A micromechanical system (MEMS) resonator includes a base substrate. A piezoelectric layer has a first electrode attached to a first surface of the piezoelectric layer and a second electrode attached to a second surface of the piezoelectric layer opposite the first electrode. The first electrode is bounded by a perimeter edge. A patterned acoustic mirror is formed on a top surface of the first electrode opposite the piezoelectric layer, such that the patterned acoustic mirror covers a border strip of the top surface of the first electrode at the perimeter edge and does not cover an active portion of the top surface of the first electrode.