JP5471450B2 - Method for producing silicon - Google Patents

Description

  The present invention relates to a method for manufacturing silicon.

  As a method for producing semiconductor grade silicon, the Siemens method in which trichlorosilane and hydrogen are reacted at a high temperature is mainly employed. In this method, extremely high-purity silicon can be obtained, but it is said that the cost is high and further cost reduction is difficult.

  As environmental problems are highlighted, solar cells are attracting attention as a clean energy source, and demand is rapidly increasing mainly for residential use. Since silicon-based solar cells are excellent in reliability and conversion efficiency, they account for about 80% of solar power generation. Solar cell silicon is mainly made of non-standard semiconductor grade silicon. Thus, in order to further reduce the power generation cost, it is desired to secure a low-cost silicon raw material.

As a silicon production method replacing the Siemens method, for example, Patent Documents 1 and 2 below disclose a method of producing silicon by reducing silicon halide with a reducing agent (for example, molten metal).
For example, when tetrachlorosilane is used as the silicon halide and aluminum is used as the reducing agent metal, the reaction represented by the following chemical reaction formula (A) proceeds by bringing tetrachlorosilane and aluminum into contact with each other, Silicon and aluminum trichloride are produced.
3SiCl ₄ + 4Al → 3Si + 4AlCl ₃ (A)

In the reduction reaction of the silicon halide, a side reaction in which an aluminum subhalide (AlCl, AlCl ₂ ) is generated is known.
The produced aluminum subhalide becomes unstable at 1000 ° C. or lower, and can be decomposed to produce aluminum and aluminum trichloride as shown in the following formulas (D-1) and (D-2). Are known.
3AlCl → AlCl ₃ + 2Al (D-1)
3AlCl ₂ → 2AlCl ₃ + Al (D-2)
When these reactions occur, aluminum as a reducing agent is not effectively used, and the reaction efficiency of the formula (A) decreases.

Japanese Patent Laid-Open No. 2-64006 JP 2007-77007 A

  An object of this invention is to provide the manufacturing method of the silicon which can improve the reaction efficiency of reaction which reduces a silicon halide with a metal and produces | generates silicon.

In order to solve the above problems, in a first aspect of the present invention, there is provided a method for producing silicon by reducing silicon halide represented by the following formula (1) with molten aluminum inside a reactor. The present invention provides a method for producing high-purity silicon in which aluminum trichloride is supplied into the reactor during reduction.
SiH _n X _4-n (1)
(In the formula, n is an integer of 0 to 3, and X is one or more halogen atoms selected from F, Cl, Br, and I.)
It is preferable that boron contained in the aluminum is 5 ppm by weight or less and phosphorus is 0.5 ppm by weight or less.
Moreover, it is preferable that the said aluminum trichloride exists in the inside of the said reactor with the density | concentration of 1 volume% or more and 30 volume% or less.
Further, the purity of the silicon halide is preferably 4N or more.
Moreover, in the 2nd aspect of this invention, the manufacturing method of high purity silicon which further has the process of refine | purifying the silicon obtained by the said manufacturing method of high purity silicon by directional solidification is provided.

  According to the silicon production method of the present invention, it is possible to improve the reaction efficiency of a reaction in which silicon halide is reduced with molten aluminum to produce silicon.

In the first aspect of the present invention, there is provided a method for producing silicon by reducing silicon halide represented by the following formula (1) with molten aluminum inside the reactor, and the inside of the reactor during the reduction: A method for producing high-purity silicon that supplies aluminum trichloride is provided.
SiH _n X _4-n (1)
Below, the case where tetrachlorosilane is used as the silicon halide will be described as an example.
The reaction for producing silicon by reducing tetrachlorosilane with aluminum is represented by the formula (A).
3SiCl ₄ + 4Al → 3Si + 4AlCl ₃ (A)

When AlCl ₃ , which is a product on the right side in the formula (A), is supplied to the inside of the reactor during the reduction, the equilibrium is generally considered to move to the left side in the formula (A).
However, in the present invention, surprisingly, by supplying AlCl ₃ into the reactor during the reduction, the equilibrium of the formula (A) is not moved to the left side, but further moved to the right side of the formula (A). Thus, the reaction efficiency of the reaction for generating silicon can be increased.

The reason is considered as follows.
The free energy of the reaction of the formula (A) is negative and the value is large (for example, −196 kJ / mol at 1000 ° C.), that is, the equilibrium constant of the formula (A) is 1.1 × 10 ⁸ and biased toward the product side Therefore, even if the partial pressure of AlCl ₃ is increased, the equilibrium of the formula (A) does not move to the left.
On the other hand, the free energy of the reaction of the formula (D-1) (for example, 75 kJ / mol at 1000 ° C.), the free energy of the reaction of the formula (D-2) (for example, −35 kJ / mol at 1000 ° C.), and the reaction formula Compared with (A), the equilibrium is not so much biased toward the product side (the right side of the equation), so AlCl ₃ is supplied to the inside of the reactor and AlCl ₃ inside the reactor is When the partial pressure is increased, the effect of trying to advance in the direction of returning to the left side of the expressions (D-1) and (D-2), that is, in the direction of suppressing the decomposition reaction of the subhalide is large.
Therefore, the reaction efficiency of the reaction of formula (A) can be improved due to the effect of suppressing the decomposition reaction of the aluminum subhalide.

