Boundary acoustic wave device having three-medium structure

1. A boundary acoustic wave device comprising:

a piezoelectric substrate;

a first dielectric layer laminated on the piezoelectric substrate and having a set thickness;

a second dielectric layer laminated on the first dielectric layer; and

an IDT electrode provided at an interface between the piezoelectric substrate and the first dielectric layer; wherein

the boundary acoustic wave device utilizes a Stoneley wave that propagates along the interface, and the thickness of the first dielectric layer is set such that Va>V 1 is satisfied, where V 1 denotes an acoustic velocity of a slow transversal bulk wave in the piezoelectric substrate and Va denotes an acoustic velocity at an anti-resonant point in a higher-order mode of the Stoneley wave.

2. The boundary acoustic wave device according to claim 1 , wherein Vp max >V 1 is satisfied where Vp max denotes an acoustic velocity at a maximum phase in the higher-order mode.

3. The boundary acoustic wave device according to claim 1 , wherein Vr>V 1 is satisfied where Vr denotes an acoustic velocity at a resonant point in the higher-order mode.

4. The boundary acoustic wave device according to claim 1 , wherein the piezoelectric substrate is made of LiNbO 3 .

5. The boundary acoustic wave device according to claim 1 , wherein the first dielectric layer is made of SiO 2 or SiON.

6. The boundary acoustic wave device according to claim 5 , wherein the first dielectric layer is made of SiO 2 , and a normalized thickness of the first dielectric layer, which is obtained by normalization of the set thickness of the first dielectric layer with a wavelength λ of the Stoneley wave, is equal to or less than about 1.4.

7. The boundary acoustic wave device according to claim 1 , wherein the first dielectric layer is made of SiO 2 , and when a duty ratio of the IDT electrode and h×a/λ obtained by multiplying a wavelength normalized thickness h/λ of the IDT electrode, which is obtained by normalization with a wavelength λ, by a ratio a of a density of the IDT electrode to a density of Pt, are within each of regions J 1 to J 13 shown in FIG. 9 , a wavelength normalized thickness H/λ of the first dielectric layer, which is obtained by normalization of the set thickness of the first dielectric layer with the wavelength λ, is equal to or less than a value described in Table 1 below:

TABLE 1

Normalized thickness of

first dielectric layer (H/λ)

J1

1.40

J2

1.39

J3

1.38

J4

1.37

J5

1.36

J6

1.35

J7

1.34

J8

1.33

J9

1.32

J10

1.31

J11

1.30

J12

1.29

J13

 1.28.

8. The boundary acoustic wave device according to claim 1 , wherein the first dielectric layer is made of SiO 2 , and when an Euler angle θ of the piezoelectric substrate and h×a/λ obtained by multiplying a wavelength normalized thickness h/λ of the IDT electrode, which is obtained by normalization with a wavelength λ, by a ratio a of a density of the IDT electrode to a density of Pt, are within each of regions K 1 to K 11 shown in FIG. 10 , a wavelength normalized thickness H/λ of the first dielectric layer, which is obtained by normalization of the set thickness of the first dielectric layer with the wavelength λ, is equal to or less than a value described in Table 2 below:

TABLE 2

Normalized thickness of

first dielectric layer (H/λ)

K1

1.39

K2

1.38

K3

1.37

K4

1.36

K5

1.35

K6

1.34

K7

1.33

K8

1.32

K9

1.31

K10

1.30

K11

 1.29.