Focused S0 Lamb Modes for Gigahertz Delay Lines in 30% Scandium Aluminum Nitride

Scandium-doped aluminum nitride (ScAlN) is an emergent piezoelectric material for acoustic wave devices, exhibiting higher electromechanical coupling $\left( {k_t^2} \right)$ compared to undoped AlN. The S0 Lamb mode in suspended ScAlN films theoretically supports $k_t^2$ up to ~40%, demonstrating comparable performance to thin film lithium niobate. The high phase velocity of the S0 mode enables gigahertz acoustic delay lines (ADLs) with low insertion loss and high fractional bandwidth. Introducing a curvature to transmitting interdigitated transducers generates a Gaussian-like beam of focused acoustic energy, which can be captured and routed through acoustic waveguides defined in the thin film. This work employs the first use of focusing interdigitated transducers (FIDTs) in suspended 30% ScAlN acoustic waveguides for gigahertz ADLs.