JPH0848512A - Polycrystalline silicon particles - Google Patents

Abstract

(57) [Summary] [Purpose] In each step of the production of granular polycrystalline silicon,
Impurity is reduced to obtain high-purity granular polycrystalline silicon. [Constitution] There is a concentration distribution of the iron element from the surface of the particle toward the inside, and there is a maximum value inside the particle in the concentration distribution of the iron element, and the maximum value is silicon in order from the surface of the particle to 3.3. Polycrystalline silicon particles having an iron element concentration of less than 10 ppba when etched by 10 ^-5 g each, and having an iron element concentration of 3 ppba or less as a whole. After crushing silicon, washing and etching are performed in this order with aqua regia-water-hydrofluoric acid to obtain high-purity crushed silicon particles, and new silicon is deposited around the silicon particles by thermal decomposition or reduction of silanes using these as nuclei. Manufactured by.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to polycrystalline silicon particles having a very low iron element content and a method for producing the same.

[0002]

2. Description of the Related Art The Czochralski method (hereinafter simply referred to as the CZ method) is generally employed for the production of single crystal silicon. In the CZ method, a single crystal is pulled by adding a dopant such as P or B into the crucible. However, due to segregation, these dopants change the concentration in the crystal at the beginning and the end of pulling of the single crystal, and as a result, there is a problem that it is difficult to obtain a single crystal product having an in-specification resistance value at a high yield. As a means for solving this problem, a continuous CZ method (hereinafter,
This is called the CCZ method. ) Has been proposed and much research has been done. The CCZ method is a method of continuously supplying a raw material and a dopant impurity while controlling each in a pulling crucible by the CZ method to continuously produce a long single crystal having a uniform dopant concentration.

Granular polycrystalline silicon (hereinafter also referred to as granular polysilicon) is most suitable for continuous supply because of its good fluidity, and is considered to be an essential polycrystalline silicon raw material in the CCZ method. . However,
In the CCZ method, the concentration of heavy metal as an impurity in the melt increases due to segregation as the pulling length of the single crystal becomes longer. As a result, the concentration of heavy metal in the single crystal increases as the pulling end of the single crystal silicon approaches. To do. Therefore CC
As the polycrystalline silicon for the Z method, it is strongly desired that the polycrystalline silicon is granular and has a lower concentration of heavy metals than the polycrystalline silicon produced by the Siemens method which is generally used at present. Among heavy metals, iron element is said to have a particularly bad influence on single crystal silicon, and the heavy metal impurity taken in most due to pollution from the surrounding environment or equipment is iron element. Therefore, the concentration of iron element is generally adopted as an index of heavy metal pollution.

FIG. 1 shows the relationship between the iron concentration in the raw material polycrystalline silicon and the iron concentration in the single crystal produced by the CCZ method. Curve 1 in the figure shows the concentration of the iron element in the single crystal when the single crystal was manufactured by the CCZ method using polycrystalline silicon containing the iron element of 3 ppba. Curve 2 shows the concentration of the iron element in the single crystal when the single crystal was manufactured using polycrystalline silicon containing the iron element of 0.3 ppba.

In single crystal silicon used for semiconductor integrated circuit devices, in order to suppress the occurrence of oxidation-induced stacking faults on the substrate surface due to thermal oxidation of the single crystal substrate, the heavy metal impurity concentration is kept low below a specific value. It is said that it is necessary. For example, in JP-A-3-80193, the Cu, Fe, Ni, and Cr concentrations are each 0.1 p.
It is said that the occurrence of the oxidation-induced stacking fault can be suppressed by being less than or equal to pta. The most abundant element of these elements is generally iron.

The iron element concentration in single crystal silicon is set to 0.1 p.
It can be seen from FIG. 1 that the iron element concentration of polycrystalline silicon allowed to be less than or equal to pta must be less than or equal to 3 ppba when a 2 m long single crystal is manufactured by the CCZ method.

On the other hand, in the case of massive polycrystalline silicon generally used in the CZ method, it is said that the lower the concentration of iron element, the better the product. For example, in the bulk silicon currently supplied to the market, the product regarded as a high grade has its iron element concentration kept at 0.5 ppba or less. CZ using this massive silicon as a raw material
The concentration of iron element in the single crystal silicon by the method is about 0.01
It is ppta. Therefore, in order to manufacture a long single crystal having the same purity as the single crystal by the CZ method by the CCZ method,
The concentration of iron element in the raw material polycrystalline silicon is
It is required to be 0.3 ppba or less.

Several attempts have been made to improve the purity of granular polysilicon. Regarding precipitation, JP-A-2-1678
No. 11 discloses a device on the surface of the container, that is, a method of forming a reaction container from graphite and coating the inner surface with high-purity silicon. Further, regarding the production of high-purity silicon seed particles serving as a nucleus for precipitation, for example, JP-A-64-5903 proposes a method of colliding silicon particles with a rotating target made of silicon to obtain crushed particles, Further, Japanese Patent Laid-Open No. 63-8045 proposes a method in which a silicon lump is crushed by a roll crusher made of a high-purity silicon rod and classified by sieving.

As a method for obtaining high-purity crushed silicon particles, the crushing environment is completely cleaned as described above, and a crushing apparatus in which all of the contacting particles are made of high-purity silicon is used, as well as a normal environment and a normal method. After crushing with the crushing device, the method of washing the crushed silicon as a post-treatment is considered. The former is simple in process and looks like a simple method at first glance, but when an actual device is made and operated, some probability of contamination is very high. For example, when colliding silicon particles with a rotating target, it is necessary to accelerate the particles at high speed, which causes severe wear, and the motor that rotates the target releases metal, mainly iron, etc. Contamination occurs. Also, in the roll crusher made of silicon, the roll axis,
Friction from bearings and motors releases metal, mainly iron, which causes pollution. In order to prevent this, it is actually impossible to make the crushing device entirely from silicon,
In addition, since the hardness of a metal material such as a bearing is extremely smaller than that of silicon, it is difficult to avoid the risk of being easily worn by contact with particles or fine powder and generating a large amount of iron-based metal. Further, in the case of this method, there are many problems other than the crushing device. For example, there are many unsolved problems, such as the development of a crushed particle recovery device that does not use metal materials at all, or the development of a non-polluting dust collection ventilation system for maintaining the crushing environment at high purity.

