Boron diffusion method

Abstract

The invention discloses a boron diffusion method. The method comprises the following steps: S1, heating a furnace tube to 950-1050 DEG C, and keeping the temperature; the pressure of the furnace tube is set to be 500-800 mbar during the period; oxygen is introduced for purging and air exhaust; cooling and restoring to normal pressure; nitrogen of 100 sccm to 2000 sccm is arranged at a furnace opening of the furnace tube; s2, feeding a silicon wafer into the furnace tube in the step S1, and sequentially carrying out source pre-deposition and redistribution; obtaining a finished product. The method can greatly reduce the problem of edge blackening of the silicon wafer.

Description

Technical Field

[0001] The invention relates to the technical field of solar cells, and in particular to a boron diffusion method. Background Art

[0002] TOPCon (Tunnel Oxide Passivated Contact) solar cells that use an ultra-thin oxide layer as a passivation layer structure are considered to be one of the current mainstream application technologies for N-type high-efficiency crystalline silicon solar cells.

[0003] At present, in industrial production lines, the front emitter structure of N-type TOPCon cells is usually formed by a low-pressure boron thermal diffusion doping process. However, since the solid solubility of boron in silicon is lower than that of phosphorus, the difficulty of boron diffusion increases and a higher process temperature is required. The boron diffusion process mainly has two stages: first, the diffusion pre-deposition process step uses a lower temperature and a shorter time to deposit a layer of impurity source on the surface of the silicon wafer with a constant surface source concentration, during which the impurities hardly diffuse into the silicon wafer; then, according to the junction depth and concentration requirements, a higher temperature and a longer time are set in the diffusion redistribution process step to allow the surface deposition source to diffuse into the silicon wafer body for a second time, complete the redistribution of impurities, and form the necessary junction depth and surface concentration. However, the existing boron diffusion process has the problem of blackening the edges of the silicon wafer after diffusion.

[0004] Therefore, it is necessary to develop a boron diffusion method that can reduce the blackening problem of silicon wafers. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a boron diffusion method, which can reduce the problem of blackening of silicon wafers after diffusion.

[0006] The boron diffusion method according to the first aspect of the present invention comprises the following steps:

[0007] S1, heating the furnace tube to 950-1050°C and keeping it warm; during this period, the furnace tube pressure is set to 500-800 mbar; oxygen is introduced for purging and exhaust; the temperature is lowered and restored to normal pressure; 100-2000 sccm of nitrogen is set at the furnace mouth of the furnace tube;

[0008] S2, sending the silicon wafer into the furnace tube in step S1 to perform source pre-deposition and redistribution in sequence; and obtaining the product.

[0009] The boron diffusion method according to the embodiment of the present invention has at least the following beneficial effects:

[0010] The present invention first heats an empty furnace tube to 950-1050°C for heat preservation treatment; then sets 100-2000sccm of nitrogen at the furnace mouth of the furnace tube, and then performs source pre-deposition and redistribution to obtain a diffused silicon wafer. This method can greatly reduce the problem of blackening of the edge of the silicon wafer.

[0011] This is because high-temperature calcination at 950-1050°C can remove water vapor and accumulated boron oxide impurities in the furnace tube, improve the black edge problem, and increase the yield.

[0012] Furthermore, 100-2000sccm of nitrogen is set at the furnace mouth of the furnace tube to form a gas protection layer between the furnace door and the quartz tube, which can reduce source corrosion and protect the furnace door on the one hand, and is conducive to better maintaining the stability of the furnace tube pressure on the other hand.

[0013] According to some embodiments of the present invention, in step S1, the insulation time is 10 min to 300 min.

[0014] According to some embodiments of the present invention, in step S1, the oxygen flow rate is 5000-30000 sccm.

[0015] According to some embodiments of the present invention, in step S1, the furnace tube pressure is set to high and low pressure alternately, for example, first set to 500-600 mbar; then set to 700-800 mbar; alternately performed several times; each time lasting 1600-2000 s.

[0016] According to some embodiments of the present invention, in step S2, the temperature of the through-source pre-deposition is 800-900°C.

[0017] According to some embodiments of the present invention, in step S2, the time of the source pre-deposition is 300s to 1500s.

