JPH0867510A - Mechanical workpiece of polycrystal silicon - Google Patents

Abstract

PURPOSE: To obtain a high-purity polycrystal silicon having the surface which is not contaminated by iron atom though it is a mechanical workpiece. CONSTITUTION: This polycrystal silicon has <=10000atom/μm<2> iron atom concentration of the surface per unit surface area. The polycrystal silicon is produced by successively cleaning a polycrystal silicon with aqua regia, water and hydrofluoric acid, e.g. aqua regia having 1:1 volume ratio of nitric acid to hydrochloric acid, water and 1-20wt.% of hydrofluoric acid in this order.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machined work of polycrystalline silicon having a high surface purity.

[0002]

2. Description of the Related Art Generally, the Czochralski method (hereinafter, simply referred to as CZ method) and the floating zone method (hereinafter, referred to as FZ method) are adopted for producing single crystal silicon. In these methods, heavy metals existing on the surface of polycrystalline silicon as a raw material, particularly iron atoms, are concentrated by segregation, and the concentration of heavy metals in single crystal silicon increases as the single crystal silicon approaches the end. Therefore, it is necessary to reduce the concentration of heavy metals on the surface of polycrystalline silicon. In particular, since polycrystalline silicon used in the CZ method has a small particle size and a large surface area, it is strongly desired to reduce the concentration of heavy metals on the surface.

Polycrystalline silicon as a raw material for the CZ method is obtained by finely crushing a polycrystalline silicon rod manufactured by a precipitation method generally called Siemens method,
Polycrystalline silicon, which is a raw material for the FZ method, is manufactured by undergoing a shaping process such as surface grinding of the above-mentioned polycrystalline silicon rod and tapering of the tip portion. In this way, the mechanically processed product of polycrystalline silicon obtained through mechanical processing such as crushing and grinding has the problem that the surface is contaminated by the surrounding environment during crushing and grinding, and the concentration of heavy metals becomes high. there were.

In order to use such a mechanically processed product of polycrystalline silicon as a raw material for single crystal silicon, it is necessary to lower the iron atom concentration on the surface. Therefore, generally, the surface of a mechanically processed product of polycrystalline silicon is etched with a mixed acid of nitric acid-hydrofluoric acid (hereinafter referred to as nitric hydrofluoric acid).
Although this method is widely used, it has a problem that the iron atom concentration on the surface cannot be sufficiently lowered. Further, in the above method, since NO _X is generated during etching, it is also a problem that measures against environmental pollution are required.

[0005]

If the mechanical work of polycrystalline silicon is made highly pure, the single crystal silicon obtained therefrom can be made highly pure. Therefore,
There has been a demand for higher-purity, particularly low-iron-atom-concentration on the surface, of mechanically processed polycrystalline silicon processed for pulling single-crystal silicon.

[0006]

SUMMARY OF THE INVENTION We have found that heavy metal impurities present on the surface of a polycrystalline silicon mechanical work,
In particular, as a result of intensive studies on reducing iron atoms, when cleaning the surface of a mechanically processed polycrystalline silicon, an oxide film is first formed, and then it is removed in two steps. As a result, they have found that a mechanically processed product of polycrystalline silicon having an extremely high degree of surface cleanliness can be obtained, and have completed the present invention and have proposed it here. That is, the present invention is a mechanically processed product of polycrystalline silicon characterized in that the concentration of iron atoms on the surface is 10,000 atoms / μm ² or less per unit surface area.

The mechanically processed product of polycrystalline silicon of the present invention is polycrystalline silicon as a raw material of single crystal silicon, and has been subjected to mechanical processing such as crushing, crushing, polishing, grinding and cutting. is there. Such a mechanically processed product of polycrystalline silicon has its surface contaminated with heavy metals due to contact with the surrounding environment and a jig used for the processing, after undergoing mechanical processing. However, according to the method of the present invention described later, it is possible to obtain a mechanically processed product of polycrystalline silicon in which the concentration of iron atoms existing on the surface is suppressed to an extremely low level. Despite being a mechanically processed product of polycrystalline silicon, the surface of which has a low iron atom concentration, its existence is not known to date, and is the first to be provided by the present invention.

