A boron diffusion process

Abstract

This invention discloses a boron diffusion process, comprising the following steps: S1: placing a small boat containing a silicon wafer into a furnace tube; S2: evacuating the furnace tube; S3: leak detection; S4: introducing nitrogen and oxygen for pre-oxidation; S5: introducing nitrogen, oxygen, and a diffusion boron source for diffusion; S6: introducing nitrogen for heating diffusion; S7: introducing nitrogen, oxygen, and a diffusion boron source under constant temperature conditions; S8: introducing nitrogen and oxygen, heating for aerobic propulsion; S9: introducing oxygen, continuing heating for post-oxidation; S10: introducing mixed oxidation with large and small oxygen molecules; S11: introducing nitrogen and large oxygen molecules and cooling; S12: breaking the vacuum to atmospheric pressure; S13: removing the boat. This invention has advantages such as simple principle, low process temperature, and low BRL layer thickness, significantly shortening process time and increasing production capacity.

Description

Technical Field

[0001] This invention belongs to the field of solar cell fabrication technology, specifically relating to a boron diffusion process. Background Technology

[0002] Currently, in boron diffusion processes, due to the relatively higher diffusion coefficient of boron compared to phosphorus diffusion, and the high melting temperature of boron and the reaction temperature of boron oxide, high-temperature propagation diffusion (900–990°C) is required during the diffusion process. This results in a large number of BRL layers on the silicon wafer surface. The formation of the BRL layer significantly increases the recombination rate on the silicon wafer surface, leading to a decrease in the photoelectric conversion efficiency of the resulting solar cells. To eliminate the BRL layer, existing technologies, after propagation diffusion, continue heating to 1040–1050°C and maintain this high temperature for 30–60 minutes, continuously introducing a large flow of oxygen to remove the BRL layer and achieve gettering. This ensures that the solar cells made from boron-diffused silicon wafers meet industry standards for conversion efficiency. The reason for using high-temperature, long-duration oxidation is primarily to completely remove the BRL layer, and the oxidation rate is directly related to temperature; the higher the temperature, the faster the oxidation rate. It also has a certain promoting effect on the depth of diffusion-doped junctions. Prolonged high temperatures can significantly increase the cooling time after high-temperature oxidation in the process, and also have an adverse effect on the service life of the furnace door seals and production capacity. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a boron diffusion process that is simple in principle, has a low process temperature, a short process time and a low BRL layer thickness.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] A boron diffusion process includes the following steps:

[0006] Step S1: Place the small boat containing the silicon wafer into the furnace tube, where the temperature is 750℃~780℃;

[0007] Step S2: Evacuate the furnace tube and simultaneously raise the temperature at an incline to 790℃~850℃ for 15~18 minutes.

[0008] Step S3: Leak detection;

[0009] Step S4: Pre-oxidation, nitrogen and oxygen are introduced, the pressure inside the furnace tube is 100-300 mbar, and the temperature is 790℃-850℃ to carry out the pre-oxidation process;

[0010] Step S5: Source deposition, nitrogen, oxygen and diffusion boron source are introduced for diffusion, the pressure inside the furnace tube is 100-300 mbar and the temperature is 790℃-850℃;

[0011] Step S6: Introduce nitrogen gas and simultaneously raise the temperature to 860-930°C for heating diffusion;

[0012] Step S7: Under constant temperature conditions of 860-930℃, nitrogen, oxygen and a diffusion boron source are introduced for diffusion;

[0013] Step S8: Introduce nitrogen and oxygen, raise the temperature to 930-980°C, and carry out oxidative propulsion;

[0014] Step S9: Introduce oxygen, raise the temperature inside the furnace tube to 1000-1020℃, and raise the pressure inside the furnace tube to 400-800 mbar to carry out the post-oxidation process;

[0015] Step S10: Introduce large and small oxygen, maintain the furnace tube temperature at 1000-1020℃ and the furnace tube pressure at 400-800mbar, and carry out mixed oxidation;

[0016] Step S11: Introduce nitrogen and oxygen, while simultaneously cooling the process to 800°C;

[0017] Step S12: Break the vacuum to atmospheric pressure;

[0018] Step S13: Depart the boat.

