U.S. patent application number 12/837876 was filed with the patent office on 2011-07-21 for silicon wafers and ingots with reduced oxygen content and methods for producing them.
This patent application is currently assigned to MEMC SINGAPORE PTE LTD.. Invention is credited to Aditya J. Deshpande, Steven L. Kimbel, Richard J. Phillips, Gang Shi.
Application Number | 20110177284 12/837876 |
Document ID | / |
Family ID | 42634798 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110177284 |
Kind Code |
A1 |
Phillips; Richard J. ; et
al. |
July 21, 2011 |
SILICON WAFERS AND INGOTS WITH REDUCED OXYGEN CONTENT AND METHODS
FOR PRODUCING THEM
Abstract
Silicon nitride coated crucibles for holding melted
semiconductor material and for use in preparing multicrystalline
silicon ingots by a directional solidification process; methods for
coating crucibles; methods for preparing silicon ingots and wafers;
compositions for coating crucibles and silicon ingots and wafers
with a low oxygen content.
Inventors: |
Phillips; Richard J.; (St.
Peters, MO) ; Kimbel; Steven L.; (St. Charles,
MO) ; Deshpande; Aditya J.; (Chesterfield, MO)
; Shi; Gang; (O'Fallon, MO) |
Assignee: |
MEMC SINGAPORE PTE LTD.
Singapore
SG
|
Family ID: |
42634798 |
Appl. No.: |
12/837876 |
Filed: |
July 16, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61226176 |
Jul 16, 2009 |
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61226175 |
Jul 16, 2009 |
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61226172 |
Jul 16, 2009 |
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Current U.S.
Class: |
428/98 ; 264/219;
264/332; 423/348 |
Current CPC
Class: |
C30B 35/002 20130101;
C04B 35/584 20130101; C04B 2235/656 20130101; C30B 11/002 20130101;
C04B 2235/3418 20130101; C04B 35/63488 20130101; C04B 2235/3217
20130101; C04B 35/6342 20130101; C04B 2235/3225 20130101; C30B
15/10 20130101; C04B 2235/721 20130101; C04B 35/6269 20130101; Y10T
428/24 20150115; C30B 29/06 20130101 |
Class at
Publication: |
428/98 ; 264/332;
264/219; 423/348 |
International
Class: |
B32B 5/00 20060101
B32B005/00; B22D 7/06 20060101 B22D007/06; B22D 7/12 20060101
B22D007/12; B22D 7/00 20060101 B22D007/00; C01B 33/02 20060101
C01B033/02 |
Claims
1. A method for preparing a multicrystalline silicon ingot, the
method comprising: loading polycrystalline silicon into a coated
crucible to form a silicon charge, the crucible having a body
having a bottom and a sidewall extending up from the bottom, the
bottom and sidewall defining a cavity for holding the charge, the
sidewall having an inner surface and an outer surface, the crucible
having a first coating on a first area of the inner surface of the
sidewall and a second coating on a second area of the inner surface
of the sidewall, wherein the second coating comprises an additive
not present in the first coating; heating the silicon charge to a
temperature above about the melting temperature of the charge to
form a silicon melt; and directionally solidifying the silicon melt
to form a multicrystalline silicon ingot.
2. A method as set forth in claim 1 wherein the first coating
extends from the bottom of the crucible to a height H.sub.1 and the
second coating extends from about H.sub.1 to a height H.sub.2.
3. A method as set forth in claim 2 wherein the distance between
the bottom of the crucible and H.sub.1 is at least about 50% of the
height of the sidewall.
4. A method as set forth in claim 2 wherein the distance between
the bottom of the crucible and H.sub.2 is at least about 60% of the
height of the sidewall.
5. A method as set forth in claim 2 wherein H.sub.2 extends above
about a solidification line, S.sub.1.
6. A method as set forth in claim 2 wherein H.sub.2 extends to
about the top of the crucible.
7. A method as set forth in claim 1 wherein the first coating
comprises silicon nitride.
8. A method as set forth in claim 7 wherein the first coating
consists essentially of silicon nitride and carbon.
9. A method as set forth in claim 7 wherein the first coating
comprises less than about 1% by weight carbon.
10. A method as set forth in claim 1 wherein the second coating
comprises silicon nitride and a sintering agent.
11. A method as set forth in claim 10 wherein the second coating
comprises at least about 40% by weight silicon nitride.
12. A method as set forth in claim 10 wherein the second coating
comprises a sintering agent.
13. A method as set forth in claim 10 wherein the second coating
comprises at least about 0.5% by weight silica.
14. A method as set forth in claim 10 wherein the second coating
comprises at least about 0.5% by weight alumina.
15. A method as set forth in claim 10 wherein the second coating
consists essentially of silicon nitride, sintering agent and
carbon.
16. A method as set forth in claim 10 wherein the second coating
comprises less than about 1% by weight carbon.
17. A method as set forth in claim 10 wherein the mass ratio of
sintering agent to silicon nitride in the second coating
composition is at least about 1:20.
18. A method as set forth in claim 1 wherein the crucible body
comprises a material selected from silica, silicon nitride and
graphite.
19. A method as set forth in claim 1 wherein the crucible body
comprises silica.
20. A method as set forth in claim 1 wherein the thickness of the
first coating is at least about 50 .mu.m.
21. A method as set forth in claim 1 wherein the thickness of the
second coating is at least about 50 .mu.m.
22. A method as set forth in claim 1 wherein the silicon charge is
heated to at least about 1410.degree. C. to form a silicon
melt.
23. A method as set forth in claim 1 wherein the silicon charge is
heated to at least about 1450.degree. C. to form a silicon
melt.
24. A method as set forth in claim 1 wherein the ingot has an
average nominal crystal grain size of from about 1 mm to about 15
mm.
25. A method as set forth in claim 1 comprising coating the
crucible with a first and second coating composition to form the
first and second coatings prior to loading polycrystalline silicon
into the crucible.
26. A method as set forth in claim 1 wherein the crucible is
sintered during the step of heating the silicon charge to a
temperature above about the melting temperature of the charge.
27. A method for preparing a multicrystalline silicon ingot, the
method comprising: loading polycrystalline silicon into a coated
crucible to form a silicon charge, the crucible having a body
having a bottom and a sidewall extending up from the bottom, the
bottom and sidewall defining a cavity for holding the charge, the
sidewall having an inner surface and an outer surface, the crucible
having a coating on a portion of the inner surface of the sidewall,
the coating comprising silicon nitride and a sintering agent
selected from yttria and silica; heating the silicon charge to a
temperature above about the melting temperature of the charge to
form a silicon melt; and directionally solidifying the silicon melt
to form a multicrystalline silicon ingot.
28. A method as set forth in claim 27 wherein the coating comprises
at least about 0.5% by weight yttria.
29. A method as set forth in claim 27 wherein the coating comprises
at least about 0.5% by weight silica.
30. A method as set forth in claim 27 wherein the coating comprises
at least about 0.5% by weight alumina
31. A method as set forth in claim 27 wherein the coating comprises
less than about 1% by weight carbon.
32. A method as set forth in claim 27 wherein the mass ratio of
sintering agent to silicon nitride in the coating is at least about
1:20.
33. A method as set forth in claim 27 wherein the crucible body
comprises a material selected from silica, silicon nitride and
graphite.
34. A method as set forth in claim 27 wherein the thickness of the
coating is at least about 50 .mu.m.
35. A method as set forth in claim 27 wherein the crucible is
prepared by coating the crucible with a coating composition to form
the coating.
36. A method as set forth in claim 27 wherein the silicon charge is
heated to at least about 1410.degree. C. to form a silicon
melt.
37. A method as set forth in claim 27 wherein the ingot has an
average nominal crystal grain size of from about 1 mm to about 15
mm.
38. A method as set forth in claim 27 wherein the crucible is
sintered during the step of heating the silicon charge to a
temperature above about the melting temperature of the charge to
form a silicon melt.
39. A method for preparing a multicrystalline silicon ingot in a
crucible, the crucible comprising a body having a bottom and a
sidewall extending up from the bottom, the bottom and sidewall
defining a cavity for holding a silicon charge, the sidewall having
an inner surface and an outer surface, the method comprising:
applying a composition to a portion of the inner surface of the
sidewall, the composition comprising a medium, silicon nitride, a
dispersant and a binder to enhance adhesion of the coating to the
crucible; vaporizing the medium from the composition to produce a
silicon nitride coating on the inner surface of the sidewall;
loading polycrystalline silicon into a coated crucible to form a
silicon charge; heating the silicon charge to a temperature above
about the melting temperature of the charge to form a silicon melt;
and directionally solidifying the silicon melt to form a
multicrystalline silicon ingot.
