CMUTs for air coupled ultrasound with improved bandwidth

We present the design, fabrication and characterization of a multi-purpose capacitive micromachined ultrasonic transducer (CMUT) for air coupled ultrasound (~300-500 kHz), with a focus on improved bandwidth. The focus of this work is to investigate whether and how wafer-bonded CMUTs can be improved in terms of bandwidth for airborne ultrasound by intentionally varying the cell radii across the entire device. Further, we consider the differences in terms of static d.c. operation point for improved frequency response and predictability by using a sensitivity-weighted distribution of cell radii groups. The pitch-catch results from one of our designs, compared to a reference device (all cell radii identical), show that the -6 db fractional bandwidth (FBW) can be improved from 1% up to 2.5%. For many applications, in which the -14 db duration signal (ASTM E1065) plays a role, the improvement goes from 1% to 18.9%. The observed improvements support the argument that such CMUTs will be beneficial for ultrasonic transit-time detection based applications.