Superconducting magnet device for single crystal pulling apparatus and magnetic field application method in single crystal pulling apparatus

Abstract

To provide a technique for preventing the crucible of a single crystal pulling apparatus from dissolving into a melt.SOLUTION: A superconductive magnet device 100 for single crystal pulling apparatuses includes a plurality of superconducting coils 120 arranged so as to surround a rotatable crucible 14 around a single crystal pulling axis 24 and for generating a synthetic magnetic field 122 orthogonal to the single crystal pulling axis 24. The superconductive magnet device 100 is constituted so as to synchronize the synthetic magnetic field 122 with the rotation of the crucible 14 to rotate the synthetic magnetic field around the single crystal pulling axis 24.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a superconducting magnet device for a single crystal pulling apparatus and a method for applying a magnetic field in the single crystal pulling apparatus. [Background technology]

[0002] The MCZ (Magnetic Field Applied Czochralski) method, in which a magnetic field is applied to a melt such as molten silicon in a crucible in a single crystal pulling apparatus, is known. A superconducting magnet device is used as the magnetic field generator for such a single crystal pulling apparatus. The strong magnetic field generated by the superconducting magnet suppresses thermal convection in the melt, thereby enabling the oxygen concentration in the pulled single crystal to be sufficiently reduced or controlled to a desired level. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-162827 Summary of the Invention [Problem to be solved by the invention]

[0004] The inventors of the present invention have conducted extensive research into superconducting magnet devices for single crystal pulling systems and have come to recognize the following problem. Single crystals are typically pulled while rotating a crucible and the single crystal being pulled from it around a pulling axis. While the crucible rotates, the convection, or movement, of the melt inside the crucible is suppressed by the magnetic field applied by the superconducting magnet device. In other words, the inner wall of the rotating crucible moves while touching the melt. As a result, dissolution of the inner wall surface of the crucible into the melt is promoted, raising concerns about a shortened crucible lifespan. Furthermore, oxygen or other impurities that may be contained in the crucible wall may dissolve in the melt and be incorporated into the pulled single crystal, potentially affecting crystal quality.

[0005] An exemplary object of an embodiment of the present invention is to provide a technique for suppressing dissolution of a crucible of a single crystal pulling apparatus into a melt. [Means for solving the problem]

[0006] According to one aspect of the present invention, a superconducting magnet device for a single crystal pulling apparatus includes a plurality of superconducting coils arranged to surround a crucible rotatable about a single crystal pulling axis, and generating a resultant magnetic field perpendicular to the single crystal pulling axis. The superconducting magnet device is configured to rotate the resultant magnetic field in synchronization with the rotation of the crucible about the single crystal pulling axis.

[0007] According to one aspect of the present invention, there is provided a method for applying a magnetic field in a single crystal pulling apparatus having a crucible rotatable about a single crystal pulling axis, the method comprising: generating a resultant magnetic field perpendicular to the single crystal pulling axis using a plurality of superconducting coils; and rotating the resultant magnetic field in synchronization with the rotation of the crucible about the single crystal pulling axis. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a technique for suppressing dissolution of the crucible of a single crystal pulling apparatus into the melt. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a top view schematically showing a cross section of a single crystal pulling apparatus according to an embodiment. [Figure 2] 1 is a side view schematically showing a cross section of a single crystal pulling apparatus according to an embodiment. [Figure 3] FIG. 2 is a circuit diagram illustrating a power supply system of a superconducting magnet apparatus according to an embodiment. [Figure 4] 4 is a waveform diagram illustrating an example of the waveform of an alternating current supplied to a superconducting coil according to the embodiment. FIG. [Figure 5]5 is a schematic diagram showing the rotation of a resultant magnetic field generated by a superconducting coil when the alternating current shown in FIG. 4 is supplied to the superconducting coil. [Figure 6] FIG. 10 is a schematic diagram showing a single crystal pulling apparatus according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description and drawings, identical or equivalent components, parts, and processes are designated by the same reference numerals, and redundant explanations will be omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience to facilitate explanation, and should not be interpreted as limiting unless otherwise specified. The embodiments are merely examples and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0011] Fig. 1 is a top view schematically showing a cross section of a single crystal pulling apparatus 10 according to an embodiment. Fig. 2 is a side view schematically showing a cross section of a single crystal pulling apparatus 10 according to an embodiment. Fig. 1 shows a BB cross section of Fig. 2, and Fig. 2 shows an AA cross section of Fig. 1.

