Electronic grade caustic alkali and preparation method thereof

Abstract

The invention relates to a preparation method of an electronic grade caustic alkali solution, the mass fraction of the caustic alkali solution is 40-50%, and the content of transition metal ions is less than 5 ppb. The invention aims to provide the preparation method of the caustic alkali solution suitable for cleaning / etching the silicon wafer, the problem that transition metal ion impurities are easy to remain after the silicon wafer is cleaned / etched is solved, and the transition metal impurities in the caustic alkali solution are deeply removed mainly through a cooling crystallization process controlled by an additive. Compared with a mode of directly carrying out cooling crystallization and directly adding a chelating agent into the caustic alkali solution, the method provided by the invention fundamentally solves the problem that the content of transition metal impurities in the caustic alkali solution for cleaning the silicon wafer is too high, so that the silicon wafer is polluted.

Description

Technical Field

[0001] The invention relates to the field of semiconductor manufacturing, and in particular to electronic-grade caustic soda and a preparation method thereof. Background Art

[0002] Silicon wafers are the base material of integrated circuit (IC) devices. With the continuous advancement of IC technology, the line width of devices has gradually decreased, and the requirements for the metal content on the surface and inside of silicon wafers have become increasingly stringent. In the process of silicon wafer manufacturing, the ingot must be processed by wire cutting, grinding, thinning and other processes, and it is difficult to avoid the introduction of metal impurities during the above processes. For example, during wire cutting and grinding, the surface of the silicon wafer can easily adsorb the residues from grinding and pollutants such as metals, particles, organic matter and metal ions in the grinding liquid. The above pollution will be retained in the damaged layer on the surface of the silicon wafer. In order to remove the damaged layer and pollutants, the current mainstream is to use alkaline cleaning / etching liquid for cleaning / etching. This is due to the fact that the etching rate of alkaline cleaning / etching liquid for silicon wafers is lower than that of acidic cleaning agents, and it is easier to obtain a surface with uniform roughness. It should be noted that although the alkaline cleaning / etching process can remove the damaged layer and retained pollutants on the surface of the silicon wafer, the damaged layer and the metal ions in the cleaning / etching solution may diffuse into the interior of the silicon wafer, especially transition metal elements such as nickel (Ni), copper (Cu), iron (Fe), and zinc (Zn). Once the metal ions diffuse into the interior of the silicon wafer, they are very difficult to remove, which will undoubtedly seriously affect the electrical properties of the silicon wafer, thereby causing product failure. Therefore, the alkaline cleaning / etching solution used in the silicon wafer industry has extremely stringent requirements on the metal ion content. The main component of the common alkaline cleaning / etching solution is caustic soda. Since caustic soda is generally produced through an ion membrane caustic soda process, it has a high content of transition metal impurities such as iron, nickel, and copper, and cannot be directly used for silicon wafer cleaning or etching. Further treatment is required to reduce the metal impurity content to reduce the residual metal ion impurities on the silicon wafer.

[0003] However, it is well known that metal ion chelating agents usually have the maximum chelating ability under neutral conditions, and the pH of alkaline cleaning / etching solutions for silicon wafers can often reach up to 14. Under such strong alkaline conditions, on the one hand, metal ions are difficult to ionize, and on the other hand, the chelating ability of various chelating agents for free metal ions decreases. These two reasons will greatly limit the chelation of metal ion impurities in alkaline cleaning / etching solutions for silicon wafers with chelating agents, thereby limiting the chelation of free metal ions in the solution by chelating agents. In addition, as a type of organic matter, chelating agents are also very likely to remain on the surface of silicon wafers after cleaning / etching, causing non-metallic pollution. Finally, as a component directly added to the cleaning / etching solution, the metal ion impurities of the chelating agent itself will also affect the total amount of metal ions in the solution.

