Modeling, fabrication, and characterization of SiC concentrically matched differential capacitance output pressure sensors

This paper describes modeling, optimization, fabrication, and characterization of silicon carbide (SiC) absolute pressure sensors that have concentrically matched differential capacitance output. The differential capacitance scheme is achieved by utilizing a concentric ring and a circular diaphragm to cancel out common-mode noise. The sensors have been fabricated by a surface micromachining process where SiC is chosen as the structural layer to sustain various high temperature environment conditions. Characterization is conducted in a custom-built temperature-controlled pressure chamber up to 180 ℃ that is similar to a low-temperature geothermal well. Results show that under an applied pressure range of 0.5-1.4 MPa, structures with a 160 μm-diameter diaphragm have sensitivities of 1.03 fF/kPa and 0.92 fF/kPa under the temperatures of 20 ℃ and 180 ℃, respectively. On the other hand, the mechanically much stiffer ring structure with the same device area as the circular diaphragm maintains a constant capacitance of 0.14 pF under the same pressure ranges of the two different temperatures. In addition, an array of circular capacitive pressure sensors are also fabricated to obtain higher capacitance outputs. The sensor array is also characterized up to 1.4 MPa and the sensitivities of 13.7 fF/kPa and 13.4 fF/kPa is observed at temperatures of 20 ℃ and 180 ℃, respectively.