TWI519682B - Electroplating baths of silver and tin alloys - Google Patents

Description

Silver and tin alloy plating bath

The present invention relates to an electroplating bath for silver and tin alloys and a method of electroplating the silver and tin alloys. More specifically, the present invention relates to an electroplating bath for silver and tin alloys and a method of electroplating the silver and tin alloys, wherein the electroplating bath contains a binder that allows for the enrichment of silver or a tin-rich alloy.

There are two major categories of silver and tin alloys. One is silver or silver-rich silver/tin alloy with a silver content of more than 50%. Compared to pure silver, these alloys exhibit higher hardness and higher abrasion resistance and are used in decorative applications. Due to its excellent electrical conductivity, it can also be used for electrical connections to reduce the amount of hard gold. Since hard gold has good abrasion resistance and corrosion resistance, it is used for electrical connection as a contact material finish. Hard gold provides the low electrical connection resistance required for charge transfer; however, the increase in gold prices has made it a limiting factor for low cost contact finishes. The silver/tin alloy industry has been used to join the finishes to replace or reduce the hard gold content of the finish. The alloys are fabricated by plating one or more layers of alternating layers of silver and tin followed by diffusion in a non-oxidizing atmosphere to form a silver/tin alloy. U.S. Patent No. 5,438,175 discloses the electrical connection with silver/tin and silver/palladium flash plating to allow a thin gold finish Layers to create low cost components.

The second type of alloy is a tin-based alloy having a silver content close to a eutectic such as about 3.5%. The tin/silver alloy is a softer alloy than silver/tin alloy and has a hardness similar to that of pure tin. These alloys can be used as a whisker, lead-free solder. Among the various electronic applications of tin/silver alloys, wafer-level packaging (WLP), now in the semiconductor manufacturing industry, has become the focus. The silicon interconnects can be fabricated together on a wafer using a wafer level package, and the complete IC module can be built on the wafer before it is diced into dies. Benefits that can be obtained using WLP include, for example, increased I/O density, improved operating speed, enhanced power density and thermal management, and reduced package size.

One of the key issues in WLP is the fabrication of flip-chip conductive interconnect bumps on a wafer. The interconnecting bumps serve as electrical or physical connections between the semiconductor component and the printed circuit board. Several methods of forming interconnect bumps on semiconductor devices have been disclosed, such as solder bumps, vapor deposited bumps, conductive paste bonds, screen printed solder bumps, stud bumps, and solder ball placement bumps. Among these techniques, the most cost-effective technique for forming micro-pitch arrays is solder disk bumps, which include a combination of temporary photoresist plating masks and electroplating. This technology is rapidly adopted as a full-area interconnect bump for high value-added assemblies such as microprocessors, digital signal processors, and application specific integrated circuits.

Typical tin/lead alloys are used for the formation of solder bumps; however, due to the toxicity of lead, the industry has attempted to find acceptable lead-free tin alloys that are susceptible to co-deposition. The problems associated with co-depositing lead-free tin alloys by electroplating are presented when the deposited material has significantly different deposition potentials. For example, when trying to deposit tin (-0.137 volts (V)) with When silver (0.799V) alloy is used, problems may arise. It is industrially desirable to effectively control the composition of the deposit to prevent the material from melting at too high or too low temperatures for a given application. Poor compositional control can result in temperatures that are too high to be tolerated for the component being processed, or at the other extreme, solder joints are not fully formed.

Another problem often encountered when plating bumps is the bump form. For example, tin/silver bumps are deposited on the copper or nickel under the bump metal through the via defined by the photoresist. The photoresist is stripped and the tin-silver is reflowed to form spherical bumps. The uniformity of the bump dimensions is important so that all of the bumps are in electrical contact with their respective flip-chip elements. In addition to the uniformity of the bump size, it is also important to form a low density and volume void during the bump reflow process. Ideally, no voids are formed during the reflow process. The voids in the bumps can also cause reliability problems in the interconnection when the bumps are connected to their respective flip chip elements. Another problem associated with plated bumps is the nodule formed on the surface of the bump, which can be easily detected by a variety of conventional scanning electron microscopes. These nodules can cause the formation of reflow voids, and the tabular deposition outside the nodules is commercially unacceptable.

Therefore, there is still a need for a stable silver-free silver/tin alloy that replaces hard gold and a silver and tin alloy plating bath that provides a tin-rich tin/silver alloy to provide a substantially knotless and void free solder. Bump.

The electroplating bath comprises one or more sources of silver ions; one or more sources of tin ions; one or more compounds having the formula: XSY wherein X and Y may be substituted or unsubstituted phenolic groups, HO-R- or -R'-SR"-OH, the restriction is that when X and Y are the same, they are substituted or unsubstituted phenol groups, otherwise X and Y are different, and wherein R, R' and R" are the same Or different and R, R' and R" are straight-chain or branched-chain alkyl radicals having 1 to 20 carbon atoms; and one or more compounds having the formula: Wherein M is hydrogen, NH ₄ , sodium or potassium and R ₁ is a substituted or unsubstituted linear or branched (C ₂ -C ₂₀ ) alkyl group, substituted or unsubstituted (C ₆ -C) ₁₀ ) aryl.

