US3009973A

US3009973A - Device for controlling the zone melting of semiconductor rods

Info

Publication number: US3009973A
Application number: US831289A
Authority: US
Inventors: Emeis Reimer
Original assignee: Siemens Corp
Current assignee: Siemens Schuckertwerke AG; Siemens Corp
Priority date: 1958-08-16
Filing date: 1959-08-03
Publication date: 1961-11-21
Anticipated expiration: 1978-11-21
Also published as: NL240020A; CH373193A; BE581687A; NL113487C; FR1232565A; GB907478A; DE1141086B

Description

Nov. 21, 1961 R. EMEIS 3,009,973

DEVICE FOR CONTROLLING THE ZONE MELTING OF SEMICONDUCTOR RODS Filed Aug. 3, 1959 4 Sheets-Sheet '1 Nov. 21, 1961 R. EMEIS 3,009,973

DEVICE FOR CONTROLLING THE ZONE MELTING OF SEMICONDUCTOR RODS Filed Aug. 3, 1959 4 Sheets-Sheet 2 20 21 8 I z l a Q g Q 25 1 18 FIG.2

R. EMEIS 3,00

DEVICE FOR CONTROLLING THE ZONE MELTING OF SEMICONDUCTOR RODS Nov. 21, 1961 4 Sheets-Sheet 3 Filed Aug. 3, 1959 4|l|lllllllll \IIIIIIIIIIIIIH FIG.4

Nov. 21, 1961 EMEIS DEVICE FOR CONTROLLING THE ZONE MELTING OF SEMICONDUCTOR RODS 4 Sheets-Sheet 4 Filed Aug. 3, 1959 United States Patent 3,009,973 DEVICE FOR CONTROLLING THE ZONE MELT- ING OF SEMICONDUCTOR RODS Reimer Emeis, Ebermannstadt, Upper Franconia, Germany, assignor to Siemens-Schuckertwerke Aktiengesellschaft, Berlin, Germany, a corporation of Germany Filed Aug. 3, 1959, Ser. No. 831,289

Claims priority, application Germany Aug. 16, 1958 I 21 Claims. (Cl. 13-1) My invention relates to devices for controlling or regulating the zone melting of rod-shaped semiconductor material for the production of semiconductor elements to be used in electronic semiconductor devices such as rectifiers, transistors, photodiodes and the like.

' The semiconductor body of such devices, in most cases, consists of germanium, silicon or an intermetallic compound of respective elements from the third and fifth groups of the periodic system, such as indium antimonide or indium arsenide. Large quantities of such materials of extremely high purity are required for'industrial production of the electronic devices. Various producing and fabricating methods have been developed for providing such high-purity-materials. Forexample, a semiconductor-rod, obtained by pulling the material out ofa melt or by any other suitable process, can be purified to the de-' sired degree by subjecting the rod to the so-called zonemelting or zone-pulling method. One way of performing this method is to heat the rod to the melting point within an axially limited zone by means of an induction winding, and to then shift the winding back and forth along the rod, to thus advance the molten zone repeatedly along the entire length of the rod, with the effect of eliminating undesired impurities by shifting them toward the rod ends. The devices used for this purpose are also applicable for converting the material into monocrystalline condition.

Ina known device for such crucible-free zone melting, particularly of silicon, the semiconductor rod is vertically mounted and firmly secured at one end, whereas the other end is turned about the rod axis when the zone is in melted condition. Considerable skill is required for the attendant to properly determine the moment when the semiconductor material is melted throughout the narrow zone. If the rotation of the rod end is commenced too early, the crystal seed being used may break off or some other disturbance may be encountered. If the rotation is started too late after the melting-through moment, the melted zone may sag and cause spherical thickening of the rod or drip away. I

It is therefore an object of my invention to devise a zone-melting apparatus which automatically indicates the accurate melting-through moment. Another, related object is to provide a device that permits automizing the zone-melting operation by signalling the melting-through moment'and, if desired, by issuing a control signal for initiating the rotating or other subsequent processing operation.

