A low-defect etching solution for silicon wafer wet etching and a preparation method thereof

Abstract

The application provides a low-defect etching solution for silicon wafer wet etching and a preparation method thereof, the low-defect etching solution comprises the following components in the total mass of the low-defect etching solution being 100 parts: 4-10 parts of hydrofluoric acid, 30-50 parts of nitric acid, 2-8 parts of an organic acid stabilizer, 0.05-1 part of a polyethylene glycol monoalkyl ether citric acid monoester, 0.002-0.02 parts of a metal ion control agent, and the balance is water; wherein the polyethylene glycol monoalkyl ether citric acid monoester is obtained by esterification reaction of polyethylene glycol monoalkyl ether and citric acid. The etching solution can reduce the surface roughness of the silicon wafer after wet etching, improve the flatness of the silicon wafer surface, and thus obtain a low-defect etching treatment effect while maintaining effective etching capacity.

Description

Technical Field

[0001] This application relates to the field of etching solution technology, specifically to a low-defect etching solution for wet etching of silicon wafers and its preparation method. Background Technology

[0002] Silicon wafers are a crucial basic material in the fabrication of semiconductor devices, integrated circuits, microelectronic components, and related functional structures. During silicon wafer processing, wafers typically undergo cutting, grinding, thinning, polishing, cleaning, and patterning. These machining or pretreatment processes may introduce micro-scratches, micro-pits, mechanical damage layers, localized stress concentration areas, or residual contaminants onto the wafer surface. If these surface defects are not effectively removed or mitigated, they can easily be amplified in subsequent processes, affecting the surface flatness of the silicon wafer and the stability of subsequent processes.

[0003] Wet etching is a common method in silicon wafer surface treatment. It typically utilizes a chemical etching solution to react with the silicon wafer surface to remove surface materials, improve surface condition, or achieve a certain degree of surface finishing. Acidic etching systems are widely used in wet etching of silicon wafers. For example, acidic etching solutions containing hydrofluoric acid and nitric acid can etch the silicon wafer surface through a continuous process of oxidation and dissolution. Nitric acid oxidizes the silicon wafer surface, while hydrofluoric acid further dissolves the oxidized surface layer, thereby removing the surface materials.

[0004] However, existing acidic wet etching solutions for silicon wafers still suffer from insufficient surface defect control during practical use. The silicon wafer surface is not completely uniform, especially after processes such as cutting, grinding, or thinning. The reactivity of surface micro-scratches, micro-pits, localized mechanical damage points, and contamination adsorption points differs from that of smooth areas. In a strong acid etching environment, these highly reactive micro-regions are prone to faster oxidation and dissolution, leading to further deepening of micro-scratches and expansion of micro-pits, ultimately increasing the surface roughness of the etched silicon wafer.

[0005] In addition, wet etching of silicon wafers can be carried out by means of immersion, spraying or liquid flow carrying to achieve contact between the etching solution and the silicon wafer surface. In the immersion or liquid flow process, the areas with micro-scratches and micro-pits on the silicon wafer surface are more likely to have insufficient local liquid renewal, bubble retention or enrichment of reaction by-products, which further amplifies the local etching differences.

[0006] In existing technologies, to improve the performance of acidic silicon wafer etching solutions, the ratio of hydrofluoric acid, nitric acid, organic acids, and water is typically adjusted, or conventional wetting agents, surfactants, and complexing agents are added to improve etching rate, liquid spreading, or metal ion control. However, these approaches often focus on adjusting overall acidity, etching rate, or general wetting properties, paying insufficient attention to the localized differences in reactions at micro-defect areas such as micro-scratches, micro-pits, and mechanical damage points on the silicon wafer surface. If wetting or adsorption is too weak, it is difficult to effectively mitigate localized over-etching in defective micro-areas; if adsorption is too strong, it may form localized organic shielding on the silicon wafer surface, affecting etching uniformity and potentially leading to residual problems during subsequent cleaning.

[0007] Therefore, there is still room for improvement in existing wet etching solutions for silicon wafers. In particular, it is necessary to solve the problems of excessively rapid reaction in micro-defect areas on the silicon wafer surface, local over-etching, and high surface roughness after etching during acidic wet etching, in order to obtain silicon wafers with better surface flatness after etching. Summary of the Invention

[0008] This application provides a low-defect etching solution for wet etching of silicon wafers and its preparation method, aiming to solve the problem that existing acidic wet etching solutions for silicon wafers are prone to excessively rapid local reactions in micro-areas of defects such as micro-scratches, micro-pits, and mechanical damage points on the silicon wafer surface, resulting in increased surface roughness and insufficient surface flatness after etching.

