Oxygen reduction process of 210N type silicon rod

Abstract

The invention provides an oxygen reduction process for a 210 type silicon rod. The oxygen reduction process comprises a welding stage, a seeding stage, a shouldering stage, a shoulder rotating stage, an equal-diameter stage and an ending stage, in the shouldering stage, the furnace pressure is 7 Torr when the shoulder length is 0-110 mm, and the furnace pressure is gradually reduced to 5 Torr after the shoulder length is 110 mm; when the shoulder length is 0-120 mm, the speed of the crucible is kept at 5 rpm, and after the shoulder length is 120 mm, the speed of the crucible is gradually reduced to 4 rpm; in the shoulder rotation stage, the furnace pressure is 5 Torr, and the crucible rotation speed is 4 rpm; in the equal-diameter stage, when the crystal length is 0-150mm, the speed of the crucible is kept to be 4rpm, when the crystal length is 150-800mm, the speed of the crucible is gradually changed to be 6rpm, and then the speed of the crucible is kept to be 6rpm; and when the crystal length is 0-300mm, the liquid opening distance is gradually reduced to a target liquid opening distance of 20mm. According to the oxygen reduction technology, the crystallization condition can be improved while the oxygen content is reduced, the equipment cost is not increased, and high popularization performance is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of single crystal preparation, and particularly relates to an oxygen reduction process for 210-type silicon rods. Background Art

[0002] N-type monocrystalline silicon has become the main substrate material for high-efficiency solar cells (such as TOPCon and HJT) due to its high minority carrier lifetime, low light-induced attenuation rate, and excellent low-light performance. Its market share is continuously expanding, and among them, the proportion of N-type TOPCon cells reaches 60%. At the same time, 210mm-sized monocrystalline silicon wafers have become the mainstream in the photovoltaic industry due to their high module power and low system cost. Their combination with N-type monocrystalline silicon ("210+N") further improves the module efficiency and power, driving the photovoltaic industry into the 700W+ era. The oxygen content has a significant impact on the performance of monocrystalline silicon. Excessive oxygen content during the preparation process will increase crystal defects, reduce the minority carrier lifetime and battery efficiency, and even cause problems such as concentric circles and black chips. Especially in N-type TOPCon cells, the problems are more prominent when the oxygen content is higher than 7ppm. Therefore, reducing the oxygen content has become a key link in the production of monocrystalline silicon. Oxygen reduction can not only improve the battery efficiency, but also reduce crystal defects and increase the crystal formation rate, thereby reducing production costs. In addition, the trend of thinning N-type monocrystalline silicon (developing from 150μm to 130μm or even thinner) further promotes the demand for oxygen reduction, because thinning requires a higher minority carrier lifetime and a lower oxygen content, and the oxygen content of the monocrystalline silicon pulled under the current process basically cannot meet the requirements of this type of battery. Summary of the Invention

[0003] The purpose of the present invention is to provide an oxygen reduction process for 210-type silicon rods. The oxygen reduction process in the present invention can improve the crystal formation while reducing the oxygen content, and has high popularization potential without increasing equipment costs.

[0004] The present invention provides an oxygen reduction process for 210-type silicon rods, including a fusion welding stage, a seed crystal pulling stage, a shoulder forming stage, a shoulder turning stage, an equal diameter stage, and a finishing stage;

[0005] In the shoulder forming stage, the furnace pressure is 7 Torr when the shoulder length is 0 - 110mm, and gradually decreases to 5 Torr after 110mm; the crucible rotation speed is maintained at 5 rpm when the shoulder length is 0 - 120mm, and gradually decreases to 4 rpm after 120mm;

[0006] In the shoulder turning stage, the furnace pressure is 5 Torr and the crucible rotation speed is 4 rpm;

[0007] In the equal diameter stage, the crucible rotation speed is maintained at 4 rpm when the crystal length is 0 - 150mm, gradually changes to 6 rpm when the crystal length is 150 - 800mm, and then remains at 6 rpm; the liquid level distance gradually decreases to the target liquid level distance of 20mm when the crystal length is 0 - 300mm.

