JP2012193070A - Method for producing raw material for silicon-based solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a raw material for a silicon-based solar cell, by which silicon oxide-based foreign matter floating on the surface of a melt of silicon sludge can be evaporated and vanished for a short time.SOLUTION: A melt prepared from silicon powder as raw material is boiled before pulling out a polycrystalline silicon ingot from the melt by the Czochralski method. By the boiling, oxygen in foreign matter comprising silicon oxide and floating on the surface of the melt is evaporated and the foreign matter is vanished. As a result, when a polycrystalline silicon ingot is pulled out by the Czochralski method, foreign matter comprising silicon oxide does not adhere to the outer peripheral surface of the ingot.

Description

  The present invention relates to a method for manufacturing a silicon-based solar cell raw material, and more particularly to a method for manufacturing a silicon-based solar cell raw material capable of highly purifying silicon powder contained in silicon sludge generated in various silicon processing processes.

  A silicon wafer which is a formation substrate of an ultra-high integrated device such as ULSI is manufactured by performing wafer processing on a single crystal silicon ingot pulled up by the Czochralski (CZ) method. Specifically, the single crystal silicon ingot is cut into blocks, and then the silicon block is subjected to peripheral grinding with a grinding wheel and slicing with a wire saw in order to obtain a large number of silicon wafers. Then, chamfering, lapping, etching, and polishing are sequentially performed on each silicon wafer to manufacture a product wafer for device formation.

Of the wafer processing processes, a large amount of silicon sludge, which is processing waste (silicon waste), is generated in the peripheral grinding step and the slicing step. Also, a large amount of silicon sludge is generated in the back grinding process of the device manufacturer. Since silicon sludge is cumbersome to handle, it was common to dispose of it as industrial waste until now, but since silicon powder contained in silicon sludge is a reusable resource, A technology for reusing silicon powder of silicon sludge as a raw material for solar cells has been developed.
However, the silicon powder contained in the silicon sludge has been contaminated by oxides in addition to metal impurities such as Fe, Cr, and Ni resulting from the wafer processing apparatus. As a result, the photoelectric conversion rate of the manufactured solar cell was reduced.

  Thus, as a conventional technique for solving this problem, for example, one described in Patent Document 1 is known. Patent Document 1 removes easily oxidizable components (iron, aluminum, titanium, tungsten, chromium, gallium, etc.) contained in the silicon sludge raw material in the melting furnace by oxidation treatment, and also inert gas atmosphere or reduced pressure / vacuum. Evaporable components (oxygen, etc.) in this raw material are removed by melting the silicon sludge raw material in a melting furnace under an atmosphere, and the metal components in this raw material are removed by unidirectional solidification treatment in the melting furnace. It is the purification method of the silicon for solar cells to remove.

JP 2003-212533 A

However, according to Patent Document 1, when removing an easily evaporable component containing oxygen, the silicon sludge raw material is dissolved, this dissolved state is maintained for a predetermined time, and oxygen is evaporated from the liquid surface for removal. Was. The term “dissolving” as used herein means that the solid silicon sludge raw material is heated to the melting point to “melt” from the description in the specification of Patent Document 1, and the molten silicon sludge raw material is further heated. The concept of “boiling” accompanied by evaporation (vaporization) not only from the melt surface but also from the inside of the melt is not included. The reason is that silicon has an extremely high boiling point of 2600 ° C., and if heated to the boiling point, generally, a crucible made of quartz (melting point: 1650 ° C.) is melted, making it difficult to purify silicon for solar cells. In order to reduce the oxygen concentration of the melt from the melt surface of the silicon sludge raw material to a level that can be used as the raw material of the silicon-based solar cell by heating near the melting point of silicon, a heat treatment exceeding 24 hours is required. It was.
In addition, Patent Document 1 includes a description “dissolution of silicon sludge in a reduced pressure atmosphere or a vacuum atmosphere”. However, the purpose is not described at all. Probably a technique for promoting the evaporation of oxygen from the melt surface.

