JP2012091964A - Method for producing polycrystalline silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing polycrystalline silicon with which cooling of ingot can be finished in a short period of time, and cracks does not occur during cutting operation.SOLUTION: In the production method of polycrystalline silicon of continuously taking out polycrystalline silicon ingot 20 from a bottomless crucible 12 by letting down silicon melt 21 formed in the bottomless crucible 12 and solidifying the melt, when the polycrystalline silicon ingot 20 heat-retained at a predetermined heat-retaining temperature of 1,000°C or above by using a heat-retaining heater 16 is cooled down to a predetermined open temperature of 300°C or below, the temperature of the heat-retaining heater is controlled using a first cooling pattern gradually increasing in inclination at least until 620°C.

Description

  The present invention relates to a method for producing polycrystalline silicon, and more particularly to a method for cooling polycrystalline silicon for solar cells cast by an electromagnetic casting method.

  An electromagnetic casting method is known as one method for producing polycrystalline silicon for solar cells. This method is also called a continuous casting method, in which the silicon raw material charged into the bottomless crucible is melted by electromagnetic induction heating, and the molten silicon is drawn out from below the bottomless crucible and cooled to grow crystals. A silicon ingot of unidirectional solidification is obtained.

  In the production of polycrystalline silicon by the electromagnetic casting method, temperature control when cooling in one direction from about 1420 ° C., which is the melting point of silicon, is very important. Patent Document 1 describes a method of controlling the cooling rate by bringing a plasma electrode of an ingot into contact with an ingot when it is cooled to 900 to 500 ° C. in a method for producing polycrystalline silicon by an electromagnetic casting method. Patent Document 2 describes a cooling method in which polycrystalline silicon grown by unidirectional solidification is allowed to pass at 15 to 25 ° C./cm when it is cooled from 1420 ° C. to 1200 ° C.

  In addition, in the electromagnetic casting method, it is also important to heat and cool the cast silicon ingot. When a unidirectionally solidified silicon ingot is rapidly cooled, the temperature gradient inside the ingot becomes too large, resulting in an ingot with a lot of residual stress, and cracks are generated during cooling, or cracks are generated during cutting of the ingot. There is a problem. Therefore, the cast silicon ingot is kept at a constant temperature (for example, 1100 ° C.), and when the ingot is taken out, it is gradually lowered to the furnace opening temperature (for example, 300 ° C.) while preventing the generation of thermal stress. Cooling is done.

JP 2001-019593 A Japanese Patent No. 3005633

  However, when cooling is performed for a long time so as not to cause cracks, there is a problem that the cooling time becomes too long and the productivity is deteriorated. In fact, the ingot was cooled over about 52 hours. On the other hand, when cooling with a linear cooling pattern from the ingot heat retention temperature to the furnace opening temperature, if the cooling time is shortened to shorten the cooling time, the temperature gradient inside the ingot becomes too large due to rapid cooling, resulting in residual stress. There is a problem that the ingot becomes a lot and cracks are generated during the cutting of the ingot.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to make it possible to complete cooling of an ingot in a short period of time and to prevent polycrystalline silicon from generating cracks during cutting processing. It is to provide a manufacturing method.

  In order to solve the above problems, the inventors of the present application have made extensive studies, and as a result, the residual stress inside the ingot that causes cracking is related to the thermal stress generated during casting and cooling of the ingot. It was found that when a high thermal stress was generated during the cooling process, a high residual stress remained. And in the conventional linear cooling pattern, since the temperature fall at the time of a cooling start was rapid, it turned out that a big thermal stress generate | occur | produces in the early stage of cooling.

  In order to reduce the stress, it has been found that it is only necessary to start cooling at a low temperature decrease rate and gradually increase the temperature decrease rate, and in particular, to realize a temperature decrease pattern using an exponential function. Furthermore, since the temperature drop at the initial stage of cooling is slow when only the exponential function is used, it has been found that in this case, a temperature drop pattern combining an exponential function and a linear function may be realized.

