Energy Harvesting

New Structure Enhances the Design and Fabrication Flexibility of MEMS Energy Harvesters

Scientists at the Tokyo Institute of Technology developed a micro-electromechanical system (MEMS) energy harvester that they say allows for more flexibility in design. Microelectronic devices including Internet of Things applications require power to run. One potential solution is to use energy-harvesting MEMS to power these tiny contraptions, making them run on ambient energy, including harnessing energy from mechanical vibrations.

Conventional MEMS energy harvesters employ an electret, the electrical equivalent of a permanent magnet. An electret has a permanent charge stored within it. In traditional MEMS energy harvesters, the electret is placed in a MEMS-tunable capacitor, which features a moving electrode that is pushed by ambient forces. These ambient forces and vibrations can thereby induce the movement of charges. Unfortunately, this design is quite restrictive because the fabrication processes for the electret and the MEMS components have to be compatible. Above, you can see schematic diagrams comparing the design of the conventional MEMS energy harvester and the new one which has the electret and MEMS components on separate chips.

A team of scientists, including Assistant Professor Daisuke Yamane from Tokyo Tech, proposed a new MEMS electret-based energy harvester that consists of two separate chips with one for the MEMS tunable capacitor, and one containing an electret and dielectric material to form another capacitor. “This allows us to physically separate MEMS structures and electrets for the first time,” stated Yamane.

Diagram of charge-inducing mechanism

The capacitance of the electret circuit is fixed (Cfix), but that of the MEMS tunable capacitor (CM) changes in relation to the stretching of the spring that results from external vibrations. When CM becomes higher than Cfix, a movement of charges is produced, and the tunable capacitor gains charge. Moreover, when Cfix is higher, charges flow in the opposite direction and the capacitor in the electret circuit accumulates charge. These movements of charges provide small amounts of electrical power that can be exploited.

“The proposed method can be a promising way to enhance the design and fabrication flexibility of both MEMS structures and electrets,” concludes Yamane. Loosening up design constraints expands the limits for engineers and will accelerate the onset of the IoT era so that we can reap its benefits.

Reference

Daisuke, Yamane, Hiroaki, Honma, and Hiroshi Toshiyoshi. A MEMS Vibratory Energy Harvester Charged by an Off-chip Electret. MEMS 2019outer (The 32nd International Conference on Micro Electro Mechanical Systems).

Tokyo Institute of Technology
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