US3172734A

US3172734A - warren

Info

Publication number: US3172734A
Authority: US
Priority date: 1957-03-07
Publication date: 1965-03-09
Anticipated expiration: 1982-03-09
Also published as: CH416572A; NL237042A; DE1191970B; DE1226539B; NL113928C; NL225605A; FR1192712A; GB889615A; DE1164982B; NL236919A; NL114078C; CH385794A; NL239559A; DE1293934B; CH435757A; BE565404A; NL233434A

Description

March 9, 1965 R. w. WARREN 3,172,734

WATER COOLED CRUCIBLE FOR ZONE REFINING Filed Jan. 10, 1958 2 Sheets-Sheet l 3 I FIG. 3. l I 2 I 0 O -o v o 4 F/G.

F 6 A 0 00 oo A "am e y March 9, 1965 R. w. WARREN 3,172,734

WATER COOLED CRUCIBLE FOR ZONE REFINING Filed Jan. 10, 1958 2 Sheets-Sheet 2 72. W. Warren y Kb I Atlor ney United States Patent 3,172,734 WATER COOLED CRUCIBLE FOR ZONE REFINING Reginald Walter Warren, London, England, assiguor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed Jan. 10, 1958, Ser. No. 708,100 Claims priority, application Great Britain, Mar. 7, 1957, 7,540/57 4 Claims. (Cl. 23-273) This invention relates to a method and apparatus for processing fusible material. The invention is particularly applicable to the processing of semiconductor material such as germanium or silicon.

The processing of semi-conductor material involves melting the material. For example, zone refining of germanium or silicon is carried out by passing a molten zone of the material from one end of a rod to another. It may be required to consolidate germanium or silicon in the form of small lumps or powder into a coherent rod by melting the lumps or powder and casting the material into the form of a rod so that the process of zone refining may be carried out on the rod.

In any process involving melting of semi-conductor material care must be exercised in the choice of the material for the crucible in which melting takes place. This is for the reason that very small quantities of impurities in semiconductor material cause large changes in the properties thereof, and hence it is important that no impurities should be introduced into the material from the crucible. Silicon, in particular, is very liable to contamination when in the molten condition. Hitherto crucibles for the treatment of germanium or silicon in the molten condition have been made of purified graphite or silica. It is, however, very difficult to ensure that graphite or quartz crucibles are entirely free from impurities.

According to the present invention a crucible for processing fusible material is made of metal of high electrical and thermal conductivity and in the form of a hollow walled vessel, fluid being circulated through the hollow walls for cooling purposes. A work coil for inducing eddy currents surrounds at least a portion of said crucible and the current through said coil is made of sufficient magnitude to melt the said material. 5

A molten zone may be caused to traverse the length of the crucible. In this way silicon or germanium in the form of granules or powder may be consolidated into the form of a rod. A rod of silicon or germanium may be refined in a similar way care being taken to observe the limitations in the width of the zone for impurities to be driven to the ends of the rod. The invention has been found to be highly successful in the treatment of silicon. It has been found that, although silicon very readily absorbs impurities when in the molten condition, no contamination of silicon occurs in the practice of the present invention.

The metals copper, silver and gold are chosen for the material of the crucible because each of these metals has high electrical and thermal conductivity and does not react with the material being treated; silver is preferred because its electrical and also thermal conductivities are the highest of the three and because it can be most readily polished to reflect heat optically into the melt. 65

One way of making the crucible is to start with a hol low cylinder of silver and to beat it into the shape of a boat. Practical tests with silver have shown that molten silicon does not wet silver and does not pick up impurities therefrom. Copper and gold may, however, be used.

The invention will be better understood from the fol- 'ice lowing description of embodiments thereof taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side view of one embodiment of apparatus according to the invention.

FIG. 2 is a cross sectional view on the line 11 of FIG. 1, and in conjunction with the accompanying drawings in which:

FIG. 3 is a plan view of another embodiment, with part shown in section.

