Transversely-excited film bulk acoustic resonator with reduced loss in the aperture direction
Acoustic resonator devices and acoustic filter devices. An acoustic resonator includes a piezoelectric plate having front and back surfaces, the back surface facing a substrate. A conductor pattern is formed on the front surface. The conductor pattern includes interleaved interdigital transducer (IDT) fingers connected alternately to first and second busbars, wherein a mark mt of the IDT fingers in margins of an aperture is greater than a mark m of the IDT fingers in a central portion of the aperture.
1. An acoustic resonator, comprising:
a substrate;
a piezoelectric layer having a front surface and a back surface facing the substrate; and
a conductor pattern on the piezoelectric layer and comprising an interleaved interdigital transducer (IDT) having fingers connected alternately to first and second busbars,
wherein a mark mt of the fingers of the IDT in margins of an aperture is a same mark m of the fingers of the IDT in a central portion of the aperture,
wherein a thickness of the fingers measured in a direction substantially perpendicular to the mark mt is greater in an area associated with the mark mt and less in an area associated with the mark m,
wherein a dimension MA of the margins, measured parallel to a length direction of the fingers of the IDT, is greater than or equal to 0.5p and less than or equal to 1.5p, where p is a pitch of the IDT that is measured as a center-to-center spacing between a pair of adjacent fingers of the IDT, and
wherein a ratio of the mark m to the pitch p is between 0.2 and 0.3.
2. The acoustic resonator of claim 1 , wherein a portion of the piezoelectric layer is a diaphragm spanning a cavity in the substrate, and the fingers of the IDT are on the diaphragm.
3. The acoustic resonator of claim 1 , further comprising an acoustic Bragg reflector between the substrate and the back surface of the piezoelectric layer.
4. The acoustic resonator of claim 1 , wherein the piezoelectric layer is Z-cut lithium niobate.
5. The acoustic resonator of claim 1 , wherein a dimension of gaps between ends of the fingers of the IDT and the first and second busbars, respectively, measured parallel to the length direction of the fingers of the IDT, is greater than or equal to 0.5p and less than or equal to 1.5p.
6. An acoustic resonator, comprising:
a substrate;
a piezoelectric layer having a front surface and a back surface facing the substrate; and
a conductor pattern of an interdigital transducer (IDT) on the piezoelectric layer, the conductor pattern comprising a first plurality of fingers extending from a first busbar and a second plurality of fingers extending from a second bus bar, wherein the first and second pluralities of fingers are interleaved,
wherein an overlap of the first and second pluralities of fingers defines an aperture of the acoustic resonator,
wherein a mark of the first and second pluralities of fingers is mt in margins of the aperture and m in a central portion of the aperture, and 1.04m≤mt≤1.13m,
wherein a dimension MA of the margins, measured parallel to a length direction of the first and second pluralities of fingers, is greater than or equal to 0 . 5 p and less than or equal to 1 . 5 p, where p is a pitch that is measured as a center-to-center spacing between a pair of adjacent fingers of the first and second pluralities of fingers of the IDT, and
wherein the piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the piezoelectric layer in which acoustic energy propagates along a direction substantially orthogonal to the front and back surfaces of the piezoelectric layer and that is transverse to a lateral direction of an electric field created by the first and second pluralities of fingers.
7. The acoustic resonator of claim 6 , wherein:
a portion of the piezoelectric layer is a diaphragm over a cavity, and
the first and second pluralities of fingers are on the diaphragm.
8. The acoustic resonator of claim 6 , further comprising an acoustic Bragg reflector between the substrate and the back surface of the piezoelectric layer.
9. The acoustic resonator of claim 6 , wherein the piezoelectric layer is Z-cut lithium niobate.
10. The acoustic resonator of claim 6 , wherein a dimension of gaps between ends of the interleaved fingers of the IDT and the oppose first and second busbars, respectively, measured parallel to the length direction of the interleaved fingers of the IDT, is greater than or equal to 0.5p and less than or equal to 1.5p.
11. A filter device, comprising:
a piezoelectric layer having front and back surfaces, the back surface facing a substrate; and
a conductor pattern at the piezoelectric layer and comprising a plurality of interdigital transducers (IDTs), each IDT of the plurality of IDTs comprising interleaved fingers connected alternately to a first busbar and a second busbar,
wherein, for at least one IDT of the plurality of IDTs, a mark mt of the interleaved fingers in margins of an aperture is the same as a mark m of the interleaved fingers in a central portion of the aperture, and a thickness of the interleaved fingers measured in a direction substantially perpendicular to the mark mt is greater in an area associated with the mark mt than in an area associated with the mark m,
wherein, for the at least one IDT, a dimension MA of the margins, measured parallel to a length direction of the interleaved fingers, is greater than or equal to 0.5p and less than or equal to 1.5p, where p is a pitch that is measured as a center-to-center spacing between a pair of adjacent fingers of the interleaved fingers of the IDT, and
wherein a dimension of gaps between ends of the interleaved fingers of the IDT and the first and second busbars, respectively, measured parallel to the length direction of the fingers of the IDT, is greater than or equal to 0.5p and less than or equal to 1.5p.
12. The filter device of claim 11 , wherein:
portions of the piezoelectric layer are one or more diaphragms spanning respective cavities in the substrate, and
the interleaved fingers of each IDT are on a respective diaphragm.
13. The filter device of claim 11 , further comprising an acoustic Bragg reflector between the substrate and the back surface of the piezoelectric layer.
14. The acoustic resonator of claim 2 , wherein the IDT is disposed on the front surface of the piezoelectric layer and opposite the cavity in the substrate.
15. The acoustic resonator of claim 7 , wherein the IDT is disposed on the front surface of the piezoelectric layer and opposite the cavity in the substrate.
16. The filter device of claim 12 , wherein the plurality of IDTs are each disposed on the front surface of the piezoelectric layer.
17. The acoustic resonator of claim 2 , wherein the piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites a primarily shear acoustic mode in the piezoelectric layer in which acoustic energy propagates along a direction substantially orthogonal to the front and back surfaces of the piezoelectric layer and that is transverse to a lateral direction of an electric field created by the fingers of the IDT.
18. The acoustic resonator of claim 11 , wherein the piezoelectric layer and the at least one IDT are configured such that a radio frequency signal applied to the at least one IDT excites a primarily shear acoustic mode in the piezoelectric layer in which acoustic energy propagates along a direction substantially orthogonal to the front and back surfaces of the piezoelectric layer and that is transverse to a lateral direction of an electric field created by the interleaved fingers.