A method for controlling metal impurities on the surface of a silicon wafer

Abstract

The present invention relates to a method for controlling metal impurities on the surface of silicon wafers. The method comprises the following steps: providing a silicon wafer; performing a wetting treatment by spraying ultrapure water on the silicon wafer to fully wet the surface of the silicon wafer; drying the silicon wafer with a fully wetted surface; repeating the steps of the wetting treatment and the drying treatment; performing an acid solution treatment by treating the surface of the silicon wafer with an acid solution to obtain a treatment solution; performing an analysis treatment by using ICPMS to analyze the metal ion content in the treatment solution; and a judgment step of comparing the detected metal ion content with a metal pollution lower limit value to judge whether the ultrapure water can continue to clean the silicon wafer. Thus, through this method, the pollution amount of metal ions in the ultrapure water to the silicon wafer can be detected, the relative influence of the increase or decrease of the metal content on the surface of the silicon wafer by the metal ions in the ultrapure water can be determined, and this method also has the advantages of simple operation and high accuracy rate.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor wafers, and particularly relates to a method for controlling metal impurities on the surface of wafers. Background Art

[0002] Wafers are widely used as substrates for semiconductor devices. In semiconductor devices, metal contamination of the substrate will reduce the performance of the device, so wafers with less metal contamination are required.

[0003] In related technologies, the metal impurity content of wafers is mainly controlled by controlling the metal impurity content of the ultrapure water used for final cleaning. The metal ion content in ultrapure water is mainly measured by ICPMS (Inductively coupled plasma mass spectrometry) or AA atomic absorption spectrophotometer. However, this detection method can only know the amount of metal impurities in ultrapure water, and cannot completely determine whether these metal ions will cause an increase or decrease in the metal content on the surface of the wafer. Therefore, it is of great significance to develop a control method that can improve the above technical problems. Summary of the Invention

[0004] To improve the above technical problems, the present invention provides a method for controlling metal impurities on the surface of wafers, the method comprising the following steps: providing a wafer; performing a wetting treatment by spraying ultrapure water on the wafer to fully wet the surface of the wafer; drying the wafer with a fully wet surface; repeating the steps of wetting treatment and drying treatment; performing an acid solution treatment by treating the surface of the wafer with an acid solution to obtain a treatment solution; performing an analysis treatment by using ICPMS to analyze the metal ion content in the treatment solution; and a judgment step of comparing the detected metal ion content with a metal contamination lower limit value to judge whether the ultrapure water can continue to clean the wafer. Thus, through this method, the contamination amount of metal ions in ultrapure water on the wafer can be detected, the relative influence of the metal ions in ultrapure water on the increase or decrease of the metal content on the surface of the wafer can be determined, and this method also has the advantages of simple operation and cheap and easily available reagents. In addition, the method of the present invention can also be used to monitor the long-term change trend of the metal ion content on the surface of the wafer. The inventor found that when the method of the present invention is used for control, the number of wafers with excessive metal impurity content on the surface of the finally shipped wafers is significantly lower than the number of wafers with excessive metal impurity content on the surface of the finally shipped wafers controlled by the existing method.

[0005] Through the method of the present invention, the metal impurity content on the surface of the wafer can be directly and accurately obtained, without indirectly predicting the metal impurity content on the surface of the wafer through the metal impurity content of ultrapure water. The result of the method of the present invention is more accurate and simple to implement.

[0006] According to an embodiment of the present invention, in the determination step, when the content of at least one of the detected metal ions is greater than or equal to the metal pollution lower limit value, the metal ion content in the ultrapure water will affect the performance of the silicon wafer at this time, and the use of ultrapure water to clean the silicon wafer is stopped; when the content of all detected metal ions is less than the metal pollution lower limit value, the ultrapure water can continue to clean the silicon wafer. Thus, the method of the present invention can directly determine whether the metal ions in the ultrapure water will affect the performance of the silicon wafer, and can directly determine whether the ultrapure water can continue to clean the silicon wafer, and the operation is simple and convenient.

