KR20190063989A - Method of preparing cerium oxide particles, cerium oxide particles and composition of slurry for polishing compring the same - Google Patents

Abstract

The present invention relates to: a method for manufacturing cerium oxide particles having a high porosity; the cerium oxide particles; and a polishing slurry composition comprising the same. According to the present invention, surface-processed cerium oxide particles can be manufactured by the following steps: obtaining a cerium precursor mixture by mixing a cerium precursor, a surfactant, an organic solvent, and water; mixing a precipitant with the cerium precursor mixture to obtain a cerium hydroxide precipitate; and calcining the cerium hydroxide to obtain cerium oxide particles. The present invention also relates to polishing cerium oxide particles with the porosity of 40 to 80%, and a polishing slurry composition comprising the same.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a cerium oxide particle, a cerium oxide particle, and a polishing slurry composition containing the cerium oxide particle,

The present invention relates to a process for producing cerium oxide particles, cerium oxide particles and a polishing slurry composition containing the cerium oxide particles.

As the semiconductor devices are diversified and highly integrated, a finer pattern forming technique is used, thereby complicating the surface structure of the semiconductor device and increasing the step difference of the surface films.

A chemical mechanical polishing (CMP) process is used as a planarization technique for removing a step in a specific film formed on a substrate in manufacturing a semiconductor device.

For example, a process for removing an insulating film excessively formed for interlayer insulation includes a process for interlayer dielectronic (ILD), a process for planarization of an insulating film for STI (shallow trench isolation) A contact plug, a via contact, or the like.

In the CMP process, the polishing rate, the degree of planarization of the polishing surface, and the degree of occurrence of scratches are important, and are determined by the CMP process conditions, the kind of slurry, the type of polishing pad, and the like. Particularly, removal of large particles directly related to the generation of scratches is a more important technique. In order to reduce the scratches, if the average particle size of the slurry is reduced, there is a problem that the production amount is decreased due to the reduction of the polishing amount.

Therefore, in the CMP process, abrasive grains having an appropriate size and distribution are required in consideration of the above polishing rate, dispersion stability, scratch, and the like.

SUMMARY OF THE INVENTION The present invention is directed to a cerium oxide particle production method, a cerium oxide particle, and a polishing slurry composition containing the cerium oxide particle, wherein the cerium oxide particle has substantially no surface defect after polishing even when the polishing rate is high.

According to an aspect of the present invention, there is provided a method for preparing a cerium precursor, comprising: obtaining a cerium precursor mixture by mixing a cerium precursor, a surfactant, an organic solvent, and water; Mixing a cerium precursor mixture with a precipitant to obtain a cerium hydroxide precipitate; And calcining the cerium hydroxide to obtain cerium oxide particles.

The present invention also provides cerium oxide particles for polishing having a porosity of 40 to 80%.

The present invention also provides a polishing slurry composition comprising the cerium particles for polishing.

The present invention can provide a method for producing cerium oxide particles in which the polishing rate is high and the surface has almost no defects after polishing, cerium oxide particles and a polishing slurry composition containing the cerium oxide particles.

Fig. 1 is an SEM image of cerium oxide particles, wherein (a) is cerium oxide particles prepared according to Example 1, and (b) is an SEM image of cerium oxide particles prepared according to Comparative Example 1. Fig.

Hereinafter, the present invention will be described in detail.

1. Method for producing cerium oxide particles

An aspect of the present invention provides a method for producing cerium oxide particles.

The cerium oxide particles are produced by mixing a cerium precursor, a surfactant, an organic solvent and water to obtain a cerium precursor mixture, mixing the cerium precursor mixture with a precipitant to obtain a cerium hydroxide precipitate, To obtain cerium oxide particles.

First, a cerium precursor, a surfactant, an organic solvent and water are mixed to obtain a cerium precursor mixture.

The cerium precursor may include at least one selected from the group consisting of cerium nitrate, ammonium cerium nitrate, cerium chloride, cerium sulfate, cerium carbonate, cerium hydroxide, and cerium acetate.

The surfactant is adsorbed on the surface of the cerium precursor to chemically surface-treat the surface of the cerium precursor. The surfactant forms micelles in a mixed solution, and a cerium precursor is bound to the micelles to obtain cerium hydroxide having a uniform shape and particle size.