  Further, when the silicon sludge raw material is melted by the conventional method, a large amount of silicon oxide (SiOx) as a by-product floats on the melt surface as foreign matter. Therefore, in accordance with the Czochralski method, when pulling up the polycrystalline silicon ingot which is the raw material for silicon-based solar cells, a large amount of silicon oxide foreign matter adheres to the outer peripheral surface of the polycrystalline silicon ingot being pulled up. There is a possibility that the polycrystalline silicon ingot pulled up due to foreign matter adhesion cannot be used as a raw material for solar cells. In order to solve this problem, conventionally, the work of extracting foreign substances made of silicon oxide has to be performed many times during the melting of the raw material, which increases the cycle time.

  Therefore, as a result of earnest research, the inventor has found that silicon sludge can be heated even at a temperature below the melting point of the crucible by adjusting the pressure reduction level in the furnace from the correlation between the boiling temperature of the melt and the pressure in the furnace. The raw material silicon powder begins to boil, and the oxygen in the silicon oxide can be evaporated in a shorter time than the conventional method in which the melt is heated at a temperature near the melting point of silicon. The present invention was completed.

  An object of the present invention is to provide a method for producing a raw material for a silicon-based solar cell capable of evaporating a foreign substance made of silicon oxide floating on the liquid surface of a silicon sludge melt in a short time.

  In the first aspect of the present invention, silicon powder containing silicon oxide and having an average particle size of 0.1 μm to 0.6 mm is charged into a crucible, and the silicon powder in the crucible is placed in a reduced pressure atmosphere of 100 to 6000 Pa at 1480. The oxygen in the silicon oxide is decomposed and evaporated by boiling the silicon powder melt by heating at a temperature not lower than 1 ° C and lower than 1650 ° C. Next, the temperature of the melt is increased from 1410 ° C to 1450 ° C. This is a method for producing a silicon-based solar cell raw material, in which boiling is stopped by lowering, and then a polycrystalline silicon ingot of the silicon-based solar cell raw material is pulled up from the melt by the Czochralski method.

  Invention of Claim 2 is a manufacturing method of the raw material for silicon-type solar cells of Claim 1 with which the said silicon powder is contained in the silicon sludge generate | occur | produced by the silicon processing process.

  Invention of Claim 3 is a manufacturing method of the raw material for silicon | silicone solar cells of Claim 1 or Claim 2 which introduce | transduces a silicon lump with the said silicon powder into the said crucible.

  According to the first aspect of the present invention, the melt is boiled before the polycrystalline silicon ingot is pulled up from the melt using silicon powder as a raw material according to the Czochralski method. As a result, oxygen in the silicon oxide foreign matter floating on the liquid surface of the melt evaporates and disappears. As a result, when the polycrystalline silicon ingot is later pulled up from the melt by the Czochralski method, silicon oxide-based foreign matter does not adhere to the outer peripheral surface of the ingot. Therefore, the problem of disqualification of the polycrystalline silicon ingot as a raw material for solar cells due to the adhesion of foreign matters does not occur.

  In particular, according to the invention described in claim 2, since the silicon powder contained in the silicon sludge generated in the silicon processing process is employed as the silicon powder, the silicon sludge that should be a waste material is used as a silicon solar cell. It can be reused as a raw material for producing a raw material.

  According to the invention described in claim 3, since a mixture of silicon powder and silicon lump is put into the crucible, the silicon lump can be easily dissolved and the melting time can be shortened.

It is a flow sheet of the manufacturing method of the raw material for silicon system solar cells concerning Example 1 of this invention. It is a graph which shows the relationship between the generation amount of the silicon oxide type | system | group foreign material which floats on a melt surface, and the compounding ratio of the silicon sludge in a silicon raw material. It is a graph which shows the relationship between the boiling temperature of a melt, and a furnace internal pressure. It is a flow sheet of the manufacturing method of the raw material for silicon | silicone solar cells which concerns on Example 2 of this invention. It is a flow sheet of the manufacturing method of the raw material for silicon type solar cells concerning Example 3 of this invention. It is a graph which shows the relationship between the loss | disappearance time of the silicon oxide type foreign material which floats on a melt surface, and the boiling temperature of a melt. It is a graph which shows the relationship between the loss | disappearance time of the silicon oxide type foreign material which floats on a melt surface, and a furnace pressure.

