Two-terminal CMUT device

1. An ultrasound system comprising:

an ultrasound array including at least one capacitive micromachined ultrasonic transducer device comprising a membrane coupled to a first electrode, a substrate opposing the membrane with a gas or vacuum cavity there between and coupled to a second and a third electrodes, wherein the second electrode opposes the first electrode in a peripheral region and the third electrode opposes the first electrode in a central region;

at least one drive circuit coupled to the array and adapted to (a) bring the membrane into a collapsed state in which the membrane is collapsed to the substrate in the central region, by applying a DC voltage over the first and the third electrodes of the at least one CMUT device, and (b) activate the CMUT device by applying an AC voltage having a CMUT operating frequency over the first and the second electrodes of the at least one said CMUT device; and

wherein

the third electrode of at least one capacitive micromachined ultrasonic transducer device of the array is electrically coupled to the second electrode via a high impedance resistor (R) having an impedance value higher than an AC impedance between the first and the second electrodes, when the membrane is in the collapsed state and the CMUT devices is activated at operating frequency.

2. The ultrasound system according to claim 1 , wherein the second electrodes and the third electrodes of two or more capacitive micromachined ultrasonic transducer devices in the array are coupled to a single high impedance resistor.

3. The ultrasound system according to claim 1 , wherein the high impedance resistor has an impedance value in between 10 kOhm and 5 MOhm or in particular in between 500 kOhm and 1 MOhm.

4. The ultrasound system according to claim 1 , wherein the drive circuit comprises a DC voltage source and an AC voltage source.

5. The ultrasound system according to claim 1 , wherein the high impedance resistor comprises a thin film layer having a thickness equal or less than 7 nm or in particular in between 2 nm and 4 nm.

6. The ultrasound system according to claim 5 wherein the thin film layer is TiN.

7. The ultrasound system according to claim 5 , wherein the thin film layer is deposited by an atomic layer deposition (ALD) technique.

8. A capacitive micromachined ultrasonic transducer device comprising a membrane coupled to a first electrode, a substrate opposing the membrane with a gas or vacuum cavity there between and coupled to a second and a third electrodes, wherein the second electrode opposes the first electrode in a peripheral region and the third electrode opposes the first electrode in a central region;

at least one drive circuit adapted to (a) bring the membrane into a collapsed state in which the membrane is collapsed to the substrate in the central region, by applying a DC voltage over the first and the third electrodes, and (b) activate the CMUT device by applying an AC voltage having a CMUT operating frequency over the first and the second electrodes; and

wherein the third electrode is electrically coupled to the second electrode via a high impedance resistor having an impedance value higher than an AC impedance between the first and the second electrodes, when the membrane is in the collapsed state and the CMUT device is activated at operating frequency.

9. A method for manufacturing a capacitive micromachined ultrasonic transducer device comprising:

providing a substrate;

providing a patterned electrode layer that defines a third and second electrodes (S 2 ) embedded in the substrate;

defining a gas or vacuum cavity;

providing a membrane opposing the substrate with the gap there between;

providing a patterned first electrode layer that defines a first electrode embedded in the membrane such that the second electrode opposes the first electrode and the third electrode opposes the first electrode in a central region;

providing an insulating layer that isolates the first electrode from the second and the third electrodes;

providing a thin film layer of a high impedance resistor by applying an atomic layer deposition technique;

electrically coupling the third electrode with the second electrode via the high impedance resistor.

10. The method for manufacturing the capacitive micromachined ultrasonic transducer device according to claim 9 , wherein the step of defining the gas or vacuum cavity comprises one of: a sacrificial layer release process and a wafer bonding process.

11. The method for manufacturing the capacitive micromachined ultrasonic transducer device according to claim 10 , wherein the high impedance layer comprises TiN layer.

12. The method for manufacturing the capacitive micromachined ultrasonic transducer device according to claim 10 , wherein the step of providing the insulating layer comprises depositing a high-k dielectric layer with the ALD technique.

13. The method for manufacturing the capacitive micromachined ultrasonic transducer device according to claim 9 , wherein all step of providing layers comprise the ALD technique.

14. The method for manufacturing the capacitive micromachined ultrasonic transducer device according to claim 9 , further comprising a step of adjusting of a thickness and a material selection of the insulating layer in order to vary a collapsed area capacitor.

15. The method for manufacturing the capacitive micromachined ultrasonic transducer device according to claim 14 , wherein the adjusting step causes an increase of the collapsed area capacitor via a reduction of the insulating layer thickness and a selection of a higher dielectric constant material.