A film bulk acoustic resonator (FBAR) structure includes a bottom cap wafer, a piezoelectric layer disposed on the bottom cap wafer, a bottom electrode disposed below the piezoelectric layer, and a top electrode disposed above the piezoelectric layer. Portions of the bottom electrode, the piezoelectric layer, and the top electrode that overlap with each other constitute a piezoelectric stack. The FBAR structure further includes a lower cavity disposed below the piezoelectric stack. A projection of the piezoelectric stack is located within the lower cavity.