Cyanide- and thiourea-free electrochemical etching of gold for microelectronics applications

Abstract

Electrochemical etching of gold using complexing agents other than cyanide and thiourea has been investigated. Through complexation with gold ions, the complexing agents such as chloride, bromide, thiocyanate, and iodide are all able to facilitate electrochemical oxidation of gold. It was found that, while chloride makes electrochemical oxidation of gold feasible, particularly at highly acidic conditions, the process appeared inadequate for applications in microelectronics, as it left gold residues on areas where gold seed layers were to be removed, most probably due to the limited complexation ability of chloride with gold ions. In contrast, electrochemical etching of gold using iodide resulted in complete removal of gold seed layers, as iodide strongly facilitates electrochemical oxidation of gold. To prevent electrochemical and chemical oxidation of iodide, we envisioned the use of a sacrificial stabilizer that is oxidizable at anodic potentials, at which gold is oxidized in the presence of iodide, and is able to convert the generated iodine, if there is any, back to iodide. With such an approach, we demonstrated that electrochemical etching of gold without using cyanide and thiourea is practically viable. Comparative evaluation using such parameters as surface roughness, undercut, line resistance, and leakage current revealed that the iodide-based process produced results comparable to or better than those obtainable with a thioureabased process, demonstrating the suitability of the novel process for gold seed layer removal in microelectronics applications. The electrochemical etching of gold using bromide was also demonstrated.