WO2009065444A1

WO2009065444A1 - A method of producing polycrystalline and single crystal silicon

Info

Publication number: WO2009065444A1
Application number: PCT/EP2007/062743
Authority: WO
Inventors: Pedro Blach Servera
Original assignee: Mar De Acedos, S.L.
Priority date: 2007-11-23
Filing date: 2007-11-23
Publication date: 2009-05-28

Abstract

A method of producing polycrystalline and single crystal silicon from quartz, employing reduction of gaseous silicon monoxide in methane atmosphere aided by electrodeless discharge plasma, comprising a three stage process: a first stage reactor wherein quartz is reduced to silicone monoxide in gaseous form by silicon; a second stage reactor wherein silicone monoxide in gaseous form is reduced by carbon to elemental silicon; and a third stage wherein the produced liquid silicon is subjected, in furnaces, to directed crystallization to obtain polycrystalline and single crystal silicon, while the silicon waste produced after the third stage of processing is returned to the first stage to serve as the reducing agent, wherein the stage twosilicon monoxide reduction is conducted in electrodeless-discharge plasma, in a plasma reactor, by atomic carbon obtained through pyrolysis of methane taking place simultaneously in the same volume of the plasma reactor, with the process chain providing for recycling of methane.

Description

A METHOD OF PRODUCING POLYCRYSTALLINE AND SINGLE

CRYSTAL SILICON

OBJECT OF THE INVENTION

A method of producing polycrystalline and single crystal silicon from quartz employing reduction of gaseous silicon monoxide in methane atmosphere aided by electrodeless discharge plasma.

Background of the Invention

The present invention relates to the chemical technology for producing high purity polycrystalline and single crystal silicon for use in production of solar PV cells.

The method for producing polycrystalline silicon (PCS) practiced world wide is the chloride method that involves hydrochlorination of powdered technical grade silicon and produces chlorine-containing compounds in gaseous forms (chlorosilanes). After scrubbing and separation of gases leaving the reactor in which hydrochlorination takes place, a high purity thchlorosilane is extracted and then subjected to hydrogen reduction. The process produces PCS that contains less than 10^~4 % of impurities by weight and is used to manufacture polycrystalline and single crystal silicon ingots for the semiconductors industry. This method is described in "Technologies of the semiconducting silicon", edited by E. S. Falkevich, Metallurgy publishing house, Moscow, 1992.

Implementation of the chloride method requires complex and expensive equipment, the technological process is complex and multi-staged. The use of chlorine necessitates taking special measures to provide for environmental safety. Another known method of producing high purity polycrystalline and single crystal silicon is the method disclosed in the article by A.A. Bakhtin, L.V. Chernyakhovskiy, LP. Kishchenko, P. S. Menshikov "Quality of the raw materials as a factor in manufacturing high purity silicon", "Non- ferrous metals", issue #1 , 1992 , pp. 29-32. There, high purity quartz with boron and phosphor content less than 1*10⁴ % by weight and total impurities content less then 1^*10³ % by weight was subjected to carbothermal reduction. Graphite of the grades ShG-SOCh and GMZ-OSCh characterized by the same as above levels of impurity content was used as the reducing agent.

However, this method has the following drawbacks:

significant loss of silicon, in the form of dust and silicon monoxide, up to 30%, in the course of processing; stringent requirements placed on the properties of electrodes of the furnace; the quality of the produced silicon, as the impurities content is concerned, is not high enough for use in PV cells; - the liquid silicon from the electric arc furnace is cast into ingots that are later melted again to produce polycrystalline and single crystal silicon ingots; like the chloride method, it also poses environmental problems stemming from the necessity to purify high temperature gases and remove dust exiting the electric arc furnace.

The method that most closely resembles the present invention is disclosed in the Russian Federation patent application # 2173738, "A Method of Producing Polycrystalline and Single Crystal Silicon". It proposes a method of producing polycrystalline and single crystal silicon from quartz as the raw material by reduction to elemental silicon with subsequent crystallization of the silicon. The process comprises three stages: in the first stage reactor, the quartz is reduced to silicone monoxide in gaseous form by chemically purified technical grade silicon; in the second stage reactor, the silicone monoxide in gaseous form is reduced by finely particulate carbon to elemental silicon; in the third stage, the produced liquid silicon is subjected, in furnaces, to directed crystallization to obtain polycrystalline and single crystal silicon.