[0004] Patent CN114774948B provides a method for preparing electronic-grade potassium hydroxide. This method first purifies industrial potassium chloride, avoiding the shortcomings of high equipment requirements, difficult operation, and complex processes in the process of directly refining potassium hydroxide solution to prepare electronic-grade potassium hydroxide. However, this method relies on chlor-alkali equipment, and the entire production process is complex, occupies a large area, has low output, and consumes a lot of energy. Patent CN216737607U provides an electronic-grade potassium hydroxide purification device, which uses an ion membrane electrolysis process to purify industrial-grade potassium hydroxide to obtain electronic-grade potassium hydroxide.

[0005] In short, whether it is industrial-grade caustic soda purification to reduce the overall metal ion content in the solution, or the use of chelating agents to chelate metal ion impurities to limit the migration of metal ion impurities from the solution to the silicon wafer surface, the purpose of the above two methods is to minimize the residual metal ion impurities on the silicon wafer during the alkaline cleaning / etching process. Summary of the invention

[0006] The present invention aims at the problem that the impurities of transition metal ions in industrial-grade caustic soda exceed the standard and make it unable to be used for cleaning / etching silicon wafers. The present invention provides an electronic-grade caustic soda solution for cleaning silicon wafers prepared by using industrial-grade caustic soda as a raw material and a preparation method thereof. The caustic soda solution prepared by the method comprises potassium hydroxide or sodium hydroxide, the mass concentration of the caustic soda is 40-50%, the additive is one or more of potassium gluconate, sodium gluconate, potassium laurate, sodium laurate, EDTA, DTPA and HEADTA, the content of each transition metal ion in the solution is less than 5 ppb, and more preferably, the content of each transition metal ion is less than 2 ppb.

[0007] The method for preparing an electronic grade caustic solution provided by the present invention mainly comprises: ① solution preparation, ② cooling crystallization, ③ crystal separation and cleaning, and ④ crystal dissolution. The specific operation steps are as follows: Step 1: Use ultrapure water to prepare industrial / food grade solid caustic soda and additives into a solution of a certain concentration, wherein the caustic soda is potassium hydroxide or sodium hydroxide, and the mass concentration of the caustic soda is 58-65wt%, preferably 60-62wt%.

[0008] Step 2: Adding an appropriate amount of additives to the caustic solution, the additives are one or more of sodium gluconate, potassium gluconate, potassium laurate, sodium laurate, EDTA, DTPA, and HEADTA, and the total concentration of the additives is 0.1-2 wt%, preferably 0.75-1.0 wt%.

[0009] Step 3, the caustic soda solution is subjected to programmed cooling crystallization in a crystallizer. When the solution temperature is greater than 30°C, the cooling rate is 3-5°C / hour, preferably 3-4°C / hour. When the solution temperature is lower than 30°C, the cooling rate is 0.1-2°C / hour, preferably 0.5-1°C / hour, to obtain a slurry containing high-purity caustic soda crystals and a mother liquor with a high impurity content. Step 4: Use a centrifuge to separate the crystals from the mother liquor in the slurry, and use ultrapure water to spray and wash during the centrifugation process to obtain high-purity crystals.

[0010] Step 5: Use an appropriate amount of ultrapure water to dissolve the high-purity crystals to obtain a 40-50 wt% electronic grade caustic soda solution.

[0011] It should be further explained that the key to the above process lies in two aspects: on the one hand, the selection of additive types and the matching of dosage in the solution preparation step; on the other hand, the control of the crystallization process during the cooling crystallization process.

[0012] The crystallization process is the result of the combined action of mass transfer and heat transfer. The morphology of the crystal product depends not only on the properties of the substance itself, but is also closely related to the crystallization conditions. Additives are introduced into the crystallization process. The additive molecules interact with the solute molecules through electrostatic forces, van der Waals forces or hydrogen bonds, thereby affecting the diffusion of the solute, the interfacial mass transfer rate, and ultimately the morphology of the crystal. The mechanism of action of additives on the crystallization process is very complex. So far, it is difficult to find a recognized and mature principle to explain it. Generally, researchers believe that additives not only change the diffusion process of solute molecules, but also inhibit the adsorption and reaction process of the crystal interface. That is, the additive molecules selectively adsorb on a certain crystal face of the crystal to occupy the adsorption sites of the solute molecules, inhibiting the reaction growth of the solute molecules to the crystal interface. At the same time, surfactants in the solution will also affect the diffusion of free solute molecules.

