JP2008143754A - Spherical silicon crystal and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To directly convert a silicon raw material into spherical silicon single crystals; and at the same time, to drastically improve the yield of the spherical silicon single crystals. <P>SOLUTION: High purity powdery silicon 22 is accommodated in recesses of a high purity ceramic, quartz glass or porous ceramic vessel each having a surface with a fine projection and recess structure and many recesses. Then, the powdery silicon 22 in each recess is melted in an atmosphere, obtained by adding a silane gas or a silicon halide gas into an inert gas or a high purity inert gas, to form liquid-repellent silicon 23 in the recess. Thereafter, the spherical silicon crystals are obtained by cooling the liquid-repellent silicon 23. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a spherical silicon crystal and a method for producing the same. More specifically, high-purity powdered silicon is used as a raw material, and it is possible to directly produce a spherical silicon crystal for solar cells without processing such as cutting. The present invention relates to a spherical silicon crystal that enables effective use of resources and significant cost reduction of a silicon solar cell by realizing an inexpensive manufacturing method, and a manufacturing method thereof.

  Securing electric energy by solar power generation is an important issue for solving serious problems such as depletion of fossil resources, environmental degradation, and energy shortages in each country, and is essential for the stable development of the world economy. Therefore, developed countries have been investing in the government to develop high-efficiency solar cells and inexpensive manufacturing methods for over 40 years since the 1960s. As a result, mass production is currently underway by private companies in each country. Especially since 2003, there has been a steep increase in demand due to preferential policies for the introduction of solar power generation in Europe, especially in Germany. Production expansion continues. However, despite such commercial production, technical problems remain that hinder the cost reduction of photovoltaic power generation. That is because the current silicon substrate manufacturing technology uses only about half of the raw material, and mass production is progressing without a prospect of cost reduction. In other words, a cheap and inexpensive manufacturing method for solar cells has not yet been realized.

  Today's solar cells can be broadly classified into three types according to the type of material, its form and structure. The first of these is crystalline silicon, which has a single crystal material and a polycrystalline material, and solar cells using these materials as a substrate account for a high proportion of about 90% of the world's total production. The second solar cell is based on thin film silicon, which uses amorphous silicon and microcrystalline silicon thin film, with a global total production ratio of 7-8%. The third solar cell is a compound semiconductor thin film system that does not use silicon. In this case, the raw material uses rare resources such as gallium arsenide, copper / indium / selenium compound, cadmium telluride, etc., and the world total production is 2 to 3%.

  Among the above three types of solar cells, solar cells using crystalline silicon as a substrate have the highest power generation efficiency, no deterioration over time, no depletion of resources, and no toxicity of materials, etc. It has been put into practical use. However, the current production methods using crystalline silicon substrates have the above-mentioned problems, leading to a shortage of silicon raw materials and soaring, and it is difficult to meet the rapidly increasing demand with the current methods in the future. It is predicted. In order to overcome this, the material is silicon, and the emergence of solar cells with new manufacturing methods and shapes that effectively use the raw materials and solar cells with other materials has become essential (see Non-Patent Documents 1 and 2). .

  The current method for producing a crystalline silicon substrate is to use a high-purity silicon raw material to pull up a single crystal ingot (diameter: 200 mm, length: about 800 mm) or a cast method to make a polycrystalline ingot (about 600 x 600 x 300 mm) make. These are once shaped into a 150 x 150 x 300 mm silicon block using a band saw, etc., and then a silicon cell substrate with a size of about 150 x 150 x 0.3 mm is cut from this silicon block with a multi-wire saw. This is a method of obtaining hundreds. The problem is that in this later step, a silicon substrate is obtained by a method of mechanically cutting the ingot. That is, the silicon crystal corresponding to the thickness (corresponding to the cutting allowance for cutting) hit by the blade used for cutting is scraped into silicon fine particles, and becomes sludge that is turbid with water, oil, abrasives and the like used for cutting. Particularly in the process of obtaining several hundred silicon substrates using a multi-wire saw, the amount obtained by multiplying the thickness of the wire by about 0.3 mm and the number of silicon substrates, that is, about half of the silicon ingot is cut sludge. Since the abrasive sludge and metal wire components (iron and nickel) are mixed in the generated sludge, it is difficult to use it again as a raw material. Even so, the refining cost increases and it is higher than new raw materials. In the end, about half of the raw material is wasted in the current method for producing a crystalline silicon substrate. In addition, the multi-wire saw used for cutting is expensive at 10 million yen, especially for materials that are difficult to cut, such as silicon, it takes a considerable amount of time to cut and process a 150 mm square substrate from an ingot. ineffective.

  Thus, the present manufacturing method which obtains a silicon substrate from a silicon block using a silicon ingot requires costs such as raw material costs, processing costs, and equipment costs, and hinders cost reduction of solar cells. FIG. 8 shows the manufacturing flow and cost breakdown from the raw material 101 to the module 105 through the ingot 102, the silicon substrate 103, and the cell 104, based on previously published materials. From this, it can be seen that the manufacturing cost of the silicon substrate 103 accounts for about 50% of the total, and the high cost of this part is a problem.

  As a method for solving these problems, a method has been proposed in which a silicon raw material is directly made into a spherical silicon crystal and used as a substrate. There are two advantages of using spherical silicon crystals as solar cell substrates. One of them is that the amount of silicon raw material used can be reduced to 1/5 to 1/8 compared to the current crystalline silicon substrate. For example, either spherical silicon with a diameter of around 1 mm is laid on a substrate such as resin or glass, or sunlight is sparsely collected using a concave mirror or lens. It has become clear that the amount of silicon raw material used is significantly reduced. (Refer nonpatent literature 3). Another advantage is that it can be manufactured directly without a cutting step. Therefore, if spherical silicon crystals can be used, it is expected that the production cost will be reduced and the solar cell system price will be lowered due to the significant reduction in the amount of raw materials used and the direct manufacturing process, which will lead to widespread use. it can.

  The spherical silicon crystal described above is manufactured by a drop method, and the outline will be described with reference to FIG. The silicon raw material put in the crucible 111 in the electric furnace 114 is heated by the heater 112 to become molten silicon 113. When pressurized with argon gas 115 from the gas pipe at the top of the crucible 111, the molten silicon 113 becomes a silicon droplet 116 from a nozzle with a diameter of about 0.5 mm opened at the bottom of the crucible 111 and is pushed out of the crucible. It is accelerated by gravity in the fall tower 117 of about 14m and falls. The silicon droplet 116 pushed out to the space outside the crucible becomes spherical due to its surface tension, and the spherical silicon crystal 118 cooled and solidified and crystallized as it falls is collected in the storage section 118 below the dropping tower 117. . In this method, it is said that several hundreds or more of spherical silicon can be obtained per second by adjusting the way of pressurization with argon gas 115, the shape of the nozzle, etc., and is excellent in mass productivity as a high-speed manufacturing method. Yes.

  Whether spherical silicon crystals obtained by such a drop method have solar cell quality is determined by measuring the type and amount of impurities, the number of crystal defects, and the carrier lifetime associated with them. Yes. As a result, the proportion of crystals that can be used for solar cells with a carrier lifetime exceeding 1 μsec (microseconds) among the obtained spherical silicon crystals is currently reported to be about 50% (see Non-Patent Document 4).

  However, it is expected that it is difficult to further increase the yield of spherical silicon crystals by the drop method. The reason is that the solidification and crystallization speed of the silicon crystal by the drop method is performed in a supercooled state at an abnormal speed of about several hundred degrees Celsius / second. This is because it is a crystal. Moreover, the melted silicon spheres falling at a high speed are solidified from the outer surface, so that about half of those containing high-density crystal defects can be formed. Since these have a carrier lifetime of 0.1 μsec or less, they cannot be used for solar cells, which causes a decrease in yield. Therefore, the drop method has an advantage that the raw material can be directly converted into a spherical silicon crystal without going through a cutting process, but has a problem that the utilization rate of the raw material is currently about 50%.

