JP4231951B2 - Polycrystalline silicon foam and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
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 to obtain granular polycrystalline silicon is extremely small.
[0002]
[Prior art]
Conventionally, various methods for producing polycrystalline silicon used as a raw material for semiconductors or photovoltaic power generation batteries are known, some of which have already been industrially implemented.
[0003]
For example, one of them is a method called a Siemens method, in which a silicon rod heated to the deposition temperature of silicon by energization is placed inside a bell jar, and trichlorosilane (SiHCl ₃ , hereinafter referred to as TCS) or monosilane (SiH ₄ ). Is deposited with a reducing gas such as hydrogen to deposit silicon.
[0004]
There is also an increasing demand for a granular material obtained by crushing the above polycrystalline silicon to a particle size of about 300 μm to 2 mm. For example, in the use for semiconductors or solar cells, the granular polycrystalline silicon is used by melting it.
[0005]
Also known is a technique for producing particulate silica having a particle diameter of about 1 μm by introducing the granular polycrystalline silicon into an oxyhydrogen flame and evaporating and evaporating it.
[0006]
Furthermore, silicon nanoparticles that are attracting attention as visible light emitting devices are manufactured by irradiating a silicon target with an excimer laser in a helium atmosphere, and granular polycrystalline silicon is easily obtained as a material for the silicon target. If possible, silicon nanoparticles can be produced efficiently.
[0007]
The granular polycrystalline silicon has been produced by a method of further pulverizing a lump of crushed material obtained by crushing a silicon rod produced by the Siemens method.
[0008]
However, when the silicon rod is crushed to obtain granular polycrystalline silicon, a large amount of flakes, needles, and fine powders called “fine particles” are generated in a large amount. Such fine particles cause generation of dust and are difficult to handle. In particular, fine particles having a particle diameter of 200 μm or less have a risk of ignition, and thus have been carefully disposed of. For this reason, not only the yield relative to the raw material is reduced, but also a great deal of labor is required for disposal.
[0009]
[Problems to be solved by the invention]
Accordingly, an object of the present invention is to provide polycrystalline silicon and a method for producing the same, in which the amount of fine particles generated in crushing for producing granular polycrystalline silicon is extremely small.
[0010]
[Means for Solving the Problems]
The inventors of the present invention have made extensive studies to achieve the above object. First, the present inventors have confirmed that the generation mechanism of fine particles is due to the disintegration of polycrystalline silicon. That is, since polycrystalline silicon is highly degradable, when the nugget obtained by crushing the silicon rod is further crushed into granular polycrystalline silicon, a large amount of fine particles broken into flakes and needles are generated. Easy to do.
[0011]
And, based on the knowledge that if a structure that is preferentially crushed by a stress smaller than the stress that exhibits such a cracking property is given to the structure of polycrystalline silicon, the generation of fine particles in crushing can be suppressed. By adopting a foam structure, which is not known as a form of conventional polycrystalline silicon, the energy when crushing silicon is given priority as acting as the breaking energy of the bubble wall rather than acting on the cracked surface of the crystal. It was possible to reduce the generation rate of waste significantly compared to normal silicon crushed material.
[0012]
Moreover, in order to fully exhibit the effect of making the bubbles present, it has been found that it is effective to make the amount of the bubbles less than a specific apparent density, and the present invention has been completed.
[0013]
That is, the present invention is a polycrystalline silicon foam characterized by having air bubbles inside and an apparent density of 2.20 g / cm ³ or less.
[0014]
In the present invention, the apparent density is a value obtained from the volume and weight of particles obtained using a pycnometer. Specifically, the method described in pages 51 to 54 of the powder engineering handbook (Nikkan Kogyo Shimbun, issued February 28, 61).
[0015]
Further, in the course of studying the method for producing the polycrystalline silicon foam, the present inventors are known that gas hardly dissolves in molten metal such as silicon melt. When hydrogen is hydrogen, it was found that it can be dissolved in a certain amount. As a result of repeated research based on such knowledge, hydrogen was brought into contact with the silicon melt and dissolved, and then the liquid was spontaneously dropped as droplets and solidified under specific cooling conditions. It has been found that a large shear force is not applied to the droplets, and that the hydrogen present in the droplets is formed as bubbles to obtain a foam encapsulated in solidified polycrystalline silicon.