The conventional method for etching polycrystalline silicon with hydrofluoric acid has a certain level of surface cleaning effect, but it cannot be said to be sufficient. In this method, when the temperature of fluorinated nitric acid rises due to the etching reaction, the effect of cleaning the surface decreases. In particular, when there are many crystal grain boundaries in the polycrystalline silicon to be etched, it is considered that the crystals at the crystal grain boundary portions are incomplete, but the progress of etching at the crystal grain boundary portions due to hydrofluoric acid is otherwise. It becomes sharper than the part. Therefore, it is considered that the etching depth differs depending on the location, and the surface area increases, so that the etching reaction temperature further rises, and as a result, the reduction of iron atoms on the surface is hindered. However, such a problem does not occur in the present invention. Therefore, according to the present invention, even if a mechanically worked product of polycrystalline silicon has many crystal grain boundaries, that is, even if the crystal grain size is small, it is possible to obtain polycrystalline silicon having a sufficiently clean surface. Here, the crystal grain size can be measured by the following method.

Polycrystalline silicon was cut at a given cross section, mirror-polished, and then etched with hydrofluoric acid until crystal grain boundaries could be observed, and the surface was observed with an optical microscope or a scanning electron microscope. Sometimes, the grain size can be defined as the distance between the grain boundaries crossed by an arbitrary straight line drawn in the observation field. Then, make an arbitrary straight line 10
The average value when the main drawing is performed can be used as the average crystal grain size. For example, in polycrystalline silicon having a crystal grain size of 500 μm, if a straight line with a length of 1000 μm or more is drawn,
It will pass through more than one grain boundary. Here, the grain boundaries also include twin boundaries.

In the present invention, the crystal grain size is 500 μm or less as an average value of the crystal grain size measured by the above measuring method,
Furthermore, even if the thickness is 300 μm or less, which is relatively small, polycrystalline silicon having a sufficiently clean surface can be obtained. In addition, polycrystalline silicon deposited from monosilane as a raw material at a temperature lower than about 800 ° C. has a very small crystal grain size, or the crystallinity of the whole polycrystalline silicon is deteriorated, so that it is possible to confirm the grain boundary. It gets harder. Since such polycrystalline silicon has a higher etching rate with nitric hydrofluoric acid than that whose grain size can be clearly confirmed, the above-mentioned problems occur.

The above-described etching rate generally correlates with the half-value width obtained by X-ray diffraction, and the larger the half-value width, the higher the etching rate. The full width at half maximum of the silicon peak (near 2θ = 28.5) by X-ray diffraction of polycrystalline silicon deposited from trichlorosilane by the Siemens method is 0.14 to 0.16 °. When dichlorosilane or monosilane is used as the raw material and the deposition temperature is lowered, the full width at half maximum is further increased to about 0.17 °.
Further, when monosilane is used as a raw material and is manufactured in a fluidized bed, the full width at half maximum is further increased to 0.20 to 0.33 °. In the present invention, when the half-width of the silicon peak is large, for example, even if it is 0.17 or more, and even 0.20 or more, no problem occurs when etching with conventional nitric hydrofluoric acid. Therefore, the present invention can provide polycrystalline silicon with a sufficiently clean surface even when the half-width of the silicon peak is large.

The size of the mechanically worked product of polycrystalline silicon in the present invention is not particularly limited. However, since the surface area of silicon becomes larger as the grain size of polycrystalline silicon becomes smaller, it is not possible to remove the liquid by ordinary etching with hydrofluoric acid. Fever occurs,
Etching becomes difficult. Therefore, the present invention is particularly effective in the case of polycrystalline silicon having a small grain size, for example, in the range of 0.1 to 100 mm, and further in the range of 0.1 to 20 mm.

