Principal Investigator Sang-Gook Kim
MEMS energy harvesting has been keenly pursued to provide perpetual power for many wireless applications including distributed sensor networks and upcoming IoT systems. However, scavenging a sufficient amount of power for wireless communication from environmentally available vibrations, typically at low frequency (<70Hz) and low acceleration (0.5g), has neither been successful nor reported at the MEMS scale. Here we present a bi-stable buckled beam MEMS energy harvester which could meet those requirements in terms of low operating frequency, wide bandwidth, and power, all packaged in the size of a coin. This new design does not rely on conventional linear or non-linear resonance of the MEMS structure, but instead operates with large snapping motions of buckled beams at very low frequencies. A fully functional piezoelectric device has been designed, monolithically fabricated, and tested to induce bi-stable buckling of ~200µm. The first batch device generated peak power of 85 nW with 50% half-power bandwidth under 70Hz at 0.5g.
The bi-stable nonlinear oscillator-based MEMS energy harvester has a clamped-clamped beam structure with a stack of thin-films having 28 pairs of beams 0.4mm wide in a silicon frame of 15mm×12mm. Each beam has approximately 500 interdigitated Au fingers over 0.2 μm thick PZT. A proof mass is located in the middle, connecting the beams to synchronize their out-of-plane motion and minimize undesirable torsion.
Thin-film layers of various stresses have an effective total compression and balanced stress with respect to the neutral axis to achieve bi-stable buckling. The residual stress and the thickness of the thin films are monitored for each deposition step, and progressive feedback control of subsequent deposition is employed to minimize deviation from the design target. The final released device shows desired bi-stable buckling of about 200µm) which is within 5% of the designed value. The dynamic testing with a laser vibrometer validates the design concept that the buckled beam device could have large-amplitude oscillations with low-frequency and low-amplitude inputs (<70Hz and 0.5g).