  In the present invention, silicon powder containing silicon oxide and having an average particle size of 0.1 μm to 0.6 mm is put into a crucible, and the silicon powder in the crucible is placed at 1480 ° C. or more and less than 1650 ° C. in a reduced pressure atmosphere of 100 to 6000 Pa The silicon powder is boiled by heating at a temperature of 2 to decompose and evaporate oxygen in the silicon oxide, and then the temperature of the melt is lowered to 1410 ° C. to 1450 ° C. to boil. Is then removed, and then the polycrystalline silicon ingot of the silicon solar cell raw material is pulled up from the melt by the Czochralski method.

  According to this invention, as a pretreatment for pulling up the polycrystalline silicon ingot by the Czochralski method, the melt in which the silicon powder is dissolved is boiled in the crucible. As a result, oxygen evaporates from the silicon oxide-based foreign matter floating on the melt surface and disappears. As a result, when pulling up the polycrystalline silicon ingot, silicon oxide-based foreign matter does not adhere to the outer peripheral surface of the ingot. Does not occur.

The “raw material for a silicon-based solar cell” is, for example, a material for a single crystal silicon-based solar cell or a material for a polycrystalline silicon-based solar cell.
As the silicon powder, silicon powder obtained from the remainder of the polysilicon crushing process, etc. can be employed in addition to those contained in the silicon sludge generated in the silicon processing process.
When the average particle diameter of the silicon powder is less than 0.1 μm, the thermal conductivity is deteriorated, so that the melting time of the silicon powder becomes long. If the thickness exceeds 0.6 mm, the silicon powder can be easily melted. However, since the powder exceeding 0.6 mm can be used as a raw material for products, the input cost is high, resulting in a high cost. If the particle size of the silicon powder is 0.1 μm to 0.6 mm, the silicon powder can be melted in a relatively short time and at a low cost.
Examples of the oxide contained in the silicon powder include a silicon oxide film (SiO ₂ film) formed on the surface layer of the silicon powder. Examples of the silicon oxide film include a natural oxide film and a thermal oxide film.

As the crucible material, quartz (SiO ₂ ) having a melting point of 1650 ° C. can be used.
A crucible is one for boiling a melt of silicon powder to evaporate oxygen from silicon oxide floating on the surface of the melt and one for growing a polycrystalline silicon ingot by the Czochralski method. One crucible may also be used (continuous use). Different crucibles may be used.
In the case of evaporating oxygen from silicon oxide, if the reduced-pressure atmosphere in the furnace is less than 100 Pa, the silicon powder is easily dissolved but the silicon powder is rolled up. Moreover, if it exceeds 6000 Pa, it will become difficult to boil on the temperature conditions whose melt is 1480 degreeC or more and less than 1650 degreeC. A preferable reduced-pressure atmosphere in the furnace is 100 to 2500 Pa. Within this range, silicon oxide-based foreign matters can be safely and completely lost under temperature conditions of 1480 ° C. or higher and lower than 1650 ° C.

  When oxygen is evaporated from silicon oxide, if the boiling temperature of the molten silicon powder is less than 1480 ° C., silicon oxide-based foreign matters are not completely lost. Moreover, if it is 1650 degreeC or more, a crucible made from quartz will deform | transform, and if it is higher temperature, it will melt | dissolve. The preferred boiling temperature of the silicon powder is 1480 ° C. to 1550 ° C. (temperature 40 ° C. to 110 ° C. higher than the melt temperature (necking temperature) at the start of pulling up the polycrystalline silicon ingot by the Czochralski method). Within this range, silicon oxide-based foreign matter can be completely eliminated while preventing deformation of the quartz crucible.

  When oxygen is evaporated from the silicon oxide, the time for keeping the molten silicon powder in a boiling state is 1 to 5 hours. If it is less than 1 hour, silicon oxide-based foreign matters are not completely lost. Further, if it exceeds 5 hours, the quartz crucible is likely to be deformed by the heat load. The preferred time for maintaining the silicon powder melt at the boiling temperature is 1 to 3 hours. Within this range, silicon oxide-based foreign matters can be completely eliminated, and the influence of heat on the quartz crucible and in-furnace components is further reduced.

