An Electret-Augmented Low-Voltage MEMS Electrostatic Out-of-Plane Actuator for Acoustic Transducer Applications

Abstract

Despite the development of energy-efficient devices in various applications, microelectromechanical system (MEMS) electrostatic actuators yet require high voltages to generate large displacements. In this respect, electrets exhibiting quasi-permanent electrical charges allow large fixed voltages to be integrated directly within electrode structures to reduce or eliminate the need of DC bias electronics. For verification, a − 40 V biased electret layer was fabricated at the inner surface of a silicon on insulator (SOI) structure facing a 2 μm gap owing to the high compatibility of silicon micromachining and the potassium-ion-electret fabrication method. A − 10 V electret-augmented actuator with an out-of-plane motion membrane reached a sound pressure level (SPL) of 50 dB maximum with AC input voltage of V i n = 5 V pp alone, indicating a potential for acoustic transducer usage such as microspeakers. Such devices with electret biasing require only the input signal voltage, thus contributing to reducing the overall power consumption of the device system.

Review of Recent Development of MEMS Speakers

“…To generate considerable SPLs and balance the above-mentioned constraints, several approaches in terms of device structure and driving voltage have been applied in the development of electrostatic MEMS speakers [ 9 , 38 , 90 , 91 , 92 ]. One approach to improve SPL is to use multiple speakers, i.e., array structures.…”

“…To reduce or eliminate the DC bias of electrostatic MEMS speakers, electrets embedded with quasi-permanent electrical charges have been integrated within the electrode structures. In 2020, Sano et al presented an electret-augmented electrostatic MEMS speaker and demonstrated its sound generation under low driving voltages [ 92 ]. The schematic and an SEM image of the fabricated MEMS speaker are shown in Figure 15 e,f, respectively.…”

“… Electrostatic MEMS speakers with special designs: ( a ) optical image of a MEMS speaker array (Reproduced with permission from IEEE [ 91 ]), ( b ) schematic and ( c ) optical image of a MEMS speaker with in-plane bending electrostatic actuators working in air chambers (Reproduced with permission from Nature Portfolio [ 9 ]), ( d ) schematic of a peripheral electrode configuration (Reproduced with permission from IEEE [ 38 ]), ( e ) schematic and ( f ) the corresponding SEM image of an electret-augmented MEMS speaker (Reproduced with permission from MDPI AG [ 92 ]). …”

Robotic micromanipulation: a) actuators and their application

“…Despite the development of energy-saving devices in various fields, MEMS electrostatic actuators still require high voltages to produce large displacements [119]. Electrostatic scratch drive actuators provide low-power, high force actuation but require power supplied by RF [120] or substrate [121] sources.…”

Negative-charge-storing mechanism of potassium-ion SiO2-based electrets for vibration-powered generators

“…5 Recently, a vibration-powered microelectromechanical generator with a potassium-ion electret has attracted increasing attention. [6][7][8][9][10][11] A potassium-ion electret can be fabricated by adding potassium atoms to amorphous silica (a-SiO2) and then subsequently removing them. A potassium-ion electret can permanently store a negative charge, and this enables the creation of maintenance-free power-generation devices.…”