U.S. patent application number 14/447031 was filed with the patent office on 2014-11-20 for high temperature furnace insulation.
The applicant listed for this patent is GTAT Corporation. Invention is credited to Ning Duanmu, Menahem Lowy, Dzung D. Nguyen, Dean C. Skelton.
Application Number | 20140338590 14/447031 |
Document ID | / |
Family ID | 46876232 |
Filed Date | 2014-11-20 |
United States Patent
Application |
20140338590 |
Kind Code |
A1 |
Duanmu; Ning ; et
al. |
November 20, 2014 |
HIGH TEMPERATURE FURNACE INSULATION
Abstract
A high temperature furnace comprising hot zone insulation having
at least one shaped thermocouple assembly port to reduce
temperature measurement variability is disclosed. The shaped
thermocouple assembly port has an opening in the insulation facing
the hot zone that is larger than the opening on the furnace shell
side of the insulation. A method for producing a crystalline ingot
in a high temperature furnace utilizing insulation having a shaped
thermocouple assembly port is also disclosed.
Inventors: |
Duanmu; Ning; (Nashua,
NH) ; Skelton; Dean C.; (Fitzwilliam, NH) ;
Lowy; Menahem; (Merrimack, NH) ; Nguyen; Dzung
D.; (Nashua, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GTAT Corporation |
Merrimack |
NH |
US |
|
|
Family ID: |
46876232 |
Appl. No.: |
14/447031 |
Filed: |
July 30, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13069027 |
Mar 22, 2011 |
8821634 |
|
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14447031 |
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Current U.S.
Class: |
117/201 ;
432/247; 432/262; 432/32; 65/355 |
Current CPC
Class: |
C30B 29/20 20130101;
F27D 1/12 20130101; C30B 11/006 20130101; F27D 1/0033 20130101;
F27D 21/0014 20130101; C30B 11/003 20130101; F23M 5/00 20130101;
C30B 29/06 20130101; Y10T 117/1004 20150115; F27B 14/00 20130101;
Y10T 117/1024 20150115; F27B 2014/004 20130101 |
Class at
Publication: |
117/201 ; 432/32;
432/247; 432/262; 65/355 |
International
Class: |
C30B 11/00 20060101
C30B011/00; C30B 29/06 20060101 C30B029/06; F27D 21/00 20060101
F27D021/00; F27B 14/00 20060101 F27B014/00; F27D 1/12 20060101
F27D001/12; F27D 1/00 20060101 F27D001/00 |
Claims
1. A furnace comprising: a furnace shell; a hot zone within the
furnace shell, the hot zone comprising at least one heating element
in the hot zone; insulation surrounding the hot zone having a
furnace shell side facing the furnace shell and a heating element
side facing the heating element; and at least one thermocouple
assembly inserted through at least one thermocouple assembly port
in the insulation, wherein the thermocouple assembly port increases
in size from the furnace shell side of the insulation to the
heating element side of the insulation.
2. The furnace of claim 1, wherein the thermocouple assembly port
comprises an opening on the heating element side of the insulation
and an opening on the furnace shell side of the insulation, and
wherein the opening on the heating element side of the insulation
is larger than the opening on the furnace shell side of the
insulation.
3. The furnace of claim 2, wherein the thermocouple assembly port
is countersunk on the heating element side of the insulation.
4. The furnace of claim 3, wherein the thermocouple assembly port
is countersunk at an angle of at least 60.degree..
5. The furnace of claim 3, wherein the thermocouple assembly port
is countersunk at an angle of at least 90.degree..
6. The furnace of claim 3, wherein the thermocouple assembly port
is countersunk at an angle of about 60.degree. to about
120.degree..
7. The furnace of claim 3, wherein the thermocouple assembly port
is countersunk through at least one half of the insulation.
8. The furnace of claim 3, wherein the thermocouple assembly port
is countersunk through the insulation.
9. The furnace of claim 8, wherein the insulation further comprises
an insulation flange surrounding the opening on the furnace shell
side of the insulation.
10. The furnace of claim 9, wherein the insulation flange is at
least about one-half as thick as the insulation.
