Solidly-mounted transversely-excited film bulk acoustic resonator
Resonator devices, filter devices, and methods of fabrication are disclosed. A resonator device includes a substrate and a single-crystal piezoelectric plate having parallel front and back surfaces. An acoustic Bragg reflector is sandwiched between a surface of the substrate and the back surface of the single-crystal piezoelectric plate. An interdigital transducer (IDT) is formed on the front surface. The IDT is configured to excite shear acoustic waves in the piezoelectric plate in response to a radio frequency signal applied to the IDT.
1. An acoustic resonator device comprising:
a substrate having a surface;
a single-crystal piezoelectric plate having front and back surfaces;
an acoustic Bragg reflector sandwiched between the surface of the substrate and the back surface of the single-crystal piezoelectric plate, the acoustic Bragg reflector configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator device; and
an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate, the IDT and the single-crystal piezoelectric plate configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the single-crystal piezoelectric plate.
2. The device of claim 1 , wherein
a direction of acoustic energy flow of the primary acoustic mode is substantially orthogonal to the front and back surfaces of the single-crystal piezoelectric plate.
3. The device of claim 2 , wherein the single-crystal piezoelectric plate is one of lithium niobate and lithium tantalate, and
a z-axis of the single-crystal piezoelectric plate is normal to the front and back surfaces.
4. The device of claim 3 , wherein
the IDT is oriented such that fingers of the IDT are parallel to an x-axis of the single-crystal piezoelectric plate.
5. The device of claim 1 , wherein a thickness between the front and back surfaces of the piezoelectric plate is greater than or equal to 200 nm and less than or equal to 1000 nm.
6. The device of claim 1 , wherein the acoustic Bragg reflector comprises:
a plurality of layers alternating between high acoustic impedance layers and low acoustic impedance layers, wherein
all of the plurality of layers are dielectric materials.
7. The device of claim 6 , wherein
the high acoustic impedance layers are one of silicon nitride and aluminum nitride, and
the low acoustic impedance layers are silicon oxycarbide.
8. The device of claim 7 wherein the plurality of layers includes at least four layers and not more than seven layers.
9. The device of claim 1 , wherein a pitch of fingers of the IDT is greater than or equal to 2 times the thickness of the piezoelectric plate and less than or equal to 25 times the thickness of the piezoelectric plate.
10. The device of claim 9 , wherein
the fingers of the IDT have a width, and
the pitch is greater than or equal to 2 times the width and less than or equal to 25 times the width.
11. 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 first surface of a single-crystal piezoelectric plate having a second surface attached to a sacrificial substrate;
bonding the single-crystal piezoelectric plate attached to the sacrificial substrate to the device substrate such that the layers of the acoustic Bragg reflector are sandwiched between the first surface of the single-crystal piezoelectric plate and the device substrate;
removing the sacrificial substrate to expose the second surface of the single-crystal piezoelectric plate;
forming a conductor pattern on the second surface of the single-crystal piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator;
depositing a first dielectric layer having a first thickness over the IDT of the shunt resonator; and
depositing a second dielectric layer having a second thickness less than the first thickness over the IDT of the series resonator, wherein
the single-crystal piezoelectric plate and all of the plurality of IDTs are configured such that respective radio frequency signals applied to the IDTs excite respective shear primary acoustic modes within the single-crystal piezoelectric plate.
12. A filter device comprising:
a substrate having a surface;
a single-crystal piezoelectric plate having front and back surfaces;
an acoustic Bragg reflector sandwiched between the surface of the substrate and the back surface of the single-crystal piezoelectric plate; and
a conductor pattern formed on the front surface of the single-crystal piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator;
a first dielectric layer having a first thickness deposited over the IDT of the shunt resonator; and
a second dielectric layer having a second thickness less than the first thickness deposited over the IDT of the series resonator, wherein
the single-crystal piezoelectric plate and all of the plurality of IDTs are configured such that respective radio frequency signals applied to the IDTs excite respective shear primary acoustic modes within the single-crystal piezoelectric plate.
13. The filter device of claim 12 , wherein
direction of acoustic energy flow of all of the primary acoustic modes is substantially orthogonal to the front and back surfaces of the single-crystal piezoelectric plate.
14. The filter device of claim 12 , wherein
the piezoelectric plate is one of lithium niobate and lithium tantalate, and
a z-axis of the single-crystal piezoelectric plate is normal to the front and back surfaces.
15. The filter device of claim 14 , wherein
all of the plurality of IDTs are oriented such that fingers of each IDT are parallel to an x-axis of the single-crystal piezoelectric plate.
16. The filter device of claim 12 , where the acoustic Bragg reflector is configured to reflect shear acoustic waves over a frequency range including the resonant and anti-resonance frequencies of all of the plurality of resonators.
17. The filter device of claim 16 , wherein the acoustic Bragg reflector comprises:
a plurality of dielectric layers alternating between high acoustic impedance layers and low acoustic impedance layers, wherein
the high acoustic impedance layers are one of silicon nitride and aluminum nitride, and
the low acoustic impedance layers are silicon oxycarbide.
18. The filter device of claim 17 wherein the plurality of layers includes at least four layers and no more than seven layers.
19. The filter device of claim 12 , wherein a thickness between the front and back surfaces of the piezoelectric plate is greater than or equal to 200 nm and less than or equal to 1000 nm.
20. The filter device of claim 12 , wherein all of the plurality of IDTs have respective pitches greater than or equal to 2 times the thickness of the piezoelectric plate and less than or equal to 25 times the thickness of the piezoelectric plate.
21. The filter device of claim 12 , wherein
the second thickness is greater than or equal to 0, and
the first thickness is less than or equal to 300 nm.
22. The filter device of claim 12 , wherein
the plurality of resonators includes two or more shunt resonators, and
the first dielectric layer is deposited over the two or more shunt resonators.
23. The filter device of claim 12 , wherein
the plurality of resonators includes two or more series resonators, and
the second dielectric layer is deposited over the two or more series resonators.
24. The filter device of claim 12 , wherein
a resonance frequency of the shunt resonator is set, at least in part, by the first thickness, and
a resonance frequency of the series resonator is set, at least in part, by the second thickness.
25. The filter device of claim 24 , wherein
a difference between the first thickness and the second thickness is sufficient to set the resonance frequency of the shunt resonator at least 100 MHz lower than the resonance frequency of the series resonator.
26. The filter device of claim 12 , wherein
the first and second dielectric layers are SiO2, and
a difference between the first thickness and the second thickness is greater than or equal to 25 nm.
27. A method of fabricating an acoustic resonator device comprising:
forming an acoustic Bragg reflector by depositing material layers on one or both of a surface of a device substrate and a first surface of a single-crystal piezoelectric plate having a second surface attached to a sacrificial substrate;
bonding the single-crystal piezoelectric plate attached to the sacrificial substrate to the device substrate such that the layers of the acoustic Bragg reflector are sandwiched between the first surface of the single-crystal piezoelectric plate and the device substrate;
removing the sacrificial substrate to expose the second surface of the single-crystal piezoelectric plate; and
forming an interdigital transducer (IDT) on the second surface of the single-crystal piezoelectric plate,
wherein the single-crystal piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the single-crystal piezoelectric plate.