CA1038559A - Electroless gold plating process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An electroless plating bath, and processes for the autocatalytic deposition of Group 1B metals upon various substrates using such a bath, are disclosed. The electroless plating bath disclosed includes an aqueous solution of an imide complex of the Group 1B metal to be plated, an alkali metal cyanide, and a reducing agent, and is maintained at a pH of from about 11 to 14 by the addition of alkali metal hydroxides. In a preferred embodiment, an electroless gold plating bath is disclosed, including an aqueous solution of an alkali metal gold imide complex. Methods for plating various substrate from such electroless plating baths are also surfaces are rendered catalytically active prior to immersion in plated, they are preferably pre-coated with a thin plate by immersion in an aqueous plating bath including a soluble gold salt, an ammonium buffering agent, and an organic chelating agent, prior to immersion in the electroless plating baths of the present invention.

Description

Th~ present invention relates to improved electroles~ ~.
plating baths f~r the autocatalytic deposition of Group IB metals upon substrat~s.
More particularly, the present invention relates to im-prove.d electroless silver, copper and gold plating baths for auto-.~. catalytic deposition upon catalyti.c surfaces, which baths provide improved plating characteristics, including improved plating . . .
~ rates.
,..
Still more parti.cularly, the present invention relates ~,. 10 to improved methods of plating various substrates utilizingelectroless plating baths of the Group IB metal to be plated .... .
. thereon.

The use oE electroless plating baths for the autocata-~ lytic deposition of gold and other Group IB metals upon various substrates has become widely known. Such bath~., AS compared to ~ conventional electroplating baths, are characterized by an ability ... ;. to deposit these metals on a wide variety of metallic and non- ~.

.;,. 1 metalic substrates without requiring the use of electricity.

~.. l For example, in a series of articles published in , ! 20 Plating, beginning in September of 1970, Dr. Okinaka has described :. I. the electroless deposition of gold using as reducing agents horo--, l hydride and dimethylamine borane, and alkali metal cyanides as ... , the source of gold, with a demonstrable deterioration in the ~ baths with increased cyanide concentrationsi parti`cularly upon ! replenishment with additional alkali metal gold cyanide. In fact, in a Novembex, 1971 article he specifically demonstrates thls (see Figure 2 thereof~, and notes that the accumulation of free cyanide ions is known to decrease the deposition (plating) rate.

. After a single replenishment the author notes some gold precipi-.:.......

,. " . ! , --:. '-, . .' ' .

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tation. In addition, Dr. Okinak;l discloses that his process results in platinc~ rat~s of up to about 2.5 microns per hour, and that ev~n as these rates were approached, serious bath insta-:. :
bility resulted.
Furthermore, in U.S. Patent No. 3,589,916, to McCormack, ~' an electroless gold plating bath is described including a water-- soluble gold salt and a complexing agent for the gold, in addi-tion to the water-soluble borohydride or amine borane reducing agent, and a stabilizing amount of a cyanide compound, all main -- 10 tained at a pH of between 10 and 14. This disclosure thus teaches ~` the use of a complexing agent to form a gold complex in an attempt to prevent gold precipitation during plating. This disclosure also teaches that the amount of water-soluble cyanide compound, between 5 micrograms and 500 milligrams per liter, is critical.
Each of the gold sources previously employed have, ~;
-~ .
t' however, not been entirely satisfactory. Thus, with the use of ; -the gold cyanide compounds there is a decided deterioration in `~ these plating baths as the cyanide ion concentration increases, particularly upon replenishment therewith. Furthermore, these compounds are not water-soluble to any great extent, and require ~ ~;
i considerable efforts to maintain in solution, in addition to the -~ fact that the use of these water-soluble gold salts, as described ",K in U.S. 3,589,916, result in the decomposition of the bath.
¦ It is therefore an ob~ect of the present invention to provide improved electroless plating baths for the autocatalytic deposition of Group IB metals, including silver, copper and gold.

