Thin-film bulk-acoustic wave (BAW) resonators
A thin-film bulk acoustic wave (BAW) resonator, such as SBAR or FBAR, for use in RF selectivity filters operating at frequencies of the order of 1 GHz. The BAW resonator comprises a piezoelectric layer ( 14 ) having first and second surfaces on opposing sides, a first electrode ( 16 ) extending over the first surface, and a second electrode ( 12 ) extending over the second surface, the extent of the area of overlap (R 1 ) of the first and second electrodes determining the region of excitation of the fundamental thickness extensional (TE) mode of the resonator. The insertion loss to the resonator is reduced by providing a dielectric material ( 18 ) in the same layer as the first electrode ( 16 ) and surrounding that electrode. The material constituting the dielectric material ( 18 ) has a different mass, typically between 5% and 15%, from the material comprising the first electrode ( 16 ) it surrounds. The mass of the dielectric material ( 18 ) can be lower or higher than the mass of the first electrode ( 16 ). Planarisation of the dielectric material ( 18 ) enhances the performance of the device.
1. A bulk acoustic wave (BAW) resonator comprising first and second layers and an intervening piezoelectric layer, the first and second layers comprising respective first and second overlapping regions of metal for use as electrodes, the first layer further comprising a region of dielectric material external to the region of the overlap and at least partially surrounding the first metal region, wherein the dielectric material has a mass different from the mass of the first metal region;
characterised in that the difference in mass of the dielectric material and the first metal region is between about 5% and about 15% and characterised in that the mass of the dielectric material is less than the mass of the first metal region.
2. A BAW resonator as claimed in claim 1 , characterised in that the first metal region comprises platinum and the dielectric material comprises tantalum pentoxide.
3. A BAW resonator as claimed in claim 1 , characterised in that the first metal region comprises molybdenum and the dielectric material comprises silicon dioxide.
4. A BAW resonator as claimed in claim 1 , characterised in that the dielectric material has the same thickness as the first metal region.
5. A BAW resonator as claimed in claim 1 , characterised in that the second layer comprises a region of dielectric material external to the region of overlap and at least partially surrounding the second metal region.
6. A BAW resonator as claimed in claim 1 , wherein the region of dielectric material is spaced from the first metal region.
7. A BAW resonator as claimed in claim 1 , characterised in that the resonator comprises a Solidly-mounted-Bulk-Acoustic wave-Resonator.
8. A BAW resonator as claimed in claim 1 , characterised in that the resonator comprises a Film-Bulk-Acoustic-wave-Resonator.
9. A filter comprising a plurality of BAW resonators as claimed in claim 1 .
10. A radio frequency apparatus including a filter comprising a plurality of BAW resonators as claimed in claim 1 .
11. A BAW resonator as claimed in claim 1 , wherein the difference in mass is selected in order to avoid overlap between a passband of the first metal region and a radiation band of the region of the dielectric material.
12. A method of reducing insertion loss in the passband of a filter comprising a plurality of bulk acoustic wave (BAW) resonators, each of the BAW resonators comprising first and second layers and an intervening piezoelectric layer, the first and second layers comprising respective first and second overlapping regions of metal for use as electrodes, the method comprising forming in the first layer a region of dielectric material external to the region of the overlap and at least partially surrounding the first metal region, wherein the dielectric material has a mass different from the mass of the first metal region;
characterised in that the difference in mass of the dielectric material and the first metal region is between about 5% and about 15% and characterised in that the mass of the dielectric material is less than the mass of the first metal region.
13. A method as claimed in claim 12 , characterised by forming the dielectric material to have the same thickness as the first metal region.
14. A method as claimed in claim 12 , wherein the difference in mass is selected in order to avoid overlap between a passband of the first metal region and a radiation band of the region of the dielectric material.