[0007] According to an embodiment of the present invention, the silicon wafer is a silicon wafer with a hydrophilic film. Thus, there is a hydrogen bond attraction between the silicon wafer with a hydrophilic film and water molecules, and the silicon wafer with a hydrophilic film has better ability to retain water. Compared with a hydrophobic silicon wafer, after drying treatment, the silicon wafer with a hydrophilic film can retain more metal ions on the silicon wafer surface and can obtain a higher signal intensity of metal ions.

[0008] According to an embodiment of the present invention, the silicon wafer with a hydrophilic film is obtained after being treated with chemicals; the chemicals include at least one of a mixed solution of NH4OH and H2O2 and an HCl / H2O solution.

[0009] According to an embodiment of the present invention, the contact angle between the silicon wafer and the ultrapure water is 5-30°. Thus, more metal ions can be retained, and the accuracy of the detection result can be further improved.

[0010] According to an embodiment of the present invention, the number of repetitions is not less than 3 times; for example, the number of repetitions is 3 times, 4 times, 5 times, 6 times or more. By repeating the steps of wet treatment and dry treatment, the accuracy of the measurement result can be improved.

[0011] Further, the number of repetitions is preferably 3 times or 4 times. At this time, the metal ion content retained on the silicon wafer surface reaches the saturation value. Continuing to repeat the steps of wet treatment and dry treatment, the measured metal ion content will not continue to increase, and the metal ion content retained on the silicon wafer surface can be detected more accurately.

[0012] According to an embodiment of the present invention, in the wet treatment, the spraying direction of the ultrapure water is perpendicular to the plane where the silicon wafer is located.

[0013] According to an embodiment of the present invention, the wet treatment includes: placing the silicon wafer in a crystal boat and placing it in a horizontal spin dryer capable of spraying ultrapure water.

[0014] According to an embodiment of the present invention, the wet treatment includes: spraying ultrapure water above the center of the silicon wafer, rotating the silicon wafer, and using centrifugal force to throw the ultrapure water to areas outside the center, so as to achieve full coverage of the surface of the silicon wafer with ultrapure water.

[0015] Specifically, after spraying ultrapure water on the surface of the silicon wafer and through drying treatment, metal ions in the ultrapure water can remain on the surface of the silicon wafer.

[0016] According to an embodiment of the present invention, the drying treatment includes: using a horizontal spin dryer to spin-dry the ultrapure water on the surface of the silicon wafer.

[0017] The acid solution includes hydrofluoric acid solution.

[0018] According to an embodiment of the present invention, the metal ions include Na + , K + , Ca 2+ and at least one of them. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a flowchart of the method for controlling metal impurities on the surface of the silicon wafer of the present invention.

[0020] Figure 2 is a correlation diagram of the Na content on the surface of the silicon wafer detected in the present invention and the number of spraying times. + content and the number of spraying times.

[0021] Figure 3 is a correlation diagram of the K content on the surface of the silicon wafer detected in the present invention and the number of spraying times. + content and the number of spraying times.

[0022] Figure 4 is a correlation diagram of the Ca content on the surface of the silicon wafer detected in the present invention and the number of spraying times. 2+ content and the number of spraying times. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] The embodiments of the present invention will be described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as a limitation to the present invention. For those not specified in the embodiments in terms of specific technologies or conditions, they shall be carried out according to the technologies or conditions described in the literature in the field or according to the product specifications. For the reagents not specified in terms of the manufacturer, they are all conventional products that can be obtained through market purchase.

[0024] Existing methods indirectly control the metal impurities on the silicon wafer surface by controlling the metal impurities in ultrapure water, and indirectly predict the metal impurity content on the silicon wafer surface based on the metal impurity content in ultrapure water. The metal impurities in ultrapure water are mainly measured by ICPMS or AA atomic absorption spectrophotometer. The existing detection methods can only indirectly predict the metal impurity content on the silicon wafer surface based on the metal impurity content in ultrapure water, and cannot determine the impact of metal ions in ultrapure water on the metal ion content of the silicon wafer. Specifically, the existing detection methods can only detect the fixed content of metal ions in ultrapure water, but cannot determine the content of metal ions in the ultrapure water retained on the silicon wafer surface after using ultrapure water to clean the silicon wafer surface. That is to say, the existing detection methods cannot determine the relative impact of metal ions in ultrapure water on the increase or decrease of the metal content on the silicon wafer surface.

