Boron diffusion process for n-type silicon wafer, n-type silicon wafer and solar cell

Abstract

Disclosed in the present invention are a boron diffusion process for an N-type silicon wafer, an N-type silicon wafer and a solar cell. The boron diffusion process for the N-type silicon wafer comprises the following steps: primary boron diffusion: introducing nitrogen and water vapor, preparing a first oxidation mask on the surface of the N-type silicon wafer, then introducing a boron source gas, and performing primary PN junction deposition; secondary boron diffusion: introducing nitrogen and water vapor, depositing a second oxidation mask, then introducing a boron source gas, and performing secondary PN junction deposition; and drive-in: in a nitrogen atmosphere, raising the temperature to 900-990ºC, and performing PN junction drive-in. The adsorption and deposition capability of the oxidation mask on a boron source is obviously improved, and more boron sources can be deposited on the surface of the oxidation mask. In cooperation with the arrangement of the two layers of oxidation masks, the boron in the N-type silicon wafer is improved to a better concentration at a relative low propulsion temperature.

Description

Boron diffusion process for N-type silicon wafer, N-type silicon wafer, and solar cell

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on September 25, 2023, with application number 2023112436792 and invention name “A Boron Diffusion Process for N-Type Silicon Wafers and N-Type Silicon Wafers and Solar Cells”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present invention relates to the field of solar cells, and in particular to a boron diffusion process for an N-type silicon wafer, an N-type silicon wafer, and a solar cell. Background Art

[0003] Boron diffusion on the surface of an N-type silicon wafer can form a PN junction, but due to the low solid solubility of boron in silicon, boron diffusion is more difficult than conventional phosphorus diffusion. To achieve optimal boron concentration in an N-type silicon wafer, the temperature must be raised to a relatively high level for thermal diffusion. Therefore, at the boron diffusion temperature for N-type silicon wafers, damage to the N-type wafer is likely to occur, reducing the minority carrier lifetime. Furthermore, this temperature approaches the maximum temperature limit of the boron diffusion equipment, causing significant wear and tear on the equipment.

[0004] Summary of the Invention

[0005] The present application provides a boron diffusion process for N-type silicon wafers, N-type silicon wafers, and solar cells. By improving the preparation process of the oxidation mask in the boron diffusion process, the improved oxidation mask has a significantly improved ability to adsorb and deposit boron sources, and more boron sources can be deposited on the surface of the oxidation mask. In addition, with the setting of two layers of oxidation masks, the boron doping concentration in the N-type silicon wafer is increased to an optimal concentration at a lower propulsion temperature. The preparation process of the first oxidation mask and the second oxidation mask also generates a large amount of H + , these H + It can passivate the defects of N-type silicon wafers and further improve the performance of N-type silicon wafers.

[0006] In a first aspect, the present invention provides a boron diffusion process for an N-type silicon wafer.

[0007] The boron diffusion process of the N-type silicon wafer includes the following steps:

[0008] Primary boron diffusion: nitrogen and water vapor are introduced to form a first oxide mask on the surface of the N-type silicon wafer, and then a boron source gas is introduced to perform a PN junction deposition on the surface of the first oxide mask;

[0009] Secondary boron diffusion: introducing the nitrogen gas and the water vapor to deposit a second oxide mask on the surface of the N-type silicon wafer after the first boron diffusion deposition process, then introducing the boron source gas to perform a secondary PN junction deposition on the surface of the second oxide mask;

[0010] Push-in: In a nitrogen atmosphere, the temperature is raised to 900℃~990℃ to push the PN junction.

[0011] As an optional implementation, in an embodiment of the present invention, the thickness of the first oxidation mask is 20 nm to 30 nm, and the thickness of the second oxidation mask is 20 nm to 30 nm.

[0012] As an optional embodiment, in an embodiment of the present invention, during the preparation process of the first oxidation mask and / or the second oxidation mask, the water vapor is nitrogen carrying water vapor, and the nitrogen carrying out the water vapor is carried out in an environment of a pressure of 500mbar to 750mbar and a temperature of 60°C to 80°C, and the flow rate of the nitrogen carrying water vapor is 100sccm to 500sccm.

[0013] As an optional implementation manner, in an embodiment of the present invention, during the preparation of the first oxidation mask and / or the second oxidation mask, the flow rate of the nitrogen gas is 1000 sccm to 3000 sccm.

[0014] As an optional embodiment, in an embodiment of the present invention, during the primary PN junction deposition and / or the secondary PN junction deposition process, the boron source gas is a mixed gas of BCl3, oxygen and nitrogen, the flow rate of BCl3 is 50sccm~300sccm, the flow rate of oxygen is 150sccm~800sccm, and the flow rate of nitrogen is 1000sccm~3000sccm.

[0015] As an optional implementation, in an embodiment of the present invention, the time for depositing the first oxidation mask is 8 minutes to 15 minutes, the time for depositing the primary PN junction is 15 minutes to 25 minutes, the time for depositing the second oxidation mask is 8 minutes to 15 minutes, and the time for depositing the secondary PN junction is 15 minutes to 25 minutes.

[0016] As an optional implementation, in an embodiment of the present invention, the primary boron diffusion and / or the secondary boron diffusion is performed under the conditions of a pressure of 100 mbar to 300 mbar and a temperature of 800° C. to 850° C.

[0017] As an optional implementation, in an embodiment of the present invention, after the advancing step is completed, a third oxide mask with a thickness of 45nm to 55nm is deposited on the surface of the second oxide mask, and nitrogen and water vapor are introduced during the deposition of the third oxide mask.