[0009] In a first aspect, this application provides a low-defect etching solution for wet etching of silicon wafers, comprising the following components based on 100 parts by mass of the total low-defect etching solution: 4-10 parts hydrofluoric acid, 30-50 parts nitric acid, 2-8 parts organic acid stabilizer, 0.05-1 part polyethylene glycol monoalkyl ether citrate monoester, 0.002-0.02 parts metal ion control agent, and the balance being water; wherein the polyethylene glycol monoalkyl ether citrate monoester is obtained by esterification reaction of polyethylene glycol monoalkyl ether and citric acid, and the polyethylene glycol monoalkyl ether includes at least one of polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether.

[0010] According to this application, by introducing polyethylene glycol monoalkyl ether citrate monoester into an acidic silicon wafer wet etching system composed of hydrofluoric acid and nitric acid, and by cooperating with an organic acid stabilizer to adjust the interfacial state of the etching solution, the etching solution can reduce the surface roughness of the silicon wafer after wet etching while maintaining effective etching ability, and improve the surface flatness of the silicon wafer, thereby obtaining a low-defect etching treatment effect.

[0011] Specifically, hydrofluoric acid and nitric acid are the main etching components of the etching solution in this application, forming a continuous oxidation-dissolution reaction process on the silicon wafer surface. Nitric acid promotes oxidation of the silicon wafer surface, while hydrofluoric acid dissolves the oxidized silicon surface layer, allowing new silicon surfaces to be exposed and participate in the etching reaction. By controlling the hydrofluoric acid content to 4-10 parts and the nitric acid content to 30-50 parts, it is beneficial to achieve a relative match between the oxidation and dissolution processes on the silicon wafer surface, avoiding insufficient etching due to an overly weak reaction, and also avoiding further amplification of micro-defect areas on the silicon wafer surface due to an overly strong reaction.

[0012] The main improvement of this application lies in the introduction of polyethylene glycol monoalkyl ether citrate monoester into the aforementioned acidic etching system. The polyethylene glycol monoalkyl ether citrate monoester is obtained through an esterification reaction between polyethylene glycol monoalkyl ether and citric acid. The polyethylene glycol monoalkyl ether includes at least one of polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether. This component is not a simple polyether wetting agent, nor is it a physical mixture of polyethylene glycol monoalkyl ether and citric acid; rather, it links polyethylene glycol segments and citrate monoester residues within the same molecule. The polyethylene glycol segments possess good hydrophilicity and hydration ability, which can improve the spreading state of the etching solution on the silicon wafer surface, enabling the acidic etching solution to form a more continuous hydration interface film near micro-scratches, micro-pits, and mechanical damage points. The citrate monoester residues retain carboxyl and hydroxyl structures, which can generate appropriate hydrogen bonding or polar interactions with silicon oxide, silanol groups, or highly hydrophilic defect micro-regions formed on the silicon wafer surface during etching. Therefore, polyethylene glycol monoalkyl ether citrate monoester can form a dynamic adsorption-desorption state near the defect micro-regions on the silicon wafer surface.

[0013] Compared to adding polyethylene glycol monoalkyl ether alone, the citrate residues in polyethylene glycol monoalkyl ether citrate monoester can enhance the interfacial orientation of the molecule to the silicon wafer oxidation / hydroxylation microregions. This allows the polyethylene glycol segments to not only be dispersed in the bulk solution but also to more easily enter the vicinity of the defect microregions on the silicon wafer surface along with the citrate residues. Compared to adding citric acid alone, the polyethylene glycol segments can also provide hydration segment support for the citrate monoester residues, enabling them to form a renewable interfacial liquid film near the defect microregions, rather than rapidly diffusing into the entire acidic system as small molecules. Therefore, this structure allows "appropriate binding of defect microregions" and "hydration liquid film regulation" to occur synergistically on the same interfacial molecule.

[0014] In the wet etching process of acidic silicon wafers, areas with micro-scratches, micro-pits, and grinding damage points typically exhibit higher local reactivity than smooth silicon surfaces. These areas are more prone to faster oxidation and dissolution in strong acid systems, leading to further defect expansion and resulting in increased surface roughness after etching. The polyethylene glycol monoalkyl ether citrate monoester in this application generates a moderate dynamic effect near these highly reactive micro-regions through citrate monoester residues, allowing the molecule to temporarily reside in the defect micro-regions. Simultaneously, polyethylene glycol segments form a hydration interface layer near these regions, mitigating the rapid local reaction of acidic components to the defect points. Therefore, the difference in reaction rates between defect micro-regions and smooth regions is reduced, mitigating the tendency for micro-scratches to deepen and micro-pits to expand, thereby reducing the surface roughness of the silicon wafer after etching.

[0015] It is worth noting that, unlike conventional surfactants which primarily rely on hydrophobic segments and hydrophilic groups to reduce liquid surface tension, the polyethylene glycol monoalkyl ether citrate monoester in this application is not solely used to improve the macroscopic wettability of the etching solution. The citrate monoester residues in this component retain carboxyl and hydroxyl groups, enabling them to interact dynamically with the oxidized / hydroxylated microregions formed during silicon wafer etching, giving them a certain interfacial orientation near the defect microregions. The polyethylene glycol segments, on the other hand, can form a hydration interfacial liquid film near this region. Therefore, this component can simultaneously play a role in liquid film regulation and defect microregion reaction buffering, rather than simply improving wettability by reducing surface tension.