It is known to take advantage of the fact that the melting-through of a semiconductor rod causes a separation of the rigid mechanical connection between the two ends of the rod, by releasing the opening or closing motion of an electric switch contact. A known device of this type thus employs an occurring longitudinal motion of one of the two rod-end holders; but such longitudinal movement may result in undesired squashing of the melted zone similar to an upsetting action. It is therefore another object of my invention to provide an automatic indicating or controlling device of the abovementioned type that reliably obviates the just-mentioned deficiency.

According to a feature of my invention, the apparatus for controlling or regulating the zone-melting operation Patented Nov. 21, 1961 "ice of the semiconductor rod mounted at both ends in ro tatable holders, is provided with a device which, in re sponse to melting-through conditions, imposes a torque upon one of the two holders, and I provide a switch contact which is controlled by the rotation of the holder caused by such torque.

According to another feature of my invention, the above-mentioned device is provided with a time-delay member which retards the signal transmission of the switching device a given interval of time. The delay member, according to a more specific feature, consists preferably of an oil-damping or dashpot device which delays the rotation of the rod holder. The above-mentioned torque may be produced for example by a spring or by the coaction of an electric-current winding with a permanent magnet. In the latter case the torque-producing device can be readily reset to the starting position simply by reversing the poling of the current winding.

According to another feature of the invention, it is preferable to locate the torque-producing device, as well as the switch contact, outside of the high-vacuum or protective-gas filled processing vessel of the zone-pulling app'aratus and to couple the torque-producing device with the movable holder inside the vessel by means of a magnetic coupling. This has the advantage that the amount 3 of material and space to be accommodated within the vessel is not increased and that the novel components to be added to the zone-pulling apparatus in accordance with the invention remain readily accessible. Consequently, any trouble is easily recognizable and can immediately be eliminated. It is particularly preferable to mount the oil-damping member outside of the vacuum chamber.

The provision of a magnetic coupling for connecting the components located outside of the vessel with the rotatable holder inside the vessel, is particularly advantageous because it obviates the need for another seal, such as a flexible gasket, and also because it operates entirely free of friction.

The design of the magnetic coupling for transmitting a rotary motion can be kept relatively simple. For this purpose, the movable holder is preferably provided with a magnetizable armature of magnetically soft iron and is mounted within a cup of non-magnetic metal connected with the vessel, whereas the torque-producing device is mounted outside of the cup and acts upon the armature by means of one or more permanent magnets.

According to another feature, more specific than the one last mentioned, a U-shaped bracket is mounted outside of the non-magnetic cup and is connected with the torque-producing device, the bracket being rotatable about the axis of the cup and carrying respective permanent magnets on its two limbs. The rotation angle of the U-shaped bracket is preferably limited by two stops which also serve as electric contacts.

According to still another feature of my invention, an

additional contact device is mounted beyond the angle of bracket rotation and is releasable by the movable rod holder, for example, with the aid of a permanent magnet which is attracted when a soft-iron armature, connected the apparatus, FIG. 2 is a top view, and FIG. 3 a lateral view. FIG. 4 shows the complete apparatus together with a schematic circuit diagram of the associated electric control component; FIG. 5 is a schematic diagram of another embodiment of control circuitry applicable with the apparatus of FIGS. 1 to 3, and FIG. 6 illustrates a signal device.

