System and method for low off angle outer guide tube pulling of n-type 12-inch single crystal

Abstract

The application discloses a system and method for drawing N-type 12-inch single crystals by using a low-inclination outer guide cylinder, and is characterized in that, based on the low-inclination outer guide cylinder, the main and auxiliary argon and the furnace pressure are linearly adjusted in the shoulder forming stage, the seed crystal quality in the crystal pulling stage is ensured, the oxygen partial pressure in the furnace is reduced after the shoulder forming, the drawing speed filtering coefficient and the drawing speed dead zone threshold value are set in the constant diameter stage, the precipitation and migration of oxygen elements to the crystal are reduced from the source, the first-time success rate of the shoulder forming is improved, and the thermal field and the oxygen distribution are stabilized.

Description

Technical Field

[0001] This application relates to the field of monocrystalline silicon manufacturing technology, specifically to a system and method for pulling N-type 12-inch monocrystalline silicon using a low-angle external guide tube. Background Technology

[0002] In the Czochralski process for producing single-crystal silicon, the oxygen in the crystal rod mainly originates from the melting of the quartz crucible. Oxygen migrates with the melt convection to the crystal growth interface and is incorporated into the crystal rod. The flow guide tube, by guiding the argon gas flow field and controlling the thermal field distribution and melt convection, has a decisive influence on the oxygen content of the crystal rod. Optimizing the flow guide tube structure to reduce the oxygen content is one of the important technological directions.

[0003] Currently, the bottom guide angle of the external guide tube widely used in the industry is mostly 30°. This process has gradually revealed performance bottlenecks in the production of N-type 12-inch single crystals, mainly in the following three aspects: Firstly, oxygen content control is insufficient. The 30° external guide tube has limited thermal field regulation capabilities, resulting in higher oxygen content in the middle and tail sections of the crystal rod. This makes it difficult to meet the stringent oxygen content requirements of N-type single crystals, thus affecting battery conversion efficiency. Studies have shown that the bottom inclination angle and structure of the guide tube have a significant impact on oxygen content, with different inclination angles corresponding to different thermal field gradients and airflow distribution patterns.

[0004] Secondly, the process window narrows and the breakage rate is high. Forcing a narrowing of the process window to reduce oxygen content (such as adjusting the range of crystal rotation, crucible rotation, argon gas, and furnace pressure) significantly affects crystal formation stability. Prolonged low-furnace-pressure crystal pulling means lower gas partial pressure, resulting in more impurities remaining on the liquid surface, leading to an increased breakage rate and abnormal crystal formation. Narrow process tolerance and even small parameter deviations can cause breakage or off-diameter deviations, resulting in a persistently high breakage rate.

[0005] Third, the process window is narrowing and the breakage rate is high. The existing 30° external guide tube process consumes a lot of power when pulling crystals, and the average operating power of the constant diameter section is relatively high, resulting in high production costs and making it difficult to meet the photovoltaic industry's need for continuous cost reduction.

[0006] Therefore, this application is submitted. Summary of the Invention

[0007] The purpose of this invention is to provide a system and method for pulling N-type 12-inch single crystals using a low-angle external guide tube, thereby solving the problems of insufficient oxygen content control, narrowing process window, and high breakage rate.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following solution: A method for pulling N-type 12-inch single crystals using a low-tilt external guide tube includes the following steps: S1, during the welding and crystal-leading stages, auxiliary argon gas is introduced to provide directional gas flow protection for the seed crystal; S2, during the shoulder formation stage, the main argon flow rate, auxiliary argon flow rate, furnace pressure, and crystal rotation speed decrease synchronously and linearly with the shoulder formation process, and the pulling speed is controlled according to the preset range of the crystal diameter. The upper and lower limits of the pulling speed increase step by step as the crystal diameter increases. S3, the shoulder turning stage and the constant diameter stage, the main argon flow rate and the furnace pressure remain constant. In the constant diameter stage, the crystal pulling speed increases with the crystal growth length, and the crystal rotation speed, crucible rotation speed, liquid outlet distance and follower ratio are adjusted in coordination according to the preset linkage relationship. S4, the final stage: the main argon flow rate and furnace pressure are switched to the final stage setting value, and the crucible speed, heating power and crystal pulling speed are adjusted in segments according to the final stage process to achieve a smooth diameter reduction.

