WO2014008271A1

WO2014008271A1 - Method of recovering elemental metal from polycrystalline semiconductor production

Info

Publication number: WO2014008271A1
Application number: PCT/US2013/049084
Authority: WO
Inventors: William Herron; Brad A. METZLER; Daniel D. SOBECK
Original assignee: Hemlock Semiconductor Corporation
Priority date: 2012-07-02
Filing date: 2013-07-02
Publication date: 2014-01-09
Also published as: TW201406966A

Abstract

A method includes the step of introducing hydrogen and a silane-containing feed gas that comprises trichlorosilane into one or more decomposition reactors that contain a polycrystalline silicon seed substrate. The trichlorosilane and hydrogen are reacted to form polycrystalline silicon in the decomposition reactor. A gaseous stream that comprises silicon tetrachloride is exhausted from the one or more decomposition reactors. The gaseous stream and a source of elemental metal that is reactive with the silicon tetrachloride is fed into a recovery reactor. The silicon tetrachloride and the elemental metal are reacted to produce polycrystalline silicon and a first composition that comprises metal chloride. Elemental metal is recovered from the metal chloride in the first composition through at least one of 1) subjecting the first composition comprising the metal chloride to hydrogenation to produce elemental metal and hydrogen chloride, or 2) reacting the metal chloride in the first composition and elemental silicon.

Description

METHOD OF RECOVERING ELEMENTAL METAL FROM POLYCRYSTALLINE SEMICONDUCTOR PRODUCTION

Cross Reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 61/667,145, filed on July 2nd, 2012, which is incorporated herewith by reference in its entirety.

Background of the Invention

1. Field of the Invention

[0002] The instant invention generally relates to a method of recovering elemental metal from semiconductor production. More specifically, the instant invention relates to a method of recovering elemental metal from metal chloride that is produced during semiconductor production.

2. Description of the Related Art

[0003] It is known that polycrystalline silicon can be formed through various processes. One type of process for forming polycrystalline silicon involves decomposing silanes. In such processes, a feed gas comprising a mixture of hydrogen and silane (SiH₄) or a mixture of hydrogen and trichlorosilane is fed to a decomposition reactor containing a polycrystalline silicon seed substrate, generally in the form of a rod (in a Siemens reactor) or pellets (in a fluidized bed reactor), with the polycrystalline silicon seed substrate kept at a temperature of more than 1000°C. Silicon is deposited on the polycrystalline silicon seed substrate and byproduct gas mixtures exit in a vent stream. When a mixture comprising hydrogen and trichlorosilane is used, such as in the Siemens process, the vent stream may include hydrogen, hydrogen chloride, chlorosilanes, silane, and silicon powder. For purposes of this application, the term "chlorosilanes" refers to any silane species having one or more chlorine atoms bonded to silicon and includes, but is not limited to monochloro silane (H₃SiCl), dichlorosilane (H₂S1CI₂), trichlorosilane (HSiCl₃), silicon tetrachloride (SiCl₄), and various chlorinated disilanes such as hexachlorodisilane and pentachlorodisilane. In the vent stream, hydrogen and chlorosilanes such as silicon tetrachloride and trichlorosilane may be present both from unreacted feed gas as well as from reaction products of the decomposition reaction. The vent stream is typically passed through a complex recovery process where condensation, scrubbing, absorption and adsorption are unit operations often used to facilitate the capture of trichlorosilane, hydrogen, and/or a diluent (e.g. inert) gas for recycle. [0004] Despite the fact that the Siemens process is useful for forming electronic grade polycrystalline silicon, one problem associated with the Siemens process is that it is difficult to achieve a high yield of polycrystalline silicon to silicon fed due to the chemical equilibria and kinetics that control this reaction process.

[0005] Another methodology for production of polycrystalline silicon is through zinc reduction of silicon tetrachloride. In particular, zinc reduction of silicon tetrachloride involves feeding silicon tetrachloride and elemental zinc into a fluidized bed reactor to thereby produce polycrystalline silicon and zinc chloride. While zinc reduction of silicon tetrachloride is also useful for forming high grade polycrystalline silicon, this process requires handling zinc chloride by-product and recovering zinc therefrom. However, recent developments in zinc recovery techniques have improved the prospects for production of polycrystalline silicon through zinc reduction of silicon tetrachloride.

[0006] In view of the foregoing, there remains an opportunity to provide improved methods of recovering elemental metal from polycrystalline silicon production processes.

Summary of the Invention and Advantages

[0007] A method is provided that includes the step of introducing hydrogen and a silane- containing feed gas that comprises trichlorosilane into one or more decomposition reactors containing a polycrystalline silicon seed substrate. The trichlorosilane and hydrogen are reacted to form polycrystalline silicon in the decomposition reactor. A gaseous stream comprising silicon tetrachloride is exhausted from the one or more decomposition reactors. The gaseous stream and a source of elemental metal that is reactive with the silicon tetrachloride are fed into a recovery reactor. The silicon tetrachloride and the elemental metal are reacted to produce polycrystalline silicon and a first composition that comprises metal chloride. Elemental metal is recovered from the metal chloride in the first composition through at least one of 1) subjecting the first composition comprising the metal chloride to hydrogenation to produce elemental metal and hydrogen chloride, or 2) reacting the metal chloride in the first composition and elemental silicon.

