JP2013212974A - Method for producing polycrystalline silicon and method for producing hydrogen gas used in the production method as reducing agent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for purifying hydrogen in a production method of polycrystalline silicon by thermal decomposition of trichlorosilane and hydrogen reduction, enabling to easily obtain high purity polycrystalline silicon of 99.999999999% or more purity.SOLUTION: As the generation source of hydrogen, electrolysis of metal inorganic acid salt and/or metal hydroxide of common salt, potassium hydroxide or the like in aqueous solution is adopted, and generated hydrogen is washed with water, and then removal of microparticles is performed by filtering. Because the electrolysis is performed in an aqueous solution of the metal inorganic acid salt and/or metal hydroxide, any carbon content does not mix in hydrogen, and the metal hydroxide (as microparticle) can be removed in very high efficiency by the washing with water and filtering with a filter.

Description

  The present invention relates to a method for producing polycrystalline silicon using trichlorosilane as a raw material and a method for producing hydrogen gas used as a reducing agent in the method.

  Polycrystalline silicon used for manufacturing semiconductor devices and the like is required to have high purity. As a method for producing such high-purity polycrystalline silicon, a method is known in which trichlorosilane is reduced to deposit polycrystalline silicon.

In this method, hydrogen gas is used as a reducing agent, and trichlorosilane (SiHCl ₃ ) is reduced at a high temperature of 1000 ° C. or higher, thereby precipitating high-purity polycrystalline silicon mainly by the reaction of the following formula. The impurity concentration can be reduced to the ppb to ppt level, and it is widely put into practical use in terms of good efficiency.

4SiHCl ₃ → Si + 3SiCl ₄ + 2H ₂
SiHCl ₃ + H ₂ → Si + 3HCl
In the production method as described above, in order to obtain high-purity polycrystalline silicon, it is necessary to use a high-purity hydrogen gas used as trichlorosilane or a reducing agent.

  For example, as a trichlorosilane, a metal silicon having a purity of about 97 to 99% is used as a raw material, and after chlorination, a highly pure one obtained through multistage distillation purification is used (patent) References 1 and 2).

  On the other hand, as hydrogen gas, it is known that what is obtained by a hydrocarbon gas decomposition method, a deep cold hydrogen purification method, etc. is high purity, and the hydrogen gas obtained by such a method is used as a reducing agent. Has been.

JP 2006-52110 A JP-A-1-145302

  By the way, with high performance required for semiconductor devices and the like in recent years, there is a demand for a method capable of stably producing high-purity polycrystalline silicon (for example, having a purity of 11N (Eleven Nine) or higher). A conventionally known method for producing polycrystalline silicon has a problem that a product having low purity may be obtained.

    As a result of studying the purity of such polycrystalline silicon, the present inventors have used hydrogen gas produced by electrolysis of water and subjected to a certain purification treatment as the hydrogen gas used as the reducing agent. Thus, it has been found that high-purity polycrystalline silicon having a purity of 11 N or more can be stably produced, and the present invention has been completed.

  That is, an object of the present invention is to provide a method capable of stably producing high purity polycrystalline silicon having a purity of, for example, 11 N or more and a method for producing hydrogen gas used as a reducing agent in the method. is there.

That is, according to the present invention,
A step of electrolyzing an aqueous solution containing an inorganic acid metal salt and / or a metal hydroxide as a solute to generate hydrogen;
A purification step in which the hydrogen gas obtained in the above step is washed with water and then passed through a mist filter;
A polycrystalline silicon deposition step in which trichlorosilane is reduced using the generated hydrogen gas to deposit polycrystalline silicon;
A method for producing polycrystalline silicon is provided.

  According to the present invention, there is also provided a method for producing hydrogen gas used as a reducing agent for producing polycrystalline silicon including the hydrogen generation step and the purification step.

In the method for producing hydrogen gas used as the reducing agent for producing polycrystalline silicon of the present invention described above,
(1) supplying water to the mist filter and passing the hydrogen gas through the filter while keeping the filter in a wet state;
(2) In the purification step, after passing the hydrogen gas through the mist filter, removing moisture and oxygen contained in the hydrogen gas;
(3) The mist filter has a performance capable of removing 100% of particles having a size of 2 μm or more and removing 99% or more of particles having a size of 1 μm,
(4) Use a polyolefin mesh or felt as the mist filter,
Is preferred.

