US 3677329 A
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.
Description (OCR text may contain errors)
United States Patent Kirkpatrick July 18, 1972 ANNULAR HEAT PIPE 1,987,119 1/1935 Long ..219/325 2,820,134 1/1958 Kobayashi... .....165/l05 X  Invent 2 '5" g' verdes 3,327,772 6/1967 Kodaira ..165/1o5 x I 3,229,759 1/1966 Grover... ..l65/105  Assngnee: TRW Inc., Redondo Beach, Calif. I OTHER PUBLICATIONS  Filed: 1970 Deverall, J. Efet 211., High Thermal Conductances Devices,  Appl. No.: 89,705 Los Alamos Scientific Lab., Univ. of Cal., 4/1965, pp. 1, 34,
35. Related US. Application Data Pn'mary Examiner-Albert W. Davis, Jr. )1v1s1on of Ser. No. 797,725, Jan. 31, 1969, Contmua- Attorney Danie| T. Andemn, Jerry A Dimudo am Donald tlon-in-part of Ser. No. 637,193, May 9, 1967, aban- R. Nyhagen doned.
 US. Cl. ..l/l05, 219/326, 219/399  ABSTRACT  Int. F2811 15/00 Spaced inner and outer tubes form a closed annular chamber 58 Field 6: Search ..165/l05; 219/325, 326, 399, whose inner surfaces contain coverings 9f wick material that 219/400 406 are interconnected through vane-like wicks. The wick material transports a vaporizable working fluid from cold areas  References cm where the vapor condenses to warm areas where the fluid vapon'zes. An isothermal working space is produced within UNITED STATES PATENTS the central volume bounded by the inner tube and along its entire length, which may be used to advantage for oven or fur- 2,616,628 1 1/1952 Guild 165/105 x ace applications for providing an isothermal jacket 3,405,299 10/1968 Hall et aL... 3,490,718 l/l970 Vary ..165/ X 7 Claim, 9 Drawing Figures Pmmanauuemz 3.677.329
SHEE 1 OF 4 Milton E. Kirkpatrick INVENTOR.
AGENT PATENTEDJULIBIQIZ 3.671.329
SHEET 2 0F 4 lnslde Hoe! Pipe 7 2-0 sec Outside Heo'Pipe Temperature, C
0 2 4 6 B IO l2 Distance, Inches Milton E. Kirkpatrick INVENTOR.
AGENT Pmmmmwmz 3,677,329,
SHEET 3 0F 4 a. gM
PATENTED JUL 1 8 m2 SHEET 0F 4 Milton E Kirkpatrick INVENTOR.
BY M G.
AGENT ANNULAR HEAT PIPE CROSS-REFERENCE TO RELATED APPLICATION This application is a division of application Ser. No. 797,725, filed Jan. 31, 1969, which in turn is a continuationin-part of application Ser. No. 637,193, filed May 9, 1967, now abandoned.
BACKGROUND OF THE INVENTION I 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 US. 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. t
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 two to three 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 1300 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.
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 as 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 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 EMBODIMENTS 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 fheat 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 detennined 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 l000 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. shows curves of temperatures taken along the length of the furnace 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 end. In contrast, the temperature on the interior surface of the inner tube 22 is uniformly at 720 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 i000 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 em ployed 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 temperature 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 con-. struction 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 furnaceis 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 com-' prise 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 crosssection. 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 preferable that the heater element 580 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. Altematively, 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 diflusion furnace 50b that is similar to that of FIG. 8. In this embodiment, the a 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 it 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.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. In combination:
a tubular member having spaced inner and outer walls and interconnecting end walls defining a closed, evacuated annular chamber surrounding a central opening;
means forming a capillary structure on the inner surface of said outer wall and a capillary structure on the outer surface of said inner wall;
additional capillary means interconnecting said capillary structures and including a plurality of groups of wick elements spacing said capillary structures, said wick elements being circumferentially and longitudinally spaced within said annular chamber to provide continuous fluid flow paths through said chamber;
and said annular chamber being evacuated of all non-condensable gases and being partially filled with a substance which is vaporizable from a liquid phase and is capable of transport through said capillary structures and additional capillary means.
2. A heat pipe comprising:
a tubular member having spaced inner and outer walls and intersecting end walls forming a closed, evacuated chamber including an annular space between said spaced walls;
a capillary structure on the inner surface of said outer wall and a capillary structure on the outer surface of said inner' wall;
additional capillary means interconnecting said capillary structures and including a plurality of groups of wick elements spacing said capillary structures, said wick elements being circumferentially and longitudinally spaced within said annular space to provide continuous fluid flows paths within said chamber; and
a working fluid partially filling said chamber and formed of a substance that is vaporizable from a liquid phase and is capable of transport through said capillary structures and said additional capillary means.
3. A heat pipe comprising:
a pair of radially spaced inner and outer walls extending longitudinally and circumferentially and closed by end walls to form a hermetically sealed chamber including an annular space between said radially spaced walls;
a capillary structure on the inner surface of said outer wall and on the outer surface of said inner wall;
members to form an elongated annular chamber surrounding an open passageway. 5. The invention according to claim 4, wherein said tubular members are circularly cylindrical.
6. The invention according to claim 3, wherein said capillary structures and said wick elements are formed of metal mesh.
7. The invention according to claim 3, wherein said working fluid is a material selected from the group consisting of potassium and lithium.