US 3278274 A
Description (OCR text may contain errors)
1966 w. LIEBMANN ETAL 3,278,274
METHOD OF PULLING MONOCRYSTALLINE SILICON CARBIDE Filed NOV. 13, 1964 ELECTRON 13- ELECTRON BEAM BEAM INVENTORS WERNER K. SPIELMANN WOLFGANG LIEBMANN ATTORNEY 1 mwm nu LIN-" United States Patent 3,278,274 METHOD OF PULLING MONOCRYSTALLINE SILICON CARBIDE Wolfgang Liebmann, Schoneberger Weg, and Werner K.
Spielmann, Post Deufringen, Germany, assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Nov. 13, 1964, Ser. No. 411,067 Claims priority, application Germany, Dec. 17, 1963,
8 Claims. (c1. 23-301 This invention relates to a method of pulling monocrystalline silicon carbide, and more particularly to the fabrication of semiconductor components by this method.
More recently, SiC has gained increasing importance in connection with the fabrication of semiconductor components in the electrical engineering art. However, the advantages of this substance, which essentially reside in its high melting point and its chemical inertness, cause unfavorable efI'ects in processing. Especially aggravating is the fact that, unlike other known semiconductor materials, SiC cannot be pulled as a bar out of the melt, as, at approximately 2300 C., SiC changes over directly out of the solid into the gaseous phase. Also, the pulling of SiC bars out of a melt containing SiC has not been possible so far since SiC dissolves only very poorly with the solvents deemed suitable until now. Thus, it has not yet been possible to pull conocrystalline bars out of a melt consisting of Si and SiC since even at 1700 C. the solubility of SiC in Si amounts only to approximately 2 percent. It has only been possible to obtain small SiC crystallites as inclusions in Si. Such crystallites are, however, only conditionally useful in fabricating semiconductor components.
In order to enable the production of the relatively large and homogenous SiC monocrystals required for the economic fabrication of semiconductor elements, this invention proposes a method of pulling monocrystalline SiC in which SiC dissolved in metallic chromium is locally heated to approximately 1700 C. in an SiC crucible under the action of a suitable radiation, e.g. under the action of a laser or an electron beam, the heated region being placed into contact with an SiC seed which, preferably rotatably, is Withdrawn at such a low speed that the chromium which has been enriched by the pulling of the SiC crystal migrates in the direction of the higher temperature away from the crystallizing front and, due to the low vapor pressure caused by the high temperature prevailing there, partly separates by changing over into the gaseous phase,
This method has the advantage that the crucible, which consists of SiC, remains relatively cold whereas the temperature required for performing the pulling operation remains-due to the suitably controlled radiationrestricted to the inner regions of the melt directly adjacent to the crystallizing front. The optimum range of temperatures that may be used for the local heating of the SiC-Cr melt is between 1650" C. and 1800 C. In accordance with a particularly advantageous embodiment of the invention, the temperature of the crucible lies between 1550 C. and 1650 C., the maximum temperature of the locally heated regions surrounding the crystallizing front lies between 1700 C. and 1800 C., and the temperature of the crystallizing front lies between 1650 C. and 1750 C. It has proved to be of particular advantage to carry out the method with the crucible having a temperature of 1600 C., with the maximum temperature of the locally heated region surrounding the crystallizing front at 1700 C. The pulling rate is appropriately 1 millimeter per hour. By a suitable additional heating of the SiC crucible it is possible to keep the SiC content of the SiC-Cr melt constant by the SiC of the crucible changing over into the SiC-Cr melt to such an extent that the SiC-Cr ratio remains constant.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.
In the drawing:
The figure represents a sectional view of the crucible and an arrangement for carrying through the method of this invention.
Represented at 1 is a crucible consisting of SiC containing a melt 2 of Cr (87 percent) and SiC (13 percent), the temperature of which is approximately 1650" C. This melt is heated to a little over l700 C. by two electron beams 13 and 14 produced by means of the electron sources 6 and 7. It is into this region where the SiC seed 11 is initially dipped, which is mounted in the clamp 10 at the lead screw 9 guided in 8. By turning the lead screw 9, the seed 11 is under constant rotation pulled out of the melt 2 upwardly at such a low speed that the chromium will migrate in the direction toward the region 4 of the melt which has been heated to slightly over 1700 C. by the electron beams 13 and 14 and which surrounds the crystallizing point hemispherically. Thereby, the bar 12 is produced which consists of monocrystalline SiC. The electron beams 13 and 14 are directed to sweep a circular region on the surface of the melt. The intensity of the electron beams is controlled so that, due to the heat reflection of the crucible and the heat reflection of that portion of the melt surface enclosed by said hemispherical surface, the temperature of the melt in the region of the hemisphere indicated by the dotted line 4 is at a maximum and decreases in a direction toward the hemispheres indicated by the dotted lines 3 and 5 to reach a value of approximately 1700 C. in the region of the SiC bar 12, i.e., in the crystallizing region. The Cr which has been enriched by the pulling process migrates in the direction of the higher temperature, i.e., into the region of the hemisphere indicated by the dotted line 4, and in doing so partly changes over into the gaseous phase since at the indicated temperature the vapor pressure of the Cr is only approximately 2 millimeters Hg. The temperature of the crucible 1 and of the lower parts of the melt 2 is controlled by means of the induction furnace 15 in such a manner that sufficient SiC will be transferred into the melt for keeping the latters SiC content constant.
While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
1. Method of pulling monocrystalline SiC from a melt at a crystallizing front therebetween comprising the steps of:
pulling a seed of monocrystalline SiC from a melt of Cr and SiC, said melt being heated by a first external heating means to form a hemispherical Zone of high Cr concentration in the region of said crystallizing front; and
heating the region of the outer surface of said hemispherical zone by a second external radiation heating means to a temperature ranging between 1650 C. and 1800 C. to partially vaporize the Cr thereat and to cause said high concentration of Cr in said region of said crystallizing front to migrate to said region of said latter heating.
2. Method according to claim 1 wherein said second external radiation heating means includes an electron beam.
3. Method according to claim 1 wherein said second external radiation heating means includes a laser.
4. Method according to claim 1 wherein said pulling is at a rate of 1 millimeter per hour.
5. Method according to claim 1 wherein said temperature of heating is approximately 1700 C.
6. Method according to claim 1 wherein said melt is established in an SiC crucible, said first external heating means heats said crucible to a temperature which lies between 1550 C. and 1650 C., the maximum temperature of the heated area of the outer surface of said hemispherical zone lies between 1700 C. and 1800 C., and the temperature of said crystallizing front lies between 1650 C. and 1750 C.
7. Method according to claim 6 wherein said crucible temperature is 1600 C., said temperature of said region 4- maximum of 1750 C., and said temperature of said crystallizing front is 1700 C.
8. Method according to claim 6 wherein said temperature of said crucible of SiC is so controlled that the transfor of the SiC into the melt of SiC and Cr takes place in such a manner that the Cr-SiC ratio remains constant.
References Cited by the Examiner UNITED STATES PATENTS 2,858,199 10/1958 Larson 148-1.6 2,968,723 1/1961 Steigerwald. 3,053,635 9/1962 Shockley 23-301 NORMAN YUDKOFF, Primary Examiner.
of the outer surface of said hemispherical zone being a 15 G. P. HINES, Assistant Examiner.