US 3036898 A
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May 29, 1962 G. E. BROCK ET AL SEMICONDUCTOR ZONE REFINING AND CRYSTAL GROWTH Filed April 30, 1959 INVENTORS GEOFFREY E. BROCK EDWARD S. WAJDA daw yawa/ ATTORNEY United rates Patent @fifice 3,935,898 Patented May 29, 1962 3,036,898 SEMlCONDUCT DR ZONE REFINENG AND CRYSTAL GRGWTH Geoffrey E. Brock and Edward S. Wag'da, Wappingers Falls, N.Y., assignors to international Business Machines Corporation, New York, N.Y., a corporation of New York Filed Apr. 38, 1959, Ser. No. 809,956 2 Claims. (Cl. 23-301) This invention relates to the zone refining of semiconductor materials and in particular to the formation of a single crystal during the zone refining of semiconductor material.
The semiconductor material used in devices such as transistors is in the form of a single crystal of a high degree of purity with a very closely controlled quantity of deliberately introduced impurities therein. The deliberately introduced impurities operate to establish the resistivity and the conductivity type of the semiconductor crystal. The order of magnitude or the relationship of the impurity atoms to semiconductor atoms in the crystal has been found to be in the vicinity of one impurity atom to ten million crystal atoms. In order to provide such purity, techniques that go beyond the standard chemical purification methods previously used in the art have been developed for the semiconductor industry. Two of these techniques have been known in the art by the terminology Zone leveling and zone refining wherein in the zone leveling technique, a given quantity of impurity is evenly distributed throughout the semiconductor material, and in the zone refining technique, the fact that an impurity with a segregation coeificient less than unity has a greater solubility in the molten state than in the solid state, is employed to sweep impurities out of a quantity of semiconductor material. The technique of zone refining has been described in the publication The Transactions of the American Institute of Metallurgical Engineers, vol. 194, page 141, 1952, by W. G. Pfann, and the technique of zone leveling is described in the Bell System Technical Journal, vol. 35, page 637, 1956, by D. C. Bonnet and B. Sawyer.
What has been discovered as a technique and structure whereby the zone refining and/ or zone leveling operation may be combined with a proper arrangement of conditions of heat application to result in the growth of a single crystal in a single operation wherein a higher degree of crystal purity than has heretofore been available in the art is attained. The technique is practiced through the use of a zone refining boat structure which permits an induc ive heating element source to heat only a portion of the semiconductor material and to prevent heat from being applied in undesired portions.
It is an object of this invention to provide a technique for growing an improved purity semiconductor single crystal in an inductive heating zone refining operation.
It is another object of this invention to provide a control for the amount of a semiconductor material melted during an inductive heating zone refining operation.
t is another object of this invention to prevent a seed crystal from being completely melted during an inductive heating zone refining operation.
It is another object of this invention to enhance the purity of semiconductor single crystals grown during an inductive heating zone refining operation.
It is still another object of this invention to provide a technique of controlling the coupling of inductive heating element into a heated object.
It is still another object of this invention to provide an improved inductive heating semiconductor refining container.
()ther objects of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings which disclose by way of example, the principle of the invention and the best mode which has been contemplated of applying that principle.
In the drawings:
FIGURE 1 is a schematic illustration of the structural conditions present in the single crystal zone refining technique of this invention.
FTGURE 2 is an illustration of the structural details of the container or boat employed in connection with the invention.
Referring to FIGURE 1, there is shown a schematic view of a quantity of semiconductor material in a controlled environment being subjected to a plurality of concentrated heat zones such that anumber of molten regions may be caused to traverse the semiconductor material from one end to another when relative motion is applied to the semiconductor material with respect to the sources of heat.
In FIGURE 1, a high purity quartz tube 1, is provided as an illustration of an environment controlling element. The purpose of the quartz tube 1 is to encase the semiconductor material in an environment such that at the elevated temperatures to be reached, no impurities will be introduced into the semiconductor material from the environment and unwanted chemical reactions will be maintained at a minimum.
