WO1991002438A1

WO1991002438A1 - Modular heater

Info

Publication number: WO1991002438A1
Application number: PCT/US1990/004230
Authority: WO
Inventors: Larry Fiel; Bobby Mayfield
Original assignee: Union Oil Company Of California
Priority date: 1989-07-31
Filing date: 1990-07-27
Publication date: 1991-02-21
Also published as: KR910004065A

Abstract

A modular resistance heater (10) is constructed of elongated elements (12) and connectors (14) which together provide a continuous electrical current path through the heater (10). The heater (10) is preferably fabricated of machined graphite parts which can be press-fit together.

Description

MODULAR HEATER

FIELD OF THE INVENTION

This invention relates to the field of semi¬ conductor processing. In one of its more particular aspects, this invention relates to a modular heater. In another of its more particular aspects, this inven¬ tion relates to a heater which is constructed of easily replaceable parts, and which is especially adapted for processing semiconducting materials

BACKGROUND OF THE INVENTION

During the fabrication of semiconducting materials, for example, in the formation of a single crystal of silicon, it is necessary to subject the semiconducting materials to high temperatures for extended periods of time in a vacuum furnace or other inert atmosphere furnace. Typically the semiconducting material is heated in a quartz-lined graphite crucible surrounded by a resistance heater. Resistance heaters are fabricated of electrically conductive materials such as graphite and are fitted with terminal blocks for passing an electrical current through the heater. Such heaters generally have slotted walls which permit the free flow of gases and ensure even heating. A common design for resistance heaters is the picket- fence heater, which has slotted sides in the form of a picket fence arranged in a cylindrical configuration.

Although the picket-fence heater has been generally satisfactory, it has been necessary to fabricate such heaters from a single block of material and to then use a threaded joint or clamp arrangement to connect electrical power to the heater. Heaters fabricated from single blocks of material have dis¬ played variation in electrical resistivity with orien¬ tation, due to anisotropy, as well as with location within the material, due to non-uniformity of the piece of graphite from which the heater is fabricated. Such resistivity variations lead to non-uniform temperature profiles in heaters fabricated in this manner. In addition, picket-fence heaters and other heaters previ¬ ously used have been relatively fragile devices. Repair has been almost impossible because of the method of fabrication, necessitating replacement of the entire heater when the heater is broken. It would be desira¬ ble to provide a heater which is fabricated in a way such that electrical resistivity can be more easily controlled and parts easily repaired or replaced.

SUMMARY OF THE INVENTION

The present invention provides a resistance heater which has the advantage of being fabricated in such a manner that the parts thereof are readily re¬ placeable. Replacement can be accomplished using common tools and does not require any special skills. The heater of the present invention is provided in modular form.. It includes a plurality of non-contigu¬ ous, elongated elements, such as rods or bars, a plu¬ rality of non-contiguous connectors, such as connector blocks, adapted to connect at least two adjacent ele¬ ments, and terminal means for application of an elec¬ trical potential to the connected elements and connec¬ tors. The elements are connected by the connectors in a way such that a continuous electrical current path is provided from the terminal means through the connected elements and connectors. The heater formed by connect¬ ing the elements and connectors can have a circular, oval or rectangular cross-section or can be provided in any other desired configuration.

In a preferred embodiment, the heater of the present invention comprises

(a) a plurality of essentially identically sized and shaped, essentially straight electrically conduc¬ tive elongated elements;

(b) a plurality of essentially identically sized and shaped, electrically conductive connector blocks, each connector block containing two essentially identi¬ cal apertures, each of which is sized and shaped to matingly engage an end of one of said elongated ele¬ ments, the number of. said connector blocks being two less than the number of said elongated elements;

(c) two terminal connector blocks containing terminals for receiving electric current but otherwise essentially identical to said connector blocks, not in physical contact with any of said connector blocks; .

wherein each of said connector blocks and terminal connector blocks matingly engages a pair of said elongated elements in said apertures and each end of each elongated element is matingly engaged into one of said apertures, with no two connector blocks or terminal connector blocks being matingly engaged to the same pair of elongated elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood by reference to the accompanying drawings where like numerals refer to like elements and in which

Figure 1 is a perspective view of the modular heater of the present invention.

