Electromagnetic casting method and apparatus for polycrystalline silicon

Abstract

Disclosed is an electromagnetic casting method of polycrystalline silicon which is characterized in that polycrystalline silicon is continuously cast by charging silicon raw materials into a bottomless cold mold, melting the silicon raw materials using electromagnetic induction heating, and pulling down the molten silicon to solidify it, wherein the depth of solid-liquid interface before the start of the final solidification process is decreased by reducing a pull down rate of ingot in a final phase of steady-state casting. By adopting the method, the region of precipitation of foreign substances in the finally solidified portion of ingot can be reduced and cracking generation can be prevented upon production of a polycrystalline silicon as a substrate material for a solar cell.

Claims

1. An electromagnetic casting method of polycrystalline silicon for continuously casting polycrystalline silicon, the method comprising: charging silicon raw materials into a bottomless cold mold; melting the silicon raw materials using electromagnetic induction heating; and pulling down the molten silicon to solidify it, wherein the depth of solid-liquid interface from an upper surface of the molten silicon before the startup of a final solidification process is reduced by reducing a pull down rate of ingot from a prescribed rate in a final phase of a steady-state casting, wherein the prescribed rate is such that molten silicon on the solidified silicon is confined in the solidified silicon when the silicon ingot is pulled at the prescribed rate in the final solidification process, and wherein the reduced pull down rate from the prescribed rate is such that molten silicon on the solidified silicon is not confined in the solidified silicon when the silicon ingot is pulled at the reduced rate in the final solidification process.

2. The electromagnetic casting method of polycrystalline silicon according to claim 1, wherein the pull down rate ingot is reduced by 0.05 to 0.2 mm/min per hour.

3. An electromagnetic casting apparatus for polycrystalline silicon including: a conductive bottomless cold mold in which a part of the mold along an axial direction is divided into a plurality of elements in a circumferential direction; an induction coil surrounding t he mold; and a heat retention heater arranged below the mold for slowly cooling a solidified silicon ingot, in which silicon melted by electromagnetic induction heating using the induction coil is pulled down and solidified, the apparatus comprising: a pull down rate controller for changing a pull down rate of ingot between a prescribed rate and a reduced rate from the prescribed rate in response to a casting stroke, wherein the pull down rate controller comprises: a casting stroke counter for measuring the casting stroke; a motor drive amount computing unit for computing a drive amount of a shaft-pull-down motor according to a predetermined deceleration rate by inputting the measured casting stroke and then outputting the computation result to the shaft-pull-down motor; and a shaft-pull-down motor that is driven based on a signal input from the motor drive amount computing unit, wherein the prescribed rate is such that molten silicon is confined in the solidified silicon when the silicon ingot is culled at the prescribed rate in the final solidification process, and wherein the reduced rate from the prescribed rate is such that molten silicon is not confined in the solidified silicon when the silicon ingot is pulled at the reduced rate in the final solidification process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a vertical sectional view showing a schematic configuration example of an electromagnetic casting apparatus for polycrystalline silicon of the invention.

(2) FIG. 2 is a diagram illustrating a relationship between a pull down rate of ingot and the depth of solid-liquid interface.

(3) FIGS. 3A and 3B are diagrams illustrating a change of a casting stroke and a pull down rate of ingot when a deceleration pattern is changed, of these, FIG. 3A shows a relationship between a pull down rate and a casting stroke, and FIG. 3B shows a relationship between elapsed time from the startup of deceleration and a pull down rate.

(4) FIGS. 4A and 4B are views for explaining a method for measuring a precipitation region of foreign substances and evaluating the absence or presence of cracking generation in a finally solidified portion of ingot, of these, FIG. 4A shows a precipitation region of foreign substances and FIG. 4B shows a state of cracking generation.

(5) FIG. 5 is a diagram showing results of EXAMPLES indicating the depth of foreign substance precipitation both in each casting condition and in each examination conducted for several times.

(6) FIG. 6 is a view schematically showing a configuration example of a principal part of an electromagnetic casting apparatus to be used for producing polycrystalline silicon.

(7) FIG. 7 is a view for describing a final solidification process in a production of polycrystalline silicon by electromagnetic induction.

(8) FIG. 8 is a view conceptually showing a precipitation state of foreign substances in a final solidification process.

(9) FIGS. 9A and 9B are views conceptually illustrating, a state of cracking in an ingot caused by the confinement of molten silicon into the finally solidified portion of ingot.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) The electromagnetic casting method of polycrystalline silicon of the invention is based on the premise that polycrystalline silicon is continuously cast by charging silicon raw materials into a bottomless cold mold, melting the silicon raw materials by electromagnetic induction heating, and pulling down the molten silicon to solidify it.

