6F-5 Characterization of Fabrication Related Gap-Height Variations in Capacitive Micromachined Ultrasonic Transducers

The gap-height directly affects the static operation point of the CMUT which in turn affects the receive sensitivity and the total possible output pressure. In this paper we report on the characterization of fabrication related gap-height variations in CMUTs. The CMUTs under investigation using the sacrificial release process were found to have lower collapse voltage, smaller maximal membrane deflection and missing pull-in behavior as compared to theory. The in-cavity deposition was examined by SEM and the surface topography on the bottom side of the gap by AFM. The AFM measurements were evaluated for devices with and without in-cavity-deposition as well as devices fabricated with doped polysilicon film or doped bulk silicon for the bottom electrode. A methodology to determine the contribution of each layer in the fabrication process to surface roughness is presented. Our results show that surface roughness reduces the actual gap-height, and the polysilicon layer is the main contributor.