JP4818137B2 - Silicon refining equipment, silicon refining method - Google Patents

Description

  The present invention relates to a silicon refining apparatus, and more particularly to a silicon refining apparatus that reduces the phosphorus concentration in a silicon raw material.

  Since polysilicon used as a raw material for semiconductor wafers requires ultra-high purity, silicon chlorides such as trichlorosilane obtained by chlorination of metallic silicon are purified and purified, and then crystallized in a furnace. The surface temperature is about 1000 ° C. The silicon seed crystal is grown by the CVD method so as to be grown by the so-called Siemens method. The purity is as extremely high as 11ine. On the other hand, since silicon for solar cells may be about 6 nines, conventionally, scrap generated in the manufacturing process of semiconductor silicon has been used as a raw material.

However, in recent years, the production of solar cells has been steadily increasing, and the demand for high-purity silicon, the main raw material for solar cells, has expanded rapidly. Silicon scrap alone does not meet the increase in demand. The shortage of silicon is prominent.
Therefore, a technique for producing high-purity silicon for solar cells by using commercially available high-purity metallic silicon as a starting material and removing impurities contained in a metallurgical process has been studied.

  A typical process is a method developed by NEDO, which is a method combining solidification and purification with electron beam melting and plasma melting. That is, since P and B have an equilibrium distribution coefficient close to 1 and are difficult to solidify and purify, P is evaporated and removed by an electron beam vacuum refining method, and B is evaporated and removed as B2O3 by a plasma melting method to which water vapor is added.

  By the way, not only the evaporation removal reaction of P but also the removal reaction by evaporation of elements having relatively high vapor pressure such as Al and Ca from silicon, the surface reaction is rate limiting, and the surface temperature is preferentially heated over the inside of the hot water. An electron beam melting and refining method that can be used is selected as an optimum method for reducing these elements.

An electron beam vacuum melting method has been developed in order to remove phosphorus having a great influence on the characteristics. When the raw material silicon is melted by irradiation with an electron beam, phosphorus is removed from the molten silicon as a vapor.
However, when the silicon vapor is generated together with the phosphorus vapor and the silicon vapor adheres to the wall surface of the vacuum vessel, the phosphorus is taken into the deposit.

  Since the silicon deposit adhered in the vacuum vessel is peeled off and falls into the molten silicon, there is a problem that the purity of the molten silicon does not increase when the electron beam vacuum melting method is applied to mass production equipment.

For example, when refining a silicon raw material having a phosphorus concentration of 25 ppm to 0.1 ppm, when the concentration of phosphorus in the deposit is four times the concentration of molten silicon, 1% of the volume of the molten silicon is near the outlet of the molten silicon. Even if / 1000 is mixed, the phosphorus concentration of the molten silicon rises by 0.1 ppm and exceeds the target value.
For example, the following documents are related to the prior art of the silicon refining method and the prior art similar to the apparatus of the present invention.
JP-A-9-309716 JP 2005-67916 A

  The present invention was created to solve the above-mentioned problems of the prior art, and is to provide a silicon refining method free from impurity contamination.

  When melting raw material silicon in a melting vessel and preferentially evaporating impurities whose vapor pressure is higher than that of silicon to increase purity, silicon vapor is also generated and released into the vacuum chamber from the opening of the melting vessel And adheres to the inner wall of the vacuum chamber.

  When the inventors of the present invention conducted a silicon refining experiment by the electron beam vacuum melting method, after the experiment was completed, the atmosphere was introduced into the vacuum chamber and the vacuum chamber door was opened. It was observed that fine metal pieces (silicon thin pieces) were dancing.

  Usually, when the deposit grown on the inner wall of the vacuum chamber becomes thick to some extent, it begins to peel off due to thermal distortion caused by the heating cycle of the vacuum chamber. In the case where the deposit is silicon, peeling occurs even if it is thinner than other substances.

This is because the difference between the thermal expansion coefficient of silicon (3.6 × 10 ⁻⁶ / ° C.) and the thermal expansion coefficient (17.3 × 10 ⁻⁶ / ° C.) of SUS304 on the wall surface of the vacuum chamber and the deposition plate is large. This is presumed to be the cause.
If such an estimation is correct, for example, it is possible to develop a technique for collecting deposits by utilizing the difference between the thermal expansion coefficients of silicon and metal.

