Miniature Pressure Sensors for the High Temperature Domain

In this thesis, the design of micromachined pressure sensors for application in fast response aerodynamic probes in turbomachinery is investigated. The requirements for pressure sensing in avionics are stringent. Sensors should be able to operate at high temperatures (> 150 °C) and under high mechanical vibrations. The accurate probing of important gas flow parameters in turbines and compressors such as the dynamic and static pressure, the gas flow angle, the level of turbulence and temperature can still be considered a serious engineering challenge. To avoid obstruction of the gas flow during measurements, the sensor should have sub-mm dimensions. In order to resolve secondary flow effects (shock waves and vortices), the bandwidth of the total probe setup should be higher than 100 kHz. As current micromachined pressure sensors are not optimised for turbomachinery measurements (size constraints, packaging issues and thermal requirements), there is a need for custom pressure sensors. Two routes have been explored for this application: a differential piezoresistive silicon-on-insulator (SOI) pressure sensor and an absolute Fabry-Pérot (FP) pressure sensor at the tip of an optical fiber. For the development of sub-mm pressure sensors with a high pressure resolution, a solid understanding of the effects of diaphragm dimensions and temperature on the final sensitivity and linearity are essential. Despite the high volumes of piezoresistive pressure sensors sold each year, a good insight in the contributing factors of nonlinearity in these sensors was not yet available in literature. Therefore, a rigorous analytical and experimental study was performed. Sensor statistics were gathered for three different splits of piezoresistive sensor dies: square versus circular diaphragms, thin versus thick diaphragms and relative versus absolute sensors. Three causes of linearity error were identified: mechanical nonlinearity due to strain stiffening and membrane stress, the nonlinearity of the piezo-effect itself and nonlinearity introduced by the bridge circuit in the electrical domain. Thanks to the opposing nonlinearity of the longitudinal and transversal piezoresistors in the pressure sensors under test, the linearity error of the total bridge output was observed to be much lower than the linearity error of the single piezoresistors. A large deflection model was developed which predicted the bridge output and the nonlinearity with reasonable accuracy. Two novel methods f