The surfactant may be at least one selected from the group consisting of a nonionic surfactant, an anionic surfactant, a cationic surfactant, an amphoteric surfactant, and a gemini surfactant. And at least one selected from the group consisting of a nonionic surfactant and an amphoteric surfactant is preferable in terms of ease of removal of the surfactant adsorbed on the surface of the reactant in the washing process of the reaction product.

The nonionic surfactant is selected from the group consisting of alkyl phenol ethoxylates (APEO), alkyl ethoxylates (AE), alkanol amides (AA), amine oxides (AO), polyethylene glycols (PEG), ethylene glycols (PVA), glycerin, polypropylene glycol (PPG), polyvinyl pyrrolidone (PVP), and combinations thereof.

The anionic surfactant may be at least one selected from the group consisting of polyacrylic acid, ammonium polyacrylate, polymethacrylic acid, polymethacrylic acid ammonium salt, polyacrylic maleic acid, polystyrenesulfonic acid, carboxylic acid, carboxylate, sulphonic ester, sulphonic ester salt, A sulfonic acid salt, a sulfonic acid salt, a phosphonic ester, and a phosphoric ester salt.

The cationic surfactant may include at least one member selected from the group consisting of quaternary ammonium, sulfoxonium, sulfonium salts, iodine compounds and arsenic compounds.

The amphoteric surfactant may include at least one selected from the group consisting of amino acid, betaine, imidazoline, and alkylpolyamino acids.

The gemini type surfactant may be selected from the group consisting of N-hexane-diyl-1,2-ethane bis ammonium bromide, N-dodecanediyl- N-dodecane-diyl-1,2-ethane bis ammonium bromide, N-hexadecane-diyl-1,2-ethane bis ammonium bromide, butane-diyl-1,4-bis dimethyl cetylammonium bromide, pentane-diyl-1,5-bisdimethyl cetylammonium bromide at least one member selected from the group consisting of -diyl-1,5-bis dimethylcetyl ammonium bromide, hexane-diyl-1,6-bis dimethyl cetylammonium bromide, . ≪ / RTI >

The organic solvent can control the degree of supersaturation of the solvent so as to precipitate the water-soluble cerium precursor with cerium hydroxide. The degree of supersaturation controls the solubility of the reaction product, and the crystal size and shape of the resulting particles can be controlled. As the organic solvent, ethylene glycol, polyethylene glycol, methyl alcohol, isopropyl alcohol, methoxy ethanol, acetone, glycerin , At least one member selected from the group consisting of toluene, methyl ethyl ketone, ethyl acetate, cyclohexane, butyl lactate, and butyl carbitol acetate.

It is preferable to use ultrapure water as the water.

The water and the organic solvent may be contained in a weight ratio of 6: 1 to 1: 1, preferably 4: 1 to 1: 1. When the mixing ratio of the water and the organic solvent is more than 6: 1, it is difficult to control the shape of the cerium oxide particles, which makes it difficult to uniformly form particles. If the ratio is less than 1: 1, it is difficult to remove the organic solvent Can be.

The cerium precursor may be contained in the cerium precursor mixture liquid at a concentration (M) of 0.1 to 1.0 mol based on the molar amount of cerium atoms, preferably 0.3 to 0.7 mol (M). If the concentration of the cerium precursor is less than 0.1 molar (M), the reaction yield decreases. If the concentration of the cerium precursor exceeds 1.0 molar (M), the size of the cerium hydroxide particles may increase and the reaction control may be difficult.

The surfactant may be contained in the cerium precursor mixture in an amount of 0.1 to 10% by weight, and preferably 0.5 to 5% by weight. If the amount of the surfactant is less than 0.1% by weight, the formation of micelles is low and the size of the particles may increase. If the surfactant is more than 10% by weight, a large amount of bubbles may be generated to control the formation of particles.

Next, a precipitant is mixed with the cerium precursor mixture to obtain a cerium hydroxide precipitate.

The precipitant may include at least one selected from the group consisting of ammonia water, potassium hydroxide, sodium hydroxide, and hydrogen peroxide.

The precipitant may be present in a molar ratio of 1 to 6 relative to the cerium precursor. If the precipitant is less than 1, the yield of cerium hydroxide production may be lowered, and the particle size and reaction time may increase. If the precipitant is more than 6, the reaction rate with the cerium precursor mixture may be drastically increased, resulting in a problem of aggregation of particles and uniformity of size.

Next, the cerium hydroxide is washed to remove the organic solvent and dried. The dried cerium hydroxide is calcined to obtain cerium oxide particles having a porosity of 40 to 80%.

