JP2023046873A - Formation method for protective film, manufacturing method of semiconductor chip and preparation method for coating liquid - Google Patents

Abstract

To provide a formation method for a protective film capable of suppressing a foreign substance in a formed protective film even after the lapse of time from manufacturing a protective film forming agent for forming the protective film on a surface of a semiconductor wafer, a manufacturing method of a semiconductor chip and a preparation method for a coating liquid.SOLUTION: The present invention relates to a formation method for a protective film for forming a protective film 24 on a surface 20a of a semiconductor wafer 2 in dicing of the semiconductor wafer 2. The formation method for the protective film includes: a protective film forming agent preparing step of preparing a protective film forming agent which contains a water soluble resin, a light absorption agent and a solvent and in which the solvent contains water and a solid content concentration is 20 mass% or more; a coating liquid preparing step of preparing a coating liquid by diluting the protective film forming agent; and a protective film forming step of forming the protective film by coating the surface of the semiconductor wafer with the coating liquid.SELECTED DRAWING: Figure 3

Description

The present invention provides a protective film forming method for forming a protective film on the surface of a semiconductor wafer in dicing of a semiconductor wafer, a semiconductor chip manufacturing method using the protective film forming method, and the protective film forming method described above. and a method for preparing a coating liquid that can be used.

A wafer formed in a semiconductor device manufacturing process is a laminate in which an insulating film and a functional film are laminated on the surface of a semiconductor substrate such as silicon, and is partitioned by grid-like dividing lines called streets. Each partitioned area is a semiconductor chip such as an IC or LSI.

A plurality of semiconductor chips are obtained by cutting the wafer along the streets. Further, in the optical device wafer, a laminated body in which gallium nitride-based compound semiconductors and the like are laminated is divided into a plurality of regions by streets. By cutting along these streets, the optical device wafer is divided into optical devices such as light emitting diodes and laser diodes. These optical devices are widely used in electrical equipment.

In the past, such cutting along the wafer streets was performed by a cutting device called a dicer. However, in this method, since the wafer having a laminated structure is a highly brittle material, when the wafer is cut and divided into semiconductor chips, etc. There is a problem that the insulating film necessary for the formed circuit element is peeled off.

In order to solve such problems, a mask containing a layer of a water-soluble material is formed on the surface of the semiconductor substrate, and then the mask is irradiated with a laser to decompose and remove a part of the mask. A method has been proposed in which after exposing the surface of a semiconductor substrate in a portion of a mask, the semiconductor substrate exposed from a portion of the mask is cut by plasma etching to divide the semiconductor substrate into semiconductor chips (ICs) ( See Patent Document 1.).

A mask (protective film) containing a layer of a water-soluble material is formed, for example, by coating a semiconductor substrate with a protective film-forming agent containing a water-soluble resin.

Japanese Patent Publication No. 2014-523112

When a protective film-forming agent is applied to the surface of a semiconductor substrate to form a protective film as a mask, the protective film-forming agent is applied not immediately after production of the protective film-forming agent, but after storage or transportation after production of the protective film-forming agent. A forming agent may be applied to the semiconductor substrate.
When the protective film-forming agent is applied to the semiconductor substrate after a lapse of time after the production of the protective film-forming agent in this way, the protective film formed is less than when the protective film-forming agent is applied to the semiconductor substrate immediately after the production of the protective film-forming agent. There is a problem that foreign matter generated in the

The present invention has been made in view of the above problems, and is a method for forming a protective film on the surface of a semiconductor wafer in dicing of a semiconductor wafer, wherein a time elapses after the production of a protective film forming agent. It can be used in a method of forming a protective film capable of suppressing an increase in foreign substances in the formed protective film, a method of manufacturing a semiconductor chip using the method of forming the protective film, and the method of forming the protective film described above. An object of the present invention is to provide a method for preparing a coating liquid.

The present inventors prepare a specific protective film-forming agent, dilute the protective film-forming agent to prepare a coating solution, and apply the coating solution on a semiconductor wafer to form a protective film. The present inventors have found that the problem can be solved, and have completed the present invention. More specifically, the present invention provides the following.

A first aspect of the present invention is a method for forming a protective film on the surface of a semiconductor wafer in dicing of a semiconductor wafer, comprising:
Protection for preparing a protective film-forming agent comprising a water-soluble resin (A), a light absorbing agent (B), and a solvent (S), wherein the solvent (S) contains water and has a solid content concentration of 20% by mass or more a film-forming agent preparation step;
a coating liquid preparation step of diluting the protective film forming agent to prepare a coating liquid;
a protective film forming step of applying the coating liquid onto a semiconductor wafer to form a protective film;
A method for forming a protective film.

A second aspect of the present invention is a method for manufacturing a semiconductor chip by processing a semiconductor wafer, comprising:
According to the method for forming a protective film according to the first aspect, a protective film forming step of forming a protective film on the semiconductor wafer; a process groove forming step of exposing the surface of the semiconductor wafer by irradiation and forming process grooves having a pattern corresponding to the shape of the semiconductor chip;
a cutting step of cutting the positions of the processed grooves in the semiconductor wafer;
A method for manufacturing a semiconductor chip, comprising:

A third aspect of the present invention is a method for preparing a coating liquid used for forming a protective film on the surface of a semiconductor wafer in dicing of a semiconductor wafer,
Dilute a protective film-forming agent comprising a water-soluble resin (A), a light absorbing agent (B), and a solvent (S), wherein the solvent (S) comprises water and has a solid content concentration of 20% by mass or more. A method for preparing a coating liquid including a dilution step.

According to the present invention, in the dicing of semiconductor wafers, a protective film forming method for forming a protective film on the surface of a semiconductor wafer, wherein the protective film is formed even after the passage of time after the production of the protective film forming agent. The present invention provides a method for forming a protective film capable of suppressing the increase of foreign matter, a method for manufacturing a semiconductor chip using the method for forming the protective film, and a method for preparing a coating liquid that can be used in the method for forming the protective film. be able to.