In the mechanically worked product of polycrystalline silicon of the present invention, the iron atom concentration on the surface is 10,000 atoms / μm ² or less per unit surface area. That is, it means that the number of iron atoms present in a square having one side of 1 μm is 10,000 or less. Here, as the method of measuring the iron atom concentration on the surface, a method of dissolving and extracting the iron atom on the surface by sequentially washing aqua regia, water and hydrofluoric acid is adopted. In this case, in order to improve the analysis accuracy, it is preferable to use nitric acid and hydrochloric acid, which are raw materials for aqua regia, for the semiconductor industry or those obtained by further distilling and refining them. Further, the surface area S (μm ² ) of the mechanically processed product of polycrystalline silicon is that of the mechanically processed product of polycrystalline silicon, which has a desired mesh size of JIS standard sieve and a mesh size which is one step larger than that. Overlapping and sieving, 100 particles with the same particle size were measured and particle 1
The average weight per piece was determined, and the average weight W (g) was calculated from the following formula in sphere conversion. S = 2.75 × 10 ⁸ × W ^{2/3 In the} present invention, the iron atom concentration on the surface can be set to 5,000 atoms / μm ² or less, and further set to 3,000 atoms / μm ² or less. It is also possible.

The polycrystal silicon mechanically processed product of the present invention is obtained by sequentially cleaning a polycrystal silicon mechanically processed product whose surface is contaminated with heavy metals with an oxidizing chemical solution, water and a chemical solution capable of decomposing an oxide film. Can be obtained by doing. It is considered that the oxidizing chemical solution has a function of eluting impurities on the surface of polycrystalline silicon and a function of forming an oxide film on the surface of polycrystalline silicon. Also, the chemical that decomposes the oxide film is
It is presumed that it has a function of removing the formed oxide film and a function of eluting impurities existing in the oxide film or at the interface between the oxide film and polycrystalline silicon. The object of the present invention can be achieved by sequentially using such two kinds of chemicals.

As a method of forming an oxide film on the surface of polycrystalline silicon, there is a method of heating silicon to form a thermal oxide film. However, according to this method, iron atoms diffuse from the surface of the polycrystalline silicon to the inside during heating, so that it cannot be removed by a chemical solution that decomposes the oxide film in a later step. Therefore, it is not preferable to heat the cleaning step in the present invention to a high temperature, for example, 200 ° C. or higher in any step, not limited to the oxide film formation.

1 to 3 show an estimated mechanism for removing iron atoms by the cleaning method of the present invention. FIG. 1 shows the surface of polycrystalline silicon that has been crushed and then washed with water. In the figure, 1 indicates the metal-state iron adhering to the surface of the polycrystalline silicon, and 2
Indicates iron in the atomic state diffused near the surface. FIG. 2 shows a state after cleaning the polycrystalline silicon shown in FIG. 1 with an oxidizing chemical solution. The reaction of oxidizing the surface of the polycrystalline silicon by the oxidizing chemical solution to form the oxide film 3 and the reaction of dissolving and removing the metallic iron 1 by the oxidizing chemical solution proceed at the same time and adhere to the surface of the polycrystalline silicon. The iron 1 in the metallic state and the iron 2 in the atomic state diffused inside are gathered at the interface between the polycrystalline silicon and the oxide film 3. FIG. 3 shows a state after removing the oxidizing chemical solution by washing with water and then removing the oxide film with a chemical solution that decomposes the oxide film. When the chemical solution that decomposes the oxide film is hydrofluoric acid, the reaction for dissolving the silicon oxide film follows the following reaction formula. SiO ₂ + 6HF → H ₂ SiF ₆ + 2H ₂ O H ₂ SiF ₆ generated with the removal of the silicon oxide film is a very strong acid, so the iron atoms taken inside the silicon oxide film are Are dissolved and removed by.

