JP3833382B2 - Refrigerator-cooled superconducting magnet device for single crystal pulling device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a refrigerator-cooled superconducting magnet apparatus, and more particularly to a refrigerator-cooled superconducting magnet apparatus suitable for applying a magnetic field to molten silicon in a single crystal growth apparatus for producing a single crystal semiconductor from molten silicon.
[0002]
[Prior art]
In the manufacture of silicon single crystals, the Czochralski method (CZ method) is known in which polycrystalline silicon is melted and grown into a single crystal seed crystal. In this method, since the molten silicon is melted in the crucible, the quality of the single crystal produced by thermal convection may be deteriorated. Therefore, for the purpose of improving the quality of the produced single crystal, a method is known in which a magnetic field is applied to molten silicon and convection is suppressed by electromagnetic braking. This method is called a magnetic field application type Czochralski method (MCZ method).
[0003]
Three types of magnetic field application methods are known: a longitudinal magnetic field method, a transverse magnetic field method, and a cusp magnetic field, and an example thereof is disclosed in JP-A-8-188493. As a means for applying a magnetic field, a normal conducting magnet or a superconducting magnet is used. However, the normal conducting magnet is heavy because the use of an iron core is indispensable, and enormous power and cooling water are required. On the other hand, superconducting magnets usually require liquid helium cooling, which complicates and enlarges the apparatus. In addition, handling of liquid helium is complicated and skill of an operator is required. Furthermore, liquid helium needs to be replenished, and the liquid helium cost is high.
[0004]
[Problems to be solved by the invention]
By the way, there is provided a superconducting magnet (helium-free superconducting magnet) device that uses an oxide superconducting current lead and conducts and cools a superconducting coil only by a small refrigerator. This type of superconducting magnet device is called a refrigerator-cooled superconducting magnet device, is easy to operate, and is considered suitable for a silicon single crystal pulling device. However, the above-described single crystal pulling apparatus using the MCZ method is required to have such performances as being capable of generating a magnetic field stably, being easy to operate, having high reliability, and having a small leakage magnetic field.
[0005]
Therefore, an object of the present invention is to provide a refrigerator-cooled superconducting magnet device for a single crystal pulling device that can satisfy the above-described performance.
[0006]
[Means for Solving the Problems]
The present invention is a refrigerator-cooled superconducting magnet device for applying a magnetic field to the molten silicon in a single crystal growth apparatus for manufacturing a single crystal semiconductor from molten silicon, and is disposed around the single crystal growth apparatus, and the magnetic field A vacuum vessel having a double cylindrical structure containing a superconducting coil for generating a gas, and a refrigerator disposed on an upper portion of the vacuum vessel for cooling the superconducting coil, wherein the superconducting coil includes the vacuum vessel It is supported by a cylindrical coil cooling heat conductor as a winding frame, and the refrigeration stage of the refrigerator is in the vacuum vessel and near a connection portion provided in the coil cooling heat conductor extends to, between the freezing stage cold head and the connecting portion of the tip are connected by a heat transfer member having flexibility, said superconducting coil, mosquito to the molten silicon Each of the split type coils is wound so as to be concentric with the center of the vacuum vessel above and below the coil cooling heat conductor, and the two split type coils are provided. The coil and the heat conductor for cooling the coil are housed in a double-cylindrical heat shield container disposed in the vacuum container together with the cold head at the tip of the refrigeration stage and the heat transfer member. In order to separate the electric motor constituting the part from the superconducting coil, the mounting part of the electric motor in the vacuum vessel is extended upward in a cylindrical shape, and the freezing stage of the refrigerator is also extended, A portion corresponding to the heat shield container is extended upward in a cylindrical shape to accommodate the extended portion of the stage .
[0010]
In this case, it is preferable that a plurality of slits be formed at intervals in the circumferential direction in the lower region of the cylindrical extension in the heat shield container.
