Boat drying process for improving EL black edge of graphite boat

Abstract

The invention discloses a boat drying process for improving EL black edges of a graphite boat, which belongs to the technical field of silicon solar cell manufacturing and comprises the following steps of: emptying an internal silicon wafer of the graphite boat of which the downtime is more than or equal to 12h; the no-load graphite boat is placed in a tubular PECVD equipment furnace tube, and nitrogen is introduced for drying; introducing silane and ammonia gas into the baked furnace tube, and plating a silicon nitride layer on the surface of the graphite boat; and after deposition is completed, subsequent coating, printing, testing and packaging procedures are continued. According to the method, the graphite boat which is crashed for more than 12 hours is emptied and is subjected to drying and film coating continuous treatment, so that the surface pollution or oxidation of the graphite boat caused by long-term shutdown is avoided, the surface microdefects of the graphite boat are repaired through a passivation effect, a black edge is prevented from forming a root, and the stability after the process is restarted is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of silicon solar cell manufacturing, and particularly relates to a baking process for improving the black edge of a graphite boat EL. Background Art

[0002] In the production of crystalline silicon cells, plasma-enhanced chemical vapor deposition (PECVD) technology deposits a blue silicon nitride film on the surface of silicon wafers, achieving multiple optimization benefits: reducing light reflection, suppressing leakage, and increasing open-circuit voltage and short-circuit current. As industry requirements for cell conversion efficiency and color uniformity increase, the physical condition of the graphite boat, the carrier used for silicon wafer coating, has a crucial impact on the quality of the silicon nitride film. While graphite's unique layered structure of carbon atoms provides excellent thermal conductivity and chemical stability, its porous nature also leads to strong adsorption. At the production site, when equipment is idle for extended periods due to troubleshooting, planned maintenance, or when the graphite boat is offline for repair, the boat continuously absorbs moisture and dust particles from the environment. This adsorption contamination directly leads to abnormalities in the loaded silicon wafers after coating: after the screen printing process, electroluminescence (EL) images show dark spots corresponding to the contact surface with the graphite boat, seriously compromising the cell's appearance consistency.

[0003] To address the inevitable equipment downtime in TOPCon battery production, the existing restart process faces a technical bottleneck: when the machine is restored, operators directly transfer the graphite boat carrying the silicon wafers into the PECVD furnace tube via a transfer paddle, where they are baked at the process temperature. However, this treatment only removes moisture from the surface of the graphite boat and lacks the ability to remove dust particles embedded in the material's pores. Actual production data shows that the defective rate of EL edge blackening caused by dust contamination has remained above 0.5% since the machine was restarted, becoming a key bottleneck restricting product yield and production costs.

[0004] The prior art discloses patents for some fiber thermoplastic composite board production equipment, among which the invention patent with publication number 110449409A discloses a treatment process to avoid graphite boat printing in the PECVD process, comprising the following steps: step 1, transporting the graphite boat that has been stored for more than 4 hours to a tubular PECVD machine; step 2, transporting the graphite boat to the furnace tube through automated equipment, loading the furnace tube with a boat washing process and running the boat washing process, the boat washing process steps are: start, boat entry, vacuuming, purging, vacuuming, leak detection, vacuuming, constant temperature, constant pressure, deposition one, vacuuming, purging, deposition two, vacuuming, purging, vacuuming, nitrogen filling, boat removal, and end; step 3, removing the graphite boat after the boat washing process from the furnace tube, loading silicon wafers, and running the coating process. Although the problem of EL graphite boat marks caused by silicon wafer contamination after coating on the graphite boat loaded with silicon wafers has been solved, there is still a problem of EL black edges of more than 0.5% caused by dust contamination of silicon wafers after the machine is restarted, which affects the overall yield and cost. Summary of the Invention

[0005] To overcome the problem in the prior art of TOPCon battery preparation that, in the coating section, the graphite boat becomes damp due to long machine restarts, dust is adsorbed on the surface, and silicon wafers are contaminated, resulting in batches of EL black edges, the present invention provides a baking process for improving the EL black edges of the graphite boat, comprising the following steps:

[0006] S1. Empty the silicon wafers inside the graphite boat if the downtime is ≥12h;

[0007] S2. Place the empty graphite boat in the furnace tube of the tubular PECVD equipment and introduce nitrogen, followed by drying;

[0008] S3, introducing silane and ammonia into the furnace tube after baking to coat a silicon nitride layer on the surface of the graphite boat;

[0009] S4. After the deposition is completed, the subsequent coating, printing, testing and packaging processes are continued.