40. A method as set forth in claim 39 wherein the silicon nitride
is a particulate that is suspended in the medium.
41. A method as set forth in claim 39 wherein the coating comprises
less than about 1% by weight carbon.
42. A method as set forth in claim 39 wherein the crucible body
comprises a material selected from silica, silicon nitride and
graphite.
43. A method as set forth in claim 39 wherein the inner surface of
the bottom of the crucible is coated with the coating.
44. A method as set forth in claim 39 wherein the thickness of the
first coating is at least about 50 .mu.m.
45. A method as set forth in claim 39 wherein the coating extends
over the entire inner surface of the sidewall.
46. A method as set forth in claim 39 wherein the silicon charge is
heated to at least about 1410.degree. C. to form a silicon
melt.
47. A method as set forth in claim 39 wherein the ingot has an
average nominal crystal grain size of from about 1 mm to about 15
mm.
48. A method as set forth in claim 39 comprising heating the
crucible to a temperature of at least about 150.degree. C. after
application of the composition.
49. A method as set forth in claim 48 wherein the crucible is
heated for at least about 1 hour.
50. A method as set forth in claim 39 comprising heating the
crucible to a temperature of at least about 1000.degree. C. after
application of the composition.
51. A method as set forth in claim 50 wherein the crucible is
heated for at least about 1 hour.
52. A method as set forth in claim 39 wherein the crucible is
sintered during the step of heating the silicon charge to a
temperature above about the melting temperature of the charge to
form a silicon melt.
53. A silicon ingot having a bottom, a top and a height, H.sub.3,
defined between the bottom and top, wherein the oxygen
concentration of the ingot at a height about 20% of H.sub.3 is less
than about 4.5 ppma.
54. A multicrystalline silicon ingot as set forth in claim 53
wherein the oxygen concentration generally decreases from the
bottom of the crucible toward the top of the crucible.
55. A multicrystalline silicon ingot as set forth in claim 53
wherein the oxygen concentration of the ingot at a height about 20%
of H.sub.3 is less than about 4.0 ppm.
56. A multicrystalline silicon ingot as set forth in claim 53
wherein the oxygen concentration of the ingot between a height
about 20% of H.sub.3 to a height about 80% of H.sub.3 is less than
about 3.0 ppma.
57. A multicrystalline silicon ingot as set forth in claim 53
wherein the oxygen concentration of the ingot between the bottom
and the top is less than about 2.5 ppma.
58. A multicrystalline silicon ingot as set forth in claim 53
wherein the ingot is rectangular in shape.
59. A multicrystalline silicon ingot as set forth in claim 53
wherein the ingot contains multicrystalline silicon.
60. A multicrystalline silicon ingot as set forth in claim 53
wherein the ingot has an average nominal crystal grain size of from
about 1 mm to about 15 mm.
61. A multicrystalline silicon wafer with an oxygen concentration
of less than about 2.5 ppma.
62. A multicrystalline silicon wafer as set forth in claim 61
wherein the wafer has an oxygen concentration of less than about 2
ppma.
63. A multicrystalline silicon wafer as set forth in claim 61
wherein the wafer has an oxygen concentration of from about 1 to
about 3 ppma.
64. A multicrystalline silicon wafer as set forth in claim 61
wherein the wafer contains multicrystalline silicon.
65. A multicrystalline silicon wafer as set forth in claim 61
wherein the wafer has an average nominal crystal size of from about
1 mm to about 15 mm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit of U.S.
Provisional Application No. 61/226,175, filed Jul. 16, 2009, U.S.
Provisional Application No. 61/226,176, filed Jul. 16, 2009 and
U.S. Provisional Application No. 61/226,172, filed Jul. 16, 2009,
each of which is incorporated herein by reference in its
entirety.
BACKGROUND
[0002] The field of the disclosure relates to coated crucibles for
holding melted semiconductor material and, particularly, for use in
preparing multicrystalline silicon ingots by a directional
solidification process. Other aspects include methods for coating
crucibles, methods for preparing silicon ingots and wafers,
compositions for coating crucibles and silicon ingots and wafers
with a low oxygen content.
[0003] Conventional photovoltaic cells, used for the production of
solar energy, utilize multicrystalline silicon. Multicrystalline
silicon is conventionally produced in a directional solidification
(DS) process in which silicon is melted in a crucible and
directionally solidified in a separate or in the same crucible. The
solidification of the ingot is controlled such that molten silicon
solidifies unidirectionally at the solidifying front of the
casting. The multicrystalline silicon produced in such a manner is
an agglomeration of crystal grains with the orientation of the
grains being random relative to each other due to the high density
of heterogeneous nucleation sites at the crucible wall. Once the
multicrystalline ingot is formed, the ingot may be cut into blocks
and further cut into wafers. Multicrystalline silicon is generally
the preferred silicon source for photovoltaic cells rather than
single crystal silicon due to its lower cost resulting from higher
throughput rates, less labor-intensive operations and the reduced
cost of supplies as compared to typical single crystal silicon
production.
[0004] During and after solidification, the solidified ingot must
be released from the crucible without causing cracking of the
ingot. Conventional crucibles are constructed of formed and
sintered silica or fused-silica (synonymously "quartz"), silicon
nitride or graphite. Silicon nitride crucibles may be reused but
are typically more expensive relative to other crucibles. Crucibles
may be coated with various coating materials; however, these
processes and the resulting coated crucibles have been found to be
deficient.
SUMMARY
[0005] In one aspect of the present disclosure, a method for
preparing a multicrystalline silicon ingot includes loading
polycrystalline silicon into a coated crucible to form a silicon
charge. The crucible has a body with a bottom and a sidewall
extending up from the bottom. The bottom and sidewall define a
cavity for holding the charge. The sidewall has an inner surface
and an outer surface. The crucible has a first coating on a first
area of the inner surface of the sidewall and a second coating on a
second area of the inner surface of the sidewall. The second
coating includes an additive not present in the first coating. The
silicon charge is heated to a temperature above about the melting
temperature of the charge to form a silicon melt. The silicon melt
is directionally solidified to form a multicrystalline silicon
ingot.
[0006] In a further aspect, a method for preparing a
multicrystalline silicon ingot includes loading polycrystalline
silicon into a coated crucible to form a silicon charge. The
crucible has a body with a bottom and a sidewall extending up from
the bottom. The bottom and sidewall define a cavity for holding the
charge. The sidewall has an inner surface and an outer surface. The
crucible has a coating on a portion of the inner surface of the
sidewall. The coating comprises silicon nitride and a sintering
agent selected from yttria and silica. The silicon charge is heated
to a temperature above about the melting temperature of the charge
to form a silicon melt. The silicon melt is directionally
solidified to form a multicrystalline silicon ingot.
[0007] One aspect of the present disclosure is directed to a method
for preparing a multicrystalline silicon ingot in a crucible. The
crucible includes a body with a bottom and a sidewall extending up
from the bottom. The bottom and sidewall define a cavity for
holding a silicon charge. The sidewall has an inner surface and an
outer surface. A composition is applied to a portion of the inner
surface of the sidewall. The composition comprises a medium,
silicon nitride, a dispersant and a binder to enhance adhesion of
the coating to the crucible. The medium is vaporized from the
composition to produce a silicon nitride coating on the inner
surface of the sidewall. Polycrystalline silicon is loaded into a
coated crucible to form a silicon charge. The silicon charge is
heated to a temperature above about the melting temperature of the
charge to form a silicon melt. The silicon melt is directionally
solidified to form a multicrystalline silicon ingot.
[0008] In another aspect, a silicon ingot has a bottom, a top and a
height, H.sub.3, defined between the bottom and top. The oxygen
concentration of the ingot at a height about 20% of H.sub.3 is less
than about 4.5 ppma.
[0009] In yet another aspect, a multicrystalline silicon wafer has
an oxygen concentration of less than about 2.5 ppma.