The electroplating method comprises: contacting a substrate with an electroplating bath, the electroplating bath comprising one or more sources of silver ions; one or more sources of tin ions; one or more compounds having the formula: XSY wherein X and Y are substituted Or unsubstituted phenolic group, HO-R- or -R'-SR"-OH, the restriction is that when X and Y are the same, they are substituted or unsubstituted phenolic groups, otherwise the X and Y systems are different. And wherein R, R' and R" are the same or different and R, R' and R" are straight-chain or branched alkyl-alkyl radicals having 1 to 20 carbon atoms; and one or more of the following formulae Compound: Wherein M is hydrogen, NH ₄ , sodium or potassium, and R ₁ is a substituted or unsubstituted linear or branched (C ₂ -C ₂₀ ) alkyl group, substituted or unsubstituted (C ₆ - C ₁₀ ) an aryl group; and electroplating silver and a tin alloy on the substrate.

The electroplating method also includes: providing a semiconductor die having a plurality of interconnecting bump pads; forming a seed layer on the interconnect bump pad; contacting the semiconductor die with a silver and tin alloy plating bath, the plating bath comprising One or more sources of silver ions; one or more sources of tin ions; one or more compounds having the formula: XSY wherein X and Y may be substituted or unsubstituted phenolic groups, HO-R- or -R'- SR"-OH, which is a substituted or unsubstituted phenolic group when X and Y are the same, otherwise X and Y are different and wherein R, R' and R" are the same or different and R, R' and R" are straight-chain or branched-chain alkyl radicals having 1 to 20 carbon atoms; and one or more compounds having the formula: Wherein M is hydrogen, NH ₄ , sodium or potassium, and R ₁ is a substituted or unsubstituted linear or branched (C ₂ -C ₂₀ ) alkyl group, substituted or unsubstituted (C ₆ - a C ₁₀ ) aryl group; electroplating silver and tin alloy interconnect bumps on the interconnect bump pads; and reflowing the silver and tin alloy interconnect bumps.

The silver and tin alloy baths are lead free and stable. It can deposit silver or tin-rich silver and tin alloys. The silver-rich silver/tin alloy provides a bright silver/tin alloy that can be used for decorative purposes and is sufficiently hard to replace hard gold as a finish layer for electrical joining. The tin-rich tin/silver alloy bath can be used to deposit eutectic or near-eutectic tin/silver alloys. In addition, the interconnect bumps deposited using the tin/silver alloy bath have a substantially uniform morphology and provide interconnective bumps that are substantially void free after reflow. The interconnect bumps are also substantially free of nodules.

Unless otherwise expressly stated in the text, the abbreviations used throughout this specification have the following meanings: °C = degrees Celsius; g = grams; mm = millimeters; cm = centimeters; mL = milliliters; L = liters; ppm = parts per million ; DI = deionization; nm = nanometer; μ m = micron; wt% = weight percent; A = amperes; A / dm ² and ASD = amperes / square decimeters; Ah = ampere hours; HV = hardness values; mN = millinewton; cps = centipoise; rpm = rotation / minute; IEC = International Electrochemical Commission; and ASTM = American Standard Test Method. The plating potential energy is provided with respect to the hydrogen reference electrode. For the electroplating process, the terms "depositing,""coating,""plating," and "plating" are used interchangeably throughout the specification. "halide" means fluoride, chloride, bromide and iodide. "Cigranene" means the lowest melting point of an alloy obtainable by varying the proportions of the components; and has a clear and lowest melting point compared to combinations of other identical metals. All percentages are by weight unless otherwise stated. Except that the numerical range interpretation is limited to a logical total of up to 100%, all numerical ranges include upper and lower limits and can be combined in any order.

The silver and tin alloy plating bath is substantially free of lead. "Substantially free of lead" means that the bath and the silver and tin alloy deposits contain 50 ppm or less of lead. Furthermore, the silver and tin alloy plating bath is preferably free of cyanide. Cyanide is primarily avoided by not using any silver or tin salts or other compounds including CN ^- anions in the bath.

The silver and tin alloy plating baths are also low foaming. The low foaming electroplating bath is very satisfactory in the metal plating industry because the more the electroplating bath is foamed during the plating process, the more components of the bath are lost per unit time during the plating process. Loss of components during the plating process can result in commercial inferior silver and tin alloy deposits. Therefore, the staff must closely monitor the component concentration and replenish the lost component to its initial concentration. Monitoring component concentrations during the plating process is both tedious and difficult because certain important components are included in the plating bath at relatively low concentrations making it difficult to accurately weigh and replenish to maintain the most favorable plating performance. The low-foam plating bath improves the consistency and thickness uniformity of the alloy composition across the surface of the substrate, and reduces organic matter and bubbles trapped in the deposit, which organic matter and bubbles can cause in the deposit after reflow The creation of voids.

The electroplating bath contains one or more sources of silver ions. The silver ion source may be provided by a silver salt such as, but not limited to, tin halide, silver gluconate, silver citrate, silver lactate, silver nitrate, silver sulfate, silver alkyl sulfonate, and silver alkoxide sulfonate. When a silver halide is used, the halide is preferably chloride. Preferably, the silver salt is silver sulfate, silver alkyl sulfonate or a mixture thereof, and More preferably, it is silver sulfate, silver methane sulfonate or a mixture thereof. The silver salt is typically commercially available or can be prepared by methods described in the literature. Preferably, the silver salt is readily soluble in water. The amount of the one or more silver salts used in the bath depends, for example, on the desired alloy composition and operating conditions to be deposited. In general, the silver salt content in the bath may range from 0.01 g/L to 100 g/L, typically from 0.02 g/L to 80 g/L. When it is desired to have a bright silver-rich alloy, the concentration of silver ions to tin ions in the plating bath is in the range of from 1 to 12, preferably from 1 to 6. When silver is a minor amount of metal in the alloy, the silver salt may range from 0.01 g/L to 20 g/L, typically from 0.01 g/L to 15 g/L.

