U.S. patent application number 12/201180 was filed with the patent office on 2009-03-05 for ribbon crystal string with extruded refractory material.
This patent application is currently assigned to EVERGREEN SOLAR, INC.. Invention is credited to Lawrence Felton, Christine Richardson.
Application Number | 20090061224 12/201180 |
Document ID | / |
Family ID | 39995448 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090061224 |
Kind Code |
A1 |
Richardson; Christine ; et
al. |
March 5, 2009 |
Ribbon Crystal String with Extruded Refractory Material
Abstract
A method of making string for string ribbon crystal provides a
substrate having an outer surface, and extrudes refractory material
over the substrate. The refractory material substantially covers
the outer surface of the substrate. The method then cures the
refractory material.
Inventors: |
Richardson; Christine;
(Northborough, MA) ; Felton; Lawrence; (Hopkinton,
MA) |
Correspondence
Address: |
BROMBERG & SUNSTEIN LLP
125 SUMMER STREET
BOSTON
MA
02110-1618
US
|
Assignee: |
EVERGREEN SOLAR, INC.
Marlborough
MA
|
Family ID: |
39995448 |
Appl. No.: |
12/201180 |
Filed: |
August 29, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60969263 |
Aug 31, 2007 |
|
|
|
Current U.S.
Class: |
428/373 ;
427/372.2; 427/397.7; 428/367; 428/391 |
Current CPC
Class: |
Y10T 428/2918 20150115;
C30B 15/36 20130101; C30B 35/00 20130101; C04B 35/62849 20130101;
Y10T 428/24628 20150115; Y10T 428/2942 20150115; C04B 35/62868
20130101; Y10T 428/2962 20150115; C30B 15/34 20130101; Y10T
428/24479 20150115; C04B 35/62871 20130101; Y10T 117/1032 20150115;
C04B 2235/5248 20130101; C04B 35/62873 20130101; Y10T 428/30
20150115; Y10T 428/2929 20150115; C04B 35/62852 20130101; C04B
35/62863 20130101; C04B 35/62894 20130101 |
Class at
Publication: |
428/373 ;
427/372.2; 427/397.7; 428/391; 428/367 |
International
Class: |
B32B 18/00 20060101
B32B018/00; B05D 3/02 20060101 B05D003/02; B05D 1/00 20060101
B05D001/00; B32B 1/00 20060101 B32B001/00 |
Claims
1. A method of making string for string ribbon crystal, the method
comprising: providing a substrate having an outer surface;
extruding refractory material over the substrate, the refractory
material substantially covering the outer surface of the substrate;
curing the refractory material.
2. The method as defined by claim 1 wherein the substrate comprises
a carbon filament.
3. The method as defined by claim 1 wherein the refractory material
comprises silicon carbide.
4. The method as defined by claim 1 wherein the substrate comprises
a tow.
5. The method as defined by claim 1 further comprising forming an
exterior reduced wetting layer radially outward of the refractory
material.
6. The method as defined by claim 1 wherein the substrate and
refractory material form a generally elongated cross-sectional
shape.
7. The method as defined by claim 1 wherein the substrate and
refractory material are generally concentric.
8. A string for forming a ribbon crystal, the string comprising: a
substrate having an outer surface; and an extruded refractory
material layer substantially covering the outer surface of the
substrate.
9. The string as defined by claim 8 wherein the substrate comprises
a carbon filament.
10. The string as defined by claim 8 wherein the refractory
material comprises silicon carbide.
11. The string as defined by claim 8 wherein the substrate
comprises a tow.
12. The string as defined by claim 8 further comprising an exterior
reduced wetting layer radially outward of the refractory
material.
13. The string as defined by claim 8 wherein the substrate and
refractory material form a generally elongated cross-sectional
shape.
14. The string as defined by claim 8 wherein the substrate and
refractory material are generally concentric.
15. The string as defined by claim 8 further comprising a handling
layer radially outward of the refractory material.
16. A string for forming a ribbon crystal, the string comprising: a
substrate; and extruded refractory means substantially covering the
substrate.
