Fabrication of multilayer 2D ultrasonic transducer microarrays by green machining

This paper presents a fabrication technique for multilayer 2D ultrasonic transducer arrays based on wet building and mechanical machining of piezoelectric PZT ceramics. The arrays are intended for microparticle trapping and manipulation using ultrasonic standing waves in microfluidic devices. The developed fabrication technique is relevant also for the miniaturization of other multilayer piezoceramic components. Array elements were fabricated monolithically on a common carrier with voids integrated behind each element, to work as acoustic reflectors. Their function was later proved by identification of thickness resonances of the supported multilayer transducers. The voids were integrated by using sacrificial material that was removed during firing of the component. Electrodes, vias, sacrificial material and finally the array itself were structured in the green state using a high precision dicing saw. The fabrication technique with integrated multilayer electrodes and electrical vias was shown to allow for miniaturized array elements working above 10 MHz with lateral dimensions down to 350 x 350 mum(2). The resolution of electrode patterning was below 10 mum with insulating spaces down to 30 mum. The smallest dimension of via interconnects was 20 mum and integrated voids were fabricated. with lateral dimensions reaching from 10 mum to 600 mum.