Modern portable micro-systems like biomedical implants and ad-hoc wireless transceiver micro-sensors continue to integrate more functions into smaller devices, which result in low energy levels and short operational lives. Researchers and industry alike are therefore considering harvesting energy from the surrounding environment as a means of offsetting this energy deficit. The fact is, even with power efficient designs, low duty-cycle operation, smart power-aware network architectures, and batteries with improved energy density, the stored energy in micro-scale systems is simply not sufficient to sustain extended lifetimes . What is more, the ubiquity of sensor nodes within a network and their limited accessibility prohibit the use of external energy supplies and the maintenance of micro-scale rechargeable batteries, creating the need for in situ long-lasting and self-renewable chip-compatible energy harvesting sources, in other words, self-sustaining and self-powered micro-scale system-in-package (SiP) solutions.
Even if the harvester has low efficiency, tapping into the energy of the surrounding environment is attractive because it is, for all practical purposes, an infinite source. Ambient solar, kinetic (vibrations), and thermal energy can be harnessed on-chip with photovoltaic cells and micro-electromechanical systems (MEMS) generators [2-3]. The amounts of energy and power levels that can be achieved, however, depend on the conditions surrounding the application and the compatibility of the available technologies. Relatively low-frequency ambient vibrations from engines, flowing water, gusting winds, moving people, and others, however, are abundant, stable, and predictable .
A Self-Sustaining System
Energy from ambient mechanical vibrations can be harvested by means of a magnetic field, an electric field, or a strain on a piezoelectric material [3-4]. Electromagnetic and piezoelectric scavengers, however, are less CMOS-compatible and less scalable. Electrostatic harvesters, on the other hand, are fully compatible with MEMS technologies and capable of generating moderate power levels without the use of exotic materials or obscure process steps. The foregoing scheme therefore harvests energy from an electrostatic generator and stores it in a thin-film polymer lithium-ion (Li-Ion) battery, which in turn powers the system, as illustrated in Figure 1. The electrostatic harvester does not convert energy continuously so an intermittent battery charger is used. To ensure the system is fully operational and self-sustaining, power-intensive tasks such as data transmission and reception are constrained to low duty-cycle operation, in other words, operate only when there is sufficient energy in the system to do so. Sensing and other low power functions may have longer duty-cycles, but for the sake of energy, they are also limited, unless they perform indispensable functions in the system.
Figure 1. Self-sustaining micro-system