US3456932A - Heat-treatment apparatus having reciprocating multiple furnaces - Google Patents

Description

TERUO KOZAI July 22, 1969 HEAT-TREATMENT APPARATUS HAVING RECIPROCATING MULTIPLE FURNACES Filed Jan. 31, 1968 2 Sheets-Sheet l FIG. I

FIG. 3A F1638 INVLZN'I'OR. TERI/O K024! FIGBC 4 rraz/vzrs TERUO KOZAI July 22, 1969 HEAT-TREATMENT APPARATUS HAVING RECIPROCATING MULTIPLE FURNACES Filed Jan. 31, 1968 2 Sheets-Sheet 2 mmdE INVliN'IUAv 7521/0 KOZAI' United States Patent Cl. 2636 3 Claims ABSTRACT OF THE DISCLOSURE A heat-treating apparatus and method are provided for rapidly heating and cooling objects such as semiconductor elements. A plurality of furnaces are provided on a common support arranged along a common axis in a predetermined direction. Each of the furnaces has a heating chamber. An elongated member passes freely through the furnaces, the member supporting an object-carrying means, e.g., a crucible. By causing the furnaces to move simultaneously back and forth, the object-carrying means is caused to negotiate each of the heating chambers, whereby it is subjected to the particular temperature profile of each chamber. Gas is fed to opposite ends of the line of furnaces to control the atmosphere and to cool rapidly the object-carrying means.

This invention relates generally to heat-treatment apparatus, and more particularly to apparatus adapted for effecting rapid heating and cooling according to a predetermined timing schedule for objects to be heated, such as semiconductor elements employed in the fabrication of semiconductor devices.

It is known to use heating apparatus of the bell-jar type for rapidly heating and cooling semiconductor elements in the fabrication thereof. However, it is difficult to accurately reproduce a desired temperature time profile by this technique, whereby uniform results could not always be obtained. The improvement over the foregoing will become apparent from the following description and the accompanying drawings, wherein:

FIG. 1 is a schematic cross section of a bell-jar type heat-treatment apparatus previously employed in producing semiconductor elements;

FIG. 2A is a schematic representation in longitudinal cross section of the essential parts of a reciprocating multiple-furnace type heat-treatment apparatus provided as a preferred embodiment of this invention;

FIG. 2B is a schematic side elevational view of the apparatus shown in FIG. 2A;

FIG. 3A is a top view of a crucible (or boat) employed as an integral unit of the apparatus of FIG. 2 for carrying objects to be processed, such as semiconductor elements, drawn to an enlarged scale to show the details; and

FIGS. 3B and 3C are respectively side and front elevational views of the crucible shown in FIG. 3A.

Heretofore, rapid heating and cooling for semiconductor elements were conducted in heat-treatment apparatus such as that shown in FIG. 1, using a ribbon-type electric heater and a controlled atmosphere in a bell-jar type enclosure illustrated.

Referring to the drawing, a ribbon-type heater 1 is shown to which electric power is supplied through leads 22 for rapidly heating a plurality of semiconductor elements mounted thereon in the middle portion thereof. A thermocouple 3 is provided soldered to the lower side of said ribbon-type heater for sensing the temperature of the heater, the bell-jar 4 being employed asan enclosure for a suitable inert gas atmosphere to prevent oxidation. In subjecting the semiconductors mounted on the heater to a scheduled temperature/time heating process in a suitable inert gas atmosphere, the apparatus had many drawbacks keenly recognized by those skilled in the art as will be enumerated.

Firstly, the apparatus is not structurally suited to follow up a sharply defined temperature/time profile of the heating process ideally called for semiconductor elements. This deficiency cannot generally be rectified, even if fully automatic and precise temperature control instrumentation are provided, because of the small thermal capacity of the ribbon-type heater, slow thermal responses due to heat conduction as a means of heat transfer adopted for this apparatus, and other causes. As a second consideration, it is diflicult to reproduce the same temperature-time profiles, whereby reproducibility of the electrical characteristics of semiconductor products could not always be obtained. Thirdly, this type of apparatus essentially imposes practical limitations on the quantity of semiconductors that can be handled at one time, the average quantity being, for example, about 10. The apparatus itself, therefore, is not suited to volume production. Hermetic scaling becomes much more difiicult with increasing size which, in turn, brings about more deviations from the ideal profile. Fourthly, as the rate of output from the apparatus is generally low, the heat-treat ment is extremely tedious and timeconsuming. This is compounded further by the fact that air must be replaced by inert gas prior to carrying out the process.

