U.S. patent number 3,651,240 [Application Number 04/797,725] was granted by the patent office on 1972-03-21 for heat transfer device.
This patent grant is currently assigned to TRW Inc.. Invention is credited to Milton E. Kirkpatrick.
United States Patent |
3,651,240 |
Kirkpatrick |
March 21, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
HEAT TRANSFER DEVICE
Abstract
Spaced inner and outer tubes form a closed annular chamber whose
inner surfaces contain coverings of wick material that are
interconnected through vane-like wicks. The wick material
transports a vaporizable working fluid from cold areas where the
vapor condenses to warm areas where the fluid vaporizes. An
isothermal working space is produced within the central volume
bounded by the inner tube and along its entire length, which may be
used to advantage for oven or furnace applications or for providing
an isothermal jacket.
Inventors: |
Kirkpatrick; Milton E. (Palos
Verdes Peninsula, CA) |
Assignee: |
TRW Inc. (Redondo Beach,
CA)
|
Family
ID: |
26215522 |
Appl.
No.: |
04/797,725 |
Filed: |
January 31, 1969 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
637193 |
May 9, 1967 |
|
|
|
|
Current U.S.
Class: |
219/78.02;
219/390; 219/406; 219/540; 118/715; 165/104.26; 219/399; 219/530;
392/394 |
Current CPC
Class: |
F28D
15/0233 (20130101); F27D 11/06 (20130101); F28D
15/046 (20130101); H05B 3/64 (20130101); F27D
11/02 (20130101); F28F 2200/005 (20130101) |
Current International
Class: |
F28D
15/04 (20060101); F27D 11/00 (20060101); F27D
11/06 (20060101); F27D 11/02 (20060101); H05B
3/64 (20060101); F28D 15/02 (20060101); H05B
3/62 (20060101); H05b 003/66 (); F28d 015/00 () |
Field of
Search: |
;219/399,406,530,540,390,413 ;165/105 ;13/22,1,24 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
RCA, The Heat Pipe, RCA Electronic Components and Devices Direct
Energy Conversion Dept., Lancaster, Penn., RCA Ref. 994-619, cover
page and pp. 6, 7; Feb., 1967.
|
Primary Examiner: Davis, Jr.; Albert W.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of application Ser. No.
637,193, filed May 9, 1967, now abandoned.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed and defined as follows:
1. A diffusion furnace, comprising:
a tubular heat pipe having radially spaced inner and outer walls
formed about a central longitudinal opening extending through both
ends thereof and end walls closing the ends of said radially spaced
walls to form a closed annular chamber;
said heat pipe including capillary means covering substantially all
radially opposing internal surfaces of said walls;
a processing tube disposed within the opening of said tubular heat
pipe substantially coextensively therewith and defining an
elongated processing region where semiconductor bodies may be
subjected to diffusion process;
said heat pipe containing a working fluid operative at a
temperature range were said semiconductor bodies are subject to
diffusion;
heating means disposed externally of said tubular heat pipe and
extending along a major portion of the outer one of said walls for
heating the same to the working temperature, whereupon the entire
surface thereof surrounding said processing tube becomes isothermal
and a flat temperature profile is established within and along the
entire length of said elongated processing region that is
coextensive with said heat pipe; and
means for transporting said working fluid in its liquid phase from
the capillary means on said inner wall to the capillary means on
said outer wall, said last mentioned means including additional
capillary means extending longitudinally between said end walls and
radially between and interconnecting the capillary means on said
radially opposing wall surfaces.
2. The invention according to claim 1, wherein said heating means
comprises a helical heating coil wound around at least a major
portion of the length of said heat pipe.
