Single crystal furnace
By designing a liftable crucible and a synchronous lifting exhaust device in a single crystal furnace, the oxygen recurrence problem caused by the hindered SiO discharge during the preparation of a single crystal silicon rod is solved, and faster and more efficient SiO discharge is achieved, improving the quality of the single crystal silicon rod.
Patent Information
- Application Number
- CN202510409763.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-02
- Publication Date
- 2025-06-27
AI Technical Summary
During the preparation of single crystal silicon rods, as the liquid level of the molten silicon material drops, the crucible position needs to be raised, resulting in the inert gas flow carrying SiO and being blocked from discharge, resulting in the oxygen content of the single crystal silicon rod being curled backward.
A single crystal furnace is designed, including a crucible that can be lifted and an exhaust device that can be lifted and lowered simultaneously with the crucible. The exhaust device includes a flow guide pipe and a flow guide ring, which has a hollow annular chamber and a plurality of air guide holes, which can guide the air flow to be quickly discharged from the bottom of the crucible to avoid the formation of air flow vortex.
By shortening the discharge path of SiO and guiding the airflow to discharge, the discharge speed of SiO is significantly accelerated, the oxygen recurrence phenomenon is avoided, and the quality of the single crystal silicon rod is improved.
Smart Images
Figure CN120210933A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of single-crystal silicon production, and particularly to a single-crystal furnace. Background Art
[0002] In the process of preparing a single-crystal silicon rod, the oxygen element content is a key process parameter determining the quality of the silicon rod. The main source of the oxygen element is gaseous SiO generated by the continuous reaction of a quartz crucible and molten silicon material at high temperature. The gaseous SiO is brought into the single-crystal silicon growth interface through melt thermal convection, and thus enters the single-crystal silicon rod. In order to reduce the oxygen content in the single-crystal silicon rod, when preparing the single-crystal silicon rod, an inert gas is usually introduced into the single-crystal furnace. The gas flow formed by the inert gas timely takes away the gaseous SiO, thereby reducing the entry of the gaseous SiO into the single-crystal growth interface and reducing the oxygen content in the single-crystal silicon rod. The inert gas flow generally carries the gaseous SiO, first transfers from the liquid surface of the molten silicon material to the edge of the crucible opening, then descends along the outer wall of the crucible to below the crucible, and finally is discharged from the exhaust port at the bottom of the furnace body out of the single-crystal furnace cavity.
[0003] During the crystal pulling process, as the liquid level of the molten silicon material drops, the position of the crucible needs to be correspondingly lifted to maintain the stability of the solid-liquid interface. However, in this stage, the discharge of the inert gas flow carrying SiO is blocked, resulting in an oxygen back-curl (a phenomenon where the oxygen content at the tail of the single-crystal silicon rod increases) in the oxygen content of the single-crystal silicon rod. Summary of the Invention
[0004] To improve the problem of oxygen back-curl in the oxygen content of the single-crystal silicon rod caused by the blocked discharge of SiO, the present invention provides a single-crystal furnace.
[0005] A single-crystal furnace according to an embodiment of the present invention includes:
[0006] A furnace body;
[0007] A crucible, which is arranged inside the furnace body in a liftable manner;
[0008] An exhaust device, which is arranged on a diversion pipeline below the crucible. The diversion pipeline includes a suction end and an exhaust end located below the suction end. The suction end is located inside the furnace body, and the exhaust end is communicated with a evacuation system outside the furnace body. At least the suction end synchronously lifts and lowers with the crucible.
[0009] In some embodiments, the exhaust device includes a vertically arranged diversion pipeline. The upper end of the diversion pipeline is the suction end, and the lower end of the diversion pipeline is the exhaust end.
[0010] In some embodiments, the exhaust end is communicated with the evacuation system outside the furnace body through an exhaust port opened at the bottom of the furnace body.
[0011] In some embodiments, the exhaust device further includes a flow guiding ring connected to the suction end, and the flow guiding ring has a flow guiding channel that diverts from below the crucible to the suction end.
