Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-10
2004-06-08
Stein, Stephen (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S034400, C428S446000, C117S951000
Reexamination Certificate
active
06746787
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention relates to manufacturing method of a single-crystal silicon carbide (SiC), especially to a method capable of restraining a micro-pipe as a crystal defect from being inherited.
2. Related Art
Heretofore, SiC single crystals are generally produced by sublimation method, however, hollow penetrating holes called micro-pipe defects (hollow penetrating defects) are formed at a degree of 100 to 1000 pieces/cm
2
.
In a case where a power device or a high frequency device is formed, an epitaxial film, which becomes regions to form devices, is formed so as to have a structure suitable for the devices using these single crystals as a substrate. When the defects exist in the substrate, the defects are inherited into the epitaxial film which is grown on the substrate, so that defects which are in the same number of the micro-pipe defects are formed in the epitaxial film. Further, when the devices are formed in this epitaxial film with these defects, the fact is reported that leak current of the devices increases while backward withstand voltage decreases. Therefore, it is very important to reduce the defects in producing the devices.
As a method for reducing the micro-pipe defects in the epitaxial film in which devices are formed, recently, methods for eliminating the micro-pipe defects in the SiC single crystals as the substrate has been proposed. The methods are disclosed in U.S. Pat. No. 5,679,153, JP-A-10-324600, JP-A-2000-44398, and “Study on dislocations of 4H-SiC thick layer grown by CVD ” (The Lecture of the 47th Japan Society of Applied Physics Related Association, Abstracts of the lecture, separate volume 1, page 407, No. 29P-YF-6, Kamata et al., March, 2000, Central Research Institute of Electric Power Industry).
According to the method in U.S. Pat. No. 5,679,153, when crystals are grown by liquid crystal epitaxy technique using melted SiC in silicon, an epitaxial film in which micro-pipe defects are reduced is grown on a seed substrate having micro-pipes.
Next, according to the method in JP-A-10-324600, formation of a polycrystalline film of a &bgr;(cubic)-SiC or &agr;(hexagonal)-SiC on a surface of an &agr;-SiC single crystal substrate (seed crystal) by thermal chemical vapor deposition (CVD) and thermal treatment of the composite body resulting from the formation are repeated a plurality of times so that a plurality of &agr;-SiC or &bgr;-SiC polycrystalline films are oriented (the kind of solid phase epitaxial growth) in the same direction of the crystal axis of the &agr;-SiC single crystal substrate (seed crystal). Thus, SiC single crystals are formed so as to have few micro-pipe defects.
On the other hand, according to JP-A-2000-44398, after a coating material is coated on a single crystal substrate having micro-pipes, thermal treatment is conducted to occlude the micro-pipe defects in the SiC substrate that exist in the SiC substrate, so that a crystal in which at least a part of the micro-pipe defects are occluded is obtained.
Further, according to the Abstracts in The Lecture of the 47th Japan Society of Applied Physics Related Association, the fact is reported that an epitaxial film is formed on a substrate in a thickness of 65 &mgr;m at a rate of 16 &mgr;m/h, so that micro-pipes are occluded.
According to the above-described first method, the epitaxial film should be grown to a thickness of about 20 to 75 &mgr;m or more by the liquid phase epitaxy method, to obtain a region where the micro-pipes are eliminated. Moreover, an epitaxial film on which devices are formed is formed on the epitaxial film by liquid phase epitaxy by a CVD method, so that a number of manufacturing processes increase.
According to the above-mentioned second method, SiC composite is obtained so as to include crystal boundaries therein since the polycrystalline film is formed on the single crystal substrate. When the composite is subjected to the thermal treatment to cause the solid phase epitaxy on the seed crystal, there is possibility that crystal defects due to internal stress at the crystal boundaries in the polycrystalline film are introduced. These defects become sources of traps, and therefore there is a problem that this substrate is not suitable for a substrate to form devices. Moreover, the formation of the film, the thermal treatment, and a surface flattening should be repeated several times to grow a substrate having a practical thickness. Thus, processes increase so that manufacturing cost becomes high.
According to the above-mentioned third method, at least the covering process with the coating material, the thermal treatment, and a surface flattening process that includes a removing process of the coating material are necessary, so that the manufacturing process increases.
According to the above-mentioned fourth method, although the micro-pipes are occluded by thickening the epitaxial film, a thickness of the epitaxial film to form devices on the substrate is about 20 to 30 &mgr;m at most. Therefore, there is a need that the micro-pipes are occluded even if the epitaxial film is thin. Besides, the growth rate only about 16 &mgr;m/h. It takes many hours, i.e., 4 hours or more to occlude the micro-pipes. That is, this method is not suitable for a commercial use as a method for forming an epitaxial film for devices or bulk.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problem, and an object thereof is to provide a technique for providing more practical occluding method of a micro-pipe in a silicon carbide substrate that has the micro-pipe.
According to a first aspect of the present invention, when an epitaxial film is formed on a silicon carbide substrate having a micro-pipe, temperature difference is applied between a front surface of the substrate and a back surface of the substrate that is disposed opposite to the front surface so that the front surface is at a low temperature.
By lowering the temperature at the front surface of the silicon carbide substrate as compared to the back surface, sublimation gas of SiC is generated at the vicinity of the back surface where the temperature is high (the micro-pipe or the back surface). The sublimated gas flows to a side of the front surface through the micro-pipe, and then recrystallized at the vicinity of the front surface where the temperature is low. At that time, the gas is recrystallized at an inside of the micro-pipe, so that an inner diameter of the micro-pipe becomes small, and finally, the micro-pipe can be occluded.
Incidentally, as described in a second aspect of the present invention, by setting a temperature of the substrate at 1650° C. or more, sublimation is apt to occur from the substrate, and occlusion of the micro-pipe is stimulated.
Moreover, as described in a third aspect of the present invention, by setting the temperature of the substrate at 1750° C. or more, sublimation is stimulated from the substrate, so that the micro-pipe is occluded easily. However, in a case where the temperature exceeds 1900° C., the sublimation is stimulated so that the sublimation and a recrystallization are balanced so as to restrain the occlusion of the micro-pipe. Therefore, the temperature of the silicon carbide substrate is preferably set to 1900° C. at most.
Moreover, as described in a fourth aspect of the present invention, since hydrogen gas or helium gas has a high heat-transmitting characteristic so as to effectively lower the temperature at the front surface of the silicon carbide substrate where the gas is supplied, so that the temperature difference between the front surface and the back surface of the substrate is sufficiently generated. Thus, the sublimation gas from the vicinity of the back surface of the silicon carbide substrate is transferred to the front surface.
Moreover, as described in a fifth aspect of the present invention, by setting a flow rate of the gas at 1 m/sec or more, the temperature at the front surface of the silicon carbide substrate where the gas is supplied is effectively
Hara Kazukuni
Hirose Fusao
Naito Masami
Onda Shoichi
Posz & Bethards, PLC
Stein Stephen
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