Stable, electroless, aqueous, acidic gold bath for depositing go

Details Stable, electroless, aqueous, acidic gold bath for depositing go Stable, electroless, aqueous, acidic gold bath for depositing go

1993-04-02

1994-06-21

Klemanski, Helene

Compositions: coating or plastic

Coating or plastic compositions

Metal-depositing composition or substrate-sensitizing...

106126, C23C 1831

Patent

active

053225520

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a stable, electroless, aqueous acidic gold bath for depositing gold and the use thereof. The bath, in accordance with the invention, containing the tetracyanogold(III) anion, a complexing agent or a mixture of several complexing agents and an acid or a mixture of acids, is suitable for the electroless deposition of gold on metals which are less noble than gold as well as on the alloys of these metals.
Gold baths for the electroless deposition of gold are already known, for example from documents such as DE patent specification 3640028, U.S. Pat. No. 4,830,668 and GB patent specification 20 99 460. These deal with gold baths which, for the most part, contain an alkali metal dicyanoaurate(I) or an alkali metal tetracyanoaurate(III), a complexing agent and a reducing agent. All of these baths have unsatisfactory stability, as a rule, and decompose with the deposition of metallic gold or gold(I) cyanide.
The present invention has the task of depositing gold from the tetracyanogold(III) on metals or their alloys in the acidic range and, at the same time, providing a more stable gold bath.
It has been found that the stability of the gold bath is increased when complexing agents or mixtures of several complexing agents are used which contain various groups per molecule which have a complexing and reducing effect. In this regard, we are dealing with such compounds that contain, per molecule, one or more carboxyl groups and one or more phosphonic acids groups. In particular, these are compounds of the general formula I and II as follows: ##STR1## whereby: A=H, --CH.sub.3, --(CH.sub.2).sub.n ------PO(OH).sub.2, ##STR2## whereby: B=H, --(CH.sub.2).sub.n ------COOH, and
N,N-bis(carboxymethylene)-1-aminoethane-1,1-diphosphonic acid and 2-phosphonobutane-1,2,4-tricarboxylic acid have proven themselves valuable as complexing agents in the baths in accordance with the invention.
A special advantage of the bath in accordance with the invention is that layers up to 0.5 .mu.m are obtained even with a gold content in the bath of 1.0 g/l of gold. As a result, the bath makes the gold plating of alloys possible, as is common in the semi-conductor industry, for example iron-nickel-cobalt alloys and nickel alloys that are coated reductively by chemical means, such as nickel-phosphorus and nickel-boron and super-pure nickel.
Surprisingly, it has also been found that the direct coating of tungsten is also possible, which leads to completely new layer structures in the chip carrier industry.
In accordance with the invention, the carboxymethyleneaminoalkylphosphonic acids and/or the phosphonoalkylcarboxylic acids are used as complexing agents which make a considerable increase in deposition speed possible as well as greater layer thicknesses, this being something which was not foreseen.
The acids used are, for example, sulfuric acid or phosphoric acid or their mixtures.
The basic composition of the bath in accordance with the invention is, for example, as follows:


______________________________________ gold as metal 0.05-30 grams/liter complexing agent 1-100 grams/liter acids 10-100 ml/liter ______________________________________
The working temperature of the bath is, as a rule, 70.degree. to 90.degree. C. Even in the case of higher temperatures, it has been found that decomposition of the bath, e.g. sedimentation of elemental gold, does not take place. An additional advantage of the bath is that it can be used repeatedly and that the gold salt can be metered out subsequently as desired.
For layer thicknesses of up to 0.5 .mu.m, the bath in accordance with the invention has a constant deposition speed, this being a function of its gold content and the temperature.
The bath in accordance with the invention can be used for the gold coating of soldered joints which are formed from crystal or wires bonds, this being something which is of particular value industrially.
The high deposition speed also makes utilization possible in the case of decorative gold plating. A uniform yellow layer can be observed

REFERENCES:
patent: 4374876 (1983-02-01), El-Shazly et al.
patent: 4474838 (1984-12-01), Halecky et al.
patent: 4830668 (1989-05-01), Wundt et al.
patent: 4838937 (1989-06-01), Oh
patent: 4919720 (1990-04-01), Stavitsky

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