U.S. patent application number 11/982748 was filed with the patent office on 2015-07-02 for laser conversion of high purity silicon powder to densified granular forms.
The applicant listed for this patent is Alleppey V. Hariharan, Jagannathan Ravi. Invention is credited to Alleppey V. Hariharan, Jagannathan Ravi.
Application Number | 20150183055 11/982748 |
Document ID | / |
Family ID | 39365094 |
Filed Date | 2015-07-02 |
United States Patent
Application |
20150183055 |
Kind Code |
A1 |
Hariharan; Alleppey V. ; et
al. |
July 2, 2015 |
Laser conversion of high purity silicon powder to densified
granular forms
Abstract
The present invention relates to a direct method to convert fine
and ultra fine silicon powder from polysilicon manufacturing
sources such as fluid bed and free space reactors into densified
granular forms. This conversion process is effected by the use of
lasers of selective wavelengths from solid state diode or
optically-pumped YAG sources to locally heat, melt and densify a
controlled quantity of silicon powder, and comprises the steps of
distributing dry silicon powder on an inert substrate, subjecting
the silicon charge to a focused laser beam to realize melted and
densified granular forms, and discharging the product. When adapted
to high purity silicon powder, the end use for the densified
silicon granular forms is primarily as feedstock for silicon-based
semiconductor and photovoltaic manufacturing industries. The
process, suitably modified, is adaptable to form other silicon body
shapes and components.
Inventors: |
Hariharan; Alleppey V.;
(Austin, TX) ; Ravi; Jagannathan; (Bedford,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hariharan; Alleppey V.
Ravi; Jagannathan |
Austin
Bedford |
TX
MA |
US
US |
|
|
Family ID: |
39365094 |
Appl. No.: |
11/982748 |
Filed: |
November 5, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60856539 |
Nov 3, 2006 |
|
|
|
Current U.S.
Class: |
65/21.1 ;
423/348; 65/142 |
Current CPC
Class: |
B23K 26/354 20151001;
C30B 29/06 20130101; C01B 33/02 20130101; C01B 33/037 20130101;
B23K 2103/56 20180801; C01B 33/021 20130101; B23K 26/0006 20130101;
C30B 15/00 20130101; C30B 11/00 20130101 |
International
Class: |
B23K 26/00 20060101
B23K026/00; C01B 33/02 20060101 C01B033/02 |
Claims
1. A method for converting high purity silicon powder into a
polysilicon granule of a desired size and of a desired shape, the
method comprising: (a) providing a dry source of high purity
silicon powder; (b) providing a process substrate comprising a
recess having a defined size and shape, the defined size and shape
being the size and shape required to form the polysilicon granule
of the desired size and the desired shape; (c) feeding a selected
amount of dry silicon powder into the recess of the process
substrate, the selected amount of dry silicon powder being the
amount required to form the polysilicon granule of the desired size
and the desired shape; (d) heating the dry silicon powder disposed
in the recess with a laser emitting light at a wavelength in the
range of ultraviolet to near infrared for a selected time and at a
selected power so as to cause local melting, densification and
solidification, whereby to form a polysilicon granule of the
desired size and the desired shape, wherein the polysilicon granule
has a diameter greater than 0.75 mm and less than 5 mm; and (e)
discharging the polysilicon granule from the process substrate.
2. The method according to claim 1, wherein the silicon powder
comprises a particle size in the range of 0.01 .mu.m to 500 .mu.m,
and a mean particle size in the range of 0.1 .mu.m to 50 .mu.m.
3. The method according to claim 1, wherein the silicon powder has
a bulk density within a range of about 0.1 g/cc to about 0.6
g/cc.
4. The method according to claim 1, wherein the process substrate
is fabricated from an inert and refractory material.
5. The method according to claim 4 wherein the inert and refractory
material is selected from the group consisting of high purity
silicon, fused quartz, silicon-nitride, silicon-nitride coated
silicon-carbide, boron-nitride and boron-nitride coated
graphite.
6. The method according to claim 1, wherein the process substrate
comprises a plurality of recesses.
7. The method according to claim 6, wherein the plurality of
recesses comprise at least one from the group consisting of grooves
and wells.
8. The method according to claim 6, wherein the plurality of
recesses have a tapered shape to collect the silicon granule after
melting.
9. The method according to claim 8, wherein the recesses have a
cross-section which comprises at least one from the group
consisting of flat, v-shaped, u-shaped, conical and
hemispherical.
10. The method according to claim 1 wherein the laser comprises a
solid state diode laser.
11. The method according to claim 1 wherein the laser comprises an
optically-pumped laser.
12. (canceled)
13. The method according to claim 1, wherein the laser emits light
at a wavelength in the range of 200 nm to 1100 nm.
14. The method according to claim 1, wherein the silicon powder is
pre-heated in the recess in the process substrate to a temperature
less than 1412.degree. C. prior to the granulation process so as to
economize and accelerate the granulation process.
15. The method according to claim 14, wherein the heat source for
pre-heating the silicon powder is selected from the group
consisting of radiant, microwave, induction and laser heating.
16. The method according to claim 1, wherein said step of heating
comprises applying a temperature higher than 1412.degree. C. to the
silicon powder.
