JP2002316813A - Polycrystalline silicon foam and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polycrystalline silicon foam in which the amount of fine powder generated during crushing for obtaining granular polycrystalline silicon is extremely small, and a method for producing the same. SOLUTION: Bubbles exist inside and the apparent density is 2.
20 g / cm ³ or less polycrystalline silicon foam,
It can be manufactured by allowing a silicon melt melted in the presence of hydrogen to fall naturally as droplets, and fixing the hydrogen bubbles in the droplets within a period of 0.2 to 3 seconds.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a novel polycrystalline silicon foam. Specifically, the present invention provides a polycrystalline silicon foam in which the amount of fine powder generated during crushing for obtaining granular polycrystalline silicon is extremely small.

[0002]

2. Description of the Related Art Conventionally, various methods for producing polycrystalline silicon used as a raw material for semiconductors or photovoltaic power generation batteries have been known, and some of them have already been industrially implemented.

For example, one of the methods is a method called a Siemens method, in which a silicon rod heated to a deposition temperature of silicon by energization is placed inside a bell jar, and trichlorosilane (SiHCl ₃ , hereinafter referred to as TCS) or monosilane (TCS) is placed therein. SiH ₄ ) is brought into contact with a reducing gas such as hydrogen to precipitate silicon.

The above polycrystalline silicon has a particle diameter of 300 μm.
There is also growing demand for granules crushed to a particle size of about m to about 2 mm. For example, in applications for semiconductors or solar cells, the granular polycrystalline silicon is used by melting it.

There is also known a technique in which the particulate polycrystalline silicon is introduced into an oxyhydrogen flame and melted and evaporated to produce particulate silica having a particle diameter of about 1 μm.

[0006] Further, silicon nanoparticles, which are attracting attention as visible light emitting devices, are produced by irradiating a silicon target with an excimer laser in a helium atmosphere.
If granular polycrystalline silicon can be easily obtained as a material for the silicon target, silicon nanoparticles can be efficiently produced.

The above-mentioned granular polycrystalline silicon has been produced by a method in which a massive crushed product, a so-called nugget, obtained by crushing a silicon rod produced by the Siemens method into a fist size, is further finely crushed.

However, when the above silicon rod is crushed to obtain granular polycrystalline silicon, a large amount of flakes, needles, and fine powders called "fine particles" are generated during the crushing. . Such fine particles cause generation of dust and are difficult to handle, and in particular, fine particles having a particle diameter of 200 μm or less have a risk of ignition, and thus have been discarded carefully. For this reason, not only the yield for the raw materials has decreased, but also a great deal of labor has been required for disposal.

[0009]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide polycrystalline silicon which produces very little fine particles during crushing for producing granular polycrystalline silicon, and a method for producing the same.

[0010]

Means for Solving the Problems The present inventors have intensively studied to achieve the above object. First, the present inventors
It was confirmed that the generation mechanism of the fine particles was due to the habit of polycrystalline silicon. That is, since polycrystalline silicon has a strong habit resolving property, when the nugget obtained by crushing the silicon rod is further crushed to obtain granular polycrystalline silicon, flaky,
Needle-shaped fine particles tend to be generated in large quantities.

Based on the finding that if a structure that is preferentially disintegrated is given to a polycrystalline silicon structure with a stress smaller than the stress exhibiting such habit resolving property, the generation of fine particles in the disintegration can be suppressed. By adopting a foam structure, which is not known as the form of conventional polycrystalline silicon, the energy at the time of crushing silicon has a higher priority than the energy acting on the crystal habit dissolving surface and the breaking energy of the bubble wall. As a result, the rate of waste generation was significantly reduced as compared with normal silicon crushed materials.

Further, it has been found that in order to sufficiently exert the effect of causing the presence of the above-mentioned bubbles, it is effective to reduce the amount of the bubbles to a specific apparent density or less, thereby completing the present invention. Was.

That is, the present invention is a polycrystalline silicon foam characterized in that bubbles are present therein and the apparent density is 2.20 g / cm ³ or less.

In the present invention, the apparent density is a value obtained from the volume and weight of particles obtained by using a pycnometer. Specifically, the method described on pages 51 to 54 of the Powder Engineering Handbook (published by Nikkan Kogyo Shimbun, February 28, 61) can be mentioned.

In the course of studying the method for producing the polycrystalline silicon foam, the present inventors have known that almost no gas is dissolved in a molten metal such as a silicon melt. However, it has been found that when the gas is hydrogen, it can be dissolved in a certain amount. Based on such findings, as a result of repeated research, hydrogen was brought into contact with and melted into the silicon melt, then dropped naturally as droplets, and solidified under a specific cooling condition, whereby the liquid was solidified. It has been found that a large shearing force is not applied to the droplet, and hydrogen present in the droplet is obtained as a bubble to obtain a foam encapsulated in the solidified polycrystalline silicon.

