Abstract:
High purity silicon usable for production of solar cells is easily produced with high production efficiency. In a rotary chamber ( 50 ) made of quartz, which is evacuated and filled with an hydrogen-argon atmosphere containing SiF 4 , a plasma area ( 60 ) is generated by supplying electric power from a coil ( 51 ) to decompose SiF 4  and produce silicon as being fine powder particles. Fine particles of seed silicon (Si) in the rotating reaction chamber are picked up and transported upward by weirs ( 52 ), and then they can fall by gravity into the plasma area where silicon elements produced by decomposition of SiF 4  are deposited onto surfaces of the silicon fine particles.

Description:
TECHNICAL FIELD TO WHICH THE INVENTION PERTAINS  
         [0001]    The present invention relates to a method and apparatus for producing high purity silicon.  
         BACKGROUND OF THE INVENTION  
         [0002]    There is a large demand for high purity silicon as materials for production of semiconductor devices and solar cells. Although siliceous sand (SiO 2 ) as a raw material of high purity silicon is one of the abundant and the chief elements in the earth crust, it must be highly purified through reducing and purifying processes adopting a high-level and complicated physicochemical technology to obtain high purity silicon usable for production of the semiconductor devices and solar cells. Some drawbacks of such processes are low productivity and high production cost of high purity silicon.  
           [0003]    In particular, metal silicon material usable for making solar cells has somewhat lightened purity requirement in comparison with silicon material for semiconductor devices but should have a large light-receiving surface area and hence should be produced at a low cost by mass production technology in connection with rising demand for solar cells most desirable for protecting the earth environments.  
           [0004]    Accordingly, a variety of production methods have been proposed and attempted. For example, NEDO (New Energy and Industrial Technology Development Organization (JAPAN)) developed a production method that comprises converting silicon dioxide (starting material) to metal silicon by reduction with carbon at 1800° C., removing phosphorus (P) from the metal silicon by electron beam melting at 2000° C. followed by directional solidification for obtaining a purified ingot, removing boron (B) from the ingot by plasma arc melting at 2500° C. in a quartz crucible followed by further directional solidification for obtaining a high purity silicon ingot of the purity of the order of six nines.  
           [0005]    However, this method includes high temperature processes to be conducted at temperature higher than 2000° C., which could not avoid mixture of impurities from the environment, as well as purification processes having low productivity. The method, therefore, could not realize low cost production of high purity silicon.  
           [0006]    Conventional silicon purification methods use metal silicon obtained by reducing silica (silicon dioxide) with carbon at high temperatures as starting materials to be purified by the purifying process and hence cannot prevent impurities from mixing in the product in the reduction process with carbon.  
           [0007]    Furthermore, the purification method for a metal silicon ingot uses a combination of complicated processes of removing impurities P and B by vaporizing at high temperatures of not lower than 2000° C. and iron (Fe) and other elements by directional solidification, all of which are conducted at high temperatures and unable to improve their productivity and further require protection against mixing-in of impurities from crucibles and chambers accommodating metal silicon ingot at high temperatures. The latter fact may increase the cost of production equipment itself.  
         SUMMARY OF THE INVENTION  
         [0008]    In consideration of the facts that the conventional purification methods are considerably loaded of removing impurities mixed in metal silicon in the process of reducing the starting silica material and the adopted high temperature purification processes are of low productivity, difficult to prevent impurities from mixing-in the product and expensive to manufacture the production apparatus, the present inventor aims at creation of a new method and apparatus for producing high purity silicon by adopting processes different from the conventional methods. Accordingly, a primary object of the present invention is to provide a new mass-production method which is simple and capable of producing high purity metal silicon at a low cost by using, as starting materials, silicon fluoride not allowing impurities to mixing therein and by obtaining metal silicon directly from the silicon fluoride by applying low-temperature plasma reaction.  