According to FIGS. 1 and 4, the zone-pulling device comprises a vacuum vessel 1 (FIG. 4), whose top 2 has an opening 2a (FIG. 1) closed by a cup structure 3 of non-magnetic material, preferably of aluminum or other light metal, a sealing gasket '4 being interposed. A shaft 5 is fastened by means of a flange 6 to the top portion of the cup structure and carries two ball bearings 7 and 8 in which the upper holder 9 for the semiconductor rod 34 is journalled. The upper end of the semiconductor rod is fastened in holder 9 by means of set screws 10 of which only one is shown. The sleeveshaped holder 9v carries on its upper end a soft-iron armature 11 (FIGS. 1, 2) for cooperation with two rod-shaped permanent magnets 12 mounted on the re,- spective limb ends of a U-shaped bracket 13. The bracket 13 is mounted by means of ball bearings 14 "on a vertical pivot shaft 15 co-axially aligned with shaft 5, so that the axis of bracket rotation coincides with the rotation axis of the rod holder 9 and hence with the longitudinal axis of the semiconductor rod 34 to be processed. The pivot shaft 15 carries a hub member 16 with vanes 17 of an oil-damping mechanism. Hub 16 and vanes 17 are located in a cavity within the top portion of the cup structure 3. The cavity is closed by a cover 18.

Mounted on the yoke portion of bracket 13 is a permanent magnet 19 which, in cooperation with a current coil 20, imposes a torque upon the bracket 13.

when coil 20 is energized. Coil 20 is. rigidly joined with the cup structure 3 by two supports 21 which also form pole shoes for the coil 20 (FIGS. 1, 2, 3,). An insulating bar 22 serves as a carrier for two stop contacts 23 which limit the angle of rotation of the bracket 13. Two contact pins 24, in electrically conducting connection with each other, are mounted in the bracket 13 but are insulated therefrom. After the bracket passes through the rotation angle a (FIG. 2), the contact pins 24 abut against the respective stop contact 23, thus performing an electric switching operation for the signalling and control purposes more fully described below. The supply of current to the contact pins is effected free of torque through a flexible conduct-or, such as a thin strip of silver 24a (FIG. 6) coming from terminal means 22a mounted on the insulating bar 22.

On one side of the cup structure 3 there is mounted another contact device which mainly consists of a support 25 (FIG. 3), a fixed contact 26 mounted on the support 25, a movable part 27, and a permanent magnet 28 joined with part 27. Any movement of the permanent magnet is transmitted by means of a pin 30 to the movable contact of the device.

The operation of the device is as follows.

The semiconductor rod 34 to be processed by zone pulling, for example for purification, is inserted into the two

holders

9 and 35 of the zone-pulling device as shown in FIG. 4. The lower holder 35 is connected through gears 37 with a drive motor M for rotating the holder 35 about the longitudinal axis of the rod, but the drive is at first inactive. After evacuating the processing vessel 1 through a conduit 1a or filling it with a desired protective atmosphere, the operation is started by energizing the current coil 20. Due to the coaction of the magnetic field of coil 20 with that of the permanent magnet 19, the magnet attempts to adjust itself in the longitudinal direction of the coil 20. Since the bracket 13 with which the magnet 19 is firmly connected, is journalled for easy rotation, a rotary movement of the permanent magnet would be possible if the bracket were not arrested by the permanent magnets 12 attracting the armature 11. The magnets 12 retain the bracket 13 as long as the armature 11, firmly joined with the rod holder 9, does not permit a rotation. Simultaneously with the coil 20, the heating device, namely the inductive heating coil IC of the apparatus, is energized so that a limited zone of the rod becomes heated. As soon as the semiconductor rod in the heated zone becomes melted throughout its entire cross section, the rigid connection between the upper and lower rod holders is eliminated, and the upper holder 9 becomes free to rotate. Due to the torque produced by the magnet fields of coil 20 and permanent magnet 19, the holder 9 and the bracket 13 now commence rotating about the longitudinal axis of the semiconductor rod. This rotary motion is terminated by one of the contact pins 24 abutting against the correlated stop contact 23. The closing of this contact is then used for effecting a signalling or control operation.