[0012] In some embodiments, the interior of the flow guiding ring has a hollow annular chamber. The end face at the top of the flow guiding ring is provided with a plurality of first air guiding holes communicating with the annular chamber, and the end face at the bottom of the flow guiding ring is provided with second air guiding holes communicating with the air inlet end. The first air guiding holes, the annular chamber, and the second air guiding holes form the flow guiding channel.
[0013] In some embodiments, the end face at the top of the flow guiding ring and the end face at the bottom of the flow guiding ring are recessed downward.
[0014] In some embodiments, the exhaust device further includes a liftable lift rod and a driver for driving the lift rod to lift and lower. The top of the lift rod is fixedly connected to the end face at the bottom of the flow guiding ring, and the bottom of the lift rod movably passes through the bottom of the furnace body and extends out of the furnace body to be connected to the driver.
[0015] In some embodiments, the exhaust end and the suction end can be lifted and lowered synchronously.
[0016] In some embodiments, the exhaust end is fixed and the flow guiding pipe is telescopic.
[0017] In some embodiments, the single crystal furnace further includes a heater and a heat preservation cylinder. The heater is disposed around the outer periphery of the single crystal furnace, and the heat preservation cylinder is disposed below the heater;
[0018] The inner diameter of the heat preservation cylinder is smaller than the outer diameter of the legs of the heater, and a notch for the legs to pass through is formed in the heat preservation cylinder.
[0019] During the process of pulling a single crystal rod in the single crystal furnace of the present invention, the liquid level of the molten silicon material in the crucible gradually decreases, the crucible gradually rises, and the suction end of the exhaust device rises following the crucible. Compared with the conventional single crystal furnace in which gaseous SiO can only be discharged from the exhaust port at the bottom of the furnace under the action of air flow after entering below the crucible, since the suction end of this embodiment can rise and fall synchronously with the crucible, the distance between the opening edge of the crucible and the suction end can be kept constant. On the one hand, it effectively shortens the discharge path of SiO from the bottom of the crucible to the exhaust device. On the other hand, it guides the discharge path of SiO from the bottom of the crucible to the bottom of the furnace, avoiding the formation of vortices below the crucible, thereby accelerating the discharge speed of SiO and being able to improve the problem that the oxygen content of the single crystal rod shows a reverse warp due to the blockage of SiO discharge. Description of the Drawings
[0020] Figure 1 It is a schematic structural diagram of the interior of the furnace body of the single crystal of this embodiment;
[0021] Figure 2 Explosion schematic diagram of the exhaust device and evacuation system of the single crystal furnace of this embodiment connected through the furnace bottom;
[0022] Figure 3 Structural schematic diagram of the connection between the diversion pipeline and the diversion ring;
[0023] Figure 4 is Figure 3 Cross-sectional structural schematic diagram at A-A in
[0024] Figure 5 Structural schematic diagram of the heat preservation cylinder;
[0025] Figure 6 Structural schematic diagram of the furnace body bottom;
[0026] Figure 7 Schematic diagram of the airflow inside the single crystal furnace of this embodiment;
[0027] Figure 8 Schematic diagram of the airflow inside the existing single crystal furnace.
[0028] In the figure: furnace body 10; exhaust port 11; crucible 20; heater 30; support leg 31; lifting shaft 40; bracket 41; heat preservation cylinder 50; notch 51; diversion pipeline 60; suction end 61; exhaust end 62; diversion ring 70; annular chamber 71; first air guide hole 72; second air guide hole 73; diversion channel 74; evacuation system 80; intake section 81; transition section 82; confluence section 83; outlet section 84; lifting rod 90. Detailed implementation manners
[0029] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention and are not used to limit the present invention.
[0030] The structures, ratios, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those familiar with this technology to understand and read, and are not used to limit the limited conditions under which the present invention can be implemented. Any modification of the structure, change of the proportional relationship or adjustment of the size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope that can be covered by the technical content disclosed in the present invention.
[0031] The directions or positional relationships indicated by the terms "upper", "lower", "left", "right", "middle", "longitudinal", "lateral", "horizontal", "inner", "outer", "radial", "circumferential" and the like in this specification are based on the directions or positional relationships shown in the drawings and are only for the convenience of simplifying the description. They do not indicate or imply that the devices or elements referred to must have a specific direction, be constructed and operate in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.
