Optimizing CMUT geometry for high power

Capacitive micromachined ultrasonic transducers (CMUTs) have demonstrated various advantages over piezoelectric transducers. However, current CMUT designs produce low output pressures with high harmonic distortions. Optimizing the transducer parameters requires an iterative solution and is too time consuming using finite element (FEM) modelling tools. In this work, we present a method of designing high output pressure CMUTs with relatively low distortion. We analyze the behavior of a membrane under high voltage continuous wave operation using a nonlinear electrical circuit model. The radiation impedance of an array of CMUTs is accurately represented using an RLC circuit in the model. The maximum membrane swing without collapse is targeted in the transmit mode. Using SPICE simulation of the parametric circuit model, we design the CMUT cell with optimized parameters such as the membrane radius (a), thickness (t_m), insulator thickness (t_i) and gap height (t_g). The model also predicts the amount of second harmonic at the output. To verify the accuracy of the results, we built a FEM model with the same CMUT parameters. The design starts by choosing t_i for the given input voltage level. First, a is selected for the maximum radiation resistance of the array at the operating frequency. Second, t_m is found for the resonance at the input frequency. Third, t_g is chosen for the maximum membrane swing. Under this condition, a frequency shift in the resonant frequency occurs. Second and third steps are repeated until convergence. This method results in a CMUT array with a high output power and with low distortion.