  The present invention is based on such technical knowledge, and the method for producing polycrystalline silicon according to the present invention drops the silicon melt formed in the bottomless crucible and solidifies it, thereby the bottomless crucible. In this method, the polycrystalline silicon ingot is continuously taken out from the substrate, and the polycrystalline silicon ingot kept at a predetermined temperature of 1000 ° C. or higher is lowered to a predetermined open temperature of 300 ° C. or lower using a heat insulating heater. When cooling, the temperature of the heat retaining heater is controlled using the first cooling pattern in which the temperature decreasing rate gradually increases to at least 620 ° C.

  According to the present invention, since the polycrystalline silicon ingot is cooled using an exponential cooling pattern in which the cooling rate gradually increases, the cooling of the ingot can be completed in a short period of time, and cracks may occur during cutting. No polycrystalline silicon can be produced.

  In the present invention, when the polycrystalline silicon ingot is cooled, a third cooling pattern consisting of a combination of the first cooling pattern and a second cooling pattern having a constant cooling rate is used up to at least 620 ° C. It is preferable to control the temperature of the heat retaining heater.

  According to the present invention, since the exponential cooling pattern and the linear cooling pattern are combined, the cooling rate can be increased without generating a large thermal stress in the initial cooling stage, particularly in the period from the start of cooling to 10 hours. The speed can be increased, and the ingot can be efficiently cooled.

  In the present invention, when the polycrystalline silicon ingot is cooled, the temperature of the heat retaining heater is controlled using the fourth cooling pattern in which the temperature decreasing rate is constant during the initial cooling period from the start of cooling to a predetermined time. After the initial cooling period, it is preferable to control the temperature of the heat retaining heater using the third cooling pattern at least up to 620 ° C. In this case, the initial cooling period is preferably a period from the start of cooling to 2.5 hours.

  According to the present invention, a cooling pattern of a linear function is used in the initial cooling period, and a combined pattern of an exponential function and a linear function is used after the initial cooling period, so that the cooling rate that has become too high in the initial cooling stage is reduced. can do.

  ADVANTAGE OF THE INVENTION According to this invention, cooling of an ingot can be completed in a short period of time, and the manufacturing method of a polycrystalline silicon which a crack does not generate | occur | produce during a cutting | disconnection can be provided.

1 is a schematic side sectional view showing a structure of a polycrystalline silicon manufacturing apparatus according to a preferred embodiment of the present invention. It is a schematic perspective view which shows the structure of a bottomless crucible. It is a graph which shows the 1st cooling method (Example 1) of a silicon ingot. It is a graph which shows the 2nd cooling method (Example 2) of a silicon ingot. It is a graph which shows the 3rd cooling method (Example 3) of a silicon ingot. It is a graph which shows the cooling method by the comparative example of a silicon ingot. It is a graph which shows the time change of the maximum value of Mises equivalent stress in Examples 1-3 and a comparative example.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic side sectional view showing the structure of a polycrystalline silicon casting apparatus according to a preferred embodiment of the present invention.

  As shown in FIG. 1, the polycrystalline silicon casting apparatus 10 according to the present embodiment electromagnetically induces a chamber 11, a bottomless crucible 12 provided in the upper center of the chamber 11, and a silicon raw material in the bottomless crucible 12. An induction coil 13 for heating, an auxiliary heater 14 for heating the silicon raw material in the bottomless crucible 12 together with the induction coil 13, a support shaft 15 for supporting the silicon ingot 20 drawn from the bottomless crucible 12, and a bottomless A heat retaining heater 16 that retains the temperature of the silicon ingot 20 obtained by cooling the silicon melt 21 in the crucible 12 is provided.

  An inert gas introduction port 11 a is provided in the upper part of the chamber 11, and a vacuum suction port 11 c is provided in the lower part of the chamber 11. In addition, a raw material inlet 11b partitioned by a blocking means is provided at the top of the chamber 11, and an ingot outlet 11d is provided at the bottom.

  FIG. 2 is a schematic cross-sectional view showing the configuration of the bottomless crucible 12.

  As shown in FIG. 2, the bottomless crucible 12 is a square tube body made of copper, and is divided in the circumferential direction by a partial vertical slit 12a. The bottomless crucible 12 is a water-cooled type, and is forcibly cooled by cooling water flowing through the inside. Although not particularly limited, the opening size of the bottomless crucible 12 can be 350 × 530 mm.

