ELECTRODEPOSITION OF COPPER
United States Patent 3674660
A copper pyrophosphate electroplating bath contains a known heterocyclic additive in conjunction with an auxiliary brightener selected from iminodiacetic acid, cinammic acid, aliphatic acid, di- and poly- carboxylic acids having at least seven carbon atoms, salts of the aforesaid acids and hydroxyethylcellulose. The auxiliary brightener prevents step plating which is normally caused by the presence of the heterocyclic brightener.
Albright & Wilson Limited (Oldbury near Birmingham, EN)
C25D3/38; (IPC1-7): C23B5/18; C23B5/46
Field of Search:
Kaplan G. L.
Parent Case Data:
This application is a
continuation-in-part application of copending application
Ser. No. 724,225, filed Apr. 25, 1968, now abandoned.
The present invention relates to improvements
in the electrodeposition of copper.
known to carry out the electrodeposition of copper from
alkaline solutions containing an alkali metal copper
pyrophosphate complex of the formula X.sub.6 Cu(P.sub.2
O.sub.7).sub.2, where X represents an alkali metal.
A characteristic feature of these plating
solutions is that in conjunction with suitable brightening
additives they may be used to form highly specular mirror
bright finishes which do not require subsequent mechanical
polishing. In this they differ from the conventional acid
copper plating systems and from copper cyanide systems,
which produce matt finishes. Brighteners used in the former
solutions are intended to improve the specularity of the
bright finish whereas those used in the latter systems are
designed to make the matt finish more amenable to mechanical
Various heterocyclic compounds have
been proposed as brightener additives for alkaline
pyrophosphate electroplating compositions and some of these
have found particular favor in the art. A group of these
heterocyclic compounds is characterized by the presence of
contained in a five- or
six-membered heterocyclic ring system where Q represents a
nitrogen atom (either as a .dbd.N--or --N--grouping) or a
sulphur atom. Examples of these mercapto-heterocyclic
compounds are described in U.K. Specifications Nos. 939,997;
940,282 and 1,051,150.
Some of the
aforementioned specifications refer to the optional presence
in the electroplating composition of other organic
compounds, such as simple aliphatic or hydroxy-aliphatic
carboxylic acids including oxalic, citric, tartaric, acetic,
propionic and phthalic acids and wetting agents. Certain of
these compounds are stated, for example in specification No.
1,051,150, to improve the grain refinement of the
electrodeposited material and to reduce the anodic
polarization. These additives were originally described in
the early work on copper pyrophosphate baths, before the
advent of heterocyclic brighteners. In practice, however, we
have discovered that although these known carboxylic acid
additives and wetting agents caused some slight improvements
in the unbrightened copper pyrophosphate baths for which
they were originally proposed, they do not produce any
noticeable effect in the presence of heterocyclic
brighteners. This may be because the heterocyclic
brighteners introduce such a striking improvement in the
specular brightness and appearance of the work as completely
to overide any effects of these previously known additives.
Certainly these additives, such as citric acid, have not in
commercial practice been used in conjunction with the more
successful heterocyclic brighteners.
heterocyclic brighteners have one disadvantage--they tend to
cause a plating defect called step plating. This arises when
in consequence of reduction of current density across the
surface of the workpiece, such as commonly occurs in
practice with contoured surfaces, the thickness of the
coating varies. In normal plating there may be a continuous
variation of the thickness of the coating across the surface
without affecting the appearance or quality of the work. In
step plating there is a sharp division between an area of
relatively thick plating of high specular brightness and an
area of relatively thin plating of inferior quality. This
tendency can be avoided by keeping the concentration of
heterocyclic brightener small, but that imposes limitations
on the amount by which the specular brightness is improved.