[0011]
The number of the refrigerators may be one, but when at least two refrigerators are arranged, they are preferably arranged adjacent to each other or at positions facing each other with respect to the diameter direction of the vacuum vessel.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will be described with reference to FIGS. The refrigerator-cooled superconducting magnet apparatus according to the first embodiment is arranged around a single crystal growth apparatus (not shown), and contains a vacuum container 10 containing superconducting coils 11a and 11b for generating a magnetic field, and a vacuum container 10 And two refrigerators 12a and 12b for cooling the superconducting coils 11a and 11b. The refrigerators 12a and 12b are connected to a compressor for compressing and supplying and circulating helium gas as a refrigerant. Briefly, the illustrated refrigerators 12a and 12b are connected to a displacer connected to an electric motor that switches a rotary valve for switching between introduction and discharge of helium gas, and the reciprocating motion is changed to a reciprocating motion. A motion conversion mechanism for setting upper and lower limits is provided. Details are disclosed in Japanese Examined Patent Publication No. 63-53469, and illustration and description thereof are omitted here. The superconducting coils 11a and 11b are supported in the vacuum vessel 10 by a coil cooling heat conductor 13 as a winding frame. Since the refrigerators 12a and 12b have exactly the same structure, only the refrigerator 12a will be described below.
[0013]
The refrigerator 12a includes a first-stage refrigeration stage 12-1 having a 50K first-stage cold head 12-11 and a second-stage refrigeration stage 12-2 having a 4K second-stage cold head 12-21. This is a two-stage refrigerator. The refrigeration stages 12-1 and 12-2 are in the vacuum vessel 10, and in particular, the refrigeration stage 12-2 extends to the vicinity of the connection portion 13-1 provided in the coil cooling heat conductor 13. The second stage cold head 12-21 at the tip of the refrigeration stage 12-2 and the connecting portion 13-1 are connected by a heat transfer member 14 having flexibility.
[0014]
The vacuum vessel 10 has a double cylindrical structure that can surround the periphery of the single crystal growth apparatus, and the heat conductor 13 for cooling the coil is also formed in a cylindrical shape. That is, the single crystal growth apparatus is disposed in a space formed inside the vacuum vessel 10. Since this single crystal growth apparatus may be the same as a conventional one, illustration and description are omitted. The superconducting coils 11a and 11b are two split coils for applying a caps-like magnetic field to the molten silicon in the crucible of the single crystal growth apparatus. The superconducting coils 11 a and 11 b formed by these two split coils are wound so as to be concentric with the center of the vacuum vessel 10 above and below the coil cooling heat conductor 13.
[0015]
Further, the two superconducting coils 11a and 11b and the coil cooling heat conductor 13 are housed in a double cylindrical heat shield container 15 disposed in the vacuum container 10 together with the refrigeration stage 12-2 and the heat transfer member 14. Has been. The heat shield container 15 is for preventing intrusion of radiant heat. The two superconducting coils 11a, 11b, the coil cooling heat conductor 13, and the heat shield container 15 are supported by a plurality of load supports 16a, 16b provided at intervals in the circumferential direction at the top and bottom in the vacuum container 10. Has been. Of course, the vacuum vessel 10 has a sealed structure, but the heat shield vessel 15 also preferably has a sealed structure from the viewpoint of preventing intrusion of radiant heat. Further, although one refrigerator may be used, when two refrigerators are arranged as in this example, they are arranged adjacent to each other as shown in the figure, or arranged at positions facing each other in the diameter direction of the vacuum vessel 10. Is preferred.
[0016]
Next, each component described above will be described.
[0017]
The vacuum vessel 10 is a vessel for maintaining an adiabatic vacuum, and serves as a fulcrum for supporting loads such as the superconducting coils 11a and 11b, the heat shield vessel 15 and the refrigerator. The vacuum vessel 10 can be formed by a welded structure or a flange structure. In the case of the flange structure as in this embodiment, it can be easily disassembled. In the case of a welded structure, it is preferable to provide a removable box in a part so that only the part where the current lead and the refrigerator are installed can be maintained.
[0018]
Superconducting coils 11a and 11b are formed of a metal-based or oxide-based superconducting wire. The superconducting coils 11a and 11b are of a split type in which two coils wound in a cylindrical shape are installed at an interval in the vertical direction, and a cusp-shaped magnetic field is formed by flowing currents in opposite directions to each coil. To do. The coil interval G is set to 0.1D <G <0.5D with respect to the winding diameter D of the superconducting coils 11a and 11b.
