Evaluation of Capacitance Changes of Capacitive Micromachined Ultrasonic Transducers for Ultrasonic Diagnoses by Charge-Injection

Abstract

This research reports capacitance-voltage (CV) changes of 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 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. There is a problem that the CV changes of CMUT occur due to charge-injection into the insulator, since a high AC and DC voltages are applied for driving the vibrating film. Thus, it is necessary to suppress the change in CV characteristics of the CMUT due to charge-injection. It is clarified that the CV drift due to charge-injection can be decreased by forming the top electrode so as to cover the vibrating film. Also, the CV drift is able to be suppressed by setting the driving voltage to be equal to or less than the collapse voltage.