[0010] Various refinements exist of the features noted in relation
to the above-mentioned aspects of the present disclosure. Further
features may also be incorporated in the above-mentioned aspects of
the present disclosure as well. These refinements and additional
features may exist individually or in any combination. For
instance, various features discussed below in relation to any of
the illustrated embodiments of the present disclosure may be
incorporated into any of the above-described aspects of the present
disclosure, alone or in any combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a perspective view of a crucible body according to
one embodiment of the present disclosure;
[0012] FIG. 2 is a perspective of a crucible according to one
embodiment with height, H.sub.1 shown;
[0013] FIG. 3 is a perspective of a crucible body according to one
embodiment with the solidification line, S.sub.1, and top, T,
shown;
[0014] FIG. 4 is a perspective of a crucible according to one
embodiment with heights, H.sub.1 and H.sub.2 shown; and
[0015] FIG. 5 is a graphical illustration comparing the oxygen
concentrations along the height of a conventionally prepared
crucible and a crucible prepared by the process of one embodiment
of the present disclosure.
[0016] Corresponding reference characters indicate corresponding
parts throughout the drawings.
DETAILED DESCRIPTION
[0017] Applicants have found that by coating a crucible such as,
for example, a silica crucible, with a coating composition of
sufficient thickness that contains minimal sources of oxygen,
multicrystalline ingots and wafers with a reduced oxygen content
and correspondingly reduced light induced degradation may be
produced. Further, oxygen-containing sintering aids may be used in
various coating compositions applied to the crucible. A first area
of the crucible body may be coated with a coating composition that
does not contain the sintering aid and a second area coated with a
composition that does contain the sintering aid to minimize the
surface area of the crucible exposed to oxygen.
[0018] By controlling the amount of oxygen in the coating
compositions, the resulting ingots have been found to be
characterized by a beneficial oxygen profile as compared to
conventional processing methods and coatings. It has been found
that lower oxygen levels may be achieved in the ingot which is
desirable in, for example, boron-doped solar silicon. High oxygen
levels in boron-doped solar silicon have been associated with
light-induced degradation over time.
Crucible Body Starting Material
[0019] Referring now to FIG. 1, a crucible body for use in
embodiments of the present disclosure is generally designated as
numeral 5. The crucible body 5 has a bottom 10 and a sidewall 14
that extends from the base or bottom 10. While the crucible body 5
is illustrated with four sidewalls 14 being shown, it should be
understood that the crucible body 5 may include fewer than four
sidewalls or may include more than four sidewalls without departing
from the scope of the present disclosure. Also, the corners 18
between sidewalls 14 may be connected to each other at any angle
suitable for forming the enclosure of the crucible body and may be
sharp as illustrated in FIG. 1 or may be rounded. In some
embodiments, the crucible body has one sidewall that is generally
cylindrical in shape. The sidewalls 14 of the crucible body 5 have
an inner surface 12 and an outer surface 20. The crucible body 5 is
generally open, i.e., the body may not include a top. It should be
noted, however, the crucible body 5 may have a top (not shown)
opposite the bottom 10 without departing from the scope of the
present disclosure.
[0020] In several embodiments of the present disclosure, the
crucible body 5 has four sidewalls 14 of substantially equal length
(e.g., the crucible has a generally square base 10). The length of
the sidewalls 14 may be at least about 25 cm, at least about 50 cm
or even at least about 75 cm. The height of the sidewalls 14 may be
at least about 15 cm, at least about 25 cm or even at least about
35 cm. In this regard, the volume of the crucible (in embodiments
wherein a square or rectangular base is used or wherein the
crucible is cylindrical or round or in embodiments wherein another
shape is used) may be at least about 0.05 m.sup.3, at least about
0.15 m.sup.3 or at least about 0.25 m.sup.3). Further in this
regard, it should be understood that crucible shapes and dimensions
other than as described above may be used without departing from
the scope of the present disclosure. In one or more particular
embodiments of the present disclosure, the crucible body 5 has four
sidewalls 14 that are each about 87.7 cm in length and 40 cm in
height and the crucible has a volume of about 0.31 m.sup.3.
[0021] The crucible body 5 may be constructed of any material
suitable for the solidification of semiconductor material. For
example, the crucible may be constructed from a material selected
from silica, silicon nitride, silicon carbide, graphite, mixtures
thereof and composites thereof. Composites may include, for
example, a base material with a coating thereon. Composite
materials include, for example, silica coated with silicon nitride
and graphite coated with calcium chloride and/or silicon nitride.
It should be noted that some crucible body materials may not
inherently be a source of oxygen contamination (e.g., graphite),
however they may have other attributes to be taken into
consideration when designing a system (e.g., cost, contamination
and the like). In addition, the material preferably is capable of
withstanding temperatures at which such semiconductor material is
melted and solidified. For example, the crucible material is
suitable for melting and solidifying semiconductor material at
temperatures of at least about 300.degree. C., at least about
1000.degree. C. or even at least about 1580.degree. C. for
durations of at least about 10 hours or even as much as 100 hours
or more.
[0022] The thickness of the bottom 10 and sidewalls 14 may vary
depending upon a number of variables including, for example, the
strength of material at processing temperatures, the method of
crucible construction, the semiconductor material of choice and the
furnace and process design. Generally, the thickness of the
crucible may be from about 5 mm to about 50 mm, from about 10 mm to
about 40 mm or from about 15 mm to about 25 mm.
Coating Compositions
[0023] At least a portion of the inner surface 12 of the sidewalls
14 of the crucible body 5 described above may be coated with a
coating composition including silicon nitride and one or more
additives and of a sufficient thickness to reduce the amount of
oxygen in an ingot subsequently formed in the crucible and/or to
enhance an ingot-release characteristic of the crucible. The
additive may be, for example, one or more of binders, dispersants,
sintering aids and a medium, diluent, solvent or combinations
thereof Ingot-release characteristics include the ability of the
ingot to release the ingot during cooling (i.e., ability of the
crucible not to adhere to the ingot) and to release the ingot
without causing ingot cracking. Evidence of ingot adhesion
includes, for example, (1) a failure of the ingot to release from
the crucible even at room temperatures, (2) the amount of ingot
cracking upon release and/or (3) the presence and amount of
semiconductor material stuck to the crucible after release of the
ingot.
[0024] The composition may include at least about 5% by weight
silicon nitride and, in other embodiments, at least about 15% by
weight or even at least about 30% by weight silicon nitride. In
various embodiments, the coating composition may include from about
5% to about 50% by weight silicon nitride, from about 15% to about
50% by weight, from about 10% to about 40% by weight or from about
30% to about 40% by weight silicon nitride. In some embodiments,
the coating composition includes from about 37.0 to 37.7% by weight
silicon nitride. For purposes of the present disclosure, percent
inclusion of components of the "coating composition" or simply
"composition" refer to the material applied to the crucible body
and not the coating itself which forms after additional processing
steps (e.g., vaporization of mediums, heating, or sintering, etc.).
Percent inclusion of the components of the "coating" (described
below under the section entitled "Coated Crucibles") refer to the
solid material covering a portion of the crucible body after all
processing steps are performed and that covers the crucible body
during preparation of an ingot. Unless described otherwise, listed
percentages are given as the percentage of the entire composition
or entire coating including the component or components being
described.
[0025] Without being bound by a particular theory, it is believed
that when used in particulate form, the size of the silicon nitride
particles can affect the rheology of the coating composition and
affect the ease of application. In some embodiments, the average
nominal diameter of the particulate silicon nitride may be less
than about 100 .mu.m. In other embodiments, the average nominal
diameter of the silicon nitride may be less than about 50 .mu.m,
less than about 25 .mu.m or even less than about 10 .mu.m.
Utilization of smaller particle sizes generally results in coating
compositions with improved fluidity.
[0026] The composition may include a medium in which the silicon
nitride remains substantially in particulate form. Generally, it
should be understood that one or more of the silicon nitride and
additives such as a binder, dispersant, sintering aid and the like
may not dissolve, partially dissolve or fully dissolve in the
medium and the terms "medium," "diluent," and "solvent" may be used
interchangeably and are not meant to limit embodiments of the
present disclosure to compositions wherein one or more components
do or do not dissolve in the medium.
[0027] The medium may include an organic compound or may be
aqueous. It should be noted, however, that the presence of water in
an aqueous solution may result in more oxygen in the cured coating
which may also result in an increase in the oxygen level of a
semiconductor ingot grown therein. Thus, while aqueous solutions
may be utilized without departing from the present disclosure, it
may be preferable in some embodiments to use a non-aqueous medium.