Nikkei Microdevice, March 2006, P25 Nikkei Electronics, March 13, 2006, P103 Opt News (2006), No.1, P24 2005 Preliminary Report on “Power Generation Technology Development and Related Projects”, September 29, 2006, New Energy and Industrial Technology Development Organization

  The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to make the silicon raw material directly into a spherical silicon crystal and to greatly improve its yield.

According to the first aspect of the present invention, a spherical layer obtained by forming a release layer that imparts liquid-liquidity to molten silicon on a substrate having a large number of depressions, and melting and solidifying high-purity silicon therein. It is a silicon crystal.
(2) The invention is based on high-purity powdered silicon, and in a high-purity inert gas atmosphere, the surface has a fine concavo-convex structure, and a high-purity ceramic or quartz glass container having a large number of depressions in the depressions. It is a spherical silicon crystal obtained from liquid silicon.
According to a third aspect of the present invention, in the spherical silicon crystal according to the second aspect, a high-purity nitride powder is coated on a surface of the high-purity ceramic or quartz glass.
The invention according to claim 4 is the spherical silicon crystal according to claim 3, wherein the high-purity ceramic is a high-purity porous ceramic.
The invention according to claim 5 is the spherical silicon crystal according to claim 3 or 4, wherein the high-purity nitride powder is a high-purity aluminum nitride powder.
A sixth aspect of the present invention is the spherical silicon crystal according to the third or fourth aspect, wherein the high-purity nitride powder is a high-purity silicon nitride powder.
The invention according to claim 7 is the spherical silicon crystal according to claim 6, wherein the high-purity silicon nitride powder is a high-purity β-type silicon nitride powder. Here, the high-purity β-type silicon nitride powder desirably has a particle size of 1 to 50 microns.
The invention of claim 8 is a high-purity nitride powder in an atmosphere in which high-purity powdered silicon is used as a raw material and a silane gas having a concentration of 10% or less or a silicon halide gas having a concentration of 10% or less is added to a high-purity inert gas. Is a spherical silicon crystal obtained from high-purity ceramics or quartz glass having a concavo-convex structure with a fine surface and having a large number of depressions or liquid silicon in the depressions of a high-purity porous ceramic container.
The invention of claim 9 is to form a release layer that imparts liquid-liquidity to molten silicon on a substrate having a number of depressions, and melt and solidify high-purity silicon in the depressions. A method for producing a spherical silicon crystal, comprising obtaining a spherical silicon crystal.
According to the invention of claim 10, high-purity powdered silicon is accommodated in the dent of a high-purity ceramic or quartz glass or high-purity porous ceramic container having a concavo-convex structure with a fine surface and a large number of dents. In an atmosphere in which a silane gas or a silicon halide gas is added to an active gas or a high-purity inert gas, powdered silicon in the recess is melted to produce liquid silicon in the recess, and the liquid silicon is cooled. Thus, a spherical silicon crystal having a diameter of 5 mm or less is obtained.
The invention of claim 11 is the method for producing a spherical silicon crystal according to claim 10, characterized in that the shape of the bottom surface of the recess is flat.
A twelfth aspect of the invention is the method for producing a spherical silicon crystal according to the eleventh aspect, wherein the cooling rate of the liquid silicon is set to 200 ° C. or less per hour.
Here, the high purity inert gas is an inert gas having a purity of 6N (99.9999%) or more. In addition, powdered silicon is a generic term for fine silicon, coarse silicon, fine powder silicon, fine powder silicon, and ultrafine powder silicon having a size of 500 microns (0.5 mm) or less. Means powdered silicon having a purity of 6N (99.9999%) or higher. Furthermore, the liquid-liquid state is a state having liquid-liquidity, and the liquid-liquid state is that elements and various materials are melted at a high temperature, and the surface of the substrate or the container that supports the liquid is in a “non-wetting state”. Means. When the melt is water, there is a term “water repellency” to indicate a state without wettability, but “liquid repellency” is different from “water repellency” and there is no state of wettability in a wider temperature range. (“Metal” August 2005 issue, p. 35, “Effect of liquid crystallinity on crystal growth”). The spherical silicon crystal is a general term for a spherical silicon single crystal and a spherical silicon polycrystal.

(Function)
According to the present invention, a release layer that imparts liquid-liquidity to molten silicon is formed on a substrate having a large number of depressions, and high-purity silicon is melted and solidified in the depressions. A silicon crystal is obtained. In addition, according to the present invention, a number of depressions are formed on the surface of a high purity ceramic or quartz glass container having fine irregularities on the surface, and high purity powder silicon powder is placed in the depressions. A molten silicon melt is produced by melting at a high temperature in a pure inert gas atmosphere, and a spherical silicon crystal is obtained by cooling it.

According to the present invention, the following effects can be obtained.
(1) Most of the raw material can be converted into a spherical silicon crystal, and the raw material utilization rate is high and there is no waste.
(2) Most of the obtained spherical silicon crystals are single crystals and have high quality.
(3) Since it is a simple process, it can be manufactured at low cost by an inexpensive mass production apparatus with a simple structure and few attached equipment.
(4) Quartz glass, silicon carbide, and silicon nitride ceramics, which are container materials, can be mass-produced by a casting method, so that inexpensive ones can be obtained.
(5) As described above, the present invention has many advantages in production, and it can be expected to effectively use resources and significantly reduce the cost of silicon solar cells.
Furthermore, the composite ceramics of the container and the release material according to the present invention can be applied to a crucible for producing a silicon ingot by the current casting method and a crucible for producing a single crystal silicon by the Bridgeman method. Can be expected.

Hereinafter, embodiments of the present invention will be described.
In this embodiment, unlike the drop method, a large number of high-quality spherical silicon crystals are produced by the multiple crucible method, almost all of them have a carrier lifetime of 1 μsec or more, and the utilization rate of the high-purity silicon raw material is increased to nearly 100%. The biggest feature. In the multiple crucible method, a small amount of high-purity powdered silicon powder is placed in a number of depressions opened on quartz glass or ceramics, and this is melted at a high temperature of 1450 ° C in a high-purity inert gas atmosphere. This is a method of obtaining a spherical silicon crystal having a diameter of about 1 mm by making the melt into a liquid and gradually cooling it.

  Here, it is technically difficult to freely obtain liquid silicon by melting silicon while maintaining high purity, and it is necessary to solve various important problems in applying the multiple crucible method to silicon. Therefore, in order to better understand the problem of obtaining liquid silicon, the basic interpretation of liquid property and an application example of the multiple crucible method to materials other than silicon will be described below.

  Regarding the wettability of liquids at room temperature, the mechanism of water and organic liquids has long been interpreted. In this case, the state in which the liquid is not wettable on the substrate is called water repellency, and whether water repellency is observed is the presence or absence of a chemically reactive group between the liquid and the substrate, or an uneven surface is formed on the surface. And how much the contact area of the liquid can be reduced. This is common also in the case of a high-temperature liquid, and the wettability and liquid-repellent property of the powder raw material in the present invention will be described with reference to FIG. As shown in FIG. 3A, the powder raw material 1 on the substrate 2 melts at a high temperature and becomes a liquid. However, if the substrate 2 is dirty or there are impurities in the powder raw material 1, these are the substrate 2 and the liquid. As shown in FIG. 2B, the liquid 3 that spreads so that the contact angle θ with the substrate 2 becomes an acute angle becomes a solid as it is. On the other hand, if there is no reactive group on the substrate 2 and the liquid and the contact surface is physically reduced by forming irregularities on the surface of the substrate 2, the liquid becomes a liquid liquid 4, the shape is close to a sphere, and the contact angle is It approaches 180 °.

