Evaluation of Thin Films Composition of Capacitive Micromachined Ultrasonic Transducers for Ultrasonic Diagnoses

Abstract

This research reports thin films composition of vibrating film on capacitive micromachined ultrasonic transducers (CMUT) for ultrasonic diagnoses. The CMUT constitutes an electrostatically vibrating film, which is fabricated by deposition processes of SiO₂ and SiN films as insulator and TiN and Al films as top electrode on an Si substrate. The ultrasonic waves are transmitted by addition of AC voltage between the top electrode of vibrating film and the Si substrate, whereas the reflective ultrasonic wave are received with the vibrating film and converted to the electrical signals for ultrasonic diagnostic images. When the vibrating film is initially deformed by an internal stress of the stacked thin films, the transmitting strength of ultrasonic waves and the capacitance change by receiving reflective ultrasonic waves are not enough to achieve a high sensitivity. Thus, the thickness of SiO₂ and SiN films was analytically and experimentally determined to achieve the flat vibrating film. Accordingly, a 300-nm-thick SiO₂ film with a compressive stress of 150 MPa and a 1700-nm-thick SiN film with a tensile stress of 100 MPa were decided as one proper films composition. Also, the prototype CMUT can acquire an image of sponge simulated a test body.