Acid etching additive, low-concentration hydrofluoric acid etching solution and etching method

Abstract

The invention discloses an acid etching additive, a low-concentration hydrofluoric acid etching solution and an etching method. The acid etching additive comprises the following components in percentage by mass: 0.01%-2% of an accelerant, 0.1%-0.5% of an interfacial agent, 1%-20% of an activating agent and the balance of deionized water. When the low-concentration etching liquid prepared by the acid etching additive is used for removing and etching the BSG on the back surface of the silicon wafer, the physical property of the silicon wafer cannot be influenced, the concentration of hydrofluoric acid for back etching can be remarkably reduced only by adding the acid etching additive in the process of removing the BSG on the back surface of the silicon wafer through acid pickling, the safety and environmental risks are reduced, and the service life of the acid etching additive is prolonged. And the production line efficiency, the productivity and the yield are not influenced.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic cell technology, and in particular to an acid etching additive for the BSG layer on the surface of a silicon wafer, a low-concentration hydrofluoric acid etching solution containing the additive, and an etching method for pre-removing the BSG layer using the etching solution. Background Technology

[0002] With the development of solar cells, TOPCon / perovskite tandem cells are one of the key directions leading the next generation of photovoltaic technology. Since TOPCon cells are PN junction cells, the emitter must first be fabricated on the front side of the silicon wafer using boron diffusion. Diffusion through a higher-temperature boron diffusion process forms a boron-rich layer. After the boron diffusion is complete, the boron atoms in the boron-rich layer undergo an oxidation reaction to form borosilicate glass (BSG), which protects the PN junction on the front side of the silicon wafer. Simultaneously, the BSG also wraps around the sides and back of the silicon wafer. Therefore, it is necessary to first remove the BSG layer on the sides and back of the silicon wafer using a chain conveyor method, followed by chemical etching, polishing, cleaning, and drying using a tank method to effectively improve the performance and safety of the cell. If the BSG layer is not completely removed from the cell, it will lead to a decrease in solar cell performance, leakage, reduced capacity, and increased internal resistance, ultimately resulting in a decrease in the conversion efficiency of the solar cell. Therefore, removing the BSG layer on the sides and back of the silicon wafer beforehand is an essential step in the photovoltaic cell manufacturing process.

[0003] However, the following problems exist when removing the BSG layer from the sides and back of the silicon wafer: 1. BSG has a very low etching rate in hydrofluoric acid, making removal very difficult. Therefore, removing BSG requires longer processing time and higher concentrations of hydrofluoric acid (25%~35%) to meet the demands of mass production. This leads to increased chemical usage costs, higher acid treatment costs, and safety hazards associated with high-concentration hydrofluoric acid. 2. As etching time progresses, products and insoluble byproducts generated during the HF solution etching of BSG deposit on the silicon wafer and BSG surface. The increased volume of insoluble byproducts accumulates and hinders the rate of fluoride ion etching of BSG, affecting cell quality and production capacity.

[0004] Therefore, overcoming the shortcomings of existing methods that pre-remove the BSG layer from the sides and back of the silicon wafer, such as increased production costs and safety hazards, and reduced product quality and production efficiency, is a problem that needs to be solved in this field. Summary of the Invention

[0005] To address the technical problems of increased production costs, safety hazards, and low production efficiency when pre-removing the BSG layer on the back and sides of silicon wafers, this invention provides an acid etching additive for removing BSG, a low-concentration HF solution containing the additive, and an etching method. This fundamentally alters the chemical reaction kinetics of low-concentration HF etching of BSG, achieving the goal of reducing concentration and increasing efficiency.

[0006] To achieve the above-mentioned technical objectives, the present invention provides an acid etching additive comprising the following components by mass percentage: Accelerator 0.01%–2%, surfactant 0.1%–0.5%, activator 1%–20%, balance deionized water.

[0007] Preferably, the accelerator component is at least one selected from sodium nitrate, sodium nitrite, sodium 3-nitrobenzenesulfonate, sodium m-nitrobenzenesulfonate, 3-nitrobenzeneic acid, sodium 4-nitrobenzeneacetate, or 4-nitrobenzenesulfonic acid.

[0008] Preferably, the surfactant component is at least one selected from sodium N-lauroyl sarcosinate, sodium dodecylbenzene sulfonate, sodium dioctyl sulfosuccinate, sodium lauroyl glutamate, and sodium secondary alkyl sulfonate.

