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

A niobium oxide (NbO_x) 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, NbO_x 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. 30 nm-thick NbO_x layer is suggested to have a lower drift coefficient after RTA in O₂ ambience compare to 5 nm-thick NbO_x layer. An acceptable pH sensing performance including a pH sensitivity of 57-60 mV/pH and a drift of -3 mV/h is achieved in the 30 nm-thick NbO_x-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 NbO_x 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.