U.S. patent application number 15/166039 was filed with the patent office on 2016-09-22 for directional solidification system and method.
The applicant listed for this patent is Silicor Materials Inc.. Invention is credited to Abdallah Nouri, Kamel Ounadjela.
Application Number | 20160271692 15/166039 |
Document ID | / |
Family ID | 45816671 |
Filed Date | 2016-09-22 |
United States Patent
Application |
20160271692 |
Kind Code |
A1 |
Nouri; Abdallah ; et
al. |
September 22, 2016 |
DIRECTIONAL SOLIDIFICATION SYSTEM AND METHOD
Abstract
The present invention relates to an apparatus and method for
purifying materials using a rapid directional solidification.
Devices and methods shown provide control over a temperature
gradient and cooling rate during directional solidification, which
results in a material of higher purity. The apparatus and methods
of the present invention can be used to make silicon material for
use in solar applications such as solar cells.
Inventors: |
Nouri; Abdallah; (Montreal,
CA) ; Ounadjela; Kamel; (Belmont, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Silicor Materials Inc. |
San Jose |
CA |
US |
|
|
Family ID: |
45816671 |
Appl. No.: |
15/166039 |
Filed: |
May 26, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13234960 |
Sep 16, 2011 |
9352389 |
|
|
15166039 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B 23/025 20130101;
C30B 11/003 20130101; C01B 33/02 20130101; C30B 23/00 20130101;
C30B 35/002 20130101; C30B 29/06 20130101; B22D 27/045 20130101;
C01B 33/037 20130101; C30B 11/002 20130101; C30B 29/36
20130101 |
International
Class: |
B22D 27/04 20060101
B22D027/04; C01B 33/02 20060101 C01B033/02 |
Claims
1. A system for directional solidification, comprising: an outer
jacket; a base lining a bottom of the outer jacket, the base
including a heat conducting material; and a wall insulation
structure lining a wall of the outer jacket, the wall insulation
structure tapering in thickness from a rim of the mold, having a
first thickness, to a bottom portion of the mold, having a second
thickness that is thinner than the first thickness.
2. The system of claim I, wherein the top cover further includes
one or more heating elements.
3. The system of claim 1, wherein the wall insulation structure
tapers in thickness from a rim of the mold to a bottom interface
with the base.
4. The system of claim 3, wherein the second thickness is 25
percent thinner than the first thickness.
5. The system of claim 1, wherein the wall insulation structure
includes a layer of refractory bricks, and an exposed layer of
substantially continuous refractory material.
6. The system of claim 5, wherein the exposed layer includes a
layer of Al.sub.2O.sub.3.
7. The system of claim 6, wherein the exposed layer is greater than
98 percent pure Al.sub.2O.sub.3.
8. A mold for directional solidification, comprising: a
substantially rectangular shaped wall structure, including a long
side and a short side; a heat conducting base positioned to
directly contact molten material, the base including a material of
a higher thermal conductivity than a wall material, coupled to the
substantially rectangular shaped wall structure; wherein the
substantially rectangular shaped wall; and wherein the
substantially rectangular shaped wall structure is positioned to
directly contact molten material, and the rectangular shaped wall
structure tapers in thickness from a rim of the mold, having a
first thickness, to a bottom interface with the heat conducting
base, having a second thickness that is thinner than the first
thickness.
9. The mold of claim 8, wherein the rectangular shaped wall
structure includes a rounded profile at wall intersections.
10. The mold of claim 8, wherein an intersection of the rectangular
shaped wall structure and the bottom includes a rounded
profile.
11. The mold of claim 8, further including: a support structure
spacing the mold above a floor surface, and defining a space
between the mold and the floor; one or more flow pathways to move
air within the space; and one or more valves to adjust a flow
through the one or more flow pathways.
12. The mold of claim 11, further including a fan to move air
within the space.
13. The mold of claim 12, further including a number of cooling
fins thermally coupled to the heat conducting base within the
space.
14. The mold of claim 8, wherein the heat conducting base includes
silicon carbide.
15. The mold of claim 8, further including a top cover.
16. The mold of claim 15, further including a vent hole in the top
cover, and a vacuum pump coupled to the vent hole to remove gases
from a surface of silicon during a cooling operation.
17. The mold of claim 16, wherein the top cover further includes
one or more heating elements.
Description
CLAIM OF PRIORITY
[0001] This application is a continuation of and claims the benefit
of priority under 35 U.S.C. .sctn.120 to U.S. patent application
Ser. No. 13/234,960, filed on Sep. 16, 2011, which is hereby
incorporated by reference herein in its entirety.
BACKGROUND
[0002] Solar cells can be a viable energy source by utilizing their
ability to convert sunlight to electrical energy. Silicon is a
semiconductor material and the raw incoming material used in the
manufacture of solar cells; The electrical properties of the cells,
namely the conversion efficiency depends critically on the purity
of the Silicon. Several techniques have been used to purify
Silicon. The most well known technique is called `Siemens process`.
This technique allows to almost remove every single impurity
present within the Silicon. However this technique requires to
produce the silicon into a gas phase and redeposit into a solid
phase in order to remove the impurities. The technique describes in
this patent allows to remove very effectively impurities by melting
the Silicon into a liquid phase and solidifying the Silicon using a
technique called `directional solidification`. While this technique
is very well known, this patent focuses on a new way to use the
directional solidification that allows to reduce considerably the
cost of this procedure.
[0003] Techniques used to make purified silicon crystals for solar
cells are known. Most of these techniques operate on the principle
that while silicon crystals are solidifying from a molten silicon
solution, undesirable impurities remain in the molten solution. A
first example technique, the float zone technique, can be used to
make silicon monocrystalline ingots using a moving liquid to urge
impurities toward an edge of a mold for removal. Another example
technique, the Czochralski technique, can be used to make silicon
monocrystalline ingots using a seed crystal that is slowly pulled
out of a molten solution, allowing the formation of a
monocrystalline column of silicon while leaving impurities in the
solution. Further example techniques, such as the Bridgeman or heat
exchanger techniques, can be used to make silicon multicrystalline
ingots through the creation of a temperature gradient with a
controlled cooling rate, causing directional solidification.
