Transversely-excited film bulk acoustic resonator with periodic etched holes
There are disclosed acoustic resonators and method of fabricating acoustic resonators. An acoustic resonator includes a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to a surface of a substrate except for portions of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. A conductor pattern on the front surface includes an interdigital transducer (IDT) with interleaved fingers of the IDT disposed on the diaphragm. A periodic array of holes is provided in the diaphragm.
1. An acoustic resonator, comprising:
a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to a surface of a substrate except for portions of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate;
a conductor pattern formed on the front surface, the conductor pattern comprising an interdigital transducer (IDT) with interleaved fingers of the IDT disposed on the diaphragm; and
a periodic array of holes in the diaphragm at the ends of alternate interleaved fingers.
2. The acoustic resonator of claim 1 , wherein
the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm; and
the piezoelectric plate is one of lithium niobate and lithium tantalate.
3. The acoustic resonator of claim 1 , wherein the holes are one of:
right circular cylinders with a diameter approximately equal to the width of the fingers,
larger or smaller than the width of the fingers, or
have a cross-sectional shape other than circular.
4. The acoustic resonator of claim 1 , wherein
the periodic array of holes consists of one hole proximate the end of every other finger of the IDT.
5. The acoustic resonator of claim 1 , wherein the interleaved fingers comprise:
a first set of parallel fingers extending from a first busbar; and
a second set of parallel fingers extending from a second busbar, wherein
the first and second sets of parallel fingers are interleaved and disposed on the diaphragm.
6. The acoustic resonator of claim 5 , wherein the periodic array of holes comprises:
one hole between the end of each alternate one of the first set of parallel fingers and the second busbar; and
one hole between the end of each alternate one of the second set of parallel fingers and the first busbar.
7. The acoustic resonator of claim 5 , wherein
the first and second busbars are perpendicular to the interleaved fingers.
8. The acoustic resonator of claim 5 , wherein
the first and second busbars are inclined by and angle θ with respect to a direction perpendicular to the interleaved fingers, the angle θ being greater than 0 and less than or equal to 25 degrees.
9. The acoustic resonator of claim 1 , wherein each of the periodic array of holes extend through the diaphragm.
10. The acoustic resonator of claim 1 , wherein each of the periodic array of holes is a blind hole not extending through the diaphragm.
11. A method for fabricating an acoustic resonator on a single-crystal piezoelectric plate having a front surface and a back surface attached to a substrate, comprising:
creating a cavity in the substrate such that a portion of the piezoelectric plate forms a diaphragm spanning the cavity;
forming a conductor pattern on the front surface, the conductor pattern comprising an interdigital transducer (IDT) with interleaved fingers of the IDT disposed on the diaphragm; and
etching a periodic array of holes in the diaphragm at the ends of alternate interleaved fingers.
12. The method of claim 11 , wherein
the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm; and
the piezoelectric plate is one of lithium niobate and lithium tantalate.
13. The method of claim 11 , wherein etching the periodic array of holes comprises:
defining the holes photolithographically, and
forming the holes using a suitable wet or dry etching process.
14. The method of claim 11 , wherein etching the periodic array of holes comprises:
etching one hole proximate the end of every other finger of the IDT.
15. The method of claim 11 , wherein the interleaved fingers comprise:
a first set of parallel fingers extending from a first busbar; and
a second set of parallel fingers extending from a second busbar, wherein
the first and second sets of parallel fingers are interleaved and disposed on the diaphragm.
16. The method of claim 15 , wherein etching the periodic array of holes comprises:
etching one hole between the end of each alternate one of the first set of parallel fingers and the second busbar; and
etching one hole between the end alternate one of each of the second set of parallel fingers and the first busbar.
17. The method of claim 15 , wherein
the first and second busbars are perpendicular to the interleaved fingers.
18. The method of claim 15 , wherein
the first and second busbars are inclined by and angle θ with respect to a direction perpendicular to the interleaved fingers, the angle θ being greater than 0 and less than or equal to 25 degrees.
19. The method of claim 11 , wherein etching the periodic array of holes comprises
etching holes extending through the diaphragm; and
wherein creating a cavity in the substrate comprises etching the cavities by using an etchant introduced through the holes.
20. The method of claim 11 , wherein etching the periodic array of holes comprises:
etching blind holes not extending through the diaphragm.