~ Itisafurther object of the present invention to provide such .,,,: . .
- electroless plating baths having improved plating rates, and ; plating lives. It is still another object of the present in-~' ~ ' t .'' $ 30 vention to provide such electroless plating baths which result ,n ,.:~, 1 ,:
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`, J"",;,. ""',' .. "' .,' . ,' "; ; ' '' ' ' ' . ' ,, ' ~. '., ' ' . ' , ' ' ', "' - ''' ,~ ' ~Q;38S~9 the de~osition of Group IB metal plates of improved c}laracteristics, inclu~.ing better adhesion, thickness, bric~htness ar~l ductility It is yet c~nother object of the present invention to provide me-thods ;~ for plating varjous metallic and non-me-tallic substrates utiliz:ing such electro-. less platiny baths.
~ In accordance with the present invention e].ectroless plating baths for - autocatalytic depositon uporl various substrates are provided, resulting in the deposition of thick silver, copper and gold plates. ~rhe electroless plating baths of this invention comprise an aqueous solution of an imide complex of the Group IB metal to be plated, an aIkali metal cyanide in an arnount sufficient to stabilize the bath,and areducing agent selected from the group consisting of water soluble alkali metal borohydrides, water soluble amine borc~nes cmd formaldehyde~
The F~I of the bath is maintained from about 11 to 14. It has thus been found ~:
:.:
that these electroless plating baths exhibi.t superior plating rates, without con-oomitant bath dec~mposition. Specifically, plati.n~ rates of greater than 2.5 ,:: .
:~ microrls per hour, and preferably gYeater than 2.7 microns per hour are ob-tc~in-. able therewith.

: In a preferred embodiment, the required pH level is mk~intained by the :.;1 .
~ -addition of alkali metal hydroxides to the bath, and preferably alkali metal ;,~
..`: 20 buffering salts are added thereto in addi.tion to the alkali metal hydroxides, in - :, order to obtain a superior degree of pH control.
In addition, certain organic chelating agents may be c~dded to these ........... electroless plating baths, particularly when metallic substrates are to be pla-ted therewith, in order to forrn complexes with the replaced metals, and prevent the precipitation thereof.

1 Further in accordance with the present invention, improved methods for -plating various substrates are provided, in which the metallic or non-metallic .
substrates are rendered qatalytically active prior to immel-sion in the improved. ..
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i59 electroless platinc3 baths hereo~. Ductile silver, copper and gold , ., - plates are thus provided, having strong adherence to the sub~trates i~
employed, and of relatively uniform thicknesses as compared to the plates obtainable by the prior art platincJ baths.
It has therefore been found that highly improved elec-r,`~ troless plating baths are obtained when an imide complex of - the Group IB metal to be plated is employed as the source of ; that metal, such as gold. Such imide compounds thus form un-. ::
~ expectedly strong complexes with the Group IB metals to be plated, ,~.,., , ' therefore preventing precipitation of the Group IB metal durlng plating, while at the same time bei.ng fully soluble in the plating solution, and being sufficiently stable so as not to react with the reducing agent prior to deposition.
The imides of the present invention, which are cabable of forming particularly strong complexes with the Group IB metals ' ';' ' ' ! .
;, to be plated, such as gold, having a general formula as follows: ~
, ',.- - ~ '.
RNHCp, ~1 wherein R is a radical selected from the group con-~ sisting of alkylene, substituted alkylene, arylene, and sub-;,~, 20 stituted arylene.
,'',:;~ .
Preferably, R will be substituted arylene, such as sulfonyl-0-phenylene ~-S02-C6H4) in which case the imide foxmed ; will be sulphobenzoic imide (i.e. saccharin, or 0-benzosulfamide) ~r' C6H4 (S02) ~C09-NH.

Thus, for the preparation of an electroless gold plating ; bath, complexes of these imides with gold are preferred, such as . J', alkali metaI gold sulphobenzoic imide.
~i PreferabIy, the imides with which the Group IB metals to be plated are complexed will have a general formula as follows:
:.. ~, . ~ .
"'~
~,:,, , ,~

ph~

. ~ 55~
whcrein R is a r~dical selected from the group com-; sistin~ of alkylene, substituted alkylenel arylene and substituted arylene.
Preferably, R will be alkylene. For example, where R
is -CH2CH~-, the imide will be succinimide, and where R is C6H4=
the imide will be phthalimide. Where the preparation of an elec-troless gold-plating bath is desired, these two compounds, suc-cinimide and phthalimide, will be particularly preferred, thus forming complexes with the gold in the form of alkali metal gold succinimide, particularly potassium gold succinimide, and alkali . :.;
metal gold phthalimide, particularly potassium gold phthalimide.
`, It is also essential that the electroless plating baths of this invention include soluble cyanide compounds in critical amounts in order to maintain the stability of the bath.
': '`~
Among these water soluble cyanide compounds are the ` `~
~ Alkali metal cyanides, such as sodium, potassium and lithium i cyanide. Among these, sodium and potassium cyanide are particu- ;
larly preferred. Other such compounds may, however, be employed, : . . .
such at nitriles, including alpha-hydroxynitriles, etc. as de-scribed in U.S. Patent No. 3,589,916.
;: ",~ .
~ It is critical, however, that the water-soluble cyanide ~. .~. . .
compounds be used in specific amounts, generally from between a-bout 2 and 20 grams per liter, and preferably from about 5 to 15 ~, grams per liter. It has thus been discovered that at below about

2 grams per liter the electroless plating baths of this invention <
are relatively unstab~, and the metal to be plated precipitates rom the bath, and ~hat when the cyanide compound concentration ;.`. .

