Thermal field structure, single crystal furnace and crystal pulling process

Abstract

The invention provides a thermal field structure which comprises a guide cylinder and a furnace cover matched with the guide cylinder, the guide cylinder is at least provided with a body, the body comprises a vertical section, a curved surface section and an inner diameter section which are connected in sequence, and the vertical section is connected with the curved surface section through a transition section; the curved surface section is constructed to be an arc surface protruding outwards. The thermal field structure is optimized, the gap between the lower bottom face of the guide cylinder and the solid-liquid interface is reduced, the volatilization speed of oxygen content taken away by gas is increased, the concentration of oxygen in the peripheral atmosphere is reduced, and the influence of shear convection on oxygen volatilization is reduced. And meanwhile, the fitting tightness of the auxiliary body and the furnace cover is improved, and the loss of inert gas is reduced, so that the utilization rate of gas can be improved, the crystal pulling quality can be improved, and the oxygen content can be reduced. The invention also provides a single crystal furnace provided with the thermal field structure and a crystal pulling process for preparing a single crystal silicon rod by adopting the single crystal furnace.

Description

Technical Field

[0001] The present application belongs to the technical field of Czochralski single crystal production, and in particular, relates to a thermal field structure, a single crystal furnace provided with the thermal field structure, and a crystal pulling process for preparing single crystal silicon rods using the single crystal furnace. Background Art

[0002] With the iteration and development of photovoltaic cells, the industry's requirements for the quality of single crystals are constantly increasing. Due to the limitations of the solar-grade direct-pull single crystal silicon production process, oxygen impurities are introduced during the crystal growth process. In the later solar cell manufacturing process, the high-temperature battery process is prone to induce oxygen precipitation defects inside the silicon wafer, resulting in a reduced lifespan and an increase in the proportion of concentric circles, which in turn affects the conversion efficiency of the battery. Oxygen concentration is the core indicator of single crystal quality control, and oxygen reduction is the direction of continuous improvement. Since the structure of the crystal pulling thermal field directly affects the oxygen content during the crystal pulling process, and the existing thermal field structure is unreasonable in design, it directly affects the generation of oxygen during the single crystal pulling process, and affects the amount of oxygen generated and the speed of volatilization. The oxygen content value directly determines the quality of the single crystal. Summary of the invention

[0003] The present application provides a thermal field structure, a single crystal furnace provided with the thermal field structure, and a crystal pulling process for preparing single crystal silicon rods using the single crystal furnace, which solves the technical problem in the prior art that excessive oxygen content during crystal pulling directly affects product quality.

[0004] In order to solve at least one of the above technical problems, the technical solution adopted in this application is:

[0005] A thermal field structure comprises a guide tube and a furnace cover matched with the guide tube, wherein the guide tube is at least provided with a main body, the main body comprises a vertical section, a curved section and an inner diameter section connected in sequence, the vertical section and the curved section are connected by a transition section; the curved section is constructed as an outwardly convex arc surface.

[0006] Furthermore, the inner diameter section is a laterally arranged straight line section, the radius of the transition section is smaller than the radius of the curved surface section; and the arc length of the curved surface section is greater than the length of the inner diameter section.

[0007] Furthermore, the angle corresponding to the arc length of the curved surface segment is greater than 20° and not greater than 60°.

[0008] Furthermore, the thickness of the end of the curved surface segment close to the inner diameter segment is thicker than the thickness of the end of the curved surface segment away from the inner diameter segment, and the thickness of the curved surface segment away from the inner diameter segment is the same as the thickness of the vertical segment;

[0009] Preferably, the end of the inner diameter section away from the curved surface section is a step surface, and the minimum thickness of the step surface in the inner diameter section is the same as the thickness of the vertical section.

[0010] Furthermore, a cylindrical auxiliary body with a variable diameter is arranged on the outer side of the upper end surface of the main body, and the auxiliary body is constructed as an upper oblique and lower straight structure;

[0011] Preferably, the inclined section of the auxiliary body is constructed as a structure inclined outward, and the horizontal angle between the inclined section and the top surface of the main body is not greater than 60°.

[0012] Preferably, the height of the inclined section in the auxiliary body is greater than the height of its straight section, and the height of the inclined section in the auxiliary body is not less than 1 / 2 of the height of the auxiliary body; the thickness of the auxiliary body is the same as the thickness of the main body.