[0019] As a further improvement of the present invention, in step S4, the oxygen flow rate is 1500-3000 sccm, the nitrogen flow rate is 0-1000 sccm, and the pre-oxidation process time is 4-8 min.

[0020] As a further improvement of the present invention, in step S5, the nitrogen flow rate is 1200-3000 sccm, the oxygen flow rate is 400-1200 sccm, the diffusion boron source flow rate is 50-200 sccm, and the diffusion deposition time is 5-15 min.

[0021] As a further improvement of the present invention, in step S6, the nitrogen flow rate is 5000-12000 sccm, and the heating time is 6-10 min.

[0022] As a further improvement of the present invention, in step S7, the flow rate of the diffused boron source is 80-100 sccm, the flow rate of oxygen is 1000-1200 sccm, the flow rate of nitrogen is 3000-5000 sccm, and the duration of gas introduction is 30-60 s.

[0023] As a further improvement of the present invention, in step S7, the flow rate ratio of oxygen to diffuse boron source is >10:1, and the ratio of diffuse boron source volume to total gas volume is ≤2.5%.

[0024] As a further improvement of the present invention, in steps S5 and S7, the boron diffusion source is BCl3 or BBr3.

[0025] As a further improvement of the present invention, in step S8, the aerobic propulsion time is 3 to 10 minutes.

[0026] As a further improvement of the present invention, in step S8, the nitrogen flow rate is 2000-12000 sccm and the oxygen flow rate is 1000-10000 sccm.

[0027] As a further improvement of the present invention, in step S9, the oxygen flow rate is 10000-20000 sccm, and the post-oxidation process time is 40-50 min.

[0028] As a further improvement of the present invention, in step S10, the large oxygen flow rate is 6-8 L / s, the small oxygen flow rate is 1-2 L / s, and the mixed oxidation time is 8-10 min; in step S11, the oxygen flow rate is 8-10 L / s, the nitrogen flow rate is 10-15 L / s, and the cooling time is 25-30 min.

[0029] Compared with the prior art, the advantages of the present invention are as follows:

[0030] 1. The boron diffusion process of the present invention effectively suppresses the formation of a BRL layer during the high-temperature propulsion process by introducing a certain proportion of oxygen, thereby significantly reducing the thickness of the final BRL layer. This, in turn, significantly shortens the time required for high-temperature oxidation to eliminate the BRL layer. The combined effect of introducing moist oxygen at high temperature further accelerates the complete elimination of the BRL layer. At the same time, by continuing short-time, low-concentration source diffusion at a relatively higher temperature, it is ensured that the doping rate is not affected while the peak temperature decreases, ultimately ensuring that the diffusion junction depth meets the process requirements.

[0031] 2. The boron diffusion process of the present invention introduces nitrogen, oxygen and a diffusion boron source at a high temperature of 930°C for an extremely short time of 30-60 seconds, and controls the volume concentration of the diffusion boron source to be below 2.5%, while the flow ratio of oxygen to diffusion boron source is greater than 10:1. The oxygen concentration far exceeds the content required for diffusion, which ensures the doping rate while effectively limiting the formation of an excessive doping BRL layer. The final BRL layer thickness is only 30nm, which is far lower than the BRL layer thickness in the prior art.

[0032] 3. The boron diffusion process of the present invention has a post-oxidation temperature of only 1020℃, which is beneficial for the oxidation removal of the BRL layer and reduces the sealing requirements of the furnace door. After oxidation, the cooling time required for the furnace tube to cool down to 800℃ is only 25-30 minutes, which greatly shortens the cooling time after high-temperature oxidation. Under the premise of ensuring the process effect, the diffusion surface concentration will not increase and the battery conversion efficiency will not be affected. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the boron diffusion process in Embodiment 1 of the present invention.

[0034] Figure 2 This is a comparison diagram of the diffusion process of Embodiment 1 and Comparative Example 1 of the present invention.

