Piezoelectric thin film resonator, filter, and multiplexer

A filter comprising:

• a piezoelectric thin film resonator including:
• a substrate;
• a piezoelectric film located on the substrate;
• a lower electrode and an upper electrode facing each other across at least a part of the piezoelectric film;
• a mass load film that is located at least one of a first side, which is closer to the upper electrode, of the piezoelectric film and a second side, which is closer to the lower electrode, of the piezoelectric film, is separated from the upper electrode and the lower electrode, and surrounds in plan view a resonance region at least in part, the lower electrode and the upper electrode facing each other across the piezoelectric film in the resonance region;
• and
• an acoustic reflection layer that includes the resonance region and the mass load film in plan view, is located in or on the substrate, and includes an air gap or an acoustic mirror in which at least two layers with different acoustic characteristics are stacked.

A multiplexer comprising:

• a filter including a piezoelectric thin film resonator, wherein
• the piezoelectric thin film resonator includes:
• a substrate;
• a piezoelectric film located on the substrate;
• a lower electrode and an upper electrode facing each other across at least a part of the piezoelectric film;
• a mass load film that is located at least one of a first side, which is closer to the upper electrode, of the piezoelectric film and a second side, which is closer to the lower electrode, of the piezoelectric film, is separated from the upper electrode and the lower electrode, and surrounds in plan view a resonance region at least in part, the lower electrode and the upper electrode facing each other across the piezoelectric film in the resonance region;
• and
• an acoustic reflection layer that includes the resonance region and the mass load film in plan view, is located in or on the substrate, and includes an air gap or an acoustic mirror in which at least two layers with different acoustic characteristics are stacked.