[0013] Furthermore, the inner side wall of the furnace cover is constructed with an inner wall surface adapted to the auxiliary body, and the inner wall surface is also an upper slanted and lower vertical structure.

[0014] A single crystal furnace is equipped with the thermal field structure as described above.

[0015] A crystal pulling process uses the single crystal furnace as described above to pull crystals, and the steps include: the initial value of the pot rotation speed of the quartz crucible during welding is 4-6rpm; and then the crystal is seeded, the shoulder is expanded, and the shoulder is rotated based on the pot rotation speed of the quartz crucible during welding.

[0016] Furthermore, when the diameter is equal, the pot rotation speed of the quartz crucible based on the shoulder rotation is increased by 1-2 rpm, and the pot rotation speed is kept unchanged until the pulling is completed.

[0017] Furthermore, during the entire crystal pulling process, the furnace pressure in the single crystal furnace remains unchanged at 5-9 torr.

[0018] A thermal field structure designed in the present application is adopted to optimize the thermal field structure, so that the gap between the bottom surface of the guide tube and the solid-liquid interface is reduced, that is, the gas flow area is reduced, thereby increasing the circulation speed of the gas per unit volume, expanding the volatilization speed of the gas carrying away the oxygen content, reducing the concentration of oxygen in the peripheral atmosphere, and reducing the effect of shear convection on oxygen volatilization. At the same time, the tightness of the fitting between the auxiliary body and the furnace cover is improved, and the loss of inert gas is reduced, thereby improving the utilization rate of the gas, and then improving the quality of crystal pulling and reducing the generation of oxygen content. The present application also proposes a single crystal furnace provided with the thermal field structure, and a crystal pulling process for preparing single crystal silicon rods using the single crystal furnace. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic diagram of a thermal field structure of an embodiment of the present application;

[0020] Figure 2 This is a structural diagram of a guide tube according to an embodiment of the present application;

[0021] Figure 3 It is a structural diagram of the main body of an embodiment of the present application;

[0022] Figure 4 is a structural diagram of an auxiliary body according to an embodiment of the present application;

[0023] Figure 5 It is a comparison of the oxygen content of silicon round rods prepared by the crystal pulling process of the present application and the crystal pulling process in the prior art.

[0024] In the figure:

[0025] 1. Guide tube 10, body 11, vertical section

[0026] 12. curved surface section 13. inner diameter section 14. transition section

[0027] 20. Auxiliary body 21. Inclined segment 22. Straight segment

[0028] 3. Furnace cover 3. Single crystal furnace DETAILED DESCRIPTION

[0029] The present application is described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] This embodiment proposes a thermal field structure, such as Figure 1 As shown, the flow guide tube 1 and the furnace cover 2 matched with the flow guide tube 1 are both arranged in the single crystal furnace 3, and the outer wall surface of the furnace cover 2 is connected to the inner wall of the single crystal furnace 3, and the top of the flow guide tube 1 is fixed on the inner avoidance surface of the furnace cover 2. Specifically, the flow guide tube 1 is equipped with a main body 10 and an auxiliary body 20 with a variable diameter cylindrical structure arranged on the outer side of the upper end surface of the main body 10. Among them, the main body 10 includes a vertical section 11, a curved section 12 and an inner diameter section 13 connected in sequence, and the vertical section 11 and the curved section 12 are connected by a transition section 14; the curved section 12 is constructed as an arc surface convex outward; and the auxiliary body 20 is constructed as an upper inclined and lower straight structure. The outwardly protruding curved surface section 12 can reduce the gap between the bottom surface of the guide tube 1 and the solid-liquid interface, that is, reduce the gas flow area, thereby increasing the circulation speed of the gas per unit volume, so as to expand the volatilization speed of the gas taking away the oxygen content, reduce the concentration of oxygen in the peripheral atmosphere, and reduce the effect of shear convection on the volatilization of oxygen. At the same time, the auxiliary body 20 of the upper inclined and lower straight structure cooperates with the inner wall surface of the furnace cover 2 of the upper inclined and lower straight structure, thereby improving the tightness of the connection between the auxiliary body 20 and the furnace cover 2, reducing the loss of inert gas, thereby improving the utilization rate of gas, and further improving the quality of crystal pulling and reducing the generation of oxygen content.

