Transversely-excited film bulk acoustic resonator with multiple diaphragm thicknesses and fabrication method
Filter devices and methods are disclosed. A filter device includes a substrate and a piezoelectric plate attached to the substrate, the piezoelectric plate forming diaphragms spanning respective cavities in the substrate. A first portion of the piezoelectric plate has a first thickness. A front surface of a second portion of the piezoelectric plate is recessed relative to a front surface of the first portion of the piezoelectric plate such that the second portion of the piezoelectric plate has a second thickness less than the first thickness. A conductor pattern is formed on the front surfaces of the first and second portions of the piezoelectric plate. The conductor pattern includes a first interdigital transducer (IDT) with interleaved fingers on a diaphragm having the first thickness, and a second IDT with interleaved fingers on a diaphragm having the second thickness.
1. A filter device, comprising:
a substrate;
at least one piezoelectric layer attached directly to the substrate or by one or more intermediate material layers, the at least one piezoelectric layer comprising a plurality of diaphragms that are over a plurality of cavities, respectively, in at least one of the substrate and the one or more intermediate material layers, wherein:
a first portion of the at least one piezoelectric layer has a first thickness, and
a surface of a second portion of the at least one piezoelectric layer is recessed relative to a surface of the first portion of the at least one piezoelectric layer such that the second portion of the at least one piezoelectric layer has a second thickness less than the first thickness; and
a conductor pattern at the first and second portions of the at least one piezoelectric layer, the conductor pattern comprising:
a first interdigital transducer (IDT) with interleaved fingers on a diaphragm having the first thickness, and
a second IDT with interleaved fingers on a diaphragm having the second thickness.
2. The filter device of claim 1 , the conductor pattern further comprising one or more additional IDTs with interleaved fingers on respective diaphragms having one of the first thickness and the second thickness.
3. The filter device of claim 1 , the conductor pattern further comprising one or more additional IDTs with interleaved fingers on respective diaphragms having thicknesses intermediate to the first thickness and the second thickness.
4. The filter device of claim 1 , wherein the at least one piezoelectric layer and all of the IDTs are configured such that a respective radio frequency signal applied to the first IDT and the second IDT excites a respective shear primary acoustic mode within the respective diaphragm.
5. The filter device of claim 4 , wherein a direction of acoustic energy flow of all of the shear primary acoustic modes is substantially orthogonal to front and back surfaces of the respective diaphragms.
6. The filter device of claim 4 , wherein the at least one piezoelectric layer is one of lithium niobate and lithium tantalate.
7. The filter device of claim 1 , wherein:
the second thickness is greater than or equal to 200 nm, and
the first thickness less than or equal to 1000 nm.
8. The filter device of claim 1 , 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.
9. The filter device of claim 8 , further comprising:
one or more additional shunt resonators and one or more additional series resonators, wherein:
interleaved fingers of IDTs of all of the shunt resonators are on respective diaphragms having the first thickness, and
interleaved fingers of IDTs of all of the series resonators are on respective diaphragms having the second thickness.
10. A method of fabricating a filter device, comprising:
attaching a back surface of at least one piezoelectric layer having a first thickness to a substrate or one or more intermediate material layers on the substrate;
selectively forming a recess in a front surface of the piezoelectric layer to thin a portion of the piezoelectric layer from the first thickness to a second thickness less than the first thickness;
forming a plurality of cavities in at least one of the substrate and the one or more intermediate material layers, such that portions of the at least one piezoelectric layer form a plurality of diaphragms spanning respective cavities; and
forming a conductor pattern at the at least one piezoelectric layer, the conductor pattern comprising
a first interdigital transducer (IDT) with interleaved fingers on a first diaphragm having the first thickness, and
a second IDT with interleaved fingers on a second diaphragm having the second thickness.
11. The method of claim 10 , further comprising forming the conductor pattern to include one or more additional IDTs with interleaved fingers on respective diaphragms having one of the first thickness and the second thickness.
12. The method of claim 10 , further comprising forming the conductor pattern to include one or more additional IDTs with interleaved fingers on respective diaphragms having thicknesses intermediate to the first thickness and the second thickness.
13. The method of claim 10 , wherein the at least one piezoelectric layer and all of the IDTs are configured such that a respective radio frequency signal applied to the first IDT and the second IDT excites a respective shear primary acoustic mode within the respective diaphragm.
14. The method of claim 13 , wherein a direction of acoustic energy flow of all of the shear primary acoustic modes is substantially orthogonal to front and back surfaces of the respective diaphragms.
15. The method of claim 13 , wherein the at least one piezoelectric layer is one of lithium niobate and lithium tantalate.
16. The method of claim 10 , wherein:
the second thickness is greater than or equal to 200 nm, and
the first thickness less than or equal to 1000 nm.
17. The method of claim 10 , 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.
18. The method of claim 17 , the conductor pattern further comprising:
IDTs of one or more additional shunt resonators and one or more additional series resonators, wherein:
interleaved fingers of IDTs of all of the shunt resonators are on respective diaphragms having the first thickness, and
interleaved fingers of IDTs of all of the series resonators are on respective diaphragms having the second thickness.
19. The filter device of claim 1 , wherein the at least one piezoelectric layer and the first and second IDTs are configured such that a radio frequency signal applied thereto excites a bulk shear wave that propagates in a direction normal to the respective surfaces of the first and second portions of the at least one piezoelectric layer.
20. The method of claim 10 , further comprising configuring the at least one piezoelectric layer such that a radio frequency signal applied to the first and second IDTs excite bulk shear waves in the respective diaphragms that propagate in a direction normal to the back surface of the at least one piezoelectric layer.