Surface Acoustic Wave Devices with Non-Contact Interdigital Transducers Ashish

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Surface acoustic wave (SAW) is a mechanical wave that travels along the surface of an elastic medium. Conventional SAW devices consist of metallic comb-shaped electrodes called interdigital transducer (IDT) patterned over the surface of a piezoelectric substrate. The IDTs convert electrical energy to mechanical energy and vice versa. The IDTs patterned over the piezo-substrate affect the propagation of SAW mainly due to conductivity (electrical load), mass load, dispersion, and other secondary effects. The device characteristics deviate if these effects are not accounted for while designing. In this thesis, a non-contact arrangement of IDT (NCIDT) is proposed where IDTs are fabricated on a non-piezoelectric holding substrate kept above facing the piezo-substrate with a small air gap, and the electric field between NCIDT and the piezo-substrate is coupled through the air gap. As the piezo-substrate is void of IDTs, the effects mentioned above are either eliminated or greatly reduced. For sensor applications, the proposed configuration provides entire piezo-substrate for sensing, in addition to the convenience of easy cleaning or coating of sensing film due to the absence of bonding wires. The SAW devices with NCIDTs are simulated by finite element method (FEM) using COMSOL Multiphysics and the most significant result encountered is that the velocity of SAW in the device is appreciably close to the free surface velocity. However, the coupling decreases as the electric field couples through the air gap between the transducer and the piezo-substrate. Thus the proposed technique has applications in the fields of instrumentation, signal processing and communication where frequency accuracy is crucial. The parameters like SAW phase velocity dispersion, reflection coefficient, and electromechanical coupling coefficient as a function of air gap between NCIDT and piezo-substrate are calculated from the simulation results. The comparison of the proposed devices with the conventional SAW devices is presented based on the device parameters, the transduction coefficient, quality factor, SAW phase velocity dispersion, reflection coefficient, and electromechanical coupling coefficient. The thesis includes a thorough analysis of the proposed SAW devices with NCIDT that investigates the effects of the permittivity of the holding substrate, separation between NCIDT and piezo-substrate, thickness of IDT fingers, metallization ratio, and aperture width.
Supervisor: Harshal B. Nemade