Electroless Plating Baths With Improved Deposition Rates

Abstract

There are provided compositions comprising imino compounds, specifically, ethoxylated cyclohexylamines and benzyliminodiacetic acid, complexed with copper ions. These compositions in aqueous alkaline media, in the presence of a reducing agent, provide baths for the autocatalytic electroless deposition of the copper metal. The new baths have improved stability and plating rates.

Description

The present invention relates to improved solutions for autocatalytic deposition of copper metal and to processes for their use. More particularly, it is concerned with solutions containing certain imino compounds complexed with the copper ion. These solutions for the electroless deposition of the metals have improved stability and increased plating rate.

BACKGROUND OF THE INVENTION

Chemical plating solutions for continuously depositing copper metal by autocatalytic chemical reduction of ions of copper dissolved in the solutions and in contact with a catalytic surface of an article to be plated are well known. Broadly, the solutions comprise an aqueous alkaline solution of a water-soluble salt of copper metal, a reducing agent and a complexing agent for copper ion. Such solutions, which do not utilize electricity, are sometimes referred to in the art as electroless metal plating solutions, to distinguish them from electroplating solutions, which do require the use of electricity. Electroless metal deposition is also to be distinguished from displacement metal plating of the type described in Metals Finishing Guide Book, 27th Edition, 1959, page 469 et seq., and metal mirroring procedures, wherein the metal plating desired and achieved is only a few millionths of an inch in thickness.

This invention is primarily concerned with the complexing agents used in electroless copper baths.

Any complexing agent in general has to form a sufficiently strong complex with the copper ions to prevent the precipitation of the copper metal or salts thereof. Further, the complexing agent must be capable of forming a complex with the copper ions which is soluble in the plating solution, and also which is sufficiently stable so that it will not react with the reducing agent in the main body of the plating solution, but only at or in the vicinity of the catalytic surface to be plated.

Somewhat surprisingly, many complexing and sequestering agents have been found to be suitable for use. However, only a very few by themselves give both good stability and rapid plating rates. One of these, cyclohexanediaminetetraacetic acid, gives an excellent combination of stability and plating rate when complexed with copper.

Unfortunately, the few known, highly efficient complexers, e.g., cyclohexanediaminetetraacetic acid, are too expensive to be used by themselves, in spite of the advantages noted.

It has now been found possible to use a family of imino compounds as complexing agents and to obtain stabilities and plating rates surprisingly approaching, and sometimes even surpassing those obtained with cyclohexanediaminetetraacetic acid. The imino compounds can also be used in mixtures with one another and with cyclohexanediaminetetraacetic acid to obtain stable baths with rapid plating rates. These advantages are obtained at unexpectedly low cost.

In addition to their influence on the rate of dissociation of the complex with copper ion, the new family of imino compounds also influence the oxidation of the reducing agent, e.g., formaldehyde. Therefore, mixtures of imino compounds with themselves or with other complexers can provide enhanced stability and deposition rate by controlling the oxidation rate of the reducing agent, which is an essential component in all electroless copper baths.

This invention is concerned therefore with providing electroless metal plating solutions containing complexes of copper ions with a complexing agent which is an imino compound of a specified type.

DESCRIPTION OF THE INVENTION

According to this invention, there are provided aqueous alkaline chemical plating solutions capable of autocatalytically depositing copper metal on a substrate in contact therewith comprising

a. water;

b. a water soluble complex of an imino compound of the formula

wherein x and y are each at least 1 and x and y together are from 2 to 25, or a mixture of said compounds, with copper ion; and

c. a reducing agent; and wherein the pH is between about 10 and 14.

One preferred family of imino compounds is depicted by FORMULA IaP:

wherein x and y are each at least 1 and x and y together are from 2 to 25. An especially preferred member of this family is cyclohexylaminediethanol, i.e., a compound of FORMULA Ia wherein x and y are each 1. These compounds can be made by techniques familiar to those skilled in the art. For example cyclohexylamine can be reacted with ethylene oxide in an amount sufficient to give polyoxyethylene chains of predetermined length.

Compounds of FORMULA Ia are commercially available under the name ABBOMEENS E-2 to E-25 from Abbott Laboratories, North Chicago, Illinois. The compound ABBOMEEN E-2 is cyclohexylaminediethanol.

Another preferred complexing agent is of FORMULA Ib:

This compound is known as N-benzyliminodiacetic acid (BiDA). It can be prepared by techniques familiar to those skilled in the art. For example, sodium cyanide and formaldehyde can be added to an alkaline solution of benzylamine. This forms the trialkali metal salt of BiDA and acidification yields BiDA.