On the other hand, the latter is theoretically reliable as a method for obtaining particles of higher purity than the former, but it is disclosed in JP-A-63
As disclosed in Japanese Patent Publication No.-8045, conventionally, a method for removing a metal component mainly containing iron from the surface of fine silicon particles to highly purify it has not been known.

[0011]

A method for producing granular polycrystalline silicon is already known. Although that technology is an excellent technology that has been industrially implemented, according to our analysis, all of the conventional products currently on the market have an iron element concentration of 3 pp.
It exceeds ba. For this reason, these are used alone or mixed with high-purity silicon produced by the Siemens method and applied to the ordinary CZ method to be single-crystallized, and the usage of the product is very limited. That is, there is a problem that it cannot be used as a raw material for an integrated circuit, which is the original use of granular polycrystalline silicon. Therefore, for the above reason, there has been a strong demand for development of polycrystalline silicon particles having an iron element concentration of 3 ppba or less, preferably 0.3 ppba or less.

Under these circumstances, the present inventors have conducted various studies on the problems of the conventional product, and as a result, have found that the problem is particularly in the manufacturing process of the seed particles which become the nucleus of precipitation. That is, it has been very difficult to reduce the iron element once attached to the surface of the fine silicon particles by the conventional techniques. Therefore, in order to obtain high-purity silicon particles having a diameter of 1 mm or less, a crushing and sieving method in a non-contaminated state has been adopted. However, in our analysis, near the center of the granular polysilicon products currently on the market,
That is, around the precipitation nucleus, 400 ppba relative to the weight of the nucleus.
It has been confirmed that the above-mentioned contamination by the iron element exists. For these reasons, as an elemental technology for highly purifying granular polysilicon, the development of a contamination prevention technology during precipitation and the development of a method for highly purifying crushed silicon particles by washing are the objectives of high purity granularity. It was strongly desired as a means of obtaining polysilicon.

[0013]

DISCLOSURE OF THE INVENTION The inventors of the present invention have conducted extensive studies as to reduce the iron element in impurities contained in granular polysilicon, and as a result, have cleaned the surface of seed silicon, which is a nucleus of precipitation. In doing so, it is possible to obtain extremely high-purity seed silicon by performing a two-step cleaning process of first forming an oxide film and then removing it, and then performing a precipitation reaction using this seed. It was found that the region where the concentration of iron element is very high can be eliminated by the inside of the grain, and the concentration of iron element in the granular polysilicon as the final product can be 3 ppba or less, and the present invention has been completed and is proposed here. Came to.

That is, in the present invention, the concentration distribution of the iron element exists from the surface of the particle toward the inside, the maximum value of the iron element concentration exists inside the particle, and the maximum value of the iron element concentration is the surface of the particle. Is a polycrystalline silicon particle having a value of less than 10 ppba when the silicon is etched by 3.3 × 10 ⁻⁵ g and the iron element concentration of the entire particle is 3 ppba or less.

The size of the polycrystalline silicon particles of the present invention is not particularly limited, but when used in the CCZ method, the average diameter is 0.5 to 0.5 due to reasons such as fluidity and easy handling.
The range of 5 mm is preferable.

Such polycrystalline silicon particles are usually
It is produced by introducing seed silicon as a nucleus into a fluidized bed reactor and depositing polycrystalline silicon by thermal decomposition or reduction of silanes on the seed silicon. in this case,
The newly-deposited polycrystalline silicon has seed silicon as nuclei and grows grains in substantially concentric circles around it. To form the deposited layer, the temperature of the seed silicon is set to the decomposition temperature of the silanes,
Alternatively, the silanes are decomposed or reduced by contacting silanes accompanied by a reducing gas or an inert gas on the surface of the precipitation nuclei while keeping the temperature above the reducing temperature. For the decomposition reaction of silanes, usually monosilane gas diluted with hydrogen gas or argon gas is preferably used. Further, dichlorosilane, and / or a mixed gas of trichlorosilane and hydrogen is preferably adopted for the reduction reaction of silanes. However, monochlorosilane or tetrachlorosilane is used by mixing with hydrogen, and further, the dichlorosilane or It is also possible to use it as a mixture with trichlorosilane. Since the polycrystalline silicon deposition layer formed around the seed silicon in the fluidized bed reactor grows almost concentrically around the seed silicon, it is often spherical in shape.

The seed silicon, which serves as a nucleus for precipitation, must have an extremely high purity. However, when seed silicon is obtained by crushing large silicon, its surface is always contaminated with iron element. When seeded silicon on the surface is used as a nucleus and new polycrystalline silicon is deposited around it, the portion that was the surface of the nucleus before the deposition,
That is, a large amount of iron element locally exists at the boundary surface between the nucleus and the deposited layer.

When such polycrystalline silicon particles are etched and analyzed step by step from the outside as if peeling the onion skin one by one, when it is approached to a portion where a large amount of iron element is locally present. , The analysis value of iron increases momentarily. More specifically, when analyzing particles where the surface of the nucleus is contaminated especially with the iron element compared to the inside of the nucleus and the deposition layer, it is possible to analyze the particles gradually by peeling from the outside. The analysis value of iron is low up to the boundary surface of the layer, the concentration of iron is high in the part including the boundary surface, and the analysis value of iron decreases again when the boundary surface is further analyzed. Therefore, such a polycrystalline silicon particle has a concentration distribution of iron element from the surface of the particle toward the inside, and when the surface of the nucleus is contaminated with iron, the concentration distribution of iron has a maximum value inside the particle. To have.

