Transversely-excited film bulk acoustic resonator with partial BRAGG reflectors

1. An acoustic resonator device comprising:

a substrate having a surface;

a single-crystal piezoelectric plate attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity over the substrate in a thickness direction of the substrate that is orthogonal to the surface of the substrate;

an interdigital transducer (IDT) on a surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm;

a back-side partial Bragg reflector on a back surface of the diaphragm, the back-side partial Bragg reflector consisting of a layer of low acoustic impedance dielectric material on the back surface of the diaphragm and a layer of high acoustic impedance dielectric material on the layer of low acoustic impedance dielectric material and opposite the diaphragm; and

a front-side partial Bragg reflector on a front surface of the diaphragm, with the front surface of the diaphragm opposing the back surface of the diaphragm, the front-side partial Bragg reflector consisting of a layer of low acoustic impedance dielectric material on the front surface of the diaphragm and a layer of high acoustic impedance dielectric material on the layer of low acoustic impedance dielectric material and opposite the diaphragm,

wherein each of the back-side partial Bragg reflector and the front-side partial Bragg reflector is structurally configured such that the respective reflectors do not reflect at least a portion of energy of an incident acoustic wave when the primary shear acoustic mode is excited in the diaphragm, and

wherein the primary shear acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the single-crystal piezoelectric plate and transverse to a direction of electric field created by the IDT.

2. The device of claim 1 , wherein each layer of the front-side partial Bragg reflector and the back-side partial Bragg reflector has a thickness equal to one-quarter of a wavelength in the respective material of the primary shear acoustic mode at a predetermined frequency.

3. The device of claim 2 , wherein the predetermined frequency is one of a resonance frequency of the device, an anti-resonance frequency of the device, and a frequency within a passband of a filter incorporating the device.

4. The device of claim 1 , wherein the low acoustic impedance material is one of silicon dioxide and silicon oxycarbide.

5. The device of claim 1 , wherein the high acoustic impedance material is one of silicon nitride and aluminum nitride.

6. The device of claim 1 , wherein the single-crystal piezoelectric plate is one of lithium niobate and lithium tantalate.

7. The device of claim 1 , wherein the back-side partial Bragg reflector extends between a surface of the piezoelectric plate and the surface of the substrate.

8. An acoustic resonator device comprising:

a substrate;

a piezoelectric layer attached to the substrate;

an interdigital transducer (IDT) on a surface of the piezoelectric layer and including interleaved fingers, the IDT being configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the piezoelectric layer;

a back-side partial Bragg reflector on a back surface of the piezoelectric layer, the back-side partial Bragg reflector including of a layer of low acoustic impedance dielectric material on the back surface of the piezoelectric layer and a layer of high acoustic impedance dielectric material on the layer of low acoustic impedance dielectric material and opposite the piezoelectric layer; and

a front-side partial Bragg reflector on a front surface of the piezoelectric layer, with the front surface of the piezoelectric layer opposing the back surface of the piezoelectric layer, the front-side partial Bragg reflector including of a layer of low acoustic impedance dielectric material on the front surface of the piezoelectric layer and a layer of high acoustic impedance dielectric material on the layer of low acoustic impedance dielectric material and opposite the piezoelectric layer,

wherein each of the back-side partial Bragg reflector and the front-side partial Bragg reflector is configured such that the respective reflectors do not reflect at least a portion of energy of an incident acoustic wave when the primary shear acoustic mode is excited in the piezoelectric layer, and

wherein the primary shear acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric layer and transverse to a direction of electric field created by the IDT.

9. The acoustic resonator device of claim 8 , wherein each layer of the front-side partial Bragg reflector and the back-side partial Bragg reflector has a thickness equal to one- quarter of a wavelength in the respective material of the primary shear acoustic mode at a predetermined frequency.

10. The acoustic resonator device of claim 9 , wherein the predetermined frequency is one of a resonance frequency of the device, an anti-resonance frequency of the device, and a frequency within a passband of a filter incorporating the device.

11. The acoustic resonator device of claim 8 , wherein the low acoustic impedance material is one of silicon dioxide and silicon oxycarbide.

12. The acoustic resonator device of claim 8 , wherein the high acoustic impedance material is one of silicon nitride and aluminum nitride.

13. The acoustic resonator device of claim 8 , wherein the piezoelectric layer is one of lithium niobate and lithium tantalate.

14. The acoustic resonator device of claim 8 , wherein the back-side partial Bragg reflector extends between a surface of the piezoelectric layer and a surface of the substrate.