Thin film piezoelectric bulk acoustic wave resonator and radio frequency filter using the same

A thin film piezoelectric bulk acoustic wave resonator having a laminated structure, said laminated structure comprising:

• a piezoelectric thin film;
• a first metal electrode film;
• and
• a second metal electrode film, wherein between the first and the second metal electrode films at least part of the piezoelectric thin film is sandwiched,
• wherein the first metal electrode film has a hole structure including a plurality of holes,
• wherein, if a combined thickness of the first metal electrode film, the second metal electrode film, and the piezoelectric thin film is ht, the covering ratio σ of an opposite area of the first metal electrode film facing the second metal electrode film satisfies the condition 0<σ<1 for every 1.28 ht pitch, and
• wherein the holes extend into the piezoelectric thin film, and have a depth equivalent to a combined thickness of the first metal electrode film and the piezoelectric thin film.

A radio frequency filter comprising:

• an input terminal;
• an output terminal;
• and
• at least one thin film piezoelectric bulk acoustic wave resonator connected to the input terminal and the output terminal,
• wherein the thin film piezoelectric bulk acoustic wave resonator includes a laminated structure, said laminated structure comprising:
• a piezoelectric thin film;
• a first metal electrode film;
• and
• a second metal electrode film, wherein between the first and the second metal electrode films at least part of the piezoelectric thin film is sandwiched,
• wherein the first metal electrode film has a hole structure including a plurality of holes, and
• wherein the holes extend into the piezoelectric thin film, and have a depth equivalent to a combined thickness of the first metal electrode film and the piezoelectric thin film,
• whereby the resonator has a resonant frequency at which an acoustic wave excited by the piezoelectric thin film cannot identify the holes of the hole structure individually.