[0032] like Figure 8 As shown in the figure, in the existing single crystal furnace, as the liquid level of the molten silicon material decreases, the crucible position increases accordingly, and the inert gas flow carrying SiO is blocked from being discharged, resulting in the oxygen content of the single crystal silicon rod being reversed. The main reasons are:
[0033] 1. After the crucible is raised, the distance between the bottom of the crucible and the exhaust port at the bottom of the furnace increases, which prolongs the discharge path of SiO gas and increases the difficulty of SiO discharge.
[0034] 2. After the crucible is lifted, the blank area formed between the crucible and the bottom of the furnace becomes larger, which can easily cause the airflow to produce cyclones, hindering the normal discharge of SiO.
[0035] like Figure 1As shown in the figure, this embodiment provides a single crystal furnace, which includes a furnace body 10, a crucible 20, an exhaust device, and a evacuation system 80. The crucible 20 is disposed inside the furnace body 10 in a liftable manner, and silicon material is contained in the crucible 20. According to an embodiment, the single crystal furnace further includes a lift shaft 40. The lift shaft 40 is disposed below the crucible 20, and a bracket 41 is provided at the top of the lift shaft 40 to support the bottom of the crucible 20. The bottom of the lift shaft 40 movably passes through the bottom of the furnace body 10 and extends out of the furnace body 10, so that a driving member for driving the lift shaft 40 to lift is connected to the bottom of the lift shaft 40. The evacuation system 80 is located outside the furnace body 10 and is used to evacuate the inside of the furnace body 10 to a vacuum. It should be noted that the furnace body 10, the crucible 20, the lift shaft 40, the driving member, and the evacuation system 80 in this embodiment are all prior arts, so the specific structures thereof will not be described in detail in this embodiment. The exhaust device 20 is disposed below the crucible 20 and includes a suction end 61 and an exhaust end 62 located below the suction end 61. The suction end 61 is located inside the furnace body 10, and the exhaust end 62 communicates with the evacuation system 80 outside the furnace body 10. At least the suction end 61 follows the crucible 20 to lift. The suction end 61 is located above the exhaust end 62, which can enable the air flow to enter the suction end 61 and then smoothly discharge from the exhaust end 62 after being guided by the exhaust device. The exhaust end 62 communicates with the external evacuation system 80, so that after the air flow discharges from the exhaust end 62, it is discharged from the single crystal furnace under the action of the evacuation system 80. Therefore, after being guided by the exhaust device, the air flow can be promoted to discharge from the single crystal furnace. In this embodiment, the suction end 61 follows the crucible 20 to lift synchronously. During the preparation process of the single crystal rod, when the crucible rises, the suction end 61 of the diversion pipe 60 follows the crucible 20 to lift, which can enable the air flow to enter the suction end 61 more quickly after flowing out from the rear edge of the crucible, and thus quickly discharge under the guidance of the exhaust device, reducing the residence time of the air flow below the crucible.
[0036] During the process of pulling the single crystal rod in this embodiment of the single crystal furnace, the liquid level of the molten silicon material in the crucible 20 gradually decreases, the crucible 20 gradually rises, and the suction end 61 of the diversion pipe 60 follows the crucible 20 to rise. That is to say, the distance between the suction end 61 of the diversion pipe 60 and the crucible 20 remains unchanged. For specific reference, see Figure 7 and Figure 8 , compared with the way of discharging SiO from the exhaust port 11 without the guidance of the exhaust device in the traditional gaseous SiO, the additional exhaust device in this embodiment effectively shortens the discharge path of SiO from the bottom of the crucible 20 to the exhaust device on the one hand, and guides the discharge path of SiO from the bottom of the crucible 20 to the bottom of the furnace on the other hand, avoiding the formation of vortices below the crucible 20, so as to accelerate the discharge speed of SiO and improve the problem that the oxygen content of the single crystal rod shows a reverse warp due to the blockage of SiO discharge.