[0018] As an optional embodiment, in an embodiment of the present invention, in the advancing step, the water vapor is nitrogen carrying water vapor, the flow rate of the nitrogen carrying water vapor is 500 sccm to 1500 sccm, and the flow rate of the nitrogen is 1000 sccm to 2000 sccm.

[0019] As an optional implementation, in an embodiment of the present invention, the third oxide mask deposition is performed at a temperature of 900° C. to 990° C. and a pressure of 100 mbar to 300 mbar.

[0020] As an optional implementation, in an embodiment of the present invention, in the advancing step, the temperature is kept at 900° C. to 990° C. for 30 min to 50 min.

[0021] As an optional embodiment, in an embodiment of the present invention, in the advancing step, after keeping the temperature at 900°C to 990°C for 30min to 50min, the temperature is lowered to 850°C to 650°C, nitrogen is introduced for purging, and normal pressure is restored.

[0022] As an optional embodiment, in an embodiment of the present invention, nitrogen purging is performed before the primary boron diffusion. The nitrogen purging is performed at 650°C to 800°C and normal pressure. During the nitrogen purging process, the nitrogen flow rate is 1500sccm to 5000sccm.

[0023] In a second aspect, the present application provides an N-type silicon wafer obtained by a boron diffusion process.

[0024] The sheet resistance of the N-type silicon wafer is 120Ω / sq to 170Ω / sq, and the surface concentration is 2.5×10 19 N / cm 2 ~6×10 19 N / cm 2 , the junction depth is 0.8μm~1.2μm.

[0025] In a third aspect, the present application provides a solar cell, comprising an N-type silicon wafer, wherein the N-type silicon wafer is an N-type silicon wafer obtained by the boron diffusion process as described in the first aspect.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The embodiment of the present invention provides a boron diffusion process for an N-type silicon wafer. By improving the preparation process of the oxide mask in the boron diffusion process, a first oxide mask and a second oxide mask with more surface pores and increased specific surface area are obtained. The first oxide mask and the second oxide mask can provide more channels or spaces for accommodating boron sources, which is conducive to the accumulation of more boron sources on the surfaces of the first oxide mask and the second oxide mask. The concentration of the boron source accumulated on the surface of the first oxide mask is increased, and in conjunction with the secondary boron diffusion treatment, the boron source is introduced again, so that more boron source accumulates on the surface of the second oxide mask. By combining the first oxide mask and the second oxide mask, the content of the boron source deposited on the surface of the first oxide mask and the second oxide mask is greatly increased. Therefore, the above-mentioned first oxide mask and the second oxide mask can not only prevent excessive boron source from directly entering the silicon surface and causing lattice damage, providing protection for the silicon wafer, but also can segregate and absorb the boron source on the surface and adjust the surface doping concentration. Too large or too small a thickness will lead to abnormal surface concentration, reduce the effective doping effect of the silicon wafer, and easily reduce the performance of the solar cell.

[0028] In the subsequent steps, the advancing temperature is raised to 900°C to 990°C, so that a large amount of boron source deposited on the first oxide mask and the second oxide mask can be further diffused into the N-type silicon wafer through thermal diffusion, and the boron doping content in the N-type silicon wafer is rapidly increased. There is no need to raise the advancing temperature to above 1000°C to increase the boron doping content in the N-type silicon wafer to a considerable concentration.

[0029] In summary, the above scheme produces a first and second oxide mask with a unique structure. These masks are then used for two boron diffusions, effectively increasing the boron source deposition concentration on the surfaces of the first and second oxide masks. This allows the boron doping concentration within the N-type silicon wafer to be increased to an optimal level even at relatively low propulsion temperatures, improving boron diffusion efficiency within the N-type silicon wafer. This helps reduce energy consumption and equipment losses, while also minimizing damage to the N-type silicon wafer caused by excessive temperatures and increasing minority carrier lifetime. Furthermore, the first and second oxide masks work together to passivate the N-type silicon wafer, representing a significant improvement. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] FIG1 is a schematic diagram of the boron diffusion process steps for an N-type silicon wafer disclosed in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0033] Boron diffusion is a challenging process for doping, requiring high energy levels. Boron diffusion relies primarily on high-temperature thermal diffusion. Processing below 1000°C results in low boron doping concentrations in N-type silicon wafers. Increasing the boron diffusion temperature to 1000°C to 1060°C allows more boron source to diffuse into the N-type silicon wafer, increasing the boron doping concentration. However, high temperatures can easily damage N-type silicon wafers, reducing minority carrier lifetimes and increasing energy consumption. The processing temperature of 1000°C to 1060°C approaches the tolerance limit of the boron diffusion equipment, potentially causing damage.

[0034] In the boron diffusion process, an oxide mask is generally deposited before boron diffusion deposition. The deposition of the oxide mask can prevent excessive accumulation of boron sources on the surface of the N-type silicon wafer during boron deposition, which leads to lattice deformation and formation of a dead layer. On the other hand, it also plays a role in reducing uneven boron distribution.

[0035] By repeatedly injecting a boron source into the oxide mask surface to deposit a PN junction, the boron source concentration on the oxide mask surface can be increased, lowering the temperature of the boron diffusion process. However, the boron source content currently deposited on the oxide mask surface is very limited, restricting further increases in the boron doping concentration in N-type silicon wafers. In this case, the boron diffusion process temperature is lowered, making it difficult to further increase the boron doping concentration in N-type silicon wafers to the optimal level.

