Film bulk acoustic resonator (FBAR) is provided. An exemplary FBAR includes a substrate; a first insulating material layer on the substrate, the first insulating material layer containing a first cavity; a second insulating material layer on the first insulating material layer, the second insulating material layer containing a second cavity and a third cavity spaced apart from the second cavity, the second cavity and the third cavity both in communication with the first cavity; a resonator sheet covering the second cavity and partially extending over the second insulating material layer; a third insulating material layer over the second insulating material layer and the resonator sheet, the third insulating material layer containing a fourth cavity, the fourth cavity in communication with the third cavity, and the fourth cavity partially overlapping the second cavity; and a capping layer on the third insulating material layer.