Transversely-excited film bulk acoustic resonators wafer-level packaging using a dielectric cover
An acoustic resonator device includes a substrate having a surface and a single-crystal piezoelectric plate bonded to the substrate surface. A portion of the piezoelectric plate forms a diaphragm that spans a cavity. A conductor pattern including an interdigital transducer (IDT) formed on a surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. A dielectric cover is disposed over the IDT and the plate, and the dielectric cover forms an air gap above the IDT and the plate.
1. An acoustic resonator device comprising:
a substrate having a surface;
a piezoelectric layer attached to the surface of the substrate via an intermediate layer, the piezoelectric layer including a portion that forms a diaphragm that is over a cavity;
a conductor pattern including an interdigital transducer (IDT) on a surface of the piezoelectric layer and having interleaved fingers on the diaphragm; and
a dielectric cover over the IDT and the piezoelectric layer, the dielectric cover including a bottom surface attached to a portion of the conductor pattern, wherein the conductor pattern includes at least one metal layer, and wherein the dielectric cover is attached to the at least one metal layer,
wherein the dielectric cover forms an air gap above the IDT and the piezoelectric layer.
2. The acoustic resonator device of claim 1 , further comprising one or more contact vias that include a conductive material and that extend through the dielectric cover.
3. The acoustic resonator device of claim 2 , wherein the conductive material in the contact vias electrically connects the IDT to external devices.
4. The acoustic resonator device of claim 1 , wherein the conductor pattern comprises a plurality of metal layers including the at least one metal layer, wherein the IDT comprises a first set of metal layers and the at least one metal layer electrically connects the IDT to vias in the dielectric cover.
5. The acoustic resonator device of claim 4 , wherein the at least one metal layer is thicker than the set of metal layers of the IDT.
6. The acoustic resonator device of claim 1 , wherein the air gap is between the bottom surface of the dielectric cover, side surfaces of the at least one metal layer and a top surface of the piezoelectric layer.
7. The acoustic resonator device of claim 1 , wherein:
the IDT includes a first set of the interleaved fingers attached to and extending from a first busbar, and a second set of the interleaved fingers attached to and extending from a second busbar; and
the conductor pattern includes the at least one metal layer on the first and second busbars.
8. The acoustic resonator device of claim 1 , wherein the dielectric cover has a height that is between 40 micrometers (μm) and 80 μm.
9. The acoustic resonator device of claim 1 , wherein a radio frequency signal applied to the IDT excites a primarily shear acoustic mode in the diaphragm, such that acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric layer, which is also normal to a direction of an electric field created by the interleaved fingers.
10. The acoustic resonator device of claim 1 , wherein the cavity is in the intermediate layer.
11. The acoustic resonator device of claim 1 , wherein the at least one metal layer comprises multiple layers of metal and dielectric material.
12. The acoustic resonator device of claim 1 , wherein a perimeter of the gap has a different area than a perimeter of the cavity.
13. The acoustic resonator device of claim 1 , wherein a thickness of the dielectric cover is between 12 percent and 60 percent of a thickness of the piezoelectric layer.
14. A method of fabricating radio frequency filters, the method comprising:
forming a plurality of filter circuits on a surface of a wafer, each filter circuit comprising a plurality of transversely-excited film bulk acoustic resonators (XBARs) that each include a conductor pattern that includes an interdigital transducer (IDT) on a surface of a piezoelectric layer and that has interleaved fingers on a diaphragm of the piezoelectric layer;
bonding a dielectric cover over the surface of the wafer to form a laminate, such that the dielectric cover forms an air gap above the IDT and the piezoelectric layer of at least one of the plurality of filter circuits, the dielectric cover including a bottom surface attached to a portion of the conductor pattern, and the conductor pattern including at least one metal layer that is attached to the dielectric cover; and
dicing the laminate to provide individual filters.
15. A radio frequency filter comprising:
a plurality of bulk acoustic wave resonators that each include:
a substrate having a surface;
a piezoelectric layer;
a conductor pattern on a surface of the piezoelectric layer, the conductor pattern including an interdigital transducer (IDT) that includes interleaved fingers; and
a dielectric cover over the interleaved fingers and the piezoelectric layer, the dielectric cover forming an air gap above the interleaved fingers of the IDT and the piezoelectric layer
wherein the dielectric cover includes a bottom surface attached to a portion of the conductor pattern, and
wherein the conductor pattern includes at least one metal layer that is attached to the dielectric cover.
16. The radio frequency filter of claim 15 , wherein the plurality of bulk acoustic wave resonators includes a plurality of series resonators and a plurality of shunt resonators.
17. The radio frequency filter of claim 16 , wherein the dielectric cover is configured to at least one of: a) reduce capacitive coupling between each of the plurality of series resonators and the plurality of shunt resonators, and b) reduce charges on a surface of the dielectric cover that act as a resistive layer.
18. The radio frequency filter of claim 15 , wherein the dielectric cover of each of the plurality of bulk acoustic wave resonators is Quartz glass and has a height that is between 40 micrometers (μm) and 80 μm.
19. The radio frequency filter of claim 15 , wherein a radio frequency signal applied to each IDT of the plurality of bulk acoustic wave resonators excites a primarily shear acoustic mode in the piezoelectric layer, such that acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric layer, which is also normal to a direction of an electric field created by the interleaved fingers of the respective bulk acoustic wave resonator.
20. The radio frequency filter of claim 15 , wherein the conductor pattern comprises a plurality of metal layers including the at least one metal layer, wherein the IDT comprises a first set of metal layers and the at least one metal layer electrically connects the IDT to vias in the dielectric cover.