Stability enhancement of polysilicon thin-film transistors using stacked plasma-enhanced chemical vapor deposited SiO2/SiNx gate dielectric

The use of a double-layer, plasma-enhanced chemical vapor deposited SiO2/SiNx film as a gate dielectric material for polysilicon thin-film transistors was investigated in order to reduce mobile ion contamination and to improve gate oxide integrity degradation. We observed that the interposed silicon nitride film between the gate electrode and the SiO2 gate insulator prevents the incorporation of mobile ions into the SiO2 film, and also increases the breakdown voltage of the gate-insulating film. The mobile ion densities for the double SiO2/SiNx and single SiO2 gate dielectrics (no interposed SiNx layer between the gate electrode and SiO2 gate insulator) were 1.3 x 10(11) and 1.7 x 10(12)/cm(2), respectively. The breakdown fields at the 50% failure points in the Weibull plots for the double and single dielectric cases were 8 and 5 MV/cm, respectively. We conclude that the silicon nitride layer of the double gate insulator film minimizes ion contamination, leading to the enhancement of breakdown characteristics.