Gold, passivity

Wafer Bonding with intermediate Layers Wafers can be bonded using intermediate layers such as metals to form hermetic seals. Polymers, in general, do not provide water-tight sealing and are hence less important in the field of MEMS packaging. Soft metals such as indium can be bonded with silicon with a gold passivation [4]. Aluminum has been used to bond with silicon dioxide [5] and silicon nitride [6] to form hermetic seals. 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

In the presence of oxygen and water the oxides of most metals are more thermodynamically stable than the elemental form of the metal. Therefore, with the exception of gold, the only metal which is thermodynamically stable in the presence of oxygen, there is always a thermodynamic driving force for corrosion of metals. Most metals, however, exhibit some tendency to passivate, ie, to form a protective oxide film on the surface which retards further corrosion. 

Palladium and gold Palladium electrodeposition is of special interest for catalysis and for nanotechnology. It has been reported [49] that it can be deposited from basic chloroaluminate liquids, while in the acidic regime the low solubility of PdCl2 and passivation phenomena complicate the deposition. In our experience, however, thick Pd layers are difficult to obtain from basic chloroaluminates. With different melt compositions and special electrochemical techniques at temperatures up to 100 °C we succeeded in depositing mirror-bright and thick nanocrystalline palladium coatings [10]. 

Passivity is the loss of chemical reactivity of certain metals and alloys under specific environmental conditions. In other words, certain metals (e.g., iron, nickel, chromium, titanium, etc.) become relatively inert and act as noble metals (e.g., gold and platinum). Figure 4-420 shows the behavior of a metal immersed in an air-free acid solution with an oxidizing power corresponding to point A... 

Fig. 6.2 Theoretical conditions of corrosion, immunity and passivation of gold, at 25°C...

In view of its susceptibility to sulphide tarnishing, silver may itself require some measure of protection in many decorative and industrial applications. Chromate passivation processes are commonly employed, but as an alternative, thin coatings of gold, rhodium or palladium may be used. 

But metallic coatings have high absorption losses. But one can remedy them with dielectric coatings, which are necessary also to passivate the metal, avoiding oxidation. Aluminum is the best metal for the ultraviolet in addition it adheres on most substrates it needs passivation. Silver is easy to deposit it has the highest reflectance in the visible and the infrared since it tarnish rapidly, passivation is mandatory. Gold is the best material beyond 700 nm, and it is considered that it does not tarnish, which is not true actually because its surface is not that stable on the long term. 

Hoffman and Kordesch74,75 have presented a series of studies on the passive films on iron with particular attention to cell design. They have employed a so-called bag cell that allows for the in situ passivation and/or cathodic protection of the iron films. These were deposited onto gold films deposited on Melinex. 

Geoghegan, W.P., Ambegaonkar, N., and Calvanico, N. (1980) Passive gold agglutination An alternative to passive hemagglutination./. Immunol. Meth. 34, 11. 

---