[0025] To address the above technical problems, the present invention provides a method for controlling metal impurities on the silicon wafer surface. As Figure 1 shown, the method includes the following steps:

[0026] S100. Provide a silicon wafer

[0027] According to an embodiment of the present invention, the silicon wafer is a silicon wafer with a hydrophilic film. Further, the contact angle between the silicon wafer and ultrapure water is 5 - 30°, for example, it can be 5°, 6°, 7°, 8°, 9°, 10°, 11°, 12°, 13°, 14°, 15°, 16°, 17°, 18°, 19°, 20°, 21°, 22°, 23°, 24°, 25°, 26°, 27°, 28°, 29°, 30°. Thus, the silicon wafer has hydrophilicity. Compared with a hydrophobic silicon wafer, a silicon wafer with a hydrophilic film can enrich more metal ions, and thus can more accurately detect the impact of metal ions in ultrapure water on the metal content on the silicon wafer surface.

[0028] Exemplarily, the silicon wafer with a hydrophilic film can be a silicon wafer with a hydrophilic silicon oxide film. Silicon oxide has a hydrogen bond attraction with water. Thus, the silicon wafer with a hydrophilic film can retain water, and then the metal ions in the water can be retained on the surface of the silicon wafer, further improving the measurement accuracy.

[0029] The silicon wafer with a hydrophilic film is obtained by chemical treatment. Specifically, the chemicals include at least one of a mixed solution of NH4OH and H2O2 and an HCl / H2O solution.

[0030] It should be noted that the above silicon wafer with a hydrophilic film has completed the final cleaning step, for example, has completed steps such as RCA, SC1, and SC2 cleaning. That is to say, the surface of the silicon wafer with a hydrophilic film has no metal ions and has no impact on the performance of the silicon wafer.

[0031] S200. Perform a wetting treatment by spraying ultrapure water onto the silicon wafer to fully wet the surface of the silicon wafer.

[0032] According to an embodiment of the present invention, in the wetting treatment, the spraying direction of the ultrapure water is perpendicular to the plane where the silicon wafer is located.

[0033] According to an embodiment of the present invention, the wetting treatment includes: placing the silicon wafer in a crystal boat and placing it in a horizontal spin dryer capable of spraying ultrapure water.

[0034] The surface of the silicon wafer being fully wet means that each area on the silicon wafer is wetted by ultrapure water and a water film is formed on the surface. This way of achieving full coverage and fully wetting the surface of the silicon wafer can improve the effective signal of the measurement compared to partial infiltration, and further improve the accuracy of the measurement result.

[0035] For example, the method of central spraying combined with rotation can be used to centrifugally throw the ultrapure water in the center to areas outside the center, and finally achieve full coverage of the silicon wafer surface. Specifically, it can be: spraying ultrapure water above the center of the silicon wafer, rotating the silicon wafer, and centrifugally throwing the ultrapure water to areas outside the center to achieve full coverage of the silicon wafer surface with ultrapure water.

[0036] S300. Dry the silicon wafer with a fully wet surface.

[0037] According to an embodiment of the present invention, the drying treatment includes: using a horizontal spin dryer to spin-dry the ultrapure water on the surface of the silicon wafer.

[0038] S400. Repeat the steps of the wetting treatment and the drying treatment.

[0039] The inventors found that if the steps of the wetting treatment and the drying treatment are only carried out once, the metal ions retained on the surface of the silicon wafer do not reach the saturated enrichment value, and there is a problem that the control method is inaccurate. If the steps of the wetting treatment and the drying treatment are repeated, the content of the metal ions retained on the surface of the silicon wafer will increase with the increase of the number of repetitions. When the saturated enrichment value is reached, continuing to increase the number of repetitions, the content of the metal ions retained on the surface of the silicon wafer will not continue to increase.

[0040] According to an embodiment of the present invention, the number of repetitions is not less than 3 times, for example, it can be 3 times, 4 times, 5 times, 6 times or more. Thus, by repeating the steps of the wetting treatment and the drying treatment, the accuracy of the detection result can be further improved.

