US2956863A

US2956863A - Apparatus for the manufacture of single crystals

Info

Publication number: US2956863A
Application number: US689197A
Authority: US
Inventors: Goorissen Jan
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1956-11-28
Filing date: 1957-10-09
Publication date: 1960-10-18
Anticipated expiration: 1977-10-18
Also published as: DE1051806B; FR1196958A; GB827466A; CH377786A; NL212548A; NL103477C

Description

1960 J- GOORlSS EN 2,956,863

APPARATUS FOR THE MANUFACTURE OF SINGLE CRYSTALS Filed Oct. 9. 1957 INVENTOR JAN GOORISSEN AGENT United States Patent APPARATUS FOR THE MANUFACTURE OF SINGLE CRYSTALS Jan Goorissen, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N .Y., a corporation of Delaware Filed Oct. 9, '1957, Ser. No. 689,197 Claims priority, application Netherlands Nov. 28, 1956 2 Claims. (Cl. 23-473) This invention relates to an apparatus for the manufacture of single crystals from a substance, for instance a semiconductor such as germanium or silicon, by drawmg a single crystal upwards from a melt of this substance in the form of a rod. In a further production stage this rod may be divided into smaller bodies; for instance in the case of semiconductors these smaller bodies are used in semi-conductive devices such as transistors or crystal diodes.

Single crystals obtained by this known crystal-pulling technique have a high content of lattice defects, so called dislocations. In general these dislocations are undesirable, since they seriously interfere with the physical properties, for example the conductivity, of a single crystal and may also adversely affect the further processing of a single crystal, for example diflusion or alloying.

It has been pointed out that dislocations are occasioned by thermal stresses in the freshly grown monocrystal rod when being at a high temperature, which stresses are due to radiation at the surface and concomitant un even cooling over the cross-sectional area of the rod. In order to counteract radiation from the surface as much as possible it has been proposed, in the method termed zone leveling, to maintain the grown monocrystal rod as a whole at a temperature only slightly lower than the melting temperature of the rod and, upon termination of the growing process, to cool the whole rod slowly and evenly.

In practice, the use of this known expedient in crystalpulling is not well possible inter alia since in the crystalpulling process the rod is subjected to tensile stress and does not rest, as in zone-levelling, in an elongated horizontal boat during the growing process. Such a method further suffers from a limitation in that it takes considerable time due to the low temperature gradient at the surface of solidification, which involves a slow growth. Moreover, such a method merely amounts to shifting the diflieulty to a later phase, that is to say the phase in which the Whole rod has to be cooled slowly and uniformly in order to prevent thermal stresses and dislocations originating therefrom.

The present invention has for its object to provide a simpler method and a device for carrying out this method, which constitutes an improvement upon the pulling method and yields good results without involving the aforesaid inconveniences.

It is based on the recognition that the heat radiated at the surface of a freshly grown rod can be largely compensated by reflecting the thermal radiation from the surface of the melt, which is otherwise largely radiated to the environment, to the surface of the rod, thus using it to advantage.

According to the invention, said rod is at least partially drawn upwards inside i.e. along the axis, of a reflecting screen which is located above the melt and shaped as a truncated cone shell, the base of which faces the melt. Th angle subtended by the generatrix of the cone and the axis of the rod is advantageously made smaller than 60", preferably 10 to 30. The screen is preferably made from a material having a high reflection-coelficient, for example molybdenum, nickel or tungsten. It may sometimes be advantageous to use a screen made from a material having a low reflection coefiicient, which satisfies other requirements imposed by the circumstances and the inner wall of which is furnished with a reflective layer.

It has been found that when using the method according to the invention the etch-pit density of the crosssectional area of a pulled-up monocrystal rod, which density is a measure of the dislocation density of the rod, can be reduced to as low a value as one-tenth of that of a monocrystal rod made by means of the conventional pulling-method.

In order that the invention may be readily carried into effect, an example will now be described in detail with reference to the accompanying drawing, which represents schematically in cross-sectional view that part of the crystal pulling apparatus according to the invention, which is relevant thereto.

A graphite crucible 1 constains a germanium melt 2 and is heated by induced high-frequency current by means of a high frequency coil 3. The upper edge of the crucible 1 carries a quartz ring 4 on which is rested a reflecting molybdenum screen 5 in the form of a truncated cone shell, the imaginary base '6 of which faces the melt 2. The monocrystal rod 7 is partially pulled up inside and more particularly in axial sense of the reflecting screen. The screen is approximately 8 centimetres high and the angle a subtended by the generatrix of the cone and the axis of the rod is approximately 20. The highfrequency coil 3 also partially surrounds the reflecting screen 5 so that this screen contributes, by radiation, to the reflected radiation. It has been found that this expedient means another appreciable improvement. The reflecting screen is provided with a number of holes 8 which permit inspection of the growing process inside the crucible. The crucible is separated from the enw'ronment by means of a quartz tube 9.

What is claimed is:

1. Crystal-pulling apparatus comprising a refractory vessel with a substantially open top for holding a quan tity of molten semi-conductive material, a hollow, truncated, conical, conductive reflecting member having open ends and mounted on and over the vessel with its wide end adjacent the vessel and with its inner reflecting surface exposed to the vessel interior, a coil surrounding the vessel for heating the vessel interior by means of high-frequency currents, and means for drawing a growing crystal upwards from the melt in the vessel and.

through the interior of the reflecting member along its axis, whereby thermal radiation from the melt surface is reflected onto the grown crystal thereby compensating radiation losses from the crystal and thus improving its quality.

2. Apparatus as set forth in claim 1 wherein the coil also surrounds a lower portion of the reflecting member and generates additional heat therein by high-frequency currents, and the angle subtended by the cone axis and its generatrix is between 10 and 30.

References Cited in the file of this patent UNITED STATES PATENTS 2,291,083 Jung July 28, 1942 2,686,212 Horn Aug. 10, 1954 2,753,280 Moore July 3, 1956 2,809,136 Mortimer Oct. 8, 1957 2,839,436 Cornelison June 17, 1958

Claims

1. CRYSTAL-PULLING APPARATUS COMPRISING A REFRACTORY VESSEL WITH A SUBSTANTIALLY OPEN TOP FOR HOLDING A QUANTITY OF MOLTEN SEMI-CONDUCTIVE MATERIAL, A HOLLOW, TRUNCATED, CONICAL, CONDUCTIVE REFLECTING MEMBER HAVING OPEN ENDS AND MOUNTED ON AND OVER THE VESSEL WITH ITS WIDE END ADJACENT THE VESSEL AND WITH ITS INNER REFLECTING SURFACE EXPOSED TO THE VESSEL INTERIOR, A COIL SURROUNDING THE VESSEL FOR HEATING THE VESSEL INTERIOR BY MEANS OF HIGH-FREQUENCY CURRENTS, AND MEANS FOR DRAWING A GROWING CRYSTAL UPWARDS FROM THE MELT IN THE VESSEL AND THORUGH THE INTERIOR OF THE REFLECTING MEMBER ALONG ITS AXIS, WHEREBY THERMAL RADIATION FROM THE MELT SURFACE IS REFLECTED ONTO THE GROWN CRYSTAL THEREBY COMPENSATING RADIATION LOSSES FROM THE CRYSTAL AND THUS IMPROVING ITS QUALITY.