The present invention was created based on the above knowledge, and is arranged in a vacuum chamber, a melting vessel, a heating and melting apparatus for heating and melting a silicon raw material arranged in the melting vessel, and the vacuum vessel. A feeding roller and a collecting roller, a cooling device for cooling the collecting roller, a deposit collecting belt disposed on the melting container and stretched between the feeding roller and the collecting roller, and the feeding roller And a motor that rotates one or both of the collection rollers and the deposit collecting belt, and the deposit collecting belt (material) is made of a material having a thermal expansion coefficient larger than that of silicon. The deposit collecting belt faces the opening of the melting container and passes over the melting container, and then the back surface is in contact with the cylindrical side surface of the collecting roller. A portion that is in close contact with the collecting roller is cooled by the cooling device and is moved above the collecting roller, and has a cooling device that cools the collecting roller, and the deposited material is collected by the cooling device. A silicon refining device configured to cool a portion of the belt that is in close contact with the collecting roller.
Further, the present invention is the silicon refining apparatus in which the deposited material collecting belt is made of metal.
Moreover, this invention is a silicon | silicone refining apparatus which has a peeling apparatus contact | abutted to the part cooled with the said collection roller of the said deposits collection belt.
Further, the present invention provides a silicon raw material in a melting vessel, heated and melted, and while generating impurity vapor and silicon vapor from the molten silicon, a deposit collecting belt is moved over the molten silicon, and the impurities Vapor and silicon vapor are allowed to reach the deposit collection belt to form a silicon deposit in which impurities are concentrated on the deposit collection belt, the deposit collection belt is partially cooled, and the deposit collection belt is In this silicon refining method, the silicon deposit is contracted to be larger than that of the silicon deposit, the silicon deposit is peeled off, and then moved again on the molten silicon.
Moreover, this invention is a silicon | silicone refining method which rubs the said cooled said deposit deposit belt surface, and peels the said silicon deposit.

  Table 1 below shows the thermal expansion coefficient (linear expansion coefficient) of main practical metals and practical alloys.

The deposit collecting belt is made of typical austenitic stainless steel such as SUS304, ferritic stainless steel such as SUS430, or general carbon steel, and is 10 × 10 ⁻⁶ / ° C. or higher, and silicon. The silicon deposits grown at a high temperature are peeled off at a low temperature.

  Table 2 below shows impurities contained in metallurgical grade silicon raw materials sold and their concentrations, and the impurities in the deposits on the inner wall of the vacuum chamber when the silicon raw material is melted and held in the vacuum chamber. Indicates the concentration.

  It can be seen that the impurity elements in Table 2 are concentrated in the deposits on the inner wall surface of the vacuum chamber when silicon is melted and held in vacuum.

  Thus, impurities are concentrated in the deposits in the vacuum chamber. In particular, when repeatedly used as a mass production facility, the deposit grows and undergoes a thermal cycle of heating by radiant heat from the molten silicon and cooling at the time of stopping, so that thermal stress is applied and begins to peel off. If it falls onto the silicon melt in operation, it will melt into the melt, causing contamination and reducing the refining effect.

  Since the peeling piece does not fall on the molten silicon, the purity of the molten silicon is prevented from being lowered, and high-purity silicon can be obtained in a short time.

The code | symbol 10 of FIG. 1 has shown the refining apparatus 10 of this invention.
The refining apparatus 10 has a vacuum chamber 11. A melting vessel 13 is disposed on the inner bottom wall of the vacuum chamber 11, and a deposit collection device 12 is disposed above the melting vessel 13 inside the vacuum chamber 11.
The deposit collecting apparatus 12 includes a deposit collecting belt 24 and a cylindrical feed roller 21 and a collecting roller 22, respectively.
The feed roller 21 and the collection roller 22 are disposed above the melting container 13 so as to face each other in parallel with each other.

