U.S. patent application number 12/669504 was filed with the patent office on 2010-08-05 for methods and apparatuses for manufacturing cast silicon from seed crystals.
This patent application is currently assigned to BP Corproation North America Inc.. Invention is credited to Roger F. Clark, James A. Cliber, Nathan G. Stoddard, Bei Wu.
Application Number | 20100197070 12/669504 |
Document ID | / |
Family ID | 39730616 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100197070 |
Kind Code |
A1 |
Stoddard; Nathan G. ; et
al. |
August 5, 2010 |
Methods and Apparatuses for Manufacturing Cast Silicon From Seed
Crystals
Abstract
Methods and apparatuses are provided for casting silicon for
photovoltaic cells and other applications. With these methods, an
ingot can be grown that is low in carbon and whose crystal growth
is controlled to increase the cross-sectional area of seeded
material during casting.
Inventors: |
Stoddard; Nathan G.;
(Gettysburg, PA) ; Wu; Bei; (Frederick, MD)
; Clark; Roger F.; (Knoxville, MD) ; Cliber; James
A.; (Emmitsburg, MD) |
Correspondence
Address: |
CAROL WILSON;BP AMERICA INC.
MAIL CODE 5 EAST, 4101 WINFIELD ROAD
WARRENVILLE
IL
60555
US
|
Assignee: |
BP Corproation North America
Inc.
Warrenville
IL
|
Family ID: |
39730616 |
Appl. No.: |
12/669504 |
Filed: |
July 16, 2008 |
PCT Filed: |
July 16, 2008 |
PCT NO: |
PCT/US08/70196 |
371 Date: |
January 18, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60951155 |
Jul 20, 2007 |
|
|
|
Current U.S.
Class: |
438/68 ;
257/E31.001; 264/308; 425/144; 425/224 |
Current CPC
Class: |
Y02E 10/547 20130101;
Y10T 428/12528 20150115; C30B 11/003 20130101; C30B 19/067
20130101; H01L 31/182 20130101; H01L 31/0312 20130101; Y02E 10/546
20130101; F27B 14/06 20130101; Y10T 117/1092 20150115; H01L 31/1804
20130101; Y02P 70/50 20151101; Y02P 70/521 20151101; C30B 29/06
20130101; H01L 31/036 20130101; C30B 11/02 20130101 |
Class at
Publication: |
438/68 ; 264/308;
425/224; 425/144; 257/E31.001 |
International
Class: |
H01L 31/18 20060101
H01L031/18; B29C 39/14 20060101 B29C039/14; B28B 1/54 20060101
B28B001/54; B29C 35/16 20060101 B29C035/16 |
Goverment Interests
[0002] This application was made with U.S. Government support under
Subcontract No.: ZAZ-6-33628-11 under prime contract with the
National Renewable Energy Laboratory awarded by the Department of
Energy. The Government has certain rights in this invention.
Claims
1. A method of manufacturing cast silicon, comprising: placing a
crucible on a layer comprising: a thermally conducting material; a
heat sink; and a thermally insulating area, where a thermally
conductive part of the layer contacts with a portion of a bottom
surface of the crucible; placing at least one seed crystal on a
bottom of the crucible; placing molten silicon in contact with the
at least one seed crystal; and forming a solid body of silicon by
extracting heat through the thermally conducting material.
2. A method of manufacturing a solar cell, comprising: providing a
solid body of cast silicon according to claim 1; slicing the solid
body of cast silicon to form at least one wafer; forming a p-n
junction by doping a surface of the at least one wafer; and forming
a surface neutralizing layer and forming electrically conductive
contacts on at least one surface of the wafer.
3. The method according to claim 1, wherein the extracting heat
expands a lateral area of the seeded crystal during
solidification.
4. The method according to claim 1, wherein the placing molten
silicon further includes placing a solid silicon feedstock in the
crucible on top of the at least one seed crystal and melting the
solid silicon feedstock while cooling the bottom of the crucible to
maintain the at least one seed crystal in an at least partially
solid state.
5. The method according to claim 4, wherein a heat flux through the
layer changes from the step of melting the silicon to the step of
forming a solid body.
6. The method according to claim 1, wherein the placing molten
silicon further includes: melting a silicon feedstock in a melt
container separate from the crucible; heating the crucible to a
melting temperature of silicon; controlling heating so that the at
least one seed crystal in the crucible does not melt completely;
and transferring the molten silicon from the melt container into
the crucible.
7. The method according to claim 1, further including forming a
portion of the solid body to include the at least one seed
crystal.
8. The method according to claim 1, wherein the thermally
conducting material contacts between about 5% to about 99% of the
bottom surface area of the crucible.
9. The method according to claim 1, wherein the thermally
conducting material corresponds to a size and a shape of the at
least one seed crystal within the crucible.
10. The method according to claim 1, wherein the heat sink
comprises a radiative heat sink, the radiative heat sink radiating
heat to walls of a water-cooled vessel.
11. The method according to claim 1, wherein the thermally
insulating area forms a perimeter around the thermally conducting
material.
12. The method according to claim 11, wherein perimeter comprises a
contoured shape wider in a middle of a side of the layer than in
corners of the layer.
13. The method according to claim 11, wherein the perimeter
thermally isolates side support walls for the crucible from the
heat sink.
14. The method according to claim 1, further comprising reducing or
enlarging the thermally conducting area in contact with the
crucible bottom by adding or removing at least a portion of the
thermally insulating area.
15. An apparatus for casting of silicon, comprising: resistive
heaters for being in thermal communication with a crucible; and a
layer comprising: a thermally conducting material; a heat sink; and
a thermally insulating area; wherein a thermally conductive part of
the layer is for contact with a portion of a bottom surface of the
crucible on a side and the heat sink on an opposite side.
16. The apparatus according to claim 15, wherein the thermally
insulating area forms a perimeter around the thermally conducting
material.
17. The apparatus according to claim 15, wherein the thermally
insulating area increases or decreases heat transferred through the
layer by moving with respect to the thermally conducting
material.
18. The apparatus according to claim 15, wherein a ratio of thermal
conductivities of the thermally conducting material to the
thermally insulating area is at least about 20:1.
19. The apparatus according to claim 15, wherein a ratio of an area
of the thermal conducting material to an area of the seed crystal
is from about 0.5 to about 2.0.
20. A process for manufacturing cast silicon, comprising: loading a
seed layer of crystalline silicon together with a solid feedstock;
melting the solid feedstock and part of the seed layer by
maintaining a solid/liquid interface essentially flat over the
center of the seed layer, but convex in the solid portion at the
edges of the seed layer; forming a solid body of silicon by
extracting heat through the seed layer while maintaining the
solid/liquid interface essentially flat over the center of the seed
layer, but convex in the solid portion at the edges of the seed
layer; bringing the solid body to a first temperature; and cooling
the solid body to a second temperature.
21. A method for manufacturing cast silicon, comprising: loading a
seed layer of crystalline silicon together with a solid feedstock;
melting the solid feedstock and part of the seed layer by
maintaining a solid/liquid interface substantially flat over the
entire seed layer; forming a solid body of silicon by extracting
heat through the seed layer while at least initially providing
extra heat in the local vicinity of at least one edge of the seed
layer; bringing the solid body to a first temperature; and cooling
the solid body to a second temperature.
22. An apparatus for casting silicon, comprising: at least one
primary resistive heater for melting of silicon for surrounding a
crucible resting on a heat sink; a means for the controlled
extraction of heat through the heat sink; a port for the
introduction of a gas; and an additional heater for encircling the
crucible to provide inductive heating at different regions within
the crucible.
23. The apparatus according to claim 22, wherein the additional
heater comprises one loop of a thermally insulated, water cooled,
electrically conductive tube residing with the at least one primary
resistive heater.
24. The apparatus according to claim 22, wherein the additional
heater moves relative to walls of the crucible.
25. The apparatus according to claim 22, further comprising at
least one seed crystal on a bottom of the crucible.
26. A method of manufacturing cast silicon, comprising: placing at
least one monocrystalline seed crystal of at least about 10 cm by
about 10 cm area on a bottom of a crucible that rests on a
partially insulating base plate; placing liquid silicon in contact
with the at least one seed crystal; forming a solid body of silicon
by extracting heat through the seed crystal in such a way that a
convex solid boundary increases a cross-sectional area of
monocrystalline growth; bringing the solid body to a first
temperature, and cooling the body to a second temperature; cutting
a slab from a side of the solid body opposite the seed crystal;
cleaning the slab using a chemical process; and using the slab as a
seed layer for a subsequent casting process.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/951,155, filed Jul. 20, 2007. The entire
disclosure of U.S. Provisional Application No. 60/951,155 is hereby
incorporated by reference into this specification.
