PhD Defense: A Vacuum Encapsulated Resonator for Humidity Measurement - Wed. 2:15pm Allen 101x

University PhD Dissertation Defense

A Vacuum Encapsulated Resonator for Humidity Measurement

Robert Hennessy
Department of Electrical Engineering
Advisor: Prof. Roger T. Howe

Wednesday, February 22, 2012
2:15 pm (refreshments at  2 pm)
Paul G. Allen Auditorium (CIS-X 101)
http://cis.stanford.edu/directions/


Relative humidity sensing is important in many applications including home appliances, semiconductor manufacturing, air conditioning, medical, automotive and meteorological. Various technologies exist to measure relative humidity, including capacitive, resistive, and resonant gravimetric sensors. For these methods, water must diffuse into a material, which limits the speed of the sensors. Instead, the surface resistance of insulators could be used. But, the resistance cannot be measured using direct measurement techniques because of the high surface resistance (10¹⁵ – 10²⁰ Ω/□ for silicon dioxide).

In our sensor, a resonator is used to indirectly measure the surface resistance of silicon dioxide.  As relative humidity increases, the surface resistance of the silicon dioxide decreases because the thickness of adsorbed water on the surface increases. This decrease in surface resistance leads to faster charge decay from a capacitor. . An electrically connected resonator converts the charge on the capacitor to a frequency via the electrostatic spring softening term of the resonator. Next, an oscillator and counter are used to measure frequency shift over time. Finally, this time-varying shift in resonant frequency is used to determine the relative humidity of the ambient.

To characterize our 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 and the charge decay characteristic of the resonator were measured. Our sensor has 50 times improvement in the minimal detectible signal over commercial sensors. Additionally, our sensor are faster than the commercial sensors.  Finally, the measured hysteresis of our sensor is <0.25% relative humidity.

Potential design improvements will be discussed including modification of the surface with Atomic Layer Deposition (ALD) films to change the surface resistance by modifying the thickness of the adsorbed water. Additionally, ground rings around the bondpads can  reduce the steady state surface resistance and decrease the drift caused by the transient response of the surface resistance. Finally, potential extensions of our sensor to other quasistatic charge measurements, including dielectric conduction, biological sensors, gas sensors and chemical sensors, will be briefly discussed.