1 is a perspective view showing a semiconductor wafer processed by a semiconductor chip manufacturing method using a protective film forming method of the present invention; FIG. FIG. 2 is an enlarged sectional view of the semiconductor wafer shown in FIG. 1; FIG. 2 is an enlarged cross-sectional view of a main part of a semiconductor wafer on which a protective film is formed; FIG. 4 is a perspective view showing a state in which a semiconductor wafer having a protective film formed thereon is supported by an annular frame via a protective tape; FIG. 2 is a perspective view of a main part of a laser processing device that carries out a laser beam irradiation process; FIG. 2 is an enlarged cross-sectional view of a semiconductor wafer having a protective film and processed grooves formed by laser light irradiation; FIG. 7 is an explanatory view showing plasma irradiation to the semiconductor wafer shown in FIG. 6; FIG. 2 is an enlarged cross-sectional view showing a state in which a semiconductor wafer is divided into semiconductor chips by plasma irradiation; FIG. 4 is an enlarged cross-sectional view showing a state in which a protective film on a semiconductor chip is removed;

<<Method for Forming Protective Film and Method for Preparing Coating Solution>>
The protective film forming method is a protective film forming method for forming a protective film on the surface of a semiconductor wafer in dicing of the semiconductor wafer.
Specifically, the method for forming a protective film includes a protective film forming step of forming a protective film on a semiconductor wafer;
A processed groove forming step of irradiating a predetermined position of one or more layers including a protective film on a semiconductor wafer with a laser beam to expose the surface of the semiconductor wafer and form processed grooves having a pattern corresponding to the shape of the semiconductor chip. and,
a cutting step of cutting the positions of the processed grooves in the semiconductor wafer;
It is suitably used for a protective film forming step in a method for manufacturing a semiconductor chip including

The method for forming the protective film comprises a water-soluble resin (A), a light absorbing agent (B), and a solvent (S), the solvent (S) containing water, and a solid content concentration of 20% by mass or more. A protective film forming agent preparing step of preparing a forming agent, a coating solution preparing step of diluting the protective film forming agent to prepare a coating solution, and a protective film forming step of coating the coating solution on a semiconductor wafer to form a protective film. and a step.

<Protective film forming agent preparation process>
In the protective film forming agent preparation step, a protective film forming agent is prepared.
The protective film forming agent prepared in the protective film forming agent preparing step contains a water-soluble resin (A), a light absorbing agent (B), and a solvent (S), and the solvent (S) contains water. Moreover, the solid content concentration of the protective film-forming agent is 20% by mass or more.
The essential or optional components contained in the protective film-forming agent and the solid content concentration are described below.

<Water-soluble resin (A)>
The water-soluble resin (A) is a base material for a protective film formed using a protective film-forming agent. The type of water-soluble resin (A) is not particularly limited as long as it is a resin that can be dissolved in a solvent such as water, applied, and dried to form a film.
Water-soluble means that 0.5 g or more of solute (water-soluble resin) dissolves in 100 g of water at 25°C.

From the viewpoint of compatibility between decomposability when irradiated with laser light and film formability, the weight average molecular weight of the water-soluble resin (A) is preferably 15,000 or more and 300,000 or less, more preferably 20,000 or more and 200. ,000 or less is more preferable.

Specific examples of types of the water-soluble resin (A) include vinyl-based resins, cellulose-based resins, polyethylene oxide, polyglycerin, and water-soluble nylon.
The vinyl-based resin is not particularly limited as long as it is a homopolymer or copolymer of a monomer having a vinyl group and is water-soluble. Vinyl resins include polyvinyl alcohol, polyvinyl acetal (including vinyl acetate copolymer), polyvinylpyrrolidone, polyacrylamide, poly(N-alkylacrylamide), polyallylamine, poly(N-alkylallylamine), partially amidated poly Allylamine, poly(diallylamine), allylamine-diallylamine copolymer, polyacrylic acid, polyvinyl alcohol-polyacrylic acid block copolymer, and polyvinyl alcohol-polyacrylate block copolymer.
The cellulose resin is not particularly limited as long as it is a water-soluble cellulose derivative. Examples of cellulosic resins include methyl cellulose, ethyl cellulose, and hydroxypropyl cellulose.
In addition, it is preferable that the water-soluble resin (A) does not have an acidic group.
These can also be used individually by 1 type, and can also be used in combination of 2 or more type.

Among the specific examples of the above water-soluble resin (A), vinyl-based resins and cellulose-based resins are preferred because the shape of the processed groove is less likely to deteriorate due to thermal sagging of the protective film, and polyvinylpyrrolidone and hydroxypropyl Cellulose is more preferred.

The protective film formed on the surface of the semiconductor wafer is usually applied to the semiconductor wafer or the semiconductor chip at an appropriate time after the formation of the processed grooves according to the method of processing the semiconductor wafer comprising the protective film and the processed grooves into a semiconductor chip. removed from the surface of the Therefore, a water-soluble resin having a low affinity for the surface of the semiconductor wafer is preferred from the standpoint of water washability of the protective film. As the water-soluble resin having low affinity with the semiconductor wafer surface, resins having only ether bonds, hydroxyl groups, and amide bonds as polar groups, such as polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, and hydroxypropyl cellulose are preferred.

When forming grooves by irradiating a laser beam on the protective film, poor opening and deterioration of the shape of the groove due to thermal sagging of the protective film are unlikely to occur, so water-soluble resins ( The ratio of the mass of the water-soluble resin (A) to the total amount of the mass of A) and the mass of the light absorber (B) is preferably 60% by mass or more and 99% by mass or less, and 85% by mass or more and 98% by mass or less. more preferred.

<Light absorbing agent (B)>
As the light absorbing agent (B), light absorbing agents generally used in protective film forming agents can be used.
As the light absorbing agent (B), it is preferable to use water-soluble light absorbing agents such as water-soluble dyes, water-soluble pigments, and water-soluble ultraviolet absorbers. A water-soluble light-absorbing agent is advantageous in making it uniformly present in the protective film. Examples of water-soluble light absorbing agents include organic acids having a carboxy group or a sulfo group; sodium salts, potassium salts, ammonium salts, and quaternary ammonium salts of organic acids; and compounds having a hydroxy group.
When a water-soluble light-absorbing agent is used, the storage stability of the protective film-forming agent is high, and problems such as phase separation of the protective film-forming agent and precipitation of the light-absorbing agent are suppressed during storage of the protective film-forming agent. Therefore, it is also advantageous in that it is easy to maintain good applicability of the protective film forming agent for a long period of time.

