Zero-group-velocity acoustic waveguides for high-frequency resonators

Abstract

The propagation of the Lamb-like modes along a silicon-on-insulator (SOI)/AlN thin supported structure was theoretically studied in order to exploit the intrinsic zero group velocity (ZGV) features to design electroacoustic resonators that don't require metal strip gratings or suspended edges to confine the acoustic energy. The ZGV resonant conditions in the SOI/AlN composite plate, i.e. the frequencies where the mode group velocity vanishes while the phase velocity remains finite, were investigated in the frequency range from few hundreds of MHz up to 1900 MHz. Some ZGV points were found that show up mostly in loworder modes. The thermal behaviour of these points was studied in the -30 to 220 °C temperature range and the temperature coefficients of the ZGV resonant frequencies (TCF) were estimated. The behaviour of the ZGV resonators operating as gas sensors was studied under the hypothesis that the surface of the device is covered with a thin polyisobutylene (PIB) film able to selectively adsorb dichloromethane (CH 2 Cl 2 ), trichloromethane (CHCl 3 ), carbontetrachloride (CCl 4 ), tetrachloroethylene (C 2 Cl 4 ), and trichloroethylene (C 2 HCl 3 ). at atmospheric pressure and room temperature. The sensor sensitivity to gas concentration in air was theoretically estimated for the first four ZGV points of the inhomogeneous plate. The feasibility of high-frequency, low TCF electroacoustic micro-resonator based on SOI and piezoelectric thin film technology was demonstrated theoretically.[5, 10], poly methyl methacrylate [11] and metal plates bonded with submillimetric glue layer [9]. Although the literature concerning ZGV Lamb modes is rather extensive, to the authors' knowledge, only a few studies consider the topic of ZGV electro-acoustic resonators on thin suspended acoustic waveguides: in the references [12 ,13] the first symmetric Lamb S 1 mode ZGV resonator based on a c-AlN thin suspended membrane (2.5 µm thick and wavelengths of about 8.0 -8.8 µm) is theoretically studied and experimentally verified. The impact of the free edges shapes (flat or biconvex) of the piezoelectric suspended rectangular membrane, the tether-to-plate angle (90º or 59° for "butterfly" shaped plate), electrode configurations, materials, and thicknesses on the performances of the AlNbased Lamb wave resonators have been investigated in references [14 -17]. As opposed to the ZGV resonators, the resonators based on a piezoelectric thin suspended membrane with free edges require a more complicated micromachining technology to acoustically isolate the resonator and mechanically couple it to the substrate by two anchors. Indeed, the ZGV points are associated with an intrinsic energy localization: the energy confinement is a natural consequence of the selected acoustic mode thus reducing the technological complexity with respect to that required by the free edges resonators. The characteristics of the ZGV resonators, such as their phase velocity, resonant frequency and Q factor, are affected by the properties of the layer materials, the thi...

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