The Characteristics and Reliability With Channel Length Dependent on the Deposited Sequence of SiO₂ and Si₃N₄ as PV in LTPS TFTs

Abstract

This study investigates the characteristics on different channel lengths for a sequence of Si3N4 and SiO2 deposition as PV of LTPS TFTs. After analyzing the subthreshold swing (SS) of the initial condition and change in the $\Delta \text{V}_{\text{TH}}$ after NBTI and PBTI operations, a degradation mechanism is identified. When Si3N4 is deposited as the first layer of passivation (PV), hydrogen diffuses into the channel owing to activation or thermal annealing. As the channel length decreases, the hydrogen concentration increases at the center of the channel for devices with Si3N4 as the first layer of PV. Elevated hydrogen concentrations in the center of short channel devices lead to a debased SS. Moreover, the more positive fixed oxide charges create a more pronounced degradation after NBTI operation. On the other hand, PBTI performance shows a milder degradation with decreasing channel length due to fewer trapping charges. Finally, the hydrogen concentration is verified using SIMS. In summary, the heightened degradation of NBTI with device scaling is attributed to excess hydrogen on channel center during Si3N4 film deposition. The uneven hydrogen distribution also contributes the different SS and the different degradation after PBTI operation with different channel length.