[0031] like Figure 2As shown, the guide tube 1 is a cylindrical structure, the inner diameter section 13 of which is a straight section structure arranged horizontally, the vertical section 11 is a straight section structure arranged vertically, and the curved section 12 is an arc-shaped curved section structure inclined downward and convex outward from one end close to the vertical section 11. Compared with the existing straight-line inclined structure, it is more convex and curved toward the solid-liquid interface side of the quartz crucible (omitted in the figure) below it, so that the gap between the curved section 12 and the solid-liquid interface becomes smaller, thereby reducing the flow area of ​​the gas overflowing from the inner side of the guide tube 1 along the lower bottom surface of the curved section 12. Under the condition that the unit volume remains unchanged, the circulation speed of the gas per unit volume can be increased to expand the volatilization speed of the oxygen content carried away by the gas, reduce the concentration of oxygen in the peripheral atmosphere, and reduce the influence of shear convection on oxygen volatilization.

[0032] In this embodiment, the transition section 14 is an arc-shaped structure, which is used to connect the vertical section 11 and the curved section 12, and the radius of the transition section 14 is smaller than the radius of the curved section 12; and the arc surface length of the curved section 12 is greater than the length of the inner diameter section 12.

[0033] Preferably, the angle θ1 corresponding to the arc length of the curved segment 12 is greater than 20° and not greater than 60°. The curved segment 12 with a curved arc structure is more conducive to the flow of gas along its outer wall surface, and also reduces the gap distance between it and the solid-liquid interface, thereby increasing the flow rate of gas per unit volume. If the angle θ1 corresponding to the arc length of the curved segment 12 is less than or equal to 20°, the length of the curved segment 12 will be shorter, and the guide tube 1 will not be able to support the water cooling jacket inside it; if the angle θ1 corresponding to the arc length of the curved segment 12 is greater than 60°, the curvature of its convex curved surface will be increased, resulting in the curved segment 12 directly intersecting with the surface of the solid-liquid interface, interfering with the flow of gas, and directly affecting the quality of crystal pulling. Preferably, when the angle θ1 corresponding to the arc length of the curved segment 12 is 30-40°, and the radius of the curved segment 12 is 280-380 mm, the curved convex effect is best, and the length of its circumference can perfectly connect the vertical segment 11 and the inner diameter segment 13, and on the basis of having a convex arc surface, it also has a structure that is inclined downwardly curved, so that the gas is more conducive to flowing upward along the outer wall surface of the curved segment 12.

[0034] like Figure 3 As shown, the thickness of the end of the curved section 12 close to the inner diameter section 13 is thicker than the thickness of the side away from the inner diameter section 13, in order to improve the strength of the bottom surface of the guide tube 1 to support the water cooling jacket. Except for the end of the curved section 12 close to the inner diameter section 13, the thickness of the curved section 12 away from the inner diameter section 13 is the same as the thickness of the vertical section 11, and the thickness of the transition section 14 is the same as the thickness of the vertical section 11, so as to improve the convenience and consistency of the processing of the guide tube 1.

[0035] Furthermore, the end of the inner diameter section 13 away from the curved section 12 is a step surface, and the minimum thickness D of the step surface in the inner diameter section 13 is the same as the thickness of the vertical section 11; and the maximum thickness D1 of the step surface in the inner diameter section 13 is 1-5mm different from the thickness of D.

[0036] like Figure 4 As shown, the inclined section 21 of the auxiliary body 20 is constructed as a structure inclined outward, and the horizontal angle θ2 between the inclined section 21 and the top surface of the main body 10 is not greater than 60°, that is, the angle between the inclined section 21 and the horizontal horizontal direction is θ2, and is not greater than 60°, that is, the angle θ3 of the inclined sections 21 on both sides inclined outward is not less than 120°. The more the inclined section 21 is inclined outward, the longer the length of the inclined section 21 is, so that the inclined surface area that cooperates with the furnace cover 3 is larger, thereby increasing the area of ​​the auxiliary body 20 and the furnace cover 2, and strengthening the tightness of the connection between the guide tube 1 and the furnace cover 2, thereby reducing the loss of inert gas through the gap between the guide tube 1 and the furnace cover 2, thereby improving the utilization rate of the gas, thereby improving the quality of the crystal pulling, and reducing the generation of oxygen content.

