Curved micromachined ultrasonic transducer membranes

1. A method of forming an ultrasonic transducer device, the method comprising:

forming a curved membrane over a transducer cavity, wherein a center portion of the curved membrane is closer to a bottom surface of the transducer cavity than with respect to radially outwardly disposed portions of the curved membrane;

forming a curved sacrificial cavity material, over a transducer bottom electrode, by chemical mechanical polishing (CMP);

forming one or more membrane layers over the curved sacrificial cavity material and the transducer bottom electrode;

removing the sacrificial cavity material to form the curved membrane,

wherein forming the curved sacrificial cavity material by CMP includes:

forming a sacrificial cavity material layer and patterning the sacrificial cavity material layer over the transducer bottom electrode;

forming a dielectric layer over the patterned sacrificial cavity material layer;

patterning the dielectric layer disposed over the patterned sacrificial layer; and

performing the CMP on the patterned dielectric layer and patterned sacrificial cavity material layer to induce dishing of the patterned sacrificial cavity material layer such that more sacrificial cavity material layer is removed at a center location than with respect to outer locations.

2. The method of claim 1 , wherein

the patterned dielectric layer includes dielectric material that protrudes above a surface of the patterned sacrificial cavity material to induce dishing of the patterned sacrificial cavity material layer during CMP.

3. A method of forming an ultrasonic transducer device, the method comprising:

forming a curved membrane over a transducer cavity, wherein a center portion of the curved membrane is closer to a bottom surface of the transducer cavity than with respect to radially outwardly disposed portions of the curved membrane;

forming a curved sacrificial cavity material over a transducer bottom electrode, by grayscale lithography;

forming one or more membrane layers over the curved sacrificial cavity material and the transducer bottom electrode; and

removing the sacrificial cavity material to form the curved membrane,

wherein the grayscale lithography includes:

forming a sacrificial cavity material layer over the transducer bottom electrode;

forming a photoresist layer over the sacrificial cavity material layer;

performing spatially modulated patterning of the photoresist layer to create a curved profile in the patterned photoresist layer;

developing the patterned photoresist layer so as to form a photoresist structure having the curved profile above the transducer bottom electrode; and

etching the curved profile of the photoresist structure and the sacrificial cavity material to transfer the curved profile into the sacrificial cavity material layer.

4. The method of claim 3 , further comprising:

using a etch recipe selected for a low etch selectivity between the photoresist and the sacrificial cavity material layer.

5. A method of forming an ultrasonic transducer device, the method comprising:

forming a curved membrane over a transducer cavity, wherein a center portion of the curved membrane is closer to a bottom surface of the transducer cavity than with respect to radially outwardly disposed portions of the curved membrane,

wherein forming the curved membrane includes:

forming the transducer cavity on a CMOS wafer;

bonding a membrane wafer to the CMOS wafer to seal the transducer cavity; and

performing an anneal to collapse the bonded membrane wafer into the transducer cavity and induce a curvature in the membrane wafer.

6. The method of claim 5 , further comprising:

patterning layers in the membrane wafer to induce film stress that biases the membrane wafer to curve toward the transducer cavity.

7. The method of claim 5 , wherein

after bonding the membrane wafer, a gas is pressurized on an exterior of the transducer cavity such that the annealing is performed at a gas pressure that induces the membrane wafer to collapse into the transducer cavity.