Electroless Deposition Of Ductile Copper

Abstract

A bath for electroless copper-plating containing a soluble copper salt, one or more complexing agents, formaldehyde and as an addition, which gives copper a satisfactory colour and makes it ductile also in case of comparatively large layer thicknesses, a polyalkylene oxidic compound including at least four alkylene oxide groups. The bath does not contain cyanide, nitrile or a metal compound of, for example, V, As or Sb.

Parent Case Text

This is a continuation, of application Ser. No. 78,740, filed Oct. 7, 1970 now abandoned.

Description

The invention relates to an alkaline aqueous electroless copper-plating bath and provides simplified methods of depositing ductile copper.

In this connection electroless copper-plating is understood to mean the deposition by chemical reduction of an adherent copper layer on a suitable surface in the absence of an external source of electricity. Such a copper-plating method is used, for example, on a large scale in the manufacture of printed circuits, conducting coatings which are subsequently to be further coated electrically, and for decorative purposes.

Different electroless copper-plating baths are already known. Such aqueous solutions generally contain cupric ions, formaldehyde, an alkali hydroxide and a complexing agent which prevents the cupric ions from being deposited in an alkaline medium. The copper deposition with the aid of such a bath is effected by reduction of cupric ions to copper by the formaldehyde, which reaction is initiated by a catalytic surface, for example, a catalytic metal or a synthetic resin which has been made catalytic (activated).

The known electroless copper-plating baths have a number of drawbacks. For example, the appearance of electroless deposited copper often greatly deviates from that of metallic copper. It is then, for example, not metallically bright but dull and has a dirty dark colour and a great brittleness, a comparatively small specific electrical conductance and a poor solderability. Among the known solutions which have a comparatively high deposition rate especially at the beginning the quality of the deposition is generally poor and it is characteristic that the deposition rate in many cases is rapidly reduced and sometimes decreases to zero. Consequently slow and hence more stable copper-plating solutions are generally used with the aid of which a basic layer having a thickness of 0.1 to approximately 0.25 .mu.um is deposited, for example, within 15 minutes, which layer is subsequently electrolytically copper-plated to the desired thickness. A layer having a thickness in the order of 25-50 .mu.um is generally desired and the electrolytic copper-plating process involves a considerable investment and a number of additional operations.

Apart from this fact it is generally difficult to use an electrolytical copper-plating process when copper is to be deposited in accordance with a pattern whose parts are not uninterruptedly coherent (for example, in additive methods of manufacturing printed circuits). A drawback of electrolytic depositions is also that these depositions often differ in thickness as a result of an irregular current density distribution. The commercially available electroless copper-plating solutions are generally of the slow type and are not suitable to deposit in an economic manner thick, ductile, adherent copper layers which are free from blisters. These solutions also generally have the unpleasant property that the deposition rate, being slow as it is, is reduced during deposition.

One of the little known solutions for electroless copper-plating by which a layer of eminent, ductile copper up to a thickness of approximately 25 .mu.um can be deposited within 24 hours contains an inorganic cyanide and/or an organic nitrile (U.S. Pat. No. 3,095,309). According to a further proposal, for example, vanadium pentoxide, sodium arsenite or potassium antimony tartrate is added instead of cyanide and/or nitrile to copper-plating solutions so as to effect the deposition of ductile copper (U.S. Pat. No. 3,310,430). All mentioned additions have the drawback that they have a highly toxic effect on the copper deposition and in certain cases the activity of a solution may stop completely and substantially instantaneously in the presence of several parts per million of the relevant compound. As a result the dosage must be very accurate so that also inspection of the deposition process requires much attention. Finally a number of the said compounds are extremely toxic.

A novel method of electroless copper-plating has recently become known (U.S. Pat. No. 3,329,512) whose major object is to deposit copper up to the thickness of 25 .mu.um or more at a rate of at least 6 .mu.um per hour and preferably more. To this end at least a great part of the complexing agent for cupric ions in the copper-plating solution consists of one or more hydroxyalkyl-substituted tertiary amines, which solution also contains a colloidal soluble non-reactive polymer chosen from the group of cellulose ethers, hydroxyethyl starch, polyvinylalcohol, polyvinylpyrrolidone, gelatin, peptones, polyamides and polyacrylamides. The polymer addition acts as a brightnening agent. The appearance of the copper layers which are deposited with the aid of such solutions is very satisfactory indeed, but the ductility of the layers is generally marginal and even insufficient when the relevant solutions are used for the manufacture of printed circuits in accordance with an additive method.

It is an object of the present invention to provide a bath for electroless deposition of copper up to the desired thickness in such a manner that the ductility of satisfactory electrolytic copper is approximated and that the above-mentioned drawbacks of the known baths and methods are obviated, and by which method smooth, brightly red, reasonably ductile, comparatively thick copper layers can be deposited in an economic manner at a rate of approximately 5 .mu.um per hour or more.

The ductility may be determined by partially loosening the deposited copper layer from the substrate and bending it in one direction over 180.degree., folding it and bending it back in its original position whereafter the fold is flattened under pressure. This completes one band. The operations are repeated until the layer breaks and thus it is possible to express the ductility in the number of bends which the layer can stand. For obtaining comparable results it is desirable to perform the measurements on copper layers of comparable layer thickness, for example, 15-20 .mu.um.

The starting point for the experiment which led to the present invention was that the ductility should be at least two bends with a view to versatile unability. This experiment showed that the ductility of electroless deposited copper depends on the rate of deposition which follows from the following Table which applies for a bath temperature of 50.degree.C.