Preferably the medium vaporizes readily during any drying
steps.
[0028] The medium may include a C1 to C10 alcohol and may be
isopropyl alcohol or ethanol. In some embodiments, at least about
10% by weight of the total composition prior to being applied to
the crucible is the medium.
[0029] In other embodiments, the composition may include at least
about 30%, at least about 50% or even at least about 70% by weight
medium. In various other embodiments, the composition includes from
about 10% to about 80% by weight medium, from about 30% to about
70%, from about 40% to about 60% or even from about 45% to about
55% by weight medium. The composition may include from about 47.9%
to about 50.6% by weight medium. The composition may include more
than one medium with the total weight fraction of medium in the
composition being as described above.
[0030] The composition may include one or more binders which act to
bind the coating and, more particularly, the silicon nitride
particles, to the crucible body and to each other after
application. Generally, the binder alters the rheology of the
coating composition and maintains the distribution of particles in
the medium throughout application and drying. In some embodiments,
the binder is dissolved in the medium. In some embodiments, the
binder is polyvinyl butyral, such as B-76 available from Solutia,
Inc. (St. Louis, Mo.). In some embodiments, at least about 0.5%, at
least about 2%, at least about 5% or at least about 10% or even at
least about 15% by weight of the total composition prior to being
applied to the crucible is one or more binders. In various
embodiments, the composition includes from about 0.5% to about 20%
by weight binder, from about 0.5% to about 10%, from about 0.5% to
about 8% by weight binder or from about 2% to about 8% by weight
binder. The composition may include from about 5.4% to about 6.8%
by weight binder. In some embodiments, the composition does not
include a binder. The composition may include more than one binder
with the total weight fraction of binder in the composition being
as described above.
[0031] The coating composition may include a dispersant. Generally,
the dispersant acts to prevent the silicon nitride particulate from
settling prior to application of the coating composition to the
crucible body. Suitable dispersants generally do not contribute
metallic impurities to the coating composition and are clean
burning and decompose during a thermal cycle. The combination of
plasticizer (described below) and dispersant may be chosen such
that drying of the coating without cracking or with minimal
cracking is realized. In some embodiments, the dispersant is a
methyloxirane polymer such as SOLSPERSE.RTM. 20000 (Lubrizol Corp.,
Wickliffe, Ohio). The composition may include at least about 0.05%
by weight dispersant and, in other embodiments, at least about
0.1%, at least about 0.5%, at least about 1% or even at least about
5% by weight dispersants. In various other embodiments, the
composition includes from about 0.05% to about 10% by weight
dispersant, from about 0.05% to about 5% or from about 0.5% to
about 2.5% by weight dispersant. The composition may include from
about 1.6% to about 2.1% by weight dispersant. In some embodiments,
the composition does not include a dispersant. The composition may
include more than one dispersant with the total amount of
dispersant in the composition being as described above.
[0032] The coating composition may include a plasticizer. Suitable
plasticizers generally do not contribute metallic impurities to the
coating composition and are clean burning and decompose during a
thermal cycle. Plasticizers and dispersant may be chosen such that
drying of the coating without cracking or with minimal cracking is
realized. The plasticizer may be, for example, polyethylene glycol.
Polyethylene glycol is commercially available as CARBOWAX.RTM. 400
(Dow Chemical Co., Midland, Mich.). The coating composition may
include at least about 0.5%, at least about 2%, at least about 5%,
at least about 10% or even at least about 15% by weight
plasticizer. In various embodiments, the composition includes from
about 0.5% to about 20% by weight plasticizer, from about 0.5% to
about 10%, from about 0.5% to about 8% or from about 2% to about 8%
by weight plasticizer. The composition may include from about 5.4%
to about 6.8% by weight plasticizer. In some embodiments, the
composition does not include a plasticizer. The composition may
include more than one plasticizer with the total amount of
plasticizer in the composition being as described above.
[0033] The composition may include one or more sintering aids, such
as for example, yttria, silica and/or alumina The sintering aid may
be in particulate form and may be suspended in one or more mediums.
The sintering aid (and in particular yttria) strengthens the
coating once applied to the crucible body and generally improves
adherence of silicon nitride particles to the crucible body and to
each other. The coating composition may include at least about 0.1%
by weight sintering aid (or sintering aids when more than one is
used), at least about 0.5%, at least about 1%, at least about 5% or
at least about 7.5% by weight sintering aid(s).
[0034] The mass ratio of sintering agent to silicon nitride may be
at least about 1:20 and, in other embodiments, is at least about
1:10, at least about 1:5, at least about 2:5, at least about 3:5 at
least about 4:5 or even at least about 1:1. In various other
embodiments, the ratio of sintering agent to silicon nitride is
from about 1:20 to about 1:1 or from about 1:5 to about 1:2.
[0035] In embodiments wherein the coating composition comprises
yttria, the coating composition may contain at least about 0.1% by
weight yttria and, in other embodiments, may contain at least about
0.5% yttria, at least about 1% yttria, at least about 5% yttria, at
least about 7.5% yttria, or even at least about 12% by weight
yttria. In various other embodiments, the coating composition
includes from about 0.1% to about 40% by weight yttria, from about
1% to about 40% by weight yttria or from about 1% to about 20% by
weight yttria.
[0036] Alternatively or in addition to yttria, the composition may
include other components which may act as sintering aids, such as
silica and/or alumina In some embodiments, the composition includes
at least about 0.1% by weight silica and, in others, at least about
0.5%, at least 1% or at least about 3% by weight silica. In various
embodiments, the composition includes from about 0.1% to about 10%
by weight silica, from 0.1% to about 5% silica or from about 1% to
about 5% silica. Alternatively or in addition, the composition may
include at least about 0.1% by weight alumina In other embodiments,
the composition includes at least about 0.5% by weight alumina, at
least about 1% or at least about 2% by weight alumina In various
embodiments, the composition includes from about 0.1% to about 10%
by weight alumina, from 0.1% to about 4% alumina or from about 1%
to about 5% alumina Generally, yttria, silica, alumina or other
oxides are used as a sintering aid to increase the strength and
adherence of the silicon nitride coating.
[0037] It should be noted that the amount of medium may be
increased in compositions containing relatively higher amounts of
oxides and, particularly, that contain relatively higher amounts of
silica and alumina. Compositions with relatively higher amounts of
medium are generally characterized by improved fluidity (see
Example 1).
Methods for Coating a Crucible
[0038] The coating compositions described in the present disclosure
may be applied to at least a portion of the inner surface of a
crucible body by chemical vapor deposition, plasma spraying,
brushing, aerosol spraying, pouring or any combination of these.
Typically, the application is done under a ventilated hood at
atmospheric pressures and at a temperature below the flash point of
the coating composition. The coating composition may be applied in
a single application or multiple times to reach a desired
thickness. Once the desired thickness is reached, the coated
crucible may be heated to vaporize the binder, medium, dispersant
and the like and to leave behind silicon nitride and any oxide
additive (e.g., yttria, silica, alumina and the like) as the
coating. The crucible may also be sintered to cause densification
and strengthening of the coating. It should be noted that in
embodiments wherein multiple applications are used, the crucible
may be dried, heated and/or sintered after one or more of the
intermediate applications in addition to the final application.
[0039] Generally, the coating compositions herein described may be
applied alone or in combination to at least a portion of the inner
surface of the sidewall of the crucible or the entire inner surface
of the sidewall of the crucible. If the crucible includes more than
one sidewall, the coating composition may be applied to at least a
portion of the inner surface of one or more sidewalls or the entire
surface of one or more sidewalls and may be applied to the entire
inner surfaces of all the sidewalls.
[0040] As shown in FIG. 2, for example, the coating may be applied
from the bottom of the crucible to a height, H.sub.1. The distance
between the bottom of the crucible and H.sub.1 may be at least
about 50% of the height of the sidewall. In other embodiments, the
distance between the bottom of the crucible and H.sub.1 is at least
about 70% of the height of the sidewall or even at least about 85%
of the height of the sidewall. In some embodiments, H.sub.1 extends
above about a solidification line, S.sub.1, described below (FIG.
3) and, in other embodiments, to about the top of the crucible.