A water-insoluble light absorbing agent such as a pigment may also be used. When using a water-insoluble light-absorbing agent, the use of the protective film-forming agent is not fatally hindered, but the laser absorption capacity of the protective film may vary, and the protective film may be formed with excellent storage stability and coating properties. In some cases, it may be difficult to obtain the agent, or it may be difficult to form a protective film having a uniform thickness.

（式（Ｂ１）中、Ｒ^１及びＲ^３は、それぞれ独立に、水酸基、又はカルボキシ基であり、Ｒ^２及びＲ^４は、それぞれ独立に、水酸基、カルボキシ基、又は－ＮＲ^５Ｒ^６で表される基であり、Ｒ^５及びＲ^６は、それぞれ独立に、水素原子、又は炭素原子数１以上４以下のアルキル基であり、ｍ及びｎは、それぞれ独立に０以上２以下の整数である。） Examples of the light absorbing agent (B) include benzophenone-based compounds, cinnamic acid-based compounds, anthraquinone-based compounds, naphthalene-based compounds, and biphenyl-based compounds.
Benzophenone-based compounds include compounds represented by the following formula (B1). The compound represented by the following formula (B1) is preferable because the protective film can efficiently absorb the energy of the laser beam and promote the thermal decomposition of the protective film.

(In formula (B1), R ¹ and R ³ are each independently a hydroxyl group or a carboxy group, and R ² and R ⁴ are each independently a hydroxyl group, a carboxy group, or —NR ⁵ R ⁶ . R ⁵ and R ⁶ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and m and n are each independently an integer of 0 to 2 .)

The compound represented by the above formula (B1) has a high absorption coefficient, and exhibits a high absorption coefficient even when added to the protective film-forming agent together with an alkali. Therefore, if a protective film is formed using a protective film-forming agent containing the compound represented by the above formula (B1) as a light absorber (B), a partial laser beam on the protective film is formed during mask formation for dicing. can be successfully carried out.

In formula (B1) above, R ² and R ⁴ may be a group represented by —NR ⁵ R ⁶ . ^R5 and ^R6 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. Alkyl groups as R ⁵ and R ⁶ may be linear or branched. Specific examples of alkyl groups for R ⁵ and R ⁶ are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl.

The group represented by -NR ⁵ R ⁶ is preferably an amino group, a methylamino group, an ethylamino group, a dimethylamino group and a diethylamino group, more preferably an amino group, a dimethylamino group and a diethylamino group.

（式（Ｂ１－１中）、Ｒ^１～Ｒ^４、ｍ、及びｎは、式（Ｂ１）中のこれらと同様である。） The compound represented by the formula (B1) is preferably a compound represented by the following formula (B1-1) because of its high absorption coefficient in the presence of a base.

(In Formula (B1-1), R ¹ to R ⁴ , m, and n are the same as those in Formula (B1).)

In the formulas (B1) and (B1-1), at least one of R ¹ and R ³ is preferably a hydroxyl group because of its high absorption coefficient in the presence of a base.

（式（Ｂ１－１ａ）～式（Ｂ１－１ｅ）中、Ｒ^１～Ｒ^４は、式（Ｂ１）中のこれらと同様である。） The compound represented by formula (B1-1) is preferably a compound represented by any one of formulas (B1-1a) to (B1-1e) below.

(In formulas (B1-1a) to (B1-1e), R ¹ to R ⁴ are the same as those in formula (B1).)

Among the compounds represented by formulas (B1-1a) to (B1-1e), the compound represented by formula (B1-1a) is preferred.
In the compounds represented by formulas (B1-1a) to (B1-1e), R ² is the aforementioned group represented by —NR ⁵ R ⁶ , and R ⁵ and R ⁶ are each independently It is preferably an alkyl group having 1 to 4 carbon atoms.

Preferred specific examples of the compound represented by formula (B1) include the following compounds.
These compounds are preferred due to their ready availability and high extinction coefficients even in the presence of bases.

When the light absorber (B) contains the compound represented by formula (B1), the ratio of the mass of the compound represented by formula (B1) to the mass of the light absorber (B) does not hinder the object of the present invention. The range is not particularly limited. The mass ratio of the compound represented by the formula (B1) to the mass of the light absorbing agent (B) is preferably 70% by mass or more, more preferably 80% by mass or more, further preferably 95% by mass or more, and 100% by mass. is particularly preferred.

Benzophenone-based compounds may also include 4,4'-dicarboxybenzophenone, benzophenone-4-carboxylic acid, and tetrahydroxybenzophenone. All of these are water-soluble ultraviolet absorbers.

（式（Ｂ２）中、Ｒ^１１は、水酸基、アルコキシ基、又は－ＮＲ^１２Ｒ^１３で表される基であり、Ｒ^１２及びＲ^１３は、それぞれ独立に、水素原子、又は炭素原子数１以上４以下のアルキル基であり、ｐは、０以上３以下の整数であり、ｐが２以上である場合、複数のＲ^１１は、同一でも異なっていてもよい。） Examples of cinnamic acid compounds include compounds represented by the following formula (B2). The compound represented by the following formula (B2) is preferable because the protective film can efficiently absorb the energy of the laser beam and promote the thermal decomposition of the protective film.

(In formula (B2), R ¹¹ is a hydroxyl group, an alkoxy group, or a group represented by —NR ¹² R ¹³ , and each of R ¹² and R ¹³ is independently a hydrogen atom, or has 1 or more carbon atoms. an alkyl group of 4 or less, p is an integer of 0 or more and 3 or less, and when p is 2 or more, a plurality of R ¹¹ may be the same or different.)

In formula (B2) above, the alkoxy group as R ¹¹ may be linear or branched. The alkoxy group as R ¹¹ is preferably an alkoxy group having 1 to 4 carbon atoms. Specific examples of alkoxy groups as R ¹¹ are methoxy, ethoxy, n-propoxy, isopropoxy and n-butoxy groups.

In formula (B2) above, R ¹¹ may be a group represented by —NR ¹² R ¹³ . R ¹² and R ¹³ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. Alkyl groups as R ¹² and R ¹³ may be linear or branched. Specific examples of alkyl groups for R ¹² and R ¹³ are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl.

