ELECTROLYTIC TIN PLATING SOLUTION

Abstract

An electrolytic tin plating solution contains a compound serving as a source of supply of tin ions and an unsaturated aldehyde compound having a heterocyclic group.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2018-177797 filed on Sep. 21, 2018, the entire disclosure of which is incorporated by reference herein.

BACKGROUND ART

The present disclosure relates to an electrolytic tin plating solution, and more particularly relates to an electrolytic tin plating solution which can reduce voids formed after reflow and which is suitable for forming a bump.

A connection bump is provided on a semiconductor chip. A solder ball or any other suitable structure is used as a connection bump. However, miniaturization of semiconductor chips increases the difficulty in using a known solder ball as the connection bump. Although microballs having a diameter of about 100 μm may also be used, finer design rules have been required. Attention has been given to the formation of a bump plated with tin (Sn) or a Sn alloy (see, for example, Japanese Unexamined Patent Publication No. 2016-106181).

If a minute bump is formed by plating, a plurality of films having a thickness of several tens of micrometers and independent of one another need to be uniformly deposited in the several-tens-of-micrometer range. To improve the joint reliability, voids to be formed need to be reduced. Furthermore, even if a step is present on a BVH (blind via hole) forming part of a film, a uniform film needs to be deposited.

SUMMARY OF THE INVENTION

However, an electrolytic tin plating solution suitable for forming a minute bump has not been present yet. In particular, even if a step is formed on the BVH, a uniform bump needs to be formed without any void.

The present disclosure attempts to provide an electrolytic tin plating solution suitable for forming, e.g., bumps.

An electrolytic tin plating solution according to one aspect of the present disclosure contains a compound serving as a source of supply of tin ions and an unsaturated aldehyde compound having a heterocyclic group. Since the electrolytic tin plating solution according to the aspect contains the unsaturated aldehyde compound having the heterocyclic group, a uniform film containing grains having a small grain size can be formed.

In the electrolytic tin plating solution according to the aspect, the heterocyclic group may be a five- or six-membered heterocyclic group containing at least one of an oxygen atom, a nitrogen atom, or a sulfur atom.

In the electrolytic tin plating solution according to the aspect, a content of the unsaturated aldehyde compound having the heterocyclic group may be from 0.01 mmol/L to 10 mmol/L.

In the electrolytic tin plating solution according to the aspect, grains to be deposited may have a maximum grain size of 10 μm or smaller.

An electrolytic tin plating solution according to an aspect of the present disclosure allows a uniform film containing grains having a small grain size to be formed.

DESCRIPTION OF EMBODIMENTS

An electrolytic tin plating solution according to an embodiment contains an unsaturated aldehyde compound having a heterocyclic group (hereinafter referred to as a heterocycle-containing unsaturated aldehyde compound). The heterocycle-containing unsaturated aldehyde compound functions as a crystal regulator, which reduces the grain size of Sn deposit by electrolytic plating. Reducing the grain size allows for formation of a dense Sn deposit having less voids. Further, the electrolytic tin plating solution containing the heterocycle-containing unsaturated aldehyde compound allows for formation of a Sn deposit having a uniform thickness also on a BVH having a step.

The unsaturated aldehyde is aldehyde having a straight chain or a branched chain of molecules each having one or more unsaturated bonds, and may have a group other than a heterocyclic group. Examples of the unsaturated aldehyde include acrolein, methacrolein, crotonaldehyde, 2-methylcrotonaldehyde, 2-ethylcrotonaldehyde, 2-ethylacrolein, 2-ethyl-2-hexenal, citronellal, 2,3-dimethyl-2-propenal, undecylenic aldehyde, 4-heptenal, 2-hexenal, 2-undecenal, 2-nonenal, 2-formylpropenenitrile, 3-ethoxy-2-methyl-2-propenal, 4-hydroxy-2-nonenal, citronellyloxyacetaldehyde, 2-heptenal, 2-octenal, 2-decenal, 2,4-nonadienal, 2,6-nonadienal, 2,4-octadienal, 2,4-decadienal, and farnesal. The unsaturated aldehyde may include a stereoisomer, which may be any stereoisomer or include various stereoisomers.