The second stage implements closed carbon cycle. There, the finely particulate carbon (soot) that is used as the reducing agent is injected into the reactor being carried by the stream of carbon monoxide. The exhaust gases produced in the second stage are divided in two streams, one of which is used to produce soot, which is then returned to the reactor as the silicon monoxide reducing agent, and the second stream, after purification, is used for transporting the soot in the second stage reactor. Either plasma or electrical heating are used as the source of thermal energy required for reduction of silicon monoxide to take place. The gases leaving the reactor are mainly comprised of carbon monoxide. The gases also contain some quantities of carbon dioxide and finely particulate dust comprised of baked decomposition products of silicon monoxide. The exhaust gases are filtered to remove solid particles and then divided in two streams. One stream passes through a layer of graphite heated to 1250-1300⁰C, where carbon dioxide is converted to carbon monoxide. Then the gas is cooled in the refrigerator, than compressed, and, after being mixed with finely particulate carbon, is returned to the second stage reactor.

Another gas stream is directed to the CO catalytic converter, where it is converted into carbon dioxide and soot that is used as the reducing agent.

The silicon waste produced in the third stage of processing is returned to the first processing stage to be used as the reducing agent. The dust filtered out from the exhaust gases is returned to the first stage as a raw material. The method proposed in the above described patent application has the following drawbacks:

1. The processing chain for recycling the reducing agent, the finely particulate carbon, involves a large number of apparatuses, some of which need refining, and is difficult to implement.

2. Loading the finely particulate carbon into the silicon monoxide reduction reactor requires adhering to stringent requirements on the precise time interval the carbon should stay in the reactor's active zone to assure full utilization of the carbon particles. 3. Loading carbon monoxide, the reduction process product, into the second stage reactor, as the transport for carbon particles, could lead to the shift of reaction equilibrium toward the reactants and to reduce their utilization. 4. The carbon monoxide entering the second stage reactor has the temperature of about 2000 C °. If plasma is used as the heat source to facilitate reduction of silicon monoxide, this gives rise to the problem of thermal protection of the plasmatron walls. Use of electrical heating, on the other hand, poses the problem of providing for transfer of intense heat flow to the reagents already heated to high temperatures.

Summary of the Invention

The present invention provides the following benefits:

- Simplification of the processing chain for recycling the reducing agent.

While, in the present invention, carbon retains its role of the reducing agent, pulverized carbon suspended in carbon monoxide is substituted by gaseous methane that also plays the role of the recycled reagent. Here, carbon used for reduction is produced by pyrolysis of methane taking place in the active zone of the plasma reactor. The processing chain of methane recycling involves a small number of standard pieces of equipment. - Reduced time required for carbon, the reducing agent, to stay in the active zone of the plasma reactor, due to extreme chemical aggressiveness of atomic carbon produced by pyrolysis of methane.

- Increased efficiency of the reduction process due to the fact that a product of the reaction, carbon monoxide, is no longer used as the transport for carbon. Hydrogen produced by pyrolysis of methane is a neutral component in the process of silicon monoxide reduction.

- The invention solves the problem of thermal protection of the plasmatron's walls by effecting rotating peripheral flow of methane moving in the direction opposite to the central flow of hot silicon monoxide.

To wit, the proposed method of producing polycrystalline and single crystal silicon from quartz as the raw material is a three stage process: in the first stage reactor, the quartz is reduced to silicone monoxide in gaseous form by silicon; in the second stage reactor, the silicone monoxide in gaseous form is reduced by carbon to elemental silicon; in the third stage, the produced liquid silicon is subjected, in furnaces, to directed crystallization to obtain polycrystalline and single crystal silicon.

