Development of MEMS Piezoelectric Energy Harvesters

Abstract

The research of powering devices in a microwatt range has been activated and developed by the emergence of low-power Very Large Scale Integration (VLSI) technology in the past few years. The powering devices require a size that is compatible with the application, sufficient power, and extended lifetime using permanent and ubiquitous energy sources. The piezoelectric energy harvester using vibration sources is attractive due to its high conversion efficiency, simple design for miniaturizing, and lack of external voltage source. While bulk piezoelectric energy harvesters produce enough power for a few tens of mW, the insufficient power is still a major issue during miniaturizing into micro size. The piezoelectric energy harvester was fabricated by micro-electro-mechanical systems (MEMS) and developed to enhance its output power. It was designed to be resonated at the frequency range of ambient vibration source (50~300 Hz) and convert the mechanical stress to electricity by piezoelectric thin film. The cantilever structure was chosen in this study due to its large strain, and a big proof mass at the end of tip was integrated for the same reason. This study focuses on three specific issues related to the robust fabrication process, including the integration of piezoelectric thin film, structure design for high power density, and the reliability of device. The Lead Zirconate Titanate (PZT) thin films were prepared by a sol-gel process and were used to fabricate energy harvesters by an optimized MEMS process. The properties of PZT thin film were studied considering the substrate effect, heat treatment, and thickness effects. The fabricated energy harvester produced 769 mVpk-pk, and 6.72 µW with the optimal resistive load of 11 kΩ at 127 Hz of resonant frequency. The device had dimensions of about 4 mm(L) x 2 mm(w) x 0.021 mm(H), and the Si proof mass had dimensions of 3 mm(L) x 2 mm(W) x 0.5 mm(H). Beyond this result, the technical platform for the robust fabrication process was established on a Deep Reactive Ion Etcher (DRIE). The plasma etching using DRIE was optimized to prevent damage of the PZT film and to obtain uniform and precise dimension control. The trapezoidal shape of the cantilever was demonstrated to enhance the power density by stress distribution on the PZT film. The geometry change in cantilever shape distributed the strain on piezoelectric film and improved the output power ~40% higher than that of the rectangular shape due to nonlinear piezoelectric properties. The multi-beam arrays were designed to obtain a multiplied electric power effect as if as number of cantilevers was used. The multi-beam arrayed design requires the uniform machining to match the unified resonant frequency of each cantilever structure. Based on the optimized fabrication process, the cantilever array that consists of four cantilevers generated 18.39 µA and 1.352 µW with 4 kΩ of optimized resistive load in parallel connection under 1 G of acceleration force. The result was exactly four times higher power and current than that of individual cantilever. Finally, reliability tests were performed for the piezoelectric MEMS energy harvester considering the number of cyclic loads and temperature, and the degradation of PZT during fabrication was also investigated.