This dissertation describes the design and fabrication of a high-resolution capacitive sensor encapsulated in poly-crystalline silicon carbide. This sensor can continuously measure the strain in the substrate under aggressive environments such as extreme ambient temperature, corrosive media and high-g shocks. This strain sensor has a sub-millimeter gauge length capable of precise assessment of strain fields. The device has a novel structural design which performs as a function generator to magnify the applied strain in the desired direction while attenuating the cross-axis strain effect to less than 10% of the readout signal. The Silicon-on-Insulator (SOI) is used to microfabricate the mechanical structure of the sensor using advanced Micro-Electro-Mechanical System (MEMS) fabrication techniques. A conformal and pinhole-free thin-film of Silicon Carbide (SiC) is used to form a passivation layer over the silicon structure. This protective layer imparts inert characteristics of SiC over the outer surfaces of the sensor to function at high temperature and corrosive ambient.
The mechanical structure of the gauge is designed to behave similar to a four-point-bending beam and uses a gap-closing capacitive sensing technique to measure the applied strain. The applied strain is transferred from the substrate to the device layer through the bonding and deforms the middle bending beam to generate the signal. The dimension of the strain-transfer beams, the differential capacitive measurement and the position of sensing pads diminish the effect of cross-axis strain sensitivity to improve the signal to noise ratio. The performance of the gauge is evaluated using the commercial-off-the-shelf (COTS) electronics.
Silicon Carbide (SiC) as structural material has shown reliable and robust performance under various operational conditions. However, the high cost of either wafers or thick film deposition (in orders of μm) of SiC is a major setback against its adoption for the high volume markets. In this work, A Low-Pressure Chemical Vapor Deposition (LPCVD) method is used to form a conformal SiC encapsulation layer over the previously released Si-based strain gauge. The nominally incremental cost associated with the SiC coating process ( ≤ 70 nm thickness) motivates the possibility of the commercialization of the Si-based harsh environment sensors. The SiC passivated strain gauge has survived the corrosive ambient test and successfully operated at 370°C in air and dry steam. The experimental results show the gauge performs with the sensitivity of 45 aF/με over the applied strain range of 1-1000 με. The mechanical bandwidth of the gauge 535 Hz qualifies it for applications such as automotive and aerospace.
December 31, 2008
Jamshidi, B. (2008). Poly-crystalline Silicon Carbide Passivated Capacitive MEMS Strain Gauge for Harsh Environments. (n.p.): University of California, Berkeley.