A 19 GHz All-Epitaxial Al₀.₈Sc₀.₂N Cascaded FBAR for RF Filtering Applications
Mingyo Park, Jialin Wang, Ding Wang, Zetian Mi, Azadeh Ansari
Abstract
Next-generation RF front-end architectures require a high Figure of Merit (FoM) of Quality factor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> ) x electromechanical coupling factor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k<sub>t</sub></i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) 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 50Ω impedance matching for the K-band resonators requires device area reduction compared to conventional FBARs operating below 6GHz, which can lower <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k<sub>t</sub></i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and power handling capability. To tackle these issues, we utilize cascaded FBARs (cas-FBARs) with all-epitaxial metal/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> N/metal layers on an oxide/Si substrate. The fabricated cas-FBARs achieve a measured <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q<sub>max</sub></i> × <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k<sub>t</sub></i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 14.71 at 19.11GHz, marking the highest <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k<sub>t</sub></i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 10.14% within the K-band FBAR devices published to date. We investigated <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k<sub>t</sub></i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while considering size scaling, crucial for 50Ω 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.