Acoustic wave filter device

1. An acoustic wave filter device comprising:

a substrate;

a first acoustic impedance layer and a second acoustic impedance layer on the substrate;

a piezoelectric layer on the first acoustic impedance layer and the second acoustic impedance layer;

a first interdigital transducer electrode on the piezoelectric layer, the first interdigital transducer electrode overlapping at least partially the first acoustic impedance layer in a plan view from a thickness direction of the piezoelectric layer;

a second interdigital transducer electrode on the piezoelectric layer, the second interdigital transducer electrode overlapping at least partially the second acoustic impedance layer in the plan view from the thickness direction;

an input terminal on the piezoelectric layer;

an output terminal on the piezoelectric layer;

a ground terminal on the piezoelectric layer;

a series arm circuit on a first path connecting the input terminal and the output terminal, the series arm circuit including the first interdigital transducer electrode and the second interdigital transducer electrode;

a parallel arm circuit on a second path connecting a node on the first path and the ground terminal;

a first electrode;

a second electrode; and

a third acoustic impedance layer; wherein

each of the first acoustic impedance layer and the second acoustic impedance layer includes:

at least one high acoustic impedance layer; and

at least one low acoustic impedance layer whose acoustic impedance is lower than that of the at least one high acoustic impedance layer;

in each of the first acoustic impedance layer and the second acoustic impedance layer, at least one of the high acoustic impedance layer and the low acoustic impedance layer is a conductive layer;

the conductive layer in the first acoustic impedance layer and the conductive layer in the second acoustic impedance layer are electrically insulated from each other;

the parallel arm circuit includes a plurality of third interdigital transducer electrodes provided on the piezoelectric layer;

each of the first interdigital transducer electrode, the second interdigital transducer electrode, and the plurality of the third interdigital transducer electrodes includes a pair of busbars;

the first electrode is provided on the piezoelectric layer and electrically connected to one of the pair of busbars on an input terminal side, the one of the pair of busbars being included in one of the first interdigital transducer electrode and the second interdigital transducer electrode;

the second electrode is provided on the piezoelectric layer and electrically connected to one of the pair of busbars on a ground terminal side, the one of the pair of busbars being included in one of the plurality of third interdigital transducer electrodes; and

the third acoustic impedance layer includes at least one conductive layer that overlaps at least a portion of the first electrode and at least a portion of the second electrode in a plan view in between the substrate and the piezoelectric layer.

2. The acoustic wave filter device according to claim 1 , wherein a thickness of the piezoelectric layer is about 1λ or less where λ is a wavelength of an acoustic wave determined by electrode finger pitches of the first interdigital transducer electrode, the second interdigital transducer electrode, and the plurality of third interdigital transducer electrodes.

3. The acoustic wave filter device according to claim 2 , wherein the acoustic wave is a plate wave.

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

the at least one high acoustic impedance layer includes a plurality of the high acoustic impedance layers;

the at least one low acoustic impedance layer includes a plurality of the low acoustic impedance layers; and

the plurality of high acoustic impedance layers and the plurality of the low acoustic impedance layers alternate in the thickness direction.

5. The acoustic wave filter device according to claim 1 , wherein the substrate is made of silicon.

6. The acoustic wave filter device according to claim 1 , wherein a thickness of the substrate is about 120 μm.

7. The acoustic wave filter device according to claim 1 , wherein the at least one high impedance layer is made of Pt.

8. The acoustic wave filter device according to claim 1 , wherein the at least one low impedance layer is made of SiO 2 .

9. The acoustic wave filter device according to claim 1 , wherein the piezoelectric layer is made of LiNbO 3 .

10. The acoustic wave filter device according to claim 1 , wherein a thickness of the piezoelectric layer is about 340 nm.

11. The acoustic wave filter device according to claim 1 , wherein each of the first and second interdigital transducer electrodes is made of Al.

12. The acoustic wave filter device according to claim 1 , wherein a thickness of each of the first and second interdigital transducer electrodes is about 85 nm.

13. The acoustic wave filter device according to claim 1 , wherein

an insulation layer is interposed between the substrate and the piezoelectric layer at a position different from the first acoustic impedance layer and the second impedance layer;

the insulation layer is made of a same material as a material from which the low acoustic impedance layer is made; and

in a direction along the thickness direction of the piezoelectric layer, a thickness of the insulation layer is the same or substantially the same as a thickness of at least one of the first acoustic impedance layer and the second acoustic impedance layer.