Toward Ku-Band Surface Acoustic Wave Delay Lines on AlScN-on-Diamond with Decoupled Phase and Group Velocities

This work reports surface acoustic wave (SAW) acoustic delay lines (ADLs) on an aluminum scandium nitride (AlScN) on diamond platform operating in the X band and approaching the Ku band. The large acoustic-velocity contrast between the AlScN film and the diamond substrate produces a strongly dispersive Sezawa branch that decouples the phase velocity from the group velocity. Delay lines with a 1 $μ$m wavelength show a Sezawa passband at 9.55 GHz with a fractional bandwidth of 1.05%, a propagation loss of 0.094 dB per wavelength, a propagation-limited quality factor of 465, and a group velocity of 5979 m/s, while co-fabricated resonators give a phase velocity of 9560 m/s, a ratio of about 1.6. Scaling the wavelength to 0.5 $μ$m moves the passband to 16.5 GHz with a fractional bandwidth of 0.7%. The platform therefore reaches an operating frequency about 1.5 times higher than AlScN on sapphire at the same lithographic pitch while preserving group delay per unit length.