[0041] Generally, the volume of semiconductor materials and the
proportional height of the material within the crucible may vary as
the semiconductor material solidifies in the crucible. For
instance, silicon increases in volume as it solidifies. The
"solidification line" of the inner surface of the crucible extends
the circumference of the inner surface of the sidewall (or
sidewalls if the crucible has more than one sidewall) and
corresponds to the expected portion of the crucible adjacent the
top of a solidified ingot or the expected elevation of the top of
the ingot. The distance between the bottom of the crucible and the
solidification line corresponds to the distance between the bottom
of the crucible and the top of an ingot solidified within the
crucible and, when the bottom of the crucible is not uniform in
dimension (such as, for example, a concave-bottomed crucible), the
distance from the bottom of the crucible at a point where it meets
the sidewall to the top of the solidified ingot.
[0042] The bottom 10, top, T, and solidification line, S.sub.1, are
generally illustrated in FIG. 3. Generally, the distance between
the solidification line, S.sub.1, and the top, T, of the crucible
sidewall is less than about 25% of the height of the sidewall
(i.e., about 25% of the distance between where the sidewall meets
the bottom of the crucible and the top of the sidewall) in order to
maximize the size of the solidified ingot. In other embodiments,
this distance is less than about 15% of the sidewall height, less
than about 10% or even less than about 5% of the height. In some
embodiments, this distance is from 0.5% to about 25% of the
sidewall height.
[0043] The inner surface of the crucible may be coated with coating
compositions of embodiments of the present disclosure across the
inner surface of the bottom of the crucible and the inner surface
of the sidewall from the bottom of the crucible to at least the
solidification line and, in other embodiments, to a distance above
the solidification line. The distance above the solidification line
to which the inner sidewall is coated may be at least about 0.5% of
the height of the sidewall, at least about 1% of the height of the
sidewall or even at least about 3% of the height of the
sidewall.
[0044] According to some embodiments of the present disclosure, a
first coating composition is applied to a first area of the inner
surface of the crucible sidewall and a second coating composition
is applied to a second area of the inner surface of the sidewall.
For instance, two coating compositions may be applied to different
portions of the crucible sidewall with one coating composition
being applied to the portions of the sidewall to which the ingot is
relatively more likely to adhere and the other coating composition
being applied to the other areas of the sidewall. The second
coating composition may contain an additive that it not present in
the first coating composition. According to some embodiments, the
first coating includes a medium, silicon nitride, dispersant and a
binder. In addition or alternatively, the second coating
composition may include a medium, silicon nitride, and an oxide
additive such as alumina, silica and yttria and/or other oxides
(e.g., lanthanides). The silicon nitride and/or oxide additive may
be suspended in the medium.
[0045] In various embodiments and as shown in FIG. 4, the first
composition may be applied to the inner surface of the sidewall
from the bottom 10 of the crucible 5 to a height H.sub.1 and the
second coating composition may be applied to the inner surface of
the crucible from about H.sub.1 to a height H.sub.2. The distance
between the bottom of the crucible and H.sub.1 may be at least
about 50% of the height of the sidewall and, in other embodiments,
is at least about 70% or even at least about 85% of the height of
the sidewall. The distance between the bottom of the crucible and
the height H.sub.2 to which the second coating composition may be
applied may be at least about 60% of the height of the sidewall
(for instance when H.sub.1 is less than about 60% of the height of
the sidewall), at least about 75% (for instance when H.sub.1 is
less than about 75% of the height of the sidewall) or even at least
about 90% of the height of the sidewall (for instance when H.sub.1
is less than about 90% of the height of the sidewall). In other
embodiments, H.sub.2 extends above the solidification line,
S.sub.1, or even extends to about the top, T, of the crucible (FIG.
3).
[0046] In some embodiments, the first coating composition and the
second coating composition may be applied to overlapping portions
of the crucible sidewall. For instance, the first coating may be
applied from the bottom 10 of the crucible 5 to a height H.sub.1
and the second coating composition may be applied to a height below
H.sub.1 to a height H.sub.2 with the distance between the bottom of
the crucible and H.sub.1 and H.sub.2 being as described previously.
In these embodiments, H.sub.1 may even extend to or above the
solidification line, S.sub.1, or even extend to about the top, T,
of the crucible.
[0047] In embodiments where the second coating composition contains
yttria or other oxides and the first coating composition does not
contain such additives, by limiting the amount of silicon-wetted
surface area to which the second composition is applied, the total
oxygen content of the coating may be reduced with a proportional
reduction in the amount of oxygen in the solidified ingot.
Generally, the first composition which, in some embodiments, does
not contain oxide additives (e.g., yttria, silica and/or alumina
and the like) may be applied to the bottom of the crucible.
[0048] Once a coating composition is applied to a portion of the
inner surface of the crucible, the composition may be dried to
vaporize the medium. Generally, the crucible may be dried under any
atmosphere including, for example, ambient air, nitrogen, argon or
mixtures thereof. Generally when ambient air is utilized, a
substantial portion if not all of the medium vaporizes after about
20 minutes and, in other embodiments, after about 30 minutes or
even after 40 minutes. By increasing ventilation (e.g., by the use
of circulating air), drying times may be proportionally decreased.
Generally, the coating is dried when the coating does not adhere or
transfer material to human fingers upon contact with the
coating.
[0049] The coating composition may be applied and dried several
times to increase the thickness of the coating. Each application
may be air-dried to remove the medium before application of another
coat. Alternatively or in addition, heating may be utilized as
described in more detail below. In some embodiments, the coating
composition is applied and dried at least about 2 times and in
other embodiments, at least about 4 times or even about 8 times. In
various embodiments, the coating composition is applied until a
thickness of at least about 50 .mu.m is achieved and, in other
embodiments, at least about 100 .mu.m, at least about 250 .mu.m, at
least about 500 .mu.m or even at least about 750 .mu.m. In several
embodiments, the coating composition is applied until a thickness
of from about 50 .mu.m to about 1000 .mu.m, from about 100 .mu.m to
about 750 .mu.m or from about 250 .mu.m to about 750 .mu.m is
achieved. Generally, increased thickness of the coating result in
decreased oxygen content in resulting ingots, particularly, if the
coating composition itself contains minimal amounts of oxygen.
[0050] After the desired thickness is achieved (or thicknesses if
more than one range of thickness is desired on the inner surface of
the crucible), the crucible may be heated to a temperature
sufficient to remove any binder, dispersant, plasticizer and the
like. Generally, heating may be achieved by heating the crucible
with the applied coating composition until the desired temperature
is achieved. The binder, dispersant and/or plasticizer may be
removed until the coating contains less than 5% by weight residual
carbon or less than about 3% or less than about 1% by weight
residual carbon. In some embodiments, the crucible is heated to at
least about 150.degree. C., to at least about 200.degree. C., to at
least about 300.degree. C., to at least about 400.degree. C. or
even to at least about 750.degree. C. to remove any binder,
plasticizer and dispersant compounds. In various other embodiments,
the crucible may be heated to from about 100.degree. C. to
750.degree. C. or from 400.degree. C. to 750.degree. C. The
crucible may be heated for at least about 1 hour and, in other
embodiments, at least about 2 hours, at least about 3 hours or from
about 1 hour to about 5 hours. In some embodiments, the crucible is
heated to at least about 300.degree. C. for at least about 2 hours.
The atmosphere may be at a pressure (vacuum) of from about 60 torr
(0.08 atm) to about 1 atmosphere or from about 150 torr (0.20 atm)
to about 1 atmosphere. In other embodiments, pressures above
atmospheric are used such as, for example, pressures of at least 1
atmosphere, at least 2 atmospheres or even at least 5
atmospheres.
[0051] The crucible may be heated in the presence of an inert gas
such as, for example, nitrogen, helium or argon. Ambient air may
also be used as the atmosphere during heating but is less preferred
as it may introduce oxygen into the coating. Further, the materials
of construction of the furnace as well as the gas flows may be
controlled to avoid oxidation of the coating as appreciated by
those of skill in the art.
