Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state
Reexamination Certificate
2001-05-21
2004-03-16
Norton, Nadine G. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
C117S088000, C117S094000, C117S097000, C117S911000, C117S951000
Reexamination Certificate
active
06706114
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the fabrication of semiconductor materials and, more particularly, to devices for use in fabricating silicon carbide crystals and the fabrication of silicon carbide crystals, for example, to produce silicon carbide boules from which a silicon carbide wafer may be provided.
BACKGROUND OF THE INVENTION
Silicon carbide (SiC) is rarely found in nature. It has, however, been manufactured for more than eighty years, in crystalline form, for abrasive products. Silicon carbide crystals found in nature and in abrasive products are generally black and not translucent because they contain substantial levels of impurity atoms.
In the 1950's the Lely process was developed at General Electric Company by which silicon carbide was sublimed and randomly deposited to produce small, thin silicon carbide crystals that were used in early silicon carbide semiconductor device development.
Because of the theoretically quite favorable electronic properties of silicon carbide, significant development activities were initiated during the 1960's and 1970's with the objective of growing large (bulk) crystals of low impurity silicon carbide for use in the production of semiconductor devices. These efforts finally resulted in the commercial availability of relatively low impurity, translucent silicon carbide crystals.
North Carolina State University was issued U.S. Pat. No. 4,866,005 and Re. 34,861 on the growth of single crystal silicon carbide through a sublimation process. The disclosure of U.S. Pat. No. 4,866,005 and Re. 34,861 are incorporated herein by reference as if set forth fully herein. Since that time, the growth of silicon carbide crystals has been described in several United States Patents, including commonly assigned U.S. Pat. No. 6,045,613 entitled “PRODUCTION OF BULK SINGLE CRYSTALS OF SILICON CARBIDE,” the disclosure of which is incorporated herein by reference as if set forth fully herein. Additional patents relating to the growth of silicon carbide or silicon carbide alloys include commonly assigned U.S. Pat. No. 6,048,813 entitled “SIMULATED DIAMOND GEMSTONES FORMED OF ALUMINUM NITRIDE AND ALUMINUM NITRIDE: SILICON CARBIDE ALLOYS,” and U.S. Pat. No. 6,086,672 entitled “GROWTH OF BULK SINGLE CRYSTALS OF ALUMINUM NITRIDE: SILICON CARBIDE ALLOYS,” the disclosures of which are incorporated herein by reference as if set forth fully herein.
One difficulty with silicon carbide is that silicon carbide may contain micropipes or other defects, such as dislocation defects. Such defects may reduce the suitability of regions of a silicon carbide wafer containing such defects for use in a semiconductor device. For example, a transistor with such defects incorporated therein may have a higher leakage current than a corresponding transistor without such defects. Accordingly, improvements may be needed in the growth of silicon carbide crystals.
In gallium arsenide (GaAs) and gallium nitride (GaN) growth through chemical vapor deposition (CVD), epitaxial lateral overgrowth (ELOG) and pendeo-epitaxial growth techniques have been utilized to reduce defects in layers of GaAs or GaN. Such techniques are, for example, illustrated in U.S. Pat. No. 4,522,661. U.S. Pat. Nos. 6,051,849 and 6,177,688. However, growth of silicon carbide crystals (e.g. boules) suitable for providing wafers or substrates is typically not carried out using CVD processes but is performed using physical vapor transport (PVT) growth, such as through a sublimation growth process as described above.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide methods for producing silicon carbide crystals, seed crystal holders and seed crystals for use in producing silicon carbide crystals and silicon carbide crystals. According to embodiments of methods according to the present invention, silicon carbide crystals are produced by forcing nucleation sites of a silicon carbide seed crystal to a predefined pattern (e.g. nonrandom) and growing silicon carbide utilizing physical vapor transport (PVT) so as to provide selective preferential growth of silicon carbide corresponding to the predefined pattern.
In further embodiments of the present invention, nucleation sites are forced to the predefined pattern by modulating a thermal profile of the seed crystal. Such a modulation may be provided by forming regions of higher thermal conductivity in a seed crystal holder. The regions of higher thermal conductivity correspond to the predefined pattern. The seed crystal is mounted on the seed crystal holder.
The regions of higher thermal conductivity may be formed by removing portions of the seed crystal holder so that the seed crystal selectively contacts the seed crystal holder. The regions of higher thermal conductivity may correspond to regions where the seed crystal contacts the seed crystal when the seed crystal is mounted on the seed crystal holder.
Alternatively, the regions of higher thermal conductivity may be formed by removing portions of the seed crystal holder so as to provide cavities in the seed crystal holder and filling the cavities in the seed crystal holder with a material having a higher thermal conductivity than a material of the seed crystal holder. Such cavities may be filled by covering the seed crystal holder with a layer of the material having a higher thermal conductivity and removing a sufficient amount of the layer of the material of higher thermal conductivity so as to expose portions of the seed crystal holder. In particular embodiments of the present invention, the material of the seed crystal holder is graphite and the material of higher thermal conductivity is silicon carbide.
In further embodiments of the present invention, the predefined pattern may be stripes in the seed crystal holder. The predefined pattern could also be a pattern of posts in the seed crystal holder. The posts may be substantially circular.
In additional embodiments of the present invention, the nucleation sites may be forced to a predefined pattern by forming a pattern on an exposed surface of the seed crystal so as to provide regions of the seed crystal which extend beyond other regions of the seed crystal. Again, the pattern may include stripes in the seed crystal, a plurality of posts, and/or a plurality of substantially circular posts.
In still further embodiments of the present invention, nucleation sites are forced to the predefined pattern by forming a pattern of material other than silicon carbide on the silicon carbide seed crystal so as to provide a pattern of regions of having a reduced sticking coefficient over other regions of the seed crystal. Possible patterns may include stripes on the seed crystal, a plurality of posts on the seed crystal and/or a layer of material having a plurality of opening therein so as to expose potions of the seed crystal. The opening in the layer may be substantially circular. Furthermore, the material other than silicon carbide may be graphite.
In yet additional embodiments of the present invention, a seed crystal holder for growing silicon carbide using physical vapor transport is provided. The seed crystal holder has a body section configured to hold a silicon carbide seed crystal and a plurality of regions of differing thermal conductivity in the graphite body section. The regions of differing thermal conductivity have a predefined pattern and, along with the body section, are configured to produce a thermal profile in the seed crystal corresponding to the predefined pattern.
The plurality of regions may be a plurality of cavities in the body section. Alternatively, the plurality of regions of differing thermal conductivity may be configured to contact the seed crystal. In such embodiments, the regions of differing thermal conductivity may be regions of a material having a different thermal conductivity than the body section provided within the body section. The regions of differing thermal conductivity may have a lower thermal conductivity than the body section of the seed holder or a higher thermal conductivity than th
Cree Inc.
Myers Bigel Sibley & Sajovec P.A.
Norton Nadine G.
Song Matthew J
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