WO2007148988A1 - Crystallization furnace - Google Patents

Crystallization furnace Download PDF

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Publication number
WO2007148988A1
WO2007148988A1 PCT/NO2007/000227 NO2007000227W WO2007148988A1 WO 2007148988 A1 WO2007148988 A1 WO 2007148988A1 NO 2007000227 W NO2007000227 W NO 2007000227W WO 2007148988 A1 WO2007148988 A1 WO 2007148988A1
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WIPO (PCT)
Prior art keywords
electromagnetic
crucible
furnace
stirring
silicon
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Application number
PCT/NO2007/000227
Other languages
French (fr)
Inventor
Stein Julsrud
Magne Rudshaug
Original Assignee
Rec Scanwafer As
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Filing date
Publication date
Application filed by Rec Scanwafer As filed Critical Rec Scanwafer As
Publication of WO2007148988A1 publication Critical patent/WO2007148988A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B28/00—Production of homogeneous polycrystalline material with defined structure
    • C30B28/04—Production of homogeneous polycrystalline material with defined structure from liquids
    • C30B28/06—Production of homogeneous polycrystalline material with defined structure from liquids by normal freezing or freezing under temperature gradient
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/02—Elements
    • C30B29/06—Silicon
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B30/00—Production of single crystals or homogeneous polycrystalline material with defined structure characterised by the action of electric or magnetic fields, wave energy or other specific physical conditions
    • C30B30/04—Production of single crystals or homogeneous polycrystalline material with defined structure characterised by the action of electric or magnetic fields, wave energy or other specific physical conditions using magnetic fields
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B35/00—Apparatus not otherwise provided for, specially adapted for the growth, production or after-treatment of single crystals or of a homogeneous polycrystalline material with defined structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic System
    • H01L31/182—Special manufacturing methods for polycrystalline Si, e.g. Si ribbon, poly Si ingots, thin films of polycrystalline Si
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/546—Polycrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a furnace for crystallization of silicon.
  • the most common substrate material for photovoltaic cells is multicrystalline silicon. Industrially, this material is commonly produced in crystallization furnaces by directional solidification (DS) by a variety of related techniques. One variant of DS is described for instance in US 2007/0044707-A1.
  • the heaters of the furnace are normally powered by a 3 -phase alternating current.
  • This alternating current also establishes a stirring force of a magnitude at least similar to the thermal buoyancy forces generated by temperature differences in the furnace.
  • This stirring force causes melt circulation which improves segregation of impurities away from the crystal and decreases temperature gradients in the silicon melt.
  • the stirring force is dependent of the heating power of the furnace.
  • One object of the invention is to provide a furnace where the quality of silicon produced by the furnace is improved.
  • One other object is to improve the control of the crystallization of silicon.
  • Fig. 1 illustrates an embodiment of the present invention seen from the side
  • Fig. 2 illustrates an embodiment of the present invention seen from above
  • Fig. 3 shows an IR-image of silicon ingots where a furnace with a stirring device was used during the crystallization.
  • Fig. 4 shows an IR-image of silicon ingots where a furnace without a stirring device was used during the crystallization.
  • Fig. 5 shows a diagram of the minority carrier lifetime from the samples shown in fig. 3 and fig. 4. It is now referred to fig. 1 and 2.
  • a furnace comprises an upper housing 1 and a lower housing 10.
  • the upper housing can be removed for loading and unloading.
  • the closed furnace is rendered vacuum tight by means of a seal 8.
  • the furnace further comprises a crucible 4 for containing a raw material for the crystallization.
  • the raw material can for example be silicon produced by the Siemens process or similar processes.
  • the raw material is placed in the crucible before the crystallization process starts.
  • FIG 1 and 2 are shown a layer of solid silicon 6 and a layer of molten silicon 5.
  • the furnace further comprises a heating device 3 for heating the crucible 4, thereby causing the raw material to melt.
  • the heating device can for example be provided as one or several heating elements near the crucible 4.