[0016] In this application, the organic acid stabilizer is mainly used to improve the reaction stability of the acidic etching system and the function of the interface regulator. On the one hand, it can regulate the dispersion environment of acidic components and organic interface regulator molecules in the etching solution, so that the system maintains a relatively uniform liquid phase state during etching. On the other hand, the carboxyl and / or hydroxyl structures in the organic acid stabilizer can also weakly interact with the oxidized / hydroxylated microregions on the silicon wafer surface, and form dynamic competitive adsorption and displacement with the citrate residues in polyethylene glycol monoalkyl ether citrate monoester. This dynamic competitive effect helps to avoid the formation of organic shielding due to long-term strong adsorption of polyethylene glycol monoalkyl ether citrate monoester on the silicon wafer surface, while maintaining its short-cycle interface regulation effect at highly active defect sites. Therefore, the combination of the organic acid stabilizer and polyethylene glycol monoalkyl ether citrate monoester is conducive to forming an interface regulation state of "brief residence-dynamic desorption-continuous renewal".

[0017] Metal ion control agents can form complexes or coordination with trace metal ions that may be present in the etching solution, reducing the adsorption or localized accumulation of metal ions on the silicon wafer surface. Since residual metal ions can become inducing points for uneven local reactions on the silicon wafer surface, adding an appropriate amount of metal ion control agent is beneficial to reducing localized roughness caused by metal contamination points.

[0018] Therefore, this application establishes a dynamic hydration interface control system during the wet etching process of acidic silicon wafers through the combination of hydrofluoric acid, nitric acid, organic acid stabilizers, metal ion control agents, and polyethylene glycol monoalkyl ether citrate monoester. This system can reduce localized over-etching at micro-defect areas on the silicon wafer surface without significantly sacrificing etching capability, and reduce the further amplification of micro-scratches, micro-pits, and mechanical damage points during the etching process, thereby reducing the surface roughness of the silicon wafer after etching and improving the surface smoothness of the silicon wafer after wet etching.

[0019] It should also be noted that the low-defect etchant of this application is preferably used as a freshly prepared etchant, with a short time from preparation to wet etching of the silicon wafer, and the wet etching process of the silicon wafer is usually a short process. Under these preparation and etching conditions, polyethylene glycol monoalkyl ether citrate monoester will not undergo extensive hydrolysis, and it can still maintain an effective interface regulation effect near the defect micro-regions on the silicon wafer surface. This allows the polyethylene glycol segments and citrate monoester residues to play a spatial synergistic role at the etching interface, thereby mitigating local over-etching at the defect micro-regions and reducing the surface roughness of the silicon wafer after etching.

[0020] Even in strong acid etching systems, the small amount of hydrolysis of polyethylene glycol monoalkyl ether citrate monoester during use will not affect the technical effects of this application. On the one hand, polyethylene glycol monoalkyl ether citrate monoester can participate in interface regulation of the silicon wafer surface in its monoester structure during the initial etching stage; on the other hand, its small amount of hydrolysis products are mainly polyethylene glycol monoalkyl ether and citric acid, both of which are compatible with acidic aqueous systems and will not form insoluble deposits or difficult-to-clean residues. Therefore, under the on-demand preparation and short-cycle etching conditions of this application, the small amount of hydrolysis of polyethylene glycol monoalkyl ether citrate monoester will not disrupt the interface regulation effect of the etching solution, nor will it hinder the achievement of low-roughness, low-defect etching effects.

[0021] In some embodiments, the polyethylene glycol monoalkyl ether citrate monoester is obtained by esterification of polyethylene glycol monoalkyl ether and citric acid under the catalysis of a solid acid catalyst, wherein the molar ratio of the terminal hydroxyl group in the polyethylene glycol monoalkyl ether to the molar ratio of the citric acid is 1:(1.01~1.1).

[0022] In some of the above embodiments, by maintaining a slight excess of citric acid relative to the terminal hydroxyl groups of polyethylene glycol monoalkyl ether (PEG-MAE), it is beneficial to promote the esterification reaction between the terminal hydroxyl groups and the carboxyl groups in citric acid, and to ensure that the reaction mainly forms PEG-MAE citrate monoester structures. This feeding ratio can ensure effective esterification of the terminal hydroxyl groups while reducing side reactions or free citric acid residues that may be caused by excessive citric acid. The citrate monoester residues retained in PEG-MAE citrate monoester are beneficial in providing appropriate carboxyl / hydroxyl interaction sites, allowing it to play a dynamic regulatory role at the silicon wafer etching interface; at the same time, fewer side reactions also help to reduce the impact of complex byproducts on the cleanliness of the etching solution and the stability of the surface treatment, thereby further reducing the surface roughness of the silicon wafer after etching.

