Electrodeposition process and a layered composite material produced thereby Number:6,797,409 from the United States Patent and Trademark Office (PTO) owispatent

Title: Electrodeposition process and a layered composite material produced thereby

Abstract: An electrodeposition process for producing a layered composite material and the layered composite material produced by the process. The layered composite material includes at least one layer of a first alloy species of an alloy and at least one layer of a second alloy species of the alloy. The first alloy species and the second alloy species have distinguishable properties. The process includes the steps of first energizing an electroplating circuit to provide a first electroplating current to deposit a layer of the first alloy species and second energizing the electroplating circuit to provide a second electroplating current to deposit a layer of the second alloy species. The alloy is preferably a gold-tin alloy, the first alloy species is preferably the Au.sub.5 Sn alloy phase and the second alloy species is preferably the AuSn alloy phase.

Patent Number: 6,797,409 Issued on 09/28/2004 to Ivey,   et al.

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Inventors:	Ivey; Douglas G. (Edmonton, CA), Djurfors; Barbara M. (Edmonton, CA), Doesburg; Jacobus Cornelius (Westbury, NY)
Assignee:	The Governors of the University of Alberta (Edmonton, CA)
Appl. No.:	10/029,456
Filed:	December 21, 2001

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Foreign Application Priority Data

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Dec 20, 2001 [CA]			2365749

Current U.S. Class:	428/635 ; 428/646; 428/672; 428/935
Field of Search:	425/615,635,646,672,935

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References Cited [Referenced By]

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U.S. Patent Documents

	3216806		November 1965		Sama et al.
	3396454		August 1968		Murdock et al.
	4391679		July 1983		Zilske et al.
	4869971		September 1989		Nee et al.
	4936927		June 1990		Grunke et al.
	5197654		March 1993		Katz et al.
	5277790		January 1994		Morrissey
	5902472		May 1999		Arai et al.
	6245208		June 2001		Ivey et al.

Foreign Patent Documents

	2268867		Oct., 2000		CA
	4406434		Aug., 1995		DE
	56 136994		Oct., 1981		JP
	58-100993		Jun., 1983		JP
	61 15992		Apr., 1994		JP
	2001-271127		Oct., 2001		JP
	WO 98/03700		Jan., 1998		WO

Other References

Kallmayer, C. et al., "Fluxless Flip-Chip Attachment Techniques Using the Au/Sn Metallurgy," IEEE/CPMT International Electronics Manfacutring Technology Symposium, 1995, pp. 20-28 (no month). . Kallmayer, C. et al., "Fluxless Flip-Chip Soldering Using the Eutectic Gold-Tin System--A Comparison between Self-Alignment and Thermode-Bonding," 10th European Microelectronics Conference, Apr. 1995, pp. 440-449. . Zakel, E. and Reichl, Herbert, "Flip-Chip Assembly Using Gold, Gold-Tin, and Nickel-Gold Metallury," Flip-Chip Technologies, ed., J. Lau, McGraw-Hill, (1995), pp. 415-468 (no month). . Mason, D.R. et al., "Alloy Gold Deposits: Have They Any Industrial Use?" Transactions of the Institute of Metal Finishing, 1974, vol. 52, pp. 143-148. . Raub, E. and Bihlmaier, K., "Galvanische Weissgoldniederschtage," Mitteilungen Des Forschungsinstituts und Probieramts fur Edelmetalle an der Staatl. 11 Jahrgang, Nr. 7/8, Okt./Nov. 1937, pp. 59-71. . Kubota, N. et al., "Electrodeposition of Gold-Tin Alloys from the Pyrophosphate Solution," J. Met. Fin. Soc. Japan, 34 (1983) No. 1, pp. 37-43 (no month). . Tanabe, Y. et al., "On the Microstructure and the Phase of Electrodeposition Au-Sn and Ag-Sn Alloys," J. Met. Fin. Soc. Japan, 34 (1983) No. 9, pp. 8-15 (no month). . Kubota, N., et al., "Conductivity and Ion Transport In Gold-Tin Pyrophosphate Baths," Plating and Surface Finishing, 71 (1984) Mar., pp. 46-49..

Primary Examiner: Zimmerman; John J.
Attorney, Agent or Firm: Kuharch; Terrence N. Rodman & Rodman

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Claims

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The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A layered composite material comprising a layer of a first alloy species of an alloy, the first alloy species having first alloy species properties and consisting essentially of a first alloy phase, and further comprising a layer of a second alloy species of the alloy, the second alloy species having second alloy species properties and consisting essentially of a second alloy phase, wherein the first alloy species properties are distinguishable from the second alloy species properties, wherein the alloy is comprised of gold and tin, wherein the first alloy phase is Au.sub.5 Sn and the second alloy phase is AuSn and wherein the layered composite material is comprised of a plurality of layers of each of the first alloy species and the second alloy species.

2. The layered composite material as claimed in claim 1 wherein the first alloy phase has a first alloy phase composition, wherein the second alloy phase has a second alloy phase composition, and wherein the first alloy phase composition is different from the second alloy phase composition.

3. The layered composite material as claimed in claim 2 wherein the material has a composite material composition and wherein the composite material composition is comprised of between about 25 at % tin and about 40 at % tin.

4. The layered composite material as claimed in claim 3 wherein the composite material composition is comprised of between about 27 at % tin and about 35 at % tin.

5. The layered composite material as claimed in claim 4 wherein the composite material composition is comprised of about 30 at % tin.

6. The layered composite material as claimed in claim 4 wherein the composite material composition is a eutectic composition.

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Description

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TECHNICAL FIELD

A layered composite material comprised of layers of an alloy and a process for producing the layered composite material.

BACKGROUND OF THE INVENTION

Gold-tin (Au--Sn) eutectic solders are commonly used in the optoelectronic and microelectronic industries for chip bonding to dies. Au--Sn solder is classified as a "hard solder" with superior mechanical and thermal properties relative to "soft" solders, such as the Pb--Sn system.

Au--Sn solder can be applied in a number of ways, i.e., as Au--Sn preforms, solder paste, by sequential evaporation and sequential electrodeposition. Compared with solder preforms and pastes, evaporated solder is cleaner and provides more precise thickness and positional control. Thin film technology, however, involves expensive vacuum systems.

