Acoustic resonators with high acoustic velocity layers

1. An acoustic resonator device comprising:

a piezoelectric layer having front and back surfaces;

a high acoustic velocity layer on the back surface, wherein the high acoustic velocity layer has a higher acoustic velocity than an acoustic velocity of the piezoelectric layer; and

an interdigital transducer (IDT) on the front surface,

wherein a thickness of the high acoustic velocity layer is smaller than a pitch of interleaved fingers of the IDT, and

wherein the pitch is a center-to-center spacing of adjacent fingers of the interleaved fingers of the IDT.

2. The device of claim 1 , wherein the piezoelectric layer is lithium niobate.

3. The device of claim 1 , wherein the high acoustic velocity layer is diamond.

4. The device of claim 1 , wherein the high acoustic velocity layer is boron nitride.

5. The device of claim 1 , further comprising a substrate, wherein the high acoustic velocity layer is directly attached to the piezoelectric layer and is between the piezoelectric layer and the substrate.

6. The device of claim 1 , wherein the interleaved fingers of the IDT are positioned over a cavity of the acoustic resonator device.

7. The device of claim 2 , wherein the IDT is configured such that a radio frequency signal applied to the IDT excite a shear horizontal acoustic mode in the piezoelectric layer, the shear horizontal acoustic mode being a bulk shear mode where acoustic energy propagates along a direction substantially parallel to the front surface of the piezoelectric layer and with displacements normal to a direction of an electric field created by the interleaved fingers.

8. The device of claim 1 , further comprising an oxide layer between the piezoelectric layer and the high velocity acoustic layer.

9. A filter device comprising:

a piezoelectric layer comprising lithium niobate and having front and back surfaces;

a high acoustic velocity layer on the back surface, wherein the high acoustic velocity layer has a higher acoustic velocity than an acoustic velocity of the piezoelectric layer; and

a conductor pattern on the front surface, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including at least one series resonator and at least one shunt resonator,

wherein a thickness of the high acoustic velocity layer is smaller than a pitch of interleaved fingers of at least one IDT of the plurality of IDTs, and

wherein the pitch is a center-to-center spacing of adjacent fingers of the interleaved fingers of the at least one IDT of the plurality of IDTs.

10. The device of claim 9 , wherein the high acoustic velocity layer is diamond.

11. The device of claim 9 , wherein the high acoustic velocity layer is boron nitride.

12. The device of claim 9 , further comprising a substrate, wherein the high acoustic velocity layer is directly attached to the piezoelectric layer and is between the piezoelectric layer and the substrate.

13. The device of claim 12 , wherein the interleaved fingers of the at least one IDT are positioned over at least one cavity of the filter device.

14. The device of claim 9 , wherein the respective IDTs of the plurality of IDTs are configured such that respective radio frequency signals applied to the IDTs excite a shear horizontal acoustic mode in the piezoelectric layer, the shear horizontal acoustic mode being a bulk shear mode where acoustic energy propagates along a direction substantially parallel to the front surface of the piezoelectric layer and with displacements normal to a direction of an electric field created by the interleaved fingers.

15. The device of claim 9 , further comprising an oxide layer between the piezoelectric layer and the high acoustic velocity layer.

16. A method of fabricating an acoustic resonator device, the method comprising:

attaching a high acoustic velocity layer to a back side of a piezoelectric layer, wherein the high acoustic velocity layer has a higher acoustic velocity than an acoustic velocity of the piezoelectric layer; and

forming an interdigital transducer (IDT) on a front side of the piezoelectric layer,

wherein the high acoustic velocity layer is formed to have a thickness that is smaller than a pitch of interleaved fingers of the IDT, the pitch being a center-to-center spacing of adjacent fingers of the interleaved fingers of the IDT.

17. The method of claim 16 , wherein the piezoelectric layer is lithium niobate.

18. The method of claim 16 , wherein the high acoustic velocity layer is diamond.

19. The device of claim 16 , wherein the high acoustic velocity layer is boron nitride.

20. The device of claim 1 , wherein at least one finger of the interleaved fingers of the IDT has a thickness that is less than 12% of λ, and wherein λ is equal to two times the pitch.