5I-6 Finite Element Analysis of Fabrication Related Thermal Effects in Capacitive Micromachined Ultrasonic Transducers

The accurate prediction of the static operation point of capacitive micromachined ultrasonic transducer (CMUT) membranes is essential for dynamic performance modeling, device design, and device fabrication. We evaluate whether fabrication-related stress effects in CMUT cells cause significant deviations observed between membrane deflection measurements and calculations. Our finite element analysis (FEA) considers the thermal structural interaction associated with the temperature stress history during device fabrication. Specific boundary conditions enable thermal expansion in all directions. Compared to conventional FEAs with one static solution step, we use seven static solution steps using the element birth and death feature of ANSYS to assemble the CMUT cell layer by layer at the appropriate deposition temperatures. We use measured static deflection profiles of metallized and non-metallized membranes with different diameters and thicknesses to extract all unknown parameters for the model. This approach improves the prediction of membrane deflections significantly, and it explains why present models often overestimate the collapse voltage