WO2019234088A1

WO2019234088A1 - Aqueous composition for depositing a tin silver alloy and method for electrolytically depositing such an alloy

Info

Publication number: WO2019234088A1
Application number: PCT/EP2019/064621
Authority: WO
Inventors: Himendra Jha; Ralf Schmidt; Christian Schwarz; Recep Kocer; Grigory VAZHENIN
Original assignee: Atotech Deutschland Gmbh
Priority date: 2018-06-08
Filing date: 2019-06-05
Publication date: 2019-12-12
Also published as: PT3578693T; TWI715991B; EP3578693A1; EP3578693B1; TW202000999A

Abstract

The present invention refers to aqueous composition for depositing a tin silver alloy, the composition comprising (a) tin ions, (b) silver ions, (c) at least one first compound independently selected from the group consisting of unsubstituted bis(aminophenyl)disulfides, substituted bis(aminophenyl)disulfides, unsubstituted dipyridyldisulfides, and substituted dipyridyldisulfides, (d) at least one second compound of formula (II), and salts thereof, wherein independently X denotes a C1 to C10 alkyl moiety comprising one or more than one sulfhydryl group, R¹ denotes hydrogen, methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substituted benzyl, and R² denotes methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substituted benzyl.

Description

Aqueous composition for depositing a tin silver alloy and method for electrolytically depos iting such an alloy

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Field of the Invention

The present invention relates to an aqueous composition for depositing a tin silver alloy, a method for electrolytically depositing such an alloy onto a substrate, and the use of said composition for electrolytically depositing tin silver alloy solder bumps or tin silver alloy solder caps onto pillars.

Background of the Invention

In modern packaging technologies a clear trend toward decreasing sizes is observed to address increased performance and improved functionality for electronic devices. Solder caps on copper pillars have turned out to be an interesting and promising alternative to conventional solder ball applications, in particular for Flip-Chip applications in microelec tronic devices.

Solder caps on copper pillars typically consist of two elements: (i) a structural element made of copper forming the pillar, e.g. a cylinder and (ii) a solder cap on top of said pillar. A large number of such capped copper pillars are typically arranged on a die, which is part of a wafer. Compared to conventional solder ball applications, solder caps on copper pil lars provide an improved thermal and electrical behaviour due to the comparatively large volume of copper in the capped pillar, which provides an excellent electrical connection and increased conductivity. The solder cap provides an electrical as well as mechanical connection between the respective pillar and a corresponding feature, for example a pillar of another die.

An additional advantage of solder caps on copper pillars is the slim shape of the pillars compared to ball-like conventional solder ball applications. This allows a reduced distance (also called pitch) between two neighbouring pillars, which is a key element for higher packaging.

The solder cap typically includes tin and in many cases is a tin silver alloy; typically being free of lead. Such an alloy usually provides an excellent solderability and is a suitable al ternative to previously used lead-containing solder caps and solder balls, respectively.

Depositing such tin silver alloys is known in the art. For example, JP 2006-265573 A dis closes a tin silver alloy plating bath without cyanide, which improves solderability and ap- pearance of an electrodeposition film obtained from the tin silver bath. EP 0 854 206 B1 relates to an acidic tin silver alloy plating bath, which is substantially free of cyanide, and a method for electroplating tin silver alloy onto a substrate.

US 2014/0251818 A1 refers to a cyanide-free tin alloy plating solution having outstanding serial stability as well as a method of plating tin alloy onto an electroconductive object us- ing the tin alloy plating solution. The tin alloy plating solution contains tin ions and one or more additional metal ions of silver, copper, bismuth, indium, palladium, lead, zinc, or nickel, and peptides with cysteine residues.

WO 03/046260 A2 relates to an electrolysis bath for electrodepositing silver-tin alloys that, in addition to water serving as a solvent with a pH value of less than 1 .5, contains a water- soluble silver compound, a water-soluble tin compound and an organic complexing agent. In order to obtain a stable electrolysis bath that enables the homogenous deposition of a compact tin silver alloy with any type of composition, an aliphatic complexing agent having a sulfide group and an amino group is used as a complexing agent, whereby said func tional groups are bound to different carbon atoms.

EP 1 553 21 1 B1 relates to a tin-silver-copper plating solution comprising 30-90 wt% of water, a sulfonic acid, tin ions, copper ions and silver ions, wherein the concentration of the silver ions is 0.01 to 0.1 mol/L, the concentration of the tin ions is 0.21 to 2 mol/L, the concentration of the copper ions is 0.002 to 0.02 mol/L and the mole ratio of the silver-ions to the copper ions is in the range of 4.5 to 5.58.

US 6,607,653 B1 relates to a tin-copper alloy plating bath, tin-copper-bismuth alloy plating bath or tin-copper-silver alloy plating bath containing a soluble metal compound and a specific sulfur-containing compound.

EP 2 221 396 A1 relates to a composition comprising one or more sources of tin ions, one or more sources of alloying metal ions, the metal ions are selected from the group consist- ing of silver ions, copper ions and bismuth ions, one or more flavone compounds, and one or more compounds having a formula: HOR(R”)SR’SR(R”)OH wherein R, R’ and R” are the same or different and are alkylene radicals having 1 to 20 carbon atoms.

Despite known approaches, depositing a tin silver alloy, typically carried out by means of electrolytic deposition, requires highly sophisticated and accurate deposition methods to obtain solder caps with a high measure of uniformity. Only then the appearance of den drites on the pillar can be effectively reduced or even completely avoided. Such dendrites are irregular, non-uniform solder caps with the potential to even bridge two adjacent pillars. In such a case the entire die carrying a high number of pillars with solder caps might be come useless and needs to be thrown away. Suitable methods and compositions, respec- tively, to obtain copper pillars with basically dendrite-free solder caps are still missing and highly demanded to decrease the number of defective goods.

Objective of the present Invention

It is an objective of the present invention to provide an aqueous composition for depositing a tin silver alloy, which overcomes the above mentioned problems. It is in particular an objective to obtain a tin silver alloy with high uniformity such that for example very uniform solder caps on pillars are obtained and dendrites (as defined above) are basically avoided; in particular at comparatively high current densities in the range from 20 A/dm² to 30 A/dm². It is furthermore an objective that said tin silver alloy can also be utilized to form solder bumps having above mentioned advantages.

It is an additional objective of the present invention to provide also a respective method for electrolytically depositing such a tin silver alloy, in particular for electrolytically depositing tin silver alloy solder bumps and tin silver alloy solder caps on pillars such that the occur rence of dendrites is suppressed for both solder caps and solder bumps.

Summary of the Invention

These objectives are solved by an aqueous composition for depositing a tin silver alloy, the composition comprising

(a) tin ions,

(b) silver ions,

(c) at least one first compound independently selected from the group consisting of unsubstituted bis(aminophenyl)disulfides, substituted bis(aminophenyl)disulfides, unsubstituted dipyridyldisulfides, and substituted dipyridyldisulfides,

(d) at least one second compound of formula (II)

(II), and salts thereof,

wherein independently

X denotes a C1 to C10 alkyl moiety comprising one or more than one sulfhydryl group, R¹ denotes hydrogen, methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substituted benzyl, and

R² denotes methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substi tuted benzyl.

