CA1151395A - High purity silane and silicone production - Google Patents
High purity silane and silicone productionInfo
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- CA1151395A CA1151395A CA000334373A CA334373A CA1151395A CA 1151395 A CA1151395 A CA 1151395A CA 000334373 A CA000334373 A CA 000334373A CA 334373 A CA334373 A CA 334373A CA 1151395 A CA1151395 A CA 1151395A
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- silane
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- trichlorosilane
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Abstract
ABSTRACT
A mini-plant for producing silane from dichlorosilane via a redistribution reaction has produced 97% pure SiH4 at 93%
efficiency The residual impurities were monochlorosilane (2.5%) and dichlorosilane (.5%). The silane purity of 97% achieved by operating the silane still partial condenser at 40 psig and -32°C
outlet temperature was significantly higher than predicted by ideal vapor-liquid equilibrium considerations. Laboratory equilibrium concentrations and order of magnitude reaction rates were confirmed in mini-plant operations. Kinetic isotope studies on the mechanism of the amine catalyzed redistribution of chloro-silanes indicated the amine hydrochloride may not be the active species as originally proposed. The hydrogenation of silicon tetrachloride to trichlorosilane appears to be a true equilibrium, approachable from either HSiCl3 or SiC14. Virtually no H2SiC12 was formed by hydrogenation of HSiCl3.
A mini-plant for producing silane from dichlorosilane via a redistribution reaction has produced 97% pure SiH4 at 93%
efficiency The residual impurities were monochlorosilane (2.5%) and dichlorosilane (.5%). The silane purity of 97% achieved by operating the silane still partial condenser at 40 psig and -32°C
outlet temperature was significantly higher than predicted by ideal vapor-liquid equilibrium considerations. Laboratory equilibrium concentrations and order of magnitude reaction rates were confirmed in mini-plant operations. Kinetic isotope studies on the mechanism of the amine catalyzed redistribution of chloro-silanes indicated the amine hydrochloride may not be the active species as originally proposed. The hydrogenation of silicon tetrachloride to trichlorosilane appears to be a true equilibrium, approachable from either HSiCl3 or SiC14. Virtually no H2SiC12 was formed by hydrogenation of HSiCl3.
Description
~513~5 Bac~ground of the Invention Field of the Invention - The invention relates to the production of high purity silane and silicon. More particularly, it relates to an improved process for enhancing the production of said desired products.
Description of the Prior Art - The development of new techniques for the utilization of non-polluting sources of energy is of paramount national and world-wide interest.
Solar energy is among the energy sources of greatest interest because of its non-polluting nature and its abundant, non-diminishing availability. One approach to the utilization of solar energy involves the conversion of solar energy into electricity by means of the photovoltaic effect upon the absorption of sunlight by solar cells.
Silicon solar cells, the most commonly employed devices based on the photovoltaic effect, have been employed reliably in space craft applications for many years For such applications and for industrial and commercial applications in general, crystals of high purity, semiconductor grade silicon are commonly employed.
Such high purity, high perfection silicon is generally prepared by procedures involving converting metallurgical grade silicon to trichlorosilane, which is then reduced to produce polycrystalline, semiconductor grade silicon from which single crystals can be grown. The costs associated with the production of such high purity, high perfection crystals are high The initial step of converting metallurgical silicon to trichlorosilane has commonly been carried out by reacting .~.
~ 39 5 metallurgical grade silicon with ~Cl in a fluid bed reaction zone at about 300C. Trichlorosilane comprises about 85%
of the resulting reaction mixture, which also contains silicon tetrachloride, dichlorosilane, polysilanes and metal halides. While this technique has been employed successfully in commercial practice, it requires the use of relatively large reaction vessels and the consumption of excess quantities of metallurgical silicon. In addition, the reaction mixture is relatively difficult to handle and has associated waste disposal problems that contribute to the cost of the overall operation.
In producing high purity polycrystalline silicon from trichlorosilane, current commercial technology is a low volume, batch operation generally referred to as the Siemens process. This technology is carried out in the controlled atmosphere of a quartz bell jar reactor that contains silicon rods electrically heated to about 1100C.
The chlorosilane, in concentraticns of less than 10% in hydrogen, is fed to the reactor under conditions of gas flow rate, composition, silicon rod temperature and bell jar temperature adjusted so as to promote the heterogeneous decomposition of the chlorosilane on the substrate rod surfaces. A general description of the Siemens type process can be found in the Dietze et al patent, US 3,979,490.
Polycrystalline semiconductor grade silicon made from metallurgical grade silicon costing a~out $0.50/lb. will, as a result of the cost of such processing, presently cost on the order of about $30/lb. and up. In growing ~ 39 5 a single crystal from this semiconductor grade material, the ends of the single crystal ingot are cut off, and the ingot is sawed, etched and polished to produce polished wafers for solar cell application, with mechanical breakage and electronic imperfection reducing the amount of useable material obtained. As a result, less than 20%
of the original polycrystalline, semiconductor grade silicon will generally be recoverable in the form of useable wafers of single crystal material. The overall cost of such useable material is, accordingly, presently on the order of about $300/lb. and up. Because of the relatively large area requirements involved in solar cell applications, such material costs are a significant factor in the overall economics of such applications.
The economic feasibility of utilizing solar cell technology for significant portions of the present and prospective needs for replenishable, non-polluting energy sources would be enhanced, therefore, if the overall cost of high purity single crystal wafers could be reduced.
One area of interest, in this regard, relates to the de~elopment of a low-cost, continuous process for the production of polycrystalline silicon from silane or chlorosilanes. The decomposition of such silanes in a fluid bed reaction zone is disclosed in Ling, US 3,012,861 and Bertrand, US 3,012,862. Another approach for the continuous production of silicon from silane is that disclosed in German Patent Specification Nos. 752,280, published May 26, 1953, and 1,180,346, published July 1, 1965. In this approach, the silane is heated to above its decomposition temperature quickly ~ 39 S
in a nozzle and is then caused to expand into a substantially cooler chamber from the bottom of which silicon product is collected. A second area of interest of the development of lower cost has been the production of silane by the disproportionation of trichlorosilane.
One suggested approach involves the use of a bed of insoluble, solid anion exchange resin from which silane is recovered at the top and from which Si C14 is withdrawn at the bottom as disclosed in Bakay, US 3,968,199.
Another area of interest resides, of course, in the initial production o~ trichlorosilane from metallurgical grade silicon. Improved processing permitting a reduction in the number or size of the reaction vessels employed, or simplifying the handling of the reaction mixture and reducing the waste disposal problems involved, would contribute significantly to the overall reduction in the cost of high purity silane and/or silicon. Such reduction in costs through simplified processing operations is desired in the art not only in the field of solar cell technology, but to enhance the prospects for the use of s-~ch high purity silicon for semiconductor applications as well. In addition to such specific areas of interest for possible processing improvement, a genuine need exists for integrated overall processing improvements to reduce overall costs, simplify fe~d material requirements and reduce waste disposal and other material disposal considerations. For example, the Bakay process referred to above produces by-product Si C14, which must be utilized, sold or otherwise disposed of in the overall processing operation. The commonly employed process ~513g5 for producing trichlorosilane, on the other hand, requires a source of HCl, adding to the cost and the material handling requirements of the process. An integrated process for the conversion of metallurgical grade silicon to high purity silane and silicon, with simplified material requirements and reduced waste disposal, is genuinely needed in the art~ therefore, to enhance the prospects for effectively utilizing high purity silicon on a commercially practical basis for solar cell and semiconductor applications.
It is an object of the present invention, therefore, to provide an improved process for the production of high purity silane.
It is another object of the invention to provide an improved process for the conversion of metallurgical grade silicon to high purity silane and high purity silicon.
It is another object to provide an integrated process for the production of high purity silane and high purity silicon with simplified feed material and reduced material disposal requirements.
It is a further object of the invention to provide a process for the production of silane from metallurgical grade silicon incorporating an enhanced process for the initial production of trichlorosilane from metallurgical grade silicon.
With these and other objects in mind, the invention is hereinafter disclosed in detail, the novel features thereof being particularly pointed out in the appended claims.
~ 39 5 Summary of the Invention High purity silane is produc,ed by an improved, integrated process utilizing hydrogen and metallurgical grade silicon as essentially the only consumed feed materials, The initial trichlorosilane production is accomplished at elevated pressure and temperature, substantially enhancing the conversion rate and the production rate obtainable in a given size reaction vessel, Unreacted silicon tetrachloride is conveniently recycled for reaction with additional quantities of hydrogen and metallurgical silicon. Material wastage is minimized, and waste disposal is readily accomplished by condensing a minor portion of unreacted silicon tetrachloride from the trichlorosilane reaction mixtu~e, said silicon tetrachloride and accompanying impurities being passed to waste without the necessity for dilution prior to hydrolysis during said waste disposal. The high purity silane thus produced can be further purified to remove residual impurities, as required, and may be decomposed on a continuous or semicontinuous basis to produce high purity sil~n, e.g. in the hot free space of a decomposition reactor or in a fluid bed reaction zone.
By-product hydrogen produced in the silane decomposition operation can conveniently be recycled to the trichlorosilane production step and/or recycled for use as a carrier gas for the silane being decomposed as desired.
Brief DescriPtion of the Drawings The invention is further described with reference to the accompanying drawing illustrating the process flow diagram of one embodiment of the invention for converting metallurgical grade silicon to high purity silicon.
Detailed Description of the Invention The process of the invention as herein described and claimed is an ~ntegrated process of enhanced performance capability for producing high purity silane or high purity silicon from metallurgical grade silicon. The invention achieves higher performance efficiency and reduced equipment size requirements than previously known techniques. The overall process includes (1) the enhanced production of trichlorosilane from metallurgical silicon and hydrogen, (2) the disprop~rtionation of trichlorosilane to produce high purity silane, and (3) the conversion of said silane to high purity silicon, if desired. The integrated process effectively recycles unreacted and by-product materials, minimizing material wastage and simplifying waste disposal operations.
The present invention is suitable for use in the c~nversion of the conventional metallurgical grade silicon materials available in the art to high purity silane or high purity silicon. Metallurgical grade silicon, as referred to herein, is a grade of silicon having a resistivity generally on the order of about 0~005 ohm-cm and up to about 3~ iron, 0.75% aluminum, 0,5% calcium and other impurities normally found in silicon produced by the carbothermic reduction of silica.
It is also within the scope of the invention to employ a ferro-silicon material containing at least about 90~/O Si and up to 10% or more of iron. It will also be 3~51395 understood by those skilled in the art that suitable grades of ferro-silicon material are included within the meaning of the term "metallurgical silicon," as used herein.
