On the exploitation of mode localization in surface acoustic wave MEMS for sensing applications

Abstract

Mode localization refers to the spatial trapping of energy in a coupled oscillatory system that occurs when a disorder is introduced into a previously ordered system. This thesis explores the exploitation of this phenomenon in surface acoustic wave (SAW) devices for sensing applications. The sensing application of primary focus within this work is a magnetic field sensor, wherein the strength of mode localization changes in proportion to an external magnetic field. In addition, application as a bio-mass sensor is suggested and briefly discussed. Utilisation of mode localization as a sense mechanism involves the use of changes in the normalised mode shape of a weakly-coupled two degree-of-freedom system as the sensor output. This is in contrast to the use of shifts in frequency, phase or amplitude as is commonplace in resonant micro-electromechanical systems (MEMS) sensor technology. The theory and principles of device operation are introduced utilising a discretised model. In particular, the use of a periodic array to couple the sensors’ two degrees-of-freedom is investigated. A generalised geometry of the SAW device is introduced, consisting of a pair of acoustically-coupled cavities. An analytical solution is found for the displacement fields within the cavities. The solution is achieved by coupling the internal cavity solutions using a ray tracing method. The results of the analytical solution are compared to a numerical solution found using commercial finite element analysis (FEA) software; good agreement is observed. The model is subsequently used to analyse and discuss device performance in the presence of noise; expressions are presented describing device operation and performance, and a case study is outlined evaluating use as a MEMS magnetometer. Finally, the design, manufacture and testing of a prototype design is discussed.