U.S. patent application number 13/653988 was filed with the patent office on 2013-02-14 for ribbon crystal end string with multiple individual strings.
This patent application is currently assigned to Max Era, Inc.. The applicant listed for this patent is Scott REITSMA. Invention is credited to Scott REITSMA.
Application Number | 20130036966 13/653988 |
Document ID | / |
Family ID | 41560364 |
Filed Date | 2013-02-14 |
United States Patent
Application |
20130036966 |
Kind Code |
A1 |
REITSMA; Scott |
February 14, 2013 |
RIBBON CRYSTAL END STRING WITH MULTIPLE INDIVIDUAL STRINGS
Abstract
A ribbon crystal has a body and end string within the body. At
least one end string has a generally concave cross-sectional shape
and is formed from at least two individual strings.
Inventors: |
REITSMA; Scott; (Shrewsbury,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
REITSMA; Scott |
Shrewsbury |
MA |
US |
|
|
Assignee: |
Max Era, Inc.
Reno
NV
|
Family ID: |
41560364 |
Appl. No.: |
13/653988 |
Filed: |
October 17, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12252557 |
Oct 16, 2008 |
8304057 |
|
|
13653988 |
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Current U.S.
Class: |
117/24 |
Current CPC
Class: |
C30B 15/007 20130101;
C30B 29/06 20130101; Y10T 428/249921 20150401; Y10T 428/24628
20150115; C30B 35/00 20130101; C30B 15/36 20130101; C30B 15/34
20130101; Y10T 428/24479 20150115; C30B 15/24 20130101 |
Class at
Publication: |
117/24 |
International
Class: |
C30B 15/34 20060101
C30B015/34 |
Claims
1. A method of forming a ribbon crystal, the method comprising:
providing a plurality of end strings, at least one of the end
strings comprising at least two spaced apart individual strings;
adding molten material to a crucible; and passing the end strings
through the molten material causing the molten material to freeze
above an interface to form a sheet of frozen material, the at least
one end string having frozen molten material between its individual
strings above the interface.
2. The method as defined by claim 1 wherein the molten material
comprises silicon.
3. The method as defined by claim 1 wherein the individual strings
each generally forms an elongated cross-sectional shape.
4. The method as defined by claim 1 wherein at least two end
strings each have a plurality of spaced apart individual
strings.
5. The method as defined by claim 4 wherein the at least two end
strings each have frozen molten material between their respective
individual strings above the interface.
6. The method as defined by claim 1 wherein the sheet has a
thickness dimension, the individual strings of the at least one end
string being spaced generally along the thickness dimension of the
sheet.
Description
PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a continuation-in-part of U.S.
patent application Ser. No. 12/201,117, filed Aug. 29, 2008,
entitled, "RIBBON CRYSTAL STRING FOR INCREASING WAFER YIELD,"
assigned attorney docket number 3153/173, and naming Scott Reitsma
as inventor, the disclosure of which is incorporated herein, in its
entirety, by reference.
[0002] The parent patent application (U.S. Ser. No. 12/201,117)
claims priority from provisional U.S. patent application Ser. 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.
[0003] This patent application also is related to the following
copending, co-owned patent applications, filed on Aug. 29, 2008,
claiming the same priority to the. U.S. provisional patent
application No. 60/969,263, and incorporated herein, in their
entireties, by reference:
[0004] Attorney Docket Number 3253/172, entitled, "REDUCED WETTING
STRING FOR RIBBON CRYSTAL," and assigned U.S. patent application
Ser. No. 12/200,996, and
[0005] Attorney Docket Number 3253/174, entitled, "RIBBON CRYSTAL
STRING WITH EXTRUDED REFRACTORY MATERIAL," and assigned U.S. patent
application Ser. No. 12/201,180.
FIELD OF THE INVENTION
[0006] 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
[0007] 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.
[0008] 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
[0009] In accordance with one embodiment of the invention, a ribbon
crystal has a body and end string within the body. At least one end
string has a generally concave cross-sectional shape and is formed
from at least two individual strings.
[0010] The noted two individual strings may be spaced apart
generally along the thickness dimension of the body to generally
form an elongated, concave cross-sectional shape. Moreover, body
material (e.g., silicon) may be between the two individual strings.
In alternative embodiments, the noted two individual strings are in
physical contact.
