Hi Everyone,

We are designing a Capacitive shear stress sensor out of Silicon which is
used in environments with high flow speeds, and varying temperature. The
device is fabricated in the top layer of an SOI wafer. During testing, the
capacitance changes from the shear stress of the fluid moving over it,
however it also changes from a varying temperature. Right now we are using
an absolute capacitance measurement which includes the comb drive (using air
as the dielectric) and parasitic effects from the substrate (SiO2 is the
dielectric). The capacitance measurement is made with the Keithely 590CV,
using a 100kHz test signal, and it assumes a resistance and capacitance in
parallel.

I am attempting to estimate how much the capacitance measurement should
change from only temperature changes with no flow applied. These changes
would be a result of thermal expansion and changes in the permittivity. I
have been able to find very little on how the permittivity of air and SiO2
vary with temperature and pressure.

>From Capacitive sensors (Baxter), I found the following values for
dielectric coefficients of air:
moist air ( 6.8 ppm / psi , and 7 ppm /  degC)

I have only found one source which quotes any values of SiO2 and that was
for sputtered SiO2 which was -8.5ppm / K.

Any help would be much appreciated.

Thanks,
Andrew O'Grady
andrew.ogrady@gmail.com
Columbia University - Mechanical Engineering Dept.
New York, NY




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