Principal Investigator Sang-Gook Kim
Project Website http://www-mtl.mit.edu/researchgroups/mems/docs/2007/APpage54.pdf
A novel thin-film lead zirconate titanate Pb(Zr,Ti)O3 (PZT) MEMS energy-harvesting device is designed and developed for powering autonomous wireless sensors. It is designed to harvest energy from parasitic vibrational energy sources and convert it to electrical energy via the piezoelectric effect. The new pie-shaped design always generates positive tension on the PZT layer and then positive charge output throughout vibration cycles. It produces mono-polarity output charge without using any additional bridge rectifier circuitry, which will be a huge cost saving for commercial production of scaled-up products. Contrary to the high-Q cantilever designs, the new design has a low-Q, doubly anchored beam design, which provides a wide bandwidth of operational frequency. This will enable more robust power generation even if the frequency spectrum of the source vibration varies unexpectedly. Furthermore, the beam shape is optimized to achieve uniform strain throughout the PZT layer.
In this new design, the whole thickness of the silicon wafer is used as the proof mass to increase the power of the generator. The fabrication includes CVD of 10-micron-thick oxide, followed by spin-coating, patterning, wet-etching, and annealing a thin ZrO2 layer as the diffusion barrier layer, followed by three layers of PZT. The top interdigitated electrodes are patterned by the lift-off method out of gold. A long BOE etching through the oxide followed by a DRIE of silicon from the wafer's back finalizes the device structure and releases the beams and proof mass. The SEM images of the released multi-beam cantilever beam design with a common heavy proof mass (an improved version of type-I PMPG) and a pie-shaped device with a center proof mass (type-II device) are imaged using scanning electron microscope (SEM).