DESCRIPTION
[0003] 1. Technical Field
[0004] The present invention generally relates to the field of
photovoltaics and to methods and apparatuses for manufacturing cast
silicon for photovoltaic applications. The invention further
relates to new forms of cast silicon that can be used to
manufacture devices, such as photovoltaic cells and other
semiconductor devices. The new silicon can have a monocrystalline,
near-monocrystalline, bi-crystal, or geometric multicrystalline
structure and can be manufactured by a casting process utilizing
seed crystals.
[0005] 2. Background Information
[0006] Photovoltaic cells convert light into electric current. One
of the most important features of a photovoltaic cell is its
efficiency in converting light energy into electrical energy.
Although photovoltaic cells can be fabricated from a variety of
semiconductor materials, silicon is generally used because it is
readily available at reasonable cost, and because it has a suitable
balance of electrical, physical, and chemical properties for use in
fabricating photovoltaic cells.
[0007] In a known procedure for the manufacture of photovoltaic
cells, silicon feedstock is doped with a dopant having either a
positive or negative conductivity type, melted, and then
crystallized by either pulling crystallized silicon out of a melt
zone into ingots of monocrystalline silicon (via the Czochralski
(CZ) or float zone (FZ) methods), or cast into blocks or "bricks"
of multi-crystalline silicon or polycrystalline silicon, depending
on the grain size of the individual silicon grains. As used herein,
the term "monocrystalline silicon" refers to a body of single
crystal silicon, having one consistent crystal orientation
throughout. Further, conventional multi-crystalline silicon refers
to crystalline silicon having centimeter scale grain size
distribution, with multiple randomly oriented crystals located
within a body of multicrystalline silicon. As used herein, however,
the term "geometrically ordered multicrystalline silicon"
(hereinafter abbreviated as "geometric multicrystalline silicon")
refers to crystalline silicon, according to embodiments of the
present invention, having a geometrically ordered centimeter scale
grain size distribution, with multiple ordered crystals located
within a body of multi-crystalline silicon. For example, in
geometric multi-crystalline silicon, the grains are typically an
average of about 0.5 cm to about 5 cm in size, and grain
orientation within a body of geometric multi-crystalline silicon is
controlled according to predetermined orientations. Further, as
used herein, the term "polycrystalline silicon" refers to
crystalline silicon with micrometer scale grain size and multiple
grain orientations located within a given body of crystalline
silicon. For example, the grains are typically an average of about
submicron to about micron in size (e.g., individual grains are not
visible to the naked eye), and grain orientation distributed
randomly throughout. In the procedure described above, the ingots
or blocks are cut into thin substrates, also referred to as wafers,
by known slicing or sawing methods. These wafers may then be
processed into photovoltaic cells.
[0008] Monocrystalline silicon for use in the manufacture of
photovoltaic cells is generally produced by the CZ or FZ methods,
both being processes in which a cylindrically shaped boule of
crystalline silicon is produced. For a CZ process, a seed crystal
is touched to a pool of molten silicon and the boule is slowly
pulled out of the pool while heat is extracted through the solid
part of the boule. As used herein, the term "seed crystal" refers
to a piece of crystalline material that is brought in contact with
liquid silicon such that, during solidification, the liquid silicon
adapts to the crystallinity of the seed. For a FZ process, solid
material is fed through a melting zone, melted upon entry into one
side of the melting zone, and re-solidified on the other side of
the melting zone, generally by contacting a seed crystal.
[0009] Recently, a new technique for producing monocrystalline or
geometric multicrystalline material in a casting station has been
invented, as disclosed in U.S. patent application Ser. Nos.:
11/624,365 and 11/624,411 and published as U.S. Patent Application
Publication Nos.: 20070169684A1 and 20070169685A1, filed Jan. 18,
2007. Casting processes for preparing multicrystalline silicon
ingots are known in the art of photovoltaic technology. Briefly, in
such processes, molten silicon is contained in a crucible, such as
a quartz crucible, and is cooled in a controlled manner to permit
the crystallization of the silicon contained therein. The block of
cast crystalline silicon that results is generally cut into bricks
having a cross-section that is the same as or close to the size of
the wafer to be used for manufacturing a photovoltaic cell, and the
bricks are sawn or otherwise cut into such wafers.
Multi-crystalline silicon produced in such manner is an
agglomeration of crystal grains where, within the wafers made
therefrom, the orientation of the grains relative to one another is
effectively random. Monocrystalline or geometric multicrystalline
silicon has specifically chosen grain orientations and (in the
latter case) grain boundaries, and can be formed by the new casting
techniques disclosed in the above-mentioned patent applications by
bringing liquid silicon in contact with a large seed layer that
remains partially solid during the process and through which heat
is extracted during solidification. As used herein, the term `seed
layer` refers to a crystal or group of crystals with desired
crystal orientations that form a continuous layer. They can be made
to conform with one side of a crucible for casting purposes.
[0010] In order to produce the best quality cast ingots, several
conditions should be met. Firstly, as much of the ingot as possible
have the desired crystallinity. If the ingot is intended to be
monocrystalline, then the entire usable portion of the ingot should
be monocrystalline, and likewise for geometric multicrystalline
material. Secondly, the silicon should contain as few imperfections
as possible. Imperfections can include individual impurities,
agglomerates of impurities, intrinsic lattice defects and
structural defects in the silicon lattice, such as dislocations and
stacking faults. Many of these imperfections can cause a fast
recombination of electrical charge carriers in a functioning
photovoltaic cell made from crystalline silicon. This can cause a
decrease in the efficiency of the cell.
[0011] Many years of development have resulted in a minimal amount
of imperfections in well-grown CZ and FZ silicon. Dislocation free
single crystals can be achieved by first growing a thin neck where
all dislocations incorporated at the seed are allowed to grow out.
The incorporation of inclusions and secondary phases (for example
silicon nitride, silicon oxide or silicon carbide particles) is
avoided by maintaining a counter-rotation of the seed crystal
relative to the melt. Oxygen incorporation can be minimized using
FZ or Magnetic CZ techniques as is known in the industry. Metallic
impurities are generally minimized by being left in the potscrap or
the tang end after the boule is brought to an end.
SUMMARY OF THE INVENTION
[0012] According to some embodiments, this invention relates to a
method and apparatus of controlling heat flow during the casting of
silicon, particularly flow of heat through a seed crystal.
Desirably, the silicon melts with a flat interface, but growth
occurs with as much curvature as possible, such as to maximize an
amount of monocrystalline silicon material. The embodiments of this
invention balance the needs of melting a feedstock and growing the
crystalline material.
[0013] During the melting steps, heat can be conducted through the
seed crystal to ensure melting of the feedstock while maintaining
at least a portion of the seed crystal as a solid to initiate
crystal growth orientation during the solidification steps. During
the solidification steps, it is desirable to reduce and/or prevent
heat loss through walls of a crucible, such as to minimize an
amount of multicrystalline material produced. Since the bottom of
the crucible is in thermal communication with a heat sink and the
material for the crucible typically includes a higher thermal
conductivity than molten silicon, the sides of the crucible cool as
well. Surprisingly and unexpectedly, a configuration of a thermally
conducting material and a thermally insulating material placed
under the crucible allows control of heat flow to improve
solidification by increasing an amount of monocrystalline silicon
produced from the seed crystals.
[0014] As used herein, the term "near-monocrystalline silicon"
refers to a body of crystalline silicon, having one consistent
crystal orientation throughout for greater than 50% by volume of
the body, where, for example, such near-monocrystalline silicon may
comprise a body of single crystal silicon next to a
multicrystalline region, or it may comprise a large, contiguously
consistent crystal of silicon that partially or wholly contains
smaller crystals of silicon of other crystal orientations, where
the smaller crystals do not make up more than 50% of the overall
volume. Preferably, the near-monocrystalline silicon may contain
smaller crystals which do not make up more than 25% of the overall
volume. More preferably, the near-monocrystalline silicon may
contain smaller crystals which do not make up more than 10% of the
overall volume. Still more preferably, the near-monocrystalline
silicon may contain smaller crystals which do not make up more than
5% of the overall volume.
[0015] As used herein, the term "bi-crystal silicon" refers to a
body of silicon, having one consistent crystal orientation
throughout for greater than or equal to 50% by volume of the body,
and another consistent crystal orientation for the remainder of the
volume of the body. For example, such bi-crystal silicon may
comprise a body of single crystal silicon having a one crystal
orientation next to another body of single crystal silicon having a
different crystal orientation making up the balance of the volume
of crystalline silicon. Preferably, the bi-crystal silicon may
contain two discrete regions within the same body of silicon, the
regions differing only in their crystal orientation.
