US3311694A

US3311694A - Diffusion furnace utilizing high speed recovery

Info

Publication number: US3311694A
Application number: US459795A
Authority: US
Inventors: Jr Cecil A Lasch
Original assignee: Electroglas Inc
Current assignee: Electroglas Inc
Priority date: 1965-05-28
Filing date: 1965-05-28
Publication date: 1967-03-28
Anticipated expiration: 1984-03-28

Description

March 28, 1967 c. A. LASCH, JR

DIFFUSION FURNACE UTILIZING HIGH SPEED RECOVERY 2 Sheets-Sheet 1 Filed May 28, 1965 3 o: 3 mom m vb NJ I v 73 b --mu Two 1 I N6 m mum wum Tmum mm F I \v E L m6 mm mw -01 To TE 6 ml .L (E? (E? (Ea INVENTOR.

Cecil A. Lasch, Jr

BY I

Attorneys March 28, 1967 c, L scH, JR 3,311,694

DIFFUSION FURNACE UTILIZING HIGH SPEED RECOVERY Filed May 28, 1965 2 Sheets-Sheet 2 s u mmmadwa Q INVENTOR.

Cecil A. Lasch, Jr:

(7% (jag bus Attorneys 3,311,694 DIFFUSION FURNACE UTILIZING HIGH SPEED RECOVERY Cecil A. Lasch, In, Los Altos, Califi, assignor to Electroglas, Incorporated, Menlo Park, Calif., a corporation of California Filed May 28, 1965, Ser. No. 459,795

. 10 Claims. (CI. 13-24) This invention relates to a diffusion furnace and method and more particularly to a diffusion furnace and method utilizing high speed recovery.

A conventional diffusion furnace will recover to a set temperature very quickly after a boat is inserted and, therefore, is essentially at a constant temperature insofar as the boat is concerned. It has been found that the boat will heat to 90% of the set temperature in approximately 20% of the total time required to heat the boat to the set temperature. The driving force determines the rate of temperature rise of the boat and is the difference in temperature between the boat and the furnace. This temperature difference becomes very small as the boat approaches the temperature of the furnace and it is for this reason that approximately 80% of the total time is required to raise the boat the, final 10% of the set temperature. Such characteristics are particularly undesirable where the diffusion process takes only a few minutes. There is, therefore, a need for a new and improved diffusion furnace and method which utilizes high speed recovery.

. In general, it is an object of the present invention to provide a diffusion furnace and method utilizing a high speed recovery system.

Another object of the invention is to provide a diffusion furnace and method of the above character in which a 'boat or other object in the furnace is rapidly brought up to the set point temperature of the furnace.

Another object of the invention is to provide a diffusion furnace and method of the above character in which the rate of temperature rise of the boat is greatly increased over conventional rates as the boat nears the set temperature.

Another object of the invention is to provide a diffusion furnace and method of the above character in which the thermal gradient produced by introductionof a boat into the furnace is automatically corrected.

Another object of the invention is to provide a diffusion furnace and method of the above character in which the thermal profile becomes flat well below the set temperature and in which the load uniformly arrives at the set diffusion temperature 'of the furnace.

Another object of the invention is to provide a diffusion furnace and method of the above character in which the boat can be brought up to the set temperature very rapidly without overshoot.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings.

. Referring to the drawings:

FIGURE 1 is a circuit diagram with certain parts schematically illustrated of a diffusion furnace incorporating the present invention.

FIGURE 2 is across-sectional view of a portion of the diffusion furnace with certain parts broken away.

FIGURE 3 is a cross-sectional view taken along the line 3-3 of FIGURE 2.

FIGURE 4 is a graph showing the operation of the diffusion furnace of the present invention and also compares the operation with aconventional diffusion furnace.

In general, the diffusion furnace consists of means forming a zone which can be heated; Heatingmeans United States Patent 3,311,694 Patented Mar. 28, 1967 is disposed in the zone. A source of power is provided for supplying energy to the heating means. Control means is also provided for connecting the source of power to the heating means. Temperature sensing means is disposed in the zone to sense the temperature of the zone.- A set point controller is connected to the temperature sensing means and provides a first predetermined reference set point and provides a signal which is proportional to the difference between the temperature measured by the temperature sensing means and the set point. The signal from the set point controller is supplied to the control means to control the application of power to the heating means to bring the temperature of the zone up to the first set point. Additional temperature sensing means is disposed in the zone for sensing when the temperature drops in said zone. Means is also provided which establishes a second predetermined set point which is above said first named set point. Timing means is connected to the additional temperature sensing means and to said means providing a second predetermined set point for causing the control means to supply power to said heating means for a predetermined time to cause the temperature in said zone to rise above said firstset point and to then terminate the supply of power to permit the temperaturein said zone to return to said first set point.

