U.S. patent application number 13/079838 was filed with the patent office on 2011-10-13 for ribbon crystal string for increasing wafer yield.
This patent application is currently assigned to EVERGREEN SOLAR, INC.. Invention is credited to Daniel Doble, Weidong Huang, Scott Reitsma, Christine Richardson, Richard L. Wallace.
Application Number | 20110247546 13/079838 |
Document ID | / |
Family ID | 39995448 |
Filed Date | 2011-10-13 |
United States Patent
Application |
20110247546 |
Kind Code |
A1 |
Richardson; Christine ; et
al. |
October 13, 2011 |
Ribbon Crystal String for Increasing Wafer Yield
Abstract
A ribbon crystal has a body with a width dimension, and string
embedded within the body. The string has a generally elongated
cross-sectional shape. This cross-section (of the string) has a
generally longitudinal axis that diverges with the width dimension
of the ribbon crystal body.
Inventors: |
Richardson; Christine;
(Northborough, MA) ; Huang; Weidong; (Bolton,
MA) ; Wallace; Richard L.; (Acton, MA) ;
Doble; Daniel; (Somerville, MA) ; Reitsma; Scott;
(Shrewsbury, MA) |
Assignee: |
EVERGREEN SOLAR, INC.
Marlborough
MA
|
Family ID: |
39995448 |
Appl. No.: |
13/079838 |
Filed: |
April 5, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12201117 |
Aug 29, 2008 |
|
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13079838 |
|
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60969263 |
Aug 31, 2007 |
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Current U.S.
Class: |
117/24 ;
65/90 |
Current CPC
Class: |
C04B 35/62873 20130101;
C04B 35/62871 20130101; Y10T 117/1032 20150115; Y10T 428/2942
20150115; C30B 35/00 20130101; C04B 35/62852 20130101; Y10T
428/24628 20150115; Y10T 428/2918 20150115; C04B 35/62863 20130101;
Y10T 428/2962 20150115; C04B 2235/5248 20130101; C30B 15/36
20130101; C04B 35/62894 20130101; Y10T 428/24479 20150115; C30B
15/34 20130101; Y10T 428/2929 20150115; C04B 35/62868 20130101;
C04B 35/62849 20130101; Y10T 428/30 20150115 |
Class at
Publication: |
117/24 ;
65/90 |
International
Class: |
C30B 15/30 20060101
C30B015/30; C03B 15/02 20060101 C03B015/02 |
Claims
1. A method of forming a ribbon crystal, the method comprising:
providing molten material having a material coefficient of thermal
expansion; providing a string having an outer surface with a
contact angle with the molten material of between about 15 and 120
degrees, the string also having a string coefficient of thermal
expansion that is substantially matched to the material coefficient
of thermal expansion; and passing the string through molten
material to form a sheet.
2. The method as defined by claim 1 wherein the string comprises a
refractory layer supported on a substrate.
3. The method as defined by claim 2 wherein the string comprises a
handling layer radially outward of the refractory layer.
4. The method as defined by claim 3 wherein the outer surface of
the string comprises the handling layer.
5. The method as defined by claim 3 wherein the string comprises a
reduced wetting layer radially outward of the handling layer, the
reduced wetting layer comprising the outer surface of the
string.
6. The method as defined by claim 1 wherein the material comprises
a silicon based material.
Description
PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a continuation application 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 3253/173 and naming Scott Reitsma
as inventor, which 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 disclosures of both these applications are incorporated herein,
in their entireties, by reference.
[0002] This patent application also is related to the following,
co-owned patent applications and patents and incorporated herein,
in their entireties, by reference:
[0003] Attorney Docket Number 3253/172, entitled, "REDUCED WETTING
STRING FOR RIBBON CRYSTAL," now U.S. Pat. No. 7,651,768,
[0004] Attorney Docket Number 3253/174, entitled, "RIBBON CRYSTAL
STRING WITH EXTRUDED REFRACTORY MATERIAL," U.S. patent application
Ser. No. 12/201,180, and
[0005] Attorney Docket Number 3253/193, entitled, "RIBBON CRYSTAL
HAVING REDUCED WETTING STRING," now U.S. Pat. No. 7,842,270.
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 string
for use in a string ribbon crystal formed from a specific crystal
material, such as one of silicon, silicon-germanium, gallium
arsenide and indium phosphide, has a substrate and a refractory
layer supported on the substrate. The string also has an externally
exposed layer having a contact angle with the crystal material of
between about 15 and 120 degrees. The externally exposed layer is
radially outward of the refractory layer.
[0010] The string also may have a handling layer radially outward
of the refractory layer. The handling layer applies a generally
radially inward force to the refractory layer. The handling layer
may include the externally exposed layer. Alternatively, the
externally exposed layer may be radially outward of the handling
layer.
