PhD Defense: A Vacuum Encapsulated Resonator for Humidity Measurement - Wed. 2:15pm Allen 101x
rhenn at stanford.edu
Sat Feb 18 11:27:56 PST 2012
University PhD Dissertation Defense
*A Vacuum Encapsulated Resonator for Humidity Measurement*
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)/
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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