FIG. 4 is a cross sectional view of the line AA of FIG. 3.

FIG. 5 is a diagrammatic view illustrating one manner of operation of the apparatus.

Referring to the drawings and first to FIGS. 1 and 2, a crucible C of silver has in cross-section the form of a hollow half torus, through which cooling water F is circulated by pipes D and E. Silicon B to be zone refined is contained within the crucible C. High frequency heating coils A are placed around the crucible in the places shown in FIG. 1.

As silicon is of very high resistivity when at room temperature a susceptor (not shown) is initially placed in a position adjacent to one of the coils A and heats the silicon B by radiant heat to a temperature suflicient to reduce its resistivity to an extent to allow eddy currents to be induced therein. This preheater is then removed. The current in the heating coil A may be adjusted so that the electromagnetic field resulting therefrom and from currents induced in the metal of the crucible C react with the currents induced in a molten zone of the material B in such manner as to raise such molten zone away from contact with the metal of the crucible. For this purpose the crucible should be so located with respect to the heating coil that the silicon lies below the centre of the coil and the reaction between the electromagnetic field of coil A and the field clue to currents induced in the wall of the crucible immediately adjacent to the silicon with the field induced in the molten zone of silicon lifts that zone upwards in the drawing.

This arrangement leads to economy in the power needed by reducing heat loss from the molten zone to the metal of the crucible.

Preferably the heat treatment is carried out in an atmosphere of a protective gas such as argon and the crucible C is therefore contained in a chamber in which such protective atmosphere can be maintained.

In the apparatus shown in FIGS. 3 and 4 the crucible 1 is formed from a hollow cylinder of silver having copper rings 2 welded on its ends. The copper rings 2 are closed at one end of each except for apertures communicating with copper pipes 3. The dimensions of one tube 1 which has been used successfully are: length between copper end pieces 15 inches, diameter 1% inches. The crucible is formed by pressing into the shape shown in the drawings the maximum depth of the depression formed being a little over half an inch. In operation cooling water is circulated through the hollow space below the depression. An induction heater 4 consisting of hollow copper tubes wound in a coil surrounds a part of the crucible 1. Cooling liquid may then be circulated through these hollow copper tubes. It is convenient to place the crucible 1 within a tube 5 made of silica in order that the material in the crucible can be surrounded by a protective atmosphere, the induction heater 4 being exterior to the tube 5. The crucible 1 is then pushed or pulled, by means described later, so that the full length of the crucible 1 passes the coil 4.

The apparatus shown in FIGS. 3 and 4 has been found to be particularly suitable in the zone refining of silicon. As silicon is of extremely high resistance when cold it is difficult to induce currents therein. Initially therefore susceptor rings 6 are placed in the neighbourhood of coil 3 4 and are heated by induction from the: coil 4. Radiant heat from the rings 6 then raises the temperature of the silicon sufliciently to lower its resistance and sufiicient eddy currents are then induced therein to melt the silicon in a limited zone immediately within the coil 4. The susceptor rings 6 are then moved away.

The temperature gradient on either side of the limited molten zone is very sharp since the silicon is cooled by the circulation of water through the hollow crucible Walls. The temperature gradient does, however, permit of the molten zone being progressively traversed along a rod of silicon lying in the crucible, the material immediately ahead of the molten zone in the direction of movement being of sufiiciently high temperature to have currents induced therein when this material reaches the centre of the coil.

One means for causing the traversal of the molten zone is shown diagrammatically in FIG. 5.

In this figure silicon 7 is shown contained within a crucible 1 of the kind above described. The copper pipes 3 pass through the center of tubular supports 8 secured to end plates 9. A silica tube surrounds the crucible 1 and is secured to the end plates 9 in gas tight manner. End plates 9 are mounted on a platform 10 which is in turn supported on wheels 11 running on a track 12. Depending from platform 10 is a nut 13 engaged by a lead screw 14 rotatable through gearing 15 from a motor 16. Flexible hose 17 serves to circulate cooling water via copper pipes 3 through the hollow Walls of crucible 1.

