Acoustic wave filter and multiplexer

1. An acoustic wave filter comprising:

a piezoelectric substrate;

one or more series resonators that are connected in series between an input terminal and an output terminal and located on the piezoelectric substrate, each of the one or more series resonators including first electrode fingers that are arranged with a first duty ratio and excite an acoustic wave;

one or more parallel resonators that are connected in parallel between the input terminal and the output terminal and located on the piezoelectric substrate, each of the one or more parallel resonators including second electrode fingers that are arranged with a second duty ratio and excite an acoustic wave, the second duty ratio in each of the one or more parallel resonators being less than the first duty ratio in each of the one or more series resonators; and

a dielectric film that has a temperature coefficient of elastic modulus that is opposite in sign to that of the piezoelectric substrate, is located on the piezoelectric substrate so as to cover the first electrode fingers and the second electrode fingers, and has a film thickness greater than those of the first electrode fingers and the second electrode fingers,

wherein:

each of the one or more series resonators includes a pair of first comb-shaped electrodes, the first duty ratio is a duty ratio in a first region in which third electrode fingers of one of the pair of first comb-shaped electrodes among the first electrode fingers overlap with fourth electrode fingers of another of the pair of first comb-shaped electrodes among the first electrode fingers, the third electrode fingers and the fourth electrode fingers are alternately arranged in the first region, only a single fourth electrode finger among the fourth electrode fingers is located between adjacent third electrode fingers among the third electrode fingers in the first region, only a single third electrode finger among the third electrode fingers is located between adjacent fourth electrode fingers among the fourth electrode fingers in the first region,

each of the one or more parallel resonators includes a pair of second comb-shaped electrodes, the second duty ratio is a duty ratio in a second region in which fifth electrode fingers of one of the pair of second comb-shaped electrodes among the second electrode fingers overlap with sixth electrode fingers of another of the pair of second comb-shaped electrodes among the second electrode fingers, the fifth electrode fingers and the sixth electrode fingers are alternately arranged in the second region, only a single sixth electrode finger among the sixth electrode fingers is located between adjacent fifth electrode fingers among the fifth electrode fingers in the second region, only a single fifth electrode finger among the fifth electrode fingers is located between adjacent sixth electrode fingers among the sixth electrode fingers in the second region,

a resonant frequency of each of the one or more parallel resonators is less than a resonant frequency of each of the one or more series resonators,

an antiresonant frequency of each of the one or more parallel resonators is less than an antiresonant frequency of each of the one or more series resonators,

no resonator other than the one or more parallel resonators and the one or more series resonators is connected between the input terminal and the output terminal, and

a difference in value between a value, expressed in percentage, of a largest second duty ratio in the one or more parallel resonators and a value, expressed in percentage, of a smallest first duty ratio in the one or more series resonators is 5% or greater.

2. The acoustic wave filter according to claim 1 , wherein

the one or more series resonators are a plurality of series resonators,

the one or more parallel resonators are a plurality of parallel resonators, and

all of the second duty ratios in the plurality of parallel resonators are less than all of the first duty ratios in the plurality of series resonators.

3. The acoustic wave filter according to claim 1 , wherein

the difference in value between the value, expressed in percentage, of the largest second duty ratio in the one or more parallel resonators and the value, expressed in percentage, of the smallest first duty ratio in the one or more series resonators is 10% or less.

4. The acoustic wave filter according to claim 1 , wherein

a pitch of the first electrode fingers is less than a pitch of the second electrode fingers.

5. The acoustic wave filter according to claim 1 , wherein

the piezoelectric substrate is a lithium niobate substrate or a lithium tantalate substrate.

6. The acoustic wave filter according to claim 1 , wherein

the piezoelectric substrate is a lithium niobate substrate and the dielectric film is a silicon oxide film.

7. A multiplexer comprising:

the acoustic wave filter according to claim 1 .

8. The acoustic wave filter according to claim 1 , wherein

the value, expressed in percentage, of the largest second duty ratio in the one or more parallel resonators is equal to or less than 0.9 times the value, expressed in percentage, of the smallest first duty ratio in the one or more series resonators.

9. The acoustic wave filter according to claim 1 , wherein the resonant frequency of each of the one or more parallel resonators is lower than a passband of the acoustic wave filter, and the antiresonant frequency of each of the one or more series resonators is higher than the passband.

