JP4800095B2 - Granular silicon manufacturing method and manufacturing apparatus - Google Patents

Description

  The present invention relates to a method and an apparatus for producing polycrystalline or single crystal granular silicon.

  In the production of granular silicon by the dropping method, the discharged silicon droplets are cooled while falling into an inert gas atmosphere or vacuum, and become supercooled, generating a large amount of crystal nuclei on the low temperature surface. , Solidifies from the outside to the inside, forming a polycrystal. In particular, during solidification, it often breaks or deforms due to the expansion force of the volume. Silicon grains produced by this method contain a large amount of crystal defects such as etch pits and dislocations.

  In order to improve the crystallinity of granular silicon, Patent Document 1 has already disclosed a seed crystal technique. In this prior art, single crystal seeds are supplied to a falling silicon melt in situ to obtain single crystal or polycrystalline granular silicon. A part of the outside of the polycrystalline silicon grains is melted, and a part of the center is left as a crystal seed, which is solidified again to grow a single crystal. This technique is problematic in terms of mass productivity because it produces granular silicon one by one.

In the production of granular silicon by other dropping methods, the discharged silicon droplets are dropped in an inert gas atmosphere in which fine powder for seed crystals is drifted to generate crystal nuclei on the surface at a low temperature, and external Is hardened from the inside toward the inside to form a polycrystal. This is known, for example, as disclosed in Patent Document 2.
A conventional apparatus for producing granular silicon is shown in FIG. A melting furnace 1 containing a melting crucible and melting a silicon raw material body, a dropping tube 3 for dropping molten silicon discharged from the melting crucible, a recovery device 4 for collecting granular silicon solidified while dropping the dropping tube, and a seed in the dropping tube It is comprised from the fine powder supply apparatus 20 for the seed crystal which supplies the fine powder for a crystal | crystallization.

As shown in FIG. 2, a quartz crucible 5 that stores the silicon melt 6 is provided inside the melting furnace 1, and a heater 11 for melting silicon is provided around the side surface of the quartz crucible 5.
High purity quartz is used for the quartz crucible 5. A nozzle 7 is attached to the bottom of the quartz crucible 5.

Hereinafter, a method for producing granular silicon will be described.
First, silicon is dissolved in a quartz crucible having a nozzle 7 at the bottom, and when the temperature of the silicon solution becomes sufficiently higher than the melting point, a small amount of extrusion gas is allowed to flow from the top of the quartz crucible, and a hole at the bottom of the crucible To discharge the melt.
At this time, the melt is discharged so as to continuously flow in a linear manner without dropping the melt discontinuously. The discharged linear silicon melt freely falls in the drop tube 3 arranged below the melting furnace, but at that time, the diameter becomes thin due to gravitational acceleration, and finally, due to the surface tension of the melt. Shredded and granular.

  At that time, since the spray pipe 14 of the fine powder supply device 20 for seed crystal generation is connected to the upper part of the drop pipe 3, the droplets of the silicon melt immediately after being shredded spout in the melt state. It is immersed in an atmospheric gas containing smoke silicon ejected from the tube 14. The droplets of the silicon melt come into contact with the fine powder for seed crystal in the atmospheric gas while being in the melt state, rapidly cooled from the contact point, and solidified in the drop tube 3 while dropping. The solidified granular silicon is recovered in the cooling oil in the recovery device 4 provided at the lower end of the drop tube 3.

Here, the fine powder supply device 20 includes a container 21 in which silicon powder is stored, an introduction pipe 13 for introducing atmospheric gas into the container 21, and a silicon powder provided in the container 21 by stirring the silicon powder. The agitation means 15 for producing silicon in the form of a gas and the ejection pipe 14 for ejecting the produced silicon in the form of smoke to the dropping pipe 3 are provided.
US Pat. No. 6,264,742 JP 2004-881 A

The conventional granular silicon manufacturing method and manufacturing apparatus have the following problems.
Part of the fine powder, such as silicon, ejected into the drop tube and smoked, falls to the lower part of the drop tube due to its own weight, or is carried to the upper part of the drop tube by the rising air current during discharge, and the drop tube There is a problem that it does not stay in a certain space inside, and for this reason, the crystallization of the silicon melt droplets in the drop tube 3 is not sufficient.
Therefore, an object of the present invention is to provide a method for manufacturing granular silicon and an apparatus for manufacturing the same, in which the above problems are eliminated.