[0008] By combining the process for producing polycrystalline silicon in a decomposition reactor, such as a Siemens reactor, through decomposition of trichlorosilane, with a process for producing polycrystalline silicon through reaction of zinc and silicon tetrachloride present in the gaseous stream that is exhausted from the decomposition reactor(s), yield of polycrystalline silicon to silicon fed to the decomposition reactor(s) can be increased while consuming components present in the gaseous stream that is exhausted from the decomposition reactor(s). Further, by recovering the elemental metal from the metal chloride in the first composition, the recovered elemental metal may be recycled within the process to effectively achieve a closed-loop process with minimized waste stream volume and maximized yield of polycrystalline silicon.

Description of the Drawings

[0009] Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

[0010] Figure 1 is a schematic flow chart illustrating one embodiment of the instant method including steps for producing trichlorosilane, decomposing trichlorosilane, reacting silicon tetrachloride and elemental metal (e.g., zinc), recovering elemental metal from a metal chloride, and recycling the elemental metal and silicon tetrachloride to the step of reacting the silicon tetrachloride and elemental metal;

[0011] Figure 2 is a schematic flow chart illustrating another embodiment of the instant method including steps for producing trichlorosilane, decomposing trichlorosilane, reacting silicon tetrachloride and elemental metal (e.g., zinc), recovering elemental metal from metal chloride, recycling the elemental metal to the step of reacting the silicon tetrachloride and elemental metal; and recycling HC1 to the step of producing trichlorosilane; and

[0012] Figure 3 is a schematic representation of a reactor that may be employed for recovering elemental metal in accordance with one embodiment of the method of the instant invention.

Detailed Description of the Invention

[0013] A method is provided that involves production of polycrystalline silicon through a combination generally of: 1) reacting hydrogen and trichlorosilane in one or more decomposition reactors, with 2) reacting silicon tetrachloride from a gaseous stream exhausted from the one or more decomposition reactors and elemental metal in one or more recovery reactors. By supplying the silicon tetrachloride from the gaseous stream that is exhausted from the one or more decomposition reactions, increased yield of polycrystalline silicon (generally of high quality, such as solar grade silicon) can be achieved.

[0014] The instant method may include the step of producing a silane-containing feed gas comprising trichlorosilane that is to be used for production of polycrystalline silicon, as represented at numeral 30 in Figures 1 and 2. One technique known in the art for producing trichlorosilane is the direct process, which involves reaction of hydrogen chloride with a source of silicon to form chlorosilanes such as trichlorosilane and silicon tetrachloride. The direct process may involve use of a fluidized bed reactor, and may be a continuous process. Silicon tetrachloride that is produced through the direct process, unreacted feed gases such as HC1, particulates entrained in the product stream, and other reaction product gases other than trichlorosilane may be separated from the product stream to produce the silane-containing feed gas that is fed into the one or more decomposition reactors. Alternatively, some gaseous products such as silicon tetrachloride may be left unseparated in the resulting silane- containing feed gas that is fed into the one or more decomposition reactors.

[0015] As alluded to above, the method includes the step of introducing hydrogen and the silane-containing feed gas comprising trichlorosilane into one or more decomposition reactors, as represented at numeral 32 in Figures 1 and 2. The decomposition reactor(s) contain a polycrystalline silicon seed substrate. For example, Siemens reactors are typically used as the decomposition reactors, with the polycrystalline silicon seed substrate being a polycrystalline silicon seed rod. Alternatively, fluidized bed reactors can be used as the decomposition reactors, in which case the polycrystalline silicon seed substrate may be in the form of polycrystalline silicon seed particles.

[0016] The Siemens reactor employed herein may be a conventional Siemens reactor. For example, the Siemens reactor may operate as follows. Polycrystalline silicon seed rods are placed upright and parallel to one another in the Siemens reactor. Two or more of these seed rods may be connected to one another by a bridge, thereby forming a U-rod. The U-rods are heated until they reach a temperature ranging from about 700°C to about 1,400°C, about 1,000°C to about 1,200°C, or about 1,100°C to about 1,150°C. The Siemens reactor may be operated at a pressure ranging from about 13 kPa (2 psig) to about 3450 kPa (500 psig), about 6 kPa (1 psig) to aboutl380 kPa (200 psig), or about 100 kPa (1 bar) to about 690 kPa (100 psig).

[0017] The silane-containing feed gas is typically fed to the Siemens reactor through an inlet in the base. As set forth above, the silane-containing feed gas comprises hydrogen and trichlorosilane, and typically comprises from about 5% to about 75% trichlorosilane. The silane-containing feed gas may comprise about 0.015 moles of trichlorosilane per mole of hydrogen to about 0.3 moles of trichlorosilane per mole of hydrogen. Alternatively, the silane-containing feed gas may comprise about 0.03 moles of trichlorosilane per mole of hydrogen to about 0.15 moles of trichlorosilane per mole of hydrogen. The silane-containing feed gas may optionally further comprise silicon tetrachloride, as also set forth above.

[0018] The instant method further includes the step of reacting the trichlorosilane and hydrogen to produce polycrystalline silicon in the decomposition reactor. When the one or more decomposition reactors are Siemens reactors, silicon is deposited from the silane- containing feed gas onto the U-rod, thereby increasing the diameter of the U-rod, under the temperature and pressure conditions set forth above. Without wishing to be bound by theory, it is thought that an amount of polycrystalline silicon product ranging from about 5% to about 50%, or about 20% to about 50%, based on the total quantity of silicon contained in the silane-containing feed gas, may be obtained from the Siemens reactor.