[0012] The single crystal pulling apparatus 10 includes a single crystal pulling furnace 12 and a superconducting magnet device 100. The single crystal pulling apparatus 10 is, for example, a silicon single crystal pulling apparatus.

[0013] As shown in Fig. 2, the single crystal pulling furnace 12 includes a crucible 14, a single crystal pulling mechanism 16, a crucible rotation mechanism 18, and a heater 20. For ease of understanding, only the crucible 14 of the single crystal pulling furnace 12 is shown in Fig. 1.

[0014] The crucible 14 is a container for storing molten material (for example, molten silicon) and is made of, for example, quartz.

[0015] The single crystal pulling mechanism 16 is a drive device that pulls the single crystal 22 upward from the molten material in the crucible 14 along a single crystal pulling axis 24, and includes a pulling drive source that is disposed above and outside the single crystal pulling furnace 12. The single crystal pulling axis 24 is an axis that extends vertically (i.e., perpendicular to a horizontal plane). The single crystal pulling mechanism 16 is configured to pull the single crystal 22 while rotating it around the single crystal pulling axis 24.

[0016] The crucible rotation mechanism 18 is a drive device that rotates the crucible 14 around the single crystal pulling axis 24, and includes a rotation drive source that is disposed below and outside the single crystal pulling furnace 12. The crucible rotation mechanism 18 can rotate the crucible 14 in synchronization with the rotation of the single crystal 22 by the single crystal pulling mechanism 16 (for example, in the same direction and at the same rotation speed).

[0017] The heater 20 is disposed around the crucible 14 in the single crystal pulling furnace 12 and heats the crucible 14. The molten material in the crucible 14 is maintained in a molten state by the heating provided by the heater 20.

[0018] The single crystal pulling apparatus 10 is also provided with a rotation control device 26 that controls the crucible rotation mechanism 18 (and the single crystal pulling mechanism 16). The rotation control device 26 determines the rotation speed of the crucible 14 (and the single crystal 22) around the single crystal pulling axis 24, and operates the crucible rotation mechanism 18 (and the single crystal pulling mechanism 16) to rotate the crucible 14 (and the single crystal 22) at this rotation speed.

[0019] The superconducting magnet device 100 includes a cryostat 110, a plurality of superconducting coils 120, and a power supply system 130. The superconducting magnet device 100 is used as a magnetic field generation source for the single crystal pulling apparatus 10.

[0020] The cryostat 110 has a cylindrical shape that defines a hollow space inside. The single crystal pulling furnace 12 is disposed in this hollow space, and the cryostat 110 is installed coaxially with the single crystal pulling shaft 24 so as to surround the single crystal pulling furnace 12. The cryostat 110 is equipped with a cryogenic refrigerator (not shown), such as a Gifford-McMahon (GM) refrigerator, for cooling the superconducting coil 120. The cryostat 110 is also connected to a vacuum pumping system (not shown) for evacuating the internal space. During operation of the single crystal pulling apparatus 10, a cryogenic vacuum environment suitable for bringing the superconducting coil 120 into a superconducting state is provided within the internal space of the cryostat 110.

[0021] A plurality of superconducting coils 120 are arranged to surround the crucible 14, which is rotatable around the single crystal pulling axis 24, and generate a resultant magnetic field 122 that is perpendicular to the single crystal pulling axis 24. As will be described in detail later, the superconducting magnet device 100 is configured to rotate the resultant magnetic field 122 in synchronization with the rotation of the crucible 14 around the single crystal pulling axis 24. The resultant magnetic field 122 rotates in the same direction as the rotation of the crucible 14. The rotation speed of the resultant magnetic field 122 is, for example, equal to the rotation speed of the crucible 14.