OVERVIEW
[0004] Various techniques for making silicon crystals for solar
cells utilize a mold to hold silicon during the molten
manufacturing stage. One challenge with directional solidification
of multicrystalline ingots is maintaining consistent progression of
a liquid-solid interface with a flat shape or concave with a small
deflection from the bottom of the mold to the top. If the
progression of the liquid-solid interface is not controlled,
impurities can become trapped in the ingot, and a large amount of
the solidified silicon may end up with lower performance, or
unacceptable performance as a result.
[0005] The present molds, mold systems, and related methods provide
means for purifying silicon using directional solidification. The
molds, mold systems, and related methods allow for control over a
temperature gradient during directional crystallization, which can
result in silicon of higher purity for use in solar cells. A mold
can include an outer jacket, a base lining a bottom portion of the
outer jacket, and a wall insulation structure lining a wall of the
outer jacket. The base can include a heat-conducting material. The
wall insulation structure can taper in thickness from a top portion
of the mold, having a first thickness, to a bottom portion of the
mold, having a second thickness that is thinner than the first
thickness. The wall insulation structure can include one or more of
an exposed layer, refractory bricks, ceramic fiber, and a
microporous refractory layer.
[0006] To better illustrate the molds, mold systems, and related
methods disclosed herein, a non-limiting list of examples is now
provided:
[0007] In Example 1, a system for directional solidification
includes an outer jacket, a base lining a bottom of the outer
jacket, the base including a heat conducting material, and a wall
insulation structure lining a wall of the outer jacket, the wall
insulation structure tapering in thickness from a rim of the mold,
having a first thickness, to a bottom portion of the mold, having a
second thickness that is thinner than the first thickness.
[0008] In Example 2, the system of Example 2 is optionally
configured such that the wall insulation structure tapers in
thickness from a rim of the mold to a bottom interface with the
base.
[0009] In Example 3, the system of any one or any combination of
Examples 1-2 is optionally configured such that the second
thickness is approximately 25 percent thinner than the first
thickness.
[0010] In Example 4, the system of any one or any combination of
Examples 1-3 is optionally configured such that the wall insulation
structure includes a layer of refractory bricks, and an exposed
layer of substantially continuous refractory material.
[0011] In Example 5, the system of any one or any combination of
Examples 1-4 is optionally configured such that the exposed layer
includes a layer of Al.sub.2O.sub.3.
[0012] In Example 6, the system of any one or any combination of
Examples 1-5 is optionally configured such that the exposed layer
is greater than about 98 percent pure Al.sub.2O.sub.3.
[0013] In Example 7, the system of any one or any combination of
Examples 1-6 is optionally configured such that the layer of
refractory bricks is rated between approximately 1540.degree. C. to
1430.degree. C.
[0014] In Example 8, the system of any one or any combination of
Examples 1-7 is optionally configured such that the exposed layer
tapers in thickness from a wide portion at the rim of the mold, to
a thinner portion at the bottom interface with the base, and the
layer of refractory bricks tapers in thickness from a wide portion
at the rim of the mold, to a thinner portion at the bottom
interface with the base.
[0015] In Example 9, the system of any one or any combination of
Examples 1-8 is optionally configured such that the wall insulation
structure further includes a microporous refractory layer.
[0016] In Example 10, the system of any one or any combination of
Examples 1-9 is optionally configured such that the microporous
refractory layer has a uniform thickness.
[0017] In Example 11, the system of any one or any combination of
Examples 1-10 is optionally configured such that the microporous
refractory layer provides a substantially uniform thickness along
the wall of the outer jacket, and at least part of the bottom of
the outer jacket.
[0018] In Example 12, the system of any one or any combination of
Examples 1-11 is optionally configured such that the wall
insulation structure further includes a ceramic fiber layer between
the microporous refractory layer and the layer of refractory
bricks.
[0019] In Example 13, the system of any one or any combination of
Examples 1-12 is optionally configured such that the wall
insulation structure further includes a ceramic fiber layer between
the microporous refractory layer and the outer jacket.
[0020] In Example 14, the system of any one or any combination of
Examples 1-13 is optionally configured such that the base includes
a layer of silicon carbide.
[0021] In Example 15, the system of any one or any combination of
Examples 1-14 is optionally configured such that the system further
includes a top heater.
[0022] In Example 16, the system of any one or any combination of
Examples 1-15 is optionally configured such that the top heater
includes 12 heating elements.
[0023] In Example 17, the system of any one or any combination of
Examples 1-16 is optionally configured such that the heating
elements are equally spaced apart over a distance of approximately
54 inches.
[0024] In Example 18, the system of any one or any combination of
Examples 1-17 is optionally configured such that a lower edge of
the heating elements is positioned to be spaced above a surface of
a melt by a distance of approximately 1.9 inches.
[0025] In Example 19, the system of any one or any combination of
Examples 1-18 is optionally configured such that the heating
elements include silicon carbide heating elements.
[0026] In Example 20, the system of any one or any combination of
Examples 1-19 is optionally configured such that the system further
includes a vent hole in the top heater.
[0027] In Example 21, the system of any one or any combination of
Examples 1-20 is optionally configured such that the vent hole
includes an approximately one inch diameter hole in a center of the
top heater, and there is only a single vent hole in the top
heater.
[0028] In Example 22, a mold for directional solidification
includes a substantially rectangular shaped wall structure,
including a long side and a short side, a heat conducting base,
coupled to the substantially rectangular shaped wall structure, and
wherein the substantially rectangular shaped wall is dimensioned to
provide a wall contact area for a given quantity of molten silicon
that is less than a wall contact area for a cylindrical mold having
a diameter substantially equal to a length of the short side of the
mold.
[0029] In Example 23, the mold of Example 22 is optionally
configured such that the rectangular shaped wall structure includes
a rounded profile at wall intersections.
[0030] In Example 24, the system of any one or any combination of
Examples 22-23 is optionally configured such that an intersection
of the rectangular shaped wall structure and the bottom includes a
rounded profile.
