Bandpass filter with frequency separation between shunt and series resonators set by dielectric layer thickness
Filter devices and methods of fabrication are disclosed. A filter device includes a piezoelectric plate attached to a substrate, portions of the piezoelectric plate forming diaphragms spanning respective cavities in the substrate. A conductor pattern formed on a surface of the piezoelectric plate includes a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators including a shunt resonator and a series resonator, interleaved fingers of each of the plurality of IDTs disposed on one of the diaphragms. Radio frequency signals applied to the IDTs excite respective primary shear acoustic modes in the respective diaphragms. A thickness of a first dielectric layer disposed on the front surface between the fingers of the IDT of the shunt resonator is greater than a thickness of a second dielectric layer disposed on the front surface between the fingers of the IDT of the series resonator.
1. A filter device, comprising:
a substrate;
a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to a surface of the substrate, portions of the single-crystal piezoelectric plate forming one or more diaphragms spanning respective cavities in the substrate;
a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators including a shunt resonator and a series resonator, interleaved fingers of each of the plurality of IDTs disposed on a respective diaphragm of the one or more diaphragms;
a first dielectric layer having a first thickness disposed on the front surface between the fingers of the IDT of the shunt resonator, and
a second dielectric layer having a second thickness disposed on the front surface between the fingers of the IDT of the series resonator, wherein
the piezoelectric plate and all of the plurality of IDTs are configured such that radio frequency signals applied to the IDTs excite respective primary shear acoustic modes within the respective diaphragms, and
the first thickness is greater than the second thickness.
2. The filter device of claim 1 , wherein
a difference between a resonance frequency of the series resonator and a resonance frequency of the shunt resonator is determined, in part, by a difference between the first thickness and the second thickness.
3. The filter device of claim 1 , wherein the first thickness is less than or equal to 500 nm, and the second thickness is greater than or equal to zero.
4. The filter device of claim 1 , wherein respective directions of acoustic energy flow of each of the excited primary shear acoustic modes are substantially normal to the surfaces of the piezoelectric plate.
5. The filter device of claim 1 , wherein a thickness between the front and back surfaces of the single-crystal piezoelectric plate is greater than or equal to 200 nm and less than or equal to 1000 nm.
6. The filter device of claim 1 , wherein each of the plurality of IDTs has a respective pitch greater than or equal to 2 times the thickness of the single-crystal piezoelectric plate and less than or equal to 25 times the thickness of the piezoelectric plate.
7. The filter device of claim 1 , wherein the first and second dielectric layers comprise at least one of silicon dioxide and silicon nitride.
8. The filter device of claim 1 , wherein the conductor pattern comprises one of aluminum, an aluminum alloy, copper, a copper alloy, beryllium, and gold.
9. The filter device of claim 1 , wherein
a z-axis of the piezoelectric plate is normal to the front and back surfaces.
10. The filter device of claim 9 , wherein
the fingers of all of the plurality of IDTs are parallel to an x-axis of the piezoelectric plate.
11. The filter device of claim 1 , wherein
the plurality of resonators includes two or more shunt resonators, and
the first dielectric layer is disposed on the front surface between the fingers of all of the two or more shunt resonators.
12. The filter device of claim 1 , wherein
the plurality of resonators includes two or more series resonators, and
the second dielectric layer is disposed on the front surface between the fingers of all of the two or more series resonators.
13. A method of fabricating a filter device on a piezoelectric plate having front and back surfaces, the back surface attached to a substrate, the method comprising:
forming one or more cavities in the substrate such that respective portions of the piezoelectric plate form one or more diaphragms suspended over the cavities;
forming a conductor pattern on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator, wherein interleaved fingers of each of the plurality of IDTs are disposed on a respective diaphragm of the one or more diaphragms;
forming a first dielectric layer having a first thickness, the first dielectric layer disposed on the front surface between the fingers of the IDT of the shunt resonator; and
forming a second dielectric layer having a second thickness, the second dielectric layer disposed on the front surface between the fingers of the IDT of the series resonator, wherein
the piezoelectric plate and all of the plurality of IDTs are configured such that radio frequency signals applied to the IDTs excite respective primary shear acoustic modes in the respective diaphragms, and
the first thickness is greater than the second thickness.
14. The method of claim 13 , wherein
a difference between a resonance frequency of the series resonator and a resonance frequency of the shunt resonator is determined, in part, by a difference between the first thickness and the second thickness.
15. The method of claim 13 , wherein the first thickness is less than or equal to 500 nm, and the second thickness is greater than or equal to zero.
16. The method of claim 13 , wherein respective directions of acoustic energy flow of each of the excited shear acoustic modes are substantially normal to the surfaces of the piezoelectric plate.
17. The method of claim 13 , wherein a thickness between the front and back surfaces of the single-crystal piezoelectric plate is greater than or equal to 200 nm and less than or equal to 1000 nm.
18. The method of claim 13 , wherein each of the plurality of IDTs has a respective pitch greater than or equal to 2 times the thickness of the single-crystal piezoelectric plate and less than or equal to 25 times the thickness of the piezoelectric plate.
19. The method of claim 13 , wherein the first and second dielectric layers comprise at least one of silicon dioxide and silicon nitride.
20. The method of claim 13 , wherein the conductor pattern comprises one of aluminum, an aluminum alloy, copper, a copper alloy, beryllium, and gold.
21. The method of claim 13 , wherein
a z-axis of the piezoelectric plate is normal to the front and back surfaces.
22. The method of claim 21 , wherein
the fingers of all of the plurality of IDTs are parallel to an x-axis of the piezoelectric plate.
23. The method of claim 13 , wherein
the plurality of resonators includes two or more shunt resonators, and
the first dielectric layer is disposed on the front surface between the fingers of all of the two or more shunt resonators.
24. The method of claim 13 , wherein
the plurality of resonators includes two or more series resonators, and
the second dielectric layer is disposed on the front surface between the fingers of all of the two or more series resonators.