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Publication numberUS3370120 A
Publication typeGrant
Publication dateFeb 20, 1968
Filing dateSep 26, 1966
Priority dateMay 28, 1965
Publication numberUS 3370120 A, US 3370120A, US-A-3370120, US3370120 A, US3370120A
InventorsLasch Jr Cecil A
Original AssigneeElectroglas Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Diffusion furnace and method utilizing high speed recovery
US 3370120 A
Abstract  available in
Images(2)
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Claims  available in
Description  (OCR text may contain errors)

- C. A. LASCH, JR

Feb. 20, 1968 2 Sheets-Sheet 1 Original Filed May 28, 1965 u 0.. 21 83 n m... Y4 N 7 m E m w m I L m mo j H 93 1 1 $3 1 a c N E J Nb qllllllll VM .6 p E v .P m mum wum 75m mm $8 mm 3 4 Nmfl V I l S m6 mam 0W N9. 3 IE U lllll d U U n 1 n N u n u n v Minn. m fl v IIIL I ll TE E. (PEP I I. Hi4 mm N- E Attorneys Feb. 20, 1968 v c. A. LASCH, JR $370,120

DIFFUSION FURNACE AND METHOD UTILIZING HIGH SPEED RECOVERY Original Filed May 28, 1965 I 2 Sheets-Sheet 2 9,, u ummadun v INVENTOR;

Cecil A. Lasch Jr Attorneys United States Patent 3,370,120 DIFFUSION FURNACE AND METHOD UTILIZING HIGH PEED RECOVERY I Cecil A. Lasch, .113, Los Altos, Calif., assignor to Electrogias, Incorporated, Menlo Park, Calif., a corporation of California Original application May 28, 1965, Ser. No. 459,795, now Patent No. 3,311,694, dated Mar. 28, 1967. Divided and this application Sept. 26, 1966, Ser. No. 598,535 3 Ciaims. (CI. 13-34) ABSTRACT OF THE DISCLOSURE Method for operating diffusion furnace which rapidly brings the furnace to the desired operating temperature 4 as soon as it is sensed that a cold load is introduced into the diffusion furnace.

This is a division of the application Ser. No. 459,795, filed May 23, 1965, now Patent No. 3,311,694 granted Mar. 28, 1967.

This invention relates to a diffusion furnace 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 thefurnace. 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 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 method utilizing high speed recovery.

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

Another object of the invention is to provide a diffusion furnace method of the above character in which the thermal profile becomes fiat 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 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.

3,37d,l Patented Feb. 20, 1968 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 I 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 first set point and to then terminate the supply of power to permit the temperature in 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, now Patent No. 3,299,196. 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 by means of ceramic tube 15 carried by fire brick 1-6 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 such 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 consists of a tube 31 which is formed of a suitable material such as Mullite which is a vitreous refractory manufactured by McDanel Refractory Porcelain Co. of Beaver Falls, Pa. As can be seen from the drawing, the tube 31 extends the length of the quartz processing tube 22 and completely surrounds the tube 22 to shield the same from radiant energy from the heating element 14. The tube 31 also acts as a heat sink.

A tube 33 is mounted adjacent the heating element 14 and is adapted to contain a thermocouple assembly as hereinafter described. The tube 33 is supported in a suitable 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. 1-8, 1963. The rings 34 are disposed between the coils of turns of the helical heating element 14 and retained thereby to support 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 power such as 208 volts single phase A-C connected to two lines L-l and L-2 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 L-1 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 TR3 are provided. The primaries of these transformers have one side connected to the line L-1 through circuit breakers CB1, CB-2 and CB3, respectively. The other sides of the primaries are connected to silicon controlled rectifier assemblies identified as SCR-l, SCR-2 and SCR-3, respectively, through the circuit breakers CB1, (DB-2 and CB3 to the line L2. 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 ES25001 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 1 4 and consists of devices identified as G-l, G-2 and 6-3, respectively. These devices can be null galvanometers of a suitable type such as the Type 407 manufactured by the Barber- Colman Company of Rockford, Ill.

