Electrochemical Microstructuring
The fabrication of microdevices is one of today’s key technologies. Applications range from sensors and electronic devices up to complete miniaturized machines. Electrochemical methods can offer various advantages for their fabrication. For example, they allow an easy supply of different materials and the experimental setup is rather simple. However, the application of electrochemical methods for microstructuring is usually hampered by the large-scale charging of the double layer and the consequently weak spatial confinement of the reactions.
We developed a method, where electrochemical reactions on electrode surfaces are localized with submicrometer precision, due to the application of ultrashort voltage pulses of only nanosecond duration. It is based on the fact that the charging time constant of the double layer capacity varies linearly with the separation between the electrodes (Fig. 1). During short pulses of about 50 ns duration, effective charging is limited to electrode regions, where the two electrodes are typically only a few micrometers apart. Since electrochemical reactions are exponentially dependent on the potential drop in the double layer, this enables the direct three-dimensional machining with micrometer resolution.
The micromachining method was demonstrated for various metals and semiconductors such as copper, nickel, tungsten, gold and silicon and real construction materials such as stainless steel. With pulse durations in the picoseond range machining precisions down to about 20 nm could be achieved.
Recent projects focus on the use of nonaqueous electrolytes i) for machining submicrometer structures in Au and ii) for the machining of construction materials like highly alloyed steel. Nonaqueous electrolytes have two advantages over aqueous ones for those applications. First, the specific electrolyte resistance is generally higher than this of aqueous electrolyte solutions, which allows the application of longer and easier to provide pulses for obtaining the same spatial resolution. Furthermore passivation of the surface often can be avoided, which helps for the machining of such heterogeneous materials like highly alloyed steels, where carbide grains might exist within an iron matrix, with both exhibiting fairly different electrochemical properties. (The report on our Project 'Elektrochemische Mikrobearbeitung von hochlegierten Stählen in nichtwässrigen Elektrolyten', supported by the Bundesministerium für Wirtschaft and Technologie via the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen "Otto von Guericke" eV (in German) is available on request).