[0009] Preferably, the activator component is one of potassium perchlorate, chlorite, ammonium perchlorate, hypochlorous acid, sodium bromate, ammonium persulfate, sodium percarbonate, or sodium chlorate.

[0010] The low-concentration hydrofluoric acid etching solution with the aforementioned acid etching additive provided by the present invention comprises the following components by mass percentage: The mixture consists of 1-10% hydrofluoric acid solution with a mass concentration of 49%, 0.1-1% acid etching additive, and the balance being deionized water.

[0011] The etching process method for BSG on the back and side surfaces of a silicon wafer provided by this invention includes the following steps: Step 1: Prepare the low-concentration hydrofluoric acid etching solution according to the specified ratio; Step 2: Place the low-concentration hydrofluoric acid etching solution in a tank equipped with an ultrasonic device, set the working frequency and power density of the ultrasonic waves, and start the ultrasonic waves. Step 3: Immerse the silicon wafer with the BSG layer on the back into the low-concentration hydrofluoric acid etching solution in the tank, and perform etching at the set temperature for the set time.

[0012] Preferably, in step 2, the operating frequency of the ultrasonic wave is set to 20~100 kHz and the power density is set to 100~1000 W / m².

[0013] Preferably, in step 3, the set temperature is 15-35℃ and the set time is 120-400s.

[0014] This invention provides an acid etching additive for formulating a low-concentration hydrofluoric acid etching solution. Using this etching solution, BSG on the sides and back of Topcon solar cell wafers is pre-etched and removed, effectively solving the technical problem of high hydrofluoric acid consumption in traditional processes. Specifically, by adding the acid etching additive provided by this invention to the hydrofluoric acid solution, the concentration of hydrofluoric acid is significantly reduced, enabling effective removal of BSG on the back of the silicon wafer in the HF bath of the wet process. This does not affect the normal operation of the wet process, ensuring the quality and efficiency of cell production and improving the overall level of the wet process. It reduces acid consumption, optimizes the process window without affecting cell yield, avoids safety risks, and does not affect the conversion efficiency of solar cells, effectively optimizing the energy-saving process of Topcon cells. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the hydrophobic effect on the back side of a silicon wafer after etching with the low-concentration hydrofluoric acid of the present invention, as shown in Example 1. Figure 2 For Comparative Example 1, a schematic diagram showing the hydrophobic effect on the back side of the silicon wafer after etching with concentrated acid from the production line. Figure 3 A schematic diagram showing the hydrophobic effect on the back side of a silicon wafer after etching with low-concentration hydrofluoric acid without additives, as shown in Comparative Example 2. Figure 4 Microscopic image of the BSG on the back of the silicon wafer before acid etching; Figure 5 This is a microscope image of the silicon wafer after the BSG on the back side was removed in Example 1; Figure 6 This is a microscope image of the back side of the silicon wafer after alkaline polishing in Example 1; Figure 7 This is a microscope image of the silicon wafer after the BSG was removed in Comparative Example 1. Figure 8 This is a microscope image of the back side of the silicon wafer after alkaline polishing in Comparative Example 1. Figure 9 This is a schematic diagram of the etching process of the present invention. Detailed Implementation

[0016] To address the drawback of high consumption of BSG acid etching solution for the back of photovoltaic cells, this application provides an acid etching additive for formulating low-concentration HF etching solution. The technical solution of this invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0017] This invention provides an acid etching additive, comprising the following components by mass percentage: Accelerator 0.01%–2%, surfactant 0.1%–0.5%, activator 1%–20%, balance deionized water, wherein: The accelerator component is at least one selected from sodium nitrate, sodium nitrite, sodium 3-nitrobenzenesulfonate, sodium m-nitrobenzenesulfonate, 3-nitrobenzeneic acid, sodium 4-nitrobenzeneacetate, or 4-nitrobenzenesulfonic acid.

[0018] This accelerator is a nitro compound in its molecular structure. It promotes rapid and characteristic acid-free etching of BSG on silicon wafers. Nitrate ions attract electrons to the silicon-oxygen bonds, lowering the activation energy of these bonds and allowing fluoride ions to better bind to silicon atoms. This increases the etching rate of BSG by fluoride ions. Combined with the accelerator, it makes Si-O and BO bonds easier to break, enabling the silicon-oxygen bonds to break in a shorter time, thus allowing fluoride ions to bond more quickly to silicon or boron atoms. The anionic carboxyl groups in the accelerator's molecular structure can separate and repel the fluorosilicate anion particles, the product of hydrofluoric acid etching, from the silicon wafer surface through electrostatic repulsion and steric hindrance, dispersing them uniformly in the aqueous system. This prevents particle aggregation and sedimentation, and avoids the product hindering the chemical reaction between hydrofluoric acid and BSG due to particle aggregation.

