US3671703A

US3671703A - Device for crucible-free, floating zone melting a crystalline

Info

Publication number: US3671703A
Application number: US23366A
Authority: US
Inventors: Wolfgang Keller
Original assignee: Siemens Corp
Current assignee: Siemens AG; Siemens Corp
Priority date: 1969-03-29
Filing date: 1970-03-27
Publication date: 1972-06-20
Anticipated expiration: 1989-06-20
Also published as: CH540716A; JPS4811441B1; DE1916317C3; DE1916317A1; SE352218B; NL7003483A; FR2035990A1; DE1916317B2; GB1257284A

Abstract

Device for zone melting a crystalline rod includes a vessel, an induction heating coil mounted in the vessel for heating a molten zone formed in a crystalline rod, and an electric current supply for the induction heating coil extending into the vessel through an opening formed in a wall of the vessel, the current supply including a plurality of mutually telescoped conductor tubes radially spaced from one another, the plurality of telescoped conductor tubes including a first sequence of alternately disposed tubes and a second sequence of tubes disposed in alternating relationship with the first sequence of alternately disposed tubes, all of the tubes of the first sequence of alternately disposed tubes being electrically connected at both ends thereof, and all of the tubes of the second sequence of alternately disposed tubes being electrically connected at both ends thereof.

Description

[ 1 June 20, 1972 [54] DEVICE FOR CRUCIBLE-FREE,

FLOATING ZONE MELTING A CRYSTALLINE [72] Inventor:

[73] Assignee:

Wolfgang Keller, Pretdeld, Germany Siemens Aktiengesellschaft, Berlin and Munich, Germany 221 Filed: March 27, 1970 21 Appl. No.: 23,366

[30] Foreign Application Priority Data March 29, 1969 Germany ..P 19 11 317.8

[52] US. Cl. ..2l9/l0.75, 174/105 R, 219/1079 [51] Int. Cl. ..H05b 9/06 [58] Field ofSearch ..2l9/10.75, 10.77, 10.43, 10.65,

OTHER PUBLICATIONS German Published Application, Nov. 15, 1957, 1 sheet drawing, 3 pages specifications, Emlis & Hans, No. 1,076,623.

Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel J. Tick 57 ABSTRACT Device for zone melting a crystalline rod includes a vessel, an induction heating coil mounted in the vessel for heating a molten zone formed in a crystalline rod, and an electric current supply for the induction heating coil extending into the vessel through an opening formed in a wall of the vessel, the current supply including a plurality of mutually telescoped conductor tubes radially spaced from one another, the plurality of telescoped conductor tubes including a first sequence of alternately disposed tubes and a second sequence of tubes disposed in alternating relationship with the first sequence of alternately disposed tubes, all of the tubes of the first sequence of alternately disposed tubes being electrically connected at both ends thereof, and all of the tubes of the second sequence of alternately disposed tubes being electrically connected at both ends thereof.

7 Claims, 4 Drawing Figures Patented June 20, 1972 3,671,703

3 Sheets-Sheet 1 Fig.1

Fig.2

Patented June 20, 1972 I 3 Sheets-Sheet 2 Fig.3

Patented June 20, 1972 s Sheets-Shed s DEVICE FOR CRUCIBLE-FREE, FLOATING ZONE MELTING A CRYSTALLINE My invention relates to device for crucible-free floatingzone melting a crystalline rod.

From the German published application 1,076,623 there is known a device for crucible-free zone melting of rod shaped semiconductor material. This device includes a high vacuum vessel wherein a molten zone produced with the aid of an induction heating coil in rod shaped semiconductor material, is passed through the material. The induction heating coil is connected to a current supply of high frequency alternating current, the current supply being displaceably mounted in a gastight passage or opening formed in the vessel wall.

The current supply is formed of three telescoping and coaxially disposed conductor tubes. Both outer conductor tubes are electrically connected to one another at both ends of the current supply, while an intermediate layer of band-shaped polytetrafluorethylene cast in paraffin is located in the intermediate space between the inner inductor tube of smallest tube diameter and the middle conductor tube of next larger tube diameter. The inner conductor tube with the smallest tube diameter as well as the middle conductor tube with the next larger tube diameter are formed of copper having a good electrical conductivity, whereas the outer conductor tube with the largest tube diameter is formed of steel having an electrical conductivity which is not as good as the copper tube. The steel tube surrounds a channel for a coolant which cools the middle conductor tube formed of copper. Moreover, the outer wall surface of the copper tube has an exceptionally smooth sliding surface which is barely susceptible to damage so that the scaling in the opening or passage formed in the vessel wall functions without any trouble, and no gas can pass into the evacuated vessel from the outer atmosphere.