[0023] In addition, the esterification reaction is catalyzed by a solid acid catalyst. After the reaction, the catalyst can be removed by filtration, which helps to reduce the impact of soluble catalyst residue on the cleanliness of the etching solution. This makes the obtained polyethylene glycol monoalkyl ether citrate monoester more suitable for use in silicon wafer wet etching systems.

[0024] In some embodiments, the number-average molecular weight of the polyethylene glycol monoalkyl ether is 600 to 1000.

[0025] In some of the above embodiments, controlling the number-average molecular weight of polyethylene glycol monoalkyl ethers to be between 600 and 1000 is beneficial for the resulting polyethylene glycol monoalkyl ether citrate monoester to possess both good aqueous dispersibility and interface control capabilities. Within this molecular weight range, the polyethylene glycol segments have appropriate chain lengths, enabling the formation of a hydrated interfacial liquid film near the silicon wafer etching interface, mitigating local reaction differences at defect micro-regions, thereby helping to reduce the surface roughness of the silicon wafer after etching.

[0026] Meanwhile, the above molecular weight range is also beneficial for controlling the system compatibility and subsequent cleaning properties of polyethylene glycol monoalkyl ether citrate monoester, making it less likely to cause excessive interfacial adsorption or increased organic residues in acidic etching solutions due to excessively long chain segments, making it more suitable for use in silicon wafer wet etching systems.

[0027] In some embodiments, the polyethylene glycol monoalkyl ether comprises polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether, wherein the mass ratio of polyethylene glycol monomethyl ether to polyethylene glycol monobutyl ether is 1:(0.4~0.6).

[0028] In some of the above embodiments, the inventors discovered that when polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether are used together in the citric acid monoesterification reaction, and their mass ratio is controlled at 1:(0.4~0.6), the surface roughness of the silicon wafer after treatment with the low-defect etching solution is further reduced. The possible reason for this is that the citric acid monoester formed by polyethylene glycol monomethyl ether has strong hydrophilicity, which is beneficial to improving its dispersion stability in the acidic etching solution and forming a more continuous hydration interface liquid film on the silicon wafer surface. The citric acid monoester formed by polyethylene glycol monobutyl ether contains butoxy group end groups, which have relatively stronger interfacial migration ability, which is beneficial to its distribution near the silicon wafer-etching solution interface and the microbubble interface, reducing the adhesion stability of microbubbles on the silicon wafer surface and reducing local etching unevenness caused by local bubble shading. The combination of the two can achieve a better balance between aqueous phase dispersibility, liquid film continuity, and microbubble desorption ability, making the acid contact state of each micro-region on the silicon wafer surface more uniform, thereby further reducing the surface roughness of the silicon wafer after etching.

[0029] In some embodiments, the solid acid catalyst comprises a sulfonic acid-type strong acid cation exchange resin.

[0030] In some of the above embodiments, the sulfonic acid-type strong acid cation exchange resin exhibits good acid catalytic activity, which can promote the esterification reaction between polyethylene glycol monoalkyl ether and citric acid, thus facilitating the acquisition of polyethylene glycol monoalkyl ether citrate monoester. Simultaneously, the sulfonic acid-type strong acid cation exchange resin is a solid catalyst, which can be removed from the reaction system after the reaction is complete by means of filtration, thereby reducing the residue of soluble acid catalyst and making the resulting product more suitable for use in silicon wafer wet etching solution systems.

[0031] In some embodiments, the organic acid stabilizer includes at least one of lactic acid and malic acid.

[0032] In some of the above embodiments, both lactic acid and malic acid can form a well-compatible acidic liquid phase system with hydrofluoric acid and nitric acid, which is beneficial to improving the dispersion state of each component in the etching solution and maintaining a relatively stable reaction environment in the acidic etching system during use. Among them, lactic acid and malic acid contain hydroxyl and carboxyl structures, which can participate in the weak interaction near the oxidation / hydroxylation micro-regions on the silicon wafer surface to a certain extent. When combined with polyethylene glycol monoalkyl ether citrate monoester, it is beneficial to maintain its dynamic regulation state at the etching interface. By selecting the above-mentioned organic acid stabilizers, the local reaction fluctuations during acidic etching can be further reduced, thereby helping to reduce the surface roughness of the silicon wafer after etching.

[0033] In some embodiments, the metal ion control agent includes at least one of hydroxyethylidene diphosphonic acid, aminotrimethylene phosphonic acid, ethylenediaminetetraacetic acid, and diethylenetriaminepentaacetic acid.

