GB2121444A

GB2121444A - Electroless gold plating

Info

Publication number: GB2121444A
Application number: GB08315556A
Authority: GB
Inventors: Mohamed Fathy El-Shazly; Alan Halecky; Kenneth Derek Baker
Original assignee: Occidental Chemical Corp
Current assignee: Occidental Chemical Corp
Priority date: 1982-06-07
Filing date: 1983-06-07
Publication date: 1983-12-21
Also published as: IT1171818B; FR2528073B1; AT380902B; DK231783A; GB8315556D0; NL8302029A; DK231783D0; IT8348420A0; CH655132A5; DE3320308C2; BE896977A; ES8407520A1; AU541923B2; JPS591668A; SE8302798D0; FR2528073A1; ATA193183A; SE8302798L; ES523032A0; CA1188458A

Abstract

An electroless or autocatalytic gold plating bath includes, as the gold ingredient, an admixture of (a) a water soluble trivalent gold component selected from an alkali metal auricyanide, an alkali metal aurihydroxide, and an alkali metal aurate; and (b) a water-soluble monovalent gold component such as an alkali metal aurocyanide. The bath contains an amino borane, alkali metal borohydride, alkali metal cyanoborohydride, hydrazine, or hyposulphite as the reducing agent; an alkaline agent such as an alkali metal hydroxide; and an alkaline buffering agent. The method of using such an electroless or autocatalytic plating bath for depositing gold on metallic substrates such as gold, copper, copper alloys, electroless copper, electroless nickel, nickel, nickel alloys, etc. and on a nonmetallic substrate is also described.

Description

SPECIFICATION Electroless gold plating The present invention relates to an improved method for the electroless or autocatalytic deposition of gold on substrates; and more particularly to the use of a novel electroless plating bath for depositing gold on metallic and non-metallic substrates.

In recent years a fairly substantial literature has developed with respect to the electroless method of gold plating on surfaces. U.S. patents of special interest both as to the electroless gold plating method and the problems associated with this procedure include 3,300,348 (Luce); 3,589,916 (McCormack); 3,389,916 (Gostir); 3,697,296 (Bellis); 3,700,469 (Okinaka); 3,798,056 (Okinaka et al.); 3,917,885 (Baker); as well as the earlier patents and articles cited therein. Relevant articles include: Rich. D.W., Proc. American Electroplating Society, (1971); Y. Okinaka, Plating 57,914 (1970); and Y. Okinaka and C. Wolowodink, Plating 58 1080 (1971).This body of literature is pertinent to the present invention insofar as it discloses alkali metal cyanides as the source of the gold or related metal in the electroless bath as well as the use of alkali metal borohydrides and amine boranes as reducing agents. Thus, for example, the 1970 article by Okinaka as well as his U.S. Patent 3,700,469 describes an electroless gold plating bath having the following ingredient: (1) soluble alkali metal gold complex; (2) excess free cyanide such as potassium cyanide: (3) An alkaline agent such as potassium hydroxide; and (4) a borohydride or an amine borane The 1971 article by Okinaka et al. as well as Baker's U.S. Patent 3,91 7,885 point out the problems associated with the use of these particular plating baths, particularly when the cyanide concentration is increased.Other problems were encountered when bath replenishment was carried out and the baths became unstable when a plating rate of about 2.5 microns was approached. The need to avoid undesirable gold precipitation from the baths is also noted.

In U.S. Patent 3,917,885 these problems were overcome by utilising, as the gold or related metals source, an alkali metal imide complex formed from certain imides. In order to maintain the electroless gold plating at the desired pH of about 11 to 14, the Baker patent suggests the addition to the bath of alkali metal buffering salts such as the citrates, etc. The need to utilise special imides in preparing the gold imide complex is an obvious commercial disadvantage.

It also has been proposed in G.B. Patent Application No. 2095292 to overcome the prior art problems by utilising a trivalent gold metal complex or compound as the source of the gold in the plating bath. Unfortunately, under certain large scale commercial conditions it has been found that the substitution of trivalent gold for the monovalent gold complexes or compounds may not entirely overcome the problems pertaining to erratic bath stability, metal deposition, and bath replenishment.