__________________________________________________________________________ Bath composition (moles/litre) Layer thickness(.mu.m) Average depo- Ductility CuSo.sub.4,5H.sub.2 0 EDTA(4Na).sup.x NaOH Na.sub.2 CO.sub.3 HCHO after x hours sition rate (bends) (.mu.m/hour) __________________________________________________________________________ 0.008 0.009 0.10 -- 0.18 20 .mu.m in 13 hours 1.5 5 - 7 0.014 0.015 0.10 0.10 0.18 20 .mu.m in 10 hours 2.0 21/2 0.020 0.025 0.10 0.10 0.18 25 .mu.m in 61/2hours 3.8 1/2 __________________________________________________________________________ x EDTA (4 Na) is the tetrasodium salt of ethylenediaminetetraacetic acid

Furthermore it was found that 20 .mu.um of copper (10 .mu.um/hour) having a ductility of two to three bends could be deposited at a comparatively high temperature (75.degree. C) within two hours from a fairly unstable bath of the following composition: CuSO.sub.4, 5H.sub.2 O 0.02 mol/litre EDTA (4Na) 0.02 do. NaOH 0.10 do. HCHO 0.18 do.

Goldie (Plating, Nov. 1964) was able to deposit 1.54 mg Cu/sq.cm (approximately 0.85 .mu.um/hour) at 20.degree.C within 2 hours from a bath of the following composition:

CuSO.sub.4, 5H.sub.2 0 0.04 mol/litre Potassiumsodiumtartrate 0.18 mol/litre NaOH 0.175 mol/litre HCHO 0.13 mol/litre

It was found that copper deposited from such a bath at this temperature did not satisfy our condition of ductility. The critical rate at which reasonably ductile copper (at least two bends) can be deposited was thus found to be greater as the temperature of the bath increased.

During the experiment which led to the present invention the starting point was chosen to be the assumption that the improvement in ductility which may be brought about by an addition of, for example, inorganic cyanide, organic nitrile, vanadium pentoxide, sodium arsenite or potassium antimony tartrate should be ascribed to the reduction of the rate of deposition caused by such an addition.

However it was stated that compounds which contain bivalent sulphur such as thiourea and 2-mercaptobenzothiazole which are sometimes used to enhance the stability of copper-plating solutions, although they reduce the rate of the copper deposition already at very small quantities, generally do not enhance the ductility but rather the contrary. The inventors also found that certain cationic surface-active compounds such as cetylpyridiniumchloride and cationic polyethylenimine and the anionic compound heptadecylbenzimidazole monosulphate (Na-salt) neither enhance the ductility although they also reduce the deposition rate to a great extent.

Unlike cyanide and nitrile, all mentioned additions have a detrimental influence on the appearance of the deposited copper.

It was surprisingly found that many polyalkylene oxidic (polyalkoxy) compounds used in electroless copper-plating solutions have the property that they both reduce the deposition rate of copper and enhance the ductility and the appearance of the deposition, while in addition they tend to shift the limit of suitable ductility to faster deposition rates. In this respect it is important that none of the essential constituents of the solutions is present in a concentration which exceeds a certain limit. However, many variations are possible within the admitted regions of concentration.

The aqueous alkaline electroless copper-plating bath according to the invention for metallizing with ductile copper and containing a soluble copper salt, one or more complexing agents and formaldehyde is characterized in that it is free from an inorganic cyanide, an organic nitrile or a compound of one of the following elements: molybdenum, niobium, tungsten, rhenium, vanadium, arsenic, antimony, bismuth, actinium, lanthanium and the rare earths and that it contains as its essential constituents 0.01 - 0.10 mol of a copper salt soluble in water, a total of 0.01 - 0.80 mol of one or more complexing agents which prevent cupric ions from being deposited from the alkaline solution, 0.05 - 0.50 mol alkalihydroxide (pH approximately: 11 - 13.5), 0.01 - 0.35 mol formaldehyde or a compound producing a formaldehyde and -- to enhance the ductility of the depositing copper -- an effective concentration of a soluble, non-ionic or ionic polyalkylene oxidic compound either or not constituting micelles which contain at least four alkylene oxide (alkoxy) groups.

An effective concentration of the polyalkylene oxidic compound is understood to mean a concentration which enhances the ductility and -- when insufficient ductile copper is deposited from the solution without the active compound -- is at least such that a ductility of at least two bends is obtained. Also, an effective copper concentration substantially always enhances the colour and the smoothness of the deposition and often reduces the number of blisters formed in the surface of the deposited copper and enhances the adhesion of the deposited layer.

Although the deposition rate of copper is generally reduced due to the presence of a polyalkylene oxidic compound according to the invention, it has been found that -- dependent on the concentrations of the essential constitutents of the copper-plating solution -- it is not always possible to reduce it to the critical rate at which copper is deposited at a ductility of two bends. For this reason a maximum but still suitable limit concentration is indicated above for each essential constituent of the solution. The minium limit concentrations are determined by the requirement that copper must be deposited at a rate which is still acceptable for practical use. Maximum and still acceptable concentrations of the other constituents are associated with each maximum limit concentration of an essential constituent, which concentrations are generally lower than the mentioned maximum limit concentrations of said constituents. Thus this means that when the limit concentration of one of the constituents is exceeded in a copper plating solution, the technical effect of the invention can no longer be realized to its full extent. This does not mean that this is indeed the case in a bath in which all concentrations correspond to the limit concentrations. It has been found that the critical deposition rate is also slightly influenced by the nature of the complexing agent for cupric ions so that a general critical rate for all baths containing polyalkylene oxidic compounds cannot be given. On the other hand, as already noted, the critical rate increases with the operating temperature of the copper-plating solution.

It is to be noted that electroless copper-plating baths of the previously described type are known from U.S. Pat. No. 3,257,215, which baths contain a cyanide compound and a mercapto compound. It is recommended to add a wetting agent to the bath, the specification mentioning a few agents which satisfy the definition according to the invention. However, it was found that addition of such a wetting agent to such a cyanide-containing bath does not provide any improvement with regard to ductility as compared with the corresponding bath without a wetting agent. These known baths have the drawbacks already referred to.