[0036] To this end, the present application provides a boron diffusion process for N-type silicon wafers. By improving the preparation process of the oxide mask in the boron diffusion process, the improved oxide mask has a significantly improved adsorption and deposition capability for the boron source, and more boron source can be deposited on the oxide mask surface. In addition, with the setting of two oxide masks, the boron doping concentration in the N-type silicon wafer is increased to an optimal concentration at a lower propulsion temperature. The preparation process of the first oxide mask and the second oxide mask also generates a large amount of H + , these H + It can passivate the defects of N-type silicon wafers and further improve the performance of N-type silicon wafers.

[0037] The technical solution of the present invention will be further described below in conjunction with embodiments and drawings.

[0038] In a first aspect, an embodiment of the present application provides a boron diffusion process for an N-type silicon wafer.

[0039] The boron diffusion process of the N-type silicon wafer includes the following steps:

[0040] Primary boron diffusion: nitrogen and water vapor are introduced to form a first oxide mask on the surface of the N-type silicon wafer. Then, a boron source gas is introduced to perform a PN junction deposition on the surface of the first oxide mask.

[0041] Secondary boron diffusion: nitrogen and water vapor are introduced to deposit a second oxide mask on the surface of the N-type silicon wafer after the first boron diffusion deposition process. Then, boron source gas is introduced to perform a second PN junction deposition on the surface of the second oxide mask.

[0042] Push-in: In a nitrogen atmosphere, the temperature is raised to 900℃~990℃ to push the PN junction.

[0043] An oxidation mask is prepared by introducing nitrogen and water vapor to react with silicon. The reaction of the first oxidation mask prepared by the water vapor and nitrogen is as follows:

[0044] Si+2H2O→SiO2+2H2

[0045] Compared to introducing oxygen as a reaction gas, the present application introduces water vapor as a reaction gas when preparing the first oxide mask. The surface of the first oxide mask prepared by the above reaction has more pores and a larger specific surface area. Then, a boron source is introduced to perform a PN junction deposition reaction. The reaction is as follows:

[0046] 4BCl3+3O2→2B2O3+6Cl2

[0047] 2B2O3+3Si→B+3SiO2

[0048] During the primary PN junction deposition process, a small portion of the boron source diffuses into the N-type silicon wafer for reaction, while the remaining majority of the boron source is deposited and accumulated on the surface of the first oxide mask. Since the surface pores of the first oxide mask prepared by the above reaction are relatively large, more boron source is deposited on the surface of the first oxide mask, and the concentration of boron source accumulated on the surface of the first oxide mask is increased. In addition, in conjunction with the secondary boron diffusion process, a second oxide mask with a relatively rich pore structure like the first oxide mask is prepared on the basis of the first oxide mask. The reaction of the second oxide mask is as follows:

[0049] Si+2H2O→SiO2+2H2

[0050] When the boron source is introduced again, the following reaction occurs:

[0051] 4BCl3+3O2→2B2O3+6Cl2

[0052] 2B2O3+3Si→B+3SiO2

[0053] After the above reaction is completed, more boron source is deposited on the surface of the second oxidation mask layer. By combining the first oxidation mask layer and the second oxidation mask layer, the content of boron source deposited on the surface of the first oxidation mask layer and the second oxidation mask layer is greatly increased.

[0054] In the subsequent steps, the advancing temperature is raised to 900°C to 990°C, so that a large amount of boron source deposited on the first oxide mask and the second oxide mask can be further diffused into the N-type silicon wafer through thermal diffusion, and the boron doping content in the N-type silicon wafer is quickly increased to a better concentration. There is no need to raise the advancing temperature to above 1000°C to increase the boron doping content in the N-type silicon wafer to the optimal concentration.

[0055] In summary, the above scheme produces a first and second oxide mask with a relatively unique structure. These masks are then applied to two boron diffusion processes, fully regulating the deposition concentration of the boron source on the surfaces of the first and second oxide masks. This allows the boron doping concentration within the N-type silicon wafer to be increased to an optimal level even at relatively low propulsion temperatures, improving the boron diffusion efficiency of the N-type silicon wafer. This helps reduce energy consumption and equipment losses, while also minimizing damage to the N-type silicon wafer caused by excessive temperatures and increasing the minority carrier lifetime. The first and second oxide masks not only prevent excessive boron source from directly entering the silicon surface and causing lattice damage, thus protecting the silicon wafer, but also absorb the surface boron source and regulate the surface doping concentration. Excessive or insufficient thickness can lead to abnormal surface concentrations, reducing the effective doping effect on the silicon wafer and potentially reducing the performance of the solar cell. Furthermore, the first and second oxide masks work together to passivate the N-type silicon wafer, representing a significant improvement.

[0056] In some embodiments, the thickness of the first oxide mask is 20 nm to 30 nm, and the thickness of the second oxide mask is 20 nm to 30 nm.

[0057] The oxide mask has two layers, increasing the total amount of boron source deposited on the oxide mask. By heating the temperature to 900°C to 990°C, the boron source deposited on the surfaces of the two oxide masks is concentrated and abundant, which helps to increase the boron doping concentration in the N-type silicon wafer to an optimal level. However, increasing the number of oxide mask layers also tends to increase their overall thickness. This increases the diffusion path length of the boron source deposited on the oxide mask surfaces, especially on the second oxide mask, and increases the loss of boron source entering the N-type silicon wafer. Therefore, by controlling the thickness of the first and second oxide masks, the boron source content on the oxide mask surfaces can be maximized while minimizing the loss of the oxide mask layers, allowing more boron source to enter the N-type silicon wafer.

