Sensing and pH-imaging properties of niobium oxide prepared by rapid thermal annealing for electrolyte–insulator–semiconductor structure and light-addressable potentiometric sensor

Abstract

A niobium oxide (NbOx) layer is firstly investigated as a pH sensing membrane in single-layer light-addressable potentiometric sensor (LAPS) in this study. To confirm basic sensing performances, NbOx layer directly deposited on silicon substrate was prepared by reactive rf sputtering and following with post-deposition rapid thermal anneal (RTA) for electrolyte insulator semiconductor (EIS) structure first. RTA processed at 500 to 700°C can be used to make the increases of calculated pH sensitivity and linearity, especially performed in O2 ambience. Increases of pH sensitivity can result from the surface site density increases causing by the re-crystallization in RTA, which be supported by the larger grain size and surface roughness in AFM analysis. In this single-layer structure, desified layer with slightly higher atom concentration by RTA and partial Si out-diffusion to surface shown in SIMS analysis could be another factor for better pH sensing performance. 30nm-thick NbOx layer is suggested to have a lower drift coefficient after RTA in O2 ambience compare to 5nm-thick NbOx layer. An acceptable pH sensing performance including a pH sensitivity of 57–60mV/pH and a drift of −3mV/h is achieved in the 30nm-thick NbOx-EIS structure with RTA in O2 ambience at 500 to 700°C, which could be a candidate of pH sensing membrane. This developed process of NbOx sensing membrane is also applied to LAPS to collect a hydrogen-ion image for area defined by photolithography and buffer solution with red laser scanning controlled by X–Y stage.