The input to the galvanometer 6-2 is connected to the output of a set point controller C-1 of a suitable type such as Model 352-T manufactured by the Barber-Colman Company of Rockford, 111.

A thermocouple assembly consisting of thermocouples T-1, T-2, T-3, T-4 and T-5 is disposed in the tube 33 and is connected to the galvanometers G-1 and 6-3, and to the set point controller C-1. Thus, as shown, the null galvanometer 6-1 is connected to the thermocouples T-l and T2; the null galvanomenter G1 is connected to the thermocouples T-3 and T-4. The set point controller C-1 is connected to the master thermocouple T-5. 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 T2, T-3 and T-S are positioned in zone B.

As described in copending application Ser. No. 317,268, filed Oct. 18, 1963, now Patent No. 3,291,969, the set point controller C-1 is manually adjusted to provide a set point voltage which represents a predetermined temperature at which it is desired to operate the furnace. The set point voltage from the controller C-1 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 C-1 and is utilized for controlling the galvanometer G2. The galvanometer G-2 serves as a master galvanometer and the galvanometers G-1 and G3 are slaved to the master galvanometer so that the temperature which is set by the set point controller C-1 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, now Patent No. 3,291,969 and Ser. No. 382,038, filed July 13, 1964, now Patent No. 3,299,196. As hereinbefore explained, the present diffusion furnace includes means for providing a high speed recovery of temperature within the processing zone whenever a load such 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 C-1 and which is utilized for providing an adjustable set point voltage representing a temperature which is above the temperature represented by the set point voltage of the set point controller C-1. As can be seen from FIGURE 1, the output of the set point controller G2 is connected in series with the input to the set point controller C-1.

Additional temperature sensing means is provided in the form of a thermocouple T-6 which is disposed in the ceramic tube 36 which is positioned so that the thermocouple T-6 is in zone B as shown in FIGURE 1 between 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. Movable contact 9 is connected to the set point controller C-2 by a conductor 53 and the stationary contact 10 is connected to the set point controller C-Z by conductor 54. The stationary contact 10 is connected to the movable 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 a terminal L-4 by conductor 59. The terminal L3 is connected to movable contact 2 of the meter relay 48 by a conductor 61. The terminals L-3 and L-4 are connected to a suitable source of voltage such as volts AC. The movable contact 9 of the timer 52 is connected to the conductor 61 by conductor 62. The stationary contact 2 of the timer 51 is connected to conductor 53 by a conductor 63. Stationary contact 2, of timer 51 is also connected to one side of the motor by a condoctor 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 G4 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 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-S produces a voltage which opposes the reference voltage produced by the digital set point controller C-l. As pointed out previously, thermocouples T4 and T-4 sense the temperatures in zones A and C, and since they are slaved to the master thermocouple T-S, they bring the zones A and C up to the same temperature as zone B.

Now let it be assumed that the temperature in the processing zone 28 within the processing tube 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 articles on which the diffusion 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 C-2 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 C-l. By way of example, the set point controller C-Z can be manually adjusted to provide an additional voltage which is equivalent to a temperature of 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 C. within 20 seconds.

As soon as the cold, i.e., in comparison to the furnace temperature, 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 introduction of the boat and the consequent 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 10 which causes the set point voltage being produced by the controller C-2 to be added to the set point voltage produced by the controller C-l to indicate to the galvanometer 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 hold ing 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 energization of the meter relay 48 that the voltage produced by the set point controller C-2 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 0-1. For that reason, until the meter relay 4% is operated, the diffusion furnace is under the control of the controller C-1.

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 6 is substantially above the temperature represented by the output from the controller C-1. Thus, it is the setting of the set point controller C-2 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 C-2 so that the control system is only under the control of the set point voltage provided by the controller C-l.