（式（Ｂ２－１）中、Ｒ^１１は、式（Ｂ２）中のＲ^１１と同様である。） The compound represented by formula (B2) is preferably a compound represented by formula (B2-1) below.

(In formula (B2-1), R ¹¹ is the same as R ¹¹ in formula (B2).)

Specific examples of cinnamic acid compounds include 4-aminocinnamic acid, 3-aminocinnamic acid, 2-aminocinnamic acid, sinapinic acid (3,5-dimethoxy-4-hydroxycinnamic acid), ferulic acid, and caffeic acid. can be mentioned.
Among these, 4-aminocinnamic acid, 3-aminocinnamic acid, 2-aminocinnamic acid and ferulic acid are preferred, and 4-aminocinnamic acid and ferulic acid are more preferred.

When the light absorbing agent (B) contains the compound represented by formula (B2), the ratio of the mass of the compound represented by formula (B2) to the mass of the light absorbing agent (B) does not hinder the object of the present invention. The range is not particularly limited. The mass ratio of the compound represented by the formula (B2) to the mass of the light absorber (B) is preferably 70% by mass or more, more preferably 80% by mass or more, further preferably 95% by mass or more, and 100% by mass. is particularly preferred.

Specific examples of anthraquinone compounds include 2-carboxyanthraquinone, 2,6-anthraquinonedisulfonic acid, and 2,7-anthraquinonedisulfonic acid.

Specific examples of naphthalene compounds include 1,2-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid. can be mentioned.

Specific examples of biphenyl compounds include biphenyl-4-sulfonic acid.

Examples of the light absorber (B) include curcumin and water-soluble amines such as EAB-F (4,4'-bis(diethylamino)benzophenone).

Specific examples of water-soluble dyes include azo dyes (monoazo and polyazo dyes, metal complex salt azo dyes, pyrazolone azo dyes, stilbene azo dyes, thiazole azo dyes), anthraquinone dyes (anthraquinone derivatives, anthrone derivatives), indigoid dyes (indigoid derivatives , thioindigoid derivatives), phthalocyanine dyes, carbonium dyes (diphenylmethane dyes, triphenylmethane dyes, xanthene dyes, acridine dyes), quinone imine dyes (azine dyes, oxazine dyes, thiazine dyes), methine dyes (cyanine dyes, azomethine dyes), quinoline Water-soluble dyes are selected from dyes, nitroso dyes, benzoquinone and naphthoquinone dyes, naphthalimide dyes, perinone dyes, and other dyes.

Examples of water-soluble pigments include Food Red No. 2, Food Red No. 40, Food Red No. 102, Food Red No. 104, Food Red No. 105, Food Red No. 106, Food Yellow NY, Food Yellow No. 4 Tartrazine, and Food Yellow. No. 5, Edible Yellow No. 5 Sunset Yellow FCF, Edible Orange AM, Edible Vermilion No. 1, Edible Vermilion No. 4, Edible Vermilion No. 101, Food Blue No. 1, Food Blue No. 2, Food Green No. 3, Food Melon Color B, and Food Egg Color No. 101. Food additive dyes such as No. 3 are preferable because of their low environmental load.

The content of the light absorbing agent (B) in the protective film-forming agent is not particularly limited as long as the object of the present invention is not impaired. The content of the light absorbing agent (B) in the protective film-forming agent is preferably 0.05% by mass or more and 10% by mass or less.
When forming grooves by irradiating a laser beam on the protective film, poor opening and deterioration of the shape of the groove due to thermal sagging of the protective film are unlikely to occur, so water-soluble resins ( The ratio of the mass of the light absorbing agent (B) to the total amount of the mass of A) and the mass of the light absorbing agent (B) is preferably 0.5% by mass or more and 20% by mass or less, and 1% by mass or more and 18% by mass or less. is more preferable, and 2% by mass or more and 15% by mass or less is even more preferable.

The content of the light absorbing agent (B) can be set so that the protective film formed using the protective film forming agent has a desired absorbance. The absorbance of the protective film formed using the protective film-forming agent is not particularly limited. It is preferably 0.8 or more, more preferably 0.8 or more, and even more preferably 1.0 or more.

<Other additives>
In addition to the water-soluble resin (A) and the light-absorbing agent (B), the protective film-forming agent may contain other ingredients as long as they do not hinder the object of the present invention. Other ingredients that can be used include, for example, preservatives, basic compounds, surfactants, and the like.

Preservatives include benzoic acid, butylparaben, ethylparaben, methylparaben, propylparaben, benzalkonium chloride, benzethonium chloride, benzyl alcohol, cetylpyridinium chloride, chlorobutanol, phenol, phenylethyl alcohol, 2-phenoxyethanol, phenyl nitrate. Dimercury, thimerosal, metacresol, lauryldimethylamine oxide, or combinations thereof can be used.

It is preferable to use an antiseptic not only from the viewpoint of preserving the protective film-forming agent, but also from the viewpoint of reducing the burden of treating the waste liquid after cleaning the semiconductor wafer. Large amounts of cleaning water are typically used for cleaning semiconductor wafers. However, in the process using the protective film-forming agent described above, there is concern about the propagation of various bacteria in the waste liquid due to the water-soluble resin (A) contained in the protective film-forming agent. Therefore, it is desirable that the waste liquid derived from the process using the aforementioned protective film-forming agent be treated separately from the waste liquid derived from the process not using the protective film-forming agent. However, when the protective film-forming agent contains a preservative, the propagation of bacteria caused by the water-soluble resin (A) is suppressed, so the waste liquid derived from the process using the protective film-forming agent and the protective film-forming agent Effluents from processes that do not use a Therefore, the load of the wastewater treatment process can be reduced.

When the protective film-forming agent contains a preservative, the content of the preservative is not particularly limited as long as the object of the present invention is not impaired. The content of the preservative in the protective film-forming agent is preferably 0.01 parts by mass or more and 5 parts by mass or less, and 0.05 parts by mass or more and 2 parts by mass or less with respect to 100 parts by mass of the water-soluble resin (A). more preferred.