If, contrary to the automatic, performance of the apparatus shown in .FIG. 4, the contact device is used only for signalling purposes, it suffices if the signal is transmitted without delay; for example when the rotation of the bracket occurs relatively rapidly, the closing of the contact between pin 24 and stop contact 23 may immediately close the, circuit of cell 23a to energize an acoustical or optical signal device 23b (FIG; 6) which calls attention of the attendant to the fact that the zone is completely melted. The attendant can then, after any interval of time as may appear desirable, switch on the advancing travel of the heating coil and, hence of the melting zone, and can also switch on the drive, for rotating the portion of the semiconductor rod located beneath the melting. zone. If desired, the; attendant may also effect a change in the amount of power supply.

If it is desired to automate the entire operation of the device, the above-mentioned closing of the electric contact between

members

24 and 23 may also be used for the actuation of relays which then set the individual operations in. motion. This requires a time delay because it may happen that although the semiconductor rod is sufliciently melted over its cross section to permit the above-mentioned rotary motion of the upper holder, the conditions may at first be such that an immediate rotation of the lowerholder may cause disturbance or faults, for example due to the fact that the cones of still solid semiconductor material protruding from both axial sidesinto the melted zone may hook into each other. The desired time delay may be effected electrically, for example by timing relays. However, the delay may also be obtained by retarding the rotary motion of the bracket 13 as is the case in the example shown in FIGS. 1 to 3. For this purpose the upper cavity of the cup structure 3, closed by cover 18, is filled with oil or other liquid in order to impede the motion of the vanes 17. In any event it is preferable to provide for damping the rotation of the upper semiconductor rod portion in order to prevent disturbance of the melted zone by a too vigorous motion.

The automatic control system illustrated in FIG. 4 provides for delaying action by the above-described hydraulic device comprising the vanes 17 as well as by an electromagnetic timing relay TR. The control system operates'as follows.

After the semiconductor rod 34 is properly .inserted into the

holders

9 and 35, and the processing vessel 1 is evacuated, the attendant depresses a push-button switch 40 in order to start the automatic performance. Switch 40 closes its normally

open contacts

41, 42, 43, 44 and is thereafter held closed by a latch releasable under control by a coil 45 whose circuit includes anormally open stop contact 46 in parallel to the freeze-

responsive contacts

26 and 29 described above.

The actuation of start button 40 applies alternating current through contacts.

torque upon the upper holder 9 as described above, but the torque is at first ineffective as long as the semiconductor rod 34 maintains a rigid connection between the two

holders

9 and 35. However, when a narrow zone of rod 54 is melted through by the heating effect of coil IC, the bracket 13 rotates until it closes the

contacts

23, 24. These contacts now energize the coil 47 of the timing relay TR which closes its normally

open contacts

48 and 49. Coil 47 remains thereafter energized through the self-holding contact 48. The closing of contact 49 energizes a drive motor M which, acting through a gearing 37, commences to rotate the rod holder 35. At the same time, the motor drives through a gear 38 a vertical screw spindle 39 with which the holder 36 of coil 10 is in threaded engagement. Consequently, the coil IC now moves slowly upward along the rod to perform the desired zone-melting operation. The operation can be stopped either at will by actuation of the stop button 46 or automatically by the

contacts

26, 29 which energize the release coil 45 in response to freezing of the melting zone.

The automatic control effected by the

contacts

26, 29 will be more fully understood from the following. If due to any disturbance, for example failure of the feed voltage applied to the zone-heating device, the melting zone commences to freeze, the upper rod holder 9 is entrained by the rotary motion of the lower rod holder 35. Since the bracket 13 cannot follow this motion, the armature 11 is forcefully torn out of its magnetic connection with the two permanent magnets 12. The armature now passes out of the range of the angle a and, after about one quarter of one rotation, passes into the range of the permanent magnet 28 of the auxiliary contact device. The magnet 28 now moves radially toward the armature 11 and thereby closes the

contacts

26 and 29. This can be used for issuing a corresponding signal and/or, as described, for disconnecting the entire apparatus or the components affected by the disturbance. Consequently, by providing the auxiliary contact device comprising the components 25 to 30, the invention afi'ords the possibility of supervising not only the melting-through of the semiconductor rod but also the freezing of the melting zone.

