US2908739A - Water cooled crucible for high frequency heating - Google Patents

Description

Oct. 13, 1959 T. RUMMEL 2,908,739 I WATER COQLED CRUCIBLE FOR HIGH FREQUENCY HEATING Filed May 15, 1957 2 Sheets-Sheet 1 oil 12 9a 9 1 I jzuew r.

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WATER COOLED CRUCIBLE FOR HIGH FREQUENCY HEATING Filed May 15, 1957 2 Sheets-Sheet 2 Inventor. I jxaalorfiwmmefi United States Patent WATER COOLED CRUCIBLE FOR rnon FREQUENCY HEATING Theodor Rummel, Munich, Germany, assignor to Siemens & Halske Aktiengesellschaft, Berlin and Munich, Germany, a German corporation The present invention relates to a device comprising a metallic crucible, particularly for use in melting, by high-frequency induction heating, highly pure semi-conductive materials, forinstance, silicon or germanium to be employed, for example, in the production of therrnistors, transistors, fieldistors, diodes and the like.

crucibles which are heated by high frequency are broadly known. Prior embodiments have, however, in general a poor efliciency, that is, only part of the energy given off by the primary coil of the high frequency device is utilized for heating the substance to be melted. The crucibles must therefore be made of a material which has a substantially higher melting point than the material to be melted and which does not react with the material to be melted.

The object of the present invention is to provide, for the above indicated purpose, a device including a crucible in which the high frequency energy is utilized with high efficiency by closely coupling the primary coil with the crucible and in which furthermore, there can be used as crucible material even metallic substances which have a lower melting point than the material to be melted and/ or could react with the material to be melted without contamination resulting therefrom by the dissolving of the crucible material in the melt or reaction with it.

In accordance with the invention, this object is achieved by the provision of a cooled secondary coil which is particularly closely coupled with the primary coil and forms in whole or in part a crucible comprising a heat-resistant insulating bottom which is preferably outside the action of the high frequency field. The invention also contemplates a method of melting semi-conductive substances in melting devices made in accordance therewith.

Part of the secondary coil may for instance directly form the side wall of a crucible. This may be obtained, for example, by winding a tube of rectangular crosssection so as to form a cylindrical coil traversed by a suitable coolant, for instance, water, and having an insulating member made of heat resistant material, for example, quartz glass or a ceramic material, forming the bottom thereof. The ends of the cylinder coil are connected with another part of the secondary coil, likewise traversed by the coolant, and cooperatively disposed with respect to the primary high-frequency coil. The spacings between the water-cooled turns of the crucible coil are so narrow that the material to be melted cannot escape therethrough. It is furthermore advisable to provide the secondary coil with substantially fewer number of turns than the primary coil so as to pass through the material to be melted the highest possible current.

One very advantageous further feature of the invention resides in forming the crucible as a part'of a cooled secondary coil comprising only a single turn, in such a manner that two hollow bodies, in particular of cylindrical shape and having double walls and open at each end which are slit open along a generatrix, are conductively connected along the common slit by the wall of one of the hollow bodies which is disposed adjacent the 2,908,739 Patented Oct. 13, 1959 primary coil, such wall passing into the inner wall of the second hollow body forming the crucible and provided with an insulating heat-resistant bottom disposed preferably outside of the high-frequency field, producing in this way a closed current path.

For the melting of semiconductor compounds, in melting devices made in accordance with the invention, it is not advantageous in all cases to connect the high-frequency heating at the very start of the melting operation since the material to be melted has a considerable resistance in cold state and therefore high frequency energy is taken up by it only with very low efiiciency. For this reason, the material to be melted is initially suitably heated by means of a gas discharge, or electric discharge, for instance with high cathode potential drop, or by an arc discharge or the like, until the conductivity thereof has become sufliciently high so that the heating may be continued with high efliciency by high-frequency until the molten state is obtained.

