PRODUCTION OF ELEMENTAL SILICON FROM IMPURE SILANE FEED

Abstract: Elemental silicon is produced by a process and apparatus wherein relatively impure silane (SiH₄) is purified in the gaseous state, while mixed with an inert carrier gas, to an impurity content no higher than that of «electronic grade» silicon, then is thermally decom­ posed to form elemental silicon of electronic grade purity, without need for purification of the elemental silicon itself. The purification is carried out by injecting the impure silane gas as a series of timed, spaced pulses into a carrier gas stream which transports the silane pulses to a gas chromatographic column, through which the pulses flow in sequence. The components of each pulse are differentially retarded by the packing so that they move through the column at different rates, as a result of which they exit at different but precisely spaced time intervals in relation to pulse input time. The emergence from the column of the silane «peak» or portion of the respective pulse is detected individually or is timed from the pulse input. When detected or timed, the eluent silane peak and admixed carrier gas are valved to a receiver separately from the other com­ ponents, which exit before and after that peak. The silane gas fraction of the peak is then thermally decom­ posed, to form elemental silicon which is of electronic purity, or it may be collected and pressurized before thermal decomposition. The admixed carrier gas is not decomposed, remains a gas, and thereby is separated from the silicon. Depending on the extent to which the gas purification is carried, the product silicon may have a measured resistivity of about 1500 ohms cm. or better, and can be used in fabrication of solar cells or semi­ conductor devices without further purification.

Description: This invention relates to the production of high purity elemental silicon from an impure thermally decomposable silicon compound, without the need to zone refine or otherwise purify the elemental silicon once formed. .Background of the Invention The fabrication of semiconductor devices is generally agreed to require silicon of at least the following purity: Table I Purity Requirements For Polysilicon Electronically active Parts per billion impurity atomic Group III elements < 0.3 (B, Al, etc.) Group V elements < 1.5 (P, As, etc.) Heavy metals < 0.1b- Carbon < 300C Oxygen < 50c a See Proceedings of 3rd International Symposium on Silicon Materials, Science & Technology, Vol. 77-2, p. 18, Electrochemical Society. bActivation analysis Ctimits of detection (infrared) This extremely high degree of purity is referred to in the industry as "electronic grade" purity. Silicon solar cells can utilize a somewhat lower degree of purity than other semiconductor devices, electronic grade silicon being roughly about 100x more pure than solar grade. "Solar grade" purity corresponds to a resistivity of about 1.0 ohm cm or better. Silicon of these very high purities is currently required in very large quantities; the total annual usage of electronic grade purity silicon alone is in excess of 2500 metric tons. The cost of purifying silicon to these extremely high purity levels, in the large quantities needed, has been a major factor -- if not, indeed, a major bottleneck -- in the industry. Such exceedingly high levels of purity are required because of the disastrous electronic affects of even low levels of certain impurities. The most critical of these are electronically active impurities, specifically the Group III elements boron and aluminum, and the Group V elements phosphorus and arsenic. Originally elemental silicon was purified to the requisite purity by zone refining of previously formed impure elemental silicon. The elemental silicon used for purification by the zone refining process was generally produced by high temperature and/or hydrogen reduction of a decomposable silicon-containing compound such as SiC14, Sic13, or SiH4. As an incident to the decomposition of the silicon-containing compound, impurities present in that compound codeposit and thus appear as impurities in the silicon formed from it. For a description of silicon zone refining purification techniques, see -Zone lting, William G. Pfann, Wiley and Sons, 1958. Zone refining can produce silicon of very high purity from such impure silicon. However, zone refining is an expensive and slow technique, requiring very high power input to produce the moving molten zone. Furthermore, due to the relatively large segregation coe'f fi c ient between boron and silicon, boron impurity is not so effectively removed from the silicon by zone refining. The method which is presently used commercially to produce polycrystalline silicon is based on the purification of trichlorosilane (SiHC13) by repetitive distillation, followed by decomposition of the purified compound on a hot silicon filament. Trichlorosilane has a boiling point of 33 or, and is commonly purified by vacuum distillation. It is a corrosive compound, which further complicates its purification. The hydrogen reduction of trichlorosilane takes place at a relatively high temperature, which is a disadvantage because of the higher energy required to sustain the chemical reaction. Another disadvantage is that trichlorosilane is only208 silicon by weight, so that relatively large weights of that compound must be processed to obtain a desired weight of silicon. The