Investigation of thermal stress influence on CMUT in standard CMOS process

Capacitive micromachined ultrasonic transducer (CMUT) fabricated in standard CMOS and post-CMOS processes undergoes a series of thermal processes at about 1000degC. Thermal stress is generated in the membrane during such process. This paper studies the most severe influence of thermal stress on the performance of CMUT, such as stiffness, collapse voltage, resonant frequency, transformer ratio, coupling coefficient, sensitivity, and bandwidth for the worst-case design, assuming no thermal stress relief by annealing. A membrane in the shape of square microbridge is modeled and some finite element analysis (FEA) is carried out using ANSYS 11.0. A convex membrane with the deformation up to 17% of the initial air gap is obtained in the worst case. Then the influence of thermal stress is investigated based on the comparison between ideal and deformed membrane.