[0010] Furthermore, in step S2, the temperature in the furnace tube is 450° C. to 500° C., the drying time is 1000 to 1500 s, the flow rate of the nitrogen is 8000 to 12000 sccm / min, and the pressure during ventilation is set to 180 to 220 mbar.

[0011] Furthermore, in step S3, the temperature in the furnace tube is maintained at 450° C. to 500° C., the power is 7500 to 8500 W, and the pressure during ventilation is set to 180 to 220 mbar.

[0012] Furthermore, in step S3, the silane flow rate is 2000-2800 sccm / min, and the ammonia flow rate is 8000-12000 sccm / min.

[0013] Furthermore, the reaction time in step S3 is determined according to the formula t=700×(d / 100), where the unit of t is s, d is the target thickness of the silicon nitride layer, and d∈[50, 300] is in nm.

[0014] Furthermore, in step S3, the deposition rate of the silicon nitride layer is 0.12-0.16 nm in thickness per 100 s.

[0015] Furthermore, in step S3, the refractive index of the silicon nitride layer is 1.95-2.05, and the film stress is ≤50 MPa.

[0016] Furthermore, the boat clamping device in step S1 clamps the graphite boat through a double-cylinder symmetrical clamping structure, and the clamping force is 50N~200N; the manipulator is equipped with a visual positioning system and a control module with a row-by-row sheet picking accuracy of ≤0.5mm.

[0017] Furthermore, the process can be extended to be applied to crystalline silicon cell production lines such as PERC and HJT, and the graphite boat is continuously processed in the furnace tube during the implementation of the process, without the need for tube cooling between the baking and coating processes.

[0018] The beneficial effects produced by the technical solution of the present invention are as follows:

[0019] (1) The present invention avoids surface contamination or oxidation of the graphite boat caused by long-term shutdown by emptying the graphite boat that has been shut down for more than 12 hours and performing continuous processing such as drying and coating it with a silicon nitride layer for isolation. The silicon nitride layer repairs microscopic defects on the surface of the graphite boat through the passivation effect, blocks the black edge formation path, reduces the root cause of EL black edge formation, and improves the stability after the process is restarted.

[0020] (2) High-temperature baking combined with a silicon nitride coating composite process and specific parameter combinations significantly improves yield by fundamentally isolating dust pollution sources. Through systematic parameter optimization and equipment innovation, the EL black edge problem caused by long-term graphite boat downtime pollution and uneven coating is solved, while taking into account efficiency, cost and compatibility, and has significant industrial value.

[0021] (3) The process temperature of the baking boat is between 450℃ and 500℃, and the reaction temperature can also be set at 450℃ to 500℃. After the baking boat is completed, there is no need to remove the tube, and the silicon nitride deposition process can continue, shortening the restart time. The production line has small movements and can be operated by one person. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Therefore, the following detailed description of the embodiments of the present invention provided in the scheme is not intended to limit the scope of the claimed invention, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0023] This embodiment avoids surface contamination or oxidation of the graphite boat caused by long-term downtime by clearing the graphite boat that has been down for more than 12 hours and performing a continuous drying-coating process. The silicon nitride layer repairs microscopic defects on the graphite boat surface through the passivation effect, blocks the black edge formation path, reduces the root cause of EL black edge formation, and improves stability after the process is restarted. The specific implementation method is as follows:

[0024] A baking process for improving the black edge of a graphite boat EL, comprising the following steps:

[0025] S1. The graphite boat with downtime ≥12h is transferred to the boat slot of the wafer inserter through the transport platform, fixed by the boat clamping device, and then emptied of the silicon wafers inside row by row by the robot;

[0026] S2. Place the empty graphite boat in the furnace tube of the tubular PECVD equipment and introduce nitrogen gas for drying;

[0027] S3, introducing silane and ammonia into the furnace tube after baking to coat a silicon nitride layer on the surface of the graphite boat;

[0028] S4. After the deposition is completed, the subsequent coating, printing, testing and packaging processes are continued.