11. The furnace of claim 2, wherein the thermocouple assembly port
is counterbored on the heating element side of the insulation.
12. The furnace of claim 11, wherein the thermocouple assembly port
is counterbored through at least one half of the insulation.
13. The furnace of claim 11, wherein the thermocouple assembly port
is counterbored at a diameter of at least twice the diameter of the
thermocouple assembly port on the furnace shell side of the
insulation.
14. The furnace of claim 2, wherein the opening on the heating
element side is circular, square or polygonal in shape.
15. The furnace of claim 1, wherein the insulation comprises top
and side insulation panels.
16. The furnace of claim 15, wherein the side insulation panel is
configured to move in a vertical direction relative to the
crucible.
17. The furnace of claim 1 further comprising a heat exchanger in
the hot zone for controlling heat extraction.
18. The furnace of claim 1, further comprising a crucible in the
hot zone on top of a crucible support block, the crucible
configured to receive at least feedstock material.
19. The furnace of claim 18, wherein the heating element heats and
melts the feedstock material in the crucible to produce a
crystalline ingot.
20. The furnace of claim 18, wherein the feedstock material
comprises polycrystalline silicon.
21. The furnace of claim 18, wherein the feedstock material
comprises aluminum oxide.
22. The furnace of claim 21, wherein the aluminum oxide further
comprises titanium.
23. The furnace of claim 19, wherein the crystalline ingot
comprises polycrystalline silicon.
24. The furnace of claim 19, wherein the crystalline ingot
comprises monocrystalline silicon.
25. The furnace of claim 19, wherein the crystalline ingot
comprises sapphire.
26. Insulation surrounding a hot zone of a furnace, wherein: the
insulation has a furnace shell side and a heating element side and
comprises at least one thermocouple assembly port through the
insulation to insert a thermocouple assembly and wherein the
thermocouple assembly port is countersunk or counterbored on the
heating element side of the insulation.
27-28. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The present invention relates to a high temperature furnace
comprising insulation having a shaped thermocouple assembly
port.
[0003] 2. Description of the Related Art
[0004] Crystallization furnaces, such as directional solidification
systems (DSS) and heat exchanger method (HEM) furnaces, involve the
melting and controlled resolidification of a feedstock material in
a crucible to produce an ingot. Production of an ingot from molten
feedstock occurs in several specific temperature steps and
temperature rates over many hours. For example, to produce a
silicon ingot by the DSS method, silicon feedstock is added to a
crucible, often contained in a graphite crucible box, placed into a
DSS furnace and then heated to fully melt the feedstock. Typically,
heating from room temperature to 1200.degree. C. occurs over
several hours at specified temperature rates before further heating
for several hours up to 1550.degree. C. The temperature, well above
the silicon melting temperature of 1412.degree. C., is maintained
for several hours at this temperature to completely melt the
silicon feedstock before applying a temperature gradient over 18
hours to directionally solidify the melt. The temperature is then
reduced below the melting point of silicon to anneal the ingot over
several hours, and, thereafter, the furnace is cooled down for
several more hours before removing the formed ingot.
[0005] The challenge in consistently producing high quality ingots
in a production scale furnace is to ensure that the melting,
solidification, and annealing temperatures, temperature rates, and
times can be accurately and consistently measured in the furnace
hot zone where ingot formation occurs. For example, the hot zone of
a furnace generally comprises a crucible containing feedstock and
at least one heating element above or beside the crucible to melt
the feedstock. Insulation typically surrounds at least the top and
sides of the crucible and heating element(s) to contain the heat
and define the hot zone, with one side of the insulation facing the
furnace shell wall and the other side facing the heating element(s)
within the hot zone. At least one thermocouple assembly, positioned
through a thermocouple assembly port in the insulation and into the
hot zone, is typically used in conjunction with a computer feedback
mechanism to the heater power source to measure, control and
maintain the correct temperature during various phases of ingot
growth. Conventionally, the size or diameter of the port opening on
the side of the insulation that faces the furnace shell is
generally substantially equal to the size of the port opening on
the side of the insulation that faces the heating element, creating
a generally cylindrical port. The thermocouple assembly, typically
comprising a thermocouple sensor encased in heat-protecting tubes
housed in a protective sheath, for example, graphite, fits into and
through this cylindrical port.