[0022] In this embodiment, three sets of superconducting coils 120 are arranged in the internal space of the cryostat 110. Each set of superconducting coils 120 consists of two superconducting coils 120 facing each other across the single crystal pulling axis 24. The superconducting coils 120 are circular coils of the same diameter, with their central axes aligned in a radial direction perpendicular to the single crystal pulling axis 24. As shown in FIG. 1, they are arranged in a regular hexagonal shape when viewed from above. When current is supplied from the power supply system 130, each superconducting coil 120 generates a magnetic field directed radially outward or radially inward. These superconducting coils 120 generate a composite magnetic field 122 perpendicular to the single crystal pulling axis 24 by superposing the magnetic fields generated by the individual superconducting coils 120.

[0023] For ease of explanation, the sets of superconducting coils 120 may be distinguished from one another by being referred to as the first set of superconducting coils 120a, the second set of superconducting coils 120b, and the third set of superconducting coils 120c. As shown in FIG. 1 , the second set of superconducting coils 120b are disposed adjacent to the first set of superconducting coils 120a in a counterclockwise direction around the single crystal pulling axis 24. Similarly, the third set of superconducting coils 120c are disposed adjacent to the second set of superconducting coils 120b in a counterclockwise direction around the single crystal pulling axis 24. Furthermore, the first set of superconducting coils 120a are disposed adjacent to the third set of superconducting coils 120c in a counterclockwise direction around the single crystal pulling axis 24.

[0024] 3 is a circuit diagram illustrating a power supply system 130 of the superconducting magnet apparatus 100 according to the embodiment. The power supply system 130 is disposed outside the cryostat 110 and includes an excitation power supply 132 that excites the superconducting coils 120. The excitation power supply 132 is a three-phase AC power supply that supplies three-phase AC current to the three sets of superconducting coils 120 at a frequency synchronized with the rotation speed of the crucible 14. Each set of superconducting coils 120 is connected to a corresponding phase of the excitation power supply 132.

[0025] The excitation power supply 132 includes a converter 134 that rectifies three-phase AC (R, S, and T phases) supplied from an external power supply 140 such as a commercial power supply and converts it into DC (pulsating current), a smoothing circuit 136 that smooths the DC converted by the converter 134 to form a waveform, and an inverter 138 that converts the DC smoothed by the smoothing circuit 136 into three-phase AC (U, V, and W phases). A U-phase AC current is supplied to the first set of superconducting coils 120a, a V-phase AC current is supplied to the second set of superconducting coils 120b, and a W-phase AC current is supplied to the third set of superconducting coils 120c. A known configuration can be appropriately adopted for such excitation power supply 132, and therefore a detailed description of the specific configuration will not be given here.

[0026] The excitation power supply 132 includes a controller 142 that controls the inverter 138. The controller 142 is communicatively connected to the rotation control device 26 of the single crystal pulling apparatus 10 so as to receive a rotation speed signal S1 indicating the rotation speed of the crucible 14 from the rotation control device 26. The controller 142 receives the rotation speed signal S1 and, based on this signal, determines the frequency of the three-phase AC current to be supplied to the three sets of superconducting coils 120 so as to synchronize with the rotation speed of the crucible 14. The controller 142 controls the inverter 138 so that a three-phase AC current of the determined frequency is generated and supplied to the superconducting coils 120.

[0027] Furthermore, the controller 142 may receive an on / off signal S2 indicating on / off of the rotation of the crucible 14 from the rotation control device 26, and may supply AC current from the excitation power supply 132 to the superconducting coil 120 in accordance with this on / off signal S2. That is, when the crucible rotation mechanism 18 is rotating the crucible 14, the controller 142 may operate the inverter 138 to supply AC current from the excitation power supply 132 to the superconducting coil 120. When the operation of the crucible rotation mechanism 18 is stopped and the crucible 14 is not rotating, the controller 142 may operate the inverter 138 to cut off the supply of AC current from the excitation power supply 132 to the superconducting coil 120.

[0028] FIG. 4 is a waveform diagram illustrating an example of the waveform of an AC current supplied to the superconducting coil 120 according to the embodiment. As described above, a three-phase AC current is supplied to the superconducting coil 120 from the excitation power supply 132. More specifically, a U-phase AC current is supplied to the first set of superconducting coils 120a, a V-phase AC current is supplied to the second set of superconducting coils 120b, and a W-phase AC current is supplied to the third set of superconducting coils 120c. In FIG. 4, the moments when the U-phase current becomes zero are designated as phases of 0 degrees and 180 degrees. Since the V-phase has a phase difference of 120 degrees from the U-phase, the current becomes zero at phases of 120 degrees and 300 degrees. Similarly, since the W-phase has a phase difference of 120 degrees from the V-phase, the current becomes zero at phases of 60 degrees and 240 degrees.

