Progresses in cMUT device fabrication using low temperature processes

In this paper, we present an original fabrication process of capacitive micromachined ultrasonic transducers (cMUTs) using a low temperature method for high frequency medical imaging applications. The process, which is limited to 400 °C, is based on surface micromachining. The material choices are adapted in order to respect the thermal specifications allowing monolithic integration. Thus, we have found alternative methods to replace the usual high temperature steps in cMUT elaboration. In this way, a nickel silicide layer, presenting good physical and electrical characteristics, is used as a bottom electrode. The membrane, silicon nitride, is deposited using a 200 °C PECVD process. Then, a metallic layer is chosen as a sacrificial layer, in order to achieve the cavity. For that, nickel has been chosen due to its low roughness and its high etching selectivity during the excavation. After their fabrication, the transducers have been tested to verify their functionality and, thus, to validate this low temperature process. Device physical properties have been determined by electrical and optical measurement in air. We evaluated resonance frequency, collapse voltage and electromechanical coupling coefficient in accordance with the simulation. Eventually, low charging effects and low initial deflections can predict good long-term use and ageing of the cMUTs. (paper)