That is, the present invention provides a method in which silicon melted in the presence of hydrogen is allowed to fall naturally as droplets, and the hydrogen bubbles are fixed in the droplets within a time period of 0.2 to 3 seconds. There is also provided a method of making the featured polycrystalline silicon foam.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The polycrystalline silicon foam of the present invention has bubbles inside. As described above, the polycrystalline silicon structure having bubbles therein has not been known so far, and is a major feature of the present invention.

That is, the polycrystalline silicon rod obtained by the Siemens method uses hydrogen gas as a raw material in production, but the polycrystalline silicon deposited is a solid, and there is no room for hydrogen to dissolve.

Further, there has been proposed a method of depositing silicon using hydrogen as one of the raw materials and recovering the silicon by a melt. In these methods, the melt is taken out of a hydrogen atmosphere and solidified. Therefore, the hydrogen gas in the solidified body diffuses and disappears in the state of the melt.

Further, there has been proposed a method of producing polycrystalline silicon particles by dropping silicon produced in a hydrogen gas in a molten state on a rotating disk and flying the same. However, in such a method, silicon containing hydrogen is used. Since the liquid droplets of the melt are subjected to a large shearing force in the initial stage of solidification, hydrogen dissolved in the silicon melt cannot be aggregated to form bubbles. The polycrystalline silicon foam of the invention cannot be obtained.

Furthermore, polycrystalline silicon obtained by using monosilane as a source gas and growing polysilicon particles in a fluidized bed takes in a relatively large amount of hydrogen, which is contained in the polycrystalline silicon. Since it exists as a hydride of silicon, it cannot exist as bubbles.

The polycrystalline silicon foam of the present invention may have any shape as long as it has air bubbles therein. For example, a shape of an irregular coarse particle is generally preferable. The size of the coarse particles is 0.01 to
3 cc, particularly preferably in the range of 0.1 to 0.5 cc. In the production method described below, the obtained coarse particles may be obtained as a partially fused mass depending on the cooling mode. Such clumps can be separated by light crushing, such fused parts,
The irregular-shaped coarse particles can be easily obtained.

In the present invention, the abundance of the bubbles in the polycrystalline silicon foam is determined based on an apparent density of 2.20 g / cm.
³ or less, especially 2.0 g / cm ³ or less, further 1.8
g / cm ³ or less.

Usually, the apparent density of polycrystalline silicon is 2.
Although it is 33 g / cm ³ , the apparent density decreases when air bubbles are contained. The polycrystalline silicon foam of the present invention contains bubbles in such an amount as to have an apparent density of 2.20 g / cm ³ or less, so that the generation of fine particles during crushing can be significantly reduced.

Since the polycrystalline silicon foam of the present invention is lightweight, when supplied as it is to silicon crucibles for producing single crystal silicon as silicon for recharging, it is said that there is little generation of silicon melt in the crucible. It also has advantages and is useful even in a state where it is not crushed.

In the above-mentioned polycrystalline silicon foam, a large number of air bubbles may be present uniformly, or one or several large air bubbles may be present. Preferably it is 50 μm or more.

In the present invention, the polycrystalline silicon foam having an excessively small apparent density is generally difficult to produce, and fine particles may be generated due to a reduction in the thickness of the cell wall. The polycrystalline silicon foam preferably has an apparent density of 1 g / cm ³ or more.

The gas present in the bubbles of the polycrystalline silicon foam of the present invention is generally hydrogen gas from the production method described below, but the present invention is not limited to this. There is no particular limitation as long as the gas can produce the foam.

The method for crushing the polycrystalline silicon foam of the present invention is not particularly limited, and the crushing method using a known crusher such as a jaw crusher or a pin mill suppresses the generation of fine particles. It is possible to obtain granular polycrystalline silicon with a high yield.

Although the method for producing a polycrystalline silicon foam of the present invention is not particularly limited, as described above, the method utilizes the fact that hydrogen gas easily dissolves into a silicon melt.
It is preferable to use a method in which silicon melted in an atmosphere of hydrogen gas is turned into droplets, dropped naturally without applying a strong shearing force, and solidified under specific conditions.

That is, according to the present invention, silicon melted in the presence of hydrogen is allowed to fall naturally as droplets,
A method for producing a polycrystalline silicon foam is provided, wherein the hydrogen bubbles are fixed in droplets within a time period of up to 3 seconds.

