(1) An improved method for operating CMUTs under high and varying pressure, and (2) Production of pre-charged CMUTs for zero-external-bias operation

Stanford researchers have developed two related inventions which advance the state-of-the-art of CMUT's (capacitive micromachined ultrasonic transducers).

The first part of the invention is a new operational mode for CMUTs, in which the plate is in permanent contact with the bottom of the cavity, even at zero DC bias and 1 atm pressure. This operational mode provides more stable resonant frequencies, and an improved coupling efficiency at lower DC bias voltages, in an environment with high and varying pressure (~1-20 atm).

The second part of the invention comprises a CMUT structure with a partial electrode, which surrounds a floating island. This structure is advantageous for both the proposed new operational mode with plate in permanent contact (reduces breakdown and improves device reliability; reduces the parasitic capacitance associated with the permanently contacting area, allowing improvement of the coupling efficiency at higher pressure) as well as the conventional operational mode ( the device can be pre-charged to realize dc bias-free operation). Bias-free CMUT's will be applicable to medical imaging, such as 3D/4D real-time ultrasonic imaging, intracardiac ultrasound imaging, and 3D photoacoustic functional imaging.

Stage of Research:

The CMUTs in the permanent contact operation mode has been successfully fabricated and validated using finite element analysis (FEA).

Devices have been characterized by measuring their electrical input impedance (1-20 atm) and displacement profile.

Pitch-catch operation has been demonstrated up to 20 atm pressure.

Some CMUTs in the conventional mode have been precharged to demonstrate bias free pitch catch. And the charge stays stable for > 1.5 years.

On-going Research:

The future work includes further simulation (FEA, transient) and characterization.