[0016] In accordance with the invention as embodied and broadly
described, there is provided a method of manufacturing cast
silicon, comprising: placing a crucible filled with silicon on a
layer, the layer comprising: a thermally conducting material in
contact with a heat sink, and a thermally insulating area, where a
thermally conductive part of the layer is in contact with about 5%
to about 99% of a bottom surface of the crucible; and solidifying
the silicon by extracting heat through the thermally conducting
layer. The heat extraction may occur after part or all of the
silicon is melted, in order to direct seeded growth by bringing the
cast silicon to a first temperature and then cooling it to a second
temperature.
[0017] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising placing
silicon in a crucible having walls tapered inwards towards a center
of the crucible, melting the silicon, solidifying the silicon by
extraction of heat through a bottom of the crucible, bringing the
cast silicon to a first temperature, cooling the silicon down to a
second temperature different from the first temperature, extracting
the cast silicon from the crucible and then cutting sections from
the cast silicon.
[0018] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising placing
silicon in a crucible having walls tapered outwards away from a
center of the crucible, melting the silicon, solidifying the
silicon by extraction of heat through a bottom of the crucible,
bringing the cast silicon to a first temperature, cooling the
silicon down to a second temperature different from the first
temperature, extracting the cast silicon from the crucible and then
cutting sections from the cast silicon.
[0019] In accordance with the present invention, there is also
provided a crucible for the casting of silicon having a bottom
surface and a plurality of side walls, wherein at least one of the
plurality of side walls tapers inwards toward a center of the
crucible at an angle from about 1.degree. to about 25.degree. with
respect to a plane perpendicular to a bottom surface of the
crucible and viewed in a direction extending upwards from the
bottom surface. The tapered side wall or walls may reduce the
vessel cross-sectional area taken in the direction away from the
bottom surface.
[0020] In accordance with the present invention, there is also
provided a crucible for the casting of silicon having a bottom
surface and a plurality of side walls, wherein at least one of the
plurality of side walls tapers outwards from a center of the
crucible at an angle greater than about 2.degree., with respect to
a plane perpendicular to a bottom surface of the crucible and
viewed in a direction extending upwards from the bottom surface.
The tapered side wall or walls may increase the vessel
cross-sectional area taken in the direction away from the bottom
surface.
[0021] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising:
coating inner side walls of a crucible with a release coating,
leaving a bottom wall uncoated; placing silicon seed crystals in
contact with the uncoated wall, placing silicon feedstock in the
crucible, melting the feedstock while maintaining the seed crystals
in at least a partially solid state, solidifying the silicon by
extracting heat through the seed crystals, bringing silicon to a
first temperature and cooling the silicon to a second
temperature.
[0022] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising:
slicing a previously cast ingot into slabs, chemically treating the
slabs to remove impurities, placing the slab in a crucible for use
as a seed layer and then filling the crucible with feedstock for
casting.
[0023] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising:
placing a layer of monocrystalline silicon seed crystals on at
least one surface in a crucible such that seed crystals in a center
region of the layer have one crystal pole direction perpendicular
to the surface and cover about 50% to about 99% of the layer area,
while the remaining seed crystals on the edges of the layer have at
least one different crystal pole direction perpendicular to the
surface and cover the remaining layer area; adding feedstock
silicon and bringing the feedstock and a portion of the seed layer
to a molten state; solidifying the silicon by extracting heat
through the seed layer; bringing the silicon to a predetermined,
for example, uniform first temperature and then preferably
uniformly cooling the silicon down to a uniform second
temperature.
[0024] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising:
placing at least one monocrystalline seed crystal having at least
about 10 cm by about 10 cm area on a bottom surface of a crucible
that rests on a partially insulating base plate; introducing solid
or liquid silicon feedstock and partially melting the seed crystal,
extracting heat through the seed crystal in such a way that a
convex solid boundary increases the cross-sectional area of
monocrystalline growth; bringing the silicon to a first temperature
and cooling it, preferably uniformly, down to a second temperature;
cutting a slab from a side of the cast silicon opposite the seed
crystal; cleaning the slab using a chemical process; and using the
large slab as a new seed layer for a subsequent casting
process.
[0025] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising:
loading a seed layer of crystalline silicon together with solid
silicon feedstock into a crucible having a lid or cover; melting
and solidifying the silicon while maintaining part of the seed
layer as solid and while flowing at least one of argon and nitrogen
gas through at least one hole in the lid or cover while at least
another hole exits the gas; and cooling the silicon preferably
uniformly.
[0026] In accordance with the present invention, there is also
provided a method of manufacturing cast silicon, comprising:
loading a seed layer of crystalline silicon into a crucible,
covering the crucible having a lid; introducing liquid silicon into
the crucible, the liquid silicon preferably being superheated;
allowing part of the seed layer to melt; solidifying the silicon
while flowing at least one of argon and nitrogen gas through at
least one hole in the lid while at least one other hole exits the
gas; and cooling the silicon.
[0027] In accordance with the present invention, there is also
provided a process for manufacturing cast silicon comprising
loading a seed layer of crystalline silicon together with solid
feedstock; melting the feedstock and part of the seed layer while
maintaining a solid/liquid interface that is essentially flat over
a center portion of the seed layer, and convex at the edges of the
seed layer; solidifying the silicon by extracting heat through the
seed layer while at least initially maintaining the same
solid/liquid interface shape; bringing the silicon to a first
temperature and cooling the silicon to a second temperature, the
heating and cooling preferably being uniform.
[0028] In accordance with the present invention, there is also
provided a process for manufacturing cast silicon comprising
loading a seed layer of crystalline silicon together with solid
feedstock; melting the feedstock and part of the seed layer while
maintaining a solid/liquid interface that is substantially flat
over the entire seed layer; solidifying the silicon by extracting
heat through the seed layer while at least initially providing
extra heat in a region comprising the edges of the seed layer;
bringing the silicon to a first temperature and preferably
uniformly cooling the silicon to a second temperature, the heating
and cooling preferably being uniform.
[0029] In accordance with the present invention, there is also
provided an apparatus for the casting of silicon comprising heaters
for surrounding a crucible resting on a heat sink, the heaters
being provided for the melting of silicon; a means for controlled
extraction of heat through the heat sink; a port for introduction
of a gas; and at least one loop of insulated, water cooled tube
residing with the primary heaters and for encircling the crucible,
wherein the loop can be energized to provide inductive heating at
different regions within the crucible.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and, together with the description, serve to explain
the features, advantages, and principles of the invention. In the
drawings:
[0031] FIGS. 1A-1B illustrate an exemplary system where a thermally
insulating layer is combined with a thermally conducting layer
below a crucible in a casting station, according to an embodiment
of the present invention;
[0032] FIGS. 2A-2D illustrate two examples of tapered crucibles
together with illustrations of the desired effects on the silicon
cast therein, according to embodiments of the present
invention;
[0033] FIG. 3 illustrates an example of silicon feedstock loaded
into a partially coated crucible, according to an embodiment of the
present invention;
[0034] FIG. 4 illustrates an example of a method for recycling seed
layer material, according to an embodiment of the present
invention;
[0035] FIG. 5 illustrates an exemplary arrangement of single
crystal silicon to form a seed layer, according to an embodiment of
the present invention;
[0036] FIG. 6 illustrates an exemplary method for creating large
single crystal seed layers, according to an embodiment of the
present invention;
[0037] FIGS. 7A-7B illustrate an exemplary apparatus for casting
low carbon monocrystalline or multicrystalline silicon, according
to an embodiment of the present invention;
[0038] FIG. 8 illustrates an exemplary apparatus for casting
monocrystalline or multi-crystalline silicon, according to
embodiments of the present invention; and
[0039] FIGS. 9A-D illustrate an exemplary system where a thermally
insulating layer is combined with a thermally conducting layer in
an alternate geometry, according to an embodiment of the present
invention.
DESCRIPTION OF THE EMBODIMENTS
[0040] Reference will now be made in detail to embodiments of the
present invention, examples of which are illustrated in the
accompanying drawings. Wherever possible, the same or similar
reference numbers will be used throughout the drawings to refer to
the same or like parts.
[0041] In embodiments consistent with the invention, the
crystallization of molten silicon is conducted by casting processes
using seed crystals. As disclosed herein, such casting processes
may be implemented so that the size, shape, and orientation of
crystal grains in the cast body of crystallized silicon is
controlled. As used herein, the term "cast" means that the silicon
is formed by cooling molten silicon in a mold or vessel used to
hold the molten silicon. By way of example, the silicon can be
formed by solidification in a crucible, where solidification is
initiated from at least one wall of the crucible, and not through a
cooled foreign object drawing silicon out of the crucible. Thus,
the crystallization of molten silicon is not controlled by
"pulling" a boule either by moving a seed or moving the mold,
vessel, or crucible. Further, consistent with an embodiment of the
present invention, the mold, vessel, or crucible includes at least
one hot side wall surface for solidifying the molten silicon. As
used herein, the term "hot-wall" refers to a surface that is
isothermal or hotter than molten silicon. Preferably, a hot-wall
surface remains fixed during processing of the silicon.
