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

Robert Hennessy rhenn at stanford.edu
Sat Feb 18 11:27:56 PST 2012


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 (10^15 -- 10^20 ?/? 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.

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