Band gap measurement of Si-based phononic crystals using layered SFIT

In recent years, there are studies aimed at phononic crystals, both experimentally and theoretically. However, the smallest scale of the phononic structures considered is in the millimeter scale and the frequency is limited in the MHz range. For the purpose toward the applications of phononic crystals to micro electromechanical system (MEMS) related components, it is necessary to reduce the lattice size to micrometer or even in nanometer scale. In order to achieve the objective, the band gap width of surface waves of micrometer scale phononic crystals has to be measured and compared with the theoretical calculation such as plane wave expansion (PWE) method. Moreover, for further integrating with the complementary metal-oxide semiconductor (CMOS) processing techniques, silicon is chosen to be the base material of the two dimensional phononic crystals in this thesis. Therefore, a high frequency wide-band surface acoustic wave (SAW) filter on a silicon substrate is needed to carry out this band gap measurement.
The general used wide-band SAW filter is the slanted finger interdigital transducer (SFIT). Nevertheless, since the silicon is not piezoelectric materials, the layered structure SFIT/ZnO/silicon is thus considered in this thesis. For the layered structure, the dispersive relation is calculated by the effective permittivity approach, and the frequency response of layered SFIT can then be simulated by the coupling-of-modes (COM) model. Since the actual parameters of a layered SFIT are different slightly from the designed parameters, the simulated frequency response has to be modified. Results show that the modified simulations are in good agreement with experimental frequency responses.
Finally, the band gap width and frequency range of 2-D air/silicon phononic crystals in micrometer-scale are successfully measured by the layered SFIT, and this experimental measurement agrees with the theoretical prediction by PWE method.