Structural properties and sensing performances of CoNxOy ceramic films for EGFET pH sensors

Abstract

This paper describes the impact of both Ar/N2 flow rate and rapid thermal annealing (RTA) on the structural properties and sensing characteristics of CoNxOy ceramic films formed through reactive sputtering and then used in extended-gate field-effect transistor (EGFET) pH sensors. The CoNxOy ceramic films were prepared under three different Ar/N2 flow rates (20/5, 20/10, and 20/15) and then annealed at three different RTA temperatures (200, 300, and 400 °C). X-ray diffraction revealed that all of the CoNxOy ceramic films featured amorphous structures. X-ray photoelectron spectroscopy demonstrated that the Co3+ content of the CoNxOy film prepared at the 20/10 flow rate was higher than those of the films prepared at the other flow rates. Atomic force microscopy indicated that the surface roughnesses of the CoNxOy films obtained at flow rates of 20/10 and 20/15 were higher than that of the film prepared at 20/5. High pH-sensitivity (64.36 mV/pH), a small drift rate (0.27 mV/h), and a low hysteresis voltage (1.4 mV) were achieved for the CoNxOy EGFET sensor that had been fabricated at a flow rate of 20/10 with subsequent RTA at 300 °C. This high performance was due to its CoNxOy film having a rough surface, a high Co3+ ion content, and a low number of oxygen vacancies or defects.