Figure 2 is a perspective view showing the connection between a rod, a connector block, and a terminal block comprising the modular heater of the present invention.

Figure 3 is a cross-section of a rod and part of two connector blocks of the modular heater of the present invention taken along the line 3-3 of Figure 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The heater of the present invention is fabri¬ cated of materials which are sufficiently durable to be readily removable for replacement or modification. In the preferred embodiment of the invention, a modular heater is comprised of a plurality of rods and blocks which are connected by press-fit to form an open cylin¬ drical configuration. Two of the blocks are provided with terminals for providing a source of electrical potential to the heater. Preferably, electrical termi¬ nals are provided by means of to diametrically opposed special terminal blocks which are substituted for two of the connector blocks forming the electrical path for the heater. If three-phase electrical potential were used, three of the connector blocks would be replaced by terminal blocks, and the geometry of the heater would be changed accordingly as is known in the art.

The heater of the present invention is pref¬ erably fabricated of graphite. Graphite, which is electrically conductive, has the desired electrical resistivity required for a resistance heater. In addition, graphite is a relatively strong and durable ^• material. However, graphite tends to be somewhat brittle. Therefore, replacement of graphite parts frequently becomes necessary. In the heater of the present invention, replacement of fragile parts is facilitated by the modular construction of the heater. It is preferred that the parts be press-fitted togeth¬ er. However, other methods for connecting the parts of the heater can be utilized as well. For example, the heater elements and heater connector blocks or terminal blocks can be connected using graphite screws or bolts, if desired. In utilizing graphite parts, it is impor¬ tant that the same coefficient of thermal expansion be required of all parts in order to ensure proper func¬ tioning of the heater and avoid unnecessary damage.

As pointed out above, graphite has the advan¬ tages of providing the properties desired for the heater. Among other advantages, it is known that graphite resists chemical attack, can be purified to contain vese^ low ash levels, i.e., less than about 5 ppm ash, and can be fabricated to precise dimensions. Graphite pajrts are dimensionally stable, thereby making press-fit connections in constructing the heater of the present invention feasible. The thermal and electrical conductivities of. graphite make it an ideal material for use as the material of construction of the heater.

Various grades of graphite are commercially available. A particularly desirable material is a fine particle, high strength, isotropic graphite available from POCO Graphite, Inc. a subsidiary of UNOCAL Corpo¬ ration, Decatur, Texas, which is sold as POCO DFP-2. Electrical resistivity of this material is in the range of about 400 to about 600 t ohm-in. Other grades of -graphite having electrical resistivities of about 200 to about 1000 ϊ ohm-in. can be used as well.

Referring to the drawings, Fig. 1 depicts the heater of this invention. The numeral 10 represents the heater, which consists of a plurality of rods 12 and a plurality of connector blocks 14, two of which are terminal blocks 16. As shown in Figure 1, the rods 12 and connector blocks 14 including terminal block 16 are connected by press-fit in an alternate fashion. The connection is more particularly seen in Figure 2, wherein block 14a is shown connected to rod 12a, which in turn is connected to terminal block 16a. Aperture 18a in terminal block 16a is shown with rod 12b removed and separated from terminal block 16a, and connector block 14b. The lower end of rod 12b is adapted to press-fit into aperture 18a in terminal block 16a and the upper end of rod 12b is adapted to be press-fit into aperture 18b in connector block 14b. Connector block 14b also contains aperture 18c. Rod 12c is shown connected to block 14c at one end thereof. The other end of rod 12c is adapted to be press-fit into aperture 18c in connector block 14b. Connector block 14c is also shown connected to rod 12d which is also fitted into aperture 18d in connector block 14d. When the parts shown in Figure 2 are connected, a continuous path for electrical current is provided through connec¬ tor terminal block 16a, rod 12b, connector block 14b, rod 12b, connector block 14c, rod 12d and connector block 14d. The current path continues in similar fashion, traversing the heater in turn through blocks 14 and rods 12 to the other terminal block. At termi¬ nal block 16a, the current path splits, part of the current following the path described above, the other part of the current flowing through connector block 16a, rod 12a, thence through block 14a, traversing the heater in turn through blocks 14 and rods 12 to ^"the other terminal block. Figure^" 3 shows a rod 12 connected at either end to connector blocks 14.

