JP7732205B2 - Electrolytic plating solution and electrolytic plating method - Google Patents

Description

The present invention relates to an electrolytic plating solution and an electrolytic plating method for forming a plating film of tin or a tin alloy. More specifically, it relates to an electrolytic plating solution and an electrolytic plating method used for tin or tin alloy plating suitable for forming solder bumps on semiconductor wafers and printed circuit boards.

Electrolytic plating (hereinafter sometimes simply referred to as plating) equipment generally includes an anode (positive electrode) and a cathode (negative electrode) positioned opposite each other in a plating tank containing an electrolytic plating solution (hereinafter sometimes simply referred to as plating solution), with the workpiece to be plated being a semiconductor wafer or substrate connected to the anode. A voltage is applied between the anode and the workpiece to be plated. This electrical current forms a plating film on the surface of the workpiece to be plated.

The anodes used in this electrolytic plating apparatus are broadly divided into soluble anodes and insoluble anodes. In tin and tin alloy plating solutions, insoluble anodes made of platinum (Pt), platinum-coated titanium (Pt/Ti), iridium oxide ( _IrO2 ), or the like are generally used as the anode to suppress reactions of plating solution components (additives and metal components more noble than tin) on the anode surface. When an insoluble anode is used, a water electrolysis reaction occurs on the anode surface during electrolysis, as shown in the following reaction formula:
2H ₂ O → 4H ⁺ + O ₂ ↑ + 4e ^-
During this process, oxygen gas (bubbles) generated on the anode surface desorbs from the anode surface and is released into the plating solution. The released oxygen bubbles float in the plating solution and then either disappear when they reach the liquid surface or dissolve in the plating solution as dissolved gas. However, if the bubbles adhere to the workpiece connected to the cathode before disappearing, they can interfere with plating deposition and easily cause pit defects. They can also be incorporated into the plating film and easily cause voids. This problem tends to become more pronounced when using a horizontal plating apparatus (also known as a CUP type, face-down type, or fountain type), and is particularly noticeable when plating is performed with an anode current density of 0.5 A/dm ² (hereinafter sometimes referred to as ASD (Ampere per Square Decimeter)) or higher.

Specifically, referring to Figure 2, when forming solder bumps on a semiconductor wafer in a horizontal plating tank 1, a semiconductor wafer (member to be plated) 4 is placed in plating solution 2, facing a horizontally placed insoluble anode 3. The semiconductor wafer 4 is connected to a cathode and positioned horizontally above the anode. During plating, the semiconductor wafer 4 rotates horizontally. The plating solution 2 is circulated via a suction port 1a, a return port 1b, and a circulation pump 1c, all located at the bottom of the plating tank 1. Air bubbles 8 detached from the surface of the anode 3 rise to the surface and adhere to the surface of the semiconductor wafer 4. Therefore, as shown in the enlarged view of Figure 2, if the air bubbles 8 are smaller than the diameter of the via 6 in the resist layer 5 formed by the resist pattern, the air bubbles 8 adhering to the bump 7, which is the plating deposition layer within the via, do not detach. This prevents plating from being deposited in the bubble-attached area, resulting in the formation of pit defects 9a. Furthermore, the air bubbles 8 remain in the bump 7, which is the plating deposition layer, and are likely to form voids 9b within the bump after reflow.

To solve this problem, a plating method has been disclosed that reduces pit defects by plating in a reduced-pressure plating tank (see, for example, Patent Document 1 (Claim 1, paragraphs [0001] to [0003])).

Japanese Patent Application Laid-Open No. 2004-43916

Conventional bump electrodes often have a diameter of over 100 μm, and air bubbles attached to the bumps in the vias could be removed by changing equipment conditions such as stirring. For this reason, the problem of pit defects caused by air bubbles was thought to be solvable by changing the plating equipment structure or stirring strength.

However, in recent years, bump electrodes have become smaller, with diameters of 100 μm or less, and the via depth relative to the via diameter (depth/diameter) in the resist layer has become a high aspect ratio. As a result, it has become difficult to remove air bubbles that have adhered to the plating deposition layer (bump) deep within the fine vias using equipment conditions. For this reason, there was a need to reduce the incidence of pit defects caused by bubbles preventing the plating deposition layer from growing, and voids within the bump after the plating deposition layer has grown. As shown in Patent Document 1, plating in a reduced-pressure plating tank is one way to reduce the incidence of pit defects and voids, but even with this plating method, the above-mentioned issues remain unresolved.

The object of the present invention is to provide an electrolytic plating solution and electrolytic plating method that reduces the incidence of pit defects in bumps and voids within bumps, which occur when plating does not deposit due to bubbles generated in the plating solution.

The inventors conducted extensive research to solve the above problems and discovered that by adding a surfactant with a specific structure to the plating bath, oxygen bubbles generated on the anode surface aggregate and grow larger on the anode surface before being released into the plating solution. These larger oxygen bubbles are then quickly carried to the liquid surface by buoyancy, preventing the oxygen bubbles from dispersing in the plating solution. As a result, adhesion of bubbles smaller than the via diameter to the surface of the plated member connected to the cathode is reduced, dramatically reducing the occurrence of pit defects and voids, leading to the present invention.

A first aspect of the present invention is an electrolytic plating solution comprising (A) a soluble salt containing at least a stannous salt, (B) an acid or a salt thereof (excluding soluble salts containing at least a stannous salt) , and (C) a surfactant, wherein when electrolysis is performed at an anode current density of 0.7 A/ ^dm2 , the maximum value of the particle size distribution of bubbles released from an insoluble anode is 150 μm or more, the surfactant (C) is a nonionic surfactant of a polyoxyethylene polyoxypropylene block polymer (PO-EO-PO) having polyoxypropylene (PO) at its terminal, the EO ratio in the block polymer (PO-EO-PO) is in the range of 35% or more and 50% or less in molar ratio, and the mass average molecular weight of the nonionic surfactant is in the range of 3,000 or more and 5,000 or less .