Since hydraulic or time-relay damping, in view of the short travel distance of the bracket 13 (rotation angle a in FIG. 2), affords too little delay for some purposes, it is in some cases desirable to provide for a further time delay between the melting-through moment of the zone and the issuance of the signal. This can be done by causing tthe bracket 13 to repeat its rotary motion once or several times befor the electric signal is transmitted.

For example, after the bracket has once passed through the travel angle at, the closing of

contacts

24, 23 may be caused to reverse the poling of the current coil 20, whereafter the bracket 13 returns in the opposite direction and thereafter closes the other contact pair with the effect of initiating the above-mentioned signalling or controlling operation. If then the rotary motion of the lower rod holder is switched on, the bracket 13 remains in the position last reached, which is also its starting position. This is so because the mechanical connections between the upper and lower rod holders are now virtually interrupted by the intermediate melting zone.

An apparatus comprising the features last described is illustrated in FIG. 5, it being understood that the zonemelting device proper is in accordance with that described above with refcrence to FIGS. 1 to 4.

In the apparatus according to FIG. the automatic operation is initiated by actuating a push-button switch 70 which closes its normally open contacts 71 to 74. The contacts 73 and 74 energize the induction heater IC. At the same time, a control relay R1 picks up because its coil 50 is energized through

contacts

71 and 72 from a source of direct current under control by the

closed contacts

23a, 24. Relay R1 closes its normally

open contacts

51, 52, 53. Contact 51 energizes the torque winding 20 with the polarity indicated by an arrow A. When the zone is melted through, the bracket 13 starts rotating as explained above. As a result,

contact

23a, 24 opens, but the coil 50 of relay R1 remains energized through self-holding contact 52. As soon as the bracket reaches the limit position where it closes the

contacts

23b, 24, another control relay R2 picks up because its coil 60 is now energized. The relay R2 temporarily interrupts at contact 64 the holding circuit for relay R1. Relay R1 drops out, and torque winding 20 is now energized through closed contact 61 with the reversed poling denoted by an arrow B. The relay R2 thereafter holds itself picked up through the self-holding contact 62 and prepares a circuit for the drive motor M by closing a contact 63.

The bracket 13 now rotates in the opposite direction. When it again closes the

contacts

23a, 24, the relay R1 again picks up, and both relays R1 and R2 remain sealed in. Now, the

contacts

63 and 53, being both closed simultaneously, energize the drive motor M for rotating the rod 34 and shifting the induction heater IC along the rod in the same manner as explained above with reference to FIG. 4. The operation is terminated either by actuation of the stop button 77 or by the closing of the freeze-

responsive contacts

26, 29 which operate in the manner described above with reference to FIG. 4.

It will be understood by those skilled in the art upon a study of this disclosure that apparatus according to my invention may be modified in various resjects relative to mechanical components as well as circuitry, and may thus be given embodiments other than particularly illustrated and described herein, without departing from the essential features of my invention, and within the scope of the claims annexed hereto.

I claim:

1. With a device for the zone melting of semiconductor rods having two coaxitlly spaced holders for attaching the respective rod ends and having a zone heater surrounding the holder axis and displaceable along said axis, the combination of a control apparatus responsive to the condition of the rod zone heated by said heater and comprising bearing means in which one of said holders is rotatable about said axis relative to said other holder when the rod zone is melted, torque means operatively coupled with said rotatable holder, and electric switch means having a switch member operatively connected with said rotatable holder for controlling said switch means to operate when said torque means cause rotation of said rotatable holder due to melting-thorugh of said rod zone.

2. Apparatus according to claim 1, comprising a timedelay device connected with said switch means for delaying the operation of said switch means beyond the melting through moment.

3. Apparatus according to claim 1, comprising a hydraulic damper having two relatively movable members and having liquid means for damping the relative motion of said members, one of said damper members being connected with said rotatable holder and the other being fixed.

