Method for improving white spot defect of silicon wafer in post-oxidation treatment

Abstract

The application provides a method for improving the white spot defect of a crystalline silicon wafer in a post-oxidation process, which comprises the following steps: (1) wafer loading; (2) first ozone purification; (3) temperature rising; (4) first vacuum pumping; (5) second ozone purification; (6) post-oxidation; (7) second vacuum pumping; (8) pressure recovery; (9) temperature falling; and (10) wafer unloading. The method utilizes ozone with strong oxidizing property to purify the air in a furnace tube, improves the cleanliness of the environment atmosphere in the post-oxidation process, reduces the proportion of pollutants in the furnace tube, and further effectively reduces the proportion of the white spot defect of the crystalline silicon wafer. The method is simple in operation, low in requirement for equipment, low in processing cost, and has a large-scale application prospect.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, and in particular to a method for improving white spot defects in crystalline silicon wafers during post-oxidation treatment. Background Technology

[0002] To improve the passivation performance and increase the resistance to induction in solar cells, oxidation treatment is necessary. However, during the oxidation process, the atmosphere and environment inside the furnace tube cannot be completely controlled, easily leading to contamination such as white spots on the silicon wafer surface, severely affecting product quality and significantly reducing the yield of solar cells. The oxidation treatment of crystalline silicon wafers mainly includes two steps: pre-oxidation and post-oxidation. The quality of the crystalline silicon wafer after post-oxidation largely determines the quality of the final finished solar cell. Therefore, effectively controlling the occurrence of white spot defects in crystalline silicon wafers during post-oxidation is a problem that urgently needs to be solved.

[0003] CN110690319A discloses an oxidation annealing process for high-efficiency monocrystalline silicon solar cells. The process changes atmospheric oxidation to low-pressure oxidation and uses constant temperature 680-710℃ and cooling temperature 620-650℃ for annealing. This allows for precise control of the gas atmosphere during the process, improves the cleanliness of the annealing environment, and enhances the efficiency of the solar cells.

[0004] CN112670373A discloses an oxidation annealing method for crystalline silicon solar cells and its application. The oxidation process is broken down into smaller steps. After the first oxide layer is formed, the temperature is rapidly increased to carry out a second oxidation. Due to the increased temperature, the required oxygen passage time is reduced, thereby significantly reducing the process time.

[0005] CN108878289A discloses a high-efficiency battery annealing process. By heating and drying the silicon wafer after back passivation, and by maintaining a negative pressure state during the heating and cooling process, the moisture is completely removed. By using a gradient cooling method, impurities in the silicon wafer are more fully precipitated, thereby reducing defects and recombination phenomena.

[0006] However, the above methods only improve the oxidation or annealing process, and do not take into account how to solve the white spot defect problem that exists in the oxidation process.

[0007] Therefore, it is of great significance to develop a simple and cost-effective method to improve the white spot defect of silicon wafers during post-oxidation treatment. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a method for improving white spot defects in silicon wafers during post-oxidation treatment. This method utilizes ozone, a highly oxidizing agent, to purify the air inside the furnace tube, improving the cleanliness of the environment during the post-oxidation process, reducing the proportion of contaminants within the furnace tube, and thus effectively lowering the proportion of white spot defects in the silicon wafers. The method described in this invention is simple to operate, requires minimal equipment, has low processing costs, and has promising prospects for large-scale application.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] This invention provides a method for improving white spot defects in silicon wafers during post-oxidation treatment, the method comprising the following steps:

[0011] (1) Entering the boat; (2) First ozone purification; (3) Heating; (4) First vacuuming; (5) Second ozone purification; (6) Post-oxidation; (7) Second vacuuming; (8) Back pressure; (9) Cooling; (10) Exiting the boat;

[0012] Step (2) The volume concentration of ozone in the first ozone purification is 50% to 70%;

[0013] In step (5), the volume concentration of ozone in the second ozone purification is 40% to 50%.