[0037] For specific reference, see Figure 2, the exhaust port 11 of the single crystal furnace in this embodiment is arranged at the bottom of the furnace body 10. The exhaust device specifically includes a vertically arranged diversion pipeline 60. The top of the diversion pipeline 60 is the suction end 61, and the bottom of the diversion pipeline 60 is the exhaust end 62. The exhaust end 62 is connected to the evacuation system 80 outside the furnace body 10 through the exhaust port 11. The diversion pipeline 60 is connected to the evacuation system 80 through the original exhaust port 11, without the need to change the layout of the original components in the traditional single crystal furnace, which can reduce the transformation cost. The number of the diversion pipelines 60 in this embodiment can be set according to actual needs. For example, two exhaust ports 11 are arranged at the bottom of the furnace body 10, so the number of the diversion pipelines 60 is two.
[0038] In some embodiments, the exhaust end 62 and the suction end 61 can be lifted and lowered synchronously. The exhaust end 61 is located outside the furnace body 10 and can be lifted and lowered relative to the exhaust port 11; in some embodiments, the exhaust end 62 is fixedly arranged, the diversion pipeline 60 is telescopic, and the diversion pipeline 60 in some embodiments is telescopic. In some other embodiments, the diversion pipeline 60 can be implemented in any form as long as the suction end 61 can be lifted and lowered.
[0039] For example, the diversion pipeline 60 is implemented as non-telescopic, the suction end 61 and the exhaust end 62 are lifted and lowered synchronously, and the exhaust end 62 can be inserted into the pipeline of the evacuation system 80 and lifted and lowered in the pipeline of the evacuation system 80.
[0040] For example, the diversion pipeline 60 can be selected to include at least two spliced rigid pipes, and at least two rigid pipes are nested to form a telescopic tubular structure, so that the suction end 61 of the diversion pipeline 60 can follow the crucible 20 to lift and lower, and the exhaust end 62 of the diversion pipeline 60 remains fixed. The diversion pipeline 60 in some embodiments can also be selected to use a flexible pipe, such as a corrugated pipe, so that the suction end 61 of the diversion pipeline 60 can follow the crucible 20 to lift and lower, and the exhaust end 62 of the diversion pipeline 60 remains fixed.
[0041] The evacuation system 80 in this embodiment includes a suction pipeline and a suction pump. The suction pipeline preferably includes an intake section 81 for connecting the exhaust port 11. The intake section 81 is vertically arranged below the exhaust port 11. The suction pipeline 80 also preferably includes a transition section 82, a manifold section 83, and an outlet section 84 that are sequentially connected to the intake section 81. The outlet section 84 is connected to the suction pump. The gas in the intake section 81 first passes through the transition section 82 and converges in the manifold section 83, and then is sucked out by the suction pump through the outlet section 84. The transition section 82 extends laterally along the intake section 81 to avoid interference between the suction pipeline 80 and the driving member at the bottom of the lifting shaft 40.
[0042] As described above, when the suction end 61 and the exhaust end 62 of the diversion pipeline 60 are lifted and lowered synchronously, the exhaust end 62 can be vertically movably inserted into the intake section 81. In this case, the diameter of the intake section 81 is slightly larger than the diameter of the diversion pipeline 60 so that the diversion pipeline 60 can be vertically movably inserted into the intake section 81. The intake section 81 is sleeved on the outer peripheral side of the part of the diversion pipeline 60 extending out of the furnace body 10, so that the diversion pipeline 60 can move vertically inside the intake section 81 to meet the lifting requirements of the entire diversion pipeline 60.
[0043] Specifically, refer to Figure 3-4 , the exhaust device of this embodiment further includes a diversion ring 70 connected to the suction end 61. The diversion ring 70 is coaxially arranged below the crucible 20 and communicated with the suction end 61 of the diversion pipeline 60. The diversion ring 70 has a diversion channel 74 for diverting from below the crucible 20 to the suction end 61.
