US3454368A

US3454368A - Apparatus for crucible-free floating zone melting of crystalline rods

Info

Publication number: US3454368A
Application number: US554805A
Authority: US
Inventors: Wolfgang Keller; Gunther Berger
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1965-06-04
Filing date: 1966-06-02
Publication date: 1969-07-08
Anticipated expiration: 1986-07-08
Also published as: DE1519872A1; DE1519872B2; CH432475A; GB1126104A; BE681815A; NL6607745A

Description

July 8,1969 WKELLER ETAL 3,454,368

APPARATUS FOR CHUClBLErFREE FLOATING ZONE MELTl NG O1" CHYSTALLINE RODS Filed June 2, 1966 Sheet 4 -of 2 T l i 1- l l f? ELECTBICALLY NON-CONDUCTIVE MOTOR I INDICATOR MOTOO.

, 3,454,368 ZONE Sheet of 2 July 8, 1969 w. KELLER ET AL APPARATUS FOR CRUCIBLE-FREE FLOATING MELTING OF CRYSTALLINE RODS Filed June 2, 1966 United States. Patent Int. 01. Btilj 17/08 US. Cl. 23-301 7 Claims ABSTRACT OF THE DISCLOSURE Apparatus for crucible-free zone melting a crystalline rod includes a clutch located between one holder of a pair of end holders vertically supporting a rod therebetween and a drive shaft for rotating the one holder. The clutch.

has two vertically aligned fractionally engageable clutch plate members, the lower member being downwardly displaceable for uncoupling the one holder from the drive shaft before a molten zone is produced in the rod by a heating device and, after the molten zone is produced in the rod, the upper member is displaceable downwardly by gravity force so as to frictionally engage the lower member.

Our invention relates to crystalline rods for cruciblefree floating zone melting of crystalline rods.

Such apparatus is well known and comprises an evacuated melting chamber or a melting chamber filled with protective gas in which the rod is located in a vertical position and is secured, for example at its upper end, in a holder which is in turn mounted on a rotary shaft. The rod is surrounded by a heating device such .as a flatly wound induction coil for example, whose windings are disposed in a plane perpendicular to the axis of the rod and supplied with high-frequency alternating current. Before starting the zone melting process, a monocrystalline seed secured in a holder is fused to the lower end of the crystalline rod with the aid of the heating device. The holder for the seed is mounted on the end of a shaft which is displaceable in the axial direction thereof. After the seed is fused with the aid of the melt-producing induction coil, the holders of the rod and the seed, respectively, are set in rotation. Simultaneously, relative motion is'effected between the induction heating coil and the rod so that the melting zone produced in the rod is displaced from the junction of the seed and rod upwardly along the rod. For this purpose, either the rod, i.e. the shafts with the holders gripping the rod, or the induction heating coil can be moved in a vertical direction. Purification of the rod is accordingly achieved in this manner. Furthermore, the recrystallized rod growing at the monocrystalline seed from the melt of the melting zone possesses a very extensive monocrystalline structure.

Often, a single pass of the melting zone through the rod is insufficient to purify the crystalline rod at the desired width or diameter. Consequently, further zone passes may be necessary. For this purpose, the heating power of the melting coil is reduced and the relative motion between the melting coil and the rod' is regulated so that an incandescent or glow zone is passed through the rod and so that the melting coil is again located at the level of the junction location of the seed and rod after termination of the relative motion between the coil and rod. Thereafter, the melting zone is subjected to the full heating power output of the coil and the seed is melted through.

It has been found that the thermal stresses occurring when the seed is reheated and melted, cause damage to the crystalline structure of the material recrystallizing from "ice the melt. Since the rod and seed are rigidly clamped between the holders, the thermal stresses cannot balance ont or become equalized, so that large forces are consequently exerted on the seed which is at that time in a plastic condition. These unbalanced forces are responsible for the poor crystal quality of the material subsequently recrystallizing from the melting zone.

It has also been found to be advantageous for the rod and seed to be placed in rotation in opposite directions as directly as possible after the melting of the seed.

