Acoustic wave device

1. An acoustic wave device comprising:

a piezoelectric substrate including a piezoelectric layer with a reverse-velocity surface having an ellipse shape;

an IDT electrode on the piezoelectric substrate; and

a dielectric film on the piezoelectric substrate and covering the IDT electrode; wherein

a Love wave is excited;

the IDT electrode includes a first busbar and a second busbar opposite to each other, first electrode fingers of which respective first ends are connected to the first busbar, and second electrode fingers of which respective first ends are connected to the second busbar and that are interdigitated with the first electrode fingers;

when a first direction denotes a propagation direction of an acoustic wave and a second direction denotes a direction orthogonally or substantially orthogonally intersecting the first direction, the IDT electrode further includes an intersecting region in which the first electrode fingers and the second electrode fingers overlap each other as viewed in the first direction;

the intersecting region includes a central region at a central portion of the intersecting region in the second direction, a first edge region adjacent to or in a vicinity of the central region at a position near the first busbar, and a second edge region adjacent to or in a vicinity of the central region at a position near the second busbar;

when λ denotes a wavelength defined by a pitch of the first and second electrode fingers of the IDT electrode, h denotes a film thickness of the IDT electrode, x (%) denotes a wavelength-normalized film thickness defined as h/λ×100(%), and y (g/cm 3 ) denotes an electrode density of the IDT electrode, the wavelength-normalized film thickness x is set, depending on the electrode density y of the IDT electrode, at a value not less than x that satisfies the following equation:

y= 0.0757 x 2 −3.9023 x+ 27.986; and

a film thickness of the dielectric film in the first edge region and a film thickness of the dielectric film in the second edge region are smaller than that of the dielectric film in the central region.

2. The acoustic wave device according to claim 1 , wherein

the IDT electrode includes a multilayer metal film in which multiple metal films are laminated; and

the electrode density y of the IDT electrode is a density of the multilayer metal film.

3. The acoustic wave device according to claim 2 , wherein the multilayer metal film includes a Pt film and an Al film, a Pt film and a Cu film, or a Mo film and an Al film.

4. The acoustic wave device according to claim 1 , wherein the IDT electrode further includes a first gap region that is adjacent to or in a vicinity of the first edge region at a position near the first busbar and in which only the first electrode fingers of the first and second electrode fingers are provided, and a second gap region that is adjacent to or in a vicinity of the second edge region at a position near the second busbar and in which only the second electrode fingers of the first and second electrode fingers are provided.

5. The acoustic wave device according to claim 4 , wherein the IDT electrode further includes first dummy electrode fingers of which respective first ends are connected to the first busbar and that oppose corresponding ones of the second electrode fingers with the first gap region located therebetween, and second dummy electrode fingers of which respective first ends are connected to the second busbar and that oppose corresponding ones of the first electrode fingers with the second gap region located therebetween.

6. The acoustic wave device according to claim 1 , wherein the piezoelectric layer is made of LiNbO 3 .

7. The acoustic wave device according to claim 6 , wherein an Euler angle of the piezoelectric layer is an Euler angle of (φ, a range of −90°±30°, ψ).

8. The acoustic wave device according to claim 1 , wherein the piezoelectric substrate includes a high-velocity layer which is laminated directly or indirectly on the piezoelectric layer and in which a bulk wave propagates faster than an acoustic wave propagating in the piezoelectric layer.

9. The acoustic wave device according to claim 8 , wherein the piezoelectric substrate includes a low-velocity layer which is laminated between the piezoelectric layer and the high-velocity layer and in which a bulk wave propagates slower than a bulk wave propagating in the piezoelectric layer.

10. The acoustic wave device according to claim 1 , wherein the acoustic wave device defines a one-port surface acoustic wave resonator.

11. The acoustic wave device according to claim 1 , wherein the dielectric film is made of silicon oxide.

12. An acoustic wave device comprising:

a piezoelectric substrate including a piezoelectric layer with a reverse-velocity surface having an ellipse shape;

an IDT electrode on the piezoelectric substrate; and

a dielectric film on the piezoelectric substrate and covering the IDT electrode; wherein

a Love wave is excited;

the IDT electrode includes a first busbar and a second busbar opposite to each other, first electrode fingers of which respective first ends are connected to the first busbar, and second electrode fingers of which respective first ends are connected to the second busbar and that are interdigitated with the first electrode fingers;

when a first direction denotes a propagation direction of an acoustic wave and a second direction denotes a direction orthogonally or substantially orthogonally intersecting the first direction, the IDT electrode further includes an intersecting region in which the first electrode fingers and the second electrode fingers overlap each other as viewed in the first direction;

the intersecting region includes a central region located at a central portion of the intersecting region in the second direction, a first edge region adjacent to or in a vicinity of the central region at a position near the first busbar, and a second edge region adjacent to or in a vicinity of the central region at a position near the second busbar;

the IDT electrode is made of at least one of Pt, Au, W, Ta, Mo, and Cu;

when λ denotes a wavelength defined by a pitch of the first and second electrode fingers of the IDT electrode, h denotes a film thickness of the IDT electrode, x (%) denotes a wavelength-normalized film thickness defined as h/λ×100(%), the wavelength-normalized film thickness x is set, depending on a material of the IDT electrode, at a value not less than a corresponding value in Table 1 below:

TABLE 1

Material

x = h/λ × 100 (%)

Pt

≥1.8

W

≥2.2

Mo

≥5.05

Ta

≥2.55

Au

≥1.9

Cu

≥5.25;

and

a film thickness of the dielectric film in the first edge region and a film thickness of the dielectric film in the second edge region are smaller than that of the dielectric film in the central region.

13. The acoustic wave device according to claim 12 , wherein the IDT electrode further includes a first gap region that is adjacent to or in a vicinity of the first edge region at a position near the first busbar and in which only the first electrode fingers of the first and second electrode fingers are provided, and a second gap region that is adjacent to or in a vicinity of the second edge region at a position near the second busbar and in which only the second electrode fingers of the first and second electrode fingers are provided.

14. The acoustic wave device according to claim 13 , wherein the IDT electrode further includes first dummy electrode fingers of which respective first ends are connected to the first busbar and that oppose corresponding ones of the second electrode fingers with the first gap region located therebetween, and second dummy electrode fingers of which respective first ends are connected to the second busbar and that oppose corresponding ones of the first electrode fingers with the second gap region located therebetween.

15. The acoustic wave device according to claim 12 , wherein the piezoelectric layer is made of LiNbO 3 .

16. The acoustic wave device according to claim 15 , wherein an Euler angle of the piezoelectric layer is an Euler angle of (φ, a range of −90°±30°, ψ).

17. The acoustic wave device according to claim 12 , wherein the piezoelectric substrate includes a high-velocity layer which is laminated directly or indirectly on the piezoelectric layer and in which a bulk wave propagates faster than an acoustic wave propagating in the piezoelectric layer.

18. The acoustic wave device according to claim 17 , wherein the piezoelectric substrate includes a low-velocity layer which is laminated between the piezoelectric layer and the high-velocity layer and in which a bulk wave propagates slower than a bulk wave propagating in the piezoelectric layer.

19. The acoustic wave device according to claim 12 , wherein the acoustic wave device defines a one-port surface acoustic wave resonator.

20. The acoustic wave device according to claim 12 , wherein the dielectric film is made of silicon oxide.