17. The method for making silicon granules by a laser process
according to claim 1, wherein the method is performed in a
controlled atmosphere gas environment.
18. The method of claim 17 wherein the controlled atmosphere gas
environment comprises at least one selected from the group
consisting of inert helium and argon gas.
19. The method of claim 17 wherein the controlled atmosphere gas
environment is mixed with hydrogen gas, with the content of
hydrogen gas in the controlled atmosphere gas environment being 1%
by volume to 10% by volume.
20. (canceled)
21. A silicon granule consisting of melted and densified silicon
powder and devoid of a binder, the silicon granules obtained by a
method comprising feeding a controlled amount of dry silicon powder
into a process substrate; and thermally treating the dry silicon
powder on the process substrate with a laser heat source so as to
cause local melting and densification of the silicon powder into
granular form.
22. Apparatus for converting high purity silicon powder to
densified silicon granules, the apparatus comprising: a process
substrate; apparatus for distributing a controlled amount of the
high purity silicon powder on the process substrate; and a laser
assembly for directing a laser beam on a selected region of the
process, whereby the laser beam causes the silicon powder in that
selected region to melt and densify into densified silicon
granules.
23. Apparatus according to claim 22, wherein the process substrate
comprises a plurality of recesses configured to hold a controlled
quantity of silicon powder within the substrate.
24. Silicon granules for use as silicon feedstock to the
photovoltaic and semiconductor industries for making silicon
crystals, the silicon granules comprising melted and densified
high-purity silicon powder, the granules being non-spherical.
25. Silicon granules according to claim 25, wherein the silicon
granules comprise a non-spherical shape including a pigtail.
26. The method according to claim 10, wherein the diode laser emits
light at a wavelength in the range of 940 nm to 980 nm.
27. (canceled)
28. The method according to claim 1 wherein the desired size and
the desired shape of the polysilicon granule is defined by at least
one selected from the group consisting of the recess in the process
substrate, the amount of dry silicon powder in the recess in the
process substrate and the amount of energy delivered by the laser
to the silicon powder.
Description
REFERENCE TO PENDING PRIOR PATENT APPLICATION
[0001] This patent application claims benefit of pending prior U.S.
Provisional Patent Application Ser. No. 60/856,539, filed Nov. 3,
2006 by Alleppey V. Hariharan et al. for LASER CONVERSION OF HIGH
PURITY SILICON POWDER TO DENSIFIED SHAPES, which patent application
is hereby incorporated herein by reference.
FIELD OF INVENTION
[0002] The present invention is directed towards conversion of fine
and ultra fine silicon powder into densified granular forms. This
conversion process is achieved by the use of lasers to melt and
densify the silicon powder fed into the process system. The
densified granular forms may then be used as, among other things,
feedstock for silicon-based semiconductor and photovoltaic
manufacturing industries.
BACKGROUND OF THE INVENTION
[0003] The photovoltaic (PV) industry continues and is projected to
have significant growth amidst the environment of fossil fuel
supply uncertainties, concerns over global warming, difficulties of
energy transport from distant centralized facilities, gradual yet
steady improvements in PV module prices, re-evaluation of true
costs for alternate energy in decentralized locations and other
such factors.
[0004] Silicon is the main material resource for making solar cells
and will remain so for a long time. The projected growth of the PV
industry is singularly hampered by the lack of silicon feedstock
material for the PV industry. While dedicated solar grade
polysilicon plants are currently in production, or under
construction, significant shortfall of silicon feedstock material
is still expected in the foreseeable future. Many of the solar
grade silicon manufacturing processes currently in use produce very
significant quantities of high purity silicon powder as a
by-product. However, despite the high purity of the silicon powder,
this silicon powder by-product is presently very difficult to use,
as will hereinafter be discussed in further detail.
[0005] More particularly, ultra fine silicon powder is currently a
by-product of two major processes.
[0006] (1) The Fluid Bed Process To Manufacture High Purity
Electronic Or PV Grade Polysilicon.
[0007] In this process, silicon is deposited by heterogeneous
thermal decomposition of silane (SiH.sub.4) gas or chlorosilane
(SiCl.sub.xH.sub.y, where y=4-x) gas on granules of silicon seed
particles. The granules grow in size from an initial seed size of
.about.0.2 mm to, typically, 1 mm in diameter. The granules are
then utilized in silicon melting and crystal growth
applications.
[0008] As stated above, the fluid bed process also results in the
production of a large quantity of ultra fine silicon powder (or
dust). More particularly, of the total amount of material produced
by the fluid bed process, approximately 15% to 20% is ultra fine
silicon powder. The particle sizes of such ultra fine silicon
powder vary over wide ranges, but are typically in the range of
approximately 0.1 microns to 20 microns in diameter. This powder is
of high purity, but cannot currently be recycled or used in silicon
melting and crystal growth applications because of the fineness of
the powder.
[0009] More particularly, the powder produced by the fluid bed
process is so fine that it becomes extremely difficult to handle.