[0058] The primary boron diffusion and the secondary boron diffusion are carried out under the conditions of a pressure of 100 mbar to 300 mbar and a temperature of 800° C. to 850° C.

[0059] The entire process of primary boron diffusion and secondary boron diffusion is carried out under the conditions of 100mbar~300mbar and 800℃~850℃. The temperature and pressure of the entire diffusion process are relatively stable, and there is no need to switch the pressure or temperature conditions.

[0060] In some embodiments, during the preparation of the first oxidation mask and / or the second oxidation mask, the water vapor is nitrogen carrying water vapor, and the nitrogen carrying out the water vapor is carried out in an environment with a pressure of 500mbar to 750mbar and a temperature of 60°C to 80°C, and the flow rate of the nitrogen carrying water vapor is 100sccm to 500sccm.

[0061] In some embodiments, during the preparation of the first oxidation mask and / or the second oxidation mask, the flow rate of nitrogen gas is 1000 sccm to 3000 sccm.

[0062] Water vapor is carried out by passing nitrogen into water. The amount of water vapor carried is affected by the amount of nitrogen introduced, the ambient temperature and pressure. Here, the nitrogen introduction is controlled to be 100 sccm to 500 sccm, the ambient temperature is 60℃ to 80℃, and the ambient pressure is 500mbar to 750mbar. Under these conditions, the amount of water vapor carried is relatively high. By controlling the ratio of water vapor and nitrogen, the oxidation reaction rate of water vapor and silicon is controlled, thereby controlling the surface morphology and thickness of the oxide mask, generating an oxide mask structure with more pores that is more conducive to the accumulation of boron sources. The combination of the first oxide mask and the second oxide mask prepared under the above conditions can not only improve the uniformity of boron diffusion and avoid the formation of a dead layer, but also facilitate the deposition and diffusion of the boron source. At the same time, in this oxidation reaction, the H2 content generated by the water vapor reaction is relatively high, and the boron diffusion is carried out at a temperature of 800℃ to 850℃. At this temperature, H2 can further decompose to generate a large amount of H + , a large amount of H + Contact with N-type silicon wafers fully passivates surface defects and increases minority carrier lifetime. Therefore, controlling the water vapor and nitrogen usage within the aforementioned ranges can simultaneously achieve oxidation mask formation and improve N-type silicon wafer quality. Furthermore, the oxidation mask generated through these reactions can accumulate more boron source on its surface, which helps lower subsequent advancement temperatures, thereby reducing machine losses and saving energy.

[0063] When the content of water vapor introduced is too low, the first oxide mask and the second oxide mask have fewer pores and lower specific surface area, and the adsorption effect on the boron source is poor, which is not conducive to increasing the concentration of the boron source in the silicon wafer; when the content of water vapor introduced is too high, the surfaces of the first oxide mask and the second oxide mask are too loose, and the thickness of the generated first oxide mask and the second oxide mask increases, which is not conducive to the diffusion of the boron source and the subsequent advancement steps, and it is easy to cause excessive boron source to accumulate on the surface of the silicon wafer, resulting in increased Auger recombination.

[0064] In some embodiments, during the primary PN junction deposition and / or secondary PN junction deposition process, the boron source gas is a mixture of BCl3, oxygen and nitrogen, the flow rate of BCl3 is 50sccm~300sccm, the flow rate of oxygen is 150sccm~800sccm, and the flow rate of nitrogen is 1000sccm~3000sccm.

[0065] During the primary PN junction deposition and / or secondary PN junction deposition, a mixture of BCl3, oxygen, and nitrogen is used as the boron source gas. Under the combined action of the boron source gas at the aforementioned flow rates, during the primary PN junction deposition and / or secondary PN junction deposition, a portion of the boron is doped into the N-type silicon wafer, and the remaining majority of the boron source is deposited on the surface of the oxide mask.

[0066] In some embodiments, the first oxide mask deposition time is 8 min to 15 min, the primary PN junction deposition time is 15 min to 25 min, the second oxide mask deposition time is 8 min to 15 min, and the secondary PN junction deposition time is 15 min to 25 min.

[0067] The deposition time of the first and second oxide masks is controlled to regulate the thickness of the first and second oxide masks, so as to increase the concentration of the boron source deposited on the surface of the oxide mask with a relatively loose structure. At the same time, the thickness of the first and second oxide masks should be avoided to be too high, which will cause the boron source diffusion path to become longer and the loss to increase, which is not conducive to the diffusion of the boron source into the N-type silicon wafer.

[0068] In some embodiments, after the advancing step is completed, a third oxide mask with a thickness of 45 nm to 55 nm is deposited on the surface of the second oxide mask. During the deposition of the third oxide mask, nitrogen and water vapor are introduced.

[0069] The reaction for preparing the third oxide mask is as follows:

[0070] Si+2H2O→SiO2+2H2

[0071] The third time nitrogen and water vapor are introduced for reaction is to add a third oxide mask with a thickness of 45nm to 55nm on the basis of the first oxide mask and the second oxide mask to increase the overall thickness of the oxide mask. The oxide mask of this thickness can effectively prevent the back tunneling layer from being deposited on the front or edge of the N-type silicon wafer to prevent short circuit.

[0072] In some embodiments, in the advancing step, the water vapor is nitrogen gas carrying water vapor, the flow rate of the nitrogen gas carrying water vapor is 500 sccm to 1500 sccm, and the flow rate of the nitrogen gas is 1000 sccm to 2000 sccm.