A large number of suitable polyalkylene oxidic compounds is defined by the following general formula

R(OC.sub.2 H.sub.4).sub.a (OC.sub.n H.sub.2n).sub.b (OC.sub.2 H.sub.4).sub.c R.sub.2 with a+b+c .gtoreq. 4. For the polyalkylene glycols not constituting micelles there applies that R.sub.1 = H and R.sub.2 =OH. For the polyethylene glycols b is equal to zero and the formula changes into H(OC.sub.2 H.sub.4).sub.a.sub.+c OH. For the polypropylene glycols a and c are both zero and n = 3 : H(OC.sub.3 H.sub.6).sub.b OH and for the polybutylene glycols both a and c are likewise zero and n = 4 : H(OC.sub.4 H.sub.8).sub.b OH.

Polyethylene glycols are satisfactorily soluble in water even when they have a high molecular weight. Thus they are very much suitable for use within the scope of the present invention. Even a compound having a molecular weight of 500,000 and a + c > 1,100 is found to be still effective. When the molecular weight of polypropylene and polybutylene glycols increases, the solubility decreases. When the compounds are completely insoluble in the copper-plating solution, they are unsuitable for the method according to the invention. Compounds which are interesting with a view to their suitability are block polymers with a > 1, b > 1, and c > 1 and n = 3. These are compounds which consist of one more hydrophobic block of propylene oxide (propoxy) groups flanked by hydrophilic blocks of ethylene oxide (ethoxy) groups. Such compounds are commercially available under the name of Pluronics (Wyandotte Chemicals Corp.) in a great variety of compositions and molecular weights. It has been found that polyalkylene oxidic compounds which contain approximately 13 or more alkylene oxide (alkoxy) groups are active within a very wide range of concentrations when they are used in copper-plating solutions which are composed in conformity with the concentration requirements formulated hereinbefore. This may be concluded from the following embodiments which also illustrate the essence of the method according to the present invention.

EXAMPLE I

An adhesive layer roughened with chromic acid-sulphuric acid and based on a mixture of polybutadieneacrylonitrile and cresol resin which is provided on hard paper was locally activated by a photochemical process for the electroless copper deposition. This activation was performed as follows. Firstly the layer was immersed for a short period in a photosensitive solution of the following composition:

0.10 mol orthomethoxybenzenediazosulphonate (Mg salt)

0.017 mol cadmium lactate

0.017 mol calcium lactate

0.017 mol lactic acid

10 gms Lissapol N

deionized water up to 1 litre.

The pH of the solution was set at 4. The adherent liquid layer was dried with the aid of a stream of cold air. The photosensitive layer was subsequently exposed behind a negative of fairly broad lines for 1 minute with the aid of a reprographic mercury vapour lamp (type HPR 150 W) which was at a distance of 42 cms from the negative. Subsequently the exposed layer was treated with a solution containing

0.075 mol mercurous nitrate

0.01 mol silver nitrate

0.15 mol nitric acid

deionized water up to 1 litre.

The latent image formed consisting of metal nuclei, was slightly intensified with silver by treating it for 1 1/2 minutes with a solution comprising

0.01 mol silver nitrate

0.025 mol metol

0.10 citric acid

deionized water up to 1 litre.

The weak image thus obtained consisting of catalytic silver nuclei was finally electroless copper-plated in a copper-plating solution of the composition:

0.028 mol copper sulphate (5 H.sub.2 O)

0.030 mol ethylenediaminetetraacetate (4 Na)

0.13 mol formaldehyde

0.10 mol sodium hydroxide

x mol polyethylene glycol

water up to 1 litre. Copper layers of approximately 20 .mu.m thickness were grown at a bath temperature of 50.degree.C on a number of substrates activated in the manner described. Without the addition of polyethylene glycol (x = 0) the deposition rate was approximately 9 .mu.m per hour. In that case non-ductile copper (1/2 bend) was deposited.

By varying x the smallest effective concentration was determined for a series of polyethylene glycols having an increasing number of ethoxy groups. The compounds used are gathered in Table I.

TABLE I __________________________________________________________________________ H(OC.sub.2 H.sub.4).sub.a.sub.+c OH Manufacturer mol. weight a + c __________________________________________________________________________ carbowax 200 Union Carbide Chemicals Co. 190 - 210 4 - 5 do. 300 do. 285 - 315 6 - 7 do. 400 do. 380 - 420 8 - 9 do. 600 do. 570 - 630 13 - 14 do. 1540 do. 1300 - 1600 29 - 36 do. 4000 do. 3000 - 3700 68 - 84 do. 20 M do. 15000 - 20000 340 - 450 __________________________________________________________________________

These minimum effective concentrations c are plotted in FIG. 1 as a function of the number of ethoxy groups n. The curve through the experimental points separates a region of satisfactory ductile copper from a region of non-satisfactory, non-ductile copper. If copper having a ductility of at least 2 bends is to be deposited, approximately 0.1 mol per litre of the compounds including fewer than 8 - 9 ethoxy groups is to be added. For the compounds having more ethoxy groups much smaller concentrations may be sufficient. Nevertheless it is recommended to use a concentration which is slightly higher than the limit concentration. In that case ductilities of 3 - 8 bends may be obtained. The ductile copper was deposited in these cases at a rate in the order of 1 .mu.m per hour. It had a better metallic appearance and a slighter oxidation sensitivity than the non-ductile copper deposited from baths containing too little polyethylene glycol. It is to be noted that the method and extent of activating the substrate as well as the composition of the copper-plating solution within the indicated concentration ranges quantitatively exert some influence on the minimum effective concentration of a polyethylene glycol, although this does not lead to a different qualitative image.

EXAMPLE II

Using substrates which were activated in the manner described with reference to Example I for the electroless copper deposition, copper layers of approximately 20 .mu.m thickness were grown at a bath temperature 50.degree.C while using copper-plating solutions containing the following constituents:

0.028 mol copper sulphate (5 H.sub.2 O)

0.030 mol ethylenediaminetetraacetate (4 Na)

0.10 mol sodium hydroxide

0.13 mol formaldehyde

y percent by weight of polyalkylene glycol

water up to 1 litre.

