Piezoelectric thin film resonator with insertion film, filter, and multiplexer
A 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; and an insertion film that is inserted between the lower electrode and the upper electrode, is located in an outer peripheral region within a resonance region where the lower electrode and the upper electrode face each other across the piezoelectric film, is located in a region that is located outside the resonance region and surrounds the resonance region, is not located in a center region of the resonance region, and includes a first part, which is located in the resonance region and has a first film thickness, and a second part, which is located outside the resonance region and has a second film thickness, the first film thickness being less than the second film thickness.
1. A piezoelectric thin film resonator comprising:
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; and
an insertion film that is inserted between the lower electrode and the upper electrode, is located in an outer peripheral region within a resonance region defined by a region where the lower electrode and the upper electrode face each other across the piezoelectric film, is located in a region that is located outside the resonance region and surrounds the resonance region, is not located in a center region of the resonance region, and includes a first part, which is located in the resonance region and along an outer circumference of the resonance region and has a first film thickness, and a second part, which is located outside the resonance region and along the outer circumference of the resonance region, is not located in the resonance region and has a second film thickness, the first film thickness being less than the second film thickness, the second part of the insertion film is not located in a region where both of the lower electrode and the upper electrode are located.
2. The piezoelectric thin film resonator according to claim 1 , further comprising:
an acoustic reflection layer that 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,
wherein the acoustic reflection layer is located in the resonance region and at least a part of the second part of the insertion film.
3. The piezoelectric thin film resonator according to claim 1 , wherein:
a wave number of a lateral mode in the center region at an antiresonant frequency in the resonance region is less than a wave number of a lateral mode in a region where the first part of the insertion film is located, and
at the antiresonant frequency in the resonance region, a wave number of a lateral mode in a region where the second part of the insertion film is located is less than the wave number of the lateral mode in the region where the first part of the insertion film is located.
4. The piezoelectric thin film resonator according to claim 1 , wherein:
the piezoelectric film includes a lower piezoelectric film and an upper piezoelectric film located on the lower piezoelectric film, and
the insertion film is inserted between the lower piezoelectric film and the upper piezoelectric film.
5. The piezoelectric thin film resonator according to claim 1 , wherein the first part of the insertion film is in contact with the second part of the insertion film.
6. The piezoelectric thin film resonator according to claim 1 , wherein the first part of the insertion film and the second part of the insertion film are located at different positions in a thickness direction.
7. The piezoelectric thin film resonator according to claim 1 , wherein the insertion film has less acoustic impedance than the piezoelectric film.
8. 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; and
an insertion film that is inserted between the lower electrode and the upper electrode, is located in an outer peripheral region within a resonance region defined by a region where the lower electrode and the upper electrode face each other across the piezoelectric film, is located in a region that is located outside the resonance region and surrounds the resonance region, is not located in a center region of the resonance region, and includes a first part, which is located in the resonance region and along an outer circumference of the resonance region and has a first film thickness, and a second part, which is located outside the resonance region and along the outer circumference of the resonance region, is not located in the resonance region and has a second film thickness, the first film thickness being less than the second film thickness, the second part of the insertion film is not located in a region where both of the lower electrode and the upper electrode are located.
9. 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; and
an insertion film that is inserted between the lower electrode and the upper electrode, is located in an outer peripheral region within a resonance region defined by a region where, the lower electrode and the upper electrode face each other across the piezoelectric film, is located in a region that is located outside the resonance region and surrounds the resonance region, is not located in a center region of the resonance region, and includes a first part, which is located in the resonance region and along an outer circumference of the resonance region and has a first film thickness, and a second part, which is located outside the resonance region and along the outer circumference of the resonance region, is not located in the resonance region and has a second film thickness, the first film thickness being less than the second film thickness, the second part of the insertion film is not located in a region where both of the lower electrode and the upper electrode are located.