A high Q acoustic BAW resonator with high coupling and improved spurious mode suppression is given. The BAW resonator comprises an active resonator region (AR) formed by an overlap of the three layers bottom electrode (BE), piezoelectric layer (PL) and top electrode layer (TE). An inner-flap (IF) is formed by a dielectric 3D structure sitting on a marginal region (MR) of the active resonator region (AR) or adjacent thereto, extending inwardly towards the center thereof and having a section that runs in parallel and distant to the top surface of the resonator keeping an inner gap (IG) thereto or an angle Θ.