  In this way, the liquid spillability can be explained very simply, but silicon in a molten state causes a chemical reaction with almost all elements and substances other than inert gas to form various silicon compounds. There are difficulties in making it happen. On the other hand, since it is relatively easy to obtain a spherical crystal by making a material other than silicon, such as germanium as a semiconductor material or calcium fluoride as an optical material, into a liquid state, the experimental results will be described first. These melts have a melting point lower than that of silicon, hardly cause wettability with container materials such as carbon and quartz glass, and have low reactivity to facilitate liquefaction. The liquefaction of materials other than silicon and the crystal shaping method associated therewith are described in Japanese Patent Application No. 2000-216319 (Japanese Patent Application Laid-Open No. 2002-29882) previously filed by the present inventor. For optical materials and gallium antimony semiconductor materials, it is possible to create a liquid crystal state using carbon as a container material to obtain a shaped crystal having a free surface. In order to confirm that it is easy to obtain spherical crystals with these materials, the present inventor made three types of spherical crystals using carbon as a crucible and a powder material as a raw material. The result will be described with reference to FIG. Fig. A shows the flow of the multiple crucible method in a simple manner. First, the powder raw material 1 is put into a crucible 5 having a number of depressions as shown in the upper diagram, and then melted as shown in the middle diagram. Then, the melt 6 becomes spherical due to the surface tension, and the spherical crystal 7 is obtained by cooling and solidifying and crystallizing as shown in the lower figure. Fig. B is a photograph of spherical crystals of 5mm diameter potassium chloride (melting point: 768 ° C), 2mm diameter germanium (melting point: 936 ° C) and calcium fluoride (melting point: 1396 ° C) obtained by such a process. is there. In this case, the material of the crucible 5 is carbon, and the atmosphere is vacuum, argon gas, hydrogen gas, or the like. These materials are useful materials for infrared and ultraviolet optical materials, but all have the purity required for their intended use and are obtained without contamination from the initial powder raw material to spherical crystals. . As described above, it is not difficult to obtain a spherical crystal having a diameter of about 5 mm or less by using a material other than silicon and using these high-purity powder raw materials as a liquid melt.

  What should be noted here is that most of the obtained spherical crystals of potassium chloride, germanium and calcium fluoride are single crystallized. In general, a single crystal is obtained by artificially preparing a seed crystal in advance, melting a part of the seed crystal, and growing the crystal to form a single crystal. However, in this experiment, it was discovered that a small spherical single crystal was spontaneously formed without using a seed crystal. This spontaneous single crystallization is interpreted as being promoted for the following reasons, and the mechanism will be described with reference to FIG. In FIG. A, a spherical liquid melt 11 is supported by a substrate 13 at a melt contact interface 12 having a small area at the lowest point of the sphere. When the temperature of the melt 11 is lowered and crystallized, the flow of latent heat of crystallization is concentrated at one point indicated by an arrow “⇒”. Therefore, the fine seed crystal nuclei where the melt 11 is solidified to start crystal growth are limited to point-like shapes, which gradually grow and single crystallization proceeds. Further, the crystal growth interface 14 proceeds outward as shown by three arrows “↑” to form a convex surface, and the entire single crystallization is likely to proceed spontaneously. On the other hand, as shown in FIG. B, when the melt 11 gets wet with the substrate 13 and the contact angle becomes an acute angle, the area of the melt contact interface 12 between the melt 11 and the substrate 13 is different from the case of A. And large area. Accordingly, the flow of latent heat of crystallization is dispersed in a large number as indicated by an arrow “⇒”, and many fine seed crystal nuclei are generated at the substrate 13 and the melt contact interface 12 to cause polycrystallization. In this case, the crystal growth interface 14 progresses inward as shown by three arrows “↑” to become a concave surface, and the whole crystallizes. The correlation that the direction of the flow of latent heat of crystallization and the growth interface become convex is single crystallization, and that when it becomes concave is polycrystallization has long been supported by the growth process of large crystals. This time, it was clarified for the first time that the same phenomenon occurred in a small spherical crystal.

  Needless to say, this spherical single crystallization mechanism can be applied to the spherical silicon crystal of the present invention. However, there are many difficulties in obtaining spherical silicon crystals that can be used for solar cells using high-purity silicon powder as a raw material. They come from the technical problems associated with the four items of silicon powder raw material, container material, atmosphere and equipment / process, which will be explained in order.

First, a high-purity silicon powder as a raw material, which is the first item, will be described.
During the high-temperature process of melting the silicon powder raw material, silicon undergoes a chemical reaction with almost all elements and substances other than inert gas to produce all kinds of silicon compounds, and this silicon-specific property makes it easy to obtain liquid silicon. It is the biggest factor that can not be. Since the raw material that can be used here requires high-purity powder of 6N (99.9999%) or higher, obtain a commercially available product for semiconductors or solar cells, and make it a raw material that has been crushed so that there is no decrease in purity. I must. An important criterion along with purity is the particle size of the powder. The reason is that the powder raw material has a larger surface area ratio relative to the volume of the particles than the bulk raw material, so that the amount of moisture, oxygen, etc. adsorbed on the surface thereof is an order of magnitude larger than that of the bulk raw material. Also, nitrogen, oxygen and carbon dioxide, which are air components, are occluded in the spaces between the powder particles. When the temperature rises in the presence of such moisture and residual gas, these are taken in, and the molten silicon is in a liquid solution. The state cannot be maintained. Therefore, the raw material for spherical silicon single crystal has high purity because it requires effective removal of moisture adsorbed on the powder surface and occluded gas between powder particles by performing vacuum heat treatment in the high temperature process leading to molten silicon. It is desirable to use a powder that has a particle size of about 10 to 300 microns.

The second item is a container that holds silicon from high-purity silicon powder to melting, liquefaction and crystallization.
Vessel materials that are highly reactive and have no wettability / reactivity and are not a source of contamination are extremely limited. For example, carbon that is useful as a general container material reacts instantaneously when it comes into direct contact with molten silicon, forming silicon carbide, and the molten silicon penetrates into the container, so it is not useful at all, and the best container for gallium arsenide Pyrolytic boron nitride (P-BN), which is a material, is not wettable with molten silicon, but it cannot be used because it dissolves boron into silicon and causes an amount of boron impurity contamination that cannot be used in solar cells. .
Therefore, the melting point of the container material is higher than the melting point of silicon, there is no wettability / reactivity with molten silicon, and it becomes various refractory materials that can be highly purified, and those that satisfy these conditions are limited as follows: Is done. They are aluminum oxide (alumina), magnesium oxide, beryllium oxide for oxide ceramics, aluminum nitride, silicon nitride for nitride ceramics, silicon carbide for carbide ceramics, and quartz glass and carbon, all of which are dense and porous. There are materials.
However, these materials, whether dense or porous, all have a surface with a fine concavo-convex structure, or a mold release material is applied to the concavo-convex surface, or a CVD (Chemical Vapor Deposition) method or the like. It is difficult to liquefy silicon unless a thin film such as aluminum nitride is formed or any surface treatment is performed. For example, dense alumina, aluminum nitride, and silicon nitride have their surfaces mechanically rolled, blasted, laser processed, etc. to make an uneven structure of several tens of microns, and quartz glass is melted with hydrofluoric acid. It is necessary to perform liquefaction by drastically reducing the contact area with the molten silicon by taking out (etching process) to form a concavo-convex structure. On the other hand, porous carbon, silicon carbide, silicon nitride, or the like can function as a container material by applying a release material having no wettability and reactivity with molten silicon on the surface thereof.
It is necessary to search for a special container material in which impurity contamination to molten silicon is reduced by selecting an optimal one among the combinations of various materials and release materials.