[0041] The inventors obtained through multiple experiments that when the number of repetitions is 3 times or 4 times, the metal ions enriched on the surface of the silicon wafer reach the saturated enrichment value of the silicon wafer. If the number of repetitions is continued to increase, the measured content of the metal ions will not continue to increase but remain unchanged.

[0042] S500. Perform acid solution treatment, treating the surface of the silicon wafer with an acid solution to obtain a treatment solution.

[0043] According to an embodiment of the present invention, the acid solution includes a hydrofluoric acid solution. Thus, metal ions can be included in the obtained treatment solution. The present invention does not limit the concentration of the acid solution, and those skilled in the art can select according to the usage requirements.

[0044] S600. Perform analysis treatment, using ICPMS to analyze the metal ion content in the treatment solution.

[0045] According to an embodiment of the present invention, the metal ions include Na + , K + , Ca 2+ and at least one of them.

[0046] It should be understood that the present invention does not particularly limit the parameters of the ICPMS analysis, and those skilled in the art can select according to the different metal ions to be detected.

[0047] S700. Judgment step, comparing the detected metal ion content with the metal pollution lower limit value to judge whether the ultrapure water can continue to clean the silicon wafer.

[0048] According to an embodiment of the present invention, when the content of at least one detected metal ion is greater than or equal to the metal pollution lower limit value, at this time, the metal ion content in the ultrapure water will affect the performance of the silicon wafer, and the ultrapure water cannot be used to clean the silicon wafer continuously. When the content of all detected metal ions is less than the metal pollution lower limit value, at this time, the metal ion content will not affect the performance of the silicon wafer, and the ultrapure water can be used to clean the silicon wafer. Among them, the metal pollution lower limit value refers to the lowest value of the metal ion content that affects the performance of the silicon wafer.

[0049] The existing method indirectly reflects the pollution amount of metal ions in the ultrapure water to the silicon wafer by measuring the metal ion content in the ultrapure water. The method of the present invention can improve this problem, and the method of the present invention can directly obtain the pollution amount of metal ions in the ultrapure water to the silicon wafer.

[0050] For the embodiments described below of the present invention, unless otherwise specified, the reagents used can be purchased from the market or can be prepared by the methods described in the present invention.

[0051] It should be noted that the conversion relationship between atoms / cm 2 and ppt is known to those skilled in the art. Specifically, for Na + , 100 ppt = 3.71×10 9 atoms / cm 2 . For K+ In terms of, 100 ppt = 2.18×10 9 atoms / cm 2 For Ca 2+ in terms of, 100 ppt = 2.13×10 9 atoms / cm 2 .

[0052] Example 1

[0053] The ultrapure water sample used in Example 1 is the ultrapure water that has washed the silicon wafer for a period of time, and the metal ion content therein is unknown. The control method for metal impurities on the silicon wafer surface is as follows.

[0054] (1) Provide a silicon wafer with a hydrophilic film. The silicon wafer with a hydrophilic film is a silicon wafer with a hydrophilic silicon oxide film, and this silicon wafer with a hydrophilic film has completed the final cleaning (has completed steps such as RCA, SC1, and SC2 cleaning). The contact angle between this silicon wafer and ultrapure water is 15°.

[0055] (2) Spray ultrapure water on the silicon wafer to fully wet the surface of the silicon wafer. Place the silicon wafer in a boat and place it in a horizontal spin dryer that can spray ultrapure water. Use the method of central spraying combined with rotation to centrifugally throw the ultrapure water in the center to the area outside the center and finally achieve full coverage of the silicon wafer surface with ultrapure water. When spraying, the spraying direction of the ultrapure water is perpendicular to the plane where the silicon wafer is located.

[0056] (3) Perform a drying treatment to dry the surface of the silicon wafer. Use a horizontal spin dryer to spin-dry the ultrapure water on the silicon wafer surface.

[0057] (4) Repeat steps (2)-(3), and the number of repetitions is 1 time. That is to say, the number of spraying times in Example 1 is 2 times.

[0058] (5) Treat the surface of the silicon wafer with a hydrofluoric acid solution to obtain a treatment solution.

[0059] (6) Use ICPMS to analyze the metal ion content in the treatment solution.

