Modeling of Metal-based Piezoelectric MEMS Energy Harvesters

We fabricated a piezoelectric MEMS energy harvester (EH) of Pb(Zr, Ti)O-3 (PZT) thin film on stainless steel cantilever. The use of metal substrates makes it possible to fabricate thin cantilevers owing to a large fracture toughness compared with Si substrates. The PZT thin film was directly deposited onto 50-mu m-thick stainless steel substrate by rf-magnetron sputtering. By attaching a tip mass (weight: 480 mg) to the substrate, the resonant frequency of the cantilever (length: 10 mm, width: 10 mm) was dropped to about 75 Hz. From X-ray diffraction (XRD) measurement, we confirmed that the PZT thin film on Pt-coated stainless steel substrate had a perovskite structure with a random orientation. The relative dielectric constant epsilon(r) and transverse piezoelectric coefficient e(31,f) were measured to be 650 and -1.7 C/m(2), respectively. From the evaluation of the power generation performance of the PZT thin-film EH, we obtained a large average output power of 1.1 mu W under vibration at a low frequency of 75 Hz (acceleration amplitude: 5 m/s(2), load resistance: 20 k Omega). Moreover, the experimental output voltages with open circuit state were in good agreement with the theoretical values calculated using theoretical equation.