[0008] The present invention reduces the friction between the silicon melt and the crucible wall by reducing the crucible rotation, thereby directly reducing the oxygen source. Further, the present invention places the node of changing the crucible rotation after 120 mm of the shoulder length to increase the temperature adjustment window while reducing oxygen release and ensure a sufficient draw survival rate.

[0009] The low furnace pressure process adopted by the present invention in the shoulder forming stage can effectively reduce the partial pressure of SiO in the gas phase, reduce the absorption amount of molten silicon to SiO, thereby reducing the oxygen content. However, the boiling point of the silicon melt decreases under low furnace pressure, and the surface of the silicon melt becomes very unstable, greatly increasing the probability of liquid surface jitter during the seeding process and affecting crystal formation. The present invention further reduces the furnace pressure after the shoulder length is greater than 110 mm to ensure the crystal formation rate while reducing the oxygen content.

[0010] Preferably, in the welding stage, the furnace pressure is 7 Torr, the crucible rotation is 5 rpm, and the liquid orifice distance is 30 mm.

[0011] Preferably, in the seeding stage, the furnace pressure is 7 Torr, the crucible rotation is 5 rpm, and the liquid orifice distance is 30 mm.

[0012] Preferably, in the shoulder forming stage, when the shoulder length reaches 100 mm, the furnace pressure is gradually reduced from 7 Torr to 5 Torr at a rate of 0.1 Torr / min.

[0013] Preferably, in the shoulder forming stage, when the shoulder length reaches 120 mm, the crucible rotation is gradually reduced from 5 rpm to 4 rpm at a rate of 1.5 rpm reduction per hour.

[0014] Preferably, in the shoulder forming stage, the liquid orifice distance is 30 mm.

[0015] Preferably, in the shoulder turning stage, the liquid orifice distance is 30 mm, and the crucible lifting rate is 12 mm / hr.

[0016] Preferably, in the constant diameter stage, the furnace pressure is maintained at 5 Torr throughout the process.

[0017] Preferably, in the constant diameter stage, before the liquid orifice distance reaches 20 mm, the crucible lifting rate is 12 mm / h.

[0018] The present invention can improve the flow rate of argon gas flowing through the surface of the silicon melt by quickly compensating the liquid orifice distance and reaching the target liquid orifice distance faster, thereby more efficiently carrying the SiO vapor away from the melt surface, reducing oxygen dissolution, and increasing the rate of decrease in the oxygen content at the head.

[0019] Preferably, in the finishing stage, the furnace pressure is 5 Torr and the crucible rotation is 5 rpm.

[0020] The present invention provides an oxygen reduction process for a 210-type silicon rod, including a welding stage, a seed crystal pulling stage, a shoulder broadening stage, a shoulder turning stage, an equal diameter stage, and a finishing stage; in the shoulder broadening stage, the furnace pressure is 7 Torr when the shoulder length is 0 - 110 mm, and gradually decreases to 5 Torr after 110 mm; the crucible rotation speed is maintained at 5 rpm when the shoulder length is 0 - 120 mm, and gradually decreases to 4 rpm after 120 mm; in the shoulder turning stage, the furnace pressure is 5 Torr and the crucible rotation speed is 4 rpm; in the equal diameter stage, the crucible rotation speed is maintained at 4 rpm when the crystal length is 0 - 150 mm, gradually changes to 6 rpm when the crystal length is 150 - 800 mm, and then remains at 6 rpm; the liquid level distance gradually decreases to the target liquid level distance of 20 mm when the crystal length is 0 - 300 mm. The present invention enables the head oxygen content to decrease at a faster rate by quickly compensating the liquid level distance to reach the target liquid level distance faster; the present invention reduces the oxygen source directly by decreasing the crucible rotation speed. Further, the present invention selects the crucible rotation speed change point after the shoulder length reaches 120 mm, which increases the temperature adjustment window while ensuring oxygen content reduction and ensuring the success rate of crystal pulling and feeding. In addition, the present invention reduces the furnace pressure after the shoulder length is greater than 110 mm to effectively reduce the oxygen content, while avoiding excessive variable caused by simultaneous change with the crucible rotation speed and affecting crystal formation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on the provided drawings without creative efforts.