A second aspect of the present invention is an electrolytic plating solution comprising (A) a soluble salt containing at least a stannous salt, (B) an acid or a salt thereof (excluding soluble salts containing at least a stannous salt), and (C) a surfactant, wherein when electrolysis is performed at an anode current density of 0.7 A/dm2 ^, the maximum value of the particle size distribution of bubbles released from an insoluble anode is 150 μm or more, the surfactant (C) is an amphoteric surfactant selected from the group consisting of alkylsulfobetaine and alkylhydroxysulfobetaine, the alkyl group of which has 10 or more and 22 or less carbon atoms, and the content of the surfactant (C) is in the range of 0.5 g/L or more and 10 g/L or less .

A third aspect of the present invention is the invention according to either the first or third aspect , wherein the content of the surfactant (C) is in the range of 0.5 g/L or more and 10 g/L or less.

A fourth aspect of the present invention is a method for electrolytically plating a member to be plated, comprising supplying the electrolytic plating solution according to any one of the first to third aspects to a plating tank in which an insoluble anode is disposed opposite a member to be plated that is connected to a cathode.

A fifth aspect of the present invention is the invention according to the fourth aspect , which is an electrolytic plating method for performing electrolytic plating using a plating tank in which the member to be plated and the insoluble anode are arranged horizontally.

In the electrolytic plating solution according to the first aspect of the present invention, as shown in Fig. 1, when electrolysis is performed at an anode current density of 0.7 A/ ^dm2 , the maximum value of the particle size distribution of bubbles 8 emitted from the insoluble anode 3 is 150 µm or more, making it difficult for bubbles 8 to enter vias 6 in a resist layer 5 formed by a resist pattern. Since the plating deposit layer grows to form bumps 7 without bubbles 8 adhering to the plating deposit layer in vias 6, the incidence of pit defects and voids in bumps caused by non-precipitation of plating can be reduced. In Fig. 1, elements that are the same as those shown in Fig. 2 are designated by the same reference numerals.

Furthermore, the surfactant (C) is a nonionic surfactant of a polyoxyethylene polyoxypropylene block polymer (PO-EO-PO) terminated with polyoxypropylene (PO), the EO ratio in the block polymer (PO-EO-PO) being in the range of 35% to 50% by molar ratio, and the mass average molecular weight of the nonionic surfactant being in the range of 3,000 to 5,000. Because such surfactants have properties intermediate between hydrophilic and hydrophobic, they have strong adsorption to the insoluble anode surface and the oxygen bubble surface. As a result, as shown in FIG. 1 , oxygen bubbles 8 generated on the surface of the anode 3 aggregate and grow large on the anode 3 surface before being released into the plating solution 2, making it difficult for bubbles released from the anode 3 to enter the vias 6 in the resist layer 5. As a result, the incidence of pit defects and voids can be reduced.

In the electroplating solution according to the second aspect of the present invention, the surfactant (C) is an amphoteric surfactant such as an alkylsulfobetaine or an alkylhydroxysulfobetaine, and therefore has the effect of exhibiting strong adsorption to the surface of the insoluble anode and the surface of oxygen bubbles.

In the electroplating solution according to the third aspect of the present invention, the surfactant (C) content is in the range of 0.5 g/L or more and 10 g/L or less, so that a sufficient amount of surfactant (C) can be adsorbed onto the surface of the insoluble anode and the surface of oxygen bubbles, thereby further reducing the incidence of pit defects and voids.

In the electroplating method according to the fourth aspect of the present invention, the maximum value of the particle size distribution of bubbles 8 released from insoluble anode 3 by electroplating solution 2 is 150 μm or more, so that when vias 6 are formed in resist layer 5 on plated member 4 disposed opposite insoluble anode 3, electroplating is performed without bubbles 8 being trapped in the vias. As a result, this plating method can reduce the incidence of pit defects and voids.

In the electroplating method according to the fifth aspect of the present invention, the member to be plated 4 and the insoluble anode 3 are arranged horizontally, and therefore even if bubbles 8 released from the insoluble anode 3 reach the member to be plated 4 above, the bubbles 8 are so large that they are not taken into the vias 6 of the resist layer 5 formed on the member to be plated 4, and electroplating is carried out. As a result, this plating method can reduce the incidence of pit defects and voids.

1 is a schematic diagram of an electrolytic plating apparatus showing a state in which electrolytic plating is being performed using an electrolytic plating solution according to the present invention, and a cross-sectional view showing an enlarged portion of a member to be plated. 1A and 1B are a schematic diagram of an electrolytic plating apparatus showing a state in which electrolytic plating is performed using a conventional electrolytic plating solution, and an enlarged cross-sectional view of a portion of a member to be plated. FIG. 2 is a diagram showing the configuration of an apparatus for measuring the maximum value of the particle size distribution of bubbles released from an insoluble anode in an example. FIG. 1 is a plan view of a semiconductor wafer having a resist layer produced in an example. 5A and 5B are cross-sectional views showing the growth of bumps (plating deposit layers) in vias (openings) before and after plating. Fig. 5A is a cross-sectional view of a plated member before plating, Fig. 2B is a cross-sectional view of a plated member on which normal mushroom-shaped bumps are formed, Fig. 2C and Fig. 2D are cross-sectional views of plated members on which pit defects are formed, and Fig. 2E is a cross-sectional view of a plated member on which voids are formed.

The following describes an electrolytic plating solution according to one embodiment of the present invention. This plating solution is used as a material for forming tin or tin alloy plating films used as solder bumps for semiconductor wafers and printed circuit boards.