[0019] The method further includes the step of exhausting a gaseous stream comprising silicon tetrachloride from the one or more decomposition reactors. In particular, silicon tetrachloride is a by-product of the decomposition reaction and, thus, is produced in the one or more decomposition reactors. The silicon tetrachloride may also be present in the silane- containing feed gas that is introduced into the one or more decomposition reactors. The exhausted gaseous stream from the one or more decomposition reactors may be fed directly to one or more recovery reactors for recovery of silicon from the silicon tetrachloride, as represented at numeral 34 in Figures 1 and 2, and may be directly fed to the one or more recovery reactors without intervening treatment steps (without any unit operations between the one or more decomposition reactors and the one or more recovery reactors). Alternatively, the exhausted gaseous stream from the one or more decomposition reactors may be treated to remove certain species before being fed into the one or more recovery reactors. The exhausted gaseous stream from the one or more decomposition reactors may be treated, for example, by feeding the exhausted gaseous stream through a dust removing apparatus, which may be cooled with fluid such as service water, thereby removing fine silicon powder, disilanes, or combinations thereof.

[0020] The silicon tetrachloride and the elemental metal are reacted in the one or more recovery reactors to produce polycrystalline silicon and a first composition comprising metal chloride. More specifically, the silicon tetrachloride in the exhausted gaseous stream is reacted with the elemental metal to produce high purity silicon. The elemental metal may be selected from the group of zinc, copper, aluminum, magnesium, sodium, and combinations thereof. The reaction can be carried out by a gas phase reaction of silicon tetrachloride gas with a gas of the elemental metal as known in the art. In particular, reaction of the silicon tetrachloride and the elemental metal can be carried out by reacting silicon tetrachloride gas with zinc gas in a recovery reactor having a temperature of from about 800 to about 1200°C, or about 900 to about 1000°C, at which conditions silicon tetrachloride gas readily reacts with zinc gas. A pressure within the recovery reactor is, for example, from about 0 to about 1723 kPa. As set forth above, polycrystalline silicon is produced as a result of reaction of the silicon tetrachloride and the elemental metal. The polycrystalline silicon is typically of high purity and may be considered solar grade silicon by those of skill in the art. Also produced is the first composition that comprises the metal chloride as a by-product of the reaction. When zinc is used as the elemental metal, zinc chloride is by-produced as shown in the following reaction formula.

SiCl₄+2Zn→Si+2ZnCl₂

[0021] The first composition, which remains after producing the polycrystalline silicon in the one or more recovery reactors, contains the metal chloride and also may contain elemental metal, silicon tetrachloride, and the like. The metal chloride may be separated and recovered in the form of a liquid by lowering the temperature to a boiling point of metal chloride or lower. Elemental metal may be recovered in the form of powder or liquid metal and can be recycled to the one or more recovery reactors. Remaining silicon tetrachloride can also be recycled to the one or more recovery reactors.

[0022] The instant method further includes the step of recovering elemental metal from the metal chloride in the first composition, as represented at numerals 36 and 38 of Figures 1 and 2, through at least one of: 1) subjecting the first composition comprising the metal chloride to hydrogenation to produce elemental metal and hydrogen chloride; or 2) reacting the metal chloride in the first composition and elemental silicon.

Recovery of the elemental metal from the metal chloride in the first composition through hydro enation to produce elemental metal and hydro en chloride

[0023] In accordance with the instant method, and as shown in Figure 2, the metal chloride present in the first composition may be subjected to hydrogenation, as shown in the following reaction formula when the metal chloride is zinc chloride, to produce hydrogen chloride and zinc. ZnCl₂+H₂→Zn+2HCl

[0024] The reduction reaction of zinc chloride with hydrogen gas is carried out at a temperature of typically from about 700 to about 1500°C, about 800 to about 1400°C, or about 900 to about 1300°C. The reduction reaction may be carried out at a mole ratio of hydrogen to zinc chloride of from about 2: 1 to about 200: 1, or about 5: 1 to about 100: 1. Reaction retention time is typically from about 0.01 to about 1 second, or about 0.03 to about 0.1 second. The hydrogenation reaction is a reversible reaction, and therefore the temperature may be lowered to a melting point of the elemental metal zinc or lower immediately after finishing the reaction. Zinc chloride is reduced by hydrogen gas on the above reaction conditions to obtain a fine powder of zinc.

[0025] As shown in Figure 2 and in the above formula, the hydrogenation reaction produces elemental metal (e.g., zinc) and HC1. The elemental metal may be recycled to the one or more recovery reactors, while the HC1 may be recycled to the direct process for producing a silane- containing feed gas comprising trichlorosilane. In this regard, yield of polycrystalline silicon, as well as reduction of waste streams, can be maximized through the instant method.

Recovery of the elemental metal from the metal chloride in the first composition through reacting the metal chloride in the first composition and elemental silicon