The third item is the atmosphere (space) in which the high-purity silicon powder raw material is melted.
The high-purity silicon powder raw material is filled in a multiple crucible at room temperature, heated in an electric furnace, and air, adsorbed gas, and moisture are exhausted by a vacuum pump. An active gas must be introduced into the electric furnace, and the temperature must be further raised to melt to form liquid silicon.
The high-purity inert gas used as the atmosphere flows from the cylinder through the piping and is introduced into the atmosphere of the electric furnace. If a small amount of oxygen, moisture, nitrogen, carbon dioxide, or other components are mixed during this period, A chemical reaction occurs instantaneously with molten silicon, oxygen and moisture form silicon dioxide, nitrogen forms silicon nitride, and carbon dioxide forms silicon carbide, and the product is a high melting point ceramic. As a result, the molten silicon loses its liquidity and gets wet with the container material, and at the same time, these are taken into the molten silicon and become impurities. Therefore, in order to prevent this, it is important to keep the contamination of the gas pipe and the impurity gas in the electric furnace and the springing out to a minimum. Also, even if a very small amount of water springs out from the inner wall of the electric furnace tank, it must be devised to make it harmless by using a chemical reaction before it comes into contact with the molten silicon. .
In this way, it is essential to prevent contamination of silicon and maintain the purity that can be used for solar cells, from the powder raw material to solidification, in order to minimize the decrease in purity of the high purity inert gas used in the atmosphere. Contamination must be prevented.

  Finally, the fourth item is an apparatus used in the process of obtaining a spherical silicon single crystal from high-purity silicon powder through molten silicon and a process for functioning it. High-purity silicon powder raw materials and container materials are placed in an electric furnace during a high temperature period from the normal temperature at the start of the process to the molten silicon and then solidified. Here, even if the above-mentioned problems are solved with respect to the raw material, the container and the atmosphere, no liquid silicon melt can be obtained. The reason for this is that if a series of processes from high-purity silicon powder to molten silicon occurs, impurity elements and impurity gas from the electric furnace accessory parts such as vacuum valves and vacuum gauges, electric furnace inner walls, and heat insulating materials If the amount is too small, liquid silicon cannot be obtained due to the chemical reaction between the molten silicon and impurities. For example, high-purity argon gas is contaminated in the middle of the piping from the gas cylinder to the electric furnace, and when introduced into the electric furnace, there is also contamination from the gas that springs out from the internal walls and heat insulating material at high temperatures. . It is necessary to devise ways to keep these to a minimum or to make them harmless even if contamination occurs. In addition, since gas is always emitted from the heater and structure of the electric furnace at high temperatures, the electric furnace is first baked by a vacuum pump at a high temperature of 1450 ° C or higher (hereinafter referred to as baking) before adding high-purity silicon powder. It is necessary to devise processes in order to minimize atmospheric pollution.

  Thus, in order to obtain a spherical silicon single crystal from the high-purity silicon powder by the “liquid crystallizing method”, it is essential to solve the individual problems described above, and these are related to each other. There must be nothing unresolved, and the best conditions must be found in it.

The problems to obtain spherical silicon crystals for solar cells are summarized as follows.
(1) First problem: Establishing conditions for freely obtaining the liquid-liquidity using silicon powder as a raw material.
(2) Second problem: To obtain spherical silicon crystals of a quality that can be used for solar cells while minimizing impurity contamination of molten silicon in the manufacturing process.
(3) Third problem: Obtaining a larger amount of spherical silicon crystals for solar cells than liquid molten silicon.
By solving these problems, it becomes possible to freely obtain a large amount of liquid silicon melt and spherical silicon crystals from high-purity silicon powder, and the means for solving the problems will be described in detail below.

A high-purity silicon powder raw material is first taken up as a means for solving the problems.
Silicon for semiconductors has been highly purified since ancient times, and ultra-high-purity products with a purity of 11N have been commercially available. This is mainly a high-quality single crystal necessary for making high-performance semiconductors such as LSIs. Used as a raw material for lifting. In the 1990s, when demand for solar cells was low, the low-quality, unusable parts at the front and rear ends of single crystal silicon for semiconductors were reused as raw materials for solar cells. However, the demand for raw materials for solar cells necessary for producing ingots has been greatly increased at present, and the amount of the offcuts is insufficient, and high-purity raw materials for semiconductors have to be used.
The silicon powder raw material required in the present invention can be used as long as a purity of 6N or more can be ensured, like the raw material for solar cells. However, since the particle size and price of the powder are important in addition to the purity here, those satisfying them can be selected from the following three types.
(A) Polycrystalline silicon by pyrolysis This is a lump product with a purity of 11N, so it needs to be crushed. Therefore, the main lump product can be used by taking measures such as reducing the pulverization cost, mixing impurities during the pulverization, and reducing the variation in particle size.
(B) Granular silicon by fluidized bed method This is a high-quality, granular material with a purity of 11N and has a track record as a high-quality material for semiconductors. Since this raw material is granular having a diameter of about 0.1 mm to 2 mm, it can be used by selecting a size of 0.3 mm (300 microns) or less with a sieve in order to use it as a raw material for producing spherical silicon of about 1 mm in the present invention. Granular silicon of a larger size can be used as a raw material by pulverization as in the case of (a). Moreover, in this raw material manufacturing process, silicon high-purity fine powder of 1 micron or less is discharged as a by-product, and it is a great advantage to use this effectively.
(C) Powdered silicon by zinc reduction method This was developed by Chisso Corporation as a raw material for solar cells, and is a needle-shaped powder crystal with a diameter of several microns and a length of about 100 microns. is there. Although this raw material is not yet commercially available, it can be used as a raw material for a granular silicon single crystal in the present invention.

Although the container material and surface structure that can be used in the present invention have been described above, in order to realize a single ceramic material that exhibits a liquid smoke without being in direct contact with and contaminating the silicon melt, It will be necessary to verify and optimize from all points of view, such as improving purity, forming surface irregularities, and forming high quality thin films by CVD.
Therefore, as an alternative to this, in the present embodiment, a mold release material that does not contaminate the surface of the ceramic material even when the silicon melt is in direct contact is searched for, and a container material having a simple structure coated with it is found and improved. Repeatedly addressed problem solving.
Table 1 below shows that Hokusan Co., Ltd. is making an attempt to directly produce a polycrystalline silicon substrate for solar cells on a substrate in which a release material is applied to the surface of carbon or a ceramic material in a project of the Sunshine Project of the Ministry of International Trade and Industry. 52 candidate release material materials were picked up on a carbon substrate, and these were individually applied to the carbon substrate, and the results of an experiment to evaluate the wettability with silicon and the release property of the release material (I. Hide etal, J. of Cryst. Growth 79 (1986) p583).

  In this experiment, when the melted silicon did not get wet with the various release materials on the carbon substrate, a drop-like silicon was obtained, and when it was slightly wet, a flat silicon was obtained. In addition, when wet or a chemical reaction occurred, the raw material soaked into the mold release material and the carbon substrate, and a shaped product could not be obtained. Here, the contact angle between each of the drop-shaped silicon and the flat-plate silicon and the substrate is obtained. Table 1 shows nine types of release materials having a large contact angle with silicon among 52 types of experimental results. Was written out. According to this table, boron nitride shows the maximum contact angle. However, as described above, this is not possible because boron is mixed into silicon, as is the case with other boron compounds in the table.

  Therefore, when we use high-purity powdered silicon as a raw material other than boron compounds in Table 1, two types of aluminum nitride and silicon nitride each have a liquid-liquid property with respect to molten silicon, It was confirmed that it was a useful release material. These excellent points are not only good releasability with respect to silicon, but also good adhesion to the container material and a practical simple structure. Furthermore, although silicon nitride has α type and β type due to the difference in internal crystal structure, it has been found through a series of experiments that β type is superior to α type in releasability from silicon. However, in practice, it is not possible to obtain a silicon nitride containing 100% β-type due to the manufacturing method of silicon nitride. Therefore, it is sufficient to use a β-type and β-type mixing ratio with a high β-type content. understood. Of course, it is needless to say that these materials require a quality that maintains a purity of 5N or more without impairing the performance of spherical silicon for solar cells.