[0060] It is measured that: as Figures 2-4 shown, the content of Na + on the silicon wafer surface is 0.00198834×10 10 atoms / cm 2 , that is, 0.54 ppt. The content of K + on the silicon wafer surface is 0.00029×10 10 atoms / cm 2 , that is, 0.133 ppt. The content of Ca 2+ on the silicon wafer surface is 0.00437199×10 10atoms / cm 2 , i.e., 2.05 ppt.

[0061] The control method further includes: (7) comparing the detected metal ion content with the lower limit value of metal contamination to determine whether the ultrapure water can continue to clean the silicon wafer.

[0062] It should be noted that different substrates have different requirements for the metal impurity content on the surface of the silicon wafer, that is, the lower limit value of metal contamination varies according to the different final products. Further, when the content of at least one metal ion detected is greater than or equal to the lower limit value of metal contamination, the metal ion content in the ultrapure water will affect the performance of the silicon wafer at this time, and the ultrapure water cannot be used to clean the silicon wafer continuously. When the content of all detected metal ions is less than the lower limit value of metal contamination, the content of metal ions will not affect the performance of the silicon wafer at this time, and the ultrapure water can be used to clean the silicon wafer.

[0063] Example 2

[0064] Refer to the method of Example 1 to measure the content of metal impurities on the surface of the silicon wafer. The difference from Example 1 is that in step (4), the number of repetitions is 3 times. That is to say, the number of spraying times in Example 2 is 4 times.

[0065] It is measured that: as Figures 2-4 shown, the content of Na + on the surface of the silicon wafer is 0.00641831x10 10 atoms / cm 2 , i.e., 1.73 ppt. The content of K + on the surface of the silicon wafer is 0.00126989×10 10 atoms / cm 2 , i.e., 0.58 ppt. The content of Ca 2+ on the surface of the silicon wafer is 0.0121022×10 10 atoms / cm 2 , i.e., 5.68 ppt.

[0066] Example 3

[0067] Refer to the method of Example 1 to measure the content of metal impurities on the surface of the silicon wafer. The difference from Example 1 is that in step (4), the number of repetitions is 4 times. That is to say, the number of spraying times in Example 3 is 5 times.

[0068] As Figures 2-4 shown, the measured contents of Na + , K + and Ca 2+ metal ions on the surface of the silicon wafer are the same as those measured in Example 2.

[0069] Example 4

[0070] Refer to the method of Example 1 to measure the content of metal impurities on the silicon wafer surface. The difference from Example 1 is that in step (4), the number of repetitions is 5 times. That is to say, the number of spraying times in Example 4 is 6 times.

[0071] As Figures 2-4 shown, the measured Na + , K + and Ca 2+ contents of metal ions on the silicon wafer surface are the same as those measured in Example 2.

[0072] Comparative Example 1

[0073] Use the method of heating and concentration to improve the accuracy, and then use ICPMS (Inductively Coupled Plasma Mass Spectrometry) to measure the content of metal ions in the ultrapure water sample in Example 1.

[0074] The measurement results show that: the content of Na + in ultrapure water is < 0.35 ppt. The content of K + is < 0.43 ppt. The content of Ca 2+ is < 0.39 ppt.

[0075] Comparative Example 2

[0076] Refer to the method of Example 1 to measure the content of metal impurities on the silicon wafer surface. The difference from Example 1 is that step (4) is omitted, that is, step (5) is directly carried out after step (3). That is to say, the number of times of repeating the wet treatment and drying treatment steps is 0 times, that is, the number of spraying times is 1 time.

[0077] The measurement results show that: as Figures 2-4 shown, the content of Na + on the silicon wafer surface is 0.0013×10 10 atoms / cm 2 , that is, 0.35 ppt. The content of K + on the silicon wafer surface is 0.00029×10 10 atoms / cm 2 , that is, 0.133 ppt. The content of Ca 2+ on the silicon wafer surface is 0.0043×10 10 atoms / cm 2 , that is, 2.02 ppt.