In order to solve the above-described problems, the present invention provides a method and apparatus for producing granular silicon so that fine powder such as silicon produced by a fine powder supply device does not fall downward and remains in a constant space in the drop tube. This is the main point.
Means for solving the problems are as follows.
(1) A step of heating and melting a silicon raw material in a melting crucible, and discharging a molten silicon from a nozzle hole provided at the bottom of the melting crucible while applying a predetermined pressure to the upper surface of the silicon melt in the melting crucible. And dropping the molten silicon discharged in the molten state in a molten state so that the fine powder for seed crystal stays in a certain space without dropping, and crystal growth using the fine powder as a nucleus And a step of solidifying into granular form.
(2) The fine powder for seed crystal is ejected from the fine powder jetting part in the dropping tube together with the inert gas, and the inert gas is supplied from below and above the fine powder jetting part, and at the same time, exhausted from the upper and lower vicinity of the fine powder jetting part Thus, the method for producing granular silicon is characterized in that the fine powder for seed crystals stays in a certain space without falling.
(3) The fine powder for seed crystal is supplied into the drop tube together with the raw material gas of plasma, and the fine powder is distributed at a constant density in the plasma, so that the fine powder for seed crystal does not fall into a constant space. (4) The fine powder for seed crystal is one of silicon, quartz glass, silicon nitride, boron nitride powder, or a mixture thereof. To produce granular silicon.
(5) The method for producing granular silicon, wherein the silicon melt discharged from the nozzle hole has a degree of supercooling within 300 ° C. in the fixed space.
(6) The above-mentioned fine powder for seed crystals is jetted in the form of smoke into the drop tube by blowing an inert gas in a pulse form from the lower part of the container containing the fine powder for seed crystals. A method for producing granular silicon.

Further, in the present invention, the above-described problems are solved by the following manufacturing apparatus.
(7) A melting furnace that includes a melting crucible having a nozzle at the bottom and melts silicon raw material, a dropping tube that drops molten silicon discharged from the melting crucible bottom in contact with fine powder for seed crystals, and dropping the dropping tube In the granular silicon production apparatus having a recovery device for recovering the granular silicon solidified during
A granular silicon production apparatus comprising a fine powder control mechanism for controlling so that fine powder for seed crystals stays in a certain space inside the drop pipe without falling on a part of the drop pipe.
(8) The fine powder control mechanism includes a fine powder ejection nozzle that ejects fine powder together with an inert gas into the drop pipe, and an upper gas supply that is provided above and below the fine powder jet nozzle and supplies the inert gas into the drop pipe A granular silicon production apparatus comprising: a nozzle and a lower gas supply nozzle; and an upper exhaust nozzle and a lower exhaust nozzle provided between the upper gas supply nozzle, the lower gas supply nozzle, and the fine powder ejection nozzle, respectively.
(9) The fine powder control mechanism includes means for supplying the fine powder for seed crystal to the space together with the inert gas, and means for generating plasma of the inert gas in a certain space inside the drop tube. A granular silicon manufacturing apparatus characterized by the above.
(10) Funnel-like container containing fine powder for seed crystal, means for spraying inert gas from the lower end of the container, and fine powder for seed crystal in the form of smoke from above the container A granular silicon production apparatus, further comprising: a seed crystal fine powder supply device having a discharge means for discharging together.
(11) The granular silicon production apparatus, wherein the seed crystal fine powder supply apparatus includes a vibrating means for vibrating the funnel-shaped container and a pulse electromagnetic valve means in an inert gas introduction path.

  According to the present invention, since the fine powder remains in a certain space in the drop tube, it is possible to efficiently produce polycrystalline or single crystal granular silicon having high crystallinity.

Embodiments of the present invention will be described below in detail with reference to the drawings.
An apparatus for producing granular silicon according to the present invention is shown in FIG. It comprises a melting furnace 1 for melting silicon raw material, a fine powder control mechanism 2 for seed crystals, a drop tube 3 and a recovery device 4.