Benzyl iminodiacetic acid is commercially available from Aldrich Chemical Company, Inc., Milwaukee, Wisconsin.

Also contemplated by the present invention are embodiments wherein in addition to a compound of FORMULA Ia or Ib, another complexing agent is present. The other sequestering or complexing agents can comprise ammonia or an organic compound containing at least one amine group, hydroxy group or carboxy group. Especially suitable for use are ammonia and organic complex-forming agents containing one or more of the following functional groups: primary amine group (--NH.sub.2), a secondary amine group (>NH), tertiary amino group (>N--), imino group (.dbd.NH), carboxy group (--COOH), and hydroxy group (--OH). Among such agents may be mentioned specifically cyclohexanediaminetetraacetic acid, ethylene diamine, diethylene triamine, triethylene tetramine, ethylenediaminetetraacetic acid, N,N,N',N'-tetrakis-2-(2-hydroxypropyl)ethylenediamine, citric acid, tartaric acid, 1,3-propane diamine, and ammonia. Related polyamines and N-carboxymethyl derivatives thereof, such as diethylenetriaminepentaacetic acid, may also be used.

Rochelle salts, the sodium salts (mono-, di- and tri-sodium) salts of N-hydroxyethyethylenediaminetriacetic acid, nitrilotriacetic acid and its alkali salts, gluconic acid, gluconates, and triethanolamine are preferred as complexing agents, but commercially available glucono-.gamma.-lactone and modified ethylenediamineacetates are also useful, and in certain instances give even better results than the pure sodium ethylenediaminetetraacetates and N-hydroxyethylethylenediaminetriacetate.

The water-soluble complex of copper used in this invention can be prepared by adding the complexing agents to an aqueous solution of the copper metal salt, e.g., the water soluble sulfates, chlorides, acetates or nitrates thereof.

According to this invention, there are also provided processes for electrolessly depositing copper on a catalytic surface which comprise contacting the surface with an aqueous alkaline solution of a complexed copper ion according to this invention; a reducing agent, and wherein the pH is between about 10 and 14.

The reducing agent may consist of formaldehyde or any water-soluble borohydride or amine borane having a good degree of solubility and stability in aqueous solutions. Aqueous formaldehyde or paraformaldehyde are preferred. Sodium and potassium borohydrides are also suitable for use in the baths. Also may be mentioned substituted borohydrides, such as sodium trimethoxy borohydride, NaB(OCH.sub.3).sub.3 H. The amine boranes, such as isopropyl amine borane and dimethyl amine borane are good reducing agents too and can be used with the copper baths.

The plating solutions described herein will ordinarily be operated at high pH, preferably at between pH 10 and 14.

When formaldehyde is used as the reducing agent, the pH will ordinarily be between 10 and 14. When sodium borohydride is used as the reducing agent, the pH will ordinarily be in the vicinity of 13-14. With potassium borohydride, the pH will generally be maintained between about 11 and 12.

In preparing the electroless copper deposition baths, it is desirable to combine the bath ingredients in such a manner as to avoid reaction between the soluble metal salt and the reducing agent.

It is therefore preferred to first add the complexing agent to the aqueous solution of the metal salt to form the water-soluble complex of the copper metal cations. The complexing agent can be added as the free acid, or base, as the case may be, or as a salt or other water-soluble derivative. The pH will determine the species present. If the reducing agent is added before the metal complex is formed, there will be a tendency for it to react instantaneously with the copper metal salt to copper metal or a salt of copper. After forming the copper complex, the pH is adjusted to the proper value, e.g., by adding an acid or a base, before adding the reducing agent. The reducing agent will ordinarily be added in the form of an aqueous solution, e.g., 37 percent aqueous formaldehyde, preferably alkaline too.

The catalytic surfaces which may be plated with the baths of this invention include metals such as nickel, cobalt, iron, steel, palladium, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin or silver. All such metals are catalytic to the reduction of the copper cations dissolved in the solutions described.

Materials such as glass, ceramic, and various plastics are, in general, noncatalytic. However, the surfaces of such noncatalytic materials can be rendered catalytic by producing a film or particles of one of the catalytic materials thereon. This can be accomplished by a variety of techniques known to those skilled in the art. One suitable sensitization procedure involves dipping noncatalytic materials in an acidic solution of stannous chloride, washing with water, and then contacting the material with an acidic solution of a precious metal salt, e.g., palladium chloride. A monolayer of precious metal is thus produced, which monolayer is catalytic to reduction of the copper ions in the electroless metal plating solutions.

