Abstract
Micro-electro-mechanical systems (MEMS) are integrated mechanical and electrical
elements realised with micro-fabrication technology and employed as sensors and actuators. The integration of reliable MEMS switches and resonators into transceiver
devices is a challenging and attractive solution to increase the efficiency and reduce the
power consumption. Silicon carbide (SiC) is an excellent candidate for developing robust and reliable high frequency MEMS for transceivers applications due to its unique
mechanical properties.
This thesis presents the design, fabrication and characterisation of 3C-SiC micromechanical vertical resonators. New device architectures have been developed for the
study of the electro-mechanical behaviour of the devices with the aim of optimising
the actuation efficiency, increasing the resonant frequency and obtaining new device
functions.
A process for the fabrication of single or poly-crystalline 3C-SiC cantilevers, bridges
and rings has been developed with the option of integrating top electrodes made of
aluminium (Al) or lead zirconium titanate (PZT). The crystal structure and quality
of the SiC layers have been evaluated with X-ray diffraction and Raman spectroscopy.
A Young's Modulus of ~ 440 GPa has been calculated for the single crystalline SiC
from the mechanical resonant frequency of the fabricated single material cantilevers.
The fabricated Al/SiC bridges and rings have been actuated and driven into resonance electro-thermally. It has been found that wide Al electrodes applied close to the
beams' anchor can maximise the induced displacement and vibration amplitude thus
improving the actuation efficiency. Resonant frequencies in the MHz range have been
obtained with the ring architectures therefore achieving higher frequencies compared
to beam architectures. In addition, electro-thermal mixing of two input frequencies has
been demonstrated and performed with the fabricated Al/SiC structures. Furthermore,
piezo-electric transduction has been used for actuating the PZT/SiC cantilevers and
for sensing the devices' resonance electrically. The design of the PZT piezo-electric
active layer has been shown to influence strongly the devices' resonant frequency and
has been optimised to enhance the electrical output by decreasing the electrodes length
thus decreasing the feedthrough capacitance.
The results obtained in this work can be used for the implementation of SiC MEMS
mixer-filters with electro-thermal actuation and piezo-electric sensing for transceiver
applications.