For example, when the commercially available polycrystalline silicon particles are analyzed by the above method, the graph shown in FIG. 2 is obtained. FIG. 2 is a graph showing the concentration distribution of iron concentration in the radial direction of polycrystalline silicon particles. The horizontal axis of the figure represents the stage of analysis. The rightmost is the surface of the polycrystalline silicon particles, and the leftmost is the center. The vertical axis of the figure shows the iron concentration at each stage. The concentration is represented by the atomic ratio of iron and silicon, and the unit is ppba. Peak 1 on the far right in the figure represents a high concentration region of iron due to surface contamination of polycrystalline silicon particles. Also, peak 2 on the left represents that the surface of the seed is contaminated with iron.

The concentration distribution referred to in the present invention means that when the polycrystalline silicon particles are analyzed stepwise, different analysis values are obtained at each step. In this case, measuring the concentration distribution of each polycrystalline silicon particle causes a large measurement error. Therefore, it is more accurate to analyze a large number of particles collectively and take the average value thereof. Therefore, in the present invention, as the concentration of iron in silicon, the analysis value of a plurality of, for example, 100 silicon particles is calculated as the particle 1
The value converted as the average value per piece is adopted.

For stepwise analysis of particles one by one (hereinafter referred to as one step of analysis), an etching solution containing a mixed acid of hydrofluoric acid and nitric acid can be used. The weight of silicon to be etched by the etching solution is determined by the amount of the etching solution. Therefore, the amount of silicon to be etched can be controlled by controlling the amount of etching liquid. Further, silicon and iron elements are dissolved in the solution after etching, and by heating this solution at a temperature of 100 ° C. or lower, only hydrofluoric acid, nitric acid, and silicon elements are evaporated,
The elemental iron in the solution can be concentrated. An ICP mass spectrometer with high sensitivity is preferably used for measuring the iron concentration in the concentrated liquid.

When analyzed by this method, the detection limit of the iron element in the measurement is 0.01 ng / ml in the state of the concentrated solution, that is, the measurement solution. That is, it represents a state in which 0.01 ng of iron atoms are present in 1 ml of the solution. Therefore, the confirmation as to whether or not there is a concentration distribution of the iron element from the surface to the inside of the particles is judged by whether or not the concentration change of the concentrated solution at each step of the analysis is larger than 0.01 ng / ml.

Further, the above-mentioned etching solution has the property of etching the surfaces of all the silicon particles in the solution with a substantially uniform thickness. Therefore, the average value per silicon particle obtained by simultaneously etching a large number of particles and analyzing the etching solution is close to the actual analysis value of one particle. In this case, if the diameter of the particle is different, the thickness of one step of the analysis is also different. Therefore, it is preferable that the particles are sieved before the analysis so that the particles have substantially the same particle size.

In the present invention, when obtaining the maximum value inside the particle in the concentration distribution of iron element, it is necessary to define the amount of silicon in one step of analysis. In the present invention, the amount of silicon corresponding to one step of analysis is 3.3 × 10 ⁻⁵ g on average per particle. This amount corresponds to 7 × 10 ¹⁷ atoms of silicon. For example, if the number of silicon particles to be measured is 10,000, the weight for one step will be 0.33 g. This weight is 4.5μ from the surface for spherical particles with a diameter of 1mm.
It is the weight of silicon for m minutes. Further, if the particles are spherical particles having a diameter of 0.3 mm, the weight of all the particles is obtained.

The fact that the concentration of iron element is 10 ppba means that the atomic ratio of silicon to iron is 10 ppb. Therefore, in one step of the analysis, 7 ×
This means that 7 × 10 ⁹ iron atoms are present in 10 17 silicon atoms. However, one step of the analysis is 3.3 × 10 ^-5 g in order to judge whether the concentration distribution of the iron element exists from the surface of the particle toward the inside.
The number of atoms in one step can be made smaller than the above value and determination can be made at a finer step interval.

The maximum value inside the particle in the concentration distribution of the iron element referred to in the present invention is due to the contamination of the surface of the nucleus,
For this reason, it exists inside the particles. By the way, one step of the analysis in the present invention is 3.3 × on average for each particle.
Since the amount is 10 ⁻⁵ g atom, the total amount of particles having a diameter of 0.3 mm or less is one stage. Here, if the diameter of the nucleus is 0.2 mm and the surface is contaminated, the maximum value inside the particle in the concentration distribution of the iron element is 0.2 m in diameter.
Appears at the position of m. However, the analysis method of the present invention is the last one.
Since the step is 0.3 mm, the value of iron in the last step of the analysis is only increased, and it is not the maximum value generally called. However, since the maximum value actually exists around the nucleus, the maximum value in the present invention is not limited to the case where the high-concentration region of iron exists in the course of the stepwise analysis, and the final step, That is, in the analysis of the innermost region including the center, the case where the iron concentration is higher than the other regions is also called the maximum value.

The iron element referred to in the present invention means iron which is considered to act as an impurity in semiconductor grade silicon regardless of its state, which may be in a metallic state or an atomic state. As a factor for invading iron, crushing of seed silicon, which is a nucleus of precipitation, and a sieving process are considered. Iron that invades in this process is taken in by abrasion between silicon particles and a metal containing iron, so it exists as metallic iron in the state of seed silicon before precipitation, and once it precipitates with that seed as a nucleus, Since a part of metallic iron is diffused, it becomes an atomic state in silicon by diffusion.