[0073] Compared to the preparation of the first and second oxidation masks, the flow rate of nitrogen carrying H2O is significantly increased during the preparation of the third oxidation mask. Therefore, the oxidation rate of the third oxidation mask is faster, which can shorten production time and improve production efficiency. Furthermore, the third oxidation mask has good affinity with the previously deposited first and second oxidation masks, and has strong adhesion stability with the first and second oxidation masks. Furthermore, the third oxidation mask is significantly more porous than the first and second oxidation masks, resulting in a more porous structure overall. During subsequent front etching, these three oxide masks are easily removed without the need for large amounts of alkaline solution.

[0074] Furthermore, the third oxide mask deposition is performed at a temperature of 900° C. to 990° C. and a pressure of 100 mbar to 300 mbar.

[0075] At this temperature and pressure, water vapor and silicon react to form a third oxide mask, and the boron doping concentration is further adjusted and increased.

[0076] In some embodiments, in the advancing step, the temperature is maintained at 900° C. to 990° C. for 30 min to 50 min.

[0077] In some embodiments, in the advancing step, after maintaining the temperature at 900° C. to 990° C. for 30 min to 50 min, the temperature is lowered to 850° C. to 650° C., nitrogen is introduced for purging, and the pressure is restored to normal.

[0078] In some embodiments, nitrogen purge is performed before the primary boron diffusion. The nitrogen purge is performed at 650° C. to 800° C. and normal pressure. During the nitrogen purge, the nitrogen flow rate is 1500 sccm to 5000 sccm.

[0079] Before the boron diffusion process, nitrogen is used to purge and remove air to avoid the introduction of impurities during the boron diffusion process. The temperature is raised to the diffusion temperature of 650℃~800℃ to achieve relative temperature stability and reduce temperature fluctuations.

[0080] In a second aspect, the present application provides an N-type silicon wafer obtained by a boron diffusion process.

[0081] The sheet resistance of the N-type silicon wafer is 120Ω / sq to 170Ω / sq, and the surface concentration is 2.5×10 19 N / cm 2 ~6×10 19 N / cm 2 The junction depth of the PN junction is 0.75μm~1.1μm.

[0082] In a third aspect, the present application provides a solar cell, which includes an N-type silicon wafer, where the N-type silicon wafer is an N-type silicon wafer obtained by the boron diffusion process as mentioned in the first aspect.

[0083] The technical solution of the present invention will be further described below in conjunction with more specific embodiments and drawings.

[0084] Example 1

[0085] The embodiment of the present application provides a boron diffusion process for an N-type silicon wafer, comprising the following steps:

[0086] Exhaust air: Open the furnace door and introduce nitrogen for continuous purge at a nitrogen flow rate of 1500sccm. Maintain the temperature at 650℃ and the pressure at atmospheric pressure. Enter the boat, close the furnace door, evacuate, and increase the temperature to perform leak detection on the furnace. Maintain the pressure at 100mbar and the temperature at 800℃.

[0087] Primary boron diffusion: including preparation of the first oxide mask and primary PN junction deposition:

[0088] Prepare the first oxide mask: H2O and nitrogen are introduced to form the first oxide mask on an N-type silicon wafer. The flow rate of the nitrogen carrying H2O is maintained at 100 sccm, the flow rate of the nitrogen is maintained at 1000 sccm, the pressure is maintained at 100 mbar, the time is 8 minutes, and the bottle pressure is 650 mbar.

[0089] Primary PN junction deposition: BCl3, oxygen, and nitrogen are introduced to perform PN junction deposition. The BCl3 flow rate is 50 sccm, the oxygen flow rate is 150 sccm, and the nitrogen flow rate is 1000 sccm. The pressure is maintained at 100 mbar for 15 minutes.

[0090] Secondary boron diffusion: including preparation of the second oxide mask and secondary PN junction deposition:

[0091] Prepare the second oxide mask: H2O and nitrogen are introduced to form the second oxide mask on the N-type silicon wafer. The flow rate of nitrogen carrying H2O is maintained at 100 sccm, and the nitrogen flow rate is generally maintained at 1000 sccm. The pressure is maintained at 100 mbar. The time is 8 minutes and the bottle pressure is 650 mbar.

[0092] Secondary PN junction deposition: BCl3, oxygen and nitrogen are introduced to perform PN junction deposition again. The BCl3 flow rate is 50 sccm, the oxygen flow rate is 150 sccm, the nitrogen flow rate is 1000 sccm, and the pressure is maintained at 100 mbar. The deposition time is 15 minutes.

[0093] Push-in: nitrogen purge was introduced with a nitrogen flow rate of 3000 sccm and the pressure maintained at 100 mbar. The temperature was raised to 900°C and the PN junction was pushed in for 60 minutes. Nitrogen was introduced at 1000 sccm and the pressure was maintained at 100 mbar.

[0094] Third oxide mask deposition and doping concentration adjustment: Continue to maintain 900 ° C, carry out oxide mask deposition and doping concentration adjustment for 30 minutes, introduce H20 and nitrogen, with the H20 and nitrogen flow rates of 500 sccm and 1000 sccm, and the pressure is maintained at 100 mbar;

[0095] Cooling: Cool to 850℃, introduce nitrogen for purging, restore the pressure to normal pressure, and then take the boat out.

[0096] Example 2

[0097] This embodiment of the present application provides a boron diffusion process for an N-type silicon wafer, which differs from the first embodiment in that:

[0098] In the step of preparing the first oxide mask, the flow rate of nitrogen gas carrying H2O is maintained at 300 sccm;

[0099] In the step of preparing the second oxide mask, the flow rate of nitrogen gas carrying H2O is maintained at 300 sccm;

[0100] The rest remains the same as in Example 1.

