High Q solidly-mounted transversely-excited film bulk acoustic resonators

1. An acoustic resonator comprising:

a substrate;

a piezoelectric layer;

an acoustic Bragg reflector between a surface of the substrate and a back surface of the piezoelectric layer; and

a conductor pattern on a front surface of the piezoelectric layer, the conductor pattern comprising:

an interdigital transducer (IDT) comprising a first busbar, a second busbar, and interleaved fingers, wherein the interleaved fingers extend alternately from the first and second busbars, and the interleaved fingers comprise a first finger and a last finger at opposing ends of the IDT;

a first reflector element proximate to the first finger; and

a second reflector element proximate to the last finger,

wherein a distance pr between the first reflector element and the first finger and between the second reflector element and the last finger is greater than a pitch p of the IDT, the pitch p being a center-to-center spacing between at least two fingers of the interleaved fingers of the IDT.

2. The acoustic resonator of claim 1 , wherein the distance pr is greater than or equal to 1.2 p and less than or equal to 1.5 p.

3. The acoustic resonator of claim 1 , wherein a width wr of each of the first and second reflector elements is configured to improve a Q-factor of the acoustic resonator at a predetermined frequency.

4. The acoustic resonator of claim 1 , wherein:

the first reflector element is connected to a same one of the first and second busbars as the first finger, and

the second reflector element is connected to a same one of the first and second busbars as the last finger.

5. The acoustic resonator of claim 1 , wherein:

the first reflector element is connected to an opposite one of the first and second busbars as the first finger, and

the second reflector element is connected to an opposite one of the first and second busbars as the last finger.

6. The acoustic resonator of claim 1 , wherein the first and second reflector elements are floating.

7. The acoustic resonator of claim 1 further comprising:

a third reflector element proximate and parallel to the first reflector element, the third reflector element disposed such that the first reflector element is between the third reflector element and the first finger; and

a fourth reflector element proximate and parallel to the second reflector element, the fourth reflector element disposed such that the second reflector element is between the fourth reflector element and the last finger.

8. The acoustic resonator of claim 7 , wherein a distance pr2 between the first and third reflector elements and between the second and fourth reflector elements is greater than or equal to 1.2 p and less than or equal to 1.5 p.

9. The acoustic resonator of claim 8 , wherein pr=pr2.

10. The acoustic resonator of claim 8 , wherein pr=(p+pr2)/2.

11. The acoustic resonator of claim 1 , wherein the first reflector element of the conductor pattern extends in a direction parallel to the first finger, and the second reflector element of the conductor pattern extends in a direction parallel to the last finger.

12. A filter device comprising:

a plurality of bulk acoustic resonators, wherein at least one of the plurality of bulk acoustic resonators comprises:

a substrate;

a piezoelectric layer;

an acoustic Bragg reflector between a surface of the substrate and a back surface of the piezoelectric layer; and

a conductor pattern on a front surface of the piezoelectric that includes an interdigital transducer (IDT) comprising a first busbar, a second busbar, and interleaved fingers extending alternately from the first and second busbars, the interleaved fingers comprise a first finger and a last finger at opposing ends of the IDT,

wherein the IDT comprises:

a first reflector element proximate to the first finger; and

a second reflector element proximate to the last finger, and

wherein a distance pr between the first reflector element and the first finger and between the second reflector element and the last finger is greater than a pitch p of the IDT, the pitch p being a center-to-center spacing between at least two fingers of the interleaved fingers of the IDT.

13. The filter device of claim 12 , wherein the first reflector element of the IDT extends in a direction parallel to the first finger, and the second reflector element of the IDT extends in a direction parallel to the last finger.

14. The filter device of claim 12 , wherein the distance pr is greater than or equal to 1.2 p and less than or equal to 1.5 p.

15. The filter device of claim 12 , wherein a width wr of each of the first and second reflector elements is configured to improve a Q-factor of the respective resonator at a predetermined frequency.

16. The filter device of claim 15 wherein:

the IDT is part of a shunt resonator, and

the width wr is configured to improve the Q-factor of the shunt resonator at a lower edge of a pass-band of the filter device.

17. The filter device of claim 15 wherein:

the IDT is part of a series resonator, and

the width wr is configured to improve the Q-factor of the series resonator at an upper edge of a pass-band of the filter device.

18. The filter device of claim 12 , wherein the IDT of the at least one bulk acoustic resonator further comprises:

a third reflector element proximate to the first reflector element, the third reflector element disposed such that the first reflector element is 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 between the fourth reflector element and the last finger.

19. The filter device of claim 18 , wherein a distance pr2 between the first and third reflector elements and between the second and fourth reflector elements is greater than or equal to 1.2 p and less than or equal to 1.5 p.

20. The filter device of claim 19 , wherein pr=pr2.

21. The filter device of claim 19 , wherein pr=(p+pr2)/2.