The tin or tin alloy plating solution of this embodiment is an electrolytic plating solution containing at least a soluble salt (A) containing a stannous salt, an acid or its salt (B), and a surfactant (C). It is characterized in that when electrolysis is performed at an anode current density of 0.7 A/ ^dm² , the maximum value of the particle size distribution of bubbles emitted from the insoluble anode is 150 µm or more. Preferably, the maximum value of the particle size distribution of these bubbles is 200 µm or more, and although there is no particular upper limit, it may be, for example, 1000 µm or less. The anode current density is limited to 0.7 ASD (A/ ^dm² ) because pit defects and voids tend to become more apparent at a current density of 0.7 ASD or more. The reason why the maximum value of the particle size distribution of bubbles released from the insoluble anode is set to 150 μm or more is that the via diameter of the resist layer formed by the resist pattern of the plated member is generally in the range of 10 μm or more and less than 100 μm, and by setting the maximum value of the particle size distribution of bubbles to 150 μm or more, the probability of bubbles being taken into the plating deposit layer in the via is reduced, and as a result, the incidence of pit defects and voids can be reduced.

The tin alloy of this embodiment is an alloy of tin and one or more predetermined metals selected from silver, copper, bismuth, nickel, antimony, indium, and zinc. Examples include binary alloys such as tin-silver alloy, tin-copper alloy, tin-bismuth alloy, tin-nickel alloy, tin-antimony alloy, tin-indium alloy, and tin-zinc alloy, and ternary alloys such as tin-copper-bismuth and tin-copper-silver alloy.

[Soluble salt (A) containing at least a stannous salt]
The soluble salt (A) of this embodiment is a stannous salt alone, or a mixture of this stannous salt and salts of one or more metals selected from the group consisting of silver, copper, bismuth, nickel, antimony, indium, and zinc.

Therefore, the soluble salt (A) of this embodiment contains Sn ²⁺ alone in the plating solution, or contains one or more soluble salts that, together with Sn ²⁺ , produce various metal ions such as Ag ⁺ , Cu ⁺ , Cu ²⁺ , Bi ³⁺ , Ni ²⁺ , Sb ³⁺ , In ³⁺ , Zn ²⁺ , etc. Examples of soluble salts include oxides, halides, and inorganic or organic acid salts of these metals.

Metal oxides include stannous oxide, silver oxide, copper oxide, nickel oxide, bismuth oxide, antimony oxide, indium oxide, and zinc oxide. Metal halides include stannous chloride, bismuth chloride, bismuth bromide, cuprous chloride, cupric chloride, nickel chloride, antimony chloride, indium chloride, and zinc chloride.

Examples of metal salts of inorganic or organic acids include copper sulfate, stannous sulfate, bismuth sulfate, nickel sulfate, antimony sulfate, bismuth nitrate, silver nitrate, copper nitrate, antimony nitrate, indium nitrate, nickel nitrate, zinc nitrate, copper acetate, nickel acetate, nickel carbonate, sodium stannate, stannous borofluoride, stannous methanesulfonate, silver methanesulfonate, copper methanesulfonate, bismuth methanesulfonate, nickel methanesulfonate, indium methanesulfonate, zinc bismethanesulfonate, stannous ethanesulfonate, and bismuth 2-hydroxypropanesulfonate.

The content of stannous salt in the plating solution of this embodiment, converted into the amount of tin, is preferably in the range of 5 g/L or more and 200 g/L or less, and more preferably in the range of 20 g/L or more and 100 g/L or less.

[Acid or salt thereof (B)]
The acid or salt thereof (B) of this embodiment is one or more acids or salts thereof selected from organic acids, inorganic acids, and salts thereof. Examples of the organic acids include organic sulfonic acids such as alkanesulfonic acids, alkanolsulfonic acids, and aromatic sulfonic acids, as well as aliphatic carboxylic acids. Examples of inorganic acids include fluoroboric acid, hydrosilicic acid, sulfamic acid, hydrochloric acid, sulfuric acid, nitric acid, and perchloric acid. Examples of the salts include alkali metal salts, alkaline earth metal salts, ammonium salts, amine salts, and sulfonates. Organic sulfonic acids are more preferred from the viewpoints of the solubility of the metal salts and ease of wastewater treatment.

The alkanesulfonic acid may be one represented by the chemical formula _{CnH2n + 1SO3H} (where n = 1 to 5, preferably 1 to 3, for example). Specific examples include methanesulfonic acid, ethanesulfonic acid, 1-propanesulfonic acid, 2-propanesulfonic acid, 1-butanesulfonic acid, 2-butanesulfonic acid, pentanesulfonic acid, as well as hexanesulfonic acid, decanesulfonic acid, and dodecanesulfonic acid.

The alkanol sulfonic acid may be one represented by the chemical formula C _p H _2p+1 -CH(OH)-C _q H _2q -SO ₃ H (for example, p = 0 to 6, q = 1 to 5, preferably p = 0 to 2, q = 1 to 2). Specific examples include 2-hydroxyethane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid, 2-hydroxybutane-1-sulfonic acid, 2-hydroxypentane-1-sulfonic acid, as well as 1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid, 4-hydroxybutane-1-sulfonic acid, 2-hydroxyhexane-1-sulfonic acid, 2-hydroxydecane-1-sulfonic acid, and 2-hydroxydodecane-1-sulfonic acid.

The above-mentioned aromatic sulfonic acids are basically benzenesulfonic acid, alkylbenzenesulfonic acid, phenolsulfonic acid, naphthalenesulfonic acid, alkylnaphthalenesulfonic acid, etc. Specific examples include 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, toluenesulfonic acid, xylenesulfonic acid, p-phenolsulfonic acid, cresolsulfonic acid, sulfosalicylic acid, nitrobenzenesulfonic acid, sulfobenzoic acid, and diphenylamine-4-sulfonic acid.

