A 19 GHz All-Epitaxial Al_0.8Sc_0.2N Cascaded FBAR for RF Filtering Applications

Next-generation RF front-end architectures require a high Figure of Merit (FoM) of quality factor (Q ) x electromechanical coupling factor (k_t2 ) acoustic resonators for K-band filtering applications. Scaling up the frequency of thin-film bulk acoustic wave resonators (FBARs) involves thickness downscaling in both the piezo films and the metal electrodes, causing mechanical and electrical losses. Furthermore, achieving 50~Ω impedance matching for the K-band resonators requires device area reduction compared to conventional FBARs operating below 6 GHz, which can lower k_t2 and power handling capability. To tackle these issues, we utilize cascaded FBARs (cas-FBARs) with all-epitaxial metal/Al_0.8 Sc_0.2 N/metal layers on an oxide/Si substrate. The fabricated cas-FBARs achieve a measured Q_max × k_t2 of 14.71 at 19.11 GHz, marking the highest k_t2 of 10.14% within the K-band FBAR devices published to date. We investigated k_t2 while considering size scaling, crucial for 50~Ω impedance matching for 5G devices. We provide a comprehensive characterization of both single FBARs and cas-FBARs, taking into account the impact of resonator sizes on device performance.