Transversely-excited film bulk acoustic resonators with narrow gaps between busbars and ends of interdigital transducer fingers
An acoustic resonator has a piezoelectric plate attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) formed on the plate has interleaved fingers on the diaphragm with first parallel fingers extending from a first busbar and second parallel fingers extending from a second busbar of the IDT. An average center-to-center distance between adjacent interleaved fingers defines an IDT pitch. The IDT has a gap distance between the ends of the first plurality of parallel fingers and the second busbar, and between the ends of the second plurality of parallel fingers and the first busbar; and the gap distance is less than ⅔ times the IDT pitch.
1 . An acoustic resonator device, comprising:
a substrate having a surface;
a piezoelectric layer having front and back surfaces, the back surface attached to the surface of the substrate and a portion of the piezoelectric layer forming a diaphragm that is over a cavity; and
an interdigital transducer (IDT) on the front surface of the piezoelectric layer such that interleaved fingers of the IDT are on the diaphragm;
wherein the interleaved fingers include a first plurality of fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT,
wherein an average center-to-center distance between all pairs of adjacent interleaved fingers defines an IDT pitch,
wherein the IDT includes a gap distance between ends of the first plurality of fingers and the second busbar, and between ends of the second plurality of fingers and the first busbar,
wherein the gap distance is between ⅔ and ½ times the IDT pitch,
wherein the acoustic resonator device is a shunt resonator of a ladder filter that includes a plurality of series resonators and a plurality of shunt resonators including the acoustic resonator device, and
wherein the IDT pitch is between 2 and 20 times a width of the interleaved fingers of the IDT.
2 . The acoustic resonator device of claim 1 , wherein the gap distance is between 1.0 μm and 5 μm.
3 . The acoustic resonator device of claim 1 , wherein the gap distance is between 1.5 μm and 5.0 μm, and the IDT pitch is between 3 μm and 7.5 μm.
4 . The acoustic resonator device of claim 1 , wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm and a thickness of the piezoelectric layer is selected to tune the primary shear acoustic mode.
5 . The acoustic resonator device of claim 4 , wherein the piezoelectric layer is a Y-cut lithium niobate.
6 . The acoustic resonator device of claim 4 , wherein:
the radio frequency signal applied to the IDT excites spurious modes in a gap region between the respective ends of the interleaved fingers and the adjacent busbars that cause undesired spurs in an admittance of the acoustic resonator device; and
the gap distance is configured to suppress the spurious modes by at least 10 dB during excitation of the primary shear acoustic mode.
7 . The acoustic resonator device of claim 1 , wherein the gap distance of between ⅔ and ½ the IDT pitch is configured to suppress spurious modes at a band edge of a filter device including the acoustic resonator device.
8 . A filter device, comprising:
a substrate having a surface;
a piezoelectric layer having front and back surfaces, the back surface attached to the surface of the substrate, the piezoelectric layer including portions that form a plurality of diaphragms that are over respective cavities; and
a conductor pattern on the front surface of the piezoelectric layer, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a corresponding plurality of resonators that includes at least one shunt resonator and at least one series resonator of a ladder filter,
wherein interleaved fingers of each IDT are on a respective diaphragm of the plurality of diaphragms,
wherein each IDT of the plurality of IDTs comprises a first plurality of fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT,
wherein, for each IDT of the plurality of IDTs, an average center-to-center distance between all pairs of adjacent interleaved fingers defines a respective IDT pitch,
wherein each IDT of the plurality of IDTs includes a respective gap distance between ends of the first plurality of fingers and the second busbar, and between ends of the second plurality of fingers and the first busbar, and
wherein, for the at least one shunt resonator of the plurality of resonators, the gap distance of the respective IDT is between ⅔ and ½ of the respective IDT pitch, and
wherein the IDT pitch of each of the plurality of IDTs is between 2 and 20 times a width of interleaved fingers of the respective IDT.
9 . The filter device of claim 8 , wherein the gap distance of the IDT of the at least one shunt resonator is between 5 μm and 1.0 μm.
10 . The filter device of claim 9 , wherein the IDT pitch of the IDT of the at least one shunt resonator is between 3 μm and 7.5 μm.
11 . The filter device of claim 8 , wherein a radio frequency signal applied to the IDT of the at least one shunt resonator excites a primary shear acoustic mode in the respective diaphragm, and a thickness of the piezoelectric layer is selected to tune the primary shear acoustic mode in the diaphragm.
12 . The device of claim 11 , wherein:
the piezoelectric layer is a Y-cut lithium niobate piezoelectric material;
the radio frequency signal applied to the IDT of the at least one shunt resonator excites spurious modes in a gap region between the respective ends of the interleaved fingers of the at least one shunt resonator and the respective first and second busbars that cause undesired spurs in an admittance of the at least one shunt resonator; and
the gap distance is configured to suppress the spurious modes by at least 10 dB during excitation of the primary shear acoustic mode.
13 . The filter device of claim 8 , wherein the gap distance of the respective IDT of the at least one shunt resonator being between ⅔ and ½ the IDT pitch is configured to suppress spurious modes at a band edge of the filter device.
14 . A method of fabricating an acoustic resonator device, comprising:
bonding a back surface of a piezoelectric layer to a substrate such that a portion of the piezoelectric layer forms a diaphragm spanning a cavity in the substrate; and
forming an interdigital transducer (IDT) on a front surface of the piezoelectric layer such that interleaved fingers of the IDT are disposed on the diaphragm,
wherein the interleaved fingers include a first plurality of fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT,
wherein an average center-to-center distance between all pairs of interleaved fingers defines an IDT pitch,
wherein the IDT includes a gap distance between ends of the first plurality of fingers and the second busbar, and between ends of the second plurality of fingers and the first busbar;
wherein the gap distance is between ½ and ⅓ times the IDT pitch,
wherein the acoustic resonator device is a shunt resonator of a ladder filter that includes a plurality of series resonators and a plurality of shunt resonators including the acoustic resonator device, and
wherein the IDT pitch is between 2 and 20 times a width of interleaved fingers of the IDT.
15 . The method of claim 14 , wherein the gap distance is between 1.0 μm and 5 μm.
16 . The method of claim 14 , wherein the gap distance is between 1.5 μm and 5.0 μm, and the IDT pitch is between 3 μm and 7.5 μm.
17 . The method of claim 14 , further comprising:
applying a radio frequency signal to the IDT to excite a primary shear acoustic mode in the diaphragm; and
selecting a thickness of the piezoelectric layer to tune the primary shear acoustic mode in the diaphragm.
18 . The method of claim 17 , wherein:
the piezoelectric layer is a Y-cut lithium niobate;
the radio frequency signal applied to the IDT excites spurious modes in a gap region between the respective ends of the respective interleaved fingers and the respective first and second busbars that cause undesired spurs in an admittance of the acoustic resonator device; and
the gap distance is configured to suppress the spurious modes by at least 10 dB during excitation of the primary shear acoustic mode.
19 . The method of claim 14 , wherein the gap distance of between ⅔ and ½ the IDT pitch is configured to suppress spurious modes at a band edge of a filter device including the acoustic resonator device.