Examples of the aliphatic carboxylic acid include acetic acid, propionic acid, butyric acid, citric acid, tartaric acid, gluconic acid, sulfosuccinic acid, and trifluoroacetic acid.

The content of the acid or salt thereof (B) selected from organic acids and inorganic acids in the plating solution of this embodiment is not particularly limited, but may be, for example, in the range of 10 g/L or more and 500 g/L or less, and preferably in the range of 50 g/L or more and 300 g/L or less.

[Surfactant (C)]
The surfactant (C) used in the plating solution of this embodiment includes a nonionic surfactant and an amphoteric surfactant. These two types of surfactants may be used alone or in combination.
As the nonionic surfactant, a polyoxyethylene polyoxypropylene block polymer (PO-EO-PO) containing polyoxyethylene (EO) and polyoxypropylene (PO) and having a polyoxypropylene (PO) terminal can be used. The EO ratio in this block polymer (PO-EO-PO) is in the range of 35% to 50% by molar ratio, and the mass average molecular weight of this nonionic surfactant is in the range of 3000 to 5000. If the EO ratio is less than 35% by molar ratio, the surfactant tends to be hydrophobic, while if it exceeds 50%, the surfactant tends to be hydrophilic. In either case, it becomes difficult to achieve a particle size distribution maximum of 150 μm or more for the bubbles released from the insoluble anode. Furthermore, if the mass average molecular weight of the surfactant is less than 3000, the effect of suppressing tin or tin alloy deposition may be insufficient, while if it exceeds 5000, the suppression of tin or tin alloy deposition may be too strong, resulting in the formation of a uniform plating film. The EO ratio is preferably in the range of 35% to 45%, more preferably 40% to 45%, in terms of molar ratio, and the mass average molecular weight is preferably in the range of 3,000 to 4,500, more preferably 3,500 to 4,500.

The nonionic surfactant of this embodiment is represented by the following formula (1). In formula (1), a, b, and c represent the numbers of repeating units of (PO), (EO), and (PO), respectively, in the block polymer.

Furthermore, as the nonionic surfactant in this embodiment, an ethylenediamine polyoxyethylene polyoxypropylene block polymer, which is an addition polymerization of ethylenediamine with polyoxyethylene (EO) and polyoxypropylene (PO) and has a polyoxypropylene (PO) terminal, can also be used. The EO ratio in this block polymer is in the range of 35% to 50% by molar ratio, and the mass average molecular weight of this surfactant is in the range of 3000 to 5000. If the EO ratio is less than 35%, the surfactant tends to be hydrophobic, while if it exceeds 50%, the surfactant tends to be hydrophilic. In either case, it becomes difficult to achieve a particle size distribution maximum of 150 μm or more for the bubbles released from the insoluble anode. Furthermore, if the mass average molecular weight of the surfactant is less than 3000, the effect of suppressing the deposition of tin or tin alloys may be insufficient, while if it exceeds 5000, the suppression of tin or tin alloy deposition may be too strong, resulting in the formation of a uniform plating film.

The nonionic surfactant of this embodiment is also represented by the following formula (2). In formula (2), m and n are the numbers of repeating units of (PO), (EO), and (PO), respectively, in the block polymer.

The nonionic surfactant represented by the above structure is a block polymer containing polyoxyethylene (EO) and polyoxypropylene (PO). When polyoxypropylene (PO) is present at the terminal, the surfactant's hydrophobicity is enhanced, resulting in increased adsorption to the insoluble anode and oxygen bubble surfaces. Conversely, when polyoxyethylene (EO) is present at the terminal of this block polymer, the surfactant's hydrophilicity is enhanced, resulting in reduced adsorption to the insoluble anode and oxygen bubble surfaces. By virtue of the surfactant's properties, the particle size distribution maximum of the bubbles released from the insoluble anode is 150 μm or greater when electrolysis is performed at an anode current density of 0.7 ASD.
The EO ratio can be measured by comparing the intensity ratio of polyoxyethylene (EO) and polyoxypropylene (PO) using NMR (Nuclear Magnetic Resonance). This measurement method is described in detail in "Surfactant Analysis Methods" (Surfactant Analysis Research Group, Saiwai Shobo, 1975).
The weight average molecular weight can also be measured by size exclusion chromatography (SEC) in comparison with a commercially available standard substance.

In order to more accurately ensure that the maximum value of the particle size distribution of the bubbles released from the insoluble anode is 150 μm or more, it is preferable that the content of the nonionic surfactant in the plating solution be in the range of 0.5 g/L or more and 10 g/L or less, that the EO ratio be in the range of 35% or more and 50% or less by molar ratio, and that the mass average molecular weight of the surfactant be in the range of 3000 or more and 5000 or less. It is even more preferable that the content of the nonionic surfactant in the plating solution be in the range of 1 g/L or more and 5 g/L or less. If the content of this surfactant is less than 0.5 g/L, the maximum value of the particle size distribution of the bubbles released from the insoluble anode may not be 150 μm or more, and if it exceeds 10 g/L, a uniform plating film may not be formed.

The nonionic surfactant of this embodiment can be obtained by purifying a commercially available product, EP-1461, manufactured by Aoki Oil & Fat Industries Co., Ltd. The surfactant of this embodiment can also be produced by known techniques. For example, it can be synthesized by adjusting the molecular weight of the raw material polyoxyethylene and the reaction amount of polyoxypropylene to be added, as described in U.S. Patent No. 4,726,909.