For example, at 1000 ° C., partial pressure _P (SiCl 4) of SiCl ₄ in the interior of the reactor based on the free energy of the reaction formula (A), to represent the partial pressure of the AlCl ₃ in P (AlCl ₃₎ ,
P (SiCl ₄ ) = 0.9 × 10 ⁻⁸ × P (AlCl ₃ )
The relationship was obtained.
On the other hand, for example, based on the free energy of the formula (D-2), said reactor _P partial pressure of AlCl ₂ inside the (AlCl 2), to represent the partial pressure of the AlCl ₃ in P (AlCl _3),
P (AlCl ₂ ) = 1.2 × 10 ³ × P (AlCl ₃ )
The relationship was obtained.
Accordingly, it was found that the effect of the partial pressure of AlCl ₃ on the partial pressure of the gas on the left side (raw material) of the reaction formula is on the order of 10 ¹⁰ , and the degree of suppressing the decomposition reaction of the subhalide is large.

As a method for producing silicon according to the present embodiment, atomized gas is blown onto molten aluminum to form molten aluminum microdroplets, and the microdroplets (liquid particles) are brought into contact with the silicon halide. Therefore, a method of obtaining silicon by reducing the silicon halide is preferable.
Specifically, a trickle of molten aluminum is ejected from the nozzle into the reactor, atomized gas is blown into the trickled stream of molten aluminum, and molten aluminum is injected into the reactor as fine droplets of molten aluminum. It is preferable to supply.

  By making aluminum into fine droplets (liquid particles), the specific surface area of aluminum is increased and the contact area of aluminum with silicon halide is increased, so that the reaction efficiency is easily improved.

As a method for supplying the aluminum trichloride, a method in which a mixed gas of an inert gas, silicon halide, and aluminum trichloride is used as the atomizing gas can be used.
Moreover, the method of using the mixed gas of an inert gas and aluminum trichloride as said atomizing gas is mentioned. In this case, a silicon halide gas is supplied into the reactor separately from the atomizing gas.
The concentration of aluminum trichloride in the atomized gas is preferably 1% by volume or more and 30% by volume or less.

  Examples of the inert gas include nitrogen, argon, and helium. Nitrogen is preferred as the inert gas in terms of cost.

From the viewpoint of obtaining high-purity silicon particles, the inert gas used for the preparation of the atomizing gas preferably has a purity of 99.9% by volume or more, more preferably 99.99% by volume or more, More preferably, it is 99.999 volume% or more, and it is especially preferable that it is 99.9995 volume% or more.
Moreover, the method of supplying aluminum trichloride gas to a reactor separately from the said atomizing gas is mentioned.

In the present invention, aluminum trichloride is supplied into the reactor in order to suppress the decomposition reaction of the aluminum subhalide. As a result, the concentration or pressure of aluminum trichloride inside the reactor increases.
By supplying aluminum trichloride to the inside of the reactor, the concentration of aluminum trichloride increased in the reactor is preferably as high as possible from the viewpoint of the reaction rate. This results in a decrease in the concentration of chlorosilane, which is not preferable in view of the volumetric efficiency of the reactor. Considering these, the preferable aluminum trichloride concentration inside the reactor is about 1% by volume or more and 30% by volume or less. If the concentration is lower than 1% by volume, the effect is not remarkable. If the concentration exceeds 30% by volume, the concentration of the raw material gas is lowered, which is not advantageous in volume efficiency.

The aluminum trichloride used in the present invention must be non-hydrated AlCl ₃ , and its purity affects the purity of the product silicon. Therefore, high purity is preferable, 3N or more, preferably 4N or more, more preferably 6N or more.

Since AlCl ₃ has a sublimation temperature of about 180 ° C., it is usually a gas at the reaction temperature used. The method of supplying AlCl ₃ inside the reactor, is supplied from the pre-gasified AlCl ₃ solid by a vaporizer heated at above 180 ° C., the AlCl ₃ gas obtained inside the reactor This is preferable in terms of controllability. When gasification is performed, the controllability is good if an inert gas such as argon is used as a carrier gas. A method of supplying solid AlCl ₃ directly into the reactor can also be employed.

  In the present invention, the total content of Fe, Cu, Ni, Pb and Sn contained as impurity components in aluminum used as the reducing agent is preferably 30 ppm by mass or less, and more preferably 10 ppm by mass or less. . Thus, by using high-purity aluminum in which Fe, Cu, Ni, Pb, and Sn, which are metal components that are not effective for reduction of silicon halide, are used as a reducing agent, the obtained silicon is Fe, Cu, Ni, Pb. And contamination by Sn can be suppressed.

  The temperature of molten aluminum at the time of reduction is not lower than the melting point (660 ° C.) of aluminum, usually 700 to 1300 ° C., preferably 700 to 1200 ° C., more preferably 700 to 1000 ° C.

