Acid gold bath for the electroless deposition of gold

Stabilized aqueous, acid gold bath, containing a dicyanogold(I)-complex, a complex former, a reducing agent and customary additives, for electroless deposition of gold onto gold and metals that are more electronegative than gold, as well as alloys of these metals, containing a salt of hydroxylamine or a hydroxylamine derivative as reducing agent and a fluoride or hydrogen fluoride as stabilizer. Preferred embodiments include the use of an alkali- or ammonium dicyanoaurate(I) as dicyanogold(I)-complex; using a salt of hydroxylamine or a hydroxylamine derivative of the general formula ##STR1## in which R.sub.1 and R.sub.2 are the same or different and represent hydrogen or alkyl of 1 to 5 carbon atoms and X represents the residue of an inorganic acid, as reducing agent; using an alkali fluoride or an alkali hydrogen fluoride as stabilizer; and a pH-value less than 3, preferably from 0.5 to 2.8.

BACKGROUND OF THE INVENTION 
The invention concerns a stabilized aqueous, acid gold bath containing a 
dicyanogold(I)-complex, a complex former, a reducing agent and customary 
additives, for the electroless deposition of gold onto gold and metals 
which are more electronegative than gold, as well as alloys of these 
metals. 
Gold baths for the electroless deposition of gold are already known. They 
involve an alkaline or acid gold bath, containing predominantly an 
alkalidicyanoaurate(I), a complex former, a reducing agent, as well as 
additives for increasing the velocity of deposition and for improvement of 
the adhesiveness (U.S. Pat. Nos. 4,091,128, 3,300,328, 4,154,877, 
3,032,436, German Offenlegungsschrifts DE-OS 2,052,787, DE-OS 2,518,559). 
All of these baths have, as a rule, an unsatisfactory stability, 
decomposing during deposition of metallic gold. 
These baths have the further disadvantage that at a pH-value less than 3 a 
decomposition of the dicyanoaurate(I)-complex into difficult to dissolve 
gold(I)-cyanide and hydrocyanic acid occurs. 
The mentioned gold baths are, moreover, suited only for the gilding of 
metals that are more electronegative than gold. An optimal electroless 
deposition of gold onto gold is, on the other hand, not possible by means 
of these baths. 
SUMMARY OF THE INVENTION 
The present invention has the object of providing a stabilized aqueous, 
acid gold bath which makes possible the electroless deposition of gold 
onto gold and metals more electronegative than gold, as well as their 
alloys. 
This object will be accomplished according to the present invention with a 
gold bath of the above described type, thereby characterized in that it 
contains a salt of hydroxylamine or a hydroxylamine derivative as reducing 
agent, and a fluoride or hydrogen fluoride as stabilizer. 
Particular embodiments include: 
that the bath contain an alkali- or ammonium-dicyanoaurate(I) as 
dicyanogold(I)-complex; 
that the bath contain a salt of hydroxylamine or a hydroxylamine derivative 
of the general formula 
##STR2## 
in which R.sub.1 and R.sub.2 are the same or different and represent 
hydrogen or alkyl of 1 to 5 carbon atoms and X represents the residue of 
an inorganic acid, preferably of hydrochloric or muriatic or sulfuric 
acid, as reducing agent; 
that the bath contains extra alkali chloride and/or bromide as well as, if 
necessary, unsaturated carbonic acids; and 
that the bath display a pH-value less than 3, preferably from about 0.5 to 
2.8. 
A particular advantage of the bath according to the present invention is 
that gold can be deposited without current from a stable bath onto gold 
surfaces. In this manner, already present gold coatings, which are too 
thin, can be optionally strengthened with the aid of the bath according to 
the present invention. The bath makes possible also the gilding of alloys, 
that are customary in the semiconductor industry, for example, 
iron-nickel- and iron-nickel-cobalt-alloys. 
The bath according to the present invention has the further advantage that 
the cementation of gold onto metals that are more electronegative than 
gold, such as for example copper and nickel, will be promoted, and indeed 
through stabilization of the dicyanogold(I)-complex to pH-values less than 
3, even at the boiling temperature of the bath. 
As dicyanogold(I)-complex, all alkali-dicyanoaurate(I), for example, the 
sodium- and potassium-complex salts and ammoniumdicyanourate(I), are 
suitable. 
Expediently, the concentration can amount to between about 0.05 and 30 g 
gold/liter. 
According to the present invention, a salt of hydroxylamine or a 
hydroxylamine derivative of the general formula 
##STR3## 
is used as reducing agent, in which R.sub.1 and R.sub.2 are the same or 
different and represent hydrogen or alkyl of 1 to 5 carbon atoms, and X 
represents the residue of an inorganic acid, preferably of hydrochloric or 
muriatic or sulfuric acid, and in which, as alkyl, may be mentioned for 
example methyl, ethyl, propyl, n-butyl and n-pentyl. The stability of this 
salt is so extraordinarily great in the acid medium of the bath according 
to the present invention, that a decomposition into ammonia and dinitrogen 
monoxide hardly occurs. 