[0006] In an effort to grow ever larger ingots with existing
production equipment, space within the hot zone must be maximized,
often requiring that the heating elements reside in close proximity
to the hot zone insulation. As such, thermocouple assembly(s)
placed through conventional ports in the insulation to monitor
process temperatures in the hot zone are typically positioned close
to the heating element, for example, less than an inch. This close
proximity places the thermocouple assembly in a large temperature
gradient zone ranging from approximately 1500.degree. C. at the
heating element side of the insulation to approximately
1600-1700.degree. C. at the heating element surface. This
temperature gradient, typically spanning less than two inches from
the insulation to the heating element surface, makes the placement
of the thermocouple assembly highly susceptible to positional
temperature measurement variability.
[0007] Difficulties in ensuring consistently repeatable heater
control temperature measurements arise when a thermocouple assembly
must be placed in a furnace at a specific distance relative to the
heating element because the measured temperature is highly
dependent on that distance, Therefore, the high positional
sensitivity of the thermocouple assembly placed in a large
temperature gradient zone can result in significant differences in
the measured temperatures if not accurately placed in the
designated position, requiring expensive and time consuming effort
to measure and compensate for the observed differences,
[0008] As such, there is an increasing need in the industry for a
simpler, more reliable, and cost effective means to position a
thermocouple assembly in a high temperature furnace so that the
resulting measured heater control temperature is substantially
insensitive to thermocouple assembly position. The present
invention reduces this high positional sensitivity and results in
more consistent and repeatable heater control temperature
measurements required for high quality ingot growth.
SUMMARY OF THE INVENTION
[0009] The present invention relates to a furnace comprising a
furnace shell, a hot zone within the furnace shell comprising at
least one heating element in the hot zone, insulation surrounding
the hot zone having a furnace shell side facing the furnace shell
and a heating element side facing the heating element, and at least
one thermocouple assembly inserted through at least one
thermocouple assembly port in the insulation. The thermocouple
assembly port increases in size from the furnace shell side of the
insulation to the heating element side of the insulation.
Preferably, the thermocouple assembly port is either countersunk or
counterbored.
[0010] The present invention further relates to insulation
surrounding a hot zone of a furnace, wherein the insulation has a
furnace shell side and a heating element side and comprises at
least one thermocouple assembly port through the insulation to
insert a thermocouple assembly. The thermocouple assembly port is
either countersunk or counterbored on the heating element side of
the insulation. The insulation can be configured to be arranged
between a furnace shell of a furnace and a heating element in a hot
zone of the furnace, with the furnace shell side facing the furnace
shell and the heating element side facing the heating element,
[0011] The present invention also relates to a method for producing
a crystalline ingot in a furnace comprising a furnace shell and a
hot zone, wherein the insulation surrounding the hot zone comprises
a shaped thermocouple assembly port. The method comprises the steps
of heating a crucible containing at least feedstock material in the
hot zone of the furnace to a temperature greater than or equal to
1000.degree. C., wherein the hot zone comprises at least one
heating element, and insulation surrounds the hot zone, the
insulation being between the heating element and the furnace shell
of the furnace and having a heating element side facing the heating
clement and a furnace shell side facing the furnace shell, and
measuring the temperature in the hot zone with at least one
thermocouple assembly inserted through a thermocouple assembly port
in the insulation to a position between the heating element and the
heating element side of the insulation. The thermocouple assembly
port increases in size from the furnace shell side of the
insulation to the heating element side of the insulation, and the
measured temperature is substantially insensitive to the position
of the thermocouple assembly.
