The Internet of Things is shaping the evolution of information society, requiring an increasing number of objects with embedded electronics, sensors and connectivity. This spurs the need for systems where summing to performance and low cost, multifunctionality has to be assured. To date, there is not a technology fulfilling these needs: for instance, thin film microelectronics enables flexibility and large area processing but fails on assuring high levels of electrical performance or energy harvesting; nanoscale Si or III-V materials, while exhibiting outstanding electrical performance, typically fail on multifunctionality and/or low-temperature & large-area processing.
In this context, TREND aimed to take transparent electronics into as-of-yet unexplored levels of functionality, by combining on flexible substrates transparent and high-speed nanocircuits with energy harvesting capabilities, based on multicomponent metal oxide materials, particularly nanowires (NWs). For this end, sustainable and recyclable materials as zinc-tin oxide (ZTO) were synthesized using low-temperature and low-cost solution processes. The precise control of the density and alignment of these nanostructures is critical for device integration and in TREND different approaches were considered to tackle this, from transfer methods to direct growth of NWs from nm-scale patterned seed layers on flexible substrates. Major applications envisaged were flexible nanotransistors, that soon will enable comparable or even superior integration densities on flexible substrates compared to sub-22 nm nodes in Si technology, as well as piezo/triboelectric nanogenerators able to scavenge energy from human body movement, demonstrated within TREND on flexible polymeric substrates and on carbon fibers weaved on textiles. But other fascinating properties of these materials were also explored for a multifunctional platform, such as photocatalytic and sensing ones. With the developed nanoscale processing technologies, we could also demonstrate transparent and flexible electrocorticography electrode arrays in in-vivo neural recordings. On the circuit level, multiple analog/digital circuit blocks were demonstrated on flexible substrates, exploring circuit design techniques to compensate for oxide transistor performance limitations compared to Si CMOS and by establishing baseline processes enabling miniaturized vias through multilevel metallization schemes (up to 4 metal levels demonstrated), fully compatible with flexible and large area electronics tools.
Tweaking the optimized processes developed for transistors/circuits, particularly multilayer insulators, we could also verify unprecedented sensitivity levels on flexible oxide transistors used as ionizing radiation detectors, which triggered an ERC Proof of Concept Grant starting in October 2022 (FLETRAD, GA 101082283) to explore commercialization routes of such technology.
TREND was a very ambitious interdisciplinary project motivating advances in materials science, engineering, physics and chemistry, with impact extending from consumer electronics to health monitoring wearable devices. By promoting new ideas for practical ends, it contributed to place Europe in a leading position of flexible electronics, where sustainability of materials and processes tackled with multifunctional concepts are key factors.