  The deposited material collecting belt 24 is stretched between the feeding roller 21 and the collecting roller 22, and the back surface of the deposited material collecting belt 24 is in close contact with half of the outer peripheral side surfaces of the feeding roller 21 and the collecting roller 22. .

  The surface of the portion located above the rollers 21 and 22 between the portion in close contact with the feed roller 21 and the portion in close contact with the collecting roller 22 faces the ceiling side of the vacuum chamber 11, and the portion located below is the bottom. It faces the wall side.

  The melting vessel 13 has an opening 31 at a portion facing the ceiling of the vacuum chamber 11. The opening is, for example, a rectangle that is at least twice as long as the width, and the feed roller 21 and the collecting roller 22 are arranged at one end and the other end in the longitudinal direction of the opening 21 and outside the both ends in the longitudinal direction. Yes. Therefore, the length between the portions of the deposit collecting belt 24 that are in close contact with the rollers 21 and 22 is longer than the longitudinal direction of the opening 31.

The width of the deposit collecting belt 24 is wider than the width of the opening 31 (1.2 times or more, more preferably 1.5 times or more). The portion between the portion in close contact with the collecting roller 22 and the portion in close contact with the collecting roller 22 is located vertically above the opening 31, and the surface of the portion below the rollers 21, 22 of the deposited material collecting belt 24 faces the opening 31. is doing.
One or both rotation shafts of the feed roller 21 and the collection roller 22 are configured to be rotationally driven by a motor (not shown) from the outside of the vacuum chamber.

  The deposit collecting belt 24 and the rollers 21 and 22 are in close contact with each other. When the rollers 21 and 22 rotate, the deposit collecting belt 24 slides between the deposit collecting belt 24 and the rollers 21 and 22. It is configured to rotate without. For example, when a motor is attached to the rotation shaft of the collection roller 22, the deposit collection belt 24 is rotated by the rotation of the collection roller 22, and the feed roller 21 is rotated by the deposit collection belt 24.

The rotation direction of each roller 21, 22 is a direction in which the portion facing the opening 31 of the deposit collection belt 24 moves from the feed roller 21 toward the collection roller 22, and the rotation of each roller 21, 22 in this direction The portion of the deposit collecting belt 24 that has moved in the direction of the collecting roller 22 passes through the position above the opening 31 while facing the opening 31 and arrives at the collecting roller 22.
The portion of the deposit collecting belt 24 that has arrived at the collecting roller 22 contacts the cylindrical side surface of the collecting roller 22 and moves upward while being in close contact with the cylindrical side surface.

  A cooling device 25 is connected to the collection roller 22, a refrigerant body cooled to a predetermined temperature is supplied into the collection roller 22, and the refrigerant body flows through the collection roller 22. The portion in close contact with the collecting roller 22 moves upward while being cooled by the refrigerant. The refrigerant body that has flowed through the collection roller 22 returns to the cooling device 25, is cooled, and is then resupplied into the collection roller 22.

The portion moved upward of the deposit collecting belt 24 is separated from the collecting roller 22 at the upper end of the collecting roller 22.
In the vicinity of the collecting roller 22, for example, a peeling device 27 may be additionally arranged. As the peeling device, for example, a wire brush or a leaf spring, which has a structure that can be held lightly, is desirable.

  The deposit collecting belt 24 separated from the collecting roller 22 moves toward the feeding roller 21, and moves downward when it reaches the feeding roller 21, and again passes through the position above the opening 31 while facing the opening 31. Arrives at the collection roller 22.

  Thus, the deposit collecting belt 24 moves infinitely between the feeding roller 21 and the collecting roller 22, and the deposit collecting belt 24 moves from the feeding roller 21 toward the collecting roller 22 at a position vertically above the opening 31. It keeps moving.

A method for purifying silicon using this production apparatus will be described.
A silicon raw material composed of silicon pieces or silicon lump is disposed in the melting vessel 13, and the vacuum chamber 11 is evacuated by a vacuum evacuation system 33 connected to the vacuum chamber 11.

