Development of graphene-based microelectromechanical systems for acoustic sensing
Graphene has been considered to be a desirable material in the application of semiconductor devices for the next generation due to its outstanding electrical and mechanical properties. In this thesis, the research focuses on the realization of graphene-based acoustic microelectromechanical systems (MEMS). The applications of acoustic MEMS include microphones, hearing aids and ultrasound identification and non-contact testing. Apart from acoustic technology, the graphene-based MEMS designs can be applied in areas for sensing and actuation purpose, such as pressure detectors, micro-drums and ultrasensitive mass sensors. The performance of the devices is determined by the structures of devices, materials properties, dimensions, and the spacing between the membranes and the substrate. In this project, for the first time, the resonant frequency of graphene-based acoustic sensors has been extended to lower ultrasonic frequency range (20 kHz to 200 kHz). Additionally, a modified dry transfer method with Kapton tape and a novel graphene transfer method with silicon dioxide sacrificial layer have been developed for millimetre-size graphene membranes. To be more specific, three types of devices′ structures, including open cavity, closed cavity and partly open cavity, have been developed, in order to detect the frequency for both audio and ultrasound range (from 11 kHz to 200 kHz). 450 nm polymethyl methacrylate (PMMA) layer has been laminated onto 6-layer graphene to support and form millimetre-size bi-layer membrane. The open cavity resonator for ultrasound sensing has been fabricated with graphene wet transfer process. For closed cavity resonators, a modified dry transfer method with the use of Kapton tape frame has been developed. Using the modified dry transfer method, it is the first time that the millimetre-size graphene/PMMA have been transferred and suspended over the closed cavity. Due to good gas encapsulation of graphene/PMMA closed cavity devices, the vibration of membrane has been prevented due to the air damping when the air gap is decreasing. For the purpose of increasing the capacitance between membrane and substrate and improving the electrical output signal, the air gap should be optimized and decreased. Thus, the partly open structure has been designed for the realization of the graphene/PMMA electrostatic sensors. The graphene/PMMA membrane has been released by etching silicon dioxide sacrificial layer. The air gap of 2 μm of between the millimetre-size graphene-based membrane and the substrate has been achieved for the first time and reported to be minimum among the literature. Furthermore, the dynamic behaviour of the devices have been characterized with laser Doppler Vibrometer (LDV), the confirmation of graphene has been detected by Raman spectroscopy. Finite element analysis has been applied for the simulation of membranes′ dynamic behaviour. The static deformation of graphene after modified dry transfer method has been measured by white light interferometry (WLI). The realization of graphene/PMMA acoustic devices paves the way to the integration of graphene with MEMS to achieve sensors with high sensitivity.