Alternatively, such materials as glass, ceramic and plastic may be sensitized or rendered catalytic by treatment with an acidic aqueous solution containing, in combination, stannous tin ions and precious metal ions.

A further sensitization procedure involves adhering to the normally noncatalytic surface, finely divided particles of metals or metal oxides which are catalytic to electroless copper solutions.

The term catalytic surface as used herein refers to the surface of any article composed of the catalytic material described hereinabove or covered therewith or to the surface of a noncatalytic material which has been sensitized by producing a film of particles of said catalytic materials on its surface.

The baths of this invention will deposit copper metal electrolessly on the catalytic surfaces of metals and nonmetals, such as paper, glass, ceramic, synthetic resins and plastics, including but not limited to silicones, phenolics, alkyds, epoxies, styrenes, acrylics, vinyl chlorides, nylon, mylar, acrylonitrile-butadiene-styrene, and the like.

The quantities of the various ingredients in the baths of this invention are subject to wide variation. Typically, however, the bath constituents will be as follows:

Water soluble copper salt 0.002 to 0.60 moles/liter Reducing Agent 0.0002 to 2.5 moles/liter Complexing Agent 0.7 to 10 times the moles of the metal salt Alkali metal hydroxide sufficient to provide a pH of 10 to 14

There is theoretically required from 1 to 2 moles of formaldehyde for every mole of copper salt. There is theoretically required one-fourth to one-eighth moles of borohydride for every mole of copper salt. These then may be considered the minimum amounts of respective reducing agents required.

The amount of complexing agent to be added to the plating solution depends upon the nature of the complexing agent and the amount of copper salt present in the bath. In alkaline solutions, the preferred ratio of copper salt to the complexing agent lies between about 1:0.7 and 1:10. A small excess of the complexing agent, based upon copper salt, generally is advantageous. The ratio of the complexing agents, when used in admixture, may be varied over rather broad limits without sacrificing the economic advantages secured by this invention.

With the baths disclosed, copper metal deposition occurs autocatalytically at a uniform rate wherever there is contact between the catalytic surface being plated and the plating solution. There is no substantial variation in the plate thickness even for the most complicated shapes. Thus, copper may be uniformly deposited in recesses, as well as on exposed parts of the object being plated, and there is no buildup of coating at points or edges. These conditions are difficult or impossible to achieve by electroplating. Because of the uniform deposition achieved, the plating process described is particularly suitable for plating objects of irregular to complicated shapes which are difficult or impossible to metallize by conventional techniques.

The process of this invention permits copper to be deposited on practically any conceivable substratum and to practically any thickness desired.

It should be understood that as the baths are used up in plating, the copper salt, and the reducing agent may be replenished from time to time, and also that it may be advisable to monitor the pH, and the solution ingredients, and to adjust them to their optimum value as the bath is used.

For best results, surfactants in an amount of less than about 5 grams per liter are added to the baths disclosed herein. Typical of suitable surfactants are organic phosphate esters, and oxyethylated sodium salts. Such surfactants may be obtained under the Tradenames "Gefac RE 610" and "Triton QS-15," respectively.

In using the autocatalytic or electroless copper solutions, the surface to be plated must be free of grease and other contaminating material.

Where a nonmetallic surface is to be plated, the surface areas to receive the deposit should first be sensitized as described hereinabove.

Where a metal surface, such as stainless steel, is to be treated, it should be degreased, and then treated with acid, such as hydrochloric acid or phosphoric acid, to free the surface of oxides.

Following pretreatment and/or sensitization, the surface to be plated is immersed in the autocatalytic bath, and permitted to remain in the bath until a copper metal deposit of the desired thickness has been built up.

Description of the Preferred Embodiments. Typical plating baths made in accordance with the teachings contained herein are given below:

Example 1 __________________________________________________________________________ Copper sulfate pentahydrate 0.050 moles/liter (CuSO.sub.4.sup.. 5H.sub. 2 O) Cyclohexylaminediethanol 0.170 moles/liter (CADE) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (5 mg.) 0.0001 moles/liter Water q.s.a.d. pH 12.80 __________________________________________________________________________

a clean copper strip is immersed in the bath at 50.degree. C. Copper is deposited at a plating rate of 115 mg./10 cm..sup.2 /hr. (0.60 mils per hour).