[0052] In addition or alternatively to the crucible heating, the
crucible may also be sintered to densify the coating. In
embodiments wherein the crucible is heated to remove any residual
medium and the binders and plasticizers as described above,
sintering may be performed after the removal of any residual
medium, solvent, binder and/or plasticizer or the sinter may
additionally be used to remove one or more of these components. In
certain embodiments, sintering is performed during meltdown of a
silicon charge to minimize processing time. To achieve sintering,
the crucible may be heated to a temperature of at least about
1000.degree. C. and, in another embodiment, at least about
1100.degree. C. The crucible may be sintered for at least about 1
hour and, in other embodiments, at least about 2 hours, at least
about 3 hours or from about 1 hour to about 5 hours. In some
embodiments, the crucible is sintered at a temperature of at least
about 1100.degree. C. for at least about 3 hours. The crucible may
be sintered in the presence of an inert gas such as, for example,
nitrogen, helium or argon. The atmosphere may be at a pressure
(vacuum) of from about 60 torr (0.08 atm) to about 1 atmosphere or
from about 150 torr (0.20 atm) to about 1 atmosphere. In other
embodiments, pressures above atmospheric are used such as, for
example, pressures of at least 1 atmosphere, at least 2 atmospheres
or even at least 5 atmospheres. The residual carbon levels left on
the crucible after different heating and/or sintering treatments
were tested in Example 6 below.
[0053] When one or more coating compositions are applied to the
inner surface of the crucible sidewall, the second composition
which may include oxide additives (yttria, silica, alumina and the
like) may be applied to the second area of the crucible and dried.
This may be repeated until the desired thickness is achieved. The
first composition that does not contain additives may then be
applied to the first area of the crucible and dried. The first
composition may be applied and dried repeatedly to achieve the
desired thickness. The first coating and second coating after
applied and/or after dried may overlap each other over a portion of
the inner surface of the crucible without departing from the scope
of the present disclosure.
Coated Crucibles
[0054] As discussed above, the coating composition is applied to a
portion or all of the inner surface using one or more applications
and the coating composition may be dried in various methods
depending on the type and number of coatings applied and the
desired composition of the resulting coating. In general, by drying
the crucible at ambient temperatures the medium vaporizes to leave
behind the silicon nitride and a portion of the binder,
plasticizer, dispersant and/or oxide additives (yttria, silica,
alumina and the like). By heating the crucible, the residual
binder, plasticizer and/or dispersant may be vaporized leaving
behind silicon nitride, any oxide additives and residual carbon. As
shown in Example 6, by heating the crucible to 400.degree. C. for
at least 2 hours, the residual carbon concentration in the coating
may be reduced to less than 1% by weight of the coating.
[0055] In some embodiments including embodiments wherein the
coating composition used to coat a portion of the crucible does not
contain an oxide additive, the crucible coating may include at
least about 90% by weight silicon nitride. In other embodiments,
the coating includes at least about 95% by weight silicon nitride
or even at least about 97.5% by weight silicon nitride. The coating
may include less than about 1% by weight carbon. It should be noted
that the coating compositions described in this section entitled
"Coated Crucibles" refer to compositions of the coating after a
heating step (e.g., heating to at least about 300.degree. C.) and,
optionally, a sintering step as described above in the section
above entitled "Methods for Coating a Crucible."
[0056] In various embodiments, the coating extends over the entire
inner surface of the crucible sidewall (and in some embodiments
sidewalls) or a portion of the inner surface of the sidewall. As
described above in relation to the area to which coating
compositions may be applied and as shown in FIG. 2, the coating may
extend across the inner surface of the bottom of the crucible and
the inner surface of the sidewall from the bottom of the crucible
to a height, H.sub.1.
[0057] In some embodiments, the coating that covers a first area of
the crucible sidewall may include silicon nitride and an amount of
a sintering agent (e.g., an oxide additive such as yttria, silica
or alumina) In such embodiments, the coating may include at least
about 40% by weight silicon nitride, at least about 60% silicon
nitride or even at least about 80% by weight silicon nitride in
addition to sintering agent.
[0058] The coating may include at least about 0.5% by weight
yttria. In other embodiments, the coating may include at least
about 1% by weight yttria, at least about 5%, at least about 10%,
at least about 15% or even at least about 20% by weight yttria. In
various embodiments, the coating includes from about 0.5% to about
25% by weight yttria, from about 0.5% to about 20% by weight, from
about 0.5% to about 10% by weight or from about 1% to about 20% by
weight yttria. The mass ratio of yttria to silicon nitride in the
coating may be at least about 1:20 and, in other embodiments, at
least about 1:10, at least about 2:5 or even at least about
4:5.
[0059] Alternatively or in addition, oxide additives other than
yttria such as silica and/or alumina may be present in the coating.
Other suitable oxide additives may include lanthanide oxides. In
some embodiments the coating comprises at least about 0.5% by
weight silica. In other embodiments, the coating may include at
least about 1% by weight silica, at least about 5%, at least about
10%, at least about 15%, or even at least about 20% by weight
silica. In various other embodiments, the coating comprises from
about 0.5% to about 25% by weight silica, from about 0.5% to about
20% by weight, from about 0.5% to about 10% by weight or from about
1% to about 20% by weight silica.
[0060] The crucible coating may include an amount of alumina and,
in some embodiments, contains at least about 0.5% by weight
alumina. In other embodiments, the coating includes at least about
1% by weight alumina, at least about 5% or even at least about 10%
by weight alumina. In various other embodiments, the coating
comprises from about 0.5% to about 25% by weight alumina, from
about 0.5% to about 20% by weight, from about 0.5% to about 10% by
weight or from about 1% to about 20% by weight alumina.
[0061] In particular embodiments, the coating on the inner surface
of the crucible sidewall may consist essentially of silicon
nitride, sintering agent (e.g., yttria, silica or alumina) and
carbon. The amount of carbon in the coating may be less than about
1% by weight carbon.
[0062] Coated crucibles of the present disclosure may include more
than one coating applied to the inner surface of the crucible
sidewall. The crucible may include a first coating on a first area
of the inner surface of the sidewall and a second coating on a
second area of the inner surface of the sidewall. In some
embodiments, the second coating comprises an additive not present
in the first coating such as, for example, yttria. Alternatively or
in addition, the first coating may include an additive not present
in the second coating.
[0063] Coatings containing sintering agent (e.g., yttria, silica or
alumina) may be applied to the entire inner surface of the crucible
sidewall or may be applied to a portion of the sidewall (for
instance between H.sub.1 and H.sub.2 as shown in FIG. 4). Such
coatings provide improved ingot-release characteristics so it may
be desirable to apply the coatings to the portion of the crucible
where improved ingot-release characteristics are desirable,
typically the portion near the top of the sidewall in upward
directional solidification applications. However, it should also be
noted that such coatings may introduce oxygen into the ingot. As
such, it may also be desirable to minimize the use of such
coatings. The amount and location of such coating may be determined
based on the desired release characteristics and ingot oxygen
content.
[0064] As described above in relation to application of the first
and second coating compositions and with reference to FIG. 4, the
first coating may extend from the bottom of the crucible to a
height H.sub.1 and the second coating may extend from about H.sub.1
to a height H.sub.2. When the coated crucible contains two
coatings, the first coating may include silicon nitride and may
include residual carbon left from any binder, plasticizer,
dispersant and the like. The first coating may include less than
about 1% by weight carbon. The first coating may even consist
essentially of silicon nitride and carbon.
[0065] The second coating may include silicon nitride and sintering
agent (e.g., yttria, silica or alumina). The second coating may
include at least about 40% by weight silicon nitride, at least
about 60% or at least about 80% by weight silicon nitride. The mass
ratio of sintering agent to silicon nitride in the second coating
may be at least about 1:20, at least about 1:10, at least about
2:5, or even at least about 4:5. The second coating may include at
least about 0.5% by weight yttria or at least about 1%, at least
about 5%, at least about 10%, at least about 15% or even at least
about 20% by weight yttria. In some embodiments, the second coating
includes from about 0.5% to about 25% by weight yttria.
[0066] The second coating may include an amount of silica and, in
some embodiments, includes at least about 0.5% by weight silica, at
least about 1%, at least about 5%, at least about 10%, at least
about 15%, at least about 20% or from about 0.5% to about 25% by
weight silica. The second coating may also include an amount of
alumina and, in some embodiments, includes at least about 0.5% by
weight alumina, at least about 1%, at least about 5%, at least
about 10% or from about 0.5% to about 25% by weight alumina
[0067] The second coating may include silicon nitride and any
combination of sintering agents (e.g., any combination of yttria,
silica and alumina). The second coating may consist essentially of
silicon nitride, sintering agent and carbon.
[0068] In embodiments where only one coating is applied to the
inner surface of the crucible sidewall or even when multiple
coatings are applied, the thickness of any or even all of the
coatings if more than one may correspond to the thicknesses
described above.