[0026] In accordance with the instant method, and as shown in Figure 1, the elemental metal may be recovered from the metal chloride in the first composition through reacting the metal chloride in the first composition and elemental silicon, which is an equilibrium reaction. In particular, the equilibrium reaction involves reaction of the metal chloride and elemental silicon (i.e., silicon atoms present in a source of silicon and that are available for reaction) to produce a silicon tetrachloride and elemental metal (i.e., metal atoms separated from the chlorine atoms of the metal chloride), which benefits from driving the equilibrium reaction in one direction or the other for purposes of recovering the elemental metal. The elemental silicon may be provided in a relatively impure source of the silicon that includes impurities in addition to the elemental silicon, and the reaction of the elemental silicon and metal chloride may be exploited to effectively separate the elemental silicon from some impurities in the source of the elemental silicon, with the resulting silicon tetrachloride possibly subjected to further processing to separate the silicon tetrachloride from impurities. [0027] It is to be appreciated that the instant method may be applicable to any equilibrium reaction that involves reaction of a metal halide (e.g. a metal chloride) and elemental silicon to produce a silicon tetrahalide (e.g. silicon tetrachloride) and elemental metal. For example, suitable elemental metals may be selected from the group of zinc, copper, aluminum, magnesium, sodium, and combinations thereof. In one specific embodiment of the instant method, the equilibrium reaction involves reaction of zinc chloride and elemental silicon to produce silicon tetrachloride and elemental zinc. In another specific embodiment of the instant method, the equilibrium reaction involves reaction of copper chloride and elemental silicon to produce silicon tetrachloride and elemental copper. In another specific embodiment of the instant method, the equilibrium reaction involves reaction of aluminum chloride and elemental silicon to produce silicon tetrachloride and elemental aluminum. Of course, each of the above-mentioned reactions are equilibrium reactions such that the reverse reaction occurs in equilibrium at least to some degree with the forward reaction, with all compounds on both the reactant side and on the product side referred to as "reactive species".

[0028] As shown in Figure 3, the step of recovering the elemental metal through reacting the metal chloride in the first composition and elemental silicon is conducted in an apparatus 10 that is typically sealed from the ambient environment. It is to be appreciated that the apparatus 10 may be unsealed for purposes of inserting or removing reactive species; however, during the reaction, the apparatus 10 is typically sealed to prevent contaminants from entering the apparatus 10, to avoid detrimental environmental effects on the reaction, and to control thermodynamics of the equilibrium reaction. A suitable apparatus 10 is described in co-pending PCT Application No. , which claims priority to

U.S. Application Serial No. 61/667,134 filed on July 2nd 2012, and is entitled "APPARATUS FOR FACILITATING AN EQUILIBRIUM REACTION AND SELECTIVELY SEPARATING REACTIVE SPECIES" (Docket No.: DC11194PSP1), filed on even date herewith, the entirety of which is hereby incorporated by reference.

[0029] Typically, as shown in Figure 3, the apparatus 10 includes a heating zone 12, an overhead temperature modulation zone 14 in fluid communication with the heating zone 12, and an overhead cooling zone 16 in fluid communication with the overhead temperature modulation zone 14. In particular, by "fluid communication", it is meant that liquids and/or gases from one zone may flow directly between the zones without unsealing the apparatus 10 (although valves, gates or other intervening structures or devices may be disposed between the various zones to control flow of apparatus 10 contents between the zones). The temperature of each zone is independently controlled, in relation to other zones in the apparatus 10, to enable thermodynamic equilibrium of the equilibrium reaction to be shifted in the various zones of the apparatus 10, thereby enabling the physical state of the reactive species in the various zones to be controlled (e.g. between solid, liquid, and/or gas states) and enabling separation of reactive species based upon physical state under the environmental conditions in the respective zones. By separation of the reactive species based upon physical state, the equilibrium reaction can be driven in one direction or another as described in further detail below. While temperature in each of the zones is independently controlled, it is to be appreciated that the zones are not necessarily isolated from each other and conditions from one zone may affect conditions in other zones so long as steps in the method that are described in further detail below may still be executed within the respective zones. To illustrate, the apparatus 10 may be a reactor having integral zones contained therein, such as the reactor shown in Figure 3, with temperature in one zone possibly affecting conditions in adjacent zones. However, it is to be appreciated that the respective zones may be represented by distinct reactors, vessels, or devices with feed lines connecting the zones, which may present advantages in commercial-scale execution of the instant method.

[0030] In accordance with one embodiment of the instant method, the first composition comprising the metal chloride, e.g., zinc chloride, is introduced into the apparatus 10 when the apparatus 10 having the various zones is used to implement the instant method. The first composition may be introduced into the heating zone 12 of the apparatus 10 in gaseous form for purposes of reacting the metal chloride and elemental silicon therein. The first composition is provided from the one or more recovery reactors and may include components other than the metal chloride. However, it is to be appreciated that the first composition typically includes substantially pure, i.e., at least about 90%, at least about 92.5%, at least about 95%, at least about 98%, at least about 99%, at least about 99.5%, at least about 99.9%, or at least about 99.99%, by weight, metal chloride.

[0031] In certain embodiments, a carrier (or diluent) gas is introduced into the heating zone 12 of the reactor 10 to strip one or more components from the first composition. The gas is typically inert, but may also be reactive towards another compound. Examples of such gases include noble gases, e.g. argon gas, and/or process gases, such as silicon tetrachloride (STC) gas, hydrogen gas (H₂), etc. As an example, the gas can be passed (e.g. bubbled) through the first composition when in a molten/liquid state to remove unwanted species therefrom, such as volatiles. Such a process may be referred to in the art as gas stripping of the first composition.

[0032] As alluded to above, in accordance with the embodiment in which a source of the elemental silicon that is reactive with the metal chloride is also introduced into the apparatus 10. Stated differently, the elemental silicon is present in the source of the elemental silicon in a form that is reactive with the metal chloride to form silicon tetrachloride. To exploit advantages of the instant method in regards to purification of the elemental silicon, and as alluded to above, the elemental silicon may be provided in a relatively impure source of the elemental silicon that includes impurities in addition to the elemental silicon. Use of relatively impure sources of the elemental silicon may be beneficial because such sources are much less expensive than purified sources of elemental silicon. For example, relatively impure sources of elemental silicon may be readily available in waste streams from some processes such that the instant method may be employed in conjunction with other processes that produce waste streams containing sources of elemental silicon.