[0016]
That is, the present invention is characterized in that silicon melted in the presence of hydrogen is spontaneously dropped as droplets, and the hydrogen bubbles are fixed in the droplets within a period of 0.2 to 3 seconds. A method for producing a polycrystalline silicon foam is also provided.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The polycrystalline silicon foam of the present invention has air bubbles inside. Thus, a polycrystalline silicon structure having bubbles inside is not known so far and is a major feature of the present invention.
[0018]
That is, the polycrystalline silicon rod obtained by the Siemens method uses hydrogen gas as a raw material in production, but the deposited polycrystalline silicon is solid and there is no room for hydrogen to dissolve.
[0019]
In addition, methods of depositing silicon using hydrogen as one of the raw materials and recovering the silicon in a melt have been proposed, but in these methods, the melt is taken out of the hydrogen atmosphere and solidified. The hydrogen gas in the solidified body diffuses and disappears in the melt state.
[0020]
Furthermore, a method of producing polycrystalline silicon particles by dropping silicon produced in hydrogen gas onto a rotating disk in a molten state and flying it has also been proposed, but in such a method, a silicon melt containing hydrogen Since the droplets are subjected to a large shearing force at the initial stage of solidification, hydrogen dissolved in the silicon melt cannot gather to form bubbles, and many of the present invention having bubbles in which the dissolved hydrogen gas has grown. Crystalline silicon foam cannot be obtained.
[0021]
Furthermore, polycrystalline silicon obtained by using monosilane as a source gas and growing polysilicon particles in a fluidized bed incorporates a relatively large amount of hydrogen, but the hydrogen is a hydrogen of silicon in the polycrystalline silicon. Because it exists as a compound, it cannot exist as bubbles.
[0022]
The polycrystalline silicon foam of the present invention may have any shape as long as it has bubbles inside. For example, the shape of irregular coarse particles is common and preferred. The size of the coarse particles is preferably in the range of 0.01 to 3 cc, particularly 0.1 to 0.5 cc by volume. Moreover, in the manufacturing method mentioned later, the said coarse particle obtained may be obtained as a lump with which it fused partially depending on the cooling mode. Such a mass can be separated into the fused portion by light crushing, and can be easily made into the above irregular shaped coarse particles.
[0023]
In the present invention, abundance of air bubbles of the polycrystalline silicon foam in the apparent density of 2.20 g / cm ³ or less, in particular, 2.0 g / cm ³ or less, more, 1.8 g / cm ³ or less Preferably there is.
[0024]
Usually, the apparent density of polycrystalline silicon is 2.33 g / cm ³ , but when bubbles are included, the apparent density decreases. And the polycrystal silicon foam of this invention can suppress generation | occurrence | production of the microparticles | fine-particles at the time of crushing by containing a bubble in the quantity used as an apparent density of 2.20 g / cm < ³ > or less.
[0025]
In addition, since the polycrystalline silicon foam of the present invention is light, when it is supplied as it is to silicon for recharging as it is to a crucible for producing single crystal silicon, there is also an advantage that the generation of silicon melt droplets in the crucible is small. It is useful even in the state that it has and does not crush.
[0026]
In the above-mentioned polycrystalline silicon foam, a large number of bubbles may be present uniformly, or one or several large bubbles may be present, but the bubble diameter per one is 50 μm or more. Preferably there is.
[0027]
In the present invention, since the polycrystalline silicon foam having an excessively small apparent density is difficult to produce and may cause generation of fine particles due to a decrease in the thickness of the bubble wall, The silicon foam preferably has an apparent density of 1 g / cm ³ or more.
[0028]
The gas present in the bubbles of the polycrystalline silicon foam of the present invention is generally hydrogen gas from the production method described later, but the present invention is not limited to this, and the foam is Any gas that can be produced is not particularly limited.
[0029]
In addition, the method for crushing the polycrystalline silicon foam of the present invention is not particularly limited, and by using a crushing method using a known crusher such as a jaw crusher or a pin mill, the generation of fine particles is suppressed and a high yield is obtained. It is possible to obtain granular polycrystalline silicon.
[0030]
The method for producing the polycrystalline silicon foam of the present invention is not particularly limited, but as described above, silicon melted in an atmosphere of hydrogen gas by utilizing the fact that hydrogen gas easily dissolves in the silicon melt. A method is preferred in which the droplets are made into droplets, allowed to fall naturally without applying a strong shearing force, and solidified under specific conditions.
[0031]
That is, according to the present invention, silicon melted in the presence of hydrogen is naturally dropped as a droplet, and the hydrogen bubbles are fixed in the droplet within a period of 0.2 to 3 seconds. A method for producing a polycrystalline silicon foam is provided.