Although shown fabricated of rods and connec¬ tor blocks -of identical sizes and shapes (except for the two terminal blocks) , the modular heater of the present invention can utilize different sized or dif¬ ferent shaped elongated elements and connectors if desired. In addition, although in the illustrated preferred embodiment, the rods and connector blocks are shown in a parallel configuration, in certain applica¬ tions it may be desirable to have only some of the rods or other elongated elements or connectors arranged in a parallel disposition. Furthermore, more than two elongated elements can be connected by means of a single connector.

The heater described above can be fabricated from machine parts by press-fitting the parts together into the configuration shown in Figure 1. It should be appreciated that the dimensions of the various compo¬ nents of the heater of the present invention control not only the size of the heater, which can cover a wide range of sizes from about 3 inches to 3 feet in diame¬ ter, typically about 12 inches tall and 12 to 18 inches in diameter, but also the resistance of the heater. Further, the number of rods utilized in the heater can be used to-control the resistance of the heater. Typically 16, 20, or 24 rods are used.

Connector blocks 14 and rods 12 are all preferably identical, although identicality is not required.

The heater of the present invention is effec¬ tive to provide uniform controlled heating over exten¬ sive periods of time. The use of high strength, iso- tropic graphite results in a strong, durable, rugged heater that can withstand handling without undue break¬ age. The modular construction facilitates repair or replacement when necessary. Providing the terminal block as a integral part of the heater allows a uniform rod to be used throughout the heater, thereby resulting in providing a uniform temperature around the periphery of the heater. The use of an integral terminal block also contributes to the ruggedness of the heater and insures uniform current flow throughout the framework of the heater.

In use a source of electrical potential is supplied to the heater by means of terminal blocks 16 or in any other desired manner. A.C. or D.C. potential can be used. If an A.C. potential is used, it can be 60 cycle or any other desired frequency. As pointed out above, two-phase or three-phase alternating current can be used.

While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto, since many obvious modifications can be made. For example, other shapes than the cylindrical shape of the heater of the present invention can be utilized for particular appli- _, _Λ

10

cations while still providing the advantages of the present invention. It is intended to include any such modifications as will fall within the scope of the appended claims.

Claims

We claim:

1. A resistance heater comprising a plural¬ ity of non-contiguous, elongated elements; a plurality of non-contiguous connectors adapted to connect at least two adjacent elongated elements; and terminal means for application of an electrical potential to the connected elongated elements and connectors; said elongated elements and said connectors being connected so as to provide a continuous path for electrical current from said terminal means through said plurality of elongated elements and said plurality of connectors.

2. A heater according to claim 1 wherein said elongated elements are rods.

3. A heater according to claim 1 wherein said elongated elements are bars.

4. A heater according to claim 1 wherein said connectors are connector blocks.

5. A heater according to claim 1 wherein at least some of said elongated elements are parallel to one another.

6. A heater according to claim 1 wherein all of said elongated elements are parallel to one another. 1 _,2 _,

7. A heater according to claim 1 wherein said plurality of connectors are disposed at both ends of said plurality of elongated elements.

8. A heater according to claim 1 wherein said plurality of elongated elements and said plurality of connectors are arrayed in a cylindrical configura¬ tion.

9. A heater according to claim 8 wherein said cylindrical configuration is an open cylindrical configuration.

10. A heater according to claim 1 wherein said plurality of elongated elements are perpendicular to said plurality of connectors.

11. A heater according to claim 10 wherein said plurality of connectors are disposed at both ends of said plurality of elongated elements.

12. A heater according to claim 1 wherein each of said plurality of connectors is connected to a pair of said plurality of elongated elements.

13. A heater according to claim 1 wherein each pair of said plurality of elongated elements is connected at one end to one of said plurality of con¬ nectors and at the other end to two adjacent connec¬ tors.