The polyalkylene glycols used and the ductilities of the copper layers deposited therewith are gathered in Table 2. All compounds with the exception of PPG 150 including two to three propoxy groups considerably improve the ductility.

Table 2 __________________________________________________________________________ H(OC.sub.2 H.sub.4).sub.a (OC.sub.n H.sub.2n).sub.b (OC.sub.2 H.sub.4).sub .c OH Manufacturer a+c b n y(% by ductility weight (number of bends) __________________________________________________________________________ Carbowax 6000 Union Carbide Chemicals Co. 140-170 0 -- 0.02 5 0.10 6 Polyox WSR-205 do. >11000 0 -- 0.10 3 PPG 150 Hodag Chemical Corp. 0 2-3 3 2.5 1/2 PPG P1025 Dow Chemical Co. 0 17 3 0.01 3 0.10 3 PPG 1200 Hodag Chemical Corp. 0 20-21 3 0.01 3 0.10 3 PPG 2025 Union Carbide Chemicals Co. 0 34-35 3 0.01 3 0.10 3 PBG B500 Dow Chemical Co. 0 6-7 4 0.10 1/2 2.5 41/2 Pluronic L31 Wyandotte Chemicals Corp. 2 16 3 0.10 41/2 Pluronic L61 do. 4 30 3 0.0001 1/2 0.001 5 0.10 4 Pluronic L64 do. 26 30 3 0.10 5 Pluronic F68 do. 159 30 3 0.0001 11/2 0.001 5 0.10 41/2 Pluronic L81 do. 5 38 3 0.10 51/2 Pluronic F88 do. 204 38 3 0.10 6 No addition -- -- -- -- <1/2 __________________________________________________________________________

When R.sub.1 is a hydrophobic final group, b = O and R.sub.2 =OH, a molecule is obtained whose solubility in water is determined by the number of ethyleneoxide groups in the hydrophilic tail. Such compounds are typical micelle-constituting surface-active compounds of which it is known that the number of molecules constituting one micelle decreases when the number of ethoxy groups in the molecule increases. They are generally non-ionic and are eminently suitable for use within the scope of the present invention.

When R.sub.1 is a paraffin tail such as, for example, a lauryl, cetyl, stearyl, myristyl or oleyl group, then ethoxylated fatty alcohols, or more correctly expressed, an ether of a fatty alcohol and a polyethylene glycol are concerned: C.sub.m H.sub.2m.sub.+1 (OC.sub.2 H.sub.4).sub.a.sub.+c OH. The alkyl group may alternatively be branched. Numerous active surface-active compounds of this type are commercially available such as, for example:

Avolan IW from Farbenfabriken Bayer AG ON Brij (series compounds) Atlas Chemical Ind. Empilan K (serie) Marchon Products Ltd. Emulgin 05 General Aniline & Film Corp. 010 Emulphor ON Henkel International GmbH Ethosperse La (serie) Glyco Chemicals, Inc. - Lipal CSA (serie) Drew Chemical Corp. LA (do.) MA (do.) OA (do.) Lorox (serie) Chem-Y Lubrol AL (serie) I.C.I. Peregal O General Aniline & Film Corp. Plurafac A (serie) Wyandotte Chemicals Corp. B (do.) RA (do.) Siponic E (serie) Alcolac Chemical Corp. L (do.) Y (do.)

For the chemical data regarding these compounds and those of the compounds which will be further mentioned reference is made to Mc.Cutcheon's Detergents and Emulsifiers 1967 Annual, J. W. Mc. Cutcheon Inc.

R.sub.1 may alternatively be an alkylaryl group such as, for example, an octylphenyl, nonylphenyl or dodecylphenyl group. Relevant compounds are alkylarylethers of polyethylene glycol: C.sub.m H.sub.2m.sub.+1 - Ar (OC.sub.2 H.sub.4).sub.a.sub.+c OH. Active ethoxylated alkylphenols which are commercially available are, for example: Androx P (series compounds) alkylphenol Chem-Y Ampilan NP alkylphenol Marchon Products Ltd. Hyonic PE (series) alkylphenol Nopco Chemical Co. Igepal CA (series) octylphenol General Aniline & Film Corp. CO ( do. ) nonylphenol DM ( do. ) alkylphenol Lessagene ( do. ) alkylphenol Chem-Y Lissapol N( do. ) nonylphenol I.C.I. Lubrol E alkylphenol I.C.I. L nonylphenol Neutronyx 600 (series) alkylphenol Onyx Chemical Co. Poly Tergent B (series) nonylphenol Olin Mathieson G ( do. ) octylphenol Chemical Corp. Retzanol NP (series) alkylphenol Retzloff Chemical Co. Renex 600 (series) nonylphenol Atlas Chemical Ind. Tergitol P (series) dodecylphenol Union Carbide Corp. NP ( do. ) nonylphenol Triton N ( do. ) nonylphenol Rohm and Haas Co. X ( do. ) octylphenol

The final hydroxyl group of alkyl and alkylary ethers of polyethylene glycol may be esterified with sulphuric acid. R.sub.2 then changes into ##SPC1##

Sulphates are produced of the alkyl and alkylaryl ethers of polyethyleneglycol, which are anionactive. Suitable sulphates are for example:

Empicol ESB -- sulphated laurylether of polyethyleneglycol (Na) - Marchon Products Ltd.

Sipex EA -- sulphated laurylether of polyethyleneglycol (NH.sub.4) - Alcolac Chemical Co.

Alipal CO-433 -- alkylphenylether of polyethyleneglycol (Na) - General Aniline & Film Corp.

Co-436 -- alkylphenylether of polyethyleneglycol (NH.sub.4) - General Aniline & Film Corp.

Neutronyx S-30 -- alkylphenylether of polyethyleneglycol (Na) - Onyx Chemical Co.

S-60 -- alkylphenylether of polyethyleneglycol (NH.sub.4) - Onyx Chemical Co.

