KR101129110B1 - Method for texturing surface of multicrystalline silicon wafers - Google Patents

Abstract

PURPOSE: A surface process texturing method of a polycrystalline silicon wafer is provided to improve the reflective efficiency of a silicon wafer by offering optimal concentration of hydrofluoric acid, nitric acid, and phosphoric acid which is included in etchant used in a texturing process. CONSTITUTION: Ultrasonic cleaning using cleaning agent is executed(S100). Volume ratio of nitric acid, hydrogen peroxide, and water of the cleaning agent is 1:2.8:114. A wafer is etched by etching solution consisting of the nitric acid, hydrofluoric acid, and phosphoric acid(S110). The etching solution is composed of the rate of 1:2:2 of the hydrofluoric acid of concentration 35%, the nitric acid of concentration 45%, and the phosphoric acid of concentration 10-30%. The etching solution is processed for 50second-5 minutes at room temperature.

Description

Texturing method for surface treatment of polycrystalline silicon wafers

The present invention relates to a method of texturing a silicon wafer for manufacturing a solar cell, and more particularly, using an etching solution composed of water, hydrofluoric acid, and phosphoric acid after treatment with a cleaning agent. A texturing method for surface treatment of a silicon wafer.

As concerns about environmental pollution have increased, research on solar cells, which are clean energy, is being actively conducted. Among these, a method of maximizing the absorption of light by texturing the wafer surface is one of the important techniques for improving the efficiency of solar cells by reducing optical loss due to light confinement. "Texturing" refers to the overall process of treating a silicon wafer surface to have an uneven structure.

On the other hand, crystalline silicon used in the solar cell manufacturing process includes a saw-damaged layer immediately after wire-sawing. The etching process for removing the crystal damage layer is generally a wet etching process, and a polycrystalline silicon wafer is a mixture of HF and HNO ₃ , an isotropic etching solution having a uniform etching rate regardless of the crystal orientation. Use a solution.

US Pat. 7,192,885 and the like have been published for this type of process. The patent discloses 20-55% water, 10-40% concentrated hydrofluoric acid (concentration: 50%), concentrated nitric acid (concentration: 65%), etc. By using the wet etching process, the self-reflection of the silicon wafer can be reduced to increase the energy conversion efficiency of the solar cell. For reference, the performance of a solar cell capable of converting sunlight into electrical energy generally measures the efficiency of conversion to light energy, the value of which is a ratio of the amount of incident light at the electrical output of the solar cell, usually expressed in%. . Therefore, lowering the self-reflection rate improves the efficiency of the solar cell.

As described above, the process of removing the crystal damage layer of the polycrystalline silicon wafer is a process that can greatly affect the energy conversion efficiency of the solar cell. Therefore, there is a demand for improvement of a process method capable of performing this process more effectively.

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and an existing method by adding the slurry wafer cleaning process prior to texturing and then presenting the optimum concentrations of hydrofluoric acid, nitric acid and phosphoric acid contained in the etching solution used in the texturing process. The purpose of the present invention is to provide a texturing method for surface treatment of a polycrystalline silicon wafer which can lower the self-reflection of the silicon wafer and improve the energy conversion efficiency of the solar cell.

In accordance with another aspect of the present invention, there is provided a method for texturing a surface of a polycrystalline silicon wafer, the method including: cleaning in a chemical etching process for texturing a silicon wafer for a solar cell, using a cleaning agent; And etching the etching solution (etching solution) consisting of hydrofluoric acid, nitric acid and phosphoric acid; wherein the cleaning step, using a detergent having a volume ratio of nitric acid, hydrogen peroxide and water of 1: 2.8: 114, at room temperature Ultrasonic cleaning treatment for 10 minutes, the etching solution used in the etching step is a mixture of 35% hydrofluoric acid, 45% nitric acid, 10-30% phosphoric acid in a ratio of 1: 2: 2, at room temperature Characterized in that the treatment for 30 seconds to 10 minutes.

Preferably, the etching step is performed at room temperature for 50 seconds to 5 minutes, wherein the etching solution is a mixture of 35% hydrofluoric acid, 45% nitric acid, and 10% phosphoric acid in a ratio of 1: 2: 2. , 1 minute treatment at room temperature is best.

According to the present invention as described above, by presenting the composition ratio of the cleaning solution used in the texturing process for the surface treatment of the polycrystalline silicon wafer and the composition ratio of the etching solution for texturing, by improving the reflection efficiency of the silicon wafer, the energy conversion of the solar cell It is possible to improve the efficiency.

In addition, the complex process, such as the existing RCA cleaning can be simplified to a single cleaning process, it is easy to apply to the actual manufacturing process.

