A bulk acoustic wave filter device and method thereof includes a first layer forming an air gap together with a substrate, a lower electrode disposed over the first layer, a piezoelectric layer disposed to cover a portion of the lower electrode, an upper electrode disposed over the piezoelectric layer, a frame layer disposed below the upper electrode, and a lower electrode reinforcing layer disposed on the lower electrode, other than portions in which the piezoelectric layer is disposed. The lower electrode reinforcing layer is formed by separating the lower electrode reinforcing layer from the upper electrode or the frame layer upon one of the upper electrode and the frame layer being formed.

A filter circuit includes a first inductor circuit connected in series with a second inductor circuit, and a first capacitor circuit connected in series with a second capacitor circuit. The first inductor circuit and the second inductor circuit are connected between a first input/output port and a second input/output port of the filter circuit. A third inductor circuit is connected between a reference node and a first node that is between the first inductor circuit and the second inductor circuit. A resonator circuit is connected to a second node between the first capacitor circuit and the second capacitor circuit. A fourth inductor circuit is connected between the resonator circuit and the reference node. In some embodiments, another resonator circuit is connected between the input port and the output port of the filter circuit.