Next, FIG. 2 shows a main part of the melting furnace 1.
The silicon raw material is stored in the quartz crucible 5, and the silicon is heated and melted to a temperature equal to or higher than the melting point by the heater 11 of the melting furnace.
A pressure is applied to the inside of the quartz crucible with an inert gas such as Ar or He, an extrusion gas is supplied, and the silicon melt 6 is discharged from the nozzle at the bottom of the crucible.

  The discharged silicon melt 6 is chopped by gravity and surface tension to form granular silicon droplets 8, which are cooled by an inert gas while falling and become supercooled. Therefore, seed crystal fine powder is supplied and bonded to the supercooled silicon droplets. The joined portion locally nucleates and solidifies while growing.

The present invention is characterized in that when the seed crystal fine powder is supplied and joined to the supercooled silicon droplet, the seed crystal fine powder stays in a certain space without falling.
The details of the present invention will be described below by exemplifying an embodiment in which the fine powder for seed crystals stays in a certain space without falling.

  FIG. 4 shows an embodiment of a fine powder control mechanism in which fine powder for seed crystals stays in a certain space without falling. In this embodiment, the seed crystal fine powder stays in a certain space without falling, so that the central axis of the fine powder control mechanism 2 coincides with the path of the falling silicon melt in a part of the drop tube 3. It is intended to be installed.

Fine powder and inert gas ejected from the outlet of the seed crystal fine powder supply apparatus as shown in FIG. 3 are transported to the fine powder control mechanism through a clean tube, and ejected from the fine powder and inert gas ejection nozzle 23 into the drop tube 3. Is done. One or more jet nozzles 23 are branched from the transport tube.
An inert gas is supplied from the upper gas supply nozzle 26 and the lower gas supply nozzle 27, and at the same time, exhaust is performed from the upper exhaust nozzle 24 and the lower exhaust nozzle 25. The gas supply rate and exhaust rate are precisely controlled so that the pressure in the drop tube is always constant.

By providing the fine powder control mechanism as described above, the fine powder ejected from the ejection nozzle 23 does not fall down, does not diffuse into the upper space, and remains in a certain space near the ejection nozzle.
Furthermore, by adjusting the gas supply amount from the upper gas supply nozzle 26 and the lower gas supply nozzle 27, the stay space range of the fine powder can be adjusted. The fine powder concentration can be adjusted by the flow rate of the fine powder transport inert gas.
In order to make the fine powder distribution uniform, a plurality of gas supply nozzles and a plurality of exhaust nozzles are preferably installed uniformly on the circumference. The gas supply amount and the exhaust speed can be adjusted independently by installing a valve in each path. The interval between the nozzles is designed according to the diameter of the drop tube and the fine powder control mechanism.
The fine powder control mechanism may use a part of the drop tube, or may be separately manufactured and inserted in the middle of the drop tube.
By the fine powder control mechanism, fine powder for seed crystals can be supplied stably and distributed uniformly in a certain space.

Apart from the above, it is possible to confine or disperse the fine powder by plasma so that the fine powder for seed crystal stays in a certain space without falling.
Another embodiment of the fine powder control mechanism will be described with reference to FIGS.

The fine powder control mechanism shown in FIG. 5 is provided with a plasma device outside the drop tube, and a plasma discharge gas is introduced therein to generate plasma. The fine powder for seed crystal is introduced into the plasma, and the fine powder for seed crystal is supplied into the drop tube together with the plasma from the opening of the plasma device. The fine powder is uniformly distributed in the restricted space of the plasma at a constant density.
It is also possible to provide a plasma device inside the drop tube.

  The fine powder control mechanism of FIG. 6 has a cylindrical microwave plasma device arranged around a drop tube. Microwave and plasma discharge gas are introduced, plasma is generated in the drop tube, and fine powder for seed crystal is introduced into the plasma. The fine powder is uniformly distributed in the restricted space of the plasma at a constant density.