[0014] The method for improving white spot defects in crystalline silicon wafers during post-oxidation processing described in this invention effectively reduces the proportion of white spot defects by adding two ozone purification treatments to the existing post-oxidation process. In the first ozone purification treatment, the ozone volume concentration is 50%–70%, primarily used to oxidize dust or other contaminants that appear during the tube opening process. In the second ozone purification treatment, the ozone volume concentration is 40%–50%, primarily used to further react with watermarks and contaminants on the surface of the crystalline silicon wafer to produce oxides, changing their original state and lightening or removing white spots. Furthermore, the crystalline silicon wafers obtained by the method described in this invention exhibit excellent electrical properties, meeting quality requirements in both photothermal induced degradation tests and module potential-induced degradation tests.

[0015] In step (2) of this invention, the volume concentration of ozone in the first ozone purification is 50% to 70%, for example, it can be 50%, 55%, 60%, 65%, 68% or 70%, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable. In step (5), the volume concentration of ozone in the second ozone purification is 40% to 50%, for example, it can be 40%, 42%, 45%, 47%, 49% or 50%, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0016] Preferably, step (1) of feeding the silicon wafer into the quartz boat includes inserting the silicon wafer into the quartz boat and feeding it into the furnace tube.

[0017] Preferably, the temperature of the first ozone purification in step (2) is 700 to 760°C, for example, it can be 700°C, 710°C, 720°C, 740°C, 750°C or 760°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0018] The present invention preferably uses a first ozone purification temperature of 700-760°C. By utilizing the principle that the decay of ozone accelerates with increasing temperature, ozone achieves 100% decomposition within a reaction time of 1-2 seconds, thereby realizing efficient purification of the gas inside the furnace tube.

[0019] Preferably, the pressure of the first ozone purification is 800 to 1100 mbar, for example, it can be 800 mbar, 850 mbar, 900 mbar, 950 mbar, 1000 mbar or 1100 mbar, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0020] Preferably, the first ozone purification time is 600 to 1200 seconds, for example, it can be 600 seconds, 700 seconds, 800 seconds, 900 seconds, 1000 seconds or 1200 seconds, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0021] The preferred time for the first ozone purification in this invention is 600-1200 seconds. If the first ozone purification time is too short, it cannot effectively purify and remove harmful elements such as potassium, calcium, and sodium, as well as dust and floating particles. If the first ozone purification time is too long, it will increase the treatment cost and reduce economic benefits.

[0022] Preferably, the volumetric flow rate of ozone in the first ozone purification is 10,000 to 15,000 sccm, for example, it can be 10,000 sccm, 11,000 sccm, 12,000 sccm, 13,000 sccm, 14,000 sccm or 15,000 sccm, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] In this invention, the preferred ozone volumetric flow rate in the first ozone purification is 10,000–15,000 sccm, which is significantly higher than the 1,000–2,000 sccm in the second ozone purification, thus offering the advantage of effectively purifying particles within the furnace tube. If the ozone volumetric flow rate in the first ozone purification is too low, it will lead to incomplete purification of particles within the furnace tube, increasing the proportion of white spots; conversely, if the ozone volumetric flow rate in the first ozone purification is too high, it will result in excessive ozone consumption, increasing costs.

[0024] Preferably, the temperature for heating in step (3) is 700 to 800°C, for example, 700°C, 720°C, 740°C, 750°C, 770°C or 800°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the heating pressure is 900 to 1060 mbar, for example, 900 mbar, 930 mbar, 980 mbar, 1000 mbar, 1030 mbar or 1060 mbar, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] Preferably, nitrogen gas with a volumetric flow rate of 10,000 to 15,000 sccm is introduced during the heating process. For example, it can be 10,000 sccm, 11,000 sccm, 12,000 sccm, 13,000 sccm, 14,000 sccm, or 15,000 sccm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0027] Preferably, the temperature of the first vacuuming in step (4) is 700 to 750°C, for example, it can be 700°C, 710°C, 720°C, 730°C, 740°C or 750°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0028] Preferably, the pressure after the first vacuum is 0.