The distribution of iron element in the polycrystalline silicon particles is
It consists of three parts: the nucleus, the deposited layer, and the grain surface. In the polycrystalline silicon particles of the present invention, the concentration of iron element in the above three parts is 3 ppba or less. That is, the ratio of the number of atoms obtained by dividing the total number of iron element atoms in the above three parts by the number of silicon atoms in the entire polycrystalline silicon particles is 3 ppb or less. This value should be as small as possible in the case of producing semiconductor grade polycrystalline silicon particles.
It is possible to obtain polycrystalline silicon particles having an extremely high purity of ppba or less, further 0.3 ppba or less.

Further, the polycrystalline silicon particles of the present invention are not limited to the above-mentioned iron element concentration, but also the elements which are problematic in use as a semiconductor raw material, such as P, B, Al, As and N.
i, Cr, Cu, Mn, and Ti are also 3 ppba or less, preferably 1 ppb each.
a or less, and more preferably 0.3 ppb each
It can be a or less.

The polycrystalline silicon particles of the present invention are composed of a central portion and a peripheral portion having a texture different from that of the central portion. This is because the seed silicon and the polycrystalline silicon deposited around the seed silicon have different structures. For example, when crushing silicon and depositing new silicon around the crushed particles as a nucleus, or when interrupting the deposition and depositing new silicon in a different reactor with the particles as a nucleus. In addition, due to the difference in the manufacturing process between the central portion and the peripheral portion, a difference in structure is observed. An example of the concrete method is shown.

When monosilane is used as the raw material gas for the deposition, the monosilane is decomposed in the gas phase to form micronuclei, so that a large amount of fine powder is produced in the reactor. The fine powder floating in the reaction gas is taken into the deposition layer during the deposition process of granular silicon. This precipitation mechanism is called scavenging. The particles precipitated by scavenging have some characteristics. The precipitated granular polysilicon is halved from the center, the cross section is polished, and the cross section is etched with a mixed acid consisting of hydrofluoric acid and nitric acid. Numerous holes are noted, indicating after the presence of fines. Further, it is recognized that the holes are arranged side by side on a plane perpendicular to the precipitation direction. That is, it is a proof that scavenging progresses concentrically. In addition, some grooves may be recognized on the etched surface in the direction along the deposition. Also, polycrystalline silicon deposited by scavenging has poor crystallinity,
The crystallinity by X-ray diffraction is usually 40 to 70%. This crystallinity improves as the amount of scavenging is reduced, that is, as it approaches a normal CVD reaction (chemical vapor deposition). This is achieved by reducing the concentration of monosilane in the reaction gas. On the other hand, the monosilane concentration is lowered and CVD
When the reaction is preferentially deposited, or when trichlorosilane is used as the source gas for deposition and the deposition is performed at a higher temperature than the deposition with monosilane, scavenging hardly occurs. Therefore, when the same etching as described above is performed, the above-mentioned holes or grooves are not observed, but the shape of the crystal grains is observed.

As described above, if the gas component or concentration during deposition is different, the central portion and the peripheral portion can be easily distinguished by etching. However, when the deposition conditions are exactly the same, for example, the polycrystalline silicon particles obtained by the precipitation are crushed, and the crushed silicon particles are charged as the nuclei into the same reactor again, and when the precipitation is performed, the central part Includes a crushing step, so that it is not easy to identify the tissue by etching, unlike the above example, even though the manufacturing steps of the central portion and the peripheral portion are different.
In this case, the most accurate identification can be made by observing the connection of the deposited layers at the time of observation after etching. For example, the precipitated particles of trichlorosilane can be determined by the above-described crystal grain direction. During precipitation, the polycrystalline grains grow radially outward from the center. Therefore, if a crystal grows inside the polycrystalline silicon particle in a direction different from the radial direction, and if the crystal suddenly changes its direction in the radial direction, the boundary surface is the fracture surface of the particle and therefore the nucleus of the particle. You can see that it is the surface. Also, using monosilane as the source gas,
For precipitated particles that include scavenging, instead of the grains in the trichlorosilane example, use holes that are in the presence of concentric fines or grooves that extend in the growth direction as the identifying material. You can

In the present invention, the high-purity polycrystalline silicon particles may be produced by any method, but the method of highly purifying the surface of the seed silicon can be most suitably adopted. That is, crushed silicon is sequentially washed and etched with aqua regia-water-hydrofluoric acid to prepare seed silicon, and polycrystalline silicon is deposited by thermally decomposing or reducing silanes in the presence of the seed silicon. The method is preferred in the present invention. Hereinafter, a specific description will be given.

The nucleus in the present invention is a seed silicon particle which becomes a precipitation nucleus of polycrystalline silicon. Seed silicon is polycrystalline,
Any single crystal may be used, and known ones can be adopted without any limitation, and it can be obtained by crushing and sieving silicon having a particle size larger than the seed. The size of the nucleus is not particularly limited, but in view of production efficiency, it is preferable that the granular silicon has a mass several tens of times that of the nucleus. Therefore, if the grain size of the product granular polysilicon is 1 mm, the diameter of the core is 0.2 mm.
The degree is preferably adopted.

The sieved silicon particles are aqua regia-water-
Sequential washing with hydrofluoric acid is important, and only by this method can highly pure nuclei be obtained. A mixed acid of aqua regia and hydrofluoric acid cannot sufficiently remove the iron element. This is because a surface oxide film is formed during etching with mixed acid,
It is presumed that this is because it acts as a barrier and inhibits the elution of iron element.

It is considered that aqua regia has a function of eluting impurities on the silicon surface and a function of forming an oxide film on the silicon surface. Further, hydrofluoric acid has a function of removing the oxide film and in the oxide film, or It is presumed that it has a function of eluting impurities existing at the interface between the oxide film and silicon. Therefore, the chemical solution used here is not limited to aqua regia, and other acids such as nitric acid can be used as long as they are acids that dissolve the iron element and that oxidize the silicon surface. However, since nitric acid has a small oxidizing ability, it is necessary to repeat the washing step many times in order to obtain the purity of the nucleus required in the present invention. Therefore, it is preferable to use aqua regia, which has the greatest effect of removing impurities, as the acid used for the main washing.