The various objects and features of the invention will appear from the description, which is rendered below, with reference to the accompanying drawing, wherein Fig. 1 shows one embodiment in sectional view;

Fig. 1a shows the structure of Fig. l in simplified perspective view;

Fig. 2 is a vertical sectional view of another embodiment;

Fig. 3 shows a transverse sectional view of the embodiment illustrated in Fig. 2; and

Fig. 4 illustrates in sectional view an embodiment comprising a crucible made of cross-sectionally rectangular tubing wound to form a cylindrical coil.

Referring now to Figs. 1 and la, the primary coil 3 of the high-frequency device, having two terminals 3 and 3", is disposed within a hollow cylinder 1 having the walls 1' and 1" slightly spaced therefrom as indicated in Fig. 1 at 7. The crucible 2, forming the melting chamber 4, is disposed outside this cylinder. It has two walls 2' and 2" and is conductively connected with the cylinder 1 by Way of walls 6', 6" and 6a, 6b forming channels which communicate with the channels 1 and 2 and also forming a central channel 5 extending from the crucible 2 to the cylinder 1. The channels 1 and 2 are traversed by a coolant, preferably water. The high frequency energy which is suitably fed to the primary coil 3 and which should have a high voltage and low amperage, is transmitted with only slight energy losses at low voltage and high amperage to the inner wall 1" and from there over the connecting walls 6 and 6" to the inner wall 2" of the crucible. The primary coil 3 may for instance have 30 turns so that the voltage in the secondary coil is stepped down to about one 30th of the voltage in the primary coil, but the current becomes about 30 times as high. In order to increase the current density within the crucible, it is furthermore advisable to keep its crosssection small by making the crucible substantially shorter than the cylinder 1 disposed adjacent to the primary coil 3. As indicated in Fig. la by numeral 16, the wall of the crucible 2 may be lined by a layer of powdered material corresponding to the material to be melted.

As is particularly shown in Fig. 1b, electrodes such as E and E may be disposed directly above the material 4 which is to be melted, such electrodes being connected to a voltage source U for producing a discharge serving for initially preheating the semiconductor material 4 so as to increase its conductivity to a value at which the high-frequency energy supplied by the source HF and given off from the high-frequency primary coil 3 to the cylinder 1 and therewith to the secondary coil 2 (crucible) and to the material 4, can be efliciently employed to continue heating the material to melt it. Numeral 15 indi cates the bottom of the crucible 2, which is made of heatr. c 3 resistant material, and 15', 15" indicate connections for the cooling medium.

In Figs. 2 and 3, numeral 3 is the high-frequency primary coil having the terminals 3' and 3".' Numerals 9" and 9" are the wallsfof a cylindrical secondary coil coupled with the high-frequency primary coil 3 and forming an inner cavity 9, the walls 9' and 9" being conductively interconnected over connecting walls 9a and 9b with the walls of another cylindrical metallic member disposed coaxially therein and having the walls 10 and 10" forming the cavity 10. The cavities 9 and 10 are traversed by a coolant by way of the channels formed by the connecting walls 9a and 9b. The inner walls such as 9a extending radially inwardly and connecting with the inner wall of the cylindrical member forming the cavity 10 are separated by a slot 12. Numeral 13 indicates the space formed between the inner wall 9" of the outer secondary coil portion and the outer wall 10' of the inner secondary coil portion forming the crucible for melting the material 4. The inner chamber of the inner cylinder having the. walls 10 and 10" is charged with the material to be melted, for instance, semiconductor material 4. It is equipped with an insulating bottom 15 in the form of a plate or a shallow dish (Fig. 2) which may consist for instance of quartz or ceramic and should preferably lie outside the field. The inner wall of the inner cylinder 10" gives off the high frequency energy to the material to be melted. The current path-of the high frequency on the secondary coil extends essentially from the outer wall 9 of the outer cylinder by way of the walls forming the slot 12 to the inner wall 10 of the inner cylinder. Numerals 15 and 15 indicate connections for the circulation of the cooling medium.

