DIELECTRIC COATED TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR (XBAR) FOR COUPLING OPTIMIZATION

1 . An acoustic resonator comprising:

a piezoelectric plate, a portion of the piezoelectric plate spanning a cavity in an intermediate layer of a substrate;

an interdigital transducer on a surface of the piezoelectric plate, interleaved fingers of the IDT on the portion of the piezoelectric plate that spans the cavity; and

a dielectric layer over the interleaved fingers and the surface of the portion of the piezoelectric plate that spans the cavity,

wherein a thickness of the dielectric layer optimizes electromechanical coupling of the acoustic resonator.

2 . The device of claim 1 , wherein

the dielectric layer is one of SiO2 and Si3N4; and

the thickness of the dielectric layer is between 10 to 30 percent of a thickness of the plate.

3 . The device of claim 2 , wherein a pitch of the fingers is 4 um / (400 nm/(400 nm + a thickness of the dielectric layer)).

4 . The device of claim 1 , wherein:

the dielectric layer is SiO2;

the thickness of the dielectric layer is between 10 to 30 percent a thickness of the plate.

5 . The device of claim 4 , wherein the thickness of the dielectric layer is 20 percent the thickness of the plate, and wherein optimizing the electromechanical coupling includes a 10 percent increase in resonator coupling as compared to a similar resonator without the dielectric layer.

6 . The device of claim 1 , wherein:

the dielectric layer is Si3N4;

a thickness of the dielectric layer is between 10 to 35 percent a thickness of the plate.

7 . The device of claim 6 , wherein the thickness of the dielectric layer is 20 percent the thickness of the plate, and wherein optimizing the electromechanical coupling includes a 10 percent increase in resonator coupling as compared to a similar resonator without the dielectric layer.

8 . The device of claim 1 , wherein:

the plate has a thickness of 400 nm and is one of Z-cut or 128-Y Cut LiNbO3;

the fingers are 10 nm thick metal electrodes with a 20 nm mark; and

the fingers have a pitch of between 4.4 and 5.2 um.

9 . An acoustic resonator device comprising:

a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of an intermediate layer of a substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity formed in the intermediate layer of the substrate;

an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm;

a dielectric layer over the interleaved fingers and the surface of the portion of the piezoelectric plate that spans the cavity,

wherein a thickness of the dielectric layer maximizes electromechanical coupling of the acoustic resonator.

10 . The device of claim 9 , wherein

the plate has a thickness of 400 nm;

the dielectric layer is one of SiO2 and Si3N4;

the thickness of the dielectric layer is between 10 to 30 percent a thickness of the plate; and

a pitch of the fingers is between 5 and 6 um.

11 . The device of claim 9 , wherein:

the dielectric layer is SiO2;

the thickness of the dielectric layer is between 10 to 30 percent a thickness of the plate.

12 . The device of claim 11 , wherein the thickness of the dielectric layer is 20 percent the thickness of the plate, and wherein optimizing the electromechanical coupling includes a 10 percent increase in resonator coupling as compared to a similar resonator without the dielectric layer.

13 . The device of claim 9 , wherein:

the dielectric layer is Si3N4;

a thickness of the dielectric layer is between 10 to 35 percent a thickness of the plate.

14 . The device of claim 13 , wherein the thickness of the dielectric layer is 20 percent the thickness of the plate, and wherein optimizing the electromechanical coupling includes a 10 percent increase in resonator coupling as compared to a similar resonator without the dielectric layer.

15 . The device of claim 9 , wherein:

the plate has a thickness of 400 nm and is one of Z-cut or 128-Y Cut LiNbO3;

the fingers are 10 nm thick metal electrodes with a 20 nm mark; and

the fingers have a pitch of between 4.4 and 5.2 um.

16 . A method of fabricating an acoustic resonator device having a dielectric layer selected to optimize electromechanical coupling, the method comprising:

attaching a back surface of a single crystal piezoelectric plate to a surface of an intermediate layer of a substrate;

forming a cavity in the intermediate layer of the substrate such that a portion of the single-crystal piezoelectric plate forms a diaphragm spanning the cavity; and

forming an interdigital transducer (IDT) on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm;

forming a dielectric layer over the interleaved fingers and the surface of the portion of the piezoelectric plate that spans the cavity,

wherein a thickness of the dielectric layer is selected to optimize electromechanical coupling of the acoustic resonator.

17 . The method of claim 16 , further comprising selecting the thickness of the dielectric layer based on a thickness of the plate.

18 . The method of claim 17 , wherein selecting is selecting the thickness of the dielectric layer to be between 10 and 30 percent the thickness of the plate.

19 . The method of claim 16 , wherein

the plate has a thickness of 400 nm;

the dielectric layer is one of SiO2 and Si3N4;

the thickness of the dielectric layer is between 10 to 30 percent a thickness of the plate; and

a pitch of the fingers is between 5 and 6 um.