Resonators with different membrane thicknesses on the same die

1 . An acoustic resonator comprising:

a first piezoelectric layer coupled to a top surface of a substrate either directly or via one or more intermediate layers, the first piezoelectric layer extending over at least a first cavity and a second cavity, the first piezoelectric layer including a planarized top surface and having a first thickness;

a bonding layer bonded to the planarized top surface of the first piezoelectric layer, the bonding layer extending over the first cavity and the second cavity; and

a second piezoelectric layer having a back surface attached to a top surface of the bonding layer, the second piezoelectric layer extending over the first cavity, but not the second cavity,

wherein the second piezoelectric layer includes a planarized top surface and has a second thickness that is less than the first thickness of the first piezoelectric layer.

2 . The acoustic resonator of claim 1 , further comprising:

a first membrane including the first piezoelectric layer, the bonding layer, and the second piezoelectric layer over the first cavity; and

a second membrane including the first piezoelectric layer and the bonding layer over the second cavity.

3 . The acoustic resonator of claim 2 , further comprising:

a first interdigital transducer (IDT) over the second piezoelectric layer of the first membrane such that interleaved fingers of the first IDT are disposed over the first cavity; and

a second IDT over the first piezoelectric layer of the second membrane such that interleaved fingers of the second IDT are disposed over the second cavity.

4 . The acoustic resonator of claim 3 , wherein the first and second membranes and the first and second IDTs are each configured such that a radio frequency signal applied to the first and second IDTs excite different first and second primary shear acoustic modes in the first and second membranes, respectively.

5 . The acoustic resonator of claim 1 , wherein the bonding layer is configured as an etch stop for the etching the second piezoelectric layer.

6 . The acoustic resonator of claim 1 , wherein:

the second piezoelectric layer comprises a first material configured to be etched by a first process, and

the bonding layer comprises a second material that is substantially impervious to the first process.

7 . The acoustic resonator of claim 1 , further comprising one or more openings through the first piezoelectric layer, the second piezoelectric layer and the bonding layer.

8 . A filter system comprising:

a first bulk acoustic resonator on a substrate comprising:

a composite piezoelectric wafer attached to the substrate either directly or via one or more intermediate layers, the composite piezoelectric wafer comprising a first piezoelectric layer and a second piezoelectric layer;

a first interdigital transducer (IDT) on a surface of the composite piezoelectric wafer; and

a second bulk acoustic resonator on the substrate comprising:

a second IDT over the second piezoelectric layer but not the first piezoelectric layer.

9 . The filter system of claim 8 , further comprising a bonding layer between the first piezoelectric layer and the second piezoelectric layer.

10 . The filter system of claim 9 , wherein the first and the second bulk acoustic resonators are configured such that a radio frequency signal applied to the first and second IDTs excite different first and second primary shear acoustic modes in the first and second bulk acoustic resonators.

11 . The filter system of claim 10 , wherein the first piezoelectric layer and the second piezoelectric layer have different thicknesses from each other.

12 . The filter system of claim 8 , wherein the first piezoelectric layer and the second piezoelectric layer have different crystal-cut orientations from each other.

13 . A filter device comprising:

a substrate;

a first piezoelectric layer disposed above the substrate and over at least a first cavity and a second cavity;

a bonding layer on the first piezoelectric layer and opposite the substrate, the bonding layer over the first cavity and the second cavity; and

a second piezoelectric layer on the bonding layer opposite the first piezoelectric layer, the second piezoelectric layer over the first cavity but not the second cavity;

a first interdigital transducer (IDT) over the second piezoelectric layer and over the first cavity; and

a second IDT over the bonding layer and the first piezoelectric layer and over the second cavity.

14 . The filter device of claim 13 , further comprising:

a first resonator that includes the first piezoelectric layer, the bonding layer, the second piezoelectric layer and the first IDT over the first cavity; and

a second resonator that includes the first piezoelectric layer, the bonding layer and the second IDT over the second cavity.

15 . The filter device of claim 14 , wherein the first resonator and the second resonator are each configured such that a radio frequency signal applied to the first and second IDTs excite different first and second primary shear acoustic modes in the first resonator and second resonator.

16 . The filter device of claim 13 , wherein the bonding layer is configured as an etch stop for etching the second piezoelectric layer.

17 . The filter device of claim 13 , wherein:

the second piezoelectric layer comprises a first material configured to be etched by a first process, and

the bonding layer comprises a second material that is substantially impervious to the first process.

18 . The filter device of claim 13 , further comprising one or more openings extending through the first piezoelectric layer, the second piezoelectric layer and the bonding layer.

19 . The filter device of claim 13 , wherein the first piezoelectric layer and the second piezoelectric layer have different thicknesses from each other.

20 . The filter device of claim 1 , wherein the second IDT is disposed directly on a surface of the bonding layer.