Surface acoustic wave device

1. A surface acoustic wave device comprising:

a piezoelectric substrate;

an IDT electrode provided on the piezoelectric substrate;

a first dielectric layer provided on the piezoelectric substrate and made of silicon oxide; and

a second dielectric layer provided on the first dielectric layer and having an acoustic velocity greater than that of the first dielectric layer; wherein

the surface acoustic wave device further includes a third dielectric layer provided between the first dielectric layer and the piezoelectric substrate so as to cover a surface of the piezoelectric substrate and an upper surface and side surfaces of the IDT electrode; and

a thickness of the third dielectric layer is at least about 0.015λ, where λ is a wavelength of the IDT electrode determined by a pitch between electrode fingers of the IDT electrode.

2. The surface acoustic wave device according to claim 1 , wherein a thickness of the third dielectric layer is equal to or less than that of the first dielectric layer.

3. The surface acoustic wave device according to claim 1 , wherein the third dielectric layer is made of at least one of silicon nitride, silicon oxynitride, diamond, alumina, or aluminum nitride.

4. The surface acoustic wave device according to claim 1 , wherein the second dielectric layer is made of at least silicon nitride, silicon oxynitride, diamond, alumina, or aluminum nitride.

5. The surface acoustic wave device according to claim 1 , wherein the piezoelectric substrate is made of one of LiNbO 3 or LiTaO 3 .

6. The surface acoustic wave device according to claim 5 , wherein the piezoelectric substrate is a 120° to 130° rotated Y-cut X-propagation LiNbO 3 substrate.

7. The surface acoustic wave device according to claim 5 , wherein the piezoelectric substrate is a −10° to 10° rotated Y-cut X-propagation LiNbO 3 substrate.

8. The surface acoustic wave device according to claim 1 , wherein the third dielectric layer is in direct contact with the upper surface and the side surfaces of the IDT electrode.

9. The surface acoustic wave device according to claim 1 , wherein the IDT electrode includes a pair of comb-shaped electrodes arranged side by side to one another.

10. The surface acoustic wave device according to claim 1 , wherein the IDT electrode is made of a metal selected from the group consisting of Al, Pt, Au, Ag, Cu, Ni, Ti, Cr, and Pd or an alloy including at least one metal selected from the group consisting of Al, Pt, Au, Ag, Cu, Ni, Ti, Cr, and Pd.

11. The surface acoustic wave device according to claim 1 , wherein the IDT electrode is defined by a laminate of electrically conductive layers.

12. The surface acoustic wave device according to claim 11 , wherein each of the electrically conductive layers is made of a metal selected from the group consisting of Al, Pt, Au, Ag, Cu, Ni, Ti, Cr, and Pd or an alloy including at least one metal selected from the group consisting of Al, Pt, Au, Ag, Cu, Ni, Ti, Cr, and Pd.

13. The surface acoustic wave device according to claim 11 , wherein the first dielectric layer is made of silicon oxide.

14. The surface acoustic wave device according to claim 10 , wherein a thickness of the first dielectric layer is in a range from approximately a thickness of the IDT electrode to about 0.5λ.

15. The surface acoustic wave device according to claim 11 , wherein a thickness of the second dielectric layer is in a range of about 0.002λ to about 0.05λ.

16. The surface acoustic wave device according to claim 11 , wherein the third dielectric layer is made of a material that is different from a material from which the first dielectric layer is made.