[0018] According to some embodiments of the present invention, in step S2, the flow rate of nitrogen gas in the through-source pre-deposition is 1000-5000 sccm.

[0019] According to some embodiments of the present invention, in step S2, the flow rate of oxygen in the through-source pre-deposition is 200-2000 sccm.

[0020] According to some embodiments of the present invention, in step S2, the amount of boron source in the through-source pre-deposition is 100-200 sccm. Thus, by controlling the amount of boron source within the above range, the problem of silicon wafer EL black edge caused by boron expansion source blackening can be improved, and the online EL yield can be improved.

[0021] According to some embodiments of the present invention, the pressure in the furnace tube during the through-source pre-deposition is 100 mbar to 300 mbar. Therefore, by controlling the furnace tube pressure in the through-source pre-deposition within the above range, the problem of EL black edges of silicon wafers caused by blackening due to boron diffusion sources can be improved, thereby improving the online EL yield.

[0022] According to some embodiments of the present invention, in step S2, the redistribution temperature is 800°C to 1000°C.

[0023] According to some embodiments of the present invention, in step S2, the redistribution time is 300s to 2000s.

[0024] According to some embodiments of the present invention, the boron diffusion method further comprises repeating step S1 several times, for example, once, twice, three times or four times.

[0025] According to some embodiments of the present invention, the boron diffusion method further comprises, before step S1, a step of evacuating the furnace tube for leak detection.

[0026] According to some embodiments of the present invention, the normal pressure mentioned in the present invention refers to a standard atmospheric pressure.

[0027] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0029] Figure 1 The figure is a graph showing the black edge ratio of the silicon wafer in-line EL according to embodiments 1 to 3 of the present invention.

[0030] Figure 2 It is a graph showing the black edge ratio of the silicon wafer in-line EL of Example 1 and Example 4 of the present invention and Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0031] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0032] In the description of the present invention, it should be understood that descriptions involving orientations, such as up, down, front, back, left, right, etc., and orientations or positional relationships indicated are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0033] In the description of the present invention, "several" means more than one, "many" means more than two, "greater than", "less than", "exceed", etc. are understood to exclude the number itself, and "above", "below", "within", etc. are understood to include the number itself. If there is a description of "first" or "second", it is only used for the purpose of distinguishing the technical features, and cannot be understood as indicating or implying the relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

[0034] In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, connecting, etc. should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention based on the specific content of the technical solution.

[0035] In the description of the present invention, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0036] The raw materials in the embodiment of the present invention are as follows:

[0037] Silicon wafer: N-type silicon wafer grown using the CZ method, with a resistivity of 0.5-1.2Ω / cm.

[0038] Control scheme

[0039] This example provides a conventional boron diffusion method, comprising the following steps:

[0040] Without processing the quartz tube, the silicon wafer is sent into the quartz furnace tube of the diffusion furnace for source pre-deposition and redistribution in sequence;

[0041] The conditions for the through-source pre-deposition are as follows:

[0042] The pressure is 200 mbar; the source time is 690 s; the source temperature is 850°C; the nitrogen flow rate is 2950 sccm; the oxygen content is 700 sccm; the boron source amount is 220 sccm;

[0043] The redistribution conditions are as follows:

[0044] The redistribution time is 960s; the redistribution temperature is 900°C.

[0045] Example 1

[0046] This example provides a boron diffusion method, comprising the following steps:

[0047] Close the door of the empty quartz furnace tube, evacuate to 500mbar and complete the leak detection;

[0048] S1. Set the leak-checked furnace tube to be heated in a stepwise manner to 1050°C; during this period, the pressure of the furnace tube is set to high and low pressures, in the following order: ① The furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 5000sccm; ② The furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 23000sccm; ③ The furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 5000sccm; ④ The furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 23000sccm; perform two exhaust and purge cycles; cool down and return to normal pressure; set 100sccm of nitrogen at the furnace mouth of the furnace tube;

[0049] S2, sending the silicon wafer into the furnace tube in step S3 to perform source pre-deposition and redistribution in sequence;

[0050] The conditions for the through-source pre-deposition are as follows:

[0051] The pressure is 300 mbar; the source time is 1500 s; the source temperature is 800°C; the nitrogen flow rate is 5000 sccm; the oxygen content is 200 sccm; the boron source amount is 100 sccm;

[0052] The redistribution conditions are as follows:

[0053] The redistribution time is 300s; the redistribution temperature is 1100°C.