[0009] Furthermore, during the shoulder formation stage, the main argon flow rate, the auxiliary argon flow rate, the furnace pressure, and the crystal rotation speed remain constant after reaching their respective termination values ​​until the shoulder formation stage ends.

[0010] Furthermore, the multiple pulling speed limiting intervals divided according to the crystal diameter range during the shoulder-forming stage include at least three intervals, each interval corresponding to a different upper and lower limit value of pulling speed. As the crystal diameter increases, the upper and lower limit values ​​of pulling speed corresponding to each interval increase progressively.

[0011] Furthermore, the interval includes the following: When the crystal diameter is 0~100mm, the pulling speed is 0.8~1.2mm / min; When the crystal diameter is 100~200mm, the pulling speed is 0.85~1.4 mm / min; When the crystal diameter is 200~300mm, the pulling speed is 0.9~1.6mm / min.

[0012] Furthermore, the process parameters for each stage are determined using linear interpolation between two adjacent control nodes. The linear interpolation is calculated as follows: ; in, and These are the length values ​​for two adjacent control nodes, respectively. and These are the process parameter values ​​corresponding to the two adjacent control nodes. This is the current length value. The process parameter value corresponding to the current length measurement value includes at least one of the following: main argon flow rate, auxiliary argon flow rate, furnace pressure, crystal rotation speed, crucible rotation speed, liquid outlet distance, crystal pulling speed, heating power, and crucible rising rate.

[0013] Furthermore, the constant diameter stage includes a pulling speed filtering coefficient and a pulling speed dead zone threshold. The pulling speed filtering coefficient is used to smooth the pulling speed correction amount, and the pulling speed dead zone threshold is used to determine whether to start the pulling speed correction. When the absolute value of the crystal diameter deviation is less than the dead zone threshold of the pulling speed, the pulling speed remains unchanged; When the absolute value of the crystal diameter deviation is greater than the pulling speed dead zone threshold, the pulling speed correction amount is output after being smoothed by the pulling speed filtering coefficient.

[0014] Furthermore, the smoothing calculation method for the pulling speed is as follows: ; in, This refers to the actual output speed. The pull speed is the output speed of the previous cycle. The theoretical required pulling speed is calculated based on the crystal diameter deviation. These are the speed-up filter coefficients.

[0015] Furthermore, the pulling speed filtering coefficient is ≤0.010, and the pulling speed dead zone threshold is 0.07mm / min.

[0016] Furthermore, the angle between the bottom of the outer guide tube and the horizontal plane is ≤15°.

[0017] A single crystal furnace control system for the above method includes a PLC controller that outputs timing control signals according to the process control parameters of each stage of the method to execute the method.

[0018] The beneficial effects of this invention are as follows: This method, based on a low-angle external guide tube, linearly adjusts the main / auxiliary argon gas and furnace pressure during the shoulder-forming stage. This ensures the quality of the seed crystal during the crystal pulling stage and reduces the oxygen partial pressure in the furnace after shoulder formation, thereby reducing the precipitation and migration of oxygen into the crystal from the source. Simultaneously, differentiated upper and lower limits for pulling speed are set for different diameter ranges to avoid crystal diameter deviations and thermal stress caused by pulling speed fluctuations. This significantly reduces the risk of edge breakage and wire breakage during shoulder formation and improves the success rate of shoulder formation on the first attempt.

[0019] The constant diameter stage is equipped with a casting speed filter coefficient and a casting speed dead zone threshold. The casting speed filter coefficient is used to smooth the casting speed correction amount; the casting speed dead zone threshold is used to determine whether to activate casting speed correction. The two work together to eliminate the invalid adjustments caused by minor noise in visual measurement and minor sloshing of the melt, reduce the frequent acceleration and deceleration of the lifting servo, and stabilize the thermal field and oxygen distribution. Attached Figure Description

[0020] Figure 1 This is a parameter table diagram of the shoulder placement stage of the present invention; Figure 2This is a table showing the parameters of crystal rotation, crucible rotation, and liquid outlet distance during the constant diameter stage of this invention; Figure 3 This is a table showing the parameters of the drawing speed and power during the constant diameter stage of this invention; Figure 4 This is a table showing the control parameters for crystal rise, main power, and crucible rotation segmentation during the final stage of this invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0022] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of the invention.