primary objective of this invention has been to provide a new and unique process whereby elemental silicon of extremely high purity can be produced on an industrial scale, at a cost competitive with or below that of other available techniques, without need for zone refining or any other purification of the elemental product once formed. Brief Description of the Invention A process and apparatus have now been discovered whereby a particular decomposable silicon-containing compound, in relatively impure form, can be purified to an impurity level as low as that of electronic grade silicon, or even purer,and whereby the compound so purified can be decomposed to produce elemental silicon without introducing other impurities and without any other purification procedure. The product can be used for the fabrication of solar cells or semiconductor devices without further purification. This technique requires silane, SiH4, as the decomposable silicon-containing compound. In carrying out the process of the invention, a silane gas supply of relatively low degree of purity can be used. The silane gas purification is carried out by automatically injecting the impure gas from the source as a series of spaced pulses into a carrier gas stream which carries them in "plug" flow into a gas chromatographic column, through which the pulses are carried in sequence by an inert carrier gas such as hydrogen. The components of the impure gas feed are retarded to different degrees by the packing as they move through the column, with the result that they exit at different but precisely spaced time intervals in relation to input time. The emergence from the column of the silane "peak" or portion of the respective pulse is detected at the column outlet, or is timed from the pulse input. The silane peak (including admixed carrier gas), when it emerges, is valved (by a valve responsive to the timer or detector) to a separate receiver from the other gas components, which exit before and after it. The silane gas fraction so separated is then thermally decomposed; or it may be collected, pressurized and then be thermally decomposed. The carrier gas is not decomposed, remains gaseous, and is thereby separated. In the preferred practice of the invention the peak valving is so short as to separate a silane and carrier gas plug of very high purity, having an impurity content no higher than that of elect'ironic grade purity. In this embodiment the silicon so produced is of electronic purity, and is suitable for use in fabrication of semiconductor devices or solar cells without further purification such as zone refining. In a modified embodiment the peak valving is somewhat longer, resulting in separation of a silane and carrier gas plug of lower purity which when decomposed produces silicon of solar grade purity, corresponding to a resistivity of at least 1.0 ohm cm. The Prior Art Techniques for purifying silicon containing compounds are taught in Jacob et al U.S. Patent No. 3,109,087, and in Shoemaker et al U.S. Patent No. 3,041,141. Various other purification techniques are known per se, by which it might be attempted to purify silane gas. These include cryogenic distillation; pressure distillation; solvent extraction in the liquid state; gaseous diffusion; selective absorption (molecular sieve); sublimation; and solid phase zone refining. However, each of those techniques presents serious limitations for ultra high purification of silane. Large scale gas chromatography is another known purification technique, see "Gas Chromatography - Analysis to Production", Bonmati et al, Chemical Engineering, March 24, 1980, pp 70-72. However, as chromatograph column diameter is increased in order to handle the greater throughput needed for production, as distinguished from the small column diameters that are sufficient for analytical separation, flow through the column increasingly departs from ideal plug flow: radial velocity non-uniformities arise, as a result of which the product is less purified than in standard analytical chromatography. These effects ¯ become increasingly adverse as column diameter increases. Thus the Chemical Engineering article merely indicates that "purities of 99.99% and yields of 98% can be attained." That degree of purity is far short of the ultra high purity required for electronic grade silicon; even solar grade purity is about 100 Ox higher than the highest purity referred to in the article, and electronic grade is about 100x higher still. Thus, until now no technique has been known for obtaining the extremely high degree of silane-gas purification required if subsequent purification of the silicon itself is to be rendered unnecessary. Description of the Drawings The invention can best be further disclosed by reference to the accompanying drawings, in which: Figure 1 is a flow diagram in schematic form, illustrating the practice of the process of the invention in accordance with a preferred embodiment; Figure 2 is a schematic diagram illustrating the segmented flow of the impure silane gas feed through the gas chromatographic column, with hydrogen as the carrier gas; and Figure 3 is a graphical representation showing the relative composition of the output stream from the gas chromatographic column as a function of time, illustrating the cyclic nature of the composition changes by reason of the repetitive segmented introduction of the impure silane into the column. As