17. The string as defined by claim 16 wherein the extruded
refractory means comprises a refractory material.
18. The string as defined by claim 16 wherein the substrate
comprises a carbon filament.
19. The string as defined by claim 16 wherein the extruded
refractory means comprises silicon carbide.
20. The string as defined by claim 16 wherein the substrate
comprises a tow.
21. The string as defined by claim 16 further comprising an
exterior reduced wetting layer radially outward of the extruded
refractory means.
Description
PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority from provisional
U.S. patent application No. 60/969,263, filed Aug. 31, 2007,
entitled, "STRING RIBBON CRYSTAL AND STRING WITH IMPROVED
EFFICIENCY," assigned attorney docket number 3253/106, and naming
Christine Richardson, Lawrence Felton, Richard Wallace, and Scott
Reitsma as inventors, the disclosure of which is incorporated
herein, in its entirety, by reference.
[0002] This patent application also is related to the following
copending, co-owned patent applications, filed on even date
herewith, claiming the same priority as noted above and
incorporated herein, in their entireties, by reference:
[0003] Attorney Docket Number 3253/172, entitled, "REDUCED WETTING
STRING FOR RIBBON CRYSTAL," and
[0004] Attorney Docket Number 3253/173, entitled, "RIBBON CRYSTAL
STRING FOR INCREASING WAFER YIELD."
FIELD OF THE INVENTION
[0005] The invention generally relates to string ribbon crystals
and, more particularly, the invention also relates to string used
to form string ribbon crystals.
BACKGROUND OF THE INVENTION
[0006] String ribbon crystals, such as those discussed in U.S. Pat.
No. 4,689,109 (issued in 1987 and naming Emanuel M. Sachs as the
sole inventor), can form the basis of a variety of electronic
devices. For example, Evergreen Solar, Inc. of Marlborough, Mass.
forms solar cells from conventional string ribbon crystals.
[0007] As discussed in greater detail in the noted patent,
conventional processes form string ribbon crystals by passing two
or more strings through molten silicon. The composition and nature
of the string can have a significant impact on the efficiency and,
in some instances, the cost of the ultimately formed string ribbon
crystal.
SUMMARY OF THE INVENTION
[0008] In accordance with one embodiment of the invention, a method
of making string for string ribbon crystal provides a substrate
having an outer surface, and extrudes refractory material over the
substrate. The refractory material substantially covers the outer
surface of the substrate. The method then cures the refractory
material.
[0009] For example, the substrate may be formed from a carbon
filament or a tow, while the extruded refractory material may
include silicon carbide. The method also may form an exterior
reduced wetting layer radially outward of the refractory material.
In some embodiments, the substrate and refractory material form a
generally elongated cross-sectional shape, and/or are generally
concentric.
[0010] In other embodiments of the invention, a string for forming
a ribbon crystal has a substrate, and an extruded refractory
material layer substantially covering the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Those skilled in the art should more fully appreciate
advantages of various embodiments of the invention from the
following "Description of Illustrative Embodiments," discussed with
reference to the drawings summarized immediately below.
[0012] FIG. 1 schematically shows a string ribbon crystal that may
be formed from strings configured in accordance with illustrative
embodiments of the invention.
[0013] FIG. 2 schematically shows an illustrative furnace used to
form string ribbon crystals.
[0014] FIG. 3 schematically shows a cross-sectional view of a
portion of a prior art ribbon crystal with a prior art string.
[0015] FIG. 4A schematically shows a string formed in accordance
with illustrative embodiments of the invention.
[0016] FIG. 4B schematically shows eight cross-sectional views of
the string of FIG. 4A along line B-B in accordance with various
embodiment of the invention.
[0017] FIG. 5 shows an illustrative process of forming a string
ribbon crystal using strings configured in accordance with
illustrative embodiments of the invention.
[0018] FIGS. 6A, 6B, and 6C schematically show cross-sectional
views of ribbon crystals in accordance with an embodiment using
strings with an elongated cross-section.
