A micro capacitive pressure sensor with two deformable electrodes : design, optimization and fabrication

Abstract

Work reported in this thesis proposes a novel micro capacitive pressure sensor for detecting a small pressure variation over a large constant load. To meet various measurement requirements in practice, this dissertation assesses the evolving structure and performance of the proposed sensor, from the perspectives of computer simulation, parameters optimization, fabrication and testing, etc. The proposed sensor fundamentally consists of a sealed chamber with a rigid substrate, and two movable diaphragms which will deform under applied pressure. Simulation experiments have been conducted to identify the theoretically sensor performance. Specifically, mechanical deformation of sensing diaphragm is simulated relied upon a Finite Element Method, and the geometric data of the deformed diaphragm is then imported into an integration method to estimate changes in the capacitance. Modelling results indicate that the deformation of a thick sensing diaphragm could be magnified after it comes into contact with a thin cantilever middle diaphragm, and thus the sensitivity could be improved by 1364% after the onset touch point. Compared to conventional parallel plate capacitive pressure sensors, the proposed sensor has more structural parameters so the task of selecting the various structural parameters is more complex. Based on the FEM simulation results, relationship between the structural parameters and sensor performance have been discussed and a graphical method has been proposed for sensor design. The feasibility of using evolutionary algorithms to optimize the structural parameters is also investigated. First, an analytical model of the proposed sensor that can be conveniently used to evaluate the fitness of the candidate solutions is first constructed using plate theory. The deflection model of the sensing diaphragm is based on energy method in order to consider the effects of internal stress. Theory of plate deflection is then used to model the deflection model of the cantilever middle plate. Results demonstrate that the accuracy of the analytical model is within 3% of the finite element approach. The analytical model is then combined with a Multi-Objective Evolutionary Algorithm package to optimize the sensor structure. After constraining the search space to satisfy fabrication limitations, an optimal structure that provide 65:8% improvement in sensitivity over a graphical design method is evolved. Finally, the concept of using mechanical amplification to improve device sensitivity is investigated experimentally. The proposed device is fabricated by forming the cantilever middle plate on a SOI wafer using surface micromachining technology, bulk micromachining a pyrex wafer to active mechanical amplification, before forming a sealed chamber using anodic bonding. Using a hydrostatic pressure system, a probe station and capacitance measuring instruments, the device is characterized. Experimental results demonstrate that the proposed device is able to provide enhanced sensitivity to small pressure fluctuations in the presence of a relatively large ambient load. The experiment done on a MS3110 measurement board is also presented to find the possibility of converting capacitance change to voltage output.