Modeling of metal-based piezoelectric MEMS energy harvesters

We fabricated a piezoelectric MEMS energy harvester (EH) of Pb(Zr, Ti)O₃ (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-μm-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 ε_r and transverse piezoelectric coefficient e_{31, f} were measured to be 650 and -1.7 C/m², 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 μW under vibration at a low frequency of 75 Hz (acceleration amplitude: 5 m/s², load resistance: 20 kΩ). Moreover, the experimental output voltages with open circuit state were in good agreement with the theoretical values calculated using theoretical equation.