  A heating and melting apparatus 15 is disposed in the vacuum chamber 11, and the silicon raw material in the melting container 13 is heated by the heating and melting apparatus 15 while operating the motor and rotating the deposit collecting belt 24. Form. Here, the heating and melting apparatus is an electron beam gun, and the silicon raw material in the melting vessel 13 is irradiated with an electron beam to be heated and melted.

When molten silicon 30 is produced, impurity vapor having a higher vapor pressure than silicon is preferentially generated.
At this time, silicon vapor is also generated, and the silicon vapor and the impurity vapor are released into the vacuum chamber 11 from the opening 31 at the upper part of the melting vessel 13 and move on the opening 31 when flying vertically upward. It collides with the surface of the collecting belt 24, adheres to the surface, and silicon deposits enriched with impurities such as P are formed. The electron beam is bent by a magnetic field, and the heating and melting apparatus 15 is configured so that the vapor does not reach directly.

  The portion of the deposit collecting belt 24 that is in close contact with the rollers 21 and 22 is stretched horizontally, so that the silicon vapor reaches the outer side surface in a flat state, and the deposit collecting belt 24 moves over the opening 31. In the meantime, silicon deposits grow on the surface of the flat deposit collection belt 24 and increase in thickness.

  As the deposit collecting belt 24 moves away from the collecting roller 22, the temperature starts to rise due to indirect radiant heat from the atmosphere. In particular, the surface temperature of the feed roller 21 also rises due to indirect radiant heat from the atmosphere, so that the temperature also rises when it comes into contact with it. When the feed roller 21 further passes, the temperature rises due to radiant heat from the molten silicon, reaches a steady state in a relatively short time, and passes over the opening 31 of the melting vessel 13. Meanwhile, silicon deposits grow.

  When the deposit collecting belt 24 comes into contact with the collecting roller 22 whose inner surface is cooled by the cooling medium, the temperature decreases. The deposit collecting belt 24 is made of a material having a thermal expansion coefficient larger than that of silicon, and contracts more than the silicon deposit when the temperature is lowered, so that a thermal stress is generated, and thus the silicon deposit is peeled off.

Further, the outer surface of the deposit collecting belt is bent convexly at a position in close contact with the collecting roller 22, whereas the silicon deposit is hard and tries to maintain a flat surface, so that the peeling proceeds more reliably.
In addition, by providing an auxiliary peeling device 27 such as a wire brush, the deposits can be more reliably removed.

  A tray 28 is disposed below the collection roller 22, and silicon deposits peeled off from the deposit collecting belt 24 fall on the tray 28. The silicon deposits falling on the tray 28 are carried out of the vacuum chamber 11 and collected.

  In addition, when the film thickness of the silicon deposit becomes too thick, it becomes easy to peel from the deposit collecting belt 24, and there is a possibility that it falls on the molten silicon. In order to prevent the purity of the molten silicon from being lowered due to the fall of the peeling piece, the lower limit of the moving speed of the deposit collecting belt 24 is set so as to reach the collecting roller 22 before reaching the film thickness that increases the possibility of peeling. Set it.

  On the other hand, if it is too fast, it will pass over the opening 31 of the melting vessel 13 without being sufficiently preheated, during which the temperature of the deposit collecting belt 24 will continue to rise, and thermal stress between the first deposited silicon film and Occurs and becomes easy to peel.

  In order to prevent contamination of molten silicon due to peeling and dropping of silicon deposits due to such factors, the belt moving speed is set to 0.02 m / min to 2 m / min, and more preferably 0.05 m / min to 1 m / min. .

  A belt heating device is provided in the feed roller 21 and in the vicinity of the feed roller 21, and the temperature of the vapor deposition material collecting belt 24 is increased before it starts to pass over the opening 31 of the melting container 13, and the temperature change thereafter. By reducing the amount, contamination due to peeling and falling of the deposited material due to the above factors can be reduced.

  The heating temperature varies depending on the output of the electron beam, the area of the opening 31 of the melting container 13, the distance between the deposit collecting belt 24 and the opening 31, and the like, but the heating temperature is in the range of 200 ° C. or more and 500 ° C. or less. desirable.

