Broadband ultrasound transducer

1. A transducer array, comprising a plurality of CMUT cells suitable for a collapsed operation mode, each CMUT cell comprising a first electrode supported by a substrate and a second electrode supported by a membrane suspended over a cavity between the first electrode and the second electrode, the plurality of CMUT cells comprising:

a first group of CMUT cells each having a membrane comprising a first layer stack; and

a second group of CMUT cells each having membrane comprising a second layer stack, the second layer stack including a layer of a material having a higher density than any of the layers in the first layer stack,

wherein the membranes of the first group of CMUT cells have a first spring constant and the membranes of the second group of CMUT cells have a second spring constant that is no more than 20% different to the first spring constant, and

wherein the plurality of CMUT cells are arranged in a staggered array comprising:

a first column of spaced CMUT cells on at least one silicon island;

a second column of spaced CMUT cells on at least one further silicon island, the second column being staggered in alignment with the first column such that cells of the second column are partially located in spaces between successive cells of the first column, the first column and the second column being spaced apart by a gap; and

a flexible foil retaining the respective silicon islands, the flexible foil comprising conductive interconnects.

2. The transducer array of claim 1 , wherein the second spring constant is no more than 10% different to the first spring constant.

3. The transducer array of claim 1 , wherein:

the first layer stack comprises a layer of a dielectric material to a first thickness; and

the second layer stack comprises a layer of the dielectric material to a second thickness that is smaller than the first thickness; or

the second layer stack comprises a layer of the dielectric material to the same thickness as the first layer stack and an additional layer comprising the layer of the material having a higher density than any of the layers in the first layer stack.

4. The transducer array of claim 1 , comprising a plurality of transducer elements, each transducer element comprising at least one CMUT cell of the first group and at least one CMUT cell of the second group, wherein each CMUT cell of a transducer element of the plurality of transducer elements is configured to be controlled in the collapsed operation mode based with a same bias voltage.

5. The transducer array of claim 1 , wherein the CMUT cells of the first group each have a first diameter and the CMUT cells of the second group each have a second diameter that is different to the first diameter.

6. The transducer array of claim 1 , wherein the layer of a material having a higher density than any of the layers in the first layer stack has a density in excess of 7 g/cm 3 , preferably a density in excess of 10 g/cm 3 and a Young's modulus of less than 200 GPa, preferably a Young's modulus of less than 100 GPa.

7. The transducer array of claim 1 , wherein the layer of a material having a higher density than any of the layers in the first layer stack is a metal layer or metal alloy layer, preferably wherein the metal layer is a gold layer or a platinum layer.

8. The transducer array of claim 1 , wherein the layer of a material having a higher density than any of the layers in the first layer stack is a patterned layer.

9. The transducer array of claim 1 , wherein the first group of CMUT cells and the second group of CMUT cells are each arranged at a constant pitch in the staggered array.

10. The transducer array of claim 1 , wherein:

the first column of spaced CMUT cells is located on a first silicon island having opposing meandering edges, each edge meandering outwardly around one of the CMUT cells and meandering inwardly into a space between the CMUT cells;

the second column of spaced CMUT cells is located on a second silicon island having opposing meandering edges, each edge meandering outwardly around one of the CMUT cells and meandering inwardly into a space between the CMUT cells; and

the first silicon island is arranged adjacent to the second silicon island such that an outwardly meandering edge portion of the first silicon island slots into an inwardly meandering edge portion of the second silicon island.

11. The transducer array of claim 1 , wherein:

the first group of CMUT cells is configured to exhibit a first resonance frequency in the collapsed operation mode,

the second group of CMUT cells is configured to exhibit a different, second resonance frequency in the collapsed operation mode,

the first group of CMUT cells and the second group of CMUT cells are configured to be controlled in the collapsed operation mode with a same bias voltage, and

the first group of CMUT cells and the second group of CMUT cells exhibit the different first and second resonance frequencies while being controlled in the collapsed operation mode with the same bias voltage.

12. A system comprising:

a transducer array comprising plurality of CMUT cells suitable for a collapsed operation mode, each CMUT cell comprising a first electrode supported by a substrate and a second electrode supported by a membrane suspended over a cavity between the first electrode and the second electrode, the plurality of CMUT cells comprising:

a first group of CMUT cells each having a membrane comprising a first layer stack; and

a second group of CMUT cells each having membrane comprising a second layer stack, the second layer stack including a layer of a material having a higher density than any of the layers in the first layer stack,

and

wherein the membranes of the first group of CMUT cells have a first spring constant and the membranes of the second group of CMUT cells have a second spring constant that is no more than 20% different to the first spring constant; and

a controller configured to control the transducer array,

wherein the plurality of CMUT cells are arranged in a staggered array comprising:

a first column of spaced CMUT cells on at least one silicon island;

a second column of spaced CMUT cells on at least one further silicon island, the second column being staggered in alignment with the first column such that cells of the second column are partially located in spaces between successive cells of the first column, the first column and the second column being spaced apart by a gap; and

a flexible foil retaining the respective silicon islands, the flexible foil comprising conductive interconnects.

13. The system of claim 12 , further comprising a power supply adapted to operate the CMUT cells in a collapse mode during at least one of an ultrasound transmission mode and an ultrasound reception mode.

14. The system of claim 12 , wherein the transducer array is included in a probe.