High temperature pressure sensing using a resonant-mode sensor

Resonant frequency-mode pressure sensors are presented as a potential solution to high temperature and harsh environment applications. Using frequency instead of amplitude based measurement is intended to decrease temperature sensitivity. Two different types of resonant technologies are explored, an electrical inductive-capacitive (LC) pressure sensor and mechanical resonant strain gage pressure sensor. Because of a simpler design and fabrication, the LC pressure sensor is modeled, fabricated and tested while the mechanical resonant strain gage design is modeled and a proposed fabrication scheme is discussed. Analytical modeling and fabrication schemes are detailed in a dedicated chapter, along with the fabrication challenges and advantages of working with silicon carbide as a MUMS material.;The LC pressure sensor comprises a silicon-glass capacitive pressure sensor paired with a planar spiral inductor. This LC pressure sensor is modeled, fabricated, and tested to determine feasibility for high temperature operation through several design iterations. The final design of the sensor is an LC resonant circuit, in which a capacitive pressure sensor is paired with an inductor to form a LC tank circuit whose resonant frequency varies with applied pressure. Modeling is done using analytical equations and numerical methods such as finite element analysis one the capacitive pressure sensor structure to characterize its mechanical operation. Simple analytical models, empirical equations, and field solver modeling are used to characterize the inductor and capacitor and the complete LC circuit. Testing is done on a chip level using a probe station and network analyzer, and on the device level using simple electrical lest equipment. An oscillator circuit is paired with the LC pressure sensor to obtain and characterize the frequency-mode output using a phase noise measurement to determine the noise floor for various measurement bandwidths. The performance of the LC sensor is compared to commercial state-of-the-art piezoresistive pressure sensors. The latest fabricated prototype in an oscillator circuit shows a pressure sensitivity up to 346kHz/kPa and operates over a range front 110.6 to 42.3 MHz. Temperature sensitivity was observed as a sensitivity change or span shift and measured approximately 2.5%/100°C.