           [0009]    The purification method for obtaining high purity silicon according to the present invention is featured by generating a plasma in a hydrogen atmosphere containing SiF 4  gas or SiH 4  gas, decomposing SiF 4  or SiH 4  in the plasma and at the same time causing silicon crystal material, preferably silicon crystal particles (Si) passing through the plasma to deposit silicon produced by decomposition of SIF 4  or SiH 4  onto surfaces of the silicon crystal particles.  
           [0010]    The high purity silicon production method according to the present invention, instead of obtaining metal silicon through high-temperature reduction of silicon dioxide with carbon as conducted in the prior art, uses gaseous silicon tetrafluoride obtained through reaction of silica with hydrofluoric acid, which fluoride does not allow the reduction of impurities (transition elements other than silicon) and enables the separation of the impurities in solid state without migrating to the silicon tetrafluoride. The silicon fluoride is gaseous at an ordinary temperature and can be easily purified by using a low-temperature compression method, which has an advantage of achieving a certain high purity before applying it as a starting material for the purification process.  
           [0011]    According to the present invention, a plasma is generated in a decreased-pressure atmosphere composed of a mixture of silicon tetra fluoride with hydrogen while allowing silicon crystal powder to freely falling through the plasma, and so-called plasma CVD (chemical vapor deposition) reaction occurs between the atmosphere and silicon crystal powder in such a way that silicon decomposed from the silicon tetra fluoride by the plasma is deposited onto the silicon crystal powder surface to homoepitaxially grow as a silicon crystal layer thereon.  
           [0012]    The homoepitaxial growth of the silicon crystal occurs on fine particles of seed silicon crystal powder with a very large entire reaction surface uniformly contacting with the reaction gas in the plasma. This ensures smooth and rapid processing with a very large deposition per unit time.  
           [0013]    Thus, the silicon crystal powder can quickly grows and be taken out of the reaction system at a stage of growth to a size suitable for use as starting materials, e.g., single-crystals usable for production of silicon wafers.  
           [0014]    The method of the present invention can easily obtain high-purity silicon materials having the purity of more than six nines since the material in the form of silicon tetrafluoride may be of high purity and does not allow environmental impurities to mix therein in the process of silicon crystal growth by the plasma CVD method. In addition, as described above, the process can achieve high reaction efficiency and high productivity.  
           [0015]    The reaction process uses the plasma CVD reaction in which the reaction gas excited as a plasma has very high activity but the atmosphere in a reaction chamber has a low temperature of about 200° C., thereby the reaction furnace has no need of having specially high heat-resistant structure and the plasma reaction area can be separated at a specified space from the wall surface of the reaction chamber not to allow impurities to mix therein from the surrounding structures.  
           [0016]    The reaction process consumes electric energy mainly for generating a plasma and endothermic reaction for decomposing the silicone tetra fluoride. Since this reaction robes the silicon tetra fluoride of fluorine and combines fluorine with hydrogen, the power consumption of the process may be deceased.  
           [0017]    On the other hand, hydrogen fluorides produced in the reaction process can be taken out of the reaction system through a closed system during a dry process and reused as starting materials for production of silicon tetrafluorides, realizing the least load to the environment.  
           [0018]    Another object of the present invention is to provide a high-purity silicon production apparatus which comprises a rotary reaction chamber of a substantially cylindrical shape with weirs made on its inside wall along the chamber rotation axis, which chamber can be shut off the outside air to control the reaction atmosphere therein and is further provided with a device for supplying gaseous starting material and hydrogen gas, a device for discharging gas produced by reaction, a device for generating a plasma in an area within the reaction chamber, a device for feeding silicon crystal powder into the chamber and a device for taking silicon crystal powder (product) from the chamber, wherein silicon crystal powder supplied into the reaction chamber is carried upward by the weirs with rotation of the reaction chamber and freely falls to pass the plasma area generated with power supply in the reaction chamber so that silicon separated in the plasma may deposit on the surface of silicon crystal powder.  
