Patent ID: 7893793

Claim:
A film bulk acoustic wave resonator comprising: a piezoelectric body made of a piezoelectric thin film; a first electrode provided on one main surface of the piezoelectric body; a second electrode provided on the other main surface of the piezoelectric body; and a first mass load material portion having an annular shape provided outside the planar region of the first electrode on the main surface of the piezoelectric body on which the first electrode is provided, with a mass load effect of the first mass load material portion being larger than that of the first electrode, the piezoelectric body being supplied with an electric field through the first electrode and the second electrode so as to generate a resonant vibration, wherein the outer periphery of the planar region of the first electrode and the inner periphery of the annular first mass load material portion are spaced apart from each other, whereby the first electrode and the first mass load material portion are electrically insulated from each other, the first mass load material portion has a laminated structure including a first auxiliary electrode and a load material layer formed on the first auxiliary electrode, the first auxiliary electrode being formed with the same material to have the same thickness as the first electrode on the piezoelectric body, and a distance g between the outer periphery of the planar region of the first electrode and the inner periphery of the first mass load material portion falls within a range defined by the expression g≧dp×ε 11/ε33 where dp is the thickness of the piezoelectric body, ε33 is the relative dielectric constant in the thickness direction of the piezoelectric body, and ε11 is the relative dielectric constant in the plane direction of the piezoelectric body.