High Temperature (800°C) MEMS Pressure Sensor Development Including Reusable Packaging for Rocket Engine Applications

For aircraft and rocket engines there is a strong need to measure the pressure in the propulsion system at high temperature (HT) with a high local resolution. Miniaturized sensor elements commercially available show decisive disadvantages. With piezoelectric-based sensors working clearly above 500°C static pressures can not be measured. Optical sensors are very expensive and require complex electronics. SiC sensor prototypes are operated up to 650°C, but require high technological efforts. The present approach is based on resistors placed on top of a 2 mm diameter sapphire membrane (8 mm chip diameter). The strain gauges are made either of antimony doped tin oxide (SnO2:Sb) or platinum (Pt). This material combination allows for matching the thermal coefficients of expansion (TCE) of the materials involved. The morphology of the SnO2:Sb layer can be optimized to reduce surface roughness on the nanometer scale and hence, gas sensitivity. Antimony doping increases conductivity, but decreases the gauge factor. With this nanotechnological knowledge it is possible to adjust the material properties to the needs of our aerospace applications. Tin oxide was shown to be very stable at HT. We also measured a 2.5% change in electrical resistivity at room temperature at maximum membrane deflection. The maximum temperature coefficient of resistivity (TCR) is less than 3.5·10−4 K−1 in the temperature range between 25°C and 640°C. In addition to the device related research work, a novel reusable packaging concept is developed as housing is the main cost driver. After the chip is destroyed the functional device can simply be replaced — housing and contacts can be reused. The MEMS device is electrically contacted with a miniaturized spring mechanism. It is loaded from the harsh environment side into the HT stable metal housing. A cap is screwed into the housing and compresses the inserted seal ring against the chip. The part for electrical contacting on the opposite housing side is not disassembled. The MEMS device is not in direct contact with the housing material, but embedded between two adaptive layers of the same material as the device (sapphire) to decrease thermally induced mechanical stress. Overall weight is 46 g. This packaging concept has been successfully optimized so that the whole assembly can withstand 800°C and simultaneously provides sealing up to 250 bar! After testing in such harsh environment, the small packaging can still be unscrewed to exchange the MEMS device. Due to the reutilization, the packaging can be used far beyond the lifetime of HT MEMS devices.