Impact of Annealing on Contact Formation and Stability of IGZO TFTs

Annealing processes were investigated on Indium-Gallium-Zinc-Oxide (IGZO) thin-film transistors (TFTs). Molybdenum and aluminum were used as contact metals which defined the working source/drain electrodes. Annealing was performed either pre-metal or post-metal deposition, in various gas ambient conditions including air, oxygen, nitrogen, forming gas (5% H2 in N2) and vacuum. Pre-metal annealing in air ambient resulted in similar I-V characteristics on Mo-contact and Al-contact devices. A post-metal anneal for Mo-contact devices resulted in higher on-state current and steeper subthreshold slope, whereas the Al-contact devices experienced severe degradation suggesting the formation of an AlOX interface layer. Oxidant exposure during anneal was vital for controlling the electronic properties of IGZO. A post-metal anneal at 400°C in N2 followed by an air ambient ramp-down yielded Mo-contact devices with channel mobility μsat ∼ 8.5 cm2/V·s and subthreshold swing SS ∼ 200 mV/dec. Electron-beam evaporated alumina was used for back-channel passivation which resulted in improved stability at the expense of slight degradation in device performance.