Methods for Preparing an Ingot
[0069] One aspect of the disclosure relates to preparation of
silicon ingots and, particularly, to preparation of silicon ingots.
In embodiments where multicrystalline silicon ingots produced by a
directional solidification process are desired, polycrystalline
silicon may be loaded into a coated crucible to form a silicon
charge. Coated crucibles to which polycrystalline silicon may be
applied are generally described above. Methods for crystallizing
are generally described by K. Fujiwara et al. in Directional Growth
Medium to Obtain High Quality Polycrystalline Silicon from its
Melt, Journal of Crystal Growth 292, p. 282-285 (2006), which is
incorporated herein by reference for all relevant and consistent
purposes.
[0070] Once loaded into the coated crucible of the present
disclosure, the silicon charge may be heated to a temperature above
about the melting temperature of the charge to form a silicon melt.
The silicon charge may be heated to at least about 1410.degree. C.
to form the silicon melt and, in another embodiment, at least about
1450.degree. C. to form the silicon melt. Once the silicon melt has
been prepared, the melt may be solidified such as, for example, in
a directional solidification process. The ingot may then be cut
into one or more pieces with dimensions matching several of the
dimensions of a desired solar cell. Wafers may be prepared by
slicing these pieces by, for example, use of a wiresaw to produce
sliced wafers.
[0071] The multicrystalline silicon produced by directional
solidification is an agglomeration of crystal grains with the
orientation of the grains relative to each other being random due
to the high density of heterogeneous nucleation sites at the
crucible wall. The resulting multicrystalline silicon ingot may
have an average nominal crystal grain size of from about 1 mm to
about 15 mm and, in other embodiments, has an average nominal
crystal grain size of from about 5 mm to about 25 mm or from about
5 mm to about 15 mm.
[0072] Silicon wafers may be produced by slicing the ingot using,
for example, a wiresaw. The resulting silicon wafers have average
nominal crystal grain sizes as described above for multicrystalline
ingots.
Ingots with Low Oxygen Content
[0073] Generally, it has been found that by utilizing a crucible
with a coating of sufficient thickness that contains minimal
sources of oxygen (i.e., yttria, silica, alumina and the like) and
by utilizing C1 to C10 alcohol mediums in coating compositions
rather than water and, optionally, by sintering these coatings in
an inert atmosphere, resulting multicrystalline ingots and wafers
are characterized by a reduced oxygen content and proportionally
reduced light induced degradation.
[0074] Referring now to FIG. 5, ingots prepared according to the
present disclosure, that is by utilizing a silicon nitride coating
in an alcohol medium with a binder and dispersant, contained less
oxygen across all fractions of the ingot (i.e., from top to
bottom).
[0075] Silicon ingots according to embodiments of the present
disclosure may have a bottom, a top and a height, H.sub.3, wherein
H.sub.3 corresponds to the average distance between the bottom and
top of the ingot. As shown in FIG. 5, the oxygen concentration in
the as grown ingot may generally decrease from the bottom of the
ingot to the top of the ingot, wherein the top and bottom of the
ingot correspond to the top and bottom as grown in the crucible. It
should be noted in this regard that the oxygen concentration
profile as described herein refers to the concentration profile of
an as grown multicrystalline ingot. In this regard, the ingot may
be free of cut marks exhibited by ingots that have been cut by a
wiresaw or otherwise. The bottom and top of the ingots as described
herein correspond to the bottom and top of the ingots after
solidification. This allows the ingots to be characterized by an
oxygen concentration along the height of the ingot as
solidified.
[0076] The silicon ingot may be produced by a directional
solidification process as described above. The ingot may contain
multicrystalline silicon with the average nominal crystal grain
size being from about 1 mm to about 15 mm. In other embodiments,
the average nominal crystal grain size is from about 5 mm to about
25 mm or from about 5 mm to about 15 mm.
[0077] In some embodiments, the oxygen concentration of the ingot
at a height about 20% of H.sub.3 is less than about 4.5 ppma. In
other embodiments, the oxygen concentration of the ingot at a
height about 20% of H.sub.3 is less than about 4.0 ppma, 3.0 ppma
or even 2.0 ppma. The oxygen concentration of the ingot between a
height about 20% of H.sub.3 to a height about 80% of H.sub.3 may be
less than about 3.0 ppma and, in other embodiments, is less than
about 2.0 ppma. In one particular embodiment, the oxygen
concentration of the ingot between the bottom and the top is less
than about 2.5 ppma and, in another embodiment, less than about 2.0
ppma.
[0078] Generally, oxygen content may be measured using Fourier
Transform Infrared Spectophotometry (FTIR). For example, the oxygen
concentration may be measured by cutting 2 mm thick slugs of
silicon from the ingot at the height of interest and measuring the
oxygen content by FTIR.
[0079] Generally, the shape of the ingot corresponds to the
crucible in which it was solidified and, in some embodiments, the
ingot is rectangular or even square in shape.
Wafers with Low Oxygen Content
[0080] As can be seen from FIG. 5, the oxygen concentrations along
the entire height of the ingot of the present disclosure are less
than the lowest oxygen concentration exhibited by the conventional
ingot. Accordingly, wafers produced along the entire height of the
ingot will exhibit a lower oxygen content that all wafers produced
from the conventional ingot. In some embodiments of the present
disclosure, a silicon wafer has an oxygen concentration of less
than about 2.5 ppma. In other embodiments, the silicon wafer has an
oxygen concentration of less than about 2.25 ppma, of less than
about 2 ppma, of less than about 1.75 ppma, of less than about 1.5
ppma or even less than about 1.25 ppma. In various other
embodiments, the wafer has an oxygen content of from about 0.1 to
about 3 ppma, from about 0.5 to about 3 ppma, from about 0.75 to
about 3 ppma, from about 1 to about 3 ppma, from about 0.75 to
about 2.5 ppma, from about 0.75 to about 2.25 ppma, from about 0.75
to about 2 ppma or from about 0.75 to about 1.75 ppma.
[0081] The wafer may contain multicrystalline silicon. The average
nominal crystal grain size may be from about 1 mm to about 15 mm,
from about 5 mm to about 25 mm or from about 5 mm to about 15 mm.
In some embodiments, the wafer is rectangular (including square
wafers). The wafer may be characterized by any shape without
departing from the scope of the present disclosure.
EXAMPLES
Example 1
Coating a Crucible with a Silicon Nitride Coating Composition
[0082] The coating composition of Table 1 below was prepared by
weighing out the amount of isopropyl alcohol in a sufficiently
large beaker. The amounts of dispersant (SOLSPERSE.RTM. 20000),
binder (PVB (B-76)), plasticizer (PEG (CARBOWAX.RTM. 400)) and
silicon nitride powder were weighed out separately. The beaker and
medium were placed onto a hot plate. A high shear impeller type
mixer was placed into the fluid. The beaker was capped to minimize
loss by evaporation. The medium was warmed to a temperature between
60 to 80.degree. C. while stirring. The PVB was added and stirred
until the binder broke down and the fluid became a tacky viscous
fluid. After mixing (about 15 minutes) the fluid color achieved a
more clear state.
[0083] Dispersant and PEG were added to the stirred fluid while
stirring. The hot plate was turned off and the mixture was stirred
for 5 minutes. The contents were poured while warm into a
polyethylene container. Silicon nitride milling balls (10 mm in
diameter) were added to the container for milling. A polyethylene
lid was screwed onto the open end of the container to minimize
leakage of the mixture or evaporation of the medium.
[0084] The milling container was opened and the pre-weighed silicon
nitride powder was added. The container was closed and placed onto
a milling apparatus that turned at a speed of 60 rpm. The mixture
was milled for 6 hours to ensure thorough mixing.
[0085] The composition was then used to coat the inner surface of a
silica crucible (68 cm.times.68 cm.times.42 cm). The slip was
brushed onto the surface using a foam applicator common to paint
application. The composition was first applied in the corners and
then applied to the vertical walls and then the bottom of the
crucible. Any puddling or dripping was brushed out before
appreciable medium evaporated and skinned the surface of the drip.
The composition was allowed to dry for about 30 to 45 minutes in
air at room temperature in a ventilated area. The composition was
applied three more times with drying in-between to build up a
four-pass coating.