The final hydroxyl group of alkylethers and alkylaryl ethers of polyethylene glycol may alternatively be esterified with phosphoric acid. In this case R.sub.2 becomes ##SPC2##

The relevant phosphate esters are anion-active. Examples are:

Antara LB-400-acid phosphate ester of an alkylether of polyethylene glycol-General Aniline & Film Corp.

Gafac RE-610-acid phosphate ester -- General Aniline & Film Corp.

Rozak BD-COC-phosphate ester of the oleylether of polyethylene glycol-Rozilda Laboratories, Inc.

Wascope 9J2 -- acid phosphate ester of alkylphenylether of polyethylene glycol -- Wasco Laboratories.

9F1 -- glycol -- Wasco Laboratories

9F2 -- glycol -- Wasco Laboratories

An active compound is also the nonionic Victawet 12 from the firm of Stauffer Chemical Co., a tertiary phosphate ester defined by the formula: ##SPC3##

It is a remarkable that fatty acid esters of polyethylene glycols, for example, compounds of the Myrj-series (Atlas Chemical Ind.) or of the Tween-series (Atlas Chemical Ind.) greatly reduce the deposition rate of copper from copper-plating solutions when the concentration increases generally before reaching a suitable improvement of the ductility. Consequently they are less suitable for use within the scope of the present invention. The activity of a number of these micelle-constituting surface-active compounds of the non-ionic and anionic type will hereinafter be described in detail.

EXAMPLE III

Two methods of activation of hard paper substrates provided with an adhesive were used:

method A: the photochemical production of silver nuclei described in Example I, and;

method B: the production of palladium nuclei,

Palladium nuclei:

Plates (25 sq.cm) of chemically roughened hard paper provided with an adhesive (Example I) were activated for the electroless copper deposition by successively treating them at room temperature in

a stationary solution of 10 grams of tin (II) chloride and 10 mls of hydrochloric acid in 1 litre of deionized water (3minutes),

deionized water (1 minute),

a stationary solution of 0.2 gram of palladium (II) chloride and 10 mls of hydrochloric acid in 1 litre of deionized water (3 minutes)

running deioinized water (10 minutes).

The plates were turned about after 30 seconds in the tin (II) chloride and palladium (II) chloride solutions. The copper-plating solution used for this Example had the following composition:

0.028 mol copper sulphate (5 H.sub.2 O),

0.030 mol ethylenediaminetetra-acetate (4 Na),

0.10 mol sodium hydroxide,

0.13 mol formaldehyde,

z percent by weight of polyoxyethylene derivate, water up to 1 litre.

This solution was used at a temperature of 50.degree.C. The polyoxyethylene derivates used and the results obtained are gathered in Table 3.

Table 3 __________________________________________________________________________ Polyoxyethylene- number z acti- Duc- derivate Class Manufacturer mol. of (% by vation tili- weight ethoxy weight method ty groups __________________________________________________________________________ Surfactant QS-44 alkylphenoxy-P.O.E.- Rohm and Haas Co. approx. appr. 0.10 B 6 phosphate-ester 800 8 Triton X - 100 octylphenylether do. 650 9-10 0.005 A 1/2 0.02 A 11/2 0.05 A 4 0.10 A 4 Lissapol N nonylphenylether I.C.I. 695 11 0.05 A 3 Triton X - 102 octylphenylether Rohm and Haas Co. 780 12-13 0.001 B 21/2 Triton X - 165 octylphenylether do. 910 16 0.005 B 3 Brij 35 Laurylether Atlas Chemical Ind. 980 20 0.001 A 2 0.02 A 4 0.10 A 7 Triton X - 305 octylphenylether Rohm and Haas Co. 1530 30 0.001 B 3 Triton QS - 15 geethoxyleerd Na-salz do. 2390 35 0.0001 B 1/2 0.01 B 4 0.1 A 5 1 A 6 Gafac RE - 610 phosphate-ester General Aniline & Film Co. 2440 38 0.02 A 4 0.10 A 5 1 A 4 __________________________________________________________________________ The compound pulp 30. a lauryl ether of polyethylene glycol having a mol. weight of approximately 320 and 3-4 ethoxy groups provides copper layers of insufficient ductility.

A further type of compounds is obtained when in the general formula R.sub.1 = H, B = O, ##SPC4##

C.sub.m H.sub.2m.sub.+1 is a paraffin tail. In that case an ethoxylated secondary amine is concerned when R.sub.3 = H, or an ethoxylated tertiary amine when R.sub.3 is, for example, a polyethoxy group: R.sub.3 = (C.sub.2 H.sub.4 O).sub.d H. Closely related therewith are ethoxylated fatty acid amides and R.sub.2 is then, for example, ##SPC5##

The ethoxylated alkylamines are potential cationactive compounds particularly in an acid medium. The cationactive character will, however, be less manifest as more ethoxy groups occur in the molecule. These compounds are eminently active within the scope of the present invention which is slightly surprising because it was found that certain non-ethoxylated cationactive compounds greatly delay the copper deposition and have a detrimental influence on the quality of the copper.

The activity of these ethoxylated fatty amines is illustrated by the following Example.