  In a conventionally known production method, hydrogen gas obtained by a hydrocarbon gas decomposition method, a cryogenic hydrogen purification method, or the like is used as a reducing agent, all of which are derived from fossil resources. For this reason, such hydrogen gas contains carbon as an impurity, though very little, and when such a hydrogen gas is used as a reducing agent to produce polycrystalline silicon, the resulting polycrystalline silicon gradually contains carbon. Will accumulate, leading to a decrease in its purity. In particular, carbon is conductive, and adversely affects the characteristics of semiconductor devices and the like even if the content in the polycrystalline silicon is very small.

  However, in the present invention, hydrogen gas obtained by electrolysis of water, which has not been studied at all in the past (hereinafter sometimes referred to as electrolytic hydrogen gas), is subjected to a certain purification treatment, and then the reducing agent. As shown in Examples to be described later, the high-purity polycrystalline silicon having a purity of 11N or more (impurity concentration of 1 ppt or less) was successfully produced.

  That is, the electrolytic hydrogen gas has a problem that metal hydroxides such as NaOH generated as a by-product during water electrolysis are mixed as impurities. When polycrystalline silicon is produced using such a hydrogen gas containing a metal hydroxide as a reducing agent, fine powder is generated with the metal hydroxide as a nucleus in the reaction vessel or is taken into the precipitated polycrystalline silicon. As a result, the purity of the obtained polycrystalline silicon is greatly reduced. Therefore, conventionally, the use of electrolytic hydrogen gas as a reducing agent has not been studied at all.

  However, since the electrolytic hydrogen gas does not use fossil resources as a raw material, it does not contain the carbon described above as an impurity. In the present invention, such an electrolytic hydrogen gas is used as a reducing agent after washing with water and purification through a mist filter, thereby effectively avoiding the problem of mixing of metal components and at the same time not containing carbon. By taking full advantage of the advantages, it became possible to stably produce high-purity polycrystalline silicon.

The figure for demonstrating the principle of the electrolysis of the water by an ion exchange membrane method.

In the method for producing polycrystalline silicon according to the present invention, hydrogen gas is used as a reducing agent, and trichlorosilane is reduced to deposit desired high-purity polycrystalline silicon.
<Manufacture of reducing agent>
The hydrogen gas used as the reducing agent is obtained through a step of generating hydrogen by electrolysis of water and a purification step of generating the hydrogen gas obtained in this step.

1. Hydrogen generation step;
Hydrogen is generated by electrolysis of water by passing an electric current through an aqueous electrolyte solution containing an inorganic acid metal salt and / or metal hydroxide as an electrolyte (ie, an aqueous solution containing an inorganic acid metal salt and / or metal hydroxide as a solute). At least it is done by electrolyzing water.

  The inorganic acid metal salt is not particularly limited as long as it is a compound that is soluble in water and ionizes in water, but alkali metals such as sodium chloride and potassium chloride are highly useful as a by-product compound. The hydrogen chloride salt (chloride) is preferred, and sodium chloride (salt) is most preferred. In the case of an alkali metal salt, there is an advantage that there are few problems such as generation of scale.

  Here, the reason for limiting to a metal salt of an inorganic acid is that when an organic acid salt such as an acetate is used, there is a high possibility that a carbon component is mixed in hydrogen gas. The same applies to organic base salts such as tetraalkylammonium salts, and therefore metal salts are used.

  Moreover, it can replace with an inorganic acid metal salt and a metal hydroxide can also be used as electrolyte. The hydroxide is preferably an alkali metal hydroxide, and specific examples include sodium hydroxide and potassium hydroxide.

  The method of electrolysis is not particularly limited, and a known electrolysis method can be appropriately employed, but electrolysis by an ion exchange membrane method is preferable.

  In the electrolysis of water by this ion exchange membrane method, referring to FIG. 1 for explaining the principle, an anode and a cathode are arranged opposite to each other with a cation exchange membrane in between, and the anode is included. An electrolyte aqueous solution (for example, a generated NaCl aqueous solution) is supplied to the anode chamber, and pure water is supplied to the cathode chamber including the cathode.

  In this state, a voltage is applied between the anode and the cathode to conduct electricity, whereby water is electrolyzed to generate hydrogen.

  In other words, an NaCl aqueous solution will be described as an example. In the anode chamber, chlorine gas is generated by the following electrode reaction.

2NaCl → 2Na ⁺ + 2Cl ⁻
2Cl ⁻ → Cl ² + 2e
On the other hand, in the cathode chamber, Na + generated in the anode chamber migrates through the cation exchange membrane. As a result, hydrogen gas is generated and NaOH is generated at the same time by the electrode reaction described below.