It will also be understood that particular grades of metallurgical silicon containing unusual concentrations of certain specific impurities, perhaps for example 1% or more of lead, may not be suitable feed material for the process.
Metallurgical grade silicon or ferrosilicon is processed, in accordance with the invention, to produce an initial reaction mixture of di- and trichlorosilane by a novel and advantageous technique that enables the overall process to be carried out in an economically advantageous manner, with the desirable by-product recycle and simplified waste disposal referred to herein. As silicon tetrachloride separated from the reaction mixture can readily be recycled for reaction with additional quantities of metallurgical silicon and hydrogen, high purity silane is produced with said metallurgical silicon and hydrogen being essentially the only major consumed feed materials.
In the process of the invention, metallurgical silicon is initially reacted with hydrogen and silicon tetra-chloride in a reaction zone maintained at a temperature of from about 400C to about 600C and at a pressure in excess of about 100 psi ,o form trichlorosilane as follows:
(1) 3 Si C14 + 2H2 + Si (me~ grade)--? 4H Si C13 with the reaction (1) mixture containing a yield of about 20-30% by weight trichlorosilane on a hydrogen-free basis, and of about 0.5% dichlorosilane with the remainder being ~1395 silicon tetrachloride together with impurities comprising mainly carryover metallurgical silicon powder, metal halides essentially without undesired polysilanes~
The elevated pressure and temperature in the reaction zone substantially enhances the production rate obtainable in a given sized reaction vessel and the feed conversion rate, thereby reducing the size requirements for reaction vessels and facilitating the overall production operation.
Additional advantages of the invention in providing an integrated process for the production of high purity silane and high p~rity silicon, with simplified feed material and reduced material disposal requirements, are hereinafter pQinted out with reference to particular aspects and embodiments of the invention.
In the embodiment of the invention illustrated in the drawing, metallurgical silicon is passed through line 1, lock hopper 2 and line 3 into hydrogenation reactor 4 in which said silicon is reacted with hydrogen and silicon tetrachloride in accordance with reaction (1~ under the reaction conditions herein disclosed. Hydrogen from line 5 and recycle silicon tetrachloride from line 6 are passed through vaporizer/preheater 7 to reach the desired reaction temperature and are passed therefrom through line 8 to said reactor 4.
The reaction mixture from reactor 4 comprises trichloro-silane, dichlorosilane, silicon tetrachloride, carryover met-allurgical silicon powder, metal halides and other impurities.
This trichlorosilane gas stream, upon leaving reactor 4 through line 9 is cooled to condense a minor portion of the unreacted s'licon tetrachloride therein, with said carryover ~1395 metallurgical silicon, metal halides and other impurities present in said trichlorosilane gas stream separating therefrom with the condensed silicon tetrachloride.
For this purpose, the trichlorosilane gas stream in line 9 passes to settling zone 10 from which the gas stream passes upward through line 11 to condenser unit 12 from which t~.e partially condensed stream exits through line 13. A minor portion of the unreacted silicon tetra-chloride, for example on the order of about 5% by weight of the overall silicon tetrachloride in said trichlorosilane gas stream, to~ether with accompanying carryover silicon powder and other impurities is returned through line 14 to settling zone L0 from which a waste stream is removed through line 15 for convenient disposal as the only waste stream of the overall integrated process of the invention. Hydrogen gas withdrawn from condenser unit 12 is passed through line 16 to hydrcgen recycle blower 17 from which said hydrogen is passed in line 18 for recycle to line 5 for passage with fresh hydrogen to reactor 4 for reaction therein with additional ~uantities of fresh metallurgical silicon.
The trichlorosilane gas stream from line 13, puri-fied of said impurities and containing said dichlorosilane and the remainder of said silicon tetrachloride, passes through line 19 to surge tank 20 from which it is pumped through line 21 to distillation zone 22 from which tri-chlorosilane and dichlorosilane are separated as overhead from the unreacted silicon tetrachloride, which is withdrawn from the bottom of said distillation zone 22 through line 23, passes to surge tank 24 and is pumped through line 6 so as to pass to reactor 4 for reaction therein with additional quantities of metallurgical silicon and hydrogen.
~1395 The trichlorosilane and dichlorosilane withdrawn as overhead from the distillation zone are then subjected to a temperature capable of causing the disproportionatiDn thereof, resulting in the formation of product silane gas and by-product monochlorosilane and dichlorosilane. For this purpose, the di-and trichlorosilane overhead stream withdrawn from distillation zone 22 through line 25, after passing through reflux condenser 26 from which reflux is returned to said zone 22, first passes through line 27, including surge tank 28, to second distillation zone 29 from which trichlorosilane is removed as a bottoms stream through line 30, including surge tank 31 and pump 32, and is passed therefrom into first disporportionation reactor or reaction zone 33. This reactor contains insoluble, solid anion exchange resin containing tertiary amino or quaternary ammonium groups bonded to carbon and is main-tained at a temperature capable of causing the dispropor-tionation of trichlorosilane according to reaction (2) below:
Description of the Prior Art - The development of new techniques for the utilization of non-polluting sources of energy is of paramount national and world-wide interest.
Solar energy is among the energy sources of greatest interest because of its non-polluting nature and its abundant, non-diminishing availability. One approach to the utilization of solar energy involves the conversion of solar energy into electricity by means of the photovoltaic effect upon the absorption of sunlight by solar cells.
Silicon solar cells, the most commonly employed devices based on the photovoltaic effect, have been employed reliably in space craft applications for many years For such applications and for industrial and commercial applications in general, crystals of high purity, semiconductor grade silicon are commonly employed.
Such high purity, high perfection silicon is generally prepared by procedures involving converting metallurgical grade silicon to trichlorosilane, which is then reduced to produce polycrystalline, semiconductor grade silicon from which single crystals can be grown. The costs associated with the production of such high purity, high perfection crystals are high The initial step of converting metallurgical silicon to trichlorosilane has commonly been carried out by reacting .~.
~ 39 5 metallurgical grade silicon with ~Cl in a fluid bed reaction zone at about 300C. Trichlorosilane comprises about 85%
of the resulting reaction mixture, which also contains silicon tetrachloride, dichlorosilane, polysilanes and metal halides. While this technique has been employed successfully in commercial practice, it requires the use of relatively large reaction vessels and the consumption of excess quantities of metallurgical silicon. In addition, the reaction mixture is relatively difficult to handle and has associated waste disposal problems that contribute to the cost of the overall operation.
In producing high purity polycrystalline silicon from trichlorosilane, current commercial technology is a low volume, batch operation generally referred to as the Siemens process. This technology is carried out in the controlled atmosphere of a quartz bell jar reactor that contains silicon rods electrically heated to about 1100C.
The chlorosilane, in concentraticns of less than 10% in hydrogen, is fed to the reactor under conditions of gas flow rate, composition, silicon rod temperature and bell jar temperature adjusted so as to promote the heterogeneous decomposition of the chlorosilane on the substrate rod surfaces. A general description of the Siemens type process can be found in the Dietze et al patent, US 3,979,490.
Polycrystalline semiconductor grade silicon made from metallurgical grade silicon costing a~out $0.50/lb. will, as a result of the cost of such processing, presently cost on the order of about $30/lb. and up. In growing ~ 39 5 a single crystal from this semiconductor grade material, the ends of the single crystal ingot are cut off, and the ingot is sawed, etched and polished to produce polished wafers for solar cell application, with mechanical breakage and electronic imperfection reducing the amount of useable material obtained. As a result, less than 20%
of the original polycrystalline, semiconductor grade silicon will generally be recoverable in the form of useable wafers of single crystal material. The overall cost of such useable material is, accordingly, presently on the order of about $300/lb. and up. Because of the relatively large area requirements involved in solar cell applications, such material costs are a significant factor in the overall economics of such applications.
The economic feasibility of utilizing solar cell technology for significant portions of the present and prospective needs for replenishable, non-polluting energy sources would be enhanced, therefore, if the overall cost of high purity single crystal wafers could be reduced.
One area of interest, in this regard, relates to the de~elopment of a low-cost, continuous process for the production of polycrystalline silicon from silane or chlorosilanes. The decomposition of such silanes in a fluid bed reaction zone is disclosed in Ling, US 3,012,861 and Bertrand, US 3,012,862. Another approach for the continuous production of silicon from silane is that disclosed in German Patent Specification Nos. 752,280, published May 26, 1953, and 1,180,346, published July 1, 1965. In this approach, the silane is heated to above its decomposition temperature quickly ~ 39 S
in a nozzle and is then caused to expand into a substantially cooler chamber from the bottom of which silicon product is collected. A second area of interest of the development of lower cost has been the production of silane by the disproportionation of trichlorosilane.
One suggested approach involves the use of a bed of insoluble, solid anion exchange resin from which silane is recovered at the top and from which Si C14 is withdrawn at the bottom as disclosed in Bakay, US 3,968,199.
Another area of interest resides, of course, in the initial production o~ trichlorosilane from metallurgical grade silicon. Improved processing permitting a reduction in the number or size of the reaction vessels employed, or simplifying the handling of the reaction mixture and reducing the waste disposal problems involved, would contribute significantly to the overall reduction in the cost of high purity silane and/or silicon. Such reduction in costs through simplified processing operations is desired in the art not only in the field of solar cell technology, but to enhance the prospects for the use of s-~ch high purity silicon for semiconductor applications as well. In addition to such specific areas of interest for possible processing improvement, a genuine need exists for integrated overall processing improvements to reduce overall costs, simplify fe~d material requirements and reduce waste disposal and other material disposal considerations. For example, the Bakay process referred to above produces by-product Si C14, which must be utilized, sold or otherwise disposed of in the overall processing operation. The commonly employed process ~513g5 for producing trichlorosilane, on the other hand, requires a source of HCl, adding to the cost and the material handling requirements of the process. An integrated process for the conversion of metallurgical grade silicon to high purity silane and silicon, with simplified material requirements and reduced waste disposal, is genuinely needed in the art~ therefore, to enhance the prospects for effectively utilizing high purity silicon on a commercially practical basis for solar cell and semiconductor applications.
It is an object of the present invention, therefore, to provide an improved process for the production of high purity silane.
It is another object of the invention to provide an improved process for the conversion of metallurgical grade silicon to high purity silane and high purity silicon.
It is another object to provide an integrated process for the production of high purity silane and high purity silicon with simplified feed material and reduced material disposal requirements.
It is a further object of the invention to provide a process for the production of silane from metallurgical grade silicon incorporating an enhanced process for the initial production of trichlorosilane from metallurgical grade silicon.
With these and other objects in mind, the invention is hereinafter disclosed in detail, the novel features thereof being particularly pointed out in the appended claims.