It is accordingly an object of our invention to provide apparatus for crucible-free zone melting of crystalline rods which avoids the foregoing disadvantages of the known apparatus.

It is a more specific object of our invention to provide such apparatus wherein the seed is relieved of thermal stresses occurring during the melting thereof before further melting zone passes are executed.

A further specific object of our invention is to provide apparatus by means of which the melting of the seed is immediately determinable so that rod and seed can be placed in rotation instantaneously, whereby a uniformly circular cross section of the rod recrystallizing from the melting zone can be achieved.

With the foregoing and other objects in view, in accordance with our invention, we provide apparatus wherein the seed is maintained free of mechanical stresses by coupling or clutch means comprising a pair of engaging members that are releasable by the displacement of one of the clutch members in the axial direction of the rod, and wherein the falling motion of one of the clutch members which occurs at the instant the seed becomes plasticized is employed for relieving the seed of thermal stresses and for registering or establishing when the seed has been melted through.

More specifically, the apparatus of the invention for crucible-free zone melting of a crystalline rod, such as of semiconductor material, for example, comprises a pair of end-supporting holders for the rod, at least one of the holders being axially displaceable and rotatably connected to a rotary drive shaft, means for heating the rod to produce a molten zone therein, the rod and said heating means being relatively displaceable so as to pass the molten zone through the rod along the axis thereof, and clutch means located between said one holder and said rotary drive shaft, said clutch means comprising a pair of engageable clutch members, at least one of said clutch members being axially displaceable for disengaging said one holder from said drive shaft.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in apparatus for crucible-free zone melting of crystalline rods, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together-with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view, partly in cross section and partly broken away, of the apparatus constructed in accordance with our invention wherein the coupling elements are disc-shaped; and

FIG. 2 is a similar view of a second embodiment of the apparatus of FIG. 1 wherein the coupling elements are conical or funnel-shaped.

Referring now to the drawings, 'there is shown in FIG. 1 an apparatus wherein a crystalline rod 3 of silicon for example, is vertically mounted in a melting chamber 1 filled with protective gas or evacuated, and is surrounded by a heating device, for example a stationary flat meltproducing induction coil 13. The upper end of the rod 3 is secured in a holder 2 mounted at the end of a shaft 4. The shaft 4 can be displaced in the axial direction thereof, that is, in a vertically upward and downward direction. At the lower end of the rod 3, there is located a monocrystalline seed 5 which is secured in a holder 6. The seed 5 is fused by the heat produced from the melting coil 13 to the lower end of the rod 3 during the first pass of the melting zone along the rod 3. The holder 6 is mounted in the center of the upper flat surface of the upper member 7 of a coupling or clutch consisting of two disc-shaped members. The upper disc-shaped half 7 of the clutch can consist of chrome-nickel-steel alloy (stainless steel), for example. In the lower fiat surface of the coupling member 7, there is formed a concentric annular groove 8 shown in dotted lines in FIG. 1. The second disc-shaped half 10 of the clutch or coupling is located beneath the coupling half 7 and is mounted at the end of a drive shaft 9. The secondcoupling half 10 can consist of ceramic, for example. Three conical pins 12 are mounted on the upper fiat surface of the lower disc-shaped element 10 of the coupling, and are uniformly distributed along an imaginary circle aligned with the aforementioned groove 8 formed in the upper disc-shaped element 7 of the coupling. The pins 12 are thus located at the corners of an equilateral triangle and can engage in the groove 8 of the upper disc-shaped element 7 of the coupling. The lengths of the conical pins 12 are selected so that they are advantageously somewhat longer than the depth of the groove 8. Two of the pins 12 consist of electrically conductive material, such as chromenickel-steel alloy for example, and are connected electrically with two slip rings 11 mounted on the shaft 9. By suitable brush means 15, in a manner well known to the man of ordinary skill in the art, when the electrically conductive pins 12 engages the electrically conductive upper half 7 of the coupling, an electric circuit through the slip rings 11 can thereby be energized to indicate engagement between the coupling halves 7 and 10, as by energizing a signal lamp, for example.

