US3354257A - Furnace heating apparatus - Google Patents

Description

Nov. 21, 1967 K. A. LANG 3,354,257

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Patented Nov. 21, 1967 3,354,257 FURNACE HEATING APPARATUS Karl A. Lang, 136 Venetia Drive, Long Beach, Calif. 90803 Fiied July 25, 1966, er. No. 567,527 5 Claims. ((31. 13-24) This invention relates to furnace heating apparatus and more particularly to apparatus for heating elongated diffusion furnaces.

The invention has particular utility with respect to a diffusion furnace of the type described in my copending patent application Ser. No. 447,343, filed Apr. 13,- 1965, and entitled Furnace. This furnace utilizes an elongated heating coil surrounded by refractory material and inter nally accommodating a hollow quartz tube coextensive with the coil. The materials to be heated in the furnace, such as semi-conductor materials and the like, upon an elongated quartz vessel or boat which is inserted into one end of the quartz tube.

The temperature gradient or profile along the length of the quartz tube is typically characterized by higher temperatures in the central portion as compared to the ends. However, it is extremely important that the higher temperatures in the center zone be as uniform as possible, the two end zones of this typical three zone furnace also being important but not presenting nearly as diflicult a temperature control problem.

Normally, in a diffusion furnace the user employs a quartz boat which may vary in length anywhere from 8 inches to 20 inches. Consequently, the center zone of such a furnace has been made approximately 24 inches to accommodate the longest boat.

The center zone is defined between a pair of intermediate taps on the heat element or coil, while the two end zones are each defined between an end tap and one of the intermediate taps. These taps are connected to corresponding taps on the secondary of a transformer, and the temperature in each zone is controlled by suitable apparatus which employs thermocouples to adjust the length of the center zone.

However, there are numerous occasions when it is desired to load a short boat, of perhaps 10 inches, into a diffusion furnace charac erized by a long center section. It was found that an overheating condition occurred at the end of the boat that was inserted into the furnace the leading end of the boat. This caused the material or load on the leading end to be overheated, and the diffusion of the semi-conductor materials and their electrical characteristics was therefore undesirably different from the load at the trailing end of the boat.

Various means have been attempted to even out the temperature along the length of the boat, despite the fact were consequently comparatively slight. One attempt was to make the boat longer at the end which was normally overheated in order to absorb the additional heat. Another attempt was to load the boat unevenly to try and equalize the heat absorption along the length of the boat. However, these special techniques require special handling not conducive to a high production operation.

Accordingly, it is an object of the present invention to provide a furnace heating apparatus which can be arranged to provide a heating element center zone of more than one length so that the furnace can accommodate boats of more than one length. By providing a center zone commensurate with boat length, the heat requireicnts throughout the center zone are more nearly the same and therefore better controlled by the associated temperature regulation means.

Other objects and features of the invention will become scription taken in connection with the accompanying drawings, in which:

FIG. 1 is a partially schematic wiring diagram of a furnace heating apparatus of the prior art;

FIG. 2 is a partially schematic wiring diagram of a furnace heating apparatus according to the present inven- The conventional apparatus 10 includes an elongated, helical heating element or coil 14 which is surrounded by refractory material (not shown) and which normally axially accommodates a quartz tube (not shown) within perature in the quartz tube.

temperature in the tube is raised by energization heating coil 14. For this purpose, the heating coil The of the cally accommodates itself to the output.

The secondary 30 includes a plurality of transformer taps 36, 38, 40 and 42; which define a plurality of windings therebetween corresponding to the heating zones 24, 26, and 28.

The heating element taps and transformer taps are connected together by circuit means which include a pair of leads 46 and 48 connecting the taps 16 and 36 and the taps 20 and 38, respectively, in a heating loop which includes the heating zone 24 and a corresponding end portion or winding of the secondary 30. A similar loop is afforded by a pair of leads 50 and 52 which connect the taps 22 and 40 and the taps 18 and 42. A third or central heating loop is also provided by the leads 48 and 50, as will be apparent.

From the foregoing, it is seen that the taps on the heating coil 14 divide the coil into three heating zones 24, 26, and 28, and that the taps on the secondary 3t divide the secondary into three windings or secondary portions. Moreover, the leads between the element taps and the transformer taps define two end heating loops and an intermednate or center heating loop, with the leads 48 and 50 forming parts of both the center loop and one of the adjacent end loops.