[0029] A complete graphite boat regeneration process framework has been established. Through a systematic process involving emptying, drying, coating, and continuous production, the EL black edge problem caused by surface contamination during long periods of downtime has been resolved. Particular emphasis has been placed on scenarios where downtime of 12 hours or longer has been applied, defining process trigger conditions. Furthermore, positioning on the transport platform and robotic clearing ensure processing accuracy, laying the foundation for subsequent processes.

[0030] As a preferred embodiment, the temperature in the furnace tube in step S2 is 450°C~500°C, the drying time is 1000~1500s, the flow rate of the nitrogen is 8000~12000sccm / min, and the pressure during ventilation is set to 180~220mbar.

[0031] Here, precise control of nitrogen purge parameters in a high-temperature environment enables efficient dehumidification and removal of volatile organic compounds. High nitrogen flow creates a turbulent effect, improving cleaning efficiency.

[0032] As a preferred embodiment, in step S3, the temperature in the furnace tube is maintained at 450°C to 500°C, the power is 7500 to 8500W, and the pressure during ventilation is set to 180 to 220 mbar.

[0033] Here, by coordinating the temperature-power-pressure control (7500~8500W RF power combined with 180~220mbar low pressure), the plasma density can be enhanced while maintaining thermal stability, thereby improving the SiH4 / NH3 reaction efficiency.

[0034] As a preferred embodiment, in step S3, the silane flow rate is 2000-2800 sccm / min, and the ammonia flow rate is 8000-12000 sccm / min.

[0035] Here, the classic ratio of NH3 / SiH4=4:1 is set to ensure sufficient N source supply while avoiding the generation of powder caused by excessive SiH4.

[0036] As a preferred embodiment, the reaction time in step S3 is determined according to the formula t=700×(d / 100), where t is in s, d is the target thickness of the silicon nitride layer, and d∈[50,300] is in nm.

[0037] Here, a linear mathematical model is established to directly relate the process time to the film thickness. By limiting the thickness of the silicon nitride layer to control the time range, different process requirements can be met while avoiding stress accumulation caused by excessive thickness. The target thickness of the silicon nitride layer is preferably 100±10nm. At this thickness, the refractive index is optimally matched to the reflectivity of the crystalline silicon surface, which can reduce the interface recombination rate to <100cm / s while maintaining the film stress within a safe threshold.

[0038] As a preferred embodiment, the deposition rate of the silicon nitride layer in step S3 is 0.12-0.16 nm in thickness per 100 s.

[0039] Here, quantifying the deposition rate provides a direct basis for process control. At this rate, 6 to 8 boats can be processed per hour, balancing production efficiency and film quality. By inverting the rate, it can be verified that the plasma density of the equipment is 1×10 10 cm -3 Magnitude

[0040] As a preferred embodiment, the refractive index of the silicon nitride layer in step S3 is 1.95-2.05, and the film stress is ≤50 MPa.

[0041] As a preferred embodiment, the boat clamping device in step S1 clamps the graphite boat through a double-cylinder symmetrical clamping structure, and the clamping force is 50N~200N; the manipulator is equipped with a visual positioning system and a control module with a row-by-row sheet picking accuracy of ≤0.5mm.

[0042] As a preferred embodiment, the process can be extended to apply to PERC, HJT and other crystalline silicon cell production lines, and the graphite boat is kept continuously processed in the furnace tube during the implementation of the process, and no cooling is required between the baking and coating processes.