Electroplating of Au--Sn eutectic solder is an attractive alternative in that it is a low cost process, offering the thickness and positional control of thin film techniques. Au--Sn solder layers have been produced sequentially by depositing Au first on a seed layer, followed by Sn (see for example C. Kallmayer, D. Lin, J Kloeser, H. Oppermann, E. Zakel and H. Reichl, 1995 IEEE/CPMT International Electronics Manufacturing Technology Symposium, (1995) 20; C. Kallmayer, D. Lin, H. Oppermann, J. Kloeser, S. Werb, E. Zakel and H. Reichl, 10.sup.th European Microelectronics Conference, (1995) 440; and E. Zakel and H. Reichl, Chapter 15, in Flip-Chip Technologies, ed., J. Lau, McGraw-Hill, (1995) 415.

Commercially available Au and Sn baths are utilized from which several microns of solder can be deposited sequentially. Co-electrodeposition or codeposition of Au and Sn from a single solution offers the same economic advantage of sequential plating relative to vacuum deposition techniques, as well as the prospect of depositing the solder in a single step without oxidation of an outer Sn layer.

One of the challenges with Au--Sn alloy plating baths is preventing the oxidation of Sn(II) to Sn(IV), as discussed in D. R. Mason, A. Blair and P. Wilkinson, Trans. Inst. Met. Finish., 52 (1974) 143. Oxidation of Sn can be minimized by using soluble Sn anodes. However, Au is deposited on the anodes unless they are isolated by semi-permeable diaphragms.

It has been reported that Au--Sn alloys containing up to 30 at (i.e. atomic) % Sn could be deposited from baths containing no free cyanide, and containing the Sn as its stannate complex formed with KOH (see E. Rau and K. Bihlimaier, Galvanische Weissgolniederschlage, Mitt. Forschungsinst. Probierants. Edelmetalle Staatl. Hoheren Fachschule Schwab. Gmund, 11 (1937) 59. Later claims concerning Au--Sn alloy plating, however, have been based on the use of alkaline and acid cyanide electrolytes, where Sn in many cases has been incorporated with the goal of obtaining brightening effects rather than producing deposits with significant amounts of Sn.

Several cyanide based systems have been reported (see T. Frey and W. Hempel, DE 4406434, (1995); W. Kuhn, W. Zilske and A.-G. Degussa, Ger. DE 4,406,434, Aug. 10, 1995: N Kubota, T. Horikoshi and E. Sato, J. Met. Fin. Soc. Japan, 34 (1983) 37; and Y. Tanabe, N. Hasegawa and M. Odaka, J. Met. Fin. Soc. Japan, 34 (1983) 8.

Frey and Hempel developed a bright Au--Sn plating bath with a pH of 3-14, comprised of potatassium dicyanoaurate, soluble Sn(IV), potassium hydroxide, potassium salt of gluconic, glucaric and/or glucaronic acid, conductivity salt, piperazine and a small amount of As. The bath was used to plate small parts with an alloy containing 5-25 wt % Sn. Bright deposits were obtained for thicknesses greater than 0.1 .mu.m and the solution exhibited long term stability without the use of soluble Sn anodes.

A.-G. Degussa, Ger. DE 4,406,434 teaches using potassium dicyanoaurate and tin chloride and claims a deposit composition of 8 wt % Sn and thickness of 5 .mu.m.

Au--Sn codeposition from a cyanide system using pyrophosphate as a buffering agent was studied by Kubota et al (N. Kubota, T. Horikoshi and E. Sato, J. Met. Fin. Soc. Japan, 34 (1983) 37; and N. Kubota, T. Horikoshi and E. Sato, Plating and Surface Finishing, 71 (1984) 46. The basic formula consisted of K.sub.4 P.sub.2 O.sub.7, Kau(CN).sub.2 and SnCl.sub.2 --2H.sub.2 O. The mass transfer was investigated to clarify reaction mechanisms between monovalent Au or bivalent Sn and pyrophosphate ions, by measuring conductivity, kinematic viscosity and limiting current density of the bath components. Two pyrophosphate ions were complexed with one stannous ion, with excess pyrophosphate acting as a supporting constituent.

Tanabe et al, referred to above, obtained various Au--Sn alloy compositions by electrodeposition from cyanide baths containing HauCl.sub.4 --4H.sub.2 O, K.sub.2 SnO.sub.3 --3H.sub.2 O, KCN and KOH. Although a linear relationship was not found between the Sn content in the bath and the Sn content in the alloy formed, a relationship was found between the two alloys which permitted formation of alloys of desired compositions. The composition of electrodeposited Au--Sn was shifted by about 10% to the Sn side in comparison with alloys at thermal equilibrium; thus exhibiting the .zeta. phase in the 25-29 at % range. AuSn, AuSn.sub.2 and AuSn.sub.4 were also electrodeposited.

Gold chloride electrolytes were used in the early days of Au plating, but today are employed almost exclusively in the electrochemical refining of Au. An extensive investigation of the cathodic behaviour of Au in chloride solutions has shown that the quality of the cathode deposit is strongly influenced by the relative amounts of Au(I) and Au(III) in the solution. The reduction of Au(III) chloride to the metal can be expected to involve the formation of Au(I) as an intermediate species. Under plating conditions, Au will be deposited from both the Au(III) and Au(I) species. Since Au(I) has a more positive plating potential (1.154 V) than Au(III) (1.002 V), a limiting current density for Au(I) will be reached first and it can be expected that the deposits will be of relatively poor quality, i.e., they tend to be bulky and porous. Gold fines will be present in the solution as a result of the following disproportionation reaction:

Detailed studies of the anodic and cathodic reactions have shown that the use of low temperatures and periodic interruption of the current are major factors that can contribute to reduced Au(I) concentration.

Japanese Patent JP 56 136994 to Masayoshi Mashiko describes a process carried out under alkaline conditions and employing a bath composition containing gold, tin and copper and sodium sulphite or potassium sulphite was used as a stabilizer for the gold.

Japanese Patent to S. Matsumoto and Y. Inomata, JP 61 15,992 [86 15.992], (Jan. 24, 1986) discloses a Au--Sn plating bath (pH=3-7) containing KauCl.sub.4, SnCl.sub.2, triammonium citrate, L-ascorbic acid, NiCl.sub.2 and peptone. A 7 .mu.m Au--Sn alloy (20.+-.2 wt % Sn) layer was plated out on a 50 mm diameter Si wafer at 208.degree. C. and a current density of 0.6 A/dm.sup.2 in 30 minutes using a Pt coated non-consumable Ti anode. The stability of the bath seemed to be the weak link in this process as stability decreased dramatically when the Sn salt was added.