The system thus far described is regulated by temperature control means which are well known in the prior art. More particularly, a number of thermocouples (not shown) or similar heat sensing devices are located adjacent the various heating zones of the heating coil 14, and are connected to a suitable temperature controller (not shown). The controller is preset to particular temperatures for the three heating zones 24, 26, and 28, and the differentials between the preset temperatures and the temperatures sensed by the thermocouples are converted into control signals which are fed to three silicon control rectifiers 54, 56, and 58 located in the circuits or heating loops of the three heating Zones 24, 26 and 28. These signals alter the phase of each of the SCRs 54, 56 and 58, to thereby change the on-time of the SCRs. As is well known to those skilled in the art, this has the effect of supplying more or less power to the heating zones 24, 26, and 28, according to the temperature differentials sensed.

The heating apparatus 18 thus far described is already known. However, as previously indicated, it is an undesirably inflexible arrangement on those occasions when it is desired to use a different length of quartz boat and still maintain a uniform temperature gradient throughout the center zone 28. In this regard, the end zones 24 and 26 of a typical diffusion furnace are approximately 8 inches long. The center zone 28 is made approximately 24 inches long to accommodate a 20 inch boat. Thus, assumto be inserted in the diffusion of the boat tends to be overing that a 10 inch boat is furnace, the leading end heated.

However, with the heating apparatus 12 of the present invention, it is possible to select either the long center zone 28 or a short center zone 68, as best illustrated in FIG. 2.

The short center zone 60 is defined between an additional pair of element taps 62 and 64 located on the heating coil 14 inwardly or intermediate the element taps 20 and 22. The taps 62 and 64 are located approximately 6 inches each side of the center of the coil 14, thereby providing a short center zone 60 measuring approximately 12 inches.

A complemental and additional pair of transformer taps 66 and 68 are also provided on the secondary 30, thereby defining a secondary winding portion corresponding to the short center zone 60. The element tap 62 and the transformer tap 66 are each connected to the opposite ends of a lead 70, and the taps 64 and 68 are each connected to the opposite ends of a lead 72.

The leads 70 and 72 are each characterized by a pair of open terminals located at a pair of junction boxes 74 and 76, respectively. Similar open terminals are provided at the junction boxes 74 and 7-6 in the leads 48 and 50, respectively. As best viewed in FIG. 2, the open terminals 4 of the respective leads 48, 58, 78, and 72 can be closed by one or more jumpers 78.

Two of the jumpers 78 are illustrated in FIG. 2 in positions completing the circuit which includes the leads 70 and 72. The circuits of the leads 48 and 50 are open at this time, so that the heating loop of the short center Zone 60 is energized. The diffusion furnace is thereby adapted to receive a shorter quartz boat. It will be noted that the end zones 24 and 26 are correspondingly increased in length by virtue of the shortening of the center zone. Thus, regardless of whether the short center zone 68 or the long center zone 28 is being energized, the full secondary windings of the transformer are always utilized.

In the circuit of FIG. 2, the temperature in the now longer end zones is controlled through the agency of the SCRs54 and 58, the short center zone temperature being controlled by the SCR 56. Since the length of the short center zone 68 more nearly corresponds with the length of the short boat, the heat load and consequently the temperature throughout the zone 60 is more nearly uniform.

FIG. 3 illustrates the apparatus 12 together with certain switches which, in combination with the arrangement of the additional leads 70 and '72 and the open terminals at the junction boxes 74 and 76, make possible achievement of various heating patterns throughout the various heating zones. More particularly, assuming that the long center zone 28 is being used, by virtue of jumpers '78 closing the circuits of leads 48 and 5t), and further assuming that the quartz boat is inserted from left to right and the leading end of the boat is hotter than the trailing end, a normally open switch 80 in the circuit of the lead 7 0 can be closed for short periods to even out the boat temperature.

If the quartz boat measures approximately 20 inches and is approximately centered in the zone 28, the trailing end of the boat will be located approximately over that section of the heating coil 14 located between the element taps 20 and 62. Consequently, by closing the switch 80 a higher voltage and current will be induced across that section of the heating coil 14 located between the taps 20 and 62, putting additional heat on the trailing end of the boat located in that region.

The same problem of overheating of the leading end of the boat can also be alleviated by closing a switch 82 located at the junction box 76.

The switch 82 is connected between the leads 72 and 50 and when closed shorts out that portion of the heating coil 14 located between the taps 64 and 22. This removes heat from the leading end of the boat located in that region.

Various other combinations of switches can be employed at the junction boxes 74 and 76 to achieve various heating patterns in the diffusion furnace, as will be apparent.

