Tin alloy plating solution

Abstract

This tin alloy plating solution comprises: (A) a soluble salt that contains at least a stannous salt; (B) a soluble salt of a metal that is nobler than tin; (C) a leveling agent that is composed of an alkanesulfonic acid which contains 9-18 carbon atoms in each molecule or a salt thereof; (D) a free acid; (E) a nonionic surfactant that contains one or more phenyl groups in each molecule; and (F) an EO / PO block polymer that has an EO group at an end. The EO ratio in the EO / PO block polymer is within the range of 20 mol% to 50 mol% inclusive, and the molecular weight of the EO / PO block polymer is within the range of 1,500 to 3,500 inclusive.

Description

Tin alloy plating solution

[0001] The present invention relates to a tin alloy plating solution for manufacturing bumps that serve as tin alloy protruding electrodes on a circuit board when mounting a semiconductor integrated circuit chip onto the board. This application claims priority based on Japanese Patent Application No. 2025-024825, filed in Japan on February 19, 2025, the contents of which are incorporated herein by reference.

[0002] Conventionally, a method has been disclosed for forming a tin or tin alloy plating layer on a substrate having vias of multiple different via diameters, using a tin or tin alloy plating solution for plating a substrate having vias of multiple different via diameters, comprising (A) a soluble salt containing at least a stannous salt, (B) an acid or a salt thereof selected from organic acids and inorganic acids, (C) a surfactant, (D) a leveling agent, and (E) an additive (see, for example, Patent Document 1 (Claim 1, paragraphs

[0020] ,

[0043] , and

[0044] )).

[0003] Leveling agent (D) is added to form a uniform and dense plating film and to smooth the plating film. Two types of leveling agents, a first leveling agent (D-1) and a second leveling agent (D-2), are used to enhance via filling properties and suppress the generation of voids. The first leveling agent (D-1) is one or more selected from the group consisting of aliphatic aldehydes, aromatic aldehydes, aliphatic ketones, and aromatic ketones, and the second leveling agent (D-2) is an α,β-unsaturated carboxylic acid or its amide, or a salt thereof. Examples of the above aromatic ketones include benzalacetone, 2-chloroacetophenone, 3-chloroacetophenone, 4-chloroacetophenone, 2,4-dichloroacetophenone, and 2,4,6-trichloroacetophenone.

[0004] In this method for forming a tin or tin alloy plating layer, the deposition of Sn ions is suppressed during plating, enabling good plating on the surface to be plated. In particular, with this plating solution, even in the case of patterns with different bump diameters, whether the bump diameter is large or small, the via filling to vias on the substrate is excellent, and the height of the formed bumps becomes uniform.

[0005] On the other hand, a method for forming low-melting-point metal bumps on semiconductor chips or packages by electroplating has been disclosed (see, for example, Patent Document 2 (Claim 1, paragraphs

[0013] and

[0019] )). In this method for forming low-melting-point metal bumps, the following components are used as the electroplating bath: (a) 5 to 300 g / l of at least one type of alkanesulfonate ion or alkanolsulfonate ion, (b) 0.01 to 10 g / l of Ag ions, and (c) Sn ２＋ and Bi ３＋ A silver-based alloy plating bath containing (d) 0.1 to 40 g / L of one selected metal ion, (d) 0.01 to 40 g / L of one sulfur-containing compound, and (e) 0.5 to 30 g / L of a nonionic surfactant is used, and the current is applied in the form of a repeating rectangular pulse wave or a multi-stage rectangular wave. Furthermore, it is necessary to use an alkanesulfonic acid or alkanolsulfonic acid as the acidic component, and preferred examples of alkanesulfonic acid or alkanolsulfonic acid include methanesulfonic acid and ethanesulfonic acid.

[0006] This method for forming low-melting-point metal bumps makes it possible to form bumps with good shape without using lead. Furthermore, since the bumps obtained by this method do not contain lead, they do not cause malfunctions such as inversion of semiconductor memory elements due to alpha radiation, and are also advantageous in terms of environmental pollution. Therefore, this method for forming low-melting-point metal bumps is extremely advantageous as a method for forming bumps on semiconductor chips and packages by electroplating.

[0007] However, in the method for forming a tin or tin alloy plating deposition layer shown in Patent Document 1, aromatic carbonyl compounds (such as benzalacetone, cinnamic acid, cinnamaldehyde, and benzaldehyde) added as leveling agents to form a uniform and dense plating film and to smooth the plating film are reduced by electrolysis in electroplating. As a result, analytical control and replenishment work to maintain the concentration becomes complicated, and there is a problem in that the plating quality deteriorates due to the accumulation of decomposition products of the leveling agent in the plating solution.

[0008] Furthermore, in the method for forming low-melting-point metal bumps described in Patent Document 2, alkanesulfonic acid or alkanolsulfonic acid is used as the acidic component. However, alkanesulfonic acid or alkanolsulfonic acid is methanesulfonic acid or ethanesulfonic acid, which have a small number of carbon atoms. Because they are highly water-soluble, they function as free acids, but they do not function as leveling agents to smooth the plated film.

[0009] Therefore, Patent Document 3 proposes a tin alloy plating solution comprising (A) a soluble salt containing at least a stannous salt, (B) a soluble salt of a metal nobler than tin, (C) a leveling agent consisting of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule, (D) a nonionic surfactant containing one or more phenyl groups, and (E) a free acid. In this tin alloy plating solution, an aromatic carbonyl compound (such as benzalacetone, cinnamic acid, cinnamaldehyde, or benzaldehyde) is not used as a leveling agent, and a uniform and dense plating film can be formed and the plating film can be made smooth without replenishing the leveling agent over a long period of time during electroplating.