[0011] In accordance with another embodiment of the invention, a
ribbon crystal has a body and a plurality of end strings within the
body. At least one end string is formed from at least a pair of
spaced apart individual strings. Body material is positioned
between the pair of individual strings.
[0012] At least one end string may have a generally concave
cross-sectional shape. In addition, the individual strings each may
generally form an elongated cross-sectional shape. Moreover, at
least one of the individual strings may have a concavity that is
generally symmetrical about the width dimension. The individual
strings of the at least one end string may be spaced generally
along the thickness dimension of the body.
[0013] In accordance with other embodiments of the invention, a
method of forming a ribbon crystal provides a plurality of end
strings. At least one of the end strings has at least two spaced
apart individual strings. The method also adds molten material to a
crucible, and then passes the end strings through the molten
material to cause the molten material to freeze above an interface,
thus forming a sheet of molten material. The at least one end
string has frozen molten material between its individual strings
above the interface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] 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.
[0015] FIG. 1 schematically shows a string ribbon crystal that may
be formed from strings configured in accordance with illustrative
embodiments of the invention.
[0016] FIG. 2 schematically shows an illustrative furnace used to
form string ribbon crystals.
[0017] FIG. 3 schematically shows a cross-sectional view of a
portion of a prior art ribbon crystal with a prior art string.
[0018] FIG. 4A schematically shows a string formed in accordance
with illustrative embodiments of the invention.
[0019] 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.
[0020] FIG. 5 shows an illustrative process of forming a string
ribbon crystal using strings configured in accordance with
illustrative embodiments of the invention.
[0021] 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.
[0022] FIGS. 7A and 7B schematically show cross-sectional views of
ribbon crystals with end string formed from multiple individual
strings.
[0023] FIGS. 8A and 8B schematically show a ribbon crystal with a
string having a generally concave cross-sectional shape.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0024] Illustrative string ribbon fabrication processes use plural
individual strings for each end of a ribbon crystal. For example,
each end string may be formed from a pair of individual spaced
apart strings. Geometric and thermal properties of such a string
should improve crystal properties, such as by forming thicker neck
regions. Details of various embodiments are discussed below.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] As known by those skilled in the art, the ribbon crystal 10
is formed from a pair of strings 12 (see FIG. 2 and later figures)
generally embedded/encapsulated by the ribbon material. As
ascertained from the process discussed below, the pair of strings
12 effectively form the edges of the ribbon crystal 10; namely,
they define the width of the ribbon crystal 10.
[0029] Accordingly, the strings may be generally referred to herein
as "end strings," or simply as strings 12. Moreover, 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.
[0030] 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.
[0031] 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.
[0032] 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 (i.e., the end strings 12). Specifically, the
furnace 14 of FIG. 2 has eight string holes 24 for receiving four
pairs of end strings 12. Each pair of strings 12 passes through
molten silicon in the crucible 18 to form a single ribbon crystal
10.
[0033] 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.
[0034] 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.
[0035] 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.
Illustrative embodiments then position the string 12 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] During experimentation, as discussed below, the inventors
were surprised to discover that forming the end strings 12 from
plural individual strings significantly improved neck size. In
other words, the end strings 12 may be formed from two or more
individual strings. 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). As inherently shown by
string seven, the end string 12 using two spaced, individual
strings comprises two individual stings, plus some ribbon body
material (e.g., polysilicon) between the two individual
strings.
[0040] It should benoted that end strings 12 using plural
individual strings may use more than two individual strings. For
example, some end strings 12 may use three or four strings to
increase their depth dimension. In addition, individual strings of
these plural string embodiments 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).
[0041] It should be noted that the specific shapes of FIG. 4B
merely are examples of a variety of different cross-sectional
string shapes. Accordingly, those skilled in the art should
understand that other string shapes fall within the scope of
various embodiments.
[0042] 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.
[0043] 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 U.S. 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.
[0044] 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 to that of the final cross-sectional
string shape.
[0045] 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. In addition, this first layer may be formed
in a number of conventional ways, such as with a conventional (and
often complex) CVD coating process.
[0046] To avoid the use of complex machinery and hazardous
chemicals of a CVD process, illustrative embodiments extrude the
refractory material directly onto the core/substrate 28. This 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.