  • the heating device 3 can for example be based on electric heating elements, for example supplied with a direct current or a single phase or three phase alternating current.
  • the heating device can for example be based on conventional heating elements or induction heating elements.
  • the furnace can for example comprise a thermal insulation device 2 for thermal insulation of the crucible.
  • a thermal insulation device 2 for thermal insulation of the crucible.
  • the thermal insulation device 2 is surrounding the heating device 3 and also the crucible 4.
  • the furnace can for example comprise a lower insulation device 12, having an opening for the guide 13.
  • the furnace further comprises a heat discharging device for discharging the heat from the crucible during the crystallization.
  • a heat discharging device for discharging the heat from the crucible during the crystallization.
  • the thermal insulation 12 can be moved, for instance downward, to increase conduction of heat away from the crucible.
  • a further possible solution is to actively remove heat by circulation of a cooled gas as disclosed in WO 2006/082085.
  • the furnace comprises a gas inlet 7 and a gas outlet 9 to provide an inert atmosphere in the furnace, as will be known for a person skilled in the art.
  • inert atmosphere means an atmosphere in contact with the materials of the furnace and silicon metal in the hot zone which is essentially chemically inert towards the materials of the furnace and the silicon metal phase, both in the solid and liquid state.
  • the term as used herein includes any gas pressure of the inert atmosphere, including vacuum.
  • the furnace further comprises a stirring device 14.
  • the stirring device 14 comprises one or several electromagnetic devices supplied with an alternating current for applying an alternating electromagnetic field to the silicon in the crucible 4.
  • the electromagnetic device can for example comprise coils or other types of electrically conducting rails suitable to provide a sufficient alternating electromagnetic field when supplied with an alternating current.
  • the electromagnetic device comprises a coil with 1 - 50 turns.
  • the stirring device is connected to a power supply.
  • the power supply can for example be a controllable power supply for controlling the frequency and/or the amplitude of the alternating current supplied to the coil(s).
  • the power supply is supplying a current with a constant frequency and/or constant amplitude.
  • the stirring force provided by the alternating current can be described as a Lorenz force given by the following expression:
  • the stirring device 14 is placed outside the thermal insulation device 2, to protect it from the heat from the heating device 3. Moreover, it would be possible to provide some or all the elements of the stirring device 14 with cooling means, for example water cooling or similar.
  • one coil of five turns, horizontally oriented, is placed around the thermal insulation device 2.
  • the coil is supplied with an AC current with frequency adjustable from approximately 10 - 100 Hz.
  • the amplitude of the current is adjustable from 0 - 3000 A.
  • the coil is connected to a controllable power supply comprising control means to control the values for the frequency and the amplitude of the current delivered to the coil.
  • control means to control the values for the frequency and the amplitude of the current delivered to the coil.
  • These values can be constant, or these values can be varying according to a preset time lapse for the process. Alternatively, these values can be controlled dependent of one or several process parameters, for example the temperature of the furnace etc.
  • sensors for sensing the process parameters will be connected to the control means of the power supply.
  • a current which generates a mean melt velocity of at least 0,5 cm/s will be suitable.
  • the mean melt velocity can not be measured directly, but based on models and simulations, it is possible to compute the current supplied to the stirring device to achieve this mean melt velocity.
  • the electromagnetic control devices can for example be a magnetic focusing material such as Fluxtrol A (trademark), supplied by Fluxtrol, Inc from Auburn Hill, MI,
  • the heating device is supplied with direct current, while the stirring device is supplied with an alternating current.
  • the electromagnetic field from the heating device will thus be static and will not induce stirring in the melt.
  • the alternating current of the respective coils can be phase-shifted in relation to each other.
  • the stirring device comprises three coils, where each coil is supplied with respective phase of a three phase alternating current.
  • Example 1 Furnace operation and characterization of material produced using an electromagnetic stirring device.
