Improvements towards a reliable fabrication process for cMUT

This paper reports the fabrication process for capacitive micromachined ultrasonic transducers (cMUT) using surface micromachining techniques. We describe the processing steps that allow control with high repeatability of the transducer geometrical features (membrane radius, membrane thickness, vertical distance between electrodes) and of the mechanical properties of the structural materials (stress in the silicon nitride layer). The transducers are fabricated with the fixed electrode patterned on top of the silicon wafer, which works as a support for the device, thus reducing parasitic capacitances. Tests of cMUTs with 0.5-μm-thick silicon nitride membranes suspended on 0.4-μm-thick cavities were performed showing resonance frequency at 5.5 MHz