[0043] Example 1

[0044] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0045] S1. After a long downtime of 12 hours, the graphite boat is transferred to the boat slot of the wafer inserter through the transport platform, the boat clamping device is ventilated, the cylinder contracts to clamp the graphite boat, and the robot inputs the wafer removal and no wafer insertion signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the silicon wafer rework box through the belt for rework.

[0046] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device. The temperature in the furnace tube is set to 450°C. Nitrogen is introduced at a pressure of 200 mbar and a nitrogen flow rate of 10,000 sccm / min for 1,200 seconds to continuously dry the surface moisture.

[0047] S3. After baking, silane and ammonia are introduced into the furnace tube at a pressure of 200 mbar. The temperature in the furnace tube is maintained at 450°C, and the flow rates of silane and ammonia are 2400 sccm / min and 10,000 sccm / min, respectively. A high-frequency reaction power of 8000 W is set to deposit a silicon nitride layer on the surface of the graphite boat. Under the high-frequency power, an electric current is generated on the surface of the graphite boat. Under the glow reaction, the silane and ammonia are ionized into high-energy radicals mixed with molecules and ions. The high-energy radicals undergo a chemical reaction on the surface of the graphite boat, and the silicon covalently bonds with the silicon nitride atoms to form silicon nitride. The excess H element is released as hydrogen. The reaction time is set to 700 s, and a 100 nm thick silicon nitride film is deposited on the surface of the graphite boat. The deposition rate of the silicon nitride layer is 0.12 nm per 100 s. The refractive index of the silicon nitride layer is 1.95, and the film stress is ≤50 MPa.

[0048] S4. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0049] Example 2

[0050] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0051] S1. After a long downtime of 13 hours, the graphite boat is transferred to the boat slot of the wafer inserter through the transport platform, the boat clamping device is ventilated, the cylinder contracts to clamp the graphite boat, and the robot inputs the wafer removal and no-wafer input signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the silicon wafer rework box for rework via a belt.

[0052] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device. The temperature in the furnace tube is set to 500°C. Nitrogen is introduced at a pressure of 180 mbar and a nitrogen flow rate of 8000 sccm / min. The graphite boat is dried for 1000 seconds to continuously dry the surface moisture.

[0053] S3. After baking, silane and ammonia are introduced into the furnace tube at a pressure of 200 mbar. The temperature in the furnace tube is maintained at 500°C, and the flow rates of silane and ammonia are 2000 sccm / min and 8000 sccm / min, respectively. A high-frequency reaction power of 8500 W is set to deposit a silicon nitride layer on the surface of the graphite boat. Under the high-frequency power, an electric current is generated on the surface of the graphite boat. Under the glow reaction, the silane and ammonia are ionized into high-energy radicals mixed with molecules and ions. The high-energy radicals undergo a chemical reaction on the surface of the graphite boat, and the silicon atoms covalently bond to form silicon nitride. The excess hydrogen element is released as hydrogen. The reaction time is set to 350 seconds, and a 50nm thick silicon nitride film is deposited on the surface of the graphite boat. The deposition rate of the silicon nitride layer is 0.13nm per 100 seconds. The refractive index of the silicon nitride layer is 2.05, and the film stress is ≤50MPa.

[0054] S4. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0055] Example 3

[0056] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0057] S1. After a long downtime of 12 hours, the graphite boat is transferred to the boat slot of the wafer inserter through the transport platform, the boat clamping device is ventilated, the cylinder contracts to clamp the graphite boat, and the robot inputs the wafer removal and no wafer insertion signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the silicon wafer rework box through the belt for rework.

[0058] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device. The temperature in the furnace tube is set to 490°C, and nitrogen is used at a pressure of 200 mbar and a nitrogen flow rate of 12000 sccm / min. The time is set to 1500 s to continuously dry the moisture on the surface.