In an appropriate environment, such as a vacuum, or of a special environment well-known in the art and, referred to as forming gas, a boat assembly labelled elements 2 and 3 to be later described in detail, in connection with FIGURE 2, carries the semiconductor charge to be purified and grown into a single crystal. The semiconductor material used for the charge is not restricted to type and may for example, be the monoatomic semiconductors such as germanium and silicon or the intermetallic compounds such as indium antimonide. It should be noted that there is an inter-relationship between the material selected for the environment controlling element 1, the environment, and the semiconductor material charge, in that there must be sufiicient compatibility be tween these elements that adverse physical property manifestation such as one element having a high vapor pressure must be controllable by the environment such as by increased pressure and the environment housing 1 must be capable of retaining the environment at the increased pressure. A forming gas atmosphere serving as the environment, is maintained in the quartz tube 1 by a fixed quantity being introduced through inlet and outlet tubes labelled elements 4 and 5. The heating of the semiconductor charge is effected by means of inductive energy coils 6a, b, and c which are spaced such that small molten zones 7a, b, and c are formed in the semiconductor charge which for a specific example will be illustrated as germanium and are caused to traverse the length of the charge longitudinally as relative motion between the sources of heat 6, and the germanium charge 7, it is applied in the direction of the arrow via element 8 which operates to pull'the container or boat along inside the housing 1. The solid portions of the semiconductor charge 7, are illustrated, in the case of the portion not yet traversed by a molten zone, as 7d shown as unrefined semiconductor material as in illustration of the fact that no purification has taken place thus far on this portion of the charge. The portions of the charge 7 that have been traversed by one or more molten zones and are in a partial state of refinement, are illustrated as solid sections and are labelled 7e and 7] respectively. A single seed crystal'is placed in the leading portion of the boat and is labelled 7g. This seed portion operates to cause a single crystal to grow from the last molten zone 7a as the refining operation progresses. The original edge of the seed crystal is illustrated schematically as a shoulder 10. The width of the molten zones 7a, is maintained constant by controlling temperatures by means of a thermocouple 9. Impurity additions, in quantities desired, are accomplished by dropping a selected amount of the impurity from a ladle 11, which for example, may be made of high purity quartz or other similar material. Upon completion of a heating operation, the crystal is placed in an after-heater which is illustrated as element 12 which tends to remove strains from the crystal and reduce crystal imperfections.
Referring next to FIGURE 2, a container, commonly known in the art as a boat for carrying the semiconductor charge is made up of an inner portion of liner 3 and an outer portion or shell 2 which covers only a portion of the liner up to a shoulder 13. As will be described in detail later, the inner liner 3 holds a semiconductor charge and the outer shell 2 does not extend along the whole length of the inner element 3, and serves to couple the induction energy in a place where desired. The outer element 2 may be made of any suitable material for coupling inductive energy so long as sufficient purity is maintained and no contamination of the semiconductor material results therefrom. High purity baked graphite has been found adequate for this purpose. The length of the inner liner 3 is covered up to the shoulder 13 is determined by the portion of the charge to which it is desired to have heat applied and to have rendered molten. The seed crystal 7g is placed in the liner 3 in the portion not covered by the inductive coupling element 2.
Operation The principle on which the operation is based is that radio frequency energy from the inductive heating elements 6 will not couple sufficiently with the solid semiconductor material to heat it to the melting point, but will couple with and heat the graphite to a temperature high enough to melt the semiconductor material it surrounds, thus where the seed crystal 7g is placed in the portion of the boat, beyond the shoulder 13 there is no graphite 2, for inductive coupling and hence heat is applied only at the places covered by the covering of graphite 2, so that the molten zones 7a, 7b and 7c are produced only in the region of the boat covered by the graphite covering.