3. A diffusion furnace, comprising:
an elongated outer casing;
a lining of thermal insulation within said outer casing and formed
with a central horizontally extending longitudinal cavity;
an open-ended tubular heat pipe nested within said cavity;
a processing tube nested within said heat pipe, coextensive
therewith, and formed with a horizontally extending opening through
which a boat-load of semiconductor bodies may be inserted for
disposition along the entire length of said processing tube covered
by said heat pipe;
said heat pipe including radially spaced elongated walls
surrounding said processing tube, end walls closing the ends of
said elongated walls and capillary means covering the radially
facing surfaces of said elongated walls;
said heat pipe containing a working fluid operative at a
temperature range where said semiconductor bodies are subject to
diffusion;
a heating element supported adjacent to and extending over at least
a major outer surface portion of said heat pipe for applying heat
thereto;
means for transporting said working fluid in its liquid phase from
the capillary means on the inner one of said radially spaced walls
to the capillary means on the outer one of said radially spaced
walls;
said last mentioned means including additional capillary means
extending radially between and interconnecting the capillary means
on said radially facing wall surfaces and longitudinally between
said end walls;
temperature sensing means thermally coupled to said heat pipe for
sensing the temperature thereof;
power supply means connected with said heating element for
furnishing heating power thereto; and
power controller means coupled between said temperature sensing
means and said power supply means for controlling the supply of
power to said heating element in response to signals from said
temperature sensing means so as to maintain the temperature of said
heat pipe substantially constant.
4. Furnace apparatus, comprising:
an outer casing;
thermal insulation covering the interior of said casing and forming
a central, elongated open cavity;
an open-ended tubular heat pipe disposed within said cavity;
said heat pipe having radially spaced walls and interconnecting end
walls forming a closed annular chamber surrounding a longitudinal
passageway, and a capillary structure covering the internal
surfaces of said radially spaced walls;
said heat pipe containing a working fluid of a metal that is
vaporizable from the liquid phase within the temperature range in
which said heat pipe is intended to operate;
a heating element disposed adjacent to and extending over at least
a major portion of the outer surface of said heat pipe for applying
thermal energy thereto;
means for transporting said working fluid in its liquid phase from
the capillary structure of the inner one of said radially spaced
walls to the capillary structure on the outer one of said radially
spaced walls, said last mentioned means including additional
capillary means extending radially between and interconnecting the
capillary structures on said radially spaced walls and
longitudinally between said end walls;
energy source means for applying heating energy to said heating
element; and
temperature control means in circuit with said energy source means
for sensing the temperature of the said heat pipe so as to modulate
the energy supplied to said heating element by said energy source
in such a manner as to maintain the temperature of said heat pipe
substantially constant.
5. In combination:
a thermal transfer device having radially spaced walls and
interconnecting end walls forming a closed, evacuated, annular
chamber surrounding a central opening;
said annular chamber being evacuated of all non-condensable gases
and being partially filled with a metallic substance that is
vaporizable from the liquid phase within the temperature range in
which said thermal transfer device is intended to operate;
capillary means covering substantially all internal surfaces of
said radially spaced walls within said chamber;
heating means disposed externally of said chamber and thermally
coupled to at least a portion of one of said walls that is covered
by said capillary means, whereby thermal input energy received from
said heating means is uniformly distributed throughout said chamber
to establish substantially isothermal conditions therein; and
means for transporting said metallic substance in its liquid phase
from the capillary means on the other one of said radially spaced
walls to the capillary means on said one wall, said last mentioned
means comprising additional capillary means interconnecting the
capillary means on said radially spaced walls intermediate said end
walls and including a plurality of groups of wick elements spacing
said capillary means, said wick elements being circumferentially
and longitudinally spaced within said annular chamber to provide
continuous fluid flow paths through said chamber.
6. The invention according to claim 5, wherein said capillary means
are arranged to provide capillary fluid transport and return of
said substance in its liquid phase.
7. THe invention according to claim 5, wherein said heating means
extends along at least a major portion of the length of said
chamber.
8. The invention according to claim 5, wherein said heating means
is uniformly distributed along the length of said chamber.
9. The invention according to claim 5, and further including means
mounting said thermal transfer device with its annular chamber
extending horizontally.
10. The invention according to claim 5, and further including an
insulation sheath surrounding said device and heating means.
11. The invention according to claim 5, wherein said heating means
comprises a helical heating coil wound around said device and
traversing the same longitudinally.
12. The invention according to claim 5, wherein said heating means
comprise sinuous electrical windings extending transversely to the
longitudinal extent of said device.
13. The invention according to claim 5, wherein said heating means
comprise sinuous electrical windings extending parallel to the
longitudinal extent of said device.
