Piezoelectric thin film resonator, filter, duplexer, and method of fabricating piezoelectric thin film resonator

A method of fabricating a piezoelectric thin film resonator, the method comprising:

• forming a piezoelectric film on a lower electrode by sputtering using a target containing aluminum, a II-group or XII-group element, and a IV-group or V-group element, the piezoelectric film including an aluminum nitride film of which a concentration of the IV-group or V-group element in a middle region in a thickness direction is higher than a concentration of the IV-group or V-group element in at least one of end regions in the thickness direction, the aluminum nitride film being formed by changing a sputtering power between the middle region and the at least one of the end regions in the thickness direction in the sputtering;
• and
• forming an upper electrode on the piezoelectric film, the upper electrode facing the lower electrode across the piezoelectric film.