The influence of isothermal annealing on tin oxide thin film for pH-ISFET sensor

The aim of this paper is to investigate pH sensitivity of tin oxide thin films prepared by thermal evaporation, and the influences of isothermal annealing on its characteristics. A series of capacitance–voltage (C–V) curves of SnO2/SiO2/Si electrolyte insulator semiconductor (EIS) diodes are used to evaluate pH sensitivity of tin oxide thin films. The results show that tin oxide thin films (as grown) have linear pH sensitivities of approximately 58 mV/pH in a concentration range between pH 2 and pH 10. However, pH sensitivity decreases after the isothermal annealing processes at 300°C, 400°C, and 500°C in N2 ambiance for 1 h; and pH sensitivity goes further down to only 33 mV/pH, after annealing in N2 ambiance for 15 h. This phenomenon influences the structure of tin oxide thin films, which will undergo phase transition from amorphous to polycrystal after the isothermal annealing process. er, the characteristics of the tin oxide gate ion-sensitive field-effect transistor (ISFET) (SnO2/SiO2 gate ISFET), where the tin oxide is formed under optimum condition, is also presented in this paper.