Thiosulfate- and Thiosulfonate-Based Etchants for the Patterning of Gold Using Microcontact Printing

The transcription of microcontact-printed self-assembled monolayer patterns into gold substrates requires a wet chemical etch process that selectively dissolves unprotected metal without attacking SAM-covered gold regions. Gold dissolution in a standard thiosulfate−ferricyanide etch solution, the most commonly used etchant containing sulfur-based ligands, shows a complex kinetic behavior. Etch-rate-determining under most conditions are the concentrations of thiosulfate, ferricyanide, and hydroxide. The etch process is accelerated under high ionic strength conditions, in particular in the presence of potassium ions. This is ascribed to the formation of potassium−reagent ion pairs, which reduce the Coulomb repulsion between the negatively charged reaction partners. The thiosulfate−ferricyanide etchant is intrinsically unstable due to the reduction of the ferricyanide oxidant during thiosulfate decomposition. The gold ligand thiosulfate was therefore substituted with benzenethiosulfonate, which was expected to be less oxidation sensitive. The new bath decomposed in the course of days instead of several hours. Since the etch efficiency was also reduced, a significantly higher concentration of the gold ligand was required to achieve a sufficiently high etch rate. The somewhat lower etch quality of the new bath could be compensated for by introducing a new defect healing additive, 1-decanesulfonamide, which was found to be more effective than 1-octanol in the new etchant.