  Next, the material of the deposit collecting belt 24 will be described. As a material of the deposit collecting belt 24, the difference from the thermal expansion coefficient of silicon is reasonably large, and it is desirable that the material is particularly twice or more. In consideration of heat resistance, fatigue resistance, and the like, and also in consideration of welding workability when manufacturing in an endless belt shape, for example, austenitic stainless steel such as SUS304 and SUS316 can be selected as an optimum material. The thickness of the deposit collecting belt 24 is 0.2 mm or more, more preferably 0.3 mm or more because welding is difficult and durability is inferior if it is too thin. On the other hand, if it is too thick, the elasticity as an endless belt becomes poor, so the thickness is 1 mm or less, more preferably 0.8 mm or less. It is desirable that the surface is moderately uneven by a method such as hairline processing or sand blasting in order to increase the retention of deposited silicon while passing over the opening 31 of the melting vessel 13.

After the release of a certain amount of vapor, the molten silicon 30 inside the melting vessel 13 has a reduced concentration of impurities such as phosphorus. When high-purity silicon is obtained, the molten silicon is solidified and used as a raw material for solar cells. Obtainable.
When the impurity concentration is higher than that of the raw material of the solar cell, the impurity concentration can be further reduced by refining using another deposit collecting belt in the same vacuum chamber 11 or refining in another refining apparatus.

  Further, the molten silicon 30 can be continuously taken out from one end in the longitudinal direction of the melting vessel 13 and the silicon raw material can be supplied to the other end. In this case, since it is desirable that the impurity concentration at the take-out position is low, the silicon raw material is supplied to the collecting roller 22 at the position 38, and there is no possibility of falling of silicon deposits, from the position 37 close to the feed roller 21 of the melting container 13. Molten silicon can be removed continuously or intermittently.

  Further, in the above embodiment, the cooling medium is allowed to flow through the collection roller 22 to cool the deposit collection belt 24, but a cooling member that contacts the deposit collection belt 24 is provided separately from the collection roller 22, and the cooling member is provided. The deposited material collecting belt 24 may be cooled by circulating a refrigerant body. In this case, the deposited material collecting belt 24 may be cooled by the collecting roller 22 before being bent, and the silicon deposit may be peeled off.

<Example 1>
A water-cooled copper crucible with an opening for metal purification having a shape of 10 cm × 40 cm is provided at the bottom of the vacuum tank, and a 0.5 mm thick SUS304 vapor deposition having a planar shape with a width of 15 cm and a length of 60 cm at the top of the vacuum tank. A deposited material collecting device was disposed in which the material collecting belt 24 was passed between the collecting roller 22 and the feeding roller 21. The surface of the belt was polished in the length direction with # 240 emery paper to obtain a hairline finish. The moving speed of the collecting belt was 0.1 m / min. The inside of the collecting roller was cooled by flowing water at a rate of 2 L / min.

  Using such a silicon refining apparatus, 2 kg of raw silicon having a purity of 99.99% and a phosphorus concentration of 25 ppm was charged into a water-cooled copper crucible, and electron beam melting was performed. After holding for 1 hour, irradiation was stopped and cooling was performed. As a result of taking out the silicon ingot after cooling and analyzing the phosphorus concentration, it was 0.05 ppm, and the phosphorus concentration sufficiently satisfied the purity level of silicon for solar cells.

  Furthermore, melt refining was repeated 5 times under exactly the same conditions. As a result, the phosphorus concentration in silicon was within the range of 0.04 to 0.07 ppm, and any of the dissolutions sufficiently satisfied the purity level of silicon for solar cells.

<Example 2>
A refining test was performed using a refining apparatus having the same configuration as in Example 1 except that a wiper made of a wire brush was installed outside the position where the deposit collecting belt contacted the collecting roller. The silicon raw material was the same, and the amount of dissolution, the electron beam irradiation pattern, etc. were the same as in Example 1, and the test was repeated five times. The phosphorus concentration in the obtained silicon was 0.03 to 0.06 ppm, and although the amount was slightly lower than that in Example 1, there was a tendency for the phosphorus concentration to further decrease.

