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

This paper describes the impact of both Ar/N₂ flow rate and rapid thermal annealing (RTA) on the structural properties and sensing characteristics of CoN_xO_y ceramic films formed through reactive sputtering and then used in extended-gate field-effect transistor (EGFET) pH sensors. The CoN_xO_y 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 CoN_xO_y ceramic films featured amorphous structures. X-ray photoelectron spectroscopy demonstrated that the Co³⁺ content of the CoN_xO_y 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 CoN_xO_y 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 CoN_xO_y 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 CoN_xO_y film having a rough surface, a high Co³⁺ ion content, and a low number of oxygen vacancies or defects.