Micromechanical pressure sensors are used in machine construction, process measurement engineering, automotive engineering, and in medical technology for numerous measuring tasks. Special measures must usually be taken for use in a corrosive measuring environment in order to ensure functionality of the sensor. In addition to the micromechanical sensor structure, it is also necessary in particular to protect the electrical circuit elements for signal detection and the connections for signal pickup, and for power supply. It is known to use gels to protect exposed metal surfaces such as bond pads, for example. In the case of pressure sensors which are used in the exhaust gas region of motor vehicles, lime-filled gels, for example, are used. The lime neutralizes the acidic exhaust gases, thus preventing corrosion of the nonprecious metal surfaces. Another option for protecting the sensor element from harmful environmental influences lies in complex packaging. Corrosion of circuit parts as well as deposition of dirt particles on the sensor structure may be prevented in this way.
Sensor elements which operate according to the piezoresistive principle are often used in practice for detecting differential pressure. These sensor elements generally include only one sensor diaphragm, which may be acted on both sides by the two measuring pressures to be compared. If there is any dependency at all of the measuring results on absolute pressure, in this measuring system it is very slight, since both measuring pressures act on the same sensor diaphragm. The resistor elements for signal detection are generally situated in the regions of maximum expansion of the diaphragm surface, so that without additional protective measures they come into contact with one of the two measuring media.
In addition, sensor elements for detecting differential pressure are known which operate capacitively. In this measuring concept it is absolutely essential to prevent penetration of dirt particles into the sensor structure, since such soiling distorts the measuring results. For this purpose, the electrodes of the measuring capacitor or the interspace between the measuring electrodes within the sensor structure is/are hermetically sealed so that they do not come into contact with any of the measuring media. As a result, the construction of these sensor elements is very robust, but as a rule is much more complex than that of sensor elements which operate according to the piezoresistive principle.