The second stage reduction, the silicone monoxide reduction, is conducted in electrodeless-discharge plasma by atomic carbon obtained through pyrolysis of methane taking place simultaneously in the same volume of the plasma reactor. The process chain provides for recycling of methane. The plasma reactor implements rotating peripheral flow of methane and the pyrolysis reaction products, hydrogen and atomic carbon, in the direction opposite to the reactor's axis centered flow of silicon monoxide, which is the center area of electrodeless discharge. The electrodeless discharge is induced either by HF induction coil or by microwave (SHF) radiation. To effect recycling of methane, the mixture of carbon monoxide and hydrogen produced in the course of reduction of silicon monoxide is transported to the methanation reactor that outputs methane and water. Methane is returned to the plasma reactor and water is decomposed, in an electrolyzer, into hydrogen and oxygen. Hydrogen is returned to the methanation reactor and oxygen is taken out of the production process. Alternatively, water produced in the methanation reactor is discarded, and the electrolyzer gets supplied by water independently. The unreacted silicon monoxide is condensed in the special apparatus, a separator, on ground technical grade silicon and quartz mixed in the molar proportion 1 :1. The content of the separator is periodically transferred for processing to the first stage reactor thus effecting recycling of the escaped silicon monoxide.

Brief Description of the Drawings

Fig.1 Block diagram of the process of production of polycrystalline and single crystal silicon by a plasma method of reduction of silicon monoxide with methane regeneration.

Fig.2 The drawing depicts the plasmatron energized by HF electromagnetic field. Fig.3 The drawing depicts the plasmatron energized by microwave (SHF) radiation.

Description of the Preferred Embodiment

The block diagram of the process (Fig.1 ) comprises reactor 1 for producing silicon monoxide from silicon and quartz; plasma reactor 2 for silicon monoxide reduction by methane; separator 3 for separation of liquid silicon from gaseous silicon monoxide, carbon monoxide, and hydrogen; crystallization vat 4 for collection of liquid silicon, its cooling and crystallization; separator 5 for separation of unreduced silicon monoxide from carbon monoxide and hydrogen achieved by cooling and condensing silicon monoxide; heat exchanger for additional cooling of carbon monoxide and hydrogen mixture; compressor 7 used to transport the carbon monoxide and hydrogen mixture to the methanation reactor; methanation reactor 8 used to produce methane from the carbon monoxide and hydrogen mixture; separator 9 for separation of water and methane produced in the process of methanation; electrolyzer 10 for producing hydrogen used in the process of methanation.

The plasmatron (the HF version embodiment, Fig. 2), which is the main element of reactor 2 that effects reduction of silicon monoxide, comprises plasmatron wall 11 made of quarts; inlet collector 12 for admitting methane into the reactor, one or more inlet blow tubes 13 that effect tangential to the wall of the plasmatron injection of flow of methane and effects spinning of the flow and gives it tangential momentum; methane's peripheral flow shaper 14; flow deflector 15; plasma generation area 16, axial channel 17 for inputting gaseous silicon monoxide; HF inductor 18, the plasmatron's output channel.

The microwave (SHF) version of the plasmatron (Fig.3) is equipped with a waveguide, instead of the inductor, for energizing plasma (Fig.3).

In the preferred embodiment, the disclosed method of producing polycrystalline and single crystal silicon is realized as follows.

The silicon producing apparatus functions in a cyclical manner. At the beginning of each cycle, the mixture of ground silicon and quartz, together with silicon monoxide that had not been reduced in the previous cycle and has condensed on the mixture, is partially unloaded from separator 5. After unloading the mixture, the separator is refilled with the raw material, that is with mixture of ground silicon and quartz in molar proportion 1 :1. During cooling and condensation, silicon monoxide may undergo either partial or complete transformation into quartz or silicon, but that would not change molar proportion of ingredients in the mix. A part of the silicon produced in the previous cycle, as well as silicon waste produced in silicon production, such as processing ingots, slicing silicon wafers, etc., are used as the raw material to refill the separator.

The mix unloaded off separator 5 is put into reactor 1 , the purpose of which is production of silicon monoxide.

Reactor 1 is an electrically heated vessel made of a carbon material of the required purity and having an outlet for exit of gaseous silicon monoxide, an outlet for venting the reactor vessel, and inlets for pumping in gases

(hydrogen, argon, etc.) for creating either an inert or chemically active atmosphere inside the reactor vessel and for measuring temperature. Reactor 1 is thermo insulated to reduce energy dissipation. During initial heating of the reactor, the reactor is vented to remove gases produced, under the influence of high temperature and chemically active atmosphere, by impurities contained in the raw material. In this way, additional purification of the raw materials is effected. To obtain flow of gaseous silicon monoxide, the reactor's temperature is increased to about 2000° C. Gaseous exiting reactor 1 is transported to reactor 2 that carries out plasma assisted reduction of C silicon monoxide by methane.