U.S. Pat. No. 6,245,208 (Ivey et al), issued on Jun. 12, 2001 describes a relatively stable, weakly acidic, non-cyanide electroplating solution for codeposition of Au--Sn alloys over a range of compositions, including the technologically important eutectic and near eutectic compositions. In the preferred embodiment, the solution consists of Au and Sn chloride salts, as well as ammonium citrate as a buffering agent and sodium sulphite and L-ascorbic acid as stabilizers.

Ivey et al discusses the use of both direct current and pulsed current power sources and describes relationships between Sn content and average current density, Sn content and pulsed current "ON time", and Sn content and pulsed current "OFF time". These relationships indicate that within certain ranges, the Sn content of the resulting Au--Sn alloy will increase with an increase in average current density, pulsed current ON time, and pulsed current OFF time.

Ivey et al also discusses the effect of current density, pulsed current "ON time" and pulsed current "OFF time" upon the quality of the alloy deposit and provides some guidance for optimizing the electroplating process to obtain an alloy deposit of desired composition and quality.

Ivey et al contemplates the application of direct current or pulsed current at a single value of electroplating current density to produce an alloy deposit having a desired Sn content. Unfortunately, however, the relationships amongst the variables, although predictive, are subject to significant scatter due to numerous influences, such as edge effects, local current effects etc. As a result, the exact Sn content of the Au--Sn alloy deposit in Ivey et al is in practice somewhat difficult to control.

As a result, there remains in the art of alloy electrodeposition a need for an electrodeposition process which is capable of providing relatively precise control over the composition or other properties of the alloy deposit.

Preferably this process should be applicable to the electrodeposition of many different alloy systems, including but not limited the gold-tin alloy system.

SUMMARY OF THE INVENTION

The present invention is based upon the broad principle that by varying an electroplating current, it is possible to electrodeposit alloy species with distinguishable properties in a controlled manner.

In one aspect the invention is therefore directed at an electrodeposition process for separately depositing layers of at least two alloy species of an alloy to produce a layered composite material. The invention is also directed at a layered composite material comprising a layer of a first alloy species and a layer of a second alloy species, wherein the first alloy species and the second alloy species have distinguishable properties.

The distinguishable properties of the alloy species are due to different alloy phases or combinations of alloy phases being deposited in the alloy species. The invention is therefore applicable to any alloy system in which the alloy is capable of electrodeposition in two or more alloy phases and in which the identity of the electrodeposited alloy phase or phases is dependent upon the electroplating current.

In this specification, the terms "alloy" and "alloy system" indicate substances containing two or more essential elements which are defined by their essential elements and the term "alloy phase" describes a particular form or phase of a substance which contains the essential elements of the alloy or alloy system. For example, the gold-tin alloy or alloy system contains gold and tin as essential elements and may be produced in several different alloy phases, including for example Au.sub.5 Sn or AuSn.

In this specification, the term "alloy species" indicates a substance which is electrodeposited by the process using a specific electroplating current, which substance may be comprised of one alloy phase or a combination of alloy phases.

More particularly, the invention may be applied to any alloy system in which two or more alloy phases of the alloy can be selectively electrodeposited by controlling the electroplating current so that an alloy can be electrodeposited as a layered composite material of two or more alloy species which together contain two or more alloy phases. The properties of each particular alloy species are controlled by controlling the electroplating current. The layered composite material is therefore comprised of two or more alloy species and the overall properties of the layered composite material are dependent upon the properties and relative proportions of the different alloy species.

A single alloy species will include those alloy phases of the alloy which are electrodeposited at a selected electroplating current so that a single alloy species may be comprised of one or more alloy phases. Preferably, however, a selected electroplating current electrodeposits primarily or essentially a single alloy phase so that any particular alloy species consists primarily or essentially of a single alloy phase.

Regardless of whether a selected electroplating current deposits one alloy phase or more than one alloy phase, a selected electroplating current should preferably result in the electrodeposition of an alloy species which has consistent properties which are distinguishable from the properties of alloy species which are electrodeposited at a different selected electroplating current. This will facilitate the combination of layers of different alloy species to produce a layered composite material having desired properties.

There is no upper limit to the total number of layers which may make up the layered composite material and the layered composite material may be comprised of as few as two layers.

Regardless of the total number of layers which make up the layered composite material, there should preferably be one or more layers of at least two different alloy species, which alloy species have different properties. The layered composite material is preferably comprised of a plurality of layers of each alloy species.

The layered composite material may be comprised of as few as two alloy phases. Although theoretically there is no maximum number of alloy phases which may be deposited in the various layers of different alloy species, the number of alloy phases present in the layered composite material should preferably be minimized.

Similarly, the layered composite material may be comprised of as few as two alloy species, and although theoretically there is no maximum number of alloy species which may be deposited in the various layers, the number of alloy species present in the layered composite material should preferably be minimized.

The layered composite material is therefore most preferably comprised of two different alloy species, a plurality of layers of each alloy species, and with each alloy species consisting primarily or essentially of a single alloy phase.

The invention may also be applied to the production of an alloy deposit which comprises a single layer of a single alloy species instead of a layered composite material comprised of a plurality of layers of different alloy species. This single alloy species may be comprised of as few as two alloy phases, and although theoretically there is no maximum number of alloy phases which make up the single alloy species, the number of alloy phases comprising the single alloy species should preferable be minimized. Where the invention is applied to the production of a single layer alloy deposit instead of a layered composite material, the single alloy species is most preferably comprised of two different alloy phases.

In a preferred process aspect of the invention, the invention is an electrodeposition process for producing a layered composite material comprised of layers of an alloy, the process using an electroplating circuit comprising a power supply, an electroplating solution comprising ions of the elements comprising the alloy, and an electrodeposition substrate, the process comprising the following steps: (a) first energizing the electroplating circuit with the power supply to provide a first electroplating current in the electroplating circuit during a first current plating time interval to deposit a layer of a first alloy species of the alloy on the substrate, the first alloy species having first alloy species properties; and (b) second energizing the electroplating circuit with the power supply to provide a second electroplating current in the electroplating circuit during a second current plating time interval to deposit a layer of a second alloy species of the alloy on the substrate, the second alloy species having second alloy species properties;

wherein the first alloy species properties are distinguishable from the second alloy species properties.

In a preferred product aspect of the invention, the invention is a layered composite material comprising a layer of a first alloy species of an alloy, the first alloy species having first alloy species properties, and further comprising a layer of a second alloy species of the alloy, the second alloy species having second alloy species properties, wherein the first alloy species properties are distinguishable from the second alloy species properties.