[0054] Example 2

[0055] This example provides a boron diffusion method, comprising the following steps:

[0056] Close the door of the empty quartz furnace tube, evacuate to 500mbar and complete the leak detection;

[0057] S1. Set the leak-checked furnace tube to be heated in a stepwise manner to 1030°C; during this period, the pressure of the furnace tube is set to high and low pressures, in the following order: ① the furnace tube pressure is set to 600mbar; the time is 2000s; the oxygen flow rate is 16000sccm; ② the furnace tube pressure is set to 700mbar; the time is 1600s; the oxygen flow rate is 23000sccm; ③ the furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 16000sccm; ④ the furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 23000sccm; perform two exhaust and purge cycles; cool down and return to normal pressure; set 800sccm of nitrogen at the furnace mouth of the furnace tube;

[0058] S2, sending the silicon wafer into the furnace tube in step S3 to perform source pre-deposition and redistribution in sequence;

[0059] The conditions for the through-source pre-deposition are as follows:

[0060] The pressure is 100 mbar; the source time is 300 s; the source temperature is 900°C; the nitrogen flow rate is 1000 sccm; the oxygen flow rate is 2000 sccm; the boron source amount is 200 sccm;

[0061] The redistribution conditions are as follows:

[0062] The redistribution time is 980s; the redistribution temperature is 895°C.

[0063] Example 3

[0064] This example provides a boron diffusion method, comprising the following steps:

[0065] Close the door of the empty quartz furnace tube, evacuate to 500mbar and complete the leak detection;

[0066] S1. Set the leak-checked furnace tube to a step-by-step temperature increase to 950°C; during this period, the furnace tube pressure is set to high and low pressures, in the following order: ① The furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 20000; ② The furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 30000sccm; ③ The furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 20000sccm; ④ The furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 30000sccm; perform two exhaust and purge cycles; cool down and return to normal pressure; set 2000sccm of nitrogen at the furnace mouth of the furnace tube;

[0067] S2, sending the silicon wafer into the furnace tube in step S3 to perform source pre-deposition and redistribution in sequence;

[0068] The conditions for the through-source pre-deposition are as follows:

[0069] The pressure is 140mbar; the source time is 540s; the source temperature is 845℃; the nitrogen flow rate is 2500sccm; the oxygen content is 700sccm; the boron source amount is 170sccm;

[0070] The redistribution conditions are as follows:

[0071] The redistribution time is 2000s; the redistribution temperature is 800°C.

[0072] Example 4

[0073] This example provides a boron diffusion method, comprising the following steps:

[0074] Close the door of the empty quartz furnace tube, evacuate to 500mbar and complete the leak detection;

[0075] S1, set the leak-checked furnace tube to stepwise temperature increase to 1030°C; during this period, the pressure of the furnace tube is set to high and low pressure, in the following order: ① the furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 20000; ② the furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 23000sccm; ③ the furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 20000sccm; ④ the furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 23000sccm; perform two exhaust and purge cycles; cool down and return to normal pressure; step S1 is repeated three times; 800sccm of nitrogen is set at the furnace mouth of the furnace tube;

[0076] S2, sending the silicon wafer into the furnace tube in step S3 to perform source pre-deposition and redistribution in sequence;

[0077] The conditions for the through-source pre-deposition are as follows:

[0078] The pressure is 150mbar; the source time is 600s; the source temperature is 825℃; the nitrogen flow rate is 2580sccm; the oxygen content is 500sccm; the boron source amount is 170sccm;

[0079] The redistribution conditions are as follows:

[0080] The redistribution time is 1100s; the redistribution temperature is 895°C.

[0081] Comparative Example 1

[0082] This example provides a boron diffusion method, and its preparation method is the same as that of Example 1, except that the step of "raising the temperature to 1050°C; during which the pressure of the furnace tube is set to high and low pressures, in sequence as follows: ① the furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 5000sccm; ② the furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 23000sccm; ③ the furnace tube pressure is set to 500mbar; the time is 2000s; the oxygen flow rate is 5000sccm; ④ the furnace tube pressure is set to 800mbar; the time is 1600s; the oxygen flow rate is 23000sccm; perform two vacuum and purge cycles; cool down and return to normal pressure" in step S1 is lacking.

