The eight-member project team of European companies, universities and research institutes fabricated a demonstrator embedding a silicon chip with 250 pixels, and its associated electronics for signal collection and post-processing. The chip was designed to demonstrate the concept and the major technological achievements, not the maximum potential nanowire integration density.
The chip featured a matrix of interconnected piezoelectric zinc-oxide (ZnO) nanowires (NW) grown on silicon at resolution of 1,000 dots per inch (DPI), with bundles of nanowires forming pixels less than 50×50µm2 each, encapsulated in a polymer. Under deformation (from a finger), the piezoelectric nanowires generate potentials whose amplitudes are proportional to the NW displacement and can be used to reconstruct the smallest minutiae (pores and ridge shapes) of human fingerprints.
In one of their implementations, the researchers narrowed down the sensing pixels to only a few standing nanowires (designed as a ‘bending mode’ force sensor), creating a 8×8 pixel array at a resolution of 5000dpi.
Coordinated by CEA Leti, the PiezoMAT project also involved Fraunhofer-Institut für Angewandte Festköperphysik (IAF), the MEMS lab, Institute for Technical Physics and
Materials Science, Universität Leipzig, Kaunas University of Technology, Specific Polymers, Tyndall National Institute, and Safran Identity and Security (Morpho).
The 44-month, €2.9 million EC-funded project also provided valuable experience and know-how in several key areas, such as optimization of seed-layer processing, localized growth of well-oriented ZnO nanowires on silicon substrates, mathematical modelling of complex charge generation, and synthesis of new polymers for encapsulation.
PiezoMAT – www.piezomat.eu