[0044] The diversion ring 70 of this embodiment is preferably hollow, and has a hollow annular chamber 71 inside. A plurality of first air guide holes 72 are arranged on the end face at the top of the diversion ring 70 and communicated with the annular chamber 71, so that the diversion ring 70 is porous. A second air guide hole 73 is arranged on the end face at the bottom of the diversion ring 70 and communicated with the suction end 61 of the diversion pipeline 60. The first air guide hole 72, the annular chamber 71 and the second air guide hole 73 form the aforementioned diversion channel 74. The connection between the diversion ring 70 and the intake end 61 can make the diversion ring 70 and the intake end 61 lift and lower synchronously, so that it can lift and lower synchronously with the crucible 20. The diversion ring 70 can not only fill the blank area between the crucible 20 and the bottom of the furnace body 10, but also effectively reduce the probability of generating air vortices with its porous structure thereon, and improve the problem of blocked discharge of SiO. It should be noted that the diversion ring 70 in some embodiments can also be designed into a horn shape or other existing diversion structure shapes according to requirements.
[0045] The end face at the top of the diversion ring 70 and the end face at the bottom of the diversion ring 70 of this embodiment are preferably recessed downward. The shape of the recess can guide SiO to enter the diversion pipeline 60 through the first air guide hole 72, the annular chamber 71 and the second air guide hole 73 in sequence, change the flow path of SiO, and further reduce the probability of generating air vortices in the blank area.
[0046] Specifically, refer to Figure 2 and Figure 6, the exhaust device of this embodiment further includes a lifting rod 90 and a driver for driving the lifting of the lifting rod 90. The top of the lifting rod 90 is fixedly connected to the end face at the bottom of the flow guide ring 70. The bottom of the lifting rod 90 movably passes through the bottom of the furnace body 10 and extends below the furnace body 10, so as to facilitate the connection between the bottom of the lifting rod 90 and the driver for driving the lifting of the lifting rod 90. The driver drives the lifting of the lifting rod 90, and the lifting rod 90 is connected to the end face at the bottom of the flow guide ring 70, so as to drive the flow guide ring 70 to lift. The end at the bottom of the flow guide ring 70 is connected to the air inlet end 61, so as to drive the air inlet end 61 to lift. The driver for driving the lifting of the air suction end 61 and the driving member for driving the lifting of the crucible 20 can be from the same power source, which can reduce costs. In some embodiments, the power source can also be set separately according to requirements to drive the lifting of the air suction end 61. The number of the lifting rods 90 can be set according to requirements. In this embodiment, two lifting rods 90 are specifically set, and the two lifting rods 90 are distributed on the opposite sides of the flow guide ring 70 to ensure the stable lifting of the flow guide ring 70.
[0047] For details, refer to Figure 1 and Figure 5 , the single crystal furnace of this embodiment further includes a heater 30 sleeved around the crucible 20 and a heat preservation cylinder 50 arranged below the heater 30. The thickened heat preservation cylinder 50 can not only improve the heat preservation effect and reduce the heat loss in the single crystal furnace, but also fill more of the blank area between the bottom of the crucible 20 and the bottom of the furnace body 10 to reduce the probability of air vortices generated in the blank area. The inner diameter of the heat preservation cylinder 50 is preferably smaller than the inner diameter of the leg 31 of the heater 30. A notch 51 is formed in the upper part of the heat preservation cylinder 50 for the leg 31 to pass through to avoid interference between the heat preservation cylinder 50 and the heater 30. The heat preservation cylinder 50 is thickened towards the inner side, which can further reduce the space below the crucible and further weaken the air flow vortices generated in the space below the crucible.
[0048] In the process of preparing the single crystal silicon rod in this embodiment, when the liquid level of the molten silicon material gradually decreases due to crystal growth, the driving member lifts the quartz crucible 20 through the lifting shaft 40 to maintain the stability of the solid-liquid interface. The lifting rod 90 drives the flow guide ring 70 and the air inlet end 61 to rise synchronously, so that the air suction end 61 of the flow guide pipe 60 always maintains a constant distance from the crucible 20. During this process, for details, refer to Figure 7The inert gas injected from the top of the furnace body 10 forms a directional airflow under the action of the suction device at the bottom of the furnace body 10. The airflow carries the gaseous SiO generated by the reaction of the silicon material liquid surface into the suction end 61 under the action of the guide ring 70, and enters the evacuation system from the exhaust end 62 under the action of the guide pipe 60, and is discharged from the furnace body 10: the inert gas flow first carries the gaseous SiO along the liquid surface to the opening edge of the crucible 20, and then descends along the outer wall of the crucible 20 to the bottom of the crucible 20, and then is guided by the guide ring 70 and the guide pipe 60 through the exhaust port 11, and finally discharged through the suction device.
