Piezoelectric thin film resonator, filter, and duplexer

A piezoelectric thin film resonator comprising:

• a substrate;
• a lower electrode and an upper electrode that are located on the substrate, and face each other in a stacking direction;
• a piezoelectric film that is sandwiched between the lower electrode and the upper electrode, and includes a lower piezoelectric film and an upper piezoelectric film, an outer outline of the upper piezoelectric film coinciding with or being located further out than an outer outline of a resonance region, in which the lower electrode and the upper electrode face each other across the piezoelectric film, in at least a part of a region surrounding the resonance region, the outer outline of the upper piezoelectric film being located further in than an outer outline of the lower piezoelectric film in the at least a part of the region surrounding the resonance region;
• an insertion film that is interposed between the lower piezoelectric film and the upper piezoelectric film, is located in at least a part of an outer peripheral region within the resonance region, is not located in a central region of the resonance region, and is located on at least a part of an upper surface of the lower piezoelectric film in the at least a part of the region surrounding the resonance region;
• and
• a protective film that is located on the upper electrode in the resonance region, and is located so as to cover an end face of the upper piezoelectric film and at least a part of an upper surface of the insertion film in the at least a part of the region surrounding the resonance region.