Transversely-excited film bulk acoustic resonators with two-layer electrodes with a wider top layer

1. An acoustic resonator device comprising:

a substrate having a surface;

a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate; and

an interdigital transducer formed on the front surface of the piezoelectric plate, interleaved fingers of the IDT disposed on the diaphragm configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm,

wherein the interleaved fingers comprise a first layer adjacent the diaphragm and a second layer over the first layer, the second layer having a greater width than the first layer.

2. The device of claim 1 , wherein the first layer is a different material than the second layer.

3. The device of claim 2 , wherein the first layer comprises a low transverse acoustic impedance metal.

4. The device of claim 2 , wherein the second layer comprises a high transverse acoustic impedance metal.

5. The device of claim 2 , wherein the first layer comprises aluminum or titanium.

6. The device of claim 2 , wherein the second layer comprises chromium or tungsten.

7. The device of claim 1 , wherein a thickness of the first layer is less than a thickness of the second layer.

8. The device of claim 1 , wherein a direction of acoustic energy flow of the primary acoustic mode is substantially normal to the front and back surfaces of the diaphragm.

9. The device of claim 1 , wherein the diaphragm is contiguous with the piezoelectric plate around at least 50% of a perimeter of the cavity.

10. A filter resonator device comprising:

a substrate having a surface;

a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate, portions of the piezoelectric plate forming one or more diaphragms spanning respective cavities in the substrate; and

a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators, interleaved fingers of each of the plurality of IDTs disposed on one of the one or more diaphragms, the piezoelectric plate and all of the IDTs configured such that respective radio frequency signals applied to each IDT excite respective shear primary acoustic modes in the respective diaphragm,

wherein the interleaved fingers comprise a first layer adjacent the one or more diaphragms and a second layer over the first layer, the second layer having a greater width than the first layer.

11. The device of claim 10 , wherein the first layer is a different material than the second layer.

12. The device of claim 11 , wherein the first layer comprises a low transverse acoustic impedance metal.

13. The device of claim 11 , wherein the second layer comprises a high transverse acoustic impedance metal.

14. The device of claim 11 , wherein the first layer comprises aluminum or titanium.

15. The device of claim 11 , wherein the second layer comprises chromium or tungsten.

16. The device of claim 10 , wherein a thickness of the first layer is less than a thickness of the second layer.

17. The device of claim 10 , wherein a direction of acoustic energy flow of the primary acoustic mode is substantially normal to the front and back surfaces of the one or more diaphragms.

18. The device of claim 10 , wherein the one or more diaphragms is contiguous with the piezoelectric plate around at least 50% of a perimeter of the cavity.

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

bonding a piezoelectric plate to a substrate;

forming a cavity in the substrate, before or after bonding the piezoelectric plate to the substrate, such that a portion of the piezoelectric plate forms a diaphragm spanning the cavity; and

forming an interdigital transducer (IDT) on the front surface of the piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the IDT configured to excite a primary acoustic mode in the diaphragm in response to a radio frequency signal applied to the IDT,

wherein the interleaved fingers comprise a first layer adjacent the diaphragm and a second layer over the first layer, the second layer having a greater width than the first layer.

20. The method of claim 19 , wherein the first layer is a different material than the second layer.

21. The method of claim 20 , wherein the first layer comprises a low transverse acoustic impedance metal.

22. The method of claim 20 , wherein the second layer comprises a high transverse acoustic impedance metal.

23. The method of claim 20 , wherein the first layer comprises aluminum or titanium.

24. The method of claim 20 , wherein the second layer comprises chromium or tungsten.

25. The method of claim 19 , wherein a thickness of the first layer is less than a thickness of the second layer.

26. The method of claim 19 , wherein a direction of acoustic energy flow of the primary acoustic mode is substantially normal to the front and back surfaces of the diaphragm.

27. The method of claim 19 , wherein the diaphragm is contiguous with the piezoelectric plate around at least 50% of a perimeter of the cavity.