Fabrication process for very-low frequency operation of guided two-beam piezoelectric energy harvester

MEMS-based piezoelectric vibration energy harvesters found suitable to power wireless sensor nodes, typically in a remote area of operation. In MEMS technology, piezoelectric type vibration energy harvesters are realized using fixed-fixed or fixed-free cantilever structures. These structures are spring mass damper systems that have a fixed resonance frequency which is to be matched with ambient vibration frequency. The low resonance frequency of the cantilever gives better output with the low-frequency mechanical vibrations. Parameters such as width, length and thickness must be critically controlled to achieve low-frequency response during device fabrication. This paper presents the optimized fabrication process for guided two-beam type piezoelectric vibration energy harvester device. The device thickness is critically controlled using tetramethylammonium hydroxide zig during wet bulk micromachining followed by beam thinning process using deep reactive ion etching on the back side of the silicon wafer. A highly c-axis oriented zinc oxide layer of 2.5 µm thickness is sandwiched between aluminum electrodes on the front side of the silicon wafer for harvesting electric potential. The laser Doppler vibrometer test gives the resonance frequency of the fabricated device around 466 Hz which is lowest reported so far for the guided two-beam device. The resonance frequency of the device reported has been reduced by 32.85% than the earlier reported guided two-beam device. The device when vibration shaker tested was found to be operational in the very low-frequency range up to 10 Hz giving a sensitivity of 1.648 mV/m/s² near resonance frequency.