A 19 GHz All-Epitaxial Al₀.₈Sc₀.₂N Cascaded FBAR for RF Filtering Applications

Next-generation RF front-end architectures require a high Figure of Merit (FoM) of quality factor (
${Q}\text {)}$
x electromechanical coupling factor (
${{k}_{t}}^{{2}}\text {)}$
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~\Omega $
impedance matching for the K-band resonators requires device area reduction compared to conventional FBARs operating below 6 GHz, which can lower
${{k}_{t}}^{{2}}$
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
${Q}_{{max}} \times {{k}_{t}}^{{2}}$
of 14.71 at 19.11 GHz, marking the highest
${{k}_{t}}^{{2}}$
of 10.14% within the K-band FBAR devices published to date. We investigated
${{k}_{t}}^{{2}}$
while considering size scaling, crucial for
$50~\Omega $
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.