  • four standard crucibles for production of multicrystalline silicon were coated with a release coating and charged with PV-grade silicon according to standard procedures.
  • the loaded crucibles were placed on graphite support plates within the furnace.
  • the furnace was then closed, evacuated and backfilled with argon.
  • the crucibles were heated by means of top and bottom resistance heaters until all silicon was melted.
  • the heat discharging device and the electromagnetic stirring device were activated, and multicrystalline silicon ingots were grown. Upon completion of crystal growth, the ingots were cooled according to standard procedures.
  • the ingots were cut into 16 156x156 mm vertical blocks.
  • the material was inspected by IR camera and the minority carrier lifetime of each block was measured using a Semilab ⁇ -PCD instrument. Images of four blocks from one of the ingots are shown in fig. 3. The IR images show no precipitation of secondary phases. The average minority carrier lifetime as a function of the position in the block is shown in Figure 5 (fully drawn curve).
  • Example 2 Model calculation of stirring by an electromagnetic stirrer.
  • a coil of 5 turns of a square copper profile, 28 x 28 x 4 mm, is placed around the outside of the thermal insulation in a four-crucible DS furnace.
  • the coil is 180 mm tall and placed at the vertical level of the ingot at a distance of 37.3 cm from the crucible wall.
  • a 2D axisymmetric FEM calculation of the furnace with the coil indicates that with a coil current of 2830 A at 50 Hz a max melt velocity of 14.3 cm/s and a mean melt velocity of 2.7 cm/s are obtained.
  • Example 3 Model calculation of stirring by an electromagnetic stirrer.
  • Example 2 The calculation described in Example 2 was repeated, but with current reduced to 20%.
  • a 2D axisymmetric FEM calculation of the furnace with the coil indicates that with a coil current of 566 A at 50 Hz, a max melt velocity of 3.0 cm/s and a mean melt velocity of 0.6 cm/s are obtained.
  • a 2D axis symmetric FEM calculation of the furnace and coil of Example 2 indicates that that with a coil current of 2830 A at 30 Hz, a maximum melt velocity of 14.2 cm/s and a mean melt velocity of 2.8 cm/s are obtained.
  • Example 5 (Comparative) Model calculation of stirring by means of a 3-phase AC heater arranged above and below the crucible zone.
  • a 3D FEM calculation is made of a DS furnace with 3-phase AC heaters arranged above and below the crucibles. Heating at full capacity, with a current of approximately 1080 A (each phase) results in a maximum melt velocity of 2.2 cm/s and a mean melt velocity of 0.6 cm/s. During crystallization the stirring effect is reduced due to reduced power input.
  • the furnace according to the invention increases the melt velocity of the melt and also that the melt velocity can be controlled in a better way.
  • the result is an improved quality of the multicrystalline silicon produced by the furnace.

Abstract

The present invention relates to a method and a furnace for crystallization of silicon. The furnace comprises a crucible or a plurality of crucibles for containing the silicon; a heating device for heating the crucible; a heat discharging device for discharging the heat from the crucible; a stirring device comprising an electromagnetic device supplied with an alternating current for applying an alternating electromagnetic field to the crucible.

Description

Crystallization furnace
FIELD OF THE INVENTION
The present invention relates to a furnace for crystallization of silicon.
BACKGROUND OF THE INVENTION The most common substrate material for photovoltaic cells is multicrystalline silicon. Industrially, this material is commonly produced in crystallization furnaces by directional solidification (DS) by a variety of related techniques. One variant of DS is described for instance in US 2007/0044707-A1.
In the known art, the heaters of the furnace are normally powered by a 3 -phase alternating current. This alternating current also establishes a stirring force of a magnitude at least similar to the thermal buoyancy forces generated by temperature differences in the furnace. This stirring force causes melt circulation which improves segregation of impurities away from the crystal and decreases temperature gradients in the silicon melt. However, the stirring force is dependent of the heating power of the furnace.