[0101] Example 3

[0102] This embodiment of the present application provides a boron diffusion process for an N-type silicon wafer, which differs from the first embodiment in that:

[0103] In the step of preparing the first oxide mask, the flow rate of nitrogen gas carrying H2O is maintained at 500 sccm;

[0104] During the step of preparing the second oxide mask, the flow rate of nitrogen gas carrying H2O is maintained at 500 sccm;

[0105] The rest remains the same as in Example 1.

[0106] Example 4

[0107] This embodiment of the present application provides a boron diffusion process for an N-type silicon wafer, which differs from the second embodiment in that:

[0108] In the step of preparing the first oxidation mask, the deposition time of the first oxidation mask is 11 minutes.

[0109] In the step of preparing the second oxide mask, the deposition time of the second oxide mask is 11 minutes.

[0110] The rest remains the same as in Example 1.

[0111] Example 5

[0112] This embodiment of the present application provides a boron diffusion process for an N-type silicon wafer, which differs from the second embodiment in that:

[0113] In the step of preparing the first oxidation mask, the deposition time of the first oxidation mask is 15 minutes.

[0114] In the step of preparing the second oxide mask, the deposition time of the second oxide mask is 15 minutes.

[0115] The rest remains the same as in Example 1.

[0116] Comparative Example 1

[0117] The comparative example of the present application provides a boron diffusion process for an N-type silicon wafer, which differs from the second embodiment in that:

[0118] The first oxide mask is prepared using a conventional dry oxygen oxidation method, specifically as follows: Oxygen and nitrogen are introduced to create the mask before silicon wafer deposition. The oxygen flow rate is maintained at 1000 sccm, the nitrogen flow rate is generally maintained at 1000 sccm, and the pressure is maintained at 100 mbar for 8 minutes.

[0119] The second oxide mask is prepared using a conventional dry oxygen oxidation method, specifically as follows: Oxygen and nitrogen are introduced to create the mask before silicon wafer deposition. The oxygen flow rate is maintained at 1000 sccm, the nitrogen flow rate is generally maintained at 1000 sccm, and the pressure is maintained at 100 mbar for 8 minutes.

[0120] The rest remains the same as in Example 1.

[0121] Comparative Example 2

[0122] The comparative example of the present application provides a boron diffusion process for an N-type silicon wafer, which differs from the second embodiment in that:

[0123] Multiple boron source depositions are performed on the first oxide mask without preparing a second oxide mask layer, specifically as follows:

[0124] Primary boron diffusion: including preparation of the first oxide mask and primary PN junction deposition:

[0125] Prepare the first oxide mask: H2O and nitrogen are introduced to form the first oxide mask on an N-type silicon wafer. The flow rate of the nitrogen carrying H2O is maintained at 300 sccm, and the nitrogen flow rate is generally maintained at 1000 sccm. The pressure is maintained at 100 mbar. The time is 8 minutes and the bottle pressure is 650 mbar.

[0126] Primary PN junction deposition: BCl3, oxygen, and nitrogen are introduced to perform PN junction deposition. The BCl3 flow rate is 50 sccm, the oxygen flow rate is 150 sccm, and the nitrogen flow rate is 1000 sccm. The pressure is maintained at 100 mbar for 15 minutes.

[0127] Secondary PN junction deposition: BCl3, oxygen and nitrogen are introduced to perform PN junction deposition again. The BCl3 flow rate is 50 sccm, the oxygen flow rate is 150 sccm, the nitrogen flow rate is 1000 sccm, and the pressure is maintained at 100 mbar. The deposition time is 15 minutes.

[0128] Push-in: nitrogen gas was introduced with a flow rate of 3000 sccm and the pressure maintained at 100 mbar. The temperature was raised to 900°C and the PN junction was pushed in for 60 minutes. Nitrogen gas was introduced at a flow rate of 1000 sccm and the pressure was maintained at 100 mbar.

[0129] The rest remains the same as in Example 1.

[0130] Experiment 1

[0131] The sheet resistance, boron doping concentration and junction depth after boron diffusion of the above embodiments and comparative examples were tested. The test results are shown in Table 1.

[0132] Experiment 2

[0133] The boron diffusion process of the N-type silicon wafer is used as the boron diffusion process for preparing the N-type Topcon cell. The N-type Topcon cell is prepared to test the cell's photoelectric conversion efficiency (Eta), open circuit voltage (Uoc), short circuit current (Isc), fill factor (FF), series resistance (Rs), and parallel resistance (Rsh). The preparation method of the N-type Topcon cell is as follows:

[0134] Texturing: Using an alkaline method to texturize N-type silicon wafers, the reflectivity after texturing is less than 11.1%, and the weight loss control range is 0.405±0.05g;

[0135] Boron diffusion is carried out using a boron trichloride tube furnace, and the boron diffusion is carried out according to the boron diffusion process described in the above examples or comparative examples;

[0136] Back etching, using alkaline method to complete the etching of the PN junction on the back side of the silicon wafer and the edge of the expansion and polishing of the back side, with a reflectivity of >40% and a weight reduction control range of 0.20±0.03g;

[0137] The tunnel oxide layer and poly-doped silicon are deposited using LP or PE methods. The thickness of the tunnel oxide layer is controlled at 1-2 nm, and the back side resistance is 35±10Ω / sq.

[0138] Front aluminum oxide deposition: Atomic layer deposition (ALD) is used to deposit aluminum oxide on the front surface to complete the field passivation of the front surface. The thickness of the aluminum oxide is 95±10nm.