The alloy species properties are distinguishable with respect to one or more properties so that by controlling the deposition of each alloy species, the properties of the layered composite material can be controlled by taking advantage of the different properties of the alloy species. The different property or properties of the alloy species may relate to any chemical or physical property. For example, the distinguishing property may be the chemical composition of the alloy species.

Preferably the first alloy species consists essentially of a first alloy phase and preferably the second alloy species consists essentially of a second alloy phase.

The first alloy phase and the second alloy phase will therefore be distinguishable with respect to one or more chemical or physical properties. Preferably the first alloy phase has a first alloy phase composition, the second alloy phase has a second alloy phase composition, and the first alloy phase composition is different from the second alloy phase composition.

The first alloy species and the second alloy species are combined in the layered composite material so that the layered composite material has composite material properties, including a composite material composition. The composite material properties include any chemical or physical properties. The composite material properties will depend upon the first alloy species properties, the second alloy species properties and the relative proportions of the first alloy species and the second alloy species comprising the layered composite material.

The first electroplating current and the second electroplating current may each either be a direct current or a pulsed current. Preferably the first electroplating current and the second electroplating current are both a direct current or both a pulsed current.

The first electroplating current and the second electroplating current are selected having regard to the particular alloy system and the particular electroplating process. The selection of the characteristics of the electroplating currents is guided by an understanding of the relationships between the properties of deposited alloys and electroplating current. Procedures for determining these relationships are taught in U.S. Pat. No. 6,245,208 (Ivey et al) with respect to the gold-tin alloy system. These relationships can be established easily for other alloy systems using the same general procedures.

The first electroplating current is preferably selected so that the first alloy species consists essentially of a first alloy phase and the second electroplating current is preferably selected so that the second alloy species consists essentially of a second alloy phase.

The relative proportions in the layered composite material of the first alloy species and the second alloy species will be dependent upon the first current plating time interval and the second plating time interval. As a result, the first current plating time interval and the second current plating time interval may be selected so that the layered composite material has a desired composite material composition which is obtained by combining the first alloy species and the second alloy species.

The alloy produced by the invention may be any alloy system which may be electrodeposited in different alloy species, which alloy species are dependent upon the electroplating current.

A preferred alloy system for use in the invention is the gold-tin alloy system. Within the gold-tin alloy system, the preferred alloy phases for use in the invention are Au.sub.5 Sn and AuSn.

The reason Au.sub.5 Sn and AuSn are preferred alloy phases is because a particularly desirable alloy composition for the optoelectronic and microelectronic industries is the eutectic gold-tin alloy composition, which comprises about 30 at % tin. Au.sub.5 Sn comprises about 15 at % tin and AuSn comprises 50 at % tin. As a result, it can be readily seen that a combination of Au.sub.5 Sn and AuSn can readily produce a layered composite material which has a composite material composition comprising anywhere between 15 at % tin and 50 at % tin, thus including the eutectic composition as well as near-eutectic compositions.

For example, by selection of the first current plating time interval and the second current plating time interval, Au.sub.5 Sn and AuSn can be electrodeposited as a layered composite material to provide a composite material composition of anywhere between about 15 at % tin and 50 at % tin, including between about 25 at % tin and about 40 at % tin, between about 27 at % tin and about 35 at % tin, as well as the eutectic composition.

Where the alloy system is the gold-tin alloy system, the first alloy species therefore consists primarily or essentially of a first alloy phase Au.sub.5 Sn and the second alloy species consists primarily or essentially of a second alloy phase AuSn.

Electroplating current density is a measure of electroplating current per unit area of electrodeposition substrate. In direct current applications, average current density and peak current density are the same. In pulsed current applications, average current density is a function of peak current density and duty cycle, and duty cycle is a function of electroplating current ON time and pulse cycle period.

It has been discovered that the relationship between average current density and alloy phase in the gold-tin alloy system is such that an average current density of less than or equal to about 1 mA/cm.sup.2 will result in the electrodeposition of an alloy species which consists essentially of Au.sub.5 Sn, while an average current density of greater than or equal to about 2 mA/cm.sup.2 will result in the electrodeposition of an alloy species which consists essentially of AuSn. It has also been discovered that an average current density within a range of between about 1 mA/cm.sup.2 and 2 mA/cm.sup.2 will result in a mixture of Au.sub.5 Sn and AuSn which varies greatly within that range.

Preferably the first electroplating current and the second electroplating current which are used with the gold-tin alloy system are both pulsed currents. Where the electroplating currents are pulsed currents, the pulsed current ON time, pulsed current OFF time and peak current density are selected first, to provide a suitable average current density to facilitate the electrodeposition of the desired alloy species and alloy phases and second, to provide an alloy deposit which has a suitable quality in terms of grain size and structure.

Fine grained and smooth alloy deposits are generally preferred over coarse grained and rough alloy deposits. The following general trends in alloy electrodeposition are noted: 1. grain structures tend to become less coarse as either average current density or peak current density increase, for current density values below a limiting current density value; 2. grain structures tend to become more coarse as either average current density or peak current density exceed a limiting current density value; 3. grain structures tend to become more coarse with increasing pulsed current ON times; and 4. grain structures tend to become less coarse with increasing pulsed current OFF times.

The limiting current density values for any particular alloy system can easily be determined. In the case of the gold-tin alloy system, it has been found that preferred ranges for the characteristics of the first electroplating current and the second electroplating current are as follows:

pulsed current ON time: greater than or equal to about 2 milliseconds per pulse cycle, most preferably about 2 milliseconds per pulse cycle; pulsed current OFF time: greater than or equal to about 4 milliseconds per pulse cycle, most preferably about 8 milliseconds per pulse cycle; pulse cycle period: about 6 milliseconds to about 12 milliseconds, most preferably about 10 milliseconds.

The electroplating solution may be any electrolytic solution which includes a suitable solvent containing ions of the elements comprising the alloy or alloy system and which has been suitably stabilized for use as an electroplating solution so that it is capable of codepositing the elements of the alloy or alloy system as two or more alloy species.

As previously indicated, one of the preferred alloy systems for use with the invention is the gold-tin alloy system. In the gold-tin alloy system, a preferred electroplating solution comprises ammonium citrate, a salt of gold soluble in the ammonium citrate, a salt of tin soluble in the ammonium citrate, a gold stabilizer and a tin stabilizer.