In the method for producing a polycrystalline silicon foam of the present invention, as a method of obtaining silicon melted in the presence of hydrogen, a method of melting silicon or bringing molten silicon into contact with hydrogen gas is employed. The most efficient method for dissolving hydrogen in a silicon melt is a method in which, in the presence of hydrogen, precipitation of silicon using chlorosilanes as a raw material and melting of the silicon are performed at the same time.

Specifically, there is an embodiment in which a mixed gas of hydrogen gas and chlorosilanes is brought into contact with the surface of a heating body heated to a temperature equal to or higher than the melting point of silicon, and the deposition and melting of silicon are simultaneously performed.

As the above-mentioned chlorosilanes, chlorosilanes containing hydrogen in the molecule, for example, trichlorosilane and dichlorosilane are preferable because the hydrogen concentration in the silicon melt can be further increased.

A known ratio of hydrogen to the chlorosilanes may be used without any particular limitation.
In order to form a higher concentration hydrogen atmosphere, it is preferable to adjust the molar ratio of hydrogen / chlorosilanes to be 5 to 50.

The silicon melt in which hydrogen has been dissolved in this manner is allowed to fall naturally as droplets, and the hydrogen bubbles are fixed in the droplets within a period of 0.2 to 3 seconds. The fixing method is not particularly limited, but a method of contacting with a coolant having a surface temperature of 1100 ° C. or lower, preferably 1000 ° C. or lower, particularly 500 ° C. or lower is effective, and is preferably used in the present invention.

In the above-described operation, the hydrogen dissolved in the droplet of the silicon melt that falls naturally accumulates as bubbles at the center of the droplet by holding for the above-mentioned specific time, and solidifies in that state. A crystalline silicon foam is obtained.

Therefore, in the above method, it is important that the silicon melt falls naturally as droplets. That is,
The supersaturated hydrogen gas present in the silicon melt gathers over time to form bubbles, but if the melt was solidified as it was, the bubbles would have moved upward due to gravity and dissolved. Is very easily released to the outside.

On the other hand, by allowing the silicon melt to fall naturally, gasified hydrogen remains in the droplet.

In this case, the mechanism by which bubbles collect in the center of the droplet is estimated as follows. That is, when the liquid drops from the substrate holding the melt, the droplet has a momentum associated with the deformation and tends to become spherical immediately from its surface tension. Therefore, the centrifugal force acts on the inside of the droplet due to the above-mentioned rotation even if it is weightless. Then, the centrifugal force replaces gravity, and the bubbles of hydrogen existing inside have buoyancy acting toward the center, and as a result, the bubbles gather at the center of the droplet.

The conditions for the above bubbles to collect at the center are as follows:
It depends on the rotational angular velocity of the droplet and the elapsed time. The initial momentum given to the droplet is such that the longer the yarn is pulled during separation, the larger the momentum of rotation and the greater the angular velocity. That is, the higher the adhesion between the silicon melt and the substrate,
Bubbles inside collect early in the center and tend to remain. In consideration of the adhesion to the silicon melt, SiO ₂ or silicon nitride can be used as the base material, but Si is more wettable.
The effect of the present invention can be more remarkably exhibited by using C or a carbon material which forms silicide easily even if the wettability is poor at first and has a high wettability.

In the above method of the present invention, the time required for leaving the heating element in which the silicon droplet is present and fixing the bubbles is at least as long as the apparent density of the present invention can be achieved. It is necessary that the time is such that it can be maintained up to the state of being collected at the center of the droplet, and it is preferably 0.2 seconds or more, more preferably 0.4 seconds or more, and further preferably 0.6 seconds or more.

Conversely, the air bubbles collected at the center of the angle are diffused and escape to the outside if cooled slowly, so that the above-mentioned time is preferably 3 seconds or less, preferably 2 seconds or less.

After leaving the heating element in which the silicon droplet is present,
The time until the bubbles are fixed depends on the material of the heating element.
The angular velocities given to the droplets are expected to be slightly different, and when silicon nitride with poor wettability is used for the substrate, the above-mentioned time should be somewhat longer than when SiC, which can increase the angular velocity sufficiently, is used for the substrate. It is preferred to take.

In the present invention, in the operation of bringing the droplet into contact with the coolant, the coolant is not particularly limited, such as a solid, a liquid, and a gas.

In a preferred embodiment using the above-described coolant, the coolant is constituted by a material that does not substantially react with silicon, for example, a member such as silicon, copper, molybdenum, and the like. A mode in which a droplet is dropped, a mode in which a liquid refrigerant that does not substantially react with silicon, for example, liquid silicon tetrachloride, liquid nitrogen, or the like is used as a coolant, and a droplet of a silicon melt is dropped into the coolant. No.

