Fabrication and Characterization of Capacitive Micromachined Ultrasonic Transducers Integrated on Ultra-thin and Flexible Substrates

• Published in: 2019 IEEE International Ultrasonics Symposium (IUS) pp. 778 - 780
• Main Authors: Lucarini, Ivano, Maiolo, Luca, Savoia, Alessandro S.
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.10.2019
• Subjects: CMUT; Copper; Electrodes; Flexible; Microfabrication; Plastics; Polyimide; SU-8 polymer; Surface treatment; Ultrasonic transducers
• ISSN: 1948-5727
• DOI: 10.1109/ULTSYM.2019.8926258

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 spun on a silicon wafer. The electrodes passivation is achieved by a thin SiO2 layer with excellent dielectric properties. A thin layer of SU-8 is used for the membrane fabrication and for sacrificial etch holes sealing. The devices are mechanically peeled-off from the wafer at the end of the process.