previously indicated, this invention requires silane gas, SiH4, as the decomposable source of silicon. Use of this particular compound is needed in order to effectively achieve a sufficient purification of the feed that the silicon, produced by decomposition of that purified feed, does not itself require further purification. The silane gas used as the feed can be produced by methods known per se, and the particular technique used to ¯ produce it is not part of the invention. See, for example, the several production methods listed in The Merck Index, 9th Ed., 1976 pp. 1098-9. The impurity content of the silane feed to the purifying column is not crucial to the invention, however higher purity of the feed leads to more economical ultra purification by the method of this invention. The degree of purity of the gas feed can be determined by common analytical gas chromatographic methods. As produced, silane gas normally contains impurities which carry over into elemental silicon upon thermal decomposition of the gas. These impurities most commonly include boron, arsenic and phosphorus-containing gases, such as diborane (B2H6), arsine (AsH3), and phosphine (PH3) which, like silane, are gases at room temperature. Referring to Figure 1, the impure silane gas is supplied from a source 15 at a constant pressure and temperature to a gas supply line 16. Silane boils at -ll20C, and thus is a gas at room temperature. Unlike trichlorosilane, it need not be specially vaporized for injection into the chromatograph; - and this avoids one factor which would adversely affect the uniformity of purification. In Figure -1, a large scale gas chromatographic column ("LSGC") is indicated at 20. By "large scale" is meant that the unit has a column with a diameter greater than 10mum, and that it is capable of handling large throughputs. The construction of such a column is known per se; see for example the Chemical Engineering article previously identified. The column preferably contains at least 500 theoretical gas chromatographic plates. A pure carrier gas which is inert (i.e., non-decomposable and unreactive) -with respect to silane, the column, the packing and the later decomposition step, for example H2, He, N2, or Ar, is supplied from a bottle or other source 17 at a constant pressure and temperature, through a carrier gas supply line 18. Carrier gas sources with electronic impurity contents far less than electronic grade silicon are commercially available. The carrier gas is supplied at the same pressure as the silane. A valve 24 permits a continuous flow of the carrier gas from line 18 to column inlet line 21. Periodically valve 24 is actuated to admit a "pulse' of silane, that is, a constant, predetermined volume from source 15 through line 16 into the flowing carrier gas. The valve 24 can be operated by a valve operator 22, which in turn automatically. energizes a solenoid. Such automatically operated valves are available commercially, for example# from Crawford Fittings, Inc. The silane pulse rate, time duration, and spacing will depend on column size, operating pressure, temperature, packing, and other factors. Injections at the rate of 5 to 20 pulses per minute are believed most useful for volume production; the interval between pulses must be long enough to ensure no cross contamination of the silane component of any one pulse, as it flows through the column, by the impurities of preceding and following pulses, but must be short enough to allow useful throughput of the silane gas. This can readily be determined from the eluent pattern for the pulses. Because of the pulsed flow, the flow entering column 20 will comprise an interval of carrier gas only, followed by a pulse of silane-carrier mixture, followed again by a carrier gas interval, and so forth. This is shown diagrammatically in Figure 2. The column contains a packing chosen to achieve adequate separation of the impurities in the silane feed pulse. A preferred form of packing is " Porop ak -- made by Waters Associates of Milford, Massachusetts. The packing is chosen for its ability differentially to retard the various gas components (other than the carrier) as they flow through the Column. For the Poropak column material B2H6, if present, is typically retarded least; it emerges first, followed by SiH4, then PH3 and AsH3. Figure 3 graphically shows the various component peaks, as they emerge, for a series of pulses. The peak heights and shapes vary with concentrations and other factors, but for a given set of conditions, they remain constant from pulse to pulse. In order to maximize the throughput, it is desirable to have the peaks no further apart than necessary for separation. Squeezing the peaks closer together is accomplished by making the injected samples larger, other things remaining constant, but for high purity it is desirable to have the peaks well separated, sufficiently to permit the desired "peak cutting" or valving, to be described. The operation of the column is thus a compromise between these two requirements. The nature of the effluent peaks can be determined, as is known, by use of a thermal conductivity sensor 25, having a detector in outlet line 26. Such detectors are well known, see "Modern Methods of Chemical Analysis," Pecsok and Shields, John Wiley and Sons, 1968, p.75. As the components elute discretely, the conductivity of the gas changes; Figure 3 reflects the changes in