Impact of Annealing on Contact Formation and Stability of IGZO TFTs

Oxide semiconductors are candidate materials to replace amorphous silicon in thin-film transistors (TFTs) for LCD and OLED backplanes where the demand on performance is increasing. Indium-Gallium-Zinc-Oxide (IGZO) has shown promise because it exhibits high electron mobility and can utilize the existing fabrication infrastructure for flat-panel display manufacturing.  However the acceptance of IGZO requires exceptional uniformity and stability in device performance.  The necessity to anneal sputtered IGZO for improved device operation has been widely established; however the temperature, gas ambient, and process integration details vary. 

In this work, bottom-gate top-contact TFTs are fabricated using sputtered IGZO.  Molybdenum and aluminum were investigated as contact metals which defined the working source/drain electrodes.  Annealing processes applied either before or after metal deposition were investigated.  Annealing was done at 400 °C in various gas ambients including air, oxygen, nitrogen, forming gas (5% H₂ in N₂) and vacuum.  Channel passivation materials including SiO₂ and Al₂O₃ were also investigated.  The annealing ambient and arrangement of process steps were found to have a significant influence on the contact behavior and the uniformity and stability of electrical characteristics.  These details have a significant impact on circuit operation, which will be discussed along with results of test circuits including inverters and ring oscillators.