[0034] In some of the above embodiments, hydroxyethylidene diphosphonic acid, aminotrimethylenephosphonic acid, ethylenediaminetetraacetic acid, and diethylenetriaminepentaacetic acid all have the ability to complex metal ions, which can control the trace metal ions that may be present in the etching solution and reduce the adsorption or local enrichment of metal ions on the silicon wafer surface. Therefore, the risk of uneven local reactions caused by metal ion contamination points can be reduced, making the etching solution more suitable for wet etching systems of silicon wafers.

[0035] In some embodiments, the water is ultrapure water with a resistivity of not less than 18 MΩ·cm at 25°C.

[0036] In some of the above embodiments, using high resistivity ultrapure water helps to reduce the impact of ionic impurities in the water on the etching solution system, and reduces the risk of impurity ions remaining on the silicon wafer surface or inducing uneven local reactions, thus making it more suitable for use in silicon wafer wet etching systems.

[0037] Secondly, this application provides a method for preparing a low-defect etching solution for wet etching of silicon wafers, comprising: Provide the components comprising a low-defect etching solution according to any embodiment of the first aspect; mix the components and filter to obtain a low-defect etching solution.

[0038] According to this application, a low-defect etching solution with uniform component dispersion and low impurity and particle content can be obtained by mixing hydrofluoric acid, nitric acid, organic acid stabilizer, polyethylene glycol monoalkyl ether citrate monoester, metal ion control agent, and water, followed by filtration. This preparation method is simple and suitable for preparing ready-to-use acidic wet etching solutions for silicon wafers. The resulting etching solution can play a dynamic interface control role during silicon wafer etching, thereby helping to reduce the surface roughness of the silicon wafer after etching and improving its surface flatness.

[0039] Thirdly, this application provides an application of the low-defect etchant according to any embodiment of the first aspect in wet etching of silicon wafers.

[0040] According to this application, when the low-defect etching solution described in the first aspect is used for wet etching of silicon wafers, hydrofluoric acid and nitric acid can effectively etch the silicon wafer surface, polyethylene glycol monoalkyl ether citrate monoester can play a dynamic regulatory role at the silicon wafer etching interface, organic acid stabilizers can improve the reaction stability of the acidic system, and metal ion control agents can reduce the adsorption or local enrichment of trace metal ions on the silicon wafer surface. Therefore, local over-etching at micro-scratches, micro-pits, and mechanical damage points on the silicon wafer surface can be reduced, and surface roughening after etching can be reduced, thereby obtaining a silicon wafer with better surface flatness.

[0041] Compared with the prior art, the beneficial effects of this application are at least as follows: This application introduces polyethylene glycol monoalkyl ether citrate monoester into an acidic silicon wafer wet etching system composed of hydrofluoric acid and nitric acid. This component simultaneously possesses polyethylene glycol segments and citrate monoester residues. The polyethylene glycol segments facilitate the formation of a relatively continuous hydration interface film on the silicon wafer surface, while the citrate monoester residues can exert a moderate dynamic interaction with the oxidation / hydroxylation defect micro-regions on the silicon wafer surface. Therefore, the polyethylene glycol monoalkyl ether citrate monoester can play an interface regulation role during the silicon wafer wet etching process, mitigating localized over-etching at micro-scratches, micro-pits, and mechanical damage points, thereby reducing the surface roughness of the etched silicon wafer and improving its surface flatness.

[0042] Meanwhile, this application utilizes an organic acid stabilizer in combination with polyethylene glycol monoalkyl ether citrate monoester, which helps to regulate the local reaction environment of the acidic etching system and maintains the interface regulator in a dynamic adsorption-desorption state on the silicon wafer surface, preventing the formation of an excessively strong or prolonged organic masking layer on the silicon wafer surface. Therefore, the etching solution of this application can improve the surface quality of the silicon wafer after wet etching while maintaining effective etching capability.

[0043] Furthermore, the polyethylene glycol monoalkyl ether citrate monoester described in this application can be obtained by esterification of polyethylene glycol monoalkyl ether with citrate. The raw materials are readily available and the preparation method is relatively simple. The resulting etching solution is suitable for use as a ready-to-use wet etching solution for silicon wafers and has good practical application feasibility. Detailed Implementation

[0044] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0045] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0046] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0047] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0048] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0049] Polyethylene glycol monomethyl ether, with a number average molecular weight of approximately 750, MPEG-750; Polyethylene glycol monobutyl ether, with a number average molecular weight of approximately 750, BPEG-750; Sulfonic acid type strong acid cation exchange resin, NKC-9.

[0050] Preparation Example 1 Preparation of polyethylene glycol monoalkyl ether citrate monoester: Polyethylene glycol monoalkyl ether (PEG) and citric acid were added to a reaction vessel. The PEG monoalkyl ether was a mixture of PEG monomethyl ether (GM) and PEG monobutyl ether (GMBE) in a mass ratio of 1:0.5, and the molar ratio of the terminal hydroxyl groups in the PEG monoalkyl ether to the molar ratio of citric acid was 1:1.06. Then, 2.0% (by mass) of a sulfonic acid-type strong acid cation exchange resin was added. The mixture was heated to 105°C under nitrogen protection and stirred for 5 hours. After the reaction was completed, the system was cooled to below 60°C, and the solid acid catalyst was removed by filtration to obtain PEG monoalkyl ether citrate monoester A.