[0022] Figure 1 It is a curve showing the change of oxygen content in the crystal rods prepared in Embodiments 3 - 5 of the present invention with the liquid level distance in the crystal pulling process. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] In order to further illustrate the present invention, the following will describe in detail an oxygen reduction process for a 210-type silicon rod provided by the present invention with reference to embodiments, but it should not be construed as a limitation to the protection scope of the present invention.

[0024] Embodiment 1

[0025] S1. Welding stage: furnace pressure 7 Torr, crucible rotation speed 5 rpm, liquid level distance 30 mm;

[0026] S2. Seed crystal pulling stage: furnace pressure 7 Torr, crucible rotation speed 5 rpm, liquid level distance 30 mm;

[0027] S3. Shoulder broadening stage:

[0028] The furnace pressure is 7 Torr from the shoulder length of 0 - 110 mm, and then gradually decreases to 5 Torr at a rate of 0.1 Torr / min after 110 mm;

[0029] The crucible rotation speed is maintained at 5 rpm from the shoulder length of 0 - 120 mm, and then gradually decreases to 4 rpm at a rate of 1.5 rpm per hour after 120 mm;

[0030] The liquid level distance is 30 mm;

[0031] S4, Shoulder transition stage: The furnace pressure is 5 Torr, the crucible rotation speed is 4 rpm, the crucible lifting speed is 12 mm / hr, and the liquid level distance is 30 mm;

[0032] S5, Equal diameter stage:

[0033] The crucible rotation speed is maintained at 4 rpm for the crystal length of 0 - 150 mm, gradually changes to 6 rpm for the crystal length of 150 - 800 mm, and then remains at 6 rpm;

[0034] The furnace pressure is maintained at 5 Torr throughout the process;

[0035] The liquid level distance gradually decreases at a crucible lifting speed of 12 mm / h within the crystal length of 0 - 300 mm until the liquid level distance reaches the target liquid level distance (20 mm) when the crystal length reaches 300 mm;

[0036] S6, Ending stage: The furnace pressure is 5 Torr, and the crucible rotation speed is 5 rpm;

[0037] Comparative example 1

[0038] S1, Welding stage: The furnace pressure is 7 Torr, the crucible rotation speed is 4 rpm, and the liquid level distance is 30 mm;

[0039] S2, Crystal seeding stage: The furnace pressure is 7 Torr, the crucible rotation speed is 4 rpm, and the liquid level distance is 30 mm;

[0040] S3, Shoulder broadening stage:

[0041] The furnace pressure is 7 Torr from the shoulder length of 0 - 110 mm, and then gradually decreases to 5 Torr at a rate of 0.1 Torr / min after 110 mm;

[0042] The crucible rotation speed is 4 rpm;

[0043] The liquid level distance is 30 mm;

[0044] S4, Shoulder transition stage: The furnace pressure is 5 Torr, the crucible rotation speed is 4 rpm, the crucible lifting speed is 12 mm / hr, and the liquid level distance is 30 mm;

[0045] S5, Equal diameter stage:

[0046] The crystal length is maintained at 4 rpm of crucible rotation from 0 to 150 mm, gradually changes to 6 rpm of crucible rotation from 150 to 800 mm, and then remains at 6 rpm of crucible rotation consistently;

[0047] The furnace pressure is maintained at 5 Torr throughout the process;

[0048] The liquid level distance is gradually decreased at a crucible lifting rate of 12 mm / h within the crystal length range of 0 - 300 mm until the liquid level distance reaches the target liquid level distance (20 mm) when the crystal length reaches 300 mm;

[0049] S6, Ending stage: Furnace pressure 5 Torr, crucible rotation 5 rpm;