The present invention enables the provision of an improved electroless or autocatalytic gold plating bath which avoids the problems and disadvantages of the baths heretofore proposed.

The present invention also enables the provision of an improved electroless or autocatalytic gold plating bath which will readily deposit gold on gold as well as on a variety of metallic and non-metallic substrates with good adhesion.

The present invention further enables the provision of an improved electroless or autocatalytic gold plating bath which will readily deposit ductile, lemon yellow pure gold on substrates at very desirable commercial rates and thicknesses.

The present invention still further enables the provision of an electroless or autocatalytic gold plating bath which has enhanced stability and can be effectively replenished.

According to a first aspect of the present invention, there is provided an aqueous electroless gold plating composition comprising a water-soluble trivalent gold component, a water-soluble monovalent gold component and a reducing agent.

In accordance with a preferred aspect of the present invention it has now been found that a further improvement in an electroless or autocatalytic gold plating bath and gold plating procedure can be achieved by utilising as the source of the gold in the plating bath an admixture (a) a trivalent gold metal complex or compound such as alkali metal auricyanides, alkali metal aurates, or alkali metal aurihydroxides; and (b) a monovalent gold metal complex or compound such as alkali metal gold aurocyanide. More particularly, the present invention pertains to autocatalytic baths and procedures, i.e. where the gold can be plated on gold as well as on other suitably treated metallic or non-metallic substrates. Thus, the term "electroless" as used in this specification is intended to encompass autocatalytic plating.

The electroless plating baths of this invention will also contain a suitable reducing agent such as an amino borane or an alkali metal borohydride, cyanoborohydride, hydrazine or hyposulphite. The baths may be at a pH of from about 10 to 1 3 and may contain additional ingredients to attain and/or maintain this pH, including an alkaline agent such as an alkali metal hydroxide, and a buffering agent such as an alkali metal citrate. An alkali metal cyanide is a further optional ingredient to improve bath stability.

In most operations the electroless plating bath of the present invention will be operated at a plating temperature of from about 500C up to a temperature at which the bath decomposes. Typically the operating temperatures will be from about 500C to 950C, and preferably from about 600 to 85 C.

The substrates to be plated in accordance with the teachings of this invention are preferably metals such as gold, copper, etc. No special pretreatments are required for these metal substrates.

Additionally, metallised ceramic, and non-metallic substrates may also be plated. Such substrates will, of course, be subjected to appropriate pretreatments, as are known in the art, before plating.

The invention also provides for replenishing the electroless plating bath with trivalent gold in the form of an alkali metal aurate or aurihydroxide solution to maintain the desired gold concentration of the bath. This is an important feature of the present invention which overcomes the prior art problem of erratic metal deposition rates associated with the use of monovalent gold cyanide. Additional alkaline agent and reducing agent may also be added during replenishment of the bath without encountering any untoward results.

As previously described, an essential feature of the present invention is to employ, as the source of gold in the electroless plating bath trivalent and monovalent water-soluble gold components. This is in contrast to the prior art teachings of using complexes where the gold was either in the monovalent state such as, for example, potassium aurocyanide, or in the trivalent state, for example, potassium auricyanide. In the present invention, the trivalent gold complex or compound may be an alkali metal aurihydroxide, with the preferred materials being the alkali metal auricyanides and alkali metal aurates; while the monovalent gold complex may be alkali metal aurocyanide. For most purposes the alkali metal is typically either potassium or sodium, and the use of potassium as the alkali metal is especially preferred.Thus, potassium auricyanide, KAu(CN)4, or potassium aurate, and potassium aurocyanide are preferably utilised in formulating the electroless gold plating baths of the present invention. It is to be appreciated however, that other alkali metal and/or ammonium trivalent gold and monovalent gold compounds or complexes may also be used and that the term "alkali metal", as used in the present specification and claims, is intended to be understood as including ammonium compounds and complexes.