[0033] In one embodiment, the source of elemental silicon comprises less than or equal to about 90.00% by weight of the silicon, which is representative of a "relatively impure" source of the elemental silicon in accordance with the instant application. However, it is to be appreciated that any source of elemental silicon may be used in accordance with the instant method without regard to purity thereof. Examples of relatively impure sources of the silicon include, but are not limited to, low grade sources of silicon such as metallurgical grade silicon; waste streams including silicon kerf, direct process residue, and fumed silica; and any combination of the aforementioned sources of elemental silicon.

[0034] As with the first composition including metal chloride, the source of the elemental silicon is typically introduced into the heating zone 12 of the apparatus 10. Unlike the first composition, the source of the elemental silicon is typically introduced into the apparatus 10 in solid form. In this regard, the heating zone 12 of the apparatus 10 typically includes a suspended tray or dish 18 disposed therein to retain the solid source of the elemental silicon. The suspended tray or dish 18 may be supported by sidewalls 20 of the apparatus 10 in the heating zone 12, or may otherwise be suspended in the heating zone 12 through other features of the apparatus 10. To promote contact between the first composition and the source of the elemental silicon, the suspended tray or dish 18 typically defines perforations 22 therethrough that allow the first composition to flow up through the suspended tray or dish 18 and contact the source of the elemental silicon. Logistically, the first composition is typically introduced into the heating zone 12 beneath the suspended tray or dish 18 to promote flow of the first composition toward the source of the elemental silicon.

[0035] The metal chloride from the first composition and the elemental silicon from the source of the elemental silicon are reacted in the apparatus 10 to produce a gaseous stream comprising silicon tetrachloride, unreacted metal chloride, and, optionally, elemental metal. The reaction may leave impurities that may be present in the source of the silicon behind in solid form, thereby effectively separating the silicon from impurities that may be present in the source of the silicon.

[0036] Typically, the reaction of the metal chloride and the elemental silicon occurs in the heating zone 12 of the apparatus 10, and the reaction typically proceeds through a gas/solid phase reaction with the first composition in gas form and the source of elemental silicon in solid form. More specifically, the metal chloride is typically in gaseous form and the elemental silicon is typically in solid form during the reaction in the heating zone 12 of the apparatus 10. The reaction is carried out under conditions that are sufficient to drive the reaction and, in this regard, temperature and pressure within the heating zone 12 of the apparatus 10 are controlled to drive the reaction of the metal chloride and elemental silicon. Physical state of the reactive species is also taken into account for purposes of establishing environmental conditions in the heating zone 12, with the temperature and pressure of the heating zone 12 controlled so as to avoid melting of the source of the silicon and to avoid condensation of the silicon tetrachloride and unreacted metal chloride.

[0037] In one embodiment, the elemental metal may have a boiling point that is greater than environmental conditions, i.e., temperature and pressure, in the heating zone 12 such that the elemental metal may condense in the heating zone 12 upon reaction of the metal chloride and the elemental silicon. In this regard, the temperature and pressure in the heating zone 12 may be controlled so as to promote condensation of the elemental metal within the heating zone 12 (thus addressing the possibility that the elemental metal may only be an optional component of the gaseous stream produced through reaction of the metal chloride and elemental silicon due to condensation of the elemental metal upon reaction of the metal chloride and elemental silicon). Alternatively, the elemental metal may have a boiling point that is lower than the environmental conditions in the heating zone 12 such that the gaseous stream produced by reaction of the metal chloride and elemental semiconductor includes the elemental metal. Temperature and pressure conditions in the heating zone 12 are typically optimized to achieve the greatest conversion of silicon to silicon tetrachloride.

[0038] As alluded to above, the elemental metal may be condensed in liquid form in the heating zone 12 after reaction of the metal chloride and elemental silicon. For this embodiment, when the environmental conditions in the heating zone 12 are below the boiling point of the elemental metal, a collector (not shown) may be provided in the heating zone 12 to collect the condensed elemental metal separate from the source of the elemental silicon. For example, when the suspended tray or dish 18 is present in the heating zone 12 and defines perforations 22, the collector may be disposed beneath the suspended tray or dish 18 and may collect liquid elemental metal that flows downward through the perforations 22. It is to be appreciated that under some circumstances, features within the heating zone 12 may have a higher temperature than the environment within the heating zone 12 such that even under environmental conditions that are sufficient to condense the elemental metal, features within the heating zone 12 may have a higher temperature that does not allow for the condensation of the elemental metal thereon. To the extent that elemental metal is condensed and collected in the heating zone 12, elemental metal in the collector may be transported out of the apparatus 10, either during or after the equilibrium reaction is conducted. As shown in Figure 1, the elemental metal (shown as zinc in these figures) may be recycled to the one or more recovery reactors to thereby form a closed loop process between the one or more recovery reactors and the apparatus within which the elemental metal is recovered from the metal chloride. Alternatively, the elemental metal may be used within the apparatus 10 for further reactions with silicon tetrachloride as described in further detail below.

[0039] In one specific embodiment in which the elemental metal is condensed in liquid form in the heating zone 12, the metal chloride is zinc chloride and the elemental metal is zinc. In this embodiment, the silicon and zinc chloride are typically reacted in the heating zone 12 at a temperature of from about 756 up to about 910°C, which conditions are sufficient to condense zinc in liquid form in the heating zone 12 while maintaining the temperature in the heating zone 12 above the boiling point of zinc chloride.

