Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2001-01-16
2002-12-24
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S105000, C117S109000, C117S951000
Reexamination Certificate
active
06497764
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for growing at least one silicon carbide (SiC) single crystal by sublimation of a SiC source material.
The sublimation of industrial-grade SiC in powder form and growing the SiC in a gas phase on a monocrystalline SiC seed crystal is a method for the production of a SiC single crystal known from German Patent DE 32 30 727 C2. The gas mixture that is formed during the sublimation primarily contains the components Si, Si
2
C, SiC
2
and SiC. The gas mixture is also referred to below as “SiC in the gas phase”. The source material used is usually industrial-grade silicon carbide in granular form and of high purity, the silicon carbide preferably having a grain size of approximately 200 to 300 &mgr;m. The SiC source material is produced prior to the growing process of the SiC single crystal and is introduced as a prepared source material into a growing chamber.
The article titled “SiC-Bulk Growth By Physical-Vapor Transport And Its Global Modeling”, by D. Hofmann et al., Journal of Crystal Growth, Vol. 174, 1979, pages 669 to 674 describes a method for the synthesis of a SiC powder which is used as a source material from elemental silicon (Si) granules and carbon (C) powder.
Another method for producing the source material in the form of SiC powder is known from the article titled “Improvement In The Growth Rate Of Cubic Silicon Carbide Bulk Single Crystals Grown By The Sublimation Method”, by H. N. Jayatirtha et al., Journal of Crystal Growth, Vol. 174, 1997, pages 662 to 668. In this case, high-purity silicon and high-purity carbon react with one another for a total of three hours at 1800° C. Then, the powder is removed from the reaction furnace and is subjected to further treatment steps. In particular, a three hour oxidation is carried out at 1200° C., in order to remove excess carbon. Then, an etching step is carried out, in order to eliminate traces of SiO
2
that have formed as a result of the oxidation.
A common feature of all the above-mentioned methods for the production of the source material in the form of the SiC powder is that the production of the SiC powder and the actual growing of the SiC single crystal represent individual processes that are carried out independently of one another.
By contrast, Published Japanese Patent Application JP 6-316 499 A describes a method for the production of an SiC single crystal which in one first process step also includes the formation of the SiC source material from silicon and carbon as an integral part of the growing process. The growing operation in the narrower sense takes place as a result of the sublimation of the SiC source material produced in the previous process step and as a result of deposition of the SiC in the gas phase on an SiC seed crystal. The starting materials used for the production of the SiC source material are carbon in the form of a graphite block or carbon (C) powder with a grain diameter of below 10 &mgr;m and granular silicon with a grain diameter of between 2 and 5 mm. The grain diameter of the silicon is given as a parameter that is not critical and, if appropriate, may also assume values outside the diameter range stated. However, for the C powder employed it is expressly stated that if the grain size is over 10 &mgr;m only the surface of the C powder reacts with silicon, and therefore the SiC source material formed is not suitable.
In a second embodiment, in which a graphite block of porous graphite with a relative density of 0.5 g/cm
3
is used instead of the C powder, the production process of the SiC source material and the growing process of the SiC single crystal do not take place within a single process sequence. The growing chamber is opened again after the SiC source material has been produced, in order for the SiC seed crystal to be introduced. Only then is the actual growing commenced. The relative density of the porous graphite used may also adopt values other than 0.5 g/cm
3
. The upper limit specified is a relative density of 1.0 g/cm
3
, since at higher levels SiC source material is no longer formed in sufficient quantities.
The process conditions during the production of the SiC source material involve a temperature in the range between 1150° C. and 1800° C. and a pressure of less than 200 Torr, corresponding to approximately 266 mbar.
When using C powder with a grain diameter of less than 10 &mgr;m, the reaction taking place during the production of the SiC source material can very easily lead to fluidization of particles, in particular of the C powder. As a result, deposits are formed on the SiC seed crystal, which has an adverse effect on the quality of the growing which is then to be carried out.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for growing SiC single crystals that overcomes the above-mentioned disadvantages of the prior art methods of this general type, which avoids the fluidization of the C powder which occurs in the prior art and therefore leads overall to an improved quality of the SiC single crystal which is grown.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for growing at least one silicon carbide (SiC) single crystal by sublimation of an SiC source material. The method includes introducing silicon (Si), carbon (C) and an SiC seed crystal into a growing chamber, the carbon being a C powder with a mean diameter of powder grains of greater than 10 &mgr;m. Producing the SiC source material from the silicon and the carbon in a synthesis step carried out before a growing step; and carrying out the growing step by growing the SiC single crystal immediately after the synthesis step.
The invention is based on the recognition that, unlike in the customary procedure used in the specialist field, which is defined by the technical teaching disclosed by Published Japanese Patent Application JP 6-316 499 A, the use of C powder with a mean grain diameter of greater than 10 &mgr;m is still possible and even advantageous. On account of the larger geometry and the consequently higher weight, this measure avoids the undesirable fluidization of the C powder during the production of the SiC source material. Therefore, during this process step there is no undesirable deposition on the SiC seed crystal, which is already in the growing chamber while the SiC source material is being produced. The recognition that C powder even with a mean grain diameter of greater than 10 &mgr;m reacts with the silicon so well that the result is a SiC source material of high quality which is eminently suitable for the subsequent growing of the SiC single crystal is particularly surprising.
The carbon used may be both synthetic and natural carbon.
In a first preferred configuration, the C powder has a mean grain diameter of greater than 20 &mgr;m, and in particular of greater than 30 &mgr;m. The larger the mean grain diameter, the more successfully the undesirable fluidization of the C powder is suppressed.
An embodiment in which the C powder is selected to have a grain size which is less than 200 &mgr;m and preferably less than 80 &mgr;m is also advantageous. If the grain size is below the upper limits given above, all the carbon inside the powder grain still reacts with the silicon. By contrast, if a C powder with a larger mean grain diameter is provided, the carbon actually only reacts with the silicon at the surface of the powder grain. As a result, there is an undesirable excess quantity of unbonded carbon and silicon atoms in the SiC source material.
Another advantageous embodiment of the method involves a type of carbon in which the reaction with the silicon to form the SiC source material takes place endothermally or at least without significant amounts of energy being released being used for the carbon in the C powder. This is because undesirable fluidization of particles, in this case of both carbon and silicon particles, may also be caused by an exothermic reaction between the silicon and the carbon. Th
Kuhn Harald
Stein Rene
Völkl Johannes
Greenberg Laurence A.
Kunemund Robert
Locher Ralph E.
Siemens Aktiengesellschaft
Stemer Werner H.
LandOfFree
Method for growing SiC single crystals does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for growing SiC single crystals, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for growing SiC single crystals will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2934349