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ph~

3~5551~
increases to about 20 grams p~r liter,th~ pla-ting rate obtainable with these baths rapidly deteriorates. Thus, the use of those amounts of water soluble cyanide compounds described in U.S~
Patent No. 3,589,916, which as described above are considerahly below those taught herein, results in the preparation of hiyhly unstable electroless plating baths.
The reducing agents employed in connection with the present electroless plating baths include any of the borohyclrides or amine boranes which are soluble and stable in aqueous solution.
~0 10 Thus, alkali metal borohydrides, preferably sodium and potassium borohydrides, are utilized, although various substituted boro-hydrodes, 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 C6 alkyl-amine boranes, preferably isopropyl amine borane and dimethy-lamine borane. In addition, formaldehyde is also an excellent reducing agent for use~ in these baths, particularly for the de-.; ;~ .
~;i position of copper and silver. ;

It is also essential that the electroless plating baths of the present invention be maintained at a pH of between about 11 and 14, principally to prevent spontaneous decomposition ~, ~
`' thereof. It is thus preferred that an alkali metal hydroxide, such as sodium or potassium hydroxide, be employed to maintain . .
j the pH at this level. It has, however, been discovered that pH
, ..~, `~ -control is considerably easier when alkali metal buffering saIts are employed in addition to the alkali metal hydroxide. Thus, ; ."~3 !~ -while the alkali metal hydroxide is necessary in order to maintain . .- . ., ~ - .
~1 the present electroless baths at the required pH level during :,.,,~ ~ . ' ~
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plating, as the pH tends to dxop, the case of p~l control is con-siderably facilitated by the addition of such alkali metal buf-fering salts. These alkali metal buffering salts thus include r~ the alkali metal phosphates, citrates, tartrates, borates, meta-borates, etc. Specifically, the alkali metal buffering salts may this include sodium or potassium phosphate, po-tassium pyrophosp}l-- ate, sodium or potassium citrate, sodium or potassium tartrate, .,; sodium or potassium borate, sodium or potassium metaborat~, etc.
, j, j ,. . .
The preferred alkali metal buffering salts are sodium or potassium citrate and sodium or potassium tartrate. In order to further ;~i improve the electroless plating baths of this invention, it is preferred to add an organic chelating agent thereto. Such chela-ting agents combine with the replaced surface metal ions, thus preventing their interference with the plating process, and the ~ consequent deterioration of the color characteristics of the '-' deposited plates, as well as other plate properties such as ad~
~ herence and thickness. These organic chelating agents thus in-'~ i clude ethylenediamine tetraacetic acid, and the di-sodium, tri-sodium and tetra-sodium and potassium salts of ethylenediamine ~, 20 tetraacetic acid, di-ethylene triamine pentacetic acid, nitrilot-riacetic acid. ~he ethylenediamine tetraacetic acid and its di-, tri-, and tetra-sodium salts are the preferred chelating agents, ~ with the tri- and tetra-sodium salts being particularly preferred.
:~ In order to autocatalytically deposit Group IB metal plates, such as gold plates, in accordance with this invention, it i is necessary to contact these electroless plating solutions with ~ ~ ~
a catalytically active substrate surface. 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. While it therefore may in some cases be preferred to further sentitize the substrate by treatments well known to those skilled in this art, the use of nickel, cobalt, iron steel pala- , dium, platinum, copper, brass, manganese, chromium, .
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s,l ~ -7-ph: ~
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-` 1038559 molyb~num, tuncJsten, titaniurn, tin, silver, etc., as metal substrates upon which th~ Group IB metal, such as gold, is to ' be plated, ar~ possible.
With the use of non-metallic substrates, however, these sur~aces must b~ rendered catalytically active by producing a film of particles of catalytic material thereon. This may be done in the method described in U.S. Patent No. 3,589,916, upon .
such surfaces as glass, ceramics, various plastics, etc. Prefer-ably, when a plastic substrate is to be plated according to the :~
,j 10 present invention, it is initally etched, preferably in a solu-tion of chromic and sulfuric 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 , paladium chloride in hydrochloric acid. Subse~uently, the now -~ catalykically active non-metallic substrate may be contacted with :i .
~ the electroless plating solutions of this invention in order to ~
.. ,.; . ~
autocatalytically deposite Group IB metal plates thereon.