[0019] FIGS. 7A and 7B schematically show cross-sectional views of
ribbon crystals with multiple strings used to perform the function
of a single string.
[0020] FIGS. 8A and 8B schematically show a ribbon crystal with a
string having a generally concave cross-sectional shape.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0021] Illustrative embodiments extrude a refractory material over
a core/substrate to form string used to grow ribbon crystals. This
process beneficially avoids use of complex prior art processes that
require hazardous chemicals (e.g., CVD processes). Details of
various embodiments are discussed below.
[0022] FIG. 1 schematically shows a string ribbon crystal 10
configured in accordance illustrative embodiments of the invention.
In a manner similar to other ribbon crystals, this ribbon crystal
10 has a generally rectangular shape and a relatively large surface
area on its front and back faces. For example, the ribbon crystal
10 may have a width of about 3 inches, and a length of about 6
inches. As known by those skilled in the art, the length can vary
significantly. For example, in some known processes, the length
depends upon a furnace operator's discretion as to where to cut the
ribbon crystal 10 as it grows. In addition, the width can vary
depending upon the separation of its two strings 12 (see FIG. 2)
forming the ribbon crystal width boundaries. Accordingly,
discussion of specific lengths and widths are illustrative and not
intended to limit various embodiments the invention.
[0023] The thickness of the ribbon crystal 10 may vary and be very
small relative to its length and width dimensions. For example, the
string ribbon crystal 10 may have a thickness ranging from about 60
microns to about 320 microns across its width. Despite this varying
thickness, the string ribbon crystal 10 may be considered to have
an average thickness across its length and/or width.
[0024] The ribbon crystal 10 may be formed from any of a wide
variety of materials (often referred to generally as "ribbon
material" or "crystal material"), depending upon the application.
For example, when grown for a photovoltaic application, the ribbon
crystal 10 may be formed from a single element, such as silicon, or
a compound, such as a silicon-based material (e.g., silicon
germanium). Other illustrative ribbon materials may include gallium
arsenide, or indium phosphide. The ribbon material may be any of a
variety of crystal types, such as multi-crystalline, single
crystalline, polycrystalline, microcrystalline or
semi-crystalline.
[0025] As known by those skilled in the art, the ribbon crystal 10
is formed from a pair of strings 12 generally embedded/encapsulated
by the ribbon material. For simplicity, the ribbon crystal 10 is
discussed as being formed from polysilicon ribbon material. It
nevertheless should be reiterated that discussion of polysilicon is
not intended to limit all embodiments.
[0026] Illustrative embodiments grow the ribbon crystal 10 in a
ribbon crystal growth furnace 14, such as that shown in FIG. 2.
More specifically, FIG. 2 schematically shows a silicon ribbon
crystal growth furnace 14 that may be used to form the string
ribbon crystal 10 in accordance with illustrative embodiments of
the invention. The furnace 14 has, among other things, a housing 16
forming a sealed interior that is substantially free of oxygen (to
prevent combustion). Instead of oxygen, the interior has some
concentration of another gas, such as argon, or a combination of
gasses. The housing interior also contains, among other things, a
crucible 18 and other components for substantially simultaneously
growing four silicon ribbon crystals 10. A feed inlet 20 in the
housing 16 provides a means for directing silicon feedstock to the
interior crucible 18, while an optional window 22 permits
inspection of the interior components.
[0027] As shown, the crucible 18, which is supported on an interior
platform within the housing 16, has a substantially flat top
surface. This embodiment of the crucible 18 has an elongated shape
with a region for growing silicon ribbon crystals 10 in a
side-by-side arrangement along its length. In illustrative
embodiments, the crucible 18 is formed from graphite and
resistively heated to a temperature capable of maintaining silicon
above its melting point. To improve results, the crucible 18 has a
length that is much greater than its width. For example, the length
of the crucible 18 may be three or more times greater than its
width. Of course, in some embodiments, the crucible 18 is not
elongated in this manner. For example, the crucible 18 may have a
somewhat square shape, or a nonrectangular shape.
