This Action focuses primarily on the development of novel electrochemical fabrication processes with strong commitment to the requirements of European micro- and nanosystems industry. The innovative elements of the proposals include the identification of target materials for critical industrial applications at micro- and nanoscale. There is a need for materials with increased electrical performance (conductivity, thermoelectricity, and piezoelectricity), magnetic properties (soft- and hard-ferromagnetism, spring magnetism, magnetostriction, magnetoresistance), high surface-area and enhanced corrosion and tribocorrosion resistance. Some materials with an urgent need for miniaturization have already been identified. Below, some examples are listed and they are grouped as a function of the targeted property or application.
Materials with electrical properties (conductivity, piezoelectricity, thermoelectricity)
Performance requirements in the micro- and nanoelectronic industry can no longer be met by downscaling Si-based CMOS devices. New 3D compact architectures are being developed, which involve 3D short metallic connections between the stacked layers. Copper damascene Through Silicon Vias (TSV) technology is the subject of current investigations. The TSV technology can be miniaturized at the chip and wafer level and can be integrated in industrial production. Copper has significant advantages over other metals but the observed increase in resistivity when the interconnect dimensions are reduced toward the nanoscale poses some drawbacks. Therefore, research on electrodeposition of high-performance nanostructured alloys that could replace copper is required.
Most of materials exhibiting piezoelectricity are oxide ceramics such as barium titanate (BT), lead zirconium titanate (PZT) or zinc oxide (ZnO). ZnO has been successfully grown by means of electrodeposition. There have been also investigations to electrophoretically deposit BT and PZT from sol-gel formulations. However, the production of piezoelectric materials by electrochemical means is still in its infancy.
Thermoelectric materials are particularly important in applications of power generation. Recently, many studies are showing that nanostructuring of thermoelectric materials leads to a considerable improvement in their thermoelectric performance. There are several works on the electrolytic production of micro- and nanostructured thermoelectric materials such as bismuth-telluride or bismuth-selenide compounds. Recently, tin-selenide has been reported as the material with the highest thermoelectric effect. However, baths for the growth of these materials are either non-developed or require substantial improvements for their long-term usage and industrial production.
Materials with magnetic properties
Most of the permanent magnets used in bulk form (rare-earth permanent magnets like NdFeB and SmCo) are brittle and therefore their miniaturization requires very restrictive processing conditions, often not compatible with micro- and nanofabrication steps. Meanwhile, Pt–M-(P) alloys (M = Fe, Co) are platable and can be processed at small scales. These alloys may exhibit high anisotropy and coercivity, but these features are typically achieved by deposition at high temperature or post-annealing above 500 °C. Although there have been attempts to electrodeposit CoPt(P) films with enhanced hard-magnetic properties already in the as-deposited state, the magnetic characteristics are still far from those exhibited by rare-earth permanent magnets. Therefore, further research on the electrosynthesis of CoPt(P) micro- and nanostructures with enhanced hard-magnetic properties is required. On the other hand, finding alternative compositions that involve neither rare-earth nor noble metals deserve investigation given the serious socio-economic and environmental concern on these elements. Most of the mines and reserves of RE are controlled by emerging countries (i.e. China), that have developed their own technological products putting limitations to the exportation of RE. Protectionist trade policies (strategic and economic stockpiling) have also been applied to the commercialization of noble metals, from countries like USA, China or Russia, dramatically increasing their prices and creating difficulties for EU industry to have access to these critical raw materials (href="http://ec.europa.eu/enterprise/policies/raw-materials/").
Soft-magnetic layers are especially important for realizing magnetic micro- and nanodevices. Many formulations have been developed to produce this type of materials by electroless or electrochemical deposition. However, the best soft-magnetic materials are iron-based alloys. The instability of iron salts in electrolytes due to formation of insoluble iron(III) hydroxides and oxides has hampered the development of industrial processes for these alloys. Novel formulations containing novel additives or solvents (e.g.: IL) are urgently required. Another issue, which requires attention, is the fact that iron-based alloys suffer from corrosion. Inclusion of other elements in the alloy or protective layers must be developed.
The production of magnetostrictive materials by electrochemical means can significantly benefit the field of micro- and nanomechanical devices and sensors. These materials are potential substitutes of piezoelectric films in several applications because they can be manipulated by means of external magnetic fields. Hence, the integration of these materials in devices would simplify their fabrication, since no electrical connections are required. There have been few attempts to manufacture magnetostrictive alloys such as galfenol (iron-gallium) by electrodeposition, but there are no realistic formulations yet that can operate at industrial scales.
Materials with high surface area (foams, cellular materials, sponges)
Micro and nanoporous materials have a wealth of applications in sensing, (electro)catalysis, filtration or batteries. Recently, the production of metallic 3D porous layers by electrochemical means (de-alloying, electrodeposition, anodization) has been demonstrated, but there is still much room for investigation, both from the point of view of the target materials and the fabrication conditions. When integrated into miniaturized devices, these materials can add a sort of functionalities to the device such as the storage of different compounds (nanoparticles, drugs, dyes) and its subsequent release, etc. Moreover, the electrodeposition of cellular metallic layers can be an important starting point for the realization of protective coatings able to support dry wear condition.
Materials with corrosion and tribocorrosion resistance at small scales
Corrosion at micro- and nanoscales is an important and timely topic that should be considered at an appropriate stage of product development. Defects related to the nanostructure of the materials and their protective layers, as well as galvanic coupling between different metallic materials, become the major causes of corrosion and subsequent failure at the micro- and nanoscales. There is a general agreement that the methodology necessary for investigating the corrosion mechanisms must operate at the nanoscale and in the relevant environment (e.g. aqueous solution). So far, researchers have combined in situ nanoprobes (STM and AFM or SKP-FM) with ex-situ surface sensitive analytical tools (SPS, Auger, SIMS). Other studies have been performed at micro-scale using the electrochemical microcell, which allows to perform electrochemical measurements in glass capillaries with a diameter of 20-50 mm. However, there is currently a lack of ‘standard’ protocols to quantitatively assess corrosion phenomena at the nanoscale. No coordinated project on this topic is running under Horizon 2020 or other European fora. Tribological issues are also relevant in MEMS/NEMS requiring intended and/or unintended relative motion. Since MEMS/NEMS are expected to perform typically in the millisecond to picoseconds range, the increase in resistive forces such as adhesion and friction upon miniaturization becomes a serious tribological concern that limits their durability and reliability. To tackle this problem, coatings can be applied with the aim to maximize adhesion, and minimize friction and wear. The nature of these protective coatings as well as the required pathways for integrating them at the micro- and nanoscale is a topic that requires the concerted effort of the electrochemical and surface finishing communities and the micro- and nanodevices research and industry. Electrochemical processing methods can provide solutions to facilitate the total or the partial integration of these materials into devices. Novel electrolytes and electrochemical processing methods will be proposed in order to synthesize these materials, while current methods of synthesis and drawbacks toward miniaturization will be reviewed. In order to create device prototypes, the compatibility and complementarity of the electrochemical fabrication methods with other manufacturing approaches will be assessed. In parallel, the corrosion resistance of the newly developed materials as well as protective measures will be investigated.