dc.description.abstract | Acoustic waves consist of longitudinal and transverse elastic waves that propagate along the surface of solids. Surface acoustic wave (SAW) usually generated on the surface of a piezoelectric material using interdigital electrodes as signal transformers. Due to its high sensitivity, reliability, portability, and capability of the room temperature operation, the surface acoustic wave device has become an important component as filters, sensors, and oscillators. In recent years, SAW devices as high sensitivity gas sensors and biological sensors have been proposed. They usually require microfabrication processes that are time consuming and difficulty to implement. In this study, we perform numerical simulations in order to understand the insight phenomena in the SAW sensors and prevent the cumbersome fabrication processes. The purpose of this study is to investigate the effects of microstructured surfaces on the resonant frequencies of SAW sensors. Different structure geometries of micro-columns are considered in this study to investigate the sensing performance, including the distribution density, the height, and the width of the columns. Besides, considering the feasibility in the SAW structure fabrication, the simulations of stepped substrate are also discussed to accounts for the wet etch and dry etch situations. Finally, a simplified model using equivalent pressure force is compared.
In order to derive the surface acoustic wave frequency in detail and realize the different vibration modes, the displacement frequency response is simulated. To compare with the measurements, the insertion loss frequency response is used, which is the ratio of electric field frequency response at the input IDTs to that at the output IDTs. Meanwhile, the simplified analytical solutions are also applied in comparing with the simulated results to estimate the extent for frequency shift and search for the appropriate sensing regions. To verify the validity of the simulation, experiments that show the frequency shifts between unload and load situations via the Network analyzer are performed. The simulated results agree well with the experimental data.
According to the simulation results, the increased active surface area is linear proportional to the frequency shift; however, this relation can only be applied to certain geometric conditions. The simulations of increasing the number of surface structures or its height are performed. Judging from the results, increasing the number of surface structure is slightly more effective than increasing its height in promoting the sensing performances. The former structure fabrication is much robust as well. Considering the stepped substrate, the features of the surface acoustic wave lose if the step height is too large. Besides, it is justified that the pyramidal structure is less sensitive to the load than columns structure.
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