Acoustic wave resonator and duplexer using same

1. An acoustic wave resonator comprising:

a piezoelectric body;

an IDT electrode on the piezoelectric body, for exciting an acoustic wave with wavelength λ; and

a dielectric thin film covering the IDT electrode, for propagating a side wave slower than a side wave propagated through the piezoelectric body,

wherein the IDT electrode includes a bus bar electrode region, a dummy electrode region, and an IDT cross region in an order from outside in a direction in which electrode fingers of the IDT electrode extend, and

wherein a film thickness of the dielectric thin film above at least one of the bus bar electrode region and the dummy electrode region is smaller than a film thickness of the dielectric thin film above the IDT cross region by 0.1λ to 0.25λ.

2. An acoustic wave resonator comprising:

a piezoelectric body;

an IDT electrode on the piezoelectric body, for exciting an acoustic wave with wavelength λ; and

a dielectric thin film covering the IDT electrode, for propagating a side wave slower than a side wave propagated through the piezoelectric body,

wherein the IDT electrode includes a bus bar electrode region, a gap region, and an IDT cross region in an order from outside in a direction in which electrode fingers of the IDT electrode extend, and

wherein a film thickness of the dielectric thin film above at least one of the bus bar electrode region and the gap region is smaller than a film thickness of the dielectric thin film above the IDT cross region by 0.1λ to 0.25λ.

3. The acoustic wave resonator of claim 1 , wherein the film thickness of the dielectric thin film above the IDT cross region is set so that a sound velocity of an acoustic wave excited by the IDT electrode is lower than a sound velocity of a side wave propagated through the piezoelectric body.

4. The acoustic wave resonator of claim 1 , wherein the IDT electrode is a normal-type, comb-shaped electrode having a cross width constant in a direction in which the acoustic wave is propagated.

5. The acoustic wave resonator of claim 1 , wherein the dielectric thin film is formed of silicon oxide.

6. The acoustic wave resonator of claim 5 , wherein the film thickness of the dielectric thin film above the IDT cross region is set so that a temperature characteristic of an acoustic wave excited by the IDT electrode is 10 ppm/° C. or lower.

7. The acoustic wave resonator of claim 1 , wherein the film thickness of the dielectric thin film above the IDT cross region is 0.27λ to 0.5λ.

8. The acoustic wave resonator of claim 1 , wherein the dummy electrode region includes a metallized dummy electrode weighting part.

9. The acoustic wave resonator of claim 1 ,

wherein the IDT electrode is apodized weighted so that a cross width of the IDT cross region becomes gradually narrower from a center of the IDT cross region toward an end of the IDT cross region in a direction in which the acoustic wave is propagated, and

wherein an angle formed by a straight line connecting front ends of the electrode fingers from the center of the IDT cross region toward the end of the IDT cross region is between 4 and 10 degrees.

10. The acoustic wave resonator of claim 1 ,

wherein the piezoelectric body is based on lithium niobate having Euler angle (φ, θ, ψ),

wherein the dielectric thin film has a film thickness above the IDT cross region greater than 0.27λ, and

wherein the Euler angle satisfies

−100°≦θ≦−60°,

1.193φ−2°≦ψ≦1.193φ+2°, and

ψ≦−2φ−3°,−2φ+3°≦ψ.

11. The acoustic wave resonator of claim 1 ,

wherein the piezoelectric body is based on lithium niobate having an Euler angle (φ, θ, ψ),

wherein the dielectric thin film has a film thickness above the IDT cross region greater than 0.2λ,

wherein the dielectric thin film has a projection on a cross section orthogonal to a direction in which the electrode fingers of the IDT electrode extend, above the electrode fingers of the IDT electrode,

wherein a width of a top of the projection is smaller than a width of the electrode fingers of the IDT electrode, and

wherein the Euler angle satisfies

−100°≦θ≦−60°,

1.193φ−2°≦ψ≦1.193φ+2°, and

ψ≦−2φ−3°,−2φ+3°≦ψ.

12. The acoustic wave resonator of claim 10 , wherein the Euler angle satisfies −20°≦φ≦20°.

13. The acoustic wave resonator of claim 11 , wherein the dielectric thin film has a shape of the cross section with a side line curved outward from the top of the projection toward a bottom of the projection.

14. The acoustic wave resonator of claim 11 , wherein the width of the top of the projection is smaller than ½ of the width of an electrode finger of the IDT electrode.

15. The acoustic wave resonator of claim 11 , wherein a center position of the top of the projection coincides substantially with a point above a center position of the electrode finger.

16. The acoustic wave resonator of claim 11 , wherein 0.03λ<T≦h is satisfied, assuming that T is a height of the projection and h is a film thickness of the IDT electrode.

17. A duplexer having a transmission filter and a reception filter, wherein the transmission filter or the reception filter includes the acoustic wave resonator of claim 1 .

18. The acoustic wave resonator of claim 2 , wherein the film thickness of the dielectric thin film above the IDT cross region is set so that a sound velocity of an acoustic wave excited by the IDT electrode is lower than a sound velocity of a side wave propagated through the piezoelectric body.

19. The acoustic wave resonator of claim 2 , wherein the IDT electrode is a normal-type, comb-shaped electrode having a cross width constant in a direction in which the acoustic wave is propagated.

20. The acoustic wave resonator of claim 2 , wherein the dielectric thin film is formed of silicon oxide.

21. The acoustic wave resonator of claim 2 , wherein the film thickness of the dielectric thin film above the IDT cross region is 0.27λ to 0.5λ.

22. The acoustic wave resonator of claim 2 , wherein the dummy electrode region includes a metallized dummy electrode weighting part.

23. The acoustic wave resonator of claim 2 ,

wherein the IDT electrode is apodized weighted so that a cross width of the IDT cross region becomes gradually narrower from a center of the IDT cross region toward an end of the IDT cross region in a direction in which the acoustic wave is propagated, and

wherein an angle formed by a straight line connecting front ends of the electrode fingers from the center of the IDT cross region toward the end of the IDT cross region is between 4 and 10 degrees.

24. The acoustic wave resonator of claim 2 ,

wherein the piezoelectric body is based on lithium niobate having Euler angle (φ, θ, ψ),

wherein the dielectric thin film has a film thickness above the IDT cross region greater than 0.27λ, and

wherein the Euler angle satisfies

−100°≦θ≦−60°,

1.193φ−2°≦ψ≦1.193φ+2°, and

ψ≦−2φ−3°,−2φ+3°≦ψ.

25. The acoustic wave resonator of claim 2 ,

wherein the piezoelectric body is based on lithium niobate having an Euler angle (φ, θ, ψ),

wherein the dielectric thin film has a film thickness above the IDT cross region greater than 0.2λ,

wherein the dielectric thin film has a projection on a cross section orthogonal to a direction in which the electrode fingers of the IDT electrode extend, above the electrode fingers of the IDT electrode,

wherein a width of a top of the projection is smaller than a width of the electrode fingers of the IDT electrode, and

wherein the Euler angle satisfies

−100°≦θ≦−60°,

1.193φ−2°≦ψ≦1.193φ+2°, and

ψ≦−2φ−3°,−2φ+3°≦ψ.

26. The acoustic wave resonator of claim 11 , wherein the Euler angle satisfies −20°≦φ≦20°.