Elastic wave device

1. An elastic wave device comprising:

a piezoelectric film;

a low-acoustic-velocity film which is stacked on the piezoelectric film and in which an acoustic velocity of a bulk wave that propagates through the low-acoustic-velocity film is lower than an acoustic velocity of a bulk wave that propagates through the piezoelectric film;

a high-acoustic-velocity film which is stacked on a surface of the low-acoustic-velocity film on a side opposite to the piezoelectric film and in which an acoustic velocity of a bulk wave that propagates through the high-acoustic-velocity film is higher than an acoustic velocity of an elastic wave that propagates through the piezoelectric film;

a substrate directly or indirectly stacked on a surface of the high-acoustic-velocity film on a side opposite to the low-acoustic-velocity film; and

a bonding layer disposed at any position from an inside of the high-acoustic-velocity film to an interface between the low-acoustic-velocity film and the piezoelectric film.

2. The elastic wave device according to claim 1 , wherein the bonding layer is located in the high-acoustic-velocity film, at an interface between the high-acoustic-velocity film and the low-acoustic-velocity film, in the low-acoustic-velocity film, or at the interface between the low-acoustic-velocity film and the piezoelectric film.

3. The elastic wave device according to claim 1 , wherein the bonding layer includes a metal oxide layer or a metal nitride layer.

4. The elastic wave device according to claim 1 , wherein the bonding layer includes a Ti layer, and the Ti layer has a thickness in a range of about 0.4 nm to about 2.0 nm.

5. The elastic wave device according to claim 4 , wherein the Ti layer has a thickness a range of about 0.4 nm to about 1.2 nm.

6. The elastic wave device according to claim 1 , wherein the piezoelectric film is made of LiTaO 3 .

7. The elastic wave device according to claim 1 , wherein the low-acoustic-velocity film is made of silicon oxide.

8. The elastic wave device according to claim 2 , wherein

the low-acoustic-velocity film is made of silicon oxide; and

the bonding layer is located in the low-acoustic-velocity film;

the low-acoustic-velocity film includes a first low-acoustic-velocity layer located on a side of the bonding layer close to the piezoelectric film and a second low-acoustic-velocity layer located on a side of the bonding layer opposite to the piezoelectric film; and

when an elastic wave used in the elastic wave device is assumed to have a wavelength λ, the first low-acoustic-velocity layer has a thickness of about 0.12λ or more.

9. The elastic wave device according to claim 8 , wherein the first low-acoustic-velocity layer has a thickness of about 0.22λ or more.

10. The elastic wave device according to claim 1 , wherein the high-acoustic-velocity film is made of aluminum nitride or silicon nitride.

11. The elastic wave device according to claim 1 , further comprising an intermediate layer stacked between the high-acoustic-velocity film and the substrate.

12. An elastic wave device comprising:

a piezoelectric film;

a low-acoustic-velocity film which is stacked on the piezoelectric film and in which an acoustic velocity of a bulk wave that propagates through the low-acoustic-velocity film is lower than an acoustic velocity of a bulk wave that propagates through the piezoelectric film;

a high-acoustic-velocity substrate which is directly or indirectly stacked on a surface of the low-acoustic-velocity film on a side opposite to the piezoelectric film and in which an acoustic velocity of a bulk wave that propagates through the high-acoustic-velocity substrate is higher than an acoustic velocity of an elastic wave that propagates through the piezoelectric film; and

a bonding layer located in the low-acoustic-velocity film or at an interface between the piezoelectric film and the low-acoustic-velocity film.

13. The elastic wave device according to claim 12 , wherein the bonding layer is located in the high-acoustic-velocity substrate, at an interface between the high-acoustic-velocity substrate and the low-acoustic-velocity film, in the low-acoustic-velocity film, or at the interface between the low-acoustic-velocity film and the piezoelectric film.

14. The elastic wave device according to claim 12 , wherein the bonding layer includes a metal oxide layer or a metal nitride layer.

15. The elastic wave device according to claim 12 , wherein the bonding layer includes a Ti layer, and the Ti layer has a thickness in a range of about 0.4 nm to about 2.0 nm.

16. The elastic wave device according to claim 15 , wherein the Ti layer has a thickness a range of about 0.4 nm to about 1.2 nm.

17. The elastic wave device according to claim 12 , wherein the piezoelectric film is made of LiTaO 3 .

18. The elastic wave device according to claim 12 , wherein the low-acoustic-velocity film is made of silicon oxide.

19. The elastic wave device according to claim 13 , wherein

the low-acoustic-velocity film is made of silicon oxide; and

the bonding layer is located in the low-acoustic-velocity film;

the low-acoustic-velocity film includes a first low-acoustic-velocity layer located on a side of the bonding layer close to the piezoelectric film and a second low-acoustic-velocity layer located on a side of the bonding layer opposite to the piezoelectric film; and

when an elastic wave used in the elastic wave device is assumed to have a wavelength λ, the first low-acoustic-velocity layer has a thickness of about 0.12λ, or more.

20. The elastic wave device according to claim 19 , wherein the first low-acoustic-velocity layer has a thickness of about 0.22λ or more.

21. The elastic wave device according to claim 12 , wherein the high-acoustic-velocity film is made of aluminum nitride or silicon nitride.

22. The elastic wave device according to claim 12 , further comprising an intermediate layer stacked between the high-acoustic-velocity film and the substrate.