[0029] Preferably, the temperature for the second ozone purification in step (5) is 700 to 850°C, for example, it can be 700°C, 720°C, 750°C, 800°C, 720°C or 850°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] Preferably, the pressure of the second ozone purification is 200 to 400 mbar, for example, it can be 200 mbar, 220 mbar, 250 mbar, 300 mbar, 360 mbar or 400 mbar, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0031] Preferably, the second ozone purification time is 600 to 1200 seconds, for example, it can be 600 seconds, 700 seconds, 800 seconds, 900 seconds, 1000 seconds or 1200 seconds, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] Preferably, the volumetric flow rate of ozone in the second ozone purification is 1000-2000 sccm, for example, it can be 1000 sccm, 1300 sccm, 1500 sccm, 1800 sccm, 1960 sccm or 2000 sccm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] The present invention preferably uses a second ozone purification time of 600-1200s and an ozone volume flow rate of 1000-2000sccm in the second ozone purification, which can effectively improve the cleanliness of the environmental atmosphere in the post-oxidation treatment and effectively reduce the proportion of white spot defects in crystalline silicon wafers.

[0034] Preferably, the post-oxidation temperature in step (6) is 700 to 850°C, for example, it can be 700°C, 730°C, 780°C, 800°C, 820°C or 850°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0035] Preferably, the post-oxidation pressure is 150 to 300 mbar, for example, it can be 150 mbar, 170 mbar, 200 mbar, 250 mbar, 280 mbar or 300 mbar, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] Preferably, oxygen with a volumetric flow rate of 1000-1500 sccm is introduced during the post-oxidation process. For example, it can be 10000 sccm, 11000 sccm, 12000 sccm, 13000 sccm, 14000 sccm or 15000 sccm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0037] Preferably, the volume concentration of oxygen is 40% to 80%, for example, it can be 40%, 50%, 60%, 65%, 70% or 80%, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0038] Preferably, the temperature of the second vacuuming in step (7) is 600 to 700°C, for example, it can be 600°C, 620°C, 650°C, 670°C, 680°C or 700°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] Preferably, the pressure after the second vacuum is 0.

[0040] Preferably, the temperature of the back pressure in step (8) is 500 to 600°C, for example, it can be 500°C, 510°C, 530°C, 550°C, 570°C or 600°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] Preferably, the back pressure is 1060 to 1100 mbar, for example, it can be 1060 mbar, 1070 mbar, 1080 mbar, 1090 mbar, 1095 mbar or 1100 mbar, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0042] Preferably, nitrogen gas with a volumetric flow rate of 11,000 to 17,000 sccm is introduced during the back pressure process. For example, it can be 11,000 sccm, 12,000 sccm, 13,000 sccm, 14,000 sccm, 15,000 sccm, or 17,000 sccm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0043] Preferably, the cooling temperature in step (9) is 400 to 600°C, for example, it can be 400°C, 430°C, 500°C, 560°C or 600°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] Preferably, the cooling pressure is 900 to 1060 mbar, for example, 900 mbar, 930 mbar, 970 mbar, 1000 mbar or 1060 mbar, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0045] Preferably, nitrogen gas with a volumetric flow rate of 10,000 to 15,000 sccm is introduced during the cooling process. For example, it can be 10,000 sccm, 11,000 sccm, 12,000 sccm, 13,000 sccm, 14,000 sccm, or 15,000 sccm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0046] Preferably, step (10) of unloading the boat includes removing the quartz boat from the furnace tube, cooling it, and then unloading the silicon wafer.

[0047] As a preferred technical solution of the present invention, the method includes the following steps:

[0048] (1) Inserting the silicon wafer into the quartz boat and sending it into the furnace tube;

[0049] (2) First ozone purification: Ozone with a volume concentration of 50% to 70% and a volume flow rate of 10,000 to 15,000 sccm is introduced into the furnace tube, and the first ozone purification is carried out at a temperature of 700 to 760°C and a pressure of 800 to 1100 mbar for 600 to 1200 seconds.