As the basic compound, both inorganic compounds and organic compounds can be used, and examples thereof include alkylamines, alkanolamines, imidazole compounds, ammonia and alkali metal hydroxides.
Specific examples of basic compounds include sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium silicate, sodium metasilicate, ammonia; ethylamine, n-propylamine, monoethanolamine, diethylamine, di-n- Propylamine, diethanolamine, triethylamine, methyldiethylamine, dimethylethanolamine, triethanolamine, tetramethylammonium hydroxide, tetraethylammonium hydroxide, imidazole, 2-methylimidazole, 1,2-methylimidazole, N-methylimidazole, 4- methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, pyrrole, piperidine, 1,8-diazabicyclo[5,4,0]-7-undecene, and 1,5-diazabicyclo[4,3,0 ]-5-nonane.

The amount of the basic compound used is not particularly limited as long as the object of the present invention is not impaired. The amount of the basic compound to be used is preferably 1 mol or more, more preferably 1 mol or more and 20 mol or less, per 1 mol of the light absorbing agent (B). The lower limit of the amount of the basic compound used may be 1.5 mol or more, 2 mol or more, or 3 mol or more per 1 mol of the light absorber (B). The upper limit of the amount of the basic compound used may be 15 mol or less, 10 mol or less, or 5 mol or less per 1 mol of the light absorber (B).

Surfactants are used, for example, to improve the defoaming properties during the production of protective film-forming agents, the stability of protective film-forming agents, and the applicability of coating liquids prepared from the protective film-forming agents. In particular, it is preferable to use a surfactant from the viewpoint of defoaming properties during the production of the protective film-forming agent.

Also, the protective film is formed by, for example, spin-coating a coating liquid prepared from a protective film-forming agent. However, when the protective film is formed, irregularities due to air bubbles may occur. In order to suppress the occurrence of such unevenness, it is preferable to use an antifoaming agent such as a surfactant.

As the surfactant, a water-soluble surfactant can be preferably used. Any of nonionic surfactants, cationic surfactants, anionic surfactants, and amphoteric surfactants can be used as surfactants. Surfactants may be silicone-based. A nonionic surfactant is preferable from the point of detergency.

<Solvent (S)>
The protective film-forming agent contains a solvent (S) for dissolving or dispersing the water-soluble resin (A) and the light absorbing agent (B). That is, the protective film-forming agent is a solution or dispersion of the water-soluble resin (A) and the light absorbing agent (B).
Solvent (S) includes water.
The solvent (S) may contain an organic solvent together with water. As the solvent (S), water and an aqueous solution of an organic solvent are preferable, and water is more preferable, because there is little risk of ignition during use, and the cost is low.

From the viewpoint of flammability, the content of the organic solvent in the solvent (S) is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less.

Solvent (S) is preferably selected such that the overcoat-forming agent does not have a flash point at 1 atm. Specifically, by adjusting the water content in the protective film-forming agent, the flash point of the protective film-forming agent and the presence or absence of the flash point are adjusted.
Protective film formers that do not have a flash point are safe and can be placed, for example, in non-explosive environments. Specifically, handling such as storage, transportation, and use of the protective film-forming agent can be performed in a non-explosion-proof environment. For example, it is possible not only to introduce a protective film forming agent into a semiconductor factory, but also to form a protective film in a non-explosion-proof environment. Therefore, a protective film-forming agent that does not have a flash point is industrially very advantageous in that it does not require an explosion-proof environment such as an explosion-proof facility that is usually expensive.

The flash point is obtained by measuring with a closed tag system at a liquid temperature of 80°C or less under 1 atm, and by measuring with a Cleveland open system at a liquid temperature of over 80°C.
In the present specification, when the flash point cannot be measured even by measuring with the Cleveland open system, it is defined as no flash point.

Examples of organic solvents that the protective film-forming agent may contain include methyl alcohol, ethyl alcohol, alkylene glycol, alkylene glycol monoalkyl ether, alkylene glycol monoalkyl ether acetate, and the like.
Alkylene glycol includes ethylene glycol, propylene glycol, and the like. Alkylene glycol monoalkyl ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether. Alkylene glycol monoalkyl ether acetates include ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate.
The protective film-forming agent may contain a combination of two or more organic solvents.

The solid content concentration of the protective film-forming agent must be 20% by mass or more. Although details will be described later, in the present invention, a protective film-forming agent having a solid content concentration of 20% by mass or more is used, and a coating liquid obtained by diluting this is applied to form a protective film.
Although the upper limit of the solid content concentration of the protective film-forming agent is not particularly limited, it is preferably 60% by mass or less, more preferably 50% by mass or less.
In addition, in this specification, solid content is a component other than a solvent (S).

<Coating liquid preparation process>
In the coating liquid preparation step, the coating liquid is prepared by diluting the above protective film forming agent.
That is, in the dicing of semiconductor wafers, a method for preparing a coating solution used for forming a protective film on the surface of a semiconductor wafer, comprising a water-soluble resin (A), a light absorbing agent (B) and a solvent (S) and the solvent (S) contains water, and the solid content concentration is 20% by mass or more.

The solid content concentration of the coating liquid may be lower than the solid content concentration of the protective film-forming agent, but is preferably 20% by mass or less, more preferably 15% by mass or less. The solid content concentration of the coating liquid is not particularly limited, but is preferably 1% by mass or more, more preferably 5% by mass or more.

The solvent used for diluting the protective film-forming agent is not particularly limited. It may be diluted with a mixed solvent of the solvent (S) contained and the solvent not containing the protective film-forming agent, but it is preferable to dilute with the same solvent as the solvent (S) contained in the protective film-forming agent.
Examples of the solvent that does not contain the protective film-forming agent include the solvent (S) described above other than the solvent (S) that the protective film-forming agent contains.

<Protective film forming process>
In the protective film forming step, a protective film is formed by applying a coating liquid onto the semiconductor wafer.
As described above, the coating liquid applied in the protective film forming step is obtained by diluting the protective film forming agent having a concentration higher than that of the coating liquid.
Examples of semiconductor wafers include silicon substrates.
It is easy to remove the protective film by washing with water after processing, and sufficient durability of the protective film against plasma irradiation when plasma irradiation is performed in the cutting process in the manufacturing method of the semiconductor chip described later. The film thickness is typically preferably 1 μm or more and 100 μm or less.
When laser irradiation is performed in the processing groove forming step and/or the cutting step in the semiconductor chip manufacturing method described later, the film thickness of the protective film is preferably 0.1 μm or more and 10 μm or less.
In the cutting step, when cutting with a blade, the film thickness of the protective film is not particularly limited. When cutting with a blade, the thickness of the protective film is preferably 0.1 μm or more and 100 μm or less, for example, because the protective film can be easily removed by washing with water after processing.