[0050] Comparative Example 2

[0051] S1, Welding stage: Furnace pressure 7 Torr, crucible rotation 5 rpm, liquid level distance 30 mm;

[0052] S2, Crystal seeding stage: Furnace pressure 7 Torr, crucible rotation 5 rpm, liquid level distance 30 mm;

[0053] S3, Shoulder forming stage:

[0054] The furnace pressure is 7 Torr when the shoulder length is from 0 to 110 mm, and is gradually decreased to 5 Torr at a rate of 0.1 Torr / min after 110 mm;

[0055] Crucible rotation is 5 rpm;

[0056] Liquid level distance is 30 mm;

[0057] S4, Shoulder turning stage: Furnace pressure 5 Torr, crucible rotation 5 rpm, crucible lifting 12 mm / hr, liquid level distance 30 mm;

[0058] S5, Equal diameter stage:

[0059] The crystal length is maintained at 5 rpm of crucible rotation from 0 to 150 mm, gradually changes to 6 rpm of crucible rotation from 150 to 800 mm, and then remains at 6 rpm of crucible rotation consistently;

[0060] The furnace pressure is maintained at 5 Torr throughout the process;

[0061] The liquid level distance is gradually decreased at a crucible lifting rate of 12 mm / h within the crystal length range of 0 - 300 mm until the liquid level distance reaches the target liquid level distance (20 mm) when the crystal length reaches 300 mm;

[0062] S6, Ending stage: Furnace pressure 5 Torr, crucible rotation 5 rpm;

[0063] Comparative Example 3

[0064] S1, Welding stage: Furnace pressure 7 Torr, crucible rotation 5 rpm, liquid level distance 30 mm;

[0065] S2, Crystal seeding stage: furnace pressure 7 Torr, crucible rotation speed 5 rpm, distance from liquid surface 30 mm;

[0066] S3, Shoulder formation stage:

[0067] The furnace pressure is 7 Torr when the shoulder length is 0 - 110 mm, and it is gradually reduced to 5 Torr at a rate of 0.1 Torr / min after 110 mm;

[0068] Crucible rotation speed 5 rpm;

[0069] Distance from liquid surface 30 mm;

[0070] S4, Shoulder transition stage: furnace pressure 5 Torr, crucible rotation speed 5 rpm, crucible lifting speed 12 mm / hr, distance from liquid surface 30 mm;

[0071] S5, Equal diameter stage:

[0072] When the crystal length is 0 - 150 mm, the crucible rotation speed is maintained at 5 rpm. When the crystal length is 150 - 800 mm, the crucible rotation speed is gradually changed to 6 rpm and then maintained at 6 rpm;

[0073] The furnace pressure is maintained at 5 Torr throughout the process;

[0074] The distance from the liquid surface is gradually reduced within the crystal length range of 0 - 600 mm until the distance from the liquid surface reaches the target distance from the liquid surface (20 mm) when the crystal length reaches 600 mm;

[0075] S6, Ending stage: furnace pressure 5 Torr, crucible rotation speed 5 rpm;

[0076] Comparative example 4

[0077] S1, Welding stage: furnace pressure 7 Torr, crucible rotation speed 5 rpm, distance from liquid surface 32 mm;

[0078] S2, Crystal seeding stage: furnace pressure 7 Torr, crucible rotation speed 5 rpm, distance from liquid surface 32 mm;

[0079] S3, Shoulder formation stage:

[0080] The furnace pressure is 7 Torr when the shoulder length is 0 - 110 mm, and it is gradually reduced to 5 Torr at a rate of 0.1 Torr / min after 110 mm;

[0081] Crucible rotation speed 5 rpm;

[0082] Distance from liquid surface 32 mm;

[0083] S4, Shoulder transition stage: furnace pressure 5 Torr, crucible rotation speed 5 rpm, crucible lifting speed 12 mm / hr, distance from liquid surface 32 mm;

[0084] S5. Equal-diameter stage:

