Transversely-excited film bulk acoustic resonators with solidly mounted resonator (SMR) pedestals
An acoustic resonator is fabricated with a substrate having a substrate top surface and a piezoelectric plate having plate front and plate back surfaces. An acoustic Bragg reflector is sandwiched between the substrate top surface and the plate back surface. The reflector has a cavity with a top surface perimeter, and the acoustic Bragg reflector is configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator. The back surface is mounted on the cavity top surface perimeter except for a portion of the plate forming a diaphragm that spans the cavity. An interdigital transducer (IDT) is formed on the plate front surface such that interleaved fingers of the IDT are disposed on the diaphragm. Two or more layers of the acoustic Bragg reflector form pedestals that support the back surface of the plate opposite some or all interleaved fingers of the IDT.
1. An acoustic resonator, comprising:
a substrate having a substrate top surface;
a piezoelectric plate having plate front and plate back surfaces;
an acoustic Bragg reflector sandwiched between the substrate top surface and the plate back surface, the acoustic Bragg reflector having a cavity with a cavity top surface perimeter, and the acoustic Bragg reflector configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator;
the piezoelectric plate back surface is mounted on the cavity top surface perimeter except for a portion of the piezoelectric plate forming a diaphragm that spans the cavity; and
an interdigital transducer (IDT) is formed on the piezoelectric plate front surface such that interleaved fingers of the IDT are disposed on the diaphragm,
wherein two or more layers of the acoustic Bragg reflector form pedestals that support the back surface of the piezoelectric plate opposite some or all interleaved fingers of the IDT.
2. The acoustic resonator of claim 1 , wherein the pedestals are aligned with every n fingers of the IDT, where n is an integer greater than or equal to two and less than or equal to twenty.
3. The acoustic resonator of claim 1 , wherein the pedestals exist opposite at least a total width and at least a total length of the interleaved fingers.
4. The acoustic resonator of claim 3 , wherein the pedestals have a width extending beyond the total width of the interleaved fingers by between 1 and 10 percent of the total width of the interleaved fingers.
5. The acoustic resonator of claim 1 , wherein the acoustic Bragg reflector has a total of six layers, a mirror portion of the reflector has four of the total six layers and the pedestals have two of the total six layers;
wherein the total of six layers alternate between layers of material having a high acoustic impedance and layers of material having a low acoustic impedance; and
wherein the mirror portion is attached to a cavity bottom surface that is the substrate top surface.
6. The acoustic resonator of claim 5 , wherein the material having the high acoustic impedance is Tungsten and the material having the low acoustic impedance is Silicon Dioxide.
7. The acoustic resonator of claim 6 , wherein the layers of material having the high acoustic impedance each have a thickness between 110 and 180 nm, and the layers of material having the low acoustic impedance each have a thickness between 150 and 230 nm.
8. The acoustic resonator of claim 5 , wherein each of the total of six layers has a floating potential, and each of two of the total of six layers has a potential equal to a potential of the interleaved fingers of the IDT it is opposite of.
9. The acoustic resonator of claim 1 , wherein the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm, and
wherein a direction of acoustic energy flow of the primary shear acoustic mode is substantially orthogonal to the front and back surfaces of the piezoelectric plate.
10. An acoustic resonator device comprising:
a substrate having a top surface;
a single-crystal piezoelectric plate having parallel front and back surfaces;
an acoustic Bragg reflector sandwiched between the top 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 configured to excite a primary acoustic mode in the single-crystal piezoelectric plate in response to a radio frequency signal applied to the IDT,
wherein two or more layers of the acoustic Bragg reflector form pedestals that support the back surface of the piezoelectric plate opposite some or all interleaved fingers of the IDT.
11. The acoustic resonator device of claim 10 , wherein the pedestals are aligned with every n fingers of the IDT, where n is an integer greater than or equal to two and less than or equal to twenty.
12. The acoustic resonator device of claim 10 , wherein the acoustic Bragg reflector comprises:
a plurality of layers alternating between high acoustic impedance layers and low acoustic impedance layers, wherein:
the high acoustic impedance layers are tungsten having a thickness between 110 and 180 nm;
the low acoustic impedance layers are silicon dioxide having a thickness between 150 and 230 nm; and
wherein each of the of the plurality of layers has a floating potential.
13. The acoustic resonator device of claim 10 , wherein the pedestals exist opposite at least a total width of and at least a portion of total length of the interleaved fingers.
14. The acoustic resonator device of claim 10 , wherein the acoustic Bragg reflector has a total number of layers, a mirror portion of the reflector has a first number of the total number of layers and the pedestals have a remaining number of the total number of layers;
wherein the total number of layers alternate between layers of material having a high acoustic impedance and layers of material having a low acoustic impedance; and
wherein the mirror portion is attached to a cavity bottom surface that is the top surface of the substrate.
15. The acoustic resonator device of claim 14 , wherein each of the total number of layers has a floating potential, and each of the remaining number of the total number of layers has a potential equal to that of the interleaved fingers of the IDT it is opposite of.
16. An acoustic resonator, comprising:
a substrate having a substrate top surface and a cavity with a cavity top surface;
a piezoelectric plate having plate front and plate back surfaces, the plate back surface is mounted on the substrate top surface except for a portion of the piezoelectric plate forming a diaphragm that spans the cavity in the substrate;
an interdigital transducer (IDT) formed on a front surface of the piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm; and
an acoustic Bragg reflector sandwiched between the substrate and a back surface of the piezoelectric plate, the acoustic Bragg reflector configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator,
wherein two or more layers of the acoustic Bragg reflector form pedestals that support the back surface of the piezoelectric plate opposite some or all interleaved fingers of the IDT.
17. The acoustic resonator of claim 16 , wherein the pedestals exist opposite at least a total width and at least a total length of the interleaved fingers.
18. The acoustic resonator of claim 17 , wherein the acoustic Bragg reflector has a total number of layers, a mirror portion of the reflector has a first number of the total number of layers and the pedestals have a remaining number of the total number of layers;
wherein the total number of layers alternate between layers of material having a high acoustic impedance and layers of material having a low acoustic impedance; and
wherein the mirror portion is attached to a cavity bottom surface that is the substrate top surface.
19. The acoustic resonator of claim 18 , wherein the material having the high acoustic impedance is Tungsten and the material having the low acoustic impedance is Silicon Dioxide; and
wherein the layers of material having the high acoustic impedance each have a thickness between 110 and 180 nm, and the layers of material having the low acoustic impedance each have a thickness between 150 and 230 nm.
20. The acoustic resonator of claim 16 , wherein the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm, and
wherein a direction of acoustic energy flow of the primary shear acoustic mode is substantially orthogonal to the plate front and plate back surfaces of the piezoelectric plate.