By special means described below, in reactor 2 there is created rotating flow of methane. Also, the inner volume of reactor 2 is irradiated by either HF or SHF electromagnetic field that generates and sustains gas plasma formation inside the reactor. Interaction of flows of silicon monoxide and methane with the plasma leads to pyrolysis of methane that produces atomic carbon and hydrogen and to simultaneous reduction of silicon monoxide by atomic carbon that produces silicon and carbon monoxide. Under conditions of thermodynamic equilibrium, hydrogen takes practically no part in the chemical process. The plasma formation serves to provide high temperature heating of the reactants and supplies thermal energy needed for the process of carbon monoxide reduction. The maximal efficiency of the reduction process does not exceed 75%, therefore the output flow of reactor 2 is a mixture of silicon, hydrogen, and unreduced silicon monoxide. To capture drops of liquid silicon contained in the output flow of reactor 2, the flow is channeled to separator 3.

Silicon separator 3 is made of high purity grades of graphite materials enclosing a heated volume filled with pieces of graphite or silicon carbide. When the reactor 2 output flow passes through the bed of material filling the vessel, liquid silicon condenses on the surfaces of the pieces of graphite or silicon carbide and flows down under its own weight into crystallization vat 4.

Temperature in the range of 1400 - 2200° C is maintained in separator 3 to assure liquid state of silicon and gaseous state of silicon monoxide. Additionally, keeping the temperature near the higher end of the range provides for a more complete reduction of silicon monoxide by taking advantage of the reaction between silicon monoxide and remnants of carbon and/or the graphite's carbon on the surface of the graphite at the upper part of the graphite bed. After passing through the separator, the gaseous components of the mixture are passed through a heated transportation channel to silicon monoxide separator 5.

Crystallization vat 4 is analogous to the crucibles used in the Czochralski method of single crystal silicon production. Crystallization vat 4 comprises a heated quartz bowl inserted to preserve its shape into a graphite bowl. The crystallization vat is equipped with temperature control devices including a controlled heater and cooler to carry out the process of directed crystallization of silicon. During the crystallization process, the impurities that may result from silicon contamination through contact with parts of equipment made of graphite or silicon carbide will float to the top of the ingot and can be subsequently removed by cutting out the top of the ingot. After cooling the ingot, it is removed from the vat and processed. A part of the produced silicon is grounded and put into separator 5 as the raw material used in the next production cycle.

Silicon monoxide separator 5 is implemented as a vessel made of high purity grades of graphite and equipped with water cooled jacket. During the first start up of the silicon production facility, separator 5 is filled with grounded silicon and quartz mixed in the molar proportion 1 :1. During operation, silicon monoxide condenses on the surface of granules of the mix and may undergo either partial or complete transformation into quartz or silicon. As noted before, this would not change molar proportion of ingredients in the mix. The heat generated in separator 5 by cooling the flow of silicon monoxide, hydrogen, and carbon monoxide, as well as by condensation of silicon monoxide and its transformation into silicon and quartz, is removed by the water cooling jacket.

The mix of hydrogen and carbon monoxide, in molar proportion 2:1 , outputted by separator 5, is channeled to heat exchanger 6 for additional cooling. Before the mix enters compressor 7, it is enriched by hydrogen generated by electrolyzer 10. The amount of hydrogen added to the mix is chosen to make the molar proportion of hydrogen and carbon monoxide in the mix close to

3:1 , which is the optimal proportion for conducting methanation of the mix that takes place in catalytic methanation reactor 8.

Design of catalytic methanation reactor 8 may vary; it may use various types of catalyst, may implement recirculation of methane flow to reduce carbon monoxide content, it may use thin catalytic layers to facilitate heat exchange, etc. The known designs of methanation reactors provide practically complete conversion of carbon monoxide, as well as conversion of the possibly present quantities of carbon dioxide, into methane. The by-product of the conversion is water that can be easily separated in the liquid form with the help of separator 9. The methane flow from separator 9 is channeled to plasmatron (plasma reactor) 10 for use in reduction of silicon monoxide. Alternatively, methane can be put in a gasholder for storage and to meet fluctuating demand during transient operating regimes of the facility.

