Characterization of silicon transducers with Si3N4sensing surfaces by an AFM and a PAB system

Silicion nitride is normally used as a sensitive surface for silicon-based transducers like ISFETs or light-addressable potentiometric sensors (LAPS), because of its capability to sense preferentially H+ions. We have investigated Si/SiO2/Si3N4heterostructures and studied the surface layers via an atomic force microscope (AFM) and a PAB (potentiometric alternating biosensor) system; in particular, the latter method allows the investigation of the temporal behaviour after input pH steps. A suitable experimental set-up has been installed in order to optimize the pH changes in the measuring chamber; data have been acquired corresponding to unit pH steps. The resulting time constants are calculated and the data related to the surface layer images acquired by the AFM. A correlation between different observed responses and the corresponding surfaces characteristics has been established.