The etching of gold is a key enabling technology in the fabrication of many microdevices and is widely used in the electronic, optoelectronic and microelectromechanical systems (MEMS) industries. In this review, we examine some of the available methods for patterning gold thin films using dry and wet etching techniques. Dry methods which utilise reactive ion etching (RIE) have a number of important advantages over other methods, but the low volatility of gold etch products has made the development of suitable processes problematic. More recently, the adoption of high-density plasma reactors with optimised chlorine-based chemistries has allowed improved processes to be developed, and etching in hydrogen plasmas also shows promise. Wet etching methods for gold have also been critically reviewed. Traditionally, iodine-and cyanide-based etch processes have been used, but in the last decade, a number of alternative etchants have been studied. Of particular interest is the recent development of a range of novel nonaqueous-based gold etchants, and the suitability of these etchants for microfabrication is assessed.

An analysis of the speciation of gold plating solutions containing sulfite and thiosulfate as complexing agents has been performed. In the case of gold sulfite baths, it was found that the observed stability and electrochemical behavior were inconsistent with the commonly assumed stability constants of ␤ = 10 10 for the Au͑SO 3 ͒ 2 3− species. The data are, however, consistent with a much larger stability constant of ␤ = 10 27 . In gold plating baths containing sulfite and thiosulfate ligands, the speciation model predicts that the dominant species at neutral or basic are mixed complexes of the form Au͑S 2 O 3 ͒͑SO 3 ͒ 3− and Au͑S 2 O 3 ͒͑SO 3 ͒ 2 5− . The thiosulfate complex Au͑S 2 O 3 ͒ 2 3− is only dominant under acidic conditions. It is shown that the electrodeposition characteristics reported in many studies can be interpreted in terms of the existence of such mixed ligand complexes. Finally, the effect of the stability of various gold plating solutions on colloidal gold formation has been explored. For free-standing solutions, decomposition is more likely to occur via the disproportionation of Au͑I͒ species such as AuOH, Au͑OH͒ 2 − , and AuCl 2 − rather than free aurous ions. The stability of gold baths during electrodeposition is mainly influenced by the formation of dithionite ions which reduce Au͑I͒ complexes to form colloidal gold.

The stability of citrate electrolytes for the electrodeposition of copper-nickel alloys and multilayers has been investigated. It was found that electrolytes operating at pH 4 are unstable due to the formation of an insoluble citrate complex. This complex had previously been thought to be copper citrate dihydrate, Cu2C6H4O7'2H,O, but elemental analysis of the precipitate revealed it to be a heteronuclear copper-nickel citrate complex. Calculations of the distribution of citrate species in a typical plating solution was undertaken to identify experimental conditions under which the insoluble citrate complex is not formed. These studies indicate that increasing the pH of the citrate solution from 4 to 6 should produce a stable electrolyte. Experimental studies show that the electrolytes at pH 6 are stable for periods of several weeks, in agreement with the predictions of the speciation calculations. The stable citrate electrolytes were used to deposit Cu-Ni alloys. It was found that alloys could be deposited from these electrolytes with almost 100% current efficiency and with a morphology and composition comparable to those.obtained from the unstable citrate electrolytes.

