Challenges in Copper Chemical Machining by Transition Metal Salts: Passivation and Dissolved Metal Stability

The chemical machining of copper is a ubiquitous process in electronics manufacturing used to create conductor patterns for printed circuits among a variety of electronic components. Popular etchants for this process are high-ionic-strength solutions of acidic ferric chloride, acidic cupric chloride, and alkaline cupric ammine chloride, which face challenges arising from passivity and stability of the dissolved metals, particularly in alkaline solutions. While these concepts are common to electrodissolution, they are not well-reviewed for systems which do not apply external voltage, as is the case for chemical machining, where complimentary redox reactions occur spontaneously and simultaneously on the same surface of the workpiece. This article serves to review the most influential challenges posed against copper chemical machining reactions through passivation and factors leading to precipitation of metal species in each of the three common etchants of transition metal salts. Academic texts are referenced in conjunction with primary evidence to introduce novel insight regarding the implications of ion transference in the electrolyte solutions and ligation effects in preventing hydrolysis in addition to opportunities for further research identified throughout the article.