[0042] Consistent with embodiments of the invention, the
crystallized silicon can be either continuous monocrystalline, or
continuous multi-crystalline having controlled grain orientations.
As used herein, the term "continuous monocrystalline silicon"
refers to single crystal silicon, where the body of silicon is one
homogeneous body of monocrystalline silicon and not smaller pieces
of silicon joined together to form a larger piece of silicon.
Further, as used herein, the term "continuous multi-crystalline
silicon" refers to multi-crystalline silicon where the body of
silicon is one homogeneous body of multi-crystalline silicon and
not smaller pieces of silicon joined together to form a larger
piece of silicon.
[0043] Casting of silicon, according to embodiments of the present
invention, can be accomplished by positioning a desired collection
of crystalline silicon "seeds" in, for example, the bottom of a
vessel, such as a quartz crucible that can hold molten silicon. The
seeds may cover all, or most, or substantially all, of the bottom
of the crucible. As used herein, the term "seed" refers to a
geometrically shaped piece of silicon with a desired crystal
structure, having a side that conforms to a surface of a vessel in
which it may be placed. Such a seed can be either a monocrystalline
piece of silicon or a piece of geometrically ordered
multi-crystalline silicon. Consistent with the present invention, a
seed may have a top surface that is parallel to its bottom surface,
although this does not have to be the case. For example, a seed can
be a piece of silicon, varying in size from about 2 mm to about 10
mm across, to about 100 mm to about 1000 mm across. The piece of
silicon may have a thickness of about 1 mm to about 1000 mm,
preferably about 10 mm to about 50 mm. A suitable size and shape of
the seed may be selected for convenience and tiling. Tiling, which
will be described in more detail below, is where silicon seed
crystals are arranged in a predetermined geometric orientation or
pattern across either the bottom or the sides and bottom surfaces
of a crucible.
[0044] Silicon feedstock may then be introduced into the crucible
over the seeds, and then the feedstock is melted. Alternatively,
molten silicon may be poured directly into the crucible and over
the seeds. When molten silicon is poured, the crucible is
preferably first brought very close to or up to the melting
temperature of silicon, and then the molten silicon is poured in.
Consistent with embodiments of the invention, a thin layer of the
seeds can be melted before solidification begins.
[0045] The molten silicon is then allowed to cool and crystallize
in the presence of the seeds, preferably in a manner such that the
cooling of the molten silicon is conducted so that the
crystallization of the molten silicon starts at or below the level
of the original top of the solid seeds and proceeds away,
preferably upwards away, from the seeds. This can be accomplished
by extracting the heat of fusion through the seed crystals to a
heat sink. As used herein, the term "heat sink" refers to a body of
material used to extract heat from another body of material. A heat
sink may extract heat by means of conduction of heat from a higher
temperature area to a lower temperature area, by convection with a
lower temperature fluid or by direct radiation of energy to a lower
temperature object. A thermal gradient is generally maintained
across a heat sink such that one side is in equilibrium with the
object to be cooled while the other exchanges energy with a cooler
area.
[0046] According to embodiments of the invention, the liquid-solid
interface between the molten silicon and the crystallized silicon,
during melting or solidification, need not be maintained
substantially flat throughout the casting process. That is, the
solid-liquid interface at an edge of the molten silicon is
controlled during the cooling so as to move in a direction that
increases a distance between the molten silicon and the silicon
seed crystal. As the solidification of the molten silicon starts,
the solidification front is initially substantially flat,
preferably with a strong curvature at the horizontal edges of the
growing solid mass of silicon. The shape of the solid-liquid
interface thus may have a controlled profile throughout the casting
process.
[0047] By conducting the crystallization of the molten silicon in a
manner consistent with embodiments of the invention, cast silicon
having specific, rather than random, grain boundaries and specific
grain sizes can be made. Additionally, by aligning the seeds in a
manner such that all seeds are oriented the same relative direction
to each other, for example the (100) pole direction being
perpendicular to a bottom of the crucible and the (110) pole
direction at 45.degree. to the sides of a rectangular or square
cross-section crucible, large bodies of cast silicon can be
obtained that are, or are essentially, monocrystalline silicon in
which the pole direction of such cast silicon is the same as that
of the seeds. Similarly, other pole directions may be perpendicular
to the bottom of the crucible. Moreover, one or more seeds may be
arranged so that any common pole direction is perpendicular to a
bottom of the crucible. Furthermore, consistent with an embodiment
of the invention, seed crystals of two or more different pole
directions can be used together to maximize the effectiveness of
the crystal growth, creating a volume of silicon as large as
possible with the desired crystal orientation.
[0048] The seeds used for casting processes, consistent with
embodiments of the invention, can be of any desired size and shape,
but are suitably geometrically shaped pieces of monocrystalline, or
geometrically ordered multi-crystalline, silicon, such as square,
rectangular, hexagonal, rhomboid or octagonal shaped pieces of
silicon. They can be shaped conducive to tiling, so they can be
placed or "tiled" edge-to-edge and conformed to the bottom of a
crucible in a desired pattern. Also consistent with embodiments of
the invention, seeds can be placed on one or more sides of the
crucible. Such seeds can be obtained, for example, by sawing a
source of crystalline silicon, such as a boule of monocrystalline
silicon, into pieces having the desired shapes. The seeds can also
be formed by cutting them from a sample of silicon made by a
process according to the embodiments of the invention, such that
seeds for use in subsequent casting processes can be made from an
initial casting process. For example, a smaller piece of
dislocation-free seed material can used to grow a large dislocation
free single crystal, sufficient to cover the entire bottom of the
crucible for use as a new seed crystal layer.
[0049] Processes and apparatuses for preparing silicon in
accordance with embodiments of the invention will now be described.
However, it is to be understood that these are not the only ways to
form silicon consistent with the embodiments of the invention.
[0050] Referring to FIGS. 1A and 1B, the cross-section of a casting
station hot zone is depicted in FIG. 1A, showing liquid silicon 100
and solid silicon 101 at the end of the melting stage of a seeded
casting process. The silicon is positioned in a bottomed and walled
crucible 110, which may be, for example, a fused quartz or silica
crucible. At this point, solid silicon 101 in crucible 110 is
entirely constituted from a seed layer of silicon previously loaded
at the bottom of the crucible. Feedstock silicon (not shown) is
introduced on top of the seed layer. Feedstock silicon can either
be loaded as a solid and then melted in the crucible, or melted in
a separate container and introduced as a liquid on top of the
seeds. In either case, the original silicon seed layer is partially
melted and solid silicon 101 is entirely composed of the remainder
of the silicon seed layer. Preferably, crucible 110 has a release
coating such as one made from silica, silicon nitride, or a liquid
encapsulant, to aid in the removal of crystallized silicon from
crucible 110.
[0051] Still referring to FIG. 1A, in this depiction of a furnace
hot zone, resistive heaters 120 provide the energy to maintain the
temperature required to melt silicon, while insulation 130 prevents
the escape of heat to an outer chamber (not shown). Consistent with
an embodiment of the invention, crucible 110 is supported by a
number of layers which also serve to conduct heat away from the
silicon in a controlled way. For example, a heat conducting block
140 radiates heat to a water cooled chamber (not shown), thereby
cooling the hot-zone components above it. A graphite support plate
142, shown in cross section in FIG. 1A and in three dimensions in
FIG. 1B, conducts heat from heat conducting layer 141, which in
turn conducts heat away from crucible 110 and silicon 100 and 101.
A thermally insulating layer 150 may surround heat conducting layer
141, in an exemplary configuration, in order to alter the heat
removal path and consequently alter the shape of the solidification
front. Solid graphite side plates 143 surround crucible 110 and
provide structural support to the crucible. Consistent with
embodiments of the invention, the casting station may have a
graphite support plate 142, though a tailored heat conduction path
controlled by conducting layer 141 and thermally insulating layer
150 is not required.
[0052] Still referring to FIGS. 1A and 1B, graphite side plates 143
may rest on graphite support plate 142, and conduct heat directly
to plate 142, which may create cold spots at the bottom edges of
the crucible. The effect of the tailored heat conduction, vis-a-vis
layers 141 and 150, however, can alter the cooling parameters, and,
hence, the shape of the liquid/solid interface by, for example,
keeping the corners of crucible 110 hotter, resulting in only a
small amount of lateral melting. For example, as shown in FIG. 1A,
solid silicon 101 has a high curvature at its left and right edges
due to the heat exchange occurring in materials below crucible 110.
