Degradation and Self-recovery of Polycrystalline Silicon TFT CMOS Inverters under NBTI Stress

Degradation and self-recovery of polycrystalline Silicon (poly-Si) Thin film transistor (TFT) by using complementary metal oxide semiconductor (CMOS) inverter were investigated. Under DC stress, degradation mechanisms were clarified by comparing the Voltage transfer characteristics (VTC) of fresh and stressed inverters. It is determined that Negative bias temperature instability (NBTI) of p-TFT dominates the degradation of the inverter under zero bias DC stress. After removing the stress, the VTC continues to be degraded, because the interface trap-states and the grain boundary trap-states increase due to hydrogen species diffusion. It is found out that the VTC is shifted to its right side severely with negative bias stress of VIN. The NBTI of p-TFT is enhanced and the NBTI of n-TFT also plays a role on the degradation. When removing the negative bias stress, the self-recovery of NBTI of n-TFT and the continuing degradation of NBTI of p-TFT become competing mechanisms, together controlling the VTC after-stress behavior. Consequently, the continuing degradation of NBTI of p-TFT is restrained by self-recovery of NBTI of n-TFT.