As stabilizer, the bath according to the present invention contains a 
fluoride or a hydrogen fluoride, preferably an alkali fluoride or an 
alkali hydrogen fluoride, for example, a sodium or potassium salt of these 
compounds. 
To increase the velocity of deposition, it has been shown to be 
advantageous to add to the bath alkali chloride and/or bromide, such as, 
for example, sodium chloride, potassium chloride or sodium bromide, as 
well as, if necessary, unsaturated carbonic acid. Suitable carbonic acids 
of this type are, for example, propionic acid, aryl acid and crotonic 
acid. Moreover, polyhydroxy carbonic acids, dicarbonic acids and other 
complex formers, such as for example succinic acid, citric acid, 
nitrilotriacetic acid or ethylene diamine tetraacetic acid, can 
expediently be added, since these work to accelerate the metal deposition. 
In order to adjust the pH-value to less than 3, preferably from 0.5 up to 
2.8, dilute sulfuric acid will be used, which is added to the bath in 
required amounts. It is to be understood, however, that the bath according 
to the present invention is also stable at higher pH-values, and displays 
advantageous effectiveness. 
The basic composition of the bath according to the present invention is as 
follows: 
______________________________________ 
gold (as metal) 0.05-30 gram/liter 
reducing agent 0.5-25 gram/liter 
fluoride 1.0-30 gram/liter 
additives 1.0-150 gram/liter. 
______________________________________ 
It is of particular advantage to select a mol ratio of gold to fluoride 
which is greater than 1:1. 
The operational temperature of the bath can be selected from about room 
temperature up to about boiling temperature, preferably from about 
60.degree. to 85.degree. C. 
The use of the bath according to the present invention follows in known 
manner, in that the appropriately pre-treated--depending upon the 
substrate--object is dipped in practical manner into the bath solution. 
It is advantageous herewith either to stir the bath solution or to agitate 
the article, in order to obtain smooth, uniform surfaces. 
The bath according to the present invention can be used, in particular, for 
the chemical gilding of metallic surfaces, such as gold, and metals more 
electronegative than gold, for example, copper, silver gold or nickel, and 
alloys of these metals. After suitable pre-treatment, non-metallic 
materials, such as, for example, made from plastic, glass or ceramic, may 
also be gilded. 
It is of particular technical advantage that the bath according to the 
present invention works with a constant deposition velocity up to about 
3.0 .mu.m/h. 
A further advantage of the bath according to the present invention is that 
the deposition velocity remains the same even after a standing period of 
several months. 
The bath according to the present invention makes it possible to produce 
foils of optional thickness. The porosity of the deposit is so slight with 
coating thicknesses of about 0.2 .mu.m, that the substrate will not be 
attacked by 1:1 diluted nitric acid. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following examples of bath compositions are given by way of example to 
enable one to deposit very uniform, well adhering and ductile coatings, 
under the given operational conditions. 
EXAMPLE 1 
______________________________________ 
Potassium dicyanoaurate-I 
0.02 mol/liter 
Citric acid 0.10 mol/liter 
Potassium hydrogen difluoride 
0.12 mol/liter 
Potassium chloride 2.00 mol/liter 
Hydroxylammoniumchloride 
0.06 mol/liter 
pH-value: 2.8 
Temperature: 70.degree. C. 
Deposition velocity: 0.8 .mu.m/h. 
______________________________________ 
EXAMPLE 2 
______________________________________ 
Ammonium dicyanoaurate-I 
0.015 mol/liter 
Succinic acid 0.250 mol/liter 
Potassium fluoride 0.120 mol/liter 
Acrylic acid 0.125 mol/liter 
Di-sodium salt of ethylene 
dinitrilotetraacetic acid 
0.010 mol/liter 
Ammonium chloride 1.200 mol/liter 
Hydroxylammoniumsulfate 
0.025 mol/liter 
pH-value: 2.3 
Temperature: 85.degree. C. 
Deposition velocity: 1.2 .mu.m/h. 
______________________________________ 
EXAMPLE 3 
______________________________________ 
Potassium dicyanoaurate-I 
0.03 mol/liter 
Citric acid 0.23 mol/liter 
Potassium fluoride 0.15 mol/liter 
Potassium chloride 1.50 mol/liter 
Hydroxyldimethylammonium chloride 
0.05 mol/liter 
pH-value: 2.8 
Temperature: 85.degree. C. 
Deposition velocity: 0.5 .mu.m/h. 
______________________________________ 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of metal 
baths differing from the types described above. 
While the invention has been illustrated and described as embodied in an 
acid gold bath for the electroless deposition of gold, it is not intended 
to be limited to the details shown, since various modifications and 
structural changes may be made without departing in any way from the 
spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.