[0085] The crystal length is maintained at 5 rpm crucible rotation from 0 to 150 mm, gradually changes to 6 rpm crucible rotation from 150 to 800 mm, and then consistently maintains 6 rpm crucible rotation;

[0086] The furnace pressure is maintained at 5 Torr throughout the process;

[0087] The liquid orifice distance is 32 mm;

[0088] S6. Ending stage: Furnace pressure is 5 Torr, crucible rotation is 5 rpm;

[0089] Comparative Example 5

[0090] S1. Welding stage: Furnace pressure is 7 Torr, crucible rotation is 5 rpm, liquid orifice distance is 30 mm;

[0091] S2. Crystal seeding stage: Furnace pressure is 7 Torr, crucible rotation is 5 rpm, liquid orifice distance is 30 mm;

[0092] S3. Shoulder broadening stage:

[0093] The furnace pressure is 7 Torr for the shoulder length from 0 to 110 mm, and gradually decreases to 5 Torr at a rate of 0.1 Torr / min after 110 mm;

[0094] The crucible rotation is maintained at 5 rpm for the shoulder length from 0 to 20 mm, and gradually decreases to 4 rpm at a rate of 1.5 rpm per hour after 20 mm;

[0095] The liquid orifice distance is 30 mm;

[0096] S4. Shoulder turning stage: Furnace pressure is 5 Torr, crucible rotation is 4 rpm, crucible lifting is 12 mm / hr, liquid orifice distance is 30 mm;

[0097] S5. Equal-diameter stage:

[0098] The crystal length is maintained at 4 rpm crucible rotation from 0 to 150 mm, gradually changes to 6 rpm crucible rotation from 150 to 800 mm, and then consistently maintains 6 rpm crucible rotation;

[0099] The furnace pressure is maintained at 5 Torr throughout the process;

[0100] The liquid orifice distance gradually decreases at a crucible lifting rate of 12 mm / h within the crystal length of 0 - 300 mm until the liquid orifice distance reaches the target liquid orifice distance (20 mm) when the crystal length reaches 300 mm;

[0101] S6. Ending stage: Furnace pressure is 5 Torr, crucible rotation is 5 rpm;

[0102] The oxygen content, head oxygen content, and drawing and seeding survival rate of the crystals in Example 1 and Comparative Examples 2 - 5 were detected, and the results are as followsFigure 1 , as shown in Tables 1 - 2.

[0103] Figure 1 It is the curve of the oxygen content in the crystal bars prepared in Comparative Examples 2 - 4 varying with the liquid - orifice distance in the crystal - pulling process. It can be seen from Figure 1 that under the same thermal - field conditions, in the crystal - pulling process, the size of the liquid - orifice distance will affect the oxygen content in the crystal bar. The smaller the liquid - orifice distance, the lower the oxygen content. The faster the compensation rate of the head liquid - orifice distance, the faster the reduction rate of the head oxygen content. Therefore, the liquid - orifice distance is positively correlated with the oxygen content.

[0104] The head oxygen content of the crystal bars obtained in Example 1 and Comparative Examples 1 - 2, 5 in Table 1

[0105]

[0106] The drawing - in and drawing - out survival rate of the crystal bars obtained in Example 1 and Comparative Examples 1 - 2, 5 in Table 2

[0107] Sample size Survival rate of induced release Comparative example 1 1917 71％ Example 1 566 65％ Comparative example 2 567 75％ Comparative example 5 100 60％