Among other things, the very fine silicon powder goes airborne
easily, making it extremely difficult to transfer from location to
location and making it hard to create "clean" (i.e., particle free)
and inert atmosphere conditions by vacuum processes. In addition,
the fineness of the powder creates significant maintenance problems
for equipment. Furthermore, the fineness of the powder creates
safety issues, since the powder can be explosive, in much the same
way that corn dust may be explosive in a silo.
[0010] Accordingly, the very fine silicon powder created by the
fluid bed process is tapped out of the reactor outlet as process
waste.
[0011] (2) Ultra Fine Silicon Powder Is The Primary Product From
Free Space Reactors Which Are Being Developed To Manufacture High
Purity Electronic Or PV Grade Polysilicon.
[0012] In this process, silicon powder is formed in the gas phase
by homogeneous decomposition of silane (SiH.sub.4) gas or
trichlorosilane (SiHCl.sub.3) gas at high temperatures. The
particle size of such formed silicon powder is typically in the
sub-microns range. The purity of the silicon powder produced by the
free space reactor process is very high when used with high purity
electronic or PV grade silane or trichlorosilane gas and when
performed in suitable reactors. However, the ultra fine nature of
the silicon powder prevents its direct use in silicon melting and
crystal growth applications. This is because the fineness of the
silicon powder creates the same transfer, "clean" and inert
atmosphere, maintenance and safety issues discussed above with
respect to the fluid bed process. Since the entire silicon
production from the free space reactor process is in the form of
such silicon powder, it needs to be processed and converted into
larger forms for use by the PV industry.
[0013] A few methods have been attempted to convert ultra fine
silicon powder (e.g., such as that produced by the two processes
recited above) to larger forms so that they can be productively
used as feedstock in manufacturing operations (e.g., such as
silicon melting and crystal growth applications).
[0014] Granulation and augmentation of the silicon powder particle
size by electron beam melting or microwave heating to 1200.degree.
C. to 1500.degree. C. are described in Japanese Patent 11199382JP.
Such procedures are mainly for making silicon nitride and other
silicon compounds and are not applicable to the high purity silicon
granules needed for the PV industry.
[0015] Typical powder metallurgical process schemes have been
suggested to convert silicon powder into larger aggregates to make
them useable in various applications. They include compacting with
a typical organic binding agent such as starch and lignin.
[0016] High-pressure densification and hot pressing of silicon
powder, with or without sintering aids, are described in J.
Materials Sci. 31 (18), 4985-4990 (1996); J. Jap. Soc. Powder
Metal. 28 (1)15-19 (1981); U.S. Pat. No. 4,040,848, and U.S. Pat.
No. 4,040,849.
[0017] Dry compaction of high purity silicon powder without the use
of binders is described in U.S. Pat. No. 7,175,685, but the
compacted material does not have the strength and integrity needed
for subsequent uses.
[0018] Compaction, using selected high purity inorganic or organic
binders, with subsequent debinding and densification by high
temperature sintering of pressed silicon bodies, in order to
realize an industrially viable process and robust product, is
described in U.S. patent application Ser. No. 11/479,735 (U.S.
Patent Application Publication No. 2007/0014682).
[0019] Several procedures to make crystalline silicon spheres are
described in various literatures. These procedures include:
shotting of molten silicon through a nozzle or orifice as described
in Asia Electronics Industry, p 45, Feb 2003, IEEE Vol. 31, p 963
(2005) and Jap. J. Appl. Phys. Vol. 46, p 5695 (2007); processing a
paste of silicon deposited on a substrate and furnace melting the
paste as described in U.S. Pat. No. 4,637,855; coating the silicon
particles with an oxide skin layer, melting and coalescing the
silicon into spheres within the oxide skin and further processes as
described in U.S. Pat. Nos. 4,425,408, 5,069,740, 5,431,127 and
5,556,791, and other variations of such techniques as described in
U.S. Pat. No. 5,817,173. Such silicon spheres, intended for spheral
solar cells, are in the sub-millimeter to one millimeter diameter
range. These processes are typically followed by some sequence of
other processes to remove contaminations and cause crystallization,
all of which make them inherently complicated for direct
application.
[0020] Where it becomes necessary to maintain or improve the level
of purity of the silicon powder for PV feedstock usage, application
of conventional processes such as direct melting, powder
compaction, sintering and densification, etc., or methods to form
silicon spheres, create considerable complexity and associated
costs, and in practice result in incorporation of unwanted
impurities into the silicon.
[0021] In spite of the various above-mentioned proposals, there are
currently no prior art industrially-practical and cost-effective
methodologies available to directly convert silicon powders to high
density granular forms which (i) maintain the purity and quality of
the polysilicon powder by-product, (ii) are reproducible in a
large-scale manufacturing environment, and (iii) may be transported
and used without form failure and without the need for additional
processes for subsequent product uses.
[0022] Accordingly, it is an object of the present invention to
provide a robust process for the conversion of high purity silicon
powder to high value densified granular forms which can be directly
used by the PV feedstock industries to grow silicon crystals.