I have discovered that certain compounds may
be used as auxiliary brighteners in conjunction with
heterocyclic brighteners to provide higher levels of
specular brightness with negligible tendency to step
plating. An object of my invention is to inhibit the
tendency of heterocyclic brighteners to cause step plating
in copper pyrophosphate electroplating baths. A further
object of my invention is to provide copper pyrophosphate
electroplating baths which produce work of improved specular
brightness. A further object of my invention is to provide
copper pyrophosphate electroplating baths of improved
1. In a copper pyrophosphate electroplating electrolyte consisting essentially of an alkaline aqueous solution containing dissolved therein a copper salt and an alkali metal pyrophosphate in an amount at least sufficient to form the complex salt X6 Cu(P2 O7)2, and a known heterocyclic brightening additive in an amount sufficient to provide bright electrodeposits selected from the group consisting of mercaptothiazoles, mercaptobenzthiazoles, mercaptothiadiazoles, mercaptopyrimidines, and mercaptoiminazoles, the improvement wherein said electrolyte also contains an auxiliary brightener selected from the group of step plating inhibitors consisting of imino diacetic acid, malonic acid, cinnamic acid, aurine tricarboxylic acid, aliphatic dicarboxylic acids having at least seven carbon atoms, salts of the aforesaid acids and hydroxyethyl cellulose in a concentration effective to inhibit step plating.
2. The electrolyte of claim 1 wherein said heterocyclic brightener is selected from the group consisting of 2-mercaptothiazole, 2-mercaptonbenzthiazole, 2-mercaptothiadiazole, 2-mercaptopyrimidines, and 2-mercapto iminazole, in a concentration of from 1 to 10 ppm, and wherein said auxiliary brightener is in a concentration of from 2 to 100 ppm.
3. The electrolyte of claim 2 wherein said auxiliary brightener is an adduct of maleic acid with a polyolephine having from 30 to 150 carbon atoms.
4. The electrolyte of claim 3 wherein said polyolephine is polyisobutylene.
5. The electrolyte of claim 2 wherein said auxiliary brightener is sebacic acid.
6. The electrolyte of claim 2 wherein said auxiliary brightener is azelaic acid.
7. The electrolyte of claim 2 wherein said auxiliary brightener is suberic acid.
8. The electrolyte of claim 2 wherein said auxiliary brightener is cinnamic acid.
9. The electrolyte of claim 2 wherein said auxiliary brightener is aurine tricarboxylic acid.
10. The electrolyte of claim 2 wherein said auxiliary brightener is hydroxyethyl cellulose.
11. The electrolyte of claim 2 wherein said auxiliary brightener is iminodiacetic acid.
12. The electrolyte of claim 9 wherein said auxiliary brightener is malonic acid.
13. In a process for electrodepositing bright copper from an aqueous copper phyrophosphate electrolyte wherein copper is electrodeposited from electrolyte consisting essentially of an alkaline aqueous solution containing dissolved therein a copper salt and an alkali metal pyrophosphate in an amount at least sufficient to form the complex salt X6 Cu(P2 O7)2, and of a known heterocyclic brightening additive in an amount sufficient to provide bright electrodeposits selected from the group consisting of mercaptothiazoles, mercaptobenzthiazoles, mercaptothiadiazoles, mercaptopyrimidines, and mercaptoiminazoles, the improvement comprising admixing in said electrolyte at least one part per million of an auxiliary brightener selected from the group of step plating inhibitors consisting of imino diacetic acid, malonic acid, cinnimac acid, aurine tricarboxylic acid, aliphatic dicarboxylic acids having at least seven carbon atoms, salts of the aforesaid acids and hydroxyethyl cellulose.
The invention provides, in brightening compositions for addition to copper pyrophosphate electroplating baths and containing a heterocyclic brightener selected from the known group consisting of 2-mercaptothiazole, 2-mercaptobenzthiazole, 2-mercaptothiadiazole, 2-mercaptopyrimidine, 2-mercaptoiminazole, substituted homologs of the aforesaid heterocyclic brighteners and derivatives that form any of the aforesaid compounds when dissolved in copper pyrophosphate electroplating baths, the improvement which consists in that the brightening composition contains, in addition to the heterocyclic brightener, an auxiliary brightener selected from the group of step plating inhibitors consisting of aliphatic dicarboxylic and polycarboxylic acids having at least seven carbon atoms, malonic acid, cinammic acid, iminodiacetic acid,salts of the said acids and hydroxyethyl-cellulose.
The invention further provides an aqueous electrolyte for copper plating having dissolved therein a copper salt, an alkali metal pyrophosphate in an amount at least sufficient to form the complex X6 Cu(P2 O7)2, where X represents an alkali metal, and a minor proportion of a brightening composition according to the invention. The invention also provides a process for the electrodeposition of copper using an aqueous electrolyte of the invention.