[0108] It can be seen from Table 1 and Table 2 that when the liquid - orifice distance is maintained at 30 mm and the compensation rate of the liquid - orifice distance is increased, from Comparative Examples 1 and 2, we can see that when the crucible rotation speed is reduced from 5 rpm to 4 rpm throughout the process, the oxygen content is reduced from 9.82 ppma to 8.87 ppma. The oxygen content of the crystal bar with reduced crucible rotation speed is significantly reduced, but the drawing - in and drawing - out survival rate drops by 10%, and the crystal - forming situation of the ingot becomes worse. This is mainly because the reduction of the crucible rotation speed reduces the lateral temperature gradient of the silicon melt, making the temperature - regulation window narrower and increasing the difficulty of drawing - in and drawing - out. Therefore, the variable - crucible - rotation process is adopted, that is, in the shoulder - forming process, the crucible rotation speed is maintained at 5 rpm before the shoulder - forming length reaches 120 mm and then slowly changed to 4 rpm, so as to reduce the oxygen content while ensuring the drawing - in and drawing - out survival rate. From Comparative Examples 1 and 2, it can be seen that the drawing - in and drawing - out survival rate reaches 71% and the average oxygen content is 9.27 ppma. The variable - crucible - rotation node must be adjusted to be greater than 120. At this time, the temperature gradient has little influence on the shoulder - forming due to the change of the crucible rotation speed. If the variable - crucible - rotation is carried out in the early stage of shoulder - forming, it will seriously affect the stability of shoulder - forming. For example, in Example 6, the drawing - in and drawing - out survival rate is reduced to 60%.

[0109] The above - mentioned is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. Oxygen reduction process for 210-type silicon rods, including the welding stage, seeding stage, shoulder expansion stage, shoulder transition stage, constant diameter stage, and ending stage; In the shoulder expansion stage, the furnace pressure is 7 Torr when the shoulder length is 0 - 110 mm, and gradually decreases to 5 Torr after 110 mm; the crucible rotation speed is maintained at 5 rpm when the shoulder length is 0 - 120 mm, and gradually decreases to 4 rpm after 120 mm; In the shoulder transition stage, the furnace pressure is 5 Torr and the crucible rotation speed is 4 rpm; In the constant diameter stage, the crucible rotation speed is maintained at 4 rpm when the crystal length is 0 - 150 mm, gradually changes to 6 rpm when the crystal length is 150 - 800 mm, and then remains at 6 rpm; The liquid level distance gradually decreases to the target liquid level distance of 20 mm when the crystal length is 0 - 300 mm.

2. The oxygen reduction process of the 210-type silicon rod according to claim 1, characterized in that, In the welding stage, the furnace pressure is 7 Torr, the crucible rotation speed is 5 rpm, and the liquid level distance is 30 mm.

3. The oxygen reduction process of the type 210 silicon rod according to claim 1, characterized in that, In the seeding stage, the furnace pressure is 7 Torr, the crucible rotation speed is 5 rpm, and the liquid level distance is 30 mm.

4. The oxygen reduction process of the 210-type silicon rod according to claim 1, characterized in that In the shoulder expansion stage, when the shoulder length reaches 110 mm, the furnace pressure is gradually decreased from 7 Torr to 5 Torr at a rate of 0.1 Torr / min.

5. The oxygen reduction process of the type 210 silicon rod according to claim 1, characterized in that, In the shoulder expansion stage, when the shoulder length reaches 120 mm, the crucible rotation speed is gradually decreased from 5 rpm to 4 rpm at a rate of 1.5 rpm per hour.

6. The oxygen reduction process of the type 210 silicon rod according to claim 1, characterized in that, In the shoulder expansion stage, the liquid level distance is 30 mm.

7. The oxygen reduction process of the 210-type silicon rod according to claim 1, characterized in that, In the shoulder transition stage, the liquid level distance is 30 mm and the crucible lifting rate is 12 mm / hr.

8. The oxygen reduction process of the type 210 silicon rod according to claim 1, characterized in that, In the constant diameter stage, the furnace pressure is maintained at 5 Torr throughout the process.

9. The oxygen reduction process of the type 210 silicon rod according to claim 1, characterized in that, In the constant diameter stage, before the liquid level distance reaches 20 mm, the crucible lifting rate is 12 mm / h.

10. The oxygen reduction process of the 210-type silicon rod according to claim 1, characterized in that, In the ending stage, the furnace pressure is 5 Torr and the crucible rotation speed is 5 rpm.

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