EXAMPLE IV

Glass plates (8 sq.cm) slightly roughened with carborundum powder were activated for the electroless copper deposition by successively treating them at room temperature, while being shaked, in:

a solution of 50 grams of tin (II) chloride and 10 mls of hydrochloric acid in 1 litre of deionized water (2 minutes)

deionized water (1 minute)

a solution of 0.25 grams of palladium (II) chloride and 10 mls of hydrochloric acid in 1 litre of deionized water (1 minute)

running deionized water (11/2 minutes)

A copper layer 15 - 20 .mu.m thickness was deposited at 50.degree.C on the plates thus pretreated. 200 mls of copper-plating solution were used for each plate. This solution had the following composition:

0.028 mol copper sulphate (5 H.sub.2 O),

0.030 mol ethylenediaminetetra-acetate (4 Na)

0.10 mol sodium hydroxyde

0.12 mol formaldehyde

0.1 percent by weight of an ethoxylated fatty amine deionized water up to 1 litre. The following polyethoxy compounds were used:

a. Sipenol 1T15 fatty amine, 15 ethoxy groups -- Alcolac Chemical Corp.

b. Sipenol 1S15 fatty amine, 15 ethoxy groups -- Alcolac Chemical Corp.

c. Sipenol 1S50 fatty amine, 50 ethoxy groups- Alcolac Chemical Corp.

d. Ethomene C/20 tertiary fatty amine, 10 ethoxy groups -- Armour Industrial Chemical Co.

e. Ethomene S/20 tertiary fatty amine, 10 ethoxy groups -- Armour Industrial Chemical Co.

f. Priminox T-15 secondary fatty amine, 15 ethoxy groups -- Rohm and Haas Co. (C.sub.m H.sub.2m.sub.+1 = C.sub.18.sub.-22 - H.sub.36.sub.-44)

g. Priminox T-25 secondary fatty amine, 25 ethoxy groups -- Rohm and Haas Co.

The copper layer deposited from a solution without polyethoxy compound had a ductility of <one-half bend. The layers deposited from solutions including an addition had the following ductilities:

Addition bends ______________________________________ a 2 b 3 c 41/2 d 21/2 e 41/2 f 31/2 g 31/2 ______________________________________

Compounds which are interesting within the scope of the present invention are the thioethers of a higher alkylmercaptane and polyethylene glycol defined by the formula: H(OC.sub.2 H.sub.4).sub.a.sub.+c SC.sub.m H.sub.2m.sub.+1 (R.sub.1 = H; R.sub.2 = SC.sub.m H.sub.2m.sub.+1 ; b = O)

It is known that organic sulphur compounds enhance the stablity of electroless copper-plating solutions. However, it is also known that these compounds increase the brittleness of the deposition and that they can generally be used only in a very small concentration because otherwise they suppress the copper deposition completely. When a sulphur atom enhancing the stability and increasing the brittleness and a number of ethoxy groups enhancing the ductility are built in in the same molecule, such as is the case in the above-mentioned ethoxylated thioethers, then it may be expected that the ductility of the deposited copper will be acceptable in the presence of sufficient ethoxy groups while the stability will be enhanced by the presence of the thioether bridge. If necessary the ductility-enhancing effect may be increased by adding a polyalkylene glycol or a surface-active polyethoxy compound of one of the types described so far.

As shown in Example V this consideration may be realized with the surprising effect that the concentration of the thioether was much less critical in connection with the effect on the deposition rate than that of other stabilising organic sulphur compounds.

EXAMPLE V

Glass plates which were activated for the electroless copper-plating process as described in the previous Example were coated with a copper layer of 15 - 20 .mu.m by treating them at 50.degree.C in a copper-plating solution of the composition:

0.05 mol copper sulphate (5 H.sub.2 O)

0.075 mol ethylenediaminetetra-acetate (4 Na)

0.30 mol sodium hydroxide

0.12 mol formaldehyde

0.01 percent by weight of Carbowax 4000

0.001 percent by weight of ethoxylated thioether deionized water up to 1 litre.

The following non-ionic ethoxylated thioethers were used:

a. Siponic SK thioether of polyethylene glycol -- Alcolac Chemical Corp.

b. Sterox SE thioether of polyethylene glycol -- Monsanto Co.

c. Nonic 260 tertiary dodecyl thioether of polyethylene glycol Pennsalt Chemicals Corp. (5,7 ethoxy groups).

The ductility of the copper layer which was deposited from a solution without polyethoxy compound was one-half bend. The ductility of the copper layers deposited from solutions including additions was at least three bends. The durability of a bath including additions was a factor of 3 to 4 better than that of a bath without an addition. Solubility permitting the concentration of the thioether may be increased to 0.1 percent by weight before the copper deposition is unacceptably delayed.

As already previously noted, a number of bivalent sulphur compounds added in a quantity which is smaller than that which completely prevents the copper deposition has a stabilising effect on an electroless copper-plating solution so that it is durable for a longer period. U.S. Pat. No. 3,361,580 describes the action of a few stabilisers (thiourea, potassium polysulphide, thioglycol acid and 2-mercaptobenzothiazole) which are used in quantities of 0.001 to 0.1 milligrams per litre of solution, although the magnitude of the technical effect is not clearly mentioned. When tracing the described examples, the stabilising effect is unmistakably present, but in so far as they can be deposited to the required thickness the copper layers have a completely insufficient ductility. An experiment on the stabilising effect of 2-mercaptobenzothiazole is described in Electronic Industries of Sept. 1962, pages 117 - 119.

In the experiment performed in connection with the present invention it was attampted to enhance the stability of copper-plating solutions which contain a polyalkoxy compound in a concentration ensuring the deposition of satisfactorily ductile copper by addition of one of the known bivalent sulphur compounds. It was found that it is very difficult to enhance the stability of these baths to an optimum extent while maintaining an acceptable ductility of the deposited copper. A number of examined compounds showed that 1-phenyl-5-mercaptotetrazole in combination with polyalkoxy compounds, which had not been previously described as a stabiliser for electroless copper-plating solutions, led to the envisaged object. The quantity of 1-phenyl-5-mercaptotetrazole which is added is to be between 0.01 and 1 milligram per litre of solution (including the limit values). The following Example illustrates this embodiment of the copper-plating bath according to the invention.

EXAMPLE VI

Plates of chemically roughened hard paper provided with an adhesive (25 sq.cm) were subjected to the formation of palladium nuclei in the manner as described with reference to Example III in order to activate these plates for electroless copper-plating. The plates were copper-plated at 50.degree.C while being stationary in 200 mls of the following solution:

0.05 mol copper sulphate (5 H.sub.2 O)

0.075 mol ethylenediaminetetra-acetate (4 Na)

0.30 mol sodium hydroxide

0.19 mol formaldehyde

0.5 mg 1-phenyl-5-mercaptotetrazole

1 g Carbowax 1540

deionized water up to 1 litre.