Furthermore, the additional objective is solved by a method for electrolytically depositing a tin silver alloy onto a substrate, the method comprising the steps

(A) providing the substrate,

(B) providing an aqueous composition according to the present invention, prefera bly as described throughout the text as being preferred,

(C) contacting said substrate with said aqueous composition and supplying an electrical current such that said tin silver alloy is electrolytically deposited onto the substrate.

In addition, the objectives are solved by the use of an aqueous composition according to the present invention, preferably as described as being preferred throughout the present text, for electrolytically depositing tin silver alloy solder bumps or tin silver alloy solder caps onto metal pillars, preferably for electrolytically depositing tin silver alloy solder caps onto copper pillars.

Brief description of the figure

Figure 1 is a top view of a section of a die comprising a plurality of copper pillars with tin silver solder caps (shown as black dots), obtained according to the present invention. For further details see the examples below in the text. The silicon-based wafer coupon com prising the die is shown in light gray.

Figure 2 is a top view of a section of a die comprising a plurality of copper pillars with tin silver solder caps (shown as black dots), obtained according to prior art (JP 2006- 265573 A, see examples below). Again, the silicon-based wafer coupon comprising the die is shown in light gray. In the upper right corner, a dendrite formed on top of a cylindrical copper pillar is emphasized by the letter“A”.

Figure 3 is a diagram showing the complexing properties of various compounds for silver ions. On the x-axis the concentration in g/L of the respective compound is given, wherein on the y-axis the overpotential in dependence on the compound’s concentration is given. In the figure, letters have the following meaning: A: 2,2'-dithiodianiline, B: 2- (dimethylamino)ethanethiol, C: L-cysteine, D: glutathione, and E: cysteamine. Detailed Description of the Invention

Own experiments show that the aqueous composition, the method, and the use as defined above, result in excellent uniform tin silver solder caps. These caps can be excellently uti lized for metal pillars, in particular copper pillars (see examples below in the text). Fur- thermore, the formation of dendrites is impressively decreased or even entirely prevented.

In the context of the present invention, the term“at least one” denotes (and is exchangea ble with)“one, two, three or more than three”.

The term“independently” denotes for example that in the at least one second compound of formula (II) X, R¹, and R² in a first compound of formula (II) are selected independently from X, R¹, and R² in a second and further compound of formula (II) in the aqueous com position of the present invention. Furthermore, for example the at least one first compound is independently selected from a number of compound groups. This means that, if for ex ample two first compounds are selected, each compound can be selected from a different compound group, for example one compound is a substituted bis(aminophenyl)disulfide, the other is a substituted dipyridyldisulfide. In other words, the selection of the first com pound of said two first compounds is independent from the selection of the second first compound of said two first compounds.

The composition of the present invention is an aqueous composition, which means that water is the primary component. Thus, more than 50 wt-% of the aqueous composition is water, based on the total weight of the aqueous composition, preferably at least 60 wt-%, even more preferably at least 70 wt-%, most preferably 80 wt-% or more. It is preferred that the aqueous composition is substantially free of organic solvents; more preferably does not contain organic solvents at all. Furthermore, the aqueous composition is prefera bly an aqueous solution, i.e. is homogeneous and thus preferably does not contain any particles.

In the context of the present invention, the term“at least” in combination with a particular value denotes (and is exchangeable with) this value or more than this value. For example, “at least 70 wt-%“ denotes (and is exchangeable with)“70 wt-% or more than 70 wt-%”.

In the context of the present invention, the term“substantially free” of a subject-matter (e.g. a compound, a material, etc.) independently denotes that said subject-matter is not pre sent at all or is present only in (to) a very little and undisturbing amount (extent) without affecting the intended purpose of the invention. For example, such a subject-matter might be added or utilized unintentionally, e.g. as unavoidable impurity.“Substantially free” pref erably denotes 0 (zero) ppm to 50 ppm, based on the total weight of the aqueous composi- tion of the present invention, if defined for said aqueous composition, or based on the total weight of the tin silver alloy, if defined for said alloy; preferably 0 ppm to 25 ppm, more preferably 0 ppm to 10 ppm, even more preferably 0 ppm to 5 ppm, most preferably 0 ppm to 1 ppm. Zero ppm denotes that a respective subject-matter is not at all comprised.

Preferred is an aqueous composition of the present invention, wherein the composition is acidic, preferably the composition has a pH in the range from -2 to +4, more preferably in the range from -1 to +2, most preferably in the range from -0.5 to +1.2. Generally, acidic compositions, in particular having a pH range as defined above, are typically free of cya nide, which is desired for environmental reasons. Thus, also the composition of the pre sent invention is preferably free of cyanide. Furthermore, above mentioned pH ranges lead to an improved quality of the deposited tin silver alloy compared to a composition with a pH significantly higher than 4 or even being alkaline. Furthermore, the stability is increased if the pH is in the above defined pH ranges. In particular undesired plate out is observed if the pH is significantly higher than 4 or even alkaline. In order to prevent plate out in com positions having a pH significantly higher than 4 or even alkaline, typically very strong complexing agents are utilized such as environmentally hazardous cyanides. However, this can be successfully avoided in the composition and method of the present invention. As a result, waste water treatment is simplified and environmental issues are prevented. In the context of the present invention, pH is referenced to a temperature of 20°C.

The aqueous composition of the present invention is for depositing a tin silver alloy, pref- erably a tin silver alloy being substantially free of sulfur.

The aqueous composition of the present invention comprises (a) tin ions and (b) silver ions in order to deposit the tin silver alloy. In the aqueous composition the tin ions are prefera bly tin (II) ions and the silver ions preferably silver (I) ions.

Preferred is an aqueous composition of the present invention, wherein in the aqueous composition the tin ions are present in a total concentration in the range from 20 g/L to 200 g/L, based on the total volume of the aqueous composition, preferably in the range from 25 g/L to 150 g/L, more preferably in the range from 30 g/L to 120 g/L, even more preferably in the range from 35 g/L to 100 g/L, most preferably in the range from 40 g/L to 90 g/L. A concentration significantly below 20 g/L often results in an undesired low deposi- tion rate. If the concentration significantly exceeds 200 g/L problems with solubility are frequently observed. An optimal compromise of deposition rate and solubility is obtained in above defined preferred concentration ranges. Best results have been obtained with a concentration in the range from 35 g/L to 100 g/L and from 40 g/L to 90 g/L, respectively.

The tin ions are from a tin ion source. The tin ion source of said tin ions is preferably at least one tin salt, more preferably at least one inorganic tin salt and/or at least one organic tin salt. Preferred inorganic tin salts are selected from the group consisting of tin oxide, tin sulfate, and tin sulfide. Preferred organic tin salts are selected from the group consisting of tin acetate, tin citrate, tin oxalate, and tin alkyl sulfonates. More preferred is an aqueous composition of the present invention, wherein the tin ions are from at least one organic tin salt, preferably from at least one tin alkyl sulfonate. In the aqueous composition of the pre sent invention most preferred is tin methane sulfonate. Preferably in each tin ion source tin is present as tin (II).

