Crystal pulling thermal field control method, crystal pulling thermal field system and application

Abstract

The invention relates to the field of semiconductor materials, in particular to a crystal pulling thermal field control method, a crystal pulling thermal field system and application. The crystal pulling thermal field control method comprises the following steps: in an equal-diameter growth stage of czochralski silicon, when the curing rate of a silicon rod is less than or equal to 30%, controlling the argon flow to be 90-120SLPM, and controlling the furnace pressure to be 800-1000Pa. According to the crystal pulling thermal field control method, in the initial stage of equal-diameter growth where the oxygen segregation effect is most sensitive, argon flow and furnace pressure are cooperatively controlled in a specific'low flow-low pressure 'window, and a process environment beneficial to oxygen volatilization instead of entering crystals is actively created. The segregation and doping of oxygen atoms to a solid-liquid interface are effectively inhibited from the aspect of dynamics, and the method is the most critical step for realizing the overall low oxygen content of the silicon rod, especially reducing the oxygen content of the head and improving the axial uniformity.

Description

Technical Field

[0001] This invention relates to the field of semiconductor materials, and more specifically, to a method for controlling the thermal field of crystal pulling, a system for controlling the thermal field of crystal pulling, and its applications. Background Technology

[0002] The Czochralski method is currently the mainstream method for preparing single-crystal silicon rods. Its basic principle is to melt polycrystalline silicon in a high-temperature thermal environment, and then gradually crystallize it into a single-crystal silicon rod through stages such as seed crystal introduction, shoulder formation, constant-diameter growth, and tailing. During this process, the quartz crucible acts as the molten silicon container. At high temperatures, it reacts with the molten silicon to generate silicon monoxide (SiO). Some of the SiO volatilizes and is carried away in the argon protective atmosphere, while the rest dissolves in the molten silicon, ultimately entering the silicon lattice as interstitial oxygen, leading to an increase in the oxygen content of the silicon rod.

[0003] As photovoltaic cell technology advances towards higher efficiency and lower cost, back-contact (BC) cells have become an important development direction for next-generation high-efficiency photovoltaic cells due to their advantages such as no electrode obstruction on the front, high optical utilization, and significant conversion efficiency (mass production efficiency has reached over 26.5%). However, BC cells have extremely stringent requirements for the quality of the silicon substrate, especially its oxygen content. When the oxygen content in the silicon wafer is too high, oxygen precipitation, thermal donors, and their derived defects are easily formed during subsequent high-temperature processes, leading to a decrease in minority carrier lifetime and a significant degradation in cell efficiency (typically exceeding 2.5%).

[0004] Currently, in conventional Czochralski single crystal growth processes, the following measures are typically taken to control oxygen content: Adjust the argon flow rate (usually controlled at 70–130 SLPM). Reduce the rotation speed of the quartz crucible; Optimize the thermal field structure to suppress melt convection.

[0005] However, the above-mentioned traditional methods have the following limitations: The process parameters are highly coupled and difficult to control: adjusting the argon flow rate or the crucible rotation speed alone can easily cause problems such as furnace pressure fluctuations and liquid surface shaking, which affect the stability of crystal growth and crystal formation rate. Uneven oxygen content distribution: The oxygen content varies significantly between the head and tail of the silicon rod, reaching more than 30%, which seriously affects the quality consistency of the entire silicon rod and the yield of subsequent silicon wafers. The thermal field structure is not adapted to the requirements of large size and low oxygen: existing thermal field designs are mostly designed for conventional size silicon rods. For large size silicon rods (such as 210mm) required by BC cells, the thermal field temperature gradient, argon flow field and other aspects have not been optimized, making it difficult to effectively control the overall oxygen content. Lack of active deoxidation methods: Traditional processes rely on passive volatilization and convection diffusion, which have limited ability to remove residual oxygen in the melt.