Further, a cooling gas generated by spraying the above-mentioned refrigerant can be used as a refrigerant and brought into contact with liquid droplets of the silicon melt.

In the embodiment using the solid coolant, the surface may be cooled directly or indirectly by a known cooling method, if necessary. Also, in some cases, the polycrystalline silicon foam is deposited by dropping and solidifying silicon melt droplets sequentially on the coolant,
In this case, the uppermost surface of the polycrystalline silicon foam acts as a coolant. Further, in order to absorb the shock when the droplet of the silicon melt falls on the surface of the coolant, it is preferable that the surface of the coolant has irregularities, for example, by pre-existing particulate matter such as silicon particles. Preferably. In this case, it is particularly preferable to use a part of the obtained polycrystalline silicon foam as the silicon particles.

Although the apparatus for carrying out the method of the present invention is not particularly limited, an apparatus having a structure shown in FIG. 1 is suitable for continuously dropping a silicon melt droplet. No. That is, (1) a cylindrical container 1 having an opening 2 serving as a silicon outlet at the lower end, and (2) heating for heating the inner wall from the lower end of the cylindrical container 1 to an arbitrary height to a temperature equal to or higher than the melting point of silicon. Apparatus 3, (3) a chlorosilanes supply pipe 4 for supplying chlorosilanes A, which is provided to open downward into a space 5 surrounded by an inner wall heated above the melting point of silicon in the cylindrical container. (4) a seal gas supply pipe 6 for supplying a raw material hydrogen gas as a seal gas B in a gap formed by an inner wall of the cylindrical vessel and an outer wall of the chlorosilanes supply pipe, and (5) the cylindrical vessel 1 A device which basically consists of a coolant 9 provided with a space below it.

In the above-mentioned apparatus, in order to efficiently collect the exhaust gas discharged from the cylindrical container 1, the cylindrical container 1 and the coolant 9 are transferred by the closed container 7 provided with the exhaust gas extraction pipe 12. It is preferable to cover.

At this time, a seal gas supply pipe 11 is provided in a gap formed by the outer wall of the cylindrical container 1 and the inner wall of the closed container 7 to supply a seal gas C such as nitrogen, hydrogen, or argon. Is preferred.

In the above-mentioned apparatus, a high-frequency coil is preferably used as the heating device 3 for heating the cylindrical container 1. Further, the material of the cylindrical container 1 is preferably a material that can be heated by high frequency and has a resistance at the melting point of silicon. Generally, carbon, silicon nitride, or the like is used.

In the above-described apparatus, chlorosilane supplied from the chlorosilane supply pipe 4 is supplied to the space 5 of the cylindrical container 1 together with hydrogen supplied from the seal gas supply pipe 6 also serving as a sealing gas. As a result, silicon is deposited on the inner wall heated above the melting point of silicon, and at the same time, the silicon is melted to form a silicon melt. The silicon melt flows down the inner wall and falls naturally from the opening 2 as a droplet 14.

By adjusting the distance from the opening 2 to the coolant 9 at the lower part of the closed vessel, the time during which the droplet of the silicon melt contacts the coolant can be controlled.
In addition, the temperature of the coolant 9 can be adjusted to an intended temperature indirectly or by directly supplying the cooling gas E from the cooling gas supply pipe 13.

The droplets in contact with the coolant form bubbles of hydrogen gas therein while falling, and solidify by contact with the coolant in that state to form a polycrystalline silicon foam 8. .

[0056]

As will be understood from the above description, the polycrystalline silicon foam of the present invention generates very few fine particles due to crushing.
The yield in the case of producing granular polycrystalline silicon of about m to 2 mm can be significantly improved.

Further, the polycrystalline silicon foam of the present invention is light in itself, and is useful as silicon for recharging during the production of a single crystal.

[0058]

Example 1 Using the apparatus shown in FIG.
After heating to 3 ° C., trichlorosilane is supplied from the chlorosilanes supply pipe 4 and hydrogen is supplied from the seal gas supply pipe 6 to the cylindrical container 1.
And silicon was precipitated and melted on the inner surface of the cylindrical container. At this time, the molar ratio of trichlorosilane to hydrogen (hydrogen / trichlorosilane) was adjusted to 20. The silicon melt was separated and dropped from the opening of the cylindrical container. At this time, the tip of the lower opening of the cylindrical container was sufficiently wet with silicon, and the surface was covered with silicon.