14. In combination:
a thermal transfer device having radially spaced walls and
interconnecting end walls forming a closed, evacuated, elongated
annular chamber surrounding a central working volume adapted to
receive a workpiece;
said annular chamber being evacuated of all non-condensable gases
and being partially filled with a metallic substance that is
vaporizable from the liquid phase within the temperature range in
which said thermal transfer device is intended to operate;
means forming a capillary structure covering substantially all
radially opposing internal wall surfaces of said annular
chamber;
additional capillary means extending radially between and
interconnecting the capillary structure on said opposing wall
surfaces, said additional capillary means including a plurality of
wick elements extending longitudinally and radially and spaced
longitudinally within said annular chamber to provide continuous
fluid flow paths through said chamber;
and heating means disposed externally of said chamber and thermally
coupled to at least a portion thereof that is coextensive with said
capillary structure, whereby thermal input energy is uniformly
distributed throughout said chamber to establish an isothermal
environment for a workpiece to be inserted within said central
working volume.
15. The invention according to claim 14, wherein said heating means
extends along at least a major portion of the length of said
chamber.
16. In combination:
a thermal transfer device having radially spaced walls and
interconnecting end walls forming a closed, evacuated, annular
chamber surrounding a central opening;
said annular chamber being evacuated of all non-condensable gases
and being partially filled with a metallic substance that is
vaporizable from the liquid phase within the temperature range in
which said thermal transfer device is intended to operate;
capillary means covering substantially all internal surfaces of
said radially spaced walls within said chamber;
heating means disposed externally of said chamber extending
circumferentially therearound and thermally coupled to at least a
major portion of one of said walls covered by said capillary means,
whereby thermal input energy received from said heating means is
uniformly distributed throughout said chamber to establish
substantially isothermal conditions therein; and
means for transporting said metallic substance in its liquid phase
from the capillary means on the other one of said radially spaced
walls to the capillary means on said one wall, said last mentioned
means comprising a plurality of groups of wick elements
interconnecting and spacing said capillary means intermediate said
end walls and circumferentially and longitudinally spaced within
said annular chamber to provide continuous fluid flow paths through
said chamber.
17. In combination:
a heat exchanger having radially spaced walls and interconnecting
end walls forming a closed, evacuated, elongated annular chamber
surrounding a central elongated opening;
said annular chamber being evacuated of all non-condensable gases
and being partially filled with a metallic substance that is
vaporizable from the liquid phase within the temperature range in
which said heat exchanger is intended to operate;
heating means disposed adjacent and thermally coupled to at least a
portion of one of said radially spaced walls for heating the
same;
means forming a capillary structure for said substance and covering
the internal surface of at least the portion of said wall that is
adjacent said heating means; and
means for transporting said substance in its liquid phase from the
other one of said radially spaced walls to said capillary
structure, said last mentioned means comprising a plurality of
groups of capillary elements extending intermediate said end walls
and circumferentially and longitudinally spaced within said annular
chamber.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to heat transfer devices, and particularly
to devices employing capillary fluid transport, which are of such
configuration that lends itself especially to oven or furnace
applications.
2. Description of the Prior Art
The concept and art of building reflux boilers is well developed
and dates back to papers on the subject during the 1930's. A "heat
pipe" works on the principle of a reflux boiler and is extremely
efficient in terms of transferring large thermal heat fluxes.
Example of "heat pipe" devices are described in U.S. Pat. Nos.
3,152,774 and 3,229,759, issued to T. Wyatt and G. M. Grover,
respectively. The basic "heat pipe" is a closed tube which has a
layer of porous wick material attached to the interior surface of
the tube wall. The tube or pipe is partially filled with a fluid,
the specific fluid being determined by the temperature range
desired, which wets the porous wick material and spreads throughout
the wick material by capillary forces.
When a sufficient heat flux is applied to any point on the surface
of the pipe, liquid will be vaporized. Energy equivalent to the
heat of vaporization is carried away from the high heat flux region
by the vapor that migrates throughout the interior regions of the
pipe. The vapor will recondense on any and all interior surfaces
which are at temperatures below that of the vaporizing surface,
thereby giving up the heat of vaporization to all cooler
surfaces.
The recondensed fluid is then transported by capillary forces back
to the vaporization region, or high heat flux input zone, to
continue the closed loop process of transporting and delivering
thermal energy to any and all cool regions of the pipe. As a result
of this action, the "heat pipe," although heated only in one small
region, quickly becomes an isothermal surface; i.e., all surface
temperatures on the pipe are equal or nearly equal no matter what
the distribution of heat flux input may be.