[0051] Preparation Example 2 The preparation method is largely the same as in Example 1, except that the polyethylene glycol monoalkyl ether is polyethylene glycol monomethyl ether, resulting in polyethylene glycol monoalkyl ether citrate monoester B.

[0052] Preparation Example 3 The preparation method is largely the same as in Example 1, except that the polyethylene glycol monoalkyl ether is polyethylene glycol monobutyl ether, resulting in polyethylene glycol monoalkyl ether citrate monoester C.

[0053] Comparative Preparation Example 1 Preparation of polyethylene glycol monoalkyl ether malate monoester: The preparation method is largely the same as in Example 1, except that citric acid is replaced with malic acid to obtain polyethylene glycol monoalkyl ether malic acid monoester.

[0054] Comparative Preparation Example 2 Preparation of polyethylene glycol monoalkyl ether succinate monoester: The preparation method is largely the same as in Example 1, except that citric acid is replaced with succinic acid to obtain polyethylene glycol monoalkyl ether succinic acid monoester.

[0055] Comparative preparation example 3 Preparation of citric acid-modified polyethylene glycol: Polyethylene glycol (number average molecular weight approximately 750) and citric acid were added to a reaction vessel, with the molar ratio of the terminal hydroxyl groups in polyethylene glycol to the molar ratio of citric acid being 1:1.06. Then, a sulfonic acid-type strong acid cation exchange resin, comprising 2.0% of the total mass of polyethylene glycol monoalkyl ether and citric acid, was added. The mixture was heated to 105°C under nitrogen protection and stirred for 5 hours. After the reaction was completed, the system was cooled to below 60°C, and the solid acid catalyst was removed by filtration to obtain citric acid-modified polyethylene glycol.

[0056] In the following examples and comparative examples, the amounts of hydrofluoric acid and nitric acid used are based on the effective acid content. When preparing the low-defect etching solution, the amount added is calculated based on the actual mass fraction of the hydrofluoric acid aqueous solution and nitric acid aqueous solution used. The water content in the acid aqueous solution is included in the amount of water used, and the total mass of the final etching solution is 100 parts by mass by adjusting the amount of added ultrapure water.

[0057] Example 1 Preparation of low-defect etching solution: Based on a total mass of 100 parts by weight of the low-defect etching solution, weigh out 6.5 parts hydrofluoric acid, 42 parts nitric acid, 5 parts lactic acid, 0.30 parts polyethylene glycol monoalkyl ether citrate monoester A, 0.008 parts hydroxyethylidene diphosphonic acid, and the remainder being ultrapure water. First, add the ultrapure water to a polytetrafluoroethylene container. Then, add the lactic acid and polyethylene glycol monoalkyl ether citrate monoester A under stirring, stirring until the system is homogeneous. Next, slowly add the nitric acid under ice-water bath cooling, followed by the slow addition of hydrofluoric acid, controlling the liquid temperature to not exceed 30°C during mixing. After the acid addition is complete, continue stirring for 20 minutes, then filter through a 0.22 μm acid-resistant filter membrane to obtain the low-defect etching solution.

[0058] Example 2 Preparation of low-defect etching solution: Similar to Example 1, except that polyethylene glycol monoalkyl ether citrate monoester B is used instead of polyethylene glycol monoalkyl ether citrate monoester A.

[0059] Example 3 Preparation of low-defect etching solution: Similar to Example 1, except that polyethylene glycol monoalkyl ether citrate monoester C is used instead of polyethylene glycol monoalkyl ether citrate monoester A.

[0060] Comparative Example 1 Preparation of low-defect etching solution: Based on a total mass of 100 parts by weight of the low-defect etching solution, weigh out 6.5 parts hydrofluoric acid, 42 parts nitric acid, 5 parts lactic acid, 0.25 parts polyethylene glycol monoalkyl ether (a mixture of polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether in a mass ratio of 1:0.5), 0.05 parts citric acid, 0.008 parts hydroxyethylidene diphosphonic acid, and the remainder being ultrapure water. First, add the ultrapure water to a polytetrafluoroethylene container. Then, under stirring, add the lactic acid, polyethylene glycol monoalkyl ether, and citric acid, stirring until the system is homogeneous. Next, slowly add the nitric acid under ice-water bath cooling, followed by the slow addition of hydrofluoric acid, controlling the liquid temperature to not exceed 30°C during mixing. After the acid addition is complete, continue stirring for 20 minutes, then filter through a 0.22 μm acid-resistant filter membrane to obtain the low-defect etching solution.

[0061] Comparative Example 2 Preparation of low-defect etching solution: Similar to Example 1, except that polyethylene glycol monoalkyl ether malate monoester is used instead of polyethylene glycol monoalkyl ether citrate monoester A.