The separated silicon melt droplet was allowed to fall naturally, and was brought into contact with the coolant 9 provided at the bottom of the closed vessel 7 for 0.5 seconds.

The coolant 9 is laid with particles of the polycrystalline silicon foam obtained in advance, and the surface temperature is set to 300.
Cooled so as to be kept at ° C.

The apparent density of the obtained polycrystalline silicon foam 10 was 1.66 g / cm ³ .

When the polycrystalline silicon foam was crushed, amorphous particles having an average particle volume of 0.1 cc were obtained.
When these particles were divided by a hammer,
Many were observed after the bubbles. The silicon particles were polished with diamond and the cross section was observed. As a result, many bubbles having a diameter of 0.5 mm to 1 mm were present at the center.

In addition, the particles 1 of the polycrystalline silicon foam
00g with jaw crusher, maximum particle size 2m
m, and the particle size of the crushed product was measured with an SK laser. As a result, the incidence of fine particles having a size of 200 μm or less was less than 0.05%.

Comparative Example 1 In Example 1, the time until contact with the coolant was set at 0.
All operations were performed under the same conditions as in Example 1 except that the time was set to 05 seconds, to obtain polycrystalline silicon. No visible bubbles were observed in the obtained polycrystalline silicon particles. The apparent density of the particles was 2.28 g / cm ³ .

When the particle size of the crushed product was measured with an SK laser in the same manner as in Example 1, the generation rate of fine particles having a size of 200 μm or less was 1%.

Comparative Example 2 In Example 1, a quartz plate heated to 1350 ° C. with a heater at the lower part was used as a cooling material, and cooled slowly.

No bubbles were present in the silicon. The apparent density of the particles was 2.33 g / cm ³ .

Further, the particle size of the crushed material which was crushed in the same manner as in Example 1 was measured with an SK laser. As a result, the incidence of fine particles having a size of 200 μm or less was 2%.

Example 2 Instead of forming a silicon melt by reacting trichlorosilane and hydrogen, solid silicon was filled in a graphite cylindrical container having a hole at the bottom, and heated to 1500 ° C. in a hydrogen atmosphere at a high frequency. Heated to form a silicon melt. Further, after maintaining the molten state in the presence of hydrogen for 30 minutes, pressure was applied with hydrogen from the upper portion of the melt, and the silicon melt was dropped from the lower hole.

The separated silicon melt liquid drops were allowed to fall naturally, and were brought into contact with the cooling material 9 provided at the bottom in 0.5 seconds.

Note that the coolant 9 is laid with particles of the polycrystalline silicon foam obtained in advance, and the surface temperature thereof is set to 300.
Cooled so as to be kept at ° C.

The apparent density of the solidified particles was measured and found to be 2.11 g / cm ³ .

When the particle size of the crushed product was measured with an SK laser in the same manner as in Example 1, the rate of generation of fine particles having a size of 200 μm or less was 0.2%.

Example 3 Example 1 was repeated under the same conditions as in Example 1 except that silicon tetrachloride was used as a raw material and the molar ratio of hydrogen / silicon tetrachloride was adjusted to 20 to form a silicon melt. A crystalline silicon foam was obtained.

The apparent density of the solidified particles was measured and found to be 2.05 g / cm ³ .

When the particle size of the crushed product was measured with an SK laser in the same manner as in Example 1, the generation rate of fine particles having a size of 200 μm or less was 0.2%.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a typical embodiment of an apparatus for producing a polycrystalline silicon foam of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cylindrical container 2 Opening 3 Heating device 4 Chlorosilane supply pipe 6, 11 Seal gas supply pipe 7 Closed container 8 Polycrystalline silicon foam 9 Coolant

Claims (5)

[Claims] 1. Bubbles are present inside and have an apparent density of 2.
A polycrystalline silicon foam having a weight of 20 g / cm ³ or less. 2. The method according to claim 1, wherein the silicon melt melted in the presence of hydrogen is allowed to fall naturally as droplets, and the hydrogen bubbles are fixed in the droplets within a period of 0.2 to 3 seconds. Of producing a polycrystalline silicon foam. 3. The method for producing a polycrystalline silicon foam according to claim 2, wherein the hydrogen bubbles are fixed in the droplet by bringing the droplet of the silicon melt into contact with a coolant having a temperature of 1100 ° C. or less. 4. The method for producing a polycrystalline silicon foam according to claim 2, wherein precipitation of silicon from chlorosilanes and melting of the silicon are simultaneously performed in the presence of hydrogen. 5. A method for producing granular polycrystalline silicon, comprising crushing the polycrystalline silicon foam according to claim 1 to an average particle diameter of 300 μm to 2 mm.