The heterocyclic group should not be specifically limited, but may be a five-, six-, or seven-membered nitrogen-, oxygen-, or sulfur-containing heterocyclic group, or a nitrogen-, oxygen-, or sulfur-containing polycyclic group. Examples of the heterocyclic group or the polycyclic group include a five-membered ring heterocyclic group, such as a pyrrolidine group, a pyrrole group, a tetrahydrofuran group, an oxolane group, a furan group, a tetrahydrothiophene group, a thiolane group, a thiophene group, an imidazole group, a pyrazole group, an imidazoline group, an oxazole group, a thiazole group, a thiazolidine group, a triazole group, a tetrazole group, a dioxolane group, an oxadiazole group, and a thiadiazole group, a six-membered ring heterocyclic group, such as a piperidine group, an azinane group, a tetrahydropyran group, an oxane group, a tetrahydrothiopyran group, a pyridine group, a pyrane group, a thiopyran group, a pyrimidine group, a pyrazine group, a pyridazine group, a thiazine group, a morpholine group, a dioxane group, a dithiin group, a thiomorpholine group, a trithiane group, and a dithiazine group, a seven-membered ring heterocyclic group, such as a thiazepine group, and a polycyclic group, such as an indole group, an isoindole group, an indolizine group, a benzimidazole group, a benzotriazole group, a purine group, a quinoline group, an isoquinoline group, a quinazoline group, a quinoxaline group, a cinnoline group, a phthalazine group, a chromene group, an isochromene group, a benzodioxole group, a benzodioxan group, a benzoxazole group, a benzothiazole group, a pteridine group, a phenothiazine group, a phenanthridine group, and a thianthrene group. The heterocyclic group may include a constitutional isomer and a stereoisomer, which may be any isomer or include various isomers.

These unsaturated aldehydes and these heterocyclic groups may be optionally combined together. Examples of the resultant compound include 3-(2-furyl)acrolein, 2-methyl-3-(2-furyl)propenal, 3-(5-nitro-2-furyl)acrolein, (4-pyridyl)acrolein, 1,3-benzodioxole-5-acrolein, 3-[3-(4-fluorophenyl)-1-isopropylindol-2-yl]acrolein, 5-hydroxytetrahydrofuran-2-acrolein, 5-(2-furyl)-2,4-pentadienal, 2-formyl-3-(2-furyl)propenenitrile, 2-cyano-3-(2-furyl)propenal, 3-(4-pyridyl)propenal, pyridine-3-propenal, 3-(1H-indole-3-yl)propenal, 3-(3,4-diethyl-2-pyrrolyl)propenal, 3-(2-thienyl)acrolein, and 3-(isoindoline-2-yl)propenal. In particular, (2-furyl)acrolein, (4-pyridyl)acrolein, and 1,3-benzodioxole-5-acrolein are preferably used for reasons such as cost, ease of availability, and stability. The heterocycle-containing unsaturated aldehyde compound may include a constitutional isomer and a stereoisomer, which may be any isomer or include various isomers. One or more types of the heterocycle-containing unsaturated aldehyde compound may be contained in the plating solution.

The concentration of the heterocycle-containing unsaturated aldehyde compound in the electrolytic tin plating solution according to this embodiment is preferably from 0.01 mmol/L to 10 mmol/L and more preferably from 0.1 mmol/L to 1 mmol/L, to maintain a small grain size and to make the film deposited on the step uniform.

To reduce the formation of voids, the grain size of a maximum one of grains forming the Sn deposit formed using the electrolytic tin plating solution according to this embodiment is preferably 10 μm or smaller and more preferably 9 μm or smaller. The maximum one of the grains preferably has a smaller grain size. However, an actually possible grain size range of the maximum grain is preferably 0.1 μm or larger, more preferably 0.5 μm or larger, still more preferably 1 μm or larger, and yet more preferably 3 μm or larger. A minimum one of the grains preferably has a smaller grain size. However, an actually possible grain size range of the minimum grain is, but not limited to, preferably 0.1 μm or larger, more preferably 0.5 μm or larger, and still more preferably 1 μm or larger.

The electrolytic tin plating solution according to this embodiment contains a compound serving as a source of supply of tin (Sn) ions, in addition to the heterocycle-containing unsaturated aldehyde compound. Examples of the compound serving as the source of supply of Sn ions include a tin salt. In particular, a first tin salt (tin salt (II)) and a second tin salt (tin salt (IV)) are preferably used.

Non-limiting examples of the first tin salt (tin salt (II)) include tin (II) alkanesulfonate, such as tin (II) methanesulfonate, organic tin (II) sulfonate, such as tin (II) alkanolsulfonate such as tin (II) isethionate, tin (II) sulfate, tin (II) fluoroborate, tin (II) chloride, tin (II) bromide, tin (II) iodide, tin (II) oxide, tin (II) phosphate, tin (II) pyrophosphate, tin (II) acetate, tin (II) citrate, tin (II) gluconate, tin (II) tartrate, tin (II) lactate, tin (II) succinate, tin (II) sulfamate, tin (II) formate, and tin (II) silicofluoride.