[0047] This step thus forms what is considered to be a base string
portion 26. It should be reiterated that the base string portion 26
may be formed from one or. more of any of a wide variety of
materials. Such materials may include a graphite fiber or tow, a
refractory material, such as tungsten or silicon carbide, or a
combination thereof. In fact, some embodiments may form the base
string portion 26 without a core 28.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] Related techniques for implementing this step include:
[0053] a sol gel dip for silica or alumina oxide or silicon
oxycarbide either at the end of a CVD furnace or during rewind,
[0054] a CVD nonwetting coating deposited by heating quartz from
the outside and induction heating the base string portion 26,
[0055] spray-on deposition with a polymer binder that subsequently
would be burned off,
[0056] shaking particles onto a base string portion 26 or tow and
then baking the into the base string portion 26 or tow, and
[0057] coating with base string portion 26 with refractory slurry
(e.g., silicon carbide/silicon dioxide) or liquid and then burning
off residual.
[0058] 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).
[0059] 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 wetting 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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. Prior art techniques
known to the inventor orient this longitudinal axis 42 generally
parallel with the width dimension of the ribbon crystal 10.
Contrary to this explicit teaching in the art, however, the
inventors discovered that orienting the longitudinal axis 42 so
that it diverges with the ribbon crystal width dimension should
increase the neck size.
[0065] 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. Either embodiment should increase
the size of the neck portion 36 when compared to the above noted
prior art technique. This neck size increase consequently should
reduce breakage, thus improving yield.
[0066] 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.
[0067] As the strings 12 move through the furnace 14, the molten
ribbon material (of each ribbon crystal 10) forms a meniscus.
During testing, the inventor discovered that raising the height of
the meniscus also generally increased the thickness of the neck
portion 36. To that end, the inventors recognized that the
principal radii of the cross-sectional string shapes should have
certain prescribed properties.
[0068] More specifically, the pressure difference across the static
interface between the gas and molten material is defined by the
Young-Laplace Equation, which is set out as follows:
P I - P II = .sigma. ( 1 r 1 + 1 r 2 ) ##EQU00001##
[0069] where:
[0070] P.sub.I is the pressure of molten material,
[0071] P.sub.II is the pressure of the gas,
[0072] r1 and r2 are principal radii of curvature of the meniscus,
and
[0073] .sigma. (rho) is the surface tension.
[0074] The inventor determined that the meniscus height should
increase if the pressure of the molten material is less than the
pressure of the gas. To accomplish this, the inventors determined
that the principal radii of curvature of the meniscus should be
small when they are positive (i.e., when the cross-sectional shape
is generally concave). Conversely, if the second radius of
curvature r2 is negative, in which case the cross-sectional shape
is generally convex, then the second radius of curvature r2 should
be large.
[0075] Early testing at least preliminarily confirms these
conclusions. Moreover, such tests yielded additional, surprising
results. Specifically, the inventor noticed the meniscus raising
phenomenon by passing two individual strings 12 through the molten
material for a single edge of a ribbon crystal 10. FIGS. 7A and 7B
schematically show ribbon crystals 10 formed using this
technique.
[0076] The inventor also noticed another surprising result when the
individual strings 12 for each edge were separated (FIG. 7B). In
particular, in one test, the two individual strings 12 forming a
single edge were separated by about 700 microns. In addition to
thickening the neck portion 36, a close examination of this edge
also showed larger grains near in that region--a result that was
completely unexpected (these individual strings 12 did not have the
above noted nonwetting layer 32). As such, the inventor believes
that such a technique, and related techniques, also should improve
the electrical efficiency of the ribbon crystal 10.
[0077] 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. This orientation
is preferred because the inventor believes that it will shape the
meniscus in a manner that promotes appropriate crystal growth.
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).
[0078] 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).
[0079] Accordingly, illustrative embodiments of the invention
orient specially configured strings 12 within a ribbon crystal 10
to increase neck thickness. Alternatively, or in addition,
specially configured strings 12 raise the height of the meniscus
within, the furnace 14 to further increase neck thickness. For
example, end strings 12 formed from multiple individual strings may
raise the meniscus portion between each individual string, in
addition to the portion of the meniscus between the end strings 12.
Ribbon crystals 10 grown using these techniques therefore should be
less prone to breaking, thus improving yields.
[0080] 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. For example, some embodiments may use
more than two end strings 12 to form a single ribbon crystal
10.
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