Such a curvature can result in the lateral expansion of the solid
and outward growth of a seeded crystal structure. In FIG. 1A,
crystal growth directions of solid silicon 101 are indicated by
black arrows.
[0053] Referring to FIGS. 2A-2D, crystal growth of silicon maybe
altered by altering the shape of the crucible. For example, crystal
growth can be accomplished in an outwardly tapered crucible 200, as
shown in FIGS. 2A and 2B, where the curvature of liquid silicon 220
to the solid silicon 221 promotes lateral expansion of the seeded
crystal (not shown), whose growth direction is indicated by arrows
in FIG. 2B. In another example, crystal growth can be accomplished
in an inwardly tapered crucible 210, as shown in FIGS. 2C and 2D,
which, like crucible 200 in FIG. 2A, also has the advantage of
maximizing the amount of usable cast silicon 222, and minimizing
the amount of unusable or undesirable silicon 223 to be removed
during cutting of the cast silicon ingot (222+223) into bricks
(shown by dashed lines). The tapered shape of undesirable silicon
223 on a side wall of the cast silicon (viewed in cross-section in
FIG. 2D) is due to the extra time that the silicon at the bottom of
the crucible spends at a high temperature state during
solidification and crystal growth compared with the silicon at the
top of the ingot, which is cooled more quickly.
[0054] FIG. 3 illustrates a cross-section of silicon (feedstock 300
and crystalline seeds 301) loaded into crucible 310 for casting.
Release coating 320, such as silicon nitride or silicon carbide,
may be applied to areas of crucible 310 where feedstock 300
contacts crucible 310, which corresponds to areas of silicon 300
that will become completely melted during casting. No coating has
been applied below crystalline seeds 301. Seeds 301 will not be
completely melted and thus will not adhere to crucible 110.
[0055] FIG. 4 illustrates a process for the reuse of a crystalline
silicon seed layer. As shown in FIG. 4, cast ingot 400 grown from
seed layer 401 is first sliced along the dotted lines to remove a
slab of material containing seed layer 401. The slab of material is
then trimmed at the dotted edges to remove excess material that
might interfere with its placement in another crucible. Trimmed
slab 402, having been trimmed to the size and shape of original
seed layer 401, is then treated, potentially with other similar
pieces of silicon, in a container 410, such as a tank or a tub
containing a suitable liquid or other material, to remove
contaminants and debris from layer 401 (and possibly other pieces
of silicon) before being placed in a new crucible 420 for use as a
seed layer in a subsequent casting process.
[0056] FIG. 5 illustrates an exemplary arrangement of single
crystal silicon pieces arranged to form a seed layer. The (001)
crystal orientation has been shown to have advantageous properties
for the manufacture of silicon solar cells. (001) silicon may be
chemically etched in such a way as to produce a pattern pyramids
covering its entire surface, which can improve the light-trapping
ability of the silicon by both decreasing reflection and increasing
the path length of light in the material. Chemical etching may be
accomplished by known methods. However, the casting of (001)
silicon is made difficult by its tendency to grow grain boundaries
at acute angles to its (001) pole direction when located next to a
multicrystalline region of silicon. To counteract the growth of
multicrystalline silicon, a geometric arrangement of a plurality of
monocrystalline silicon seed crystals can be placed on at least one
surface in a crucible (not shown), e.g., a bottom surface of a
crucible, wherein the geometric arrangement includes close-packed
polygons. As shown in FIG. 5, a piece of (001) silicon 500 is
surrounded by a periphery of rectangles of (111) silicon 501. The
pole orientation of the peripheral silicon 501 is shown as (111),
but it could be any crystal orientation that is competitively
favored when grown next to a multicrystalline region. In this way,
the majority of a resulting cast ingot (not shown) will be composed
of (001) silicon, and the competitively favored (111) grains grown
from silicon 501 will limit the growth of multicrystalline silicon
in the region occupied by (001) silicon over silicon 500.
Similarly, silicon crystal grains produced by casting a body of
multi-crystalline silicon, consistent with embodiments of the
invention, may be grown in a columnar manner. Further, such crystal
grains may have a cross section that is, or is close to, the shape
of the seed from which it is formed, instead of having an (001)
cross-sectional area that shrinks as solidification proceeds. When
making silicon that has such specifically selected grain
boundaries, preferably the grain boundary junctions only have three
grain boundaries meeting at a corner, a condition met in the
arrangement shown in FIG. 5.
[0057] FIG. 6 illustrates a process for manufacturing large area,
dislocation-free single crystals for use as seed layers. In this
process, depicted in cross-section, polycrystalline feedstock 600
is loaded together with a single crystal seed 601 which may have
lateral dimensions from about 25 cm.sup.2 to about 10,000 cm.sup.2
in area and a thickness from about 3 mm to about 1000 mm. Feedstock
600 is placed in crucible 610, which is then placed in a station
(not shown) on top of layers 620, 621, and 630, composed of
thermally conducting (620) and thermally insulating (630) parts.
The area of the thermally conducting parts 620 should preferably be
about the same shape of bottom of crucible 610, having a lateral
area from about 50% to about 150% that of seed crystal 601. During
melting, heat is extracted through thermally conducting area 620 to
a support plate 621, while heat is prevented from passing through
thermally insulating layer 630. Heat is conducted out through
thermally conducting area 620 even during the melting phase of
casting, in order to prevent the complete melting of seed crystal
601. Once all feedstock 600 and a small portion of seed crystal 601
are melted into liquid silicon 602, remaining solid silicon 603
then acts as the nucleation layer for the solidification process.
The presence of insulating layer 630 helps control the shape of
solid silicon 603 during nucleation and growth, as well as the
direction of solidification, indicated by arrows in FIG. 6. The
strong curvature in the solidification surface causes an outward
growth of solid silicon 603, while multicrystalline regions 605 are
minimized. Once ingot 604 is cast, horizontal layers may be cut
(dashed lines) from the upper parts of the ingot to be used as new
seed slabs 606. Slabs 606 can be cleaned, trimmed, and used as a
complete seed layer for a new ingot in a new crucible 610, or as a
starting point for an even larger single crystal, again using the
process just described.
[0058] FIGS. 7A and 7B are depictions of the cross-section of an
apparatus for the casting of low carbon monocrystalline or
multicrystalline silicon in a seeded ingot. As shown in FIG. 7A,
seed crystal 700 is loaded together with feedstock 701 in crucible
710 located in a furnace hot zone (unlabeled). Crucible 710, though
illustrated as covered with ceramic lid 711 (also shown in FIG.
7B), may be uncovered and completely open to the surrounding
atmosphere. In casting, carbon can be incorporated into an ingot
from detached pieces of graphite insulation 720 which may fall into
crucible 710, or by a gas phase reaction where oxygen from crucible
710 dissolves into the silicon melt and then evaporates as SiO
molecules (not shown). These molecules can adhere to graphite parts
720, 750, 760 of the furnace and react via the reaction
SiO+2C.fwdarw.SiC+CO.
[0059] The CO gas molecule enters the liquid where SiC forms and O
is again liberated to repeat the cycle. By introducing ceramic lid
711 (shown in FIG. 7B) to crucible 710, and carefully controlling
process gas 730 (which may be, for example, argon), both mechanisms
of carbon incorporation can be effectively stopped, or severely
restricted. Ceramic lid 711 can be made of a number of materials
including, for example, fused silica, quartz, silicon carbide,
silicon nitride, and the like. It is desirable for the design that
a fresh supply of an inert gas, such as argon, come in through
channel 740 and exit through another channel (not shown) in order
to prevent the above-described carbon gas reaction.
[0060] Still referring to FIGS. 7A and 7B, the casting process can
be operated either by loading one or more seeds 700 and feedstock
701 prior to installing crucible 710 in the furnace, or by loading
only one or more seeds 700 and later introducing liquid silicon 750
into the crucible from a separate melt chamber.
[0061] FIG. 8 illustrates an apparatus consistent with embodiments
of this invention for modifying the shape of the solid-liquid
interface during casting. As shown in FIG. 8, primary heaters 820
and an additional heater 840 are placed in the hot zone (shown
surrounded by insulation 831) of a casting station to introduce
targeted heating to material 800, 801. Liquid material 800 on top
of solid seed material 801 has an interface that is curved at the
edges, near the side walls of crucible 810. Primary heaters 820
together with primary heat sink 860 normally work to produce a
substantially flat solid-liquid interface (not shown). However,
additional heater 840 couples an electric current directly to
material 800, 801, introducing inductive heating to the edges of
the material 800, 801 near the walls of crucible 810, and thereby
melts solid material 801 in its vicinity.