[0013] In this case, the inventors found through a large number of experiments that adding an appropriate amount of additives during the crystallization process can promote nucleation and improve the distribution of crystal particle size to a certain extent, which is conducive to obtaining crystal products with good crystal habits. In addition, the active functional groups of the additives can chelate free metal ions in the solution to form macromolecular chelates, thereby reducing the inclusion of metal ion impurities in and between crystals.

[0014] It should be noted that the preparation method of the electronic grade caustic solution provided by the present invention involves a solution crystallization process. During the growth process, the caustic crystals spontaneously exclude other non-caustic solutes in the solution to the outside of the crystals, which means that this scheme does not have excessive requirements for the purity of the additives, and conventional analytical purity is sufficient. The inventor of this method conducted a large number of experiments on the purity of the additives and finally screened out key information such as the preferred type and content of the additives.

[0015] In the process of solution crystallization, a too slow cooling rate will lead to low efficiency of the crystallization process, excessive crystal growth, increased impurity entrainment, low crystal supersaturation, irregular crystal morphology, solution scaling and blockage, and poor crystal uniformity. If the cooling rate is too fast, it will easily lead to excessive solution supersaturation, frequent nucleation of the solution and "scaling" of the crystallizer, which will seriously affect the quality and yield of the crystal product. In order to obtain high-quality crystals, it is necessary to avoid the negative effects of too fast cooling and the problems caused by too slow cooling. Reasonable control of the cooling rate is the key to ensure the efficiency, stability and high quality of the crystallization process. Through a large number of experiments, the inventor finally determined the process parameters of the additive-controlled caustic solution crystallization process.

[0016] In the process of solution crystallization, a cooling rate that is too fast can easily lead to a surge in solution supersaturation, causing problems such as explosive nucleation and "scarring" of the crystallizer, while a cooling rate that is too slow will seriously affect production efficiency and is not conducive to obtaining crystal products with ideal crystal habits. Therefore, in order to obtain high-quality crystal products, it is necessary to avoid the negative effects of too fast cooling and the problems caused by too slow cooling. Reasonable control of the cooling rate is the key to ensuring the efficiency, stability and high quality of the crystallization process. Through a large number of experiments, the inventors finally determined the process parameters of the additive-controlled caustic solution crystallization process.

[0017] The separation and cleaning of crystals and the dissolution of crystals are conventional processes in the industry and will not be elaborated on herein.

[0018] The concentration of the chelating agent or the metal ion concentration in the final electronic grade caustic solution can be monitored by methods known in the art.

[0019] In summary, the above process mainly removes metal ion impurities in the caustic soda solution through the solution crystallization process controlled by additives, and finally realizes the preparation of electronic grade caustic soda solution products.

[0020] The present invention is different from the previously reported process of reducing metal ion residues on the surface of silicon wafers by purifying caustic soda and adding a chelating agent to the caustic soda. The present invention realizes the preparation of an electronic grade caustic soda solution for cleaning / etching silicon wafers through a single additive-controlled crystallization process. The method realizes the deep removal of metal ion impurities in industrial grade caustic soda, fundamentally reducing the residues of metal ion impurities on silicon wafers after cleaning / etching the silicon wafers with caustic soda solutions.

[0021] The main innovations of the present invention are: ① Additives are used to control the processes of caustic soda pre-nucleation, stabilization of amorphous precursors, nucleation, secondary nucleation, crystal growth, etc., so as to obtain crystal products with high crystal purity and uniform size distribution; ② Additives are used to achieve chelation of transition metal ions in industrial-grade caustic soda, thereby limiting the content of free metal ions in the solution, reducing the inclusion of metal ion impurities in and between crystals, and improving the purity of the final crystal product; ③ Through a single cooling crystallization, while achieving the purification of caustic soda, the influence of the introduction of metal impurities in the additive on the purity of the final caustic soda solution is avoided, and the requirement for the purity of the additive is also reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is an image of the crystallization process of Example 1.

