Film bulk acoustic resonator and method of forming the same

A film bulk acoustic resonator (FBAR) includes an insulation layer on a substrate to prevent a signal from being transmitted to a substrate. The FBAR includes a portion of a membrane layer corresponding to an activation area to adjust a resonance frequency band and improve a transmission gain of the resonance frequency band, the portion of the membrane layer being partially etched to have a thickness less than the other portion of the membrane layer. A method of forming the FBAR includes forming an sacrificing layer made of polysilicon, forming an air gap using a dry etching process, and forming a via hole. The method prevents structural problems occurred in a conventional air gap forming process and provides locations and the number of the via holes to be controlled.