[0078] Combining the measurement results of the examples and comparative examples, as Figures 2-4 shown, when the number of repetitions is 3 times, that is, when the number of spraying times is 4 times, the metal ions (Na + , K+ and Ca 2+ ) reaches saturation. Continuing to increase the spraying times, the metal ions (Na + , K + and Ca 2+ ) enriched on the silicon wafer surface will not continue to increase but remain unchanged.

[0079] The method of the present application improves the defects of the existing detection methods. When the performance of the substrate decreases, the method of the present invention can determine whether the ultrapure water for cleaning the silicon wafer will contaminate the silicon wafer, and can directly determine whether the reason for the decrease in the substrate performance is caused by the ultrapure water for cleaning the silicon wafer, and can quickly find the reason for the decrease in the substrate performance.

[0080] Moreover, the method of the present invention can determine the relative influence of metal ions in the ultrapure water on the increase or decrease of the metal content on the silicon wafer surface.

[0081] The method of the present invention can detect whether the metal ion content in the ultrapure water for cleaning the silicon wafer will affect the performance of the silicon wafer, that is, whether the ultrapure water can continue to clean the silicon wafer. For example, after using the ultrapure water to clean the silicon wafer for a period of time, the metal ion content in the ultrapure water is unknown. Using the method of the present invention to detect the ultrapure water used for a period of time, the metal ion content in the ultrapure water enriched on the silicon wafer surface can be directly measured to determine whether the ultrapure water can continue to be used to clean the silicon wafer and whether the ultrapure water will affect the performance of the silicon wafer. When the content of at least one metal ion detected is greater than or equal to the metal pollution lower limit value, the metal ion content in the ultrapure water will affect the performance of the silicon wafer, and the ultrapure water cannot continue to be used to clean the silicon wafer.

[0082] In the description of this specification, the descriptions referring to terms such as "one embodiment", "another embodiment", "yet another embodiment", "one example", "another example", etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment are included in at least one embodiment of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples.

[0083] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for controlling metal impurities on the surface of a silicon wafer, characterized in that, The method includes the following steps: Provide a silicon wafer, the silicon wafer being a silicon wafer with a hydrophilic film, the silicon wafer with the hydrophilic film being obtained after being treated with chemicals, the chemicals including at least one of a mixed solution of NH4OH and H2O2 and an HCl / H2O solution; Perform a wetting treatment by spraying ultrapure water on the silicon wafer to fully wet the surface of the silicon wafer. The full wetting of the surface of the silicon wafer means that each area on the silicon wafer is wetted by the ultrapure water and a water film is formed on the surface. In the wetting treatment, the spraying direction of the ultrapure water is perpendicular to the plane where the silicon wafer is located. The wetting treatment includes: spraying ultrapure water above the center of the silicon wafer, and the silicon wafer rotates, and the ultrapure water is thrown to areas outside the center by centrifugal force to achieve full coverage of the surface of the silicon wafer with ultrapure water; Dry the silicon wafer with a fully wetted surface; Repeat the steps of the wetting treatment and the drying treatment, and the number of repetitions is 3 or 4 times; Perform an acid solution treatment by treating the surface of the silicon wafer with an acid solution to obtain a treatment solution; Perform an analysis treatment by using ICPMS to analyze the metal ion content in the treatment solution; A judgment step of comparing the detected metal ion content with the metal pollution lower limit value to judge whether the ultrapure water can continue to clean the silicon wafer; 2. The control method according to claim 1, wherein In the judgment step, when the content of at least one of the detected metal ions is greater than or equal to the metal pollution lower limit value, stop using the ultrapure water to clean the silicon wafer; When the content of all detected metal ions is less than the metal pollution lower limit value, the ultrapure water can continue to clean the silicon wafer; 3. The control method according to claim 1, characterized in that The contact angle between the silicon wafer and the ultrapure water is 5-30°; 4. The control method according to claim 1, wherein The wetting treatment includes: placing the silicon wafer in a boat and placing it in a horizontal spin dryer capable of spraying ultrapure water; 5. The control method according to claim 1, characterized in that The drying treatment includes: using a horizontal spin dryer to spin-dry the ultrapure water on the surface of the silicon wafer; The acid solution includes a hydrofluoric acid solution.

6. The control method according to claim 1, wherein The metal ions include Na + , K + , Ca 2+ and at least one of them.

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