[0012] The present invention further relates to a method for
reducing the positional temperature sensitivity of a thermocouple
assembly inserted in a thermocouple assembly port and placed into a
furnace hot zone. The method comprises the steps of providing a
furnace comprising a furnace shell, a hot zone within the furnace
shell wherein the hot zone comprises at least one heating element
in the hot zone, insulation surrounding the hot zone having a
furnace shell side facing the furnace shell and a heating element
side facing the heating element, at least one thermocouple
assembly, and at least one thermocouple assembly port in the
insulation wherein the thermocouple assembly port comprises an
opening on the heating element side of the insulation and an
opening on the furnace shell side of the insulation, and the
opening on the heating element side of the insulation is larger
than the opening on the furnace shell side of the insulation. The
thermocouple assembly is inserted through a thermocouple assembly
port from the furnace shell side of the insulation to a position
between the heating element and the heating element side of the
insulation to measure temperature in the hot zone.
[0013] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are intended to provide further
explanation of the present invention, as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross-sectional schematic view of a high
temperature furnace of the present invention comprising a hot zone
surrounded by insulation having a shaped thermocouple assembly
port.
[0015] FIG. 2 shows a cross-sectional schematic view of insulation
comprising a thermocouple assembly positioned in a conventional
thermocouple assembly port known in the art.
[0016] FIG. 3A, 3B, 3C and 3D are cross-sectional schematic views
of embodiments of insulation of the present invention having a
conical shaped thermocouple assembly port wherein the port is
countersunk at the heating element side of the insulation at
symmetrical and asymmetrical angles.
[0017] FIG. 4A and 4B are cross-sectional schematic views of
embodiments of insulation of the present invention having a shaped
thermocouple assembly port wherein the port is counterbored.
[0018] FIG. 5A 5B, and 5C are cross-sectional schematic views of
embodiments of insulation of the present invention having a shaped
thermocouple assembly port and an insulation flange at the furnace
shell side of the insulation.
[0019] FIG. 6 is a cross-sectional schematic view of an embodiment
of a hybrid shaped thermocouple assembly port produced by
counterboring and countersinking.
[0020] FIG. 7 graphically compares temperature changes associated
with retraction of a thermocouple assembly in a conventional
thermocouple assembly port through conventional insulation and a
conical shaped thermocouple assembly port through insulation of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention relates to a furnace, a hot zone
within the furnace comprising a heating element, insulation
surrounding and defining the hot zone having a shaped thermocouple
assembly port, and to a method of producing a crystalline ingot
using this furnace and insulation.
[0022] The furnace of the present invention comprises a furnace
shell and a hot zone within the furnace shell. The furnace shell
can be any known in the art used for high temperature
crystallization furnaces, including a stainless steel shell
comprising an outer wall and an inner wall defining a cooling
channel for circulation of a cooling fluid, such as water. The hot
zone of the furnace, comprising at least one heating element within
the hot zone, is surrounded by and defined by, insulation. The
insulation will be described in more detail below. In addition, the
hot zone of the furnace may further comprise a crucible in a
crucible support box atop a crucible support block. In one
embodiment, the hot zone comprises a top heating element,
positioned in the upper region of the hot zone above the crucible,
and at least one side heating element positioned below the top
heating element and along the sides of the hot zone and the
crucible. Any crucible known in the art may be used. The crucible
may be made of various heat resistant materials, for example,
quartz, silica, graphite or molybdenum, can be cylindrical or
square in dimension or tapered, and optionally, may be coated to
prevent cracking of the ingot after solidification. The crucible
support box and crucible support block are typically made of
graphite.
[0023] The crucible containing feedstock is heated by at least one
heating element in the hot zone. Melting of the feedstock
preferably is controlled by regulating the power to the at least
one heating element. Directional solidification of the melt is
achieved through controlled heat extraction from the crucible by
increasing radiant heat losses to the water-cooled chamber, such as
through the bottom of the hot zone. This can be achieved, for
example, by moving the insulation relative to the crucible so as
not to disturb the solid-to-liquid interface of the growing
ingot.
[0024] The insulation that surrounds and defines the hot zone of
the furnace of the present invention can be made of any material
known in the art that possesses low thermal conductivity and is
capable of withstanding the temperatures and conditions in the
furnace, including, for example, graphite. The insulation may
comprise flat panels, for example, when a square crucible is used,
though other insulation dimensions may be used commensurate with
other furnace hot zone shapes, for example, a cylindrical hot zone.