(11) Such an application of electromagnetic induction technique is taken as the premise because molten silicon can be cast in a mold almost without contacting the mold to continuously cast polycrystalline silicon, which is not contaminated with metal from the mold and capable of favorably maintaining a conversion efficiency upon production of polycrystalline silicon to be used as a substrate material of a solar cell. Furthermore, by using plasma arc heating in combination, a rapid and uniform melting of silicon raw materials can be promoted to further reduce a production cost and as well contribute to an improvement in quality to be used for a solar cell.

(12) The electromagnetic casting method of polycrystalline silicon of the invention is characterized in that the depth of solid-liquid interface before the startup of the final solidification process is reduced by reducing a pull down rate of ingot in a final phase of steady-state casting.

(13) Conventionally, a pull down rate of ingot after the start of casting was in a steady-state, and after reaching a predetermined casting stroke (e.g., approximately 7 m in ingot length), the casting was completed by promptly reducing the pull down rate to zero (0 mm/min), and then shifted to the final solidification process.

(14) On the other hand, in the electromagnetic casting method of the invention, a process for reducing a pull down rate of ingot is arranged in a final phase of steady-state casting (this process is referred to as a deceleration process or a melt cutback process), and after the depth of solid-liquid interface is decreased to an expected level by using a proportional relationship between the depth of solid-liquid interface and the pull down rate (see FIG. 2 above), the casting operation is shifted to the final solidification process. It is noted that, the final solidification process is a process after the casting operation is completed, and in this process, charging of raw materials, induction heating and plasma arc heating of the charged materials are all stopped, whereas in the deceleration process (melt cutback process), a casting operation is performed, and thus charging of raw materials, plasma arc heating of the charged materials and the like are still continued.

(15) A pull down rate of ingot is reduced in a final phase of steady-state casting. With this, time duration of steady-state casting operation is extended, and whereby high production efficiency can be maintained. It is desirable that the pull down rate is reduced at a timing such that the pull down rate becomes just zero (0 mm/min) (pull down is stopped) at the time that a predetermined casting stroke is achieved, and immediately after this, the casting operation is shifted to the final solidification process.

(16) The mode (pattern) of reducing a pull down rate of ingot is not particularly limited. For example, either of a stepwise deceleration or a continuous deceleration can be adopted. However, from a viewpoint of making possible efforts for expelling disturbances to the environment where the solidification (in other words, a crystal growth) is performed, it is desirable to reduce the pull down rate linearly with respect to the elapsed time.

(17) FIGS. 3A and 3B are diagrams illustrating changes of a casting stroke and a pull down rate of ingot when a deceleration pattern is changed, of these FIG. 3A shows a relationship between a pull down rate and a casting stroke, and FIG. 3B shows a relationship between elapsed time from the start of deceleration and a pull down rate. As an example, the phrase 0.05 mm/min per hour described in FIGS. 3A and 3B means that a pull down rate is reduced linearly at an hourly rate of 0.05 mm/min (0.5 mm/min for ten hours). The degree of the reduction of pull down rate is also referred to as deceleration rate.

(18) As shown in FIG. 3A, a pull down rate of ingot is 1.7 mm/min at steady-state casting. In a conventional casting method, no deceleration process is arranged, and a pull down rate is immediately reduced to 0 mm/min and shifted to the final solidification process at a time when a predetermined casting stroke (e.g., an ingot length of approximately 7 m) is achieved. Contrary to this, in a casting process of the invention, a deceleration process is set just before shifting to the final solidification process. In FIG. 3A, a broken line indicated as 0.05 mm/min per hour corresponds to a portion that the deceleration rate is set to 0.05 mm/min per hour.

(19) Temporal change of pull down rate in such a deceleration processes is shown in FIG. 3B. It is seen that a pull down rate is reduced linearly with respect to elapsed time when a deceleration rate is set to 0.2 mm/min per hour or 0.05 mm/min per hour. In a conventional casting method, since no deceleration process is adopted, a pull down rate of ingot is reduced instantaneously from 1.7 mm/min to zero (0 mm/min) Moreover, for example, when the deceleration rate is set to 0.05 mm/min per hour, it is confirmed that it will take 26 hours to reduce a pull down rate of ingot from 1.7 mm/min of steady-state casting to 0.4 mm/min.

(20) In an electromagnetic casting method of the invention, if the deceleration of the pull down rate of the ingot is set in the range of 0.05 to 0.2 mm/min per hour, as shown in an example below, the region of precipitation of foreign substances in the finally solidified portion of ingot can be reduced and generation of cracking can be effectively prevented.

