Bulk acoustic wave resonator having piezoelectric layer with varying amounts of dopant

1. A bulk acoustic wave (BAW) resonator structure, comprising:

a first electrode disposed over a substrate;

a piezoelectric layer disposed over the first electrode, the piezoelectric layer comprising a single layer, which comprises: a doped piezoelectric material, and an undoped piezoelectric material, wherein the doped piezoelectric material is variously doped at varying atomic percentages of at least one rare earth element for improving piezoelectric properties of the piezoelectric layer and reducing compressive stress; and

a second electrode disposed over the piezoelectric layer.

2. The BAW resonator structure according to claim 1 , wherein the piezoelectric layer comprises aluminum nitride (AlN).

3. The BAW resonator structure according to claim 2 , wherein the at least one rare earth element comprises at least one of scandium (Sc) and yttrium (Y).

4. The BAW resonator structure according to claim 2 , wherein a concentration of each of the at least one rare earth element is less than approximately 10 atomic percent of the doped piezoelectric material.

5. The BAW resonator structure according to claim 2 , wherein a concentration of each of the at least one rare earth element is in a range of approximately 30 atomic percent to approximately 40 atomic percent of the doped piezoelectric material.

6. The BAW resonator structure according to claim 1 , wherein the doped piezoelectric material has an incrementally increasing atomic percentage of the at least one rare earth element in a direction away from the substrate.

7. The BAW resonator structure according to claim 1 , wherein the doped piezoelectric material has an incrementally decreasing atomic percentage of the at least one rare earth element in a direction away from the substrate.

8. A thin film bulk acoustic resonator (FBAR) structure, comprising:

a substrate defining a cavity;

a first electrode disposed over the substrate, at least a portion of the first electrode being formed over the cavity in the substrate;

a single piezoelectric layer disposed over the first electrode, the single piezoelectric layer comprising: an undoped portion comprising undoped aluminum nitride (AlN) material; and a doped portion comprising doped AlN material, the doped portion being doped with a rare earth element; and

a second electrode disposed over the single piezoelectric layer.

9. The FBAR structure according to claim 8 , wherein the rare earth element comprises either scandium (Sc) and yttrium (Y).

10. The FBAR structure according to claim 8 , wherein a concentration of the rare earth element is less than approximately 10 atomic percent of the doped AlN material.

11. The FBAR structure according to claim 8 , wherein a concentration of each of the rare earth element is in a range of approximately 30 atomic percent to approximately 40 atomic percent of the doped AlN material.

12. A solidly mounted resonator (SMR) structure, comprising:

a substrate;

an acoustic reflector formed on the substrate, the acoustic reflector comprising a plurality of acoustic impedance layers, wherein at least two of the acoustic impedance layers have different acoustic impedances;

a first electrode disposed over the acoustic reflector;

a single piezoelectric layer disposed over the first electrode, the single piezoelectric layer comprising: an undoped portion comprising undoped aluminum nitride (AlN) material; and a doped portion comprising doped AlN material, the doped portion being doped with a rare earth element; and

a second electrode disposed over the single piezoelectric layer.

13. The SMR structure according to claim 12 , wherein the rare earth element comprises either scandium (Sc) and yttrium (Y).

14. The SMR structure according to claim 12 , wherein a concentration of the rare earth element is less than approximately 10 atomic percent of the doped AlN material.

15. The SMR structure according to claim 12 , wherein a concentration of each of the rare earth element is in a range of approximately 30 atomic percent to approximately 40 atomic percent of the doped AlN material.

16. A thin film bulk acoustic resonator (FBAR) structure, comprising:

a substrate defining a cavity;

a first electrode disposed over the substrate, at least a portion of the first electrode being formed over the cavity in the substrate;

a piezoelectric layer disposed over the first electrode, the piezoelectric layer comprising: an undoped sub-layer comprising undoped aluminum nitride (AlN) material; a first doped sub-layer comprising a first doped AlN material, the first doped AlN material being doped with a first atomic percentage of a first rare earth element; a second doped sub-layer comprising a second doped AlN material, the second doped AlN material being doped with a second atomic percentage of a second rare earth element, the second atomic percentage being different from the first atomic percentage; and

a second electrode disposed over the piezoelectric layer.

17. The FBAR structure according to claim 16 , wherein the first rare earth element comprises either scandium (Sc) and yttrium (Y).

18. The FBAR structure according to claim 16 , wherein a concentration of the first rare earth element is less than approximately 10 atomic percent of the first doped AlN material.

19. The FBAR structure according to claim 16 , wherein a concentration of the second rare earth element is less than approximately 10 atomic percent of the second doped AlN material.

20. The FBAR structure according to claim 16 , wherein a concentration of the first rare earth element is in a range of approximately 30 atomic percent to approximately 40 atomic percent of the first doped AlN material.

21. The FBAR structure according to claim 16 , wherein a concentration of the second rare earth element is in a range of approximately 30 atomic percent to approximately 40 atomic percent of the second doped AlN material.