Abstract

Piezoelectric microelectromechanical systems (PiezoMEMS) are attractive for developing next generation self-powered microsystems. PiezoMEMS promises to eliminate the costly assembly for microsensors/microsystems and provide various mechanisms for recharging the batteries, thereby, moving us closer towards batteryless wireless sensors systems and networks. In order to achieve practical implementation of this technology, a fully assembled energy harvester on the order of a quarter size dollar coin (diameter=24.26 mm, thickness=1.75 mm) should be able to generate about 100 μW continuous power from low frequency ambient vibrations (below 100 Hz). This paper reviews the state-of-the-art in microscale piezoelectric energy harvesting, summarizing key metrics such as power density and bandwidth of reported structures at low frequency input. This paper also describes the recent advancements in piezoelectric materials and resonator structures. Epitaxial growth and grain texturing of piezoelectric materials is being developed to achieve much higher energy conversion efficiency. For embedded medical systems, lead-free piezoelectric thin films are being developed and MEMS processes for these new classes of materials are being investigated. Non-linear resonating beams for wide bandwidth resonance are also reviewed as they would enable wide bandwidth and low frequency operation of energy harvesters. Particle/granule spray deposition techniques such as aerosol-deposition (AD) and granule spray in vacuum (GSV) are being matured to realize the meso-scale structures in a rapid manner. Another important element of an energy harvester is a power management circuit, which should maximize the net energy harvested. Towards this objective, it is essential for the power management circuit of a small-scale energy harvester to dissipate minimal power, and thus it requires special circuit design techniques and a simple maximum power point tracking scheme. Overall, the progress made by the research and industrial community has brought the energy harvesting technology closer to the practical applications in near future.

Piezoelectric properties of the c-axis oriented Pb(Zr,Ti)O3 (PZT) thin films were investigated. The PZT films with a composition near the morphotropic phase boundary were epitaxially grown on (100)Pt-coated MgO substrates by rf-magnetron sputtering. The PZT films exhibited excellent ferroelectricity with a remanent polarization more than 50 μC/cm2. In order to examine intrinsic piezoelectric properties, cantilever structures were microfabricated with the PZT films. The piezoelectric coefficient d31 of PZT films, which were not subjected to poling treatments, was measured directly from the transverse expansion of the cantilever beams. The measurements revealed that the PZT films were naturally polarized and had a relatively large piezoelectric coefficient d31 of 100×10−12 m/V without poling.

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High-piezoelectricity lead-free films of (K,Na)NbO3 (KNN) were successfully deposited on Pt/MgO and Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. The KNN film was epitaxially grown on Pt/MgO with a high <001> orientation in the pseudo-cubic perovskite structure. The KNN film on Pt/Ti/SiO2/Si was polycrystalline with a preferential <001> orientation in the pseudo-cubic perovskite structure. The piezoelectric properties of the KNN films were determined from the tip displacement of KNN/Pt/MgO or KNN/Pt/Ti/SiO2/Si unimorph cantilevers. The transverse piezoelectric coefficients e31* (d31/s11E) of the KNN films on Pt/MgO and Pt/Ti/SiO2/Si were calculated to be -3.6 and -5.5 C/m2, respectively, which are amongst the highest values for KNN films ever reported.