Back-End-of-Line-Compatible Scaled InGaZnO Transistors by Atomic Layer Deposition

In this work, we report on back-end-of-line (BEOL)-compatible InGaZnO indium gallium zinc oxide (IGZO) thin film transistors (TFTs) with extreme scaled device dimension including channel thickness ( ${T}_{\text {ch}}{)}$ down to 1.5 nm and channel length ( ${L}_{\text {ch}}{)}$ down to 60 nm. These IGZO channels with a high In atomic ratio of 92% were derived by atomic-layer-deposition (ALD), where the IGZO thickness could be precisely controlled by ALD cycles. These TFTs were subjected to a mild O2 annealing at 250 °C, the effect of which is also systematically investigated. It is found that both ${T}_{\text {ch}}$ and O2 annealing have significant effects on TFT performance. By using optimized O2 annealing conditions, the ALD IGZO TFTs with scaled ${T}_{\text {ch}}$ of 1.5 nm and ${L}_{\text {ch}}$ of 60 nm exhibit desirable electrical performance including a high ON/ OFF ratio ( ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}{)}~\sim ~10^{{11}}$ , a decent high Ion of 354 $\mu \text{A}/\mu \text{m}$ under ${V}_{\text {DS}}$ of 1.2 V, a steep subthreshold swing (SS) of 68 mV/dec, a small drain-induced-barrier-lowering (DIBL) of 30 mV/V, and a normal-off operation, which is comparable to the state-of-art sputtered IGZO TFTs. Furthermore, the optimized TFTs also exhibit significantly resolved threshold voltage ( ${V}_{\text {T}}{)}$ roll-off and a remarkably high degree of stability to the positive gate bias stress (PBS). A trap model with its possible microscopic origin is proposed, which explains well the dependence of electrical performance on both ${T}_{\text {ch}}$ and O2 annealing, thus providing a new insight into the reliability of IGZO TFTs.