(21) The reduction of pull down rate is set to 0.05 mm/min per hour or higher because a production efficiency (efficiency of manufacturing ingot) can be maintained in an allowable range. As apparent from FIG. 2 described above, the depth of solid-liquid interface decreases as a pull down rate of ingot is reduced, whereby the rising of solid-liquid interface in association with the solidification of silicon from below becomes easy to be completed before the solidification of the upper surface of molten silicon, thereby making the confinement of molten silicon less likely to occur. Moreover, since foreign substances are concentrated on the side of the molten silicon in a process of solidification, as the depth of solid-liquid interface decreases, the depth of the region of precipitation of foreign substances is also reduced, whereby the region is reduced. However, as shown in FIG. 3B, as the deceleration rate is smaller, a longer duration of time is required to reduce a pull down rate, and thus when the deceleration rate is less than 0.05 mm/min per hour, the deceleration process requires so much time that the production efficiency of silicon ingot is markedly lowered.

(22) On the other hand, the reduction of pull down rate is set to not more than 0.2 mm/min per hour in order to ensure an effect for reducing the region of precipitation of foreign substances as well as an effect for preventing cracking generation in the finally solidified portion of ingot. As apparent from FIG. 2 described above, the depth of solid-liquid interface increases as a pull down rate of ingot is increased, when the deceleration rate exceeds 0.2 mm/min per hour, the effect of the invention cannot be fully expected because of a shortened deceleration process.

(23) As described above, according to the electromagnetic casting method of the invention, by arranging a deceleration process (a melt cutback process) before shifting to the final solidification process, the depth of solid-liquid interface is decreased so as to reduce the region of precipitation of foreign substances in the finally solidified portion of ingot, and whereby cracking generation can be prevented.

(24) As mentioned above, the electromagnetic casting apparatus for polycrystalline silicon of the invention includes a conductive bottomless cold mold, an induction coil, and a heat retention heater for slowly cooling a solidified silicon ingot, in which molten silicon that is melted by electromagnetic induction heating using the induction coil is pulled down and solidified, and is characterized in that the apparatus comprises a pull down rate controller for changing a pull down rate of ingot in response to a casting stroke, wherein the pull down rate controller comprises: a casting stroke counter for measuring a casting stroke; a motor drive amount computing unit for computing the drive amount (the number of revolutions) of a shaft-pull-down motor according to a predetermined deceleration rate by inputting the measured casting stroke and then outputting a computation result to the shaft-pull-down motor; and a shaft-pull-down motor that is driven based on a signal input from the motor drive amount computing unit.

(25) FIG. 1 is a vertical sectional view showing a schematic configuration example of the electromagnetic casting apparatus for polycrystalline silicon of the invention. Similarly to an ordinarily used electromagnetic casting apparatus, the electromagnetic casting apparatus of the invention comprises a conductive bottomless cold mold 1 and an induction coil 2 surrounding the same. Below the induction heating coil 2, a heat retention heater 4 for heating a solidified silicon ingot 3 to prevent its abrupt cooling is installed, and below the heat retention heater 4, soaking heaters 5 are arranged in multiple stages.

(26) These devices are installed in a sealed chamber 7 so that molten silicon 6 and a high-temperature silicon ingot 3 are not brought into direct contact with the atmosphere. The sealed chamber 7, as shown in FIG. 1, comprises a main chamber 7-1 and a sub-chamber 7-2 connected to the main chamber 7-1, and normally, an interior of the chamber 7 is substituted with an inert gas so as to perform a continuous casting in a slightly pressurized state.

(27) Above the chamber 7, a raw material tank 9 and a dopant tank 10 for storing raw materials or the like to be charged into the mold 1 are disposed. A support stand 11 for holding an ingot with its lower end surface secured is disposed fixedly to a vertically movable shaft 12. Further, in this exemplary apparatus, a plasma torch 8 for heating silicon raw materials as necessary is attached above the mold 1.

(28) The electromagnetic casting method of the invention characteristically has a pull down rate controller 13 for changing a pull down rate of the ingot 3 in response to a casting stroke. The pull down rate controller 13 comprises a casting stroke counter 14, a motor drive amount computing unit 15 and a shaft-pull-down motor 16.

(29) The casting stroke counter 14 measures a casting stroke and then outputs a signal of the measured casting stroke to the motor drive amount computing unit 15. The motor drive amount computing unit 15 computes a drive amount of the shaft-pull-down motor 16 from an input of the measured casting stroke according to the predetermined deceleration rate (amount of reduction of pull down rate), and outputs a computation result to the shaft-pull-down motor. The shaft-pull-down motor 16 is driven based on a signal input therefrom.

(30) According to the electromagnetic casting apparatus of the invention, an electromagnetic casting method of the invention can easily be applied, and thus the region of precipitation of foreign substances in a finally solidified portion of ingot can be reduced and cracking generation can be prevented.

(31) Examples

(32) By using an apparatus having a configuration described in FIG. 1 above, and applying an electromagnetic casting method of the invention, a silicon ingot (sectional shape: 345 mm512 mm, length: 6,900 mm) was cast to examine the region of precipitation of foreign substances and the absence or presence of cracking generation in the finally solidified portion of ingot. For comparison, a similar examination was also performed in a case of applying a conventional casting method in which a pull down rate was not reduced.