Here, when the protective film forming agent is applied onto the semiconductor wafer to form the protective film, the protective film forming agent is applied onto the semiconductor wafer after a certain amount of time has passed after the production of the protective film forming agent. There is Specifically, for example, after the protective film-forming agent is manufactured, the protective film may be formed using the protective film-forming agent that has been transported over a long period of time or stored for a long period of time.
When the protective film-forming agent is applied to the semiconductor wafer after a certain amount of time has passed since the production of the protective film-forming agent in this way, the formation of the protective film-forming agent is less than when the protective film-forming agent is applied onto the semiconductor wafer immediately after production. foreign matter generated on the protective film may increase.
Foreign matter in the protective film is considered to originate from foreign matter generated in the protective film-forming agent over time after the production of the protective film-forming agent.

In general, generation of foreign matter in a solution over time is often caused by aggregation of solutes or reactions between solutes.
However, according to the study of the present inventor, unexpectedly, the solid content concentration of the protective film-forming agent containing the water-soluble resin (A), the light absorbing agent (B), and the solvent containing water (S) It has been clarified that when the concentration is as high as 20% by mass or more, foreign matter is less likely to occur over time.
Specifically, in the present invention, a protective film-forming agent containing a water-soluble resin (A), a light-absorbing agent (B), and a solvent (S) containing water, and having a solid content concentration of 20% by mass or more. is prepared, a coating liquid is prepared by diluting this, and the coating liquid is coated on a semiconductor wafer to form a protective film. As a result, as will be shown in Examples described later, it is possible to suppress an increase in the amount of foreign substances in the formed protective film even after a long period of time has elapsed after the production of the protective film-forming agent.
Therefore, even if the protective film is formed after the protective film-forming agent is transported or stored after the production of the protective film-forming agent, the protective film can be formed with less foreign substances.

The reason why it is possible to suppress the increase of foreign matter on the protective film is unknown, but it is presumed as follows.
As shown in the comparative examples described later, a protective film-forming agent containing a water-soluble resin (A), a light-absorbing agent (B), and a solvent (S) containing water and having a low solid content concentration (e.g., solid content When a protective film-forming agent having a concentration of 10% by mass or less is used, when the protective film-forming agent is applied to the semiconductor wafer after a certain period of time has elapsed after the production of the protective film-forming agent, foreign matter in the protective film formed increases significantly.
On the other hand, a protective film is formed using a protective film-forming agent containing the water-soluble resin (A), the light absorbing agent (B), and the solvent (S) containing water, and having a solid content concentration of 20% by mass or more. When a diluted coating liquid is prepared after a certain period of time has passed since the preparation of the formulation, and the coating liquid is coated on the semiconductor wafer, the increase in the amount of foreign substances in the formed protective film can be suppressed.
In this way, when a protective film-forming agent with a low solid content concentration is used, the increase in foreign matter is greater. However, it is thought that it may be derived from other factors, such as bacteria and other fungi.

The shorter the time from the preparation of the coating liquid to the application of the coating liquid onto the semiconductor wafer, the better. After preparation of the coating liquid, it is preferably applied within one day, more preferably within one hour, and even more preferably immediately after preparation (for example, within 10 minutes). If it takes a long time to apply the coating liquid onto the semiconductor wafer after preparation of the coating liquid, foreign matter tends to increase in the formed protective film.

<Storage/transfer process>
The method for forming a protective film of the present invention can suppress the increase of foreign matter in the protective film to be formed even after a certain period of time has passed after the production of the protective film-forming agent. A storage/transport step of storing or transporting the protective film forming agent may be included before the coating liquid preparation step. Examples of transportation include transportation by transportation and transportation by means of transportation such as automobiles such as ships, aircraft, railroad vehicles, and trucks.
The storage or transfer time is not particularly limited, and may be one month or more or three months or more.
The storage or transfer temperature is not particularly limited, but is preferably -10°C or higher and 40°C or lower.

<<Method for manufacturing semiconductor chip>>
A semiconductor chip manufacturing method is a method including processing a semiconductor wafer to manufacture a semiconductor chip.
More specifically, the method of manufacturing a semiconductor chip includes:
A protective film forming step of forming a protective film on a semiconductor wafer by the method for forming a protective film described above;
A processed groove forming step of irradiating a predetermined position of one or more layers including a protective film on a semiconductor wafer with a laser beam to expose the surface of the semiconductor wafer and form processed grooves having a pattern corresponding to the shape of the semiconductor chip. and,
a cutting step of cutting the positions of the processed grooves in the semiconductor wafer;
is a method that includes

<Protective film forming process>
The protective film forming step is as described above.
Also, the shape of the processing surface of the semiconductor wafer on which the protective film is formed is not particularly limited as long as the semiconductor wafer can be processed as desired. Typically, the processing surface of a semiconductor wafer has many irregularities. A recess is formed in a region corresponding to the street.
On the processing surface of the semiconductor wafer, a plurality of regions corresponding to semiconductor chips are defined by streets.

Below, referring to the drawings, a semiconductor chip manufacturing method in which a semiconductor wafer having a plurality of semiconductor chips partitioned by grid-like streets is diced using the protective film forming method described above. A preferred embodiment of the method for manufacturing a semiconductor chip will be described.

FIG. 1 shows a perspective view of a semiconductor wafer to be processed. FIG. 2 shows an enlarged cross-sectional view of the main part of the semiconductor wafer shown in FIG. In the semiconductor wafer 2 shown in FIGS. 1 and 2, a laminate 21 is provided on a surface 20a of a semiconductor substrate 20 made of silicon or the like. In the laminate 21, a plurality of semiconductor chips 22 such as ICs and LSIs are formed in a matrix.
Here, the shape and size of the semiconductor chip 22 are not particularly limited, and can be appropriately set according to the design of the semiconductor chip 22 .

