Transversely-excited film bulk acoustic resonators with solidly mounted resonator (SMR) pedestals

1. A method of fabricating an acoustic resonator with a Bragg reflector having pedestals and configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator, comprising:

forming plural Bragg reflector layers on a top surface of a silicon substrate;

masking over the plural Bragg reflector layers at a plurality of pedestal locations for the pedestals of the Bragg reflector;

removing one or more top layers of the Bragg reflector layers from between the pedestal locations to form the pedestals and to form a cavity with a cavity top surface perimeter;

bonding a top surface of the pedestals and the cavity top surface perimeter to a plate front surface of a piezoelectric plate having the plate front surface, a plate back surface, and a sacrificial substrate bonded to the plate back surface;

removing the sacrificial substrate from the plate back surface to leave the piezoelectric plate bonded to the pedestals and standing over the cavity; and

forming at least one conductor pattern on the plate front surface.

2. The method of claim 1 , wherein:

forming plural Bragg reflector layers includes layering a total number of blanket layers that alternate between layers of material having high acoustic impedance and layers of material having low acoustic impedance;

masking over pedestal locations includes forming a patterned mask over the locations for pedestals and over the cavity top surface perimeter; and

removing one or more top layers of the Bragg reflector layers includes selectively etching the material having high acoustic impedance and the material having low acoustic impedance.

3. The method of claim 1 , wherein masking of over pedestal locations includes patterning a top surface of the Bragg reflector layers;

wherein removing one or more top layers of the plural Bragg reflector layers comprises performing one or more selective etches of the plural Bragg reflector layers to create air cavities between the pedestals; and

wherein the cavity top surface perimeter has the plural Bragg reflector layers disposed around the cavity.

4. The method of claim 3 , wherein bonding the pedestals to the piezoelectric plate includes mounting a piezoelectric plate back surface onto the Bragg reflector top surface except for a portion of the piezoelectric plate disposed over the air cavities.

5. The method of claim 4 , further comprising:

prior to bonding the piezoelectric plate, forming a sacrificial material in the air cavities; and

after forming the conductor pattern, forming the holes in a substrate perimeter beside the piezoelectric plate, and etching away the sacrificial material in the air cavities through the holes.

6. The method of claim 1 , wherein:

the Bragg reflector has the pedestals in a pedestal portion and has a mirror portion;

removing one or more top layers of the plural Bragg reflector layers includes leaving the mirror portion of Bragg reflector layers; and

the Bragg reflector is configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator.

7. The method of claim 1 , wherein removing the sacrificial substrate includes selectively etching away a material of the sacrificial substrate with respect to the plate and the silicon substrate to leave a piezoelectric membrane fixed on the pedestals and standing over the cavity.

8. The method of claim 7 , wherein forming at least one conductor pattern comprises forming an interdigital transducer (IDT) with interleaved fingers disposed on the membrane to form a diaphragm over a cavity.

9. The method of claim 8 , wherein:

the piezoelectric plate and the conductor pattern are configured such that radio frequency signals applied to the conductor pattern excites a primary shear acoustic mode in the piezoelectric plate over the cavity, wherein a thickness of the diaphragm is selected to tune the primary shear acoustic modes in the piezoelectric plate.

10. The method of claim 8 , wherein the pedestals are each formed from two or more of the plural Bragg reflector layers; support the back surface of the piezoelectric plate opposite the interleaved fingers of the IDT; and are attached to the diaphragm at locations under where the interleaved fingers are disposed on the membrane.

11. A method of fabricating an acoustic resonator with a Bragg reflector having pedestals and configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator, comprising:

forming plural pedestal portion Bragg reflector layers on a bottom surface of a piezoelectric plate, the piezoelectric plate having a plate front surface, a plate back surface, and a sacrificial substrate bonded to the plate back surface;

masking over the plural pedestal portion Bragg reflector layers at pedestal locations for the pedestals;

removing all of the plural pedestal portion Bragg reflector layers from between the pedestal locations to form the pedestals and to form a cavity with a cavity top surface perimeter;

forming plural mirror portion Bragg reflector layers on a top surface of a silicon substrate;

bonding a bottom surface of the pedestals formed on the plate having the substrate to a top surface of the mirror portion;

removing the sacrificial substrate from the plate back surface to leave a piezoelectric membrane bonded to the pedestals and standing over the cavity; and

forming at least one conductor pattern on the plate front surface.