It is an object of the invention in the instant application to provide an improvement over the aforementioned known device for crucible-free zone melting of rod shaped semiconductor material, wherein the power loss in the current supply for the induction heating coil is reduced. The result thereof is that a single-winding induction heating coil having low ohmic and inductive resistance will transmit a high heating power to the molten zone in the crystalline rod.

With the foregoing and other objects in view I provide in accordance with the invention a device for zone melting a crystalline rod comprising a vessel, an induction heating coil mounted in the vessel for heating a molten zone formed in a crystalline rod, and an electric current supply for the induction heating coil extending into the vessel through an opening formed in a wall of the vessel, the current supply comprising a plurality of mutually telescoped conductor tubes radially spaced from one another, the plurality of telescoped conductor tubes including a first sequence of alternately disposed tubes and a second sequence of tubes disposed in alternating relationship with the first sequence of alternately disposed tubes, all of the tubes of the first sequence of alternately disposed tubes being electrically connected at both ends thereof, and all of the tubes of the second sequence of alternately disposed tubes being electrically connected at both ends thereof.

In accordance with another feature of the invention, the conductor tubes are provided with a channel for a coolant.

In accordance with a further feature of the invention, the intermediate space between the conductor tubes is filled with insulating material having a Shore hardness within the range of to 100 and an extensibility or expansion of at least percent, and a gas-tight adhesive connection is provided between the insulating material and the surface of the partial conductOl'S.

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 a device for crucible-free, floating-zone melting a crystalline rod, 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 partly schematic and partly sectional view of a vessel wall of a device for crucible-free zone melting having an opening or passage through which a current supply extends in accordance with the invention;

FIG. 2 is a substitute circuit diagram of the current supply shown in FIG. 1;

FIG. 3 is a perspective view, partly broken away and partly in section, of a current supply of a device for crucible-free zone melting in accordance with the invention; and

FIG. 4 is a longitudinal sectional view of another embodiment of the current supply of FIG. 3.

Referring now to the drawings and first particularly to FIG. 1 thereof, there is shown schematically therein a current supply A which is formed of five telescoping conductor tubes disposed coaxially to and radially spaced from one another. The current supply A is disposable in axial direction in an opening or passage formed in a wall B of a vessel within which an induction heating coil (not shown in FIG. 1) is disposed, and within which a zone melting operation is performed. The opening or passage formed in the wall B is sealed by an oil seal In FIG. 1, the sequence of Arabic numerals l to 5 represents the numbering of the conductor tubes in accordance with their increasing tube diameter while the sequence of Roman numerals I to V represents the numbering of the conductor tubes in accordance with their decreasing tube diameter.

At both ends of the current supply A, the conductor tubes having uneven numbers i.e. the

conductor tubes

1, 3 and 5 or I, III and V, are electrically conductively connected to one another by electrical conductors D, E and F, G, and the tubes of even numbers i.e. 2 and 4 or II and IV are electrically conductively connected to one another by the electrical conductors H and K.

In FIG. 2 there is shown a circuit diagram. of the current supply A, shown in FIG. 1. The ohmic resistance or resistivity R (Arabic numeration) signifies the ohmic resistance or resistivity of the coaxial conduction formed from the conductor tubes 2 and 3 (Arabic numeration), and the conductivity L represents the conductivity of the coaxial conduction formed from the conductor tubes 2 and 3 (Arabic numeration). This applies analogously to the ohmic resistances or resistivities R R R and the conductivities L L and L In the substitute circuit diagram of FIG. 2, the capacitances of the current supply A of FIG. 1 are not taken into consideration because these capacitances are very small for devices for crucible-free zone-melting with respect to the capacitance of the condenser or capacitor in the heating circuit which contains the induction heating coil for heating the molten zone in the crystalline rod.

As is shown by the circuit diagram of FIG. 2, the coaxial conductors formed from respectively two adjacent conductor tubes are connected in parallel to one another. Therefore, the voltage drop at the current supply A having many telescoping conductor tubes which are connected as shown in FIG. 1, is smaller than at a current supply having only two conductor tubes nested within or telescoping one another. Therefore, the power loss for the current supply A with many conductor tubes telescoping or nested within one another is lower than the power loss of a current supply with only two conductor tubes nested within or telescoping one another. With the cur-' rent supply A according to FIG. 1 it is adequate to supply sufficient energy for heating the molten zone in a crystalline rod to a single-winding induction heating coil. The current supply A can even have a considerable length, such as 1,600 mm for example, without the power loss therein becoming significant.