[0028] As shown in FIG. 2 and discussed in greater detail below,
the furnace 14 has a plurality of holes 24 (shown in phantom) for
receiving string 12. Specifically, the furnace 14 of FIG. 2 has
eight string holes 24 for receiving four pairs of strings 12. Each
pair of strings 12 passes through molten silicon in the crucible 18
to form a single ribbon crystal 10.
[0029] Many conventional ribbon crystal growth processes form
ribbon crystals with a thin neck portion near the string. More
specifically, FIG. 3 schematically shows a cross-sectional view of
a portion of a prior art ribbon crystal 10P having a prior art
string 12P. This prior art ribbon crystal 10P has a thin neck
portion 36 between the string 12P and a wider portion 38 of the
ribbon crystal 10. If the neck portion 36 is too thin, then the
ribbon crystal 10P may be very fragile and more prone to breaking,
thus leading to yield losses. For example, if the coefficient of
thermal expansion differential between the string 12 and ribbon
material forming the ribbon crystal 10P (e.g., polysilicon) is
sufficiently large, the ribbon crystal 10P may be more prone to
breaking at the neck portion 36.
[0030] To increase the neck thickness, those skilled in the art
have added equipment to the ribbon growth process. For example, one
such solution adds gas jets (not shown) to the furnace 14. These
gas jets direct relatively cool gas streams toward the neck portion
36, thus decreasing the temperature in that area to increase neck
thickness. Other solutions involve adding specialized meniscus
shapers.
[0031] Rather than use such additional external measures,
illustrative embodiments of the invention engineer the
cross-sectional dimension of the string 12 in a prescribed manner.
The string 12 then is positioned within the crystal growth furnace
14 in a manner that increases the size of the neck portion 36 of
the growing ribbon crystal 10. For example, the resulting ribbon
crystal 10 with an average thickness of about 190 microns may have
a neck portion 36 with a minimum thickness of about 60 microns,
which may suffice in certain applications. This innovation
consequently should reduce yield loss, thus reducing production
costs.
[0032] FIG. 4A schematically shows a string 12 that may be formed
in accordance with illustrative embodiments of the invention.
Although this figure appears to show a generally convex or rounded
cross-section, it should be considered merely schematic and not
representative of any specific cross-sectional shape. To that end,
FIG. 4B schematically shows eight different possible
cross-sectional views of the string 12 of FIG. 4A along cross-line
B-B in accordance with a number of different embodiments of the
invention. For example, some of the shapes are generally elongated,
such as the irregular shape of string one, the rectangular shape of
string two, and the somewhat elliptical shape of string three.
[0033] Whether or not they are elongated, the various strings 12
may be categorized as being either generally concave or generally
convex. As used herein, a cross-sectional shape is generally
concave when any portion of its perimeter forms at least one
non-negligible concavity. Thus, string one is considered to be
generally concave despite its other convex portions. Conversely, a
cross-sectional shape is considered to be generally convex when its
perimeter forms no non-negligible concavities. Thus, string two and
string three of FIG. 4B a generally convex.
[0034] FIG. 4B shows a number of other cross-sectional string
shapes that are generally concave. In fact, some may be considered
elongated and concave. For example, string four is generally "C"
shaped, concave, and elongated, while string five is generally
cross shaped, concave, but not elongated. The shape of string five
(cross shaped) is not elongated because it is generally
symmetrical--both the horizontal and vertical portions of the cross
are about the same size. Depending upon its actual dimensions,
string eight, which is generally "T" shaped, may or may not be
considered elongated. For example, if the portion of the "T" shape
extending downwardly is longer than its horizontal portion, then
string eight may be considered elongated. In either case, string
eight is considered to be generally concave.
[0035] Some embodiments use plural strings 12 to form one edge of a
ribbon crystal 10. Strings six and seven show two such embodiments.
Specifically, string six shows one embodiment where the individual
strings 12 physically contact each other in the final ribbon
crystal 10, while string seven shows another embodiment where the
individual strings 12 are spaced from each other in the final
ribbon crystal 10. It should be noted that embodiments using plural
strings 12 may use more than two strings 12. In addition,
individual strings 12 of this plural string embodiment may have the
same or different cross-sectional shapes (e.g., a first
elliptically shaped string 12 and another cross or circular shaped
string 12).
