Piezoelectric thin film resonator, filter, and multiplexer

1. A piezoelectric thin film resonator comprising:

a substrate;

a piezoelectric film located on the substrate and mainly composed of aluminum nitride;

a lower electrode and an upper electrode facing each other across at least a part of the piezoelectric film; and

a protective film covering an upper surface of the piezoelectric film, a side surface of the piezoelectric film, and an inner surface of a penetration hole penetrating through the piezoelectric film and the lower electrode, the protective film entirely covering an upper surface of the upper electrode in a resonance region, where the upper electrode faces the lower electrode across the piezoelectric film, and the inner surface of the piezoelectric film in the penetration hole, an inner surface of the lower electrode in the penetration hole being exposed from the protective film to an air in the penetrating hole, the protective film being a SiON film with a refractive index of a light with a wavelength of 632.8 nm of 1.7 or greater, an aluminum oxide film with a density of 3 g/cm 3 or greater or a diamond-like carbon film,

wherein:

an air gap is located between the substrate and the lower electrode and overlaps with a resonance region and a region outside the resonance region in a plan view, and

the penetration hole overlaps with the air gap in the plan view and is directly connected to the air gap.

2. A piezoelectric thin film resonator comprising:

a substrate;

a piezoelectric film located on the substrate and mainly composed of aluminum nitride;

a lower electrode and an upper electrode facing each other across at least a part of the piezoelectric film; and

a protective film covering an upper surface of the piezoelectric film, a side surface of the piezoelectric film, and an inner surface of a penetration hole penetrating through the piezoelectric film and the lower electrode, the protective film entirely covering an upper surface of the upper electrode in a resonance region, where the upper electrode faces the lower electrode across the piezoelectric film, and the inner surface of the piezoelectric film and the lower electrode in the penetration hole, the protective film being a SiON film with a refractive index of a light with a wavelength of 632.8 nm of 1.7 or greater, an aluminum oxide film with a density of 3 g/cm 3 or greater or a diamond-like carbon film,

wherein:

an air gap is located between the substrate and the lower electrode and overlap with the resonance region and a region outside the resonance region in a plan view, and

the protective film extends from the penetration hole to an upper surface of the substrate in the air gap and are located between the penetration hole and the air gap.

3. A piezoelectric thin film resonator comprising:

a substrate;

a piezoelectric film located on the substrate and mainly composed of aluminum nitride;

a lower electrode and an upper electrode facing each other across at least a part of the piezoelectric film; and

a protective film covering an upper surface of the piezoelectric film, a side surface of the piezoelectric film, and an inner surface of a penetration hole penetrating through the piezoelectric film and not penetrating through the lower electrode, the protective film entirely covering an upper surface of the upper electrode in a resonance region, where the upper electrode faces the lower electrode across the piezoelectric film, and the inner surface of the piezoelectric film in the penetration hole, the protective film being a SiON film with a refractive index of a light with a wavelength of 632.8 nm of 1.7 or greater, an aluminum oxide film with a density of 3 g/cm 3 or greater or a diamond-like carbon film,

a pad located inside the penetration hole, directly connected to an upper surface of the lower electrode and covers the inner surface of the protective film in the penetration hole.

4. The piezoelectric thin film resonator according to claim 1 , wherein

the protective film covers a lower surface of the piezoelectric film.

5. The piezoelectric thin film resonator according to claim 1 , wherein

an aperture is located in the protective film on the upper electrode, and a pad is located so as to cover an inner surface of the aperture and the upper electrode exposed from the aperture.

6. The piezoelectric thin film resonator according to claim 1 , wherein

a portion of protective film located on the upper electrode within the resonance region is thinner than another portion of the protective film located on the upper surface of the piezoelectric film in a region other than the resonance region.

7. A filter comprising the piezoelectric thin film resonator as set forth in claim 1 .

8. A multiplexer comprising the filter as set forth in claim 7 .

9. The piezoelectric thin film resonator according to claim 1 , wherein

the protective film is the SiON film with a refractive index of a light with a wavelength of 632.8 nm of 1.7 or greater.

10. The piezoelectric thin film resonator according to claim 1 , wherein

the protective film is the aluminum oxide film with a density of 3 g/cm 3 or greater.

11. The piezoelectric thin film resonator according to claim 1 , wherein

the protective film is the diamond-like carbon film.

12. The piezoelectric thin film resonator according to claim 1 , wherein

the piezoelectric film is not in direct contact with the air.

13. The piezoelectric thin film resonator according to claim 2 , wherein

the protective film covers a lower surface of the piezoelectric film.

14. The piezoelectric thin film resonator according to claim 3 , wherein

the protective film covers a lower surface of the piezoelectric film.

15. A filter comprising the piezoelectric thin film resonator according to claim 2 .

16. A multiplexer comprising the filter according to claim 15 .

17. A filter comprising the piezoelectric thin film resonator according to claim 3 .

18. A multiplexer comprising the filter according to claim 17 .