[0050] (3) Heating: The temperature inside the furnace tube is controlled at 700-800℃ and the pressure is controlled at 900-1060mbar. During the heating process, nitrogen gas with a volume flow rate of 10000-15000sccm is introduced.

[0051] (4) First vacuuming: Under the condition of a temperature of 700-750℃, the furnace tube is evacuated for the first time until the pressure of the furnace tube is 0;

[0052] (5) Second ozone purification: Introduce ozone with a volume concentration of 40% to 50% and a volume flow rate of 1000 to 2000 sccm into the furnace tube, and perform second ozone purification at a temperature of 700 to 850°C and a pressure of 200 to 400 mbar for 600 to 1200 s.

[0053] (6) Post-oxidation: Post-oxidation is carried out at a temperature of 700-850℃ and a pressure of 150-300mbar; during the post-oxidation process, oxygen with a volume concentration of 40%-80% and a volume flow rate of 1000-1500sccm is introduced.

[0054] (7) Second vacuuming: Under the condition of a temperature of 600-700℃, the furnace tube is subjected to a second vacuuming until the pressure of the furnace tube is 0;

[0055] (8) Back pressure: Back pressure is carried out at a temperature of 500-600℃ and a pressure of 1060-1100mbar; during the back pressure process, nitrogen gas with a volume flow rate of 11000-17000sccm is introduced.

[0056] (9) Cooling: The temperature of the furnace tube is controlled at 400-600℃ and the pressure is controlled at 900-1060mbar. During the cooling process, nitrogen gas with a volume flow rate of 10000-15000sccm is introduced.

[0057] (10) Unloading the boat: Remove the quartz boat from the furnace tube, cool it, and then unload the silicon wafers.

[0058] Compared with the prior art, the present invention has at least the following beneficial effects:

[0059] The method for improving white spot defects in silicon wafers during post-oxidation processing provided by this invention adds two ozone purification treatments to the existing post-oxidation process, improving the cleanliness of the environmental atmosphere during post-oxidation and thus effectively reducing the proportion of white spot defects in silicon wafers. The method described in this invention is simple to operate, has low equipment requirements, and produces silicon wafers with excellent electrical properties, making it suitable for large-scale industrial application. Detailed Implementation

[0060] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0061] The present invention will now be described in further detail. However, the examples described below are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0062] Example 1

[0063] This embodiment provides a method for improving white spot defects in silicon wafers during post-oxidation treatment, the method comprising the following steps:

[0064] (1) Boat entry: Control the temperature inside the furnace tube to 600℃, the pressure to 1060mbar, and introduce nitrogen gas with a volume flow rate of 15000sccm for 700s. Insert the silicon wafer into the quartz boat and send it into the furnace tube.

[0065] (2) First ozone purification: Ozone with a volume concentration of 58% and a volume flow rate of 15000 sccm is introduced into the furnace tube for first ozone purification at a temperature of 700℃ and a pressure of 1060 mbar for 600s.

[0066] (3) Heating: The temperature inside the furnace tube is controlled at 700℃ and the pressure is 1060mbar. During the heating process, nitrogen gas with a volume flow rate of 15000sccm is introduced for 500s.

[0067] (4) First vacuuming: Under the condition of 700℃, the furnace tube is evacuated for 200s until the pressure of the furnace tube is 0;

[0068] (5) Second ozone purification: Ozone with a volume concentration of 45% and a volume flow rate of 1500 sccm is introduced into the furnace tube for second ozone purification at a temperature of 700℃ and a pressure of 300 mbar for 600 seconds.

[0069] (6) Post-oxidation: Post-oxidation is carried out at a temperature of 700℃ and a pressure of 300mbar; during the post-oxidation process, oxygen with a volume concentration of 66% and a volume flow rate of 1000sccm is introduced.

[0070] (7) Second vacuuming: Under the condition of 700℃, the furnace tube is subjected to a second vacuuming for 200s until the pressure of the furnace tube is 0;

[0071] (8) Back pressure: Back pressure is carried out at a temperature of 600℃ and a pressure of 1060mbar; during the back pressure process, nitrogen gas with a volume flow rate of 15000sccm is introduced.