Inasmuch as the present invention may advantageously be used to
provide a diffusion furnace for the semiconductor industry, a brief
description of such diffusion furnaces will be given. In the
present state of the art, a diffusion furnace has a long processing
tube 2 to 3 feet in length and several inches in diameter. The
processing tube is surrounded along its length by a long helical
heating coil, which is divided into three coil portions, namely a
long central portion and two shorter end portions. The three coil
portions are separately supplied with electrical heater power so as
to produce three separately thermostatically controllable heating
zones within the processing tube. The three zones are necessary to
achieve a flat temperature profile along the longest possible
length of the processing tube. A flat temperature profile is
necessary to assure that all the semiconductor wafers, which are
placed in a boat within the processing tube, will be subjected to
the same thermal diffusion processing conditions. The diffusion
process consists of introducing a gaseous impurity or dopant
material into the processing tube while the boatload of
semiconductor wafers are heated at about 1,300.degree. C.
Despite such an elaborate three zone heating arrangement,
substantially less than the entire length of the processing tube
attains a flat temperature profile. Furthermore, some rather
complex electrical control circuitry is required to control or
modify the temperatures of the three zones so that the furnace not
only attains a flat temperature profile but also maintains it under
different boatload conditions.
Reference may be had to the following U.S. Patents for fuller
descriptions of diffusion furnaces and the problems associated
therewith:
3,291,969 B. J. Speransky et al. Dec. 13, 1966
3,299,196 C. A. Lasch, Jr., et al. Jan. 17, 1967
3,311,694 C. A. Lasch, Jr. March 28, 1967
3,370,120 C. A. Lasch, Jr. Feb. 20, 1968
3,385,921 G. P. Hampton May 28, 1968
3,387,078 W. S. Montgomery Jr., et al. June 4, 1968
3,396,955 R. G. Martinek Aug. 13, 1968
SUMMARY OF THE INVENTION
The present invention resides in a uniquely configured structure
utilizing the basic "heat pipe" concept, and in the recognition
that such structures advantageously can be used and ought to be
used in certain types of furnaces, such s diffusion furnaces.
Interior surfaces of an annular pipe are provided with a porous
wick material, such as sintered metals, wire screens, or other
porous compacts, to provide complete interconnection of fluid flow
paths. When charged with a working fluid which wets the wick
material and has a suitable vapor pressure matching the desired
temperature range of interest, the isothermal annular "heat pipe"
may be used for a variety of heat transfer applications. In
particular, when employed in a diffusion furnace, it will produce a
flat temperature profile along the entire length of the processing
tube or chamber by the use of a single electrical heater coil and a
single temperature controller .
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 is a partially broken away diagrammatic view in perspective
of a tube furnace employing a structure in accordance with the
invention;
FIG. 2 is a side view, partly in section, of the structure;
FIG. 3 is a sectional view taken along line 3--3 of FIG. 2;
FIG. 4 is a diagrammatic view showing an alternative form of heater
for the structure of FIG. 2;
FIG. 5 is a graph of curves showing the temperature, as a function
of distance along the furnace of two different regions thereof;
FIG. 6 is a perspective view, with portions removed, of a diffusion
furnace employing an annular heat pipe according to the
invention;
FIG. 7 is a cross-sectional view of the furnace assembly of FIG. 6;
and
FIGS. 8 and 9 are perspective views of alternative forms of
diffusion furnaces employing annular heat pipes of rectangular
cross section.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown an oven or furnace 10 provided
with a central isothermal working space 12 formed within an annular
"heat pipe" 14. An electrical heater coil 16 is wound around one
end of the "heat pipe" 14 and receives electrical power from a
voltage source 18. The heater coil 16 may be embedded in a thermal
insulation sheath 20 that surrounds the "heat pipe" 14 along its
length. The insulation sheath 20 serves to minimize heat loss from
the "heat pipe" 14 to the surrounding atmosphere.