It is the principal object of this invention, therefore, to provide a new and improved heat-treatment apparatus which is easy-to-handle, reliable in heat treatment, adapted for volume production and, yet, which affords substantial advantages as compared with previously known constructions by eliminating as much as possible the above-mentioned drawbacks.

The other object of this invention is to provide a heattreatment apparatus with reciprocating multiple furnaces for realizing a scheduled temperature-time profile requiring rapid heating and cooling for an object or objects to be heated.

Among outstanding physical, mechanical, and constructional features of the apparatus according to this invention are the following:

(1) Adaption of a multiple-furnace type heating source having a sufficient thermal capacity which will be unaffected by the heating load conditions and of heat radiation instead of heat conduction as a means for heat transfer.

(2) The construction of the heating apparatus, or the multiple-furnace assembly, is designed to move back and forth in a reciprocating motion, with a mutfie passing through but integral with the walls of the multiple-furnace chambers. A crucible (or a boat) for carrying objects to be processed is supported by an elongated supporting member, for example a wire or wires, which is adapted to pass freely through the muffle of the heating source and fixedly supported regardless of movement of the heating source. By moving the heating source relative to the crucible, the crucible can pass freely and unhindered during a firing cycle through the muffie or through the multiplefurnace heating chambers.

(3) A well-coordinated action is provided between rapid start-stop operations in linear back and forth movement of the multiple-furnace arrangement with a scheduled temperature profile. This permits the provision of clear-cut and reproducible temperature/time profiles for objects to be heated for consistent results'with the aid of fully automatic instrumentation incorporated in the apparatus. v

(4) A gas supply means is provided at the entrance and exit ends of the mufile to form protective gas-curtains at 3 both ends for preventing the air from getting into" the mufile and for rapidly cooling the product.

Automatic time and temperature setting facilities are incorporated for presetting almost any type of tem* perature/ time profile required for various kinds of semiconductors.

The above-mentioned and other objects and features of this invention will be apparent from the following description of a preferred embodiment of this invention as illustrated in FIGS.' 2A, 2B, 3A, 3B and 3C of the accompanying drawings.

Referring to FIG. 2A and FIG. 2B, which depict the rnade 2,500 mm. long in the preferredembodiment. These heat-treatment apparatus as a preferred embodiment of this invention with a part of the control console cut away and a part thereof shown in front view, the heating source is composed of two furnaces: preheating furnace 31 of 375 mm. in length, 260 mm. in diameter, and a main heating furnace 32 of 424 mm. in length, 260 mm. in diameter, each mounted rigidly on a common base 33. The clearance between the two furnaces can be suitably adjusted as required. In the embodiment, the clearance is made 30 mm. A scheduled temperature profile precisely adapted for a particular kind of object to be processed is provided for muffle 6 along its length by the furnaces. The tubular muflle *6 made of fused quartz or other heat-resistant refractory and good heat-transmittive materials may be 1,200 mm. in length and 60 mm. in inner diameter, the mufile passing through but being integral with the walls of the preheating and main heating furnaces 31 and 32. In other words, the mufile is supported by the furnace walls. Each furnace is built of a refractory material 34 in which sectionalized heating elements (not shown) are imbedded and of a light-weight heat insulating material 35. Two thermocouples 21 are provided for sensing furnace temperatures through the top walls of the furnaces, the sensing ends of the thermocouples 21 being made to contact closely the external wall surface of the muffle tube 6 at approximately the centers of the respective heating chambers. With the aid of temperature regulators 32 and automatic control circuitry associated therewith (not shown), these thermocouples can sense and control the furnace temperatures independently.