[0023] Laser-based processes are currently being developed in the
realm of rapid tooling and rapid prototyping for use with metal
powders. These processes include laser-assisted metal processes
(LAMP), laser forming processes (LASFORM), solid freeform
fabrication processes (SFF), shape deposition manufacturing
processes (SDM), selective laser sintering processes (SLS) and
laser engineered net shaping processes (LENS). Both CO.sub.2 and
Nd-YAG lasers are utilized for such applications. A variety of
metal powders, such as stainless steel, bronze, titanium, aluminum,
copper and INVAR have been processed to make from small to complex
net-shaped articles for industrial use. Additionally, high power
laser systems are also being utilized for metal forming operations
such as cutting, drilling, welding, micromachining, etc. Many such
laser systems concentrate power through fiber optic and other
focusing aids. High power solid state lasers are utilized for
several such applications. They can be used on a continuous wave
(CW), quasi-CW or pulsed mode depending on the application.
[0024] However, it is believed that, heretofore, no one has
considered using lasers to convert high purity silicon powder to
silicon granules and thereby to render this powder into viable
feedstock.
[0025] Elemental silicon is uniquely positioned to utilize laser
processing schemes. The optical absorption of elemental silicon
increases exponentially when the incident radiation has wavelengths
shorter than one (1) micrometer. This is illustrated in FIG. 1,
where the optical absorption coefficient of silicon is plotted
against the incident wavelength of the radiation.
[0026] The optical data for selected incident radiations are shown
in Table 1. The optical absorption of silicon powder is expected to
be a little higher than that of a polished wafer. Efficient
absorption of short wavelength radiation will cause the silicon to
heat and finally melt. See Table 1.
[0027] Several existing types of lasers have lasing wavelengths
that can be applicable for processing silicon. These laser types
include, for example, optically-pumped lasers of wavelengths
slightly over 1 .mu.m based on matrices of yttrium aluminum garnet
(Nd:YAG, Yb:YAG), yttrium orthvanadate (Nd:YVO4) and lithium
yttrium fluoride (Nd: LiYF4). However, the efficiencies of these
lasers are below 30%. Because of their low efficiencies, their long
wavelengths where silicon absorption is poor, and their high cost,
these lasers are not ideal for melting of silicon powder on an
industrial scale.
[0028] Diode lasers are also becoming more feasible for silicon
processing, with larger power lasers becoming available. At the
near infrared lasing wavelengths, very high power conversion
efficiencies of greater than 65% have been achieved, since these
devices are directly energized and electrically activated, rather
than being optically pumped. The diode lasers utilizing
gallium-arsenide (GaAs) and indium gallium arsenide (InGaAs), in
the 940 nm to 980 nm range, which are used for optically pumping
Yb:YAG or Yb:glass lasers, would themselves be highly efficient
energy sources for silicon heating and melting.
SUMMARY OF THE INVENTION
[0029] It is, therefore, an object of the present invention to
provide a viable and practical process and technology to convert
silicon powder into a form, typically melted and densified granular
shapes, that can be easily manufactured, transported and utilized
to produce silicon feedstock for other applications.
[0030] It is a further object of the present invention to provide a
process and technology that will maintain the purity of the silicon
granule at substantially the same level as, or even better than,
the starting silicon powder.
[0031] It is another object of the present invention to provide a
method of locally melting and densifying a controlled quantity of
silicon powder into granular shapes.
[0032] It is yet another object of the present invention to provide
a system and facility for conducting a powder-to-densified granular
shape conversion at a commercially useful production rate, such as
high speed melting and densification, and processing in the order
of a kilogram or more of silicon powder per hour per process
unit.
[0033] One of the most important aspects of the present invention
is the development of a direct process that, when used with silicon
powder of high purity, adds significant form and value to the
material, and provides feedstock for the electronic or photovoltaic
industries.
[0034] Hence, another object of the present invention is to
produce, from silicon powder, dense forms (such as granules) of
sufficiently high purity that can be used directly for the
production of solar cells by methods such as ribbon or EFG crystal
growth processes, without the need for additional large volume
melting and crushing operations.
[0035] The process of the present invention generally uses
selective laser wavelengths and energies to locally heat, melt and
densify small charges of the silicon powder into granular
shapes.
[0036] In one embodiment of the present invention, a charge of dry
silicon powder is submitted to appropriate laser radiation, such as
the primary and/or frequency-doubled wavelengths from direct
semiconductor diode lasers or pumped solid state lasers, such as
Yb:YAG, Yb doped glass, Nd:YAG, Nd:YVO.sub.4, etc., to cause the
silicon to heat and finally melt. If the charge is appropriately
located on an inert substrate or in a recess formed in or on the
inert substrate, the silicon melt coalesces into a regular or
irregular granular shape. If the melting occurs on a dynamically
forming silicon granule, the new melt incorporates into the silicon
granule, becomes part of it and the granule grows in size. By
appropriate combination of the substrate shape, amount of silicon
powder charge, melt location, and laser power intensity and
duration, a continuous larger size silicon body of regular or
irregular shape, such as spherical or pear shaped granules, etc.,
can be realized.
[0037] The operation of conveying the silicon powder to the laser
beam and locally melting the powder as described above is done in
an inert and controlled atmosphere environment that will prevent
oxidation, nitridation or carburization of the silicon at the high
process temperatures. In addition, the in-process melting of the
silicon powder can also favorably enhance the purity of the
densified compact product through the selective vaporization and
removal of specific impurities. Such purification will result from
vaporization of silicon monoxide, SiO, thus reducing the oxygen
impurity typically present in high surface area silicon powder, and
by removal of other volatile metal impurities. Reductions of
impurity levels of selective elements by a factor of 10 to 100 are
feasible.