[0062] Comparative Example 3 Preparation of low-defect etching solution: Similar to Example 1, except that polyethylene glycol monoalkyl ether succinate monoester is used instead of polyethylene glycol monoalkyl ether citrate monoester A.

[0063] Comparative Example 4 Preparation of low-defect etching solution: Similar to Example 1, except that citric acid-modified polyethylene glycol is used instead of polyethylene glycol monoalkyl ether citrate monoester A.

[0064] Test section The etching solutions prepared in the examples and comparative examples were used for wet etching of silicon wafers. The silicon wafers to be processed were P-type wafers that had undergone mechanical polishing pretreatment. <100> Before use, monocrystalline silicon wafers were rinsed with ultrapure water and dried with nitrogen. Silicon wafer samples of the same specifications were immersed in the etching solutions prepared in the respective examples and comparative examples, and etched at 25°C for 120 seconds. During etching, a low-speed oscillation was maintained to ensure liquid renewal. Immediately after etching, the silicon wafers were removed, thoroughly rinsed with ultrapure water, and then dried with nitrogen to obtain the etched silicon wafer samples.

[0065] The arithmetic mean roughness Ra of the etched silicon wafer surface was tested using a non-contact white light interferometer. Nine test points were selected from the central, middle, and edge regions of each silicon wafer. The test area of ​​each test point was 100 μm × 100 μm, and the average value of the nine test points was taken as the surface roughness Ra of the sample. The test results are shown in Table 1.

[0066] Table 1

[0067] According to Table 1, each embodiment has a lower Ra value compared to Comparative Examples 1-4, indicating that the low-defect etching solution provided in this application can effectively reduce the surface roughness of silicon wafers after wet etching and improve the surface smoothness of silicon wafers after etching. The possible reason is that this application forms polyethylene glycol monoalkyl ether citrate monoester by reacting polyethylene glycol monoalkyl ether with citric acid, so that polyethylene glycol segments and citrate monoester residues are linked in the same molecule. The polyethylene glycol segments are conducive to forming a hydration interface liquid film on the silicon wafer surface, while the citrate monoester residues are conducive to forming a moderate dynamic effect near the micro-regions of silicon wafer oxidation / hydroxylation defects, thereby mitigating local over-etching at micro-scratches, micro-pits and mechanical damage points, and reducing the surface roughness after etching. In Comparative Example 1, although both polyethylene glycol monoalkyl ether and citric acid were added, they were only physically combined; the polyethylene glycol segments and citric acid residues were not linked in the same molecule. The polyethylene glycol monoalkyl ether mainly dispersed in the liquid phase to exert a wetting effect, while citric acid mainly participated in acidic environment regulation in a small molecule form. It was difficult for the two to achieve a stable spatial synergistic effect at the silicon wafer etching interface. Therefore, Comparative Example 1 had a weak dynamic control ability on the micro-regions of defects on the silicon wafer surface, resulting in a high Ra value after etching. In Comparative Example 2, citric acid was replaced with malic acid to prepare polyethylene glycol monoalkyl ether malic acid monoester. Although malic acid residues contain carboxyl and hydroxyl groups, the number and spatial configuration of their action sites differ from those of citric acid residues, resulting in a relatively insufficient moderate dynamic effect on the micro-regions of silicon wafer oxidation / hydroxylation defects. Therefore, although Comparative Example 2 showed some improvement compared to Comparative Example 1, its effect on reducing surface roughness after etching was still weaker than in the other examples. In Comparative Example 3, citric acid was replaced with succinic acid to prepare polyethylene glycol monoalkyl ether succinic acid monoester. The succinic acid residues do not contain hydroxyl groups, resulting in weaker hydrogen bonds or polar interactions with the silicon wafer oxidation / hydroxylation micro-regions. This structure is closer to the interface regulation structure of ordinary polyether carboxylic acids, thus providing insufficient buffering for local reactions in defect micro-regions, leading to a higher Ra value after etching compared to the other examples.

[0068] In Comparative Example 4, citric acid-modified polyethylene glycol was used to replace polyethylene glycol monoalkyl ether citrate monoester A. While citric acid-modified polyethylene glycol also contains polyether segments and citric acid residues, its Ra value was still significantly higher than that of Examples 1-3. This may be because citric acid-modified polyethylene glycol is obtained by reacting dihydroxy polyethylene glycol with citric acid. Both ends of the polyethylene glycol segments may be connected to citric acid residues or form hydrophilic polar end groups mainly composed of carboxyl / hydroxyl groups, giving the molecule strong hydrophilic / polar interaction capabilities at both ends. This makes it easy to form tighter bonds with the silicon wafer oxidation / hydroxylation defect micro-regions, resulting in insufficient desorption and interface renewal capabilities at the defect micro-regions. This can easily lead to longer local organic molecule residence times or localized masking. In contrast, polyethylene glycol monoalkyl ether citrate monoester has a structure with one end capped with an alkyl ether and the other end containing citrate monoester residues. Its interfacial interaction is relatively moderate, which is more conducive to forming an interfacial regulation state of "brief residence-dynamic desorption-continuous renewal", thereby more effectively reducing the surface roughness of silicon wafers after etching.