The temperature for calcining the cerium hydroxide may be 400 to 1200 ° C, preferably 500 to 1000 ° C. If the calcining temperature is less than 400 ° C, the oxidizing power of the cerium hydroxide particles is lowered and the ratio of the trivalent cerium oxide particles is increased. At this time, if the ratio of the trivalent cerium oxide particles is increased, the dispersibility in the slurry preparation may be lowered. If the calcining temperature exceeds 1200 ° C, the primary particle size of the cerium oxide particles increases, making it difficult to control the secondary particle size of the slurry in the slurry dispersing step. In addition, .

2. Cerium oxide particles

Another aspect of the present invention provides cerium oxide particles for polishing.

The cerium oxide particles for polishing according to the present invention may comprise the cerium oxide particles produced by the above-mentioned method for producing cerium oxide particles. The cerium oxide particles may have a porosity of 40 to 80%.

Here, the porosity of the cerium oxide particles is a physical property measured by the following method.

Porosity (%) = (B / A) x100%

In this case, A means the volume (mL) indicated by the scale indicated by the uppermost cerium oxide particles when a certain amount of cerium oxide particles are weighed and charged evenly to the measuring cylinder, and B indicates the volume Means the volume (mL) of distilled water (DIW) injected into the cylinder when the cerium oxide particles are injected until the particles are locked. The porosity of the present invention is defined as the amount of distilled water that 100 ml volume of cerium oxide particles can contain. As the porosity of the particles increases, the content of distilled water that can be contained may increase. It is defined as a value that includes the volume of the particle's foreground volume and the volume between the particle and the particle.

3. Slurry composition for abrasive

Another aspect of the present invention provides a polishing slurry composition (hereinafter referred to as a " slurry composition ").

The slurry composition includes the above-mentioned cerium oxide particles, and may further comprise a solvent and a dispersant.

 The slurry composition may include 0.1 to 10 parts by weight of the cerium oxide particles, 90 to 99.9 parts by weight of the solvent, and 0.01 to 5 parts by weight of the dispersing agent.

If the content of the cerium oxide particles is less than 0.1 part by weight, a sufficient polishing rate may not be secured. If the content of the cerium oxide particles exceeds 10 parts by weight, superficial phenomenon occurs due to abrasive grains, and defects on the surface of the polishing target film due to abrasive grains may occur.

The solvent in the slurry composition may comprise water or an organic solvent. Preferably, the water is ultrapure water, and the organic solvent is selected from the group consisting of ethylene glycol (EG), polyethyleneglycol (PEG), methyl alcohol, isopropyl alcohol, methoxyethanol, acetone, glycerin, toluene, At least one selected from the group consisting of acetone, cyclohexane, butyl lactate, and butyl carbitol acetate may be included.

The dispersant serves to coat the cerium oxide particles to improve the degree of dispersion of the cerium oxide particles in the dispersion. There may be mentioned polyacrylic acid, polyacrylic acid ammonium salt, polymethacrylic acid, polymethacrylic acid ammonium salt, polyacrylic maleic acid, polystyrenesulfonic acid, carboxylic acid, carboxylate, sulphonic ester, sulphonic ester salt, sulphonic acid, sulphonic acid salt, And an anionic polymer containing at least one selected from the group consisting of esters and phosphonic ester salts.

As the anionic polymer is used as the dispersant, the slurry composition can be uniformly applied to an oxide film that is a film to be polished having a substantially partial negative charge.

The dispersant may include an anionic polymer having a weight average molecular weight of about 2,000 to about 100,000. When the weight average molecular weight of the dispersant is less than about 2,000, the degree of dispersion of the polishing composition may be reduced, resulting in local dishing. If the weight average molecular weight of the dispersant exceeds about 100,000, the polishing rate may be excessively reduced.

If the content of the dispersant is less than 0.01 parts by weight, the content of the dispersant capable of adsorbing on the surface of the cerium oxide is low, and the dispersibility becomes insufficient. If the content of the dispersant is more than 5 parts by weight, there is a problem that the slurry is reacted in the slurry composition to cause aggregation due to adsorption with abrasive grains, resulting in defects or scratches.

The polishing slurry composition may further include a pH adjusting agent in the solvent and in the dispersion excluding the dispersion.

The pH adjuster controls the dispersion of the cerium oxide particles by adjusting the pH of the slurry composition from pH 6 to pH 12. The pH adjusting agent may include at least one selected from the group consisting of an inorganic acid or an inorganic acid salt, and an organic acid or an organic acid salt.