Non-limiting examples of the second tin salt (tin salt (IV)) include sodium stannate and potassium stannate.

In particular, tin (II) alkanesulfonate, such as tin (II) methanesulfonate, and organic tin (II) sulfonate, such as tin (II) alkanolsulfonate such as tin (II) isethionate are preferably used.

To reduce burning, the concentration of tin salt as Sn²⁺ is preferably 5 g/L or higher and more preferably 10 g/L or higher. To improve the stability of plating solution and reduce precipitation, the concentration of the tin salt is preferably 120 g/L or lower and more preferably 90 g/L or lower. This concentration also helps reduce cost.

Tin salt having a low lead (Pb) concentration of 1.0 ppm or lower may be used as the tin salt. Using tin salt having a low Pb concentration allows the plating solution to have a lower Pb concentration.

The electrolytic tin plating solution according to this embodiment may contain any one of an inorganic acid, an organic acid, and soluble salts of these acids. Adding the acid or the soluble salt to the plating solution allows the pH of the surface of a plated object and the pH of the surface of Sin forming the Sn deposit to be kept constant, thus providing a uniform surface potential. This can retard the eutectoid reaction of Pb.

Non-limiting examples of the acid or the soluble salt of the acid include a sulfuric acid, a hydrochloric acid, a nitric acid, a phosphoric acid, a sulfamic acid, an organic sulfonic acid (an alkanesulfonic acid, such as a methanesulfonic acid, or an alkanolsulfonic acid, such as an isethionic acid), and a carboxylic acid (an aromatic carboxylic acid, an aliphatic saturated carboxylic acid, or an amino carboxylic acid). If necessary, a neutralized salt of any one of these soluble salts may be used. In particular, a methanesulfonic acid is preferably used for ease of handling.

To improve the stability of the plating solution and reduce the precipitation, the concentration of the acid or the soluble salt of the acid is preferably 50 g/L or higher and more preferably 100 g/L or higher. This concentration also helps substantially prevent an appropriate Pb deposition potential. To reduce cost, the concentration of the acid or the soluble salt is preferably 500 g/L or lower, more preferably 300 g/L or lower, and still more preferably 200 g/L or lower.

The electrolytic tin plating solution according to this embodiment may contain a surfactant. One or more selected from an anionic surfactant, a cationic surfactant, and a nonionic surfactant may be used as the surfactant. In particular, a nonionic surfactant is preferably used, and an alkylene oxide-based surfactant is more preferably used. Adding the surfactant to the plating solution allows the surface of a plated object and the surface of a Sn crystal forming a film to have a uniform current density, thus maintaining a uniform deposition potential at the surface. This can retard the eutectoid reaction of Pb.

Non-limiting examples of the alkylene oxide-based surfactant include polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl amine, polyoxyethylene alkyl amide, polyoxyethylene fatty acid ester, polyoxyethylene polyhydric alcohol ether, an ethylene oxide-propylene oxide block copolymer compound, an ethylene oxide-propylene oxide random copolymer compound, and a propylene oxide polymer compound. In particular, polyoxyethylene alkyl phenyl ether is preferably used.

The concentration of the surfactant is preferably 0.05 g/L or higher, and more preferably 0.5 g/L or higher. Even if plating is performed at a high current density to shorten the plating time, the surfactant having such a concentration or higher can reduce burning at an area having a high current density. To reduce color irregularities arising from blackening of the Sn deposit, the concentration of the surfactant is preferably 100 g/L or lower.

The electrolytic tin plating solution according to this embodiment contains an acid or a soluble salt thereof and a surfactant. The acid or the soluble salt thereof is more preferably one or more acids or soluble salts thereof selected from a sulfuric acid, a hydrochloric acid, a nitric acid, a phosphoric acid, a sulfamic acid, an organic sulfonic acid, a carboxylic acid, or salts of these acids. The surfactant is more preferably selected from one or more surfactants selected from an anionic surfactant, a cationic surfactant, and a nonionic surfactant.

The electrolytic tin plating solution according to this embodiment may contain an organic solvent, an antioxidant, and a chelating agent. Non-limiting examples of the organic solvent include monohydric alcohols, such as methanol and 2-propanol, and dihydric alcohols, such as ethylene glycol, diethylene glycol, and triethylene glycol. Non-limiting examples of the antioxidant include catechol, hydroquinone, 4-methoxyphenol, and ascorbic acid. Non-limiting examples of the chelating agent include oxalic acid, succinic acid, malonic acid, glycolic acid, gluconic acid, gluconolactone, glycine, ethylenediamine-acetic acid, pyrophosphoric acid, and tripolyphosphoric acid.

