Efficiency of capacitive micromachined ultrasound transducers For large signal non-collapsed operation

Abstract

Although capacitive micromachined ultrasonic transducers (CMUTs) are mostly considered for imaging applications for their broad bandwidth, these devices can also prove useful for high intensity focused ultrasound (HIFU) applications for therapeutics. For these purposes, energy conversion efficiency is especially significant for high intensity and high duty cycle ultrasound applications. The usual small signal coupling coefficient analysis based on capacitance and resonance frequency measurement is not adequate for large signal application when nonlinearity comes to affect device performance. Therefore, an energy conversion ratio (ECR) based on a nonlinear large signal model has been proposed to analyze high power CMUT operation with and without DC bias (AC only). The results on a particular CMUT operating around 5 MHz show that AC only operation at half of device working frequency provides a higher level of pressure (0.8 dB more) as compared to DC biased case and can achieve 90% ECR. Since the input and output frequencies are not the same for AC only operation, Insertion loss (IL) is defined as the ratio of mechanical output power divided by the available electrical power in this case. With that definition, AC only and DC biased operation shows about the same IL in line with the ECR calculations.