The inorganic acid or salt may be at least one selected from the group consisting of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, hydrofluoric acid, bromic acid, iodic acid and salts thereof.

The organic acid or organic acid salt may be at least one selected from the group consisting of formic acid, malonic acid, maleic acid, acetic acid, adipic acid, citric acid, adipic acid, acetic acid, propionic acid, fumaric acid, lactic acid, salicylic acid, pimelic acid, benzoic acid, succinic acid, phthalic acid, At least one selected from the group consisting of tartaric acid, glutamic acid, glycolic acid, lactic acid, aspartic acid, tartaric acid and salts thereof.

If the pH of the slurry composition is less than 6, the zeta potential of the slurry composition may approach the IEP point and the dispersibility may deteriorate. If the pH of the slurry composition exceeds 12, corrosion of the polishing equipment may increase due to high basicity.

In particular, the inclusion of the pH adjusting agent can improve the dispersion stability and polishing performance of the cerium oxide particles, prevent precipitation or agglomeration of the cerium oxide particles that may occur due to a rapid pH change (pH shock) It is possible to minimize the occurrence of micro-scratches that can be caused in the process. When the pH of the polishing slurry composition is increased, a large amount of hydroxide ions (OH-) are present around the cerium oxide particles, and the zeta potential value of the cerium oxide particles is decreased. If the zeta potential value is decreased, the dispersion stability is lowered and micro scratches due to precipitation or aggregation of cerium oxide particles may occur, or the polishing rate for the oxide film may be lowered. Accordingly, it is preferable to maintain the pH of the polishing slurry composition at an appropriate value.

Hereinafter, the present invention will be described in detail with reference to examples. However, the following examples are illustrative of the present invention, and the present invention is not limited by the following examples.

[ Example  1-1]

Polyethylene glycol-polypropylene glycol-polyethylene glycol (PEG-PPG-PEG) copolymer was added in an amount of 2 wt% to a mixed solution of distilled water and ethylene glycol in a weight ratio of 3: 1 by weight, followed by rapid stirring to form micelles. After dissolving 5 mol of cerium nitrate in the aqueous solution in which the micelle was formed, the solution was stirred for 30 minutes while raising the temperature of the aqueous solution to 80 캜. To the aqueous solution was added 2 moles of hydrogen peroxide to 1 mole of cerium nitrate to prepare cerium hydroxide. The prepared cerium hydroxide was washed with distilled water and dried. The dried cerium hydroxide was calcined at 650 ° C for 4 hours to prepare cerium oxide particles.

[ Example  1-2]

Cerium oxide particles were prepared in the same manner as in Example 1-1 except that the mixing ratio of distilled water to ethylene glycol was 4: 1 in Example 1-1.

[ Example  1-3]

Cerium oxide particles were prepared in the same manner as in Example 1-1, except that cerium acetate was used as the cerium precursor in Example 1-1.

[ Example  1-4]

Cerium oxide particles were prepared in the same manner as in Example 1-1, except that ammonia water was used as the peroxide aqueous solution in Example 1-1.

[ Comparative Example  1-1]

The cerium carbonate precursor was vacuum dried at 80 DEG C and then calcined at 875 DEG C for 4 hours to prepare cerium oxide particles having a grain size of 52 nm.

[ Comparative Example  1-2]

A cerium oxide particle having a grain size of 38 nm was prepared in the same manner as in Comparative Example 1-1, except that the calcination temperature was set to 650 ° C in the above Comparative Example 1-1.

[ Comparative Example  1-3]

A cerium oxide particle having a grain size of 21 nm was produced in the same manner as in Comparative Example 1-1, except that the calcination temperature was set to 450 ° C in the above Comparative Example 1-1.

[ Example  2-1]

5 parts by weight of the cerium oxide particles obtained in Example 1-1 were mixed with 3 parts by weight of an anionic polymeric polyacrylic acid to prepare a polishing slurry composition.

[ Example  2-2]

A polishing slurry composition was prepared in the same manner as in Example 2-1, except that the cerium oxide particles obtained in Example 1-2 were used.

[ Example  2-3]

A polishing slurry composition was prepared in the same manner as in Example 2-1, except that the cerium oxide particles obtained in Example 1-3 were used.