[0037] Preferably, the overall height H of the auxiliary body 20 is 80-100 mm, wherein the height H1 of the inclined section 21 is greater than the height of its straight section 22, and the height H1 of the inclined section 21 in the auxiliary body 20 is not less than 1 / 2 of the height H of the auxiliary body 20, and preferably, the height of the inclined section 21 is 42-65. The thickness of the auxiliary body 20 is the same as the thickness D of the vertical section 11 in the main body 10. The higher the height of the inclined section 21, the longer the length of its inclined surface, which can also increase the area of ​​the auxiliary body 20 and the furnace cover 2, and strengthen the tightness of the matching connection between the guide tube 1 and the furnace cover 2.

[0038] Furthermore, in order to improve the strength of the fit between the furnace cover 2 and the guide tube 1, the inner wall of the furnace cover 2 is constructed with an inner wall surface that is compatible with the auxiliary body 20, that is, the inner wall surface of the furnace cover 2 is also an upper inclined and lower straight structure, and is tightly fitted with the outer wall surface of the auxiliary body 20.

[0039] A single crystal furnace is equipped with the thermal field structure as described above.

[0040] A crystal pulling process, using the single crystal furnace 3 as above to pull crystals, comprises the following steps:

[0041] The initial value of the pot rotation speed of the quartz crucible during welding is 4-6rpm; then the crystal seeding, shoulder expansion and shoulder rotation are carried out based on the pot rotation speed of the quartz crucible during welding.

[0042] Before pulling the crystal, it is necessary to use the seed crystal and the molten silicon single crystal for welding in preparation for seeding and growing the required silicon single crystal on the seed crystal. During welding, the speed of the pot rotation directly affects the quality of the crystal pulling and the oxygen content of the silicon round rod. In the present application, a lower pot rotation starting speed is set so that the pot rotation speed during welding is lower than the pot rotation speed in the prior art, so that the seed crystal can be slowly welded, and then the pot rotation speed during welding is used as a reference to continue seeding, shoulder expansion and shoulder turning. That is, during seeding, shoulder expansion and shoulder turning, the pulling is still carried out at the starting pot rotation speed of 4-6rpm during welding, and the seeding, shoulder expansion and shoulder turning are continued under the condition of low pot rotation.

[0043] Furthermore, when the diameter is equal, the pot rotation speed of the quartz crucible based on the shoulder rotation is increased by 1-2 rpm, and the pot rotation speed is kept unchanged until the pulling is completed.

[0044] After the shoulder is turned, equal diameter pulling begins. At this time, the pot rotation speed needs to be increased. Based on the pot rotation speed during shoulder turning, that is, on the basis of 4-6rpm, it is quickly increased by 1-2rpm to directly increase the pot rotation speed to the stable pot rotation speed during equal diameter, that is, the pot rotation speed is 5-8rpm, and then the equal diameter production of crystal pulling is continued at the cap pot rotation speed, and other dimensions remain unchanged. After the equal diameter is completed, the pot rotation speed is used to finish until the pulling is completed.

[0045] Furthermore, during the entire crystal pulling process, whether it is welding, seeding, shoulder expansion or shoulder rotation, or equal diameter and tailing, the furnace pressure in the single crystal furnace 3 remains unchanged and is always in a low pressure state, that is, its range is 5-9 torr.

[0046] The present application proposes a process for pulling silicon ingots using the single crystal furnace, in which the melting crucible rotation speed is 4-6rpm, which is different from the conventional process of 6-8rpm, and the pot rotation speed is always maintained in a low range of 5-8rpm when the diameter is equal, which is different from the conventional high pot rotation speed when the diameter is equal. This can slow down the generation of strong convection inside the melt, and further delay the oxygen generated by the decomposition of the quartz crucible, thereby achieving the control of the oxygen concentration in the solution.

[0047] The process of the present application is used to pull a single crystal silicon round rod, and the conventional high pot rotation crystal pulling process is used to pull a silicon round rod of the same specification. The length and oxygen content of the obtained silicon round rod are compared, as shown in FIG. Figure 5 As shown, a is a silicon rod pulled by the crystal pulling process of the prior art, and b is a silicon rod pulled by the crystal pulling process of the present application. The obtained silicon rods are cut into segments and the average oxygen content of each segment after cutting is obtained. It can be seen from the figure that the crystal pulling process of the present application can reduce the oxygen content in each segment by 1.1-1.3ppma.