[0038] The powder processing system and machinery can be
semiautomatic or automated for control of operation. The entire
process facility of silicon powder handling, transport, and laser
process systems will be within a controlled ingress and filtered
egress facility for environmental safety.
[0039] It is the combination of the ability to convert silicon
powder into densified granular shapes and in-process conservation
of, or even improvement in, the purity level of the densified
product silicon that enables subsequent value-added use of the
silicon powder, especially high purity silicon powder, for example
to critical uses such as feedstock materials for electronic and
photovoltaic applications. The purity level of the silicon material
feedstock for photovoltaic applications should be >99.99%.
[0040] In one form of the present invention, there is provided a
method for converting small amounts of high purity silicon powder
into melted and densified granules of polysilicon for use as
silicon feedstock, the method comprising:
[0041] (a) providing a dry source of high purity silicon
powder;
[0042] (b) feeding a controlled amount of dry silicon powder onto a
process substrate;
[0043] (c) thermally treating the dry silicon powder with a laser
heat source so as to cause local melting and densification of the
silicon powder into granular form;
[0044] (d) discharging the silicon granules from the process
substrate; and
[0045] (e) repeating steps (a) through (d) so as to produce a
sufficient quantity of silicon granules.
[0046] In an additional form of the present invention, there is
provided a silicon granule consisting of melted and densified
silicon powder and devoid of a binder, the silicon granules
obtained by a method comprising feeding a controlled amount of dry
silicon powder into a process substrate; and thermally treating the
dry silicon powder on the process substrate with a laser heat
source so as to cause local melting and densification of the
silicon powder into granular form.
[0047] In another form of the present invention, there is provided
apparatus for converting high purity silicon powder to densified
silicon granules, the apparatus comprising:
[0048] a process substrate;
[0049] apparatus for distributing a controlled amount of the high
purity silicon powder on the process substrate; and
[0050] a laser assembly for directing a laser beam on a selected
region of the process, whereby the laser beam causes the silicon
powder in that selected region to melt and densify into densified
silicon granules.
[0051] In an additional form of the present invention, there is
provided silicon granules for use as silicon feedstock to the
photovoltaic and semiconductor industries for making silicon
crystals, the silicon granules comprising melted and densified
high-purity silicon powder, the granules being non-spherical.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] These and other objects and features of the present
invention will be more fully disclosed or rendered obvious by the
following detailed description of the preferred embodiments of the
invention, which is to be considered together with the accompanying
drawings wherein like numbers refer to like parts, and further
wherein:
[0053] FIG. 1 illustrates the optical absorption coefficient of
silicon as a function of the wavelength of incident radiation, in
the range 0.3 microns to 1.2 microns;
[0054] FIG. 2 shows a typical flow sheet for the laser process
scheme of the present invention;
[0055] FIG. 3 is a schematic illustration of the process for
densifying the silicon powder;
[0056] FIG. 4 is a simplified laser process system for forming
silicon granules in accordance with the present invention;
[0057] FIG. 5 is a powder feeder for silicon powder which can be
used with the process of the present invention;
[0058] FIG. 6 illustrates examples of silicon product granule
shapes produced in accordance with the present invention;
[0059] FIG. 7 is a schematic illustration of a multi-station,
rotary-indexing process platform for the laser processing of
silicon powder into small melted and densified granules in
accordance with the present invention;
[0060] FIG. 8 is a schematic view of a multi-station X-Y indexing
process platform for laser processing silicon powder into small
melted and densified granules in accordance with the present
invention;
[0061] FIG. 9 is another schematic view illustrating an X-Y
indexing process platform (with V-shaped grooves) for laser
processing silicon powder into small melted and densified
granules;
[0062] FIG. 10 illustrates examples of typical silicon product
shapes formed in accordance with the present invention; and
[0063] FIG. 11 is a simplified laser process system for forming
silicon shapes in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0064] As shown in FIG. 1, silicon has excellent absorption
characteristics for optical radiation between the ultraviolet and
visible wavelengths, and the absorption tapers off to negligible
levels in the near infrared wavelength (i.e., >1.2 .mu.m) range.
Solid state lasers such as Yb:YAG and Nd:YAG lasers typically have
lasing wavelengths around 1 .mu.m and can be effective in melting
silicon powder. Diode lasers, which typically operate in the
wavelength range of 970 nm, are more effective than solid state
lasers in melting silicon, from both efficiency and energy
absorption considerations. Silicon energy absorption leading to
melting is even more effective if the wavelengths are shifted into
the visible or UV regions of the optical spectrum, either by means
of frequency doubling or by use of new lasers which are currently
being developed.
[0065] A process flow sheet of converting silicon powder to
densified silicon shapes (e.g., granules) is disclosed in FIG.
2.
[0066] FIG. 3 shows a schematic of the processes involved in the
densification of silicon powder into silicon granules.
[0067] A simplified laser process system for silicon powder
processing into granules is described in FIG. 4.