  A duct 18 is provided in the chamber 11, and a granular or massive silicon raw material 22 supplied via a raw material inlet 11 c from a raw material hopper (not shown) provided outside the chamber 11 is formed in this duct. 18 through the bottomless crucible 12.

  The induction coil 13 is disposed around the bottomless crucible 12 and is connected to a power source via a coaxial cable. In addition, in order to heat and melt the silicon raw material together with the induction coil 13, an auxiliary heater 14 by means of a plasma arc torch is provided above the bottomless crucible 12 so that it can be raised and lowered. It has become so.

  The support shaft 15 is provided so as to be able to move up and down through the ingot outlet 11d. The tip of the support shaft 15 can reach the lower end of the bottomless crucible 12, thereby forming a movable bottom surface with respect to the bottomless crucible 12, and a silicon ingot drawn downward from the bottomless crucible 12 Can be supported.

  The heat retaining heater 16 is, for example, a resistance heater, and is provided below the bottomless crucible 12 and heats the ingot pulled down from the bottomless crucible 12 to keep the temperature at a constant temperature of 1100 ° C., for example. The heat retaining heater 16 according to the present embodiment uniformly heats the ingot without giving a predetermined temperature gradient in the axial direction. In order to increase the heat insulation efficiency of the heat insulation heater 16, a heat insulating material 17 is provided on the outer periphery of the heat insulation heater 16.

  Next, a method for casting polycrystalline silicon will be described.

  In the production of polycrystalline silicon by electromagnetic casting, first, the inside of the chamber 11 is evacuated from the vacuum suction port 11c, and then an inert gas such as argon is introduced into the chamber 11 from the inert gas introduction port 11a. Is an inert gas atmosphere. Next, the support shaft 15 with the initial silicon raw material attached to the tip is raised and inserted from below the bottomless crucible 12, and the opening at the bottom of the bottomless crucible 12 is closed with the initial silicon raw material.

  Next, electromagnetic force is applied to the initial silicon raw material by the induction coil 13, and the initial silicon raw material in the bottomless crucible 12 is melted using the induction coil 13 and the auxiliary heater 14 to generate a silicon melt 21. At this time, the silicon melt 21 in the bottomless crucible 12 is held in a non-contact state with respect to the inner surface of the bottomless crucible 12. Thereafter, the silicon melt 21 is gradually lowered together with the support shaft 15 to be solidified. At the same time, the silicon raw material 22 is added to the silicon melt 21 and the additional raw material is dissolved by the combined use of electromagnetic induction heating by the induction coil 13 and plasma heating by the auxiliary heater 14.

  By continuing this operation, the silicon ingot 20 is continuously manufactured in the bottomless crucible 12 by electromagnetic induction heating by the induction coil 13 and is continuously drawn out from the bottomless crucible 12. The silicon ingot 20 is pulled out below the chamber 11 while being kept warm by the warming heater 16.

  The silicon ingot 20 is continuously cast until it reaches a predetermined length (for example, 7000 mm), and during that period, the silicon ingot 20 is kept at a constant temperature (for example, 1100 ° C.) by heating from the heat retaining heater 16. When the silicon ingot 20 that has reached a predetermined length is taken out of the chamber 11, the temperature of the heat retaining heater 16 is gradually lowered so that the silicon ingot 20 is not excessively stressed so that the silicon ingot 20 is gradually removed. Need to cool down. At this time, by controlling the temperature of the heat retaining heater 16, it is possible to complete the cooling in a short period without giving high residual stress to the silicon ingot 20.

  FIG. 3 is a graph showing a method for cooling a silicon ingot according to the first embodiment of the present invention, in which the horizontal axis represents time from the start of cooling, the left vertical axis represents the temperature of the heat retaining heater, and the right vertical axis represents the heat retaining heater. Shows the power of. In the figure, line A1 indicates the target temperature (cooling pattern) of the heat retaining heater 16, line A2 indicates the actual temperature of the heat retaining heater 16, and line A3 indicates the power of the heat retaining heater 16.

