Piezoelectric micro-machined ultrasonic transducer (pMUT) for energy harvesting
Résumé
Micro-electromechanical devices dedicated to energy scavenging purpose have yield an increasing interest for a few years. In this paper we report on the fabrication of PZT/Si piezoelectric micro-machined ultrasonic transducers (pMUT) first designed to ultrasonic imaging applications that may be used as a mechanical to electrical energy transformer for energy harvesting. This work aims to demonstrate the ability of pMUT to convert inertial energy into electrical energy through the piezoelectric layer deposited atop silicon membrane. The diameter of the membrane ranges from 132 µm to 600 µm and the thickness of silicon and PZT are respectively set to 1 and 2 µm. It is shown that the membrane exhibit a deformed shape, as the PZT is under lateral compression, with a maximum deflection equal to more than 1.5 times the equivalent membrane thickness. We first aimed to design a bistable micro power generator as the device could take two stable states that respectively corresponds to the case of PZT under lateral compression and the case of PZT under lateral extension (the symmetric deformation state). First experiments consist in testing the capability of the pMUT to change from one state to the other by a simple and weak mechanical excitation ranging from 0.5g to 2g acceleration. The experiment results have demonstrated two typical mechanical behaviours, linear (elastic) and non-linear (bistable). The pMUT device can generate electricity along both mechanical behaviours. The elastic mode has been emphasized as we observed different levels of generated voltages corresponding to different levels of mechanical excitation. The membrane is presumably deformed by the inertial excitation at a level less or equal than the threshold enabling to change state. In this case the membrane should then return to its initial stable state along an elastic behaviour. The bistable behaviour has been emphasized as we observed two state changes, i.e. two very sharp opposite and equal signals (larger than 180 mV on a 1 M. input impedance oscilloscope), corresponding to the stress inversion (compression to extension and extension to compression) with both respective flow of generated electrical charges. It should be noted that first results were limited by air damping and electrical damping. As a consequence we have developed a piezoelectric finite element model that takes into account the electrical load in the pMUT design. Further simulations with this finite element model should enable to optimize the impedance load of the pMUT for harvesting the maximum electrical energy.