effluent gas conductivity for a series of pulses. The pulse injections are spaced so that no measurable impurities will overlap the purified silane pulses as they emerge from the column. Column outlet line 26 contains a separating valve generally designated at 27, having a valve operator 30 which is responsive to. a solenoid. Separating valve 27 is operable to direct the outlet gas from line 26 to any of two or more separate outlets. In the preferred embodiment shown, valve 27 is operated by the separating valve operator 30 to direct the silane (and admixed carrier) component when it reaches the valve, to a silane collector 35. The other impurities are directed through lines 36 to separate collectors. The carrier gas (i.e., the flow other than the silane peak and the impurity peaks) is directed to a hydrogen recycle line 40 which contains a conventional hydrogen purification device 41. Referring to Figure 3, the separating valve (27 in Fig. 1) is opened by operator 30 for an interval such as that designated by X, corresponding to the passage of the silane peak11 through valve 27, to direct the silane and admixed carrier to receiver 35. As the impurities elute, valve 27 is actuated by operator 30 to direct them to the impurity outlets 36. It will be appreciated that since the impurities elute separately from the silane, the valve can be operated for the elution of each to collect them separately, if desired. These individually collected impurities may have sufficient value that their collection may be economically attractive. Between the elution of the components of the respective pulses, the carrier gas elutes continuously from the column. -As indicated, separating valve 27 is desirably operated to direct the carrier (e.g.-, hydrogen) through a carrier recycle line 40, back to carrier gas source 17. Separating valve operator 30 is preferably responsive to a detector 25 which responds to the elution of a component at a particular time interval after injection of the respective pulse. For this purpose a controller 31 (which may be a timer) establishes an appropriate time window within which the silane elutes; when, within this window, detector 25 detects a sharp change in thermal conductivity, it signals valve operator 30 to open the valve to direct the silane to receiver 35. Alternatively, if the gas chromatographic parameters are sufficiently well controlled, valve operator 30 may be triggered solely by a timer at 31. Timers and detectors suitable for such use are known per se, and do not comprise the invention. Other things #remaining constant, the purity of the silane collected will depend upon the relative period during which valve 27 directs the output gas to receiver 35, in comparison to the duration of the actual silane peak. In other words, the shorter the period of time during which the silane is directed to the receiver, in relation to the width of the silane peak, the more precise the separation will be; but correspondingly, the smaller the yield of purified silane. This factor ultimately controls the purity of the final product silicon. From what has been said, it will be seen that preferably the impurity concentrations of the admixed silane and carrier peak should not exceed the impurity levels of electronic grade silicon set forth previously. If, at given gas purification conditions the resistivity of the silicon product is, say, 100 ohms cms, its purity can be increased by decreasing the period of time in which the silane gas component is directed to the receiver, The precision of the chromatographic separation diminishes with increasing pressure. It is normally desirable to operate the column at the lowest practical positive pressure to obtain efficient utilization of the chromatographic column. Silane can be thermally decomposed by techniques known per se, for example, heated filament free space reactor, fluidized bed, etc. The decomposition technique which is especially useful and which is preferred because it minimizes the introduction of the impurities during decomposition, is that of decomposing the gas in a fluidized bed of fine silicon particles. Apparatus which can be used for this type of thermal decomposition is taught in DOE/JPL Final Report, Contract No. 954334, dated June 1979, the disclosure of which is incorporated by reference. In that technique, the bed on which the silane is decomposed itself must comprise silicon beads previously formed. The silicon particles act as a heat transfer medium to decompose the gas, which deposit additional layers of silicon on the particles. In this decomposition, the inert carrier gas is not decomposed and is thereby separated. (In any event the carrier atoms do not constitute an electronically active impurity in the product silicon itself.) Silane is uniquely suited for purification to an ultra high degree by large scale gas chromatography. Moreover, because it is a gas, it can be injected into the carrier gas stream at reproducible, uniform flow rates, without the irregularities that would accompany vaporization of a solid or liquid. A solid column packing has been found by which high purification can be achieved simultaneously with useful throughput rates. Moreover, the purified gas can be converted to solid silicon pyrolytically, directly following elution from the purification step, which avoids cryogenic trapping that is often required in order to collect other gaseous outputs.

