MEMS Energy Harvester with solidified powder magnet
"Miniaturized power supplies are an important building block for the realization of next generation mobile sensor applications. The work presented here focuses on a bulk micromachined energy harvester based on a 3 mm x 1 mm x 50 µm sized piezoelectric cantilever. Two micrometer thick AlN, AlScN or PZT layers are applied as the piezoelectric component. Whereas the AlN and AlScN layers are sputter deposited using a reactive pulsed-DC method, the PZT layer is grown by means of chemical solution deposition. To drive the harvester magnetically, a miniaturized permanent magnet is attached to the cantilever. This magnet is produced by filling a Si cavity with NdFeB powder (particle size of the order of a few microns) using the doctor blade method. Subsequently, atomic layer deposition of Al2O3 agglomerates the particles resulting in a high energy density magnet . Four-point bending measurements reveal piezoelectric coefficients of e31,f = 1.4 C/m², 3.0 C/m² and 12.6 C/m² for the three different materials AlN, AlScN and PZT [2,3]. However, the maximum figure of merit FOM = e31,f^2/ε0ε33 = 60 GJ/m³ is measured for AlScN. Consequently, energy harvesters comprising AlScN exhibit the best performance yielding a power output of 13 µW for an alternating magnetic field with a 1 G amplitude and a frequency of 3.3 kHz matching the mechanical resonance. This corresponds to a high area-normalized power output of 4.3 µW/mm². As a first demonstration, the harvesters and a magnet wheel are tested in a drilling tool. The purpose of the harvester is to enable an integrated sensor system, which measures the position of the cutting edge in next generation tools.  Reimer, T., Lofink, F., Lisec, T., Thede, C., Chemnitz, S., & Wagner, B., Temperature-stable NdFeB micromagnets with high-energy density compatible with CMOS back end of line technology. MRS Advances, 1(3), 209-213, 2016.  Yarar, E., Hrkac, V., Zamponi, C., Piorra, A., Kienle, L. and Quandt, E., 2016. Low temperature aluminum nitride thin films for sensory applications. AIP Advances, 6(7), p.075115.  Fichtner, S., Reimer, T., Chemnitz, S., Lofink, F. and Wagner, B., 2015. Stress controlled pulsed direct current co-sputtered Al1−xScxN as piezoelectric phase for micromechanical sensor applications. APL Materials, 3(11), p.116102."