To form a Sn deposit using the electrolytic tin plating solution according to this embodiment, the pH of the plating solution is preferably strongly acidic. The temperature at which the Sn deposit is formed should not be specifically limited. However, the temperature is preferably from 25° C. to 40° C. The current density at which the Sn deposit is formed is preferably from 1 A/dm² to 20 A/dm², and more preferably from 2 A/dm² to 6 A/dm².

The electrolytic tin plating solution according to the present disclosure can be used, for example, to form plated bumps on a semiconductor chip. To form a plated bump, a Sn deposit having a predetermined size is formed at a predetermined position, and then a reflow process is performed. The reflow process should not be specifically limited, but may be performed using a known reflow apparatus.

EXAMPLES

The electrolytic tin plating solution according to the present disclosure will now be described in more detail with reference to examples. The following examples are illustrative, and are not intended to limit the present disclosure.

Formation of Sn Deposit

A substrate was electrolytically plated with Ni (an electrolytic nickel plating solution: Thru-Nic AMT, manufactured by C. Uyemura & Co., Ltd., liquid temperature: 50° C., current density: 1 A/dm², plating time: 10 min). A Sn deposit was formed on the Ni surface, where an electrolytic tin plating solution having a predetermined composition was set to have a liquid temperature of 30° C. and a current density of 4 A/dm².

Estimation of Grain Size

The grain size of Sn grains forming the resultant Sn-plated film was measured by an electron emission scanning electron microscope (JSM-7800F, manufactured by JEOL Ltd.). An IPF mapping image obtained under conditions of an accelerating voltage of 20 kV and an illumination current of 13 nA was analyzed to calculate a distribution range of the grain sizes of the Sn grains.

Evaluation of Void

The obtained Sn deposit was reflowed at 260° C., and then the presence or absence of a void was evaluated by an X-ray nondestructive testing system (XD7600NT Diamond FP, manufactured by Nordson DAGE). The X-ray nondestructive testing system had a tube voltage of 60 kV and an output of 1.5 W.

First Example

As a heterocycle-containing unsaturated aldehyde compound, 3-(2-furyl)acrolein was used. The concentration of the heterocycle-containing unsaturated aldehyde compound was 0.2 mmol/L. Tin (II) alkanesulfonate as a tin salt, methanesulfonic acid as an orgainc acid, and polyoxyethylene bisphenol A ether as a surfactant were added. The Tin (II) alkanesulfonate was added such that the concentration thereof was 70 g/L as Sn(Sn²⁺), and the methanesulfonic acid, and the polyoxyethylene bisphenol A ether in the compound were respectively 100 g/L, and 50 g/L. The resultant plating solution was set to have a liquid temperature of 30° C. and a current density of 4 A/dm².

Grain size of Sn deposit obtained had a maximum grain size of 8 μm and a minimum grain size of 1 μm. No voids were observed in the Sn deposit after reflow.

Second Example

A second example was similar to the first example except that (4-pyridyl)acrolein was used as a heterocycle-containing unsaturated aldehyde compound and that the concentration of the (4-pyridyl)acrolein was 0.8 mmol/L.

Grain size of Sn deposit obtained had a maximum grain size of 7 μm and a minimum grain size of 1 μm. No voids were observed in the Sn deposit after reflow.

Third Example

A third example was similar to the first example except that 1,3-benzodioxole-5-acrolein was used as a heterocycle-containing unsaturated aldehyde compound and that the concentration of the 1,3-benzodioxole-5-acrolein was 0.4 mmol/L.

Grain size of Sn deposit obtained had a maximum grain size of 6 μm and a minimum grain size of 1 μm. No voids were observed in the Sn deposit after reflow.

First Comparative Example

A first comparative example was similar to the first example except that no heterocycle-containing unsaturated aldehyde compound was added.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm and a minimum grain size of 4 μm. Voids were observed in the Sn deposit after reflow.

Second Comparative Example

A second comparative example was similar to the first example except that 1.0 mmol/L of benzaldehyde was added instead of a heterocycle-containing unsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm and a minimum grain size of 3 μm. Voids were observed in the Sn deposit after reflow.

Third Comparative Example

A third comparative example was similar to the first example except that 1.0 mmol/L of cinnamaldehyde was added instead of a heterocycle-containing unsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm and a minimum grain size of 3 μm. Voids were observed in the Sn deposit after reflow.

