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An acoustic resonator structure includes an acoustic reflector over a cavity formed in a substrate, the acoustic reflector including a layer of low acoustic impedance material stacked on a layer of high acoustic impedance material. The acoustic resonator further includes a bottom electrode on the la

An acoustic resonator structure includes an acoustic reflector over a cavity formed in a substrate, the acoustic reflector including a layer of low acoustic impedance material stacked on a layer of high acoustic impedance material. The acoustic resonator further includes a bottom electrode on the layer of low acoustic impedance material, a piezoelectric layer on the bottom electrode, a top electrode on the piezoelectric layer, and a collar formed outside a main membrane region defined by an overlap between the top electrode, the piezoelectric layer and the bottom electrode. The collar has an inner edge substantially aligned with a boundary of or overlapping the main membrane region. The layer of the low acoustic impedance material includes a temperature compensating material having a positive temperature coefficient for offsetting at least a portion of a negative temperature coefficient of the piezoelectric layer, the bottom electrode and the top electrode.

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1. An acoustic resonator, comprising: an acoustic reflector disposed on a substrate over a cavity formed in the substrate, the acoustic reflector comprising a layer of low acoustic impedance material stacked on a layer of high acoustic impedance material;a bottom electrode disposed on the layer of l

1. An acoustic resonator, comprising: an acoustic reflector disposed on a substrate over a cavity formed in the substrate, the acoustic reflector comprising a layer of low acoustic impedance material stacked on a layer of high acoustic impedance material;a bottom electrode disposed on the layer of low acoustic impedance material of the acoustic reflector;a piezoelectric layer disposed on the bottom electrode;a top electrode disposed on the piezoelectric layer; anda collar formed outside a main membrane region defined by an overlap between the top electrode, the piezoelectric layer and the bottom electrode, the collar having an inner edge substantially aligned with a boundary of or overlapping the main membrane region,wherein the layer of the low acoustic impedance material comprises a temperature compensating material having a positive temperature coefficient for offsetting at least a portion of a negative temperature coefficient of the piezoelectric layer, the bottom electrode and the top electrode. 2. The acoustic resonator of claim 1, wherein the low acoustic impedance material comprises borosilicate glass (BSG) or tetra-ethyl-ortho-silicate (TEOS). 3. The acoustic resonator of claim 1, wherein the collar defines a collar region having a cutoff frequency that is substantially the same as a cutoff frequency of the main membrane region. 4. The acoustic resonator of claim 3, wherein the collar is formed on a top surface of the top electrode and a planarization layer adjacent the top electrode. 5. An acoustic resonator, comprising: an acoustic reflector disposed over a cavity formed in a substrate, the acoustic reflector comprising a layer of low acoustic impedance material stacked on a layer of high acoustic impedance material;a bottom electrode disposed on the layer of low acoustic impedance material of the acoustic reflector;a piezoelectric layer disposed on the bottom electrode;a top electrode disposed on the piezoelectric layer; anda collar formed outside a main membrane region defined by an overlap between the top electrode, the piezoelectric layer and the bottom electrode, the collar having an inner edge substantially aligned with a boundary of or overlapping the main membrane region,wherein the layer of the low acoustic impedance material comprises a temperature compensating material having a positive temperature coefficient for offsetting at least a portion of a negative temperature coefficient of the piezoelectric layer, the bottom electrode and the top electrode, andwherein the collar is formed between the bottom electrode and the piezoelectric layer. 6. The acoustic resonator of claim 1, further comprising: at least one frame disposed within the main membrane region and having an outer edge substantially aligned with the boundary of the main membrane region. 7. The acoustic resonator of claim 6, wherein the at least one frame comprises an add-on frame. 8. The acoustic resonator of claim 6, wherein the at least one frame comprises a composite frame. 9. The acoustic resonator of claim 6, wherein the at least one frame comprises a frame disposed at a bottom portion of the top electrode. 10. The acoustic resonator of claim 9, wherein the at least one frame comprises another frame disposed at one of a top portion or a bottom portion of the bottom electrode. 11. The acoustic resonator of claim 6, wherein the at least one frame comprises a frame disposed at a bottom portion of the bottom electrode. 12. The acoustic resonator of claim 1, wherein the collar comprises borosilicate glass, carbon-doped silicon oxide, silicon carbide, silicon nitride, aluminum oxide, aluminum nitride, zinc oxide, lead zirconium titanate, diamond or diamond-like carbon. 13. The acoustic resonator of claim 6, wherein the frame comprises a layer of copper, molybdenum, aluminum, tungsten, iridium, borosilicate glass, carbon-doped silicon oxide, silicon carbide, silicon nitride, aluminum oxide, aluminum nitride, zinc oxide, lead zirconium titanate, diamond or diamond-like carbon. 14. The acoustic resonator of claim 1, wherein the piezoelectric layer is doped with at least one rare earth element for offsetting at least a portion of degradation of an electromechanical coupling coefficient of the acoustic resonator caused by the temperature compensating material. 15. A thin-film bulk acoustic resonator (FBAR), comprising: a substrate defining a cavity;a distributed Bragg reflector (DBR) disposed on a top surface of the substrate over the cavity, the DBR comprising at least one layer of low acoustic impedance material having a positive temperature coefficient;an acoustic stack arranged on the DBR over the cavity, the acoustic stack comprising a piezoelectric layer sandwiched between bottom and top electrode layers, and having a main membrane region defined by an overlap between the bottom electrode, the piezoelectric layer, and the top electrode; anda collar arranged outside the main membrane region, the collar defining a collar region having a cutoff frequency that is substantially the same as a cutoff frequency of the main membrane region,wherein the positive temperature coefficient of the at least one layer of low acoustic impedance material offsets at least a portion of a negative temperature coefficient of the piezoelectric layer, the bottom electrode layer and the top electrode layer. 16. The FBAR of claim 15, further comprising: a frame formed in or on at least one of the bottom and top electrodes, and disposed within the main membrane region. 17. The FBAR of claim 16, wherein the collar is formed on a surface of the bottom electrode. 18. The FBAR of claim 16, wherein the collar is formed on a surface of the top electrode and a planarization layer adjacent to the top electrode. 19. The acoustic resonator of claim 1, wherein the piezoelectric layer is doped with at least one rare earth element for offsetting at least a portion of degradation of an electromechanical coupling coefficient of the acoustic resonator caused by the low acoustic impedance material having the positive temperature coefficient. 20. The acoustic resonator of claim 5, wherein the collar defines a collar region having a cutoff frequency that is substantially the same as a cutoff frequency of the main membrane region.