As shown more clearly in FIGS. 2 and 3, the "heat pipe" 14 includes
concentric inner and outer cylindrical metal tubes 22 and 24
respectively. The space between the tubes 22 and 24 forms an
annular chamber 25. The surfaces of the tubes 22 and 24 disposed
within the annular chamber 25 are covered with linings 26 and 28 of
porous wick material. The two wick linings 26 and 28 are spaced
apart and joined together by short spacer elements 30 of wick
material that are spaced along the length of the tubes 22 and
24.
The annular chamber 25 is closed at both ends by cover plates 32,
such as the one shown in FIG. 1, which leave the isothermal working
space 12 open for easy access from the outside. The annular chamber
25 is evacuated of non-condensable gases, such as air, and contains
a vaporizable fluid 34 of sufficient quantity to wet the entire
wick material by capillary action. The specific fluid depends upon
the operating temperature desired for the "heat pipe" 14.
The wick material for the linings 26 and 28 and spacer elements 30
may be in the form of sintered metal, wire screens, or other porous
compacts having voids or openings of capillary size and capable of
transporting the vaporizable fluid 34.
In the operation of the furnace 10, the heater coil 16 is energized
to heat the portion of the annular "heat pipe" 14 surrounded
thereby. The fluid 34 heated thereby vaporizes and the vapor
carries away from the high heat flux region thermal energy
equivalent to the heat of vaporization. The vapor migrates through
the annular chamber 25 where it condenses on all interior surfaces
that are below the temperature of the vaporizing surface, thereby
giving up the heat of vaporization to and raising the temperature
of all the cooler surfaces. Continuous vapor flow paths are
provided along the annular extent of the annular chamber 25 by
means of the linear spacing between the spacer elements 30. The
condensed fluid 34 is then transported by capillary action through
the wick material from these condensing regions to the vaporizing
region or high heat flux input zone, where the fluid 34 again
vaporizes.
By means of this closed loop process, thermal energy supplied by
the heater coil 16 is transported and delivered to any and all
cooler interior regions of the chamber 25. The result is that the
entire surface of the "heat pipe" 14 quickly becomes an isothermal
surface when operating in the temperature range specified by the
working fluid, and the volume within the isothermal working space
12 of the furnace 10 is uniform in temperature along the entire
length of the "heat pipe" 14.
For a specific working fluid, there is a range of equilibrium
temperatures over which the device of this invention will provide
isothermal conditions. The lower limit of the equilibrium
temperature range is determined by the thermodynamic properties of
the working fluid, namely the vapor pressure and the heat of
vaporization. The upper limit of the equilibrium temperature range
is determined by the mechanical ability of the device to withstand
the positive pressures of the vapor relative to the surrounding
atmosphere.
The working space 12, being devoid of fluid 34, can be used as an
oven to process various articles of manufacture, such as
semiconductive devices, without danger of contamination by the
working fluid 34. In addition, the furnace 10 may be used to
provide an isothermal environment for various components requiring
uniform thermal distribution, with the oven shaped in conformity
therewith. Accordingly, whereas the furnace 10 has been shown as
having a circular, cylindrical shape, it may have a rectangular
cross section or even a complex cross-sectional shape.
In accordance with an exemplary operative embodiment, the tubes 22
and 24 consisted of stainless steel cylinders having lengths of 12
inches, inside diameters of 1.5 inches and 1.9 inches respectively
and outside diameters of 1.6 inches and 2.0 inches respectively.
The wick material was fabricated from four multiple layers of 100
mesh stainless steel screen. The vaporizable fluid was potassium
metal.
For applications where temperatures in excess of 1,000.degree. C.
are required, such as in the treatment of semiconductor devices,
working fluids such as lithium or other liquid metals having the
desired vaporization temperature can be employed. For applications
requiring high operating temperatures it will prove advantageous to
utilize silicon carbide rods as heating elements, rather than
heater coils. In FIG. 4, for example, two or more such rods 35 may
be arranged side by side beneath the annular "heat pipe" 14 as
shown. The rods 35 may be connected in parallel with the voltage
source 18.
FIG. 5 shows curves of temperatures taken along the length of the
furnace 10 at two different regions thereof. Curve 36 refers to the
region indicated in FIG. 1 by dashed line 38 as occupying the space
adjacent to the inside surface of the insulation sheath 20. Curve
40 refers to the region on the interior surface of the inner tube
22; that is, the surface that bounds the central working space 12.
