Small transversely-excited film bulk acoustic resonators with enhanced Q-factor
An acoustic resonator device includes a conductor pattern formed on a surface of a piezoelectric plate. The conductor pattern includes a first busbar, a second busbar, and n interleaved parallel fingers of an interdigital transducer (IDT), where n is a positive integer. The fingers extend alternately from the first and second busbars. A first finger and an n'th finger are disposed at opposing ends of the IDT. The conductor pattern also includes a first reflector element proximate and parallel to the first finger and a second reflector element proximate and parallel to the n'th finger. When an RF signal is applied between the first and second busbars, the first reflector element is at substantially the same potential as the first finger and the second reflector element is at substantially the same potential as the n'th finger.
1. An acoustic resonator device comprising:
a piezoelectric layer; and
a conductor pattern at a surface of the piezoelectric layer, the conductor pattern comprising:
an interdigital transducer (IDT) including a first busbar, a second busbar, and n interleaved fingers, where n is a positive integer, wherein the n interleaved fingers extend alternately from the first and second busbars, and the n interleaved fingers include a first finger and an n'th finger at opposing ends of the IDT;
a first reflector element proximate to the first finger; and
a second reflector element proximate to the n'th finger,
wherein a center-to-center spacing from the first reflector element to the first finger is different than a center-center spacing of at least a pair of the n interleaved fingers, 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, the primary shear acoustic mode being a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric layer and transverse to a direction of an electric field created by the IDT.
2. The acoustic resonator device of claim 1 , wherein the first reflector element and the first finger are connected to a same one of the first and second busbars.
3. The acoustic resonator device of claim 1 , wherein
the first reflector element is capacitively coupled to the first finger, and
the second reflector element is capacitively coupled to the n'th finger.
4. The acoustic resonator device of claim 1 , wherein a distance pr between a center of the first reflector element and a center of the first finger and between a center of the second reflector element and a center of the n'th finger is greater than or equal to 1.2 times the center-to-center spacing of at least the pair of the n interleaved fingers of the IDT.
5. The acoustic resonator device of claim 1 , wherein a mark mr of the first and second reflector elements is configured to improve a Q-factor of the device at a predetermined frequency.
6. The acoustic resonator device of claim 5 , wherein
the acoustic resonator device is a shunt resonator in a ladder bandpass filter circuit having a passband, and
mr is selected to improve a Q-factor of the device at a lower edge of the passband.
7. The acoustic resonator device of claim 5 , wherein
the acoustic resonator device is a series resonator in a ladder bandpass filter circuit having a passband, and
mr is selected to improve a Q-factor of the device at an upper edge of the passband.
8. The acoustic resonator device of claim 1 , the conductor pattern further comprising:
a third reflector element proximate to the first reflector element, the third reflector element disposed such that the first reflector element is centered between the third reflector element and the first finger; and
a fourth reflector element proximate to the second reflector element, the fourth reflector element disposed such that the second reflector element is centered between the fourth reflector element and the n'th finger.
9. The acoustic resonator device of claim 8 , wherein
the first and third reflector elements and the first finger are connected to the same one of the first and second busbars, and
the second and fourth reflector elements and the n'th finger are connected to the same one of the first and second busbars.
10. The acoustic resonator device of claim 8 , wherein the first and third reflector elements are connected to each other and capacitively coupled to the first finger, and
the second and fourth reflector elements are connected to each other and capacitively coupled to the n'th finger.
11. The acoustic resonator device of claim 8 , wherein a distance pr between a center of the first reflector element and a center of the first finger, between respective centers of the first and third reflector elements, between respective centers of the second and fourth reflector elements, and between a center of the second reflector element and a center of the n'th finger is greater than or equal to 1.2 times the center-to-center spacing of at least a pair of the n interleaved fingers of the IDT.
12. The acoustic resonator device of claim 8 , wherein a mark mr of the first, second, third, and fourth reflector elements is configured to improve a Q-factor of the device at a predetermined frequency.
13. The acoustic resonator device of claim 12 , wherein
the acoustic resonator device is a shunt resonator in a ladder bandpass filter circuit having a passband, and
mr is selected to improve a Q-factor of the device at a lower edge of the passband.
14. The acoustic resonator device of claim 12 , wherein
the acoustic resonator device is a series resonator in a ladder bandpass filter circuit having a passband, and
mr is selected to improve a Q-factor of the device at an upper edge of the passband.
15. The acoustic resonator device of claim 8 , wherein a center-to-center spacing from the first reflector element to the third reflector element is different than the center-to-center spacing of at least the pair of the n interleaved fingers.
16. The acoustic resonator device of claim 1 , wherein a center-to-center spacing from the second reflector element to the n'th finger is different than the center-to-center spacing of at least the pair of the n interleaved fingers.
17. An acoustic resonator device comprising:
a substrate;
a piezoelectric layer coupled directly or by one or more intermediate layers to the substrate and including a portion that forms a diaphragm over a cavity; and
a conductor pattern at a surface of the piezoelectric layer, the conductor pattern comprising:
an interdigital transducer (IDT) including a first busbar, a second busbar, and n interleaved fingers, where n is a positive integer, wherein the n interleaved fingers extend alternately from the first and second busbars, and the n interleaved fingers include a first finger and an n'th finger at opposing ends of the IDT;
a first reflector element proximate to the first finger; and
a second reflector element proximate to the n'th finger,
wherein the n interleaved fingers, the first reflector element, and the second reflector element are disposed on the diaphragm, 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 diaphragm, the primary shear acoustic mode being a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric layer and transverse to a direction of an electric field created by the IDT.
18. The acoustic resonator device of claim 17 , wherein a mark mr of the first and second reflector elements is configured to improve a Q-factor of the device at a predetermined frequency.
19. The acoustic resonator device of claim 17 , wherein a center-to-center spacing from the first reflector element to the first finger is different than the center-to-center spacing of at least a pair of the n interleaved fingers.
20. The acoustic resonator device of claim 17 , wherein a distance pr between a center of the first reflector element and a center of the first finger and between a center of the second reflector element and a center of the n'th finger is greater than or equal to 1.2 times a center-to-center spacing of at least a pair of the n interleaved fingers of the IDT.
21. The acoustic resonator device of claim 20 , wherein the conductor pattern further comprises:
a third reflector element proximate to the first reflector element, the third reflector element disposed such that the first reflector element is centered between the third reflector element and the first finger; and
a fourth reflector element proximate to the second reflector element, the fourth reflector element disposed such that the second reflector element is centered between the fourth reflector element and the n'th finger, wherein
the third and fourth reflector elements are disposed on the diaphragm.
22. The acoustic resonator device of claim 21 , wherein a center-to-center spacing from the first reflector element to the third reflector element is different than the center-to-center spacing of at least the pair of the n interleaved fingers.