[0023] Figure 2 This is an image of the crystallization process of Example 5.

[0024] Figure 3 This is the crystallization process image of Comparative Example 1.

[0025] Figure 4 This is the crystallization process image of Comparative Example 4. DETAILED DESCRIPTION

[0026] The above contents of the present invention are further described in detail below through specific embodiments, but it should not be understood that the scope of the above subject matter of the present invention is limited to the following embodiments. All technologies realized based on the above contents of the present invention belong to the scope of the present invention. The present invention is further described below in conjunction with the embodiments.

[0027] Example 1 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 60%.

[0028] 2) Additive addition: The types of additives are EDTA and potassium laurate, the ratio is 1:1, and the total content of additives is 0.75wt%.

[0029] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 4°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 1°C / hour.

[0030] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0031] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45 wt% solution.

[0032] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0033] Example 2 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 60%.

[0034] 2) Additive addition: The types of additives are sodium gluconate and sodium laurate, the ratio is 1:1, and the total content of additives is 1.0 wt%.

[0035] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 3°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 1°C / hour.

[0036] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0037] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45 wt% solution.

[0038] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0039] Example 3 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of sodium hydroxide, and the mass concentration of potassium hydroxide in the prepared solution is 60%.

[0040] 2) Additive addition: The types of additives are EDTA and sodium gluconate, the ratio is 1:1, and the total content of additives is 1.0wt%.

[0041] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 3°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 1°C / hour.

[0042] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0043] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the sodium hydroxide crystals into a 45wt% solution.

[0044] The content of the sodium hydroxide product was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained sodium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0045] Example 4 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 58%.

[0046] 2) Additive addition: The type of additive is sodium laurate, and the total content of additives is 1.0 wt%.

[0047] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 3°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 1°C / hour.

[0048] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0049] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45 wt% solution.

[0050] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0051] Example 5 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 62%.

[0052] 2) Additive addition: The type of additive is HEDTA, and the total content of additives is 2.0 wt%.

[0053] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 4°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 1°C / hour.

[0054] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0055] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45wt% solution.

[0056] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0057] Comparative Example 1 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 60%.

[0058] 2) Additives added: No additives are added.

[0059] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 3°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 1°C / hour.

[0060] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0061] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45wt% solution.

[0062] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0063] Comparative Example 2 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 70%.

[0064] 2) Additive addition: The type of additive is EDTA, and the total content of additives is 0.5 wt%.

[0065] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 3°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 3°C / hour.

[0066] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0067] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45 wt% solution.

[0068] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0069] Comparative Example 3 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 60%.

[0070] 2) Additive addition: The type of additive is HEDTA, and the total content of additives is 2.0 wt%.

[0071] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 3°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 2°C / hour.

[0072] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0073] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45 wt% solution.

[0074] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0075] Comparative Example 4 The main process includes the following main steps: 1) Solid alkali dissolution: Use ultrapure water to dissolve an appropriate amount of potassium hydroxide. The mass concentration of potassium hydroxide in the prepared solution is 60%.

[0076] 2) Additive addition: The type of additive is HEDTA, and the total content of additives is 2.0wt%.

[0077] 3) Cooling crystallization: When the solution temperature is greater than 30°C, the cooling rate is 5°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 0.1°C / hour.

[0078] 4) Crystal separation and cleaning: Use a centrifuge to separate the crystals from the mother liquor, and use low-temperature ultrapure water to spray and wash the crystals.

[0079] 5) Crystal dissolution: Use an appropriate amount of ultrapure water to dissolve the potassium hydroxide crystals into a 45 wt% solution.

[0080] The potassium hydroxide product content was tested using a fully automatic potentiometric titrator, and the metal ion content of the obtained potassium hydroxide product was analyzed using ICP-MS. Detailed results are shown in Table 1.