One object of the invention is to provide a furnace where the quality of silicon produced by the furnace is improved. One other object is to improve the control of the crystallization of silicon.
SUMMARY OF THE INVENTION The present invention is disclosed in claim 1. Further features of the present invention are disclosed in the dependent claims.
DETAILED DESCRIPTION
In the accompanying drawings:
Fig. 1 illustrates an embodiment of the present invention seen from the side; Fig. 2 illustrates an embodiment of the present invention seen from above;
Fig. 3 shows an IR-image of silicon ingots where a furnace with a stirring device was used during the crystallization.
Fig. 4 shows an IR-image of silicon ingots where a furnace without a stirring device was used during the crystallization. Fig. 5 shows a diagram of the minority carrier lifetime from the samples shown in fig. 3 and fig. 4. It is now referred to fig. 1 and 2.
A furnace comprises an upper housing 1 and a lower housing 10. The upper housing can be removed for loading and unloading. The closed furnace is rendered vacuum tight by means of a seal 8. The furnace further comprises a crucible 4 for containing a raw material for the crystallization. The raw material can for example be silicon produced by the Siemens process or similar processes. The raw material is placed in the crucible before the crystallization process starts. In figure 1 and 2 are shown a layer of solid silicon 6 and a layer of molten silicon 5. The furnace further comprises a heating device 3 for heating the crucible 4, thereby causing the raw material to melt. The heating device can for example be provided as one or several heating elements near the crucible 4. The heating device 3 can for example be based on electric heating elements, for example supplied with a direct current or a single phase or three phase alternating current. The heating device can for example be based on conventional heating elements or induction heating elements.
The furnace can for example comprise a thermal insulation device 2 for thermal insulation of the crucible. In fig. 1 it is shown that the thermal insulation device 2 is surrounding the heating device 3 and also the crucible 4. In addition, the furnace can for example comprise a lower insulation device 12, having an opening for the guide 13.
The furnace further comprises a heat discharging device for discharging the heat from the crucible during the crystallization. There are several alternative embodiments of the heat discharging device. In one embodiment, the thermal insulation 12 can be moved, for instance downward, to increase conduction of heat away from the crucible. Alternatively, it may be possible in some embodiments to lower the crucible support 11 and crucible 4 relative to the heating device 3, for example by means of a guide 13. It would also be possible to decrease the power to the heating device 3 during the cooling. A further possible solution is to actively remove heat by circulation of a cooled gas as disclosed in WO 2006/082085.
Moreover, the furnace comprises a gas inlet 7 and a gas outlet 9 to provide an inert atmosphere in the furnace, as will be known for a person skilled in the art. The term "inert atmosphere" as used herein means an atmosphere in contact with the materials of the furnace and silicon metal in the hot zone which is essentially chemically inert towards the materials of the furnace and the silicon metal phase, both in the solid and liquid state. The term as used herein includes any gas pressure of the inert atmosphere, including vacuum. The furnace further comprises a stirring device 14. The stirring device 14 comprises one or several electromagnetic devices supplied with an alternating current for applying an alternating electromagnetic field to the silicon in the crucible 4. The electromagnetic device can for example comprise coils or other types of electrically conducting rails suitable to provide a sufficient alternating electromagnetic field when supplied with an alternating current. For example, the electromagnetic device comprises a coil with 1 - 50 turns.
The stirring device is connected to a power supply. The power supply can for example be a controllable power supply for controlling the frequency and/or the amplitude of the alternating current supplied to the coil(s). Alternatively, the power supply is supplying a current with a constant frequency and/or constant amplitude.
The stirring force provided by the alternating current can be described as a Lorenz force given by the following expression:
Re(J x (B + BJ) = -Re(J x B) + Re(J) xBs + -Re(JxB)
The Lorentz force v v ' Time dependent part v v '
Stationary part of with frequency ω of Time dependent part Lorentz force Lorentz force with frequency 2ω of
Lorentz force
The theory of the Lorentz force is further described in textbooks, for example P. A. Davidson, "An introduction to Magnetohydrodynamics", Cambridge Texts In Applied Mathematics, Cambridge University Press, 2001, and R. Moreau, "Magnetohydrodynamics", Kluwer Academic Publishers, London 1990.
