A vacuum encapsulated resonator for humidity measurement [electronic resource]

Relative humidity sensing is everywhere. Current applications include home appliances, semiconductor manufacturing, air conditioning, medical, automotive, and meteorology. Various technologies are being used to measure relative humidity, including capacitive, resistive and resonant gravimetric sensors. In resonant gravimetric sensors, the quality factor is limited by air damping because the resonator must be exposed to the ambient. The sensor presented in this work decouples the ambient of the resonator from the monitored ambient. Therefore, the sensor achieves a high quality factor (~10,000). This study uses surface resistance of silicon dioxide and a charge-biased, microshell encapsulated, single-anchored double-ended tuning fork resonator (DETF) to measure relative humidity. The vacuum encapsulated resonator is charge biased and electrically connected to a metal bond pad on the surface silicon dioxide, which is exposed to the ambient. The surface resistance of the silicon dioxide decreases as relative humidity increases. Smaller surface resistance leads to faster decay from the resonator. This variation in decay is monitored by measuring the shift in the resonant frequency. As the charge decays, the resonant frequency increases because of the electrostatic spring softening. The relative humidity is determined by the time constant of the decay, or the initial slope. The time constant and the initial slope depend on the relative humidity. In summary, variation in surface resistance causes changes in the charge decay characteristic of the resonator and ultimately, the resonant frequency of the resonator. The sensing principal of this new sensor is expanded upon. The variation of surface resistance with respect to relative humidity and temperature is discussed. Additionally, analytical models are derived for the steady state and transient surface resistance. A charge biased resonator is compared to a voltage bias resonator. The charge biased resonator is more linear than the voltage biased resonator. However, the charge biased resonator has internal feedthrough. The noise sources in the sensor are explored. Equations are developed to convert Allan deviation to limit of detection of relative humidity. Scaling laws are also derived to improve the limit of detection of the sensor. To characterize the sensor, a custom experimental test setup, including environmental chamber and oscillator board, was built. The effect of relative humidity and temperature on the surface resistance was measured. The limit of detection was 1.3·10^-3 % RH when slope fitting is used. Additionally, the limit of detection was 1.4·10^-4 % RH when exponential fitting is used. The rise time of the sensor was one third of the rise time of the commercial capacitive sensor. The measured hysteresis of the sensor was less than 0.15% relative humidity. This is significantly less than the hysteresis of the commercial capacitive sensor. In conclusion, a new relative humidity sensor was developed.

Keywords: Electrical Engineering
Advisor: Howe Roger Thomas primary advisor. Thesis advisor
Kenny Thomas William advisor. Thesis advisor
Solgaard Olav advisor. Thesis advisor