  Also, as the container material, quartz glass, high-purity porous quartz, and high-purity porous silicon carbide whose surface is subjected to a concavo-convex structure by sandblasting have excellent adhesion to aluminum nitride and silicon nitride, and are durable It was found that it becomes a simple structure container material. The release material, aluminum nitride, silicon nitride, and quartz glass, high-purity porous quartz, and high-purity porous silicon carbide, all of which are picked up here, are all available on the market and can be reduced in mass production. Since it is a simple material, it can be put into practical use in any combination. In addition, since the release material is a fine powder of about 1 to 10 microns, it can be dissolved in methanol or polyvinyl alcohol and applied to the surface of the container material by brushing or spraying, thus eliminating the need for expensive equipment. .

  In order to create an atmosphere that melts high-purity silicon powder raw materials, first of all, the materials used for the place where the atmospheric gas is touched, such as the gas piping material, the inner surface material of the electric furnace vessel, and the high temperature member in the electric furnace, must be of different types and surface finishes. The level used in semiconductor manufacturing equipment, such as the degree, should be adopted, and the impurity gas must be purged in advance by vacuum baking. However, even if these steps are taken, there is always a trace amount of moisture and oxygen in the high purity inert gas pipe and the electric furnace. Therefore, a small amount of the following silicon compound is introduced into the electric furnace and gas piping, changed to another substance using a chemical reaction, and detoxified so that the atmosphere around the molten silicon is completely free of moisture and oxygen. Maintain a highly clean environment. Table 2 shows silicon compounds that can be used in the present invention, their melting points, boiling points, and chemical reaction formulas with water and oxygen. These reactions occur even at room temperature, but become more intense at higher temperatures and are extremely effective in removing moisture and oxygen and making them harmless.

One characteristic of all reaction formulas in Table 2 is that silicon dioxide is one of the reaction products with moisture and oxygen. Therefore, before these moisture and oxygen come into contact with molten silicon, It is to be converted. The reaction product of monosilane with moisture or oxygen is hydrogen, which has no adverse effect even when it comes into contact with molten silicon. In the other four types of silicon halides, the reaction product with moisture and oxygen is hydrogen halide or halogen in addition to silicon dioxide. Of these, halogen gases such as fluorine and chlorine, when brought into contact with molten silicon, newly produce silicon halide, which is harmless because it becomes a new killer of moisture and oxygen. In addition, the reaction between hydrogen halides such as hydrogen chloride and hydrogen bromide and molten silicon is not clear, but experimental results show that there is no adverse effect on the process. However, the hydrogen halide or halogen produced here hardly undergoes a chemical reaction with metals or other substances in an atmosphere where moisture has disappeared, but once the work is completed, the electric furnace is temporarily opened to the atmosphere by taking out the prepared silicon sample. When exposed to moisture, it immediately reacts with stainless steel and other metals, and the resulting product becomes a source of contamination for high-purity silicon raw materials. In order to prevent this, it is essential that the dry nitrogen gas is sufficiently flowed into the electric furnace before the electric furnace is opened to the atmosphere, thereby replacing and replacing silicon halide, hydrogen halide and halogen. Here, an example of using a simple compound of silicon and hydrogen or halogen is shown, but other silicon compounds such as trichlorosilane (SiHCl ₃ ) and dichlorosilane (SiH ₂ Cl ₂ ) also detoxify trace amounts of moisture and oxygen. It can be used as a reactive gas.

Here, the structure and size of a hollow hole opened in a container material, which is important as a means for obtaining a large amount of spherical silicon single crystals for solar cells from liquid molten silicon, which is a third problem, will be described.
When a solar cell is made using a spherical silicon single crystal having a diameter of about 1 mm, about 2,000 spherical silicons are required to generate 1 watt (Non-patent Document 3). Therefore, 6 million (2,000 x 3,000 W) spherical silicon single crystals are required to generate 3 kilowatts, which is the scale of power generation in a general solar cell system. In order to mass-produce such a large amount of spherical silicon single crystal by the multiple crucible method, it is natural that the production equipment needs to be devised, but before that, it is necessary to fully consider the container material. It is. For example, in order to obtain a spherical silicon single crystal having a diameter of 1 mm, a hollow hole of a multiple crucible into which a powder raw material enters is set to a diameter of 1.5 mm and a depth of 1.5 mm. If this is processed into a plate-like container of 200 × 200 mm and a thickness of 2 mm at intervals of 2 mm, 100 pieces can be opened vertically and horizontally. One plate can produce 10,000 spherical silicon single crystals of 1 mm (100 × 100), but the thickness of this plate is 2 mm and the total height is 200 mm, which is not so large. If this is 1 million units (100 x 100 x 100) of containers for mass production, if we develop a manufacturing device that can process one unit in about 10 minutes, the production capacity will be 6 million per hour It becomes a piece (100 pieces x 100 pieces x 100 pieces / 10 minutes x 60 minutes). From this, it can be seen that the spherical silicon single crystal of the present invention has a sufficient possibility of producing a large quantity of about 1 mm in diameter.

Next, the shape of the hollow hole of the multiple crucible in which the powder raw material enters will be described. The mass productivity of spherical silicon single crystals depends on how the hollow shape is determined and how many can be processed into plate-like containers. FIG. 4A shows a state where a hollow shape with a diameter of 1.5 mm and a depth of 1.5 mm and a state in which silicon powder 22 is filled in an upper stage, and a state in which powder silicon melts together with an arrow below to form liquid silicon 23. Show. The hollow hole in this case is processed to a depth of 1.5 mm using a round ball end mill with a 1.5 mm diameter blade. The case where the depth is 1 mm with the same blade is shown in B. At this time, since the filling amount of the powder raw material 22 is reduced, the diameter of the spherical silicon single crystal 23 is reduced to about 0.8 mm. In the case where the tip of the processing blade is flat, C shows a case where the cross section is a square shape and D shows a case where the cross section is a trapezoid, and in both cases, the filling amount of the raw material is determined by the diameter and depth.
In addition, it is necessary to design these indentations so that the distance between them is as small as possible so that the maximum number of processing can be performed on the plane of the multiple crucible.

  Here, the relationship between the ease of single crystallization of spherical silicon described above and the shape of the bottom surface of each hollow hole of the multiple crucible shown in FIG. 4 will be described with reference to FIG. 4A and 4B is a curved surface having a radius of curvature of 0.75 mm, but in C and D, it is a flat surface. Spherical silicon is easily crystallized when the contact area between the bottom surface of the liquid silicon and the container is as small as possible. Since the bottom surfaces of the C and D hollows are flat, the bottom of the liquid spherical silicon is in contact with the substrate at points. On the other hand, since the bottom of the liquid spherical silicon is in contact with the curved surface in the hollow holes A and B whose bottoms are curved, the contact area is larger than in the case of C and D. From this, it can be said that the shape of the bottom surface of the hollow is preferably a C or D shape in order to increase the single crystallization rate of spherical silicon and obtain more high quality materials.

Next, an apparatus for solving the above-described three problems and producing a spherical silicon single crystal for solar cells will be described.
FIG. 5 shows an electric furnace and attached equipment and apparatus for converting high-purity silicon powder into liquid silicon and solidifying it to obtain a small amount of spherical silicon single crystals for solar cells.
Inside the electric furnace 31, there are a heater 32 and a heat insulating material 33. The heater 32 can be heated and controlled by a thermocouple 37 and a temperature control device 38 so that the silicon powder raw material 35 is precisely set to a specified temperature. The silicon powder raw material 35 is placed on a substrate 36 and surrounded by a liner tube 34 so that the inert gas 39 is isolated from the heat insulating material 33 and the heater 32 as much as possible so that there is no decrease in purity. The substrate 36 is for confirming a liquid silicon drop having a diameter of about 10 mm. The material is quartz glass, and the release material applied to the substrate is silicon nitride powder.