~ 39 5 Summary of the Invention High purity silane is produc,ed by an improved, integrated process utilizing hydrogen and metallurgical grade silicon as essentially the only consumed feed materials, The initial trichlorosilane production is accomplished at elevated pressure and temperature, substantially enhancing the conversion rate and the production rate obtainable in a given size reaction vessel, Unreacted silicon tetrachloride is conveniently recycled for reaction with additional quantities of hydrogen and metallurgical silicon. Material wastage is minimized, and waste disposal is readily accomplished by condensing a minor portion of unreacted silicon tetrachloride from the trichlorosilane reaction mixtu~e, said silicon tetrachloride and accompanying impurities being passed to waste without the necessity for dilution prior to hydrolysis during said waste disposal. The high purity silane thus produced can be further purified to remove residual impurities, as required, and may be decomposed on a continuous or semicontinuous basis to produce high purity sil~n, e.g. in the hot free space of a decomposition reactor or in a fluid bed reaction zone.
By-product hydrogen produced in the silane decomposition operation can conveniently be recycled to the trichlorosilane production step and/or recycled for use as a carrier gas for the silane being decomposed as desired.
Brief DescriPtion of the Drawings The invention is further described with reference to the accompanying drawing illustrating the process flow diagram of one embodiment of the invention for converting metallurgical grade silicon to high purity silicon.
Detailed Description of the Invention The process of the invention as herein described and claimed is an ~ntegrated process of enhanced performance capability for producing high purity silane or high purity silicon from metallurgical grade silicon. The invention achieves higher performance efficiency and reduced equipment size requirements than previously known techniques. The overall process includes (1) the enhanced production of trichlorosilane from metallurgical silicon and hydrogen, (2) the disprop~rtionation of trichlorosilane to produce high purity silane, and (3) the conversion of said silane to high purity silicon, if desired. The integrated process effectively recycles unreacted and by-product materials, minimizing material wastage and simplifying waste disposal operations.
The present invention is suitable for use in the c~nversion of the conventional metallurgical grade silicon materials available in the art to high purity silane or high purity silicon. Metallurgical grade silicon, as referred to herein, is a grade of silicon having a resistivity generally on the order of about 0~005 ohm-cm and up to about 3~ iron, 0.75% aluminum, 0,5% calcium and other impurities normally found in silicon produced by the carbothermic reduction of silica.
It is also within the scope of the invention to employ a ferro-silicon material containing at least about 90~/O Si and up to 10% or more of iron. It will also be 3~51395 understood by those skilled in the art that suitable grades of ferro-silicon material are included within the meaning of the term "metallurgical silicon," as used herein.
It will also be understood that particular grades of metallurgical silicon containing unusual concentrations of certain specific impurities, perhaps for example 1% or more of lead, may not be suitable feed material for the process.
Metallurgical grade silicon or ferrosilicon is processed, in accordance with the invention, to produce an initial reaction mixture of di- and trichlorosilane by a novel and advantageous technique that enables the overall process to be carried out in an economically advantageous manner, with the desirable by-product recycle and simplified waste disposal referred to herein. As silicon tetrachloride separated from the reaction mixture can readily be recycled for reaction with additional quantities of metallurgical silicon and hydrogen, high purity silane is produced with said metallurgical silicon and hydrogen being essentially the only major consumed feed materials.
In the process of the invention, metallurgical silicon is initially reacted with hydrogen and silicon tetra-chloride in a reaction zone maintained at a temperature of from about 400C to about 600C and at a pressure in excess of about 100 psi ,o form trichlorosilane as follows:
(1) 3 Si C14 + 2H2 + Si (me~ grade)--? 4H Si C13 with the reaction (1) mixture containing a yield of about 20-30% by weight trichlorosilane on a hydrogen-free basis, and of about 0.5% dichlorosilane with the remainder being ~1395 silicon tetrachloride together with impurities comprising mainly carryover metallurgical silicon powder, metal halides essentially without undesired polysilanes~
The elevated pressure and temperature in the reaction zone substantially enhances the production rate obtainable in a given sized reaction vessel and the feed conversion rate, thereby reducing the size requirements for reaction vessels and facilitating the overall production operation.
Additional advantages of the invention in providing an integrated process for the production of high purity silane and high p~rity silicon, with simplified feed material and reduced material disposal requirements, are hereinafter pQinted out with reference to particular aspects and embodiments of the invention.
In the embodiment of the invention illustrated in the drawing, metallurgical silicon is passed through line 1, lock hopper 2 and line 3 into hydrogenation reactor 4 in which said silicon is reacted with hydrogen and silicon tetrachloride in accordance with reaction (1~ under the reaction conditions herein disclosed. Hydrogen from line 5 and recycle silicon tetrachloride from line 6 are passed through vaporizer/preheater 7 to reach the desired reaction temperature and are passed therefrom through line 8 to said reactor 4.
The reaction mixture from reactor 4 comprises trichloro-silane, dichlorosilane, silicon tetrachloride, carryover met-allurgical silicon powder, metal halides and other impurities.
This trichlorosilane gas stream, upon leaving reactor 4 through line 9 is cooled to condense a minor portion of the unreacted s'licon tetrachloride therein, with said carryover ~1395 metallurgical silicon, metal halides and other impurities present in said trichlorosilane gas stream separating therefrom with the condensed silicon tetrachloride.
For this purpose, the trichlorosilane gas stream in line 9 passes to settling zone 10 from which the gas stream passes upward through line 11 to condenser unit 12 from which t~.e partially condensed stream exits through line 13. A minor portion of the unreacted silicon tetra-chloride, for example on the order of about 5% by weight of the overall silicon tetrachloride in said trichlorosilane gas stream, to~ether with accompanying carryover silicon powder and other impurities is returned through line 14 to settling zone L0 from which a waste stream is removed through line 15 for convenient disposal as the only waste stream of the overall integrated process of the invention. Hydrogen gas withdrawn from condenser unit 12 is passed through line 16 to hydrcgen recycle blower 17 from which said hydrogen is passed in line 18 for recycle to line 5 for passage with fresh hydrogen to reactor 4 for reaction therein with additional ~uantities of fresh metallurgical silicon.
The trichlorosilane gas stream from line 13, puri-fied of said impurities and containing said dichlorosilane and the remainder of said silicon tetrachloride, passes through line 19 to surge tank 20 from which it is pumped through line 21 to distillation zone 22 from which tri-chlorosilane and dichlorosilane are separated as overhead from the unreacted silicon tetrachloride, which is withdrawn from the bottom of said distillation zone 22 through line 23, passes to surge tank 24 and is pumped through line 6 so as to pass to reactor 4 for reaction therein with additional quantities of metallurgical silicon and hydrogen.
~1395 The trichlorosilane and dichlorosilane withdrawn as overhead from the distillation zone are then subjected to a temperature capable of causing the disproportionatiDn thereof, resulting in the formation of product silane gas and by-product monochlorosilane and dichlorosilane. For this purpose, the di-and trichlorosilane overhead stream withdrawn from distillation zone 22 through line 25, after passing through reflux condenser 26 from which reflux is returned to said zone 22, first passes through line 27, including surge tank 28, to second distillation zone 29 from which trichlorosilane is removed as a bottoms stream through line 30, including surge tank 31 and pump 32, and is passed therefrom into first disporportionation reactor or reaction zone 33. This reactor contains insoluble, solid anion exchange resin containing tertiary amino or quaternary ammonium groups bonded to carbon and is main-tained at a temperature capable of causing the dispropor-tionation of trichlorosilane according to reaction (2) below:
(2) 2 H S~ C13 ~ H2 Si C12 + Si C14 The resulting dichlorosilane and silicon tetrachloride disproportionation products of reactor 33 are passed through line 34 to said first distillation colunn 22 for separation therein together with the trichlorosilane gas stream from hydrogenation reactor 4 as discussed above.
The overhead stream withdrawn from said second distillation zone 29 comprises dichlorosilane, monochloro-silane and product silane gas. This strQam passes through line 35 to condenser unit 36 from which liquid dichlorosilane and monochlorosilane, apart from a reflux portion, are ~51395 passed through line 37 to second disproportionation reactor or reaction zone 38 containing said anion exchange resin and in which dichlorosilane is diAssociated according to reactions (3) and (4) below:
~3) 4H2 Si C12 - - 2H3 Si Cl + 2H Si C13 (4) 2H3 Si Cl- Si H4 + H2 Si C12 The resulting product silane, and by-product mono-chlorosilane and trichlorosilane produced in reactor 38 are passed through line 39 to said second distillation zone 29 for separation therein together with di-and trichlorosilane overhead stream withdrawn from first distillation zone 22 as discussed above. Thus, by-product mono-, di- and tri-chlorosilane withdrawn from either or both of said dis-proportionation reactors 33 and/or 38 are recycled either to distillation zone 22, or to said distillation zone 29 that constitutes a part of t'ne overall disproportionation zone of the illustrated embodiment and that also includes, of course, said disproportionation reactors 33 and 38, for fur~her separation and/or reaction. All of the by-product materials are recycled back for ~urther use, therefore, with silicon tetrachloride being returned to hydrogenation reactor 4, with trichlorosilane being returned to first disproportionation reactor 33 and with dichlorosilane and monochlorosilane being returned to second disproportionation reactor 38. Silane gas thus becomes the only product of the overall process, with metallurgical silicon and hydrogen being essentially the only consumed materials.
High purity silane product is recovered from the disproportionation zone as the overhead in line 40 leaving condenser unit 36. While such high purity silane has ~1395 utility as recovered, it is within the scope of the invention to subject said high purity silane to further treatment in a purification zone capable of assuring that the impurity content of the silane is at a semiconductor level. For this purpose, the high purity silane in line 40 is passed to purification zone 41 from which purified silane is withdrawn through line 42. Such purification zone may comprise a bed of activated carbon or a bed of silica gel. Alternately, the high purity silane may be distilled under pressure in said purification zone 41 comprising a cryogenic distillation zone. In this latter embodiment, the purified silane is removed as an overhead product from said cryogenic distilla-tion zone, with trace quantities of monochlorosilane and residual impurities that may not have been removed by the ion exchange resins in reactors 33 and 38 of the distillation zone being separated from said purified silane in said cryogenic distillation zone.
One embodiment of the present invention involves the improved, integrated process for the production of polycrystalline silicon from metallurgical grade silicon.