The diffusion furnace shown in the drawings is similar in many respects to the diffusion furnaces disclosed in copending applications Ser. No. 317,268, filed Oct. 18, 1963, now Patent No. 3,291,969 and Ser. No. 382,038, filed July 13, 1964. As described therein and as shown in the drawings, each diffusion furnace consists of one or more furnace assemblies 11 and a control system 12 for the furnace assemblies. Each of the furnace assemblies consists of a rectangular'casing 13 which forms a zone to be heated. A helical heating element 14 extends longitudinally of. the casing 13 in the zone and is supported within the casing in a suitable manner such as bymeans of a ceramic tube 15 carried by fire brick 16 positioned within the casing 13. The heating element serves as means for heating the zone within the heating element. Ceramic separators 17 are used for maintaining separation between the turns of the heating element and are cemented to the tube 15. Flanges 19 of a suitable material suoh as ceramic are mounted in the casing .13 on opposite ends of the heating element and each is provided with an opening 21 which is substantially the same size as the inner diameter of the heating element 14.

A processing tube 22 formed of a suitable material such as quartz is disposed within the heating element 14 and extends longitudinally thereof. The processing tube 22 is provided with an open end 24 and a smaller opening 26 which is provided in a neck 27 of the processing tube. -This opening 26 is provided for introducing gases into the processing tube 22. A processing zone 28 is defined by the processing tube 22 and is disposed within and surrounded by the heating element 14.

Means is provided for shielding the processing zone 28 from radiant energy from the heating element 14 and consists of a tube 31 which is formed of a suitable masuitable manner adjacent to the heating element 14 such as by the rings 34 which are of the type described in copending application Ser. No. 317,268, filed Oct. 18, 1963. The rings 34 are disposed between the coils of turns of the helical heating element 14' and retained thereby to sup port the tube 33 which is disposed in the rings adjacent the heating element 14.

An additional thermocouple tube 36 is disposed within the tube 31 and adjacent the processing tube 22 and is adapted to receive additional temperature sensing means hereinafter described. The tube 36 is provided with a pair of parallel holes 37. The tube 36 can be formed of any suitable material such as ceramic.

The heated zone which is formed within the heating element 14 is divided into three separate zones as shown particularly in FIGURE 1 identified as zone A, zone B and zone C, respectively. The control system 12 for the diffusion furnace is connected to a suitable source of powersuch as 208 volts single phase A.-C. connected to two lines L-l and L-Z as shown in FIGURE 1 of the drawing. The control system 12 includes control means for each of the zones A, B and C connected to the lines L1 and L-2 to supply power to the heating element 14. The section of the heating element associated with the respective zone is connected to the secondary of a transformer. Thus, as shown, three transformers TR-l, TR-2 and TR-3 are provided; The primaries of these transformers have one side connected to the line L-l through circuit breakers OB-l, CB-2 and CB-3, respectively; The other sides of the primaries are connected to silicon controlled rectifier assemblies identified as SCR1,

SCR-Z and SCR-3, respectively, through the circuitbreakers CB-l, CB-2 and CB-3 to the line L-2. As can be seen from the drawing, each of the silicon controlled rectifier assemblies consists of two silicon controlled rectifiers which are arranged in opposite directions to provide full wave rectification. Control circuits FC-l, FC-2 and FC-3 are provided for controlling the firing of the silicon controlled rectifiers. The fire control circuits can be of any suitable type such as a Model ES2-5001 SCR Controller manufactured by the Barber Colman Company of Rockford, Ill.

Means is provided'for controlling each of the fire control circuits FC-l, FC-2 and FC3 in order to control the temperature of the heating element 14 and consists of devices identified as G-l, G-2 and G-3, respectively.

These devices can be null galvonometers of a suitable type such as the Type 407 manufactured by the Barber Colman Company of Rockford, Ill.

The input to the galvonometer G-2 is connected to the output of a set point controller C1 of a suitable type such as Model 352-T manufactured by the Barber Colman Company of Rockford, Ill.