4. Apparatus according to claim 1, said torque means comprising a ferromagnetic member and an electric winding inductively coupled with each other of which one is fixed and the other is mechanically connected with said rotatable holder, and electric current supply means con-. nected with said winding for energizing it.

5. Apparatus according to claim 1, said torque means comprising a permanent magnet and an electric winding inductively coupled with each other of which one is fixed and the other is mechanically connected with said rotatable holder, a direct-current energizing circuitconnected with said winding, a polarity-reversing switch in said circuit for reversing the sense of the torque produced by said torque means, and limit control means connecting said reversing switch with said rotatable holder for causing said torque means to reverse the rotation of said rotatable holder when said holder reaches a given limit position of rotation.

6. With a device for the zone melting of semiconductor rods having a processing vessel sealed when in operation, two coaxially spaced rod-end holders in said vessel, and a zone heater surrounding the holder axis in said vessel and displaceable along said axis, the combination of a control apparatus responsive to the condition of the rod zone heated by said heater and comprising bearing means in which one of said holders is rotatable about said axis relative to said other holder when the rod zone is melted, torque producing means mounted outside said vessel, torque transmitting means inside said vessel connected with said rotatable holder and magnetically coupled with said torque producing means, and electric switch means mounted outside said vessel and having a switch control member connected with said torque producing means for controlling said switch means to operate when said torque producing means are permitted to rotate by said rotatable holder due to melting through of said rod zone.

7. A device according to claim 6, said vessel comprising a cup-shaped housing structure of non-magnetic material disposed in coaxial relation to said rotatable holder, said torque transmitting member having a magnetizable armature located in said cup structure in coaxial relation to said rotatable holder and forming two diametrically opposite pole faces, and said torque producing means having a substantially U-shaped bracket coaxially rotatable relative to said cup structure and straddling said cup structure, and two permanent magnets mounted on the respective legs of said bracket opposite said respective pole faces for magnetically coupling said torque producmg means with said armature.

8. A device according to claim 6, comprising stationary stop means engageable by said torque producing means for limiting its angle of rotation, and auxiliary electric switching means mounted outside said vessel and magneti- 'cally responsive to said torque transmitting means when said torque transmitting means, due to excessive rotation of said rotatable holder, passes beyond said limited rotation angle of said torque producing means, whereby said auxiliary switching means are controlled to operate upon occurrence of faulty operation of the zone-melting device.

9. A zone-melting device according to claim 7, comprising two stops engageable with said bracket for limiting its angle of rotation, auxiliary switching means for response to faulty operation of the zone melting device, said auxiliary switching means having a switch control member comprising a permanent magnet mounted on said bracket near the pat-h of said armature but at a location beyond said angle of bracket rotation, whereby said permanent magnet becomes magnetically coupled to cause operation of said auxiliary switching means when said armature rotates beyond said limited angle of bracket rotation.

10. With a device for the zone melting of semiconductor rods having two coaxially spaced holders for attaching the respective rod ends and having a zone heater surrounding the holder axis and displaceable along said axis, one of said holders having drive means for turning it about said axis during zone melting operation and the other holder being coaxially rotatable relative to said one holder when the rod zone is melted through by said heater, the combination of control apparatus responsive to the condition of said zone and comprising torque means operatively connected with said rotatable holder for subjecting it to torque about said axis, electric switch means having a switch member operatively connected with said rotatable holder for controlling said switch means to operate when said torque means cause rotation of said rotatable holder due to melting through of said rod zone, stop means engageable with said torque means for limiting its angle of rotation, and auxiliary electric switching means responsive to rotation of said rotatable holder beyond said angle of rotation of said torque producing means, whereby said auxiliary switching means are caused to operate when said rotatable holder becomes rigidly coupled with said drive means due to freezing of said rod zone.