[0036] The specific shapes of FIG. 4B merely are examples of a
variety of different cross-sectional string shapes. For example,
some embodiments use strings that have a generally circular
cross-sectional shape. Accordingly, those skilled in the art should
understand that other string shapes fall within the scope of
various embodiments.
[0037] FIG. 5 shows an illustrative process of forming a string
ribbon crystal 10 with strings 12 configured in accordance with
illustrative embodiments of the invention. For simplicity, this
process is discussed with reference to string two of FIG. 4B
only--because string two is the only string 12 in that figure
explicitly showing various string layers discussed in this process.
It nevertheless should be noted that the discussed principles apply
to strings 12 having other cross-sectional shapes, or other strings
formed by other processes.
[0038] The process begins at step 500 by forming a core/substrate
28, which acts as a substrate to receive a refractory material
layer. As discussed in greater detail in co-pending US patent
application having attorney docket number 3253/172 and entitled,
"REDUCED WETTING STRING FOR RIBBON CRYSTAL," (which is incorporated
by reference above), the core 28 can be formed from carbon by
conventional extruding processes. In other embodiments, however,
the core 28 may be a wire, filament, or plurality of small
conductive fibers wound together as a tow. For example,
post-fabrication processes could form a monofilament through a
known fabrication process, such as oxidation, carbonization, or
infiltration.
[0039] The core 28 may have the desired cross-sectional shape. For
example, as shown in FIG. 4B, the core 28 of string two is
generally rectangular. Alternatively, the core 28 may have a
different cross-sectional shape, while refractory material
application equipment may be specially configured to form the
desired cross-sectional shape. For example, the extrusion equipment
may be specially configured to form the cross-sectional shape from
a core material having a prespecified cross-sectional shape that is
the same as or different than that of the final cross-sectional
string shape.
[0040] After forming the core 28, the process forms a first
coating/layer, which acts as the above noted refractory material
layer 30 (step 502). Among other things, the first coating 30 may
include silicon carbide, tungsten, or a combination of silicon
carbide and tungsten. Conventional wisdom dictates that this outer
surface 30 should be very smooth to minimize nucleations that may
occur when it contacts molten ribbon material within the furnace
24. Fewer nucleations desirably should produce fewer grains and
thus, fewer grain boundaries. Consequently, such strings 12 should
be more electrically efficient than those with more grains and more
grain boundaries.
[0041] To those ends, one commonly used prior art process known to
the inventors uses chemical vapor deposition (i.e., "CVD") to form
the refractory material layer 30. Accordingly, such prior art
strings should have smoother outer surfaces and thus, produce fewer
grains and grain boundaries. Undesirably, however, such a process
is complex and uses hazardous chemicals.
[0042] Illustrative embodiments solve these problems. Specifically,
to avoid the use of such complex machinery and hazardous chemicals
of a CVD process (or other similar process), illustrative
embodiments extrude the refractory material directly onto the
core/substrate 28, thus covering substantially the entire outer
(circumferential) surface of the core 28. This is contrary to prior
art teachings, however, because it is expected to yield a less
smooth surface. The inventors nevertheless anticipate that such a
string can produce satisfactory results in a much less costly
manner and with fewer safety risks.
[0043] Formation of the extruded refractory material layer 30 may
involve, among other things, a pulltrusion process, or both
spinning of a refractory material with a polymer component, which
subsequently is baked off. Processes may use at least one component
of carbon, silicon, silicon carbide, silicon nitride, aluminum,
mullite, silicon dioxide, BN particles, or fibers mixed with a
polymer binder, coupled with extrusion/pulltrusion. This also may
involve bicomponent extrusion of a core 28 with at least one
silicon carbide, carbon, silicon, and a sheath with a least one of
oxide, mullite, carbon, and/or silicon carbide. Accordingly, as
noted above, the core 28 effectively acts as a substrate for
supporting the refractory material layer 30. For example, the
refractory material layer 30 may be, or may not be, generally
concentric with the core 28. After it is extruded onto the core 28,
the refractory material layer 30 is allowed to harden/cure for a
sufficient amount of time.