[0010] Japanese Patent No. 6635139, Japanese Unexamined Patent Publication No. 2000-100850, Japanese Unexamined Patent Publication No. 2022-083489

[0011] Incidentally, as described in Patent Document 3, plating solutions to which alkanesulfonic acid or its salts are added tend to have poor defoaming properties, and bubbles sometimes accumulate near the liquid surface. In face-down type plating equipment (with membrane) commonly used as wafer plating equipment, bubbles generated on the liquid surface may enter bump holes and inhibit the growth of the plating film. Furthermore, if minute bubbles are incorporated into the plating film, they may cause voids. Moreover, if bubbles are incorporated into the surface of the plating film, the bump smoothness may deteriorate.

[0012] This invention has been made in view of the circumstances described above, and aims to provide a tin alloy plating solution that can sufficiently ensure defoaming properties, suppress the occurrence of defects caused by foam, and stably form a smooth plating film, even in a plating solution to which alkanesulfonic acid or a salt thereof is added as a leveling agent.

[0013] To solve the above problems, the tin alloy plating solution of embodiment 1 of the present invention comprises (A) a soluble salt containing at least a stannous salt, (B) a soluble salt of a metal nobler than tin, (C) a leveling agent consisting of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule, (D) a free acid, (E) a nonionic surfactant containing one or more phenyl groups in its molecule, and (F) an EO / PO block polymer having an EO group at its terminus, wherein the EO ratio in the EO / PO block polymer is in the range of 20 mol% to 50 mol%, and the molecular weight of the EO / PO block polymer is in the range of 1500 to 3500. Note that EO is an oxyethylene group (-O-CH ２ -CH ２ -) indicates that PO is an oxypropylene group (-O-CH ２ -CH(CH ３ )-) indicates.

[0014] According to the tin alloy plating solution of Embodiment 1 of the present invention, (C) it contains a leveling agent consisting of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule. Therefore, a uniform and dense plating film can be formed and the plating film can be made smooth without replenishing the leveling agent for a long period of time during electroplating. Furthermore, in the tin alloy plating solution of Embodiment 1 of the present invention, (F) it contains an EO / PO block polymer with an EO group at the end, the EO ratio in the EO / PO block polymer is in the range of 20 mol% to 50 mol%, and the molecular weight of the EO / PO block polymer is in the range of 1500 to 3500. Therefore, the defoaming effect can be improved without inhibiting the atomization effect of the leveling agent consisting of an alkanesulfonic acid or a salt thereof, and the occurrence of defects caused by foam can be suppressed.

[0015] The tin alloy plating solution of Embodiment 2 of the present invention is characterized in that, in the tin alloy plating solution of Embodiment 1, the metal nobler than tin is silver or copper. According to the tin alloy plating solution of Embodiment 2 of the present invention, since the metal nobler than tin is silver or copper, it has excellent solder wettability, mounting strength, bendability and reflowability, and whisker formation is less likely.

[0016] The tin alloy plating solution of embodiment 3 of the present invention is characterized in that, in the tin alloy plating solution of embodiment 1 or embodiment 2, the content of the EO / PO block polymer is in the range of 1.0 g / L to 10 g / L. According to the tin alloy plating solution of embodiment 3 of the present invention, since the content of the EO / PO block polymer is in the range of 1.0 g / L to 10 g / L, it is possible to reliably improve the defoaming effect without inhibiting the atomization effect of the leveling agent consisting of alkanesulfonic acid or its salt, and to further suppress the occurrence of defects caused by foam.

[0017] The tin alloy plating solution of embodiment 4 of the present invention is characterized in that, in any one of the tin alloy plating solutions of embodiment 1 to 3, the content of the leveling agent consisting of alkanesulfonic acid or its salt is in the range of 0.01 g / L to 1.00 g / L. According to the tin alloy plating solution of embodiment 4 of the present invention, since the content of alkanesulfonic acid or its salt is 0.01 g / L to 1.00 g / L, the phenyl group-containing nonionic surfactant can easily solubilize the alkanesulfonic acid, interaction is more likely to occur, and the atomization effect can be fully achieved.

[0018] According to an aspect of the present invention, even in a plating solution to which alkanesulfonic acid or a salt thereof is added as a leveling agent, sufficient defoaming properties can be ensured, the occurrence of defects caused by foam can be suppressed, and a tin alloy plating solution that can stably form a smooth plating film can be provided.

[0019] This is an explanatory diagram showing a state in which a leveling agent consisting of alkanesulfonic acid or a salt thereof is arranged on the foam surface. This is a schematic diagram showing the defoaming mechanism by an EO / PO block polymer with EO groups at the ends. This is a schematic diagram showing a normal bump as an evaluation criterion for bump shape in the examples. This is a schematic diagram showing an example of a defective bump with pit defects as an evaluation criterion for bump shape in the examples. This is a schematic diagram showing another example of a defective bump with pit defects as an evaluation criterion for bump shape in the examples. This is a photograph showing an example of the evaluation results of bump shape in the examples, and is the bump appearance of Comparative Example 8 (when a leveling agent consisting of alkanesulfonic acid or a salt thereof is added and an EO / PO block polymer is not used). This is a photograph showing an example of the evaluation results of bump shape in the examples, and is the bump appearance of Example 1 of the present invention (when a leveling agent consisting of alkanesulfonic acid or a salt thereof is added and an EO / PO block polymer is used).