[0019] The surfactant is at least one of N-lauroyl sarcosinate sodium, sodium dodecylbenzene sulfonate, sodium dioctyl sulfosuccinate, sodium lauroyl glutamate, and sodium secondary alkyl sulfonate.

[0020] This surfactant reduces the surface tension of water while significantly improving the wettability of HF on the BSG surface, thus promoting the timely reaction of fluoride ions with Si-O bonds. It also enhances the spreadability of HF on the silicon wafer surface, allowing fluoride ions to react more fully with Si-O bonds, ultimately forming hydrophobic Si-H bonds and rapidly stripping the poorly soluble reaction product, fluorosilicic acid, from the BSG surface. Simultaneously, it facilitates the spread of pickling solutions on the silicon wafer surface, increasing the effective contact area between HF and BSG. Hydrogen ions (H+) in the HF molecule attack oxygen atoms on the silicon dioxide surface, forming SiF4- and water, significantly increasing the etching initiation points, thereby increasing the reaction rate, improving the efficiency of fluoride ion utilization, significantly reducing HF usage, and shortening the BSG etching time, thus reducing costs while providing superior etching performance. The anions in the surfactant molecule structure can repel fluorosilicate anion products, preventing their aggregation and precipitation on the silicon wafer surface, further promoting and accelerating the chemical etching reaction rate of the silicon oxide layer formed on the surface by fluoride ions. The surfactant also prevents the etching solution from creeping along the edges of the silicon wafer, which could damage the BSG on the front side of the silicon wafer.

[0021] The activator is one of potassium perchlorate, sodium chlorite, ammonium perchlorate, hypochlorous acid, sodium bromate, ammonium persulfate, sodium percarbonate, or sodium chlorate.

[0022] The activator's strong oxidizing properties continuously oxidize the bare silicon and boron atoms formed on the surface during BSG etching, creating nano- or micron-sized gaps. It also promotes the formation of hydrophilic hydroxyl groups from hydrogen atoms in the outer layer during etching. Furthermore, the accelerator, in conjunction with the surfactant, forms a more robust hydrophilic interface, promoting the adsorption of fluoride ions onto the silicon wafer surface, increasing the fluoride ion concentration at the silicon wafer-etching solution interface, and accelerating the breaking of silicon-oxygen bonds by fluoride ions (F⁻). Simultaneously, the accelerator oxidizes metal ions in the BSG, forming defect interstitial sites, which facilitates the penetration of fluoride ions for deeper etching of the BSG, further increasing the etching rate.

[0023] The low-concentration hydrofluoric acid etching solution with the aforementioned acid etching additive provided by the present invention comprises the following components by mass percentage: The solution consists of 1-10% hydrofluoric acid solution with a mass concentration of 49%, 0.1-1% acid etching additive, and the balance being deionized water.

[0024] The etching process method for BSG on the back and side surfaces of a silicon wafer provided by this invention includes the following steps: Step 1: Prepare the low-concentration hydrofluoric acid etching solution according to the specified ratio.

[0025] Step 2: Place the low-concentration hydrofluoric acid etching solution in a tank equipped with an ultrasonic device, set the ultrasonic operating frequency and power density, and start the ultrasonic process. The set ultrasonic operating frequency is 20~100 kHz, and the power density is 100~1000 W / m².

[0026] Step 3: Immerse the silicon wafer with the BSG layer on the back side into the low-concentration hydrofluoric acid etching solution in the tank, and perform etching at a set temperature for a set time. The set temperature is 15-35℃, and the set time is 120-400s.

[0027] This invention utilizes the aforementioned promoter, activator, surfactant, and ultrasound to achieve the removal of metal contamination through the following synergistic effect: Promoting oxidation: For example, oxidizing metallic impurities with hypochlorous acid to form porous active sites suitable for hydrofluoric acid etching on or inside the BSG surface.