[0059] S3. After baking, silane and ammonia are introduced into the furnace tube at a pressure of 220 mbar. The temperature in the furnace tube is maintained at 490°C, and the flow rates of silane and ammonia are 2500 sccm / min and 11000 sccm / min, respectively. A high-frequency reaction power of 8000 W is set to deposit a silicon nitride layer on the surface of the graphite boat. Under the high-frequency power, an electric current is generated on the surface of the graphite boat. Under the glow reaction, the silane and ammonia are ionized into high-energy radicals mixed with molecules and ions. The high-energy radicals undergo a chemical reaction on the surface of the graphite boat, and the silicon covalently bonds with the silicon nitride atoms to form silicon nitride. The excess H element is released as hydrogen. The reaction time is set to 1400 s, and a 200 nm thick silicon nitride film is deposited on the surface of the graphite boat. The deposition rate of the silicon nitride layer is 0.14 nm per 100 s. The refractive index of the silicon nitride layer is 2.05, and the film stress is ≤50 MPa.

[0060] S4. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0061] Example 4

[0062] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0063] S1. After a long downtime of 12 hours, the graphite boat is transferred to the boat slot of the wafer inserter through the transport platform, the boat clamping device is ventilated, the cylinder contracts to clamp the graphite boat, and the robot inputs the wafer removal and no wafer insertion signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the silicon wafer rework box through the belt for rework.

[0064] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device. The temperature in the furnace tube is set to 460°C. Nitrogen is introduced at a pressure of 200 mbar and a nitrogen flow rate of 11,000 sccm / min. The drying time is set to 1,300 s to continuously dry the surface moisture.

[0065] S3. After baking, silane and ammonia are introduced into the furnace tube at a pressure of 180 mbar. The temperature in the furnace tube is maintained at 460°C, and the flow rates of silane and ammonia are 2400 sccm / min and 10,000 sccm / min, respectively. A high-frequency reaction power of 8000 W is set to deposit a silicon nitride layer on the surface of the graphite boat. Under the high-frequency power, an electric current is generated on the surface of the graphite boat. Under the glow reaction, the silane and ammonia are ionized into high-energy radicals mixed with molecules and ions. The high-energy radicals undergo a chemical reaction on the surface of the graphite boat, and the silicon covalently bonds with the silicon nitride atoms to form silicon nitride. The excess H element is released as hydrogen. The reaction time is set to 1050 s, and a 150 nm thick silicon nitride film is deposited on the surface of the graphite boat. The deposition rate of the silicon nitride layer is 0.16 nm per 100 s. The refractive index of the silicon nitride layer is 1.95, and the film stress is ≤50 MPa.

[0066] S4. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0067] Example 5

[0068] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0069] S1. After a long downtime of 20 hours, the graphite boat is transferred to the boat slot of the wafer inserter through the transport platform, the boat clamping device is ventilated, the cylinder contracts to clamp the graphite boat, and the robot inputs the wafer removal and no wafer insertion signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the silicon wafer rework box through the belt for rework.

[0070] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device. The temperature in the furnace tube is set to 450°C. Nitrogen is introduced at a pressure of 200 mbar and a nitrogen flow rate of 10,000 sccm / min for 1,200 seconds to continuously dry the surface moisture.

[0071] S3. After baking, silane and ammonia are introduced into the furnace tube at a pressure of 200 mbar. The temperature within the furnace tube is maintained at 450°C, with silane and ammonia flow rates of 2400 sccm / min and 10,000 sccm / min, respectively. A high-frequency reaction power of 8000 W is set to deposit a silicon nitride layer on the surface of the graphite boat. Under the high-frequency power, an electric current is generated on the surface of the graphite boat. The glow reaction ionizes the silane and ammonia into high-energy radicals, a mixture of molecules and ions. These high-energy radicals chemically react on the surface of the graphite boat, covalently bonding silicon and silicon nitride atoms to form silicon nitride. Excess hydrogen is released as hydrogen. The reaction time is set to 2100 seconds, resulting in a 300 nm thick silicon nitride film deposited on the surface of the graphite boat. The deposition rate of the silicon nitride layer is 0.15 nm per 100 seconds. The refractive index of the silicon nitride layer is 1.95, and the film stress is ≤50 MPa.

