US3291570A - Apparatus for crucible-free zone melting of crystalline rods, particularly of semiconductor rods - Google Patents

Description

Dec. 13, 1966 L. MlCHL ETAL 3,291,570

APPARATUS FOR CRUCIBLE-FREE ZONE MELTING OF CRYSTALLINE RODS, PARTICULARLY OF SEMICONDUCTOR RODS Filed Oct. 22, 1963 United States atent APPARATUS FOR CRUCIBLE-FREE ZONE MELT- ING F CRYSTALLHNE RODS, PARTICULARLY 0F SEMICONDUCTOR RODS Ludwig Michl, Miesbach, and Werner Spielmann, Dachtel, Kreis Calw, Germany, assiguors to Siemens & Halske Aktiengesellschaft, Berlin, Germany, a corporation of Germany Filed Oct. 22, 1963, Ser. No. 318,011 6 Claims. (Cl. 23273) Our invention relates to an apparatus for carrying out crucible-free zone or floating-zone melting, and more particularly zone melting of rods consisting of semiconductor material.

It is expedient for a series of tasks that are accomplished by using crucible-free zone melting, that the heat source which produces the melting zone is stationary and the semiconductor rod that is being worked is led in the direction of its axis past the heat source, which preferably consists of a .short induction coil surrounding the rod concentrically at the location of the melting zone that is to be produced. It is moreover desirable and an object of this invention to provide a zone-melting apparatus in which the two holders of the rod that is to be melted also run toward each other synchronously at change in direction of movement of the rod. It is an additional object of our invention to provide a zone-melting apparatus of this type in which the spacing of both rod holders relative to each other is controllable as required independently of the existing variable travel speed of the melting zone dur ing the zone melting process, as is for example necessary with thin drawing processes in Which a thin crystal is produced out of a thicker rod by means of zone melting.

With the above and other related objects in view, and in accordance with our invention, we provide an apparatus for crucible-free zone melting in which, on the one hand, the portions of a vertically held rod, that are separated from one another by the melting zone, are displaceable with respect to one another as needed and, on the other hand, the rod is displaceable as a unit, the spacing between both of its holders remaining unchanged, in the direction of its axis with respect to the heat source which produces the melting zone.

According to further aspects of our invention, the apparatus includes a pair of holders secured to both rod ends and slidingly displaceably arranged by means of a rigid support arm on a stationary guide, for example a guide rail, extending parallel to the rod axis. The arms carrying the rod holders are provided with respective screw threads extending parallel to the guide, and each threaded=ly engaging a corresponding screw thread (worm) respectively carried by a rotary shaft that extends parallel to the aforementioned guide, so as to produce reciprocation of the respective rod holder in the direction of the rod axis by rotation of the coordinated rotary shaft. Furthermore, both rotary shafts coordinated respectively with both rod ends are parallel one to another, or prefer ably coaxially aligned and are each respectively coupled with -a motor connected between the respective rotary shaft and its mounting, the motor making possible a rotation of the respective rotary shaft and therewith a recipro' cation of the rod end that is connected with the respective rotary shaft. Both rotary shafts are coupled to each other in such a way mechanically by a pair of serially arranged, differential gear mechanisms connected one with another by an additional rotary shaft which extends parallel to and preferably coaxial with both of the aforementioned rotary shafts, that by means of an additional motor connected 'betwen the rotary shaft intermediate to both differential mechanisms and a fixed location, this rotary shaft can be rotated with respect to its surroundings so as to produce rotation of the outer bevel gears of both equally-dimensioned differentials in the same direction and at the same speed by the fact that the respective outer bevel gears .are rigidly connected to each other and the respective cooperating rotary shafts are coupled with the rod holders, when both of the remaining motors are switched off.

Other features which are considered as characteristic for the invention are set forth in the appended claims. The invention, both as to its instruction and method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the acompanying drawing in which the sole figure shows an elevational view partly in section and partly diagrammatic of a zone melting assembly constructed in accordance with the invention.