In the present embodiment, the stirring device 14 is placed outside the thermal insulation device 2, to protect it from the heat from the heating device 3. Moreover, it would be possible to provide some or all the elements of the stirring device 14 with cooling means, for example water cooling or similar.
In the present embodiment one coil of five turns, horizontally oriented, is placed around the thermal insulation device 2. The coil is supplied with an AC current with frequency adjustable from approximately 10 - 100 Hz. The amplitude of the current is adjustable from 0 - 3000 A.
In the present embodiment the coil is connected to a controllable power supply comprising control means to control the values for the frequency and the amplitude of the current delivered to the coil. These values can be constant, or these values can be varying according to a preset time lapse for the process. Alternatively, these values can be controlled dependent of one or several process parameters, for example the temperature of the furnace etc. Here, sensors for sensing the process parameters will be connected to the control means of the power supply. In the example above, it has been found that a current which generates a mean melt velocity of at least 0,5 cm/s will be suitable. The mean melt velocity can not be measured directly, but based on models and simulations, it is possible to compute the current supplied to the stirring device to achieve this mean melt velocity. It would be possible to provide the furnace with electromagnetic control devices to concentrate the electromagnetic field from the coil to the crucible. The electromagnetic control devices can for example be a magnetic focusing material such as Fluxtrol A (trademark), supplied by Fluxtrol, Inc from Auburn Hill, MI,
USA. In the present embodiment, the heating device is supplied with direct current, while the stirring device is supplied with an alternating current. The electromagnetic field from the heating device will thus be static and will not induce stirring in the melt.
In one alternative embodiment, the alternating current of the respective coils can be phase-shifted in relation to each other. In yet an alternative embodiment, the stirring device comprises three coils, where each coil is supplied with respective phase of a three phase alternating current.
In one alternative embodiment, there are several crucibles 4 in the furnace.
Verification of the invention According to the invention, an improved control of the crystallization process is achieved, since the stirring means 14 can be controlled independently of the heating means 3.
Example 1 : Furnace operation and characterization of material produced using an electromagnetic stirring device. First, four standard crucibles for production of multicrystalline silicon were coated with a release coating and charged with PV-grade silicon according to standard procedures. The loaded crucibles were placed on graphite support plates within the furnace. The furnace was then closed, evacuated and backfilled with argon. The crucibles were heated by means of top and bottom resistance heaters until all silicon was melted. The heat discharging device and the electromagnetic stirring device were activated, and multicrystalline silicon ingots were grown. Upon completion of crystal growth, the ingots were cooled according to standard procedures. The ingots were cut into 16 156x156 mm vertical blocks. The material was inspected by IR camera and the minority carrier lifetime of each block was measured using a Semilab μ-PCD instrument. Images of four blocks from one of the ingots are shown in fig. 3. The IR images show no precipitation of secondary phases. The average minority carrier lifetime as a function of the position in the block is shown in Figure 5 (fully drawn curve).
For comparison, the same procedure was performed without using the electromagnetic stirring device. Corresponding images of four blocks from one of the ingots are shown in fig. 4. The IR-images show precipitation of secondary phases. The average minority carrier lifetime as a function of the position in the block is shown in Figure 5 (dashed curve).
Two quality requirements for solar grade silicon ingots are a high minority carrier lifetime and absence of precipitation. As can be seen in fig. 4 and 5, the present invention improves both these properties.
Example 2: Model calculation of stirring by an electromagnetic stirrer.
A coil of 5 turns of a square copper profile, 28 x 28 x 4 mm, is placed around the outside of the thermal insulation in a four-crucible DS furnace. The coil is 180 mm tall and placed at the vertical level of the ingot at a distance of 37.3 cm from the crucible wall. A 2D axisymmetric FEM calculation of the furnace with the coil indicates that with a coil current of 2830 A at 50 Hz a max melt velocity of 14.3 cm/s and a mean melt velocity of 2.7 cm/s are obtained.