The switches 80 and 82 can be operated manually or by any suitable automatic means, but are preferably operated by the associated temperature controller of the diffusion furnace in response to some predetermined condition. For example, cooling of the boat leading end or heating of the boat trailing end would preferably be automatically accomplished by a solenoid or the like associated with the particular switch 88 or 82 and actuated by the temperature controller immediately upon sensing by the controller of the characteristic temperature drop occurring when the quartz boat is inserted into the furnace.

Although only the addition of leads 70 and 72 and their associated heating coil and transformer taps have been discussed, it will be apparent that still more such leads and taps can be provided to thereby afford even greater flexibility in adjusting the temperature profile of the heating zones of the furnace. Moreover, the temperature profile can be varied still further by, for example, closing leads 48 and 72 with jumpers 7 8 and leaving leads 70 and 50 open. This provides an unsymmetrical center zone which may be advantageous in certain applications.

Thus, the heating apparatus 12 is not only adapted to provide a means for adjusting the length of the center zone of a diffusion furnace, but also affords a means for adjusting the heating pattern through the dilfusion furnace to a degree not heretofore accomplished without complex circuitry, relatively expensive transformer arrangements, or the like.

Various modifications and changes may be made with regard to the foregoing detailed description without departing from the spirit of the invention or the scope of the following claims.

I claim:

1. Furnace heating apparatus comprising:

an elongated heating element;

element taps on said heating element defining a plurality of heating zones therebetween, including a short center zone between a first pair of said element taps, and a long center zone between a second pair of said element taps located outwardly of said first pair of element taps;

a transformer;

transformer taps on the secondary of said transformer defining a plurality of windings therebetween corresponding to said heating zones;

first circuit means including a pair of first leads extending between said first pair of element taps and the corresponding ones of said transformer taps, said first leads being open and adapted to be closed to energize said heating element in said short center zone;

second circuit means including a pair of second leads extending between said second pair of element taps and the corresponding ones of said transformer taps, said second leads being normally open and adapted to be closed to energize said heating element in said long center zone; and

means for selectively closing various ones of said first and second leads.

2. Furnace heating apparatus according to claim 1 wherein said last-mentioned means includes a pair of jumpers closing said first leads.

3. Furnace heating apparatus according to claim 1 wherein said last-mentioned means includes a switch located in one of said first leads and adapted on closure to complete said one of said first leads.

4. Furnace heating apparatus according to claim 1 wherein said heating zones comprise said short center zone, said long center zone, and a pair of end zones, each of said end zones being defined by one of said element taps at an end of said heating element and by the adjacent one of the pair of element taps of the operative one of the center zones.

5. Furnace heating apparatus according to claim 1 and including a switch connected between one of said first leads and one of said second leads and operative on closure to short out a portion of the heating zone between the element taps associated therewith.

References Cited UNITED STATES PATENTS 1,282,330 10/1918 VanAllen 219-490 1,670,846 5/1928 Cope 13 24 2,249,993 7/1941 Upton 13-24 3,311,694 3/1967 Lasch 13-24 FOREIGN PATENTS 570,139 2/1933 Germany.

RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

Claims (1)

1. FURNACE HEATING APPARATUS COMPRISING: AN ELONGATED HEATING ELEMENT; ELEMENT TAPS ON SAID HEATING ELEMENT DEFINING A PLURALITY OF HEATING ZONES THEREBETWEEN, INCLUDING A SHORT CENTER ZONE BETWEEN A FIRST PAIR OF SAID ELEMENT TAPS, AND A LONG CENTER ZONE BETWEEN A SECOND PAIR OF SAID ELEMENTS TAPS LOCATED OUTWARDLY OF SAID FIRST PAIR OF ELEMENT TAPS; A TRANSFORMER; TRANSFORMER TAPS ON THE SECONDARY OF SAID TRANSFORMER DEFINING A PLURALITY OF WINDINGS THEREBETWEEN CORRESPONDING TO SAID HEATING ZONES; FIRST CIRCUIT MEANS INCLUDING A PAIR OF FIRST LEADS EXTENDING BETWEEN SAID FIRST PAIR OF ELEMENT TAPS AND THE CORRESPONDING ONES OF SAID TRANSFORMER TAPS, SAID FIRST LEADS BEING OPEN AND ADAPTED TO BE CLOSED TO ENERGIZE SAID HEATING ELEMENT IN SAID SHORT CENTER ZONE;

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