[0086] The binder, dispersant and plasticizer were removed by
heating to a temperature between 300 to 400.degree. C. for two
hours. The crucible was then heated to a temperature of
1100.degree. C. for three hours to sinter the coating. The
thickness of the coating was 400 .mu.m before sintering and from
290 .mu.m to 325 .mu.m after sintering. A silicon charge of 270 kg
was added to the coated crucible and directionally solidified. The
solidified ingot released well from the crucible.
TABLE-US-00001 TABLE 1 Percentage Inclusion by Weight of Components
of the Coating Composition of Example 1 Component Percentage
Inclusion (wt %) Silicon Nitride 37.5 Dispersant (SOLSPERSE .RTM.
20000) 1.8 Medium (isopropyl alcohol) 49.5 Binder (PVB (B-76)) 5.6
Plasticizer (PEG (CARBOWAX .RTM. 400)) 5.6
Example 2
Coating a Crucible with a Silicon Nitride Coating Composition that
Contains Yttria
[0087] Compositions 1-3 shown in Table 2 were prepared with
increasing amounts of yttria. The portions of the desired mixture
were weighed out taking into account the desired silicon nitride to
additive mass ratio. The silicon nitride and yttria were mixed and
the medium was added followed by additions of the dispersant, PVB
and PEG. The mixture was mechanically mixed for five minutes.
TABLE-US-00002 TABLE 2 Percentage Inclusion by Weight of Components
of the Coating Compositions of Example 2 Composition 1 Composition
2 Composition 3 Component (wt %) (wt %) (wt %) Silicon Nitride
36.14 34.88 32.61 Dispersant 1.73 1.67 1.57 (SOLSPERSE .RTM. 20000)
Medium (isopropyl 47.71 46.05 43.04 alcohol) Binder (PVB (B-76))
5.40 5.21 4.87 Plasticizer (PEG 5.40 5.21 4.87 (CARBOWAX .RTM.
400)) Yttria 3.61 6.98 13.04
[0088] The compositions were then used to coat the inner surface of
several crucibles. Each composition was brushed onto a silica
crucible to achieve a 400 .mu.m thickness. The coated-crucible was
heated to 400.degree. C. for three hours and was then heated to
1100.degree. C. for four hours. A 60 torr argon gas atmosphere was
used during heating. Not taking residual carbon into account, the
coating contained silicon nitride and yttria in the amounts shown
in Table 3.
TABLE-US-00003 TABLE 3 Percentage Inclusion by Weight of Components
of the Coating of Example 2 Composition 1 Composition 2 Composition
3 Component (wt. %) (wt. %) (wt. %) Silicon Nitride 90.9 83.3 71.4
Yttria 9.1 16.7 28.6
[0089] Increasing the concentration of yttria improved bonding of
the coating to the crucible. The coating produced from Composition
3 was found to adhere best to the crucible.
Example 3
Adjustment of the Amount of Medium to Maintain Fluidity
[0090] Compositions A-D shown in Table 4 were prepared with
increasing amounts of silica, alumina and yttria. The compositions
were prepared and applied to a crucible as described in Example 2.
The amount of dispersant was increased in each composition until
the composition was characterized by sufficient fluidity.
TABLE-US-00004 TABLE 4 Percentage Inclusion by Weight of Components
of the Coating Compositions of Example 3 Compo- Compo- Compo-
Compo- sition A sition B sition C sition D Component (%) (%) (%)
(%) Silicon Nitride 34.04 22.42 18.01 14.53 Dispersant 1.63 1.08
0.86 0.70 (SOLSPERSE .RTM. 20000) Medium (isopropyl 50.30 59.63
63.49 67.25 alcohol) Binder (PVB (B-76)) 5.08 3.35 2.69 2.17
Plasticizer (PEG 5.08 3.35 2.69 2.17 (CARBOWAX .RTM. 400)) Silica
1.37 3.61 4.35 4.68 Alumina 0.77 2.04 2.46 2.65 Yttria 1.72 4.52
5.45 5.86
[0091] The compositions were then used to coat a portion of the
inner surface of several silica crucibles to a thickness of 100
.mu.m. The coated-crucible was heated to 400.degree. C. for three
hours and was then heated to 1100.degree. C. for four hours. A 60
torr argon gas atmosphere was used during heating. Not taking
residual carbon into account, the coating contained silicon
nitride, yttria, silica and alumina in the weight percentages shown
in Table 5.
TABLE-US-00005 TABLE 5 Percentage Inclusion by Weight of Components
of the Coating of Example 3 Compo- Compo- Compo- Compo- sition A
sition B sition C sition D Component (wt %) (wt %) (wt %) (wt %)
Silicon Nitride 89.8 68.8 59.5 52.4 Silica 3.6 11.1 14.4 16.9
Alumina 2.0 6.3 8.1 9.6 Yttria 4.5 13.9 18.0 21.1
[0092] Only the top 10% of the inner surface of the crucibles was
coated with the yttria-containing composition. The remainder of the
inner surface of each crucible was coated with the coating
composition of Table 1.
[0093] Increasing the amounts of yttria, silica and alumina in the
coating improved bonding of the coating to the crucible. The
coating produced from Composition D was found to adhere best to the
crucible.
Example 4
Preparation of a Coating Composition without Oxide Additives and a
Coating Composition that Contains an Oxide Additive (Silica)
[0094] The compositions shown in Table 6 were used as starting
compositions that may be adjusted to provide the proper fluidity so
as to avoid defects in the coating (e.g., to assure continuity of
the coating on the crucible and to avoid pinholes). The first
coating composition did not contain any oxide additives and is
suitable for use on the lower potions of the crucible. The second
coating composition contained silica as an additive and is suitable
for use on upper portions of the crucible.
TABLE-US-00006 TABLE 6 Percentage Inclusion by Weight of Components
of the Coating Compositions of Example 4 First Second Component
Composition (%) Composition (%) Silicon Nitride 21.8 20.9
Dispersant 4.3 4.1 (SOLSPERSE .RTM. 20000) Medium (isopropyl 64.2
61.6 alcohol) Binder (PVB 3.3 3.1 (Butvar .RTM.)) Plasticizer (PEG
6.4 6.2 (CARBOWAX .RTM. 400)) Silica -- 4.2
Example 5
Comparison of the Oxygen Content of Commercially Available Ingots
and Ingots Prepared Using Coated Crucibles of the Present
Disclosure
[0095] The oxygen content of a conventional multicrystalline ingot
was compared against multicrystalline ingots solidified in a silica
crucible coated with the composition of Table 1 and dried and
sintered in an argon atmosphere and according to the third to last
row of Table 7 (below). The coating was 400 .mu.m thick. 2 mm thick
samples of material cut horizontally from the bricks were taken at
various solidified fractions (heights) of the ingot. Data points
are interpolated by straight lines in the plots to generate the
oxygen concentrations at various points along the height of the
crucible as shown in FIG. 5.
Example 6
Determination of the Carbon Content of the Coating During
Drying
[0096] The coating composition of Table 1 was applied to several
silica crucibles and different heat treatments were applied. The
results are shown in Table 7 below. Heat treatment temperatures (to
remove plasticizer, binder, dispersant and the like) were achieved
over a two hour ramp. Sintering temperatures were achieved over a 2
hour ramp from the burn-off temperature.
TABLE-US-00007 TABLE 7 Residual Carbon within a Crucible Coating
after Different Heat Treatments Heat Sinter Residual Air Heat
Treatment Temp. Carbon Dried? Treatment? Temp. (.degree. C.)
Sinter? (.degree. C.) (wt %) Yes No -- No -- 14.00 Yes Yes 180 No
-- 12.30 Yes Yes 210 No -- 11.00 Yes Yes 400 No -- 0.57 Yes Yes 210
Yes 1100 0.16 Yes Yes 300 Yes 1100 0.20 Yes Yes 400 Yes 1100
0.24
[0097] As can be seen from Table 7, a heat treatment at 400.degree.
C. is generally sufficient to remove most carbon from the crucible
coating.
[0098] When introducing elements of the present disclosure or the
preferred embodiments(s) thereof, the articles "a", "an", "the" and
"said" are intended to mean that there are one or more of the
elements. The terms "comprising", "including" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements.
[0099] As various changes could be made in the above apparatus and
methods without departing from the scope of the disclosure, it is
intended that all matter contained in the above description and
shown in the accompanying figures shall be interpreted as
illustrative and not in a limiting sense.
* * * * *