The electroplating bath contains one or more sources of tin ions. Sources of tin ions include, but are not limited to, salts such as tin halides, tin sulfate, tin alkyl sulfonates, tin alkoxide sulfonates, and acids. When a tin halide is used, the halide is typically a chloride. The tin compound is preferably tin sulfate, tin chloride or tin alkane sulfonate, and more preferably tin sulphate or tin methane sulfonate. The tin compound is typically commercially available or can be prepared by methods known in the literature. Preferably, the tin salt is readily soluble in water. The amount of the tin salt used in the bath depends on the desired composition and operating conditions of the alloy to be deposited. In general, the tin salt can range from 1 g/L to 100 g/L, typically from 5 g/L to 80 g/L. When the alloy is rich in tin, the tin salt is typically in the range of from 30 g/L to 100 g/L. When tin is a relatively small amount of the two metals, the weight ratio of the silver ions to tin ions is as described above.

The silver and tin alloy electroplating bath comprises one or more compounds having the formula: XSY(I) wherein X and Y may be substituted or unsubstituted phenolic, HO-R- or -R'-SR"- OH, the restriction condition is that when X and Y are the same, they are substituted or unsubstituted phenol groups, otherwise X and Y are different and wherein R, R' and R" are the same or different and R, R' and R" is a linear or branched chain alkyl radical having 1 to 20 carbon atoms. The substituent on the phenol includes, but is not limited to, a linear or branched (C ₁ -C ₂ ) alkyl group. The compound is included in the bath in an amount from 0.1 g/L to 25 g/L, typically from 0.5 g/L to 10 g/L.

Examples of the compound of the variable X and Y phenol group are 4,4'-thiodiphenol and 4,4'-thiobis(2-methyl-6-tert-butylphenol). Preferably, the compound is 4,4'-thiodiphenol.

When the X and Y systems are different, the compound preferably has the following formula: HO-RS-R'-SR"-OH (II) wherein R, R' and R" are the same or different and have from 1 to 20 carbon atoms, preferably a linear or branched chain alkyl radical from 1 to 10 carbon atoms, more preferably R and R" has 2 to 10 carbon atoms and R' has 2 carbon atoms. These compounds are conventional dihydroxybissulfide compounds. Preferably, the dihydroxybissulfide compound contained in the alloy bath exceeds the phenolic compound.

Examples of such dihydroxybissulfide compounds are 2,4-dithia-1,5-pentanediol, 2,5-dithia-1,6-hexanediol, 2,6-dithia- 1,7-heptanediol, 2,7-dithia-1,8-octanediol, 2,8-dithia-1,9-nonanediol, 2,9-dithia-1, 10-decanediol, 2,11-dithia-1,12-dodecanediol, 5,8-dithia-1,12-dodecanediol, 2,15-dithia-1,16-hexadecanediol, 2,21-dithia-1,22-docosane di Alcohol, 3,5-dithia-1,7-heptanediol, 3,6-dithia-1,8-octanediol, 3,8-dithia-1,10-decanediol, 3,10-dithia-1,8-dodecanediol, 3,13-dithia-1,15-pentadecanediol, 3,18-dithia-1,20-eicosanediol , 4,6-dithia-1,9-nonanediol, 4,7-dithia-1,10-decanediol, 4,11-dithia-1,14-tetradecanediol, 4,15-dithia-1,18-octadecanediol, 4,19-dithia-1,22-docosanediol, 5,7-dithia-1,11-undecane Alcohol, 5,9-dithia-1,13-tridecanediol, 5,13-dithia-1,17-heptadecanediol, 5,17-dithia-1,21-twenty-one Alkanediol and 1,8-dimethyl-3,6-dithia-1,8-octanediol.

The silver and tin alloy bath also comprises a mercaptotetrazole compound having the formula: Wherein M is hydrogen, NH ₄ , sodium or potassium and R ₁ is a substituted or unsubstituted linear or branched (C ₂ -C ₂₀ ) alkyl group, substituted or unsubstituted (C ₆ -C) ₁₀ ) an aryl group, preferably a substituted or unsubstituted linear or branched (C ₂ -C ₁₀ ) alkyl group and a substituted or unsubstituted (C ₆ ) aryl group, more preferably substituted or Unsubstituted linear or branched (C ₂ -C ₁₀ ) alkyl. Substituents include, but are not limited to, alkoxy, phenoxy, halogen, nitro, amine, substituted amine, sulfo, sulfonyl, substituted sulfonyl, sulfophenyl , sulfonyl-alkyl, fluorosulfonyl, sulfoguanidinophenyl, sulfoguanamine-alkyl, carboxyl, carboxylate, ureidomethylmercapto, amine-mercapto-phenyl, Aminoformylalkyl, carbonylalkyl and carbonylphenyl. Preferred substituents include an amine group and a substituted amine group. An example of a mercaptotetrazole is 1-(2-diethylaminoethyl)-5-mercapto-1,2,3,4-tetrazole, 1-(3-ureidophenyl)-5-mercapto-4 Isozolium, 1-((3-N-ethyl-oxalylamino)phenyl)-5-mercaptotetrazole, 1-(4-acetamidophenyl)-5-mercapto-tetrazole and 1-( 4-carboxyphenyl)-5-mercaptotetrazole. In general, the amount of the mercaptotetrazole compound of formula (III) contained in the bath is from 1 g/L to 200 g/L, typically from 5 g/L to 150 g/L.

