Pull-in Analysis of CMUT Elements

This paper presents a novel characterization method for the pull-in voltage of a Capacitive Micromachined Ultrasonic Transducer (CMUT) element. The presented method allows pull-in estimation of all CMUT cells contained in an element, which is in contrast to conventional methods that only allows for a pull-in average across an element. The methodology has been conducted on four different CMUT elements with a varying distance between the element separation and the CMUT cells closest to the edge. This distance was designed to be 3 µm, 5 µm, 10 µm, and 20 µm for the four CMUT elements, and the pull-in voltage of all the individual CMUT cells was determined using the presented methodology. The pull-in voltage of the CMUT cells at the edge of the element is observed to be significantly influenced as the distance to the element separation is lowered. The relative difference in the pull-in voltage between CMUT cells at the edge and the center are observed to be 14.8% and 2.8% for the designs with an edge distance of 3 µm and 5 µm, respectively. This edge effect is not observable for the two other designs where the relative difference is less than 0.5 %. Hence, this work demonstrates how the configuration CMUT cells can influence the pull-in voltage.