In this embodiment, the high purity silane from line 40, or advantageously, the purified silane from line 42, is passed advantageously to a silane decomposition zone repre-sented generally by the numeral 43. The decomposition zone is maintained at a temperature within the decomposition range of silane, thereby causing said silane to decompose and to form high purity polycrystalline silicon and by-product hydrogen. The high purity silicon, which is separated -from by-product hydrogen, is shown in the drawing as being withdrawn from said decomposition zone 43 through line 44 ~1395 for further processing and use. By-pro-duct hydrogen, which is shown being withdrawn from decomposition zone 43 by line 45, can advantageously ~e employed in the overall integrated process of the invention. For example, said by-product hydrogen, or at least a portion thereof, can be passed to said reaction zone, i.e. hydrogenation reactor 4, for reaction therein with metallurgical silicon and silicon tetrachloride to form trichlorosilane as described above.
In another application in the integrated process, said by-product hydrogen, or a portion thereof, can be employed todilute the silane gas prior to its being introduced into said silane decomposition zone. In accordance with such overall, integrated processing features of the invention, silicon is recovered as a low-cost, high purity, polycrys-talline product capable of being produced in decomposition zones, such as free space reactors or fluid bed reaction zones, at relatively high production rates on a semicontinuous or continuous basis. Therefore, metallurgical silicon is essentially the only consumed material apart from make-up of losses, in the process employing elevated pressure and temperature in the initial hydrogenation reaction zone, enhancing the production rate obtainable in said reaction zone and thus e~hancing the overall process. The invention minimizes material wastage and simplifies waste disposal operations, thereby further enhancing the overall conversion of metallurgical grade silicon to high purity silicon for solar cell and semiconductor silicon applications.
A significant aspect of the present invention is the enhanced initial production of tr~chlorosilane from metallur-gical grade silicon. Apart from the inherent advantages ~1395 of the reaction of metallurgical silicon, hydrogen and silicon tetrachloride, said reaction is carried out at elevated pressure and temperature levels that substantially enhance the production rate obtainable in the hydrogenation reaction zone. The reaction zone is maintained at a tem-perature of from about 400C to about 600C, preferably at from about 500C to about 550C. The reaction zone, which may comprise a fluid bed, fixed bed or stirred bed, is maintained at pressures in excess of 100 psi, e.g.
from about 300 psi to about 600 psi, preferably from about 400 psi to about 600 psi, although even greater yields may be obtained at pressures above 600 psi. Under such conditions, it has been found that the yleld of trichlorosilane is significantly improved, said yield being on the order of 15-20 mole% at atmospheric reaction conditions, about 20-25V/o at 60 psi and over 30% at pressures greater than 100 psi. Larger quantit~es of the desired trichlorosilane are thus obtainable from smaller size reactors, this feature contributing significantly to the production of low-cost silane and silicon as compared with conventional processing. The production of trichlorosilane by the reaction of metallurgical silicon with H Cl at about 300C, by comparison, requires relatively large reaction vessels and produces a reaction product mixture containing appreciable quantities of polysilane, which results in additional processing costs not encountered in the practice of the present invention.
While not essential, it is within the scope of the invention to employ a copper catalyst in the hydrogenation reactor zone. For this purpose, metallic copper or a mixture of said metallic copper and copper oxides, such as obtained ~ ~ 5~ 39 Sby conventional copper precipitation processing, can be employed. Metallic copper will generally be employed at about 150 mesh, similar to ground up silicon, with said copper oxides being of fine range, such as ~bout 10 microns in size. Cu C12 is also operable for such purposes. The copper catalyst is employed in an amount within the range of from about 0.1% to about 5% by weight based on the overall weight of metallurgical silicon and said copper catalyst employed in the reaction zone.
The hydrogenation reaction zone of the invention constitutes a relatively small first stage of the overall process, utilizing an energy efficient sized reactor having decreased utility costs as compared w;th those that would be required at lower, more conventional, reaction pressure levels. It should be noted that, although relatively high reaction pressures are employed, such pressures do not require the additional level of construction techniques, complexity and costs encountered in the construction of reaction vessels for operation at pressures greater than 600 psi. The simplified waste disposal operations of the invention are accomplished, as indicated above, by condensing a minor portion of the unreacted silicon tetrachloride in the trichlorosilane gas stream removed from the reaction zone prior to its passage to the distillation zone in which silicon tetrachloride is separated from di-and trichlorosilane for recycle to the hydrogenation reaction zone. As such recycle is an important aspect of the overall processing advantages inherent in the integrated and improved process of the invention, it will be understood by those skilled in the art that the tempera-tures and pressures are adjusted so that as little silicon ~ 39 5 tetrachloride as possible is condensed to achieve the desired impurity removal, thereby maximizing the silicon tetrachloride available for recycle. By this convenient means, carryover metallurgical silicon powder, metal halides and other impurities present in the trichlorosilane gas stream from the hydrogenation reaction zone separate therefrom with said condensed silicon tetrachloride. Such impurities include metal halides, and small amounts of copper if such copper catalyst is employed. Because of the absence of appreciable quantities of the more hazardous polysilanes ehcountered in conventional processing, said unreacted silicon tetrachloride and accompanying silicon powder and impurit~es can be passed to waste without the necessity of diluting the waste stream with additional silicon tetrachloride prior to hydrolysis thereof during waste disposal.
It should be noted that the disproportionation reac-tions set forth above are, of themselves,well known in the art although the disproportionation reaction zone comprising reactors 33 and 38, second distillation zone 29 and related processing units, and its interaction with said first distillation zone 22, as shown in the drawing and as disclosed herein represents a novel and advantageous process-ing arrangement intended further to facilitate the objects of a low-cost, fully integrated process.
The disporportionation reactions are carried out in reactors 33 and 38 in the presence of ~nsoluble, solid anion exchange resins containing tertiary amino or quaternary ammon-ium groups bonded to carbon, with such resins that are macroreticular and that contain tertiary amino groups being generally preferred. Such resins, including the commercial ~1395 macroreticular tertiary amine ion exchange resin produced by Rohm and Haas Company and sold under its Amberlyst A-21 trademark, are described in detail in U.S. Patent No. 3,968,199, in the name of Carl J. Bakay. The Bakay patent also describes the generally known features of the disproportionation reactio~s. It should be noted that it is convenient, for purposes of the present invention to employ said resin in an amount generally on the order of about 20 lbs. of resin per lb./hr. of product silane produced in said disproportionation `iO zone. It should also be noted that the disproportionation reactors can be operated in either vapor phase or liquid phase although it has been found generally advantageous to operate reactor 33 in the vapor phase, with temperatures of about 8~C and up to about 150C, and to operate reactor 38 in the liquid phase, with temperatures of about 55-60C
and down as low as about 0C.
The high purity silane removed from the disporpor-tionation zone is of about 97-99% purity and may contain from about 1-3% monochlorosilane. Such chlorine material is the only significant impurity normally present in the product silane, which is essentially free of electronically active impurities such as boron. As indicated above, said high purity silane may be purified, by passage through purification zone 41, to assure that the silane is of semiconductor purity quality. The purification zone can be in the form of carbon traps, comprising beds of activated carbon, that will remove monochlorosilane from the silane product gas stream. Silica gel or other commercially available adsorbents may also be employed for such purposes.
Alternately, said purification zone 41 may comprise a ~ ~5139 5 cryogenic distillation zone in which the high purity silane is distilled under pressure, with purified silane being removed as an overhead product. Trace quantities of mono-chlorosilane and residual impurities that may not have been removed by said ion exchange resin in the disproportionation zone are separated from said purified silane in said cryogenic distillation zone. The cryogenic zone would be operated to assure that diborane, B2H6, the lowest boiling electronically active impurity, boiling point - 86~C, is separated from the silane product, boiling point - 112C.
More broadly, the practice of the invention results in all by-products, intermediate products and/or unreacted materials, together with impurities present in the system, being rejected back into the system, with high purity or purified silane being separated therefrom. Thus, silicon tetrachloride passes back ultimately to hydrogenation reactor 4, trichlorosilane eventually returns to disp-~opor-tionation reactor 33, and mono-and dichlorosilane to reactor 38~ with most impurities being withdrawn from the system in the sludge of material removed from settler zone 10 through line 150 Any boron trichloride, boiling point 12C, that was not removed in said sludge or retained on the ion exchange resin in reactors 33 and/or 38 could be removed from the silane in said purification zone 41. It will be appreciated by those skilled in the art that various other steps could be cmployed to further assure that the product silane is of semiconductor purity, having impurities present only at parts per billion levels, rather than at levels on the order of about 0.05% or 500 parts per million. It will also be appreciated that such purification steps as ~ 3~ 5 indicated herein might, in practice, constitute redundant features useful primarily on that basis. Eor example, the cryogenic distillation step is useful to remove boron impurities if such impurities are present in the product stream removed from the disproportionation zone. Such impurities may not be present in significant amounts, however, as a result of removal by the ion exchange resins of the disproportion reactors as indicated above. In addition, it is possible to subject the trichlorosilane stream to purification prior to disproportionation by means of severe distillation in an extra distillation zone and boron removal by water vapor treatment utilizing the technique of US 3,540,861 or by other known techniques, including those utilizing amine ion exchange resins such as those suitable as disproportionation catalysts in the present invention. The removal of residual chlorine materials is desired, on the other hand, to avoid undesired corrosion in the subsequent pyrolysis of silane to form high purity polycrystalline silicon. The use of possibly redundant purification steps merit consideration in determining an appropriate, practical commercial operation as a balance or trade off between a desire to minimize the cost of producing silane and silicon products on a relatively large scale and the need to take prudent steps to assure the desired quality of the products on a continuous or semi-continuous basis in commercial scale operations~
In the embodiments of the invention in which high purity silicon is to be produced, the silane obtained as disclosed above is passed to a silane decomposition zone, represented generally by the numeral 43 in the drawing, ~ ~ S~ 39 5 in which the silane is decomposed to form high purity polycrystalline silicon and by-product hydrogen according to the following reaction:
(5) Si H4- Si ~ 2H2 The silicon thus obtained can readily be separated from by-product hydrogen and recovered :Eor further refine-ment or use. In one embodiment, said high purity poly,-crystalline silicon can be passed, preferably directly, to a melting zone maintained at a temperature above the melting point of silicon, thereby obtaining a high purity, polycrystalline silicon melt from which high purity single crystal silicon can be obtained by known crystal pulling techniques for use in solar cell or semiconductor appli-cations.
It is within the scope of the invention to decompose silane on a continuous or semicontinuous basis at relatively high production rates, overcoming the inherent disadvan-tages and limitations of the Siemens process, utilizing either a free space reactor or a fluidized bed reactor.