[0031] In Example 25, a system for directional solidification of
silicon, includes a mold to contain a volume of molten silicon,
including a heat insulating wall structure, and a heat conducting
base, a support structure spacing the mold above a floor surface,
and defining a space between the mold and the floor, one or more
flow pathways to move air within the space, and one or more valves
to adjust a flow through the one or more flow pathways.
[0032] In Example 26, the mold of Example 25 is optionally
configured such that the system further includes a top heater.
[0033] In Example 27, the system of any one or any combination of
Examples 25-26 is optionally configured to further include a fan to
move air within the space.
[0034] In Example 28, the system of any one or any combination of
Examples 25-27 is optionally configured to further include a number
of cooling fins thermally coupled to the heat conducting base
within the space.
[0035] In Example 29, the system of any one or any combination of
Examples 25-28 is optionally configured such that the heat
conducting base includes a metal heat spreading layer coupled to
the number of cooling fins.
[0036] In Example 30, the system of any one or any combination of
Examples 25-29 is optionally configured such that the heat
conducting base includes a silicon carbide layer.
[0037] In Example 31, the system of any one or any combination of
Examples 25-30 is optionally configured such that the one or more
valves includes a movable wall sized to cover both the heat
insulating wall structure and the space between the floor and the
mold.
[0038] In Example 32, the system of any one or any combination of
Examples 25-31 is optionally configured such that the one or more
valves includes a movable wall sized to cover the heat insulating
wall structure, an interface between the mold and the top heater,
and the space between the floor and the mold.
[0039] In Example 33, a system for directional solidification of
silicon, includes an outer jacket, a silicon carbide layer lining a
bottom of the outer jacket, and a composite wall insulation
structure lining a wall of the outer jacket, wherein the composite
wall insulation structure includes a first ceramic fiber layer
adjacent to a wall of the outer jacket, a microporous refractory
layer adjacent to the first ceramic fiber layer, a second ceramic
fiber layer adjacent to the microporous refractory layer, a layer
of refractory bricks adjacent to the second ceramic fiber layer, a
substantially continuous aluminum oxide layer adjacent to the layer
of refractory bricks, the aluminum oxide layer forming an exposed
interior surface of the composite wall insulation structure,
wherein the composite wall insulation structure tapers in thickness
from a rim of the mold, having a first thickness, to a bottom
interface with the silicon carbide layer, having a second thickness
that is thinner than the first thickness, a top heater to fit over
a top of the mold, the top heater including, a plurality of silicon
carbide heating elements, a vent hole, and a vacuum pump coupled to
the vent hole to remove gases from a surface of silicon during a
cooling operation.
[0040] In Example 34, the system of Example 33 is optionally
configured such that the outer jacket includes a stainless steel
outer jacket.
[0041] In Example 35, the system of any one or any combination of
Examples 33-34 is optionally configured such that the second
thickness is approximately 25 percent thinner than the first
thickness.
[0042] In Example 36, the system of any one or any combination of
Examples 33-35 is optionally configured such that the substantially
continuous aluminum oxide layer includes a greater than about 98
percent pure Al.sub.2O.sub.3 layer.
[0043] In Example 37, a directional solidification system, includes
an outer jacket, a base lining a bottom of the outer jacket, the
base including a heat conducting material, a wall insulation
structure lining a wall of the outer jacket, wherein the wall
insulation structure tapers in thickness from a rim of the mold,
having a first thickness, to a bottom interface with the base,
having a second thickness that is thinner than the first thickness,
wherein the mold is rectangular in shape, with a wall that is
dimensioned to provide a wall contact area for a given quantity of
molten silicon that is less than a wall contact area for a
cylindrical mold having a diameter substantially equal to a length
of the short side of the mold, a removable top heater, a support
structure spacing the mold above a floor surface, and defining a
space between the mold and the floor, one or more flow pathways to
move air within the space, and one or more valves to adjust a flow
through the one or more flow pathways.
[0044] In Example 38, the system of Example 37 is optionally
configured such that the base of the mold includes a metal heat
spreading layer coupled to a number of cooling fins that extend
into the space between the mold and the floor.
[0045] In Example 39, a system for directional solidification,
includes an outer jacket, a base lining a bottom of the outer
jacket, the base including a heat conducting material, a wall
insulation structure lining a wall of the outer jacket, and a
removable heat spreader coupled to the bottom of the outer
jacket.
[0046] In Example 40, the system of Example 39 is optionally
configured such that the wall insulation structure tapers in
thickness from a rim of the mold, having a first thickness, to a
bottom portion of the mold, having a second thickness that is
thinner than the first thickness.
[0047] In Example 41, the system of any one or any combination of
Examples 39-40 is optionally configured to further include a number
of cooling fins coupled to the removable heat spreader.
[0048] In Example 42, the system of any one or any combination of
Examples 39-41 is optionally configured such that the outer jacket
is substantially cylindrical in shape.
[0049] In Example 43, the system of any one or any combination of
Examples 39-42 is optionally configured such that the outer jacket
is substantially rectangular in shape.
[0050] These and other examples and features of the present molds,
mold systems, and related methods will be set forth in part in the
following detailed description. This overview is intended to
provide non-limiting examples of the present subject matter it is
not intended to provide an exclusive or exhaustive explanation. The
detailed description below is included to provide further
information about the present molds, mold systems, and methods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] In the drawings, like numerals can be used to describe
similar elements throughout the several views. Like numerals having
different letter suffixes can be used to represent different views
of similar elements. The drawings illustrate generally, by way of
example, but not by way of limitation, various embodiments
discussed in the present document.
[0052] FIG. 1 shows an isometric view of a system according to at
least one embodiment of the invention.
[0053] FIG. 2 shows a cross section of a mold according to at least
one embodiment of the invention.
[0054] FIG. 3 shows a top view of a heater according to at least
one embodiment of the invention.
[0055] FIG. 4 shows a side view of a heater according to at least
one embodiment of the invention.
[0056] FIG. 5 shows a series of modeled cooling profiles for
silicon using a mold according to at least one embodiment of the
invention.
[0057] FIG. 6 shows a top view of a rectangular system according to
at least one embodiment of the invention.
[0058] FIG. 7 shows a top view of a cylindrical system according to
at least one embodiment of the invention.
