A mems variable capacitor with barium titanate (BaTiO3) dielectric layer for energy harvesting
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] In the research, a MEMS variable capacitor for electrostatic energy harvester was developed. The device uses the undesirable environmental mechanical vibrations to power microelectronics and wireless sensors. Although the generated power is small, it can have a significant impact on the usage of sensors. The self-powered sensors will no longer require battery replacement or maintenance. The design, fabrication, and characterization of our variable capacitor are presented in detail. The device consists of 2x2 mm2 movable plate with a thickness of 30 [mu]m suspended above the fixed electrode plate, the air gap is 15 [mu]m between the movable proof mass plate and the fixed electrode plate. The movable plate is supported by four serpentine springs connected to the substrate via 4 anchors which are made of electroplated nickel. The suspension beams are made with a thickness, width and total length of 5 [mu]m, 15 [mu]m, and 1485 [mu]m. Gold stoppers with 1.5 [mu]m height were electroplated on the fixed bottom plate to prevent pull-in behavior of the movable plate by excessive electrostatic force. High dielectric constant of barium titanate thin dielectric layer is deposited on the fixed bottom plate to insulate the stopper and improve the dialectic property of capacitive cavity. The MEMS variable capacitor has been designed and fabricated for electrostatic energy harvester using environmental mechanical vibration. The device uses nickel as the main material for structure. The variable single-cavity capacitor of electrostatic energy harvester has been fabricated by surface micromachining technology. The device testing has been performed by a PCB board mounted on a shaker and data acquisition (NI USB-6218 DAQ). The resonance behavior of the moving plate was observed by mechanical and electrical measurements. The measured resonance frequency was 520 Hz. An optimum value for the load resistor that is connected in series with the power harvesting device was determined to be 460 k[omega]. The output voltage was measured was measured across the load resistor. A maximum power of 130.5 nW was measured under a DC bias voltage 15V and 5 g.
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