[0032]
In the method for producing a polycrystalline silicon foam according to the present invention, as a method for obtaining silicon melted in the presence of hydrogen, a method in which silicon is melted or the molten silicon is brought into contact with hydrogen gas may be employed. However, as a method of dissolving hydrogen most efficiently into the silicon melt, there is a method of simultaneously performing silicon deposition and melting of silicon using chlorosilanes as a raw material in the presence of hydrogen.
[0033]
Specifically, a mode in which a mixed gas of hydrogen gas and chlorosilanes is brought into contact with the surface of a heating body heated to a melting point of silicon or higher, and silicon is deposited and melted at the same time is exemplified.
[0034]
As the chlorosilanes, chlorosilanes containing hydrogen in the molecule, such as trichlorosilane and dichlorosilane, are preferable because the hydrogen concentration in the silicon melt can be further increased.
[0035]
In addition, as for the use ratio of hydrogen with respect to the chlorosilanes, a known ratio is employed without any particular limitation, but in order to form a higher concentration hydrogen atmosphere, the molar ratio of hydrogen / chlorosilanes is 5-50. It is preferable to adjust so that it becomes.
[0036]
Further, the silicon melt in which hydrogen is dissolved in this manner is naturally dropped 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.
[0037]
In the above-described operation, the hydrogen dissolved in the droplet of the silicon melt that falls naturally gathers as a bubble at the center of the droplet by holding for the specified time, and solidifies in that state to foam the polycrystalline silicon. The body is obtained.
[0038]
Therefore, in the above method, it is important that the silicon melt is naturally dropped as droplets. In other words, the supersaturated hydrogen gas present in the silicon melt gathers as time passes to form bubbles, but if the melt is solidified as it is, the bubbles move upward and dissolve under the influence of gravity. The hydrogen gas that should have been released is released very easily.
[0039]
On the other hand, when the silicon melt is naturally dropped, the gasified hydrogen remains in the droplet.
[0040]
In this case, the mechanism for collecting bubbles in the center of the droplet is estimated as follows. That is, when dropping from the substrate on which the melt is held, the droplet has a momentum associated with the deformation and tends to become a sphere immediately from its surface tension, so that the momentum derived from the deformation becomes the angular momentum of rotation. Therefore, even if it is weightless, a centrifugal force acts on the inside of the droplet due to the rotational motion. The centrifugal force is used as a substitute for gravity, and the hydrogen bubbles existing in the inside act as buoyancy toward the center, and as a result, the bubbles gather at the center of the droplet.
[0041]
The conditions for the bubbles to collect at the center depend on the rotational angular velocity of the droplet and the elapsed time. The initial momentum applied to the droplets is such that the longer the yarn is pulled during separation, the greater the momentum of rotation and the greater the angular velocity. That is, the higher the adhesion between the silicon melt and the base material, the faster the inner bubbles gather at the center and remain. When considering adhesion to the silicon melt, SiO ₂ or silicon nitride can also be used as the base material, but SiC having higher wettability, or silicide is easily formed even if the wettability is poor at first, The effect of the present invention can be shown more remarkably by using a carbon material with higher wettability.
[0042]
In the above method of the present invention, the time required to leave the heating body where the silicon droplets exist and fix the bubbles is at least sufficient to achieve the apparent density of the present invention. It is necessary for the time to be able to maintain the state gathered at the center, and it is preferably 0.2 seconds or longer, more preferably 0.4 seconds or longer, and further preferably 0.6 seconds or longer.
[0043]
On the other hand, since the bubbles collected at the center of the corner also diffuse and escape to the outside if cooled slowly, the above time is preferably 3 seconds or less, preferably 2 seconds or less.
[0044]
The time required to leave the heating body where the silicon droplets exist and fix the bubbles is predicted to be slightly different from the angular velocity applied to the droplets depending on the material of the heating body, and is based on SiC that can sufficiently increase the angular velocity. When using silicon nitride having poor wettability for the base material as compared with the case of using it for the material, it is preferable to take the time slightly longer.
[0045]
In the present invention, in the operation of bringing the droplet into contact with the coolant, the coolant is not particularly limited to a solid, liquid, gas or the like.
[0046]
To illustrate a preferred embodiment using the above coolant, a coolant is composed of a material that does not substantially react with silicon, for example, a member such as silicon, copper, or molybdenum, and a droplet of silicon melt is placed on the coolant. Examples include a mode of dropping, a mode of using a liquid refrigerant that does not substantially react with silicon, for example, liquid silicon tetrachloride, liquid nitrogen or the like as a coolant, and dropping a droplet of silicon melt therein.