As another embodiment, a plasma electrode is installed in a drop tube, high-frequency power is supplied to generate an inert gas plasma, and fine powder for seed crystals is supplied to the plasma space at a constant flow rate. It can also be dispersed between the electrodes by the plasma potential.
The plasma electrode comprises one of a parallel plate type in the vertical direction, a parallel O-ring type in the horizontal direction, and a coil type in the vertical direction. In any case, the fine powder is designed to be uniform on the central axis of the falling silicon melt droplet passage, and a cylindrical space of 50 mmφ or more is secured for the silicon melt to pass through.
As described above, the fine powder can be uniformly distributed in the restricted space of the plasma with a constant density.

Next, the manufacturing method of the granular silicon which employ | adopted the fine powder control mechanism shown in FIG. 4 is demonstrated in detail.
First, 120 g of silicon raw material was placed in a quartz crucible and heated to 1500 ° C., which is higher than the melting point of silicon. When this temperature was maintained for about 30 minutes, the silicon was completely dissolved, and discharge was started in 40 minutes.

  Before starting the discharge, the fine powder for seed crystal is adjusted. As the fine powder for seed crystal, single crystal silicon fine powder having a size of 1 to 100 μmφ was used. The purity of the silicon fine powder was 5N (nine) or more. 3 g of silicon fine powder was put into a quartz funnel-shaped fine powder container 21 shown in FIG. 3, Ar gas having a flow rate of 1 L / min was supplied from the bottom of the container, and smoke was formed in the upper space of the container. The fine powder container was vibrated by an ultrasonic oscillator so that the fine powder did not harden. Further, a pulse electromagnetic valve was provided in the Ar gas path, and the electromagnetic valve was operated at a frequency of 5 Hz to supply Ar gas. By doing so, the silicon fine powder was not hardened and was continuously ejected from the outlet above the container. If the fine powder does not harden, the vibration of the fine powder container may be caused by a low frequency.

  The ejected silicon fine powder is transported and ejected by the Teflon (registered trademark) tube to the fine powder and inert gas ejection nozzle 23 of the fine powder control mechanism. At the same time, Ar gas was supplied from the upper gas supply nozzle 26 and the lower gas supply nozzle 27, and the respective flow rates were 5 L / min and 10 L / min. The valve connected to the exhaust nozzle was gradually opened to adjust the pressure in the drop tube to 1 atm. Then, the silicon fine powder stayed within a height range of about 50 mm around the surface of the fine powder and the inert gas ejection nozzle, and became a stable distribution.

  Discharge was started after the fine powder for seed crystals reached the above stable state. Before discharging, an electromagnetic valve for the same pressure was provided so that the inside and outside of the crucible had the same pressure. At the time of discharge, the solenoid valve for the same pressure was closed, and high-purity (6N) Ar gas for extrusion was supplied to the inner space of the crucible. When the pressure was increased to 0.02 MPa, the nozzle was continuously discharged. Silicon is ejected linearly from the nozzle, but is chopped into particles by surface tension and gravity.

  The installation location of the fine powder control mechanism is basically installed in a location where the silicon particles are already supercooled. However, the crystallinity of granular silicon is greatly affected by the installation location. When the degree of supercooling is 50 ° C. or less, the silicon granule becomes a single crystal, becomes polycrystalline when it is between 50 and 150 ° C., and tends to be dendrite growth when it exceeds 150 ° C. The installation place of the fine powder control mechanism is preferably a place where the degree of supercooling of the silicon granular material is 300 ° C. or less.

  As the fine powder for the seed crystal, high purity silicon, quartz glass (glass beads), silicon nitride, boron nitride powder, or a mixture thereof is used. As soon as the silicon particles are bonded to the fine powder, the temperature locally decreases, nuclei are generated in the silicon, and crystal growth begins. Since powders of glass beads, silicon nitride, and boron nitride have low solid solubility in silicon, impurities are not brought into the silicon. In the case of silicon fine powder, in addition to nucleation, it also has a role of providing crystal orientation. However, since the silicon fine powder is dissolved in the silicon granular material, the impurities in the fine powder are brought into the granular silicon. Therefore, the use of high-purity fine powder is important for reducing impurities in granular silicon.