Water separated from methane in separator 9 could be transported to electrolyzer 10 for hydrogen generation. Alternatively, it can be discarded. In the latter case, electrolyzer 10 is fitted with a water supply line and a water treatment facility.

The plasmatron of reduction reactor 2, both in its HF and SHF embodiments, operates as follows. The flow of silicon monoxide enters the plasmatron through axial channel 17. Methane is supplied to the plasmatron through blow tubes 13 and inlet collector 12, and, with the help of flow shaper 14, it forms rotating flow 20 along the wall of the plasmatron moving in the direction opposite to the direction of flow of monoxide entering through axial channel 17. The peripheral flow of methane along the wall of the plasmatron provides thermal insulation of the wall against the high temperature plasma. When reaching flow deflector 15, the methane flow turns back and shrinks to the center of the plasmatron, so that when flows of methane and silicon monoxide start mixing they will be moving in the same direction. High temperature should be maintained inside the channel transporting silicon monoxide to the plasmatron to prevent condensation of silicon monoxide on its walls. The methane flow, when moving along the surface of deflector 15, absorbs heat from the reflector, which provides for a gradual deflector temperature rise from the point of its contact with the dielectric (quartz) wall of the plasmatron to its other end at axial channel 17. The flows of silicon monoxide and methane begin to mix after the latter reverses its direction at deflector 15. A part of the mixing flows penetrates plasmoid 16 formed by electrodeless discharge, and another part forms flow 21 that moves along its outer limits in the direction of output channel 19. Both the reactants that penetrate the plasmoid and the reactants in flow 21 are intensively heated by the plasma which provides for intensive pyrolysis of methane and intensive reduction of silicon monoxide by atomic carbon. Sustaining life of electrodeless discharge plasma 16 is accomplished by continuous energizing of the plasma by an external source of electromagnetic energy. For the HF plasmatron, energizing is accomplished with the help of inductor coil 18. For the SHF plasmatron, energizing is accomplished with the help of waveguide 22.

For those skilled in the art, it is obvious that the various embodiments of the proposed method are not restricted to the examples described above, but may vary within the scope of the appended claims.

Claims

1. A method of producing polycrystalline and single crystal silicon from quartz as the raw material in a three stage process: in the first stage reactor, the quartz is reduced to silicone monoxide in gaseous form by silicon; in the second stage reactor, the silicone monoxide in gaseous form is reduced by carbon to elemental silicon; in the third stage, the produced liquid silicon is subjected, in furnaces, to directed crystallization to obtain polycrystalline and single crystal silicon, while the silicon waste produced after the third stage of processing is returned to the first stage to serve as the reducing agent, such that the stage two silicon monoxide reduction is conducted in electrodeless-discharge plasma, in a plasma reactor, by atomic carbon obtained through pyrolysis of methane taking place simultaneously in the same volume of the plasma reactor, with the process chain providing for recycling of methane.

2. A method as claimed in 1 , such that the plasma reactor implements rotating flow of methane and the pyrolysis reaction products, hydrogen and atomic carbon, along the walls of the plasma reactor moving in the direction opposite to the reactor's axis centered flow of gaseous silicon monoxide directed to the center area of electrodeless discharge.

3. A method as claimed in 1 - 2, such that electrodeless HF electromagnetic field induced discharge is effected in the silicon monoxide reduction reactor.

4. A method as claimed in 1 - 2, such that electrodeless microwave (SHF) electromagnetic field induced discharge is effected in the silicon monoxide reduction reactor.

5. A method as claimed in 1 - 4, such that the mixture of carbon monoxide and hydrogen produced in the course of reduction of silicon monoxide is transported to a methanation reactor that outputs methane and water; methane is returned to the silicon monoxide reduction reactor and water is decomposed, in an electrolyzer, into hydrogen and oxygen; hydrogen is returned to the methanation reactor, and oxygen is taken out of the production process.

6. A method as claimed in 5, such that the water produced in the methanation reactor is discarded, and the electrolyzer gets supplied by water independently.

7. A method as claimed in 1 , such that the unreacted silicon monoxide is condensed in the special apparatus on ground technical grade silicon and quartz mixed in the molar proportion 1 :1 , and the mix containing condensed silicon monoxide is periodically transferred for processing to the first stage quartz reduction reactor, thus effecting recycling of the escaped silicon monoxide.