Claims: Claims: 1. In the production of elemental silicon by decomposition of silane, the improvement comprising, providing an impure silane gas feed containing undesirable impurities, injecting said feed as a series of regularly spaced pulses into a stream of an inert carrier gas, passing the resulting stream through a gas chromatograph column, the column having a packing which differentially retards the silane and the said impurities such that the silane gas component of each respective pulse elutes from the column as a silane peak at a different time than the said impurities, operating a valve during elution of the silane peak of each respective pulse to direct that peak to a separate outlet line from the impurities; said valve being opened only during elution of the silane peak, for so brief a period that the purity of the valved silane and admixed carrier gas is no less than that of electronic grade silicon, and decomposing the thus separated silane, in the presence of the carrier gas, to produce elemental silicon of at least electronic grade purity. 2. The process of Claim 1 wherein the valved silane and admixed carrier gas contain no more than the following impurities: Total Parts Per Purity Billion Atomic Group III elements < 0.3 Group V elements < 1.5 Heavy metals < 0.1 Carbon < 300 Oxygen < 50 All others < 0.001 3. The process of Claim 1 wherein the silane feed contains impurities of the group#AsX3, BX3, SiHX3, SiH3X, and PX3#where X is a halogen. 4. The process of Claim 3 wherein the silane feed contains one or more gaseous compounds of the grOUp AsH3 B2H6' and PH3. 5. The process of Claim 1 wherein the carrier gas flows continuously through the column and wherein the silane gas is admitted to the carrier gas stream as square wave pulses separated from start to start by an interval sufficient for elution of all the respective components of the pulse. 6. The process of Claim 1 wherein said valve is open only for a central portion of the silane peak. 7. The process of Claim 1 wherein no step of purifying said elemental silicon is present. 8. In the production of elemental silicon by decomposition of silane, the improvement comprising, providing an impure silane gas feed containing undesirable impurities, injecting said feed as a series of regularly spaced pulses into a stream of an inert carrier gas, passing the resulting stream through a gas chromatograph column, the column having a packing which differentially retards the silane and the said impurities such that the silane gas component of each respective pulse elutes from the column at a different time than the said impurities, operating a valve during elution of the silane gas component of each respective pulse to direct the silane gas component of the respective pulse to a separate outlet line from the impurities; ; said valve being opened only during elution of the silane component to direct said silane component, and admixed carrier gas, to said outlet line, and decomposing the thus separated purified silane, to produce elemental silicon, without other purification, said elemental silicon having a purity corresponding to a resistivity of at least 1.0 ohm cms. 9. Apparatus for producing silicon comprising, a source of silane gas which contains undesirable gaseous impurities, a source of an inert carrier gas, a gas chromatograph column having a diameter of at least 10 mm., an inlet - line connecting said source of carrier gas to said column, a silane flow segmenting valve for admitting said silane gas to said inlet line as a series of timed, uniformly spaced pulses, said column having a packing which differentially retards the flow of the silane and the said impurities therethrough such that the silane component of each respective pulse elutes from an outlet of the column at a different time than the said impurities, a separating valve connected at the column outlet, and means for operating said separating valve during the flow of the silane gas component therethrough to direct at least a portion of the silane gas component of each sequential pulse to a silane line and for closing said valve no later than the conclusion of the flow of the silane gas component therethrough, thereby to separate the silane component from the impurities, and means for thermally decomposing the silane gas component so separated. 10. The apparatus of Claim 9 further including a detector for detecting the passage of the silane gas component of each pulse. 11. The apparatus of Claim 10 wherein said valve is responsive to said detector, to valve the flow of the silane gas component. 12. The apparatus of Claim 9 wherein operation of said separating valve is responsive to a timer which is actuated by the silane flow segmenting valve, to operate said separating valve at a predetermined time thereafter. 13. The apparatus of Claim 9 including a separating valve controller which is responsive both to a signal from a timer actuated by said flow segmenting valve and to a signal from a detector which detects the passage of the silane component from said column within a preestablished time period after actuation of said timer.

Inventors: POTTS, THOMAS M., WALKER, JOE M.
Applicants: EAGLE-PICHER INDUSTRIES, INC.
Application year: 1982
Main classification: C01B33
IPC codes: H01L21, B01D53, C01B33