[0023] At the same time, it should be understood that, for ease of description, the dimensions of the various parts shown in the accompanying drawings are not drawn according to actual scale.

[0024] Furthermore, for clarity and brevity, descriptions of well-known structures, functions, and configurations may have been omitted. Those skilled in the art will recognize that various changes and modifications can be made to the examples described herein without departing from the spirit and scope of this disclosure.

[0025] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0026] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0027] This invention employs a carbon-carbon composite external guide tube with a 15° inclination angle at its lower end. The carbon-carbon material possesses excellent high-temperature stability and uniform thermal conductivity. Combined with the optimized 15° inclination angle, it can reconstruct the thermal and airflow fields within the furnace, suppressing oxygen migration to the crystals at its source. Compared to existing 30° external guide tubes, the 15° inclination angle effectively avoids localized backflow at the end of the inclination angle, resulting in smoother airflow diffusion across the liquid surface and facilitating oxygen volatilization.

[0028] Example 1 A method for pulling N-type 12-inch single crystals using a low-tilt external guide tube includes the following steps: (1) Welding stage: Crucible rotation speed: 5 r / min; main argon flow rate: 120 slm; auxiliary argon flow rate: 10 slm; furnace pressure: 1.3 kPa; crystal rotation speed: 8 r / min. Liquid outlet distance: 32±1 mm.

[0029] During this stage, secondary argon gas is continuously introduced, and the directional airflow is used to isolate high-temperature silicon vapor, which is specifically used to suppress oxidation on the seed crystal surface and prevent the seed crystal from developing color defects.

[0030] (2) Crystallization stage: The process parameters in this stage remain consistent with those in the welding stage, maintaining stable airflow, pressure, and melt flow within the furnace to ensure continuous and reliable crystal pulling.

[0031] (3) Shoulder relaxation stage: During this stage, the crucible rotation speed remains constant at 5 r / min throughout; the standard shoulder height is 300±10mm, and the shoulder setting time is 200±10min.

[0032] The parameters of main argon flow rate, auxiliary argon flow rate, furnace pressure, and crystal rotation speed are adopted in a continuous linear gradual variation mode, referring to... Figure 1 The main argon flow rate was linearly reduced from 120 slm to 100 slm throughout the process; the auxiliary argon flow rate was linearly reduced from 10 slm to 0 slm throughout the process; the furnace pressure was linearly reduced from 1.3 kPa to 0.8 kPa throughout the process; and the crystal rotation speed was linearly reduced from 8 r / min to 7 r / min throughout the process. All parameters remained constant after reaching their termination values ​​until the shoulder formation stage ended.

[0033] The above linear variation is calculated using a two-point linear difference formula. Taking the main argon flow rate as an example, let the starting point of the shoulder be the length. =0mm, main argon gas flow rate = 120slm, shoulder length for 200min corresponding node length Corresponding main argon flow rate =100slm, and The main argon flow rate corresponding to any intermediate gauge length L is: The auxiliary argon flow rate, furnace pressure, and crystal rotation speed are calculated similarly using the formulas described above.

[0034] During the shoulder-forming stage, the crystal diameter, diameter rate, crucible rising rate, and heating power are controlled in a coordinated manner according to preset parameters. Starting with a crystal diameter of 1 mm and ending at 300 mm, the diameter rate gradually increases from 2 mm / min to 6 mm / min, the crucible rising rate gradually increases from 0 to 0.111 mm / min, and the heating power decreases by a cumulative 12 kW throughout the shoulder-forming process. All parameters exhibit a phased linear change with the increase of crystal diameter.

[0035] To improve the stability of the shoulder-forming stage, the crystal is divided into three intervals according to its real-time diameter, and the upper and lower limits of the pulling speed are defined for each interval. The specific parameters are shown in Table 1. Table 1 shows the upper and lower limits of the pulling speed for each interval during the shoulder relaxation phase. The purpose of setting a pulling speed limit is to constrain the pulling speed range at different crystal diameter expansion stages, avoid drastic fluctuations in pulling speed that could cause thermal stress and diameter deviation, and significantly reduce the risk of shoulder breakage and wire breakage.

