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 film prepared by thermal evaporation, and the influences of characteristics by isothermal annealing. 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 film. The results have shown that tin oxide thin films(as grown) have a linear pH sensitivity of approximately 58mV/pH in a concentration range between pH2 and pH10. However, pH sensitivity is decreasing after the isothermal annealing processes at 300 degrees C,400 degrees C and 500 degrees C in N-2 ambiance for one hour, and pH sensitivity is even only about 33mV/pH after annealing in Nz ambiance for fifteen hours. This phenomenon may be believed that the structure of tin oxide thin film, which is after isothermal annealing process, will result in the phase transition from amorphous to polycrystal. Moreover, the characteristics of the tin oxide gate ISFET(SnO2/SiO2 gate ISFET), which the tin oxide is formed by the optimum condition is also presented in this paper.