[0019]
Although not shown, current leads are used as means for supplying current from outside the vacuum vessel 10 to the superconducting coils 11a and 11b inside. As this current lead, a current lead made of an oxide superconductor is used, and current is supplied to the superconducting coils 11a and 11b while reducing the intrusion heat into the heat shield container 15. As the material, bismuth or yttrium is used. Further, a bulk body or a wire shape can be applied as the oxide superconductor. In this case, since it is a material with low mechanical strength, the junction with the superconducting coil that needs to reduce heat generation is fixed, and the other end is anchored to the first stage refrigeration stage temperature of the refrigerator, The free end is used to relieve stress due to heat shrinkage. That is, it joins by the flexible member which applied the laminated board, the flat net wire, etc. Silver oxide is coated on both ends of the oxide superconductor by thermal spraying, and electrodes are joined thereon by soldering. As a result, a current lead with low contact resistance and low Joule heat generation is possible.
[0020]
As the refrigerators 12a and 12b, a highly reliable small refrigerator such as a GM refrigerator is used. When applying a two-stage refrigerator as in this embodiment, the first stage cold head 12-11 is attached to the heat shield container 15, and the heat shield container 15 is attached to the first stage freezing stage 12-1. After cooling, the second stage cold head 12-21 is attached to the connecting portion 13-1, and the superconducting coils 11a and 11b can be cooled by the second stage refrigeration stage 12-2. A separate refrigerator may be added exclusively for the heat shield container 15. The cold head of each refrigeration stage and the superconducting coils 11a and 11b or the heat shield container 15 may be joined directly, but the stress is relieved as in this embodiment in order to prevent the occurrence of stress due to heat shrinkage. It is effective to interpose a heat transfer member 14 having the function of
[0021]
The coil cooling thermal conductor 13 is made of a material having a high thermal conductivity such as copper or aluminum. When it is made of a material having poor thermal conductivity, such as stainless steel or FRP, a foil-like high thermal conductive material such as copper or aluminum is attached to the surface.
[0022]
For example, a copper plate and a high-purity aluminum plate are laminated on the heat transfer member 14. As an example, it is assumed that a laminate of two copper plates having a thickness of 1 mm and a high-purity aluminum plate is bent into a substantially U shape as shown. One end of the heat transfer member 14 is bolt-coupled to the second-stage cold head 12-21 of the second-stage refrigeration stage 12-2, and the other end is bolt-coupled to the connection portion of the coil cooling heat conductor 13. When the heat transfer member 14 having such a laminated structure is used, the time required for cooling is the same as that using a copper-only plate, and the cooling characteristics are better than those using a copper-only plate when kept at a low temperature (about 4K). The number of laminated copper plates and high-purity aluminum plates is not limited to two, but it is preferable that the overall plate thickness is small.
[0023]
The heat shield container 15 surrounds the periphery of the superconducting coils 11a and 11b to reduce heat radiation in order to reduce heat intrusion into the superconducting coils 11a and 11b. The heat shield container 15 is joined to the first stage call head 12-11 of the first cooling stage 12-1 of the refrigerator. In this case as well, it is preferable to realize a good thermal connection with a flexible member so that excessive thermal stress is not generated in the cylinder of the refrigerator due to thermal contraction. In addition, a multilayer heat insulating material (super insulation) can also be used together for the purpose of enhancing the effect of reducing heat radiation. The heat shield container 15 is made of a material having excellent heat transfer characteristics, such as copper or aluminum. When mechanical strength is required, stainless steel can be used as the strength member, and copper or aluminum can be used in combination as the heat member. Furthermore, in order to prevent eddy currents caused by quenching of the superconducting coils 11a and 11b or sudden magnetic field fluctuations, slits may be partially provided in the length direction as will be described later. In this case, the slit width is set to several mm in consideration of the influence of thermal radiation.