In the free space reactor approach!, high purity polycrys-talline powder is conveniently produced by introducing silane into the hot free space of a decomposition zone maintained at a temperature within the decomposition tem-perature range of the silane and below the melting point temperature of silicon, i.e. from about 390C to about 1400C, preferably from about 800C to about 1000C. As a result of the homogeneous decomposition of the silane within the free space reactor, polycrystalline silicon powder is formed together with by-product hydrogen. The decomposition can be carried out at essentially atmospheric conditions or f~51395 at elevated pressures up to 100 psi or above, with elevated pressures tending to form higher silicon production rates and the formation of larger particles, which generally range from submicron to low micron size, e.g. 5 11. The silane feed gas is preferably introduced into the free space zone turbulently, as by injector means positioned at the top of the reactor, with the turbulence tending to minimize hetcr-ogeneous decomposition at the reactor wall and consequent silicon wall deposit build-up. The silicon powder of high purity, upon discharge from the decomposition reactor, may be consolidated or melted for further processing by conventional means to produce a low-cost, high purity single crystal material. In passing the silicon powder from the settling zone in which it is separated from by-product hydrogen, typically within the reactor itself, the silicon powder can advantageously be passed directly to a melting zone, without outside contact, so as to minimize exposure of the product polycrystalline silicon to sources of potential impurities, thereby assuring the high quality of the product siliconO Alternately, the silicon powder may be passed to a consolidation zone to form larger sized silicon particles for subsequent treatment or use.
In another embodiment of the invention, the silane decomposition zone comprises a fluid bed silicon reaction zone. In thIs embodiment, silane is injected into the reaction chamber containing particles of elemental silicon small enough to be fluidized by the injected silane gas. The fluidized bed of silicon particles are maintained at a temperature within the thermal decomposition range ~ ~513~5 and below the melting point of silicon. By the heterogeneous decomposition of the silane, the desired silicon product is deposited on the fluidized bed particles, which increase in size until removed from the reaction chamber as product.
Seed particles for the fluidized bed are formed by the grinding of a portion of the product silicon particles in a manner avoiding the introauction of impurities into said seed particles.
The by-product hydrogen formed upon decomposition of silane in the free space reactor or in a fluid bed reactor can be effectively utilized in the integrated process of the invention. For example, the silane feed to the decomposition zone is advantageously diluted with at least a portion of said by-product hydrogen prior to being introduced into said zone. Likewise, said by-product hydrogen, or at least a portion thereof, can be effectively utilized by being passed to the hydrogenation reaction zone for reaction therein with metallurgical grade silicon and silicon tetrachloride in the initial step of forming the trichlorosilane gas stream from which silane is produced as herein described.
The process of the invention has been employed in the production of silaP~e in accordance with the embodiment illustrated in the drawing, with metallurgical grade silicon, hydrogen and silicon tetrachloride being reacted in reactor 4 with said silicon and hydrogen being employed in approxi-mately a 1:2 mole ratio. The hydrogen and silicon tetra-chloride, employed in a 1:1 mole ratio, were preheated to 500C and pressurized to 325 psig prior to being introduced into reactor 4. The trichlorosilane gas stream leaving ~51395 reactor 4 was at 500C and 300 psig. Condenser unit 12 was employed to condense a portion of the silicon tetra-chloride content, i.e. about 5%, said silicon tetrachloride carryover silicon powder and impurities being withdrawn from the system through settler 10, the waste stream being discharged through line 15. One portion of condenser unit 12 was operated at 25C fpr this purpose, the other portion thereof being operated at about -15C with recycle hydrogen in line 16 thus being recycled at -15C. The trichlorosilane 10 s~ream in line 21 was fed to distillation column 22 at 25C and 50 psig., with recycle silicon tetrachloride exiting from said column 22 at 124C and 50 psig. The di- and trichlorosilane stream removed from column 22 is passed, at 65C and 50 psig, through line 27 to tank 28 and into column 29 from which trichlorosilane is passed through line 30 at 70C and 40 psig for redistribution in disproportionation reactor 33, from which dichlorosilane and silicon tetra-chloride are recovered at 80C and 50 psig. The overhead stream from column 29 passes through line 35 to condenser 20 unit 36 that is operated, in the first stage thereof at -30C and in the second at -60C, so as to separate and recover product silane in line 40 and to return all other materials to the system as reflux to column 29 and as the liquid stream in line 37 passed to disproportionation reactor 38. The stream in line 37 was at 0C and 40 psig, with the redistribution mixture leaving said reactor 38 being at 55C and 40 psig. High purity silane was recovered in line 40 at -40C and 40 psig~-and was passed to purification zone 41 in which carbon traps were employed to remove monochloro-silane from the high purity silane product.
~51395 It will be appreciated by those skilled in the art that various changes and modifications can be made in the particular arrangement of disproportionation reactors and supporting apparatus as shown in the disproportionation zone and its relationship to the distillation zone of the illustrated embodiment of the invention. Said embodiment, however, represents an advantageous arrangement intended to simplify th0 necessary processing apparatus and thus to contribute to the overall object of enabling high purity silane and silicon to be produced at relatively large production rates on a semicontinuous or continuous basis at relatively low cost.
The invention achieves such objective through the integrated nature of the process, with simplified waste disposal, the recycle of all by-product and unreacted materials other than produ¢t silane, and, significantly, by the overall improvements to the overall process and the cost thereof achieved by carrying out the initial hydrogenation of metallurgical silicon and by-product silicon tetrachloride under conditions of elevated temperature and pressure such as to reduce the necessary apparatus size and to increase the yield of trichlorosilane. This latter feature, which is an essential feature of the inven-tion as claimed herein, thus contributes in an appreciable manner to the necessary decrease in the cost of producing polycrystalline semiconductor grade silicon so as to enhance the prospects for employing such silicon ~n the production of solar cells for commercial application and in other desirable commercial semiconductor applications. ~y means of the integrated process of the present invention, ~513~5 high purity silane and silicon can be produced, in an economically feasible manner, from metallurgical grade silicon with such metallurgical grade material and hydrogen being essentially the only consumed feed material and with waste disposal being facilitated in an environmentally attractive manner.
The overhead stream withdrawn from said second distillation zone 29 comprises dichlorosilane, monochloro-silane and product silane gas. This strQam passes through line 35 to condenser unit 36 from which liquid dichlorosilane and monochlorosilane, apart from a reflux portion, are ~51395 passed through line 37 to second disproportionation reactor or reaction zone 38 containing said anion exchange resin and in which dichlorosilane is diAssociated according to reactions (3) and (4) below:
~3) 4H2 Si C12 - - 2H3 Si Cl + 2H Si C13 (4) 2H3 Si Cl- Si H4 + H2 Si C12 The resulting product silane, and by-product mono-chlorosilane and trichlorosilane produced in reactor 38 are passed through line 39 to said second distillation zone 29 for separation therein together with di-and trichlorosilane overhead stream withdrawn from first distillation zone 22 as discussed above. Thus, by-product mono-, di- and tri-chlorosilane withdrawn from either or both of said dis-proportionation reactors 33 and/or 38 are recycled either to distillation zone 22, or to said distillation zone 29 that constitutes a part of t'ne overall disproportionation zone of the illustrated embodiment and that also includes, of course, said disproportionation reactors 33 and 38, for fur~her separation and/or reaction. All of the by-product materials are recycled back for ~urther use, therefore, with silicon tetrachloride being returned to hydrogenation reactor 4, with trichlorosilane being returned to first disproportionation reactor 33 and with dichlorosilane and monochlorosilane being returned to second disproportionation reactor 38. Silane gas thus becomes the only product of the overall process, with metallurgical silicon and hydrogen being essentially the only consumed materials.
High purity silane product is recovered from the disproportionation zone as the overhead in line 40 leaving condenser unit 36. While such high purity silane has ~1395 utility as recovered, it is within the scope of the invention to subject said high purity silane to further treatment in a purification zone capable of assuring that the impurity content of the silane is at a semiconductor level. For this purpose, the high purity silane in line 40 is passed to purification zone 41 from which purified silane is withdrawn through line 42. Such purification zone may comprise a bed of activated carbon or a bed of silica gel. Alternately, the high purity silane may be distilled under pressure in said purification zone 41 comprising a cryogenic distillation zone. In this latter embodiment, the purified silane is removed as an overhead product from said cryogenic distilla-tion zone, with trace quantities of monochlorosilane and residual impurities that may not have been removed by the ion exchange resins in reactors 33 and 38 of the distillation zone being separated from said purified silane in said cryogenic distillation zone.
One embodiment of the present invention involves the improved, integrated process for the production of polycrystalline silicon from metallurgical grade silicon.
In this embodiment, the high purity silane from line 40, or advantageously, the purified silane from line 42, is passed advantageously to a silane decomposition zone repre-sented generally by the numeral 43. The decomposition zone is maintained at a temperature within the decomposition range of silane, thereby causing said silane to decompose and to form high purity polycrystalline silicon and by-product hydrogen. The high purity silicon, which is separated -from by-product hydrogen, is shown in the drawing as being withdrawn from said decomposition zone 43 through line 44 ~1395 for further processing and use. By-pro-duct hydrogen, which is shown being withdrawn from decomposition zone 43 by line 45, can advantageously ~e employed in the overall integrated process of the invention. For example, said by-product hydrogen, or at least a portion thereof, can be passed to said reaction zone, i.e. hydrogenation reactor 4, for reaction therein with metallurgical silicon and silicon tetrachloride to form trichlorosilane as described above.
In another application in the integrated process, said by-product hydrogen, or a portion thereof, can be employed todilute the silane gas prior to its being introduced into said silane decomposition zone. In accordance with such overall, integrated processing features of the invention, silicon is recovered as a low-cost, high purity, polycrys-talline product capable of being produced in decomposition zones, such as free space reactors or fluid bed reaction zones, at relatively high production rates on a semicontinuous or continuous basis. Therefore, metallurgical silicon is essentially the only consumed material apart from make-up of losses, in the process employing elevated pressure and temperature in the initial hydrogenation reaction zone, enhancing the production rate obtainable in said reaction zone and thus e~hancing the overall process. The invention minimizes material wastage and simplifies waste disposal operations, thereby further enhancing the overall conversion of metallurgical grade silicon to high purity silicon for solar cell and semiconductor silicon applications.
A significant aspect of the present invention is the enhanced initial production of tr~chlorosilane from metallur-gical grade silicon. Apart from the inherent advantages ~1395 of the reaction of metallurgical silicon, hydrogen and silicon tetrachloride, said reaction is carried out at elevated pressure and temperature levels that substantially enhance the production rate obtainable in the hydrogenation reaction zone. The reaction zone is maintained at a tem-perature of from about 400C to about 600C, preferably at from about 500C to about 550C. The reaction zone, which may comprise a fluid bed, fixed bed or stirred bed, is maintained at pressures in excess of 100 psi, e.g.
from about 300 psi to about 600 psi, preferably from about 400 psi to about 600 psi, although even greater yields may be obtained at pressures above 600 psi. Under such conditions, it has been found that the yleld of trichlorosilane is significantly improved, said yield being on the order of 15-20 mole% at atmospheric reaction conditions, about 20-25V/o at 60 psi and over 30% at pressures greater than 100 psi. Larger quantit~es of the desired trichlorosilane are thus obtainable from smaller size reactors, this feature contributing significantly to the production of low-cost silane and silicon as compared with conventional processing. The production of trichlorosilane by the reaction of metallurgical silicon with H Cl at about 300C, by comparison, requires relatively large reaction vessels and produces a reaction product mixture containing appreciable quantities of polysilane, which results in additional processing costs not encountered in the practice of the present invention.