[0139] Front and back film deposition: Plasma-enhanced chemical vapor deposition (PECVD) is used to introduce SiH4, NH3 and other gases to complete the deposition of front and back silicon nitride films, forming enhanced hydrogen passivation and light absorption. The thickness of the front film is 74±3nm and the refractive index is 2.07±0.6. The thickness of the back film is 79±3nm and the refractive index is 2.09±0.6.

[0140] Screen printing and sintering, use screen printing to print the metallized patterns on the front and back, complete the current conduction, and sintering to complete the contact with the substrate, complete the current conduction, the printing weight of the front is 53mg, and the printing weight of the back is 47mg.

[0141] The controlled conditions are standard solar spectrum of AM1.5, temperature of 25℃, and light intensity of 1000W / cm 2 The photoelectric conversion efficiency (Eta), open circuit voltage (Uoc), short circuit current (Isc), fill factor (FF), series resistance (Rs), and parallel resistance (Rsh) of the above-mentioned battery were tested using an IV tester.

[0142] The test results of Experiment 2 are shown in Table 2.

[0143] Table 1

[0144] Table 2

[0145] It can be seen from the data in Table 1 that the sheet resistance of Example 2 is reduced by 30Ω / sq compared with that of Comparative Example 1, and the boron doping concentration is increased by 1.5×10 19 N / cm 2, the junction depth is increased by 0.4μm, which proves that the use of water vapor and nitrogen as the gas for preparing the oxidation mask can change the surface morphology of the oxidation mask. The obtained oxidation mask has a strong adsorption capacity for the boron source and can promote more boron sources to diffuse into the N-type silicon wafer. After boron diffusion, the square resistance in the silicon wafer is reduced, the doping concentration of the boron source is increased, and the junction depth is increased. In the process of preparing the oxidation mask in comparative example 1, water vapor was not introduced, and the obtained oxidation mask was relatively dense, and the adsorption capacity for the boron source was weak, resulting in a lower boron source concentration in the silicon wafer. Combined with the data in Table 2, it can be seen that Eta of Example 2 is increased by 0.2% compared with that of Comparative Example 1, Uoc is increased by 0.0011 V compared with that of Comparative Example 1, Isc is increased by 0.01 A compared with that of Comparative Example 1, FF is increased by 0.91% compared with that of Comparative Example 1, Rs is decreased by 0.0008 Ω compared with that of Comparative Example 1, and Rsh is increased by 235 Ω compared with that of Comparative Example 1. This proves that the boron doping effect in the N-type silicon wafer in Example 2 is better, and the doping concentration of the boron source, junction depth, and sheet resistance in the silicon wafer are within the optimal range.

[0146] By comparing the data of Example 2 and Comparative Example 2, it can be seen that the sheet resistance of Example 2 is reduced by 40Ω / sq compared with that of Comparative Example 2, and the boron doping concentration is reduced by 0.5×10 19 N / cm 2 , the junction depth is increased by 0.5μm, which proves that the mode of depositing the first oxide mask - introducing the boron source - the second oxide mask - and then introducing the boron source is adopted to increase the boron source deposition concentration in the silicon wafer and diffuse it to a better concentration and depth. In comparative example 2, the boron source is deposited twice on an oxide mask layer, and the boron doping concentration is significantly reduced. At the same time, the square resistance is improved, the junction depth is reduced, and the diffusion effect of the boron source in the N-type silicon wafer is poor, which proves that the deposition and diffusion effects of the boron source on the single-layer oxide mask are poor, the effective doping concentration of boron is reduced, and the boron source that can be accommodated on the surface of the first oxide mask is limited. The number of boron source depositions is increased, and the boron source content on the surface of the first oxide mask is not effectively improved, resulting in a decrease in the effective doping concentration of boron in the N-type silicon wafer. When the two oxide mask layers work together, the boron source is deposited on the first oxide mask and the second oxide mask, respectively, which greatly increases the total amount of boron source on the surfaces of the first oxide mask and the second oxide mask. As a result, the effective boron doping concentration in the N-type silicon wafer is increased. As shown in Table 2, Eta of Example 2 is increased by 0.4% compared to Comparative Example 2, Uoc is increased by 0.0013V compared to Comparative Example 2, Isc is increased by 0.070A compared to Comparative Example 2, FF is increased by 1.21% compared to Comparative Example 2, Rs is decreased by 0.0009Ω compared to Comparative Example 2, and Rsh is increased by 345Ω compared to Comparative Example 2.

[0147] By comparing the data of Example 1, Example 2 and Example 3, it can be seen that the sheet resistance, boron doping concentration and junction depth of Example 2 are all within the optimal range. The boron doping concentration of Example 1 is relatively low, the sheet resistance of Example 3 is relatively high, and the junction depth is relatively small. The Eta of the solar cell measured in Example 2 in Table 2 is improved compared with Examples 1 and 3, Rs is reduced, and Rsh is improved, which proves that as the water vapor intake increases, the thickness of the first oxide mask and the second oxide mask increases, the surface morphology changes, and the adsorption and deposition ability of the boron source is stronger. However, if the water vapor intake is too high, the thickness of the first oxide mask and the second oxide mask generated by the reaction is too high, partial doping forms Auger recombination, the effective doping rate of the boron source becomes low, and the junction depth is reduced.