[0011] The externally exposed layer may be formed from any of a
number of materials that reduces wetting, such as pyrolytic carbon,
oxide, and nitride. For example, the externally exposed layer may
have a contact angle with the crystal material of greater than
about 25 degrees. Moreover, the substrate may be formed from
carbon, while the refractory layer may be formed from silicon
carbide.
[0012] Various embodiments generally match the coefficient of
thermal expansion. For example, the substrate, refractory layer,
and exposed layer have a combined coefficient of thermal expansion
that is substantially matched to the coefficient of thermal
expansion of the crystal material. To further thermal matching, the
exposed layer is thinner than the refractory layer. In a more
specific embodiment, the string may have a coefficient of thermal
expansion that is generally matched to the coefficient of thermal
expansion of polysilicon.
[0013] In accordance with another embodiment, a string for use in a
string ribbon crystal has a base portion with a refractory
material, and an externally exposed layer radially outward of the
refractory material. The base portion has a coefficient of thermal
expansion that is generally matched with the coefficient of thermal
expansion for silicon. The externally exposed layer has a contact
angle with silicon of between about 15 and 120 degrees.
[0014] In accordance with other embodiments, a ribbon crystal has
1) a string with an outer surface, and 2) a body with a body
material having a body coefficient of thermal expansion. The body
coefficient of thermal expansion is generally matched to the
coefficient of thermal expansion of the string. The string outer
surface (i.e., the circumferential outer surface) also is partially
exposed.
[0015] In accordance with yet other embodiments of the invention, a
method of forming a string for use with a ribbon crystal forms a
refractory layer on a substrate, and applies a reduced wetting
material radially outward of the refractory layer. The reduced
wetting material has a contact angle with silicon of between about
15 and 120 degrees.
[0016] In accordance with still other embodiments of the invention,
a method of forming a ribbon crystal provides 1) molten material
having a material coefficient of thermal expansion and 2) a string
having an outer surface with a contact angle with the molten
material of between about 15 and 120 degrees. The string also has a
string coefficient of thermal expansion that is substantially
matched to the material coefficient of thermal expansion. To form a
sheet, the method passes the string through molten material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] 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.
[0018] FIG. 1 schematically shows a string ribbon crystal that may
be formed from strings configured in accordance with illustrative
embodiments of the invention.
[0019] FIG. 2 schematically shows an illustrative furnace used to
form string ribbon crystals.
[0020] FIG. 3A schematically shows a string formed in accordance
with illustrative embodiments of the invention.
[0021] FIG. 3B schematically shows a cross-sectional view of the
string of FIG. 3A along line B-B in accordance with one embodiment
of the invention.
[0022] FIG. 3C schematically shows a cross-sectional view of the
string of FIG. 3A along line B-B in accordance with another
embodiment of the invention.
[0023] FIG. 4A schematically shows a cross-sectional view of a
ribbon crystal using a prior art string.
[0024] FIG. 4B schematically shows a cross-sectional view of a
ribbon crystal using a string configured in accordance with
illustrative embodiments of the invention.
[0025] FIG. 5 shows an illustrative process of forming a string
ribbon crystal using strings configured in accordance with
illustrative embodiments of the invention.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0026] In illustrative embodiments, a string has a reduced wetting,
outer exposed layer to increase grain size near the edge of a
ribbon crystal. To that end, the string may have a contact angle of
between about 15 and 120 degrees with the ribbon crystal material,
such as single crystal or multi-crystal silicon. To improve ribbon
robustness, the coefficient of thermal expansion of the string
generally matches that of the material forming the ribbon crystal
(e.g., silicon). Details of various embodiments are discussed
below.
[0027] 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.
[0028] 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 range,
the string ribbon crystal 10 may be considered to have an average
thickness across its length and/or width.
[0029] 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.
[0030] As known by those skilled in the art, the ribbon crystal 10
is formed from a pair of strings 12 generally encapsulated by the
ribbon material (e.g., polysilicon). Although it is surrounded by
the ribbon material (in the prior art), the string 12 and ribbon
material outwardly of the string 12 generally form the edge of the
ribbon crystal 10. For simplicity, the ribbon crystal 10 is
discussed as being formed from polysilicon. It nevertheless should
be reiterated that this discussion of polysilicon is not intended
to limit all embodiments.
[0031] 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.
[0032] 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.
[0033] 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 ribbon crystal 10.
[0034] The string 12 must have a wetting angle with silicon that is
low enough to cause the molten ribbon material to adhere to its
outer surface. Accordingly, some prior art ribbon crystals used in
commercial solar panels, for example, use string having a wetting
angle with silicon that is very small, such as on the order of
about 11 degrees or less. While sufficient for many applications,
those in the art noticed long ago that such string creates many
small grains of ribbon material in the final product. Undesirably,
these small grains reduce the electrical efficiency of the solar
cell the crystal 10 ultimately forms.