As a method of forming an oxide film on the silicon surface, there is a method of heating silicon to form a thermal oxide film. However, in this method, the iron element diffuses from the silicon surface to the inside during heating, so that it cannot be removed by hydrofluoric acid in a later step. Therefore, it is not preferable to heat any cleaning step in the present invention, not limited to the oxide film formation.

FIG. 3 shows an estimated mechanism for removing the iron element by this cleaning method. State 1 represents the surface of silicon particles that were crushed and then washed with water. In the figure, 1 indicates a state in which metallic iron is attached to the silicon surface, and 2 indicates a state in which it is diffused in the vicinity of the surface. State 2 is a state in which the silicon particles are washed in aqua regia. The reaction of oxidizing the silicon surface by aqua regia to form the oxide film 2 and the reaction of dissolving and removing the metallic iron 1 by aqua regia simultaneously proceed, and the metallic iron adhered to the silicon surface and the inside The diffused atomic iron is collected at the interface between the silicon of 3 and the oxide film. State 3 is a state in which after removing the aqua regia component by washing with water, the oxide film is removed with hydrofluoric acid. The reaction of dissolving the silicon oxide film by hydrofluoric acid follows the following reaction formula. SiO ₂ + 6HF → H ₂ SiF ₆ + 2H ₂ O H ₂ SiF ₆ generated by the removal of the silicon oxide film is a very strong acid, so the iron element taken inside the film is dissolved by this acid. To be removed. If further improvement in purity is required, states 2 and 3 may be repeated.

The aqua regia used to wash the nuclei may be any mixture ratio of nitric acid and hydrochloric acid,
The ratio of nitric acid to hydrochloric acid is 1: 3 for strong cleaning.
A composition of ~ 3: 1 is preferably adopted. If the composition is out of the above range or if the mixed solution is diluted with water or the like, there arises a problem that the ability of the aqua regia to dissolve the iron element is reduced, or the life of the solution is reduced. The purity of nitric acid and hydrochloric acid is required to be high, but commercially available nitric acid and hydrochloric acid for the semiconductor industry can be used without problems.

Prior to the aqua regia washing, more effective washing can be performed by removing fine powder and foreign matters in the crushed particles by washing with water. After washing with aqua regia, the particles must be washed once with water. This is because when aqua regia in the previous step remains in the stage of treatment with hydrofluoric acid, a mixed acid is formed by aqua regia and hydrofluoric acid, and an etching reaction of silicon occurs. In this state, the silicon surface becomes rough due to local etching, and when a large amount of aqua regia remains, a film that cannot be completely removed by hydrofluoric acid is formed on the surface, which hinders sequential cleaning of aqua regia and hydrofluoric acid. Therefore, it is necessary to remove aqua regia as much as possible in the water washing step between the aqua regia washing and the hydrofluoric acid washing. If the above film is formed due to a mistake in the process,
By dipping in hot water at 70 ° C. or higher for several minutes, the above film can be changed to a normal oxide film and the cleaning process can be continued.

The hydrofluoric acid used for cleaning in the present invention is
Any composition may be used as long as the surface oxide film of silicon can be removed, but in order to release the oxide film removed from the surface and the iron element liberated with the removal of the oxide film to the outside of the system without adhering again to the silicon surface. In particular, the form of hydrofluoric acid is preferably an aqueous solution. Further, in order to prevent recontamination with a chemical solution, the hydrofluoric acid is preferably a grade for the semiconductor industry. The concentration of the aqueous solution of hydrofluoric acid can be adopted without any problem as long as the composition has the effect of removing the oxide film.
A 0%, preferably 3-10%, aqueous solution is employed. Since impurities are not redeposited on the surface washed with aqua regia and hydrofluoric acid, not only water used for diluting hydrofluoric acid, but all water used for main washing is preferably pure water. The pure water referred to here is preferably ion-exchanged water having an iron concentration of 1 ppba or less. Further, nuclei of higher purity can be obtained by sequentially performing aqua regia washing, water washing, hydrofluoric acid etching, and water washing, and then repeating the same steps again.

The polycrystalline silicon particles of the present invention can be obtained by using the high-purity seed silicon obtained above as a nucleus to deposit new silicon around the nucleus. For depositing new silicon around, silanes such as monosilane and trichlorosilane can be used. Among them, a raw material such as trichlorosilane that generates hydrogen chloride in the course of the reaction has a problem that the metal part of the device material is corroded by the generated hydrogen chloride or a problem that the silicon material in the high temperature part is etched. Occurs. In addition, since the starting material trichlorosilane itself is also corrosive, it corrodes the apparatus material in the steps of storing and transporting the starting material. Although it is possible to manufacture high-purity granular polysilicon by suppressing such corrosion of the device material as much as possible, a great deal of labor is required to prevent contamination due to corrosion. On the other hand, in the case of deposition with monosilane, since the decomposition reaction of monosilane is used, hydrogen chloride is not generated, and the raw material gas itself is not corrosive, so that the corrosion of the material of the device is unlikely to occur.

For the above reasons, the decomposition reaction of monosilane gas, which is unlikely to cause contamination, is preferably adopted as the precipitation method in the present invention. At this time, it is natural that the raw material monosilane has as few impurities as possible, and for example, one in which the specific resistance value of the silicon deposited by the raw material gas is 1000 Ωcm or more is suitably adopted.

Any reactor may be used for depositing the granular polysilicon, for example, a moving bed type such as a stirring tank or a rotary kiln may be used.
In order to produce high-purity polycrystalline silicon particles, it is preferable that the structure is as simple as possible and that no driving unit is present. Therefore, in the present invention, a fluidized bed type reactor is preferably adopted as the reactor, for example, a vertically installed cylindrical reactor is suitable.