[0036] (4) Shoulder rotation phase: During this stage, the main argon gas flow rate is kept constant at 100 slm, the furnace pressure is 0.8 kPa, and the crystal rotation speed is 7 rpm. The crucible rotation speed is 5 rpm; when the crystal diameter is between 294.9 and 301 mm, the crystal lifting rate increases linearly from 1.80 mm / min to 2.30 mm / min; when the crystal diameter is between 1 and 301 mm (these two diameters refer to what diameter), the crystal lifting rate decreases linearly from 0.350 mm / min to 0.120 mm / min.

[0037] By coordinating the crystal lift-up amount with the crystal lift-up amount, a smooth transition of the crystal diameter from the shoulder profile to the 12-inch constant diameter is achieved, avoiding thermal stress concentration and abrupt diameter changes, laying the foundation for stable growth in the subsequent constant diameter stage.

[0038] (5) Equal diameter stage: This stage is the core stage of crystal growth. The main argon flow rate, furnace pressure, and parameters are kept consistent with those in the shoulder-turning stage. The crystal rotation speed and crucible rotation speed are dynamically adjusted according to the crystal growth length: the crystal rotation speed is gradually adjusted from 7.5 r / min to 7.0 r / min, and the crucible rotation speed is gradually increased from 5.0 r / min to 7.0 r / min and then decreased back to 5.0 r / min, with the rotation speed and follower ratio increasing linearly. The follower ratio increases linearly from 0.102 to 0.168 with crystal growth. The liquid outlet distance decreases linearly from 30 mm to 23 mm with the crystal length, referring to... Figure 2 .

[0039] By coordinating the crystal rotation and crucible rotation, the convection state of the silicon melt is stabilized, and the migration of oxygen elements into the crystal is suppressed; by gradually reducing the liquid outlet distance, the thermal field at the growth interface is kept constant, and the crystal diameter fluctuation is reduced.

[0040] The crystal pulling speed was gradually increased from 1.20 mm / min to 1.50 mm / min with increasing growth length to adapt to the natural decay of the thermal field within the furnace; the heating power was simultaneously and gradually increased to compensate for thermal losses and ensure stable crystal growth interface temperature, referring to... Figure 3 .

[0041] Meanwhile, a three-stage speed limiting method is adopted in the constant diameter stage, as shown in Table 2: Table 2 shows the speed limit for the three-stage equal-diameter stage. The constant diameter stage also includes a speed filtering coefficient and a speed dead zone threshold. The speed filtering coefficient K=0.010 is used to smooth the speed correction amount; the speed dead zone threshold is 0.07 mm / min, used to determine whether to activate speed correction.

[0042] The calculation method for speed filtering is as follows: ; This is the theoretical required pulling speed calculated by the PID controller based on the diameter deviation.

[0043] The working rule of the speed dead zone is as follows: the machine vision measures the difference between the actual diameter of the crystal and the target diameter in real time. When the absolute value of the diameter deviation is less than the dead zone value (0.07 mm / min), it is determined that the size is qualified, the PID does not output the speed adjustment command, and the speed remains unchanged; when the absolute value of the diameter deviation is greater than the dead zone value, the filtering and speed correction logic is activated.

[0044] The combination of the filter coefficient and the dead zone of the pulling speed forms a dual suppression mechanism that first determines whether the dead zone has been entered, and then smooths the output through filtering. This eliminates the ineffective adjustment caused by the small noise of visual measurement and the small sway of the melt, reduces the frequent acceleration and deceleration of the pulling servo, and stabilizes the thermal field and oxygen distribution.

[0045] In addition, the equal diameter stage is also equipped with auxiliary control parameters such as 60-minute liquid outlet distance PID delay recovery time, 0.00 mm liquid outlet distance start-up length, 0.00 kg residual weight after shutdown, 2.0 mm calculated diameter deviation, 237.5 kg residual weight at the end, and 25.0% at the end.

