Analytical Expressions for the Electromechanical Coupling Coefficient of Capacitive Micromachined Ultrasonic Transducers (CMUTs) and Its Effects on Power Consumption

Capacitive micromachined ultrasonic transducers (CMUTs) are promising in applications such as portable ultrasonic imaging, ultrasonic therapy, ultrasonic diagnostic for homecare and ultrasound-based touchless human-machine interface. Development of CMUTs with low power consumption and high electromechanical coupling coefficient is the key to meeting the requirements from the aforementioned applications. The analytical expressions for the CMUTs with circular and square membranes based on the ratio of the fixed capacitance to free capacitance are established. The finite element simulation (FEM) and experiment testing on the fabricated CMUTs chips are carried out to validate the analytical expressions. Parametric studies using the proposed analytical expressions are done to study the effects of CMUTs parameters on the electromechanical coupling coefficient. The relationship between the power consumption of CMUTs and the bias voltage is established. The results show that the analytical expressions for the CMUTs with circular and square membranes can accurately predict the electromechanical coupling coefficients under the bias voltages lower than 96% of the corresponding collapsed voltages. The electromechanical coupling coefficients decrease with the increasing height of the CMUTs cavities, while increase with the increasing radius under the same bias voltages. CMUTs with different parameters have the same electromechanical coupling coefficient under the same ratios of bias voltages to collapse voltages. In addition, from the study on the relationship between the power consumption and electromechanical coupling coefficient, it can be concluded that the performance requirements for both low power consumption and high electromechanical coupling coefficient can be coordinately achieved through reducing the collapse voltages or increasing the electromechanical coupling coefficient under low bias voltages.