Reverse electrical behavior of N-channel and P-channel LTPS-TFTs by N₂O plasma surface treatment

In this paper, the opposite impacts of N₂O plasma surface treatment on the n- and p-channel low-temperature poly-Si thin-film transistors (LTPS-TFTs) with HfO₂ gate dielectric are investigated. Significant performance improvement of n-channel LTPS-TFT by N₂O plasma surface treatment is observed, including threshold voltage reduction ΔV_TH ∼ -0.57V, 1.55× higher transconductance G_m , and 1.82× higher driving current I_drv. However, p-channel LTPS-TFT shows serious performance degradation after N₂O plasma surface treatment, including threshold voltage increase Δ V_TH ∼-0.99 V, transconductance G_m reduction, and driving current I_drv degradation. The difference of the two mechanisms of N₂O plasma surface treatment, namely, plasma induced interfacial layer (PIL) growth effect and trap passivation effect on poly-Si, is elaborated by a process of the PIL removal after the PIL growth. As a result, the trap passivation effect benefits both n- and p-channel LTPS-TFTs. The PIL growth effect shows the opposite impacts on the n- and p-channel LTPS-TFTs, that benefits n-channel LTPS-TFT and degrades p-channel LTPS-TFT seriously. Therefore, the negative impacts of N₂O plasma surface treatment on the p-channel LTPS-TFTs can be eliminated by a PIL removal step in the process. These results would be critical for the applications of system-on-panel and 3-D integrated circuit.