[0059] FIG. 8 shows a top view of a rectangular system according to
at least one embodiment of the invention.
[0060] FIG. 9 shows a side view of the system from FIG. 8.
[0061] FIG. 10 shows a side view of a system according to at least
one embodiment of the invention.
[0062] FIG. 11 shows a system according to at least one embodiment
of the invention.
DETAILED DESCRIPTION
[0063] In the following detailed description, reference is made to
the accompanying drawings. The drawings form a part of the
description and are provided by way of illustration, but not of
limitation. The drawing embodiments are described in sufficient
detail to enable those skilled in the art to practice the present
subject matter. Other embodiments may be utilized and mechanical,
structural, or material changes may be made without departing from
the scope of the present patent document.
[0064] Reference will now be made in detail to certain examples of
the disclosed subject matter, some of which are illustrated in the
accompanying drawings. While the disclosed subject matter will
largely be described in conjunction with the accompanying drawings,
it should be understood that such descriptions are not intended to
limit the disclosed subject matter to those drawings. On the
contrary, the disclosed subject matter is intended to cover all
alternatives, modifications, and equivalents, which can be included
within the scope of the presently disclosed subject matter, as
defined by the claims.
[0065] References in the specification to "one embodiment", "an
embodiment", "an example embodiment", etc., indicate that the
embodiment described can include a particular feature, structure,
or characteristic, but every embodiment may not necessarily include
the particular feature, structure, or characteristic. Moreover,
such phrases are not necessarily referring to the same embodiment.
Further, when a particular feature, structure, or characteristic is
described in connection with an embodiment, it is submitted that it
is within the knowledge of one skilled in the art to affect such
feature, structure, or characteristic in connection with other
embodiments whether or not explicitly described.
[0066] In this document, the terms "a" or "an" are used to include
one or more than one and the term "or" is used to refer to a
nonexclusive "or" unless otherwise indicated. In addition, it is to
be understood that the phraseology or terminology employed herein,
and not otherwise defined, is for the purpose of description only
and not of limitation.
[0067] The present subject matter relates to molds, mold systems,
and related methods for purifying silicon using directional
solidification techniques, while maintaining a consistent
progression of a solid-liquid interface throughout a mold. The
purified silicon resulting from the directional solidification can
be used in solar cells. It has been found that by controlling the
temperature gradient within the mold, a highly controlled
directional solidification can be accomplished. Although
purification of silicon is described in the most detail in examples
below, systems and methods described can also be used for
directional solidification and purification of other materials such
as sapphire.
[0068] Directional crystallization generally proceeds from bottom
to top, thus the desired temperature gradient has a lower
temperature at the bottom and a higher temperature at the top. High
degrees of control over the temperature gradient and the
corresponding directional crystallization can advantageously allow
a more effective directional solidification, resulting in silicon
of higher purity.
[0069] FIG. 1 illustrates a specific embodiment of a mold system
100 for directional solidification of silicon. The system can
include a top heater 110 positioned on or near a top portion of a
mold 120. The top heater 110 can be support by one or more chains
101, which are engaged on first ends with holes 102 of vertical
structural members 103. The chains 101 in this example form a
bridle, allowing the top heater 110 to be moved by the use of a
crane or other lifting system. The system 100 can also be moved,
for example, by placing the mold 120 on a fork or scissor lift
while leaving the top heater 110 over the mold 120. Screen boxes
106 can enclose ends of the top heater's 110 heating members that
protrude from an outer jacket, protecting users from the heat and
electricity present in and near the ends of these members.
[0070] The vertical structural members 103 can extend from a bottom
edge of the top heater 110 to a top edge of the top heater 110. The
vertical structural members 103 can be located on an outer surface
of the top heater's 110 outer jacket and can extend outward in a
direction perpendicular to the outer surface. The top heater can
also include a horizontal structural member 104 located on the
outer surface of the top heater's 110 outer jacket and can extend
outward in a direction perpendicular to the outer surface.
[0071] The top heater 110 can include a lip 105 that forms a part
of the heater's outer jacket. The lip can protrude outward from the
outer jacket and can extend inward toward a center axis of the top
heater 110 such that it covers a thickness of any present
insulation. Alternatively, the lip 105 can extend inward only,
enough to cover the bottom edge of the top heater's 110 outer
jacket.
[0072] In the embodiment depicted in FIG. 1, insulation 111 from
the mold 120 can extend between the top heater 110 and the mold
120. In various examples, at least part of one or more insulating
layers 111 of the mold 120 can extend above the height of the
mold's outer jacket. Similar to the top heater 110, the mold 120
can include vertical structural members 112. The vertical
structural members 112 can be located on an outer surface of the
mold's 120 outer jacket and can extend outward in a direction
perpendicular to the outer surface. The vertical structural members
112 can extend from the bottom edge of the mold 120 to the top edge
of the mold 120 The mold 112 can also include one or more
horizontal structural members 113. The horizontal structural
members 113 are shown on the outer surface of the outer jacket of
the mold 120, extending outward from the outer jacket. The
horizontal structural members 113 can extend horizontally around a
circumference of a cylindrical mold 112 or along one or more sides
of a non-cylindrical mold. The mold 112 can also include
criss-crossing bottom structural members 114, 115. The bottom
structural members 114, 115 can extend across the bottom of the
mold 112. Some of the bottom structural members 115 can be shaped
and sized to allow a fork or scissors lift or other machine to
physically manipulate (e.g., move) the system 100.
[0073] FIG. 2 shows a mold 200 according to an embodiment of the
invention. The mold 200 includes an outer jacket 210, including
side walls 201 and a floor 202, that encloses a bottom material 212
and a wall insulation structure 220. In one example, the side walls
201 and the floor 202 are integrally formed. In another example,
the side walls 201 and the floor 202 are bolted, or otherwise
mechanically connected together to form the outer jacket 210.
[0074] The mold 200 defines an interior portion 201 within the mold
200 to contain an amount of molten silicon. In one example, the
wall insulation structure 220 is a composite structure including a
number of different materials positioned next to each other. One
advantage of a composite wall insulation structure 220 includes the
ability to control a thermal gradient by selection of materials and
geometry of the individual components of the composite structure.