The size of the fine powder for seed crystal is about 1 to 100 μm. The discharged silicon granular material passed through the fine powder dispersion space 28 for seed crystals. The distribution state of the fine powder was not affected by the passage of the silicon granule and kept stable.
The supercooled silicon particles join with the fine powder to nucleate and then start to grow rapidly. Grows in one direction from the nucleated place. Since the heat of solidification (latent heat) of silicon is large, a part of silicon grows again while melting near the melting point in the growth direction. Through this process, tear-like granular silicon with high crystallinity is formed.

  The solidified granular silicon is cooled while falling into the drop tube. However, the granular silicon reaches the recovery device at a high temperature. Since it is rapidly cooled when entering the recovery device, the performance of the recovery medium, such as heat resistance, thermal conductivity coefficient, and viscosity, affects dislocation in granular silicon and grain boundary generation. Therefore, silicone oil with low vapor pressure and low volatility and high heat resistance is used for the recovery device. Silicone oil has a role of sufficiently reducing impact at the time of landing of granular silicon, absorbing heat of granular silicon, and reducing the density of dislocations and grain boundaries.

  Silicone oil having a heat resistant temperature of 250 ° C. or higher was used in the recovery device. When the silicon particles landed on the silicone oil, combustion, decomposition, and color change were not observed, and the silicon particles were kept in a stable state.

  The diameter of the granular silicon thus produced is determined by the diameter of the nozzle and the pressure of Ar to be extruded. As the nozzle diameter increases from 0.2 to 0.5 mm, the diameter of the granular silicon gradually increases. Even if the nozzle diameter was kept constant, the discharge speed increased and the granular silicon diameter increased as the pressure of the extruded Ar increased from 0.02 MPa to 0.05 MPa. In general, the nozzle diameter for producing granular silicon of about 1.0 mm was 0.2 to 0.5 mm.

Further, the granular silicon produced under the above conditions became a teardrop shape and showed high crystallinity as shown in the photograph on the right side of FIG. Here, FIG. 7 is a cross-sectional electron micrograph and a cross-sectional enlarged photograph after the cross-section polishing of the teardrop-shaped granular silicon produced according to the present invention. The surface of the silicon grains is smooth and there are no grain boundaries or irregularities. It can also be seen that there are few etch pits, crystal defects, and grain boundaries even when the cross section is polished and selectively etched.
According to the present invention, the percentage of tears that are regarded as non-defective products is greatly improved from about 10% when the fine powder control mechanism is not used to 60% or more.
FIG. 8 is a cross-sectional electron micrograph and a cross-sectional enlarged photograph after cross-sectional polishing of granular silicon, which is regarded as defective for reference. A large amount of grain boundaries and irregularities are seen on the surface due to thermal expansion and stress, and after high-pressure polishing and selective etching, high-density etch pits and grain boundaries are seen. It has been confirmed that such granular silicon has a short carrier life and low performance as a solar cell.

It is a schematic diagram of the manufacturing apparatus of the granular silicon which concerns on this invention. It is a schematic diagram which shows the principal part of a melting furnace. It is a schematic diagram which shows a fine powder supply apparatus. It is a schematic diagram which shows a fine powder control mechanism. It is a schematic diagram which shows the fine powder control mechanism using plasma. It is a schematic diagram which shows the other fine powder control mechanism using plasma. It is a cross-sectional electron micrograph after carrying out cross-sectional polishing of the teardrop type granular silicon produced by this invention, and the enlarged photograph of a cross section. It is a cross-sectional electron micrograph after carrying out cross-sectional polishing of the produced defective granular silicon, and an enlarged photograph of a cross section. It is a schematic diagram of the manufacturing apparatus of the conventional granular silicon.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Melting furnace 2 Fine powder control mechanism 3 Falling tube 4 Collection | recovery apparatus 5 Quartz crucible 6 Silicon melt 7 Nozzle 8 Silicon drop 11 Heater 12 Melting furnace core tube 13 Introducing pipe 14 Ejection pipe 15 Stirring means 20 Seed crystal fine powder supply apparatus 21 Fine powder Container 22 Fine powder for seed crystal 23 Fine powder and inert gas ejection nozzle 24 Upper exhaust nozzle 25 Lower exhaust nozzle 26 Upper gas supply nozzle 27 Lower gas supply nozzle 28 Seed crystal fine powder dispersion space

Claims (9)