In this embodiment, an amphoteric surfactant can be used as the surfactant (C). Specifically, for example, alkyl sulfobetaine or alkylhydroxysulfobetaine can be used. Alkyl sulfobetaine is represented by the following formula (3). Examples include lauryl sulfobetaine and stearyl sulfobetaine. Alkyl hydroxysulfobetaine is represented by the following formula (4). Examples include lauryl hydroxysulfobetaine. In formulas (3) and (4), R represents an alkyl group. Furthermore, the number of carbon atoms in R in the alkyl sulfobetaine or alkyl hydroxysulfobetaine is preferably in the range of 10 to 22. Furthermore, the content of these amphoteric surfactants in the electroplating solution is preferably in the range of 0.5 g/L to 10 g/L in order to more accurately achieve a particle size distribution maximum of 150 μm or more for the bubbles released from the insoluble anode. It is even more preferable that the content of the amphoteric surfactant in the electroplating solution be in the range of 1 g/L to 5 g/L inclusive. The reason for setting the preferred range for the content of amphoteric surfactant in the electroplating solution to 0.5 g/L or more and 10 g/L or less is the same as the reason for setting the preferred range for the content of nonionic surfactant.

The surfactant (C) of this embodiment has the above characteristics, and when electrolysis is performed at an anode current density of, for example, 0.7 ASD, the maximum value of the particle size distribution of bubbles released from the insoluble anode is 150 μm or more. As a result, even in vias where the bump electrode is miniaturized to a diameter of 100 μm or less, bubbles are less likely to enter the vias in the resist layer, allowing the plating deposit layer in the via to grow smoothly without being hindered by bubbles, making pit defects less likely to occur. Furthermore, voids are less likely to form in the plating deposit layer in the vias. This results in a uniform bump height when the plating deposit layer is reflowed, and reduces voids in the bump.

By using the above-described surfactant (C), the maximum value of the particle size distribution of bubbles released from the insoluble anode can be made 150 μm or more when electrolysis is performed at an anode current density of 0.7 A/dm ² .

The plating solution of this embodiment may further contain an antioxidant, a complexing agent, a pH adjuster, and a brightener, as needed.

[Antioxidants]
The antioxidant is intended to prevent the oxidation of Sn2 ⁺ in the plating solution. 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, and hydroquinone sulfonic acid or its salts. For example, hydroquinone sulfonic acid or its salts is preferred for acidic baths, and ascorbic acid or its salts is preferred for neutral baths.

Antioxidants may be used singly or in combination of two or more. The amount of antioxidant added to the plating solution of this embodiment is generally in the range of 0.01 g/L or more and 20 g/L or less, preferably 0.1 g/L or more and 10 g/L or less, and more preferably 0.1 g/L or more and 5 g/L or less.

[Complexing Agent]
The plating solution of this embodiment can be applied to tin or tin alloy plating baths of any pH range, including acidic, weakly acidic, and neutral. ^Sn ions are stable at strongly acidic (pH: <1), but tend to form white precipitates at acidic to near-neutral (pH: 1 to 7). Therefore, when the tin or tin alloy plating solution of this embodiment is applied to a near-neutral tin plating bath, it is preferable to add a tin complexing agent to stabilize the ^Sn ions.

Oxycarboxylic acids, polycarboxylic acids, and monocarboxylic acids can be used as complexing agents for tin. 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. Gluconic acid, citric acid, glucoheptonic acid, gluconolactone, glucoheptolactone, or salts thereof are preferred. Additionally, polyamines and aminocarboxylic acids such as ethylenediamine, 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 of these are also effective as complexing agents.

Tin complexing agents may be used singly or in combination of two or more. The amount of tin complexing agent added to the plating solution of this embodiment is generally preferably in the range of 0.001 mol or more and 10 mol or less per mol of tin in the soluble tin salt compound contained in the tin or tin alloy plating solution. It is more preferably in the range of 0.01 mol or more and 5 mol or less, and even more preferably in the range of 0.5 mol or more and 2 mol or less.

Furthermore, when the tin alloy plating solution is a SnAg plating solution, a water-soluble sulfide compound or a water-soluble thiol compound can be used as a complexing agent for silver.

[pH adjuster]
The 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. 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 hydroxycarboxylic acids such as lactic acid and tartaric acid.

[Brightening Agent]
The plating solution of this embodiment may contain a brightening agent, if necessary. Aromatic carbonyl compounds are effective as brightening agents. Aromatic carbonyl compounds have the effect of reducing the size of tin alloy crystal grains in tin alloy plating films. Aromatic carbonyl compounds are compounds in which a carbonyl group (-CO-X, where X represents a hydrogen atom, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms) is bonded to a carbon atom of an aromatic hydrocarbon. Aromatic hydrocarbons include benzene rings, naphthalene rings, and anthracene rings. Aromatic hydrocarbons may have a substituent. Examples of the substituent include halogen atoms, hydroxyl groups, alkyl groups having 1 to 6 carbon atoms, and alkoxy groups having 1 to 6 carbon atoms. The carbonyl group may be directly bonded to the aromatic hydrocarbon or may be bonded via an alkylene group having 1 to 6 carbon atoms. Specific examples of aromatic carbonyl compounds include benzalacetone, cinnamic acid, cinnamaldehyde, and benzaldehyde.

Aromatic carbonyl compounds may be used singly or in combination of two or more. The amount of aromatic carbonyl compound added to the tin alloy plating solution of this embodiment is generally preferably in the range of 0.01 mg/L or more and 500 mg/L or less. It is more preferably in the range of 0.1 mg/L or more and 100 mg/L or less, and even more preferably in the range of 1 mg/L or more and 50 mg/L or less.

