Optimizing CMUT geometry for high power

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) have demonstratedvarious advantages over piezoelectric transducers. However, current CMUT designsproduce low output pressures with high harmonic distortions. Optimizing thetransducer parameters requires an iterative solution and is too time consumingusing finite element (FEM) modelling tools. In this work, we present a method ofdesigning high output pressure CMUTs with relatively low distortion. We analyzethe behavior of a membrane under high voltage continuous wave operation using anonlinear electrical circuit model. The radiation impedance of an array ofCMUTs is accurately represented using an RLC circuit in the model. The maximummembrane swing without collapse is targeted in the transmit mode. Using SPICEsimulation of the parametric circuit model, we design the CMUT cell withoptimized parameters such as the membrane radius (a), thickness (tm),insulator thickness (ti) and gap height (tg). The modelalso predicts the amount of second harmonic at the output. To verify theaccuracy of the results, we built a FEM model with the same CMUT parameters. Thedesign starts by choosing ti for the given input voltage level.First, a is selected for the maximum radiation resistance of the array at theoperating frequency. Second, tm is found for the resonance at theinput frequency. Third, tg is chosen for the maximum membrane swing.Under this condition, a frequency shift in the resonant frequency occurs. Secondand third steps are repeated until convergence. This method results in a CMUTarray with a high output power and with low distortion. © 2010 IEEE.