Investigation of Potential Platforms for Low Frequency MEMS-based Piezoelectric Energy Harvesting (2015)

MEMS based energy harvesters have recently been investigated for scavenging, otherwise useless, ambient vibration energy. Piezoelectric materials are fabricated on micro-devices to convert the mechanical vibration energy into electrical energy. The main focus for these harvesters is low frequency (under 500 Hz) ambient vibration which is the source of a fundamental challenge with MEMS oscillators. The smaller the oscillator is, the higher its natural frequencies will become. Various techniques have been proposed to decrease the natural frequency of micro-energy harvesters such as increasing the length of the devices or assembling extra proof mass to the fabricated devices which could potentially affect the mass production of the MEMS devices. Another challenge is that most of the reported piezoelectric energy harvesters in the literature have cantilever designs. These structures have a high mechanical quality factor providing a sharp peak at their resonant frequency. Since microfabricating resonators with a resonant frequency exactly matching their designed value is very challenging, linear cantilever designs seem to be less practical for real applications where excitation frequency could change. Therefore, some techniques in vibration have been adapted to widen the frequency bandwidth of the harvesters. One of the most effective methods to broaden the frequency bandwidth is taking advantage of large deflection effect of oscillators. However, some of the proposed designs such as a fixed-fixed beam design have high resonant frequencies (≥1 kHz), whereas the focus for energy harvesters is low frequency range. In this work, a silicon based structure has been designed and fabricated to carry an electronic chip and potentially provide in-situ supplementary power for it. This design provides capability of harvesting at three different frequencies because the resonant frequencies of this structure at its first three mode shapes are within the low ambient vibration frequency range. The widening frequency bandwidth has been investigated for this design. Natural frequencies as low as 71.8, 84.5, and 188.4 Hz have been measured using a laser vibrometer. A frequency bandwidth of ~10 Hz has been obtained for the 2nd mode shape of the structure under the base excitation of 0.2g. A maximum open circuit voltage of ~1V and maximum power output of 136nW have been obtained using this harvester. In addition, as opposed to the conventional silicon-based harvesters, polymeric materials have been investigated as the main structural material for energy harvesters. Due to the much lower stiffness of polymers compared to silicon, the resonant frequency of the harvesters could be reduced. To prove the concept, a SU-8 (ESU-8=5GPa vs. ESi=160GPa) membrane has been designed and fabricated with Aluminum Nitride harvesting elements. The membrane configuration provides the capability to widen the harvester’s frequency bandwidth. Testing results reveal a linear resonant frequency of 381 Hz, frequency bandwidth of 146Hz, maximum output power of 1.37µW, and power density of 3.81 µW/cm2 at the base excitation of 4g with this design. The much lower resonant frequency of polymeric structures compared to the similar silicon-based structures (more than 5 times lower) makes them a strong candidate for the future harvesters. The objective of this thesis is to develop a platform using silicon-based and polymer-based energy harvesters to improve the performance of the energy harvesters by reducing the resonant frequencies and widening the frequency bandwidth. Throughout this research, all stages including design, fabrication, packaging, testing, and characterization of both silicon- and polymer-based harvesters have been developed or adapted for the purpose of this work. Finite element simulations have been conducted to examine the mechanical response of the structures as well as their electrical output at the design stage. A scalable microfabrication process flow has been developed in this work to fabricate piezoelectric layers on SU-8 micro-structures. An improved approach for cleaving fabricated devices from the silicon substrate has been developed to overcome challenges of the dicing process. Various 3-D micro-assembly techniques have been adapted to package the fabricated harvesters. In addition, 3-D printed parts were used to enhance the yield of the packaging and testing stages. This technique could potentially be used for bio-compatible packaging, as well. In conclusion, the polymer-based and wideband energy harvesters seem promising for real applications at low ambient vibration frequencies. This research introduces opportunities to further improve the performance of the harvesters by decreasing their resonant frequencies.