           [0019]    The apparatus can maintain the specified reaction conditions of a plasma reaction area and high reaction efficiency by continuously feeding gaseous starting materials such as silicon tetra fluoride (gas) and hydrogen gas and discharging gaseous reaction products. In the reaction chamber, silicon powder (seed crystals) is transported upward by the weirs with rotation of the reaction chamber so that it may freely fall toward the plasma reaction area generated the chamber. Silicon separated in the plasma area can be deposited directly, without contacting with the chamber wall surface (i.e., without being contaminated with other elements), on the surfaces of the silicon crystal particles falling therein, achieving high efficiency of epitaxial growth of silicon thereon as observed in the semiconductor production process. The weir may be linear, helical, or any other suitable pattern in respect to the rotation axis of the reaction chamber and may have a section suitable for picking up the silicon powder.  
           [0020]    In the production apparatus, the plasma is generated in the near center portion of the reaction chamber with the atmosphere maintained under a certain decreased pressure and the silicon crystal powder moves upwards along the chamber wall as the chamber rotates, then freely falls from the top side of the chamber, passes the central plasma area and returns to the bottom of the chamber. Therefore, a silicon crystalline layer homoepixially deposited on surface of each silicon crystal particle becomes thicker by repeatedly passing though the plasma during the rotation of the reaction chamber. When the high purity silicon layer deposited on the silicon crystal powder has grown to a specified thickness, the reaction chamber is tilted to discharge the silicon product from the opposite end thereof.  
           [0021]    In the apparatus, the reaction rate is determined depending upon a feed rate of hydrogen radicals and hence the reaction efficiency may be further improved by providing a separate system for generating hydrogen radicals.  
           [0022]    The hydrogen radical generating system may be such that hydrogen radicals are generated by any of known methods, for example, for effectively ionizing hydrogen gas by glow discharging Ar+H 2  gas or injecting electrons from a hollow cathode electron gun. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a flow diagram depicting processes of a high-purity silicon production method according to the present invention.  
         [0024]    [0024]FIG. 2 is a cross-sectional view of a reaction chamber of the high-purity silicon production method according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    [0025]FIG. 1 is a flow diagram of a silicon purifying process of the present invention. In FIG. 1, there is shown a starting material (siliceous sand)  10 , a hopper  11 , a reaction drum  12 , a gas cooler  13 - 1 , an evaporator  13 - 2 , a rotary compressor  14 , a tank  15 , an expansion tank  16 , a surge tank  17 , a roughing vacuum pump  18 , a pressure control tank  20 , a SiF 4  gas bomb  21 , a H 2 gas bomb  22 , a roughing vacuum pump  23 , a plasma reaction device  30 , a reactor (reaction chamber)  30 - 1 , fine silicon powder  31 , a hopper  32 , a vacuum chamber  33 , a electron beam generator  34 , a polysilicon ingot  35 , a turbomolecular pump  40 , a Roots pump  41 , a gas cooler  42 , a tank  43 , a rotary compressor  44 , an expansion tank  45 , a hydrofluoric acid tank  46  and a roughing vacuum pump  47 .  
         [0026]    In the silicon purification process of the present invention, the starting material (siliceous sand)  10  is loaded from the hopper  11  into the reaction drum  12  in which the silica reacts with hydrogen fluoride to form silicon tetrafluoride (SiF 4  gas). The gasification is prompted by evacuating the reaction drum  12  by using the roughing vacuum pump  18 - 1 . Humidity is removed by using the gas cooler  13 - 1 , the hydrogen fluoride (HF) is liquefied by using the rotary compressor  14  and then the gaseous silicon tetrafluoride is fed to the expansion tank  16  in which the gas is purified from other impurities such as nitrogen gas and then fed and stored in the surge tank  17 . In the evaporator  13 - 2 , the gas is heated with hot water to form silicon tetrafluoride gas and fed to the pressure control tank  20  in which it is mixed with silicon tetrafluoride gas fed from the SiF 4 gas bomb  21  until the mixture gas reaches a specified pressure. In the plasma reaction device  30 , plasma is applied to the silicon tetrafluoride gas and hydrogen gas to obtain silicon powder by the plasma reaction. In this process, prepared silicon powder  31  is fed from the hopper  36 , which is used as seed crystals allowing the rapid homoepitaxial growth of a new silicon layer thereon. Thus, silicon crystal material, e.g. silicon powder of excellent crystal quality can be obtained at a high deposition rate. The gas after the reaction is discharged by the turbomolecular pump  40  and fed through the Roots pump  41  to the gas cooler in which hydrogen fluoride (HF) is liquefied and recovered. The gas is then compressed by the rotary compressor  44  to obtain liquefied tetrafluoride that is then expanded in the expansion tank  45  to separate H 2  gas by vaporization and obtain high purity silicon tetrafluoride (liquid). The liquefied silicon tetrafluoride stored in the expansion tank  45  is fed through a high-pressure line to the surge tank  17  and then reused.  
