Dec. 12, 1961 H. W. LNG 3,012,861 PRODUCTION OF SILICON Filed Jan. 15, 1960 zzzzz INVENTOR HARRY W. LING ATTORNEY 3,012,861 United States Patent Office Patented Dec. 12, 1961 1. 2 3,012,861 . Referring now to this drawing, there is shown a ver PRODUCTION OF SELICON tically disposed cylindrical or tubular reactor 1 with a Harry W. Ling, Wilmington, Del assignor to E. I. du conical bottom 3 provided with inlet 4 for entrance of Pont de Nemours and Company, Wilmington, Dei, a reactant fluidizing gas. The reactor 1 is sufficiently high corporation of Delaware to provide for a 2-3 times expansion of the bed 6 and also Filed Jan. 15, 1960, Ser. No. 3,708. a disengaging space 7 above the bed 6 to reduce solids 9. Claims. (C. 23-223.5) blowover to a minimum. A valved inlet 8 is provided for introduction of silicon bed particles. The reaction This invention relates to the production of elemental by-product gases are removed from the reactor through silicon. More particularly, it relates to a new and im 10 the outlet 9 and cyclone 13 which can be kept hot by a proved method of thermally decomposing volatile silicon furnace (not shown). These gases then pass to a con compounds. densing system 10 and cold trap (not shown) through Silicon can be produced by thermally decomposing sili which the vacuum pumping system (not shown) is con con iodide at a reduced pressure -on an electrically nected. The reactor 1 is maintained at the desired reactor heated, incandescent metal filament. In such a process 5 temperature by external heating means 11 surrounding the hot filament is a source of contamination for the the vertical walls of the reactor 1. Product silicon is deposited material. This contamination is especially removed from the reactor after being deposited on the undesirable in semiconductor grade silicon where im initial elemental silicon bed particles through the valved purity levels for certain elements must be less than a few outlet 14. parts per billion. Moreover, in a thermal decomposition 20 The reactor 1 and associated auxiliaries such as the process which uses a metal filament, the silicon must be boiler 12 for silicon iodide vaporization, the cyclone separated from the filament by cutting or by dissolving separator 13 for trapping any solids blown past the dis the filament with strong acids, which results in further engaging space 7, the by-product condensing system 10 contamination. and cold trap, are made of fused silica, and welded It is among the objects of this invention to overcome 25 vacuum tight connections are used throughout the system. these and other disadvantages of prior methods for pro In operating such a process with the apparatus de ducing silicon and to provide novel procedures for scribed above utilizing a reactor 1 where dimensions are accomplishing such objects. It is among the particular 4 inches inside diameter and 5 feet in height, the reactor objects of this invention to provide an improved process is first filled to a depth of 16 inches (static measurement) for obtaining elemental silicon in a state of high purity 30 with elemental silicon particles that have been sized to by thermal decomposition of a vaporized compound of -20-25 mesh. The system is then pumped down in silicon. A further object is to provide an improved pressure to less than .1 micron, while heating the reactor method for readily producing silicon of semiconductor 1 and bed 6 by electric resistance heaters 1 to about purity by a process adaptable to a continuous type of 1000° C. Highly purified silicon iodide vapor from the operation. Other objects and advantages of the inven 35 boiler 12 is passed into the reactor 1 through the inlet 4 tion will be apparent from the following description. at the rate of 35 pounds per hour, as the reactant and the These and other objects of the invention are accom fluidizing medium for the silicon bed particles. The bed plished by the process comprising thermally decomposing is thus expanded to a volume equivalent to about 32 a vaporized compound of silicon in the presence of a bed inches in the reaction space 6. The reaction by-products of elemental silicon particles fluidized by said vaporized 40 silicon compound, and depositing the silicon product comprising iodine vapor and unreacted silicon tetraiodide from said decomposition on said elemental silicon bed are removed from the disengaging space 7 to the by particles. product condensing system, collecting any very fine silicon For example, the objects of the invention are accom blowover in the cyclone separator 13. After operating plished by the process comprising thermally decomposing 45 5 hours, the particles are found