Lithium niobate single crystal thin film resonator with benzocyclobutene as a reflective layer

Abstract

The intrinsic electromechanical coupling coefficient (kt2) of the piezoelectric material is the decisive factor limiting the band width of the thin film bulk acoustic (FBAR) filters. In the present work, single crystalline LiNbO3 thin films are transferred by crystal ion slicing. Instead of the often used silicon dioxide, Benzocyclobutene is used as the bonding layer, which significantly increases the tolerance of wafer bonding for the surface flatness, thus a patterned bottom electrode is easily buried underneath the LiNbO3 thin film during the ions slicing process. More importantly, this single Benzocyclobutene layer also acts as an effective acoustic reflector, and a large kt2 exceeding 21% is obtained in the LN-based bulk acoustic wave resonator. The Mason model is built up according to the experimental results, and the fitting shows that the Benzocyclobutene has a low acoustic impedance, which effectively confines the acoustic energy within the piezoelectric layer and results in the high kt2.