Example 2 Copper sulfate pentahydrate 0.050 moles/liter (CuSO.sub.4.sup.. 5H.sub.2 O) Cyclohexylaminediethanol 0.20 moles/liter (CADE) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (30 mg.) 0.0006 moles/liter Water q.s.a.d. pH 13.00 __________________________________________________________________________

the plating rate at 60.degree. C. is 100 mg./10 cm..sup.2 /hr. (0.44 mils per hour).

Example 3 __________________________________________________________________________ Copper sulfate pentahydrate 0.050 moles/liter (CuSO.sub.4.sup. . 5H.sub.2 O) Cyclohexylaminediethanol 0.40 moles/liter (CADE) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (30 mg.) 0.0006 moles/liter Water q.s.a.d. pH 13.00 __________________________________________________________________________

the plating rate at 60.degree. C. is 100 mg./10 cm..sup.2 /hr. (0.44 mils per hour).

Example 4 __________________________________________________________________________ Copper sulfate pentahydrate 0.025 moles/liter (CuSO.sub.4.sup. . 5H.sub.2 O) Cyclohexylaminediethanol 0.20 moles/liter (CADE) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (30 mg.) 0.0006 moles/liter Water q.s.a.d. pH 13.00 __________________________________________________________________________

the plating rate at 60.degree. C. is 55 mg./10 cm..sup.2 /hr. (0.24 mils per hour).

Example 5 __________________________________________________________________________ Copper sulfate pentahydrate 0.050 moles/liter . . 5H.sub.2 O) Cyclohexylaminediethanol 0.20 moles/liter (CADE) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (30 mg.) 0.0006 moles/liter Ammonium Sulfate (0.5 g.) 0.0038 moles/liter Water q.s.a.d. pH 13.00 __________________________________________________________________________

the plating rate at 60.degree. C. is 45 mg./10 cm..sup.2 /hr. (0.20 mils per hour).

Example 6 __________________________________________________________________________ Copper sulfate pentahydrate 0.050 moles/liter (CuSO.sub.4.sup.. 5H.sub.2 O) Cyclohexylaminediethanol 0.20 moles/liter (CADE) Abbomeen E-25 0.01 molelliter (Abbott Laboratories, Ill.) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (20 mg.) 0.0004 moles/liter Water q.s.a.d. pH 13.00 __________________________________________________________________________

the plating rate at 60.degree. C. is 24 mg./10 cm..sup.2 /hr. (0.11 mils per hour).

Example 7 __________________________________________________________________________ Copper sulfate pentahydrate 0.050 moles/liter CuSO.sub.4.sup.. 5H.sub.2 O) Cyclohexylaminediethanol 0.20 moles/liter (CADE) o-Cyclohexanediaminetetraacetic 0.05 moles/liter acid (CDTA) Formaldehyde 0.074 moles/liter Sodium cyanide (10 mg.) 0.0002 moles/liter Water q.s.a.d. pH 12.80 __________________________________________________________________________

the plating rate at 60.degree. C. is 40 mg./10 cm..sup.2 /hr. (0.18 mils per hour).

Example 8 __________________________________________________________________________ Copper sulfate pentahydrate 0.05 moles/liter (CuSO.sub.4.sup.. 5H.sub.2 O) N-Benzyliminodiacetic acid 0.20 moles/liter (BIDA) Formaldehyde 0.074 moles/liter Water q.s.a.d. pH 12.50 __________________________________________________________________________

the plating rate at 25.degree. C. is 40 mg./10 cm..sup.2 /hr. (0.18 mils per hour).

Example 9 __________________________________________________________________________ Copper sulfate pentahydrate 0.05 moles/liter (CuSO.sub.4.sup.. 5H.sub.2 O) N-Benzyliminodiacetic acid 0.10 moles/liter (BIDA) Ethylenediaminetetraacetic 0.10 moles/liter acid (EDTA) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (30 mg.) 0.0006 moles/liter Water q.s.a.d. pH 12.50 __________________________________________________________________________

the plating rate at 60.degree. C. is 35 mg./10 cm..sup.2 /hr. (0.15 mils per hour).

Example 10 __________________________________________________________________________ Copper sulfate pentahydrate 0.05 moles/liter (CuSO.sub.4.sup.. 5H.sub.2 O) Cyclohexylaminediethanol 0.10 moles/liter (CADE) N-Benzyliminodiacetic acid 0.15 moles/liter (BIDA) Formaldehyde (6 ml. of 37%) 0.074 moles/liter Sodium cyanide (30 mg.) 0.0006 moles/liter Water q.s.a.d. pH 13.50 __________________________________________________________________________

the plating rate at 55.degree. C. is 40 mg./10 cm..sup.2 /hr. (0.18 mils per hour).

