Evaluation of low-acceleration MEMS piezoelectric energy harvesting devices

Microelectromechanical systems-based piezoelectric energy harvesting device research is continuing to increase due to high demands in powering wireless sensor networks. This paper compares three different cantilever structures that have been the most widely used designs in MEMS energy harvesting devices. The cantilever structures consist of a wide beam, narrow beam, and trapezoidal beam structure. Aluminium nitride was used as the piezoelectric material because of its CMOS compatibility. Finite element modelling was used to investigate the theoretical outputs of the devices prior to fabrication. The three different structures were fabricated using standard micro-fabrication techniques on SOI wafers in order to verify the results experimentally. The finite element modelling results agree with the experimental results. The AlN deposited on the experimental wafers had a (002) FWHM rocking curve value of 1.7°. The power density based on the volume of space needed to fabricate the structures was 2.5, 0.78, and 0.65 mW/cm³/g² at resonant frequency for the wide, trapezoidal, and narrow beam structures respectively. The bandwidth of the devices is also an important parameter when selecting the cantilever structure. An array of the cantilevers over a 4 cm² area resulted in a bandwidth of was 4.8, 9, and 26.4 Hz for the wide, trapezoidal, and narrow beam structures respectively.