Miniature High-Coupling Lithium Niobate Thin Film Bulk Acoustic Wave Resonators at 10-30 GHz

Thin film bulk acoustic wave resonators (FBARs) leveraging sputtered aluminum nitride (AlN) and scandium aluminum nitride (ScAlN) films, are a leading commercial solution for compact radio frequency (RF) filters in mobile devices. However, as 5G/6G bands extend beyond 6 GHz, achieving the required operational bandwidth presents a significant challenge due to the moderate electro-mechanical coupling <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(k^{2})$</tex> of AlN/ScAlN. More recently, transferred ultra-thin single-crystal piezoelectric lithium niobate (LN) has enabled lateral field excited bulk acoustic wave resonators (XBAR) at 10-30 GHz. While these devices boast a high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$k^{2}$</tex>, they face challenges with low capacitance density, large footprint, and significant electromagnetic (EM) effects. On the other hand, thickness-field excited LN FBARs face challenges with bottom electrode integration. In this work, we demonstrate a high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$k^{2}$</tex> LN FBAR without the need for a patterned bottom electrode. The resonators show first order symmetric mode (S1) at 10.5 GHz with a 3-dB series resonance quality factor (Q<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf>) of 38 and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$k^{2}$</tex> of 14.1%, alongside third order symmetric mode (S3) at 27 GHz with a 3-dB <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q_{s}$</tex> of 22 and a high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$k^{2}$</tex> of 11.3%. Further analysis shows that higher <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q$</tex> could be achieved by adjusting the low-loss piezoelectric to lossy metal volume ratio.