In order to obtain a high current density in the crucible, it is advisable, as described in the previous embodiment, to make the crucible (10, 10', 10") shorter than the surrounding cylinder (9, 9', 9").

The advantage of the arrangement according to Figs. 2 and 3 is in particular that in addition to a very close coupling between the high-frequency primary coil and the secondary coils, one of which forms the crucible, with efliciencies of close to 1, it can also be constructed in an extremely compact manner. This is advantageous particularly when the melting is to be effected in a protective gas atmosphere.

Fig. 4 shows a structure wherein the side wall of the crucible is formed by a crossrsectionally rectangular tubing 21 wound to form a cylindrical body provided with a bottom 15 made of heat resistant insulating material such as quartz glass or ceramic and the like. Numeral 4 again indicates the material to be melted. The ends of the tubing 21, forming a portion of the secondary coil, are conductively connected with another cooperating secondary coil portion 2111 which is closely coupled with the high frequency primary coil 3, and the latter is connected with the high-frequency source HF. Both portions ofthe secondary coils 21a and 21 (the latter forming the crucible 20) are in operation traversed by a cooling medium such as water, the inlet and outlet being indicated by arrows. The secondary coil portion 21a has advantageously a smaller number of turns than the highfrequency primary coil 3 so as to provide for high current flow through the crucible 20. The turns of the tubing 21 are closely adjacent so as to reliably prevent flow of molten material through the corresponding slot formed thereby.

The slot provided in all embodiments is so narrow that the molten material cannot flow out of the crucible. This is prevented in particular by the high surface tension of the molten material. The .cooling may also be made so strong that the material to be melted and seeping into the slot is not heated to the melting point and therefore remains thickly viscous or solid. Furthermore, the property of the semiconductor material tobe melted, of being a poor conductor in-cold condition, is favorable.

The current will preferably flow through the heated inner part of good conductivity of the material to be melted and heat it, while the poorly conductive cooled outer layers are passed through by a small amount of the current. It has been found by experience that the width of the slot may be made about 0.5 to 2 mm.

For the cooled crucible there may also be used metallic substances which have a lower melting point than the material which is to be melted. For instance, copper or silver are very well suited for this purpose, but other metals of good conductivity such as aluminum may also be employed.

The wall thickness of the material to be used for making the cylindrical members may be about 0.5 to 2 mm.

The melting is preferably carried out ina gas protective atmosphere, for instance in a hydrogen atmosphere.

Changes may be made within the scope and spirit of the appended claims.

I claim: v V

1. In apparatus forheat-treating highly pure semiconductor material deposited in a metallic crucible provided with means for cooling wall parts thereof, a device for inducing high-frequency current into said material deposited in said crucible to elfect melting thereof, said device comprising a primary coil connected to a high frequency current source, a secondary coil closely coupled with said primary coil, and means for conductively interconnecting said metallic crucible with said secondary coil to form a closed circuit therewith for the flow of high-frequencyenergy therethrough and through said material deposited in said crucible.

2. A structure and cooperationof parts according to claim 1, comprising a crucible having a narrow slot formed in a cooled wall part thereof, molten material initially seeping into said slot being cooled therein and plugging said slot to prevent outflow of molten material from said crucible.

3. A structure and cooperation of parts according to claim 1, wherein said crucible comprises a bottom wall made of heat resistant insulating material and a metallic side wall extending from said bottom wall, said side wall being hollow and being cooled by a coolant flowing therethrough, a narrow slot being formed in said side wall, molten material initially seeping into said slot being cooled therein and plugging said slot to prevent outflow of molten material from said crucible.

4. A structure and cooperation of parts according to claim 1, comprising means for producing an electric discharge to preheat said semiconductor material deposited in said crucible prior to applying high-frequency energy thereto which is inducedinto said material to eifect the melting thereof.

