Titanium oxide (TiOx) passivation layer was employed and optimized to stabilize the performance of the bottom gate amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). A molybdenum/titanium (Mo/Ti) source-drain electrode was deposited on an a-IGZO layer, and the TiOx passivation layer was formed by oxidizing the Ti layer using oxygen plasma after etching the Mo layer. By increasing the oxygen plasma treatment time, the subthreshold slope and leakage current of the a-IGZO TFTs were improved to 0.78 V decade(-1) and 0.3 pA, respectively, and the degradation of the TFT performance was not observed, even after thermal treatment at 280 degrees C for 1 h.