In order to make the silicon particles flow in the reactor, a fluidizing gas is introduced from the lower part of the reactor. Silanes such as monosilane and trichlorosilane, which are raw materials, can be introduced into the fluidized bed reactor together with the fluidized gas. The seed silicon particles introduced into the reactor are in a fluidized state in the reactor and are heated to the temperature inside the reactor. The surface temperature of the heated seed silicon particles becomes equal to or higher than the decomposition temperature of monosilane or the reduction temperature of chlorosilanes, and a new silicon layer is deposited around the nuclei with the seed silicon particles as nuclei. The supply of seed silicon in this step may be continuous or intermittent.

The inside of the deposition reactor and the particle transportation pipe is preferably made into a non-polluting environment, and the material is not particularly limited as long as it guarantees non-polluting. For example, if the inner wall of the reactor or the pipe is made of high-purity polycrystalline silicon, the particles do not come into contact with substances other than silicon, so that the particles are not contaminated. Further, as the valve provided in the middle of the pipe, a plug valve or the like in which the powder contact portion and the particle contact portion are made of silicon and the seal portion is made of fluorine resin can be used. When using polycrystalline silicon as a device material, it is also possible to adopt a method in which only the inner wall corresponding to the powder contact part and the grain contact part is made of silicon and a specific metal material is used for the outer wall. That is, since the metal material releases an impurity gas containing P, B, etc. when heated, there is a high possibility that the particles will be indirectly contaminated even if they do not directly contact the wall.
However, contamination of the granular polysilicon can be prevented by appropriately selecting and using a material that emits less gas, such as a heat-resistant alloy containing 28% by weight or more of nickel, for the heating portion such as the deposition reactor wall. .

[0047]

In general, a mixed acid of hydrofluoric acid and nitric acid is used for etching silicon, but this etching solution cannot achieve effective removal of iron element adhering to the surface. The present inventors believe that the reason is that the elution of the iron element is hindered by the formation of the oxide film barrier. That is, in the mixed acid, an etching film of silicon occurs and an oxide film is always formed on the silicon surface. Since the crystal strain is large at the interface between the oxide film and silicon, it is considered that iron atoms having a larger atomic radius than atoms such as silicon and oxygen are likely to gather at such positions. As a result, it is presumed that iron atoms cannot be removed because they are always collected at the interface between the oxide film and the silicon inside the oxide film and the elution is suppressed by the oxide film barrier.

Considering why the cleaning in the present invention is extremely efficient, in the present invention, in the result, iron atoms are easily collected at the interface between the silicon and the oxide film. , It is considered that the effective processing was performed. That is, 1. Formation of oxide film on silicon surface 2. Collection and uptake of iron atoms into silicon-silicon oxide film interface 3. Removal of oxide film and iron element. It is considered that the iron component was surely removed by repeating the process repeatedly.

[0049]

According to the method of the present invention, the content of the iron element in the core which is the seed particle can be reduced to 10 ppba or less with respect to the weight of the core. This is because the core obtained by crushing in a conventional non-polluting atmosphere has a Fe content of 400
It can be said that this is a significant improvement compared to the high level of ppba or higher contamination. By using the nucleus, polycrystalline silicon particles having an iron element concentration of 3 ppba or less can be manufactured for the first time. Further, it was revealed that the method of the present invention can reduce the contents of elements other than iron. Here, according to the present invention, granular polycrystalline silicon that can sufficiently withstand the requirements as granular polycrystalline silicon for the CCZ method has been provided for the first time.

[0050]

EXAMPLES The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples. Example 1 [Production of Polycrystalline Silicon Nucleus] A polycrystalline silicon rod deposited by the Siemens method was crushed by a crusher made of stainless steel,
The average particle size was adjusted to 0.2 mm with a stainless steel sieve. The crushed silicon was sequentially washed with aqua regia, water, and hydrofluoric acid in a Teflon container. First, the debris contained in the seed particles was washed and separated with pure water, and the silicon fine particles adhering to the crushed silicon particles were roughly removed. After washing with water for 15 minutes, the iron content of the silicon particles was measured. As a result, the iron concentration was 6000
It was 0 ppba. Next, after discharging the pure water in the container,
The aqua regia was poured and the mixture was stirred for 15 minutes so that the silicon particles did not overflow in the aqua regia. The iron content of the silicon particles at this point was 320 ppba. After discharging the aqua regia, fresh aqua regia was supplied again and the mixture was further stirred for 15 minutes.
The iron content of the silicon particles at this point is 140 pp
It was ba. After discharging the aqua regia, the aqua regia components were almost completely removed by washing with pure water, and then the pure water was drained. Then, a 5% aqueous solution of hydrofluoric acid was injected and the mixture was stirred for 5 minutes to remove the oxide film on the surface and the iron element collected at the interface between the oxide film and silicon, and washed again with water. The iron content of the silicon particles at this point was 20 ppba. Even with this purity, it can be used as a seed for granular silicon, but in consideration of the case where the weight ratio of seed silicon in the product is large or the case where the seed is contaminated during the drying process, the aqua regia-washing is repeated. Repeated washing with hydrofluoric acid. The iron content of the silicon particles at this point was 1.4 ppba. It is possible to further reduce the iron element content by repeating further washing, but it was determined that the above value was sufficient as the seed purity.

The washed silicon particles were quickly drained to avoid contamination due to the formation of a surface oxide film, and then rapidly dried at a high temperature of 130 ° C. or higher. After completion of the drying, the heavy metal concentration of the particles was measured by a shield torch system I
It was measured by analysis using a CP mass spectrometer (ICP-MS, Yokogawa Analytical Systems, PMS2000). The detection limit of iron in this analysis method is 0.05p
It is pba. Further, the silicon particles were made into polycrystalline rods in a quartz tube and then single crystallized by the FZ method, and then the amount of impurities other than heavy metals was measured. The results of these measurements are shown in Table 1 as seed sample 1.

