An acoustic resonator with improved quality factor and electro-mechanical coupling is disclosed. In one embodiment, the acoustic resonator includes an acoustic mirror formed on the top surface of a substrate or in the substrate, a first electrode having a end portion, formed on the acoustic mirror, a piezoelectric layer formed on the first electrode; and a second electrode formed on the piezoelectric layer, where at least one of the first electrode and the second electrode and the piezoelectric layer define a gap in a region that overlaps the end portion of the first electrode. In one embodiment, a dielectric film is deposited on the surface of the end portion of the first electrode to form completely planarized surface before the piezoelectric layer deposition. In another embodiment, a gap between the second electrode and the piezoelectric layer, so that the piezoelectric coupling in the end portion area of the first electrode is minimally contributed into the whole resonator.