[Electrolytic plating method]
The electrolytic plating method of this embodiment is performed using an electrolytic plating apparatus shown in FIG. 1 . In a plating tank 1 of this apparatus, an insoluble anode 3 is placed opposite a member to be plated, such as a semiconductor wafer 4, which is connected to the cathode. The above-described electrolytic plating solution 2 is supplied to this plating tank 1 to perform electrolytic plating. The effects of the present invention are more pronounced in a horizontal plating apparatus in which the insoluble anode 3 is placed horizontally relative to the semiconductor wafer 4. In this electrolytic plating apparatus, it is preferable that the opposing surfaces of the member to be plated 4 and the insoluble anode 3 are arranged substantially parallel to each other. Furthermore, the vertical relationship between the member to be plated and the insoluble anode is not limited, but from the perspective of mass productivity, it is preferable to place the insoluble anode on the lower side and the member to be plated on the upper side, as shown in FIG. 1 , because this makes it easier to replace the member to be plated.
In this embodiment, the anode current density during the formation of the plating film is preferably in the range of 0.1 A/dm2 or ^more and 5 A/dm2 or ^less . The solution temperature is preferably in the range of 10°C or more and 50°C or less, and more preferably in the range of 20°C or more and 40°C or less.

Next, examples of the present invention will be described in detail along with comparative examples.

(Nonionic surfactants used in Examples and Comparative Examples)
The structural formulas of polyoxyethylene polyoxypropylene block polymers (PO-EO-PO) or (EO-PO-EO) containing polyoxyethylene (EO) and polyoxypropylene (PO) used in Examples 1 to 5 and Comparative Examples 1 to 5, the EO ratios, and the mass average molecular weights of the surfactants are shown in Table 1.

(Nonionic surfactants and amphoteric surfactants used in the examples)
The surfactants used in Examples 6 to 9 are shown in Table 2.

(Preparation of Sn plating solution)
Example 1
To the aqueous solution of tin methanesulfonate, methanesulfonic acid as the free acid, a nonionic surfactant having the structural formula (PO-EO-PO) shown in Table 1 above, with an EO ratio of 35% (molar ratio) and a mass-average molecular weight of 4,000, and benzalacetone as a brightener were added. Finally, ion-exchanged water was added to prepare a tin plating bath with the following composition. The aqueous solution of tin methanesulfonate was prepared by electrolyzing a metal tin plate in an aqueous solution of methanesulfonic acid.

(Composition of Sn plating solution)
Sn methanesulfonate (as Sn ²⁺ ): 50 g/L
Methanesulfonic acid (as free acid): 100 g/L
Surfactant: 5g/L
Benzalacetone (as a brightener): 10 mg/L
Ion-exchanged water: balance

<Examples 2 to 4, Comparative Examples 1 to 5>
In Examples 2 to 4 and Comparative Examples 1 to 5, the surfactant used was a nonionic surfactant having the structural formula (PO-EO-PO) or (EO-PO-EO), EO ratio, and mass average molecular weight shown in Table 1. Except for these, the Sn plating solutions of Examples 2 to 4 and Comparative Examples 1 to 5 were prepared in the same manner as in Example 1.

(Preparation of SnAg plating solution)
Example 5
Methanesulfonic acid as a free acid and thiodiethanol as a complexing agent were mixed and dissolved in an aqueous solution of tin methanesulfonate, followed by the addition of an aqueous solution of silver methanesulfonate. After mixing to form a uniform solution, a nonionic surfactant having the structural formula (PO-EO-PO) shown in Table 1 above, an EO ratio of 50% (molar ratio), and a mass-average molecular weight of 5,000, and benzalacetone as a brightener were added. Finally, ion-exchanged water was added to prepare a SnAg plating bath with the following composition. The aqueous solution of tin methanesulfonate and the aqueous solution of silver methanesulfonate were prepared by electrolyzing a metal tin plate in an aqueous solution of methanesulfonic acid, respectively.

(Composition of SnAg plating solution)
Sn methanesulfonate (as Sn ²⁺ ): 50 g/L
Methanesulfonic acid Ag (as Ag ⁺ ): 0.2 g/L
Methanesulfonic acid (as free acid): 100 g/L
Thiodiethanol (as a complexing agent): 5 g/L
Surfactant: 5g/L
Benzalacetone (as a brightener): 10 mg/L
Ion-exchanged water: balance

Example 6
As surfactants, nonionic surfactant 1 shown in Comparative Example 4 in Table 1 above and amphoteric surfactant 2, lauryl hydroxysulfobetaine (carbon chain number: 12) shown in Table 2 above, were used.
Methanesulfonic acid as a free acid and thiodiethanol as a complexing agent were mixed and dissolved in an aqueous solution of tin methanesulfonate, and then an aqueous solution of silver methanesulfonate was added and mixed. After mixing to form a uniform solution, surfactant 1, surfactant 2, and benzalacetone as a brightener were added. Finally, ion-exchanged water was added to prepare a SnAg plating bath having the following composition. The aqueous solution of tin methanesulfonate and the aqueous solution of silver methanesulfonate were prepared by electrolyzing a metal tin plate and a metal silver plate, respectively, in an aqueous solution of methanesulfonic acid.

(Composition of SnAg plating solution)
Sn methanesulfonate (as Sn ²⁺ ): 50 g/L
Methanesulfonic acid Ag (as Ag ⁺ ): 0.2 g/L
Methanesulfonic acid (as free acid): 100 g/L
Thiodiethanol (as a complexing agent): 5 g/L
Nonionic surfactant 1: 5 g/L
Amphoteric surfactant 2: 2 g/L
Benzalacetone (as a brightener): 10 mg/L
Ion-exchanged water: balance

Example 7
A SnAg plating bath having the same composition as in Example 6 was prepared, except that amphoteric surfactant 2 in Example 6 was replaced with lauryl sulfobetaine.

Example 8
A SnAg plating bath having the same composition as in Example 6 was prepared, except that amphoteric surfactant 2 in Example 6 was replaced with stearyl sulfobetaine.

Example 9
A SnAg plating bath having the same composition as in Example 6 was prepared, except that Ag methanesulfonate and the nonionic surfactant were omitted.