[0040] All temperatures referred to herein are internal temperatures of the specified zones unless indicated otherwise (as opposed to temperatures of walls, heating elements, or other features of the apparatus 10). For example, a jacket disposed around the heating zone 12 may be set at a temperature of about 1000°C which imparts the heating zone 12 with a temperature of from about 756 to about 910°C. It is expected that at least some amount of heat is lost between such heating elements and the heating zone 12. The same logic applies to the other zones of the reactor, e.g. the cooling zone 16 where the cooling means may be at a temperature lower than that of the cooling zone 16 itself. It is to be appreciated that some zinc can still be present in the gaseous stream under such conditions, even though some zinc may condense in the heating zone 12. It is also to be appreciated that features present within the heating zone 12 may be too hot to allow for the condensation of the elemental zinc thereon such that the elemental zinc may remain in the gaseous stream.

[0041] Any unreacted metal chloride and, when present, elemental metal is condensed from the gaseous stream that results from the reaction of the elemental silicon and the metal chloride, with the silicon tetrachloride remaining in the gaseous stream after condensation of the unreacted metal chloride and any elemental metal. More specifically, when the metal chloride and elemental silicon are reacted under environmental conditions sufficient to maintain the elemental metal in the gaseous stream, both the elemental metal and the unreacted metal chloride in the gaseous stream are condensed from the gaseous stream while maintaining the silicon tetrachloride in gaseous form. Alternatively, when the metal chloride and elemental silicon are reacted under environmental conditions that are insufficient to maintain the elemental metal in the gaseous stream, only the unreacted metal chloride may be condensed from the gaseous stream. Ultimately, the aim is to separate the silicon tetrachloride from the other reactive species by condensing most reactive species from the gaseous stream and maintaining the silicon tetrachloride in the gaseous stream. Yield of various reactive species, and dynamics of the equilibrium reaction, can be theoretically be impacted by removal of certain reactive species from the system shown in Figure 3.

[0042] In the apparatus 10 including the various zones, condensation of the unreacted metal chloride and elemental metal (when present) may be conducted in a separate zone from reaction of the metal chloride and elemental silicon in view of the fact that the unreacted metal chloride may not be condensable at desired environmental conditions at which the metal chloride and elemental silicon are reacted. More specifically, the gaseous stream including the unreacted metal chloride and, optionally, elemental metal may be fed into the overhead temperature modulation zone 14, and the unreacted metal chloride and any elemental metal present in the gaseous stream may be condensed in the overhead temperature modulation zone 14 that is in fluid communication with the heating zone 12. Because the environmental conditions in the overhead temperature modulation zone 14 are independently controlled in relation to the heating zone 12, the environmental conditions in the overhead temperature modulation zone 14 can be established to selectively condense the unreacted metal chloride and the elemental metal (when present) while maintaining the silicon tetrachloride in the gaseous stream. Because the elemental metal and metal chloride typically have different melting points, it may be possible to further separate the elemental metal and the metal chloride depending upon the environmental conditions at which the overhead temperature modulation zone 14 is operated. For example, it may be possible to solidify the elemental metal while condensing the metal chloride into liquid form (with the liquid being less viscous than the elemental metal). The condensed metal chloride may be returned from the overhead temperature modulation zone 14 to the heating zone 12 to participate in the reactions conducted therein with the elemental silicon, thereby effectively recovering the unreacted metal chloride and enhancing conversion efficiency of metal chloride to the elemental metal. The unreacted metal chloride can theoretically be recovered through the above-summarized mechanism until the gaseous stream is free of unreacted metal chloride. In this regard, the equilibrium reaction is typically run with excess silicon such that this cycle can be repeated until the metal chloride is depleted.

[0043] In one specific embodiment, the metal chloride is zinc chloride and the elemental metal is zinc. The silicon and zinc chloride may be reacted in the heating zone 12 at a temperature of at least about 910°C at ambient pressure, at which conditions zinc is present in the gaseous stream. The zinc is condensed in liquid or solid form in the overhead temperature modulation zone 14. To condense the zinc at ambient pressure, the temperature in the overhead temperature modulation zone 14 must be less than about 910°C. While it is to be appreciated that the zinc and the unreacted zinc chloride may be condensed from the gaseous stream in different zones, the unreacted zinc chloride may also be condensed in the overhead temperature modulation zone 14 along with the zinc. Under such circumstances, the unreacted zinc chloride and zinc may be condensed from the gaseous stream at a temperature of from about 275 up to about 756°C at ambient pressure in the overhead temperature modulation zone 14, which conditions are less than a boiling point of the zinc chloride and enable condensation thereof. Alternatively, the unreacted zinc chloride and zinc may be condensed from the gaseous stream at a temperature of from about 275 up to about 420°C at ambient pressure, under which conditions the zinc is solid and the zinc chloride is liquid, which may enable easier separation of the zinc chloride and zinc. Alternatively, the unreacted zinc chloride and zinc may be condensed from the gaseous stream at a temperature of from about 420 up to about 756°C at ambient pressure.

[0044] For purposes of driving the equilibrium reaction toward yield of silicon tetrachloride and elemental metal at least a portion of the gaseous stream including the silicon tetrachloride is isolated from the condensed metal chloride and elemental metal after condensing the metal chloride and, when present, elemental metal from the gaseous stream. By isolating at least a portion of the gaseous stream that includes the silicon tetrachloride after the condensing step described above, at least a portion of the silicon tetrachloride is effectively removed from the equilibrium reaction to drive yield of the equilibrium reaction toward production of the silicon tetrachloride and the elemental metal. Ultimately, such isolation can possibly shift the equilibrium reaction to much higher yields such as at least about 90% by mole conversion and even approaching 100% by mole conversion.