The insulation typically surrounds the feedstock-charged crucible
and at least one heating element, above and on all four sides of
the crucible. Insulation may also be employed below the crucible,
for example, below the crucible support block. The insulation has a
furnace shell side facing the shell of the furnace and a hot zone
side facing the hot zone. Preferably, the shape and dimension of
the insulation panels conform to the shape and size of the crucible
used. For example, where a square crucible of a given dimension is
used to melt feedstock, then square insulation panels at least as
large as the crucible top and sides are used. In one embodiment,
the insulation comprises top and side insulation panels, and
preferably the side insulation is configured to move vertically
relative to a crucible within the hot zone.
[0025] In another embodiment of the present invention, a heat
exchanger may be employed in the furnace, either alone or in
conjunction with insulation configured to be moved relative to a
crucible, to control heat extraction. A gas-cooled heat exchanger,
for example, a helium-cooled heat exchanger, can be arranged
beneath the crucible, to promote solidification of the melted
feedstock.
[0026] The furnace of the present invention further comprises at
least one thermocouple assembly inserted through at least one
thermocouple assembly port in the insulation, which will be
described in more detail below. The thermocouple assembly can
comprise a thermocouple assembly sensor encased in heat-protecting
tubes housed in a protective sheath, for example, made of graphite.
The thermocouple assembly is used to measure the temperature in the
hot zone surrounded and defined by insulation in the furnace and
can be any thermocouple assembly known in the art used to measure
high temperatures typically associated with heating, melting and
resolidifying of feedstock material. At least one thermocouple
assembly is used in one embodiment of the present invention to
measure temperature in the hot zone near the heating element above
the crucible, where feedstock is melted and resolidified to form an
ingot.
[0027] The thermocouple assembly port in the insulation of the
present invention through which the thermocouple assembly is
inserted to measure the temperature in the hot zone increases in
size from the furnace shell side of the insulation to the heating
element side of the insulation. The port opening on the furnace
shell side is shaped and sized to accommodate the thermocouple
assembly. Thermocouple assembly ports can be configured in
insulation at least above and at the sides of the crucible. At
least one port is configured in the top insulation of the furnace
of the present invention to measure temperature near the top
heating element used to melt feedstock in the crucible. The
thermocouple assembly port opening on the heating element side of
the insulation is enlarged in the present invention to expose more
of the surface area of the thermocouple assembly to heat. Specific
embodiments of the shaped thermocouple assembly port are shown in
FIG. 1 and FIGS. 3A-FIG. 6 and discussed below.
[0028] FIG. 1 shows a cross-sectional view of a furnace of the
present invention comprising a furnace shell 1 and hot zone 2
surrounded and defined by insulation 15 having a shaped
thermocouple assembly port 13 wherein the size of the port opening
on the heating element side 17 of the insulation is larger than the
size of the port opening on the furnace shell side 16 of the
insulation. Thermocouple assembly 10 is inserted into thermocouple
assembly port 13 through the furnace shell side 16 of insulation 15
and beyond the heating element side 17 of insulation 15 to near
heating element 18 to measure the temperature in the hot zone
during the melting and resolidification of feedstock, for example,
silicon or alumina oxide-based feedstock, to produce crystalline
silicon or sapphire ingots, respectively. As shown for this
embodiment of the furnace of the present invention, at least one
heating element is positioned above and along side of crucible 11
in crucible box 12 atop crucible support block 14, to melt
feedstock in the crucible.
[0029] FIG. 2 shows a conventional thermocouple assembly and
thermocouple assembly port shape known in the art. Structurally,
thermocouple assembly 20 generally comprises an outer
heat-protecting sheath 24, typically made of graphite, covering
heat-protecting tubes 21 that further enclose and isolate
thermocouple sensor 22. Thermocouple assembly 20 is typically
inserted through a thermocouple assembly port 23 appropriately
configured to the size of the thermocouple assembly and positioned
in close proximity to heating element 28. In general, the size or
diameter of the port opening at the furnace shell side 26 of
insulation 25 generally equals the size of the opening on the
heating element side 27 of the port. Because insulation 25
comprises low thermal conductivity material, for example, graphite,
a large thermal gradient exists across thermal gradient zone 29
from the heating element side 27 of insulation 25 (approximately
1500.degree. C.) to the surface of heating element 28
(approximately 1600-1700 .degree. C.). This large thermal gradient
across a short distance makes thermocouple assembly 20 highly
sensitive to positioning in thermocouple assembly port 23 which can
lead to large variations and decreased repeatability in the
temperatures measured.