[0046] (6) Final stage: When the crystal grows to the set point where the remaining molten material in the furnace reaches a final weight of 237.5 kg, the finishing process begins. During the finishing stage, the basic process parameters are fixed: constant argon gas at 90 slm, constant furnace pressure at 1.0 kPa, constant crystal rotation speed at 7 r / min, and a total finishing length of 190 mm. By dynamically adjusting the crystal growth rate, heating power, and crucible rotation speed, the crystal can be smoothly finished with reduced diameter, avoiding sudden shrinkage and breakage during the finishing process.

[0047] The crucible rotation speed was kept constant at 5 r / min during the initial 1–50 min of the finishing stage to ensure stable melt flow and avoid violent surface disturbance. Subsequently, the speed was gradually and linearly reduced from 4.5 r / min to 3.0 r / min, gradually slowing down the crucible stirring intensity. This, combined with the continuous reduction of melt on the surface, suppressed melt tumbling and abrupt diameter reduction at the end of the finishing stage. (Refer to...) Figure 4 .

[0048] The heating power is gradually reduced in stepwise increments according to the length of the crystal, from an initial single reduction of 3.0 kW to a single reduction of 2.0 kW, precisely matching the thermal demand during the diameter reduction process, avoiding local remelting of the crystal due to excessive power, or rapid wire breakage due to insufficient power.

[0049] The crystal pulling rate was first slightly reduced from 1.7 mm / min to 1.3 mm / min, and then gradually increased to 2.4 mm / min at the end. This gradual increase in pulling speed resulted in a smooth shrinkage of the crystal diameter, optimized the forming quality of the tapered surface at the end, and significantly reduced the probability of wire breakage at the end stage.

[0050] It should be noted that the length calculation for the constant diameter stage and the length calculation for the terminal stage refer to the axial length of the crystal growth at this stage, calculated with the starting point of each stage as zero.

[0051] Application Examples Using the same type 1600 single crystal furnace, consistent charge amount, and N-type doped raw material, a comparative experiment was conducted by continuously producing 30 furnaces using the existing 30° external guide tube process and the 15° carbon-carbon external guide tube of this invention, based on the process of Example 1. The actual application results are as follows: After adopting the process of this invention, the average operating power of the constant diameter section is 44.8kW, which is 2.9kW lower than the 47.7kW real-time power consumption of a single furnace in the 30° external guide tube benchmark process. Under long-cycle continuous production conditions, steady-state operation can save 1.5kW~2kW of electricity. At the same time, the segmented step power reduction scheme adopted in the final stage further reduces the power consumption of the entire furnace production compared with the traditional one-time sudden power reduction process, effectively reducing the cost of monocrystalline manufacturing.

[0052] The original 30° guide tube required compressing the adjustable range of crystal rotation speed, crucible rotation speed, argon gas, and furnace pressure to control oxygen content, resulting in a narrow process tolerance. Even slight deviations in parameters could lead to breakage or diameter deviation. This new solution utilizes a 15° guide tube to optimize the thermal field, combined with segmented pulling speed limiting and linear gradual adjustment of parameters in each process. A fixed crucible rotation speed of 5 r / min allows for stable production, and slight fluctuations in argon gas and furnace pressure will not cause crystal breakage. The overall process window is significantly widened, and the breakage rate is reduced by 5% compared to the baseline process. In the final stage, a combination of crucible rotation speed reduction, power graded reduction, and crystal lifting speed variation significantly improves the breakage problem. At the same time, the addition of auxiliary argon gas protection in the welding stage solves the problem of high-temperature oxidation and color development of seed crystals, resulting in a near 100% crystal pulling success rate.

[0053] The thermal and airflow fields are reconstructed by a 15° carbon-carbon guide tube, which reduces the transport efficiency of silicon and oxygen dissolved from the crucible sidewall to the crystal. The measured average oxygen content of the reverse-cut crystal rod is 9.74 ppma, which is 0.6 ppma lower than the 10.34 ppma of the 30° guide tube process. The average oxygen content at the tail of the crystal rod is 10.01 ppma, which is 0.56 ppma lower than the original process of 10.57 ppma. The uniformity of oxygen content throughout the rod is improved, which fully meets the low-oxygen single crystal technology standard for N-type high-efficiency batteries.