[0048] In addition, as the length of the crystal pulling increases, the pot rotation speed decreases. When the diameter is constant, the pot rotation speed is 5-8rpm, which improves the diffusion capacity of SiO in the inert gas, weakens the effect of suppressing the evaporation of SiO from the solid-liquid interface, and is conducive to reducing the oxygen content in the melt. When the furnace pressure is maintained at a low level, especially within 5-9torr, it is conducive to reducing the oxygen concentration inside the melt. At the same time, according to the current status of the single crystal furnace 3, while reducing the pot rotation and furnace pressure, the dry pumping rate is increased, and the flow of the inert gas in the single crystal furnace 3 is accelerated, which can reduce the oxygen concentration in the peripheral atmosphere.

[0049] A thermal field structure designed in the present application is adopted to optimize the thermal field structure, so that the gap between the bottom surface of the guide tube and the solid-liquid interface is reduced, that is, the gas flow area is reduced, thereby increasing the circulation speed of the gas per unit volume, expanding the volatilization speed of the gas carrying away the oxygen content, reducing the concentration of oxygen in the peripheral atmosphere, and reducing the effect of shear convection on oxygen volatilization. At the same time, the tightness of the fitting between the auxiliary body and the furnace cover is improved, and the loss of inert gas is reduced, thereby improving the utilization rate of the gas, and then improving the quality of crystal pulling and reducing the generation of oxygen content. The present application also proposes a single crystal furnace provided with the thermal field structure, and a crystal pulling process for preparing single crystal silicon rods using the single crystal furnace.

[0050] The above detailed description of the embodiments of the present application is only a preferred embodiment of the present application and cannot be considered to limit the scope of implementation of the present application. All equivalent changes and improvements made within the scope of the present application should still fall within the scope of the patent coverage of the present application.

Claims

1. A thermal field structure, characterized in that: It includes a guide tube and a furnace cover matched with the guide tube, the guide tube is equipped with at least a main body, the main body includes a vertical section, a curved section and an inner diameter section connected in sequence, the vertical section and the curved section are connected by a transition section; the curved section is constructed as an arc surface protruding outward.

2. A thermal field structure according to claim 1, characterized in that: The inner diameter section is a straight line section arranged transversely, the radius of the transition section is smaller than the radius of the curved surface section; and the arc length of the curved surface section is greater than the length of the inner diameter section.

3. A thermal field structure according to claim 1 or 2, characterized in that: The angle corresponding to the arc length of the curved surface segment is greater than 20° and not greater than 60°.

4. A thermal field structure according to claim 4, characterized in that: The thickness of the end of the curved surface segment close to the inner diameter segment is thicker than the thickness of the end of the curved surface segment away from the inner diameter segment, and the thickness of the end of the curved surface segment away from the inner diameter segment is the same as the thickness of the vertical segment; Preferably, the end of the inner diameter section away from the curved surface section is a step surface, and the minimum thickness of the step surface in the inner diameter section is the same as the thickness of the vertical section.

5. A thermal field structure according to any one of claims 1-2 and 4, characterized in that: A cylindrical auxiliary body with a variable diameter is also arranged on the outer side of the upper end surface of the main body, and the auxiliary body is constructed as an upper oblique and lower straight structure; Preferably, the inclined section of the auxiliary body is constructed as a structure inclined outward, and the horizontal angle between the inclined section and the top surface of the main body is not greater than 60°. Preferably, the height of the inclined section in the auxiliary body is greater than the height of its straight section, and the height of the inclined section in the auxiliary body is not less than 1 / 2 of the height of the auxiliary body; the thickness of the auxiliary body is the same as the thickness of the main body.

6. A thermal field structure according to claim 5, characterized in that: The inner side wall of the furnace cover is constructed with an inner wall surface matched with the auxiliary body, and the inner wall surface is also an upper inclined and lower vertical structure.

7. A single crystal furnace, characterized in that: The thermal field structure is provided as described in any one of claims 1 to 6.

8. A crystal pulling process, characterized in that: The single crystal furnace as claimed in claim 7 is used for crystal pulling, and the steps include: the initial value of the pot rotation speed of the quartz crucible during welding is 4-6rpm; and then the crystal is seeded, the shoulder is expanded and the shoulder is rotated based on the pot rotation speed of the quartz crucible during welding.

9. A crystal pulling process according to claim 8, characterized in that: When the diameter is equal, the pot rotation speed of the quartz crucible based on the shoulder rotation is increased by 1-2rpm, and the pot rotation speed is kept unchanged until the end of pulling.

10. A crystal pulling process according to claim 8 or 9, characterized in that: During the entire crystal pulling process, the furnace pressure in the single crystal furnace remains unchanged at 5-9 torr.

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