While not essential, it is within the scope of the invention to employ a copper catalyst in the hydrogenation reactor zone. For this purpose, metallic copper or a mixture of said metallic copper and copper oxides, such as obtained ~ ~ 5~ 39 Sby conventional copper precipitation processing, can be employed. Metallic copper will generally be employed at about 150 mesh, similar to ground up silicon, with said copper oxides being of fine range, such as ~bout 10 microns in size. Cu C12 is also operable for such purposes. The copper catalyst is employed in an amount within the range of from about 0.1% to about 5% by weight based on the overall weight of metallurgical silicon and said copper catalyst employed in the reaction zone.
The hydrogenation reaction zone of the invention constitutes a relatively small first stage of the overall process, utilizing an energy efficient sized reactor having decreased utility costs as compared w;th those that would be required at lower, more conventional, reaction pressure levels. It should be noted that, although relatively high reaction pressures are employed, such pressures do not require the additional level of construction techniques, complexity and costs encountered in the construction of reaction vessels for operation at pressures greater than 600 psi. The simplified waste disposal operations of the invention are accomplished, as indicated above, by condensing a minor portion of the unreacted silicon tetrachloride in the trichlorosilane gas stream removed from the reaction zone prior to its passage to the distillation zone in which silicon tetrachloride is separated from di-and trichlorosilane for recycle to the hydrogenation reaction zone. As such recycle is an important aspect of the overall processing advantages inherent in the integrated and improved process of the invention, it will be understood by those skilled in the art that the tempera-tures and pressures are adjusted so that as little silicon ~ 39 5 tetrachloride as possible is condensed to achieve the desired impurity removal, thereby maximizing the silicon tetrachloride available for recycle. By this convenient means, carryover metallurgical silicon powder, metal halides and other impurities present in the trichlorosilane gas stream from the hydrogenation reaction zone separate therefrom with said condensed silicon tetrachloride. Such impurities include metal halides, and small amounts of copper if such copper catalyst is employed. Because of the absence of appreciable quantities of the more hazardous polysilanes ehcountered in conventional processing, said unreacted silicon tetrachloride and accompanying silicon powder and impurit~es can be passed to waste without the necessity of diluting the waste stream with additional silicon tetrachloride prior to hydrolysis thereof during waste disposal.
It should be noted that the disproportionation reac-tions set forth above are, of themselves,well known in the art although the disproportionation reaction zone comprising reactors 33 and 38, second distillation zone 29 and related processing units, and its interaction with said first distillation zone 22, as shown in the drawing and as disclosed herein represents a novel and advantageous process-ing arrangement intended further to facilitate the objects of a low-cost, fully integrated process.
The disporportionation reactions are carried out in reactors 33 and 38 in the presence of ~nsoluble, solid anion exchange resins containing tertiary amino or quaternary ammon-ium groups bonded to carbon, with such resins that are macroreticular and that contain tertiary amino groups being generally preferred. Such resins, including the commercial ~1395 macroreticular tertiary amine ion exchange resin produced by Rohm and Haas Company and sold under its Amberlyst A-21 trademark, are described in detail in U.S. Patent No. 3,968,199, in the name of Carl J. Bakay. The Bakay patent also describes the generally known features of the disproportionation reactio~s. It should be noted that it is convenient, for purposes of the present invention to employ said resin in an amount generally on the order of about 20 lbs. of resin per lb./hr. of product silane produced in said disproportionation `iO zone. It should also be noted that the disproportionation reactors can be operated in either vapor phase or liquid phase although it has been found generally advantageous to operate reactor 33 in the vapor phase, with temperatures of about 8~C and up to about 150C, and to operate reactor 38 in the liquid phase, with temperatures of about 55-60C
and down as low as about 0C.
The high purity silane removed from the disporpor-tionation zone is of about 97-99% purity and may contain from about 1-3% monochlorosilane. Such chlorine material is the only significant impurity normally present in the product silane, which is essentially free of electronically active impurities such as boron. As indicated above, said high purity silane may be purified, by passage through purification zone 41, to assure that the silane is of semiconductor purity quality. The purification zone can be in the form of carbon traps, comprising beds of activated carbon, that will remove monochlorosilane from the silane product gas stream. Silica gel or other commercially available adsorbents may also be employed for such purposes.
Alternately, said purification zone 41 may comprise a ~ ~5139 5 cryogenic distillation zone in which the high purity silane is distilled under pressure, with purified silane being removed as an overhead product. Trace quantities of mono-chlorosilane and residual impurities that may not have been removed by said ion exchange resin in the disproportionation zone are separated from said purified silane in said cryogenic distillation zone. The cryogenic zone would be operated to assure that diborane, B2H6, the lowest boiling electronically active impurity, boiling point - 86~C, is separated from the silane product, boiling point - 112C.
More broadly, the practice of the invention results in all by-products, intermediate products and/or unreacted materials, together with impurities present in the system, being rejected back into the system, with high purity or purified silane being separated therefrom. Thus, silicon tetrachloride passes back ultimately to hydrogenation reactor 4, trichlorosilane eventually returns to disp-~opor-tionation reactor 33, and mono-and dichlorosilane to reactor 38~ with most impurities being withdrawn from the system in the sludge of material removed from settler zone 10 through line 150 Any boron trichloride, boiling point 12C, that was not removed in said sludge or retained on the ion exchange resin in reactors 33 and/or 38 could be removed from the silane in said purification zone 41. It will be appreciated by those skilled in the art that various other steps could be cmployed to further assure that the product silane is of semiconductor purity, having impurities present only at parts per billion levels, rather than at levels on the order of about 0.05% or 500 parts per million. It will also be appreciated that such purification steps as ~ 3~ 5 indicated herein might, in practice, constitute redundant features useful primarily on that basis. Eor example, the cryogenic distillation step is useful to remove boron impurities if such impurities are present in the product stream removed from the disproportionation zone. Such impurities may not be present in significant amounts, however, as a result of removal by the ion exchange resins of the disproportion reactors as indicated above. In addition, it is possible to subject the trichlorosilane stream to purification prior to disproportionation by means of severe distillation in an extra distillation zone and boron removal by water vapor treatment utilizing the technique of US 3,540,861 or by other known techniques, including those utilizing amine ion exchange resins such as those suitable as disproportionation catalysts in the present invention. The removal of residual chlorine materials is desired, on the other hand, to avoid undesired corrosion in the subsequent pyrolysis of silane to form high purity polycrystalline silicon. The use of possibly redundant purification steps merit consideration in determining an appropriate, practical commercial operation as a balance or trade off between a desire to minimize the cost of producing silane and silicon products on a relatively large scale and the need to take prudent steps to assure the desired quality of the products on a continuous or semi-continuous basis in commercial scale operations~
In the embodiments of the invention in which high purity silicon is to be produced, the silane obtained as disclosed above is passed to a silane decomposition zone, represented generally by the numeral 43 in the drawing, ~ ~ S~ 39 5 in which the silane is decomposed to form high purity polycrystalline silicon and by-product hydrogen according to the following reaction:
(5) Si H4- Si ~ 2H2 The silicon thus obtained can readily be separated from by-product hydrogen and recovered :Eor further refine-ment or use. In one embodiment, said high purity poly,-crystalline silicon can be passed, preferably directly, to a melting zone maintained at a temperature above the melting point of silicon, thereby obtaining a high purity, polycrystalline silicon melt from which high purity single crystal silicon can be obtained by known crystal pulling techniques for use in solar cell or semiconductor appli-cations.
It is within the scope of the invention to decompose silane on a continuous or semicontinuous basis at relatively high production rates, overcoming the inherent disadvan-tages and limitations of the Siemens process, utilizing either a free space reactor or a fluidized bed reactor.
In the free space reactor approach!, high purity polycrys-talline powder is conveniently produced by introducing silane into the hot free space of a decomposition zone maintained at a temperature within the decomposition tem-perature range of the silane and below the melting point temperature of silicon, i.e. from about 390C to about 1400C, preferably from about 800C to about 1000C. As a result of the homogeneous decomposition of the silane within the free space reactor, polycrystalline silicon powder is formed together with by-product hydrogen. The decomposition can be carried out at essentially atmospheric conditions or f~51395 at elevated pressures up to 100 psi or above, with elevated pressures tending to form higher silicon production rates and the formation of larger particles, which generally range from submicron to low micron size, e.g. 5 11. The silane feed gas is preferably introduced into the free space zone turbulently, as by injector means positioned at the top of the reactor, with the turbulence tending to minimize hetcr-ogeneous decomposition at the reactor wall and consequent silicon wall deposit build-up. The silicon powder of high purity, upon discharge from the decomposition reactor, may be consolidated or melted for further processing by conventional means to produce a low-cost, high purity single crystal material. In passing the silicon powder from the settling zone in which it is separated from by-product hydrogen, typically within the reactor itself, the silicon powder can advantageously be passed directly to a melting zone, without outside contact, so as to minimize exposure of the product polycrystalline silicon to sources of potential impurities, thereby assuring the high quality of the product siliconO Alternately, the silicon powder may be passed to a consolidation zone to form larger sized silicon particles for subsequent treatment or use.
In another embodiment of the invention, the silane decomposition zone comprises a fluid bed silicon reaction zone. In thIs embodiment, silane is injected into the reaction chamber containing particles of elemental silicon small enough to be fluidized by the injected silane gas. The fluidized bed of silicon particles are maintained at a temperature within the thermal decomposition range ~ ~513~5 and below the melting point of silicon. By the heterogeneous decomposition of the silane, the desired silicon product is deposited on the fluidized bed particles, which increase in size until removed from the reaction chamber as product.
Seed particles for the fluidized bed are formed by the grinding of a portion of the product silicon particles in a manner avoiding the introauction of impurities into said seed particles.
The by-product hydrogen formed upon decomposition of silane in the free space reactor or in a fluid bed reactor can be effectively utilized in the integrated process of the invention. For example, the silane feed to the decomposition zone is advantageously diluted with at least a portion of said by-product hydrogen prior to being introduced into said zone. Likewise, said by-product hydrogen, or at least a portion thereof, can be effectively utilized by being passed to the hydrogenation reaction zone for reaction therein with metallurgical grade silicon and silicon tetrachloride in the initial step of forming the trichlorosilane gas stream from which silane is produced as herein described.
