Improvement of Stress Stability in Back Channel Etch-Type Amorphous In-Ga-Zn-O Thin Film Transistors with Post Process Annealing

The electronic structures and electronic states near the back channel surfaces in amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) were investigated. The X-ray photoelectron spectroscopy (XPS) analysis revealed that, due to insufficient wet-etching of source/drain (S/D) of Mo metal, continuous sub-gap states formed throughout the bandgap, which induced degradation of the subthreshold swing of the TFTs with a back channel etch (BCE) structure. The stability under the negative bias thermal illumination stress (NBTIS) in the a-IGZO BCE-type TFTs was worse than that of conventional etch stop layer (ESL)-type TFTs. The Vth shift (ΔVth) increased from −2.5 V to −4.0 V with a hump-shaped variation. According to photoinduced transient spectroscopy (PITS) analysis, a broader shoulder of around 160 K clearly appeared in PITS spectra from the a-IGZO thin films with the BCE structure. The effect of post process annealing applied as the final process step of the TFT fabrication (post-annealing) was also examined against the a-IGZO BCE-TFTs. As the post-annealing temperature was increased, the peak at around 160 K which is associated with the hydrogen-related traps in a-IGZO thin film was markedly decreased. The results for the post-annealing at 300°C suggested a decrease in density of trap states which originated from depletion of Zn. A marked improvement of the stress stabilities under the positive bias, thermal stress (PBTS) was also observed as well as NBTIS.