For comparison, a silicon roll crusher was prepared, and the silicon was crushed and analyzed. The crushed silicon particles are sieved in a Teflon container,
After washing with pure water, sedimentation separation was performed. The results are shown in Table 1.
Seed as Sample Sample 2 below.

[0053]

[Table 1]

Further, Table 2 shows the difference in surface cleanliness when the crushed silicon particles having a particle diameter of 0.15 mm are sieved by a stainless mesh and then etched by another method. It can be seen from Table 2 that the elemental iron was not sufficiently removed by the aqua regia-hydrofluoric acid mixed acid. Under conditions where the concentration of hydrofluoric acid is low, sufficient etching reaction to remove the iron element does not occur, and under conditions where the concentration of hydrofluoric acid is high, there is a reaction that inhibits the removal of the iron element, that is, a reaction of incorporating impurities by the oxide film. It is believed that The iron element, which could not be removed by using the acid alone or in the mixed acid, was almost removed by repeating the stepwise etching with aqua regia and hydrofluoric acid. In addition, in the oxide film formation by nitric acid and the thermal oxide film formation in the air, which was examined as a method of forming an oxide film,
It was not as effective as aqua regia.

[0055]

[Table 2]

[Production of Polycrystalline Silicon Particles] The polycrystalline silicon nuclei obtained by the above method were charged into the deposition reactor shown in FIG. 4, and vapor phase deposition reaction was carried out using seed silicon as nuclei. The reactor is a vertical fluidized bed with a diameter of 8 inches and a height of 2 m, and the inner wall of the reactor is made of polycrystalline silicon obtained by the Siemens method and cut into a plate with a thickness of 10 mm. After being inserted into a deposition reactor, the polycrystalline silicon plate 1 is heated to 750 ° C. by an external heater, and Ar is heated to 1
A monosilane gas diluted to 100% was flowed and a silicon plate completely adhered was used. Since the outer wall 2 of the reactor is heated by the external heater 3, a heat-resistant alloy containing 28% by weight or more of nickel was used as the material for the purpose of preventing the release of the dopant impurity gas from the outer wall during heating.

Seed feeding and product withdrawal were carried out continuously during the precipitation reaction. Seed supply line 4 and seed hopper 5
The structure was such that a cylinder made of polycrystalline silicon was connected to the inner wall made of stainless steel, and the seed particles did not come into contact with anything other than silicon. Since the temperature of this part is close to room temperature, an epoxy adhesive was used to bond the silicon cylinders together. A plug valve 6 having a powder contact portion made of polycrystalline silicon and a fluororesin was prepared and used for cutting the edge of the reactor and the seed supply pipe.

The seed silicon supplied from the seed supply line 4 is mixed in the fluidized bed 7 inside the reactor to be in a fluidized state. Silane gas diluted with hydrogen gas was introduced from the gas introduction pipe 8 to deposit new silicon on the surface of the fluidized particles. The gas introduction pipe 8 is made of metal. Therefore, in order to avoid contact with particles, a bottom plate formed by processing and shaping a silicon rod was provided at the bottom of the deposition reactor, a hole 9 was opened at the center, and a pipe 8 was passed through it. A hole 10 was provided next to the hole 9, and a product particle extraction line consisting of a stainless steel pipe having a silicon pipe inserted therein was provided below the hole 10 to intermittently withdraw during the reaction. The cycle of withdrawal is every 2 hours, and the withdrawn particles are cooled by a cooling hopper having a silicon cylinder inserted at the bottom of the reactor. The outer wall of the hopper 11 is a jacket, and the cooling water 13 is flown inside the jacket. To prevent contamination of particles, no valve is used between the reactor and the cooling hopper 11, and the hopper 1
Particles cooled by 1 are plug valves similar to 6
Stopped by 2.

FIG. 5 shows the time change of the iron content in the product extracted from the reactor. Iron content at the beginning of operation is 4p
It was close to pba, but after driving for 6 hours, the target was 0.3.
It fell below ppba. It is considered that the reason why the purity improved with the lapse of time was that the surface of the product withdrawing pipe and the surface of the product storage hopper were less contaminated by the co-washing with the product particles in the transportation process after the precipitation. The above precipitation reaction is continuously carried out for 100 hours to obtain an average particle size of 10
Polycrystalline silicon particles of 00 μm were obtained.

[Analysis of Polycrystalline Silicon Particles] Table 3 shows the analytical values of the main elements of the polycrystalline silicon particles (Sample 1) produced by the above method. The analyzed product is one obtained when 100 hours have passed since the start of operation of the apparatus.
For comparison, Sample 8 shows the analysis values of the main elements of commercially available polycrystalline silicon particles. In the analysis, heavy metals were measured by chemical analysis, and other impurities were analyzed by Fourier transform infrared spectrophotometer (FT-) after single-crystallizing the particles.
IR), and Fourier transform photoluminescence (FT-
PL). It is not clear in what process the commercially available polycrystalline silicon particles are produced, but the concentration of iron element around the nucleus is much higher than that of the method of the present invention. Further, the analysis values of the iron concentration in the polycrystalline silicon particles are shown in Table 4 for Sample 1 and Sample 8 by dividing them for each analysis step.

[0061]

[Table 3]

[0062]

[Table 4]

* 1 The particle size before and after etching in that step is shown. * 2 Not screened. Others were adjusted in particle size by sieving. * 3 Comparative example (commercial product) * 4 ND shows <0.05 ppba. * 5 Samples 1 to 6 were washed with aqua regia-water-hydrofluoric acid in advance and then etched with nitric hydrofluoric acid. The analysis value of step 1 is not shown here because of the influence of surface contamination due to sieving of the sample. * 6 The analysis value of Step 1 of Sample 8 represents the surface analysis value before sieving.