Referring to the drawing there is shown a pair of vertical guide rails 1, 1' which are supported parallel to the axis of a silicon rod 6 that is to be melted. The melting zone 6a is produced by a fixed induction coil 7 which concentrically surrounds the rod and is supplied with a high frequency current. The ends of therod 6 are secured with set screws or the like at the ends of two vertically arranged elongated holders 4, 5 which are vacuumtightly and glidingly displaceably guided through apertures in the walls of a perfectly stationary receiving chamber or rece ver 30 which surrounds the rod. The manner in which the rod holders 4, 5 may be guided through the wall can be effected as described in Patent No. 3,160,- 478 to T. Rummel et al. and issued December 8, 1964.

In order to maintain a good vacuum in the receiver 30, it is recommended further to insert the rod holders 4, 5 through antechambers 31 which are also evacuated to effectively prevent the inlet or entry of air into the receiver 30 itself. The rod holders 4, 5 suitably consist of heat resistant metal which preferably does not give off any gaseous impurities due to the heating that is required for the zone melting. VZa-sheet (German) containing about 19% chromium and about 9% nickel is an example of a metal that is suitable for this purpose. The receiver 39 and the antechambers 31 are suitably connected during operation to a continously running vacuum pump (not shown), If desired the receiver 30 and the antechambers 31 can also be filled with a protective gas.

The rod holders 4, 5 which lie along extensions of the axis of the silicon rod 6 that is to be processed, are thus vertically oriented in the embodiment that is illustrated in the drawing, and are rigidly connected at their ends to support arms 2, 3 which extend perpendicular to their longitudinal extension. The arms 2, 3 that are disposed advantageously parallel to one another directly engage the guide rails 1, 1' and are able to move only in the direction of these guide rails, i.e. in the vertical direction of the illustrated embodiment. T 0 effect such a movement, the support arms 2, 3 are provided with threaded tap holes 2', 3 whose axes extend parallel to the guide rails 1, 1 and consequently to the axis of the rod 6. Rotary shafts 18 and 19 arranged coaxial to one another extend through the tap holes 2 and 3' respectively, parallel to the guide rails 1, 1'. The shafts 18 and 19 extend through externally threaded sleeves or worms 18 and 19' respectively which are securely held thereon by pressfitting splines or keys, or the like. The worms 18' and 19 correspond to and mesh with the threads of the holes 3', the length of the worm thread 18', 19' being a multiple of the length of the holes 2', 3. Both thread systems 2', 18 or 3', 19' have the same pitch and the same thread. When the rotary shafts 18, 19 are rotated in the same rotary direction with the same rotary speed about their axes, then the spacing of both arms 2, 3 remains unchanged. However, both arms are displaced vertically upward or downward so that the rod 6 experiences a uniform or steady translatory movement in its longitudinal direction with respect to the heat source 7, and the melting zone 6a accordingly creeps further or travels along the rod.

Both rotary shafts 18, 19 are at no time rigidly connected to one another but are rather coupled by means of a pair of differential gear mechanisms 16, 17 connected one behind the other. Both differential gear mechanisms consist of a core 16a, 17a each of which carries a bevel gear 16b, 17b which meshes with the bevel gears 16c, 16d or 17c, 17d that are mounted opposite one another at the sides of both differential gear mechanisms. As shown in the figure, the rotary shaft 18 is connected with the core 16a of the upper differential gear mechanism 16, and the rotary shaft 19 is connected with the core 17a of the lower differential gear mechanism 17. The gears 16:: and 28 are carried on a common hollow shaft 18a rotatably mounted in the core 16a of the differential mechanism 16, the shaft 18 extending through the hollow shaft 18a and being secured to the core 16a. A similar arrangement is afforded by the hollow shaft 19a for the differential mechanism 17. The gears 16c and 17d on the one hand and the gears 16d and 17c on the other hand are in each case of similar dimensions. As a matter of fact, in the illustrated embodiment of this application all of the spur gears are of equal dimension. Both of the spur gears 160?, 17c are rigidly fitted on their sides respectively on a connecting shaft 13 which is shown as coaxial to the rotary shafts 18, 19 in the illustrated embodiment, but which may be arranged parallel to them, and is locatedbetween both differential gears 16, 17. The connecting shaft 13 is, however, neither rigidly connected with the shafts 18, 19 nor with the cores 16a, 16b of the differential gears 16, 17.