[0006] Therefore, developing a novel thermal field structure and process method that can systematically control oxygen content, adapt to the growth of large-size silicon rods, and possess active deoxygenation capability is key to improving the performance of monocrystalline silicon materials for BC batteries.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] The purpose of this invention is to provide a crystal pulling thermal field control method, crystal pulling thermal field system and application to solve the efficiency degradation problem caused by oxygen precipitation in BC cell silicon wafers, while taking into account the crystallization rate and process stability of large-size silicon rods.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: One aspect of the present invention relates to a method for controlling the thermal field of crystal pulling, comprising the following steps: During the constant diameter growth stage of Czochralski single crystal silicon, when the silicon rod solidification rate is ≤30%, the argon flow rate is controlled at 90~120 SLPM and the furnace pressure is controlled at 800~1000 Pa.

[0010] The described crystal pulling thermal field control method, in the initial stage of constant diameter growth when oxygen segregation is most sensitive, actively creates a process environment conducive to oxygen volatilization rather than its entry into the crystal by coordinating the argon gas flow rate and furnace pressure within a specific "low flow-low pressure" window. This effectively suppresses the segregation and incorporation of oxygen atoms into the solid-liquid interface from a kinetic perspective, and is the most critical step in achieving overall low oxygen content in the silicon rod, especially reducing the oxygen content at the head and improving axial uniformity.

[0011] Another aspect of the present invention relates to a crystal pulling thermal field system for implementing the crystal pulling thermal field control method, comprising a Czochralski single crystal furnace, a main heater, a quartz crucible, and a flow guide tube; The main heater is fixedly installed inside the furnace cavity of the Czochralski single crystal furnace and is arranged around the outside of the quartz crucible; the flow guide tube is suspended inside the furnace cavity of the Czochralski single crystal furnace and is located radially inside the main heater; the axial center line of the flow guide tube coincides with the center line of the quartz crucible, and its cylinder is sleeved on the outside of the single crystal silicon rod. The main heater has a first slit and a second slit on its cylinder wall, and the axial length of the first slit is greater than that of the second slit.

[0012] The crystal pulling thermal field system described above features slits of varying lengths on the main heater, providing a hardware structure that allows for precise control of the thermal field temperature distribution. This structural design is a physical prerequisite for altering the internal thermal balance of the thermal field and achieving targeted temperature control. The use of an unequal-slit main heater reduces the temperature gradient in the lower part of the crucible, thereby decreasing the quartz crucible's melting rate.

[0013] Another aspect of the present invention relates to a method for preparing a low-oxygen single-crystal silicon rod, including the aforementioned method for controlling the crystal pulling thermal field.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The crystal pulling thermal field control method provided by this invention achieves systematic and precise control of oxygen content by introducing a staged dynamic coordinated control strategy of argon flow rate and furnace pressure, and applying ultrasonic-assisted deoxidation during silicon melting. This method effectively suppresses the oxygen segregation effect by setting the optimal "low flow rate-low pressure" process window at different stages of crystal growth (especially the initial stage of constant diameter); at the same time, the active cavitation effect of ultrasonic waves breaks through the oxygen reduction limit of traditional passive volatilization. It significantly reduces the overall oxygen content of silicon rods and greatly improves their axial uniformity, fundamentally solving the efficiency decay problem of BC cell silicon wafers caused by oxygen precipitation, while ensuring the process stability and high crystallization rate of large-size single crystal silicon rod growth.