[0020] Below, an embodiment of the present invention, a tin alloy plating solution, will be described with reference to the attached drawings.

[0021] An embodiment of the present invention is a tin alloy plating solution comprising: (A) a soluble salt containing at least a stannous salt; (B) a soluble salt of a metal nobler than tin; (C) a leveling agent consisting of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule; (D) a free acid; (E) a nonionic surfactant containing one or more phenyl groups in its molecule; and (F) an EO / PO block polymer with EO groups at its termini.

[0022] [Tin Alloy] The tin alloy produced by the tin alloy plating solution of this embodiment is an alloy of tin (Sn) and a predetermined metal selected from silver (Ag), copper (Cu), gold (Au), and bismuth (Bi). Examples include binary alloys such as SnAg alloy, SnCu alloy, SnAu alloy, and SnBi alloy, and ternary alloys such as SnCuAg alloy.

[0023] [Soluble Salt (A) Containing at Least a First Tin Salt] The soluble salt (A) containing at least a first tin salt used in the tin alloy plating solution of the present embodiment is a salt that dissolves in water to generate divalent tin ions. Examples of the soluble salt include halides, sulfates, oxides, alkanesulfonates having less than 9 carbon atoms, arylsulfonates, and alkanolsulfonates. Specific examples of the alkanesulfonate include methanesulfonate and ethanesulfonate. Specific examples of the arylsulfonate include benzenesulfonate, phenolsulfonate, cresolsulfonate, and toluenesulfonate. Specific examples of the alkanolsulfonate include isethionate.

[0024] The soluble salt (A) containing at least a first tin salt (hereinafter referred to as soluble tin salt (A)) may be used alone or in combination of two or more. The content of the soluble tin salt (A) in the tin alloy plating solution of the present embodiment is preferably in the range of 5 g / L or more and 200 g / L or less in terms of the amount of tin. When the content of the soluble tin salt (A) is 5 g / L or more, abnormal precipitation of tin is less likely to occur in the range of current density of 1 to 20 ASD (ampere per square decimeter) generally used for bump plating, and stable and good bumps can be formed. On the other hand, when the content of the soluble tin salt (A) is 200 g / L or less, an increase in the viscosity of the plating solution is suppressed, bumps can be formed stably, and the tin content is not contained more than necessary, so that the cost of the plating bath can be reduced. In addition, the content of the soluble tin salt (A) in the tin alloy plating solution is more preferably 10 g / L or more, and even more preferably 15 g / L or more. On the other hand, the content of the soluble tin salt (A) in the tin alloy plating solution is more preferably 180 g / L or less, and even more preferably 160 g / L or less.

[0025] [Soluble Salt of a Metal More Noble Than Tin (B)] The soluble salt of a metal more noble than tin (B) used in the tin alloy plating solution of this embodiment is a salt that dissolves in water. Examples of metals more noble than tin include at least one or more metals selected from silver, copper, gold, and bismuth. The examples of these soluble salts of metals more noble than tin (B) are the same as the examples of soluble tin salts (A). Among these metals, it is preferable to include silver or copper. This is because they have excellent solder wettability, mounting strength, bendability, and reflowability, and are less prone to whisker formation. The tin-silver alloy (SnAg alloy) has a low melting point of 221°C in its eutectic composition (Sn-3.5 mass%Ag), and the tin-copper alloy (SnCu alloy) has a low melting point of 227°C in its eutectic composition (Sn-1.7 mass%Cu). Both alloys have advantages such as excellent solder wettability, mounting strength, bendability, and reflowability, and are less prone to whisker formation. The soluble salt (B) of a metal nobler than tin may be used alone or in combination of two or more types.

[0026] In this embodiment, the content of the soluble salt (B) of a metal more noble than tin in the plating solution is preferably in the range of 0.01 g / L to 10 g / L, when converted to the amount of metal. By setting the content of the soluble salt (B) of a metal more noble than tin to the range of 0.01 g / L to 10 g / L, the composition of the precipitated solder alloy can be reliably made into a eutectic composition, and the properties of a solder alloy can be obtained. Furthermore, the content of the soluble salt (B) of a metal more noble than tin is more preferably 0.1 g / L or more, and more preferably 0.2 g / L or more. On the other hand, the content of the soluble salt (B) of a metal more noble than tin is more preferably 7 g / L or less, and more preferably 5 g / L or less.

[0027] [Leveling Agent (C)] The leveling agent (C) composed of an alkanesulfonic acid containing 9 to 18 carbon atoms in the molecule or a salt thereof used in the tin alloy plating solution of the present embodiment is added to form a plating film uniformly and densely, smooth the plating film, further enhance the via filling property, and suppress the generation of voids. The alkanesulfonic acid containing 9 to 18 carbon atoms in the molecule is represented by the following formula (1) or formula (2), and the alkanesulfonate containing 9 to 18 carbon atoms in the molecule is represented by the following formula (3) or formula (4). R １ -SO ３ H …(1) R ２ -HC(SO ３ H)-R ３ …(2) R １ -SO ３ Na …(3) R ２ -HC(SO ３ Na)-R ３ …(4) In formula (1) and formula (3), R １ is an alkyl group, and the number of carbon atoms in the molecule of R １ (total carbon atoms in the molecule) is 9 to 18, preferably 10 to 17. Also, in formula (2) and formula (4), R ２ and R I ３ are alkyl groups, and the total number of carbon atoms in the molecules of R ２ and R ３ (total carbon atoms in the molecule) is 9 to 18, preferably 10 to 17.

