Filter circuit

1. A filter circuit, comprising

a first and a second bulk acoustic wave (BAW) resonator, the first BAW resonator having a first piezoelectric layer structured such that the first BAW resonator has a lower resonant frequency than the second BAW resonator, wherein the first piezoelectric layer has a plurality of recesses formed in the first piezoelectric layer, wherein at least one of the plurality of recesses extends through an entirety of a thickness of the first piezoelectric layer and at least one of the recesses does not extend through the entirety of the thickness of the first piezoelectric layer, and wherein:

the first BAW resonator and the second BAW resonator are series resonators that are connected in series with a signal path of the filter circuit, or

the first BAW resonator and the second BAW resonator are parallel resonators that are connected to the signal path of the filter circuit such that one electrode of each of the BAW resonators is connected to the signal path.

2. The filter circuit according to claim 1 , wherein the first piezoelectric layer of the first BAW resonator is structured such that when an electric field is applied in a spatial direction, a vibration mode with vibrations in at least two spatial directions is induced.

3. The filter circuit according to claim 1 , wherein the second BAW resonator comprises a second piezoelectric layer which is unstructured or which is structured by recesses passing through the second piezoelectric layer.

4. The filter circuit according to claim 3 , wherein the second piezoelectric layer of the second BAW resonator is structured such that when an electric field is applied in a spatial direction, a vibration mode with vibrations in at least two spatial directions is induced.

5. The filter circuit according to claim 3 , wherein the first piezoelectric layer and the second piezoelectric layer have the same thickness.

6. The filter circuit according to claim 1 , wherein the filter circuit is a ladder-type circuit or a bridge circuit.

7. The filter circuit according to claim 1 , wherein the filter circuit is a bandpass filter or a bandstop filter.

8. The filter circuit according to claim 1 , wherein the first BAW resonator or the second BAW resonator is a film bulk wave acoustic resonator (FBAR) or a solid mounted bulk acoustic resonator (SMR).

9. The filter circuit according to claim 1 , wherein the plurality of recesses are formed by photolithographic patterning the respective piezoelectric layer.

10. The filter circuit according to claim 1 , wherein the filter circuit is arranged on a chip.

11. The filter circuit according to claim 1 , further comprising at least a third BAW resonator and a fourth BAW resonator,

wherein the first BAW resonator and the third BAW resonator are serially connected as series resonators to the signal path of the filter circuit,

wherein the second BAW resonator and the fourth BAW resonator are connected as parallel resonators to the signal path of the filter circuit in such a way that one electrode of each of the resonators is connected to the signal path,

wherein the third BAW resonator comprises a third piezoelectric layer structured such that the third BAW resonator has a lower resonant frequency than the fourth BAW resonator, wherein the third piezoelectric layer is structured by recesses passing through the first piezoelectric layer.

12. The filter circuit according to claim 11 , wherein the piezoelectric layers of the first BAW resonator, the second BAW resonator, the third BAW resonator and the fourth BAW resonator have the same thickness.

13. A filter circuit, comprising

a first and a second resonant bulk acoustic wave (BAW) resonator, the first BAW resonator having a first piezoelectric layer structured such that the first BAW resonator has a lower resonant frequency than the second BAW resonator, wherein the first piezoelectric layer is structured by recesses passing through the first piezoelectric layer, wherein the second resonator comprises a second piezoelectric layer which is structured by recesses passing through the second piezoelectric layer, and wherein the first piezoelectric layer has more recesses than the second piezoelectric layer, and wherein:

the first BAW resonator and the second BAW resonator are series resonators that are connected in series with a signal path of the filter circuit, or

the first BAW resonator and the second BAW resonator are parallel resonators that are connected to the signal path of the filter circuit such that one electrode of each of the BAW resonators is connected to the signal path.

14. The filter circuit according to claim 13 , wherein the first piezoelectric layer of the first BAW resonator is structured such that when an electric field is applied in a spatial direction, a vibration mode with vibrations in at least two spatial directions is induced.

15. The filter circuit according to claim 13 , wherein the second piezoelectric layer of the second BAW resonator is structured such that when an electric field is applied in a spatial direction, a vibration mode with vibrations in at least two spatial directions is induced.

16. The filter circuit according to claim 13 , wherein the first piezoelectric layer and the second piezoelectric layer have the same thickness.

17. The filter circuit according to claim 13 , wherein the filter circuit is a ladder-type circuit or a bridge circuit.

18. The filter circuit according to claim 13 , wherein the filter circuit is a bandpass filter or a bandstop filter.

19. The filter circuit according to claim 13 , wherein the first or the second resonator is a film bulk wave acoustic resonator (FBAR) or a solid mounted bulk acoustic resonator (SMR).

20. The filter circuit according to claim 13 , wherein the recesses are formed by photolithographic patterning the respective piezoelectric layer.