Piezoelectric micromachined ultrasonic transducers and methods for fabricating thereof

1. A piezoelectric micromachined ultrasonic transducer (PMUT) device comprising:

a wafer;

an active layer comprising a piezoelectric stack;

an intermediate layer having a cavity formed therein, the intermediate layer disposed between the wafer and the active layer such that the cavity is adjoining the piezoelectric stack;

a via formed through the active layer and the intermediate layer to reach the wafer; and

a metallic layer disposed over the active layer and over surfaces of the via;

wherein the intermediate layer comprises an interposing material surrounding the cavity, and further comprises a sacrificial material surrounding the via;

wherein the sacrificial material is different from the interposing material; and

wherein the metallic layer comprises:

a first member at least overlapping the piezoelectric stack;

a second member extending from the first member to the cavity through the active layer, filling up a release via passing through the active layer and disposed above the cavity, and not passing through the cavity; and

a third member extending into the active layer to contact an electrode within the active layer.

2. The PMUT device of claim 1 , wherein the wafer comprises:

a substrate;

a first wafer electrode and a second wafer electrode on the substrate; and

a dielectric material between the first wafer electrode and the second wafer electrode.

3. The PMUT device of claim 2 , wherein the cavity is formed directly over the first wafer electrode.

4. The PMUT device of claim 2 , wherein a thickness of the dielectric material is less than a thickness of each of the first wafer electrode and the second wafer electrode.

5. The PMUT device of claim 2 , wherein the sacrificial material in the intermediate layer adjoins the dielectric material to surround part of the second wafer electrode.

6. The PMUT device of claim 1 , wherein the wafer is a Complementary Metal-Oxide-Semiconductor (CMOS) device wafer.

7. The PMUT device of claim 1 , wherein the sacrificial material is a dielectric material.

8. The PMUT device of claim 1 , wherein the piezoelectric stack comprises a piezoelectric material sandwiched between a pair of electrodes of the active layer.

9. The PMUT device of claim 1 , further comprising:

a passivation layer disposed over the metallic layer to encapsulate the PMUT device.

10. The PMUT device of claim 9 , wherein the passivation layer extends onto side walls of the via.

11. The PMUT device of claim 1 , wherein the first member electromagnetically shields the piezoelectric stack.

12. A method for fabricating a PMUT device, the method comprising:

providing an intermediate layer on a wafer, the intermediate layer comprising an interposing material and a sacrificial material different from the interposing material;

forming a cavity in the intermediate layer;

arranging an active layer comprising a piezoelectric stack on the intermediate layer such that the piezoelectric stack in the active layer is adjoining the cavity;

forming a via through the active layer and the intermediate layer to reach the wafer, wherein the sacrificial material in the intermediate layer surrounds the via;

forming a release via passing through the active layer and above the cavity;

providing a metallic layer over the active layer and over surfaces of the via, the metallic layer comprising:

a first member at least overlapping the piezoelectric stack;

a second member extending from the first member to the cavity through the active layer, filling up the release via, and not passing through the cavity; and

a third member extending into the active layer to contact an electrode within the active layer.

13. The method of claim 12 , wherein providing the intermediate layer on the wafer comprises:

forming alignment features on the wafer by etching back the wafer to expose a first wafer electrode and a second wafer electrode.

14. The method of claim 12 , wherein providing the intermediate layer on the wafer comprises:

depositing the interposing material over the wafer;

removing a first portion of the interposing material lying directly over a first wafer electrode to form a first wafer electrode opening;

removing a second portion of the interposing material lying directly over a periphery of the second wafer electrode to form a second wafer electrode opening;

depositing the sacrificial material into the first wafer electrode opening and the second wafer electrode opening; and

planarizing the interposing material and the sacrificial material.

15. The method of claim 14 , wherein forming a cavity in the intermediate layer comprises:

etching the active layer to form the release via and a contact via,

wherein the release via extends to the sacrificial material lying directly over the first wafer electrode, and

wherein the contact via extends to the sacrificial material lying directly over one or more active layer bottom electrodes; and

removing the sacrificial material lying directly over the first wafer electrode.

16. The method of claim 15 , wherein providing the metallic layer comprises:

depositing metal over the active layer,

wherein the metal coats a section of the active layer corresponding to the piezoelectric stack to form the first member;

wherein the metal seals the release via to form the second member,

wherein the metal coats the contact via to form the third member.

17. The method of claim 12 , wherein arranging an active layer on the intermediate layer comprises:

providing an active layer bottom electrode layer;

providing a piezoelectric material over the active layer bottom electrode layer;

providing an active layer top electrode over the piezoelectric material; and

patterning each of the active layer top electrode layer and the active layer bottom electrode layer to form the piezoelectric stack.

18. The method of claim 12 , further comprising:

disposing a passivation layer over the metallic layer and into the via, such that the passivation layer covers side walls of the via.

19. The method of claim 18 , further comprising:

removing part of the passivation layer in the via to expose the metallic layer on a via bottom surface, the via bottom surface being at least perpendicular to the side walls.