[0024]
The load supports 16a and 16 are members that support the superconducting coils 11a and 11b, the coil cooling heat conductor 13, and the heat shield container 15, and need to have excellent heat insulation performance while maintaining mechanical strength. . As the material, for example, GFRP, CFRP, stainless steel or the like can be applied. As an example of a support structure, a configuration in which a superconducting coil is supported from a vacuum vessel by a plurality of pipe-like GFRP materials is known. This pipe-shaped GFRP material takes a heat shield container and an anchor in the middle portion to reduce intrusion heat by conduction. In this embodiment, a plurality of pipe-like load supports 16a and 16b are provided above and below the vacuum vessel 10 at intervals in the circumferential direction, and the superconducting coils 11a and 11b are insulated and supported. With such a simple support structure, not only the load in the vertical direction but also the function of supporting the load in the lateral direction can be provided. Although there are examples of using wires and pipes that protrude from the lateral direction to support the lateral load, the structure that supports the superconducting coil with only the members in the vertical direction is simple, easy to manufacture, and reliable. It becomes a high quality thing.
[0025]
In addition to the above-described components, if necessary, a magnetic shield body made of a ferromagnetic material is provided on the outer side (upper and / or outer peripheral surface, or both) of the vacuum vessel 10, so that the peripheral portion of the vacuum vessel 10 is provided. The leakage magnetic field can be reduced.
[0026]
With the above configuration, it is possible to provide a refrigerator-cooled superconducting magnet apparatus that is simple in structure, resistant to thermal stress, lightweight and compact, easy to operate, and highly reliable and suitable for the magnetic field application type Czochralski method.
[0027]
Next, a second embodiment of the present invention will be described with reference to FIGS. The feature of the second embodiment is that the installation locations of the motors 12a-1 and 12b-1 constituting the refrigerators 12a and 12b described above are separated from the superconducting coil as compared with the first embodiment. It is in. This is because a refrigerator, particularly an electric motor, may malfunction due to leakage magnetic flux from the superconducting coil. Therefore, it is the same as that of the first embodiment except for the configuration for separating the installation locations of the electric motors 12a-1 and 12b-1 from the superconducting coil, and the same parts as those in FIGS. Description is omitted. Here, the refrigerator 12a will be described.
[0028]
In order to separate the electric motor 12a-1 of the refrigerator 12a from the superconducting coil 11a, the mounting portion of the electric motor 12a-1 in the vacuum vessel 10 is configured to extend upward in a cylindrical shape. That is, the cylindrical vacuum vessel extension 10-1 is provided at the upper end of the vacuum vessel 10 corresponding to the mounting portion of the electric motor 12a-1. In accordance with this, a heat shield extension 15-1 is also provided at the upper end of the heat shield container 15. Furthermore, the second stage cold head 12-21 of the second stage refrigeration stage 12-2 of the refrigerator 12a is provided with a cold head extension 12-22, and the tip of the cold head extension 12-22 is connected to the coil cooling heat. The conductor 13 is positioned near the connection portion 13-1. This is because the length of the heat transfer body 14 is preferably as short as possible. In other words, if the length of the heat transfer body 14 is increased, it is necessary to increase the plate thickness in order to improve heat conduction. And if plate | board thickness becomes large, rigidity will rise and it will become impossible to respond | correspond to heat shrink. Note that the heat shrinkage is caused by the shrinkage of the superconducting coil when cooled, and since the temperature difference is large, the distortion with respect to heat increases.
[0029]
The heat shield extension 15-1 is made of a material having good thermal conductivity such as copper or aluminum. Here, the heat shield extension 15-1 has a cylindrical shape with a thickness of 20 mm, and eight slits are formed in the lower region at intervals in the circumferential direction. Then, it is bolted to the upper end portion of the heat shield container 15 at the lower end portion of the heat shield extension portion 15-1 divided into eight equal parts. The reason why the slits are provided as described above is that a large distortion due to thermal contraction occurs in the heat shield container 15 made of a thin plate, and this distortion acts on the heat shield extension 15-1, and this distortion is absorbed. Because. In other words, if the shield extension 15-1 is a cylindrical rigid body, the difference in thermal shrinkage between the cylindrical rigid body and the heat shield container 15 is large, and the cylindrical rigid body is distorted. In order to absorb such distortion, a slit is provided in the shield extension 15-1.
[0030]
The cold head extension 12-22 also uses a material having good thermal conductivity such as copper or aluminum, but is not made hollow but is made in a block shape.