[0015] (2) The crystal pulling thermal field system provided by this invention, through core hardware innovations such as the use of a main heater with unequal slits (first slit 210-220mm, second slit 150-160mm) and a 20-30mm longer guide tube, provides an optimized physical carrier for the above control method. The unequal slit design actively reduces the temperature gradient in the lower part of the quartz crucible from the source, reducing the introduction of oxygen; while the extended guide tube increases the argon gas flow rate and enhances the SiO carry-out efficiency. A stable and efficient thermal field environment that can adapt to the growth of large-size, low-oxygen silicon rods is constructed. This hardware system not only physically ensures the realization of the low-oxygen process, but also has excellent temperature uniformity and gas flow efficiency, thereby improving the compatibility of the equipment and the production yield. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the main heater provided in an embodiment of the present invention. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0019] One aspect of the present invention relates to a method for controlling the thermal field of crystal pulling, comprising the following steps: During the constant-diameter growth stage of Czochralski single-crystal silicon, when the silicon rod solidification rate is ≤30% (e.g., it can be any value or a range between any two of 0%, 5%, 10%, 15%, 20%, 25%, or 30%), the argon flow rate is controlled at 90~120 SLPM (e.g., it can be any value or a range between any two of 90 SLPM, 100 SLPM, 110 SLPM, or 120 SLPM), and the furnace pressure is controlled at 800~1000 Pa (e.g., it can be any value or a range between any two of 800 Pa, 850 Pa, 900 Pa, 950 Pa, or 1000 Pa). Performing the above operations can suppress the oxygen atom segregation effect.

[0020] The described crystal pulling thermal field control method, in the initial stage of constant diameter growth when oxygen segregation is most sensitive, actively creates a process environment conducive to oxygen volatilization rather than its entry into the crystal by coordinating the argon gas flow rate and furnace pressure within a specific "low flow-low pressure" window. This effectively suppresses the segregation and incorporation of oxygen atoms into the solid-liquid interface from a kinetic perspective, and is the most critical step in achieving overall low oxygen content in the silicon rod, especially reducing the oxygen content at the head and improving axial uniformity.

[0021] Furthermore, in the final stage, the argon flow rate is controlled at 100~120 SLPM (e.g., any value or range between 100 SLPM, 105 SLPM, 110 SLPM, 115 SLPM, or 120 SLPM), and the furnace pressure is controlled at 0.8~1.2 kPa (e.g., any value or range between 0.8 kPa, 0.9 kPa, 1.0 kPa, 1.1 kPa, or 1.2 kPa). This avoids a sudden increase in oxygen content at the tail end. By optimizing the process parameters in the final stage, the problem of a sudden increase in oxygen content at the tail end, which is easily caused by the decrease in melt volume and the relative increase in oxygen concentration at the end of crystal growth, is solved. Appropriately increasing the argon flow rate and furnace pressure can enhance the gas flow sweeping, prevent oxygen enrichment at the tail end, thereby ensuring the uniformity and stability of oxygen content throughout the silicon rod from head to tail, and improving the yield of the entire silicon rod.

[0022] Furthermore, during the melting stage, the argon flow rate is controlled at 100~120 SLPM (e.g., any value or range between 100 SLPM, 105 SLPM, 110 SLPM, 115 SLPM, or 120 SLPM), and the furnace pressure is controlled at 0.8~1.5 kPa (e.g., any value or range between 0.8 kPa, 1 kPa, 1.1 kPa, 1.2 kPa, 1.3 kPa, 1.4 kPa, or 1.5 kPa). This allows for rapid removal of SiO volatiles. The higher argon flow rate and furnace pressure establish an efficient airflow channel in the early stages of SiO volatilization, quickly carrying away the SiO volatiles generated during the melting of the quartz crucible. This reduces the initial oxygen content of the silicon melt, laying a good foundation for low-oxygen control in subsequent stages.

[0023] Furthermore, during the silicon melting process, ultrasonic waves with a frequency of 20-40 kHz (e.g., any point value or range between 20 kHz, 25 kHz, 30 kHz, 35 kHz, or 40 kHz, or any value between two of these) are applied to the surface of the molten silicon. An ultrasonic transducer is implanted into the molten silicon surface, generating a localized cavitation effect that promotes the escape of oxygen bubbles, further reducing the oxygen content by 15%-20%. The cavitation effect of the ultrasonic waves can directly promote the aggregation and escape of oxygen atoms within the melt, complementing the traditional passive evaporation method for oxygen reduction. This breaks through the oxygen reduction limits of traditional processes from another dimension, significantly reducing the overall oxygen content, and is particularly suitable for applications with extremely high oxygen content requirements.