[0081] The process implemented in the above embodiment and the comparative example is different in the following aspects: the initial caustic solution concentration, the type and amount of additives, and the programmed cooling rate. The initial caustic solution concentration determines the supersaturation at the beginning of crystallization, which is one of the key process control points affecting the quality of the crystal; the selection and amount of additives are the key to controlling the crystal growth process. Appropriate additives are essential for obtaining crystals with good crystal habits. Excessive use of additives will have an adverse effect on crystal growth, while too little additive will not play its due role in regulating crystal growth; the programmed cooling rate is the key to determining the crystal nucleation and crystal growth process. Too high a cooling rate will promote rapid crystal growth, which is not conducive to impurity separation, while too low a cooling rate will significantly increase the crystallization time, thereby affecting the overall crystallization efficiency.

[0082] Table 1. Analysis and test results

[0083] It is not difficult to see from the results in Table 1 that the method described in this patent can achieve deep removal of transition metal ion impurities in industrial-grade caustic soda and can meet the relevant specifications for caustic soda for silicon wafer cleaning / etching.

[0084] To further illustrate the regulatory effect of additives on the crystallization process, the inventors used an online crystallization detection system to obtain key crystal morphology images during the crystallization process, see the attached drawings in the specification for details.

[0085] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. Matters not covered by the present invention are known technologies.

Claims

1. A method for preparing an electronic grade caustic solution, characterized in that: Here are the steps: (1) Solid alkali dissolution: Use ultrapure water to dissolve caustic soda and stir evenly to obtain caustic soda solution; (2) Adding additives: adding an appropriate amount of additives to the caustic soda solution obtained in step (1), stirring evenly under heat preservation conditions, to obtain a crystallization starting solution; (3) Cooling crystallization: The crystallization starting solution obtained in step (2) is subjected to programmed cooling crystallization to obtain a slurry containing crystals and mother liquor; (4) Crystal separation and washing: The crystal slurry obtained in step (3) is centrifuged and washed to obtain crystals; (5) Crystal dissolution: Use an appropriate amount of ultrapure water to fully dissolve the crystals obtained in step (4) to obtain an electronic grade caustic soda solution.

2. The method for preparing an electronic grade caustic solution according to claim 1, characterized in that: The electronic grade caustic alkali is potassium hydroxide or sodium hydroxide.

3. The method for preparing an electronic grade caustic solution according to claim 1, characterized in that: The mass concentration of the caustic soda solution prepared in step (1) is 58-65%, preferably 60-62%.

4. The method for preparing an electronic grade caustic solution according to claim 1, characterized in that: The additive in step (2) is one or more of potassium gluconate, sodium gluconate, potassium laurate, sodium laurate, ethylenediaminetetraacetic acid EDTA, diethylenetriaminepentaacetic acid DTPA, and hydroxyethylethylenediaminetriacetic acid HEDTA.

5. The method for preparing an electronic grade caustic solution according to claim 1, characterized in that: The total amount of the additive added in step (2) is 0.1-2 wt%, preferably 0.75-1.0 wt%.

6. The method for preparing an electronic grade caustic solution according to claim 1, characterized in that: The programmed cooling condition of step (3) is that when the solution temperature is higher than 30°C, the cooling rate is 3-5°C / hour, preferably 3-4°C / hour; when the solution temperature is lower than 30°C, the cooling rate is 0.1-2°C / hour, preferably 0.5-1°C / hour.

7. The method for preparing an electronic grade caustic solution according to claim 1, characterized in that: In step (4), the crystal is cleaned online using low-temperature ultrapure water with a temperature below 10°C.

8. The electronic grade caustic soda solution obtained by the preparation method according to any one of claims 1 to 7, characterized in that: The electronic grade caustic soda obtained in step (5) has a content of 40-50%, and the content of each transition metal ion is less than 5 ppb, and more preferably, the content of each transition metal ion is less than 2 ppb.

9. A method for preparing an electronic grade caustic solution for cleaning / etching silicon wafers, characterized in that: The electronic grade caustic soda solution is obtained by the preparation method described in any one of claims 1 to 7.

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