[0044] As discussed below, some embodiments form one or more layers
radially outward of the refractory material layer 30. Such layers
can be smoother, or take on a roughness that is similar to that of
this layer 30.
[0045] This step thus forms what is considered to be a base string
portion 26. At this point in the process, the base string portion
26 has a combined coefficient of thermal expansion that preferably
generally matches the coefficient of thermal expansion of the
ribbon material. Specifically, the thermal expansion
characteristics of the string 12 should be sufficiently well
matched to the ribbon material so that excessive stress does not
develop at the interface. Stress is considered excessive if the
string 12 exhibits a tendency to separate from the ribbon during
reasonable subsequent ribbon crystal handling and processing steps,
or if the string 12 exhibits a tendency to curl outwardly or
inwardly from the ribbon crystal edge. In other embodiments,
however, the coefficient of thermal expansion of the base string
portion 26 does not generally match that of the ribbon
material.
[0046] As noted above, some embodiments of the invention may have
one or more additional layers, depending upon the application. For
example, as discussed in greater detail in the above noted
incorporated patent application having attorney docket number
3253/172, the string 12 may have a non-wetting/reduced wetting
layer 32 to increase the grain size of the ribbon material. In that
case, the process continues to step 504, which forms an exposed
non-wetting/reduced layer 32 on the base string portion 26. In
applications sensitive to coefficient of thermal expansion
differences, this layer 32 preferably is very thin so that it has a
negligible impact on the overall string coefficient of thermal
expansion. For example, the reduced wetting layer 32 should be much
thinner than that of the refractory material layer 30.
[0047] In embodiments using this non-wetting layer 32, the contact
angle with the ribbon material of its exterior surface should be
carefully controlled to cause the molten ribbon material to adhere
to it--otherwise, the process cannot form the ribbon crystal 10. In
applications using molten polysilicon, for example, it is
anticipated that contact angles with silicon of between about 15
and 120.degree. degrees should produce satisfactory results. Such
angles of greater than 25 degrees may produce better results.
[0048] Among other ways, the non-wetting layer 32 may be formed by
CVD processes, dip coating or other methods. For example, the base
string portion 26 may be CVD coated by applying electrical contacts
in a deposition chamber while it is being fed through the
chamber--thus heating the base string portion 26 itself.
Alternatively, the base string portion 26 may be heated by
induction heating through the chamber.
[0049] Related techniques for implementing this step include:
[0050] a sol gel dip for silica or alumina oxide or silicon
oxycarbide either at the end of a CVD furnace or during rewind,
[0051] a CVD nonwetting coating deposited by heating quartz from
the outside and induction heating the base string portion 26,
[0052] spray-on deposition with a polymer binder that subsequently
would be burned off, [0053] shaking particles onto a base string
portion 26 or tow and then baking the into the base string portion
26 or tow, and [0054] coating with base string portion 26 with
refractory slurry (e.g., silicon carbide/silicon dioxide) or liquid
and then burning off residual.
[0055] The string 12 also may have a handling layer 34 radially
outward of the refractory material layer 30 to maintain the
integrity of the base string portion 26. To that end, if included,
the handling layer 34 provides a small compressive stress to the
base string portion 26, thus improving robustness to the overall
string 12. Accordingly, if the base string portion 26 develops a
crack, the compressive stress of the handling layer 34 should
reduce the likelihood that the string 12 will break. Among other
things, the handling layer 34 may be a thin layer of carbon (e.g.,
one or two microns thick for strings 12 having generally known
sizes).
[0056] Accordingly, prior to performing step 504, some embodiments
may form a handling layer 34 that is separate from the produced
nonwetting layer 32 (e.g., see string two of FIG. 4B). Thus, in
such an embodiment, the nonwetting layer 32 substantially covers
the handling layer 34. More specifically, the nonwetting layer 32
covers the outer, circumferential surface of the handling layer 34.