[0062] Additional heater 840, as shown in FIG. 8, is a coil of
conductive metal, which may be, for example, copper, that is cooled
with circulating liquid 850 and thermally insulated from primary
heaters 820 by surrounding layer 830. Additional heater 840 may be
a single turn coil surrounding crucible 810 in a loop, as
illustrated in FIG. 8, or it may have multiple loops forming a
helix having any desired spacing between loops constituting the
helix. Additional heater 840 may also be configured so that it can
move relative to the walls of crucible 810 in order to affect the
solid-liquid interface (not shown). Additional heater 840 operates
by electrical current flowing through the copper pipe while the
water cools it so the current through the pipe forms a strong
magnetic field which couples with the liquid silicon, inducing a
corresponding current in the silicon. Resistive heat from the
current in and/or through the silicon provides the heating action
in a localized way and/or manner.
[0063] Alternately, resistive heaters could be used as additional
heaters 840, but resistive heaters may not be as efficient in
targeting the heat application to a specific volume of material,
such as material 800, 801. During casting with the apparatus shown
in FIG. 8, additional heater 840 would only be activated near the
end of the melting cycle, so as not to overly melt seed material
801. Additional heater 840 would continue to apply heat to crucible
810 through at least about the first 20% of the solidification
process. Additional heater 840 may also continue to apply heat to
crucible 810 through the entire solidification process until
implementation of the cooling stage.
[0064] As disclosed herein, embodiments of the invention can be
used to produce large bodies of monocrystalline silicon,
near-monocrystalline silicon, bi-crystal silicon, or geometric
multi-crystalline silicon, by a simple and cost-effective casting
process. The silicon feedstock used in processes consistent with
embodiments of the invention, and thus the silicon produced, can
contain one or more dopants selected from a list including: boron,
aluminum, lithium, gallium, phosphorus, antimony, arsenic, and
bismuth. The total amount of such dopant or dopants can be about
0.01 parts per million (ppm) by atomic % (ppma) to about 2 ppma.
Preferably, the amount of dopant or dopants in the silicon is an
amount such that a wafer made from the silicon has a resistivity of
about 0.1 to about 50 ohm-cm, preferably of about 0.5 to about 5.0
ohm-cm. Alternately, other materials having a suitable liquid phase
can be cast using the processes and apparatuses disclosed here. For
example, germanium, gallium arsenide, silicon germanium, sapphire,
and a number of other III-V or II-VI materials, as well as metals
and alloys, could be cast according to embodiments of the present
invention.
[0065] Moreover, although casting of silicon has been described
herein, other semiconductor materials and nonmetallic crystalline
materials may be cast without departing from the scope and spirit
of the invention. For example, the inventors have contemplated
casting of other materials consistent with embodiments of the
invention, such as germanium, gallium arsenide, silicon germanium,
aluminum oxide (including its single crystal form of sapphire),
gallium nitride, zinc oxide, zinc sulfide, gallium indium arsenide,
indium antimonide, germanium, yttrium barium oxides, lanthanide
oxides, magnesium oxide, calcium oxide, and other semiconductors,
oxides, and intermetallics with a liquid phase. In addition, a
number of other group III-V or group II-VI materials, as well as
metals and alloys, could be cast according to embodiments of the
present invention.
[0066] According to some embodiments, suitable insulating materials
for the thermally insulating area may include carbon fiber
insulation board, carbon bonded carbon fiber (CBCF), alumina fiber,
silica fiber, fused silica, fused quartz, radiation reflector,
carbon fiber composite, and or any other substance having a
relatively high thermal conductivity and a stability at the
operating temperatures of the casting processes.
[0067] According to some embodiments, suitable conducting materials
for the thermally conducting material may include graphite, high
temperature metals, high temperature alloys, tungsten, molybdenum,
tantalum, silicon carbide, ceramics with sufficient thermal
conductivity and/or any other substance having a lower thermal
conductivity than the insulating material and a stability at
operating temperatures of the casting processes.
[0068] According to some embodiments, materials in the layer
include a ratio of thermal conductivities of at least about 20:1
(conductor/insulator), desirably at least about 50:1 and more
desirably at least about 100:1. For example, at the working
temperature range of about 1400.degree. C., graphite has a thermal
conductivity of 48 W/m/K conductivity while CBCF has a thermal
conductivity of 0.7 W/m/K, resulting in a ratio of about 68:1. When
measured at room temperature, the same materials have a ratio of
about 260:1.
[0069] According to some embodiments, the thermal conducting
material is framed by the thermally insulating area, such as to
form a square and/or a rectangular shape. Desirably, a conduction
window is at least generally congruent and/or conesponds with a
shape of the seed crystal arrangement. Alternately and as shown in
FIGS. 9A-D, extra cooling area is applied in the corners because
they are heated from two sides. FIG. 9A shows thermally conducting
layer 141 with a contoured thermally insulating layer and/or area
150, such as a width of insulating layer 150 in a middle of a side
is about double a width of insulating layer 150 in a corner. FIG.
9B shows crucible 110 in relation to thermally conducting layer 141
and thermally insulating layer 150. FIG. 9C shows an outline in
dashed lines of support walls 142 (typically graphite) placed on
the insulating layer 150. FIG. 9D shows an outline in dashed lines
of solid silicon 101 placed in crucible 110 and with respect to
thermally conducting layer 141 and thermally insulating layer
150.
[0070] According to some embodiments a method of shaping the
thermal conducting material and/or the thermal insulating material
includes the use of saws, routers and/or any other suitable device.
Any suitable configuration of the thermal conducting material
and/or the thermal insulating material is possible, such as
including lips, ledges, interlocking pieces, chamfers, rounded
corners, and the like.
[0071] Desirably, a solid perimeter of the partially melted seed
crystal remains roughly square. Any suitable ratio of an area of a
conducting window to a seed crystal area is possible, such as from
about 0.5 to about 2.0, desirably about 1.0, and even more
desirably about 0.9 to about 1.1.
[0072] According to some embodiments, the additional heater and/or
the water tube heater is movable relative the a height of the
crucible, such as to apply localized heat to the silicon during
solidification and adjust upwards and/or downwards with respect to
the solid-liquid interface. This dynamic capability allows for flat
melt/solid interface shapes during melting, with control of heat
input during solidification, such as to keep the walls warm and
minimize growth of multicrystalline material, while maximizing
growth of the desired monocrystalline silicon, near-monocrystalline
silicon, and/or geometric multicrystalline silicon.
[0073] According to other embodiments of this invention, dynamic
capabilities allow the apparatus to vary the heat flow, such as in
the melting segment and in the growth segments. The insulating area
may be increased and/or decreased by inserting and/or removing the
thermally insulating area from under the crucible and/or support
walls, for example. The layer may include markings, notches, pegs
and/or any other suitable devices to aid in positioning various
components. In other embodiments, the static insulation balances
between the needs and/or characteristics in melting and in
growth.
[0074] According to some embodiments, the invention includes a
method of manufacturing cast silicon, comprising placing a crucible
on a layer. The layer comprising a thermally conducting material, a
heat sink, and a thermally insulating area, where a thermally
conductive part of the layer contacts with a portion of a bottom
surface of the crucible. The method further comprises placing at
least one seed crystal on a bottom of the crucible, placing molten
silicon in contact with the at least one seed crystal, and forming
a solid body of silicon by extracting heat through the thermally
conducting material. Desirably, the method further includes forming
a portion of the solid body to include the at least one seed
crystal.
[0075] According to some embodiments, the invention includes a
method of manufacturing a solar cell comprising providing a solid
body of cast silicon, slicing the solid body of cast silicon to
form at least one wafer, forming a p-n junction by doping a surface
of the at least one wafer, and forming a surface neutralizing layer
and/or a back surface field and forming electrically conductive
contacts on at least one surface of the wafer.
[0076] According to some embodiments, a heat flux through the layer
changes from the step of melting the silicon to the step of forming
a solid body, such as to provide asymmetric melting and optimize
the casting process. Desirably, a minimum heat transfer to the heat
sink occurs during melting, such as is barely sufficient for
retaining solid silicon seed material on the crucible bottom.
During the melting, however, the area of heat transfer is as wide
as possible to encourage a flat melt/solid interface. During
cooling or solidification the heat sink experiences a higher heat
flux to cause solidification of the ingot, but the thermally
insulating area is increased to at least partially isolate the
graphite support walls and the side walls of the crucible from the
thermally conducting material and/or the heat sink. The effect of
this arrangement is to keep the sides warm and maintain a domed
melt/solid interface, minimizing growth of multicrystalline silicon
from the sidewalls.
[0077] Optionally, the step of extracting heat expands a lateral
area of the seeded crystal during solidification. The embodiment
may further include placing a solid silicon feedstock in the
crucible on top of the at least one seed crystal and melting the
solid silicon feedstock while cooling the bottom of the crucible to
maintain the at least one seed crystal in an at least partially
solid state.