12. The method of claim 11 , wherein:

forming the plural pedestal portion and plural mirror portion Bragg reflector layers each include layering a number of layers that alternate between layers of material having high acoustic impedance and layers of material having low acoustic impedance;

masking over pedestal locations includes forming a patterned mask over the locations for pedestals and over the cavity top surface perimeter; and

removing all of the plural pedestal portion Bragg reflector layers includes performing selective etches with respect to the material having high acoustic impedance and the material having low acoustic impedance.

13. The method of claim 11 , wherein masking of over pedestal locations includes patterning a top surface of the plural pedestal portion Bragg reflector layers;

wherein removing all the plural pedestal portion Bragg reflector layers comprises performing one or more selective etches of the plural Bragg reflector layers to create air cavities between the pedestals; and

wherein the cavity top surface perimeter has the plural pedestal portion Bragg reflector layers disposed around the cavity.

14. The method of claim 13 , further comprising:

prior to bonding the piezoelectric plate, forming a sacrificial material in the air cavities; and

after forming the conductor pattern, forming the holes in a substrate perimeter beside the piezoelectric plate, and etching away the sacrificial material in the air cavities through the holes.

15. The method of claim 11 , wherein forming at least one conductor pattern comprises forming an interdigital transducer (IDT) with interleaved fingers disposed on the membrane to form a diaphragm over the cavity;

wherein the piezoelectric plate and the conductor pattern are configured such that radio frequency signals applied to the conductor pattern excites a primary shear acoustic mode in the piezoelectric plate over the cavity, wherein a thickness of the diaphragm is selected to tune the primary shear acoustic modes in the piezoelectric plate; and

wherein the Bragg reflector is configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator.

16. A method of fabricating an acoustic resonator with a Bragg reflector having pedestals and configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator, comprising:

forming plural Bragg reflector layers on a bottom surface of a piezoelectric plate, the piezoelectric plate having a plate front surface, a plate back surface, and a sacrificial substrate bonded to the plate back surface;

masking over the plural Bragg reflector layers at a plurality of pedestal locations for the pedestals of the Bragg reflector;

removing all of the plural layers of the Bragg reflector layers from between the pedestal locations and from a perimeter of a cavity location to form the pedestals;

forming a substrate cavity in a top surface of a silicon substrate; then

bonding a bottom surface of the pedestals to a top surface of the substrate cavity;

removing the sacrificial substrate from the plate back surface to leave a piezoelectric membrane bonded to the pedestals and standing over the substrate cavity; and

forming at least one conductor pattern on the plate front surface.

17. The method of claim 16 , wherein bonding includes bonding a perimeter of the substrate cavity top surface to a bottom surface of the plate at the perimeter of the cavity location.

18. The method of claim 16 , wherein:

forming plural Bragg reflector layers includes blanket layering a number of blanket layers that alternate between layers of material having high acoustic impedance and layers of material having low acoustic impedance;

masking over pedestal locations includes forming a patterned mask over the locations for pedestals but not over the perimeter of the cavity location; and

removing includes performing selective etches with respect to the material having high acoustic impedance and the material having low acoustic impedance.

19. The method of claim 16 , wherein masking of over pedestal locations includes patterning a top surface of the pedestal portion Bragg reflector layers;

wherein removing all the plural Bragg reflector layers comprises performing one or more selective etches of the plural Bragg reflector layers to create air cavities between the pedestals; and

wherein the perimeter of the substrate cavity top surface has the substrate material disposed around the cavity.

20. The method of claim 16 , further comprising:

prior to bonding, forming a sacrificial material in the air cavities; and

after forming the conductor pattern, forming the holes in a silicon substrate perimeter beside the piezoelectric plate, and etching away the sacrificial material in the air cavities through the holes.

21. The method of claim 16 , wherein forming at least one conductor pattern comprises forming an interdigital transducer (IDT) with interleaved fingers disposed on the membrane to form a diaphragm over the cavity;

wherein the piezoelectric plate and the conductor pattern are configured such that radio frequency signals applied to the conductor pattern excites a primary shear acoustic mode in the piezoelectric plate over the cavity, wherein a thickness of the diaphragm is selected to tune the primary shear acoustic modes in the piezoelectric plate; and

wherein the Bragg reflector is configured to reflect shear acoustic waves at a resonance frequency of the acoustic resonator.