ScAlN-Based LCAT Mode Resonators Above 2 GHz With High FOM and Reduced Fabrication Complexity

This letter reports the first implementation of a new class of laterally coupled alternating thickness (LCAT) mode resonators operating above 2 GHz, realised by replacing the bottom interdigitated electrodes of the conventional LCAT mode resonator with an electrically floating plate. Besides saving one mask process and relaxing the tight resolution and alignment requirements for defining small features, the flat unpatterned bottom electrode metal is also more favorable for the growth of highly oriented piezoelectric thin film, which results in an improved resonator quality factor (Q). A group of 8% scandium doped aluminium nitride-based modified LCAT mode resonators with frequency difference over 100 MHz by varying the top interdigitated electrode pitches are fabricated achieving coupling coefficient (k
_eff
²
) of up to 6.45% and figure of merit of up to 54, making them promising candidates for integrated band selection filter banks.