A copper layer of 20 .mu.m was deposited within 5 hours and had a ductility of at least three bends. When the polyalkoxy compound was omitted a brittle copper layer was deposited (<1/2 bend). The durability of the bath was enhanced by a factor of 8 due to the addition of 1-phenyl-5-mercaptotetrazole. Similar results are obtained when instead of 1 g of Carbowax 1540, 2 grams of Triton QS-15 are added.

The choice of the water-soluble copper salt for electroless copper-plating solutions is mainly determined by economic considerations. Copper sulphate is preferred, but the nitrate, halogenides, acetates and other soluble salts of copper may alternatively be used.

In the electroless copper-plating solutions cupric ions and complexing agent as a rule constitute complexes at a molecular ratio of 1 : 1, but generally an excess of complexing agent will preferably be used. There are many known complexing agents for cupric ions. Preferably the invention employs Rochelle salt (potassium-sodium tartrate), triethanolamine and alkali salts of N-hydroxyethylethylenediaminetriacetic acid, ethylenediaminetetra-acetic acid and diethylenetriaminepentacetic acid and mixtures thereof.

Reducing agents which are used in alkaline electroless copper-plating solutions are formaldehyde and compounds or derivates producing formaldehyde such as paraformaldehyde, glyoxale and trioxane.

Among the alkalihydroxydes, sodium hydroxyde is preferably used for reasons of economy. The baths may contain less essential constituents such as, for example, buffers and sodium carbonate.

As regards the operating temperature of the baths, the fact should be taken into account that the rate by which copper is deposited increases as the temperature increases and that then also the critical rate by which copper having a ductility of at least two bends is deposited increases. Thus it is economically advantageous to use the baths at a temperature which is higher than that in the working space. On the other hand it should be taken into account that the stability of the baths rapidly decreases at comparatively high temperatures. The highest temperature at which a copper-plating solution can still be used is determined to a great extent by the nature of the complexing agent for the cupric ions. Baths containing weak complexing agents must be used at a comparatively low temperature with a view to the stability of the solutions, whereas baths containing strong complexing agents will be used preferably at comparatively high temperatures, in some cases even up to 90.degree.C. Thus, solutions in which potassium-sodium tartrate, triethanolamine and/or N-hydroxyethylethylenediaminetriacetate (3Na) are used as complexing agents may be used at temperatures of 15.degree.C to 40.degree.C; solutions based on ethylenediaminetetraacetate (4 Na) may be used at temperatures of 20.degree.C to 80.degree.C and solutions based on diethylenetriaminepentaacetate (5 Na) may be used at temperatures of 60.degree.C to 90.degree.C. The last-mentioned baths have the advantage that they do not deposit at room temperature and are then very stable. When not in use, these baths are therefore preferably allowed to cool down to room temperature. The following examples illustrate the invention when using the different complexing agents in a temperature range of from 15.degree. to 90.degree.C.

EXAMPLE VII

Similar activated substrates as those used in Example VI were electroless copper-plated with a layer of 15 - 20 .mu.m thickness.

__________________________________________________________________________ Bath compositions and results CuSO.sub.4 (5H.sub.2 O) KNA tartrate HEDTA NaOH HCHO Na.sub.2 CO.sub.3 Surfactant Carbowax Rate Bends (3N a) QS-44 6000 __________________________________________________________________________ 0.028mol/1 0.15 mol/1 -- 0.40mol/1 0.32mol/1 0.10mol/1 -- -- 0.7 1/2 m/h 0.028 0.15 -- 0.40 0.32 0.10 10g/1 -- 0.3 4 0.028 0.15 -- 0.40 0.32 0.10 -- 20g/1 0.2 3 0.02 -- 0.80 0.20 0.32 0.80 -- -- not 1/2 rmined 0.02 -- 0.80 0.20 0.32 0.80 5 -- 0.45 21/2 __________________________________________________________________________ (HEDTA (3Na) is the trisodium salt of N-hydroxyethylethylenediametriacetat e)

At 20.degree.C a deposition rate of 0.7 .mu.m/hour is hypercritical, but a rate of 0.45 .mu.m/hour is almost critical.

EXAMPLE VIII

Using the same kind of activated substrates the following results were obtained with the copper-plating solution mentioned in the table while using a bath temperature of 35.degree.C (see tables page . . . . ). ##SPC6##

EXAMPLE IX

Copper layers of approximately 20 .mu.m thickness were deposited on glass plates which were activated for the electroless copper deposition in the manner as described with reference to Example IV by treating these plates for 4 hours (deposition rate 5 .mu.m/hour) with the below-mentioned copper-plating solution which was maintained at 35.degree.C and which was not stirred. Composition of copper-plating solution:

0.028 mol copper sulphate (5 H2O)

0.065 mol triethanolamine

0.20 mol sodium hydroxide

0.19 mol formaldehyde

10 g Carbowax 4000 (deionized water up to 1 litre).

The deposited copper layers could stand three full bends. A similar bath without Carbowax 4000 yielded 10 .mu.m of copper deposition per hour, but the ductility was then< 1/2 bend.

EXAMPLE X

Glass plates (8 sq.cm) were superficially roughened for 11/2 minutes with carborundum powder (particle diameter approximately 37 .mu.m), rins with deionized water and dried. Subsequently the plates were activated for electroless copper-plating by treating them successively with a solution of 50 grams of tin (II) chloride and 10 mls of hydrochloric acid in 1 litre of deionized water (2 minutes at 22.degree.C).

deionized rinsing water (11/2 minutes)

a solution of 0.25 grams of palladium (II) chloride and 10 mls of hydrochloric acid in 1 litre of deionized water (1 minute at 22.degree.C).

deionized rinsing water (2 minutes)

The copper deposition rate was varied by means of the concentration of the copper salt in the copper-plating solution. The composition thereof was as follows:

x mol copper sulphate (5 H.sub.2 O)

0.20 mol ethylenediaminetetraacetate (4 Na)

0.10 mol sodium hydroxyde

0.18 mol formaldehyde

1 g Carbowax 4000

deionized water up to 1 litre.

