Gold etchants

A second, more sophisticated type of devices was fabricated in the following way. First a temporary gold layer, which closed the nonconducting gap between the two chromium electrodes, was evaporated. Subsequently a voided double-gyroid-structured template was prepared on this continuously metal-coated substrate and refilled with PPy. Then the redundant template was dissolved with xylene. Finally the superficial gold layer was etched with a wet gold etchant, which reestablished the gap between the chromium electrodes. 

Masking is required for many micromechanical applications. While Si3N4 is only suitable for a small etching depth because of its significant etch rate in HF, noble metals like gold are sufficient mask materials. In contrast to alkaline etchants, organic materials like certain resists or even some adhesive tapes are well suited to protect the silicon surface in isotropic etchants. 

A prerequisite for all etch-stop techniques discussed so far is an electrical connection to an external power supply. However, if the potential required for passivation in alkaline solutions is below 1 V, it can be generated by an internal galvanic cell, for example by a gold-silicon element [As4, Xil]. An internal galvanic cell can also be realized by a p-n junction illuminated in the etchant, as discussed in the next section. Internal cells eliminate the need for external contacts and make this technique suitable for simple batch fabrication. 

Wet chemical etching is also a commonly used process to pattern the metallic film structure. In these cases, a suitable solvent for the metallic film is used to dissolve or etch away the exposed area of the metallic film. This wet etching technique has been used for various metallic films, such as gold, copper, aluminum, and others. However, there are cases where chemical etching of the metal can be difficult. For instance, the etching of the platinum film is not easy because of the required etchant for the platinum film. One of the patterning processes to overcome the potential... 

After photolithographic pattern definition (which is covered in more detail in the next section), gold can be etched in a 30% solution of KI in DI water, then rinsed. The chromium adhesion layer can be etched using any of several etching cocktails [62] [63] or pre-mixed commercial reagents (e.g. Transene Chromium Mask Etchant), then rinsed and stripped. 

Alkanethiolate coating of the gold particles protects them against action of chemical etchants. Protected colloids dissolve much more slowly in aqua regia than their unprotected counterparts. The level of protection observed from monolayers on the flat gold surface, however, is never achieved, probably because of the high curvature of the surface of gold... 

Nahidi M, Kolasinski KW (2006) Effects of stain etchant composition on the photoluminescence and morphology of porous silicon. J Electrochem Soc 153 C19 Najar A, Charrier J, Pirasteh P, Sougrat R (2012) Ultra-low reflection porous silicon nanowires for solar cell applications. Opt Expr 20 16861-16870 Peng K, Zhu J (2003) Simultaneous gold deposition and formation of silicon nanowire arrays. J Electroanal Chem 558 35-39... 

Gold. With an underplate of nickel or tin/nickel, gold provides excellent resistance to ah the common copper etchants. Some etchants have a slight dissolving effect on gold. 

Ferric chloride can be used with screen ink, photoresist, and gold patterns, but it cannot be used with tin or tin/lead resists. However, ferric chloride is an attractive spray etchant because of its ease of use, holding capacity for copper, and ability to be used on an infrequent batch application basis. 

These etchants for solder- and tin-plated boards were preferred for many years. More recently, their use has been completely eliminated due to Cr(VI) listing as a critical environmental hazard. Other problems with chromic-sulfuric etchants are difficulty in regeneration, inconsistent etch rate, the low limit of dissolved copper (4 to 6 oz/gal), and dangerous degradation of PVC and polypropylene equipment. Chromic add etchant is suitable for use with solder, tin/nickel, gold, screened vinyl lacquer, and dry or liquid film photoresists. Chromic-sulfuric mixtures etch copper slowly, and additives are needed to increase the etch rate. For example, sodium sulfate and iodine have been used for rate increase. Alkaline etchants have become so well controlled that there is no justification for the risks and costs of chromic acid formulations. 

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