Droplets, vapours and clouds - A new approach to capacitive transducer manufacture

Capacitive ultrasonic transducers (cUT) comprise a substrate patterned with a regular array of uniformly dimensioned cavities above which a membrane is positioned. Transmission and reception of ultrasound is effected via controlled electrical or mechanical stimulus of the membrane, respectively. The dimensions of the cavity and the mechanical properties of the membrane determine the vibrational behavior of the resultant transducer. This paper employs a facile process for the micropatterning of polymer substrates for the manufacture of capacitive transducers. A positive mask of the desired cavity microstructure is deposited onto a polystyrene substrate. The substrate is then exposed to a saturated toluene vapour which is absorbed by the polystyrene causing swelling of the surface except in the areas where the droplets are situated resulting in the formation of micro-cavities at the position of each droplet. The PZFlex finite element (FE) software, deployed on custom cloud computing architecture allowing for 1000's of simultaneous parallel simulations, has been employed to explore the potential for the use of the micropatterned substrates in the manufacture of capacitive ultrasonic transducers. This new cloud approach has facilitated reducing months of intensive FE modelling to a few days. The results have been employed to guide the optimisation of the patterning process in order to manufacture devices suited to air-coupled non-destructive testing. The paper describes the results of the finite element modelling, the optimisation of the manufacturing route and the characterisation of the resultant devices.