<Example 3>
The same refining deposit collection apparatus as in Example 2 was arranged, and the test was conducted using the silicon refining apparatus having the same crucible shape dimensions. However, in Example 2, the melting and refining was performed in a batch type, whereas the silicon raw material was continuously supplied from the collecting roller side of the deposit collecting apparatus at 0.02 kg / min and continuously overflowed from the feeding roller side. A refining test was conducted by this method. The crucible was prepared so that the amount of silicon staying in the crucible was 2 kg. The same raw material silicon as in Example 1 and Example 2 was used, and the power of the electron beam was also set to 80 kW. The phosphorus concentration of the obtained silicon was 0.06 to 0.08 ppm, which was higher than Examples 1 and 2, but satisfied the purity level of solar cell silicon.

<Comparative Example 1>
Using a water-cooled copper crucible having the same dimensions and shape as in Examples 1 to 3, the amount of dissolution was 2 kg, and the electron beam power was irradiated at 80 kW for 1 hour. However, no deposit collector was installed above the opening of the water-cooled copper crucible.
The phosphorus concentration in the silicon after being dissolved in this way was a relatively low concentration of 0.2 ppm in the first dissolution, but when it was repeatedly dissolved, there was a tendency to increase with the number of times. Then, it increased to about 0.5 ppm.

  In order to prevent such a reduction in the refining effect, it was necessary to clean and remove the deposits deposited in the vacuum tank for each melting batch by hand, and it was found that the workability was extremely poor.

<Comparative example 2>
A comparative test was conducted in the same manner as in Example 3 by supplying the silicon raw material continuously. However, it was carried out by placing a fixed type deposit collecting plate made of SUS304 on the upper part. In addition, the retention amount of silicon in the crucible, the supply amount of silicon per unit time, the electron beam, etc. were all set to the same conditions as in Example 3.

  After about 2 hours from the start of refining, silicon having the same phosphorus concentration as in Example 3 was obtained. However, as time increased, the phosphorus concentration increased, and after 5 hours, 1.5 ppm, which was almost doubled. It became the value of.

The figure for demonstrating an example of the refining apparatus of this invention The figure for demonstrating the positional relationship of a melting container and a deposit collection belt

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Vacuum tank 13 ... Melting container 15 ... Heating-melting device 21 ... Feeding roller 22 ... Collecting roller 24 ... Deposited material collecting belt 25 ... Cooling device 30 ... Molten silicon 31 ... Opening

Claims (5)

A vacuum chamber;
A melting vessel;
A heating and melting apparatus for heating and melting the silicon raw material disposed in the melting container;
A feed roller and a collection roller disposed in the vacuum chamber;
A cooling device for cooling the collecting roller;
A deposit collecting belt disposed on the melting container and stretched between the feeding roller and the collecting roller;
A motor that rotates one or both of the feed roller and the collection roller, and rotates the deposited material collection belt;
The deposited material collecting belt is made of a material having a thermal expansion coefficient larger than that of silicon,
The deposit collecting belt faces the opening of the melting container and passes over the melting container, and then the back surface is in contact with the cylindrical side surface of the collecting roller and is in close contact with the collecting roller of the deposit collecting belt. The portion is cooled by the cooling device and is moved above the collecting roller,
A cooling device for cooling the collecting roller;
A silicon refining device configured to cool a portion of the deposit collecting belt that is in close contact with the collecting roller by the cooling device.   The silicon refining apparatus according to claim 1, wherein the deposit collecting belt is made of metal.   3. The silicon refining apparatus according to claim 1, further comprising a peeling device in contact with a portion cooled by the collecting roller of the deposited material collecting belt. A silicon raw material is placed in a melting container, heated and melted, and while generating impurity vapor and silicon vapor from the molten silicon, a deposit collecting belt is moved over the molten silicon, and the impurity vapor and silicon vapor are moved. Reaching the deposit collection belt, forming a silicon deposit enriched in impurities on the deposit collection belt;
A silicon refining method in which the deposit collecting belt is partially cooled, the deposit collecting belt is contracted more than the silicon deposit, the silicon deposit is peeled off, and then re-moved on the molten silicon.   The silicon refining method according to claim 4, wherein the surface of the cooled deposit collecting belt is rubbed to separate the silicon deposit.

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