Before the beginning of a melting zone pass through the rod 3 and the seed 5 fused thereto, the shaft 9 is displaced axially downwardly so that the coupling halves 7 and 10 are spaced from one another from 0.5 to 2 mm. To ensure the fact that secure coupling or clutch engagement will take place, this spacing is so selected that, even after the shaft 9 is downwardly displaced, the points of the conical pins 12 remain inserted in the groove 8 Without contacting the walls thereof, i.e. during disengagement of the clutch members. Thereafter, the bused seed 5 is remelted with the aid of the melting coil 13. Since the seed 5 is actually not rigidly clamped at the lower end thereof as seen in FIG. 1, it remains free of thermal stresses during the heating operation. The instant the seed is melted through, the coupling half 7 drops down onto the coupling half 10 and the conical pins 12 become inserted into the groove 8 of the coupling half 7. Since the length of the pins 12 is greater than the depth of the groove 8, the coupling half 7 does not lie directly on the coupling half 10. Nevertheless, because the pins 12 are uniformly distributed circumferentially on the lower coupling half 10, e.g. the pins 12 are located at the points of an imaginary equilateral triangle, the connection between the coupling halves 7 and 10 thus formed is free of any play.

When the coupling half 7 drops down so as to come into engagement with the lower coupling half 8, the circuit of an electric control device or an electric signal device which, as shown schematically in FIG. 1, for example is connected through brushes with the slip rings 11 and with the two electrically conductive pins 12, can be closed, so as to actuate a relay 16 for example to 4 energize a motor or motors setting both shafts 4 and 9 instantaneously into rotation. The dropping of the coupling half 7 can also be observed through a viewing window 14 in the melting chamber by servicing personnel for the apparatus, whereby the rotation of the shafts 4 and 9 can then be effected or instituted by manual means.

In the embodiment shown in FIG. 2, a crystalline rod 23, which can consist of silicon for example, is vertically mounted in an evacuated melting chamber 21 or a melting chamber filled with protective gas, and the rod 23 is surrounded by a stationary flat melt-producing induction coil 33, The upper end of the rod 23 is secured in a holder, not shown in FIG. 2, which is in turn mounted at the end of a shaft, which is also not shown in FIG. 2. The non-illustrated shaft can, if desired, be displaced in the axial direction of the rod 23. At the lower end of the rod 23 there is located a monocrystalline seed 25 which is secured in a holder 26 and which is fused to the lower end of the rod during the first pass of the melting zone which is effected by heating with the melt-producing induction coil 33. The holder 26 is mounted at the upper female or hollow member 27 of a friction type coupling or clutch formed with conical or funnel-shaped friction surfaces. The lower coupling half or male member 30 is secured to a drive shaft 21, also displaceable in the axial direction of the rod 23.

Before the beginning of a melting zone pass through the rod 23 with the seed 25 fused thereto, the drive shaft 29 of apparatus embodiment shown in FIG. 2 is displaced axially downwardly so that the coupling halves 27 and 30 are spaced from one another, the spacing being selected in order to ensure a secure coupling or clutch engagement by providing for a portion of the coupling half 30 to project into the hollow coupling half 27 without engaging any portion of the latter. Then, with the aid of the meltproducing coils 33, the feed 25 is melted through. At the moment that the melting of the seed occurs, the upper coupling half 27 drops down onto the lower coupling half 30 and effects a connection free of play between the seed crystal 25 and the drive shaft 29. This operation can be observed, for example by servicing personnel through an observation window 34 so they can immediately effect the rotation of the shafts 24 and 29 as the coupling half 27 drops down. The distance through which the coupling half 27 falls can be substantially between 0.5 and 2 mm.

By means of clutches or couplings, such as those of FIGS. 1 and 2, which are releasable by axial movement thereof, zone-melting apparatuses can also be employed wherein the crystalline rod is fixed and the heating device, for example the induction heating coil, can be suitably mounted so that it is displaceable in a vertical direction. Above all then, the drive shaft with which the coupling or clutch has to effect a connection, must be movable in the axially downward direction of the rod as viewed in the figures so that the upper coupling half can drop at the moment the seed is melted through.