The process of the invention has been employed in the production of silaP~e in accordance with the embodiment illustrated in the drawing, with metallurgical grade silicon, hydrogen and silicon tetrachloride being reacted in reactor 4 with said silicon and hydrogen being employed in approxi-mately a 1:2 mole ratio. The hydrogen and silicon tetra-chloride, employed in a 1:1 mole ratio, were preheated to 500C and pressurized to 325 psig prior to being introduced into reactor 4. The trichlorosilane gas stream leaving ~51395 reactor 4 was at 500C and 300 psig. Condenser unit 12 was employed to condense a portion of the silicon tetra-chloride content, i.e. about 5%, said silicon tetrachloride carryover silicon powder and impurities being withdrawn from the system through settler 10, the waste stream being discharged through line 15. One portion of condenser unit 12 was operated at 25C fpr this purpose, the other portion thereof being operated at about -15C with recycle hydrogen in line 16 thus being recycled at -15C. The trichlorosilane 10 s~ream in line 21 was fed to distillation column 22 at 25C and 50 psig., with recycle silicon tetrachloride exiting from said column 22 at 124C and 50 psig. The di- and trichlorosilane stream removed from column 22 is passed, at 65C and 50 psig, through line 27 to tank 28 and into column 29 from which trichlorosilane is passed through line 30 at 70C and 40 psig for redistribution in disproportionation reactor 33, from which dichlorosilane and silicon tetra-chloride are recovered at 80C and 50 psig. The overhead stream from column 29 passes through line 35 to condenser 20 unit 36 that is operated, in the first stage thereof at -30C and in the second at -60C, so as to separate and recover product silane in line 40 and to return all other materials to the system as reflux to column 29 and as the liquid stream in line 37 passed to disproportionation reactor 38. The stream in line 37 was at 0C and 40 psig, with the redistribution mixture leaving said reactor 38 being at 55C and 40 psig. High purity silane was recovered in line 40 at -40C and 40 psig~-and was passed to purification zone 41 in which carbon traps were employed to remove monochloro-silane from the high purity silane product.
~51395 It will be appreciated by those skilled in the art that various changes and modifications can be made in the particular arrangement of disproportionation reactors and supporting apparatus as shown in the disproportionation zone and its relationship to the distillation zone of the illustrated embodiment of the invention. Said embodiment, however, represents an advantageous arrangement intended to simplify th0 necessary processing apparatus and thus to contribute to the overall object of enabling high purity silane and silicon to be produced at relatively large production rates on a semicontinuous or continuous basis at relatively low cost.
The invention achieves such objective through the integrated nature of the process, with simplified waste disposal, the recycle of all by-product and unreacted materials other than produ¢t silane, and, significantly, by the overall improvements to the overall process and the cost thereof achieved by carrying out the initial hydrogenation of metallurgical silicon and by-product silicon tetrachloride under conditions of elevated temperature and pressure such as to reduce the necessary apparatus size and to increase the yield of trichlorosilane. This latter feature, which is an essential feature of the inven-tion as claimed herein, thus contributes in an appreciable manner to the necessary decrease in the cost of producing polycrystalline semiconductor grade silicon so as to enhance the prospects for employing such silicon ~n the production of solar cells for commercial application and in other desirable commercial semiconductor applications. ~y means of the integrated process of the present invention, ~513~5 high purity silane and silicon can be produced, in an economically feasible manner, from metallurgical grade silicon with such metallurgical grade material and hydrogen being essentially the only consumed feed material and with waste disposal being facilitated in an environmentally attractive manner.
Claims (55)
1. An improved process for the production of silane from metallurgical grade silicon comprising:
(a) reacting metallurgical silicon with hydrogen and silicon tetrachloride in a reaction zone maintained at a temperature of from about 400°C to about 600°C and at pressures in excess of about 100 psi to form trichlorosilane and dichlorosilane;
(b) separating said trichlorosilane and dichlorosilane as overhead from unreacted silicon tetrachloride in a distillation zone;
(c) recycling separated silicon tetrachloride from said distillation zone to said reaction zone for reaction with additional quantities of metallurgical silicon and hydrogen;
(d) subjecting said trichlorosilane and dichloro-silane to a temperature capable of causing the disproportion-ation thereof in a disproportionation reaction zone containing insoluble, solid anion exchange resin containing tertiary amino or quaternary ammonium groups bonded to carbon, said disproportionation constituting reactions resulting in the formation of product silane gas and by-product mono-, di-and trichlorosilane and silicon tetrachloride;
(e) recycling said by-product mono-, di- and tri-chlorosilane and silicon tetrachloride to said distillation zone and said disproportionation reaction zone for further separation and disproportionation therein; and (f) recovering high purity silane from said dispro-portionation zone, whereby high purity silane is conveniently produced, with metallurgical silicon and hydrogen being essentially the only consumed feed materials, said elevated pressure and temper-ature in the reaction zone substantially enhancing the production rate obtainable in said reaction zone, said process facilitating the low-cost production of said high purity silane, minimizing material wastage and simplifying waste disposal operations in the overall conversion of metallurgical grade silicon to high purity silane.
(a) reacting metallurgical silicon with hydrogen and silicon tetrachloride in a reaction zone maintained at a temperature of from about 400°C to about 600°C and at pressures in excess of about 100 psi to form trichlorosilane and dichlorosilane;
(b) separating said trichlorosilane and dichlorosilane as overhead from unreacted silicon tetrachloride in a distillation zone;
(c) recycling separated silicon tetrachloride from said distillation zone to said reaction zone for reaction with additional quantities of metallurgical silicon and hydrogen;
(d) subjecting said trichlorosilane and dichloro-silane to a temperature capable of causing the disproportion-ation thereof in a disproportionation reaction zone containing insoluble, solid anion exchange resin containing tertiary amino or quaternary ammonium groups bonded to carbon, said disproportionation constituting reactions resulting in the formation of product silane gas and by-product mono-, di-and trichlorosilane and silicon tetrachloride;
(e) recycling said by-product mono-, di- and tri-chlorosilane and silicon tetrachloride to said distillation zone and said disproportionation reaction zone for further separation and disproportionation therein; and (f) recovering high purity silane from said dispro-portionation zone, whereby high purity silane is conveniently produced, with metallurgical silicon and hydrogen being essentially the only consumed feed materials, said elevated pressure and temper-ature in the reaction zone substantially enhancing the production rate obtainable in said reaction zone, said process facilitating the low-cost production of said high purity silane, minimizing material wastage and simplifying waste disposal operations in the overall conversion of metallurgical grade silicon to high purity silane.
2. The process of Claim l in which said reaction zone temperature is from about 500°C to about 550°C .
3. The process of Claim l in which said reaction zone pressure is from about 400 to about 600 psi.
4. The process of Claim l and including carrying out said trichlorosilane and dichlorosilane formation in said reaction zone in the presence of a copper catalyst.
5. The process of Claim 4 in which said copper catalyst is employed in an amount within the range of from about 0.1% to about 5% by weight based on the overall weight of metallurgical silicon and said copper catalyst employed in said reaction zone.
6. The process of Claim 4 in which said reaction zone is maintained at a pressure of from about 400 to about 600 psi and at a temperature from about 500°C to about 550°C.
7. The process of Claim 6 in which said copper catalyst is employed in an amount within the range of from about 0.1% to about 5% by weight based on the overall weight of metallurgical silicon and said copper catalyst employed in said reaction zone.
8. The process of Claim 7 in which said reaction zone comprises a fluid bed reaction zone.
9. The process of Claim 1 and including condensing a minor portion of said unreacted silicon tetrachloride in the trichlorosilane stream removed from said reaction zone prior to passage thereof to said distillation zone, carry-over metallurgical silicon powder and other impurities present in said trichlorosilane gas stream separating therefrom with the condensed silicon tetrachloride, said impurities thereby being removed from said trichlorosilane stream before it passes to said distillation zone.
10. The process of Claim 9 and including passing said minor portion of unreacted silicon tetrachloride and accompanying carryover silicon powder and other impurities to waste disposal without the necessity to dilute said waste with additional silicon tetrachloride prior to hydrolysis thereof during waste disposal.
11. The process of Claim 9 in which said reaction zone pressure is from about 300 to about 600 psi.
12. The process of Claim 11 and including forming said trichlorosilane and dichlorosilane in the presence of a copper catalyst.
13. The process of Claim 1 and including subjecting said high purity silane to further treatment in a purification zone capable of assuring that the impurity content of said silane is at a semiconductor purity level.
14. The process of Claim 13 in which said high purity silane is passed through a purification zone comprising a bed of activated carbon.
15. The process of Claim 13 in which said high purity silane is passed through a purification zone comprising a bed of silica gel.
16. The process of Claim 13 in which said high purity silane is distilled under pressure in a cryogenic distillation zone, said purified silane being removed as an overhead product from said cryogenic distillation zone, trace quantities of monochlorosilane and residual impurities not removed by said ion exchanger resin in said dispropor-tionation zone being separated from said purified silane in said cryogenic distillation zone.
17. The process of Claim 1 in which said ion exchange resin is macroreticular and contains tertiary amino groups.
18. The process of Claim 6 in which said ion exchange resin is macroreticular and contains tertiary amino groups.
19. An improved process for the production of polycrystalline silicon from metallurgical grade silicon comprising:
(a) reacting metallurgical silicon with hydrogen and silicon tetrachloride in a reaction zone maintained at a temperature of from about 400°C to about 600°C and at pressures in excess of 100 psi to form trichlorosilane and dichlorosilane;
(b) separating said trichlorosilane and dichlorosilane as overhead from unreacted silicon tetrachloride in a distillation zone;
(c) recycling separated silicon tetrachloride from said distillation zone to said reaction zone for reaction with additional quantities of metallurgical silicon and hydrogen;
(d) subjecting said trichlorosilane and dichlorosilane to a temperature capable of causing the disproportionation thereof in a disproportionation reaction zone containing insoluble, solid anion exchange resin containing tertiary amino or quaternary ammonium groups bonded to carbon, said disproportionation constituting reactions resulting in the formation of silane gas and mono-, di- and trichlorosilane and silicon tetra-chloride;
(e) recycling said mono-, di-, and trichlorosilane and silicon tetrachloride and unreacted trichlorosilane to said distillation zone and said disproportionation reaction zone for further separation and disproportionation therein;
(f) passing silane formed in said disproportionation reaction zone to a silane decomposition zone maintained at a temperature within the decomposition temperature range of silane, thereby causing said silane to decompose and to form high purity polycrystalline silicon and by- product hydrogen;
(g) separating said high purity silicon from said by-product hydrogen, whereby said silicon is recoverable as a low-cost, high purity, polycrystalline product capable of being produced at relatively high production rates on a semicontinuous or continuous basis with metallurgical silicon and hydrogen being essentially the only consumed feed materials, said elevated pressure and temperature in the reaction zone substantially enhancing the production rate obtainable in said reaction zone, the process minimizing material wastage and simplifying waste disposal operations and enhancing the overall conversion of metallurgical grade silicon to high purity silicon for solar cell and semiconductor silicon applications.
