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 ambients including air, oxygen, nitrogen, forming gas (5% H-2 in N-2) 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. A post-metal anneal at 400 degrees C in N-2 followed by an air ambient ramp-down yielded Mo-contact devices with SS similar to 200 mV/dec, channel mobility mu(sat) similar to 8.5 cm(2)/V.s, and improved stability over other anneal conditions.