[0045] To isolate at least a portion of the gaseous stream including the silicon tetrachloride from the condensed metal chloride and elemental metal after condensing the metal chloride and, when present, elemental metal from the gaseous stream, the gaseous stream remaining after condensation in the overhead temperature modulation zone 14 may be fed to the overhead cooling zone 16, with at least a portion of the gaseous stream condensed in the overhead cooling zone 16. More specifically, conditions in the overhead cooling zone 16 may be such that the silicon tetrachloride is condensed therein, while conditions in the overhead temperature modulation zone 14 maintain the silicon tetrachloride in gaseous form. A barrier in the apparatus 10 is present to separate or isolate the condensate in the overhead cooling zone 16 from the overhead temperature modulation zone 14. As illustrated in Figure 3, the overhead cooling zone 16 may be configured to prevent the condensed silicon tetrachloride from flowing back into the overhead temperature modulation zone 14, with the barrier being established through the angle at which the overhead cooling zone 16 extends in the apparatus 10 in relation to the overhead temperature modulation zone 14. However, it is to be appreciated that valves or other features (not shown) could also be employed as the barrier to prevent such flow. In any event, the overhead cooling zone 16 prevents intermingling of the condensate produced therein and other condensate, gaseous stream, or other reactive species that are present in the other zones of the reactor, thereby effectively isolating the condensate that is produced in the overhead cooling zone 16 from reactive species that are present in other zones of the apparatus 10. The condensate produced in the overhead cooling zone 16 may be collected in a collection chamber 24, as shown in Figure 3, and recycled to the recovery reactor used in the step indicated by numeral 34. In this regard, the step of isolating at least a portion of the gaseous stream including the silicon tetrachloride may include drawing at least a portion of the gaseous stream from the apparatus 10 either for use in another process or for disposal.

[0046] In addition to isolating at least a portion of the gaseous stream that includes the silicon tetrachloride from the condensed metal chloride and elemental metal, and as alluded to above, silicon tetrachloride and the elemental metal from reaction of the metal chloride and elemental silicon may be reacted to form high purity elemental silicon at a location separate from the reaction of the elemental silicon and the metal chloride. For example, as set forth above, the elemental metal and the silicon tetrachloride may be recycled to the one or more recovery reactors used in the step indicated by numeral 34. The silicon tetrachloride that may be recycled to the one or more recovery reactors may be the isolated silicon tetrachloride. As referred to herein, "high purity" elemental silicon typically refers to a silicon composition having a purity of greater than about 90.00% by weight, or at least about 99.00% by weight, thereby distinguishing "high purity" elemental silicon from the silicon that is typically included in the heating zone 12 for reaction with the metal chloride.

[0047] Many modifications and variations of the present invention are possible in light of the above teachings, and the invention may be practiced otherwise than as specifically described within the scope of the appended claims. It is to be understood that the appended claims are not limited to express and particular compounds, compositions, or methods described in the detailed description, which may vary between particular embodiments which fall within the scope of the appended claims. The subject matter of all combinations of independent and dependent claims, both single and multiple dependent, is herein expressly contemplated. With respect to any Markush groups relied upon herein for describing particular features or aspects of various embodiments, it is to be appreciated that different, special, and/or unexpected results may be obtained from each member of the respective Markush group independent from all other Markush members. Each member of a Markush group may be relied upon individually and or in combination and provides adequate support for specific embodiments within the scope of the appended claims. [0048] It is also to be understood that any ranges and subranges relied upon in describing various embodiments of the present invention independently and collectively fall within the scope of the appended claims, and are understood to describe and contemplate all ranges including whole and/or fractional values therein, even if such values are not expressly written herein. One of skill in the art readily recognizes that the enumerated ranges and subranges sufficiently describe and enable various embodiments of the present invention, and such ranges and subranges may be further delineated into relevant halves, thirds, quarters, fifths, and so on. As just one example, a range "of from 0.1 to 0.9" may be further delineated into a lower third, i.e., from 0.1 to 0.3, a middle third, i.e., from 0.4 to 0.6, and an upper third, i.e., from 0.7 to 0.9, which individually and collectively are within the scope of the appended claims, and may be relied upon individually and/or collectively and provide adequate support for specific embodiments within the scope of the appended claims. In addition, with respect to the language which defines or modifies a range, such as "at least," "greater than," "less than," "no more than," and the like, it is to be understood that such language includes subranges and/or an upper or lower limit. As another example, a range of "at least 10" inherently includes a subrange of from at least 10 to 35, a subrange of from at least 10 to 25, a subrange of from 25 to 35, and so on, and each subrange may be relied upon individually and/or collectively and provides adequate support for specific embodiments within the scope of the appended claims. Finally, an individual number within a disclosed range may be relied upon and provides adequate support for specific embodiments within the scope of the appended claims. For example, a range "of from 1 to 9" includes various individual integers, such as 3, as well as individual numbers including a decimal point (or fraction), such as 4.1, which may be relied upon and provide adequate support for specific embodiments within the scope of the appended claims.