[0028] Here, when an alkanesulfonic acid or a salt thereof having a total carbon number of 8 or less in the molecule is used, for example, when methanesulfonic acid or ethanesulfonic acid is used, the water solubility of the alkanesulfonic acid is high and adsorption to the surface of the object to be plated does not occur, so it does not act as a leveling agent and there is a problem that it has no effect of smoothing the plating film. Also, when an alkanesulfonic acid or a salt thereof having a total carbon number of 19 or more in the molecule is used, there is a problem that the alkanesulfonic acid cannot dissolve in the plating solution because the water solubility of the alkanesulfonic acid is low.

[0029] The alkanesulfonic acid is preferably a primary alkanesulfonic acid, a secondary alkanesulfonic acid, or a salt thereof, and more preferably a secondary alkanesulfonic acid or a salt thereof. Here, the reason why it is even more preferable to use a secondary alkanesulfonic acid or a salt thereof as the leveling agent (C) is that its solubility in the plating solution is slightly improved compared to that of a primary alkanesulfonic acid or a salt thereof, and the interaction in which the phenyl group-containing nonionic surfactant solubilizes the alkanesulfonic acid or its salt occurs more quickly in the plating solution. Examples of primary alkanesulfonic acids represented by formula (1) include 1-nonanesulfonic acid, 1-decanesulfonic acid, 1-dodecylsulfonic acid, 1-tetradecanesulfonic acid, 1-hexadecanesulfonic acid, and 1-octadecanesulfonic acid. Furthermore, examples of primary alkanesulfonates represented by formula (3) include sodium 1-nonanesulfonate, potassium 1-decanesulfonate, sodium 1-dodecylsulfonate, potassium 1-dodecylsulfonate, potassium 1-tetradecanesulfonate, 1-hexadecanesulfonic acid trimethim, and sodium 1-octadecanesulfonate.

[0030] On the other hand, examples of secondary alkanesulfonic acids represented by formula (2) include nonane-sec-sulfonic acid, dodecyl-sec-sulfonic acid, tetradecyl-sec-sulfonic acid, heptadecyl-sec-sulfonic acid, and octadecane-sec-sulfonic acid. Furthermore, examples of secondary alkanesulfonates represented by formula (4) include sodium nonane-sec-sulfonate, calcium dodecyl-sec-sulfonate, sodium tetradecyl-sec-sulfonate, potassium heptadecyl-sec-sulfonate, and sodium octadecane-sec-sulfonate.

[0031] Here, it is preferable that the content of the leveling agent (C), which consists of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule, is 0.01 g / L to 1.00 g / L. When the content of the leveling agent (C), which consists of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule, is 0.01 g / L or more, the above-mentioned effects can be fully achieved. On the other hand, when the content of the leveling agent (C), which consists of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule, is 1.00 g / L or less, it is possible to stably smooth the plating film. Furthermore, it is even more preferable that the content of the leveling agent (C), which consists of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule, is 0.02 g / L or more, and more preferably 0.05 g / L or more. On the other hand, the content of the leveling agent (C), which consists of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule, is more preferably 1.00 g / L or less, and more preferably 0.70 g / L or less.

[0032] [Free Acid (D)] Examples of free acid (D) used in the tin alloy plating solution of this embodiment include hydrogen chloride, hydrogen bromide, sulfuric acid, alkanesulfonic acid (number of carbon atoms: 1 to 6), arylsulfonic acid, or alkanolsulfonic acid. Specific examples of alkanesulfonic acid include methanesulfonic acid (number of carbon atoms: 1) and ethanesulfonic acid (number of carbon atoms: 2). However, alkanesulfonic acid with 7 to 18 carbon atoms used as the leveling agent (C) in this embodiment is not used as free acid (D). Specific examples of arylsulfonic acid include benzenesulfonic acid, phenolsulfonic acid, cresolsulfonic acid, or toluenesulfonic acid. Specific examples of alkanolsulfonic acid include isethionic acid. Free acid (D) has the effect of increasing the conductivity of the tin alloy plating solution. Furthermore, free acid (D) may be used alone or in combination of two or more types. In this embodiment, the free acid (D) content in the tin alloy plating solution is preferably 5 g / L or more, and more preferably 30 g / L or more. On the other hand, the free acid (D) content in the tin alloy plating solution is preferably 500 g / L or less, and more preferably 300 g / L or less.

[0033] [Nonionic surfactant (E) containing one or more phenyl groups in the molecule] The nonionic surfactant (E) (hereinafter referred to as phenyl group-containing nonionic surfactant (E)) used in the tin alloy plating solution of this embodiment has the effect of increasing the affinity between the tin alloy plating solution and the object to be plated, and by adsorbing onto the surface of the plating film during the formation of the tin alloy plating film, it suppresses the crystal growth of the tin alloy within the plating film and refines the crystals, thereby improving the appearance of the plating film, improving adhesion to the object to be plated, and making the film thickness uniform. The number of phenyl groups in the molecule is preferably 1 or more and 5 or less.