Preferred is an aqueous composition of the present invention, wherein in the aqueous composition the silver ions are present in a total concentration in the range from 0.1 g/L to 10 g/L, based on the total volume of the aqueous composition, preferably in the range from 0.2 g/L to 8 g/L, more preferably in the range from 0.3 g/L to 6 g/L, even more preferably in the range from 0.4 g/L to 4 g/L, most preferably in the range from 0.5 g/L to 2 g/L. A con centration significantly below 0.1 g/L often results in an insufficient amount of silver in the tin silver alloy leading to undesired mechanical and electrical properties in respective sol- der caps and solder bumps. If the concentration significantly exceeds 10 g/L the composi tion is not sufficiently stable and plate out is sometimes observed. In Addition, too much silver is usually incorporated into the tin silver alloy, significantly increasing the melting point of a respective solder cap, which is undesired in subsequent reflow processes. Opti mal melting points are obtained in the above defined preferred concentration ranges, most preferably in the ranges from 0.4 g/L to 4 g/L and 0.5 g/L to 2 g/L, respectively.

The silver ions are from a silver ion source. The silver ion source of said silver ions is pref erably at least one silver salt, more preferably at least one inorganic silver salt and/or at least one organic silver salt. Preferred inorganic silver salts are selected from the group consisting of silver oxide, silver sulfate, and silver nitrate. Preferred organic silver salts are selected from the group consisting of silver acetate, silver citrate, silver oxalate, and silver alkyl sulfonates. More preferred is an aqueous composition of the present invention, wherein the silver ions are from at least one organic silver salt, preferably from at least one silver alkyl sulfonate. In the aqueous composition of the present invention most preferred is silver methane sulfonate. Preferably in each silver ion source silver is present as silver (I). Most preferred, the silver ion source as well as the tin ion source comprise alkylsulfonates, more preferably are alkylsulfonates. This is in particular preferred if the pH in the aqueous composition of the present invention is (re)adjusted by means of alkylsulfonic acids. In this case only one type of organic acid anion is present in the composition of the present inven tion. Preferred is an aqueous composition of the present invention, wherein the molar ratio of the tin ions to the silver ions is in the range from 200:1 to 5:1 , preferably in the range from 150:1 to 10:1 , more preferably in the range from 125:1 to 12:1 , most preferably in the range from 100:1 to 15:1. If the molar ratio is significantly exceeding 200:1 usually silver is insufficiently incorporated into the tin silver alloy. If the molar ratio is significantly below 5:1 in some cases the tin silver alloy comprises too much silver. However, if the molar ratio is as defined above, in particular in the range from 125:1 to 12:1 and from 100:1 to 15:1 , re spectively, an excellent uniform tin silver alloy is obtained and dendrites are very much suppressed in solder caped pillars.

Very preferred is a composition of the present invention, wherein the composition is sub stantially free of, preferably does not comprise, halide ions, most preferably chloride ions. First, silver chloride is very insoluble in an aqueous environment. Second, it has been ob served in some cases that in particular chloride ions, although present in low amounts and not necessarily leading to immediate precipitation of insoluble chlorides, result in deposi- tion defects in the tin silver alloy. Thus, it is preferred to not utilize tin ion sources and silver ion sources comprising halide anions, most preferably chloride ions.

In some cases an aqueous composition of the present invention is preferred, wherein the composition additionally comprises copper (II) ions, preferably in a total concentration from 0.06 g/L to 5 g/L, based on the total volume of the aqueous composition, more preferably in a total concentration from 0.3 g/L to 4 g/L, even more preferably in a total concentration from 0.8 g/L to 3 g/L. Such a composition is for basically depositing a tin silver copper al loy.

In some of these cases an aqueous composition of the present invention is preferred, wherein the composition comprises copper (II) ions in a total concentration from 0.06 g/L to 0.6 g/L, based on the total volume of the aqueous composition, preferably from 0.2 g/L to

0.5 g/L, more preferably from 0.3 g/L to 0.4 g/L. In such cases copper is incorporated into the tin silver alloy only to a very limited extent.

In alternative cases an aqueous composition of the present invention is preferred, wherein the composition comprises copper (II) ions in a total concentration from 0.7 g/L to 5 g/L, based on the total volume of the aqueous composition, preferably from 1 g/L to 4.2 g/L, more preferably from 1 .3 g/L to 3.6 g/L, even more preferably from 2.1 g/L to 3.1 g/L. In such cases copper is incorporated into the tin silver alloy to a more significant extent.

However, in most cases it is in particular desired to not include copper ions in the aqueous composition of the present invention such that the obtained tin silver alloy is substantially free of, preferably does not comprise, copper. Thus, in such cases an aqueous composi- tion of the present invention is preferred, wherein the composition is substantially free of, preferably does not comprise, copper ions, preferably is substantially free of, preferably does not comprise, copper ions and bismuth ions.

Preferred is an aqueous composition of the present invention being substantially free of, preferably not comprising, nickel ions, zinc ions, iron ions, lead ions, and aluminium ions.

Preferably, said tin ions and said silver ions are the only depositable metal ions in the aqueous composition. This means that these metal ions are the only metal ions that are deposited in the tin silver alloy. Preferred is an aqueous composition of the present inven tion, wherein in the aqueous composition the weight of said tin ions and said silver ions in total represents 80 wt.-% to 100 wt.-% of all group 3 to group 15 metal cations in the aqueous composition, based on the total weight of all group 3 to group 15 metal cations in the aqueous composition, preferably at least 90 wt.-%, more preferably at least 95 wt.-%, even more preferably at least 98 wt.-%, most preferably at least 99 wt.-%. Group 3 to 15 refers to the 18 groups in the periodic table of elements.

The aqueous composition of the present invention comprises, besides (a) tin ions and (b) silver ions, (c) at least one (preferably one) first compound (as described throughout the present text) and additionally (d) at least one (preferably one) second compound of formu la (II).

The at least one first compound is independently selected from the group consisting of unsubstituted bis(aminophenyl)disulfides, substituted bis(aminophenyl)disulfides, unsubsti tuted dipyridyldisulfides, and substituted dipyridyldisulfides, preferably is independently selected from the group consisting of unsubstituted bis(aminophenyl)disulfides, substituted bis(aminophenyl)disulfides, and unsubstituted dipyridyldisulfides, more preferably is inde pendently selected from the group consisting of unsubstituted bis(aminophenyl)disulfides and substituted bis(aminophenyl)disulfides, most preferably is independently selected from the group consisting of unsubstituted bis(aminophenyl)disulfides. In the context of the pre sent invention, unsubstituted denotes that no additional substituents (i.e. functional groups) are included, wherein substituted denotes that additional substituents (i.e. functional groups) are present.

In the unsubstituted bis(aminophenyl)disulfides preferably each phenyl moiety comprises a single amino group. Preferably, the unsubstituted bis(aminophenyl)disulfides are selected from the group consisting of 4,4’-diaminodiphenyl disulfide (also called 4,4'-dithiodianiline) and 2,2’-diaminodiphenyl disulfide (also called 2,2'-dithiodianiline). More preferred is an aqueous composition of the present invention, wherein the at least one first compound is 2,2’-diaminodiphenyl disulfide. Most preferred is an aqueous composition of the present invention, wherein 2,2’-diaminodiphenyl disulfide is the only first compound.