[0035] Figure 3 This is a comparison chart of the ECV test results of Example 1 and Comparative Example 1 of the present invention.

[0036] Figure 4 This is a performance comparison chart of the product batteries prepared in Example 1 and Comparative Example 1 of the present invention.

[0037] Figure 5 These are scanning electron microscope (SEM) images comparing the products prepared in Example 1 and Comparative Example 1 of the present invention. In Figure (a), the SEM image of the product prepared in Example 1 is shown, and in Figure (b), the SEM image of the product prepared in Comparative Example 1 is shown. Detailed Implementation

[0038] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0039] BRL stands for Boron Rich Lay, which is a diffusion "dead layer" that increases the recombination rate on the silicon wafer surface, thereby reducing the final conversion efficiency of the battery.

[0040] BSG stands for Boron Silicon Glass, a mixture of silicon oxide and boron oxide doped together.

[0041] Example 1

[0042] like Figure 1 As shown, the boron diffusion process of the present invention includes the following steps:

[0043] Step S1: Place the small boat containing the silicon wafer into the furnace tube, where the temperature is 750°C.

[0044] Step S2: Evacuate the furnace tube and simultaneously raise the temperature to 820°C over a period of 15 minutes.

[0045] Step S3: Leak detection.

[0046] Step S4: Pre-oxidation. Nitrogen and oxygen are introduced, the pressure inside the furnace tube is 100 mbar, and the temperature is maintained at 820℃ for the pre-oxidation process. The oxygen flow rate is 2200 sccm, the nitrogen flow rate is 500 sccm, and the pre-oxidation process time is 6 min.

[0047] Step S5: Source deposition. Nitrogen, oxygen, and a diffusing boron source are introduced for diffusion. The pressure inside the furnace tube is maintained at 100 mbar, and the temperature is maintained at 820°C. The diffusing boron source is BCl3, the nitrogen flow rate is 2300 sccm, the oxygen flow rate is 800 sccm, the diffusing boron source flow rate is 120 sccm, and the diffusion deposition time is 10 min. It is understood that in other embodiments, the diffusing boron source can also be BBr3.

[0048] Step S6: Introduce nitrogen gas and simultaneously raise the temperature to 930°C for thermal diffusion. The nitrogen gas flow rate is 8000 sccm, and the heating time is 8 minutes.

[0049] Step S7: Under constant temperature conditions of 930℃, nitrogen, oxygen, and a diffusing boron source are introduced, wherein the flow rate of the diffusing boron source is 80 sccm, the flow rate of oxygen is 1000 sccm, and the flow rate of nitrogen is 5000 sccm; short-time, low-concentration source replenishment and diffusion are performed. The flow rate ratio of oxygen to diffusing boron source is 12.5, the volume ratio of diffusing boron source to total gas volume is 1.32%, the gas introduction time is 30 s, and the diffusing boron source is BCl3. It is understood that in other embodiments, the diffusing boron source can also be BBr3.

[0050] Step S8: Introduce nitrogen and oxygen, raise the temperature to 980°C, and maintain the pressure inside the furnace tube at 100 mbar for aerobic propulsion. The nitrogen flow rate is 7000 sccm, the oxygen flow rate is 5000 sccm, and the aerobic propulsion time is 8 minutes. It is understood that in other embodiments, the nitrogen to oxygen flow rate ratio can be other values, as long as it meets the nitrogen and oxygen reference requirements for aerobic propulsion, slows down the doping rate, and reduces the total amount of doping.

[0051] Step S9: Oxygen is introduced, the temperature inside the furnace tube rises to 1020℃, and the pressure inside the furnace tube increases to 400 mbar, to carry out the post-oxidation process. The oxygen flow rate is 15000 sccm, and the post-oxidation process takes 45 minutes.

[0052] Step S10: Introduce both large and small oxygen streams. The furnace tube temperature is maintained at 1020℃, and the furnace tube pressure is maintained at 400 mbar. The mixing oxidation time is 8 minutes. The large oxygen flow rate is 6 L / s, and the small oxygen flow rate is 1 L / s. In this process, large oxygen refers to directly introducing oxygen (dry oxygen) into the tube through a pipeline. Small oxygen refers to introducing oxygen into a water-filled container outside the tube, generating oxygen mixed with water, which is then introduced into the tube through the pipeline (wet oxygen).