[0024] Another aspect of the present invention relates to a crystal pulling thermal field system for implementing the crystal pulling thermal field control method, comprising a Czochralski single crystal furnace, a main heater, a quartz crucible, and a flow guide tube; The main heater is fixedly installed inside the furnace cavity of the Czochralski single crystal furnace and is arranged around the outside of the quartz crucible; the flow guide tube is suspended inside the furnace cavity of the Czochralski single crystal furnace and is located radially inside the main heater; the axial center line of the flow guide tube coincides with the center line of the quartz crucible, and its cylinder is sleeved on the outside of the single crystal silicon rod. The main heater has a first slit and a second slit on its cylinder wall. The axial length of the first slit is greater than that of the second slit. Figure 1 As shown.

[0025] The crystal pulling thermal field system described above features slits of varying lengths (unequal slits) on the main heater, providing the ability to precisely control the temperature distribution within the thermal field from a hardware perspective. This structural design is the physical prerequisite for altering the internal thermal balance of the thermal field and achieving targeted temperature control. The use of an unequal-slit main heater reduces the temperature gradient in the lower part of the crucible, thereby decreasing the melting rate of the quartz crucible.

[0026] This invention effectively improves the coupling of process parameters, enhances the uniformity of oxygen content within the silicon rod, and strengthens the equipment compatibility of the thermal field system for large-size silicon rods through the synergistic optimization of the crystal pulling thermal field system and process parameters.

[0027] Furthermore, the axial length of the first slit is 210~220mm (for example, it can be a point value or a range between any two of 210mm, 215mm, or 220mm, and the axial length of the second slit is 150~160mm (for example, it can be a point value or a range between any two of 150mm, 152mm, 154mm, 156mm, 158mm, or 160mm, for example). This specific combination of dimensions can optimize the axial resistance distribution of the heater, thereby precisely achieving the design goal of "reducing the temperature gradient in the lower part of the crucible", effectively reducing the dissolution rate of the quartz crucible, and reducing the introduction of oxygen at the source.

[0028] Furthermore, the first slit is located above the main heater cylinder wall, and the second slit is located below the main heater cylinder wall. Defining the vertical position and orientation of the slits ensures that the unequal slit design produces the desired symmetrical thermal field effect.

[0029] Furthermore, the opening direction of the first slit faces the top of the main heater, and the opening direction of the second slit faces the bottom of the main heater. This upward / downward opening design aligns with the natural direction of heat flow and airflow within the thermal field, further enhancing the control precision and uniformity of the axial temperature field, preventing the generation of localized hot spots, and improving the overall stability of the thermal field.

[0030] Furthermore, the first and second slits are arranged alternately in the circumferential direction of the main heater. This alternating arrangement of slits ensures the uniformity of heating in the circumferential direction and the axial symmetry of the thermal field. This is a crucial structural guarantee that prevents the crystal from developing stress, dislocations, or even transforming into a polycrystalline state due to uneven heating, and is essential for the stable growth of large-diameter single-crystal silicon rods.

[0031] Furthermore, the guide tube is constructed to have an elongated cylindrical body, the axial length L of which satisfies: L=L0+L1; Where L0 is the axial length of the conventional guide tube; L1 is the extended length, which is 20~30mm (for example, it can be any point value or any range between 20mm, 22mm, 24mm, 26mm, 28mm or 30mm).

[0032] The length of the guide tube was increased, raising the argon flow rate to 1.2 m / s and enhancing the efficiency of SiO volatilization and removal. Precisely controlling the lengthening of the guide tube optimized the thermal and flow field environment. The extended guide tube reduced the cross-sectional area of ​​the flow channel in key areas, thus significantly increasing the argon flow rate over the silicon rod and liquid surface at the same inlet flow rate. This greatly enhanced the "purge" and removal efficiency of SiO volatilities by argon, providing crucial flow field assurance for achieving low oxygen content.

