High quality factor transducers for surface acoustic wave devices
The present disclosure relates to acoustic wave devices, and particularly to high quality factor (Q) transducers for surface acoustic wave (SAW) devices. An exemplary SAW device includes an interdigital transducer (IDT) between two reflective gratings to form a resonator. The resonator operates through shear horizontal mode acoustic waves, and therefore suppression of transverse modes (parallel to electrode fingers of the IDT) is desired. A piston mode can be formed in the resonator to suppress transverse modes, which may also increase energy leakage and result in a lower Q. A higher Q is achieved by adding a fast region at an end of one or more of the electrode fingers of the IDT.
1. A surface acoustic wave (SAW) device, comprising:
a piezoelectric substrate; and
an interdigital transducer (IDT) disposed over the piezoelectric substrate, comprising:
a first electrode;
a second electrode;
a first set of electrode fingers coupled to the first electrode, each comprising:
a first inside region;
a first end defining a first gap with the second electrode;
a first fast region adjacent to the first end and defining a first propagation velocity on the piezoelectric substrate which is faster than the first inside region; and
a first slow region between the first inside region and the first fast region and defining a second propagation velocity on the piezoelectric substrate which is slower than the first inside region; and
a second set of electrode fingers interleaved with the first set of electrode fingers and coupled to the second electrode, each comprising:
a second inside region; and
a second end defining a second gap with the first electrode.
2. The SAW device of claim 1 , wherein the first propagation velocity on the piezoelectric substrate is achieved by at least one of reducing an electrode width of the first fast region compared to the first inside region, reducing a metal thickness of the first fast region compared to the first inside region, increasing a dielectric thickness over and confined within the first fast region compared to the first inside region.
3. The SAW device of claim 1 , wherein the second propagation velocity on the piezoelectric substrate is achieved by at least one of increasing an electrode width of the first slow region compared to the first inside region, increasing a metal thickness of the first slow region compared to the first inside region, and reducing a dielectric thickness over and confined within the first slow region compared to the first inside region.
4. The SAW device of claim 1 , wherein:
an electrode width of the first fast region, defined transverse to the first gap, is smaller than an electrode width of the first inside region; and
an electrode width of the first slow region, defined transverse to the first gap, is greater than the electrode width of the first inside region.
5. The SAW device of claim 1 , wherein:
a metal thickness of the first fast region over the piezoelectric substrate is smaller than a metal thickness of the first inside region; and
a metal thickness of the first slow region over the piezoelectric substrate is greater than the metal thickness of the first inside region.
6. The SAW device of claim 1 , wherein:
an electrode width of the first fast region, defined transverse to the first gap, is smaller than an electrode width of the first inside region; and
a metal thickness of the first slow region over the piezoelectric substrate is greater than the metal thickness of the first inside region.
7. The SAW device of claim 1 , further comprising a dielectric layer over the piezoelectric substrate and the IDT.
8. The SAW device of claim 7 further comprising a passivation layer over the dielectric layer.
9. The SAW device of claim 8 , further comprising a metallic layer embedded in the dielectric layer, wherein:
the metallic material is confined in the first slow region; and
the metallic material is placed on top of the IDT, on top of the dielectric layer, or on top of the passivation layer.
10. The SAW device of claim 8 wherein:
the passivation layer has a greater thickness over the first fast region than the first inside region; and
the passivation layer has a smaller thickness over the first slow region than the first inside region.
11. The SAW device of claim 8 , wherein the passivation layer comprises at least one of silicon nitride, aluminum nitride, aluminum oxide, or diamond.
12. The SAW device of claim 1 further comprising a dielectric layer that is inserted between the piezoelectric substrate and the first set of electrode fingers in the first fast region.
13. The SAW device of claim 12 , wherein the dielectric layer is a fast layer comprising at least one of silicon nitride or aluminum nitride.
14. The SAW device of claim 1 , wherein the piezoelectric substrate comprises at least one of lithium tantalate or lithium niobate.
15. The SAW device of claim 1 , wherein a width of the first fast region, defined parallel to the first gap, is at least one wavelength.
16. The SAW device of claim 1 , wherein acoustic waves are guided in the first inside region and the second inside region.
17. The SAW device of claim 1 , wherein each of the second set of electrode fingers further comprises a second fast region adjacent to the second end defining a third propagation velocity on the piezoelectric substrate which is faster than the second inside region.
18. The SAW device of claim 17 , wherein each of the second set of electrode fingers further comprises a second slow region between the second inside region and the second fast region and defining a fourth propagation velocity on the piezoelectric substrate which is slower than the second inside region.
19. The SAW device of claim 18 , wherein:
the first propagation velocity on the piezoelectric substrate is achieved by at least one of reducing an electrode width of the first fast region compared to the first inside region, reducing a metal thickness of the first fast region compared to the first inside region, increasing a dielectric thickness over and confined within the first fast region compared to the first inside region; and
the third propagation velocity on the piezoelectric substrate is achieved by at least one of reducing an electrode width of the second fast region compared to the second inside region, reducing a metal thickness of the second fast region compared to the second inside region, increasing a dielectric thickness over and confined within the second fast region compared to the second inside region.
20. The SAW device of claim 19 , wherein:
the second propagation velocity on the piezoelectric substrate is achieved by at least one of increasing an electrode width of the first slow region compared to the first inside region, increasing a metal thickness of the first slow region compared to the first inside region, reducing a dielectric thickness over and confined within the first slow region compared to the first inside region; and
the fourth propagation velocity on the piezoelectric substrate is achieved by at least one of increasing an electrode width of the second slow region compared to the second inside region, increasing a metal thickness of the second slow region compared to the second inside region, and reducing a dielectric thickness over and confined within the second slow region compared to the second inside region.
21. The SAW device of claim 20 , further comprising a dielectric layer over the piezoelectric substrate and the IDT.
22. The SAW device of claim 21 further comprising a passivation layer over the dielectric layer, wherein the passivation layer comprises at least one of silicon nitride, aluminum nitride, aluminum oxide, or diamond.
23. The SAW device of claim 22 , further comprising a metallic layer embedded in the dielectric layer, wherein:
the metallic material is confined in the first slow region and the second slow region; and
the metallic material is placed on top of the IDT, on top of the dielectric layer, or on top of the passivation layer.
24. The SAW device of claim 1 , wherein the first set of electrode fingers and the second set of electrode fingers define a mode within 10% of a flat curve in the first inside region.