10. An acoustic wave filter comprising:

a piezoelectric substrate;

one or more series resonators that are connected in series between an input terminal and an output terminal and located on the piezoelectric substrate, each of the one or more series resonators including first electrode fingers that are arranged with a first duty ratio and excite an acoustic wave;

one or more parallel resonators that are connected in parallel between the input terminal and the output terminal and located on the piezoelectric substrate, each of the one or more parallel resonators including second electrode fingers that are arranged with a second duty ratio and excite an acoustic wave, the second duty ratio in each of the one or more parallel resonators being less than the first duty ratio in each of the one or more series resonators;

a first dielectric film that has a temperature coefficient of elastic modulus that is opposite in sign to that of the piezoelectric substrate, is located on the piezoelectric substrate so as to cover the first electrode fingers, and has a first film thickness greater than those of the first electrode fingers; and

a second dielectric film that has a temperature coefficient of elastic modulus that is opposite in sign to that of the piezoelectric substrate, is located on the piezoelectric substrate so as to cover the second electrode fingers, and has a second film thickness that is greater than those of the second electrode fingers and is substantially equal to the first film thickness,

wherein:

each of the one or more series resonators includes a pair of first comb-shaped electrodes, the first duty ratio is a duty ratio in a first region in which third electrode fingers of one of the pair of first comb-shaped electrodes among the first electrode fingers overlap with fourth electrode fingers of another of the pair of first comb-shaped electrodes among the first electrode fingers, the third electrode fingers and the fourth electrode fingers are alternately arranged in the first region, only a single fourth electrode finger among the fourth electrode fingers is located between adjacent third electrode fingers among the third electrode fingers in the first region, only a single third electrode finger among the third electrode fingers is located between adjacent fourth electrode fingers among the fourth electrode fingers in the first region,

each of the one or more parallel resonators includes a pair of second comb-shaped electrodes, the second duty ratio is a duty ratio in a second region in which fifth electrode fingers of one of the pair of second comb-shaped electrodes among the second electrode fingers overlap with sixth electrode fingers of another of the pair of second comb-shaped electrodes among the second electrode fingers, the fifth electrode fingers and the sixth electrode fingers are alternately arranged in the second region, only a single sixth electrode finger among the sixth electrode fingers is located between adjacent fifth electrode fingers among the fifth electrode fingers in the second region, only a single fifth electrode finger among the fifth electrode fingers is located between adjacent sixth electrode fingers among the sixth electrode fingers in the second region,

a resonant frequency of each of the one or more parallel resonators is less than a resonant frequency of each of the one or more series resonators,

an antiresonant frequency of each of the one or more parallel resonators is less than an antiresonant frequency of each of the one or more series resonators,

no resonator other than the one or more parallel resonators and the one or more series resonators is connected between the input terminal and the output terminal, and

a difference in value between a value, expressed in percentage, of a largest second duty ratio in the one or more parallel resonators and a value, expressed in percentage, of a smallest first duty ratio in the one or more series resonators is 5% or greater.

11. The acoustic wave filter according to claim 10 , wherein the first dielectric film and the second dielectric film are made of substantially identical materials.

12. The acoustic wave filter according to claim 10 , wherein

the one or more series resonators are a plurality of series resonators,

the one or more parallel resonators are a plurality of parallel resonators, and

all of the second duty ratios in the plurality of parallel resonators are less than all of the first duty ratios in the plurality of series resonators.

13. The acoustic wave filter according to claim 10 , wherein the difference in value between the value, expressed in percentage, of the largest second duty ratio in the one or more parallel resonators and the value, expressed in percentage, of the smallest first duty ratio in the one or more series resonators is 10% or less.

14. The acoustic wave filter according to claim 10 , wherein a pitch of the first electrode fingers is less than a pitch of the second electrode fingers.

15. The acoustic wave filter according to claim 10 , wherein the piezoelectric substrate is a lithium niobate substrate or a lithium tantalate substrate.

16. The acoustic wave filter according to claim 10 , wherein the piezoelectric substrate is a lithium niobate substrate and the dielectric film is a silicon oxide film.

17. A multiplexer comprising:

the acoustic wave filter according to claim 10 .

18. The acoustic wave filter according to claim 10 , wherein the value, expressed in percentage, of the largest second duty ratio in the one or more parallel resonators is equal to or less than 0.9 times the value, expressed in percentage, of the smallest first duty ratio in the one or more series resonators.

19. The acoustic wave filter according to claim 2 , wherein the resonant frequency of each of the one or more parallel resonators is lower than a passband of the acoustic wave filter, and the antiresonant frequency of each of the one or more series resonators is higher than the passband.