[0078] Alternately, the step of placing molten silicon further
includes melting a silicon feedstock in a melt container separate
from the crucible, heating the crucible to melting temperature of
silicon, controlling heating so that the at least one seed crystal
in the crucible does not melt completely, and transferring the
molten silicon from the melt container into the crucible.
[0079] According to some embodiments, the thermally conducting
material contacts between about 5% to about 99% of the bottom
surface area of the crucible, and desirably at least about 90%.
Alternately, the thermally conducting material corresponds to a
size and a shape of the at least one seed crystal within the
crucible, such as having a ratio of an area of the thermal
conducting material to an area of the seed crystal from about 0.5
to about 2.0, and desirably from about 0.9 to about 1.
[0080] The method of manufacture may further include reducing
and/or enlarging the thermally conducting material and//or
thermally conducting area in contact with a crucible bottom by
adding and/or removing at least a portion of the thermally
insulating area, for example.
[0081] Desirably, but not necessarily, the heat sink comprises a
radiative heat sink, radiating heat to walls of a water-cooled
vessel. According to some embodiments, the thermally insulating
area forms a perimeter or a border around the thermally conducting
material. Alternately, the perimeter comprises a contoured shape
wider in a middle of a side of the layer than in corners of the
layer. The perimeter may thermally isolate graphite side support
walls for the crucible from the heat sink, such as to reduce
cooling and slow multicrystalline growth from the walls. The
perimeter may sometimes be referred to as a thermal ring.
[0082] According to some embodiments, the invention includes an
apparatus for casting of silicon comprising optionally a crucible,
optionally at least one seed crystal on a bottom of the crucible,
resistive heaters in thermal communication with the crucible, and a
layer. The layer comprises a thermally conducting material, a heat
sink, and a thermally insulating area, wherein a thermally
conductive part of the layer is for contact with a portion of a
bottom surface of the crucible on a side and the heat sink on an
opposite side. Desirably, the thermally insulating area forms a
perimeter around the thermally conducting material. Optionally, the
thermally insulating area is movable, such as comprising four or
more discrete pads or blocks, so that the thermally insulating area
increases, decreases and/or changes heat transferred through the
layer by moving with respect to the thermally conducting
material.
[0083] According to some embodiments, a ratio of thermal
conductivities of the thermally conducting material to the
thermally insulating area is at least about 20:1. In other
embodiments, a ratio of an area of the thermal conducting material
to an area of the seed crystal is from about 0.5 to about 2.0.
[0084] The invention also may include a process for manufacturing
cast silicon comprising the steps of loading a seed layer of
crystalline silicon together with a solid feedstock, melting the
solid feedstock and part of the seed layer by maintaining a
solid/liquid interface essentially flat over the center of the seed
layer, but convex in the solid portion at the edges of the seed
layer, forming a solid body of silicon by extracting heat through
the seed layer while maintaining the solid/liquid interface
essentially flat over the center of the seed layer, but convex in
the solid portion at the edges of the seed layer, bringing the
solid body to a first temperature, and cooling the solid body to a
second temperature.
[0085] The first temperature, such as a range of between about
1410.degree. C. and about 1300.degree. C. usually includes a
temperature gradient across and/or through the solid body. The
second temperature, such as about 1350.degree. C. on average
usually includes a reduced temperature gradient and/or a uniform
temperature profile across and/or through the solid body. The
reducing the temperature gradient may be referred to sometimes as
annealing in the context of this disclosure. Annealing may include
closing up the insulation, for example.
[0086] The invention also may include a method for manufacturing
cast silicon comprising loading a seed layer of crystalline silicon
together with a solid feedstock, melting the solid feedstock and
part of the seed layer by maintaining a solid/liquid interface
substantially flat over the entire seed layer, forming a solid body
of silicon by extracting heat through the seed layer while at least
initially providing extra heat in the local vicinity of at least
one edge of the seed layer, bringing the solid body to a first
temperature, and cooling the solid body to a second
temperature.
[0087] According to some embodiments, the invention includes an
apparatus for casting silicon comprising at least one primary
resistive heater for melting of silicon for surrounding a crucible
resting on a heat sink, a means for the controlled extraction of
heat through the heat sink, a port for the introduction of a gas;
and an additional heater for encircling the crucible to provide
inductive heating at different regions within the crucible.
Desirably, the additional heater comprises one loop of a thermally
insulated, water cooled, electrically conductive tube residing with
the at least one primary resistive heater. Also desirably, the
additional heater moves relative to walls of the crucible. The
apparatus may also include at least one seed crystal on a bottom of
the crucible.
[0088] The following examples are experimental results consistent
with embodiments of the invention. These examples are presented for
merely exemplifying and illustrating embodiments of the invention
and should not be construed as limiting the scope of the invention
in any manner.
EXAMPLE 1
[0089] Crucible preparation: A crucible was placed on a supporting
structure consisting of two layers. The bottom layer of the
supporting structure is a solid isomolded graphite plate measuring
80 cm by 80 cm by 2.5 cm which supported a composite layer. The
upper composite layer had an inner region that was a thermally
conducting isomolded graphite plate measuring 60 cm by 60 cm by 1.2
cm, and was surrounded on all sides by a 10 cm perimeter of
thermally insulating graphite fiber board of 1.2 cm thickness. In
this way, the composite layer completely covered the bottom
layer.
[0090] Seed preparation: A boule of pure Czochralski (CZ) silicon
(monocrystalline) obtained from MEMC, Inc. and having 0.3 ppma of
boron, was cut down along its length using a diamond coated band
saw so that it had a square cross section measuring from 140 mm per
side. The resulting block of monocrystalline silicon was cut
through its cross section using the same saw into slabs having a
thickness of about 2 cm to about 3 cm. These slabs were used as
monocrystalline silicon seed crystals, or "seeds." The (100)
crystallographic pole orientation of the silicon boule was
maintained. The resulting single crystal silicon slabs were then
arranged in the bottom of a quartz crucible so that the (100)
direction of the slabs faced up, and the (110) direction was kept
parallel to one side of the crucible. The quartz crucible had a
square cross section with 68 cm on a side and a depth of about 40
cm. The slabs were arranged in the bottom of the crucible with
their long dimension parallel to the bottom of the crucible and
their sides touching to form a single, complete layer of such slabs
on the bottom of the crucible.
[0091] Casting: The crucible was loaded with the seed plates and
then filled up to a total mass of 265 kg of solid silicon feedstock
at room temperature. A few wafers of highly boron doped silicon
were added to provide enough boron for a total ingot doping of
.about.0.3 ppma. The filled crucible was first surrounded with
graphite support plates that rested on the thermally insulating
portion of the support structure, and was then loaded into an
in-situ melting/directional solidification casting station used to
cast multi-crystalline silicon. The melt process was run by heating
resistive heaters to approximately 1550.degree. C., and the heaters
were configured so that the heating came from the top while heat
was allowed to radiate out the bottom by opening the insulation a
total of 6 cm. This configuration caused the melting to proceed in
a top-down direction towards the bottom of the crucible. The
passive cooling through the bottom caused the seed crystals to be
maintained in solid state at the melting temperature, as was
monitored by a thermocouple. The extent of melting was measured by
a quartz dip rod that was lowered into the melt every ten minutes.
The dip rod height was compared with a measurement taken on an
empty crucible in the station to determine the height of the
remaining solid material. By dip rod measurement, first the
feedstock melted, and then the melting phase was allowed to
continue until only a height of about 1.5 cm of the seed crystals
remained. At this point, the heating power was dropped to a
temperature setting of 1500.degree. C., while the radiation from
the bottom was increased by opening the insulation to 12 cm. One or
two additional millimeters of seed crystals melted before
solidification began, as observed by dip-rod measurements. Then
seeded single crystal growth proceeded until the end of the
solidification step. The growth stage and the remainder of the
casting cycle were performed with the normal parameters where the
top-to-bottom thermal gradient is evened out, and then the entire
ingot is slowly cooled to room temperature. The cast silicon
product was a 66 cm by 66 cm by 24 cm ingot. The region of
crystallinity consistent with the seeds began at the bottom and
conformed with the edge of the unmelted material, and from there
grew laterally outwards toward the crucible walls as growth began,
and stabilized to a constant size towards the end of
crystallization. The monocrystalline silicon structure was evident
from visually inspecting the faces of bricks cut from the
ingot.