Claims

1. A method comprising the steps of:

introducing hydrogen and a silane-containing feed gas comprising trichlorosilane into one or more decomposition reactors containing a polycrystalline silicon seed substrate;

reacting the trichlorosilane and hydrogen to form polycrystalline silicon in the decomposition reactor;

exhausting a gaseous stream comprising silicon tetrachloride from the one or more decomposition reactors;

feeding the gaseous stream and a source of elemental metal that is reactive with the silicon tetrachloride into one or more recovery reactors;

reacting the silicon tetrachloride and the elemental metal to form polycrystalline silicon and a first composition comprising metal chloride; and

recovering elemental metal from the metal chloride in the first composition through at least one of 1) subjecting the first composition comprising the metal chloride to hydrogenation to produce elemental metal and hydrogen chloride, or 2) reacting the metal chloride in the first composition and elemental silicon.

2. The method as set forth in claim 1, wherein the step of recovering elemental metal from the metal chloride is further defined as subjecting the first composition comprising the metal chloride to hydrogenation to produce elemental metal and hydrogen chloride.

3. The method as set forth in claim 1, further comprising the steps of:

introducing the first composition comprising the metal chloride into an apparatus that is sealed from the ambient environment;

introducing a source of an elemental silicon that is reactive with the metal chloride into the apparatus;

reacting the metal chloride and the elemental silicon in the apparatus to produce a gaseous stream comprising silicon tetrachloride, unreacted metal chloride and, optionally, elemental metal, wherein the reaction is an equilibrium reaction;

condensing the unreacted metal chloride and, when present, elemental metal from the gaseous stream with the silicon tetrachloride remaining in the gaseous stream; and isolating at least a portion of the gaseous stream including the silicon tetrachloride from the condensed metal chloride and elemental metal after condensing the metal chloride and, when present, elemental metal from the gaseous stream.

4. The method as set forth in any one of claims 1 through 3, wherein the elemental metal is selected from the group of zinc, copper, aluminum, magnesium, sodium, and combinations thereof.

5. The method as set forth in claim 3 or 4, wherein the apparatus includes a heating zone, an overhead temperature modulation zone in fluid communication with the heating zone, and an overhead cooling zone in fluid communication with the overhead temperature modulation zone, and wherein the temperature of each zone is independently controlled.

6. The method as set forth in claim 5, wherein the source of the elemental silicon is introduced into the heating zone of the apparatus and wherein the metal chloride and the elemental silicon are reacted in the heating zone of the apparatus.

7. The method as set forth in claim 6, wherein the metal chloride is in gaseous form and the elemental silicon is in solid form during the reaction in the heating zone of the apparatus.

8. The method as set forth in any one of claims 5 through 7, further comprising the step of feeding the gaseous stream into the overhead temperature modulation zone and condensing unreacted metal chloride in the overhead temperature modulation zone.

9. The method as set forth in claim 8 further comprising the step of returning the condensed metal chloride from the overhead temperature modulation zone to the heating zone.

10. The method as set forth in claim 8 or 9, wherein the gaseous stream remaining after condensation in the overhead temperature modulation zone is fed to the overhead cooling zone and wherein at least a portion of the gaseous stream is condensed in the overhead cooling zone.

11. The method as set forth in any one of claims 8 through 10, wherein a barrier in the apparatus separates the condensate in the overhead cooling zone from the overhead temperature modulation zone.

12. The method as set forth in any one of claims 8 through 11, wherein the gaseous stream comprises the elemental metal and wherein the elemental metal is condensed in liquid or solid form in the overhead temperature modulation zone.

13. The A method as set forth in claim 12, wherein the metal chloride is zinc chloride and the elemental metal is zinc, and wherein the silicon and zinc chloride are reacted in the heating zone at a temperature of at least 910°C at ambient pressure with zinc condensed in liquid or solid form in the overhead temperature modulation zone.

14. The method as set forth in claim 13, wherein unreacted zinc chloride and zinc are condensed and/or solidified from the gaseous stream at a temperature of from 275 up to 756°C at ambient pressure in the overhead temperature modulation zone.

15. The method as set forth in claim 14, wherein the unreacted zinc chloride is condensed, and the unreacted zinc is solidified, from the gaseous stream at a temperature of from about 275 up to about 420°C at ambient pressure.

16. The method as set forth in claim 14, wherein unreacted zinc chloride and zinc are condensed from the gaseous stream at a temperature of from about 420 up to about 756°C at ambient pressure.

17. The method as set forth in any one of claims 8 through 11, wherein the elemental metal is condensed in liquid form in the heating zone after reaction of the metal chloride and elemental silicon.

18. The method as set forth in claim 17, wherein the metal chloride is zinc chloride and the elemental metal is zinc, and wherein the silicon and zinc chloride are reacted in the heating zone at a temperature of from about 756 up to about 910°C.

19. The method as set forth in any one of claims 3, 5 through 15, 17, or 18, further comprising the step of reacting the silicon tetrachloride and the metal from reaction of the metal chloride and elemental silicon to form high purity elemental silicon at a location separate from the reaction of the elemental silicon and the metal chloride.

20. The method as set forth in claim 19, wherein the high purity elemental silicon has a purity of greater than about 90.00% by weight.

21. The method as set forth in any one of the prior claims, wherein the step of isolating at least a portion of the gaseous stream including the silicon tetrachloride is further defined as drawing at least a portion of the gaseous stream from the apparatus.

22. The method as set forth in any one of claims 3, 5 through 15, or 17 through 21, wherein isolated silicon tetrachloride from the gaseous stream is recycled to the one or more recovery reactors.

23. The method as set forth in any one of claims 3, or 5 through 22, wherein the source of elemental silicon comprises less than or equal to about 90.00% by weight of the silicon.

24. The method as set forth in any one of the prior claims, wherein recovered elemental metal from the metal chloride in the first composition is recycled to the one or more recovery reactors.