[0035] Others have tried and failed to solve this long felt need in
the art. For example, an article by Ciszek et al., entitled,
"Filament Materials For Edge-Supported Pulling of Silicon Sheet
Crystals," published by the Solar Energy Research Institute, Golden
Colo., dated 1982 and 1983 and submitted with this patent
application, addresses the phenomenon. In particular, this article
discusses high contact angles of the string with the ribbon
material (fifteen degrees or higher), but acknowledges yield
problems with materials having such high contact angles. More
specifically, the article notes breakage problems caused by
coefficient of thermal expansion mismatches between the string and
ribbon material. The article does not recognize any solution to
this problem.
[0036] Rather than solve the problem, the Ciszek et al. article
attempts to manage it. In particular, the article teaches use of a
high contact angle (with silicon) string formed from a material
having a coefficient of thermal expansion that is not matched to
the ribbon material (e.g., quartz used with silicon ribbon
material). They reason that a significant coefficient of thermal
expansion mismatch will cause the string to break off, which it
suggests is a good result. Such a solution, however, is very
undesirable for many applications.
[0037] Accordingly, years after the Ciszek research, the inventors
discovered how to obtain the benefits of a string with both a
matched coefficient of thermal expansion and high contact angle
with the ribbon material. To that end, generally speaking of
various embodiments, the inventors solved of this long felt need by
pursuing a solution that Ciszek teaches away from; namely,
engineering string to have both a matched coefficient of thermal
expansion and high contact angle. To that end, the inventors
applied an exterior layer of material with a favorable contact
angle onto a base string portion 26 having a matched coefficient of
thermal expansion.
[0038] More specifically, FIG. 3A schematically shows a string 12
that may be formed in accordance with illustrative embodiments of
the invention. FIG. 3B schematically shows a cross-sectional view
of the string 12 of FIG. 3A along cross-line B-B in accordance with
one embodiment, while FIG. 3C schematically shows a cross-sectional
view of the string 12 in accordance with another embodiment. As
shown, the string 12 in both embodiments has a base string portion
26 formed of a central core 28 and a substantially concentric,
refractory material layer 30 (also referred to herein as a
"refractory layer 30").
[0039] In some embodiments, the central core 28 is a conductive
carbon formed from conventional extrusion processes. In other
embodiments, the central core 28 is formed from a plurality of
small conductive fibers (e.g., carbon fibers) that are wound
together to form a tow. Moreover, the refractory material layer 30
may be formed from any conventional refractive material suitable
for a given application. For example, the refractory material layer
30 may be formed from silicon carbide if used to form a
photovoltaic cell from a silicon.
[0040] Illustrative embodiments of the string 12 also have an
exposed, nonwetting layer 32 that, when used with string 12 having
a circular or similarly symmetric cross-sectional shape, also is
generally concentric with the core 28. Among other things, this
nonwetting layer 32 may be a carbon, pyrolytic carbon/carbide
(e.g., graphite), an oxide, or a nitride. More specifically,
appropriate materials may include aluminum oxide, or silica. It is
preferable that the materials selected to form this nonwetting
layer 32 have no more than a negligible contaminating impact on the
molten silicon within the crucible 18.
[0041] The inventors discovered that a very thin nonwetting layer
32 should minimize its impact on the coefficient of thermal
expansion of the base string portion 26. Moreover, the nonwetting
layer 32 should be thick enough to provide a robust outer surface
that can withstand the demands of the ribbon crystal fabrication
process. For example, a string 12 having a total cross-sectional
dimension of about 140 microns can have a nonwetting layer 32 with
a thickness of about one micron.
[0042] Although not explicitly discussed above, 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, the handling layer 34 is formed to provide
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
having generally known sizes).
[0043] The nonwetting layer 32 may be integrated directly into the
handling layer 34, as shown in FIG. 3B. Alternative embodiments,
however, may form the nonwetting layer 32 radially outward of the
handling layer 34, as shown in FIG. 3C. Yet other embodiments may
form the other layers between the nonwetting layer 32 and the base
string portion 26, or omit the handling layer 34.
[0044] FIGS. 3B and 3C show string with generally circular
cross-sectional shapes. Various embodiments of the string 12,
however, may have cross-sectional shapes that are not generally
circular. For example, the string 12 may have a concave
cross-sectional shape (e.g., a cross or "c" shape), an elongated
cross-sectional shape (e.g., an ellipse or rectangle), or other
regular or irregular cross-sectional shape. As discussed in greater
detail in co-pending patent applications having docket numbers
3253/173 and 3253/174 (identified and incorporated above), these
embodiments may improve the robustness of the resulting ribbon
crystals 10.
