Single-crystal – oriented-crystal – and epitaxy growth processes; – Apparatus
Reexamination Certificate
2002-01-07
2004-08-03
Norton, Nadine G. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Apparatus
C117S084000, C117S951000
Reexamination Certificate
active
06770136
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a device for producing at least one SiC single crystal using a sublimation process that is performed in a crucible.
Published German Patent DE 32 30 727 C2 discloses a method and a device for the sublimation growth of an SIC single crystal. For this purpose, solid silicon carbide (SiC), which is situated in a storage area, is heated to a temperature of between 2000° C. and 2500° C. and is thereby sublimed. A SiC gas phase, which forms through the sublimation, contains components that include inter alia, pure silicon (Si) and the carbide compounds Si
2
C, SiC
2
, and SiC. The gas mixture of this SiC gas phase diffuses through a porous graphite wall into a reaction or crystal area in which an SiC seed crystal is situated. Silicon carbide crystallizes out of the SiC gas phase on this seed crystal at a crystallization temperature of between 1900° C. and 2000° C. In addition to the gas mixture of the SiC gas phase, in the crystal area, there is also a shielding gas that is preferably argon (Ar). A pressure of between 1 mbar and 5 mbar, which is desired in the crystal area, is set by suitably introducing this argon gas. The overall pressure in the crystal area is composed of the vapor partial pressure of the SiC gas phase and the vapor partial pressure of the argon gas.
Published International Patent Application WO 94/23096 A1 describes a method and a device for sublimation growth using the modified Lely method. In each case disclosed, there is a gas passage arranged between the SiC storage area and the crystal area. This gas passage enables the sublimed gas mixture of the SiC gas phase to be supplied to the SiC seed crystal in the crystal area in a controlled and targeted manner.
Moreover, an arrangement of a plurality of such gas passages inside a crucible allows a plurality of SiC single crystals to be simultaneously produced. The sides of the crucible walls that face the inner zone of the crucible may also be provided with a heat-resistant coating, which is preferably produced by pyrolysis. Published International Patent Application WO 94/32096 does not give any precise details about the composition and mode of action of this coating.
Silicon carbide is a compound of silicon and carbon that is sublimed or vaporized in an incongruent form. An incongruent compound is understood as meaning a compound of at least two components in which, at a predetermined temperature, one of the two components (silicon) has a higher vapor pressure than the other (carbon). Therefore, considerably more silicon atoms than carbon atoms sublime out of the stock of solid SiC. This leads to the presence of an excess of silicon in the gas mixture of the SiC gas phase and the presence of an excess of carbon in the remaining stock. It is also said that the stock is carburized. The gas mixture of the SiC gas phase, at the process temperature of over 2000° C., is highly aggressive because of the pure silicon component. The silicon atoms that are highly excessively present in the SiC gas phase have a very considerable tendency to react with other material that is present in the crucible, for example, the material of the crucible wall. Particularly if the crucible wall, as in the prior art, consists of graphite, some of the silicon in the SiC gas phase is lost through a reaction with the carbon of the crucible wall. A further loss of silicon atoms results from the diffusion of atoms out through pores in the crucible wall or through joints between mechanically separate crucible elements from which the crucible is assembled. This lost fraction of silicon atoms is then no longer available for growing the SiC single crystal. In the crystal area, the stoichiometric ratio between silicon and carbon consequently does not correspond to the value required for growing a high-quality crystal.
Published International Patent Application WO 97/27350 A1 and the article Inst. Phys. Conf. Serial No. 142: Chapter 1, Silicon Carbide and Related Materials 1995 Conference, Kyoto, 1996 IOP Publishing Ltd., pages 29 to 32 describe a crucible made from a solid tantalum material. Since tantalum is a very heat-resistant material that is chemically stable even at high temperatures, there is barely any reaction between the tantalum of the crucible wall and the silicon of the aggressive SiC gas phase even at the growth temperature of over 2000° C., so that this source of silicon loss is eliminated in the tantalum crucible disclosed. However, silicon atoms continue to be lost via the joints between the individual crucible elements. Moreover, the tantalum crucible disclosed is so small that it is only possible to grow an SiC single crystal up to 3 mm long. In addition, a solid tantalum crucible is very complex and therefore also expensive to produce.
Further devices for producing an SiC single crystal are disclosed by U.S. Pat. No. 5,895,526 and Published Japanese Patent Application JP 10291899 A. Moreover, published German Patent Application DE 36 44 746 A1 describes a general method for growing a crystal from the melt and a device that can be used for this method.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a device for producing at least one SiC single crystal which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide a device for producing at least one SiC single crystal in which, compared to the prior art, the device loses fewer silicon atoms in the SiC gas phase, and the device is suitable for growing a longer SiC single crystal.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for producing at least one SiC single crystal. The device includes a crucible and a heater device configured outside of the crucible. The crucible has a crucible inner zone, at least one storage area for holding a stock of solid SiC, and at least one crystal area for holding at least one SiC seed crystal onto which an SiC single crystal grows. The crucible has a side that faces the crucible inner zone. The side that faces the crucible inner zone is lined with a foil that includes a material selected from the group consisting of tantalum, tungsten, niobium, molybdenum, rhenium, iridium, ruthenium, hafnium, and zirconium.
The invention is based on the discovery that, at the high process temperature involved in growing the SiC single crystal, the foil that is used to line the crucible is carburized. Carburization of this type involves a change in volume of the foil. When using a foil, this change in the foil, in contrast to the situation when using a coating that is applied in fixed form to the outer crucible wall, results in a change in length and thickness. In particular the change in length of the foil may amount to up to 10%. By contrast, a coating that would be fixed to the inner wall of the crucible would grow primarily in the thickness direction during the carburization, because of the adhesion to the crucible wall. However, this very change in the length of the foil advantageously seals gaps or pores that are present in the crucible wall. Closing off these unsealed locations has the positive consequence that it is virtually impossible for any silicon of the SiC gas phase to diffuse out of the crucible inner zone. Since the material of the foil is also chemically stable with respect to the aggressive SiC gas phase and there is no significant reaction with the silicon component of the SiC gas phase, both main sources of loss of silicon atoms in the SiC gas phase are thereby eliminated. Consequently, there is no longer any significant loss of silicon atoms.
Rather, a small proportion of carbon atoms are removed from the SiC gas phase as part of the carburization of the foil. However, this only takes place at the beginning of the growth process and lasts until the foil has been completely carburized. The proportion of carbon that is withdrawn is so small that, when considered over the entire d
Kuhn Harald
Stein Rene
Voelkl Johannes
Anderson Matthew A.
Greenberg Laurence A.
Locher Ralph E.
Norton Nadine G.
Siemens Aktiengesellschaft
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