         [0027]    Silicon powder  31  obtained by the present process is loaded from the hopper  32  into the electron-beam melting device (consisting of the vacuum chamber  33  and the electron beam generator  34 ) by which a high-purity silicon ingot  35  can be obtained.  
         [0028]    [0028]FIG. 2 is a schematic cross-sectional view of a reaction chamber (corresponding to the reaction camber  30 - 1  as shown in FIG. 1) according to the present invention.  
         [0029]    In FIG. 2, there is shown a reaction chamber  50 , a coil  51 , weirs  52  and a rotation support ring  53 . The reaction chamber is driven into rotation by supporting rollers  55 . Although an induction type plasma generator is used in the shown embodiment, a capacitor type plasma generator composed of externally disposed electrodes may be also used if the chamber has an enough space therein.  
         [0030]    In the reaction chamber  50 , high-frequency power from the coil  51  is applied to generate a plasma area  60  in a substantially center portion of a decreased-pressure atmosphere of silicon tetrafluoride gas and hydrogen gas. As shown in FIG. 2, the plasma area is formed in the substantially center portion at a space separated from the wall surface of the reaction chamber. The plasma area is heated by plasma generation heat to a temperature of 200° C. to 400° C.  
         [0031]    In this area, the starting gaseous material (silicon tetrafluoride) reacts with hydrogen to dissociate silicon according to the following reaction.  
         SiF 4 +2H 2 →Si+4HF   (1)  
         [0032]    This reaction is endothermic but the atmosphere temperature can be maintained at 200° C. to 400° C. by heat generated by the plasma.  
         [0033]    During rotation of the reaction chamber, silicon crystal powder  61  loaded into the reaction chamber through one end thereof is pickup by weirs  52  formed on the chamber inner wall and carried to the top position of respective weirs, from which it falls by gravity as shown at  62  and passes the plasma area  60  while dissociated silicon deposits by epitaxy onto the surface of the falling silicon powder. Although a typical weir shape is illustrated, it may be modified to any convex shape suitable for picking up silicon powder.  
         [0034]    The atmosphere temperature is relatively low, i.e., its plasma area has a relatively low temperature plasma. However, since very active radicals may be produced in the atmosphere in the state exited by the plasma, the reaction rapidly proceeds and silicon crystal layer effectively deposits and grows on the surface of silicon powder freely falling in the plasma by the well-known effect of the homoepitaxial CVD reaction as adopted for producing semiconductor devices.  
         [0035]    This reaction process can be conducted under the conditions of: RF frequency of 13.56MHz, input power of 4KW, gas pressure of 0.1-30 Torr and starting-gas flow rate 0.1-1 l/min (SiF 4 ) and 0.1-2 l/min (H 2 ).  
         [0036]    In the reaction process, silicon crystal particles freely fall, being uniformly dispersed over the plasma area, thereby silicon produced according to the reaction (1) is evenly swept attaining a high productivity.  
         [0037]    The weir system  52  acting as the above-described silicon crystal powder spreading mechanism may be arranged linearly along the rotary axis of the chamber. Alternatively, weirs may be formed helically to smoothly vary the dispersion of silicon powder or it may be of different cross-sectional shape suitable for a specified powder feed rate.  