A thermocouple assembly consisting of thermocouples T1, T- 2, T-3, T 4 and T-5 is disposed in the tube 33 and is connected to the galvonometers G-1 and G-3, and to the set point controller C1. Thus, as shown, the null galvonometer G1 is connected to the thermocouples T-1 and T-2; the null galvonometer G-l is connected to the thermocouples T-3 and T4. The set point controller C1 is connected to the master thermocouple T-S. As can be seen from the drawing, the thermocouple T-1 is positioned in zone A and thermocouple T-4 is positioned in zone C. The thermocouples T-2, T-3 and T-S are positioned in zone B.

As described in copending application Ser. No. 317,268, filed Oct. 18, 1963, the set point controller C1 is manually adjusted to provide a set point voltage which repre- 'sents a predetermined temperature at which it is desired to operate the furnace. The set point voltage from the controller C1 serves as a constant reference voltage which is presented in opposition to the varying input voltage supplied by the master thermocouple T-5 positioned in zone B. The difference in deviation between these two voltages is the output of the digital set point controller C1 and is utilized for controlling the galvonometer G 2. The galvonometer G2 serves as a master galvonometer and the galvonometers G-1 and G-3 are slaved to the master galvonometer so that the temperature which is set by the set point controller C1 for zone B is also obtained in zones A and C.

The portion of the diffusion furnace thus far described is very similar to that disclosed in copending applications Ser. No. 317,268, filed Oct. 18, 1963 and Ser. No. 382,038, filed July 13, 1964. As hereinbefore explained, the present diffusion furnace includes means for providing a high speed recovery of temperature within the processing zone whenever a load as, for example, a boat containing work pieces, is positioned in the processing zone 28 within the processing tube 22. This means consists of an additional set point controller C-2 which can be of a type similar to the set point controller C1 and which is utilized for providing an adjustable set point voltage representing a temperature which is above the temperature represented the processing tube 22 and tube 31. The thermocouple T 6 is connected to leads 46 and 47 which extend through the holes 37 in the tube 36 and is connected to a meter relay 48 of a suitable type such as the Model No. 195 meter relay pyrometer manufactured by the General Electric Company. Such a meter relay includes

stationary contacts

1 and 3 and a movable contact 2. It also includes a coil 49 which is energized when a predetermined temperature drop is sensed by the meter relay 48 to open the

contacts

2 and 3 and

close contacts

1 and 2.

A timer 51 of a suitable type such as Type No. HP54 manufactured by the Eagle Signal Co. of Moline, 111;, is provided. As shown in the drawing, this timer includes a motor M which is provided with cam means (not shown) for opening

contacts

1 and 2, and 4 and 5, and

closing contacts

3 and 4. It also includes a coil 52 which, when energized, is adapted to close contacts 6 and 7, and 8 and 9, and to open contacts 9 and 10'.

able contact 4 by conductor 56 and the movable contact 9 is connected to the stationary contact 3 by conductor 57. Stationary contact 1 of the meter relay 48 is connected to one side of the coil 52 of the timer 51 by a conductor 58 and the other side of the coil 52 is connected to aterminal L-4 by conductor 59. The terminal L-3 is connected to movable contact 2 of the meter relay 48 by a conductor 61. The terminals L-3 and L4 are connected-to a suitable source of voltage such as volts A.C. The movable contact 9 of the timer 51 is connected to the conductor 61 by conductor 62. The stationary contact 2 of the timer 51 is connected to conductor 5 8 by a conductor 63. Stationary contact 2 of timer 51 is also connected to one side of the mot-or by a conductor 64 and the other side of the motor M is connected to conductor 59 by conductor 66-. Movable contact 1 of the timer 51 is connected to stationary contact 6 of the timer 51 by conductor 67.

Operation of the diffusion furnace in performing the present method may now be briefly described as follows. Let it be assumed that the set point controller C1 has been set so that the diffusion furnace normally operates at a predetermined temperature as, for example, 1000 C. When this is the case, power is supplied to the heating element 14 by the control system 12 until all three zones A, B and- C arrive at the 1000 C. temperature. As

Movable contact 9 is connected to the set point controller pointed out in copending applications Ser. No. 317,268, filed Oct. 18, 1963, and Ser. No. 382,038, filed July 13, 1964, power is supplied to the heating element 14 until the master thermocouple T-5 produces a voltage which opposes the reference voltage produced by the digital set point controlled Cl. As pointed out previously, thermocouples T-1 and T-4 sense the temperatures in zones A and C, and since they are slaved to the master thermocouple T -5, they bring the zones A and C up to the same temperature as zone B. r