As the oxidizing chemical solution used in the present invention, a known chemical solution can be used as long as it can oxidize the surface of polycrystalline silicon to produce silicon dioxide. Specific examples thereof include nitric acid and aqua regia. Can be listed in. Particularly, aqua regia can be preferably used because of its large oxide film forming ability. The aqua regia may have any mixing ratio as long as it is a mixed solution of nitric acid and hydrochloric acid, but a nitric acid: hydrochloric acid = 1: 3 to 3: 1 (volume) composition is preferably adopted for strong washing. To be done. 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 cleaning with the oxidizing chemical solution, more effective cleaning can be performed by removing fine powder and foreign matters in the crushed particles by washing with water. After cleaning with the oxidizing chemical solution, the polycrystalline silicon must be washed once with water. This is because, in the step of treating with the chemical solution that decomposes the oxide film in the subsequent step, if the oxidizing chemical solution in the previous step remains, mixed acid is formed by the oxidizing chemical solution and the chemical solution that decomposes the oxide film, and An etching reaction of crystalline silicon occurs. In this state, the polycrystalline silicon surface becomes rough due to local etching, and if a large amount of oxidizing chemicals remain, a film that cannot be completely removed by the chemical that decomposes the oxide film is formed on the surface, and the oxidizing chemicals , Inhibits the sequential cleaning effect of the chemical solution that decomposes the oxide film. Therefore, in the water washing process between the oxidizing chemical cleaning and the chemical cleaning that decomposes the oxide film, it is necessary to remove the oxidizing chemical as much as possible. If the film is formed due to a mistake in the process, the film can be changed to a normal oxide film by immersing it in warm water at 70 ° C. or higher for several minutes, and the cleaning process can be continued.

The chemical solution for decomposing the oxide film used for cleaning in the present invention may be liquid or gas as long as the surface oxide film of polycrystalline silicon can be removed, but the oxide film and oxide film removed from the surface can be removed. The aqueous solution of hydrofluoric acid is preferable in order to discharge the iron atoms liberated along with the above to the outside of the system without adhering to the surface of the polycrystalline silicon again. Also,
In order to prevent contamination with hydrofluoric acid itself, it is preferable that the hydrofluoric acid is of a semiconductor industrial grade. The concentration of the aqueous solution of hydrofluoric acid can be adopted without any problem as long as it has an effect of removing the oxide film, but in view of the removal rate of the oxide film and the production cost, it is industrially 1 to 20% by weight, preferably 3 to 10% by weight.
A weight% aqueous solution is employed. Since impurities are not redeposited on the surface washed by the oxidizing chemical solution and the chemical solution that decomposes the oxide film, not only the water used for diluting the chemical solution that decomposes the oxide film, but all the water used for the cleaning of the present invention is pure. It is preferably water. Pure water has an iron concentration of 1 ppb each
Ion-exchanged water having a or less is preferably adopted. Also,
After sequentially performing oxidizing chemical cleaning, water cleaning, and chemical cleaning that decomposes an oxide film, water cleaning is performed, and this cleaning method is repeated twice or more to obtain a mechanically processed polycrystalline silicon product with higher purity. be able to.

[0019]

According to the method of the present invention, the content of iron atoms existing on the surface of polycrystalline silicon can be reduced to a level lower than that achieved by the conventional nitric hydrofluoric acid etching. Became. Further, according to the present invention, not only the above-mentioned iron atom concentration but also the atoms which are problematic for use as a semiconductor raw material, for example, Ni, Cr and Cu, have a surface concentration of 10,000 atoms / unit surface area / μ
m ² or less, preferably 5,000 atoms / μm ² each
Or less, more preferably 3,000 atoms / μm ² each
It can also be: Therefore, according to the method of the present invention, high-purity polycrystalline silicon useful as a raw material for producing high-purity single crystal silicon can be obtained.