The unsubstituted dipyridyldisulfides are preferably selected from the group consisting of 4,4'-dipyridyldisulfide and 2,2'-dipyridyldisulfide.

Preferred is an aqueous composition of the present invention, wherein the substituted bis(aminophenyl)disulfides and substituted dipyridyldisulfides have at least one substituent independently selected from the group consisting of C1 to C4 alkyl, C1 to C4 alkoxy, hy droxyl, sulfhydryl, carboxyl, amino, nitro, and halide.

In case of substituted bis(aminophenyl)disulfides an amino substituent denotes a second amino group in at least one of the two phenyl moieties.

In particular preferred is an aqueous composition of the present invention, wherein the substituted bis(aminophenyl)disulfides have at least one substituent independently select ed from the group consisting of C1 to C4 alkyl, C1 to C4 alkoxy, hydroxyl, sulfhydryl, car boxyl, amino, nitro, and halide, preferably selected from the group consisting of C1 to C3 alkyl, C1 to C4 alkoxy, hydroxyl, and sulfhydryl, more preferably selected from the group consisting of C1 to C3 alkyl, C1 to C3 alkoxy, and hydroxyl.

In particular preferred is an aqueous composition of the present invention, wherein the substituted dipyridyldisulfides have at least one substituent independently selected from the group consisting of C1 to C4 alkyl, C1 to C4 alkoxy, hydroxyl, sulfhydryl, carboxyl, amino, nitro, and halide, preferably selected from the group consisting of C1 to C3 alkyl, C1 to C4 alkoxy, hydroxyl, sulfhydryl, and amino, more preferably selected from the group consisting of C1 to C3 alkyl, C1 to C3 alkoxy, hydroxyl, and amino.

Even more preferred is an aqueous composition of the present invention, wherein the sub stituted dipyridyldisulfides have at least one substituent independently selected from the group consisting of methoxy, ethoxy, methyl, ethyl, hydroxyl, and amino.

Most preferably the substituted dipyridyldisulfides have at least one amino substituent.

Preferred is an aqueous composition of the present invention, wherein the total concentra tion of all first compounds is in the range from 1 mmol/L to 100 mmol/L, based on the total volume of the aqueous composition, preferably from 5 mmol/L to 80 mmol/L, more prefer- ably from 10 mmol/L to 60 mmol/L, even more preferably from 15 mmol/L to 50 mmol/L, most preferably from 20 mmol/L to 40 mmol/L. Preferably, the total concentration as de fined above is formed by only one first compound, most preferably by 2,2’-diaminodiphenyl disulfide (2,2 -dithiodianiline). If the total concentration is significantly below 1 mmol/L or significantly above 100 mmol/L undesired variations in the tin silver alloy are frequently observed. Furthermore, in many cases issues with the bath stability were observed.

The at least one second compound of formula (II)

(II), and salts thereof,

comprises one or more than one, preferably one, sulfhydryl group (SH group) in residue X.

In the compound of formula (II), X denotes a C1 to C10 alkyl moiety comprising said one or more than one, preferably one, sulfhydryl group, preferably a C1 to C8 alkyl moiety, more preferably a C1 to C6 moiety, even more preferably a C1 to C5 alkyl moiety, most prefera- bly a C1 to C4 alkyl moiety, each comprising said one or more than one, preferably one, sulfhydryl group. For example, if X is a C1 alkyl moiety comprising one sulfhydryl group, X denotes -CH₂-SH.

Among said C1 to C10 alkyl moiety, alkyl moieties with three or more than three carbon atoms are linear or branched, preferably linear. Linear alkyl moieties are preferably cova- lently connected with the nitrogen atom in formula (II) via a terminal carbon atom in the linear alkyl moiety. Thus, linear alkyl moieties are preferably n-alkyl moieties.

In the compound of formula (II), R¹ denotes hydrogen, methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substituted benzyl, preferably R¹ denotes hydrogen, methyl, ethyl, linear C3 to C5 alkyl, or branched C3 to C5 alkyl, more preferably R¹ denotes hydrogen, methyl, ethyl, or linear C3 to C5 alkyl, most preferably R¹ denotes methyl or ethyl. In the context of the present inven tion, branched C3 alkyl denotes its /so-form.

In the compound of formula (II), R² denotes methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substi- tuted benzyl, preferably R² denotes methyl, ethyl, linear C3 to C5 alkyl, or branched C3 to C5 alkyl, more preferably R² denotes methyl, ethyl, or linear C3 to C5 alkyl, most prefera bly R² denotes methyl or ethyl.

Preferred substituents in said substituted phenyl and substituted benzyl are independently selected from the group consisting of hydroxyl, C1 to C3 alkyl, C1 to C3 alkoxy, amino, nitro, and carboxyl. The at least one second compound of formula (II) serves as complexing agent in the aqueous composition of the present invention. Preferred is an aqueous composition of the present invention, wherein compounds of formula (II) are the only complexing agents in the composition. It is in particular preferred that the aqueous composition of the present inven- tion is substantially free of, preferably does not comprise, glycine, cysteine, and glutathi one, more preferably is substantially free of, preferably does not comprise, amino acids with at least one sulfhydryl group and peptides with at last one sulfhydryl group, most pref erably is substantially free of, preferably does not comprise, amino acids and peptides at all. Own experiments indicate that amino acids and peptides have negative influence on the shelf life of the aqueous composition of the present invention, finally causing stability problems. In some cases it has been observed that the shelf life does not go beyond six month, which is not desired. In contrast, a shelf life exceeding six month is desired and compounds of formula (II) do not negatively affect the shelf life, preferably resulting in a shelf life of more than six month.

In the aqueous composition of the present invention, compounds of formula (II) surprisingly serve a second advantage. Typically, during utilization of the aqueous composition of the present invention, compounds of the at least one first compound get reduced, which re sults in their monomerization. This monomerization is basically desired. Usually this results in a very unpleasant odor due to the aromatic ring with a sulfhydryl group. However, in the context of the present invention it has been observed that compounds of formula (II) re duce compounds of the at least one first compound only to such an extent that unpleasant odors are largely prevented. Instead a progressive and self-regulated reducing is obtained such that the total amount of monomeric first compounds is kept very low but sufficiently high to ensure operational capabilities of the aqueous composition. This is advantageous for people working in close proximity to a respective composition.

Therefore, the combination of the at least one first compound and the at least one second compound of formula (II) results in a number of unexpected advantages.

Preferred is an aqueous composition of the present invention, wherein the at least one second compound of formula (II) is a compound of formula (lla)

(lla), and salts thereof,

wherein independently R¹ denotes hydrogen, methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substituted benzyl, preferably R¹ denotes hydrogen, methyl, ethyl, lin ear C3 to C5 alkyl, or branched C3 to C5 alkyl, more preferably R¹ de- notes hydrogen, methyl, ethyl, or linear C3 to C5 alkyl, most preferably

R¹ denotes methyl or ethyl,

R² denotes methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substi tuted benzyl, preferably R² denotes methyl, ethyl, linear C3 to C5 alkyl, or branched C3 to C5 alkyl, more preferably R² denotes methyl, ethyl, or linear C3 to C5 alkyl, most preferably R² denotes methyl or ethyl, and n denotes 1 , 2, 3, or 4, preferably n denotes 1 , 2 or 3.