Now let it be assumed that the temperature in the processing zone 28 within the processingtube is up to the desired temperature, e.g. 1000 C., and that it is desired to carry out a diffusion operation. Let it also be assumed that the article on which the diffffusion operation is to be performed as, for example, silicon wafers, are disposed on a suitable carrying device such as a boat 71 formed of a suitable material such as quartz. Before loading the boat into the processing zone 28, the digital set point controller C2 is manually adjusted to provide a second set point voltage which can be added to the set point voltage provided by the set point controller C1 to thereby provide a voltage which represents a temperature which is substantially above the temperature represented by the set point voltage from the controller Cl. By way of example, the set point controller C2 can be manually adjusted to provide an additional voltage which is equivalent to a temperature of 15 C. above the 1000 C. temperature to give a temperature of 1015 C.

Let it also be assumed that the thermocouple T-6 is positioned so that it will readily sense any temperature drop within the zone B as, for example, a temperature drop of 25 C. within 20 seconds.

As soon as the boat 71 is introduced into the processing zone 28, it will absorb heat within the furnace and cause the temperature within the processing zone to drop. This temperature drop is quickly sensed by the meter relay 48 which closes its

contacts

1 and 2 to supply power to the coil 52 and the motor M in the timer 51. The supplying of power to the coil' 52 opens contacts 9 and which causes the set point voltage being produced by the controller C2 to be added to the set point voltage produced by the controller Cl to indicate to the galvonometer G-Z that the temperature of the processing zone must be immediately raised. Closing of contacts 6 and 7 of the timer 51 establishes a holding circuit for the coil 52 and the motor M in the event that

contacts

1 and 2 of the meter'relay are opened before the timer has timed out.

It can be seen that prior to the energizationof the meter relay 48-that the voltage produced by the set point controller C2 is shunted out by the normally closed contacts 9 and 10 so that voltage has no effect upon the set point provided by the controller C-1. For that reasons, until the meter relay 48 is operated, the diffusion furnace is under the control of the controller 0-1. V Thus, it can be seen that as soon as the meter relay 48 is operated, the control system 12 is operated to supply additional power to the heating element 14 to raise the temperatures in zones A, B and C to a temperature which is substantially above the temperature represented by the output from the controller Cl. Thus, it is the setting of the set point controller C2 which determines the amount of. boost in furnace temperature when a boat is inserted into the furnace.

After a predetermined interval of time, the motor M of the timer 51 opens

contacts

1 and 2 to deenergize the holding circuit of the motor M and the coil 52.

Contacts

3 and 4 are closed to shunt out the voltage from .the controller C2 so that the control system is only under the control of the set point voltage provided by the controller C-1.

From the foregoing, it can be seen that once the timer 51 is started in operation by the closing of

contacts

1 and 2 of the meter relay, the timer will finish its time cycle regardless of whether the

meter relay contacts

1 and 2 open during the cycle. If the meter relay does open its

contacts

1 and 2 during the timer cycle, the timer will reset immediately upon completion of its time cycle. If the time taken for the meter relay to open its contacts exceeds the timers cycle time, the timer will wait until the meter relay has opened its contacts before automatically resetting itself. Thus, it can be seen that the set point voltage of the controller C2 will only be added to the set point voltage of the controller C 1 only during the timer cycle.

As soon as the timer cycles out, the control system will be under the control of the controller C-l so that power is no longer supplied to the winding 14 to permit the furnace to cool down to the temperature represented by the set point of the controller 0-1.

The method of operation of the present diffusion furnace in comparison with conventional diffusion furnaces is graphically illustrated in FIGURE 4. The wall temperature of a conventional diffusion furnace is represented by the broken line 81, whereas the temperature of the boat inserted into a conventional diffusion furnace is represented by the broken line 82. The wall temperature of the present diffusion furnace is represented by the solid line 83, whereas the temperature of a boat introduced into the present diffusion furnace is represented by the solid line 84.

From the graph in FIGURE 4, it can be seen that a boat in a conventional furnace will heat to approximately 90% of the set temperature in approximately 20% of the total time required to heat to the set temperature. The difference in temperature between the boat and the furnace represents the driving forcewhich determines the rate of temperature rise of the boat. As can be seen from FIG- URE 4, this temperature differential becomes smaller and smaller and it is for this reason that approximately 80% of the total time is required .to raise the boat the final 10% of the set temperature.