The embodiment of a current supply for a device for crucible-free zone-melting according to the invention, which is shown in FIG. 3, is formed of three coaxially nested or telescoped

copper tubes

1, 2 and 3. This current supply is located in a non-illustrated opening or passage in the wall of a vessel in a device for crucible-free zone-melting. The conductor tubes 1 to 3 are numbered in accordance with their increasing tube diameter. At the upper end of the current supply of FIG. 3, electrically conductive metal tongues 9 to 11, for example of copper, are applied to the conductor tubes 1 to 3, respectively, while at the lower end, as shown in FIG. 3, which is located outside the vessel of the device for crucible-free zone-melting, clamps 34 to 36 of electrically conductive metal such as silver for example, are fastened to the individual tubes 1 to 3. An induction heating coil 13 having, for example, a single winding, is conductively secured to the tongue 10 and the tongue 11. The conductor tubes 1 and 3 and the

tongues

9 and 11 are connected to one another edgewise by a silver bridge 12 for keeping stray or scattered inductance low.

At the lower end of the current supply which is connected to a non-illustrated high frequency generator,

clamps

34 and 36 and, accordingly, the conductor tubes 1 and 3 are electrically conductively connected to one another by three rails 37 which are spaced circumferentially at an angle of l20 from one another about the axis of the tubes and are insulated from the clamp 35.By means of this disposition of the rails 37, uniform distribution of the current flow is produced on the conductor tubes. Parallel to the

clamps

35 and 36 and thereby parallel to the conductor tube 2 and the conductor tubes 1 and 3, three capacitors 38 are connected and serve as oscillating capacitors in the heating circuit containing the induction heating coil 13 and have terminals that are secured for example to the

clamps

35 and 36. Four or more rails 37 and capacitors 38 can also be provided having terminals at the

clamps

34 and 36 or 35 and 36 with respectively the same annular spacing therebetween. Moreover, a non-illustrated high frequency generator is connected to the

clamps

35 and 36.

The intermediate spaces between the

conductor tubes

1, 2 and 3 can contain a gas or can be evacuated. Advantageously it is filled with insulating material, however. Especially when the vessel of a device for crucible-free zone-melting, wherein the upper end of the current supply is provided with the induction heating coil 13, is evacuated or filled with protective gas such as a noble gas, it is advantageous for the intermediate spaces between the conductor tubes 1 to 3 to be filled with insulating material which has a Shore hardness in the range of l to I00 and an extensibility of at least 20 percent, and when a gas-tight adhesive connection is provided between the insulating material and the surface of the conductor tubes. It is assured thereby that no tears will occur at the connections of the insulating material with the walls of the conductor tubes 1 to 3 due to changes in temperature, through which the vacuum in the vessel of the device for crucible-free zone-melting is reduced. A suitable insulating material is cold-vulcanized silicon rubber, for example silicon rubbers of the German firm Wacker known by the trade names Sil-Gel and Vergussmasse K." The walls of the conductor tubes 1 to 3 are advantageously provided with a silicon resin coating, for example a silicon resin from the aforementioned firm Wacker having the trade name G 718. The silicon rubber forms with the silicon resin coating a gas-tight and lasting adhesive connection. Silicon rubber has the further advantage that the dielectric loss factor thereof is smaller than and that the disruptive or dielectric strength thereof is between 30 and 40 kV/mm. It is thereby possible to operate the induction heating coil 13 with relatively high voltages so that a relatively high power is transmitted to the molten zone in the crystalline rod.

FIG. 4 shows an embodiment of the current supply for a device for crucible-free zone-melting in accordance with the invention, which has three coaxial nesting or telescoping conductor tubes 401-403 of copper, the walls thereof being pro vided respectively with a channel for a coolant. The current supply is provided with a steel casing 404.

The

conductor tubes

402 and 403 have a double wall respectively, and a coolant, for example water, flows into the space between both wall portions of each of the

conductor tubes

402 and 403, through a tube 405 and flows out of the same through a tube 406. In the conductor tube 401, which has the smallest tube diameter of the three conductor tubes 401 to 403, there is located an additional tube 409 with a still smaller tube diameter, the end of the tube 409 located in the tube 401 being open and having at the other end thereof an additional tube 410 for supplying a coolant, such as water for example. The coolant supplied through the tube 410 flows back in the intermediate space between the conductor tube 401 and the tube 409 to an outlet tube 408 through which it discharges from the current supply.