[0034] Specific examples of phenyl group-containing nonionic surfactants (E) include those obtained by adding and condensing 5 to 50 moles of ethylene oxide (starting material for the EO group) and / or propylene oxide (starting material for the PO group) to 1 mole of a phenyl group-containing structure such as phenol, ethylphenol, nonylphenol, dodecylphenol, bisphenol A, bisphenol B, bisphenol E, bisphenol F, cumylphenol, polystyrene-containing phenol, polystyrene-containing cresol, tripenzylphenol, β-naphthol, and naphthyl ether. Two or more of these phenyl group-containing nonionic surfactants (E) may be mixed and used in combination. Furthermore, the amount of phenyl group-containing nonionic surfactant (E) added to the tin alloy plating solution of this embodiment is preferably 0.01 g / L or more, more preferably 0.1 g / L or more, and even more preferably 1 g / L or more. On the other hand, the amount of phenyl group-containing nonionic surfactant (E) added to the tin alloy plating solution is preferably 50 g / L or less, more preferably 40 g / L or less, and even more preferably 30 g / L or less.

[0035] [EO / PO block polymer (F) with EO groups at the end] The EO (oxyethylene group) / PO (oxypropylene group) block polymer used in the tin alloy plating solution of this embodiment has EO groups at the end, the EO ratio in the EO / PO block polymer ((m1 + m2) / (m1 + m2 + n) in the following structural formula (5)) is in the range of 20 mol% to 50 mol%, the molecular weight is in the range of 1500 to 3500, and it has the structure shown in the following structural formula (5). In structural formula (5), m1 and m2 represent the number of repeating units of EO, and n represents the number of repeating units of PO. The PO ratio is n / (m1 + m2 + n).

[0036]

[0037] The EO / PO block polymer (F) with EO groups at the ends, having the structure described above, can improve the defoaming properties of the tin alloy plating solution without impairing the effect of the leveling agent (C). The EO ratio is preferably 20 mol% or more, and more preferably 25 mol% or more. On the other hand, the EO ratio is preferably 50 mol% or less, and more preferably 40 mol% or less. The molecular weight of the EO / PO block polymer is preferably 1500 or more, and more preferably 2000 or more. On the other hand, the molecular weight of the EO / PO block polymer is preferably 3500 or less, and more preferably 3000 or less. Furthermore, the amount of EO / PO block polymer (F) with EO groups at the ends added to the tin alloy plating solution of this embodiment is preferably 1.0 g / L or more, and more preferably 2.0 g / L or more. On the other hand, the amount of EO / PO block polymer (F) with EO groups at the end added to the tin alloy plating solution is preferably 10 g / L or less, and more preferably 7.5 g / L or less.

[0038] [Additives] The tin alloy plating solution of this embodiment may further contain additives such as antioxidants, tin-specific complexing agents, and pH adjusters.

[0039] [Antioxidant] The tin alloy plating solution of this embodiment may contain an antioxidant as needed. The antioxidant is Sn in the tin alloy plating solution. ２＋The purpose is to prevent oxidation. Examples of antioxidants include ascorbic acid or its salts, pyrogallol, hydroquinone, phloroglucinol, trihydroxybenzene, catechol, cresol sulfonic acid or its salts, catechol sulfonic acid or its salts, hydroquinone sulfonic acid or its salts, etc. For example, hydroquinone sulfonic acid or its salts are preferred in acidic baths, and ascorbic acid or its salts are preferred in neutral baths. Furthermore, one type of antioxidant may be used alone, or two or more types may be used in combination. The amount of antioxidant added to the tin alloy plating solution in this embodiment is generally in the range of 0.01 g / L to 20 g / L, preferably in the range of 0.1 g / L to 10 g / L, and more preferably in the range of 0.1 g / L to 5 g / L.

[0040] [Complexing agent for tin] The tin alloy plating solution of this embodiment can be applied to tin alloy plating baths in any pH range, such as acidic, weakly acidic, or neutral. Sn ２＋ Ions are stable in strongly acidic conditions (pH: <1), but tend to form a white precipitate in acidic to neutral conditions (pH: 1-7). Therefore, when applying the tin alloy plating solution of this embodiment to a tin plating bath near neutral pH, Sn ２＋ It is preferable to add a tin-specific complexing agent to stabilize the ions.

[0041] For tin, oxycarboxylic acids, polycarboxylic acids, and monocarboxylic acids can be used as complexing agents. Specific examples include gluconic acid, citric acid, glucoheptonic acid, gluconolactone, acetic acid, propionic acid, butyric acid, ascorbic acid, oxalic acid, malonic acid, succinic acid, glycolic acid, malic acid, tartaric acid, or salts thereof. Preferably, gluconic acid, citric acid, glucoheptonic acid, gluconolactone, glucoheptolactone, or salts thereof. Furthermore, polyamines and aminocarboxylic acids such as ethylenediamine, 3,6-dithia-1,8-octanediol, ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), iminodipropionic acid (IDP), hydroxyethylethylenediaminetriacetic acid (HEDTA), triethylenetetraminehexaacetic acid (TTHA), ethylenedioxybis(ethylamine)-N,N,N',N'-tetraacetic acid, mercaptotriazoles, mercaptotetrazoles, glycines, nitrilotrimethylphosphonic acid, 1-hydroxyethane-1,1-diphosphonic acid, or salts thereof are also effective as complexing agents for tin.

[0042] The tin-containing complexing agent may be used alone or in combination of two or more types. The amount of tin-containing complexing agent added to the tin alloy plating solution in this embodiment is in the range of 0.01 g / L to 20 g / L, preferably in the range of 0.1 g / L to 10 g / L.

[0043] [pH Adjuster] The tin alloy plating solution of this embodiment may contain a pH adjuster as needed. Examples of pH adjusters include various acids such as hydrochloric acid and sulfuric acid, and various bases such as aqueous ammonia, potassium hydroxide, sodium hydroxide, and sodium bicarbonate. Other effective pH adjusters include monocarboxylic acids such as acetic acid and propionic acid, boric acids, phosphoric acids, dicarboxylic acids such as oxalic acid and succinic acid, and oxycarboxylic acids such as lactic acid and tartaric acid.

