CMOS compatible low-frequency aluminium nitride MEMS piezoelectric energy harvesting device

Piezoelectric materials are widely used in various applications including sensors, actuators, and energy harvesting devices. Energy harvesting devices can be used to power autonomous wireless sensors that are placed in remote or difficult to reach areas, where replacing a battery is not practical or feasible. In this paper the authors present work on the fabrication and design of a CMOS compatible Aluminium Nitride (AlN) piezoelectric based MEMS cantilever structure for harvesting vibrational energy. In order for AlN to be piezoelectric it needs to be highly structured in the c-axis (002) crystal orientation. The deposition of highly structured AlN and its polarity is dependent on the underlying films and their crystal orientation. XRD rocking curve results from this paper show a highly oriented (002) AlN film with a FWHM value of 2.1°. The MEMS cantilever structures were fabricated using standard MEMS fabrication techniques using SOI wafers. By optimising the AlN material deposition process and the stress values in the cantilever structures the authors were able obtain a power density of 2.55 mW/ cm 3 /g 2 for a single MEMS structure with 500 nm thick AlN. The cantilever structure had a resonant frequency of approximately 150 Hz. In this paper the authors also investigated methods to increase the bandwidth of the cantilever structures, by building an array of devices with slightly varying length masses.