[0030] The large variations attributable to inconsistent
thermocouple assembly placement in a large temperature gradient can
surprisingly be minimized using insulation of the present invention
having a shaped thermocouple assembly port. FIG. 3A, FIG. 3B, FIG.
3C and FIG. 3D show embodiments of the present invention in which
the thermocouple assembly port 33 is conically-shaped, exposing
more surface area of thermocouple assembly 30 to the high
temperatures of the hot zone and reducing its sensitivity to
positional changes in thermocouple assembly port 33 by expanding
the temperature gradient zone 39, Referring to FIG. 3A, the opening
of the thermocouple assembly port on the furnace shell side 36 of
insulation 35 is configured to receive a standard size thermocouple
assembly 30, typically approximately one inch in diameter although
any diameter can be used. The thermocouple assembly port 33 has an
opening on the heating element side 37 of insulation 35 that is
larger than the opening on the furnace shell side 36 of insulation
35. The thermocouple assembly port can be enlarged by
countersinking the opening, forming a conical shape that exposes
more of the thermocouple assembly 30 to heat generated by heating
element 38 without significant loss of insulation or insulating
capability. Thus, as shown in FIG. 3A, the enlarged, conical
opening in thermocouple assembly port 33 at the heating element
side 37 of insulation 35 results in more of the surface area of
thermocouple assembly 30 being exposed to high temperatures, thus
expanding temperature gradient zone 39 along its length, from the
tip of thermocouple assembly 30 to the point where insulation 35
meets thermocouple assembly 30.
[0031] Conical shapes may be created in insulation 35 by various
means known in the art, including, for example, by countersinking
with a countersink tool, The cross-sectional area of the conical
opening of thermocouple assembly port 33 on the heating element
side 37 of insulation 35 may vary depending on the desired angle
and the depth of the countersunk port. For example, the port can be
countersunk through at least approximately one-half of the
thickness of the insulation, though other depths are contemplated.
Furthermore, various countersink angles may be used. For example,
the port can be countersunk at an angle of at least about
60.degree., at least about 90.degree., or at least about
120.degree., such as from about 60.degree. to about 120.degree. .
Examples of specific countersink angles are shown in FIG. 3A
(.alpha.=60.degree.), FIG. 3B (.beta.=90.degree.) and FIG. 3C
(.gamma.=120.degree.). Other angles may also be contemplated so
long as they expose more of the surface area of thermocouple
assembly 30 to the heating environment to form an expanded
temperature gradient zone 39 along the length of thermocouple
assembly 30. Furthermore, while the thermocouple assembly port
openings as shown in FIG. 3A, FIG. 3B and FIG. 3C are symmetrical
on each side of the center thermocouple assembly axis, in an
alternative embodiment, the conical shape may also be angularly
asymmetrical in relation to the center axis of the thermocouple
assembly. For example, as shown in FIG. 3D, the thermocouple
assembly port can have an opening having two different countersink
angles, such as .alpha.=60.degree. and .beta.=90.degree. (half of
these angles are shown).
