Air loading on ultrathin graphene membranes for microphone application

Abstract

This thesis explores the challenges of using ultrathin graphene membranes in microelectronic mechanical sensors (MEMS) technology, specifically in the development of MEMS microphones. Graphene, being only an atom thin and one of the strongest known materials, offers promising potential for further miniaturization of MEMS microphones. However, the mass loading effect on the graphene membrane can impact the device’s resonance frequency and bandwidth.To address this issue, this study analyzes the dynamic response of the graphene membrane under different pressures and membrane parameters, such as diameter and thickness. The membranes are fabricated by using chemical vapour deposition, after which the membranes are transferred over a cavity. The measuring setup uses a laser Doppler vibrometer to measure the deflection of the membranes and investigate their dynamic response. Pressure-dependent experiments are performed to measure the resonance frequency of the membrane’s response, and the relationship between the radius, thickness, and resonance frequency is explored. By doing these measurements we explore the effect of air loading to investigate the ultimate performance limits of graphene membranes for microphone application.
The experimental results show a clear presence of the air loading effect and align with earlier models of the resonance frequency with mass loading effects. The thesis validates the accuracy and reliability of the measurements and contributes to the body of knowledge surrounding the topic. The limitations of the study include fabrication imperfections and setup difficulties like a limited bandwidth of the piezo-shaker used for the actuation of the membranes. The assumptions made about limited cavity effects are also discussed. Future research could explore the impact of an added backplate with venting holes for capacitive readout. Next to that, the influence of air loading on the system’s damping could be researched.