[0033] Another aspect of the present invention relates to a method for preparing a low-oxygen single-crystal silicon rod, including the aforementioned method for controlling the crystal pulling thermal field.

[0034] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0035] Example 1 The crystal pulling thermal field control method provided in this embodiment uses the crystal pulling thermal field system of Embodiment 4, including: Crucible rotation speed: 5 rpm during the shoulder-setting stage, and reduced to 4 rpm during the constant diameter stage; Argon flow rate: 110 SLPM in the melting stage, 110 SLPM in the crystal pulling stage, 95 SLPM in the constant diameter stage when the silicon rod solidification rate is ≤30%, and 110 SLPM in the finishing stage. Furnace pressure: 1.3 kPa during the melting stage, 1 kPa during the equal diameter middle stage, and 1 kPa during the final stage; During the silicon melting process, ultrasonic waves with a frequency of 30 kHz are applied to the surface of the molten silicon. All other parameters and crystal pulling steps adopt conventional crystal pulling methods in this field; Results: The oxygen content of the silicon rod was 9.42 ppma, and the minority carrier lifetime was >7200 μs, which met the battery technology requirements.

[0036] Example 2 The crystal pulling thermal field control method provided in this embodiment uses the crystal pulling thermal field system of Embodiment 5, including: Crucible rotation speed: 5 rpm during the shoulder-setting stage, and reduced to 4 rpm during the constant diameter stage; Argon flow rate: 100 SLPM in the melting stage, 110 SLPM in the crystal pulling stage, 90 SLPM in the constant diameter stage when the silicon rod solidification rate is ≤30%, and 120 SLPM in the finishing stage. Furnace pressure: 1 kPa during the melting stage, 800 Pa during the equal diameter intermediate stage, and 0.8 kPa during the final stage; During the silicon melting process, ultrasonic waves with a frequency of 20 kHz are applied to the surface of the molten silicon. All other parameters and crystal pulling steps are performed using conventional crystal pulling methods in the field, as in Example 1; Results: The oxygen content of the silicon rod was 9.6 ppma, and the minority carrier lifetime was >7600 μs, which met the battery technology requirements.

[0037] Example 3 The crystal pulling thermal field control method provided in this embodiment uses the crystal pulling thermal field system of Embodiment 6, including: Crucible rotation speed: 5 rpm during the shoulder-setting stage, and reduced to 4 rpm during the constant diameter stage; Argon flow rate: 120 SLPM in the melting stage, 110 SLPM in the crystal pulling stage, 120 SLPM in the constant diameter stage when the silicon rod solidification rate is ≤30%, and 100 SLPM in the finishing stage. Furnace pressure: 1.5 kPa during the melting stage, 900 Pa during the equal diameter intermediate stage, and 1.2 kPa during the final stage; During the silicon melting process, ultrasonic waves with a frequency of 40 kHz are applied to the surface of the molten silicon. All other parameters and crystal pulling steps are performed using conventional crystal pulling methods in the field, as in Example 1; Results: The oxygen content of the silicon rod was 9.42 ppma, and the minority carrier lifetime was >7200 μs, which met the battery technology requirements.

[0038] Example 4 The crystal pulling thermal field system provided in this embodiment adopts a 36-inch thermal field Czochralski single crystal furnace, equipped with an isostatic graphite heater and an ultrasonic generator, including a Czochralski single crystal furnace, a main heater, a quartz crucible and a flow guide tube. The main heater is fixedly installed inside the furnace cavity of the Czochralski single crystal furnace and surrounds the outside of the quartz crucible; the guide tube is suspended inside the furnace cavity of the Czochralski single crystal furnace and is located radially inside the main heater; the axial centerline of the guide tube coincides with the centerline of the quartz crucible, and its cylinder is sleeved on the outside of the single crystal silicon rod. The main heater has a first slit and a second slit on its cylinder wall. The axial length of the first slit is 215 mm and the axial length of the second slit is 155 mm. The first slit is located above the main heater cylinder wall, and the second slit is located below the main heater cylinder wall; the opening direction of the first slit faces the top of the main heater, and the opening direction of the second slit faces the bottom of the main heater; the first slit and the second slit are arranged alternately in the circumferential direction of the main heater.