The feasibility of using a thiosulfate-sulfite bath to electrodeposit soft gold for microelectronic applications has been investigated. This bath is stable at near-neutral pH, shows good compatibility with positive photoresists, and does not contain any additives. It was found that the bath produced gold bumps with straight sidewalls, flat top surfaces, and a good reproduction of the resist pattern was achieved. The thickness uniformity, roughness, stress, hardness, adhesion, and shape of the plated structures were found to be compatible with the requirements for a wide range of microelectronic applications, including wafer bumping.Electrodeposition of soft gold has numerous applications in microelectronics and microsystem technology. 1-8 In the electronic packaging industry, for example, interconnects between integrated circuits ICs and external devices are performed using tape automated bonding ͑TAB͒, chip-on-glass ͑COG͒, and chip-on-flex ͑COF͒ techniques. 1-3 A key process in all of these technologies is gold wafer bumping. In addition to connecting driver ICs to flat panel displays, gold bumping is used in many other packaging applications -particularly where a high density of input/output ͑I/O͒ connections is required. 4 Electrodeposition of gold is also used in the fabrication of transmission lines and air bridges on gallium arsenide ICs. 5 The electrodeposition of soft gold has also been used to fabricate X-ray masks for the deep X-ray lithography electroforming, and plastic molding ͑LIGA͒ process. [6][7][8] All of the above technologies utilize the through-mask plating technique. 9 Typically this involves depositing a conductive seed layer on a wafer substrate, followed by patterning with a photoresist material. The wafer is then gold plated, followed by resist removal and seed layer etching. Although the requirement for each application varies slightly, usually, gold deposits are required to have high purity, low porosity, low stress, and good adhesion to the substrate. 1-8 It is also important that the fabricated features exhibit straight sidewalls, a relatively planar top surface and are uniform in height across the wafer. 1-3 For reliable thermocompression bonding ͑e.g., in TAB͒ low deposit hardness is also required. 2,3 Similarly, in transmission-line applications, 5 a high electrical conductivity and low surface roughness is desirable.For most applications requiring soft gold deposition through a photoresist mask, sulfite-based plating baths are employed. 1,2,5,7,8,10,11 Cyanide-based electrolytes can also be used to deposit soft gold, 3 but due to their toxicity and poor compatibility with many photoresists, they are rarely used. For example, the use of gold cyanide baths often leads to the delamination of the resist from the seed layer. 3,5 This leads to gold deposition underneath the resist, which is highly undesirable in most applications. In contrast, sulfitebased baths exhibit better resist compatibility, and underplating is not usually a problem. 5,11 However, as gold sulfite baths are typica...

This study has examined the effect of water on the electrodeposition of copper from a deep eutectic solvent (DES). Initial physiochemical measurements showed that the viscosity and resistivity of the DES decreased with added water in the range 1 -15 wt%. This reduction in viscosity resulted in an increase in the mass transfer limiting current, without narrowing the electrochemical window or altering the speciation of the copper chloro-complexes. This shows that metal deposition rates can be increased by water additions. The effect of water on the electrochemical kinetics of the Cu(I) and Cu(II) chloro-complexes was also studied. It was found that the kinetics of the Cu(I)/Cu(0) reaction is largely irreversible, while the Cu(II)/Cu(I) couple was quasi-reversible. The rate constants for Cu(II)/Cu(I) and Cu(I)/Cu(0) reactions were accelerated by water additions, although the transfer coefficients remained unchanged. The effect of increased deposition rates, electrolyte conductivity and reaction kinetics on deposit uniformity was estimated and subsequently verified by experiments. It was found that, although higher deposition rates could be achieved, the thickness of the deposit was non-uniform since the Wagner numbers remained relatively low.

The luminescence emitted by amorphous H2O ice at 78 K when irradiated with a continuous source of 260 nm light has been examined. The emission spectrum was similar to that from crystalline ice, exhibiting two features around 340 and 420 nm that had short and long lifetimes, respectively. The rise and fall times of the 420 nm band were the same within experimental error, and the decay was equally well fit by either a biexponential decay or a second-order decay with initial concentrations in a ratio of 3.2:1. However, the identical nature of the luminescence rise and decay profiles strongly suggests biexponential kinetics. The 420 nm emission was tentatively attributed to the 4Σ- → X2Π transition of excited OH radicals. A reaction mechanism that explains the preceding observations is presented, and the biexponentiality of the 420 nm decay is attributed to excited OH radicals located in two distinct environments within the ice lattice.

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An analysis of the anodic reaction occurring at soluble copper anodes during the electrodeposition of copper from an ethaline-based deep eutectic solvent (DES) has been performed. It was shown by UV-Vis spectroscopy and electrochemical measurements that the dominant anodic species produced is the CuCl 2 -complex. In pure ethaline the current efficiency of the anodic process is 100% and the dissolution valency is one. However, in the presence of Cu(II) species the apparent dissolution valency measured gravimetrically was typically less than unity, corresponding to an observed mass loss greater than that expected from Faraday's law.Moreover, the apparent dissolution valency showed a marked dependence on the electrode