[0049] Since the present embodiment adds an exhaust device below the crucible 20, the distance between the opening edge of the crucible 20 and the air intake end 61 of the exhaust device is kept constant. On the one hand, the discharge path of SiO from the bottom of the crucible 20 to the exhaust device is effectively shortened; on the other hand, the blank area between the bottom of the crucible 20 and the bottom of the furnace body 10 can be filled to improve the probability of airflow forming a vortex in the blank area; on the third hand, the discharge path of SiO from the bottom of the crucible 20 to the bottom of the furnace can be guided to avoid the formation of a vortex below the crucible 20, thereby accelerating the discharge speed of SiO, and improving the problem of reverse warping of the oxygen content of the single crystal silicon rod due to the obstruction of SiO discharge.
[0050] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.
[0051] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.
Claims
1. A single crystal furnace, characterized in that: include: Furnace body (10); A crucible (20) is arranged inside the furnace body (10) in a liftable manner; An exhaust device is arranged below the crucible (20), comprising an air intake end (61) and an exhaust end (62) located below the air intake end (61), wherein the air intake end (61) is located inside the furnace body (10), and the exhaust end (62) is connected to an evacuation system (80) outside the furnace body (10), and at least the air intake end (61) rises and falls synchronously with the crucible (20).
2. The single crystal furnace according to claim 1, characterized in that: The exhaust device comprises a vertically arranged flow guiding pipe (60), the upper end of the flow guiding pipe (60) is an air intake end (61), and the lower end of the flow guiding pipe (60) is an air exhaust end (62).
3. The single crystal furnace according to claim 1, characterized in that: The exhaust end (62) is connected to the evacuation system (80) outside the furnace body (10) through an exhaust port (11) opened at the bottom of the furnace body (10).
4. The single crystal furnace according to any one of claims 1 to 3, characterized in that: The exhaust device further comprises a guide ring (70) connected to the air intake end (61), wherein the guide ring (70) has a guide channel (74) for guiding air from the bottom of the crucible (20) to the air intake end (61).
5. The single crystal furnace according to claim 4, characterized in that: The guide ring (70) has a hollow annular chamber (71) inside, the top end surface of the guide ring (70) is provided with a plurality of first air guide holes (72) connected to the annular chamber (71), the bottom end surface of the guide ring (70) is provided with second air guide holes (73) connected to the air inlet end, and the first air guide holes (72), the annular chamber (71), and the second air guide holes (73) form the guide channel (74).
6. The single crystal furnace according to claim 5, characterized in that: The end surface of the top of the guide ring (70) and the end surface of the bottom of the guide ring (70) are recessed downwards.
7. The single crystal furnace according to claim 4, characterized in that: The exhaust device also includes a lift rod (90) that can be raised and lowered and a driver that drives the lift rod (90) to be raised and lowered, wherein the top of the lift rod (90) is connected to the end surface of the bottom of the guide ring (70), and the bottom of the lift rod (90) can be moved through the bottom of the furnace body (10) and extend out of the furnace body (10) to be connected to the driver.
8. The single crystal furnace according to claim 2, characterized in that: The exhaust end (62) and the intake end (61) can be raised and lowered synchronously.
9. The single crystal furnace according to claim 2, characterized in that: The exhaust end (62) is fixed and the flow guide pipe (60) is retractable.
10. The single crystal furnace according to any one of claims 1 to 3, characterized in that: It also includes a heater (30) and a heat preservation tube (50), wherein the heater (30) is arranged around the outer circumference of the crucible (20), and the heat preservation tube (50) is arranged below the heater (30); The inner diameter of the heat-insulating tube (50) is smaller than the outer diameter of the supporting leg (31) of the heater (30), and a notch (51) is provided in the heat-insulating tube (50) for the supporting leg (31) to pass through.