[ Example  2-4]

A polishing slurry composition was prepared in the same manner as in Example 2-1 except that the cerium oxide particles obtained in Example 1-4 were used.

[ Comparative Example  2-1]

A polishing slurry composition was prepared in the same manner as in Example 2-1, except that the cerium oxide particles obtained in Comparative Example 1-1 were used.

[ Comparative Example  2-2]

A polishing slurry composition was prepared in the same manner as in Example 2-1, except that the cerium oxide particles obtained in Comparative Example 1-2 were used.

[ Comparative Example  2-3]

A polishing slurry composition was prepared in the same manner as in Example 2-1, except that the cerium oxide particles obtained in Comparative Example 1-3 were used.

[ Experimental Example ]

a) Measurement of porosity

50 g of the cerium oxide particles prepared according to the above Examples and Comparative Examples were evenly charged into a measuring cylinder and the volume of the cerium oxide particles filled in the measuring cylinder was measured. Then, DI water was added to the measuring cylinder, When the uppermost part of the particle was injected until it was locked, the volume of the injected distilled water was measured to measure the porosity. The results are shown in Table 1 below.

b) Secondary particle size measurement

The measurement was performed using a Malvern Zetasizer measuring instrument employing the identification light scattering measurement principle. The results are shown in Table 1 below.

c) Measurement of polishing rate

The initial wafer thickness was measured with a thickness measuring apparatus using a non-contact optical reflection measurement principle on the thickness of a wafer on which an oxide film having a thickness of 13,000 A was formed on a silicon (Si) wafer by CVD vapor deposition.

Thereafter, the oxide films formed on the wafers were polished by a CMP process using the slurry compositions of the above-described Examples and Comparative Examples according to the following polishing conditions. After the CMP process, the oxide film polishing rate, defects, and scratches were measured. The results are shown in Table 2 below.

[Polishing condition]

- Slurry mixing ratio (slurry: distilled water (DI Water) = 1: 10)

- Wafer: 300 mm TEOS blanket wafer

- Pressure: Head 3.0 psi / R-Ring 3.6 Psi

- Carrier / Platen Speed (RPM): 100/101

- Flow rate: 250ml

Porosity Secondary particle size Example 1-1 56 104 nm Examples 1-2 48 155 nm Example 1-3 62 187 nm Examples 1-4 78 274 nm Comparative Example 1-1 35 275 nm Comparative Example 1-2 26 156 nm Comparative Example 1-3 18 105 nm

The polishing rate (A) of the silicon oxide film Defects / Scratches Example 2-1 3564 297/8 Example 2-2 3784 784/21 Example 2-3 4321 567/14 Examples 2-4 4986 645/16 Comparative Example 2-1 4758 3476/248 Comparative Example 2-2 3845 2498/214 Comparative Example 2-3 3641 2142/198

Referring to Table 1 and Table 2, when the porosity of the cerium oxide particles is 40 to 80% even though the cerium oxide particles have similar particle diameters, it is confirmed that the cerium oxide particles have a higher polishing rate and a lower scratch generation than the cerium oxide particles that do not satisfy the porosity .

Claims (26)

