Reliability Improvement of High Mobility Oxide TFTs Based on Hydrogen-Resistant PEALD Al₂O₃ Gate Insulators Grown with N₂O Plasma

High quality aluminum oxide (Al₂O₃) dielectric films were fabricated based on plasma-enhanced atomic layer deposition (PEALD) and applied as gate insulators in high mobility oxide thin film transistors (TFTs). N₂O plasma was used as the oxidizing reactant during the PEALD process, which resulted in the incorporation of nitrogen in the growing layers. The nitrogen content in Al₂O₃ could be adjusted by varying the N₂O plasma power between 100 and 250 W. An optimum power of 200 W was observed, at which a 3% improvement in the hard breakdown and a 90% reduction in the trap density could be achieved, as compared with Al₂O₃ grown at an N₂O plasma power of 100 W. However, as the power was increased up to 250 W, the film properties were compromised owing to the dominant plasma radiation damage. High mobility top gate oxide TFTs were next fabricated using indium-rich indium-gallium-zinc oxide (IGZO) as the active layer, and the PEALD grown Al₂O₃ films as the gate insulators. At an N₂O plasma power of 200 W, a peak field effect mobility of 53.45 cm²/(V s) and a threshold voltage (<i>V</i>_th) of -0.03 V were achieved. During positive bias temperature stress (PBTS), the devices exhibited only slight negative <i>V</i>_th shifts of less than 0.18 V as the N₂O plasma power was increased up to 200 W, which may be interpreted to be due to the improved hydrogen resistance of the Al₂O₃ film. The out-diffusion of hydrogen from the gate insulator is suppressed, and the retained hydrogen atoms are anticipated to diffuse into IGZO to generate free electrons during bias stress. This effect dominates the generally observed electron trapping phenomenon, which results in highly stable devices. To substantiate this hypothesis, the TFTs were annealed at 350 °C for 3 h in a hydrogen forming gas. The devices fabricated with Al₂O₃ at an N₂O plasma power of 200 W exhibited changes of 0.28 cm²/(V s) in field effect mobility and 0.04 V in <i>V</i>_th after the anneal process. This is indicative of a suppressed hydrogen diffusion from the ambient into the active layer, thus demonstrating the hydrogen resistance of the Al₂O₃ dielectrics under consideration.