Thinned-PZT on SOI process and design optimization for piezoelectric inertial energy harvesting

This paper presents the design, fabrication, and testing of a thinned-PZT/Si unimorph for vibration energy harvesting. It produces a record power output and has state-of-the-art efficiency. The harvester utilizes thinning of bulk-PZT pieces bonded to an SOI wafer, and takes advantage of the similar thermal expansion between PZT and Si to minimize beam bending due to residual stress. Monolithic integration of a tungsten proof mass lowers the resonance frequency and increases the power output. The harvester dimensions, including the PZT/Si thickness ratio and the proof-mass/total-beam length ratio, are optimized via parametric multi-physics FEA. Additionally, a fabrication process for hermetic packaging of the harvester is introduced. It uses vertical Si vias for electrical feed-throughs. An unpackaged harvester with a tungsten proof mass produces 2.74 μW at 0.1 g (167 Hz), and 205 μW at 1.5 g (154 Hz) at resonance (here, g = 9.8 m/s²). The active device volume is 27 mm³ (7 × 7 × 0.55 mm³). We report the highest power output, Normalized Power Density (N.P.D.), and Figure of Merit (N.P.D. × Bandwidth) amongst reported microfabricated vibration energy harvesters.