The combination of one or more compounds of formula (I) and (II) with one or more mercaptotetrazole compounds of formula (III) provides stability of the alloy bath during storage or plating and stability within the range of applicable current densities An alloy composition that results in the deposition of a hard silver-rich silver/tin alloy as a substitute for hard gold; or a tin-rich tin/silver alloy that can be deposited to provide good morphology, reduced or no nodules and Solder bumps with reduced or no voids after soldering. In addition, the silver and tin alloy deposits are more resistant to rust than silver. In general, the weight ratio of the compound of formula (III) to the compound of formula (I) is from 3 to 300. When the variables X and Y in the formula (I) are the same, the weight ratio of the mercaptotetrazole of the formula (III) to the compound of the formula (I) is from 25 to 300, preferably from 25 to 150. When the compound of the formula (II) is contained in the bath, the weight ratio of the mercaptotetrazole to the compound of the formula (II) is from 3 to 30, preferably from 3 to 15.

Any aqueous acid solution that does not adversely affect the bath can be used. Suitable acids include, but are not limited to, arylsulfonic acids, alkylsulfonic acids (methanesulfonic acid, ethanesulfonic acid, and propanesulfonic acid), arylsulfonic acids (such as phenylsulfonic acid and toluenesulfonic acid), and inorganic Acids (such as sulfuric acid, sulfamic acid, hydrochloric acid, hydrobromic acid, and fluoroboron) acid). Typically, the acid is an alkyl sulfonic acid and an aryl sulfonic acid. Although a mixture of acids can be used, a single acid is typically used. The acid used in the present invention is generally commercially available or can be prepared by methods known in the art.

The acid content in the electrolyte composition may range from 0.01 to 500 g/L, such as from 10 to 400 g/L, or such as from 100 to 300 g/L, depending on the desired alloy composition and operating conditions. . When the silver ion and tin ion are derived from a metal halide, it is preferred to use the corresponding acid. For example, when one or more of tin chloride or silver chloride is used, it is preferred to use hydrochloric acid as the acid component. A mixture of acids can also be used.

Optionally, the bath may include one or more inhibitors. Typically, the inhibitor is present in an amount from 0.5 to 15 g/L or from 1 to 10 g/L. Such inhibitors include, but are not limited to, alkanolamines, polyethylenimines, and alkoxylated aromatic alcohols. Suitable alkanolamines include, but are not limited to, substituted or unsubstituted methoxylated, ethoxylated, and propoxylated amines, such as hydrazine (2-hydroxypropyl) ethylenediamine, 2- {[2-(Dimethylamino)ethyl]-methylamino}ethanol, N,N'-bis(2-hydroxyethyl)-ethylenediamine, 2-(2-aminoethylamine) - Ethanol and combinations thereof.

Suitable polyamidides include, but are not limited to, substituted or unsubstituted linear or branched chain stretched imines having a molecular weight of from 800 to 750,000 or mixtures thereof. Suitable substituents include carboxyalkyl groups such as carboxymethyl or carboxyethyl.

Alkoxylated aromatic alcohols useful in the present invention include, but are not limited to, ethoxylated bisphenols, ethoxylated beta-naphthols, and ethoxylated nonylphenols.

Add one or more reducing agents to the bath as needed The auxiliary tin is kept in a soluble divalent state. Suitable reducing agents include, but are not limited to, hydroquinone, hydroquinone sulfonic acid, potassium salts, and hydroxylated aromatic compounds such as resorcinol and catechol. The content of the reducing agent when used in the composition is from 0.01 to 20 g/L or such as from 0.1 to 5 g/L.

For applications requiring good wettability, the bath may include one or more surfactants. Suitable surfactants are well known to those skilled in the art and include any which produces excellent solderability, desirable matte or glossy finish, satisfactory grain refinement, and stability in the acid plating bath. a surfactant for the deposit. It is preferred to use a low foaming surfactant. Conventional amounts of surfactants can be used.

One or more brighteners may be included as needed. Such brighteners are well known to those skilled in the art. Suitable brighteners include, but are not limited to, aromatic aldehydes such as chlorobenzaldehyde or derivatives thereof such as benzalkonium. Suitable amounts of such brightening agents are well known to those skilled in the art.

Other optional compounds can be added to the bath for further grain refinement. Such compounds include, but are not limited to, alkoxylates (such as polyethoxylated amines JEFFAMINE T-403 or TRITON RW) or sulfated alkyl ethoxylates (such as TRITON QS-15) and gelatin or gelatin derivative. Alkoxylated amine oxides can also be included. A wide variety of alkoxylated amine oxide surfactants are known, preferably low foaming amine oxides. The preferred alkoxylated amine oxide surfactants have a viscosity of less than 5000 cps when measured using a Brookfield LVT viscosity agent at #2 axis. Typically this viscosity is measured at ambient temperature. Conventional amounts of such grain refiners can be used. Typically contained in the bath in an amount from 0.5 g/l to 20g/L.

A flavonoid compound may also be included as a grain refiner in the bath. Such flavonoid compounds include, but are not limited to, pentahydroxyflavone, mulberry pigment, chrysin, quercetin, quercetin, myricetin, rutin, and quercetin. The flavonoid compound may be present in an amount from 1 to 200 mg/L, typically from 10 to 100 mg/L, more typically from 25 to 85 mg/L.