5. A structure and cooperation of parts according to claim 1, comprising a crucible having a slot formed therein, the width of said slot being about 0.5 to 2 mm., molten material initially seeping into said slot beingcooled therein and plugging said slot to prevent outflow of molten material from said crucible.

6. A structure and cooperation of parts according to claim 1, wherein the inner wall of said crucible is coated with a layer of powder consisting of the material to be melted and about one-tenth to about one-hundredth millimeter thick. 7

7. A structure and cooperation of parts according to claim 1, wherein' said crucible is made of material of good metallic conductivity with a wall thickness of about 0.5to2 mm. l e I 8. A structure and cooperation of parts according to claim 1, wherein said crucible comprises 'a' bottom wall made of heat resistant insulating material, a hollow metallic member extending fromsaid bottom wallin the manner of a single turn and formingthe side wall of said crucible, said hollow side wall being cooled by a coolant flowing therethrough, a narrow axially extending slot being formed in saidside wall, molten material in itially seeping into said slot being cooled therein and plugging said slot to prevent outflow of molten material from said crucible.

9. A structure and cooperation of parts according to claim 1, comprising a secondary coil made of a doublewalled hollow member extending in the manner of a single turn which is closely coupled with said primary coil, said crucible comprising a bottom wall made of heat resistant insulating material, another double-walled hollow metallic member extending from said bottom wall in the manner of a single turn and forming the side wall of said crucible, wall means for metallically interconnecting the respective walls of said double-walled members to form a closed circuit for the flow of high-frequency energy including said crucible and said secondary coil and also forming a narrow slot extending from the inside of said crucible to the inside of said secondary coil, molten material initially seeping into said slot being cooled therein and plugging said slot to prevent outflow of molten material from said crucible.

10. A structure and cooperation of parts according to claim 9, wherein the axial length of said secondary coil appreciably exceeds the axial length of said crucible.

11. A structure and cooperation of parts according to claim 9, wherein said secondary coil is disposed radially spaced from said crucible.

12. A structure and cooperation of parts according to claim 9, wherein said crucible is disposed within said secondary coil coaxially thereof.

13. A structure and cooperation of parts according to claim 1, wherein said crucible comprises a wall formed of a generally spirally coiled metallic tube, said tube being electrically conductively interconnected with said secondary coil and being cooled by a cooling medium flowing therethrough, the turns of said coiled tube being mutually closely disposed forming narrow slots therebetween, molten material initially seeping into said slots being cooled therein and plugging said slots against outflow of molten material from said crucible.

14. A structure and cooperation of parts according to claim 1, wherein said crucible comprises a bottom wall made of heat resistant insulating material and a side wall extending from said bottom wall, said side wall being formed of a generally spirally coiled metallic tube, said tube being electrically conductively interconnected with said secondary coil and being cooled by a cooling medium flowing therethrough, the turns of said coiled tube being mutually closely disposed forming narrow slots therebetween, molten material initially seeping into said slots being cooled therein and plugging said slots against outflow of molten material from said crucible.

15. A structure and cooperation of parts according to claim 1, wherein said crucible comprises a generally spirally coiled cross-sectionally rectangilar metallic tube, said secondary coil being formed of a coiled tubing which is closely coupled with said primary coil and metallically interconnected with said coiled tube forming said crucible, means for conducting a cooling medium for circulation through said secondary coil and through the coiled tube forming said crucible, the turns of said coiled tube being mutually closely disposed forming a continuous narrow slot, molten material initially seeping into said slot being cooled therein and plugging said slot against outflow of molten material from said crucible.

References Cited in the file of this patent UNITED STATES PATENTS 760,057 Cowles May 17, 1904 1,378,187 Northrup May 17, 192.1 1,855,750 Long Apr. 26, 1932 2,686,864 Wroughton et al. Aug. 17, 1954 2,693,498 Penberthy Nov. 2, 1954

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