have any number of levels, stages, or
plates for the purification of the SiI.sub.4. The column may have
one or more recycle streams to improve the efficiency of the
separation process. Additional distillation columns and/or any
other distillation devices, e.g., vapor compression distillation
devices, known in the art may be used may also be used to further
purify the SiI.sub.4. Stream 20 and/or the column can be preferably
operated in a vacuum or low-pressure mode that reduces overall
temperature and heat requirements and thus can lead to less
corrosion of the column internals. Stream 20 can carry an inert gas
that assists with mixing within the column. The pressure in the
distillation column can be less than about 101.3 kPa (1 atm), 75,
kPa, 50 kPa, 25 kPa, or 5 kPa. The distillation column can be a
tray distillation column, a packed distillation column, a
vapor-compression distillation column or any other type of
distillation column known to one skilled in the art.
[0068] The impurity iodides can be removed through stream 21 and
processed in any manner known or later developed in the art. For
instance, the impurity iodides may be processed according to the
methods, devices and systems described in U.S. Provisional Patent
Application No. 61/044,342 and U.S. Patent Publication No.
20080044337, each incorporated herein by reference in their
entirety.
[0069] In addition to distillation, stream 19, carrying SiI.sub.4,
can be purified using vapor stripping and/or crystallization. Vapor
stripping can be performed by mixing an inert gas with liquid
SiI.sub.4 such that light iodides like boron iodide can be removed
by the inert gas stream. Crystallization can be performed by mixing
the liquid SiI.sub.4 with an organic liquid, such that the
SiI.sub.4 precipitates in the form of crystals. Seed crystals may
be used. The additional processes, e.g., vapor stripping and/or
crystallization, can be performed before or after the distillation
process.
[0070] The distillation and/or other processes may yield a
gas-vapor stream 22 comprising purified SiI.sub.4. The purified
SiI.sub.4 stream can have a purity of at least 7N, 8N, 9N, 10N,
11N, or greater. The purified SiI.sub.4 in gas-vapor stream 22 can
be injected either into the dense phase of a fluidized bed (Reactor
2) or in the entrance to the distributor plate of said fluidized
bed as illustrated in FIG. 3. Reactor 2 can be maintained at a
temperature throughout its volume in the range of between about
900-1300.degree. C. and at substantially less than atmospheric
pressure. Examples of fluidized bed reactors include those
described in U.S. Pat. No. 4,444,811, incorporated herein by
reference in its entirety. The fluidized bed reactor for silicon
deposition can be a bubbling or spouted-bed type fluidized bed
reactor.
[0071] The purified SiI.sub.4 may react to form SiI.sub.2 (silicon
di-iodide) which then breaks down into a solid silicon atom and an
iodine vapor molecule (I.sub.2) in Reactor 2. In this example, the
solid silicon may be preferably deposited onto the particulate
phase of the fluidized bed that may consist of a silicon seed
material. As the bed particles of silicon grow into granules of a
desired size, e.g., about 0.5-10 millimeters or about 2 mm), they
can be removed from the bed by appropriate mechanical means and
enter stream 24. The silicon granules are then cooled down to room
temperature and form the saleable product. A gas-vapor phase 23 may
exit the fluidized bed and may consist of mostly iodine, un-reacted
SiI.sub.4 and inert gas. After suitable conditioning the iodine can
be recycled back to Reactor 1 and SiI.sub.4 may be recycled back to
the distillation process, or to Reactor 2. The recycled SiI.sub.4
can increase the amount of silicon that can be deposited in Reactor
2.
[0072] It shall be understood that the reactors or vessels provided
in accordance with the invention including those for cyclone and
fluidized bed vessel can be made of construction material typically
composed of an outer metal alloy shell that provides structural
strength and an inner ceramic shell that is exposed to the bed
particles that is resistant to high temperature corrosion by the
halogen-bearing vapors contained therein.
[0073] The methods, systems, and apparatus described herein can be
employed in either continuous, semi-continuous, batch, or fed-batch
modes. In some embodiments of the invention, some of the processes
are batch and others are continuous mode. For example, the
separation steps used for the production of the silicon-rich stream
can be performed in a batch-wise manner and the steps for producing
deposited silicon from the silicon-rich stream can be performed in
semi-continuous manner.
[0074] The foregoing is considered as illustrative only of the
principal of the invention. It shall be understood that the
concepts of the invention herein may be applied to known silicon
processing or recovery systems including but not limited to any of
the following, which are hereby incorporated by reference in their
entirety: pending U.S. patent application Ser. No. 11/893,980
(Fallavollita), now published as US Patent Publication No.
20080044337; U.S. Pat. No. 3,006,737 to Moates et al; U.S. Pat. No.
3,020,129 to Herrick; U.S. Pat. No. 4,388,080 to Kapur et al.; U.S.
Pat. No. 4,388,286 to Kapur et al.; U.S. Pat. No. 4,910,163 to
Jain; U.S. Pat. No. 5,772,900 to Yorita et al.; U.S. Pat. No.
6,113,473 to Costantini et al.; U.S. Pat. No. 6,231,628 to
Zavattari et al.; U.S. Pat. No. 6,281,098 to Wang et al.; U.S. Pat.
No. 6,322,710 to Katsumata et al.; WO 00-01519 to Zavattari et al.;
WO2002 040407 to Henriksen; U.S. Pat. No. 6,615,817 to Horio; U.S.
Pat. No. 6,780,665 to Billiet et al.; U.S. Pat. No. 6,929,537 to
Kajimoto; U.S. Pat. No. 6,838,047 to Billiet et al.; WO 2006-137098
to Frangiacomo; and U.S. Pat. No. 7,223,344 to Zavattari et al.
Further, since numerous modifications and changes will occur to
those persons skilled in the art, it is not desired to limit the
invention to the exact construction and operation shown and
described, and accordingly all suitable modifications and
equivalents may be resorted to falling within the scope of the
invention as defined by the claims which follow.
* * * * *