Transversely-excited film bulk acoustic resonator with symmetric diaphragm
Acoustic resonator devices and filters are disclosed. An acoustic resonator includes a substrate having a surface. A back surface of a single-crystal piezoelectric plate is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) is formed on a front surface of the piezoelectric plate with interleaved IDT fingers of the IDT disposed on the diaphragm. Back-side fingers are formed the back surface of the diaphragm. A pitch of the IDT fingers and a pitch of the back-side fingers are substantially equal.
1. An acoustic resonator device comprising:
a substrate having a surface;
a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate;
an interdigital transducer (IDT) formed on the front surface of the piezoelectric plate, interleaved IDT fingers of the IDT disposed on the diaphragm; and
back-side fingers formed on the back surface of the diaphragm, a pitch of the IDT fingers and a pitch of the back-side fingers are substantially equal.
2. The device of claim 1 , wherein
each back-side finger is aligned with a corresponding one of the IDT fingers.
3. The device of claim 1 , wherein a mass per unit length of each back-side finger is effectively equal to a mass per unit length of the corresponding IDT finger.
4. The device of claim 1 , wherein
an aperture of the acoustic resonator device is defined by the overlapping portions of the IDT fingers, and
each back-side finger extends across the aperture.
5. The device of claim 1 wherein the back-side fingers are conductors.
6. The device of claim 5 , wherein the back-side fingers connect alternately to first and second back-side busbars to form a back-side IDT.
7. The device of claim 6 , wherein the back-side IDT is electrically connected to the IDT formed on the front surface of the piezoelectric plate such that each back-side finger is electrically connected to the corresponding IDT finger.
8. The device of claim 5 , wherein the IDT fingers and the back-side fingers have the same structure and substantially equal width and thickness.
9. The device of claim 5 , further comprising:
a front-side dielectric layer formed between the IDT fingers; and
a back-side dielectric layer formed between the back-side fingers.
10. The device of claim 9 , wherein
the IDT fingers and back-side fingers are substantially aluminum, and
the front-side dielectric layer and the back-side dielectric layer are silicon dioxide.
11. The device of claim 9 , wherein
the IDT fingers and back-side fingers are substantially copper, and
the front-side dielectric layer and the back-side dielectric layer are tantalum pentoxide.
12. The device of claim 1 , wherein the back-side fingers are dielectric.
13. The device of claim 12 , wherein
the IDT fingers are substantially aluminum, and
the back-side fingers are silicon dioxide.
14. The device of claim 12 , wherein
the IDT fingers are substantially copper, and
the back-side fingers are tantalum pentoxide.
15. The device of claim 12 , wherein
a thickness of the IDT fingers and a thickness of the back-side fingers are substantially equal, and
a width of the IDT fingers and a width of the back-side fingers are substantially equal.
16. A filter device comprising:
a substrate having a surface;
a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for portions of the piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate;
a conductor pattern including a plurality of interdigital transducers (IDT) of a corresponding plurality of acoustic resonators formed on the front surface of the piezoelectric plate, interleaved IDT fingers of each IDT disposed on a respective diaphragm from the plurality of diaphragms; and
back-side fingers formed on the back surface of one or more diaphragms from the plurality of diaphragms,
wherein, for each diaphragm having back-side fingers, a pitch of the back-side fingers is substantially equal to the pitch of the IDT fingers on the front surface of the diaphragm.
17. The device of claim 16 , wherein
each back-side finger is aligned with a corresponding one of the IDT fingers.
18. The device of claim 17 , wherein a mass per unit length of each back-side finger is effectively equal to a mass per unit length of the corresponding IDT finger.
19. The device of claim 16 wherein the back-side fingers are conductors.
20. The device of claim 19 , further comprising for each diaphragm having back-side fingers:
a front-side dielectric layer formed between the IDT fingers; and
a back-side dielectric layer formed between the back-side fingers.
21. The device of claim 16 , wherein the back-side fingers are dielectric.