Frequency tuning of silicon carbide microelectro-mechanical resonators and pressure sensors for harsh environments

Zhibang Chen

In this thesis, bidirectional and location-dependent frequency tuning of single crystal 4H silicon carbide (4H-SiC) cantilever and bridge resonators was investigated. 4H-SiC is a superior material platform for microelectromechanical systems (MEMS) operated in harsh environments. A passive tuning method was employed using focused ion beam (FIB) deposition of Platinum (Pt) and milling of Pt and SiC material at different locations on cantilever beams. And Pt deposition was applied to do the frequency tuning of bridges. Linear downward shifts in resonant frequency were observed with Pt films deposited at the free end of cantilever and the center of bridge, which is attributed to the increase in effective mass. Subsequent FIB milling reduced Pt mass and therefore restored the resonant frequency to previous values in cantilever, showing the advantage of FIB for fine-tuning of frequency by precise adjustment of Pt mass. Location dependence of frequency tuning was investigated by measuring frequency shifts when removing SiC material at different locations by FIB milling, which leads to changes in effective mass and effective stiffness. Results show that resonant frequency increases when removing SiC at the free end, and decreases when close to the fixed anchor. These results demonstrate the capability of bidirectional frequency tuning of SiC cantilever resonators. Another work is the design of new capacitive pressure sensor for harsh environment. Based on the simulation results, a higher sensitivity was achieved by the new pressure sensor compared to the traditional pressure structure. The superior material properties of SiC ensure robustness of the new sensor to withstand large-scale pressure at high temperature and in chemical/biological medium. The sensor structure consists of a circular SiC diaphragm suspended by four arms over a SiC substrate. They deflect nearly uniformly with applied pressure. With high sensitivity and operation capability in hostile environment, this new pressure sensor is promising for use in a wide range of applications such as automotive, nuclear station, aerospace, and oil/gas exploration, etc.

Frequency tuning of silicon carbide microelectro-mechanical resonators and pressure sensors for harsh environments

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