It is very difficult to determine what takes place when a transparent material such as a quartz boat is in a nonequilibrium condition in the furnace because of heat loss by radiation. It is believed that in the diffusion furnace the principal mechanism of heat transfer is by radiation. This is further complicated by the fact that the silicon wafers often carried by the quartz boats are not transparent to radiation and, therefore, tend to pick up more heat and be at a higher temperature than the quartz boat or carrier.

In order to" avoid this difference in sensitivity to radiation, the tube 31 is provided which prevents heat from being transferred by any substantial. radiation to the quartz boat or the Wafers carried thereon. This thus prevents the wafers from being heated to a higher temperature than the quartz boat or carrier.

By utilization of the high speed recovery systemhereinbefore described, it can be seen that as soon as a load is introduced into the furnace, the wall temperature of the furnace drops. The wall temperature of the furnace in recovery is then deliberately allowed to overshoot the set temperature by an appreciable amount which has the effect of greatly shortening the time for the boat or load to reach the set point. In order to prevent the boat from also overshooting the set point, the furnace itself must be returned to the set temperature just before the boat reaches that point.

Thus, as can be seen in FIGURE 4, in the portion of the curve identified as 83a, the wall furnace of the temperature is lowered-because of the introduction of the load consisting of the boat and the wafers carried thereon. Thereafter, as soon as the meter relay senses the temperature drop, power is supplied to the windings 14 to cause the wall temperature of the furnace to rapidly recover as, for example, within two minutes, as represented by the portion 83b of the curve83. The wall temperature of the furnace then overshoots the set point to arrive heating from the unloaded ends of the furnace.

at a new set point and to remain at this set point for a period of time as represented by the portion 830 of the curve 83. This new set point is represented by the sum of the voltages from the controllers C-1 and C-2. At an appropriate time as determined by the timer 51, the timer times out and removes the additional voltage from the galv-onometer 6-2 as hereinbefore described to place the furnace under the control of the controller -1. This step is represented by the point 83d on the curve 83. Thereafter, the wall temperature of the furnace drops gradually down to the set point as represented by the portion 83e of the curve 83. During this time, the temperature of the boat is rising relatively rapidly. The rate of temperature rise for the boat decreases as the wall temperature of the furnace drops gradually to the first set point so that both the boat and the furnace approach the' set point asymptotically and then arrive at the set point temperature of 1000 C. at approximately the same time.

Thus, with the present furnace and method, it can be seen that the boat can be brought up to the set point temperature within a much shorter period of time as, for example, a period of five minutes rather than ten minutes or more in conventional furnaces. This substantial decrease in the time required to bring the boat up to the set point temperature Without overshoot is very important in many diffusion operations, and in particular for short diffusion runs.

In order to achieve optimum results with the present diffusion furnace, it is desirable to utilize a thermally balanced boat. Normally, when a boat is placed in a furnace, the ends normally reach the set temperature before the center of the boat because they receive additional In order to overcome these effects, masses are added to each end of the boat in the form of masses 71a and 71b of a sufficient size to prevent overshooting of the ends of the working portion of the boat. As can be seen from FIGURE 2 of the drawing, the front end of the boat requires a larger mass than the rear end of the boat because the front end of the boat is introduced first'into the furnace. By utilizing this thermally balanced boat, all points in the working area of the boat will be subject to the same thermal conditions and will all reach the set temperature simultaneously to thereby make possible very uniform diffusion operations with repeatability.

It is apparent from the foregoing that I have provided a new and improved diffusion furnace and method utilizing high speed recovery which makes it possible to greatly shorten the time required to bring a load up to the set.

point temperature.

It should be appreciated that although in the arrangement shown in the drawings in which the voltages from controllers C-1 and C-2 are added, it is possible to merely switch from a first digital set point controller to a second digital set point controller and obtain the second set point voltage solely from the second controller.

I claim:

1. In a diffusion furnace, means forming a zone, heating means disposed in the zone, a source of power, control means for connecting the source of power tothe heating means, temperature sensing means disposed in said zone, means providing a first predetermined reference set point representing a first temperature connected to said temperature sensing means for providing a'signal proportional to the difference between the temperature measured by the temperature sensing means and the first temperature, means for supplying said signal to the control means to control the application of power to said heating means to bring the temperature in said zone up to said first temperature, additional temperature sensing means disposed in said zone for sensing when the temperature drops in said zone, means providing a second predetermined reference set point representing a second temperature which is higher than said first temperature, and timing means controlled by said additional temperature sensing means and connected to said means providing a second predetermined set point and to said control means to cause said control means to supply power to said heating means to cause the temperature in said zone to rise above said first temperature to said second temperature and then to terminate the supply of power to said heating means to permit the temperature in said zone to return to said first temperature.

