The Redistribution Layer-First Embedded Fan-Out Wafer Level Packaging for 2-D Ultrasonic Transducer Arrays

In this letter, the redistribution layer-first embedded fan-out wafer-level packaging (RDL-first FO-WLP) for 2-D ultrasonic transducer arrays with individual electrical connection is proposed. This process provides the capability of manufacturing thin film circuit with multilayer RDLs and allows signals of elements in arrays being fanned-out to a region larger than the array size. An anisotropic conductive film (ACF) with low curing temperature is employed to provide the electrical and mechanical connection between RDLs and ultrasonic transducer arrays. The thin film circuit with multilayer RDLs and ultrasonic transducer arrays were respectively fabricated and the curing temperature of ACF is close to half of the Curie temperature of the array. This process avoids ultrasonic transducer arrays losing their piezoelectric property, for ultrasonic transducer arrays are not suffering from the high curing temperature of the passivation layers and of the bonding process.