The inside of the electric furnace 31 is preliminarily vacuum-baked to prevent vacuum leakage and sufficiently exhaust the adsorbed gas from the internal heater 32, the heat insulating material 33, the liner tube 34, and the substrate 36.
After such preparatory work, the silicon powder raw material 35 is placed in the liner tube 34 inside the electric furnace 31 after being handled so as not to be contaminated, the valve 43 is opened, and the valves 42, 45, 47 and 48 are opened. The valve 41 is slowly opened by operating the vacuum pump 40. The air and moisture inside the electric furnace 31 and the liner tube 34 are exhausted by the action of the vacuum pump 40. The vacuum pump 40 uses a rotary pump for roughing, and exhausts to a high vacuum with a cryopump. Specifically, when the degree of vacuum reaches 0.05 Torr by exhausting with a rotary pump, switching to exhausting with a cryopump is performed.
Thereafter, the internal vacuum degree is kept below 10 ^-5 Torr by evacuation by a cryopump, heating of the silicon powder raw material 35 is started by the temperature control device 38 and the heater 32, and the internal temperature of the electric furnace 31 is about 400 ° C. per hour. The temperature rises at a rate of. At this time, if the temperature rises too early, the degree of vacuum deteriorates. In this case, the temperature rise is temporarily stopped, and the process waits for a while until the degree of vacuum is improved in the hold state. The silicon powder raw material 35 is heated to 800 ° C. with the degree of vacuum kept at 10 ⁻⁵ Torr or less to sufficiently remove the adsorbed moisture in the raw material. When the heating in vacuum is 10 ⁻⁵ Torr or less, the silicon powder raw material 35 placed in the vacuum hardly oxidizes even when heated to 800 ° C.
When the silicon powder raw material 35 reaches 800 ° C., the valve 41 is closed to disconnect the vacuum pump side, the valve 48 is opened, and the silicon tetrachloride 46a in the container 46 made of a glass tube is moved into the electric furnace 31 and the liner tube 34. Introduce about 5 Torr. Silicon tetrachloride 46a in the container 46 has a vapor pressure of about 400 Torr at room temperature, and the pressure can be accurately adjusted by gradually opening the valve 48. Thereafter, the valve 47 is opened, and high purity argon gas is introduced into the electric furnace 31 and the liner tube 34 from the argon gas cylinder 44, and the internal pressure is raised slightly higher than 1 atm (˜790 Torr). Next, the valve 42 is opened to bring the inside of the electric furnace 31 to 1 atm, and high-purity argon gas is allowed to flow through the silicon tetrachloride 46a container 46 at a flow rate of about 1 liter / min into the electric furnace 31 and the liner tube 34. . This is continued until the silicon powder raw material is melted and solidified. The gas exiting the valve 42 passes through the sodium hydroxide neutralized aqueous solution 49 and is discharged to the atmosphere. At this time, silicon tetrachloride in the argon gas is neutralized with sodium hydroxide neutralized aqueous solution 49 by the reaction of “SiCl ₄ + 4NaOH = SiO ₂ + 4NaCl”, which changes into silicon dioxide and sodium chloride. Thus, since the high purity argon gas always flows inside the liner tube 34 with a few Torr of silicon tetrachloride 46 contained therein, the silicon powder raw material is completely cut off from moisture and oxygen, and the liquid silicon is It will surely be realized.

As described above, silicon with a diameter of about 10 mm is realized by special consideration for high-purity silicon powder, substrate (container material) and atmospheric gas, and device and process in order to realize liquid silicon. Drops can be obtained with good reproducibility. When a silicon drop of this size can be made under the same conditions as a spherical silicon single crystal with a diameter of about 1 mm, the resulting sample is measured for resistivity by the four-probe method, infrared transmittance measurement by the FTIR method, and dislocation density by the etching method. Measurements that are difficult to evaluate with small spherical silicon can be effectively performed.
When the silicon drop process is stable, high-purity silicon powder is filled in a small multiple crucible shown in FIG. 4, and tens to hundreds of spherical silicon single crystals are obtained by the same process as described above.

FIG. 6 shows the configuration of an apparatus for mass production of spherical silicon single crystals. Here, FIG. 6B shows the overall configuration, and FIG. 6A shows the upper surface of the container unit storage chamber.
The apparatus includes a process chamber 61, a container unit supply chamber 62, a temperature control unit 63, a gas supply unit 64, a vacuum exhaust unit 65, and a container unit storage chamber 66. In the process chamber 61, four heaters Ha, Hb, Hc, Hd are arranged at equal intervals, and a fixed heat insulating material 67 surrounds them. A liner pipe 68 is provided inside the heaters Ha, Hb, Hc, and Hd. The liner pipe 68 is used to indirectly block the inside of the liner pipe 68 from the heater, the heat insulating material, and the inner wall of the process chamber to prevent contamination of the internal atmosphere. In the liner tube 68, a guide rail 69 for sequentially transporting the container unit from the right side to the left side is disposed horizontally. The process chamber 61, the container unit supply chamber 62, and the container unit storage chamber 66 are connected and fixed by an inlet gate valve 72 and an outlet gate valve 73 that can completely shut off each other. The temperature control unit 63 individually controls the heaters Ha, Hb, Hc, and Hd to raise the temperature inside the process chamber 61, change the temperature distribution, or keep the temperature constant. In addition to the rotary pump, a cryopump 65 for creating a high vacuum is connected to the process chamber 61 as a vacuum exhaust unit 65, and a silicon halide storage unit 74 is connected to the process chamber 61 as well as a high purity argon gas cylinder. Yes. When the silane gas is used in the gas supply unit 64, the silicon halide storage unit 74 is not necessary, and instead, a high purity argon gas mixing cylinder containing 10% or less of silane gas may be used as the gas cylinder.

Since the liner tube 68 and the guide rail 69 are located closest to the container unit, it is necessary to select materials having high purity and durability. As the material, pyrolytic carbon thin film coated carbon, glassy carbon film coated carbon, silicon nitride CVD film coated silicon nitride ceramics, silicon carbide CVD film coated silicon carbide ceramics and quartz glass can be used.
In the figure, container units La, Lb, Lc, and Ld are disposed on the guide rail 69 in the liner tube 68, and container units Le, Lf, Lg, and Lh are waiting in the container unit supply chamber 62. Each container unit has one unit obtained by stacking 100 sheets of the above-described 200 × 200 mm, 2 mm thick plate-like containers.

An outline of a process for mass-producing spherical silicon crystals using the apparatus having the above configuration will be described below. The feature of the production system here is that a container unit pre-filled with high-purity powdered silicon flows from the container unit supply chamber 62 through the process chamber 61 to the container unit storage chamber 66, and a spherical silicon single crystal is continuously formed. Can be obtained.
First, a large number of container units are housed in the container unit supply chamber 62, and several of the units Le, Lf, Lg, Lh,... And expel other gas components. The inside of the container unit supply chamber 62 is evacuated and then replaced with high-purity argon gas containing several torr of silicon tetrachloride or high-purity argon gas containing 1% or less of silane gas (hereinafter referred to as mixed gas). Make the atmosphere without. During this time, the process chamber 61 is in a highly clean environment that has been vacuum-baked in advance, and the inside is set to 1 atm with a mixed gas, and a mixed gas of several liters per minute is constantly supplied from the gas supply unit 64. Among a large number of container units, the container units La, Lb, Lc, and Ld that have been first subjected to the above processing in the container unit supply chamber 62 are arranged in a high-temperature process chamber 61 having a temperature distribution 89. The first container unit La already contains 1 million liquid silicon, the silicon of the next container unit Lb has just become liquid, and the next container unit Lc is just before melting, and the container unit Ld Then, it is in the state of the raw material reflecting the temperature distribution 89 and the staying time in the process chamber 61 that the powder silicon remains.
The temperature distribution 89 from the beginning to the end of the container unit in the process chamber 61 can be determined when optimizing the process. Here, the part where the container units La and Lb are arranged is defined as the melting point of silicon. It was set to 1470 ° C or higher, sufficiently higher than 1414 ° C.