Mixing a cerium precursor, a surfactant, an organic solvent and water to obtain a cerium precursor mixture;
Mixing a cerium precursor mixture with a precipitant to obtain a cerium hydroxide precipitate; And
And calcining the cerium hydroxide to obtain cerium oxide particles. The method according to claim 1,
Wherein the cerium precursor comprises at least one selected from the group consisting of cerium nitrate, ammonium cerium nitrate, cerium chloride, cerium sulfate, cerium carbonate, cerium hydroxide and cerium acetate. The method according to claim 1,
Wherein the surfactant comprises at least one selected from the group consisting of a nonionic surfactant, an anionic surfactant, a cationic surfactant, an amphoteric surfactant, and a gemini type surfactant. The method of claim 3,
The nonionic surfactant is selected from the group consisting of alkyl phenol ethoxylates (APEO), alkyl ethoxylates (AE), alkanol amides (AA), amine oxides (AO), polyethylene glycols (PEG), ethylene glycols Wherein the cerium oxide particles comprise at least one selected from the group consisting of alcohol (PVA), glycerin, polypropylene glycol (PPG), polyvinyl pyrrolidone (PVP), and combinations thereof. The method of claim 3,
The anionic surfactant may be at least one selected from the group consisting of polyacrylic acid, ammonium polyacrylate, polymethacrylic acid, polymethacrylic acid ammonium salt, polyacrylic maleic acid, polystyrenesulfonic acid, carboxylic acid, carboxylate, sulphonic ester, sulphonic ester salt, A sulfonic acid salt, a sulfonic acid salt, a phosphonic ester, and a phosphoric ester salt. The method of claim 3,
Wherein the cationic surfactant comprises at least one member selected from the group consisting of quaternary ammonium, sulfoxonium, sulfonium salts, iodine compounds and arsenic compounds. The method of claim 3,
Wherein the amphoteric surfactant comprises at least one selected from the group consisting of amino acid, betaine, imidazoline, and alkylpolyamino acids. The method of claim 3,
The gemini type surfactant may be selected from the group consisting of N-hexane-diyl-1,2-ethane bis ammonium bromide, N-dodecanediyl- Di-n-hexadecane-diyl-1,2-ethane bisammonium bromide, N-hexadecane-diyl- ethane bis ammonium bromide, butane-diyl-1,4-bis dimethyl cetylammonium bromide, pentane-diyl-1,5-bisdimethyl cetylammonium bromide Pentane-diyl-1,5-bis dimethylcetyl ammonium bromide), hexane-diyl-1,6-bisdimethyl cetylammonium bromide (at least one selected from the group consisting of hexane- ≪ / RTI > The method according to claim 1,
Examples of the organic solvent include ethylene glycol, polyethylene glycol, methyl alcohol, isopropyl alcohol, methoxyethanol, acetone, glycerin, toluene, methyl ethyl ketone, ethyl acetate, cyclohexane, butyl lactate and butyl carbitol acetate Wherein the cerium oxide particles contain at least one selected from cerium oxide particles and cerium oxide particles. The method according to claim 1,
Wherein the precipitating agent comprises at least one selected from the group consisting of ammonia water, potassium hydroxide, sodium hydroxide and hydrogen peroxide. The method according to claim 1,
And the temperature for calcining the cerium hydroxide is 400 to 1200 ° C. The method according to claim 1,
Wherein the water and the organic solvent are contained in a weight ratio of 6: 1 to 1: 1. The method according to claim 1,
Wherein the cerium precursor is contained in the cerium precursor mixture at a concentration (M) of 0.1 to 1.0 mole based on the mole number of cerium atoms in the cerium precursor mixture. The method according to claim 1,
Wherein the surfactant is contained in the cerium precursor mixture solution in an amount of 0.1 to 10 wt%. A cerium oxide particle produced by the manufacturing method according to any one of claims 1 to 14. A porosity of 40 to 80% for cerium oxide particles for polishing. A polishing slurry composition comprising the cerium oxide particles of claim 16. 18. The method of claim 17,
A solvent and a dispersant. 19. The method of claim 18,
Wherein the slurry composition comprises 0.1 to 10 parts by weight of the cerium oxide particles, 90 to 99.9 parts by weight of a solvent and 0.01 to 5 parts by weight of a dispersing agent. 19. The method of claim 18,
Wherein the polishing slurry composition further comprises a pH adjusting agent. The method of claim 20,
Wherein the polishing slurry composition has a pH of 6 to 12. 19. The method of claim 18,
The dispersant may be selected from the group consisting of polyacrylic acid, ammonium polyacrylate, polymethacrylic acid, ammonium polymethacrylate, polyacrylic maleic acid, polystyrene sulfonic acid, carboxylic acid, carboxylate, , An anionic polymer comprising at least one selected from the group consisting of a phosphonic ester and a phosphonic ester salt. 19. The method of claim 18,
Wherein the solvent comprises water or an organic solvent. The method of claim 20,
The pH adjusting agent may be an inorganic acid or an inorganic acid salt; And at least one selected from the group consisting of an organic acid or an organic acid salt. 27. The method of claim 24,
Wherein the inorganic acid or inorganic acid salt includes at least one selected from the group consisting of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, hydrofluoric acid, bromic acid, iodic acid and salts thereof. 27. The method of claim 24,
The organic acid or organic acid salt may be at least one selected from the group consisting of formic acid, malonic acid, maleic acid, acetic acid, adipic acid, citric acid, adipic acid, acetic acid, propionic acid, fumaric acid, lactic acid, salicylic acid, pimelic acid, benzoic acid, succinic acid, phthalic acid, At least one member selected from the group consisting of tartaric acid, glutamic acid, glycolic acid, lactic acid, aspartic acid, tartaric acid and salts thereof.

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