Transversely-excited film bulk acoustic resonators with piezoelectric diaphragm supported by piezoelectric substrate
Acoustic resonators and filter devices, and methods for making acoustic resonators and filter devices. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces. The back surface is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. A conductor pattern formed is formed on the front surface of the piezoelectric plate, including an interdigital transducer (IDT) with interleaved fingers of the IDT on the diaphragm. An insulating layer is formed between the piezoelectric plate and portions of the conductor pattern other than the interleaved fingers.
1. A filter 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 of the piezoelectric plate,
wherein an insulating layer is disposed between the piezoelectric plate and portions of the conductor pattern other than the interleaved fingers, and wherein
the piezoelectric plate and interleaved fingers are configured such that a radio frequency signal applied to the plurality of IDTs excites a primary shear acoustic mode in the one or more diaphragms.
2. The filter device of claim 1 , wherein
the piezoelectric plate is one of lithium niobate and lithium tantalate.
3. The filter device of claim 2 , wherein
the piezoelectric plate is one of Z-cut, rotated Z-cut, and rotated YX-cut.
4. The filter device of claim 1 , wherein the insulating layer is comprised of one or both of SiO 2 and Si 3 N 4 .
5. The filter device of claim 4 , wherein the insulating layer has a thickness in a range between 1 nm and a thickness of the conductor pattern.
6. The filter device of claim 1 , wherein the portions of the conductor pattern comprise at least one busbar, and the insulating layer is between the at least one busbar and the piezoelectric plate.
7. The filter device of claim 6 , wherein the insulating layer is configured to reduce acoustic coupling between the at least one busbar and the piezoelectric plate.
8. The filter device of claim 1 , wherein the insulating layer does not extend across the cavities.
9. The filter device of claim 1 further comprising a front-side dielectric formed on the front surface of the piezoelectric plate between the interleaved fingers.
10. A method of fabricating a filter device comprising:
attaching a piezoelectric plate to a substrate, the piezoelectric plate having front and back surfaces, the back surface attached to the substrate, portions of the piezoelectric plate forming one or more diaphragms spanning respective cavities in the substrate;
forming an insulating layer on the front surface of the piezoelectric plate; and
forming a conductor pattern comprising a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators on the piezoelectric plate, interleaved fingers of each of the plurality of IDTs on the one or more diaphragms of the piezoelectric plate,
wherein the insulating layer is disposed between the piezoelectric plate and portions of the conductor pattern other than the interleaved fingers, and wherein
the piezoelectric plate and interleaved fingers are configured such that a radio frequency signal applied to the plurality of IDTs excites a primary shear acoustic mode in the diaphragm.
11. The method of claim 10 , wherein the insulating layer is comprised one or both of SiO 2 and Si 3 N 4 .
12. The method of claim 11 , wherein the insulating layer has a thickness in a range between 1 nm and a thickness of the conductor pattern.
13. The method of claim 10 , wherein
the piezoelectric plate is one of lithium niobate and lithium tantalate.
14. The method of claim 13 , wherein
the piezoelectric plate is one of Z-cut, rotated Z-cut, and rotated YX-cut.
15. The method of claim 10 , wherein the conductor pattern comprises at least one busbar, and the insulating layer is between the at least one busbar and the piezoelectric plate.
16. The method of claim 15 , wherein the insulating layer is configured to reduce acoustic coupling between the busbar and the piezoelectric plate.
17. The method of claim 10 , wherein the insulating layer does not extend across the cavity.
18. The method of claim 10 further comprising forming a front-side dielectric on the front side of the piezoelectric plate between the interleaved fingers.