Low loss transversely-excited film bulk acoustic resonators and filters

1. An acoustic resonator device comprising:

a diaphragm comprising a portion of a piezoelectric layer that is over a cavity; and

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

an interdigital transducer (IDT) comprising a first busbar, a second busbar, and a plurality of interleaved fingers, wherein the plurality of interleaved fingers extend alternately from the first and second busbars and overlapping portions of the plurality of interleaved fingers are on the diaphragm;

first and second reflector elements on the diaphragm proximate to a first finger of the plurality of interleaved fingers; and

third and fourth reflector elements on the diaphragm proximate to a last finger of the plurality of interleaved fingers,

wherein:

pr1 is a center-to-center distance between the first and second reflector elements and a center-to-center distance between the third and fourth reflector elements,

p is a pitch that is a center-to-center distance between two adjacent interleaved fingers of the plurality of interleaved fingers, and

1.1p≤pr1≤1.5p.

2. The acoustic resonator device of claim 1 , wherein:

the first finger and the first and second reflector elements extend from the first busbar,

the last finger and the third and fourth reflector elements extend from a same busbar of the first or second busbar.

3. The acoustic resonator device of claim 1 , wherein:

the first finger extends from the first busbar,

the first and second reflector elements extend from the second busbar,

the last finger extends from one busbar of the first and second busbars, and

the third and fourth reflector elements extend from the other busbar of the first or second busbar.

4. The acoustic resonator device of claim 1 , wherein:

pr2 is a center-to-center distance between the first reflector element and the first finger and a center-to-center distance between the third reflector element and the last finger, and

p≤pr2≤pr1.

5. The acoustic resonator device of claim 4 , wherein pr2=(pr1+p)/2.

6. The acoustic resonator device of claim 4 , wherein pr2=pr1.

7. The acoustic resonator device of claim 1 , wherein a mark mr of the first, second, third, and fourth reflector elements is configured to improve a Q-factor of the acoustic resonator device at a predetermined frequency.

8. The acoustic resonator device of claim 1 , wherein the first and second reflector elements of the conductor pattern extend in a direction parallel to the first finger, and the third and fourth reflector elements of the conductor pattern extend in a direction parallel to the last finger.

9. A bandpass filter, comprising:

a plurality of acoustic resonators comprising one or more series resonators and one or more shunt resonators connected in a ladder filter circuit, wherein:

each of the plurality of acoustic resonators comprises:

a diaphragm comprising a portion of a piezoelectric layer that is over a cavity, and

an interdigital transducer (IDT) on a surface of the piezoelectric layer, the IDT comprising a first busbar, a second busbar, and a plurality of interleaved fingers extending alternately from the first and second busbars, wherein overlapping portions of the plurality of interleaved fingers are on the respective diaphragm,

wherein at least one series resonator of the one or more series resonators further comprises:

first and second reflector elements on the respective diaphragm proximate to a first finger of the plurality of interleaved fingers; and

third and fourth reflector elements on the respective diaphragm proximate to a last finger of the plurality of interleaved fingers,

wherein:

pr1 is a center-to-center distance between the first and second reflector elements and a center-to-center distance between the third and fourth reflector elements,

p is a pitch that is a center-to-center distance between two adjacent interleaved fingers of the interleaved fingers, and

1.1p≤pr1≤1.5p.

10. The bandpass filter of claim 9 , wherein, for the at least one series resonator:

the first finger and the first and second reflector elements extend from the first busbar,

the last finger and the third and fourth reflector elements extend from the same busbar of the first or second busbar.

11. The bandpass filter of claim 9 , wherein, for the at least one series resonator:

the first finger extends from the first busbar,

the first and second reflector elements extend from the second busbar,

the last finger extends from one busbar of the first or second busbar, and

the third and fourth reflector elements extend from the other busbar of the first and second busbars.

12. The bandpass filter of claim 9 , wherein:

pr2 is a center-to-center distance between the first reflector element and the first finger and a center-to-center distance between the third reflector element and the last finger, and

p≤pr2≤pr1.

13. The bandpass filter of claim 12 , wherein pr2=(pr1+p)/2.

14. The bandpass filter of claim 12 , wherein pr2=pr1.

15. The bandpass filter of claim 9 , wherein, for the at least one series resonator:

a mark mr of the first, second, third, and fourth reflector elements is configured to improve an input/output transfer function of the bandpass filter at a predetermined frequency.

16. The bandpass filter of claim 9 , wherein, for the at least one series resonator, the first and second reflector elements of the conductor pattern extend in a direction parallel to the first finger, and the third and fourth reflector elements of the conductor pattern extend in a direction parallel to the last finger.

17. The bandpass filter of claim 9 , wherein all of the one or more series resonators comprise the respective first, second, third and fourth reflector elements.

18. The bandpass filter of claim 9 , wherein at least one shunt resonator of the one or more shunt resonators comprises:

respective first and second reflector elements on the respective diaphragm proximate and parallel to a first finger of the plurality of interleaved fingers; and

third and fourth reflector elements on the respective diaphragm proximate and parallel to a last finger of the plurality of interleaved fingers.

19. A bandpass filter including a series resonator and a shunt resonator connected in a ladder filter circuit, each of the series resonator and the shunt resonator comprising:

a piezoelectric layer that includes a diaphragm that is over a cavity, and

an interdigital transducer (IDT) on a surface of the piezoelectric layer, the IDT comprising:

a first busbar and a second busbar;

a plurality of interleaved fingers extending alternately from the first and second busbars and having overlapping portions on the diaphragm;

first and second reflector elements on the diaphragm proximate to a first finger of the plurality of interleaved fingers; and

third and fourth reflector elements on the diaphragm proximate to a last finger of the plurality of interleaved fingers,

wherein:

pr1 is a center-to-center distance between the first and second reflector elements and a center-to-center distance between the third and fourth reflector elements of the series resonator,

pr2 is a center-to-center distance between the first and second reflector elements and a center-to-center distance between the third and fourth reflector elements of the shunt resonator, and

pr1 and pr2 are different.

20. The bandpass filter of claim 19 , wherein:

for each of the series resonator and the shunt resonator, p is a pitch that is a center-to-center distance between two adjacent interleaved fingers of the plurality of interleaved fingers,

1.1p≤pr1≤1.5p, and

1.1p≤pr2≤1.5p.