Example 3. Model calculation of stirring by an electromagnetic stirrer.
The calculation described in Example 2 was repeated, but with current reduced to 20%. A 2D axisymmetric FEM calculation of the furnace with the coil indicates that with a coil current of 566 A at 50 Hz, a max melt velocity of 3.0 cm/s and a mean melt velocity of 0.6 cm/s are obtained.
Example 4. Model calculation of stirring by an electromagnetic stirrer.
A 2D axis symmetric FEM calculation of the furnace and coil of Example 2 indicates that that with a coil current of 2830 A at 30 Hz, a maximum melt velocity of 14.2 cm/s and a mean melt velocity of 2.8 cm/s are obtained.
Example 5. (Comparative) Model calculation of stirring by means of a 3-phase AC heater arranged above and below the crucible zone.
A 3D FEM calculation is made of a DS furnace with 3-phase AC heaters arranged above and below the crucibles. Heating at full capacity, with a current of approximately 1080 A (each phase) results in a maximum melt velocity of 2.2 cm/s and a mean melt velocity of 0.6 cm/s. During crystallization the stirring effect is reduced due to reduced power input.
Example 6. (Comparative*) Model calculation of stirring by buoyancy forces (thermal stirring)
In a crucible containing molten silicon, the maximum temperature difference between the centre of the crucible and the wall is 10 K. Model calculations indicate a mean melt velocity in the range 0.1-0.5 cm/s.
Consequently, it has been shown that the furnace according to the invention increases the melt velocity of the melt and also that the melt velocity can be controlled in a better way. The result is an improved quality of the multicrystalline silicon produced by the furnace.

Claims

1. Furnace for crystallization of silicon, characterized in that it comprises: a crucible or a plurality of crucibles for containing the silicon; a heating device for heating the crucible; - a heat discharging device for discharging the heat from the crucible; a stirring device comprising an electromagnetic device supplied with an alternating current for applying an alternating electromagnetic field to the crucible.
2. Furnace according to claim 1, characterized in that the stirring device is connected to a controllable power supply, for controlling the frequency and/or the amplitude of the alternating current supplied to the electromagnetic device.
3. Furnace according to claim 2, characterized in that the alternating current from the controllable power supply is a single phase AC current with frequency from 5 — 200 Hz, more preferably 10 - 100 Hz.
4. Furnace according to one of claims 1 - 3, characterized in that the stirring device comprises several electromagnetic devices, where each electromagnetic device is supplied with an alternating current in phase or phase-shifted in relation to the current supplied to the other electromagnetic devices.
5. Furnace according to one of claims 1 - 3, characterized in that the stirring device comprises three electromagnetic devices, where each electromagnetic device is supplied with respective phase of a three phase alternating current.
6. Furnace according to one of claims 1 - 5, characterized in that the electromagnetic device comprises a coil with 1 - 50 turns.
7. Furnace according to one of claims 1 - 6, characterized in that the electromagnetic device is provided near the crucible.
8. Furnace according to claim 1, characterized in that the stirring device comprises electromagnetic control devices to concentrate the electromagnetic field from the electromagnetic device to the crucible.
9. Furnace according to claim 1, characterized in that the furnace further comprises a thermal insulation device surrounding the crucible and the heating device.
10. Furnace according to claim 9, characterized in that the stirring device is located on the outside of the thermal insulation device.
11. Furnace according to claim 1 , characterized in that the heating device is powered by a direct current power source.
12. Furnace according to claim 1, characterized in that the silicon is multicrystalline silicon.
13. Furnace according to claim 1, characterized in that the stirring device is providing a mean melt velocity of at least 0,5 cm/s.