In the method for producing a raw material for a silicon solar cell according to the present invention, silicon sludge generated by a silicon processing process or fine powder generated by crushing polysilicon is used as silicon powder. As a result, the powder that originally becomes waste can be reused.
Examples of silicon processing processes that involve the generation of silicon sludge include block cutting of single crystal silicon ingots or polycrystalline silicon ingots, peripheral grinding of silicon blocks with grinding wheels, orientation flat processing or notching of silicon blocks with grinding wheels, and wire saws. Each process such as slicing of a silicon block, chamfering of a silicon wafer, lapping of a silicon wafer, and the like can be given. Further, a back grinding process performed on the wafer after device formation is also included.
Silicon sludge is a cocoon in which silicon powder, impurities, and water are mixed in a mud. Impurities are, for example, alumina, silica, corundum, Cu, Fe, Ni, C, barium oxide, magnesium oxide, dust, etc. generated by wear of a grinding wheel.

In the method for producing a raw material for a silicon-based solar cell according to the present invention, it is desirable to put silicon lump in addition to silicon powder in the crucible. The silicon lump is easily melted and easily becomes a melt. If the melt is formed, the melting of the silicon powder is promoted, so that the melting time is shortened.
As a silicon lump as a raw material for silicon-based solar cells, for example, silicon lumps generated in various silicon processing processes (including ingot processing processes for single crystal solar cells) can be employed. Specifically, after pulling up the silicon ingot using a Czochralski-type silicon crystal growth device, the remaining silicon remaining at the bottom of the crucible, a polycrystal cast by an ingot casting device (mold, electromagnetic casting device, etc.) The top part (final solidification part), end plate (casting surface part), etc. of a silicon-type solar cell are mentioned.
In the mixture (100 parts by weight) of silicon powder and silicon lump charged in the crucible, the proportion of silicon powder is 1 to 100% by weight. A preferable ratio of the silicon powder is 30 to 70% by weight. Within this range, the dissolution time is short and the cost can be reduced.

The polycrystalline silicon ingot pulled up from the crucible becomes a melting raw material for silicon-based solar cells after crushing. For example, this molten raw material is put into an ingot casting apparatus for polycrystalline silicon solar cells and melted, and, for example, a prismatic polycrystalline silicon ingot is cast. Thereafter, the cast ingot is processed into a wafer, and a PN junction is formed by a predetermined method, whereby a silicon-based solar cell is obtained.
It should be noted that the foreign matter made of silicon oxide is easily melted when the melt surface is brought closer to the heating center of the heater. This is because silicon oxide-based foreign matter floats on the melt surface, and the vicinity of the melt surface needs to be at the maximum temperature in order to eliminate the foreign matter.

  Examples of the present invention will be specifically described below.

With reference to the flow sheet of FIG. 1, the manufacturing method of the raw material for silicon solar cells which concerns on Example 1 of this invention is demonstrated.
First, 50% by weight of silicon sludge generated in a silicon processing process and containing silicon powder having an average particle diameter of 2 to 4 μm, and a top portion (final solidified portion) of a polycrystalline silicon solar cell cast by an ingot casting apparatus, A silicon lump consisting of 50% by weight of an end plate (cast skin part) is loaded into a crucible having a diameter of 18 inches in a Czochralski 6-inch single crystal pulling apparatus.
As described above, since silicon powder contained in silicon sludge generated in the silicon processing process is used, recycling of silicon sludge, which is waste material generated in the silicon processing process, and cost reduction of raw materials for silicon-based solar cells An effect is obtained. Moreover, since not only silicon powder but also silicon lump is put into the crucible, the melting time of silicon powder can be shortened. This is because the silicon lump is easier to melt than the silicon powder, and if the silicon lump becomes a melt, the melting of the silicon powder is promoted and the melting time of the silicon powder is shortened.

  Next, the pressure in the furnace is reduced to 6000 Pa, and the silicon in the crucible is melted by heating to 1450 ° C. to 1470 ° C. while flowing argon gas at a rate of 10 liter / min. At this time, a large amount of silicon oxide-based foreign matters resulting from the silicon oxide film of silicon powder of silicon sludge floats on the surface of the melt. The amount of silicon oxide-based foreign matter generated increases as the blending ratio of silicon sludge in the total silicon raw material increases (graph in FIG. 2).