[0072] (9) Cooling: The temperature of the furnace tube is controlled at 600℃ and the pressure is 1060mbar. During the cooling process, nitrogen gas with a volume flow rate of 15000sccm is introduced.

[0073] (10) Unloading the boat: Nitrogen gas with a volume flow rate of 15000 sccm is introduced into the furnace tube for 700 seconds. The quartz boat is then removed from the furnace tube and unloaded after cooling.

[0074] Example 2

[0075] This embodiment provides a method for improving white spot defects in silicon wafers during post-oxidation treatment, the method comprising the following steps:

[0076] (1) Boat entry: Control the temperature inside the furnace tube to 600℃, the pressure to 1060mbar, and introduce nitrogen gas with a volume flow rate of 15000sccm for 700s. Insert the silicon wafer into the quartz boat and send it into the furnace tube.

[0077] (2) First ozone purification: Ozone with a volume concentration of 50% and a volume flow rate of 12000 sccm is introduced into the furnace tube and the first ozone purification is carried out at a temperature of 730℃ and a pressure of 800 mbar for 1200s.

[0078] (3) Heating: The temperature inside the furnace tube is controlled at 750℃ and the pressure at 900mbar. During the heating process, nitrogen gas with a volume flow rate of 10000sccm is introduced.

[0079] (4) First vacuuming: Under the condition of a temperature of 750℃, the furnace tube is evacuated for the first time until the pressure of the furnace tube is 0;

[0080] (5) Second ozone purification: Ozone with a volume concentration of 50% and a volume flow rate of 1000 sccm is introduced into the furnace tube for second ozone purification at a temperature of 850℃ and a pressure of 200 mbar for 1200 s.

[0081] (6) Post-oxidation: Post-oxidation is carried out at a temperature of 850℃ and a pressure of 150mbar; during the post-oxidation process, oxygen with a volume concentration of 40% and a volume flow rate of 1500sccm is introduced.

[0082] (7) Second vacuuming: Under the condition of a temperature of 650℃, the furnace tube is subjected to a second vacuuming until the pressure of the furnace tube is 0;

[0083] (8) Back pressure: Back pressure is carried out at a temperature of 500℃ and 1100mbar; during the back pressure process, nitrogen gas with a volume flow rate of 17000sccm is introduced.

[0084] (9) Cooling: The temperature of the furnace tube is controlled at 400℃ and the pressure is 900mbar. During the cooling process, nitrogen gas with a volume flow rate of 10000sccm is introduced.

[0085] (10) Unloading the boat: Nitrogen gas with a volume flow rate of 15000 sccm is introduced into the furnace tube for 700 seconds. The quartz boat is then removed from the furnace tube and unloaded after cooling.

[0086] Example 3

[0087] This embodiment provides a method for improving white spot defects in silicon wafers during post-oxidation treatment, the method comprising the following steps:

[0088] (1) Boat entry: Control the temperature inside the furnace tube to 600℃, the pressure to 1060mbar, and introduce nitrogen gas with a volume flow rate of 15000sccm for 700s. Insert the silicon wafer into the quartz boat and send it into the furnace tube.

[0089] (2) First ozone purification: Ozone with a volume concentration of 70% and a volume flow rate of 10000 sccm is introduced into the furnace tube and the first ozone purification is carried out at a temperature of 760℃ and a pressure of 1100 mbar for 900 seconds.

[0090] (3) Heating: The temperature inside the furnace tube is controlled at 800℃ and the pressure is 1000mbar. During the heating process, nitrogen gas with a volume flow rate of 11000sccm is introduced.

[0091] (4) First vacuuming: Under the condition of a temperature of 720℃, the furnace tube is evacuated for the first time until the pressure of the furnace tube is 0;

[0092] (5) Second ozone purification: Ozone with a volume concentration of 40% and a volume flow rate of 2000 sccm is introduced into the furnace tube, and a second ozone purification is carried out at a temperature of 810℃ and a pressure of 400 mbar for 700 seconds.