(a) reacting metallurgical silicon with hydrogen and silicon tetrachloride in a reaction zone maintained at a temperature of from about 400°C to about 600°C and at pressures in excess of 100 psi to form trichlorosilane and dichlorosilane;
(b) separating said trichlorosilane and dichlorosilane as overhead from unreacted silicon tetrachloride in a distillation zone;
(c) recycling separated silicon tetrachloride from said distillation zone to said reaction zone for reaction with additional quantities of metallurgical silicon and hydrogen;
(d) subjecting said trichlorosilane and dichlorosilane to a temperature capable of causing the disproportionation thereof in a disproportionation reaction zone containing insoluble, solid anion exchange resin containing tertiary amino or quaternary ammonium groups bonded to carbon, said disproportionation constituting reactions resulting in the formation of silane gas and mono-, di- and trichlorosilane and silicon tetra-chloride;
(e) recycling said mono-, di-, and trichlorosilane and silicon tetrachloride and unreacted trichlorosilane to said distillation zone and said disproportionation reaction zone for further separation and disproportionation therein;
(f) passing silane formed in said disproportionation reaction zone to a silane decomposition zone maintained at a temperature within the decomposition temperature range of silane, thereby causing said silane to decompose and to form high purity polycrystalline silicon and by- product hydrogen;
(g) separating said high purity silicon from said by-product hydrogen, whereby said silicon is recoverable as a low-cost, high purity, polycrystalline product capable of being produced at relatively high production rates on a semicontinuous or continuous basis with metallurgical silicon and hydrogen being essentially the only consumed feed materials, said elevated pressure and temperature in the reaction zone substantially enhancing the production rate obtainable in said reaction zone, the process minimizing material wastage and simplifying waste disposal operations and enhancing the overall conversion of metallurgical grade silicon to high purity silicon for solar cell and semiconductor silicon applications.
20. The process of Claim 19 in which said reaction zone is maintained at a temperature of from about 500°C to about 550°C.
21. The process of Claim 19 in which said reaction zone pressure is from about 400 to about 600 psi.
22. The process of Claim 19 and including carrying out said trichlorosilane and dichlorosilane formation in-said reaction zone in the presence of a copper catalyst.
23. The process of Claim 22 in which said copper catalyst is employed in an amount within the range of from about 0.1% to about 5% by weight based on the overall weight of metallurgical silicon and said copper catalyst employed in said reaction zone.
24. The process of Claim 22 in which said reaction zone is maintained at a pressure of from about 400 to about 600 psi and at a temperature of from about 500°C to about 550°C.
25. The process of Claim 24 in which said copper catalyst is employed in an amount within the range of from about 0.1% to about 5% by weight based on the overall weight of metallurgical silicon and copper catalyst employed in said reaction zone.
26. The process of Claim 25 in which said reaction zone comprises a fluid bed reaction zone.
27. The process of Claim 19 and including condensing a minor portion of said unreacted silicon tetrachloride in the trichlorosilane stream removed from said reaction zone, carryover metallurgical silicon powder and other impurities present in said trichlorosilane gas stream separating there-from with the condensed silicon tetrachloride, said impurities thereby being removed from said trichlorosilane stream before it passes to said distillation zone.
28. The process of Claim 27 and including passing said minor portion of said unreacted silicon tetrachloride and accompanying carryover silicon powder and other impuri-ties to waste disposal without the necessity to dilute said waste with additional silicon tetrachloride prior to hydrolysis thereof during waste disposal.
29. The process of Claim 27 in which said reaction zone pressure is from about 300 to about 600 psi.
30. The process of Claim 29 and including forming said trichlorosilane in the presence of a copper catalyst.
31. The process of Claim 19 and including subjecting said silane formed in said disproportionation reaction zone to further treatment in a purification zone capable of assuring that the impurity content of said silane is at the semiconductor grade silane level.
32. The process of Claim 31 in which said silane from the disproportionation reaction zone is passed through a purification zone comprising a bed of activated carbon.
33. The process of Claim 31 in which said high purity silane is passed through a purification zone comprising a bed of silica gel.
34. The process of Claim 31 in which said silane is distilled under pressure in a cryogenic distillation zone, said purified silane being removed as an overhead material from said cryogenic distillation zone, trace quantities of monochlorosilane and residual impurities not removed by said ion exchange resin in said disproportionation zone being separated from said purified silane in said cryogenic distillation zone.
35. The process of Claim 19 in which said ion exchange resin is macroreticular and contains tertiary amino groups.
36. The process of Claim 24 in which said ion exchange resin is macroreticular and contains tertiary amino groups.
37. The process of Claim 19 and including passing said high purity silicon separated from by-product hydrogen to a melting zone maintained at a temperature above the melting point of silicon, thereby obtaining a high purity, polycrystalline silicon melt from which high purity single crystal silicon can be obtained for solar cell or semi-conductor applications.
38. The process of Claim 37 in which said silane decomposition zone comprises the hot free space zone of a decomposition reactor, said silicon being formed as a silicon powder and including passing said silicon powder and by-product hydrogen from said free space zone to a separation zone for said separation of silicon powder from by-product hydrogen, said silicon powder being passed from said separation zone to said melting zone 50 as to minimize exposure of said product silicon to sources of potential impurities, thereby assuring the high quality of the product-polycrystalline silicon.
39. The process of Claim 19 and including passing at least a portion of said by-product hydrogen to said reaction zone for reaction therein with metallurgical silicon and silicon tetrachloride to form said trichloro-silane.
40. The process of Claim 37 in which said silane decomposition zone comprises a fluid bed silicon reaction zone containing a bed of silicon particles upon which silicon is decomposed by the heterogeneous decomposition of silane within said decomposition zone,
41. The process of Claim 40 and including grinding a portion of said silicon product formed in said silane decomposition zone to form seed particles for said fluid bed silicon reaction zone, at least a portion of said by-product hydrogen being passed to said fluid bed reaction zone for reaction in the preparation of additional quantities of trichlorosilane.
42. The process of Claim 19 in which said silane decomposition zone comprises the hot free space zone of a decomposition reactor, said silicon being formed therein as a silicon powder and including passing said silicon powder and by-product hydrogen from said free space zone to a settling zone for said separation of silicon powder from by-product hydrogen, said silicon powder being passed to a consolidation zone to form larger sized silicon particles for subsequent treatment or use.
43. The process of Claim 19 in which said silane decomposition zone comprises a fluid bed silicon reaction zone.
44. The process of Claim 19 and including diluting said silane with at least a portion of said-by product hydrogen prior to being introduced into said silane decompo-sition zone.
45. The process of Claim 38 and including distilling said silane under pressure in a cryogenic distillation zone, said purified silane being removed as an overhead material from said cryogenic distillation zone, said reaction zone for trichlorosilane production being maintained at a pressure of from about 400 to about 600 psi.
46. The process of Claim 45 and including diluting said purified silane with at least a portion of said by-product hydrogen prior to being introduced into said silane decomposition zone.
47. The process of Claim 40 and including distilling said silane under pressure in a cryogenic distillation zone, said purified silane being removed as an overhead material from said cryogenic distillation zone, said reaction zone for trichlorosilane production being maintained at a pressure of from about 400 to 600 psi.
48. The process of Claim 45 and including passing at least a portion of said by-product hydrogen to said fluid bed reaction zone for use in the formation of said trichlorasilane.
49. The process of Claim 45 and including diluting said purified silane with at least a portion of said by-product hydrogen prior to being introduced into said silane decomposition zone.
50. The process of Claim 49 and including carrying out said trichlorosilane and dichlorosilane formation in the presence of a copper catalyst.
51. The process of Claim 19 and including condensing a minor portion of said unreacted silicon tetrachloride in said trichlorosiIane stream removed from said reaction zone prior to passage thereof to said distillation zone, carryover metallurgical silicon powder and other impurities present in said trichlorosilane gas stream separating therefrom with the condensed silicon tetrachloride, said impurities thereby being removed from said trichlorosilane before it passes to said distillation zone.
52. The process of Claim 38 in which said high purity silane is passed through a purification zone compri-sing a bed of activated carbon, said reaction zone for trichlorosilane production being maintained at a pressure of from about 400 to about 600 psi.
53. The process of Claim 38 in which said high purity silane is passed through a purification zone compri-sing a bed of silica gel, said reaction zone for trichloro-silane production being maintained at a pressure of from about 400 to about 600 psi.
54. The process of Claim 40 in which said high purity silane is passed through a purification zone comprising a bed of activated carbon, said reaction zone for trichloro-silane production being maintained at a pressure of from about 400 to about 600 psi.
55. The process of Claim 40 in which said high purity silane is passed through a purification zone comprising a bed of silica gel, said reaction zone for trichlorosilane production being maintained at a pressure of from about 400 to about 600 psi.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US93609178A | 1978-08-23 | 1978-08-23 | |
| US936,091 | 1986-11-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1151395A true CA1151395A (en) | 1983-08-09 |
Family
ID=25468157
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000334373A Expired CA1151395A (en) | 1978-08-23 | 1979-08-23 | High purity silane and silicone production |
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| Country | Link |
|---|---|
| CA (1) | CA1151395A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2013110247A1 (en) * | 2012-01-28 | 2013-08-01 | Xi Chu | The method and system for production of silicon and devicies |
| CN110078080A (en) * | 2019-04-26 | 2019-08-02 | 天津科技大学 | A kind of chlorosilane high-boiling components recovery process of combination slurry processing and cracking reaction |
-
1979
- 1979-08-23 CA CA000334373A patent/CA1151395A/en not_active Expired
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2013110247A1 (en) * | 2012-01-28 | 2013-08-01 | Xi Chu | The method and system for production of silicon and devicies |
| CN104271504A (en) * | 2012-01-28 | 2015-01-07 | 储晞 | The method and system for production of silicon and devicies |
| CN110540208A (en) * | 2012-01-28 | 2019-12-06 | 储晞 | Method for producing silicon |
| CN110078080A (en) * | 2019-04-26 | 2019-08-02 | 天津科技大学 | A kind of chlorosilane high-boiling components recovery process of combination slurry processing and cracking reaction |
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