[0069] As shown in Examples 1-3, the type of polyethylene glycol monoalkyl ether has a certain influence on the effect of low-defect etchant on reducing the surface roughness of silicon wafers after etching. Specifically, the Ra value of Example 1 is lower than that of Examples 2 and 3, indicating that when polyethylene glycol monoalkyl ether citrate monoester is prepared using a compound of polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether, the resulting etchant has a better effect on improving the surface smoothness of the silicon wafer.

[0070] The possible reason for this is that in Example 1, polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether participated in the esterification reaction together at a mass ratio of 1:0.5, resulting in polyethylene glycol monoalkyl ether citrate monoester containing both a highly hydrophilic methoxy polyether structure and a butoxy polyether structure with moderate interfacial migration ability. The former is beneficial for improving the dispersion stability of this component in the acidic etching solution and forming a more continuous hydration interface liquid film on the silicon wafer surface; the latter is beneficial for enhancing the distribution ability of this component at the silicon wafer-etching solution interface and near the microbubble interface, reducing localized etching unevenness caused by localized microbubble adhesion and insufficient liquid renewal.

[0071] In contrast, Example 2, which uses only polyethylene glycol monomethyl ether to prepare polyethylene glycol monoalkyl ether citrate monoester, exhibits good aqueous dispersion, but its interfacial migration and microbubble desorption assistance are relatively insufficient. Example 3, which uses only polyethylene glycol monobutyl ether to prepare polyethylene glycol monoalkyl ether citrate monoester, shows relatively enhanced interfacial interaction capabilities, but its aqueous dispersion stability and continuous liquid film formation ability are inferior to the composite system. Example 1, through the combination of two polyether structures, achieves a better balance between aqueous dispersion, continuous liquid film formation, and interfacial migration, resulting in a more uniform acid contact state in various micro-regions on the silicon wafer surface during etching. This more effectively mitigates localized over-etching in defect areas such as micro-scratches and micro-pits, ultimately resulting in a lower post-etching surface roughness.

[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A low-defect etching solution for wet etching of silicon wafers, characterized in that, The low-defect etching solution comprises the following components based on a total mass of 100 parts by weight: 4-10 parts hydrofluoric acid, 30-50 parts nitric acid, 2-8 parts organic acid stabilizer, 0.05-1 part polyethylene glycol monoalkyl ether citrate monoester, 0.002-0.02 parts metal ion control agent, and the balance being water; wherein the polyethylene glycol monoalkyl ether citrate monoester is obtained by esterification reaction of polyethylene glycol monoalkyl ether and citric acid, and the polyethylene glycol monoalkyl ether includes at least one of polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether.

2. The low-defect etching solution according to claim 1, characterized in that, The polyethylene glycol monoalkyl ether citrate monoester is obtained by esterification of polyethylene glycol monoalkyl ether and citric acid under the catalysis of a solid acid catalyst, wherein the molar ratio of the terminal hydroxyl group in the polyethylene glycol monoalkyl ether to the molar ratio of the citric acid is 1:(1.01~1.1).

3. The low-defect etching solution according to claim 2, characterized in that, The number-average molecular weight of the polyethylene glycol monoalkyl ether is 600-1000.

4. The low-defect etching solution according to claim 3, characterized in that, The polyethylene glycol monoalkyl ether includes polyethylene glycol monomethyl ether and polyethylene glycol monobutyl ether, wherein the mass ratio of polyethylene glycol monomethyl ether to polyethylene glycol monobutyl ether is 1:(0.4~0.6).

5. The low-defect etching solution according to claim 2, characterized in that, The solid acid catalyst includes a sulfonic acid-type strong acid cation exchange resin.

6. The low-defect etching solution according to claim 1, characterized in that, The organic acid stabilizer includes at least one of lactic acid and malic acid.

7. The low-defect etching solution according to claim 1, characterized in that, The metal ion control agent includes at least one of hydroxyethylidene diphosphonic acid, aminotrimethylene phosphonic acid, ethylenediaminetetraacetic acid, and diethylenetriaminepentaacetic acid.

8. The low-defect etching solution according to claim 1, characterized in that, The water is ultrapure water with a resistivity of not less than 18 MΩ·cm at 25℃.

9. A method for preparing a low-defect etching solution for wet etching of silicon wafers, characterized in that, include: Provide the components included in the low-defect etching solution according to any one of claims 1 to 8; mix the components and filter to obtain the low-defect etching solution.

10. The application of a low-defect etchant according to any one of claims 1 to 8 in wet etching of silicon wafers.

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