From the foregoing, it can be seen that once the timer 51 is started in operation by theclosin g 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 C-Z 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-1 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 C-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 r 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 force which determines the rate of temperature rise of the boat. As can be seen from FIGURE 4, this temperature differential becomes smaller and smaller and it is for this reason that approximately 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 system hereinbefore 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 curve 83. The wall temperature of the furnace then overshoots the set point to arrive 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 -2. At an appropriate time as determined by the timer 51, the timer times out and removes the additional voltage from the galvanometer G-Z as hereinbefore described to place the furnace under the control of the controller C-l. This step is represented by the joint 83d on the curve 83. Thereafter, the wall temperature of the furnace drops gradually down to the set point as represented by the portion 832 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 heating from the unloaded ends of the furnace. 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 C2 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 method for operating a diffusion furnace having a processing zone, heating the processing zone to a first temperature, sensing when a cold load is introduced into the processing zone causing the temperature in said processing zone to drop below said first temperature, supplying additional heat to the processing zone continuously after introduction of the load to accelerate the recovery of said furnace and to cause the temperature in said zone to rise rapidly above said first temperature and terminating the application of heat to said zone after a predetermined period of time to permit said zone to return to said first temperature.

2. In a method for operating a diffusion furnace having a processing zone, supplying heat to the processing zone to maintain the same at a first temperature, introducing a cold load into the processing zone causing the temperature in the processing zone to drop, sensing when the load is introduced into the processing zone by determining that there has been a transfer of heat from the furnace to the load, supplying additional heat to the processing zone continuously as soon as the introduction of the load is sensed to cause the temperature in said processing zone to recover and to overshoot beyond the first temperature to cause the load to rapidly increase in temperature, and terminating the application of heat to said zone after a predetermined period of time and permitting the temperature in the zone to drop to said first temperature so that it reaches said first temperature at aproximately the same time that the load reaches the first temperature.

3. A method as in claim 2 wherein the temperature, when it overshoots, rises to a second temperature which is higher than said first temperature and remains at said second temperature for a predetermined period of time.

References Cited UNITED STATES PATENTS 2,422,734 6/1946 Jung "13-24 2,600,490 6/1952 De v66 13 34 2,874,906 2/1959 Nossen 236-15 3,171,018 2/1965 Lawler 219 494 3,311,694 3/1967 Lasch 13 24 RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Examiner.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
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US2600490 *Apr 21, 1947Jun 17, 1952Corning Glass WorksGlass melting method
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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3543005 *May 18, 1967Nov 24, 1970Kelemen Leslie AndrewTemperature control system for an electrically heated blanket
US3952945 *Nov 18, 1974Apr 27, 1976United States Borax & Chemical CorporationFurnace temperature control system
US4061870 *Jul 28, 1975Dec 6, 1977Kokusai Electric Co., Ltd.Temperature control system
US4276603 *Oct 30, 1979Jun 30, 1981Btu Engineering CorporationDiffusion furnace microcontroller
US4348580 *May 7, 1980Sep 7, 1982Tylan CorporationEnergy efficient furnace with movable end wall
US4849608 *Feb 11, 1988Jul 18, 1989Dainippon Screen Mfg. Co., Ltd.Apparatus for heat-treating wafers
US4952780 *Oct 31, 1988Aug 28, 1990Grumman Aerospace CorporationComputerized multi-zone crystal growth furnace
US5228114 *Sep 17, 1991Jul 13, 1993Tokyo Electron Sagami LimitedHeat-treating apparatus with batch scheme having improved heat controlling capability
US5415535 *Jan 6, 1994May 16, 1995Kabushiki Kaisha ToshibaSemiconductor resin sealing apparatus
Classifications
U.S. Classification373/136, 236/15.00R, 236/15.0BF, 219/494
International ClassificationF27D11/02, H05B3/00, F27D11/00
Cooperative ClassificationH05B3/0028, F27D11/02
European ClassificationF27D11/02, H05B3/00C2