  The container unit made of liquid silicon opens the outlet opening / closing heat insulating material 71 and the outlet gate valve 73, and is quickly moved to the cooling base 84 in the left direction by the pulling mechanism 91. The outlet opening / closing heat insulating material 71 and the outlet gate valve Close 73. A slow cooling heater 92 is provided around the cooling table 84, and the single crystallization of liquid silicon proceeds from the bottom upward without the container unit being rapidly cooled. Further, by controlling the temperature of the slow cooling heater 92, the growth rate of the liquid silicon in the container unit can be freely controlled and optimized. When the container unit is taken in and out, the container unit storage chamber 66 is naturally replaced with a mixed gas after evacuation in advance and maintained at 1 atm. Since the container unit cooling table 84 is cooled by a water cooling shaft (a columnar cooling body having a water cooling pipe disposed therein) 85, the bottom surface of the container unit 86 is cooled efficiently, and the unidirectional direction of liquid silicon Crystal growth can be performed effectively.

  The container unit 26, which has been cooled and crystallized in the container unit storage chamber 66, is turned 90 degrees by the storage push-out mechanism 88, moved to the low temperature part in the container unit storage chamber 66, and stored in the container unit until it is close to room temperature. It is stored in the chamber 66. In this way, a large amount of spherical silicon single crystals are sequentially crystallized and sent out once every 10 minutes in the container unit. A new container unit is sent from the container unit supply chamber 62 to the process chamber 61 in synchronism with this delivery.

FIG. 6 shows the state after the container units La, Lb, Lc, and Ld are sent out to the process chamber 61 in the container unit supply chamber 62. The container units Le, Lf, Lg, and Lh are the support bases. It is placed on top of 90. When the container unit Ld moves to the position of the container unit Lc in the process chamber 61, the inlet opening / closing heat insulating material 70 and the inlet gate valve 72 are opened, and the container unit Le is quickly pushed leftward by the pushing mechanism 83. And the inlet opening / closing heat insulating material 70 and the inlet gate valve 72 are closed. The container unit Le is rapidly heated from several hundred degrees C to 1400 degrees C, and in 30 minutes until it moves to the position of the heater Hb, the powdered silicon in the container unit melts into liquid silicon.
Since the location of the container unit Le is emptied in the container unit supply chamber 62, the container push-up mechanism 82 pushes up the height of the container unit Lf, Lg, Lh on the support base 90 by one, and the next operation Wait until. During the series of operations, the inside of the process chamber 61, the container unit supply chamber 62, and the container unit storage chamber 66 is evacuated and then mixed gas is introduced to maintain a highly clean atmosphere of 1 atm. Nor. In particular, in the process chamber 61, the rotary pump of the vacuum exhaust unit 65, the cryopump 75, the open / close valves 76, 77, 78, 79, 80, the gas supply unit 64, and the high purity silicon halide storage unit 74 cooperate with each other, It is important to perform the operation described in “0039”.
As described above, using the continuous production apparatus shown in FIG. 6, it becomes possible to produce a large amount of spherical silicon single crystals for solar cells at a low cost by the operation procedure described here.

〔Example〕
Details of the present invention will be described in the examples. The present invention is not limited by these examples.
In this embodiment, the apparatus for obtaining a spherical silicon crystal uses FIG. 5 showing the basic configuration. For this reason, the details of the process from filling the raw material to raising the temperature, evacuation and filling with high-purity atmospheric gas are almost the same as those explained in FIG. 5 (“0038”, “0039”). Further, it will not be described in the examples. Therefore, in the following examples, the contents and the obtained results are important parameters for obtaining spherical silicon crystals from liquid silicon: a: raw material, b: container material / release material, and c: atmospheric gas. Read about. In addition, as an indication of whether or not spherical silicon crystals can be used for solar cells, the carrier lifetime value is 1 μsec or more, and the target value of resistivity without boron additive is 10 Ω · cm or more. The suitability of the conditions was judged.

Example 1
In this example, a commercially available material having a purity of 4N and a powder particle size of 75 microns was used. The atmosphere gas was argon gas with a purity of 6N (99.9999%) or more, and silicon tetrachloride was used to remove moisture and oxygen. The multi-crucible container was coated with α-type silicon nitride powder having a purity of 5N and a particle size of 0.5 microns on a porous silicon carbide ceramic plate having a purity of 5N. Since this powder was put in a methanol liquid to a concentration of about 20%, brush coating or spraying was used for application to the porous silicon carbide ceramic plate. The applied silicon nitride powder layer has a thickness of about 150 microns. The porous silicon carbide ceramics has a thickness of 3 mm, and approximately 900 depressions having a diameter of 2 mm and a depth of 1 mm are opened in an area of 80 × 80 mm. Crystal growth of spherical silicon was performed at a solidification rate cooling rate from liquid silicon of 100 ° C./H.
The spherical silicon crystals obtained under these conditions have about 850, and the surface of most of them is somewhat dull and contaminated. Therefore, a mixture of hydrofluoric acid and nitric acid is used until the surface is shiny. Etching was performed for about 2 minutes with the liquid. In addition, on a porous silicon carbide ceramic plate, a small container with an inner diameter of 12 mm and a height of 15 mm made of the same material and coated with the same silicon nitride powder is placed, and a silicon drop with a diameter of about 12 mm and a height of about 10 mm is placed. Made simultaneously with spherical silicon crystals. As a result of measuring the resistivity by the four-point probe method using this silicon drop, the resistivity was 0.2 to 0.5 Ω · cm. The spherical silicon crystal was measured for carrier lifetime by the micro PCD method, and the value was about 0.05 μsec.
In this embodiment, the resistivity value is smaller than the target value, and the carrier lifetime is significantly worse. This is because the influence of the purity of the raw material is the largest, and the next conceivable reason is the porous silicon carbide ceramic plate and the α-type silicon nitride powder applied thereto. Therefore, the conditions in Example 1 are not suitable for obtaining spherical silicon crystals for solar cells.

Example 2
In this example, a fluidized bed granular silicon having a purity of 11 N for semiconductors was used as a raw material. However, since most of these have a large particle size, those having a size of 0.3 mm or less were selected using a sieve as raw materials. The atmosphere gas used was an argon gas with a purity of 6N (99.9999%) or higher, and silicon tetrachloride was used to remove moisture and oxygen. The multi-crucible container was made of high-purity quartz glass with 64 depressions having a diameter of 2 mm and a depth of 1.5 mm, and the surface was coated with β-type silicon nitride powder with a purity of 5N. Crystal growth of spherical silicon was performed at a cooling rate from liquid silicon of 120 ° C./H.
The obtained spherical silicon crystals had about 60 surface glosses and almost all had a surface gloss. However, before the carrier lifetime measurement, an etching treatment for 30 seconds was performed with a mixed solution of hydrofluoric acid and nitric acid. Also, a quartz glass small container with an inner diameter of 12 mm and a height of 15 mm is placed beside the quartz glass multiple crucible, and the same 5N β-type silicon nitride powder is applied, and a silicon drop with a diameter of about 12 mm and a height of about 10 mm is formed into spherical silicon. Made at the same time as crystals. As a result of measuring the resistivity by the four-probe method using silicon drop, the resistivity was about 30 Ω · cm. The spherical silicon crystal was subjected to carrier lifetime measurement by the micro PCD method, and the value was 2 to 2.4 μsec. Furthermore, about the sample in a present Example, 5 pieces were chosen at random and the Laue photography by X-rays was implemented, and crystallinity was evaluated. As a result, in all five photographs, most of the Laue spots are clear dot-like and their arrangement is curved, which is unique to single crystals. FIG. 7 shows four of the five Laue photos. Therefore, these facts suggest that most of the spherical silicon crystals obtained by the multiple crucible method are single crystals.
In Example 2, the resistivity value exceeds the target value, and the carrier lifetime also exceeds the target value. This factor seems to be the largest because the purity of the raw material is greatly improved. Therefore, the conditions of Example 2 are suitable for obtaining spherical silicon crystals for solar cells, and most of them are single crystals. Therefore, there is a possibility that the quality can be improved by improving the purity of the material and improving the process in the future. There is enough.