[0028] Activation: For example, the nitrate ions of nitrobenzoic acid attract electrons on the silicon-oxygen bond by inducing the activation energy of the silicon-oxygen bond, reducing the reaction energy of fluoride ions combining with Si atoms, and promoting the breaking of Si-O bonds.

[0029] Spreading and stripping: For example, by wetting with sodium dodecylbenzenesulfonate, the hydrofluoric acid molecules it can carry can more easily wet the microstructure of the BSG surface of the silicon wafer, increase the etching sites, and accelerate the etching efficiency; at the same time, the stripping utilizes its own surface activity to enhance the wetting and dispersing effect of the solution, stripping the etching products from the surface and stably dispersing them in the solution.

[0030] The localized high temperature and pressure generated by ultrasonic cavitation accelerates the production of more reactive oxygen species and free radicals (·OH) by the activator, enhancing the oxidation step of BSG and making selective etching more sensitive. Ultrasound promotes more controllable cavitation bubble collapse regulated by surfactants, and the generated microjets can precisely impact the by-product accumulation layer. Optimized cavitation microfluidics help the etching inhibitor molecules adsorb more uniformly on the silicon surface and preferentially wash away the protective layer of the already reacted BSG area, exposing fresh glass surface for limited HF reaction. The acoustic flow effect maintains the stability of the H⁺ concentration at the silicon wafer interface, thereby ensuring the stability of the HF molecular state ratio and keeping the BSG etching rate constant. Ultrasound significantly increases the collision frequency between HF molecules and the BSG surface, allowing a low amount of limited HF molecules to exert maximum etching effect, directly improving the intrinsic reaction rate. Additives chemically "stabilize and amplify" the effectiveness of limited HF, while ultrasound physically "forcefully opens" the channels for reactant transport and product removal. The two work together to transform the traditional diffusion-controlled reaction into a highly efficient interfacial reaction under physically forced mass transfer, thereby achieving rapid and uniform exfoliation with low acid dosage.

[0031] The integration of the oxidizing power of the promoter, the inducing power of the activator, and the dispersing and carrying wetting effect of the surfactant forms a triple synergistic mechanism of "oxidation-activation-detachment", which significantly reduces the concentration of hydrofluoric acid, optimizes the surface morphology, and ultimately achieves reduced consumption and improved etching effect.

[0032] The present invention will be further illustrated below by comparing embodiments of the present invention with comparative examples of the prior art.

[0033] Example 1 1. Preparation of acid etching additives (raw material amounts are by mass percentage) Weigh out 0.4% sodium N-lauroyl sarcosinate, 0.1% sodium 3-nitrobenzene, 10% sodium chlorite, and 89.5% deionized water according to the specified ratio, and mix them evenly to obtain the acid etching additive.

[0034] (0.4% surfactant + 10% accelerator + 0.1% activator + 89.5% water) 2. Preparation of low-concentration hydrofluoric acid etching solution Add 120 liters of ultrapure water to the HF pickling tank of the chain-type BSG removal equipment, and then slowly add 6.0 liters of concentrated hydrofluoric acid with a mass concentration of 49% until the hydrofluoric acid volume concentration is 5%. At the same time, add 2 liters of acid etching additive provided by the present invention. Control the temperature of the prepared etching solution at 28°C and stir evenly to obtain the prepared low-concentration hydrofluoric acid etching solution.

[0035] 3. Acid tank etching process like Figure 9 As shown, after the boron-diffused silicon wafer enters the hydrofluoric acid bath via a conveyor belt, the front side of the silicon wafer is protected by a sprayed water film. During the chain-transfer cleaning of BSG in the HF bath, the etching solution prepared in this invention contacts the back side of the silicon wafer via rollers, causing the silicon wafer to float on the surface of the etching solution, thereby etching away the BSG on the back side and edges of the silicon wafer (please refer to...). Figure 4 , Figure 5 After etching, observe the hydrophobicity on the back of the silicon wafer (e.g., ...). Figure 1 (As shown), then the back of the silicon wafer is alkaline polished, cleaned, washed with water, dried, and sampled for testing. The results after treatment are as follows. Figure 6 As shown.

[0036] Example 2 1. Preparation of acid etching additives (raw material amounts are by mass percentage) Weigh 0.8% of 4-nitrobenzenesulfonic acid, 0.2% of sodium dioctyl sulfosuccinate, 5% of potassium perchlorate, and 94% of deionized water according to the specified ratio, and mix them evenly to obtain the acid etching additive.