[0020]

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 A polycrystalline silicon rod deposited by the Siemens method was crushed by a crusher made of stainless steel, and the particle diameter was adjusted to 0.155 mm on average by a sieve made of stainless steel. The average crystal grain size of this polycrystalline silicon was 30 μm, and the half width of the peak of silicon by X-ray diffraction was 0.16 °. This polycrystalline silicon was sequentially washed with aqua regia, water and hydrofluoric acid in a Teflon container. First, with pure water,
While separating the waste contained in polycrystalline silicon with water,
The polycrystalline silicon fine particles adhering to the polycrystalline silicon were roughly removed. After washing with water for 15 minutes, the iron atom concentration on the surface of this polycrystalline silicon was measured. As a result, the iron atom concentration was 7 × 10 ⁷ atoms / μm ² . Next, pure water in the container was discharged, and then aqua regia (nitric acid: hydrochloric acid (volume ratio) = 1: 1) was injected, and the mixture was stirred for 15 minutes so that polycrystalline silicon did not overflow in the aqua regia. The iron atom concentration of the polycrystalline silicon at this time was 4 × 10 ⁵ atoms / μm ² . After discharging the aqua regia, fresh aqua regia was supplied again and the mixture was further stirred for 15 minutes. The iron atom concentration of the polycrystalline silicon at this time was 1.7 × 10 ⁵ atoms / μm ² . 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 5% by weight
By injecting an aqueous solution of hydrofluoric acid and stirring for 5 minutes, the oxide film on the surface and iron atoms collected at the interface between the oxide film and polycrystalline silicon were removed, and the film was washed again with water. At this point, the concentration of iron atoms in the polycrystalline silicon was 25,000 atoms / μm ²
Met. Then, as a result of repeating the washing with aqua regia, washing with water, and washing with hydrofluoric acid again, the concentration of iron atoms in the polycrystalline silicon was 1,700 atoms / μm ² . Further, as a result of repeated washing with aqua regia, water and hydrofluoric acid, the iron atom concentration became 100 atoms / μm ² or less, which is the detection limit. As a precaution, the total amount of polycrystalline silicon thus obtained was dissolved in hydrofluoric nitric acid, the concentration of iron atoms in fluoronitric acid was measured, and the result was divided by the surface area of polycrystalline silicon before dissolution. 1
It was less than 00 atoms / μm ² .

The washed polycrystalline silicon was drained quickly 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. An ICP mass spectrometer (ICP-MS, Yokogawa Analytical Systems, PMS2000) that employs a shield torch system for the iron atom concentration and heavy metal concentration of polycrystalline silicon that has been washed twice with aqua regia-water-hydrofluoric acid is used. It was measured by the chemical analysis used. The measurement results are shown in Table 1. The iron atom concentration and heavy metal concentration on the surface of the crushed polycrystalline silicon rod before cleaning were also analyzed, and the results are also shown in Table 1.

[0022]

[Table 1]

Comparative Examples 1 to 5 The polycrystalline silicon rods used in Example 1 were crushed with an iron hammer and a crusher to produce various particle sizes. Table 2 shows the grain size of polycrystalline silicon and the optimum acid mixing ratio when this crushed polycrystalline silicon was etched with hydrofluoric acid, and the iron atom concentration on the surface when etching was performed with that composition. Indicated.

[0024]

[Table 2]

Examples 2 to 4 The polycrystalline silicon rods used in Example 1 were crushed with an iron hammer and a crusher to produce various particle sizes. Table 1 shows the iron atom concentration and heavy metal concentration in the same manner as in Comparative Example 1 when the washing with aqua regia-water-hydrofluoric acid was repeated twice using this crushed polycrystalline silicon. In the etching with the hydrofluoric acid of Comparative Example 1, the iron atom concentration on the surface was greatly changed depending on the grain size of the polycrystalline silicon. However, in the present invention, the surface iron atom concentration differs depending on the grain size of the polycrystalline silicon. But it has hardly changed.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a surface of polycrystalline silicon which has been crushed and then washed with water.

FIG. 2 is an explanatory diagram showing a state after cleaning polycrystalline silicon with an oxidizing chemical solution.

FIG. 3 is an explanatory diagram showing a state after removing an oxidizing chemical solution by washing with water and then removing the oxide film with a chemical solution that decomposes the oxide film.

[Explanation of symbols]

 1 Iron in metallic state 2 Iron 3 in atomic state Oxide film

Claims (1)

[Claims] 1. The surface iron atom concentration is 1 per unit surface area.
A mechanically processed product of polycrystalline silicon, which has a content of not more than 000 atoms / μm ² .