The functional enhancement of devices during the last decade is primarily related to the downscaling of their components. Clear evidence of this technological development is the increasing number of microelectromechanical system (MEMS) devices that can be found in the form of sensors or actuators in automobiles, phones, airplanes and biomedical devices. Industry needs and miniaturization trends are pushing the development of new materials and processes. Electrochemical processing methodologies are fundamental fabrication tools for the implementation of small functional parts in novel complex micro- and nanosystems.
Since electrochemical processing was incorporated in the electronics industry during the 1940s, its impact on boards and packages, storage devices, chip-to-package interconnections, microelectromechanical systems (MEMS) (sensors, actuators and micromotors) and microprocessors is undeniable [Ref.]. Such progress has been possible due to the advancement in other areas: improvements in lithographic and other patterning procedures, implementation of advanced characterization tools, fundamental understanding of thermodynamics and kinetics phenomena at small scales. Despite tremendous advancements in the field, there is still an urgent need for miniaturized materials with increased electric performance (e.g.: high conductivity, piezoelectricity, thermoelectricity), magnetic properties (e.g.: soft- and hard-ferromagnetism, magnetostriction, magnetoresistance), high-surface area characteristics and enhanced corrosion resistance.
These materials have to be processed at the micro- and nanoscale and subsequently controllably assembled and/or integrated into functional devices. To accomplish this goal, one or a combination of the following techniques can be used: electrodeposition (in aqueous and non-aqueous electrolytes), micro- and nanoelectroforming, electroless deposition, electrochemical machining, electrophoretic deposition and anodization. Despite the versatility in terms of materials (metals, alloys, composites, semiconductors, polymers) and cost-effectiveness of these techniques, most of them were overshadowed for many years. In particular, electrochemical processing techniques offer several advantages over other material deposition techniques, such as sputtering, evaporation, physical and chemical vapor deposition or atomic layer deposition. For example, electrochemical processing techniques work at room pressure (and very often at room temperature or near room temperature). Also, substrates of complex shapes (even parts with undercuts) can be coated, and relatively thick films can be obtained at reasonable deposition rates. Moreover, they do not usually require very expensive equipment. During the last decade, the research in the field of nanomaterials has revolutionized the fabrication techniques and the template-assisted electrodeposition is listed prominently among them. Using this approach, nanowire and nanotube arrays of metals, alloys, semiconductors and conducting polymers have been deposited in the pores of anodized alumina and polycarbonate membranes. Similarly, arrays of dots, pillars and other material geometries have been obtained onto e-beam lithographed substrates. However, these micro- and nanoarchitectures have not been integrated into real functional devices yet and have not thus reached the market. An important drawback in device miniaturization is that the choice of materials for a target application poses further issues due to incompatibilities between materials. As a consequence, tedious and complex fabrication steps are often needed in micro- and nanofabrication. Moreover, the selected materials can be used only in any real industrial application if they are stable against corrosion (in air or in liquids depending on the end application). Although atmospheric corrosion is a well-known phenomenon in metals, there is a lack of information about the stability of metallic micro- and nanostructures against it. The loss of material due to corrosion, which is considered as reasonable in conventional materials (i.e., mm/year) is no longer acceptable and must be limited to values less than 10–100 nm in micro- and nanomaterials. This includes all products incorporating micro- and nanosized metallic components and ultrathin layers conferring the relevant property (or function) to the material surface. To ensure good stability, nanometer-thick protective films can be applied, but the protective effect should not compromise other surface functionalities. To increase the durability of the protection, self-repair of the ultrathin protective films is also a key factor.