Structural characteristics and sensing capabilities of stacked Ti/Ni sensitive film for extended-gate field-effect transistor solid-state pH sensors

This study describes a straightforward approach for fabricating stacked Ti/Ni sensitive membranes on n⁺-type Si substrates using dc sputtering to build solid-state extended-gate field-effect transistor (EGFET) pH sensors. The effects of rapid thermal annealing (RTA) temperature (500–700 °C) and ambient (N₂ or O₂) on the structural characteristics of stacked Ti/Ni sensitive films were fully explored. We used X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, X-ray diffraction, atomic force microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy to characterize the chemical compositions, element profiles, film structures, surface morphologies, film microstructures, and film compositions of the stacked Ti/Ni sensitive films. The relative structural characteristics of the stacked Ti/Ni sensitive films have a significant impact on their sensing capabilities. The most effective sensing performance, such as pH sensitivity, drift, and hysteresis, was achieved at an RTA temperature of 600 °C and in the presence of N₂ gas among various RTA temperatures and annealing ambient. Due to its compact size and strong stability, the stacked Ti/Ni sensitive film shows great potential for use in future pH sensor and biosensor applications.