This letter reports on the design and experimental demonstration of a microscale fractal-like phononic bandgap (PBG) structure in aluminum nitride (AlN). The micro-fabricated fractal phononic crystals (PnCs) exhibit two frequency stop bands for symmetric lamb waves in the Γ-Χ direction centered about 900 MHz (bandwidth of 11.1%) and 1.10 GHz (bandwidth of 9.1%) with maximum acoustic rejection of 40 dB. Differently from the conventional phononic bandgap designs, the unit cell consists of a center air square scatterer with four side air square scatterers repeating at its corners. The presence of these side squares essentially shortens the scattering distance between the unit cells and translates into the suppression of higher frequency vibrational modes. In other words, this design is capable of extending the frequency of operation of the PBGs for a given unit cell and minimum feature size. For the purpose of the demonstration, AlN lamb wave transducers were utilized to launch symmetric lamb waves into the PBG structure. The evidence of direct in-plane integration between the transducers and the PnC structure operating in the ultra high frequency range lays the foundation for the development of ultrasonic devices based on PBGs.
This work presents the design and demonstration of a microscale inverse acoustic band gap (IABG) structure in aluminum nitride (AlN) with a frequency stop band for bulk acoustic waves in the very high frequency range. Conversely to conventional microscale acoustic band gaps, the IABG is formed by a two-dimensional periodic array of unit cells consisting of a high acoustic velocity material cylinder surrounded by a low acoustic velocity medium. The periodic arrangement of the IABG array induces scattering of incident acoustic waves and generates a stop band, whose center frequency is primarily determined by the lattice constant of the unit cell and whose bandwidth depends on the cylinder radius, the film thickness, and the size of the tethers that support the cylinder. A wide band gap (>13% of the center frequency) is formed by the IABG even when thin AlN films are used. The experimental response of an IABG structure having a unit cell of 8.6 μm and an AlN film thickness of 2 μm confirms the existence of a frequency band gap between 185 MHz and 240 MHz.
This paper presents an experimental study on the impact of the metal electrodes on the level of the first spurious mode, electromechanical coupling (k t 2 ) and quality factor (Q) of laterally vibrating AlN contour mode resonators (CMRs). Pt, Ni, and Al are selected for the fabrication of the device top electrode since they exhibit a broad range of values of Young's modulus, density and resistivity, which are the most relevant parameters that have an impact on the electromechanical characteristics of the device. The results on the level of the first spurious mode, and k t 2 suggest that the acoustic mismatch between electrode and nonelectroded regions affect mostly these parameters. The extracted Q (after subtracting the effect of the electrode resistance) exhibits a trend in line with the theory of interfacial dissipation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.