[0093] (6) Post-oxidation: Post-oxidation is carried out at a temperature of 790℃ and a pressure of 200mbar; during the post-oxidation process, oxygen with a volume concentration of 80% and a volume flow rate of 1200sccm is introduced.

[0094] (7) Second vacuuming: Under the condition of a temperature of 600℃, the furnace tube is subjected to a second vacuuming until the pressure of the furnace tube is 0;

[0095] (8) Back pressure: Back pressure is carried out at a temperature of 580℃ and a pressure of 1080mbar; during the back pressure process, nitrogen gas with a volume flow rate of 17000sccm is introduced.

[0096] (9) Cooling: The temperature of the furnace tube is controlled at 580℃ and the pressure is 960mbar. During the cooling process, nitrogen gas with a volume flow rate of 14000sccm is introduced.

[0097] (10) Unloading the boat: Nitrogen gas with a volume flow rate of 15000 sccm is introduced into the furnace tube for 700 seconds. The quartz boat is then removed from the furnace tube and unloaded after cooling.

[0098] Example 4

[0099] This embodiment provides a method to improve the white spot defect of silicon wafers in the post-oxidation treatment. The method is the same as in Embodiment 1 except that the volume flow rate of ozone in the second ozone purification step (5) is replaced with 1000 sccm instead of 1500 sccm.

[0100] Example 5

[0101] This embodiment provides a method to improve the white spot defect of silicon wafers in the post-oxidation treatment. The method is the same as in Embodiment 1 except that the volume flow rate of ozone in the second ozone purification step (5) is replaced with 2000 sccm instead of 1500 sccm.

[0102] Example 6

[0103] This embodiment provides a method to improve the white spot defect of silicon wafers in post-oxidation treatment. The method is the same as in embodiment 1 except that the second ozone purification time of 600s in step (5) is replaced with 800s.

[0104] Example 7

[0105] This embodiment provides a method to improve the white spot defect of silicon wafers in post-oxidation treatment. The method is the same as that in embodiment 1, except that the second ozone purification time in step (5) is replaced with 1200s instead of 600s.

[0106] Example 8

[0107] This embodiment provides a method to improve the white spot defect of silicon wafers in post-oxidation treatment. The method is the same as in embodiment 1 except that the second ozone purification time of 600s in step (5) is replaced with 800s.

[0108] Example 9

[0109] This embodiment provides a method to improve the white spot defect of silicon wafers in post-oxidation treatment. The method is the same as in embodiment 1 except that the ozone purification time in step (2) is changed from 600s to 1200s.

[0110] Example 10

[0111] This embodiment provides a method to improve the white spot defect of silicon wafers in post-oxidation treatment. The method is the same as that in embodiment 7, except that the ozone purification time in step (2) is changed from 600s to 1200s.

[0112] Example 11

[0113] This embodiment provides a method for improving the white spot defect of silicon wafers in post-oxidation treatment. The method is the same as that in embodiment 7, except that the ozone purification time of 600s in step (2) is replaced with 800s.

[0114] Comparative Example 1

[0115] This comparative example provides a method for post-oxidation of crystalline silicon wafers, the method comprising the following steps:

[0116] (1) Preparatory stage: control the temperature inside the furnace tube to 600℃ and introduce nitrogen gas with a volume flow rate of 9000sccm for 15s;

[0117] (2) Boat entry: Control the temperature inside the furnace tube to 600℃, the pressure to 1060mbar, and introduce nitrogen gas with a volume flow rate of 15000sccm for 700s. Insert the silicon wafer into the quartz boat and send it into the furnace tube.

[0118] (3) Vacuuming: Under the condition of 700℃, the furnace tube is evacuated for 200s until the pressure of the furnace tube is 0;

[0119] (4) Purging: At a temperature of 700℃, nitrogen gas with a volume flow rate of 9000 sccm is introduced for 300s for purging.

[0120] (5) Heating: The temperature inside the furnace tube is controlled at 700℃ and the pressure is 1060mbar. During the heating process, nitrogen gas with a volume flow rate of 15000sccm is introduced for 500s.

