Desigment And Fabrication Of MEMS-Based Vibration Energy Havester

Traditionally electrical power has been generated from fossil fuels in large, which produce exhaust gas polluted atmosphere, soil and ocean. Now a variety of different methods exist for harvesting energy from large-scale ambient, such as wind energy, ocean waves, solar power, electrostatic, piezoelectricity, thermoelectricity, and physical motion. There are widely available and uncontaminated but difficult to harvest.Ambient energy harvesting of MEMS (Micro Electro Mechanical System) -based devices which convert mechanical energy into electrical energy have attracted much interest in both the military and commercial sectors. For example, random vibration can be converted into electricity which can be used by automotive sensors, microprocessors, RF transmitters and such as those developed using MEMS technology, which are often very small and require only a little power, but their applications are limited by the depending on battery power. So micro-generators are definitely needed by a completely self-powered wireless and battery-less sensor node. In the past several years, there is an increasing interest in the development of device which can generate electrical energy by using vibrations from the environment. Traditionally, there are three main strategies for the vibration energy harvesting: piezoelectric, electrostatic and magnetic [2]. Piezoelectric energy harvesting converts mechanical energy to electrical by straining a piezoelectric material, but the power collective efficiency lies on the quality of PZT. Electromagnetic energy harvesting has relatively lower voltage and power density level and no long-term stability. For electrostatic energy harvesting, capacitive principle can be quite easy implemented in MEMS (Micro-Electro-Mechanical System) technology. Therefore, capacitive method is strategically used in electrostatic energy converter device. The capacitive energy harvesting relies on the changing capacitance of vibration-dependent variable capacitor. Variable capacitor is initially charged and, as vibrations separate its plates, mechanical energy transforms to electrical energy. The attractive feature of this method is its IC-compatible nature, since MEMS variable capacitors are fabricated with relatively mature silicon micromachining techniques. But it needs complicated and high speed and precision dedicated circuitry. In addition, an external power source is needed to initially charge the capacitors. This disadvantage baffles the capacitive energy harvesting developing progress.Base on the changing capacitance of vibration-dependent variable capacitor, a capacitive energy harvesting devices has been designed. The two electrode plates of the capacitor are two different metals that have different work functions. When they are contact via external connection, the contact voltage or build-in-voltage will be created across the capacitor, so there is no need for the additional power source. In addition, the MEMS technology involved is relatively simple.In this work, the capacitive energy harvester is designed as a sandwich structure. SOI (Silicon-On-Insulator) wafer and Pyrex 7740 glass are used to make the function elements and vacuum sealing package. SOI wafer is in the middle and glass wafers are bonded on both the upper and the lower side of SOI wafer. The SOI wafer consists of beam, mass and the movable electrode. The counter electrode and the capacitor gap are formed in the upper glass wafer. The lower glass wafer defines the beam vibration range. The two glass wafers encapsulates device, and keep the device operating under low pressure conditions. A capacitive vibration-to-electrical energy harvester has been designed and optimized by suing ANSYS simulation tool to meet the 1KHz variation available in environment. The Pt and Al are used as the two electrode plates of the capacitor. An eight-beam-structure after the dimension optimization results in the best performance of the capacitive harvester. The process flow has been developed and the prototype harvester was successfully produced in SAH MEMS research center at Xiamen University.