Temperature is plotted along the ordinate and distance along the
furnace 10 is plotted along the abscissa. It is seen that the
temperature external of the annular "heat pipe" 14 increases from
700.degree. C. on one end adjacent to the heater coil 16 to a
maximum of over 780.degree. C. just 2 inches away, and then drops
to below 600.degree. C. at the opposite end. In contrast, the
temperature on the interior surface of the inner tube 22 is
uniformly at 720.degree. C. along the entire length.
The present invention provides special advantages when used for
processing semiconductor devices. For example, in the art of
semiconductor device manufacture, it is necessary to heat wafers of
silicon in the presence of dopant materials in a furnace or oven at
temperatures of the order of 1,000.degree. C. With furnaces in
present use, the temperature is fairly uniform in the central
portion but drops substantially at the ends. As a result, about 60
percent of the furnace length is unusable. With the present
invention, substantially the entire length of the furnace is
uniform in temperature, and a much greater length of the furnace
zone may be used for treating semiconductor devices. Consequently,
in a particular application, the equivalent power consumption for
the processing can be significantly reduced.
Furnaces presently in use utilize several electrical heaters
distributed along the furnace length, and each of the heater coils
may be individually thermostatically controlled. Maintenance
problems arise from the fact that failure can occur from one of the
number of heating coils and control circuits. Maintenance problems
as well as systems costs are reduced in the present invention in
that only a single heater source and control circuit are
required.
The present invention provides additional special advantages when
used for elevated temperature mechanical property testing. In the
art of property testing, a furnace is employed to heat the subject
specimen. Furnaces in present use in the art, employ a multiplicity
of heater coils along the length of the furnace which are
individually controlled and adjusted to provide semi-uniform
temperature over the active region. When adjusted, such a furnace
will provide temperatures uniformity of several degrees variance
over the length of interest. During the test sequence, any change
in heat balance due to changing test conditions will effect and
degrade the thermal uniformity within the furnace volume. The
present invention eliminates the need for any manual or
semiautomatic adjustment of the position of thermal input or
temperature uniformity within the volume of the isothermal working
space. A single automatic temperature control is thus all that is
required to maintain uniformity, throughout the working volume,
over any desired temperature within the working range of the heat
transfer fluid.
The invention will now be described as applied to the construction
of a diffusion furnace for processing semiconductors. Referring to
FIGS. 6 and 7, there is shown a diffusion furnace consisting of a
furnace assembly 50 and a power control system 52. The furnace
assembly 50 includes an outer cabinet or casing 54 lined with
thermal insulation 56. Extending longitudinally and centrally of
the casing 54 and supported by the insulation 56 is a helical
heating coil 58 that is wound around a cylindrical ceramic support
tube 60.
The heating coil 58 serves the same function as the heater coil 16
of FIG. 1, namely that of supplying heating energy to the annular
heat pipe 20, which in this diffusion furnace is supported within
the support tube 60. To this end, the heating coil 58 may be formed
of high resistance wire that will heat to a high temperature when
supplied with electrical current of 60 cycle frequency.
Alternatively, the heating coil 58 may comprise metal tubing of
high electrical conductivity which, when furnished with radio
frequency current, will cause the annular heat pipe 20 to heat up
by electromagnetic induction.
A cylindrical processing tube 62 made of suitable material such as
quartz is supported within the annular heat pipe 20 and extends
longitudinally through both ends thereof. The processing tube 62 is
provided with an open end 64 through which may be inserted a boat
66 containing wafers 68 of silicon or other semiconductor. The
other end of the processing tube 62 is provided with a smaller
opening 70 through which a suitable gaseous dopant material may be
introduced into the processing tube for diffusion into the
semiconductor wafers 68.
It will be observed that the wafer-loaded boat 66, or a plurality
thereof arranged end to end, may extend substantially the entire
length of the annular heat pipe 20 and even beyond the extremities
of the heating coil 58. The reason for this is that the effective
heating zone for heating the semiconductor wafers 68 is determined
by the interior of the annular heat pipe 20 rather than the heating
coil 58. The effective heating zone has a flat temperature profile
along the entire length of the annular heat pipe 20.