[0044] Here, we will explain the defoaming mechanism by EO / PO block polymer (F) with EO groups at the end. When performing plating in a plating tank, the plating solution in the tank is stirred with a stirrer and the solution is circulated using a circulation pump. At this time, if a plating solution is used that contains surfactants and has high foaming properties and low defoaming properties, there is a concern that bubbles will be generated and accumulate on the liquid surface due to stirring and circulation of the plating solution, affecting bump plating.

[0045] In this embodiment of the tin alloy plating solution, alkanesulfonic acid or its salt is used as a leveling agent. Because this alkanesulfonic acid or its salt has both hydrophobic and hydrophilic groups, as shown in Figure 1, air is trapped with the hydrophobic side facing inward, and a thin liquid film surrounds the air with the hydrophilic side facing inward, forming bubbles. At this time, the alkanesulfonic acid molecules are arranged regularly on the bubble surface. However, as shown in Figure 2, if an EO / PO block polymer with EO groups at the ends is added, a portion of the molecular arrangement on the bubble surface is replaced by the EO / PO block polymer. While the thin film of the bubbles maintains a certain thickness and elastic properties, the replacement by the EO / PO block polymer causes the film to lose its elasticity, and the film thickness becomes thinner than the limit, leading to bubble rupture. (Defoaming effect)

[0046] In this embodiment of the tin alloy plating solution, which has the above configuration, a leveling agent (C) consisting of an alkanesulfonic acid or its salt containing 9 to 18 carbon atoms in its molecule is included. Therefore, a uniform and dense plating film can be formed and the plating film can be made smooth without replenishing the leveling agent for a long period of time during electroplating. Furthermore, in this embodiment of the tin alloy plating solution, an EO / PO block polymer (F) with EO groups at the ends is included. The EO ratio in the EO / PO block polymer is in the range of 20 mol% to 50 mol%, and the molecular weight of the EO / PO block polymer is in the range of 1500 to 3500. Therefore, the defoaming effect can be improved without hindering the atomization effect of the leveling agent consisting of alkanesulfonic acid or its salt, and the occurrence of defects caused by foam can be suppressed. Thus, even in a plating solution to which alkanesulfonic acid or its salt is added, sufficient foaming / defoaming properties can be ensured, the occurrence of defects caused by foam can be suppressed, and a smooth plating film can be stably formed.

[0047] In this embodiment of the tin alloy plating solution, when the metal nobler than tin is silver or copper, it exhibits excellent solder wettability, mounting strength, bendability, and reflowability, and is less prone to whisker formation.

[0048] In the tin alloy plating solution of this embodiment, when the EO / PO block polymer content is within the range of 1.0 g / L to 10 g / L, the defoaming effect can be reliably improved without inhibiting the atomization effect of the leveling agent consisting of alkanesulfonic acid or its salt, and the occurrence of defects caused by foam can be further suppressed.

[0049] In the tin alloy plating solution of this embodiment, when the content of alkanesulfonic acid or its salt is in the range of 0.01 g / L to 1 g / L, the phenyl group-containing nonionic surfactant can easily solubilize the alkanesulfonic acid, making interactions easier and allowing the atomization effect to be fully achieved.

[0050] Although embodiments of the present invention have been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical requirements of the invention.

[0051] The results of the verification experiments conducted to confirm the effectiveness of the present invention are described below.

[0052] <Example 1 of the Invention> In an aqueous solution of tin methanesulfonic acid, methanesulfonic acid was dissolved as a free acid, 3,6-dithia-1,8-octanediol as a silver complexing agent, polyoxyethylene phenyl ether (obtained by adding and condensing 5 moles of ethylene oxide (a starting material that becomes an EO group) to 1 mole of a phenyl group-containing nonionic surfactant), sodium 1-nonanesulfonate as a leveling agent consisting of an alkanesulfonic acid or a salt thereof, and polyoxyethylene polyoxypropylene glycol as an antifoaming agent (EO / PO block polymer). Further, silver methanesulfonic acid solution was added and mixed. Finally, deionized water was added to prepare a SnAg plating solution (tin alloy plating solution) with the following composition. Note that the aqueous solution of tin methanesulfonic acid was prepared by electrolyzing a tin metal plate in an aqueous solution of methanesulfonic acid. The aqueous solution of silver methanesulfonic acid was prepared by electrolyzing a silver metal plate in an aqueous solution of methanesulfonic acid.

[0053] ・Tin methanesulfonate (Sn ２＋ As): 60 g / L • Silver methanesulfonate (Ag ＋ As: 0.5 g / L • Methanesulfonic acid (free acid): 120 g / L • 3,6-Dithia-1,8-Octanediol (complexing agent for silver): 1 g / L • Polyoxyethylene phenyl ether (phenyl group-containing nonionic surfactant): 5 g / L • Sodium 1-nonanesulfonate (leveling agent consisting of alkanesulfonates): 1.00 g / L • Polyoxyethylene polyoxypropylene glycol (EO / PO block polymer): 5.0 g / L • Ion-exchanged water: remainder

[0054] <Examples 2-14 of the Invention and Comparative Examples 1-14> Table 1 shows phenyl group-containing nonionic surfactants classified as A-O, and Table 2 shows leveling agents consisting of alkanesulfonic acids or their salts classified as A-S. Then, an SnAg plating solution (tin alloy plating solution) was prepared in the same manner as in Example 1 of the Invention, except that the type of phenyl group-containing nonionic surfactant, the type and content of the leveling agent consisting of alkanesulfonic acids or their salts, and the type and content of the EO / PO block polymer were changed as shown in Tables 3 and 4.

