Thermal-Mechanical Noise Modeling and Measurements of a Row-Column Addressed CMUT Probe

Thermal-Mechanical (T-M) noise is a natural phenomenon occurring in Capacitive Micromachined Ultrasonic Transducers (CMUT). T-M noise is a value of great interest because it is linked to the minimal detectable pressure of a transducer and can also serve as a convenient characterization tool. Indeed, the general behavior of a CMUT array is translated through its T-M noise which does not require any external applied source to be assessed. However, T-M noise is difficult to measure, often time requires a dedicated measurement chain and is mostly based on spectrum analyzers in a carefully controlled environment. In this paper, we present a temporal technique to characterize the T-M noise of CMUT-based arrays with a commercially available amplifier and a digital oscilloscope. The approach is applied to an air coupled Row-Column Addressed (RCA) matrix array, for which the elements cannot be measured with traditional micro-probes systems. This task is performed using a Printed Circuit Board (PCB) dedicated to the characterization of RCA arrays and designed to drive rows and columns individually. Noise Power Spectral Density (PSD) modeling of the complete measurement chain is achieved using a lumped-parameter model of the RCA array element and using the amplifier gain, electrical impedance, and noise characteristics. Measurements obtained with the signal analyzer and the temporal method are in good agreement with the model. The presented characterization technique can be extended to other micromachined ultrasonic transducer probe architectures, technologies, and amplification systems.