2H ₂ O + 2e → H ₂ + 2OH ⁻
2Na + 2OH ⁻ → 2NaOH
That is, this electrolysis electrode reaction is generally expressed by the following equation.

_{2NaCl + 2H 2 O → 2NaOH +} Cl 2 + H 2
Thus, in the electrolysis of water by the ion exchange membrane method, generation of oxygen in the cathode chamber can be suppressed, and hydrogen gas can be efficiently generated and recovered.

  In the present invention, electrolysis using a so-called zero gap electrolytic cell is preferable among the electrolysis by the ion exchange membrane method performed as described above. That is, this method is such that the electrolysis is performed in a state where both the anode and the cathode are provided in close contact with the ion exchange membrane, and is highly energy efficient and extremely industrially advantageous.

2. Hydrogen gas purification process;
The purity of the hydrogen gas obtained as described above is generally about 99% and contains metal impurities (most of which are metal hydroxides such as NaOH). For this reason, in the present invention, it is necessary to purify the hydrogen gas by washing with water and further passing through a mist filter to remove the metal impurities.

  The method for washing hydrogen gas with water is not particularly limited, and for example, a known method such as a bubbling method, a packed tower method, or a shower method can be employed.

  In the bubbling method, water is washed by blowing hydrogen gas into water stretched on a tank or the like and bubbling.

  The packed tower system uses a packed tower having a coarse packed bed, and water is dropped from the upper part of the packed tower, and at the same time, hydrogen gas is introduced from the lower part, And hydrogen gas are counter-contacted to wash the hydrogen gas with water. The filler is preferably made of a material that does not contaminate hydrogen gas, and is preferably made of polypropylene or vinyl chloride. The type, size, surface area, and porosity of the packing are not particularly specified, and may be those generally used in a gas / liquid countercurrent contact system. For example, it is commercially available from Tsukishima Environmental Engineering Co., Ltd. under the trade name “Terraret”, and in particular, a resin filler made of heat-resistant hard vinyl chloride resin (HTPVC) is preferably used.

  Furthermore, the shower system is a method in which hydrogen gas passes through water sprayed in a shower shape.

  In particular, the bubbling method and the packed tower method are preferable from the viewpoint of performing water washing effectively, and the packed tower method is most preferable in consideration of the efficiency of the cleaning, the installation area of the equipment, the versatility of the equipment, and the like.

  In addition, the water washing as described above can be performed in multiple stages. For example, it is more effective to perform metal washing in two stages of washing with a bubbling method after washing with a packed tower method. It is preferable in that it can be removed. In other words, the hydrogen gas washed with water in the packed tower system contains a small amount of water mist, and the water mist contains metal impurities. For this reason, it is preferable to perform the water washing by bubbling after the water washing by the packed tower method because the metal impurities in the water mist can be removed.

  Although hydrogen gas is washed as described above, such water washing has a limit in removing metal impurities, and is insufficient as a reducing agent for obtaining polycrystalline silicon having a purity of 11N or more, for example. That is, most of the metal impurities can be removed, but according to the study by the present inventors, it has been found that trace amounts of metal impurities such as NaOH remain in the hydrogen gas in a mist form.

  For this reason, in the present invention, the hydrogen gas after washing is passed through a mist filter, and the remaining trace amount of metal impurities is removed by filtration.

  Such a mist filter has a mesh or felt shape, and by passing through it, particulate impurities can be removed from the hydrogen gas. Although the performance varies depending on the size of the holes formed in the mesh or felt, in the present invention, for example, it is preferable to remove particles of 2 μm or more by 100%, particles of 1 μm by 99% or more, and particles of 1 μm or more by More preferable are those capable of removing 95% or more of 100% and 0.5 μm particles, and particularly preferable are those capable of removing 100% of 1 μm or more particles and 98% or more of 0.5 μm particles.

  The filter material is preferably made of polyolefin in that it is unlikely to become a gas contamination source, and polypropylene felt is most preferred in view of durability and removal efficiency.

  Such a polypropylene felt mist filter is commercially available, for example, under the name of a candle filter from Matsui Machine.

  The above-described mist filter improves the filtration and removal efficiency of fine particles by using it in a wet state rather than using it in a dry state.

  Therefore, in the present invention, water is constantly supplied to the surface of the mist filter held in the housing by a spray method or the like to hold the mist filter in a wet state, and hydrogen gas is supplied into the housing in this state. It is preferable to pass through a mist filter.