Another advantage of a composite wall insulation structure 220
includes the ability to reduce cost of the mold. Higher cost, high
heat resistance materials are used on exposed surfaces where molten
silicon will be contacted, while lower cost materials with lower
heat resistance are layered as the composite structure progresses
away from the molten silicon.
[0075] In one example, the wall insulation structure 220 tapers in
thickness from a rim 222 of the mold 200, having a first thickness
224, to a bottom interface 226 with the base 212, having a second
thickness 228 that is thinner than the first thickness 224. In one
example the second thickness is approximately 25 percent thinner
than the first thickness. In operation, the taper of the wall
insulation structure 220 provides a thermal gradient that provides
a desirable progression of the liquid-solid interface in the
silicon during directional solidification. In one example, the
taper of the wall insulation structure 220 provides more insulation
near a top of the melt to keep the surface of the melt in a liquid
state, while less insulation is provided near the bottom interface
with the base 212, to facilitate cooling at the bottom of the mold
200. More detailed results of the thermal gradient are discussed
below, regarding FIG. 5.
[0076] In one example, the wall insulation structure 20 includes an
exposed layer 230 that will be in direct contact with the molten
silicon in a directional solidification operation. In one example,
the exposed layer 230 is substantially continuous (in contrast, to
an assembled brick layer or the like) to contain the molten
silicon. One material that includes good thermal properties
includes aluminum oxide in a form such as Al.sub.2O.sub.3. In one
example, the aluminum oxide is substantially pure Al.sub.2O.sub.3,
which minimizes impurities from the exposed layer 230 into the
silicon melt. Due to the ionic nature of the bond structure in
Al.sub.2O.sub.3, aluminum stays in exposed layer 230, and is not
released as an impurity into the melt. In one example, the
Al.sub.2O.sub.3 is greater than 98 percent pure Al.sub.2O.sub.3. In
one example, the exposed layer 230 wraps over a top surface 231 of
the wall insulation structure 220. A feature of this configuration
includes protection for inner structures or layers within the wall
insulation structure 220 that may not have thermal resistance to
temperatures as high as the exposed layer 230.
[0077] In one example, the wall insulation structure 220 further
includes a layer of refractory bricks 232 forming an interface with
the exposed layer 230. In one example, the layer of refractory
bricks 232 include Al.sub.2O.sub.3. Refractory bricks 232 can
provide desirable structural properties such as toughness to the
wall insulation structure 220. In one example, the layer of
refractory bricks 232 includes bricks that are rated to
1540.degree. C. or less. In one example, the layer of refractory
bricks 232 includes bricks that are rated to 1430.degree. C. or
less. A combination of properties are taken into consideration in
selection of refractory bricks 232, such as thickness and thermal
properties needed for the desired gradient of the molten silicon,
and other properties such as cost, strength, and toughness.
[0078] In one example, the wall insulation structure 220 further
includes a microporous refractory layer 236. In one example, the
microporous refractory layer 236 is rated to 1000.degree. C. or
less. Pores in the microporous refractory layer 236 provide good
insulating properties. In selected examples layers of ceramic fiber
insulation are further included between selected layers. In one
example, a first ceramic fiber layer 238 is included between the
outer jacket 210 and the microporous refractory layer 236. In one
example, a second ceramic fiber layer 234 is included between the
microporous refractory layer 236 and the layer of refractory bricks
232. Examples of ceramic fiber layers include boards, flexible
fabrics, or other configurations.
[0079] In one example one or more layers in the wall insulation
structure 220 taper from a rim 222 of the wall insulation structure
220 to a bottom 226 of the wall insulation structure 220, where the
wall insulation structure 220 forms an interface with the base 212.
In the example shown in FIG. 2, both the exposed layer 230 and the
layer of refractory bricks 232 taper from the rim 222 to the bottom
226, while the microporous refractory layer 236 maintains a
consistent thickness 242. In one example the microporous refractory
layer 236 maintains a consistent thickness 242 along the outer
jacket 210, and also around a corner 240 over at least part of the
bottom of the outer jacket 210. This configuration provides precise
control of the cooling rate of the silicon.
[0080] In one example, the base 212 includes a silicon carbide
material. Silicon carbide includes the desirable property of high
thermal conductivity, with high heat resistance. It conducts heat
from the silicon melt out the bottom of the mold 200, without
melting itself, or contaminating the silicon. With heat flowing out
the bottom of the mold, to begin the directional solidification,
wall insulation structure 220 design choices such as thicknesses,
material choice, and tapers work together to provide the desired
consistent progression of the liquid-solid interface from the
bottom of the mold to the top.
[0081] FIG. 3 illustrates a top heater 300 that may further
facilitate keeping a top surface of a silicon melt in a liquid
state, while solidification desirably progresses from the bottom to
the top of the mold. The top heater 300 can include one or more
heating members 310. Each of the one or more heating members can
independently include any suitable material. For example, each of
the one or more heating members 310 independently can include a
heating element, where the heating element can include silicon
carbide, molybdenum disilicide, graphite, copper, or a combination
thereof; and, each of the one or more heating members can
alternatively independently include an induction heater. In one
embodiment, the one or more heating members 310 are positioned at
approximately the same height. In another embodiment, the one or
more heating members are positioned at different heights.
[0082] In one example, the top heater 300 includes 12 heating
members 310. In one example, the 12 heating members 310 are spaced
approximately equally apart over a distance 312. In one example,
the distance 312 is approximately 54 inches. Variables such as the
positioning of heating members and the number of heating members
are important to the thermal gradients generated in the silicon
during processing. Small variations in the thermal gradient may
cause undesirable progression of portions of the liquid-solid
interface in the silicon during directional solidification. For
example it is undesirable to have a surface of the molten silicon
solidify and trap a molten interior portion within the ingot. A
trapped molten portion of silicon may contain an undesirable level
of impurities that adversely affect performance of the resulting
silicon material.