[0053] Step S11: Introduce nitrogen and oxygen while simultaneously cooling the process to 800℃ for 25 minutes. The oxygen flow rate is 8 L / s and the nitrogen flow rate is 10 L / s.

[0054] Step S12: Introduce nitrogen gas to break the vacuum.

[0055] Step S13: Pull out the small boat containing the silicon wafer; the process is complete.

[0056] In this embodiment, by introducing a certain proportion of oxygen during the high-temperature propulsion process at 980°C in step S8, the formation of a BRL layer during the boron diffusion process is effectively suppressed, resulting in a significant reduction in the thickness of the final BRL layer. This, in turn, significantly shortens the time required for high-temperature oxidation to eliminate the BRL layer. Combined with the effect of introducing moist oxygen at a high temperature of 1020°C in step S10, the complete elimination of the BRL layer is further accelerated. At the same time, by performing short-time, low-concentration source diffusion at a high temperature of 930°C in step S7, it is ensured that while the peak temperature decreases, the total amount of doped into the silicon wafer remains unaffected, ultimately ensuring that the diffusion junction depth meets the process requirements.

[0057] In this embodiment, nitrogen, oxygen, and a diffusion boron source are introduced at a high temperature of 980°C for an extremely short period of 30–60 seconds. The volume concentration of the diffusion boron source is controlled below 2.5%, and the flow rate ratio of oxygen to diffusion boron source is greater than 10:1. The oxygen concentration far exceeds the amount required for diffusion, ensuring the doping rate while effectively limiting excessive doping. Furthermore, by simultaneously introducing nitrogen and oxygen in step S8 for aerobic diffusion, the doping rate can be slowed down and the total doping amount can be reduced. The final BRL layer thickness is only 30 nm, which is far lower than the BRL layer thickness in the prior art.

[0058] In this embodiment, the post-oxidation process temperature is only 1020℃, which is beneficial for oxidizing and removing the BRL layer and reduces the sealing requirements of the furnace door. After oxidation, the cooling time required for the furnace tube to cool down to 800℃ is only 25-30 minutes, which greatly shortens the cooling time after high-temperature oxidation. Under the premise of ensuring the process effect, the diffusion surface concentration does not increase and the battery conversion efficiency is not affected.

[0059] Comparative Example 1

[0060] In Comparative Example 1, in step (4) through-source deposition, 0.2 slm BCl3, 0.5 slm O2, and 2 slm N2 were introduced for diffusion, with a time of 15 min, a pressure of 200 mbar, and a temperature of 800 °C.

[0061] Step (5) Introduce 2L of nitrogen gas and simultaneously raise the temperature to 980℃ for anaerobic propulsion for 20 minutes;

[0062] Step (6) Introduce 1L of nitrogen gas and simultaneously open the valve of the large oxygen flow meter to introduce 15L of oxygen gas. Simultaneously raise the temperature to 1040℃, set the pressure inside the pipe to 400mbar, and carry out the post-oxidation process for 45min.

[0063] Step (7) The process temperature is lowered to 980℃, and 15L of oxygen is introduced during the cooling process. The process time is 10min.

[0064] Step (8) Keep 2L of oxygen flowing in, and at the same time open the small oxygen flow valve to let 1L of oxygen flow in. The oxygen flows through the pure water bottle and carries water vapor. The temperature of the pure water bottle constant temperature bath is 40℃, the process temperature inside the furnace is 980℃, and the time is 10min.

[0065] Step (9) Stop the flow of large oxygen and small oxygen carrying water vapor, and introduce 10L of nitrogen for 40 minutes. At the same time, the process is cooled down and the temperature is set to 800℃.

[0066] The other implementation steps in Comparative Example 1 are the same as those in Example 1.