Each semiconductor chip 22 is partitioned by streets 23 formed in a grid pattern. In the illustrated embodiment, the insulating film used as the laminate ₂₁ is a SiO2 film, an inorganic film such as SiOF or BSG (SiOB), or a polymer film such as polyimide or parylene. It consists of a low dielectric constant insulating film (Low-k film) made of a certain organic film.

The surface of the laminate 21 described above corresponds to the surface 2a, which is the processed surface. A protective film is formed on the surface 2a using the protective film forming method described above.

In the protective film forming step, for example, the protective film 24 is formed by coating the surface 2a of the semiconductor wafer 2 with a coating liquid prepared by diluting a protective film forming agent using a spin coater. The method of applying the coating liquid is not particularly limited as long as a protective film having a desired thickness can be formed. You may dry the liquid coating liquid which coat|covers the surface 2a as needed.
As a result, a protective film 24 is formed on the surface 2a of the semiconductor wafer 2 as shown in FIG.

After the protective film 24 is formed on the front surface 2a of the semiconductor wafer 2 in this way, the protective tape 6 attached to the annular frame 5 is adhered to the rear surface of the semiconductor wafer 2 as shown in FIG. be.

<Processing groove formation process>
In the process groove forming step, a predetermined position of one or more layers including the protective film 24 on the semiconductor wafer 2 is irradiated with a laser beam so that the surface 20a of the semiconductor substrate 20 is exposed, and a shape corresponding to the shape of the semiconductor chip 22 is exposed. A patterned groove is formed.

Specifically, the surface 2 a (street 23 ) on the semiconductor wafer 2 is irradiated with laser light through the protective film 24 using the laser processing apparatus 7 . This laser beam irradiation is carried out using a laser beam irradiation means 72 as shown in FIG.
The laser is preferably an ultraviolet laser having a wavelength of 100 nm or more and 400 nm or less in terms of intensity. A YVO4 laser with a wavelength of 266 nm, 355 nm, etc., and a YAG laser are also preferable.

The laser light irradiation in the process groove forming step is performed, for example, under the following processing conditions. Note that the focused spot diameter is appropriately selected in consideration of the width of the processed groove 25 .
Laser light source: YVO4 laser or YAG laser
Wavelength: 355nm
Repetition frequency: 50 kHz to 100 kHz
Output: 0.3W to 4.0W
Processing feed rate: 1 mm/sec or more and 800 mm/sec or less

By performing the above-described process groove forming step, process grooves 25 are formed along the streets 23 in the laminate 21 having the streets 23 in the semiconductor wafer 2 as shown in FIG. Since the protective film 24 is formed using the method for forming the protective film described above, it is a protective film containing few foreign matter even after a long time has passed since the production of the protective film-forming agent. Therefore, interference of the laser beam by foreign matter is suppressed, and a groove having a desired shape can be easily formed in the protective film 24 by irradiating the protective film 24 with the laser beam as described above.

After the laser beam is irradiated along the predetermined streets 23 as described above, the semiconductor wafer 2 held on the chuck table 71 is indexed and moved in the direction indicated by the arrow Y by the distance between the streets, and the laser beam is irradiated again. carry out

After all the streets 23 extending in the predetermined direction have been irradiated with laser light and indexed and moved in this manner, the semiconductor wafer 2 held on the chuck table 71 is rotated by 90 degrees to move in the predetermined direction. Along each street 23 extending at a right angle to the surface, irradiation of laser light and indexing movement are performed in the same manner as described above. In this manner, processing grooves 25 can be formed along all the streets 23 formed in the laminate 21 on the semiconductor wafer 2 .

<Cutting process>
In the cutting step, the semiconductor wafer 2 having the processed grooves 25 at positions corresponding to the positions of the streets 23 is cut. Preferred methods include a method of cutting the semiconductor wafer 2 by irradiating the semiconductor wafer 2 with the protective film 24 and the processed grooves 25 with a laser or plasma, or a method of cutting the semiconductor wafer 2 with the protective film 24 or with the protective film 24. A method of cutting the peeled semiconductor wafer 2 with a blade can be used. When irradiating the laser, the processing groove 25 is irradiated with the laser so as to cut the semiconductor wafer 2 . When irradiating plasma, a part or the entire surface of the semiconductor wafer 2 provided with the protective film is irradiated with the plasma so that the surface of the processing groove 25 is exposed to the plasma. When the blade is used for cutting, the blade cuts the semiconductor wafer 2 along the positions of the processing grooves 25 while supplying pure water to the cutting portion.
A cutting method by plasma irradiation, which is a preferable cutting method, will be described below.

As shown in FIG. 7, the semiconductor wafer 2 having the protective film 24 and the processed grooves 25 is irradiated with plasma. By doing so, as shown in FIG. 8, the position of the processing groove 25 in the semiconductor wafer 2 is cut.
Specifically, in the semiconductor wafer 2 covered with the protective film 24, as described above, after the processing grooves 25 are formed, the protective film 24 and the surface 20a of the semiconductor substrate 20 exposed from the processing grooves 25 are subjected to , the semiconductor wafer 2 is cut according to the shape of the semiconductor chips 22 by performing plasma irradiation, and the semiconductor wafer 2 is divided into the semiconductor chips 22 .

The plasma irradiation conditions are not particularly limited as long as the semiconductor wafer 2 can be cut satisfactorily at the position of the processing groove 25 . The plasma irradiation conditions are appropriately set within the range of general plasma etching conditions for semiconductor substrates, taking into account the material of the semiconductor wafer 2, the type of plasma, and the like.
A gas used for generating plasma in plasma irradiation is appropriately selected according to the material of the semiconductor wafer 2 . Typically, _SF6 gas is used for plasma generation.
Also, according to the so-called BOSCH process, the semiconductor wafer 2 may be cut by alternately performing side wall protection by supplying C ₄ F ₆ or C ₄ F ₈ gas or the like and etching the semiconductor wafer 2 by plasma irradiation. good. According to the BOSCH process, etching with a high aspect ratio is possible, and the semiconductor wafer 2 can be easily cut even when the semiconductor wafer 2 is thick.

Next, as shown in FIG. 9, the protective film 24 covering the surface of the semiconductor chip 22 is removed. As described above, since the protective film 24 is formed using the protective film-forming agent containing the water-soluble resin (A), the protective film 24 can be washed away with water (or hot water).