1 is a flowchart illustrating a texturing method for surface treatment of a polycrystalline silicon wafer according to the RCA method according to the prior art;
2 is a flow chart showing a texturing method for surface treatment of a polycrystalline silicon wafer according to the present invention;
3 is a graph showing the surface reflectivity of a sample etched after ultrasonic cleaning;
4 and 5 are SEM images of the textured polycrystalline silicon surface after ultrasonic cleaning.

Hereinafter, the texturing method of the polycrystalline silicon wafer for solar cells according to the present invention will be described with the drawings.

FIG. 1 is a flowchart illustrating RCA cleaning according to the prior art, and FIG. 2 is a flowchart illustrating a part of a surface treatment process of a polycrystalline silicon wafer according to the present invention.

The general RCA cleaning corresponds to the steps S10 to S40, and each step is configured as follows.

Step of _{SPM (H 2 SO 4 + H} 2 O 2 + H 2 O) S10 cleaning is a step of removing organic matter adhering to the wafer surface by the strong oxidizing power of hydrogen peroxide and lactic acid. DHF (HF + H ₂ O) cleaning in step S20 is a process for removing unnecessary natural oxide film on the silicon surface. SC-1 (NH ₄ OH + H ₂ O ₂ + H ₂ O) cleaning of step S30 is a process of lifting off by removing the particles of the surface with hydrogen peroxide, and by removing the oxide film with ammonia. SC-2 (HCI + H ₂ O ₂ + H ₂ O) cleaning in step S40 is a process of dissolving and removing heavy metals (Fe, Ni, Cr, Cu etc) attached to the surface with HCI. Conventionally, after performing this process, the process proceeds to step S50 to perform an etching process with a hydrofluoric acid, nitric acid, phosphoric acid etching solution.

However, RCA cleaning, as mentioned above, is complex and requires considerable processing time and equipment for commercial applications. Therefore, the present invention is to replace the above steps S10 to S40, as shown in Figure 2, characterized in that the cleaning using an optimized cleaning agent (cleaning agent). The cleaning operation is ultrasonically cleaned at room temperature for 10 minutes using a cleaner having a volume ratio of nitric acid (HNO ₃ ), hydrogen peroxide (H ₂ O ₂ ), and water (H ₂ O) in a ratio of 1: 2.8: 114 ( Step S100). In step S100, since the step S10 to S40 corresponding to the RCA cleaning is replaced with one step, the process can be performed more simply.

When the step S100 is completed, the etching solution is composed of a ratio of 1: 2: 2 of hydrofluoric acid (HF) having a concentration of 35%, nitric acid (HNO ₃ ) having a concentration of 45%, and phosphoric acid (H ₃ PO ₄ ) having a concentration of 10-30%. As a mixed liquid, it processes for 30 second-10 minutes at normal temperature. According to a preferred embodiment of the present invention, when the concentration of phosphoric acid is 10% and the texturing time is 1 minute, fine pores are formed relatively uniformly and densely. When the microstructure is densely formed as described above, low reflectance can be obtained. The concentration of 35% hydrofluoric acid (HF), the concentration 45% nitric acid (HNO ₃ ), and the concentration 10% phosphoric acid (H ₃ PO ₄ ) are 1 When texturing is performed with a mixed solution composed of a ratio of 2: 2, it can be seen that a 7% reflectance is obtained (step S110).

Table 1 compares the surface reflectivity of samples etched with a ratio of 1: 1,2 of hydrofluoric acid (HF), nitric acid (HNO ₃ ), and pure water (deionized water, DIW) after cleaning. After performing the etching liquid etching process without performing the cleaning operation, B is to perform the etching liquid etching process after the RCA cleaning operation through the steps S10 to S40, C is the etching liquid etching after the S100 step according to the present invention This is a table comparing the reflectance when the process is performed. In comparison with the surface reflectance of the cleaning agent pretreatment ultrasonic cleaning process according to the present invention and the texturing after the conventional RCA cleaning process, it can be seen that it has a relatively excellent surface reflectivity (see Table 1). .

For reference, texturing No. The concentration ratio of hydrofluoric acid and nitric acid of 1 is 40% / 50%, and texturing No. The concentration ratio of hydrofluoric acid and nitric acid of 2 is 30% / 40%, texturing no. The concentration ratio of hydrofluoric acid and nitric acid of 3 is 20% / 30%.