[0067] like Figure 2 As shown, the process of Comparative Example 1 is carried out at a high temperature above 900°C, using an oxidation method combining stepwise dry oxygen + water oxygen + dry oxygen. In contrast, Example 1 of the present invention is carried out at a high temperature above 900°C, using an oxidation method combining "nitrogen + dry oxygen" + "dry oxygen" + "dry oxygen + wet oxygen" + "nitrogen + dry oxygen". Furthermore, it employs a propulsion diffusion method that replenishes low-concentration source in a short time. This ensures the thickness of the oxide layer growth while limiting the doping rate and suppressing the BRL layer thickness, thus ensuring the complete removal of the BRL layer. This shortens the process time by approximately 30 minutes and improves the conversion efficiency.

[0068] The cells prepared using the processes of Comparative Example 1 and Example 1 were characterized in terms of junction structure. The ECV electrical performance data are as follows: Figure 3 As shown, by Figure 3As can be seen, in Example 1, the surface source concentration of the product in the junction test remained low (the difference was not significant compared with the boron diffusion process in Comparative Example 1), and the junction depth was slightly shallower than the existing wet oxygen process in Comparative Example 1, but it still met the process requirements for battery fabrication. In Example 1, the dry oxygen + wet oxygen short-time rapid oxidation method accelerated the oxidation rate, ensured the complete removal of the BRL layer, and further promoted diffusion, so that the junction depth met the process requirements.

[0069] Figure 4 The graph shows a performance comparison of the final product batteries obtained using the methods in Example 1 and Comparative Example 1. In the graph, Eta represents the conversion efficiency, Uoc represents the open-circuit voltage, Isc represents the short-circuit current, FF represents the fill factor, Rsh represents the parallel resistance of the solar cell, Rs represents the series resistance of the solar cell, and Irev2 represents the reverse saturation current of the PN junction. Figure 4 The electrical performance data are average values. Electrical performance data are as follows: Figure 4 As shown, compared with the existing wet oxygen process in Comparative Example 1, in Example 1, the reduction in surface concentration slightly increases Uoc, slightly decreases FF, slightly decreases Irev2, increases Rs contact, and improves the overall battery conversion efficiency by 0.007%.

[0070] Figure 5 These are scanning electron microscope (SEM) comparison images of products prepared using the methods in Example 1 and Comparative Example 1, as shown. Figure 5 As shown in (a), in the silicon wafer product obtained by diffusion in Example 1, the BSG thickness is 380 nm and the BRL thickness is 30 nm. Figure 5 As shown in (b), in the silicon wafer product obtained by diffusion in Comparative Example 1, the BSG thickness is 300 nm and the BRL thickness is 90 nm. By comparison, it can be seen that the product obtained in Example 1 has a larger BSG thickness and a smaller BRL thickness than the product obtained in Comparative Example 1, indicating that the diffusion effect in Example 1 is better than that in Comparative Example 1.

[0071] Compared to Comparative Example 1, in Example 1, step S8 was changed from oxygen-free propulsion to oxygen-enriched propulsion. A certain amount of oxygen and nitrogen were introduced, and a certain flow rate of oxygen was set to limit the formation of a BRL layer due to excessive and rapid doping. As a result, the thickness of the BRL layer formed before high-temperature oxidation at 1020°C was much lower than that in Comparative Example 1. Since the formation of the BRL layer was suppressed in Example 1, it was not necessary to completely remove the BRL layer through prolonged high-temperature oxidation. In Example 1, during the heating diffusion process in step S8, nitrogen and oxygen were simultaneously added for aerobic propulsion. The presence of oxygen slowed the doping rate, resulting in a decrease in the total doping amount. Combined with step S7, which appropriately increased the diffusion temperature and propagated diffusion at 930°C, introducing a source with a volume concentration below 2.5% within a very short time (30-60 seconds) for high-temperature supplementary source diffusion, and utilizing a low-concentration boron source and excess oxygen to suppress excessively rapid doping, this compensated for the relatively low total doping amount and the increased process time caused by the slowed doping rate, while also suppressing the rapid increase of the BRL layer. This resulted in a shorter process time than Comparative Example 1 before the high-temperature oxidation (1020°C) step. Furthermore, Example 1 used a lower process temperature during the high-temperature oxidation process than Comparative Example 1, and the required cooling time after the process was completed was shorter. Ultimately, this achieved similar doping concentrations and junction depths on the diffused surface, and similar cell conversion efficiencies after silicon wafer diffusion, significantly reducing process time and increasing production capacity.