[0121] (6) Post-oxidation: Post-oxidation is carried out at a temperature of 700℃ and a pressure of 300mbar; during the post-oxidation process, oxygen with a volume concentration of 66% and a volume flow rate of 1000sccm is introduced.

[0122] (7) Unloading the boat: Nitrogen gas with a volume flow rate of 15000 sccm is introduced into the furnace tube for 700 seconds. The quartz boat is then removed from the furnace tube and unloaded after cooling.

[0123] The silicon wafers obtained in the above embodiments and comparative examples were subjected to performance tests.

[0124] The results of electrical performance parameters and the proportion of white spot defects are shown in Table 1.

[0125] Table 1

[0126]

[0127]

[0128] As can be seen from Table 1:

[0129] In Examples 1-11, the proportion of white spot defects in silicon wafers obtained by using the method provided by the present invention to improve the white spot defect rate in the post-oxidation treatment of silicon wafers can reach less than 0.92%, and under better conditions, the proportion of white spot defects can reach less than 0.35%. Moreover, the method has little impact on the electrical performance of the silicon wafers. The open-circuit voltage, short-circuit current, series resistance, parallel resistance and fill factor are basically equivalent to those of Comparative Example 1, and the conversion efficiency of the silicon wafers is slightly improved.

[0130] A comparison of Examples 7 and Examples 10-11 shows that the first and second ozone purification times in Example 10 were both longer, and the white spot defect rate of the resulting silicon wafer was only 0.35%.

[0131] The crystalline silicon wafers obtained in the above embodiments and comparative examples were subjected to electrical performance tests. At the same time, the passivated emitter and the solar cell (PERC cell) with rear contact obtained by electrical injection were subjected to photothermal induced degradation (LeTID) tests. The LeTID test conditions were: temperature 110℃, current 0.5A, time 8h. The results are shown in Table 2.

[0132] Table 2

[0133] LeTID test value Example 1 0.82％ Example 2 0.53％ Example 3 0.72％ Example 4 0.57％ Example 5 0.65％ Example 6 0.63％ Example 7 0.56％ Example 8 0.68％ Example 9 0.72％ Example 10 0.75％ Example 11 0.63％ Comparative Example 1 0.86％

[0134] The silicon wafers obtained in the above embodiments and comparative examples were subjected to module potential-induced degradation (PID) testing. PID test conditions: 85%RH, 85℃, 96h, -1500V, electrical performance degradation ≤3%, 1000W / m 2 The results are shown in Table 3.

[0135] Table 3

[0136]

[0137]

[0138] The results in Tables 2 and 3 show that:

[0139] The LeTID test value of the silicon wafers obtained in Examples 1 to 11 is ≤1, which is considered qualified, and the PID test value meets the quality requirements.

[0140] In summary, the method for improving white spot defects in silicon wafers during post-oxidation treatment provided by this invention results in silicon wafers with a low proportion of white spot defects and excellent electrical properties. The method is simple to operate, has low equipment requirements, and is suitable for large-scale industrial application.

[0141] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for improving white spot defects in silicon wafers during post-oxidation treatment, characterized in that, The method includes the following steps: (1) Entering the boat; (2) First ozone purification; (3) Heating; (4) First vacuuming; (5) Second ozone purification; (6) Post-oxidation; (7) Second vacuuming; (8) Back pressure; (9) Cooling; (10) Exiting the boat; Step (2) The volume concentration of ozone in the first ozone purification is 50%~70%; the temperature of the first ozone purification is 700~760℃, the pressure is 800~1100mbar, and the time is 600~1200s; the volume flow rate of ozone in the first ozone purification is 10000~15000sccm. Step (5) The volume concentration of ozone in the second ozone purification is 40%~50%; the temperature of the second ozone purification is 700~850℃, the pressure is 200~400mbar, and the time is 600~1200s; the volume flow rate of ozone in the second ozone purification is 1000~2000sccm.

2. The method according to claim 1, characterized in that, Step (1) involves inserting the silicon wafer into the quartz boat and feeding it into the furnace tube.

