Fabrication of capacitive ultrasonic transducers by a low temperature and fully surface-micromachined process

The fabrication of capacitive ultrasonic transducers by means of surface micromachining techniques and a low-temperature process is presented. Investigation of main process steps is reported. The use of polyimide as sacrificial layer, possible as the process is at low temperature, guarantees precise control of active transducer cells, thanks to its etching selectivity against the structural materials employed, and to the lithographic definition of the sacrificial layer into islands before the deposition of the membrane layer (pre-patterning). Control of the mechanical properties of free-standing membranes has been gained with the optimization of silicon nitride deposition and following thermal annealing steps. Transducers have been characterized by electrical impedance analysis, and their behavior agrees with theoretical modeling.