Improvement of negative bias temperature instability of LTPS TFTs by high pressure H2O annealing

Abstract

In this study, high pressure H2O annealing (HPA) was performed to improve the reliability of low temperature poly-Si thin film transistors (LTPS TFTs) fabricated on a polyimide (PI) substrate. Due to the low heat resistance of the PI substrate, the pressure was high to reduce the temperature of the heat treatment. HPA was performed at 130 °C and 0.3 Mpa in an H2O ambient for 2 h. After HPA treatment, the threshold voltage (VT) shifted to the positive direction from −2.27 V to −1.75 V, and the hysteresis (ΔVT) decreased from 0.67 V to 0.24 V. The density of states (DOS) measurement shows that the interface trap decreased after HPA processing, hence, it is assumed that stability for bias stress has improved. Before and after HPA, the VT shift for negative bias temperature instability (NBTI) stress was 1.19 V and 0.2 V, respectively. After HPA treatment, ΔVT decreased significantly for bias stress. It seems that the decomposed oxygen from H2O diffused to the gate insulator and combined with Si to reduce the oxygen vacancy. The strength of the gate insulator improved after HPA.