Optimization of the fabrication of sealed capacitive transducers using surface micromachining

Processing issues for the fabrication of capacitive micromachined ultrasonic transducer (cMUT) arrays have been studied using surface micromachining. This work focuses on the critical steps of process fabrication such as membrane formation, sacrificial layer properties and vacuum sealing performance of cavity. We describe a four-mask process for the realization of sealed cMUT. We demonstrate that the use of a sacrificial layer with a columnar structure gives a fast etching rate (29 nm s−1) in a buffered hydrofluoric acid solution. The mechanical stress of LPCVD silicon nitride (SiNx), used as membrane, was evaluated. We compared the vacuum sealing performance of different materials in order to find the best material in terms of lateral deposition inside the cavity. Functional transducers have been obtained. We proposed an electrical test in order to evaluate the vacuum sealing of the cavity based on the collapse voltage determination. Laser interference measurements were used to characterize the dynamic displacement of the membrane.