[0037] 2. Preparation of low-concentration hydrofluoric acid etching solution Add 120 liters of ultrapure water to the HF pickling tank of the chain-type BSG removal equipment, and then slowly add 7.2 liters of concentrated hydrofluoric acid until the hydrofluoric acid concentration is 6%. At the same time, add 1.8 liters of acid etching additive provided by this invention. Control the temperature of the prepared etching solution at 28°C and stir evenly to obtain the prepared low-concentration hydrofluoric acid etching solution.

[0038] 3. Acid tank etching process like Figure 9 As shown, after boron diffusion, the silicon wafer enters the hydrofluoric acid bath via a conveyor belt. The front side of the silicon wafer is protected by a sprayed water film. During the chain-transfer cleaning of BSG in the HF bath, the etching solution prepared in this invention contacts the back side of the silicon wafer by being driven by rollers and floats the silicon wafer on the surface of the etching solution to etch away the BSG on the back side and edges of the silicon wafer. After etching, the silicon wafer passes through a post-cleaning, water washing, and drying bath.

[0039] Comparative Example 1 Taking traditional etching solutions and acid etching processes on the production line as an example, the BSG on the back of the silicon wafer is removed by a chain acid cleaning machine using a "floating on water" method. The back of the silicon wafer on the chain machine is etched by contacting a 25% concentration hydrofluoric acid solution until the removal is achieved (observe the hydrophobicity, see details). Figure 2 The back of the silicon wafer undergoes alkaline polishing, cleaning, rinsing, drying, and sample taking. Please refer to the results after processing. Figure 7 , Figure 8 .

[0040] Comparative Example 2 Comparative Example 2 used the same low-concentration hydrofluoric acid etching solution as in Example 1, but without the acid etching additive of the present invention.

[0041] After boron diffusion, the silicon wafer enters the hydrofluoric acid bath via a conveyor belt. The front side of the silicon wafer is protected by a sprayed water film. The silicon wafer undergoes chain-transfer cleaning and BSG etching in the HF bath (the hydrofluoric acid concentration of the bath solution is 5%, the same as the acid concentration in Example 1). The etching solution is applied to the back of the silicon wafer via a roller, causing the wafer to float on the surface of the etching solution. This process removes the BSG from the back and edges of the silicon wafer. After etching, the hydrophobicity is observed (e.g., ...). Figure 3 Then, the silicon wafers are alkaline polished, cleaned, washed with water, dried, and sampled for testing.

[0042] The hydrophobic effect on the back side of the silicon wafer etched with low-concentration HF etching solution in Example 1 of the present invention ( Figure 1 (As shown) and Comparative Example 1: Hydrophobic effect on the back side of a silicon wafer etched with conventional high-concentration HF etchant (as shown) Figure 2(as shown) and the hydrophobic effect on the back side of the silicon wafer after hydrofluoric acid etching without using the acid etching additive of the present invention in Comparative Example 2 (as shown). Figure 3 As shown in the figure, a comparison of these three silicon wafer samples shows that cleaning BSG with the low-concentration HF etching solution containing acid etching additives of the present invention (Example 1) still has a hydrophobic effect. Figure 1 ), and compared with the traditional concentrated acid etching effect ( Figure 2 The results are basically the same. If no additives are used, and only a low-acid etching solution (Comparative Example 2) is used for etching, BSG cannot be completely removed. Figure 3 ), with no hydrophobic effect.

[0043] Microscopic images of the back side BSG of the silicon wafer after acid etching and alkaline polishing in Example 1 of the present invention (e.g.) Figure 5 , Figure 6 The microscopic images (as shown) of the back surface of the silicon wafer after acid etching (BSG) and alkaline polishing, compared with those of Comparative Example 1 (as shown) Figure 7 , Figure 8 As shown in the figure, the size of the base of the silicon wafer after alkaline polishing is not much different from that of the traditional method, indicating that the residual BSG that hinders alkaline etching of single crystal silicon has been completely removed, and the hydrophobic effect can still be achieved. Moreover, it is basically the same as the effect of traditional acid washing. If the additive of the present invention is not used, but only low acid is used, there is no hydrophobic performance.