A protective gas, such as argon is circulated through the silca tube 5 by flexible hose 18. By adjusting the speed of the motor 16 the platform 10 carrying the crucible 1 within the protective gas atmosphere may be traversed past a stationary heating coil 4 to carry out the process of zone refining at any speed found necessary or desirable and by reversing the direction of rotation of the motor 16 passes of the silicon 7 may be given in alternate directions.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. Apparatus for processing fusible material comprising an open elongated hollow walled vessel of a substantially pure metal of high electrical and thermal conductivity which is substantially non-reactive with the material being treated, means for circulating cooling fluid through the hollow walls of said crucible, a heating coil surrounding a portion of said vessel, means for passing through said coil alternating current of such value as to melt a limited zone of said fusible material and means for causing relative movement between said coil and said crucible to melt successive limited zones of said material.

2. Apparatus for zone refining an elongated body of semi-conductor material comprising an open elongated hollow walled crucible for containing said body, said crucible being made of a substantially pure metal of high electrical and thermal conductivity which is substantially non-reactive with said semi-conductor material, means for circulating cooling fluid through the hollow walls of said crucible, an induction heating coil surrounding a portion of said crucible, means for passing alternating current through said coil of such frequency and power as to induce suiiicient current in a limited zone of said body as to melt the material in said zone and means for causing such relativ movement between said body and said coil that said molten zone traverses at least a portion of the length of said body.

3. Apparatus as claimed in claim 2 in which said coil is placed around the crucible with the center above the center of said material so that the electromagnetic fields of the currents in said coil and those induced in the metal of the crucible tend ot lift the material in the molten zone out of contact with said crucible.

4. Apparatus for processing a fusible material comprising a crucible of substantially pure metal from the group consisting of copper, silver and gold, for containing said fusible material in contact therewith said crucible being in the form of a hollow walled vessel, means for circulating a cooling fluid through said hollow walls to maintain said crucible below its melting point and below the melting point of said fusible material and means for inducing currents in said crucible and in at least a part of the fusible material contained therein suflicient to melt said fusible material through a cross sectional zone.

References Cited by the Examiner UNITED STATES PATENTS 2,686,865 8/54 Kelly 23273 2,719,799 10/55 Christian 148l.6 2,768,074 10/56 Staugger 23223.5 2,785,058 3/57 Buehler 23-301 2,836,412 5/58 Krieger 266--39 2,870,309 1/59 Capita 23301 2,872,299 2/59 Celmer 23-301 2,880,117 3/59 Hamlet 2330l 2,897,329 7/59 Matare 23-301 OTHER REFERENCES Pfann: Transistor Technology, Bell Tel. Lab. Inc. and Western Electric Sp., 1952, pp. 44 to 54.

NORMAN YUDKOFF, Primary Examiner.

MAURICE A. BRINDISI, GEORGE D. MITCHELL,

Examiners.

Claims

1. APPARATUS FOR PROCESSING FUSIBLE MATERIAL COMPRISING AN OPEN ELONGATED HOLLOW WALLED VESSEL OF SUBSTANTIALLY PURE METAL OF HIGH ELECTRICAL AND THERMAL CONDUCTIVITY WHICH IS SUBSTANTIALLY NON-REACTIVE WITH THE MATERIAL BEING TREATED, MEANS FOR CIRCULATING COLLING FLUID THROUGH THE HOLLOW WALLS OF SAID CRUCIBLE, A HEATING COIL SURROUNDING A PORTION OF SAID VESSEL, MEANS FOR PASSING THROUGH SAID COIL ALTERNATING CURRENT OF SUCH VALUE AS TO MELT A LIMITED ZONE OF SAID FUSIBLE MATERIAL AND MEANS FOR CAUSING RELATIVE MOVEMENT BETWEEN SAID COIL AND SAID CRUCIBLE TO MELT SUCCESSIVE LIMITED ZONES OF SAID MATERIAL.