[0072] S4. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0073] Example 6

[0074] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0075] S1. After a long downtime of 24 hours, the graphite boat is transferred to the boat slot of the wafer inserter through the transport platform, the boat clamping device is ventilated, the cylinder contracts to clamp the graphite boat, and the robot inputs the wafer removal and no wafer insertion signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the silicon wafer rework box for rework via a belt.

[0076] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device. The temperature in the furnace tube is set to 470°C. Nitrogen is introduced at a pressure of 200 mbar and a nitrogen flow rate of 10,000 sccm / min for 1,200 seconds to continuously dry the surface moisture.

[0077] S3. After baking, silane and ammonia are introduced into the furnace tube at a pressure of 200 mbar. The temperature in the furnace tube is maintained at 470°C, and the flow rates of silane and ammonia are 2400 sccm / min and 10,000 sccm / min, respectively. A high-frequency reaction power of 8000 W is set to deposit a silicon nitride layer on the surface of the graphite boat. Under the high-frequency power, an electric current is generated on the surface of the graphite boat. Under the glow reaction, the silane and ammonia are ionized into high-energy radicals mixed with molecules and ions. The high-energy radicals undergo a chemical reaction on the surface of the graphite boat, and the silicon atoms covalently bond to form silicon nitride. The excess H element is released as hydrogen. The reaction time is set to 1750 s, and a 250 nm thick silicon nitride film is deposited on the surface of the graphite boat. The deposition rate of the silicon nitride layer is 0.16 nm per 100 s. The refractive index of the silicon nitride layer is 1.95, and the film stress is ≤50 MPa.

[0078] S4. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0079] Comparative Example 1

[0080] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0081] S1. After a long downtime of ≥12h, the graphite boat is transferred to the boat slot of the wafer inserter via the transport platform. The boat clamping device is ventilated, the cylinder contracts, and the graphite boat is clamped. The robot inputs the wafer removal and no-wafer input signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the wafer rework box via a belt for rework.

[0082] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device, and nitrogen gas is introduced into the furnace tube at 10,000 sccm / min with the pressure set to 200 mbar.

[0083] S3: Silane and ammonia are introduced into the furnace tube at a pressure of 200 mbar, with flow rates of 2400 sccm / min and 10,000 sccm / min, respectively. A high-frequency power of 8000 W is set for the reaction. Under the high-frequency power, a current is generated on the surface of the graphite boat. The glow reaction ionizes the silane and ammonia into high-energy radicals, a mixture of molecules and ions. These radicals chemically react on the surface of the graphite boat, covalently bonding silicon and silicon nitride atoms to form silicon nitride. Excess hydrogen is released as hydrogen. The reaction time is set to 700 s, depositing a 100 nm thick silicon nitride film on the surface of the graphite boat. The deposition rate of the silicon nitride layer is 0.14 nm per 100 s. The refractive index of the silicon nitride layer is 1.95-2.05, and the film stress is ≤50 MPa.

[0084] S4. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0085] Comparative Example 2

[0086] A solar cell prepared by a baking boat process for improving the black edge of a graphite boat EL comprises the following steps:

[0087] S1. After a long downtime of ≥12h, the graphite boat is transferred to the boat slot of the wafer inserter via the transport platform. The boat clamping device is ventilated, the cylinder contracts, and the graphite boat is clamped. The robot inputs the wafer removal and no-wafer input signals. After receiving the signals, the robot empties the silicon wafers in the graphite boat row by row and transfers them to the wafer rework box via a belt for rework.

[0088] S2. The empty graphite boat is placed in the furnace tube of the tubular PECVD equipment through the transfer device. The temperature in the furnace tube is set to 450°C. Nitrogen is introduced into the furnace tube at a pressure of 200 mbar and a nitrogen flow rate of 10,000 sccm / min. The drying time is set to 1,200 s to continuously dry the surface moisture.

[0089] S3. After completing the above operations, the graphite boat insert completes the remaining coating, printing, testing, and packaging processes.

