Method for gettering of n-type silicon wafer

Abstract

A gettering method for N-type silicon wafers involves annealing and gettingtering the chemically cleaned and polished silicon wafer body in an annealing furnace at a high temperature of 700-850°C. During the annealing and gettingtering process, oxygen is introduced into the annealing furnace. Before entering the annealing furnace, the oxygen is passed through a water bath in a low-temperature pure water bottle, allowing the oxygen to carry water vapor into the annealing furnace for an oxidation reaction. Subsequently, nitrogen gas is introduced into the annealing furnace. Before entering the annealing furnace, the nitrogen gas is passed through a constant-temperature phosphorus oxychloride bottle at 20°C, allowing the nitrogen gas to carry phosphorus oxychloride into the annealing furnace. This high-temperature annealing treatment of the silicon wafer body effectively prevents the deposition of metal ion impurities into the silicon wafer to form defects, improves the quality of N-type silicon wafers, and makes the cell conversion efficiency more concentrated, with an overall efficiency improvement of more than 0.2%.

Description

Technical Field

[0001] This invention belongs to the field of solar cell technology, and specifically relates to a gettering method for N-type silicon wafers used in N-type crystalline silicon solar cells. Background Technology

[0002] Currently, the production process of N-type monocrystalline silicon solar cells commonly uses N-type monocrystalline silicon wafers. Because phosphorus is used as a dopant in N-type silicon, and phosphorus has a low segregation coefficient, the resistivity and lifespan distribution of N-type silicon wafers are quite diverse during continuous crystal pulling. Areas with excessively low resistivity have a higher impurity content within the wafer. Excessive phosphorus doping intensifies the recombination of photogenerated carriers, and residual metal ions form deep-level recombination centers in the silicon material, resulting in lower cell conversion efficiency. Furthermore, directly recycling silicon wafers with low resistivity leads to significant cost waste. Summary of the Invention

[0003] To address the problems existing in the prior art, this invention provides a gettering method for N-type silicon wafers. By performing high-temperature annealing on the silicon wafer body, it effectively prevents the deposition of metal ion impurities into the silicon wafer to form defects, improves the quality of N-type silicon wafers, and makes the conversion efficiency of the battery more concentrated, with an overall efficiency improvement of more than 0.2%.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: a gettering method for N-type silicon wafers, comprising chemical cleaning and polishing of the silicon wafers after diamond wire cutting, annealing and gettering the chemically cleaned and polished silicon wafers in an annealing furnace at a high temperature of 700-850°C, wherein oxygen is introduced into the annealing furnace during the annealing process, and before the oxygen enters the annealing furnace, the oxygen passes through a water bath in a low-temperature pure water bottle, so that the oxygen carries water vapor into the annealing furnace for oxidation reaction; subsequently, nitrogen is introduced into the annealing furnace, and before the nitrogen enters the annealing furnace, the nitrogen passes through a constant-temperature phosphorus oxychloride bottle at 20°C, so that the nitrogen carries phosphorus oxychloride into the annealing furnace.

[0005] Furthermore, a mixture of hydrogen chloride and oxygen gas is introduced into the annealing furnace.

[0006] Furthermore, the amount of oxygen introduced into the annealing furnace is 100-5000 sccm, and the amount of nitrogen introduced into the annealing furnace is 100-3000 sccm.

[0007] Furthermore, the mixing ratio of hydrogen chloride and oxygen is 100-1000 sccm for hydrogen chloride and 100-3000 sccm for oxygen.

[0008] Furthermore, the combined reaction time of oxygen and nitrogen in the annealing furnace is 200-6000 seconds.

[0009] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0010] 1. Because the present invention uses pure water in a water bath before the oxygen source enters the annealing furnace, a certain amount of water vapor is formed after the oxygen source enters the annealing furnace. This water vapor, together with oxygen, forms a silicon dioxide layer on the silicon wafer, while effectively increasing the reaction between phosphorus oxychloride and oxygen. The phosphorus atoms generated by the reaction diffuse into the interior of the silicon wafer under high temperature conditions. During the diffusion process, the phosphorus atoms adsorb metal ions on the surface and inside of the silicon wafer, confining them from the free state to the defect sites of the silicon wafer oxidation reaction axis, effectively achieving the purpose of gettering.

[0011] 2. While phosphorus atoms are being attracted, this invention also introduces hydrogen chloride gas, thereby further increasing the gettering effect, improving the quality of the N-type silicon wafer, making the battery conversion efficiency more concentrated, and improving the overall efficiency by more than 0.2%, which has great economic value. Detailed Implementation

[0012] To further illustrate the present invention, specific embodiments are listed below;

[0013] I. Chemical cleaning and polishing of silicon wafers cut by diamond wire can be achieved using the following steps:

[0014] Step 1: At 60-70℃, use a mixed solution of sodium hydroxide (or potassium hydroxide) with a mass concentration of 0.1%-2% and hydrogen peroxide with a mass concentration of 0.1%-10%. Use a chemical solution circulation method and assist nitrogen bubbling to stir the chemical solution. The process time is 60-300 seconds.

[0015] Step 2: Clean the silicon wafer with pure water at room temperature for 60-300 seconds;

[0016] Step 3: Clean with a sodium hydroxide (or potassium hydroxide) solution with a mass concentration of 0.1%-5% at 75-85℃. During the cleaning process, the solution is circulated and stirred with nitrogen bubbling. The process time is 60-300 seconds.