We claim:

1. Apparatus for crucible-free zone melting of a crystalline rod, comprising a pair of substantially vertically aligned holders for respectively supporting the rod substantially vertically at the ends thereof, at least one of said holders being displaceable in the axial direction of the rod and being rotatably connected to a rotary drive shaft, means for heating the rod to produce a molten zone therein, the rod and said heating means being relatively displaceable so as to pass the molten zone through the rod along the axis thereof, and clutch means located between said one holder and said rotary drive shaft, said clutch means comprising a pair of frictionally engageable clutch members located one above the other, the lower one of said clutch members being displaceable downwardly in the axial direction of the rod for uncoupling said one holder from said drive shaft prior to producing the molten zone in the rod, and the upper one of said clutch members, after the molten zone is produced in the rod, being displaceable by force of gravity to a lower position wherein it frictionally engages said lower one of said clutch members.

2. Apparatus according to claim 1, including electrical conductive means on said clutch means adapted, to conduct an electrical current upon frictional engagement of said clutch members, and electrical control means connected to said electrical conductive means for rotating said rod end holders in response to clutching engagement of said clutch members.

3. Apparatus according to claim 2, including electrical indicating means connected to said electrical conductive means for indicating clutching engagement of both of said clutch members.

4. Apparatus according to claim 1, wherein said clutch members are disc-shaped, each having a face opposing a face of the other, one of said clutch members being formed with an annular groove on said face thereof, and the other of said clutch members having at least three conical pins extending from said respective face thereof, said pins having a length greater than the depth of said annular groove, said pins being substantially equally spaced circumferentially on said face of said other clutch member in alignment with said annular groove of said one clutch member, whereby said pins are adapted to be inserted into said annular groove, said pins, in disengaged condition of said clutch members, extending into said annular groove though spaced from the walls of said groove, said heating means effecting a release of one of said clutch members by producing the melting zone in the rod.

5. Apparatus according to claim 1, wherein said clutch means is a friction clutch and said clutch members are respective male and female conical members having mutually engageable friction surfaces.

6. Apparatus for crucible-free zone melting of a crystalline rod, comprising a pair of holders for respectively supporting the rod at the ends thereof, at least one of said holders being displaceable in the axial direction of the rod and being rotatably connected to a rotary drive shaft, means for heating the rod to produce a molten zone therein, the rod and said heating means being relatively displaceable so as to pass the molten zone through the rod along the axis thereof, and clutch means located between said one holder and said rotary drive shaft, said clutch means comprising a pair of engageable clutch members,

at least one of said clutch members being displaceable in the axial direction of the rod for uncoupling said one holder from said drive shaft, said clutch members being :disc-shaped, each having a face opposing a face of the other, one of said clutch members being formed of electrically conductive material and with an annular groove on said face thereof, and the other of said clutch members being nonconductive and having at least three conical pins, of which at least two are electrically conductive, extending from said respective face thereof, said pins having a length greater than the depth of said annular groove and being substantially equally spaced circumferentially on said face of said other clutch member in alignment with said annular groove of said one clutch member, whereby said pins are adapted to be inserted into said annular groove, said pins being disposed initially so as to extend within said groove but out of eletcrical and frictional engagement with the walls of said groove, said heating means effecting a release of one of said clutch members by uncoupling said one holder to cause completion of an electrical circuit through said pins.

7. Apparatus according to claim 6, including an electric drive motor for rotating one of said rod holders about said substantially vertical axis, said electric drive motor being electrically connected to said electrically conductive pins and being actuable for rotating said one rod holder when said electrically conductive pins are in engagement with the walls of the groove formed in said clutch member of electrically conductive material.

References Cited UNITED STATES PATENTS 2,743,200 4/1956 Hannay 23-301 2,870,309 1/ 1959 Capita 23-273 2,989,378 6/ 1961 Sterling 23273 3,046,379 7/1962 Keller et al 23-301 FOREIGN PATENTS 629,412 10/ 1961 Canada.

NORMAN YUDKOFF, Primary Examiner.

US. Cl. X.R.