The crucible 9 for carrying the object (or objects) to be processed, and to which a scheduled rapid heating and cooling cycle is to be effected, is mounted on a pair of wires 10, which are in turn made of a heat-resistant metal and adapted to pass freely through mufile 6. The inside diameter of the mufile 6 should, therefore, be suitably designed to allow free passage of the crucible 9.

The "pair of wires 10 are stretched and fixed between the two support frames 11 and 12, in such a manner that .vibrations or shocks which occur in the start-stop movement of the furnace assembly are inhibited from being transmitted to the wires. An appropriate constant tension is maintained in each wire by means of a pulley 13 and a counterweight 14. A lug 17 is rigidly fixed to the base 33 .this case clutch 24 is energized and brake 25 is deenergized. This causes the assembly of preheating furnace 31 and main heating furnace 32 to move quickly in the direction of the arrow 41 at a constant speed from the starting position on the extreme right-hand side of the forward stroke. When the midpoint of crucible 9 reaches the position of thermocouple 21 in preheating furnace 31, the furnace arrangement stops for a predetermined time interval. Nozzles 7 meanwhile supply an inert gas continuously into the muffle 6, while nozzles 8 provide inert gas curtains at all times at the entrance and exit ends of the mufile and also to cool the heated crucible 9 containing the heated objects to room temperature when it reaches a stop position beneath nozzle 8 on the righthand side of the mutlle.

After the elapse of a specified time, the furnace ar rangement resumes rapidly the forward motion (in the direction of arrow 41) and stops suddenly when the midpoint of the crucible 9 reaches a similar position in the main heater 32. After a specified elapse of time, the arrangement resumes the forward motion (in the direction 41) and stops quickly at a point where the midpoint of the crucible 9 is aligned with the axis of the nozzle 8 I on the righthand side. In this location, the objects and as shown, the lug having a threaded hole with which 4 The automatic timers 26 and the limit switches 27 are employed for automatically controlling the startstop operation schedule for the furnace assembly. The limit switch 28 is to confirm whether or not the crucible 9 is properly positioned with respect to the furnace assembly, 'while the limit switches 27 are to determine the extreme ends of the reciprocating motion of the furnace assembly. The interval between the two switches 27 is the crucible 9 are rapidly cooled by an inert gas blowing out from the nozzle 8 through a diffuser (not shown).

After the cooling process is over, the furnace assembly, comprising furnaces 31 and '32, is displaced in the same direction of the arrow 41 a small distance to enable removal of the crucible 9 from wires 10. The assembly then assumes the return stroke, moving in the direction shown bythe arrow 42 and reaching the reset position without stopping.

Crucible 9 will now be discussed with reference to FIG. 3. It is made of heat-resistant metal such as nickel having a small thermal capacity and having a flat recessed portion for holding objects to be processed (not shown), such as semiconductor elements. The two pairs of flangelike portions 9' are for mounting the crucible 9 on a pair of wires 10 to assure stability against any vibrations or shocks of the wires. The crucible 9 may be 20 mm. in length, 3.9 mm. in maximum width and 1.0 mm. in depth. The wires 10 are made of heat-resistant material, such as Ni containing a small amount of Cr, Al and Fe of 1.0 mm. in diameter, the wires being separated by about 3.5 mm.

An alloying process is known for producing germanium pellets with indium dots in the fabrication of germanium Esaki diodes, wherein rapid heating and cooling is required. The necessity of the rapid heating and cooling treatment in the alloying process exemplified above is to prevent the p-n junction formed during the alloying process from being too enlarged. In one example in which the alloying process of Esaki diodes was carried out by use of the heat-treatment apparatus according to the above preferred embodiment of this invention, n-type germanium pellets (containing arsenic of more than 10 cmr mounted 'with indium dots (containing 0.5 wt. percent gallium) were put in crucible 9, and the following temperature/time profiles considered the most appropriate for the alloying process of' the Esaki diodes were employed: 1st and 2nd stopping periods approximately 10 sec. and 20 sec. respectively; heating time (from room temperature to approximately 390 C.) 12-13 seconds heating at temperature of 390 C. -0.5 C. for 20 seconds in the main furnace 32; cooling time (down to room temperature) 20-30 seconds. The constant speed of the furnace assembly was set at 500 mm./sec. Further, the steady state temperature distribution in muflie 6 was as follows: high temperature level (in preheating furnace 31) 700800C. :03 C., over the length of approximately 200 mm.; low temperature level (in the main heating furnace 32) 390 C. :0.5 C. over approximately 100 mm.