The electroplating bath is typically prepared by adding the following ingredients to a bottle: one or more acids, one or more compounds of formula (I), (II), and one or more compounds of formula (III), followed by one or more Solution soluble silver and tin compounds, one or more optional additives, and the balance of water. Preferably, the compounds of formula (I), (II) and (III) are added to the bottle prior to the addition of the solution soluble silver and tin compounds. Once the aqueous bath is prepared, the final volume of the bath can be adjusted by removing the undesired material, such as by filtration, and typically adding water. The bath can be agitated by any conventional means, such as agitation, suction or recirculation to increase the plating speed. The bath is acidic, i.e. has a pH of less than 7, typically less than 1, more typically from less than or equal to 1 to 2.

The bath can be used in a variety of plating methods requiring silver and tin alloys and low foaming. Plating methods include, but are not limited to, horizontal or vertical wafer plating, roller plating, rack plating, and high speed plating (such as roll-to-roll plating and sputtering). Silver and tin alloys can be deposited on the substrate by contacting the bath with the substrate and energizing the bath to deposit the silver and tin alloy on the substrate. The substrate that can be plated includes, but is not limited to, copper, copper alloys, nickel, nickel alloys, brass-containing materials, electronic components such as electrical connections, and semiconductor wafers such as germanium wafers. The bath can be used for electronic components (such as electrical connections), jewelry, decoration and wafer interconnects Bump plating applications. The substrate can be contacted with the bath by any method known in the art.

The current density used to plate silver and tin alloys depends on the particular plating method. In general, the current density is 0.05 A/dm ² or higher, or such as from 1 to 25 A/dm ² . Typical low current densities range from 0.05 A/dm ² to 10 A/dm ² . High current densities such as those with high-speed agitation exceed 10 A/dm ² and can be as high as 25 A/dm ² .

The silver and tin alloy may be electroplated at a temperature from room temperature to 55 ° C, or such as from room temperature to 40 ° C, or such as from room temperature to 30 ° C. Typically, silver/tin plating is carried out from room temperature to 55 ° C and tin/silver is carried out from room temperature to 40 ° C.

The bath can be used to deposit silver and tin alloys of various compositions. When the alloy is bright silver-rich silver/tin alloy, the silver content can range from greater than 50% to 95% with the balance being alloy tin, which typically ranges from 60% to 90%. The tin-rich alloy contains from more than 50% tin to 99% tin and the balance is silver. Typically, the tin-rich alloy comprises 80% to 99% tin and the balance silver. These weights are based on atomic adsorption spectroscopy ("AAS"), X-ray fluorescence ("XRF"), inductively coupled plasma ("ICP"), or differential scanning calorimetry ("DSC"). For a variety of applications, the eutectic composition of the alloy can be used. These tin alloys do not substantially contain lead and cyanide.

In general, the silver-rich alloy provides a deposit that is harder than the tin-rich alloy. These silver-rich alloys are used to replace hard gold finishes such as decorative applications for jewelry or other accessories and for hard finishes that require a combination of abrasion resistance and rust resistance. Typical thicknesses of such finishes are 0.4 μm to 5 μm.

Tin-rich tin/silver alloys are typically used for interconnect bump formation for wafer level packaging. This involves providing a semiconductor die having a plurality of interconnected bump pads, forming a seed layer on the interconnect bump pads, and depositing a tin/silver alloy interconnect bump layer on the interconnect bump pads by: The bath is contacted with the semiconductor mold and the bath is energized to deposit the tin/silver alloy interconnect bump layer on the substrate, and to reflow the interconnect bump layer.

In general, a device includes a semiconductor substrate on which a plurality of conductive interconnect bump pads are formed. The semiconductor substrate can be a single crystal germanium wafer, a sapphire upper (SOS) substrate, or a silicon-on-insulator (SOI) substrate. The conductive interconnect bump pad can be one or more layers of metal, composite metal or metal alloy typically formed by physical vapor deposition (PVD), such as sputtering. Typical conductive interconnect bump pad materials include, but are not limited to, aluminum, copper, titanium nitride, and alloys thereof.

A passivation layer is formed on the interconnect bump pad, and an opening extending to the interconnect bump pad is formed in the passivation layer by etching (typically dry etching). The passivation layer is typically an insulating material such as tantalum nitride, hafnium oxynitride or hafnium oxide (such as phosphonium silicate glass (PSG)). The materials can be deposited by a chemical vapor deposition (CVD) process such as plasma enhanced CVD (PECVD).

A sub-bump metallization (UBM) structure, typically formed of a plurality of metal layers or metal alloy layers, is deposited on the device. The UBM serves as an adhesive layer and an interconnect bump to be formed for electrically connecting the bottom (seed layer). The layer forming the UBM structure can be formed by PVD (such as sputtering or evaporation) or CVD Process deposition. The UBM structure may include, but is not limited to, a composite structure including a bottom chrome layer, a copper layer, and an upper tin layer in this order.

The photoresist layer is applied to the device and subjected to standard photolithographic exposure and development techniques to form a plated mask. The plating mask defines the size and location of the plated through holes on the I/O pads and UBM. Without limitation, the plating process typically uses a relatively thin thickness (typically 25 to 70 μm ) of the photoresist layer, while the via internal plating process typically uses a relatively thick thickness (typically 70). Photoresist layer up to 120 μm ). Photolithographic materials are commercially available and are generally known in the art.

The interconnect bump material is deposited on the device by an electroplating process using the tin-rich tin/silver alloy plating bath described above. It may be desirable to use a tin/silver alloy bath that provides a eutectic concentration. The bump material is plated in a region defined by the plated through holes. To this end, horizontal or vertical wafer plating systems, such as jet plating systems, typically use direct current (DC) or pulse plating techniques. In the plating process, the interconnect bump material completely fills the via and extends upwardly over a portion of the top surface of the plating mask. This ensures that a sufficient volume of interconnecting bump material is deposited to achieve the desired ball size after reflow. In the through-hole plating process, the photoresist thickness is sufficiently thick to include an appropriate volume of interconnect bump material in the plated through-via. A layer of copper or nickel may be electroplated in the plated through holes prior to plating the interconnecting bump material. This layer acts as a wettable substrate for the interconnect bump during reflow.