In FIG. 3, not the temperature of the silicon ingot but the temperature of the heat retaining heater 16 is an object of evaluation. This is because the temperature of the silicon ingot substantially matches the temperature of the heat retaining heater 16. The thermal conductivity of polycrystalline silicon is as extremely high as 162 W · m ^{? 1} · K ^{? 1,} and as shown in FIG. This is because it receives direct radiant heat. The temperature of the heat retaining heater 16 can be measured using a temperature sensor such as a thermocouple, and the power of the heat retaining heater 16 is feedback controlled based on the output result of the temperature sensor. In order to eliminate the temperature difference between the ingot and the heat retaining sensor 16, it is preferable to install a temperature sensor between the heat retaining heater 16 and the ingot.

  As shown in FIG. 3, the silicon ingot is kept at about 1100 ° C. by heating from the heat insulation heater 16 during the heat insulation operation, but at least until the silicon ingot (heat insulation heater 16) reaches 620 ° C. during cooling. Controls the temperature of the heat retaining heater 16 so that the silicon ingot is cooled along a cooling pattern (first cooling pattern) in which the temperature decrease rate gradually increases. Finally, the temperature of the silicon ingot is lowered to a furnace opening temperature of 300 ° C. or lower. In order for the silicon ingot to be cooled along the first cooling pattern, it is preferable that the temperature drop rate of the heat retaining heater 16 is substantially constant from 1100 ° C. to 620 ° C., and thereafter the power of the heat retaining heater 16 is turned off. . The cooling pattern in which the temperature decreasing rate gradually increases may use an exponential function or may be approximated by a plurality of straight lines having different slopes.

  Further, as shown in FIG. 3, the actual temperature A2 of the heat retaining heater 16 gradually decreases from the time when it reaches 620 ° C. (about 16 hours from the start of cooling), and deviates from the target temperature A1. However, this is due to the influence of the heat insulating material 17 provided outside the heat retaining heater 16. In other words, the temperature sensor that measures the temperature of the heat retaining heater 16 is affected by the heat insulating material 17 to indicate a high temperature. The heat insulating material 17 is necessary to increase the heat insulation efficiency by the heat insulation heater 16, but becomes a factor that hinders rapid cooling during cooling. Therefore, the cooling control by the heat retaining heater 16 is an effective method for accelerating the cooling in the high temperature region rather than accelerating the cooling in the low temperature region. On the contrary, in the low temperature region side, the silicon ingot can be gradually cooled using only the heat insulating material 17 without energizing the heat retaining heater 16, so that it is possible to suppress power consumption.

  When the silicon ingot is cooled according to a linear cooling pattern as in the prior art, a large thermal stress is generated at the initial stage of cooling, so that a high residual stress remains inside the ingot. However, when cooling starts with a small cooling pattern at the beginning and the cooling rate is gradually increased, cooling can be completed in a relatively short time, and no large thermal stress is generated in the initial stage of cooling. It is possible to prevent cracks from occurring when the ingot is cut.

  FIG. 4 is a graph showing a method for cooling a silicon ingot according to the second embodiment of the present invention, where the horizontal axis is the time from the start of cooling, the left vertical axis is the temperature of the heat retaining heater, and the right vertical axis is the heat retaining heater. Shows the power of. In the figure, line A1 indicates the target temperature (cooling pattern) of the heat retaining heater 16, line A2 indicates the actual temperature of the heat retaining heater 16, and line A3 indicates the power of the heat retaining heater 16.

  As shown in FIG. 4, in this cooling method, a combined pattern (third cooling pattern) of a cooling pattern (first cooling pattern) in which the temperature decreasing rate gradually increases and a cooling pattern (second cooling pattern) in which the temperature decreasing rate is constant. The temperature of the heat retaining heater 16 is controlled so that the silicon ingot is cooled along the cooling pattern. In order to cool the silicon ingot along the third cooling pattern, it is necessary to gradually decrease the temperature lowering rate of the heat retaining heater 16.

  In the cooling method using the first cooling pattern shown in FIG. 3, since the temperature lowering rate is a little slower in the initial stage of cooling from the start of cooling to 10 hours, there is still room for cooling to be completed in a shorter time. . The third cooling pattern is an improvement of the first cooling pattern, and is a combination of an exponential cooling pattern and a linear cooling pattern. According to the third cooling pattern, the cooling can be efficiently performed in the initial cooling stage from the start of cooling to 10 hours.