In the context of the present invention, linear C3 to C5 alkyl and branched C3 to C5 alkyl preferably and explicitly includes n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert- butyl, n-pentyl, 2-methylbutan-2-yl, 2,2-dimethylpropyl, 3-methylbutyl, pentan-2-yl, pentan- 3-yl, 3-methylbutan-2-yl, and 2-methylbutyl.

Most preferred is a composition of the present invention, wherein the at least one second compound of formula (II) is 2-(dimethylamino)ethanethiol.

In the aqueous composition of the present invention, compounds of formula (II) are prefer- ably positively charged at the nitrogen atom due to the preferred highly acidic pH (for pH see details above in the text).

Preferred is an aqueous composition of the present invention, wherein the total concentra tion of all said second compounds of formula (II) is in the range from 5 mmol/L to 100 mmol/L, based on the total volume of the aqueous composition, preferably from 8 mmol/L to 80 mmol/L, more preferably from 10 mmol/L to 60 mmol/L, even more preferably from

15 mmol/L to 50 mmol/L, most preferably from 20 mmol/L to 40 mmol/L. Preferably, the total concentration as defined above is formed by only one second compound, most pref erably by only 2-(dimethylamino)ethanethiol. If the total concentration is significantly below 5 mmol/L, silver ions are not sufficiently complexed and undesired plate out is frequently observed.

Preferred is an aqueous composition of the present invention, wherein the molar ratio of all first compounds to all second compounds is in the range from 10:1 to 1 :10, preferably in the range from 7:1 to 1 :7, more preferably in the range from 5:1 to 1 :5, even more prefera bly in the range from 4:1 to 1 :4, most preferably in the range from 3:1 to 1 :3, even most preferably in the range from 2:1 to 1 :2. If the molar ratio is significantly above 10:1 or sig- nificantly below 1 :10 the stability of the aqueous composition is in some cases negatively affected. Furthermore, in some cases in the tin silver alloy the total amount of silver was negatively affected, leading to a tin silver alloy comprising too high or too little amounts of silver.

The aqueous composition of the present invention preferably comprises further com pounds.

Preferred is an aqueous composition of the present invention furthermore comprising

(e) at least one organic acid anion, preferably an alkyl sulfonic acid anion, most preferably methane sulfonic acid anions.

Said at least one organic acid anion is preferably obtained from the source of tin ions and/or the source of silver ions.

Also preferably, said at least one organic acid anion is obtained from an organic acid, pref erably from at least one alkyl sulfonic acid, most preferably from methane sulfonic acid.

Most preferably, said at least one organic acid anion is obtained from the source of tin ions, the source of silver ions, and an organic acid. In such a case, the aqueous composi tion of the present invention preferably contains only one type of organic acid anion, which is very preferred. Therefore, most preferred is an aqueous composition of the present in vention, wherein in the composition alkyl sulfonate, preferably methane sulfonate, is the only organic acid anion. This provides a number of advantages because solubility of metal ions is typically high in the presence of alkyl sulfonic acids. Furthermore, undesired oxida tion of tin (II) ions to tin (IV) ions is significantly reduced. In addition it has been observed that the corrosive effect of alkyl sulfonic acids is reduced compared to strong inorganic acids.

Alkyl sulfonic acids are preferred acids because they serve as optimal pH adjuster and typically result in a very strong acidic pH. If other organic acid anions are utilized, prefera bly acetate, oxalate, and citrate, either as organic acid or in the source of tin ions and/or silver ions, an additional strong acid is usually needed to obtain the preferred strong acidic pH, for example strong inorganic acids, which include additional inorganic anions. This is less preferred, e.g. for the reasons stated above. Thus, preferred is an aqueous composi- tion of the present invention, wherein the composition is substantially free of, preferably does not comprise, inorganic acids. Instead strong organic acids are preferred in the com position of the present invention.

Preferred is an aqueous composition of the present invention, wherein in the aqueous composition the total concentration of all organic acid anions is in the range from 0.5 mol/L to 4.0 mol/L, based on the total volume of the aqueous composition, preferably from 0.6 mol/L to 2.5 mol/L, more preferably from 0.7 mol/L to 2 mol/L, even more preferably from 0.8 mol/L to 1 .5 mol/L, most preferably from 0.9 mol/L to 1.3 mol/L. Preferably, the total concentration as defined above is formed by only one type of organic acid anions, more preferably by only alkyl sulfonates, most preferably by only methane sulfonate. If the total concentration is significantly exceeding 4.0 mol/L the composition is not sufficiently stable anymore. Furthermore, a too high total concentration creates severe issues or even damages on the substrate. If the total concentration is significantly below 0.5 mol/L an in sufficient conductivity of the aqueous composition is usually obtained.

In order to increase the wettability, the aqueous composition of the present invention pref erably comprises at least one surfactant. The kind of surfactant is not particularly limited. Thus, preferred is an aqueous composition of the present invention furthermore comprising

(f) at least one surfactant, preferably selected from the group consisting of nonionic surfactants, amphoteric surfactants, cationic surfactants, and anionic surfactants, preferably nonionic surfactants and cationic surfactants, more preferably alkoxylated cationic surfactants and polyether nonionic surfactants, even more preferably alkylene oxide co-polymers and alkoxylated amines, most preferably ethylene oxide/propylene oxide co-polymers and ethoxylated amines.

Preferred is an aqueous composition of the present invention, wherein the total concentra tion of all surfactants is in the range from 0.1 g/L to 20 g/L, based on the total volume of the aqueous composition, preferably from 0.4 g/L to 10 g/L, more preferably from 0.6 g/L to 8 g/L, most preferably from 1 g/L to 5 g/L. If the total concentration is significantly below 0.1 g/L in many cases the wettability is insufficient. If the total concentration is significantly exceeding 20 g/L it is often observed that undesired foam is formed.

In order to avoid or at least to suppress foaming, the aqueous composition of the present invention furthermore preferably comprises, at least in many cases, at least one anti-foam agent.

The aqueous composition of the present invention preferably comprises tin (II) ions. How- ever, tin (II) ions are susceptible to oxidation, which results in tin (IV) ions. Such an oxida tion is not desired because it promotes precipitation of tin (IV) species, such as tin (IV) oxide. To avoid oxidation, an anti-oxidizing agent is preferably utilized. Thus, preferred is an aqueous composition of the present invention furthermore comprising

(g) at least one anti-oxidizing agent, preferably selected from the group consisting of compounds comprising a hydroxylated ring moiety, most preferably selected from the group consisting of catechol, hydroquinone, resorcinol, phenolsulfonic acids, ascorbic acid, and naptholsulfonic acids.

Preferred hydroxylated ring moieties are hydroxylated aromatic compounds, more prefera bly benzene diols, even more preferably selected from the group consisting of catechol, hydroquinone, and resorcinol, most preferably selected from the group consisting of resor cinol and hydroquinone. Most preferred is an aqueous composition of the present inven tion, wherein the at least one anti-oxidizing agent is resorcinol, more preferably resorcinol is the only anti-oxidizing agent in the aqueous composition of the present invention. Anti oxidizing agents as defined above are strong enough to avoid oxidation of tin (II) ions to tin (IV) ions without reducing tin (II) ions to metallic tin.