Metallized surfaces produced by utilizing the autocatalytic metal deposition solution of this invention are useful as ornamental designs, markings, and the like. Similarly, using these baths copper may be deposited in predetermined patterns on insulating substrata and serve as electrical conductors. Electrically conductive circuit patterns, e.g., printed circuits, may thus be selectively plated on the activated areas of an inexpensive sheet of a wide variety of insulating material.

It has been shown that mixing of complexers in different proportions in electroless baths provides new and unexpected results in the variation of plating rates and bath stabilities.

There have been provided novel, alternative and simultaneous pathways for the reduction of copper metal complexes. Also provided are novel species in the bath (mixed metal complexes, two or more different complexers on the same metal ion, and all permutations thereof). Novel intermediates and new chemical reactions are also obtained with the teachings herein.

The invention in its broader aspects is not limited to the specific steps, processes and compositions described and departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

Claims

What is claimed is:

1. In an aqueous alkaline chemical plating solution capable of autocatalytically depositing copper metal on a substrate in contact therewith comprising

a. water;

b. a water-soluble complexing agent

c. copper ion, and

d. a reducing agent; wherein the pH is between about 10 and 14; the improvement which comprises the use as a complexing agent an effective amount of a compound selected from the group consisting of:

wherein x and y are each at least 1 and x and y together are from 2 to 25, or a mixture of said compounds.

2. A solution as defined in claim 1 wherein said imino compound is of the formula

wherein x and y are each at least 1 and x and y together are from 2 to 25.

3. A solution as defined in claim 2 wherein said imino compound is cyclohexylaminediethanol.

4. A solution as defined in claim 1 wherein said imino compound is N-benzyliminodiacetic acid.

5. A solution as defined in claim 1 wherein ammonia or an organic compound other than an imino compound of the formula defined is present, which organic compound contains at least one amine group, hydroxy group or carboxy group.

6. A solution as defined in claim 5 wherein said organic compound is cyclohexanediaminetatraacetic acid, ethylene diamine, diethylene triamine, triethylene tetramine, ethylenediaminetetraacetic acid, N,N,N',N'-tetrakis-2-(2-hydroxypropyl)ethylenediamine, citric acid, tartaric acid, 1,3-propane diamine, diethylenetriamine pentaacetic acid, Rochelle salts, sodium salts (mono-, di- and tri-sodium) of N-hydroxyethylenediaminetriacetic acid, nitrilotriacetic acid and alkali metal salts thereof, gluconic acid and alkali metal salts thereof or triethanolamine.

7. In a process for electrolessly depositing a copper metal on a catalytic surface which comprises contacting said catalytic surface with an aqueous alkaline chemical plating bath comprising

a. water;

b. a water soluble complexing agent,

c. copper ion, and

d. a reducing agent, wherein the pH is between about 10 and 14, the improvement which comprises the use as a complexing agent an effective amount of a compound selected from the group consisting of:

wherein x and y are each at least 1 and x and y together are from 2 to 25 or a mixture of said compounds.

8. A process as defined in claim 7 wherein said imino compound is of the formula

wherein x and y are each at least 1 and x and y together are from 2 to 25.

9. A process as defined in claim 8 wherein said imino compound is cyclohexylaminediethanol.

10. A process as defined in claim 9 wherein said imino compound is N-benzyliminodiacetic acid.

11. A process as defined in claim 7 wherein ammonia or an organic compound other than an imino compound of the formula defined is present, which organic compound contains at least one amine group, hydroxy group or carboxy group.

12. A process as defined in claim 11 wherein said organic compound is cyclohexanediaminetetraacetic acid, ethylene diamine, diethylene triamine, triethylene tetramine, ethylenediamine tetraacetic acid, N,N,N',N'-tetrakis-2-(2-hydroxypropyl)ethylene diamine, citric acid, tartaric acid, 1,3-propane diamine, diethylenetriaminepentaacetic acid, Rochelle salts, sodium salts (mono-, di- and tri-sodium) of N-hydroxyethylenediamine triacetic acid, mitrilotriacetic acid and alkali metal salts thereof, gluconic acid and alkali metal salts thereof or triethanolamine.