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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. |
