Leveraging the superior properties of silicon carbide (SiC) for harsh environment and demanding applications, a series of SiC capacitive pressure sensors have been developed and characterized for use up to 550°C. To address the related high-temperature-capable metallization need, a thermally stable Ti/TaSi2/Pt Ohmic contact was developed for heavily-doped polycrystalline 3C-SiC films used in the construction of the pressure sensors. These contacts showed very good linearity with specific contact resistance as low as 5.7´10-6 Ω-cm2 without thermal annealing following metal sputtering. The metallization demonstrated robustness after being subjected to an aging test in air at 550°C for 1000 hours. Specific contact resistance remained stable in the range of ~10-5 Ω-cm2.
A finite element analysis tool was used to model diaphragm displacement and study the thermal mismatch stress effect on sensor behavior for different substrates. The simulation confirmed that dielectric constant variation with temperature, rather than thermal expansion mismatch stress, dominated the capacitance change. SiC Sensors were fabricated on both SiC and Si substrates, and characterized at room temperature and 500°C. Both sets of sensors demonstrated similar performance, which were in agreement with the simulation results.
The capacitance versus pressure behavior of fabricated capacitive SiC pressure sensors were characterized from 2.1 MPa (300 psi) to 8.3 MPa (1200 psi) and for temperatures up to 550°C. The measurement results were found to be in agreement with the analytical calculations.
Comparison tests were also performed for sensing capacitance gap sizes and operating modes (i.e., small deflection and touch mode of the diaphragm). Sensors with 0.5 μm sensing capacitance gap showed improved sensitivity, while allowing for smaller diaphragms for a given pressure. With respect to operating modes, contact mode sensors exhibited superior performance to small deflection mode sensors in terms of sensitivity, nonlinearity and resolution. Thus, for a single sensor on a SiC substrate with 0.5 μm gap and 70 μm diaphragm radius, a repeatable performance of 0.06 fF/Pa (0.39 pF/psi) sensitivity and 2.0% nonlinearity was obtained at 550°C for contact mode operation.