  FIG. 5 is a graph showing a method for cooling a silicon ingot according to a third embodiment of the present invention, in which the horizontal axis represents time from the start of cooling, the left vertical axis represents the temperature of the heat retaining heater, and the right vertical axis represents the heat retaining heater. Shows the power of. In the figure, line A1 indicates the target temperature (cooling pattern) of the heat retaining heater 16, line A2 indicates the actual temperature of the heat retaining heater 16, and line A3 indicates the power of the heat retaining heater 16.

  As shown in FIG. 5, in this cooling method, in the initial cooling period from the start of cooling to, for example, 2.5 hours, a silicon ingot (insulating temperature) along a cooling pattern (fourth cooling pattern) in which the temperature decreasing rate is constant. The temperature of the heat retaining heater 16 is controlled so that the heater 16) is cooled, and after the initial cooling period, the first cooling pattern in which the temperature decreasing rate gradually increases and the second cooling pattern in which the temperature decreasing rate is constant. The temperature of the heat retaining heater 16 is controlled so that the silicon ingot is cooled along the third cooling pattern, which is a combined pattern.

  In the cooling method using the third cooling pattern shown in FIG. 4, since the rising portion of the exponential function appears earlier in time, the cooling rate becomes faster and a time zone in which the thermal stress becomes too large is generated. However, if the fourth cooling pattern is used, the cooling rate that has become too high in the initial cooling stage by combining the exponential function and the linear function can be improved by delaying the phase of the exponential term.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Needless to say, it is included.

  For example, in the above embodiment, a resistance heater is used as the heat retaining heater, but the type of the heat retaining heater is not particularly limited. Further, the heat insulating material 17 may be omitted, and the ingot may be kept warm only by the heat of the warming heater 16.

Example 1
Using the above-described polycrystalline silicon casting apparatus 10, a polycrystalline silicon ingot was cast by a normal electromagnetic casting method. At this time, the cooling pattern shown below was used as the cooling pattern of the silicon ingot. That is, in Example 1, when cooling a silicon ingot from 1100 ° C.,
1100−exp (2.3 / 20 × t + 17.7) / exp (20) × 889.1446−89.1446 (1)
The cooling pattern represented by t is the time from the start of cooling.

  Thereafter, the silicon ingot was cut out as a silicon block having a predetermined size, and then the presence or absence of cracks in the silicon block was inspected, and the defect rate (number of blocks with cracks / total number of blocks) was obtained. As a result, the defect rate of the silicon ingot was 3%.

  Moreover, the thermal stress generated inside the ingot was calculated by the following procedure. First, heat transfer analysis was performed on the temperature distribution inside the ingot in the cooling process from 1100 ° C., which is the heat retention temperature of the ingot, to 300 ° C., which is the final temperature of cooling, and calculation was performed at certain temperature steps. Next, based on the obtained temperature distribution, stress analysis was performed using general-purpose stress analysis software “ABAQUS (trade name)” to calculate thermal stress. Furthermore, thermal stress was converted to Mises equivalent stress, and the maximum value of Mises equivalent stress was plotted for each time step during cooling.

  A graph showing the maximum value of Mises equivalent stress is shown in FIG. In the graph of FIG. 7, the horizontal axis indicates the time from the start of cooling, and the vertical axis indicates the maximum value of Mises equivalent stress. In FIG. 7, not only this example but also the results of other examples and comparative examples described later are plotted.

  As can be seen from FIG. 7, in Example 1, the peak value during the cooling of Mises equivalent stress by thermal stress analysis was 6.56 MPa, which was a small value of 7 MPa or less. Moreover, the peak value of Mises equivalent stress occurred on the central surface on the long side of the ingot in all cases. As described above, in Example 1, the maximum value of Mises equivalent stress could be made zero in 23 hours from the start of cooling by using an exponential cooling pattern.

(Example 2)
The second embodiment is an improvement over the first embodiment. As can be seen from FIG. 7, in Example 1, the peak value of Mises equivalent stress appeared after 10 hours from the start of cooling. This indicates that there is room for increasing the cooling rate in the initial stage from the start of cooling to 10 hours.