[0068] More particularly, the basic steps of a preferred method for
making high purity densified and robust silicon granules is as
follows. First, the ultra fine silicon powder is transferred into a
drying system, where the powder is dried at a temperature of
between 100.degree. C. and 150.degree. C., and preferably at
.about.100.degree. C., to remove absorbed and adsorbed water and
other environmental impurities and improve powder flowability.
Drying may be effected by methods such as radiant, microwave or
laser heating. The apparatus where the silicon powder resides or
flows through during the drying operation is configured
appropriately. It is important to note that the silicon powder does
not have to be dried before it moves through the powder feeder,
however, this step is preferred in order to ensure the highest
purity in the final product (i.e., the silicon granules).
[0069] The silicon powder may also be fed into the laser process
system at ambient temperatures.
[0070] After the foregoing (optional) drying step, the dried powder
is conveyed to a feed hopper attached to a powder feeder system
(FIG. 5). This feed hopper and powder feeder system will feed a
defined and controlled amount, by weight or by volume, of the
silicon powder per unit process time to a laser process system such
as the one shown in FIG. 4. The amount of silicon powder conveyed
into the laser process system is determined by, among other things,
the size and energy of the laser beam(s) impinging on the
powder.
[0071] The laser process system (FIG. 4) comprises (i) a process
substrate of appropriate design, (ii) a focused beam of laser
radiation, with a wavelength preferably in the ultraviolet to near
infrared ranges, (iii) appropriate inert gas for protection against
unwanted chemical reactions (e.g., oxidation), and (iv) auxiliary
cooling systems. The laser beam will cause the silicon charge to
locally heat and melt as is, or locally heat, melt and fuse with
the forming silicon granule. The melt typically coalesces to form
either a pear-shaped, a near spherical granule or a spherical
granule (see FIG. 6) that solidifies when the laser heat is
removed. The laser beam can be used either as a continuous wave
(CW) form, modulated CW or pulsed form, depending on the particle
size and the initial form of the silicon powder. The choice and
methodology used is based on effective heat transfer into the
silicon particles and overall heating of the silicon body.
[0072] Although the above description of the invention is directed
to the use of a laser to locally heat the silicon powder to a
temperature higher than 1412.degree. C. (i.e., above the melting
point of silicon) so that the powder melts into granules, for cost
considerations, it is important to note that the silicon powder may
also be pre-heated by auxiliary sources prior to utilizing laser
heating for the rapid fusing and melting of the powder. These
auxiliary sources may include laser, radiant, microwave, or
induction heating to high temperatures (e.g., from hundreds of
degrees Celsius up to the melting point of silicon, i.e.,
1412.degree. C.).
[0073] By design, the components of the process systems of the
present invention are of sufficient quality to handle high purity
materials. For example, the substrate platform may be made of high
purity silicon, fused quartz, silicon-nitride, silicon-nitride
coated silicon-carbide, boron-nitride, or boron-nitride coated
graphite. The choice of such substrates is to be on the basis of
minimized wetting by the silicon melt during the laser-effected
thermal process.
[0074] Appropriate gases, e.g., inert gases, may include high
purity helium, argon, etc., and are preferably mixed with hydrogen
to reduce or prevent oxidation, nitridation or carburization of the
silicon powder and formed granules. When the inert gases are mixed
with hydrogen, the hydrogen content may comprise 1% to 10% by
volume, and preferably comprises 5% by volume. Performing the
granulation process under a controlled, inert gas environment
prevents contamination from ambient gases and ensures high purity
of the process.
[0075] Special high purity powder feeders may be required. These
may be rotary screw feeders, vibratory feeders, disc feeders,
fluidized bed feeders, etc. In such powder feeders, the materials
which contact the silicon powder are preferably made of high purity
silicon or silicon-nitride.
[0076] The silicon powder itself is conveyed from the hopper to the
process platform (i.e., the substrate platform) either by gravity
flow or with the aid of a transporting inert gas or with a
mechanical transport system, etc.
[0077] The density of the silicon powder on the substrate may be
increased for more efficient processing. This may be achieved by
vibration, tamping or by means of electrostatic charging of the
powder feed.
[0078] The invention is amenable to many embodiments. In a
preferred embodiment, the present invention utilizes fine silicon
powder having a particle size in the range of about 0.01 microns to
about 500 microns, and preferably a particle size in the range of
0.1 microns to about 100 microns, and a mean particle size in the
range of about 0.1 microns to about 50 microns, and preferably a
mean particle size in the range of about 0.5 microns to about 10
microns. The present invention preferably converts the fine silicon
powder into granules approximately 2 mm in diameter.
[0079] In one preferred embodiment, the fine silicon powder has a
bulk density in the range of approximately 0.1 g/cc to
approximately 0.6 g/cc, and preferably has a bulk density in the
range of approximately 0.2 glee to approximately 0.4 g/cc.
[0080] In accordance with the present invention, the fine silicon
powder may be a by-product from the fluid bed reactors, the free
space reactors or other silicon process reactors that manufacture
feedstock quality silicon for the PV industries. Also in accordance
with the present invention, the fine silicon powder may be a
by-product from the crushing of silicon chunks or recovered from
the wafer processing and ingot shaping in the semiconductor and/or
PV industries. In a preferred embodiment, the silicon powder
consists of substantially pure silicon particles.
