Fabrication of capacitive micromechanical ultrasonic transducers by low-temperature process

This work is focused on the development of an innovative process for the realization of capacitive micromachined ultrasonic transducers (cMUTs) using surface micromachining on silicon with the possibility of integrating front-end electronic. We describe the processing steps and the materials used to obtain an entirely low-temperature (<510 °C) fabrication process. The structural SiOx layer and the SiNx membrane layer are deposited, respectively, by thermal evaporation and by plasma-enhanced chemical vapor deposition (PECVD). These techniques are compatible with the introduction of polyimide as the sacrificial layer. The 100% etch selectivity of the polyimide with respect to SiOx and SiNx allows the fabrication of membranes with a wide variety of shapes and air-gap dimensions. We have optimized thermal annealing treatment to control stress of the PECVD silicon nitride film to obtain optimal mechanical properties of membranes. Transducers have been characterized by electrical impedance analysis, showing resonance frequencies ranging from 4 to 6 MHz.