[0055] In the SnAg plating solutions of Examples 2-9, 12, 13 and Comparative Examples 2-14 of the present invention, the phenyl group-containing nonionic surfactant was obtained by adding and condensing ethylene oxide (starting material for the EO group) to 1 mole of the phenyl group-containing structure in the number of moles (number of moles of EO added) shown in Tables 3 and 4, but propylene oxide (starting material for the PO group) was not added. Furthermore, in the SnAg plating solutions of Examples 10, 11, and 14 of the present invention, the phenyl group-containing nonionic surfactant was obtained by adding and condensing both ethylene oxide (EO) and propylene oxide (PO) to 1 mole of the phenyl group-containing structure in the number of moles (number of moles of EO added and number of moles of PO added) shown in Tables 3 and 4. Note that the phenyl group-containing structure includes not only phenol but also bisphenol A, naphthyl ether, etc.

[0056] In Tables 3 and 4, the “formula” in the column for alkanesulfonic acid or its salt is one of the following formulas (1) to (4). １ -SO ３ H ... (1) R ２ -HC (SO ３ H)-R ３ …(2) R １ -SO ３ Na … (3) R ２ -HC (SO ３ Na)-R ３ …(4) R in equations (1) and (3) １ And, R in equations (2) and (4) ２ and R ３This is an alkyl group. Also, the "content" in the column for alkanesulfonic acid or its salt in Tables 3 and 4 refers to the content (g / L) of alkanesulfonic acid or its salt in 1 L of SnAg plating solution.

[0057] In Tables 3 and 4, the "EO ratio" in the EO / PO block polymer column represents the molar ratio of EO to the total amount of PO in the EO / PO block polymer. The "molecular weight" in the EO / PO block polymer column in Tables 3 and 4 represents the number-average molecular weight. Furthermore, the "content" in the EO / PO block polymer column in Tables 3 and 4 represents the content (g / L) of EO / PO block polymer in 1 L of SnAg plating solution.

[0058] The various tin alloy plating solutions described above were evaluated according to the following procedure. The evaluation results are shown in Tables 5 and 6.

[0059] <Plating Method> A seed layer for electrical conductivity was formed on the surface of a 300 mm diameter silicon wafer by sequentially laminating a 0.1 μm titanium layer and a 0.3 μm copper layer using the sputtering method. A dry film resist (thickness 50 μm) was laminated on top of the seed layer, and the dry film resist was partially exposed and developed through an exposure mask. In this way, a resist layer was formed having a pattern in which 1.6 million openings with a diameter of 75 μm were formed with a distance of 150 μm between the center points of two adjacent openings. The SnAg plating solution (tin alloy plating solution) obtained as described above was used as the electrolyte, and the temperature of this electrolyte was adjusted to 30°C. The silicon wafer was immersed in the electrolyte, and electroplating was performed at a current density of 4 ASD to form 40 μm thick bumps on the circular exposed areas of the silicon wafer. The resist was peeled off from the silicon wafer surface where these bumps were formed using an organic solvent.

[0060] <Temporal Stability of Plating Solution> The temporal stability of the plating solution was evaluated using the following method. The target solution sample was sealed in three transparent glass bottles. The appearance of the solution was visually inspected immediately after preparation. Next, these bottles were stored standing at low temperature (5°C), room temperature (25°C), and high temperature (40°C), respectively, and the appearance of the solution was visually inspected after one month. In the visual inspection, if the solution was transparent compared to deionized water at all temperatures of 5°C, 25°C, and 40°C, it was judged as "good," and if there was turbidity or precipitation at any of the temperatures, it was judged as "poor." In addition, even if it appeared transparent to the naked eye, if scattered light was observed when light was incident on it at any of the temperatures, it was judged as "slightly poor."

[0061] <Defoaming Properties> The defoaming properties were evaluated in accordance with the foaming power and foam stability (Ross-Miles test method) of JIS K 3362:2008. 200 mL of plating solution was dropped onto the liquid surface from a height of 900 mm for 30 seconds at 30°C, and the height of the bubbles (mm) immediately afterward was measured visually. The height of the bubbles (mm) was measured visually again 5 minutes after dropping. This procedure was repeated three times, and the average of the measured values ​​was calculated. The defoaming properties were examined by the ratio of the bubble height after 5 minutes to the bubble height immediately after dropping. If the ratio of the bubble height after 5 minutes to the bubble height immediately after dropping was less than 20%, the defoaming properties were judged as "good," if it was between 20% and 35%, it was judged as "acceptable," and if it was 35% or more, it was judged as "poor."

[0062] <Bump Appearance> When the bump appearance was visually inspected after plating, it was judged as "good" if there were no abnormal deposits such as nodules or discoloration, and no indentations in the bump. It was judged as "poor" if abnormal deposits were observed in the center or edges of the bump, or if indentations were observed in the bump.

[0063] <Surface Smoothness> Using a laser microscope (Olympus OLS3000), the arithmetic mean surface roughness Ra of a 10 μm square area near the center of the bump top was calculated. Surfaces with an Ra of less than 0.5 μm were classified as "good," those with an Ra of 0.5 μm or more but less than 0.7 μm were classified as "acceptable," and those with an Ra of 0.7 μm or more were classified as "unacceptable."

