Towards high power output electrostatic energy converters

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The field of wireless sensors for various applications including applications related to the Internet of Things is rapidly growing. Powering wireless devices with replaceable batteries will become unsustainable due to their finite lifespan and high maintenance costs. Thus, there is a great need for micro-scale power generators, which are inconspicuous and inexpensive while generating sufficient power to match the requirements of wireless sensors. Silicon-based electrostatic power generators employ variable capacitors to harvest mechanical vibration energy. They have significant advantages over other mechanical vibration harvesting technologies because of their compatibility with processes for fabrication of integrated circuits (IC), which minimize costs and also allow for on-chip integration with wireless sensors and control circuitry. Currently, the level of power output of state-of-the art harvesters is still an order of magnitude lower than that required by most advanced ultra-low power wireless systems. In this work, new concepts will be investigated to create silicon-based electrostatic harvesters with increased power output. As part of the project, a consistent effort will be made to attract students from underrepresented minorities in the research. The research results will also be communicated to a broad audience including high school students, through summer programs and other outreach efforts at Rensselaer Polytechnic Institute. The objective of this proposal is to investigate new methods for silicon-based electrostatic vibration energy harvesters in order to increase their power output. Such devices typically employ variable capacitors with interdigitated electrodes, where a set of electrodes moves in response to vibration. The energy converted depends on the level of capacitance variation during motion. The proposed work explores two transformative approaches to increase the capacitance variation, and consequently the power level, by investigating: i) a dual-level stopper, in conjunction with highly dense packed electrodes in gap-closing interdigitated in-plane harvesters and ii) out-of-plane interdigitated harvesters. The first approach employs two different stopping mechanisms, a nanostopper and a soft-stopper, that work together to boost the maximum capacitance and produce frequency up-conversion, increasing the number of energy conversion cycles per unit time for a gap closing in-plane harvester. The nanostopper is defined by an insulating film with nanometer thickness deposited on the electrode sidewalls, which sets the absolute minimum gap. The thin film stopper will provide a separation gap that is smaller by at least an order of magnitude, as compared to stoppers defined via lithography and etching. This in turn results in an order of magnitude increase in capacitance variation and consequently in power output. The role of the soft-stoppers is to trigger frequency up-conversion, increasing the harvesting frequency, along with increasing device bandwidth and prolonging the lifespan of the nanostopper by minimizing impact forces. The second proposed approach employs a structure not previously explored for vibration energy harvesting with silicon-based devices. This structure oscillates out-of-plane and has high conversion potential due to the ability to undergo extreme capacitance changes. To validate these approaches, theoretical models will be developed to predict performance as a function of input parameters. Design optimization will be carried out for both types of devices, followed by fabrication and testing.