[0044] The samples from Examples 1 and 2 of this invention were applied to a battery production line and compared with the sample from Comparative Example 1. The photoelectric conversion performance parameters were monitored and their efficiency measured, and the results are shown in Table 1. The recorded test results were obtained using a formula: Eta=Pout / Pin = (FFIscVoc) / Pin In the formula, Eta represents the photoelectric conversion efficiency, Uoc represents the open-circuit voltage, Isc represents the short-circuit current, FF represents the fill factor, and Pout and Pin represent the incident output and input power of sunlight. In Table 1, Rs represents the series resistance, Rsh represents the parallel resistance, and IRev2 represents the reverse current.

[0045] During testing, the production line efficiency remained stable, maintaining a single variable: the only difference between Examples 1 and 2 was the addition of the acid etching additive of this invention; all other parameters remained unchanged. The comparison method used a production line employing a traditional chain etching process as the baseline. Specifically, the battery electrical performance and yield were compared between the production line using a traditional acid washing process (Comparative Example 1) and Examples 1 and 2 treated with the acid etching additive, as shown in Table 2. Table 2

[0046] Based on the feedback electrical performance data, the short-circuit current in Examples 1 and 2 is no different from that in the production line, indicating that the low-concentration hydrofluoric acid prepared using the acid etching additive in this invention can completely remove the BSG on the back side. At the same time, the electrical performance is basically consistent with that of the acid washing process used in the production line, and the efficiency and yield are stable.

[0047] Existing technologies using chain-type wet etching processes with concentrated hydrofluoric acid (25%~30%) cause safety, environmental, and low production efficiency problems. This invention significantly reduces hydrofluoric acid consumption simply by adding a low-cost acid etching additive to the acid solution. Specifically, the concentration of the hydrofluoric acid solution can be reduced to 1~10% of a 49% (w / w) hydrofluoric acid solution, thus achieving the removal of BSG on the back side of the silicon wafer using low-concentration hydrofluoric acid while ensuring etching quality.

[0048] All aspects not detailed in this invention are well-known to those skilled in the art. The above specific embodiments are only used to illustrate the technical solutions of this invention and are not intended to limit it. Although this invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications and equivalent substitutions can be made to the technical solutions of this invention without departing from the spirit and scope of the technical solutions of this invention, and all such modifications and substitutions should be covered within the scope of the claims of this invention.

Claims

1. An acid etching additive, comprising the following components in weight percentage: Accelerator 0.01%–2%, surfactant 0.1%–0.5%, activator 1%–20%, balance deionized water.

2. The acid etching additive as described in claim 1, characterized in that, The accelerator component is at least one selected from sodium nitrate, sodium nitrite, sodium 3-nitrobenzenesulfonate, sodium m-nitrobenzenesulfonate, 3-nitrobenzeneic acid, sodium 4-nitrobenzeneacetate, or 4-nitrobenzenesulfonic acid.

3. The acid etching additive as described in claim 1, characterized in that, The surfactant component is at least one of N-lauroyl sarcosinate sodium, sodium dodecylbenzene sulfonate, sodium dioctyl sulfosuccinate, sodium lauroyl glutamate, and sodium secondary alkyl sulfonate.

4. The acid etching additive as described in claim 1, characterized in that, The activator component is one of potassium perchlorate, chlorite, ammonium perchlorate, hypochlorous acid, sodium bromate, ammonium persulfate, sodium percarbonate, or sodium chlorate.

5. A low-concentration hydrofluoric acid etching solution comprising the acid etching additive as described in any one of claims 1 to 4, comprising the following components by mass percentage: The mixture consists of 1-10% hydrofluoric acid solution with a mass concentration of 49%, 0.1-1% acid etching additive, and the balance being deionized water.

6. An etching method using the low-concentration hydrofluoric acid etching solution as described in claim 5, comprising the following steps: Step 1: Prepare the low-concentration hydrofluoric acid etching solution according to the specified ratio; Step 2: Place the low-concentration hydrofluoric acid etching solution in a tank equipped with an ultrasonic device, set the working frequency and power density of the ultrasonic waves, and start the ultrasonic waves. Step 3: Immerse the silicon wafer with the BSG layer on the back into the low-concentration hydrofluoric acid etching solution in the tank, and perform etching at the set temperature for the set time.

7. The etching method as described in claim 6, characterized in that, In step 2, the working frequency of the ultrasonic wave is set to 20~100 kHz and the power density is set to 100~1000 W / m².

8. The etching method as described in claim 6, characterized in that, In step 3, the set temperature is 15-35℃ and the set time is 120-400s.

Images