The aqueous composition of the present invention is preferably for electrolytically deposit ing a tin silver alloy and is preferably not for electroless deposition.

Preferred is an aqueous composition of the present invention, wherein the total concentra tion of all anti-oxidizing agents is in the range from 1 .0 mmol/L to 100 mmol/L, based on the total volume of the aqueous composition, preferably from 2.0 mmol/L to 70 mmol/L, more preferably from 4.0 mmol/L to 40 mmol/L, even more preferably from 5.0 mmol/L to 20 mmol/L, most preferably from 6.0 mmol/L to 15 mmol/L. Preferably, the total concentra tion as defined above is formed by only one type of anti-oxidizing agent, more preferably by only benzene diols, most preferably by only resorcinol. If the total concentration as de- fined above is significantly exceeding 100 mmol/L in many cases it is observed that the plating performance is insufficient. If the total concentration is significantly below 1 .0 mmol/L oxidation of tin (II) ions to insoluble tin (IV) species is not sufficiently prevented and undesired precipitation can be observed in some cases

If the at least one anti-oxidizing agent is known as a typical anti-oxidizing agent, the amount of such a compound is preferably counted among the above mentioned total con centration of all anti-oxidizing agents. In particular, if the at least one anti-oxidizing agent is an organic acid and comprises a hydroxylated ring moiety, the amount of said compound is counted among above mentioned total concentration of all anti-oxidizing agents.

As mentioned above, the present invention also refers to a method for electrolytically de- positing a tin silver alloy onto a substrate, the method comprising the steps

(A) providing the substrate,

(B) providing an aqueous composition according to the present invention, prefera bly as described above as being preferred, (C) contacting said substrate with said aqueous composition and supplying an electrical current such that said tin silver alloy is electrolytically deposited onto the substrate.

The above mentioned regarding the aqueous composition of the present invention applies likewise to the method of the present invention.

In step (A) of the method of the present invention, the substrate is provided, preferably a conductive substrate. In some cases, preferred is a method of the present invention, wherein the substrate is a semiconductor base substrate, preferably comprising a conduc tive or semi-conductive layer thereon. Such a conductive or semi-conductive layer is also often referred to as seed layer. The kind of seed layer is not particularly restricted. A most preferred seed layer is a copper seed layer.

The substrate preferably is or comprises at least one metalloid and/or gallium, more pref erably is or comprises at least one element selected from the group consisting of silicon, germanium, and gallium, most preferably is or comprises silicon. If the substrate is not in itself sufficiently conductive, it preferably comprises above mentioned conductive or semi- conductive layer.

Most preferred is a method of the present invention, wherein the substrate is a wafer, pref erably a wafer comprising a conductive or semi-conductive layer thereon, most preferably comprising a copper seed layer.

In other cases, preferred is a method of the present invention, wherein the substrate is a non-conductive substrate, preferably a substrate being or comprising glass or plastic. In such a case, the substrate preferably comprises already at least partly a conductive seed layer, or a conductive seed layer is deposited in a subsequent processing step.

Preferred is a method of the present invention, wherein the substrate (as defined before) comprises a plurality of individual structural features. Preferred is a method of the present invention, wherein the substrate comprises a plurality of metal features, preferably a plural ity of metal pillars and/or areas with at least one metal layer, more preferably a plurality of copper pillars and/or areas with at least one copper layer.

Preferably, the metal features have an aspect ratio in the range from 1 :40 to 15:1 (heighhwidth), preferably in the range from 1 :30 to 10:1 , more preferably in the range from 1 :20 to 7:1. Metal pillars and copper pillars, respectively, have a preferred aspect ratio in the range from 1 :1 to 10:1 , more preferably in the range from 1 :1 to 7:1 . Metal bumps and copper bumps, respectively, have a preferred aspect ratio in the range from 1 :40 to 1 :1 , preferably in the range from 1 :30 to 1 :10. In the context of the present invention, individual structural features and metal features, respectively, include individual geometrical forms such as pillars, spots and areas, which are accessible for tin silver alloy deposition by means of the method of the present invention.

Preferred is a method of the present invention, wherein said metal pillars and copper pil lars, respectively, have a cylindrical shape. Preferably, said metal pillars and copper pil- lars, respectively, have a height in the range from 3 pm to 100 pm, preferably from 5 pm to 90 pm, more preferably from 9 pm to 80 pm, even more preferably from 15 pm to 70 pm, most preferably from 20 pm to 70 pm. Preferably, said metal pillars and copper pillars, respectively, have a width in the range from 5 pm to 100 pm, preferably from 10 pm to 80 pm, more preferably from 10 pm to 30 pm.

In the method of the present invention said metal bumps and copper bumps, respectively, can have any shape; preferably a cylindrical, polygonal, and/or rectangular shape.

Preferably, the metal bumps and copper bumps, respectively, have a height in the range from 0.5 pm to 50 pm, preferably from 1 pm to 30 pm, more preferably from 2 pm to 25 pm, even more preferably from 3 pm to 20 pm, most preferably from 4 pm to 15 pm. Pref- erably, bumps have a width of at least 50 pm, preferably of at least 80 pm, more preferably of at least 100 pm.

In step (B) an aqueous composition of the present invention, preferably as described throughout the text as being preferred, is provided, typically in a tank for electrolytic metal deposition.

In step (C) of the method of the present invention the tin silver alloy is electrolytically de posited onto the substrate. Step (C) requires that the substrate is sufficiently conductive in order to electrolytically deposit the tin silver alloy. Either the substrate provided in step (A) is already sufficiently conductive or the substrate provided in step (A) is processed in sub sequent steps such that it is sufficiently conductive prior to step (C). Thus, the substrate in step (C) preferably comprises at least one conductive or semi-conductive layer such that the tin silver alloy is electrolytically deposited thereon. Preferably, the at least one conduc tive or semi-conductive layer is a conductive metallic, conductive semi-metallic, or conduc tive non-metallic layer. Most preferably it is a conductive metallic layer, preferably a metal lic copper layer, typically a copper seed layer.

A method of the present invention is preferred, wherein in step (C) the tin silver alloy is deposited as solder caps on pillars and/or as solder bumps.

In the method of the present invention, the substrate is operated as a cathode in order to deposit the tin silver alloy in step (C). Preferred is a method of the present invention, wherein the electrical current is a direct current, preferably with a cathodic current density in the range from 1 A/dm² to 100 A/dm², more preferably with a cathodic current density in the range from 3 A/dm² to 70 A/dm², most preferably with a cathodic current density in the range from 5 A/dm² to 50 A/dm². In some cases it is preferred that the direct current in step (C) is not supplemented by current pulses. This means that preferably in step (C) the direct current is the only electrical cur rent.