200 mls of copper-plating solution were used for each plate. The operating temperatures of the baths were 50.degree.C, 60.degree.C and 75.degree.C.

The following results were obtained.

__________________________________________________________________________ Bath temp. x Thickness of Average deposition Number the deposited rate of .degree.C mol/1 copper layer (.mu.m/hour) bends (.mu.m) __________________________________________________________________________ 50 0.03 24 1.4 4 0.05 20 3.3 4 0.06 20 5 3 0.07 23 10 1/2 60 0.03 20 2.8 5 0.05 18 4 6 0.07 20 9 11/2 - 2 75 0.03 21 6 8 0.05 31 13 5 0.07 21 21 2 __________________________________________________________________________ These baths showed signs of spontaneous decomposition at the end of the deposition period.

It is clearly apparent that the critical rate at which copper having a ductility of at least two bends is deposited increases as the bath temperature increases.

EXAMPLE XI

The copper deposition from a bath of the composition below was compared using three kinds of activated substrates:

a. chemically roughened hard-paper substrates provided with an adhesive (Example I), photochemically activated for the copper deposition in the manner described in Example I (formation of silver nuclei);

b. the same hard paper substrates activated in the manner described in Example X (formation of palladium nuclei);

c. mechanically roughened glass substrates (Example X) activated in the manner described in Example X (formation of palladium nuclei). Composition of the copper-plating solution:

0.028 mol copper sulphate (5 H.sub.2 O)

0.030 mol ethylenediaminetetraacetate (4 Na)

0.10 mol sodium hydroxyde

0.12 mol formaldehyde

1 g Carbowax 4000

deionized water up to 1 litre.

Layers of approximately 20 .mu.m thickness were again deposited. The results may be summarized as follows:

Bath temp. Substrate Average depo- Number of .degree.C sition rate bends (.mu.m/hour) ______________________________________ 50 glued hard paper/Ag 1 5 50 glass/Pd 1 6 60 glued hard paper/Ag 2.2 6 60 glass/Pd 2.1 6 75 glued hard paper/Ag 4 7 75 glued hard paper/Pd 4 8 75 glass/Pd 4,4 8 ______________________________________

No significant differences could be found in the deposition on the different substrates. However, the tendency of a better ductility at a higher temperature of the bath becomes clearly manifest.

EXAMPLE XII

Copper was deposited for 4 hours on glass plates which were roughened and activated in the manner described in Example X from a copper-plating solution of the composition:

0.028 mol copper sulphate (5 H.sub.2 O) ) ) ) 0.04 mol diethylenetriaminepentaacetate (5 Na) ) ) 0.10 mol sodium hydroxyde ) ) bath temperature: 70.degree. C 0.19 mol formaldehyde ) 5 grams surfactant QS-44 deionized water up to 1 litre. )

The layers which had a thickness of approximately 15 .mu.m could stand 41/2 bends. After deposition the bath was cooled down to room temperature and it was brought to 70.degree.C again after 24 hours whereafter glass plates were copper-plated again.

Without the addition of surfactant QS-44 dark copper was deposited which could only stand 11/2 bends.

Claims

1. An alkaline aqueous electroless copper-plating bath for depositive ductile copper, which bath is free of inorganic cyanides, organic nitriles and compounds of La, the rare earths Mo, Nb, W, Re, V, As, Sb, Bi and Ac, said bath having a pH of about 11-13.5 and containing per liter of water 0.01 - 1.10 mols of a water-soluble copper salt, 0.01-0.35 mols of formaldehyde or a formaldehyde producing compound, 0.01 - 0.80 mols of at least one coupling compound capable of forming a soluble complex with cupric ions in an alkaline solution, 0.05 - 0.50 mols of an alkali metal hydroxide and a water soluble polyalkylene oxide compound containing at least four alkylene oxide groups of two to four carbons per molecule in an

2. The copper-plating bath of claim 1 wherein the polyalkylene oxide compound is defined by the formula:

R.sub.1 (C.sub.2 H.sub.4 O).sub.a (C.sub.n H.sub.2n O).sub.b (C.sub.2 H.sub.4 O).sub.c R.sub.2 in which formula a + b + c .gtoreq. 4, n = 0-4, R.sub.1 being H and R.sub.2 being OH when n is 0, 3 or 4, R.sub.1 being alkyl or alkaryl and R.sub.2 being OH, esterified or unesterified hydroxy, sulfate or phosphate groups when n = 0 and when n = 6, R.sub.1 is H and R.sub.2 is fatty acid amino, fatty acid amino or an alkyl substituted

3. The copper-plating bath of claim 2 wherein the polyalkylene oxide compound is a polyalkylene glycol containing at least 4 ethoxy or propoxy

4. The copper-plating bath of claim 2 wherein the polyalkylene oxide compound is an ethoxylated fatty acid amine or an ethoxylated fatty acid

5. The copper-plating bath of claim 2 wherein the polyalkylene oxide

6. The copper-plating bath of claim 1, wherein in addition 0.01-1 mg per

7. The copper-plating bath of claim 1 wherein the complexing agent for cupric ions is selected from the group consisting of potassium-sodium tartrate, triethanolamine and N-hydroxyethylethylene diamine-trisodiumacetate and the temperature of the bath is between

8. The copper-plating bath of claim 1 wherein the complexing agent for cupric ions is ethylenediamine-tetrasodium-acetate, and the temperature of

9. The copper-plating bath of claim 1 wherein the complexing agent for cupric ions is diethylenetriamin-epenta-sodiumacetate, and the temperature of the bath is between 60.degree. and 90.degree.C.