[0054] Improved process stability led to increased output per furnace. Excluding furnaces with sudden equipment malfunctions, the daily output of finished products per furnace increased from 185.0 kg to 198.6 kg, and the single furnace capacity increased by 13.6 kg, taking into account the comprehensive production advantages of low oxygen, low energy consumption and high output.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for pulling N-type 12-inch single crystals using a low-angle external guide tube, characterized in that, Includes the following steps: S1, during the welding and crystal-leading stages, auxiliary argon gas is introduced to provide directional gas flow protection for the seed crystal; S2, during the shoulder formation stage, the main argon flow rate, auxiliary argon flow rate, furnace pressure, and crystal rotation speed decrease synchronously and linearly with the shoulder formation process, and the pulling speed is controlled according to the preset range of the crystal diameter. The upper and lower limits of the pulling speed increase step by step as the crystal diameter increases. S3, the shoulder turning stage and the constant diameter stage, the main argon flow rate and the furnace pressure remain constant. In the constant diameter stage, the crystal pulling speed increases with the crystal growth length, and the crystal rotation speed, crucible rotation speed, liquid outlet distance and follower ratio are adjusted in coordination according to the preset linkage relationship. S4, the final stage: the main argon flow rate and furnace pressure are switched to the final stage setting value, and the crucible speed, heating power and crystal pulling speed are adjusted in segments according to the final stage process to achieve a smooth diameter reduction.

2. The method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 1, characterized in that, During the shoulder formation stage, the main argon flow rate, auxiliary argon flow rate, furnace pressure, and crystal rotation speed remain constant after reaching their respective termination values ​​until the shoulder formation stage ends.

3. The method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 1, characterized in that, The shoulder-forming stage is divided into multiple speed limiting intervals according to the crystal diameter range, including at least three intervals. Each interval corresponds to a different upper and lower speed limit value. As the crystal diameter increases, the upper and lower speed limits of each interval increase progressively.

4. The method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 3, characterized in that, The interval includes the following: When the crystal diameter is 0~100mm, the pulling speed is 0.8~1.2mm / min; When the crystal diameter is 100~200mm, the pulling speed is 0.85~1.4 mm / min; When the crystal diameter is 200~300mm, the pulling speed is 0.9~1.6mm / min.

5. The method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 1, characterized in that, The process parameters for each stage are determined using linear interpolation between two adjacent control nodes. The linear interpolation is calculated as follows: ; in, and These are the length values ​​for two adjacent control nodes, respectively. and These are the process parameter values ​​corresponding to the two adjacent control nodes. This is the current length value. The process parameter value corresponding to the current length measurement value includes at least one of the following: main argon flow rate, auxiliary argon flow rate, furnace pressure, crystal rotation speed, crucible rotation speed, liquid outlet distance, crystal pulling speed, heating power, and crucible rising rate.

6. The method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 1, characterized in that, The constant diameter stage includes a speed filtering coefficient and a speed dead zone threshold. The speed filtering coefficient is used to smooth the speed correction amount, and the speed dead zone threshold is used to determine whether to start speed correction. When the absolute value of the crystal diameter deviation is less than the dead zone threshold of the pulling speed, the pulling speed remains unchanged; When the absolute value of the crystal diameter deviation is greater than the pulling speed dead zone threshold, the pulling speed correction amount is output after being smoothed by the pulling speed filtering coefficient.

7. The method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 6, characterized in that, The smoothing calculation method for the pulling speed is as follows: ; in, This refers to the actual output speed. The pull speed is the output speed of the previous cycle. The theoretical required pulling speed is calculated based on the crystal diameter deviation. These are the speed-up filter coefficients.

8. The method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 6, characterized in that, The pull speed filtering coefficient is ≤0.010, and the pull speed dead zone threshold is 0.07mm / min.

9. A method for pulling N-type 12-inch single crystals using a low-angle external guide tube according to claim 6, characterized in that, The angle between the bottom of the outer guide tube and the horizontal plane is ≤15°.

10. A single crystal furnace control system for the method according to any one of claims 1 to 9, characterized in that, The method includes a PLC controller that outputs timing control signals for the process control parameters at each stage of the method according to any one of claims 1 to 9, in order to execute the method.

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