EXAMPLE 2
[0092] Seeding was accomplished as in Example 1, and an ingot was
cast containing a large monocrystalline volume. After cooling, the
ingot was stood on its side and loaded into a band saw with fixed
diamond abrasive for cutting. The bottom of the ingot was cut off
as a single layer with a thickness of 2 cm. This layer was then
fixed horizontally on a cutting table. In the same band saw, the
edges of the layer were trimmed such that approximately 1.5 cm was
removed from each side. The slab was then sandblasted to remove
glue and foreign materials, after which it was etched in a hot
sodium hydroxide bath, rinsed, and dipped in a HCl bath to remove
metals. The slab was then placed on the bottom of a standard
crucible of the same size as the previous ingot. Silicon feedstock
was loaded to a total mass of 265 kg and the casting process was
repeated, producing a second seeded ingot.
EXAMPLE 3
[0093] Seed preparation: A seed layer was prepared, starting with
18 kg of square, (100), plates used to line the bottom of a
crucible, providing a coverage area of 58 by 58 cm and a thickness
ranging from 2-3 cm. These plates were placed together into a
larger square that was centered in the crucible. Next, this square
was surrounded by a 2 cm thick layer of (111) oriented seed
crystals, making the total seed layer a 63 cm by 63 cm square.
[0094] Casting: The crucible containing the seeds was filled with
silicon to a total mass of 265 kg and placed in a casting station.
Casting was performed as in Example 1, monitoring the process to
assure that the seed layer remained intact through the end of melt
and beginning of solidification. The resulting ingot was cut into a
5.times.5 grid of 12.5 cm bricks. Optical inspection of the crystal
structure of the bricks showed that the (111) crystals acted as a
buffer layer, preventing the ingress of randomly nucleated grains
into the (100) volume.
EXAMPLE 4
[0095] Crucible preparation: A standard 69 cm.sup.2 crucible was
placed on a support structure composed of two layers. The layers
were composed as in Example 1 except that the dimensions of the
composite layer were different. The bottom solid graphite layer had
dimensions of 80.times.80.times.2.5 cm.sup.3 as before, but the
heat conducting portion of the composite layer measured only
20.times.20.times.1.2 cm.sup.3, centered on top of the bottom
layer. The remainder of the bottom layer was covered with heat
insulating graphite fiber board.
[0096] Seed preparation: A single piece of (100)-oriented single
crystal silicon with a size of 21 cm by 21 cm by 2 cm was centered
in the bottom of the crucible. The crucible was then filled with a
balance of silicon feedstock to a total mass of 265 kg.
[0097] Casting: The crucible and support plates were placed in a
casting station and cycled as in Example 1, except that additional
time was allowed for the solidification of the silicon, given the
smaller heat extraction area. After cooling down, the ingot was
sectioned. Visual inspection of the sectioned ingot verified the
strong outwards growth of the crystals from the controlled heat
extraction.
EXAMPLE 5
[0098] Crucible preparation: A standard 69 cm.sup.2 crucible was
placed on a graphite support plate and loaded with a seed layer,
feedstock and dopant as in Example 1, except that the feedstock
contained no silicon recycled from previous ingots. A fused silica
lid that had dimensions of 69.times.69.times.12 cm.sup.3 was then
placed on the crucible. A casting station was modified such that a
telescoping tube was attached to the hole in the top insulation
where the process gas is introduced. The charge was then loaded
into the station and raised up to engage the telescope. The casting
station was run using an altered recipe to allow better gas control
and altered solidification settings to compensate for the effects
of the crucible lid. The resulting ingot was measured to have
1/10.sup.th of the carbon concentration found in a typical ingot,
and additionally had a mirror-like top surface and fewer included
foreign particles than typical ingots.
[0099] Thus, consistent with embodiments of the invention and the
examples described above, wafers made from the silicon consistent
with embodiments of the invention are suitably thin and can be used
in photovoltaic cells. For example, wafers can be about 10 microns
thick to about 300 microns thick. Further, the wafers used in the
photovoltaic cells preferably have a diffusion length (L.sub.p)
that is greater than the wafer thickness (t). For example, the
ratio of L.sub.p to t is suitably at least 0.5. It can, for
example, be at least about 1.1, or at least about 2. The diffusion
length is the average distance that minority carriers (such as
electrons in p-type material) can diffuse before recombining with
the majority carriers (holes in p-type material). The L.sub.p is
related to the minority carrier lifetime .tau. through the
relationship L.sub.p=(D.tau.).sup.1/2, where D is the diffusion
constant. The diffusion length can be measured by a number of
techniques, such as the Photon-Beam-Induced Current technique or
the Surface Photovoltage technique. See for example, "Fundamentals
of Solar Cells", by A. Fahrenbruch and R. Bube, Academic Press,
1983, pp. 90-102, which is incorporated by reference herein, for a
description of how the diffusion length can be measured.
[0100] The wafers can have a width of about 100 millimeters to
about 600 millimeters. Preferably, the wafers have at least one
dimension being at least about 50 mm. The wafers made from the
silicon of the invention, and consequently the photovoltaic cells
made by the invention can, for example, have a surface area of
about 100 to about 3600 square centimeters. The front surface of
the wafer is preferably textured. For example, the wafer can be
suitably textured using chemical etching, plasma etching, or laser
or mechanical scribing. If a monocrystalline wafer is used, the
wafer can be etched to form an anisotropically textured surface by
treating the wafer in an aqueous solution of a base, such as sodium
hydroxide, at an elevated temperature, for example about 70.degree.
C. to about 90.degree. C., for about 10 to about 120 minutes. The
aqueous solution may contain an alcohol, such as isopropanol.
[0101] Thus, solar cells can be manufactured using the wafers
produced from cast silicon ingots according to the embodiments of
the invention, by slicing the solid body of cast silicon to form at
least one wafer; optionally performing a cleaning procedure on a
surface of the wafer; optionally performing a texturing step on the
surface; forming a p-n junction by doping the surface; optionally
depositing an anti-reflective coating on the surface; optionally
forming a back surface field with, for example, an aluminum
sintering step; and forming electrically conductive contacts on at
least one surface of the wafer.
[0102] In a typical and general process for preparing a
photovoltaic cell using, for example, a p-type silicon wafer, the
wafer is exposed on one side to a suitable n-dopant to form an
emitter layer and a p-n junction on the front, or light-receiving
side of the wafer. Typically, the n-type layer or emitter layer is
formed by first depositing the n-dopant onto the front surface of
the p-type wafer using techniques commonly employed in the art such
as chemical or physical deposition and, after such deposition, the
n-dopant, for example, phosphorus, is driven into the front surface
of the silicon wafer to further diffuse the n-dopant into the wafer
surface. This "drive-in" step is commonly accomplished by exposing
the wafer to high temperatures. A p-n junction is thereby formed at
the boundary region between the n-type layer and the p-type silicon
wafer substrate. The wafer surface, prior to the phosphorus or
other doping to form the emitter layer, can be textured. In order
to further improve light absorption, an anti-reflective coating,
such as silicon nitride, is typically applied to the front of the
wafer, sometimes providing simultaneous surface and or bulk
passivation.
[0103] In order to utilize the electrical potential generated by
exposing the p-n junction to light energy, the photovoltaic cell is
typically provided with a conductive front electrical contact on
the front face of the wafer and a conductive back electrical
contact on the back face of the wafer, although both contacts can
be on the back of the wafer. Such contacts are typically made of
one or more highly electrically conducting metals and are,
therefore, typically opaque.
[0104] Thus, solar cells consistent with the embodiments described
above may comprise a wafer sliced from a body of continuous
monocrystalline silicon being substantially free of
radially-distributed defects, the body having at least two
dimensions each being at least about 35 cm, a p-n junction in the
wafer, an anti-reflective coating on a surface of the wafer; and a
plurality of electrically conductive contacts on at least one
surface of the wafer, wherein the body is substantially free of
swirl defects and substantially free of oxygen-induced stacking
fault defects.
[0105] Also, solar cells consistent with the embodiments described
above may comprise a wafer sliced from a body of continuous
multi-crystalline silicon being substantially free of
radially-distributed defects, the body having a predetermined
arrangement of grain orientations with a common pole direction
being perpendicular to a surface of the body, the body further
having at least two dimensions each being at least about 10 cm, a
p-n junction in the wafer; an anti-reflective coating on a surface
of the wafer, and a plurality of electrically conductive contacts
on at least one surface of the wafer, wherein the multi-crystalline
silicon includes silicon grains having an average grain boundary
length of about 0.5 cm to about 30 cm, and wherein the body is
substantially free of swirl defects and substantially free of
oxygen-induced stacking fault defects.
[0106] It will be apparent to those skilled in the art that various
modifications and variations can be made in the disclosed
structures and methods without departing from the scope or spirit
of the invention. Other embodiments of the invention will be
apparent to those skilled in the art from consideration of the
specification and practice of the invention disclosed herein. It is
intended that the specification and examples be considered
exemplary only, with a true scope and spirit of the invention being
indicated by the following claims.
* * * * *