Development of micro and nano resonators for acoustic sensing applications

Abstract

Suspended vibrating structures play a significant role as basic building blocks for mechanical resonators and form the foundation of modern acoustic transducers. The practical use of mechanical resonators is not limited to acoustic technology but also includes a wide range of applications for sensing and actuation purposes. The ultimate goal of this project has been set to realise highly tunable and sensitive resonators that have operating frequencies covering the audible range (20 Hz – 20 kHz). In this thesis, two distinct types of mechanical resonators have been developed, dedicated mainly to hearing assistive devices and acoustic microphones. The overall performance of mechanical resonators is governed by their structural elements design, material properties, and dimensions. Inspired by their unique mechanical properties, a refractory metal of tantalum and a two-dimensional (2D) material of graphene have been utilised as vibrating structural elements for the developed resonators. In the first parts of this project, mechanical resonators of tantalum tunable to audio frequencies have been developed. First, a comprehensive investigation of the influence of fabrication process parameters on the residual stress of tantalum thin-films has been implemented. Based on the residual stress characterisation, an array of suspended microbeams of tantalum has been created and their mechanical static deflection has been investigated. Accordingly, the design and fabrication process of the resonators have been optimised, and hence straight and undeformed free-standing microbeams with lengths of 1 – 3.4 mm have been created and actuated electrostatically. The resonators have achieved a low resonant frequency (1.4 kHz) tuned over the audio range. Unlike the conventional microphones that have their vibrating membranes made of stressed and stiff materials, the graphene-based resonators developed here from ultra-large and thin bilayer membranes have the advantages of possessing enhanced durability and high frequency tuning sensitivity. A simple and reproducible fabrication process has been demonstrated to create millimetric membranes composed of a multilayer graphene and a thin polymeric film. The novelty of the developed resonators lies in the exceptional area to thickness aspect ratios of ~ 10,000, and the implementation of electrothermal actuation to drive the membranes into resonance and tune their resonant frequencies.