  The hydrogen gas that has passed through the mist filter as described above does not substantially contain metal impurities, and can be used for the reaction with trichlorosilane as a reducing agent. However, this hydrogen gas may contain gaseous impurities such as oxygen and water vapor, and these gaseous impurities may react with trichlorosilane. Therefore, it is preferable to remove such gaseous impurities prior to use as a reducing agent. For example, the oxygen concentration is desirably reduced to 4.0 ppm or less, preferably 1.0 ppm or less, more preferably 0.8 ppm or less, and water vapor (water) has a dew point of −50 ° C. or less, preferably −60 ° C. In the following, it is more desirable to decrease until it becomes −70 ° C. or less.

  As a method for removing oxygen and moisture, a known method known for obtaining industrial hydrogen can be employed.

  For example, moisture can be removed by condensation using deep cooling or adsorption using a desiccant (silica gel, zeolite (molecular sieves), activated alumina, etc.). When using a desiccant, it is necessary to pay attention so that these fine powders of the desiccant do not enter the hydrogen gas. For this reason, it is desirable to pass the hydrogen gas after moisture removal through a fine powder removing filter, if necessary. This fine powder removing filter has a performance capable of removing fine particles derived from a desiccant or the like. For example, a resin impregnated glass fiber ZH13 type of trade name “Ultipore GF Plus” manufactured by Pall Japan is suitable for this. Yes.

  Further, oxygen can be removed by converting it into water by reaction with hydrogen using a catalyst such as palladium and removing the water by the above method.

  The hydrogen gas obtained through the purification step described above has a purity of, for example, 99.99 vol% or more, and by using such a high purity hydrogen gas as a reducing agent for the reduction reaction of trichlorosilane described below, Polycrystalline silicon having a purity of 11N or higher can be obtained.

3. Polycrystalline silicon deposition process;
The polycrystalline silicon is deposited by using the high purity hydrogen gas obtained above and reducing trichlorosilane at a high temperature using the high purity hydrogen gas as a reducing agent.

  Trichlorosilane used in this reaction is produced by a known method.

  For example, trichlorosilane can be produced by the following formula by reacting metal silicon having a purity of 98% or more with hydrogen chloride in a fluidized bed using an iron or Al-containing catalyst.

Si + 3HCl → SiHCl ₃ + H ₂
Further, in the above reaction, tetrachlorosilane is produced as a by-product. By using such tetrachlorosilane and reacting with hydrogen gas in a fluidized bed in the presence of a copper silicide catalyst together with the above metal silicon, the following formula is obtained. Thus, trichlorosilane can also be produced.

3SiCl ₄ + 2H ₂ + Si → 4SiHCl ₃
As the hydrogen gas used above, high-purity hydrogen gas obtained by the above-described method can be used, or hydrogen gas obtained from a fossil resource by a known method can be used.

  The trichlorosilane obtained as described above removes impurities such as P and B by distillation, and is subjected to a reaction with hydrogen gas, for example, as high-purity trichlorosilane having a purity of 99.9% or more.

  The above reaction (reduction reaction) between the high purity trichlorosilane and the high purity hydrogen gas is as described above. According to the following formula at a high temperature of 1000 ° C. or higher, the reduction of trichlorosilane occurs along with the thermal decomposition of trichlorosilane. Crystalline silicon is deposited.

4SiHCl ₃ → Si + 3SiCl ₄ + 2H ₂
SiHCl ₃ + H ₂ → Si + 3HCl
A method for producing polycrystalline silicon by the reaction as described above is well known as, for example, the Siemens method, but as an example, it is performed as follows.

  That is, a core wire made of high-purity silicon is mounted in a bell jar and heated by a heater, and when heated to a predetermined temperature or higher, current is passed through the silicon core wire to generate heat, and the surface temperature of the silicon core wire is set to 900. The high purity trichlorosilane and the high purity hydrogen gas are introduced into the bell jar in a predetermined ratio at a high temperature of about 1 ° C. to 1100 ° C. Thereby, polycrystalline silicon in which trichlorosilane is reduced is deposited on the core wire, and polycrystalline silicon is obtained as a rod-shaped material.

  Thus, according to the present invention, high-purity polycrystalline silicon having a purity of 11N or more can be stably produced.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to these Examples.

In the following examples, analysis of the obtained metal silicon and analysis of hydrogen gas and trichlorosilane were performed by the following methods.
Analysis of metallic silicon;
Metallic silicon was dissolved in hydrofluoric acid, evaporated to dryness, an analytical solution dissolved in a nitric acid solution was prepared, and the measurement was performed using an inductively coupled plasma mass spectrometer (ICP-MS).
Analysis of hydrogen gas;
Hydrogen gas was analyzed by a thermal conductivity detector type gas chromatograph (GG-TCD).
Analysis of trichlorosilane;
After liquid trichlorosilane was evaporated to dryness, an analysis solution dissolved in a nitric acid solution was prepared, and an inductively coupled plasma mass spectrometer (ICP-MS) was used.