Example 3
In this example, a material obtained by pulverizing an 11N-purity polycrystalline silicon lump by a pyrolysis method and having a purity of 6N due to mixing of alumina impurities was used. The atmosphere gas used was a high purity argon gas added with 0.5% silane gas (SiH ₄ ). As in Example 1, the multi-crucible container is made of porous silicon carbide ceramics with an opening of 80 mm x 80 mm with a diameter of 2 mm and a depth of 1 mm and approximately 900 depressions, and the surface is β-type nitrided with a purity of 4N. Silicon powder was applied. Crystal growth of spherical silicon was performed at a cooling rate from liquid silicon of 120 ° C./H.
The obtained spherical silicon crystals had about 800 pieces and almost all had a surface gloss. However, before the carrier lifetime measurement, an etching treatment for 30 seconds was performed with a mixed solution of hydrofluoric acid and nitric acid. In addition, on a porous silicon carbide ceramic plate, a small container with an inner diameter of 12 mm and a height of 15 mm made of the same material and coated with the same silicon nitride powder is placed, and a silicon drop with a diameter of about 12 mm and a height of about 10 mm is placed. Made simultaneously with spherical silicon crystals. As a result of measuring the resistivity by the four-probe method using silicon drop, the resistivity was about 10 to 20 Ω · cm. The spherical silicon crystal was measured for carrier lifetime by the micro PCD method, and the value was 0.8 to 1.8 μsec.
In this embodiment, the resistivity value exceeds the target value, and the carrier lifetime is a value within the target value range. The reason why these values are slightly inferior to those of Example 2 seems to be the largest when the purity of the β-type silicon nitride powder coated on the porous silicon carbide ceramic plate is as low as 4N. Therefore, the conditions in Example 3 are appropriate if the release material is improved in order to obtain spherical silicon crystals for solar cells.

Example 4
In this example, fluidized bed granular silicon having a purity of 11N for semiconductors and a diameter of 0.3 mm or less was used as a raw material. The atmosphere gas used was a high purity argon gas added with 0.5% silane gas (SiH ₄ ). As in the case of Example 2, the multi-crucible container was made of high-purity quartz glass with 64 depressions having a diameter of 2 mm and a depth of 1.5 mm, and the surface was coated with 4N purity aluminum nitride powder (AlN). Crystal growth of spherical silicon was performed at a cooling rate of 60 ° C./H from liquid silicon.
The obtained spherical silicon crystals had about 50 surface glosses and almost all had a surface gloss. However, before the carrier lifetime measurement, an etching treatment for 30 seconds was performed with a mixed solution of hydrofluoric acid and nitric acid. Next, a quartz glass small container with an inner diameter of 12 mm and a height of 15 mm is placed beside the quartz glass multiple crucible, and an aluminum nitride powder (AlN) with the same purity of 4N is applied to a silicon drop with a diameter of about 12 mm and a height of about 10 mm. Made simultaneously with spherical silicon crystals. As a result of measuring the resistivity by the four-point probe method using silicon drop, the resistivity was 1 to 5 Ω · cm. The spherical silicon crystal was measured for carrier lifetime by the micro PCD method, and the value was 0.4 to 0.8 μsec.
In this embodiment, the resistivity value is about half of the target value, and the carrier lifetime has not reached the target value. The reason why these values are somewhat worse than in Example 2 seems to be the largest when the purity of the aluminum nitride powder coated on quartz glass is as low as 4N. Therefore, the conditions in Example 4 are appropriate if the release material is improved in order to obtain spherical silicon crystals for solar cells.

  As described above, the data obtained in the examples of the present invention are clearly different from the data obtained in the spherical silicon crystals for solar cells obtained by the conventional drop method. About 50% of the spherical silicon crystals produced by the drop method have a carrier lifetime value of 0.1 μsec or less and cannot be used for solar cells, and the utilization rate of silicon raw materials is difficult to exceed 50%. On the other hand, it was confirmed that almost all spherical silicon crystals obtained by the multiple crucible method have similar resistivity, carrier lifetime and crystallinity.

It is explanatory drawing about the wettability and liquid-repellent property of material. It is the figure which showed the flow of manufacture of the spherical crystal by the multiple crucible method using a powder raw material, and the photograph of three types of obtained spherical crystals. It is a figure which shows the spontaneous single crystallization from a liquid melt, and the polycrystallization from a wettability melt. It is a figure which shows the shape of the hollow of a multiple crucible. It is a figure which shows the electric furnace for attaching a high purity silicon powder to liquid silicon, and solidifying it, and obtaining a small amount of spherical silicon crystals for solar cells, and an attached instrument and apparatus. It is sectional drawing of the apparatus and accessory member for mass-producing a spherical silicon crystal. It is a figure of the Laue photograph which shows the single crystallinity of a spherical silicon crystal. It is a figure which shows the flow of manufacture and cost breakdown from the raw material of a crystalline silicon to a module. It is a figure which shows the outline of the manufacturing apparatus of the spherical silicon crystal by a dropping system.

Explanation of symbols

  35 ... silicon powder raw material, 31 ... electric furnace, 32, Ha, Hb, Hc, Hd ... heater, 36 ... substrate, 39 ... inert gas, 40 ... vacuum pump, 44: Argon gas cylinder, 64: Gas supply unit, La to Lh: Container unit.

Claims (12)

  A spherical silicon crystal obtained by forming a release layer that imparts liquid-liquidity to molten silicon on a substrate having a large number of depressions, and melting and solidifying high-purity silicon therein.   Using high-purity powdered silicon as a raw material, it is obtained from high-purity ceramics or quartz glass container in the hollow of the high-purity ceramics or quartz glass container having a fine concavo-convex structure and a large number of depressions in a high-purity inert gas atmosphere. Spherical silicon crystal.   3. The spherical silicon crystal according to claim 2, wherein a high-purity nitride powder is applied to a surface of the high-purity ceramic or quartz glass.   4. The spherical silicon crystal according to claim 3, wherein the high-purity ceramic is a high-purity porous ceramic.   5. The spherical silicon crystal according to claim 3, wherein the high-purity nitride powder is a high-purity aluminum nitride powder.   5. The spherical silicon crystal according to claim 3, wherein the high-purity nitride powder is a high-purity silicon nitride powder.   7. The spherical silicon crystal according to claim 6, wherein the high-purity silicon nitride powder is a high-purity β-type silicon nitride powder.   High purity powdered silicon is used as a raw material, and high purity nitride powder is applied in an atmosphere of high purity inert gas with silane gas of 10% or less concentration or silicon halide gas of 10% concentration or less. A spherical silicon crystal obtained from the liquid silicon in the recess of a high-purity ceramic or quartz glass or high-purity porous ceramic container having a concavo-convex structure and a large number of recesses.   A spherical silicon crystal is obtained by forming a release layer that imparts liquid-liquidity to molten silicon on a substrate having a large number of depressions, and melting and solidifying high-purity silicon in the depressions. A method for producing a spherical silicon crystal.   High-purity powdered silicon is accommodated in the recess of a high-purity ceramic or quartz glass or high-purity porous ceramic container having a fine concavo-convex structure on the surface and a large number of recesses, and inert gas or high-purity inert In a gas atmosphere, silane gas or silicon halide gas is added to melt powdered silicon in the recess to produce liquid silicon in the recess, and the liquid silicon is cooled to a spherical shape having a diameter of 5 mm or less. A method for producing a spherical silicon crystal, comprising obtaining a silicon crystal.   11. The method for producing a spherical silicon crystal according to claim 10, wherein the shape of the bottom surface of the recess is flat.   12. The method for producing a spherical silicon crystal according to claim 11, wherein a cooling rate of the liquid silicon is set to 200 ° C. or less per hour.