In Example 2, an exponential cooling pattern and a linear cooling pattern were combined in order to increase the cooling rate in the initial cooling stage. That is, in Example 2, when cooling the heater temperature from 1100 ° C.,
1100−exp (2.3 / 20 × t + 17.7) / exp (20) × 889.1446−89.1446−13.3t (2)
The cooling pattern represented by Other conditions were the same as in Example 1.

  As a result, the defect rate of the silicon ingot was 6%. Moreover, as shown in FIG. 7, the peak value during the cooling of Mises equivalent stress by thermal stress analysis was 6.98 MPa.

(Example 3)
The third embodiment is an improvement over the second embodiment. As can be seen from FIG. 7, in Example 2, it was possible to efficiently cool in the initial cooling stage from the start of cooling to 10 hours by combining the cooling pattern of the exponential function and the cooling pattern of the linear function. . However, since the rising portion of the exponential function appears earlier in time, it has been found that the cooling rate increases and a time zone in which the thermal stress becomes excessively large occurs.

In the third embodiment, the combination of the exponential function and the linear function in the second embodiment improves the cooling rate that is too high in the initial cooling stage by delaying the phase of the exponential function term. That is, in Example 3, when cooling the heater temperature from 1100 ° C.,
When 0 ≦ t <2.5,
1100-13.3t
When 2.5 ≦ t,
1100−exp (2.3 / 20 × (t−2.5) +17.7) / exp (20) × 889.1446−89.1446−13.3t
The cooling pattern represented by Other conditions were the same as in Example 1.

  As a result, the defect rate of the silicon ingot was 2%. Moreover, as shown in FIG. 7, the peak value during the cooling of Mises equivalent stress by thermal stress analysis was 6.51 MPa.

(Comparative example)
In the comparative example, as shown in FIG. 6, when cooling the heater temperature from 1100 ° C.,
1100-40t
The cooling pattern represented by Other conditions were the same as in Example 1.

  As a result, the defect rate of the obtained silicon ingot was 65%, which was a very high defect rate. Moreover, as shown in FIG. 7, the peak value during the cooling of Mises equivalent stress by thermal stress analysis was 9.26 MPa.

DESCRIPTION OF SYMBOLS 10 Polycrystalline silicon casting apparatus 11 Chamber 11a Inert gas introduction port 11b Raw material introduction port 11c Vacuum suction port 11d Ingot drawing port 12 Bottomless crucible 12a Slit 13 Induction coil 14 Auxiliary heater 15 Support shaft 16 Heat insulation heater 17 Heat insulation material 18 Duct 20 Silicon ingot 21 Silicon melt 22 Silicon raw material

Claims (4)

A method for producing polycrystalline silicon, in which a silicon melt formed in a bottomless crucible is lowered and solidified to continuously take out a polycrystalline silicon ingot from the bottomless crucible,
When the polycrystalline silicon ingot kept at a predetermined temperature of 1000 ° C. or higher is cooled to a predetermined furnace opening temperature of 300 ° C. or lower by using a heat insulating heater and cooled, the temperature decreasing rate is gradually increased to at least 620 ° C. A method for producing polycrystalline silicon, wherein the temperature of the heat retaining heater is controlled using the first cooling pattern.   When the polycrystalline silicon ingot is cooled, the heat retaining heater is used by using a third cooling pattern composed of a composite pattern of the first cooling pattern and a second cooling pattern having a constant cooling rate until at least 620 ° C. The method for producing polycrystalline silicon according to claim 1, wherein the temperature of the polycrystalline silicon is controlled.   When cooling the polycrystalline silicon ingot, in the initial cooling period from the start of cooling to a predetermined time, the temperature of the heat retaining heater is controlled using a fourth cooling pattern with a constant temperature drop rate, and the initial cooling period 3. The method for producing polycrystalline silicon according to claim 2, wherein after the elapse of time, the temperature of the heat retaining heater is controlled using the third cooling pattern at least up to 620 ° C. 3.   The method for producing polycrystalline silicon according to claim 3, wherein the initial cooling period is a period from the start of cooling to 2.5 hours.

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