[0081] The specific size and shape of the densified silicon
granules is generally not critical for substantially all end-use
applications, since the granules will be used as feedstock and
re-melted during the subsequent end-use application. All that is
generally required is that the densified silicon granules have a
size and shape which is conducive to easy handling, e.g., a mass
substantial enough to prevent the granules from becoming airborne,
a size which facilitates transport by typical material handling
systems (e.g., bins and hoppers, screw-feeders, etc.), and an
integrity to permit the granules to be transported without breaking
down.
[0082] In the process of the present invention, preferably, a
quantity by weight, or by volume, of the silicon powder is fed into
the specific substrate platform and locally heated and melted by
the focused laser beam to achieve the desired dimensions. The
powder melts under the laser beam and solidifies to granules of
varying dimensions, with the size of the granules depending on,
among other things, the power, duration and area of the laser beam.
The melted shape is then ejected from the machine through a
take-off system.
[0083] The ejection of the formed solid silicon pieces for
collection can be achieved by various means such as vibration or
tapping. Mechanical or electromechanical devices, including
solenoids or piezoelectric actuators, can be used to effect such
means.
[0084] The substrate may have a flat surface and the silicon powder
may lie on the top surface of the substrate during processing.
Alternatively, and more preferably, the substrate may be designed
with a series of recesses (e.g., grooves, wells, etc., with or
without tapered bottoms) and filled with silicon powder to a
certain depth. The powder melts under the laser beam and solidifies
to granules of varying dimensions, depending on, among other
things, the power, duration and area of the laser beam, and also on
the size of the recesses.
[0085] The size and shape of the recesses in the substrate may be
varied depending on the desired granule dimensions. The recesses
may have a flat bottom or may have a tapered shape to collect the
silicon granule during and after melting. The tapered recesses may
have a cross-section which is v-shaped or u-shaped or otherwise
tapered and, where the recesses are wells, the tapered recesses may
have a bottom shape which is conical, hemispherical or otherwise
tapered. The recesses may also have any other shape which would
produce the desired granule characteristics.
[0086] It should be appreciated that the recesses may be formed as
openings extending into the depth of the substrate or by
receptacles disposed on the top surface of the substrate.
[0087] By way of example but not limitation, the multi-station
process platforms shown in FIGS. 7 through 9 (i.e., rotary indexing
process platforms, X-Y indexing process platforms, etc.) may be
used for laser processing silicon powder into small melted and
densified granules.
[0088] The process efficiency can be increased by scanning or
oscillating the laser beam to cover more of the powder surface area
on the substrate. The laser beam movement may be achieved by
electro-optical means.
[0089] The machinery may be configured to operate multiple lines of
laser processors (e.g., by using multiple lasers or multiple beams
split from individual lasers) to meet high volume requirements.
Such machinery shall also be located within the confines of an
inert gas (e.g., argon) chamber to generally confine the silicon
powder and the process, and ensure clean oxygen-free and
moisture-free thermal processing.
[0090] The present invention provides for the utilization of the
melted and densified silicon granules as direct feed material for
different industries. More particularly, densified silicon granules
of high purity (i.e., >99% and preferably >99.99%) may be
used as polysilicon feedstock in semiconductor and photovoltaic
materials industries to make high purity silicon crystals. In
addition, densified silicon granules of nominal purity may be used
for auxiliary ferrosilicon, aluminosilicon and other alloy
manufacturing operations.
[0091] In one embodiment of the present invention, the silicon
granules have a diameter in the range of about 1 millimeter to
about 5 millimeters, and preferably about 2 millimeters.
[0092] In one embodiment, the granules produced by the process of
the present invention have a density of greater than approximately
80% of the theoretical density of elemental silicon and have a
weight within the range of approximately 1 milligram to
approximately 30 milligrams.
[0093] In one embodiment of the present invention, the silicon
granules have a weight of approximately 10 milligrams.
[0094] In another embodiment of the present invention, the silicon
granules have a weight within the range of approximately 0.1
milligrams to approximately 150 milligrams.
[0095] As a consequence of the localized heating and melting of the
silicon powder on the substrate with a directed laser energy
source, the granules produced in accordance with the present
invention may have regular and irregular spherical shapes.
Specifically, the silicon granules may also have a "pigtail"
feature appended to the body of the granule.
[0096] High purity silicon crystals may be made from the densified
silicon granules in ways which are known in the art of crystal
growth. Some of the crystal growth processes include methods such
as Czochralski (CZ), edge defined film growth (EFG), heat exchanger
method (HEM), directional solidification system (DSS), etc.
[0097] There is a further particular advantage of the process of
the present invention. Continuous crystal growing methods confer
technical and economic benefits. In fact, some methods (such as
EFG) already use or require continuous feed where the silicon is in
granular form or composed of small pieces from crushed silicon
chunks. Using the process of the present invention, small size
solid silicon pieces, such as granules of regular or irregular
shapes, can be directly made. These silicon pieces may then be used
as polysilicon feedstock for continuous crystal growth
operations.
