| Capacitive sensors with
integrated Faraday Cage: Are specially susceptible to stray
electric/magnetic fields and noise. While a robust
enclosure can protect the sensor it will falsify at the
same time the received signal considerably. This is not
the case with the patented Capacitive Microsensor , which
is self protected in the middle of a Faraday Cage. The sensor outlet is monocrystalline silicon, which is quite suitable for
this function due to its elastic property.
 Through an etching process, a
cavity is etched inside this silicon cell to produce a
highly elastic fatigue free silicon membrane. Basically
the Sensor is built as a MOS-Sandwich
(Metal/Oxide/Semiconductor). The membrane and the base plate,
both made out of silicon, are then fused together on the
outside periphery, forming a mechanical and electrical
entity.
Both sides of membrane and the
silicon base plate of this measuring cell (the cavity),
are covered with an isolating layer of silicon oxide (SiO2),
over which the measuring electrodes are placed,
surrounded all around by silicon. The conducting silicon
builds a Faraday Cage around the measuring electrodes.
Upon request, the outside surface of the silicon could
also be coated to protect against aggressive environment.
The lateral dimensions of the
Sensor are 0,17 inch x 0,20 inch (4,32 mm x 5,08 mm). The total
thickness is 0,056 inch (1,4 mm). Electrodes are placed
in an etched chamber isolated from the silicon. The distance
between the measuring electrodes is 0,0002 inch (5 µm) this will yield a starting
Capacitance "Cx0" of 5 pF. The max
deflection of the measuring electrodes, due to the
loading of the membrane will be less than 0,0002 inch,
therefore a displacement free measurement is guarantied.
By changing the membrane thickness, a measuring range
between < 0,15 PSI to 2200 PSI
(<10 mbar to 1500 bar) could be achieved.
The measuring electrodes are
routed to the outside through the opening between the
membrane and the base silicon plate, where both the
electrodes and the shielded silicon are contacted with
two 0,012 inch (0,3 mm) thick coax cables (three-wires
version). It is also possible to connect the sensor with
coax cable (two-wires version), where the measuring
electrode and the silicon are shorted together. By
comparison, the three wire version, offers a higher
resolution and accuracy at a higher price.
The max resolution
at the moment stands at 0,002% of the max measuring
range.
Through the opening, the chamber
between the two electrodes is exposed to the surrounding
pressure, therefore it acts as a Relative
Pressure Sensor.
An Absolute
Pressure Sensor could
also be built by evacuating the chamber and hermetically
sealing the opening with Solder under vacuum (under
development).
The sensor response is solely
dependent upon its mechanical construction. The measuring
cell itself is not linear. By increasing the pressure
this non-linearity will increase. However if the cell
membrane is deflected only to 60%, the error in linearity
will stay less than 1%.
No temperature dependant
"semi-conductor effect" will happen in the
sensor itself, because the silicon is used as a unitized
crystal and not as a P-N- Conductor. The eventual
temperature changes will only be due to the thermal
stress coefficient. Therefor in most applications , no
temperature compensation is necessary (temperaturecoefficient is less than
0,01 % / Kelvin). And service
temperatures are possible to 350 °C.
The world wide patented sensor
with integrated Faraday Cage, is a product of
many years of trials, before we were able to produce it
in series. (under license)
By using the principle of a
Capacitive sensor, low power electronics can be used for
measurement
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