[0039] The guide tube is constructed to have an elongated cylindrical body, the axial length L of which satisfies: L=L0+L1; Where L0 is the axial length of the conventional guide tube; L1 is the extended length, which is 25mm.

[0040] Example 5 The difference between this embodiment and embodiment 4 is that the axial length is 220mm, the axial length of the second slit is 150mm, and L1 is 30mm.

[0041] Example 6 The difference between this embodiment and embodiment 4 is that the axial length is 210mm, the axial length of the second slit is 160mm, and L1 is 20mm.

[0042] Comparative Example 1 The conventional process, namely: using an equal-slit main heater, a conventional flow guide tube, a constant argon flow rate of 105 SLPM, a constant furnace pressure of 1.2 kPa, and without applying ultrasound, is the same as in Example 1, to prepare a single crystal silicon rod, and the oxygen content was measured to be 10.8 ppma.

[0043] The silicon wafers prepared in Example 1 and Comparative Example 1 were used to fabricate BC cells for efficiency comparison. The results showed that the cells prepared by the process of the present invention have a significant advantage in efficiency.

[0044] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling the thermal field during crystal pulling, characterized in that, Includes the following steps: During the constant diameter growth stage of Czochralski single crystal silicon, when the silicon rod solidification rate is ≤30%, the argon flow rate is controlled at 90~120 SLPM and the furnace pressure is controlled at 800~1000 Pa.

2. The crystal pulling thermal field control method according to claim 1, characterized in that, During the final stage, the argon flow rate is controlled at 100~120 SLPM and the furnace pressure is controlled at 0.8~1.2 kPa.

3. The crystal pulling thermal field control method according to claim 1 or 2, characterized in that, During the melting stage, the argon flow rate is controlled at 100~120 SLPM and the furnace pressure is controlled at 0.8~1.5 kPa.

4. The crystal pulling thermal field control method according to claim 1, characterized in that, During the silicon melting process, ultrasonic waves with a frequency of 20~40kHz are applied to the surface of the molten silicon.

5. A crystal pulling thermal field system for implementing the crystal pulling thermal field control method according to any one of claims 1 to 4, characterized in that, Includes a Czochralski single crystal furnace, main heater, quartz crucible, and flow guide tube; The main heater is fixedly installed inside the furnace cavity of the Czochralski single crystal furnace and is arranged around the outside of the quartz crucible; the flow guide tube is suspended inside the furnace cavity of the Czochralski single crystal furnace and is located radially inside the main heater; the axial center line of the flow guide tube coincides with the center line of the quartz crucible, and its cylinder is sleeved on the outside of the single crystal silicon rod. The main heater has a first slit and a second slit on its cylinder wall, and the axial length of the first slit is greater than that of the second slit.

6. The crystal pulling thermal field system according to claim 5, characterized in that, The axial length of the first slit is 210~220mm, and the axial length of the second slit is 150~160mm.

7. The crystal pulling thermal field system according to claim 6, characterized in that, The first slit is located above the main heater cylinder wall, and the second slit is located below the main heater cylinder wall; And / or, the opening direction of the first slit faces the top of the main heater, and the opening direction of the second slit faces the bottom of the main heater.

8. The crystal pulling thermal field system according to claim 7, characterized in that, The first slit and the second slit are arranged alternately in the circumferential direction of the main heater.

9. The crystal pulling thermal field system according to claim 5, characterized in that, The guide tube is constructed to have an elongated cylindrical body, the axial length L of which satisfies: L=L0+L1; Where L0 is the axial length of the conventional guide tube; L1 is the extended length, which is 20~30mm.

10. A method for preparing a low-oxygen single-crystal silicon rod, characterized in that, The method for controlling the thermal field of crystal pulling as described in any one of claims 1 to 4.

Images