Texturing No. A (no cleaning) B (RCA cleaning) C (invention) One 15% 13% 12% 2 33% 30% 25% 3 37% 35% 33%

[Table 2] is a table showing the surface reflectivity of a sample etched with a volume ratio of hydrofluoric acid, nitric acid and phosphoric acid 1: 2: 2, with a volume ratio of nitric acid, hydrogen peroxide, and water being 1: 2.8: 114. In this case, the concentration ratios of all the samples were 35% and 45%, respectively, and only the phosphoric acid concentration was changed to 10% to 30%, and the reflectance was measured.

Sample No. Phosphoric Acid Concentration (10%) Phosphoric Acid Concentration (20%) Phosphoric Acid Concentration (30%) Response time (minutes) One 7% 8% 16% One 2 6.6% 16% 15% 3 3 6% 17% 16% 5 4 5.6% 16% 18% 7 5 8% 11% 14% 10

Meanwhile, under the above conditions, the reflectance is measured in the wavelength range of 200-900 nm using a UV-visible spectrometer to measure the reflectance according to the structure formation of the textured substrate surface, and the scanning electron microscope for observation of the etched tissue. Observe the surface with (Scanning Electron Microscope, SEM, Quanta 200-FEG from FEI).

As a result of the experiment, HF / HNO ₃ textured polysilicon after the cleaning process showed a low reflectivity compared to the texturing without the cleaning process, it can be seen that the cleaning treatment is an important parameter to obtain a low reflectance. This is due to the fact that when the texturing is directly performed without undergoing the cleaning process, the texturing is performed while the foreign materials remain on the surface, so that the reaction rate is slow and the texturing is not efficiently performed.

The experimental results are as shown in the graph shown in FIG. That is, the graph is ultrasonic cleaning with a cleaning solution consisting of nitric acid, hydrogen peroxide, and water and then subjected to an etching process for 1 minute in a volume ratio of 1: 2: 2 of 35% hydrofluoric acid, 45% nitric acid, and 10% phosphoric acid. The reflectance of the surface reflectance of a sample is shown. The horizontal axis of the graph is the wavelength expressed in nanometers, and the vertical axis is the% reflectance. As can be seen from the graph, when the etching process is performed under the above conditions, it can be seen that approximately 7% of the optimized reflectance value can be obtained.

Figure 4 shows HNO ₃ , H ₂ O ₂ , H ₂ O (1: 2.8: 114) after ultrasonic cleaning, concentration 35% HF, concentration 45% HNO ₃ , concentration 10% H ₃ PO ₄ (1: 2 SEM image of the surface of the polycrystalline silicon textured for 1 minute under: 2). FIG. 5 shows concentrations of HNO ₃ , H ₂ O ₂ , H ₂ O (1: 2.8: 114) after ultrasonic cleaning. SEM image of polycrystalline silicon surface textured at 35% HF, 45% HNO ₃ , 10% H ₃ PO ₄ (1: 2: 2) for 5 minutes. As shown, it can be confirmed that homogeneous micropores are sufficiently generated so that the reflectance is lowered.

Therefore, it is better to perform the etching process after the pre-cleaning of the polycrystalline silicon substrate using an appropriate cleaning agent in the pre-texturing stage to obtain a better surface shape, thereby increasing the reflection efficiency of the solar cell. Can be.

The technical object of the present invention is not limited to the above-mentioned technical objects and other technical objects which are not mentioned can be clearly understood by those skilled in the art from the following description will be.

Claims (3)

In the chemical etching process for texturing a silicon wafer for solar cells,
A cleaning step using a cleaning agent; And
It includes; etching the etching solution (etching solution) consisting of hydrofluoric acid, nitric acid and phosphoric acid;
In the washing step, by using a detergent having a volume ratio of nitric acid, hydrogen peroxide and water in a ratio of 1: 2.8: 114, ultrasonic cleaning treatment at room temperature for 10 minutes,
Etching liquid used in the etching step is a mixed liquid consisting of 1: 2: 2 ratio of hydrofluoric acid at a concentration of 35%, nitric acid at a concentration of 45%, and phosphoric acid at a concentration of 10-30%, which is treated at room temperature for 30 seconds to 10 minutes. A texturing method for surface treatment of a polycrystalline silicon wafer, characterized in that.
The method of claim 1, wherein the etching step,
A texturing method for surface treatment of a polycrystalline silicon wafer, which is performed at room temperature for 50 seconds to 5 minutes.
The method of claim 1, wherein the etching step,
The etching solution is a mixed solution consisting of a hydrofluoric acid at a concentration of 35%, nitric acid at a concentration of 45%, and phosphoric acid at a concentration of 10% of 1: 2: 2, and is treated at room temperature for 1 minute for surface treatment of a polycrystalline silicon wafer. Texturing method.

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