[0083] In addition to a number of heating members 310 and a lateral
spacing of heating members 310, in one example, the number of
heating members 310 are positioned to be a distance 314 of
approximately 1.9 inches above a surface of a melt. In one example,
a diameter of the heating members 310 is approximately 2 inches. As
with a selected number of heating members 310 and lateral spacing
of heating members 310, chosen dimensions such as diameter of the
heating members 310 and distance above a surface of a melt have
been discovered in the present disclosure to provide a desirable
progression of portions of the liquid-solid interface in the
silicon during directional solidification.
[0084] In one example, a vent hole 302 is provided in the top
heater 300 to remove gases such as oxygen from the heating member
310 and a surface of the silicon melt. In one example, a vacuum
pump (not shown) is coupled to the vent hole 302 to remove the
unwanted gases through the vent hole 302. In one example, only a
single vent hole 302 sized between approximately one to two inches
in diameter is used in the top heater 300. In one example, the
single vent hole 302 is approximately one inch in diameter. It has
been discovered that variables such as a single vent hole of
appropriate dimensions effectively removes unwanted gases without
leading to unwanted cooling of a surface of the silicon melt.
[0085] In one example, the heating elements include silicon
carbide, which has certain advantages. For example, silicon carbide
heating elements do not corrode at high temperatures in the
presence of oxygen. Oxygen corrosion can be reduced for heating
elements including corrodible materials by using a vacuum chamber,
but silicon carbide heating elements can avoid corrosion without a
vacuum chamber. Additionally, silicon carbide heating elements can
be used without water-cooled leads, having multiple working zones,
such as a cool zone at the edges, and a hot zone at the center of
the heating element. In one embodiment, the heating elements are
used in a vacuum chamber, with water-cooled leads, or both. In
another embodiment, the heating elements are used without a vacuum
chamber, without water-cooled leads, or without both.
[0086] In one embodiment, the one or more heating members 310 are
induction heaters. The induction heaters can be cast into one or
more refractory materials. The refractory material containing the
induction heating coil or coils can then be positioned over the
mold. The refractory material can be any suitable material. For
example, the refractory material can include aluminum oxide,
silicon oxide, magnesium oxide, calcium oxide, zirconium oxide,
chromium oxide, silicon carbide, graphite, or a combination
thereof. In another embodiment, the induction heaters are not cast
into one or more refractory materials.
[0087] The top heater can include insulation, for example top
heater 300 shown in FIG. 4 includes insulation 316. The insulation
can include any suitable insulating material. The insulation can
include one or more insulating materials. For example, the
insulation can include insulating brick, a refractory, a mixture of
refractories, insulating board, ceramic paper, high temperature
wool, cast insulating materials, or a mixture thereof. Insulating
board can include high temperature ceramic board. In one example,
the insulation 316 is cast around the heating members 310, making
them more robust, and resistant to thermal shock to prevent
deformation of parts around the heating members 310. One suitable
cast material includes Kaolite.RTM. 3300 from Morgan Thermal
Ceramics, Inc.
[0088] The top heater can include an outer jacket, for example top
heater 300 shown in FIG. 3 includes outer jacket 304. The outer
jacket can include any suitable material. For example, the outer
jacket can include steel or stainless steel. In another embodiment,
the outer jacket includes steel, stainless steel, copper, cast
iron, a refractory material, a mixture of refractory materials, or
a combination thereof. The insulation 316 is disposed at least
partially between the one or more heating members and the outer
jacket. In FIG. 4, the bottom edge of the outer jacket 304 is shown
to be approximately even with the bottom edge of the
insulation.
[0089] Variations in the top heater are possible, within the scope
of the invention. For example, the edge of outer jacket 304 can
extend below the edge of the insulation 316 and the one or more
heating members 310. In another example, the edge of the outer
jacket 304 can extend below the edge of the insulation 316, below
the one or more heating members, or a combination thereof. In one
example, the outer jacket 304 can extend below the bottom edge of
the insulation 316 and continue across either fully or partially
covering the bottom edge of the insulation. In some embodiments,
the portion of the outer jacket 304 that covers the edge of the
insulation can include a material with a relatively low
conductivity, such as a suitable refractory, such as aluminum
oxide, silicon oxide, magnesium oxide, calcium oxide, zirconium
oxide, chromium oxide, silicon carbide, graphite, or a combination
thereof. In another example, the outer jacket 304 does not extend
below the bottom edge of the insulation or the height of the one or
more heating members. In another embodiment, the outer jacket 304
extends below the height of the one or more heating members 310,
but is still above the bottom edge of the insulation 316.
[0090] As discussed above, by controlling the temperature gradient
in the apparatus, a highly controlled directional solidification
can be accomplished. High degrees of control over the temperature
gradient and the corresponding directional crystallization can
allow a more effective directional solidification, providing a
silicon of high purity. In the present invention, the directional
crystallization proceeds from approximately bottom to top, thus the
desired temperature gradient has a lower temperature at the bottom
and a higher temperature at the top. In embodiments with a top
heater, the top heater is one way to control the entry or loss of
heat from the top of the directional solidification mold.
[0091] FIG. 5 shows a thermal model of directional solidification
using a mold 550 and top heater 554 similar to embodiments such as
mold 200 and top heater 300 described above. An amount of silicon
is shown, contained in the mold 550, the silicon having a liquid
portion 502, a solid portion 504, and a liquid-solid interface 506
progressing upward in the mold 550 desirably as described
above.
[0092] Graph 510 of FIG. 5 shows the silicon at 2 hours into a
directional solidification operation. Graph 520 of FIG. 5 shows the
silicon at 7 hours into the directional solidification operation.
The liquid-solid interface 506 is shown progressing upward into the
liquid portion 502. Graph 530 of FIG. 5 shows the silicon at 15
hours into the directional solidification operation. The
liquid-solid interface 506 is still progressing upward into the
liquid portion 502, and atop surface 508 of the liquid portion 502
is still desirably in a liquid state, where impurities can collect,
and be later removed, or example by sawing from the resulting
ingot, or skimming the liquid portion 502 after an appropriate
amount of time to allow a bulk of the solid portion 504 to
form.