In the embodiment illustrated in the figure, an additional plate-shaped gear 20 is mounted on the rotary hollow shaft 18a surrounding the shaft 18, an additional plate-shaped gear 21 is mounted on the rotary hollow shaft 19a surrounding the shaft 19, and an additional plate-shaped gear 22 is mounted on the shaft 13, all rigidly connected to their respective shafts. All of these gears 20, 21, 22 mesh respectively with similar gears 23, 25, 24 on separately rotatable shafts 10, 12, 11 that are mounted in bearings located on the gear housing 14, these shafts 10, 12, 11 also respectively carrying a worm gear 26, 27, 9 driven by respective motors 28, 29, 8. A consequence of this arrangement is that the shaft 13 and therewith the gears 16d, 170 are rotated when the motor '8 rotates. Also, as a result of the rotation of motor 28-,

the shaft 18 and the bevel gear 160 of the differential gear mechanism 16 is set in motion, and by the rotation of motor 29, the shaft 19 and the bevel gear 17d of the differential gear mechanism 17 are set in rotation.

If the motor 8 is alone in operation, then the cores 16a, 17a of the differential gears 16, 17 are not rotated by the rotation of both bevel gears 16d, 17c. The spur gears 16c, 17d nevertheless experience an opposing rotation and indeed witht the same speed of rotation as both spur gears 16d and 17c when the spur gears 16c, 16d, 17c, 17d are of the same dimensions. The shafts 18, 19 are then rotated with the same speed as the shaft 13, although in the opposite rotary direction. As a result, both shafts 18, 19 behave like a single rigid shaft and cause displacement of the rod 6 as a unit relative to the heat source 7, whereby the spacing between both rod holders 4, remains unchanged.

On the other hand, if the motor 28 only is in operation, by means of which the upper shaft 18 is driven, the core 16a of the upper differential gear 16 rotates at half the speed as the shaft 18 in the same direction. The gear 16d, the shaft 13 and the lower differential 17 on the contrary remains at rest. Consequently the lower portion of the rod 6 does not move while the upper portion of the rod 6 is either brought closer to or drawn away 1 from the lower rod portion and the heat source 7 which produces the melting zone. In a similar manner, actuation of the motor 29 only, causes displacement of the lower rod portion while the upper rod portion remains at rest.

Simultaneous actuation of all three motors 8, 28, 29 causes both axial displacement of the rod 6 as a unit toward or away from the heat source 7 as Well as also an approach toward one another or a separation from one another, respectively, of both ends of the rod and therewith a compression or elongation of the melting zone 6a.

The speed of the individual motion components is arrived at by selectively proportioning the ratio of the differential gears, by choosing a suitable rotary speed of the motors 8, 28, 29, and by appropriate design or arrangement of the screw thread systems 2', 18', or 3, 19'. These screw thread systems are suitably coordinated with or adjusted to one another so that they produce the same displacement speed of the arms 2, 3 with the same rotary speed of the shafts 18, 19, or, in other words, the screw thread systems have the same pitch or range. The screw thread systems are interchangeable in suitable manner in that the arms 2, 3 which are formed with the holes 2', 3' respectively and the worm screws 18' and 19' mounted on the rotary shafts 18, 19, respectively, are interchangeable. In order to achieve a simple means of regulating the rotary speed of the shafts 13, 18 and 19, it is advisable that the motors 8, 28, 29 be direct-current motors. With motors of this type the speed and the direction of rotation can be regulated in a simple manner by the voltage that is fed to the field coils and the rotor as well as by the choice of polarity of these voltages. It is then possible to switch the motors and thereby the rotary speed of the rotary shaft continuously from a maximum speed in one direction to a maximum speed in the other direction.

While the invention has been illustrated and described as embodied in a particular apparatus for crucible-free zone melting of crystalline rods, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing from the spirit of the present invention and within the scope and range of equivalents'of the following claims.