11. An apparatus for zone melting a rod of meltable material, comprising a vessel enclosing a zone-melting chamber, upwardly-downwardly axially spaced holder members in said vessel, the holder members being adapted to support opposite end portions of said rod, a heating eans, power-operated means for turning a first one of the holder means about the axis of the holders, means for relative longitudinal movement of the heating means lengthwise of the rod, means for rotatably supporting the second one of the holder members for rotation about said axis, means for establishing a magnetic flux in a fixed direction, a magnet means located near said flux, the magnet means being adapted to adjust itself in the flux direction, means operative so that torque applied by said flux is temporarily inoperative to turn the magnet means until the second one of the holder members is made free to turn by melting-through of said rod by said heating means, and signal means operative when the melting-through of the rod permits said torque to turn said magnet means.

12. An apparatus for zone melting a rod of meltable material, comprising a vessel enclosing a zone-melting chamber, upwardly-downwardly axially spaced holder members in said vessel, the holder members being adapted to support opposite end portions of said rod, a heating means, power-operated means for turning a first one of the holder members about the axisof the holders, means for relative longitudinal movement of the heating means lengthwiseof the rod, means for rotatably supporting the second one of the holder members for rotation about said axis, means for establishing a magnetic flux in a fixed direction, a magnet means located near said fiux, the magnet means being adapted to adjust itself in the flux direction, means operative so that torque applied by said flux is temporarily inoperative to turn the magnet means until the second one of the holder members is made free to turn by melting-through of said rod by said heating means, and control means operative at a time not before the melting-through of the rod permits said torque to turn the second holder member, said control means being operably connected to automatically initiate the said turning of the first one of the holder members.

13. The apparatus defined in claim 12, said control means including a time delay device to delay the automatic initiating of said rotating.

-14. An apparatus for zone melting a rod of meltable material, comprising a vessel enclosing a zone-melting chamber, coaxial holder members in said vessel, the holder members being adapted to support opposite end portions of said rod, a heating coil in said vessel, poweroperated means for turning a first one of the holder members about the axis of the holders, means for relative longitudinal movement of the heating coil lengthwise of the rod, bearing means for rotatably supporting the second one of the holder members for rotation about said axis, a coil carrying current for establishing a magnetic flux in a fixed direction, a first magnet means located near said flux, current supply means connectable to said coil toenergize it and to thereby initiate the operation of said power-operated means, the first magnet means being adapted to adjust itself in the flux direction, a second magnet means, which is outside of the chamber enclosed by said vessel, a third magnet means in the chamber enclosed by said vessel and mounted upon the second one of the holder members, the second and third magnet means being juxtaposed to act on each other so that the torque applied by said flux is temporarily inoperative to turn the said first magnet means until the second one of the holder members, and consequently the third magnet means, is made free to turn by melting-through of said rod by said coil,

and signal means operative When'the melting-through of the rod permits said torque to turn said second hold- I operated means for turning a first one of the holder.

members about the axis of the holders, means for relative longitudinal movement of the heating coil length- Wise of the rod, bearing means for rotatably supporting the second one of the holder members for rotation about said axis, a coil carrying current for establishing a magnetic flux in a fixed direction, a first magnet means located near said flux, current supply means connectable to said coil to energize it and to thereby initiate the operation of said power-operated means, the first magnet means being adapted to adjust itself in the flux direction,

, a second magnet means, which is outside of the chamber enclosed by said vessel, a third magnet meansin the chamber enclosed by said vessel and mounted upon the second one of the holder members, the second and third magnet means being juxtaposed to act on each other so that the torque applied by said flux is temporarily inoperative to turn the said first magnet means until the second one of the-holder members, and consequently the third magnet means, is made free to turn by meltingthrough of said rod by said coil, and control means operative at a time not before the melting-through of the rod permits said torque to turn the second holder memher, said control means being operably connected to automatically initiate the said turning of the first one of the holder members.