         [0038]    Practical reaction conditions are as follows:  
         [0039]    The reaction process can be implemented at a RF frequency of 13.56MHz, input electric power of 4KW, gas pressure of 0.1 to 30 Torr, starting-gas flow rates of.0.1 to 1 l/min (SiF 4 ) and 0.1 to 2 l/min (H 2 ).  
         [0040]    The seed crystal acting as a nucleus for growing the new crystal thereon was obtained by this process. The depositing rate of fine silicon crystal powder in the process was in the rage of 0.5 to 5 g/h.  
         [0041]    It is also possible to add SiH 4  as the starting material gas.  
         [0042]    Since the present invention does not aim at obtaining a planar polycrystalline layer, fine crystal particles irregularly formed depending upon actual reaction conditions or fine crystal particles separated after production may be allowed and thus deposited particles may be also used as seed crystal particles for growing thereon new crystal layers by the process.  
         [0043]    Silicon wafer crushed to fine particles may be also used as seed crystal powder.  
         [0044]    The production of silicon and the silicon-forming rate of this reaction is determined depending upon the feed rate of atomic hydrogen or hydrogen radicals. Therefore, when a constant feed rate of SiF 4  gas is preset for forming a silicon crystal on the surface of each silicon particle, the decomposition rate may be determined in accord with a feed rate of hydrogen radicals.  
         [0045]    To further improve the production efficiency of the production apparatus, the following method may be adopted to effectively generate hydrogen radicals.  
         [0046]    (1) In addition to the existing reaction chamber, a separate reaction chamber is provided for generating hydrogen radicals to be effectively fed to the main reaction chamber.  
         [0047]    (2) As widely adopted in the semiconductor manufacturing processes, a hot wire cell method may be used for generating hydrogen radicals by heating a metal filament catalyst (W, Mo, Si) to a temperature of 1500° C. to 2000° C.  
         [0048]    (3) Electrons are injected into the plasma by using a neutralizer or a hollow cathode to effectively generate hydrogen radicals.  
         [0049]    In the above description, SiF 4  is decomposed with hydrogen radicals by using the low-temperature plasma to produce silicon powder. A method for decomposing SiH 4  and rapidly obtaining thin polycrystalline silicon layers usable for manufacturing solar cells by using the heat plasma CVD is well-known. SiF 4  gas (instead of SiH 4 ) is used as the starting gas and rapidly decomposed by using the heat plasma to obtain fine silicon crystal particles. The deposition conditions are considerably lightened as compared with those required for forming polycrystalline thin layers of uniform thickness and structure. The power consumption may be also saved.  
         [0050]    The deposition conditions are the same in principle as those required for forming polycrystalline thin layers. For example, the process can be implemented under the following conditions:  
                                                                                       Gas pressure:   100-1000   Torr           Electric energy:   10-50   KW                Gas components and flow rates:                SiF 4     0.1-10   m 3 /min.           Ar   50-100   m 3 /min.           H 2     0.1-10   m 3 /min.                      
 
         [0051]    The growing mechanism of thin polycrystalline layers is determined depending upon the feed rate of SiF 4  gas. On the other hand, the deposition rate is 0.3 g/sec at electric energy of 10KW and a SiF 4  feed rate of 0.1 m 3 /min and it is 5 g/sec at the same electric energy and a SiF 4  feed rate of 1 m 3 /min. Irrespective of layer forming conditions, powder is allowed to use and therefore the deposition rate can be further increased, for example, to about 150 g/sec at a SiF 4  feed rate of 10 m 3 /min.  
         [0052]    As be apparent from the foregoing, these reaction processes can be implemented commonly in principle. This means that the high-purity silicon production method and apparatus according to the embodiment of the present invention, which has been described with the starting SiF 4  gas, may be also operated with SiH 4  gas (instead of SiF 4  gas) and under the same basic reaction conditions.