Reduced temperature coefficient of frequency at filter transition band while retaining pass-band width

1. An electronic filter comprising:

a plurality of series arm acoustic wave resonators electrically connected in series between an input port and an output port;

a plurality of parallel arm acoustic wave resonators electrically connected in parallel and electrically connected on first sides between respective ones of the plurality of series arm acoustic wave resonators and electrically connected on second sides to ground; and

at least one additional acoustic wave resonator including a first additional acoustic wave resonator electrically connected in parallel to a first one of the plurality of series arm acoustic wave resonators and a second additional acoustic wave resonator electrically connected in parallel to a second one of the plurality of series arm acoustic wave resonators, at least one of the first and second additional acoustic wave resonators having a temperature coefficient of frequency (TCF) lower than a TCF of the acoustic wave resonator to which it is electrically connected in parallel, the plurality of series arm acoustic wave resonators, the plurality of parallel arm acoustic wave resonators, and the at least one additional acoustic wave resonator being surface acoustic wave (SAW) resonators having interdigital transducer (IDT) electrodes disposed on a piezoelectric substrate, the IDT electrodes of the plurality of series arm acoustic wave resonators, the plurality of parallel arm acoustic wave resonators, and the at least one additional acoustic wave resonator being covered by silicon dioxide (SiO 2 ), the IDT electrodes of the at least one additional acoustic wave resonator being covered by a thicker layer of SiO 2 than the IDT electrodes of the plurality of series arm acoustic wave resonators and the IDT electrodes of the plurality of parallel arm acoustic wave resonators.

2. The filter of claim 1 wherein the at least one additional acoustic wave resonator has a resonant frequency lower than a lower edge of a passband of the filter.

3. The filter of claim 1 further comprising at least one additional acoustic wave resonator electrically connected in parallel to one of the plurality of parallel arm acoustic wave resonators and having a resonant frequency above an upper edge of a passband of the filter.

4. The filter of claim 3 wherein the at least one additional acoustic wave resonator includes a third additional acoustic wave resonator electrically connected in parallel to a first one of the plurality of parallel arm acoustic wave resonators and a fourth additional acoustic wave resonator electrically connected in parallel a second one of the plurality of parallel arm acoustic wave resonators.

5. The filter of claim 4 wherein each of the third additional acoustic wave resonator and the fourth additional acoustic wave resonator has a TCF lower than the first one of the plurality of parallel arm acoustic wave resonators and the second one of the plurality of parallel arm acoustic wave resonators.

6. The filter of claim 4 wherein only one of the third additional acoustic wave resonator and the fourth additional acoustic wave resonator has a TCF lower than the first one of the plurality of parallel arm acoustic wave resonators and the second one of the plurality of parallel arm acoustic wave resonators.

7. The filter of claim 1 wherein each of the first additional acoustic wave resonator and the second additional acoustic wave resonator has a TCF lower than the first one of the plurality of series arm acoustic wave resonators and the second one of the plurality of series arm acoustic wave resonators.

8. The filter of claim 1 wherein only one of the first additional acoustic wave resonator and the second additional acoustic wave resonator has a TCF lower than the first one of the plurality of series arm acoustic wave resonators and the second one of the plurality of series arm acoustic wave resonators.

9. The filter of claim 1 wherein the IDT electrodes of at least one of the plurality of series arm acoustic wave resonators are covered by a thinner layer of SiO 2 than the IDT electrodes of the plurality of parallel arm acoustic wave resonators.

10. The filter of claim 1 wherein the IDT electrodes of at least one of the plurality of parallel arm acoustic wave resonators are covered by a thinner layer of SiO 2 than the IDT electrodes of the plurality of series arm acoustic wave resonators.

11. The filter of claim 1 further comprising a layer of a silicon nitride covering the SiO 2 over each of the IDT electrodes of the plurality of series arm acoustic wave resonators, the IDT electrodes of the plurality of parallel arm acoustic wave resonators, and the IDT electrodes of the at least one additional acoustic wave resonator.

12. The filter of claim 1 wherein the IDT electrodes of the plurality of series arm acoustic wave resonators and the IDT electrodes of the plurality of parallel arm acoustic wave resonators have a greater pitch than the IDT electrodes of the at least one additional acoustic wave resonator.