14. A heater according to claim 1 wherein said terminal means comprises at least two terminal blocks substantially identical to said plurality of connectors, the number of said terminal blocks being two where said electrical potential is a D.C. potential and corresponding to the number of phases of said electrical potential where said electrical potential is an A.C. potential, said terminal blocks being disposed so as to provide equidistant current paths therebe¬ tween.

15. A heater according to claim 14 wherein said number of terminal blocks is two.

16. A heater according to claim 14 wherein said number of terminal blocks is three.

17. A heater according to any of claims 1-16 which is fabricated of graphite.

18. A heater according to any of claims 1-16 which is fabricated of graphite having an electrical resistivity in the range of about 200 to about 1000 ϊ ohm-in.

19. A heater according to any of claims 1-16 which is fabricated of graphite having an electrical resistivity in the range of about 400 to about 600 I ohm-in.

20. A heater according to claim 1 wherein said plurality of elongated elements and said plurality of connectors are press-fit together.

21. A resistance heater comprising:

(a) a plurality of essentially identically sized and shaped, essentially straight, electrically conductive elongated elements;

-) a plurality of essentially identically sized and shaped, electrically conductive connector blocks, each connector block containing two essentially identical apertures, each of which is sized and shaped to matingly engage an end of one of said elongated elements, the number of said connector blocks being two less than the number of said elongated elements;

(c) two terminal connector blocks containing terminals for receiving electric current but otherwise essentially identical to said connector blocks,

wherein each of said connector blocks and terminal connector blocks matingly engages a pair of said elongated elements in said apertures and each end of each elongated element is matingly engaged into one of said ap&rtures, with no two connectors or terminal connectors being matingly engaged to the same pair of elongated elements.

22. A heater according to claim 21 wherein said elongated elements are rods.

23. A heater according to claim 21 wherein each elongated element, connector block, and terminal connector block is comprised of graphite.

24. A heater according to claim 23 wherein each of said matingly engaged elongated elements is essentially parallel to every other elongated element.

25. A heater according to claim 23 wherein one half of said matingly engaged elongated elements are parallel with each other and the other half are likewise parallel with each other but in a different direction.

26. A heater according to claim 24 wherein the two terminal blocks lie diametrically opposed to each other but in a different plane to that defined by one- half the connector blocks.

27. A heater according to claim 26 wherein each connector block and terminal block has a flat surface perpendicular to the axes of said apertures.

28. A heater according to claim 26 wherein each connector block and terminal block has two flat surfaces perpendicular to the axes of said apertures. ID

23 . A heater according to claim 28 wherein said mating engagement is by press-fit.

30. A resistance heater comprising

(a) A plurality of essentially identically sized and shaped straight graphite rods, none of which is in physical contact with another of said rods;

(b) A plurality of essentially identically sized and shaped graphite connector blocks, none of which is in physical contact with another of said blocks, the number of said connector blocks being two less than said rods, each of which contains two aper¬ tures for receiving the ends of two of said rods;

(c) two graphite terminal connector blocks containing a terminal for electrical current to be applied, each of said terminal connector blocks other¬ wise being essentially identical in size and shape as said connector blocks.

31. A heater comprising

(a) a plurality of rods, none of which is in physical contact with another;

(b) A plurality of connectors, none of which is in physical contact with another;

wherein, for each rod, the two terminal portions are connected to different connectors, with each connector being connected to the terminal portions of at least two rods, with the connectors and rods being so connected that at least one continuous pathway is formed wherein said rods and connectors are composed of electrically conductive material.

32. A resistance heater comprising:

(1) a plurality of rods, no two of which are in physical contact;

(2) a plurality of connector blocks, no two of which are in physical contact, said connector blocks adapted to engage one end of two of said rods;

wherein said rods and connector blocks are engaged such that each connector engages two rods but no two connectors are engaged to the same pair of rods, with said rods and connector blocks being composed of a material which, when an electric current is applied, releases heat.

33. A heater according to claim 32 wherein two of said connector blocks are each equipped with a terminal for application of an electrical potential thereto, said rods and connector blocks being engaged to provide a continuous electrical current path from one connector block equipped with a terminal to the

/other connector block equipped with a terminal.