The plating mask is stripped using a suitable solvent after depositing the interconnecting bump material. Such solvents are generally known in the art. Thereafter, the UBM structure is selectively etched using conventional techniques to remove the interconnect All metals between the bumps and the areas around them.

Thereafter, the wafer is optionally melted and heated in a reflow oven to a temperature at which the interconnecting bump material melts and flows into a truncated substantially spherical shape. Heating techniques are well known in the art and include, for example, infrared, thermal and convection techniques, and combinations thereof. The reflowed interconnect bumps are typically coextensive with the edges of the UBM structure. The heat treatment step can be carried out in an inert atmosphere or in air, and the particular process temperature and time depend on the particular composition of the interconnecting bump material.

The silver and tin alloy baths are lead free and stable. It is preferred that it does not contain a cyanide compound. The silver and tin alloys of the present invention typically remain stable for at least 8 months of idle time compared to conventional silver and tin cyanide baths which typically degrade upon 2 months of idle time. It can deposit silver or tin-rich silver and tin alloys. The silver-rich silver/tin alloy provides a bright silver/tin alloy that can be used for decorative purposes and is sufficiently hard to replace hard gold as a finish layer for electrical connection. The tin-rich tin/silver alloy bath can be used to deposit eutectic or near-eutectic tin/silver alloys. In addition, the interconnect bumps deposited using the tin/silver alloy bath have a substantially uniform morphology and provide interconnective bumps that are substantially void free after reflow. The interconnect bumps are also substantially free of nodules.

The following examples are intended to further illustrate the invention, but are not intended to limit the scope of the invention.

Example 1

A plurality of silver-rich silver/tin alloy plating baths having the two formulations in the table below were prepared.

Perform electrolysis of Formulation I. This test was conducted to show the electrochemical stability of the bath. A 7.5 cm x 10 cm brass panel was weighed and fixed in the plating tank. A low current density of 2 ASD was applied to the panel for 15 minutes. The panel was removed from the plating bath and weighed. The amount of silver/tin plated in 15 minutes was measured. And record the ampere-hours of electricity per liter of bath. The results indicate that the amount of silver/tin deposited on the brass panel per liter per amp hour is substantially the same at 2 ASD for 100 Ah/L bath age and the bath is stable during plating. The current efficiency is stable during the electrolysis and is close to 100%. No degradation of the bath was observed during electrolysis; however, in order to maintain the turbidity and performance of the bath fixation, the tin (IV) precipitate was filtered off. The plated panel has a bright silver/tin deposit.

The Hull cell test of Formulation I was carried out at a total current of 2 amps at bath ages of 0 Ah/L, 30 Ah/L, 70 Ah/L, and 100 Ah/L. It is determined whether the current density can be applied to maintain stability in the bath age range. The 7.5 cm x 10 cm brass panel was plated for 3 minutes in this current density range. This current density range is from 0.05 ASD to 10 ASD. Bright silver/tin deposits were obtained under all conditions.

Example 2

Silver/tin alloys were electroplated on brass, copper, copper/bismuth alloys, copper/nickel alloys, copper/tin alloys, nickel plated brass and nickel plated copper panels using the two formulations in Table 1. Each panel is 7.5 cm x 10 cm. The thickness of the panel is 0.25 mm. The nickel plated brass and nickel plated copper panels contain a silver strike layer of less than 100 nm to promote the silver/tin layer with the nickel. The silver plating is plated on the panel using a silver bottom plating bath containing 2 g/L of silver ions, 15 g/L of 3,6-dithia-1,8-octanediol, and 67 g/L of methanesulfonic acid. on. The plating is carried out at 0.5 ASD to 1 ASD for 10 seconds to 30 seconds.

The panel is placed in a plating bath containing Formulation I or Formulation II. The temperature of the bath was 50 ° C and the anode was an insoluble anode of cerium oxide. Connect each electrode to a rectifier. Electroplating was carried out at 2 ASD. After the panel was electroplated, the physical appearance of the silver/tin deposit was tested. Each has a shiny deposit.

Example 3

A brass cylinder having a diameter of 12 mm and a height of 8 mm was mounted on the shaft of the electric motor and immersed in a plating bath containing Formulation I in Table 1. The rotation speed of the motor was set to 1000 rpm. An insoluble cerium oxide electrode was used as the anode. Connect the electrode to the rectifier. The plating bath was agitated by cathode rotation and the temperature of the bath was maintained at 50 °C. Electroplating is at 0.5, 1, 2, 4, 6, and 8. 10, 12, 14, 16, 18 and 20 ASD. The cylinder is changed when moving from one current density to another, and the plating time is adjusted to maintain the same film thickness. All of the silver/tin alloy deposits in the current density range from 0.5 ASD to 16 AS D have a shiny appearance. A small number of burned edges were observed at 18 ASD and 20 ASD.

The silver and tin contents of the deposits were determined by X-ray fluorescence (XRF) using the FISCHERSCOPE X-Ray mode XDV-SD analysis from Helmut Fischer AG. XRF analysis showed that the silver/tin alloy layer contained from 75 wt% to 80 wt% silver and from 20 wt% to 25 wt% tin. The average alloy content of the determined silver/tin alloy is 78 ± 2% silver and 22 ± 2% tin. The silver and tin contents did not change significantly over the entire range of applicable current densities.