Own experiments show that in particular at comparatively high current densities the meth od of the present invention gives excellent results. This means that at such current densi- ties excellent and very uniform tin silver alloys are deposited and additionally the number of dendrites is impressively low. This is in particular the case compared to compositions comprising amino acids with at least one sulfhydryl group, e.g. cysteine (see examples below). Therefore, particular preferred is a method of the present invention, wherein the electrical current comprises a direct current with a cathodic current density in the range from 10 A/dm² to 40 A/dm², more preferably in the range from 12 A/dm² to 35 A/dm².

A method of the present invention is preferred, wherein in step (C) the contacting and sup plying of electrical current is carried out for 3 seconds to 400 min, preferably for 5 seconds to 200 min, most preferably for 6 seconds to 100 min. If the contacting is carried out for significantly less than 3 seconds typically an incomplete tin silver alloy is deposited and the uniformity of the solder caps is insufficient.

In step (C) of the method of the present invention, preferably a tin silver alloy layer is de posited, preferably with a layer thickness in the range from 1 pm to 150 pm, more prefera bly in the range from 4 pm to 100 pm, even more preferably in the range from 7 pm to 90 pm, most preferably in the range from 10 pm to 80 pm.

Preferred is a method of the present invention, wherein the silver content in the electrolyti- cally deposited tin silver alloy is in the range from 0.1 wt-% to 10 wt-%, based on the total weight of the electrolytically deposited tin silver alloy, preferably is in the range from 0.5 wt-% to 5 wt-%, most preferably is in the range from 1.5 wt-% to 3.5 wt-%. Most pref erably the rest of the tin silver alloy is tin. Thus, the major metal in the tin silver alloy is tin. As a result, a method of the present invention is preferred, wherein the tin content in the electrolytically deposited tin silver alloy is at least 60 wt-%, based on the total weight of the electrolytically deposited tin silver alloy, preferably at least 70 wt-%, more preferably at least 80 wt-%, even more preferably at least 90 wt-% or at least 95 wt.-%, most preferably at least 96.5 wt-%, and even most preferably at least 98.5 wt-% or at least 99.5 wt.-%. A tin silver alloy as described above is very much desired and suitable for solder caps on pillars and as solder bumps because of its excellent solderability.

Only in some cases a method of the present invention is preferred, wherein the tin silver alloy comprises copper.

In most cases it is very preferred that the tin silver alloy obtained with the method of the present invention is not a ternary alloy, more preferably not a tin silver copper alloy. Pref erably, the tin silver alloy is substantially free of, preferably does not comprise, one, more than one or all elements of the group consisting of zinc, nickel, iron, copper, bismuth, alu minium, and lead. In particular, the tin silver alloy obtained with the method of the present invention is substantially free of, preferably does not comprise, lead.

Preferred is a method of the present invention, wherein in step (C) the aqueous composi tion has a temperature in the range from 5°C to 90°C, preferably in the range from 15°C to 60°C, more preferably in the range from 20°C to 50°C, most preferably in the range from 22 °C to 40°C. If the temperature is significantly below 5°C no adequate deposition speed is obtained. If the temperature is significantly above 90°C undesired evaporation and bath instability occurs.

The present invention also refers to a use of the aqueous composition of the present in vention for electrolytically depositing tin silver alloy solder bumps or tin silver alloy solder caps onto metal pillars, preferably for electrolytically depositing tin silver alloy solder caps onto copper pillars. The aforementioned regarding the aqueous composition of the present invention and the method of the present invention, respectively, applies likewise to the aforementioned use.

The present invention is described in more detail by the following non limiting examples.

Examples

1. Aqueous composition for depositing a tin silver alloy:

In a first step different aqueous compositions are prepared as follows:

1 .1 Aqueous composition E-1 (according to the present invention):

Aqueous composition E-1 according to the present invention comprises 45 g/L tin(ll) ions, 1 g/L silver(l) ions, approximately 1 mol/L methane sulfonic acid, approximately 10 mmol/L resorcinol, 20 mmol/L to 40 mmol/L 2,2'-dithiodianiline, 20 mmol/L to 40 mmol/L 2- (dimethylamino)ethanethiol, and ethylene oxide/propylene oxide co-polymers and ethox- ylated amines as surfactants. The pH is approximately 0.

1.2 Comparative aqueous composition C-1 (not according to the present invention): For comparative purposes, comparative aqueous composition C-1 comprises 40 g/L tin(ll) ions, 1 g/L silver (I) ions, approximately 1 mol/L methane sulfonic acid, approximately 5 mmol/L hydroquinone, 10 mmol/L to 20 mmol/L 2,2'-dithiodianiline, approximately 20 mmol/L cysteine, and surfactants. The pH is approximately 0. Comparative aqueous com- position C-1 represents prior art JP 2006-265573 A.

2. Method for electrolytically depositing a tin silver alloy onto a substrate (deposition experiment):

In a second step depositions are carried out and a tin silver alloy is deposited in each dep osition experiment.

As substrates silicon-based wafer coupons, rendered conductive by means of an approxi mately 500 nm copper seed layer, each comprising nine dies with a plurality of cylindrical copper pillars (height: 10 pm) were provided. The copper pillars were formed within a re spective photoresist. Without removing the photoresist, tin silver alloys were deposited on top of the copper pillars to form respective solder caps. As a result, copper pillars with tin silver solder caps were obtained.

For that, 1000 ml of a respective aqueous composition prepared in the first step was pro vided.

Subsequently, the respective substrates were immersed into the respective composition for varying contact times according to the supplied current density such that in each case an approximately 15 pm tin silver solder cap was deposited (immersion for 60 seconds to 120 seconds depending on applied current density). The temperature of each composition was 25°C and stirring was set to 300 rpm. After each deposition experiment the number and average area of dendrites as well as the uniformity of the solder caps were deter mined. The parameters for each deposition experiment and the respective results are summarized in Table 1.

Table 1 , parameters and results of deposition experiments

^* within pillar non-uniformity

^** within die non-uniformity (nine pillars from each die were evaluated)

Table 1 shows that in a deposition experiment utilizing the aqueous composition E-1 the number of dendrites is insignificant, even at comparatively high current densities such as 30 A/dm². This is also observed for the average area of dendrites, which is significantly higher in comparative aqueous composition C-1.

The number of dendrites and the average area of dendrites were determined by a light microscope (Olympus LEXT OLS4000) using software Olympus Stream Enterprise Desk top 2.2”.

The benefit of the aqueous composition of the present invention is furthermore demon strated in Fig. 1 and Fig. 2, wherein Fig. 1 shows the result utilizing aqueous composition E-1 and Fig. 2 the result of comparative aqueous composition C-1 , both at 30 A/dm². In Fig. 1 almost no dendrites are present, wherein in Fig. 2 significantly more dendrites can be seen.

Furthermore, copper pillars with solder caps, obtained with aqueous composition E-1 ex hibit a higher uniformity compared to respective capped copper pillars obtained with com parative aqueous composition C-1 , in particular at a current density of 30 A/dm². This can be seen by parameters WIP and WID, which are well known in this technical field.

The same improvement has been observed with copper bumps instead of copper pillars (data not shown).

3. Complexing properties determined by titration experiments:

In order to evaluate the complexing properties of various compounds for silver ions several titrations were carried out. The results are summarized in Fig. 3.