[0148] By comparing Examples 2, 4, and 5 in Tables 1 and 2, it can be seen that the square resistance, boron doping concentration, and junction depth of Example 2 are all within a preferred range, the boron doping concentration of Example 4 is relatively low, and the junction depth is relatively large, while the boron doping concentration of Example 5 is relatively high, and the junction depth is relatively large. The Eta of the solar cell measured in Example 2 is improved compared to those in Examples 4 and 5, Rs decreases, and Rsh increases, proving that as the deposition time of the first oxide mask and the second oxide mask increases, the thickness of the first oxide mask and the second oxide mask changes, which easily leads to partial doping to form Auger recombination, and the effective doping efficiency of the boron source decreases.

[0149] The boron diffusion process for an N-type silicon wafer and the N-type silicon wafer disclosed in the embodiments of the present invention are introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only intended to help understand the boron diffusion process for an N-type silicon wafer and the N-type silicon wafer and their core concepts of the present invention. At the same time, for those skilled in the art, according to the concepts of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A boron diffusion process for an N-type silicon wafer, characterized in that: The following steps are involved: Primary boron diffusion: nitrogen and water vapor are introduced to prepare a first oxide mask on the surface of the N-type silicon wafer, and then a boron source gas is introduced to perform a PN junction deposition on the surface of the first oxide mask; Secondary boron diffusion: introducing the nitrogen gas and the water vapor to deposit a second oxide mask on the surface of the N-type silicon wafer after the primary boron diffusion deposition treatment, and then introducing the boron source gas to perform secondary PN junction deposition on the surface of the second oxide mask; Pushing: In a nitrogen atmosphere, the temperature is raised to 900℃~990℃ to push the PN junction.

2. The boron diffusion process for N-type silicon wafer according to claim 1, characterized in that: The thickness of the first oxide mask is 20 nm to 30 nm, and the thickness of the second oxide mask is 20 nm to 30 nm.

3. The boron diffusion process for N-type silicon wafer according to claim 2, characterized in that: During the preparation of the first oxidation mask and / or the second oxidation mask, the water vapor is nitrogen carrying water vapor, and the nitrogen carrying out the water vapor is carried out under an environment of pressure of 500mbar~750mbar and temperature of 60℃~80℃, and the flow rate of the nitrogen carrying water vapor is 100sccm~500sccm.

4. The boron diffusion process for N-type silicon wafer according to claim 3, characterized in that: During the preparation of the first oxidation mask and / or the second oxidation mask, the flow rate of the nitrogen gas is 1000 sccm to 3000 sccm.

5. The boron diffusion process for an N-type silicon wafer according to any one of claims 1 to 4, characterized in that: During the primary PN junction deposition and / or the secondary PN junction deposition process, the boron source gas is a mixed gas of BCl3, oxygen and nitrogen, the flow rate of BCl3 is 50sccm~300sccm, the flow rate of oxygen is 150sccm~800sccm, and the flow rate of nitrogen is 1000sccm~3000sccm.

6. The boron diffusion process for an N-type silicon wafer according to any one of claims 1 to 5, characterized in that: The deposition time of the first oxide mask is 8 min to 15 min, the deposition time of the primary PN junction is 15 min to 25 min, the deposition time of the second oxide mask is 8 min to 15 min, and the deposition time of the secondary PN junction is 15 min to 25 min.

7. The boron diffusion process for an N-type silicon wafer according to any one of claims 1 to 6, characterized in that: The primary boron diffusion and / or the secondary boron diffusion is performed under the conditions of a pressure of 100 mbar to 300 mbar and a temperature of 800° C. to 850° C.

8. The boron diffusion process for an N-type silicon wafer according to any one of claims 1 to 7, characterized in that: After the advancing step is completed, a third oxide mask with a thickness of 45 nm to 55 nm is deposited on the surface of the second oxide mask. During the deposition of the third oxide mask, nitrogen and water vapor are introduced.

9. The boron diffusion process for an N-type silicon wafer according to claim 8, characterized in that: The third oxide mask deposition is performed at a temperature of 900° C. to 990° C. and a pressure of 100 mbar to 300 mbar.

10. The boron diffusion process for an N-type silicon wafer according to any one of claims 1 to 9, characterized in that: In the advancing step, the water vapor is nitrogen gas carrying water vapor, the flow rate of the nitrogen gas carrying water vapor is 500 sccm to 1500 sccm, and the flow rate of the nitrogen gas is 1000 sccm to 2000 sccm.

11. The boron diffusion process for an N-type silicon wafer according to any one of claims 1 to 10, characterized in that: In the advancing step, the temperature is kept at 900° C. to 990° C. for 30 min to 50 min.

12. The boron diffusion process for N-type silicon wafer according to claim 11, characterized in that: In the advancing step, after being kept at 900° C. to 990° C. for 30 min to 50 min, the temperature is lowered to 850° C. to 650° C., nitrogen is introduced for purging, and normal pressure is restored.

13. The boron diffusion process for an N-type silicon wafer according to any one of claims 1 to 12, characterized in that: Before the primary boron diffusion is performed, nitrogen purging is performed in advance. The nitrogen purging is performed at 650° C. to 800° C. and normal pressure. During the nitrogen purging process, the nitrogen flow rate is 1500 sccm to 5000 sccm.

14. An N-type silicon wafer obtained by the boron diffusion process of the N-type silicon wafer according to any one of claims 1 to 13, characterized in that: The square resistance of the N-type silicon wafer is 120Ω / sq~170Ω / sq, and the surface concentration is 2.5×10 19 N / cm 2 ~6×10 19 N / cm 2 The junction depth of the PN junction is 0.75 μm to 1.1 μm.

15. A solar cell, characterized in that: The solar cell comprises an N-type silicon wafer, and the N-type silicon wafer is an N-type silicon wafer obtained by a boron diffusion process of the N-type silicon wafer according to any one of claims 1-13.

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