A MEMS Sensor for Strain Sensing in Downhole Pressure Applications Based on a Double Mass Stricture

Mircoelectromechanical (MEM) resonators have been widely used as sensors and accelerometers as the resonators’ resonant frequencies shift when one of its properties, namely stiffness and mass change. The devices that employ stiffness sensing have been developed in many areas, including pressure sensors, accelerometers and force sensors. The double-mass resonator for pressure sensing has been the focus of many researches in recent year. By introducing the dual mass structure onto the traditional double-end tuning fork (DETF), it has been shown that this type of structural design has: 1) lower natural resonant frequency for easier detection mechanism; 2) improving the Quality factor (Q) due to lower total energy loss. However, the area of stress induction mechanism is under research.This thesis introduces a novel stress induction mechanism to work with the double-mass structure, namely centrally located anchor points on diaphragm. The structure is intended to maximise the engagement of pressure induced stress in generating strain in the resonator while minimise the risk of structural failure in high pressure environment. In addition, I have investigated several practical aspects of double-mass resonator that have not been under intensively researched namely the sensor behaviour in high pressure environment (1000 Bar) and the risk of piezoresistor-on-chip detection mechanism.I also investigated the disadvantage of traditional diaphragm structure. The diaphragm only engages shear stress in induction mechanism. To provide an alternative solution, the novel lateral stress induced structure (LSIS) is proposed. By using the LSIS, it was shown in simulation that the compressive stress can also be engaged in induction mechanism on the same level of magnitude with shear stress in diaphragm structure.Finally, I have simulated the effect of high temperature condition have on resonator stiffness, hence its resonant frequency. Furthermore, based on these simulation result, I have proposed a novel dual double-mass structure, which is capable to be used as temperature compensation mechanism