The heterocyclic brighteners for present use are exemplified by the mercapto-thiazole compounds described in specification No. 940,282, for example 2-mercapto-1, 3-thiazole and 2-mercapto-benthiazole; by the 2-mercapto-1,3,4-thiadiazole compounds described in specification No. 939,997, for example 2,5-dimercapto-1,3,4-thiadiazole, 2-mercapto-5-methylmercapto-1,3,4-thiadiazole and 2-mercapto-5-n-butylmercapto-1,3,4-thiadiazole; and by the 2-mercapto-iminazole and 2-mercapto pyrimidines described in specification No. 1,051,150, for example 2-mercapto-1-methyliminazole, 2-mercaptopyrimidine, 6-hydroxy-2 mercaptopyrimidine and 6-hydroxy-2-mercapto-4-methylpyrimidine. Also included among the classes of brighteners are precursors of the compounds described above. By "precursor" is meant herein a compound which when dissolved in the copper pyrophosphate electrolyte provides in solution a compound containing the structure (II). For example there may be used mercapto salts, or heterocyclic disulphides containing the grouping:
The auxiliary brighteners for present use are for the most part acids which will normally be used as sodium, potassium, or ammonium salts.
There may be used, usually in the form of water-soluble salts, organic di- and poly-carboxylic acids or anhydrides thereof having more than seven carbon atoms. As examples of these compounds there can be mentioned suberic acid, azelaic acid, and sebacic acid as well as adducts of the type formed when a dienophilic carboxylic acid such as maleic anhydride is condensed with a conjugally unsaturated hydrocarbon such as polyiso-butylene, for example the alkyl- or alkenyl-substituted succinic acids and anhydrides wherein the alkyl or alkenyl group contains from 30-150 carbon atoms.
Normally the heterocyclic brightening agent is employed in a proportion of at least 0.001 grams per liter of the total weight of electrolyte and preferably from 1 to 10 mg. per liter. The optimum concentration may vary from 2 to 4 mg. per liter depending upon the nature of the auxiliary brightener present. Normally the auxiliary brightener is employed in a proportion of from 1 ppm to saturation, preferably from 2 to 100 ppm, for example 3 to 6 ppm.
It is preferred that the copper plating electrolytes of the invention have a similar constitution with respect to the concentration of copper salt and pyrophosphate as those conventionally employed, for example as described in the above-mentioned specifications. These electrolytes may contain other additives that are conventionally employed in this type of electrolyte in addition to the heterocyclic brighteners. The electrolyte may be employed for plating metal articles in accordance with known procedures.
The invention is particularly surprising because the compounds which have been found effective are similar chemically to a number of compounds which had already been tested and found totally ineffective. These ineffective compounds include oxalic acid, tartaric acid, citric acid, formic acid, acetic acid, propionic acid, benzoic acid and phthalic acid.
The invention is illustrated by the following examples wherein there were employed electrolytes of the following constitution:
Copper pyrophosphate 94 g/l, potassium pyrophosphate 300 g/l, ammonia 1 g/l. Standard solutions were made by using 3 heterocyclic brighteners, namely 4 ppm. 2,5-dimercapto-1,3,4-thiadiazole, 6 ppm. 2-mercaptobenzimidazole and 10 ppm.2-mercaptobenzthiazole. The solutions were obtained at a pH of 8.8 at a temperature of 55° C. Plating was carried out on brass panels in a standard Hull cell with a mean current density of 30 amp per sq. ft. under air agitation.
Tests were repeated with the three standard electrolytes containing the following auxiliary brighteners at a concentration of 10 ppm;
1. Cinnamic acid
2. Aurine tricarboxylic acid
3. Hydroxyethyl cellulose
4. Iminodiacetic acid
5. Malonic acid
6. Sebacic acid
7. Azelaic acid
8. Suberic acid
9. Maleic anhydride/polyisobutylene adduct (Na salt)
10. Oxalic acid
11. Citric acid
12. Tartaric acid
13. Formic acid
14. Acetic acid
15. Propionic acid
16. Benzoic acid
17. Phthalic acid
Examples 1 to 9 provided excellent specular brightness with improved leveling and negligible step plating in each of the electrolytes. Comparative Examples 10 to 17 on the other hand, although providing high specular brightness in regions of high current density, gave rise to serious step plating.
It was found that Examples 1 to 9, when tested in an electrolyte containing no heterocyclic brightener, either gave no detectable improvement, or else were actually disadvantageous.