The power control system 52 includes a power supply 72 for
furnishing electrical energy to the heating coil 58. The power
supply 72 is connected to the heating coil 58 through a controller
74. A thermocouple 76 contacting the annular heat pipe 20 is
connected to the controller 74. The thermocouple 76, which may be
supported in a tube 77, as shown in FIG. 7, senses changes in heat
pipe temperature above and below a given set point for which
circuits in the controller 74 are set. The circuits in the
controller 74 operate to turn on power to the heating coil 58 when
the temperature falls below the set point and to turn off power to
the heating coil 58 when the temperature rises above the set
point.
Temperature control systems for diffusion furnaces are well known
in the art and therefore the controller 74 requires no further
detailed description. It will suffice to say that the controller 74
may be one of the kind disclosed in U.S. Pat. No. 3,291,969 issued
Dec. 13, 1966, to B. J. Speransky et al., for controlling the
central zone B of the heating coil 11 of that patent.
The power supply 72 may be designed to furnish 60 cycle alternating
current to the heating coil 58 if the latter operates on the
principle of resistance heating. On the other hand, if the heating
coil 58 is an electromagnetic induction heating coil, the power
supply 72 may be designed to furnish radio frequency current to the
heating coil 58.
It will be seen that the diffusion furnace thus described is much
simpler in the construction of its furnace assembly 50 and its
control system 52 than the corresponding structure of conventional
diffusion furnaces. The inclusion of an annular heat pipe according
to the invention permits the use of a single heater coil instead of
three and a single temperature control system instead of three. The
annular heat pipe 20 provides a flat temperature profile along its
entire interior length, thereby increasing the capacity of the
semiconductor processing zone. Furthermore, whenever it is desired
to change the temperature of the furnace, the temperature will rise
or fall uniformly along the entire length of the heating zone.
Referring now to FIG. 8, there is shown a modified form of
diffusion furnace assembly 50a which has a rectangular cross
section. Thus, the processing tube 62a and annular heat pipe 20a
are rectangular instead of circular. A heater element 58a of flat
sinuous form is mounted adjacent to a surface of the heat pipe 20a,
such as the top surface thereof. The windings of the heater element
58a extend at an angle to the longitudinal axis of the heat pipe
20a and processing tube 62a. With this flat configuration, it is
preferably that the heater element 58a be of the resistance wire
heating type. The heater element 58a may be designed for direct
thermal contact with the annular heat pipe 20a. For example, the
heater element 58a may comprise a central current carrying
conductor 78 spaced from an outer metal sheath 80 by electrical
insulation 82. Alternatively, for convenience in assembly or
disassembly, the heater element 58a may be mounted on a flat
support member 60a, which itself is mounted on the heat pipe 20a.
For ease in illustration, the remaining parts of the furnace
assembly 50a are not shown, it being understood that it contains
similar parts corresponding to the insulation 56 and casing 54 of
FIGS. 6 and 7. Likewise, a control system 52 similar to that
already described in connection with FIGS. 6 and 7 may be used with
the rectangular furnace assembly 50a.
An additional advantage of incorporating an annular heat pipe in a
diffusion furnace is apparent in FIG. 8. That is, the heating
element 58a need not envelope the processing tube 62a, as is
required in conventional diffusion furnaces. It is sufficient to
apply all the required thermal input energy to a localized area of
the heat pipe 20a, such as the top surface or a portion thereof,
and through the operation of the heat pipe 20a, the entire surface
area thereof will attain an isothermal condition. Furthermore, it
is not necessary, in the design of the heater element 58a, that
great regard be given to precise spacing between turns or windings,
or in uniformity in the lengths of the windings.
FIG. 9 shows another form of rectangular diffusion furnace 50b that
is similar to that of FIG. 8. In this embodiment, the heater
element 58b has sinuous windings that extend parallel to the
longitudinal axis of the heat pipe 20b and process tube 62b. The
heater element 58b, which may be mounted on a support tube 60b, may
cover all four sides of the heat pipe 20b both longitudinally and
circumferentially, as shown, or it may cover a less number of sides
or only portions thereof.
A principal advantage of a rectangular configuration for the
diffusion furnace assembly is that is minimizes the cross-sectional
area of the processing tube required for any boat and semiconductor
load configuration. This minimizes the heat loss from the open ends
of the furnace and improves the temperature profile thereof.
* * * * *