Some embodiments, however, may integrate the non-wetting layer 32
into the handling layer 34.
[0057] It then is determined at step 506 if the coated string 12
has filaments extending through the nonwetting layer 32 (such
filaments are referred to herein as "whiskers"). This can occur,
for example, when a tow of filaments forms the core 28. If the
coated string 12 has whiskers, then the process shaves them off at
step 508. The process then may loop back to step 504, which
re-applies the nonwetting layer 32.
[0058] Alternatively, if the string 12 has no whiskers, the process
continues to step 510, which provides the string 12 to the furnace
14 as shown in FIG. 2. To that end, some embodiments provide a
single string 12 for each ribbon crystal edge, or multiple strings
12 for each ribbon crystal edge (e.g., strings six and seven of
FIG. 6B). The term "string," unless explicitly modified to the
contrary (e.g., by the words "single" or "multiple"), when
mentioned with reference to forming a boundary/width of a ribbon
crystal 10, generally means one or more strings.
[0059] Rather than using the methods above for forming the string
12, some embodiments machine or bore a concavity into a rounded or
other otherwise generally convex string 12. Accordingly, the string
12 may be formed by other methods.
[0060] Illustrative embodiments orient the strings 12 in the
furnace 14 in a manner that increases the thickness of the ribbon
crystal neck portion 36. For example, FIGS. 6A-6C schematically
show cross-sectional views of three ribbon crystals 10 with strings
12 having elongated, generally elliptical, generally convex
cross-sectional shapes. To increase the thickness of the neck
portion 36, these embodiments orient their respective generally
longitudinal axes 42 so that they diverge with the width dimension
of their respective ribbon crystals 10. In other words, to diverge,
the longitudinal axis 42 is not parallel with the width
dimension--instead, the longitudinal axis 42 and width dimension
intersect.
[0061] More specifically, the cross-section of each string 12 has a
largest dimension, each of which is shown as double-head arrows in
FIGS. 6A-6C. For reference purposes, the longitudinal axis 42 of
each of these elongated cross-sectional shapes thus is considered
to be co-linear with the largest dimension. For example, FIG. 6A
orients the longitudinal axis 42 substantially perpendicular to the
width dimension, while FIG. 6C orients the longitudinal axis 42 to
form a shallow angle with the width dimension. FIG. 6B orients the
longitudinal axis 42 between the extremes of FIGS. 6A and 6C.
[0062] It should be noted that orientations other than those shown
in FIGS. 6A-6C also should provide satisfactory results. For
example, orienting the longitudinal axis 42 in a manner so that is
rotated about 90 degrees (either clockwise or counterclockwise)
from the angle shown in FIG. 6B also should increase neck size.
[0063] FIGS. 8A and 8B schematically show two ribbon crystals 10
with strings 12 having a generally concave cross-sectional shape.
As shown, the strings 12 are oriented so that their concavities
either are oriented completely toward or completely away from the
wafer width (i.e., in the X-direction). In particular, the
concavity is generally symmetrically oriented, e.g., the concavity
forms a mirror image above and below the X-axis. Significant
rotation from these orientations (either clockwise or
counterclockwise), however, may impact the meniscus shape to impede
appropriate crystal growth. Those in the art can apply this concept
to a string 12 having multiple concavities or concavities on
opposing sides of the cross-sectional shape (e.g., a
cross-shape).
[0064] At this point, for each ribbon crystal 10 being grown, the
process passes two strings 12 (together forming the ultimate ribbon
crystal width) through the furnace 14 and crucible 18, thus forming
the string ribbon crystal 10 (step 512).
[0065] Accordingly, illustrative embodiments of the invention
extrude the refractory material layer 30 on the core 28, thus
avoiding problems associated with prior art deposition techniques
and reducing production costs.
[0066] Although the above discussion discloses various exemplary
embodiments of the invention, it should be apparent that those
skilled in the art can make various modifications that will achieve
some of the advantages of the invention without departing from the
true scope of the invention.
* * * * *