[0083] Comparative Example 2

[0084] This example provides a boron diffusion method, and its preparation method is the same as that of Example 1, except that the step of setting 100 sccm of nitrogen at the furnace port of the furnace tube in step S1 is missing.

[0085] Performance Testing

[0086] The boron-diffused silicon wafers prepared in the embodiments of the present invention and the comparative examples were subjected to the following tests:

[0087] The test method for the black edge of silicon wafers: Electroluminescence (EL) injects unbalanced carriers by applying a forward bias to the cell, observes the self-luminescence of the silicon wafer under electric field excitation, and uses a high-resolution CCD camera to capture the image of the cell under near-infrared light, thereby detecting and determining its defects.

[0088] The calculation formula for the online EL black border ratio is: the number of degraded pieces determined by the corresponding EL black border / the total number of online EL test pieces × 100%.

[0089] The results are as follows Figure 1 He Ru Figure 2 As shown, Figure 1 and 2 The horizontal axes D and N in the graph represent the day shift and night shift respectively, with a total of seven consecutive days of production. Figure 1 It can be seen that the corresponding online EL black edge ratio of the control solution continues to be high, and the EL black edge ratio of Examples 1, 2, and 3 remains at a relatively low level, which can indicate that the process has advantages in production stability, can improve the EL black edge caused by boron diffusion-induced blackening, and improve the final online EL yield.

[0090] from Figure 2It can be seen that Comparative Example 1 did not adopt the cyclic calcination step to continue normal production, and its corresponding online EL black edge ratio was high and had a trend of continuous increase. Comparative Example 2 adopted a cyclic calcination step, but did not set the furnace mouth nitrogen. Its corresponding online EL black edge ratio was significantly reduced on the first day and remained below 0.05%. As production continued, its ratio increased and the peak value reached more than 0.5%, indicating that the setting of furnace mouth nitrogen will affect the yield stability to a large extent. The EL black edge ratio of Example 4 has a significant advantage, indicating that performing multiple cyclic calcination steps can reduce the corresponding online EL black edge ratio to a certain extent, improve the yield, and maintain the stability of the production line.

[0091] The above is a detailed description of the embodiments of the present invention, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A boron diffusion method, characterized in that: The steps include: S1, heating the furnace tube to 950-1050°C and keeping the temperature; during this period, the furnace tube pressure is set to 500-800 mbar; introducing oxygen for purging and exhausting; cooling and returning to normal pressure; setting 100-2000 sccm of nitrogen at the furnace port of the furnace tube; S2, sending the silicon wafer into the furnace tube in step S1 to perform source pre-deposition and redistribution in sequence; and obtaining the product.

2. The boron diffusion method according to claim 1, characterized in that: In step S1, the insulation time is 10 minutes to 300 minutes.

3. The boron diffusion method according to claim 1, characterized in that: In step S1, the oxygen flow rate is 5000-30000 sccm.

4. The boron diffusion method according to claim 1, characterized in that: In step S2, the temperature of the through-source pre-deposition is 800-900°C.

5. The boron diffusion method according to claim 1, characterized in that: In step S2, the time of the source pre-deposition is 300s to 1500s.

6. The boron diffusion method according to claim 1, characterized in that: In step S2, the flow rate of nitrogen gas in the through-source pre-deposition is 1000-5000 sccm; Preferably, the flow rate of oxygen is 200 to 2000 sccm; Preferably, the amount of the boron source is 100-200 sccm.

7. The boron diffusion method according to claim 1, characterized in that: During the source pre-deposition, the pressure in the furnace tube is 100 mbar to 300 mbar.

8. The boron diffusion method according to claim 1, characterized in that: In step S2, the redistribution temperature is 800°C to 1100°C.

9. The boron diffusion method according to claim 1, characterized in that: In step S2, the redistribution time is 300s to 2000s.

10. The boron diffusion method according to claim 1, characterized in that: The boron diffusion method further includes repeating step S1 several times.

Images