Fourth Comparative Example

A fourth comparative example was similar to the first example except that 0.4 mmol/L of acrylic acid was added instead of a heterocycle-containing unsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm and a minimum grain size of 3 μm. Voids were observed in the Sn deposit after reflow.

Fifth Comparative Example

A fifth comparative example was similar to the first example except that 0.8 mmol/L of acrolein was added instead of a heterocycle-containing unsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm and a minimum grain size of 3 μm. Voids were observed in the Sn deposit after reflow.

TABLE 1
	Examples	Comparative Examples
	1	2	3	1	2	3	4	5
Heterocycle-	3-(2-furyl)	(4-pyridyl)	1,3-benzodioxole-	None	(benz-	(cinnam-	(acrylic	(acrolein)
containing	acrolein	acrolein	5-acrolein		aldehyde)	aldehyde)	acid)
Unsaturated
Aldehyde
Compound
(mmol/L)	0.2	0.8	0.4	—	1.0	1.0	0.4	0.8
Grain Size	1-8	1-7	1-6	4-12	3-12	3-12	3-12	3-12
(μm)
Void	None	None	None	Formed	Formed	Formed	Formed	Formed

Table 1 summarizes the conditions and result of each of the examples and comparative examples. Using the electrolytic tin plating solution containing the heterocycle-containing unsaturated aldehyde compound reduced the grain size, and allowed for formation of uniform bumps without any void.

The electrolytic tin plating solution according to the present disclosure allows for formation of a Sn deposit having a uniform and small grain size, and is useful for forming bumps, for example.

Claims

1. An electrolytic tin plating solution comprising:

a compound serving as a source of supply of tin ions; and

an unsaturated aldehyde compound having a heterocyclic group.

2. The electrolytic tin plating solution of claim 1, wherein

the heterocyclic group is a five- or six-membered heterocyclic group containing at least one of an oxygen atom, a nitrogen atom, or a sulfur atom.

3. The electrolytic tin plating solution of claim 1, wherein

a content of the unsaturated aldehyde compound having the heterocyclic group in the electrolytic tin plating solution is from 0.01 mmol/L to 10 mmol/L.

4. The electrolytic tin plating solution of claim 1, wherein

grains to be deposited have a maximum grain size of 10 um or smaller.

5. The electrolytic tin plating solution of claim 1, wherein

the heterocyclic group is one, two, or more groups selected from a pyrrolidine group, a pyrrole group, a tetrahydrofuran group, an oxolane group, a furan group, a tetrahydrothiophene group, a thiolane group, a thiophene group, an imidazole group, a pyrazole group, an imidazoline group, an oxazole group, a thiazole group, a thiazolidine group, a triazole group, a tetrazole group, a dioxolane group, an oxadiazole group, a thiadiazole group, a piperidine group, an azinane group, a tetrahydropyran group, an oxane group, a tetrahydrothiopyran group, a pyridine group, a pyrane group, a thiopyran group, a pyrimidine group, a pyrazine group, a pyridazine group, a thiazine group, a morpholine group, a dioxane group, a dithiin group, a thiomorpholine group, a trithiane group, a dithiazine group, a thiazepine group, an indole group, an isoindole group, an indolizine group, a benzimidazole group, a benzotriazole group, a purine group, a quinoline group, an isoquinoline group, a quinazoline group, a quinoxaline group, a cinnoline group, a phthalazine group, a chromene group, an isochromene group, a benzodioxole group, a benzodioxan group, a benzoxazole group, a benzothiazole group, a pteridine group, a phenothiazine group, a phenanthridine group, and a thianthrene group.

6. The electrolytic tin plating solution of claim 1, wherein

the unsaturated aldehyde compound having the heterocyclic group is one, two, or more selected from 3 -(2-furyl)acrolein, 2-methyl-3-(2-furyl)propenal, 3-(5-nitro-2-furyl)acrolein, (4-pyridyl)acrolein, 1,3-benzodioxole-5-acrolein, 3-[3-(4-fluorophenyl)-1-isopropylindol-2-yl]acrolein, 5-hydroxytetrahydrofuran-2-acrolein, 5-(2-furyl)-2,4-pentadienal, 2-formyl-3-(2-furyl)propenenitrile, 2-cyano-3-(2-furyl)propenal, 3-(4-pyridyl)propenal, pyridine-3-propenal, 3-(1H-indole-3-yl)propenal, 3-(3,4-diethyl-2-pyrrolyl)propenal, 3-(2-thienyl)acrolein, and 3-(isoindoline-2-yl)propenal.