[0047]
Moreover, the cooling gas generated by spraying the refrigerant can be used as a refrigerant and brought into contact with the silicon melt droplets.
[0048]
In the embodiment using the solid coolant, the surface thereof may be directly or indirectly cooled by a known cooling method, if necessary. In addition, there is a case where the polycrystalline silicon foam is deposited by dropping and solidifying the droplets of the silicon melt on the coolant sequentially. In this case, the uppermost surface of the polycrystalline silicon foam is cooled. Acts as a material. Further, in order to absorb the impact when the silicon melt droplet falls on the coolant surface, the surface of the coolant preferably has irregularities, for example, particles such as silicon particles are present in advance. It is preferable to keep it. In this case, it is particularly preferable to use a part of the obtained polycrystalline silicon foam as the silicon particles.
[0049]
An apparatus for carrying out the method of the present invention is not particularly limited, but an apparatus having a structure shown in FIG. 1 may be mentioned as an apparatus suitable for continuously dropping silicon melt droplets. That is,
(1) A cylindrical container 1 having an opening 2 serving as a silicon outlet at the lower end;
(2) A heating device 3 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,
(3) In the cylindrical container, a chlorosilane supply pipe 4 that is provided so as to open downward in a space 5 surrounded by an inner wall heated to a melting point of silicon or higher, and supplies chlorosilanes A;
(4) a seal gas supply pipe 6 for supplying a raw material hydrogen gas as a seal gas B into a gap formed by the inner wall of the cylindrical container and the outer wall of the chlorosilanes supply pipe;
And (5) a coolant 9 provided with a space below the cylindrical container 1.
A more basic device is mentioned.
[0050]
In the above apparatus, in order to efficiently collect the exhaust gas discharged from the cylindrical container 1, the cylindrical container 1 and the coolant 9 are covered with the sealed container 7 provided with the exhaust pipe 12 for exhaust gas D. preferable.
[0051]
At that 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 sealed container 7 to supply a seal gas C such as nitrogen, hydrogen, and argon. preferable.
[0052]
Moreover, in the said apparatus, the high frequency coil is used suitably for the heating apparatus 3 for heating the cylindrical container 1. FIG. The cylindrical container 1 is preferably made of a material that can be heated by a high frequency and is resistant at the melting point of silicon. Generally, carbon, silicon nitride, or the like is used.
[0053]
In the above apparatus, chlorosilane supplied from the chlorosilanes 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 seal gas. Silicon is deposited on the inner wall heated above the melting point, and at the same time, the silicon melts to form a silicon melt. The silicon melt flows down the inner wall and spontaneously falls as a droplet 14 from the opening 2.
[0054]
By adjusting the distance from the opening 2 to the coolant 9 at the bottom of the hermetic container, it is possible to control the time during which the droplets of the silicon melt contact the coolant. Further, the temperature of the coolant 9 can be adjusted to a desired temperature indirectly or by directly supplying the cooling gas E from the cooling gas supply pipe 13.
[0055]
The droplets in contact with the coolant form bubbles of hydrogen gas while falling, and solidify by contacting with the coolant in that state to form the polycrystalline silicon foam 8.
[0056]
【The invention's effect】
As can be understood from the above description, the polycrystalline silicon foam of the present invention generates very few fine particles due to crushing, and crushes this to produce granular polycrystalline silicon having an average particle size of about 300 μm to 2 mm. The yield can be significantly improved.
[0057]
Further, the polycrystalline silicon foam of the present invention is light in itself and is useful as silicon for recharging at the time of producing a single crystal.
[0058]
【Example】
Example 1
The apparatus shown in FIG. 1 is used, and after heating the inner wall of the cylindrical container 1 made of carbon as a base material to 1500 ° C. by the high frequency coil of the heating device 3, trichlorosilane is supplied from the chlorosilanes supply pipe 4 and hydrogen is sealed gas. It introduced into the space part 5 of the cylindrical container 1 from the supply pipe 6, and silicon was deposited and melted on the inner surface of the cylindrical container. At this time, the molar ratio of trichlorosilane and 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.
[0059]
The separated and dropped silicon melt droplets were naturally dropped and brought into contact with the coolant 9 provided at the bottom of the sealed container 7 in 0.5 seconds.
