Mass Loading Effect of Resonant Structures in Surface Acoustic Wave Devices Suitable for Sensing Applications

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Surface acoustic wave (SAW) devices are widely used in sensors, actuators, filters, and telecommunication systems. Surface acoustic waves are elastic waves launched on a piezoelectric substrate when an electrical signal is applied to metallic comb-like electrodes patterned on the surface of the substrate. SAW devices used in sensing applications extensively employ mass loading effect of a sensing film coated over the surface of the device. Changes in the properties of the sensing film due to the measurand cause variation in mass loading that alters the velocity of the surface wave and results in deviation in the electrical properties of the device, e.g. change in resonance frequency in a SAW resonator. Some other properties of sensing films that can affect SAW velocity are conductivity, permittivity, viscosity, and temperature; hence sensing films can be developed accordingly. In this thesis, mass loading effect of resonant structures (in place of sensing films) has been investigated. High aspect ratio pillars are proposed as the resonant structures to be fabricated on SAW resonators. Finite Element Method (FEM) simulation shows that when the resonance frequency of the pillars is close to the resonance frequency of the resonator, extraordinary changes in mass loading occur. The pillars and the SAW resonator form a system of coupled resonators, and the deviations in the system frequency are large when the resonance frequencies of the two coupled systems are close. The rapid variation in the mass loading characteristics at the resonance frequency can be used to develop highly sensitive sensors. It is observed that the sensitivity with resonant pillars is at least 10 times that obtained by thin film as the sensing medium. FEM simulation is carried out for wide range of pillar dimensions and it is observed that the thinner pillars offer greater sensitivity implying that nano size pillars will be the most desirable sensing medium. As an example, a hydrogen sensor using nano size pillars of palladium as sensing medium has been simulated and the results show high sensitivity for sensing hydrogen at concentrations below 1%. Experiments are carried out by fabricating SAW resonators with SU-8 micro-pillars of varying dimensions. Experimental results and the FEM simulation show typical characteristics of a system of coupled resonators, viz. increase in frequency in certain cases and the state of zero mass loading. The present work is not restricted to any specific sensor; rather introduces a technique for the development of a novel class of SAW sensors. In addition to the highly sensitive sensors, the technique has potential applications in filters and other SAW devices....
Supervisor: H. B. Nemade & Roypaily