Impact of Passivation Conditions on Characteristics of Bottom-Gate IGZO Thin-Film Transistors

The electrical characteristics of bottom-gate amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) are reported. The a-IGZO TFTs without (w/o) a passivation layer have shown unstable electrical properties in air, such as negative shift of threshold voltage. This degradation is probably due to the IGZO environmental instability, especially in rich-oxygen and/or hydrogen environments. In this paper, the electrical behavior of TFTs passivated by various materials are presented. It has been observed that the passivation condition strongly affects device performance. The effect of passivation process, such as plasma-enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD) on TFTs characteristics is investigated. In both passivation cases, a negative shift of threshold voltage has been observed by increasing the silane (SiH₄) flow rate in the first case, or by increasing the Al₂O₃ thickness in the later. By analyzing these TFTs with time of flight secondary ion mass spectroscopy (ToF-SIMS), hydrogen was detected. It appears that there is a significant correlation between hydrogen and TFT electrical degradation. The mechanisms leading to this degradation and the solutions to eliminate it are proposed.