Single-crystal – oriented-crystal – and epitaxy growth processes; – Apparatus
Reexamination Certificate
2000-12-29
2002-05-21
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Apparatus
C117S906000
Reexamination Certificate
active
06391109
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of making an SiC single crystal and apparatus for making an SiC single crystal in which high-quality SiC suitable for semiconductor electronic components is grown.
2. Related Background Art
Being a material excellent in resistance to chemicals such as acids and alkalis, less likely to be damaged by high energy radiation, and yielding a high durability, SiC has been used as a semiconductor material.
In order for SiC to be used as a semiconductor material, it is necessary to obtain a high-quality single crystal thereof having a certain order of dimensions. Conventionally utilized as a method of growing an SiC single crystal of the aimed scale is Acheson method employing a chemical reaction or Lely method employing sublimation/recrystallization technique.
In particular, as a method of growing a bulk of SiC single crystal, Japanese Patent Publication No. 59-48792, for example, discloses so-called modified Lely method in which, in a crucible made of graphite, an SiC single crystal of appropriate dimensions is used as a seed crystal, and material SiC powder is sublimed in an atmosphere under a reduced pressure, so as to be recrystallized on the seed crystal, whereby an SiC single crystal of the aimed scale is grown.
SUMMARY OF THE INVENTION
Of the above-mentioned conventional methods, the Acheson method heats a mixture of silica and coke in an electric furnace and deposits the crystal due to naturally occurring nucleation, thus yielding a large amount of impurities and making it difficult to control the form of resulting crystal and crystal faces, whereby it is hard to produce high-quality SiC single crystals.
Also, in the case where an SiC single crystal is made by the Lely method, since the crystal is grown due to naturally occurring nucleation, it is difficult to control the form of crystal and crystal faces.
On the other hand, an SiC single crystal having a considerably good quality can be obtained in accordance with the invention disclosed in the above-mentioned Japanese Patent Publication No. 59-48792, which belongs to the modified Lely method. When the SiC single crystal is obtained by this method, however, SiC crystals naturally occur from the graphite crucible during the crystal growth period. Using these SiC crystals as nuclei, crystals rapidly grow and inhibit the crystal growth from the seed crystal, thus making it difficult to yield a crystal with a high homogeneity.
Further, there is a problem that, under the influence of heat radiation, the temperature of the upper face of the material becomes higher than that within the material, whereby the amount of sublimation is large at the early stage of growth and gradually decreases as the surface is graphitized. In order to overcome this problem, Japanese Patent Application Laid-Open No. 5-105596 proposes to make a material contain carbon and further form a surface portion of the material with a layer containing carbon, thereby preventing heat radiation from occurring from the upper part of the crucible. Even in this method, however, it is difficult to effect such control that the material consistently reaches the seed crystal under the same vapor pressure during the synthesis, whereby the production of a high-quality SiC single crystal cannot be expected.
Also, the area where the material is sublimed upon heating by the heat conduction or heat radiation from the crucible gradually expands from the material in the vicinity of the part in contact with the side face or bottom face of the crucible to the material located at the center part. Since the part of material in the vicinity of the side face or bottom face of the crucible sublimed in the early stage changes into highly heat-insulating soot-like powder as the sublimation area expands, however, the heat conduction and heat radiation to the material at the center part would decrease drastically, whereby the sublimation of the material at the center part may diminish suddenly or fail to occur. In particular, for synthesizing a single crystal having a large area, the crucible for charging the material is required to have a large diameter as well, whereby the radial alteration of material would be a severe problem. Though the growth apparatus disclosed in Japanese Patent Application Laid-Open No. 5-58774 aims at uniformly heating the material by installing a heat conductor within the crucible, it cannot restrain the SiC material from subliming so as to change into soot-like powder, thus failing to keep crystallizing speed from changing over time in principle, whereby the manufacture of high-quality SiC single crystal cannot be expected in this apparatus, either.
FIG. 5
is a graph showing vapor pressure curves of carbon (C) and SiC, in which the ordinate (on a logarithmic scale) and the abscissa indicate pressure (Pa) and temperature (°C.), respectively. As shown in
FIG. 5
, the vapor pressure of Si is higher than that of SiC
2
or Si
2
C occurring during the generation of SiC by one digit. For enhancing the SiC-forming speed, it is necessary to supply a sufficient amount of Si and C to the seed crystal. In this case, however, there is a problem that, if the material temperature is raised so as to increase the partial pressures of SiC
2
and Si
2
C, which have low vapor pressures, in order to sufficiently supply C, the partial pressure of the Si system will be so high that the stoichiometry (stoichiometric composition) of the material and synthesized crystal may shift.
WO9713013A discloses an epitaxial growth method in which a high-speed jet of silane gas is sprayed onto an SiC substrate within high-temperature hot walls. The SiC single crystal can be grown at a high speed in this technique. Since Si is supplied by a gas, however, there occurs a problem that hydrogen etches SiC within the high-temperature hot walls. Also, the silane gas may form particles in the vapor phase, thus contaminating the inside of the apparatus and degrading the SiC single crystal.
FIG. 6
shows the temperature dependence of Si partial pressure in a major reaction in which SiC grows in thermal CVD of SiC. From this graph, it can be seen that, as the hydrogen partial pressure rises, the reverse reaction for SiC growth proceeds, whereby SiC is etched. Namely, when the hydrogen partial pressure is high, it becomes difficult to form a high-quality SiC single crystal.
In view of such conventional problems, it is an object of the present invention to provide a method of making an SiC single crystal and an apparatus for making an SiC single crystal in which a high-quality SiC single crystal can be obtained.
In order to overcome the above-mentioned problems, the present invention provides a method of making an SiC single crystal, the method comprising a disposing step of disposing solid Si with in a first temperature area T
1
, and disposing a seed crystal of SiC within a second temperature area T
2
that is higher than the first temperature area T
1
; an Si-evaporating step of evaporating Si from the first temperature area T
1
; an SiC-forming-gas-generating step of generating an SiC-forming gas by reacting thus evaporated Si and carbon; and a single-crystal-forming step of causing the SiC-forming gas to reach the seed crystal so as to form the SiC single crystal.
First, in the method of making an SiC single crystal in accordance with the present invention, solid Si is evaporated as being heated by the first temperature area T
1
. Here, as the temperature of the first temperature area T
1
, is regulated, the partial pressure of Si can be adjusted. Subsequently, thus evaporated Si is reacted with carbon, where by an SiC-forming gas is generated. As the SiC-forming gas reaches the seed crystal of SiC, the SiC single crystal is formed. Here, if the partial pressure of carbon to combine with the evaporated Si is made substantially the same as the partial pressure of Si determined by the temperature of the first temperature area T
1
, a high-quality SiC single crystal can be obtained.
Also, since a solid source of
Nishino Shigehiro
Shiomi Hiromu
Anderson Matthew
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
Utech Benjamin L.
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