Preferably the gold salt is a gold chloride and the tin salt is a tin chloride. More preferably the gold salt is potassium gold chloride (KAuCl.sub.4) and the tin salt is tin chloride (SnCl.sub.2).

Preferably the gold salt is present in the electroplating solution in the amount of between about 5 g/L and about 15 g/L and the tin salt is present in the amount of between about 5 g/L and about 15 g/L.

Preferably the ratio of gold to tin in the electroplating solution is in the range of about 0.5 to about 3.0 (by weight).

Preferably the gold and the tin are present in a ratio to form the alloy phases Au.sub.5 Sn and AuSn and are present in a ratio conducive to producing a layered composite material which may contain anywhere between about 15 at % Sn and about 50 at % Sn.

The gold stabilizer and the tin stabilizer may be any substances which will improve the stability of the electroplating solution and facilitate electrodeposition of the layered composite material. Exemplary gold stabilizers include sodium sulfides such as Na.sub.2 SO.sub.3 (sodium sulphite) and Na.sub.2 S.sub.2 O3, with Na.sub.2 SO.sub.3 (sodium sulphite) being most preferred, particularly where the gold salt is KAuCl.sub.4. A preferred tin stabilizer is ascorbic acid, and in particular L-ascorbic acid.

The preferred electroplating solution may, for example, be prepared in accordance with the method described in U.S. Pat. No. 6,245,208 (Ivey et al) by dissolving a suitable tin salt in ammonium citrate to form a tin solution, dissolving a suitable gold salt in ammonium citrate to form a gold solution, and then combining and mixing the tin solution and the gold solution.

Preferably the gold stabilizer is added to the gold solution and the tin stabilizer is added to the tin solution before the gold and tin solutions are combined.

The layers of the layered composite material may be any thickness, as determined by the lengths of the plating time intervals. Preferably the thickness of the layers is kept relatively small so that the alloy species and alloy phases in the various layers will approximate a homogeneous or completely interspersed structure. Most preferably the thickness of the layers ranges from submicron dimensions (<10 nm) to several microns.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an electroplating arrangement suitable for use in the invention.

FIG. 2 is a plot of Sn concentrations in gold-tin alloy deposits when obtained at different average current densities under direct current (DC) and pulsed current (PC) conditions.

FIG. 3 is a copy of scanning electron microscope (SEM) top view images of a plurality of PC and DC plated deposits of gold-tin alloys.

FIG. 4 is a copy of enlarged SEM top view images showing edge effects on gold-tin alloy deposits obtained at different average current densities.

FIG. 5 is a plot showing the effect of ON time in PC plating on gold-tin alloy composition, for a fixed average current density and cycle period.

FIG. 6 is a copy of SEM top view images of a plurality of gold-tin alloy deposits resulting from PC plating at various ON times, corresponding to the ON times depicted in FIG. 5.

FIG. 7 is a plot showing the effect of ON time on gold-tin alloy composition at a constant peak current density.

FIG. 8 is a copy of SEM top view images of a plurality of different PC gold-tin alloy deposits resulting from different ON times at constant peak current density, corresponding to the ON times depicted in FIG. 7.

FIG. 9 is a series of cleaved cross section images of gold-tin alloy deposits resulting from different ON times and constant peak current density, corresponding to the ON times depicted in FIG. 7.

FIG. 10 is a plot of gold-tin alloy deposit composition at different OFF times and constant peak current density.

FIG. 11 is a copy of SEM top view images of gold-tin alloy deposits obtained with different OFF times, corresponding to the OFF times depicted in FIG. 10.

FIG. 12 is a copy of SEM cross section images for gold-tin alloy deposits obtained at selected OFF times, corresponding to the OFF times depicted in FIG. 10.

FIG. 13 is a copy of SEM images of a polished and cleaved gold-tin alloy deposit obtained in a reproducibility test.

FIG. 14 is a copy of backscattered electron (BSE) images of several polished cross sections of a gold-tin alloy deposit obtained in a reproducibility test.

FIG. 15 is a plot showing the at % Sn content of gold-tin alloy deposits at locations across the deposit measured from the semiconductor/solder interface outwards.

FIG. 16 is a schematic plan illustration of an electroplating substrate depicting a gold contact area, a stop-off lacquer area and an exposed gold seed layer area for plating.

FIG. 17 is a plot showing the at % Sn content of gold-tin alloy deposits obtained at different values of average current density.

FIG. 18 is a plot showing the at % Sn content of alloy deposits containing Au.sub.5 Sn, AuSn and mixtures thereof as obtained at different values of average current density.

FIG. 19(a) through FIG. 19(c) are diffraction patterns showing spectra obtained from the Au.sub.5 Sn region, the AuSn region and the Au.sub.5 Sn--AuSn region as depicted in FIG. 18.

FIG. 20 is a phase diagram for the gold-tin alloy system showing the Au.sub.5 Sn alloy phase, the AuSn alloy phase and the melting points for gold-tin alloys containing between 0 at % Sn and 50 at % Sn.

FIG. 21(a) and FIG. 21(b) are SEM top view images of an Au.sub.5 Sn alloy phase deposit and an AuSn alloy phase deposit respectively.

FIG. 22 is a BSE image of layers of a gold-tin alloy deposit showing a layer of the Au.sub.5 Sn alloy phase and a layer of the AuSn alloy phase on top of a gold seed layer.

FIG. 23(a) is a low magnification BSE image of a gold-tin alloy layered composite material comprising a plurality of layers of each of the Au.sub.5 Sn alloy phase and the AuSn alloy phase in which the first current plating time interval is 21 minutes and the second current plating time interval is 5 minutes.

FIG. 23(b) is a high magnification BSE image of a gold-tin alloy layered composite material comprising a plurality of layers of each of the Au.sub.5 Sn alloy phase and the AuSn alloy phase in which the first current plating time interval is 21 minutes and the second current plating time interval is 5 minutes.

DETAILED DESCRIPTION

In the preferred embodiment the present invention is an electrodeposition process for producing a layered composite material comprised of layers of an alloy, wherein the layered composite material includes at least one layer of a first alloy species and at least one layer of a second alloy species.

The invention is intended for use with any alloy system in which the alloy is capable of being electrodeposited as different alloy species, the deposition of which is dependent upon the electroplating current, but is hereafter described with reference to the gold-tin alloy system as a preferred embodiment, in which Au.sub.5 Sn is the first alloy species and AuSn is the second alloy species.