Example 4

The following will in the following layer or plating bath 5cm x 2.5cm brass panel: preformed from formulations containing 750mL / L RONOVAL ^TM CM-97 is, 17.7g / L (68.2%) of gold potassium cyanide, 20mL / L RONOVAL ^TM CM Cobalt concentrate (cobalt Concentrate, available from Dow electronic materials) and a sufficient amount of DI water to make the bath to a desired volume of gold/cobalt alloy plating bath to obtain a hard gold layer; or from the above Table 1 formula I silver / tin alloy plating bath plating. The brass panel is placed in a plating chamber having an electroplating bath of either of the two. An insoluble platinized titanium electrode was used as the anode. The silver/tin plating was carried out at 50 ° C and the gold/cobalt plating was carried out at 60 ° C. The current density of the hard gold bath is 4 ASD and the current density of the silver/tin bath is 2 ASD. Electroplating was carried out until 5 μm of a hard gold layer containing 0.2 wt% of cobalt or a silver/tin alloy layer of 79 wt% of silver and 21 wt% of tin was deposited on the brass.

Vickers Hardness was tested with diamond probes on each plated brass panel using a CSM instrument Nano-Indentation Tester at room temperature. The depth of penetration of the indented probe is less than or equal to 10% of the thickness of the hard gold or silver/tin alloy layer on the brass panel. This ensures that the brass below does not affect the hardness results. The average hardness of the hard gold was determined to be 175 HV and the hardness of the silver/tin alloy layer was determined to be 240 HV. The silver/tin layer is harder than the hard gold layer.

The plated brass panels were then annealed at 150 ° C for one hour in a conventional convection oven. The hardness of the hard gold and the silver/tin alloy layer was tested again. The hardness of the hard gold layer has an average hardness value of 200 HV and the silver/tin alloy has an average hardness value of 225 HV. Although the annealing process softens the silver/tin alloy layer, the silver/tin alloy layer still has a harder surface than the hard gold. The silver/tin alloy exhibits an improved hardness beyond the traditional hard gold.

Example 5

A 5 cm x 10 cm and 0.25 mm thick brass panel was plated with silver-rich silver/tin alloy or gold/cobalt hard gold according to the method of Example 4 above. Electroplating was performed to form a layer of 3 μm on the panel. The ductility of each of the plated brass panels was tested according to ASTM Standard B 489-85 using a bending tester taken from SHEEN Instruments Ltd. The ductility of the hard gold is from 4 to 5% and the ductility of the silver/tin alloy is greater than 7.8%. There is no crack signal in the extension of the silver/tin alloy sample. The hard gold sample was partially cracked after 4% extension, and the remaining portion was cracked after 5% extension. The silver/tin layer has improved ductility beyond the hard gold sample.

Example 6

Two of 5cm x 2.5cm 0.25mm thick brass panel SILVERON ^TM GT-101 silver plating bath (available from Dow Electronic Materials, Marlboro Massachusetts City) or silver is plated in the above Table 1 Formulation Silver/tin alloy bath plating of I. Each panel was electroplated in a plating chamber at 50 °C. The anode of the silver/tin bath is a silver anode which is plated with an insoluble electrode of titanium and which is soluble in the silver bath. The current density is 0.5 ASD. The plating was performed by depositing a 3 μm thick silver or silver/tin layer on the panel.

Each panel was then immersed in a 2 wt% potassium sulfide (K ₂ S) solution for 10 minutes to test the rust resistance of each coating layer. The silver coating layer has a deep blue appearance, indicating the formation of Ag ₂ S and severe rust. The silver/tin coating layer has a bright brown color indicating that the polysulfide system reacted with the silver/tin coating layer is very slow relative to the substantially pure silver coating layer in the presence of sulfur. The silver/tin coating provides more rust resistance than the silver coating.

Example 7

Silver-rich silver/tin alloy layers were electroplated on a 1 cm x 3 cm brass panel using Formulation I. The plating was carried out in a plating chamber at a temperature of 1 ASD and 50 °C. The anode is an insoluble electrode of platinum-plated titanium. The electroplating was carried out to deposit a 3 μm silver/tin alloy layer on the panel. Brass panels 1cm x 3cm second lines using bright SOLDERON ^TM BT-280 tin plating bath (available from Dow Electronic Materials) bright tin plating. The plating was performed at 1 °C at 30 ° C until a 3 μm bright tin layer was deposited on the panel.

The solderability performance of the silver/tin alloy is then tested in accordance with IEC 60068-2-69 ("SMD" for surface mount devices" by the wetting balance method) The solderability of bright tin is compared. First, each sample was immersed in a non-activated flux C25R supplied by Metronelec (France) to remove dirt, oil and oxides prior to soldering. The sample was then mounted on a Metronelec/MENESCO ST 50 solderability device (available from Metronelec) and the sample was pulled into the solder at a speed of 20 mm/sec. The solder is a lead-free solder containing 95.5 wt% tin, 3.8 wt% silver, and 0.7 wt% copper. The test time was 10 seconds and at 245 °C. The programmed software attached to the device provides a means of measuring the force as the sample contacts the solder and the force with which the solder wets the surface of the sample. All measurements are set as a function of time. The zero-crossing time, the time of 5 mN, the average force after 5 seconds, and the critical parameter of the angle at 5 seconds are known from the measured values of the force calculated by the software program. The test was performed five times and the average results of the five tests are shown in Table 2 below.