[0093] FIG. 6 shows a mold 600 according to another embodiment of
the invention. The top view of the mold 600 shows a wall structure
601, including long side walls 602 and short side walls 604. In one
example, both the long side walls 602 and the short side walls 604
are formed from thermally insulating material, such as a refractory
material. As discussed in embodiments, above, in one example, the
refractory material includes aluminum oxide, substantially in the
form Al.sub.2O.sub.3.
[0094] A number of heating elements 606 are shown in a
configuration to be included within a rectangular top heater to
cover the mold 600 during a directional solidification process. In
one example the rectangular top heater can be used in a manner
similar to the top heater described in FIGS. 3 and 4 above.
Configurations of molds 600 and corresponding top heaters with a
substantially rectangular shape have an advantage of improving
efficiency and scalability of a directional solidification
process.
[0095] Heating element 606 may include silicon carbide heating
elements as described in embodiments above. Silicon carbide
includes desirable properties such as electrical conductivity, and
high heat resistance to provide resistive heating without melting
or otherwise damaging the heating elements themselves. Heating
elements 606 are commonly manufactured in linear segments. One
common linear size of heating elements 606 includes 2 meter
lengths.
[0096] Substantially rectangular molds provide the ability to
increase a capacity of the mold 600 by increasing the length 603 of
the long side walls 602. The width 605 of the short side walls 604
can remain at a standard size e.g. 2 meters in selected examples)
and additional heating elements 606 can be added to the mold and
top heater configurations to accommodate the increased length 603.
This configuration provides an easy method to scale up a
manufacturing process, and make larger silicon ingots with reduced
manufacturing costs and time. In one example, a rectangular mold
provides a single batch size of 5-6 metric tons, where a
cylindrical mold using similar size heating elements (such as shown
in FIG. 7) provides a batch size of approximately 1.4 metric
tons.
[0097] In addition, the substantially rectangular shape of the mold
600 better matches an areal pattern of the heating elements 606.
Cylindrical mold designs with a circular top area are more
difficult to cover with linear heating elements 606. FIG. 7 shows
gaps 612 that lead to less consistent heat control of a surface of
molten silicon in cylindrical mold 610.
[0098] In addition, the substantially rectangular shape of the mold
600 provides for the ability to adjust an amount of contact that
the ingot has with side walls of the mold 600 during the
directional solidification process. In some processes,
contamination from side walls of a mold reduces a quality of the
ingot. One example includes aluminum contamination from aluminum
oxide refractory walls of a mold. By making the mold 600 longer,
using a substantially rectangular configuration, the ingot can be
made with increased contact with a bottom of the mold, and reduced
contact with side walls of the mold 600. In one example, a bottom
of the mold 600 is made from a material, such as silicon carbide,
that has a much lower, or non-existant contribution to
contamination of the silicon.
[0099] For example, a 2 meter diameter cylindrical mold provides a
surface area of .pi.(r.sup.2)=.pi.(d/2).sup.2=.pi.. Because wall
contact area is directly related to a height (h) of the molten
silicon, for a given volume of silicon (.pi.h for a cylindrical
mold), (xyh for a rectangular mold), a substantially rectangular
mold having a surface area (xy) greater than or equal to .pi. will
have a wall contact area that is smaller than the wall contact area
for the cylindrical mold. For a substantially rectangular mold
having one dimension equal to 2 meters, a second dimension larger
than .pi./2 meters provides a smaller wall contact area than a
cylindrical mold with a diameter equal to 2 meters.
[0100] FIG. 8 shows a mold 700 according to another embodiment of
the invention. The top view of the mold 700 shows a wall structure
701, including long side walls 702 and short side walls 704. In one
example, both the long side walls 702 and the short side walls 704
are formed from thermally insulating material, such as a refractory
material. As discussed in embodiments, above, in one example, the
refractory material includes aluminum oxide, substantially in the
form Al.sub.2O.sub.3. A number of heating elements 706 are shown in
a configuration to be included within a rectangular top heater to
cover the mold 700 during a directional solidification process. The
mold in FIG. 8 includes a rounded profile at corners 708.
[0101] In one example, the addition of rounded corners provides a
reduction in stress concentration at the corners. During a
directional solidification process, forces, such as thermal
expansion and contraction of silicon, can strain the mold 700. The
addition of rounded corners 708 reduces cracking or other failure
in the mold, which frequently includes brittle refractory materials
such as aluminum oxide, etc. as discussed in embodiments above.
[0102] FIG. 9 shows a side view of the mold 700 from FIG. 8. In
addition to the rounded corners 708, in one embodiment, the mold
700 includes a rounded profile 710 at the intersection of the
rectangular shaped wall structure 701 and a base 703. In one
example, a height 707 of the mold 700 is approximately 40
centimeters, with an ingot height within the mold 700 of
approximately 33 centimeters.
[0103] FIG. 10 shows a directional solidification system 800
according to an embodiment of the invention. A mold 801 is defined
by a wall structure 802 and a base 804. The mold 801 is adapted to
contain a volume of molten silicon 803, and to cool the molten
silicon 803 in a directional solidification process. The system 800
also includes a top heater 820 to control the thermal gradient and
cooling rate of the molten silicon 801. The top heater 820 is shown
with a number of heating elements 822, similar to top heaters
described above in other embodiments. Similar to examples described
above, in one configuration the wall structure 801 includes a taper
in thickness from a rim of the mold 801 to an interface with the
bottom 804.
[0104] A support structure 810 is shown holding the mold 801 a
distance above a floor surface 812 to define a space 811 In one
example, a flow of air or other cooling media is controlled within
the space 811 to control the rate of cooling at the base 804 of the
mold 801. In one example, one or more flow pathways are provided to
move air or other cooling media within the space 811. In FIG. 8, a
first flow pathway 840 and a second flow pathway 842 are shown.
Although two flow pathways (e.g. inlet and outlet) are used in
selected examples, other examples include more than two flow
pathway, or only a single flow pathway. A single flow pathway may
serve as an inlet and an outlet at the same time.
[0105] FIG. 10 also includes a circulation device 838 such as a fan
or other active mover of air or other cooling media. Also shown in
FIG. 10 are a first valve 830 and a second valve 834. The first
valve 830 is movable in direction 831 to adjust a space 832 and
provide metered access to the space 811 beneath the mold 801.