The reasons why the admixture of trivalent gold and monovalent gold functions better than trivalent gold or monovalent gold alone in these plating baths and in this autocatalytic plating process are not fully understood at this time. Possible explanations may be that use of the mixture (1) leads to better deposition rates through ease of the reduction of the mixed gold metals, (2) enhanced stability of the bath, and (3) ease of replenishment of the bath utilising the trivalent alkali metal aurate or aurihydroxide which overcomes the prior art problems caused by high concentrations of cyanide ions in the plating bath. Moreover, the bath is exceptionally stable toward high concentrations of reducing agents thereby achieving faster plating rates than heretofore attained while maintaining bath stability.

It will be understood that the alkali metal gold (mono and trivalent) cyanides employed in the practice of this invention are water-soluble. However, a variety of compounds which can provide the gold constituent in the trivalent and monovalent state may be employed in formulating the baths.

The amount of trivalent gold used in admixture with the monovalent gold will tend to be at least sufficient to provide stability to the bath and prevent cyanide build up in the bath as it is replenished.

Generally, the weight ratio of trivalent gold to monovalent gold in the bath will be at least about 0.2:1, with ratios from about 0.5:1 to about 4:1 being typical and ratios of from about 1:1 to about 3:1 being preferred. The maximum amounts of trivalent gold have not been found to be critical. Thus, although weight ratios up to about 4:1 are typically used, weight ratios of 10-1 5:1 and even higher, can also be used without encountering adverse effects in the operation of the process.

The reducing agents employed in connection with the present electroless plating baths include any of the borohydrides, cyanoborohydrides or amine boranes which are soluble and stable in aqueous solution. Thus, alkali metal borohydrides, preferably sodium and potassium borohydrides are preferred aithough various substituted borohydrides, such as sodium or potassium trimethoxyborohydride, ((Na(K)B)OCH3)3H), may also be employed. Also preferred are the amine boranes such as mono- and di-lower alkyl, e.g. up to Ce aikyl amine boranes, preferably, isopropyl amine borane and dimethylamine borane. Other reducing agents such as hydrazine and hyposulphite may also be employed.

The electroless mixed valent gold plating baths of the present invention should be maintained at a pH of between about 10 and 13, in order to achieve the desired results. It is thus preferred that an alkali metal hydroxide, such as sodium or potassium hydroxide, be employed to maintain the pH at this level. However, pH control is considerably easier when alkali metal buffering salts are employed in addition to the alkali metal hydroxide.

Suitable alkali metal buffering salts include the alkali metal phosphates, citrates, tartrates, borates, metaborates, etc. Specifically, the alkali metal buffering salts may include sodium or potassium phosphate, potassium pyrophosphate, sodium or potassium citrate, sodium potassium tartrate, sodium or potassium borate, sodium or potassium metaborate, etc. The preferred alkali metal buffering salts are sodium or potassium and sodium or potassium tartrate.

In order to improve further the electroless plating baths of this invention, it is desirable in some instances to provide further chelating capacity by the addition of an organic chelating agent such as ethylenediamine tetraacetic acid, and the di-sodium, tri-sodium and tetra-sodium and potassium salts of ethylenediamine tetraacetic acid, di-ethylene triamine pentacetic acid, nitrilotriacetic acid. The ethylenediamine tetraacetic acid, and its di-, tri- and tetrasodium salts are the preferred chelating agents, with the tri- and tetra-sodium salts being particularly preferred.

In addition to the foregoing ingredients, the electroless plating baths of this invention may also contain alkali metal cyanides, and more particularly the potassium or sodium cyanides. Such ingredients are added when greater stability for the autocatalytic process is required. When employed, the amount of alkali metal cyanide may range from about 1 to 20 grams per litre which is far in excess of the minor critical amounts employed by McCormack, which at a maximum were 500 milligrams per litre.