Many advantages of the present heat-treatment apparatus, such as reproducibility of temperature schedules, repeatability of the same type temperature/time profile, improvements in temperature control accuracy, automatization of the rapid heating-cooling process, and ease of volume production of semiconductor devices, notably tunnel diodes, will be obvious from the foregoing descriptions. The heat-treatment apparatus according to this invention finds application not only for semiconductor devices, but also for all kinds of objects to be heated for which precise and reproducible temperature/time profiles requiring rapid heating and cooling are needed. Although an inert or inactive gas such as nitrogen is preferred for the atmosphere in the mufile or for the cooling gas or the gas curtain in the thermal processing of semiconductor devices, it will be appreciated that an oxidizing or a reducing gas may be used as required. Further, there should be no objection for constructing the furnace assembly with more than three furnaces in cases where the temperature profile along the muffle requires more than three temperature levels. Alternatively, a pair of rigid rods of heat resistant metal may be used in place of the pair of wires 10.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes and modifications in form and details may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. A heat-treating apparatus comprising, a plurality of furnaces arranged along a common axis in a predetermined direction and supported on a common base, each of said furnaces having a heating chamber, an elongated heat-resistant member passing axially and freely through each of the chambers of said furnaces in said predeter mined direction, said member being supported in a fixed position, a heat-resistant object-carrying means supported by said elongated member, means for simultaneously moving said furnace together with their common base along their axis in said predetermined direction, whereby said object-carrying means passes through each of the heating chambers, and means for feeding a cooling gas at the outer ends of said furnaces.

2. A heat-treating apparatus comprising, a plurality of furnaces arranged along a common axis in a predetermined direction and supported rigidly on a common base, said apparatus having at least one preheating chamber and at least one main heating chamber, a tubular muffle of refractory material passing through the walls of said furnaces along the common axis thereof, said muffle passing through said heating chambers and being supported by said furnaces, a, crucible of heat-resistant material for carrying objects to be heat treated, said crucible being dimensioned to pass freely through said muffle, an elongated support means for supporting said crucible, said support means passing axially and freely through the muffle in said predetermined direction and being supported in a fixed position, means for simultaneously displacing said furnaces and supported mufile back and forth along said common axis involving a plurality of scheduled stopping periods relative to said crucible, and gas feeding means provided at the entrance and exit ends of said mufile for maintaining the desired atmosphere in said muflle, for maintaining protective gas curtains at both ends of the mufile and for rapidly cooling the crucible.

3. A method for rapidly heating and cooling semiconductor elements which comprises, providing a plurality of furnaces on a common support arranged along a common axis in a predetermined direction, each of said murnaces having a heating chamber characterized by a particular temperature profile, supporting a semiconductor carrying means on an elongated heatresistant member passing freely through said furnaces, said elongated member being held in fixed position so as not to contact said furnaces, then simultaneously moving said furnaces and their common support back and forth along said common axis, whereby said carrying means is caused to negotiate each of the heating chambers, and feeding a gas to the ends of said furnaces to provide a protective atmosphere in said furnaces and for rapidly cooling said carrying means.

References Cited UNITED STATES PATENTS 2,880,982 4/1959 Kuhnapfel et al 2636 2,994,522 8/1961 Albers-Schoenberg 26341 3,016,443 1/1962 Fliezar et al 263-6 X 3,020,032 2/ 1962 Casey 263-42 3,043,575 7/1962 Emeis 263--41 3,405,924 10/1968 Beck 263-41 JOHN J. CAMBY, Primary Examiner US. Cl. X.R.

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