[0090] The performance of the cells obtained by the above-mentioned examples 1 to 6 and comparative examples 1 to 2 was tested, and the results are as follows:

[0091] Table 1 Test results of cells prepared in various embodiments and comparative examples

[0092] project Baking boat temperature (℃) Thickness of silicon nitride film (nm) EL black border ratio (%) Heterogeneous color ratio (%) Cycle life (times) Example 1 450 100 0.0012 0.0037 87 Example 2 500 50 0.002 0.003 80 Example 3 490 200 0.0017 0.0049 85 Example 4 460 150 0.0017 0.0036 84 Example 5 450 300 0.0017 0.0029 80 Example 6 470 250 0.0018 0.0028 84 Comparative Example 1 0 100 0.0337 0.0059 75 Comparative Example 2 450 0 0.0278 0.0052 76

[0093] As shown in Table 1 above, when a baking boat is combined with a silicon nitride film coating on the graphite boat surface, the EL black edge, color shift ratio, and cycle life of the graphite boat are all significantly improved compared to the prior art. These improvements are achieved when the silicon nitride film thickness is 100 nm. In the embodiment, a high-temperature baking boat process combined with an isolation layer coating on the graphite boat surface fundamentally avoids contamination or oxidation of the graphite boat surface, thereby resolving the EL black edge issue on the silicon wafer. In the comparative example, only the high-temperature baking boat process or coating process was performed on the graphite boat surface. As shown in Table 1, the EL black edge ratio is still significantly higher, not as good as that achieved in the embodiment, i.e., this solution.

[0094] The present invention is further described above with the aid of specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the essence and scope of the present invention. Various modifications made to the above embodiments by ordinary technicians in this field after reading this specification are all within the scope of protection of the present invention.

Claims

1. A baking process for improving the black edge of graphite boat EL, characterized in that: The following steps are involved: S1. Empty the silicon wafers inside the graphite boat if the downtime is ≥12h; S2. Place the empty graphite boat in the furnace tube of the tubular PECVD equipment and introduce nitrogen, followed by drying; S3, introducing silane and ammonia into the furnace tube after baking to coat a silicon nitride layer on the surface of the graphite boat; S4. After the deposition is completed, the subsequent coating, printing, testing and packaging processes are continued.

2. The drying boat process according to claim 1, characterized in that: In step S2, the temperature in the furnace tube is 450° C. to 500° C., the drying time is 1000 to 1500 seconds, the flow rate of the nitrogen is 8000 to 12000 sccm / min, and the pressure during ventilation is set to 180 to 220 mbar.

3. The drying boat process according to claim 1, characterized in that: In step S3 , the temperature in the furnace tube is maintained at 450° C. to 500° C., the power is 7500 to 8500 W, and the pressure during ventilation is set to 180 to 220 mbar.

4. The drying boat process according to claim 1, characterized in that: In step S3, the silane flow rate is 2000-2800 sccm / min, and the ammonia flow rate is 8000-12000 sccm / min.

5. The drying boat process according to claim 1, characterized in that: The reaction time in step S3 is determined according to the formula t=700×(d / 100), where t is in s, d is the target thickness of the silicon nitride layer, and d∈[50, 300] is in nm.

6. The drying boat process according to claim 1, characterized in that: The deposition rate of the silicon nitride layer in step S3 is 0.12-0.16 nm in thickness per 100 s.

7. The drying boat process according to claim 1, characterized in that: The refractive index of the silicon nitride layer in step S3 is 1.95-2.05, and the film stress is ≤50 MPa.

8. The drying boat process according to claim 1, characterized in that: In step S1, the boat clamping device clamps the graphite boat through a double-cylinder symmetrical clamping structure with a clamping force of 50N~200N; the manipulator is equipped with a visual positioning system and a control module with a row-by-row sheet picking accuracy of ≤0.5mm.

9. The drying boat process according to any one of claims 1 to 8, characterized in that: The process can be extended to be applied to crystalline silicon cell production lines such as PERC and HJT, and the graphite boat is continuously processed in the furnace tube during the implementation of the process, without the need for tube cooling between the baking and coating processes.