Air-backed Al/ZnO/Al film bulk acoustic resonator without any support layer

An air-backed, Al/ZnO/Al film-bulk-acoustic resonator (FBAR) which is free-standing has been fabricated. Unlike a conventional FBAR structure, the newly fabricated resonator does not employ any supporting layer below or above it, but the whole resonator body (consisting of a ZnO piezoelectric layer sandwiched by two aluminum layers) suspends by itself in the air. Its electromechanical coupling constant (k_t²) is twice larger than that of a similarly made FBAR with 0.9 μm thick silicon-nitride support layer. Moreover, its quality factors at series resonant frequency and parallel resonant frequency around 1.5 GHz have been measured to be 1,322 and 513 (six times and three times larger than those in the conventional FBAR with 0.9 μm thick Si₃N₄ diaphragm, respectively). This newly fabricated resonator will be excellent for wide bandwidth filters because of its large electromechanical coupling constant and high quality factor (and thus high figure of merit). The free-standing air-backed FBAR is sturdy enough to survive from a shock test of being dropped on to a desk from one meter height. A layer of 0.5 μm thick parylene deposited and patterned over the Al bridge is proven to enhance the sturdiness of the structure greatly.