I tried a Google translation from German:
Ättzzen of Golld ...
... Mechanisms and etching mixtures
Last updated on 15 03. 2006
n Properties of Gold
The 19.3 g/cm3 with very dense precious metal crystallized face-centered cubic.
With a standard potential of 1.5, it is one of the precious metals.
By its electron configuration [Xe] 4f14 6s1 5D10 gold can very
are oxidized difficult: the unpaired valence electron is by
the completely filled 5d orbital of potential reactants
largely shielded and therefore very difficult to split.
Wet chemical etching of gold, therefore, requires two conditions:
A very strong oxidizing reactants, and
a complex images, which holds the etched gold in solution and - energetically favorable - Replacement
prevented.
n etching of gold with HCl/HNO3
Aqua regia
Mixtures of nitric acid and hydrochloric acid (in a ratio of 1:3 also called aqua regia) etch
Gold even at room temperature. The strong oxidative effect of this mixture depends on the formation
about the reaction of nitrosyl HNO3 +3 HCl à NOCl + 2Cl + 2H2O, while also in the process
formed free chlorine atoms III dissolved precious metal as chloride complex (tetrachlorogold-() -
acid HAuCl4 =) hold in the solution.
King water consumed itself and breaks down to form nitrous gases and chlorine gas.
Etch rate and selectivity
The etch rate of aqua regia for gold is around 10 microns / min (room temperature) and can by heating
the mixture can be increased to some 10 microns / min.
Palladium, aluminum, copper and molybdenum are also at room temperature in aqua regia
etched. For the etching of platinum and rhodium, the aqua regia must to achieve a significant
Etching rate can be heated, Iridium is etched (boiling) only strongly heated aqua regia.
Silver is not etched by aqua regia, since in this case, a protective layer of silver chloride forms.
Chromium, titanium, tantalum, zirconium, hafnium and niobium form thin, very dense passivating (mostly oxide)
layers on its surface which - at least at room temperature - the metal from an appreciable
Protect attack in aqua regia. Tungsten is aqua regia for the same reason
etched only slowly.
n etching of gold with KI/I2
Mechanism
Iodine is the equation 2Au + I2 à 2AUI Gold Iodide, its aqueous solubility by the dissolved AI
is significantly increased. The iodide and iodine may be replaced by other halogens and their salts -
except fluorine, which does not form a soluble gold compound.
Etch rate and selectivity
In the ratio KI: I2: H2O = 4g: 1g: 40ml this solution at room temperature achieved an etch rate
for gold of about 1 micron / minute. To copper No selectivity, which is nickel
only in the case of direct (key) connection attacked with gold.
n etching of gold with cyanide
Mechanism
With aqueous solutions of highly toxic sodium cyanide (NaCN) and also very toxic potassium cyanide
(KCN) is gold with the formation of highly soluble cyano complex [Au (CN) 2] - resolved. This reaction requires
either oxygen or oxygen-cleaved, which added to the etching solution of H2O2
is.
Positive resists negative resists reversal resists developers
Thinner Solvent etchants process chemicals
page 2
Selectivities
Cyanide attack besides gold also, for example, silver and copper,
which are water-soluble cyanide,
Complex form.
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n Caveat Emptor
All information contained in this fact sheet numbers and descriptions are given in good faith and
Conscience together. However, we can not guarantee the correctness of this information.
Particularly with respect to the formulas for chemical (etching) process, we can offer no warranty
for the correct indication of the ingredients, mixing ratios, the production of the approaches and their
Application. Secure order of mixing of ingredients in a recipe usually corresponds
not the order in which they are listed in tables etc. Normally acids in the
Thinner given (eg water), added stronger acids in weaker, and oxidizing materials last.
We do not guarantee the accuracy or completeness of references to (including health staff and equipment)
Risks that are described in the application of this information sheet
Chemical yield (can).
Basically, each employee is encouraged in any doubt about the appropriate specialist reading
described chemicals and their impact on advance information sufficient to damage
Exclude persons and material.
What I see above is really not much information except gold can be etched or dissolved into solution with certain well known processes, it also sounds like they are talking about an etch solution to etch electronic parts possibly in making the electronic parts.
When we want to make circuit boards we can get a fiberglass circuit board with a layer of thin clean copper metal, we can draw or trace a resist onto the copper, where we do not want to dissolve the copper (there are also photo resists that can work with a copier and light), we then can etch the circuit board in a solution that dissolves copper metal, but cannot get to the copper under the resist easily to attack it, the solution to etch the circuit board can be the copper (II) chloride solution we use, or ferrous chloride etch, the resist can be something as simple as a black felt tip Sharpie marking pen. This sounds like something of what they may be discussing in the article, (but etching gold instead of copper).
Lino asked me for proofs, so I showed, where I got those asumptions from :
