Solidly-mounted transversely-excited film bulk acoustic filters with multiple piezoelectric plate thicknesses
Filters and methods of making filters are disclosed. A filter device includes a substrate, a piezoelectric plate, and an acoustic Bragg reflector between a surface of the substrate and a back surface of the piezoelectric plate. A first portion of the piezoelectric plate has a first thickness, and a second portion of the piezoelectric plate has a second thickness less than the first thickness. A conductor pattern on front surfaces of the first and second portions of the piezoelectric plate includes a first interdigital transducer (IDT) with interleaved fingers on the first portion, and a second IDT with interleaved fingers on the second portion.
1. A filter device comprising:
a substrate;
a piezoelectric layer having a first portion that has a first thickness and a second portion that has a second thickness less than the first thickness;
an acoustic Bragg reflector between a surface of the substrate and a back surface of the piezoelectric layer; and
a conductor pattern on front surfaces of the first and second portions of the piezoelectric layer, the conductor pattern comprising:
a first interdigital transducer (IDT) with interleaved fingers on the first portion, and
a second IDT with interleaved fingers on the second portion,
wherein at least a portion of interleaved fingers of each of the first IDT and the second IDT have a width that is greater than or equal to 0.2 times a pitch of the respective IDT and less than or equal to 0.3 times the pitch of the respective IDT, and
wherein the pitch of the interleaved fingers is a center-to-center spacing between two adjacent interleaved fingers that extend from different busbars of the conductor pattern.
2. The filter device of claim 1 , wherein the conductor pattern further comprises one or more additional IDTs with interleaved fingers on respective portions of the piezoelectric layer having one of the first thickness and the second thickness.
3. The filter device of claim 1 , wherein the conductor pattern further comprises one or more additional IDTs with interleaved fingers on respective portions of the piezoelectric layer having thicknesses intermediate to the first thickness and the second thickness.
4. The filter device of claim 1 , wherein the piezoelectric layer and the first and second IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective primary shear acoustic mode within the respective portion of the piezoelectric layer.
5. The filter device of claim 4 , wherein the piezoelectric layer is one of lithium niobate and lithium tantalate.
6. The filter device of claim 1 , wherein:
the second thickness is greater than or equal to 50 nm, and
the first thickness less than or equal to 1500 nm.
7. The filter device of claim 1 , wherein the first IDT is a part of a first shunt resonator and the second IDT is a part of a first series resonator in a ladder filter circuit.
8. The filter device of claim 7 , further comprising:
one or more additional shunt resonators and one or more additional series resonators, wherein:
interleaved fingers of the IDTs of all of the shunt resonators are on respective portions of the piezoelectric layer having the first thickness, and
interleaved fingers of the IDTs of all of the series resonators are on respective portions of the piezoelectric layer having the second thickness.
9. A method of fabricating a filter device, the method comprising:
forming an acoustic Bragg reflector by depositing material layers on one or both of a surface of a device substrate and a back surface of a piezoelectric plate;
bonding the piezoelectric plate to the device substrate such that the acoustic Bragg reflector is between the back surface of the piezoelectric plate and the device substrate;
removing material from one or more portions of a front surface of the piezoelectric plate to reduce a thickness of the one or more portions from a first thickness to a second thickness less than the first thickness; and
forming a conductor pattern on the front surface of the piezoelectric plate, the conductor pattern comprising:
a first interdigital transducer (IDT) with interleaved fingers on a first portion of the piezoelectric plate having the first thickness surface, and
a second IDT with interleaved fingers on a second portion of the piezoelectric plate having the second thickness,
wherein at least a portion of interleaved fingers of each of the first IDT and the second IDT have a width that is greater than or equal to 0.2 times a pitch of the respective IDT and less than or equal to 0.3 times the pitch of the respective IDT, and
wherein the pitch of the interleaved fingers is a center-to-center spacing between two adjacent interleaved fingers that extend from different busbars of the conductor pattern.
10. The method of claim 9 , wherein the forming of the conductor pattern further comprises forming one or more additional IDTs with interleaved fingers on respective portions of the piezoelectric plate having one of the first thickness and the second thickness.
11. The method of claim 9 , wherein the forming of the conductor pattern further comprises forming one or more additional IDTs with interleaved fingers on respective portions of the piezoelectric plate having thicknesses intermediate to the first thickness and the second thickness.
12. The method of claim 9 , wherein the piezoelectric plate and the first and second IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective primary shear acoustic mode within the respective portion of the piezoelectric plate.
13. The method of claim 9 , wherein
the piezoelectric plate is attached to a sacrificial substrate during the bonding, and
the method further comprises removing the sacrificial substrate after the bonding to expose the front surface of the piezoelectric plate.
14. The method of claim 9 , wherein the removing of the material further comprises at least one of:
etching the one or more portions of the front surface of the piezoelectric plate through a mask, and
ion milling the one or more portions of the front surface of the piezoelectric plate.
15. The filter device of claim 1 , wherein the piezoelectric layer and the first and second IDTs are configured such that a radio frequency signal applied to each IDT excites a primary shear acoustic mode in the piezoelectric layer in which acoustic energy propagates along a direction substantially orthogonal to a surface of the piezoelectric layer and orthogonal to a predominantly lateral direction of an electric field in the piezoelectric layer created by the interleaved fingers of the respective IDT.
16. A filter device comprising:
an acoustic Bragg reflector between a surface of a substrate and a back surface of a piezoelectric plate; and
a conductor pattern on a front surface of the piezoelectric plate, the conductor pattern comprising:
a first interdigital transducer (IDT) with interleaved fingers on a first portion of the piezoelectric plate having a first thickness, and
a second IDT with interleaved fingers on a second portion of the piezoelectric plate having a second thickness less than the first thickness,
wherein at least a portion of interleaved fingers of each of the first IDT and the second IDT have a width that is greater than or equal to 0.2 times a pitch of the respective IDT and less than or equal to 0.3 times the pitch of the respective IDT, and
wherein the pitch of the interleaved fingers is a center-to-center spacing between two adjacent interleaved fingers that extend from different busbars of the conductor pattern.
17. The filter device of claim 16 , wherein the conductor pattern further comprises one or more additional IDTs with interleaved fingers on respective portions of the piezoelectric plate having one of the first thickness and the second thickness.
18. The filter device of claim 16 , wherein the conductor pattern further comprises one or more additional IDTs with interleaved fingers on respective portions of the piezoelectric plate having thicknesses intermediate to the first thickness and the second thickness.
19. The filter device of claim 1 , wherein the piezoelectric plate and the first and second IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective primary shear acoustic mode within the respective portion of the piezoelectric plate.
20. The filter device of claim 16 , wherein the first IDT is a part of a shunt resonator and the second IDT is a part of a series resonator in a ladder filter circuit.