The invention may be practiced with alloy systems other than the gold-tin alloy system. The first step in practicing the invention with another alloy system is to select as an electroplating solution an electrolytic solution which includes a suitable solvent containing ions of the elements comprising the alloy or alloy system and which has been suitably stabilized for use as an electroplating solution so that it is capable of codepositing the elements of the alloy or alloy system as two or more alloy species. The second step in practicing the invention with other alloy systems is to select electroplating currents which will produce desired alloy species of the alloy system in order to form the layered composite material. The electroplating currents may be selected with reference to the phase characteristics of the alloy system, which phase characteristics may be represented as a phase diagram similar to the phase diagram for the gold-tin alloy system which is shown in FIG. 20.

One of the lead-free solders currently being used in optoelectronic and microelectronic packaging applications is the eutectic gold-tin alloy (approximately 30 at % Sn). In addition to the obvious environmental advantages of not containing lead, gold tin alloys also have excellent thermal and mechanical properties making gold-tin alloys a hard solder well suited for packaging applications in which long-term device reliability is important. In addition, the comparatively low melting temperature of 280.degree. C. for the eutectic gold-tin alloy makes gold-tin alloys ideally suited for applications in which the materials are temperature sensitive.

Presently, most eutectic gold-tin alloys are prepared as solder preforms. The major drawback of this technique is that it requires expensive robots to place the preforms or it must be done manually, which is very labor intensive. Thin film deposition by evaporation or sputtering of the solder is an attractive alternative, since the oxide content is reduced relative to preforms and process control is better in terms of thickness uniformity and solder alignment. However, standard thin-film equipment is costly from a production viewpoint.

An alternate thin film deposition technique is electrodeposition. The benefits include reduced oxide formation, thickness uniformity, improved solder alignment (relative to performs) and significantly reduced capital costs, suggesting a strong commercial viability for this technique. Electrodeposition of an alloy solder can be either done sequentially of simultaneously. With sequential deposition, a pure tin layer is deposited on top of a pure gold layer. The disadvantage of this technique is that a post-deposition anneal is required to homogenize the composition through inter-diffusion. In addition to being a time consuming, multi-step process, such treatments often lead to segregation of the tin to the surface of the alloy layer resulting in the formation of an oxide layer that interferes with bonding.

One important advantage of direct alloy co-electrodeposition is that it is a one-step deposition procedure that requires no further heat treatment of diffusion during bonding.

An electroplating solution for use in co-electrodepositing gold-tin alloys and a method for co-electrodepositing gold-tin alloys has previously been developed and is described in U.S. Pat. No. 6,245,208 (Ivey et al). U.S. Pat. No. 6,245,208 (Ivey et al) is hereby incorporated by reference into this specification for its guidance in preparing electroplating solutions and for its guidance in electroplating methodology generally.

Expanding upon and refining the work which formed the basis of U.S. Pat. No. 6,245,208 (Ivey et al) it has now been shown that two distinct alloy phases, Au.sub.5 Sn and AuSn, can be deposited separately over a range of current densities at compositions of 15 at % Sn and 50 at % Sn respectively. By adjusting the electroplating current, it is possible to deposit both alloy phases in a layered composite material thereby achieving any desired composition between 15 at % Sn and 50 at % Sn, including the commercially important eutectic composition. Notably, this further work based upon U.S. Pat. No. 6,245,208 (Ivey et al) has demonstrated a composition plateau of 50 at % Sn for gold-tin alloys at average current densities exceeding about 2 mA/cm.sup.2, whereas in U.S. Pat. No. 6,245,208 (Ivey et al) a composition plateau of about 37-42 at % Sn was observed at similar average current densities.

As a result, in a preferred embodiment, the present invention is a method of depositing eutectic and near eutectic gold-tin alloys from a single electroplating solution as a layered composite material using the principles of alloy co-electrodeposition. In this way, deposition of the gold-tin alloy can occur directly on the wafer substrate without the need for any further homogenization treatments. The process may be tailored to produce any gold-tin alloy composition between about 15 at % tin and 50 at % tin without having to adjust the composition of the electroplating solution. By minimizing the thickness of the layers comprising the layered composite material, a completely interspersed structure can be approximated which will exhibit essentially the same physical properties as an equivalent alloy composition which does possess a true interspersed structure.

In the preferred embodiment pertaining to the gold-tin alloy system, a single electroplating solution is utilized for the deposition of any layered composite material in the gold-tin alloy system which has a composite material composition of between about 15 at % tin and 50 at % tin.

1. The Preferred Electroplating Solution

The electroplating solution of the preferred embodiment is composed of ammonium citrate (H.sub.2 NO.sub.2 CCH.sub.2 C(OH)(CO.sub.2 NH.sub.2)CH.sub.2 CO.sub.2 NH.sub.2), preferably triammonium citrate which functions as a buffering agent and in which a gold salt and a tin salt as well as stabilizing compounds for the gold and tin salts are dissolved. The gold and tin salts are preferably chlorides, most preferably potassium gold chloride KAuCl.sub.4 and SnCl.sub.2 respectively.

It is believed that other gold or tin salts may be suitable for use in the present invention; for example tin sulfate and HAuCl.sub.4 are possibilities.

In the preferred embodiment a suitable stabilizer is used for the gold salt and another suitable stabilizer is used for the tin salt. It has been found that suitable stabilizers for the gold salts are Na.sub.2 SO.sub.3 (sodium sulphite) and Na.sub.2 S.sub.2 O.sub.3, although Na.sub.2 SO.sub.3 is more effective at reducing gold precipitation during the addition of tin salt. Ethylene diamine has also been tried as a gold stabilizer, but in testing has been found to provide only marginal improvement in electroplating solution (i.e. bath) stability. When the preferred gold salt KAuCl.sub.4 is used, the preferred gold stabilizer is sodium sulphite (Na.sub.2 SO.sub.3).

A suitable stabilizer for the tin salt is ascorbic acid. When the preferred tin salt namely SnCl.sub.2 is used, the preferred stabilizer is ascorbic acid, more specifically L-ascorbic acid (HOCH.sub.2 CH(OH)(C(H)OC(O)C(OH)C(OH)).

The KAuCl.sub.4 and SnCl.sub.2.2H.sub.2 O salts are the sources of the initial Au (III) and Sn (II) ions, some of which immediately form the other possible valence states: Au (I) and Sn (TV). The tri-ammonium citrate functions as a buffer to maintain a nearly neutral solution pH. Sodium sulphite acts as a complexing agent for the gold, and to some degree for the tin. The following reactions are the most likely complexing reactions according to the specific stereochemistry of the Au (I), Au (III), Sn (II), and Sn (IV) ions [7]. The electroplating solution likely contains a mixture of all possible ions.