In the electroless plating baths of the present invention the gold compounds or complexes will be present in an amount at least sufficient to deposit gold on the substrate to be plated, up to their maximum solubility in the plating bath. The reducing agent is present in an amount at least sufficient to reduce the gold, again up to its maximum solubility in the bath. The alkaline agent and buffering agent may each be present in an amount sufficient to provide and maintain the desired bath pH.

More specifically, the components of the electroless plating baths of this invention may be present in amounts within the following ranges: Amountswrams/lítre Components Typical & gt; Preferred (1) Gold (III) as the alkali metal auricyanide, aurate 0.5-4 1-4 or aurihydroxide (2) Gold (I) as the alkali metal aurocyanide 0.5-3 1-2 (3) Reducing agents, as amino borane, alkali metal 1-1 5 2-10 borohydride, cyanoborohydride, etc 10-50 4 (4) Alkaline agent 10--40 20--30 (5) Buffering agent, as alkali metal salt 1040 2030 (6) Alkali metal cyanide (when present) (7) Organic chelating agent (when present) 2-25 3-1 5 (8) Water to make one litre As previously set forth, the pH of the bath may be maintained at a range of about 10-1 3, and in some instances between about 11 to 13. The typical operational temperature during plating is from about 500 to 950C., preferably from 600 to 850 C. For most purposes, the plating rates will be up to 8 microns per hour, and preferably at least'about 2 microns per hour.

Although this invention has been described above primarily in conjunction with electroless gold baths, it should be understood that one or more alloying metals such as copper, zinc, indium, tin etc.

may be added to the electroless baths. Where these are employed, they may be added to the bath as a suitable soluble salt in amounts sufficient to provide up to about 20 percent by weight of the alloying metal or metals in the gold deposit.

In accordance with the preferred features of the present invention the substrates to be plated by the electroless gold baths are metals such as gold, copper, copper alloy, electroless copper, nickel, electroiess nickel, nickel alloys, and the like. Thus, where a metallic substrate is employed, such surfaces include all metals which are catalytic to the reduction of the metal cations dissolved in the described baths. In some cases it is preferred to sensitise further the substrate by treatments well known to those skilled in this art. Moreover, it is possible to use nickel, cobalt, iron, steel, palladium, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin, silver, etc. as the metal substrates upon which the gold is to be plated.

With the use of non-metallic substrates, however, these surfaces can be rendered catalytically active by producing a film of catalytic particles thereon. This may be done by the method described in U.S. Patent No. 3,589,916, upon such surfaces as glass, ceramics, various plastics, etc. When a plastic substrate is to be plated according to the present invention, it is initially etched preferably in a solution of chromic and sulphuric acid. After rinsing, the substrate is immersed in an acidic solution of stannous chloride, such as stannous chloride and hydrochloric acid, rinsed with water and then contacted with an acid solution of a precious metal, such as palladium chloride in hydrochloric acid. Subsequently, the now catalytically active non-metallic substrate may be contacted with the electroless plating solutions of this- invention.

According to a second aspect of the present invention, there is provided an electroless plating method for plating gold on a substrate which comprises contacting the substrate with an aqueous electroless gold plating composition comprising a water-soluble trivalent gold component, a watersoluble monovalent gold component and a reducing agent and maintaining the substrate in contact with the composition for a period sufficient to deposit the desired amount of gold thereon.

The method of utilizing the present invention involves preferably the immersion of the metallic or non-metallic substrates into electroless plating baths. These baths are maintained at the pH described above, while the plating is carried out at the aforementioned temperatures. Commercially desirable thicknesses of gold metal deposits have been achieved without encountering the bath instability and other prior art problems. The necessary adhesion characteristics were also readily achieved by the practice of the present invention.

A still further aspect of the present invention is the ability to replenish the bath without encountering difficulties. It has been found, for example, that aside from adding additional alkaline agent, such as potassium hydroxide, and reducing agent, replenishment of the gold content may be accomplished by adding an alkali metal aurihydroxide or alkali metal aurate to the bath. This replenishment of the bath with water-soluble components is accomplished without adverse effect on either the bath plating rate or the bath stability.