2. A diffusion furnace as in claim 1 wherein said means providing a second predetermined set point includes means for providing an output which is added to the output of the means for providing a first predetermined reference set point.

3. A diffusion furnace as in claim 1 wherein said additional temperature sensing means includes a meter relay and wherein said timing means is connected to said meter relay and is placed in operation by said meter relay when the additional temperature sensing means senses a drop in temperature insaid zone.

4. A diffusion furnace as in claim 3 wherein said timing means includes means for causing said control means to be controlled by said means providing a second predetermined set point.

5. In a diffusion furnace, means forming a zone, heating means disposed in the zone, a source of power, control means for connecting the source of power to the heating means, temperature sensing means disposed in said zone, means providing a first predetermined reference set point representing a first temperature connected to said temperature sensing means for providing a signal proportional to the difference between the temperature measured by the temperature sensing means and'the first temperature, means for supplying said signal to the control means to control the application of power to said heating means to bring the temperature of said zone up to the first temperature, additional temperature sensing means disposed in said Zone for sensing when the temperature drops in said zone below said first temperature, means. providing a second predetermined set point representing a second temperature which is higher than said first temperature, and means connected to said additional temperature sensing means for connecting said means providing a second predetermined set point to-said first named temperature sensing means to thereby cause the control means to supply power to said heating means to cause the temperature in said zone to rise above said first temperature.

6. A diffusion furnace as in claim 5 wherein said last named means includes timing means initiated when the additional temperature sensing means senses a drop in temperature in said zone and terminating the connection of said means providing a second predetermined set point temperature to said first named temperature sensing means after a predetermined interval of time.

7. A diffusion furnace as in claim 5 wherein said heating means is substantially in a cylindrical form together with a processing tube substantially transparent to radiation disposed in said heating means and an additional tube disposed in said heating means and surrounding said processing tube,.said additional tube being formed of a material preventing the transmission of substantial radiant energy to the processing zone.

8. A diffusion furnace as in claim 6 wherein said timing means includes means for switching from said first predetermined reference set point to said means for providing a second predetermined set point.

9. In a diffusion furnace, means forming a zone to be heated, heating means disposed in the zone, a source of power, control means for connecting the source of power to the heating means, temperature sensing means disposed in said zone, additional temperature sensing means disposed in said zone, means providing first and second reference set points representing first and second temperatures in which the second set point represents a temperature substantially higher than the first set point, and means connecting said additional temperature sensing means to said means providing first and second predetermined reference set points to said control means whereby said control means is normally under the control of said first set point to maintain said zone at said first temperature represented by the first set point and said control means is under the control of said second set point when the additional temperature sensing means senses a drop below said first temperature to cause the temperature in said zone to rise to said first temperature and then to overshoot said first temperature. V

10. A diffusion furnace as in claim 10 wherein said last named means includes a timer for terminating the control of the control means by said second set point after a predetermined period of time.

References Cited by the Examiner UNITED STATES PATENTS RICHARD M. WOOD, Primary Examiner. V. Y. MAYEWSKY, Assistant Examiner.

Claims

9. IN A DIFFUSION FURNACE, MEANS FORMING A ZONE TO BE HEATED, HEATING MEANS DISPOSED IN THE ZONE, A SOURCE OF POWER, CONTROL MEANS FOR CONNECTING THE SOURCE OF POWER TO THE HEATING MEANS, TEMPERATURE SENSING MEANS DISPOSED IN SAID ZONE, ADDITIONAL TEMPERATURE SENSING MEANS DISPOSED IN SAID ZONE, MEANS PROVIDING FIRST AND SECOND REFERENCE SET POINTS REPRESENTING FIRST AND SECOND TEMPERATURES IN WHCIH THE SECOND SET POINT REPRESENTS A TEMPERATURE SUBSTANTIALLY HIGHER THAN THE FIRST SET POINT, AND MEANS CONNECTING SAID ADDITIONAL TEMPERATURE SENSING MEANS TO SAID MEANS PROVIDING FIRST AND SECOND PREDE-