Single-crystal – oriented-crystal – and epitaxy growth processes; – Apparatus
Reexamination Certificate
2002-01-07
2004-04-20
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Apparatus
C117S202000, C117S900000
Reexamination Certificate
active
06723166
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a seed crystal holder for holding a silicon carbide (SiC) seed crystal inside a crucible while an SiC bulk single crystal is grown onto the front surface of the SIC seed crystal. The crystal is grown using a sublimation method.
Currently, two forms of crucibles having fundamentally different designs are used for growing silicon carbide in single crystal form using sublimation. Using a first crucible design, a stock of solid silicon carbide that is to be sublimed is situated in an upper region of the crucible and the SIC seed crystal is situated in a lower crucible region. The silicon carbide that is sublimed from the stock grows in the form of an SIC bulk single crystal on the SIC seed crystal. A first crucible design of this type is disclosed, for example, in Issued German Patent DE 32 30 727 C2. Since the SiC seed crystal can simply rest on the base of crucible, there is no need for any specially designed seed crystal holder. However, in this crucible design, there is a risk of a solid SiC particle, for example from the stock, dropping onto the growing SiC bulk single crystal and disturbing the crystal growth.
In contrast, Published International Patent Application WO 94/23096 A1 and also the article Materials Science and Engineering, Vol. B29, 1995, pages 90 to 93 describe a second crucible design that avoids this drawback by arranging the SiC seed crystal in the upper region of the crucible and the SiC stock in the lower region of the crucible. However, there are no precise details as to how to attach the SiC seed crystal of this type in the upper region of the crucible in a suspended manner.
A further seed crystal holder for holding an SiC seed crystal in a suspended position is described in Japanese Patent Application JP
02-030699
A.
Published International Patent Application WO 97/07265 A1 discloses a coating for the SiC seed crystal that is chemically stable with respect to the material of the SiC seed crystal and with respect to the SiC vapor phase formed form the sublimed SiC of the stock. Furthermore, this coating does not melt or sublime even at a temperature of over 2000° C., which prevails during the growth. In addition, this coating is also suitable for a suspended arrangement of the SiC seed crystal. This is because the SiC seed crystal is adhesively bonded to the seed crystal holder, using a sugar solution. However, since a sugar solution of this type has relatively poor thermal coupling to the seed crystal holder, there is the risk that an undesirable temperature gradient will be established within the SiC seed crystal, thus resulting in disruption of the crystal growth.
In U.S. Pat. No. 5,679,153, an SiC substrate, which is situated in a holder, is immersed from above into a melt that contains, inter alia, silicon carbide and pure silicon. The disclosed process is not a process for growing an SiC bulk single crystal, but rather is a liquid-phase epitaxy (LPE) process, in which only a thin epitaxial SiC layer is produced on an SiC substrate. Since, during a sublimation process for producing an SiC bulk single crystal, the SiC seed crystal is not immersed into a melt, and moreover, since the demands with regard to thermally coupling the SiC seed crystal to the holder are fundamentally different from those imposed in an LPE process, the holder disclosed in this patent cannot be used in a sublimation process for producing an SiC bulk single crystal.
Even clamping the SiC bulk single crystal to suspend the crystal inside the crucible, which is usually made from graphite, may lead to difficulties with obtaining a long-term connection in such a clamped arrangement because of the very low coefficient of thermal expansion of SiC, of 3·10
−6
K
−1
. Since the graphite of the crucible has a higher expansion coefficient, the possibility exists that the clamped SiC seed crystal may become detached from the holder and drop down. However, even if the SiC seed crystal only slips slightly in the clamped holder, the crystal growth may be impaired as a result.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a seed crystal holder, which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
It is an object of the invention to provide a seed crystal holder for holding an SiC seed crystal inside a crucible while growing an SiC bulk single crystal on the front surface of the SiC seed crystal. This holder ensures that the SiC seed crystal is held securely and that there is good thermal coupling with the SiC seed crystal that is being held. Moreover, the seed crystal holder is suitable for growing a low-defect, high-quality SiC bulk single crystal. In particular, the seed crystal holder is also suitable for attaching the SiC seed crystal in a suspended position.
With the foregoing and other objects in view there is provided, in accordance with the invention, a seed crystal holder for holding a SiC seed crystal inside a crucible while growing a SiC bulk single crystal onto a front surface of the SiC seed crystal. The seed crystal holder includes: a back-surface body having a bearing surface for bearing against a back surface of the SiC seed crystal; and a lateral mount cooperating with the back-surface body. The lateral mount can be attached to the back-surface body. The lateral mount includes at least one projection providing a resting place for the edge region of the front surface of the SiC seed crystal. The lateral mount has a layer made from either tantalum, tungsten, niobium, molybdenum, rhenium, iridium, ruthenium, hafnium, or zirconium. Alternatively, the layer can be made from a material that includes tantalum, tungsten, niobium, molybdenum, rhenium, iridium, ruthenium, hafnium, and/or zirconium.
The invention is based on the discovery that a seed crystal holder, which is suitable for growing an SiC bulk single crystal, should ensure good thermal coupling with the back surface of the SiC seed crystal as well as ensure sufficient mechanical fixing. The SiC crystal, in particular, is in the form of a disk. The thermal coupling results from the back-surface body that is provided and that is in contact with the back surface of the SiC seed crystal. Since it does not fulfill any mechanical holding function, the back-surface body can be designed exclusively for achieving good thermal coupling with the SiC seed crystal. In particular, the back-surface body has a heat-dissipating action, i.e. it ensures that sufficient heat is dissipated from the SiC seed crystal and the SiC bulk single crystal growing thereon.
In contrast, the lateral mount mechanically fixes the SiC seed crystal. The SiC seed crystal rests on the projection of the lateral mount so that the SiC seed crystal can be fixed securely in the upper region of the crucible. The geometric dimensions of the lateral mount and of the back-surface body are matched to the SiC seed crystal that will be held such that the back-surface body is in contact with the SiC seed crystal. The back-surface body may, for example, simply rest on the SiC seed crystal, although it is also possible, by suitably selecting the geometric dimensions, to establish a predetermined contact pressure between the SiC seed crystal and the back-surface body. In any event, good thermal coupling of the SiC seed crystal to the back-surface body is ensured.
The projection of the mount can hold the SiC seed crystal either on the front surface, in an edge region, or on an edge shoulder that has been ground into the SiC seed crystal. The lateral mount runs substantially parallel to the direction of thickness of the SiC seed crystal and therefore also substantially parallel to the direction of growth of the SiC bulk single crystal.
To be able to fit the SiC seed crystal into the seed crystal holder, the holder can be taken apart and reassembled, i.e. it includes at least two elements that are mechanically independent of one another. In one embodiment, the back-surface body and the lateral mount are designed
Kuhn Harald
Stein Rene
Voelkl Johannes
Zintl Wolfgang
Anderson Matthew
Greenberg Laurence A.
Kunemund Robert
Locher Ralph E.
Siemens Aktiengesellschaft
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