[0029] 5 is a schematic diagram showing the rotation of a resultant magnetic field 122 generated by the superconducting coil 120 when the AC current shown in FIG. 4 is supplied to the superconducting coil 120. First, at phase 0 degrees, as shown in FIG. 4, the U-phase current is zero, and the V-phase and W-phase currents are equal in magnitude but in opposite directions. At this time, the first set of superconducting coils 120a does not generate a magnetic field, while the second set of superconducting coils 120b and the third set of superconducting coils 120c each generate a magnetic field within each coil as shown by the arrows in FIG. 5. As a result, the resultant magnetic field 122 generated by the superconducting coil 120 at phase 0 degrees is directed downward in FIG. 5.

[0030] At the 60-degree phase, as shown in Fig. 4, the W-phase current is zero, and the U-phase and V-phase currents are equal in magnitude but in opposite directions. The third set of superconducting coils 120c does not generate a magnetic field, while the first set of superconducting coils 120a and the second set of superconducting coils 120b each generate a magnetic field as shown by the arrows in Fig. 5. As a result, the composite magnetic field 122 generated by the superconducting coils 120 at the 60-degree phase is obtained by rotating the composite magnetic field 122 at the 0-degree phase by 60 degrees clockwise around the single crystal pulling axis 24.

[0031] Similarly, at phase 120 degrees, the V-phase current is zero, and the U-phase and W-phase currents are equal in magnitude but in opposite directions. The second set of superconducting coils 120b does not generate a magnetic field, while the first set of superconducting coils 120a and the third set of superconducting coils 120c each generate a magnetic field as shown by the arrows in Figure 5. As a result, the composite magnetic field 122 generated by the superconducting coils 120 at phase 120 degrees is obtained by rotating the composite magnetic field 122 at phase 60 degrees by 60 degrees clockwise around the single crystal pulling axis 24.

[0032] Similarly, the composite magnetic field 122 generated by the superconducting coils 120 at phase 180 degrees is the composite magnetic field 122 at phase 120 degrees rotated 60 degrees clockwise around the single crystal pulling axis 24, and is in the opposite direction (upward in FIG. 5) to the composite magnetic field 122 at phase 0 degrees. At phase 240 degrees, the composite magnetic field 122 is rotated 60 degrees clockwise around the single crystal pulling axis 24 with respect to the phase 180 degrees, and further, at phase 300 degrees, the composite magnetic field 122 is rotated 60 degrees clockwise around the single crystal pulling axis 24 with respect to the phase 240 degrees. In this way, by supplying three-phase AC current to the three sets of superconducting coils 120, the composite magnetic field 122 can be rotated around the single crystal pulling axis 24.

[0033] Therefore, according to the embodiment, the resultant magnetic field 122 generated by the superconducting coil 120 can be rotated in synchronization with the rotation of the crucible 14 (and the single crystal 22). The resultant magnetic field 122 always acts in the same direction as seen from the rotating crucible 14. This suppresses the relative movement between the inner wall of the crucible 14 and the molten material therein, thereby suppressing the dissolution of the inner wall of the crucible into the molten material. This helps to extend the life of the crucible 14. It also suppresses oxygen or other impurities that may be contained in the crucible wall from dissolving into the molten material and being incorporated into the single crystal 22, leading to improved crystal quality.

[0034] The multiple superconducting coils 120 may be multiple high-temperature superconducting coils. A high-temperature superconducting coil is a superconducting coil formed from a high-temperature superconducting wire. The high-temperature superconducting wire is formed from a superconductor having a superconducting transition temperature of, for example, 25 K or higher, or, for example, the temperature of liquid nitrogen (77 K) or higher. Generally, when an AC current is supplied to the superconducting coil 120, AC losses occur, which cause the superconducting coil 120 to generate heat. By using a high-temperature superconducting coil, the risk of quenching of the superconducting coil 120 due to this heat generation can be reduced.

