MEMS pressure sensor chip with a chip-level overpressure block
In some practical situations pressure sensors are subject to unforeseen oscillations, hydraulic pressure surges (water/fluid hammer) that may bring pressure far above nominal value and destroy the sensor diaphragm. The most efficient protection against this is the one built directly into the sensor chip and, like the chip itself, fabricated by MEMS technology. This protection limits the displacement of the diaphragm and keeps it below the braking level under extreme changes of external pressure. We developed a new generation of piezoresistive pressure sensors with a built-in overpressure protection in the form of a micromachined inset placed below the diaphragm and serving as a limiter against excessive displacement. Under a pressure surge far exceeding the nominal value diaphragm reaches the top surface of the inset and leans on it, thus preventing diaphragm breakage. The inset is processed by MEMS procedures by etching in 30% KOH aqueous solution. The inset, as well as the pressure sensor itself, is bonded to Pyrex glass substrate. The gap between the bottom surface of the diaphragm and the top surface of the inset must be accurately adjusted in dependence on a particular measurement range. Using such an inset, the allowed overload of the sensor diaphragm is up to several hundred times larger than the nominal measurement range.