The L-ascorbic acid is used to prevent the hydrolysis of the tin in water. It acts as a chelating agent for the tin, thereby preventing its reaction with water. Although no specific reaction mechanism has been reported in the literature, the following reactions are suggested as possible complexing reactions between the tin and the L-ascorbic acid:

In the preferred embodiment of the invention the five principal constituents of the electroplating solution are preferably present in the ranges as set forth in Table A.

TABLE A Broad range Preferred range grams/Liter (g/L grams/Liter (g/L of electroplating solution of electroplating solution ammonium citrate 100 to 800 100 to 200 gold salt 5 to 20 5 to 10 tin salt 5 to 20 5 to 10 gold stabilizer 20 to 120 40 to 80 tin stabilizer 15 to 60 15 to 30

Optionally, nickel chloride (NiCl.sub.2) may be added to the electroplating solution as a leveler, preferably in an amount of between about 0 and 2 g/L.

Eutectic or near eutectic gold-tin alloy compositions are attractive for microelectronic/optoelectronic applications because of their relatively low melting temperatures.

The eutectic composition for the gold-tin alloy system is approximately 70 at % Au and 30 at % Sn. This eutectic composition provides the lowest melting temperature for subsequent bonding applications. Near eutectic compositions, particularly hypereutectic (greater than 30% Sn) are also desirable, because gold-tin alloy solder may be used to bond gold coated wafers and chips which when combined with the solder lowers the overall tin content in the solder. Also, tin-rich solders do not increase the melting point as much as gold-rich solders (gold-rich relative to the eutectic composition). Generally the desired composite material composition will range from 25 to 40 at % Sn and more preferably from 27 to 35 at % Sn and most preferably for some applications at or very near to the eutectic composition.

For a given electroplating solution composition, the composite material composition can be controlled by controlling the electrodeposition conditions, including type of current (DC or PC), current ON time, current OFF time, average current density and peak current density.

In the examples that follow, a 1:1 ratio of Au salt to Sn salt was used in the electroplating solution.

A possible alternate electroplating solution for the gold-tin alloy system is the chloride system taught in the Matsumoto Japanese Patent JP 61 15,992. Preliminary experiments were carried out on the solution described in the patent, but the solution deteriorated immediately when Sn salt was added to the ammonium citrate buffered Au solution.

2. Preparation of the Preferred Electroplating Solution

The starting solution of the preferred compounds as above indicated was based on the Matsumoto Patent JP 61 15,992 and are listed below:

200 g/L ammonium citrate (H.sub.4 NO.sub.2 CCH.sub.2 C(OH)(CO.sub.2 NH.sub.4)CH.sub.2 CO.sub.2 NH.sub.4) 20 g/L KAuCl.sub.4 13 g/L SnCl.sub.2.2H.sub.2 O 30 g/L L-ascorbic acid (HOCH.sub.2 CH(OH)(C(H)OC(O)C(OH)C(OH)) 1 g/L NiCl.sub.2 5 g/L peptone

The electroplating solution was prepared according to the various techniques summarized in Table I.

TABLE I Electroplating Solution Preparation. Solution # Solution Observations A 13 g/L SnCl.sub.2.2H.sub.2 O dissolved in Clear solution with pH = 1.7 30 g/L L-ascorbic acid solution Precipitation after 1 week B 13 g/L SnCl.sub.2.2H.sub.2 O dissolved in 200 Clear solution with pH = 6.5 g/L ammonium citrate solution Solution still clear after 1 week but turned to dark yellow C 10 g/L KAuCl.sub.4 dissolved in water Solution turned black and turbid on standing. Precipitated fine black powder. D 10 g/L KAuCl.sub.4 dissolved in water Solution turned black and turbid on in darkness. standing. Precipitated find black powder. E 10 g/L KAuCl.sub.4 dissolved in a 200 Clear solution and stable in light. g/L ammonium citrate solution F Solution E added to B. Solution turned black and turbid on standing. Precipitated fine black powder. G 10 g/L KAuCl.sub.4 dissolved in a 800 Same phenomena as Solution F. g/L ammonium citrate solution and then Solution B added. H 1. 10 g/L KAuCl.sub.4 dissolved in a Clear solution with dark green colour. 800 g/L ammonium citrate Precipitation after a few hours. solution 2. 13 g/L SnCl.sub.2.2H.sub.2 O dissolved in 400 g/L ammonium citrate solution 3. Solution (2) added to Solution (1) drop-by-drop with vigorous agitation

Initial electroplating solution preparation results are shown in Table I. If Sn chloride is mixed with water, without any additives, the bivalent Sn chloride salt undergoes hydrolysis according to:

with a solubility product for Sn(OH).sub.2 of 3.times.10.sup.-27.

Solution A in Table I contained 30 g/L of L-ascorbic acid, while Solution B contained 200 g/L of ammonium citrate. Both solutions were acidic, which helps to minimize hydrolysis preventing hydroxide precipitation. After one week Solution A became turbid, while Solution B changed to dark yellow from colourless, but remained clear. The difference may imply that ammonium citrate is a complexing agent for Sn.sup.2+ ions; however, no information was found in the literature concerning the complexing ability of ammonium citrate with bivalent Sn ions. Although the actual chemistry for the change in the solutions is not well understood, the change is attributed to the oxidation of stannous ions (II) by dissolved air to stannic ions (IV) and the formation of stannic compounds. Higher temperatures than room temperature result in increased oxidation rates. It can therefore be concluded that without any anti-oxidant additives, Solutions A and B are only stable for about a week. The behaviour of bivalent Sn ions in water is very complex. Possible forms of Sn ions in a chloride solution include [SnCl].sup.+, [SnCl.sub.2 ], [SnCl.sub.3 ].sup.- and [SnOH].sup.+ with stability constants of 14, 15, 50 and 10.sup.10, respectively.

KAuCl.sub.4 is soluble in aqueous solutions and is light sensitive. It is used for toning silver photographic prints. Preparation of Solutions C and D (Table I) shows that KAuCl.sub.4 undergoes hydrolysis both in light and in darkness. The solutions precipitate a fine black powder, which gradually changes to a gold color on standing. The powder was determined by EDX analysis to be metallic Au. In aqueous solution, AuCl.sub.4.sup.- ions are hydrolyzed to some extent forming (AuCl.sub.3)H.sub.2 O. This in turn acts as a weak acid forming species such as AuCl.sub.4-n (OH).sub.n (where n varies from 0 to 4 and increases with increasing alkalinity) in alkaline solutions.

