Abstract
The field of microelectronics had a remarkable progress since its beginnings in 1960s, which led to the advent of myriad new electronic devices that found widespread usage in daily life. Continuous advances in CMOS and MEMS technologies reduced the cost, size, weight, and power requirements of these devices, enabling the realization of distributed systems such as wireless sensor networks. However, due to much slower pace of innovation, currently available battery technologies continue to dictate the size, weight and cost of these systems. There are further concerns brought by the batteries regarding the environmental effects or feasibility of dead battery replacement in distributed or embedded systems. As a result of this problem, there has been a growing research impetus on energy harvesting technologies, which are expected to alleviate the problems brought by the fixed capacity energy sources in electronic devices. This dissertation proposes a new class of MEMS-scale piezoelectric energy harvesters that have the potential to be monolithically integrated with CMOS circuits. Proposed devices will utilize polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), a piezoelectric polymer with an impressive electromechanical coupling factor of 0.3. Its energy harvesting potential was evaluated using theoretical analyses and finite element method (FEM) simulations and compared with other CMOS compatible piezoelectric materials. Various architectural options for the mechanical and electrical structure of the energy harvester were examined and most promising options were determined. The process for the fabrication of PVDF-TrFE thin films was optimized to yield high quality films with strong ferroelectric and piezoelectric properties. A comprehensive characterization study was performed to measure the dielectric, ferroelectric, and piezoelectric properties of the fabricated films. Cantilever type MEMS scale piezoelectric energy harvesters (PEH) were fabricated and characterized. Maximum power output density on purely resistive loads in response to a 1.0 g input acceleration was measured as 27.8 nW/mm2 from a (1800 µm × 2000 µm) device at its resonance frequency of 192.5 Hz. A power conditioning circuit, based on synchronous switching on inductor technique, was also designed and integrated with the fabricated prototypes. The circuit, which draws 250 nW power from ±1 V dual supplies at 200 Hz, improved the DC power output of the PEHs by 165%. Using the same (1800 µm × 2000 µm) prototype in combination with the circuit, a maximum power of 140 nW was transferred to a DC load under 1.0 g acceleration. The results obtained throughout the course of this dissertation work proved that PVDF-TrFE can be used in MEMS scale energy harvesting devices. CMOS compatible fabrication process of the polymer makes it possible to integrate these energy harvesters with CMOS circuits on the same substrate. This monolithic integration approach would improve the unit cost, size, and reliability compared to integration at higher levels and therefore, can find use in applications such as wireless sensors networks, structure health monitoring systems, and wide area surveillance applications.