16. The apparatus defined in claim 15, and an auxiliary device for supervising the accidental freezing of the molten zone due to faulty operationof the heating coil, said auxiliary device including means operative to signal when freezing of the molten zone causes the second holder member to rotate with the rotated first holder member. 7

17. An apparatus for zone melting a rod of meltable material, comprising a vessel enclosing a zone-melting chamber, upwardly-downwardly axially spaced holder members in said vessel, the holder members being adapted to support opposite endportions of said rod, a

heating means, power-operated means for turning a first one of the holder members about the axis of the holders, means for relative longitudinal movement of the heating means lengthwise of the rod, means for rotatably supporting the second one of the holder members for rotation about said axis, means for establishing a magnetic flux in a fixed direction, a magnet means located near said .flux, the magnet means being adapted to adjust itself in the flux direction, means operative so that torque applied by said flux is temporarily inoperative to turn the magnet means until the second one of the holder members is made free to turn by meltingthrough of said rod by said heating means, and a device for automatically initiating the rotating of said first holder'member subsequently/to the melting-through, the device comprising means governed according to the angular position of the magnet means, said device including a time delay means to delay the automatic initiating of said rotating.

18. An apparatus for zone melting a rod of meltable material, comprising a vessel enclosing a zone-melting chamber, upwardly-downwardly axially spaced holder members in said vessel, the holder members being adapted to support opposite end portions of said rod, a heating means, power-operated meansfor turning a first 10 ing means lengthwise of the rod, meansfor rotatably supporting the second one of the holder members for rotationabout said axis, and an auxiliary device for supervising the accidental freezing of the molten zone due to faulty operation of the heating means, said auxiliary device including control means governed by the rotation of the second holder member with the first f holder member, said rotation being caused by said freezone of the holder members about the axis of the holders,

means for relative longitudinal movement of the heating of the molten zone, said control means comprising a device to stop the operation of the apparatus.

19. An apparatus for zone melting a rod of meltable material comprising a vessel enclosing a zone-melting chamber, coaxial holder members in said vessel, the holder members being adapted to support opposite end portions of said rod, a heating coil in said vessel, power-operated means for turning a first one of the holder members about the axis of the holders, means for relative longitudinal movement of the heating coil lengthwise of the rod, means for rotatably supporting the second one of the holder members for rotation about said axis, a coil carrying current mounted outside the vessel for establishing a magnetic flux in a fixed direction, a first magnet means outside said vessel and located near said flux, means supporting the first magnet means, said -means for supporting being mounted for movement about said axis, current supply means carried by said means for supporting and connectable to said coil to energize it and to thereby initiate the operation of said apparatus, the first magnet means being adapted to adjust itself in the flux direction, so that a torque is applied to the means for supporting the first magnet means, a second magnet means, which is outside of the chamber enclosed by said vessel, a third magnet meansin the chamber enclosed by said vessel and mounted upon the second one ofthe holder members, the second and third magnet means being juxtaposed to act on each other so that said torque is temporarily inoperative to turn the said means supporting the first magnet until the second one of the holder members, and consequently the third magnet means, is made free to turn by meltingthrough of said rod by said coil, and control means operative at a time not before the melting-through of the rod permits said torque to turn the second holder member, said control means being operably connected to automatically initiate the said turning of the first one of the holder members, and an auxiliary device for supervising the accidental freezing of the molten zone due to faulty operation of the heating coil, said auxiliary device including second control means governed by the rotation of the second holder member with the first holder member, said rotation being caused by the freezing'of the molten zone, said second control means comprising a device to stop the operation of the apparatus.

20. The apparatus defined in claim 12, said control means including a time delay device to delay the automatic initiating of said rotating, the device comprising means governed according to the angular position of said magnet means.

- 21. The apparatus defined in claim 11, said signal means automatically indicating the moment of meltingthrough, and means governing the operation of the signal means according to the angular position of the magnet means. I

References Cited in the file of this patent UNITED STATES PATENTS -Emeis et al. Aug. 8, 1959