Similarly, the second valve 834 is movable in direction 835 to
adjust a space 836 and provide metered access to the space 811
beneath the mold 801. In operation, one or more valves such as
valves 830 and 834 are moved to change circulation and cooling
conditions within the space 811. In one example a rate of the
circulation device 838 can also be varied along with the one or
more valves such as valves 830 and 834.
[0106] In one example, a number of cooling structures (e.g. fins)
are located within the space 811 and coupled to the base 804 of the
mold 801. The cooling structures increase the ability to conduct
heat away from the base 804 of the mold. However, if the space 811
is closed by valves 830 and 834, less cooling will occur at the
base 804.
[0107] In one example, a metal heat spreading layer 806 is further
included within the base 804. In one example, the base 804 includes
a silicon carbide layer for contact with the molten silicon 803,
and the metal heat spreading layer 806 is located apart from the
molten silicon 803. In operation, a metal heat spreading layer 806,
such as a steel layer, conducts heat more quickly than silicon
carbide, and is included to enhance the cooling process at the base
804. In one example, the metal heat spreading layer 806 includes a
layer approximately 3 centimeters thick. A thick metal heat
spreading layer 806 provides a good pathway to conduct heat away
from the base 804 and to encourage directional solidification in a
desired solidification profile, such as the example profiles in
FIG. 5.
[0108] As shown in FIG. 10, in one example the valves 830 and 834
include insulating walls that cover both the walls 802 of the mold
801 and optionally cover access to the space 811 beneath the mold
801. The additional coverage adjacent to the walls 802 of the mold
801 further insulates the molten silicon 803 in the region against
the walls 802 and prevents less desirable solidification on the
walls before solidification at the base 804. In one example the
insulating walls of the valves 830 and 834 also cover an interface
824 between the mold 801 and the top heater 820. This configuration
provides additional protection against thermal loss at this
interface 824. Examples of valve materials include refractory
materials or other thermally insulating materials such as those
described in embodiments above.
[0109] In operation, the valves 830 and 834 can be controlled in a
number of ways. In one example the valves 830 and 834 provide an
adjustable space 832, 804 that is set once during the directional
solidification process. In one embodiment, one or more valves 830
and 834 are set to move at a continuous rate in directions 831, 835
to progressively open the pathways 840, 842. Movement of the valves
830 and 834 may be at the same rate for each valve, or the valves
may operate at different rates. The rate of the valves 830 and 834
may be linear, or the rate may vary during the directional
solidification process.
[0110] By using variables such as the circulation device 838, the
valves 830 and 834, the top heater 820, the tapered wall structure
802, the metal heat spreader 806, and fins 808, a rate and profile
of cooling for the molten silicon 803 can be precisely controlled.
Control of the rate and profile of cooling provides a number of
advantages, such as increased efficiency of impurity segregation.
Using configurations an methods described above, silicon can be
processed in fewer directional solidification steps, and larger
batches of silicon can be processed to produce larger ingots with
higher purity and at a faster processing rate.
[0111] FIG. 11 shows a system 900 including a mold 902 and a
removable heat spreader 910. The mold 902 includes a rim 904 and a
bottom 906. In some directional solidification operations, the
large thermal gradients experienced by molds 902 can cause warping,
and damage to heat spreading structures located on a bottom of the
mold 902. If repairs are needed on a warped mold with an integral
heat spreader, the entire mold must be taken out of production
while it is being repaired. In the example configuration
illustrated in FIG. 11, a warped or damaged removable heat spreader
910 can be removed for repair, and a spare removable heat spreader
910 can be quickly attached to keep the mold 902 available for
production while the warped or damaged removable heat spreader 910
is repaired.
[0112] In the bottom view of the removable heat spreader 910. In
one example, a number of cooling fins 914 are included. In one
example, a transport system, such as slots for a fork lift, or
other transport system are included on the removable heat spreader
910. Attachment points 912 can be used to couple the removable heat
spreader 910 to the bottom 906 of the mold 902. Examples of
coupling methods include using latches, fastening hardware, or
other fastening systems. Examples of fastening hardware include,
but are not limited to, bolts, screws, nuts, rivets, or other
suitable fastener where the removable heat spreader 910 is
separately removable from the bottom 906 of the mold. In one
example, a sacrificial fastener, such as a rivet, is used, wherein
the removable heat spreader 910 is removed by cutting, or otherwise
destroying the sacrificial fastener, and new sacrificial fasteners
are used to re-attach the removable heat spreader 910.
Configurations using a sacrificial fastener are easier to remove
than a welded integral heat spreader, yet secure in holding the
removable heat spreader 910 against the bottom 906 of the mold
902.
[0113] In addition to the ability to quickly replace a warped or
damaged removable heat spreader 910, because the removable heat
spreader 910 is only attached to the mold 902 at a selected number
of attachment points 912, the removable heat spreader 910 is
allowed a certain amount of freedom to expand and contract under
thermal stress, independent of the mold 902, which reduces the
cause of warping. In selected examples, the attachment points 912
include slotted, or oversized openings, which allow the removable
heat spreader 910 additional freedom of movement to expand and
contract around fasteners (latches, fastening hardware, etc.),
independent of the mold 902.
[0114] A removable heat spreader 910 and mold 902 of a number of
different shapes are within the scope of the invention. For
example, a substantially cylindrical mold and corresponding heat
spreader may be used. In another example, a rectangular removable
heat spreader may be coupled to a rectangular mold, similar to
molds 600, 700, or 801 as described above.
[0115] While a number of embodiments of the present subject matte
have been described, the above embodiments are not intended to be
exhaustive. It will be appreciated by those of ordinary skill in
the art that any arrangement configured to achieve silicon
purification using directional solidification techniques, while
maintaining consistent progression of a solid-liquid interface
throughout a mold can be substituted for the specific embodiment
shown. Combinations of the above embodiments, and other
embodiments, will be apparent to those of skill in the art upon
studying the above description. This application is intended to
cover any adaptations or variations of the present subject matter.
It is to be understood that the above description is intended to be
illustrative and not restrictive.
* * * * *