Polymer-based microfabrication approaches have been recently proposed as a low-cost alternative to traditional Capacitive Micromachined Ultrasonic Transducers (CMUT) fabrication methods. In most of the CMUT structures fabricated using such approaches, the electrodes passivation is achieved by a spin-coated polymer layer, with dielectric properties typically unsuitable to withstand the high in-cavity electrical fields. Moreover, typical layer thicknesses achievable by spin-coating bring to a significant increase of the effective gap height, inducing a very high collapse voltage and thus the need to use unpractically high operating voltages. In this paper, we investigate a process aimed at fabricating flexible CMUTs, potentially enabling high-performance, low-cost, curved, and ultra-miniaturized transducer configurations. In the proposed process, CMUT fabrication is carried out on an ultra-thin Polyimide substrate …