to have gained 29 a vaporized compound of silicon selected from the group pounds in weight. The yield of silicon, based on the consisting of silane, halosilanes of chlorine, bromine and silicon iodide fed is about 30%. The product particles, iodine, and silicon iodides. The vaporized silicon.com consisting of high purity silicon are suitable for use in pound is passed into an enclosed reaction chamber con manufacturing electronic semiconductor devices, such as taining particles of elemental silicon Small enough to be 50 transistors, rectifiers, and the like. The unreacted silicon fluidized and maintained in ebullient motion by the va iodide may be recovered and reused if desired. porized silicon compound. The reaction chamber and For a clearer understanding of the invention, the fol the silicon bed particles are maintained at the decom lowing specific examples are given. These examples are position temperature of the reaction involved by external intended to be merely illustrative of the invention and heating elements surrounding the reaction chamber. The 55 not in limitation thereof. - temperature of the fluidized bed of silicon particles is Example I maintained at least equal to the thermal decomposition A fused quartz reactor of the type shown in the fig temperature of the specific reactant gas under the con ure, 4 inches in diameter and 6 feet high, was filled ditions of operation, and below the melting point of to a depth of 16 inches with high purity silicon particles silicon. Generally this is within the range of 300° C. to 60 about 42 of an inch in size. The system was pumped 1400° C. The silicon product of the decomposition re down to a pressure of less than 0.1 micron and heated action is deposited on the fluidized bed particles of silicon to 1000° C. Silicon tetraiodide was fed to the reactor thus increasing their size until they are removed from the at a rate of 35 pounds per hour. After operating 100 reaction chamber as product. If desired, a semi hours, the apparatus was cooled and the bed discharged. continuous type of operation can be attained by with 65 It was found that the silicon particles had about doubled drawing the enlarged particles from the fluidized reaction in size, and that 58 pounds of new elemental silicon-Were bed and replacing the bed particles on a scheduled basis. deposited on the bed particles. This is a yield of about The smaller replacement particles may be advantageously 30% based on silicon iodide passed into the bed. The made from the product particles of silicon by grinding or silicon product particles were of high purity, and they other form of comminution. 70 were suitable for the production of transistors, rectifiers The figure of the attached drawing illustrates an and other semiconductor devices. The silicon product apparatus for carrying out the process of this invention. was ground and sized to 42 inch size, and 8 pounds re 3,012,861 3 4 turned to the reactor. After another 100 hours under crystalline powder /3 to 1 micron in diameter was recov the above reaction conditions, there were deposited on ered in the cyclone separator and identified as silicon. the bed another 58 pounds of high purity semiconductor Example VI silicon. Using the reactor described in Example V, 100 grams Example II of -60--100 mesh silicon particles were added to the The same reactor system as described in Example I reactor and purged with argon for 8 hours. With the was used. The silicon product from the second 100 reactor at 970° C. and the bed fluidized with argon, hour run in Example I was ground and sized to give SiHCl was carried to the bed by diverting a portion of particles of about 42 inch diameter. Eight pounds of 10 the argon through the SiHCls reservoir. The total argon these 42 inch silicon particles were charged into the was supplied to the reactor at a rate of 2 liters/minute. reactor 1 which was maintained at 1100° C. A stream Approximately 450 grams of SiHCls were passed through of argon gas was passed through the silicon tetraiodide the bed in 7 hours of operation. Upon completion of boiler 12 by means of auxiliary inlet 16, and the silicon the run the bed was recovered and the total Si deposited tetraiodide in the boiler vaporized into the argon stream 5 (including a thin wall scale) was determined to be 10.3 at such a rate that the partial pressure of the silicon grams. This weight gain represents an 1.1% yield, and iodide was less than 75 mm. The silicon tetraiodide a "make rate' of 1.47 g/hr. The material was charac in the argon stream then flowed to the reactor at a rate of terized by converting it to single crystal form and evaluat 16.5 pounds per hour.