The intermediate spaces between the

conductor tubes

401, 402 and 403 as well as the intermediate space between the conductor tube 403 and the steel casing 404 are filled with cold-vulcanized

silicon rubber

410 or 411. The outer surface of the conductor tube 401, the outer and the inner surfaces of the

conductor tubes

402 and 403 as well as the inner surface of the steel casing 404 are provided with a nonillustrated coating of silicon resin.

At the upper end of the current supply, metallic

terminal parts

412 and 413 are located and are connected with an electrically conductive induction heating coil, not illustrated in FIG. 4. The terminal part 412 is soldered to the conductor tube 401 at the location 416 and with the conductor tube 403 by a soldering location 422 and a metallic ring disc 417 which is soldered to the conductor tube 403 and the terminal part 412. The terminal part 412 is formed with channels 415 wherein the coolant channel formed by the double wall of the conductor tube 403 communicates. The terminal part 413 is provided with a coolant channel 414 wherein the coolant channel formed by the double wall of the conductor tube 402 communicates. To the

terminal parts

412 and 413, an induction heating coil, not shown in FIG. 4, is electrically conductively connected so that the

coolant channels

414 and 416 terminate in the coolant channel of the induction heating coil. The coolant which flows through one of the tubes 405 in the surrent supply flows through the coolant channel of the induction heating coil and finally leaves the current supply again through the other tube 406.

At the lower end of the conductor tubes 401 to 403, as shown in FIG. 4, metal terminal rings 419 to 421 are provided for non-illustrated electrical conductors which electrically conductively connect these terminal rings and consequently the conductor tubes with the clamps of non-illustrated heating circuit capacitors and a non-illustrated high frequency generator. The terminal rings 419 and 421 are connected to one another through an electrical conductor 423.

I claim:

1. Device for crucible-free zone melting of a rod consisting of semiconductor material, comprising a vessel accommodating the semiconductor rod; an induction heating coil positioned coaxially with the rod inside the vessel; an AC current source; and an AC current supply for the induction heating coil comprising electrical conducting means connected to the AC current source and passing gas-tight through the wall of the vessel, the part of the electrical conducting means passing through the wall of the vessel comprising a plurality of tubular single electrical conductors coaxially positioned relative to each other with the tubular single conductors divided into two groups, each of the groups consisting of single conductors connected in parallel and each single conductor being adjacent only to single conductors of the other group.

2. Device according to claim 1, wherein said tubular conductors are mutually telescoped and are formed with a channel for a coolant.

3. Device according to claim 1, wherein said tubular conductors have radial spacing therebetween and said radial spacing between said tubular conductors is filled with insulating material having a Shore hardness in the range of 10 to and an extensibility of at least 20 percent, a gas-tight adhesive connection being provided between said insulating material and the surface of said tubular conductors.

4. Device according to claim 3 wherein said insulating material is silicon rubber, and the surface of said tubular conductors in said radial spacing has a coating of silicon resin.

5. Device for zone-melting a crystalline rod comprising a vessel, an induction heating coil mounted in said vessel for heating a molten zone formed in a crystalline rod, and an electric current supply for the induction heating coil extending into said vessel through an opening formed in a wall of said vessel, said current supply comprising a plurality of mutually telescoped conductor tubes radially spaced from one another,

said plurality of telescoped conductor tubes, respectively, numbered sequentially, in radial direction, having all of the tubes of uneven number, on the one hand, and all of the tubes of even number, on the other hand, electrically conductively connected at both ends of said current supply.

6. Device according to claim 5 wherein said conductor tubes are numbered sequentially in order of their increasing diameter.

7. Device according to claim 5 wherein said conductor tubes are numbered sequentially in order of their decreasing diameter.

Claims

3. Device according to claim 1, wherein said tubular conductors have radial spacing therebetween and said radial spacing between said tubular conductors is filled with insulating material having a Shore hardness in the range of 10 to 100 and an extensibility of at least 20 percent, a gas-tight adhesive connection being provided between said insulating material and the surface of said tubular conductors.

5. Device for zone-melting a crystalline rod comprising a vessel, an induction heating coil mounted in said vessel for heating a molten zone formed in a crystalline rod, and an electric current supply for the induction heating coil extending into said vessel through an opening formed in a wall of said vessel, said current supply comprising a plurality of mutually telescoped conductor tubes radially spaced from one another, said plurality of telescoped conductor tubes, respectively, numbered sequentially, in radial direction, having all of the tubes of uneven number, on the one hand, and all of the tubes of even number, on the other hand, electrically conductively connected at both ends of said current supply.