The resulting data shows that the weldability of the silver-rich silver/tin alloy is similar to that of bright tin. The silver/tin alloy and the bright tin crossover time and the time of 5mN, the average force after 5 seconds and the angle at 5 seconds fall into the traditional Class I classification, and any zero crossing time less than 1.2 seconds It is considered to be very suitable for the industry. Class II is considered good and has a zero crossing time of less than 1.5 seconds, Class III is medium, and has a zero crossing time of less than 2 seconds, and Class IV is considered bad and has no zero crossing time. The good solderability of the silver/tin alloy indicates that it can be an acceptable substitute for the coated hard gold on the electrical connection.

Example 8

Three tin-rich tin/silver alloy plating baths as shown in Table 3 below were prepared.

¹ nonionic surfactant, 13 ethylene oxide unit

The pH of the three tin/silver plating baths is less than one. Solder bumps are plated in the vias using a respective tin/silver plating bath. A 4 cm x 4 cm wafer slice having a photoresist pattern of 75 μm (diameter) x 75 μm (deep) via and a copper seed layer is immersed in the plating bath in the plating bath and plated Tin/silver bumps. The samples were plated at 8 ASD in each bath. The temperature of each bath was 30 °C. An insoluble cerium oxide electrode was used as an anode in each of the examples. The plating was carried out until a bump of 60 μm was plated.

The resulting tin / silver layers form S2460 ^TM-based scanning electron microscope Hitachi used. The deposits are consistent, smooth, compact and free of nodules.

The silver concentration of the resulting tin-silver layer of this sample was determined using a conventional AAS method. The AAS device used for the measurement was manufactured by Varian, Inc. (Paluotu, California). The method comprises the steps of: 1) removing the photoresist; 2) measuring the weight of each tin-silver bump, for example, an average of 10 mg; 3) then dissolving each tin-silver bump in each of 10 to 20 mL of 30 to In a 40% nitric acid container (add more nitric acid if necessary to dissolve the tin/silver); 4) Transfer the dissolved tin/silver from each beaker to each 100 mL flask and add deionized water to the Volume and mixing; and 5) determining the amount of silver in each solution and determining the concentration of silver in the deposit using the following equation: %Ag = [10 x AAS _{Ag (ppm)} ] / weight _(mg) . The amount of each metal of the plated alloy is as shown in Table 4 below.

All baths provide tin-rich tin/silver deposits with good morphology and no observable nodules.

Claims (7)

An electroplating bath comprising one or more sources of silver ions; one or more sources of tin ions; one or more compounds having the formula: XSY(I) wherein X and Y may be substituted or unsubstituted phenolic groups, HO -R- or -R'S-R"-OH, the restriction is that when X and Y are the same, they are substituted or unsubstituted phenol groups, otherwise X and Y are different and wherein R, R' and R" A linear or branched alkylene radical having the same or different and R, R' and R" having from 1 to 20 carbon atoms; and one or more compounds having the formula: Wherein M is hydrogen, NH ₄ , sodium or potassium, and R ₁ is a substituted or unsubstituted linear or branched (C ₂ -C ₂₀ ) alkyl group, or substituted or unsubstituted (C ₆ -C ₁₀ ) aryl. The electroplating bath of claim 1, wherein the ratio of silver ions to tin ions ranges from 1 to 12. The electroplating bath according to claim 2, wherein the ratio of the silver ions to the tin ions ranges from 1 to 6. The electroplating bath of claim 1, wherein X and Y are different and are HO-R- or -R'-S-R"-OH. An electroplating bath as described in claim 1, wherein the formula (III) The ratio of the compound to the compound of the formula (I) is from 3 to 300. A method of electroplating comprising: a) contacting a substrate with an electroplating bath comprising one or more sources of silver ions; one or more sources of tin ions; one or more compounds having the formula: XSY wherein X and Y are The substituted or unsubstituted phenolic group, HO-R- or -R'S-R"-OH, is a substituted or unsubstituted phenolic group when X and Y are the same, otherwise X and Y Is different, and wherein R, R' and R" are the same or different and R, R' and R" are linear or branched alkyl radicals having 1 to 20 carbon atoms; and one or more Compound of the formula: Wherein M is hydrogen, NH ₄ , sodium or potassium, and R ₁ is a substituted or unsubstituted linear or branched (C ₂ -C ₂₀ ) alkyl group, or substituted or unsubstituted (C ₆ -C ₁₀ ) aryl; and b) electroplating silver and tin alloys on the substrate. A method of electroplating comprising: a) providing a semiconductor die having a plurality of interconnected bump pads; b) forming a seed layer on the interconnect bump pad; c) contacting the semiconductor die with an electroplating bath, the plating The bath comprises one or more sources of silver ions; one or more sources of tin ions; one or more compounds having the formula: XSY wherein X and Y may be substituted or unsubstituted phenolic groups, HO-R- or -R '-SR'-OH, the restriction is that when X and Y are the same, they are substituted or unsubstituted phenolic groups, otherwise X and Y are different, and wherein R, R' and R" are the same or different And R, R' and R" are straight-chain or branched-chain alkyl radicals having 1 to 20 carbon atoms; and one or more compounds having the formula: Wherein M is hydrogen, NH ₄ , sodium or potassium, and R ₁ is a substituted or unsubstituted linear or branched (C ₂ -C ₂₀ ) alkyl group, or substituted or unsubstituted (C ₆ -C ₁₀ ) aryl; d) electroplating silver and tin alloy interconnect bumps on the interconnect bump pads; and e) reflowing the silver and tin alloy interconnect bumps.