In practicing the invention, referring to FIGURE 1, unrefined charge 7 of semiconductor material such as germanium is placed in the quartz liner 3 of the boat, a germanium single crystal seed 7g is placed in that part of the liner beyond the shoulder 13 not surrounded by the graphite 2, so that it butts against the unrefined charge. The boat is placed in the zone refining apparatus with the interface between the seed crystal and the unrefined charge just under the first inductive heating coil 6a. A molten zone is formed and the boat is then slowly drawn through the tube by pulling element 8. As the molten zone effectively passes down the germanium charge, the germanium which solidifies does so as a single crystal which has been nucleated by the seed crystal 7g. When the boat assembly reaches the second set of coils 6b, a new molten zone is formed at the junction of the seed and the germanium charge which again will solidify as a single crystal as the boat moves thru the apparatus. In this way the germanium is zone refined in the condition of a single crystal. As many sets of inductive heating coils 6 as is pratical may be employed. This number will be determined by the degree of purification desired and the physical dimensions of the refining operations. The rate of pull of the boat thru the apparatus up to the last coil, may be just as fast as in normal zone refining. Upon arrival at the last set of inductive heating coils, indicated substantially as element 60, the rate of pull should be decreased since this is the last single crystallizing pass and the possibility of nucleating spurious crystal grains should 4 be avoided. A slower pull rate is also of assistance in achieving greater crystalline perfection. The width of the zone is maintained constant by controlling the temperature by means of a thermocouple 9.
In order to aid in understanding and practicing the invention, the following set of specifications are provided to provide a starting point for one skilled in the art in practicing the invention. It being understood that no limitation should be construed thereby, since a wide variety of such sets of particular specifications will be readily apparent to one skilled in the art.
Semiconductor charge Unrefined germanium in powder or bar form.
Radio frequency energy 450 kc.
Width of molten zones Approx. 1.25 inch.
Separation between zones Approx. 2 inches.
Overall length of charge Approx. 10 inches.
Encapsulating container Quartz tube.
Refining control environment Forming gas.
Rate of relative motion betweer charge and heating coils 3 inches per hr.
What has been described is a technique of semiconductor molten region refining into a single crystal in the same operation wherein means are provided to prevent coupling of inductive energy into the semiconductor in portions adjacent to a seed crystal and to enhance coupling at places where the refining is to take place. A refining boat for controlling the coupling of inductive energy into the semiconductor material is employed for the protection of a seed crystal in a zone refining opera tion.
While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that the various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention therefore to be limited only as indicated by the scope of he following claims.
What is claimed is:
l. A zone refining process for producing single crystal material which comprises disposing a quantity of semiconductor material including a seed crystal in a container, moving said container past a plurality of inductive heating stages so as to create in said quantity of semiconductor material a succession of discretely spaced molten zones separated by solidified portions of the material, said container comprising two sections, an inner non-inductive section in which said quantity of semiconductor material is disposed and an outer inductive section engageable with said inner section and surrounding all of said quantity of semiconductor material except said seed crystal whereby when said container is moved past said plurality of inductive heating stages heat is coupled only to the quantity of semiconductor material other than said seed crystal.
2. In a zone refining process for controlling the distribution of solutes in crystal material wherein a quantity of semiconductor material is disposed in a container and said container is moved past a plurality of inductive heating stages so as to create in said quantity of semiconductor material a succession of discretely spaced molten zones separated by solidified portions of the material, the improvement which comprises including a seed crystal as part of said quantity of semiconductor material and using a container having two sections, an inner noninductive section in which said quantity of semiconductor material is disposed and an outer inductive section in contact with said inner section ant surrounding all of said quantity of said semiconductor material except said seed crystal whereby when said container is moved past said plurality of inductive heating stages heat is coupled only to the quantity of semiconductor material other than said seed crystal so that zone refined single crystal material is 2,739,088 produced thereby. 2,789,039 2,902,350 References Cited in the file of this patent 5 UNITED STATES PATENTS 2,526,687 Reams Oct. 24, 1950 6 Williams May 12, 1953 Pfann Mar. 20, 1956 Jensen Apr. 16, 1957 Jenny Sept. 1, 1959 GTHER REFERENCE-S Pfann: Zone Melting (1958), pages 75 and 76.