Ultrasonic transducers in complementary metal oxide semiconductor (CMOS) wafers and related apparatus and methods

1. A method, comprising:

forming an acoustic membrane of an ultrasonic transducer in a complementary metal oxide semiconductor (CMOS) wafer at least in part by stacking multiple layers of the CMOS wafer including at least one dielectric layer, a first metallization layer and a second metallization layer of the CMOS wafer;

forming at least one access hole to the first metallization layer of the CMOS wafer, the first metallization layer comprising an inner metal layer bounded by first and second conductive liner layers;

forming a cavity in the CMOS wafer by removing at least a portion of the inner metal layer of the first metallization layer through the at least one access hole using a selective etch, thereby releasing the acoustic membrane while substantially retaining the first and second conductive liner layers, wherein the first conductive liner layer is disposed between the cavity and a semiconductor substrate of the CMOS wafer, and the second conductive liner layer is disposed in the acoustic membrane between the cavity and the second metallization layer;

sealing the at least one access hole with an insulating material without filling the cavity; and

coupling the first and second conductive liner layers to integrated circuitry of the CMOS wafer.

2. The method of claim 1 , wherein forming the acoustic membrane further comprises coupling the second metallization layer of the CMOS wafer to the second conductive liner layer of the first metallization layer with one or more conductive vias.

3. The method of claim 1 , wherein coupling the first and second conductive liner layers to integrated circuitry of the CMOS wafer comprises coupling the first conductive liner layer to integrated circuitry in a semiconductor substrate of the CMOS wafer by one or more conductive vias.

4. The method of claim 1 , wherein the inner metal layer comprises aluminum, and wherein using a selective etch comprises using a hydrofluoric acid etch.

5. The method of claim 4 , wherein the first and second conductive liner layers comprise titanium nitride (TiN).

6. The method of claim 5 , wherein the first conductive liner layer comprises a first electrode of the ultrasonic transducer, and the second conductive liner layer comprises a second electrode of the ultrasonic transducer.

7. A method of forming an ultrasound-on-chip device, the method comprising:

forming complementary metal oxide semiconductor (CMOS) integrated circuitry in a semiconductor substrate;

forming a layer stack over the CMOS integrated circuitry, the layer stack comprising a plurality of dielectric layers and a plurality of metallization layers;

at least partially removing a first metallization layer of the plurality of metallization layers, thereby forming a cavity of an ultrasonic transducer and an acoustic membrane of the ultrasonic transducer, wherein the cavity is disposed between first and second electrodes of the ultrasonic transducer, and wherein the acoustic membrane comprises at least one of the plurality of dielectric layers and a second metallization layer of the plurality of metallization layers, with the second electrode being disposed in the acoustic membrane between the cavity and the second metallization layer;

sealing the cavity while leaving the cavity unfilled; and

wherein the integrated circuitry is coupled to the ultrasonic transducer and configured to control operation of the ultrasonic transducer.

8. The method of claim 7 , wherein at least partially removing the first metallization layer further comprises:

forming at least one access hole to the first metallization layer; and

performing a selective etch to remove an inner metal layer of the first metallization layer.

9. The method of claim 8 , wherein the first and second electrodes comprise liner layers of the first metallization layer.

10. The method of claim 9 , wherein the inner metal layer comprises aluminum (Al), and the liner layers comprise titanium nitride (TiN).

11. The method of claim 8 , further comprising sealing the at least one access hole following the selective etch.

12. The method of claim 7 , further comprising forming a plurality of access holes around a perimeter defining the cavity.

13. The method of claim 12 , further comprising sealing the plurality of access holes with plasma enhanced chemical vapor deposition (PECVD) silicon nitride.

14. A method of forming an ultrasound-on-chip device, the method comprising:

forming, in a complementary metal oxide semiconductor (CMOS) wafer, a cavity of an ultrasonic transducer and an acoustic membrane of the ultrasonic transducer, the CMOS wafer having integrated circuitry formed in a semiconductor substrate and a layer stack formed over the integrated circuitry, the layer stack comprising a plurality of dielectric layers and a plurality of metallization layers, wherein the forming the cavity further comprises:

at least partially removing a first metallization layer of the plurality of metallization layers through at least one access hole and sealing the at least one access hole while leaving the cavity unfilled, wherein the cavity is disposed between first and second electrodes of the ultrasonic transducer, and wherein the acoustic membrane comprises at least one of the plurality of dielectric layers and a second metallization layer of the plurality of metallization layers, with the second electrode being disposed in the acoustic membrane between the cavity and the second metallization layer; and

wherein the integrated circuitry is coupled to the ultrasonic transducer and configured to control operation of the ultrasonic transducer.

15. The method of claim 14 , wherein at least partially removing the first metallization layer further comprises:

Forming the at least one access hole to the first metallization layer; and

performing a selective etch to remove an inner metal layer of the first metallization layer.

16. The method of claim 15 , wherein the first and second electrodes comprise liner layers of the first metallization layer.

17. The method of claim 16 , wherein the inner metal layer comprises aluminum (Al), and the liner layers comprise titanium nitride (TiN).