A step of heating and melting the silicon raw material in a melting crucible, and a step of discharging the molten silicon from a nozzle hole provided at the bottom of the melting crucible while applying a predetermined pressure to the upper surface of the silicon melt in the melting crucible. In addition, the discharged molten silicon is dropped in a molten state in a drop tube in which the fine powder for seed crystal stays in a certain space without falling, and the fine powder is used as a nucleus to grow crystals, and into a granular shape. A method for producing granular silicon comprising a solidifying step,
By spraying the fine powder for seed crystal together with the inert gas from the fine powder jet part in the drop tube, supplying the inert gas from below and above the fine powder jet part, and exhausting from the upper and lower vicinity of the fine powder jet part, method for producing a grain-like silicon you characterized in that so as to stay in a certain space without fine powder for seed falls. A step of heating and melting the silicon raw material in a melting crucible, and a step of discharging the molten silicon from a nozzle hole provided at the bottom of the melting crucible while applying a predetermined pressure to the upper surface of the silicon melt in the melting crucible. In addition, the discharged molten silicon is dropped in a molten state in a drop tube in which the fine powder for seed crystal stays in a certain space without falling, and the fine powder is used as a nucleus to grow crystals, and into a granular shape. A method for producing granular silicon comprising a solidifying step,
By supplying the fine powder for seed crystals into the drop tube together with the plasma source gas and distributing the fine powder at a constant density in the plasma, the fine powder for seed crystals stays in a constant space without falling. the method of producing grain-shaped silicon you characterized in that the Fines silicon for the seed crystal, quartz glass, silicon nitride, method for producing granular silicon according to claim 1 or 2 characterized in that it is a one, or a mixture of the powdered material boron nitride. The silicon melt discharged from the nozzle hole, a manufacturing method of the granular silicon according to any one of claims 1 to 3, wherein the degree of supercooling in the certain space is within 300 ° C.. The fine powder for seed crystal is blown out in the form of smoke in the drop tube by blowing inactive gas from the lower part of the container containing the fine powder for seed crystal. Item 5. The method for producing granular silicon according to any one of Items 1 to 4 . A melting furnace that includes a melting crucible having a nozzle at the bottom and melts silicon raw material, a dropping tube that drops molten silicon discharged from the melting crucible bottom in contact with the fine powder for seed crystal, and while dropping the dropping tube In a granular silicon production apparatus having a recovery device for recovering solidified granular silicon,
A part of the drop tube is provided with a fine powder control mechanism for controlling so that the fine powder for seed crystal stays in a certain space inside the drop tube without dropping,
The fine powder control mechanism includes a fine powder ejection nozzle that ejects fine powder together with an inert gas into the drop pipe, an upper gas supply nozzle that is provided above and below the fine powder jet nozzle, and supplies an inert gas into the drop pipe, and a lower part. a gas supply nozzle, the particle-like silicon manufacturing apparatus you characterized in that an upper exhaust nozzle and a lower exhaust nozzle respectively provided between the upper gas supply nozzle and the lower gas supply nozzle and the pulverized jet nozzle. A melting furnace that includes a melting crucible having a nozzle at the bottom and melts silicon raw material, a dropping tube that drops molten silicon discharged from the melting crucible bottom in contact with the fine powder for seed crystal, and while dropping the dropping tube In a granular silicon production apparatus having a recovery device for recovering solidified granular silicon,
A part of the drop tube is provided with a fine powder control mechanism for controlling so that the fine powder for seed crystal stays in a certain space inside the drop tube without dropping,
The fine powder control mechanism comprises means for supplying fine powder for seed crystals together with an inert gas, and means for generating plasma of an inert gas in a certain space inside the drop tube, particle shaped silicon manufacturing apparatus you. A funnel in which the seed crystal fine powder is placed, a means for spraying an inert gas from the lower end of the container, and the seed crystal fine powder in the form of smoke from above the container are discharged together with the inert gas. The granular silicon production apparatus according to claim 6 or 7 , further comprising a seed crystal fine powder supply device having a discharging means. 9. The granular silicon production apparatus according to claim 8 , wherein the fine powder supply device for seed crystals has a vibration means for vibrating the funnel-shaped container and a pulse electromagnetic valve means in an inert gas introduction path.