[0017] Step 4: Clean the silicon wafer with pure water at room temperature for 60-300 seconds;

[0018] Step 5: Clean the silicon wafer with a mixed solution of sodium hydroxide (or potassium hydroxide) with a mass concentration of 0.1-5% and hydrogen peroxide with a mass concentration of 0.1%-10% at 60-70 degrees Celsius for 60-600 seconds.

[0019] Step 6: Clean the silicon wafer with pure water at room temperature for 60-300 seconds;

[0020] Step 7: Use a mixture of hydrofluoric acid and hydrochloric acid with a mass concentration of 0.1%-5% at room temperature. During the cleaning process, the solution is circulated and stirred with nitrogen bubbling. The process time is 60-300 seconds.

[0021] Step 8: Clean the silicon wafer with pure water at room temperature for 60-300 seconds;

[0022] Step 9: Dehydrate the silicon wafer at room temperature using a full-pull method;

[0023] Step 10: Dry the silicon wafer at 60-90℃ under nitrogen protection for 60-900 seconds;

[0024] The above-mentioned chemical cleaning and alkaline polishing methods can effectively remove the metal ions remaining on the surface of the silicon wafer during the diamond wire cutting process. The polishing effect of the alkaline solution removes part of the mechanical damage layer on the surface, which is beneficial to the uniformity of gettering in the subsequent gettering process.

[0025] II. Annealing and gettering of the silicon wafer body after chemical cleaning and polishing can be achieved using the following steps:

[0026] Step 1: Place the cleaned silicon wafers into a high-temperature diffusion annealing furnace;

[0027] Step 2: At 700-850℃, 100-5000 sccm of oxygen is introduced. Before entering the equipment, the oxygen is passed through a pure water bath at 20 degrees Celsius to ensure that it carries a certain amount of water vapor into the furnace tube. The reaction lasts for 100-2000 seconds. Under the combined action of water vapor and oxygen, a silicon dioxide layer of a certain thickness is produced on the surface of the silicon wafer.

[0028] Step 3: At 700-850℃, introduce 100-3000 sccm of nitrogen gas. Before entering the annealing furnace, the nitrogen gas passes through a phosphorus oxychloride constant temperature bottle at 20℃, carrying a certain concentration of phosphorus oxychloride into the reaction furnace tube for 100-3000 seconds. At the same time, 100-3000 sccm of oxygen (carrying a certain amount of water vapor through a pure water bottle at 20℃) can be continuously introduced into the furnace tube. The phosphorus atoms generated by the reaction under high temperature conditions diffuse into the silicon wafer. During the diffusion process, the phosphorus atoms adsorb metal ions on the surface and inside of the silicon wafer, confining them from the free state to the defect sites of the silicon wafer oxidation reaction axis, thus achieving the purpose of gettering. The addition of water vapor in this process can effectively enhance the reaction between phosphorus oxychloride and O2, improving the gettering effect.

[0029] Step 4: Perform continuous high-temperature gettering on the silicon wafer at 700-850℃ for 100-2000 seconds;

[0030] Step 5: At 700-850 degrees Celsius, introduce 100-1000 sccm of HCl gas and 100-3000 sccm of O2 for 100-2000 seconds; introducing HCl gas can effectively enhance the attractant effect of metal ions.

[0031] Step 6: Introduce 1000-30000 sccm of nitrogen to purge the harmful gases inside the furnace tube for 100-1000 seconds.

[0032] Step 7: Remove the silicon wafer from the furnace tube after the reaction is complete, cool it down, and the process is finished;

[0033] By performing phosphorus gettering and HCl gettering on silicon wafers, the quality of N-type silicon wafers can be effectively improved, resulting in more concentrated cell conversion efficiency and an overall efficiency improvement of over 0.2%, which has great economic value.

[0034] Obviously, the above embodiments are merely one example of the present invention, and any simple improvements on the principles provided by the present invention are within the scope of protection of the present invention.

Claims

1. A gettering method for N-type silicon wafers, comprising chemical cleaning and polishing processes on the silicon wafers after diamond wire cutting, characterized in that... The following steps are adopted: Step 1: Place the cleaned silicon wafer into a high-temperature diffusion annealing furnace; Step 2: At 700-850℃, 100-5000 sccm of oxygen is introduced. Before entering the equipment, the oxygen is passed through a pure water bottle at 20 degrees Celsius in a water bath to ensure that a certain amount of water vapor is carried into the furnace tube. The reaction lasts for 100-2000 seconds. Step 3: At 700-850℃, introduce 100-3000 sccm of nitrogen gas. Before the nitrogen gas enters the annealing furnace, it passes through a phosphorus oxychloride constant temperature bottle at 20℃. The nitrogen gas carries the phosphorus oxychloride into the inside of the reactor tube for 100-3000 seconds. Step 4: Perform continuous high-temperature gettering on the silicon wafer at 700-850℃ for 100-2000 seconds.

2. The gettering method for an N-type silicon wafer according to claim 1, characterized in that... It further includes step five: at 700-850 degrees Celsius, hydrogen chloride gas at 100-1000 sccm and oxygen at 100-3000 sccm are introduced for 100-2000 seconds.

3. The gettering method for an N-type silicon wafer according to claim 1, characterized in that... The combined reaction time of oxygen and nitrogen in the annealing furnace is 200-6000 seconds.