Thereafter, the amount of heat of the heater is increased, and the melt is heated to about 1550 ° C. Thereby, vaporization of silicon occurs in the melt, and the melt is brought into a boiling state (graph in FIG. 3). By maintaining this state for 3 hours, oxygen in the silicon oxide-based foreign material evaporated and the foreign material disappeared completely.
Thereafter, the amount of heat of the heater is restored, the seed crystal is immersed in a melt at 1450 ° C. to 1470 ° C., and the ingot pulling is started after melting the seed crystal itself. Thereby, a polycrystalline silicon ingot having a diameter of 6 inches is grown. In the obtained polycrystalline silicon ingot, the adhesion of the foreign matter was not confirmed on the outer peripheral surface thereof. Therefore, the problem of disqualification of polycrystalline silicon ingot as a raw material for solar cells due to the occurrence of defects when this silicon oxide system adheres to the outer surface of foreign ingots and is used as a raw material is solved. It was done.

  In addition, since the impurities of the polycrystalline silicon ingot are purified by pulling from the melt in accordance with the Czochralski method, the polycrystalline silicon ingot becomes a high-quality polycrystalline solar material. Specifically, first, a polycrystalline silicon ingot is crushed into a fist size with a hammer or the like, and this is used as a melting raw material for a silicon-based solar cell, and is put into an ingot casting apparatus for a polycrystalline silicon-based solar cell. Here, the crushed material is melted and cast into a polycrystalline silicon ingot for a silicon-based solar cell. The cast ingot is then processed into a wafer, and a PN junction is formed by a predetermined method, whereby a silicon-based solar cell is manufactured.

Next, with reference to the flow sheet of FIG. 4, the manufacturing method of the raw material for silicon solar cells which concerns on Example 2 of this invention is demonstrated.
The feature of the method for producing the silicon-based solar cell raw material of Example 2 is that the melt is boiled, so that the melt is not raised to about 1550 ° C. as in Example 1, but the melt temperature of 1470 ° C. is increased. The method of maintaining the furnace pressure to 100 Pa and further reducing the pressure inside the furnace was adopted (graph in FIG. 3). As a result, silicon was vaporized inside the melt, and the melt was vigorously shaken to a boiling state.
Other configurations, operations, and effects are the same as those in the first embodiment, and thus description thereof is omitted.

Next, with reference to the flow sheet of FIG. 5, the manufacturing method of the raw material for silicon solar cells which concerns on Example 3 of this invention is demonstrated.
The feature of the method for producing the silicon solar cell raw material of Example 3 was that the melt was boiled at 1500 ° C. and the furnace pressure was reduced to 2000 Pa (graph of FIG. 3). . As a result, silicon was vaporized inside the melt, and the melt was vigorously shaken to a boiling state. As is apparent from the graphs of FIGS. 6 and 7, the time until the disappearance of foreign matters correlates with the furnace pressure and the melt temperature. That is, the lower the furnace pressure, the higher the melt temperature, and the easier the melt will boil and the foreign matter will disappear.
Other configurations, operations, and effects are the same as those in the first embodiment, and thus description thereof is omitted.

  The present invention is useful, for example, as a silicon waste recycling technique.

Claims (3)

Silicon powder containing silicon oxide and having an average particle size of 0.1 μm to 0.6 mm is put into a crucible,
The silicon powder in the crucible is heated at a temperature of 1480 ° C. or higher and lower than 1650 ° C. in a reduced-pressure atmosphere of 100 to 6000 Pa to decompose and evaporate oxygen in the silicon oxide by boiling the melt of the silicon powder. Let
Next, boiling is stopped by lowering the temperature of the melt to 1410 ° C. to 1450 ° C.,
Then, the manufacturing method of the raw material for silicon solar cells which pulls up the polycrystalline silicon ingot of the raw material for silicon solar cells from the melt by the Czochralski method.   The method for producing a silicon-based solar cell material according to claim 1, wherein the silicon powder is contained in silicon sludge generated in a silicon processing process.   The method for producing a silicon-based solar cell material according to claim 1 or 2, wherein a silicon lump is introduced into the crucible together with the silicon powder.

Images