Cross-coupling in sealed cMUT arrays for immersion applications

Capacitive micromachined ultrasonic transducers (cMUTs) are becoming increasingly important as a future technology in imaging arrays, and are being proposed for applications such as biomedical ultrasound and high frequency sonar. This paper will present the results of a wide-ranging investigation into the nature and extent of mechanical cross-coupling in sealed cMUT arrays. In particular, the work has characterised the operation of small sealed multi-cavity devices specifically designed for immersion-based applications. An extensive modelling program has been undertaken using finite element via the PZFlex finite element code, in an attempt to understand the precise nature of cross-coupling within a single transducer element, comprising multiple sealed cMUT cavities. The results show that there is evidence of cross coupled signals propagated through the medium in the small sealed device, which increases under immersion applications. An additional waterproofing layer is then added to the cell for immersion purposes. The performance of the devices is compared for coatings including Parylene C and PDMS. The influence of this layer has been analysed theoretically, with the objective to minimise the influence on cMUT operation. The work is then extended to show experimental characterisation of such cMUT devices, configured in the form of 2D arrays. The cross-coupling present in the different array configurations is measured using a Polytec laser vibrometer.