Variations Of The Invention
[0098] Although the process descriptions of the present invention
pertain to forming silicon granules, it is equally applicable to
form other densified shapes of silicon. If the silicon powder feed
and melting occurs on a dynamically forming silicon shape, the new
melt incorporates into the silicon shape and becomes part of it. By
appropriate combination of silicon powder feed, substrate shape,
melt location and, and laser power intensity and duration, a
continuous silicon body shape can be realized. The term "densified
shape" as used herein is inclusive of any form factor and is a
descriptive term that implies a compacted small volume of the
material. Its shape includes irregular and regular granules, and
may include irregular chunk, lump, etc., or regular spheres,
cylindrical shapes, squares, rectangular blocks, disks, flats,
slabs, wafers, etc. (FIG. 10), and sizes that are practical for
process machinery and handling.
[0099] Although intended primarily to use high purity silicon
powder as it is manufactured (either with or without the pre-drying
and pre-heating steps) as feed for the laser melting and forming
process, the invention is equally applicable to process silicon
compacts made with selective high purity binders. When binders are
utilized, the intermediate feed material shape, formed in auxiliary
equipment, may be as cylindrical pellets, spherical granules,
blocks, etc. Such shapes, when processed with the laser system,
will initially lose the binders by evaporation, and then lead to
melting into the desired final shapes. The process will also
require the environment of the silicon compact to be efficiently
purged with inert gases to remove all traces of the binder material
from the silicon prior to the silicon melting step.
[0100] The laser forming method as described in this invention, and
suitably modified to form shaping, is also amenable to make
specific components of the type typically used in the semiconductor
process industries, such as wafer carriers, wafer supports, RTP/EPI
rings, susceptors and plasma etch chamber components, etc. A
typical process platform for laser processing silicon powder into
such shaped articles is shown in FIG. 11.
EXAMPLE 1
[0101] High purity silicon powder with particle size in the range
of 0.1 microns to 3 microns, and with mean diameter of 0.5 microns,
was utilized. The bulk density of the powder was 0.3 grams per
cubic centimeter, and which tapped to a density of 0.5 grams per
cubic centimeter. A high purity semiconductor grade graphite block
with drilled chambers approximately 5 mm in diameter and 6 mm in
depth, and coated with boron nitride was used as the substrate. The
silicon powder was placed in the holes and tamped. An IPG Photonics
YLR-1700 Yb:YAG laser was used. The laser setup utilized a focus of
150 mm, 60 mm collimator and 100 microns diameter fiber with
defocused beam just below the focal plane. An argon cover gas was
used around the process area to prevent oxidation. When using a
laser power of 85 W CW, and with an approximate beam spot size of 4
mm to 5 mm, for 5 to 6 seconds, the silicon charge melted,
coalesced and formed a granule in the bottom of the chamber. The
granule sizes varied in the range of 2 mm to 3 mm diameter in
various trials.
[0102] In another series of trials, when the laser was used for 3
to 4 seconds instead of the 5 to 6 seconds discussed above, the
resulting granule sizes varied in the range of 1 mm to 2 mm
diameter.
[0103] Several sets of granules with diameters in the range of 1 mm
to 3 mm were made.
EXAMPLE 2
[0104] The high purity silicon powder of Example 1, and a second
supply of high purity silicon powder with a particle size in the
range of 0.1 microns to 20 microns, and a mean diameter of 1
micron, was used. The bulk density of the powder of the second
supply was 0.4 grams per cubic centimeter and the tap density was
>0.5 grams per cubic centimeter. The substrate was a high purity
h-boron nitride block having drilled conical chambers approximately
10 mm in diameter and 6 mm in depth, and with hemispherical bottoms
which taper into approximately 5 mm in diameter, similar to the
platform shown in FIG. 8. The silicon powder was distributed on the
substrate and tamped into the chambers.
[0105] An IPG Photonics diode laser DLR-175 was utilized. The laser
setup utilized a focus of 230 mm, 50 mm collimator and 400 microns
diameter fiber, and the beam was defocused to provide a large spot
of heat. Two off-axis inert argon gas jets, leading and trailing
the laser beam, were used to protect the silicon charge from
oxidation.
[0106] Two procedures were utilized. In one, the powder lumps were
scanned under a fixed beam of diode laser at a rate of 2 mm per
second. The laser power was 65 W. Granules in the range 1 mm to 2
mm formed at the bottom of the pits.
[0107] In the second procedure, the diode laser was aimed at each
powder lump in the multicavity substrate for 5 seconds, and then
the substrate was indexed to process the next lump. The laser power
was 65 W. Granules in the range 2 mm to 3 mm formed at the bottom
of the pits.
[0108] Several sets of granules with diameters in the range of 1 mm
to 3 mm were made using the two sources of silicon powder.
Modifications
[0109] Other and various embodiments will be evident to those
skilled in the art, from the descriptions, figures and claims
provided herein. Numerous other changes, modifications and
revisions in the apparatus and process will occur to those skilled
in the art in view of the present disclosure. Thus, the following
appended claims should not be strictly construed to their expressed
terms, but should be broadly construed in a manner consistent with
the spirit, breadth and scope of the inventors' contribution to the
process and equipment described herein.
* * * * *