[0031]
The refrigerator-cooled superconducting magnet apparatus according to the present invention described above is suitable for generating a cusp-type magnetic field as a magnetic field source in a silicon single crystal pulling apparatus using a magnetic field application type Czochralski method.
[0032]
【The invention's effect】
The refrigerator-cooled superconducting magnet apparatus according to the present invention does not require any operation related to cooling after the refrigerator is started, and does not require skill for cooling. Then, after several hours to several tens of hours, the superconducting coil reaches the superconducting temperature. When necessary, the excitation power supply can be operated to generate a predetermined magnetic field.
[0033]
Further, the generated cusp magnetic field suppresses the thermal convection of the molten silicon, and a silicon single crystal having a stable quality can be obtained.
[0034]
Furthermore, since only the power of the compressor for the refrigerator, water, and the power of the superconducting coil excitation power source are required, the running cost can be greatly reduced.
[0035]
And since a magnet apparatus is lightweight and compact, when changing the height of a magnet in the raising process, operation with low power is possible.
[Brief description of the drawings]
FIG. 1 is a plan view of a refrigerator cooled superconducting magnet apparatus according to a first embodiment of the present invention.
FIG. 2 is a longitudinal sectional view taken along line AA in FIG.
FIG. 3 is a view showing the structure around the refrigerator in FIG. 2;
FIG. 4 is a plan view of a refrigerator cooled superconducting magnet device according to a second embodiment of the present invention.
5 is a longitudinal sectional view taken along line BB in FIG.
6 is a view showing the structure around the refrigerator in FIG. 4;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Vacuum container 10-1 Vacuum container extension part 11a, 11b Superconducting coil 12a, 12b Refrigerator 12a-1, 12b-1 Electric motor 12-1 First stage refrigeration stage 12-2 Second stage refrigeration stage 12-11 1st First-stage cold head 12-21 Second-stage cold head 12-22 Cold head extension 13 Coil cooling heat conductor 13-1 Connection 14 Heat transfer member 15 Heat shield container 15-1 Heat shield extension 16a, 16b Load Support

Claims (3)

In a refrigerator-cooled superconducting magnet apparatus for applying a magnetic field to the molten silicon in a single crystal growth apparatus for manufacturing a single crystal semiconductor from molten silicon, the apparatus is disposed around the single crystal growth apparatus and generates the magnetic field. A vacuum vessel having a double cylindrical structure containing a superconducting coil, and a refrigerator disposed on the top of the vacuum vessel for cooling the superconducting coil, the superconducting coil serving as a winding frame in the vacuum vessel Are supported by a cylindrical coil cooling heat conductor, the freezing stage of the refrigerator is in the vacuum vessel and extends to the vicinity of the connection provided in the coil cooling heat conductor, A connection between the cold head at the tip of the refrigeration stage and the connection portion is made with a heat transfer member having flexibility ,
The superconducting coil is composed of two split type coils for applying a cusp-like magnetic field to the molten silicon, and each split type coil is concentric with the center of the vacuum vessel above and below the coil cooling heat conductor. Is wound as
The two split coils and the coil cooling heat conductor are housed in a double cylindrical heat shield container disposed in the vacuum container together with a cold head at the tip of the refrigeration stage and the heat transfer member,
In order to separate the electric motor constituting a part of the refrigerator from the superconducting coil, the mounting portion of the electric motor in the vacuum vessel is configured to extend upward in a cylindrical shape, and the refrigeration stage of the refrigerator is also configured A refrigerator-cooled superconducting magnet device for a single crystal pulling device , wherein the portion corresponding to the heat shield container is extended and accommodated by extending the extended portion of the refrigeration stage upward in a cylindrical shape. . 2. The single crystal according to claim 1, wherein a plurality of slits are formed in a lower region of the cylindrical extension portion in the heat shield container at intervals in the circumferential direction. Refrigerator-cooled superconducting magnet device for pulling device. The single crystal according to claim 1 or 2, wherein at least two of the refrigerators are arranged adjacent to each other or at positions facing each other in the diameter direction of the vacuum vessel. Refrigerator-cooled superconducting magnet device for pulling device.

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