FARADAY SENSOR - TECHNOLOGY
|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.
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
is a Faraday cage? return
Two silicon plates are fused at high temperature to effectively form a Monocrystal and thus a Faraday Cage which shields the measuring electrodes from external interference. It is composed of an electric conducting material, acting as a completely inclosed envelope or cage. The inside of this cage is isolated from all outside electric fields. The outside electric field/noise induces a current which is allowed to flow back to the ground. The cage can be made out of sheet-metal or wire-mesh for example. What is important; it has to be closed on itself. However the Faraday Cage can offer no protection against magnetic fields.