~: In a highly preferred embodiment o~ the present in--;i 20 vention, where a metallic substrate is to be plated a pre-plating step is employed. Thus, it has been discovered that when the catalytically active metallic substrates of this invention are ~' employed directly with the electroless plating baths hereof a .
.-:,:
rapid contamination of the plating bath with the base metals re-placed by the Group IB metals plated thereon occurs, thus af-fecting the purity of the deposit obtained, and the stability of ;
the electroless plating solution. It is therefore preferred that, in such situations a pre-plating step for the provision of a thin plate o~ gold be carried out. This may be accomplished by using the , .
, :
~ -8-,. : .. : , . . ~ , .. .. - ~ - . .. , , - ., ~
.: , , . , ~ . .

.~tom~Y." process as d~scribed in U.S. Paterl~ No. 3,230,098, wherein the metallic substrate is immersed in an aqueous gold-plating bath includi~y a soluble gold salt selected from the group consisting of the alkali metal gold cyanides, an ammonium buffering agent capable of maintaining the pH of the bath from between about . .
5.5 and 14, and an organic chelating agent capable of chelating with i ` the metal ions of the substrate employed.

~`- After a thin film, i.e. from between about 2 to 10 micro-; inches, preferably 5 to 10 microinches of gold, is plated by such a . . .
-~ 10 method,~he plated substrate may be immersed in the electroless plating baths of the present invention in order to provide a thick, ~i ductile plate of the Group IB metal having the improved properties ;
obtainable in accordance with the present invention.
"
~ ~ Typical plating baths made in accordance with the present ;~... :., .
invention follow:
,.,.. ~, ~
~ EXAMPLE 1 ;~ A gold solution is obtained by dissolving potassium; .~.
gold succinimide in water, and heating to between about 70 and 95C. Potassium cyanide, tripotassium citrate, potassium hydroxide, and ethylene diamine tetraacetic acid are added thereto, the pH

being maintained at about 13 and the temperature at about 80C.

Dimethylamine borane is then dissolved in water, and added to this ::.:., :::
solution. In order to continue the gold plating, gold is replenished ,:, . . .
from an a~ueous solution containing gold, as potassium gold succini-mide. Additional amounts of dimethyl~mine borane may be required,as .
~ ; -well as potassium hydroxide in order to maintain the proper pH.
,~
The initial plating bath thus prepared has the following composition: -. ~:.,~',i ' `