We claim:

1. An apparatus for effecting crucible-free zone melting of a semiconductor rod, comprising guide means extending parallel to the axis of the rod; rod-holder means secured to opposite end portions of the rod and slidahle along said guide means; heating means for melting a zone of the rod intermediate the end portions of the rod; and means for sliding said rod-holder means and consequently the rod end portions along said guide means, said sliding means comprising a pair of independently rotatable first and second shaft screw thread means for respectively displacing each of said rod-holder means and the respective rod end portion, a differential mechanism operatively connected to each of said first and second shaft screw thread means, rotary shaft means coaxial with said first and second shaft screw thread means serially coupling said differential mechanisms to each other, first and second motor means for independently rotating said first and second shaft screw thread means so as to selectively displace one of said rod-holder means and the respective rod end portion relative to the other, and third motor means for imparting rotation to said rotary shaft means and, through said differential mechanisms, to said first and second shaft screw thread means independently of said first and second motor means so as to simultaneously displace both of said rod-holder means and respective rod end portions in the same direction and at the same rate of speed.

2. An apparatus for effecting crucible-free zone melting of a semiconductor rod, comprising a pair of guide rails extending parallel to the axis of the rod; a pair of spaced support arms extending transversely to said rails and slidably mounted thereon; a .pair of rod-holders secured respectively to opposite end portions of the rod and fixed respectively to said support arms for sliding therewith; heating means for melting a zone of the rod intermediate the end portions of the rod; and means for sliding said support arms and consequently the rod end portions along said guide rails, said sliding means comprising a pair of independently rotatable first and second shaft screw thread means for respectively displacing each of said support arms and the respective rod end portion, a differential mechanism operatively connected to each of said first and second shaft screw thread means, rotary shaft means serially coaxial with said first and second shaft screw thread means coupling said differential mechanisms to each other, first and second motor mean for independently rotating said first and second shaft screw thread means so as to selectively displace one of said support arms and the respective rod end portion relative to the other, and third motor means for imparting rotation to said rotary shaft means and, through said differential mechanisms, to said first and second shaft screw thread means independently of said first and second motor means so as to simultaneously displace both of said support arms and respective rod end portions in the same direction and at the same rate of speed.

3. An apparatus for effecting crucible-free zone melting of a semiconductor rod, comprising a pair of guide rails extending parallel to the axis of the rod; a pair of spaced support arms extending transversely to said rails and slidably mounted thereon; a pair of rod-holders secured respectively to opposite end portions of the rod and fixed respectively to said support arms for sliding movement therewith; heating means for melting a zone of the rod intermediate the end portions of the rod; and means for sliding said support arms and consequently the rod end portions along said guide rails, said sliding means comprising a pair of similar threaded bores formed in said support arm and having an axis parallel to said guide rails, and a pair of independently rotatable shafts each having an externally threaded portion cooperating with the respective threaded bores for selectively displacing each of said support arms and the respective rod end portion, a differential mechanism operatively connected to each of said externally threaded shafts, rotary shaft means coaxial with the externally threaded portions of said pair of shafts serially coupling said differential mechanisms to each other, first and second motor means for independently rotating said externally threaded shafts in said threaded bores so as to selectively displace one of said support arms and the respective rod end portion relative to the other, and third motor means for imparting rotation to said rotary shaft means and through said differential mechanisms, to said externally threaded shafts independently of said first and second motor means so as to simultaneously displace both of said support arms and respective rod end portions in the same direction and at the same rate of speed.

4. An apparatus according to claim 3 wherein said guide rails are vertically disposed, and said rod-holders are rod-shaped and extend coaxial with the rod.

5. An apparatus for efiecting crucible-free zone melting of a semiconductor rod, comprising a pair of guide rails extending parallel to the axis of the rod; a pair of spaced support arms extending transversely to said rails and slidably mounted thereon; a pair of rod-holders secured respectively to opposite end portions of the rod and fixed respectively to said support arms for sliding therewith; heating means for melting a zone of the rod intermediate the end portions of the rod; a receiving chamber in which the rod is heated, said chamber having evacuated antechambers at either end through which said rod-holder means are slidingly reciprocable; and means for sliding said support arms along said guide rails, said sliding means comprising a pair of independently rotatable first and second shaft screw thread means for respectively displacing each of said support arms and the respective rod end portion, a differential mechanism operatively connected to each of said first and second shaft screw thread means, rotary shaft means coaxial with said first and second shaft screw thread means serially coupling said differential mechanisms to each other, first and second motor means for independently rotating said first and second shaft screw thread means so as to selectively displace one of said support arms and the respective rod end portion relative to the other, and third motor means for imparting rotation to said rotary shaft means and, through said differential mechanisms, to said first and second shaft screw thread means independently of said first and second motor means so as to simultaneously displace both of said support arms and respective rod end portions in the same direction and at the same rate of speed.

