A MEMS Microphone Using Repulsive Force Sensors

Abstract

We present a MEMS microphone that converts the mechanical motion of a diaphragm, generated by acoustic waves, to an electrical output voltage by capacitive fingers. The sensitivity of a microphone is one of the most important properties of its design. The sensitivity is proportional to the applied bias voltage. However, it is limited by the pull-in voltage, which causes the parallel plates to collapse and prevents the device from functioning properly. The presented MEMS microphone is biased by repulsive force instead of attractive force to avoid pull-in instability. A unit module of the repulsive force sensor consists of a grounded moving finger directly above a grounded fixed finger placed between two horizontally seperated voltage fixed fingers. The moving finger experiences an asymmetric electrostatic field that generates repulsive force that pushes it away from the substrate. Because of the repulsive nature of the force, the applied voltage can be increased for better sensitivity without the risk of pull-in failure. To date, the repulsive force has been used to engage a MEMS actuator such as a micro-mirror, but we now apply it for a capacitive sensor. Using the repulsive force can revolutionize capacitive sensors in many applications because they will achieve better sensitivity. Our simulations show that the repulsive force allows us to improve the sensitivity by increasing the bias voltage. The applied voltage and the back volume of a standard microphone have stiffening effects that significantly reduce its sensitivity. We find that proper design of the back volume and capacitive fingers yield promising results without pull-in instability.

Modeling and Characterization of a Pull-in Free MEMS Microphone

“…They also reveal that the acoustic sensitivity increased up to a specific bias voltage, which suggests that there is an optimum bias to achieve the highest acoustic sensitivity. However, our earlier studies proposed that the acoustic sensitivity improved as bias voltage changed from 40 to 100 Volts [13], [14]. To explain the difference between the two studies, we need to investigate electrical and mechanical sensitivities individually.…”

“…However, these studies mostly exploited the mechanical response of these devices when used for actuation. Besides, we presented a numerical study [13] and preliminary experimental work [14] on a MEMS microphone design using the same electrostatic levitation design. The numerical study was a feasibility investigation on the design and fabrication parameters of the microphone.…”

Design Approaches of MEMS Microphones for Enhanced Performance

“…Second, the bias voltage is not required to pull the diaphragm to the back plate, and third, the stiction issues between the membrane and back plate can be prevented. Due to these advantages, researchers have proposed some designs with no back plate [140,[142][143][144][145][146]. In [142], the authors have proposed a design with in-plane gap-closing sensing electrodes to detect acoustic pressure.…”

“…The microphone was further improved in [144] by using a polysilicon trench-refilled process with a high aspect ratio (HAR) sensing electrodes. In [146], the authors also proposed a capacitive finger microphone. To avoid pull-in instability between the membrane and the substrate, this membrane was biased by repulsive force instead of attractive force to achieve higher sensitivity by increasing the bias voltage.…”