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 for improving the linearity of the sensor output at the die level were put forward: (1) linearisation by placing extra piezo-elements on the pressure sensing diaphragm that create an output proportional to the nonlinear membrane stress; (2) linearisation by means of capacitive sensing. The solution with extra piezo-elements was fabricated and experimentally verified. The optimal position of the extra elements was identified to be the 'neutral' position on top of the diaphragm, i.e. the position where the bending stress crosses from tensile to compressive stress. A ten-fold improvement of the sensitivity-over-nonlinearity (S/NL) figure of merit of the compensated over the original sensing bridge was demonstrated. The operational temperature of piezoresistive pressure sensors based on conventional diffused or ion-implanted piezoresistors is limited to 125 °C due to junction leakage. To overcome the excessive leakage currents in these type of sensors at temperatures higher than 125 °C, the piezoresistors were isolated from the substrate by a buried oxide using the silicon-on-insulator (SOI) topology. The design and fabrication of a sub-mm piezoresistive pressure sensor based on a two times stacked SOI or BiSOI structure was elaborated for aerodynamic probe application up to 550 °C. The second buried oxide took the function of etch-stop layer to accurately control the thickness of the pressure sensing diaphragm. The proposed pressure sensor had outer dimensions of 1000 μm by 400 μm by 100 μm such that integration in an aerodynamic probe with a tip diameter smaller than 1 mm would be feasible. The size, position and shape of the SOI piezoresistor were optimised for maximum sensitivity. Also it was demonstrated by finite element modelling that the pressure sensitivity of transversal SOI piezo-elements could potentially be increased two-fold by the introduction of stress augmenting beams around the isolated piezoresistor on top of the silicon diaphragm. Important microfabrication steps including the dry etching of the SOI piezoresistors and the high temperature metallisation were investigated and optimised. For high spatial resolution and minimal blockage artefacts, the sensor should be as small as possible. When decreasing the in-plane size of the pressure sensing diaphragm of piezoresistive sensors below 100 μm, the piezoresistors are required to have sub-micron dimensions in order to maintain sensitivity, presenting limits in their proper alignment and manufacturability. With the aim of accomplishing sensors with sub 100 μm diaphragms, fiber-optic sensing was investigated. Fabry-Pérot cavity sensors fabricated directly on a cleaved fiber end are attractive because of their small size (as small as the diameter of the optical fiber) and high temperature capabilities due to their monolithic nature. However past co-axial configurations presented a non-ideal positioning of the membrane with respect to the gas flow when inserting the sensor in the small spaces between the stator and rotor blades of an aircraft engine. Therefore, a novel sensor with the pressure sensing diaphragm perpendicular to the axis of the fiber (cross-axial configuration) was developed. The sensor was fabricated by thin film deposition techniques and focused ion beam (FIB) microfabrication. A modulation sensitivity of 30%/bar of the reflected optical power of the FP cavity was demonstrated.status: publishe