<Comparative testing and evaluation>
Using each of the 14 types of plating solutions prepared in Examples 1 to 9 and Comparative Examples 1 to 5, (1) the maximum value of the particle size distribution of bubbles emitted from the insoluble anode, (2) the incidence of pit defects after plating, and (3) the incidence of voids in the bump were measured by the following methods.
The results are shown in Table 1 above and Table 3 below.

(1) Measurement of the Maximum Value of the Particle Size Distribution of Bubbles Emitted from an Insoluble Anode First, the apparatus for measuring the maximum value of the particle size distribution of bubbles will be described with reference to FIG. 3 . The plating tank 11 was a tank for performing face-down plating (cup type, fountain type) and was cylindrical with an inner diameter of 35 cm and a depth of 50 cm. When the prepared plating solution 12 was placed in the tank, the height from the bottom to the liquid surface was 35 cm. A 300 mm diameter Pt/Ti disk, consisting of a 1.5 mm thick Ti plate clad with a 3 μm thick Pt layer, was used as the insoluble anode (positive electrode) 13 and was installed in contact with the bottom of the plating tank 11. A pipe 11a serving as the suction port of a circulation pump 11c was installed 5 cm above the bottom of the plating tank 11. A return pipe 11b was installed on the side wall opposite the pipe 11a, 5 cm above the bottom. The circulation pump 11c was connected to the return pipe 11b via pipe 11d. A sampling pipe 11e was branched off from this pipe 11d, and a transparent measurement cell 11f was installed midway along the sampling pipe 11e. A VisiSize particle size distribution analyzer (manufactured by Oxford Lasers, model number: SF, analysis software: SOLO) 11g was installed in this measurement cell 11f to measure the size (particle size) of bubbles passing through the measurement cell. The inner diameters of pipes 11a, 11b, and 11d were 60 mm, and the inner diameter of the sampling pipe 11e was 40 mm.

After the plating solution 12 was poured into the plating tank 11, a 300 mm diameter silicon wafer 14 was secured in the plating tank 11 using a cylindrical jig (not shown) with an outer diameter of 33 cm, as shown in Figure 3. Specifically, the wafer 14 was immersed and secured so that its underside was 2 cm below the surface of the plating solution 12. The wafer 14 was connected to the cathode, and the plating solution 12 was agitated by rotating it horizontally at a speed of 50 rpm to prevent the generation of bubbles due to agitation.

To distinguish between bubbles generated by the circulation of the plating solution and oxygen bubbles generated by the insoluble anode due to electrolysis, the circulation pump 11c was operated for 10 minutes without electrolysis (plating). Pipes 11d and 11e were filled with plating solution 12. The circulation pump 11c was then stopped and the solution was left to stand for one hour for degassing. The plating solution was then circulated by the pump for one hour, and it was confirmed that the number of bubbles with a diameter of 10 μm or less passing through the measurement cell had dropped to 100/mL or less. The flow rate in pipe 11d was controlled to 5 L/min, and the flow rate in sampling pipe 11e was controlled to 0.5 L/min, while the plating solution 12 was circulated. The temperature of plating solution 12 was then set to 25°C, and the anode current density was set to 0.7 ASD, and electroplating was performed for 30 minutes.

During electrolytic plating, oxygen bubbles 18 were generated from the insoluble anode 13, as shown in Figure 3. The bubbles 18 were sucked into the suction port pipe 11a, passed through pipes 11d and 11e, and floated up through the plating solution toward the wafer 14 via the return port 11b. The size (particle size) of the bubbles 18 passing through the measurement cell 11f was measured using the particle size distribution measuring device 11g described above. The maximum values of the particle size distribution of the measured bubbles are shown in Tables 1 and 3 above. If the particle size distribution has two or more maximum values (i.e., two or more peaks), the largest peak is taken as the maximum value.

(2) Measurement of Pit Defect Incidence Rate After Plating A 0.1 μm titanium layer and a 0.3 μm copper layer were deposited in this order on the surface of a 300 mm diameter silicon wafer by sputtering to form an electrically conductive seed layer, and a dry film resist (50 μm thick) was deposited on the seed layer. The dry film resist was then partially exposed to light through an exposure mask and subsequently developed. Thus, as shown in FIG. 4 , a resist layer 5 having a pattern in which 1.6 million vias 6, which are openings with a diameter of 75 μm, were formed at a 150 μm pitch on the surface of the wafer 4 was formed.

Using the plating apparatus shown in Figure 1, the plating solution temperature was set to 25°C and the anode current density to 0.7 ASD, and vias 6 in resist layer 5 were electrolytically plated to a target plating thickness of 75 μm. Next, wafer 4 was removed from plating tank 1, washed, and dried, and then resist layer 5 was stripped using an organic solvent. In this way, a bumped wafer was produced on one die, with a pattern of 1.6 million 75 μm diameter bumps arranged at equal intervals with a 150 μm pitch.

Figure 5(a) shows the wafer 4 before plating, and Figures 5(b) to 5(e) show the wafer 4 after plating. In Figure 5, the same elements as in Figures 1, 2, and 4 are designated by the same reference numerals. In Figures 5(b) to 5(e), reference numeral 7 denotes bumps, which are plating deposition layers. The heights of 1.6 million bumps on this wafer were measured using an automated visual inspection system (Camtek, Model Falcon). The pit defect occurrence rate in the bumps was calculated from the measured bump heights using the following formula. As shown in Figure 5(b), bumps that were plated thicker than the surface of the resist 5 and had a mushroom-shaped cross section were counted as "normal bumps." As shown in Figures 5(c) and 5(d), undeveloped bumps that did not reach the surface of the resist 5 and had a non-mushroom-shaped cross section (plating film thickness less than 50 μm, i.e., less than approximately 67% of the target plating film thickness) were counted as defective bumps with "pit defects." The pit defect occurrence rate was then calculated using the following formula. The results are shown in Tables 1 and 3 above.
Pit defect occurrence rate (ppm) = (number of defective bumps/total number of bumps) x 10 ⁶