<Example 1>
Electrolysis of water;
Using the same zero gap electrolytic cell as disclosed in JP 2001-262387 A, continuous operation at a current density of 40 A / dm ² , an electrolysis temperature of 85 ° C., a caustic soda concentration of 32 wt%, and a brine concentration of 195 g / l. To generate hydrogen.

Filtration with a mist filter;
The generated hydrogen gas was introduced into the tower where the packing was installed from the bottom of the tower, water was sprayed from the top of the tower in a shower-like manner, and water washing was carried out by countercurrent contact between the gas and water in the packing. Further, bubbling was passed through the tank filled with water, and the hydrogen gas was sufficiently washed with water.

  The hydrogen gas washed with water as described above was passed through a polypropylene felt mist filter (Matsui Machine "BECOFIL" candle filter) to remove sodium hydroxide fine particles.

  This mist filter has a 1 μm particle removal rate of 100% and a 0.5 μm particle removal rate of 98% or more.

Deoxygenation;
The hydrogen gas that passed through the mist filter was introduced into a column packed with a palladium catalyst for removing oxygen to perform deoxygenation.

  In order to increase the reaction efficiency, the temperature of hydrogen gas was raised to about 85 ° C. and then introduced into the tower.

Moisture removal;
Most of the moisture contained in the deoxygenated hydrogen was removed by two-stage cooling, and passed through an adsorption tower packed with alumina and molecular sieve as the final moisture removal to a dew point of −70 ° C. or lower.

  The purity of the purified hydrogen gas after the above steps was 99.99% or higher.

Polysilicon production (polysilicon deposition);
Using the high-purity hydrogen gas obtained above and trichlorosilane having a purity of 11N produced separately, polycrystalline silicon was deposited by the Siemens method. The obtained polycrystalline silicon had an extremely high purity with a purity of 99.999999999% (11N) or more.

<Comparative Example 1>
Purified hydrogen gas was obtained in the same manner as in Example 1 except that the mist filter was not passed. The concentration of Na in the purified hydrogen gas was 0.5 mg / Nm ³ , and it was judged that this was not suitable for obtaining polycrystalline silicon having a purity of 99.999999999% (11N) or higher by the Siemens method.

Claims (10)

A step of electrolyzing an aqueous solution containing an inorganic acid metal salt and / or a metal hydroxide as a solute to generate hydrogen;
A purification step in which the hydrogen gas obtained in the above step is washed with water and then passed through a mist filter;
A polycrystalline silicon deposition step in which trichlorosilane is reduced using the generated hydrogen gas to deposit polycrystalline silicon;
A method for producing polycrystalline silicon comprising:   The method for producing polycrystalline silicon according to claim 1, wherein the hydrogen gas is passed through the filter while supplying water to the mist filter to keep the filter in a wet state.   3. The method for producing polycrystalline silicon according to claim 1, wherein in the purification step, after the hydrogen gas is passed through the mist filter, moisture and oxygen contained in the hydrogen gas are removed.   4. The polycrystalline silicon according to claim 1, wherein the mist filter has a performance capable of removing 100% of particles having a size of 2 μm or more and removing 99% or more of particles having a size of 1 μm. Manufacturing method.   The method for producing polycrystalline silicon according to claim 4, wherein a polyolefin mesh or felt is used as the mist filter. A step of electrolyzing an aqueous solution containing an inorganic acid metal salt and / or a metal hydroxide as a solute to generate hydrogen;
A purification step in which the hydrogen gas obtained in the above step is washed with water and then passed through a mist filter;
A method for producing hydrogen gas used as a reducing agent for producing polycrystalline silicon containing   The method for producing hydrogen gas according to claim 6, wherein the hydrogen gas is passed through the filter while supplying water to the mist filter to keep the filter in a wet state.   The method for producing hydrogen gas according to claim 6 or 7, wherein, in the purification step, after passing the hydrogen gas through the mist filter, moisture and oxygen contained in the hydrogen gas are removed.   9. The hydrogen gas according to claim 6, wherein the mist filter has a performance capable of removing 100% of particles having a size of 2 μm or more and removing 99% or more of particles having a size of 1 μm. Production method.   The method for producing hydrogen gas according to claim 9, wherein a polyolefin mesh or felt is used as the mist filter.

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