[0035] When the cooling capacity of the cryostat 110 that cools the superconducting coil 120 (specifically, the cooling capacity of the cryogenic refrigerator provided in the cryostat 110) is sufficiently large, a low-temperature superconducting coil may be used as the superconducting coil 120.

[0036] In the above-described embodiment, three-phase AC current is given as an example of the polyphase AC current supplied to the superconducting coil 120, but the present invention is not limited to this. The polyphase AC current may be two-phase AC current or polyphase AC current with four or more phases. In this case, the polyphase AC power supply may be configured to supply the polyphase AC current at a frequency synchronized with the rotation speed of the crucible around the single crystal pulling axis, and the multiple superconducting coils may include multiple sets of superconducting coils, each set consisting of two superconducting coils facing each other across the single crystal pulling axis, and the superconducting coils of each set may be connected to a corresponding phase of the polyphase AC power supply.

[0037] 6 is a schematic diagram showing a single crystal pulling apparatus 10 according to another embodiment. In the above-described embodiment, the composite magnetic field 122 of the superconducting coil 120 is rotated electrically, but as will be described below, the composite magnetic field 122 may be rotated by mechanically rotating the superconducting coil 120.

[0038] Therefore, the superconducting magnet apparatus 100 is provided with a rotation mechanism 150 that rotates the multiple superconducting coils 120 around the single crystal pulling axis 24 at the rotation speed of the crucible 14 around the single crystal pulling axis 24. By driving the rotation mechanism 150, the composite magnetic field 122 can be rotated in synchronization with the rotation of the crucible 14 around the single crystal pulling axis 24.

[0039] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment created by combining embodiments will have the combined effects of the respective combined embodiments.

[0040] The single crystal pulling apparatus 10 according to the embodiment may be a single crystal pulling apparatus for producing single crystals of semiconductor materials other than silicon or other materials.

[0041] The present invention has been described using specific terms based on the embodiments, but the embodiments merely illustrate one aspect of the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the concept of the present invention as defined in the claims. [Explanation of symbols]

[0042] 10 Single crystal pulling apparatus, 14 Crucible, 22 Single crystal, 24 Single crystal pulling shaft, 100 Superconducting magnet apparatus, 120 Superconducting coil, 122 Combined magnetic field, 150 Rotation mechanism.

Claims

1. A superconducting magnet device for a single crystal pulling device, a plurality of superconducting coils arranged to surround a crucible that is rotatable around a single crystal pulling axis, and that generate a composite magnetic field perpendicular to the single crystal pulling axis; a polyphase AC power supply that supplies polyphase AC current at a frequency synchronized with the rotation speed of the crucible around the single crystal pulling axis; the plurality of superconducting coils include a plurality of sets of superconducting coils, each set consisting of two superconducting coils facing each other across the single crystal pulling axis, and the superconducting coils of each set are connected to a corresponding phase of the multi-phase AC power supply; A superconducting magnet device configured to rotate the composite magnetic field in synchronization with the rotation of the crucible around the single crystal pulling axis.

2. 2. The superconducting magnet apparatus according to claim 1, wherein the resultant magnetic field is rotated in the same direction as the rotation of the crucible.

3. 2. The superconducting magnet apparatus according to claim 1, wherein the multi-phase AC power supply is a three-phase AC power supply, and the plurality of sets of superconducting coils are three sets of superconducting coils.

4. 4. The superconducting magnet apparatus according to claim 1, wherein the plurality of superconducting coils are a plurality of high-temperature superconducting coils.

5. A method for applying a magnetic field in a single crystal pulling apparatus, the single crystal pulling apparatus having a crucible that is rotatable around a single crystal pulling axis, the method comprising: generating a resultant magnetic field perpendicular to the single crystal pulling axis using a plurality of superconducting coils; and rotating the composite magnetic field in synchronization with the rotation of the crucible around the single crystal pulling axis; the plurality of superconducting coils include a plurality of sets of superconducting coils, each set consisting of two superconducting coils facing each other across the single crystal pulling axis, and the superconducting coils of each set are connected to a corresponding phase of a polyphase AC power supply; the rotating step comprises supplying a multiphase alternating current from the multiphase alternating current power supply to the plurality of superconducting coils at a frequency synchronized with a rotation speed of the crucible about the single crystal pulling axis.

Images