[0060]
The coolant 9 was laid with polycrystalline silicon foam particles obtained in advance and cooled so that the surface temperature was maintained at 300 ° C.
[0061]
The apparent density of the obtained polycrystalline silicon foam 10 was 1.66 g / cm ³ .
[0062]
When the polycrystalline silicon foam was crushed, amorphous particles having an average particle volume of 0.1 cc were obtained. When this particle was divided with a hammer, a lot of bubbles were observed on the fracture surface. Further, when the silicon particles were polished with diamond and the cross section was observed, many bubbles having a diameter of 0.5 mm to 1 mm were present in the center.
[0063]
Further, when 100 g of the above-mentioned particles of polycrystalline silicon foam were crushed with a jaw crusher until the maximum particle size was 2 mm or less, and the particle size of the crushed material was measured with an SK laser, the incidence of fine particles of 200 μm or less was 0. .05% or less.
[0064]
Comparative Example 1
In Example 1, polycrystalline silicon was obtained by operating under the same conditions as in Example 1 except that the time until contact with the coolant was 0.05 seconds. No visible bubbles were observed in the obtained polycrystalline silicon particles. The apparent density of the particles was 2.28 g / cm ³ .
[0065]
Moreover, when the particle diameter of the crushed material as measured in Example 1 was measured with an SK laser, the generation rate of fine particles of 200 μm or less was 1%.
[0066]
Comparative Example 2
In Example 1, a quartz plate provided with a heater at the bottom and heated to 1350 ° C. was used as the coolant, and was cooled slowly.
[0067]
There were no bubbles in the silicon. The apparent density of the particles was 2.33 g / cm ³ .
[0068]
Moreover, when the particle diameter of the crushed material as measured in Example 1 was measured with an SK laser, the incidence of fine particles of 200 μm or less was 2%.
[0069]
Example 2
Instead of reacting trichlorosilane with hydrogen to form a silicon melt, solid graphite is filled in a cylindrical tube with a hole in the bottom, and heated to 1500 ° C. in a hydrogen atmosphere at high frequency to obtain a silicon melt. Formed. Further, after maintaining the molten state for 30 minutes in the presence of hydrogen, pressure was applied with hydrogen from the upper part of the melt, and the silicon melt was dropped from the lower hole.
[0070]
The separated and dropped silicon melt droplets were naturally dropped and brought into contact with the coolant 9 provided in the lower portion in 0.5 seconds.
[0071]
The coolant 9 was laid with polycrystalline silicon foam particles obtained in advance and cooled so that the surface temperature was maintained at 300 ° C.
[0072]
The apparent density of the solidified particles was measured and found to be 2.11 g / cm ³ .
[0073]
Moreover, when the particle diameter of the crushed material as measured in Example 1 was measured with an SK laser, the incidence of fine particles of 200 μm or less was 0.2%.
[0074]
Example 3
In Example 1, a polycrystalline silicon foam was obtained under the same conditions as in Example 1 except that silicon tetrachloride was used as a raw material and the hydrogen / silicon tetrachloride molar ratio was adjusted to 20 to form a silicon melt. It was.
[0075]
When the apparent density of the solidified particles was measured, it was 2.05 g / cm ³ .
[0076]
Moreover, when the particle diameter of the crushed material as measured in Example 1 was measured with an SK laser, the incidence of fine particles of 200 μm or less was 0.2%.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a typical embodiment of a polycrystalline silicon foam production apparatus according to the present invention.
DESCRIPTION OF SYMBOLS 1 Cylindrical container 2 Opening part 3 Heating apparatus 4 Chlorosilane supply pipes 6 and 11 Seal gas supply pipe 7 Sealed container 8 Polycrystalline silicon foam 9 Coolant

Claims (4)

A polycrystalline silicon foam having bubbles therein and an apparent density of 2.20 g / cm ³ or less.   The polycrystalline silicon foam according to claim 1, wherein the bubbles have one or more bubbles each having a diameter of 50 μm or more. The silicon melt melted in the presence of hydrogen is allowed to fall spontaneously as droplets and contacted with a coolant having a surface temperature of 500 ° C. or less without applying a shearing force, for a time of 0.2 to 2 seconds. A method for producing a polycrystalline silicon foam, wherein the hydrogen bubbles are fixed in a droplet.   4. The method for producing a polycrystalline silicon foam according to claim 3, wherein the precipitation of silicon using chlorosilanes as a raw material and the melting of the silicon are simultaneously performed in the presence of hydrogen.

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