[0032] FIG. 4A and FIG. 4B show additional embodiments of the
present invention in which thermocouple assembly port 43 is
counterbored on the heating element side 47 of insulation 45 to
expose more of the surface area of thermocouple assembly 40 to heat
generated by heating element 48 and to expand the size of
temperature gradient zone 49 in which thermocouple assembly 40
resides. Counterbored shapes may be created by various means known
in the art, including, for example, by counterboring thermocouple
assembly port 43 on the heating element side 47 of insulation 45
with a circular counterbore tool, thereby forming a cylindrical
port opening without significant loss of insulating capability. The
width of the counterbore can vary, as shown by d and d' in FIG. 4A
and FIG. 4B respectively. The port can also be counterbored to
various depths, such as through at least approximately one-half of
the thickness of the insulation, though other depths are
contemplated by the present invention. Other counterbore shapes are
also contemplated by the present invention, including, but not
limited to, square and other polygonal shapes that can be cut into
the heating element side 47 of insulation 45 or produced using
multiple layers of insulation. The alternative shapes can be
created using various cutting means known in the art. In an
alternative embodiment shown in FIG. 6, a hybrid thermocouple
assembly port can be formed wherein the thermocouple assembly port
is first counterbored, such as with a width d, on the heating
element side of the insulation and then countersunk at the various
angles contemplate herein. Thus, various counterbored shapes,
widths and depths are contemplated in the present invention so long
as they result in more thermocouple assembly 40 surface area
exposure to high temperatures and expand temperature gradient zone
49 along its length, from the tip of thermocouple assembly 40 to
the point where insulation 45 meets thermocouple assembly 40.
[0033] FIG. 5A, FIG. 5B and FIG. 5C show alternative embodiments of
the present invention wherein the heating element side 57 of
insulation 55 is countersunk (FIG. 5A and FIG. 5C) or counterbored
(FIG. 5B) to a depth substantially greater than one-half of the
thickness of insulation 55 to expose more of its overall surface
area to heat generated by heating element 58. In particular, FIG.
5A shows a conical-shaped thermocouple assembly port 53 that is
countersunk at a 90.degree. angle (.beta.=90.degree.) from the
heating element side 57 of insulation 55 and extending through to
the furnace shell side 56 of the insulation, with the port opening
on the furnace shell side 56 coinciding with the diameter of
thermocouple assembly 50. The conical shape may be created by
various means known in the art, including, for example, with a
countersink tool. Other angles are also contemplated in the present
invention so long as they expose more of the surface area of
thermocouple assembly 50 to heat generated by heating element 58,
thus expanding the size of temperature gradient zone 59 along its
length, from the tip of thermocouple assembly 50 to the point where
insulation 55 meets thermocouple assembly 50.
[0034] Because the countersunk port of FIG. 5A extends completely
through insulation 55, the present invention also contemplates
using an insulation flange 60 to provide additional structural
support to thermocouple assembly 50 at the furnace shell side 56 of
the insulation and to minimize heat leakage from the hot zone into
the furnace chamber, thereby maintaining insulating capability.
Insulation flange 60 typically comprises the same material used for
furnace insulation, including, for instance, graphite, but may be
made of any material known in the art that possesses low thermal
conductivity and is capable of withstanding the temperatures and
conditions within the furnace. The flange may have various shapes,
dimensions and thicknesses so long as it adequately supports the
thermocouple assembly and minimizes heat leakage from the hot zone.
For example, the insulation flange can be flat and at least about
one-half the thickness of the insulation. The flange may be affixed
to the furnace shell side 56 of the insulation 55 by various means
known in the art or, where the thermocouple assembly 50 is placed
in insulation directly above the crucible, the flange may rest on
the surface of the furnace shell side 56 of the insulation. FIG. 5B
of the present invention also contemplates using a flat insulation
flange 60 to structurally support the thermocouple assembly and
minimize heat leakage from the hot zone where a thermocouple
assembly port is counterbored with a width d and to a depth that is
greater than one-half of the thickness of the insulation. In
another embodiment of the present invention, FIG SC shows that a
solid, conical shaped insulation flange 60 can be used in
conjunction with a countersunk thermocouple assembly port to
provide structural support to thermocouple assembly 50 and minimize
heat leakage from the hot zone, though its use with counterbored or
other shaped thermocouple assembly port shapes is also
contemplated.