[Appearance of Polycrystalline Silicon Particles] FIGS. 6 and 7 show appearance photographs showing the particle shapes of the polycrystalline silicon particles produced by the method of the present invention and the commercially available polycrystalline silicon particles, respectively. Both shapes are almost the same,
You can see that it is roughly spherical.

[Cross Section of Polycrystalline Silicon Particles] FIGS. 8 and 9 are sectional photographs showing the particle structures of the polycrystalline silicon particles produced by the method of the present invention and the commercially available polycrystalline silicon particles, respectively. In these photographs, a cross section was formed so as to include the vicinity of the central portion of polycrystalline silicon particles, the cross section was mirror-polished, and then 70% nitric acid and 50% hydrofluoric acid were mixed in a volume ratio of 10: 1. The surface was etched with a mixed acid for about 30 seconds, dried, and then photographed with a scanning electron microscope. FIG. 8 shows the use of crushed particles of polycrystalline silicon produced by using trichlorosilane for the core. Further, FIG. 9 shows the use of crushed particles of polycrystalline silicon produced by using monosilane for the core.

The morphology of the grain boundaries of the nuclei in FIG. 8 is clearly different from that of the surrounding precipitation layer. The centrally located nucleus has a diameter of about 200 μm. Since the nucleus is made of polycrystalline silicon using trichlorosilane, fine powder of silicon is not incorporated by scavenging. Therefore, after the etching of the cross section, minute holes which are traces of fine powder are not observed. On the other hand, many minute holes are observed in the precipitation layer located around the nucleus, and these holes are arranged in a direction perpendicular to the precipitation direction (direction parallel to the grain surface). Also, the length extending along the precipitation direction is several tens of μm.
Many grooves are observed.

In FIG. 9, the manufacturing conditions of the central portion and its peripheral portion, that is, the nucleus and the deposited portion are the same. Therefore, there is no essential difference between the two. However, the holes formed after etching are arranged around the nucleus in a direction perpendicular to the precipitation direction around the nucleus as in FIG. 8. However, since the nucleus is a crushed particle, the holes can be arranged in any direction. Line up in the direction. At the boundary between the nuclei of such particles and the surrounding precipitates, the arrangement of holes suddenly changes at a certain position. Further, the direction of extension of a groove having a length of several tens of μm, which is similar to that observed in FIG. 8, is different between the nucleus and its surroundings.

Examples 2 to 7 Etching and precipitation reaction of seed particles were carried out in the same manner as in Example 1 except that the seed particle size, product particle size and deposition reaction time were changed as shown in Table 5. . Regarding the obtained polycrystalline silicon particles and Samples 2 to 7, the analysis values of the main elements are shown in Table 3, and the analysis values of the iron concentration are shown in Table 4 for each analysis step. All of the polycrystalline silicon particles were substantially spherical, similar to those obtained in Example 1, and had the same particle structure. That is, since the core portion is made of polycrystalline silicon using trichlorosilane, minute holes are not observed. On the other hand, many minute holes are observed in the precipitation layer located around the nucleus, and these holes are arranged in a direction perpendicular to the precipitation direction (direction parallel to the grain surface). In addition, many grooves having a length of several tens of μm extending along the precipitation direction are observed.

[0069]

[Table 5]

[0070]

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between an iron concentration in a raw material polycrystalline silicon and an iron concentration in a CCZ single crystal.

FIG. 2 is a graph showing the concentration distribution of iron concentration in the radial direction of polycrystalline silicon particles.

FIG. 3 is a schematic diagram showing a presumed mechanism when iron element is removed by cleaning polycrystalline silicon nuclei.

FIG. 4 is a schematic diagram of a deposition reactor used to produce polycrystalline silicon particles.

FIG. 5 is a graph showing the relationship between time and iron content in the precipitation reaction of polycrystalline silicon particles.

FIG. 6 is a photograph showing the particle shape of the polycrystalline silicon particles of the present invention.

FIG. 7 is a photograph showing the particle shape of commercially available polycrystalline silicon particles.

FIG. 8 is a cross-sectional photograph showing the particle structure of polycrystalline silicon particles of the present invention.

FIG. 9 is a cross-sectional photograph showing the particle structure of commercially available polycrystalline silicon particles.

[Explanation of symbols]

 1 inner wall of reactor 2 outer wall of reactor 3 external heater 4 type supply line 5 hopper 6 plug valve 7 fluidized bed 8 gas introduction pipe 9 hole 10 hole 11 hopper

Claims (5)

[Claims] 1. A concentration distribution of iron element exists from the surface of the particle toward the inside thereof, and a maximum value of the iron element concentration exists inside the particle, and the maximum value of the iron element concentration is 3 from the surface of the particle to silicon. The value when etching by 3 x 10 ^-5 g each is 10p
smaller than pba and the iron element concentration of the whole particle is 3 pp
A polycrystalline silicon particle having a particle size of ba or less. 2. Polycrystalline silicon particles according to claim 1, wherein the polycrystalline silicon is generally spherical. 3. The polycrystalline silicon particle according to claim 1, comprising a central portion and a peripheral portion having a texture different from that of the central portion. 4. P, B, Al, As, Ni, Cr, C
The polycrystalline silicon particle according to claim 1, wherein none of the elements selected from the group consisting of u, Mn, and Ti is contained in an amount greater than 3 ppba. 5. Polycrystalline silicon is prepared by sequentially crushing and crushing crushed silicon with aqua regia-water-hydrofluoric acid to prepare seed silicon, and thermally decomposing or reducing silanes in the presence of the seed silicon. The method for producing polycrystalline silicon particles according to claim 1, characterized by: