A cleaning agent for single crystal silicon wafers and a method for cleaning single crystal silicon wafers using the same

Abstract

The application belongs to the technical field of semiconductor material processing and solar cell manufacturing, and discloses a single crystal silicon wafer cleaning agent and a method for cleaning a single crystal silicon wafer by using the same. The single crystal silicon wafer cleaning agent comprises choline, an organic acid, a non-ionic surfactant and benzotriazole; the mass ratio of choline to the organic acid is (1.0-2.5):1; and the non-ionic surfactant is an alkyl phenol polyoxyethylene ether non-ionic surfactant. The cleaning agent can effectively remove organic contaminants, metal impurity contaminants and damage layers on the surface of a silicon wafer, so that a silicon wafer with a clean surface, uniform color, no pyramid suede structure, smooth linear edge, and no new cracks or edge collapse is obtained; and the cleaning agent can avoid the introduction of high-risk strong acid and alkali metal pollution, and improve the quality of the silicon wafer. The cleaning process is easy to control, the process window is wide, the requirement for equipment is low, the operation cost and the operation complexity are reduced by using the cleaning agent to clean the single crystal silicon wafer.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor material processing and solar cell manufacturing technology, specifically relating to a single-crystal silicon wafer cleaning agent and a method for cleaning single-crystal silicon wafers using the same agent. Background Technology

[0002] During the fabrication of monocrystalline silicon wafers, a damage layer with a depth of approximately 10-20 micrometers is formed near the surface of the wafer obtained by diamond wire cutting. This damage layer contains numerous microcracks, dislocations, and amorphous silicon structures, which not only significantly reduce the mechanical strength of the silicon wafer but also form a large number of recombination centers within the semiconductor, severely affecting minority carrier lifetime and thus reducing the photoelectric conversion efficiency of the final solar cell. Therefore, this damage layer must be removed through a cleaning process before subsequent processing.

[0003] Currently, the industry commonly uses traditional alkaline anisotropic wet cleaning technology to remove damaged layers and prepare textured surfaces. This method typically uses high-temperature, certain concentrations of sodium hydroxide (NaOH) or potassium hydroxide (KOH) solution to treat silicon wafers. Its core working principle is based on the strong anisotropic corrosion characteristics of single-crystal silicon in alkaline solutions, i.e., there are huge differences in cleaning rates on different crystal planes. The physical essence of this difference stems from the different surface density and chemical bond states of silicon atoms on different crystal planes: the atomic arrangement of the (100) crystal plane in crystalline silicon is relatively sparse, and silicon atoms have two dangling bonds, which are easy to bond with OH. - The reaction is fast, and the cleaning rate is fast; however, the (111) crystal plane has a very dense atomic arrangement, with each silicon atom having only one dangling bond, which requires a higher energy back bond to be broken to be removed, so the cleaning rate is very slow and it is often regarded as a stop crystal plane. When cleaning silicon wafers with the (100) crystal orientation, a pyramid-shaped textured structure surrounded by four slowly cleaning (111) planes will spontaneously form on the surface. In the manufacture of solar cells, this textured structure can effectively trap light and reduce surface reflectivity, and is therefore widely used. However, although alkaline anisotropic cleaning technology has been used for a long time, it has the following inherent defects and limitations: (1) Poor controllability of surface morphology, making it difficult to obtain a smooth surface: The intrinsic characteristics of this method determine that a pyramid textured surface will inevitably be generated on the (100) silicon wafer. For high-end semiconductor devices that require an absolutely flat surface or new high-efficiency battery technologies that pursue extremely low interfacial recombination, such a rough morphology is unacceptable. Although the uniformity of the pyramid can be improved by adding alcohols or special surfactants, it is impossible to fundamentally transform anisotropic cleaning into isotropic cleaning to obtain a smooth surface. (2) High risk of alkali metal ion pollution: Sodium ions (Na+) in NaOH or KOH + ) and potassium ions (K +(3) Process sensitivity and complex uniformity control: The cleaning rate and pyramid morphology are highly dependent on the concentration of cleaning agent, temperature, type and concentration of additives, and stirring conditions. The process window is narrow and requires precise environmental control to ensure batch-to-batch consistency, which poses a challenge to large-scale production. (4) Safety and environmental burden: High-temperature concentrated alkaline solution is highly corrosive and poses a safety hazard to equipment and operators. At the same time, waste liquid treatment also increases environmental protection costs.

[0004] Therefore, there is an urgent need to develop a cleaning method that can effectively remove the damaged layer on the silicon wafer surface, while avoiding the introduction of alkali metal contamination, obtaining a smooth or controllable surface morphology, and creating a safer and more environmentally friendly process environment. Summary of the Invention

[0005] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a single-crystal silicon wafer cleaning agent and a method for cleaning single-crystal silicon wafers using the same. The cleaning agent provided by the present invention can not only effectively remove the damaged layer on the surface of the silicon wafer, obtaining a smooth, pyramid-free textured surface and a high contaminant removal rate, but also avoid the introduction of alkali metal contamination, thus improving the quality of the silicon wafer.

[0006] This invention provides a cleaning agent for monocrystalline silicon wafers.

[0007] Specifically, a single-crystal silicon wafer cleaning agent comprises 0.5wt%-5.0wt% choline, 0.5wt%-3.0wt% organic acid, 0.001wt%-0.01wt% nonionic surfactant, and 0.005wt%-0.03wt% benzotriazole; the mass ratio of choline to organic acid is (1.0-2.5):1; the nonionic surfactant is an alkylphenol polyoxyethylene ether nonionic surfactant.

[0008] In some embodiments of the present invention, the mass ratio of choline to organic acid is (1.2-2.0):1.

[0009] In some embodiments of the present invention, the monocrystalline silicon wafer cleaning agent comprises 1.0wt%-5.0wt% choline, 1.0wt%-3.0wt% organic acid, 0.002wt%-0.008wt% nonionic surfactant and 0.005wt%-0.02wt% benzotriazole.

[0010] In some embodiments of the present invention, the alkylphenol polyoxyethylene ether nonionic surfactant is polyethylene glycol octylphenyl ether and / or nonylphenol polyoxyethylene ether. Polyethylene glycol octylphenyl ether and / or nonylphenol polyoxyethylene ether can reduce the surface tension of the system, allowing it to fully wet and penetrate the microcracks in the damaged layer; simultaneously, it can promote the selective distribution and adsorption of benzotriazole on the silicon wafer surface.

[0011] In some embodiments of the present invention, the choline is choline chloride and / or choline hydroxide. Choline chloride or choline hydroxide provides a moderately alkaline environment to the system and, as a hydrogen bond disruptor and surface activity promoter, effectively penetrates and breaks down organic pollutant membranes.

[0012] In some embodiments of the present invention, the organic acid includes at least one selected from citric acid, tartaric acid, malic acid, and oxalic acid, preferably citric acid and / or oxalic acid. In this system, citric acid and oxalic acid can react with Fe... 2+ Ni 2+ This allows for the formation of more stable water-soluble complexes, thereby stripping metallic contaminants from the silicon wafer surface.

[0013] It should be noted that the single-crystal silicon wafer cleaning agent also includes ultrapure water, which is used to dissolve or disperse choline, organic acids, nonionic surfactants and benzotriazole to form an aqueous cleaning agent.

[0014] The present invention also provides a method for preparing the above-mentioned single-crystal silicon wafer cleaning agent.

[0015] Specifically, the preparation method of the above-mentioned single-crystal silicon wafer cleaning agent includes the following steps: A single-crystal silicon wafer cleaning agent is prepared by adding choline, organic acid, nonionic surfactant and benzotriazole to ultrapure water and dissolving or dispersing them thoroughly.

[0016] The present invention also provides a method for cleaning monocrystalline silicon wafers using the above-mentioned monocrystalline silicon wafer cleaning agent.

[0017] Specifically, a method for cleaning monocrystalline silicon wafers involves cleaning the monocrystalline silicon wafers using the aforementioned monocrystalline silicon wafer cleaning agent at 50-70°C.

[0018] More specifically, a cleaning method for a single-crystal silicon wafer includes the following steps: The monocrystalline silicon wafer is immersed in ultrapure water for megasonic treatment; then the monocrystalline silicon wafer is placed in the above-mentioned monocrystalline silicon wafer cleaning agent for cleaning treatment; finally, it is rinsed with hot water overflow and dried.

[0019] In some embodiments of the present invention, the frequency of the megasonic treatment is 800 kHz to 1.5 MHz, and the treatment time is 3 to 10 minutes. Utilizing the cavitation effect and physical force of high-frequency megasonic waves, most of the silicon powder, abrasive particles, and some loose organic contaminants adhering to the silicon wafer surface are pre-stripped and removed without relying on chemicals, which is beneficial for subsequent cleaning steps.

[0020] In some embodiments of the present invention, the cleaning process is as follows: the monocrystalline silicon wafer is placed in the above-mentioned monocrystalline silicon wafer cleaning agent and treated at 50-70°C with stirring for 5-30 minutes. In specific operations, slow mechanical stirring may be used as appropriate.

[0021] In some embodiments of the present invention, the hot water overflow rinsing process involves transferring the cleaned silicon wafer to ultrapure water at 60-80°C and continuously overflow rinsing for 5-10 minutes. The hot water helps dissolve and rinse away residual choline and organic acid salts, ensuring no residue remains.

[0022] In some embodiments of the present invention, the drying process is as follows: the silicon wafer, after being rinsed with hot water overflow, is removed and the surface of the silicon wafer is dried using high-purity nitrogen (N2) or dry air. The high-purity nitrogen has a purity ≥99.999% (5N grade) and is filtered before use to ensure it is free of oil and water.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) In the cleaning agent system described in this invention, choline, organic acid, alkylphenol polyoxyethylene ether nonionic surfactant, and benzotriazole work together to effectively remove contaminants and subsurface damage layers from the silicon wafer surface and protect the intact substrate. Specifically, firstly, choline, organic acid, and surfactant work together to remove contaminants from the silicon wafer surface. Choline penetrates and breaks down the bond between organic matter and the silicon surface under mild conditions, and the alkaline environment it provides helps to saponify grease; organic acid then complexes and dissolves metal ions on the surface to form soluble complexes; surfactant reduces the surface tension of the system, promotes wetting of the cleaning agent, and assists in emulsifying and dispersing the removed grease contaminants in the solution. This process removes organic matter and metal impurities simultaneously under mild conditions. Subsequently, the components further target the crystalline damage layer on the subsurface. Surfactants promote the full penetration of the cleaning solution into the microcracks of the damage layer, while assisting benzotriazole in achieving selective distribution and adsorption on the silicon wafer surface. Benzotriazole molecules, acting as corrosion inhibitors, preferentially and firmly adsorb onto the intact crystal lattice surface to form a protective film, effectively inhibiting the etching of silicon by choline and organic acids. However, in defect-rich damaged layer regions, its adsorption is weaker, allowing the cleaning active ingredients to target the damaged layer and rapidly remove defects. Ultimately, while completely removing surface contaminants and the damaged layer, substrate loss is significantly reduced, resulting in a smooth, flat silicon wafer surface free of alkali metal contamination.

[0024] (2) The cleaning agent provided by the present invention can effectively remove organic pollutants, metal impurities and damaged layers from the surface of silicon wafers by selecting the components and controlling the dosage, so as to obtain silicon wafers with clean surface, uniform color, no pyramidal textured surface, smooth edges of line marks and no new cracks or chipping; and can avoid the introduction of high-risk strong acid and alkali metal contamination, thus improving the quality of silicon wafers.

[0025] (3) This invention avoids the use of high-risk strong acids such as HF and HNO3, as well as the introduction of alkali metal ions (such as Na+). + K + This invention provides a cleaning method with a wide process window, easy control, stable etching rate, and high safety, which addresses pollution. The cleaning wastewater does not contain difficult-to-treat fluorides, strong acids, or strong alkalis, and can be easily treated biochemically after neutralization, significantly reducing environmental pressure and treatment costs.

[0026] (3) The cleaning method provided by the present invention is carried out in a near-neutral, mild chemical environment, with an extremely low etching rate on the silicon wafer body, effectively protecting the silicon wafer surface and avoiding excessive etching and surface roughening. The cleaning process is easy to control, the operation is carried out at low temperature and normal pressure, the process window is wide, the equipment requirements are low, and the operating cost and operation complexity are reduced. Detailed Implementation

[0027] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0028] Unless otherwise specified, the raw materials, reagents or apparatus used in the following examples and comparative examples are available from conventional commercial sources or can be obtained by existing known methods.

[0029] Example 1 A single-crystal silicon wafer cleaning agent comprises 3.0 wt% choline hydroxide, 1.8 wt% citric acid, 0.006 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 1.7:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0030] Example 2 A single-crystal silicon wafer cleaning agent comprises 3.0 wt% choline chloride, 1.8 wt% oxalic acid, 0.006 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline chloride to oxalic acid is 1.7:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0031] Example 3 A single-crystal silicon wafer cleaning agent comprises 4.0 wt% choline hydroxide, 2.5 wt% citric acid, 0.003 wt% nonionic surfactant and 0.010 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 1.6:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0032] Example 4 A single-crystal silicon wafer cleaning agent comprises 3.6 wt% choline hydroxide, 3.0 wt% citric acid, 0.006 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 1.2:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0033] Example 5 A single-crystal silicon wafer cleaning agent comprises 3.0 wt% choline hydroxide, 1.2 wt% citric acid, 0.006 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 2.5:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0034] Example 6 A single-crystal silicon wafer cleaning agent comprises 2.0 wt% choline hydroxide, 2.0 wt% citric acid, 0.0016 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 1.0:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0035] Example 7 A single-crystal silicon wafer cleaning agent comprises 3.0 wt% choline hydroxide, 1.8 wt% malic acid, 0.006 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 1.7:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0036] Comparative Example 1 A single-crystal silicon wafer cleaning agent comprises 4.0 wt% choline hydroxide, 1.0 wt% citric acid, 0.006 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 4:1; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0037] Comparative Example 2 A single-crystal silicon wafer cleaning agent comprises 1.0 wt% choline hydroxide, 3.0 wt% citric acid, 0.006 wt% nonionic surfactant and 0.012 wt% benzotriazole; the mass ratio of choline hydroxide to citric acid is 1:3; the nonionic surfactant is polyethylene glycol octylphenyl ether (Dow Triton X-100).

[0038] Comparative Example 3 A single-crystal silicon wafer cleaning agent comprises 3.0 wt% choline hydroxide, 1.8 wt% citric acid and 0.006 wt% nonionic surfactant; the mass ratio of choline hydroxide to citric acid is 1.7:1; the nonionic surfactant is a branched alcohol ethoxylate (Dow TERGITOL TMN-10).

[0039] Comparative Example 4 The alkaline corrosion solution provided in Example 1 of Patent CN102108557A.

[0040] Application examples The cleaning agents provided in the above embodiments and Comparative Examples 1-3 were used to clean the monocrystalline silicon wafers. The specific steps are as follows: The cut monocrystalline silicon wafers from the same batch were grouped and immersed separately in an ultrapure water bath equipped with a megohmonic ultrasonic device. The megohmonic ultrasonic device was turned on, with the frequency controlled at 1MHz, and the treatment lasted for 5 minutes. The silicon wafers were then transferred to the main cleaning tank and immersed in the cleaning solution prepared in each embodiment or comparative example.

[0041] The temperature was controlled at 60℃ for 20 minutes, supplemented by slow mechanical stirring during the process. After cleaning, the silicon wafers were transferred to a hot ultrapure water bath maintained at 70℃ and continuously overflowed for 10 minutes for rinsing. Finally, the rinsed silicon wafers were removed from the hot water and immediately dried with filtered, oil-free, and water-free high-purity nitrogen gas (purity ≥99.999%) to complete the cleaning process.

[0042] Comparative Example 4 was processed according to the method described in Example 1 of Patent CN102108557A.

[0043] Product effectiveness test 1. Perform visual inspection on the processed silicon wafers. Macroscopic inspection: Under uniform white light (cleanroom lamp), visually inspect the silicon wafer by tilting and rotating it to check for visible stains, watermarks, color differences, or blemishes. A satisfactory surface should have a uniform and consistent color throughout. Using a strong side light (LED flashlight), observe whether the surface has a smooth, mirror-like reflection. A hazy or matte finish indicates the presence of micro-roughness.

[0044] Microscopic observation: Using a standard laboratory optical microscope (equipped with a 200x to 500x objective lens and a ring light source), observe whether regular pyramidal or triangular pyramidal protrusions appear; observe whether the original cutting lines become smooth and rounded, or are etched wider and deeper; at the same time, check for any new cracks or gaps (edge ​​chipping) on ​​the silicon wafer edges. The samples of each embodiment and comparative example must be observed under exactly the same conditions.

[0045] 2. Test the thinning amount of silicon wafers before and after cleaning. The high-precision mass difference subtraction method was used for measurement. First, the silicon wafer to be tested was dried in an oven at 110±2°C for 60 minutes to completely remove adsorbed water. Then, the silicon wafer was transferred to a constant temperature and humidity environment (25±1°C, relative humidity 50%±5%) and allowed to equilibrate for 4 hours to stabilize its temperature and humidity. The initial mass (m1) of the treated silicon wafer was measured using an analytical balance with a precision of 1 / 1,000,000. After cleaning using the above process, the wafer underwent the same drying, constant temperature and humidity, and weighing process again to measure its post-cleaning mass (m). 2 Based on the mass difference (Δm=m1-m2) and silicon density (2.33g / cm³), 3 ) and the single-sided area of ​​the silicon wafer, calculate the average thinning thickness (Δd).

[0046] 3. Measurement of metal pollutant removal rate The determination was performed using an acid elution combined with trace element analysis. Silicon wafers were placed in a solution containing known concentrations of metal ions (Fe, Cu) to adsorb contaminants. For control samples, surface metals were completely eluted with hot concentrated acid to a fixed volume solution, and the metal concentration (C0) was determined using inductively coupled plasma mass spectrometry (ICP-MS). Test samples were cleaned with a cleaning agent, rinsed, and dried, then subjected to the same acid elution treatment and brought to a fixed volume. The residual metal concentration (C1) was measured. The metal contaminant removal rate was calculated using the formula (C0 - C1) / C0 × 100%.

[0047] The results of the inspection or test are shown in Table 1.

[0048] Table 1

[0049] The single-crystal silicon wafer cleaning agents provided in Examples 1-7 of this invention all use components and proportions within the protection range, and all achieve relatively excellent comprehensive cleaning effects, with Examples 1 and 2 showing the best results. Tests show that all examples, while removing a damage layer of approximately 10-20 micrometers, effectively control the silicon wafer thinning amount to a low level of 0.15-0.40 μm, and maintain the metal contaminant removal rate above 97.5%, reaching a maximum of 99.5%. This confirms that a system with a choline to organic acid mass ratio of (1.0-2.5):1, and using alkylphenol polyoxyethylene ether surfactants and benzotriazole, can achieve efficient cleaning while providing good protection for the silicon substrate. In contrast, Comparative Examples 1 and 2, due to an imbalance in the choline to organic acid mass ratio (4:1 and 1:3, respectively), exceed the scope of this invention, resulting in poor etching selectivity on the silicon substrate, a significant increase in thinning amount to above 0.65 μm, and a significant decrease in metal removal efficiency. In Comparative Example 3, replacing the key surfactant polyethylene glycol octylphenyl ether with branched alcohol ethoxylate significantly reduced the metal contaminant removal rate to 95.0%, demonstrating the irreplaceable synergistic effect of alkylphenol polyoxyethylene ether surfactants in achieving high removal rates in this system. The traditional alkaline process (Comparative Example 4), due to its strong anisotropic etching, resulted in a thinning of up to 1.20 μm and posed a risk of alkali metal contamination.

[0050] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A cleaning agent for monocrystalline silicon wafers, characterized in that, It includes 0.5wt%-5.0wt% choline, 0.5wt%-3.0wt% organic acid, 0.001wt%-0.01wt% nonionic surfactant and 0.005wt%-0.03wt% benzotriazole; the mass ratio of choline to organic acid is (1.0-2.5):1; the nonionic surfactant is an alkylphenol polyoxyethylene ether nonionic surfactant.

2. The single-crystal silicon wafer cleaning agent according to claim 1, characterized in that, The mass ratio of choline to organic acid is (1.2-2.0):

1.

3. The single-crystal silicon wafer cleaning agent according to claim 1 or 2, characterized in that, The monocrystalline silicon wafer cleaning agent comprises 1.0wt%-5.0wt% choline, 1.0wt%-3.0wt% organic acid, 0.002wt%-0.008wt% nonionic surfactant and 0.005wt%-0.02wt% benzotriazole.

4. The single-crystal silicon wafer cleaning agent according to claim 3, characterized in that, The alkylphenol polyoxyethylene ether nonionic surfactant is polyethylene glycol octylphenyl ether and / or nonylphenol polyoxyethylene ether.

5. The single-crystal silicon wafer cleaning agent according to claim 3, characterized in that, The choline is choline chloride and / or choline hydroxide; and / or, the organic acid includes at least one of citric acid, tartaric acid, malic acid, and oxalic acid.

6. The method for preparing the single-crystal silicon wafer cleaning agent according to any one of claims 1-5, characterized in that, Includes the following steps: A single-crystal silicon wafer cleaning agent is prepared by adding choline, organic acid, nonionic surfactant and benzotriazole to ultrapure water and dissolving or dispersing them thoroughly.

7. A method for cleaning single-crystal silicon wafers, characterized in that, The monocrystalline silicon wafer is cleaned at 50-70°C using the monocrystalline silicon wafer cleaning agent as described in any one of claims 1-5.

8. The cleaning method according to claim 7, characterized in that, Includes the following steps: The monocrystalline silicon wafer is immersed in ultrapure water for megasonic treatment; then the monocrystalline silicon wafer is placed in the above-mentioned monocrystalline silicon wafer cleaning agent for cleaning treatment; finally, it is rinsed with hot water overflow and dried.

9. The cleaning method according to claim 8, characterized in that, The frequency of the megasonic wave processing is 800KHz-1.5MHz, and the processing time is 3-10 minutes.

10. The cleaning method according to claim 8, characterized in that, The hot water overflow rinsing process is as follows: the cleaned silicon wafer is transferred to ultrapure water at 60-80℃ and continuously overflowed for 5-10 minutes.

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