According to a third aspect of the present invention, there is provided a plated substrate whenever plated by means of a composition in accordance with the first aspect and/or by a method in accordance with the second aspect.

The invention will be more fully understood by reference to the following illustrative embodiments.

Example I An electroless plating bath was formulated from the ingredients set forth below: Ingredients Amount gel Gold, as KAu(CN)4 2 Gold, as KAu(CN)2 2 Potassium Hydroxide 35 Tripotassium Citrate 30 Dimethyl Amino Borane 6 The pH of the resulting bath was about 11 to 1 2.

The bath was used to plate gold on gold, copper, and copper alloys (48 square inches per litre, i.e.

309 cm2 per litre) at 80 degrees C. The plating rate was 5 microns/hour. Deposits from this bath were ductile, lemon yellow, pore-free pure gold with excellent adhesion to the substrates.

During a number of runs the bath can be replenished by the addition of gold as a gold (III) complex with alkali metal hydroxide or aurate. No cyanide buildup in the bath was observed and a consistent metal deposition rate was maintained. As compared to the known baths, excellent stability was achieved here as evidenced by the consumption of about 100% of the gold metal content in the bath, with 5 to 6 replenishments, without encountering erratic metal deposition or bath instability.

Example II An electroless plating bath was formulated as follows: Ingredients Amount gel Gold, as KAu(CN)4 3 Gold, as KAu(CN)2 Tripotassium citrate 30 Potassium Hydroxide 35 Dimethylamine Borane 15 Deposits were obtained on copper and copper alloys at a plating rate approaching 2 to 8 microns per hour with the bath at a pH of 11-12 and a temperature of 800--850C. The replenishment procedure of Example I was employed with equally good results.

Example Ill Another electroless plating bath was formulated as follows: Ingredients Amount g/l Gold, as KAuO2 3 Gold, as KAu(CN)2 Potassium Cyanide 10 Potassium Hydroxide 30 Dimethylamine Borane 4 Deposits were obtained on gold at a rate of 3 microns per hour with the bath at 820C and at a pH of 12-1 3. The replenishment procedure of Example 1 was followed using gold (III) hydroxide to attain a consistent deposition rate and to avoid an undesirable build up of cyanide ions.

The above data show that the improved electroless bath of this invention leads to superior results and avoids the problems or the commercial disadvantages associated with the previously proposed electroless gold metal baths.

It will be further understood that the foregoing examples are illustrative only and that variations and modifications may be made without departing from the scope of this invention. Thus, for example, the plating bath may be initially formulated with-monovalent gold i.e. an alkali metal gold aurocyanide, and then replenished with an alkali metal aurate or aurihydroxide whereby a bath containing bath monovalent and trivalent gold constituents is produced.

Claims

1. An aqueous electroless gold plating composition comprising a water-soluble trivalent gold component, a water-soluble monovalent gold component and a reducing agent.

2. An aqueous electroless gold plating composition comprising a water-soluble trivalent gold component selected from alkali metal auricyanides, alkali metal aurates and alkali metal aurihydroxides; a water soluble alkali metal aurocyanide; and a reducing agent selected from alkylamino boranes, alkali metal borohydrides, alkali metal cyanoborohydrides, hydrazine, and hyposulphite; the gold components being present in an amount at least sufficient to deposit gold on the substrate to be plated and the reducing agent being present in an amount at least sufficient to reduce the gold in the bath, the plating composition having a pH within the range of from 10 to 1 3.

3. A composition as claimed in Claim 2, wherein there is also included an alkaline agent and an alkaline buffering agent in amounts sufficient to maintain the bath pH within the specified range.