(3) Measurement of Void Generation Rate in Bumps After calculating the pit defect generation rate, the void generation rate was measured. The electrically conductive seed layer of the plated wafer was removed by etching, and the wafer was then reflowed. Figure 5(e) shows a void 9a formed in a bump 7 before reflow. Using a transmission X-ray device (manufactured by Dage), 5,000 bumps on the reflowed wafer were observed from the top to check for the presence of voids. Here, bumps with a void area of 1% or more relative to the bump area (the maximum horizontal cross-sectional area of the bump) were counted as "bumps with voids," and the void generation rate was calculated based on the following formula.
Void occurrence rate (%) = (number of bumps with voids/total number of bumps) x 10 ²
The results are shown in Tables 1 and 3 above.

As is clear from Table 1, in the plating solution of Comparative Example 1, the maximum value of the particle size distribution of bubbles released from the insoluble anode was a small value of 51 μm. As a result, out of a total of 1.6 million bumps, 762 had pit defects, resulting in a high pit defect rate of 476 ppm. In addition, there were four bumps with voids with a void area of 1% or more, resulting in a void rate of 0.1% for a total of 5,000 bumps.

Furthermore, in the plating solution of Comparative Example 2, the maximum value of the bubble particle size distribution was small, at 40 μm. As a result, the number of bumps with pit defects was 857, and the pit defect occurrence rate was high at 536 ppm. Furthermore, the number of bumps with voids with a void area of 1% or more was 53, and the void occurrence rate was 1.1%.

Furthermore, in the plating solution of Comparative Example 3, the maximum value of the bubble particle size distribution was small, at 25 μm. As a result, the number of bumps with pit defects was 1,073, and the pit defect occurrence rate was high, at 671 ppm. Furthermore, the number of bumps with voids with a void area of 1% or more was 126, and the void occurrence rate was 2.5%.

Furthermore, in the plating solution of Comparative Example 4, the maximum value of the bubble particle size distribution was small at 83 μm. As a result, the number of bumps with pit defects was 135, and the pit defect occurrence rate was high at 84 ppm. Furthermore, the number of bumps with voids with a void area of 1% or more was 6, and the void occurrence rate was 0.1%.

Furthermore, the plating solution of Comparative Example 5 contained a surfactant with the structural formula PO-EO-PO, but the maximum value of the bubble particle size distribution was a small 62 μm. As a result, the number of bumps with pit defects was 387, and the pit defect occurrence rate was high at 242 ppm. Furthermore, the number of bumps with voids with a void area of 1% or more was 18, and the void occurrence rate was 0.4%.

In contrast, as shown in Tables 1 and 3, the plating solutions of Examples 1 to 9 had a large bubble size distribution with a maximum value of 150 μm or more, and therefore the pit defect occurrence rate in bumps formed using these plating solutions was extremely low, ranging from 0 ppm to 15 ppm. The void occurrence rate was also 0%.

These results confirm that, according to the present invention, even when bump electrodes are miniaturized to a diameter of 100 μm or less and the depth relative to the diameter of the vias in the resist pattern has a high aspect ratio, gas bubbles generated in the plating solution do not hinder plating deposition, reducing the incidence of pit defects and voids in tin-containing bumps.

The plating solution of the present invention can be used to form parts of electronic components such as bump electrodes on semiconductor wafers and printed circuit boards. Therefore, it has industrial applicability.

1 Plating tank 1a Suction port 1b Return port 1c Circulation pump 2 Plating solution 3 Insoluble anode 4 Wafer (member to be plated)
5 resist layer 6 via (opening)
7 Bump (plated deposit layer)
8 Air bubbles 9a Pit defects 9b Voids

Claims (5)

An electrolytic plating solution comprising (A) a soluble salt containing at least a stannous salt, (B) an acid or a salt thereof (excluding soluble salts containing at least a stannous salt) , and (C) a surfactant,
When electrolysis is performed at an anode current density of 0.7 A/ ^dm2 , the maximum value of the particle size distribution of bubbles released from the insoluble anode is 150 μm or more,
the surfactant (C) is a nonionic surfactant of a polyoxyethylene polyoxypropylene block polymer (PO-EO-PO) having polyoxypropylene (PO) groups at its terminals,
the EO ratio in the block polymer (PO-EO-PO) is in the range of 35% or more and 50% or less by molar ratio,
The electrolytic plating solution is characterized in that the mass average molecular weight of the nonionic surfactant is in the range of 3,000 or more and 5,000 or less . An electrolytic plating solution comprising (A) a soluble salt containing at least a stannous salt, (B) an acid or a salt thereof (excluding soluble salts containing at least a stannous salt), and (C) a surfactant,
When electrolysis is performed at an anode current density of 0.7 A/dm2 ^, the maximum value of the particle size distribution of bubbles released from the insoluble anode is 150 μm or more,
the surfactant (C) is an amphoteric surfactant such as an alkyl sulfobetaine or an alkyl hydroxy sulfobetaine ;
the number of carbon atoms in the alkyl group of the amphoteric surfactant is 10 or more and 22 or less, and the content of the surfactant (C) is in the range of 0.5 g/L or more and 10 g/L or less . 3. The electroplating solution according to claim 1, wherein the content of the surfactant (C) is in the range of 0.5 g/L or more and 10 g/L or less. A method for electrolytically plating a member to be plated, comprising supplying the electrolytic plating solution according to claim 1 to a plating tank in which an insoluble anode is disposed opposite a member to be plated, the member being connected to a cathode. 5. The electrolytic plating method according to claim 4 , wherein the electrolytic plating is carried out using a plating tank in which the member to be plated and the insoluble anode are arranged horizontally.