(33) As described below, casting conditions were the same as those illustrated in FIGS. 3A and 3B above. Condition 1: Decelerate at a rate of 0.05 mm/min per hour from a casting stroke of 5,222.0 mm Condition 2: Decelerate at a rate of 0.2 mm/min per hour from a casting stroke of 6,450.5 mm Conventional method: Decelerate immediately to 0 mm/min at a casting stroke of 6,860 mm

(34) Methods for measuring and evaluating the region of precipitation of foreign substances and the absence or presence of cracking generation in the finally solidified portion of ingot are as follows. Specific techniques will be described with reference to FIGS. 4A and 4B.

(35) [Region of Precipitation of Foreign Substances]

(36) Evaluated by a depth (mm) below a top-side (a finally solidified portion side) end surface of ingot down to an end of a portion where the precipitation of foreign substances was observed.

(37) [Absence or Presence of Cracking Generation]

(38) Evaluated by visually examining the absence or presence of fragments of peel-off on a top-side of ingot.

(39) FIGS. 4A and 4B are views for describing a method for measuring and evaluating a precipitation range of foreign substances and a presence of cracking generation in the finally solidified portion of ingot, of these, FIG. 4A shows the region of precipitation of foreign substances and FIG. 4B shows a state of cracking generation.

(40) As shown in FIG. 4A, after a sample ingot was cut from the top-side of ingot so as to include a portion where the precipitation of foreign substances is observed, the sample ingot was cut along its center axis to visualize the precipitated foreign substances by etching a cut surface with acid. A shaded area in FIG. 4A indicates the region of precipitation of foreign substances, and the region of precipitation (precipitation depth) of foreign substances can be determined by measuring a depth d (mm) below the top-side end surface of ingot. Meanwhile, it is understood that the precipitated foreign substances are normally distributed entirely inside the solid-liquid interface (that is, inside the liquid molten silicon) just before shifting to the final solidification process. Therefore, the shaded area in FIG. 4A corresponds to a portion which was not solidified just before shifting to the final solidification process, the region of precipitation (precipitation depth) of foreign substances corresponds to a depth of solid-liquid interface at that time point.

(41) As shown in FIG. 4B, cracking caused by the confinement of molten silicon is discerned in the vicinity of the top-side end surface of ingot, where the ingot is cracked to exfoliate fragments thereof. Therefore, when a 10 kg or more of fragments is exfoliated, it is determined that cracking due to the confinement of molten silicon is generated.

(42) Examination results concerning the region of precipitation of foreign substances are shown in Table 1 and FIG. 5.

(43) TABLE-US-00001 TABLE 1 (Scale: mm) Casting No. 1 2 3 4 5 Mean Inventive Condition 1 440 450 460 450 445 449 Example (decelerate by 0.05 mm/min per hour) Condition 2 590 570 560 550 580 570 (decelerate by 0.2 mm/min per hour) Conventional method 765 740 750 730 740 745 (reduce a pull down rate instantaneously to 0 mm/min)

(44) FIG. 5 is a diagram showing results of Table 1 indicating a depth of foreign substance precipitation both in each casting condition and in each casting conducted for several times.

(45) As shown in Table 1 and FIG. 5, when an examination was performed by applying a conventional casting method, Comparative Example, in which a pull down rate was not reduced, the region of precipitation (precipitation depth) of foreign substances was 745 mm as the mean of five examinations. Contrary to this, when the electromagnetic casting method of the invention (Condition 1, Condition 2) is applied, the depth of foreign substance precipitation was further decreased and in Condition 1 in which a pull down rate was slowly reduced (decelerated at 0.05 mm/min per hour), the extent of decrease of depth was further improved.

(46) An examination result of cracking generation is shown in Table 2.

(47) TABLE-US-00002 TABLE 2 Casting No. 1 2 3 4 5 Inventive Condition 1 Not Not Not Not Not Example (decelerated by 0.05 found found found found found mm/min per hour) Condition 2 Not Not Not Not Not (decelerated by 0.2 found found found found found mm/min per hour) Conventional method Not Not Found Not Found (reduce a pull down rate found found found instantaneously to 0 mm/min)

(48) As shown in Table 2, when an examination was performed by applying a conventional casting method, cracking generation was found in two of five casting examinations, however when the electromagnetic casting method of the invention was applied, no cracking generation was observed.

(49) According to the electromagnetic casting method of polycrystalline silicon and the electromagnetic casting apparatus of the invention, the range of precipitation of foreign substances in the finally solidified portion of an ingot and the region of generation of crystal defects caused therefrom can be reduced and cracking generation can be prevented so as to improve a production yield of the ingot. Therefore, the present invention can be effectively utilized in the field of production of a solar cell.