4. A composition as claimed in Claim 1, 2 or 3, wherein the pH is maintained within the range of from 11 to 12.

5. A composition as claimed in any one of Claims 1 to 4, wherein the reducing agent is a dialkylamine borane.

6. A composition as claimed in Claim 5, wherein the dialkylamino borane is dimethylamino borane.

7. A composition as claimed in any one of Claims 1 to 4, wherein the reducing agent is an alkali metal borohydride.

8. A composition as claimed in Claim 7, wherein the alkali metal borohydride is potassium borohydride.

9. A composition as claimed in any one of Claims 1 to 4, wherein the reducing agent is an alkali metal cyanoborohydride.

1 0. A composition as claimed in Claim 9, wherein the alkali metal cyanoborohydride is potassium cyanoborohydride.

11. A composition as claimed in Claim 3, wherein the alkaline agent is sodium hydroxide or potassium hydroxide.

12. A composition as claimed in Claim 3, wherein the alkaline buffering agent is selected from one or more alkali metal phosphate, citrates, tartrates, borates and metaborates.

13. A composition as claimed in any one of Claims 1 to 12, wherein from 1 to 20 g/l of an alkali metal cyanide is added as an additional ingredient.

14. A composition as claimed in any one of Claims 1 to 13, wherein the trivalent gold component is an alkali metal auricyanide.

15. A composition as claimed in Claim 14, wherein the alkali metal auricyanide is potassium auricyanide.

1 6. A composition as claimed in any one of Claims 1 to 13, wherein the trivalent gold component is an alkali metal aurate.

17. A composition as claimed in Claim 16, wherein the alkali metal aurate is potassium aurate.

1 8. A composition as claimed in any one of Claims 1 to 13, wherein the trivalent gold component is an alkali metal aurihydroxide.

19. A composition as claimed in any one of Claims 1 to 1 8, wherein the monovalent gold component is an alkali metal aurocyanide.

20. A composition as claimed in Claim 19, wherein the alkali metal aurocyanide is potassium aurocyanide.

21. A composition as claimed in any one of Claims 1 to 20, the composition comprising an organic chelating agent.

22. An aqueous electroless gold plating composition having a pH within the range of from 10 to 1 3 and comprising the following: Component Amount gel (a) Trivalent gold, as an alkali metal auricyanide, aurate or aurihydroxide 0.5 to 4 (b) Monovalent gold, as an alkali metal aurocyanide 0.5 to 3 (c) an alkali metal hydroxide 10 to 50 (d) an alkali metal buffering agent 10 to 40 (e) an amino borane, an alkali metal borohydride, or an alkali metal cyanoborohydride 1 to 15

23. A composition as claimed in Claim 22, wherein the alkali metal is sodium or potassium.

24. A composition as claimed in Claim 22 or 23, wherein the alkali metal is potassium.

25. A composition as claimed in Claim 22, 23 or 24, wherein component (a) is potassium auricyanide; Component (b) is potassium aurocyanide; Component (c) is potassium hydroxide; Component (d) is tripotassium citrate; and Component (e) is dimethylamino borane.

26. An electroless plating method for plating gold on a substrate which comprises contacting the substrate with a composition according to any one of Claims 1 to 25, and maintaining the substrate in contact with the composition for a period sufficient to deposit the desired amount of gold thereon.

27. An electroless plating method for plating gold on a substrate which comprises contacting the substrate with an aqueous electroless gold plating composition comprising a water-soluble trivalent gold component, a water-soluble monovalent gold component and a reducing agent and maintaining the substrate in contact with the composition for a period sufficient to deposit the desired amount of gold thereon.

28. A method as claimed in Claim 26 or 27, which method comprises replenishing the composition with trivalent gold.

29. A method as claimed in Claim 28, wherein the trivalent gold is in the form of an alkali metal aurate or aurihydroxide.

30. A method as claimed in Claim 28 or 29, which method comprises adding alkaline agent and/or reducing agent during replenishment.

31. A composition substantially as described with reference to anv one of the examples.

32. An electroless gold plating method substantially as described with reference to any one of the examples.

33. A plated substrate whenever plated using a composition as claimed in any one of Claims 1 to 25 and 31 and/or by a method as claimed in any one of Claims 26 to 30 and 32.