The method of manufacturing a semiconductor chip by processing a semiconductor wafer has been described above based on the embodiments.
The method of forming a protective film according to the present invention, the method of manufacturing a semiconductor chip using the method of forming the protective film, and the method of manufacturing a coating liquid that can be used in the method of forming the protective film described above include: Any method that includes forming a protective film and forming processing grooves at positions corresponding to streets on the surface of the semiconductor wafer provided with the protective film can be applied to various semiconductor chip manufacturing methods.

EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples. The present invention is in no way limited to the following examples.

また、防腐剤として、メチルパラベン（Ｃ－１）を用いた。
また、溶媒（Ｓ）として、水（Ｓ１）９０質量部と、プロピレングリコールモノメチルエーテル（ＰＧＭＥ）（Ｓ２）１０質量部との混合溶媒を用いた。 (Examples 1-11 and Comparative Examples 1-2)
In Examples 1 to 11 and Comparative Examples 1 and 2, polyvinylpyrrolidone (A-1) and hydroxypropylcellulose (A-2) were used as the water-soluble resin (A).
B-1 (ferulic acid) represented by the following formula was used as the light absorber (B).

In addition, methylparaben (C-1) was used as an antiseptic.
A mixed solvent of 90 parts by mass of water (S1) and 10 parts by mass of propylene glycol monomethyl ether (PGME) (S2) was used as the solvent (S).

<Production of protective film forming agent>
Water-soluble resin (A), light-absorbing agent (B) and preservative in the types and amounts listed in Table 1, and the amount of solvent (S) that gives the solid concentration listed in Table 1 are put into a container. and stirred for 8 hours to obtain protective film-forming agents for each of Examples and Comparative Examples.

<Formation of protective film (coating film)>
In Examples 1 to 10, the protective film-forming agent was diluted with the solvent (S) immediately after production (within 10 minutes) to prepare a coating liquid having a solid content concentration of 10% by mass, and immediately after preparation of the coating liquid (10 Within minutes), the coating liquid was applied onto the silicon substrate by a spin coating method (spin coating conditions: 1000 rpm, 60 seconds) to form a protective film 1 having a thickness of 1 μm.
In Examples 1 to 10, the protective film-forming agent was stored at 40° C. for 3 months and then diluted with the solvent (S) to prepare a coating solution having a solid content concentration of 10% by mass. immediately after preparation (within 10 minutes), the coating liquid was applied onto the silicon substrate by spin coating (spin coating conditions: 1000 rpm, 60 seconds) to form a protective film 2 having a thickness of 1 μm.
In Example 11, the solid content concentration of each coating liquid was set to 20% by mass, and the spin coating conditions were set to 2000 rpm for 60 seconds. A protective film 1 and a protective film 2 were formed.
In each comparative example, protective films 1 and 2 each having a thickness of 1 μm were formed in the same manner as in Examples 1 to 10, except that the operation of dilution with the solvent (S) was not performed.

<Evaluation of Foreign Matter in Protective Film>
The formed protective film 1 and protective film 2 were observed with NSX-220 (manufactured by Rudolph), the number of foreign substances was counted, and the rate of increase (%) of foreign substances was obtained by the following formula. When the increase rate of the foreign matter was less than 5%, it was judged as ◯, and when it was 5% or more, it was judged as x. Table 1 shows the results.
Contaminant increase rate (%)=[{(Number of contaminants on protective film 2)/(Number of contaminants on protective film 1)}−1]×100

From Table 1, a coating liquid obtained by diluting a protective film-forming agent containing a water-soluble resin (A), a light absorbing agent (B), and a solvent (S) containing water and having a solid content concentration of 20% by mass or more. was applied to the semiconductor wafer (silicon substrate), the increased rate of foreign matter in the formed protective film was low. Therefore, it can be seen that Examples 1 to 11 can suppress the generation of foreign matter in the protective film formed even after a long period of time has passed since the production of the protective film-forming agent.
On the other hand, from Table 1, in Comparative Examples 1 and 2 in which no dilution operation was performed, the increase rate of foreign substances was higher than in Examples.

2: Semiconductor wafer 20: Substrate 21: Laminate 22: Semiconductor chip 23: Street 24: Protective film 25: Laser processed groove 26: Cut groove 3: Spin coater 5: Annular frame 6: Protective tape 7: Laser processing device 71 : Chuck table of laser processing device 72 : Laser beam irradiation means

Claims (6)

A protective film forming method for forming a protective film on the surface of a semiconductor wafer in dicing of a semiconductor wafer, comprising:
Protection for preparing a protective film-forming agent comprising a water-soluble resin (A), a light absorbing agent (B), and a solvent (S), wherein the solvent (S) contains water and has a solid content concentration of 20% by mass or more a film-forming agent preparation step;
a coating liquid preparation step of diluting the protective film forming agent to prepare a coating liquid;
a protective film forming step of applying the coating liquid onto a semiconductor wafer to form a protective film;
A method of forming a protective film. 2. The method of forming a protective film according to claim 1, wherein said protective film-forming agent is diluted with said solvent (S) in said coating solution preparing step. 3. The method of forming a protective film according to claim 1, wherein the solvent (S) contains an organic solvent. 4. The method according to any one of claims 1 to 3, further comprising a storage/transfer step of storing or transferring the protective film-forming agent after the protective film-forming agent preparing step and before the coating solution preparing step. A method for forming the described protective film. A method for manufacturing a semiconductor chip by processing a semiconductor wafer,
A protective film forming step of forming a protective film on the semiconductor wafer by the method for forming a protective film according to any one of claims 1 to 4; and one or more layers including the protective film on the semiconductor wafer. a processed groove forming step of irradiating a predetermined position with a laser beam to expose the surface of the semiconductor wafer and form a processed groove having a pattern corresponding to the shape of the semiconductor chip;
a cutting step of cutting the positions of the processed grooves in the semiconductor wafer;
A method of manufacturing a semiconductor chip, comprising: A method for preparing a coating liquid used for forming a protective film on the surface of a semiconductor wafer in dicing of a semiconductor wafer, comprising:
Dilute a protective film-forming agent comprising a water-soluble resin (A), a light absorbing agent (B), and a solvent (S), wherein the solvent (S) comprises water and has a solid content concentration of 20% by mass or more. A method for preparing a coating liquid, comprising a dilution step.

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