3. The method according to claim 1, characterized in that, The temperature for heating in step (3) is 700~800℃.

4. The method according to claim 1, characterized in that, The pressure for heating is 900~1060 mbar.

5. The method according to claim 1, characterized in that, During the heating process, nitrogen gas with a volumetric flow rate of 10,000 to 15,000 sccm is introduced.

6. The method according to claim 1, characterized in that, Step (4) The temperature of the first vacuuming is 700~750℃.

7. The method according to claim 1, characterized in that, The pressure after the first vacuum is 0.

8. The method according to claim 1, characterized in that, The post-oxidation temperature in step (6) is 700~850℃.

9. The method according to claim 1, characterized in that, The post-oxidation pressure is 150~300 mbar.

10. The method according to claim 1, characterized in that, During the post-oxidation process, oxygen is introduced at a volumetric flow rate of 1000~1500 sccm.

11. The method according to claim 10, characterized in that, The volume concentration of oxygen is 40% to 80%.

12. The method according to claim 1, characterized in that, Step (7) The temperature of the second vacuuming is 600~700℃.

13. The method according to claim 1, characterized in that, The pressure after the second vacuum is 0.

14. The method according to claim 1, characterized in that, The temperature of the back pressure in step (8) is 500~600℃.

15. The method according to claim 1, characterized in that, The back pressure is 1060~1100 mbar.

16. The method according to claim 1, characterized in that, During the back pressure process, nitrogen gas with a volumetric flow rate of 11,000 to 17,000 sccm is introduced.

17. The method according to claim 1, characterized in that, The temperature for cooling in step (9) is 400~600℃.

18. The method according to claim 1, characterized in that, The cooling pressure is 900~1060 mbar.

19. The method according to claim 1, characterized in that, During the cooling process, nitrogen gas with a volumetric flow rate of 10,000 to 15,000 sccm is introduced.

20. The method according to claim 1, characterized in that, Step (10) involves removing the quartz boat from the furnace tube, cooling it, and then unloading the silicon wafer.

21. The method according to claim 1, characterized in that, The method includes the following steps: (1) Boat loading: Insert the silicon wafer into the quartz boat and send it into the furnace tube; (2) First ozone purification: Ozone with a volume concentration of 50%~70% and a volume flow rate of 10000~15000sccm is introduced into the furnace tube, and the first ozone purification is carried out at a temperature of 700~760℃ and a pressure of 800~1100mbar for 600~1200s. (3) Heating: The temperature inside the furnace tube is controlled at 700~800℃ and the pressure is 900~1060mbar. During the heating process, nitrogen gas with a volume flow rate of 10000~15000sccm is introduced. (4) First vacuuming: Under the condition of a temperature of 700~750℃, the furnace tube is evacuated for the first time until the pressure of the furnace tube is 0; (5) Second ozone purification: Introduce ozone with a volume concentration of 40%~50% and a volume flow rate of 1000~2000 sccm into the furnace tube and perform second ozone purification at a temperature of 700~850℃ and a pressure of 200~400 mbar for 600~1200s. (6) Post-oxidation: Post-oxidation is carried out at a temperature of 700~850℃ and a pressure of 150~300mbar; during the post-oxidation process, oxygen with a volume concentration of 40%~80% and a volume flow rate of 1000~1500sccm is introduced. (7) Second vacuuming: Under the condition of a temperature of 600~700℃, the furnace tube is subjected to a second vacuuming until the pressure of the furnace tube is 0; (8) Back pressure: Back pressure is carried out at a temperature of 500~600℃ and 1060~1100mbar; during the back pressure process, nitrogen gas with a volume flow rate of 11000~17000sccm is introduced; (9) Cooling: The temperature of the furnace tube is controlled at 400~600℃ and the pressure is controlled at 900~1060mbar. During the cooling process, nitrogen gas with a volume flow rate of 10000~15000sccm is introduced. (10) Unloading the boat: Remove the quartz boat from the furnace tube, cool it, and then unload the silicon wafer.

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