[0045] It should be noted that use of the term "nonwetting" layer
is somewhat of a misnomer because if it truly was nonwetting, then
the molten ribbon material would not adhere to it. The nonwetting
layer 32 thus can be referred to as a reduced wetting layer, and
alternatively may be referred to in that manner below.
[0046] FIGS. 4A and 4B graphically show a primary difference
between prior art string and string 12 configured in accordance
with illustrative embodiments of the invention. Specifically, FIG.
4A schematically shows a cross-sectional view of a portion of a
prior art ribbon crystal 10P with 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. As shown, the ribbon material
contacts and appears to substantially cover the entire outer
surface of the string 12P. Undesirably, this significant surface
contact produces many nucleation sites that, consequently, form a
relatively high volume of small grains.
[0047] FIG. 4B schematically shows a new string 12 within a ribbon
crystal 10. In a manner unlike the ribbon crystal 10P of FIG. 4A,
the ribbon material of this figure does not contact the entire
outer surface of the string 12. Instead, the ribbon material
contacts only a portion of the outer surface, thus exposing a
significant remaining portion. Accordingly, this embodiment
presents fewer nucleation sites, thus favorably reducing the number
of small grains near the string 12. In other words, this embodiment
promotes larger grains near the strings 12. As a result, the
efficiency of the string ribbon crystal 10 should be improved when
compared to string ribbon crystals using strings 12 without a
nonwetting layer 32.
[0048] Moreover, as noted above, the string 12 preferably has a
diameter that is greater than the diameters of commonly used prior
art strings. For example, larger strings 12 may have diameters
ranging from about 0.75 to 2.0 times the average thickness of its
corresponding string ribbon crystal 10. This larger diameter should
effectively increase the thickness of the string ribbon crystal 10
in the region near the string 12. Consequently, the string ribbon
crystal 10 should be less prone to breaking than some prior art
designs using strings with smaller diameters. As an example, a
single crystal 10 may have a thickness that varies between about
140 microns and 250 microns, and the string 12 may have a thickness
that is between 0.75 and 2.0 times such thickness.
[0049] 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. The process begins at
step 500 by forming the core/substrate 28. As noted above, the core
28 can be formed from carbon by conventional spinning 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 of a tow could form a
monofilament through a known fabrication process, such as
oxidation, carbonization, or infiltration.
[0050] 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. This first layer may be formed in a number of
conventional ways, or by means of an extrusion process, a
pulltrusion process, or both spinning of a refractory material with
a polymer component, which subsequently is baked off. Among other
things, 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, the core 28
effectively acts as a substrate for supporting the refractory
material layer 30.
[0051] This step thus forms the base string portion 26. It should
be reiterated that the base string portion 26 may be formed from
any of a wide variety of materials, such as 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.
[0052] At this point in the process, the base string portion 26 has
a combined coefficient of thermal expansion that should generally
match 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.
[0053] The process then continues to step 504, which forms the
exposed nonwetting/reduced layer 32 on the base string portion 26.
As discussed above, this layer could also serve as the handling
layer 34 and 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.
[0054] As also discussed above, the contact angle with the ribbon
material of the exterior surface formed by this layer should be
carefully controlled to cause the molten ribbon material to adhere
to a portion of it only (as shown in FIG. 4B). 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 even better results.
[0055] Among other ways, the nonwetting 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.
[0056] Related techniques for implementing this step include:
[0057] a sol gel dip for silica or alumina oxide or silicon
oxycarbide either at the end of a CVD furnace or during rewind,
[0058] a CVD nonwetting coating deposited by heating quartz from
the outside and induction heating the base string portion 26,
[0059] spray-on deposition with a polymer binder that subsequently
would be burned off, [0060] shaking particles onto a base string
portion 26 or tow and then baking the into the base string portion
or tow, and [0061] coating with base string portion 26 with
refractory slurry (e.g., silicon dioxide) or liquid and then
burning off residual.
[0062] Prior to performing step 504, some embodiments form a
handling layer 34 that is separate from the produced nonwetting
layer 32, as discussed above. Accordingly, 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.
[0063] 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.
[0064] 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. At this point, for each ribbon crystal being
formed, the process passes two strings 12 through the furnace 14
and crucible 18, thus forming the string ribbon crystal 10 (step
512).
[0065] Accordingly, illustrative embodiments increase the grain
sizes near the string 12, thus improving electrical efficiency of
the ribbon crystals. By using a technique for matching the
coefficient of thermal expansion with the ribbon material, the
inventors were able to achieve this goal without increasing yield
loss.
[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.
* * * * *