The pH value of Solution E containing 200 g/L of ammonium citrate falls in the range of a weak acid. The hydrolysis of KAuCl.sub.4 is prevented by the presence of concentrated ammonium citrate. (NH.sub.4).sup.+ hydrolyzes in water,

and produces a significant amount of NH.sub.3 that dissolves in the solution. NH.sub.3 can form complex Au(NH.sub.3).sup.3+ cations with simple Au(III) ions (if any are present) in the solution. The stability of Au(III) ions in the solution is further improved. The stability constant for AuCl.sub.4.sup.- is 10.sup.26 ; however, no stability constant data for Au(NH.sub.3).sup.3+ is available in the literature.

Preparation of Solution F (Table I) was the first attempt to make a Au--Sn solution. It turned black and turbid immediately after the Au solution (E) was added to the Sn solution (B). The exact chemistry responsible for the instantaneous precipitation of fine black powder is not clear because of the lack of relevant information. Still, it is reasonable to surmise that a chemical interaction between Au ions and Sn ions causes the problem. The chemical processes for Au precipitation when Sn salt and Au salt are mixed can be AuCl.sub.4.sup.- ion reduction to AuCl.sub.2.sup.- ions, followed by AuCl.sub.2.sup.- ion dissociation.

Since ammonium citrate is able to complex Au ions, solutions with more concentrated ammonium citrate should be more stable. Preparation of Solutions G and H is the result of such an attempt. No improvement was found for Solution G, while Solution H was the first solution that remained clear after preparation. Solution H was prepared by adding the Au solution drop-by-drop instead of by pouring the entire Au solution in the Sn solution. This implies that a high concentration of ammonium citrate is needed to eliminate the chemical reaction between Au(III) ions and Sn(II) ions. The way that ammonium citrate works may be twofold, i.e., as either a Au complexing agent or a Sn complexing agent. Since a very high concentration of ammonium citrate is needed to stabilize Au or Sn ions, it can be surmised that it is not a strong complexing agent for either Au(III) or Sn(II) ions. Solution H has two major problems in terms of being used as a practical plating solution. One problem is its short lifetime; the solution deteriorated by precipitating only a few hours after preparation. The other problem is the high viscosity of the solution, due to the high concentration of ammonium citrate. High viscosity results in a slow mass transport rate and therefore a lower limiting current density. Although the improvement in Solution H relative to the other solutions was minor, the key to developing a stable Au--Sn solution seems to lie in finding a more efficient Au complexing agent to decrease the oxidizing ability of Au ions when mixed with the reducing agent, bivalent Sn.

It will be apparent that to obtain a stable solution may require the use of a specific mixing sequence, as without it the results may not be acceptable. As shown in Table 1, the procedure defined in H was the only one that succeeded and it required that the gold salt be dissolved in the ammonium citrate and then a solution of the tin salt in ammonium citrate be added drop (volume less than about 5 mL) after drop to the gold salt solution while under continuous vigorous agitation. Although the specific mixing sequence is believed to be important, further testing has suggested that the gold solution and the tin solution may be combined and mixed in bulk (i.e., not drop by drop) with satisfactory results.

While procedure H showed the most promise, it still did not provide the stability required for most commercial operations.

To compensate for this deficiency in stability, three candidate stabilizers were reviewed namely, Na.sub.2 SO.sub.3 (20-100 g/L), Na.sub.2 S.sub.2 O.sub.3 (20-100 g/L) and Na.sub.2 H.sub.2 EDTA.2H.sub.2 O(5-40 g/L). The stabilizers were added separately to a solution of 300 g/L of ammonium citrate and 10 g/L of KAuCl.sub.4. The solution preparation procedure was to add chemicals in the following sequence: ammonium citrate, Au salt, stabilizer and then the Sn chloride salt (5 g/L). Each solution was stirred thoroughly after each step to ensure complete dissolution.

Na.sub.2 SO.sub.3 was more effective than Na.sub.2 S.sub.2 O.sub.3 at reducing Au precipitation during the addition of Sn salt. The Na.sub.2 SO.sub.3 containing solution was clear and stable for several days, while Au precipitation occurred within a few minutes for the Na.sub.2 S.sub.2 O.sub.3 containing solution. Na.sub.2 H.sub.2 EDTA is a complexing agent for many base metal impurities in plating baths. However, it fails to prevent interaction between Au and Sn ions; Au precipitates on the wall of the beaker within a few minutes of mixing the Au and Sn solutions.

In the method of preparing Au--Sn sulphite solutions of the present invention, Au is added in the form of solid KAuCl.sub.4 salt that is dissolved in a concentrated ammonium citrate solution. When Na.sub.2 SO.sub.3 is added to the solution, no precipitation occurs. It is presumed that the Au(III) ions have been reduced to Au(I) ions. The stability of the Au--Sn solution was substantially improved; no Au precipitation occurred when Sn salt was added.

Based on the screening tests, Na.sub.2 SO.sub.3 (sodium sulphite) was selected as a Au stabilizer for additional tests. L-ascorbic acid was chosen as the Sn stabilizer to prevent Sn hydrolysis.

Experiments were carried out according to Table II to test solution lifetime for different concentrations of additives.

TABLE II Solutions Utilized for Bath Stability Tests. S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 Ammonium 200 200 200 200 100 200 200 200 200 200 citrate (g/L) KAuCl.sub.4 5 5 5 5 7 7 7 10 14 (g/L) Na.sub.2 SO.sub.3 60 60 60 60 30 60 60 (g/L) L-ascorbic 15 15 15 15 15 15 30 Acid (g/L) SnCl.sub.2. 2H.sub.2 O 5 5 5 7 7 7 10 14 (g/L) Solution 0 0 4 15 11 9 3 7 8 7 Stability (days)

Solutions S1 and S2, which contained no sodium sulphite, deteriorated immediately when Sn salt was added. With 60 g/L of Na.sub.2 SO.sub.3, Solution S3 remained clear and stable for four days; after which it began to gradually precipitate fine Au particles. Solution S4 was the same as S3, except for the addition of 15 g/L of L-ascorbic acid. The solution stability was improved to fifteen days. Its stabilizing effect is quite surprising since L-ascorbic acid was originally added to prevent Sn hydrolysis. L-ascorbic acid only changed the pH from 6.5 to 6.0, si