In this paper, we experimentally demonstrate a new approach to improve the output power of electrostatic energy harvester with an electret skin. A symmetric threeports combelectrode mechanism is used to reduce the binding electrostatic force, thereby allowing a smooth mechanical motion of the suspended electrodes of a highaspect ratio in a small acceleration range. Given the same device footprint, two energy harvesters having different aspect ratios are prepared to compare the power generation performances. The output power is increased 6.4 times by increasing the aspectratio from the 7.1 to 33.3. At the same time, the volumetric power density is also improved from 62.5 μW cm −3 to 270.2 μW cm −3 . These results suggest a possibility to further enhance the aspect ratio to shorten the charge time of a large storage capacitor for autonomous internet of things wireless sensor node.
We propose a compensated mesh pattern filling method to achieve highly uniform wafer depth etching (over hundreds of microns) with a large-area opening (over centimeter). The mesh opening diameter is gradually changed between the center and the edge of a large etching area. Using such a design, the etching depth distribution depending on sidewall distance (known as the local loading effect) inversely compensates for the overcentimeter-scale etching depth distribution, known as the global or within-die(chip)-scale loading effect. Only a single DRIE with test structure patterns provides a micro-electromechanical systems (MEMS) designer with the etched depth dependence on the mesh opening size as well as on the distance from the chip edge, and the designer only has to set the opening size so as to obtain a uniform etching depth over the entire chip. This method is useful when process optimization cannot be performed, such as in the cases of using standard conditions for a foundry service and of short turn-around-time prototyping. To demonstrate, a large MEMS mirror that needed over 1 cm 2 of backside etching was successfully fabricated using as-is-provided DRIE conditions.
We develop an equivalent circuit model for a MEMS vibrational energy harvester that uses electrets or permanent electrical charges to generate electrostatic induction currents from mechanical vibrations. An electrode pair of periodically arranged comblike fingers is electrically biased by the built-in potential of an electret, and the distribution of electrostatically induced charges is altered by the relative mechanical motion of the electrodes. The electrostatic force as well as the induction charges are described as a function of the boundary condition and implemented into a multiphysics equivalent-circuit model by using the nonlinear current sources of the simulation software LTspice. As a practical solution for avoiding computational error, we have eliminated the use of polygonal approximations for the conditional analytical model and newly introduced a geometrical modulation function based on sigmoidal functions, by which the analytical model has become mathematically smooth and twice-differentiable with respect to the displacement. The short-circuit waveforms of vibrational energy harvesting are reproduced by simulation and are in good agreement with the experimental results.
We report a design method to enhance the output power of vibrational microelectromechanical system (MEMS) electrostatic energy harvesters by reducing the reactive power that does not contribute to the net output. The mechanism of enhancing the active current while reducing the reactive current is analytically studied using an equivalent circuit model of electret-based velocity-damped resonant-generator. Reduction of the internal parasitic capacitance associated to the contact pads and electrical interconnections significantly improves the power factor and increases the deliverable power. The design strategy is applied to an actual device that produces 1.3 mW from the vibrations of 0.65 G (1 G = 9.8 m s−2) at 158 Hz, suggesting a 2.9-fold enhancement of output power by increasing the buried oxide layer thickness from 1 µm to 3 µm.
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