Sensitivity enhancement in micro-electromechanical systems for sensor applications

Micro-mechanical sensors are typically fabricated both in large numbers and economically using the photolithographic processes that were originally developed in the integrated circuit industry. The magnitude of a change in resonant frequency of a micro-me chanical structure can be used to quantify a change in mass of such a device. Hence, when packaged with integrated measurement, actuation and control electronics, it is possible to deliver a low-cost and small system in a package using fabrication techniq ues that are both mature and widely available. A micro-mechanical resonator has been designed for this project and samples of the prototype resonator were used to investigate various methods for detecting a change in resonant frequency using discrete elec tronic components. The system that has been designed can eventually be integrated with a small micro-mechanical structure to create a mass sensor. Resonators have been fabricated at QinetiQ as part of the Europractice Foundry Access Program and characteri sation of typical devices is described in this thesis. A popular method for controlling the behaviour of resonant micro-mechanical sensors is a force feedback technique designed to increase the effective quality factor of the resonant system. In this thesis, an increase in the effective quality factor of the prototype system has been demonstrated. When the resonator operates in air at atmospheric pressure, an improvement in the effective quality factor of two orders of magnitude was achievable. This meant that it was possible to assess the potential benefits offered by the force feedback technique by testing the various detection schemes that have been implemented at the natural quality factor and also at a high effective quality factor. A prototype control system has been built using simple digital electroni cs, a key component of which is a direct digital frequency synthesis chip used to provide a stable and accurate driving frequency. Methods for determining a change in the resonant frequency of a micro-mechanical resonator using this control system have be en investigated. A method has been developed for determining the magnitude of a shift in resonance when the frequency of the excitation force is fixed. This thesis contains a description of the technique and also results demonstrating the corresponding de tection capability of the prototype sensor.