[0072] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention, or modify them into equivalent embodiments, without departing from the spirit and technical essence of the invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the content of the present invention, shall still fall within the scope of protection of the present invention.

Claims

1. A boron diffusion process, characterized in that, Includes the following steps: Step S1: Place the small boat containing the silicon wafer into the furnace tube, where the temperature is 750℃~780℃; Step S2: Evacuate the furnace tube and simultaneously raise the temperature at an incline to 790℃~850℃ for 15~18 minutes. Step S3: Leak detection; Step S4: Pre-oxidation, nitrogen and oxygen are introduced, the pressure inside the furnace tube is 100-300 mbar, the temperature is 790℃-850℃, and the pre-oxidation process is carried out; wherein, the oxygen flow rate is 1500-3000 sccm, the nitrogen flow rate is 0-1000 sccm, and the pre-oxidation process time is 4-8 min. Step S5: Source deposition, nitrogen, oxygen and diffusion boron source are introduced for diffusion, the pressure inside the furnace tube is 100-300 mbar and the temperature is 790℃-850℃; Step S6: Introduce nitrogen gas and simultaneously raise the temperature to 860-930℃ for heating and diffusion, with a heating time of 6-10 minutes; Step S7: Under constant temperature conditions of 860–930℃, nitrogen, oxygen, and a diffusion boron source are introduced for diffusion; the gas introduction time is 30–60 s, the flow rate ratio of oxygen to diffusion boron source is >10:1, and the volume ratio of diffusion boron source to total gas volume is ≤2.5%; Step S8: Introduce nitrogen and oxygen, raise the temperature to 930-980°C, and carry out oxidative propulsion; Step S9: Introduce oxygen, raise the temperature inside the furnace tube to 1000-1020℃, and raise the pressure inside the furnace tube to 400-800 mbar to carry out the post-oxidation process; Step S10: Introduce large and small oxygen, maintain the furnace tube temperature at 1000-1020℃ and the furnace tube pressure at 400-800mbar, and carry out mixed oxidation; Step S11: Introduce nitrogen and oxygen, while simultaneously cooling the process to 800°C; Step S12: Break the vacuum to atmospheric pressure; Step S13: Depart the boat.

2. The boron diffusion process according to claim 1, characterized in that, In step S5, the nitrogen flow rate is 1200–3000 sccm, the oxygen flow rate is 400–1200 sccm, the diffusion boron source flow rate is 50–200 sccm, and the diffusion deposition time is 5–15 min.

3. The boron diffusion process according to claim 1, characterized in that, In step S6, the nitrogen flow rate is 5000-12000 sccm.

4. The boron diffusion process according to claim 1, characterized in that, In step S7, the flow rate of the diffused boron source is 80-100 sccm, the flow rate of oxygen is 1000-1200 sccm, and the flow rate of nitrogen is 3000-5000 sccm.

5. The boron diffusion process according to claim 1, characterized in that, In steps S5 and S7, the boron diffusion source is BCl3 or BBr3.

6. The boron diffusion process according to claim 1, characterized in that, In step S8, the aerobic propulsion time is 3 to 10 minutes.

7. The boron diffusion process according to claim 1, characterized in that, In step S9, the oxygen flow rate is 10,000 to 20,000 sccm, and the post-oxidation process takes 40 to 50 minutes.

8. The boron diffusion process according to claim 1, characterized in that, In step S10, the high oxygen flow rate is 6-8 L / s, the low oxygen flow rate is 1-2 L / s, and the mixed oxidation time is 8-10 min; in step S11, the oxygen flow rate is 8-10 L / s, the nitrogen flow rate is 10-15 L / s, and the cooling time is 25-30 min.

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