[0064] <Voids> A bump-attached wafer, plated using the "plating method" described above and with the resist removed, was placed in a reflow apparatus and heated to 250°C to melt the bumps. After cooling, transmission X-ray images were taken at 50x magnification for 6000 bumps with a diameter of 75 μm. The captured images were visually observed, and if one or more voids of 1% or more relative to the size of the bump were observed, it was determined to be "void present," and if no voids were observed, it was determined to be "void-free."

[0065] <Pit Defect Occurrence Rate> Two silicon wafers were continuously electroplated using the method described in "Plating Method". On the silicon wafers, which had bumps formed in the resist openings by continuous electroplating, the height of 1.6 million bumps was measured using an automated visual inspection device (Camtek, model number Falcon). As shown in Figure 3A, bumps that were plated thicker than the surface of the resist layer and had a mushroom-shaped cross-section were defined as "normal bumps". As shown in Figures 3B and 3C, ungrown bumps whose film thickness did not reach the resist surface and whose cross-section did not have a mushroom shape were defined as defective bumps with "pit defects". The pit defect occurrence rate was then calculated using the following formula: Pit defect occurrence rate (ppm) = (Number of defective bumps / Total number of bumps) × 10 ６

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072] In Comparative Example 1, the nonionic surfactant did not contain a phenyl group, and the surface smoothness was judged to be "unacceptable". In Comparative Examples 2 and 4, the leveling agent had a small number of carbon atoms (8), and the surface smoothness was judged to be "unacceptable". In Comparative Examples 3 and 5, the leveling agent had a large number of carbon atoms (19), and the surface smoothness was judged to be "unacceptable". In Comparative Example 6, the leveling agent was not composed of an alkanesulfonic acid or its salt, and the surface smoothness was judged to be "unacceptable". In Comparative Example 7, there was no leveling agent, and the surface smoothness was judged to be "unacceptable".

[0073] In Comparative Example 8, the EO / PO block polymer was not included, and the defoaming performance was judged to be "poor," resulting in void formation and a high pit defect rate of 850 ppm. In Comparative Example 9, the EO / PO block polymer had PO at one end, and the defoaming performance was judged to be "poor," resulting in void formation and a high pit defect rate of 310 ppm. In Comparative Example 10, one end of the EO / PO block polymer was PO, and the defoaming performance was judged to be "poor," resulting in void formation and a high pit defect rate of 420 ppm.

[0074] In Comparative Example 11, the EO ratio of the EO / PO block polymer was low at 15 mol%, resulting in a "poor" assessment of the plating solution's stability over time and a "poor" assessment of the bump appearance. Furthermore, the pit defect rate was 75 ppm. In Comparative Example 12, the EO ratio of the EO / PO block polymer was high at 60 mol%, resulting in a "poor" assessment of the bump appearance. Furthermore, the pit defect rate was 30 ppm. In Comparative Example 13, the number-average molecular weight of the EO / PO block polymer was low at 1300, resulting in a "poor" assessment of surface smoothness. In Comparative Example 14, the number-average molecular weight of the EO / PO block polymer was high at 4000, resulting in a "poor" assessment of the bump appearance and a "poor" assessment of surface smoothness.

[0075] In contrast, in Examples 1 to 14 of the present invention, the changes in the plating solution over time, defoaming properties, bump appearance, and surface smoothness were all judged to be "good." Furthermore, there was no void formation, and the pit defect occurrence rate was kept low. It should be noted that there was no significant difference in bump smoothness between Comparative Example 8 in Figure 4A (where a leveling agent consisting of alkanesulfonic acid or its salt was added and EO / PO block polymer was not used) and Example 1 of the present invention in Figure 4B (where a leveling agent consisting of alkanesulfonic acid or its salt was added and EO / PO block polymer was used), confirming that the addition of EO / PO block polymer does not change the effect of improving smoothness by the leveling agent consisting of alkanesulfonic acid or its salt.

[0076] From the results of the above verification experiments, it was confirmed that, according to the present invention, even in a plating solution to which alkanesulfonic acid or its salt is added as a leveling agent, sufficient defoaming properties can be ensured, the occurrence of defects caused by foam can be suppressed, and a tin alloy plating solution that can stably form a smooth plating film can be provided.

[0077] The tin alloy plating solution of this embodiment is suitably applied to the process of manufacturing bumps that will serve as tin alloy protruding electrodes on a substrate.

Claims

1. A tin alloy plating solution comprising: (A) a soluble salt containing at least a stannous salt; (B) a soluble salt of a metal nobler than tin; (C) a leveling agent consisting of an alkanesulfonic acid or a salt thereof containing 9 to 18 carbon atoms in its molecule; (D) a free acid; (E) a nonionic surfactant containing one or more phenyl groups in its molecule; and (F) an EO / PO block polymer having an EO group at its terminus, wherein the EO ratio in the EO / PO block polymer is in the range of 20 mol% to 50 mol%; and the molecular weight of the EO / PO block polymer is in the range of 1500 to 3500.

2. The tin alloy plating solution according to claim 1, characterized in that the metal nobler than tin is silver or copper.

3. The tin alloy plating solution according to claim 1, characterized in that the content of the EO / PO block polymer is in the range of 1.0 g / L or more and 10 g / L or less.

4. The tin alloy plating solution according to claim 1, wherein the content of the leveling agent consisting of the alkanesulfonic acid or a salt thereof is in the range of 0.01 g / L to 1.00 g / L.

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