14. Method for crystallization of silicon, characterized in that it comprises the following steps: applying the silicon to a crucible or a plurality of crucibles; heating the crucible to melt the silicon; discharging the heat from the crucible to initiate the crystallization; - stirring the melt by applying an alternating electromagnetic field to the crucible by means of a stirring device comprising an electromagnetic device.
15. Method according to claim 14, characterized in that the stirring comprises controlling the frequency and/or the amplitude of an alternating current which is supplied to the electromagnetic device by means of a controllable power supply.
16. Method according to claim 15, characterized in that the stirring comprises supplying a single phase AC current with frequency from 5 - 200, more preferably 10 - 100 Hz.
17. Method according to claim 15, characterized in that the stirring device comprises several electromagnetic devices, where each electromagnetic device is supplied with an alternating current in phase or phase-shifted in relation to the current supplied to the other electromagnetic devices.
18. Method according to one of claims 14 - 17, characterized in that the stirring device comprises three electromagnetic devices, where each electromagnetic device is supplied with respective phase of a three phase alternating current.
19. Method according to one of claims 14 - 18, characterized in that the electromagnetic device comprises a coil with 1 - 50 turns.
20. Method according to one of claims 14 - 19, characterized in that the electromagnetic device is provided near the crucible.
21. Method according to claim 14, characterized in concentrating the electromagnetic field from the electromagnetic device to the crucible.
22. Method according to claim 14, characterized in that the furnace further comprises a thermal insulation device surrounding the crucible and the heating device.
23. Method according to claim 22, characterized in that the stirring device is located on the outside of the thermal insulation device.
3W- Method according to claim 14, characterized in that the heating device is powered by a direct current power source.
=2Sfc Method according to claim 14, characterized in that the silicon is multicrystalline silicon.
^C',, Method according to claim 14, characterized in providing a mean melt velocity of at least 0,5 cm/s.
PCT/NO2007/000227 2006-06-23 2007-06-22 Crystallization furnace WO2007148988A1 (en)

Applications Claiming Priority (2)

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US81585906P 2006-06-23 2006-06-23
US60/815,859 2006-06-23

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Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102008029951A1 (en) 2008-06-26 2009-12-31 Schott Ag Heat-insulating arrangement for crucibles and their use, as well as apparatus and method for the production of monocrystalline or multicrystalline materials
DE102008039457A1 (en) 2008-08-25 2009-09-17 Schott Ag Apparatus for oriented solidification of melts in crucibles, especially production of mono- or multi-crystalline silicon, comprises induction heater mounted on base of crucible or plate which supports it
WO2011032594A1 (en) * 2009-09-18 2011-03-24 Abb Ab Apparatus and method for crystallization of silicon
US20120167629A1 (en) * 2009-09-18 2012-07-05 Jan-Erik Eriksson Apparatus and method for crystallization of silicon
US20130104601A1 (en) * 2009-09-18 2013-05-02 Jan-Erik Eriksson Apparatus And Method For Crystallization Of Silicon
US8632632B2 (en) 2009-09-18 2014-01-21 Abb Ab Apparatus and method for crystallization of silicon
US8721789B2 (en) 2009-09-18 2014-05-13 Abb Ab Apparatus and method for crystallization of silicon
DE102010028173A1 (en) 2010-04-26 2011-10-27 Forschungsverbund Berlin E.V. Crystallization method for producing block made of material whose melt is electrically conductive, comprises e.g. providing crystallization system, loading container with material and melting it and initiating crystallization
WO2012139362A1 (en) * 2011-04-11 2012-10-18 上海普罗新能源有限公司 Polysilicon ingot casting furnace and polysilicon ingot casting method
WO2012171307A1 (en) * 2011-06-15 2012-12-20 安阳市凤凰光伏科技有限公司 Device for improving temperature gradient when casting quasi-monocrystalline crystal silicon ingot
JP2015155371A (en) * 2015-03-02 2015-08-27 エービービー エービー Device and method for crystallizing silicon

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