13. The filter of claim 1 wherein the filter is a radio frequency filter.

14. The filter of claim 1 included in a fifth-generation radio frequency circuit.

15. The filter of claim 1 included in an electronic device module.

16. The filter of claim 15 included in a transmit and receive module.

17. The filter of claim 15 wherein the electronic device module is a radio frequency device module.

18. The filter of claim 17 included in an electronic device.

19. A method of forming an electronic filter, the method comprising:

forming a plurality of series arm acoustic wave resonators electrically connected in series between an input port and an output port;

forming a plurality of parallel arm acoustic wave resonators electrically connected in parallel and electrically connected on first sides between respective ones of the plurality of series arm acoustic wave resonators and electrically connected on second sides to ground; and

forming at least one additional acoustic wave resonator with a resonant frequency below a lower edge of a passband of the filter, the at least one additional acoustic wave resonator being electrically connected in parallel to one of one of the plurality of series arm acoustic wave resonators and having a temperature coefficient of frequency (TCF) lower than a TCF of the acoustic wave resonator to which it is electrically connected in parallel.

20. The method of claim 19 wherein the plurality of series arm acoustic wave resonators, the plurality of parallel arm acoustic wave resonators, and the at least one additional acoustic wave resonator are bulk acoustic wave (BAW) resonators.

21. The method of claim 19 wherein forming the at least one additional acoustic wave resonator further includes electrically connecting at least one additional acoustic wave resonator with a resonant frequency above an upper edge of a passband of the filter in parallel to one of the plurality of parallel arm acoustic wave resonators.

22. The method of claim 19 wherein the plurality of series arm acoustic wave resonators, the plurality of parallel arm acoustic wave resonators, and the at least one additional acoustic wave resonator are surface acoustic wave (SAW) resonators having interdigital transducer (IDT) electrodes disposed on a piezoelectric substrate and the method further comprises depositing a film of silicon dioxide on the IDT electrodes of each of the plurality of series arm acoustic wave resonators, the plurality of parallel arm acoustic wave resonators, and the at least one additional acoustic wave resonator.

23. The method of claim 22 wherein forming the film of silicon dioxide on the IDT electrodes of the at least one additional acoustic wave resonator comprises forming a thicker film of silicon dioxide on the IDT electrodes of the at least one additional acoustic wave resonator than the films of silicon dioxide on the plurality of series arm acoustic wave resonators and the plurality of parallel arm acoustic wave resonators.

24. The method of claim 23 further comprising forming films of silicon dioxide on the plurality of series arm acoustic wave resonators that are thinner than films of silicon dioxide formed on the plurality of parallel arm acoustic wave resonators.

25. An electronic filter comprising:

a plurality of series arm acoustic wave resonators electrically connected in series between an input port and an output port;

a plurality of parallel arm acoustic wave resonators electrically connected in parallel and electrically connected on first sides between respective ones of the plurality of series arm acoustic wave resonators and electrically connected on second sides to ground; and

at least one additional acoustic wave resonator electrically connected in parallel to one of the plurality of series arm acoustic wave resonators and having a temperature coefficient of frequency (TCF) lower than a TCF of the acoustic wave resonator to which it is electrically connected in parallel, the plurality of series arm acoustic wave resonators, the plurality of parallel arm acoustic wave resonators, and the at least one additional acoustic wave resonator being surface acoustic wave (SAW) resonators having interdigital transducer (IDT) electrodes disposed on a piezoelectric substrate, the IDT electrodes of the plurality of series arm acoustic wave resonators, the plurality of parallel arm acoustic wave resonators, and the at least one additional acoustic wave resonator being covered by silicon dioxide (SiO 2 ), the IDT electrodes of the at least one additional acoustic wave resonator being covered by a thicker layer of SiO 2 than the IDT electrodes of the plurality of series arm acoustic wave resonators and the IDT electrodes of the plurality of parallel arm acoustic wave resonators, the IDT electrodes of at least one of the plurality of series arm acoustic wave resonators being covered by a thinner layer of SiO 2 than the IDT electrodes of the plurality of parallel arm acoustic wave resonators.