Single-crystal – oriented-crystal – and epitaxy growth processes; – Apparatus – For crystallization from liquid or supercritical state
Reexamination Certificate
2001-01-16
2002-02-05
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Apparatus
For crystallization from liquid or supercritical state
C117S951000
Reexamination Certificate
active
06344085
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a device and a method for producing at least one silicon carbide (SiC) single crystal. A device and a method of this type are known, for example, from International Patent Disclosure WO 94/23096 A1.
International Patent Disclosure WO 94/23096 A1 discloses a device and a method for producing an SiC single crystal which use the sublimation of SiC in solid form, for example of industrial-grade SiC in powder form, and the deposition of the SiC in a gas phase formed as a result of the sublimation on a single-crystalline SiC seed crystal. A reaction vessel in crucible form is used, which contains a storage region and a reaction region which are connected to one another by a gas duct. Alternatively, an additional homogenization region may be connected between the storage region and the reaction region, which homogenization region is likewise in communication with the storage region and the reaction region, in each case via a gas duct. The storage region contains the solid SiC, whereas the single-crystalline SiC seed crystal on which the SiC single crystal grows is disposed in the reaction region. In the document, embodiments in which a plurality of SiC single crystals are deposited on in each case associated SiC seed crystals are also described. In addition, various configurations of the storage regions are disclosed. Outside the reaction vessel there is a heater device which in particular may also be of a multi-part structure in accordance with the division of the reaction vessel into the storage region and the reaction region. The heater device heats the stock of solid SiC in the storage region to a temperature of from 2000° C. to 2500° C. As a result, the solid SiC is sublimed. The gas mixture which is formed in the process primarily contains the components Si (silicon), Si
2
C, SiC
2
and SiC. The gas mixture is also referred to below as “SiC in the gas phase”.
As a result of a temperature gradient being established between the stock of solid SiC and the SiC seed crystal or the SiC single crystal which has already grown, the sublimed gas mixture is conveyed from the storage region into the reaction region, in particular to the SiC seed crystal. In this case, the flow of the SiC in the gas phase is set in terms of its conveying rate and also its direction by the geometry of the gas duct.
The individual constituents of the reaction vessel preferably are formed of a high-purity electrographite. This is isostatically pressed graphite. These types of graphite are commercially available in various densities. They differ in terms of their relative density and different porosity. Even very highly pressed graphites still have a pore volume of at least 8 to 12%. The residual porosity is of importance for the silicon carbon growth, since the gases which are present during the silicon carbide growth, in particular the silicon-containing gases, penetrate into the pores, where they react with the graphite.
The article titled “Formation of Macrodefects in SiC”, by R. A. Stein, Physica B, Vol. 185, 1993, pages 211 to 216, describes a phenomenon which relates to the reaction of solid SiC with the carbon in the graphite forming the vessel material. According to this, small pores or dislocations in the region of an interface between a base material made from graphite and an SiC seed crystal disposed thereon form the starting point initially for the formation and then also the subsequent growth of cavities in the SIC seed crystal. Under the conditions that prevail in the reaction vessel, these cavities extend beyond the seed crystal and also into the SiC single crystal to be produced. These cavities lead to a reduced quality of the SiC single crystal being produced.
As is known from International Patent Disclosure WO 94/23096 A1, material is conveyed from the stock of solid SiC to the SiC seed crystal as a result of a temperature gradient being established and a heat flux which forms as a result. When controlling heat fluxes in the crucible using parts or inserts made from graphite, the difficulties that have already been mentioned above occur again, on account of the reaction between the SiC in the gas phase and the graphite.
Published, Soviet Patent Application SU 882247 A1 discloses the use of tantalum as a suitable material for the crucible or at least for an insert inside the crucible. However, tantalum also reacts with SiC in the gas phase. In particular, carbides are formed, so that the dimensions of the device containing the tantalum change. For example, if tantalum thicknesses of several millimeters are provided, this may lead to mechanical stresses in the crucible.
U.S. Pat. No. 5,667,587 discloses a crucible for sublimation growth of an SiC single crystal, the inner walls of which crucible are coated with a thermally anisotropic coating. In particular, the coating is formed of pyrolitic graphite. The thermal anisotropy of the pyrolitic graphite in this case serves to control heat fluxes inside the crucible as referred to above. However, since the coating, just like the gas duct disclosed in International Patent Disclosure WO 94/23096 A1, is formed of a graphite material, it also has undesirable reactions with the SiC in the gas phase occur. In this context, it is irrelevant whether electrographite or pyrolitic graphite is used.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a device and a method for producing at least one SiC single crystal, that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which allow heat fluxes in the crucible to be controlled and, at the same time, avoid the undesirable reactions of the materials used in the prior art with solid SiC or also with SiC in the gas phase.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for producing at least one silicon carbide (SiC) single crystal. The device contains a crucible having at least one storage region for holding a stock of solid SiC and at least one crystal region for holding in each case one SiC seed crystal on which the SiC single crystal grows. A heater device is disposed outside the crucible, and at least one insert made from glassy carbon is disposed in the crucible.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method for producing at least one silicon carbide (SiC) single crystal. The method includes the steps of:
a) introducing a stock of solid SiC into at least one storage region of a crucible;
b) introducing at least one SiC seed crystal into the crucible;
c) providing at least one insert made from glassy carbon in the crucible for controlling heat flux;
d) heating the solid SiC such that the solid SiC is sublimed and results in SiC in a gas phase being generated; and
e) conveying the SiC in the gas phase to the at least one SiC seed crystal, on which it grows forming the SiC single crystal.
The invention is based on the recognition that glassy carbon, on account of its excellent properties, is eminently suitable for use in a crucible that is used to produce SiC single crystals. Glassy carbon is an amorphous, isotropic material which has a melting point which lies considerably above the temperature of up to 2500° C. which is customarily employed during the production of SiC single crystals. Since, moreover, glassy carbon has a higher density and, with a pore volume of virtually 0%, a significantly lower porosity than all types of graphite, the glassy carbon also presents a considerably reduced tendency to react with both solid SiC and with SiC in the gas phase compared to graphite. Moreover, the thermal conductivity of glassy carbon is lower than that of graphite by a factor of approximately 10. For this reason, glassy carbon is a better thermal insulator than graphite. Therefore, heat fluxes in the crucible can be guided in specific directions by inserts made from glassy carbon.
On account of its high thermal insulating properties,
Kuhn Harald
Rupp Roland
Stein Rene
Völkl Johannes
Anderson Matthew
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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