[0035] The furnace of the present invention, comprising a shaped
thermocouple assembly port in the insulation that increases in size
from the furnace shell side of the insulation to the heating
element side of the insulation, surprisingly reduces the high
positional temperature measurement sensitivity typically observed
from a thermocouple assembly inserted through the insulation in
conventional furnaces having conventional thermocouple assembly
ports. As a result, small positional movements of the thermocouple
assembly from a starting position result in minimal variation in
measured temperature. FIG. 7 graphically compares the effect of
changes in the vertical position of a top insulation-mounted
thermocouple assembly (TC) on the recorded temperature measurements
for a conventional insulation with a standard thermocouple assembly
port and insulation of the present invention having a shaped
countersunk port (90.degree. angle). In each port, the tip of the
thermocouple assembly was placed at a starting position
approximately 0.5 inches from the top heating element and the
heating element was powered on until a temperature of approximately
1560.degree. C. was reached and maintained. Thereafter, the TC was
retracted in 0.125 inch increments and the temperature measured at
each position. FIG. 7 shows that, after retracting the tip of TC
approximately 0.625 inches in incremental steps from its starting
position, the temperature observed for insulation having the
standard thermocouple assembly port dropped continuously to nearly
40.degree. C. in contrast to a less than 10.degree. C. drop in the
observed temperature when the tip of TC was retracted the same
distance through the shaped thermocouple assembly port in
insulation of the present invention. The results surprisingly show
that the shaped thermocouple assembly port minimizes the variation
in measured temperature associated with the high positional
sensitivity of a thermocouple assembly, compared to conventional
insulation having thermocouple assembly ports typically used in the
art.
[0036] The present invention further relates to insulation
surrounding a furnace hot zone. The insulation is configured to be
arranged between the furnace shell wall and at least one heating
element in the furnace hot zone, and comprises a furnace shell side
and a heating element side and at least one thermocouple assembly
port to accommodate a thermocouple assembly. The cross-sectional
area of the opening of thermocouple assembly port is greater on the
heating element side of the insulation than the furnace shell side
and can be any of those described in more detail above. For
example, the opening can be circular, square or polygonal in shape
on the surface of heating element side and countersunk or
counterbored to form a conical or cylindrical shape
cross-sectionally to at least half the thickness of the
insulation.
[0037] The present invention also relates to a method of producing
a crystalline ingot in a furnace comprising a furnace shell, a hot
zone and insulation surrounding and defining the hot zone wherein
the insulation has at least one shaped thermocouple assembly port.
The method comprises the steps of heating a crucible containing at
least feedstock material in the hot zone of the furnace to a
temperature greater than or equal to 1000.degree. C. The hot zone,
comprising at least one heating element, is surrounded by and
defined by, insulation between the at least one heating element and
the furnace shell of the furnace, and the insulation has a heating
element side facing the heating element and a furnace shell side
facing the furnace shell. The method further comprises the step of
measuring the temperature in the hot zone with at least one
thermocouple assembly inserted through a thermocouple assembly port
in the insulation that is larger in cross-sectional area on the
heating element side of the insulation than on the furnace shell
side of the insulation. The measured temperature is substantially
insensitive to the position of the thermocouple assembly.
[0038] The present invention further relates to a method for
reducing the sensitivity of a thermocouple assembly to temperature
measurement variability associated with its position in the
thermocouple assembly port in the insulation of a furnace. The
method comprises the steps of providing a furnace, furnace shell, a
hot zone within the furnace shell wherein the hot zone comprises at
least one heating element in the hot zone, insulation surrounding
the hot zone having a furnace shell side facing the furnace shell
and a heating element side facing the heating element, at least one
thermocouple assembly, and at least one thermocouple assembly port
in the insulation wherein the thermocouple assembly port comprises
an opening on the heating element side of the insulation and an
opening on the furnace shell side of the insulation, and the
opening on the heating element side of the insulation is larger
than the opening on the furnace shell side of the insulation. The
method further comprises the step of inserting a thermocouple
assembly through a thermocouple assembly port from the furnace
shell side of the insulation to a position between the heating
element and the heating element side of the insulation to measure
temperature in the hot zone.
[0039] The foregoing description of preferred embodiments of the
present invention has been presented for the purposes of
illustration and description. It is not intended to be exhaustive
or to limit the invention to the precise form disclosed.
Modifications and variations are possible in light of the above
teachings, or may be acquired from practice of the invention. The
embodiments were chosen and described in order to explain the
principles of the invention and its practical application to enable
one skilled in the art to utilize the invention in various
embodiments and with various modifications as are suited to the
particular use contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto, and their
equivalents.
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