Fabrication of 2d Material Based Resonators

Improvements in manufacturing processes inspired by the semiconductor integrated circuit industry have seen a sharp reduction in dimensions of microelectromechanical systems (MEMS), leading to the emergence of its submicron counterpart – nanoelectromechanical systems (NEMS). NEMS resonators, nano-scale vibrating structures, have proven to be extremely sensitive sensors of physical phenomena which affect its resonance behaviour. The arrival of graphene and other 2D materials with ultrahigh surface to volume ratio, ultralow mass and a diverse range of superior electrical and mechanical properties marked an important moment in NEMS resonators’ history as the reality of having extremely sensitive NEMS resonator based sensors seems imminent. Although a fair amount of research has demonstrated the potential of 2D materials for NEMS resonators, the employed fabrication methods either are not scalable or involve too many fabrication steps. An efficient and mass-reproducible fabrication process for 2D material based NEMS resonators has been successfully demonstrated. This fabrication method is predicted to be compatible with any 2D materials although graphene was chosen for availability and cost purposes. Metal electrodes for electrostatic actuation and detection are lithographically patterned on a silicon wafer with wet thermal oxide which was then diced into small substrates to host graphene. Graphene was transferred using a wet-transfer method on these electrode chips before being optimally patterned with photolithography and oxygen plasma etch. Freestanding graphene beams were obtained by wet etching the underlying oxide and critical point drying. Compared to the existing large-scale fabrication methods of 2D material based NEMS resonators, suspended graphene is clamped on top of metal electrodes fabricated on the wafer level, resulting in a smaller number of fabrication steps and minimal exposure of graphene to sacrificial layers. The fabricated devices were electrically characterised and the graphene’s sheet resistances were derived and benchmarked against those found in literature. The characteristic bipolar field effect transistor (FET) behaviour of graphene was confirmed. The generic fabrication approach demonstrated in this thesis holds great promise for 2D material based resonators being fabricated with high reproducibility and scalability.