6. An apparatus for effecting crucible-free zone melting of a semiconductor rod, comprising guide means extending parallel to the axis of the rod; rod-holder means secured to opposite end portions of the rod and slidable along said guide means; heating means for melting a zone of the rod intermediate the end portions of the rod; and means for sliding said rod-holder means and consequently the rod end portions along said guide means, said sliding means comprising a pair of independently rotatable first and second shaft screw thread means for respectively displacing each of said rod-holder means and the respective rod end portion, a differential mechanism op eratively connected to each of said first and second shaft screw thread means, rotary shaft means coaxial with said first and second shaft screw thread means serially coupling said differential mechanisms to each other, first and second motor means for independently rotating said first and second shaft screw thread means so as to selectively displace one of said rod-holder means and the respective rod end portion relative to the other, and third motor means for imparting rotation to said rotary shaft means and, through said differential mechanisms, to said first and second shaft screw thread means independently of said first and second motor means so as to simultaneously displace both of said rod-holder means and respective rod end portions in the same direction and atthe same rate of speed, each of said motor means comprising a direct current motor, the rotary speed and rotary direction of said motor being controllable by varying the energizing voltage.

References Cited by the Examiner UNITED STATES PATENTS 2,389,047 11/1945 Reinz 74-675 X 2,493,828 1/1950 Patsons 74675 X 3,160,478 12/1964 Rummel et al 23-301 X NORMAN YUDKOFF, Primary Examiner.

ROBERT F. BURNETT, Examiner.

G. P. HINES, Assistant Examiner.

Claims (1)

1. AN APPARATUS FOR EFFECTING CRUCIBLE-FREE ZONE MELTING OF A SEMICONDUCTOR ROD, COMPRISING GUIDE MEANS EXTENDING PARALLEL TO THE AXIS OF THE ROD; ROD-HOLDER MEANS SECURED TO OPPOSITE END PORTIONS OF THE ROD AND SLIDABLE ALONG SAID GUIDE MEANS; HEATING MEANS FOR MELTING A ZONE OF THE ROD INTERMEDIATE THE END PORTIONS OF THE ROD; AND MEANS FOR SLIDING SAID ROD-HOLDER MEANS AND CONSEQUENTLY THE ROD END PORTIONS ALONG SAID GUIDE MEANS, SAID SLIDING MEANS COMPRISING A PAIR OF INDEPENDENTLY ROTATABLE FIRST AND SECOND SHAFT SCREW THREAD MEANS FOR RESPECTIVELY DISPLACING EACH OF SAID ROD-HOLDER MEANS AND THE RESPECTIVE ROD END PORTION, A DIFFERENTIAL MECHANISM OPERATIVELY CONNECTED TO EACH OF SAID FIRST AND SECOND SHAFT SCREW THREAD MEANS, ROTARY SHAFT MEANS COAXIAL WITH SAID FIRST AND SECOND SHAFT SCREW THREAD MEANS SERIALLY COUPLING SAID DIFFERENTIAL MECHANISMS TO EACH OTHER, FIRST AND SECOND MOTOR MEANS FOR INDEPENDENTLY ROTATING SAID FIRST AND SECOND SHAFT SCREW THREAD MEANS SO AS TO SELECTIVELY DISPLACE ONE OF SAID ROD-HOLDER MEANS AND THE RESPCTIVE ROD END PORTION RELATIVE TO THE OTHER, AND THIRD MOTOR MEANS FOR IMPARTING ROTATION TO SAID ROTARY SHAFT MEANS AND, THROUGH SAID DIFFERENTIAL MECHANISMS, TO SAID FIRST AND SECOND SHAFT SCREW THREAD MEANS INDEPENDENTLY OF SAID FIRST AND SECOND MOTOR MEANS SO AS TO SIMULTANEOUSLY DISPLACE BOTH OF SAID ROD-HOLDER MEANS AND RESPECTIVE ROD END PORIONS IN THE SAME DIRECTION AND AT THE SAME RATE OF SPEED.

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