9 _ s ,"~l ~
~ Ph ~` ~L038S~i9 Gold, as potassiuJn gold succinimide - 3.0 grams per liter Potassium cyanide - 5.0 grams per li-ter EDT~ (tetra sodium salt) - 2.0 grams per liter Tripotassium citrate - 25.0 grams per ]itex ;
~J Potassium hydroxide - 10.0 grams per li~er - Dimethylamine borane - 10.0 grams per liter ". ~.. ..
EX~MPL~ 2 -l A silver solution is obtained by dissolving the sodium :S~,i ~ silver phthalimide in water, along with sodium potassium tartrate, ....
-i 10 potassium cyanide, and potassium hydroxide. Ihe solution is then ;~ heated to approximately 55C, and a 37% formaldehyde solution is ... .
added thereto. ~-~
~ The silver plating bath thus prepared has the follo~ing ; composition: i~
i; Silver, as sodium silver phthalimide - 2.0 grams per liter Potassium cyanide - 10 grams per liter Sodium potassium tartrate - 25 grams per liter Potassium hydroxide - 5 grams per liter Formaldehyde (37%) - 50 milliliters per liter . ~,, ~ .
~'t 2 EXAMPLE 3 ,.~ O
i' A copper solution is obtained by dissolving potas~ium copper o-benzosulfimide in an aqueous solution of sodium potas-sium tartrate, potassium cyanide, and potassium hydroxide. This ^~, solution is then heated to approximately 30C, and a 37% solution of formaldehyde is added there~o.
- The copper bath thus prepared has the following composi- ;
;.. ~, ~
-' tion:
Copper, as potassium copper 0-benzosulfimide - 3 ,:- grams per liter Potassium cyanide - 2 grams per liter ~s~ Sodium potassium tartrate - 30 grams per liter -r Potassium hydroxide - 10 grams per liter i Formaldehyde (37~) - 20 milliliters per liter.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electroless plating bath for autocatalytic deposition of Group IB metals upon a substrate comprising an aqueous solution of an imide complex of the Group IB metal to be plated, said imide complex of the Group IB metal to be plated including an imide selected from the group consisting of imides having the formula , and cyclic imides having the formula , wherein R is selected from the group consisting of alkylene, sub-stituted alkylene, arylene and substituted arylene, and mixtures thereof, an alkali metal cyanide in an amount ranging from about 2 to 20 grams per liter, sufficient to stabilize said bath, and a reducing agent selected from the group consisting of water soluble alkali metal borohydrides, water soluble amine boranes and form-aldehyde, said bath maintained at a pH of from about 11 to 14.
2. The electroless plating bath of Claim 1 wherein said bath is maintained at a pH of from about 11 to 14 by the addition of alkali metal hydroxide thereto.
3. The electroless plating bath of Claim 2, including an alkali metal buffering salt selected from the group consisting of alkali metal phosphates, citrates, tartrates, borates, meta-borates, and mixtures thereof.
4. The electroless plating bath of Claim 1, wherein said imide is selected from the group consisting of succinimide and phthalimide.
5. An electroless gold plating bath for the autocatalytic deposition of gold from a substrate, comprising an aqueous solu-tion of an alkali metal gold imide complex, said alkali metal gold imide complex including an imide selected from the group consisting of imides having the formula , and cyclic imides having the formula , wherein R is selected from the group consisting of alkylene, substituted alkylene, arylene, substituted arylene, and mixtures thereof, and alkali metal cyanide in an amount ranging from about 2 to 20 grams per liter sufficient to stabilize said bath, a reducing agent selected from the group consisting of water soluble alkali metal borohydrides, water solu-ble amine boranes, and formaldehyde, said bath maintained at a pH of from about 11 to 14.
6. The electroless plating bath of Claim 5 wherein said imide comprises sulfobenzoicimide.
7. The electroless gold plating bath of Claim 5, wherein said cyclic imide is selected from the group consisting of succinimide and phthalimide.
8. The electroless gold plating bath of Claim 5 including an organic chelating agent capable of forming a chelate with the metal of said substrate.
9. The electroless gold plating bath of Claim 5 wherein said bath is maintained at a pH of from 11 to 14 by the addition of an alkali metal hydroxide thereto.
10. The electroless gold plating bath of Claim 9, including an alkali metal buffering salt selected from the group consisting of an alkali metal phosphates, citrates, tartrates, borates, metaborates, and mixtures thereof.
he electroless gold plating bath of Claim 5, wherein said reducing agent is selected from the group consisting of dimethylamine borane, potassium borohydride, and formaldehyde.
12. A method of plating a metallic substrate with a Group IB metal comprising:

(a) immersing said substrate in an aqueous gold plating bath comprising a soluble gold salt selected from the group consisting of the alkali metal gold cyanides, and ammonium buffer-ing agent capable of maintaining the pH of said bath between 5.5 ?nd 14, an organic chelating agent capable of chelating the metal ions of said substrate, for sufficient time to plate said sub-strate with a thin layer of gold; and (b) subsequently immersing said partially gold plated metal substrate in an electroless plating bath comprising an aqueous solution of a Group IB metal imide complex, said Group IB metal imide complex comprising an imide selected from the group consist-ing of the imide having the formula , and the cyclic imides having the formula , wherein R is selected from the group consisting of alkalyne, substituted alkalyne,arylene, and sub-stituted arylene and mixtures thereof, alkaline metal cyanide in an amount of from 2 to 20 grams/liter, sufficient to stabilize said bath, and the reducing agents selected from the group con-sisting of water soluble alkali metal borohydrides, water soluble amine boranes and formaldehyde, said bath maintained at a pH of from 11 to 14.
13. The method of Claim 12, wherein said layer of gold plated in step (a) is from 2 to 10 microinches.
14. The method of Claim 12 wherein said pH of from 11 to 14 is maintained by the addition of alkali metal hydroxide thereto.
15. The method of Claim 14 wherein said electroless plating bath includes an alkali metal buffering salt.
16. The method of Claim 12 wherein said electroless plating bath includes an organic chelating agent capable of forming a chelate with the metal of said substrate.
17. The method of Claim 12 wherein said imide comprises sulphobenzoic imide.
18. The method of Claim 12 wherein said imide comprises an imide selected from the group consisting of succinimide and phthalimide.