  In order to allow molten aluminum to flow out stably from the nozzle, the temperature of the nozzle is preferably about 700 to 1300 ° C. In addition, by installing a nozzle heat retaining device such as a heater attached to the nozzle, the temperature can be secured and the nozzle can be prevented from being blocked.

  From the viewpoint of obtaining high-purity silicon particles, the purity of tetrachlorosilane is preferably 99.99% by mass or more, more preferably 99.999% by mass or more, and 99.9999% by mass or more. More preferably.

  When the atomized gas containing tetrachlorosilane is used, the concentration of tetrachlorosilane in the atomized gas is preferably 10% by volume or more, more preferably 20% by volume or more, and further preferably 50% by volume or more. preferable. There exists a tendency for reaction of the said Formula (A) not to fully advance that the density | concentration of tetrachlorosilane is less than 10 volume%.

  The temperature of the atomizing gas sprayed on the molten aluminum trickle is preferably 200 to 1200 ° C, more preferably 200 to 1000 ° C. Atomizing efficiency can be improved by increasing the temperature of the atomizing gas. That is, molten aluminum can be atomized with a smaller amount of atomizing gas.

  The pressure of the atomizing gas blown to the molten aluminum trickle is preferably 0.1 to 15 MPa, more preferably 1 to 10 MPa, and further preferably 2 to 10 MPa. When the pressure of the atomizing gas is less than 0.1 MPa, formation of aluminum microdroplets tends to be insufficient and the reaction represented by the formula (A) tends to be insufficiently progressed. On the other hand, in order to use the atomized gas exceeding 15 MPa, it is necessary to use various devices having high pressure resistance, and the cost tends to increase.

As shown in the formula (A), the stoichiometric ratio of the number of moles of tetrachlorosilane and the number of moles of molten aluminum in the reaction is 3: 4, but is supplied to the reaction field from the viewpoint of productivity and the like. The ratio (M ₁ / M ₂ ) of the number of moles M ₁ of tetrachlorosilane per unit time and the number of moles M ₂ of molten aluminum supplied (M ₁ / M ₂ ) is preferably 0.75 to 20, and preferably 0.75 to 10 Is more preferable, and it is still more preferable that it is 0.75-7.5. If the value of M ₁ / M ₂ is less than 0.75, the progress of the reaction tends to be insufficient, whereas if it exceeds 20, the amount of tetrachlorosilane that does not contribute to the reaction tends to increase.

  Moreover, although the case where the micro droplet (liquid particle) of aluminum was used as a metal was illustrated in the said embodiment, the form of aluminum may be a molten thin film or a molten pool. Alternatively, vaporized aluminum gas may be used.

In the said embodiment, although the case where tetrachlorosilane was used as a silicon halide was illustrated, silicon halide is not limited to this, Of silicon halide shown by said Formula (1), other than tetrachlorosilane is shown. These may be used alone, or two or more of the silicon halides represented by the above formula (1) may be used in appropriate combination.
In addition, when using combining tetrachlorosilane and another silicon halide, the composition and compounding ratio are not limited, However, It is preferable that the compounding ratio of tetrachlorosilane is larger than the compounding ratio of another kind of silicon halide.

  If necessary, it may be subjected to treatment with acid or alkali to remove the residue of the metal component adhering to the obtained silicon, unreacted metal component, segregation such as directional solidification, dissolution under high vacuum, etc. . Among such operations, particularly by directional solidification, impurity elements contained in silicon are further reduced, and silicon can be further purified.

  Directional solidification is performed, for example, by melting silicon in a mold and solidifying sequentially from the bottom while controlling the solidification rate by heat removal. Since impurities collect around the final solidified part, cutting and removing the part can achieve high purity of silicon and simultaneously control the crystal structure. By repeating directional solidification several times, higher purity silicon can be produced.

  The ingot obtained by directional solidification is usually sliced by cutting the inner peripheral edge, etc., then both sides are lapped using loose abrasive grains, and further immersed in an etching solution such as hydrofluoric acid to remove the damaged layer Is done. A silicon substrate is obtained through the above steps.

  The silicon particles obtained by the manufacturing method according to the present embodiment are suitable as silicon used for manufacturing a solar cell.

Claims (5)

A method for producing silicon by reducing tetrachlorosilane with molten aluminum inside a reactor, wherein aluminum trichloride is supplied into the reactor during the reduction.   The method for producing high-purity silicon according to claim 1, wherein boron contained in the aluminum is 5 ppm by weight or less and phosphorus is 0.5 ppm by weight or less.   The method for producing high-purity silicon according to claim 1 or 2, wherein the aluminum trichloride is present in the reactor at a concentration of 1% by volume or more and 30% by volume or less. The method for producing high-purity silicon according to any one of claims 1 to 3, wherein the purity of the tetrachlorosilane is 4N or more. The manufacturing method of the high purity silicon which further has the process of refine | purifying the silicon obtained by the manufacturing method of the high purity silicon in any one of Claims 1-4 by directional solidification.