In this thesis, the design, fabrication and characterisation of graphene
electromechanical resonators have been presented. Graphene features ultrahigh
Young’s modulus and large surface to volume ratio that make it ideal for radio
frequency (RF) components, sensors and other micro/nano-electromechanical
systems (MEMS/NEMS).
A novel batch fabrication process for graphene electromechanical resonators has
been developed by using poly-Si film as sacrificial layer. Previously reported
fabrication processes of graphene resonators use SiO2 as sacrificial layer only
because graphene is visible on 300nm SiO2/Si substrate. However, the wet etching of
SiO2 involves HF, which is not compatible with metal connections or SiO2 serving as
dielectric or passivation layer in graphene NEMS devices. Moreover, the liquid
surface tension during drying after wet etching could damage graphene bridges even
critical point drying is used. Therefore, in this work, poly-Si is adopted as the
sacrificial material. To facilitate the fabrication of graphene resonators, a
poly-Si/SiO2/Si substrate has been designed and optimised to make graphene visible
under optical microscope for the first time to the author’s knowledge.
Chemical vapour deposition (CVD)-grown monolayer graphene sheet has been
transferred onto the optimised poly-Si/SiO2/Si substrate and patterned into strips.
Metal electrodes have been deposited by lift-off process to make electrical
connections, which is prerequisite for integrating graphene resonator into practical
devices. The graphene bridges have been released by etching the poly-Si layer with
XeF2 in vapour phase, which completely avoids the capillary force induced damage
to the graphene bridges. De-fluorination process has been performed by hydrazine
reduction to recover graphene’s conductivity. This fabrication process is scalable for
massive production of graphene electromechanical resonators, thus furthering their
practical application.
One-source current mixing characterisation setup has been constructed to test the
graphene resonators. Besides the fundamental peak, the activation and enhancement
of the second mode of doubly clamped resonator by electrostatic actuation have been
observed for the first time. The second mode amplitude reaches 95% of the
fundamental mode, whereas only odd higher modes of small intensity have been
reported before in other MEMS/NEMS resonators actuated by electrostatic force or
magnetomotive force. The findings in this thesis could lead to substantial increase of
the sensitivity of sensors based on the graphene resonators. Modal analysis based on
Euler-Bernoulli equation has been performed to understand the mechanism behind
the activation and enhancement of the second mode. Finite element analysis agrees
very well with experimental results and complies with the theoretical model.
Finally, a set of novel alignment marks has been designed, which can be incorporated
to process mechanically exfoliated 2D material flakes of micron size and irregular
shape with conventional photolithography tools, as have been demonstrated by the
successful fabrication of a graphene transistor. This optical alignment technique
provides an alternative for prototype device development besides electron beam
lithography to prevent electron-induced damage to fragile 2D materials