The titrations were performed as follows:

In a first step an aqueous stock solution (concentration of complexing compound: 50 g/L) comprising the respective compound(s) for silver ions was prepared.

In a second step, portions from the stock solution were added stepwise to a titration solu tion comprising silver methanesulfonate (concentration corresponding to 1 g/L Ag (I) ions) and a silver rotating disk electrode (RDE) (1000 rpm). The change in overpotential upon adding portions from the stock solution was determined in reference to a reference elec trode. The temperature of the titration solution was 25°C.

Fig. 3 shows that the complexing properties vary significantly among the tested com pounds.

Compound A alone (2,2'-dithiodianiline) does not provide any significant complexing prop erties. The overpotential remains almost zero in the range from 0 g/L to 6 g/L of compound A.

Compound B alone (2-(dimethylamino)ethanethiol) shows significant complexing proper ties toward silver ions. According to Fig. 3, a maximum overpotential of approximately -250 mV was obtained at approximately 3 g/L compound B.

Similar to compound B, compound D alone (glutathione) shows significant complexing properties and a similar maximum overpotential of approximately -250 mV. However, com pared to compound B, compound D is less strongly forming respective complexes. This can be seen in the higher concentration of compound D which is required in order to obtain an overpotential especially in the range from -50 mV to -200 mV. This means that com pound D is less effective than compound B at lower concentrations. Furthermore, as an organic compound, glutathione in some cases suffers the disadvantage that it is not suffi ciently stable over time in respective aqueous compositions (shelf-life).

Compound C alone (L-cysteine) also shows significant complexing properties towards sil- ver ions and is a typical complexing compound in known compositions (compare JP 2006- 265573 A). Fig. 3 shows that compound C is a very capable complexing compound. A maximum of approximately -250 mV is already obtained at comparatively low concentra tions of less than 2 g/L. Generally, if complex formation takes place at already very low concentrations the working concentration in a respective aqueous composition is typically also low. Under such circumstances maintaining this low working concentration is chal lenging and requires very sensitive analytical tools, which is usually not desired.

Compound E also shows a maximum overpotential of approximately -250 mV, and, thus, in this regard is very similar compared to compound B, C, and D. In contrast, compound E very quickly forms strong complexes. This is seen in the fact that at already 1 g/L almost the maximum overpotential is obtained. Although compound D is very capable in forming complexes the working concentration is typically too low and maintaining it in a respective aqueous composition is demanding.

The combinations of A and B as well as of A and C are the only means to obtain very strong complexes. In each case a maximum overpotential of approximately -400 mV is obtained. Such strong complex formation is desired in order to avoid undesired plate out of silver. Plate out typically occurs if silver ions are insufficiently complexed such that either insoluble silver species precipitate in the aqueous composition or silver is deposited too quickly on a respective substrate even in the absence of an electrical current. However, only the combination A and B additionally shows a desired working concentration range to obtain an overpotential in the range from -250 mV to -400 mV. Furthermore, the combina tion A and C suffers the disadvantage that respective solder caps are more susceptible to dendrite formation (compare item 2 above). In the stock solutions of combinations A and B and A and C, respectively, compound A was present in an approximately equimolar amount compared to 50 g/L of compound B and C, respectively. The total concentration of compound A in the stock solution is identical to the concentration of compound A for com binations A and B and A and C, respectively.

Claims

1. An aqueous composition for depositing a tin silver alloy, the composition comprising

(a) tin ions,

(b) silver ions,

(d) at least one second compound of formula (II)

(II), and salts thereof,

wherein independently

X denotes a C1 to C10 alkyl moiety comprising one or more than one sulfhydryl group,

R¹ denotes hydrogen, methyl, ethyl, linear C3 to C5 alkyl, branched C3 to

C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substituted benzyl, and

R² denotes methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substi- tuted benzyl.

2. The composition of claim 1 , wherein the at least one second compound of formula (II) is a compound of formula (lla)

(lla), and salts thereof,

wherein independently R¹ denotes hydrogen, methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substituted benzyl,

R² denotes methyl, ethyl, linear C3 to C5 alkyl, branched C3 to C5 alkyl, unsubstituted phenyl, substituted phenyl, unsubstituted benzyl, or substi tuted benzyl, and

n denotes 1 , 2, 3, or 4.

3. The composition of claim 1 or 2, wherein the substituted bis(aminophenyl)disulfides and substituted dipyridyldisulfides have at least one substituent independently se- lected from the group consisting of C1 to C4 alkyl, C1 to C4 alkoxy, hydroxyl, sulfhy- dryl, carboxyl, amino, nitro, and halide.

4. The composition of any of the aforementioned claims, wherein the composition is acidic, preferably the composition has a pH in the range from -2 to +4, more prefera bly in the range from -1 to +2, most preferably in the range from -0.5 to +1.2.

5. The composition of any of the aforementioned claims, wherein the molar ratio of all first compounds to all second compounds is in the range from 10:1 to 1 :10, prefera bly in the range from 7:1 to 1 :7, more preferably in the range from 5:1 to 1 :5, even more preferably in the range from 4:1 to 1 :4, most preferably in the range from 3:1 to 1 :3, even most preferably in the range from 2:1 to 1 :2.

6. The composition of any of the aforementioned claims, wherein the molar ratio of the tin ions to the silver ions is in the range from 200:1 to 5:1 , preferably in the range from 150:1 to 10:1 , more preferably in the range from 125:1 to 12:1 , most preferably in the range from 100:1 to 15:1.

7. The composition of any of the aforementioned claims furthermore comprising

8. The composition of any of the aforementioned claims furthermore comprising

9. The composition of any of the aforementioned claims furthermore comprising

10. The composition of any of the aforementioned claims, wherein the composition does not comprise copper ions, preferably does not comprise copper ions and bismuth ions.

1 1. The composition of any of the aforementioned claims, wherein in the aqueous com position the weight of said tin ions and said silver ions in total represents 80 wt.-% to 100 wt.-% of all group 3 to group 15 metal cations in the aqueous composition, based on the total weight of all group 3 to group 15 metal cations in the aqueous composition, preferably at least 90 wt.-%, more preferably at least 95 wt.-%, even more preferably at least 98 wt.-%, most preferably at least 99 wt.-%.

1 2. A method for electrolytically depositing a tin silver alloy onto a substrate, the method comprising the steps

(A) providing the substrate,

(B) providing an aqueous composition according to any of claims 1 to 1 1 ,

13. The method of claim 12, wherein the silver content in the electrolytically deposited tin silver alloy is in the range from 0.1 wt-% to 10 wt-%, based on the total weight of the electrolytically deposited tin silver alloy, preferably is in the range from 0.5 wt-% to 5 wt-%, most preferably is in the range from 1.5 wt-% to 3.5 wt-%.

14. The method of claim 12 or 13, wherein the substrate comprises a plurality of metal features, preferably a plurality of metal pillars and/or areas with at least one metal layer, more preferably a plurality of copper pillars and/or areas with at least one cop per layer.

15. Use of an aqueous composition according to any of claims 1 to 1 1 for electrolytically depositing tin silver alloy solder bumps or tin silver alloy solder caps onto metal pil lars, preferably for electrolytically depositing tin silver alloy solder caps onto copper pillars.